Abstract
During the Twin Rockets to Investigate Cusp Electrodynamics (TRICE-2)
High-Flyer rocket’s passage through the cusp the high frequency (HF)
radio wave receiver observed three intervals of banded Upper-Hybrid (UH)
waves. The bands begin at the UH frequency
($\sim$1.2–1.3 MHz), descending to as low as 1.1 MHz,
with amplitudes of hundreds of mV/m. The spacing of the bands are
$\sim$4.5–6 kHz and the number of bands ranges from
three to ten. Simultaneously, the very low frequency (VLF) radio wave
receiver observed Lower-Hybrid (LH) waves with amplitudes ranging from
1–10 mV/m and frequencies of 4.5-6 kHz. Slight variations of the
spacings of the bands in the UH waves were closely correlated with
variations in the LH peak frequencies. Two possible wave-wave
interactions are explored to explain this phenomenon: decay of an UH
wave into a lower frequency UH wave and a LH wave, and coalescence of
independent UH waves and LH waves that spawn UH waves. Using a
dispersion relation calculator with electron and ion distribution
functions based off those observed by the particle instruments suggests
that UH waves, and to a lesser degree LH waves, can be excited by linear
instabilities. Kinematic analysis of the waves dispersion relations and
the wave matching conditions show that wave-wave interactions linking UH
and LH modes are possible through either decay or coalescence. This
analysis along with comparisons of the energy densities of the waves,
and the ratio of their occupation numbers suggest that the decay process
is more likely than coalescence.