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The properties of explosive events in the solar transition zone are presented by means of detailed examples and statistical analyses. These events are observed as regions of exceptionally high velocity (~ 100 km s 1) in profiles of C1v, formed at 102 K, observed with the High Resolution Telescope and Spectrograph (HRTS). The following average properties have been determined from observations obtained during the third rocket flight of the HRTS: full width at half maximum extent along the slit -1.6 • 103 kin; maximum velocity -110 km s-1; peak emission measure -4 x 1041 cm-3; lifetime -60 s; birthrate -4 x 10 -21 cm -2 s-1 in a coronal hole and 1 x 10 -20 cm -2 s-1 in the quiet Sun; mass -6 x 108 g; and, kinetic energy-6 x 1022 erg. The 6 examples show that there are considerable variations from these average parameters in individual events. Although small, the events show considerable spatial structure and are not point-like objects. A spatial separation is often detected between the positions of the red and blue shifted components and consequently the profile cannot be explained by turbulence alone. Mass motions in the events appear to be isotropic because the maximum observed velocity does not show any correlation with heliographic latitude. Apparent motions of the 100 km s -~ plasmas during their 60 s lifetime should be detected but none are seen. The spatial frequency of occurrence shows a maximum near latitudes of 40-50 ~ but otherwise their sites seem to be randomly distributed. There is enough mass in the explosive events that they could make a substantial contribution to the solar wind. It is hard to explain the heating of typical quiet structures by the release of energy in explosive events.

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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).

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