摘要:

The plasma flow in Jupiter's magnetosphere observed during the Ulysses flyby is compared with previous Pioneer, Voyager and ground‐based observations. These data show that near‐rigid corotation is enforced in the inner magnetosphere, but that the azimuthal flow plateaus at 150–300 km s−1in the middle magnetosphere plasma sheet beyond ∼20RJ. Such flows extend through the prenoon plasma sheet to ∼45RJin the compressed magnetosphere observed by the Voyagers and to ∼70RJin the expanded system observed by Ulysses. Higher speeds of ∼500 km s−1occur in the postmidnight plasma sheet at 75–125RJin Voyager data, while preliminary Ulysses evidence is presented for anticorotation in the dusk plasma sheet beyond ∼50RJ. In the outer magnetosphere the dawn and dusk flank flows are antisunward at several hundred kilometres per second, while in the prenoon sector the flow appears to depend magnetospheric state, being corotational at 250–600 km s−1when compressed and anticorotational (and radially in) at ∼250 km s−1when expanded. These observations are compared with theories proposed by Hill and Vasyliunas, augmented to include the effects of solar wind coupling. This model accounts qualitatively for many features, but not for the anticorotation flows observed. We also compare the flows with the bending of the magnetic field out of meridian planes. Given the subcorotation‐anticorotation nature of the observed flow, a pervasive “lagging” configuration is expected. This accords with in situ field data, except for the dusk outer magnetosphere observed by Ulysses, where a “leading” configuration was observed in the presence of subcorotating (downtail) flow. We conclude that field bending due to the tail‐magnetopause current system dominates that due

ISSN:0148-0227    

DOI:10.1029/96JA00461

年代:1996

数据来源: WILEY

摘要:

We present a comprehensive analysis of Voyager 1 and 2 electron observations within Saturn's magnetosphere. This analysis entails the merging of electron observations from the Plasma Science (PLS) experiment, the Low Energy Charged Particle (LECP) experiment and the Cosmic Ray System (CRS) experiment. For each encounter, the three instruments combined allow us to compute the electron energy spectra over a wide range of energies from 10 eV to ∼ 2MeV between the closest approaches andL= 18.5. The instruments use different technologies, different sensitivities, and different fields of view; however, we observe a surprisingly good matching of the data sets on a 15‐min timescale. The PLS‐LECP‐CRS spectra include the low‐energy thermal component of the magnetospheric plasma, the keV suprathermal electrons, and the high‐energy tail extending into the MeV energy range. From the combined spectra, we compute a comprehensive set of macroscopic parameters (electron density, pressure, beta factor, and electron current at the spacecraft): the analysis reveals a variety of radial gradients for these quantities and the corresponding electron populations. We also compute phase space densities over a wide range in energy and radial distances, analyzing local time symmetries, electron source distributions, and temporal variations of Saturn's magnetosphere. The ultimate goal of this study is to provide a comprehensive empirical model of the charged particle population within Saturn's magnetosphere. It will be used to support the development of the Cassini mission and to allow detailed planning of the tour design with regard to charged particle science and radiat

ISSN:0148-0227    

DOI:10.1029/96JA00765

年代:1996

数据来源: WILEY

摘要:

First results of a three‐dimensional multiscale MHD model of the interaction of an expanding cometary atmosphere with the magnetized solar wind are presented. The model starts with a supersonic and super‐Alfvènic solar wind far upstream of the comet (25 Gm upstream of the nucleus) with arbitrary interplanetary magnetic field orientation. The solar wind is continuously mass loaded with cometary ions originating from a 10‐km size nucleus. The effects of photoionization, electron impact ionization, recombination, and ion‐neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our new multiscale upwind scalar conservation laws‐type numerical technique (MUSCL). We have named this the multiscale adaptive upwind scheme for MHD (MAUS‐MHD). The combination of the adaptive refinement with the MUSCL‐scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. The main findings are the following: (1) Mass loading decelerates the solar wind flow upstream of the weak cometary shock wave (M≈ 2,MA≈ 2), which forms at a subsolar standoff distance of about 0.35 Gm. (2) A cometary plasma cavity is formed at around 3 × 103km from the nucleus. Inside this cavity the plasma expands outward due to the frictional interaction between ions and neutrals. On the nightside this plasma cavity considerably narrows and a relatively fast and dense cometary plasma beam is ejected into the tail. (3) Inside the plasma cavity a teardrop‐shaped inner shock is formed, which is terminated by a Mach disk on the nightside. Only the region inside the inner shock is the “true” diamagnetic cavity. (4) The model predicts four distinct current systems in the inner coma: the density peak current, the cavity boundary current, the inner shock current, and finally the cross‐tail current. (5) The calculated plasma parameters (magnetic field, plasma density, speed, and temperature) are in very good agreement with pu

ISSN:0148-0227    

DOI:10.1029/96JA01075

年代:1996

数据来源: WILEY

摘要:

This article presents a theoretical study of the dependence of the magnetopause shape and field on the dynamic pressure of the solar wind. The magnetopause surface (assumed to be axisymmetric) is determined by requiring the plasma pressure outside, estimated by the “Newtonian approximation,” to be balanced by the magnetic pressure inside. The magnetic field inside is given by the sum of (1) the Earth's internal field, (2) the field from various magnetospheric current systems (cross‐tail current, ring current, etc.) as given by an empirical model, and (3) the field due to magnetopause currents. The magnetopause field is a function of the magnetopause shape and is updated each time this shape is adjusted toward pressure balance. In previous studies of this kind, only dipole and magnetopause fields were incorporated. This is the first three‐dimensional solution of the problem to include the cross‐tail current and thus to obtain a realistic magnetotail configuration. The results of this study are twofold. First, the shape of the magnetopause surface is computed under a variety of conditions. The resulting shapes are compared with fits to observed magnetopause crossings, and the scaling of magnetopause position on solar wind dynamic pressure is determined. Second, the model that results can be viewed as an improved version of the empirical model that was used as input, in that the magnetic field in the vicinity of the magnetopause (where empirical models have less guidance from data) is made more

ISSN:0148-0227    

DOI:10.1029/96JA01084

年代:1996

数据来源: WILEY

摘要:

A technique to measure the magnetotail reconnection rate from the ground is described and applied to 71 hours of measurements from 20 nights. The reconnection rate is obtained from the ionospheric flow across the polar cap boundary in the frame of reference of the boundary, measured by the Sondrestrom incoherent scatter radar. For our measurements, the polar cap boundary is located using 6300 Å auroral emissions andEregion electron density. The average experimental uncertainty of the reconnection rate measurement is 11.6 mV m−1in the ionospheric electric field. By using a large data set, we obtain the dependence of the reconnection rate on magnetic local time, the interplanetary magnetic field, and substorm activity, with much higher accuracy. We find that two thirds of the average polar cap potential drop occurs over the 4‐hour segment of the separatrix centered on 2330 MLT, that the linear correlation between the reconnection electric field and the half‐wave rectified dawn‐dusk solar wind electric fieldVBspeaks between 1.0 and 1.5 hours, with a maximum linear correlation coefficient of 0.46 at 70 min; and that following substorm expansion phase onset, the reconnection electric field becomes larger than the experimental uncertainty, with an average delay of 23 min. The 70‐min delay of the reconnection rate with respect toVBsis a typical convection time for a flux tube across the polar cap. This result indicates that reconnection in the magnetotail is influenced by the solar wind electric fieldVBson the field line being r

ISSN:0148-0227    

DOI:10.1029/96JA00414

年代:1996

数据来源: WILEY

摘要:

The two distant ISEE 3 geomagnetic tail passes have been examined to identify all slow‐mode shocks present in the data. We find a total of 86 events from 439 plasma sheet/lobe crossings, using five criteria based on relations between the upstream lobe and the downstream plasma sheet magnetic field and plasma measurements. The statistical results of slow‐mode shock parameters such as the angle between magnetic field and shock normal, θBn, Alfven Mach number along the normal direction,MAn, and electron beta, βe, are calculated and reported. On average, the magnetic field decreases by a factor ∼2.7, the electron density increases by ∼1.7, temperature increases by ∼1.8, and the plasma flow velocity increases by ∼ 3.8 across the shocks. The average upstream θBnis ∼76°, while the downstream angle is ∼ 50°. The shocks have an averageMAn∼ 0.87 along the normal direction, and an upstream βe∼ 0.04. In the downstream plasma sheet region, the dominant plasma flow associated with the shocks is in the tailward direction with an average speed ∼ 585 km/s. Only a few cases of Earthward downstream plasma flow have been detected. The slow shocks have thicknesses, on average, of −5380 km (about 7 ion inertial lengths) and an average tilt angle of ∼ ±22.4° between the shock normal andzaxis. Using the Petschek slow shock model, the average location of the neutral lines is located in a range of ±40REfrom observation sites. About half the slow mode shock events are detected during southward IMF intervals and half during northward intervals. There is a weak substorm dependence of slow mode shocks and plasmoids, a dependence which is most obvious whenBz>+2 nT andBz<−2 nT intervals are intercompared. We see a substorm dependence for plasma sheet/lobe crossings which suggests that the deep tail become more dynamic during substorm intervals. We have also sought the existence of large wave trains downstream of slow shocks that have been theoretically predicted byCoroniti[1971] and simulation studies. No such wave trains were observed throughout the two ISEE 3 passes of the distant tail. However, we do detect some medium amplitude transverse waves in the shock ramp regions. The waves have frequencies and polarizations similar to the plasma sheet boundary layer waves reported byTsurutani et al. [1985]. The waves present in the shock ramp are also right‐hand ion cyclotron waves in the plasma frame. We believe that these waves are generated by the ion beams flowing

ISSN:0148-0227    

DOI:10.1029/96JA00545

年代:1996

数据来源: WILEY

摘要:

Experimental results from Geotail and Galileo, as well as results from large‐scale kinetic modeling, demonstrate the characteristic structuring of ion distribution functions, especially in the distant and middle parts of the magnetotail. This structuring of the distributions is associated with nonadiabatic acceleration processes in the magnetotail. This paper suggests that the complexity of ion distributions can be quantified by using the analog of the fractal dimensionDof the isodensity contours of the ion velocity distributions. It is found thatDis usually larger than 1 for various locations within the tail and that as a result of the smearing of the structuring, the value gradually approaches the topological dimensionD= 1 as the Earth is approached. The use of the parameterDmay be helpful in comparing the degree of granularity of the distribution functions obtained in different regions of the magnetotail, as well as in comparing experimental and theoretical result

ISSN:0148-0227    

DOI:10.1029/96JA01085

年代:1996

数据来源: WILEY

摘要:

Dayside large‐scale and intermediate‐scale field‐aligned current (FAC) signatures are examined with multi‐instrument measurements from Viking and DMSP‐F7 at magnetic conjunctions. The present paper reports four such conjunction events, with an emphasis on an event that occurred on October 13, 1986. In these four events both Viking and DMSP‐F7 crossed prenoon FAC systems approximately along meridians. The altitude of DMSP‐F7 was 835 km, whereas that of Viking ranged from 8500 to 12,000 km. The electric to magnetic field ratio measured by Viking indicates that intermediate‐scale FAC systems, as well as large‐scale FAC systems, are often quasi‐stationary. This is also supported by the comparison between the Viking and DMSP‐F7 magnetic measurements. The only obvious exception is the equatorward part of the October 13 event, in which the Viking and DMSP‐F7 measurements are better explained in terms of Alfvén waves. In two other events the Viking signature projected to the DMSP‐F7 altitude is significantly more structured than the DMSP‐F7 signature, although the electric to magnetic field ratio observed by Viking suggests that the associated FACs were quasistationary. This apparent discrepancy is possibly ascribed to the fact that Viking stays longer in FAC systems and therefore has more chance to observe temporal changes in FACs. However, such temporal effects must operate longer than the Alfvén transit time so that FAC systems become quasi‐stationary. Although the generation mechanism(s) of intermediate‐scale FAC systems remains an open question, possibilities include a localized shear of plasma convection and a localized merging between the solar wi

ISSN:0148-0227    

DOI:10.1029/96JA00686

年代:1996

数据来源: WILEY

摘要:

The development of the ring current ions in the inner magnetosphere during the main phase of a magnetic storm is studied. The temporal and spatial evolution of the ion phase space densities in a dipole field are calculated using a three dimensional ring current model, considering charge exchange and Coulomb losses along drift paths. The simulation starts with a quiet time distribution. The model is tested by comparing calculated ion fluxes with Active Magnetospheric Particle Tracer Explorers/CCE measurement during the storm main phase on May 2, 1986. Most of the calculated omnidirectional fluxes are in good agreement with the data except on the dayside inner edge (L<2.5) of the ring current, where the ion fluxes are underestimated. The model also reproduces the measured pitch angle distributions of ions with energies below 10 keV. At higher energy, an additional diffusion in pitch angle is necessary in order to fit the data. The role of the induced electric field on the ring current dynamics is also examined by simulating a series of substorm activities represented by stretching and collapsing the magnetic field lines. In response to the impulsively changing fields, the calculated ion energy content fluctuates about a mean value that grows steadily with the enhanced quiescent field.

ISSN:0148-0227    

DOI:10.1029/96JA01274

年代:1996

数据来源: WILEY

摘要:

Electron temperature variations in the Earth's plasmasphere are studied using the Exos D satellite observations and the Sheffield University plasmasphere‐ionosphere model. The observations made during the years 1989–1994 are analyzed to investigate the local time and altitude (1000–8000 km) variations of the electron temperature at magnetic latitudes 0°–45°N. The observed electron temperatureTeis almost constant during both day and night and is found to have large day‐to‐night differences that vary with altitude and latitude; the largest day‐to‐night ratio inTe(≈6500 K/2600 K) occurs at the highest altitude at equatorial latitudes and the smallest ratio (≈3300 K/2300 K) at the lowest altitude at midlatitudes. During daytime,Teincreases rapidly with altitude in the lower plasmasphere (altitude<2500 km) and slowly in the upper plasmasphere with mean gradients of 1.33 and 0.22 K km−1, respectively. At night, on the other hand, the lower plasmasphere is in thermal equilibrium, and in the upper plasmasphereTeincreases slowly with a mean gradient equal to the daytime value. The electron temperature shows maximum latitude variation at medium plasmaspheric altitudes (around 4000 km) and smaller variations at lower and higher altitudes, particularly during daytime. At the altitude of maximum latitude variation,Teincreases by about 1200 K between the equator and 40°N during both day and night. The model reproduces the observations reasonably well and is used to explain the occurrence of a prominent morning peak inTe. The morning peak becomes prominent in the lower plasmasphere at low latitudes because of the verticalE×Bdrift in the equatorialFregion, which increasesTeduring morning hours and decreasesTeduring daytime hours. The observations and model results also show the occurrence of an afternoon peak inTe, which becomes prominent with increasing altitude and latitude; the peak could be caused by the dayt

ISSN:0148-0227    

DOI:10.1029/96JA00823

年代:1996

数据来源: WILEY