Driving mechanisms for the solar wind

Abstract

In this review, we consider the central physical aspects pertinent to the acceleration of the solar wind. Special importance is placed on the high-speed streams since the properties of these structures seem to strain the various theoretical explanations the most. Heavy emphasis is also given to the observations -particularly as to what constraints they place on the theories. We also discuss certain sporadic events such as spicules, macrospicules, X-ray bright points, and outflows seen in the EUV associated with the explosive events, jets, and coronal bullets which could be of relevance to this problem.Three theoretical concepts pertaining to the solar wind acceleration process are examined -purely thermal acceleration with and without extended heating, acceleration due to Alfvrn wave pressure, and diamagnetic acceleration. Emphasis is given to how well these theories meet the constraints imposed by the observations. Diamagnetism is argued to be a powerful ingredient in solar wind theory, both in the light of observed sporatic outflows seen in the chromosphere and transition region and also because of its effectiveness in increasing the flow speed and producing strong acceleration near the Sun in line with coronal hole observations.

Plasmoids in reconnecting current sheets: Solar and terrestrial contexts compared

“…There may be some similarity between these scenarios and older models of diamagnetic acceleration of the solar wind via buoyant plasmoids that may fill some 0fraction of the corona [e.g., Schlüter , 1957; Pneuman , 1986; Mullan , 1990]. The additional momentum deposited by this process is given approximately by an effective buoyancy‐driven pressure gradient where f d is the ratio of mass flux in the plasmoids to the total wind mass flux and w d is the most‐probable speed in the plasmoids [see also Pneuman , 1983; Yang and Schunk , 1989; Tamano , 1991].…”

The ?melon-seed? mechanism and coronal transients

“…Prior to the coronal transient, the large-scale magnetic field configuration usually contains a prominences. For this largescale configuration, we adopt the model of Pneuman (1983d) which is a 'figure 8' configuration such as shown in Figure 1. The model presupposes that some magnetic reconnection occurs during the prominence formation process and, since the reconnecting field is likely to be sheared in some way, the isolated loops in Figure 1 are actually helices connected at their ends to the photosphere.…”

“…The configuration which we adopt here is a 'figure 8' field structure such as shown in Figure 1 although that figure depicts the configuration at a later time during an actual two-ribbon flare. This basic configuration has been studied by Pneuman (1983d) and also as one of a class of current sheet prominence models by Malherbe and Priest (1983). We argue that this configuration is a natural consequence of the distention of a bipolar region upward into the corona while pushing the existing ambient magnetic field to the side.…”

“…Except for a small error near ri, the field strength around the circular part is just that of a circular cylinder placed in a diverging radial field and can be found by a superposition method described in Pneuman (1983b), i.e., B e=2B asin0 l+sRcos0 (lnBa) ,…”

On the Acceleration of the Solar Wind: Role of the Inhomogeneous Flow

“…It is known that a velocity Ðeld in the solar wind changes along the radial distance from the Sun : the Ñow speed increases from very low values in the inner corona and eventually becomes beyond the radius, Within this super-Alfve nic Alfve n r a . radius the magnetic Ðeld dominates over the plasma dynamics, while beyond it the magnetic Ðeld is carried by the plasma (see, for example, reviews by Axford 1985 ;Parker 1986 ;Pneuman 1986). Most two-dimensional theoretical works consider a radially symmetrical geometry of the solar wind, but this assumption is misleading near the Sun.…”