Ildiko Horvath

The Weddell sea anomaly observed with the Topex satellite data

This paper introduces the complete image of the Weddell Sea Anomaly, observed with the over-the-ocean ionospheric total electron content (TEC) values obtained from the TOPEX satellite data with an almost unlimited coverage over the oceans, the first time according to the literature; and investigates its development. With a series of TOPEX TEC maps, this paper demonstrates the diurnal variations of both the night-time and the day-time Weddell Sea Anomaly, which appeared as a night-time TEC enhancement and as a day-time TEC depletion, during the near sunspot maximum period of 1998 and 1999 investigated. Several TOPEX passes, plotted in geomagnetic latitudes, are also presented to demonstrate the longitudinal variations of the Weddell Sea Anomaly, and also to show other ionospheric features appearing such as the southern-hemisphere mid-latitude day-time and night-time trough, the northern-hemisphere mid-latitude night-time trough and the equatorial anomaly. This paper demonstrates how large the anomaly is in reality situated west of the Faraday ionosonde station over the Bellinghausen Sea and not over the Weddell Sea that is east of Faraday. Thus the correct name should be Bellinghausen Sea Anomaly. Based upon the review paper of Dudeney and Piggott (1978), the development of the Weddell Sea Anomaly is explained with the combined effects of solar ultraviolet radiation and thermospheric neutral winds.

A total electron content space weather study of the nighttime Weddell Sea Anomaly of 1996/1997 southern summer with TOPEX/Poseidon radar altimetry

This paper reports on a total electron content space weather study of the nighttime Weddell Sea Anomaly, overlooked by previously published TOPEX/Poseidon climate studies, and of the nighttime ionosphere during the 1996/1997 southern summer. To ascertain the morphology of spatial TEC distribution over the oceans in terms of hourly, geomagnetic, longitudinal and summer-winter variations, the TOPEX TEC, magnetic, and published neutral wind velocity data are utilized. To understand the underlying physical processes, the TEC results are combined with inclination and declination data plus global magnetic field-line maps. To investigate spatial and temporal TEC variations, geographic/magnetic latitudes and local times are computed. As results show, the nighttime Weddell Sea Anomaly is a large (∼1,600(°)2; ∼22 million km2 estimated for a steady ionosphere) space weather feature. Extending between 200°E and 300°E (geographic), it is an ionization enhancement peaking at 50°S–60°S/250°E–270°E and continuing beyond 66°S. It develops where the spacing between the magnetic field lines is wide/medium, easterly declination is large-medium (20°–50°), and inclination is optimum (∼55°S). Its development and hourly variations are closely correlated with wind speed variations. There is a noticeable (∼43%) reduction in its average area during the high magnetic activity period investigated. Southern summer nighttime TECs follow closely the variations of declination and field-line configuration and therefore introduce a longitudinal division of four (Indian, western/eastern Pacific, Atlantic). Northern winter nighttime TECs measured over a limited area are rather uniform longitudinally because of the small declination variation. TOPEX maps depict the expected strong asymmetry in TEC distribution about the magnetic dip equator.

Software developed for obtaining GPS‐derived total electron content values

This paper describes a complex technique with its built-in cycle slip correction procedures that have been developed for ionospheric space research to obtain high-quality and high-precision GPS-derived total electron content (TEC) values. Thus, to correct GPS anomalies while the signatures of space weather features detected in the dual-frequency 30-s rate GPS data are preserved is the main aim of this technique. Its main requirement is to complete fully automatically all the tasks required to turn the observational data to the desired final product. Its major tasks include curve fitting, cycle slip detection and correction in the slant relative TEC data, residual error detection and correction in the vertical TEC data, and vertical TEC data filtering for quantifying data smoothness and GPS phase fluctuations. A detailed description of these two data correction methods is given. Validation tests showing weaknesses and strengths of the methods developed are also included and discussed. Versatility and accuracy of the methods are demonstrated with interesting and real-world examples obtained from smooth midlatitude and from dynamic low- and high-latitude data. Results indicate that errors can be detected and corrected more reliably in the vertical TEC data than in the slant TEC data because of the lower rate of change of vertical TEC over a 30-s sampling period. Future work includes the development of a complex software package wherein the individual FORTRAN algorithms, described in this paper, will be incorporated into one main (FORTRAN, Matlab, or C++) program to provide professional and customized GPS data processing for ionospheric space research.

Using observations from the GPS and TOPEX satellites to investigate night-time TEC enhancements at mid-latitudes in the southern hemisphere during a low sunspot number period

Abstract The state of the ionization of the upper atmosphere at low and mid latitudes in the Australian region has been studied by investigating the total electron content (TEC) obtained by a dual-frequency group path and phase path GPS technique. For the low sunspot number time period of March 1995–February 1996, one week of data centred on the Priority Regular World Day for each month have been used to investigate night-time mid-latitude peaks occurring around midnight in the Australian region. TEC from TOPEX provided additional information related to the formation of the night-time peaks. Although night-time TEC enhancements have been observed previously, there is no general agreement on their origin. From the results of the present study, the development of midnight TEC enhancements coincided with the low latitude processes occurring at around the time of vertical E×B drift velocity reversal. The TOPEX results confirmed that the upward E×B drift velocity reversal and the downward plasma flow from greater heights producing the night-time peaks at mid latitudes are triggered from a common source: the westward electric field.

Distinctive plasma density features of the topside ionosphere and their electrodynamics investigated during southern winter

This study utilizes a novel technique to map the Defense Meteorological Satellite Program (DMSP) data across the two hemispheres to learn about the morphology and plasma composition of the topside ionosphere, and the underlying ionospheric dynamics. In the southern winter hemisphere, the regional maps tracked a heavy-ion (Ni-O+) trough,aurora zone, polar hole, and large plasma density depletion. The latter appeared in the region of the South Atlantic Magnetic Anomaly (SAMA). The electron temperature (Te) map detected the thermal characteristics of these features, while the plasma drifts and flux maps tracked their dynamics. Results show that there were special electrodynamic effects in the SAMA region due to the low magnetic field and high conductivity. These increased the vertical downward (VZ) and the westward (VY) drifts. Independently, the VZ and VY maps registered the affected area that was depleted in heavy ions and rich in light ions. Some field-aligned profiles tracked the impact of these SAMA effects on the heavy-ion trough, which was a stagnation trough and appeared markedly differently at different longitudes. At trough latitudes ((56 ± 4)S (geomagnetic) when Dstav = 0 nT), the elevated electron temperatures forming a Te peak indicated subauroral heating effects. A statistical study modeled the magnetic activity dependence of the Te peak’s magnitude and location and revealed their linear correlation with the activity level. Statistically, the Te peak increased [10.226 ± 1.355]K and moved equatorward [0.051 ± 0.009] (geomagnetic) per 1 nT decrease in the averaged Dst index. Per 1 nT increase in the averaged AE index, its magnitude increased [1.315 ± 0.444]K and the equatorward movement was [0.014 ± 0.003].

Large‐scale traveling ionospheric disturbances impacting equatorial ionization anomaly development in the local morning hours of the Halloween Superstorms on 29–30 October 2003

This study investigates the development of EIA‐like features during the Halloween Superstorms. These features are similar to a well‐developed equatorial ionospheric anomaly (EIA) and therefore suggest superfountain effects. We tracked EIA‐like features in the early‐morning sector of 29–30 October 2003 and at around midday on 30 October. Their plasma environment was studied with field‐aligned plasma density,vertical plasma drift, electron temperature, and vertical plasma flow profiles. Coupled thermosphere‐ionosphere plasmasphere (CTIP) simulations reproduced storm‐generated equatorward wind surges. When these EIA‐like features appeared early morning on 29–30 October, there was no forward fountain circulation; only large‐scale traveling ionospheric disturbances (TIDs) and strong equatorward winds were present. These provide observational evidence that large‐scale TIDs created large plasma depletions over the dip equator and with the aid of equatorward winds some large enhancements at ∼±30°N (geomagnetic), resulting in the development of early‐morning EIA‐like features. We identified TID signatures in the equatorial and midlatitude ionosonde and magnetometer data, and studied EIA‐like features at around midday on 30 October utilizing non‐field‐aligned TOPEX TEC (2000 UT) and published CHAMP TEC (2030 UT; 2200 UT) line plots. According to our analysis, the EIA‐like features seen in the TEC data were created by large‐scale TIDs at 2000 and 2030 UT and by the combined effects of large‐scale TIDs and forward superfountain at 2200 UT. CTIP simulations demonstrate the crucial role of the neutral winds’ mechanical or direct effects in the development of high plasma densities at low midlatitudes.

An investigation of the northern hemisphere midlatitude nighttime plasma density enhancements and their relations to the midlatitude nighttime trough during summer

This study has utilized regional surface maps and field-aligned latitudinal profiles derived from multiinstrument DMSP-F15 data. It investigates northern midlatitude plasma enhancements in the nighttime topside ionosphere during summer and their relation to the midlatitude heavy ion stagnation trough. The tracked midlatitude summer enhancements showed significant longitudinal variations. Their best development occurred in the 50°N-60°N/110°E- 170°E (geographic) region. Proven by Coupled Thermosphere-Ionosphere Plasmasphere wind velocity simulations, equatorward winds were strongest there. This region is the northern hemisphere equivalent of the Weddell Sea Anomalys (WSA) locality. There, northern summer plasma enhancements resembled the nighttime WSA, a southern summer equinoctial phenomenon, and thus were named as WSA-like feature. Their plasma density increased mainly owing to the mechanical or direct effects of the equatorward winds. However, as results show, the WSA-like feature was an ordinary northern midlatitude nighttime enhancement occurring in summer. This was further confirmed by the WSA-like features relation to the trough and by the troughs movement. Over the northern hemisphere, the summer trough appeared at (62.461 ± 2.93) °N (geomagnetic), poleward of the WSA-like feature and other midlatitude plasma enhancements, and moved equatorward with a rate of (0.097 ± 0.04) °N/nT when magnetic activity increased. As our previous WSA investigation indicates, the southern summer trough develops at lower latitudes, at (47.1564 ± 2.16) °S, equatorward of the nighttime WSA and moves poleward with a rate of (0.0497 ± 0.003) °S/nT with increasing magnetic activity under the combined influence of the nighttime WSA and South Atlantic Magnetic Anomaly. Copyright 2009 by the American Geophysical Union.

The southern-hemisphere mid-latitude day-time and night-time trough at low-sunspot numbers

The diurnal, seasonal, spatial and magnetic activity variations of the southern-hemisphere mid-latitude trough has been studied by using GPS and TOPEX satellite techniques during the low-sunspot number period of February 1995–February 1996. The ionospheric total electron content (TEC) values were obtained from the raw satellite data to observe the trough both locally in the Australian longitude region and globally over the world oceans. Various trough features were observed and investigated under different magnetic conditions, and were compared to the results of other researchers employing different techniques at Macquarie Island (−54.5°N; 154.95°E, geographic and the magnetic shell parameter (L) is 5.38). The ionization build-up on the equatorward trough wall, which has not been investigated since Foster (1993), is discussed in detail with the ΔTEC parameter utilized to characterize it. Some of the GPS findings were confirmed with the TOPEX results. Comparisons with the model generated TEC plots indicated that the model did not reproduce the trough. Several day-time and night-time global TOPEX TEC maps, showing also the geomagnetic and dip equators, were constructed for the season of the 1995 autumnal equinox to observe the trough globally, the first time in the literature, with other large-scale ionospheric formations. The magnetic alignment of these large-scale ionospheric formations is obvious. On the night-time maps the empirical position of the trough were plotted for various local time values at Kp=0 and 6, and indicated a very good agreement between the experimental and theoretical results.

Formation and evolution of the ionospheric plasma density shoulder and its relationship to the superfountain effects investigated during the 6 November 2001 great storm

This study investigates the 6 November 2001 great storm’s impact on the topside ionosphere utilizing data from the onboard TOPEX/Poseidon-NASA altimeter, Defense Meteorological Satellite Program–Special Sensor Ions, Electrons and Scintillation instruments and ACE interplanetary observatory. A set of field-aligned profiles demonstrate the storm evolution, caused by the precursor and promptly penetrating interplanetary eastward electric (E) fields, and strong equatorward winds reducing chemical loss, during the long-duration negative BZ events. At daytime-evening, the forward fountain experienced repeated strengthening, as the net eastward E field suddenly increased. The resultant symmetrical equatorial anomaly exhibited a continuous increase,while the energy inputs at both auroral regions were similar. In both hemispheres, by progressing poleward, a midlatitude shoulder exhibiting increased plasma densities, a plasma-density dropoff (steep gradient) and a plasma depletion appeared. These features were maintained while the reverse fountain operated. At the dropoff, elevated temperatures indicated the plasmapause. Consequently, the plasma depletion was the signature of plasmaspheric erosion. In each hemisphere, an isolated plasma flow, supplying the minimum plasma, was detected at the shoulder. Plasmaspheric compression, due to the enhanced E fields, could trigger this plasma flow. Exhibiting strong longitudinal variation at evening-nighttime, the shoulder increased 306% over the southeastern Pacific, where the nighttime Weddell Sea Anomaly (WSA) appeared before the storm. There, the shoulder indicated the storm-enhanced equatorward section of the quiet time WSA. Owing to the substantial equatorward plasmapause movement, a larger poleward section of the quiet time WSA eroded away, leaving a large depletion behind. This study reports first these (northern, southern) plasma flows and dramatic storm effects on a nighttime WSA.

Storm-enhanced plasma density features, daytime polar cap plasma enhancements, and their underlying plasma flows investigated during superstorms

This study investigates the effects of November superstorms on the daytime-evening northern topside ionosphere. We utilize multi-instrument interplanetary and geomagnetic data to analyze the interplanetary shock/electric (E) field events. To assess their impact, field-aligned DMSP (Defense Meteorological Satellite Program) passes are employed. We tracked evening storm-enhanced density (SED) features and daytime polar plasma enhancements occurring regularly that allowed us to learn about the basic mechanisms responsible for their development. Results show that large vertical downward plasma flows supplied the plasma building up the SED features and polar plasma enhancements detected. These plasma formations occurred during the main phase with the forward superfountain and during a sub-storm with the reverse fountain. These provide observational evidence that primary downward plasma flows, triggered by detachment processes caused by the westward sub-auroral polarization stream (SAPS) E-fields, existed and became transported sunward. At SED-latitudes, these primary downward plasma flows were intensified by the forward superfountain when its upward plasma flow spilled over into the equatorial anomaly and cascaded down the magnetic field lines. When the reverse fountain was active, such intensification was not available to the primary downward plasma flows at SED-latitudes. As the downward streaming plasma found its way to the polar cap, it resulted in the development of daytime polar plasma enhancements. Statistically, daytime polar plasma enhancements occurred at all longitudes contradicting the myth that the North American region is better suited for the development of large plasma enhancements in the polar cap.