Spectropolarimetric analysis of the solar corona during the 12 November 1966 total eclipse

Spectropolarimetric observations of the solar corona during the 12 November, 1966 total eclipse are analyzed. The trend of the intensity, the polarization degree p, and the polarization angle α of the coronal continuum are given as functions of ρ and compared with those of other authors. The H and K Caii lines have been observed in emission once again and, through a quantitative analysis, are attributed to scattered prominence light instead of to the corona as previously made.     

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.     

The coronal condensation observed at the 1973 eclipse

The flash spectrograms obtained at the June 30, 1973 eclipse contain the monochromatic images of a coronal condensation in three coronal lines of Fexiv 5303, Fex 6374 and Fexi 7892 and Hα line. The assumption of the axially-symmetric distribution of the emissivity in the coronal lines allows us to find the density and temperature structure of the coronal condensation. While the electron density in the central axis of the condensation is about ten times as high as that of the normal corona at each height, the temperature is not so high (T?2.3×10⁶K). This seems to be a representative nature of a coronal active region in the post maximum phase of activity. It is found that there exists a cool and dense core (T = 10⁶K, N _e =6 × 10⁹ cm^-3 at 17000 km) at the lower part of the coronal condensation, which is in a close geometrical coincidence with the small active prominence protruding from the underlying plage region.     

Eit and LASCO Observations of the Initiation of a Coronal Mass Ejection

We present the first observations of the initiation of a coronal mass ejection (CME) seen on the disk of the Sun. Observations with the EIT experiment on SOHO show that the CME began in a small volume and was initially associated with slow motions of prominence material and a small brightening at one end of the prominence. Shortly afterward, the prominence was accelerated to about 100 km s^-1 and was preceded by a bright loop-like structure, which surrounded an emission void, that traveled out into the corona at a velocity of 200–400 km s^-1. These three components, the prominence, the dark void, and the bright loops are typical of CMEs when seen at distance in the corona and here are shown to be present at the earliest stages of the CME. The event was later observed to traverse the LASCO coronagraphs fields of view from 1.1 to 30 R⊙. Of particular interest is the fact that this large-scale event, spanning as much as 70 deg in latitude, originated in a volume with dimensions of roughly 35" (2.5 x 10⁴ km). Further, a disturbance that propagated across the disk and a chain of activity near the limb may also be associated with this event as well as a considerable degree of activity near the west limb.     

Fabry-Pérot line profiles in the λ5303 å and λ6374 å coronal lines obtained during the 1983 Indonesian eclipse

During the total solar eclipse of 11 June, 1983, an imaging dual-channel Fabry-Pérot interferometer was used to obtain line profiles simultaneously in the green 5303 Å [Fe xiv] and the red 6374 Å [Fe x] coronal lines at various positions in the corona. Extensive microdensitometry followed by multi-Gaussian curve-fitting analysis has resulted in the determination of coronal temperatures and velocity separations between different pockets of coronal gas in the line of sight over a large extent of the corona. Fewer high temperature zones are to be found in the corona of 1983 compared with our similar green-line measurements of the solar maximum corona of 1980. The data are consistent with a temperature maximum occurring at 1.2 R , as found at the 1980 eclipse, but our new data are insufficient to observe farther out than this radius and so determine the position of a maximum. The velocity field in the corona at the 1983 eclipse is less structured compared with that at the 1980 eclipse and is mainly confined to the zone 20–30km s^–1.     

The April 8, 2005, Eclipse White-Light Corona

The hybrid solar eclipse of April 8, 2005, provided a good opportunity to observe the white-light solar corona, even though the eclipse lasted just 30 seconds and could be seen only from ships in the Pacific Ocean. During the eclipse, we detected a unique ‘cloud’ of particles in the white-light corona above the west limb ≈260°–270°. We compare this feature with EUV images from SOHO. The feature’s density and temperature seem comparable to a coronal condensation, and, like a coronal condensation, it is connected to the emergence of material from the solar surface without a flare. However, the morphology of the feature shows clear differences from a classical coronal condensation.     

Brightness,polarization and electron density of the solar corona of 1980 february 16

During the eclipse of 1980 February 16 we photographed the solar corona at an effective wavelength of 6300 å. Using a quadruple camera we also obtained the coronal pictures in polarized light for four Polaroid orientations. We have used these observations to derive the coronal brightness and polarization and from these the electron densities in the corona out to a distance of about 2.5 R⊙ from the centre of the disc. The coronal brightness matches well with that of the corona of 1958 October 12.     

Prominence Cavity Regions Observed Using SWAP 174 Å Filtergrams and Simultaneous Eclipse Flash Spectra

SWAP images from PROBA2 taken at 174 Å in the Fe ix/x lines are compared with simultaneous slitless flash spectra obtained during the solar total eclipse of 11 July 2010. Myriad faint low-excitation emission lines together with the He i and He ii Paschen α chromospheric lines are recorded on eclipse spectra where regions of limb prominences are obtained with space-borne imagers. We analyzed a deep flash spectrum obtained by summing 80 individual spectra to evaluate the intensity modulations of the continuum. Intensity deficits are observed and measured at the prominences boundaries in both eclipse and SWAP images. The prominence cavities interpreted as a relative depression of plasma density, produced inside the corona surrounding the prominences, and some intense heating occurring in these regions, are discussed. Photometric measurements are shown at different scales and different, spectrally narrow, intervals for both the prominences and the coronal background.     

Coronal Mass Ejections Observed at the Total Solar Eclipse on 13 November 2012

White-light observations of the total solar eclipse on 13 November 2012 were made at two sites, where the totality occurred 35 min apart. The structure of the corona from the solar limb to a couple of solar radii was observed with a wide dynamic range and a high signal-to-noise ratio. An ongoing coronal mass ejection (CME) and a pre-CME loop structure just before the eruption were observed in the height range between 1?–?2 R_⊙. The source region of CMEs was revealed to be in this height range, where the material and the magnetic field of CMEs were located before the eruption. This height range includes the gap between the extreme ultraviolet observations of the low corona and the spaceborne white-light observations of the high corona, but the eclipse observation shows that this height range is essential for the study of CME initiation. The eclipse observation is basically just a snapshot of CMEs, but it indicates the importance of a continuous coverage of CME observations in this height range in the future.     

Spectroscopic Coronal Observations During the Total Solar Eclipse of 11 July 2010

The flash spectra of the solar chromosphere and corona were measured with a slitless spectrograph before, after, and during the totality of the solar eclipse of 11 July 2010, at Easter Island, Chile. This eclipse took place at the beginning of Solar Cycle 24, after an extended minimum of solar activity. The spectra taken during the eclipse show a different intensity ratio of the red and green coronal lines compared with those taken during the total solar eclipse of 1 August 2008, which took place toward the end of Solar Cycle 23. The characteristic coronal emission line of forbidden Fe xiv (5303 Å) was observed on the east and west solar limbs in four areas relatively symmetrically located with respect to the solar rotation axis. Subtraction of the continuum flash-spectrum background led to the identification of several extremely weak emission lines, including forbidden Ca xv (5694 Å), which is normally detected only in regions of very high excitation, e.g., during flares or above large sunspots. The height of the chromosphere was measured spectrophotometrically, using spectral lines from light elements and compared with the equivalent height of the lower chromosphere measured using spectral lines from heavy elements.     

The properties of coronal voids

Observations of the outer solar corona obtained by the High Altitude Observatory's coronagraph aboard Skylab reveal the presence of dark, ray-like structures in the corona. A systematic identification of these voids, which exist for periods of about 24 hr, is presented and their existence as a coronal phenomenon, as opposed to a subtle photographic effect, verified. Photometric analysis indicates that these features represent reductions in the coronal radiance on the order of 5% - or about 2–3 × 10^?10 B _⊙ at 3 R _⊙. The use of a previously determined model of the electron component of the corona permits specification of the electron density in the voids over the range 2.5–4.5 R _⊙. In spite of the inevitable uncertainties regarding their longitudinal extent, we estimate that their electron density is comparable to, or less than, that in coronal holes at similar heights. Projection of the phenomena onto synoptic surface maps indicates a close relationship with filaments and neutral lines; a potentially significant temporal correlation between the void formation and that of the underlying prominence is noted. The spatial and temporal resolution of the data set places stringent restrictions on any model which may be used to infer the physical processes of formation or decay of voids; several possibilities are suggested which involve either changes in the coronal base temperature or the magnetic flux.     

A concentric ellipse multiple-arch system in the solar corona

A typical concentric ellipse multiple-arch system was observed in the solar corona during the February 4, 1962 eclipse in New Guinea. The following results have been obtained from analysis of a white-light photograph taken by N. Owaki (see Owaki and Saito, 1967a).

1. The arches are composed of four equidistant components, elliptical in shape, and almost concentric with a prominence at the common center of the ellipses.
2. The prominence and arch system appears to be the lower region of a helmet-shaped streamer.
3. The widths of the arches are observed to increase with height.
4. Analysis was made in the light of three models for the coronal structures that could lead to the observed arches: (a) rod-like concentrations of electrons; (b) tunnel-shaped elliptical shells of electrons; and (c) dome-like ellipsoidal shells of electrons. Electron densities are derived for the models, and the dome-like model is excluded as a possibility for arch systems exhibiting a coronal cavity.
5. The scale height in the arch-streamer region is found to be almost the same as that of the K-corona, suggesting equal temperatures, density distributions, etc. in each region.
6. There is a dark space (a coronal cavity) between the innermost arch and the prominence. The brightness of this cavity is 1/5 that of the adjacent arch. It is 3% brighter than the background corona of the arch-streamer system.
7. A comparison is made between the deficiency of electrons in the coronal cavity and the excess of electrons in the prominence. It is found that the ratio of the excess to the deficiency lies between 0.9 and 40.
8. A comparison between the electron efflux from the ‘leaky magnetic bottle’ possibly formed by rod-shaped coronal arches and the electron influx into those arches from the chromosphere leads us to the conclusion that the rod model is probably valid and that spicules appear to be an adequate supply for the electrons observed in the arches. The tunnel model may be valid, but in that case spicules are probably not the sources of the electrons observed in coronal arches.

     

A Comparison of the Red and Green Coronal Line Intensities at the 29 March 2006 and the 1 August 2008 Total Solar Eclipses: Considerations of the Temperature of the Solar Corona

During the total solar eclipse at Akademgorodok, Siberia, Russia, on 1 August 2008, we imaged the flash spectrum with a slitless spectrograph. We have spectroscopically determined the duration of totality, the epoch of the second and third contacts and the duration of the flash spectrum. Here we compare the 2008 flash spectra with those that we similarly obtained from the total solar eclipse of 29 March 2006, at Kastellorizo, Greece. Any changes of the intensity of the coronal emission lines, in particularly those of Fe x and Fe xiv, could give us valuable information about the temperature of the corona. The results show that the ionization state of the corona, as manifested especially by the Fe xiv emission line, was much weaker during the 2008 eclipse, indicating that following the long, inactive period during the solar minimum, there was a drop in the overall temperature of the solar corona.     

The coronal and interplanetary magnetic fields at the time of the solar eclipse of 7 March, 1970

The magnetic field in the outer corona and in interplanetary space has been calculated from the photospheric magnetic fields measured around the time of the 7 March, 1970 eclipse. The field-line maps are compared with eclipse photographs showing coronal structures out to about 12 r . The projected field lines as well as the observed streamers appear straight. This is caused by the rapid expansion of the outer corona and is not an indication of corotation. The calculations show that the angular velocity of the coronal plasma decreases rapidly with distance.The relation between magnetic fields and density enhancements is discussed. The field strength in the photosphere seems to determine the amount of mechanical heating of the lower corona. The density structure higher up in the corona will, however, depend decisively on the topology of the field, particularly on whether we are on open or closed field lines, and not simply on field strength.The calculations show a sector structure of the interplanetary field, which agrees well with spacecraft observations. Also the magnitudes of the observed and calculated interplanetary field agree after the Mt. Wilson magnetograph data have been corrected to account for the temperature and saturation effects in the spectral line Fei 5250 Å.On leave from the Astronomical Observatory, Lund, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

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.     

Filamentary Structures of Coronal White-Light Images

In the absence of magnetic field measurements of the solar corona, the density structure of white-light images has provided important insight into the coronal magnetic field. Recent work sparked by highly sensitive radio occultation measurements of path-integrated density has elucidated the density structure of unprocessed solar eclipse pictures. This paper does the same for processed images that reveal low-contrast small-scale structures, specifically Koutchmy’s edge-enhanced white-light image of the 11 August 1999 solar eclipse. This processed image provides visual evidence for two important results deduced from radio occultation measurements of small-scale density variations. First, in addition to the closed loops readily seen at the base of the corona in high-resolution EUV and soft X-ray images, open filamentary structures permeate the corona including active regions generally thought to be magnetically closed. Observed at the image resolution, the filamentary structures are 1° wide in latitude and an order of magnitude smaller than polar plumes. Second, although inhomogeneities that are convected along with the solar wind are also present, filamentary structures dominate the image because of their steeper density gradients. The quantitative profile of polarized brightness (pB) at the base of the corona shows that the filamentary structures have transverse density gradients that are proportional to their density. This explains why edge-enhanced images, limited in sensitivity to density gradients, tend to detect filamentary structures more readily in high-density regions (e.g., active regions, streamer stalks, and prominences) than in low-density polar coronal holes, and why filamentary structures seem more prevalent in solar eclipse pictures during solar maximum. The pB profile at the base of the corona also fills the gap in Doppler measurements there, reinforcing that open ultra-fine-scale filamentary structures observed by the radio measurements are predominantly radial and that they are an integral part of the radial expansion of the solar wind.     

A search at two eclipses for short-period waves that heat the corona

As part of a study of the cause of solar coronal heating, we searched for high-frequency (1 Hz) intensity oscillations in coronal loops in the [Fexiv] coronal green line. We summarize results from observations made at the 3 November 1994, total solar eclipse from the International Astronomical Union site in Putre, Chile, through partly cloudy skies, and at the 26 February 1998 total solar eclipse from Nord, Aruba, through clear skies. We discuss the image reduction and analysis of two simultaneous series of coronal CCD images digitized at 10 Hz for a total time of 160 s in Chile. One series of images was taken through a filter isolating the 5303 Å[Fexiv] coronal green line and the other through a 100 Å filter in the nearby K-corona continuum. We then discuss the modifications made for the 1998 eclipse, and the image reduction and analysis for those image sequences. After standard calibrations and image alignment of both data sets, we use Fourier analysis to search in the [Fexiv] channel for intensity oscillations in loops at the base of the corona. Such oscillations in the 1-Hz range are predicted as a result of density fluctuations from the resonant absorption of MHD waves. The dissipation of a significant amount of mechanical energy from the photosphere into the corona through this mechanism could provide sufficient energy to heat the corona. At neither eclipse do we find evidence for oscillations in the [Fexiv] green line at a level greater than 2% of coronal intensity.     

Magnetic fields and the structure of the solar corona

During the eclipse of 12 November 1966, the solar corona was photographed at an effective wavelength of 6500 Å with an f/16, 11.1 cm aperture camera. Reduction of the observations yields coronal radiances and polarizations from 1.4 to 3.5 solar radii. Standard techniques are used for the separation of F and K-coronas, determination of coronal electron densities and temperatures, and estimation of the orientation of the major streamers in space.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Oscillatory Modes of a Prominence – PCTR – Corona Slab Model

Oscillations of magnetic structures in the solar corona have often been interpreted in terms of magnetohydrodynamic waves. We study the adiabatic magnetoacoustic modes of a prominence plasma slab with a uniform longitudinal magnetic field, surrounded by a prominence – corona transition region (PCTR) and a coronal medium. Considering linear small-amplitude oscillations, we deduce the dispersion relation for the magnetoacoustic slow and fast modes by assuming evanescentlike perturbations in the coronal medium. In the system without PCTR, a classification of the oscillatory modes according to the polarisation of their eigenfunctions is made to distinguish modes with fastlike or slowlike properties. Internal and external slow modes are governed by the prominence and coronal properties, respectively, and fast modes are mostly dominated by prominence conditions for the observed wavelengths. In addition, the inclusion of an isothermal PCTR does not substantially influence the mode frequencies, but new solutions (PCTR slow modes) are present.