Southward Turnings Research Articles

Ionospheric Responses to IMF Southward Turnings in Mid- and Low-latitudes

利用卫星和地面台站的历史数据, 研究了中低纬电离层f0F2 对强行星际磁场南向翻转的响应. 结果表明, 行星际磁场南向翻转能引起电离层扰动式响应, 响应特性与纬度、季节和翻转时刻的地方时有关. 在中纬, 发生在夏分季和夜间的翻转能造成较强的电离层负响应, 其幅度随纬度的降低而变弱, 在恢复过程中存在不规则振荡; 在低纬, 南向翻转引起的电离层响应在夏分季较强, 在冬季则较弱, 且易被淹没在电离层自身的扰动中. 分析指出电离层最大负响应与翻转后南向磁场极大值之间有着较好的线性关系.

Poker flat radar observations of the magnetosphere–ionosphere coupling electrodynamics of the earthward penetrating plasma sheet following convection enhancements

The plasma sheet moves earthward (equatorward in the ionosphere) after enhancements in convection, and the electrodynamics of this response is strongly influenced by Region 2 magnetosphere–ionosphere coupling. We have used Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) observations associated with two relatively abrupt southward turnings of the IMF to provide an initial evaluation of aspects of this response. The observations show that strong westward sub-auroral polarization streams (SAPS) flow regions moved equatorward as the plasma sheet electron precipitation (the diffuse aurora) penetrated equatorward following the IMF southward turnings. Consistent with our identification of these flows as SAPS, concurrent DMSP particle precipitation measurements show the equatorial boundary of ion precipitation equatorward of the electron precipitation boundary and that westward flows lie within the low-conductivity region between the two boundaries where the plasma sheet ion pressure gradient is expected to drive downward R2 currents. Evidence for these downward currents is seen in the DMSP magnetometer observations. Preliminary examination indicates that the SAPS response seen in the examples presented here may be common. However, detailed analysis will be required for many more events to reliably determine if this is the case. If so, it would imply that SAPS are frequently an important aspect of the inner magnetospheric electric field distribution, and that they are critical for understanding the response of the magnetosphere–ionosphere system to enhancements in convection, including understanding the earthward penetration of the plasma sheet. This earthward penetration is critical to geomagnetic disturbance phenomena such as the substorm growth phase and the formation of the stormtime ring current. Additionally, for one example, a prompt electric field response to the IMF southward turnings is seen within the inner plasma sheet.

A Numerical Study of the Track Deflection of Supertyphoon Haitang (2005) Prior to Its Landfall in Taiwan

Abstract A series of numerical simulations are conducted using the advanced research version of the Weather Research and Forecasting model with a 4-km fine mesh to examine the physical processes responsible for the significant track deflection and looping motion before the landfall of Supertyphoon Haitang (2005) in Taiwan, which poses a unique scientific and forecasting issue. In the control experiment, a low-level northerly jet induced by the channeling effect forms in the western quadrant of the approaching storm, where the inner-core circulation is constrained by the presence of Taiwan’s terrain. Because of the channeling effect, the strongest winds of the storm are shifted to the western portion of the eyewall. The northerly advection flow (averaged asymmetric winds within 100-km radius) results in a sharp southward turn of the westward-moving storm. The time series of the advection flow shows that the advection wind vectors rotate cyclonically in time and well match the motion of the simulated vortex during the looping process. A sensitivity study of lowering the Taiwan terrain elevations to 70% or 40% of those in the control experiment reduces the southward track deflection and loop amplitude. The experiment with the reduced elevation to 10% of the control experiment does not show a looping track and thus demonstrates the key role of the terrain-induced channeling effect. Experiments applying different values of the structure parameter α illustrate that increasing the strength, size, and translation speed of the initial storm results in a smaller interaction with Taiwan’s terrain and a smaller average steering flow caused by the asymmetric circulation, which leads to a proportionally smaller southward track deflection without making a loop.

Low-energy ion precipitation during the Halloween storm

During the Halloween storm of October 29–31, 2003, four defense meteorological satellite program (DMSP) satellites detected fluxes of low-energy ions precipitating well equatorward of auroral electrons in the dawn/morning magnetic local time sector during the main phase. There were three southward turnings of the interplanetary magnetic field (IMF), leading to three intensifications of the ring current to DST values of −125, −310 and −350 nT. In each case the ion fluxes weakened dramatically and/or vanished during DST recoveries. DMSP only encountered short-lived episodes of subauroral ion precipitation in the dusk/evening sector. A brief survey of the DMSP database reveals that near-dawn, ion precipitation is a main-phase characteristic of all large magnetic storms. DMSP satellites also detected similar ion precipitation during the main phase of the March 1991 magnetic storm. During an outbound pass of the combined release radiation effect satellite (CRRES) through the inner magnetosphere it detected low energy (<1 keV) ion fluxes that were collocated with, but spectrally separated in energy from the ring current “nose structure”. As CRRES moved toward perigee in the dawn sector, it crossed similar low-energy (⩽500 eV) ions embedded in a broad ion population, located earthward of plasma sheet electrons. Simultaneously the DMSP F8 satellite detected spectrally similar low-energy ions precipitating at subauroral latitudes near dawn at the same invariant latitudes. To reconcile DMSP/CRRES observations with elementary concepts of allowed ion drift paths, the data suggest two source populations. The lowest-energy ions, of ionospheric origin, were initially energized earthward of the plasma sheet electron boundary in the evening local-time sector then co-rotated eastward. Higher-energy ions originated in the plasma sheet and drifted close to the Earth under the combined influences of time-varying convective electric fields and azimuthal gradients in the Earth's magnetic field generated by the stormtime ring current [Tsyganenko, N.A., Singer, H. J., Kasper, J. C., 2003. Storm-time distortion of the inner magnetosphere: how severe can it get? Journal of Geophysical Research, 108 (A5), 1209].

Origin of geosynchronous relativistic electron events

We have investigated the characteristics of solar wind and magnetospheric conditions associated with the occurrence of geosynchronous relativistic electron events. Most of the geosynchronous relativistic events for April 1999 to December 2002 are found to occur during prolonged (a number of days) quiet intervals following the appearance of high‐speed solar wind streams. In a typical relativistic event, the electron fluxes begin to increase by orders of magnitude when the solar wind density drops after reaching a sharp peak at the leading edge of a high‐speed stream. The increased fluxes stay at a high level until the quiet solar wind conditions cease. In addition, enhanced ULF wave activity and substorm injections of 10s to 100s keV electrons are observed at the time of the large flux increases in the events. We found that geosynchronous relativistic events can be observed only when both the solar wind and magnetospheric wave/substorm injection conditions are favorable regardless of whether or not a magnetic storm takes place. These observations suggest the following scenario for the occurrence of a geosynchronous relativistic electron event: (1) Quiet solar wind conditions (i.e., no strong solar wind pressure and large southward turnings of IMF Bz) can lead to stable and more dipole‐like magnetospheric configurations in which the geosynchronous orbit is located well inside the trapping boundary of the energetic electrons. (2) If a large population of MeV electrons is generated (by some acceleration process(es) involving enhanced ULF wave and substorm injections) in the inner magnetosphere, it can be trapped and effectively accumulated to a high intensity. (3) The high electron flux can persist for a number of days in the geosynchronous region as long as the solar wind conditions remain quiescent. The occurrence of a geosynchronous relativistic electron event requires not only the proper acceleration process and sufficient seed electrons but also no significant loss process that dominates over any acceleration/source.

Ion precipitation in the dawn sector during geomagnetic storms

Three southward turnings of the IMF during the magnetic storm of 29–31 October 2003 led to ring current intensifications characterized by Dst minima. Defense Meteorological Satellite Program (DMSP) satellites detected low‐energy ions precipitating equatorward of auroral electrons in the dawn/morning local time sector. These ion fluxes weakened and/or vanished subsequent to Dst recoveries. A survey of the DMSP database reveals that near‐dawn ion precipitation is a main‐phase characteristic of all large magnetic storms. DMSP and Combined Release and Radiation Effects Satellite (CRRES) satellites also detected similar ion precipitation during the main phase of the March 1991 magnetic storm. To reconcile these observations with elementary concepts of ion drifts, the data suggest two source populations. The lowest‐energy ions were initially energized earthward of the plasma sheet electron boundary in the evening sector then corotated eastward. Higher‐energy ions originated in the plasma sheet and drifted close to the Earth under the combined influences of time‐varying convective electric fields and azimuthal gradients in the Earth's magnetic field. We developed a modified Volland Stern model to study test‐particle trajectories in asymmetric magnetic fields and variable electric fields compatible with CRRES measurements. Ions with magnetic moments μ ≥ μc ≈ 14 eV/nT drift duskward. Ions with μ &lt; μc are injected to low L shells during a spike in the convective electric field. As the electric field diminishes and shielding increases, these ions drift toward dawn much closer to Earth than any test electron trajectories. Precipitation in the dawn sector results from a combination of pitch‐angle scattering by intense ambient waves and the dictates of ion drift paths.

Variations of polar cap index in response to solar wind changes and magnetospheric substorms

The polar cap (PC) index is used to measure the geomagnetic activity over the polar caps. Changes in the solar wind are able to cause disturbances in the magnetospheric‐ionospheric currents, which in turn cause variations in the PC index. In order to understand how different processes in the solar wind and magnetosphere influence the PC index, it is necessary to separate the effects of solar wind pressure impulses, interplanetary magnetic field (IMF) reorientations, and magnetospheric substorms. We study the variations of the PC index under different solar wind conditions and during substorms and present case analyses and statistical results. The main conclusions are as follows. A solar wind pressure impulse alone, without IMF southward turning or substorm, is to cause a negative spike of ∼1 in the PC index within a time interval of 2–6 min and a subsequent increase to a positive value of ∼1 within 30 min. The PC index enhances significantly after the IMF turns southward, and the increase of the PC index is 3–5 in most cases and can be as large as 8–9. When a solar wind pressure impulse occurs with a simultaneous southward turning of the IMF, the large increase of the PC index is mainly a response to the southward IMF but not to the solar wind pressure impulse. Magnetospheric substorms have significant effects on the PC index. The response of the PC index to substorms is an increase of 2–4 in most cases and can reach 6 or larger. The effects of IMF southward turnings and substorms on the PC index are much stronger than that of solar wind pressure impulses. On average, the increases of the PC index caused by solar wind pressure impulses, substorm onsets, and IMF southward turnings are 0.8, 3.2, and 3.6, respectively.

Statistical identification of solar wind origins of magnetic impulse events

We have compiled a complete list of magnetic impulse events (MIEs) during the 8‐year period of 1995–2002 covering solar minimum to solar maximum using fluxgate magnetometer data obtained at the South Pole. Wavelet analysis enables us to detect 825 distinct MIEs automatically with high confidence. Interplanetary magnetic field (IMF) discontinuities are also detected automatically for the same period from Wind and ACE satellite data. From an examination and comparison of the two lists, we find that monthly and seasonal variations in IMF discontinuity occurrences have a significant correlation (0.4–0.5) with those of MIEs. We also find that MIEs tend to occur during intervals of low density, high‐speed solar wind streams, and/or radial IMF. No preferences for MIE occurrences are found for solar wind pressure jumps or IMF Bz southward turnings. From detailed minimum variance analysis of 36 IMF discontinuities with one‐to‐one correspondence to MIEs, ∼70% of the IMF discontinuities are found to be tangential discontinuities. The hot flow anomaly (HFA) mechanism can explain at most ∼50% of the MIEs; bursty reconnection and pressure pulses can explain the production of at most ∼30% and ∼20% of the MIEs, respectively. All of the observations and associations are consistent with HFAs or foreshock cavities being the main cause of MIEs.

Periodic magnetospheric substorms: Multiple space‐based and ground‐based instrumental observations

Quasi‐periodic, sawtooth‐like variations of energetic plasma particle fluxes are often measured at geosynchronous orbit during magnetic storms. The outstanding problems are whether the sawtooth flux variations represent particle injections from the tail to the inner magnetosphere, whether the sawtooth variations correspond to substorm onsets, and what mechanism is responsible for the generation of periodic particle injections (or periodic substorms). In this paper, we present the measurements of multiple space‐based and ground‐based instruments in the magnetosphere and ionosphere during two magnetic storms. The Geotail satellite was located in the near tail between XGSM = −20 and −30 RE and detected periodic southward turnings of the magnetospheric magnetic field. The southward turnings of Bz are interpreted as the signature of periodic near‐tail reconnection and plasmoid formations. Geosynchronous satellites measured periodic gradual dropouts and rapid increases of energetic charged fluxes and magnetic field dipolarization. The IMAGE satellite measured enhanced emissions of energetic neutral atoms in the ring current and auroral brightenings. The Canadian Auroral Network for the OPEN Program Unified Study's photometers measured intensifications and latitudinal motion of auroral emissions. The auroral electrojet index showed periodic increases. These magnetospheric and ionospheric phenomena have the same periodicity and indicate the occurrence of periodic substorms. There is an excellent correspondence among the substorm signatures from all the measurements. The results show that periodic magnetospheric substorms can indeed occur during continuous southward interplanetary magnetic field, and the mean period in these two cases is ∼2.7 hours. The sawtooth variations of energetic plasma fluxes at geosynchronous orbit represent true particle injections from the tail to the inner magnetosphere. The sawtooth injections are related to the near‐tail reconnection and correspond to the onsets of periodic substorms.

Evidence of periodic (2–3 hour) near‐tail magnetic reconnection and plasmoid formation: Geotail observations

During magnetospheric substorms, magnetic reconnection occurs in the near tail and causes the formation of plasmoids. In this paper, we present Geotail observations of periodic magnetic reconnection and plasmoids in the near tail. On April 17–18, 2002, Geotail was located in the northern lobe and detected ten cycles of magnetic field compressions within 24 hours, and southward turnings of the magnetic field Bz component or formations of plasmoids were clearly identified in the first seven cycles. The average period of the plasmoids was ∼2.5 hours. A solar wind pressure impulse appeared to trigger the first magnetic compression (or plasmoid). Plasmoids continued to occur for many cycles after the solar wind pressure and interplanetary magnetic field had become stable. The observations suggest that substorms can be periodic and that the periodic substorms can occur without requirement of continuous external triggering.

Prediction of Earth arrival times of interplanetary southward magnetic field turnings

ACE magnetic field and velocity data, taken in L1 halo orbit, are used to predict the arrival time at the Wind spacecraft of sharp southward turnings of the interplanetary magnetic field. If such events are assumed to be tangential discontinuities, their orientations can be deduced from the cross product of the field immediately upstream and downstream of them. It is shown that such an assumption results in better predictions of arrival times than using minimum variance or simply assuming a sunward normal. Of the 70% of events observed at ACE that could be identified at Wind, 93% of arrival times were predicted within 10 min using the cross product, compared to 56% using minimum variance and 67% assuming a sunward normal. The most important factor controlling the likelihood of observing events at Wind was the angle of the discontinuity normal from the flow direction, although weak dependencies on solar wind speed, flow‐perpendicular separation, and normal‐perpendicular separation were also found. These results suggest that the cross product is an accurate estimator of discontinuity orientations and that using L1 monitor data, it would be possible to predict arrival times of around 86% of southward turnings at Earth with an accuracy of 10 min.

Global electrodynamics observed during the initial and main phases of the July 1991 magnetic storm

Data acquired by seven fortuitously positioned satellites in the interplanetary medium, the magnetosphere, and the topside ionosphere, as well as from numerous ground magnetometers have allowed us to follow the evolution of global currents and electric fields during the geomagnetic storm on July 8–9, 1991. An interplanetary disturbance collided with the magnetosphere at ∼1636 UT on July 8th, compressing the magnetopause inside of geostationary orbit for about 3 hours, as observed by magnetometers on two GOES satellites. The resulting storm developed in four segments with a 7 hour lull separating the initial and main phases. The initial phase was characterized by (1) an extensive, DP 2 current system that produced an AE perturbation of ∼3500 nT, with reporting stations on the dayside, (2) a twenty‐fold increase in auroral electron fluxes, and (3) the immediate appearance of electric fields in the inner magnetosphere. The main phase intensification and earthward movement of the ring current are associated with southward turnings of the interplanetary magnetic field (IMF) and a polar cap potential increase above 200 kV. A significant fraction of the ring current growth was observed prior to the first of several substorms that occurred during the main and recovery phases. The beginning of recovery was characterized by a diminution of the southward IMF component, the cross polar cap potential and magnetospheric electric fields. It was also marked by a substorm onset. Considering the timing of Dst modulations relative to substorm occurrence, we conclude that during this magnetic storm, the electric fields responsible for ring current transport/energization were mostly associated with the DP 2 current system. Finally, in the evening‐sector penetration electric fields of the initial and main phases led to the formation of upward moving equatorial plasma bubbles that were detected by two DMSP satellites in the topside ionosphere.

First simultaneous observations of flux transfer events at the high-latitude magnetopause by the Cluster spacecraft and pulsed radar signatures in the conjugate ionosphere by the CUTLASS and EISCAT radars

Abstract. Cluster magnetic field data are studied during an outbound pass through the post-noon high-latitude magnetopause region on 14 February 2001. The onset of several minute perturbations in the magnetospheric field was observed in conjunction with a southward turn of the interplanetary magnetic field observed upstream by the ACE spacecraft and lagged to the subsolar magnetopause. These perturbations culminated in the observation of four clear magnetospheric flux transfer events (FTEs) adjacent to the magnetopause, together with a highly-structured magnetopause boundary layer containing related field features. Furthermore, clear FTEs were observed later in the magnetosheath. The magnetospheric FTEs were of essentially the same form as the original "flux erosion events" observed in HEOS-2 data at a similar location and under similar interplanetary conditions by Haerendel et al. (1978). We show that the nature of the magnetic perturbations in these events is consistent with the formation of open flux tubes connected to the northern polar ionosphere via pulsed reconnection in the dusk sector magnetopause. The magnetic footprint of the Cluster spacecraft during the boundary passage is shown to map centrally within the fields-of-view of the CUTLASS SuperDARN radars, and to pass across the field-aligned beam of the EISCAT Svalbard radar (ESR) system. It is shown that both the ionospheric flow and the backscatter power in the CUTLASS data pulse are in synchrony with the magnetospheric FTEs and boundary layer structures at the latitude of the Cluster footprint. These flow and power features are subsequently found to propagate poleward, forming classic "pulsed ionospheric flow" and "poleward-moving radar auroral form" structures at higher latitudes. The combined Cluster-CUTLASS observations thus represent a direct demonstration of the coupling of momentum and energy into the magnetosphere-ionosphere system via pulsed magnetopause reconnection. The ESR observations also reveal the nature of the structured and variable polar ionosphere produced by the structured and time-varying precipitation and flow.Key words. Ionosphere (auroral ionosphere) Magentospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)

Polar observations and model predictions during May 4, 1998, magnetopause, magnetosheath, and bow shock crossings

During the rise to the maximum phase of solar cycle 23, several periods of extreme solar wind conditions have occurred. During such an example on May 4, 1998, the solar wind monitors observed a period of strong southward interplanetary magnetic field (IMF) accompanied by a solar wind dynamic pressure that was 30 times higher than average. During this period the Polar spacecraft crossed the magnetopause and bow shock and experienced its first solar wind encounter. This case provides a rare opportunity to study the magnetopause, low‐latitude boundary layer, magnetosheath, and bow shock and to test our ability to model the dynamic behavior of these boundary regions under extreme and highly variable solar wind conditions. In this study we use the gas dynamic convected field model to predict the time‐dependent magnetic field and plasma properties upstream from the magnetopause and the location of the Polar spacecraft relative to the magnetopause and bow shock during the event. To test the accuracy of the prediction, model magnetic field characteristics are compared to the fields observed along the satellite track by the magnetometer on Polar. The predicted model plasma characteristics (density, velocity, and temperature) are compared to moments derived from TIDE observations, extrapolated to account for the higher energy portion of the magnetosheath distributions. Where ambiguities occur in identifying the satellite location, plasma distribution functions from two additional ion detectors (TIMAS and HYDRA) are used to resolve the observed location of Polar relative to the boundaries. With this procedure, carried out separately for ACE and Wind and for two different magnetopause models, observed features at Polar can be traced back to drivers in the solar wind, providing a unique opportunity to assess the evolution of the solar wind and its predictability from the solar wind monitors to the magnetopause. When the Polar apogee drifts to low latitudes in the future, it will provide more and more observations in this region. Therefore what we learn from this case can be indicative of what we will see in the future in Polar operations. The high‐level correlation between the predictions and in situ measurements indicates that the solar wind monitors often provide adequate and useful solar wind conditions near the Earth. The tests indicate that in this event the vacuum dipole field magnetosphere model predicts a significantly larger magnetosphere than observed. While the empirical magnetopause model predictions are more consistent with the observations, they overpredict the response of the magnetosphere to transient southward turnings of the IMF, indicating that the magnetosphere does not respond to the southward IMF turnings on small timescales. Sometimes, there are significant differences at relatively small time scales between the measurements from the two solar wind monitors. It is possible for the solar wind conditions near the Earth to be different from the measurements made by at least one of the solar wind monitors. Cautions should be taken when interpreting near‐Earth observations if the observations are inconsistent at small timescales with the predictions based on one solar wind monitor.

A coupled map as a model of the dynamics of the magnetotail current sheet

A magnetic field model of the magnetotail current sheet in the form of a coupled-map lattice (CML) is presented. It is continuously driven (“running”) and based on the MHD diffusion equation. Solar wind vB S data (solar wind speed multiplied by the southward component of IMF) are used for driving the model, and it is shown to exhibit perturbations (avalanches) with power-law scalings in their distributions of duration and size. Such distributions may indicate self-organized critical (SOC) behavior. Furthermore, it is shown that the power spectra of the model outputs are of bicolor power-law form with different slopes for high and low frequencies. Although the “running” model gives power-law distributions independent of the strength of the driver, it is argued that the model is in SOC state only when driven slowly. When the driver is strong the model acts more like an input–output system, and the power laws are possibly due to the statistics of the driver. This is supported by the fact that durations of southward turnings of IMF have also a power-law distribution. It is also shown that by driving the model very strongly we can reproduce AE data with correlation 0.74 by using large resistivity, and Dst with correlation 0.79 by using small resistivity.

Characteristics of the postnoon auroras inferred from EISCAT radar measurements

Svalbard European Incoherent Scatter (EISCAT) radar and Polar satellite ultraviolet imager data have been used to investigate structure and dynamics of the postnoon auroral oval (in the so‐called “14 MLT” region). Southward turnings of the interplanetary magnetic field were associated with bright afternoon and evening arcs which drifted equatorward. Increase in the equatorward arc velocity occurred about 27 ± 7 min after the IMF Bz turned southward at the subsolar point. The observed equatorward motion may occur independently of the convection: In one of the cases, the velocity of the drifting arc was observed to be about 100 – 200 m/s southward whereas the ionospheric plasma drifted northward at a velocity of 160 m/s. The arcs were located in the region of convection shear that was shown by the ground magnetometers and the EISCAT data on ion velocity and temperature. In the cases when electric field data were available the meridional component of the electric field varied from about −60 mV/m (southward) equatorward of the arc to +30 mV/m (northward) poleward of the arc. Ionospheric plasma parameters were measured during the passes of the arcs through the radar beam. Downward field‐aligned ion flow at the 300 – 550 km altitude was observed poleward of the arc, whereas upward flows were observed equatorward of the arc. This variation is explained by the difference in the vertical gradient of plasma density. Convection flows associated with arc, and differences in electron precipitation and sunlight conditions create the vertical gradients of plasma density that determine the ambipolar diffusion and observed ion motion.

Central polar cap convection response to short duration southward Interplanetary Magnetic Field

Abstract. Central polar cap convection changes associated with southward turnings of the Interplanetary Magnetic Field (IMF) are studied using a chain of Canadian Advanced Digital Ionosondes (CADI) in the northern polar cap. A study of 32 short duration (~1 h) southward IMF transition events found a three stage response: (1) initial response to a southward transition is near simultaneous for the entire polar cap; (2) the peak of the convection speed (attributed to the maximum merging electric field) propagates poleward from the ionospheric footprint of the merging region; and (3) if the change in IMF is rapid enough, then a step in convection appears to start at the cusp and then propagates antisunward over the polar cap with the velocity of the maximum convection. On the nightside, a substorm onset is observed at about the time when the step increase in convection (associated with the rapid transition of IMF) arrives at the polar cap boundary.Key words: Ionosphere (plasma convection; polar ionosphere) - Magnetospheric physics (solar wind - magnetosphere interaction)

Solar wind magnetic field control of magnetospheric response delay and expansion phase onset timing

Variability in the response of the magnetosphere under different interplanetary magnetic field (IMF) conditions is investigated by utilizing a high‐resolution, three‐dimensional, global magnetohydrodynamic (MHD) model. Changes in the MHD model response time delay are found by performing a parameter search based on the strength of the IMF Bz component, which is held constant for 4 hours prior to and following southward turnings. The parameter search reveals that variations in the growth phase duration and, thus in the timing of expansion phase onset, follow a simple pattern governed by the relative strengths of IMF Bz before and after the southward turnings. If the IMF northward and southward values are equal in magnitude, the expansion phase onset occurs ∼60 min after dayside merging begins. However, if the north IMF setting is smaller (larger) than the subsequent southward field intensity, the expansion phase onset can occur after a much shorter (longer) time delay. The range of onset delay times found from the simulation is ∼60±30 min. The MHD results are tested by means of an empirical analysis, which is based on the assumption that the solar wind response of the magnetosphere, as represented by the AL index, is bimodal, owing to the different solar wind response timescales of the convection current system and the substorm current wedge. The empirical analysis utilizes short duration time intervals including solar wind input time series and corresponding AL index data. The empirical results agree favorably with the results found via MHD simulation.

MHD simulations of the response of high‐latitude potential patterns and polar cap Boundaries to sudden southward turnings of the interplanetary magnetic field

3‐D MHD simulations were used to investigate the behavior of the high‐latitude convection and the polar cap variations during two events characterized by sudden southward IMF turnings. In agreement with recent observations the simulation results indicate that the convection pattern across the entire polar cap begins to change a few minutes after the arrival of the southward IMF. In contrast, the onset of the equatorward motion of the open closed field‐line boundary depends on the local time, with equatorward motion of the midnight boundary delayed by about 20 minutes relative to the the onset of the boundary motion at noon. We interpret this delay as the time required to convect newly merged flux from the dayside to the nightside. We belive that these two different responses can reconcile apparent contradictions in studies of ionospheric reconfigurations in response to changes in the IMF.

High‐altitude polar cap electric field responses to southward turnings of the interplanetary magnetic field

Interplanetary electric field coupling with the magnetosphere has been analyzed predominantly using data from the Wind magnetometer and the Polar electric field instrument. The coupling was investigated using the Polar Electric Field Instrument (EFI) to measure the electric field in the northern polar cap immediately following sharp southward turnings of the IMF as observed by Wind. Southward turnings were chosen which exhibited a sudden change of the IMF north–south component from BZ &gt; 0 to BZ &lt; 0 (GSM coordinates) after an hour or more of relatively stable conditions, and for which Polar was in the northern polar cap. These BZ changes correspond to EY changes in the interplanetary electric field. For each of the 30 identified events, a time was estimated for the arrival of the IMF change at the magnetopause using the solar wind speed observed by the Wind Solar Wind Experiment (SWE), and Polar electric field data were examined to identify responses. For many of the selected events (about one third), abrupt changes of state in the magnetospheric electric field were evident with timing that matched the expected solar wind arrival time at Earth. For events for which additional data were available, we conducted in‐depth examination of the individual events using IMP 8, Geotail, and GOES 9. In one such event, GOES 9 data showed a substorm growth phase and onset which also corresponded to features in the solar wind observed by Wind, Geotail, and IMP 8. In addition to the individual event studies, a superposed epoch analysis of all available events revealed a consistent rise in the mean polar cap electric field about 15 min following sharp IMF southward turnings.