ISSN:0148-0227    

DOI:10.1029/JA100iA01p00001

年代:1995

数据来源: WILEY

摘要:

We discuss an example of confinement of fast solar electrons by a discrete solar wind‐interplanetary magnetic field structure on February 22, 1991. The structure is about 190,000 km in width and is clearly defined by changes in the direction of the magnetic field at the Ulysses spacecraft. This structure carries electrons moving toward the Sun as well as away from the Sun. A loss cone in the angular distribution of the fast electrons shows that mirroring, presumably magnetic, takes place downstream from the spacecraft. Following passage of this narrow structure, the return flux vanishes for 21 min after which time the mirroring resumes and persists for several hours. We identify the enhanced magnetic field region lying downstream from the Ulysses spacecraft that is responsible for the mirroring to be a corotating stream interaction region. Backstreaming suprathermal electron measurements by the Los Alamos National Laboratory plasma experiment on the Ulysses spacecraft support this interpretatio

ISSN:0148-0227    

DOI:10.1029/94JA01811

年代:1995

数据来源: WILEY

摘要:

Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a polytropic approximation. This paper discusses the properties of the polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the polytropic index of the free‐streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The polytropic index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the polytropic index has an average value of 1.5

ISSN:0148-0227    

DOI:10.1029/94JA02420

年代:1995

数据来源: WILEY

摘要:

A new model of the coronal and interplanetary magnetic field can predict both the interplanetary magnetic field strength and its polarity from measurements of the photospheric magnetic field. The model includes the effects of the large‐scale horizontal electric currents flowing in the inner corona, of the warped heliospheric current sheet in the upper corona, and of volume currents flowing in the region where the solar wind plasma totally controls the magnetic field. The model matches the MHD solution for a simple dipole test case better than earlier source surface and current sheet models. The strength and polarity of the radial interplanetary magnetic field component predicted for quiet time samples in each year from 1977 to 1986 agree with observations made near the Earth's orbit better than the hybrid MHD‐source surface model (Wang and Sheeley,1988). The results raise the question of whether coronal holes are the only solar source of the interplanetary magnetic field in the solar wind. If some interplanetary flux originates outside coronal holes, the model can match the observed field using the accepted 1.8 saturation correction factor for λ5250 Å magnetograph observations. Requiring open flux to come exclusively from coronal holes requires an additional factor o

ISSN:0148-0227    

DOI:10.1029/94JA02266

年代:1995

数据来源: WILEY

摘要:

We present a case study of the magnetospheric and ionospheric response to a series of repetitive (7–8 min) solar wind dynamic pressure and interplanetary magnetic field (IMF) strength variations generated at the bow shock. During a period of duskward and antisunward IMF orientation, the magnetospheric and ionospheric observations indicate transient events moving dawnward and duskward away from a point of origin at or after local noon. In contrast, during a subsequent period of dawnward and antisunward IMF orientation, the observations indicate events moving duskward away from a point of origin at or prior to 0900 LT. We consider explanations in terms of pressure pulse driven riplets on the magnetopause or directly driven bursts of antiparallel mergin

ISSN:0148-0227    

DOI:10.1029/94JA02296

年代:1995

数据来源: WILEY

摘要:

ISEE 1 and IMP 8 data are used to identify 19 crossings of Earth's bow shock during a 30‐hour period following 0000 UT on September 24, 1987. Apparent standoff distances for the shock are calculated for each crossing using two methods and the spacecraft location; one method assumes the average shock shape, while the other assumes a ram pressure‐dependent shock shape. The shock's apparent standoff distance, normally ∼ 14RE, is shown to increase from near 10 REinitially to near 19 REduring an 8‐hour period, followed by an excursion to near 35 RE(where two IMP 8 shock crossings occur) and an eventual return to values smaller than 19 RE. The AlfvénMAand fast magnetosonicMmsMach numbers remain above 2 and the number density above 4 cm−3for almost the entire period. Ram pressure effects produce the initial near‐Earth shock location, whereas expansions and contractions of the bow shock due to low Mach number effects account, qualitatively and semiquantitatively, for the timing and existence of almost all the remaining ISEE crossings and both IMP 8 crossings. Significant quantitative differences exist between the apparent standoff distances for the shock crossings and those predicted using the observed plasma parameters and the standard model based on Spreiter et al.'s (1966) gasdynamic equation. These differences can be explained in terms of either a different dependence of the standoff distance on Mach number at lowMAandMms, or variations in shock shape withMAandMms(becoming increasingly “puffed up” with decreasingMAandMms, as expected theoretically), or by a combination of both effects. Global MHD simulations, to be presented elsewhere, confirm that both effects occur and are significant. Ram pressure‐induced changes in the shock's shape are discussed but found to be quantitatively unimportant for the shock crossings analyzed. Approximate estimates of both the deviation of the shock's standoff distance from the standard model and of the shock's shape are determined independently (but not consistently) forMms∼ 2.4. The estimates imply substantial changes in standoff distance and/or shock shape at lowMAandMms. Mach number effects can therefore be quantitativwely important in determining and predicting the shape and location of the bow shock, even whenMAandMmsremain above 2. This study confirms and generalizes previous studies of Mach number effects on Earth's bow shock. Statistical studies and simulations of the bow shock's shape and location should be performed as a function of Mach number, magnetic field orientati

ISSN:0148-0227    

DOI:10.1029/94JA01978

年代:1995

数据来源: WILEY

摘要:

The poloidal mode field line resonance in the Earth's dipole magnetic field is investigated using cold plasma ideal MHD simulations in dipole geometry. In order to excite the poloidal mode resonance, we use either an initial or a continuous velocity perturbation to drive the system. The perturbation is localized at magnetic shellL= 7 with plasma flow in the radial direction (electric field component in the azimuthal direction). It is found that with the initial perturbation alone, no poloidal mode resonance can be obtained and the initially localized perturbation spreads out across all magneticLshells. With the continuous perturbation, oscillating near the poloidal resonance frequency, a global‐scale poloidal cavity mode can be obtained. For the first time, a localized guided poloidal mode resonance is obtained when a radial component of electric field is added to the initial perturbation such that the curl of the electric field is everywhere perpendicular to the background dipole magnetic field. During the localized poloidal resonance, plasma vortices parallel/antiparallel to the background dipole magnetic fieldB0lead to circular plasma flow perpendicular toB0. This circular flow, elongated radially, results in twisting of magnetic field flux tubes, which, in turn, leads to the slowdown of the circular plasma flow and reversal of the plasma vortices. The energy associated with the localized poloidal resonance is conserved as it shifts back and forth between the oscillating plasma vortices and the alternately twisted magnetic flux tubes. In the simulations the eigenfunctions associated with the localized poloidal resonance are grid‐scale singular functions. This result indicates that ideal MHD is inadequate to describe the underlying problem and nonideal MHD effects are needed for mode broaden

ISSN:0148-0227    

DOI:10.1029/94JA02265

年代:1995

数据来源: WILEY

摘要:

A theory of auroral substorm dynamics is constructed on the basis of MHD wave processes in the ionosphere‐magnetosphere system. The basic view is that the substorm commences in the nightside near‐Earth magnetosphere through a collapse of plasma equilibrium. The collapse releases a significant amount of free energy embedded initially in a collection of compressional waves. It is suggested that substorm dynamics after the collapse are determined by the evolution of these waves. We first investigate the quantitative ramifications of the waves in a two‐dimensional box in the GSMyzcross section of the magnetotail. The model is constructed to allow the study of radiation of substorm wave energy into the solar wind and also encompasses the essential elements of resonant interaction in the plasma sheet boundary layer. The natural boundary condition leading to radiative loss is introduced. It is found that wave radiation into the solar wind can relax the magnetospheric system in less than a hour. The resonant Alfvén modes driven by the normal compressional modes in the box are studied through the construction of proper dispersion equation. By studying the field‐aligned current generated by resonances, we establish the auroral pattern expected to result from the coupling. Following the theoretical study, we examine an auroral substorm observed by the CANOPUS photometer array on February 20, 1990. It is found that, among the testable theoretical predictions, there exists a general agreement with the observations. We did find, however, that electron‐ and proton‐induced aurora oscillate essentially in phase, thus implying a more complicated precipita

ISSN:0148-0227    

DOI:10.1029/94JA02036

年代:1995

数据来源: WILEY

摘要:

From a study of the 21 largest geomagnetic storms during solar cycle 21, a strong correlation is established between the ring current indexDstand the time‐weighted accumulation of the 1‐hour auroral electrojet indices,AEandAL. The time‐weighted accumulation corresponds to convolution of the auroral electrojet indices with an exponential weighting function with ane‐folding time of 9.4 hours. The weighted indicesAEwandALwhave correlation coefficients againstDstranging between 0.8 and 0.95 for 20 of the 21 storms. Correlation over the entire solar cycle 21 database is also strong but not as strong as for an individual storm. A set of simpleDstprediction functions provide a first approximation of the inferred dependence, but the specific functional relationship ofDst(AEw) orDst(ALw) varies from one storm to the next in a systematic way. This variation reveals a missing parametric dependence in the transfer function. However, our results indicate that auroral electrojet indices are potentially useful for predicting storm time enhancements of ring current intensity with a few hours le

ISSN:0148-0227    

DOI:10.1029/94JA02347

年代:1995

数据来源: WILEY

摘要:

One of the most important quantities that can be obtained through test particle calculations is the pitch angle scattering of particles by interaction with current sheet magnetic fields. Pitch angle scattering is a factor that helps determine the dawn‐to‐dusk current, controls particle energization, and it has also been used as a remote probe of the current sheet structure. Previous studies have interpreted their results under the expectation that randomization will be greatest when the ratio of the two timescales of motion (gyration parallel to and perpendicular to the current sheet) is closest to one. In a simple parabolic current sheet, the ratio of timescales is proportional to κ, where κ = (Rc/ρz)1/2,Rcis the field line radius of curvature at the current sheet midplane and ρzis the gyroradius at the current sheet midplane. Recently, the average exponential divergence rate (AEDR) has been calculated for particle motion in a hyperbolic current sheet (Chen, 1992). It is claimed that this AEDR measures the degree of chaos and therefore may be thought to measure the randomization. In contrast to previous expectations, the AEDR is not maximized when κ ∼ 1 but instead increases with decreasing κ. Also contrary to previous expectations, the AEDR is dependent upon the parameterbz. In response to the challenge to previous expectations that has been raised by this calculation of the AEDR, we have investigated the dependence of a measure of particle pitch angle scattering on both the parameters κ andbz. We find that, as was previously expected, particle pitch angle scattering is maximized near κ = 1 provided that κ/bz>1. In the opposite regime, κ/bz<1, we find that particle pitch angle scattering is still largest when the two timescales are equal, but the ratio of the timescales is proportional tobz. In this second regime, particle pitch angle scattering is not due to randomization, but is instead due to a systematic pitch angle change. This result shows that particle pitch angle scattering need not be due to randomization and indicates how a measure of pitch angle scattering can exhibit a different behavior than a m

ISSN:0148-0227    

DOI:10.1029/94JA02221

年代:1995

数据来源: WILEY