### 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 (22). 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 26)

where is the nondimensional reconnection rate given in Equation (19) and (length of the
diffusion region, see Figure 26) 10^{9} cm. On the other hand, the Petschek-type reconnection gives
where is the nondimensional reconnection rate given in Equation (21). 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 43. The dynamics of an ejecting plasmoid has been
investigated in numerical simulations (Magara et al., 1997; Choe and Cheng, 2000; Shibata and Tanuma,
2001).

Figure 43a 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 (16) is also plotted at this panel. Figure 43b 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 43c).

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

where and represent the initial velocity of a plasmoid and Alfvén velocity. In Equation (26),
represents the velocity growth rate of a plasmoid, defined as
where is the density of ambient plasma while and are the density and length of a
plasmoid.