MHD Waves and Heating in Coronal Holes

Coronal holes have been identified as source regions of the fast solar wind, and MHD wave activity has been detected in coronal holes by remote sensing, and in situ in fast solar wind streams. I review some of the most suggestive wave observations, and discuss the theoretical aspects of MHD wave heating and solar wind acceleration in coronal holes. I review the results of single fluid 2.5D MHD, as well as multi-fluid 2.5D MHD models of waves in coronal holes, the heating, and the acceleration of the solar wind be these waves.     

Solar Wind Models from the Sun to 1 AU: Constraints by in Situ and Remote Sensing Measurements

There are three major types of solar wind: The steady fast wind originating on open magnetic field lines in coronal holes, the unsteady slow wind coming probably from the temporarily open streamer belt and the transient wind in the form of large coronal mass ejections. The majority of the models is concerned with the fast wind, which is, at least during solar minimum, the normal mode of the wind and most easily modeled by multi-fluid equations involving waves. The in-situ constraints imposed on the models, mainly by the Helios (in ecliptic) and Ulysses (high-latitude) interplanetary measurements, are extensively discussed with respect to fluid and kinetic properties of the wind. The recent SOHO observations have brought a wealth of new information about the boundary conditions for the wind in the inner solar corona and about the plasma conditions prevailing in the transition region and chromospheric sources of the wind plasma. These results are presented, and then some key questions and scientific issues are identified. This revised version was published online in June 2006 with corrections to the Cover Date.     

Slow Solar Wind: Observations and Modeling

While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations.However, the following questions remain open: What are the source regions and their contributions to the SSW? What is the role of the magnetic topology in the corona for the origin, acceleration and energy deposition of the SSW? What are the possible acceleration and heating mechanisms for the SSW? The aim of this review is to present insights on the SSW origin and formation gathered from the discussions at the International Space Science Institute (ISSI) by the Team entitled “Slow solar wind sources and acceleration mechanisms in the corona” held in Bern (Switzerland) in March 2014 and 2015.     

Propagating MHD Waves in Coronal Holes

Coronal holes are the coolest and darkest regions of the upper solar atmosphere, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. During the years of the solar minima, coronal holes are generally confined to the Sun??s polar regions, while at solar maxima they can also be found at lower latitudes. Waves, observed via remote sensing and detected in-situ in the wind streams, are most likely responsible for the wind and several theoretical models describe the role of MHD waves in the acceleration of the fast solar wind. This paper reviews the observational evidences of detection of propagating waves in these regions. The characteristics of the waves, like periodicities, amplitude, speed provide input parameters and also act as constraints on theoretical models of coronal heating and solar wind acceleration.     

On the Role of Interchange Reconnection in the Generation of the Slow Solar Wind

The heating of the solar corona and therefore the generation of the solar wind, remain an active area of solar and heliophysics research. Several decades of in situ solar wind plasma observations have revealed a rich bimodal solar wind structure, well correlated with coronal magnetic field activity. Therefore, the reconnection processes associated with the large-scale dynamics of the corona likely play a major role in the generation of the slow solar wind flow regime. In order to elucidate the relationship between reconnection-driven coronal magnetic field structure and dynamics and the generation of the slow solar wind, this paper reviews the observations and phenomenology of the solar wind and coronal magnetic field structure. The geometry and topology of nested flux systems, and the (interchange) reconnection process, in the context of coronal physics is then explained. Once these foundations are laid out, the paper summarizes several fully dynamic, 3D MHD calculations of the global coronal system. Finally, the results of these calculations justify a number of important implications and conclusions on the role of reconnection in the structural dynamics of the coronal magnetic field and the generation of the solar wind.     

Coronal Holes and the High-Speed Solar Wind

Coronal holes are the lowest density plasma components of the Sun's outer atmosphere, and are associated with rapidly expanding magnetic fields and the acceleration of the high-speed solar wind. Spectroscopic and polarimetric observations of the extended corona, coupled with interplanetary particle and radio sounding measurements going back several decades, have put strong constraints on possible explanations for how the plasma in coronal holes receives its extreme kinetic properties. The Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft has revealed surprisingly large temperatures, outflow speeds, and velocity distribution anisotropies for positive ions in coronal holes. We review recent observations, modeling techniques, and proposed heating and acceleration processes for protons, electrons, and heavy ions. We emphasize that an understanding of the acceleration region of the wind (in the nearly collisionless extended corona) is indispensable for building a complete picture of the physics of coronal holes.     

Sources of Solar Wind at Solar Minimum: Constraints from Composition Data

In this discussion of observational constraints on the source regions and acceleration processes of solar wind, we will focus on the ionic composition of the solar wind and the distribution of charge states of heavy elements such as oxygen and iron. We first focus on the now well-known bi-modal nature of solar wind, which dominates the heliosphere at solar minimum: Compositionally cool solar wind from polar coronal holes over-expands, filling a much larger solid angle than the coronal holes on the Sun. We use a series of remote and in-situ characteristics to derive a global geometric expansion factor of?～5. Slower, streamer-associated wind is located near the heliospheric current sheet with a width of 10–20°, but in a well-defined band with a geometrically small transition width. We then compute charge states under the assumption of thermal electron distributions and temperature, velocity, and density profiles predicted by a recent solar wind model, and conclude that the solar wind originates from a hot source at around 1 million?K, characteristic of the closed corona.     

The Expansion of Coronal Plumes in the Fast Solar Wind

Coronal plumes are believed to be essentially magnetic features: they are rooted in magnetic flux concentrations at the photosphere and are observed to extend nearly radially above coronal holes out to at least 15 solar radii, probably tracing the open field lines. The formation of plumes itself seems to be due to the presence of reconnecting magnetic field lines and this is probably the cause of the observed extremely low values of the Ne/Mg abundance ratio. In the inner corona, where the magnetic force is dominant, steady MHD models of coronal plumes deal essentially with quasi-potential magnetic fields but further out, where the gas pressure starts to be important, total pressure balance across the boundary of these dense structures must be considered. In this paper, the expansion of plumes into the fast polar wind is studied by using a thin flux tube model with two interacting components, plume and interplume. Preliminary results are compared with both remote sensing and solar wind in situ observations and the possible connection between coronal plumes with pressure-balance structures (PBS) and microstreams is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Acceleration of the solar wind

In this review, we discuss critically recent research on the acceleration of the solar wind, giving emphasis to high-speed solar wind streams emanating from solar coronal holes. We first explain why thermally driven wind models constrained by solar and interplanetary observations encounter substantial difficulties in explaining high speed streams. Then, through a general discussion of energy addition to the solar wind above the coronal base, we indicate a possible resolution of these difficulties. Finally, we consider the question of what role MHD waves might play in transporting energy through the solar atmosphere and depositing it in the solar wind, and we conclude by examining, in a simple way, the specific mechanism of solar wind acceleration by Alfvén waves and the related problem of accelerating massive stellar winds with Alfvén waves.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.On leave from the Auroral Observatory, Institute of Mathematical and Physical Sciences, University of Tromsø, N-9001 Tromsø, Norway.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Semi-empirical MHD Model of the Solar Wind and its Comparison with Ulysses

We have developed a 2D semi-empirical model (Sittler and Guhathakurta 1999) of the corona and the interplanetary medium using the time independent MHD equations and assuming azimuthal symmetry, utilizing the SOHO, Spartan and Ulysses observations. The model uses as inputs (1) an empirically derived global electron density distribution using LASCO, Mark III and Spartan white light observations and in situ observations of the Ulysses spacecraft, and (2) an empirical model of the coronal magnetic field topology using SOHO/LASCO and EIT observations. The model requires an estimate of solar wind velocity as a function of latitude at 1 AU and the radial component of the magnetic field at 1 AU, for which we use Ulysses plasma and magnetic field data results respectively. The model makes estimates as a function of radial distance and latitude of various fluid parameters of the plasma such as flow velocity V, temperature T_eff, and heat flux Q_eff which are derived from the equations of conservation of mass, momentum and energy, respectively, in the rotating frame of the Sun. The term "effective" indicates possible wave contributions. The model can be used as a planning tool for such missions as Solar Probe and provide an empirical framework for theoretical models of the solar corona and solar wind. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Solar Origin of Corotating Interaction Regions and Their Formation in the Inner Heliosphere

Corotating Interaction Regions (CIRs) form as a consequence of the compression of the solar wind at the interface between fast speed streams and slow streams. Dynamic interaction of solar wind streams is a general feature of the heliospheric medium; when the sources of the solar wind streams are relatively stable, the interaction regions form a pattern which corotates with the Sun. The regions of origin of the high speed solar wind streams have been clearly identified as the coronal holes with their open magnetic field structures. The origin of the slow speed solar wind is less clear; slow streams may well originate from a range of coronal configurations adjacent to, or above magnetically closed structures. This article addresses the coronal origin of the stable pattern of solar wind streams which leads to the formation of CIRs. In particular, coronal models based on photospheric measurements are reviewed; we also examine the observations of kinematic and compositional solar wind features at 1 AU, their appearance in the stream interfaces (SIs) of CIRs, and their relationship to the structure of the solar surface and the inner corona; finally we summarise the Helios observations in the inner heliosphere of CIRs and their precursors to give a link between the optical observations on their solar origin and the in-situ plasma observations at 1 AU after their formation. The most important question that remains to be answered concerning the solar origin of CIRs is related to the origin and morphology of the slow solar wind. This revised version was published online in August 2006 with corrections to the Cover Date.     

Associating the Solar Wind Measured by Ulysses with its Source at the sun

Radio occultation, ultraviolet, and white-light measurements have expanded our knowledge of the morphology of density and velocity in polar coronal holes, and made it possible to carry out the first systematic comparisons between the Ulysses solar wind measurements and quantitative white-light observations of the solar corona. This paper summarizes the rationale and salient features of this new approach which has been used to relate the solar wind observed by Ulysses in 1993–1995 to the inner corona. The statistical characteristics (average, standard deviation, and autocorrelation function) of the Ulysses density measurements of the fast wind are found to be mirrored in those of polarized brightness measurements of path-integrated density made by the High Altitude Observatory (HAO) Mauna Loa K-coronagraph at 1.15 R _⊙. These results reinforce the conclusions from comparisons between measurements of the outer and inner corona. They show that the polar coronal hole extends radially into the solar wind, and that sources of the fast wind are not limited to coronal holes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fractionation of SI, NE, and MG Isotopes in the Solar Wind as Measured by Soho/Celias/MTOF

Using the high-resolution mass spectrometer CELIAS/MTOF on board SOHO we have measured the solar wind isotope abundance ratios of Si, Ne, and Mg and their variations in different solar wind regimes with bulk velocities ranging from 330 km/s to 650 km/s. Data indicate a small systematic depletion of the heavier isotopes in the slow solar wind on the order of (1.4±1.3)% per amu (2σ-error) compared to their abundances in the fast solar wind from coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pass the inner corona with the same charge state distribution. The influence of particle mass on the acceleration of minor solar wind ions is discussed in the context of theoretical models and recent optical observations with other SOHO instruments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soho Observations of Density Fluctuations in Coronal Holes

In recent UVCS/SOHO White Light Channel (WLC) observations we found quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes at heliocentric distances of 1.9 to 2.45 solar radii. The motivation for the observation is the 2.5D MHD model of solar wind acceleration by nonlinear waves, that predicts compressive fluctuations in coronal holes. In February 1998 we performed new observations using the UVCS/WLC in the coronal hole and obtained additional data. The new data corroborate our earlier findings with higher statistical significance. The new longer observations show that the power spectrum peaks in the 10–12 minute range. These timescales agree with EIT observations of brightness fluctuations in polar plumes. We performed preliminary LASCO/C2 observations in an effort to further establish the coronal origin of the fluctuations. This revised version was published online in June 2006 with corrections to the Cover Date.     

UVCS Observations of Temperature and Velocity Profiles in Coronal Holes

The spectroscopic observations of the Ultraviolet Coronagraph Spectrometer (UVCS), on board the SOHO observatory, allow the study and the full characterization of the expansion of the solar atmosphere by means of measurements of the outflow speeds and the physical properties of the wind, directly in the region where the solar plasma is heated and accelerated: the extended corona. During solar minimum, when the magnetic configuration of the corona is rather simple, the open magnetic fields emerging from the wide polar coronal holes channel toward the heliosphere both the fast and the slow wind. The fast wind flows along flux tubes with lower areal divergence than the slow wind which is guided by flux tubes characterized by non-monotonic areal expansion functions. Differences in the physical properties, such as kinetic temperature, electron density, composition and density fluctuations, of the fast and slow wind in the corona are discussed.     

Unsolved problems of solar wind expansion: Can we learn anything from SOHO?

Some of the main problems of solar wind expansion are addressed. Emphasis is placed on solar wind acceleration and the mass flux problem. It is demonstrated how these two properties of the flow depend on other plasma parameters such as temperature, density and helium abundance. The importance of placing constraints on a given solar wind flow in the inner corona and at larger distances from the sun simultaneously, is also shown. Whether and how these constraints can be derived from observations carried out by SOHO instruments is then discussed.     

Working Group 4 Report: Composition and Elemental Abundance Variations in the Solar Atmosphere and Solar Wind

This paper contains a summary of the topics treated in the working group on abundance variations in the solar atmosphere and in the solar wind. The FIP bias (overabundance of particles with low First Ionization Potentials over photospheric abundances) in coronal holes and coronal hole associated solar wind amounts to values between 1 and 2. The FIP bias in the slow solar wind is typically a factor 4, consistent with optical observations in streamers. In order to distinguish between different theoretical models which make an attempt to explain the FIP bias, some observable parameters must be provided. Unfortunately, many models are deficient in this respect. In addition to FIP fractionation, gravitational settling of heavy elements has been found in the core of long lived streamers. The so-called electron 'freeze in' temperatures derived from in situ observed ionization states of minor ions in the fast wind are significantly higher than the electron temperatures derived from diagnostic line ratios observed in polar coronal holes. The distinction between conditions in plumes and interplume lanes needs to be further investigated. The 'freeze in' temperatures for the slow solar wind are consistent with the electron temperatures derived for streamers. This revised version was published online in June 2006 with corrections to the Cover Date.     

Wind in the Solar Corona: Dynamics and Composition

The dynamics of the solar corona as observed during solar minimum with the Ultraviolet Coronagraph Spectrometer, UVCS, on SOHO is discussed. The large quiescent coronal streamers existing during this phase of the solar cycle are very likely composed by sub-streamers, formed by closed loops and separated by open field lines that are channelling a slow plasma that flows close to the heliospheric current sheet. The polar coronal holes, with magnetic topology significantly varying from their core to their edges, emit fast wind in their central region and slow wind close to the streamer boundary. The transition from fast to slow wind then appears to be gradual in the corona, in contrast with the sharp transition between the two wind regimes observed in the heliosphere. It is suggested that speed, abundance and kinetic energy of the wind are modulated by the topology of the coronal magnetic field. Energy deposition occurs both in the slow and fast wind but its effect on the kinetic temperature and expansion rate is different for the slow and fast wind.     

Progress, Challenges, and Perspectives of the 3D MHD Numerical Modeling of Oscillations in the Solar Corona

Recent high temporal and spatial resolution satellite observations of the solar corona provide ample evidence of oscillations in coronal structures. The observed waves and oscillations can be used as a diagnostic tool of the poorly known coronal parameters, such as magnetic field, density, and temperature. The emerging field of coronal seismology relies on the interpretation of the various coronal oscillations in terms of theoretically known wave modes, and the comparison of observed and theoretical wave mode properties for the determination of the coronal parameters. However, due to complexity of coronal structures the various modes are coupled, and the application of linear theory of idealized structures to coronal loops and active regions limits the usefulness of such methods. Improved coronal seismology can be achieved by the development of full 3D MHD dynamical model of relevant coronal structures and the oscillation phenomena. In addition to improved accuracy compared to linear analysis, 3D MHD models allow the diagnostic method to include nonlinearity, compressibility, and dissipation. The current progress made with 3D MHD models of waves in the corona is reviewed, and the challenges facing further development of this method are discussed in the perspective of future improvement that will be driven by new high resolution and high cadence satellite data, such as received from Hinode and STEREO, and expected from SDO.