Edisher Kh. Kaghashvili

On the Acceleration of the Solar Wind: Role of the Inhomogeneous Flow

It is shown that an inhomogeneous flow is capable of converting Alfven waves escaping from the solar atmosphere into other types of MHD waves that can be efficiently dissipated. The efficiency of this process depends on local characteristics of the medium. Using the geometry of the solar wind, it is shown how this mechanism operates in different regions of the solar wind and what the preferred way of the coupling process is in those regions. It is suggested that mode conversion induced by inhomogeneous flow, particularly by shear velocity flow, could be the basic mechanism required for the solar wind acceleration in the coronal holes. It is shown that this mechanism is most efficient in the fast-expanding regions of polar coronal holes and how it contributes to the detected long-period Alfven waves and density fluctuations in the solar wind. The results demonstrated by numerical simulations coincide with observations.

Ion-cyclotron wave dissipation channel for Alfvén waves

The effect of inhomogeneous velocity flow on Alfven wave dynamics in the solar wind is investigated. Unlike existing studies in this field that model energy deposition from high-frequency waves to solar wind particles, we consider here the possibility of obtaining high-frequency waves (in this case fast magnetosonic waves) from the ordinary Alfven wave spectrum. It is shown that the spatial inhomogeneity of the velocity field can lead to dissipation of Alfven waves through ion-cyclotron resonance. Recent Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) observations indicate that preferential heating and acceleration of heavy ions occurs close to the Sun. The process described here provides a possible interpretation of such solar wind observations.

ALFVÉN WAVES IN SHEAR FLOWS REVISITED

We revisit our earlier study of the evolution of an initial propagating Alfven wave in a magnetic-field-aligned flow with a cross-field velocity shear. Our goal is to show how the Alfven wave drives up plasma density fluctuations which might be observed and serve as a signature of the presence of Alfven waves in regions such as the solar corona which are inaccessible to direct observations. Here, we introduce a new initial condition which takes into account the initial distortion of the streamlines by the Alfven wave, and we present new analytical results for the driven waves. We find that the density fluctuations of a properly placed linearly polarized Alfven wave in a shear flow are much smaller than we originally estimated.

DRIVEN WAVES AS A DIAGNOSTICS TOOL IN THE SOLAR CORONA

Detecting the signature of Alfven waves in the solar atmosphere remains an observational challenge. At the same time, it could also be an important key to gaining critical understanding of the solar wind and especially of the near-Earth space weather formation. Here, we investigate the plausibility of using inhomogeneous flow-driven compressional fluctuations as a diagnostics tool for Alfven waves in the solar corona. The nature of the fluctuations driven by transverse Alfven waves in inhomogeneous flows was recently investigated by Kaghashvili et al., and analytical solutions that accurately link driven waves to the Alfvenic driver were found. The novelty of this mechanism is that the analysis of the detected compressional fluctuations can provide a clue about the Alfven waves that are otherwise difficult to detect. We review this physical process in a low-β approximation relevant to solar coronal conditions and outline basic reasons why it can be one of the major processes that comes about as outflowing plasma emerges from divergent coronal holes. After establishing a quantitative link, we consider an example with coronal hole plasma parameters similar to the ones reported recently where evidence for Alfven waves in solar X-ray jets was discussed. We show how this diagnostics tool can be used to analyze the detected intensity fluctuations.

VELOCITY-SHEAR-INDUCED MODE COUPLING IN THE SOLAR ATMOSPHERE AND SOLAR WIND: IMPLICATIONS FOR PLASMA HEATING AND MHD TURBULENCE

We analytically consider how velocity shear in the corona and solar wind can cause an initial Alfven wave to drive up other propagating signals. The process is similar to the familiar coupling into other modes induced by non-WKB refraction in an inhomogeneous plasma, except here the refraction is a consequence of velocity shear. We limit our discussion to a low-beta plasma, and ignore couplings into signals resembling the slow mode. If the initial Alfven wave is propagating nearly parallel to the background magnetic field, then the induced signals are mainly a forward-going (i.e., propagating in the same sense as the original Alfven wave) fast mode, and a driven signal propagating like a forward-going Alfven wave but polarized like the fast mode; both signals are compressive and subject to damping by the Landau resonance. For an initial Alfven wave propagating obliquely with respect to the magnetic field, the induced signals are mainly forward- and backward-going fast modes, and a driven signal propagating like a forward-going Alfven wave but polarized like the fast mode; these signals are all compressive and subject to damping by the Landau resonance. A backward-going Alfven wave, thought to be important in the development of MHD turbulence, is also produced, but it is very weak. However, we suggest that for oblique propagation of the initial Alfven wave the induced fast-polarized signal propagating like a forward-going Alfven wave may interact coherently with the initial Alfven wave and distort it at a strong-turbulence-like rate.

Linear mechanism of Alfvén wave dissipation induced by velocity shear: Phase mixing and damping

Alfven waves have been considered as candidates for coronal heating and solar wind acceleration since their presence was identified in situ solar wind. Unfortunately, because of the incompressible nature of these waves, difficulties are associated with their damping in a high temperature plasma. In this study, we discuss in detail the linear wave-conversion mechanism that can efficiently convert Alfven waves into other types of MHD waves. The phase mixing introduced by this mechanism is associated with inhomogeneous flow, in particular, with a velocity shear. To show how the mechanism might operate in different regions of the solar wind, we have considered three cases: A. β=0.133 (condition in coronal holes), B. β=1.2 (solar wind beyond the Alfvenic point) and C. β=4.8.

Density fluctuations in polar coronal holes: Mechanism and observational consequences

Recent Ultraviolet Coronagraph spectrometer observations (UVCS) indicate quasi-periodic variations in the polarized brightness in polar coronal holes. The importance of these observations arises from the fact that these fluctuations might be the signature of compressional waves at heliocentric distances of about 2 R⊙. Such observations are a clear indication that some kind of mechanism or mechanisms operate near the Sun transforming Alfven waves into compressible magnetosonic waves. In the present paper we investigate, whether a linear wave conversion of Alfven waves induced by velocity shear can explain the observations.