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.     

Connections Between the White-Light Eclipse Corona and Magnetic Fields over the Solar Cycle

Observations of ten solar eclipses (1973–1999) enabled us to reveal and describe mutual relations between the white-light corona structures (e.g., global coronal forms and most conspicuous coronal features, such as helmet streamers and coronal holes) and the coronal magnetic field strength and topology. The magnetic field strength and topology were extrapolated from the photospheric data under the current-free assumption. In spite of this simplification the found correspondence between the white-light corona structure and magnetic field organization strongly suggests a governing role of the field in the appearance and evolution of local and global structures. Our analysis shows that the study of white-light corona structures over a long period of time can provide valuable information on the magnetic field cyclic variations. This is particularly important for the epoch when the corresponding measurements of the photospheric magnetic field are absent.     

The Flattening Index of the Eclipse White-Light Corona and Magnetic Fields

From observations of the solar white-light corona at 65 eclipses from 1851 to 2015 we confirm earlier findings that the flattening index of the white-light corona depends on the phase, rather than the magnitude of solar cycles, which is in contrast with behavior of other major solar activity indices like the sunspot number, the 2800 MHz radio flux, etc. This indicates that mechanisms responsible for creation and distribution of helmet streamers, the most essential coronal structures influencing the flattening index, could be of different magnetic nature from those of other manifestations of solar surface activity.     

Intricacies of the fine structure of the eclipse corona

The inner white-light corona (up to 2 solar radii) can only be observed during total solar eclipses. New mathematical methods of the corona image processing and digital photo cameras or CCD cameras allow us to detect very faint structures (of a few arcseconds) in this part of the corona, even from images taken with relatively small telescopes (1–2 meters in the focal length). In the present paper we will discuss such structures as observed during the last few solar eclipses, mainly those of 2001 and 2006. Obtained results show that the white-light corona is highly structured not only in the sense of a variety of different types of its classical “objects”, e.g., polar plumes, helmet streamers, threadlike streamers, etc, but also within these objects themselves. Voids, loops, radial and non-radial threads, and other yet-undefined dark structures (“empty space”?) are well visible especially inside helmet streamers. This strongly indicates that the classical picture of the corona characterized by a hydrostatic distribution of density and temperature is no longer a sufficient assumption. It is magnetic forces that play a dominant role in shaping and structuring this part the corona. Given a remarkable similarity between the EUV corona as observed by SOHO and the white-light corona observed by us during the above-mentioned eclipses up to two solar radii. We suggest that the “missing” observations of the white-light corona should be replaced by those of the EUV one. Moreover, the last eclipse’s observations also indicate that the knots of some prominences extend well into the white-light corona. So, the next total eclipses of the Sun, of 1 August 2008 and 22 July 2009, offer an excellent opportunity for preparing joint observations for space-borne and ground-based eclipse teams.     

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.     

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.     

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.     

White-Light Observations and Polarimetric Analysis of the Solar Corona During the Eclipse of 1 August 2008

In order to study the solar corona during eclipses, a new telescope was constructed. Three coronal images were obtained simultaneously through a single objective of the telescope as the coronal radiation passed through three polarizers (whose transmission directions were turned 0°, 60°, and 120° in the chosen direction); one image was obtained without a polarizer. The telescope was used to observe the solar corona during the eclipse of 1 August 2008. We obtained the distributions of polarization brightness, K-corona brightness, the degree of K-corona polarization and the total polarization degree; the polarization direction, depending on the latitude and radius in the plane of the sky, was also obtained. We calculated the radial distributions of electron density depending on the latitude. The properties of all these distributions were compared for different coronal structures. We determined the temperature of the coronal plasma in different coronal structures assuming hydrostatic equilibrium.     

Polarization of the white-light corona and its large-scale structure in the period of solar cycle maximum

Distributions of brightness and polarization,p, were obtained for the February 16, 1980 solar corona. Isophotes have a circular shape, typical for the period of the solar cycle maximum. A variety of structural features are distinctly seen in the distribution ofp. The polarization reaches 55%, and thep values are comparatively high, not only in the well-defined streamers, but in the overlapping faint rays and small streamers, as well. A theoretical interpretation of the observed high degrees of polarization, taking into account the data on coronal brightness, is very difficult. This cannot be done within the scope of spherically symmetric models of the corona; the assumption of a high concentration of coronal matter into the plane of the sky is needed. With the most extreme densities in coronal structures, it is not, however, possible to exceed the observed valuep = 55%. Taking into account the accuracy of the polarization measurements, there are no reasons to reject the Thomson scattering as a basic mechanism to explain the origin of the white-light corona.     

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.     

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.     

Helmet Streamers with Triple Structures: Simulations of resistive dynamics

Recent observations of the solar corona with the LASCO coronagraph on board of the SOHO spacecraft have revealed the occurrence of triple helmet streamers even during solar minimum, which occasionally go unstable and give rise to large coronal mass ejections. There are also indications that the slow solar wind is either a combination of a quasi-stationary flow and a highly fluctuating component or may even be caused completely by many small eruptions or instabilities. As a first step we recently presented an analytical method to calculate simple two-dimensional stationary models of triple helmet streamer configurations. In the present contribution we use the equations of time-dependent resistive magnetohydrodynamics to investigate the stability and the dynamical behaviour of these configurations. We particularly focus on the possible differences between the dynamics of single isolated streamers and triple streamers and on the way in which magnetic reconnection initiates both small scale and large scale dynamical behaviour of the streamers. Our results indicate that small eruptions at the helmet streamer cusp may incessantly accelerate small amounts of plasma without significant changes of the equilibrium configuration and might thus contribute to the non-stationary slow solar wind. On larger time and length scales, large coronal eruptions can occur as a consequence of large scale magnetic reconnection events inside the streamer configuration. Our results also show that triple streamers are usually more stable than a single streamer.     

Ultraviolet spectroscopic observations of coronal streamers in the SOHO era

Measurements made with the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory can be used to determine physical parameters in the solar corona such as hydrogen and ion kinetic temperatures, electron densities, and absolute elemental abundances. Hydrogen and ion outflow velocities can be determined by combining the UV spectroscopic measurements with white light polarized brightness measurements. These combined measurements can be used to reveal physical characteristics of coronal streamers. To date we have studied plasma properties, such as the variation of plasma outflows in quiescent streamers, primarily in classic helmet streamers at solar minimum. Out-flows have not been observed in the centers of coronal streamers suggesting that these are closed magnetic field regions. We propose to study all of the coronal streamers in the UVCS synoptic dataset in order to investigate different types of streamers and their long-term evolution.     

Three-dimensional coronal structures using clark lake observations

We have undertaken a study of coronal features observed at meter-decameter wavelengths using the Clark Lake radioheliograph. Among the coronal structures we have studied are the radio manifestations of coronal streamers on the solar disk and above the solar limb. We have analyzed the radio data quantitatively, using ray-tracing models for comparison with the maps. Our study provides information about the streamers' three-dimensional shapes, scales, and density profiles, for comparison with related observations using white-light coronagraphs.     

Long-lived coronal structures and recurrent geomagnetic patterns in 1974

Daily measurements of the intensity distribution of the Sun's white-light corona over the height range $\text{1.1–2.7. R}{}_{\mathrm{?}}$ show that the global structure became quite stable (constant over periods of several months) in late 1973 and throughout 1974, as flares, ascending prominences and other transient activity became less frequent with the decline of the solar activity cycle. A highly persistent pattern of geomagnetic activity prevailed for much of this time. Bright coronal structures in the ecliptic plane were associated with geomagnetically quiet conditions, and faint coronal regions (“holes”) with geomagnetic disturbance, after a delay of about three days. These results confirm the “cone-of-avoidance” model for M-regions and reinforce the postulate that high-speed streams in the solar wind originate from coronal holes. Identification of coronal holes from ground-based K-coronal observations corresponds well with those made from spacecraft EUV and X-ray experiments on OSO-7 and Skylab.     

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.     

A study of the background corona near solar minimum

The white light coronagraph data from Skylab is used to investigate the equatorial and polarK andF coronal components during the declining phase of the solar cycle near solar minimum. Measurements of coronal brightness and polarization brightness product between 2.5 and 5.5R during the period of observation (May 1973 to February 1974) lead to the conclusions that: (1) the equatorial corona is dominated by either streamers or coronal holes seen in projections on the limb approximately 50% and 30% of the time, respectively; (2) despite the domination by streamers and holes, two periods of time were found which were free from the influences of streamers or holes (neither streamers nor holes were within 30° in longitude of the limb); (3) the derived equatorial background density model is less than 15% below the minimum equatorial models of Newkirk (1967) and Saito (1970); (4) a spherically symmetric density model for equatorial coronal holes yields densities one half those of the background density model; and (5) the inferred brightness of theF-corona is constant to within ±10% and ±5% for the equatorial and polar values, respectively, over the observation period. While theF-corona is symmetric at 2R it begins to show increasing asymmetry beyond this radius such that at 5R the equatorialF-coronal brightness is 25% greater than the polar brightness.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Streamer Belt and Chains as the Main Sources of Quasi-Stationary Slow Solar Wind

Synoptic maps of white-light coronal brightness from SOHO/LASCO C2 and distributions of solar wind velocity obtained from interplanetary scintillation are studied. Regions with velocity V≈300 – 450 km s⁻¹ and increased density N>10 cm⁻³, typical of the “slow” solar wind originating from the belt and chains of streamers, are shown to exist at Earth’s orbit, between the fast solar wind flows (with a maximum velocity V _max≈450 – 800 km s⁻¹). The belt and chains of streamers are the main sources of the “slow” solar wind. As the sources of “slow” solar wind, the contribution from the chains of streamers may be comparable to that from the streamer belt.     

The effects of large- and small-scale density structures on the radio emission from coronal streamers

The radio observations of the coronal streamers obtained using Clark Lake radioheliograph at 73.8, 50.0, and 38.5 MHz during a period of minimum activity in September 1986 are presented. Streamers appear to correlate with two prominent disk sources whose intensities fluctuated randomly. The variations in half-power diameter of the radio Sun are found to correspond with the variations in the white-light extents of the coronal streamers. It appears that the shape of the radio Sun is not a function of the phase of the solar cycle; instead it depends on the relative positions of the streamers in the corona. The observed peak brightness temperatures,T _B , of the streamers are found to be very low, being 6 × 10⁴ K.We compute the brightness temperature distribution along the equator by tracing the rays in the coronal plasma. The rays are deflected away by the streamers before reaching the critical density level, whereas they penetrate deeper into the coronal hole for small angles between the line of sight and the streamer axis. As a consequence, it is found that the streamers and coronal holes appear in the calculated equatorial brightness distribution as irregular brightness depressions and enhancements, respectively. The fine structures are found to disappear when the scattering due to small-scale density inhomogeneities is included in the ray-tracing calculations. The required relative level of density fluctuations, ₁ = N/N, is found to be greater than 12% to reduce the peak brightness temperature from 10⁶ K to 6 × 10⁴ K for all the three frequencies.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.