Sounder‐accelerated particles observed on ISIS

The soft‐particle spectrometers aboard the spacecrafts ISIS I and ISIS II detect sounder‐accelerated particles, i.e., fluxes of electrons and ions energized by the 100‐μs transmitter pulse (nominal peak power: 400 W). Fluxes of up to 108 st−1 eV−1 cm−2 s−1 are observed. Typical highest electron and ion energies are several hundred electronvolts and 100–200 eV, respectively. Sounder‐accelerated electron fluxes detected on ISIS II are energized near the major electron resonant frequencies: ƒpe (plasma frequency), ƒce (gyrofrequency), 2ƒce and the oblique resonance frequency domains. Ion fluxes are present from the lowest sounder frequency (0.1 MHz) up to the greater of ƒpe and ƒce. Electrons are observed at pitch angles near 90° while ions are present at all pitch angles. The observations can be interpreted using a model of particle motion and spacecraft dc potential both induced by the intense rf field (∼100 V/m). The ion results indicate that at ƒ < ƒpe, ƒce, a negative potential of about 100 V is on the spacecraft, whereas at ƒ > ƒpe, ƒce, the potential is much smaller. ISIS I data from equatorial perigee conditions show that electrons remain energized for a few milliseconds after the end of the rf pulse.