Oxygen abundance in coronal streamers during solar minimum

Abstract

Abstract. We present a study of the oxygen abundance relative to hydrogen in the equatorial streamer belt of the solar corona during the recent period of activity minimum. The oxygen abundance is derived from the spectroscopic observations of the outer corona performed during 1996 with the Ultraviolet Coronagraph Spectrometer (SOHO) in the ultraviolet region. This study shows that the depletion of oxygen, by almost one order of magnitude with respect to the photospheric values, found in the inner part of streamers by Raymond et al. (1997a) is a common feature of the solar minimum streamer belt, which exhibits an abundance structure with the following characteristics. In the core of streamers the oxygen abundance is 1.3×10 −4 at 1.5 R , then it drops to 0.8×10 −4 at 1.7 R , value which remains almost constant out to 2.2 R . In the lateral bright structures that are observed to surround the core of streamers in the oxygen emission, the oxygen abundance drops monotonically with heliodistance, from 3.5×10 −4 at 1.5 R to 2.2×10 −4 at 2.2 R . The oxygen abundance structure found in the streamer belt is consistent with the model of magnetic topology of streamers proposed by Noci et al. (1997). The composition of the plasma contained in streamers is not the same as observed in the slow solar wind. Even in the lateral branches, richer in oxygen, at 2.2 R the abundance drops by a factor 2 with respect to the slow wind plasma observed with Ulysses during the declining phase of the solar cycle. Hence the slow wind does not appear to originate primarily from streamers, with the exception perhaps of the plasma flowing along the heliospheric current sheet.

Elemental Abundances for the 1996 Streamer Belt

“…A discrepancy between the collisional and radiative derived results has been known and discussed in previous publications (e.g., Raymond et al 1997;Parenti et al 2000;and Marocchi et al 2001). The same discrepancy was also found for this data, and no new insights were discovered concerning a possible explanation.…”

“…For both Figures 4 and 5, the direction of solar north is indicated. The oxygen intensity signature identified as a quiescent streamer has been investigated in several previous publications (e.g., Raymond et al 1997Raymond et al , 1998Marocchi, Antonucci, & Giordano 2001). While several other possible explanations for this difference between the Lyman series and other line intensity profiles have been explored, for example, Noci et al (1997), the abundance argument seems to be the strongest.…”

“…The first and last boundaries, B1 and B6, are located at the spatial position where the intensities of Max1 and Max2 have dropped by a factor of 1/e, respectively. Marocchi et al (2001) used the same criteria for defining their streamer boundaries. For active-region streamers (see the example in Fig.…”

Slow wind and magnetic topology in the solar minimum corona in 1996–1997

“…As for the slow wind, UVCS observations of the streamer belt at solar minimum have revealed an abundance anomaly (e.g. Noci et al 1997;Raymond et al 1997;Marocchi et al 2001;Ko et al 2002;Uzzo et al 2003) that might be related to the origin of the low-speed streams. Quiescent streamers show a more marked oxygen depletion in the core than in the streamer's bright lateral branches Raymond et al 1997).…”

An information-theoretical methodology for profile parameter estimation of solar coronal spectra

“…6. The reduction of the O vi doublet intensities in a streamer core is accounted for by a decrease in the O 5+ abundance ratio there (Raymond et al 1997;Ofman 2000;Marocchi et al 2001;Vasquez & Raymond 2005), while the reduction at a coronal hole is attributed to a Doppler dimming of O vi lines (Li et al 1998;Kohl et al 1998;Cranmer et al 1999;Zangrilli et al 2002).…”