Quiet-time suprathermal ions in the inner heliosphere during the rising phase of solar cycle 25

Context. The Solar Orbiter spacecraft made its ﬁrst close perihelion passes in 2022, reaching 0.32au on 26 March and 0.29au on 12 October. Transient activity was relatively low, making it possible to perform measurements of the quiet-time suprathermal ion pool over multi-day periods. Aims. The inner heliosphere suprathermal ion pool is a source of seed particles accelerated by coronal mass ejection-driven shocks. Determining its constituents and their dependence on solar activity, location, and time is therefore critical to building physical models of particle acceleration from solar events. Methods. By selecting low-activity periods on Solar Orbiter during perihelia passes, and comparing them with a nearly identical monitoring instrument at 1 au, the observed di ﬀ erences in intensities can be related to factors such as distance from the Sun, transient events, and interacting solar wind streams. Results. Below ∼ 1MeV / nucleon, the observed quiet-time spectra at 0.32au for H, 4 He, 3 He, and Fe rise toward low energies, as observed previously during longer periods at 1au, and they show a heavy ion composition that has markers of impulsive solar ﬂare material, such as relatively high 3 He: 4 He, and a high Fe / O ratio. The proton and helium abundances are much higher, consistent with a source in corotating interaction regions. Surveying all semi-quiet times during the mission, there is only a modest ( ∼ 15%) increase in the ﬂuences in the inner heliosphere compared to 1au, indicating small gradients in these populations between 1 and 0.3au.


Introduction
Energetic particles accelerated by coronal mass ejection (CME) driven interplanetary shocks arise from a seed population of ions in the interplanetary medium (IPM). The presence of rare ions (e.g., 3 He and He + ) and other details of the composition in the accelerated particles provide evidence that suprathermal ions with speeds above the bulk solar wind play a key role (e.g., Chotoo et al. 2000;Desai et al. 2006a;Mason et al. 2008;Mewaldt et al. 2007). At 1 au the seed population varies over solar cycle (SC) scales, as well as over shorter multi-day periods when transient events such as solar energetic particles (SEPs) or corotating interaction regions (CIRs) fill the inner heliosphere with suprathermals and energetic particles. One goal of the Solar Orbiter mission is to systematically map the suprathermal ion pool in the inner heliosphere both in and out of the ecliptic plane (Müller et al. 2020). Although some recent surveys have begun to probe this region (Leske et al. 2020;Mason et al. 2021a;Rankin et al. 2022;Wiedenbeck et al. 2020), it is nevertheless largely unexplored since the technology available in prior missions such as HELIOS could not measure the particle composition below a few MeV/nucleon.

Observations
The energetic particle observations discussed here are from the Suprathermal Ion Spectrograph (SIS), which is part of the Solar Orbiter Energetic Particle Detector (EPD) suite (Rodríguez-Pacheco et al. 2020;Wimmer-Schweingruber et al. 2021). SIS is a time-of-flight mass spectrometer that measures the ion composition from ∼0.1−10 MeV/nucleon, using sunward-facing (a) and anti-sunward facing (b) telescopes. Each ion measurement consists of two separate determinations of particle mass, which are compared for consistency, thus allowing elimination of background events. This background elimination is an important consideration since events from penetrating cosmic rays can mask the low quiet-time intensities. At 1 au, measurements are from the Ultra-Low Energy Isotope Spectrometer (ULEIS; Mason et al. 1998) on board the Advanced Composition Explorer spacecraft (ACE; Stone et al. 1998) that was launched in 1997. ULEIS is also a time-of-flight mass spectrometer with an energy range, mass resolution, and mass coverage similar to that of SIS. The suprathermal ion energy range lies above the thermal solar wind distribution and ends roughly at a few MeV/nucleon. In this study, the threshold begins at instrument threshold of 20−100 keV/n depending on species, similar to earlier surveys (e.g., Dayeh et al. 2017;Mason et al. 1979;Mason & Gloeckler 2012;Richardson et al. 1990). The magnetogram data are from the Solar Dynamics Observatory (SDO) Heliospheric and Magnetic Imager (HMI; Scherrer et al. 2012).
2.1. Survey of 2020-2023 data for all activity levels Figure 1 shows the daily average 273 keV/nucleon O intensities (filled circles) from the time SIS became operational (April 2020) following the launch of Solar Orbiter in February 2020 through January 2023. The blue line shows the Solar Orbiter heliocentric distance, with five perihelia to date. Perihelia 4 and 5 came significantly closer to the Sun than the earlier ones, and are discussed below. The dashed green line shows the monthly sunspot number, which rose from deep solar minimum values to numbers close to the prior SC (24) peak, but still only about one-half the SC 23 sunspot peak during ∼2000−2003. The general correlation between solar activity and the O daily intensities is clear from the figure: Event-to-event variations greatly exceed any obvious dependence on heliocentric distance. Generally speaking, the most intense events occurred when the spacecraft was closer to 1 au, and in particular, the intensities are orders of magnitude below the peak events around perihelia 1, 2, and 5.
The intensity increases in Fig. 1 are due to a mixture of event types: Impulsive solar energetic particle (ISEP) events, large gradual solar energetic particle Events (GSEP), corotating interaction regions (CIRs), and interplanetary (IP) shocks. Figure 2 shows the Fe/O ratio for the period in Fig. 1 plotted versus heliocentric radius, along with lines that show the mean Fe/O values for different event types from surveys at 1 au, namely ISEPs (Mason et al. 2002), GSEPs (Desai et al. 2006b), IP shocks (Desai et al. 2003) and CIRs (Mason et al. 2008). The figure also shows the number of observing days at each distance (right axis). It is apparent from the figure that the Fe/O ratio  covered all types of events beyond ∼0.7 au, while in the inner heliosphere, ISEP and CIR events were relatively more common. The tendency of ISEP and CIRs to dominate at closer distances is influenced by their being more numerous than large events, which means that they are relatively more likely to be observed in the fewer days at close distances. Figure 3 shows a histogram of the points in Fig. 2 Reames 1988). Although there have been several sizeable GSEP events during the period (Fig. 1), they do not stand out since they covered relatively few days. The yellow line shows the average 3 He: 4 He ratio for the days in each histogram bin, which shows the large enhancements due to the ISEPs.

Quiet period survey
In order to estimate the overall suprathermal ion intensity inside 1 au, we removed periods dominated by GSEP events and focused on semi-quiet periods when ISEPs and CIRs dominate the intensities. This is necessary because Solar Orbiter and the 1 au monitoring instrument on ACE often have large longitudinal separations, and a large GSEP or IP shock event seen on one may not be observed by the other instrument, or else be observed at a much different intensity. For example, the 5 September 2022 GSEP and IP shock produced suprathermal O intensities at Solar Orbiter that were more than 1000 times higher than at ACE. Since such events can dominate a cumulative fluence calculation, they may easily mask other trends. For the Solar Orbiter data set, a threshold for semi-quiet was set at a daily average fluence of 2 × 10 4 particles cm −2 sr MeV/nuc, which allowed inclusion of events such as the 3 He-rich event of 21 July 2020 (Mason et al. 2021b), but not of appreciably larger events. This threshold is shown by the orange line in Fig. 1. In addition to ISEP events, this intensity level includes numerous CIRs (Allen et al. 2021) as well at the late-decay periods of some GSEPs. This threshold removed 64 of 883 days of SIS observing time, and 41 of 929 days for ACE. Because of their longitudinal separation, the days of high fluence that were removed were not the same at the two spacecraft. Figure 4 shows integrated semi-quiet daily fluences from Solar Orbiter-SIS and ACE-ULEIS beginning with the operational phase of the Solar Orbiter mission. The two curves are surprisingly similar during the first 15 months, even though Solar Orbiter was widely separated from Earth, and had two perihelia during the period. After the first 15 months, the Solar Orbiter semi-quiet fluences exceed ACE with a maximum of ∼50% around July 2022, and then decrease to ∼15% higher at the end. The main features of the two curves do not coincide with the perihelia, suggesting that the different events sampled by the two missions are the primary source of the differences. To first order, the figure shows only modest difference between the semi-quiet suprathermal ion pool at 1 au versus that sampled in as close as 0.3 au. In order to illustrate the sensitivity to the choice of the semi-quiet threshold, the dashed lines on either side of the Solar Orbiter cumulative fluence line in the figure show the effect of raising or lowering the threshold by 20%. The curves with different thresholds do not modify the qualitative description given for the nominal threshold.

First close perihelion passes in 2022
Unlike the case for galactic cosmic rays above ∼10 MeV/nucleon, at energies below a few MeV/nucleon, energetic particle intensities in the inner Solar System do not have an obvious nearly constant background level. Rather, there is a continuum of intensities from the instrument threshold to very high levels, making a definition of "quiet" periods somewhat arbitrary. One approach is to select the lowest hourly or daily values in some period, or alternatively, periods in which the intensity of a particular species and energy falls below a specified level (e.g., Mason et al. 1980Mason et al. , 2021aReames 1999;Richardson et al. 1990). Another approach used for long time-period surveys is to choose a "quiet" level that yields a reasonable number of hours over which the intensities can be studied (Dalal et al. 2022;Dayeh et al. 2009). Since the Solar Orbiter data set is still only a small fraction of a solar cycle, we used the approach of selecting the quietest periods in our L12, page 3 of 5 interval of interest, namely the 2022 periods when Solar Orbiter moved inside 0.4 au. Figure 5 shows the location of the spacecraft in the Earthcentered system at the times of the two perihelia. For the 26 March 2022 (day of year 085) perihelion, Solar Orbiter was connected to the western hemisphere, not far from the nominal ACE connection point. Using a more accurate connection tool (Rouillard et al. 2020) that includes potential field source modeling (PFSS) of the coronal field, the ACE connection was ∼30 • east of Solar Orbiter for 24−26 March (days 83−85) and then moved to about 80 • east of Solar Orbiter for 27−28 March (days 86−87). For the 12 October 2022 perihelion, Solar Orbiter and its nominal connection point were behind the east limb, far from the ACE connection point. Figure 6 shows hourly average ∼275 keV/nucleon proton (red) and Fe (blue) intensities during the Solar Orbiter perihelion passes. In the figure, Fe increases are primarily indicators of GSEP and ISEP events, while proton intensities also include CIRs. The lowest-intensity periods near perihelion were chosen to sample the spectra. The yellow boxes in Fig. 6 show the periods, which nevertheless include some proton activity and very low Fe intensities. By comparing the spectra collected for the March versus October period, we detected small increases in 3 He on ACE during the October period that were not in the Solar Orbiter observations due to the large longitudinal separation. It was decided to therefore use the March perihelion data alone because the two spacecraft had reasonably close magnetic connection to the corona. The yellow shaded areas in the left panels of Fig. 6 show the selected periods, namely days 83.0−86.0 for ACE and 84.0−87.0 for SIS. The inset in the lower left panel is the SDO HMI magnetogram on day 85, showing that the connection points to the western hemisphere lay in areas that were nearly devoid of active regions. In our prior study of very quiet days (Mason et al. 2021a), the quietest periods were also those that were devoid of active regions. Nevertheless, it is clear from the upper left panel of Fig. 6 that during our selected quiet period, the Solar Orbiter proton intensities were still decaying from a GSEP event on day 80. Figure 7 shows spectra for the March 2022 quiet period. The ACE 3 He point has been displaced slightly leftward in energy to separate it from the Solar Orbiter point. The proton and helium spectra are similar, although the ACE proton data has large uncertainties. The Solar Orbiter 4 He spectral slope is −2.25. The 3 He spectrum is measurable at 0.32 au, with only a single count at 1 au. The O spectra statistical uncertainties make a comparison difficult. Below 100 keV/nucleon, Fe at 0.32 au is roughly a factor of 10 above the 1 au points.

Discussion and conclusions
There are several goals for studying and characterizing the suprathermal ion in the inner heliosphere, including (1) establishing the seed population available for acceleration to high energies by CME-driven shocks, (2) determining the sources of the seed population (prior solar events, CIRs, anomalous cosmic rays, etc.) so that the pool constituents can roughly be estimated in the absence of in situ observations, and (3) searching for processes that are not apparent at 1 au that may produce new sources of suprathermals. Since the contents of the pool can be expected to vary with solar activity cycle, distance from the Sun, latitude off the ecliptic, and the presence or absence of episodic SEP and CIR events, separating these effects requires comprehensive surveys.
L12, page 4 of 5 At 1 au, the quiet-time low-energy spectra below ∼1−2 MeV/ nucleon have been systematically surveyed over SC 23 (1995−2008) by Dayeh et al. (2009). The authors showed that the yearly averaged quiet-time low-energy heavy ion ratios of C/O, Fe/O, and 3 He/ 4 He transitioned from CIR-like during solar minimum to GSEP-and ISEP-like during solar active periods (see also Richardson et al. 1990 During the first year of the Solar Orbiter mission during solar minimum (Fig. 1), quiet-time spectra were measured by SIS (Mason et al. 2021a). For this study, periods in between ISEPs and CIRs were chosen as "quiet", and the low level of solar activity made a substantial collection time possible even with this criterion. The resulting spectra for H and 4 He are about 100 times lower than those in Fig. 7. Interestingly, the heavy ion C-Fe spectra are only about ten times lower than Fig. 7, indicating that the contributors to H and 4 He behave differently than those for the heavy ions. This likely reflects the important role of CIRs, where H and 4 He are relatively far more abundant than heavy ions.
By comparing the 0.32 and 1.0 au spectra in Fig. 7,we would expect that ions from CIRs would decrease on average (Allen et al. 2021;Fisk & Lee 1980;Van Hollebeke et al. 1978), and ions from ISEPs and GSEP would increase from 1 au to 0.32 au. For H and 4 He, the spectra are close. However, since the proton intensity at Solar Orbiter was clearly decaying from a prior transient event (as was 4 He, not shown) that was not seen on ACE, it is likely that the comparability of the 0.32 and 1 au H and 4 He is due to the GSEP decay at Solar Orbiter. We speculate that if the GSEP particles were somehow removed, the Solar Orbiter H and 4 He would be lower than at 1 au. While comparisons of oxygen are difficult, it seems clear from Fig. 7 that the 0.32 au 3 He and Fe are both significantly higher than at 1 au. In particular, at energies below 100 keV/nucleon, the Fe intensity at 0.32 au is roughly a factor of 10 higher than at 1 au, as would be roughly expected if there were a 1/r 2 increase in ISEP intensities at smaller heliocentric distances.
Estimations of radial gradients in the inner solar system for solar energetic particles suggest gradients scaling as roughly r −2.5 (e.g., Dayeh et al. 2010;Lario & Decker 2011). For the case of Fe, the observed increase of a factor of ∼10 from 1.0 to 0.32 au is roughly consistent with expectations for solar energetic particles. Thus, the spectra in Fig. 7 are reasonably consistent with our expectations about ISEP, GSEP, and CIR ions, although the uncertainties are large. Additionally, there is no evidence in these data for any new population of energetic particles beyond those already studied.