David S. Coco

Combined Ionospheric Campaign 1: Ionospheric tomography and GPS total electron count (TEC) depletions

Results from the June 1998 combined ionospheric campaign (CIC) are presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, the University of Texas at Austin (ARL:UT), has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. In this paper we compare CIT data, GPS TEC data and data from the incoherent scatter radar at Arecibo to try to obtain an understanding of the temporal and spatial distribution of ionospheric structure observed during the campaign. We use the three data sets as inputs to the 3DVAR tomography algorithm developed at CIR and present results of the 3DVAR “objectively analyzed” electron density field. An ionization wall was found near 40° latitude in agreement with previous Millstone Hill and DMSP observations in high Kp. Several elongated density depletions were also detected.

Ionospheric observations of the November 1993 storm

Complementary measurements from three different electron density measurement techniques are presented for the time period of the November 4, 1993, storm. Computerized ionospheric tomography (CIT) data from an array of nine ground stations operating as part of the Mid-America CIT Experiment (MACE 93) is presented along with data from a digital ionosonde operating near the midpoint of the CIT array. Corroborating data from the DMSP satellites are also presented. Taken together the data provide evidence of a strongly disturbed ionosphere with rapid variations of electron density structures in space and time. The CIT data show a deep equatorward surge of the the midlatitude trough to nearly 50° geomagnetic latitude, while the ionosonde shows dramatic variations in the virtual height of the ionosphere. DMSP data confirm the equatorward surge of the trough and also display a number of sharp latitudinal variations in vertical drift velocities. An interesting occurrence of spread F also occurred during this time period.

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.

Computerized ionospheric tomography analysis of the Combined Ionospheric Campaign

Results from the June 1998 Combined Ionospheric Campaign (CIC) will be presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. One of the objectives of these campaigns is to develop suitable data sets for ingestion into data assimilative models and also to serve as validation sets for these models. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, University of Texas at Austin, has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. Analysis of the CIT data from the first CIC will be presented, and comparison with other data sets will be made. Analysis will initially focus on examining the total electron content (TEC) data from the CIT receivers and ground-based GPS TEC data and correlating it with other data sets. Subsequently, the analysis will shift to performing four-dimensional electron density estimations using the Ionospheric Data Assimilation 3D (IDA3D) algorithm developed at CIR. The resulting electron density estimates will be compared with other data sources both for accuracy of the technique and scientific investigations.

Incoherent scattering from the collision dominated D-region. Comparison of theories with experimental data

Abstract There has been a long standing controversy as to the introduction of the collisional term in the incoherent scattering theory. This has profound implications in interpreting the incoherent scatter spectra from the collision dominated D- and E-regions. Among the different theoretical formulations, those by Dougherty and Farley and Waldteufel have been used for the interpretation of incoherent scatter data. Waldteufels formulation apparently improves on the theoretical model of Dougherty and Farley by allowing for momentum conservation and temperature changes during collisions. Using incoherent scatter data from D-region heights over Arecibo we show for the first time that Dougherty and Farleys formulation is the correct one. This is due to the fact that during a collision ions lose both momentum and energy to neutrals and so far as the ion gas is concerned, neither should be conserved. Once the correct formulation has been established, it can be seen that the incoherent scatter technique offers a unique advantage in inferring mesospheric parameters with very high precision. For example, the ratio of the number density to neutral temperature in the D-region over Arecibo could be measured to accuracies of a few per cent. The results of such measurements are in excellent agreement with the CIRA 1972 model and show a day-to-day variability limited to within 10–15%.

Differential Doppler measurements of the ionosphere during a solar eclipse

Abstract The effects of the solar eclipse of 26 February 1979 on the ionosphere were measured using differential Doppler techniques. Nayy navigation satellite passes were monitored at 12 sites located across the North American continent. These data yield a measurement of the vertical columnar electron content along a north-south line. Different sites monitoring the same pass provide simultaneous observations of ionospheric variations along different longitude lines. Two satellite passes occurred during or just after the eclipse. These data show a shoulderjust northward of the umbra region and a trough just behind the umbra containing large horizontal gradients. This sharp trough recovered quickly with a half-life of about 10 min.