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Register a new userIonosphere-thermosphere global time response to geomagnetic storms
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In this study, we investigate thermospheric neutral mass density heating associated with 168 CME-driven geomagnetic storms in the period of May 2001 to September 2011. We use neutral density measured by two low-Earth orbit satellites: CHAMP and GRACE. For each storm, we superpose geomagnetic and density data for the time when the IMF B$_mathrm{z}$ component turns sharply southward chosen as the zero epoch time. This indicates the storm main phase onset. We find that the average SYM-H index reaches the minimum of $-$42 nT near 12 hours after storm main phase onset. The Joule heating is enhanced by approximately 200% in comparison to quiet values. In respect to thermosphere density, on average, high latitude regions (auroral zones and polar caps) of both hemispheres are highly heated in the first 1.5 hour of the storm. The equatorial response is presumably associated with direct equator-ward propagation of TADs (traveling atmospheric disturbances). A slight north-south asymmetry in thermosphere heating is found and is most likely due to a positive B$_mathrm{y}$ component in the first hours of the storm main phase.
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We have performed an analysis of case events and statistics of positive ionospheric storms in the dayside region of the equatorial ionization anomaly during recurrent geomagnetic storms (RGSs), which dominate in geomagnetic and ionospheric conditions on the declining phase of solar activity in 2004 to 2008. It is shown that total electron content (TEC) has a tendency to minimize before the beginning of RGSs and to peak 3 to 4 days after, i.e. on the RGS recovery phase produced by high-intensity long-duration continuous auroral activity. The maximum of TEC coincides with the maximum of solar wind velocity within high-speed solar wind streams. An analysis of electron content vertical profiles, derived from two independent methods using ionosondes and COSMIC/FORMOSAT-3 radio occultation, showed that in the maximum of an ionospheric storm on 28 March 2008, the F2 layer thickens, NmF2 increases by ~50% and hmF2 elevates by a few tens of kilometers. The response of positive ionospheric storms to solar, heliospheric and geomagnetic drivers reveals a prominent longitudinal asymmetry. In the longitudinal range from -90 deg to 90 deg, the solar illumination plays a major role, and in the range from 90 deg to -120 deg, the influence of heliospheric and geomagnetic drivers becomes significant. The highest correlations of the TEC enhancements with the heliospheric and geomagnetic drivers were found during December - February period (r increased from ~ 0.3 to ~0.5). We speculate that the dynamics controlling this might result from an effect of solar zenith angle, storm-time effects of thermospheric Sum(O/N2) enhancement, and penetrating electric fields of interplanetary and magnetospheric origin.
Low-energy ions of ionospheric origin constitute a significant contributor to the magnetospheric plasma population. Measuring cold ions is difficult though. Observations have to be done at sufficiently high altitudes and typically in regions of space where spacecraft attain a positive charge due to solar illumination. Cold ions are therefore shielded from the satellite particle detectors. Furthermore, spacecraft can only cover key regions of ion outflow during segments of their orbit, so additional complications arise if continuous longtime observations, such as during a geomagnetic storm, are needed. In this paper we suggest a new approach, based on a combination of synoptic observations and a novel technique to estimate the flux and total outflow during the various phases of geomagnetic storms. Our results indicate large variations in both outflow rates and transport throughout the storm. Prior to the storm main phase, outflow rates are moderate, and the cold ions are mainly emanating from moderately sized polar cap regions. Throughout the main phase of the storm, outflow rates increase and the polar cap source regions expand. Furthermore, faster transport, resulting from enhanced convection, leads to a much larger supply of cold ions to the near-Earth region during geomagnetic storms.
Geomagnetically induced currents (GICs) are a well-known terrestrial space weather hazard. They occur in power transmission networks and are known to have adverse effects in both high and mid-latitude countries. Here, we study GICs in the Irish power transmission network (geomagnetic latitude 54.7--58.5$^{circ}$ N) during five geomagnetic storms (06-07 March 2016, 20-21 December 2015, 17-18 March 2015, 29-31 October 2003 and 13-14 March 1989). We simulate electric fields using a plane wave method together with two ground resistivity models, one of which is derived from magnetotelluric measurements (MT model). We then calculate GICs in the 220, 275 and 400~kV transmission network. During the largest of the storm periods studied, the peak electric field was calculated to be as large as 3.8~V~kmtextsuperscript{-1}, with associated GICs of up to 23~A using our MT model. Using our homogenous resistivity model, those peak values were 1.46~V~kmtextsuperscript{-1} and 25.8~A. We find that three 400 and 275~kV substations are the most likely locations for the Irish transformers to experience large GICs.
Geomagnetic activity is often described using summary indices to summarize the likelihood of space weather impacts, as well as when parameterizing space weather models. The geomagnetic index $text{K}_text{p}$ in particular, is widely used for these purposes. Current state-of-the-art forecast models provide deterministic $text{K}_text{p}$ predictions using a variety of methods -- including empirically-derived functions, physics-based models, and neural networks -- but do not provide uncertainty estimates associated with the forecast. This paper provides a sample methodology to generate a 3-hour-ahead $text{K}_text{p}$ prediction with uncertainty bounds and from this provide a probabilistic geomagnetic storm forecast. Specifically, we have used a two-layered architecture to separately predict storm ($text{K}_text{p}geq 5^-$) and non-storm cases. As solar wind-driven models are limited in their ability to predict the onset of transient-driven activity we also introduce a model variant using solar X-ray flux to assess whether simple models including proxies for solar activity can improve the predictions of geomagnetic storm activity with lead times longer than the L1-to-Earth propagation time. By comparing the performance of these models we show that including operationally-available information about solar irradiance enhances the ability of predictive models to capture the onset of geomagnetic storms and that this can be achieved while also enabling probabilistic forecasts.
Eruptive events of solar activity often trigger abrupt variations of the geomagnetic field. Through the induction of electric currents, human infrastructures are also affected, namely the equipment of electric power transmission networks. It was shown in past studies that the rate of power-grid anomalies may increase after an exposure to strong geomagnetically induced currents. We search for a rapid response of devices in the Czech electric distribution grid to disturbed days of high geomagnetic activity. Such disturbed days are described either by the cumulative storm-time $Dst$ or $d(textit{SYM-H})/dt$ low-latitude indices mainly influenced by ring current variations, by the cumulative $AE$ high-latitude index measuring substorm-related auroral current variations, or by the cumulative $ap$ mid-latitude index measuring both ring and auroral current variations. We use superposed epoch analysis to identify possible increases of anomaly rates during and after such disturbed days. We show that in the case of abundant series of anomalies on power lines, the anomaly rate increases significantly immediately (within 1 day) after the onset of geomagnetic storms. In the case of transformers, the increase of the anomaly rate is generally delayed by 2--3 days. We also find that transformers and some electric substations seem to be sensitive to a prolonged exposure to substorms, with a delayed increase of anomalies. Overall, we show that in the 5-day period following the commencement of geomagnetic activity there is an approximately 5--10% increase in the recorded anomalies in the Czech power grid and thus this fraction of anomalies is probably related to an exposure to GICs.
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