The solar atmosphere contains a wide variety of small-scale transient features. Here, we explore the interrelation between some of them such as surges, explosive events, and blinkers via simultaneous spectral and imaging data taken with the TRACE imager, the SUMER and Coronal Diagnostics Spectrometer (CDS) on board SOHO, and Swedish Vacuum Solar Telescope La Palma. The features were observed in spectral lines with formation temperatures from 10,000 K to 1 MK and with the TRACE Fe IX/X 171 A filter. The Hα filtergrams were taken in the wings of the Hα 6365 A line at ±700 mA and ±350 mA. The alignment of all data in both time and solar XY shows that SUMER line profiles, which are attributed to explosive events, are due to a surge phenomenon. The surge's up- and downflows, which often appear simultaneously, correspond to the blue- and redshifted emission of the transition region N V 1238.82 A and O V 629.77 A lines as well as radiance increases of the C I, S I, and S II and Si II chromospheric lines. Some parts of the surge are also visible in the TRACE 171 A images which could suggest heating to coronal temperatures. The surge is triggered, most probably, by one or more Elerman bombs which are best visible in Hα ± 350 A but were also registered by TRACE Fe IX/X 171 A and correspond to a strong radiance increase in the CDS Mg IX 368.07 A line. With the present study, we demonstrate that the division of small-scale transient events into a number of different subgroups, for instance explosive events, blinkers, spicules, surges or just brightenings, is ambiguous, implying that the definition of a feature based only on either spectroscopic or imaging characteristics as well as insufficient spectral and spatial resolution can be incomplete.

http://iopscience.iop.org/article/10.1088/0004-637X/701/1/253/pdf

Explosive events associated with a surge

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.

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

We analyze an M9.1 two-ribbon solar flare which occurred on 2004 July 22 using the TRACE white-light and 1700 A images, the RHESSI, and the SOHO/MDI data. We find many small-scale fast-varying brightenings that appeared in the white-light and 1700 A images along the flare ribbons. Some of them underwent rapid motions in weak magnetic field regions. We identify these short-lived brightenings as UV continuum enhancement. Our preliminary result shows that the brightenings are closely related to the HXR emission. They have a lifetime of 30–60 s and a typical size of about 1''–2''. The intensity enhancement is about 150–200 times the mean value of the quiet-Sun. According to previous works, we infer that the 1700 A enhancement may be dominated by the increased emission of 1680 A continuum coming from the temperature minimum region. The impulsive feature in the 1700 A light curves of the small-scale brightenings may be due to the irradiation of the impulsive CIV line intensity caused by the bombardment of non-thermal electron beams.

Small-Scale Brightenings in the UV Continuum of an M9.1 Solar Flare

The optical depth at the head of the Lyman continuum, tau sub H, was determined at several positions in three hedgerow prominences using spectroheliograms of C III 977 A, LC 896 A, and O IV 554 A, observed with the Harvard experiment on Skylab. It is found that tau sub H varies over a wide range at positions even in a single prominence. At the central part of the prominences where the opacity is largest, tau sub H reaches a maximum value of 30 to 50 for the three prominences. In general, tau sub H decreases with height in the prominences. Most positions near the outer boundary of the prominences are optically thin in the LC (tau sub H less than 1). The determination of tau sub H for P23 is consistent with the data of the Mg X spectroheliograms

Analysis of extreme-ultraviolet spectroheliograms of solar prominences

Slitless spectra of the chromosphere, observed cinematographically at the total solar eclipse of 10 July 1972, were reduced. The surface brightness distribution of the helium D3 line in the undisturbed chromosphere was obtained in agreement with results by other observers. The available eclipse data on the D3 absolute brightness was analysed by means of theoretical curves of growth. Intensity data by some observers were found to be certainly too high. A trend was found that the D3 absolute brightness in the quiet chromosphere decreases with the increasing solar activity (sunspot number). This perhaps indicates a variation of the spicule number over the solar surface during a sunspot cycle.

The Brightness of the Helium D 3 Line in the Undisturbed Chromosphere from Eclipse Observations

Spectrographic observations of the flash spectrum were made by the Kwasan Observatory at the total solar eclipse on 7 March, 1970. The integrated intensities of Fexiv λ5303, Fex λ6374, and the continuum were measured on the spectrograms as a function of height above the Sun's limb. It was found that a large amount of emission in the coronal lines originates in the interspicular regions of the chromosphere. Analysis of the data yielded that the interspicular regions consist of coronal material of T
e = 1.6 × 106–1.2 × 106 and log N
e = 8.5–9.5, and that a decrease in T
e and an increase in N
e occur with decreasing height.

On the coronal lines in the chromosphere at the 1970 eclipse

Weakening of the Solar EUV Line Emission in Wavelengths Shortward of 912A

The flash spectra of the partial Sun and the chromosphere were obtained at the total solar eclipse on 7 March, 1970. We studied the distributions of the surface brightness of the continuum at six wavelengths in the visual region to compare them with the previous observations and the existing model atmospheres. All of the distributions show a shallow dip and a small hump similar to those of Heintze's observation at the 1954 eclipse. But the hump in our results is of quite a different type from that given by Heintze's revised model. It was found that none of the existing model atmospheres can reproduce this hump. The intensity distribution in the low chromosphere was also examined.

Mitsuo Kanno

Papers

On the coronal lines in the chromosphere at the 1970 eclipse

Weakening of the Solar EUV Line Emission in Wavelengths Shortward of 912A

The continuum of the extreme limb and the chromosphere at the 1970 eclipse

Wavelength Dependence of the Weakening of the Solar EUV Line Emission

Analysis of EUV Limb-Brightening Observations in a Solar Coronal Hole