ner_tags: O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Mission O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Mission O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O
tokens: in the legend. The silicon line is solid, the oxygen line is dashed (quiet Sun adjustment) and dotted–dashed (active region adjustment). The two events above were not associated with Fe xxi emission. However, in the same raster set there is a flare with Fe xxi emission. We picked the non-flare data because the explosive events in these data are more impressive than in the flare. We have also examined the flare discussed by Polito et al. ( 2016 ). The spectrum for exposure 5 and raster 15 in Figure 7 of Polito et al. ( 2016 ) shows dynamical Si iv activity at apparent footpoints of a loop or loops emitting in Fe xxi . One footpoint region has a strong continuum, and visual inspection of the Si iv spectrum reveals just a small downflow extension to the stationary component. The other footpoint shows a fairly symmetric and highly broadened Si iv line. Thus, the nature of explosive events appears to be highly variable from event to event. The events we have discussed also include visual inspection of the C ii and Mg ii lines observed by IRIS , and these lines show strong dynamical effects. So the events seen at Si iv and O iv temperatures also extend to lower temperatures. 4. DISCUSSION AND CONCLUSIONS The IRIS spectra indicate that explosive events in active regions contain plasma at densities near 10 12 cm −3 flowing outwards at speeds on the order of 100 km s −1 . The early Skylab observations butress the high density explanation. The behavior of density with upflow speed in Figure 9 and Figure 13 can be qualitatively explained by an evaporation scenario. In this scenario the evaporation initially occurs in a high density region near 10 12 cm −3 and flows upwards into an expanding flux tube which causes the density to decrease. The speed decreases as the plasma reaches the top of the loop or loop segment. However, the amount of plasma at the loop top builds up in volume, which results in a higher intensity than at the site of evaporation. Therefore, the intensity in the line profile increases toward zero upflow speed. The mystery problem is the high electron densities. This was encountered by Peter et al. ( 2014 ) in explosive events that appear like Ellerman bombs. Peter et al. ( 2014 ) simply found that the O iv line was so weak relative to the Si iv line that a very high density was implied. So he put the event in the photosphere, which was challenged by Judge ( 2015 ) for many reasons. Judge ( 2015 ) correctly pointed out that the O/Si ratio is not an ideal density diagnostic because of its temperature dependence. However, this issue has been addressed extensively above with the Skylab data, and these data seem to make clear that very small O/Si ratios are due to high electron densities, and perhaps not an FIP effect (e.g., Laming 2015 ) or non-ionization equilibrium effect (e.g., Olluri et al. 2013 ; Dudík et al. 2014 ; Martínez-Sykora et al. 2016 ). However, if a non-ionization equilibrium calculation were applied to all the ions