Robert Sheehan

Dynamics of equatorial F region irregularities from spaced receiver scintillation observations

Spaced receiver observations of amplitude scintillations on a 244 MHz signal, at an equatorial station, have been used to study random temporal changes associated with the scintillation-producing irregularities and the variability of their motion. The computed drift of the scintillation pattern shows the presence of velocity structures associated with equatorial bubbles in the early phase of their development. On magnetically quiet days, after 22:00 LT, the estimated drifts fall into a pattern which is close to that of the ambient plasma drift. There is considerable decorrelation between the two signals until 22:00 LT. The power spectra of the most highly correlated scintillations recorded by spaced receivers indicate that the associated irregularities are confined to a thin layer on the bottomside of the equatorial F region. This suggests that the convection pattern associated with bottomside irregularities is stable due to the dominance of ion-neutral collisions over ion inertia.

GPS observations of mid-latitude TEC depletions

We have used measurements of TEC values gathered by several networks of GPS receivers, which operated in Central America and the Caribbean region between 2010 and 2013 to investigate the morphology of TEC depletions that develop at magnetic latitudes higher than 20°. We found that these TEC depletions occur mainly during magnetic active conditions, persist for very long periods and are seen even during afternoon hours especially when the level of magnetic activity is high (Kp > 5o). The large spatial coverage provided by the GPS receivers in these regions (>150 receivers) allows us to identify their origin and in some cases track their paths. We also employ TEC values from South America to study possible links with the formation of TEC depletions at low latitudes and TEC measurements from North America to determine their latitudinal extensions. The mid-latitude depletions do not appear to be related to auroral plasma processes or to storm enhanced densities (SED). We are studying the possibility that their initiation process is associated with the disturbance dynamo or prompt penetrating electric fields that develop during storm conditions.

Correlations of Auroral Kilometric Radiation with Visual Auroras and with Birkeland Currents

The first satellite measurements of intense radio emissions from the earth’s magnetosphere were made seventeen years ago by Benediktov et al. (1965, 1968). They observed 725 kHz and 2.3 MHz radio emissions with the Elektron satellite receivers during geomagnetic storms. Five years later Dunkel et al. (1970) reported OGO 1 observations of strong radio wave intensities at frequencies below 100 kHz also related to magnetic disturbances. Brown (1973), using the Goddard Space Flight Center (GSFC) radio astronomy experiment on the IMP 6 satellite, observed earth related radio emissions in the frequency range 150–300 kHz. It was not, however, until the University of Iowa plasma wave experiments, on the IMP 6 and IMP 8 satellites, had measured intense electromagnetic radiation (in the frequency range of 50–500 kHz) that the first complete observational study of terrestrial kilometric radiation was produced. Gurnett (1974) was able to present a comprehensive documentation of the location of the radio source near the earth because his instruments afforded him directional resolution, dynamic range, high sensitivity, and broad frequency range. Other authors contributing to the completion of the picture of radio emissions from the earth’s magnetosphere were Stone (1973) and Frankel (1973).