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A&A
Volume 673, May 2023
Solar Orbiter First Results (Nominal Mission Phase)
Article Number L12
Number of page(s) 5
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202345978
Published online 18 May 2023

© The Authors 2023

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1. 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., 3He 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.

2. 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.

thumbnail Fig. 1.

Daily average EPD/SIS 226−320 keV/nucleon O intensities (filled red circles). The orange line is the threshold defining the semi-quiet time (see text). Right axis: heliospheric distance with five perihelia (blue line). The monthly sunspot number is shown by the dashed green line.

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.

thumbnail Fig. 2.

Ratio of 226−320 keV/nucleon Fe/O for each of the daily averages shown in Fig. 1. The horizontal lines show survey average values for ISEP events, GSEP events, IP shock events, and CIRs. The dashed olive line (right axis) shows the observing days. See the text for a discussion.

Figure 3 shows a histogram of the points in Fig. 2, showing a clear peak at Fe/O ∼1 due to 3He-rich ISEP events, and then a separate grouping of lower Fe/O values peaking around ∼0.1−0.2, near the CIR and IP shock values (see also 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 3He:4He ratio for the days in each histogram bin, which shows the large enhancements due to the ISEPs.

thumbnail Fig. 3.

Histogram of Fe/O ratios for the data in Fig. 2. The average values for the different particle populations are the same as in the caption of Fig. 2. Right axis: average 3He/4He ratios for the days in each histogram bin.

2.2. 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 × 104 particles cm−2 sr MeV/nuc, which allowed inclusion of events such as the 3He-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.

thumbnail Fig. 4.

Integrated 273 keV/nucleon daily oxygen fluences from Solar Orbiter (red) and ACE (green) for semi-quiet periods during the Solar Orbiter mission to date. The dashed black lines show the effect of raising or lowering the Solar Orbiter semi-quiet threshold by 20% (see the text for details). Right axis: heliocentric distance of Solar Orbiter (blue). Asterisks mark aphelia, when Solar Orbiter was in the vicinity of Earth.

2.3. 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. 1980, 2021a; Reames 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 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 Earth-centered 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.

thumbnail Fig. 5.

Location of Solar Orbiter and ACE during the two close perihelia in 2022 when the spacecraft moved to ∼0.3 au. In the March 2022 period, Solar Orbiter and ACE had a much closer magnetic connection than for the October 2022 period.

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. The 2022 time intervals for the yellow shaded intervals are Solar Orbiter, days 84.0−87.0, 276.0−279.0, and 286.0−289.0, and ACE, days 83.0−86.0, 281.5−282.8, 283.0−283.5, and 293.5−295.5.

thumbnail Fig. 6.

Hourly averaged ∼275 keV/nucleon proton (red) and Fe (blue) intensities for the 2022 perihelion passes. The perihelia were on 26 March 2022 (day 085, 0.32 au) and 12 October (day 285, 0.29 au). The yellow boxes show periods that were examined for quiet-time spectra (see the discussion). The inset in lower left panel is the SDO HMI magnetogram on day 085. Right axis upper panels: Solar Orbiter heliocentric distance (green).

By comparing the spectra collected for the March versus October period, we detected small increases in 3He 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 3He 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 4He spectral slope is −2.25. The 3He 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.

thumbnail Fig. 7.

Energy spectra during the March 2022 periods marked in Fig. 6 during the Solar Orbiter perihelion. Filled circles: Solar Orbiter SIS. Open circles: ACE ULEIS. O has been divided by 10 and Fe by 100 to separate the points.

3. 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.

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 3He/4He transitioned from CIR-like during solar minimum to GSEP- and ISEP-like during solar active periods (see also Richardson et al. 1990). Dayeh’s quiet-time O spectrum for the 2001 solar active period was about a factor of 6 higher than during the 2007 solar minimum. Compared to the ACE O spectrum in Fig. 7, the Dayeh et al. (2009) O spectrum for 2001 is similar. Likewise, the 3He:4He ratio in Fig. 7 at 1 au (∼0.1) is similar to Dayeh’s yearly results for solar active periods.

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 4He 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 4He behave differently than those for the heavy ions. This likely reflects the important role of CIRs, where H and 4He 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 4He, the spectra are close. However, since the proton intensity at Solar Orbiter was clearly decaying from a prior transient event (as was 4He, not shown) that was not seen on ACE, it is likely that the comparability of the 0.32 and 1 au H and 4He is due to the GSEP decay at Solar Orbiter. We speculate that if the GSEP particles were somehow removed, the Solar Orbiter H and 4He would be lower than at 1 au. While comparisons of oxygen are difficult, it seems clear from Fig. 7 that the 0.32 au 3He 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/r2 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.

Acknowledgments

Solar Orbiter is a mission of international cooperation between ESA and NASA, operated by ESA. The Suprathermal Ion Spectrograph (SIS) is a European facility instrument funded by ESA under contract number SOL.ASTR.CON.00004. We thank ESA and NASA for their support of the Solar Orbiter and other missions whose data were used in this Letter. Solar Orbiter post-launch work at JHU/APL is supported by NASA contract NNN06AA01C and at CAU by German Space Agency (DLR) grant number 50OT2002. The UAH team acknowledges the financial support by the Spanish Ministerio de Ciencia, Innovacion y Universidades FEDER/MCIU/AEI Projects ESP2017-88436-R and PID2019-104863RB-I00/AEI/10.13039/501100011033 and by the European Union’s Horizon 2020 research and innovation program under grant agreement No. 101004159 (SERPENTINE). Monthly sunspot numbers are from the Royal Observatory of Belgium, Brussels.

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All Figures

thumbnail Fig. 1.

Daily average EPD/SIS 226−320 keV/nucleon O intensities (filled red circles). The orange line is the threshold defining the semi-quiet time (see text). Right axis: heliospheric distance with five perihelia (blue line). The monthly sunspot number is shown by the dashed green line.
In the text
thumbnail Fig. 2.

Ratio of 226−320 keV/nucleon Fe/O for each of the daily averages shown in Fig. 1. The horizontal lines show survey average values for ISEP events, GSEP events, IP shock events, and CIRs. The dashed olive line (right axis) shows the observing days. See the text for a discussion.
In the text
thumbnail Fig. 3.

Histogram of Fe/O ratios for the data in Fig. 2. The average values for the different particle populations are the same as in the caption of Fig. 2. Right axis: average 3He/4He ratios for the days in each histogram bin.
In the text
thumbnail Fig. 4.

Integrated 273 keV/nucleon daily oxygen fluences from Solar Orbiter (red) and ACE (green) for semi-quiet periods during the Solar Orbiter mission to date. The dashed black lines show the effect of raising or lowering the Solar Orbiter semi-quiet threshold by 20% (see the text for details). Right axis: heliocentric distance of Solar Orbiter (blue). Asterisks mark aphelia, when Solar Orbiter was in the vicinity of Earth.
In the text
thumbnail Fig. 5.

Location of Solar Orbiter and ACE during the two close perihelia in 2022 when the spacecraft moved to ∼0.3 au. In the March 2022 period, Solar Orbiter and ACE had a much closer magnetic connection than for the October 2022 period.
In the text
thumbnail Fig. 6.

Hourly averaged ∼275 keV/nucleon proton (red) and Fe (blue) intensities for the 2022 perihelion passes. The perihelia were on 26 March 2022 (day 085, 0.32 au) and 12 October (day 285, 0.29 au). The yellow boxes show periods that were examined for quiet-time spectra (see the discussion). The inset in lower left panel is the SDO HMI magnetogram on day 085. Right axis upper panels: Solar Orbiter heliocentric distance (green).
In the text
thumbnail Fig. 7.

Energy spectra during the March 2022 periods marked in Fig. 6 during the Solar Orbiter perihelion. Filled circles: Solar Orbiter SIS. Open circles: ACE ULEIS. O has been divided by 10 and Fe by 100 to separate the points.
In the text

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