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Solar Impulsive Energetic Electron Events


The Sun is capable of accelerating ions from ∼ tens of keV up to tens of GeV and electrons from ∼ tens of eV up to hundreds of MeVs in transient events such as flares and fast coronal mass ejections (CMEs). The energized particles escaping into the interplanetary medium are referred to as Solar Energetic Particle (SEP) events. The great majority of SEP events are impulsive SEP events that are dominated by ∼1-100 keV electrons and ∼MeV/nucleon ion emissions, with enhanced 3He/4He ratios up to 104 times the coronal values (also called electron/3He-rich SEP events). This thesis is focused on solar impulsive energetic electron events, the electron part of impulsive SEP events, using electron observations from the 3-D Plasma and Energetic Particle instrument (3DP) on the WIND spacecraft near the Earth.

First, I present the first comprehensive statistical study of solar energetic electron events over almost one solar cycle. I find that the occurrence rate of solar electron events shows a strong solar-cycle variation; after correction for the background effect, the estimated occurrence frequency exhibits a good power-law distribution, and the estimated occurrence rate near the Earth is ∼1000/year at solar maximum and ∼30/year at solar minimum for the instrumental sensitivity (∼2.9×10-4 (cm3 s str eV)-1 for the 40 keV channel) of WIND/3DP, about one order of magnitude larger than the observed occurrence rate. Solar energetic electron events have a one-to-one association with type III radio bursts and a poor association with flares, but a close association with 3He-rich ion emissions. These 3He-rich electron events also have a poor association with flares but a close (∼ 60%) association with west-limb CMEs.

Then I present two case studies: one investigating the temporal relationship between solar impulsive electrons and type III radio emissions, and the second studying the temporal relationship between solar impulsive electrons and 3He-rich ions. For both studies, I chose nearly scatter-free electron events and developed a forward-fitting method that assumes an isosceles triangular injection profile (equal rise and fall times) at the Sun. I find that in electron/3He-rich SEP events, the low-energy (∼0.4 to 6-9 keV) electron injection starts ∼9 min before the coronal release of the type III radio burst; the high-energy (∼13 to ∼300 keV) electron injection starts ∼8 min after the type III burst; and the injection of ∼MeV/nucleon, 3He-rich ions begins ∼1 hour later. I also find that the selected electron/3He-rich SEP events have a remarkable one-to-one association with fast west-limb CMEs, and most of the associated CMEs are narrow.

Finally, I present a case study to investigate the propagation of different energy electrons in solar impulsive electron events. I find that in the interplanetary medium, low-energy (<∼ 10-30 keV) and high-energy (>∼ 10-30 keV) electrons propagate differently, with more scattering at high energies. Such scattering appears to be caused by resonance with waves/turbulence at scale greater than ∼ the thermal proton gyroradius in the solar wind. Although a transition to more scattering occurs at energies where the electron injection delays are detected, I show that the scattering is not enough to produce these delays.

Based on the results of this thesis, a coherent picture of electron/3He-rich SEP events can be built up. At the Sun, the low-energy (∼0.4 to 6-9 keV) electrons may be accelerated in jets that are ejected upward from magnetic reconnection sites between closed and open field lines; these low-energy electrons generate the type III radio bursts. The jets may appear as CMEs high in the corona, and the high-energy (∼13 to ∼300 keV) electrons may then be accelerated at >∼ 1 RS by CMEs, acting on the seed electrons provided by the low-energy injection. The ∼MeV/nucleon, 3He-rich ions may be accelerated by selective resonance with electron-beam generated waves and/or by fast, narrow CMEs. In the interplanetary medium, both low and high energy electrons often propagate nearly scatter-free, but the high-energy electrons experience more scattering than the low-energy electrons, likely by waves/turbulence generated by solar wind ions. In the future, the upcoming missions to visit the inner heliosphere within ∼0.2 AU (Solar Orbiter) or ∼10 RS (Solar Probe Plus) will provide the near-Sun measurements to improve our understanding of SEP events.

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