The "zebra stripes": An effect of F region zonal plasma drifts on the longitudinal distribution of radiation belt particles
We examine a characteristic effect, namely, the ubiquitous appearance of structured peaks and valleys called zebra stripes in the spectrograms of energetic electrons and ions trapped in the inner belt below L ~ 3. We propose an explanation of this phenomenon as a purely kinematic consequence of particle drift velocity modulation caused by F region zonal plasma drifts in the ionosphere. In other words, we amend the traditional assumption that the electric field associated with ionospheric plasma drives trapped particle distributions into rigid corotation with the Earth. An equation based on a simple first-order model is set up to determine quantitatively the appearance of zebra stripes as a function of magnetic time. Our numerical predictions are in agreement with measurements by the Radiation Belt Storm Probes Ion Composition Experiment detector onboard Van Allen Probes, namely: (1) the central energy of any peak identified in the spectrum on the dayside is the central energy of a spectral valley on the night side, and vice versa; (2) there is also an approximate peak-to-valley inversion when comparing the spectrum of trapped electrons with that of trapped ions in the same place; and (3) the actual energy separation between two consecutive peaks (or number of stripes) in the spectrogram of a trapped population is an indicator of the time spent by the particles drifting under quiet conditions.