In Sutu Ion Density and Composition Measurements with the Radio Receiver Instrument on e-POP Using Excitation of Plasma Waves by Photo Electrons

Knowledge of the electron density in the topside ionosphere is essential for interpretation of HF radio signals recorded by satellite receivers. A new technique has been developed to record the spectrum of plasma waves excited by daytime photo electrons for determination of the local plasma density and ion composition. Incoherent scatter radar, rocket and satellite measurements have shown that the 10 to 60 eV photo electron spectrum is found between 100 and 600 km altitude during the daytime. The e-POP satellite with the wide-band radio receiver instrument (RRI) that should be able detect photo electron excited plasma waves in situ. Each of these waves have a resonant frequency that is a function of the local properties of the plasma such as electron and ion density, electron and ion mass, electron and ion temperature and background magnetic field.

For the e-POP measurement altitudes of 320 to 1500 km altitude, the plasma is mainly composed of electrons, and two ions (e.g. protons and O⁺). With this composition, three waves have been selected for radio spectrum observations with the RRI sensor flying through the F-Layer during the sunlit hours. For the first plasma wave, the wave spectra of the electron-plasma (Langmuir) will be examined as previously seen by in situ and radar techniques. The second wave is the lower-hybrid mode with a resonance that involves the ion-ion hybrid gyro frequency using density weights of the H⁺ and O⁺ ion gyro frequencies, and the Pythagorean sum of the electron plasma frequency and the electron gyro frequency. The third wave is the low frequency two-ion hybrid wave with resonance given by the density and mass-weighted geometric mean of the individual ion cyclotron frequencies; it may be detected by the RRI at low frequency range of the instrument. The in situ electron density may be directly inferred from the electron-plasma wave. The relative two-ion plasma composition can be directly determined with ion-ion hybrid frequencies. The lower-hybrid resonance in the plasma can be used to tie together the electron density through plasma frequency and the relative ion composition. This technique has been tested with daytime operations of the RRI showing strong emissions from the plasma. The measured electron density and ion composition is in good agreement with the SAMI-2 physics based model of the F-layer.