Cold Electron Temperature in the Inner Magnetosphere Estimated Through the Dispersion Relation of ECH Waves From the Arase Satellite Observations

We have analyzed Electrostatic Electron Cyclotron Harmonic (ECH) waves observed using interferometry observation mode performed by the Arase satellite to estimate low‐energy electron temperatures. Interferometry can be used to calculate velocities, but the Arase satellite can only perform interferometry observations in a one‐dimensional direction. We proposed a method to estimate the wave vector of the observed ECH waves from the observed electric fields and calculated the phase velocity for each frequency. We determined the particle parameters from the particle detector and the upper hybrid resonance and estimated the unknown low‐energy electron temperature from the agreement between the observed ECH dispersion relation and the theoretical dispersion curves. We performed our analysis for six events and found that the low‐energy electron temperature in the observed region is on the order of 1 eV. Plain Language Summary Low‐energy electrons, which are dominant in the Earth's inner magnetosphere, are difficult to observe directly with particle instruments due to satellite charging. We have successfully estimated the plasma environment by deriving the frequency‐wavenumber relations of Electrostatic Electron Cyclotron Harmonic (ECH) waves in the inner magnetosphere and comparing them with the theoretical relations. To derive the frequency‐wavenumber relationship of the ECH waves, it is necessary to obtain the phase velocity of the ECH waves by satellite observations. We proposed a method to determine the unobserved components of the ECH wave using interferometry observations by the Arase satellite and calculated the phase velocity using this method. Thus, the obtained low‐energy electron temperatures were all 1 eV, indicating that there are many low‐energy electrons of the order of 1 eV in the inner magnetosphere. Key Points Phase velocities of electron cyclotron harmonic (ECH) waves were calculated using interferometry observations from the Arase satellite The low‐energy electron temperature was estimated based on the dispersion relation of the ECH wave It is found that low‐energy electrons of the order of 1 eV account for the majority of the electron density distribution