Sensing of ELF signatures arising from space vehicle disturbances of the ionosphere

Observations of Extremely Low Frequency (ELF) signatures during exit or reentry of space vehicles through the ionosphere are presented. The two modes regularly observed gave signals that peaked at 5.6 Hz and 11.2 Hz. The evidence points to the lower ionosphere, i.e., the D- and E-layers, as the generator of the signals. The measurements were performed using ground-based multi-turn coil sensors located in Reno and San Diego. The nature of these signals is unclear at present but it is surmised that we are detecting either the evanescent fields of hydromagnetic waves traveling in the ionosphere or the oscillating geomagnetic field associated with these hydromagnetic waves.