Ground Magnetic Field Research Articles

Statistical study of geomagnetic field line resonances observed by CHAMP and on the ground

Studies of single events showing the signatures of Pc3 pulsation field line resonances (FLRs) observed in magnetic field data by CHAMP above the E‐region ionosphere and at conjugate ground stations at L < 2 have previously been reported. Notable features of FLRs observed in these investigations were a Doppler shift in the satellite azimuthal component and a 90° rotation of the polarization ellipse between CHAMP and the ground. In this report, we extend the previous work by means of the first statistical investigation of the same features for a large number of events using CHAMP and ground magnetic field data. Our statistical study confirms the Doppler shifts to higher or lower frequencies relative to the ground FLR frequencies for equatorward or poleward passes of CHAMP over the ground station. Our statistical investigation shows the amount of rotation of the FLR polarization ellipses between the ionosphere and the ground to be ~100° on average, rather than the theoretically predicted 90°. This is explained by the polarization ellipses on the ground being oriented close to N‐S irrespective of the orientation above the ionosphere, which is −13° on average. We find no evidence of ionospheric conductivity gradients associated with sunrise and sunset affecting the amount of rotation of the polarization ellipses as predicted by more recent theoretical advances. We are not able to provide conclusive evidence on the relationship between the polarization ellipse rotation and azimuthal wave number as predicted in recent numerical modeling.

Modeling of the ionospheric Alfvén resonator in dipolar geometry

A new model for the propagation of ultralow‐frequency (ULF) waves in dipolar geometry has been developed. This model features a full height‐resolved ionosphere including finite Pedersen, Hall, and parallel conductivities. By using a nonorthogonal coordinate system, this model is capable of calculating the ground magnetic field produced by ULF waves and comparing these fields to those measured in the ionosphere and magnetosphere. This model has been used to investigate the properties of the ionospheric Alfvén resonator (IAR) in a dipolar magnetosphere. Although the IAR mode frequencies are not strongly affected by the finite magnetic zenith angle, the damping of these waves is enhanced by the presence of the height‐resolved ionosphere. Pedersen conductivity shields higher frequency ULF waves such as Pc1 from penetrating through the ionosphere, limiting the magnitude of the ground magnetic field, whereas Hall conductivity and finite azimuthal wave number enhance the coupling to the ground. Results for runs in which a wave packet is introduced into the model show that the ground magnetic field is enhanced when the central frequency of the wave packet matches a resonant frequency of the IAR. Including a more realistic height‐resolved ionosphere yields a more direct calculation of ionospheric fields, allowing a comparison between ground and ionospheric fields.

Evolution of negative SI‐induced ionospheric flows observed by SuperDARN King Salmon HF radar

The spatial evolution of vortex‐like flow structures induced by a negative sudden impulse (SI−) is studied on the basis of SuperDARN King Salmon HF radar (KSR) with other ground and satellite data. A large dip in the solar wind density induced a fairly large SI− with a SYM‐H amplitude of ∼40 nT. The SI‐induced ionospheric flow signatures in the evening sector (MLT ∼ 19 h) were observed by KSR as a westward flow associated with the preliminary impulse (PI) followed by a more intense eastward flow with the main impulse (MI) in the sub‐auroral region of the magnetic latitude ∼60–70 deg, consistent with the local ground magnetic field observations. Following the first PI‐MI flow sequence, KSR saw a second and possibly third sequence of flow variation which were much smaller in flow amplitude than the first pair but showed qualitatively very similar flow variations and latitudinal/longitudinal propagation characteristics. These observations can be interpreted as aftershocks of the first PI‐MI; the same sequence of vortices and field‐aligned currents were generated and then drifted anti‐sunward with the same mechanism, namely the pumping motion of the dayside magnetosphere. These results are qualitatively consistent with predictions suggested by recent numerical simulations.

The SuperMAG data processing technique

In this paper I outline the data processing technique which is used in the SuperMAG initiative. SuperMAG is a worldwide collaboration of organizations and national agencies that currently operate more than 300 ground based magnetometers. SuperMAG provides easy access to validated ground magnetic field perturbations in the same coordinate system, identical time resolution and with a common baseline removal approach. The purpose of SuperMAG is to provide scientists, teachers, students and the general public easy access to measurements of the magnetic field at the surface of the Earth. Easy access to data, plots and derived products maximizes the utilization of this unique data set. It is outlined how SuperMAG processes the observations obtained by the individual data provider. Data are rotated into a local magnetic coordinate system by determining a time dependent declination angle. This angle displays a slow gradual change and a yearly periodic variation attributed to changes in the Earth main field and season temperature variations. The baseline is determined from the data itself in a three step process: (1) a daily baseline, (2) a yearly trend, and (3) a residual offset. This technique does not require so‐called quiet days and thus it avoids all the well‐known problems associated with their identification. The residual offset for the N‐ and Z‐components shows a distinct latitudinal dependence while the E‐component is independent of the latitude. This result is interpreted as being due to a weak ring current (likely asymmetric) which is present even during official quiet days. For the purpose of M‐I research using 1‐min data I find no difference between observatories and variometers. I finally argue that there is no correct baseline determination technique since we do not have a set of ground‐truth observations required to make an objective evaluation. Instead, the user must keep in mind the assumptions on which the baseline was determined and draw conclusions accordingly.

Rocket borne in-situ Electron density and Neutral Wind measurements in the equatorial ionosphere—Results from the January 2010 annular solar eclipse campaign from India

One of the recent developments in the form of a novel probe for simultaneous in-situ measurements of Electron density and Neutral Wind (ENWi) parameters was extensively made use of during the annular solar eclipse that passed right over the magnetic dip equatorial station, Trivandrum in India. The response of the equatorial ionosphere to the annular solar eclipse of January 2010 is investigated using the data from ENWi and other ground based instruments like digital Ionosonde, HF radar, magnetometer etc. Significant effects on the neutral wind components and electron densities during the peak phase of the eclipse are brought out by the ENWi data. A clear reversal in the zonal neutral wind is observed during the rocket flights at the peak phase of the eclipse as well as in the flight conducted 3h later. The ground magnetic field measurements revealed the reversal of electrojet during the eclipse alongwith blanketing Es layers vindicating the earlier hypothesis that blanketing Es layers manifest over magnetic equatorial regions only during periods of either weak or reversed electrojet (counter electrojet). The HF radar results independently substantiate the in situ measurements with regard to the location of the irregularities and their Doppler velocities. The regional scale changes that have been brought about by the Solar eclipse both in the neutral and ionized medium and the consequent electrodynamics are highlighted.

Forcing the TIEGCM model with Birkeland currents from the Active Magnetosphere and Planetary Electrodynamics Response Experiment

Geomagnetic field‐aligned currents from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellite mission are used to drive the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). We present a comparison between ground magnetic signatures computed by the model and observations at four different geomagnetic observatories, for different magnetic disturbance levels. Results show the ability of the model to pick up the gross features of the magnetic variations, improving its performance with increasing disturbance level and from low to high latitudes. During geomagnetically quiescent conditions a baseline noise of about 5 nT is evident in reconstructed ground magnetic field signatures, which we attribute to the baseline noise level in the AMPERE currents. For variations shorter than about 30 min the modeled signals are often significantly lower than observed by a factor up to 3 to 4, possibly reflecting localized ionization structures not captured in the TIEGCM conductance modules, or missing small‐scale and rapid temporal variations in auroral currents. While the observed horizontal field variations are reflected in the model, the vertical component is consistently underestimated, possibly indicating errors in the estimates for ground induction currents. Comparison with the standard version of the TIEGCM is also carried out, showing that time variations shorter than 6 h and down to the 10 min resolution of the AMPERE data (which do not appear in the standard version of TIEGCM) are now reflected in the AMPERE‐driven model.

Large‐scale current systems and ground magnetic disturbance during deep substorm injections

We present a detailed analysis of the large‐scale current systems and their effects on the ground magnetic field disturbance for an idealized substorm event simulated with the equilibrium version of the Rice Convection Model. The objective of this study is to evaluate how well the bubble‐injection picture can account for some classic features of the substorm expansion phase. The entropy depletion inside the bubble is intentionally designed to be so severe that it can penetrate deep into geosynchronous orbit. The results are summarized as follows: (1) Both the region‐1‐sense and region‐2‐sense field‐aligned currents (FACs) intensify substantially. The former resembles the substorm current wedge and flows along the eastern and western edges of the bubble. The latter is connected to the enhanced partial ring current in the magnetosphere associated with a dipolarization front earthward of the bubble. In the ionosphere, these two pairs of FACs are mostly interconnected via Pedersen currents. (2) The horizontal ionospheric currents show a significant westward electrojet peaked at the equatorward edge of the footprint of the bubble. The estimated ground magnetic disturbance is consistent with the typical features at various locations relative to the center of the westward electrojet. (3) A prominent Harang‐reversal‐like boundary is seen in both ground ΔH disturbance and plasma flow pattern, appearing in the westward portion of the equatorward edge of the bubble footprint, with a latitudinal extent of ∼5° and a longitudinal extent of the half width of the bubble. (4) The dramatic dipolarization inside the bubble causes the ionospheric map of the inner plasma sheet to exhibit a bulge‐like structure, which may be related to auroral poleward expansion. (5) The remarkable appearance of the westward electrojet, Harang‐reversal‐like boundary and poleward expansion starts when the bubble reaches the magnetic transition region from tail‐like to dipole‐like configuration. We also estimate the horizontal and vertical currents using magnetograms at tens of ground stations for a deep injection substorm event occurred on April 9, 2008, resulting in a picture that is qualitatively consistent with the simulation. Based on the simulations and the observations, an overall picture of the ionospheric dynamics and its magnetospheric drivers during deep bubble injections is obtained.

Pi2 pulsations driven by ballooning instability

With the evidence set forth in this study, we argue that ballooning modes are viable candidates for the generation of high‐latitude Pi2 pulsations. In the scenario proposed here, it is the spatial separation ballooning wave perturbations along an energized boundary in the near‐Earth plasma sheet combined with a westward ion diamagnetic drift, which imprint a temporal signature, namely, Pi2 pulsation, on conjugate, stationary ground stations. The ballooning wave perturbations are connected via field‐aligned currents to the ionosphere causing Pi2 perturbations in the ground magnetic field. The observational evidence for this mechanism was conjugate spacecraft and ground data, showing correlated diamagnetic plasma perturbations and ground magnetic field oscillations, for two spatially separated, simultaneously occurring Pi2 events on 23 March 2007. The plasma perturbations were westward‐drifting waves with azimuthal mode number, m, of approximately 40 consistent with drift ballooning modes. During the Pi2 events, particle energy and energy flux gradually increased until the onset of major particle injections, causing auroral intensifications and additional larger‐amplitude Pi2s. The ground Pi2s, also shown to be westward‐traveling, were superposed on a slowly decreasing H component which abruptly turned into a substorm bay. A ballooning‐driven, high‐latitude Pi2 differs from a high‐latitude Pi2 that starts synchronized with substorm onset and that has been attributed to the transient response mechanism in that it is the frequency of drift ballooning mode in the near‐Earth plasma sheet that determines the Pi2 frequency and not the bounce frequency of Alfvén waves along a field line. The geophysical context for this type of Pi2 is a class of substorms that develop from a plasma instability in the near‐Earth plasma sheet. If this instability does not lead to a substorm breakup, it could lead to a pseudo‐breakup. Hence, it is also likely that some Pi2s associated with pseudo‐breakup are caused by the mechanism described here.

Ultra‐low frequency wave coupling in the ionospheric Alfvén resonator: Characteristics and implications for the interpretation of ground magnetic fields

In cold magnetospheric plasmas, ultra‐low frequency (ULF) waves occur in either the Alfvén or magnetosonic mode. Owing to the strong inhomogeneity of the near‐Earth plasma, these waves can produce cavity‐like or waveguide‐like eigenmodes which are mutually coupled by wave‐driven ionospheric currents and perpendicular plasma inhomogeneity. Here we present results from a numerical study of the ionospheric Alfvén resonator (IAR) and the ULF waves generated by wave‐driven field‐aligned currents (FACs) in the ionosphere. We demonstrate the excitation of cavity and waveguide eigenmodes of the IAR by a localized field‐aligned current, and examine the polarization patterns of the resultant ground magnetic fields. We find that the distribution of ellipticity reflects the configuration of the magnetospheric source while the distribution of polarization angles depends primarily on the localization of the signal. The pattern of magnetosonic waves generated by ionospheric Hall currents is generally isotropic, but its configuration is representative of the structure of the incident FAC. Apparent preferences for wave propagation eastward and to lower latitudes are identified, and implications for the interpretation of ground magnetic signatures are discussed.

Coupling of dipolarization front flow bursts to substorm expansion phase phenomena within the magnetosphere and ionosphere

Recent THEMIS spacecraft observations have shown longitudinally narrow, earthward moving, plasma sheet flow channels with large, abrupt magnetic field dipolarizations that have been referred to as dipolarization fronts. We undertake a detailed study of the subset of these events that had good quality auroral imaging. We find that many occurred during the substorm expansion phase after onset (determined by auroral intensification) and were related to auroral streamers. Both the fronts' relation to streamers and their plasma sheet changes are similar to what has been previously shown for other plasma sheet flow channels and to other flow channels seen during the expansion phases of the events considered here. Surprisingly, the dipolarization fronts examined made major contributions to substorm onset ground and space magnetic signatures. These include substorm near‐Earth dipolarization, which indicates that the current wedge may develop via a series of well‐defined, narrow, post‐onset wedge‐like structures. These also include the auroral zone ground magnetic field, which showed only modest responses to the onsets but abrupt, large responses to post‐onset dipolarization‐front‐related streamers, and midlatitude positive bays, which started near the time of streamer formation. Our observations suggest that traditional magnetic signatures of substorm onset, such as near‐Earth dipolarization, auroral‐zone negative bays, and midlatitude positive bays, may misidentify the expansion phase onset determined by auroral intensification by up to 10s of minutes for some events. It should be interesting in the future to determine the generality of the above relations between dipolarization front signatures and onset signatures.

Local time‐dependent Pi2 frequencies confirmed by simultaneous observations from THEMIS probes in the inner magnetosphere and at low‐latitude ground stations

Using electric and magnetic field data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) fast mode waves away from midnight. probes (TH‐A, TH‐D, and TH‐E) acquired in the inner magnetosphere (L < 4) and low‐latitude ground magnetic field data from Bohyun (BOH, L = 1.35) and Hermanus (HER, L = 1.83) stations, we studied longitudinal variations of Pi2‐associated fast mode waves away from midnight. We selected 48 nightside Pi2 events at BOH identified when THEMIS probes were in the inner magnetosphere for 1 month (February 2008). During that period the probes were located between 4.0 and 20.0 magnetic local time (MLT), i.e., at and on either side of the dayside sector. This choice was motivated by our interest in studying loss of Pi2 wave energy away from its nominal source at the nightside, all the way to the dayside. Between 4.0 and 8.0 MLT the probes often observed poloidal oscillations in space during BOH Pi2 events. The poloidal oscillations had high coherence (>0.6) with BOH Pi2s and radially standing fast mode structures. Thus, these fast mode waves are explained by plasmaspheric resonance. On the duskside, however, few events at THEMIS probes had high coherence with BOH Pi2s. Furthermore, the THEMIS probe data showed no evidence of Pi2 signals at 9.0–18.0 MLT, which is consistent with previous studies. Most of the high‐coherence events were detected when the local time separation between the THEMIS probes and BOH was less than 3 h. These observations suggest that Pi2 wave energy is lost as it propagates azimuthally from a source region localized in longitude. From longitudinally separated simultaneous multipoint observations at THEMIS probes and BOH and HER stations, we found that the Pi2 frequency varies with longitude both in space and on the ground. This implies that although plasmaspheric fast mode waves establish a standing wave structure on a given meridional plane, their frequency changes with longitude if the plasmasphere is not axisymmetric. Finally, we show that a low‐latitude daytime Pi2 is not a fast mode wave propagating to the dayside through the magnetosphere.

Determination of the wave mode contribution into the ULF pulsations from combined radar and magnetometer data: Method of apparent impedance

To identify the physical nature of Pc5 pulsations and to determine relative contributions of different MHD modes into their structure, we introduce the method of apparent impedance. An approximate analytical relationship from the theory of ULF wave transmission through a thin ionosphere can be compared with the measured ratio between simultaneous ionospheric electric and ground magnetic fields. The analytical expressions show that the impedances of Alfven and compressional modes should be essentially different. To validate this technique, we have analyzed global Pc5 pulsations during the recovery phase of the strong magnetic storm on October 31, 2003 using ground magnetometricdata and tri-static EISCAT radar. This radar facility gives a possibility to calculate vectors of the F-region ionospheric plasma flow and electric field. From these observations we conclude that the global Pc5 pulsations during storm conditions above the ionosphere are predominantly composed from Alfven waves with a small contribution of fast compressional mode.

Ultralow-frequency variations of the magnetic field during the propagation of acoustic-gravitaty waves in the ionosphere

The effect of the generation of ULF variations of the magnetic field during the propagation of acoustic-gravitaty waves (AGW) in the ionosphere is analyzed within a simple model. It is shown that AGW-induced ionospheric irregularities can significantly contribute to the ground magnetic field variations. Some changes in ULF variations associated with seismic events are considered as application of this mechanism.

Magnetic field depression at the Earth's surface during energetic neutral atom emission fade-out in the inner magnetosphere

[1] Using data from the high-energy neutral atom (HENA) imager onboard the IMAGE satellite, we examined the relation between the SYM-H index and the ring current energy during a storm main phase. The energy range of the energetic neutral atom (ENA) flux data used here is 16–120 keV for hydrogen and <180 keV for oxygen. From the data for the period 2000–2002, we selected 24 storm main phase events during which the IMAGE satellite was located at a geomagnetic latitude of ≥45° and a geocentric distance of ≥6 RE. According to the Dessler-Parker-Sckopke (DPS) equation, the ring current energy is expected to increase as the SYM-H index decreases. When the ENA energy flux is superimposed as a function of the SYM-H index for all 24 events, their overall correlation is negative; that is, the relation between the ENA energy flux and the SYM-H index is generally consistent with the DPS equation. However, an analysis of individual events showed only 10 events (42%) in which the ENA energy flux was negatively correlated with the SYM-H index (negative correlation events). There were 10 events showing no clear correlation between the ENA energy flux and the SYM-H index (no correlation events) and 4 events which contradicted the DPS equation (positive correlation events). In the superimposed plot, we noted that a smooth curve can be drawn for an upper limit of the data distribution, and data from the no correlation or positive correlation events create downward branches in the distribution. These observational results are not explained by the conventional DPS equation but by the “generalized” DPS equation, which includes a term representing energy stored in the stretched magnetic field. We can reasonably presume that the stretched magnetic field prevents energetic particles from being injected into the ring current. From the generalized DPS equation, we conclude that the total (kinetic and magnetic) energy stored in the stretched field and ring current loss mechanisms are important for understanding the relation between the ground magnetic field variation and ring current energy variation.

Far tail (255RE) fast response to very weak magnetic activity

[1] We present an original study of the dynamical changes measured in the far tail at XGSM ≈ −255 RE, onboard STEREO-B, related to very weak substorm activity (AE < 100 nT). Three weak auroral electrojet perturbations are well correlated with motions of the far tail in which the spacecraft passes from the lobe to the boundary layer or to the magnetosheath. These boundary motions can hardly be related to a plasmoid as a widening of the tail is expected from such a high-pressure structure. Furthermore, for one of the AE enhancements, ground measurement of auroral luminosity and ground magnetic field provided a precise timing of the substorm onset, thus allowing estimation of the propagation speed of the tail disturbance, supposing it is initiated at ∼20 RE in the midtail. The computed velocity, 1800 km/s, much greater than the typical plasmoid propagation speed, implies that the tail disturbance is due to a large-scale wave propagating inside the lobe, initiated inside the inner plasma sheet at substorm onset and linked to current disruption and/or magnetic reconnection.

Geospace Environment Modeling 2008–2009 Challenge: Ground magnetic field perturbations

Acquiring quantitative metrics‐based knowledge about the performance of various space physics modeling approaches is central for the space weather community. Quantification of the performance helps the users of the modeling products to better understand the capabilities of the models and to choose the approach that best suits their specific needs. Further, metrics‐based analyses are important for addressing the differences between various modeling approaches and for measuring and guiding the progress in the field. In this paper, the metrics‐based results of the ground magnetic field perturbation part of the Geospace Environment Modeling 2008–2009 Challenge are reported. Predictions made by 14 different models, including an ensemble model, are compared to geomagnetic observatory recordings from 12 different northern hemispheric locations. Five different metrics are used to quantify the model performances for four storm events. It is shown that the ranking of the models is strongly dependent on the type of metric used to evaluate the model performance. None of the models rank near or at the top systematically for all used metrics. Consequently, one cannot pick the absolute “winner”: the choice for the best model depends on the characteristics of the signal one is interested in. Model performances vary also from event to event. This is particularly clear for root‐mean‐square difference and utility metric‐based analyses. Further, analyses indicate that for some of the models, increasing the global magnetohydrodynamic model spatial resolution and the inclusion of the ring current dynamics improve the models' capability to generate more realistic ground magnetic field fluctuations.

A statistical investigation of the Cowling channel efficiency in the auroral zone

[1] The Cowling channel mechanism describes the creation of a secondary polarization electric field at sharp conductance boundaries in the ionosphere due to excess charges for the case in which the release of these charges to the magnetosphere is fully or partially impeded. The secondary currents generated by the polarization electric field effectively modify the effective ionospheric conductivity inside the Cowling channel. While the Cowling mechanism is generally accepted for the equatorial electrojet, there is a long-standing discussion about the importance of this mechanism and its efficiency in the auroral electrojet. We present a statistical investigation that enables us to identify the most probable geospace conditions and MLT locations for a high Cowling efficiency. This investigation is based on more than 1600 meridional profiles of data from the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment (MIRACLE) network in Scandinavia, in particular, ground magnetic field data from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network and electric field data from the Scandinavian Twin Auroral Radar Experiment (STARE) radar, supported with pointwise ionospheric conductance measurements from the European Incoherent Scatter (EISCAT) radar. We analyze the data in the framework of a 3-D ionospheric model, but our data set is filtered so that only electrojet-type situations are included so that the gradients of all measured quantities in longitudinal direction can be neglected. The analysis results in a steep peak of high Cowling channel efficiency probability in the early morning sector (0245–0645 MLT), with the largest probability around 0500 MLT and for medium and high geomagnetic activity. In agreement with an earlier single-event study by Amm and Fujii (2008), this indicates that the Cowling mechanism may be most effective in the early morning part of the central substorm bulge. Further, our analysis results in an almost monotonic increase of the probability of high Cowling channel efficiency with increasing geomagnetic activity.

Inductive ionospheric solver for magnetospheric MHD simulations

Abstract. We present a new scheme for solving the ionospheric boundary conditions required in magnetospheric MHD simulations. In contrast to the electrostatic ionospheric solvers currently in use, the new solver takes ionospheric induction into account by solving Faraday's law simultaneously with Ohm's law and current continuity. From the viewpoint of an MHD simulation, the new inductive solver is similar to the electrostatic solvers, as the same input data is used (field-aligned current [FAC] and ionospheric conductances) and similar output is produced (ionospheric electric field). The inductive solver is tested using realistic, databased models of an omega-band and westward traveling surge. Although the tests were performed with local models and MHD simulations require a global ionospheric solution, we may nevertheless conclude that the new solution scheme is feasible also in practice. In the test cases the difference between static and electrodynamic solutions is up to ~10 V km−1 in certain locations, or up to 20-40% of the total electric field. This is in agreement with previous estimates. It should also be noted that if FAC is replaced by the ground magnetic field (or ionospheric equivalent current) in the input data set, exactly the same formalism can be used to construct an inductive version of the KRM method originally developed by Kamide et al. (1981).

Characteristics of Magnetic Variations and Current Wedge in the Sawtooth Event on 30 September 2000

AbstractThe purpose of this paper is to study the response of auroral electrojets during the sawtooth event observed by LANL satellites on 30 September 2000. Firstly, we establish a method to determine large‐scale 2‐dimensional ionospheric equivalent current systems in the high‐latitude ionosphere using the spherical elementary current system method. Then we apply this method and magnetic data of high‐latitude magnetometers in the Northern Hemisphere to study the variations of auroral electrojets during the sawtooth event. By comparing characteristics of enhancements of nightside ionospheric westward electrojets with those of positive magnetic bays of H components in the middle/low‐latitude ground magnetic field after each sawtooth injection, it is demonstrated that there are current wedges formed during this sawtooth event. The local time width of current wedges suggested by the two observations are both around 11 hours (MLT). In addition, time durations of middle/low‐latitude magnetic bays to reach maximum variations were generally longer than those for high‐latitude electrojets, implying that source currents for middle/low‐latitude magnetic bays are multiple.

Propagation of interplanetary shock excited ultra low frequency (ULF) waves in magnetosphere-ionosphere-atmosphere—Multi-spacecraft “Cluster” and ground-based magnetometer observations

The ultra low frequency (ULF) wave in magnetosphere can act as an important means for solar wind energy inward transmission. This paper quantitatively analyzes the propagation process of the ULF wave triggered by the interplanetary shock propagating from inner magnetosphere equatorial plane along magnetic field lines to the top of the ionosphere and below ionosphere propagating process and establishes a relatively complete magnetosphere-ionosphere-atmosphere propagation model which can be used to study the relationship between the amplitude of the ULF waves triggered by the interplanetary shock wave in magnetospheric space and the magnetic effect caused by the ULF waves. After a comparison with recent observations, we found that: in the event during November 7, 2004 that an interplanetary shock wave interacted with the magnetosphere, Cluster satellites observed that electric field fluctuations and the band-pass filtered result of ground stations meridional component had similar characteristics. Comparing with the geomagnetic measurement near the footprints, we found that the electric field disturbance in the magnetosphere spread along the ground magnetic field lines in the form of the ULF waves and changed into geomagnetic disturbance. The result reveals that the ULF wave is in contact with the ground geomagnetic observation. The ULF waves couple with ionized components in ionosphere and spread to the ground in the form of electromagnetic waves. In this research, we believe that the magnetosphere, ionosphere and ground magnetic effects caused by interplanetary shock wave are the same physical phenomena responding in different locations. Based on the overall consideration of entire electromagnetic response to the interplanetary shock wave, we found that the correlation between CLUSTER multi-satellite observation and geomagnetic station observation is due to the ULF wave propagated in magnetosphere-ionosphere-atmosphere system, and we quantitatively interpreted this response process.