Dynamics of the Inner Radiation Belt Near the Trapping Limit

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This proposal plans to investigate the processes that trap high-energy protons in a torus called the inner radiation belt at altitudes of 1-3 Earth radii. These trapped protons represent a hazard for satellites traveling through the inner magnetosphere, damaging solar cells, sensors and integrated circuits, exacerbated by the progress in miniaturization and digitization of satellite electronics. The trapping of high-energy protons in the inner belt results because the magnetic field configuration in this region is analogous to a magnetic bottle. The energetic protons spiral along magnetic field lines towards the magnetic poles, where the increase in magnetic field strength causes them to be reflected. As a result they bounce back and forth between these "mirror points" for long periods of time and are effectively trapped. For a dangerous inner belt to form, high-energy protons must be supplied to the region of the magnetic bottle. Protons that are created as a by-produce of collisions between cosmic rays and atmospheric particles appear deep within the magnetosphere and are immediately trapped in the magnetic bottle. Solar protons released in solar flares or accelerated by interplanetary shock waves must penetrate into the magnetic bubble (called the magnetosphere) that shields the Earth. These energetic protons follow curved paths around interplanetary magnetic field lines and if the radius of curvature is large enough they are able to penetrate into the magnetosphere. The extent of this penetration is described by a parameter called the geomagnetic cutoff. The interaction with the solar wind disturbs the magnetic field continually changing the location of the geomagnetic cutoff. During space storms, the geomagnetic cutoff moves earthward allowing solar protons to penetrate deeper into the magnetosphere and as the activity calms, the geomagnetic cutoff retreats leaving the protons behind in the magnetic bottle. The formation of the inner belt depends on the dynamical interplay between the changing location of the geomagnetic cutoff, the configuration of the magnetic bottle, and the sources of energetic protons. The present proposal uses models and observations to better understand this interplay. As a broader impact, this work will contribute to the training of a graduate student and a female postdoc at Dartmouth College.Digging more deeply into the details of the investigation, the proposal undertakes a numerical study of the effects of solar wind driven changes in the Earth's magnetic field and the resultant changes in the location of geomagnetic cutoffs on the growth and decay of the inner radiation belt. Energetic protons in the inner belt are followed backwards in time in these changing magnetic fields to determine whether or not they originated in the un-trapped solar protons outside of Earth's magnetosphere. Comparison of the models to changes in the trapped inner belt proton flux during magnetic storms by spacecraft (most notably NASA's Van Allen Probes) will be used as stringent tests of the model realism. The increased sophistication of the numerical models along with the more detailed observations by the Van Allen Probes are the new elements in this investigation that are expected to produce advances in our understanding of the dynamics of the inner radiation belt.