Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N₂] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions.     

Observations of thermospheric neutral winds within the polar cusp and the auroral oval using a Doppler imaging system (DIS)

Two Doppler imaging systems (DIS) or wide-field imaging Fabry-Perot interferometers (FPI), have recently been commissioned, one at the Auroral Station, Adventdalen, Longyearbyen, Svalbard, and the second at the IRF, Kiruna, Sweden. These instruments can provide wide-field (600 * 800 km) images of neutral wind flows in the upper thermosphere, by measuring the Doppler shift of the atomic oxygen forbidden near 630 nm, which is emitted from an altitude of approximately 240 km. From the instrument in Svalbard, at mid-winter, it is possible to observe the dayside polar cusp and the polar cap throughout the entire day, whereas from Kiruna, the night-time auroral oval is observable during the hours of darkness. Measurements of thermospheric dynamics from the DIS can be used in conjunction with observations of ionospheric plasma flows and thermal plasma densities by the EISCAT-Svalbard radar (ESR) and by EISCAT, along with other complementary observations by co-located instruments such as the auroral large-scale imaging system (ALIS). Such combined data sets will allow a wide range of scientific studies to be performed concerning the dynamical response of the thermosphere and ionosphere, and the important energetic and momentum exchange processes resulting from their complex interactions. These processes are particularly important in the immediate vicinity of the polar cusp and within the auroral oval. Early results from Svalbard in late 1995 will be discussed. The DIS in Kiruna observed two interesting geomagnetic disturbances in early 1997, the minor geomagnetic storm of 10, 11 January, and the disturbed period from 7–10 February. During these events, the thermospheric wind response showed some interesting departures from the average behaviour, which we attribute to the result of strong and variable Lorenz forcing (ion drag) and Joule and particle heating during these geomagnetic disturbances.     

EISCAT observations of unusual flows in the morning sector associated with weak substorm activity

A discussion is given of plasma flows in the dawn and nightside high-latitude ionospheric regions during substorms occurring on a contracted auroral oval, as observed using the EISCAT CP-4-A experiment. Supporting data from the PACE radar, Greenland magnetometer chain, SAMNET magnetometers and geostationary satellites are compared to the EISCAT observations. On 4 October 1989 a weak substorm with initial expansion phase onset signatures at 0030 UT, resulted in the convection reversal boundary observed by EISCAT (at \sim0415 MLT) contracting rapidly poleward, causing a band of elevated ionospheric ion temperatures and a localised plasma density depletion. This polar cap contraction event is shown to be associated with various substorm signatures; Pi2 pulsations at mid-latitudes, magnetic bays in the midnight sector and particle injections at geosynchronous orbit. A similar event was observed on the following day around 0230 UT (\sim0515 MLT) with the unusual and significant difference that two convection reversals were observed, both contracting poleward. We show that this feature is not an ionospheric signature of two active reconnection neutral lines as predicted by the near-Earth neutral model before the plasmoid is “pinched off”, and present two alternative explanations in terms of (1) viscous and lobe circulation cells and (2) polar cap contraction during northward IMF. The voltage associated with the anti-sunward flow between the reversals reaches a maximum of 13 kV during the substorm expansion phase. This suggests it to be associated with the polar cap contraction and caused by the reconnection of open flux in the geomagnetic tail which has mimicked “viscous-like” momentum transfer across the magnetopause.     

Seasonal effects in the ionosphere-thermosphere response to the precipitation and field-aligned current variations in the cusp region

The seasonal effects in the thermosphere and ionosphere responses to the precipitating electron flux and field-aligned current variations, of the order of an hour in duration, in the summer and winter cusp regions have been investigated using the global numerical model of the Earths upper atmosphere. Two variants of the calculations have been performed both for the IMF B_y < 0. In the first variant, the model input data for the summer and winter precipitating fluxes and field-aligned currents have been taken as geomagnetically symmetric and equal to those used earlier in the calculations for the equinoctial conditions. It has been found that both ionospheric and thermospheric disturbances are more intensive in the winter cusp region due to the lower conductivity of the winter polar cap ionosphere and correspondingly larger electric field variations leading to the larger Joule heating effects in the ion and neutral gas temperature, ion drag effects in the thermospheric winds and ion drift effects in the F2-region electron concentration. In the second variant, the calculations have been performed for the events of 28–29 January, 1992 when precipitations were weaker but the magnetospheric convection was stronger than in the first variant. Geomagnetically asymmetric input data for the summer and winter precipitating fluxes and field-aligned currents have been taken from the patterns derived by combining data obtained from the satellite, radar and ground magnetometer observations for these events. Calculated patterns of the ionospheric convection and thermospheric circulation have been compared with observations and it has been established that calculated patterns of the ionospheric convection for both winter and summer hemispheres are in a good agreement with the observations. Calculated patterns of the thermospheric circulation are in a good agreement with the average circulation for the Southern (summer) Hemisphere obtained from DE-2 data for IMF B_y < 0 but for the Northern (winter) Hemisphere there is a disagreement at high latitudes in the afternoon sector of the cusp region. At the same time, the model results for this sector agree with other DE-2 data and with the ground-based FPI data. All ionospheric and thermospheric disturbances in the second variant of the calculations are more intensive in the winter cusp region in comparison with the summer one and this seasonal difference is larger than in the first variant of the calculations, especially in the electron density and all temperature variations. The means that the seasonal effects in the cusp region are stronger in the thermospheric and ionospheric responses to the FAC variations than to the precipitation disturbances.     

Numerical modelling of the thermospheric and ionospheric effects of magnetospheric processes in the cusp region

The thermospheric and ionospheric effects of the precipitating electron flux and field-aligned-current variations in the cusp have been modelled by the use of a new version of the global numerical model of the Earths upper atmosphere developed for studies of polar phenomena. The responses of the electron concentration, ion, electron and neutral temperature, thermospheric wind velocity and electric-field potential to the variations of the precipitating 0.23-keV electron flux intensity and field-aligned current density in the cusp have been calculated by solving the corresponding continuity, momentum and heat balance equations. Features of the atmospheric gravity wave generation and propagation from the cusp region after the electron precipitation and field-aligned current-density increases have been found for the cases of the motionless and moving cusp region. The magnitudes of the disturbances are noticeably larger in the case of the moving region of the precipitation. The thermospheric disturbances are generated mainly by the thermospheric heating due to the soft electron precipitation and propagate to lower latitudes as large-scale atmospheric gravity waves with the mean horizontal velocity of about 690 ms^–1. They reveal appreciable magnitudes at significant distances from the cusp region. The meridional-wind-velocity disturbance at 65° geomagnetic latitude is of the same order (100 ms^–1) as the background wind due to the solar heating, but is oppositely directed. The ionospheric disturbances have appreciable magnitudes at the geomagnetic latitudes 70°–85°. The electron-concentration and -temperature disturbances are caused mainly by the ionization and heating processes due to the precipitation, whereas the ion-temperature disturbances are influence strongly by Joule heating of the ion gas due to the electric-field disturbances in the cusp. The latter strongly influence the zonal- and meridional-wind disturbances as well via the effects of ion drag in the cusp region. The results obtained are of interest because of the location of the     

Longitudinal effects of ionospheric responses to substorms at middle and lower latitudes: a case study

An ionospheric model is used to simulate total electron content (TEC) disturbance events observed at middle and lower latitude sites near 75○W and 7○E longitudes. Within this longitudinal range, daytime TEC disturbances show patterns that are correlated with substrom activity seen in both auroral electrojet and ring current behavior. In modeling studies of the observed ionospheric effects, both electric field and neutral wind perturbations are examined as possible mechanisms. The morphological features of the required electric field perturbations near drawn and dusk are compared with those at other times to examine the local time characteristics of magnetospheric influence. Large-scale traveling atmospheric disturbances (TADs), an alternative candidate for the disturbance source, are also characterized and compared with known thermospheric behavior.     

The response of ionospheric convection in the polar cap to substorm activity

We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°-78°. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71° by the time of the expansion phase onset. A westward electrojet, centred at 65.4°, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1° at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the “distant” neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.     

Ionospheric effects of magnetospheric and thermospheric disturbances on March 17–19, 2015

Using vertical and oblique radio-sounding data, we analyze the ionospheric and thermospheric disturbances during the magnetic storm that occurred in northeastern Russia on March 17–19, 2015. We consider the heliospheric sources that induced the magnetic storm. During the main and early recovery phases, the midlatitude stations are characterized by extremely low values of electron density at the F2 layer maximum. Using oblique sounding data, we recorded signals that propagated outside the great circle arc. In evening and night hours, no radio signals were found to pass along the Norilsk–Irkutsk and Magadan–Irkutsk paths. The observed ionospheric effects are shown to be caused by a sharp shift of the boundaries of the main ionospheric trough to the invariant latitude 46° N during the main phase of the magnetic storm. The negative ionospheric disturbance during the recovery phase of the storm, which was associated with significant variations in the composition of the neutral atmosphere, led to a change in the mode composition of received radio signals and a decline in observed maximal frequencies in daytime hours of March 18, 2015 by more than 2 times.     

Ionospheric convection during the magnetic storm of 20–21 March 1990

We report on the response of high-latitude ionospheric convection during the magnetic storm of March 20–21 1990. IMP-8 measurements of solar wind plasma and interplanetary magnetic field (IMF), ionospheric convection flow measurements from the Wick and Goose Bay coherent radars, EISCAT, Millstone Hill and Sondrestrom incoherent radars and three digisondes at Millstone Hill, Goose Bay and Qaanaaq are presented. Two intervals of particular interest have been identified. The first starts with a storm sudden commencement at 2243 UT on March 20 and includes the ionospheric activity in the following 7 h. The response time of the ionospheric convection to the southward turning of the IMF in the dusk to midnight local times is found to be approximately half that measured in a similar study at comparable local times during more normal solar wind conditions. Furthermore, this response time is the same as those previously measured on the dayside. An investigation of the expansion of the polar cap during a substorm growth phase based on Faraday’s law suggests that the expansion of the polar cap was nonuniform. A subsequent reconfiguration of the nightside convection pattern was also observed, although it was not possible to distinguish between effects due to possible changes in B_y and effects due to substorm activity. The second interval, 1200–2100 UT 21 March 1990, included a southward turning of the IMF which resulted in the B_z component becoming -10 nT. The response time on the dayside to this change in the IMF at the magnetopause was approximately 15 min to 30 min which is a factor of \sim2 greater than those previously measured at higher latitudes. A movement of the nightside flow reversal, possibly driven by current systems associated with the substorm expansion phases, was observed, implying that the nightside convection pattern can be dominated by substorm activity.     

Dynamics of North American sector ionospheric and thermospheric response during the November 2004 superstorm

We present a study of ionospheric and thermospheric response during a November 9–10, 2004 major geomagnetic storm event (DsT ～?300 nT). We utilize the North American sector longitude chain of incoherent scatter radars at Arecibo, Millstone Hill, and Sondrestrom, operating as part of a coordinated international mesosphere/lower thermosphere coupling study experiment. Total electron content (TEC) determinations from global positioning system (GPS) ground receivers, ground magnetometer traces from the Canadian CANOPUS array, Defense Meteorological Satellite Platform (DMSP) topside data, and global convection patterns from the SuperDARN radar network are analyzed to place the detailed radar data in proper mesoscale context. The plasmaspheric boundary layer (PBL) expanded greatly in the dusk sector during ring current intensification to span more than 25° of magnetic latitude, reaching as far south as 30° invariant latitude. Strong sub-auroral polarization stream velocities of more than 1 km/s were accompanied by large upwards thermal O+ fluxes to the overlying magnetosphere. The large PBL expansion subsequently exposed both Millstone Hill and Sondrestrom to the auroral convection pattern, which developed a complex multicell and reverse convection response under strongly northward IMF conditions during a period of global interplanetary electric field penetration. Large traveling atmospheric and ionospheric disturbances caused significant neutral wind and ion velocity surges in the mid-latitude and tropical ionosphere and thermosphere, with substorm activity launching equatorward neutral wind enhancements and subsequent mid-latitude dynamo responses at Millstone Hill. However, ionosphere and thermosphere observations at Arecibo point to significant disturbance propagation modification in the post-dusk sector PBL region.     

Interplanetary control of thermospheric densities during large magnetic storms

During the main phase of large magnetic storms significant energy can be deposited in the ionosphere but produce no commensurate magnetic perturbations on the ground. Consequently, models designed to predict and specify thermospheric energy budgets based on ground magnetic data are negatively impacted. To quantify these effects we compare thermospheric densities predicted by the MSIS model with those inferred from accelerometer measurements by the Gravity Recovery and Climate Experiment (GRACE) satellites during two magnetic storm periods in 2004. Although predictions and measurements are in substantial agreement during quiet times, the model significantly underpredicts densities during storms. Also, the model's maxima occur several hours after observed stormtime peaks. We show that polar cap potentials and magnetospheric electric fields derived from interplanetary parameters measured by the Advanced Composition Explorer satellite are roughly proportional to neutral densities observed by GRACE with lead times of ∼4 h. Finally, ion drift meter data from Defense Meteorological Satellite Program spacecraft suggest that unpredicted positive and negative spikes found in high latitude accelerometer data reflect encounters with strong head and tail thermospheric winds driven by anti-sunward convecting plasma.     

Long-term thermospheric neutral wind observations over the northern polar cap

We study the solar dependence of the thermospheric dynamics based on more than 20 years Fabry–Perot interferometer O 6300 Å emission observation of polar cap thermospheric wind from three stations: Thule (76.53°N, 68.73°W, MLAT 86N), Eureka (80.06°N, 86.4°W, MLAT 89N), and Resolute (74.72°N, 94.98°W, MLAT 84N) in combination with the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). All three stations showed a dominant diurnal oscillation in both the meridional and zonal components, which is a manifestation of anti-sunward thermospheric wind in the polar cap. The three-station observations and the TIEGCM simulation exhibit varying degree of correlations between the anti-sunward thermospheric wind and solar F10.7 index. The diurnal oscillation is stronger at Eureka (∼150 m/s) than that at Resolute (∼100 m/s) according to both observations and TIEGCM simulation. The semidiurnal oscillation is stronger at Resolute (∼20 m/s) than that at Eureka based (∼10 m/s) on data and model results. These results are consistent with a two-cell convection pattern in the polar cap thermospheric winds. The Thule results are less consistent between the model and observations. The simulated meridional wind diurnal and semidiurnal oscillations are stronger than those observed.     

Twilight anomaly,midday recovery and cutoff latitudes during the intense polar cap absorption event of March 1991

A study was made of the polar cap absorption (PCA) event on 23–24 March 1991 produced by the largest solar proton event at E>10 MeV since August 1972. This PCA event was related to a solar flare in the eastern hemisphere lasting only 2 days and exhibiting a long time delay between the flare and the increase of ionospheric absorption. Midday recovery occurred regularly each PCA day near the cutoff lati- tudes during the noontime hours and is attributed to the daily variation in the proton cutoff latitudes. The maximum absorption during the PCA event was observed at high latitudes or near the cutoff latitudes where ionization may be due to both solar protons and trapped particles. The minimum in the absorption values during the night-time hours would appear to be caused by the chemistry of the D-region as well as access of the solar protons into the polar cap area.     

大尺度电离层行扰的GPS观测

利用日本境内的高空间分辨率的双频GPS台站资料,观测研究了发生于2000年7月中旬太阳强活动期间的一次大尺度电离层行扰. 结果表明:在7月15日11:00UT-1:00UT期间观测区域的电离层中出现了大尺度电离层行扰. 在15:00UT之前,扰动周期为2h左右,在15:00UT以后,扰动周期为1h左右;总电子含量扰动幅度的变化范围约为1-2TECU;通过对15:00-17:00UT之间总电子含量扰动曲线同相位点的分析,发现这期间的电离层行扰的扰动速度约为600-700m/s,扰动波长在2200km左右,扰动传播的方向几乎沿着经线从高纬向低纬传播. 该行扰与此次强太阳活动有直接的关系,因其发生在7月15日的磁暴急始之前数小时,因此与磁暴本身没有因果关系,应与磁暴之前先期到达地球空间的高能质子流有关.     

Combined ESR and EISCAT observations of the dayside polar cap and auroral oval during the May 15, 1997 storm

The high-latitude ionospheric response to a major magnetic storm on May 15, 1997 is studied and different responses in the polar cap and the auroral oval are highlighted. Depletion of the F₂ region electron density occurred in both the polar cap and the auroral zone, but due to different physical processes. The increased recombination rate of O⁺ ions caused by a strong electric field played a crucial role in the auroral zone. The transport effect, however, especially the strong upward ion flow was also of great importance in the dayside polar cap. During the main phase and the beginning of the recovery phase soft particle precipitation in the polar cap showed a clear relation to the dynamic pressure of the solar wind, with a maximum cross-correlation coefficient of 0.63 at a time lag of 5 min.     

Geomagnetic pulsations and magnetic disturbances during the initial phase of a strong magnetic storm of May 15, 2005

A very strong magnetic storm of May 15, 2005, was caused by an interplanetary magnetic cloud that approached the Earths’ orbit. The sheath region of this cloud was characterized by a high solar wind density (～25–30 cm^?3) and velocity (～850 km/s) and strong variations (to ～20 nT) in the interplanetary magnetic field (IMF). It has been indicated that an atypical bay-like geomagnetic disturbance was observed during the initial phase of this storm in a large longitudinal region at high latitudes: from the morning to evening sectors of the geomagnetic local time. Increasing in amplitude, the magnetic bay rapidly propagated to the polar cap latitudes up to the geomagnetic pole. An analysis of the global space-temporal dynamics of geomagnetic pulsations in the frequency band 1–6 mHz indicated that most intense oscillations were observed in the morning sector in the region of the equivalent ionospheric current at latitudes of about 72°–76°. The wavelet structure of magnetic pulsations in the polar cap and fluctuations in IMF was generally similar to the maximum at frequencies lower than 4 mHz. This can indicate that waves directly penetrated into the polar cap from the solar wind.     

study of ionospheric response to magnetic superstorms in the East Asian sector

In this paper, we present analyses of the great geomagnetic storms observed during last two cycles of solar activity. This study is based on data from a network of ionosondes located within the longitudinal sector of 80–150°Е. it was found that the superstorms were observed predominantly in equinox. Long-lasting severe decreases of ionization at high and middle latitudes were the most impressive storm effect. A short-time positive phase occurred in response to the onset of both ssc and recovery phases of the magnetic storm in the daytime at high and middle latitudes. Large time-varying rates of foF2 were observed at low latitudes. Modeling results of the ionospheric response to two superstorms are also presented. It is established that the storm effect at middle latitudes was controlled predominantly by disturbed thermospheric composition. At high latitudes, the impact of magnetospheric processes and thermospheric composition on the ionosphere was the same.     

Comparison of ionospheric parameters calculated with UAM and measured at Voeykovo observatory

The measurements of the critical frequencies of the ionospheric F2 layer based on vertical radiosounding, which was performed with a CADI digital ionosonde at the Voeykovo magnetic–ionospheric observatory in February 2013, have been considered. The observations have been compared with the upper atmosphere numerical model (UAM) data for three days that differ in the amplitude and the character of solar and magnetic activity and correspond to quiet and moderately disturbed states of the ionosphere. The work was performed in order to improve the methods for determining the ionospheric state by vertical sounding ionograms. The time variations in the F2 layer critical frequency, electric field vector zonal component, and thermospheric wind velocity meridional component have been analyzed. Calculations were performed with three UAM variants. The UAM version providing the best agreement with the CADI ionosonde data was the version in which the neutral temperature, neutral composition, and pressure gradients are calculated according to the MSIS empirical model and the horizontal neutral wind velocity is determined by the equation of motion with pressure gradients from MSIS. The calculated values corresponded to the measurements, except those for the evening, because the electron density at the ionospheric F2 layer maximum depends more strongly on electric fields and thermospheric wind velocities during this period. Thus, the indicated UAM version with the above limitations can be used to determine the state of the subauroral ionosphere.     

极点热层密度变化特征及其受太阳风扇形结构调制

本文基于2002年至2010年的GRACE卫星的观测密度统计分析南北极点的热层大气密度的世界时（即磁地方时）变化.研究发现：在9—11月份地球处于行星际磁场为背向太阳的扇区内（背向扇区）时,南极点热层密度在约17∶00 UT（13∶30 MLT）达到最大值,比日平均值高约22%;而在6—8月份,当地球处于行星际磁场为面向太阳的扇区内（面向扇区）时,北极点热层密度在06∶00 UT（12∶30 MLT）达到最大值,比日平均值高约13%.南极点的磁纬是-74°,其在15∶30 UT处于磁地方时正午,恰与极尖区位置重合.北极点在5∶30 UT处于磁地方时正午,此时北极点与极尖区位置最靠近.因此,极点热层大气密度的磁地方时变化可能是其周期性靠近极尖区的结果.南北极点热层密度的磁地方时变化分别在背向和面向扇区内更明显,这可能与行星际磁场B_y分量对南北半球密度的不同影响有关.统计结果还表明,极点热层大气密度的磁地方时变化在冬季半球内不明显.这可能是由于在冬季半球,沉降于极尖区的粒子相比夏季半球少、沉降高度低,因而能量沉降所引起的热层上部的密度增强较小.