CAPER-2 and TRICE-2 Sounding Rocket Investigations: Whistler and Upper-Hybrid Waves in the Polar Cusp

The CAPER-2 and TRICE-2 sounding rockets launched 4 January 2019 and 8 December 2018, respectively, into the cusp of earth's magnetic field as part of the Grand Challenge Initiative to study cusp dynamics. Both of these missions carried receivers to examine auroral radio emissions ranging from a few hertz to several megahertz allowing for fine structures within the spectra to be observed. For instance, on field lines from 300-1300 km equator-ward of the estimated cusp boundary, CAPER-2 VLF receivers observed saucer-like features in its time-frequency spectra. Although superficially similar to those seen by James [1976], these features extend over a far greater spatial extent implying a much more distant source, perhaps > 10,000 km above the space- craft. Because the plasma and the magnetic field are not uniform over such a large altitude range, ray tracing will be required to interpret these features, and determine whether or not they originate in the cusp. HF receivers aboard TRICE-2 observed several intervals of intense non-thermal upper-hybrid waves with banded-structures, having 4-12 bands with spacings and bandwidths of about 5 kHz. Two possible explanations for these features are: 1) modulation from wave-wave interactions of upper-hybrid waves with lower-hybrid/whistler waves, or 2) the HF antennas have a separation equal to an integral number of the observed wavelength resulting in nulls within the spectra. Data from the UCB VLF receivers shows lower-hybrid cutoff near 5 kHz, suggesting the presence of a wave-wave interaction. Consideration of rocket probe attitude is required to test these and other mechanisms. Both CAPER-2 and TRICE-2 observed a host of other wave and particle phenomena in the cusp that need to be investigated further to enhance our understanding of the auroral ionosphere and magnetospheric-ionospheric interactions.