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.

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).

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.