978 - 0 - 521 - 88201 - 9 - The Solar Corona , Second Edition

The corona is a high-temperature portion of the Sun’s outer atmosphere, beginning slightly above the visible surface and extending hundreds of thousands of kilometers, or further, into interplanetary space. A precise definition of the term “corona” is to some extent dependent on one’s theoretical bias: if the solar atmosphere is viewed as spherically symmetric and homogeneous (“plane-parallel”), then the corona is easily defined as the portion of the atmosphere above a certain height, with perhaps some small variation from place to place. As we shall see, the corona is far from homogeneous and such models are of limited applicability. One may then choose instead to focus on coronal structures, which seem to be omnipresent and which are found to be relatively independent of each other. The observed structuring takes the form of long, thin loops or strands either extending out into interplanetary space or curving back and reentering the photosphere. Both hot and cool structures are seen in the outer solar atmosphere. The “footpoint” of such a loop is defined as the portion of the structure that is anchored in the photosphere, and footpoints are generally cooler than the coronal portion. This implies that there is a horizontal interface between low and high temperature regions of atmosphere. However, with both hot and cool structures present in the outer solar atmosphere, in some locations there may be loops of different temperature situated next to each other. The interface between the hot and cool regions is therefore not always horizontal. Thus, height alone is not an adequate defining coordinate for coronal conditions. In this type of “loop model atmosphere,” the corona is built up from an assemblage of relatively isolated structural building blocks, which must then be joined together to form a coherent whole. The extent to which the corona departs from a spherically symmetric, planeparallel atmosphere may be seen in Fig. 1.1, an image taken by the TRACE satellite in the highly ionized Fe IX/X emission lines near the 173 Å central wavelength of this channel. These lines emit EUV radiation at ≈1 million K