Explosive events in the solar transition zone

Abstract

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.

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

“…Explosive events have been observed with high spectral and spatial resolutions by HRTS and SUMER (cf., Dere et al 1989b;Kjeldseth-Moe and Cheng 1994;Moses et al 1994;Innes et al 1997aInnes et al , 1997bInnes 2001). Observations of the line profile of C IV at 154.8 nm with HRTS revealed that, in general, the shapes of the line during the events can be described by three Gaussian profiles with one component at rest and the others shifted by ≈10 pm to either red or blue (Dere et al 1981).…”

Spectroscopic evidence for helicity in explosive events

“…Dere et al (1989) already noted "... the lack of detectable apparent motions of such high-velocity events". Innes (2004) again mention this conflict that is still unsolved.…”

“…In spectroscopic data EEs are easily detected by the redshift and blueshift of the observed transition region (TR) line. The terminology "explosive event" has first been introduced by Dere et al (1984) based on the analysis of high-resolution spectra of TR emission lines obtained by the HRTS instrument on Spacelab, but it turned out that this term is quite debatable (e.g., Dere et al 1989) and may be misleading.With the advent of SoHO-SUMER (Wilhelm et al 1995) began a revival of this field of research. Typical EEs were found to be short-lived (60 s to 200 s), small-scale (1500 km to 2500 km), high-velocity (±50 km s −1 to ±150 km s −1 ) flows that occur very frequently, sometimes in bursts.…”

Simultaneous observations of solar transition region blinkers and explosive events by SUMER, CDS and BBSO

“…The main characteristics of the explosive events were studied by Brueckner & Bartoe (1983), Dere et al (1989), Porter & Dere (1991), Innes et al (1997Innes et al ( , 2001), Chae et al (1998), Pérez et al (1999), Teriaca et al (2001), and Teriaca et al (2002).…”