Barbara J. Falkowski

Extremely intense ELF magnetosonic waves: A survey of polar observations

A Polar magnetosonic wave (MSW) study was conducted using 1 year of 1996–1997 data (during solar minimum). Waves at and inside the plasmasphere were detected at all local times with a slight preference for occurrence in the midnight-postmidnight sector. Wave occurrence (and intensities) peaked within~±5° of the magnetic equator, with half maxima at ~±10°. However, MSWs were also detected as far from the equator as +20° and 60° MLAT but with lower intensities. An extreme MSW intensity event of amplitude Bw = ~± 1 nT and Ew = ~± 25 mV/m was detected. This event occurred near local midnight, at the plasmapause, at the magnetic equator, during an intense substorm event, e.g., a perfect occurrence. These results support the idea of generation by protons injected from the plasma sheet into the midnight sector magnetosphere by substorm electric fields. MSWs were also detected near noon (1259 MLT) during relative geomagnetic quiet (low AE). A possible generation mechanism is a recovering/expanding plasmasphere engulfing preexisting energetic ions, in turn leading to ion instability. The wave magnetic field components are aligned along the ambient magnetic field direction, with the wave electric components orthogonal, indicating linear wave polarization. The MSW amplitudes decreased at locations further from the magnetic equator, while transverse whistler mode wave amplitudes (hiss) increased. We argue that intense MSWs are always present somewhere in the magnetosphere during strong substorm/convection events. We thus suggest that modelers use dynamic particle tracing codes and the maximum (rather than average) wave amplitudes to simulate wave-particle interactions.

Plasmaspheric hiss properties: Observations from Polar

In the region between L = 2 to 7 at all Magnetic Local Time (MLTs) plasmaspheric hiss was detected 32% of the time. In the limited region of L = 3 to 6 and 15 to 21 MLT (dusk sector), the wave percentage detection was the highest (51%). The latter plasmaspheric hiss is most likely due to energetic ~10–100 keV electrons drifting into the dusk plasmaspheric bulge region. On average, plasmaspheric hiss intensities are an order of magnitude larger on the dayside than on the nightside. Plasmaspheric hiss intensities are considerably more intense and coherent during high-solar wind ram pressure intervals. A hypothesis for this is generation of dayside chorus by adiabatic compression of preexisting 10–100 keV outer magnetospheric electrons in minimum B pockets plus chorus propagation into the plasmasphere. In large solar wind pressure events, it is hypothesized that plasmaspheric hiss can also be generated inside the plasmasphere. These new generation mechanism possibilities are in addition to the well-established mechanism of plasmaspheric hiss generation during substorms and storms. Plasmaspheric hiss under ordinary conditions is of low coherency, with small pockets of several cycles of coherent waves. During high-solar wind ram pressure intervals (positive SYM-H intervals), plasmaspheric hiss and large L hiss can have higher intensities and be coherent. Plasmaspheric hiss in these cases is typically found to be propagating obliquely to the ambient magnetic field with θkB0 ~30° to 40°. Hiss detected at large L has large amplitudes (~0.2 nT) and propagates obliquely to the ambient magnetic field (θkB0 ~70°) with 2:1 ellipticity ratios. A series of schematics for plasmaspheric hiss generation is presented.

Two Sources of Dayside Intense, Quasi‐Coherent Plasmaspheric Hiss: A New Mechanism for the Slot Region?

A study of dayside plasmaspheric hiss at frequencies from ~22 Hz to ~1.0 kHz was carried out using one year of Polar data. It is shown that intense, dayside plasmaspheric hiss is correlated with solar wind pressure with P > 2.5 nPa. The dayside effect is most prominent in the ~300 to ~650 Hz range. Intense dayside waves are also present during SYM-H < -5 nT. The latter is centered at local noon, with the greatest intensities in the L =2 to 3 region. Assuming drift of ~25 keV electrons from midnight to the wave MLT, plasmaspheric hiss is shown to be highly correlated with precursor AE* and SYM-H* indices, indicating that the hiss is associated with substorms and small injection events. Our hypothesis is that both sets of waves originate as outer zone (L = 6 to 10) chorus and then propagate into the plasmasphere. Fourteen high intensity dayside plasmaspheric hiss events were analyzed to identify the wave k, polarization and the degree of coherency. The waves are found to be obliquely propagating, elliptically polarized and quasi-coherent (~0.5 to 0.8 correlation coefficient). It is hypothesized that the dayside plasmaspheric hiss is quasi-coherent because the chorus has been recently generated in the outer magnetosphere and have propagated directly into the plasmasphere. It is possible that the quasi-coherency of the dayside hiss at L = 2 to 3 may be an alternate explanation for the generation of the energetic particle slot region.