5.2 Mass ejection

5.2.1 Reconnection jet

Magnetic reconnection produces oppositely directed, bidirectional high-speed flows (called reconnection jets) emanating from the reconnection point. The velocity of reconnection jet is given by the Alfvén velocity in Equation (22View Equation). In the case of the Sweet–Parker-type reconnection, the width of reconnection jet is nearly constant, comparable to the width of the diffusion region (see Figure 26View Image)

wjet ≃ MAL ∼ 100 cm, (24 )
where MA is the nondimensional reconnection rate given in Equation (19View Equation) and L (length of the diffusion region, see Figure 26View Image) ∼ 109 cm. On the other hand, the Petschek-type reconnection gives
wjet ≃ MAL ∼ 100 –1000 km , (25 )
where MA is the nondimensional reconnection rate given in Equation (21View Equation). Since the Petschek-type reconnection is accompanied by the slow MHD shocks that extend from the diffusion region (explained below) and contribute to accelerating plasma, the Petschek-type reconnection is more dynamic than the Sweet–Parker-type reconnection. It should be noted that the width of the reconnection jets produced by the Petschek-type reconnection is not constant, rather it increases as the jet leaves away from the reconnection point.

5.2.2 Plasmoid ejection

Magnetic reconnection mainly converts magnetic energy into thermal and kinetic energy, and part of the kinetic energy is used for plasmoid ejection. An observational result on plasmoid ejection and its comparison to theoretical modeling are presented in Figure 43View Image. The dynamics of an ejecting plasmoid has been investigated in numerical simulations (Magara et al., 1997Jump To The Next Citation Point; Choe and Cheng, 2000Jump To The Next Citation Point; Shibata and Tanuma, 2001Jump To The Next Citation Point).

View Image

Figure 43: (a) Hight-time relation of a magnetic island in a two-dimensional numerical simulation, which is supposed to be the two-dimensional counterpart of a plasmoid. Time variation of the conductive electric field defined by Equation (16View Equation) is also plotted (from Magara et al., 1997Jump To The Next Citation Point). (b) Time variations of the height of an observed plasmoid as well as hard X-ray intensity (modified from Ohyama and Shibata, 1997Jump To The Next Citation Point). (c) Multiple ejection of plasmoids (from Choe and Cheng, 2000Jump To The Next Citation Point).

Figure 43View Imagea shows a result from a two-dimensional MHD simulation, in which magnetic reconnection produces an ejecting magnetic island (two-dimensional counterpart of a plasmoid). The time variation of the convective electric field defined by Equation (16View Equation) is also plotted at this panel. Figure 43View Imageb shows the height-time relations of an observed plasmoid as well as hard X-ray intensity (Ohyama and Shibata, 1997). When comparing these simulation and observation, we assume that the time variation of the convective electric field is closely related to the time variation of hard X-ray emissions because the electric field can accelerate particles which contribute to producing hard X-ray emissions. The comparison suggests that the plasmoid ejection drives fast magnetic reconnection. More detailed investigations of plasmoid ejection are given by Choe and Cheng (2000), where multiple ejection of plasmoids and associated HXR bursts are discussed (see Figure 43View Imagec).

Shibata and Tanuma (2001) gives a rough estimate on the velocity of an ejecting plasmoid as follows:

-----vAexp-(ωt)----- v = exp (ωt) − 1 + v ∕v , (26 ) A 0
where v0 and vA represent the initial velocity of a plasmoid and Alfvén velocity. In Equation (26View Equation), ω represents the velocity growth rate of a plasmoid, defined as
ω = ρ0vA, (27 ) ρpL
where ρ0 is the density of ambient plasma while ρp and L are the density and length of a plasmoid.
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