Physics – Plasma Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm33a2143b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM33A-2143
Physics
Plasma Physics
[2471] Ionosphere / Plasma Waves And Instabilities, [6964] Radio Science / Radio Wave Propagation, [6984] Radio Science / Waves In Plasma
Scientific paper
The auroral zone is the source of multiple kinds of radio emissions that can be observed on the ground. The study of radio emissions offers a way to remotely sense space plasma processes and, in the case of auroral emissions, to use the auroral ionosphere as a large-scale plasma physics laboratory. Medium frequency (MF) burst is an impulsive radio emission at 1.5-4.5 MHz observed on the ground. Its generation mechanism is unknown, and it is often associated with the onset of substorms. Using continuous wave measurements, Bunch and LaBelle [2009] reported that MF burst is made up of both structured and unstructured features. The most commonly observed structured feature appears as a "backwards seven" on a time-frequency spectrogram. Recently, LaBelle [2011] proposed that MF bursts originate as Langmuir waves on the topside of the ionosphere that subsequently mode-convert into electromagnetic waves that are observed on the ground. We report two experimental tests of this theory. First, the theory predicts that the maximum frequency of MF burst must lie below the maximum ionospheric plasma frequency along the source magnetic field line. We have identified eleven instances where MF bursts were observed during operations of the Sondrestrom incoherent scatter radar near Kangerlussuaq, Greenland. A preliminary analysis of these data suggests that for all or nearly all eleven cases the maximum frequency of the MF burst lies below the maximum F-region plasma frequency inferred from the radar data. The second prediction of the theory concerns the "backwards seven" fine structures. The theory predicts that the lower frequency of a "backwards seven" fine structure must lie above the L-mode cutoff along the wave propagation path. Assuming a slab ionosphere, LaBelle [2011] found that this prediction held for the six fine structures reported by Bunch and LaBelle [2009]. In 2010, continuous wave measurements were made at South Pole Station, yielding over one hundred observations of "backwards seven" fine structures. Preliminary analysis of these data indicates that nearly all of the fine structures have a lower edge above the L-mode cut-off if a slab ionospheric model is assumed where the maximum plasma frequency equals the upper frequency of the fine structure. Continuous measurements at Toolik Lake, Alaska, in 2008 yielded sixty additional observations of MF burst fine structures to supplement this study. These two experiments are consistent with the idea that MF bursts originate as Langmuir waves in the F region of the Earth's ionosphere.
Broughton M.
Bunch Nicholas L.
Labelle James W.
McCready M. A.
Roberg-Clark G. T.
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