The PCTR is an unavoidable consequence of the thermally insulating magnetic fields.
Sun corona. skin#
That hot plasma resides in a thin, high-temperature-gradient skin around the prominence called the prominence–corona transition region. Observed EUV and UV spectral lines from highly ionized carbon, nitrogen, sulfur, oxygen, and iron in prominences imply the existence of plasma ranging in temperature from 10 000 K to 2 000 000 K. The smallest measured angular widths of such threads-0.3”, which correspond to about 250 km on the Sun-strain the spatial resolution of today’s best instruments. High-resolution observations of the solar disk leave little doubt that the filaments consist of thin, largely parallel, thread-like structures, as shown in figure 4a. In other words, filaments and prominences are not amorphous blobs they appear as sharp, detailed portraits of the corresponding magnetic fields. The partly ionized prominence plasma is thereby “frozen” to the magnetic field and may flow freely only in the direction of the field. That field range implies that the local magnetic pressure inside the structures is generally larger than the local gas pressure. Wu, eds., Nature of Prominences and Their Role in Space Weather, Cambridge U. By comparison, the magnetic fields in filaments and prominences are typically in the range of 5–30 G (see B. The average magnetic field on Earth’s surface is 0.5 gauss (10 000 G = 1 tesla). The interrelated handedness or chirality of filaments, channels, and coronal arcades 13 13. Thus the channel field lines and associated filaments are generally flat and run horizontally along the channel direction. Channel fields are nonpotential, which means that some factor-here, the overlying coronal arcade-prevents the field lines from taking on the familiar arc shape. Magnetic fields provide support against gravity and thermal shielding from the surrounding hot coronal plasma.
The magnetic environments of solar filaments and prominences influence their birth, existence, and disappearance. Only a fraction of the total length contains observable filaments. Magnetic data of the solar disk show that PILs and the associated channels circle over long stretches of the solar surface, much like seams on tennis balls. In a nonideal plasma with finite electrical resistance, ion acoustic waves-sound waves that interact with the electromagnetic fields present-and other types of waves are possible.
Because they combine longitudinal compression of the plasma and the transverse displacement of the lines of force, they propagate at an angle to the field lines. Magnetosonic waves are hybrids of sound waves and Alfvén waves. Alfvén waves travel at a speed that increases as the local plasma density decreases and as the magnetic field strength increases. The surrounding plasma wiggles along with field lines but it is not compressed as happens with sound waves. Alfvén waves are periodic transverse displacements of the lines of force. They are just sound waves, albeit in a gas of charged particles. Longitudinal plasma-density waves propagate at the local speed of sound in the direction of the field. Three types of waves are possible in an ideal plasma that has zero electrical resistance.
When shaken by external forces, a bundle of magnetic field lines and the plasma filling them can develop traveling waves.
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