Is the Solar Wind Electron Strahl a Seed Population for the Earth's Electron Radiation Belt?

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The near-Earth space is filled with charged particles from the solar wind and the Earth's upper atmosphere. However, how these particles are accelerated and transported into the radiation belt remains a mystery. This Magnetospheric project will explore a possible pathway to discern the ultimate source population of the Earth's electron radiation belt and discern the controlling factors for that source population. The reasoning is as follows. (1) Because the electron radiation belt is lost on occasion, a seed population of energetic electrons is required to replace the belt. (2) It is commonly accepted that the radiation-belt seed population is the population of energetic electrons injected into the dipole by substorms. (3) These substorm-injected electrons are suprathermal electrons in the Earth's magnetotail plasma sheet that are adiabatically delivered from the magnetotail into the dipole by substorms. (4) A resulting question is: where do the magnetotail suprathermal electrons come from, and what factors control them? This project argues that the magnetotail suprathermal electrons come from the energetic electron strahl in the solar wind.The objectives of this NSF project are: (1) To determine whether (and by how much) the solar-wind electron strahl acts as a seed population for the Earth's electron radiation belt, (2) To determine the controlling factors for this process, (3) To trace the electron strahl from the solar wind, into the Earth's magnetotail plasma sheet, into substorm injections, and evolving into the electron radiation belt, and (4) To discern whether there are other sources of the suprathermal electrons in the magnetotail plasma sheet that become energetic substorm- injected electrons. In the diverse data analysis of this project, the strahl electron population in the solar wind at Earth will be statistically followed into the magnetotail plasma sheet, into substorm electron injections, and into the electron radiation belt. The data analysis will be guided by computer simulations to answer questions about the evolution of the strahl electron population as it enters into and traverses through the magnetosphere-ionosphere system.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.