Solar energetic electrons events

Abstract

Solar energetic electron events are one of the most common solar particle accelerations observed in the interplanetary medium (IPM). According to the different dominant species, solar particle events can be divided into proton-dominated large solar energetic particle events and electron-dominated 3He/electron-rich solar energetic particle events. The main difference is that in the proton-dominated large solar energetic particle event, the ratio of 3He /4He ~5×10−4 is the same as that of the corona, and the electron-dominated 3He/electron-rich solar energetic particle event, 3He /4He >0.01, much higher than that of the corona. The release time of solar energetic electron events on the sun can be divided into two groups: low-energy (below ~10 keV) electrons and high-energy (above ~15 keV) electrons. Compared with low-energy electrons, the release time of high-energy electrons is delayed by ~20 minutes, which corresponds to the coronal mass ejection height being about 2 solar radii away from the center of the sun. The release of 3He ions is delayed by about an hour compared to electron release, which corresponds to the coronal mass ejection height being about 5.7 solar radii away from the center of the sun. The energy spectrum of solar energetic electron events is generally a double power-law shape, with a low-energy spectral index of 1.9±0.3 and a high-energy spectral index of 3.6±0.7 with a break energy of ~60 keV. The low-energy spectral index and the high-energy spectral index show a significant positive correlation, while the low-energy and high-energy spectral index show no significant correlation with the break energy. Some events show a single power-law spectrum with an index of 3.5 ±1.2. Previous statistical studies have found that ~45% of the observed solar high-energy electron events above 15 keV are related to hard X-ray flares. By comparing energies above 50 keV, the high-energy hard X-ray spectral index in these events is positively correlated with the high-energy spectral index of electron events, while the index relation disagrees with the prediction of classic bremsstrahlung theory; and by estimating the total number of electrons in high-energy electron events, it is found that the total number of electrons in high-energy electron events is only the total number of electrons that produce hard X-rays in the flare of ~0.1%～1%. In this paper, we further investigate 16 electron events with both good electron observations (with an energy coverage of 5～200 keV) and hard X-ray observations (with an energy coverage of 3～80 keV). The energy spectrum of Hard-X-ray-producing electrons can be deduced based on that the electrons generate X-rays through the relativistic thick-target bremsstrahlung mechanism, by comparing the electron spectrum index and the electron spectrum index of hard X-ray generation, it is found that the low-energy electron spectrum index is positively correlated with the electron spectrum index of hard X-ray generation, while the low-energy electron spectrum index of all events is obviously less than the Hard-X-ray-producing electron spectral index; The comparison of the high energy electron spectral index shows that in half of the events, the high energy electron spectral index is consistent with Hard-X-ray-producing electron spectral index, while in the other half of the events, the Hard-X-ray-producing electron spectral index is steeper than high energy electron spectral index observed in in-situ. These 16 events were also accompanied by strong 3He emissions, 13 were 3He-rich electron events with obvious 3He/4He>0.01, and the other 3 3He/4He<0.01. 15 out of 16 events have coronagraph observations and 14 of them are accompanied with coronal mass ejection. By comparing the simulated spectrum of electron events considering the energy loss during interplanetary transportation with in situ observations, it can be known that the source region of electron events should be located in the high corona (~1.3 solar radius). The location of source region of electron events should still be high in the corona (~1.1～1.3 solar radius) considering the variation of density model implemented, and based on these results, this paper proposes a new acceleration scenario for the acceleration of solar energetic electron events.

Keywords

• solar energetic electrons
• particle acceleration
• solar flares