Polarization measurements of unusual cases of medium frequency burst emissions extending below 1.5 MHz

Abstract Auroral medium frequency burst (MFB) is a radio emission of natural auroral origin associated with substorm expansion phase and observable at ground level. The emission usually occurs at frequencies above 1500 kHz, but occasionally it extends to a sharp lower cutoff frequency at 1300–1500 kHz depending on the observing site, with a frequency gap below the cutoff and sporadic emission below the gap extending to frequencies as low as 1000 kHz. These low-frequency MFB components lie below the electron gyrofrequency and hence could represent either whistler or LO-modes. Recently, using crossed antennas and a two-channel receiver at Toolik Lake, Alaska, polarization of these low-frequency MFB components was measured for the first time and found to be left-hand. This observation eliminates whistler mode as a possibility and requires the low-frequency components be LO-mode in the ionosphere, which constrains their source location since it requires that the frequency exceeds the L-cutoff frequency. In these occasional events marked by a cutoff and low-frequency MFB components, the latter probably originate at high altitudes ($$>\,800$$ >800 km) and reach the ground through extraordinary low-density polar cap ionosphere