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Spectroscopic diagnostic and modelling of coronal structures

D. Spadaro and R. Ventura investigated the properties of the interface between streamers anc coronal holes at low heliocentric distances, observing the extended solar corona in the North-West quadrant by UVCS/SOHO. They measured the line profiles of the H I Ly$\alpha$ and O VI resonance doublet and the visible linearly polarized radiance at heliocentric distances ranging from 1.4 to 2.5 R$_{\odot}$ and colatitudes spanning from the North pole to the West equator with steps of about 10 deg. The results show that both the line intensities and the line widths, in particolar those of the O VI, exhibit sharp variations across the streamer boundaries, with a clear anticorrelation between intensities and widths (Fig. 1). The authors also notice a positive correlation for Ly$\alpha$ in the region close to the equator. The steep changes in the O VI line profiles occur in a narrow transition region (5 deg - 10 deg), right at the borders of the streamers, from 1.5 R$_{\odot}$ onwards. The O VI resonance doublet line ratio steeply increases outside of the streamer as well, but this occurs at higher heliocentric distances (above 2 R$_{\odot}$). Hence the marked broadenong of the O VI lines and the considerable rise of their intensity ratio are an evident signature of the transition fromn closed to open field lines in streamer magnetic field topologies. This behaviour also implies that a strong and preferential non-thermal heating of O VI ions in the direction coinciding with the line of sight and the turn-on of a significant outflow occur in the open magnetic field region near or just outside of the streamer edges. Such conclusion may have important consequences for realistic theoretical models of streamers and the source regions of the slow solar wind (D. Spadaro, R. Ventura, G. Cimino M. Romoli, "UVCS/SOHO investigation of the interface between streamers and coronal holes" 2005 A&A 429, 353).

R.Ventura and D. Spadaro studied the physical properties of the extended solar corona such as ion kinetic temperature, electron density and outflow velocity, inside and outside streamers and their dependence on the phase of solar cycle. They analyzed UVCS/SOHO observations of the ionOvi resonance doublet and ionHi Ly$\alpha$ line intensities and profiles, together with measurements of the visible linearly polarized radiance, carried out during two MEDOC campaigns, in 1997 and 2000, i.e. near solar minimum and approaching the solar maximum phase, respectively. During both observational runs mid-latitude coronal regions in the West limb of the Sun have been scanned over a range of heliocentric distance from 1.39 to 4.1 R$_{\odot}$. Besides confirming some previous findings on significant differences between open and closed field-line structures at solar minimum, the results provide some evidence for differences in kinetic temperature among mid-latitude solar minimum streamers observed on different days from about 2 R$_{\odot}$ outwards, as well as in their dynamical conditions at heliocentric distances greater than 3.6 R$_{\odot}$. For observations carried out in 2000, conversely, the mid-latitude coronal streamers and their surroundings are about 3 times, and more than one order of magnitude brighter, respectively, than their solar minimum counterparts and exhibit very similar kinetic and dynamical conditions. The kinetic temperatures in adjacent regions are higher than in streamers (by about a factor of 2) only within 2 R$_{\odot}$, while at greater heights such differences vanish, making it difficult to discriminate between open and closed structures. This is opposite to the behaviour detected at solar minimum, when adjacent regions appear to be characterized by kinetic temperatures progressively higher and higher than in streamers with increasing height, from 2 R$_{\odot}$ outwards. Therefore, a clear characterization of open and closed configurations near the solar maximum might be quite difficult, probably due both to the intrisically more complex magnetic configuration of the corona in this phase of the solar activity and the line-of-sight contamination effects that in a highly structured solar corona may strongly mix background and foreground plasma with different properties. The transition from the solar minimum to maximum also seems characterized by a global increase in the electron density inside streamers of about a factor of 4 at 1.7 R$_{\odot}$ and then it progressively decreases with height (R. Ventura, D. Spadaro, G. Cimino, M. Romoli, "Streamers and adjacent regions observed by UVCS/SOHO: A comparison between different phases of solar activity" 2005, A&A 430, 701).

Figure: Image of the extended solar corona obtained by the LASCO C1 coronagraph on board SOHO on October 30, 1997, through a narrow-band Fabry-Pèrot filter at the centre of the ion Fe XIV 5303 $\AA$  line. Superposed on the LASCO image are the UVCS slit fields of view (white lines) at nominal slit height of 1.75 R$_{\odot}$, for P.A. = 330 deg (a) and P.A. = 270 deg (b). The nominal heliocentric distance and P.A. for each slit refer to the distance and P.A. of the point along the slit that is closest to the limb (shown as a white dot). The Ly$\alpha$ and O VI 1032 $\AA$  total line intensities and 1/2 half-widths, as well as the O VI line ratios, vs. position angle alonf the slit (a) and (b) are shown at the bottom of the figure.
\begin{figure}\centerline{\psfig{file=sun/0313fig8.eps,width=8cm}}\end{figure}


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Next: Irradiance variations of the Up: Solar Physics Previous: Solar Physics   Contents   Index
Innocenza Busa' 2005-11-14