³He-Rich Solar Energetic Particle Events with No Measurable ⁴He Intensity Increases

Abstract

We investigated ³He-rich solar energetic particle (SEP) events in the current solar cycle starting in 2009 through 2017. Both “impulsive” (flare-related) ³He-rich and CME-related “gradual” events are included. In the former solar cycle, we found the number of observed ³He-rich events correlated with solar activity. The same correlation is seen again in Cycle 24. Because of the comparatively weak activity, both the occurrence of ³He-rich events and their intensities are significantly less than those from Cycle 23. Interestingly, we found in several of the ³He-rich events that there is no measurable ⁴He intensity increase above the instrument background. Previously, we found that there is a limit on the number of ³He ions that can be released from the Sun in an impulsive SEP event, while there is no such limit on the ⁴He ions (Ho, Roelof, and Mason in Astrophys. J., 621, L862, 2005). In this paper, we examine several of these ³He-rich events in detail and discuss the lack of observable ⁴He intensity increases and the implications for the enhancement and acceleration mechanism of this special type of SEP events.

1 Introduction

³He-enhanced solar energetic particle (SEP) events show an intriguing isotopic enrichment. The average solar wind plasma ³He/⁴He ratio is about $5\times10^{-4}$ (Gloeckler and Geiss 1998), and on rare occasions can be as high as $7\times10^{-3}$ (Ho et al. 2000). However, in certain 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 (Reames, Meyer, and von Rosenvinge 1994). They were often found to be accompanied with energetic electrons (10 – 100 keV) and type III radio emission. But there is no correlation to be found between the measured ³He/⁴He ratio with other accompanied observations (e.g. flare class, electron intensity). One conclusive result from previous studies is that the occurrence of ³He-rich SEP events is associated with scatter-free nonrelativistic electron beams (Reames, von Rosenvinge, and Lin 1985), but not the ³He/⁴He ratio enhancement factor itself (Ho et al. 2001). Ho, Roelof, and Mason (2005) investigated the helium fluence, and found while the ⁴He fluences can vary by 5 – 6 orders of magnitude the ³He fluences in the same SEP events range is limited to only 2 orders of magnitude. This apparent limit of the ³He fluence and its distribution has been suggested as an indicator of the size of the acceleration region (Reames 1999; Ho, Roelof, and Mason 2005), and the underlying isotope enhancement mechanism (Petrosian et al. 2009). Hence, measurements of the fluence distribution of the helium isotopes are important to characterize this type of SEP event. In this paper, we report several ³He-rich SEP events with no measurable ⁴He intensity increase.

2 Observations

The energetic particle data presented in this paper are from the Ultra-Low Energy Isotope Spectrometer (ULEIS) on the Advanced Composition Explorer (ACE) spacecraft. ACE was launched in August 1997 and is currently in a halo orbit around the Sun-Earth L1 libration point ($\sim 200$ Re) upstream of the Earth (Stone et al. 1998). The ULEIS instrument is a high-resolution ion mass spectrometer that measures elemental and isotopic ion composition from 50 keV/nucleon to a few MeV/nucleon (Mason et al. 1998). The ³He-rich events described in this paper were selected using the ULEIS pulse-height-analysis (PHA) data from 2009 to 2017. We initially followed criteria similar to Ho, Roelof, and Mason (2005) to select all ³He-rich events during the time period, namely: 1) the 0.2 – 2.0 MeV/nucleon event-averaged ³He/⁴He ratio must exceed 0.004 and have uncertainty less than 50% of the helium intensities; 2) the event is isolated and shows a measurable increase from the instrument background level; and 3) the event must last more than 1 h. From these criteria alone, there were 144 events during these time period. This is sharply lower than what was reported in the previous solar cycle. Using the same criteria, Ho, Roelof, and Mason (2005) reported 201 events from 1997 to 2002. Widenbeck and Mason (2014) studied ³He in the interplanetary medium and found the fraction of time with ³He present is significantly lower in the present cycle. We then further down-selected those events that have no noticeable ⁴He intensity increases above ambient background (i.e. greater than 50% ⁴He intensity increase within 1-h interval) associated with the ³He intensity enhancement, which narrowed the list down to 16 events. Upon closer examination of these 16 events, a majority of them have elevated ⁴He intensity, such that we could not definitively rule out whether there are no associated ⁴He intensity increases with the ³He time periods.

Table 1 lists the four events we selected in this study that have no measurable ⁴He increase during the time period. Figure 1 shows the two events in 2013. The three panels in Figure 1 show: (top panel) nonrelativistic (38 – 103 keV) electron from ACE/EPAM (Gold et al. 1998); (middle panel) ULEIS hourly averaged ion composition data at $\sim 1$ MeV/nucleon; and (bottom panel) ULEIS pulse-height-analyzed (PHA) helium data. Enhanced ³He time periods can be identified from the ULEIS PHA data as distinct increases in count rates in the ³He mass track (i.e. at 3 amu), which is clearly separated from the more dominant ⁴He mass track (i.e. at 4 amu). Several enhanced ³He periods are clearly seen in the listed time periods. For example, there are clear ³He ion events at day of year (DOY) 187, 191, 198, 207 and 211. The majority of the ³He-rich events (e.g. DOY 187 and 198) are observed to also have corresponding ⁴He intensity increases. The event on DOY 191 is one that has no noticeable ⁴He increase above the 50% level from ambient. As shown by Ho et al. (2001), the nonrelativistic electrons are correlated with the ³He-rich ion events. That correlation can also be seen in some of the events shown in Figure 1. Most noticeable is the event on DOY 198, where you can also see the nonrelativistic electron injection accompanying the ion enhancement.

Figure 1

Energetic electron and ion composition measurement by ACE/EPAM and ACE/ULEIS during day of year (DOY) 180 to 240 in 2013. Top panel: 5-min averaged energetic electrons (38 – 113 keV); middle panel: hourly averaged low energy ($\sim 1$ MeV/nucleon) ion composition; bottom panel: high-resolution helium isotope data showing arrival times and masses of each ion detected. Boxed periods are discussed in the text.

Table 1 ³He-rich periods that have no measurable ⁴He intensity increase.

However, the two ³He events that we identified on DOY 207 and 211 (square boxes in the bottom panel) have none of the usual characteristics. During both events there are no corresponding ⁴He intensity increases or electron injections. Both the proton and helium intensities are at background level and the $>38$ keV electrons show no new injection during these two ³He-rich time periods. Figure 2 shows the event-averaged energy spectra of both helium isotopes for the event on 2013 DOY 211. The spectral slope of both the ³He and ⁴He are almost identical, but the intensity of ³He are on average factor of 5 higher than ⁴He above 200 keV/nucleon.

Figure 2

Event-averaged helium spectra for the event in 2013 on DOY 211.

Figure 3 shows another event we identified in 2010 in the same format as in Figure 1. A sequence of three ³He-rich periods can be seen between DOY 244 to 250. The first and last of these three events have both corresponding ⁴He and electron intensity increases during the ³He-rich period. However, the event on DOY 245 which came shortly after the initial event on 244 and immediately prior to another one on DOY 247 has no measurable ⁴He and electron increase above background level.

Figure 3

Same format as in Figure 1 but for time period in DOY 220 – 260 in 2010.

Figure 4 shows the in-situ solar wind plasma and interplanetary magnetic field (IMF) measurements from ACE during the event on 2010 DOY 245. The solar wind speed (first panel) was steady at slow speed ($<450$ km/s). Both the IMF (second and third panel) and the strahl electrons (last panel) pitch-angle data from DOY 245 to 247 also indicated that we were connected to a uniform solar wind source region since there is no change within the event interval. We note the solar wind (up to DOY 246.7) is more Alfvenic (fourth panel) than the typical slow wind, where we define $C_{vB}\equiv \frac{\Delta B\cdot \Delta v}{\vert \Delta B \vert \vert \Delta \nu \vert }$, which could be used as a proxy for cross-helicity, a property that is a measure for turbulence (see Ko, Aaron, and Lepri 2018); but otherwise nothing was unusual about the in-situ solar wind during the period when the ³He was detected.

Figure 4

The in-situ solar wind data from ACE during the ³He-rich period identified in 2010 DOY 245 – 247 (dashed lines). The solar wind velocity is shown in the top panel, while the IMF data are shown in second ($|B|$, $B_{\mathrm{T}}$, and $B_{\mathrm{N}}$) and third panels ($\lambda$, which is the angle defined as the angle from $B_{\mathrm{R}}$ towards $B_{\mathrm{T}}$). The fourth panel shows the solar wind Alfvenicity (a black dot is 64-s data point and the blue curve is the 1-h running mean; see the text for further details). The bottom panel shows the halo electron (272 eV) pitch-angle data, 0 – 180° at 18° increments from black (0 – 18°, outward IMF) to red (162 – 180°, inward IMF).

3 Discussion

We have shown cases of enhanced ³He intensity with no measurable ⁴He ion enhancement. These are unusual as we could only identity four from approximately one hundred ³He events during the same time period. The enhancement of ³He in certain SEP events has been studied since its discovery in the 1970s (see review by Reames 1999). There are competing theories in explaining the remarkable enrichment of ³He/⁴He of $10^{2}$ – $10 ^{4}$ times the typical solar wind abundance of 3 – $4 \times 10^{-4}$. The prevailing theories all involve some form of wave–particle resonate interaction. Fisk (1978) used ⁴He generated electrostatic ion cyclotron waves in order to heat the ³He and Temerin and Roth (1992) suggested that the excited electromagnetic ion cyclotron waves that could resonate with ³He and thereby accelerate the ³He directly. Recently, Liu, Petrosian, and Mason (2006) and Petrosian et al. (2009) have shown that stochastic acceleration by plasma wave turbulence can produce some of the observed ³He and ⁴He energy spectra. In their model, the relative abundances and spectra of the two helium isotopes are different because they interact with different wave modes.

In a large ³He study, Ho, Roelof, and Mason (2005) found that there is an upper limit of the amount of ³He ions can be accelerated in these events. They found that the fluence distribution of the ³He is limited to a narrow range, while the ⁴He fluence is not. Petrosian et al. (2009) argued that this steep variation of the fluence ratio could be explained by the level of turbulence in their model, and they have successfully reproduced the observed fluence distributions from Ho, Roelof, and Mason (2005).

In this study, we followed the same criteria as defined by Ho, Roelof, and Mason (2005) and found over 144 ³He-enhanced events from 2009 to 2017. The number of ³He event is a lot lower than in the previous solar cycle which was as noted by Ho and Mason (2016) and Widenbeck and Mason (2014). Four ³He-rich events were found without measurable ⁴He increases. The observed ³He fluence is small but well within the sensitivity of the ULEIS instrument. The ³He fluences from these four events are measured to be $10^{3}$ – $10^{4}$ particles/(cm²-sr MeV/nucleon), while the ⁴He are all less than 10³ particles/(cm²-sr MeV/nucleon) (Table 1). These observed fluence levels provide further limits; this was done in Ho, Roelof, and Mason (2005) by establishing observable ³He and ⁴He fluences in these types of events. The measured fluences as shown by Ho, Roelof, and Mason (2005) and Petrosian et al. (2009) provide important information on the possible enrichment mechanism of ³He. The events that we report here provide a new constraint on the possible ³He enrichment/release mechanism because any wave–particle resonant interaction that is based on specific charge-to-mass ratio also has to explain why there could be abundant ³He particles but no measurable ⁴He above background.