An axisymmetric magnetohydrodynamic model for the interaction of the solar wind with the local interstellar medium

We numerically analyze a magnetohydrodynamic, steady-state model for the interaction of a spherically symmetric solar wind with a three-component local interstellar medium (LISM), which is composed of plasma, hydrogen atoms, and a magnetic field. The magnetic field is assumed to be parallel to the velocity in the LISM. In this case, the model is axisymmetric. We study the effects of magnetic field on the plasma-flow geometry and on the distribution of hydrogen-atom parameters. In particular, we show that the presence of hydrogen atoms does not affect the qualitative change in the shape of the bow shock, the heliopause, and the solar-wind shock with increasing strength of the interstellar magnetic field. The presence of a magnetic field in the LISM can strongly affect the parameters of the energetic hydrogen atoms originated in the solar wind, although its effect on the “hydrogen wall” observed with the GHRS instrument onboard the HST spacecraft (Linsky and Wood 1996) is marginal.     

The termination shock in a striped pulsar wind

The origin of radio emission from plerions is considered. Recent observations suggest that radio-emitting electrons are presently accelerated rather than having been injected at early stages of the plerion evolution. The observed flat spectra without a low-frequency cut-off imply an acceleration mechanism that raises the average particle energy by orders of magnitude but leaves most of the particles at an energy of less than approximately a few hundred MeV. It is suggested that annihilation of the alternating magnetic field at the pulsar wind termination shock provides the necessary mechanism. Toroidal stripes of opposite magnetic polarity are formed in the wind emanating from an obliquely rotating pulsar magnetosphere (the striped wind). At the termination shock, the flow compresses and the magnetic field annihilates by driven reconnection. Jump conditions are obtained for the shock in a striped wind. It is shown that the post-shock magnetohydrodynamic parameters of the flow are the same as if the energy of the alternating field had already been converted into plasma energy upstream of the shock. Therefore, the available estimates of the ratio of the Poynting flux to the matter energy flux, σ, should be attributed not to the total upstream Poynting flux but only to that associated with the average magnetic field. A simple model for the particle acceleration in the shocked striped wind is presented.     

Dispersive shock waves in the solar wind

Compressional waves in the solar wind propagating over large distances are likely to steepen into shock waves where the increase in the amplitude is balanced by dissipation. Dispersive effects caused by, e.g. Hall currents perpendicular to the ambient magnetic field can influence the generation and propagation of shock waves. In the present study the dispersion is considered weak but in time its importance can grow. When the effect of dispersion is strong enough, it can balance the nonlinear steepening of waves leading to the formation of solitons. The obtained results show that the weak dispersion will alter the amplitude and propagation speed of the shock wave. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Acceleration of pickup interstellar protons by shock waves in corotating solar wind regions

It follows from numerous measurements of the differential fluxes of energetic charged particles in corotating interaction regions between solar wind streams with different speeds that the spectra of particles accelerated by reverse shocks are harder than those of particles accelerated by forward shocks. The measurements cannot be explained in terms of the theory of diffusive acceleration (first-order Fermi acceleration). We show that the measurements can be easily explained in terms of the theory of drift acceleration of charged particles by shock waves with allowance made for their multiple scattering from the front.     

Geo-effectiveness of solar wind extremes

Examples of extreme events of solar wind and their effect on geomagnetic conditions are discussed here. It is found that there are two regimes of high speed solar wind streams with a threshold of ∼ 850 km s^-1. Geomagnetic activity enhancement rate (GAER) is defined as an average increase in Ap value per unit average increase in the peak solar wind velocity (Vp) during the stream. GAER was found to be different in the two regimes of high speed streams with +ve and-ve IMF. GAER is 0.73 and 0.53 for solar wind streams with +ve and -ve IMF respectively for the extremely high speed streams (< 850 km s^-1). This indicates that streams above the threshold speed with +ve IMF are 1.4 times more effective in enhancing geomagnetic activity than those with -ve IMF. However, the high speed streams below the threshold with -ve IMF are 1.1 times more effective in enhancing geomagnetic activity than those with +ve IMF. The violent solar activity period (October–November 2003) of cycle 23 presents a very special case during which many severe and strong effects were seen in the environment of the Earth and other planets; however, the z-component of IMF (Bz) is mostly positive during this period. The most severe geomagnetic storm of this cycle occurred when Bz was positive.     

Slow solar wind: Sources and components of the stream structure at the solar maximum

We study the sources and components of the solar-wind spatial stream structure at the maximum of the solar cycle 23. In our analysis, we use several independent sets of experimental data: radio-astronomical observations of scattered radiation from compact sources with the determination of the distance from the Sun to the inner boundary of the transonic-flow transition region (R_in); calculated data on the magnetic-field intensity and structure in the solar corona, in the solar-wind source region, obtained from optical measurements of the photospheric magnetic-field intensity at the Stanford Solar Observatory (USA); and observations of the white-light corona with the LASCO coronograph onboard the SOHO spacecraft. We show that at the solar maximum, low-speed streams with a transition region located far from the Sun dominate in the solar-wind structure. A correlation analysis of the location of the inner boundary R_in and the source-surface magnetic-field intensity |B _R | on a sphere R=2.5R_S (R_S is the solar radius) has revealed the previously unknown lowest-speed streams, which do not fit into the regular relationship between the parameters R_in and |B _R |. In the white-light corona, the sources of these streams are located near the dark strip, a coronal region with a greatly reduced density; the nonstandard parameters of the streams probably result from the interaction of several discrete sources of different types.     

Acceleration of anomalous cosmic rays at the heliospheric termination shock

Astronomy Letters - The acceleration of anomalous cosmic rays (ACRs) at the heliospheric termination shock and their influence on the shock structure and location are analyzed in terms of a...     

Influence of solar heliospheric parameters on cosmic rays anisotropy

In the present work the cosmic ray data of three different neutron monitoring stations, Deep River, Inuvik, and Tokyo, located at different geomagnetic cutoff rigidities and altitudes have been harmonically analyzed for the period 1980–95 for a comparative study of diurnal semi-diurnal and tri-diurnal anisotropies in cosmic ray intensity in connection with the change in interplanetary magnetic field Bz component and solar wind velocity on 60 quietest days. It is observed that the amplitudes of all the three harmonics increase during the period 1982–84 at all the stations during the high speed solar wind stream epoch and remain low during the declining phase of the stream. The amplitudes of the three harmonics have no obvious characteristics associated with the time variation of magnitude of the Bz component. The phases of all the three harmonics have no time variation characteristics associated with solar wind velocity and Bz. Published in Astrofizika, Vol. 49, No. 4, pp. 651–664 (August 2006).     

A bimodal model of the solar wind speed

Until the ULYSSES spacecraft reached high latitude, the only means for measuring the solar wind velocity in the polar regions was from radio scattering observations (IPS), and these remain the only way to measure the velocity near the sun. However, IPS, like many remote sensing observations, is a line-of-sight integrated measurement. This integration is particularly troublesome when the line-of-sight passes through a fast stream but that stream does not occupy the entire scattering region. Observations from the HELIOS spacecraft have shown that the solar wind has a bimodal character which becomes more pronounced near the sun. Recent observations from ULYSSES have confirmed that this structure is clear at high latitudes even at relatively large solar distances. We have developed a method of separating the fast and slow contributions to an IPS observation which takes advantage of this bimodal structure. In this paper I will describe the technique and its application to IPS observations made using the receiving antennas of the EISCAT incoherent backscatter radar observatory in northern Scandinavia.     

Remote sensing of the heliospheric solar wind using radio astronomy methods and numerical simulations

The ground-based radio astronomy method of interplanetary scintillations (IPS) and spacecraft observations have shown, in the past 25 years, that while coronal holes give rise to stable, reclining high speed solar wind streams during the minimum of the solar activity cycle, the slow speed wind seen more during the solar maximum activity is better associated with the closed field regions, which also give rise to solar flares and coronal mass ejections (CME&amp;#x2019;s). The latter events increase significantly, as the cycle maximum takes place. We have recently shown that in the case of energetic flares one may be able to track the associated disturbances almost on a one to one basis from a distance of 0.2 to 1 AU using IPS methods. Time dependent 3D MHD models which are constrained by IPS observations are being developed. These models are able to simulate general features of the solar-generated disturbances. Advances in this direction may lead to prediction of heliospheric propagation of these disturbances throughout the solar system.     

Coronal expansion and the transfer of solar angular momentum to the interplanetary space

We discuss the question of loss of angular momentum through coronal expansion. From a large volume of data on Type-1 cometary tails we have confirmed the presence of a tangential component in the coronal expansion, which has not only a stochastic component but also a constant component of 9.8 km/s. Through coronal expansion the Sun has lost 80% of its angular momentum since it evolved on to the main sequence and the angular velocity of the Sun is decreasing exponentially. This result should have a large effect on the dynamical evolution of the Sun.     

Magnetosonic waveguide model of solar wind flow tubes

We consider solar wind flow tubes as a magnetosonic wave-guide. Assuming a symmetric expansion in edges of slab-modelled wave-guide, we study the propagation characteristics of magnetosonic wave in the solar wind flow tubes. We present the preliminary results and discuss their implications.     

行星际磁通量绳尺度及能量研究

行星际磁通量绳是太阳风中一种重要的磁结构.从1995-2001年的Wind卫星的观测资料中认证了144个行星际磁通量绳.其时间尺度介于几十分钟到几十小时之间,其空间尺度呈现连续分布.通过估算磁通量绳单位长度的能量和总能量发现:磁通量绳的能量分布和耀斑的类似都呈现很好的幂率谱.通过讨论行星际磁通量绳和太阳活动爆发的关系,建议所有的小、中、大尺度通量绳都直接起源于太阳上的爆发,和磁云对应于通常的日冕物质抛射一样,中、小尺度的通量绳对应相对较小的日冕物质抛射.     

Heliospheric magnetic fields, energetic particles, and the solar cycle

The heliosphere is the region filled with magnetized plasma of mainly solar origin. It extends from the solar corona to well beyond the planets, and is separated from the interstellar medium by the heliopause. The latter is embedded in a complex and still unexplored boundary region. The characteristics of heliospheric plasma, fields, and energetic particles depend on highly variable internal boundary conditions, and also on quasi-stationary external ones. Both galactic cosmic rays and energetic particles of solar and heliospheric origin are subject to intensity variations over individual solar cycles and also from cycle to cycle. Particle propagation is controlled by spatially and temporally varying interplanetary magnetic fields, frozen into the solar wind. An overview is presented of the main heliospheric components and processes, and also of the relevant missions and data sets. Particular attention is given to flux variations over the last few solar cycles, and to extrapolated effects on the terrestrial environment.     

Phasespace transport of a quasi-neutral multi-fluid plasma over the solar wind MHD termination shock

We study the physics of a multi-ion MHD shock, i.e. an MHD shock feature that forms when a supersonic flow of mixed ion populations is forced to adapt itself to a pressure obstacle further downstream. We shall describe this situation by using a multi-fluid approach for a mixture of ion populations with different specific masses and charges per ion species. First we calculate the effective electric potential that forces the plasma bulk to decelerate to the downstream bulk flow velocity which also then defines that system into which the downstream magnetic field is frozen-in. Then we calculate the unavoidable ion-specific overshoot velocities and gain from them, requesting energy conservation, the ion-specific contributions to the downstream thermal energies and pressures. The aim thereby is to find the solution for the MHD status of the downstream flow of the plasma mixture, specifically for a proton-electron plasma. We derive an implicit equation for the effective compression ratio and explicit relations for the different, downstream ion and electron temperatures as function of the multi-fluid compression ratio s. The resulting actual multi-fluid compression ratio s _eff is found by adding up all the partial downstream pressures and comparing it with the upstream ram pressure. As we can show, the electron pressure is the dominant contribution to the total downstream plasma pressure.     

Parametric generation of magnetoacoustic-gravity waves in the solar atmosphere

Based on a plane-parallel isothermal solar model atmosphere permeated by a horizontal magnetic field whose strength is proportional to the square root of the plasma density and in the approximation of a specified field for vertically propagating and nonpropagating magnetoacoustic-gravity waves, we consider the nonlinear interaction between the corresponding disturbances, to within quantities of the second order of smallness. We investigate the efficiency of the nonlinear generation of waves at difference and sum frequencies and of an acoustic flow (wind) as a function of the magnetic-field strength and the excitation frequency of the initial disturbances at the lower atmospheric boundary.     

Determining the rotation law of the solar convection zone

The condition of minimum total dissipation is used to derive stationary rotation and azimuthal magnetic field distributions in the bulk of the solar convection zone with an upper boundary at which the relative radius is r/R=0.95. General equilibrium con figurations with symmetric and antisymmetric (about the equator) angular-velocity and field components are determined. The calculated rotation law matches the observed one in general parameters, but the decrease in angular velocity at high latitudes in theory is larger than that in observations. Besides, there are additional sharp variations in the rotation and field distributions in the theoretical curves near the generation zone of solar torsional waves. The possible cause of the latter discrepancy is discussed. The change in equilibrium distributions due to the presence of an inverse molecular-weight gradient at the base of the convection zone is also studied. This gradient is known to be produced by accelerated gravitational helium settling in the convection zone.