摘要:

The interplanetary shocks which generate detectable low‐frequency (50%) belong to the structural classes described by the Solwind researchers as “curved front” or “halo.” Evidence presented suggests that these are the same class viewed from a different perspective. Our results are consistent with there being a close relationship between interplanetary shocks an

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09869

年代:1987

数据来源: WILEY

摘要:

Interest in slow‐mode shocks in space physics applications has increased in recent years. These shocks, which can arise from the steepening of linear slow‐mode magnetohydrodynamic waves, might form ahead of objects moving through a magnetized, highly conducting gas at speeds greater than the slow‐mode speed. In particular, slow shocks are the only shocks possible for speeds between those of the slow and fast modes. This appears to be the speed range, for example, of many and perhaps most coronal mass ejections. Despite the interest in and detection of slow‐mode shocks in the heliosphere, there appear to date to be no calculations of flows involving slow shocks. Basic properties like shock configuration and standoff distance are therefore unknown, although several authors have pointed out that slow shocks should extend upstream of the obstacle giving rise to the shock. This paper presents a simple method for computing slow shock flows in an infinitely conducting plasma, based on the solution of a free boundary problem for the shock configuration that matches a post shock potential flow to a uniform preshock flow. Such potential‐flow methods have been used in ordinary gasdynamics to obtain crude approximations to blunt‐body flows containing shocks; arguments given here suggest that the potential‐flow approximation should be more accurate in the case of slow MHD shocks. The method is applied to shocks arising from flow about spherical and paraboloidal obstacles. A parametric study of shock formation as a function of sonic and Alfvén Mach numbers shows standoff distance increasing as either Mach number decreases. Streamlines and shock configurations for several slow shock flows are

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09875

年代:1987

数据来源: WILEY

摘要:

This paper presents observations of the radial and latitudinal variations of the interplanetary magnetic field measured by the Voyager 1 (V1) and Voyager 2 (V2) spacecraft from mid‐1977 to mid‐1985. The data extend from 1 to 20 AU and from −5° to 26° in heliographic latitude. Data obtained at 1 AU are used to separate temporal variations from radial variations, and plasma measurements from V2 are used to consider the effect of temporal variations in the bulk speed. Observations of the radial variation of the large‐scale magnetic field strength in the ecliptic agree with the predictions of Parker's model when temporal variations in the magnetic field and bulk speed are taken into account. The latitudinal variation of the magnetic field observed by V1 is in agreement with the predictions of Parker's model to first approximation. The magnetic field strength at higher latitudes is somewhat lower than expected on the basis of observations made in the ecliptic, but this could be due to an increase in bulk speed and/or a decrease of solar magnetic field strength with latitude. Fluctuations in the strength of the magnetic field are small compared to the large‐scale field itself, and they decrease in amplitude with increasing distance approximately asR−1/4. Fluctuations in the components are relatively large, and they make a significant contribution to the mean field that is not described by P

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09885

年代:1987

数据来源: WILEY

摘要:

A cross‐correlation analysis between different parameters of the solar wind plasma based on Voyager 1 and 2 data from 1 to 9.5 AU evidences, as a predominant feature of the microscale structure of the solar wind, a persistent presence of time periods characterized by high anticorrelation between the magnetic field magnitude and the proton density. These events are present at least 25% of the total time, and their frequency of occurrence progressively increases with increasing heliocentric distance. These results have been satisfactorily interpreted as being due to the presence of nonpropagating structures with internal pressure balance and characteristic time scale of ≲10 hours. Alfvénic fluctuations are confirmed to dominate the microscale at 1–2 AU, but they seem to become less important at larger heliocentric dis

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09893

年代:1987

数据来源: WILEY

摘要:

From a set of approximately 7000 spectra provided by the ISEE 3 ion composition instrument at the time near the maximum of solar cycle 21 we have been able to derive iron velocities and to compare them with velocities of helium ions obtained with the same instrument. We find a strong correlation among these velocities (rcor= 0.975 ± 0.001). Whereas no significant velocity differences between helium and iron are found at low solar wind speeds, it appears that iron tends to flow at a somewhat lower speed (δυ ∼ 10 km/s) in high‐speed s

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09901

年代:1987

数据来源: WILEY

摘要:

Electric field noise bursts detected from the Pioneer Venus Orbiter across the nightside of Venus have been interpreted in a number of papers as providing evidence of extensive lightning at Venus. Moreover, the asserted clustering of these 100‐Hz signals over mountainous topography has encouraged widespread speculation that present‐day volcanism is prevalent. Earlier studies have noted, however, that the 100‐Hz signals were often associated with ionospheric perturbations, prompting the suggestion that the electric field signals were being produced in situ and were thus unrelated to the lower atmosphere or surface. In this more detailed analysis we find a statistically persistent correlation between the 100‐Hz noise and ion troughs marking the solar wind interaction with the nightside ionosphere. Clusters of closely spaced 100‐Hz noise bursts attributed to lightning often “turn on” in the troughs, simultaneously with sharp density gradients in which the plasma concentration drops by an order of magnitude or more and the direction of the magnetic field changes significantly, within a few seconds. Analysis of the 100‐Hz noise also shows two other important statistical characteristics, namely (1) the noise is most frequently observed near 180 km and above but much less frequently at lower altitudes and (2) the noise is most often detected when the angle between the spacecraft velocity vector and the direction of theBfield approaches 90°. We interpret the foregoing characteristics as evidence that the 100‐Hz noise is produced from solar wind/interplanetary magnetic field related plasma perturbations occurring near the spacecraft and that the noise is unrelated to the lower atmosphere and surface of Venus. Unfortunately, we do not have access to sufficient high‐resolution data to specifically identify the type of ionosp

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09907

年代:1987

数据来源: WILEY

摘要:

The interaction of the solar wind with Venus is influenced by the pickup of newly born exospheric oxygen ions by the convecting magnetosheath plasma. The flow and field configuration of the magnetosheath plasma, together with the large gyroradius of the pickup ions, cause mass loading to occur preferentially on one side of the magnetosheath. The observed hemispherical asymmetry in the magnetic field in the near‐planet magnetosheath, attributed to this pickup process, is confirmed by direct observation of the picked‐up planetary particles. Test particle calculations show that a current system created by ion pickup has the appropriate location and magnitude to account for the magnetic field asymmetry. The results indicate that a fluid treatment of the Venus mass‐loading problem is not entirely appropriate; a hybrid or kinetic model is necessary to incorporate the finite Larmor radius of the pickup particles which produces the observed asym

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09920

年代:1987

数据来源: WILEY

摘要:

A local field stress technique, developed previously in a study of the Saturnian magnetosphere, is introduced to the problem of determining the radial force balance characteristics of Jupiter's magnetosphere. We begin by estimating the near‐equatorial, radial magnetic force densities using the data obtained by Voyager 1 principally on the dayside (inbound) portion of its trajectory (7.4 ≤R(RJ) ≤ 42). Using the low‐energy charged particle data (≳30 keV) and other published data we then explore ways in which the field forces might be balanced. Inside 22RJthe magnetic radial forces match in both magnitude and radial variation the hot particle pressure gradient forces, assuming a mix of H+and On+ions. Outside 22RJwe confirm qualitatively a previously reported force balance problem, and we suggest two possible solutions. The magnetic radial forces could be balanced in part either by the hot particle pressure anisotropy forces (withP∥/P⊥∼ 1.5 for On+) or by the centripetal acceleration of the cool particles that may be streaming in bulk (υs) parallel to the magnetic field within the small field line curvature region of the neutral sheet (with υs∼ υcorot). Comparing present results with the results of our previous study, we note that contrary to common expectations, Saturn rather than Jupiter is unique in having the corotation centrifugal forces dominate over other sources of radial particle force in some regions of the middle (ring current) equa

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09931

年代:1987

数据来源: WILEY

摘要:

We have combined the Voyager 2 low‐energy plasma data from the Plasma Science Experiment (PLS) and the magnetic field data from the Magnetometer Experiment (MAG) with the Voyager 2 Low Energy Charged Particle Experiment (LECP) ion data (E>28 keV) for the previously described distant magnetotail observations (5000

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09943

年代:1987

数据来源: WILEY

摘要:

The interaction between Io and the plasma torus is simulated by a three‐dimensional numerical model which allows the calculation of electric fields, current density distributions, and magnetic fields in Io's vicinity and in the Alfvén wings. The model is self‐consistent in the sense that the influence of magnetic field disturbances on the electric field is taken into account and vice versa. A two‐dimensional elliptic differential equation for the electric potential is derived by using Euler potentials. The current density in Io's vicinity is decomposed into poloidal and toroidal fields. This representation allows an effective integration to get the magnetic field disturbances. Pedersen, Hall, inertial (or polarization) and field‐aligned currents are calculated. We show results for different upstream plasma conditions and atmosphere/ionosphere models and discuss the consequences of variations in neutral density, Alfvénic Mach number, and other parameters. A series of simple spherically symmetric atmosphere/ionosphere models with neutral gas densities between 1016m−3and 1017m−3are investigated. By comparison with Voyager magnetic field measurement we get agreement for a column density of 7.5 × 1021m−2(surface densityN0= 5×1016m−3, scale

ISSN:0148-0227    

DOI:10.1029/JA092iA09p09949

年代:1987

数据来源: WILEY