Coronal holes as sources of solar wind

We investigate the association of high-speed solar wind with coronal holes during the Skylab mission by: (1) direct comparison of solar wind and coronal X-ray data; (2) comparison of near-equatorial coronal hole area with maximum solar wind velocity in the associated streams; and (3) examination of the correlation between solar and interplanetary magnetic polarities. We find that all large near-equatorial coronal holes seen during the Skylab period were associated with high-velocity solar wind streams observed at 1 AU.Harvard College Observatory-Smithsonian Astrophysical Observatory.A substantial portion of this work was done while a visiting scientist at American Science and Engineering.     

Reproducible characteristics of the solar wind acceleration

In experiments that were regularly carried out in 1999–2002 with Pushchino radio telescopes (Russian Academy of Sciences), the study of the radial dependence of the scattering of radio emission from compact natural sources was extended to regions of circumsolar plasma farther from the Sun. Based on a large body of data, we show that, apart from the standard transonic acceleration region located at distances of 10–40 R_⊙ from the Sun, there is a region of repeated acceleration at distances of 34–60 R_⊙ attributable to the equality between the solar wind velocity and the Alfvénic velocity. The repetition in the trans-Alfvénic region of the characteristic features of the radial stream structure observed in the transonic region (the existence of a precursor, a narrow region of reduced scattering that precedes a wide region of enhanced scattering) suggests that the main characteristic features of the resonant acceleration of solar wind streams are preserved up to distances of the order of 60 R_⊙.     

Coronal heating

The prospects for observational tests of coronal heating theories are examined. Three heating mechanisms (Alfvén waves, D.C. electric currents, and MHD turbulence) are reviewed from this point of view. Some suggestions are offered for future tests but none, so far, would give unambiguous critical results.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

Non-thermal solar wind heating by supra-thermal ions

The effect of a new energy source due to energies transferred from supra-thermal secondary ions on the temperature profile of the solar wind has been considered. For this purpose a solution of a tri-fluid model of the solar wind including solar electrons, protons, and -particles, and starting with the boundary conditions of Hartle and Barnes at 0.5 AU is given. On the base of the assumption that suprathermal He⁺-ions which have four times the temperature of suprathermal protons are predominantly coupled to solar -particles by Alfvén waves, it is shown that the temperature T of solar -particles should be appreciably higher than those T _p of solar protons beyond the orbit of the Earth. For 1 AU a temperature excess T over T _p according to that which has been found in some solar wind ion spectrograms can only be explained for a small part of the orbit of the earth which is inside the cone of enhanced helium densities. Around 1 AU the temperatures T and T _p are found to decrease much slighter with solar distance than given in the two-fluid model of Hartle and Barnes. Beyond 1.7 and 2.2 AU the temperatures T and T _p even start increasing with solar distance and come up to about 10⁵ at about 10 AU. These predictions should lend some support to future temperature measurements with deep-space probes reaching Solar distances of some AU.Forschungsberichte des Astronomischen Institutes, Bonn, 72-10.     

Ion cyclotron waves,instabilities and solar wind heating

The effect of alpha particles on the dispersion relation of ion cyclotron waves and its influence on the heating of the solar wind plasma are investigated. The presence of alpha particles can dramatically change the dispersion relation of ion cyclotron waves, and significantly influence the way that ion cyclotron waves heat the solar wind plasma. We find that a spectrum of ion cyclotron waves affects the thermal anisotropy of the solar wind protons and other ions differently in interplanetary space: When alpha particles have a speed u _α>0.5v _A, and both protons and alpha particles have a thermal anisotropy T _⊥/T _∥>1, ion cyclotron waves heat protons in the direction perpendicular to the magnetic field, cool them in the parallel direction, and exert the opposite effect on alpha particles.     

On acceleration and motion of ions in corona and solar wind

Assuming a stationary, radial, spherically symmetric solar wind and a radial magnetic field direction in the vicinity of the sun, an equation of motion for ions heavier than protons in the solar wind is derived. The general properties of this equation are discussed and the results of numerical integrations are given. These results are based on the assumption of maxwellian velocity distribution functions for electrons, protons and ions, but the effects of first order deviations from such distributions are also presented and discussed. It is shown that dynamical friction, i.e. momentum transfer from protons to heavier ions accounts for the observed fact that heavier ions - if accelerated at all - normally reach the same velocity as the protons in the solar wind. Because of the non-linear relation between dynamical friction and proton-ion velocity difference a minimum proton flux is required to carry a certain ion species in the solar wind. Formulae comparing the minimum fluxes for different ions are given. It is shown that elements up to and beyond iron will be carried along in the solar wind as long as helium is carried along. Substantial isotopic fractionation is possible, in particular in the case of helium. The effects of ion motion and escape on abundances in the corona and in the outer convective zone of the sun are discussed.     

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.     

Resonant wave acceleration of minor ions in the solar wind

This paper extends some previous work on the acceleration of minor ions in the solar wind to include the effects of wave acceleration and heating arising from minor ions interacting via the gyroresonance with ion cyclotron waves. Resonant wave acceleration is made up of two contributions, the first, and generally the more important, is a local acceleration which is proportional to the wave power and the number of resonant particles and is also sensitive to the details of the distribution function; while the other contribution is basically fluid dynamic in character, arises from the inhomogeneity of the medium and is proportional to the radial gradient of the resonant wave power. Under suitable cir-cumstances both contributions exhibit the feature that heavier ions receive greater acceleration than lighter ones. Also the kinematics of the resonance shows that the resonance wave acceleration switches off above a maximum differential speed, between ions and protons, which increases with increasing ratio of mass to charge. We also examine briefly possible beam instabilities driven by the streaming of minor ions relative to protons.     

Five-minute magnetic field fluctuations in the solar wind acceleration region

A spectral analysis of coronal Faraday rotation (FR) data obtained with the linearly polarized signals of the two Heliosspacecraft reveals that about one-third of the temporal FR spectra contain a distinct spectral line superposed onto the background power-law spectrum. The most prevalent frequency of this quasi-harmonic component (QHC) is about 4 mHz, corresponding to a 4–5 min periodic oscillation of the coronal magnetic field. Physical reasons for the existence of QHC Alfvén fluctuations in the inner solar wind are discussed. FR fluctuations (FRF) are considered to arise from both a turbulent background as well as an isolated Alfvén wave train of finite extent and duration. An estimate can be made for the conditions under which the isolated wave train is observed above the ever present background. It is shown that the wave train must have a sufficiently long duration and transverse wavelength. It is suggested that the QHC at periods near 4–5 min in the FRF spectra are most probably produced by outward-propagating Alfvén waves excited initially in the anisotropic structures of the chromospheric network.     

The dynamics of velocity fluctuations in the solar wind – I. Coronal mass ejections

In this paper we present a simple, analytic model for the dynamical evolution of supersonic velocity fluctuations at the base of the ambient solar wind. These fluctuations result in the formation of dense working surfaces that travel down the wind. It is shown how the initial parameters of the fluctuations (velocity, density and duration) are related to the characteristics of the working surfaces far from the Sun (for instance at the Earth). We apply the model to the evolution of the coronal mass ejections in the IP medium, finding that the model is in good agreement with satellite observations of these phenomena, thus providing physical insight into their dynamical evolution. Our model may contribute to future 'space weather forecasting' on the Earth, based on detailed satellite monitoring of the solar corona.     

Coronal holes,solar wind streams,and geomagnetic disturbances during 1978 and 1979

We have extended our long-term study of coronal holes, solar wind streams, and geomagnetic disturbances through the rising phase of sunspot cycle 21 into the era of sunspot maximum. During 1978 and 1979, coronal holes reflected the influence of differential rotation, and existed within a slowly-evolving large-scale pattern despite the relatively high level of sunspot activity. The long-lived 28.5-day pattern is not produced by a rigidly-rotating quasi-stationary structure on the Sun, but seems to be produced by a non-stationary migratory process associated with solar differential rotation. The association between coronal holes and solar wind speed enhancements at Earth continues to depend on the latitude of the holes (relative to the heliographic latitude of Earth), but even the best associations since 1976 have speeds of only 500–600 km s^-1 rather than the values of 600–700 km s^-1 that usually occurred during the declining phase of sunspot cycle 20.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Interstellar pickup protons and solar wind heating in the outer heliosphere

The ionization of hydrogen atoms that penetrate into the heliosphere from the interstellar medium gives rise to a peculiar population of energetic protons (interstellar pickup protons) in the solar wind. The short-wavelength Alfvènic turbulence in the outer heliosphere is entirely attributable to the source associated with the instability of the initial anisotropic pickup proton velocity distribution. The bulk of the generated turbulent energy is subsequently absorbed by the pickup protons themselves through the cyclotron-resonance particle-wave interaction, and only an insignificant fraction of this energy can be transferred to the solar wind protons and heat them up.     

Hydrogen fluence in Genesis collectors: Implications for acceleration of solar wind and for solar metallicity

NASA's Genesis mission was flown to capture samples of the solar wind and return them to the Earth for measurement. The purpose of the mission was to determine the chemical and isotopic composition of the Sun with significantly better precision than known before. Abundance data are now available for noble gases, magnesium, sodium, calcium, potassium, aluminum, chromium, iron, and other elements. Here, we report abundance data for hydrogen in four solar wind regimes collected by the Genesis mission (bulk solar wind, interstream low‐energy wind, coronal hole high‐energy wind, and coronal mass ejections). The mission was not designed to collect hydrogen, and in order to measure it, we had to overcome a variety of technical problems, as described herein. The relative hydrogen fluences among the four regimes should be accurate to better than ±5–6%, and the absolute fluences should be accurate to ±10%. We use the data to investigate elemental fractionations due to the first ionization potential during acceleration of the solar wind. We also use our data, combined with regime data for neon and argon, to estimate the solar neon and argon abundances, elements that cannot be measured spectroscopically in the solar photosphere.     

First phase heating and particle acceleration during solar flares by fast tearing modes

We develop a simple, but physically consistent, model of heating and particle acceleration by fast tearing modes, for modeling compact loop flares or erupting prominences. It is shown that there is a slow preheating, over many e -foldings of the instability, after which a rapid heating takes place in approximately one e-folding. The role of anomalous resistivity excited by the induced electric field during tearing is discussed, and how both thermal conduction and plasma expansion may play a role in cooling. Estimates for the total number of thermal and non-thermal electrons generated by one fast tearing mode are given, and it is argued that collisional tearing modes give rise to a primarily thermal plasma.     

Coronal heating and photospheric boundary conditions

We present a simple model that demonstrates that regions of high current density cannot exist within the solar atmosphere in a quasi-stationary state if they do not already exist at the photospheric boundary. This result demonstrates that theoretical treatments of coronal heating by electrodynamic processes must take proper account of the photospheric spatial distribution of the forces that generate the currents (or equivalently waves) and not just the power contained in those waves that result in coronal heating.     

Coronal holes,solar wind streams,and recurrent geomagnetic disturbances: 1973–1976

Observations of coronal holes, solar wind streams, and geomagnetic disturbances during 1973–1976 are compared in a 27-day pictorial format which shows their long-term evolution. The results leave little doubt that coronal holes are related to the high-speed streams and their associated recurrent geomagnetic disturbances. In particular, these observations strongly support the hypothesis that coronal holes are the solar origin of the high-speed streams observed in the solar wind near the ecliptic plane.Visiting Scientist, Kitt Peak National Observatory, Tucson, Arizona.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Coronal heating by prominence turbulence

The generation of magnetohydrodynamic waves in the corona by the observed random motions in prominences is considered. The associated energy input into the corona may be a significant source of heating for the coronal loops overlying prominences, especially during the onset of flares. Some relevant observations are discussed.     

Coronal heating by magnetic reconnection

The theory of magnetic reconnection has advanced substantially over the past few years. There now exists a new generation of fast two-dimensional models known as almost-uniform reconnection and nonuniform reconnection, depending on the boundary conditions. Also, we are beginning to explore the uncharted region of three-dimensional reconnection, where regimes of “spine reconnection” and “fan reconnection” have been discovered. Furthermore, part of the coronal heating problem appears to have been solved with recent observational support for the Converging Flux Model in which heating is produced by coronal reconnection driven by footpoint motions.