Magnetospheric Magnetic Field Research Articles

Magnetopause stand-off distance in dependence on the magnetosheath and solar wind parameters

Abstract. A model of the magnetosheath structure proposed in a recent paper from the authors is extended to estimate the magnetopause stand-off distance from solar wind data. For this purpose, the relationship of the magnetopause location to the magnetosheath and solar wind parameters is studied. It is shown that magnetopause erosion may be explained in terms of the magnetosheath magnetic field penetration into the magnetosphere. The coefficient of penetration (the ratio of the magnetospheric magnetic field depression to the intensity of the magnetosheath magnetic field Bm⊥z=–Bmsin2Θ/2, is estimated and found approximately to equal 1. It is shown that having combined a magnetosheath model presented in an earlier paper and the magnetosheath field penetration model presented in this paper, it is possible to predict the magnetopause stand-off distance from solar wind parameters.Key words. Magnetospheric physics · Magnetopause · Cusp and boundary layers-Magnetosheath

Concerning the location of magnetopause merging as a function of the magnetopause current strength

We start from an assumption that merging occurs in regions of the magnetopause where current strengths are greater than some threshold value which corresponds to the total jump in the field across the magnetopause greater than 50 nT. Because time and cost constraints preclude running numerical simulations for a wide variety of interplanetary magnetic field (IMF) orientations to determine these locations, we adopt an analytical model based on previously derived formulations for magnetospheric and magnetosheath magnetic fields. The magnetospheric magnetic field is confined within a paraboloid. The magnetosheath magnetic field is derived from that in the solar wind and lies between the magnetopause and a paraboloid bow shock. We allow a slight diffusion of the magnetosheath magnetic field into the magnetosphere. The results of the model show that during periods of due southward IMF orientation, merging occurs (as expected) in a wide region centered on the subsolar magnetopause. During periods of northward IMF, connection continues near the subsolar point but also poleward of the cusps. Magnetic energy is only released to the plasma in the latter regions. During periods of strongly northward IMF (By = 0), reconnection ceases on the subsolar magnetopause but continues poleward of the cusp. If the IMF points northward but By is nonzero, reconnection continues near the subsolar point and poleward of the cusps. During periods of sunward IMF orientation, merging nearly ceases on the northern hemisphere (except in the vicinity of the subsolar point) but continues outside the southern lobes. Dawnward and duskward IMF orientations produce tilted patches of enhanced current densities in the subsolar region. We compare the results of our model with previous predictions of the “component” and “antiparallel” merging models.

Global configuration of the magnetotail current sheet as derived from Geotail, Wind, IMP 8 and ISEE 1/2 data

Based on fine‐resolution Geotail magnetometer data, a set of 5‐min magnetic field averages was compiled for the period 1993–1997 and merged with 5‐min average solar wind parameters, measured by IMP 8 and Wind spacecraft. Using this data set, the shape of the tail current sheet was studied in the interval −100 < XGSM < − 10 RE as a function of the Earth's dipole tilt angle and of the By component of the IMF. The tilt‐related warping of the current sheet and its twisting around the magnetotail axis in response to the IMF were modeled by analytical functions, whose parameters were found by least squares fitting to the data, for several bins of XGSM. A similar modeling was also done for the near‐tail region −20 < XGSM < − 10 RE using a set of 5‐min ISEE 1/2 data, tagged by corresponding solar wind parameters from IMP 8, for the entire duration of the ISEE magnetometer experiment (1977–1987). The IMF‐related twisting steadily increases down the tail and is quite conspicuous even at close geocentric distances (−20 ≤ X ≤ −10 RE). A simple and flexible mathematical method is suggested, which allows quantitative modeling on a global scale of the IMF‐related deformation of the cross‐tail current by means of a “twist transformation” of the tail field. The method allows for a wide variety of possible geometries of the current sheet and keeps the total field confined within the magnetotail boundary. The results of the study are intended to be used in the development of an improved global model of the magnetospheric magnetic field, incorporating the effects of the IMF upon the magnetotail structure.

Radio Pulse Properties of the Millisecond Pulsar PSR J0437−4715. I. Observations at 20 Centimeters

We present a total of 48 minutes of observations of the nearby, bright millisecond pulsar PSR J0437-4715 taken at the Parkes Observatory in Australia. The data were obtained at a central radio frequency of 1380 MHz using a high-speed tape recorder that permitted coherent Nyquist sampling of 50 MHz of bandwidth in each of two polarizations. Using the high time resolution available from this voltage recording technique, we have studied a variety of single-pulse properties, many for the first time in a millisecond pulsar. We show that individual are broad band, have pulse widths ranging from ~10 (~06 in pulse longitude) to ~300 μs (~20°) with a mean pulse width of ~65 μs (~4°), exhibit a wide variety of morphologies, and can be highly linearly polarized. Single pulse peaks can be as high as 205 Jy (over ~40 times the average pulse peak), and have a probability distribution similar to those of slow-rotating pulsars. We observed no single pulse energy exceeding ~4.4 times the average pulse energy, ruling out giant pulses as have been seen for the Crab and PSR B1937+21 pulsars. PSR J0437-4715 does not exhibit classical microstructure or show any signs of a preferred timescale that could be associated with primary emitters; single pulse modulation has been observed to be consistent with amplitude-modulated noise down to timescales of 80 ns. We observe a significant inverse correlation between pulse peak and width. Thus, the average pulse profile produced by selecting for large pulse peaks is narrower than the standard average profile. We find no evidence for diffractive quantization effects in the individual pulse arrival times or amplitudes as have been reported for this pulsar at lower radio frequency using coarser time resolution. Overall, we find that the single-pulse properties of PSR J0437-4715 are similar to those of the common slow-rotating pulsars, even though this pulsar's magnetosphere and surface magnetic field are several orders of magnitude smaller than those of the general population. The pulsar radio emission mechanism must therefore be insensitive to these fundamental neutron star properties.

The effect of local irregularities on large-scale ionospheric currents

Measurements performed by the low-altitude satellites MAGSAT and TRIAD have discovered small-scale (L ⊥<80 km) magnetic fluctuations at high latitudes. A relation between the fluctuation intensity, ionospheric conductivity and Birkeland currents was also revealed. We assume that the magnetic perturbations are caused by small-scale ionospheric irregularities. We discuss here the influence of random ionospheric irregularities on the effective conductivity. Through analytical consideration, it is concluded that the Pedersen effective conductivity may noticeably deviate from the average conductivity. The effect depends primarily on conditions of Hall current closing. Wherever the Hall currents flow is impossible for any reason, the influence of weak irregularities is negligible. Zones of the equatorial and polar electrojets with electric field directed along electrojets are examples. Analytical consideration conducted for the regions exterior to these zones has shown that random weak ionospheric irregularities double the total Pedersen effective integral conductivity increases. It is usually suggested in the description of the ionospheric conductivity that the ionosphere is either homogeneous or exhibits local irregularity. The background sporadic irregularities in the ionosphere usually small scale is also well known. The research concentrated on two main items: the contribution of a random wind and conductivity field to the background and magnetospheric quasi-stationary electric and magnetic field, and the influence of small stochastic ionospheric irregularities on the effective ionospheric conductivity.

Local time asymmetry of drift shells for energetic electrons in the middle magnetosphere of Saturn

Saturn encounter trajectories of Pioneer 11, Voyager 1, and Voyager 2 passed through the noon-dusk quadrant of the middle magnetosphere during approach and through dawn-midnight outbound. Intensities of energetic electrons (≥0.6 MeV) were an order of magnitude higher inbound than outbound at 6–15 R s , even though the same (Pioneer 11) or higher (Voyager 1 and 2) intensities were expected outbound for stably trapped electrons with azimuthally symmetric distributions. Explanations include electron drift shell asymmetry arising from (1) magnetopause compression by the solar wind and/or (2) dusk-to-dawn electric field associated with solar wind driven convection within the magnetosphere. Theoretical and numerical calculations indicate that maximum effects of the electric field are expected at L-dependent drift resonance energies in the range of the maximum observed asymmetries. Maximum intensities would then be expected on the dusk side of the middle magnetosphere where “bannana”-shaped drift shells are expected near the resonance energies. Since some of the asymmetric drift shells would not extend around to the dawn side, minimum intensities would be expected there as observed. Further analysis and modeling will be needed to assess the relative effects of the electric field magnetopause compression, and other possible causes. Full local time coverage of energetic particles by the Cassini Orbiter mission would be invaluable for additional constraints on the magnetospheric electric and magnetic fields.

Magnetopause stand-off distance in dependence on the magnetosheath and solar wind parameters

A model of the magnetosheath structure proposed in a recent paper from the authors is extended to estimate the magnetopause stand-off distance from solar wind data. For this purpose, the relationship of the magnetopause location to the magnetosheath and solar wind parameters is studied. It is shown that magnetopause erosion may be explained in terms of the magnetosheath magnetic field penetration into the magnetosphere. The coefficient of penetration (the ratio of the magnetospheric magnetic field depression to the intensity of the magnetosheath magnetic field Bm⊥z=–Bmsin2Θ/2, is estimated and found approximately to equal 1. It is shown that having combined a magnetosheath model presented in an earlier paper and the magnetosheath field penetration model presented in this paper, it is possible to predict the magnetopause stand-off distance from solar wind parameters.Key words. Magnetospheric physics · Magnetopause · Cusp and boundary layers-Magnetosheath

The missing dimension in Magsat and POGO anomaly studies

The development of a comprehensive model of the near‐Earth magnetic field in terms of sources in the core, ionosphere, and magnetosphere has allowed the isolation of heretofore unknown north‐south trending anomalies of lithospheric origin. Previously, along‐track filtering of satellite magnetic field data had been necessary in order to remove unmodeled magnetospheric and ionospheric magnetic fields. These fields change between adjacent passes by tens of nanoTeslas and have wavelengths which overlap the crustal spectrum. An example from Australia illustrates the additional information that can be extracted from the Magsat data set when this ‘missing dimension‧ is included. A north‐south trending low covers most of eastern Australia, in agreement with the aeromagnetic map of Australia. This feature is seen in both the dawn and dusk subsets collected from Magsat. The center of this low is roughly coincident with the Eromanga Basin but extends for almost 2000 km in a N‐S direction. The low corresponds to a region of almost featureless magnetic field on the aeromagnetic map. It is also consistent with an elevated geotherm previously proposed from studies of xenoliths in southeastern Australia. Filtering the new map in a north‐south direction reproduces the previous best estimate of the Australian satellite anomaly field. We expect that this comprehensive model will provide a new window into the magnetic field of the earth's lithosphere. The model continues to be enhanced.

Dayside isotropic precipitation of energetic protons

Abstract. Recently it has been shown that isotropic precipitation of energetic protons on the nightside is caused by a non-adiabatic effect, namely pitch-angle scattering of protons in curved magnetic field lines of the tail current sheet. Here we address the origin of isotropic proton precipitation on the dayside. Computations of proton scattering regions in the magnetopheric models T87, T89 and T95 reveal two regions which contribute to the isotropic precipitation. The first is the region of weak magnetic field in the outer cusp which provides the 1–2° wide isotropic precipitation on closed field lines in a ~2–3 hour wide MLT sector centered on noon. A second zone is formed by the scattering on the closed field lines which cross the nightside equatorial region near the magnetopause which provides isotropic precipitation starting ≈ 1.5–2 h MLT from noon and which joins smoothly the precipitation coming from the tail current sheet. We also analyzed the isotropic proton precipitation using observations of NOAA low altitude polar spacecraft. We find that isotropic precipitation of &gt;30 to &gt;80 keV protons continues around noon forming the continuous oval-shaped region of isotropic precipitation. Part of this region lies on open field lines in the region of cusp-like or mantle precipitation, its equatorward part is observed on closed field lines. Near noon it extends ~1–2° below the sharp boundary of solar electron fluxes (proxy of the open/closed field line boundary) and equatorward of the cusp-like auroral precipitation. The observed energy dispersion of its equatorward boundary (isotropic boundary) agrees with model predictions of expected particle scattering in the regions of weak and highly curved magnetic field. We also found some disagreement with model computations. We did not observe the predicted split of the isotropic precipitation region into separate nightside and dayside isotropic zones. Also, the oval-like shape of the isotropic boundary has a symmetry line in 10–12 MLT sector, which with increasing activity rotates toward dawn while the latitude of isotropic boundary is decreasing. Our conclusion is that for both dayside and nightside the isotropic boundary location is basically controlled by the magnetospheric magnetic field, and therefore the isotropic boundaries can be used as a tool to probe the magnetospheric configuration in different external conditions and at different activity levels.

Drift‐shell splitting of energetic ions injected at pseudo‐substorm onsets

One feature of a magnetospheric substorm is the injection of energetic particles into closed drift orbits. Injections are routinely observed by geosynchronous satellites and have been used to identify the occurrence of substorms and the local time of particle energization. In this study we examine pitch angle distributions of ion injections in the 50‐ to 300‐keV energy range observed by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) satellite, hereinafter CCE. In a dipole field, all pitch angles follow the same drift shell, but the day‐night asymmetry of the magnetospheric magnetic field introduces a pitch angle dependence in particle drift orbits, so that particles with different pitch angles disperse radially as they drift. The effect is known as drift‐shell splitting. For satellite observations near noon at a fixed geocentric distance, the guiding center orbits of ions detected at small pitch angles intersect the midnight meridian at larger geocentric distances than do ions with near‐90° pitch angles. The ion pitch angle distributions detected on the dayside therefore provide information about the radial distance of the nightside acceleration region. We apply this principle to study ion injection events observed on September 17–18, 1984, in association with pseudo‐substorm onsets. CCE was at 13 hours local time near its apogee (8.8 RE) and observed a series of ion flux enhancements. Energy dispersion of the timing of the flux increases assures that they are due to injections on the nightside. The flux increases were observed only at pitch angles from 0° to 60°. We calculate drift orbits of protons using the Tsyganenko 89c magnetic field model and find that the drift orbits for 60° pitch angle protons observed at the satellite pass through midnight at 9 RE, well outside of geostationary orbit, indicating that the ion injections occurred tailward of 9 RE. Energetic ion data from geostationary satellites for the same time interval show no evidence of injections at 6.6 RE, consistent with the calculated inner boundary of the injections. Model calculations are presented demonstrating that dispersive injections observed near noon outside geosynchronous orbit provide the greatest sensitivity to drift‐shell splitting effects and are therefore most suitable for remote sensing the radial boundaries of substorm injections.

Fully adiabatic changes in storm time relativistic electron fluxes

It has been suggested that much of the drop and subsequent recovery of storm time relativistic electron fluxes at geosynchronous orbit can be explained in terms of a fully adiabatic response (all three adiabatic invariants conserved) to magnetic field changes. To calculate this effect, we assume a prestorm electron flux distribution constructed from CRRES satellite data, we use modular magnetospheric magnetic field models to represent the magnetic field configuration before and during the storm, and we use Liouville's theorem to evolve the prestorm electron flux. In this work we focus on the important special case of equatorially mirroring electrons. During the main phase of a storm with a Dst minimum of −100 nT we find that the fully adiabatic effect can cause a flux decrease of up to 2 orders of magnitude, consistent with observed flux decreases. We also find that the magnitude of the fully adiabatic flux decrease is larger for lower energies, again in agreement with observations. The contribution of prestorm electron fluxes to the recovery phase flux increase at synchronous orbit is expected to be small because of losses to the dawnside magnetopause. A comparison of fully adiabatic fluxes with measured electron fluxes for the November 2–5, 1993, storm indicates that for this event the fully adiabatic effect may be contributing to the observed decrease but that nonadiabatic effects are clearly important. Overall we conclude that the fully adiabatic effect can account for a significant fraction of observed flux decreases and that differences between the observed and the fully adiabatic fluxes help to clarify when and where additional loss and source mechanisms exist.

A statistical study of the magnetic field structure in the inner magnetosphere

Three‐dimensional magnetic field structure in the inner magnetosphere is statistically analyzed using data obtained by the DE‐1 magnetometer. The main results are as follows: (1) The magnetic field structure of the rotational pattern associated with an eastward current is clearly seen in the dayside equatorial region at geocentric distances around 2.5 Re but not on the nightside. The current density is approximately 0.05μ A/m2, and does not show any clear dependence on geomagnetic disturbance, but shows a solar activity dependence which possibly suggests that a substantial part of the closure of the eastward current is through the dayside mid‐latitude ionosphere. (2) The azimuthal component of the magnetic field associated with field‐aligned currents is seen in the low‐latitude magnetosphere. The intensity of this component is about 5 to 10 nT, and has a dependence on geomagnetic activity. (3) Compared with the currently available magnetospheric models, the observed inner magnetospheric magnetic fields have much more complex structures than the models indicate. The necessity of including the field‐aligned current system and an eastward current in the models is evident. (4) A noon‐midnight asymmetry of the magnetic structure is clearly seen, particularly in the southward component associated with the ring current. The asymmetry is much larger in this meridian than that in the dawn‐dusk meridian. This is in contrast with the asymmetry observed on the ground or by low‐altitude satellites over the ionosphere.

High‐latitude ionospheric transient events in a global context

This paper presents a comprehensive global study of bow shock motion, magnetosheath dynamic pressure variations, dayside magnetospheric magnetic field perturbations, and cosmic noise absorption events attending a sequence of transient events observed in high‐latitude global ground magnetograms on February 14, 1986. ISEE 2 spacecraft observations indicated that most of the transient ground magnetometer events were associated with abrupt inward and outward motions of the dawn bow shock during a prolonged period of strongly northward and orthospiral interplanetary magnetic field orientation. Geosynchronous GOES 5 and 6 magnetometer observations provide evidence for corresponding compressions and expansions, which propagated duskward through the prenoon magnetosphere. The magnetopause moved inward past the CCE spacecraft during some of the compressions. The compressions scattered magnetospheric particles and thereby produced quasi‐periodic oscillations in the cosmic noise absorption recorded by high‐latitude ground riometers. We infer that (unobserved) strong transient variations in the solar wind dynamic pressure produced the various phenomena and that the ground magnetometer signatures are best explained in terms of the pressure pulse model of Southwood and Kivelson [1990], in which a single sharp change in the solar wind dynamic pressure generates a pair of field‐aligned currents.

Comparison of observed and model magnetic fields at high altitudes above the polar cap: POLAR initial results

Data obtained from the high altitude, polar orbiting spacecraft, POLAR, are compared with the latest version of the data‐based magnetospheric magnetic field model. The data generally agree well with the model. The major directional discrepancies at high altitudes are near the dayside cusp and on the “open” field lines over the polar cap, especially close to the boundary of the polar cap. Near the cusp, the agreement is improved if a stronger solar wind dynamic pressure and more negative IMF By and Bz are used as the model input parameters, than was actually observed. The field measured in the vicinity of the polar cusps is generally weaker than predicted by the model. Close to noon the spacecraft enters a region of additional structured field depression that appears to be the polar cusp proper. Within the limited statistics presented here, the invariant latitude of the cusp appears to be controlled by the north‐south component of the IMF and the broad depression appears to be controlled by the tilt of the dipole.

Exterior and interior polar cusps: Observations from Hawkeye

Hawkeye plasma, magnetic field, and plasma wave instruments directly sampled the throat of the northern polar cusp as the orientation of the interplanetary magnetic field (IMF) changed from southward to northward on July 3, 1974. Two distinct regions in the polar cusp were identified based on magnetic field, plasma flow and magnetic and electric noise: the interior and exterior cusps. The observations show highly variable flows in the exterior portion of the cusp and constantly strong dawn‐dusk flows in the interior portion during periods of strong IMF By component. Results of a minimum variance analysis of the magnetic field at each cusp interface crossing provides evidence that the magnetopause surface normal deviated highly from empirical models. During intervals of relatively steady solar wind dynamic pressure, the motion of the cusp relative to the slow moving spacecraft was modulated by the varying IMF clock angle as observed by IMP 8 in the upstream solar wind. The motion did not show a correlation with internal processes monitored by the AE index. We propose that observed plasma flow patterns and cusp motion are results of reconnection between the IMF and the magnetospheric magnetic field. Flow velocity observed in the interior cusp is consistent with stress balance for a reconnection process. This unique interval provides an opportunity for detailed studies of the plasma, magnetic field, and plasma wave properties in both the exterior and interior cusp.

Internal structure of flux transfer events produced by the onset of merging at a single X line

We present a new, higher‐order, two‐dimensional, magnetohydrodynamic model for the detailed internal structure of bubbles produced by the onset of merging along a single subsolar X line. The key feature of the model is to implement a nonlinear filter to postprocess results provided by the pseudospectral element method. The devised nonlinear filter (1) eliminates only the high‐frequency oscillations, (2) retains conservation laws, and (3) enforces a total variation diminished criterion to guarantee the elimination of spurious oscillations near discontinuities. For realistic parameters, the model predicts that satellites which remain in the magnetosheath will observe asymmetric bipolar magnetic field signatures normal to the magnetopause, magnetic field strength increases, density and temperature decreases, and velocities opposite the motion of the events. Satellites in the magnetosheath which enter the events observe asymmetric bipolar signatures normal to the magnetopause, magnetic field strength decreases, densities intermediate between those in the magnetosheath and magneto‐sphere, temperature increases, and velocities in the direction of event motion. Satellites initially located just outside the the magnetopause observe greatly depressed densities and enhanced temperatures (but no significant bipolar signatures normal to the magnetopause) followed by velocity jets of magnetosheath‐like plasma on northward magnetospheric magnetic field lines. Satellites in the magnetosphere which enter the events observe depressed magnetic field strengths, enhanced densities and temperatures, and accelerated flows surrounding a core region of accelerated magnetosheath‐like plasma. Neither they nor satellites which remain in the magnetosphere observe any significant bipolar signatures normal to the nominal magnetopause.

Magnetospheric structure with a southward interplanetary magnetic field

A magnetospheric magnetic field that has a topological structure with one pair of neutral points and one segmental neutral line is studied. Such a magnetic field forms when the interplanetary field is southward. There is a surface current on an open‐ended complete flux surface in this magnetospheric magnetic field. This current sheet on which the induced Chapman‐Ferraro current flows is the outer separatrix, which separates field lines attached to the Earth and field lines detached from the Earth. In addition, there is an inner separatrix, which separates closed and open field lines. The two neutral points are at the north and south apices of the common wall shared by the two separatrices, on which the surface current is intense. The surface current is weak or even insignificant on other parts of the outer separatrix. The neutral line is on the equatorial plane with two end points at the east and west apices of the common wall shared by the two separatrices. It is a separator at which closed, open, and detached field lines meet. The magnetic topology indicates that field line reconnection, which can take place only at a separator, must occur away from the subsolar part of the dayside magnetopause in order for an open magnetosphere to remain in a quasisteady state.

Mapping of the ionospheric field‐aligned currents to the equatorial magnetosphere

The assimilative mapping of ionospheric electrodynamics (AMIE)‐derived ionospheric field‐aligned current patterns during the ninth Coordinated Data Analysis Workshop substorm event have been mapped to the magnetospheric equatorial plane, using the data‐based magnetospheric magnetic field model [Tsyganenko, 1995]. We find that at the onset of the substorm expansion phase, a pair of field‐aligned currents emerges about 8 RE tailward of the Earth, near the x axis. This pair of field‐aligned currents is consistent with the substorm current wedge model; it lines up in the east–west direction across the near–Earth tail current and has the sense of the nightside region 1 current, i.e., flowing into the ionosphere on the east side and out of the ionosphere on the west side. The ionospheric counterpart of the substorm wedge currents in the midnight sector, on the other hand, has a northeast‐to‐southwest alignment. During the expansion phase, the substorm wedge currents intensify and expand in spatial size; however, their peaks remain nearly at the same location in the equatorial magnetosphere, about 8 RE tailward from the Earth. We also find that the intensification of the substorm wedge currents during the expansion phase is accompanied by the intensification of the region 2‐sense currents located closer to the Earth, and the boundary between the region 2 and substorm wedge currents lies around x = −6 RE.

Cosmic ray cutoff rigidities in the Tsyganenko magnetospheric field model at middle latitudes

The cutoff rigidities of cosmic rays are determined in the magnetospheric magnetic field simulated by the model of external current system proposed by Tsyganenko (1987) (Ts87) [ Planet Space Sci. 35, 1347]. The diurnal variations of cosmic ray vertical cutoffs are obtained for the midlatitude stations Moscow, Irkutsk, Tbilisi and Alma-Ata, at the moment of summer solstice. The cutoffs received are compared with those obtained using the model by Tsyganenko and Usmanov (1982) (TsU82) [ Planet Space Sci. 30, 985] for K p = 0, 2 and 4. The results show that the differences between daily averaged geomagnetic cutoff rigidities in the magnetospheric fields of the Ts87 and TsU82 models are not more than 0.03 GV.

Magnetic storms and magnetotail currents

The magnetospheric magnetic field is highly time‐dependent and may have explosive changes (magnetospheric substorms and geomagnetic storms) accompanied by significant energy input into the magnetosphere. However, the existing stationary magnetospheric models can not simulate the magnetosphere for disturbed conditions associated with the most interesting magnetospheric physics events (intensive auroras, particle injection in the inner magnetosphere, and precipitations at the high latitudes, etc.). We propose a method for constructing a nonstationary model of the magnetospheric magnetic field, which enables us to describe the magnetosphere during the disturbances. The dynamic changes of the magnetosphere will be represented as a sequence of quasistationary states. The relative contributions to the Dst index by various sources of magnetospheric magnetic field are considered using a dynamic model of the Earth's magnetosphere. The calculated magnetic field is obtained by using the solar wind and geomagnetic activity empirical data of the magnetic storm of March 23–24, 1969 and the magnetic disturbance of July 24–26, 1986. The main emphasis is on the current system of the magnetospheric tail, the variations of which enable a description of the fast changes of Dst.