The spectrum of LF/MF/HF radio noise at ground level during substorms



During the 1993–1994 Winter, simultaneous measurements were conducted in Alaska of the radio spectrum from 0.1 to 4.8 MHz and geomagnetic activity. Night-time geomagnetic disturbances, consisting of bays in the DC magnetic field and rapid variations recorded with the induction coil, were found to be correlated with prompt absorption features in the LF/MF/HF radio spectrum and with three types of natural emissions at LF/MF/HF frequencies. During February–March, 1994, 66 out of 73 magnetic signatures meeting certain criteria were associated with LF/MF/HF absorption events and 30 of the 73 were associated with natural emissions. The radio spectrum was sometimes observed to be sensitive to relatively weak magnetic activity.