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(The SOHO/LASCO data are produced by a consortium of the Naval Research Laboratory (USA), Max-Planck-Institut fur Sonnensystemforschung (Germany), Laboratoire d’Astrophysique de Marseille (France), and the University of Birmingham (UK). The field of view of the image is 30 solar diameters. The Sun, indicated by the white circle, has been occulted within the instrument. (a) A CME as imaged by the LASCO/C3 coronagraph on SOHO on 20 April 1998. Since observed solar wind speeds near 1 AU are never less than ∼280 km/s, the slowest CMEs are further accelerated enroute to 1 AU.įIGURE 12.7. The average CME speed at ∼5 solar radii is close to the median ecliptic solar wind speed of ∼440 km/s. Ejection speeds near the Sun range from less than 50 km/s in some of the slower events to greater than 2500 km/s in the fastest ones.

During a typical CME, somewhere between 10 15 and 10 16 g is ejected into the heliosphere. Few CMEs ever appear to sever completely their magnetic connection with the Sun. The outer edges of CMEs often have the optical appearance of closed loops such as the event shown in Figure 12.7(a). Typically these closed field regions are found in the coronal streamer belt that encircles the Sun and that underlies the heliospheric current sheet. CMEs originate in closed field regions in the corona where the magnetic field normally is sufficiently strong to constrain the coronal plasma from expanding outward. (See The Sun.) The most rapid and dramatic evolution in the corona occurs in events known as coronal mass ejections or CMEs ( Figure 12.7(a)). The solar corona evolves on a variety of timescales closely connected with the evolution of the coronal magnetic field. Magnetic reconnection operates in two quite distinct physical parameter domains: (1) in the chromosphere during magnetic flux emergence, magnetic flux cancellation, and the so-called explosive events and (2) under coronal conditions during microflares, flares, and CMEs. The sheared magnetic field configurations and the existence of coronal and chromospheric null points, which are now inferred more commonly in solar flares, require ultimately 3D reconnection models, possibly involving null point coalescence, spine reconnection, fan reconnection, and separator reconnection. A more realistic approach seems to be bursty 2D reconnection models ( Figure 11.15, bottom), which involve the tearing mode and coalescence instability and can reproduce the sufficiently fast temporal and small spatial scales required by solar flare observations. Only steady 2D reconnection models can be formulated analytically they provide basic relations for inflow speed, outflow speed, and reconnection rate, but represent oversimplifications for most (if not all) observed flares. Theory and numerical simulations of magnetic reconnection processes in the solar corona have been developed for steady 2D reconnection ( Figure 11.15, top), bursty 2D reconnection, and 3D reconnection. Magnetic reconnection processes can occur in a slowly changing quasi-steady way, which may contribute to coronal heating ( Section 5.9), but more often happen as sudden violent processes that are manifested as flares and CMEs.

SUN CORONA TEMP FREE

Topological changes in the form of magnetic reconnection always liberate free nonpotential energy, which is converted into heating of plasma, acceleration of particles, and kinematic motion of coronal plasma. in filaments, in twisted sigmoid-shaped loops, and along sheared neutral lines).

Of course, such magnetic restructuring processes occur wherever magnetic stresses build up (e.g. These three dynamic boundary conditions are the essential reasons why the coronal magnetic field is constantly stressed and has to adjust by restructuring the large-scale magnetic field by topological changes, called magnetic reconnection processes.

the tachocline) which rises by buoyancy and emerges through the photosphere into the corona, (2) the differential rotation as well as convective motion at the solar surface continuously wrap up the coronal field, and (3) the connectivity to the interplanetary field has to constantly break up to avoid excessive magnetic stress. The solar corona has dynamic boundary conditions: (1) the solar dynamo in the interior of the Sun constantly generates new magnetic flux from the bottom of the convection zone (i.e. Aschwanden, in Encyclopedia of the Solar System (Third Edition), 2014 6.1 Magnetic Reconnection

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