# The State of the White-Light Corona over the Minimum and Ascending Phases of
Solar Cycle 25 – Comparison with Past Cycles

Philippe Lamy  Hugo Gilardy  Laboratoire Atmosphères, Milieux et Observations
Spatiales, CNRS & UVSQ, 11 Bd d’Alembert, 78280 Guyancourt, France

###### keywords:

Corona, K-corona, Activity

##  1 Introduction

The white-light images obtained with the “LASCO-C2” Large-Angle Spectrometric
COronagraph (Brueckner et al. (1995)) of the Solar and Heliospheric
Observatory (SOHO: Domingo, Fleck, and Poland (1995)) have allowed an
unprecedented continuous coverage of the activity of the solar corona,
starting in 1996 and still on-going. A first analysis of the temporal
evolution of the white-light corona over the first 18.5 years (1996.0 –
2014.5) based on these images was performed by Barlyaeva, Lamy, and Llebaria
(2015). They showed that the K-corona tracks solar activity at all time scales
up to the solar cycle, including mid-term quasi-periodicities (also known as
quasi-biennial oscillations or QBOs). Among the various indices and proxies
that they considered, the strongest correlation of the integrated coronal
radiance was found with the total magnetic field. Lamy et al. (2014) compared
in detail the solar minima of Solar Cycles (SC) 23 and 24 and found a 24 %
decrease of the integrated radiance of the latter minima, but noted a very
different behaviour of the northern and southern hemispheres with decreases of
17 % and 29 %, respectively. Phasing the ascending branches of the radiance of
SC 23 and 24, they estimated the duration of SC 23 at 12 years and 3 months.
The eruptive activity of the corona over 23 years (1996.0 – 2019.0) was
investigated by Lamy et al. (2019) in their review of coronal mass ejections
(CMEs) that compared their properties reported by five catalogs, one manual
(CDAW) and four automated (ARTEMIS, CACTus, SEEDS, and CORIMP). They found
that the occurrence and mass rates track the indices/proxies of solar activity
likewise the radiance of the corona, but that the strongest correlation was
with the radio flux F10.7. However the correlation coefficients were different
during the two solar cycles, implying that the CME rates were relatively
larger during SC 24 than during SC 23. Another striking feature of SC 24 was
the significant deficit in both occurrence and mass rates of CMEs in the
southern hemisphere in comparison with the northern one.

With new LASCO-C2 data now extending to the beginning of 2022, thus covering
the complete minimum phase of SC 25 and its ascending phase, we are in
position to extend our past analysis. We are particularly interested in
characterizing these phases and comparing them with those of the past solar
cycles on the basis of the evolution of the radiance of the K-corona and the
CME activity. Another valuable aspect of describing and quantifying the
present state of the corona consists in presenting the context for the on-
going solar space missions and particularly, the instruments imaging the
corona: the Wide Field Imager for Solar Probe (WISPR; Vourlidas et al. (2016))
on Parker Solar Probe (PSP; Fox et al. (2016)), the Metis coronagraph
(Antonucci et al., 2020) and the Solar Orbiter Heliospheric Imager (SoloHI;
Howard et al. (2020)) on Solar Orbiter (SOLO; Müller et al. (2020)).

The present article makes use of images of the radiance of the K-corona and of
the ARTEMIS-II catalog part of the LASCO-C2 Legacy Archive111http://idoc-
lasco-c2-archive.ias.u-psud.fr hosted at the Integrated Data and Operation
Center (formerly MEDOC) of Institut d’Astrophysique Spatiale. It is organized
as follows: In Section Sec:Op, we briefly summarize the operations of SOHO and
of LASCO and its performance. Section Sec:Kcor is devoted to the analysis of
the state of the K-corona and Section Sec:CME to the properties of CMEs. In
Section Sec:discussion, we discuss our results in the broader context of solar
activity and the predictions for Solar Cycle 25, and we conclude in Section
Sec:conclusion.

##  2 SOHO and LASCO Operations

Sec:Op As far as LASCO is concerned, the operation of SOHO during the past
years was nominal. SOHO continues to be periodically (every three months)
rolled by 180∘ so that solar north periodically alternates between up and down
on the LASCO images in the instrument reference frame. The SOHO attitude is
such that its reference orientation is the perpendicular to the ecliptic plane
causing the projected direction of the solar rotational axis to oscillate
between $\pm$ 7∘ 15’ around the “vertical” direction on the LASCO images.

The in-flight performances of C2 slowly evolved as a consequence of the aging
of the instrument. As an example, Figure Fig:calib illustrates the evolutions
of the offset bias of the CCD detector and of the calibration factor updated
to the end of 2021. The rapid evolution of the offset bias that prevailed
during the first years of operation continues to level off so that the change
during the last few years was very limited. The calibration factor for the
orange filter derived from thousands measurements of stars present in the C2
field of view (Llebaria, Lamy, and Danjard (2006); Gardès, Lamy, and Llebaria
(2013)) exhibits a general increasing trend translating the continuous decline
of the sensitivity of the C2 detector at a rate of typically 0.3 % per year.
Apart from the sudden jump in 1999 linked to the “hibernation” interval when
SOHO lost its pointing, the two decreases that took place in (2011 – 2013) and
(2019 – 2021) have probably their origin in the electronics of the instrument.
This led us to introduce six linear functions to represent the temporal
evolution of the factor. Inside each of the six regimes, the deviations of the
measurements from the linear fits do not exceed 1 %.

Figure 1.: Upper panel: temporal variation of the offset bias of the LASCO-C2
camera. Lower panel: temporal variation of the LASCO-C2 calibration factor for
the orange filter in units of $10^{-12}\,\overline{{\rm B}_{\odot}}/({\rm
DN}\,{\rm sec}^{-1}\,{\rm pix}^{-1})$.Fig:calib

The cadences of the routine full-frame images of 1024 $\times$ 1024 pixels
used to detect and characterize the CMEs and of the polarization sequences
generating the images of the radiance of the K-corona at the format of 512
$\times$ 512 pixels, all taken with the orange filter, have remained at about
the same past levels. As illustrated in Figure Fig:cadence, the monthly
averaged daily rates amounted to $\approx$ 120 and $\approx$ 4 for the full-
frame images and the polarization sequences, respectively.

Figure 2.: Monthly averaged daily rates of the routine radiance images (upper
panel) and of the polarization sequences (lower panel) obtained with the
LASCO-C2 coronagraph with the orange filter. The out-of-scale values of the
latter rate correspond to high cadence polarization campaigns. Those that took
place during the last two years were performed during successive perihelion
passages of the Parker Solar Probe.Fig:cadence

##  3 Characterization of the K-Corona

Sec:Kcor The polarization sequences were processed following the method
developed by Lamy et al. (2020) to produce calibrated images of the polarized
radiance $pB$, of the radiance of the K-corona $B_{K}$, and of the electron
density. As these three quantities behave in a similar way, our present
analysis is limited to $B_{K}$.

###  3.1 Structure of the K-corona

Sec:K_struc An overview of the temporal evolution of the global structure of
the corona is given by the multi-annual synoptic map of the radiance of the
K-corona over 26.2 years (1996 –- 2022.2) at an elongations of 3.5 R⊙ (Figure
Fig:smap). The difference between the maxima of SC 23 and of SC 24 is visually
overwhelming and has already been addressed in many past publications. Equally
striking are the differences between the three minima in both strength and
latitude extent of the streamer belt; this will be further explored below. A
final noteworthy feature is the fast development of the activity in the rising
phase of SC 25 in comparison with the two past cycles, but this will become
more obvious when considering the temporal evolution of the global radiance
which is the topics of the next section.

Figure 3.: Multi-annual synoptic map of the radiance of the K-corona expressed
in units of $10^{-10}$ $\overline{B_{\odot}}$ at an elongation of 3.5
R⊙.Fig:smap

###  3.2 Temporal Evolution of the Radiance of the K-corona

Sec:K_evolution Following our past works (e.g. Barlyaeva, Lamy, and Llebaria
(2015)), the radiance of the K-corona was globally integrated in an annular
region extending from 2.7 to 5.5 R⊙ and the individual values were averaged
over the Carrington rotations. These authors compared the temporal variation
with six indices and proxies of solar activity, but we limit the present
comparison to the three most relevant: Sunspot Number
(SSN)222http://sidc.oma.be/silso/datafiles, Total Photospheric Magnetic Flux
(TMF)333Courtesy Y.-M. Wang, and Decimetric Radio Flux at 10.7 cm
(F10.7)444http://www.ngdc.noaa.gov/stp/space-weather/solar-data/solar-
features/solar-radio/. The first two are photospheric indices, whereas F10.7
combines chromospheric and coronal activities. Due to an equipment failure at
the Wilcox Solar Observatory, the monthly reductions of the photospheric maps
have been delayed and, as a consequence, the TMF data are presently available
only until CR 2249 (21 October 2021). Figure Fig:Bk_compare confirms the
conclusion of Barlyaeva, Lamy, and Llebaria (2015) that the integrated
radiance tracks the indices of solar activity, the highest correlation being
with the TMF followed by the F10.7 and SSN. Unlike these last two indices, TMF
and $B_{K}$ agree in many small scale variations and further, in the
remarkable increase that took place at the end of 2014 and persisted to early
2015. It resulted from an unusual configuration of the magnetic field
following the emergence of the large sunspot complex AR 12192 in October 2014
as analyzed by Sheeley and Wang (2015). It caused the coronal plasma to be
trapped at low latitudes and prevented CMEs from erupting. This process
inflated a bulge in the corona creating an anomalous surge of brightness
extensively analyzed by Lamy et al. (2017).

The last few years of the descending phases of SC 23 and 24 are remarkably
similar reaching the same base level during the following minima as will be
discussed in the next section. Whereas we correlated the ascending branches of
SC 23 and 24 to set the duration of SC 23 at 12 years and three months, we
correlated their descending branches to set the duration of SC 24 at 11.0
years (i.e. the canonical duration of a solar cycle), in agreement with the
value determined by the WDC-SILSO data center.

Figure Fig:Bk_NS displays the temporal variation of the integrated radiance
separately in the northern and southern hemispheres. Whereas marked
differences were present during SC 24 particularly during its rising and
maximum phases, they tended to disappear thereafter and the two variations are
remarkably similar during the minimum and ascending phases of SC 25.

Figure 4.: Comparison of the temporal variation of the global radiance of the
K-corona integrated from 2.7 to 5.5 R⊙ with those of the total magnetic field
(TMF expressed in units of Gauss), the sunspot number (SSN), and the radio
flux at 10.7 cm (F10.7) expressed in Solar Flux Unit ($10^{-22}\,{\rm W}\,{\rm
m}^{-2}\,{\rm Hz}^{-1})$). All quantities are averaged over the Carrington
rotations.Fig:Bk˙compare Figure 5.: Comparison of the temporal variation of
the global radiance of the K-corona integrated from 2.7 to 5.5 R⊙ and averaged
over the Carrington rotations in the northern and southern
hemispheres.Fig:Bk˙NS

###  3.3 Comparison of the Minima of Solar Cycles 23, 24, and 25

Sec:minimaK Our detailed analysis of the three minima starts with the
comparison of three typical images of the K-corona as illustrated in Figure
Fig:3minimaK. The most striking difference already alluded to in Section
Sec:K_struc concerns its structure during the prolonged anomalous minimum of
SC 24. As analyzed by Lamy et al. (2014), the observed latitudinal extent of
the streamer belt remained large, consistent with a large tilt angle of the
heliocentric current sheet (Manoharan, 2012) and a low polar field (Petrie,
2013). In contrast, the minima of SC 23 and 25 are characterized by the quasi
similar structure of a flat streamer belt corresponding to the simple “dipole”
geometry of the large-scale magnetic field of solar minima. Conversely, the
coronal holes reached their usual low latitudes of approximately $15^{o}$ to
$20^{o}$ suggesting that the polar field was restored to its “nominal” level
during the SC 25 minimum.

Figure 6.: Three images of the K-corona obtained with the LASCO-C2 coronagraph
at three consecutive minima of solar activity: SC 23 (left panel), SC 24
(middle panel), and SC 25 (right panel). The radiance is expressed in units of
$10^{-10}$ $\overline{B_{\odot}}$ and its logarithm is coded according to the
color bar.Fig:3minimaK

The comparison is pursued by considering the detailed temporal evolution of
the integrated radiance of the K-corona between 2.7 and 5.5 R⊙ over eight
years centered on the three minima conveniently named first (SC 23), second
(SC 24), and third (SC 25). In addition, different sectors were considered as
defined by Barlyaeva, Lamy, and Llebaria (2015): the northern and southern
sectors are centered on the polar direction and the equatorial sector combines
the east and west sectors centered on the equatorial direction; all of them
have a full angular width of 30∘. Following their procedure, we shifted the
time intervals in order to phase the minima according to the duration of the
solar cycles as already determined: 12 years and 3 months for SC 23 and 11.0
years for SC 24 (Figure Fig:phasing). The behaviours and the base levels of
the radiance temporal profiles of the second and third minima are remarkably
similar in the case of the global corona and of the southern and northern
sectors with a very slight increase in the latter case, the third minima
exceeding the second one by $\approx$10 %. In contrast, the base level of the
first minimum was significantly larger, exceeding that of the second one by
$\approx$32 % as determined by Lamy et al. (2014). The situation is completely
different in the equatorial sector with much larger differences. Averaging the
local fluctuations, we estimated an increase of the base levels of $\approx$30
% between the first and third minima and up to $\approx$50 % between the first
and second minima. The onset of the ascending branch of SC 25 took place in
early May 2020 on the basis of the temporal profiles of the global coronal and
the south sector, but clearly the north sector is lagging by at least 1.5 year
with a hint of a take-off in October 2021.

Figure 7.: Results of phasing the temporal variations of the radiance of the
K-corona during the minima of Solar Cyles 23, 24, and 25. The radiance is
globally integrated from 2.7 to 5.5 R⊙ and then in different sectors:
southern, northern, and equatorial. The gaps in the blue curves (SC 23)
correspond to the loss of SOHO.Fig:phasing

##  4 Coronal Mass Ejections over the Minimum and Ascending Phases of Solar
Cycle 25

Sec:CME This analysis makes use of the ARTEMIS-II catalog which relies on the
detection of CMEs on synoptic maps based on their morphological appearance
(Boursier et al. (2009); Floyd et al. (2013)). It lists CMEs detected since
June 1996 with the following parameters: time of detection at 3 R⊙, central
apparent latitude, angular width, and intensity. The intensity of a CME is
calculated by integrating its radiance on the synoptic maps at 3 R⊙ and
subtracting the local coronal background. It does not strictly corresponds to
the total radiance as recorded on the images, but it offers a valuable
estimate of the CME strength (Lamy et al., 2019). For a large fraction of the
CMEs ($\approx$ 60 %), the catalog further lists three different velocities
(“propagation”, “global”, and “median”), mass, and kinetic energy. We consider
below the last two velocities obtained by cross-correlating the detected CMEs
on the original synoptic maps at 3 and 5.5 R⊙. A global cross-correlation
yields the global velocity whereas a line by line cross-correlation produces a
distribution of velocities whose median value is taken as the median velocity.
The global velocity gives more weight to the front and central parts whereas
the median velocity gives an equal weight to every angular section of the CME.
As a consequence, the former is systematically larger than the latter. For the
time interval considered in this study, from 6 June 1996 to 7 February 2022,
the ARTEMIS-II (hereafter abbreviated to ARTEMIS for simplicity) catalog lists
a total of 42,165 CMEs, of which 23,885 have their velocities, mass, and
kinetic energy determined.

###  4.1 Occurrence and Mass

Sec:occur We first consider the temporal variation of the occurrence rate of
the whole set of CMEs corrected for the LASCO duty cycle as described by Lamy
et al. (2019) and calculated per Carrington rotation. Likewise the case of the
radiance of the corona, a comparison was performed with the selected indices
and proxies of solar activity: SSN, TMF, and F10.7 whose variations were
adequately scaled and shifted so as to best fit that of the CMEs during SC 24.
Indeed and as shown in Figure Fig:rate, it is not possible to perform the fits
over both SC 23 and 24 as the relationships were different during the two
cycles. In their review, Lamy et al. (2019) used SC 23 as a reference, but we
now favor SC 24 because of its immediate proximity to SC 25. Their conclusion
obviously still holds: the CME occurrence rate was relatively larger during SC
24 than during SC 23 as also found by Gopalswamy, Tsurutani, and Yan (2015)
using the sunspot number. The new data reveals that the occurrence rate
started to diverge from the common trend of the indices and proxies of solar
activity in the last years of the declining phase of SC 24 implying an excess
of CMEs with respect to the evolution of solar activity. This divergence
persisted during the minimum and ascending phases of SC 25, particularly
pronounced when comparing with the radio flux. Section Sec:minimaCME below
analyzes in detail the situation of this minimum compared with the two past
ones. Regarding the situation of the ascending phase of SC 25, two years after
its minimum the CME occurrence rate has reached a high of 5.2 CME per day
whereas it was only 3.7 CME per day two years after the minimum of SC 24, a
significant increase.

Figure 8.: Temporal variation of the CME occurrence rate per Carrington
rotation compared with those of the total magnetic field (TMF), the sunspot
number (SSN), and the radio flux at 10.7 cm (F10.7) expressed in Solar Flux
Unit ($10^{-22}\,{\rm W}\,{\rm m}^{-2}\,{\rm Hz}^{-1})$)

. Fig:rate

We next consider the temporal variation of the occurrence rate of the whole
set of CMEs separately in the northern and southern hemispheres (upper panel
of Figure Fig:rate_NS). There was a slight excess of southern CMEs by a mere
2.3 % during SC 23 followed by a vigorous reversal during SC 24 with an excess
of northern CMEs by an astonishing 28 % (see also Table Table:stat). The same
trend persisted during the early years of SC 25, but at a much reduced level
of 11 % until early February 2022 and this resulted from the combined effect
of a larger base level during the minimum ($\approx$ 0.9 versus $\approx$ 0.7
CME per day) and a steeper rate of occurrence in the northern versus the
southern hemispheres. However, it appears that the southern rate is catching
up during the latest CRs.

Figure 9.: Temporal variation of the CME occurrence rates per Carrington
rotation in the northern and southern hemispheres. The upper panel corresponds
to the whole set of CMEs and the lower one is restricted to the CMEs with
known mass.Fig:rate˙NS

The set of CMEs with known mass presently amounts to 57 % of the global set,
the missing 43 % corresponding to faint CMES for which complete
characterization could not be achieved (e.g. lack of detection on three
consecutive synoptic maps). The temporal evolution of the occurrence rate of
these CMEs is displayed in the upper panel of Figure Fig:mass together with
that of the radio flux F10.7. Their correlation during SC 24 is better than in
the case of the whole set of CMEs except during two restricted time intervals,
from late 2012 to early 2013 and from mid-2014 to mid-2015. Unlike the case of
the whole set of CMEs, the occurrence rate of CMEs with known mass closely
tracks the radio flux during the minimum and ascending phases of SC 24. This
implies that the relative excess found for the whole set prominently results
from the relative overabundance of faint CMEs.

We next consider the temporal variation of the occurrence rate of the set of
CMEs with known mass separately in the northern and southern hemispheres
(lower panel of Figure Fig:rate_NS and Table Table:stat). There was a slight
excess of northern CMEs of 4 % during SC 23 followed by a vigorous reversal
during SC 24 with an excess of northern CMEs of 30 %. This is very much
consistent with the whole set of CMEs so that the two sets of CMEs followed a
similar trend during SC 23 and 24, namely a quasi equilibrium of northern and
southern CMEs during SC 23 and a large excess of northern CMEs of
approximately 30 % during SC 24. The situation is entirely different during
the early years of SC 25 with an excess of the whole set of northern CMEs of
11 % and an excess of southern CMEs with known mass of 15 %. As pointed out
above, this results from the relative overabundance of faint CMEs in the
northern hemisphere, but this is probably a temporary effect as the southern
rate is catching up during the latest CRs. Ultimately, we expect a situation
opposite to that of SC 24, that is a large excess of southern CMEs during SC
25.

Turning to the mass of CMEs accumulated per Carrington rotation, the lower
panel of Figure Fig:mass shows that it better tracks the radio flux than the
occurrence rate. This is especially true over SC 24 (note the similar local
drop centered on 2013.0) and during the minimum and ascending phases of SC 24.

Figure 10.: Temporal variations of the occurrence rate of CMEs with known mass
(upper panel) and of the CME mass rate (lower panel) per Carrington rotation
compared with that of the F10.7 radio flux expressed in Solar Flux Unit
($10^{-22}\,{\rm W}\,{\rm m}^{-2}\,{\rm Hz}^{-1}$).Fig:mass

###  4.2 Comparison of the Minima of Solar Cycles 23, 24, and 25

Sec:minimaCME ikewise the case of the radiance of the K-corona, we consider
the detailed temporal evolution of the occurrence rates of the CMEs over eight
years centered on the three minima. The time intervals were shifted in order
to phase the minima according to the determined duration of the solar cycles.
Figure Fig:phasingCMEall illustrates the case of the whole set of CMEs. A
first striking feature is the quasi systematically enhanced rates of CMEs
during the last minimum in comparison with the previous two, with only a few
local exceptions. As a consequence, the base level of the global (N+S)
population during the minimum of SC 25 amounts to $\approx$ 1.5 CME per day,
significantly higher than during the two previous minima, $\approx$ 0.85 and
$\approx$ 0.67 CME per day during the minima of SC 23 and 24, respectively. A
second striking feature is the steeper increase of the rate during the
ascending phase of SC 25 in comparison with the two past similar phases, this
effect being entirely produced by the southern CMEs. Figure Fig:phasingCMEmass
illustrates the case of the set of CMEs with known mass. Unlike the whole set
of CMEs, the variations of the occurrence rates over the eight years are
approximately similar implying a common base level of $\approx$ 0.37 (N+S) CME
per day. A steeper increase of southern CMEs during the ascending phase of SC
25 is suggested, but less pronounced than in the case of the whole set of
CMEs. The combination of these results confirms that the CMEs in excess number
during the minimum of SC 25 are prominently faint events, but that the overall
population of CMEs contributes to the steep increase of the occurrence rate in
the southern hemisphere.

Figure 11.: Results of phasing the temporal variations of the occurrence rate
of the whole set of CMEs during the minima of Solar Cyles 23, 24, and 25. The
rates were calculated globally and separately in the southern and northern
hemispheres. The gaps in the blue curves (SC 23) correspond to the loss of
SOHO.Fig:phasingCMEall Figure 12.: Results of phasing the temporal variations
of the occurrence rate of the set of CMEs with known mass during the minima of
Solar Cyles 23, 24, and 25. The rates were calculated globally and separately
in the southern and northern hemispheres. The gaps in the blue curves (SC 23)
correspond to the loss of SOHO).Fig:phasingCMEmass

###  4.3 Angular Width

Sec:width Figure Fig:yearly_aw_180 displays the temporal variation of the
annualized mean and root-mean-squared values of the apparent angular width $W$
of CMEs narrower than 180∘, thus extending a similar figure produced by Lamy
et al. (2019). It now appears that the mean width was significantly larger
during the minimum of SC 25 than during the minimum of SC 24, approximately
43∘ versus 25∘. In addition, the usual trend of increasing widths as solar
activity develops has not yet materialized during the rising phase of SC 25.

Regarding the distribution of angular widths, the number of CMEs during the
last three years was relatively small so that these CMEs do not affect our
past result based on the previous 23 years, prominently controlled by the two
maxima. As a reminder, the distribution of angular width follows an
exponential law characterized by a mean value of 42∘ and a constant slope of
-0.0107 in the range 40∘ to 300∘. The restriction to CMEs of known mass
marginally changes these values to 44∘ and -0.0106, respectively. There is a
clear turnover in the distributions at $\approx$ 300∘ used to define the
regime of halo CMEs. We introduce two intervals of width, $>$180∘ and $>$300∘,
to account for partial halos (Gopalswamy et al., 2003), and Figure Fig:halo
displays the monthly occurrence rate of ARTEMIS CMEs in these two intervals.
The much larger rates during SC 24 compared with SC 23 was already highlighted
by Lamy et al. (2019), but we now see that this trend persists during the
ascending phase of SC 25. Even more striking is the presence of full and
partial halo CMEs throughout the minimum of SC 25 whereas they were merely
absent during the past two minima.

Figure 13.: Temporal variation of the annualized mean and root-mean-squared
values of the apparent angular width of CMEs narrower than
180∘.Fig:yearly˙aw˙180

Figure 14.: Monthly occurrence rates of CMEs with width $>$180∘ (upper panel)
and with width $>$300∘ (lower panel).Fig:halo

###  4.4 Apparent Latitude

Sec:lat The spatial distribution of apparent latitudes of CMEs is best
perceived on the heliolatitudinal maps displayed in Figure Fig:CMElat where
CMEs are counted in boxes defined by a Carrington rotation and a latitude
interval of 2∘. We superimposed the evolution of the tilt angle of the
heliospheric current sheet (HCS) provided by the Wilcox Solar
Observatory555http://wso.stanford.edu/Tilts.html using the “classic” potential
field model as recommended. The two maps corresponding to the whole set of
CMEs and to the set of CMEs with known mass are highly consistent, implying
that the restriction on mass does not induce any bias. Concentrating on SC 25,
a noteworthy feature is the fact that the distribution of apparent latitudes
remains well bounded by the tilt of the HCS which was not the case during the
minimum of SC 24. A synthetic view is offered by Figure Fig:CMElat_mean which
displays the evolution of the mean value per Carrington rotation of the
apparent latitude separately in the northern and southern hemispheres. In the
case of the whole set, there appears during the last six months a hint of an
asymmetry between the two hemispheres with CMEs present at larger southern
latitudes than at north latitudes in agreement with the tilt angle. If
confirmed in the near future, this trend would be consistent with the faster
development of CME activity in the southern hemisphere pointed out in the
above sections.

Figure 15.: Heliolatitudinal distributions of the whole set of CMEs (upper
panel) and the set of CMEs with known mass (lower panel). The counts are
calculated per Carrington rotation and per latitude interval of 2∘ and are
color coded according to the color bar. The red lines correspond to the tilt
angle of the heliospheric current sheet in the northern and southern
hemispheres.Fig:CMElat

Figure 16.: Temporal variation of the mean value per Carrington rotation of
the CME apparent latitude separately in the northern (red line) and southern
(blue line) hemispheres. The upper panel corresponds to the whole set of CMEs
and the lower one is restricted to the CMES with known mass. The symmetric of
the two curves (light red and light blue lines) are over-plotted to facilitate
the comparison.Fig:CMElat˙mean

###  4.5 Kinematics

Sec:kine Figure Fig:speed displays the annual variation of the mean and
standard deviation values of the global and median speed distributions
reported by the ARTEMIS catalog. We used bi-monthly average values in order to
smooth the short-scale fluctuations while preserving the detail of the
variation during the solar cycles. The trends of i) the speeds tracking solar
activity and ii) larger speeds during SC 23 compared with the weaker SC 24
have already been underlined (e.g. Lamy et al. (2019)). The new salient
feature concerns the last two minima of solar activity: whereas both speeds
experienced a drastic reduction during the minimum of SC 24, it was less
pronounced during that of SC 25 and furthermore, nearly absent in the case of
the median speed. The cumulative distributions of the two speeds calculated
until 7 February 2022 are displayed in Fig. Fig:speed_cumul. As expected, the
spread of the global speeds is larger than that of the median speeds and the
median values reach 280 and 220 km s-1 for the global and median speeds,
respectively.

Fig. Fig:ke displays the temporal evolution of the kinetic energy of CMEs per
Carrington rotation. Curiously, the minimum of SC 25 witnessed episodes of
rather energetic CMEs and the following rising phase is particularly abrupt
when compared with that of the previous cycle.

Figure 17.: Temporal variation of the bi-monthly average values of the global
(upper panel) and median (lower panel) speeds of CMEs derived from the ARTEMIS
catalog.Fig:speed Figure 18.: Cumulative distribution functions of the
apparent global and median speeds of CMEs derived from the ARTEMIS
catalog.Fig:speed˙cumul Figure 19.: Temporal evolution of the kinetic energy
of CMEs per Carrington rotation.Fig:ke

###  4.6 Summary Statistics of Coronal Mass Ejections

Sec:stat Table Table:stat presents an updated version of Table 9 of Lamy et
al. (2019) synthesizing the statistical properties of the whole set of CMEs
reported by the ARTEMIS catalog until 7 February 2022. The results for SC 23
remain unchanged whereas those for SC 24 were completed to the end of the
cycle rounded to 31 December 2019; those for SC 25 are naturally limited to
its minimum and ascending phases. The last column sums up the above results
and gives an overview of the properties of CMEs during the past 26 years.

SC 23 featured a relatively well-balanced activity in the two hemispheres, the
only exception being a modest 10 % difference concerning those CMEs with width
larger then 30∘. This was no longer the case during SC 24 as the northern
hemisphere was much more active than the southern one. For instance, the
occurrence rates of northern CMEs exceed the southern ones by 28 % and 30 %
for the whole set of CMEs for that of CMEs with known mass, respectively. The
same situation prevailed for the other properties with unbalances ranging from
22 % to 37 % and culminating to a factor of 21 in the case of halo CMEs. The
first two years of SC 25 offer a different picture. The census of the whole
set of CMEs exhibits a 12 % surplus of northern CMEs whereas the opposite
situation prevails for the set of CMEs with known mass with a 15 % surplus of
southern CMEs. This implies that the northern hemisphere ejects a relatively
large number of faint CMEs whose mass could not be determined by our
procedure. This is confirmed by the statistics on the mass for which the
southern hemisphere systematically outperforms the northern one, and it
further stands out by a larger population of fast CMEs with speeds $>$350 km
s-1, the count of southern ones exceeding the northern one by an astonishing
51 %.

Table 1.: Statistical properties of the LASCO CMES listed in the ARTEMIS
catalog until 7 February 2022.

ALL CMEs
---
Count | SC23 | SC24 | SC25 | SC24+SC23+SC25
Total count | 20194 | 19732 | 2415 | 42341
Total count (north) | 9985 | 11092 | 1273 | 22350
Total count (south) | 10209 | 8640 | 1142 | 19991
Angular width $\leq 30^{o}$ | 11340 | 10835 | 1397 | 24572
Angular width $\leq 30^{o}$ (north) | 5764 | 5944 | 754 | 12462
Angular width $\leq 30^{o}$ (south) | 5576 | 4891 | 643 | 11110
Angular width $>30^{o}$ | 8854 | 8897 | 1018 | 18769
Angular width $>30^{o}$ (north) | 4221 | 5148 | 519 | 9888
Angular width $>30^{o}$ (south) | 4633 | 3749 | 499 | 8881
Angular width $>300^{o}$ (halos) | 11 | 231 | 59 | 301
Speed $\leq 350$ km s-1 | 7397 | 6594 | 668 | 14659
Speed $\leq 350$ km s-1(north) | 3659 | 3767 | 328 | 7754
Speed $\leq 350$ km s-1(south) | 3738 | 2827 | 340 | 6905
Speed $>350$ km s-1 | 5443 | 3599 | 269 | 9311
Speed $>350$ km s-1(north) | 2646 | 1981 | 107 | 4734
Speed $>350$ km s-1(south) | 2797 | 1618 | 162 | 4577
CMEs WITH KNOWN MASS
Count/Mass | SC23 | SC24 | SC25 | SC24+SC23+SC25
Total count | 12840 | 10193 | 937 | 23970
Total count (north) | 6305 | 5748 | 435 | 12488
Total count (south) | 6535 | 4445 | 502 | 11482
Total mass (g) | 1.5E+19 | 1.1E+19 | 7.8E+17 | 2.7E+19
Total mass (g) (north) | 7.6E+18 | 6.9E+18 | 3.4E+17 | 1.5E+19
Total mass (g) (south) | 7.7E+18 | 4.3E+18 | 4.4E+17 | 1.2E+19
Mean mass (g) | 1.2E+15 | 1.1E+15 | 8.3E+14 | 1.1E+15
Mean mass (g) (north) | 1.2E+15 | 1.2E+15 | 7.7E+14 | 1.2E+15
Mean mass (g) (south) | 1.2E+15 | 9.7E+14 | 8.8E+14 | 1.1E+15
Median mass (g) | 3.0E+14 | 2.6E+14 | 2.5E+14 | 3.3E+14
Median mass (g) (north) | 3.0E+14 | 2.6E+14 | 2.4E+14 | 2.8E+14
Median mass (g) (south) | 3.0E+14 | 2.5E+14 | 2.7E+14 | 2.9E+14

Table:stat

##  5 Discussion

Sec:discussion The prediction of amplitude and timing of SC 25 has been the
subject of many articles using a variety of techniques, and compilations may
be found in Courtillot, Lopes, and Le Mouël (2021) covering the time interval
(2011 – 2019) extended to 2021 by Burud et al. (2021) and Javaraiah (2022). It
is obviously beyond the scope of the present article to discuss this copious
literature, but we may attempt to identify a general trend and note that the
most recent publications converge to a strength of SC 25 comparable to SC 24
or slightly larger (Kumar et al., 2021). A few of them (e.g. Burud et al.
(2021); Chowdhury et al. (2021); Courtillot, Lopes, and Le Mouël (2021))
however concluded on a slightly weaker SC 25 and suggested that it will
witness the beginning of the upcoming Gleissberg cycle. The forecast consensus
of the NOAA/NASA co-chaired, international panel of 9 December 2019 concluded
on a peak in July 2025 ($\pm$ 8 months), with a smoothed sunspot number of
115, hence similar to SC 24. However, the ascending phase of SC 25 appears
much steeper than this prediction666https:// www. swpc. noaa. Gov/ news/solar-
cycle-25-forecast-update. In fact, the SNN monthly values of December 2021
(67.6), January (54) and February (59.7) 2022 are well above the forecast
values of 26.6, 29.0, and 31.5, respectively.

The question of which solar hemisphere would be dominant was considered by a
dozen articles recently reviewed by Javaraiah (2022). The consensus goes
towards a marked north–south asymmetry with activity dominant in the southern
hemisphere. Particularly interesting are the converging results on the peak
amplitudes in the northern and southern hemispheres using different methods:
66 and 83, respectively according to Pishkalo (2021), 64.3 and 83.8,
respectively according to Gopalswamy et al. (2022). The present hemispheric
sunspot numbers777http://sidc.oma.be/silso/datafiles go in that direction with
an excess of southern sunspots, the turnover having occurred at the onset of
the rising phase of SC 25.

The temporal evolution of the integrated radiance of the K-corona is in
excellent agreement with the current steep increase of solar activity as it
closely tracks the selected indices and proxies (Figure Fig:Bk_compare). The
correlations with the TMF during the terminal part of the descending branch of
SC 24, and with the minimum and rising phases of SC 25 are particularly
impressive. Furthermore, Figure Fig:Bk_NS reveals that the radiance in the
southern hemisphere started to exceed the northern one during the past year.

We are aware of only one work, that of Möstl et al. (2020), attempting to
predict the CME ejection rate for SC 25 or more precisely, the ICME rate with
direct implication for Parker Solar Probe in situ observations. They did so by
linking the SSN to the observed ICME rates in SC 23 and 24 with the list of
Richardson and Cane (2010) and their own ICME catalog. They then determined
linear relationships that they extended to SC 25 using nearly extreme
predictions of its strength. By construction, their temporal evolution of the
ICMEs rate tracks the SSN, but the rates themselves are very low, ranging from
15 per year (1 per CR) to 23 per year (1.7 per CR) at the beginning of 2020,
compared with $\approx$ 140 per CR detected by LASCO. This inherently results
from the fact that the number of ICMEs detected in situ are far less than that
of CMEs detected on high cadence coronagraphic images.

The occurrence rate of CMEs with known mass and the mass rate per CR closely
track the radio flux (Figure Fig:mass) and therefore the current steep
increase of solar activity, very much like the radiance. Regarding the
hemispheric occurrence rate of CMEs, the global set indicates an excess of
northern CMEs of 12 %. In strong contrast, the occurrence rate of southern
CMEs of known mass outperforms the northern one by 15 %; this percentage
increases to 30 % when considering the mass rate per CR and even to 51 % for
CMEs with speeds $>$350 km s-1(Table Table:stat). Clearly, the southern
hemisphere develops an overwhelming activity as predicted.

Finally, the persistent large number of halo CMEs following the trend already
observed during SC 24 in comparison with SC 23 (Figure Fig:halo) is best
explained by the weak total pressure in the heliosphere that prevailed after
the anomalous minimum of SC 24 and which facilitates the widening of CMEs so
as to become halo more frequently (Gopalswamy, Akiyama, and Yashiro (2020);
Gopalswamy et al. (2022)).

##  6 Conclusion

Sec:conclusion In this article, we have presented the state of the white-light
corona over the minimum and ascending phases of SC 25 on the basis of the
analysis of the temporal variation of its radiance $B_{K}$ and its CME
production rates and properties. Both closely track the indices/proxies of
solar activity, prominently the total magnetic field for $B_{K}$ and the radio
flux for the CMEs. Their evolution confirms the steep increase of the rising
phase of SC 25, much steeper than anticipated on the basis of predicted
sunspot numbers quasi similar to those of SC 24. This is obviously of utmost
interest for the forthcoming observations of the corona by SOLO and PSP. We
highlight below our most significant results.

  * •

The global radiance of the corona integrated between 2.7 and 5.5 R⊙ reached
the same base level during the minima of SC 24 and 25, but the latitudinal
extent of the streamer belt differed, being flatter during the latter minimum
and in fact more similar to that of the minimum of SC 23.

  * •

The correlation of the descending branches of SC 23 and 24 led to a duration
of SC 24 of 11.0 years similar to that given by the sunspot number.

  * •

The occurrence rate of the global set of CMEs, when adjusted to the variation
of the indices/proxies during SC 24, started to diverge at the onset of the
minimum of SC 25, the base level being significantly larger than during the
previous minima.

  * •

This is not the case of the occurrence rate of the set of CMEs with known mass
which closely sticks to the variation of the indices/proxies, and so does the
rate of CME mass per CR. This implies that the excess CMEs are faint, modestly
contributing to the mass budget.

  * •

The southern hemisphere appears significantly more active than the northern
one in agreement with several predictions and the current evolution of the
hemispheric sunspot numbers. In particular, the occurrence rate of the set of
CMEs with known mass, the total mass of CMEs, and the number of CMEs with
speeds larger than 350 km s-1in the southern hemisphere exceeds by far the
respective values in the northern hemisphere.

  * •

The mean apparent width of CMEs during the minimum of SC 25 did not drop to
the low values reached during the previous minimum and it remains at a nearly
constant level throughout the early phase of SC 25. A similar trend is
observed in the case of the occurrence rate of halo CMES.

Acknowledgments We thank Y.-M. Wang for providing the total magnetic field
data. The LASCO-C2 project at the Laboratoire Atmosphères, Milieux et
Observations Spatiales is funded by the Centre National d’Etudes Spatiales
(CNES). LASCO was built by a consortium of the Naval Research Laboratory, USA,
the Laboratoire d’Astrophysique de Marseille (formerly Laboratoire
d’Astronomie Spatiale), France, the Max-Planck-Institut für
Sonnensystemforschung (formerly Max Planck Institute für Aeronomie), Germany,
and the School of Physics and Astronomy, University of Birmingham, UK. SOHO is
a project of international cooperation between ESA and NASA.

Disclosure of Potential Conflicts of Interest The authors declare that they
have no conflicts of interest.

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