Clayton Coker

Detection of auroral activity using GPS satellites

GPS (Global Positioning System) satellites and a receiver located at Fairbanks, Alaska are used to detect auroral activity. A technique for using GPS total electron content (TEC) data to detect auroral-E ionization (AEI) at all satellite line-of-sight elevations is presented. The location of AEI during auroral substorms is determined and is consistent with simultaneous magnetometer data. Maps of detected AEI events reveal the distribution of AEI in space and time. Additionally, a technique is presented for identifying the effects of the auroral oval E-layer on the TEC data. Particle precipitation measured by the TIROS satellite is closely related to variations in the TEC data. The effects of the oval are consistently seen in the TEC data for a variety of magnetic conditions. The location of the equatorward edge of the oval is determined during auroral substorms and compares well with a model of the oval and with individual TIROS passes.

IRI data ingestion and ionospheric tomography

Abstract We present a method by which we combine IRI-95 predictions of electron density with ionospheric tomography data to provide an improved electron density estimate. We discuss the observation that IRI-95 produced ionospheres have a topside description which is too thick when compared to CIT reconstructions. A technique for ionospheric data ingestion is discussed. The algorithm is capable of ingesting GPS, CIT, ionosonde, and ISR data. The method is extensible to other types of data as long as a characterization of the errors can be obtained. We also discuss the study of latitudinal and longitudinal correlation in the ionosphere. Results of this correlation are shown for mid-latitude ionospheres over the Western US.

Verification of ionospheric sensors

Ionospheric products from sensors and models were compared to investigate strengths and limitations of each. Total electron content data from computerized ionospheric tomography (CIT) and TOPEX sensors in the Caribbean region in 1997 were compared to estimates produced by models Parameterized Ionospheric Model (PIM) and Raytrace/ICED-Bent-Gallagher (RIBG) and global maps from GPS. A 5 total electron content unit (TECU) bias was observed in TOPEX. CIT and TOPEX confirmed the location and structure of the equatorial anomaly. A GPS map confirmed the location of the anomaly but did not reproduce structure less than 1000 km in latitude and 1500 km in longitude and underestimated TEC by at least 11 TECU or 25%. PIM positioned the anomaly 13° equatorward of its observed location and greatly underestimated (∼50%) the rise in content over 5°-25°N range. RIBG overestimated the latitudinal extent of the anomaly and underestimated TEC at the peak by 40%. Additional comparisons were made using CIT and ionosonde sensors at midlatitude during the summer of 1998. Fourteen days of TEC, hmF2, NmF2, and half-thickness comparisons showed reasonable agreement between CIT and ionosonde for TEC and NmF2. The hmF2 and half-thickness comparisons were contaminated by noise, which accounted for a significant portion of the ionospheric variation. Daytime cases where CIT overestimated maximum density were attributed to underestimating layer thickness. Finally, TOPEX and multiple GPS sensors were compared to verify regional ionospheric conditions associated with occurrence of nighttime ionospheric depletions in the Caribbean during Combined Ionospheric Campaigns in June of 1998. From 0300 to 0800 UT on June 26, GPS and TOPEX showed elevated nighttime content over the entire Caribbean region. Vertical TEC approached 25 TECU in some places with interspersed depletions, which in some cases evacuated nearly the entire ionospheric content.

Passive detection of sporadic E using GPS phase measurements

A passive sporadic E detection technique based on a Global Positioning System (GPS) receiving system has been developed and tested in a midlatitude environment. This system detects the small-scale total electron content (TEC) variations believed to be produced by electron density structures associated with sporadic E. The current GPS detection technique was able to detect ionosonde-detected sporadic E conditions for 73% of the cases at high-elevation look angles in a set of midlatitude summer observations. Several approaches have been identified that may significantly improve this detection ratio. These approaches include reducing GPS phase multipath, implementing time and space averaging, and investigating the use of high-speed GPS TEC measurements. This technique provides a basic sporadic E detection functionality for applications where an ionosonde is not available. It also provides complementary ionospheric information in regions outside the ionosonde viewing area for applications where an ionosonde is available.

An investigation of the feasibility of utilizing GPS/TEC "signatures" for near-real time forecasting of auroral-E propagation at high-HF and low-VHF frequencies

VHF propagation on /spl les/5300 km polar paths has been documented during the maximum phase of sunspot cycle 19. Mode analysis on these polar paths has shown that auroral-E ionization (AEI) supported some modes. Electron densities and plasma frequencies which could support AEI modes at frequencies up to 46 MHz have also been measured. Long distance VHF propagation from AEI has also been reported by radio amateurs using frequencies in the 2 m band in a sidescatter mode. An AEI experiment has been in operation between Wales (Alaska) and Fairbanks (Alaska) where a 75-watt CW transmitter located in Wales transmits the Morse letter R every 5 s, and a receiver in Fairbanks detects the 25.5 MHz signal whenever AEI is present near the midpoint of the 960 km path. Another experiment is underway using a GPS total electron content (TEC) receiving station at Fairbanks also using AEI data from the Wales-Fairbanks experiment. From this, the authors examine 58 passes of GPS satellites whose E-layer penetration points lie close to the midpoint of the Wales-Fairbanks path and find that there is a threshold value of TEC above which auroral-E (AE) propagation occurs. They also find that AEI propagation is strongly correlated with large- and medium-scale E-region structures in TEC determined by the GPS measurements. When TEC ionospheric structures are not present near the Wales-Fairbanks path midpoint, no AEI signal is received. The authors tentatively conclude that the occurrence of these specific TEC signatures may be utilized as predictors of AEI forward propagation on paths within and parallel to the auroral oval. >