Effect of parallel refraction on magnetospheric upper hybrid waves

Large amplitude (⩾10 mV/m) electrostatic plasma waves near the upper hybrid (UH) frequency have been observed from 0° to 50° magnetic latitude (MLAT) during satellite plasma‐pause crossings. A three‐dimensional numerical ray tracing calculation, based on an electron distribution measured during a GEOS 1 dayside intense upper hybrid wave event, suggests how UH waves might achieve such large amplitudes away from the geomagnetic equator. Refractive effects largely control the wave amplification and, in particular, the unavoidable refraction due to parallel geomagnetic field gradients restricts growth to levels below those observed. However, a cold electron density gradient parallel to the field can lead to upper hybrid wave growth that can account for the observed emission levels.