摘要:

We have used a new high‐resolution global magnetohydrodynamic simulation model to investigate the onset of reconnection in the magnetotail during intervals with southward interplanetary magnetic field (IMF). After the southward IMF reaches the dayside magnetopause reconnection begins and magnetic flux is convected into the tail lobes. After about 35 min, reconnection begins within the plasma sheet near midnight atx= −14RE. Later the x line moves toward dawn and dusk. The reconnection occurs just tailward of the region where the tail attaches onto the dipole‐dominated inner magnetosphere. The simulation shows that prior to the onset of reconnection, the Poynting flux is concentrated in this region. The time required for the start of reconnection depends on the component of the magnetic field normal to the equator (BZ). Reconnection occurs only after theBZcomponent has been reduced sufficiently for the tearing mode to grow. Later, when all the plasma sheet field lines have reconnected, a plasmoid moves down the

ISSN:0148-0227    

DOI:10.1029/93JA01321

年代:1993

数据来源: WILEY

摘要:

A new MHD simulation scheme is developed on an unstructured grid system using the finite volume total variation diminishing scheme and applied to the problem of solar wind‐planet interaction, assuming a perfect‐conducting ionosphere around the planet. It is shown that the scheme presented here enables one to calculate effectively the configuration of three‐dimensional MHD flow near planets, together with the draping process of magnetic field. Results of the calculation give a general confirmation for the formations of the bow shock, the magnetic barrier, the magnetosheath, the magnetopause, the wake region, the plasma sheet, and the magnetotail. Because of theJ×Bforces, flow near the planet is accelerated in the polar regions and decelerated in the equatorial plane in the solar wind velocity magnetic field (VB) coordinate system. In the tail region, theJ×Bforces act to accelerate the flow in the midnight meridian plane and suppress the formation of the trailing shock. However, the present calculation gives a weaker magnitude for the tail magnetic field than that given by the observations and suggests the role of some additional mechanisms for the pileup of the magnetic field in t

ISSN:0148-0227    

DOI:10.1029/93JA01516

年代:1993

数据来源: WILEY

摘要:

A global model of substorms is proposed on the basis of observational synthesis and theoretical modeling. Since the theoretical basis of the present model is the magnetosphere‐ionosphere coupling (MIC) process, it will be called the MIC model of substorms. Substorms can occur in the MIC model without a new X line formed in the near‐Earth plasma sheet, in contrast to the highly popular near‐Earth neutral line (NENL) model of substorms. Following enhanced dayside reconnection, the ionosphere overloads both the solar wind on open field lines and the plasma sheet on closed field lines. The solar wind responds to the overload by providing more driven energy from the dynamo action on open field lines. The plasma sheet responds to the overload by collapsing itself, i.e., dipolarizing its field configuration to form the substorm current wedge. The explosive intensification during the expansion phase is powered by releasing the magnetic energy stored on closed field lines in the plasma sheet. The stored energy is released by the unloading instability driven by a positive feedback in the substorm current

ISSN:0148-0227    

DOI:10.1029/93JA01168

年代:1993

数据来源: WILEY

摘要:

Since the Voyager 2 encounter in early 1986, several investigators have attempted to localize the source regions of the smooth high‐frequency radio emission which was observed by the planetary radio astronomy experiment at the nightside of Uranus. The various studies (most of them are based on the offset tilted dipole (OTD) model of the Uranian magnetic field) yielded significantly different source locations around the southern magnetic pole of Uranus. This may be a consequence of the individual a priori assumptions of the source model. However, the simplicity of the OTD model (Ness et al., 1986) also cannot adequately represent the complexity of the magnetic field at the radio source locations near the planet. The aim of this study is twofold. (1) We reanalyze the various source locations given in the literature (most of them are based on the OTD model) in the frame of the Q3 magnetic field model (Connerney et al., 1987). Our analysis moves some of the previously determined source locations from open toward closed field lines; however, the uncertainty due to the nonuniqueness of the Q3 model remains too large to exclude the possibility that open field lines are the source of smooth Uranian kilometric radiation. (2) We calculate the uncertainty of the radio source locations imposed by the nonuniqueness of the Q3 and OTD magnetic field models. We construct solutions by using generalized inversion techniques (Connerney, 1981) to obtain estimates of those magnetic field parameters (spherical harmonic coefficients up to degree and order 6) that are constrained by the magnetometer observations. The nonuniqueness of the resulting magnetic field models translates into an uncertainty about the radio source locations of some 20° in Uranocentric coordinates at altitudes of about 1.5 Uranian radii (RU). The present results are important for radio source locations at all the outer planets whose magnetic field geometries are represented by nonunique magnetic field mode

ISSN:0148-0227    

DOI:10.1029/93JA01610

年代:1993

数据来源: WILEY

摘要:

We have determined characteristics of magnetospheric equatorial currents during substorms from the vector magnetic field data acquired with the GOES 5 and GOES 6 satellites, separated about 1.9 hours in MLT in geosynchronous orbit. These data have been used to determine the local time (azimuthal) and radial variation of the equatorial current. The divergence of the equatorial current was computed from these variations, and systems of field‐aligned currents were deduced. During the growth phase to the maximum phase of the taillike reconfiguration of the near‐Earth magnetic field, a positive divergence (away from the equatorial plane) of the westward equatorial current occurs in the late evening to premidnight MLT sector, and a negative divergence (away from the equatorial plane) occurs in the premidnight to early morning MLT sector. The field‐aligned current associated with these divergences flows into the ionosphere in the late evening to premidnight MLT sector and flows away from the ionosphere in the premidnight to early morning MLT sector. This flow direction pattern is the same as that of the region 2 field‐aligned current system. During the expansion phase a field‐aligned current that is distinctive to the growth phase field‐aligned current is generated in the same near‐Earth plasma sheet region. The field‐aligned current flows away from the ionosphere in the late evening to premidnight MLT sector and flows into the ionosphere in the premidnight to morning MLT sector. These field‐aligned currents are due to a change in a sign of the divergence of the westward equatorial current. This flow direction pattern is same as that of the region 1 field‐aligned current system and also of the current‐wedge model. This region 1 sense field‐aligned current develops first near midnight at about 5 min after the expansion phase onset (as determined from the ground‐based magnetometer data), is delayed by 10‐25 min farther away from midnight in the evening and morning MLTs, and continues until the end of geomagnetic dipolarization at the site of either GOES 5 or GOES 6, whichever is located closer to midnight. We have also determined the presence of a radial current that flows toward the earth in the late evening to premidnight sector and flows away from the Earth in the midnight to morning sector. The intensity of the radial currents increases before the expansion phase. Consequently, the patterns of field‐aligned currents associated with various substorm phases are the superposition of currents driven by multiple sources with different temporal variations. We have identified at least three different, but related sources of field‐aligned currents during the growth and expansion phases. These sources are related to the divergence of the westward flowing equatorial current and to distributions of pressure and magnetic field gradients that evolve in the magnetotail. These patterns include the current‐wedge model during the expansion phase. When combined, these complicated systems support the basic region 1 to region 2 f

ISSN:0148-0227    

DOI:10.1029/93JA01644

年代:1993

数据来源: WILEY

摘要:

Using the magnetic field and the plasma measurements acquired with the DMSP F7 satellite at an altitude of ∼840 km, and in the premidnight (∼2000‐0100 MLT (magnetic local time)) sector, we have determined the characteristics of field‐aligned currents and particle precipitation during the substorm growth phase. Characteristics obtained here include the following: (1) plasma sheet structure diagnosed by the ion precipitation consists of two parts, namely, the inner plasma sheet (IPS) that fills the inner part of magnetotail plasma sheet with hot particles (average energy ≈ 8.5‐12 keV), and the outer plasma sheet (OPS) that occurs tailward of and adjacent to the former and carries weaker energy flux (<2.5 × 1010eV cm−2sr−1s−1at 840 km altitudes); (2) from the latitudinal profile in the total energy flux of ion precipitation, it is statistically confirmed for the first time from the low‐altitude observation that during the growth phase,L‐dependent profile in the ion plasma pressure associates a definite earthward ▽Pspreading all over the IPS and further into the OPS; (3) through the so‐called Harang discontinuity MLT sector, above mentioned plasma features are invariable, however, spatial distribution of field‐aligned current pattern changes systematically and includes the following. The classic evening‐type region 2 plus region 1 field‐aligned current system occurs in the late‐evening MLT sector, while classic morning‐type region 2 plus region 1 system in the midnight sector. In either case, the region 2 roughly coincides with the IPS and the region 1 with the OPS. In addition to region 2 plus region 1 system, a current system whose current polarity is directed opposite to region 1 system often occurs near the poleward limit of the OPS and poleward of the adjacent region 1 system for both the morning‐type and the evening‐type current system, which we tentatively refer to region 0 system here. It is strongly suggested that three‐region current pattern (evening‐type region 2 plus region 1 plus region 0) doesn't mean a westward intrusion of the morning‐type region 2 and region 1 system into the premidnight sector but is totally attributable to source mechanism that is proper to the evening domain; (4) the intensity of field‐aligned current is almost balanced latitudinally even when the currents are multiple sheets (imbalance is less than 20% of total amplitude). The value of intensity is (255 ± 100) nT in transverse magnetic disturbance (equivalent to (0.203 ± 0.080) A/m). From these facts, we propose here a stress balance model in the magnetotail plasma sheet, in which the inhomogeneity in plasma pressure and magnetic field intensity causes the azimuthal and radial current loop, respectively, in the inner and middle part of the plasma sheet. The azimuthal current loop associates the region 2 sense field‐aligned current and the radial current loop associates the region 2 plus region 1 sense field‐aligned currents in pair. In addition, two‐cell convective motion is thought to occur in the OPS and associates the region 1 sense field‐aligned current. We suggest that during the growth phase, field‐aligned current system includes multiple‐source mechanisms and the observed magnetic disturbances at low latitudes (so‐called region 2, region 1 field‐aligned current, etc.) are manifestation of superimposed effect of th

ISSN:0148-0227    

DOI:10.1029/93JA01474

年代:1993

数据来源: WILEY

摘要:

Neutralized ring current particles are thought to be responsible for a number of different disturbance effects in the upper atmosphere. Here the energy deposition and radiation production by these particles are investigated. It is found that the particle heating is not sufficient to explain the geomagnetic activity effect at low latitudes. In contrast, the Hαand Hβemission rates predicted by our model calculations are above the general background level and compare well with the observation

ISSN:0148-0227    

DOI:10.1029/93JA01500

年代:1993

数据来源: WILEY

摘要:

The high‐pressure plasma trapped in the geomagnetic tail strongly shapes the surrounding magnetic field. Force balance in a steady state requires a balance between the divergence of the total particle momentum stress tensor and the divergence of the magnetic field momentum stress tensor which is the Lorentz force. This condition of stress balance allows strict constraints on the total particle pressure tensor to be derived from the magnetic field structure. TheKpparameterized empirical magnetospheric fields of the Tsyganenko model with local two‐dimensional approximations are used to derive the pressure tensor components implied for the central plasma sheet. The small anisotropiesp⊥/p∥in the gyrotropic pressure tensor required for equilibrium in the central plasma sheet are calculated from the nonpotential values found for thej × Bforce. The required ratiop⊥/p∥approaches the marginal stability criterion for the magnetohydrodynamic (MHD) mirror mode and the Alfvén ion cyclotron mode, and we suggest that these instabilities may play a role in establishing the equilibrium structure of the geomagnetic tail. From this perspective the equilibrium and stability constraints for the maximum allowed anisotropy suggest that the Tsyganenko magnetotail field parameterization needs to be modified to support self‐consistent gyrotr

ISSN:0148-0227    

DOI:10.1029/93JA00794

年代:1993

数据来源: WILEY

摘要:

At the end of April 2, 1978, the ISEE 1 and 2 spacecraft moved inbound at ∼11REon the nightside (0130 MLT). Due to a flapping motion of the plasma sheet the spacecraft crossed the neutral sheet region (central region of the plasma sheet) more than 10 times in the hour between 2115 and 2215 UT. This provided a unique opportunity to study the structure of the plasma/current region and its evolution during substorm growth and early expansion before the final disruption of the current sheet. Using minimum variance analysis of the magnetic field variations during the crossings as well as finite ion gyroradius diagnostics, we determine the orientation of the current sheet (CS) and then estimate the CS thickness as well as the value of its normal component, Bn. Typically, the current distribution was inferred to be very inhomogeneous with a current concentrated in a very thin CS (only 0.2 to 0.8REas thick) embedded inside the thicker plasma sheet. Current sheet crossings could be classified as regular or turbulent. The first type prevailed during the growth phase and at the initial stage of expansion when the spacecraft were well outside (in longitude) of the active region of the substorm and no large plasma flow was detected. The normal field component Bnwas typically very small (∼1 nT) in the CS center in comparison to the larger shear magnetic Bycomponent. In the course of the growth phase we inferred an increase of the lobe field Bxand a decrease of the CS half thickness h (from h∼3000 km to ∼800 km just before the expansion onset), i.e., a very large increase (up to an order of magnitude) of the current density. At the same time, in disagreement with the usual cartoon picture of magnetic reconfiguration, the magnetic field magnitude in the CS center increased (instead of decreased) at the expense of the shear component. Three turbulent crossings were found during substorm expansion within the longitude range of the substorm current wedge (SCW). The second of them was detected ∼1 min before the main dipolarization and was characterized by a rather small CS thickness (h<600 km), by strong earthward plasma flow and by a positive normal magnetic field component. That period showed signatures of concentration of both cross‐B and field‐aligned current at the outer edge of CS and may indicate a nearby reconnection region. The main result of this study is that the region of very thin current sheet (thickness of the order of the gyroradius of thermal protons in the field just outside the current sheet), which contained a very small normal component, clearly appeared in the near tail prior to the sudden onset of current disruption as predicted by some quantitative models of quasi‐static evolution of earthward convecting plasma sheet flux tubes. Comparing these observations to theoretical results, we find that the threshold conditions for the growth of the tearing mode instability in sheared magnetic fields were apparently satisfied in this case, but the growth rate was too slow for sudden initiation of sub

ISSN:0148-0227    

DOI:10.1029/93JA01151

年代:1993

数据来源: WILEY

摘要:

The low‐latitude boundary layer (LLBL) comprises a large fraction of the magnetospheric boundary layer making it a potentially important site for the transport of mass, momentum, and energy from the magnetosheath into the magnetosphere. We have used an ideal magnetohydrodynamic computer simulation with nonperiodic inflow and outflow boundary conditions to investigate the processes responsible for the spatial development of a 6.4 Earth radii (RE) long section of the dayside LLBL. Initializing the simulation with a realistic boundary layer configuration, slightly modified for computational efficiency, leads to the development of a LLBL which broadens with downstream distance from a thin (0.12REthick) and laminar boundary layer to a broad (∼0.7REthick) and turbulent one capable of significant transport. Broadening occurs through the action of Reynolds and Maxwell stresses that develop during the roll‐up and merging of vortices generated by the Kelvin‐Helmholtz instability in the simulated boundary layer and indicates the transport of mass, momentum, and energy into the LLBL. The steep gradients that develop at the magnetopause between the vortices may further aid the transport by encouraging wave‐particle diffusion processes capable of transporting magnetosheath plasma into the LLBL. The presence of a flow‐aligned component of the magnetic field, on the other hand, can hinder downstream development of the boundary layer by slowing and then stopping boundary layer vortices through transfer of their rotational kinetic energy into deformations of the mag

ISSN:0148-0227    

DOI:10.1029/93JA01524

年代:1993

数据来源: WILEY