Elvira Astafyeva

Ionospheric response to the 2015 St. Patrick's Day storm: A global multi‐instrumental overview

We present the first multi-instrumental results on the ionospheric response to the geomagnetic storm of 17–18 March 2015 (the St. Patricks Day storm) that was up to now the strongest in the 24th solar cycle (minimum SYM-H value of A233 nT). The storm caused complex effects around the globe. The most dramatic positive ionospheric storm occurred at low latitudes in the morning (~100–150% enhancement) and postsunset (~80–100% enhancement) sectors. These significant vertical total electron content increases were observed in different local time sectors and at different universal time, but around the same area of the Eastern Pacific region, which indicates a regional impact of storm drivers. Our analysis revealed that this particular region was most concerned by the increase in the thermospheric O/N 2 ratio. At midlatitudes, we observe inverse hemispheric asymmetries that occurred, despite the equinoctial period, in different longitudinal regions. In the European-African sector, positive storm signatures were observed in the Northern Hemisphere (NH), whereas in the American sector, a large positive storm occurred in the Southern Hemisphere, while the NH experienced a negative storm. The observed asymmetries can be partly explained by the thermospheric composition changes and partly by the hemispherically different nondipolar portions of the geomagnetic field as well as by the IMF By component variations. At high latitudes, negative ionospheric storm effects were recorded in all longitudinal regions, especially the NH of the Asian sector was concerned. The negative storm phase developed globally on 18 March at the beginning of the recovery phase.

Dependence of waveform of near-field coseismic ionospheric disturbances on focal mechanisms

Using Total Electron Content (TEC) measurements with Global Positioning System we studied ionospheric responses to three large earthquakes that occurred in the Kuril Arc on 04 October 1994, 15 November 2006, and 13 January 2007. These earthquakes have different focal mechanisms, i.e. high-angle reverse, low-angle reverse, and normal faulting, respectively. TEC responses to the 2006 and 2007 events initiated with positive and negative changes, respectively. On the other hand, the initial TEC changes in the 1994 earthquake showed both positive and negative polarities depending on the azimuth around the focal area. Such a variety may reflect differences in coseismic vertical crustal displacements, which are dominated by uplift and subsidence in the 2006 and 2007 events, respectively, but included both in the 1994 event.

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 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.

Topside ionospheric irregularities as seen from multisatellite observations

We use in situ data from CHAMP and DMSP satellites, along with data of GPS receiver onboard CHAMP satellite and ground-based GPS receivers to study the occurrence and global distribution of ionospheric irregularities during the main phase of the geomagnetic storm of 29–31 August 2004 (minimum Dst excursion of A128 nT). Using the CHAMP GPS measurements, we created maps of GPS phase fluctuation activity and found two specific zones of the most intense irregularities: (1) the region of the auroral oval at high latitudes of both hemispheres and (2) the low latitudes/equatorial region between Africa and South America. At high latitudes, the topside ionospheric irregularities appeared to be more intensive in the southern hemisphere, which is, most likely, due to seasonal variations in the interhemispheric field-aligned currents system. An analysis of multi-instrumental observations reveals reinforcement of the equatorial ionization anomaly after sunset in Atlantic sector on 30 August and formation of the significant plasma depletions and irregularities over a large longitudinal range. Equatorial irregularities were also found in the morning sector at the recovery phase of the storm. In addition to low Earth orbit (LEO) GPS measurements, we analyze the LEO in situ measurements, and we show that these two techniques cannot be interchangeable in all cases because of the altitudinal extent of plasma irregularities. Overall, we demonstrate that the LEO GPS technique can serve a useful tool for detection of the topside ionospheric irregularities during space weather events and may essentially contribute to other methods based on various instruments.

Opposite hemispheric asymmetries during the ionospheric storm of 29–31 August 2004

By making use of multiple ground-based and spaceborne instruments, we study ionospheric and thermospheric behavior during the moderately intense geomagnetic storm of 29–31 August 2004 (minimum Dst excursion of A128 nT). Although this storm was far from the strongest in solar cycle 23, it provoked quite interesting effects in the ionosphere, such as opposite hemispheric asymmetries in the ionospheric F layer and in the topside ionosphere and a development of the ionospheric superfountain effect in the postsunset sector. Data from ground-based GPS receivers and ionosondes revealed large increase in total electron content (TEC) and in N m F 2 in the southern hemisphere, whereas in the northern hemisphere, very weak or no effect was observed. On the contrary, the topside measurements indicated the occurrence of a positive storm in the northern hemisphere. Overall, the strongest storm time disturbances were observed in the postsunset sector (~20:30–21:30 LT), where satellite radar altimeters TOPEX and Jason 1, along with the CHAMP satellite showed ~250–400% TEC increase in the middle-and low-latitude regions. The signatures of the ionospheric plasma enhancement were seen up to the height of the Defense Meteorological Satellite Program (DMSP) satellites (~840 km). As for the thermospheric storm, data of the Gravity Recovery and Climate Experiment (GRACE) satellite mission revealed no asymmetry in neutral density data in the evening sector (~17 UT); however, very strong hemispheric asymmetry was observed in the postsunset sector by CHAMP (~21 UT). Overall, neutral density increase in the postsunset sector was found to be much stronger than in the evening sector.

Early morning irregularities detected with spaceborne GPS measurements in the topside ionosphere: A multisatellite case study

We present observations of the equatorial plasma bubbles (EPB) in the topside ionosphere at early morning hours (05–08 LT) in the recovery phase of the 18–19 February 2014 geomagnetic storm. This rare type of irregularities was detected in the Pacific sector using GPS measurements on board several low-Earth-orbit (LEO) satellites. We use a multisatellite constellation consisted of the three Swarm and one TerraSAR-X satellites, that on 19 February flew in the same region and at similar altitudes ~500 km. The EPB occurrence in the LEO GPS data was observed for several consecutive orbits from ~11 UT to 16–17 UT on 19 February 2014, which suggests the following: (1) rather long duration (hours) of favorable conditions for EPB generation, (2) formation and evolution of EPB over wide longitude range of the Pacific Ocean, and (3) possible movement of the EPB region in the westward direction (with dawn). Registration of the early morning EPB in LEO GPS data was supported by concurrent in situ (Swarm and DMSP (Defense Meteorological Satellite Program)) and ground-based (ionosonde and GPS) measurements. LEO-based GPS technique is found to be essential and promising data source to study the topside EPB over regions with lack of the ground-based facilities. In addition, we use the Prompt Penetration Model and the Thermosphere-Ionosphere Electrodynamics Global Circulation Model (TIE-GCM) to identify the possible mechanisms responsible for the observed phenomenon. The model simulation results indicate the occurrence of the zone with the enhanced vertical plasma drift (~40–45 m/s) owing to the disturbance dynamo action in the predawn/dawn sector during 09–17 UT.

GPS & GLONASS observations of large‐scale traveling ionospheric disturbances during the 2015 St. Patrick's Day storm

Using a comprehensive database of ~5300 ground-based GNSS stations we have investigated large-scale traveling ionospheric disturbances (LSTIDs) during 17-18 March 2015 (St. Patricks Day storm). For the first time, the high resolution two-dimensional maps of the total electron content (TEC) perturbation were made using not only GPS but also GLONASS measurements. Several LSTIDs originated from the auroral regions in the Northern and Southern Hemispheres were observed simultaneously over Europe, North America and South America. This storm is considered as a two-step main phase storm. During the first main phase LSTIDs propagated over the whole daytime European region and over high latitudes of North America. During the second main phase we report: 1) intense LSTIDs propagated equatorward in North America and Europe; 2) convergence of several LSTIDs originated from the opposite hemispheres in the interference zone over geomagnetic equator in South America; 3) “super” LSTIDs with the wavefront length of more than 10000 km observed simultaneously in North America and Europe. LSTIDs observed in three sectors had wavelength of ~1200-2500 km and wave periods of ~50-80 min. During the recovery phase on the background of the negative ionospheric storm developed over North America we detect signatures of the stream-like structures alongated within the latitudinal range of 29°N-42°N across the USA. These structures persisted through the nighttime to the early morning from 04 UT till 13 UT on 18 March 2015 and they were associated with the subauroral polarization streams (SAPS)-induced nighttime ionospheric flows.

Global Ionospheric and Thermospheric Effects of the June 2015 Geomagnetic Disturbances: Multi‐Instrumental Observations and Modeling

Abstract By using data from multiple instruments, we investigate ionospheric/thermospheric behavior during the period from 21 to 23 June 2015, when three interplanetary shocks (IS) of different intensities arrived at Earth. The first IS was registered at 16:45 UT on 21 June and caused ~50 nT increase in the SYM‐H index. The second IS arrived at 5:45 UT on 22 June and induced an enhancement of the auroral/substorm activity that led to rapid increase of thermospheric neutral mass density and ionospheric vertical total electron content at high latitudes. Several hours later, topside electron content and electron density increased at low latitudes on the nightside. The third and much larger IS arrived at 18:30 UT on 22 June and initiated a major geomagnetic storm that lasted for many hours. The storm provoked significant effects in the thermosphere and ionosphere on both dayside and nightside. In the thermosphere, the dayside neutral mass density exceeded the quiet time levels by 300–500%, with stronger effects in the summer hemisphere. In the ionosphere, both positive and negative storm effects were observed on both dayside and nightside. We compared the ionospheric observations with simulations by the coupled Sami3 is Also a Model of the Ionosphere/Rice Convection Model (SAMI3/RCM) model. We find rather good agreement between the data and the model for the first phase of the storm, when the prompt penetration electric field (PPEF) was the principal driver. At the end of the storm main phase, when the ionospheric effects were, most likely, driven by a combination of PPEF and thermospheric winds, the modeling results agree less with the observations.

Ionospheric detection and localization of volcano eruptions on the example of the April 2015 Calbuco events

Using data from ground-based GNSS-receivers located in southern Chile, we study the ionospheric total electron content (TEC) response to two eruptions of the Calbuco volcano that occurred on 22-23 April 2015. In both cases, the TEC response showed quasi-periodic signals with several consecutive wave-trains. The averaged amplitude of the observed co-volcanic TEC-perturbations amounted 0.45 TECU for the first eruption and 0.16 TECU for the second one. We compare amplitudes of the TEC response to volcano eruptions of different intensity from our and previously published data, and we show that both the intensity and the background ionospheric conditions define the amplitude of ionospheric co-volcanic disturbances (CVID). The relative contribution, however, scales with the eruption intensity. The travel-time diagrams allowed to estimate the propagation speed of the observed co-volcanic TEC perturbations as ~900-1200 m/s, which is close to the acoustic (or shock-acoustic) waves speed at the ionospheric height. The spectrograms are consistent with the conclusion on the acoustic nature of the observed TEC perturbations. Finally, we use the approximation of a spherical wave propagating at a constant velocity from a point source and, for the first time, we calculate the location of the volcanic source and the onset time of the volcano eruption from ionospheric measurements. We show that even from 30-sec ionospheric GPS data it is possible to “localize” the eruptive source within several degrees of latitude/longitude.