4.1.2.6 Coronal active regions

Abstract

The active corona is characterized by magnetic loop structures of various sizes and temperatures [c, d, b, 96Low, 00Cha, f, 05Asc], (see also Fig. 2 of Sect. 4.1.2.1). Many active-region loops have much higher electron densities than expected from theoretical considerations – they are called “overdense” loops [99Len, 01Asc, 03War1, 03War2]. The most active parts of the corona are magnetically connected to active regions (AR) on the Sun (cf., Sect. 4.1.2.1). The photospheric and coronal response to magnetic field emergence of twisted flux tubes was investigated by [06Mag]. Whenever an AR produces a flare (see Sect. 4.1.2.7) and/or a coronal mass ejection (Sect. 4.1.2.5.6) the active corona is severely affected and, in all likelihood, plays an active role as well. A flux rope formation in the outer corona was observed by [06Wan]. The coronal temperature above an AR was studied by [97Ste2], who found a steady plasma component with Te≈3 MK, and a hotter, transient one caused by microflares. Relatively cool AR loops with a temperature near 1 MK in a hotter, diffuse environment were observed by [03Del]. The large-scale temperature structure of the corona for quiet and active conditions was investigated by [02Wil, 07Ben]. With increasing solar activity, equatorial coronal holes (CH) appear, which, in some cases, are still connected to the disintegrating polar CHs of the solar minimum corona. Reconnection events, as evidenced by enhanced magnetic activity at the CH boundaries, probably produce the required reconfiguration [04Mad]. At the same time, the solar X-ray and extreme-ultraviolet emissions increase substantially [97Har]. In general, the plasma parameters of equatorial CHs are not very much different from those of polar CHs [99Dob, 99Chi, 01Mir]. Both types are expanding super-radially. Spectra of the active (and quiet) corona are presented by [96Bro, 07Bro]. X-ray observations of microflare activity in an AR rotating from the front side to the rear side of the Sun showed that the emission was restricted to low heights [97Ste1]. Coronal plasma flows and the magnetic field topology of ARs were studied by [04Mar]. Inter-connections between remote ARs are discussed by [03Sve]. Synchronous microwave brightenings of ARs more than 1 R apart confirm the existence of such large-scale interconnecting loops [04Gol]. The smallest coronal loops yet detected occur in coronal bright points (BP) treated in Sect. 4.1.1.5.2.4. The size distribution of X-ray emitting regions indicated that magnetic structures of all dimensions were present down to the instrumental resolution limit of ≈500 km [93Gom1, 93Gom2]. Heating of the active X-ray emitting corona is thought to be accomplished by the dissipation of many tangential discontinuities in the entangled magnetic field of the solar atmosphere. The related events have been called “nanoflares” by [88Par]. However, more work is required before a final answer can be given. Observations of the corona with the help of recent space missions have been reviewed by [a]. Coronal active regions 11 1