Interplanetary Ion Flux Dropouts Across Multiple ³He-Rich Events

Abstract

Solar Orbiter, a joint ESA/NASA mission, is studying the Sun and inner heliosphere in greater detail than ever before. Launched in February 2020, Solar Orbiter has already completed its first three orbits, reaching perihelia of 0.5 au from the Sun in June 2020, February and August 2021. During the first 2 years in orbit, Solar Orbiter observed multiple ³He-rich Solar Energetic Particle (SEP) events inside 1 au. Even though these events were small, their spectral forms, ³He content, and association with energetic electrons and type III bursts convincingly identifies them as ³He-rich SEP events with properties similar to those previously observed at 1 au, and promising new insights as Solar Orbiter moves much closer to the Sun in 2022. In May 2021, we observed six ³He-rich SEP events in close succession within 48 h when Solar Orbiter was at 0.95 au. These events were likely released from the same active region at the Sun, and the particles arrived at Solar Orbiter in two batches with various abundances and intensities, showing strong anisotropies throughout. Multiple ion flux dropouts were also observed with these six ³He-rich SEP events. The fact that we observed so many ion injections in such a short period of time indicates the ³He enrichment and acceleration mechanism can produce SEP from the same region very efficiently and with varying enrichment levels and intensities. In addition, we report for the first-time dropout features that spanned multiple ion events simultaneously. This implies the field line random walk that we observe at 1 au still maintains magnetic connections to a small region back at the Sun up to the entire duration of these events (∼48 h).

Introduction

In one class of solar energetic particle (SEP) events, the ³He isotope is substantially enriched relative to the more abundant ⁴He, representing an intriguing isotopic enrichment phenomenon. The average solar wind plasma ³He/⁴He ratios are 5 × 10⁻⁴ (Gloeckler and Geiss, 1998) and on rare occasion can be as high as 7 × 10⁻³ (Ho et al., 2000). However, in some SEP events, the ³He/⁴He ratio in energetic and suprathermal ions can be three to four orders of magnitude higher than the solar wind value (Mason et al., 1986). The exact mechanism by which the ³He isotope is enhanced is still unknown. Most proposed theories involve some form of resonant wave-particle interaction that preferentially enhances the ³He but not ⁴He (e.g., Fisk 1978; Temerin and Roth 1992; Roth and Temerin 1997; Petrosian et al., 2009); in some models, a second stage mechanism (e.g., stochastic acceleration) provides the bulk of particle energization that accelerates the ions into the keV-MeV range (Kahler et al., 1985). Investigating the helium fluence in these ³He-rich SEP events, Ho et al. (2005) found that while the ⁴He fluences can vary by 5-6 orders of magnitude, the variations of the ³He fluences in the same SEP events are limited to only 2 orders of magnitude. This apparent limit of the ³He fluence variability and its distribution has been suggested as an indicator of the small size of the acceleration region (Reames 1999; Ho et al., 2005), and the underlying isotope enhancement mechanism (Petrosian et al., 2009).

These ³He-rich SEP events are often found to be accompanied by energetic electrons (10s–100s of keV), type III radio emission, and enhancements of heavy ions. Some events also appear to be associated with solar jets (reviews by Reames 2021; Bučík, 2020). But no correlation has been found between the enrichment of ³He with other accompanying observations. In fact, the only conclusive result from previous studies is that the occurrence of ³He-rich SEP events is associated with scatter-free nonrelativistic electron beams (Reames et al., 1994), but with no relation to the ³He/⁴He ratio itself (Ho et al., 2001).

Most of these ³He-rich SEP events are typically of short duration (i.e., impulsive) with dispersive onsets showing direct magnetic connection to the solar source. If the ions are accompanied by energetic electrons, a solar release time can be extrapolated reliably because of the fast transit times of the nonrelativistic electrons. Another interesting feature of these ³He-rich SEPs is the observation of “dropouts” (Mazur et al., 2000) in some events. Mazur et al. (2000) found in certain impulsive events, the ion flux will exhibit dispersionless “dropouts” lasting up to several hours and followed by reappearance of intensities from the same event. Mazur et al. (2000) and Chollet and Giacalone, 2008 attributed these dropouts to the spacecraft encountering magnetic flux tubes that are filled or not filled with particles, depending on the magnetic connection to the solar source. Analysis from Giacalone et al. (2000) further suggested that when SEPs are released from a small acceleration region on the Sun, the effect of field-line random walk because of supergranulation network in the solar photosphere could lead to observation of repeated dropouts in ion intensities out in the interplanetary medium. All of the dropout events identified in Chollet and Giacalone, 2008 are isolated and excluded those that had multiple impulsive injections at the same time. In an alternate explanation of dropouts by Ruffolo et al., 2003, the field line separation of filled and empty flux tubes takes place in interplanetary space due to random walk in the interplanetary medium (see also Chibber et al., 2021 and references therein).

In this paper we report six ³He-rich events, observed by Solar Orbiter in May 2021, that contained multiple dropouts. These events occurred in less than 48 h and arrived at the spacecraft location (∼0.95 au) in two batches, with particles from multiple events arriving simultaneously. During the time period there was only one active region (AR 12824) at the Sun. All the events have varying ³He/⁴He ratios and energy spectra, but have the same dropout signatures. This has implications for the sources of these enhanced ³He-rich SEP events and their propagation from the Sun through interplanetary space.

Observations

Event Overview

The ³He-rich events described in this paper were observed by the Solar Orbiter Energetic Particle Detector (EPD)/Suprathermal Ion Spectrograph (SIS). The SIS instrument is a high-resolution ion mass spectrometer that measures elemental and isotopic ion composition from 50 keV/nucleon to 10 MeV/nucleon (Rodríguez-Pacheco et al., 2020). SIS is based on the same design principle as the ULEIS instrument currently on ACE (Mason et al., 1998), but with modern electronics and the addition of another telescope to allow pitch-angle distribution measurements of suprathermal ions in the inner heliosphere for the first time.

Solar Orbiter was launched from Cape Canaveral in February 2020, and EPD was commissioned in April the same year. Since commissioning, EPD/SIS has measured the energetic and suprathermal ion composition in both large and small SEP events (Mason et al., 2021a; Mason et al., 2021b; Bučík et al., 2021); and Corotating Interaction Regions (Allen et al., 2021). The excellent sensitivity of the sensor allows us to measure elemental and isotopic composition in suprathermal particles with high precision. Hence, we are able to continue a similar study of ³He-rich events as reported by ACE ULEIS (e.g., Ho et al., 2005).

Figure 1 (left) shows spacecraft locations during this sudy, and (right) shows a 193Å image from the Solar Dynamics Observator (SDO) Atmospheric Imaging Assembly (AIA), with a blue circle marking the Solar Orbiter subsolar point. We used the Solar Orbiter magnetic connection tool (Rouillard et al., 2020) to locate the connection points during the period, assuming a 300 km/s solar wind speed, which was close to speed measured by the Solar Orbiter Solar Wind sensor. The calculated connections (shown by overlapping blue crosses on the figure and in the zoom in insert) use the Air Force Data Assimilative Photospheric flux Transport (ADAPT) model with synoptic magnetograms captured during 22 May 2021, and show the magnetic connection at Active Region 12824 throughout the study period. AR 12824 was the single active region on the visible disk, exhibiting multiple jets and accompanied by many type III radio bursts observed by STEREO-A WAVES. Thus this region exhibited the critical observational markers for sources of impulsive SEP events that show large enrichments of ³He, along with heavy ion enhancements and electrons in the 10 s of keV range. (e.g., Wang et al., 2006; Nitta et al., 2008; Gómez-Herraro et al., 2021).

FIGURE 1

Figure 2 shows Solar Orbiter particle and field observations during May 21-25, 2021. Ions (mostly protons) of 10 keV–100 MeV from the EPD/STEP, EPD/EPT and EPD/HET sensors are shown in the first two panels, while the ion composition measurements from SIS are shown in the third and fourth panel. All energetic particle panels use data from the sunward-pointing EPD sensors (STEP, sunward EPT, sunward HET, and SIS-A) which aim roughly along the interplanetary magnetic field (IMF) direction (Rodríguez-Pacheco et al., 2020). The bottom panel shows the IMF in RTN coordinates from the Solar Orbiter magnetometer (MAG; Horbury et al., 2020). The six ion events on May 22-23 labeled 1-6 in Figure 2 are all impulsive and ³He-rich as shown by the measured helium ion masses from SIS (panel 4), and had dispersive onsets shown on the inverse velocity plots (panels 2, 3). The sunward facing- and anti-sunward facing SIS telescopes showed extremely strong anisotropies during these events, with the sunward-facing: anti-sunward facing intensity ratios >10 throughout, and reaching values of several hundred during the peak intensity periods (not shown).

FIGURE 2

During the period Solar Orbiter was 98° east of the Earth-Sun line at ∼0.95 au, and at a heliographic latitude of −0.74 to −0.78°. Table 1 lists additional details of the 6 events. Line 1 shows the injection times at the Sun estimated from extrapolating the leading edge of the heavy ion 1/v plots (Figure 2 panel 3). Line 2 shows the Carrington Longitude (CL) of Solar Orbiter, and line 3 shows the separation from the subsolar points to AR 12824. Even though the nominal separation changed by 21°during the period, the connection tool shows the spacecraft still connected to AR 12824 (at CL 194-5°), due spreading of the Potential Field Source Surface lines above the active region. The observation of events from a single AR by spacecraft separated by sizable longitudinal distances has been seen for other impulsive events, and shows that such regions can be magnetically connected to a broad range of longitudes (Wiedenbeck et al., 2013; Klassen et al., 2015; Nitta et al.,. 2015).

Observations

Figures 3, 4 show events #1-4 and #5-6 in greater detail. These are in the same format as Figure 2 but with an expanded time scale to focus on the two batches of events; additionally, we added the 5 keV–350 keV electron data from the EPD/STEP, EPT, and HET sensors. There are five electron events (a, b, c, d, and e) with dispersive onsets clearly seen in Figure 3, and three (a, b, c) of those were closely associated with the extrapolated ion solar release time in the inverse velocity plots. We estimated the ion release time by fitting the first arriving particles (i.e., not the peak flux) that was based on Ho et al. (2003). Two dropout periods can be identified in the ion measurements, the first is highlighted with the vertical dashed lines, the second by the dash-dotted line. One occurred shortly before 22 May 0500 UT and can be seen as an abrupt drop in both ion and electron intensities for events #1-3, but particle intensities came back about 18 min later. A second one happened before 22 May 0800 UT that cut off the ion intensities entirely for events #1-4. In this case the ion intensities never recovered, but there was minimal effect on an on-going electron event (e). Line 4 of Table 1 lists the average energies below which the specta of events 1-4 were cut off by the second dropout, with progressively higher energy cutoffs for events with later injection times.

FIGURE 3

FIGURE 4

Vertical pairs of dashed lines and black rectangles in Figure 4 show four ion dropouts during the time period for events #5 and #6. The second dropout between 0400-0500 UT is very similar to the dropout on previous day at 0500 UT when the ion intensities went from high values [∼100/(cm² s sr MeV)] to minimum. But other dropouts only slightly affected the ion intensities and none of these ion dropout signatures could be identified in the electron data. Four distinct electron events (f, g, h, and i) can be identified during this time period, but only two (f and g) of these could be associated with the ion events (5 and 6) based on the timing.

Differential fluence spectra for oxygen in all six events are shown in Figure 5 (left). Because there are multiple ion events overlapping at Solar Orbiter at the same time, we used a separate dispersive box (Mason et al., 2000) for each event in order to calculate the contributions from different events separately. The spectra are generally similar, showing range of a factor of ∼10 for oxygen, while the range for ⁴He is larger (Table 1 line 5). As shown earlier in Figure 2, all of these events are rich in ³He, and line 6 of Table 1 shows the ³He/⁴He for each event. The ratios are typical for impulsive events. Figure 5 (right) shows Fe/O ratios for each of the events, except #4 whose cutoff precluded a measurement. At 385 keV/nuc all events show Fe/O comparable to or larger than values typical of large SEP events (0.404±0.047; Desai et al., 2006), while at lower energies Fe/O increases further still in events 5 and 6.

FIGURE 5

Discussion

It is remarkable that we observed these six ³He-rich events in such short period of time. They were all associated with a single, isolated active region (AR 12824) on the Sun and it therefore appears very likely that all six ³He-rich events originated from this single active region. The events have different energy spectra and abundances which means that the underlying mechanism for enhancing ³He-rich events from a single small region may tap into different sources of particles and/or have varying efficiency in producing different intensities and abundances in a short period of time. Equally remarkable is that we observed dropouts simultaneously in all these events, with the same individual dropouts occurring for multiple injections.

Ion dropouts in impulsive SEP events were first reported by Mazur et al. (2000) on ACE at 1 au. Giacalone et al. (2000) suggested that if particles are released from the Sun in a small region, the meandering of IMF lines could quantitively reproduce the ACE observations. Chollet and Giacalone, 2008 later examined two spacecraft measurements of these dropouts and found the dropout features are almost identical if the spacecraft are within the correlation length of the IMF turbulence, consistent with particle motion along random walking magnetic field lines. Both Giacalone et al. (2000) and Chollet and Giacalone, 2008 assumed that the energetic particles originated from a small region (field coherence or supergranulation scale) on the solar surface, and random motion of the solar supergranules led to separation of the IMF flux tubes because the footprints of the IMF are embedded in the photospheric plasma.

Ruffolo et al., 2003 proposed an alternate model, where the field line meandering does not take place on the solar surface, but rather in the turbulent solar wind itself by means of field line random walk (Jokipii 1966; Jokipii and Parker 1969). In such a model, dropouts are the results of particles being temporarily trapped within small-scale topological structures in statistically homogenous magnetic turbulence that has not diffused away yet (e.g., see Figure 3 in Ruffolo et al., 2003).

The above models share two common assumptions: 1) the particles originate in a small area near the solar surface; and 2) field line meandering causes the ion dropouts either through IMF footpoint motion (e.g. Giacalone et al., 2000) or solar wind turbulence (Ruffolo et al., 2003). The dropout events that have been reported so far are individual events or closely following one another (Chollet and Giacalone, 2008). Some had energetic electron and/or in-situ plasma signatures that coincided with the dropout (Gosling et al., 2004). The dropout events that we report in this paper are similar to prior studies with respect to the correlated concurrent in-situ plasma signatures. The events that we show here are separate ion injections that were released at separate times back at the Sun and arrived at Solar Orbiter with clear velocity dispersion. However, it is the first time we observed multiple ion injections that have the same dropouts across separate ion events.

During these ³He-rich events, the Sun had only one active region (AR 12824) on the visible solar disk as seen from Earth. If we assume particles from these ³He-rich events all originated from a single small region (i.e., AR 12824) at the Sun and were released separately, this implies that the IMF lines connected to Solar Orbiter with the source when they were released close to the Sun. However subsequent dropouts across multiple events at all energies implies that the field lines were connected/disconnected to the source multiple times and field line meandering happened even in shorter time intervals during the 2-day periods. Our observations showed within a 2-day period, a minimum of six ³He-rich events could be produced by the same AR and the footpoint of the IMF out at 0.95 au cannot be far removed from this region. In the Giacalone et al. (2000) model, the assumption is that the source region has to be small (∼coherence scale), so stimulation of a larger source region (several coherence scales) showed that the dropouts are nearly absent. One possibility is that the Solar Orbiter was connected to the source of this AR for up to 48 h with a bundle of flux tubes, some were filled with particles while others not (we note that the Sun will rotate by ∼25 degrees in these 48 h which is much larger than the active region). Hence, as these flux tubes passed Solar Orbiter at 0.95 au, the in-situ observations could detect all six of these ³He-rich events and showed multiple dropouts simultaneously. For the multiple events that we have shown here, they provide a constraint on time (up to 48 h) for which field lines at 0.95 au maintain their connectivity back at the Sun up to small region (∼coherence scale).

We note that Ruffolo et al., 2003 argued in their model that topological structures that develop in the solar wind turbulence could also explain the similar time-intensity profiles for many SEP events while at opposite side of the heliosphere (McKibben et al., 2001), which they attributed to rapid lateral diffusion of particles throughout the inner heliosphere. They argued that because ³He-rich events have rather narrow longitudinal distribution, they are limited within small-scale topological structures. However the larger SEP event “in a few days” could undergo broad lateral motion during their transport from the Sun to Earth, and they do not exhibit any dropouts. In the 2-day period that we observed these six ³He-rich events at 0.95 au, we found multiple ion dropouts but do not find any evidence of such rapid lateral diffusion within the duration of these events. We do not see how the present observations can distinguish between field line separation due to super-granule random walk on the solar surface, versus random walk in interplanetary space between the Sun and ∼0.95 au. However, when Solar Orbiter explores the inner heliosphere much closer to the Sun later in its mission, we expect new observations of similar dropouts may give us new data to reveal their exact cause and mechanism (e.g., Wang et al., 2014).

Conclusion

We observed six ³He-rich SEP events on Solar Orbiter while at 0.95 au in May 2021. These six events all showed dispersion onsets and half of these had associated energetic electrons during the extrapolated released time. All the events were measured by Solar Orbiter within 48 h and arrived in two batches, with ions from different events arriving at Solar Orbiter at the same time but at different energies. They were all likely to be associated with the same AR at the Sun. Interestingly, multiple dropouts were also observed with these six ion events with concurrent dropouts across all ion (and some electron) energies. Assuming all these ions were released from the same source at the Sun, this implies that the ³He enrichment and subsequent acceleration mechanism is able to generate this type of event in quick succession (∼several hours) with varying abundancees and intensities. The dropout features that we observed with these events are consistent with those reported by prior studies that reported the connectivity could be severed and reconnected at timescales of less than an hour. In addition, the multiple dropouts that we observed throughout these ion events indicate that field line random walk could allow magnetic connections to small region back at the Sun over 0.95 au radial distance during a time span of more than 48 h. All of these have implications on the source and enrichment mechanism of ³He-rich SEP events, and how SEPs propagate from the Sun into and through the interplanetary medium.

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