Lower hybrid waves excited through linear mode coupling and the heating of ions in the auroral and subauroral magnetosphere

Abstract

Experimental observations on the ISIS 1, ISIS 2, ISEE 1, and DE 1 spacecraft demonstrate that strong lower hybrid (LH) waves can be excited by VLF electromagnetic (em) whistler mode waves as the em waves propagate through regions of the ionosphere and magnetosphere where small‐scale magnetic‐field‐aligned irregularities exist in the mean plasma density. There is strong evidence that the LH waves are excited by linear mode coupling as the em waves scatter from the irregularities. The present paper considers two related aspects of the linear mode conversion mechanism: (1) the controlled heating of suprathermal ions in the ionosphere and magnetosphere over a powerful VLF/ELF transmitter using LH waves excited by the em transmitter signals through linear mode conversion; (2) the excitation of intense LH waves in the auroral regions through the linear conversion of VLF/ELF em auroral hiss, and the subsequent heating of ions by the excited LH waves. In addressing both aspects a critical feature is the behavior of the mode coupling mechanism at frequencies less than 10.2 kHz, the lowest value observed in earlier experiments. Using the results of controlled experiments carried out at Siple Station, Antarctica, it is demonstrated that strong LH waves can be excited by em waves down to frequencies as low as 2 kHz in the subauroral low‐altitude magnetosphere. Extrapolating from these observations and making use of ISEE 1 satellite wave amplitude data, we conclude that a ∼250 kW VLF/ELF transmitter operating in the subauroral region at 2 kHz could excite sufficiently intense LH waves to heat suprathermal 1 eV H+ ions and 16 eV O+ ions to roughly 50 eV in a region extending in altitude from 1000 to 5000 km above the transmitter with a horizontal scale of ∼500 km. Furthermore, if coherent wave stochastic heating occurs, the energy gain of O+ ions could be as large as 200 eV. This effect would be readily measurable with available satellite instrumentation. Using a recently developed model of the linear mode coupling mechanism, we furthermore conclude that a significant portion of the LH waves observed in the auroral zone in regions of ion conic development may be excited by em VLF/ELF auroral hiss as the hiss propagates through the irregular background plasma commonly observed in the auroral regions.