X- and XUV-radiation of the solar corona

First a survey of the ionization states and emission lines of the ions existing in the corona is given. Then instruments for taking pictures of the Sun in the X- and the XUV-region as well as for measuring spectra emitted in interesting locations on the Sun are presented. Methods of plasma diagnostics, in particular for the determination of the mean temperature and the differential emission measure are described.In the following review of observations, which are related to the topic of the workshop, types of coronal structures especially coronal holes, active regions and large scale structures are described. Their relations to the photospheric magnetic fields are dealt with; methods to calculate coronal magnetic fields are briefly discussed. As for temporal variations results of the analysis of expanding X-ray arches and of structures becoming visible in the outer corona in white light are mentioned. Finally, plasma diagnostics by means of high-resolution spectra are dealt with, in particular methods for the determination of the particle density by lines of He-like ions and of the local temperature by Li-like satellites lines. Thus non-thermal random velocities and outward moving plasma can be inferred during flares.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Microscopic structure of the solar wind

As the solar wind flows out from the coronal base the coulomb collision frequencies rapidly become small and particle-particle collisions can no longer maintain local statistical equilibrium. At 1 AU the particle distribution functions have important non-Maxwellian characteristics and the firehose instability, a cyclotron resonance whistler-mode instability, and several heat flux current instabilities should be operative. Superthermal particle populations also provide large wave levels, and other forms of enhanced plasma turbulence develop at shock fronts and discontinuities. This report contains a review of the theoretical concepts and a progress report on the experimental study of interplanetary wave-particle interactions.Prepared for Space Science Reviews.     

Modeling the coronal magnetic field in a polar hole

The coronal magnetic field in the northern polar coronal hole in 1986 is predicted on the basis of the photospheric magnetic field observations and the horizontal current-current sheet coronal model (Zhao and Hoeksema, 1993). The predicted magnetic field intensity is stronger near the center of the hole than near the edge. The calculated expansion factor for the entire hole does not match the expansion factor of any flux tube in the hole, suggesting that it would not be appropriate to use the expansion factor for entire hole to represent the divergence of the flux tube in analyzing the acceleration and heating of the plasma in coronal holes.     

Observational evidence for non-equilibrium ionization in the solar corona

We investigate whether temperature sensitive EUV line ratios can be used as observational signatures for the presence of non-equilibrium ionization in transition region plasma. We compute the total intensity of some EUV lines of carbon and oxygen expected from coronal loop models with a steady-state flow and which are known to have significant departures from ionization equilibrium, selecting lines whose intensity ratios are useful for deducing the electron temperature in the coronal plasma. We calculate the intensity ratios with and without the approximation of ionization equilibrium, in order to determine the effects of any deviations from equilibrium on the numerical values of the line ratios examined.     

Working Group 3 Report: Coronal Hole Boundaries and Interactions with Adjacent Regions

Summarized below are the discussions of working group 3 on "Coronal hole boundaries and interactions with adjacent regions" which took place at the 7th SOHO workshop in Northeast Harbor, Maine, USA, 28 September to 1 October 1998. A number of recent observational and theoretical results were presented during the discussions to shed light on different aspects of coronal hole boundaries. The working group also included presentations on streamers and coronal holes to emphasis the difference between the plasma properties in these regions, and to serve as guidelines for the definition of the boundaries. Observations, particularly white light observations, show that multiple streamers are present close to the solar limb at all times. At some distance from the sun, typically below 2 R, these streamers merge into a relatively narrow sheet as seen, for example, in LASCO and UVCS images. The presence of multiple current sheets in interplanetary space was also briefly addressed. Coronal hole boundaries were defined as the abrupt transition from the bright appearing plasma sheet to the dark coronal hole regions. Observations in the inner corona seem to indicate a transition of typically 10 to 20 degrees, whereas observations in interplanetary space, carried out from Ulysses, show on one hand an even faster transition of less than 2 degrees which is in agreement with earlier Helios results. On the other hand, these observations also show that the transition happens on different scales, some of which are significantly larger. The slow solar wind is connected to the streamer belt/plasma sheet, even though the discussions were still not conclusive on the point where exactly the slow solar wind originates. Considered the high variability of plasma characteristics in slow wind streams, it seems most likely that several types of coronal regions produce slow solar wind, such as streamer stalks, streamer legs and open field regions between active regions, and maybe even regions just inside of the coronal holes. Observational and theoretical studies presented during the discussions show evidence that each of these regions may indeed contribute to the solar slow wind. 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.     

Coupling of the coronal he abundance to the solar wind

Models of the transition region — corona — solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the the solar wind proton flux. The thermal force on -particles in the transition region sets the flow of helium into the corona. The frictional coupling between -particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content.     

Coronal streamers' theories

Some theoretical aspects of solar coronal streamers are discussed with emphasis on the current sheet and reconnection processes going on along the axis of the streamer. The dynamics of the streamer is a combination of MHD and transport, with acceleration of particles due to reconnection and leakage of plasma outwards as a slow solar wind as the observable results. The presence of the almost-closed magnetic bottles of streamers that can store high-energy particles for significant times provides the birdcage for solar cosmic rays, the reconnection in the sheet feeds medium-energy protons into the corona for the large-scale storage needed for certain flare models, and the build-up of excess density sets the stage for coronal mass ejections.     

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.     

Coronal plumes and fine scale structure in high speed solar wind streams

We present a solar wind model which takes into account the possible origin of fast solar wind streams in coronal plumes. We treat coronal holes as being made up of essentially 2 plasma species, denser, warmer coronal plumes embedded in a surrounding less dense and cooler medium. Pressure balance at the coronal base implies a smaller magnetic field within coronal plumes than without. Considering the total coronal hole areal expansion as given, we calculate the relative expansion of plumes and the ambient medium subject to transverse pressure balance as the wind accelerates. The magnetic flux is assumed to be conserved independently both within plumes and the surrounding coronal hole. Magnetic field curvature terms are neglected so the model is essentially one dimensional along the coronal plumes, which are treated as thin flux-tubes. We compare the results from this model with white-light photographs of the solar corona and in-situ measurements of the spaghetti-like fine-structure of high-speed winds.     

Coronal index of solar activity – Solar-terrestrial research

The purpose of this paper is to introduce a coronal index of solar activity (CI) as computed from ground-based observations of the green coronal line intensities (Fe?xiv; 530.3 nm). This index, expressed in W s^?1, is derived for the period 1939–1998 and belongs to the class of ground-based indices used to study solar activity and its influence on the heliosphere. The smoothed peak solar cycle intensity of the CI increased monotonically during the period of research. On the other hand, comparative studies have shown relatively good agreement with similar solar indices. CI can be used to study, among other things, the rotation of the Sun as a star, and long-term, intermediate- and short-term periodicities. The CI is inferred from a homogeneous coronal data set that can be used to study such topics as the 2D distribution of the green corona and photospheric/chromospheric activity, the differential rotation of the emission green corona, and the relationship between the green corona and cosmic rays.     

Source Region of High and Low Speed Wind during the Spartan 201-05 Flight

The large-scale coronal magnetic fields of the Sun are believed to play an important role in organizing the coronal plasma and channeling the high and low speed solar wind along the open magnetic field lines of the polar coronal holes and the rapidly diverging field lines close to the current sheet regions, as has been observed by the instruments aboard the Ulysses spacecraft from March 1992 to March 1997. We have performed a study of this phenomena within the framework of a semi-empirical model of the coronal expansion and solar wind using Spartan, SOHO, and Ulysses observations during the quiescent phase of the solar cycle. Key to this understanding is the demonstration that the white light coronagraph data can be used to trace out the topology of the coronal magnetic field and then using the Ulysses data to fix the strength of the surface magnetic field of the Sun. As a consequence, it is possible to utilize this semi-empirical model with remote sensing observation of the shape and density of the solar corona and in situ data of magnetic field and mass flux to predict values of the solar wind at all latitudes through out the solar system. We have applied this technique to the observations of Spartan 201-05 on 1–2 November, 1998, SOHO and Ulysses during the rising phase of this solar cycle and speculate on what solar wind velocities Ulysses will observe during its polar passes over the south and the north poles during September of 2000 and 2001. In order to do this the model has been generalized to include multiple streamer belts and co-located current sheets. The model shows some interesting new results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observations of the solar wind from coronal holes

The solar wind emanating from coronal holes (CH) constitutes a quasi-stationary flow whose properties change only slowly with the evolution of the hole itself. Some of the properties of the wind from coronal holes depend on whether the source is a large polar coronal hole or a small near-equatorial hole. The speed of polar CH flows is usually between 700 and 800 km/s, whereas the speed from the small equatorial CH flows is generally lower and can be <400 km/s. At 1 AU, the average particle and energy fluxes from polar CH are 2.5×10⁸ cm^–2 sec^–1 and 2.0 erg cm^–2 s^–1. This particle flux is significantly less than the 4×10⁸ cm^–2 sec^–1 observed in the slow, interstream wind, but the energy fluxes are approximately the same. Both the particle and energy fluxes from small equatorial holes are somewhat smaller than the fluxes from the large polar coronal holes.Many of the properties of the wind from coronal holes can be explained, at least qualitatively, as being the result of the effect of the large flux of outward-propagating Alfvén waves observed in CH flows. The different ion species have roughly equal thermal speeds which are also close to the Alfvén speed. The velocity of heavy ions exceeds the proton velocity by the Alfvén speed, as if the heavy ions were surfing on the waves carried by the proton fluid.The elemental composition of the CH wind is less fractionated, having a smaller enhancement of elements with low first-ionization potentials than the interstream wind, the wind from coronal mass ejections, or solar energetic particles. There is also evidence of fine-structure in the ratio of the gas and magnetic pressures which maps back to a scale size of roughly 1° at the Sun, similar to some of the fine structures in coronal holes such as plumes, macrospicules, and the supergranulation.     

Observed Aspects of Reconnection in Solar Eruptions

The observed magnetic field configuration and signatures of reconnection in the large solar magnetic eruptions that make major flares and coronal mass ejections and in the much smaller magnetic eruptions that make X-ray jets are illustrated with cartoons and representative observed eruptions. The main reconnection signatures considered are the imaged bright emission from the heated plasma on reconnected field lines. In any of these eruptions, large or small, the magnetic field that drives the eruption and/or that drives the buildup to the eruption is initially a closed bipolar arcade. From the form and configuration of the magnetic field in and around the driving arcade and from the development of the reconnection signatures in coordination with the eruption, we infer that (1) at the onset of reconnection the reconnection current sheet is small compared to the driving arcade, and (2) the current sheet can grow to the size of the driving arcade only after reconnection starts and the unleashed erupting field dynamically forces the current sheet to grow much larger, building it up faster than the reconnection can tear it down. We conjecture that the fundamental reason the quasi-static pre-eruption field is prohibited from having a large current sheet is that the magnetic pressure is much greater than the plasma pressure in the chromosphere and low corona in eruptive solar magnetic fields.     

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.     

Electrodynamics of the outer solar atmosphere

The structure of the outer solar atmosphere and its magnetic coupling to the photospheric motions indicate the existence of large-scale current systems. The heating and the dynamics of coronal structures is therefore governed by electrodynamic coupling of these structures to the underlying photosphere. In a structured corona, the heating is enhanced because of several processes such as resonance absorption of Alfvénic surface waves, anomalous Joule heating, reconnection and the related topological dissipation. The global thermal and dynamic behaviour of coronal structures can be fruitfully described in terms of equivalent electrodynamic circuits, taking into account the paramount role of the photospheric boundaries. Coronal current systems may be stable, as in the case of coronal loops, but occassionally they show catastrophic behaviour if the current intensity surpasses a critical threshold.     

Coronal holes and solar magnetic fields

Since 1972, nearly continuous observations of coronal holes and their associated photospheric magnetic fields have been made using a variety of satellite and ground-based equipment. The results of comparisons of these observations are reviewed and it is demonstrated that the structure and evolution of coronal holes is basically governed by the large-scale distribution of photospheric magnetic flux. Non-polar holes form in the decaying remnants of bipolar magnetic regions in areas with a large-scale flux imbalance. There is strong indirect evidence that the magnetic field in coronal holes is always open to interplanetary space but not all open-field regions have associated coronal holes. The well-observed declining phase of the last solar cycle was characterized by stable magnetic field and coronal hole patterns which were associated with recurrent, high-speed wind streams and interplanetary magnetic field patterns at the Earth. The ascending phase of the current cycle has been characterized by transient magnetic field and coronal hole patterns which tend to occur at high solar latitudes. This shift in magnetic field and coronal hole patterns has resulted in a less obvious and more complicated association with high-speed wind streams at the Earth.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting Scientist, Kitt Peak National Observatory.     

Manifestation of magnetic reconnection in coronal streamer current sheets

We investigate the possibility of observing the effects of magnetic reconnection inside a current sheet forming in a coronal streamer in the extended corona. In particular we study the possibility to observe with the UVCS of SOHO the excitation of the tearing instability in the current sheet.