John A. Klobuchar

Model studies of the latitudinal extent of the equatorial anomaly during equinoctial conditions

The latitudinal extent of the equatorial anomaly has been studied for equinoctial conditions using a theoretical model of the ionosphere which incorporates measured values of vertical E × B drift at the Earths magnetic equator. Realistic values of neutral winds are also included. The equatorial anomaly region, typically between ±20° magnetic latitude, is that part of the world where the highest values of electron density and total electron content (TEC) normally occur and hence is very important to high-frequency propagation and to transionospheric propagation effects. During the daytime, upward E × B drift at the magnetic equator drives the ionization across field lines to higher latitudes, causing crests in ionization to occur at approximately ±15° dip latitude. The E × B drift mechanism is explained in detail by Hanson and Moffett (1966). The latitude range over which the anomaly makes a significant difference in values of ƒ0F2 and TEC is calculated as a percent departure from the case with no equatorial electric field. Results from the model studies with different values of realistic electric fields show that the effects of the anomaly can be more highly variable and widespread in latitude and local time than is generally assumed.

SIMULTANEOUS STORM-TIME INCREASES OF THE IONOSPHERIC TOTAL ELECTRON CONTENT AND THE GEOMAGNETIC FIELD IN THE DUSK SECTOR.

Abstract Using the Faraday rotation technique with the ATS-3 satellite, it has been possible to monitor changes in the total electron content ( N T ) of the mid-latitude ionosphere during the first day of 20 geomagnetic storms. Our analysis has shown that during the positive phase ( ΔN T > 0) of ionospheric storms the absolute magnitude of the increase in N T exhibits a very pronounced maximum near sunset. The mean value of Δ N T at 17:00 LT is more than five times the average Δ N T value at local noon. This effect is basically independent of the storm commencement time and is usually associated with substantial local enhancements of the total geomagnetic field. The N T enhancements are discussed in terms of a contraction and draining of the plasmasphere. A model is presented in which the dawn-dusk electric field responsible for the magnetospheric convection slows down the corotational motion of the plasmaspheric ionization in the dusk sector. This braking action causes a ‘pile up’ of the plasma and the magnetic field along the entire dusk sector.

Real‐time ionospheric science: The new reality

Until the early 1990s, near-real-time updates to monthly climatological ionospheric models were driven primarily by military requirements. Since then, civilian research and commercial requirements for corrections to precise satellite ranging systems have become the driving forces for real-time ionospheric data, and they likely will continue to be major reasons for the availability of worldwide transionospheric observations in the future. This paper outlines the development of some of these requirements and describes the likely future availability of ionospheric observations for near-real-time updating of ionospheric monthly climatology.

A large-scale hole in the ionosphere caused by the launch of skylab.

A dramatic ionospheric phenomenon, unique in magnitude and in spatial and temporal extent, occurred along the Atlantic Coast of North America after the launch of the NASA Skylab Workshop on 14 May 1973. The effect was a large and rapid decrease in the total number of ionospheric electrons within a distance of 1000 kilometers of the burning engines of the Saturn V launch vehicle. The observations are interpreted in terms of exceptionally enhanced chemical loss rates due to the molecular hydrogen and water vapor contained in the Saturn second-stage exhaust plume.

Ionospheric disturbances: Evidence for the contraction of the plasmasphere during severe geomagnetic storms

Abstract Continuous measurements of the ionospheric total electron content (TEC) provide a parameter well suited for the study of F-region disturbance effects. In this investigation, five cases of large and rapid drops in TEC observed near the sunset period are interpreted as being due to the contraction of the plasmasphere to L values less than 3. A model calculation is performed for the specific case of 1 November 1968 using simultaneous Alouette I data to define the position and magnitude of the ionospheric trough. The results indicate that the motion of a deep trough across the ray path from a geostationary satellite to an observing station can cause drastic changes in the measured amounts of Faraday rotation and therefore in the derived values of TEC. All of the TEC data sources available for the five events are then examined in an attempt to describe more completely the latitude dependence of the effects. It is suggested that during severe geomagnetic storms, the large and rapid decays in TEC during the 18–21 LT period to values significantly below normal can be used as a criterion to determine the approximate latitudinal extent of the contracted plasmasphere.

Evaluation of six ionospheric models as predictors of total electron content

We have gathered total electron content (TEC) data from a range of mid-latitudes and low latitudes and longitudes for a wide range of solar activity. This data was used to evaluate the performance of six publicly available ionospheric models as predictors of total electron content. TEC is important for correcting modern DoD space systems, which propagate radio waves from the earth to satellites, for time delay effects of the ionosphere. The TEC data were obtained from polarimeter receivers located in North America, the Pacific, and the East coast of Asia. The ionospheric models evaluated are (1) the International Reference Ionosphere, (2) the Bent model, (3) the Ionospheric Conductivity and Electron Density model, (4) the Penn State model, (5) the Fully Analytic Ionospheric Model, and (6) a hybrid model consisting of the Union Radio Scientifique Internationale 88 (URSI-88) coefficients coupled with the Damen-Hartranft profile model. We will present extensive comparisons between monthly median TEC and model TEC obtained by integrating electron density profiles produced by the six models. These comparisons demonstrate that although most of the models do very well at representing ƒ0F2, none of them do very well with TEC, probably because of inaccurate representation of the topside profile. We suggest that one approach to obtaining better representations of TEC is the use of ƒ0F2 from the CCIR or URSI-88 coefficients coupled with a good climatological slab thickness model.

Ionospheric equivalent slab thickness and its modeling applications

Abstract : A database of equivalent slab thickness observations covering nearly two complete solar cycles has been obtained at a mid-latitude site (Hamilton, MA). This database has been studied for correlations between the various parameters as well as for dependencies on observable quantities. The mean variations of tau are described both qualitatively, in terms of changes to profile shapes, and quantitatively, in terms of a simple numerical model. A preliminary investigation of the day-to-day variations of tau is also described. The similarity of the mean numerical tau model to one obtained using data from a site in Wales (U.K.) suggests that longitude variations of tau may well be small at mid-latitudes. This property has in turn been used to convert a network of ionosonde observations to a database of TEC, for comparisons with a number of currently available ionospheric models. These same ionospheric models were also used to predict values of tau over Hamilton, MA, and the comparisons provided additional model validation. Adjustments to the model profiles were suggested in a couple of cases. Keywords: Ionospheric models; Slab thickness; Total electron content; Day-to-day variations; Peak density.

Total electron content over the Pan‐American longitudes: March‐April 1994

An experimental campaign to measure diurnal changes in total electron content (TEC) over the wide latitude range from approximately 50°N to 40°S was carried out from March 28 through April 11, 1994, by monitoring the differential carrier phase from the U.S. Navy Navigation Satellite System using a chain of ground stations aligned along the approximate 70°W longitude meridian. This Pan-American campaign was conducted primarily to study the day-to-day variability of the equatorial anomaly region. The experimental plan included using the received values of TEC from the chain of stations to construct profiles of electron density versus latitude using tomographic reconstruction techniques and, then, to compare these reconstructions against a theoretical model of the low-latitude ionosphere. The diurnal changes in TEC along this latitude chain of stations showed a high degree of variability from day to day, especially during a magnetic storm which occurred near the beginning of the campaign. The equatorial anomaly in TEC showed large changes in character in the two hemispheres, as well as differences in magnitude from day to day. The latitudinal gradients of TEC, especially in the lower midlatitudes, also showed large differences between magnetic storm and quiet conditions. Comparisons of the TEC data with the theoretical model illustrate the sensitivity of the model calculations to changes in magnetic E×B drift and serves to validate the strong influence that these drifts have on the formation and the strength of the equatorial anomaly regions.