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

Abstract

Abstract. UVCS/SOHO observations of the O  resonance doublet and H  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 midlatitude 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.

Physical parameters of a mid-latitude streamer during the declining phase of the solar cycle

“…The densities fitting our O vi data are only similar to those obtained by Ventura et al (2005) in some mid-latitude streamers at solar minimum, but considerably lower than the values reported in other investigations.…”

“…5). This different behavior has already been described in the literature (e.g., Strachan et al 2002;Ventura et al 2005;Uzzo et al 2006) and hints at mechanisms that preferentially heat O vi ions more than protons.…”

“…The Lyα intensity radial profile exhibits a shallower decrease than that of the O vi line observed on May 21 and 22: in the same range of heights, we notice a decrease slightly larger than one order of magnitude for the former and about two orders of magnitude for the latter. Besides the possible differences due to the solar rotation, it is worth noting that, as discussed in previous papers (e.g., Ventura et al 2005), the two lines fall off differently with height because Lyα emission is dominated by resonance scattering, proportional to the density, while the O vi line also has a collisionally excited component, which depends on density squared. Figure 7 shows the radial profile of the H i perpendicular kinetic temperature derived as in the case of the O vi ions, together with their related uncertainties.…”

“…Conversely, the equatorial streamers of Gibson et al (1999) and Strachan et al (2002), the equatorial and mid-latitude active region streamers of Parenti et al (2000) and the depleted-core equatorial streamer of Frazin et al (2003) were observed during a minimum phase of the solar activity. The results presented by Ventura et al (2005) concern a collection of streamer observations performed at different latitudes in 1997 and 2000, hence near solar minimum and maximum respectively.…”

Physical parameters along the boundaries of a mid-latitude streamer and in its adjacent regions

“…The behavior of the O vi kinetic temperature radial profiles might suggest that the same heating mechanism acting inside the streamer is working along its boundaries, and even more efficiently in the region outside the streamer, probably due to the different magnetic configuration (open vs. closed) of this region (see, e.g. Frazin et al 2003;Ventura et al 2005). …”