Mechanisms of the Sudden Loss of Inner-belt Protons: Insights from Observations

Abstract:

Energetic protons trapped in the inner Van Allen belt region are generally stable except when disturbed by transient events such as interplanetary (IP) shocks. When a strong IP shock accompanied by solar energetic protons impinges the Earth’s magnetosphere, a new proton belt is often observed to emerge at the outer rim (with L-shells ~2.2-3.5). One prevailing and plausible explanation for these new belts is that, after the penetrating solar protons place a seed population at large L-shells, given drift resonance conditions being satisfied, part of protons are promptly transported inward to low L-shells by impulsive shock-induced electric fields and adiabatically accelerated to very high energies. However, the mechanism for the sudden loss—i.e., the new proton belt may disappear with another impinging IP shock—is still a puzzling and open question. There currently exist 3 hypotheses: (H1) the loss due to strengthened scattering from the build-up of the ring current; (H2) the shock-induced electric field will further move preexisting protons toward the Earth, causing the apparent sudden losses at some L-shells; and (H3) ULF waves may outward diffuse protons against the direction of radial gradient in distribution and thus have a new flattened-out distribution. A systematic examination of particle and field observations is needed to test these loss hypotheses, and here we analyzed several proton loss events, selected from both the current operating Van Allen Probes mission period and the past CRRES era, aiming to identify observational signatures consistent with any above hypothesized mechanisms. Both data sets enable us to investigate the pitch-angle- and energy-dependences of losses in the inner belt region, and also allow us to derive proton phase space density profiles before and after loss events. In addition, in-situ magnetic field measurements from Van Allen Probes facilitate detection of field line stretching as well as the existence and intensity of induced electric fields and ULF waves. It is interesting to find proton losses have a persistent and strong dependence of on pitch angles in all selected events. The conclusion is that a new mechanism, that is the field line scattering works in association with the radial displacement caused by such as ULF waves, is the most likely cause of the sudden loss of protons.