Ground‐based observations of MF/HF radio noise in the auroral zone

Broadband noise enhancements in the frequency range of ∼1.4‐4.8 MHz have been observed with a ground‐based receiver located at Two Rivers, Alaska (near Fairbanks). During the 5‐month period from November 1991 to March 1992, eight broadband noise enhancements were recorded. A correlation is observed between the radio noise enhancements and magnetic activity recorded with the magnetometer in College, Alaska (45 km away). Initial examination of college ionosonde data also suggests that sporadic E is associated with the termination of some of the events. The enhancements are characterized by a low‐frequency cutoff at ∼1.4 MHz and a decrease in intensity near ∼2.8 MHz, which roughly correspond to 1 and 2 times the ionospheric electron cyclotron frequency, respectively. Although some discrete man‐made signals are observed to be enhanced during the events, the bulk of the broadband enhancements may be composed of natural signals. If natural, the observed spectral shape is consistent with calculations of synchrotron radiation combined with cyclotron absorption; in this case, these signals would be diagnostic of the hardness of the auroral electron energy spectrum.