The PI plans a comprehensive analysis of solar electron bursts observed during solar cycle 23 by the WIND, ACE, and Genesis spacecraft. In this investigation, the PI and his collaborators will compare in situ measurements with published simulation results. The PI will calculate the widths of electron pitch angle distributions (PADs) before, during, and after bursts for a wide variety of electron energies. The PI will also characterize the solar wind plasma and interplanetary magnetic field (IMF) before, during, and after the electron bursts. He will then determine if these PAD, solar wind plasma, and IMF signatures are consistent with pitch-angle scattering by the electron/electron instability. In non-broadened bursts, the PI will determine if the PAD, solar wind plasma, and IMF characteristics are consistent with pitch-angle scattering by broadband whistler turbulence. Since the electron/electron instability can generate Type III radio bursts, the PI and his collaborators will determine the correlation between broadened solar electron bursts and Type III radio bursts.
The PI notes that an improved understanding of electron propagation during solar bursts addresses the ongoing controversy concerning the acceleration of electrons in the solar corona and their injection into the interplanetary medium. Electron pitch-angle scattering during bursts can also serve as a proxy for electron scattering during other forms of solar activity, including CMEs, and it may be important in resolving the solar magnetic flux budget.
During this research project, the PI plans to mentor summer students and involve them in the analysis of the WIND, ACE, and Genesis spacecraft observations. The students will be participants in the Boulder Research Experience for Undergraduates program, which is jointly managed by the University of Colorado and NOAA's Hollings Undergraduate Scholarship program.
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.
