摘要:

Previous studies have established that large merged interaction regions (MIRs) in the outer heliosphere are the main cause of the 11‐year modulation cycle of galactic cosmic rays in the ecliptic. Those simulations were successful despite their use of a constant convection speed. In this study, we show that a large rise in solar wind speed, seen at higher heliographic latitudes during 1985–1987, combined with MIRs, can generate the difference between the Voyager 1 and Voyager 2 count rates in this period. Also, the difference between the starting times of the new cycle at both spacecraft is directly attributable to the difference between the pattern of MIRs measured at the two spacecraft. In addition, the Voyager 2 simulation exhibits the correct behavior of the 11‐year cycle in the cosmic ray radial gradients between points in the outer helios

ISSN:0148-0227    

DOI:10.1029/92JA02365

年代:1993

数据来源: WILEY

摘要:

It is shown that the long‐term modulation of 0.2‐ to 3‐GV galactic and anomalous cosmic rays over the 22‐year heliomagnetic cycle is principally a combination of two solar related processes, the cumulative effect of long‐lived global merged interaction regions (GMIRs) and large‐scale particle gradient and curvature drifts in the interplanetary magnetic field. This paradigm for cosmic ray modulation is based on the observed changes in cosmic ray intensity from solar minimum to solar maximum over successive solar cycles (21 and 22) using data from 1 AU and from the outer heliosphere. For cycle 21 (when positive ions flow in over the solar pole and out along the heliospheric neutral current sheet;qA>0), the 1977–1980 modulation is dominated by GMIRs. While drifts may play a role in particle transport in the heliosphere at this time, the changing inclination of the heliospheric neutral sheet has a negligible effect on the intensity of cosmic ray nuclei. In cycle 22 when this flow pattern is reversed, it is shown that drifts are an important but not dominant factor for galactic cosmic ray modulation with the current sheet related drift effects decreasing with increasing rigidityRand heliocentric distancer. Anomalous cosmic rays are much more sensitive to changes in the current sheet inclination. The observed changes in the galactic cosmic ray intensity over the 1987–1988 period due to the increasing inclination of the current sheet, α, are a factor of 3–5 smaller than predicted by the time‐dependent model of Potgieter and Le Roux (1992). The latitude variation at Voyager 1 (heliolatitude 32°) of the dependence of galactic cosmic ray intensity on α is consistent with theoretical expectations. The strong role of GMIRs and their episodic nature requires a long lifetime (1.5–1.8 years) and a magnetic structure that effectively extends over the solar poles. Such a long GMIR lifetime implies a modulation boundary

ISSN:0148-0227    

DOI:10.1029/92JA02205

年代:1993

数据来源: WILEY

摘要:

A two‐component incompressible fluctuation model is presented to explain the radial evolution of the solar wind fluctuations. The basic idea is to consider the small‐scale fluctuations in the solar wind as being composed of Alfvén waves and convective structures. The major Alfvén waves are believed to be created near the coronal base and to propagate outward along the magnetic field lines. The convective structures are defined as the small‐scale variations perpendicular to the local magnetic field direction. They are either quasi‐static or turbulent and slowly evolving in the plasma frame of reference. The small‐scale perpendicular variations are connected, in the parallel direction, with large‐scale magnetic field variations, which are convected by the solar wind as quasi‐static structures during the wind expansion time. The decomposition of the original fluctuations can be done by using special space and time averages, which are defined by space averaging along the directions parallel and perpendicular to the local magnetic field vector and by time averaging in the plasma frame of reference. The equations of motion of the fluctuations and of the correlation functions for both Alfvén waves and convective structures have been derived from the one‐fluid MHD equations. A combination of the correlation functions of these two components is then used for a comparison with observational results. The influence of the angle between the sampling direction and the magnetic field vector on the final results has also been considered. As a first step to apply these equations, a simple model has been suggested that is based on the assumption that the fluctuations are only composed of outward propagating Alfvén waves and static magnetic structures. For comparison with the observations, new statistical results from data obtained by Helios 1 during days 1–95, 1975, and Helios 2 during days 19–109, 1976, are presented. The numerical solutions are shown to describe well the basic evolution trend of the fluctuation energy, the normalized cross helicity, and the Alfvén ratio. It is also shown that the basic physical process of the evolution of the convective structures is the convection of the fluctuating velocity vortex lines and the magnetic field lines by

ISSN:0148-0227    

DOI:10.1029/92JA01947

年代:1993

数据来源: WILEY

摘要:

The evolution of rotational discontinuities (RDs) is followed using a hybrid numerical code. An extensive parameter variation is carried out, with particular emphasis on β,Ti/Te, θB(the angle between the normal and total magnetic field), and the helicity of the RD. The RD structure is shown to have features in common with the evolution of both strongly modulated, nonlinear wave packets and linear dispersive wave propagation in oblique magnetic fields. For small θB, the RD disperses linearly giving fast and Alfvén waves upstream and downstream, respectively, and the familiar S‐shaped hodograms. At larger θB(≈ 30°), nonlinearity becomes important and strong coupling to a compressional (sonic) component can occur in the main current layer. When the ions are cold, there is a critical value of βe(=β*) when the intermediate wave train moves from the downstream side of the RD to the upstream and is replaced on the downstream side by slow modes. This is reflected in the hodograms by a change of the wave polarization on both sides, and represents an important modification to the original Goodrich and Cargill (1991) wave model of RDs. AsTi/Teincreases, the spreading rate of the current layer increases for moderate θB. For large θB(≈ 60°), RDs with electron (ion) sense of rotation show increased (decreased) spreading with increasingTi/Te. These results are applied to RDs observed in the solar wind and at

ISSN:0148-0227    

DOI:10.1029/92JA02364

年代:1993

数据来源: WILEY

摘要:

A collisionless parallel shock model is presented which is based on solitary‐type solutions of the modified derivative nonlinear Schrödinger equation (MDNLS) for parallel Alfvén waves. We generalize the standard derivative nonlinear Schrödinger equation in order to include the possible anisotropy of the plasma distribution function and higher‐order Korteweg‐de Vies type dispersion. Stationary solutions of MDNLS are discussed. The new mechanism, which can be called “adiabatic,” of ion reflection from the magnetic mirror of the parallel shock structure is the natural and essential feature of the parallel shock that introduces the irreversible properties into the nonlinear wave structure and may significantly contribute to the plasma heating upstream as well as downstream of the shock. The anisotropic nature of “adiabatic” reflections leads to the asymmetric particle distribution in the upstream as well in the downstream regions of the shock. As a result, nonzero heat flux appears near the front of the shock. It is shown that this causes the stochastic behavior of the nonlinear waves which can significantly contribute to the shock thermalization. The number of adiabaticaly reflected ions define the threshold conditions of the fire‐hose and mirror type instabilities in the downstream and upstream regions and thus determine a parameter region in which the described laminar parallel shock structure can exist. The threshold conditions for the fire hose and mirror‐type instabilities in the downstream and upstream regions of the shock are defined by the number of reflected particles and thus determine a parameter region in which the described laminar parallel shock

ISSN:0148-0227    

DOI:10.1029/92JA02264

年代:1993

数据来源: WILEY

摘要:

The location of the Martian magnetopause and that of the bow shock are studied on the basis of three‐dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits. The clear and strong dependence of the areomagnetopause position on solar wind ram pressure was revealed, while the position of the bow shock was practically independent of this parameter. In the power law expression telling the dependence of the Martian magnetotail thicknessDon the solar wind ram pressure:D∼(ϱυ²)−1/k, the power index turned out to bek∼5.9±0.5. The close coincidence of this index withk= 6 for a dipole geomagnetic field, and the large areomagnetotail thickness compared with the planetary diameter, suggest that an intrinsic dipole magnetic field is likely to be an important factor in the solar wind interaction with Mars. On the other hand, the relatively stable position of the subsolar point of the Martian magnetopause and unambiguous induction effects observed by the Phobos 2 MAGMA magnetic experiment in the magnetotail indicate the essential role of an induced magnetic field, too. The weak dependence of the terminator bow shock position on the solar wind ram pressure may be related to the relatively stable position of the subsolar m

ISSN:0148-0227    

DOI:10.1029/92JA01666

年代:1993

数据来源: WILEY

摘要:

A quasi‐linear diffusion model including both pitch angle and energy diffusion, adiabatic compression and convective motion with the solar wind flow has been used to investigate the cometary ion pickup process along the Sun‐comet line at comet Halley. The total pickup ion densities and magnetic turbulence spectrum levels observed by Giotto were used to constrain the quasi‐linear model. Comparisons of the model results were made with energetic ion distributions observed by instruments onboard the Giotto spacecraft. The observed power spectrum index of magnetic turbulence γ is about 2–2.5. However, our simulation shows that when γ was 2, the calculated proton distributions were much more isotropic than the observed ones; hence we have chosen γ = 2.5 in our study. Furthermore, we assumed that only about 5% of the total low frequency wave power propagates away from the comet. The numerical solutions of the quasi‐linear diffusion equations show that the isotropization of the pickup ion distribution, particularly at the pickup velocity, is not complete even close to the bow shock (but upstream), which agrees with the observations. The calculated and observed proton distributions are somewhat more isotropic than the water group ion distributions in the region far upstream of the shock, but the opposite is true near the bow shock. We find that given the observed turbulence level, quasi‐linear theory gives pickup ion energy distributions that agree with the observed ones quite well and easily produces energetic ions with energies up to h

ISSN:0148-0227    

DOI:10.1029/92JA02035

年代:1993

数据来源: WILEY

摘要:

The wave measurements at the spacecraft encounters with comet Halley have shown intense wave activity at the lower hybrid frequency. The excitation of the lower hybrid instability by the pickup cometary ions (protons and water group) in the bow wave region and the quasi‐linear diffusion of the ions in these fluctuations are discussed. The quasi‐linear diffusion of the pickup protons takes place over a scale length shorter than that of the heavier water group ions. This enhances damping of the waves by protons, and when the pickup proton density is large enough, it can result in the saturation of the instability as this damping balances the heavy ion driven growth. The observed electric field amplitude and the scale length of proton relaxation are in agreement with the theory. For small pickup proton density the instability can saturate due to the wave energy cascade arising from the modulation instability of the large‐amplitude lower hybrid waves. This saturation mecahnism leads to electron acceleration and suprathermal tail form

ISSN:0148-0227    

DOI:10.1029/92JA01729

年代:1993

数据来源: WILEY

摘要:

The present sunspot cycle (number 22) is now in decline, having had its onset in September 1986, its primary maximum in July 1989, and a secondary maximum (of lower value) in 1991. Dependent upon whether cycle 22 is a short‐ or long‐period cycle, onset for cycle 23 will occur, respectively, either prior or subsequent to about July 1997. A relation is described, involving the slopes (i.e., the average rates of change in smoothed sunspot number values) as seen during the ascending and descending portions of the sunspot cycle, which appears to clarify, at least for cycle 22, the ambiguity of cycle length. In particular, the relation strongly suggests that cycle 22 is a short‐period cycle and that onset for cycle 23 will come early rather than late, with the most probable date for cycle 23 onset being May–Novemb

ISSN:0148-0227    

DOI:10.1029/92JA02557

年代:1993

数据来源: WILEY

摘要:

We present a modified version of the Toffoletto and Hill (1989) open magnetosphere model that incorporates a tail‐like interconnection field with a discontinuity to represent the slow‐mode expansion fan that defines the high‐latitude tail magnetopause. (The interconnection field is defined as the perturbation on an initially closed magnetosphere model to make it open.) The expansion fan controls the open field line region in the tail, and the intersection of the fan with the tail current sheet is, by design, the x line. The new interconnection field allows greater control of the tail field structure; in particular, it enables us to eliminate the nightside mapping singularity that occurs in previous models when the interplanetary magnetic field is nonsouthward. Also, in contrast to earlier models, the far tail x line extends farther downstream on the flanks than in the center of the tail, consistent with observa

ISSN:0148-0227    

DOI:10.1029/92JA02342

年代:1993

数据来源: WILEY