Browsing by Author "Hei, Matthew A."
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Item Restricted Measurement of the characteristics of TIDs using small and regional networks of GPS receivers during the campaign of 17-30 July of 2008(Hindawi, 2012-02-17) Valladares, Cesar E.; Hei, Matthew A.This scientific report presents the results of a dedicated experiment that was conducted within the framework of the Low-latitude ionospheric Sensor Network (LISN) observatory to measure the characteristics of medium-scale (hundreds of km) Traveling Ionospheric Disturbances (TIDs) as they transit through the low-latitude ionosphere. A small array of 3 GPS receivers separated by 4-5 km placed in a triangular configuration was installed near Huancayo in Peru possessing several characteristics of a radio-interferometer. During the campaign days, 17–30 July 2008, TIDs were observed daily. On July 20, 2008 between 22 and 24 UT several TIDs moved across the small array of GPS receivers with a velocity near 130 m/s, were directed northward and had wavelengths close to 450 km. Other GPS receivers that were operating hundreds of km away from Huancayo show also similar TEC traces and provide a phase velocity equal to 150 m/s. This value was measured using the GPS at Piura, Cuzco and Huancayo. Based on this positive result, we conclude that small and/or regional arrays of GPS receivers can be used at low latitudes to study the role that gravity waves may have on seeding plasma bubbles.Item Restricted Predictions of HF system performance for propagation through disturbed ionospheres measured using low‐Earth‐orbit satellite radio beacon tomography(American Geophysical Union, 2014-07) Bernhardt, Paul A.; Hei, Matthew A.; Siefring, Carl L.; Wilkens, Matthew R.The CERTO radio beacon on the C/NOFS satellite sends VHF/UHF radio signals at 150 and 400 MHz to provide measurements of integrated electron density or Total Electron Content (TEC) by an east‐west chain of ground receivers in Peru. Computerized Ionospheric Tomography (CIT) is used to convert the TEC data into two‐dimensional images of electron densities with maximum 5 × 5 km resolution in Longitude‐Altitude space. These images are updated every 95 min as the C/NOFS satellite passes over the receiver network in its low‐latitude orbit with an inclination of 12°. The 2‐D, high‐resolution images of the ionosphere are used to predict the impact of equatorial plasma structures on HF propagation of radar and radio signals. Electron density measurements from the NRL radio tomography chain across Peru are used for simulations of the performance by HF one‐way links. HF rays from transmitter to receiver are traced through the electron density images produced by radio beacon tomography. Eight separate paths are found between a transmitter and ground receiver separated by 2000 km. A total of 36 backscatter echoes are found with unique group delay, Doppler frequency shift, phase delay, and echo amplitude. This multipath effect explains the range and Doppler spreading of observations for HF monostatic radar propagation through F layer irregularities. This type of analysis is useful for prediction and interpretation of range and Doppler observations from HF systems including over‐the‐horizon and SuperDARN radars, HF Geolocation Arrays, and HF communications networks.Item Open Access Radio-tomographic images of postmidnight equatorial plasma depletions(American Geophysical Union, 2014-01-15) Hei, Matthew A.; Bernhardt, Paul A.; Siefring, Carl L.; Wilkens, Matthew R.; Huba, Joseph D.; Krall, Jonathan F.; Valladares, Cesar E.; Heelis, Roderick A.; Hairston, Marc R.; Coley, W. Robin; Chau Chong Shing, Jorge Luis; De la Jara, CésarFor the first time, equatorial plasma depletions (EPDs) have been imaged in the longitude-altitude plane using radiotomography. High-resolution (~10 km) reconstructions of electron density were derived from total electron content (TEC) measurements provided by a receiver array in Peru. TEC data were obtained from VHF/UHF signals transmitted by the C/NOFS CERTO beacon. EPDs generated pre-midnight were observed near dawn. On one night, the bubble densities were highly reduced, 100-1000 km wide, and embedded within a layerlike ionosphere. Three nights later, the EPDs exhibited similar features, but were embedded in a locally uplifted ionosphere. The C/NOFS in-situ instruments detected a dawn depletion where the reconstruction showed lifted EPDs, implying that the postmidnight electric fields raised sections of ionosphere to altitudes where embedded/reactivated fossil-EPDs were detected as dawn depletions. Satellites flying under domelike distortions of the ionosphere may observe these distortions as Broad Plasma Decreases (BPDs).Item Restricted The November 2004 superstorm: Comparison of low-latitude TEC observations with LLIONS model results(Elsevier, 2009-04-07) Hei, Matthew A.; Valladares, Cesar E.We investigate the effects of penetration electric fields, meridional thermospheric neutral winds, and composition perturbation zones (CPZs) on the distribution of low-latitude plasma during the 7–11 November 2004 geomagnetic superstorm. The impact on low-latitude plasma was assessed using total electron content (TEC) measurements from a latitudinally distributed array of ground-based GPS receivers in South America. Jicamarca Radio Observatory incoherent scatter radar measurements of vertical E×B drift are used in combination with the Low-Latitude IONospheric Sector (LLIONS) model to examine how penetration electric fields and meridional neutral winds shape low-latitude TEC. It is found that superfountain conditions pertain between ∼1900 and 2100 UT on 9 November, creating enhanced equatorial ionization anomaly (EIA) crests at ±20° geomagnetic latitude. Large-amplitude and/or long-duration changes in the electric field were found to produce significant changes in EIA plasma density and latitudinal location, with a delay time of ∼2–2.5 h. Superfountain drifts were primarily responsible for EIA TEC levels; meridional winds were needed only to create hemispherical crest TEC asymmetries. The [O/N2] density ratio (derived from the GUVI instrument, flown on the TIMED satellite) and measurements of total atmospheric density (from the GRACE satellites), combined with TEC measurements, yield information regarding a likely CPZ that appeared on 10 November, suppressing TEC for over 16 h.