BANDED ION MORPHOLOGY (SPACECRAFT CHARGING)
FRAHM, RUDY ALLAN
Doctor of Philosophy thesis
Bands of ions have been observed at constant pitch angle by the Dynamics Explorer High and Low Altitude Plasma Instruments at auroral latitudes. The observed ion dispersion pattern shows lower energy ions toward the equatorward side of the band and higher energy ions toward the poleward side of the band. Ion bands have their highest energy flux at small pitch angles. The observed bands have been correlated with storm phase (by Dst) and substorm phase (by AE). Bands are more likely to occur during main storm phase than during recovery storm phase. Substorm correlations are statistically significant, but there is a hint that most bands occur during substorm recovery phase. Two models have the potential of producing ion signatures that are similar to the band feature. They are the time-of-flight mechanism and the convective dispersion mechanism. Under a time-of-flight mechanism, ions are dispersed along a magnetic field line with higher energy particles outrunning lower energy particles. Ions are dispersed perpendicular to the magnetic field under convective dispersion. A time-of-flight effect does not explain the band energy-latitude dependence observed in the southern night or northern day very well, whereas the convective dispersion mechanism easily accomplishes this. Simulations have been performed on ions under the influence of the time-of-flight and convective dispersion mechanisms. The time-of-flight simulation determines the distance to the ion source. The requirement on its position is that two different energy ions, which are detected at different times, must have been released from the source simultaneously. A composite of the computed source distances shows that source motion had to be unphysical in order to create the signature of a sample band. Simulating convective dispersion required knowledge of ion drifts. Using the measured drifts on a satellite pass which showed a band, ions were traced from one hemisphere to the other. A composite of arrival latitudes from an inverted "V" simulated source in the opposite hemisphere, nearly approximates the banded ion pattern. Thus, we can conclude that convective dispersion is the correct mechanism for formation of ion bands.
Physics, Astronomy and Astrophysics