Energetics of Explosive Events Observed with SUMER

Observations of solar chromosphere-corona transition region plasma show evidence of small-scale, short-lived dynamic phenomena characterized by significant nonthermal broadening and asymmetry in the wings of spectral line profiles. These impulsive mass motions (explosive events) are thought to be the product of magnetic reconnection and to be similar in driving mechanism (though larger in size) to nanoflares, the small-scale events proposed to heat the corona. In this paper, we present a statistical analysis of the energetics of explosive events to address the viability of the nanoflare heating theory. We consider high spectral, spatial, and temporal resolution spectra of the C III λ977, N IV λ765, O VI λ1032, and Ne VIII λ770 lines observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) telescope and spectrometer. Each line profile exhibiting explosive event characteristics was analyzed using the velocity differential emission measure (VDEM) technique. A VDEM is a measure of the emitting power of the plasma as a function of its line-of-sight velocity and hence provides a method of accurately measuring the energy flux associated with an explosive event. We find that these events globally release ~4 × 104 ergs cm-2 s-1 toward both the corona and chromosphere. This implies that explosive events themselves are not energetically significant to the solar atmosphere. However, the distribution of these explosive events as a function of their energy has a power-law spectral index of α = 2.9 ± 0.1 for the energy range 1022.7-1025.1 ergs. Since α is greater than 2, the energy content is dominated by the smallest events. Hence, if this distribution is representative of the size distribution down to lower energy ranges (~1022 ergs), such small and (currently) undetectable events would release enough energy to heat the solar atmosphere.