Unfolding spectral patterns induced by artificial weakly relativistic beam in the ionosphere

Very diverse set of HF spectra was induced by artificial electron beam injections from the APEX satellite. The spectra were registered in unplanned absence of Xe+ plasma jet, aimed to protect spacecraft against overcharging. Acceleration voltage of ~10 kV, current modulation frequency ranging from d.c. to 250 kHz, low divergence of ~4 deg, slow variations of pitch angle and maximum instantaneous intensity of ~0.15 A characterize electron beam. In these time intervals a radiospectrometer operated in a survey mode providing one spectrum every 2 s or 8 s. The single spectrum was measured in Is with an equally spaced mesh of 200 frequencies starting from 100 kHz with a step of 50 kHz. The receiver with a bandwidth of 15 kHz was connected to a tubular dipole antenna having half length of 7.5 m. Total number of ~400 spectra is not impressive but they are characterized by reproducibility of spectral patterns. From reproducibility or slow evolution of the spectra, it may be inferred that distinct interactions prevail for some ranges of ambient electron gyro (fc) and plasma (fn) frequencies, injection pitch angles and beam intensities. It appears that discrete emission can be identified at least on ambient plasma frequency or ambient upper hybrid frequency (fu). One class of arguments supporting such identifications is provided by interrelation between spectral signatures of local plasma density in passive mode and beam induced spectra. Another class of arguments is provided by interrelations between spectral structures induced by electron beam. For large angles of e-beam injection, electromagnetic character of gyroharmonics radiation from weakly relativistic beam and emissions at harmonics of upper hybrid frequency were reported. In this presentation we extend investigation to pitch angle dependence of electron beam induced spectra. Few examples of spectra induced by very weak beam and by strong beam draw attention to relevance of its intensity. We estimate relevance of pro- - pagation for spectral bandwidth with ray tracing of inferred emission modes. The diversity and reproducibility of spectral patterns support viability of simulation of astrophysical plasma in controlled electron beam-space plasma experiments. Narrow single mode emission induced in weak beam regime can be used for monitoring of ambient plasma density.