Two sources of dayside intense, quasi-coherent plasmaspheric hiss: A new mechanism for the slot region?

Abstract

A study of dayside plasmaspheric hiss at frequencies from ~22 Hz to ~1.0 kHz was carried out by using 1 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 magnetic local time, 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.