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Propagation analysis of plasmaspheric hiss using Polar PWI measurements

We have analyzed high‐rate waveform data, taken by the POLAR Plasma Wave Instrument at high altitudes in the equatorial plasmasphere, to study plasmaspheric hiss in the range of frequencies between 100 Hz and several kHz. These emissions are found almost everywhere in the plasmasphere, and their origin is still controversial. Our analysis of several cases shows that most of the waves were propagating more or less parallel to the Earth's magnetic field, but sometimes a few of them were propagating obliquely with their normals near the Gendrin angle. Evidence of amplification was found near the geomagnetic equator. The results suggest that waves with normals both parallel and anti‐parallel to the magnetic field were being amplified by the classical mechanism that involves gyroresonant interaction with energetic electrons.