Showing papers by "Cesar E. Valladares published in 2015"

Ionosonde observations of ionospheric disturbances due to the 15 February 2013 Chelyabinsk meteor explosion

Abstract: We report the results of our investigations on ionospheric effects potentially caused by the 15 February 2013 Chelyabinsk meteor explosion. We used the observation data from a number of digisonde s...

An effective TEC data detrending method for the study of equatorial plasma bubbles and traveling ionospheric disturbances

Abstract: Using a mechanical analogy of rolling a cylindrical barrel on a rough uneven surface, we developed a special method for detrending the GPS-derived total electron content (TEC) data. This method is specifically designed to recognize the presence of depletions in the TEC time series data and handle them differently from wavelike features. We also demonstrate a potential application of this technique to map the detailed geographic profile of TEC depletions over the equatorial region, using the South American sector as an example.

Polar cap patches observed during the magnetic storm of November 2003: observations and modeling

Abstract: . We present multi-instrumented measurements and multi-technique analysis of polar cap patches observed early during the recovery phase of the major magnetic storm of 20 November 2003 to investigate the origin of the polar cap patches. During this event, the Qaanaaq imager observed elongated polar cap patches, some of which containing variable brightness; the Qaanaaq digisonde detected abrupt NmF2 fluctuations; the Sondrestrom incoherent scatter radar (ISR) measured patches placed close to but poleward of the auroral oval–polar cap boundary; and the DMSP-F13 satellite intersected topside density enhancements, corroborating the presence of the patches seen by the imager, the digisonde, and the Sondrestrom ISR. A 2-D cross-correlation analysis was applied to series of two consecutive red-line images, indicating that the magnitude and direction of the patch velocities were in good agreement with the SuperDARN convection patterns. We applied a back-tracing analysis to the patch locations and found that most of the patches seen between 20:41 and 21:29 UT were likely transiting the throat region near 19:41 UT. Inspection of the SuperDARN velocities at this time indicates spatial and temporal collocation of a gap region between patches and large (1.7 km s−1) line-of-sight velocities. The variable airglow brightness of the patches observed between 20:33 and 20:43 UT was investigated using the numerical Global Theoretical Ionospheric Model (GTIM) driven by the SuperDARN convection patterns and a variable upward/downward neutral wind. Our numerical results indicate that variations in the airglow intensity up to 265 R can be produced by a constant 70 m s−1 downward vertical wind.

Forecasting the Appearance and Evolution of Ionospheric Irregularities and Structures: Their Effects on AF Systems

Abstract: : Work performed under this contract includes scientific and technological research to provide an understanding of the origin an devolution of small to large-scale structures that develop in the ionosphere at polar and equatorial latitudes. These structures and irregularities produce detrimental disturbances on Air Force communication, navigation and surveillance systems. Systems operating at polar latitudes experience disruptions due to the high variability of patch activity that depends on universal time, season, magnetic conditions, solar cycle, and hemisphere. At low latitudes, unwanted effects on systems are even more pronounced as Global Positioning System (GPS) receivers experience loss of signal lock when the ray path traverses an ionospheric plasma bubble. The GPS systems also experience substantial errors in the position on the order of tens of meters due to ionospheric density variability. Our studies have spanned the ascending phase of solar cycle 24, a phase characterized by: (1) an increase of the worldwide background density that degrades the accuracy of navigation systems; and (2) a strengthening of the intensity of scintillations that produces communications outages. The experimental, modeling and assimilation studies described in this report have improved our overall understanding of the high and low-latitude ionospheric processes that may ultimately lead to more reliable and complete forecasts. This contract also supported the operations, maintenance, installation and logistic support of ionospheric instruments that have been deployed around the world in support of the SCINDA and LISN Networks.

Observations of Conjugate MSTIDs using Networks of GPS Receivers in the American Sector

Abstract: This study has used total electron content (TEC) values from an extended network of GPS receivers and a highly developed processing to characterize the conjugacy of medium-scale traveling ionospheric disturbances (MSTIDs) over the American continent. It was found that midlatitude nighttime MSTIDs, also named electrobuoyancy waves, map into the opposite hemisphere but the amplitude of the TEC disturbance in the Southern Hemisphere is between 8 and 13% of the amplitude in the original hemisphere. The periods of the MSTIDs vary between 50 and 65 min. MSTID dynamics is presented for two days: 20 August 2012 and 17 June 2012. On the first day, MSTIDs entered into the American sector shortly before 4 UT, last for 3 h, drifted at an average speed of 200 m/s, and dissipated in the Caribbean region. In the Northern Hemisphere, the MSTIDs were directed southwestward (SW) and 60° from south. In the Southern Hemisphere, they moved northwestward (NW) or ~60° from north. The MSTID velocity changed through the night from ~300 m/s to ~150 m/s, but the propagation direction did not vary. On 17 June 2012 a series of wide MSTIDs were seen traveling across the Caribbean region that exited through the western coast of Central America. These MSTIDs last for ~5 h. Number density measured with the DMSP-F15 and DMSP-F17 satellites confirm the notion that the MSTIDs consist of rising and falling sheets of plasma density driven by electric fields likely set by a Perkins-type instability. These observations support the notion that gravity waves can seed and boost the growth of the nighttime MSTIDs.