Negative Ionospheric Storm Effects Research Articles

Direct observational evidence for disturbance dynamo on the daytime low‐latitude ionosphere: A case study based on the 28 June 2013 space weather event

AbstractA case of the westward disturbance dynamo (DD) electric field, influencing the daytime equatorial and low‐latitude ionosphere, during a geomagnetic storm that occurred on 28–29 June 2013 is presented. The GPS total electron content (TEC) observations from a network of stations in the Indian equatorial, low and middle latitude regions along with the radio beacon TEC, ionosonde, and magnetic field observations are used to study the storm time behavior of the ionosphere. Negative ionospheric storm effects were seen over the low and middle latitudes during the storm time due to the presence of a westward DD electric field. Observations show that the suppression of the equatorial ionization anomaly (EIA) from the morning hours itself on 29 June 2013 took place due to the prevailing westward DD electric field, providing evidence for the model calculations by Balan et al. (2013). Simulations using the GITM model also agree well with our results. The present study gains importance as the direct observational evidences for disturbance dynamo effects on the daytime low‐latitude ionosphere and the EIA are sparse, as it has been difficult to delineate it from the compositional disturbances.

Long‐lasting negative ionospheric storm effects in low and middle latitudes during the recovery phase of the 17 March 2013 geomagnetic storm

AbstractIn this paper, an ionospheric electron density reanalysis algorithm was used to generate global optimized electron density during the 17–18 March 2013 geomagnetic storm by assimilating ~10 low Earth orbit satellites based and ~450 ground global navigation satellite system receiver‐based total electron content into a background ionospheric model. The reanalyzed electron density could identify the large‐scale ionospheric features quite well during storm time, including the storm‐enhanced density, the positive ionospheric storm effect during the initial and main phases, and the negative ionospheric storm effect during the recovery phase. The simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model can reproduce similar large‐scale ionospheric disturbances as seen in the reanalysis results. Both the reanalysis and simulations show long‐lasting (>17 h) daytime negative storm effect over the Asia sector as well as hemispheric asymmetry during the recovery phase. Detailed analysis of the Global Ultraviolet Imager‐derived O/N2 ratio and model simulations indicate that the polar ward meridional wind disturbance, the downward E × B drift disturbance and O/N2 depletion might be responsible for the negative storm effect. The hemispheric asymmetry is mainly caused by the geomagnetic field line configuration, which could cause hemispheric asymmetry in the O/N2 depletion.

Solar wind driving of ionosphere‐thermosphere responses in three storms near St. Patrick's Day in 2012, 2013, and 2015

AbstractWe identify interplanetary plasma regions associated with three intense interplanetary coronal mass ejections (ICMEs)‐driven geomagnetic storm intervals which occurred around the same time of the year: day of year 74–79 (March) of 2012, 2013, and 2015. We show that differences in solar wind drivers lead to different dynamical ionosphere‐thermosphere (IT) responses and to different preconditioning of the IT system. We introduce a new hourly based global metric for average low‐latitude and northern middle‐latitude vertical total electron content responses in the morning, afternoon, and evening local time ranges, derived from measurements from globally distributed Global Navigation Satellite System ground stations. Our novel technique of estimating nitric oxide (NO) cooling radiation in 11° latitudinal zones is based on Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED)/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measurements. The thermospheric cooling throughout the storm phases is studied with this high latitudinal resolution for the first time. Additionally, TIMED/Global Ultraviolet Imager (GUVI) observations of the dynamical response of the thermospheric composition (O/N2 ratio) are utilized to study negative ionospheric storm effects. Based on these data sets, we describe and quantify distinct IT responses to driving by ICME sheaths, magnetic clouds, coronal loop remnants, plasma discontinuities, and high‐speed streams following ICMEs. Our analysis of coupling functions indicates strong connection between coupling with the solar wind and IT system response in ICME‐type storms and also some differences. Knowledge of interplanetary features is crucial for understanding IT storm dynamics.

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

AbstractWe present the first multi‐instrumental results on the ionospheric response to the geomagnetic storm of 17–18 March 2015 (the St. Patrick's Day storm) that was up to now the strongest in the 24th solar cycle (minimum SYM‐H value of −233 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/N2 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.

Statistical analysis of the ionospheric response during geomagnetic storm conditions over South Africa using ionosonde and GPS data

AbstractThis paper presents a statistical analysis of ionospheric response over ionosonde stations Grahamstown (33.3°S, 26.5°E, geographic) and Madimbo (22.4°S, 30.9°E, geographic), South Africa, during geomagnetic storm conditions which occurred during the period 1996–2011. Such a climatological study is important in establishing local ionospheric behavior trend which later forms a basis for accurate modeling and forecasting electron density and critical frequency of the F2 layer (foF2) useful for high‐frequency communication. The analysis was done using foF2 and total electron content (TEC), and to identify the geomagnetically disturbed conditions, the Dst index with a storm criterion of Dst ≤−50 nT was used. Results show a strong solar cycle dependence with negative ionospheric storm effects following the solar cycle and positive ionospheric storm effects occurring most frequently during solar minimum. Seasonally, negative and positive ionospheric storm effects occurred most in summer (63.24%) and in winter (53.62%), respectively. An important finding is that only negative ionospheric storms were observed during great geomagnetic storm activity (Dst ≤−350 nT). For periods when both foF2 and TEC data (from colocated ionosonde and GPS receiver stations) were available, a similar response in terms of variational trend was observed. Hence, GPS data can be used to effectively identify the ionospheric response in the absence of ionosonde data.

A relationship between solar proton events, ionospheric uplift observed at VLF and negative ionospheric storms

Three consecutive magnetic storms during the month of September 1982 were found to be associated with solar proton events (SPE) observed over a number of high latitude VLF propagation paths. The penetration of solar protons into the auroral zone produced a marked reduction in reflection height at night for high latitude VLF paths resulting in a reduced diurnal phase shift. This effect has been known for some 50 years. However in this paper, a previously unidentified response is described consisting of an increase in the night 90km reflection height over middle latitude and transequatorial VLF paths. Solar protons do not penetrate to these latitudes and this slight increase in VLF reflection height was associated with typical negative ionospheric storm effects in the F2 region. Dynamics at the 90km base of the night ionosphere are little known and difficult to investigate except at VLF. These results are the first to suggest a response of the night ionospheric base to events leading to the well known negative ionospheric storm seen at greater heights. Such negative storms seen in the F2 region have been associated with an equatorward wind surge and change in neutral atmospheric chemistry driven by joule heating in the auroral zone produced by solar proton precipitation.

Multi-station observation of ionospheric irregularities over South Africa during strong geomagnetic storms

This paper presents results pertaining to the response of the mid-latitude ionosphere to strong geomagnetic storms that occurred from 31 March to 02 April 2001 and 07–09 September 2002. The results are based on (i) Global Positioning Systems (GPSs) derived total electron content (TEC) variations accompanying the storm, (ii) ionosonde measurements of the ionospheric electrodynamic response towards the storms and (iii) effect of storm induced travelling ionospheric disturbances (TIDs) on GPS derived TEC. Ionospheric data comprising of ionospheric TEC obtained from GPS measurements, ionograms, solar wind data obtained from Advanced Composition Explorer (ACE) and magnetic data from ground based magnetometers were used in this study. Storm induced features in vertical TEC (VTEC) have been obtained and compared with the mean VTEC of quiet days. The response of the mid-latitude ionosphere during the two storm periods examined may be characterised in terms of increased or decreased level of VTEC, wave-like structures in VTEC perturbation and sudden enhancement in hmF2 and h′F. The study reveals both positive and negative ionospheric storm effects on the ionosphere over South Africa during the two strong storm conditions. These ionospheric features have been mainly attributed to the travelling ionospheric disturbances (TIDs) as the driving mechanism for the irregularities causing the perturbations observed. TEC perturbations due to the irregularities encountered by the satellites were observed on satellites with pseudo random numbers (PRNs) 15, 17, 18 and 23 between 17:00 and 23:00 UT on 07 September 2002.

Neutral composition effects on ionospheric storms at middle and low latitudes

The two‐dimensional structure of thermospheric neutral composition, specifically, the atomic oxygen to molecular nitrogen column density ratio, [O/N2], is studied during the 17–24 April 2002 geomagnetic storms to understand the cause of ionospheric storms in regions equatorward of the auroral oval on an instantaneous large scale. The [O/N2] ratio is derived from the dayglow emission ratio of O I 1356 Å to N2 Lyman‐Birge‐Hopfield (1600–1800 Å) acquired from the Polar ultraviolet imager (UVI) and the total electron content (TEC), which is used to infer ionospheric storms, is derived from the phase delays of dual‐band global positioning satellite (GPS) accumulated around the globe. It is found that the regions of decreasing [O/N2] generally coincided with the regions of depleted TEC during and after the development of the storms. This is consistent with previous theoretical and experimental analysis in which composition changes play a major role in the negative ionospheric storm effects. At lower latitudes, long‐lived positive storm effects predicted by empirical and general circulation models were not observed. In fact, there was no noticeable change in [O/N2]. The TEC data also showed no noticeable change, except a few “short‐lived,” localized positive TEC perturbations. For this particular event, the equatorward expansion of the decreased [O/N2] at the onset of the first storms was estimated to be more than 600 m/s, much faster than the typical thermospheric wind, and reached ∼30°N in 2 hours much equatorward (>25° MLAT) beyond the auroral electrojets. This result suggests that midlatitude (negative) ionospheric storms are caused by direct equatorward penetration of a reduced thermospheric [O/N2], at least during the first few hours of storms.

Modeling negative ionospheric storm effects caused by thermospheric disturbances observed in satellite UV images

The ionospheric behavior over Podkamennaya, Russia, during the geomagnetic storm of October 14, 1981, is modeled subject to the constraints provided by ƒoF2 measurements and by the thermospheric O/N2 ratio deduced from a DE 1 FUV image taken at 0540 UT. The (nonunique) diurnal variation of O/N2 deduced from the model is consistent with the thermospheric composition disturbance zone observed in the DE 1 image having moved over Podkamennaya during the early morning hours, producing a reduction of O/N2 (referred to a column density of N2 of 1017 cm−2) to a value of less than 0.2 before recovering to a value of 0.6 during the afternoon hours. Although the data are insufficient to provide a unique solution, the results demonstrate the utility of satellite‐based FUV images in the study of thermospheric and ionospheric disturbances produced by geomagnetic storms.

Negative ionospheric storm coincident with DE 1‐observed thermospheric disturbance on October 14, 1981

The DE 1 far ultraviolet (FUV) imaging instrument recorded a large thermospheric disturbance over Europe and Russia on October 14, 1981, during a geomagnetic storm commencing near the end of the previous day. The disturbance is quantified in terms of O/N2, the column density ratio referenced to a fixed depth in N2. The ratio is derived from DE 1 dayglow observations with a filter whose signal transmission is dominated by O I 130.4‐nm emission. Within the disturbed region, O/N2 is less than that in surrounding regions, with minimum values being more than a factor of 2 smaller than undisturbed values. Images of O/N2 are compared to ground‐based ionosonde data from 13 sites, some within and others outside of the disturbed region. The data are in the form of Nmax, the maximum electron density, which in the presence of an F2 layer is equivalent to NmF2. The data exhibit negative ionospheric storm effects (reduced Nmax) within the disturbed region and normal behavior elsewhere. The degree of correlation between O/N2 and Nmax suggests that the approximate geographical extent of dayside regions experiencing negative ionospheric storm effects can be determined from global FUV imaging of the disturbed and surrounding regions. The analysis is extended to the following day to address the recovery of the disturbed region. The O/N2 ratio has returned to undisturbed values except in a smaller disturbance region containing weak reductions in O/N2. This is either a remnant of the strong disturbance from the day before or a newly formed disturbance from substorm activity several hours prior to the new observations. Only the ionosonde sites within this region recorded values of Nmax below their monthly median values.

Study of the thermospheric-ionospheric response to intense geomagnetic storms at middle latitudes

The ionospheric response to enhanced geomagnetic activity was studied in terms of two ionospheric parameters, foF2 and h'F2, aiming the investigation of the middle latitude positive and negative ionospheric storm effects characteristics. Simultaneous observations from a chain of several middle latitude stations distributed in longitude around the earth during two intense geomagnetic storms (Dst < −100 nT), were analyzed in this work. The correlation results between the foF2 and h'F2 disturbances and the Dst geomagnetic index verified that the morphology of middle latitude ionospheric disturbances in response to geomagnetic storms shows a great degree of variability and a strong dependence on local time observation. Negative deviations constitute the typical ionospheric response. In general it is reported that the F2 layer virtual height h'F2, which is observed only during daytime in conjunction with F1 layer presence, shows clear response to magnetic storm activity. On the other hand, positive ionospheric disturbances are accompanied by weak height enhancements. Real observations were also compared to the thermospheric-ionospheric view associated with positive and negative storm effects proposed by Prolss (1993). This test clearly demonstrates that such a model can capture most of the basic aspects of ionospheric storms, taking into consideration that any new burst of activity is associated to a new disturbance zone, resulting to the formation of a large zone of disturbance that covers the post-midnight and the forenoon sector as well. This mechanism may be the cause of long lasting negative storm effects observed as the typical ionospheric response to geomagnetic storm activity.

Positive and negative ionospheric disturbances at middle latitudes during geomagnetic storms

The morphology of middle latitude ionospheric disturbances in response to geomagnetic storms has been investigated to determine the phenomenological differences between positive and negative ionospheric storm effects, using foF2 observations from azimuthal chain of stations. To better organize the disturbance signatures, two ionospheric indices were introduced to describe the maximum positive (Dfu‐index) and negative (Dfl‐index) deviation observed during an ionospheric storm. A systematic appearance of nighttime positive effects was determined with a 24‐hour recurrence. The thermospheric‐ionospheric view associated with positive and negative storm effects proposed by Prolss (1993) was extended to encompass our observations. This test clearly demonstrates that such a model can capture most of the basic aspects of ionospheric storms, nevertheless the prominent feature of large nighttime enhancements in the ionization density have yet to be explained.

A theoretical study of thermospheric composition perturbations during an impulsive geomagnetic storm

The compositional response of the neutral thermosphere to an impulsive geomagnetic storm has been investigated using a numerical simulation made with the National Center for Atmospheric Research thermospheric general circulation model (NCAR‐TGCM). Calculated time‐dependent changes in neutral thermospheric composition have been studied, together with detailed neutral parcel trajectories and other diagnostic information from the model, to gain a greater understanding of the physical mechanisms responsible for composition variability during geomagnetic storms and, in particular, to investigate the causes of the positive and negative ionospheric storm effects. The following principal results were obtained from this study. (1) Calculated perturbations in thermospheric composition following the onset of an impulsive geomagnetic storm were found to be in good qualitative agreement with the previous experimental statistical study of storm time thermospheric morphology by Prölss (1981). (2) During the initial (onset) phase of the simulated storm, upward vertical winds occurred in the auroral zone and downward winds occurred in the central magnetic polar cap. (3) The largest perturbations in mass mixing ratio of nitrogen (ΨN2) at F region altitudes were found to be associated with parcels of neutral gas that travelled through the cusp region and with parcels that were trapped within the auroral zone for a long time. (4) Storm time enhancements in ΨN2 were found to occur in the midnight and early morning sectors both within and equatorward of the auroral zone, and these were determined to be associated with the advective effects of the large antisunward polar cap neutral winds. The physical size of the region of enhanced ΨN2 values was determined to be related to the time taken for individual parcels of air to corotate out of the large day‐to‐night thermospheric wind jet, and thus to the magnitudes of the antisunward polar cap winds. (5) Decreases in ΨN2 were found to occur in regions of downward vertical winds and in regions into which relatively nitrogen‐poor air was advected by the horizontal wind. (6) Calculated compositional perturbations following the onset of the storm were found to propagate to mid‐latitudes due to the advective effects of equatorward winds, leading to the mid‐latitude positive ionospheric storm effects. This last result is in disagreement with the earlier study of Rishbeth et al. (1985), who found the compositional disturbance to be largely restricted to the auroral zone on a constant‐pressure surface. However, it is consistent with the more recent study undertaken by Fuller‐Rowell et al. (1991), who demonstrated that composition changes were seen well beyond the auroral oval.

An influence of IMF-By on the ion density planetary distribution during distrubed period 20–23 October 1981: Satellite and ground-based results

The planetary distribution of the disturbed ion density (Ni) at fixed magnetic latitudes in both hemispheres, and in the evening and morning sectors, is examined using DE - B satellite and ground-based vertical sounding data. It is found that 1. the amplitude or (in some cases) the shape changes of the disturbed Ni longitude variations are asymmetrical in both hemispheres as well as in the morning and evening sectors; 2. the amplitude of “storm” variations is different at height of F2-layer peak and in the upper ionosphere (800–900 km); These irregularities in the ionospheric storm reaction are interpreted in terms of IMF influence on the magnetospheric convection pattern and its asymmetry - not only between evening and morning sectors in a given hemisphere, but also between northern and southern hemispheres. This hypothesis satisfactorily explains the different gradients of the equatorward wall of the trough and the lowest latitude reached by its minimum. The different statistical results concerning the positive and negative ionospheric storm effects, the isolated positive phase etc., can not be understood and correctly interpreted without reporting on the longitudinal variations of the ion density during magnetic disturbances and their specific manifestations.

Numerical simulation of negative ionospheric storms using observed neutral composition data

A numerical simulation of negative ionospheric storm effects has been performed using actually observed neutral composition data. The agreement between calculated and observed variations in the F-region plasma density demonstrates that the magnetic storm associated changes in the neutral composition are indeed sufficient to explain negative storm effects at middle latitudes.

Esro 4 Gas Analyzer results 2. Direct measurements of changes in the neutral composition during an ionospheric storm

Direct measurements of the neutral composition at ionospheric heights with the gas analyzer aboard Esro 4 reveal large decreases in the atomic oxygen to molecular nitrogen ratio during the ionospheric storm in late February 1973. A comparison shows that these changes in the neutral composition correlate well with decreases in the F2 layer critical frequency observed at different ionosonde stations. The data presented therefore strongly support previous theoretical studies that delineate neutral composition changes as the major cause for negative ionospheric storm effects. It is suggested that ionospheric and neutral upper atmospheric storm data could be used to supplement each other.