Journal of Geophysical Research: Space Physics

Cover image for Vol. 120 Issue 12

Impact Factor: 3.426

ISI Journal Citation Reports © Ranking: 2014: 19/175 (Geosciences Multidisciplinary)

Online ISSN: 2169-9402

Associated Title(s): Journal of Geophysical Research

Conflicting accounts of plasmaspheric hiss reconciled


Arcing from pole to pole, the Van Allen radiation belts are formed from a large number of protons or electrons trapped high above the Earth, the high-energy particles locked in paths dictated by the terrestrial magnetic field. Separating the outer electron Van Allen belt from the inner belt is the so-called slot region, a safe zone with reduced radiation levels. Not a necessary feature of the radiation belts, the safe zone is formed when low-frequency radio waves known as plasmaspheric hiss kick the high-energy electrons out of their trapped orbits. Understanding the safe zone depends on sorting out the hiss waves' basic properties, including their spatial and frequency distributions, and their typical direction of propagation-not an easy task, as indicated by decades of conflicting observations. Some previous researchers observed hiss waves propagating parallel to Earth's magnetic field lines, while others observed the waves flying away at oblique angles. Yet a simple simulation by Bortnik et al. (2011) seems to have reconciled these divergent detections. The model suggests that the behavior of plasmaspheric hiss varies with both latitude and altitude. At high latitudes, hiss propagates at oblique angles. Near the equator at low altitudes, the waves are either parallel or antiparallel to the field lines. However, at high altitudes above the equator the behavior bisects, with some waves following the magnetic field lines and some taking divergent paths. The model suggests that previous researchers' conflicting observations were instead detections of different behaviors in different regions. Interestingly, the authors' model used another type of wave, known as chorus waves, as the sole generating mechanism of the plasmaspheric hiss. That the model seemed to accurately reproduce various hiss properties lends weight to the connection between chorus waves and plasmaspheric hiss, a hypothesis first put forward by some of this study's authors only a few years ago.

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