Progress of solar physics through eclipse observations

The state of solar physics changed dramatically as a result of astronomi­cal observations during eclipses, especially after the mid-nineteenth century. Prominences were reported as "red flames" at the eclipses ofl 706 and 1733, but not until 1851 were prominences correctly interpreted as gaseous clouds floating around in the Sun's corona. The chromosphere was described as a red burning sierra; and the corona was called a white glory seen out to a height of nearly half the Moon's diameter. The corona was re­ported to be different from eclipse to eclipse, but the eclipse of 1860 was the first (and perhaps only) one in which several different observers reported changes in coronal structure from widely separate places along the eclipse path. The sketches drawn by the observers show what solar physicists today call a "coronal tran­sient", short-term variations usually visible only from spacecraft.

Following the year 1859, when the techniques of spectral analysis became available, a series of eclipses revolu­tionized our concept of solar physics. For example, in the eclipse of 1868 in India, it was discovered that the prom­inence spectrum was essentially one of emission lines, and that most of these emission lines coincided in posi­tion with the red and blue lines of hydrogen known to exist in absorp­tion. It was also discovered that a bright yellow line exists close to where the normal sodium lines are com­monly seen in laboratory spectra. The prominence spectra were so bright that they were subsequently observed by Pierre Janssen, a French astron­omer, independent of eclipses. Janssen also noted that the yellow line did not coincide with the familiar posi­tion of the lines of sodium, but instead it was shifted more towards the violet. The significance of this was not realized at the time, but a few months later it was obvious that this line was due to some unknown element on the Sun. Since this was first seen on the Sun it was named after Helios, the Greek name for the Sun. Thirty-five years later, the first of the "rare" gases was isolated in the laboratory on Earth and was found to have this spectral line. Hence it was called helium, the "solar" gas.

The path of totality of the eclipse of 1868 included Guntur, a district very close to Palem (near Hyderabad), lying in the central path of totality of the present eclipse. It was here that Janssen established that one can actually examine several aspects of solar behavior without waiting for an eclipse. "And, therefore, in a sense, one can say that solar physics was born on the 17th August 1868, in the tobacco fields of Guntur", said Prof. M.K.V. Bappu, Director of the Indian Institute of Astrophysics in Bangalore, and President of the International Astronomical Union.

In the total eclipse of 1869, observed in the U.S.A., Harkness and Young independently discovered the green coronal line whose true origin remained unknown for quite some time. Following the example of helium, it was called coronium and it was not until 1941 that Edlen realized that the green line originated from a transition in an iron atom that has lost thirteen of its outer electrons. This proved that the corona must have a temperature in the millions of degrees.

In the 1870 total eclipse observed in southern Spain and the northern coast of Africa, Young observed the flash spectrum after predicting the previous year that it should be visible. During the five or six seconds before totality, a bright flash is observed when emis­sion lines are seen in the spectrum almost exactly where the absorption lines are. These lines were shown to arise in a layer only 1 to 2 thousand km thick above the visible surface of the Sun. In the 1871 total eclipse, again in India, Janssen, with a sixteen-inch telescope which gave him con­siderable light-gathering power, dis­covered what we now know as the Fraunhofer corona. The appearance in the corona of the same dark lines seen in the photosphere of the Sun proved that the corona was seen at least par­tially by reflected sunlight. In the eclipse of 1898 in West India, the famous solar physicist J. Evershed, using an ultraviolet spectrograph with quartz optics, obtained the flash spectrum and the coronal spectrum down to wavelengths as short as 3000A.

The contribution of eclipses to the radio physics of the Sun has not been as dramatic as its optical counterpart. This is primarily because when solar radio astronomy became firmly es­tablished in the mid-l 940s, we already had a fairly good idea of the physical nature of the Sun as an astrophysical object. In particular the macroscopic or global aspects of the Sun were beginning to be known. However, the radio observations during eclipses are potentially very important, because the radio emission does not come from the visible photosphere, but originates from higher levels in the Sun's atmosphere, the chromosphere, the corona and the transition region — precisely the atmosphere that is left after the Sun's disk is completely blocked by the Moon.

It is not surprising then that since 1950 radio astronomers have been in the forefront of several eclipse expedi­tions. In the 1952 eclipse in North Africa, from observations at meter wavelengths it was discovered that the radio corona had a shape which was not symmetrical like the optical Sun, but elliptical instead. The observations at centimeter and millimeter wavelengths made at this and other eclipses showed for the first time that the radio Sun was not uniformly bright from the center to the limb. Instead, the brightness increased sharply over an extent of 30 seconds of arc at the limb. Unfortunately, there has never been any unique interpretation for the radio eclipse data; but in spite of these theoretical difficulties, the current observations were used by astro­nomers to obtain the variation of electron temperature and density with height in the Sun's corona. Most importantly, the radio eclipse observa­tions have demonstrated the immense complexity in the brightness distribu­tion of the Sun; they have also shown the existence of fine structures on the disk and a mixture of cold and hot material in the low corona, similar to what one observes at optical and ultraviolet wavelengths. However, since the radio emissions originate from different physical conditions, (temperature, density and magnetic field), the radio observations have proved to be extremely valuable and complementary to the optical and ultraviolet observations.