Formation of the high-energy ion population in the earth's magnetotail: spacecraft observations and theoretical models

Abstract. We investigate the formation of the high-energy (E ∈ [20,600] keV) ion population in the earth's magnetotail. We collect statistics of 4 years of Interball / Tail observations (1995–1998) in the vicinity of the neutral plane in the magnetotail region (X <-17 R_E, |Y| ≤ 20 R_E in geocentric solar magnetospheric (GSM) system). We study the dependence of high-energy ion spectra on the thermal-plasma parameters (the temperature T_i and the amplitude of bulk velocity v_i) and on the magnetic-field component B_z. The ion population in the energy range E ∈ [20,600] keV can be separated in the thermal core and the power-law tail with the slope (index) ~ −4.5. Fluxes of the high-energy ion population increase with the growth of B_z, v_i and especially T_i, but spectrum index seems to be independent on these parameters. We have suggested that the high-energy ion population is generated by small scale transient processes, rather than by the global reconfiguration of the magnetotail. We have proposed the relatively simple and general model of ion acceleration by transient bursts of the electric field. This model describes the power-law energy spectra and predicts typical energies of accelerated ions.