E. L. Afraimovich

Determining parameters of large-scale traveling ionospheric disturbances of auroral origin using GPS-arrays

Abstract The intention in this paper is to investigate the form and dynamics of large-scale traveling ionospheric disturbances (LS TIDs) of auroral origin. We have devised a technique for determining LS TID parameters using GPS-arrays whose elements can be selected from a large set of GPS stations forming part of the International GPS Service network. The method was used to determine LS TID parameters during a strong magnetic storm of September 25, 1998. The North-American sector where many GPS stations are available, and also the time interval 00:00–06:00 UT characterized by a maximum value of the derivative Dst were used in the analysis. The study revealed that this period of time was concurrent with the formation of the main ionospheric trough with a conspicuous southward wall in the range of geographic latitudes 50–60° and the front width of no less than 7500 km. The auroral disturbance-induced large-scale solitary wave with a duration of about 1 h and the front width of at least 3700 km propagated in the equatorward direction to a distance of no less than 2000–3000 km with the mean velocity of about 300 m/s. The wave front behaved as if it ‘curled’ to the west in longitude where the local time was around afternoon. Going toward the local nighttime, the propagation direction progressively approximated an equatorward direction.

GPS radio interferometry of travelling ionospheric disturbances

Abstract This paper presents some results investigating the new possibilities of radio interferometry of Travelling Ionospheric Disturbances (TIDs) that are based on exploiting standard measurements of transionospheric radio signal characteristics and coordinate-time measurements using dual-frequency multichannel receivers of the Global Positioning System (GPS). A Statistical Angle-of-arrival and Doppler Method for GPS radio interferometry (SADM-GPS) is proposed for determining the characteristics of the TIDs dynamics by measuring variations of GPS phase derivatives with respect to time and spatial coordinates. These data are used to calculate corresponding values of the velocity vector, in view of a correction for satellite motions based on the current information available regarding the angular coordinates of the satellites. Subsequently, velocity and direction distributions are constructed and analyzed to verify the hypothesis of whether there is a predominant displacement. If it exists, then the pattern can be considered to be travelling, and the mean travel velocity can be determined from the velocity distribution. Through a computer simulation it was shown that multi-satellite GPS radio interferometry in conjunction with the SADM-GPS algorithm allows the detection and measurement of the velocity vector of TIDs in virtually the entire azimuthal range of possible TID propagation directions. The use of the proposed method is exemplified by an investigation of TIDs during the solar eclipse of 9 March 1997, using the GPS-radio interferometer GPSINT at Irkutsk.

Effects of the August 11, 1999 total solar eclipse as deduced from total electron content measurements at the GPS network

Abstract We present the results derived from measuring fundamental parameters of the ionospheric response to the August 11, 1999 total solar eclipse. Our study is based on using the data from about 100 GPS stations located in the neighborhood of the eclipse totality phase in Europe. The eclipse period was characterized by a low level of geomagnetic disturbance (Dst-variation from −10 to −20 nT ), which alleviated significantly the problem of detecting the ionospheric response to the eclipse. Our analysis revealed a well-defined effect of a decrease (depression) of the total electron content (TEC) for all GPS stations. The delay between minimum TEC values with respect to the totality phase near the eclipse path increased gradually from 4 min in Greenwich longitude (10:40 UT, LT) to 8 min at the longitude 16° (12:09 LT). The depth and duration of the TEC depression were found to be 0.2–0.3 TECU and 60 min , respectively. The results obtained in this study are in good agreement with earlier measurements and theoretical estimates.

Ionospheric effects of the solar flares of September 23, 1998 and July 29, 1999 as deduced from global GPS network data

Abstract This paper presents data from first global positioning system (GPS) measurements of global response of the ionosphere to solar flares of September 23, 1998 and July 29, 1999. The analysis used novel technology of a global detection of ionospheric effects from solar flares as developed by one of the authors (Afraimovich, Radio Sci. 35 (2000) 1417). The essence of the method is that use is made of appropriate filtering and a coherent processing of variations in total electron content (TEC) in the ionosphere which is determined from GPS data, simultaneously for the entire set of visible (over a given time interval) GPS satellites at all stations used in the analysis. It was found that fluctuations of TEC, obtained by removing the linear trend of TEC with a time window of about 5 min, are coherent for all stations and the line-of-sight to the GPS satellites on the dayside of the Earth. The time profile of TEC responses is similar to the time behavior of hard X-ray emission variations during flares if the relaxation time of electron density disturbances in the ionosphere of order 50–100 s is introduced. No such effect on the nightside of the Earth has been detected yet.

Long-distance traveling ionospheric disturbances caused by the great Sumatra-Andaman earthquake on 26 December 2004

By using data from the GPS network, we observed exceptional intensive quasi-periodical perturbations of the total electron content (TEC) caused by the great Sumatra-Andaman earthquake on 26 December 2004. The time period of the variations was about 15 min, their duration was about 1 hour. The amplitude of the TEC oscillations exceeded the amplitude of “background” fluctuations in this range of periods by one order of magnitude, at a minimum. They were registered 2–7 hours after the main shock at a distance from 1000 to 5000 km, both on the northwest and northeast outward from the epicenter. The most probable source of the observed oscillations appeared to be a seismic airwave generated by the sudden vertical displacement of the Earth’s surface near the epicenter.

TEC response to the 2008 Wenchuan Earthquake in comparison with other strong earthquakes

We registered near-field global positioning system (GPS) total electron content (TEC) response to the Wenchuan Earthquake on 12 May 2008. The Wenchuan Earthquake (magnitude 8.0) occurred at 06:28 UT as the result of motion on a northeast striking reverse fault (thrust fault) on the northwestern margin of the Sichuan Basin. The earthquake reflects tectonic stresses resulting from the convergence of crustal material slowly moving from the high Tibetan Plateau, to the west, against a strong crust underlying the Sichuan Basin and southeastern China. We found that intensive N-shaped shock-acoustic waves with a plane waveform and with a half-period of about 200 s propagated south-eastwards with a velocity 580 m s − 1 for a distance of about 1000 km from the epicentre. The wavefront of N-shaped disturbance was parallel with the earthquake rupture direction (from southwest to northeast). The main directional lobe of shock-acoustic wave emitter is directed southeastwards, i.e. transversely to the rupture. We speculate that the above properties of TEC response are determined by the geodynamics of the Wenchuan Earthquake. No noticeable TEC response to that earthquake was found in far-field regions in South Korea and Japan. We compared TEC response to the 2008 Wenchuan earthquake with other strong earthquakes.

TEC anomalies—Local TEC changes prior to earthquakes or TEC response to solar and geomagnetic activity changes?

A number of papers have reported on deviations of daily values of the maximum electron concentration of the ionospheric F2 layer and/or total electron content (TEC) in the vicinity of an earthquake’s epicenter some time prior to the quake. Owing to the importance of this problem, a question of a “locality” of those effects is emerging. To study this issue we have developed a method based on the calculation of global electron content and of local electron content in “check-region” with low seismic activity. The effect of TEC day-to-day changes before strong earthquakes is analyzed in this work. It is shown that in some cases this effect might be a reflection of global changes of the ionization caused by the 27-day variations as well as other fast alterations due to solar and geomagnetic activity changes. We discuss the problem of certain data corrections that permit local changes to be distinguished from global ones.

Co-seismic ionospheric and deformation signals on the 2008 magnitude 8.0 Wenchuan Earthquake from GPS observations

The 8.0 magnitude Wenchuan Earthquake occurred at the Longmenshan Fault along the eastern boundary between the Tibetan Plateau and the western Sichuan Basin in southwestern China on 12 May 2008, killing tens of thousands of people in several cities along the western Sichuan Basin. In this paper, co-seismic ionospheric and deformation signals from the mainshock of this event are extracted from national global positioning system (GPS) network observations, which provide unique insights into this event. The co-seismic deformation moves towards the earthquake epicentre, and the largest magnitude reaches 2.3 m in Beichuan. The total moment of the co-seismic rupture is 2.4 × 1021 nm, equivalent to a magnitude of 8.1 and nearly identical to the seismological estimate. Furthermore, co-seismic ionospheric disturbances indicate a shock-acoustic wave propagation at a mean velocity of about 600 m s−1 towards the rupture direction.

Simultaneous radio and optical observations of the mid-latitude atmospheric response to a major geomagnetic storm of 6–8 April 2000

Basic properties of the mid-latitude traveling ionospheric disturbances (TIDs) during the maximum phase of a major magnetic storm of 6 –8 April 2000 are shown. Total electron content (TEC) variations were studied by using data from GPS receivers located in Russia and Central Asia. The nightglow response to this storm at mesopause and termospheric altitudes was also measured by optical instruments FENIX located at the observatory of the Institute of Solar-Terrestrial Physics (51:9 ◦ N; 103:0 ◦ E), and MORTI located at the observatory of the Institute of Ionosphere (43:2 ◦ N; 77:0 ◦ E). Observations of the O (557:7 and 630:0 nm) emissions originating from atmospheric layers centered at altitudes of 90 and 250 km were carried out at Irkutsk and of the O2(b 1 + g −X 3 − g ) (0-1) emission originating from an atmospheric layer centered at altitude of 94 km was carried out at Almaty. Our radio and optical measurement network observed a storm-induced solitary large-scale wave with duration of 1 h and a wave front width of no less than 5000 km, while it traveled equatorward with a velocity of 200 m=s from 62 ◦ Nt o 38 ◦ N geographic latitude. The TEC disturbance, basically displaying an electron content depression in the maximum of the F2 region, reveals a good correlation with growing nightglow emission, the temporal shift between the TEC and emission variation maxima being diAerent for diAerent altitudes. A comparison of the auroral oval parameters with dynamic spectra of TEC variations and optical 630 nm emissions in the frequency range 0.4 –4 mHz (250 –2500 s periods) showed that as the auroral oval expands into mid-latitudes, also does the region with a developed medium-sale and small-scale TEC structure. c

The use of the international GPS network as the global detector (GLOBDET) simultaneously observing sudden ionospheric disturbances

We developed a new technology for global detection of ionospheric disturbances, on the basis of phase measurements of the total electron content (TEC) along the line-of-sight (LOS) between the receiver on the ground and transmitters on the GPS satellites using an international GPS networks. Temporal dependencies of TEC with the time resolution of 30 s are obtained for a set of spaced receivers of the GPS network simultaneously for the entire set of visible satellites. These series are subjected to filtering in the selected range of oscillation periods using known algorithms for spatio-temporal analysis of signals. An analysis is made of the possibilities of using the GLOBDET when detecting the ionospheric response of solar flares. In this case it is best to make the coherent summation of the filtered series of TEC. Because of a statistical independence of the background fluctuations, the signal/noise ratio, when the flare effect is detected, is increased due to a coherent processing by at least the times, where N is the number of LOS.