Coronal streamers

This work extends a previous analysis of helmet streamers into the somewhat higher range of coronal temperature where streamer geometries are shown to be open, in the sense that there is solar wind expansion everywhere. It is shown that, for a given photospheric field distribution, a certain minimum temperature is required for this type of streamer - this minimum temperature coinciding with the maximum temperature compatible with a helmet streamer. Near this minimum temperature, the streamer is very constricted and the critical point in the streamer core lies at the point of minimum cross-section. Hence the throat, under these conditions, becomes a true geometrical throat rather than the conventional gravitational throat. As the temperature is increased, the streamer shape becomes correspondingly more radial and the location of the throat becomes asymptotically more gravitationally determined. Residual manifestations of coronal streamers at large distances are investigated. It is found that lateral density variations at the earth's orbit tend to be small but velocity variations can become appreciable (100–200 km/sec) for streamers originating in regions where the photospheric magnetic field is strong. At large distances, either streamer or interstreamer regions can dominate, the former occurring at high temperature (2 × 10⁶K) and the latter being favored at lower temperature (1.5 × 10⁶K). In all cases the cross-section becomes essentially radial just above the point where it is a minimum. The marked sensitivity of these shapes to coronal temperature is pointed out - computations indicating that streamers can vary from helmet configurations to almost radial filaments for a very slight increase in temperature. This behavior suggests a strong solar cycle influence upon coronal form.     

Helmet Streamers with Triple Structures: Weakly Two-Dimensional Stationary States

Recent observations of the solar corona with the LASCO coronagraph on board the SOHO spacecraft have revealed the occurrence of triple helmet streamers even during solar minimum, which occasionally go unstable and give rise to particularly huge coronal mass ejections. We present a method to calculate (semi-)analytically self-consistent stationary configurations of triple helmet streamers which can serve as input for stability considerations and dynamical calculations. The method is based on an asymptotic expansion procedure using the elongated structure of the streamers. The method is very flexible and can be used in both Cartesian and spherical geometry. We discuss the effects of magnetic shear, gravity and field-aligned flow on open field lines. Example solutions illustrating the influence of each of these features on the solution structure are presented.     

INFERENCES ON CORONAL MAGNETIC FIELDS FROM SOHO UVCS OBSERVATIONS

The characteristics of the magnetic field ubiquitously permeating the coronal plasma are still largely unknown. In this paper we analyze some aspects of coronal physics, related to the magnetic field behavior, which forthcoming SOHO UVCS observations can help better understand. To this end, three coronal structures will be examined: streamers, coronal mass ejections (CMEs), and coronal holes.As to streamers and CMEs, we show, via simulations of the Ly- and white-light emission from these objects, calculated on the basis of recent theoretical models (Wang et al., 1995), how new data from SOHO can help in advancing our knowledge of the streamer/CME magnetic configuration. Our discussion highlights also those observational signatures which might offer clues on reconnection processes in streamers' current sheets.Coronal holes (CHs) are discussed in the last section of the paper. Little is known about CH flux tube geometry, which is closely related to the behavior of the solar wind at small heliocentric distances.Indirect evidence for the flux tube spreading factors, within a few solar radii, is here examined.     

Study of the structure of streamer belts and chains in the solar corona

A comparison is made of polarization brightness distributions of the white-light corona based on the data from Mark III (MLSO) with calculated magnetic configurations in the corona (a potential-field approximation) between adjacent coronal holes (or associated open magnetic tubes) with magnetic fields of single polarity. It is shown that in these coronal regions, which were referred to as chains of streamers in earlier papers of these authors, magnetic structures in meridional and azimuthal cross-sections typically have the aspect of helmet streamers. The calculated magnetic field configurations under the helmets of chains of streamers have an even number of systems of arched features closely adjacent to each other (unlike the structure of the streamer belt with a neutral line where their number is odd). The height of the helmet top of chains of streamers remains smaller than that of the source surface but changes along the length of the chain and reaches maximum values at the ends of the chain.     

Interplanetary Signatures of Unipolar Streamers and the Origin of the Slow Solar Wind

Unipolar streamers (also known as pseudo-streamers) are coronal structures that, at least in coronagraph images, and when viewed at the correct orientation, are often indistinguishable from dipolar (or “standard”) streamers. When interpreted with the aid of a coronal magnetic field model, however, they are shown to consist of a pair of loop arcades. Whereas dipolar streamers separate coronal holes of the opposite polarity and whose cusp is the origin of the heliospheric current sheet, unipolar streamers separate coronal holes of the same polarity and are therefore not associated with a current sheet. In this study, we investigate the interplanetary signatures of unipolar streamers. Using a global MHD model of the solar corona driven by the observed photospheric magnetic field for Carrington rotation 2060, we map the ACE trajectory back to the Sun. The results suggest that ACE fortuitously traversed through a large and well-defined unipolar streamer. We also compare heliospheric model results at 1 AU with ACE in-situ measurements for Carrington rotation 2060. The results strongly suggest that the solar wind associated with unipolar streamers is slow. We also compare predictions using the original Wang–Sheeley (WS) empirically determined inverse relationship between solar wind speed and expansion factor. Because of the very low expansion factors associated with unipolar streamers, the WS model predicts high speeds, in disagreement with the observations. We discuss the implications of these results in terms of theories for the origin of the slow solar wind. Specifically, premises relying on the expansion factor of coronal flux tubes to modulate the properties of the plasma (and speed, in particular) must address the issue that while the coronal expansion factors are significantly different at dipolar and unipolar streamers, the properties of the measured solar wind are, at least qualitatively, very similar.     

Large-Scale Active Coronal Phenomena in YOHKOH SXT Images. III. Enhanced Post-Flare Streamer

We demonstrate several events where an eruptive flare close to the limb gave rise to a transient coronal streamer visible in X-rays in Yohkoh SXT images, and analyze one of these events, on 28–29 October 1992, in detail. A coronal helmet streamer began to appear 2 hours after the flare, high above rising post-flare loops; the streamer became progressively narrower, reaching its minimum width 7–12 hours after the flare, and widened again thereafter, until it eventually disappeared. Several other events behaved in a similar way. We suggest that the minimum width indicates the time when the streamer became fully developed. All the time the temperature in the helmet streamer structure was decreasing, which can explain the subsequent fictitious widening of the X-ray streamer. It is suggested that we may see here two systems of reconnection on widely different altitudes, one giving rise to the post-flare loops while the other creates (or re-forms) the coronal helmet streamer. A similar interpretation was suggested in 1990 by Kopp and Polettofor post-flare giant arches observed on board the SMM; indeed, there are some similarities between these post-flare helmet streamers and giant arches and, with the low spatial resolution of SMM instruments, it is possible that some helmet streamers could have been considered to be a kind of a giant arch.     

Small Solar Wind Transients and Their Connection to the Large-Scale Coronal Structure

It has been realized for some time that the slow solar wind with its embedded heliospheric current sheet often exhibits complex features suggesting at least partially transient origin. In this paper we investigate the structure of the slow solar wind using the observations by the Wind and STEREO spacecraft during two Carrington rotations (2054 and 2055). These occur at the time of minimum solar activity when the interplanetary medium is dominated by recurrent high-speed streams and large-scale interplanetary coronal mass ejections (ICMEs) are rare. However, the signatures of transients with small scale-sizes and/or low magnetic field strength (comparable with the typical solar wind value, ～?5 nT) are frequently found in the slow solar wind at these times. These events do not exhibit significant speed gradients across the structure, but instead appear to move with the surrounding flow. Source mapping using models based on GONG magnetograms suggests that these transients come from the vicinity of coronal source surface sector boundaries. In situ they are correspondingly observed in the vicinity of high density structures where the dominant electron heat flux reverses its flow polarity. These weak transients might be indications of dynamical changes at the coronal hole boundaries or at the edges of the helmet streamer belt previously reported in coronagraph observations. Our analysis supports the idea that even at solar minimum, a considerable fraction of the slow solar wind is transient in nature.     

Composition of Coronal Streamers from the SOHO Ultraviolet Coronagraph Spectrometer

The Ultraviolet Coronagraph Spectrometer on the SOHO satellite covers the 940–1350 Å range as well as the 470–630 Å range in second order. It has detected coronal emission lines of H, N, O, Mg, Al, Si, S, Ar, Ca, Fe, and Ni, particularly in coronal streamers. Resonance scattering of emission lines from the solar disk dominates the intensities of a few lines, but electron collisional excitation produces most of the lines observed. Resonance, intercombination and forbidden lines are seen, and their relative line intensities are diagnostics for the ionization state and elemental abundances of the coronal gas. The elemental composition of the solar corona and solar wind vary, with the abundance of each element related to the ionization potential of its neutral atom (First Ionization Potential–FIP). It is often difficult to obtain absolute abundances, rather than abundances relative to O or Si. In this paper, we study the ionization state of the gas in two coronal streamers, and we determine the absolute abundances of oxygen and other elements in the streamers. The ionization state is close to that of a log T = 6.2 plasma. The abundances vary among, and even within, streamers. The helium abundance is lower than photospheric, and the FIP effect is present. In the core of a quiescent equatorial streamer, oxygen and other high-FIP elements are depleted by an order of magnitude compared with photospheric abundances, while they are depleted by only a factor of 3 along the edges of the streamer. The abundances along the edges of the streamer (‘legs’) resemble elemental abundances measured in the slow solar wind, supporting the identification of streamers as the source of that wind component.     

Magnetic configurations of streamer structures in the solar atmosphere

We consider two types of streamer structures observed in the solar atmosphere. Structures of the first type are medium-scale configurations with scale lengths comparable to the scale height in the corona, kT/mg = 100 thousand km, which appear as characteristic plasma structures in the shape of a dome surrounding the active region with thin streamers emanating from its top. In configurations of this type, gravity plays no decisive role in the mass distribution. The plasma density is constant on magnetic surfaces. Accordingly, the structure of the configurations is defined by the condition ψ = const, where ψ is the flux function of the magnetic field. Structures of the second type are large-scale configurations (coronal helmets, loops, and streamers), which differ from the above structures in that their scale lengths exceed the scale height in the corona. For them, gravity plays a decisive role; as a result, instead of the magnetic surfaces, the determining surface is BgradΦ = 0. We constructed three-dimensional images of these structures. Some of the spatial curves called “visible contours” of the B_r = 0 surface are shown to be brightest in the corona. We assume that the helmet boundaries and polar plumes are such curves.     

Time–Latitudinal Development of the White-Light Coronal Structures over a Solar Cycle

Large-scale coronal structures (helmet streamers) observed in the white-light corona during total solar eclipses and/or with ground-based coronagraphs are mostly located only above quiescent types of prominences. These helmet streamers are maintained due to the magnetic fields of the Sun. Time–latitudinal distribution of prominences during a solar cycle, however, shows both the poleward and equatorward migrations, similar to the 530.3 nm emission corona (the green corona) intensities. Distribution of observed coronal helmet streamers during total solar eclipses, enlarged with the helmet streamers as were obtained by the ground-based coronagraph observations, are compared with the heliographic distribution of prominences and the green corona intensities for the first time. It is shown that the distribution of above-mentioned helmet streamers, reflects – roughly – the time–latitudinal distribution of prominences and emission corona branches, and migrates together with them over a solar cycle.     

Pre-CME Onset Fuses – Do the STEREO Heliospheric Imagers Hold the Clues to the CME Onset Process?

Understanding the onset of coronal mass ejections (CMEs) is surely one of the holy grails of solar physics today. Inspection of data from the Heliospheric Imagers (HI), which are part of the SECCHI instrument suite aboard the two NASA STEREO spacecraft, appears to have revealed pre-eruption signatures which may provide valuable evidence for identifying the CME onset mechanism. Specifically, an examination of the HI images has revealed narrow rays comprised of a series of outward-propagating plasma blobs apparently forming near the edge of the streamer belt prior to many CME eruptions. In this pilot study, we inspect a limited dataset to explore the significance of this phenomenon, which we have termed a pre-CME ‘fuse’. Although, the enhanced expulsion of blobs may be consistent with an increase in the release of outward-propagating blobs from the streamers themselves, it could also be interpreted as evidence for interchange reconnection in the period leading to a CME onset. Indeed, it is argued that the latter could even have implications for the end-of-life of CMEs. Thus, the presence of these pre-CME fuses provides evidence that the CME onset mechanism is either related to streamer reconnection processes or the reconnection between closed field lines in the streamer belt and adjacent, open field lines. We investigate the nature of these fuses, including their timing and location with respect to CME launch sites, as well as their speed and topology.     

Some aspects of coronal streamer dynamics

Observations of coronal streamers suggest that these configurations are stationary in terms of the convective time scale, but not with respect to the diffusive processes. In this paper, the diffusive time scale is estimated and the thickness of a stationary streamer where convective and diffusive processes are balanced turns out to be very small.Instead of balancing the terms in the magnetic equation to obtain a zero time derivative, we suggest that the diffusive part is negligible and therefore, the convective term must be zero in the largest part of the streamer. This can be obtained by introducing flux transport through the streamer.We show that the arising convective model is to be understood from evolutionary arguments; the upward flux transport leads to a time scale for the life of a streamer, related to the evolution of the underlying magnetic field. Slight differences occur between stationary models and the present model; these differences are shown not to be in conflict with radio observations.     

MHD shock interactions in coronal structures

We consider the magnetohydrodynamic (MHD) interactions of solar coronal fast shock waves of flare and/or nonflare origin with the boundaries of coronal streamers and coronal holes. Boundaries are treated as MHD tangential discontinuities (TD). Different parameters of the observed corona are used in the investigation. The general case of the oblique interaction is studied.It is shown that a solar fast shock wave must be refracted usually as a fast shock wave inside the coronal streamer. For the special case of the velocity shear across TD, a slow shock wave is generated. On the contrary, the shock wave refracted inside the coronal hole is indeed a slow shock wave.The significance of different effects due to the interaction of fast and slow shock waves on the coronal magnetic field is noticed, especially at the time of a coronal mass ejection (CME). It is also shown, that an oblique fast MHD coronal shock wave may trigger an instability at the boundary of a streamer considered as a TD. It might have a relation with the observed process of abrupt disappearance of the streamer's boundary in the solar corona.On leave from the Academy of Sciences, Central Astronomical Observatory Pulkovo, 196140, St. Petersburg, Russia.     

Magnetic and Velocity Shear Driven Instabilities in the Heliospheric Plasma

We have addressed the problem of combined magnetic and velocity shear driven instabilities in the context of the heliospheric plasma. New high-order numerical methods have been used to analyze the instability dynamics of the heliospheric current-sheet interacting with the structure determined by the slow component of the solar wind on the solar equatorial plane above the helmet streamers. Preliminary results are presented.     

Coronal holes

Coronal holes are extensive regions of extremely low density in the solar corona within 60° of latitude from the equator. (They are not to be confused with the well-known coronal cavities which surround quiescent prominences beneath helmet streamers.) We have superposed maps of the calculated current-free (potential) coronal magnetic field with maps of the coronal electron density for the period of November 1966, and find that coronal holes are generally characterized by weak and diverging magnetic field lines. The chromosphere underlying the holes is extremely quiet, being free of weak plages and filaments. The existence of coronal holes clearly has important implications for the energy balance in the transition region and the solar wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A coronal magnetic field model with horizontal volume and sheet currents

When globally mapping the observed photospheric magnetic field into the corona, the interaction of the solar wind and magnetic field has been treated either by imposing source surface boundary conditions that tacitly require volume currents outside the source surface (Schatten, Wilcox, and Ness, 1969) or by limiting the interaction to thin current sheets between oppositely directed field regions (Wolfson, 1985). Yet observations and numerical MHD calculations suggest the presence of non-force-free volume currents throughout the corona as well as thin current sheets in the neighborhoods of the interfaces between closed and open field lines or between oppositely directed open field lines surrounding coronal helmet-streamer structures. This work presents a model including both horizontal volume currents and streamer sheet currents. The present model builds on the magnetostatic equilibria developed by Bogdan and Low (1986) and the current-sheet modeling technique developed by Schatten (1971). The calculation uses synoptic charts of the line-of-sight component of the photospheric magnetic field measured at the Wilcox Solar Observatory. Comparison of an MHD model with the calculated model results for the case of a dipole field and comparison of eclipse observations with calculations for CR 1647 (near solar minimum) show that this horizontal current-current-sheet model reproduces polar plumes and axes of corona streamers better than the source-surface model and reproduces coronal helmet structures better than the current-sheet model.     

Observational Features of Large-Scale Structures as Revealed by the Catastrophe Model of Solar Eruptions

Large-scale magnetic structures are the main carrier of major eruptions in the solar atmosphere. These structures are rooted in the photosphere and are driven by the unceas-ing motion of the photospheric material through a series of equilibrium configurations. The motion brings energy into the coronal magnetic field until the system ceases to be in equilib-rium. The catastrophe theory for solar eruptions indicates that loss of mechanical equilibrium constitutes the main trigger mechanism of major eruptions, usually shown up as solar flares, eruptive prominences, and coronal mass ejections (CMEs). Magnetic reconnection which takes place at the very beginning of the eruption as a result of plasma instabilities/turbulence inside the current sheet, converts magnetic energy into heating and kinetic energy that are responsible for solar flares, and for accelerating both plasma ejecta (flows and CMEs) and energetic particles. Various manifestations are thus related to one another, and the physics behind these relationships is catastrophe and magnetic reconnection. This work reports on re- cent progress in both theoretical research and observations on eruptive phenomena showing the above manifestations. We start by displaying the properties of large-scale structures in the corona and the related magnetic fields prior to an eruption, and show various morphological features of the disrupting magnetic fields. Then, in the framework of the catastrophe theory, we look into the physics behind those features investigated in a succession of previous works, and discuss the approaches they used.     

WHITE-LIGHT POLARIZATION AND LARGE-SCALE CORONAL STRUCTURES

The results of the white-light polarization measurements performed during three solar eclipses (1973, 1980, 1991) are presented. The eclipse images were processed and analysed by the same technique and method and, consequently, the distributions of the polarization and coronal intensity around the Sun were obtained in unified form for all three solar eclipses. The mutual comparisons of our results, and their comparison with the distributions found by other authors, allowed the real accuracy of the current measurements of the white-light corona polarization, which is not worse than ±5%, to be estimated. We have investigated the behaviour of the polarization in dependence on heliocentric distance in helmet streamers and coronal holes. Simultaneous interpretation of the data on polarization and intensity in white-light helmet streamers is only possible if a considerable concentration of coronal matter (plasma) towards the plane of the sky is assumed. The values obtained for the coronal hole regions can be understood within the framework of a spherically symmetrical model of the low density solar atmosphere. A tendency towards increasing polarization in coronal holes, connected with the decrease of the hole's size and with the transition from the minimum to the maximum of the solar cycle, was noticed. The problem of how the peculiarities of the large-scale coronal structures are related to the orientation of the global (dipole) solar magnetic field and to the degree of the goffer character of the coronal and interplanetary current sheet is discussed briefly.     

The arch systems,cavities and prominences in the helmet streamer observed at the solar eclipse,November 12, 1966

Arch systems lying above quiescent prominences in the solar corona have long drawn the attention of eclipse observers, and such formations have been investigated since the end of the last century. Almost every eclipse photograph shows one or more arches, and in most cases the arch system is accompanied by a quiescent prominence below it and a helmet streamer above it. Also, in some cases there is a dark cavity between the arch system and the prominence.On large-scale photographs obtained at the November 12, 1966 eclipse, detailed photometry has been carried out on a formation in the corona composed of a helmet streamer straddling two multiple-arch systems each with a dark cavity and a quiescent prominence. The excess of electrons in the arches and the deficiency in the cavities are evaluated. We find that the formation of a prominence requires much more material than available in the cavity before depletion. Consequently the condensation theory of coronal matter into prominences seems to have difficulties explaining the necessary amount of matter in the cases where coronal arches - delineating magnetic field lines above the cavity - may exclude inflow of material from the corona. We comment on the low velocity of solar wind in the helmet streamer.The National Center for Atmospheric Research is sponsored by the National Science Foundation.