Dipolar Region Research Articles

On the formation of near‐Earth X‐line at substorm expansion onset

Convection in the plasma sheet can be enhanced by the MDXL (middle‐to‐distant X‐line) during the growth phase and by the NEXL (near‐Earth X‐line) during the expansion phase. Cowling current intensifies by the blockage of the northward Hall current due to lack of the radial currents or azimuthal pressure gradient in the midnight sector of the near‐Earth plasma sheet. Interaction of intensified Cowling current loop with the near‐Earth plasma sheet causes dipolarization. Dipolarization‐induced thinning tailward of the dipolarizing region can cause the NEXL formation to drive the substorm expansion phase. Further intensifications of the Cowling current in the substorm current wedge, the tailward expansion of dipolarization region, and the expansion of the auroral bulge are all driven by the NEXL during the expansion phase.

Analysis of dipole behavior in PMN–PT single crystal from modeling of dielectric spectrum

Some diverse characters between relaxor ferroelectric ceramics and their single crystals remain unknown. Based on dual polarization model used in ceramics, we modeled and analyzed dielectric spectrum of PMN–PT single crystals above characteristic temperature T m. An indistinct characteristic temperature T 2 was made clear. The results of simulation for single crystals fitted with experiments. It is shown that high temperature model in the dual polarization model is also valid for single crystals, and a characteristic temperature T 2 may signify the onset of dipolar regions freezing. The relaxor materials experience different polarization processes in different temperature ranges divided by T 2. The reasons resulted in the discrepancy between ceramics and single crystals were analyzed. We believe that the divergence between ceramics and single crystals is induced by the difference of polar region concentration, electric interaction and inner elastic stress among polar regions.

Magnetic dipolarization with substorm expansion onset

We compare the timing and spatial extent of dipolarization with auroral breakup by using magnetic field data (1‐min time resolution) from GOES 8 and GOES 9 and global auroral images (37‐s frame rate) from the Polar ultraviolet imager. We survey the entire year of 1997 data and obtain 32 unambiguous dipolarization events that had ionospheric footprints within 2 hours in magnetic local time (MLT) from their associated auroral breakups. It is found that the auroral breakup preceded the arrival of dipolarization at GOES by an average of 1.7 ± 2.7 min. (1.0 ± 2.1 min for events ±1 hour from the auroral breakup). The delay of geosynchronous dipolarization represents a propagation time, as the substorms rarely initiated at the GOES location. Propagation velocities derived from the propagation time are ∼60 km s−1 eastward, ∼75 km s−1 westward, and ∼270 km s−1 earthward at geosynchronous orbit. We also show for the first time that auroral bulge and dipolarization region closely map; the bulge is the region of currents out of the ionosphere. The eastern edge of the dipolarization region, where field‐aligned currents are flowing into the ionosphere, is, based on the T89 magnetic field model, located at ∼0.5 hour in MLT eastward of the eastern edge of the auroral bulge. Within the initiation site meridian the dipolarization onset was observed ∼1 min prior to the auroral breakup at the GOES altitude. This time is associated with Alfvén transit time and therefore depends on the radial location of the onset. The azimuthal scale of the initiation site is found to be ∼0.5 hour in MLT in the ionosphere or less than ∼1 Earth's radius at 6.6 RE. These results support current diversion into the ionosphere as a critical part of substorms. Finally, a simple timing analysis of specific substorm onset time features suggests that near‐Earth reconnection cannot occur before current disruption and therefore cannot be the cause of substorm.

Substorm Trigger Processes in the Magnetotail: Recent Observations and Outstanding Issues

The present article reviews recent studies about near-Earth substorm processes. A focus is placed on the relationship between two fundamental processes, that is, tail current disruption (TCD) and the formation of a near-Earth neutral line (NENL). The former is inferred to cause dipolarization, and the latter is often associated with the fast plasma flow in the plasma sheet. Whereas it is inferred from the directions of fast plasma flows that the NENL is formed at 20–30 RE from the Earth, dipolarization is most manifest in the near-Earth (6.6–12 RE) region. The observation of the fast plasma flow prior to substorm (Pi2) onsets favors the idea that the NENL is formed first and dipolarization is the effect of the pile-up of magnetic flux convected earthward from the NENL, which is called the pile-up model. The present paper addresses several outstanding issues regarding this model, including (1) the interpretation of plasma flow deceleration in terms of the flux pile up, (2) highly irregular magnetic fluctuations observed in the near- Earth region, (3) the spatial coherency of the fast plasma flow, (4) the spatial structure and expansion of dipolarization region, and (5) the explosive growth phase. The paper also proposes the possibility that TCD is an independent process, but the formation of the NENL sets a favorable condition for it.

A scenario covering all phases of nightside convection with the roles of ion drifts and reconnection determined from the observed field‐aligned currents

The rate of transfer of magnetic flux into the nightside dipole‐like region is a function of the field‐aligned current (FAC) intensity and Nt–Nd, where Nt and Nd are the total number of plasma sheet ions on flux tubes in the near‐Earth tail and just inside the dipole‐like region. In addition, for slowly varying convection, the FAC intensity is a function of the magnetic flux in the tail. The above two relationships provide the basis for a scenario for convection. Convection bays and steady state occur if there is an ample supply of flux tubes with a small value of Nt in the near‐Earth tail. The growth phase occurs when a major increase in dayside reconnection leads to an increase in the amount of tail flux, which in turn causes increased convection into the nightside dipolar region. The plasma sheet thins until the predipolarization phase starts (reconnection or some similar process). The reconnection provides magnetic flux with reduced Nt, and also produces a change in the field‐aligned currents, both of which increase the dipolarization rate after electrical coupling with the ionosphere is established. At that time, either a pseudobreakup or a substorm onset will occur. The recovery phase is a convection toward a steady state, in which the plasma undergoes redistribution in latitude, local time, radial distance, and energy. It can be either fairly localized or magnetosphere‐wide, depending on the changes that have occurred in the dayside reconnection rate.

Field‐aligned currents generated in magnetotail reconnection: 3D Hall‐MHD simulations

Understanding the field aligned current (FAC) system in substorms is one of the central themes of magnetospheric physics. Three‐dimensional (3D) magnetohydrodynamic (MHD) simulations of magnetotail reconnection has shown that FAC structures similar to the wedge‐current system are indeed reproduced either with or without a dipolar region situated at the earthward end of a current sheet. In this study, however, by noting that the ion inertia length is not negligibly small compared to the current sheet width in the magnetotail, we investigate the FAC generation mechanism in the Hall‐MHD system. For an idealized current sheet geometry without the dipolar region, we find that the results obtained to be totally different from the MHD results.

Can substorms produce relativistic outer belt electrons?

In an effort to explain how magnetic storms can cause strong enhancements in outer belt MeV electrons, we have studied the substorm‐associated acceleration of energetic (tens of keV) plasma sheet electrons and their injection into the outer‐trapped region of the magnetosphere. The study is based on tracing test particles in three‐dimensional MHD simulations of substorm dipolarization. The simulation‐based electric and magnetic fields [Birn and Hesse, 1996] exhibit a strong earthward collapse around local midnight, leading to magnetic field dipolarization along with a corresponding induction electric field. The test particle traces show that tens of keV plasma sheet electrons can be transported from about x ≈ −20 RE to x ≈ −10 RE and can gain about a factor of 10 in energy. If these particles are further transported inward to L ∼ 6 while conserving the first adiabatic invariant, they will have energies of an MeV or more. In the substorm acceleration process the dominant energy gain occurs during the earthward radial transport by E × B drift in the strong dipolarization region. In our calculations the electrons that gain the most energy are ones that circle a local maximum in the equatorial magnetic field strength and thus spend a relatively long time in the region of collapsing field. The first adiabatic invariant is broken at the beginning of the particle trajectory, near the neutral line, where the magnetic curvature radius can be comparable to the particle gyroradius. The second adiabatic invariant is also broken later in the process, when particles can be temporarily trapped in off‐equatorial magnetic minima associated with MHD waves on the dipolarizing field lines. Estimation of the number of accelerated plasma sheet electrons indicates that the Birn‐Hesse substorm, which is not particularly large, produces only ∼ 2% of the number of MeV electrons observed in a typical post storm outer belt electron enhancement. A series of substorms, some of them large, might produce a large enough enhancement, but it should be noted that this is an order of magnitude estimate, and it is rather sensitive to plasma sheet and substorm parameters.

Waves and fluctuations in the jovian magnetosphere

The dynamics of the jovian magnetosphere are dominated by the mass loading of the 10 torus. Sulfur dioxide and possibly other gases such as hydrogen are sputtered from 10 and are ionized. These ions in turn are accelerated by the corotational electric field. The hot torus moves slowly outward and then ever more rapidly until reconnection releases the ions down the tail. The outward transport is far from steady. Near 10 are intense ion cyclotron waves. As observed by Galileo these waves weakened rapidly on the side of 10 closest to Jupiter. Beyond 10 10 radii (RI,) at periods less than 150s there were no discernable waves above the instrument noise level. On the side of 10 away from Jupiter the 10 cyclotron waves disappeared at about 20 Rr,, and the noise level in the torus reached values almost as low as those inside the 10 orbit. However, as the radial distance increased so did the noise levels. The noise levels from 0.01 to 0.1 Hz are about 2 orders of magnitude greater at 7.7 RJ than inside the 10 orbit at 5.5 RJ. The noise appears to be due to steps in the magnetic field strength and direction perhaps due to the interchange of tubes of flux with differing plasma content. The transverse power spectral density is greater than the compressional power in this region. The next region in which we have observations is the orbit of Europa and beyond. At frequencies of about 1 Hz the waves are more strongly compressional than transverse in the equatorial regions and more transverse than compressional off the equator. The outer edge of the dipolar region is about 24 RJ. The noise just inside this distance is quite variable from rotation to rotation. Outside this distance in the magnetodisk the field crossing the current sheet varies significantly from orbit to orbit as if the magnetodisk were globally unstable. Beyond about 40 Rr the location of the current sheet oscillates with a period of about 10 minutes. Beyond 50 RJ the plasma enters a regime of

Substorm observations in the early morning sector with Equator-S and Geotail

Abstract. Data from Equator-S and Geotail are used to study the dynamics of the plasma sheet observed during a substorm with multiple intensifications on 25 April 1998, when both spacecraft were located in the early morning sector (03–04 MLT) at a radial distance of 10–11 RE. In association with the onset of a poleward expansion of the aurora and the westward electrojet in the premidnight and midnight sector, both satellites in the morning sector observed plasma sheet thinning and changes toward a more tail-like field configuration. During the subsequent poleward expansion in a wider local time sector (20–04 MLT), on the other hand, the magnetic field configuration at both satellites changed into a more dipolar configuration and both satellites encountered again the hot plasma sheet. High-speed plasma flows with velocities of up to 600 km/s and lasting 2–5 min were observed in the plasma sheet and near its boundary during this plasma sheet expansion. These high-speed flows included significant dawn-dusk flows and had a shear structure. They may have been produced by an induced electric field at the local dipolarization region and/or by an enhanced pressure gradient associated with the injection in the midnight plasma sheet.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet; storms and substorms)

Substorm‐associated pressure variations in the magnetotail plasma sheet and lobe

Simultaneous pressure measurements by Interball‐Tail in the high‐latitude lobe and by Geotail in the equatorial plasma sheet were analyzed for 30 substorms which exhibited significant pressure changes. At the onset of a few substorms we observed equatorial pressure peaks with magnitudes up to 50% higher than those in the lobe. These pileups are probably rather localized, and their properties are consistent with plasma sheet thickening between two active regions in the tail. During expansion and recovery phases of more than half of substorms, we observed equatorial pressure depletions relative to the high‐latitude lobe pressure. These depletions can last more than 2 hours and are likely formed during the substorm expansion phase near the equatorial plane behind (tailward of) the strongly dipolar near‐Earth magnetotail region. The observed pressure gradient is probably a nonstationary feature and can be compensated partially by magnetic tension on the curved field lines. Magnitude and history of the solar wind dynamic pressure appear to significantly influence substorm scenarios in the magnetotail. Possible existence of the pressure difference should be taken into account in single‐spacecraft substorm studies.

Auroral bulge formation as the result of isoline mapping of magnetic flux tube volume

The process of auroral bulge formation is modeled taking into account disruption of the near Earth tail current sheet and dipolarization of the magnetic field. The disruption of the near Earth tail current results in a considerable increase of magnetic flux tube volumes in that region. As a result, magnetic flux tube volume isolines in the equatorial plane move earthward. Their mapping to ionospheric altitudes leads to a configuration having a characteristic bulge near midnight. Thus, auroral bulge formation may be considered as the result of dipolarization of near midnight magnetic field. The increase in magnetic tube volume causes the decrease of particle pressure and formation of a pressure hole in the dipolarization region. On the east and west borders of such a region large plasma pressure gradients along magnetic flux tube volume isolines arise. Such gradients lead to the generation of powerful field-aligned currents of the Birkeland current wedge. The strength of such currents are estimated and compared with observations. Mechanisms of current disruption and field line dipolarization are discussed.

Substorm observations in the early morning sector with Equator-S and Geotail

Data from Equator-S and Geotail are used to study the dynamics of the plasma sheet observed during a substorm with multiple intensifications on 25 April 1998, when both spacecraft were located in the early morning sector (03–04 MLT) at a radial distance of 10–11 RE. In association with the onset of a poleward expansion of the aurora and the westward electrojet in the premidnight and midnight sector, both satellites in the morning sector observed plasma sheet thinning and changes toward a more tail-like field configuration. During the subsequent poleward expansion in a wider local time sector (20–04 MLT), on the other hand, the magnetic field configuration at both satellites changed into a more dipolar configuration and both satellites encountered again the hot plasma sheet. High-speed plasma flows with velocities of up to 600 km/s and lasting 2–5 min were observed in the plasma sheet and near its boundary during this plasma sheet expansion. These high-speed flows included significant dawn-dusk flows and had a shear structure. They may have been produced by an induced electric field at the local dipolarization region and/or by an enhanced pressure gradient associated with the injection in the midnight plasma sheet.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet; storms and substorms)

The fluctuating magnetic field in the middle jovian magnetosphere: initial Galileo observations

The region of the jovian magnetosphere from 10–24 planetary radii is a region of strong, nearly dipolar, magnetic field through which the plasma added to the Io torus must pass as it convects and diffuses outward to the magnetodisk region and ultimately to be lost down the tail. Magnetic fluctuations can be used to diagnose the processes present in this plasma. Herein we examine magnetic fluctuations at frequencies below 0.04 Hz observed in the middle magnetosphere on a single pass of the Galileo spacecraft. These frequencies are generally well below the ion gyro-frequencies in this portion of the magnetosphere. Near perijove, about 10.7 R J on this orbit, the equatorial region is not more disturbed than off the equator, but it is quite disturbed at the outer edge of this dipolar region near 24 R J where the magnetic field becomes weak. These fluctuations are quasi-periodic, strongly compressional, and incoherent resembling somewhat the mirror mode. Conversely, the region off the equator is quiet in this outer portion of the middle magnetosphere. In the quasi-dipolar region there are irregular transverse fluctuations everywhere Galileo travels. Compressional transient events are also observed that may mark flux tube interchange. An unusual structure that could be due to the outward convection of Europas wake is also seen. In toto, these fluctuations indicate that the middle magnetosphere is a very dynamic region.

A globally integrated substorm model: Tail reconnection and magnetosphere‐ionosphere coupling

We present a globally integrated substorm (GIS) model in which the patchybursty tail reconnection process at ∼20–35 RE and the magnetosphere‐ionosphere (M‐I) coupling process form a global feedback system to cause substorms. We identify the necessary and sufficient conditions for the expansion onset in the near‐Earth plasma sheet. The necessary condition for substorm onset is an abrupt convection braking in the near‐Earth plasma sheet at a rate ≥∼3 (mV/m)/RE, which is equivalent to dipolarizing the taillike field at a rate ≥∼0.5 nT/s. The sufficient condition requires the disrupted tail current to be carried away by the Alfvén wave to form the substorm current wedge. These conditions must be satisfied by any onset mechanism. The Alfvén wave launched by the dipolarization‐induced electric field must carry away the disrupted tail current from the dipolarization region. As an example, we show that substorm expansion onset can be triggered by abruptly braking a bursty bulk flow of ∼3 mV/m within ∼1 RE. The disrupted tail current at the expansion onset is ∼105 A/RE carried away by the Alfvén wave launched by the dipolarization‐induced electric field of ∼14 mV/m. Substorm expansion onset in the ionosphere occurs ∼1 min later when the Alfvén wave is reflected at the ionosphere. The dipolarization‐induced tail reconnection and the M‐I coupling form a feedback system to power the ongoing expansion phase which typically lasts ∼30 min. As the dipolarizing region expands tailward, the expansion phase turns into recovery phase when the local tail current becomes too weak to sustain the expansion phase and the aurora activities start to retreat equatorward.

Event study of deep energetic particle injections during substorm

The origin of multiple energetic particle injections into the inner magnetosphere is addressed using a rare opportunity of measuring the energetic particle fluxes at different radial distances under known electric and magnetic fields. During a strong substorm on February 10, 1991, the CRRES spacecraft measured E and B fields and high‐energy particle fluxes near the magnetic equator at r ∼ 5 Re, whereas particle injections, their azimuthal locations, and some other parameters were simultaneously monitored by three geostationary spacecraft and ground networks. We show a multitude of impulsive short duration injection events which correlate with 1–2 min long pulses of dawn‐dusk electric field. The observations suggest that some E field pulses recorded deep in the inner magnetosphere were fast magnetosonic waves radiated by the current disruption region. This supports the concept of impulsive dissipation event as an elementary building block of substorm expansion. Furthermore, our modeling results indicate that most of the flux variations of energetic particles can be explained by the global convective transport and corresponding particle acceleration. However, we emphasize that, depending on particle spectra and radial flux gradient, one can observe either flux increase, or decrease, or no variation (often seen in different energy ranges simultaneously and at the same point) as a response to the electric field pulse. Both the cloud of injected particles and magnetic field dipolarization region had a sharp inner boundary (injection front) which propagated inward at the convection speed. We document the complicated structure of this front, consisting of a diamagnetic hot proton layer followed by the dipolarization front which contains enhanced energetic electron fluxes. Further study is required to understand how common this structure is and, if common, how it may be formed.

Azimuthal pressure gradient as driving force of substorm currents

We have studied the azimuthal pressure gradient in the central plasma sheet during substorms using plasma and magnetic field data obtained by the AMPTE/IRM satellite at nightside in radial distances of 9–15 RE. The pressure gradient is statistically estimated for the interval when the magnetic field shows a dipolar configuration (elevation angle >45°). It is found that by this criterion, most data are obtained during and after the passage of high‐speed ion flow in the vicinity of the neutral sheet during magnetically active times. We show that there is an azimuthal gradient of plasma pressure in the dipolar field region. The pressure gradient can drive a substantial amount of field‐aligned current (4.1×105 A per 2‐hour local time sector). We suggest that this current is a source of the substorm current system after the high‐speed ion flow stops.

Are Prominences Formed by Flux Convergence?

AbstractNumerical 3D resistive MHD experiments are used to investigate the dynamical properties of a quadrupolar magnetic field, as the two dipolar regions are pushed together. A current concentration is formed between the approaching flux concentrations and, when reconnection starts, material is lifted against gravity. As it rises the material spreads out along the polarity inversion line and forms a prominence-like structure.

Details of current disruption and diversion in simulations of magnetotail dynamics

Properties of current disruption and diversion are investigated on the basis of resistive MHD simulations which include the inner tail region with the transition toward a dipole field and the dynamic formation of a thin current sheet in the inner tail. As in earlier simulations, magnetotail instability leads to reconnection and plasmoid formation and ejection, accompanied by features of the substorm current wedge. In contrast to earlier simulations of the breakup of a thick current sheet, the presence of the thin current sheet leads to a much faster evolution on the timescale of few minutes and to larger electric fields (up to 20 mV/m), concentrated mostly in the dipolarizing region earthward of the reconnection site. The fast growth is relatively insensitive to the resistivity model used. The current “disruption” is largely a consequence of an expansion of the near‐Earth current sheet, associated with the dipolarization. It starts well earthward of the reconnection site in association with a pileup of magnetic flux and propagates tailward. Field‐aligned currents of region 1 type (toward the Earth on the dawnside and away on the duskside) at the near‐Earth boundary are found to increase both during the driven phase of the thin current sheet formation and during the energy release and current disruption phase. They result from a diversion of perpendicular into parallel currents, which is associated with a gradient of the magnetic flux tube volume directed away from midnight at the flank edges of the dipolarization region. Locally, the current diversion takes place earthward of the reconnection site, predominantly in shear layers above and below the neutral sheet rather than close to the neutral sheet. The peaks of the field‐aligned current densities associated with the substorm wedge do not follow field lines very closely, because of the presence of strong perpendicular currents, so that care must be taken in using these features for a mapping between the Earth and the equatorial plane.

Experimental study of second harmonic generation by dipolar configurations in pure and Li-doped KTaO 3 and its variation under electric field

In the incipient ferroelectric crystal KTaO 3, due to the inversion symmetry of the lattice no second harmonic generation (SHG) is expected. However, even in nominally pure crystals a weak intensity S zzz of frequency doubled light is observed at low temperatures. The SHG intensity S zzz increases with applied electric field. Our results are compared with theoretical predictions [Prusseit-Elffroth and Schwabl, Appl. Phys. A51, 361 (1990)] and previous results [Fujii and Sakudo, Phys. Rev. B13, 1161 (1976)] under applied electric field E. Doping of KTaO 3 with off-center Li-ions leads, at low temperature, to higher SHG intensity compared with nominally pure KTaO 3. Various samples K 1−xLi xTaO 3 with Li-concentrations x = 0.008−;0.063 are measured and the spatial distribution and the total integrated SHG intensity S zzz (calibrated with respect to LiNbO 3) was determined as a function of temperature, with or without applied electric field. For small Li-concentrations x < 0.02, a gradual increase of S zzz with decreasing temperature appears. Heating and cooling cycles follow the same S zzz curve without thermal hysteresis for these small Li-concentrations. The spatial distribution of S zzz shows a narrow forward scattered peak. When cooling under applied electric field E the intensity S zzz(E) increases drastically at low temperatures indicating the alignment of dipolar regions. The maximum intensity saturates at relatively low electric field E sat with ratios S zzz max(E sat) S zzz max(0) varying from 250 for x = 0.008 to 50 for x = 0.022. The fact th S zzz max(E sat) S zzz max(0) of the SHG intensity are much larger than the corresponding birefringence rati Δn max(E) Δn max(0) indicates the alignment of antiparallel ordered regions. S zzz max( E sat) as a f Li-concentration is maximum at x ≈ 0.03. The ratios S zzz max(E sat) S zzz max(0) decrease drastically increasing Li-concentration corresponding to a strong increase of the mean size of ordered dipolar regions for E = 0 at low temperature. This is supported by the appearance (for x > 0.02) of a thermal hysteresis of S zzz and of lateral peaks in the spatial distribution. The variation of S zzz with the polarization direction of the Nd: YAG laser light shows, at low temperatures, a periodicity pointing to dipolar regions with tetragonal symmetry. The ratios of the SHG tensor components in all cases turn out to be ( d 33 d 31 ) 2 = 6.3 ± 0.6 and ( d 31 d 15 ) 2 ≈ 1 . The time dependence of S zzz after a electric field ( E 3 0) reveals for the small Li-concentrations both fast (τ ⩽ 0. l s) and slow relaxation processes. The slow relaxation process can be described by an Arrhenius-like behavior with a concentration dependent activation energy ( E A = k ⋯ 170 K for x = 0.008 to E A = k ⋯ 1000 K for x ⩾ 0.022).

Interplanetary magnetic field control of magnetotail magnetic field geometry: IMP 8 observations

Four years of IMP 8 magnetic field measurements, 1978‐1982, when ISEE 3 took upstream interplanetary magnetic field (IMF) measurements, have been analyzed to produce full cross‐section magnetic maps of the magnetotail at about 33RE downwind from Earth. This paper describes how the field geometry in the cross‐sectional plane responds to different IMF orientations: dominant By, dominant +Bz, and dominant −Bz. The dominant By case exhibits marked departures from bilateral symmetry that have the sense of superimposing a fraction of the IMF on the symmetrical tail field. However, the “superimposed” perturbation field, measured as a fraction of the IMF, is highly nonuniform: It is maximum in the flanks of the plasma sheet and minimum in the lobes. There is also a rotation of the current sheet which varies with IMF strength and with distance. The two dominant Bz cases show no systematic departures from bilateral symmetry. However, the shapes and the relative sizes of the dipolar and flaring field regions of their cross sections are markedly different. The difference field, obtained by subtracting the positive Bz case from the negative Bz case, shows that the strongest perturbations run the north‐south extent of the flanks, instead of residing in the lobes, as might be expected from the dayside reconnection model.