Fifty Years of 3He-Rich Events

Donald V Reames

doi:10.3389/fspas.2021.760261

Abstract

The early 1970s saw a new and surprising feature in the composition of solar energetic particles (SEPs), resonant enhancements up to 10,000-fold in the ratio 3He/4He that could even make 3He dominant over H in rare events. It was soon learned that these events also had enhancements in the abundances of heavier elements, such as a factor of ∼10 enhancements in Fe/O, which was later seen to be part of a smooth increase in enhancements vs. mass-to-charge ratio A/Q from H to Pb, rising by a factor of ∼1000. These events were also associated with streaming 10–100 keV electrons that produce type III radio bursts. In recent years we have found these “impulsive” SEP events to be accelerated in islands of magnetic reconnection from plasma temperatures of 2–3 MK on open field lines in solar jets. Similar reconnection on closed loops traps the energy of the particles to produce hot (&gt;10 MK), bright flares. Sometimes impulsive SEP intensities are boosted by shock waves when the jets launch fast coronal mass ejections. No single theory yet explains both the sharp resonance in 3He and the smooth increase up to heavier elements; two processes seem to occur. Sometimes the efficient acceleration even exhausts the rare 3He in the source region, limiting its fluence.

Highlights

The solar energetic particle (SEP) events observed first (Forbush 1946) were the largest and most energetic examples we know, where GeV protons produce a shower of particles cascading through the atmosphere to ground level in excess of that from the galactic cosmic rays (GCRs)
Reames (1988) looked at daily averages of SEPs to measure the overall distribution of abundances
How could the average abundances of Ne, Mg, and Si get enhanced if they were all fully ionized with A/Q 2 during acceleration, just like He, C, and O? This dilemma was resolved when DiFabio et al (2008) found that the ionization states in the impulsive SEP events increased with ion speed, suggesting that the ions had come to ionization equilibrium during traversal of a small amount of material after acceleration

Summary

INTRODUCTION

The solar energetic particle (SEP) events observed first (Forbush 1946) were the largest and most energetic examples we know, where GeV protons produce a shower of particles cascading through the atmosphere to ground level in excess of that from the galactic cosmic rays (GCRs). Instruments flown on satellites measured 40 keV electrons associated with X-ray bursts at the Sun and type III radio bursts in space (Lin, 1970, 1974) These events were clearly different from the large proton events and some seemed to be “pure” electron events, i.e., lacking measurable ion intensities. It was not until 1985 that “pure” electron events turned out to be 3He-rich events (Reames et al, 1985). 3He-rich events were strongly associated with type III bursts, both the metric radio events, near the Sun (Reames et al, 1985), and the kilometric events below 2 MHz produced as the electrons continued out beyond ∼6 solar radii (Reames and Stone, 1986)

Abundances of Elements

Ionization States

Findings

Acceleration Theory