Three-dimensional computer modeling of dynamic reconnection in the magnetotail     

Kyoung W. Min 《Astrophysics and Space Science》1990,163(1):95-107

Magnetospheric substorm in the magnetotail region is studied numerically by means of a three-dimensional MHD code. The analytic solution for the quiet magnetotail is emloyed as an initial configuration. The localized solar wind is modeled to enter the simulation domain through the boundaries located in the magnetotail lobe region. As a result of the interaction between the solar wind and the magnetosphere, the magnetic field lines are stretched, and the plasma sheet becomes thinner and thinner. When the current-driven resistivity is generated, magnetic reconnection is triggered by this resistivity. The resulting plasma jetting is found to be super-magnetosonic. Although the plasmoid formation and its tailward motion is not quite clear as in the two-dimensional simulation, which is mainly because of the numerical model chosen for the present simulation, the rarification of the plasmas near thex-point is observed. Field-aligned currents are observed in the late expansive stage of the magnetospheric substorm. These field-aligned currents flow from the tail toward the ionosphere on the dawn side and from the ionosphere toward the tail on the dusk side, namely in the same sence of the region 1 current. As the field-aligned currents develop, it is found that the cross-tail current in the Earthside midnight section of the magneticx-point is reduced.  相似文献   

Neutral line motion due to reconnection in two-ribbon solar flares and magnetospheric substorms     

T.G. Forbes  E.R. Priest 《Planetary and Space Science》1982,30(11):1183-1197

Two kinematic models of line-tied reconnection are considered which describe the motion of a magnetic neutral line (NL) during the main phase of a two-ribbon solar flare and during the recovery phase of a magnetospheric substorm in the geomagnetic tail. The models are kinematic in that they use only the magnetic induction equation, which suffices to determine the position and velocity of the NL as functions of time if the rate of reconnection is prescribed. The solar flare model shows that the observed large decrease in the rate at which “post”-flare loops rise upward from the photosphere during the main phase does not require a corresponding decrease in the rate of reconnection. Instead it is found that a constant rate of reconnection can account for the motion of the loops for almost the entire period during which they are observed. By contrast, application of the same procedures to the recovery phase of the magnetospheric substorm in the tail predicts a slightly increasing speed of NL motion if the rate of reconnection is constant. Furthermore, it is found that the motion of the NL relative to the ambient medium may account for much of the observed asymmetry in the magnetic field in the plasma sheet during recovery. Due to this motion, the plasma sheet thickness may be up to 4 times smaller and the normal magnetic field component up to 2 times weaker in the region tailward of the NL than in the corresponding region earthward of the NL.  相似文献   

Electron counts in the neutral sheet and magnetic variations at a geomagnetic longitude associated ground station     

F.R. Bond  P.H. Sulzberger  H.A. Thelander 《Planetary and Space Science》1973,21(11):2013-2025

A comparison of the variations in the count of electrons E > 36 keV on the satellite Vela 4A, and in the Macquarie Island magnetometer H trace, shows for a time lag of 22-8 min a correlation, r = 0.95, over a 90 min period of the recovery phase of a magnetospheric substorm on 17 August 1968. All-sky camera data suggest that during the correlation period the auroral electrojet showed very little latitudinal movement. Each peak in electron count relates to a current surge in the electrojet as shown by a deepening of the negative bay at Macquarie Island.Using the Fairfield (1968) model of the location of auroral shells in the solar magnetic equatorial plane, and the known location of the satellite, an estimate of the velocity of tail to Earth plasma convection in the plasma sheet of about 0·33 R_e/min is obtained for the recovery phase.The relationship is discussed between plasma sheet thinning and subsequent broadening, and the extension of the magnetic field lines into the tail region and their subsequent return. This discussion makes use of the estimated time lags between electron count at the satellite and the time of arrival of auroral particles at the antisolar meridian.From a somewhat speculative explanation, but one largely supported from the literature, of the magnetospheric processes involved in this auroral substorm, a plasma velocity estimate of 0·42 R_e/min for the initial phase of the substorm is obtained. These velocities are of the same order as the 0·5 R_e/min obtained by Lezniak and Winkler (1970) at 6·6 R_e.  相似文献   

Substorm related changes in the geomagnetic tail: the growth phase     

R.L. McPherron 《Planetary and Space Science》1972

Changes in the configuration of the geomagnetic tail are known to play a fundamental role in magnetospheric substorms. Observations with the UCLA magnetometer on the eccentric orbiter OGO-5 indicate that the most pronounced changes in the midnight meridian occur in the cusp between 8 and 11 R_e. In order to organize the observations it is necessary to separate effects on the tail due to the solar wind magnetic field and effects due to substorms. Provided there are no changes in the solar wind there are two distinct phases of a substorm in the near tail: a growth phase and an expansion phase. During each phase the observations depend on the location of the satellite relative to the plasma sheet boundaries. Far behind the Earth is the pure tail region which consists of the lobe and the plasma sheet. In the lobe the field magnitude is characteristically enhanced relative to the dipole. Closer to the Earth is a region of transition. The field magnitude is close to that of the dipole but its orientation is distorted forming a cusp-like field line. Near the Earth is a region of depressed field. Here the field magnitude is much less than that of the dipole, but its orientation is similar. The growth phase of a substorm appears to be the direct consequence of the onset of a southward solar wind magnetic field. In the pure tail region the lobe field begins to increase in magnitude and the plasma sheet thins. The transition region moves earthward and the field lines become more tail-like as the field magnitude increases. In the inner region of depressed field, the field magnitude decreases rapidly. The onset of the expansion phase appears to be a process internal to the magnetosphere and independent of the solar wind. In the depressed field region there is a rapid, turbulent increase in field magnitude. In the transition region there is a sudden decrease in the field magnitude and a return to dipolar orientation. In the tail region the plasma sheet expands rapidly with the field becoming quite dipolar, decreasing slowly in the lobe of the tail.  相似文献   

A model for thinning of the plasma sheet     

J.K. Chao  J.R. Kan  A.T.Y. Lui  S.-I. Akasofu 《Planetary and Space Science》1977,25(8):703-710

A one-dimensional model for thinning of the plasma sheet is developed on the basis of launching a fast mode MHD rarefaction wave propagating in the tailward direction along the plasma sheet. Behind the rarefaction wave the pressure is reduced, leading to thinning of the plasma sheet and also to an Earthward plasma flow with a speed on the order of the sound speed a₀. The plasma sheet thickness is reduced by a factor of 2 if an Earthward plasma flow speed of 0.8a₀ is induced. The predictions of the model are in reasonable agreement with observations.  相似文献   

The Aurora and the magnetosphere: The Chapman memorial lecture     

S.-I. Akasofu 《Planetary and Space Science》1974,22(6):885-923

Magnetospheric physics owes its beginnings to the seventeenth- and eighteenth-century scientists who were fascinated by one of the most spectacular natural phenomena, the aurora. In the first section, a brief historical account of the growth of magnetospheric physics and solar-terrestrial physics is given.The main part of the paper reviews recent progress in magnetospheric physics, in particular, in understanding the magnetospheric substorm. A number of magnetospheric phenomena can now be understood by viewing the solar wind-magnetosphere interaction as an MHD dynamo; auroral phenomena are powered by the dynamo. We have also succeeded in identifying magnetospheric responses to variations of the north-south and east-west components of the interplanetary magnetic field.The magnetospheric substorm is entirely different from the responses of the magnetosphere to the southward component of the interplanetary magnetic field. It may be associated with the formation of a neutral line within the plasma sheet and with an enhanced reconnection along the line. A number of substorm-associated phenomena can be understood by noting that the new neutral line formation is caused by a short-circuiting of a part of the magnetotail current.  相似文献   

The ionospheric electric field during substorms — An interpretation based on non-uniform reconnection in the geomagnetic tail     

S. W. H. Cowley 《Astrophysics and Space Science》1973,20(2):491-497

It is suggested that changes in the electric field in the night-side auroral zone and polar cap observed during the expansion phase of a substorm are related to a change in the magnetospheric flow pattern. During the substorm growth phase the flow appears to be fairly uniform across the width of the magnetosphere (uniform electric field across the tail), while at expansion the observations are consistent with the magnetospheric potential drop in the tail falling across a narrow region near the dusk magnetopause. Such non-uniform electric fields in the tail have been predicted by recent theoretical work. A rather speculative interpretation of events during a magnetospheric substorm is presented.  相似文献   

The mechanism of magnetospheric substorm and the MHD waves of the solar wind     

Hu Wen-Rui 《Planetary and Space Science》1981,29(6):695-702

A mechanism of the Earth's magnetospheric substorm is proposed. It is suggested that the MHD waves may propagate across the magnetopause from the magnetosheath into the magnetotail and will be dissipated in the plasma sheet, heating the plasma and accelerating the particles. When the solar wind parameters change, the Poynting flux of the waves transferred from the magnetosheath into the tail, may be greater than 10¹⁸ erg s^?1. The heated plasma and accelerated particles in the plasma sheet will be injected into the inner magnetosphere, and this may explain the process of the ring current formation and auroral substorm.The Alfvén wave can only propagate along the magnetic force line into the magnetosphere in the open magnetosphere, but the magnetosonic wave can propagate in both the open and closed magnetosphere. When the IMF turns southward, the configuration of the magnetosphere will change from a nearly closed model into some kind of open one. The energy flux of Alfvén waves is generally larger than that of the magnetosonic wave. This implies that it is easy to produce substorms when the interplanetary magnetic field (IMF) has a large southward component, but the substorm can also be produced even if the IMF is directed northward.  相似文献   

Magnetospheric surface flow with strong discontinuities     

K. G. Ivanov 《Astrophysics and Space Science》1974,31(2):287-294

The theory of strong discontinuities in plasma with anisotropic pressure is applied for interpretation of the recent plasma and magnetic experiments on the boundary and in the tail of the magnetosphere. The properties of the discontinuities are described. It is supposed that on the boundary and in the tail of the magnetosphere anisotropic discontinuities occur with a nonzero normal component of the magnetic field. The general consequence of this assumption is the existence of the magnetospheric surface flow expanding from the subsolar point. The consequence does not contradict the data obtained on IMP 5. For the investigation of the low latitude part of the flow, the use of electrostatic analysers is desirable with the entrance oriented along the magnetospheric surface. The well-known qualitative scheme of the hydrodynamical flow with strong discontinuities in the tail is generalized by means of the three anisotropic discontinuities: the contact one coinciding with the magnetic neutral sheet, and the two discontinuities with the nonzero normal flow of the mass simulating the boundary of the plasma sheet. The result of the scheme of the components of the bulk velocity in the plasma sheet agrees with the recent observations on the Vela 4B. The scheme connects the well-known phenomena of the blocking and the thinning of the plasma sheet in the initial phase of the substorm. According to the general principles of the reconnection the dynamical dissipation is decreased by the blocking of the flow in the plasma sheet. The decrease leads to a drift of the plasma sheet boundary in the neutral sheet direction. The reverse picture is probably a result of a relaxation of the blocking.  相似文献   

A simple semi-empirical model for the magnetospheric substorm     

V.S. Semenov  V.A. Sergeev 《Planetary and Space Science》1981,29(3):271-281

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Flapping motions of the tail plasma sheet induced by the interplanetary magnetic field variations     

Toichi Tsutomu  Miyazaki Teruki 《Planetary and Space Science》1976,24(2):147-159

Flapping motions of the magnetotail with an amplitude of several earth radii are studied by analysing the observations made in the near (x = ?25 ~ ?30 R_E and the distant (x? ?60 R_E) tail regions. It is found that the flapping motions result from fluctuations in the interplanetary magnetic field, especially Alfvénic fluctuations, when the magnitude of the interplanetary magnetic field is larger than ~10 γ and they propagate behind the Earth with the solar wind flow. Flappings tend to be observed in early phases of the magnetospheric substorm, and they have two fundamental modes with periods of ~200 and ~500 sec. In some limited cases a good correspondence with the long period micropulsations (Pc5) in the polar cap region is observed. These observational results are explained by the model in which the Alfvénic fluctuations in the solar wind penetrate into the magnetosphere along the connected interplanetary-magnetospheric field lines. The characteristics of the flapping reveal that the geomagnetic tail is a good resonator for the hydromagnetic disturbances in the solar wind.  相似文献   

Plasma-sheet dynamics and magnetospheric substorms     

T.W. Hill  P.H. Reiff 《Planetary and Space Science》1980,28(4):363-374

We present a conceptual model of the formation of the plasma sheet and of its dynamical behavior in association with magnetospheric substorms. We assume that plasma mantle particles $\text{E}\text{～}\text{×}\text{B}\text{～}$ drift toward the current sheet in the center of the tail where they are accelerated by magnetic-field annihilation to form the plasma sheet. Because of the velocity-dependent access of mantle particles to the current sheet, we argue that the convection electric field and the corresponding rate of field annihilation decrease with increasing radial distance. As a consequence, there exists no steady-state configuration for the plasma sheet, which must instead shrink continuously in thickness until the near-earth portion of the current sheet is disrupted by the formation of a magnetic neutral line. The current-sheet disruption launches a large-amplitude hydromagnetic wave which is largely reflected from the ionosphere. The reflected wave sets the neutral line in motion away from the earth; the neutral line comes to rest at a distance (which we estimate to be a few hundred earth radii) where the incoming mantle particles enter the current sheet at the local Alfvén velocity. At this “Alfvén point” reconnection ceases and the thinning of the plasma sheet begins again. Within this model, the magnetospheric substorm (which is associated with the current-sheet disruption) is a cyclical phenomenon whose frequency is proportional to the rate of convection in the magnetospheric tail.  相似文献   

A theory of filament eruptions before the impulsive phase of solar flares     

Jun-Ichi Sakai  Shinji Koide 《Solar physics》1992,137(2):293-306

We present a theory of filament eruption before the impulsive phase of solar flares. We show that the upward motion of the magnetic X-point tracing the filament eruption begins several minutes before the impulsive phase of the flare, where the explosive magnetic reconnection starts at the X-point magnetic field configuration located under the filament. No change occurs in the character of the motion of the X-point during the onset of the explosive magnetic reconnection. The upward speed of the X-point is about 110 km s^-1 at the onset of the impulsive phase. We give an important condition leading to filament eruptions, which relate to the state of the current sheet under the filament, where the magnetic energy can be released.  相似文献   

The solar wind-magnetosphere dynamo and the magnetospheric substorm     

S.-I. Akasofu 《Planetary and Space Science》1975,23(5):817-823

In a quiet condition, the solar wind kinetic energy is converted into electrical energy. A small part of this energy is dissipated as heat energy in the polar ionosphere. We identify at least three types of magnetospheric disturbances which are not associated with an increase of the heat production and call them reversible disturbances, while the magnetospheric substorm is an irreversible disturbance which is associated with a large increase of the heat production.The magnetosphere appears to have an inherent internal instability by which a large amount of heat energy is sporadically produced in the polar upper atmosphere at the expense of the magnetic energy in the magnetotail. A positive feed-back process may be responsible for the growth of the instability and for the expansive phase, while the recovery phase sets in when some process begins to suppress the positive feed-back process.  相似文献   

Solar wind speed and its acceleration inferred using the interplanetary scintillation method in carrington rotation 1753     

Kazuyuki Hakamada  Masayoshi Kojima 《Solar physics》1994,153(1-2):419-435

Solar wind speeds (SWSs) estimated by interplanetary scintillation (IPS) observations during Carrington rotation 1753 are projected onto the so-called source-surface of 2.5 solar radii along magnetic field lines in interplanetary space. The following two working hypotheses are examined from different points of view: (1) The SWS is a weighted mean along the line of sight to a radio source; the weight for the SWS depends on the distance from theP-point, the closest approach to the Sun on the line of sight. (2) The weighting function has a very sharp peak at theP-point, so that the SWS shows a real solar wind speed at theP-point. In both the two cases, the SWSs projected onto the source surface are further projected onto the photosphere along magnetic field lines in the corona. Footpoints of these field lines are inferred as photospheric source regions of the solar wind. The intensity of the Hei (1083 nm) absorption line (HEI) in the chromosphere corresponding to these photospheric sources is interpolated from observational data. The weighted mean of the HEI is calculated in case 1. The HEI corresponding to theP-point is used in case 2. The SWS is compared with the HEI in both the two cases. It is found that the correlation between the SWS and the HEI is better in case 2 than in case 1. It is further inferred by correlation analysis between the SWS and the HEI that the solar wind is accelerated within 27 solar radii on average. Although the data examined in this paper were limited to just one solar rotation, these results suggest that the SWS estimated by the IPS technique corresponds to the solar wind speed near theP-point and the weighting function along the line of sight may have a very sharp peak near theP-point.  相似文献   

Thinning of the near-earth (10∼15 R_E) plasma sheet preceding the substorm expansion phase     

A. Nishida  K. Fujii 《Planetary and Space Science》1976,24(9):849-853

The timing of the plasma-sheet thinning relative to the onset of the expansion phase of substorms is examined by the analysis of the OGO 5 electron (79 ± 23 keV) and proton (100～150 keV) data with the aid of simultaneous magnetic field observations. It is found that the timing of the thinning is significantly dependent on the distance. At $\text{x}{}^2\text{+ y}{}^2\text{? 15 R}{}_{\mathrm{E}}$ the thinning often starts before the onset, while at $\text{x}{}^2\text{+ y}{}^2\text{? 15 R}{}_{\mathrm{E}}$ it tends to occur after the onset, where x and y refer to solar magnetospheric coordinates. The thinning that precedes the expansion-phase onset has been found to reduce the thickness to ～1 R_E, and further thinning may occur in a spatially limited region. Hence it is conceivable that the formation of the neutral line characterizing the substorm expansion phase is the consequence of the thinning of the plasma sheet in the near-Earth region.  相似文献   

A mechanism for the growth phase of magnetospheric substorms     

T.W. Hill 《Planetary and Space Science》1973,21(8):1307-1316

A mechanism is presented whereby the rate of energy dissipation in the magnetosphere is controlled by the particle density in the plasma sheet in the near geomagnetic tail. The mechanism is based on a model in which the plasma sheet is sustained by injection of solar-wind particles into the dayside magnetosphere. The efficiency of the injection is controlled by solarwind parameters, in particular, the north-south component of the interplanetary magnetic field; the maximum injection rate occurs when the interplanetary field is northward. During geomagnetically quiet times, this source balances the loss of particles from the edges of the tail current sheet. If the dayside source rate is reduced (e.g. by a southward-turning interplanetary magnetic field), then the plasma sheet is depleted and the rate of magnetic merging is enhanced in the earthward portion of the tail current sheet. This period of steadily-enhanced merging is associated with the growth phase, i.e. the period of enhanced magnetospheric convection for about one hour preceding the breakup of a polar magnetic or auroral substorm. The breakup can be understood as the result of the collapse of a portion of the tail current sheet following the local depletion of the plasma sheet.  相似文献   

Observations in the earth's magnetotail relating to magnetic merging     

Edward W. Hones Jr. 《Solar physics》1976,47(1):101-113

For more than a decade there has been growing conviction that the burst of energy from a solar flare is first stored in magnetic fields and is then released rapidly by magnetic field annihilation (magnetic merging). There has also been recognition that magnetic merging may be responsible for the energy release manifested in auroral phenomena at the Earth. The most substantial evidence that magnetic merging does indeed occur in the Earth's magnetosphere and causes the auroral phenomena is provided by recent observations, in the magnetotail, of very rapid (500 km s^–1) tailward, then earthward, flow of plasma during magnetospheric substorms. The observations, made with the Vela and IMP satellites, reveal also that the component of the tail magnetic field perpendicular to the tail neutral sheet changes polarity at the time of the reversal of plasma flow. These features are interpreted as indicative of passage of a magnetic neutral line, at which magnetic merging is proceeding, past the observing satellite. This paper describes an example of such observations made with IMP 6. It is anticipated that such systematic measurements of the plasma, energetic particles and magnetic field in the neighborhood of the passing neutral line on many such occasions will provide a general understanding of the magnetic merging process which can then be applied to studies of solar flares and other astrophysical phenomena.Work performed under the auspices of the U.S. Energy Research and Development Administration.  相似文献   

On solar wind interaction with the earth's magnetosphere     

M.S. Kovner  Y.I. Feldstein 《Planetary and Space Science》1973,21(7):1191-1211

Hydrodynamic and electrodynamic problems of solar wind interaction with the Earth's magnetosphere on the day-side are investigated.The initial fact, well established, is that the density of the magnetic field energy in the solar wind is rather small. Magnetic field intensity and orientation are shown to determine the character of the solar wind flow around the magnetosphere. For mean parameters of the wind, if the tangential component of the magnetic field is more or equal 5γ, the flow in the magneto-sheath will be laminar. For other cases the flow is of a turbulent type.For turbulent flow, typical plasma parameters are estimated: mean free path, internal scale of inhomogeneities and dissipated energy. The results obtained are compared with experimental data.For the case of laminar flow, special attention is paid to the situation when magnetic fields of the solar wind and Earth are antiparallel. It is suggested, on the basis of solid arguments, that the southward interplanetary field diffuses from the magnetosheath into the Earth's magnetosphere. These ideas are used for the estimation of the distance to the magnetopause subsolar point. A detailed comparison with results of observation is made. The coincidence is satisfactory. Theoretical investigation has been made to a great extent for thin magnetopause with thickness δ ～ R_He-gyroradius of an electron.It is shown that during magnetospheric substorms relaxation oscillations with the period τ = 100–300 sec must appear. A theorem is proved about the appearance of a westward electrical field during the substorm development, when the magnetosphere's day-side boundary moves Earthward and about the recovery phase, when the magnetopause motion is away from the Earth, when there is an eastward electrical field.In the Appendix, plasma wave exitation in the magnetopause is considered and conductivity magnitudes are calculated, including the reduction due to the scattering by plasma turbulence.  相似文献   

Temperature variation of the plasma sheet during substorms     

A.T.Y. Lui  C.-I. Meng  L.A. Frank  K.L. Ackerson  S.-I. Akasofu 《Planetary and Space Science》1981,29(8):837-842

The temperature and density of the plasma in the Earth's distant plasma sheet at the downstream distances of about 20–25 R_e are examined during a high geomagnetic disturbance period. It is shown that the plasma sheet cools when magnetospheric substorm expansion is indicated by the AE index. During cooling, the plasma sheet temperature, T, and the number density, N, are related by $\text{T ∝ N}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ (adiabatic process) in some instances, while by T ∝ N^?1 (isobaric process) in other cases. The total plasma and magnetic pressure decreases when $\text{T ∝ N}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ and increases when T ∝ N^?1. Observation also indicates that the dawn-dusk component of plasma flow is frequently large and comparable to the sunward-tailward flow component near the central plasma sheet during substorms.  相似文献