Pre-storm Enhancement Research Articles

Time delay of ionospheric TEC storms to geomagnetic storms and pre-storm disturbance events in East Asia

Using the TEC data at Beijing (39.61°N, 115.89°E)/Yakutsk (62.03°N, 129.68°E) stations of East Asia regions and relevant geomagnetic data from 2010 to 2017, we have studied the time delay of ionospheric storms to geomagnetic storms and compare it with our previous results of Taoyuan (25.02°N, 121.21°E) station (Zhang et al., 2020). The data shows a well-known local time dependence of the time delay, and seasonal dependences are different at these stations. In addition, there is no correlation between the time delay and the magnetic storm intensity /solar activity, except the time delay of negative storms has weakly negative dependence on the solar activity. Comparing with the results of Taoyuan station which is located at EIA region in East Asia, we find that the time delay increases nonlinearly as the latitude decreases due to different ionospheric backgrounds at these places. Moreover, the pre-storm disturbance events are found to have similar statistical characteristics as the pre-storm enhancement in Europe middle latitudes (Burešová and Laštovička, 2007). By subtracting the common features of the pre-storm disturbance events, we preliminarily infer that auroral activity might be main driver of the pre-storm disturbance events.

Statistical Analysis and Interpretation of High-, Mid- and Low-Latitude Responses in Regional Electron Content to Geomagnetic Storms

Geomagnetic storm is one of the most powerful factors affecting the state of the Earth’s ionosphere. Revealing the significance of formation mechanisms for ionospheric storms is still an unresolved problem. The purpose of the study is to obtain a statistical pattern of the response in regional electron content to geomagnetic storms on a global scale to interpret the results using the upper atmosphere model (the Global Self-consistent Model of the Thermosphere, Ionosphere, and Protonosphere), to make the detailed comparison with the thermospheric storm concept, and to compare the obtained pattern with results from previous statistical studies. The regional electron content is calculated based on the global ionospheric maps data, which allows us to cover the midlatitude and high-latitude zones of both hemispheres, as well as the equatorial zone. Most of the obtained statistical pattern agrees with the thermospheric storm concept and with the previous statistical studies: ionospheric responses at ionospheric storm main phases including their seasonal dependences for the high- and midlatitudes and some features of ionospheric responses at recovery phases. However, some of the statistical patterns are inconsistent with the thermospheric storm concept or contradicts the previous statistical studies: negative midlatitude ionospheric responses at recovery phases in the local winter, the domination of the spring response in the equatorial zone, seasonal features of the positive after-effects, the interhemispheric asymmetry of ionospheric responses, and the prestorm enhancement. We obtained that the contribution of electric field to the interpretation of the zonal and diurnal averaged storm-time regional electron content (REC) disturbances is insignificant. The positive after-storm effects at different latitudes are caused by n(O) disturbances.

Longitudinal Variation of the Ionospheric Response to the 26 August 2018 Geomagnetic Storm at Equatorial/Low Latitudes

We have studied the ionospheric response at near-equator latitudes during the geomagnetic storm of 26 August 2018, which was the strongest geomagnetic disturbance that year (minimum Dst value: −174 nT). For the analysis, we considered the F2-layer critical frequency (foF2) and peak height (hmF2), as well as total electron content (TEC) data for Jicamarca (geographic coordinates: 12° S, 283.2° E; geomagnetic coordinates: 2.26° S, 4.09° W), Saoluis (geographic coordinates: 2.6° S, 315.8° E; geomagnetic coordinates: 5.94° N, 28.5° E), Guam (geographic coordinates: 13.4° N, 144.8° E; geomagnetic coordinates: 5.73° N, 143.2° W) and Libreville (geographic coordinates: 0.39° N, 9.45° E; geomagnetic coordinates: 1.64° N, 82.6° W). First, we observed pre-storm improvements, which could be due to previous moderate geomagnetic activity. Second, only one station (Jicamarca) clearly revealed a prompt penetration electric field (PPEF) effect when Bz turned strongly negative during the initial phase of the storm. Over the stations Saoluis and Guam, a PPEF effect was not evident. These stations presented pre-storm enhancements in foF2. In this case study, disturbed dynamo electric fields appear not to have played a crucial role in increasing electron density near equatorial regions during the recovery phase because the observed disturbances did not correspond with those produced by these electric fields, that is, negative (positive) storm effects on the dayside (night). Third, the increases in electron density observed during recovery are most likely caused by neutral composition changes.

The response of the ionosphere over Ilorin to some geomagnetic storms

The effects of some geomagnetic storms on the F2 layer peak parameters over Ilorin, Nigeria (Lat. 8:53°N, Long. 4.5°E, dip angle, −2.96°) have been investigated. Our results showed that the highest intensity of the noon bite-out occurred during the March equinox and lowest during the June Solstice on quiet days. Quiet day NmF2 disturbances which appeared as a pre-storm enhancement, but not related to the magnetic storm event that followed were observed at this station. These enhancements were attributed to the modification of the equatorial electric field as a result of injection of the Auroral electric field to the low and equatorial ionosphere. For disturbed conditions, the morphology of the NmF2 on quiet days is altered. Daytime and nighttime NmF2 and hmF2 enhancements were recorded at this station. Decreases in NmF2 were also observed during the recovery periods, most of which appeared during the post-noon period, except the storm event of May 28–29. On the average, enhancements in NmF2 (i.e. Positive phases) are the prominent features of this station. Observations from this study also indicate that Dst, Ap and Kp which have been the most widely used indices in academic research in describing the behavior of geomagnetic storms, are not sufficient for storm time analysis in the equatorial and low latitude ionosphere.

Traveling ionospheric disturbances observed at South African midlatitudes during the 29–31 October 2003 geomagnetically disturbed period

This paper presents traveling ionospheric disturbances (TIDs) observations from GPS measurements over the South African region during the geomagnetically disturbed period of 29–31 October 2003. Two receiver arrays, which were along two distinct longitudinal sectors of about 18°-20° and 27°-28° were used in order to investigate the amplitude, periods and virtual propagation characteristics of the storm induced ionospheric disturbances. The study revealed a large sudden TEC increase on 28 October 2003, the day before the first of the two major storms studied here, that was recorded simultaneously by all the receivers used. This pre-storm enhancement was linked to an X-class solar flare, auroral/magnetospheric activities and vertical plasma drift, based on the behaviour of the geomagnetic storm and auroral indices as well as strong equatorial electrojet. Diurnal trends of the TEC and foF2 measurements revealed that the geomagnetic storm caused a negative ionospheric storm; these parameters were depleted between 29 and 31 October 2003. Large scale traveling ionospheric disturbances were observed on the days of the geomagnetic storms (29 and 31 October 2003), using line-of-sight vertical TEC (vTEC) measurements from individual satellites. Amplitude and dominant periods of these structures varied between 0.08–2.16 TECU, and 1.07–2.13h respectively. The wave structures were observed to propagate towards the equator with velocities between 587.04 and 1635.09m/s.

Ionospheric F-region response to geomagnetic disturbances

The F2-region reaction to geomagnetic storms usually called as an ionospheric storm is a rather complicated event. It consists of so called positive and negative phases, which have very complicated spatial and temporal behavior. The main morphological features of ionospheric storms and the main processes governing their behavior were understood at the end of the 1900s and described in a series of review papers. During the recent decade there were many publications dedicated to the problem of ionospheric storms. In this paper a concept of ionospheric storm morphology and physics formulated at the end of the 1990s is briefly summarized and the most interesting results obtained in the 2000s are described. It is shown that the main features of the studies of the previous decade were: the use of GPS TEC data for analyzing the ionospheric storm morphology, attraction of sophisticated theoretical models for studying the processes governing ionospheric behavior in disturbed conditions, and accent to analysis of ionospheric behavior during prominent events (very strong and great geomagnetic storms). Also a special attention was paid to the pre-storm enhancements in foF2 and TEC.

Pre-storm behaviour of NmF 2 and TEC (GPS) over equatorial and low latitude stations in the Indian sector

The pre-storm behavior of NmF 2 and TEC over an equatorial station, Trivandrum (8.47°N, 76.91°E, dip 0.6°S) and a low latitude station, Waltair (17.7°N,83.3°E, dip 20°N) has been studied for a total of 18 strong geomagnetic storms with DST ⩽ −100 nT. The simultaneous measurements of GPS-TEC and NmF 2 over Trivandrum and Waltair during the period 2000–2005 have been considered for the present study. It is found that there is a substantial increase in NmF 2 and TEC before the onset of the storm over Waltair, while the increase is not present at Trivandrum. The origin of pre-storm enhancements in electron density still remains unresolved owing to several conditions in their potential sources and occurrence mechanisms. In the present study an attempt is made to identify the possible mechanisms responsible for such enhancements in electron density of the F-region.

Pre-storm <I>Nm</I>F2 enhancements at middle latitudes: delusion or reality?

Abstract. A critical analysis of recent publications devoted to the NmF2 pre-storm enhancements is performed. There are no convincing arguments that the observed cases of NmF2 enhancements at middle and sub-auroral latitudes bear a relation to the following magnetic storms. In all cases considered the NmF2 pre-storm enhancements were due to previous geomagnetic storms, moderate auroral activity or they presented the class of positive quiet time events (Q-disturbances). Therefore, it is possible to conclude that there is no such an effect as the pre-storm NmF2 enhancement as a phenomenon inalienably related to the following magnetic storm. The observed nighttime NmF2 enhancements at sub-auroral latitudes may result from plasma transfer from the plasma ring area by meridional thermospheric wind. Enhanced plasmaspheric fluxes into the nighttime F2-region resulted from westward substorm-associated electric fields is another possible source of nighttime NmF2 enhancements. Daytime positive Q-disturbances occurring under very low geomagnetic activity level may be related to the dayside cusp activity.

Pre-storm electron density enhancements at middle latitudes

Substantial increases of the F2 region peak electron density several hours to a day before the geomagnetic storm onset, the so-called pre-storm enhancements, belong to still not clear and hardly predictable features of the ionospheric disturbances. This paper presents analysis of the pre-storm enhancements observed at middle latitudes for 15 storms out of 65 strong-to-severe geomagnetic storms of the period 1995–2005. All 15 events were accompanied by significant (>20%) increases of foF2 before the storm onset over European area. We focus on the longitudinal extent and height profile of the pre-storm enhancements, particularly on their effects on the F1 and E regions of the ionosphere. Possible origin of such enhancements is also partly discussed. We observe no systematic effect of pre-storm enhancements of foF2 in electron density profiles in the F1 region. The E region (foE) appears to be insensitive to pre-storm enhancements. We find the pre-storm enhancements to be confined to the F2 region. The longitudinal extent of the pre-storm enhancements seems to be 120–240° based on comparison of simultaneous foF2 measurements in Europe, northern USA, and Eastern Asia.

Prestorm enhancements in NmF2 and total electron content at low latitudes

The enhancement of electron concentrations in the ionosphere before geomagnetic storms is one of the open questions. Using ionosonde observations and total electron content (TEC) from Global Positioning System (GPS) measurements along longitude 120°E, we analyzed three low latitude pre‐storm enhancement events that occurred on 21 April (day 111) 2001, 29 May (day 149) 2003, and 22 September (day 265) 2001, respectively, in the Asia/Australia sector. All three events (and other two cases on 9 August 2000 and 10 May 2002) show quite similar features. The strong prestorm enhancements during these events are simultaneously presented in foF2 and TEC and enhancements have latitudinal dependence, tending to occur at low latitudes with maxima near the northern and southern equatorial ionization anomaly (EIA) crests and depletions in the equatorial region. This is quite different from what reported by Burešová and Laštovička (2007) for middle latitudes. They found no systemic latitudinal dependence in prestorm enhancements over Europe. It is argued that solar flares are not the main drivers for the enhancements, at least for low‐latitude events. Main features of low‐latitude prestorm enhancements do not coincide with the solar flare effects. We postulate that the vertical plasma drift or zonal electric field is a likely cause for the low‐latitude prestorm enhancements. Its existence is supported by the facts of stronger EIA, the latitudinal coverage of the enhancements as well as the lift of the F layer peak height at an equatorward station during the prestorm enhancements. Moreover, the behaviors of hmF2 at low latitudes during the prestorm enhancements may possibly be explained in terms of the coupling nature of parallel and perpendicular dynamics at low latitudes (see, e.g., Behnke and Harper, 1973; Rishbeth et al., 1978).

Pre-storm enhancements of foF2 above Europe

In spite of many years of investigations of effects of geomagnetic storms on the F region ionosphere, still many open questions remain, one of them being the pre-storm enhancements of NmF2, the maximum electron concentration in the ionosphere. All 65 strong geomagnetic storms observed over the period 1995–2005 are analyzed as for the occurrence of the pre-storm enhancements over Europe using data of eight ionosonde stations covering European area. About 20–25% of storms are accompanied by sufficiently strong pre-storm enhancements. The pre-storm enhancements occur both day and night, more often in summer half of the year, do not exhibit a systematic latitudinal dependence, and are not accompanied by a corresponding change of hmF2. Several constraints for their mechanism were established, several potential sources were excluded, but the origin of pre-storm enhancements remains to be uncovered.