The acceleration of minor ion species in the solar wind

This paper provides a comprehensive analysis of the dynamics of the flow of minor ion species in the solar wind under the combined influences of gravity, Coulomb friction (with protons), rotational forces (arising from the Sun's rotation and the interplanetary spiral magnetic field) and wave forces (induced in the minor ion flow by Alfvén waves propagating in the solar wind). It is assumed that the solar wind can be considered as a proton-electron plasma which is, to a first approximation, unaffected by the presence of minor ions. In the dense hot region near the Sun Coulomb friction accelerates minor ions outwards against the gravitational force, part of which is cancelled by the charge-separation electric field. Once the initial acceleration has been achieved, wave and rotational forces assist Coulomb friction in further increasing the minor ion speed so that it becomes comparable with, or perhaps even exceeds, the solar wind speed. A characteristic feature of the non-resonant wave force is that it tends to bring the minor ion flow into an equilibrium where the radial speed matches the Alfvén speed relative to the solar wind speed, whereas Coulomb friction and rotational forces tend to bring the flow into an equilibrium where the radial speed of the minor ions equals the solar wind speed. Therefore, provided that there is sufficient wave energy and Coulomb friction is weak, the minor ion speed can be trapped between these two speeds. This inteststing result is in qualitative agreement with observational findings to the effect that the differential flow speed between helium ions and protons is controlled by the ratio of the solar wind expansion time to the ion-proton collision time. If the thermal speeds of the protons and minor ions are small compared to the Alfvén speed, two stable equilibrium speeds can exist because the rapid decrease in the Coulomb cross-section with increasing differential flow speed allows the non-resonant wave force to balance Coulomb friction at more than one ion speed. However, it must be emphasized that resonant wave acceleration and/or strong ion partial pressure gradients are required to achieve radial speeds of minor ions in excess of the proton speed, since, as is shown in Section 4, the non-resonant wave acceleration on protons and minor ions are identical when their radial speeds are the same, with the result that, in the solar wind, non-resonant wave acceleration tends (asymptotically) to equalize minor ion and proton speeds.     

Viscous flow properties in the transport of solar wind momentum to the Venus upper ionosphere

A comparative study of the viscous transport of solar wind momentum to the upper layers of the Venus ionosphere with that occurring within the trans-terminator flow leads to estimates of the ratio of the viscosity coefficients that are applicable to both cases. Support for viscous forces between the solar wind and the ionospheric plasma in the trans-terminator flow derives from the momentum flux balance between the momentum flux in the latter flow and the deficiency of solar wind momentum along the flanks of the ionosheath. By comparing the relative width of the viscous boundary layer in the Venus ionosheath and the width of the trans-terminator flow we find that the transport of momentum within the upper ionosphere proceeds at a rate similar to that at which momentum is delivered to the upper ionosphere from the solar wind. Comparable values are obtained for the viscosity coefficient of the solar wind that streams over the ionosphere and that implied from momentum transport within the ionospheric trans-terminator flow. It is further suggested that despite the different nature of the processes that give place to the viscous transport of the solar wind momentum to the upper ionosphere (wave-particle interactions) and those responsible for its distribution within the ionosphere (through coulombian collisions) there is a similar response in the behavior of both plasmas to momentum transport. Calculations show that with comparable values of the viscosity coefficient in the ionosheath and in the upper ionospheric plasma the mean free path suitable to wave-particle interactions in the ionosheath is of the same order of magnitude as the mean free path of the planetary O⁺ ions that interact through coulombian collisions in the upper ionosphere. The effects of this similarity are considered in the discussion.     

Energy balance of the corona and the origin of quasi-stationary high-speed solar wind streams

The energy balance of open-field regions of the corona and solar wind and the influence of the flow geometry in the corona upon the density and temperature, are analyzed. It is found that the energy flux arriving at the corona is constant for the corona's open regions with different flow geometries. For the waves heating the corona and solar wind, the dependence of the absorption coefficient on the corona's plasma density is found to be within the range of distances r=1.05–1.5R . It is shown that the wave absorption is more dependent on electron density than the coronal emission. It is this difference that causes lower-density coronal holes to be colder than quiet regions. It is found that the additional energy flux necessary for providing energy balance of the corona and for producing solar wind is a flux of Alfvén waves, which can provide the energy needed for producing quasi-stationary high-speed solar wind streams. Theoretical models of coronal holes and the question of why the high-speed solar wind streams are precisely flowing out of coronal holes, are discussed.     

The influence of the anomalous cosmic-ray component on the dynamics of the solar wind

It is well known that both the galactic and anomalous cosmic rays show positive intensity gradients in the outer heliosphere which are connected with corresponding pressure gradients. Due to an efficient dynamical coupling between the solar wind plasma and these highly energetic media by means of convected MHD turbulences, there exists a mutual interaction between these media. As one consequence of this scenario the enforced pressure gradients influence the distant solar wind expansion. Here we concentrate in our theoretical study on the interaction of the solar wind only with the anomalous cosmic-ray component. We use the standard two-fluid model in which the cosmic-ray fluid modifies the solar wind flow via the cosmic-ray pressure gradient. Then we derive numerical solutions in the following steps: first we calculate an aspherical pressure distribution for the anomalous cosmic rays, describing their diffusion in an unperturbed radial solar wind. Second, we then consider the perturbation of the solar wind flow due to these induced anomalous cosmic-ray pressure gradients. Within this context we especially take account of the action of a non-spherical geometry of the heliospheric shock which may lead to pronounced upwinddownwind asymmetries in the pressures and thereby in the resulting solar wind flows. As we can show in our model, which fits the available observational data, radial decelerations of the distant solar wind by between 5 to 11% are to be expected, however, the deviations of the bulk solar wind flow from the radialdirections are only slightly pronounced.     

Stream interaction as a heat source in the solar wind

Proton heating at stream interaction regions in the solar wind is investigated based on the solar wind data obtained by Suisei spacecraft between 0.68 and 1.01 AU from the Sun. The deflection angle of the solar wind flow in the ecliptic plane is used to identify the interaction region. In the solar wind flows coming from east of the Sun in low-speed streams and coming from west in high-speed streams, the radial gradient of proton temperature is flattened owing to heating in the interaction region. From comparison of the best-fitted power law dependence of proton temperature on the radial distance in the deflected flow with that in the non-deflected flow, it is suggested that heating in the interaction regions starts around 0.6–0.7 AU from the Sun.     

Compressive Instability Driven by Ion-Cyclotron Dissipation in the Solar Wind

Recent models of the heating and acceleration of the fast solar wind invoke the dissipation of relatively high-frequency waves by ion-cyclotron absorption. It is shown that modulations in the heating rate induced by compressive perturbations in the flow may drive the system unstable in the supersonic region. Therefore it is possible that density fluctuations generated by this instability and maintained at a level determined by the marginal stability condition are manifest in the wind.     

Manifestation of solar activity in solar wind particle flux density

An analysis has been made of the origin of long-term variations in flux density of solar wind particles (nv) for different velocity regimes. The study revealed a relationship of these variations to the area of the polar coronal holes (CH). It is shown that within the framework of the model under development, the main longterm variations of nv are a result of the latitude redistribution of the solar wind mass flux in the heliosphere and are due to changes in the large-scale geometry of the solar plasma flow in the corona.

A study has been made of the variations of nv for high speed solar wind streams. It is found that nv in high speed streams which are formed in CH, decreases from minimum to maximum solar activity. The analysis indicates that this decrease is attributable to the magnetic field strength increase in coronal holes.

It has been found that periods of rapid global changes of background magnetic fields on the Sun are accompanied by a reconfiguration of coronal magnetic fields, rapid changes in the length of quiescent filaments, and by an increase in the density of the particle flux of a high speed solar wind. It has been established that these periods precede the formation of CH, corresponding to the increase in solar wind velocity near the Earth and to enhancement of the level of geomagnetic disturbance.     

A comment on the detection of closed magnetic structures in the solar wind

It has recently been suggested that the large scale structure of the interplanetary magnetic field can be deduced solely from solar wind speed measurements. Here it is emphasized that, in addition to speed measurements, direct measurements of the interplanetary field and indirect diagnostics such as measurements of the solar wind kinetic temperature and galactic and solar energetic particle modulations and anisotropics are required to distinguish between open and closed magnetic structures in the solar wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Small Solar Wind Transients and Their Connection to the Large-Scale Coronal Structure

It has been realized for some time that the slow solar wind with its embedded heliospheric current sheet often exhibits complex features suggesting at least partially transient origin. In this paper we investigate the structure of the slow solar wind using the observations by the Wind and STEREO spacecraft during two Carrington rotations (2054 and 2055). These occur at the time of minimum solar activity when the interplanetary medium is dominated by recurrent high-speed streams and large-scale interplanetary coronal mass ejections (ICMEs) are rare. However, the signatures of transients with small scale-sizes and/or low magnetic field strength (comparable with the typical solar wind value, ～?5 nT) are frequently found in the slow solar wind at these times. These events do not exhibit significant speed gradients across the structure, but instead appear to move with the surrounding flow. Source mapping using models based on GONG magnetograms suggests that these transients come from the vicinity of coronal source surface sector boundaries. In situ they are correspondingly observed in the vicinity of high density structures where the dominant electron heat flux reverses its flow polarity. These weak transients might be indications of dynamical changes at the coronal hole boundaries or at the edges of the helmet streamer belt previously reported in coronagraph observations. Our analysis supports the idea that even at solar minimum, a considerable fraction of the slow solar wind is transient in nature.     

RADIO MAPS OF THE SOLAR WIND TRANSITION REGION

Occultation studies of near-Sun plasmas using several natural sources simultaneously result in large-scale patterns, radio maps of the solar wind flow. Large radio telescopes of the P.N. Lebedev Physical Institute, Pushino, were used. Previously it had been shown that the plasma acceleration and traverse of the sound barrier proceed in an extended region, the transition region of the solar wind, located at radial distances of about 10–40 solar radii from the Sun. The 1989–1994 experiments showed that the evolution of the transition region geometry is very close to that of the optical corona. On the other hand, the plasma flow structures characteristic of the transition region persist in the course of the 11-year cycle, which demonstrates the existence of some specific mechanism of the solar wind acceleration, independent of wide variations of the general solar activity state. These experimental facts are discussed in connection with the existing theoretical approaches.     

Long time delay between the peaks of intense solar hard X-ray and microwave bursts

New solar wind data from Helios-2 are used to study, in a statistical fashion, the relation between proton number density n, flow speed u and heliocentric distance r. It is shown that the average of nu ² r ² does not depend on flow speed nor on distance, verifying the previously established invariance of momentum flux density (mnu²) carried by the solar wind. Averages of mnu² from different spacecraft do not show correlation with the solar cycle. Rather, the close agreement (to within 1.8%) of values from Helios-1 and Helios-2 suggests that the momentum flux density carried by the solar wind may be also constant during the solar cycle.     

太阳风离子尺度湍流与太阳风加热

太阳风源自太阳大气,在行星际空间传播过程中被持续加热,然而究竟是何种能量加热了太阳风至今未研究清楚.太阳风普遍处于湍动状态,其湍动能量被认为是加热太阳风的重要能源.然而,太阳风湍流通过何种载体、基于何种微观物理机制加热了太阳风尚不明确,这是相关研究的关键问题.将回顾人类对太阳风加热问题的研究历史,着重介绍近年来我国学者在太阳风离子尺度湍流与加热方面取得的研究进展,展望未来在太阳风加热研究中有待解决的科学问题和可能的研究方向.     

The iron,silicon and oxygen abundance in the solar wind measured with SOHO/CELIAS/MTOF

From data collected with the MTOF sensor of the CELIAS instrument on board the SOHO spacecraft we derived the elemental abundance ratios for Si/O and Fe/O in the solar wind with high time resolution. Since Si and Fe are elements with a low first ionization potential (FIP) and oxygen is a high FIP element, these abundance ratios are valuable diagnostic tools for the study of the FIP fractionation process. The abundance ratios we find for slow and fast solar wind are commensurate with published values for interstream and coronal hole type solar wind. Between these two extreme cases of solar wind flow we find a continuous decrease of the abundance ratios for increasing solar wind speed, from a high value indicative of solar wind originating from the streamer belt to low values associated with flow from coronal holes.     

Electrostatic ion cyclotron waves in an anisotropic plasma

In the solar wind, electrostatic ion cyclotron waves can be excited, by electrons or ions when the flow velocity becomes supersonic. The instability of these waves is investigated for a situation in which ions are streaming in opposite directions along the interplanetary magnetic field in a uniform background of relatively stationary electrons. Many modes become unstable under the existing conditions. It is conjectured that the excitation of this instability may lead to a steady state electrostatic turbulence in the solar wind.     

On solar wind interaction with the earth's magnetosphere

Hydrodynamic and electrodynamic problems of solar wind interaction with the Earth's magnetosphere on the day-side are investigated.The initial fact, well established, is that the density of the magnetic field energy in the solar wind is rather small. Magnetic field intensity and orientation are shown to determine the character of the solar wind flow around the magnetosphere. For mean parameters of the wind, if the tangential component of the magnetic field is more or equal 5γ, the flow in the magneto-sheath will be laminar. For other cases the flow is of a turbulent type.For turbulent flow, typical plasma parameters are estimated: mean free path, internal scale of inhomogeneities and dissipated energy. The results obtained are compared with experimental data.For the case of laminar flow, special attention is paid to the situation when magnetic fields of the solar wind and Earth are antiparallel. It is suggested, on the basis of solid arguments, that the southward interplanetary field diffuses from the magnetosheath into the Earth's magnetosphere. These ideas are used for the estimation of the distance to the magnetopause subsolar point. A detailed comparison with results of observation is made. The coincidence is satisfactory. Theoretical investigation has been made to a great extent for thin magnetopause with thickness δ ～ R_He-gyroradius of an electron.It is shown that during magnetospheric substorms relaxation oscillations with the period τ = 100–300 sec must appear. A theorem is proved about the appearance of a westward electrical field during the substorm development, when the magnetosphere's day-side boundary moves Earthward and about the recovery phase, when the magnetopause motion is away from the Earth, when there is an eastward electrical field.In the Appendix, plasma wave exitation in the magnetopause is considered and conductivity magnitudes are calculated, including the reduction due to the scattering by plasma turbulence.     

Constraints on the solar coronal temperature in regions of open magnetic field

It is shown that the simultaneous consideration of observed values of the solar wind proton flux density at 1 AU and of the electron pressure at the base of the solar corona leads to relatively strong constraints on the coronal temperature in the region of subsonic solar wind flow. The extreme upper limit on the mean coronal temperature in the subsonic region is found to be about 2.6 × 10⁶ K, but this upper limit is reduced to about 2.0 × 10⁶ K if reasonable, rather than extreme, assumptions are made; the limit on the maximum temperature is about 0.5 × 10⁶ K greater than the limit on the mean. It is also found that the same two observations limit the rate of momentum addition possible in the region of subsonic solar wind flow.On leave from The Auroral Observatory, Institute of Mathematical and Physical Sciences, University of Troms0, N-9001 Tromsø, Norway.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

On the classification of comet plasma tails

The investigation of plasma tails of comets is an important part of comet research. Different classifications of plasma tails of comets are proposed. Plasma acceleration in the tails is investigated in sufficient detail. Several cometary forms are explained. Plasma tails of Mars and Venus were observed during the first studies of these planets. They are associated with the capture of ionized atoms and exosphere molecules by the solar wind magnetized plasma flow. Distinct plasma tails of Mars and Venus are caused by the mass loading of the solar wind with heavy ions. It was shown that the transverse dimension of the tails of Mars, Venus, and comets can be quite accurately determined by production rate of the obstacle to the solar wind flow. While plasma tails of Mars and Venus are investigated by in situ measurements from spacecraft, observations of comet tails from the Earth make it possible to see the entire object under study and to monitor changes in its structure. A certain similarity of cometary and planetary tails can be explained by the nonmagnetic nature of both types of bodies. Thus, it is reasonable to suppose that investigations of plasma tails of comets can supplement the information obtained by in situ methods of the study of the planets. In this paper, plasma tails of comets, presumably analogous to the plasma tails of Mars and Venus, have been identified on modern photographs of comets (more than 1500 photographs viewed). Only quasi-steady laminar tails are considered. They are divided into two types: double structures and outflows. The paper attempts to define the 3D structure of double structures and to determine certain characteristics of outflows.     

Phase mixing of standing Alfvén waves with shear flows in solar spicules

Alfvénic waves are thought to play an important role in coronal heating and solar wind acceleration. Here we investigate the dissipation of such waves due to phase mixing at the presence of shear flow and field in the stratified atmosphere of solar spicules. The initial flow is assumed to be directed along spicule axis and to vary linearly in the x direction and the equilibrium magnetic field is taken 2-dimensional and divergence-free. It is determined that the shear flow and field can fasten the damping of standing Alfvén waves. In spite of propagating Alfvén waves, standing Alfvén waves in Solar spicules dissipate in a few periods. As height increases, the perturbed velocity amplitude does increase in contrast to the behavior of perturbed magnetic field. Moreover, it should be emphasized that the stratification due to gravity, shear flow and field are the facts that should be considered in MHD models in spicules.