Excitation of Alfvén waves by modulated HF heating of the ionosphere, with application to FAST observations

During the operation of the EISCAT high power facility (heater) at Tromsø, Norway, on 8 October 1998, the FAST spacecraft made electric field and particle observations in the inner magnetosphere at 0.39 Earth radii above the heated ionospheric region. Measurements of the direct current electric field clearly exhibit oscillations with a frequency close to the modulated frequency of heater ( ~ 3 Hz) and an amplitude of ~ 2 - 5 mV m-1. Thermal electron data from the electrostatic analyser show the modulation at the same frequency of the downward electron fluxes. During this period the EISCAT UHF incoherent scatter radar, sited also at Tromsø, measured a significant enhancement of the electron density in E-layer up to 2 · 1012 m-3. These observations have prompted us to make quantitative estimates of the expected pulsations in the inner magnetosphere caused by the modulated HF heating of lower ionosphere. Under the conditions of the strong electron precipitation in the ionosphere, which took place during the FAST observations, the primary current caused by the perturbation of the conductivity in the heated region is closed entirely by the parallel current which leaks into the magnetosphere. In such circumstances the conditions at the ionosphere-magnetosphere boundary are most favourable for the launching of an Alfvén wave: it is launched from the node in the gradient of the scalar potential which is proportional to the parallel current. The parallel electric field of the Alfvén wave is significant in the region where the electron inertial length is of order of the transverse wavelength of the Alfvén wave or larger and may effectively accelerate superthermal electrons downward into the ionosphere.