A&A 408, L1-L4 (2003)
DOI: 10.1051/0004-6361:20031090

Common overabundance of 3He in high-energy solar particles

J. Torsti - J. Laivola - L. Kocharov

Department of Physics and Väisälä Institute for Space Physics and Astronomy, University of Turku, 20014 Turku, Finland

Received 31 March 2003 / Accepted 15 July 2003

Solar energetic particle (SEP) observations of the ERNE instrument (the Energetic and Relativistic Nuclei and Electron experiment) onboard the Solar and Heliospheric Observatory enable measurements of the 3He intensity in the high-energy range, ${>}15~{\rm MeV~nucleon^{-1}}$, with a good statistical resolution. We report an overview of the ERNE observations for the period from 8 February 1999 to 7 December 2000. Significant intensities of 3He are registered in all events with 4He intensity exceeding ${{\approx }}0.5$ ion per ${\rm (m^2~s\;sr~MeV~nucleon^{-1})}$. The abundance ratio 3He/4He is measured to vary in the range $\sim$0.003-2, well above solar wind values. A histogram of the daily 3He/4He ratio reveals a sharp maximum at 3He/4He $\approx$ 0.015. This is not expected in the impulsive-gradual paradigm of solar energetic particle events. We argue that the abundance 3He/4He $\approx$ 0.015 should be regarded as a "normal composition'' of high-energy solar particles, instead of the previously assumed 3He/4He  $\approx 5\times10^{-4}$. The ERNE registered also a number of 3He-rich events with 3He/4He $\sim$ 0.1-1, which however are less frequent. We consider the common overabundance of 3He as a signature of impulsive processes, which seem unavoidable during the development of the SEP-productive solar eruptions comprising coronal mass ejections and flares.

Key words: Sun: corona - Sun: flares - Sun: particle emission

1 Introduction

The 3He-rich solar energetic particle (SEP) events are usually observed in the low-energy range, ${\sim}1$ MeV nucleon-1, to have the 3He/4He abundance ratio as high as $\sim$0.03-3 (Kocharov & Kocharov 1984; Reames et al. 1994, and references therein). It is widely accepted that the 3He-rich composition is produced by wave-particle interactions in flares, whereas a "normal'' composition with nearly solar wind abundance, 3He/4He  $\approx 5\times10^{-4}$, was expected for the shock-accelerated SEPs in gradual events (Reames 1995). Nevertheless, new observations in the low-energy range revealed a number of 3He-rich events associated with interplanetary CME-driven shocks (Mason et al. 1999; Desai et al. 2001). However, the general statistics of the 3He-rich events has not been updated since mid-1990s.

A few measurements of 3He beyond 10 MeV nucleon-1 are available (Cohen et al. 1999; Clayton et al. 2000; Bakaldin et al. 2002; Torsti et al. 2002,2003, and references therein). In particular, the most recent SOHO/ERNE observations have revealed 3He enrichments in the energy range 15-30 MeV nucleon-1 with 3He/4He$\ >0.2$, produced in solar eruptions comprising impulsive flares and CMEs. For the first time, the extremely high values of 3He-to-4He ratio were observed in the high-energy energy range. However no statistical overview was reported. In this Letter, we examine this issue further by surveying 3He abundances in the high-energy range in all events observed over a 22 month period, from 8 February 1999 to 7 December 2000.

2 Observations and analysis

The particle telescope ERNE/HED identifies 3He nuclei in the energy range 15-120 MeV nucleon-1. The accurate measurement of the particle flight trajectory, and at least four independent energy loss measurements make the separation of 3He and 4He isotopes confident. During conditions of low count rate ERNE is capable of analyzing all arriving particles. When the count rate is more than 150-200 particles in a minute, a priority system saves only a sample of protons (at least 10 protons). Instead it tends to save as many helium nuclei as possible. In still higher count rates only a sample of He nuclei is collected, and heavy nuclei are stored with highest priority. When ERNE is working in the sample mode, the ERNE on-board computer controls in several ways the development of real and sample number rates, and of the dead time, which makes the correction of sample rates into absolute particle intensities reliable. The description and details of the ERNE instrument are given by Torsti et al. (1995). In the present study we chose the energy channel 15-30 MeV nucleon-1 for both helium isotopes, 3He and 4He. The channel is close to the lower energy threshold of ERNE/HED. In the selected 3He-energy channel the average geometric factor is 30.5 cm2 sr.

  \begin{figure}\par\includegraphics[clip,width=\textwidth]{Fc311_f1.eps} \end{figure} Figure 1: Daily count rates of 3He and 4He and the abundance ratio 3He/4He on the 3He event days in the energy range 15-30 MeV nucleon-1. The slowly-changing line in the uppermost panel shows a selection limit for an 3He event. A pair of dashed horizontal lines in the lowermost panel shows the levels 3He/4He = 0.005 and 0.05, between which the majority of the 3He event days fall.
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We have carried out a survey based on the particle measurements by ERNE/HED from 8 February 1999 till 7 December 2000. During the survey period there were only a few periods when either the spacecraft, or more often, the instrument was not in the observation mode. In the case of extreme count rates, as was the case during the 14 July and 9 November events in 2000, the analysis capacity was exceeded. In those occasions, the data have been excluded from the analysis. Figure 1 plots time profiles of 3He and 4He count rates during 1999-2000. The broken, nearly horizontal line in the uppermost panel displays a lower limit for the selection of 3He events representing a gliding-average of the preceding non-event period raised by a threefold statistical error. During 1999-2000 the ERNE had 620 days with observations of at least 12 hours duration. Total number of observation days was 636. The 3He and 4He count rates exceeded the event selection limit during 167 (26%) and 243 (38%) days respectively. Figure 1 shows also the 3He/4He ratios on the 3He-event days. On a few days the 3He count rate in the high-energy channel is in excess of 1000 counts per day. For instance in the beginning of 14 July 2000 high 3He-intensities were measured right before the temporary loss of measurement capacity and the intensities remained high also after the recovery of the instrument. On 30 October 2000 the absolute amount of measured 3He ions was exceptionally high and the daily 3He/4He was close to 1. The corresponding particle event started on 29 October 2000 at about 18 UT and the 3He/4He ratio remained above 1 for the first 30 hours from the event onset. This event was described in detail by Torsti et al. (2002).

  \begin{figure}\par\includegraphics[clip,width=\textwidth]{Fc311_f2.eps} \end{figure} Figure 2: Scatter plot of 3He intensity vs. 4He intensity, with background subtracted, on the 3He event days in the energy channel 15-30 MeV nucleon-1 a) and the corresponding day-number distribution over the abundance ratio 3He/4He b). The dashed lines in panel a) indicate the 0.005, 0.05, and 0.5 levels of the 3He/4He-ratio. The horizontal dotted line in the panel a) exemplifies an average 3He background level. All 4He enhancements with 4He intensity exceeding the threshold level that is shown with the vertical dotted line were accompanied by a measurable amount of 3He. Curve in panel b) shows a possible fit to the data histogram ( $\chi ^{2}=46.4$ at 43 degrees of freedom). It reveals a main peak around 3He/4He = 0.015 and a high-abundance tail at 3He/4He > 0.1.
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The left panel of Fig. 2 plots the daily average 3He intensities versus the 4He intensities on the 3He event days, with galactic background subtracted. One can discern a systematic organization of points in the scatter plot. The organization indicates a clear correlation between the 3He and 4He intensities. Most of the events are located between the lines 3He/4He = 0.005 and 0.05, especially in strong events. On several days the 3He/4He ratio was in excess of 0.5. From the same figure one can see that no 3He-event days with daily intensity less than 0.002 (m2 s sr MeV nucleon-1)-1 were found. This is because such intensities are below the typical background level. Consequently, there have not been measurable 3He intensities in many weak 4He enhancements with the 4He intensity <0.2 (m2 s sr MeV nucleon-1)-1. However for the strong enough events with 4He intensity in excess of $\approx$0.5 (m2 s sr MeV nucleon-1)-1, the 3He association is found to be absolute: all the helium events contain a measurable amount of the isotope 3He. A statistical distribution of the 3He enhancement days over the 3He/4He ratio is shown in the right panel of the figure. The distribution apparently consists of a narrow peak at 3He/4He $\sim$ 0.015 and a wide substratum extending through 3He/4He $\sim$ 1.

Based on the previous observations of 3He-rich events in the low-energy range, the distribution of 3He/4He ratios is relatively flat in the range 3He/4He $\approx$ 0.03-2 and gradually declines as 3He/4He increases from $\approx$2 to 10 and beyond (see Fig. 2 in Reames et al. 1994 and Fig. 8 in Kocharov & Kocharov 1984). Recently Mason et al. (1999) discussed some low-energy events with 3He/4He $\approx$ 0.002, but the event number distribution has not been updated. We report on the 3He-rich events recently observed at higher energies. Figure 2 indicates a very wide distribution of the high-energy events, which exhibits a narrow peak at moderate enhancements, 3He/4He $\approx$ 0.015, but changes slowly in the high enhancement range, 3He/4He $\approx$ 0.1-2. With the high-energy data in hand and based upon the previous low-energy experience, we fit the distribution shown in Fig. 2 with a sum of a narrow, Gaussian type distribution and a wide, quadratic function, which corresponds to the GAUSSFIT routine of IDL in respect to the variable log(3He/4He). The quadratic component of the distribution is qualitatively similar to the aforementioned low-energy distribution, but the Gaussian-type peak at 3He/4He $\approx$ 0.015 is new. Clearly, it is not what one might expect based on the impulsive-gradual SEP paradigm in the Reames' (1995) formulation.

3 Discussion and conclusions

Reames et al. (1985) found that virtually all low-energy 3He-rich events are accompanied by impulsive 2-100 keV electron events. The electron-3He rich events are widely known as impulsive events, because they are associated with impulsive solar soft X-ray bursts. However, the 3He/4He distribution is quite wide. For this reason, the representative values 3He/4He $\sim$ 0.1-1 for impulsive events (Lin 1994) seem more appropriate than the widely quoted 3He/4He $\sim$ 1 (Reames 1995). In the SEP classification tables by Lin (1994) and Reames (1995), a "normal'' solar abundance 3He/4He  $\sim 5\times 10^{-4}$ is shown for the gradual SEP events. Unlike impulsive events, this value was not measured in SEPs but refers to the solar wind abundance. However, there are some observations of 3He intensities in gradual events, and the abundances were found to be much higher, e.g., 3He/4He $\sim$ 0.03 by Evenson et al. (1990). Recently Mason et al. (1999) reported for 12 large SEP events the average value 3He/4He $=(1.9\pm 0.2)\times10^{-3}$, also well above the solar value that was assumed to be "normal'' for SEP events.

In the energy range of our present study, 15-30 MeV nucleon-1, we observe the frequency of 3He-rich events to decrease slowly as 3He/4He increases from $\sim$0.05 to $\sim$1 (Fig. 2). This part of distribution is qualitatively similar to the above-mentioned distribution of the low-energy impulsive events. Torsti et al. (2002,2003) carefully studied the high-energy 3He-rich events situated at the high 3He-abundance end, 3He/4He > 0.2. Those events were associated with impulsive X-ray flares and CMEs, but onsets of 3He events were delayed for many hours in respect to the flares. The delays as well as the observed spectrum of 3He can be explained in a model in which the impulsive SEPs of MeV energies were reaccelerated at interplanetary shocks (Kocharov & Torsti 2003).

The most prominent feature of the distribution shown in Fig. 2 is a narrow peak at 3He/4He $\approx$ 0.01-0.02. In previous measurements of 3He above 10 MeV nucleon-1, the 3He/4He ratios in the range 0.001-0.01 could not be well resolved and no statistical study was performed. It is important to note that 3He is measured in all sufficiently strong 4He events. We find the most common abundance to be 3He/4He $\sim$ 0.01. Surprisingly, almost no room is left for the expected "normal'' composition, 3He/4He  $\approx 5\times10^{-4}$. A couple of strong events, which might appear at the low abundance range of Fig. 2, are missed in the ERNE data, but the majority of the SEP events has been measured, and the overall distribution clearly peaks at 3He/4He $\approx$ 0.015. Recall that 3He enhancements at $\approx$0.5-2 MeV nucleon-1 in large SEP events were previously measured to be ${\approx }0.002$ (Mason et al. 1999).

In conclusion, we have analyzed the 3He abundance observed with the high-energy detector of the ERNE instrument onboard SOHO during the 22 month period in 1999-2000 and find the following:

In all 4He events with intensity exceeding ${{\approx }}0.5$ ion of 4He per (m2 s sr MeV nucleon-1), significant fluxes of 3He ions have been also detected.
The most frequent 3He abundance is 3He/4He $\sim$ 0.01.
At a lower frequency, the 3He/4He ratio is observed to be as high as $\sim$0.1-1.
We cannot rule out that in exceptional cases 3He/4He may be $\lesssim$0.001, but such a composition is definitely not typical in the energy range 15-30 MeV nucleon-1. From an empirical standpoint, the most likely abundance, 3He/4He $\approx$ 0.015, should be regarded as "normal'' for SEPs. We do not call into question other properties of gradual SEP events, but the abundance ratio 3He/4He  $\approx 5\times10^{-4}$ that is shown in the widely known SEP-classification tables finds no support in the experimental data.

Recently, Kocharov et al. (2001) performed a statistical study of the ${\sim}10$ MeV proton event occurrences in association with different types of CMEs. The results support an idea that production of SEPs depends not only on the final speed of CME but also on the magnitude of acceleration that CME experiences during its liftoff, and the SEP-producing CMEs are typified by impulsively accelerating CMEs accompanied by soft X-ray flares of different magnitude and coronal type II radio bursts. Cane et al. (2002) also performed a correlative study between >20 MeV solar proton events, CMEs, flares, and radio bursts. They found that essentially all of the proton events are preceded by groups of type III bursts (that are a flare phenomenon) and all are preceded by CMEs. The overabundance of 3He in the majority of SEP events observed by the ERNE instrument during 1999-2000 means for us that the impulsive, flare-type processes always participate in production of SEPs. We consider accelerated 3He ions, along with energetic electrons and radio bursts, as a signature of magnetic reconnection, which is unavoidable during development of solar eruptions comprising flares and the CME liftoff/aftermath processes at different coronal altitudes and locations.

This work was supported by the Academy of Finland, grant number 53543. SOHO is an international cooperation project between ESA and NASA.


Copyright ESO 2003