Interplanetary ions during an energetic storm particle event: The distribution function from solar wind thermal energies to 1.6 MeV

Abstract

Data from the Los Alamos Scientific Laboratory/Max‐Planck‐Institut fast plasma experiment on Isee 2 have been combined with data from the European Space Agency/Imperial College/Space Research Laboratory low‐energy proton experiment on Isee 3 to obtain for the first time an ion velocity distribution function f(ν) extending from solar wind energies (∼1 keV) to 1.6 MeV during the postshock phase of an energetic storm particle (ESP) event. This study reveals that f(ν) of the ESP population is roughly isotropic in the solar wind frame from solar wind thermal energies out to 1.6 MeV. Emerging smoothly out of the solar wind thermal distribution, the ESP f(ν) initially falls with increasing energy as E−2.4 in the solar wind frame. Above about 40 keV no single power law exponent adequately describes the energy dependence of f(ν) in the solar wind frame. Above ∼200 keV in both the spacecraft frame and the solar wind frame, f(ν) can be described by an exponential in speed (f(ν) ∝ e−ν/ν0) with ν0=1.05 × 108 cm s−1. The ESP event studied (August 27, 1978) was superposed upon a more energetic particle event which was predominantly field‐aligned and which was probably of solar origin. Our observations suggest that the ESP population is accelerated directly out of the solar wind thermal population or its quiescent suprathermal tail by a stochastic process associated with the shock wave disturbance. The acceleration mechanism is sufficiently efficient that ∼1% of the solar wind population is accelerated to suprathermal energies. These suprathermal particles have an energy density of ∼290 eV cm−3.