On the Slow Solar Wind

A theory for the origin of the slow solar wind is described. Recent papers have demonstrated that magnetic flux moves across coronal holes as a result of the interplay between the differential rotation of the photosphere and the non-radial expansion of the solar wind in more rigidly rotating coronal holes. This flux will be deposited at low latitudes and should reconnect with closed magnetic loops, thereby releasing material from the loops to form the slow solar wind. It is pointed out that this mechanism provides a natural explanation for the charge states of elements observed in the slow solar wind, and for the presence of the First-Ionization Potential, or FIP, effect in the slow wind and its absence in fast wind. Comments are also provided on the role that the ACE mission should have in understanding the slow solar wind. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Composition of the Solar Wind in Polar Coronal Holes

The solar wind charge state and elemental compositions have been measured with the Solar Wind Ion Composition Spectrometers (SWICS) on Ulysses and ACE for a combined period of about 25 years. This most extensive data set includes all varieties of solar wind flows and extends over more than one solar cycle. With SWICS the abundances of all charge states of He, C, N, O, Ne, Mg, Si, S, Ar and Fe can be reliably determined (when averaged over sufficiently long time periods) under any solar wind flow conditions. Here we report on results of our detailed analysis of the elemental composition and ionization states of the most unbiased solar wind from the polar coronal holes during solar minimum in 1994–1996, which includes new values for the abundance S, Ca and Ar and a more accurate determination of the ²⁰Ne abundance. We find that in the solar minimum polar coronal hole solar wind the average freezing-in temperature is ∼1.1×10⁶ K, increasing slightly with the mass of the ion. Using an extrapolation method we derive photospheric abundances from solar wind composition measurements. We suggest that our solar-wind-derived values should be used for the photospheric ratios of Ne/Fe=1.26±0.28 and Ar/Fe=0.030±0.007.     

Semiempirical Models of the Slow and Fast Solar Wind

Coronal holes can produce several types of solar wind with a variety of compositional properties, depending on the location and strength of the heating along their open magnetic field lines. High-speed wind is associated with (relatively) slowly diverging flux tubes rooted in the interiors of large holes with weak, uniform footpoint fields; heating is spread over a large radial distance, so that most of the energy is conducted outward and goes into accelerating the wind rather than increasing the mass flux. In the rapidly diverging open fields present at coronal hole boundaries and around active regions, the heating is concentrated at low heights and the temperature maximum is located near the coronal base, resulting in high oxygen freezing-in temperatures and low asymptotic wind speeds. Polar plumes have a strong additional source of heating at their bases, which generates a large downward conductive flux, raising the densities and enhancing the radiative losses. The relative constancy of the solar wind mass flux at Earth reflects the tendency for the heating rate in coronal holes to increase monotonically with the footpoint field strength, with very high mass fluxes at the Sun offsetting the enormous flux-tube expansion in active region holes. Although coronal holes are its main source, slow wind is also released continually from helmet streamer loops by reconnection processes, giving rise to plasma blobs (small flux ropes) and the heliospheric plasma sheet.     

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.     

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.     

Ulysses' Second Orbit: Remarkably Different Solar Wind

By the time of the 34th ESLAB symposium, dedicated to the memory of John Simpson, Ulysses had nearly reached its peak southerly latitude in its second polar orbit. The global solar wind structure observed thus far in Ulysses' second orbit is remarkably different from that observed over its first orbit. In particular, Ulysses observed highly irregular solar wind with less periodic stream interaction regions, much more frequent coronal mass ejections, and only a single, short interval of fast solar wind. Ulysses also observed the slowest solar wind seen thus far in its ten-year journey (∼270 km s⁻¹). The complicated solar wind structure undoubtedly arises from the more complex coronal structure found around solar activity maximum, when the large polar coronal holes have disappeared and coronal streamers, small-scale coronal holes, and frequent CMEs are found at all heliolatitudes. This revised version was published online in August 2006 with corrections to the Cover Date.     

The 3-D Heliosphere from the Ulysses and ACE Solar Wind Ion Composition Experiments

The source region of solar wind plasma is observed to be directly reflected in the compositional pattern of both elemental and charge state compositions. Slow solar wind associated with streamers shows higher freeze-in temperatures and larger FIP enhancements than coronal hole associated wind. Also, the variability of virtually all compositional parameters is much higher for slow solar wind compared to coronal hole associated wind. We show that these compositional patterns persist even though stream-stream interactions complicate the identification based on in situ plasma parameters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Langmuir Wave Activity: Comparing the Ulysses Solar Minimum and Solar Maximum Orbits

We examine the occurrence and intensity of Langmuir wave activity (electrostatic waves at the electron plasma frequency) during the solar minimum and solar maximum orbits of Ulysses. At high latitudes during the solar minimum orbit, occurrences of Langmuir waves in magnetic holes were frequent; in the second orbit, they were less common. This difference, in comparison with observations from the first Ulysses fast heliolatitude scan, suggests that Langmuir wave activity in magnetic holes is enhanced in solar wind from polar 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.     

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.     

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.     

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.     

Recent Observations of Plasma and Alfvénic Wave Energy Injection at the Base of the Fast Solar Wind

We take stock of recent observations that identify the episodic plasma heating and injection of Alfvénic energy at the base of fast solar wind (in coronal holes). The plasma heating is associated with the occurrence of chromospheric spicules that leave the lower solar atmosphere at speeds of order 100?km/s, the hotter coronal counterpart of the spicule emits radiation characteristic of root heating that rapidly reaches temperatures of the order of 1?MK. Furthermore, the same spicules and their coronal counterparts (“Propagating Coronal Disturbances”; PCD) exhibit large amplitude, high speed, Alfvénic (transverse) motion of sufficient energy content to accelerate the material to high speeds. We propose that these (disjointed) heating and accelerating components form a one-two punch to supply, and then accelerate, the fast solar wind. We consider some compositional constraints on this concept, extend the premise to the slow solar wind, and identify future avenues of exploration.     

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.     

O5+ in High Speed Solar Wind Streams: SWICS/Ulysses Results

Recent observations with UVCS on SOHO of high outflow velocities of O⁵⁺ at low coronal heights have spurred much discussion about the dynamics of solar wind acceleration. On the other hand, O⁶⁺ is the most abundant oxygen charge state in the solar wind, but is not observed by UVCS or by SUMER because this helium-like ion has no emission lines falling in the wave lengths observable by these instruments. Therefore, there is considerable interest in observing O⁵⁺ in situ in order to understand the relative importance of O⁵⁺ with respect to the much more abundant O⁶⁺. High speed streams are the prime candidates for the search for O⁵⁺ because all elements exhibit lower freezing-in temperatures in high speed streams than in the slow solar wind. The Ulysses spacecraft was exposed to long time periods of high speed streams during its passage over the polar regions of the Sun. The Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses is capable of resolving this rare oxygen charge state. We present the first measurement of O⁵⁺ in the solar wind and compare these data with those of the more abundant oxygen species O⁶⁺ and O⁷⁺. We find that our observations of the oxygen charge states can be fitted with a single coronal electron temperature in the range of 1.0 to 1.2 MK assuming collisional ionization/recombination equilibrium with an ambient Maxwellian electron gas. This revised version was published online in June 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.     

Working Group 1 Report: Solar Wind Models from the Sun to 1 AU: Constraints by "in situ" and Remote Sensing Measurements

The goal of Working Group 1 was to discuss constraints on solar wind models. The topics for discussion, outlined by Eckart Marsch in his introduction, were: (1) what heats the corona, (2) what is the role of waves, (3) what determines the solar wind mass flux, (4) can stationary, multi-fluid models describe the fast and slow solar wind, or (5) do we need time dependent fluid models, kinetic models, and/or MHD models to describe solar wind acceleration. The discussion in the working group focused on observations of "temperatures" in the corona, mainly in coronal holes, and whether the observations of line broadening should be interpreted as thermal broadening or wave broadening. Observations of the coronal electron density and the flow speed in coronal holes were also discussed. There was only one contribution on observations of the distant solar wind, but we can place firm constraints on the solar wind particle fluxes and asymptotic flow speeds from observations with Ulysses and other spacecraft. Theoretical work on multi-fluid models, higher-order moment fluid models, and MHD models of the solar wind were also presented. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar Wind Sources and Their Variations Over the Solar Cycle

In this paper I will briefly summarize the present status of our knowledge on the four different sorts of solar wind, their sources and their short- and long-term variations. First: the fast solar wind in high-speed streams that emerges from coronal hole regions. Second: the slow solar wind emerging from the non-active Sun near the global heliospheric current sheet above helmet streamers and underlying active regions. Third: the slow solar wind filling most of the heliosphere during high solar activity, emerging above active regions in a highly turbulent state, and fourth: the plasma expelled from the Sun during coronal mass ejections. The coronal sources of these different flows vary dramatically with the solar activity cycle.     

Constraints on the FIP Mechanisms from Solar Wind Abundance Data

In the slow solar wind, elements with (first) ionisation potential (FIP) between ∼10 eV and 22 eV are depleted by a factor of about 4 relative to their abundances in the Outer Convective Zone (OCZ), and helium (FIP = 24.5 eV) is further depleted by a factor of ∼1.8. This depletion, called the FIP effect, is much less pronounced in the high speed streams coming out of coronal holes. The systematics of element depletion suggests that the FIP effect is produced at a temperature ∼104 K and that it is controlled by the time of ionisation at the solar surface. At the boundary of the polar coronal holes, the transition from a strong to a weak FIP effect is relatively sharp and coincides with the change in coronal electron temperature, indicating a profound change in coronal as well as chromospheric properties at this boundary. This revised version was published online in June 2006 with corrections to the Cover Date.