Computer Science – Sound
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsm31e..07c&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SM31E-07
Computer Science
Sound
2407 Auroral Ionosphere (2704), 2487 Wave Propagation (0689, 3285, 4275, 4455, 6934), 6969 Remote Sensing, 7845 Particle Acceleration, 7867 Wave/Particle Interactions (2483, 6984)
Scientific paper
Two recent sounding rockets were launched into active auroral substorms from Poker Flat, AK: CHARM on February 27, 2007 and SIERRA on January 14, 2002. Both payloads included HF wave receivers which recorded highly structured dispersed features called "stripes", intense short-lived features at 200-500 kHz, lasting 0.2-0.5 s, though the payloads employed different sensors with different effective antenna lengths and payload orientations. The frequency-time signature of these emissions is given by a distinct, repeating signature whereby the highest frequencies arrive at the detector first, followed in a linear fashion by the lower frequencies which are progressively more and more delayed as the frequency decreases. On SIERRA, the stripes coincided with Alfvenically accelerated electrons and ~100 eV downward going ions; unfortunately no particle data was recorded on CHARM due to a payload system failure. The occurrence of such structured waves in the auroral ionosphere provides an opportunity not only to confirm the wave physics but also to exploit the structured waves to indirectly measure characteristics of the local plasma and to remotely sense plasmas through which the waves have propagated, compelling us to look further into possible mechanisms for their generation. One possibility stems from the fact that the frequency at which waves meet the electron cyclotron resonant condition depends on the electron plasma and electron cyclotron frequencies, which depend on altitude. Specifically, consider the conditions for a wave to cyclotron resonate with an upward going electron beam in the topside: for wave frequencies less than the cyclotron frequency, an upward beam of fixed energy will cyclotron resonate with successively lower frequency waves as it ascends. The different frequency waves would then arrive at the rocket at different times, with the time delay dependent on the speed, and therefore the energy, of the electron. There may be one particular electron beam energy for which the delay times for the various frequency waves exactly matches that of the observed stripe features. Using a variety of possible density profiles, we investigate the frequency time profile resulting from a set of electron cyclotron resonant waves generated at different altitudes and frequencies for a specific fixed electron energy. We find that for certain electron energies and density profiles consistent with the observed energies of the downgoing ions and the typical topside density, the shape of the observed stripe features can be recreated with this method. This suggests that an accelerating mechanism above the rocket may generate the downgoing ions and an unseen upward electron beam that generates the stripe emissions. If this mechanism is responsible for the generation of the stripe features, it would be possible to use observations of stripes to remotely sense the plasma density at high altitudes. We further investigate the reconstruction of a remote plasma density profile using the frequency-time profile of stripe features. The observations of these waves and the investigation of their possible generation mechanisms is therefore important to help us understand the wave-particle physics in the auroral ionosphere and also for remote sensing of the plasma characteristics elsewhere.
Colpitts C. A.
LaBelle James
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