CONTROL OF VLF BURST ACTIVITY IN THE NIGHTSIDE IONOSPHERE OF VENUS BY THE MAGNETIC-FIELD ORIENTATION

In order to determine whether VLF bursts observed in the nightside ionosphere on Venus are better interpreted as whistler mode waves or ion acoustic waves we have performed both case studies and statistical studies of the occurrence of these signals. The correlation between burst occurrence and the angle between the magnetic field and the radial direction, as well as the spacecraft flight direction are examined. We find that narrow-band 100 Hz bursts are more frequently associated with radial magnetic fields while wideband signals are more frequently associated with horizontal fields. Although most 100 Hz bursts appear when the field is perpendicular to the flight direction, there is no obvious dependence on the field orientation with respect to the velocity for the wideband signals. Furthermore, narrow-band 100 Hz bursts occur more often at all altitudes when the expected propagation direction is inside the whistler mode resonance cone, assuming vertical propagation. Finally, the 100 Hz burst rate inside the resonance cone decreases slowly with increasing altitude, while the burst rate outside the resonance cone and at higher frequencies decreases with a scale height of about 20 km. The dependence on the magnetic field orientation is consistent with propagation of the narrow-band signals in the whistler mode from an atmospheric source, such as lightning. The wideband source may also be lightning, but the mode of entry of these signals into the ionosphere remains undetermined.