Observations of the solar corona in polarized white light during the total solar eclipse of February 16, 1980: Preliminary results

Photographic observations of the corona were made by a Belgian expedition to Kenya during the eclipse of 1980, February 16, using a polaroïd filter oriented in three directions 60 ° apart. The preliminary results of the reduced data are presented here; these results consist in the analysis of the three following parameters: total intensity, degree of polarization, direction of the electric vector. The local variation of these parameters reveals, among other features, a deep coronal hole at the south pole and peculiar streamer (P.A. 280 °) associated to a transient, (the so-called tennis racket) also observed by other teams, and a density enhancement at P.A. 200 °, possibly associated with a transient observed with the K-coronameter of the High Altitude Observatory 12 hr before the eclipse.     

Observations of mode coupling in the solar corona and bipolar noise storms

We review high-spatial-resolution observations of the Sun which reflect on the role of mode coupling in the solar corona, and present a number of new observations. We show that typically polarization inversion is seen at 5 GHz in active region sources near the solar limb, but not at 1.5 GHz. Although this is apparently in contradiction to the simplest form of mode coupling theory, in fact it remains consistent with current models for the active region emission. Microwave bursts show no strong evidence for polarization inversion. We discuss bipolar noise storm continuum emission in some detail, utilizing recent VLA observations at 327 MHz. We show that bipolar sources are common at 327 MHz. Further, the trailing component of the bipole is frequently stronger than the leading component, in apparent conflict with the leading-spot hypothesis. The observations indicate that at 327 MHz mode coupling is apparently strong at all mode-coupling layers in the solar corona. The 327 MHz observations require a much weaker magnetic field strength in the solar corona to explain this result than did earlier lower-frequency observations: maximum fields are 0.2 G. This is a much weaker field than is consistent with current coronal models.On leave from the Indian Institute for Astrophysics, Bangalore, India.     

Hard X-ray emission from the solar corona

This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration.     

The polarization of the inner solar corona at the eclipse of 10 July 1972

Photographs of the corona in the continuum spectrum (580–700 nm) have been obtained with a doublet camera (F=4m) through rotating sector polarizers with the vibrational directions oriented radially and tangentially to the solar limb. Isophotes of the total emission and its polarized component, as well as diagrams giving the degree of polarization for the (K+F)-corona and the electron corona proper (P _K), have been plotted up to the distance of 1R from the limb. In some inner regions the polarizationP _K exceeds the highest value possible for Thomson scattering under the given conditions. The reality of this anomally is dubious; systematic photometrical errors may account for it. Three-dimensional forms of 15 different coronal rays have been ascertained by comparing the course of polarization along the rays to the family of Baumach curves. The rays are found to deflect substantially from the radial directions out of the plane of the sky. Mean values of the coronal brightness and polarization versus the distance from the Sun have been determined. Contrary to well-known models (Van de Hulst) the mean polarization of the electron corona (P _K) decreases with distance after reaching the maximum (50%) at (1.4–1.6)R from the Sun's centre. This decrease can be explained by deflection of the streamers from radial directions.     

Pulsating radio emissions from the solar corona

Three solar outbursts which show pulsating radio emissions at metric waves (239 MHz) are examined. The behaviour of the single frequency, high-time resolution records and the spectral diagrams seem to indicate that such phenomena are peculiar phases of type IV radiation, perhaps connected with absorptions in the solar corona. The spectral analysis of the low-frequency modulation of the emissions show a very definite spectral line with a period ranging from 1^s.7 to 3^s.1.     

Observations and discussions concerning ‘high’ polarization features in the solar corona

Photographic observations were obtained of the radial and tangential polarization of the solar corona for the 1970, March 7, solar eclipse. The corona was photographed using a neutral density filter and rotating linear polaroid sectors to allow the polarization structure to be seen from 1 to 6 solar radii. Anomalously high polarizations were found for structures with the E-tangential intensity being predominantly larger than the E-radial intensity. These structures are generally filamentary in nature and radial in direction. One case with a high radial polarization was also found. The photographs were calibrated accurately against the Earth shine from the Moon. Possible source mechanisms are discussed that may explain this new component in the solar corona. Most sources may be ruled out on physical grounds. One possibility appears to be synchrotron radiation from 10 GeV electrons in a 0.4 G field. The existence of these electrons, however, is unlikely in that spacecraft observations at 1 AU do not confirm their presence.     

Heating of the solar corona

The suggestion is advanced that heating of the solar corona results from Landau damping of ion-acoustic waves generated in the motion of photospheric granules. Laboratory experiments relevant to the question of corona heating are discussed, together with the available observational information on the extent of energy deposition in the corona.Of the European Space Research Organization (ESRO).     

The spectrum of the inner corona observed during the total solar eclipse of 30 May 1965

A series of spectrograms of the inner solar corona were obtained at the total solar eclipse of 30 May 1965 using a fast spectrograph with a circular slit that recorded the spectrum from 3000 to 9000 at all position angles around the limb simultaneously. In this paper absolute intensity is given as a function of position angle for the stronger lines and the continuum. In the coronal enhancement or condensation centered at heliocentric position angle 293°, absolute intensity is given for 34 forbidden emission lines and the continuum.     

Observations of the infrared FeXIII lines in the solar corona of 12 November, 1966

The purpose of this paper is to report on some intensity measurements of the Fe xiii lines at 10 747 Å and 10 798 Å made during the total eclipse of 12 November, 1966. Infrared spectra were taken of the solar corona at a dispersion of 90 Å per mm, using an RCA image converter and spectrograph aboard the NASA CV 990 aircraft off the coast of southern Brazil. The spectra have been reduced to equivalent width in terms of the coronal continuum and values derived for different points in the corona.The observed equivalent widths of the lines lie in the range 10 to 30 Å for the 10 747 line and 5 to 12 Å for the 10 798 line. The ratio of these equivalent widths is found to vary from 2.3 in the inner corona to 6 at a point 1.36 solar radii from the center of the Sun.The above results are discussed in terms of the excitation mechanisms involved in producing the lines. In particular, the results are compared with the recent theoretical calculations of Chevalier and Lambert, who are the first to include the effects of proton collisions in the excitation of the 3p ^{2 3} P levels of Fexiii. Our observations are consistent with an electron density of 4 × 10⁸ in the inner corona; a value which compares favorably with those derived by other observers from the strength of the K continuum. These are, to our knowledge, the first eclipse observations of the infrared Fe xiii lines which indicate that proton collisions are important in the excitation of the coronal lines. The coronal abundance of iron is estimated from the equivalent width of the 10 747 line, and in common with other observers we find an overabundance as compared to the photospheric abundance by a factor of 10.     

Expansion of the solar wind from a two-hole corona

A one-fluid model is employed to study the global expansion of the solar wind from a two-hole corona, under the assumptions that the holes are confined to polar caps within 30° of heliographic colatitude, the flow is steady and axisymmetric, and the geometry of streamlines is prescribed. The boundary conditions are adjusted in such a way that the calculated solar wind properties at 1 AU are in a reasonable agreement with observational results. A series of numerical solutions are obtained, the series produces a maximum terminal speed of 829 km s^?1 at the pole. The calculated solar wind speeds are strongly latitude dependent and are positively correlated with local divergence factor of a stream tube. The solutions imply that most plasma properties are highly inhomogeneous at the polar caps. The flow velocity, the temperature, the proton number flux and the conduction heat flux all increase towards the hole center.     

Photometry of the outer solar corona from lunar-based observations

Two-dimensional isophotes of the extreme solar corona (r _max 45 R ) have been derived from integrated vidicon pictures taken from the Moon's surface by the unmanned probes Surveyors 6 and 7. These data were calibrated through use of previously published values for the coronal brightness gradient along the ecliptic. The resulting structure of the outer corona is compared to ground-based observations of the innermost corona 1.125 r/R 2.0 made by the High Altitude Observatory K-coronameter. The possible existence of a streamer seen by Surveyor 7 is analyzed over the region 15 r 22.5 R .     

The physical conditions in inner corona derived from spectral data of the solar eclipse on 7 March, 1970

The spectrum of the solar corona obtained during the eclipse of 7 March, 1970 has been studied. From the analysis of the line halfwidths it is concluded that the emission of coronal lines of various groups arises in different volumes of the corona. The lines 5303 and 4231 are analyzed in detail and by the use of the equations of excitation and ionization balance. The distribution of electron temperature T _e and density n _e vs height in the solar corona has been found. The usually adopted n _e and T _e are noticeably less than those values obtained in present analysis. This discrepancy may be due to the fact that the considered area of the corona seems to be in an unusual condition.     

Dynamical processes in the solar corona and interplanetary space (invited review)

Observations of the solar corona collected over the past decade are discussed from the point of view of short-term (<1 day) and long-term (>1 year) effects. Various phenomena are described, including coronal mass ejections, shock waves, and magnetic sector structures. It is argued that emerging magnetic flux is probably the prime cause of these phenomena, although the details of the interaction processes in the corona are not fully understood.Short-term changes are caused by the sudden release of energy in the lower corona or chromosphere. The prime cause is thought to be due to the build-up of highly sheared magnetic flux. Theoretical work of late has concentrated on attempting to explain the time-scales of flare events (<10 s) in terms of magnetic reconnection. Other work has concentrated on attempting to explain observed features of coronal mass ejections which last for periods of several hours. Long-term changes last for several years and are characterized by the slow evolution of coronal structures, especially magnetic sectors, which extend into interplanetary space out and beyond the Earth. Recent observations place new restraints on the solar dynamo which is thought to be responsible for the emerging magnetic flux involved in these long-term changes.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Temperature gradients in the inner corona

Emission gradient curves for extreme ultraviolet (EUV) resonance lines of O vi and Mg x have been constructed from spectroheliograms of quiet limb regions observed with the Harvard experiment on Skylab. An analysis of these data suggests that the coronal temperature rises throughout the height range 1.03R r1.3R . This result implies that in quiet regions there is significant coronal heating beyond r = 1.3R .Now at E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, D.C., U.S.A.     

Electrons in the solar corona

During a balloon flight in France on September 13, 1971, at altitude 32 000 m, the solar corona was cinematographed from 2 to 5R during 5 hr, with an externally occulted coronagraph.Motions in coronal features, when they occur, exhibit deformations of structures with velocities not exceeding a few 10 km s^–1; several streamers were often involved simultaneously; these variations are compatible with magnetic changes or sudden reorganizations of lines of forces.Intensity and polarization measurements give the electron density with height in the quiet corona above the equator. Electron density gradient for one of the streamers gives a temperature of 1.6 × 10⁶ K and comparisons with the on-board Apollo 16 coronal observation of 31 July, 1971 are compatible with the extension of this temperature up to 25 R _bd.Three-dimensional structures and localizations of the streamers are deduced from combined photometry, polarimetry and ground-based K coronametry. Three of the four coronal streamers analysed have their axis bent with height towards the direction of the solar rotation, as if the upper corona has a rotation slightly faster than the chromosphere.     

Electrons in the solar corona

We discuss a model for the formation of the chromospheric Ca ii K line which does not make the usual assumption of complete redistribution. Using a physically reasonable scattering model, we find significant departures due to the frequency dependence of the line source function, particularly in the relative intensity and centre-to-limb behaviour of the K₁ parts of the line and in the asymmetry produced by differential velocity fields. We conclude that the frequency dependence of the K line source function must be considered in quantitative models for the formation of the K line.     

Variation in Ovii X-ray line-emission from the solar corona

The variation in intensity of the solar X-ray resonance (1s ^{2 1} S ₀ - 1s2p ¹ P ₁), intercombination (1s ^{2 1} S ₀ - 1s2p ³ P ₁), and forbidden (1s^{2 1} S ₀ - 1s2s ³ P ₁) lines of helium-like Ovii with 2800 MHz solar radio flux is presented for three solar rotations. A high correlation (r 0.80) exists between the intensities of all three X-ray lines and the 2800 MHz solar flux. The ratio of the forbidden to the intercombination line intensities is found to be essentially independent of long term solar activity. This ratio is used to determine upper limits on the coronal electron density and to make inferences concerning the change in density with solar activity.     

Structure in the solar corona from radio scintillation measurements

Since the 1950s, a wide variety of radio observations based on scattering by electron density fluctuations in the solar wind has provided much of our information on density fluctuations and solar wind speed near the source region of the solar wind. This paper reviews recent progress in the understanding of the nature of these density fluctuations and their relationship to features on the Sun. The results include the first measurements of fine-scale structure within coronal streamers and evidence for structure in solar wind speed in the inner corona.     

Electrons in the solar corona

The polarimetric survey of electrons in the K-corona initiated at Pic-du-Midi and Meudon Observatories in 1964 now covers a full solar cycle of activity. The measurements are photometrically calibrated in an absolute scale.In June 1967 a persistent coronal feature was fan-shaped as a lame coronale above quiescent prominences. We deduce an electron density of N ₀ = 1.5 × 10⁸ at 60 000 km above the photosphere, a total number of 14 × 10³⁹ electrons, a hydrostatic temperature of 1.7 × 10⁶ K, and a total thermal energy 3N _eKT = 1.0 × 10³¹ ergs. When a center of activity appeared, a major localized condensation developed to replace the old elongated feature, with N ₀ = 4.5 × 10⁸, a total of 4.5 × 10³⁹ electrons and the same temperature of 1.7 × 10⁶ K.Also, a fan-shaped feature of exceptional intensity was analysed on 8 September 1966, with N ₀ = 6 × 10⁸ and a total of 24 × 10³⁹ electrons.Fan-shaped features are frequent above quiescent prominences. They degenerate above a height of 2R into thinner isolated columns or blades with temperatures also around 1.7 × 10⁶ K.     

Shape parameters of the solar corona from 1991 to 2016

The global structure of the solar corona observed in the optical window is governed by the global magnetic field with different characteristics over a solar activity cycle. The Ludendorff flattening index has become a popular measure of global structure of the solar corona as observed during an eclipse. In this study, 15 digital images of the solar corona from 1991 to 2016 were analyzed in order to construct coronal flattening profiles as a function of radius. In most cases, the profile can be modeled with a 2nd order polynomial function so that the radius with maximum flattening index(Rmax) can be determined. Along with this value, Ludendorff index(a + b) was also calculated. Both Ludendorff index and Rmax show anti-correlation with monthly sunspot number, though the Rmax values are more scattered. The variation in Rmax can be regarded as the impact of the changing coronal brightness profile over the equator.