# Jovian sodium nebula and Io plasma torus S+ and brightnesses 2017 – 2023:
insights into volcanic vs. sublimation supply

###### Abstract

We present first results derived from the largest collection of
contemporaneously recorded Jovian sodium nebula and Io plasma torus (IPT) in
[S II] 6731 Å images assembled to date. The data were recorded by the
Planetary Science Institute’s Io Input/Output observatory (IoIO) and provide
important context to Io geologic and atmospheric studies as well as the Juno
mission and supporting observations. Enhancements in the observed emission are
common, typically lasting 1 – 3 months, such that the average flux of material
from Io is determined by the enhancements, not any quiescent state. The
enhancements are not seen at periodicities associated with modulation in solar
insolation of Io’s surface, thus physical process(es) other than insolation-
driven sublimation must ultimately drive the bulk of Io’s atmospheric escape.
We suggest that geologic activity, likely involving volcanic plumes, drives
escape.

JGR: Space Physics

Planetary Science Institute 1700 East Fort Lowell, Suite 106 Tucson, AZ
85719-2395, USA Center for Space Physics Boston University Boston, MA 02155,
USA University Of Colorado, Boulder Boulder, CO 80309, USA

Jeffrey P<EMAIL_ADDRESS>
A large set of Jovian sodium nebula and Io plasma torus S+ images provides
context for Io and Jovian magnetospheric studies

Enhancements in Na and S+ emission last 1 – 3 months, ruling out insolation-
driven sublimation as their driver

Volcanic plumes likely play a key role in atmospheric escape

## Plain Language Summary

The Planetary Science Institute’s Io Input/Output observatory (IoIO) is
composed almost entirely of off-the-shelf parts popular with amateur
astronomers. IoIO uses special filters to isolate emission from two gasses
found around Jupiter: neutral sodium and ionized sulfur. The sodium is thrown
out from Io in a vast cloud called the Jovian sodium nebula. The ionized
sulfur collects into the Io plasma torus (IPT), a ring-shaped structure
centered around Jupiter that wobbles around Io’s orbital path. These gasses
ultimately come from Jupiter’s highly volcanic moon, Io. We see the Na nebula
and IPT brighten frequently. This demonstrates that the majority of the
material leaving Io comes from whatever drives the frequent brightening
events, with volcanic plumes likely playing a key role. Our results challenge
a widely held belief in the scientific community, that the majority of the
material in the Na nebula and IPT comes from Io’s tenuous global atmosphere,
which is fed by the sublimation of surface frosts and is relatively stable in
time. Our dataset also provides important context for NASA’s Juno mission and
supporting observations that focus on Io volcanism, the material’s likely
source, and Io’s magnetosphere, the material’s ultimate destination.

##  1 Introduction

One of the first hints that Io was somehow releasing material into the Jovian
magnetospheric environment in large amounts compared to the other Galilean
satellites came from spectroscopic observations of sodium D1 (5896 Å) and D2
(5890 Å) emissions that were time variable, broad, and in a ratio suggesting
optically thick gas [RA. Brown Chaffee (1974)]. kupo76 conducted spectroscopic
studies of S+ in the [S II] 6717 Å and 6731 Å doublet in the orbital plane of
Io and detected extended emission corotational with Jupiter. As Voyager I
approached Jupiter, Extreme ultraviolet (EUV) emission from S III, S IV, and O
III resolved into a torus-like structure encircling Jupiter, dubbed the Io
plasma torus [<]IPT;¿broadfoot79. The potential source of the material
escaping Io was first hinted at when Io itself was also seen to be
intermittently bright at a particular orbital phase angle in the 3 – 5 $\mu$m
region of the infrared spectrum, with volcanic activity being one of several
possible explanations offered [Witteborn . (1979)]. Volcanic activity on Io
was subsequently unambiguously confirmed by Voyager 1 images of plumes and
volcanic surface features [Morabito . (1979), Smith . (1979)]. This volcanism
gives rise, either directly or indirectly, to an SO2 dominated atmosphere
[Pearl . (1979), Kumar (1979), de Pater . (2021)] with minor constituents,
including NaCl [Lellouch . (2003), McGrath . (2004), Moullet . (2010), Redwing
. (2022)].

Io’s atmosphere undergoes several reactions with material in the IPT,
resulting in detectable effects. For majority species S and O, change
exchange, sputtering, and electron impact ionization are important processes
for removing atmospheric material [<]e.g.,¿mcgrath87, smyth88a, smyth88,
thomas04, schneider07, dols08, dols12, smith22, resulting in a roughly torus-
shaped neutral cloud confined to Io’s orbital plane and mapped in the EUV at O
I 1304 Å [Koga . (20181)]. IPT electron impact ionization of this neutral
cloud is the primary process by which the IPT receives new material, with
direct ionization in Io’s atmosphere providing only a minor component [Dols .
(2008), Dols . (2012)]. The canonical value of $\sim$1 ton s-1 of material
flowing into the IPT from Io has been estimated using the IPT’s total EUV
power output and a simple geometric model of the EUV emission region
[<]e.g.,¿broadfoot79, schneider07.

The path that sodium-bearing material takes as it escapes Io’s atmosphere is
different, thanks to the low ionization potential of any sodium-containing
molecule, NaX. For these molecules, impact ionization and charge exchange
processes are very efficient [<]e.g.,¿schneider07. Pickup NaX ions generated
in Io’s exosphere that promptly neutralize and dissociate create a directional
feature, the “jet,” that points radially outward from Io and flaps up and down
in synchrony with the IPT [Pilcher . (1984), Wilson Schneider (1999), Burger .
(1999), see also animations accompanying Figure 1]. A more extended structure,
dubbed the “stream,” has a similar radial morphology and behavior relative to
IPT modulation, but extends for several hours in Jovian local time downstream
of Io’s position. This comes from NaX+ ionized in Io’s exosphere by IPT plasma
and swept downstream in the plasma flow before they neutralize [Schneider .
(1991), Wilson Schneider (1994)]. IPT NaX+ ions that dissociate produce
neutral fragments that are ejected from the IPT at an average of $\sim$70 km
s-1, which is the Jovian corotational velocity at the IPT. This velocity is
above Jupiter’s escape velocity. Thus, neutral Na is detected at distances
$>$500 Jovian radii (Rj) from Jupiter [Mendillo . (1990)]. All these neutral
sodium emission features are known collectively as the Jovian sodium nebula
and are well-described by Monte Carlo modeling techniques [Wilson . (2002)].
The Jovian sodium nebula has been the subject of several long-term studies
using ground-based coronagraphic techniques [Mendillo . (2004), Yoneda .
(2009), Yoneda . (2015), Roth . (2020), Morgenthaler . (2019), and this work].

Except for the study presented here, long-term observations of the IPT from
ground-based observatories have been limited. The longest continuous study to
date covered a full Jovian opposition using a spectroscopic technique [ME.
Brown Bouchez (1997)]. Ground-based IPT imaging campaigns using coronagraphic
techniques have typically lasted a few weeks per opposition, though some have
extended over several oppositions [Schneider Trauger (1995), Woodward .
(2000), Nozawa . (2004), Kagitani . (2020)]. These ground-based observations
concentrate on the bright [S II] emissions of the 6717 Å, 6731 Å doublet,
which are excited by IPT thermal electrons. A significant amount of structure
is seen in high-resolution IPT images: a dense “ribbon” near Io’s orbit is
separated by a gap from the more disk-like “cold torus,” closer to Jupiter
[<]e.g.¿schneider95. There is evidence that diffusion proceeds inward from the
ribbon to the cold torus [Herbert . (2008)]. Long-term spectro-imaging
observations of EUV emission of the IPT have been conducted from Voyager,
Cassini, and Hisaki [Broadfoot . (1979), Steffl . (2004), Yoshioka . (2014)].
This emission, known as the “warm torus,” is more extended radially and
vertically than the ribbon. The EUV emissions are excited by suprathermal
electrons and have been used to study radial transport in the IPT, providing
evidence that the total residence time of material in the IPT is 20 – 80 days
[Bagenal Delamere (2011), Hess . (2011), Copper . (2016), Tsuchiya . (2018)].
Once material has left the IPT, it rapidly spirals outward in a few days
[Bagenal Delamere (2011)]. Thus, long-term monitoring of the IPT, such as that
presented here, provides critical context to any study of Jupiter’s broader
magnetosphere, such as that conducted by NASA’s Juno mission [Bolton . (2017)]
and supporting observations [Orton . (2020), Orton . (2022)].

In this work, we present the first results of a combined Jovian sodium nebula
and IPT monitoring campaign, conducted since March 2017 by the Planetary
Science Institute’s Io Input/Output observatory (IoIO). The coronagraphic
observations are described in §2 and §3 provides the methodology used to
reduce the data. Section 4 presents the primary results of our study, which is
a time history of the surface brightnesses of the Na nebula and IPT (Figure
3.3). This time history shows 1 – 2 brightness enhancements per 7-month
observing season, each lasting 1 – 3 months, such that emissions are seldom
found in a quiescent state. In §5 we compare our results to previous studies,
noting that although none of the previous workers reported such frequent
activity in the Na nebula and IPT, all are consistent with it. In §6, we use
the IoIO data to rule out solar insolation-driven sublimation of Io’s surface
frosts as the primary driver of material from Io’s atmosphere, showing instead
that geologic processes must be involved. We then review the existing evidence
that connects enhancements in material escape from Io’s atmosphere with
volcanic plume activity and discuss implications for the transport of
material. A summary and concluding remarks are provided in §7. In §8, we
suggest additional uses for the IoIO dataset, including providing support for
current and planned missions to Jupiter.

##  2 Observations

All observations presented here were conducted with the Planetary Science
Institute’s Io Input/Output observatory (IoIO). IoIO consists of a 35 cm
Celestron telescope feeding a custom-built coronagraph, described by
morgenthaler19. Since the publication of that work, both the observatory
hardware and control software have been upgraded, enabling fully robotic
acquisition of Jovian sodium nebula and IPT [S II] on- and off-band images,
regular photometric observations of burnashev85 spectrophotometric standard
stars in all filters, and observations of telluric sodium foreground emission.
Bias, dark and sky flat images are also periodically recorded. Since
2017-03-09, IoIO has contemporaneous recorded Na 5890 Å nebula and IPT [S II]
6731 Å observations on over 550 nights, with over 2300 Na images and over 8300
[S II] images collected. The observatory has been operated on another
$\sim$500 nights in support of other Planetary Science Institute projects
[<]e.g.,¿adams23DPS and pilot studies, increasing the number of
spectrophotometric calibrations and time coverage of telluric Na emission,
which provides a time-variable foreground emission [<]e.g.,¿plane18.

##  3 Data Reduction

###  3.1 General Considerations

All IoIO data are reduced pipeline-style using the software enumerated in the
Open Research section. The burnashev85 spectrophotometric observations show
each filter in our filter library provides stable zero points and extinction
coefficients over the length of our study, modulo random nightly variations
due to variation in atmospheric transparency, which we ignore in our current
analyses, using instead the biweight location [Tukey (1977)], of all the
measurements of each filter. The biweight location, $\zeta_{biloc}$, is
defined as:

$\zeta_{biloc}=M+\frac{\sum_{|u_{i}|<1}\
(x_{i}-M)(1-u_{i}^{2})^{2}}{\sum_{|u_{i}|<1}\ (1-u_{i}^{2})^{2}}$ (1)

where $x$ is the data, $M$ is the median of the data. The quantity $u_{i}$ is:

$u_{i}=\frac{(x_{i}-M)}{c*MAD}$ (2)

where $c$ is a tuning constant, set to 9 in our case, and $MAD$ is the median
absolute deviation:

$MAD=\mathrm{median}(|(x_{i}-\overline{x}|)$ (3)

The biweight location is a more robust statistical measure of the central
location of a distribution than the median, particularly for data not
distributed as a Gaussian [Beers . (1990)]. Surface brightnesses are expressed
in rayleighs (R), where:

$1~{}R={10^{6}\over 4\pi}~{}\mathrm{photons~{}cm^{-2}~{}sec^{-1}~{}sr^{-1}}$
(4)

Astrometric solutions of our images, together with high-quality JPL HORIZONS
ephemerides [Giorgini . (1996)] enable high-precision alignment of on- and
off-band images before subtraction of the off-band images. Subtraction of the
off-band images effectively removes Jupiter’s scattered continuum light from
the on-band images. When astrometric solutions using field stars fail, the
position of Jupiter on the coronagraph neutral density (ND) filter is used to
establish the astrometric center of the image, with the clock angle determined
by the previous successfully solved image. As expected from our stellar
calibrations, we found the ratio between our on- and off-band sky flats gave
stable results over the lifetime of the project. Thus, we used the biweight
location of all ratios to scale the off-band images before subtraction from
the on-band. Sample reduced images are shown in Figure 1.

Figure 1: Sample IoIO Na nebula (left column) and IPT [S II] 6731 Å (right
column) images. The top row shows images recorded just before the large 2018
enhancements in Na nebula and IPT emission (see Figure 3.3), the middle row
images recorded near the 2018 Na nebula and IPT peaks in emission and the
bottom row images recorded near the 2022 peaks. Animations are provided in the
on-line journal.

We note that our calibration procedure is a significant improvement over the
technique used by morgenthaler19, which relied on the image of Jupiter through
the ND filter for surface brightness calibration. As discovered after the
installation of a larger and filter wheel in 2019, Jupiter’s detected
brightness is subject to an unexpected Fabry Pérot effect between the narrow-
band and ND filters, with each narrow-band filter providing a different
magnitude of effect. Our current procedure avoids this issue by using the
stellar and flat-field calibrations described above.

In order to establish a time-sequence of the Na nebula and IPT brightnesses,
we first rotate the images reduced by the procedure above into the plane of
the IPT centrifugal equator using the relation:

$\alpha=-A\times\ cos(\lambda_{\mathrm{III}}-P)$ (5)

where $\alpha$ is the angle between the Jovian rotational axis and the
perpendicular to the IPT centrifugal equator, $\lambda_{\mathrm{III}}$ is the
sub-observer System III longitude, $A$ is the amplitude of the oscillation of
the centrifugal equator and $P$ is the $\lambda_{\mathrm{III}}$ longitude of
the intersection of the magnetic and equatorial planes. For this work, we used
$A$ = 6.8∘ [Moirano . (2021)] and $P$ = 290.8∘ [Connerney . (1998)]. Values of
$A$ = 6.3∘ [Phipps Bagenal (2021)] and $P$ = 286.61∘ [Connerney . (2018)]
could also be used, and would result in trivial differences in our extracted
surface brightnesses.

###  3.2 Na Nebula

As shown by previous work (§1) and the Na nebula animations accompanying
Figure 1 in the online journal, the bulk of the bright jet and stream emission
follow Io in its 42 hour orbit and flap up and down with each 9.925 hr Jovian
rotation. To minimize the effects of this high variability when extracting
surface brightnesses from individual Na nebula images, we rotate each image by
$\alpha$, as described above, and divide the resulting image into horizontal
apertures distributed vertically from the IPT centrifugal plane, as shown in
Figure 2. The ND filter and area beyond the edge of the narrow-band filters
are masked, as are pixels with values above the non-linear point of the CCD,
with a larger mask area applied around Galilean satellites. The average
surface brightness in each aperture is calculated by totaling the individual
surface brightnesses of the unmasked pixels and dividing by the total number
of unmasked pixels. The final surface brightness for a given distance from the
IPT centrifugal plane is the average of the surface brightnesses of the pair
of apertures located at that distance above and below the plane.

Figure 2: Sample Na nebula image illustrating reduction steps described in
§3.2. The blue lines indicate boundaries between apertures used to extract
surface brightness values as a function of vertical distance from the IPT
centrifugal plane. The boundaries between apertures are defined by the
following vertical distances from the IPT centrifugal plane: 4 Rj – 8 Rj, 8 Rj
– 16 Rj, and 16 Rj – 32 Rj, with the average distances from the plane of each
pair of apertures used for subsequent identification (e.g., see legend of
Figure 3.3, top). Masked areas are shown in white.

####  3.2.1 Removal of Telluric Sodium Contamination

Telluric sodium emission provides a time-varying and, at times, substantial
field-filling component to our Na nebula images. We attempted to remove this
emission using an empirical model constructed from our multi-year dataset of
telluric sodium emission observations. The model accounted for airmass
effects, solar scattering angle, and seasonal effects. However, after
subtraction of the model, the time sequence of Na nebula surface brightnesses
was still quite noisy. Thus, we instead subtract the average surface
brightness of emission $>$32 Rj above and below the centrifugal plane from the
extracted surface brightnesses of each image. As a result, the variation
induced by telluric emission was greatly diminished. A final step in the Na
nebula reduction is to compute the biweight location of all of the
measurements at each distance on each night. The results are plotted together
with 21-point moving median filter in Figure 3.3 (top).

A byproduct of our telluric sodium removal technique is to induce an
intensity-dependent error of the order $\sim$ 5 R – $\sim$ 25 R in our quoted
Na nebula surface brightnesses. This is because our telluric removal procedure
effectively assumes that the brightness of the Na nebula is zero at the edge
of the IoIO FOV. However, as shown by larger-field images [Mendillo . (2004),
Yoneda . (2009), Yoneda . (2015)], the emission $>$30 Rj above and below
Jupiter’s equatorial plane varies from $<$ 5 R to $\sim$ 25 R, depending on
whether or not the nebula is enhanced. This effect could be corrected using a
model of the Na nebula emission, however, doing so would not affect the
results of our current study.

###  3.3 IPT

As shown by comparing Figure 1 (lower right) to the IPT image in Figure 3,
rotating by Equation 5 provides a natural coordinate system for extracting
brightness values of the ansas (edges, Latin: “handles”) of the IPT. As
described in §1, the [S II] 6731 Å ansas primarily capture the IPT ribbon
emission. We extract the average surface brightness from each ribbon feature
using the two-step process shown graphically in Figure 3. Specifically,
starting from the rotated image, we define ansa extraction regions that extend
radially 4.75 Rj to 6.75 Rj from Jupiter and $\pm$1.25 Rj above and below the
IPT centrifugal equator (white boxes). Radial profiles of the emission in the
white boxes are shown in the bottom row. These profiles are generally well-fit
by a Gaussian plus sloping continuum of the form:

$f(x)=Ae^{-{(x-x_{0})^{2}}\over{2\sigma^{2}}}+P_{0}+P_{1}x$ (6)

where $A$ is the peak surface brightness of the Gaussian component of the
radial profile, $x_{0}$ is the ribbon radial distance from Jupiter, $\sigma$
is the width of the ribbon, and $P_{0}$ and $P_{1}$ are the coefficients of
the linear background. The $\pm$1-$\sigma$ limits of these Gaussians are used
to define the radial limits of the region used to extract vertical profiles,
shown in the top row, outside plots. Equation 6, with $P_{1}$ = 0, is used to
fit the vertical profiles. The Gaussian component of this function is
integrated to arrive at an average ribbon intensity of $A\sigma\sqrt{2\pi}$.
This is converted to an average surface brightness by dividing by $\sigma$.
Occasionally, the data are of high enough quality and the torus configured
such that cold torus is resolved. This is the case for the dawn (left) ansa
and results in a small peak inside of the ribbon. We ignore the effect of this
feature on our fits, since the simple sloping continuum plus Gaussian provides
an adequate a foundation for determining the region over which to extract
vertical profiles. As shown in the Figure, the vertical profiles are well-
described by Equation 6, with $P_{1}$ = 0. The total area of the Gaussian
components of each fit is then used to establish the average surface
brightness of each ribbon. If an extraction area contains saturated pixels
from any nearby Galilean satellite, it is excluded from the analysis. Fits
that result in ribbon peak positions outside of the range 5.0 Rj – 6.5 Rj or
peak widths outside the range 0.1 Rj – 0.5 Rj are discarded. In this way, our
extractions are able to adjust for varying observing conditions and the
intrinsic variability in the IPT ansa morphology [<]e.g.,¿schneider95 and
reliably discard pathological cases. The time history of the average ribbon
surface brightnesses, together with a 21-point running median filter of the
dusk ribbon points are plotted in Figure 3.3 (bottom). On timescales of weeks
to months, all other parameters of the fits roughly scale with ribbon surface
brightness, except for the radial peak positions of the ribbon. This behavior
is expected because all of the parameters of the fits, except the radial peak
positions, are sensitive to the total amount of material in the IPT, whereas
the radial positions of the ribbon are determined by physical effects outside
of the IPT [<]e.g.,¿[see also §8]barbosa83, ip83.

Figure 3: Graphical depiction of [S II] ribbon surface brightness extraction
process described in §3.3. An IPT image rotated into the reference frame of
the IPT centrifugal equator is shown in the top, middle panel. Radial profiles
of the ansas are shown in the bottom row, fit by Equation 6. The 1-$\sigma$
limits indicated on these plots define the edges of regions between which the
vertical profiles are computed (top row, outer plots). The average surface
brightness of each ribbon is the integral of the Gaussian component of the fit
of the corresponding vertical profile.

We note that, as derived, the absolute values of the dawn ribbon surface
brightness values shown in Figure 3.3 (bottom) are artificially low because,
at the blueshift of the dawn ribbon, IPT emission falls outside of the central
bandpass of the [S II] 6731 Å filter as measured in a collimated beam. This
effect can be corrected using a velocity-dependent IPT map, the [S II] filter
transmission curve, and consideration of the effects of the telescope’s F/11
light paths on the filter’s total transmission. However, making these
corrections would not affect the results of our current study.

Top: Time sequence of surface brightnesses in Jovian sodium nebula at three
distances from the plasma torus equatorial plane. Bottom: Time sequence of IPT
ribbon average surface brightnesses. A running median filter, 21 points wide,
is applied to each Na aperture and the dusk ribbon brightnesses.

##  4 Results

We anticipate the IoIO data will be very useful for correlative studies for
observations focusing on Io and the effect that material escaping Io has on
Jupiter’s magnetosphere, such as afforded by NASA’s Juno mission [Bolton .
(2017)] and supporting observations [Orton . (2020), Orton . (2022)]. To that
effect, the surface brightness points shown in Figure 3.3 have been archived
at Zenodo [Morgenthaler . (2023)]. In this paper, we focus on what the data
themselves can say about the physical processes that drive material escape
from Io’s atmosphere.

Our 6-year time sequence of Jovian Na nebula and IPT [S II] 6731 Å ribbon
brightnesses (Figure 3.3) shows considerable modulation in each emission line
as a function of time. During each $\sim$7-month observing window, at least 1
– 2 enhancements, each lasting 1 – 3 months are seen. Very little time is
spent in a quiescent state. Visual inspection of Figure 3.3 reveals that the
average values of the Na nebula and IPT surface brightnesses are determined by
the enhancements, rather than any quiescent value. To quantify this finding we
compute the tukey77 biweight distribution (Equation 1) of the measurements
presented in Figure 3.3. Recall from §3.1 that the biweight distribution is a
more robust statistical measure of the central location of a distribution than
the median or average. The biweight distribution values of the Na nebula
points are 80 R, 50 R and 15 R in the 6 Rj, 12 Rj, and 24 Rj apertures,
respectively. Compare this to low values of approximately 30 R, 20 R, and 5 R.
Similarly, the biweight distributions of the ribbon brightnesses are 50 R and
90 R for dawn and dusk, respectively, with minima of 15 R and 30 R.

Visual inspection of Figure 3.3 also shows a quasi-contemporaneous
relationship between the Na nebula and IPT enhancements. For instance, the
relative timing between the peaks of the 2018 Na nebula and IPT enhancements
is significantly different than that seen in 2020. And the fall 2022 Na nebula
enhancement is particularly bright compared to other Na nebula enhancements,
yet the IPT enhancement during that time period is particularly weak compared
to other years. This type of behavior has not been reported before.

We discuss the implications of our results in §6. But first, we compare our
results to those of previous studies, which provide valuable context to our
discussions.

##  5 Comparison to previous studies

IoIO occupies a unique niche in sensitivity, which ideally suits it to study
of the modulation of material flow from Io into the broader Jovian
magnetosphere. The 35 cm telescope aperture of IoIO was chosen to be
comparable to the smallest apertures that have successfully imaged the IPT
[Nozawa . (2004)]. This has allowed us to reliably capture, at modest cost, a
6-year history of the modulation in the IPT [S II] 6731 Å ribbon brightnesses
(presented here) and positions (to be presented in a subsequent work). Our
equipment choice limited the FOV of the instrument to 0.4∘, which is much
smaller than the 2.5∘ – 7∘ FOV of long-term previous coronagraphic Na nebula
studies [Mendillo . (2004), Yoneda . (2009), Yoneda . (2015), Roth . (2020)].
However, the narrower FOV of IoIO affords it much greater sensitivity to
emission close to Io, as evident by comparing the left columns of our Figure 1
to Figure 1 of mendillo04 and Figures 2 of yoneda09, yoneda15. This feature of
the IoIO Na nebula observations will allow us to conduct detailed
morphological studies of the jet and stream in future work.

###  5.1 Sodium-related studies

The outer portions of the IoIO FOV overlap with the inner portions of the
images recorded by other wide-field Na nebula studies [Mendillo . (2004),
Yoneda . (2009), Yoneda . (2015), Roth . (2020)], allowing direct comparison.
For instance, the peak intensity of the fall 2022 Na nebula enhancement
detected by IoIO roughly compares to the peak intensities in the 2007 and 2015
enhancements captured by yoneda09, yoneda15. The roth20 study is useful, since
it provides a time-history of Na nebula brightnesses measured with the same
coronagraph used in the yoneda15 work for a 4-month interval in 2017 during
which no enhancement was reported. Nevertheless, modulation at the $\sim$10 R
level in daily values ($\sim$5 R in the half-month averages) is seen. This is
comparable to the variation seen in the IoIO dataset during the 2018
enhancement. The greater sensitivity of the IoIO coronagraph to emission
closer to Jupiter makes variation of this magnitude much easier to detect.
This implies that periods formerly identified as quiescent in the yoneda15
dataset may, in fact, contain enhancements. By that interpretation, the period
highlighted in the roth20 appears to be capturing the low point between two
enhancements.

Spectroscopic observations conducted at the Lick observatory over the entire
1995 Jovian opposition [ME. Brown (1994), ME. Brown Bouchez (1997)] are also
useful for comparison. This work captured an enhancement in both the Na 5890 Å
doublet and [S II] 6731 Å (see also Sections 5.2 – 5.3). The 10′′ spectrograph
slit was aligned along the centrifugal equator, with peak emission averaged
along the slit reaching levels of 400 R – 800 R. In order to compare to our
data, we extend the aperture extraction procedure outlined in §3.2 using
apertures progressively closer to the centrifugal plane following the same
geometric sequence. We stopped decreasing the aperture size when the emission
brightness increased by $<$10%. The resulting aperture extended 0.5 Rj above
and below the centrifugal plane and resulted in peak brightnesses of 200 R –
300 R during the years 2017 – 2021 and 630 R in 2022. This suggests that the
1995 Na enhancement captured by brown97 was comparable in size to the 2022
enhancement shown in Figure 3.3 (upper panel).

Also important to mention are the Galileo dust detector measurements acquired
1996 – 2003 [Krüger, Geissler . (2003)]. There is evidence that the dust comes
from Io [Graps . (2000), Krüger, Horányi Grün (2003)], is composed almost
entirely of NaCl [Postberg . (2006)] and has its origin from Io volcanic
plumes [Krüger, Geissler . (2003)]. Further evidence shows that NaCl+ is an
important pathway for Na escape from Io’s atmosphere [Grava . (2014), Schmidt
. (2023)]. This suggests that variation seen in our Na nebula dataset and
others should be echoed in the Galileo dust detector data. krueger03 used a
simple geometric model of dust emission from Io to translate dust detector
count rates into the flux of dust from Io (their Figure 2). As discussed by
krueger03, the Galileo orbit precluded continuous measurements of the dust
streams before mid 2000. However, beginning after this time, there was a
large, well-covered enhancement that lasted $\sim$6 months. Subsequent
enhancements in the calculated Io dust flux last $\sim$1 – $\sim$3 months and
have smaller amplitudes than the 2000 enhancement. The magnitudes of the
enhancements are 1 – 4 orders of magnitude, which is much larger than those
seen in sodium nebula data. Full treatment of the reasons for the difference
in magnitude seen between the different measurement methods is beyond the
scope of this work. Rather, we point out that the durations of the
enhancements in the derived dust flux from Io is comparable to those observed
in the Jovian sodium nebula [ME. Brown Bouchez (1997), Yoneda . (2009), Yoneda
. (2015), and this work].

###  5.2 IPT studies

Previous studies of IPT [S II] 6731 Å emission show peak ribbon brightness
values in the $\sim$100 R – $\sim$1000 R range in individual measurements
[<]e.g.,¿morgan85a, oliversen91, jockers92, woodward94, schneider95, thomas01,
nozawa04_no_SII, yoneda09, schmidt18. As described in §3.3, the values shown
in Figure 3.3 are the average surface brightness of each ribbon derived using
a two-step Gaussian fitting procedure, with one Gaussian used to isolate
emission in the radial direction and one to compute the average surface
brightness in the vertical. To convert from averages over the Gaussian
functions to peak values, we multiply by $2\pi$, one factor of $\sqrt{2\pi}$
for the integral over the vertical Gaussian and another factor of
$\sim\sqrt{2\pi}$ to account for the summation between the $\pm 1\sigma$
limits of the radial Gaussian. Following our 21-point moving averages, this
yields peak values of $\sim$200 R – $\sim$900 R, with individual points
ranging from $\sim$50 R to $\sim$1200 R. We take this to be good agreement
with previous studies and therefore independent validation of our stellar
calibration procedure.

The only other published study of IPT [S II] 6731 Å emission lasting more than
a few weeks during a single Jovian opposition is the companion of the
spectroscopic Na nebula observations collected in 1995 at the Lick
observatory, discussed in §5.1 [ME. Brown (1994), ME. Brown Bouchez (1997)].
That study captured an IPT enhancement that lasted $\sim$2.5 months. The
emission pre- and post enhancement was $\sim$200 R and the emission was
$\sim$400 R at its peak. The factor of $\sim$2 difference between the pre/post
and peak values is comparable to the broad enhancement partially captured at
the beginning of the 2019 Jovian opposition. In other words, within the range
observed over our 6-year study.

Also useful for comparison are the two long-term studies of the warm torus
that have been conducted in the EUV, one by Cassini and one by Hisaki
[<]e.g.,¿steffl04a, yoshikawa17. Comparison of the surface brightnesses seen
in the EUV warm torus observations to the surface brightness of the [S II]
6731 Å observations of the ribbon region would require detailed IPT modeling
that is beyond the scope of this work. Thus, we limit our discussion to the
duration of the enhancements. During the duration of its observations, Cassini
captured two enhancements in emissions from ionization states of S and O
lasting of order 1 – 3 months, one in late 2000, the other in early 2001.
During its multi-year observing campaign Hisaki saw one large enhancement that
lasted $\sim$3 months in 2015. Smaller amplitude modulations in the Hisaki
data have also been noted [Roth . (2020)]. These enhancement durations are
comparable to those seen in the IoIO data.

###  5.3 Contemporaneous Na nebula and IPT studies

Two previous studies reported contemporaneous Na nebula and IPT enhancements
[ME. Brown Bouchez (1997), Tsuchiya . (2018)]. These studies, and related
work, concentrated on the detailed behavior of the observed emission during
the enhancements and the implications for physical processes occurring within
the IPT and broader Jovian magnetosphere [<]e.g.,¿yoshikawa17, kimura18,
hikida18, yoshioka18, tsuchiya18, hikida20, roth20, tao16a, tao16b, tao18,
tao21. Such in-depth study of individual enhancements is beyond the scope of
our current work. Rather, we note for comparison to our data, that for both
the 1995 and 2015 enhancements, there was delay of $\sim$4 weeks between the
peak in the Na nebula and IPT S+ emissions, even though in the 2015 case, the
S+ emissions were from the ribbon region and detected via the [S II] 6731 Å
line and in the 2015 case, the emissions were from the warm torus and detected
via the S II 765 Å line. Because different regions of the torus were studied
in the two cases, comparison of the relative strengths of the IPT enhancements
requires modeling that is beyond the scope of this effort. Thus, we are not
currently able to use these studies to corroborate our observation that the
relative strengths of the Na nebula and S+ enhancements can vary significantly
with time.

##  6 Discussion

Our study has a unique combination of sensitivity, cadence and duration that
has enabled it to determine that the Na nebula and IPT are frequently in
states of enhancement and that the enhancements in the two species have a
quasi-contemporaneous relationship. When interpreted within the context of
previous studies, the former result allows us to rule out solar insolation-
driven sublimation as the primary mechanism driving Io atmospheric escape; the
later provides insights into the most likely mechanism – volcanism – and the
subsequent path sodium- and sulfur-containing materials take through and out
of Io’s atmosphere. Our discussion begins with atmospheric escape.

###  6.1 Response of sodium nebula and IPT to Io atmospheric escape

As reviewed in §1, Io’s atmosphere is removed by interaction with the IPT via
charge exchange, sputtering, and electron impact ionization to fill the
neutral clouds on timescales of hours [<]e.g.,¿smyth88, dols08, dols12,
smith22. The apertures used to extract Na nebula surface brightnesses (Figure
2) are primarily filled by sodium traveling near IPT’s $\sim$70 km s-1
corotation velocity. The residence time of this material in the IoIO FOV is
$\sim$11 hours. Furthermore, we have chosen the apertures that integrate over
the effect of Jupiter’s $\sim$10 hr rotation period and Io’s $\sim$40 hr
orbit. Thus, to the accuracy of $\sim$1 day, the Na nebula surface
brightnesses shown in Figure 3.3 (top panel) provide a good indicator of the
modulations in the escape rate of sodium-bearing material from Io’s
atmosphere.

The response of the IPT to Io atmospheric escape is somewhat more complicated
than that of the Na nebula. The neutral clouds described in the previous
paragraph are shaped by interaction with the IPT through the processes of
impact ionization and charge-exchange [<]e.g.,¿smyth03. Impact ionization
results in the addition of new material and proceeds on timescales of $\sim$ 1
day [Smyth Marconi (2003)]. The residence time of plasma in the IPT, is 20 –
80 days, with the shorter residence times corresponding to times of higher
total plasma density [Bagenal Delamere (2011), Hess . (2011), Copper . (2016),
Tsuchiya . (2018)]. Thus, when there is an enhancement in the escape of
sulfur-bearing material from Io’s atmosphere, the peak in the IPT S+ 6731 Å
ribbon will lag by an amount dependent on the IPT plasma density. A model is
being developed that could, in principle, calculate the precise IPT ribbon
response [D. Coffin . (2020), DA. Coffin . (2022), DA. Coffin Withers (2023),
Nerney Bagenal (2020), Nerney . (2022), Nerney . (2023)], but its completion
and application to the IoIO dataset is beyond the scope of our current
project. Thus, we take the $\sim$4 week delay between the peaks in Na nebula
and IPT S+ emission in the previous two studies that captured contemporaneous
enhancements [<]§5.3;¿brown97, tsuchiya18 as indicative of the plasma
transport time during a typical IPT enhancement. Four weeks is also similar to
the transport time deduced from the larger IPT enhancement captured by Cassini
[Steffl . (2004), Copper . (2016)].

###  6.2 Interpretation of quasi-contemporaneity of Na nebula and IPT
enhancements

Within the context of the discussion in Sections 5.3 and 6.1, we can now offer
an interpretation the quasi-contemporaneous nature of the Na nebula and IPT
enhancements seen in Figure 3.3. In all cases except 2020, each major Na
nebula enhancement has a companion enhancement seen in the IPT nebula that is
delayed by $\sim$4 weeks. This $\sim$4 week delay is consistent with that seen
in previous studies and is indicative of simultaneous release of sodium- and
sulfur-bearing material from Io’s atmosphere. In 2020, the delay between the
Na nebula and IPT enhancements peaks is almost twice as long, however, the
profiles of both enhancements are more complicated than the enhancements in
other years: the Na nebula enhancement has a shoulder on its trailing edge and
the IPT enhancement appears to consist of a broad, main peak, followed by a
small, sharp peak. Thus, we offer the suggestion that in mid 2020, there are
two overlapping sets of Na nebula and IPT enhancements, with the earlier set
being larger than the later. A similar relationship may exist between other,
smaller enhancements, such as the shoulder on the early 2020 Na nebula peak
and the small IPT peak in mid 2019. We discuss the implications of the
variation in the relative Na nebula and IPT peak _sizes_ seen in each
contemporaneous pair (e.g., fall 2022 being the most extreme) in §6.6.

###  6.3 Ruling out solar insolation-driven sublimation

The current paradigm holds that the bulk of the escaping material from Io’s
atmosphere is supplied by Io’s global sublimation atmosphere
[<]e.g.,¿schneider07, dols08, dols12. This paradigm suggests that, in the
absence of some other perturbing effect on Io’s atmosphere, the variations
seen in the Na nebula and IPT should be dominated by variations in solar
insolation, that is, Io’s 42 hour orbit or Jupiter’s 12 year orbit
[<]e.g.,¿depater20_post_eclipse, tsang12. Enhancement in the escape of
material from Io’s atmosphere may also be modulated by Jupiter’s magnetic
rotational period (9.925 hr) due to Io’s apparent motion within the IPT
[<]e.g.,¿smyth11. Even when considering the timescales of the responses of the
Na nebula and IPT to material escape from Io’s atmosphere, discussed in
Sections 6.1 – 6.2, solar insolation and magnetic periodicities are not
compatible with the behavior of the enhancements seen in Figure 3.3. Thus, we
argue that one or more other physical mechanisms are driving atmospheric
escape during enhancements. Since enhancements dominate the average supply of
material from Io’s atmosphere (Sections 4, 6.1), the process(es) driving
enhancements provide the bulk of the material in the Jovian sodium nebula and
IPT.

###  6.4 The case for volcanism

The initial claim of a link between Io volcanism and material release from
Io’s atmosphere was made using Jovian sodium nebula images recorded with a
cadence of weeks to months and disk-integrated infrared observations [Mendillo
. (2004)]. A subsequent study using Jovian sodium nebula observations recorded
with a near-nightly cadence and a much more extensive set of disk-resolved Io
infrared observations, has failed to validate that initial claim [Roth .
(2020)]. In this work, we take a different approach and use the time behavior
of material release from Io’s atmosphere to suggest the most likely driver for
atmospheric escape is volcanic plumes.

Recall from §1, that based on EUV brightness of the IPT, $\sim$1 ton s-1 of
material must be flowing into it from Io’s atmosphere. We can now attribute
this amount of flux to the mechanism(s) that cause enhancements. Io’s
atmosphere is itself tenuous and, without resupply from the surface or
subsurface, cannot act as a reservoir for supplying enhancements in escape
that last weeks to months. We have ruled out variation due to solar
insolation-induced sublimation and magnetospherically enhanced escape of Io’s
global atmosphere, in §6.3. Thus, geologic processes of some sort must
ultimately be involved in the dominant process(es) of atmospheric escape from
Io’s atmosphere.

In §6.2, we showed that enhancements in the escape of sodium and sulfur-
bearing material from Io’s atmosphere occur simultaneously. This simultaneity,
together with the geologic nature of the processes driving escape imply that
there is a single geographical location responsible for driving atmospheric
escape in each pair of enhancements, with that location not necessarily being
the same for each pair. Finally, we note that behavior of the Na nebula and
IPT surface brightnesses appears to be stochastic, with large and small
enhancements interleaved. The picture that emerges is that geologic activity
at discrete sites on Io results in enhancements in the escape of sodium- and
sulfur-bearing materials that last 1 – 3 months, with 1 – 2 enhancements seen
during each 7 month Jovian opposition observing window and with the relative
amount of material escape varying between individual events.

Of the known geologic processes on Io that match the above criteria, volcanism
is the most likely to result in the stochastic perturbation of Io’s
atmosphere, via processes involving plumes. Observational support for Io
volcanic plume-driven atmospheric escape comes from the correlation between
plumes observed by Galileo [P. Geissler . (2004), PE. Geissler McMillan
(2008)] and enhancements in the Jovian dust streams [Krüger, Geissler .
(2003)]. As reviewed in §5.1, the dust streams are composed almost entirely of
NaCl and come from Io [Graps . (2000), Krüger, Horányi Grün (2003), Postberg .
(2006)]. Furthermore, NaCl+ has been shown to be an important pathway for Na
escape from Io [Grava . (2014), Schmidt . (2023)]. Based on this evidence, it
is plausible to suggest that volcanic plumes play a key role in the supply of
the Jovian sodium nebula. We use the very large Jovian dust stream enhancement
that peaked in early September 2000 and the IPT enhancement observed by
Cassini that peaked about a month later as observational evidence of the
connection between IPT enhancements and volcanic plumes [Krüger, Geissler .
(2003), Steffl . (2004), see also §5.2].

The difficulty with suggesting that volcanic plumes themselves are responsible
for launching material out of Io’s atmosphere is that plume vent velocities
are far below Io’s gravitational escape velocity, even when considering local
atmospheric heating by plume dynamics [<]e.g.,¿schneider07, mcdoniel17,
depater20_post_eclipse, redwing22. Thus, if plumes are implicated in material
escape from Io, the mechanism must be indirect. Plume models show that shocks
in the plume canopy impede upward flow of material, redirecting it outward and
toward the surface [Zhang . (2003), Zhang . (2004)]. zhang03 suggest that the
material that is redirected forcibly toward the surface enhances sublimation
of SO2 frost over a large area. Sublimation of SO2 frosts by hot
surface/subsurface lavas would provide a similar localized enhancement in Io’s
SO2 atmosphere. This SO2 would then be available to interact with the IPT via
the pathways of sputtering, electron impact ionization and change exchange.
The difficulty with this scenario is that it does not provide a comparable
mechanism for enhancing the escape of NaCl, since NaCl has a much higher
sublimation temperature than SO2. However, the amount of NaCl provided by the
plume itself and/or NaCl lofted from Io’s surface by sublimation may be
sufficient to drive escape, when considering interaction with the IPT over a
large area (see also §6.5).

Perhaps more a more plausible path of escape for both SO2 and NaX is to
consider the ability of plumes to loft material to sufficient altitude to
enable enhanced interaction between IPT plasma and the tops of plumes. An
extension of the zhang03, zhang04 model of Pele’s plume shows that by
including interaction between the top of the plume and the IPT, better
agreement is found with the distribution of material seen on the surface
[McDoniel . (2015), McDoniel . (2017), McDoniel . (2019)]. The mcdoniel19 work
provides theoretical validation of the ability of plume tops and IPT plasma to
interact, but stops short of a full quantitative calculation of the amount of
material that could be removed by this interaction. Thus, although there is
observational evidence that connects volcanic plumes to enhancements in the
escape of sodium- and sulfur-bearing material from Io’s atmosphere, current
state-of-the-art theoretical calculations have not been able to determine the
exact pathway taken by the material to Io’s exosphere.

###  6.5 Implications for the transport of sodium- and sulfur-bearing material
through Io’s atmosphere

Atacama Large Millimeter/submillimeter Array (ALMA) observations of Io’s
atmosphere reveal collections of hot NaCl, KCl and SO2 gases, that are
interpreted as plumes [Redwing . (2022)]. Redwing . note that the highest
column density collections of alkali and SO2 gasses are consistently _not_
found to be coincident with each other (Figure 9 of that work). As discussed
by Redwing ., these results are difficult to explain given that SO2, the
primary volatile on Io, is expected to be associated with all plume activity.
These results are also in apparent conflict with our result that sodium- and
sulfur-bearing material are consistently seen to escape at the same time,
implying a common geographic source (Sections 6.2 – 6.4). In this Section, we
discuss some mechanisms that might contribute to this effect, including those
not considered by Redwing ..

redwing22 provide two potential reasons for the lack of spatial coincidence
between alkali and SO2 plumes in Io’s atmosphere which rely on the difference
in vaporization pressures of these materials. (1) SO2 gas is produced
primarily by hot lava vaporizing frost deposits, with these deposits found
primarily at low- to mid-latitudes. Alkalis sublime at a much higher
temperature and therefore will not be released into Io’s atmosphere by this
effect. (2) The alkalis observed by ALMA are released in the plumes of high-
temperature volcanoes. Redwing . note that these high-temperature plumes
should also produce SO2 but that these alkali-producing volcanoes are
consistently located at high latitudes where atmospheric temperatures may be
low enough to freeze SO2 within the plumes. In this way, (1) explains why SO2
plumes appear in the absence of alkali plumes (low latitudes) and (2) suggests
that SO2 is always collocated with the high-latitude alkali plumes, however,
this high-latitude SO2 is largely invisible to ALMA because it is in solid
form. Evidence of solid-phase transport of SO2 through Io’s atmosphere comes
from Galileo detection of very high mass-to-charge ratio ions, interpreted as
clusters of SO2 molecules, or “snowflakes,” when it flew over Io’s north pole
Frank Paterson (2002). This explanation requires that the SO2 gas in the
plumes at low latitudes not contribute significantly to SO2 escape, and
requires NaCl to be primarily sourced from the polar plumes, possibly by the
mechanism discussed in the next paragraph. Plume models have yet to be
constructed that would test this “snowflakes” hypothesis.

Another way to “hide” SO2 and/or NaCl from ALMA is to ionize them. The
behavior of Io’s auroral Na, O, SO2, and SO emission while Io transitions to
and from eclipse behind Jupiter provides evidence that (a) photoionization is
the primary mechanism for producing SO${}_{2}^{+}$ and (b) SO${}_{2}^{+}$
plays an important role on the pathway of NaCl escape from Io’s atmosphere via
charge-exchange Schmidt . (2023). Io’s polar atmosphere is exposed to the sun
for longer periods of time than that above the rest of Io, thus increasing the
average rate of photoionization at the poles. Furthermore, Io’s collisional
atmosphere is thinner in these regions <e.g.,¿walker10, providing more access
of plume material to the exosphere, where IPT-driven escape processes are the
most efficient. This suggests that plumes at the poles will have enhanced
escape (see also §6.6). Like the SO2 “snowflake” atmospheric transport
mechanism suggested in the previous paragraphs, this mechanism favors the
NaCl-rich high-latitude plumes detected by ALMA as the source of the sodium-
and sulfur-bearing material contributing to the Na nebula and IPT
enhancements.

For logical completeness, we also consider the suggestion that NaCl-containing
volcanic dust and ash (“dust bunnies”) may transport NaCl through Io’s
atmosphere in a form not visible to ALMA. To simultaneously explain the ALMA
and IoIO data, this would imply that these large particles are driven through
the atmosphere by the SO2 plumes at low latitudes and that those particles are
quickly charged by interaction with the IPT and removed from ALMA’s FOV by
Jupiter’s magnetic field. Importantly, under this interpretation, the high-
latitude NaCl plumes seen by ALMA would not contribute to escape and the SO2
from these plumes must still be invisible to ALMA, necessitating one or more
of the mechanisms in the previous paragraphs.

In support of the “dust bunny” hypothesis, we recall the observational
evidence we used to connect plumes to the Jovian sodium nebula
<§6.4;¿krueger03, grava14, schmidt23. Krüger, Geissler . found that dust
stream enhancements were likely associated with the plumes of volcanoes such
as Pele, Tvashtar, a region near the north pole, and a region south of Karei
<now known as Grian;¿geissler08. The plume deposits of these eruptions are
primarily SO2 rich, with minor contributions from silicate ash PE. Geissler .
(1999); P. Geissler . (2004). The association between atmospheric escape of
sodium- and sulfur-bearing material and these large, SO2-rich plumes is
suggestive that the large atmospheric disturbances caused by these plumes may
be the key to driving escape. The lack of association between SO2-dominated
plumes and NaCl emission in the ALMA data then becomes support for transport
of NaCl through the atmosphere in these plumes in a form such as dust or ash.

Finally, we suggest that there may be no need to “hide” SO2 from ALMA in the
regions where bright NaCl emission is seen. In the ALMA observation of NaCl
and SO2 that has the highest resolution, relatively faint concentrations of
SO2 gas are seen in the vicinity of the brightest NaCl emission <see
2016-07-26 in Figure 9 of¿redwing22. More detailed analysis of the ALMA data
would be needed to determine if the NaCl and SO2 seen in this observation is
consistent with a single geographic source. Plume models could be used to
determine if the amount of SO2 is, in fact, less than what is expected for the
range volcanic activity that has been observed on Io <see, e.g. review
by¿depater21. Additional high-resolution ALMA images, ideally recorded
contemporaneously with IoIO data, would also be useful. If the NaCl to SO2
ratio were to be found to be reasonable in NaCl plumes, this could imply that
atmospheric escape is primarily tied to NaCl-rich and/or high-latitude plumes
and that the collections of high column density SO2 gas seen at lower
latitudes in the ALMA data contribute at most a minor, steady amount to the
IPT (e.g., baseline in Figure 3.3, bottom panel.)

###  6.6 Processes that modulate Na nebula and IPT enhancement sizes

Having established that volcanic plumes are the likely precipitating agent for
material escape from Io’s atmosphere and discussed possible pathways of
material transport through Io’s atmosphere, we return to our discussion of the
quasi-contemporaneous nature of the Na nebula and IPT enhancements, begun in
§6.2, to offer possible causes for the modulation seen in the _sizes_ of Na
nebula and IPT enhancements. These suggested causes divide into four general
categories: (1) variation in the content of sodium- and sulfur-bearing
material in volcanic plumes, such as produced by different magmas <e.g.,
see¿redwing22; (2) processes involving the interaction of the plumes with the
atmosphere, such as shocks <§6.4;¿zhang03, zhang04 (3) modulation in material
transport through the atmosphere <§6.5;¿schmidt23; and (4) variation in the
efficiency of escape. In this Section, we concentrate on category (4), because
understanding effects in this category greatly enhances the ability to use
Jovian sodium nebula and IPT data to make progress understanding physical
effects in categories (1) – (3).

One of the scenarios discussed in §6.5 suggested that enhanced SO2 ionization
in Io’s polar regions may play a role in enhancing NaCl escape Schmidt .
(2023). This would favor the Tvashtar plume (63∘N 124∘W) over that of Pele
(19∘S 255∘W) or Surt (45∘N, 336∘W) for the source of the very large Jovian
dust stream enhancement observed by Galileo in late 2000, even though Surt,
active at the time, had a much larger infrared output and Pele was the most
active large plume during the Galileo era Marchis . (2002); Porco . (2003);
Krüger, Geissler . (2003); P. Geissler . (2004). Because SO2 ionization and
subsequent rapid dissociation Huddleston . (1998) may be enhanced in the polar
regions Schmidt . (2023), the efficiency of S and O production, and thus
escape, may be enhanced at the poles as well.

Because NaCl+ has an important role in the pathway of Na escape from Io’s
atmosphere <Sections 5.1, 6.4, 6.5;¿schmidt23, sodium-bearing material in Io’s
anti-Jovian equatorial exosphere may have an exaggerated escape efficiency
over that of sulfur-bearing material. The sodium “jet” was seen to be rooted
in the anti-Jovian equatorial region of Io’s exosphere the one time it has
been imaged in sufficient spatial detail for detection near Io Burger .
(1999). Burger . offered two hypotheses to explain this behavior: (i) Material
was being injected into the exosphere from below at this location, e.g., by
volcanism. (ii) Ionization is enhanced in this region due to Io equatorial
auroral activity <e.g.,¿roesler99, geissler99, roth14. Case (ii) would provide
a mechanism for enhancing material flow into the jet, over material flow to
the IPT, since the jet is formed by prompt neutralization of pickup ions
within Io’s exosphere Schneider . (1991); Wilson Schneider (1994), whereas
direct ionization of material in Io’s exosphere has been shown to be a minor
contributor to the influx of plasma to the IPT <§1;¿dols08, dols12. The prompt
neutralization of the sodium forming the jet also explains why it is not
expected to be seen rooted at the sub-Jovian auroral spot: the initial
gyration of the ions directs them into Io’s surface. We note that hypotheses
(i) and (ii) are not necessarily mutually exclusive: the “jet” may always be
rooted in the region of the anti-Jovian equatorial auroral spot, but its
response to atmospheric escape may be exaggerated if that atmospheric escape
is located in that region.

Finally, dust, which acquires a negative charge Zook . (1996), enhances the
initial escape of Na-bearing material on Io’s sub-Jovian hemisphere Grava .
(2021). As the dust particles are destroyed and the constituent molecules and
atoms are released, they acquire a positive charge and join the “jet” feature.
A positive identification between a Na nebula enhancement and a plume on the
sub-Jovian hemisphere, could thus potentially be used in support of the “dust
bunnies” hypothesis discussed in §6.5.

###  6.7 Toward a connection with Io infrared observations

Since the early 1980s, synoptic Io infrared observations have been used to
help understand Io volcanic processes and their geologic implications <e.g.,
see review by¿depater21. As noted in §6.4, these infrared observations have
been compared to sodium nebula observations in an attempt to establish a
correlation Mendillo . (2004) which has not stood the test of time Roth .
(2020). The initial attempt to connect infrared indicators of Io volcanic
activity to enhancements in the sodium nebula made the implicit assumption
that the brightest infrared events should be correlated to the brightest
nebula images. Here we suggest that instead, the dimmer infrared events may be
more likely to be correlated to the brighter sodium nebula enhancements.

One of the fundamental results of our study is that the Jovian sodium nebula
and IPT show contemporaneous enhancements of varying relative amplitudes that
last 1 – 3 months (Figure 3.3; Sections 4, 6.1 – 6.2). A long-term study of
the time variability of Io’s hotspots found that they divided into two groups:
those with persistent activity and those that exhibited sudden brightening,
followed by a steady decay de Kleer de Pater (2016). For those hotspots that
exhibited sudden brightening events, the brighter the event, the shorter the
decay <see, e.g., Figure 11 of¿dekleer16. The hot spots with decay times of
order 1 month or longer were dimmer, by a factor of 5 or more, than the
brightest outbursts. If we make the very simplistic assumption that in a
volcano where plume activity is found, plume activity will persist over
roughly the same time period as infrared activity, we can support the argument
that infrared hotspots exhibiting eruption phases lasting 1 – 3 months are the
more likely to be correlated to atmospheric release events. Thus, dim infrared
outbursts may be the most likely to be correlated with enhancements in
material release from Io’s atmosphere.

##  7 Summary and Conclusions

We have used IoIO, an observatory composed almost entirely off-the-shelf
equipment <§2 and¿morgenthaler19 to collect the largest set of
contemporaneously recorded Jovian sodium nebula and Io plasma torus (IPT) in
[S II] 6731 Å images assembled to date (see examples in Figure 1 and
accompanying animations). Using simple image analysis techniques (§3), we
construct a time history of the brightnesses of the Na nebula and IPT [S II]
emission (Figure 3.3). Qualitative inspection of this Figure shows 1 – 2
enhancements in the Na nebula and IPT [S II] emission per $\sim$7-month
observing window, such that a quiescent state of emission is rare (§4). The
minimum and maximum surface brightness values seen in the IoIO Na nebula and
IPT images compare favorably with previous studies (§5.1 – 5.2). Most large
IPT enhancements peak $\sim$4-weeks after the corresponding enhancement in the
Na nebula, as seen in previous studies (§5.3) and as expected from plasma
transport within the IPT (§6.1). The exception to this, seen in mid 2020, is
likely caused by the overlap of multiple enhancements (§6.2). We rule out
sublimation as the primary driver of material escape from Io’s atmosphere in
§6.3. This is our most definitive result.

Having ruled out sublimation as the primary driver of atmospheric escape from
Io, we show that geologic activity in some form, likely volcanic plumes,
drives escape, either directly or indirectly (§6.4). In light of other
published results, this has implications on the transport of material through
Io’s atmosphere (§6.5). In §6.6, we review the processes that can modulate the
relative sizes of contemporaneous Na nebula and IPT enhancements, focusing on
processes that might modulate the efficiency of Io atmospheric escape as a
function of geographic location. Finally, in §6.7, we note that Io’s dimmer
infrared outbursts have durations and time profiles similar to Na nebula and
IPT enhancements, suggesting that it these 1 – 3 month-long infrared outbursts
may be the more likely to show correlation with the release of material from
Io’s atmosphere.

In conclusion, our work shows that off-the-shelf equipment with minimal
customization, together with simple analysis techniques can be used to collect
data that provides valuable insights into the processes which produce material
on Io’s surface, transport it through its atmosphere, and release it into
Jupiter’s broader magnetospheric environment.

##  8 Future Plans

We have pointed out the existing observational evidence that links plume
activity on Io to atmospheric escape in §6.4. Further confirmation of this
link may be accomplished by accumulating additional contemporaneous IoIO
observations of the Na nebula and IPT together with ALMA observations of Io’s
atmosphere – currently there is only overlap during March 2018 Redwing .
(2022). Disk-integrated observations conducted by the NOrthern Extended
Millimetre Array (NOEMA) interferometer of the Institut de Radioastronomie
Millimetrique (IRAM), such as those conducted by roth20, may also be useful.
Continued theoretical work on the effect that plumes have on Io’s atmosphere
and exosphere, as well as the interaction between the exosphere and the IPT is
also needed <e.g.,¿bloecker18, mcdoniel19, dols08, dols12, dols23,
adeloye23DPS. These observational and theoretical studies can also be useful
to help differentiate between the hypotheses offered in 6.5 concerning
material transport through Io’s atmosphere.

Continued disk-resolved observations of Io IR activity, such as those carried
out at the Keck and Gemini telescopes de Kleer . (2019) will also be
interesting as they might lead to validation of the correlation that was
initially claimed between Na nebula, IPT and IR brightnesses Mendillo .
(2004); Yoshikawa . (2017); Yoshioka . (2018); Tao . (2018); Koga . (20182),
but has subsequently proven elusive de Kleer . (2016); Roth . (2020).

We are also planning to conduct more detailed analysis of the IoIO images. For
instance, the IoIO images of the Na nebula contain three distinct features –
the “banana,” “jet,” and “stream” – that can be used to estimate the neutral
sodium source rate from Io Wilson . (2002). The IPT ribbon positions, which
are detectable with IoIO (Figure 3, lower panels), are related to the dawn-
dusk electric field, which is modulated by a combination of material flow
toward the magnetotail and solar wind pressure Barbosa Kivelson (1983); Ip
Goertz (1983). When combined with the analysis presented here, the IPT ribbon
positions retrieved from the IoIO data will provide a significant amount of
information regarding the production of material on Io and its subsequent flow
through and out of Jupiter’s magnetosphere.

Finally, the unique sensitivity of IoIO to Na nebula and IPT [S II] 6731 Å
enhancements, together with reliable robotic operation and $<$24 hour
turnaround for pipeline reduction ideally suits it to provide real-time alerts
of enhancements in the departure of material from Io’s atmosphere. These can
inform planned observations of Io from both ground- and space-based platforms.
In particular, nearly all of the plasma found in Jupiter’s magnetosphere comes
from Io and makes its way through the IPT in 20 – 80 days before rapidly
spiraling out through the rest of the magnetosphere Bagenal Delamere (2011);
Hess . (2011); Copper . (2016); Tsuchiya . (2018). The modulations seen in the
lower panel of Figure 3.3 therefore precede modulation in plasma density
throughout the Jovian magnetosphere, a feature that can be used to enhance the
science operations of the Juno mission Bolton . (2017) and supporting
observations Orton . (2020, 2022). NASA’s Europa Clipper Howell Pappalardo
(2020) and ESA’s JUICE Grasset . (2013) missions will benefit from planned
IoIO observations, because of the record of exogenic material impinging on
Europa, Ganymede and Callisto during those missions. Also, because
enhancements in the Jovian dust streams can induce detector fatigue in Europa
Clipper’s SUrface Dust Analyzer <SUDA;¿goode23, IoIO observations can be used
to inform SUDA operations while Europa Clipper is sampling the broader Jovian
magnetospheric environment and thus optimize detector performance for that
mission’s primary target.

## Open Research Section

The following software was used in this project: Astrometry.net Lang . (2010),
AstroPy Astropy Collaboration . (2022), Astroquery Ginsburg . (2019),
BigMultiPipe Morgenthaler (2022), Burnashev Morgenthaler (20232), CCDMultiPipe
Morgenthaler (20233), ccdproc Craig . (2017), IoIO control software
Morgenthaler (20231), matplotlib Hunter (2007), moviepy Zulko . (2021), NumPy
Oliphant (2006); Harris . (2020), photutils Bradley . (2022), precisionguide
Morgenthaler (20234), Python 3 Van Rossum Drake (2009), reproject Robitaille .
(2020), SciPy Virtanen . (2020), specutils Earl . (2022)

The reduction products used to create Figure 3.3 are archived with Zenodo
Morgenthaler . (2023).

###### Acknowledgements.

IoIO is hosted at the San Pedro Valley Observatory, near Benson, Arizona, and
has benefited greatly from the services provided by observatory manager, Dean
Salman. Gavin Nelson, Andy Sorenson, Elisabeth Adams, and the staff of
Starizona have also provided invaluable assistance in observatory maintenance.
The authors are grateful for the insightful comments received during the
manuscript review process. This work has been supported by NSF grants 1616928
and 2109219 and NASA grant 80NSSC22K0317 to the Planetary Science Institute
and NASA grant 80NSSC20K0559 to Boston University.

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