On the Creation of Apparent Local Peaks in Electron Phase Space Density Data by Rapid Inward ULF Wave Radial Transport

Abstract:

The recently completed NASA Van Allen Probes mission provided more than seven years of continuous multipoint Van Allen radiation belt observations of unprecedented quality and resolution. However, the relatively long orbital period of the Van Allen Probes and the availability of only two spacecraft restricts the ability of the constellation to resolve rapid spatio-temporal radiation belt dynamics on sub-orbital timescales. In this study, we use multipoint electron flux measurements and phase space density (PSD) data from the Van Allen Probes and GPS satellites, as well as ULF wave radial diffusion simulations, to disentangle the spatio-temporal dynamics of the belts during an intense geomagnetic storm on September 7-10, 2017. First, we show how the timescale of the last closed drift shell related losses is overestimated by the Van Allen Probes but revealed very clearly to be ~1-hour in the combined data from the constellation of GPS satellites. Second, on its in-bound trajectory, Van Allen Probe B shows the growth of a subsequent local radial peak in PSD, which might be taken to be indicative of local acceleration. However, an extremely fortuitous conjunction with the out-bound Van Allen Probe A at exactly the right time and L* reveals instead that the apparent local peak in PSD seen along the world-line of Probe B is in fact the signature of rapid inwards transport, and that in fact there is no local peak in PSD. Instead, a growing profile of an inwards PSD gradient driven by inward ULF wave transport generates the false signature of a local peak in PSD data. This is important because the local peaks in PSD are commonly used to distinguish between local electron acceleration and inward ULF wave transport during the recovery phase of geomagnetic storms. In this case, the fortuitous spatial and temporal conjunction of the two probes in the heart of the radiation belt reveals the true dynamics to be caused in inwards radial transport. This conclusion is validated using a ULF wave radial diffusion model which recreates the observed local peak in PSD along the world-line of Probe B. The results verify the importance of considering fast spatio-temporal radiation belt dynamics when seeking to identify the processes responsible for Van Allen belt acceleration in data from relatively slowly moving satellites in geosynchronous transfer orbits.