Frequency Dependence of Ionospheric Electron Heating Around the Third Double Resonance

Abstract

The partly ionized ionosphere responds differently to high power, high frequency radio waves based on the wave frequency, the wave power, the wave polarization, and the propagation angle relative to the magnetic field. Ionospheric modification experiments are conducted to investigate the responses and their dependencies, setting experimental constraints on the contributions from non-resonant collisional interactions and resonant wave-plasma processes. The objective of the work detailed in this thesis was to examine the time and altitude variation of ionospheric electron heating around the third double resonance frequency, that is where the HF pump frequency matches the Upper Hybrid frequency as well as the third multiple of the electron gyration frequency, and determine possible hysteresis effects as the HF pump wave frequency is stepped up or down through the third double resonance. This thesis presents three new EISCAT Heating experiments where the frequency is stepped in 4.79 kHz steps up and down through the third double resonance frequency, which varied on the interval [4.151, 4.187] MHz during the three experiments. Electron temperature enhancements of up to 2000 K were achieved. The time and altitude variation of the HF heat source were estimated by assuming parameterizations for the HF heat source and solving a simplified electron energy equation for model temperatures. The parameterizations were estimated by non-linear least squares optimization of the model temperatures against the observed temperatures for each frequency step in each pulse, yielding a set of parameters per step in frequency. Due to a high level of measurement noise in the measurements we were not able to draw firm conclusions. However, the parameter estimates show indications of possible asymmetry in the HF heat source and its parameters when the HF pump frequency is stepped up and down through the third double resonance frequency. Most notable is the indication that the hysteresis effect observed by Carozzi et al. in 2002 [8] and Najmi et al. in 2014 [28] in Broad Upshifted Maxima strength might also be present in the column integrated heating rates when stepping the HF pump frequency up and down through the third double resonance frequency.