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

Abstract
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.

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.

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.

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:

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.

Contents

Common overabundance of ³He in high-energy solar particles

1 Introduction

2 Observations and analysis

3 Discussion and conclusions

References

Contents

Common overabundance of 3He in high-energy solar particles

1 Introduction

2 Observations and analysis

3 Discussion and conclusions

References

Common overabundance of ³He in high-energy solar particles