IS⊙IS Solar γ-Ray Measurements: Initial Observations and Calibrations

High-energy neutral solar radiation in the form of γ-rays and neutrons is produced as secondary products in solar flares. The characteristics of this emission can provide key information regarding the energization of charged particles, particularly when primary particles remain trapped in the corona. The Integrated Science Investigation of the Sun (IS⊙IS) suite on Parker Solar Probe is composed of instruments primarily intended to measure energetic charged particles. However, the High Energy Telescope (HET) in IS⊙IS was also designed with a supplementary neutral mode intended to measure γ-rays and neutrons. HET observed its first clear solar γ-ray event in connection with a hard X-ray flare, the eruption of a coronal mass ejection, and a solar energetic particle event on 2022 September 5. The X-ray spectral shape was observed to harden over the course of the event, culminating with the observation of γ-rays by HET. A coincident enhancement in the lower-energy Energetic Particle Instrument (EPI-Lo) was also observed, likely produced by incident solar γ-rays despite the EPI-Lo instrument not having any special neutral measurement capabilities. We use Monte Carlo modeling to reconstruct the incident γ-ray spectrum based on the measured spectrum to demonstrate that the combination of IS⊙IS instruments can measure hard X-rays and γ-rays from ∼60 keV–7 MeV. Despite the fact that this is a supplemental science goal of the mission, the capability of the IS⊙IS instruments to measure γ-rays is important for the study of this population due to the very limited instruments currently observing the Sun in γ-rays.


Introduction
Solar flares are eruptions produced by magnetic reconnection, the coming together and reconfiguration of magnetic field lines of opposite polarity, resulting in a release of the large amount of energy stored within the local coronal magnetic field (e.g., Priest & Forbes 2002, and references therein).It is estimated that 10%-50% of the magnetic energy released in a solar flare is converted into kinetic energy of accelerated charged particles (Emslie et al. 2012;Aschwanden et al. 2017).When the magnetic reconnection process occurs between open and closed magnetic field lines, called interchange reconnection (Fisk & Schwadron 2001;Crooker et al. 2002;Schwadron 2002;Wang & Sheeley 2004), the charged particles accelerated in the flare have an avenue to escape the solar corona along open field lines into interplanetary space.However, in reconnection between sets of closed magnetic field lines, the accelerated charged particles are confined to coronal loops.Even for interchange reconnection events, a portion of the charged particles are accelerated toward the chromosphere or trapped along closed magnetic field lines.In these cases, when the accelerated charged particles reach the dense ambient plasma of the chromosphere and photosphere at their magnetic footpoints, interactions between the plasma and the accelerated particles can produce various forms of neutral radiation, including X-rays, γ-rays, and neutrons (e.g., Ramaty et al. 1975;Murphy et al. 1987).
Solar γ-rays are produced through a variety of processes.Between ∼5 keV and ∼1 MeV, hard X-rays and γ-rays are produced via electron bremsstrahlung as power-law continuum emission with spectra similar to that of the accelerated parent electron spectrum.At higher energies, accelerated protons and α particles excite heavy ions in the ambient chromospheric and photospheric plasma, which, in turn, de-excite and produce γ-ray emission from ∼1-10 MeV (e.g., Ramaty et al. 1983;Murphy et al. 2009).While strong γ-ray de-excitation lines can often be individually identified with dedicated γ-ray spectrometers (e.g., Ramaty et al. 1975;Share & Murphy 1995;Vestrand et al. 1999;Rank et al. 2001), there is also a continuum of weaker lines that are not typically individually resolvable (Murphy et al. 2009).Additional γ-ray lines are frequently observed at 511 keV due to electron-positron annihilation (e.g., Share et al. 2004) and at 2.223 MeV due to neutron capture and the formation of deuterium (e.g., Prince et al. 1982;Hua & Lingenfelter 1987).At energies 100 MeV, neutral pions are produced via inelastic proton-proton and proton-α scattering.These neutral pions subsequently decay into Doppler-shifted 67.5 MeV γ-rays (e.g., Chupp & Ryan 2009).Between ∼10 and 100 MeV, γ-ray production is reduced.This energy range, however, can be well covered by the secondary neutron population produced via Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
interactions between the flare-accelerated ions and heavy ions in the ambient plasma (e.g., Ligenfelter et al. 1965;Chupp 1988).Solar neutron measurements are not addressed in this work but will be the topic of future studies utilizing this instrumentation.
A number of missions have studied solar γ-rays using dedicated solar observatories, including The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI; Lin et al. 2002) and the Gamma Ray Spectrometer (GRS) on board the Solar Maximum Mission (SMM; Forrest et al. 1980).Astrophysical γ-ray observatories, including the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-Ray Space Telescope (Atwood et al. 2009;Meegan et al. 2009), the Imaging Compton Telescope (COMPTEL) and the Energetic Gamma-Ray Experiment Telescope (EGRET) on board the Compton Gamma-Ray Observatory (CGRO), instruments on the International Gamma-Ray Astrophysics Laboratory (Winkler et al. 2003), and Phebus on the Granat International Astrophysical Observatory (Talon et al. 1993) have also made important contributions to our understanding of solar γ-rays despite their primary science objectives focusing on astrophysics.Mewaldt et al. (1977) studied the measurement of γ-rays and neutrons by the Caltech Electron/Isotope Spectrometer (EIS) silicon solid-state detector (SSD) telescope on board the International Monitoring Platform 7 (IMP 7) spacecraft.EIS included a neutral mode designed to monitor levels of neutral radiation.These authors concluded that γ-rays and neutrons produce an appreciable background in SSD telescopes designed primarily to measure charged particles.The EIS neutral mode inspired a similar triggering configuration implemented for the High Energy Telescope (HET) within the Integrated Science Investigation of the Sun (ISeIS) (McComas et al. 2016) high-energy Energetic Particle Instrument (EPI-Hi) suite (Wiedenbeck et al. 2017) on board Parker Solar Probe (Fox et al. 2016).While the primary objective of the implementation of the neutral mode on EIS was the examination of neutral radiation background, the HET neutral mode was conceived with the goal of measuring solar neutral radiation directly.Indeed, Mewaldt (2019) discussed that the inclusion of a neutral mode in HET may yield valuable solar γray and neutron science.
In this work, we present a calibration for the instrumental response to incident γ-rays of the HET neutral triggering mode utilizing Geant4 (Agostinelli et al. 2003) Monte Carlo modeling.We also perform analogous instrumental calibration of the lower-energy Energetic Particle Instrument Channel E (EPI-Lo ChanE; the instrument channel primarily used for measurements of suprathermal and energetic electrons, see below for further details) due to the discovery that EPI-Lo also responds to incident γ-rays despite having no specific neutral triggering mode.We present the first clear observations of solar γ-rays measured by the HET neutral mode, and complementary measurements from EPI-Lo and the Spectrometer/Telescope for Imaging X-rays (STIX; Krucker et al. 2020) on board Solar Orbiter (SolO; Müller et al. 2020), produced by a backside solar flare on 2022 September 5 and analyze the unique solar γ-ray measurements observed by ISeIS.

Instrumentation
The ISeIS instrument suite is described in detail in McComas et al. (2016), with Wiedenbeck et al. (2017) and Hill et al. (2017) providing additional detail about the higher-energy (EPI-Hi) and lower-energy (EPI-Lo) sensors, respectively.Further details regarding the design of the EPI-Hi/HET telescope are given in Appendix A in this work.The EPI-Hi/ HET neutral mode triggering configuration is shown graphically in Figure 1, which provides a cutaway view of the full HET SSD telescope.The inner SSD segments, shown in magenta, are the active detectors in the neutral mode.All detector guard rings, as well as the outer two detectors of the center detector stack on each side (referred to as H2A and H2B), are in anticoincidence with the center stack such that any detection in those outer layers is assumed to be from charged particles.The anticoincidence detector (ACD) segments are shown in cyan in Figure 1.The outermost SSD layers (H1A and H1B shown in green) are not included in the triggering algorithm.All other elements shown in Figure 1 are either structural (e.g., telescope housing and detector mounting rings) or protective Kapton aperture windows (orange).
Neutral mode events are identified as events above the threshold in the central element stack with no signal in the outer anticoincidence segments.Because the anticoincidence segments do not form a full 4π sr enclosure around the active segments, some charged particle contamination is expected in the neutral mode.As such, enhancements produced by incident neutral radiation are best identified outside of intensifications of solar energetic particles (SEP) or charged particles accelerated by stream interaction regions.To date, we have only observed a single clear γ-ray enhancement with the EPI-Hi/HET neutral mode (described in this work).However, with the increasing solar activity at the time of writing, we expect to observe additional events in the future.EPI-Lo does not have a triggering mode designed to identify measurements of neutral radiation.However, while EPI-Lo ChanE is designed primarily to measure electrons (see, e.g., Mitchell et al. 2021), it can also be used to measure secondary electrons produced by interactions of γ-rays with the SSD or the aluminum flashing that suppresses measurements of ions by that detector.The lower-energy range of EPI-Lo compared with EPI-Hi can be used to extend the measurements of incident photons down to hard X-ray energies (∼100 keV).
A detailed description of the calibration of the instrumental response to incident γ-rays utilizing Geant4 Monte Carlo modeling for both EPI-Hi/HET and EPI-Lo is given in Appendix B.

2022 September 5 Solar γ-Ray Event
The first indication of the EPI-Hi/HET neutral mode detecting a solar γ-ray event occurred in conjunction with an M9 class hard X-ray flare (J.T. Vievering et al. 2024, in preparation) just prior to the onset of the large SEP event observed by ISeIS on 2022 September 5.A fast coronal mass ejection (CME) was observed to reach a visible altitude of the corona at 16:36 UTC based on estimates provided by the Space Weather Database of Notifications, Knowledge, Information (DONKI)7 from the NASA Community Coordinated Modeling Center (CCMC).The CCMC estimate of the CME speed is ∼1400 km s −1 ; however, the estimate based on images from the Wide-Field Imager for Parker Solar Probe (Vourlidas et al. 2016) is significantly higher at >2200 km s −1 , placing it within the top 1% of the CMEs observed since 1996 (Paouris et al. 2023).Based on the CME observations, the eruption occurred at 177°Heliocentric Earth Equatorial (HEEQ) longitude (Thompson 2006) on the backside of the Sun from the perspective of the Earth.At this time, Parker Solar Probe was near perihelion at 0.07 au (∼15 R s ) from the Sun at an HEEQ longitude of ∼117°.SolO was located near the longitude of Parker Solar Probe at ∼149°HEEQ longitude and at a heliocentric distance of ∼0.7 au.The locations of Parker Solar Probe (magenta) and SolO (cyan) are shown in Figure 2, along with nominal Parker spirals and the direction of propagation of the CME (orange arrow).In this figure, Earth is located at 1 au along the 0°line.
An overview of the energetic proton and electron data is shown in Figure 3, along with the radio data from the Parker Solar Probe FIELDS Radio Frequency Spectrometer (RFS; Bale et al. 2016).Panel (a) shows the highest-energy protons from the EPI-Hi/HET A-side aperture (measuring protons coming from the direction of the Sun).Panel (b) shows the highest-energy electrons from the EPI-Hi/HET A-side aperture.Panel (c) shows the EPI-Lo ChanE data from wedge 3 (measuring electrons and ions coming from the direction of the Sun).EPI-Lo ChanE is the primary channel used for the measurement of suprathermal and energetic electrons in EPI-Lo; however, at higher energies (600 keV), this channel is often dominated by energetic ions (Mitchell et al. 2021).A clear enhancement in energetic protons and electrons can be seen in the top three panels of Figure 3, commencing at ∼16:48 UTC.The data shown in panel (c) of Figure 3 are a combination of energetic ions and electrons.The features of this SEP event are discussed in greater detail by Cohen et al. (2024).Panels (d) and (e) show the low-and high-frequency radio data from FIELDS/RFS.Examining the type II and III radio bursts shown in Figure 3, the solar eruption that produced the energetic particle enhancement took place at ∼16:11 UTC.
A brief enhancement was observed by the EPI-Hi/HET neutral mode just prior to the onset of the CME eruption, as inferred by the start of the type II and type III radio bursts shown in Figure 3.An overview of this neutral mode enhancement is shown in Figure 4. Panel (a) shows the single-coincidence neutral mode counts (i.e., only one detector in the central stack above the threshold-see the magenta segments in Figure 1 and all cyan detector elements in Figure 1 in anticoincidence).Panel (b) shows the double-coincidence HET neutral mode counts (i.e., two adjacent detectors above the threshold and all cyan elements in anticoincidence).Panel (c) shows time profiles of the single (black), double (orange), and triple (cyan) coincidence counts around the 2022 September 5 solar eruption.It is clear from panels (a)-(c) that a significant increase in neutral mode counts is observed from ∼15:52-16:19 UTC (marked by the vertical red dashed line in Figure 3), peaking at ∼16:10 UTC.Referring to Figure 3, this time period is long before the onset of the SEP event at Parker Solar Probe, indicating that this enhancement in the HET neutral mode was not produced either by secondary background radiation from interactions of SEPs in the spacecraft or instrument housing or by primary charged particles entering the central stack through the small gaps in the outer segments and being erroneously counted as neutral radiation.Panel (d) of Figure 4 shows the same neutral mode data as panel (c) but with a constant background (calculated from the previous hour of observations) subtracted.Panel (d) is also shown on a logarithmic scale to make comparisons between the different rates more clear.
Coincident with the clear increase in the HET neutral mode, an increase was also observed in EPI-Lo ChanE.This enhancement is shown as the dark red line in panel (d) of Figure 4 and as a spectrogram in panel (e).As discussed above, while EPI-Lo does not have a detector or triggering configuration dedicated to the measurement of neutral radiation, the timing of the enhancement in EPI-Lo compared with the neutral mode enhancement implies that EPI-Lo was measuring secondary electrons produced by interactions of incident hard X-rays and γ-rays.Thus, we can use the lower-energy threshold of EPI-Lo to extend the ISeIS neutral measurement capabilities when a neutral radiation enhancement is observed outside of an energetic particle event, as was the case in this event.During this time period, the Fermi spacecraft was in night and thus not viewing the Sun (the orbital period of the Fermi spacecraft is ∼95 minutes).As a result, Fermi/GBM and Fermi/LAT do not have measurements of γ-rays produced by this event.Even if the orientation of the Fermi spacecraft was ideal, the orientation of this flare on the backside of the Sun from the perspective of the Earth may have precluded the γrays from reaching the Earth.The SolO spacecraft, on the other hand, was located at a similar longitude to Parker Solar Probe on the backside of the Sun (see Figure 2).While SolO does not carry a γ-ray instrument, the STIX instrument produces hard X-ray light curves with energies from 4 keV (Krucker et al. 2020).The background-subtracted time profile of the highestenergy count rate band (50-84 keV) from SolO/STIX is shown in violet in Figure 5(a) with the scale on the right ordinate.A time profile of the summed single, double, and triple detector coincidence neutral mode data from HET is shown in black in Figure 5(a) on the left ordinate, along with the ∼54-154 keV EPI-Lo data.Note that the data shown in this plot are from EPI-Lo Channel F, which has shorter integration times but wider energy bins than ChanE.The time profiles have been shifted back to the Sun according to the respective spacecraft heliocentric distance (0.7 and 0.07 au for SolO and Parker Solar Probe, respectively).The onset and conclusion of the enhancement appear to be in relatively good agreement between the time profiles from the two instruments.Particularly noteworthy is the fact that the EPI-Lo data shows a very similar double-peaked structure to that of the STIX time profile.There are, however, clear differences in the peaks of low-and highenergy time profiles, with the γ-rays showing only a single peak well timed with the second hard X-ray peak.An overall hardening of the spectral index over the course of the event is supported by analysis of the full SolO/STIX data set by J. T. Vievering et al. (2024, in preparation), who found the powerlaw spectral index of the X-ray measurements to harden from −8.5 around 15:40 UTC to −4 at the peak of the highestenergy hard X-ray emission.
Analogous time profiles comparing the HET neutral mode to high-frequency radio emission from the Parker Solar Probe FIELDS/RFS are shown in Figure 5(b).A time profile of the 13.7 MHz frequency bin is shown in green also time shifted back to the Sun based on the light travel time to Parker Solar Probe (with the scale on the right ordinate).The hard X-ray emission from EPI-Lo (red) and SolO/STIX (violet-scaled up by a factor of 2 to put it on a comparable scale with the EPI-Lo data) is shown in Figure 5(c) with the scale on the left ordinate, along with a lower-frequency FIELDS/RFS channel (7.73 MHz-blue) with the scale on the right ordinate.The higher-frequency radio bin appears to be dominated by the prominent type II radio burst shown in panel (d) of Figure 3 and panel (e) of Figure 4.The lower-frequency bin shows the onset of the type III radio burst shown in panel (d) of Figure 3 and panel (e) of Figure 4.The type II radio burst shown by the green curve commences during the trailing edge of the γ-ray enhancement.The type II radio burst is a reliable indicator of the CME-driven plasma shock propagating out of the corona (e.g., Cliver et al. 1999).The type II radio burst is followed closely by the start of the main type III radio burst, indicating the streaming of suprathermal electrons out of the corona along open field lines (e.g., Lin et al. 1981).Prior to the type II emission, there was a single, relatively weak, type III radio burst.
Closer examination of Figure 4 indicates that while the primary impulsive HET neutral enhancement lasted for ∼10-15 minutes, a prolonged lower-level enhancement above the background persisted for another ∼30 minutes.This protracted enhancement is best observed in the single-coincidence time profile (black trace in Figure 4(c)).While not high enough above the background to analyze, this observation likely indicates that the γ-ray emission was not limited to only this brief impulsive phase.That said, we cannot rule out that this continuous emission is due to incident solar neutrons.The neutral rates discussed in this work cannot measure energetic neutral atoms (ENAs) as ENAs would be stripped either in the instrument foils or in the outermost detector layer.As such, ENAs could not penetrate into the central stack without being measured in one of the outer detector layers that serves as an ACD for the neutral mode.

Analysis
The incident HET neutral mode and EPI-Lo ChanE spectra were calculated using the response matrix technique described in Appendix B for the time period of the impulsive γ-ray enhancement (15:52-16:19 UTC shown by the vertical red dashed lines in Figure 4).The time interval over which the spectrum was integrated includes the main γ-ray peak observed by HET as well as the prior ∼10 minutes because the enhancement in EPI-Lo began significantly earlier than the HET peak.This ensured that we took into account the hard X-rays measured by EPI-Lo that contribute to the underlying power-law spectrum.This also captured the small HET enhancement above the background observed just prior to the event from ∼15:53-16:00 UTC.The combined backgroundsubtracted incident spectrum for this time period is shown in Figure 6, omitting the lowest energy points from both HET and EPI-Lo and the highest-energy point from HET due to response matrix edge effects, low detection efficiency at these energies and possible threshold effects in the lowest energy bins.The error bars on the data points from the HET neutral mode are significantly smaller than those for EPI-Lo due to the significantly higher geometry factor and detection efficiency of the EPI-Hi/HET neutral mode to incident γ-rays compared to EPI-Lo.This difference is made clear by the color bar ranges in Figures 9 and 11, which represent the probability of detection in a particular energy bin.At their peaks, the EPI-Hi detection probability is a factor of ∼10 larger than that of EPI-Lo.As described further in Appendix B, the detection efficiency of EPI-Lo for incident γ-rays is low in the energy range at which the dominant interaction process transitions from the photoelectric effect to Compton scattering (∼85-180 keV).As described in more detail in Appendix B, the absolute detection efficiency in both instruments is dependent on the incident direction of the γ-rays and the material in front of the detectors.Thus, while we show both the EPI-Lo and HET spectra together in the same figure, we do not claim that the two instruments are intercalibrated.The fact that the two spectra line up so well is fortuitous, but we will need to study future events to perform a true intercalibration.Solar γ-ray flares with higher statistics are necessary to faithfully crosscalibrate the instruments, ideally, an event observed by another instrument.
The spectral shape of the EPI-Lo data in Figure 6 is consistent with a power-law bremsstrahlung spectrum.At energies 0.7 MeV, the HET data (and the highest EPI-Lo point) are not consistent with a single power law.We interpret this excess as being due to nuclear de-excitation emission (nuclear de-excitation γ-ray continuum and line emission) from accelerated protons (and other ions) that precipitate onto the chromosphere during the flare, as seen in numerous γ line flares (e.g., Murphy et al. 2009).The HET neutral mode does not have the energy resolution (∼22% FWHM at 2.223 MeV) necessary to distinguish individual emission lines except perhaps in extreme cases (see Figure 13 and the associated discussion in Appendix B).We note that there is no clear indication of a 0.511 MeV positron annihilation signature or a 2.223 MeV neutron capture line in this event.However, the lack of these features could be a result of the energy resolution of the instruments or a soft proton spectrum that is not intense enough above the β emitter and neutron production thresholds.That said, this is not unprecedented.Indeed, of the 185 SMM/ GRS solar γ-ray events reported by Vestrand et al. (1999), only 48 events showed a discernible 2.223 MeV line and 23 events revealed a discernible 0.511 MeV line (though many flare observations suffer from low measurement statistics, as is the case here).The lack of a 2.223 MeV line in some of these cases may be due to limb darkening of that line.
We simulated the response of HET to the standard unnormalized broad and narrow nuclear line and continuum emission calculated by Murphy et al. (2009).For this we used the HET Geant4 response matrix model described in detail in Appendix B. This is shown as the red dashed line in Figure 6.A power law representing electron bremsstrahlung (blue dashed line in Figure 6) was added to the expected nuclear emission by fitting all of the EPI-Lo points apart from the last (that is in the nuclear emission energy range) and anchoring the fit with the two lowest HET points.The R 2 coefficient of determination for the fit was 0.978, indicating a good agreement between the data points and the fit.The total spectrum expected from the combination of these components is represented by the gray line.The theoretical spectrum and the spectrum reconstructed by ISeIS agree for the majority of points to within the uncertainty.
The time profile of the γ-rays measured by EPI-Hi/HET can be used to make inferences regarding the evolution of the particle acceleration throughout the event.This is of particular interest when compared with complementary hard X-ray  measurements such as those from SolO/STIX.As mentioned above, a detailed analysis of the SolO/STIX data during this event by J. T. Vievering et al. (2024, in preparation) showed an overall spectral hardening of the hard X-ray data over the course of the nonthermal emission corresponding to a softhard-harder (SHH) behavior (Kiplinger 1995;Grigis & Benz 2008).A number of studies have examined the SHH behavior in hard X-rays and found close relationships between events that exhibit this spectral hardening and the observation of in situ solar protons (e.g., Saldanha et al. 2008;Grayson et al. 2009).The continued spectral hardening out of the hard X-ray energy range from SolO/STIX and the appearance of γrays measured by EPI-Hi/HET at the end of the hard X-ray enhancement may imply a turning-on of the acceleration of ions by the flare to produce 1 MeV γ-rays, assuming the excess seen at >0.7 MeV is due to nuclear emission.A comparison of the hard X-ray and γ-ray time profiles over the peak of the enhancement may imply a change in the accelerated electron-proton ratio over that time period (e.g., Rieger et al. 1998).
Similarly, the time profile of the HET γ-rays can be compared to the radio bursts shown in Figure 3 and singlefrequency-bin time profiles in Figure 5 to understand how the hard X-ray and γ-ray emission relates to the formation of a CME-driven shock and in situ energetic particles observed in connection with this event.Based on the relative timing of the hard X-rays, γ-rays, and radio emission, trapped particle energization appears to have built up, judging by the progressively hardening X-ray spectrum, over the course of roughly 40 minutes.The highest-energy hard X-rays and γ-rays then peaked immediately before the formation of a fast CMEdriven plasma shock (indicated by the type II radio burst).This supports the idea of a local acceleration and precipitation prior to the high-altitude effects from the launch of the CME.A weak type III radio burst is observed in near-coincidence with the peak of the γ-ray enhancement and secondary hard X-ray peak, as is often observed when comparing type III radio emission and hard X-rays (e.g., Christe et al. 2008).That said, the strongest type III emission occurs after the onset of the type II emission, perhaps indicating electrons streaming out along previously inaccessible open field lines.
Figure 7 shows an image of the Sun taken by the Solar Orbiter Extreme Ultraviolet Imager (EUI; Rochus et al. 2020) at a wavelength of 174 A ̊near the time of the peak hard X-ray and γ-ray emission.The inset shows a blowup of the active region with contours indicating the source of 25-84 keV hard X-ray emission as measured by SolO/STIX.The time corresponding to the peak of the hard X-ray and γ-ray emission is indicated by the purple box in the data panels on the right of Figure 7.

Discussion
This study demonstrates that the neutral mode of EPI-Hi/ HET works as planned and provided the closest ever observation of a solar γ-ray event to its solar source (with the Parker Solar Probe spacecraft located at 0.07 au/15 R S ).The proximity of the spacecraft to the Sun has improved our ability to measure ion-produced γ-rays from this M9 class flare despite the relatively small effective areas of these instruments to incident γ-rays.Young et al. (2001) analyzed the nuclear line fluence of events observed by SMM/GRS and cataloged by Vestrand et al. (1999) as a function of GOES flare classification and found that M class flares occasionally have appreciable nuclear γ-ray emission (though this may be due to a sensitivity effect).They also found only one C-class flare with measurable nuclear line emission.The proximity of Parker Solar Probe to the Sun at perihelion provides the opportunity to measure solar γ-rays produced by flares of a variety of sizes and potentially identify other small flares that produce nuclear emission.Indeed, based on simple 1/R 2 approximations, when Parker Solar Probe is near perihelion, a C1-class flare will be roughly equivalent to seeing an X2-class flare at 1 au.
This work also described the neutral triggering mode of ISeIS/EPI-Hi/HET and the calibration of this mode to γ-rays using the response matrix technique.An analogous response matrix calibration was presented for EPI-Lo ChanE due to the discovery that it responded to solar γ-rays and could be utilized to extend the γ-ray measurement capabilities of the ISeIS instrument suite to lower energies when significant energetic particle fluxes are not present.
While the EPI-Hi/HET neutral mode does not have the energy resolution required to resolve individual γ-ray line emission in most cases, the deviation of the spectrum from a simple power law produced by electron bremsstrahlung is used to infer the acceleration of ions by a solar flare.This demonstrates that for sufficiently large γ-ray flares, ISeIS instruments can perform a certain amount of spectral analysis.Other γ-ray line emission (i.e., the 0.511 MeV positron annihilation line and the 2.223 MeV neutron capture line) may be discernible in future events, depending on the relative strength of those and other lines.
The HET neutral mode also opens up the possibility of comparing the hard X-rays and γ-rays produced by electrons and ions, respectively, to examine the relative timing and intensity of the flare acceleration of electrons and ions.Comparisons of this nature have been performed with COMPTEL (e.g., Rank et al. 2001) and RHESSI (e.g., Shih et al. 2009).ISeIS, however, has the opportunity to explore this relationship and compare it with in situ measurements of flare-accelerated charged particles.This unique capability, particularly when these observations are made close to the Sun, provides measurements of these two populations and thereby stands to improve our understanding of the relationship of the trapped particle population with that of escaping particles.

Conclusions
This work presented an analysis of the time profile and spectrum of the first solar γ-ray flare observed using the HET neutral mode.Discussion with members of the Fermi team has helped us locate another small γ-ray enhancement observed both by HET and the Fermi instruments on 2022 September 29 at 11:50 UTC associated with a GOES C5.8 class flare (Pesce-Rollins et al. 2024).However, the counting statistics observed by HET are significantly lower than in the event highlighted in this work due to the spacecraft's ∼0.62 au distance from the Sun.The temporal characteristics of this event were compared with complementary hard X-ray data from SolO/STIX and radio data from Parker Solar Probe/FIELDS.The nearly coincident timing of the nonthermal SolO/STIX hard X-rays compared to the HET neutral mode enhancement confirmed that HET was observing incident solar γ-rays for the first time.Further comparison demonstrated that the relatively rare SHH spectral behavior observed in this and other events can extend up to ∼7 MeV γ-ray energies.Studies have shown a consistent relationship between hard X-ray events that exhibit a SHH behavior and the in situ observation of energetic protons (Saldanha et al. 2008;Grayson et al. 2009).Based on the STIX and ISeIS measurements, we confirm the association between hard X-rays exhibiting a SHH behavior with higher-energy γrays, likely produced by accelerated ions, during the period of the hardest X-ray spectrum in this event.Since these ∼0.7-7MeV γ-rays are likely produced by flare-accelerated ions, this implies an important temporal relationship between the energization of protons and electrons by this flare.The end of the primary peak of the hard X-ray and γ-ray signals coincide with the formation of a shock driven by a fast CME (indicated by the type II radio burst).The full hard X-ray, γray, and radio time profiles indicate that progressively more energy was imparted by the flare to the accelerated charged particles that produce the hard X-ray and γ-ray emission up until the enormous release of energy in the form of the CME.This observation reflects the evolution of the pre-eruption acceleration processes and their development through the launch of the CME.
In the Heliophysics Systems Observatory, the community does not currently have any dedicated solar γ-ray instruments.Indeed, the only mission currently measuring solar γ-rays is the Fermi Gamma-Ray Space Telescope, an astrophysics mission that measures solar γ-rays as supplemental science.While the EPI-Hi/HET neutral mode and complementary measurements from EPI-Lo ChanE cannot supplant the need for a dedicated solar γ-ray observatory, they can provide more information on a currently under-observed phenomenon.In particular, the ISeIS γ-ray capabilities provide unique opportunities to observe backside solar γ-ray events and front-side events when Fermi is occulted.Thus, this supplemental capability of ISeIS will be valuable in the ongoing study of solar γ-rays.
Planned future studies of the HET neutral mode include a Monte Carlo calculation of the neutral mode response to incident neutrons, a comparative study of solar γ-ray flares, particularly extended emission reminiscent of long-duration gamma-ray flares (Ryan 2000), observed by EPI-Hi/HET with direct measurements of associated in situ protons, and a hunt for solar neutrons to understand more about this rarely measured secondary radiation.As the delayed γ-ray emission often exhibits intensities ∼1-2 orders of magnitude lower than the impulsive phase (e.g., Ryan 2000, and references therein), it will be challenging for the ISeIS instruments to measure sufficient counts; however, the right mix of close spacecraft proximity to the flare site and high intensity may allow us to study this phenomenon.A Monte Carlo calculation of the response of HET to incident neutrons is currently underway.Once the response of the neutral mode is fully characterized for both incident γ-rays and neutrons, we will have the ability to measure the entire interacting ion spectrum from the Sun.This capability provides key measurements to study events that would be otherwise inaccessible depending on the orientation of the Fermi spacecraft and the location of the flare with respect to the Earth.

Appendix A EPI-Hi/HET Instrument
As this work focuses on a newly explored capability of the EPI-Hi/HET instrument, here we provide a brief description of the details of the instrument.More detailed descriptions are provided in McComas et al. (2016) and Wiedenbeck et al. (2017).A cross-sectional view of HET is shown in Figure 1.HET is designed to be symmetric on either side of its center point.The outermost detectors (shown in green in Figure 1) are 0.5 mm thick and located ∼11 mm from the central stack of detectors.The 14 detectors that make up the central stack are all 1 mm thick and spaced ∼1.5 mm center to center.The central elements of the HET detectors that make up the neutral mode have radii of 5.65 mm.

Appendix B Instrument Response Calibration
As neither EPI-Hi/HET nor EPI-Lo were designed specifically for neutral radiation measurements, in-flight calibration of the response of both instruments to neutral radiation was required to interpret the data.A set of limited tests was performed prior to launch on the HET neutral mode to verify the triggering algorithms; however, a full characterization of the instrument response was outside the scope of the calibrations.Instead, the instrumental response was studied using a detailed Geant4 (Agostinelli et al. 2003) mass model of the EPI-Hi HET instrument (see Figure 1).To study the energy-dependent instrumental response to incident γ-rays, a plane beam distribution of γ-rays was simulated in a flat spectrum from 100 keV-20 MeV.The incident γ-ray beam was rotated ∼20°off the HET axis to account for the fact that the HET apertures are ∼20°rotated off the Sun-spacecraft line (in order to lie along the nominal Parker spiral for charged particle measurements).A cartoon of the geometry is shown in Figure 8 (not to scale).Based on the flare location, the true angle of the photons entering HET is ∼16.7°off the instrument axis.The photons in the simulations used to generate the response matrix were generated at 20°with respect to the HET instrument axis, as this is the average direction in which solar photons will enter the instrument.Investigations indicated that small deviations from this incident angle, as was observed in this event, do not result in a perceptible difference to the response matrix or resultant reconstructed spectra.The response of the instrument is dictated by the physics of the photon interactions in the detector, thus, other incident photon trajectories (e.g., isotropic), do not significantly change the shape of the reconstructed spectra, but serve to increase or decrease the magnitude of the spectrum.The output data from the simulation was processed using an analysis code designed to match the decision-making processes of the flight software (described in the main text) to ensure that the events identified as neutral mode triggers from the simulation data would match the onboard logic.
A detector response matrix was then calculated by binning the events that passed the criteria to be categorized as neutral based on their energy deposit in the active detector segments (i.e., magenta segments in Figure 1).Response matrices generated through Monte Carlo simulation have been used to great effect in understanding the response of instruments such as RHESSI (Smith et al. 2002) and Fermi/GBM (Bissaldi et al. 2009;Meegan et al. 2009).Each column of the response matrix  was normalized according to the number of simulated particles with incident energies within that bin.Thus, each element of the response matrix is essentially a probability that a γ-ray of a given incident energy will be measured in a specific energy bin.Diagonal elements of the response matrix correspond to the probability that all of the incident energy of the photon will be measured by the instrument.Off-diagonal elements below the diagonal correspond to photons that are measured in bins corresponding to lower energies than their true energies.The forward detector response matrix, P ij , therefore, relates a true incident energy spectrum, μ j , to the energy spectrum measured by the instrument, ν i , according to A visualization of the HET γ-ray neutral mode detector response matrix is shown in Figure 9(a).The matrix elements are represented by the logarithmic color bar shown in that figure.As such, the color bar represents the probability that a photon of a given incident energy will be measured in a particular energy bin.Each column of the response matrix can be thought of as an individual response histogram for the range of energies in that particular bin.The response shown in Figure 9(a) uses the energy bins in the EPI-Hi/HET flight software as this represents the resolution of the instrument with the two lowest energy bins combined and the six highest bins combined into groups of two due to lower detection efficiency at the edges of the response.Figure 9(b) shows the same data as the above response matrix but is rebinned with significantly smaller energy bins to more clearly show the physics of the instrumental response.Figure 9(b) demonstrates that Compton scattering is the dominant process in the HET energy range, as expected based on the interaction cross sections of silicon (e.g., Choppin et al. 2002).A cyan dashed line is shown in Figure 9(b) along the one-to-one line representing events that interacted via the photoelectric effect (in which the entire photon energy is absorbed).Clearly, photoelectric absorption plays a minimal role in the response of EPI-Hi/HET.The black dashed line shown in Figure 9 shows the theoretical location of the Compton edge (i.e., the maximum energy a photon can impart to an electron via Compton scattering) for each set of incident energies (column of the response matrix) based on where E e,max is the maximum energy that can be obtained by the Compton-scattered electron, E γ is the energy of the incident γ-ray, and m e is the rest mass of the electron.The agreement between the theoretical value of the Compton edge and the most probable bin in each column of the response matrix demonstrates that the photoelectric effect and pair production do not play significant roles in the photon interactions at these energies in HET.Indeed, in the HET energy range (∼0.4-10MeV), it is expected that Compton scattering is the only photon interaction process that will play a role (see, e.g., Chapter 25, Figure 1.1 of Evans & Beiser 1956).Despite the fact that EPI-Lo does not have a mode designed to trigger only on neutral radiation, the first clear γ-ray flare measured by the EPI-Hi/HET neutral mode had a coincident enhancement in ChanE, the EPI-Lo channel primarily intended for the measurement of energetic electrons.The measurements in ChanE likely came from interactions of incident photons with the EPI-Lo SSDs (or other instrumental material), thereby producing secondary electrons measured by the instrument.After the recognition that EPI-Lo could also detect neutral radiation, a detector response matrix was also calculated for the EPI-Lo electron channel response to incident photons (spanning hard X-rays to γ-rays due to the lower-energy range of the EPI-Lo detectors) using a detailed Geant4 mass model of one of the EPI-Lo instrumental wedges (see Figure 10).The magenta detector (referred to as SSDp) in Figure 10 is the detector intended primarily for the measurement of electrons.This detector has a 3.2 μm aluminum flashing designed to suppress contamination from low-energy ions.The cyan   detector (referred to as SSDa) located behind the SSD shown in magenta is the anticoincidence detector designed to remove measurements of penetrating particles.As in HET, analysis software was written to analyze the simulation data and identify measurements that would be counted as electrons based on the triggering algorithm.Due to the low measurement statistics from EPI-Lo in this event, groups of two energy bins were combined.A visualization of the normalized EPI-Lo response matrix is shown in Figure 11(a).Just as in Figure 9, the black dashed line in Figure 11 corresponds to the location of the Compton edge for a given incident photon energy.The cyan dashed line in Figure 11 corresponds to the one-to-one line expected for photons completely absorbed via the photoelectric effect by the SSD in  shows the same data as in the response matrix but with significantly smaller energy bins to more clearly show the physical processes that contribute to the instrumental response.The response of EPI-Lo to photons shows the expected transition from the photoelectric effect being the dominant interaction process to Compton scattering dominating at ∼100 keV.As discussed above, due to the instrumental threshold, the EPI-Lo detection efficiency at the transition between these two interaction processes drops precipitously.This is exemplified in Figure 12, which shows the energy-dependent effective area for γ-rays for HET (left) and EPI-Lo (right).The gray-hatched region between ∼85 and 180 keV has a factor of ∼10 lower detection efficiency than the surrounding energies.As a result, the data point in that region in Figure 6 is an upper limit.Close examination of the response matrix color bars in Figures 9 and 11 shows a much higher detection probability for incident γ-rays in the HET neutral mode than in EPI-Lo.
In order to invert Equation (B1) such that the incident spectrum of photons, μ j , can be inferred from the measured spectrum, ν i , one must unfold the measured spectrum using the instrument response matrix.There are a variety of methods to perform this unfolding developed primarily for high-energy physics applications.For this work, we employed the wellestablished Bayesian unfolding technique originally outlined in D'Agostini (1995) and later updated in D'Agostini (2010).The PyUnfold package was utilized to perform the unfolding (Bourbeau & Hampel-Arias 2018).To verify the unfolding algorithm, the original data used to construct the forward response matrices was unfolded and found to match the incident spectrum apart from small deviations (less than a factor of 2) at energies toward the edges of the instrumental energy range.It should be noted that due to the physics of Compton scattering, adjacent energy bins will be somewhat correlated.
In order to examine the ability of the HET neutral mode to resolve individual features in a γ-ray spectrum (i.e., deexcitation line emission in which the individual line could be identified), we performed several Geant4 runs simulating γray spectra with varying relative intensities of features.A comparison of the input spectrum (shown by the blue points in Figure 13 with fine bins to show individual features) and the reconstructed HET neutral spectra (shown as the green diamonds in Figure 13 and binned using the HET energy bins) is shown for three input spectra with varying relative intensities of spectral features.Panel (a) shows the resulting reconstructed spectrum from the simulation of a generic power law without any nuclear emission.With the removal of the lowest-two and highest-three energy bins (which are known to be outside the range of high detection efficiency), the reconstructed spectrum is a single power law without any indication of other features.Panel (b) shows the reconstructed spectrum from the 1989 October 24 γ-ray event measured by SMM/GRS (Share & Murphy 1995), which has line emission with low relative intensity compared to the underlying power law and nuclear continuum.We see that the reconstructed spectrum deviates from a simple power law, but we are not able to resolve individual line emission (as is the case in the 2022 September 5 flare analyzed in this work).Finally, we simulated the 1986 February 6 γ-ray event measured by SMM/GRS (Share & Murphy 1995), which featured strong 2.223 MeV neutron capture line emission.The results shown in panel (c) of Figure 13 deviate from a power law with a particular enhancement in the HET bin near 2 MeV.This implies that if we observe an event with individual line emission that is sufficiently higher than the underlying power law and surrounding line and continuum emission, we may be able to infer its presence using the neutral mode of HET despite the instrumental limitations in its bin widths and energy resolution.
As mentioned in the text, while we have studied the instrumental response of both EPI-Lo and EPI-Hi/HET to incident γ-rays, we have not yet performed an intercalibration between the two instruments.An intercalibration between the two instruments will require a significantly larger γ-ray enhancement than was observed in this event such that both instruments have high enough counting statistics to reduce the magnitude of the uncertainty.This is particularly important due to the differing overburden produced by the Parker Solar Probe spacecraft structure and other science instruments when comparing EPI-Lo and EPI-Hi/HET.As the thermal protection system and other support structures lie between the Sun and the instrument and γ-rays follow straight-line trajectories (as opposed to the charged particles that the ISeIS instruments were designed to measure), accounting for the differences in overburden will be required to truly intercalibrate the two instruments.As such, while the reconstructed spectra from EPI-Lo and EPI-Hi/HET are plotted on the same figure in Figure 6, we do not propose that their overall magnitude should be directly compared.That said, their relative shapes are expected to be independent of these considerations as the shape is dictated by the interaction processes in the instrument.

Figure 1 .
Figure 1.Schematic view of the HET neutral mode triggering conditions.Magenta center SSD-stack segments are active and all outer guard rings and center-stack detectors (cyan) are in anticoincidence.Descriptions of the other detector elements can be found in the text.

Figure 2 .
Figure 2. Locations of Parker Solar Probe (magenta) and SolO (cyan) during the eruption of the fast CME on 2022 September 5 at ∼16:30 UTC (CME propagation direction indicated by the orange arrow) in HEEQ coordinates.

Figure 3 .
Figure 3. Overview of the onset of the 2022 September 5 SEP event observed by Parker Solar Probe.Panel (a) shows protons from the A-side aperture of EPI-Hi/ HET.Panel (b) shows energetic electrons from the A-side aperture of EPI-Hi/HET.Panel (c) shows ChanE, a combination of energetic ions and electrons, from wedge 3 of EPI-Lo.Panels (d) and (e) show the low-and high-frequency radio data from Parker Solar Probe/FIELDS/RFS.Vertical red dotted lines indicate the approximate time interval during which HET measured solar γ-rays.

Figure 4 .
Figure 4. Overview of the solar γ-ray measurements of the 2022 September 5 event from Parker Solar Probe/ISeIS.Panel (a) shows the single-coincidence EPI-Hi/ HET neutral mode spectrogram.Panel (b) shows the double-coincidence EPI-Hi/HET neutral mode spectrogram.Panel (c) shows time profiles of the counts during each integration time of the single-, double-, and triple-coincidence EPI-Hi/HET neutral mode data summed over all energies.Panel (d) shows the same counts as panel (c) background subtracted and on a log scale.The EPI-Lo data integrated over all energies is shown in red.Panel (e) shows the EPI-Lo ChanE data demonstrating that this channel also responded to the γ-ray flare.Panel (f) shows the high-frequency radio data from FIELDS showing the characteristics of the radio bursts.The vertical red dashed lines indicate the time period over which the spectrum was calculated.

Figure 5 .
Figure 5. (a) Time profile of the solar γ-rays measured by EPI-Hi/HET (black), EPI-Lo (red), and SolO/STIX 50-84 keV hard X-rays (violet).(b) Time profile of the solar γ-rays measured by EPI-Hi/HET (black) and FIELDS/RFS 13.7 MHz radio emission (green).(c) Time profile of hard X-rays measured by EPI-Lo (red), SolO/STIX (violet), and FIELDS/RFS 7.7 MHz radio emission (blue).All time profiles have been shifted back to the Sun based on the location of each spacecraft at the time of these events.

Figure 6 .
Figure 6.Differential energy spectrum of the main γ-ray enhancement (15:52-16:19 UTC) as measured by EPI-Lo ChanE (magenta) and the EPI-Hi/ HET neutral mode (green).The blue dashed line represents a power law that corresponds to the EPI-Lo data and the two lowest energy HET points.The red dashed line represents the modeled spectrum of combined broad and narrow nuclear emission.The gray solid line represents the total spectrum.

Figure 7 .
Figure 7. Summary figure showing a Solar Orbiter EUI image of the Sun near the time of the peak of the hard X-ray and γ-ray emission.The inset shows a blowup of the active region that produced the observed emission along with 25-84 keV hard X-ray contours (blue) at the time of maximum hard X-ray emission from SolO/ STIX.The top four panels of Figure 4 are duplicated on the right with an arrow indicating the peak of the hard X-ray and γ-ray emission.

Figure 8 .
Figure 8. Cartoon showing the geometry of the flare and HET pointing angles with respect to the Sun-spacecraft line (not to scale).

Figure 9 .
Figure 9. (a) Visualization of the HET γ-ray neutral mode response matrix.The black dashed line indicates the theoretical Compton edge, demonstrating that the γ-ray interactions in HET are predominantly in the Compton scattering regime.(b) The same data as in the response matrix visualization in panel (a) but in much finer energy bins to show the physical processes that contribute to the instrumental response more clearly.

Figure 10 .
Figure 10.Image of the mass model of a single EPI-Lo instrumental wedge.SSD shown in magenta (SSDp) is the primary SSD used for electron measurements.SSD shown in cyan (SSDa) located behind the black SSD is the anticoincidence detector.

Figure 11 .
Figure 11.(a) Visualization of the EPI-Lo incident photon response matrix.The black dashed line indicates the theoretical Compton edge.The cyan dashed line indicates the expected energy deposit in the case of photon absorption via the photoelectric effect.(b) The same data as in the response matrix visualization in panel (a) but in much finer energy bins to show the physical processes that contribute to the instrumental response more clearly.

Figure 12 .
Figure 12.Effective area for γ-rays for HET (left) and EPI-Lo (right) as a function of incident γ-ray energy.The gray-hatched region in the EPI-Lo figure represents the transition between the photoelectric effect and the Compton scattering being the dominant interaction process.Detection efficiency is low in this region, due to instrumental threshold.

Figure 13 .
Figure13.Comparison of three simulated γ-ray spectra with varying relative intensities of line features compared with the underlying bremsstrahlung power law.Panel (a) shows the reconstructed spectrum from the HET neutral mode for a generic power-law spectrum (i.e., no line emission).Panel (b) shows the reconstructed spectrum from the 1989 October 24 event observed by SMM, which had line emission with low relative intensity compared with the underlying power law.Panel (c) shows the reconstructed spectrum from the HET neutral mode for the 1986 February 6 event observed by SMM, which had a strong 2.223 MeV neutron capture line emission.