The observation of the corona during totality should be made with the greatest care so that experiments function without fail. It is preferable to undertake reliable experiments than to attempt some special feat. Even if the resulting photograph
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Figure 10.4: A schematic diagram of an experiment designed to measure the 2nd and 3rd contacts of an eclipse.
is unique it cannot be used unless properly calibrated and all the instrumental factors are well known. Fig. 10.5 shows the origin of the sky illumination during an eclipse. A represents primary scattering by the ground and the lower atmosphere of sunlight, which then illuminates B, the line of sight in the Earth’s atmosphere.
This further scatters some of this radiation toward the observer at C.
Figure 10.5: The origin of the sky illumination during an eclipse.
Fig. 10.6 shows why the solar corona may be observed during totality. The sky becomes sufficiently dark for the contrast between the various features to be perceptible. Even the faint Earthshine on the face of the Moon may be measured at that time.
10.5.1 Structures in the Inner Corona
In addition to the data acquired at total solar eclipse numerous experiments in space and on the ground, particularly with large coronagraphs (200-mm diameter or more), have discovered very complex features in this region of the solar atmosphere, which reveals a wide range of plasma phenomena. Fig. 10.7 shows the principal features of this part of the corona, which has both quite cool regions such as prominence material,
Figure 10.6: The radial distribution of intensity close to the Sun of the daytime sky and of various objects observed during an eclipse.
super-spicules, etc., and very hot ones like coronal arches and flares. These extremes are separated by only a few hundred kilometers. Recent observations show highly variable features, which may change rapidly with waves of ionization, excitation, condensation, etc. This is particularly noticeable when they are cool and low in density. Others are violent ejections of material at velocities of several hundred km/s forming very narrow, long jets. Above active regions, coronal enhancements show that changes are continually taking place especially in the shape of temporary loops that form and dissipate repeatedly and that may even be the site of violent
Figure 10.7: Diagram showing the main structures in the inner corona and the chromospheric extensions over various quiet (A and B), or active areas (C).
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explosions. This is essential if we are to understand how the corona is heated to such high temperatures and why such a corona exists at all. Thermodynamic properties that vary with time are important factors to be taken into consideration.
10.5.2 Observations at the Extreme Limb
Observations at the extreme limb are similar to those made at the contacts, except that selective filters should be used. The details depend on whether the desired spectra are choromospheric or photospheric (the latter are emission spectra).
To study the extreme outer limit of the photosphere it is preferable to work in the red and the near infra-red, where there are spectral windows that are totally free from lines. These permit the true continuum to be measured and cover fairly wide spectral regions. It thus ensures that there is a good signal-to-noise ratio for photometric measurements. If the photographic method is applied, exposure times should be chosen to give a correct rendition of the solar limb. To ensure that measurements may be made at the extreme limb, where the intensity is less than 1% of that at the center of the disk, contrast should be kept low (γ < 1). Also, careful calibration should be carried out, so that the profile of the brightness at the extreme edge of the solar photosphere may be deduced.
The narrow layer, the chromosphere and the spicules, up to about 10 arc-seconds from the limb, may be studied with an appropriate filter. Investigation of the super-spicules that extend out to 20 to 30 arc-seconds may also be attempted.
But this is too difficult to do photographically. Visual methods may be tried with a telescope, using the optimum magnification. This type of observation could also be made with a telescope fitted with a spectro-helioscope, but the resolution is likely to be less. The best way is to use a very broad-pass filter perhaps even a Wratten gelatine one that transmits Hα lines, and concentrate on studying extremely fine detail.
10.5.3 Observations of the Inner Corona
There are different features that may be studied, provided good resolution is attainable. For these purposes photography gives the best results. To distinguish cool materials, whose radiations are dominated by the orange and red emission lines of D1, D2, and D3, and (above all) Hα from ionized material at 2 × 106 K with white emissions, color emulsions (i.e., color sheet-film) should be used. With a refractor designed for visual use, radiation in the blue is more diffuse because of chromatic aberration. Cool material emits some strong lines in the blue. If these wavelengths are removed by filtration, better discrimination between coronal and cooler material is achieved. Using this method, the green layer of the emulsion will be dominated by coronal radiation and the red layer by radiation from cooler
material. The cool material is in ceaseless motion. It is easy to detect changes during totality. The same does not hold for the ionized corona, which is fixed by the magnetic field. It is not likely to show any variations during totality, except the extremely low amplitude oscillations that are predicted by certain theories.
To obtain the maximum coverage, observations should begin several seconds before 2nd contact, when most of the corona is visible on the opposite side of the Sun, to a few seconds after 3rd contact. If the shutter release mechanism does not allow repeated exposures to be made at a sufficiently high rate, it is preferable to wait until the disappearance of the last Baily’s Bead at 2nd contact before making the first exposure. The success depends on the care with which the photographic equipment has been set up. Ensure that the image is of high quality and has an adequate scale, so that the resolution will not be too dependent on the film’s grain size. A long focal length is also required, preferably at least 5 m. A focal length of 10 m with 13 × 18 sheet film will give better results. A simple achromatic doublet will give better results because the focal ratio is so small (around f/100). With fine-grain film, exposure times between 1/4 and 1 second are required. This means that guiding must be accurate if second-of-arc resolution is to be preserved.