Ground based studies of auroral medium frequency burst radio emissions

MF burst is an impulsive auroral radio emission at ∼1.3-4.5 MHz commonly detected by ground-based instruments for a few minutes at substorm onsets. The Dartmouth College MF radio interferometer at Toolik Field Station, Alaska (68.51° invariant latitude) measured spectra, amplitudes, and directions of arrival (DOA) of naturally occurring auroral radio waves between between 0.1 and 5 MHz beginning in the fall of 2006. DOA case studies reveal temporal and spatial coincidence with expanding auroral arcs for substorms observed on 23 March and 20 November 2007. DOA statistical studies show that MF bursts are observed predominantly from the south and east of Toolik, which can be attributed to propagation effects. Statistical and case studies of DOA data of MF burst show that higher-frequency components of MF burst arrive at higher elevation angles than lower-frequency components. Ray-tracing analysis shows that this trend implies that sources of the higher-frequency components of the MF burst are at higher altitudes than those of the lower-frequency components. Full resolution observations show that MF burst consists of a superposition of "structured" (showing sharp time-frequency edges/features), and "unstructured" (more diffuse) features. Structured features show a repeatable frequency-time structure in which a leading edge decreases, from a well-defined upper bound, in frequency of 100s of kHz over ∼100ms. Despite several suggested theories, the exact generation mechanism for MF burst still remains a mystery. The foremost theory suggests MF burst originates from mode conversion of Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside F-region. However, analysis of plasmas with two electron components implies existence of additional normal modes, called the electron acoustic (EA) and electron cyclotron sound (ECS) modes, which have previously been suggested to play a role in MF burst. Further analytical calculations, using WHAMP and other dispersion solving codes, suggest that instability of these modes in the presence of an auroral electron distribution is possible. The same analysis suggests that LO-cannot be directly excited. The above recent experimental and analytical studies combined with a discussion on viable normal wave modes available in the auroral ionosphere allow us to constrain potential modes of generation for MF burst.