New Observations of Multi-harmonic Mode Field Line Resonances Detected during Low Altitude Perigee Passes of the Van Allen Probes

Abstract:

We report new observations of multi-harmonic mode field line resonances detected by the Electric Field and Waves (EFW) instrument on-board the Van Allen Probes during low altitude perigee passes. The waves were observed as the probes dropped below L≲2, and during the perigee lowering period towards the end of the Van Allen Probes mission. The harmonic waves are detected at frequencies ranging from 0.5 Hz to 3 Hz, and with frequencies which vary with L-shell and altitude through the perigee passes. Here we compare data from two different events from January 1st 2018 and September 10th 2018. During the January 1st 2018 event the frequencies of the ULF waves gradually increased as the probe moved onto lower L-shells. Conversely, for the September 10th 2018 event each wave frequency profile additionally exhibited a dip as the probe moved onto the very lowest L-shells. Here we show that the time evolution of the observed ULF wave frequency profiles can be explained using a standing guided Alfven wave eigenfrequency model constrained by plasma mass density values along the field line given by the International Reference Ionosphere (IRI) model. Our results indicate that the observed ULF waves are standing guided Alfven waves occurring at L<2, and that the frequency of the waves is as expected controlled by the magnetic field and the plasma mass density along the field line. Observations of these waves in combination with the standing guided Alfven wave eigenfrequency model presented here allows the plasma mass density to be inferred down to very low L-shells and at altitude below 600 km. This approach offers a potentially new application of magneto-seismology for examining the dynamics of plasma mass densities at equatorial latitudes on very low L-shells, and for assessing the accuracy of existing models of the plasmasphere and ionosphere.