Large-Amplitude Electromagnetic Proton Cyclotron Waves throughout the Earth’s Magnetosheath: Cassini and Wind Observations

Abstract:

A rare and unique observation of electromagnetic ion cyclotron waves has been studied for the Cassini and WIND satellites during the Cassini Earth flyby on 18^th August, 1999, across the Earth’s magnetosheath. Magnetic field data from Cassini and WIND for the time interval 0152-0226 UT are analyzed to characterize the wave modes when the satellites were present in the subsolar and dusk side magnetosheath, respectively. A new technique/program called Rosetta Automatic Wave Analysis (RAWA) has been developed based on the method initiated by Tsurutani et. al., 2013 to study the wave cycles. Various wave mode characteristics like frequency, ellipticity, propagation angle, and wave polarization are determined and are characterized statistically. Cassini and WIND wave cycle analysis suggest that almost all the waves (> 80%) were left hand circularly polarized waves with frequencies lying at or below the proton cyclotron frequency. This indicates abundance of ion (proton) cyclotron mode propagation in the Earth's magnetosheath for the aforementioned interval. No obvious mirror mode indications were found as there were no linearly polarized waves detected. The waves which were either right hand polarized or had frequencies greater than the proton cyclotron frequency were consistent with their being left hand waves with frequencies less than proton cyclotron frequency in the plasma frame. We thus conclude that the waves detected at both Cassini and WIND are electromagnetic left hand polarized proton cyclotron waves. There is no evidence of mode conversion to (plasma frame) right hand waves, even though the wave amplitudes are exceptionally large (10 nT). Majority of the waves were found to propagate parallel (<30^o) to the ambient magnetic field and were circularly polarized. However it is also found that for waves propagating at oblique angles to B₀, the polarization is still circular. This is not understood at this time. Proton cyclotron waves detected at Cassini and WIND were found to be quasi-coherent and compressive. The Earth's magnetosheath was found to have unusually low plasma beta (β=0.35) during the Cassini traversal which would have possibly favored the ion cyclotron modes over mirror modes.