Patricia H. Doherty

Characteristics of plasma structuring in the cusp/cleft region at Svalbard

Satellite scintillation, all-sky optical imager, and digisonde observations were coordinated during a cusp campaign conducted at Ny Alesund, Svalbard (78.9°N, 11.8°E 75.7°N corrected geomagnetic latitude, over the period January 4–15, 1997. This paper is focused on a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region. This study has been performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250-MHz transmissions from a quasi-stationary polar satellite as well as the amplitude and phase scintillation of 1.6-GHz signals from Global Positioning System (GPS) satellites. These GPS scintillation measurements were augmented by the use of dual-frequency (1.2 and 1.6 GHz) GPS phase data acquired at the same station by the Jet Propulsion Laboratory for the International GPS Geodynamic Service (IGS). The continuous 250-MHz scintillation observations explored the daytime auroral ionosphere 2° poleward of Ny Alesund and showed that the scintillation spectra are often broad, as may be expected for irregularities in a turbulent flow region. Such irregularity dynamics were detected poleward of the nominal cusp region over the interval of 0600–1500 magnetic local time. The period of observations included the magnetic storm of January 10–11, 1997, when GPS observations of the IGS detected polar cap patches with total electron contents of 3×1016 m−2 and large-scale (tens of kilometers) phase variations at the GPS frequency of 1.6 GHz that corresponded to temporal gradients of 2×1016 m−2 min−1. However, amplitude scintillations at the GPS frequency of 1.6 GHz could not be detected in association with these large-scale phase variations, indicating that the irregularities with wavelengths less than the Fresnel dimension of 400 m were below the detectable limit. This is shown to be consistent in the context of enhanced ionospheric convection determined by digisonde and scintillation spectra.

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.

Comparison between calculated and observed F region electron density profiles at Jicamarca, Peru

Electron density profiles and isodensity coutours derived from Jicamarca incoherent scatter radar observations in Peru for October 1-2, 1970, are compared in detail with results from the Phillips Laboratory global theoretical ionospheric model. This model solves the ion continuity equation for O(+) concentration through production, loss and transport of ionization. The primary factor controlling the peak plasma density at Jicamarca is the vertical E x B drift, which drives the ionization upward during the day and downward at night. They illustrate the sensitivity of the low-latitude plasma density calculations to changes in the vertical E x B drift and changes in the neutral winds. They also compare the calculated profiles and peak parameters with an empirical model, the International Reference Ionosphere (IRI). They illustrate several limitations associated with the IR` that contribute to its limited capability at the magnetic equator.

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.

A Look Ahead: Expected Ionospheric Effects on GPS in 2000

he next maximum in the approximate 11-year cycle of solar ultraviolet (UV) activity is expected to occur near 2000. Two major ionospheric effects on GPS signals are closely related to long-term solar UV activity, and also will maximize at that time. These are ionospheric range delays and amplitude fading and phase scintillation effects. Dual-frequency GPS receivers automatically correct for the ionospheric range delay by measuring the difference in this dispersive effect on both frequencies. Civilian users of singlefrequency LI GPS receivers must either rely on the ionospheric correction algorithm sent as part of the user message, designed to correct for only 50% rms of the range delay, or they must use a nearby, in time and space, actual measurement of the ionospheric range delay to provide a correction for the ionospheric range error. Ionospheric range delays are directly proportional to the total electron content (TEC), encountered along the path from each GPS satellite to the user. The TEC increases with increasing solar cycle activity. As the absolute values of range delay increase with the solar cycle, the need for improved corrections also will increase. Irregularities in the ionosphere that produce amplitude fading and phase scintillation effects on GPS frequencies can become significant as the solar cycle

Large‐scale ionospheric irregularities occurrence at Ilorin, Nigeria

Ionospheric irregularities are a regular occurrence at the equatorial latitude during the postsunset hours especially during high-solar activity. These irregularities could pose serious challenges to satellite-based navigation and positioning applications by causing fading and degradation of transionospheric signals passing through these irregularities. We have investigated large-scale ionospheric irregularity occurrence at Ilorin, Nigeria (latitude = 8.48°N, longitude = 4.67°W, dip = 4.1°S), a station located within the equatorial region in the African sector. The index used in this study is the rate of change of total electron content (rate of change) derived from 30 s receiver-independent exchange data obtained using a dual frequency GPS receiver (i.e., NovAtel GPStation-2). The study covers a period of 4 years (2009–2012). The results obtained showed that large-scale irregularities occur between March and November and are more pronounced between 1900 LT and 2400 LT. The irregularities were observed to show two peaks: one in March and the other in September. Solar activity trend was also observed. The irregularity level around the peaks seems to increase with solar activity. Although the study covered a period of 4 years, the period could be regarded as the increasing phase of the solar cycle 24.

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.

Eye on the Ionosphere: The Correlation between Solar 10.7 cm Radio Flux and Ionospheric Range Delay

Patricia Doherty joins the regular contributors of this column to discuss the correlation between measurements of solar 10.7 cm radio flux and ionospheric range delay effects on GPS. Mrs. Doherty has extensive experience in the analysis of ionospheric range delays from worldwide systems and in the utilization and development of analytical and theoretical models of the Earths ionosphere.Ionospheric range delay effects on GPS and other satellite ranging systems are directly proportional to the Total Electron Content (TEC) encountered along slant paths from a satellite to a ground location. TEC is a highly variable and complex parameer that is a function of geographic location, local time, season, geomagnetic activity, and solar activity. When insufficiently accounted for, ionospheric TEC can seriously limit the performance of satellite ranging applications. Since the ionosphere is a dispersive medium, dual-frequency Global Positoning System (GPS) users can make automatic corrections for ionospheric range delay by computing the apparent difference in the time delays between the two signals. Single-frequency GPS users must depend on alternate methods to account for the ionospheric range delay. Various models of the ionosphere have been used to provide estimates of ionospheric range delay. These models range from the GPS systems simple eight-coefficient algorithm designed to correct for approximately 50% rms of the TEC, to state-of-the-art models derived from physical first principles, which can correct for up to 70 to 80% rms of the TEC but at a much greater computational cost.In an effort to improve corrections for the day-to-day variability of the ionosphere, some attempts have been made to predict the TEC by using the daily values of solar 10.7 cm radio flux (F10,7). The purpose of this article is to show that this type of prediction is not useful due to irregular, and sometimes very poor, correlation between daily values of TEC and F10.7. Long-term measurements of solar radio flux, however, have been shown to be well correlated with monthly mean TEC, as well as with the critical frequency of the inonospheric F2 region (foF2), which is proportional to the electron density at the peak of the ionospheric F2 region.

Interhemispheric propagation and interactions of auroral traveling ionospheric disturbances near the equator

We present the results of our GPS total electron content and ionosonde observations of large-scale traveling ionospheric disturbances (LSTIDs) during the 26 September 2011 geomagnetic storm. We analyzed the propagation characteristics of these LSTIDs from the auroral zones all the way to the equatorial region and studied how the auroral LSTIDs from opposite hemispheres interact/interfere near the geomagnetic equator. We found an overall propagation speed of 700 m/s for these LSTIDs and that the resultant amplitude of the LSTID interference pattern actually far exceeded the sum of individual amplitudes of the incoming LSTIDs from the immediate vicinity of the interference zone. We suspect that this peculiar intensification of auroral LSTIDs around the geomagnetic equator is facilitated by the significantly higher ceiling/canopy of the ionospheric plasma layer there. Normally, acoustic-gravity waves (AGWs) that leak upward (and thus increase in amplitude) would find a negligible level of plasma density at the topside ionosphere. However, the tip of the equatorial fountain at the geomagnetic equator constitutes a significant amount of plasma at a topside-equivalent altitude. The combination of increased AGW amplitudes and a higher plasma density at such altitude would therefore result in higher-amplitude LSTIDs in this particular region, as demonstrated in our observations and analysis.

Magnetic storm effects on the variation of TEC over Ilorin an equatorial station

We have used total electron content (TEC) derived from dual-frequency GPS receivers to study magnetically quiet and storm time variations of the ionosphere at Ilorin(8.47°N, 4.68°E), an equatorial station in the African sector. Four years (2009–2012) data were used for the study. The result on the quiet time variation of the ionosphere showed that the diurnal variation of TEC is not symmetrical about noon. This is a departure from a typical Chapman variation. Daytime maximum occurred after local noon (13–16 LT) for all the seasons and at all solar activity levels considered. A significant effect of solar activity variation was observed on the seasonal trend in 2011. The tendency for magnetic storms to cause increases in TEC is much greater than those of decreases. Daytime maximum TEC usually occurred closer to the noon time during storm periods when compared to those of quiet periods. Maximum percent change in TEC on storm days varied from about 25 to 131%.