Volume 594, October 2016
|Number of page(s)||20|
|Published online||13 October 2016|
Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS⋆
1 Department of Applied Mathematics and
Theoretical Physics, CMSUniversity of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
2 Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic
3 STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon. OX11 0QX, UK
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 01238, USA
Accepted: 18 July 2016
The intensity of the O iv 2s2 2p 2P–2s2p24P and S iv 3 s2 3p 2P–3s 3p24 P intercombination lines around 1400 Å observed with the Interface Region Imaging Spectrograph (IRIS) provide a useful tool to diagnose the electron number density (Ne) in the solar transition region plasma. We measure the electron number density in a variety of solar features observed by IRIS, including an active region (AR) loop, plage and brightening, and the ribbon of the 22-June-2015 M 6.5 class flare. By using the emissivity ratios of O iv and S iv lines, we find that our observations are consistent with the emitting plasma being near isothermal (logT[K] ≈ 5) and iso-density (Ne ≈ 1010.6 cm-3) in the AR loop. Moreover, high electron number densities (Ne ≈ 1013 cm-3) are obtained during the impulsive phase of the flare by using the S iv line ratio. We note that the S iv lines provide a higher range of density sensitivity than the O iv lines. Finally, we investigate the effects of high densities (Ne ≳ 1011 cm-3) on the ionization balance. In particular, the fractional ion abundances are found to be shifted towards lower temperatures for high densities compared to the low density case. We also explored the effects of a non-Maxwellian electron distribution on our diagnostic method.
Key words: Sun: transition region / Sun: UV radiation / techniques: spectroscopic / atomic data
The movie associated to Fig. 3 is available at http://www.aanda.org
© ESO, 2016
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