11institutetext: Laboratory for Atmospheric and Space Physics, University of
Colorado at Boulder, 3665 Discovery Drive, Boulder, CO, USA
11email<EMAIL_ADDRESS>22institutetext: NOAA/National Centers
for Environmental Information, 325 Broadway, Boulder, CO, USA
33institutetext: High Altitude Observatory, National Center for Atmospheric
Research, P.O. Box 3000, Boulder, CO, USA
44institutetext: Naval Research Laboratory, Washington, DC, USA
55institutetext: Colorado Research Associates Division, NorthWest Research
Associates, 3380 Mitchell Lane, Boulder, CO, USA
66institutetext: NASA Goddard Space Flight Center, 8800 Greenbelt Road,
Greenbelt, MD, USA
77institutetext: Institute of Physics & Kanzelhöhe Observatory for Solar and
Environmental Research, University of Graz, A-8010 Graz, Austria
88institutetext: Royal Observatory of Belgium, Avenue Circulaire 3, 1180
Uccle, Belgium
99institutetext: Reflective X-ray Optics LLC, New York, NY, USA

# SunCET: The Sun Coronal Ejection Tracker Concept

James Paul Mason 11 Phillip C. Chamberlin 11 Daniel Seaton 22 Joan Burkepile
33 Robin Colaninno 44 Karin Dissauer 55 Francis G. Eparvier 11 Yuhong Fan 33
Sarah Gibson 33 Andrew R. Jones 11 Christina Kay 66 Michael Kirk 66 Richard
Kohnert 11 W. Dean Pesnell 66 Barbara J. Thompson 66 Astrid M. Veronig 77
Matthew J West 88 David Windt 99 Thomas N. Woods 11

###### Abstract

The Sun Coronal Ejection Tracker (SunCET) is an extreme ultraviolet imager and
spectrograph instrument concept for tracking coronal mass ejections through
the region where they experience the majority of their acceleration: the
difficult-to-observe middle corona. It contains a wide field of view (0–4
$R_{\odot}$) imager and a 1 Å spectral-resolution-irradiance spectrograph
spanning 170–340 Å. It leverages new detector technology to read out different
areas of the detector with different integration times, resulting in what we
call “simultaneous high dynamic range”, as opposed to the traditional high
dynamic range camera technique of subsequent full-frame images that are then
combined in post-processing. This allows us to image the bright solar disk
with short integration time, the middle corona with a long integration time,
and the spectra with their own, independent integration time. Thus, SunCET
does not require the use of an opaque or filtered occulter. SunCET is also
compact — $\sim$15 $\times$ 15 $\times$ 10 cm in volume — making it an ideal
instrument for a CubeSat or a small, complementary addition to a larger
mission. Indeed, SunCET is presently in a NASA-funded, competitive Phase A as
a CubeSat and has also been proposed to NASA as an instrument onboard a 184 kg
Mission of Opportunity.

###### keywords:

EUV instrument – Coronal Mass Ejections – high dynamic range – CubeSat

##  1 Introduction and Science Drivers

The primary science question that the Sun Coronal Ejection Tracker (SunCET)
instrument concept is designed to address is: What are the dominant physical
mechanisms for coronal mass ejection acceleration as a function of altitude
and time?

In the standard model configuration of a coronal mass ejection (CME; Figure
1), a CME must overcome the constraint of overlying field in order to escape.
Perhaps the simplest model of this defines a 1D, horizontal background
magnetic field that declines in strength with height, characterized by the
“decay index” (Bateman, 1978; Kliem and Török, 2006). If the background field
decays too rapidly, the so-called torus instability of the embedded flux rope
occurs, meaning the flux rope erupts. The decay index has a direct impact on
the CME kinematics. The acceleration curves in the bottom of Figure 2, derived
from magnetohydrodynamic (MHD) simulations by Török and Kliem (2007),
correspond to decay index profiles, with each increase in acceleration
corresponding to an increase in in the decay index profile and the final CME
speed. Thus, the acceleration profile of a CME acts as a natural probe of the
surrounding magnetic field. There are many complications layered on top of
this simple model in reality, described later in this introduction.

Figure 1: Standard cartoon CME model. The flux rope extends through the page.
Overlying fields resist the flux rope’s elevation and expansion. Magnetic
reconnection releases the energy stored in the field to accelerate the flux
rope, producing a CME. Adapted from Forbes et al. (2018).

The bulk of the CME acceleration profile in all cases occurs either in the
observational gap or in the region where existing instruments are not
optimized. This gap exists between extreme ultraviolet (EUV) imagers (widest
outer field of view [FOV] of 1.7 $R_{\odot}$) and coronagraphs (typical inner
FOV of 2.5 $R_{\odot}$ but effectively higher due to diffraction-degraded
spatial resolution). Some instruments observe only part of the low-middle
corona (Solar TErrestrial RElations Observatory [STEREO; Kaiser et al. 2007] /
Coronagraph-1 [COR1; Howard et al. 2008], Geostationary Operational
Environmental Satellite [GOES] / Solar Ultraviolet Imager [SUVI; Martínez-
Galarce et al. 2010] , Project for On-Board Autonomy [PROBA2] / Sun Watcher
with Active Pixels and Image Processing [SWAP; Seaton et al. 2013]). Some have
low signal to noise in the middle corona (SUVI, SWAP). Some are ground-based
with duty cycles $<20$% (K-Cor). Some have limitations on cadence (COR1).
SunCET, however, avoids all of these issues because it is specifically
optimized for this study of CMEs. Directly observing the CME height-time
profile through the whole low and middle corona allows the derivation of
complete speed-time and acceleration-time profiles, and thus strong model
constraints, requiring accurate modeling of the magnetic environment to obtain
the observed profiles. Such constraints do not presently exist, but SunCET can
provide them.

Figure 2: Top: Composite of SDO/AIA 171 Å image and SOHO/LASCO/C2 white-light
coronagraph image. The longstanding observational gap is shown in dark grey.
Bottom: Modeled acceleration profiles of torus instability CMEs, adapted from
Török and Kliem (2007) Fig. 3. The different curves result from different
background magnetic field decay index profile assumptions, with each higher
acceleration peak corresponding to a larger decay index profile. Most of the
acceleration occurs in the observational gap that SunCET fills.

The torus instability is not the only mechanism involved in CME eruptions.
Complicating factors are introduced by, e.g., the 3D structure of the erupting
material and the surrounding magnetic field, by potential drainage of dense
plasma, and by continued magnetic reconnection freeing more energy to drive
the CME. The influence of these factors also evolve with altitude and time, as
the CME dynamics play out. There have been at least 26 review papers on the
topic over the last two decades (Green et al. 2018, and references therein) —
a testament to the sustained, intense interest in this topic.

Figure 3: Simulated CME kinematic profiles. Solid lines indicate the
unperturbed torus instability. Dashed lines from right to left correspond to
increasing durations (6 $\tau_{A}$ up to 10$\tau_{A}$) of an upward, linearly
rising velocity perturbation, resulting in fundamentally different
acceleration profiles. The SunCET FOV (0–4 $R_{\odot}$; indicated in light
blue) covers and extends beyond this simulation. Adapted from Schrijver et al.
(2008) Fig. 7.

For example, a relatively modest complication to layer into the torus
instability model is to add an upward velocity perturbation with finite
duration. MHD simulations by Schrijver et al. (2008) showed that simply
changing the duration of this perturbation results in fundamentally different
acceleration profiles (Figure 3). With brief perturbations, the profile is
single-peaked and occurs at later times. Increasing the duration of the
perturbation does not simply result in an earlier peak, but in two peaks. Just
as in Figure 2, the heights that these acceleration profiles differentiate
themselves occurs across the Heliophysics System Observatory (HSO) measurement
gap. SunCET observations can discriminate between single-peak versus double-
peak CME acceleration profiles, which then determines the duration of a
velocity perturbation in the torus instability model.

Another CME initiation mechanism arises from the magnetic field topology of
the flux rope. Hood and Priest (1981) showed that if the total twist in a flux
rope exceeds a critical threshold (448°), a “helical kink” instability will
occur, causing the flux rope to erupt. Such contortions lead to an impulsive
acceleration and a large rotation of the flux rope (Fan 2016, Figure 4). Note
the substantial differences in the simulated acceleration profiles between
Figures 2, 3, and 4; and that they all occur in the under observed region.

Figure 4: MFE simulation containing the helical kink instability, resulting in
impulsive CME acceleration. The SunCET FOV (0–4 $R_{\odot}$; indicated in
light blue) captures the impulse and small jerks. Adapted from Fan (2016).

The other aspect of acceleration is direction: CMEs can be deflected away from
“pure” radial propagation by as much as $\sim$30$\degree$, which is again
determined primarily by Bex (Figure 5). This force has a non-radial component
because the field is not perfectly symmetric about the flux rope, causing a
magnetic gradient on the CME’s sides as the loops drape around the rising CME.
The Forecasting a Coronal Mass Ejection’s Altered Trajectory (ForeCAT)
analytical model accounts for these and other forces on a CME to determine its
non-radial velocity (Kay et al., 2013, 2015, 2016; Kay and Gopalswamy, 2018).
Furthermore, Kay and Opher (2015) modeled 200 CMEs in ForeCAT and found that
deflection occurring in the middle corona accounts for nearly all of the
deflection that occurs between initiation and 1 AU. The background magnetic
field and radial CME speed are two free parameters in ForeCAT that are
critical to get right; SunCET observations can strictly constrain them via
forward modeling.

Figure 5: ForeCAT simulations of a CME propagating through background magnetic
fields (PFSS) of various strengths. R is radial distance. CMEs experience
greater non-radial velocity in middle corona environments with stronger
magnetic fields. The SunCET FOV (0–4 $R_{\odot}$; indicated in light blue)
captures the majority of CME deflection. Adapted from Kay (2016).

Additionally, coronal dimming often occurs as a result of CMEs. The faster a
CME departs, the steeper the decline in coronal emission. The more mass the
CME takes with it, the deeper the drop in coronal emission. A large number of
studies have demonstrated this link with coronal imagers (e.g., Aschwanden
2009; Aschwanden et al. 2009; Dissauer et al. 2018, 2019; Thompson et al.
2000) and with spectral irradiance data (Woods et al., 2011; Mason et al.,
2014, 2016, 2019). A major advantage of dimming measurements is that they are
effective measures of CME kinematics even when they occur at disk center.
Coronagraphs and imagers suffer from the problem of determining halo CME speed
and/or mass. Dimming is an effective measure of CME kinematics both on and
off-disk (Dissauer et al., 2019; Chikunova et al., 2020). Thus, instrument
suites that can capture both the dimming and direct observations of limb CMEs
are ideal for CME observation. This is precisely what SunCET does.

SunCET will be the first mission that allows continuous measurements of CMEs
during their initial acceleration phase using only a single instrument. This
is advantageous compared to currently used instruments, where, e.g. EUV
imagers in the low corona are combined with white-light coronagraphs higher up
to track this phase. Artifacts can be introduced in the resulting CME
kinematics using this combined data due to the tracking of different
structures in the different instruments, since the observed emission is
generated by different physical processes. SunCET is not dependent on other
instruments to observe CME initiation and acceleration but does have a
sufficiently wide field of view to overlap with coronagraphs for further
expanded studies. The same challenges with different CME structures in EUV
versus white light will be present, but SunCET’s broader temperature response
should mitigate this somewhat.

##  2 Instrument Design

SunCET is an instrument with a Ritchey-Chrétien, wide-field-of-view telescope
(4 $R_{\odot}$), an off-rowland-circle EUV spectrograph, and a novel,
simultaneous-high-dynamic-range detector. This new detector technology allows
us to image the bright solar disk and CMEs through the dim middle corona
simultaneously. It also allows us to measure solar irradiance spectra on the
unused portion of the same detector with an integration time independent of
the telescope image. The entire design is compact, fitting in a $\sim$15
$\times$ 15 $\times$ 10 cm volume; or about 2.5 CubeSat Units. This makes it
ideal as a CubeSat or as a compact instrument suite to include on larger
spacecraft that requires few physical resources.

SunCET observes in the EUV rather than white light because 1) CMEs have
already been demonstrated to be visible in the EUV and 2) it allows for major
simplifications in the technical design of the instrument. While white-light
observations are independent of temperature since they rely on light Thomson
scattered from free electrons, SunCET observations do have the caveat that
their temperature dependence (emission from ions at particular temperatures)
means that CMEs whose plasma is not at ambient coronal temperatures will not
be visible. The dynamic range between on and off disk in the EUV is already
large ($\sim$105 by 2 $R_{\odot}$) but this is orders of magnitude larger in
white light ($\sim$108), increasing the technical challenge. Moreover, the
absolute brightness are vastly different; there are far more visible light
photons. This presents a major challenge with scattered light: even small
imperfections in optics would result in enough of the numerous disk photons to
land on the part of the detector with the exceptionally faint middle corona,
swamping out CME observations. This is further exacerbated by the fact that
most surfaces scatter light more efficiently in visible light than in EUV
light. Therefore, SunCET observes CMEs in the EUV.

##  3 Imager Design

The SunCET imager was designed to provide high-dyamic range with moderate
spatial resolution while providing a large field-of-view not heard of in
historical on-disk EUV imagers out to 4 $R_{\odot}$. This section describes
the technical design details that were traded in order to close on the science
question.

###  3.1 Dynamic Range

The SunCET imager requires a dynamic range of at least
$7\text{\times}{10}^{4}$, based on GOES-16/SUVI observations of CMEs and
SunCET’s design optimizations. The dimmest target of interest is a CME at the
outer FOV, and the brightest is the coronal loops of an active region
associated with a CME.

SUVI-observed radiances are used to estimate brightness in SunCET (see Section
3.7). At 3.5 $R_{\odot}$, CMEs are $6.9\text{\times}{10}^{-4}$ W/m2/sr. A few
of the brightest pixels in active regions reach $\sim$70 W/m2/sr, but are
typically $\sim$4.8 W/m2/sr in SunCET. Another factor of 10 is included to
distinguish the loops from the background solar disk. Thus, we have a required
dynamic range of (4.8 / $6.9\text{\times}{10}^{-4}$) $\times$ 10 =
$7\text{\times}{10}^{4}$. We allow solar flares and a small number of the
brightest pixels inside active regions to saturate because 1) they are not our
target of interest, 2) our entrance filter mesh mitigates diffraction (Section
3.4), and 3) the blooming in our detector is modest: only a few percent
ranging across a few pixels (verified during the 33.336 NASA sounding rocket
flight and in the lab).

Projected performance: CME brightness at the outer SunCET FOV of 4 $R_{\odot}$
is $2.1\text{\times}{10}^{-4}$ W/m2/sr. That implies a dynamic range of
$2.3\text{\times}{10}^{5}$. From 0–1.05 $R_{\odot}$, we run exposures of 0.025
seconds and from 1.05–4 $R_{\odot}$ the exposures will be 10 seconds — a
factor of 400$\times$ dynamic range. Our detector has a native dynamic range
of $\sim$$5\text{\times}{10}^{3}$. 2$\times$2 pixel binning provides an
additional factor of 4. Combining these, we obtain SunCET’s high dynamic range
of $8\text{\times}{10}^{6}$, well above the required range of
$7\text{\times}{10}^{4}$. For comparison, the SDO/AIA dynamic range is
$1\text{\times}{10}^{4}$ (Lemen et al., 2012).

###  3.2 Field of View

Most CMEs accelerate through the low and middle corona (Bein et al., 2011;
D’Huys et al., 2014). We set our required minimum field of view (FOV) at 0.5
$R_{\odot}$, corresponding to $\pm$30$\degree$ from disk-center. Lower than
this and the events tend to be halo CMEs, which are difficult to obtain
height-time profiles from. The outer FOV requirement is set to 3.5
$R_{\odot}$. SunCET covers the gap between existing instruments and includes
enough overlap to ensure a smooth transition in any complementary height-time
profiles. SOHO/LASCO’s inner FOV is 2.4 $R_{\odot}$ and its upcoming
replacement, NOAA’s GOES-U/CCOR and SWFO/CCOR, will have an inner FOV of 3
$R_{\odot}$.

The aforementioned traditional CME measurements, which are from white-light
coronagraphs, use occulters that are mechanically restricted to be a limited
distance away; therefore these observations have significantly degraded
spatial resolution in their inner FOV that is much worse than their stated
plate-scale resolution, sometimes upwards of 1 arc-min in the inner FOV. These
effects are primarily due to vignetting (e.g. Koutchmy 1988; Aime et al.
2019). This is not the case with SunCET as it does not require an occulter to
observe the CMEs in the low- and middle-corona, so its spatial resolution is
not diffraction limited and is superior even in the FOV region that overlaps
with the coronagraphs.

Projected performance: The FOV of SunCET is 0–4 $R_{\odot}$ (5.6 $R_{\odot}$
in image corners).

###  3.3 Temporal Resolution: Exposure and Cadence

SunCET is required to observe CMEs with speeds up to at least 1000 km/s, which
accounts for 98% of all CMEs (Gopalswamy et al., 2009; Barlyaeva et al.,
2018). Given the cadence described below and the field of view, SunCET’s
projected performance is to observe CMEs with speeds up to 3900 km/s. The
fastest CME in the CDAW catalog is $\sim$3400 km/s, meaning that SunCET will
be able to track CMEs with any previously observed speed.

SunCET requires an exposure time $\leq$23 seconds in order to avoid motion
blur of the CME. Combining the fastest required CME to observe (1000 km/s),
our required spatial resolution of 30/resolution-element, and the conversion
of angular to spatial resolution at 1 AU ($\sim$750 km/arcsec), we obtain 750
$\times$ 30 / 1000 $\approx$ 23 seconds/resolution-element. Projected
performance - exposure: SunCET’s exposure times are 0.025 seconds from 0–1.05
$R_{\odot}$ and 10 seconds beyond that.

SunCET requires a cadence $\leq$3.2 minutes. SunCET must be able to track a
1000 km/s CME from the solar limb through its FOV, a range of 2.5 $R_{\odot}$,
or $1.74\text{\times}{10}^{6}$ km. Therefore, the minimum time a CME would be
in the FOV is 29 minutes. We require at least 9 height-time samples to
distinguish acceleration profiles (Figure 3). Thus, our cadence must be less
than 29 minutes / 9 samples = 3.2 minutes.

Projected performance - cadence: The SunCET mission is designed to downlink 1
minute cadence data. The designed FOV actually extends to 4 $R_{\odot}$,
meaning we will capture 38 height-time points for limb-CMEs traveling at a
speed of 1000 km/s and more points for CMEs that start slightly on disk and/or
with slower speeds. For example, the average CME speed is 490 km/s (Webb and
Howard, 2012) and if it crosses from 0.7–4 $R_{\odot}$, we will obtain 78
height-time points.

###  3.4 Bandpass: Coatings and Filters

Table 1: Strong emission lines in the SunCET bandpass. Irradiance measured by SDO/EVE (Woods et al., 2012) [HTML]1A73C9  Ion | $\uplambda$ [Å] | log10(T [K]) |  Quiet Sun Irradiance [$\upmu$W/m2/Å]
---|---|---|---
Fe IX | 171.1 | 5.9 | 67
Fe X | 174.5 | 6.1 | 73
Fe X | 177.2 | 6.1 | 48
Fe XI | 180.4 | 6.2 | 77
| Fe XI
---
(doublet)
188.2 | 6.2 | 61
Fe XII | 193.5 | 6.2 | 45
Fe XII | 195.1 | 6.2 | 63

CMEs have been routinely identified in narrowband EUV imagers sensitive to
temperatures between $\sim$0.6–1.6 MK (e.g., GOES/SUVI). Therefore, SunCET is
required to observe at least one of the emission lines identified in Table 1.

Projected performance: SunCET’s baseline bandpass is 170–200 Å — capturing all
of the emission lines in Table 1, which boosts the signal (Section 3.7). The
telescope mirrors employ reflective multilayer coatings designed to provide
broad spectral response spanning the instrument bandpass. These coatings
follow an aperiodic design, and comprise 15 repetitions of alternating layers
of B4C, Mo, and Al, with individual layer thicknesses ranging from $\sim$5–100
Å. The aperiodic coating design provides an average reflectance of $\sim$33%
from 170–200 Å, as shown in Figure 6. For reference, periodic multilayer
coatings operating in this portion of the EUV are generally used for narrow-
band response: for example, the periodic Si/Mo coatings used for the 195 Å
channel of the GOES/SUVI instrument, also shown in Figure 6, achieve a peak
reflectance of $\sim$34% with a spectral bandpass of $\sim$9.5 Å full-width-
half-max (FWHM). Figure 6 also shows the periodic Al/Zr coatings used for the
Hi-C rocket instrument (Kobayashi et al., 2014), which achieve a peak
reflectance of $\sim$50% with a spectral bandpass of $\sim$8.5 Å FWHM. The
aperiodic B4C/Mo/Al multilayer coatings are currently under development with
funding from the NASA H-TIDeS program.

Figure 6: Calculated reflectance near normal incidence (5°) of the broad-band,
aperiodic B4C/Mo/Al multilayers used for the SunCET telescope mirrors (green),
and for reference, the narrow-band, periodic Si/Mo multilayer coatings used
for the GOES/SUVI instrument (red), and the Al/Zr multilayer coatings used for
the Hi-C rocket instrument (blue).

The C/Al/C entrance filter from Luxel Corporation prevents visible light from
entering the chamber and has high heritage (24 of them in GOES/EXIS). It is
supported by a 5 lines/inch mesh, which has heritage from the Hi-C sounding
rocket flights and avoids the diffraction issues of the 70 lines/inch mesh
used on SDO/AIA and TRACE (Lemen et al., 2012; Lin et al., 2001). A second
C/Al/C filter directly in front of the detector eliminates visible light from
possible pinholes in the primary filter or from stray light in the instrument.

###  3.5 Spatial Resolution

SunCET requires spatial resolution better than 30. CME flux ropes often
manifest observationally as a cavity which trails behind a bright front
(Forsyth et al., 2006). The smallest cavities have a diameter of 0.2
$R_{\odot}$ (180) and are approximately circular, which corresponds to a
circumference of $\sim$600 (Fuller and Gibson, 2009). To account for non-
circularities, we require $\sim$20 points outlining the cavity, which results
in our spatial resolution requirement of 600/20 = 30. Figure 7 shows a cavity
observed in PROBA2/SWAP (3.16 resolution) binned down to demonstrate that
cavities can be resolved at this resolution in practice.

Projected performance: SunCET provides 20 resolution. Its plate scale is
4.8/pixel so 2$\times$2 binning can be applied, which meets the Nyquist
sampling criterion.

Figure 7: CME cavity observed in PROBA2/SWAP 174 Å binned down to SunCET
required resolution of 30 (projected performance is 20). The cavity remains
easily identifiable. SunCET’s SNR will be 9–30$\times$ higher off disk, making
CME identification even easier. The 1.7 $R_{\odot}$ FOV shown here, the
largest of any solar EUV imager to date, is SWAP’s; SunCET’s extends to 4
$R_{\odot}$. Adapted from Byrne et al. (2014).

###  3.6 Mirrors

Figure 8: SunCET’s compact Ritchey-Chrétien telescope, which fits inside a 6U
CubeSat with all typical bus components.

SunCET contains a Ritchey-Chrétien (RC) telescope encased in a vacuum chamber
with a one-time-release door (Figure 8). This type of telescope has good
performance for wide fields of view (Figure 9) and has been used frequently
for similar instruments (e.g., SOHO/EIT, STEREO/EUVI, GOES/SUVI). Despite its
compact size, the telescope achieves nearly flat resolution across the wide
FOV. The mount for the secondary mirror is designed with a coefficient of
thermal expansion matching the mirror to account for focus sensitivity.

Figure 9: Left: Ray trace of SunCET optics. Right: 80% encircled spot diameter
over the FOV. This simple design yields excellent performance, with a mean
resolution of 20 that is flat across nearly the entire FOV.

###  3.7 Signal to Noise Ratio (SNR)

SunCET requires a signal to noise ratio (SNR) $\geq$10\. This is the
international standard that defines digital image quality as “acceptable” (ISO
12232, 2019). The same standard defines SNR of 40 as “excellent”. These
numbers are in line with the expectations of experts that have done CME image
processing with coronagraph and EUV imager data.

Table 2: SunCET SNRs for on-disk features and CME loops above the limb.
Radiances are from GOES/SUVI 195 Å images of the 2017-09-10 CME (Seaton and
Darnel, 2018) and are extrapolated beyond its FOV of 1.7 $R_{\odot}$. SNR at
all heights is above the level that ISO 12232 defines as “excellent”.

[HTML]1A73C9 |  Quiet Sun |  Active Region | Flare | 1.05 $R_{\odot}$ | 1.5 $R_{\odot}$ | 2 $R_{\odot}$ | 3 $R_{\odot}$ | 3.5 $R_{\odot}$ | 4 $R_{\odot}$
---|---|---|---|---|---|---|---|---|---
| Radiance
---
[W/m2/sr]
0.1 | 10 | 40 | 0.2 | $1.5\text{\times}{10}^{-2}$ | $3\text{\times}{10}^{-3}$ | $3\text{\times}{10}^{-4}$ | $1\text{\times}{10}^{-4}$ | $3\text{\times}{10}^{-5}$
Effective exposure [s] | 0.025 | 0.025 | 0.025 | 0.025 | 10 | 10 | 10 | 10 | 10
e-/res-element | $1.48\text{\times}{10}^{4}$ | $1.48\text{\times}{10}^{6}$ | $5.94\text{\times}{10}^{6}$ | $2.97\text{\times}{10}^{4}$ | $8.9\text{\times}{10}^{5}$ | $1.78\text{\times}{10}^{5}$ | $1.78\text{\times}{10}^{4}$ | $5.94\text{\times}{10}^{3}$ | $1.78\text{\times}{10}^{3}$
| Saturation limit
---
[e-/res-element]
$1.08\text{\times}{10}^{5}$ | $1.08\text{\times}{10}^{5}$ | $1.08\text{\times}{10}^{5}$ | $1.08\text{\times}{10}^{5}$ | $1.08\text{\times}{10}^{6}$ | $1.08\text{\times}{10}^{6}$ | $1.08\text{\times}{10}^{6}$ | $1.08\text{\times}{10}^{6}$ | $1.08\text{\times}{10}^{6}$
SNR | 122 | Saturated | Saturated | 172 | 944 | 422 | 133 | 77 | 42

Projected performance: Table 2 shows the SunCET SNR as a function of distance
from the sun, based on the parameters shown in Table 3. Conservative radiance
estimates come from GOES/SUVI 195 Å images of a CME that was tracked all the
way to the edge of the SUVI 1.7 $R_{\odot}$ FOV (Seaton and Darnel, 2018). For
the solar disk, the effective exposure is the median of three 0.025-second
images; for 1.05–4 $R_{\odot}$, it is the median of ten 1-second exposures.
This removes energetic particle tracks and, for the long exposure, increases
the full-well saturation limit of the detector by a factor of 10. These
conservative estimates show that SunCET CME measurements would have an
excellent SNR of 42 even out at 4 $R_{\odot}$.

Table 3: SunCET instrument parameters needed to calculate SNR.

[HTML]1A73C9  Instrument parameter | Value | Description
---|---|---
Wavelength | 170–200 Å | Broadband response defined by mirror coating
| Aperture
---
size
44.9 cm2 |  | 9.6 cm diameter truncated on two sides
---
to a height of 7.62 cm and a 4.8 cm
diameter secondary mirror obscuring its center
| Weighted factor
---
for broadband
6.88 |  | 7 emissions in the bandpass weighted by their
---
quiet-Sun intensity to the 195 Å emission line
(see Table 1)
Pixel size | 7 $\upmu$m $\times$ 7 $\upmu$m | e2v CIS115 datasheet and confirmed in house
Pixel array | 1500 $\times$ 1500 | Full array is 1504 $\times$ 2000; $\sim$5 rows dedicated to dark
FOV | 4 $R_{\odot}$ | Design FOV (requirement is 3.5 $R_{\odot}$)
Plate scale | 4.8/pixel |  | From pixel size, number of pixels, and FOV;
---
Note that 2$\times$2 binning will be applied,
resulting in 9.6/resolution-element
| Optics
---
throughput
0.06 |  | 2 mirrors with B4C/Mo/Al coatings (0.35 each),
---
entrance Al/C filter (0.6) with 5 lpi filter mesh (0.98),
Al secondary/pinhole filter (0.85)
Quantum yield | 18.3 e-/ph | Average over 170–200 Å bandpass
Dark noise |  $<$0.08 e-/pixel/sec | At -10°C, from LASP lab tests
Readout noise | 5 e-/pixel | From LASP lab tests
Fano noise | 1.3 e-/pixel | Fano factor of 0.1 for Si
Max read rate |  | 0.1 sec (full frame)
---
0.025 (up to 500 rows)
In SunCET, 500 rows corresponds to 0–1.33 $R_{\odot}$

Few observations of the extended corona above $\sim$2 $R_{\odot}$ have been
made in the EUV, but among these there is clear evidence that the CME signal
will be detectable (Tadikonda et al. 2019, Figure 10). At about 3 $R_{\odot}$,
noise in SUVI becomes comparable to solar signal. SunCET, however, is
optimized for this large FOV. SunCET has a larger primary mirror geometric
area (3.5$\times$), broadband wavelength response (6.88$\times$), and larger
pixel solid angle (16$\times$) for a total 385$\times$ boost in signal
compared to SUVI. Furthermore, the SunCET mirrors will be polished to highest
degree possible, up to 3 times the smoothness of SUVI’s, to minimize scattered
light.

Figure 10: Composite of GOES/SUVI 195 Å off-point images that shows solar
structure out to 3 $R_{\odot}$ — even without a bright CME — before straylight
in the instrument becomes comparable with the coronal signal. Adapted from
Tadikonda et al. (2019).

##  4 Spectrograph Design

The SunCET irradiance spectrograph channel is a high-heritage off-Rowland
circle design based on the SDO/EVE Multiple EUV Grating Spectrographs A2
(MEGS-A2) channel (Crotser et al., 2007). It provides the full-Sun solar
irradiance from 170-340 Å at 1 Å spectral resolution. This EUV range is
important for overlapping with the SunCET imager EUV bands for calibration
purposes and provides additional science capability. It observes Fe IX through
Fe XVI emission lines that often experience coronal dimming during CMEs (Woods
et al., 2011; Mason et al., 2014, 2016, 2019). This allows for halo CME
kinematics to be tracked even if SunCET is not deployed on multiple platforms
with stereoscopic viewing angles. It also enables study of the energetics
powering the CME as a function of time. It shares the vacuum door and detector
with the SunCET imager, but has its own optical path including the entrance
slit, filters, and grating. These measurements are especially pressing because
EVE/MEGS-A experienced a CCD electronics anomaly in 2014 May, preventing the
continued solar observations by MEGS-A. While other EVE channels and new GOES
EUV Sensor (EUVS) channels are continuing solar EUV observations in the
170-340 Å range, they are only broadband measurements that are not optimized
for coronal dimming irradiance observations nor for detailed calibration of
solar EUV imagers.

###  4.1 Spectrograph Dynamic Range

The solar irradiance values, as measured from SDO/EVE (Woods et al., 2012),
from 170-340 Å range from ${10}^{-6}$–${10}^{-2}$ W/m2/nm due to variations in
the peaks of the emission line in this range, the reduced irradiance values
between the strong emission lines, as well as solar activity including solar
minimum times and during the largest solar flares; therefore, the required
dynamic range of the spectrograph is $1\text{\times}{10}^{4}$.

Projected performance: The $8\text{\times}{10}^{6}$ dynamic range discussed in
Section 3.1 is more than two orders of magnitude better than needed for the
spectrograph.

###  4.2 Spectrograph Spectral Range and Resolution

The SunCET spectrograph requires a spectral range between 170-340 Å and 1 Å
spectral resolution. The entrance to the spectrograph is a 3 $\times$ 0.028 mm
in order to maximize the slit image height (cross-dispersion direction) on the
allotted 500 pixel height of the detector to maximize the SNR, while the width
is optimized to meet the 1 Å spectral resolution requirement — it is this slit
width and the grating ruling that limits the spectral resolution. The grating
ruling, distance and curvature are all optimized in order to meet the spectral
range and resolution as well.

The optical path after being dispersed from the grating goes through the hole
in the secondary imager mirror and onto the common detector. The grating is a
Type-I concave imaging grating in order to image the slit onto the detector.
There is an Al/C entrance filter mounted to the entrance slit in order to
limit the spectral bandpass close to the required range, and an additional Al
filter prior for additional bandpass rejection at the entrance to the imager
optical cavity as well as to reduce any stray light or pinholes that may
develop in the first filter.

Given the 1500 allotted pixels in the dispersion range, this gives a plate-
scale resolution of approximately 0.11 Å per pixel; therefore the spectrograph
will oversample the spectral resolution by about a factor of 9$\times$, or
4.5$\times$ with the 2$\times$2 pixel binning. This allows for fits to
spectral lines to be performed and allow for Doppler shift measurements of
emission lines and plasma velocity flows during flares to be calculated
(Chamberlin, 2016; Hudson et al., 2011)

Projected performance: SunCET provides 1 Å spectral resolution across the
fully observed 170-340 Å spectral range.

###  4.3 Spectrograph Signal to Noise Ratio (SNR)

The SunCET spectrograph also requires a SNR of 10 or better as discussed in
3.7. This is achieved by using a long-slit and minimal optical elements, along
with the high QE detector. The slit was also sized, and filter thickness
optimized, to maximize the SNR without while conservatively not saturating or
even go beyond the linear full well capacity of the the CMOS sensor. Even with
a very large factor of 10 increase (Chamberlin et al., 2008, 2018) during
flares for these lines given in Table 4, they will still be almost another
factor of 2 below the full-well of this sensor.

Table 4: The SunCET spectrograph SNRs for various strong emission lines.
Irradiances are from SDO/EVE (Woods et al., 2011). SNR at all heights is above
the level that ISO 12232 defines as “excellent”.

[HTML]1A73C9  Wavelength (Å) | 171 | 193.5 | 195 | 304 | 335
---|---|---|---|---|---
| Irradiance
---
[W/m2/sr]
$6.7\text{\times}{10}^{-4}$ | $4.5\text{\times}{10}^{-4}$ | $6.3\text{\times}{10}^{-4}$ | $1.0\text{\times}{10}^{-3}$ | $1.0\text{\times}{10}^{-4}$
Integration [s] | 10 | 10 | 10 | 10 | 10
Counts/Pixel | 737 | 495 | 693 | 1100 | 110
SNR | 272 | 237 | 282 | 444 | 145

Projected performance: Table 4 shows the SunCET spectrograph SNR for five
strong emission lines, based on the parameters shown in Table 5. These
estimates show that SunCET solar spectral irradiance measurements would have
an excellent SNR of better than 100.

Table 5: SunCET spectrograph instrument parameters needed to calculate SNR.

[HTML]1A73C9  Instrument parameter | Value | Description
---|---|---
Wavelength | 170–340 Å |  | Contains various strong emission lines,
---
including some that show coronal dimming.
Defined by grating equation.
| Aperture
---
size
0.0098 cm2 | 3.0 mm tall $\times$ 28 $\upmu$m wide
| Number of Pixels
---
per emission line
2000 |  | 500 pixels tall $\times$ 4 pixels wide
---
(defined by slit)
Pixel size | 7 $\upmu$m $\times$ 7 $\upmu$m | Teledyne e2v CIS115 datasheet and confirmed in house
Pixel allocation | 500 $\times$ 1500 | Full array is 1504 $\times$ 2000; $\sim$5 rows dedicated to dark
FOV | Full Sun | Solar Irradiance, image the slit
Plate scale | 0.011 nm |  | From pixel size, number of pixels, wavelength range;
---
Note: oversampling spectral resolution of 0.1nm
| Optics
---
throughput
0.0122 |  | Grating Efficiency (0.06), Pt Grating Coating (0.4),
---
Al/C entrance filter (0.6), Al secondary/pinhole filter (0.85)
Quantum yield | 18.3 e-/ph | Average over 170-200 Å bandpass
Dark noise |  $<$0.08 e-/pixel/sec | At -10°C, from LASP lab tests
Readout noise | 5 e-/pixel | From LASP lab tests
Fano noise | 1.3 e-/pixel | Fano factor of 0.1 for Si

##  5 Detector

Figure 11: The Teledyne e2v CIS115 detector and LASP Compact Camera and
Processor (CCAP) that flew successfully on a NASA sounding rocket in 2018;
CCAP is now flying on the CSIM CubeSat launched in 2018.

SunCET uses a Teledyne e2v CIS115 back-illuminated, back-thinned CMOS sensor
(Table 3, Figure 11). This sensor is a 1504$\times$2000 pixel array, where a
square area of 1500$\times$1500 pixels will be dedicated to the image while
the remaining 500$\times$1500 pixels will record the spectrally dispersed slit
image from the irradiance spectrograph. Using a single detector to record data
from two technically different but scientifically complementary channels
significantly reduces the technical resources needed while maximizing science
potential.

In 2017, LASP developed custom electronics for readout of this sensor that
enables independent exposure control per row. A per-pixel readout is now being
developed. LASP’s “Compact Camera and Processor” (CCAP; Figure 11) system with
this detector was successfully flown in 2018 on the NASA 36.336 sounding
rocket (PI: T. Woods, U. of Colorado/LASP) and more recently in January 2020
on the NASA 36.356 sounding rocket (PI: S. Bailey, Virginia Tech). CCAP
includes a Xilinx Kintex-7 FPGA with an embedded 32-bit processor and
dedicated image compression core.

##  6 Instrument Requirements on Spacecraft

The instruments described above place requirements on the performance and
capabilities of whatever spacecraft hosts them. They are primarily driven by
the imager. Pointing accuracy must be better than 30with stability better than
30 RMS over 23 seconds and knowledge better than 10. This ensures that the
center of the sun stays in the center of portion of the detector dedicated to
the imager and does not drift significantly during or between integrations.
This pointing performance is achievable even on CubeSat platforms as
demonstrated by the Miniature X-ray Solar Spectrometer (MinXSS), Arcsecond
Space Telescope Enabling Research in Astrophysics (ASTERIA), Compact Spectral
Irradiance Monitor (CSIM), and others (Mason et al., 2017; Pong, 2018). Prime
science data generation is heavily dependent on CME occurrence rates, but
downlink schemes can easily be designed for flexibility and the “poorest” CMEs
can be ignored if there are bandwidth limitations. For CME occurrence rates at
the middle of the rising phase of the solar cycle, SunCET generates $\sim$28
MB/day for the imager, and $\sim$65 MB/day of data for the spectrograph. These
data are compressed using a lossless JPEG-LS scheme.

##  7 Conclusions

The SunCET instrument fills a crucial, historically under observed region of
the Sun — the middle corona — precisely the region where CMEs experience the
majority of their acceleration. This region is inherently very difficult to
observe because of the extreme intensity dynamic range between the bright
solar disk and the dim corona. SunCET introduces a new technology that avoids
the limitations of previous instruments. By developing a detector that can
vary exposure time across its surface, we can simultaneously observe the disk
without saturating and the dim middle corona; allowing us to track CMEs from
their initiation all the way through their primary acceleration phase.
Moreover, we can image spectra on the same detector with their own,
independent integration time.

Figure 12: Tracking a very fast CME in GOES/SUVI 195 Å base-difference images.
The CME quickly extended beyond the FOV of SUVI. SunCET’s FOV (light blue
shading) is more than twice as large. Adapted from Veronig et al. (2018).

There is a large body of knowledge for tracking CMEs in coronagraphs and EUV
imagers (Sarkar et al., 2019; O’Hara et al., 2019; Veronig et al., 2018; Byrne
et al., 2014; Mierla et al., 2013; Bein et al., 2011; Gopalswamy et al., 2009;
Vršnak et al., 2007). SunCET data processing will employ the techniques
already developed for other observatories but improve the results because of
its wider FOV (e.g., Veronig et al. 2018; Figure 12) and that it does not
require the serendipitous alignment between instrument off-point campaigns and
CME occurrence (e.g., O’Hara et al. 2019).

Below we summarize:

  1. 1.

The majority of CME acceleration occurs in a historical observational gap: the
middle corona

  2. 2.

Observations of full CME acceleration profiles provide tight constraints on
models and thus our physical understanding of how the magnetically-dominated
corona influences CME kinematics

  3. 3.

SunCET provides these observations, overcoming the limits of traditional
technologies with a novel simultaneous high-dynamic-range detector

  4. 4.

SunCET is compact and thus suitable for CubeSat missions or an instrument on a
larger spacecraft

SunCET is presently in a NASA-funded, competitive Phase A as a 6U CubeSat and
has also been proposed to NASA as an instrument onboard a 184 kg Mission of
Opportunity.

##  8 Acknowledgements

J.P.M thanks the numerous people who contributed to the development of the
SunCET concept design and the reviewers for their commments that made this
paper stronger. A.M.V. and K.D. acknowledge the Austrian Space Applications
Programme of the Austrian Research Promotion Agency FFG (ASAP-11 4900217
CORDIM and ASAP-14 865972 SSCME, BMVIT).

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