Negative Interplanetary Magnetic Field Bz Research Articles

Interhemispheric Comparisons of Large Nighttime Magnetic Perturbation Events Relevant to GICs

AbstractNearly all studies of impulsive magnetic perturbation events (MPEs) with large magnetic field variability (dB/dt) that can produce dangerous geomagnetically induced currents (GICs) have used data from the Northern Hemisphere. Here we present details of four large‐amplitude MPE events (|ΔBx| > 900 nT and |dB/dt| > 10 nT/s in at least one component) observed between 2015 and 2018 in conjugate high‐latitude regions (65–80° corrected geomagnetic latitude), using magnetometer data from (1) Pangnirtung and Iqaluit in eastern Arctic Canada and the magnetically conjugate South Pole Station in Antarctica and (2) the Greenland West Coast Chain and two magnetically conjugate chains in Antarctica, AAL‐PIP and BAS LPM. From one to three different isolated MPEs localized in corrected geomagnetic latitude were observed during three premidnight events; many were simultaneous within 3 min in both hemispheres. Their conjugate latitudinal amplitude profiles, however, matched qualitatively at best. During an extended postmidnight interval, which we associate with an interval of omega bands, multiple highly localized MPEs occurred independently in time at each station in both hemispheres. These nighttime MPEs occurred under a wide range of geomagnetic conditions, but common to each was a negative interplanetary magnetic field Bz that exhibited at least a modest increase at or near the time of the event. A comparison of perturbation amplitudes to modeled ionospheric conductances in conjugate hemispheres clearly favored a current generator model over a voltage generator model for three of the four events; neither model provided a good fit for the premidnight event that occurred near vernal equinox.

Environmental Conditions During the Reported Charging Anomalies of the Two Geosynchronous Satellites: Telstar 401 and Galaxy 15

AbstractCharging and subsequent electrostatic discharge is recognized as a serious operational threat to spacecraft. This paper concentrates on an analysis of the environmental conditions (geomagnetic measurables, but not the particle fluxes, that were associated with the geomagnetic disturbances) and the interplanetary signatures prior to the anomalies that affected the two geostationary satellites Telstar 401 and Galaxy 15, which were believed to have been caused by spacecraft charging/electrostatic discharge. In terms of geomagnetic conditions, the planetary Kp index was very different around the two events, despite Kp commonly being used to characterize periods that are hazardous for spacecraft charging. On the contrary, local geomagnetic records, both from GOES satellites and from ground records, reveal that each anomaly took place during the expansion phase of a triggered substorm. Combined with a large and south directed interplanetary magnetic field (IMF) Bz, northward turnings of negative IMF Bz and large fluctuations in IMF By played major roles in the magnetospheric responses of the substorms. The interplanetary features of the two events were also similar: the interplanetary counterpart of coronal mass ejections interacted with high‐speed streams, favoring the substorm triggers. Although there are several differences in the preconditioning and the inner conditions of the magnetosphere before the anomalies, in each case the respective spacecraft crossed the plasma sheet in the magnetotail during enhanced substorm activity, with a dipolarization event that took place close to the spacecraft location, indicating that each satellite was in the wrong place at a critical moment.

IMF Control of Alfvénic Energy Transport and Deposition at High Latitudes

AbstractWe investigate the influence of the interplanetary magnetic field (IMF) clock angle ϕIMF on high‐latitude inertial Alfvén wave (IAW) activity in the magnetosphere‐ionosphere transition region using Fast Auroral SnapshoT (FAST) satellite observations. We find evidence that negative IMF Bz coincides with nightside IAW power generation and enhanced rates of IAW‐associated electron energy deposition, while positive IMF Bz coincides with enhanced dayside wave and electron energy deposition. Large ( nT) negative IMF By coincides with enhanced postnoon IAW power, while large positive IMF By coincides with enhanced but relatively weaker prenoon IAW power. For each ϕIMF orientation we compare IAW Poynting flux and IAW‐associated electron energy flux distributions with previously published distributions of Alfvénic Poynting flux over ∼2–22 mHz, as well as corresponding wave‐driven electron energy deposition derived from Lyon‐Fedder‐Mobarry global MHD simulations. We also compare IAW Poynting flux distributions with distributions of broad and diffuse electron number flux, categorized using an adaptation of the Newell et al. (2009) precipitation scheme for FAST. Under negative IMF Bz in the vicinity of the cusp (9.5–14.5 magnetic local time), regions of intense dayside IAW power correspond to enhanced diffuse electron number flux but relatively weaker broadband electron precipitation. Differences between cusp region IAW activity and broadband precipitation illustrate the need for additional information, such as fields or pitch angle measurements, to identify the physical mechanisms associated with electron precipitation in the vicinity of the cusp.

Electron Fluxes at Geostationary Orbit From GOES MAGED Data

AbstractElectron behavior in energies below 200 keV at geostationary orbit has significance for satellite operations due to charging effects on spacecraft. Five years of keV energy electron measurements by the geostationary GOES‐13 satellite's MAGnetospheric Electron Detector (MAGED) instrument has been analyzed. A method for determining flight direction integrated fluxes is presented. The electron fluxes at the geostationary orbit are shown to have significant dependence on solar wind speed and interplanetary magnetic field (IMF) BZ: increased solar wind speed correlates with higher electron fluxes with all magnetic local times while negative IMF BZ increases electron fluxes in the 0 to 12 magnetic local time sector. A predictive empirical model for electron fluxes in the geostationary orbit for energies 40, 75, and 150 keV was constructed and is presented here. The empirical model is dependent on three parameters: magnetic local time, solar wind speed, and IMF BZ.

Solar wind controls on Mercury's magnetospheric cusp

AbstractThis study assesses the response of the cusp to solar wind changes comprehensively, using 2848 orbits of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observation. The assessment entails four steps: (1) propose and validate an approach to estimate the solar wind magnetic field (interplanetary magnetic field (IMF)) for MESSENGER's cusp transit; (2) define an index σ measuring the intensity of the magnetic disturbance which significantly peaks within the cusp and serves as an indicator of the cusp activity level; (3) construct an empirical model of σ as a function of IMF and Mercury's heliocentric distance rsun, through linear regression; and (4) use the model to estimate and compare the polar distribution of the disturbance σ under different conditions for a systematic comparison. The comparison illustrates that the disturbance peak over the cusp is strongest and widest extending in local time for negative IMF Bx and negative IMF Bz, and when Mercury is around the perihelion. Azimuthal shifts are associated with both IMF By and rsun: the cusp moves toward dawn when IMF By or rsun decrease. These dependences are explained in terms of the IMF Bx‐controlled dayside magnetospheric topology, the component reconnection model applied to IMF By and Bz, and the variability of solar wind ram pressure associated with heliocentric distance rsun. The applicability of the component reconnection model on IMF By indicates that at Mercury reconnection occurs at lower shear angles than at Earth.

Van Allen Probes observation and modeling of chorus excitation and propagation during weak geomagnetic activities

AbstractWe report correlated data on nightside chorus waves and energetic electrons during two small storm periods: 1 November 2012 (Dst≈−45) and 14 January 2013 (Dst≈−18). The Van Allen Probes simultaneously observed strong chorus waves at locations L = 5.8–6.3, with a lower frequency band 0.1–0.5fce and a peak spectral density ∼10−4 nT2/Hz. In the same period, the fluxes and anisotropy of energetic (∼10–300 keV) electrons were greatly enhanced in the interval of large negative interplanetary magnetic field Bz. Using a bi‐Maxwellian distribution to model the observed electron distribution, we perform ray tracing simulations to show that nightside chorus waves are indeed produced by the observed electron distribution with a peak growth for a field‐aligned propagation approximately between 0.3fce and 0.4fce, at latitude <7°. Moreover, chorus waves launched with initial normal angles either θ<90° or >90° propagate along the field either northward or southward and then bounce back either away from Earth for a lower frequency or toward Earth for higher frequencies. The current results indicate that nightside chorus waves can be excited even during weak geomagnetic activities in cases of continuous injection associated with negative Bz. Moreover, we examine a dayside event during a small storm C on 8 May 2014 (Dst≈−45) and find that the observed anisotropic energetic electron distributions potentially contribute to the generation of dayside chorus waves, but this requires more thorough studies in the future.

THEMIS Na exosphere observations of Mercury and their correlation with in-situ magnetic field measurements by MESSENGER

The Na exosphere of Mercury is being studied since its discovery in mid ‘80s from Earth-based telescopes, and it has revealed a high dynamics and variability. Although the processes and their relationships characterising the Hermean exosphere generation and dynamics are still not exhaustively understood, there are no doubts on a tight interconnection among the planet's surface, exosphere, intrinsic magnetic field, the solar wind and the Interplanetary Magnetic Field (IMF). In this paper we analyze an extended dataset of images of the exospheric Na emission, collected from 2009 to 2013, by means of the THEMIS ground-based telescope, in order to perform a comprehensive statistical study of the recurrent Na emission patterns, and also their potential relationship with the IMF variability. For this purpose, we take advantage of a subset (years 2011–2013) of contemporary in-situ measurements of the IMF obtained by the MAG instrument on-board the MESSENGER spacecraft. We found that the high latitude double peak is the most common Na emission pattern, supporting the view that the solar wind ion precipitation through the polar cusps has an important role in the generation of the observed Na exospheric configuration. Moreover, the lack of a statistically significant North–South asymmetry seems to disfavor the existence of an asymmetric and/or shifted intrinsic magnetic dipole. By analyzing a subset of quasi-full disk images, we found that the double peak Na emission is typically aligned along the meridian, mostly occurring in the pre-noon sector (53%), about 1/3 close to the noon meridian (36%), whereas only 11% takes place in the post-noon sector. Finally, the comparison with the IMF data seems to indicate that the contribution of the IMF BX component to the magnetic reconnection is generally weak, even if we found a noticeable correlation between positive IMF BX and symmetric double peak pattern. Negative IMF BZ values are usually connected to double peak emission, whereas positive IMF BZ values are more frequently associated to single peaked equatorial Na emission.

Intensity asymmetries in the dusk sector of the poleward auroral oval due to IMF Bx

AbstractIn the exploration of global‐scale features of the Earth's aurora, little attention has been given to the radial component of the Interplanetary Magnetic Field (IMF). This study investigates the global auroral response in both hemispheres when the IMF is southward and lies in the xz plane. We present a statistical study of the average auroral response in the 12–24 magnetic local time (MLT) sector to an x component in the IMF. Maps of auroral intensity in both hemispheres for two IMF Bx dominated conditions (± IMF Bx) are shown during periods of negative IMF Bz, small IMF By, and local winter. This is obtained by using global imaging from the Wideband Imaging Camera on the IMAGE satellite. The analysis indicates a significant asymmetry between the two IMF Bx dominated conditions in both hemispheres. In the Northern Hemisphere the aurora is brighter in the 15–19 MLT region during negative IMF Bx. In the Southern Hemisphere the aurora is brighter in the 16–20 MLT sector during positive IMF Bx. We interpret the results in the context of a more efficient solar wind dynamo in one hemisphere. Both the intensity asymmetry and its location are consistent with this idea. This has earlier been suggested from case studies of simultaneous observations of the aurora in both hemispheres, but hitherto never been observed to have a general impact on global auroral brightness in both hemispheres from a statistical study. The observed asymmetries between the two IMF Bx cases are not large; however, the difference is significant with a 95% confidence level. As the solar wind conditions examined in the study are rather common (37% of the time) the accumulative effect of this small influence may be important for the total energy budget.

Climatology of zonal wind and large-scale FAC with respect to the density anomaly in the cusp region: seasonal, solar cycle, and IMF <i>B</i><sub><i>y</i></sub> dependence

Abstract. We investigate the relationship of the thermospheric density anomaly (ρrel) with the neutral zonal wind velocity (Uzonal), large-scale field-aligned current (FAC), small-scale FAC, and electron temperature (Te) using the superposed epoch analysis (SEA) method in the cusp region. The dependence of these variables on the sign of the interplanetary magnetic field (IMF) By component and local season is of particular interest. Also, the conditions that lead to larger relative density enhancements are investigated. Our results are based on CHAMP satellite data and OMNI online data of IMF for solar maximum (March 2002–March 2007) and minimum (March 2004–March 2009) conditions in the Northern Hemisphere. In the cusp region the SEA technique uses the time and location of the mass density anomaly peaks as reference parameters. On average, the amplitude of the relative density anomaly, ρrel, does not depend on the solar cycle phase, local season, and IMF By sign. Also, it is apparent that the amplitude of IMF By does not have a large influence on ρrel, while the negative IMF Bz amplitude prevailing about half an hour earlier is in good correlation with ρrel. Both the zonal wind velocity and the large-scale FAC (LSFAC) distribution exhibit a clear dependence on the IMF By sign. Uzonal is directed towards dawn for both positive and negative IMF By at all local seasons and for solar maximum and minimum conditions. There is a systematic imbalance between downward (upward) and upward (downward) large-scale FACs peaks equatorward and poleward of the reference point, respectively, for the IMF By+ (By−) case. Relative density enhancements appear halfway between region 1 and region 0 currents in closer proximity to the upward FAC region. FAC densities and mass density anomaly amplitudes are not well correlated, but it is apparent that there is a close spatial relationship between ρrel and LSFAC. At this point we cannot offer any simple functional relation between these two variables, because there seem to be additional quantities controlling this relation.

Saturn's dayside ultraviolet auroras: Evidence for morphological dependence on the direction of the upstream interplanetary magnetic field.

We examine a unique data set from seven Hubble Space Telescope (HST) “visits” that imaged Saturn's northern dayside ultraviolet emissions exhibiting usual circumpolar “auroral oval” morphologies, during which Cassini measured the interplanetary magnetic field (IMF) upstream of Saturn's bow shock over intervals of several hours. The auroras generally consist of a dawn arc extending toward noon centered near ∼15° colatitude, together with intermittent patchy forms at ∼10° colatitude and poleward thereof, located between noon and dusk. The dawn arc is a persistent feature, but exhibits variations in position, width, and intensity, which have no clear relationship with the concurrent IMF. However, the patchy postnoon auroras are found to relate to the (suitably lagged and averaged) IMF Bz, being present during all four visits with positive Bz and absent during all three visits with negative Bz. The most continuous such forms occur in the case of strongest positive Bz. These results suggest that the postnoon forms are associated with reconnection and open flux production at Saturn's magnetopause, related to the similarly interpreted bifurcated auroral arc structures previously observed in this local time sector in Cassini Ultraviolet Imaging Spectrograph data, whose details remain unresolved in these HST images. One of the intervals with negative IMF Bz however exhibits a prenoon patch of very high latitude emission extending poleward of the dawn arc to the magnetic/spin pole, suggestive of the occurrence of lobe reconnection. Overall, these data provide evidence of significant IMF dependence in the morphology of Saturn's dayside auroras.Key PointsWe examine seven cases of joint HST Saturn auroral images and Cassini IMF dataThe persistent but variable dawn arc shows no obvious IMF dependencePatchy postnoon auroras are present for northward IMF but not for southward IMF

Planetary distribution of geomagnetic pulsations during a geomagnetic storm at solar minimum

We investigate the features of the planetary distribution of wave phenomena (geomagnetic pulsations) in the Earth’s magnetic shell (the magnetosphere) during a strong geomagnetic storm on December 14–15, 2006, which is untypical of the minimum phase of solar activity. The storm was caused by the approach of the interplanetary magnetic cloud towards the Earth’s magnetosphere. The study is based on the analysis of 1-min data of global digital geomagnetic observations at a few latitudinal profiles of the global network of ground-based magnetic stations. The analysis is focused on the Pc5 geomagnetic pulsations, whose frequencies fall in the band of 1.5–7 mHz (T ∼ 2–10 min), on the fluctuations in the interplanetary magnetic field (IMF) and in the solar wind density in this frequency band. It is shown that during the initial phase of the storm with positive IMF Bz, most intense geomagnetic pulsations were recorded in the dayside polar regions. It was supposed that these pulsations could probably be caused by the injection of the fluctuating streams of solar wind into the Earth’s ionosphere in the dayside polar cusp region. The fluctuations arising in the ionospheric electric currents due to this process are recorded as the geomagnetic pulsations by the ground-based magnetometers. Under negative IMF Bz, substorms develop in the nightside magnetosphere, and the enhancement of geomagnetic pulsations was observed in this latitudinal region on the Earth’s surface. The generation of these pulsations is probably caused by the fluctuations in the field-aligned magnetospheric electric currents flowing along the geomagnetic field lines from the substorm source region. These geomagnetic pulsations are not related to the fluctuations in the interplanetary medium. During the main phase of the magnetic storm, when fluctuations in the interplanetary medium are almost absent, the most intense geomagnetic pulsations were observed in the dawn sector in the region corresponding to the closed magnetosphere. The generation of these pulsations is likely to be associated with the resonance of the geomagnetic field lines. Thus, it is shown that the Pc5 pulsations observed on the ground during the magnetic storm have a different origin and a different planetary distribution.

Response of thermosphere density to changes in interplanetary magnetic field sector polarity

[1] A systematic analysis of the thermospheric density response to changes in the interplanetary magnetic field (IMF) sector polarity is carried out. For this purpose we use a high-latitude southern thermospheric total mass density near 400 km altitude, derived from the high-accuracy accelerometer on board the Challenging Minisatellite Payload (CHAMP) spacecraft in 2003, a period of a well-defined IMF sector polarity change. The IMF sector polarity changes appear to strongly influence the high-latitude thermospheric density variations, especially in equinox seasons. After normalization to a constant solar flux level, densities in the Southern Hemisphere near the March equinox show a significant differences, depending on whether the IMF field polarity is toward the Sun (“toward sector,” i.e., +Bx and −By) or away from the Sun (“away sector,” i.e., −Bx and +By). Densities in the toward sector near the March equinox increase before the sector boundary passes the Earth, with strong enhancements in the cusp region and the premidnight sector. Densities in the away sector near the March equinox decrease before the sector boundary passes the Earth, with a significant decrease in the early morning hours. On the other hand, near the September equinox, densities in the Southern Hemisphere do not show significant changes associated with the IMF sector polarity changes. The IMF By and the Bz offsets associated with the IMF sector polarity changes are related to specific behaviors in terms of thermospheric densities. In the toward (away) sector near the March equinox, IMF conditions that increase (decrease) the high-latitude southern thermospheric densities, the negative (positive) By and the negative (positive) Bz offsets, are maintained. On the other hand, in the toward (away) sector near the September equinox, the negative (positive) IMF By condition, which increases (decreases) the high-latitude southern thermospheric densities, and the positive (negative) IMF Bz offset condition, which decreases (increases) the high-latitude southern thermospheric densities, coexist.

Initial development of HF radar polar patch caused by azimuthal flow burst in the cusp

In this study, we report evidence proving the existence of a close relationship between the initial development of a poleward propagating feature of the HF radar backscatter, that is, HF radar polar patch, and a fast azimuthal flow in the cusp. We focus on two events in which the HF radar polar patch was formed and was identified by the Super Dual Auroral Radar Network radars at Saskatoon, Canada, and Prince George, Canada, on 27 March 2001. We examine their spatial and temporal development. The two events occurred at the interface between the interplanetary magnetic field (IMF) BY‐controlled azimuthal flow and the antisunward flow typical of a negative IMF BZ; they developed in a longitudinally asymmetric manner. The increase in ∣BY/BZ∣ when BZ was negative triggered the fast azimuthal flow. We interpret the temporal and spatial development of the HF radar polar patch in terms of the plasma density gradient created by the azimuthal flow and in terms of the subsequent development of the gradient in the direction of both the azimuthal flow and the antisunward flow.

Storm-time related mass density anomalies in the polar cap as observed by CHAMP

Abstract. Strong and localized thermospheric mass density events are observed in the polar cap region by the CHAMP satellites at about 400 km altitude during geomagnetic storms. During the 4 years considered (2002–2005) 29 storms with Dst<−100 nT occurred, in 90% of them polar cap density anomalies were detected. Based on the altogether 56 anomaly events a statistical analysis was performed. The anomalies are of medium scale (500–1500 km) and seem to have a short dwell-time (<1.5 h) in the polar cap. The relative density enhancement is found to range around 2 in both hemispheres. The peak density is in the Northern Hemisphere by a factor of 1.4 larger than in the southern. Also the number of detected events in the north is twice as large as that in the south (37 vs. 19). Mass density anomalies in the polar cap occur under all interplanetary magnetic field (IMF) directions. Numerous strong anomalies have been detected in positive and negative IMF Bz conditions when the magnetic field strength is above 5 nT. Rather few events occurred for small |Bz| (<5 nT) or for positive Bz combined with vanishing By. Some of the density anomalies are accompanied by intensive small-scale field-aligned currents (FACs). But about as many show no relation to FACs. If FACs are present there, the current density is believed to be correlated with the strength of the IMF Bz. Although this paper concentrates on the presentation of the observations, we show for one event that the ion outflow mechanism could be responsible for the mass density anomalies in the polar cap.

Ion heating in high-speed flow channel within the duskside cell of the polar cap ion convection under large IMF-Bycondition

[1] F region strong sunward ion flow embedded in the duskside cell of expanding polar cap ion convection and coincidental increase in the ion temperature were observed from about 73° to 69° in geomagnetic latitude between 16:00 and 16:48 MLT on 19 August, 2006 from an experiment using the European Incoherent Scatter (EISCAT) radars at Tromso, Sodankyla and Longyearbyen together with the CUTLASS HF radars. The spatiotemporal structure of the convection map obtained by the SuperDARN HF radars showed that the high-speed flow channel elongated in 14–17 MLT was moving equatorward. The flow channel appeared during an interval of negative interplanetary magnetic field Bz associated with large positive By. In order to evaluate ion frictional heating and resultant ion temperature increase quantitatively, the experiment was conducted to estimate the ion velocity vector and the ion temperature at the common scattering volume of the EISCAT radars. While the flow channel moved equatorward, the ion temperature increase became less evident at higher part of latitudes within the flow channel, which means that ion frictional heating was depressed where the neutral gases had already responded to the high-speed ions. The increase in the ion temperature at particular geomagnetic latitude continued for about 20 min, and then ceased while the ion velocity was still enhanced. Ion frictional heating lasted until the neutral wind was accelerated to the equivalent level of the surrounded ion flow.

Characteristics of ion upflow and downflow observed with the European Incoherent Scatter Svalbard radar

We have investigated how geomagnetic activity, the solar wind (SW), and the interplanetary magnetic field (IMF) influence the occurrence of the F‐region/topside ionospheric ion upflow and downflow. Occurrence of dayside ion upflow observed with the European Incoherent Scatter Svalbard radar (ESR) at 75.2° magnetic latitude is highly correlated with the SW density, as well as with the strength of the IMF By component. We suggest that this correlation exists because the region where ion upflow occurs is enlarged owing to SW density and IMF By magnitude, but it does not move significantly in geomagnetic latitude. The occurrence frequency of dayside ion upflow displays peaks versus the geomagnetic activity index (Kp), SW velocity, and negative IMF Bz component; that is, ion upflow is less frequently seen at the highest values of these parameters. Dayside ion downflow in the F‐region/topside ionosphere occurs only when the Kp index and/or SW velocity are high or when IMF Bz is largely negative. The ion downflow is likely due to ballistic return of the ion upflow. We suggest that the region of ion upflow not only becomes larger but also moves equatorward with increasing Kp, SW velocity, and negative IMF Bz. The ESR can so be poleward of the upflow region and observe ions convecting poleward and returning ballistically downward.

Analysis of the Correlations between the Occurrence of Substorm Injections and Interplanetary Parameters during the Declining Phase of Solar Cycle 23

We have examined the relationship between magnetospheric substorms identi ed by LANL (Los Alamos National Laboratory) particle injections and daily interplanetary parameters observed by ACE (Advanced Composition Explorer) and Geotail during the second half of 2003, which is the declining phase of solar cycle 23. We found a good correlation (cc = 0.71) between daily substorm injection occurrence and the daily median solar wind speed, implying that solar wind speed strongly modulates substorm injection. Because an IMF (Interplanetary Magnetic Field) Alfv enic structure accompanies a high-speed solar wind, we also investigated the relationship between IMF Bz parameters and substorm occurrences. We found that neither the average strength of a negative IMF Bz nor its duration was primarily responsible for the increase in substorm injections with increasing solar wind speed, but we found that the number of north-south turnings of the IMF Bz increased with increasing solar wind speed, implying the possible role of the IMF Bz northward triggering of substorms. We tested various types of northward turning of an IMF for the correlation with substorm occurrence. We found a range of correlation coe cient from 0.6 to 0.8 for most trigger types that we tested. In particular, the correlation coe cient was 0.71 for the trigger type of IMF Bz northward turning by an amount by 2 nT after a growth phase with an average IMF Bz = 2 nT and a duration of 20 min. This is same as that obtained between the daily occurrence of substorms and the daily median solar wind speed, implying that the either solar wind speed or the IMF northward turning or both may play a signi cant role in leading to repetitive substorm occurrence during high-speed streams.

Comparison of simulated and observed large-scale, field-aligned current structures

Abstract. Recently, a model of large-scale, field-aligned current (FAC) structures, based on zero-frequency MHD surface wave (SW) modes that can emerge from the solar wind-Earth's magnetosphere interaction, has been proposed. The FAC polarity and intensity distribution are quantified as a function of the solar wind parameters and the interplanetary magnetic field (IMF) magnitude that enter as input parameters. Besides, there are input parameters intrinsic to the Earth's magnetosphere – the size of the polar cap and the boundary regions and their plasma density variations. Influence of the IMF By component on the FAC structure is examined here. Depending on the IMF By magnitude, the predicted six-cell FAC structure tends to evolve in a spiral-like fashion. This large-scale FAC model is compared with experimental evidences and empirical FAC models based on DE-2 satellite data and high-precision Oersted and Magsat satellite magnetometer data. Among the various achievements of these long-term satellite measurements, an observation/discovery of a ground-based state of FACs which includes a pair of large-scale FACs in the polar cap under both positive and negative IMF Bz has been pointed out. The FAC pattern is qualitatively and quantitatively consistent with experimental data for both polar cap FAC and Region 1 and Region 2 FAC systems.

Solar wind disturbances responsible for geomagnetic storms

The superposed epoch method is used to construct a typical behavior of solar wind parameters before and during strong isolated geomagnetic storms. A total of 130 geomagnetic storms during the period of 1966–2000 were analyzed. The results obtained show that the averaged disturbance in the solar wind responsible for geomagnetic storms is associated with the compression of ambient solar wind plasma and interplanetary magnetic field (IMF) ahead of a high‐speed plasma flow. The magnetic field strength and plasma density start to increase several hours before geomagnetic storm onset. However, the negative IMF Bz (which is responsible for the onset of strong negative Dst) starts to increase approximately 4–5 hours after the maximum variations in plasma density and IMF By. The time delays between peaks in negative IMF Bz and plasma density as well as IMF ∣By∣ (in the solar‐magnetospheric coordinate system) may be a result of the compression and three‐dimensional draping of the ambient magnetic field about high‐speed flows as discussed earlier by some authors. The combined effect of the compression and draping of the ambient magnetic field may lead to an increase in plasma density and IMF ∣By∣ ahead of a high‐speed flow followed by an increase in IMF ∣Bz∣ in its central portion. The motion of such structure leads to a time delay between the peaks in the IMF ∣By∣ and ∣Bz∣ observed at satellites.

Double cusp: Model prediction and observational verification

Recent modeling of the entry of solar wind plasma into the magnetosphere and ionosphere has adequately simulated the large‐scale particle precipitation features in the observed cusp, mantle, polar rain, and open‐field line low‐latitude boundary layer regions. The assumption of a simple dawn‐dusk electric field limited the models to the near‐noon region and southward interplanetary magnetic field (IMF) case. Here, we present an improved model that incorporates the electric field obtained from statistical convection patterns. When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of a double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower‐latitude cusp ions originate from low‐latitude magnetosheath, whereas the higher‐latitude ions originate from the high‐latitude magnetosheath. The lower‐latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp, as would be observed from a typical meridional trajectory of a polar‐orbiting satellite. The higher‐latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher‐latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. This prediction was subsequently confirmed in a large case study with Defense Meteorological Satellite Program (DMSP) data. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF By and small negative IMF Bz ,30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double‐cusp result is consistent with the following new statistical results: (1) the cusp latitudinal width increases with |IMF By| and (2) the cusp equatorward boundary moves to lower latitude with increasing |IMF By|.