摘要:

The relatively abrupt onset of cosmic ray modulation, observed in May 1987 on Earth and in September 1987 by the Pioneer 10 and 11 spacecraft, enables further investigation of the possible effect of the heliospheric current sheet (sector boundary) on modulation. According to the gradient drift theory of cosmic ray transport, modulation is associated with solar cycle changes in the current sheet, whose inclination increases systematically from solar minimum to solar maximum. The measure used in testing this hypothesis is the difference in the maximum latitudinal extent of the current sheet in the northern and southern solar hemispheres. This difference is obtained from contours of the current sheet produced by extrapolating photospheric magnetic field measurements to a solar wind source surface. When the latitude difference, divided by 2 and called the pseudoinclination or pseudotilt angle, is compared with cosmic ray count rates measured on Earth, an excellent correlation is found, as in a previous study of modulation in the years 1976–1986. The abruptness of the onset can be attributed to the greater sensitivity of the cosmic ray intensity to changes in the current sheet when the solar‐heliospheric magnetic field is inward in the northern hemisphere (above the current sheet) as predicted by the model. Thus the observations are found to be consistent with the predictions of the gradient drift model. A preliminary analysis is also carried out to test an alternative, but not necessarily incompatible, hypothesis that large‐scale transient solar wind structures (coronal mass ejections, shocks, etc.) form barriers to the inward transport of cosmic rays. Available spacecraft magnetic field measurements (ICE, Pioneer 11), geomagnetic activity, including storm sudden commencements, and Forbush decreases do not reveal an obvious change in solar wind stream structure within several months of the modulation onset. It appears that in this instance, promoted by the abruptness of the onset, it may be possible to discriminate between the effect on modulation of the heliospheric current sheet and the possible effect of solar wind shocks and/or e

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18731

年代:1990

数据来源: WILEY

摘要:

Data are presented on the properties of picked‐up cometary protons and water group (WG) ions observed upstream of the bow shock of comet Halley by the ion mass spectrometer and Johnstone plasma analyzer experiments on the Giotto spacecraft. The number of WG ions exceeded the number of cometary protons at cometocentric distancesr<1.3×106km, while the WG mass density exceeded the proton mass density whenr<6×106km. The small scale variations of proton and WG densities were well correlated, which argues against the "expanding halo" model which has been used to explain the quasi‐periodicity of energetic particles observed by VEGA and Giotto. Two parameters are used to describe the pitch angle distributions of the picked‐up ions: a 10% width, which is the full angular width at 10% of the maximum of the pitch angle distribution, and a mean width. The cometocentric distance profiles of both parameters exhibited a great deal of scatter, and both parameters showed a steeper average radial gradient for WG ions than for protons. The 10% widths of WG ions were consistent with less than 10∶1 anisotropies forr<2.5×106km, but the proton anisotropy did not drop below 10∶1 until the spacecraft entered the “foreshock” region atr= 1.4×106km. After subtraction of the variation in field direction, the mean width of the proton pitch angle distribution was nearly independent of distance everywhere outside the shock. The WG mean width, on the other hand, increased with increasing WG density (as expected) and with increasing angle α between the interplanetary field and the solar wind velocity vector (an unexpected result). No increase in shell radii, due to either adiabatic compression or first‐order Fermi acceleration, could be discerned for either ion species until the spacecraft was very close to the bow shock. The thickness of the picked‐up proton shell slowly increased asrdecreased inside 2.5×106km, whereas the water group shell thickness remained fairly constant until just outside the shock. While some of the differences between the two ion species can be attributed to the greater ionization scale length of cometary H atoms compared to WG atoms and molecules, the very different dependences of their pitch angle and energy diffusion rates on ion densities and on α are difficult to understand in terms of presently avail

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18745

年代:1990

数据来源: WILEY

摘要:

A one‐dimensional single‐fluid MHD model was used by Cravens (1989a) to predict the existence of a narrow (50 km) layer of enhanced plasma density (factor of 3 over background) at the boundary of the comet Halley diamagnetic cavity. The existence of such a layer was confirmed by measurements made by the Giotto ion mass spectrometer (Goldsteinet al.,1989), although with a relative density enhancement of more than a factor of 4. We solve the time‐dependent coupled continuity equations for several species including H3O+, H2O+, OH+, O+, NH4+, NH3+, CH4+, and CH3+. For several ion species we investigate factors affecting the magnitude of this enhancement and its relationship to the thickness of the transition layer. For example, ion species with short chemical lifetimes are shown to have smaller density enhancements than species with longer lifetimes. The cometocentric distance of the cavity boundary also strongly affects the magnitude of the density enhancement; the enhancement increases with increasing dis

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18755

年代:1990

数据来源: WILEY

摘要:

We present a study of all shocks observed from Pioneers and Voyagers in 1973–1982. The average shock strength increases with the heliocentric distance outside 1 AU, reaches a maximum near 5 AU, and then decreases with the distance. The increase in the entropy of the solar wind protons across shocks also reaches a maximum near 5 AU. When an average shock propagates through the solar wind, the shock heating increases the entropy of the solar wind protons by approximately 0.8×0−23J/K/proton. We also use plasma data from Voyagers and Pioneers between 1 and 30 AU and data from IMP at 1 AU to calculate the increase in the average entropy of solar wind protons with the heliocentric distance. When the distance increases by a factor of 10, the entropy increases by about 4×10−23J/K/proton. In order to evaluate the role played by shocks for the heating of the solar wind, we use a MHD simulation model to calculate the entropy changes for the November, 1977 event. Shock heating is the only heating mechanism included in the model. The calculated entropy increase agrees reasonably well with that calculated from observational data. The simulation result suggests that shocks are chiefly responsible for the heating of the solar wind plasma between 1 an

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18769

年代:1990

数据来源: WILEY

摘要:

The coronal slow shock has been predicted to exist embedded in large coronal holes at 4–10 solar radii. We use a three‐fluid model to study the jumps in minor ion properties across a slow shock such as the coronal slow shock. We formulate the jump conditions in the de Hoffmann‐Teller frame of reference. The Rankine‐Hugoniot solution determines the MHD flow and the magnetic field across the shocks. For each minor ion species, the fluid equations for the conservation of mass, momentum and energy can be solved to determine the velocity and the temperature of the ions across the shock. We also obtain a similarity solution for heavy ions. The results show that on the downstream side of the slow shock the ion temperatures are nearly proportional to the ion masses for He, O, Si, and Fe in agreement with observed ion temperatures in the inner solar wind. This indicates that the possibly existing coronal slow shock can be responsible for the observed heating of minor ions in the sol

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18781

年代:1990

数据来源: WILEY

摘要:

The structure of intermediate shocks is studied on the basis of the resistive, nonviscous two‐fluid equations. Electron inertia effects are neglected so that the generalized Ohm's law contains only the Hall current and the electron pressure terms in addition to the usual resistive term and the electric field. As for the case of purely resistive MHD, reported recently by Hau and Sonnerup (Journal of Geophysical Research, 94,6539, 1989), fixed‐point analysis is performed to examine the nature of the magnetic structure near the upstream and downstream states of the intermediate shock. The one‐dimensional, steady state, resistive Hall MHD equations are then integrated numerically to generate complete shock structures which are presented in the form of magnetic hodograms. These hodograms describe fast and slow shocks in addition to intermediate shocks. As expected, the calculations show that the main effect of Hall currents is to remove the symmetry between left‐hand and right‐hand polarized shock structures found in the purely resistive case and sometimes to convert the smooth shock transitions obtained from the resistive MHD model into transitions that incorporate oscillatory standing wave train structures at their upstream and/or downstream edge. The magnetic structure in the plane of the shock near the possible upstream and downstream states of the intermediate shock, which in the case of purely resistive MHD is either a node or a saddle, can be either a node, a saddle or a spiral point, the latter corresponding to a standing wave train, when the Hall term is included. As a result, the number of possible types of magnetic hodogram topology increases from 3 in the resistive case, to a total of 20. However, it appears that the constraints provided by the shock jump conditions make certain of these topologies unattainable: only 13 of the 20 cases have been found and are reported in the paper. The relationship between small‐amplitude dispersive waves in the flow upstream or downstream of a shock and the nature of the corresponding fixed point is als

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18791

年代:1990

数据来源: WILEY

摘要:

Hybrid (particle ion, massless fluid electron) simulations are carried out in one and two spatial dimensions to show that the supercritical quasi‐parallel shock is not steady. Instead of a well‐defined narrow transition region (shock ramp) between upstream and downstream states that remains at a fixed position in the flow, the ramp periodically steepens, broadens, and then re‐forms upstream of its former position. During part of the re‐formation cycle, the number of ions backstreaming from the shock increases to a significant fraction (up to 40%) of the incoming ions. This dense ion beam then couples to the incident ions to form a heated region, the leading edge of which becomes the new shock ramp. Two‐dimensional simulations demonstrate that this process is not a consequence of restricting the calculations to one spatial dimension and show that the re‐formation cycle is not in phase along the shock surface, although the surface itself remains fairly laminar. Generally, the downstream wave turbulence is comparable in all three magnetic components (unlike one dimensional whereBn= const) and shows no tendency to be aligned in any particular direction. Both the two‐dimensional simulations as well as the one‐dimensional runs indicate that the downstream plasma is a complex mixture of regions of merged incident and reflected ion beams. The simulations are consistent with recent observational studies at the quasi‐parallel bow shock showing the presence of magnetic pulsations, cold ion beams, and a downstream consisting of alternating regions of low‐density hot ions and higher

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18809

年代:1990

数据来源: WILEY

摘要:

One‐dimensional hybrid (particle ion, massless fluid electron) simulations of quasi‐parallel shocks and two stream interactions are carried out in order to study the mechanism by which large‐amplitude electromagnetic waves are generated at the shock and maintained in a quasi‐steady manner. It is concluded that a resonant interaction at the interface between the incoming ions and the heated downstream ions (“interface instability”) is the most likely source of the waves that ultimately comprise the quasi‐parallel shock. The effect of the generation of these waves at the shock transition results locally in a nonsteady shock ramp, which propagates downstream with respect to the average shock position and is then replaced by a new steepened ramp at the original front position. This process is complicated by several competing effects. First, backstreaming ions excite the beam cyclotron resonant instability at wavelengths longer than those generated at the shock interface which gives rise to compressive upstream perturbations that are then carried into the shock and interact with the shock‐generated waves. Second, the re‐formation process sometimes leads to the appearance of cold dense ion beams just upstream from the shock which gyrate in the upstream magnetic field to produce local density enhancements that have some effect on where the new shock front forms. Third, whistlers at wavelengths shorter than the waves associated with the re‐formation process are also present that can scatter the backstreaming ions into a hotter, more diffuse population. In order to separate these various processes, numerical experiments involving quasi‐parallel shocks and the interaction of two plasma streams have been carried out for various upstream parameters. In these studies the upstream conditions can be controlled to some extent by eliminating the backstreaming ions and suppressing the shorter wavelength modes, and in the case of the two stream interactions the upstream and downstream plasmas can also be distinguished. The interface instability that results from the interaction of the incident ion stream and a less dense population of heated downstream ions generates intermediate scale (kc/ωi∼ 1) magnetosonic waves with group velocities pointing upstream, but phase fronts carried downstream, consistent with the waves observed in the shock simulations. A linear analysis indicates the wave numbers of the most unstable modes decrease with increasing upstream θBnand Mach number, in agreement with the simulations. Areas where further work is

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18821

年代:1990

数据来源: WILEY

摘要:

The structure of the dissipation region during magnetic reconnection in collisionless plasma has been investigated. We consider a prescribed two‐dimensional magneticxline configuration with an imposed reconnection electric fieldEy. Particles are injected onto a computational grid, their orbits are integrated, and the moments of the distribution function are stored. The structure of the dissipation region depends on only two variables: a normalized reconnection electric field Ê and the opening angle θ of the separatrices of the magnetic field. An important conclusion of the work is that there is no linear relationship between the current sheet velocity νyand the electric field. For a small normalized electric field the maximum νyis localized away from thexline, is diamagnetic in origin (produced by local pressure gradients), and is independent of Ê as Ê → 0. For a large normalized electric field the effective life‐time of particles in the reconnection region scales as Ê−1/3and νy, ∝ Ê2/3. In this limit, particles are ejected from the reconnection region as high‐velocity, gyrophase‐bunched beams. These beams produce an irregular filamentary current distribution in the outflow region. The beams which are ejected along the center of the outflow region are eventually trapped by the magnetic field while the beams ejected just downstream from the separatrix continue to move with high velocity out of the computational region. Analytic expressions for the current sheet velocity (νy), inflow and outflow velocities, and the scale size of the dissipation region are derived. Over the entire range of Ê, significant temperature anisotropies are produced, withT⊥

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18833

年代:1990

数据来源: WILEY

摘要:

We investigate the role of the plasma mantle in the dynamics of the Earth's distant magnetotail. The plasma mantle can exert a substantial influence on the current‐sheet force equilibrium by transporting both momentum and mass from the dayside magnetopause to the tail current sheet. Such influences are particularly strong when the interplanetary magnetic field contains a significant southward component for a prolonged period. We find that a number of processes characterizing a substorm growth phase can be attributed, at least in part, to the plasma mantle. Among these processes are the thinning of the plasma sheet, storage of magnetic energy in the tail, and formation of plasmoids. The rates of these processes are found to increase nonlinearly with the momentum density of the mantle plasma. The present model applies only to the distant magnetotail (x ≲ −30RE) because of various approximations that we make to allow an analytical treatment of the governing equa

ISSN:0148-0227    

DOI:10.1029/JA095iA11p18849

年代:1990

数据来源: WILEY