Streamers and adjacent regions observed by UVCS/SOHO: A comparison between different phases of solar activity

UVCS/SOHO observations of the O VI resonance doublet and H I Lyα line intensities and profiles, together with measurements of the visible linearly polarized radiance, have been performed 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$_{⊙}$, to study the plasma properties of streamers and adjacent regions, such as ion kinetic temperature, electron density and outflow velocity, paying particular attention to comparing plasma conditions deduced for different ions in coronal structures observed on different days and during different phases of solar activity. Besides confirming some previous findings on significant differences between open and closed field-line structures at solar minimum, our results provide some evidence for differences in kinetic temperature among mid-latitude solar minimum streamers observed on different days from about 2 R$_{⊙}$ outwards, as well as in their dynamical conditions at heliocentric distances greater than 3.6 R$_{⊙}$. 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$_{⊙}$, 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$_{⊙}$ 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$_{⊙}$ and then it progressively decreases with height.