The overall goal of this project was to increase our understanding of the scattering of electrons as they propagated from the Sun to Earth and interacte with the magnetic field embedded in the solar wind.  We did this by analyzing the angular distribution of electrons emitted during solar electron bursts.  A solar electron burst is a solar active process that produces a rapid rise in the electron intensity, followed by a gradual decay.  Figure 1 displays the electron flux measured by Wind/3DP as a function of time for a wide range of energies from 0.65--515 keV during the burst of 2002 March 22.  Except for the few highest and lowest plotted energies, the onset of the solar electron burst is clearly visible as a rapid increase in the flux.  At 108 keV the onset of the burst occurred at 1112 UT, while at increasingly lower energies the burst onsets occur progressively later, as expected if electrons of all energies were simultaneously accelerated near the Sun and traveled the same distance to reach Wind.

Angular distributions of solar electrons have been measured for several decades.  Such distributions are centered on the direction of the magnetic field embedded in the solar wind.  The width of the distribution, which is proportional to the amount of scattering, varies with electron energy.  A compilation of distribution widths at various energies, measured during many different solar electron bursts over the past 30 years, is shown in Fig 2.  This schematic diagram shows that the distribution width increases with energy up to approximately 0.5-1 keV, and that from 2 to 15 keV, the beam narrows significantly before broadening again for energies greater than 15 keV.  The turnover from a broad distribution around 1 keV to a narrow distribution above 2 keV implies the existence of a local maximum in the burst beam width somewhere between 0.5 and 2 keV.  However, up to this time, such a width vs energy profile had never been constructed over a broad range of energies; therefore, a profile similar to the schematic diagram had never been observed during a single event.  The major outcome of our research was to measure a width vs energy profile during several solar electron bursts that qualitatively matched the schematic diagram; an example of profile from the burst of 2002 Mar 22 is shown in Fig. 3.

One theoretical scattering mechanism that may be able to explain the observed width-energy profile is the electron/electron instability.  Inspired by our observational results, a separate research group has performed particle-in-cell simulations of the electron/electron instability that manifest a characteristic maximum around 1-1.5 keV.

Last Modified: 09/12/2014
Modified by: Curt A De Koning