Explosive events and magnetic reconnection in the solar atmosphere

Abstract

Explosive events are highly energetic, small‐scale phenomena which are frequently detected throughout the quiet and active Sun. They are seen in profiles of spectral lines formed at transition zone temperatures as exceptionally Doppler‐shifted features, typically at 100 km s−1 to the red and/or blue of the rest wavelength. Sufficient observational evidence has now been developed to demonstrate that some explosive events are associated with the emergence of new magnetic flux. In these cases it is likely that the acceleration of plasma is caused by the magnetic reconnection resulting from flux emergence. We take as a working hypothesis the proposal that all explosive events are the result of magnetic reconnection. Since explosive events tend to occur on the edges of high photospheric magnetic field regions, we identify them with reconnection that occurs during the cancellation of photospheric magnetic flux (Martin, 1984; Livi et al., 1985). The combined observational characteristics of photospheric flux cancellation and transition zone explosive events provide powerful diagnostic information concerning the nature of magnetic reconnection. Reconnection in the quiet solar atmosphere apparently proceeds in bursts at sites much smaller than the boundary between opposite polarity flux elements that are observed to cancel in magnetograph sequences. Equating the velocity of the expelled transition zone plasma with the Alfvén speed yields magnetic field strengths of 20 G at the site of reconnection. The speed at which the reconnection proceeds is commensurate with the rapid rates predicted by Petschek (1964).

Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations

“…They are not associated with X-ray bright points (Moses et al 1994). Cancellations of photospheric magnetic fields observed in relation to explosive events (Dere et al 1991;Chae et al 1998c) and bidirectional jets (Innes et al 1997a) or chromospheric upflow events (Chae et al 1998d) are strong indications of a generation process by reconnection. The spectroscopic signature of bidirectional jets seems to be a characteristic feature of explosive events.…”

“…The first density results of n e ≈ 7 × 10 10 cm −3 were obtained by Dere et al (1991). Line-ratio studies later indicated that there is a density increase by a factor of about three and, in some cases, a significant electron temperature increase (Wilhelm et al 1998c;Teriaca et al 2001Teriaca et al , 2002aMendoza-Torres et al 2005).…”

Relationship between supergranulation flows, magnetic cancellation and network flares

“…De Pontieu et al 2007;Hasan & van Ballegooijen 2008;van Ballegooijen et al 2011;Meyer et al 2013;De Pontieu et al 2014) but at present there is no clear consensus about which dominates. Above the chromosphere, the slightly hotter transition region network is characterised by small, highly dynamic brightenings and jets Aiouaz 2008) that are thought to result from magnetic reconnection (Dere et al 1991;Innes et al 1997b). At higher temperatures, the network mixes with diffuse coronal emission and loops (Feldman et al 2000) in which sudden small-scale X-ray brightenings are seen at a rate of about one every three seconds for the whole Sun (Krucker et al 1997).…”

Chromospheric magnetic reconnection caused by photospheric flux emergence: implications for jet-like events formation

“…Dere et al (1991) suggested that they are produced by bi-directional jets as a result of magnetic reconnection. To date, these jetlike events (often called explosive events) are mainly observed in spectral lines formed at transition region temperatures (Dere 1994;Chae et al 1998;Innes et al 2001;Madjarska & Doyle 2003), although observations of explosive events in chromospheric lines are also reported.…”