Thesischapter 5

This file is part of the following reference: Pearson, John (2009) The role of the 40 foot Schaeberle camera in the Lick Observatory investigations of the solar corona. PhD thesis, James Cook University. Access to this file is available from: http://eprints.jcu.edu.au/10407 CHAPTER 5 THE LICK OBSERVATORY STUDIES THE SOLAR CORONA AND PARADIGM SHIFTS IN SOLAR PHYSICS 5.1 Introduction This chapter reviews coronal studies at the Lick Observatory which began with the first eclipse expedition and continued through to the last expedition. The LO accumulated the world’s finest archive of direct photographic eclipse plates over their expedition time-frame. The fine quality and large image scale of plates made coronal structure, form and motion analysis possible at a more complex level. The year 1889 was when the Lick Observatory initiated its eclipse expeditions. It was a year of several firsts in the quest for knowledge about the solar corona. The Observatory contributed significantly to the breadth of knowledge and new ideas gained during this first year. It was the first eclipse that the nation’s astronomical community produced an abundance of fine quality eclipse images (Clerke 1908: 185). The LO and the Harvard College Observatory, accounted for a majority of the images. Just the Harvard plates alone, numbered more than had ever been produced at previous solar eclipses on an international basis (Turner 1889b: 108). Previously, photography could not record the furthest coronal streamers as could be produced by skilled drawers who managed to visually observe and show a wealth of detail in their drawings. Using photography, comparison of the coronal images from three different eclipse stations clearly demonstrated for the first time, that major coronal features coincided. This was the first definitive photographic proof that the corona was not an artifact of the Earth’s atmosphere. Pritchett and Bigelow both remarked that the coronal extensions looked as though electrical or magnetic forces could be present. As the year of 1889 ended, Schaeberle produced his Mechanical Theory of the solar corona (Bigelow 1891: 50-52; Pritchett 1891: 158-160). Interest in the use of photography in all aspects of an observer’s work to record and reveal coronal secrets was rapidly gaining favor as the first Lick expeditions were in progress in 1889. Solar investigator H.S. Pritchett (1891: 161) brought to the table a couple of his ideas regarding coronal photography: 1. The desirability of obtaining photographs of the outer coronal streamers, whose delineation would go far towards a real knowledge of the structure of the corona. This involved the problem of designing the photographic outfit especially for this work. 160 2. The desirability of photographing the corona from points as widely separated as possible. The progress of the use of spectrographic instruments in the advancement of coronal science during the expeditions by the LO scientists was of paramount importance as will be witnessed by the discussion of Menzel’s chromospheric research that resulted in a paradigm shift in astrophysics. Because the thesis is focused on the direct photographic work, especially from the 40 foot Camera, the spectrographic work will be referenced to only on an occasional basis. The testing of the Einstein’s general Theory of Relativity was the primary focus of the three Lick Observatory expeditions sent out in 1914, 1918, and 1922. A brief discussion of the testing and results is presented showing how the LO came close to being first to secure the definitive star deflection measurements called for by the Theory. Additional information on the Einstein cameras is included in Appendix 2. 5.2 The Constitution of the Solar Corona The brightness of the corona was figured to be 10-6 as bright as the Sun and nearly as bright as the full Moon, yet the corona could not be detected by the unaided eye outside of a total eclipse. This was due to its faintness, fainter than the overall daylight. However, early experienced eclipse viewers, some using small telescopes, such as F. Arago in 1842, G.P. Bond in 1851, M.F. Petit in1860, N. Lockyer and A.R. Dawson in 1871, and S.M. Baird Gemmill in 1905, reported seeing the corona moments before and after totality (O’Meara 2005: 75-76). 5.2.1 Coronal Structure and Form The appearance of the solar corona was broken down into general structural elements and boundary conditions by the Lick staff, who used terms that were in common use by earlier investigators. By the eclipse of 1893, the Lick Observatory had adopted a couple of new terms with two new coronal features being defined by Schaeberle – the ‘arch’ and ‘streamer.’ Several of the features are discussed in more detail in Schaeberle’s Mechanical Theory of the solar corona; see Section 5.4 (Schaeberle 1895: 99-102). 1. Streamers – If an arch disappears, before reaching its aphelion point, it is called a ‘streamer.’ Streamers are subject to a variety of descriptive terms; heavy, stubby, parallel, rectilinear, curved, radial, well or sharply-defined. 2. Arches – An ‘arch’ is a visible streamer made of ejected particles traced beyond its aphelion. 161 3. Rays – A ‘ray’ is defined as the visual apparition caused by the overlapping of streamers seen in projection. Rays can take the appearance of ‘false’ streamers. 4. Beams – The term ‘beam’ was commonly found in the nineteenth century eclipse reports. The mid- to late-nineteenth century drawings and woodcuts for publication images gave the clear impression of large ‘beams’ of bright coronal light radiating from the surface of the Sun. It was replaced with the term ‘streamer’ or even by a well defined ‘ray.’ 5. Polar Rays – Polar rays, if they exist, were considered by Schaeberle to be caused by unknown forces within the solar surface. There are few rays or originating streamers in the pole regions because of the lack of sunspots. They may be just intersections of overlapping equatorial streamers viewed from a viewer’s perspective of above and below the poles. 6. Rifts – Dark areas among the rays are known as ‘rifts,’ which result from inactivity on the surface of the Sun. Rifts were commonly observed in the polar regions of the corona. 7. Gaps – ‘Gaps’ are the darker areas between the wings. 8. The coronal ‘Wings’ and the ‘Trumpets’ describing the ‘great wings’ extend to several lunar diameters. 9. Polar Gaps or Rifts – The rays are brightest near the poles and the apparent extinction of the polar rays occurs at increasing polar distances appearing as polar gaps or rifts. 10. A ‘Boundary’ describes several observed conditions – The border between two regions of unequal density – A number of different streamers having slightly different inclinations, but tangential to each other at various distances from the Sun – Osculating streamers appearing as a single streamer – A darker area marking the shape of a wing – Or in a general way, describes an area between close groupings of structure. 5.2.2 The ‘Inner’ – ‘Middle’ – ‘Outer’ Corona Basic terminology was used by Schaeberle after the 1893 eclipse to describe what appeared in the ‘inner’ corona. Found in the inner corona were streamers, arches, boundaries, and gaps. The area of the ‘middle’ corona contains extensions of the structures originating at, near or within the surface of the chromosphere. It is not uncommon for some of the large structures such as the rays to seem to originate within the ‘middle’ corona. In describing the outer corona, Schaeberle included the extensions of the features in the ‘inner’ corona. He found no characteristic differences between the ‘inner’ and ‘outer’ coronal forms other than they became fainter, more diffused and lose their sharp boundaries with 162 increasing distance from the solar limb. The wings formed whenever the streamers were most numerous in projection. Many of these streamers crossed one another. The mostly parallel wing boundaries tended to converge at a point. The diverging areas of rays would become too faint to distinguish against the background sky at their furthest extensions (ibid.). 5.2.3 Coronal Matter Coronal matter existed in a gaseous state, incandescent particle state, solid particle state, and at the atomic and ionized atomic level. Coronal matter was believed to contain unknown elements not found in laboratories though spectroscopy by the end of the LO expeditions. When the Lick Observatory expeditions began in 1889, the corona was believed to consist of hot and cold solid particles. Some attributed these particles to incoming swarms of meteorites or meteoritic clouds in orbit about the Sun. The nature of the coronal particles during the expeditions, is presented in Section 5.11. 5.2.4 Chromospheric Surface Features In the last half of the nineteenth century, the chromosphere was thought of as a thin layer separated by a small boundary area (reversing layer) from the photosphere. Airy labeled this layer ‘sierra’ in 1842. Some likened it to ‘stuff that dreams are made of’ or referred to chromosphere as a ‘coronal atmosphere.’ Frankland and Lockyer coined the term chromosphere in 1869. At the base of the chromosphere was a very bright region that appeared in what Langley referred to as ‘like a prairie on fire’ (Young 1910: 192). The red color was attributed to large concentrations of hydrogen flames. 1. Protuberances and Prominences – Large cloud-like masses that appeared above and from the surface of the chromosphere were labeled in 1842 as ‘protuberances’ and ‘prominences’ with no apparent differences noted between the terms. Within the corona, these features were described as being like floating clouds being ‘torn to pieces.’ It was believed that prominences were the result of the splash up of photospheric material from matter falling into the Sun.

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