Irina Zakharenkova

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.

Physical Mechanism and Mathematical Modeling of Earthquake Ionospheric Precursors Registered in Total Electron Content

The physical mechanism by which the regions with increased or decreased total electron content, registered by measuring delays of GPS satellite signals before strong earthquakes, originate in the ionosphere has been proposed. Vertical plasma transfer in the ionospheric F2 region under the action of the zonal electric field is the main disturbance formation factor. This field should be eastward, generating the upward component of plasma electromagnetic drift, in the cases of increased total electron content at midlatitudes and deepened minimum of the F2 layer equatorial anomaly. Upward plasma drift increases electron density due to a decrease in the O+ ion loss rate at midlatitudes and decreases this density above the equator due to an enhancement of the fountain effect (plasma discharge into the equatorial anomaly crests). The pattern of the spatial distribution of the seismogenic electric field potential has been proposed. The eastward electric field can exist in the epicentral region only if positive and negative electric charges are located at the western and eastern boundaries of this region, respectively. The effectiveness of the proposed mechanism was studied by modeling the ionospheric response to the action of the electric field generated by such a charge configuration. The results of the numerical computations indicated that the total electron content before strong earthquakes at middle and low latitudes is in good agreement with the observations.

Dynamics of the high‐latitude ionospheric irregularities during the 17 March 2015 St. Patrick's Day storm: Ground‐based GPS measurements

We report first results on the study of the high-latitude ionospheric irregularities observed in worldwide GPS data during the St. Patricks Day geomagnetic storm (17 March 2015). Multisite GPS observations from more than 2500 ground-based GPS stations were used to analyze the dynamics of the ionospheric irregularities in the Northern and Southern Hemispheres. The most intense ionospheric irregularities lasted for more than 24 h starting at 07 UT of 17 March. This period correlates well with an increase of the auroral Hemispheric Power index. We find hemispheric asymmetries in the intensity and spatial structure of the ionospheric irregularities. Over North America, the ionospheric irregularities zone expanded equatorward below ~45°N geographic latitude. Additionally, the strong midlatitude and high-latitude GPS phase irregularities in the auroral oval were found to be related to the formation of storm enhanced density and deepening of the main ionospheric trough through upper atmosphere ionization by energetic particle precipitation. Significant increases in the intensity of the irregularities within the polar cap region of both hemispheres were associated with the formation and evolution of the storm enhanced density/tongue of ionization structures and polar patches.

Observation of the ionospheric irregularities over the Northern Hemisphere: Methodology and service

Observation and analysis of the ionospheric irregularities at the high latitudes using GPS measurements represent very actual task for both scientific point of view and Global Navigation Satellite Systems (GNSS) applications, as the occurrence of the ionospheric irregularities can impact a variety of communication and navigation systems. In this paper we describe methodology and service for continuous generation of high-resolution maps of the ionospheric irregularities. To observe the high-latitude ionospheric irregularities, data collected from three ground-based GPS networks of the Northern Hemisphere are processed and analyzed. Here we used parameters ROT (rate of total electron content (TEC) change) and ROTI (index of ROT) to study the occurrence of TEC fluctuations Pi et al. (1997). ROTI maps are constructed with the grid of 2° × 2° resolution as a function of the magnetic local time and corrected magnetic latitude. The ROTI maps allow to estimate the overall fluctuation activity and auroral oval evolutions, in general, the ROTI values are corresponded to the probability of the GPS signals phase fluctuations. We demonstrate that the occurrence and magnitude of TEC fluctuations, measured using GNSS networks, increase dramatically during space weather events. The irregularities oval expands considerably equatorward with simultaneous increase of the fluctuation intensity.

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.

Dependence of the high-latitude plasma irregularities on the auroral activity indices: a case study of 17 March 2015 geomagnetic storm

The magnetosphere substorm plays a crucial role in the solar wind energy dissipation into the ionosphere. We report on the intensity of the high-latitude ionospheric irregularities during one of the largest storms of the current solar cycle—the St. Patrick’s Day storm of 17 March 2015. The database of more than 2500 ground-based Global Positioning System (GPS) receivers was used to estimate the irregularities occurrence and dynamics over the auroral region of the Northern Hemisphere. We analyze the dependence of the GPS-detected ionospheric irregularities on the auroral activity. The development and intensity of the high-latitude irregularities during this geomagnetic storm reveal a high correlation with the auroral hemispheric power and auroral electrojet indices (0.84 and 0.79, respectively). Besides the ionospheric irregularities caused by particle precipitation inside the polar cap region, evidences of other irregularities related to the storm enhanced density (SED), formed at mid-latitudes and its further transportation in the form of tongue of ionization (TOI) towards and across the polar cap, are presented. We highlight the importance accounting contribution of ionospheric irregularities not directly related with particle precipitation in overall irregularities distribution and intensity.

Physical interpretation and mathematical simulation of ionospheric precursors of earthquakes at midlatitudes

The paper presents the results of studying anomalous variations in the total electron content (TEC) of the ionosphere as probable precursors of strong seismic events. The vertical drift of the F2 layer’s ionospheric plasma under the effect of seismically generated zonal electric field is considered as a likely reason for the observed variations in the TEC. An estimation of this drift effects is made by mathematical simulation utilizing the global numerical model of the Earth’s upper atmosphere (UAM). Midlatitude ionospheric effects were simulated. Two types of seismogenerated electric fields (dipole and monopole) were used with various magnitudes and spatial configurations. The derived results were compared with the TEC data of GPS observations from the IGS for the Kitira earthquake in southern Greece (January 8, 2006; M 6.8). It was shown that variations generated by additional sources of the dipole type are consistent with the observed data; monopole-type sources did not reproduce some typical peculiarities of these observations and systematically underestimated the deviation value.

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.

How can GOCE and TerraSAR‐X contribute to the topside ionosphere and plasmasphere research?

In this study we analyze concurrent observations derived from two low-Earth orbit satellite missions—the GOCE (Gravity field and steady state Ocean Circulation Explorer) satellite with unprecedented low orbit of 250 km and the TerraSAR-X satellite with orbit altitude of 515 km. The science payloads of both satellites have no instruments for ionospheric research, but they include a dual-frequency GPS (Global Positioning System) receiver. GPS measurements from the precise orbit determination GPS antenna on board these satellites can be used to determine TEC (total electron content) at the topside ionosphere-plasmasphere system for different conditions. Results derived from these missions are presented for the first time. The comparison was done for June and December solstice conditions at low and moderate solar activity levels for quiet geomagnetic activity. We obtain the quantitative estimates of electron content and its percentage contribution to the ground-based GPS TEC for topside ionosphere/plasmasphere at altitudes 250–500 km and above 500 km. Similarities and differences in such electron content distribution on a global scale are discussed. Distinctive features of the Weddell Sea Anomaly and equatorial ionization anomaly are evident in the topside TEC derived from GOCE GPS data. We report the Weddell Sea Anomaly presence in TEC at early morning local time.

Observation of the ionospheric storm of October 11, 2008 using FORMOSAT-3/COSMIC data

The electron density profiles retrieved from the COSMIC radio occultation measurements were examined in order to estimate the possibility of its use as additional data source to study changes in electron density distribution occurred during ionospheric storms. The ionosphere behaviour during moderate geomagnetic storm which occurred on October 11, 2008 was analysed. The short-duration positive effect was revealed distinctly in GPS TEC and ionosonde measurements. For the European mid-latitude region it reached the factor of 2 or more relative to the undisturbed conditions. COSMIC data were analyzed and their validity was tested by comparison with ground-based measurements. It was shown the good agreement between independent measurements both in quiet and disturbed conditions. Analysis of COSMIC-derived electron density profiles revealed changes of the bottom-side and topside parts of the ionosphere.