The First Results of Plasma Waves Observations in the Topside Ionosphere from the Ionosphere-M1, -M2 Satellites

As a part of the Ionosonde-2025 satellite system development for space weather monitoring, LAERT topside ionosondes were installed onboard the four Ionosphere-M satellites. The devices can operate in several modes, including the passive mode, as an HF radio spectrometer in the 0.1–20 MHz frequency band, and the active sounding mode. During sounding sessions, the radio noise intensity at every frequency is measured before and after the pulse emission. It allows determination of the characteristic frequencies and excitation levels of plasma waves in the undisturbed conditions as well as in the environment, excited by the ionosonde. Thus, in addition to the sounding mode, the device operates in the relaxation sounder mode. This mode has been used on a number of foreign satellites in the ionosphere and magnetosphere (for example, the Whisper instrument on the Cluster satellites). In addition to emission at the characteristic frequencies of the surrounding plasma, the Ionosphere-M satellites recorded signals from external sources: solar radio bursts and signals from ground-based radio transmitters penetrating the ionosphere at frequencies above the ionospheric critical frequency foF2. In addition, the devices observed emissions in the whistler range (f < fhe) and upper hybrid range (fpe ≤ f ≤ fuh), generated by the beams of energetic electrons in the auroral region (fhe is gyrofrequency, fpe – the electron plasma frequency and fuh – the upper hybrid frequency). This provides an opportunity to monitor the position of the auroral oval and its dynamics. Monitoring the local plasma frequency along the satellite’s orbit provides global electron density maps at an altitude of approximately 820 km.