The outflow of solar wind from the active regions

The numerical integration of hydrodynamics equations with an allowance for thermal conductivity was made using the temperature distribution in the corona situated above the active regions obtained from the damping time of solar radio bursts of Types III and V. It is essential that for the integration path serve the magnetic field lines along which exciters of bursts are moving and accelerated coronal plasma can move freely too.The main result is the discovery of such regions, where the high temperature gradient precludes the possibility of a continuous flow of coronal plasma. These regions, where intense heating and rapid acceleration of the coronal plasma take place, were situated at distances of about 2 R from the Sun's center. They probably possess the character of weak detonation waves. The waves of cooling can also be present in these regions of discontinuity of the flow. The observations of bursts of Type V at distances up to 6.3 R gives some evidence that discontinuities of flow of the solar wind of the same nature can possibly arise also in the more remote parts of the solar corona.It is important that the similar jumps of velocity and other parameters of coronal plasma were also discovered earlier in a quite independent way as a result of the interpretation of the solar radio echo data. It can be anticipated that the nonthermal heating of coronal plasma, which was postulated to remove discrepancies between the existing models and observations of solar wind, was localized mainly in these regions thus playing an important role in the formation of the fundamental properties of the interplanetary medium.The obtained results are of preliminary character since there are no reliable and homogeneous data on bursts of Types III and V especially at 20-10 MHz, where the work is difficult due to the man-made interference and also at still lower frequencies, observed by the cosmic probes. We can hope that the filling of this gap allows us to construct a realistic model of outflow of coronal plasma from active regions, which can be successfully compared with the results of direct measurement of parameters of solar wind.     

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.     

Magnetically trapped particles in the lower solar atmosphere

The trapping of energetic electrons and protons in a simple, arched magnetic field imbedded in the lower solar atmosphere was considered. The lifetime of electrons with kinetic energies up to about 1.5 MeV was found to be completely determined by the motion of the mirror points, provided the gyro-synchrotron loss can be neglected. The same motion also influences the lifetimes of more energetic electrons, up to 10 MeV. This was not found to be the case for protons in the range from 1 MeV to 100 GeV. Some fluid and streaming instabilities were also considered; they pull the particles upward, raise their mirror points, and increase their lifetime. The emission of gyro-synchrotron radiation and bremsstrahlung in this model has been related to observations. Using the duration of non-thermal X-ray peaks given by Kane (1969), the altitude of injection of energetic particles was estimated.     

Wave-trains in the solar wind

Applying an Alfvén-Wave-Extended-QRH-approximation and the method of characteristics, we solve the equations of motion for outwardly propagating Alfvén waves analytically for three different cases of an azimuthal dependence of the background solar wind, (a) for a pure fast-slow stream configuration, (b) for the situation where the high-speed stream originates from a diverging magnetic field region, and (c) for the case of (b) and an initially decreasing density configuration (‘coronal hole’). The reaction of these waves on the background state as well as mode-mode coupling effects are neglected. These three solar wind models are discussed shortly. For the superimposed Alfvén waves we find, on an average, that there is a strong azimuthal dependence of all relevant wave parameters which, correlated with the azimuthal distributions of the solar wind variables, leads to good agreements with observations. The signature of high-speed streams and these correlations could clearly indicate solar wind streams originating from ‘coronal holes’. Contrary to the purely radial dependent solar wind, where outwardly propagating Alfvén waves are exclusively refracted towards the radial direction, we now find a refraction nearly perpendicular to the direction of the interplanetary magnetic field in the compression region and closely towards the magnetic field direction down the trailing edge and in the low-speed regime.     

Wave-trains in the solar wind

The equations of motion of all relevant parameters of Alfvén waves propagating from the sun outwardly into the expanding interplanetary medium are discussed for the case of a quiet, ideal, isotropic, one-fluid solar wind plasma. It is found that the frequency of the wave reamains constant, while the wave vector and the amplitudes depend, in general, on the evolution of the background medium and on the angle between the wave vector and the interplanetary magnetic field. This latter dependence cancels approximately for the evolution of the amplitudes in the case of a pure, overall spiral magnetic field. It is shown that in this case the results of earlier discussions can be derived by less decisive restrictions.     

Wave-trains in the solar wind

The main results of Whitham's averaged Lagrangian method for the treatment of linear wave-trains in a weakly inhomogeneous, moving medium are presented briefly. This method is then applied to an ideal, isotropic, one-fluid plasma which can be taken for the lowest order approximation for the interplanetary solar wind expansion.     

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.     

Hydromagnetic shocks in the solar wind

The Rankine-Hugoniot relations are applied to shock-like discontinuities measured by both magnetic field and plasma instruments on the satellite Explorer 34 between May 30, 1967 and Jan. 11, 1968.Shock normals were either determined from the magnetic field observations, or from the times of occurrence of the discontinuity at Explorers 33, 34 and 35. The Rankine-Hugoniot relations are obeyed to the accuracy of the observations, and the values of shock velocities, density ratios, and Mach numbers indicate that at 1 AU the typical interplanetary shock is not strong, although all the events studied caused geomagnetic impulses.     

Cyclotron waves in the solar wind

Cyclotron waves in the solar wind near 1 AU with frequencies well below the electron cyclotron frequency and wavelengths much larger than the electron cyclotron radius but less than the proton cyclotron radius are considered. The cyclotron radii are defined from parallel thermal velocity of electron component and proton component with respect to the interplanetary magnetic field. No LH cyclotron waves are found to propagate for _p < 0, where _p 1 –T _p/T _p is the temperature anisotropy of the proton component with respect to the interplanetary magnetic field. The damping or growth of RH cyclotron waves is found to depend on the frequency range and the temperature anisotropy of the proton component. The RH cyclotron waves are damped in the frequency range _r | _p | _p for _p < 0, where _p is the proton cyclotron frequency. RH cyclotron instabilities occur in the frequency range | _p | _p > _r > | _p | _p /(1– _r ) for _p < 0. The marginal state is at _r =| _p | _p .Abstract presented at theInternational Symposium on Solar-Terrestrial, São Paulo, Brazil, 17–22 June, 1974     

Heat transport in the solar wind

Heat transport is considered both for quiet and disturbed solar winds. It is shown that heat may be transferred during solar flares by sharp fronted thermal wave pulses. Energy dissipation in the wave front arises from the firehose instability excitation. The effects of ionosonic turbulence on heat transport in a quiet solar wind are also investigated. A quasi-steady state, in which there is a balance between wave-particle interations and particle collisions is found. It is shown that the effect of wave-particle ‘collisions’ is to produce a significant decrease of the electron heat flow and electron temperature, and increase of the ion temperature relative to calculations which take into account particle particle collisions only.     

Helium abundance in the solar wind

Observations of hydrogen and helium ions in the solar wind have been carried out by the Goddard Space Flight Center - University of Maryland plasma instrument on Explorer 34. These ions are completely separated by means of electrostatic and magnetic fields. The average value of the ratio of number densities is 0.051 ± .02, derived from over 3000 h of measurement. Variations about this value from about 0.01 up to greater than 0.15 occur, and there are more high values than can be explained by random variation. A tentative association with some geomagnetic storms is suggested. The above value of abundance, assuming that plasma emitted in the ecliptic plane is a fair sample of the output of the sun, combined with other recent work by other methods indicate that the solar abundance may be about half the previously quoted estimates of approximately 0.1.     

Corotating structure in the solar wind

The hydrodynamic equations which describe the radial solar wind expansion are linearized and specialized to treat corotating perturbations. Approximate solutions are found which are time stationary in the corotating reference frame. The solutions predict the behavior of corotating structures for a given boundary condition close to the sun. In particular, the structure resulting from the interaction of fast and slow streams is described. Comparison with sector structure data shows reasonable qualitative and quantitative agreement.     

Tangential discontinuities in the solar wind

This paper considers six discontinuity surfaces which were observed by magnetometers on 3 spacecraft in the solar wind. It is shown that the actual surface orientations, determined from the measured time delays and solar wind speed, are consistent with the theoretical orientations which were computed from the relation , where is the normal to the surface of a hydromagnetic tangential discontinuity across which the magnetic field direction changes from to . The plasma and magnetic field data for these discontinuities are consistent with the pressure balance condition, and the magnetic field vectors in the associated current sheets are parallel to the discontinuity surface, as required theoretically. The 6 discontinuity surfaces extended without much distortion over ∼ 0.002 AU. A seventh surface is discussed which satisfies the condition but which extended without much distortion over 0.01 AU. This latter is not a typical surface, however, and its curvature is larger than average. Most of the surfaces tended to lie along the spiral direction, but one was nearly perpendicular to the spiral direction.     

Minor ions in the solar wind

Ions heavier than ⁴He are treated as “minors” in the solar wind. This is justified for many applications since minor ions have no significant influence on the dynamics of the interplanetary plasma. However, minor ions carry information on many aspects of the formation, on the acceleration and on the transfer of solar plasma from the corona into the interplanetary space. This review concentrates on various aspects of minor ions as diagnostic tracers. The elemental abundance patterns of the solar wind are shaped in the chromosphere and in the lower transition region by processes, which are not fully understood at this moment. Despite this lack of detailed understanding, observed abundance patterns have been classified and are now commonly used to characterize the sources, and to trace back solar-wind flows to their origins in the solar atmosphere. Furthermore, the solar wind is the most important source of information for solar isotopic abundances and for solar abundances of volatile elements. In order to fully exploit this information, a comprehensive understanding of elemental and isotopic fractionation processes is required. We provide observational clues to distinguish different processes at work.     

Geoactive zones in the solar wind

Three parameters of the solar wind, proton number density n, Z-component of frozen-in magnetic field, in solar ecliptic coordinates and magnetic field variability ΔB, may be called geoactive parameters since each of them is responsible for a certain phase or stage of a geomagnetic storm.An undisturbed solar corpuscular stream differs from the quiet solar wind mainly in higher bulk velocity v; other parameters, in particular, n, Z and ΔB, are not enhanced in the stream. However, the examination of a number of geomagnetic storms shows that v is not a geoactive parameter. Hence the corpuscular stream itself is not more geoactive than the quiet solar wind.The retarding of corpuscular stream by the quiet solar wind results in various plasma deformations (compression, torsion, shear). This, in turn, leads to the creation, in the stream and ambient quiet solar wind, of geoactive zones. Each zone is characterized by the enhancement of some geoactive parameter. The entry of the Earth into a geoactive zone causes a corresponding phase or stage of a geomagnetic storm.The concept of geoactive zones is applied to the analysis of the geomagnetic storm of 8–10 July 1966.     

Magnetic dips in the solar wind

Using magnetic data from the HELIOS-1 fluxgate magnetometer, with a 0.2 s resolution, we have investigated the structure of several interplanetary discontinuities involving magnetic dips and rotations of the magnetic field vector. A minimum variance analysis illustrates the behaviour of the magnetic field through the transition. Using this analysis, quite different structures have been isolated and, in particular, narrow transitions resembling almost one dimensional reconnected neutral sheets. For the thinner cases (scale lengths of the magnetic rotation of the order or smaller than 10³ km), we find that the observed structures can be the nonlinear effect of a resistive tearing mode instability having developed on an originally one dimensional neutral sheet at the solar corona.     

Variable solar wind

The solar wind in the heliosphere is a variable phenomenon on all spatial and time scales. It has been shown that there are two basic types of solar wind by the Strouhal number S = L/VT, which characterizes relative variations in the main parameters of the solar wind on the given time interval T and linear scale L for velocity V, which is never zero. The first type is transient (S > 1), which is usually the basic type for sufficiently small values of T and large values of L. The second type is quasi-stationary, when 1 > S > 0. The constant solar wind is nonexistent. The extreme case of S = 0 is physically impossible, as is the case of S = ∞. It is always necessary to indicate and justify the range of applicability for a special quasi-stationary case 1 ? S > 0. Otherwise, to consider the case of S = 0 is incorrect. Regarding this, the widely-spread views on the stationary state of the solar wind are very conditional. They either lack physical sense, or have a very limited range of applicability for time T and scale L.     

The solar wind

The current status of our understanding of the nature and origin of the solar wind is briefly reviewed, with emphasis being placed on the need for wave-particle interactions to account for the main energy source as well as details of the particle distribution functions. There has been considerable progress in the theoretical treatment of various aspects of the physics of the solar wind but a complete understanding is not yet in sight. Arguments concerning the ultimate fate of the solar wind are reviewed, in particular those concerning the distance to the shock wave which marks the termination of supersonic flow. This is of particular significance in view of recent observations suggesting that the termination might occur at about 50 AU from the Sun.     

Coronal heating and solar wind acceleration; gyrotropic electron-proton solar wind

In coronal holes the electron (proton) density is low, and heating of the proton gas produces a rapidly increasing proton temperature in the inner corona. In models with a reasonable electron density in the upper transition region the proton gas becomes collisionless some 0.2 to 0.3 solar radii into the corona. In the collisionless region the proton heat flux is outwards, along the temperature gradient. The thermal coupling to electrons is weak in coronal holes, so the heat flux into the transition region is too small to supply the energy needed to heat the solar wind plasma to coronal temperatures. Our model studies indicate that in models with proton heating the inward heat conduction may be so inefficient that some of the energy flux must be deposited in the transition region to produce the proton fluxes that are observed in the solar wind. If we allow for coronal electron heating, the energy that is needed in the transition region to heat the solar wind to coronal temperatures, may be supplied by heat conduction from the corona.