Magnetic structure of the solar transition region as observed in various ultraviolet lines emitted at different temperatures

Abstract

Aims. The structure of the solar transition region (TR) in a polar coronal hole of the Sun is studied. In particular, the detailed association of the coronal magnetic field (carpet) with the radiance patterns of the TR, when seen in various far ultraviolet (FUV) emission lines, is investigated. Methods. A detailed comparison is made of the coronal magnetic field, as obtained by extrapolation of the NSO/Kitt-Peak photospheric field to heights of several tens of megameters, with the radiances of many FUV lines, which are emitted by ions of various elements at different ionization stages, corresponding to different local coronal temperatures. By a correlation analysis of the emission pattern with the magnetic field (network and carpet of loops), the so-called correlation height of the emission can be determined. By its help and through a correlation analysis the magnetic nature of the emission regions and the temperature structure of the TR can be better revealed and understood. Results. In particular, at mesoscopic scales of several megameters the regions with strong emission (originating from multiple small closed loops) are found to be located at low heights, whereas weak emissions (coming from locally open, i.e. far reaching fields) appear to originate at greater heights. These findings are consistent with similar results obtained at large scales for large-size loops and big coronal holes. Conclusions. Our correlation-height analysis of the emission lines confirms the notion that plasma at different temperature can coexist at the same height. The TR is not thermally stratified but strongly nonuniform and magnetically structured.

Solar wind origins in coronal holes and in the quiet Sun

“…Small scale closed loops dominate in the QS and confine the plasma in the lower atmosphere. Such influence of the magnetic-field structure on the emission height of EUV lines is also confirmed by Marsch et al (2006). They investigated the emission correlation heights of 12 EUV lines in two sub-regions with different intensity in a polar region.…”

“…These studies were based on observations made by SUMER, MDI and EIT onboard SOHO (Tu et al, 2005a,b,c;Marsch et al, 2006;He et al, 2007He et al, , 2008Tian et al, 2008a,b). We will not discuss here the issue of the solar wind origin in the vicinity of active regions, which was studied recently by Sakao et al (2007).…”

Sizes of transition-region structures in coronal holes and in the quiet Sun

“…After removing the instrumental and temperature broadening, we thus obtained the image of the non-thermal velocity for each line in the first data set. In order to investigate the links between the 3-D magnetic structures and the long-lasting transition-region features, we successfully reconstructed the magnetic structure above the photosphere by using the force-free model proposed by Seehafer (1978), following the work of many authors Wiegelmann et al 2005;Tu et al 2005a,b;Marsch et al 2006;He et al 2007;Tian et al 2007Tian et al , 2008). Here we reconstructed a potential 3-D magnetic field, by applying the same method and using the observed magnetograms which correspond to the area observed by SUMER in each data set.…”

Intermittent outflows at the edge of an active region – a possible source of the solar wind?

“…During solar minimum, the fast solar wind is recognized to originate in the (polar) coronal holes (CHs), in particular from the magnetic funnels according to both observations (Xia et al 2003;Tu et al 2005a,b;Marsch et al 2006;Tian et al 2010) and theoretical models Hackenberg et al 2000;Esser et al 2005;He et al 2008). The slow solar wind has several possible source regions, such as the boundaries of polar CHs (Wang et al 1990), the helmet streamers , the funnel-like structures in the quiet regions (He et al 2007;Tian et al 2008aTian et al , 2009, and the edges of active regions (ARs) (e.g., Sakao et al 2007;Harra et al 2008).…”