Explosive Events

High spectral, spatial and temporal resolution UV observations of the quiet Sun transition region show a highly structured and dynamical environment where transient supersonic flows are commonly observed. The extremely high quality of our observations allows us to identify tens of explosive events from which we estimate an average size of 1800km and a birthrate of 2500 per second over the entire Sun. Estimates of the kinetic and enthalpy fluxes associated with these events show that explosive events are not important so far as solar coronal heating is concerned. The relationship with the underlying photospheric magnetic field is also studied, revealing that explosive events generally occur in regions with weak (and, very likely, mixed polarity) magnetic flux.

**Figure 5:** A detailed view of the loop flow velocity at different locations (Teriaca et al. 2004). **(a)** Logarithmically-scaled radiance image. Isocontours of the negative polarity of the longitudinal magnetic flux at (, and ) Gauss (G) are shown with black solid lines (no positive flux above 10G is present in the displayed area). Locations where non-Gaussian line profiles were found are marked with a black +. The dashed dark-grey (red) line indicates the projection (on the plane perpendicular to the line-of sight [LOS]) of a semi-circular loop with a diameter of 13arcsec. The loop is inclined by 18 with respect to the LOS and the footpoint line rotated by 12.5 clockwise. The black dots indicate the measured position of the observed loop. **(b)** Line profile at a location on the northern leg of the loop. The bars indicate the data points. The thin solid lines show the three components used to fit the data while the resulting fitting profile is represented by the thick solid line. **(c)** Line profile at the top of the loop. The dotted line shows the profile obtained averaging over the whole raster times 4.9. **(d)** The same as for panel **(b)** but for the southern leg of the loop.

By studying the structure of upward and downward flows exceeding those associated with average quiet Sun profiles, we find a clear correlation between the `excess' flows and the magnetic network. However, although explosive events are always associated with flow patterns often covering areas larger than the explosive event itself, the contrary is not true. In particular, almost all flows associated with the stronger concentrations of photospheric magnetic flux do not show non-Gaussian line profiles. In some cases, non-Gaussian line profiles are associated with supersonic flows in small magnetic loops. The case of a small loop showing a supersonic siphon-like flow of 130km s is studied in detail. This is, to our knowledge, the first detection of a supersonic siphon-like flow in a quiet Sun loop. In other cases, the flow patterns associated with explosive events may suggest a relation with UV spicules (Teriaca et al. 2004).

In a second dataset we obtained simultaneous observations of the chromospheric SiII 1251.16Å and CI 1251.17Å, the transition region NV 1238.8Å and coronal MgX 625Å lines using SUMER. We show an example of a UV explosive event observed in the chromospheric and the transition region lines but not showing any detectable signature in the coronal line. The phenomenon, however, was also clearly detected by the TRACE imager with the 171Å filter. This discrepancy is explained with a non-Maxwellian electron distribution which makes a significant fraction of the plasma in the TRACE 171Å pass-band to be derived from temperatures around 300,000K, as opposed to 800,000K. This could have implications for other phenomena observed in the TRACE pass-bands, including the transition region `moss' and the three- and five-minute oscillations (Doyle et al. 2004).