Global Studies of GPS Scintillation

During geomagnetic disturbances, intense storm-time electric fields of magnetospheric origin extend across mid-latitudes and create the sub-auroral polarization stream (SAPS). The SAPS electric fields erode the outer plasmasphere producing detached plasmas, plumes, and large plasma fluxes to the dayside magnetopause. Within the ionosphere, these intense electric fields drive plasma through the neutral atmosphere. Regions of extremely large total electron content (TEC) values, approximately 100 TEC units or more, are seen to form in the southeast portion of the United States. These regions of enhanced TEC values are the source of storm enhanced density (SED). SED is characterized by a narrow plume of greatly enhanced TEC values (>50 TEC units) and is associated with very steep electron density gradients and high ion flux values. The development of SED is clearly illustrated in the global maps of TEC generated with data from the world-wide distributed array of GPS receivers. SED is typically observed to form between 17:00 - 23:00 UT over the East Coast of US during moderate to severe geomagnetic storms. From our limited analysis of geomagnetic storm days, it appears that SED forms in a preferential longitude section (the North American sector and its conjugate location). The storm enhanced density (SED) plume, and associated SAPS electric fields and density gradients, often provide the conditions necessary in the E and F-regions of the ionosphere for the onset of different types of instabilities and the generation of a wide range of perturbations in the underlying plasma. Past studies have shown that the SED phenomenon is one of the major sources of mid-latitude scintillation. During the October 2003 storms, scintillations were clearly observed in Calgary and other sites in Canada during the time periods that correspond to presence of SED. Separate studies have shown a direct relationship between the tongue of ionization (TOI) that forms across the polar cap and the SED plume as it enters the polar region. In this international study, we are combining scintillation data from specially equipped GPS scintillation receivers located in Canada and Europe with the global TEC maps for several storm periods, including the large October and November 2003 storms and the more recent July 2004 storm. This will provide an opportunity to study the global morphology of the generation and duration of the scintillation events. The GPS TEC plots will allow us to track scintillations as they form on the edges of the TEC enhancements, first as the SED region comes up into Canada, and then as it crosses the polar cap, and finally as the enhanced TEC reaches northern Europe. We hope to demonstrate that the SED phenomenon, is not only one of the major sources of mid-latitude scintillation, but also one of the sources of polar-cap scintillation. By identifying the relationship between large TEC gradients, mid-latitude scintillations, and polar structures/scintillations, this study will allow for prediction of potential impact on satellite-based navigation systems. Examples include development of scintillation models to assess reliability of Galileo receivers, quantifying performance of the WAAS under increased ionospheric activity, and assessing potential impact on marine DGPS users.

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