Tail Current System Research Articles

Decrease in SYM-H during a storm main phase without evidence of a ring current injection

Changes in the Dst index, or the similarly constructed high-resolution SYM-H index, are thought to indicate changes in the total energy content of the ring current. However, this is not always the case. In this paper we examine an intense (SYM-H ∼ −435nT) magnetic storm that occurred on March 31, 2001. The arrival at Earth of strongly southward IMF produced an immediate negative response in the SYM-H index. While energetic particle and magnetometer data from geosynchronous orbit and inner magnetosphere energetic neutral atom imaging indicate that two substorm injections took place during the main phase, there was about one hour when the SYM-H decreased more than 200nT with no evidence in the data for ring current enhancement. Instead the near-Earth magnetotail exhibited a growth phase indicative of a strong, growing cross-tail current, with the large substorm expansion phase and the associated injection of energetic particles coming significantly later. Data from the DMSP spacecraft demonstrate that the polar cap flux grew rapidly in response to the strongly southward IMF. We present observations showing that the decrease in SYM-H occurred when polar cap flux was increasing and there was no evidence of injection into the ring current. Our findings strongly support the relationship between Dst and the polar cap flux proposed by theoretical studies that determined that the tail current system could be a significant contributor to Dst.

Hemispheric asymmetries of the Venus plasma environment

We study the Venus‐solar wind interaction and the hemispheric asymmetries of the Venus plasma environment in the global HYB‐Venus hybrid simulation. We concentrate especially on the role of the flow‐aligned interplanetary magnetic field (IMF) component (i.e., the Parker spiral angle or the IMF cone angle) and analyze the dawn‐dusk and Esw asymmetries between four magnetic quadrants around Venus. Using the simulation model, we study two upstream condition cases in detail: the perpendicular IMF to the solar wind flow case and the nominal Parker spiral case (dominant flow‐aligned IMF component). Several differences and similarities were found in these two simulation runs. Common features of the Venus plasma environment between the two cases include asymmetric magnetic barrier and tail lobes and asymmetric planetary ion escape in the direction of the solar wind convection electric field. Further, protons of planetary origin and of solar wind origin were found to follow similar velocity patterns in the Venus plasma wake in both cases. The differences when the IMF flow‐aligned component is dominating compared to the perpendicular IMF case, the so‐called (magnetic) dawn‐dusk asymmetries, include the parallel bow shock and the foreshock region, the asymmetric magnetic barrier, the asymmetric tail current system, and the asymmetric central tail current sheet. Further, the escaping planetary H+ and O+ ion fluxes are concentrated more on the hemisphere of the parallel bow shock. When interpreting in situ plasma and magnetic observations from Venus, the features of at least these two basic IMF configurations should be considered.

Characteristics of plasma ring, surrounding the Earth at geocentric distances ∼7–10RE, and magnetospheric current systems

There are strong experimental evidences of the existence of plasma domain forming a closed plasma ring around the Earth at geocentric distances ∼7–10RE. In this work, we analyze the main properties of this ring, using the data of the THEMIS satellite mission, acquired between April 2007 and September 2011. We also analyze the contribution of this ring to the storm dynamics. In particular, it is shown that the distribution of plasma pressure at ∼7–10RE is nearly azimuthally symmetric. However, the daytime compression of the magnetic field lines and the shift of the minimal value of the magnetic field to higher latitudes lead to the spreading of the transverse current along field lines and splitting of the daytime integral transverse current into two branches in Z direction. The CRC is the high latitude continuation of the ordinary ring current (RC), generated by plasma pressure gradients, directed to the Earth. We evaluated the contribution of the azimuthally symmetric part of the plasma ring to the Dst index for strong geomagnetic storms using the AMPTE/CCE radial profiles of plasma pressure published before, and showed that the contribution of the ring current including both RC and CRC is sufficient to obtain the observed Dst variation without the necessity to include the tail current system.

Comparison between the two basic modes of magnetospheric convection

[1] Magnetic flux in the magnetosphere can circulate in a forward sense (southward IMF) and a reverse sense (northward IMF). The point of this paper is to compare and contrast these two modes of circulation. Both have transient and persistent phases in response to a suddenly applied southward or northward IMF that then is held steady. In the initial, transient phases, reconnection voltage exceeds transpolar potential in the ionosphere because for forward circulation there is an inductive-like time lag while the region 1 current builds up in association with a net flow of magnetic flux from the dayside to the tail (magnetospheric erosion). For the transient phase of reverse convection, reconnection voltage exceeds transpolar potential because instead of over-the-pole, night-to-day transport to undo preexisting magnetospheric erosion, magnetic flux accretes directly onto the dayside magnetosphere. The persistent phase of forward circulation almost always manifests substorms, which modulate the amounts of open and closed flux on the nightside by about 25% in a zero-sum game that leaves the dayside flux mostly unaltered. Energy to power the transient and persistent phases of forward circulation comes from the solar wind flow through magnetic stresses created as the field advects tailward. Energy to power the transient phase of reverse convection, however, comes from tapping magnetic energy stored inductively in association with the region 1 and tail current systems. The persistent phase of reverse circulation is driven possibly by the Song-Russell mechanism, which operates on a day-to-night pressure gradient of plasma entrained by dayside flux accretion. The resulting circulation is slow.

Paraboloid model of Mercury's magnetosphere

A new “Paraboloidal” model of Mercury's magnetospheric magnetic field based upon the earlier terrestrial model and using similar techniques is developed. The model describes the field of Mercury's dipole, which is considered to be offset from the planet's center; the magnetopause currents driven by the solar wind; and the tail current system including the cross‐tail currents and their closure currents at the magnetopause. The effect of the interplanetary magnetic field (IMF) is modeled as a partial penetration of the IMF into the magnetosphere. The goals of the present work are (1) to develop an easily usable, yet robust model of Mercury's magnetospheric magnetic field and (2) to produce an improved “unified” determination of Mercury's magnetic dipole moment which fits the measurements taken during both Mariner 10's first and third flybys. This new model of Mercury's magnetosphere is described and used to determine a best Mercury magnetic dipole moment of 192 nT RM3, from the two Mariner 10 flybys, a value which is intermediate between the various estimates produced by previous models. The best fit to the Mariner 10 measurements gives the dipole offset 0.18 RM above the equatorial plane. The new Paraboloidal model is used to predict the configuration of this miniature magnetosphere under average and extreme solar wind conditions.

Observed tail current systems associated with bursty bulk flows and auroral streamers during a period of multiple substorms

Abstract. We present a multi-instrument study of a substorm bursty bulk flow (BBF) and auroral streamer. During a substorm on 25 August 2003, which was one of a series of substorms that occurred between 00:00 and 05:00 UT, the Cluster spacecraft encountered a BBF event travelling Earthwards and duskwards with a velocity of ~500 km s−1 some nine minutes after the onset of the substorm. Coincident with this event the IMAGE spacecraft detected an auroral streamer in the substorm auroral bulge in the Southern Hemisphere near the footpoints of the Cluster spacecraft. Using FluxGate Magnetometer (FGM) data from the four Cluster spacecraft, we determine the field-aligned currents in the BBF, using the curlometer technique, to have been ~5 mA km−2. When projected into the ionosphere, these currents give ionospheric field-aligned currents of ~18 A km−2, which is comparable with previously observed ionospheric field-aligned currents associated with BBFs and auroral streamers. The observations of the BBF are consistent with the plasma "bubble" model of Chen and Wolf (1993). Furthermore, we show that the observations of the BBF are consistent with the creation of the BBF by the reconnection of open field lines Earthward of a substorm associated near-Earth neutral line.

A global magnetic model of Saturn's magnetosphere and a comparison with Cassini SOI data

A re‐scaling of a recent model of Jupiter's magnetosphere incorporating ring current, magnetopause, and tail current systems is used as a starting‐point for modeling Saturn's magnetospheric field. The model is compared with observations obtained by Cassini during its Saturn orbit insertion fly‐through of Saturn's magnetosphere, and is shown to give a good description. Comparison of ring current parameters obtained on the inbound and early outbound passes, when the magnetosphere was expanded due to low solar wind pressure, with those obtained by Voyager‐1 under more usual conditions, and by Pioneer‐11 when the magnetosphere was compressed, suggests that the ring current magnetic moment increases with system size. The effect is proposed to be due to radial stress balance conditions in a rapidly rotating magnetosphere, and has consequences for the dependence of magnetosphere size on solar wind pressure.

Signature of the quiet-time magnetospheric magnetic field and its electromagnetic induction in the rotating Earth

SUMMARY Accurate models of the magnetospheric field during magnetically quiet times are essential for high-resolution mapping of core field dynamics, mantle and ocean induction, crustal fields and ionospheric currents. Satellite data sampled at low-Earth orbit allow for a separate determination of the external contributions from currents in the magnetosphere. We have used Orsted and CHAMP data from the years 1999‐2004 to investigate this field component. In contrast to earlier studies, the field is decomposed here into contributions from sources in the solar-magnetic (SM) frame and those in the geocentric-solar-magnetospheric (GSM) frame. Fo ra nobserver on the Earth, stable fields in those frames generate different diurnal and annual variations which, in response, induce currents in the subsurface. All of these effects have been modelled here. Our primary findings are: in the GSM frame, there is a dominant constant magnetic field of about 13 nT, pointing due southward. This field component is attributed to the quiet-time tail current system. The interplanetary magnetic field (IMF) contributes to the near-Earth field with 10 per cent of its Bx and about 25 per cent of its By component. For the SM frame, we obtain a constant field of 7.6 nT and a variable part which can be parametrized by the DST index. The field in SM is attributed to the combined effect of the magnetopause and ring current. A comparison of the external field variations, predicted by our satellite-derived model, with the measurements of five latitudinally distributed ground observatories shows a remarkable agreement.

Modeling of the Jovian Magnetosphere

Abstract. This paper presents a global model of the Jovian magnetosphere which is valid not only in the equatorial plane and near the planet, as most of the existing models are, but also at high latitudes and in the outer regions of the magnetosphere. The model includes the Jovian dipole, magnetodisc, and tail current system. The tail currents are combined with the magnetopause closure currents. All inner magnetospheric magnetic field sources are screened by the magnetopause currents. It guarantees a zero normal magnetic field component for the inner magnetospheric field at the whole magnetopause surface. By changing magnetospheric scale (subsolar distance), the model gives a possibility to study the solar wind influence on the magnetospheric structure and auroral activity. A dependence of the magnetospheric size on the solar wind dynamic pressure psw (proportional to psw-0.23) is obtained. It is a stronger dependence than in the case of the Earth's magnetosphere (psw-1/6). The model of Jupiter's magnetospheric which is presented is a unique one, as it allows one to study the solar wind and interplanetary magnetic field (IMF) effects.

MHD simulations of quadrupolar paleomagnetospheres

During geomagnetic polarity transitions the Earth's internal magnetic field is expected to deviate significantly from the dipolar configuration observed today, and higher‐order multipoles can become important. Their effects on the magnetosphere are investigated in this study by means of MHD simulations. We look at quadrupolar configurations and vary the parameters that are expected to be most important in determining the large‐scale current systems and the field line topology of such paleomagnetospheres, namely, (1) the orientation of the internal field as well as (2) the orientation of the interplanetary magnetic field (IMF) with respect to the solar wind flow, and (3) the quadrupole shape parameter which controls the topology of the internal field. Zero IMF conditions yield reference cases which allow to locate the magnetopause that otherwise can be obscured by field line merging. Magnetopause currents and cross‐tail currents in quadrupolar configurations are contrasted with the tail current system in the present‐day dipolar magnetosphere. Interplanetary magnetic fields with a nonzero component perpendicular to the solar wind flow are found to have a much stronger effect on the global magnetospheric configuration than a purely parallel or antiparallel IMF. Since a quadrupole field is more structured than a dipole field, conditions that favor field line merging at the dayside are met regularly in quadrupolar magnetospheres. Solar wind‐magnetosphere coupling triggered by reconnection should thus be persistent rather than strongly controlled by solar wind parameters. The investigation presented in this report is part of a multi‐institutional collaboration to model the effects of geomagnetic polarity transitions on the Earth's environment in space and the upper and middle atmosphere.

Azimuthal magnetic fields in Saturn’s magnetosphere: effects associated with plasma sub-corotation and the magnetopause-tail current system

Abstract. We calculate the azimuthal magnetic fields expected to be present in Saturn’s magnetosphere associated with two physical effects, and compare them with the fields observed during the flybys of the two Voyager spacecraft. The first effect is associated with the magnetosphere-ionosphere coupling currents which result from the sub-corotation of the magnetospheric plasma. This is calculated from empirical models of the plasma flow and magnetic field based on Voyager data, with the effective Pedersen conductivity of Saturn’s ionosphere being treated as an essentially free parameter. This mechanism results in a ‘lagging’ field configuration at all local times. The second effect is due to the day-night asymmetric confinement of the magnetosphere by the solar wind (i.e. the magnetopause and tail current system), which we have estimated empirically by scaling a model of the Earth’s magnetosphere to Saturn. This effect produces ‘leading’ fields in the dusk magnetosphere, and ‘lagging’ fields at dawn. Our results show that the azimuthal fields observed in the inner regions can be reasonably well accounted for by plasma sub-corotation, given a value of the effective ionospheric Pedersen conductivity of ~ 1–2 mho. This statement applies to field lines mapping to the equator within ~ 8 RS (1 RS is taken to be 60 330 km) of the planet on the dayside inbound passes, where the plasma distribution is dominated by a thin equatorial heavy-ion plasma sheet, and to field lines mapping to the equator within ~ 15 RS on the dawn side outbound passes. The contributions of the magnetopause-tail currents are estimated to be much smaller than the observed fields in these regions. If, however, we assume that the azimuthal fields observed in these regions are not due to sub-corotation but to some other process, then the above effective conductivities define an upper limit, such that values above ~ 2 mho can definitely be ruled out. Outside of this inner region the spacecraft observed both ‘lagging’ and ‘leading’ fields in the post-noon dayside magnetosphere during the inbound passes, with ‘leading’ fields being observed both adjacent to the magnetopause and in the ring current region, and ‘lagging’ fields being observed between. The observed ‘lagging’ fields are consistent in magnitude with the sub-corotation effect with an effective ionospheric conductivity of ~ 1–2 mho, while the ‘leading’ fields are considerably larger than those estimated for the magnetopause-tail currents, and appear to be indicative of the presence of another dynamical process. No ‘leading’ fields were observed outside the inner region on the dawn side outbound passes, with the azimuthal fields first falling below those expected for sub-corotation, before increasing, to exceed these values at radial distances beyond ~ 15–20 RS , where the effect of the magnetopause-tail currents becomes significant. As a by-product, our investigation also indicates that modification and scaling of terrestrial magnetic field models may represent a useful approach to modelling the three-dimensional magnetic field at Saturn.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

Energetics of the magnetosphere during the magnetic storm

The most detailed studies of the energetic budget of the magnetosphere during the magnetic storms were done on the basis of the paraboloid model using the November 23–27, 1986 and May 6–8, 1988 magnetic storms. Calculations have shown that the energy injected in the course of the magnetic storms into the inner magnetosphere and ionosphere of both hemispheres amounts to ∼ 0.9 –2.2% of the solar wind kinetic energy on the magnetospheric cross section. The total energy injected into the magnetosphere from a distance of 60 R E in the tail down to the ionosphere is ∼4.0–7.5% of the solar wind kinetic energy during the main phase of the two magnetic storms. The injected energy into the tail E TL is 1.03–1.18 of the total energy input into the inner magnetosphere and ionosphere of both hemispheres at the main phase of the two storms. The decay parameter for the energy stored in the magnetospheric tail is ∼5 h . The total energy dissipated in the ionosphere of both hemispheres, in the inner magnetosphere and in the tail during the two storms, is 1.85×10 17 and 3.24×10 17 J , respectively. The total energy input into the magnetosphere is calculated to be 1.77×10 17 and 3.16×10 17 J . The discrepancies of 0.08×10 17 and 0.10×10 17 J amount to 4.3% and 3.1% of the total energy input and characterize the accuracy of the magnetospheric energy budget calculation. In the magnetotail the balance between the injected and dissipated energy of ∼1.09×10 17 J for one storm and ∼1.7×10 17 J for the other is preserved as well. We conclude that one-half of the energy which is injected into the magnetosphere from the solar wind during the storms enters the magnetotail and dissipates there. The coupling parameter ε PA is widely considered to be a measure of the energy dissipation in the inner magnetosphere. The dissipation energy in the inner magnetosphere U T= U J+ U A+ U DR is defined as the sum of the contributions of the Joule dissipation U J, the energy of auroral particle precipitation U A, and the energy injection into the ring current U DR . In this paper, we find that ε PA in the two storms investigated is substantially different from U T. The energy injected into the ring current region at the main phase of the storm amounts to ∼10 16 J . It is nearly 3 or 4 times smaller than the energy input into the magnetosphere via the field-aligned currents or the energy dissipated in the ionosphere by Joule dissipation. The energy injected into the magnetosphere is transferred mainly into processes different from the ring current generation. During the development of intensive auroral electrojets, the energy dissipation in the magnetotail and the increase in the energy of the tail current system occur simultaneously. The energy dissipation in the inner magnetosphere and ionosphere U S occurs not only at the expense of energy previously stored in the magnetotail, but rather at the expense of energy that is injected into the near-Earth tail. This energy transfer from the solar wind into the magnetotail and the energy dissipation in the ionosphere increased simultaneously. Thus, during disturbances in the magnetosphere, simultaneously loading–unloading and directly driven processes occur. Loading- and unloading processes manifest themselves both in the storage of the solar wind energy in the magnetotail and the ring current, and subsequent dissipation. The directly driven processes become manifest in the direct dissipation of the energy which enters into the ionosphere through large-scale field-aligned current systems.

Force balance and substorm effects in the magnetotail

A model of the quiet time middle magnetotail is developed using a consistent orbit tracing technique. The momentum equation is used to calculate geocentric solar magnetospheric components of the particle and electromagnetic forces throughout the current sheet. Ions generate the dominant x and z force components. Electron and ion forces almost cancel in the y direction because the two species drift earthward at comparable speeds. The force viewpoint is applied to a study of some substorm processes. Generation of the rapid flows seen during substorm injection and bursty bulk flow events implies substantial force imbalances. The formation of a substorm diversion loop is one cause of changes in the magnetic field and therefore in the electromagnetic force. It is found that larger forces are produced when the cross‐tail current is diverted to the ionosphere than would be produced if the entire tail current system simply decreased. Plasma is accelerated while the forces are unbalanced resulting in field lines within a diversion loop becoming more dipolar. Field lines become more stretched and the plasma sheet becomes thinner outside a diversion loop. Mechanisms that require thin current sheets to produce current disruption then can create additional diversion loops in the newly thinned regions. This process may be important during multiple expansion substorms and in differentiating pseudoexpansions from full substorms. It is found that the tail field model used here can be generated by a variety of particle distribution functions. However, for a given energy distribution the mixture of particle mirror or reflection points is constrained by the consistency requirement. The study of uniqueness also leads to the development of a technique to select guiding center electrons that will produce charge neutrality all along a flux tube containing nonguiding center ions without the imposition of a parallel electric field.

Physics of the magnetopause

A quantitative model of the magnetic field from the large-scale system I of Birkeland currents in the Earth's magnetosphere is proposed. In the near-Earth space the electric current flow lines follow the dipolar magnetic field lines, while at larger distances they enter the high-latitude magnetosheath near mid-day, the low-latitude boundary layer along the magnetospheric flanks, and the plasma sheet boundary layer at the nightside. The model takes into account the local time dependence of the latitude of Birkeland current zone at the topside ionospheric level reported by Iijima and Potemra (1976, J. geophys. Res.81, 2165.) The effects of the geodipole tilt in the overall geometry of the field-aligned current system are also incorporated; The model is sufficiently flexible; in particular, it permits an arbitrary choice of the current sheet thickness as well as of the M.L.T. distribution of the current intensity by using an appropriate combination of model coefficients in the corresponding Fourier expansion. One more possibility is to take into account the asymmetry in the net intensity and M.L.T. distribution of Birkeland currents between Northern and Southern Hemispheres which arises due to diurnal and seasonal variations of the geodipole tilt angle. The proposed analytical representation is relatively simple, which allows it to be incorporated in semi-empirical statistical magnetospheric models with parameters to be determined from spacecraft databases.Two other approaches to the modeling of warped current sheets aimed at quantitative approximation of the magnetic field of Birkeland and tail current systems are considered.

Analysis of Magnetotail Flux Ropes with Strong Core Fields: ISEE 3 Observations

A set of nineteen specific magnetic field structures in distant (≈220 Re) ISEE 3 magnetotail plasma sheet observations are analyzed. These were initially chosen on the basis of strong central (core) fields compared to nearby lobe field strengths by Slavin et al. (1995); the selection criteria by design did not include field directional changes. This present study starts with the same set of events as Slavin et al. Further discrimination is done on the basis of how well the signatures are fit by a force-free flux rope field model. This model was earlier applied successfully to magnetotail flux ropes where GEOTAIL field data were used. In general the ISEE 3 study yields average properties for the flux ropes that are similar to those of the GEOTAIL study, which had only about half the number of events, and hence, it strongly supports that earlier study. One difference in the results of these studies, however, is the greater longitudinal spread of rope-axes in the present study, and this feature, related flux rope current systems, and other properties are discussed. The results of this study should provide rigid constraints on global models of tail current systems associated with tailward moving flux ropes related to substorms.

Mapping of auroral arcs during substorm growth phase

The substorm growth phase is manifested in the auroral oval by brightening and equatorward motion of auroral arcs, prior to the formation of the westward traveling surge at substorm onset. In the magnetosphere the cross‐tail current enhances and the current sheet becomes very thin, and at substorm onset the tail current system disrupts, connecting the magnetosphere and the ionosphere through field‐aligned currents. The relation between the auroral and tail processes is studied here by magnetic‐field‐aligned mapping from the ionosphere to the current sheet using the Tsyganenko 1989 field model. The dynamical changes in the field and current distributions are modeled by stepwise modification of the field model to include the current sheet thinning and enhancement of the tail current. In addition, the field signatures of a current meander suggested to form in the tail are demonstrated. These signatures are compared with a model where line currents are added to represent the effects of the current wedge forming at substorm onset. Using the Tsyganenko model, the mapping of a growth phase arc recorded by Finnish all‐sky cameras moves earthward from 12 RE at the beginning of the growth phase to about 6 RE by the time of the substorm onset, corresponding to the equatorward motion of the arc. If the observed motion of the arc is interpreted to be entirely due to the stretching of the tail field during the growth phase, the enhancement of the cross‐tail current and the changes in the field configuration can be estimated. In this particular case, the current sheet has to be thinned to 10% of the original value at X = −8 RE, and the cross‐tail current enhanced by a factor of 1.3 to correspond to the equatorward motion of the arc.

Methods for quantitative modeling of the magnetic field from Birkeland currents

A quantitative model of the magnetic field from the large-scale system I of Birkeland currents in the Earth's magnetosphere is proposed. In the near-Earth space the electric current flow lines follow the dipolar magnetic field lines, while at larger distances they enter the high-latitude magnetosheath near mid-day, the low-latitude boundary layer along the magnetospheric flanks, and the plasma sheet boundary layer at the nightside. The model takes into account the local time dependence of the latitude of Birkeland current zone at the topside ionospheric level reported by Iijima and Potemra (1976, J. geophys. Res. 81, 2165.) The effects of the geodipole tilt in the overall geometry of the field-aligned current system are also incorporated; The model is sufficiently flexible; in particular, it permits an arbitrary choice of the current sheet thickness as well as of the M.L.T. distribution of the current intensity by using an appropriate combination of model coefficients in the corresponding Fourier expansion. One more possibility is to take into account the asymmetry in the net intensity and M.L.T. distribution of Birkeland currents between Northern and Southern Hemispheres which arises due to diurnal and seasonal variations of the geodipole tilt angle. The proposed analytical representation is relatively simple, which allows it to be incorporated in semi-empirical statistical magnetospheric models with parameters to be determined from spacecraft databases. Two other approaches to the modeling of warped current sheets aimed at quantitative approximation of the magnetic field of Birkeland and tail current systems are considered.

Response of dayside net downward field‐aligned current to changes in the interplanetary magnetic field and to substorm perturbations

We have studied the response of the auroral eastward and westward electrojets and of the downward field‐aligned currents across the noon sector which feed them to southward turnings of the interplanetary magnetic field (IMF) and to substorm onsets in the nightside magnetosphere. We have found that while southward turnings of the IMF lead to electrojet intensifications and low‐level substorm activity, northward turnings of the IMF appear capable of triggering large substorm disturbances. Both northward and southward turnings of the IMF cause the downward field‐aligned current across the noon sector, as well as the auroral electrojets, to strengthen within a few minutes of the impact of the changed IMF at the magnetopause. Since large substorms involve the sudden release of energy stored in the magnetotail, we conclude that the tail current system links directly to the overall magnetosphere‐ionosphere current system involving the auroral electrojets. In this context a substorm associated with a northward turning of the IMF reflects a reconfiguration of the magnetotail in response to a change in the level of the solar‐terrestrial interaction involving a decrease in the rate at which solar wind energy penetrates into the magnetosphere.

Saturn's magnetic tail: structure and dynamics

Voyager 1 magnetic field observations have provided evidence of a saturnian magnetic tail. Tail current system distributions are inferred through comparison of the observations with a realistic magnetotail current system model. Temporal variations observed in the tail were probably produced by solar wind variations.

A mathematical magnetospheric field model with independent physical parameters

A quantitative magnetospheric magnetic field model has been calculated in three dimensions. The model is based on an analytical solution of the Chapman-Ferraro problem. For this solution, the magnetopause was assumed to be an infinitesimally thin discontinuity with given geometry. The shape of the dayside magnetopause is in agreement with measurements derived from spacecraft boundary crossings. The magnetic field of the magnetopause currents can be derived from scalar potentials. The scalar potentials result from solutions of Laplace's equation with Neumann's boundary conditions. The boundary values and the magnetic flux through the magnetopause are determined by all magnetic sources which are located inside and outside the magnetospheric cavity. They include the Earth's dipole field, the fields of the equatorial ring current and tail current systems, and the homogeneous interplanetary magnetic field. In addition, the flux through the magnetopause depends on two constants of interconnection which provide the possibility of calculating static interconnection between magnetospheric and interplanetary field lines. Realistic numerical values for both constants have been derived empirically from observed displacements of the polar cusps which are due to changes in the orientation of the interplanetary field. The transition from a closed to an open magnetosphere and vice versa can be computed in terms of a change of the magnetic boundary conditions on the magnetopause. The magnetic field configuration of the closed magnetosphere is independent of the amount and orientation of the interplanetary field. In contrast, the configuration of the open magnetosphere confirms the observational finding that field line interconnection occurs primarily in the polar cusp and high latitude tail regions. The tail current system reflects explicitly the effect of dayside magnetospheric compression which is caused by the solar wind. In addition, the position of the plasma sheet relative to the ecliptic plane depends explicitly on the tilt angle of the Earth's dipole. Near the tail axis, the tail field is approximately in a self-consistent equilibrium with the tail currents and the isotropic thermal plasma. The models for the equatorial ring current depend on the D st -parameter. They are self-consistent with respect to measured energy distributions of ring current protons and the axially symmetric part of the magnetospheric field.