Statistical modeling of in situ hiss amplitudes using ground measurements

Plasmaspheric hiss is a naturally occurring extremely low frequency electromagnetic emission that is often observed within the Earth's plasmasphere. Plasmaspheric hiss plays a major role in the scattering and loss of electrons from the Earth's radiation belts, thereby contributing to the maintenance of the slot region between the inner and outer electron belt. Traditionally, in situ satellite observations have been the measurement modality of choice for studies of plasmaspheric hiss due to their ability to directly measure the hiss source region. However, satellite studies are relatively short‐lived and very few satellite receivers remain operational for an entire 11‐year solar cycle. Ground stations, in contrast, may collect multiple solar cycles' worth of data during their lifetime, yet they cannot directly measure the hiss source region. This study aims to determine the extent to which measurements of hiss at midlatitude ground stations may be used to predict the mean amplitude of in situ measurements of plasmaspheric hiss. We use coincident measurements between Palmer Station, Antarctica ( L  = 2.4, 50°S invariant latitude) and the THEMIS spacecraft from June 2008 through May 2010, during solar minimum. Using an autoregressive multiple regression model, we show that in the local time sector from 00 < MLT < 12, when the ionosphere above Palmer Station is in darkness and hiss is observed at Palmer, the amplitude of plasmaspheric hiss observed by the THEMIS spacecraft is 1.4 times higher than when hiss is not observed at Palmer. In the same local time sector when the ground station is in daylight and hiss is observed, the THEMIS observed amplitudes are not significantly different from those when hiss is not observed on the ground. A stronger relationship is found in the local time sector from 12 < MLT < 24 where, when Palmer is in daylight and hiss is observed, THEMIS plasmaspheric hiss amplitudes are 2 times higher compared to when hiss is not observed at Palmer. There are insufficient statistics for the 12 < MLT < 24 sector during nighttime conditions. These results suggest that hiss emissions observed at Palmer in the dusk sector are likely plasmaspheric hiss, while those observed in the dawn sector may in fact be an emission other than plasmaspheric hiss, such as either ELF hiss or dawn chorus which has originated at high L ‐shells. Though these results suggest that ground measurements of plasmaspheric hiss are not likely to be a viable replacement for in situ measurements, we believe that the predictive ability of our 12 < MLT < 24 sector model may be improved by including measurements taken during geomagnetically disturbed intervals that are characteristic of solar maximum.