Periods Of Southward Interplanetary Magnetic Field Research Articles

Response timescales of the magnetotail current sheet during a geomagnetic storm: Global MHD simulations

The response of the Earth’s magnetotail current sheet to the external solar wind driver is highly time-dependent and asymmetric. For example, the current sheet twists in response to variations in the By component of the interplanetary magnetic field (IMF), and is hinged by the dipole tilt. Understanding the timescales over which these asymmetries manifest is of particular importance during geomagnetic storms when the dynamics of the tail control substorm activity. To investigate this, we use the Gorgon MHD model to simulate a geomagnetic storm which commenced on 3 May 2014, and was host to multiple By and Bz reversals and a prolonged period of southward IMF driving. We find that the twisting of the current sheet is well-correlated to IMF By throughout the event, with the angle of rotation increasing linearly with downtail distance and being more pronounced when the tail contains less open flux. During periods of southward IMF the twisting of the central current sheet responds most strongly at a timelag of ∼ 100 min for distances beyond 20 RE, consistent with the 1–2 h convection timescale identified in the open flux content. Under predominantly northward IMF the response of the twisting is bimodal, with the strongest correlations between 15 and 40 RE downtail being at a shorter timescale of ∼ 30 min consistent with that estimated for induced By due to wave propagation, compared to a longer timescale of ∼ 3 h further downtail again attributed to convection. This indicates that asymmetries in the magnetotail communicated by IMF By are influenced mostly by global convection during strong solar wind driving, but that more prompt induced By effects can dominate in the near-Earth tail and during periods of weaker driving. These results provide new insight into the characteristic timescales of solar wind-magnetosphere-ionosphere coupling.

Diffusive Plasma Transport by the Magnetopause Kelvin-Helmholtz Instability During Southward IMF

At the Earth’s low-latitude magnetopause, the Kelvin-Helmholtz (KH) waves, which are driven by the super-Alfvénic velocity shear across the magnetopause, have been frequently observed during periods of northward interplanetary-magnetic-field (IMF) and believed to contribute to efficiently transporting the solar wind plasmas into the magnetosphere. On the other hand, during southward IMF periods, the signatures of the KH waves are much less frequently observed and how the KH waves contribute to the solar wind transport has not been well explored. Recently, the Magnetospheric Multiscale (MMS) mission successfully detected signatures of the KH waves near the dusk-flank of the magnetopause during southward IMF. In this study, we analyzed a series of two- and three-dimensional fully kinetic simulations modeling this MMS event. The results show that a turbulent evolution of the lower-hybrid drift instability (LHDI) near the low-density (magnetospheric) side of the edge layer of the KH waves rapidly disturbs the structure of the layer and causes an effective transport of plasmas across the layer. The obtained transport rate is comparable to or even larger than that predicted for the northward IMF. These results indicate that the diffusive solar wind transport induced by the KH waves may be active at the flank-to-tail magnetopause during southward IMF.

Multi-scale evolution of Kelvin–Helmholtz waves at the Earth's magnetopause during southward IMF periods

At the Earth's low-latitude magnetopause, the Kelvin–Helmholtz instability (KHI), driven by the velocity shear between the magnetosheath and magnetosphere, has been frequently observed during northward interplanetary magnetic field (IMF) periods. However, the signatures of the KHI have been much less frequently observed during southward IMF periods, and how the KHI develops under southward IMF has been less explored. Here, we performed a series of realistic 2D and 3D fully kinetic simulations of a KH wave event observed by the Magnetospheric Multiscale (MMS) mission at the dusk-flank magnetopause during southward IMF on September 23, 2017. The simulations demonstrate that the primary KHI bends the magnetopause current layer and excites the Rayleigh–Taylor instability (RTI), leading to penetration of high-density arms into the magnetospheric side. This arm penetration disturbs the structures of the vortex layer and produces intermittent and irregular variations of the surface waves which significantly reduces the observational probability of the periodic KH waves. The simulations further demonstrate that in the non-linear growth phase of the primary KHI, the lower-hybrid drift instability (LHDI) is induced near the edge of the primary vortices and contributes to an efficient plasma mixing across the magnetopause. The signatures of the large-scale surface waves by the KHI/RTI and the small-scale fluctuations by the LHDI are reasonably consistent with the MMS observations. These results indicate that the multi-scale evolution of the magnetopause KH waves and the resulting plasma transport and mixing as seen in the simulations may occur during southward IMF.

Identification of Auroral Zone Activity Driving Large‐Scale Traveling Ionospheric Disturbances

AbstractWe have used all‐sky imaging to relate different types of auroral oval disturbances to large‐scale traveling ionospheric disturbances (LSTIDs). We selected eight nights with good all‐sky imaging and Global Positioning System total electron content coverage, including five non–storm time periods with isolated initiations of geomagnetic activity and three storm main phase periods with continuous activity. Periods with LSTIDs generally started and stopped with initiation and cessation of activity. We found evidence that individual LSTIDs often show 1‐1 correspondence with identifiable auroral disturbances, disturbances either being related to a substorm onset or to auroral streamers without a substorm. Since substorm ground magnetic depressions are directly related to the electric fields and electron precipitation of auroral streamers, we hypothesize that streamers may be the primary drivers of individual nightside LSTIDs with or without a substorm. Additionally, we found evidence that (1) LSTIDs detection is more likely near the longitude range of the initiating disturbance than further away, (2) the orientation of LSTID phase fronts depends on location relative to disturbance longitude, and (3) disturbance ionospheric current and magnetic latitude may influence whether a given disturbance leads to a detectable LSTID. Numerous LSTIDs (10 to 12 over 7‐ to 8‐hr periods) were detected during southward interplanetary magnetic field periods of coronal mass ejection storm main phases, the vast majority reflecting streamers in the absence of substorms. Less LSTIDs were seen during the one examined high‐speed‐stream storm. We have also found evidence that omega band disturbances may drive interesting TIDs that are distinct from the LSTIDs driven by the substorm and streamer disturbances.

The 17 March 2013 storm: Synergy of observations related to electric field modes and their ionospheric and magnetospheric Effects

AbstractThe main phase of the 17 March 2013 storm had excellent coverage from ground‐based instruments and from low‐ and high‐altitude spacecraft, allowing for evaluation of the relations between major storm time phenomena that are often considered separately. The shock impact with its concurrent southward interplanetary magnetic field (IMF) immediately drove dramatic poleward expansion of the poleward boundary of the auroral oval (implying strong nightside reconnection), strong auroral activity, and strong penetrating midlatitude convection and ionospheric currents. This was followed by periods of southward IMF driving of electric fields that were at first relatively smooth as often employed in storm modeling but then became extremely bursty and structured associated with equatorward extending auroral streamers. The auroral oval did not expand much further poleward during these two latter periods, suggesting a lower overall nightside reconnection rate than that during the first period and approximate balance with dayside reconnection. Characteristics of these three modes of driving were reflected in horizontal and field‐aligned currents. Equatorward expansion of the auroral oval occurred predominantly during the structured convection mode, when electric fields became extremely bursty. The period of this third mode also approximately corresponded to the time of largest equatorward motion of the ionospheric trough, of apparent transport of high total electron content (TEC) features into the auroral oval from the polar cap, and of largest earthward injection of ions and electrons into the ring current. The enhanced responses of the aurora, currents, TEC, and the ring current indicate a common driving of all these storm time features during the bursty convection mode period.

A Statistical Study of the Plasma Sheet in the Near and Middle Earth Magnetotail

AbstractMagnetotail plasma sheet is the most active area in Earth's magnetosphere. It has been found that the features of plasma sheet are controlled by the conditions of solar wind and Interplanetary Magnetic Field (IMF). Some previous statistical studies have found that some parameters of the plasma sheet are dawn‐dusk asymmetric. But the thickness of the magnetotail plasma sheet in the near and mid‐tail region and how they are influenced by the IMF is still unclear. In this paper, the probability of the Cluster‐C1 satellite encountering the plasma sheet is examined statistically by utilizing the proton flux and ß data from the CODIF and FGM equipment on board the Cluster‐C1. Using data from July to November of year 2001–2004, the distributions of the probability of satellite in the plasma sheet are mapped on the Y‐Dz plane (Dz denotes the distance between the satellite and the neutral sheet) during the southward and northward IMF periods, respectively. By comparison, we found that the plasma sheet is thinner during southward IMF periods than that during northward IMF periods. It is more obvious in the flank regions of the plasma sheet. We also found that the plasma sheet in the dusk side is thinner than that in the dawn side.

GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 1: The North American sector

Abstract. The interval of geomagnetic storms of 7–17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere–ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap.

Simulation of interplanetary magnetic field By penetration into the magnetotail

Based on our global 3D magnetospheric MHD simulation model, we investigate the phenomena and physical mechanism of the By component of the interplanetary magnetic field (IMF) penetrating into the magnetotail. We find that the dayside reconnected magnetic field lines move to the magnetotail, get added to the lobe fields, and are dragged in the IMF direction. However, the By component in the plasma sheet mainly originates from the tilt and relative slippage of the south and north lobes caused by plasma convection, which results in the original Bz component in the plasma sheet rotating into a By component. Our research also shows that the penetration effect of plasma sheet By from the IMF By during periods of northward IMF is larger than that during periods of southward IMF.

Simulated ring current response during periods of dawn‐dusk oriented interplanetary magnetic field (B y )

[1] Ring current formation is mainly attributed to enhanced global magnetospheric convection and particle injection. One of the indicators of enhanced global magnetospheric convection is the transpolar potential. The transpolar potential has been shown to respond to dawn-dusk oriented interplanetary magnetic field (IMF), enhancing as the IMF magnitude grows. This suggests that the ring current should respond to dawn-dusk oriented IMF. This work examines the ring current response during periods of dawn-dusk oriented IMF using the Lyon-Fedder-Mobarry and Comprehensive Ring Current Model simulations. Exploring three hypotheses, this work shows through simulation results that the ring current does respond during periods of dawn-dusk oriented IMF, that the inner magnetospheric response is different for periods of dawn-dusk oriented IMF than for periods of southward IMF, and that these differences are attributed to both lower magnetospheric convection on closed field lines and the location of the reconnection region on the nightside. Specifically, the simulation results show that as the magnitude of the dawn-dusk oriented IMF increases, producing a corresponding increase in the transpolar potential, the ring current response increases. The response is always much less than a comparable southward IMF would produce. This lower response is due to both magnetospheric convection on closed field lines, which builds the ring current but at a slower rate, and flank reconnection, which allows energy to flow through the inner magnetosphere without building the ring current plasma population (as a conduit).

Storm time electric fields in the equatorial ionosphere observed near the dusk meridian

We compare the dynamics of plasma drifts (VP) and ionospheric electric fields (EI), measured near the dusk meridian at the magnetic equator by the Defense Meteorological Satellite Program (DMSP) F13 satellite, with those of the dawn‐to‐dusk component of the interplanetary electric field (IEF) Ey during four large magnetic storms. Presented data indicate that during the main phase of the magnetic storms when the interplanetary magnetic field (IMF) had a southward component, VP had upward and westward components that ranged up to 120 and 210 m s−1. The corresponding eastward and upward components of EI were approximately 5% and 10% of Ey. The persistence of eastward EI for many hours indicates that electric field shielding of the inner magnetosphere is ineffective during prolonged periods of southward IMF. During periods of northward IMF (Ey < 0) ionospheric responses depended on prior history. No ionospheric response is apparent after strong northward IMF turnings preceded by geomagnetic quiet. If they occurred after a long southward interval, EI often had westward and downward components. The polarity of the zonal component of EI remained well correlated with that of Ey for up to 21 h during periods of slowly varying IMF. Phenomena associated with southward IMF periods are consistent with models that allow for prolonged penetration electric field from high latitudes superposed on those of the Sq current system. However, it is not understood why the remarkable correspondence between the polarities of the zonal component of EI and the IEF Ey occurs during northward IMF. In some Ey < 0 periods, the vertical component of EI remained upward. DMSP detections of upward EI (westward VP) during periods of Ey < 0 are not predicted by existing models.

Observations of magnetic reconnection in the magnetotail associated with substorms

Magnetic reconnection is one of the most important, dynamic phenomena in the magnetotail in terms of magnetic field line configuration change and energy release. It is believed to occur in the distant magnetotail mainly during southward interplanetary magnetic field periods and in the near-Earth magnetotail in association with substorms. In the present paper, we discuss several important issues concerning magnetic reconnection in the magnetotail associated with substorms, such as reconnection signatures, location, timing, spatial scale, and behavior, from the macroscopic, observational point of view.

Evidence that solar wind fluctuations substantially affect the strength of dayside ionospheric convection

Ionospheric convection is occasionally observed to be substantially enhanced even when the interplanetary magnetic field (IMF) is not strongly southward and the IMF By is not large. Such enhanced convection flows tend to exhibit large oscillations with ∼10–30 min periodicity. We have considered the solar wind characteristics that lead to these oscillatory convection enhancements. We have used an extensive set of Sondrestrom radar observations of ionospheric convection within the dayside polar cap. We find that IMF ULF power is closely associated with the strength of dayside convection. Convection flows during periods of large north–south IMF fluctuations are observed to be as strong as for steady and large southward IMF periods. Enhanced convection is also observed during northward IMF intervals when the interplanetary magnetic field exhibits high ULF power. We find that ULF power enhances the convection strength, independent of an observed direct effect from the solar wind speed. These observations thus suggest that IMF ULF fluctuations can significantly influence ionospheric convection. Therefore, in addition to the well‐established contributions from the direction and magnitude of the IMF and the solar wind dynamic pressure, ULF fluctuations may also be an important contributor to coupling of the solar wind to the magnetosphere‐ionosphere system. We speculate that resonance between IMF fluctuations and natural magnetospheric oscillation frequencies or magnetopause boundary oscillations might be responsible for the connection between ionospheric convection and IMF ULF power. We have also found evidence for a connection between the ULF power in the solar wind dynamic pressure and the strength of convection.

Concerning the occurrence pattern of flux transfer events on the dayside magnetopause

Abstract. We present an analytical model for the magnetic field perturbations associated with flux transfer events (FTEs) on the dayside magnetopause as a function of the shear between the magnetosheath and magnetospheric magnetic fields and the ratio of their strengths. We assume that the events are produced by component reconnection along subsolar reconnection lines with tilts that depend upon the orientation of the interplanetary magnetic field (IMF), and show that the amplitudes of the perturbations generated during southward IMF greatly exceed those during northward IMF. As a result, even if the distributions of magnetic reconnection burst durations/event dimensions are identical during periods of northward and southward IMF orientation, events occurring for southward IMF orientations must predominate in surveys of dayside events. Two factors may restore the balance between events occurring for northward and southward IMF orientations on the flanks of the magnetosphere. Events generated on the dayside magnetopause during periods of southward IMF move poleward, while those generated during periods of northward IMF slip dawnward or duskward towards the flanks. Due to differing event and magnetospheric magnetic field orientations, events that produce weak signatures on the dayside magnetopause during intervals of northward IMF orientation may produce strong signatures on the flanks.

Plasma plume circulation and impact in an MHD substorm

We investigate the fate of a plasmaspheric plume generated by a discrete period of southward interplanetary magnetic field (IMF) to assess its contribution to plasma sheet and ring current pressure and compare with that for other sources. We use test particle motions in Lyon‐Fedder‐Mobarry (LFM) global circulation model fields. The inner magnetosphere is simulated with the Comprehensive Ring Current Model (CRCM) model of Fok and Wolf, driven by the transpolar potential developed by the LFM magnetosphere. A variant of the Ober plasmasphere model is embedded within the models and driven by them. Global circulation is stimulated by a period of southward IMF embedded within a long interval of northward IMF. This leads to the production of a well‐defined plasmaspheric plume, enhancing the plasma density sunward of the plasmasphere. Test particles are launched with the properties of plasmaspheric ions on the L = 6.6 RE shell and weighted with densities as specified by the Ober model, as it responds to convection imposed by CRCM. Particles are tracked until they are lost from the system downstream or into the atmosphere, using the Delcourt full equations of motion, implemented for finite element fields. Results are compared with earlier computations of polar and auroral wind outflows. The plume produces an enhanced flow of plasma ∼10 times the normal polar wind global fluence. However, we find that most of the “plasmaspheric wind” is lost from the magnetosphere such that its contribution to the ring current energy density is comparable to that of the normal polar wind for this type of event.

Comparison of Birkeland current observations during two magnetic cloud events with MHD simulations

Abstract. Low altitude field-aligned current densities obtained from global magnetospheric simulations are compared with two-dimensional distributions of Birkeland currents at the topside ionosphere derived from magnetic field observations by the constellation of Iridium satellites. We present the analysis of two magnetic cloud events, 17–19 August 2003 and 19–21 March 2001, where the interplanetary magnetic field (IMF) rotates slowly (~10°/h) to avoid time-aliasing in the magnetic perturbations used to calculate the Birkeland currents. In the August 2003 event the IMF rotates from southward to northward while maintaining a negative IMF By during much of the interval. During the March 2001 event the IMF direction varies from dawnward to southward to duskward. We find that the distributions of the Birkeland current densities in the simulations agree qualitatively with the observations for northward IMF. For southward IMF, the dayside Region-1 currents are reproduced in the simulation, but appear on average 5° further poleward of their observed location, while the nightside Region-1 currents and the Region-2 currents are largely under-represented. Comparison of the observed and simulated Birkeland current distributions, which are intimately related to the plasma drifts at the ionosphere, shows that the ionospheric convection pattern in the MHD model and its dependence on the IMF orientation is essentially correct. The Birkeland total currents in the simulations are about a factor of 2 larger than observed during southward IMF. For Bz>0 the disparity in the total current is reduced and the simulations for purely northward IMF agree with the observations to within 10%. The disparities in the magnitudes of the Birkeland currents between the observations and the simulation results are a combined effect of the simulation overestimating the ionospheric electric field and of the Iridium fits underestimating the magnetic perturbations. The marginal difference in the latitude resolution of the ionospheric grids in the simulations and the observations is shown to have only secondary effect on the magnitudes of the Birkeland currents. The electric potentials in the simulation for southward IMF periods are twice as large as those obtained from measurements of the plasma drift velocities by DMSP, implying that the reconnection rates in the simulation are too large.

Energy filter effect for solar wind particle entry to the plasma sheet via flank regions during southward interplanetary magnetic field

Several mechanisms have been used to explain solar wind plasma entry into the plasma sheet in the magnetotail. In this paper, we focus on the gradient drift entry (GDE) process in the equatorial flanks of the magnetosphere, based on the magnetopause picture of a tangential discontinuity with a small tangential electric field as was suggested by Alfvén (1968). We discuss the GDE efficiency in different conditions using the adiabatic theory. It can be clearly shown that the GDE efficiency is much lower during southward interplanetary magnetic field (IMF), with a strong energy filter effect for incoming solar wind particles. Given a typical condition, a critical energy for particle entry is calculated to be several kiloelectron volts. Only those particles with higher energy can penetrate the magnetopause, a condition which can be also proved by test particle simulations. The lower efficiency than that during northward IMF during periods of southward IMF is in agreement with the different properties of the plasma sheet observed, i.e., hot and tenuous when the IMF is southward, cold and dense for northward IMF.

High‐latitude Joule heating response to IMF inputs

We evaluate the response of the high‐latitude Joule heating to orientation and magnitude of the interplanetary magnetic field (IMF). Approximately 9000 individual Joule heating patterns derived from data assimilation for the northern hemisphere were used to develop averaged and hemispherically integrated Joule power maps for the northern hemisphere north of 40° magnetic latitude. Hemispherically integrated Joule heating increases with IMF magnitude when the IMF is southward, but is relatively unchanged with increased IMF magnitude when the IMF is north. Only a small IMF By effect is evident in the Joule heating patterns. We quantify the IMF effects with linear fits of integrated Joule heating as a function of IMF magnitude for eight different IMF clock angle bins in the GSM Y‐Z plane. During extreme northward IMF conditions, Joule heating is restricted to the high latitude dayside. During extreme southward IMF conditions, Joule heating is located predominantly in the auroral region, with increased heating in the morning sector compared to the evening sector. This additional heating may lead to the increased incidence of gravity waves reported in the morning sector. Our estimate of the increase of hemispherically integrated Joule heating with IMF magnitude during southward IMF periods is 13GW/nT.

Comparison of intense nightside shock‐induced precipitation and substorm activity

Sudden variations of the solar wind dynamic pressure frequently induce dayside enhancements of auroral activity with features such as high‐latitude arcs, low‐latitude proton flashes, and enhancement of auroral precipitation propagating dawnward and duskward from noon to the night sector. In some cases, these shocks also induce enhanced activity during which the power precipitated into the night sector may reach values as high as observed during substorms. Several studies have shown that the triggering of nightside‐enhanced precipitation is more likely during periods of southward interplanetary magnetic field (IMF) components. Early works showed that substorm‐like activity is not frequent after a shock and suggested that shocks may not be considered as substorm triggers. We examine up to what point substorm‐like nightside activity triggered by a shock is comparable to an isolated substorm. For this purpose, we analyze three events morphologically similar to substorms and occurring within less than 20 min after the arrival of a pressure pulse on the front of the magnetosphere. Different features of these events such as the mean energy of precipitated electrons, the latitudinal motion of boundaries before and after onset, and the power precipitated into the nightside sector are compared with isolated substorms. We conclude that the characteristics of shock‐induced substorms appear very similar to those of isolated substorms. Shocks are able to trigger substorms when they hit an unstable magnetosphere. The interpretation is that the perturbation due to the shock induces a substorm by closure of the plasma sheet magnetic field. For the events presented in this study, the instability result from a period of southward IMF or stretching of the magnetic tail induced by a previous shock.

On continuous versus discontinuous neutral lines at the dayside magnetopause for southward interplanetary magnetic field

During intervals of southward interplanetary magnetic field (IMF), it is thought that magnetic reconnection occurs along an extended region of the dayside magnetopause. The reconnected magnetic field lines are believed to transport solar wind plasma into the magnetosphere and into the high‐latitude ionosphere. The precipitation pattern therefore provides information on the location and extent of reconnection at the magnetopause. This study uses observations of the energy and local time distribution of energetic proton precipitation into the dayside ionosphere during periods of southward IMF from the Far UltraViolet (FUV) imager on board the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft. These precipitation patterns suggest that the magnetic neutral line can be discontinuous across the dayside magnetopause.

Solar wind control of magnetospheric energy content: Substorm quenching and multiple onsets

In this paper we report coordinated multispacecraft and ground‐based observations of a double substorm onset close to Scandinavia on November 17, 1996. The Wind and the Geotail spacecraft, which were located in the solar wind and the subsolar magnetosheath, respectively, recorded two periods of southward directed interplanetary magnetic field (IMF). These periods were separated by a short northward IMF excursion associated with a solar wind pressure pulse, which compressed the magnetosphere to such a degree that Geotail for a short period was located outside the bow shock. The first period of southward IMF initiated a substorm growth phase, which was clearly detected by an array of ground‐based instrumentation and by Interball in the northern tail lobe. A first substorm onset occurred in close relation to the solar wind pressure pulse impinging on the magnetopause and almost simultaneously with the northward turning of the IMF. However, this substorm did not fully develop. In clear association with the expansion of the magnetosphere at the end of the pressure pulse, the auroral expansion was stopped, and the northern sky cleared. We will present evidence that the change in the solar wind dynamic pressure actively quenched the energy available for any further substorm expansion. Directly after this period, the magnetometer network detected signatures of a renewed substorm growth phase, which was initiated by the second southward turning of the IMF and which finally lead to a second, and this time complete, substorm intensification. We have used our multipoint observations in order to understand the solar wind control of the substorm onset and substorm quenching. The relative timings between the observations on the various satellites and on the ground were used to infer a possible causal relationship between the solar wind pressure variations and consequent substorm development. Furthermore, using a relatively simple algorithm to model the tail lobe field and the total tail flux, we show that there indeed exists a close relationship between the relaxation of a solar wind pressure pulse, the reduction of the tail lobe field, and the quenching of the initial substorm.