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 ac tivity. Nighttime geomagnetic disturbances, consisting of bays in the DC magnetic field, and rapid variations recorded with the induction co il, 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 magnet ic signatures meeting certain criteria were associated with LF/MF/HF a bsorption events and 30 of the 73 were associated with natural emissio ns. The radio spectrum was sometimes observed to be sensitive to relat ively weak magnetic activity.