Solar wind elemental abundances related to the Sun's open magnetic flux

¹ Key Laboratory of Solar Activity, The National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, 100012 Beijing, PR China e-mail: xywang@nao.cas.cn
² Max-Planck-Institut für extraterrestrische, Physik, 85740 Garching, Germany e-mail: berndt.klecker@mpe.mpg.de
³ Physikalisches Institut der Universität Bern, 3012 Bern, Switzerland e-mail: peter.wurz@space.unibe.ch

Received: 19 November 2008
Accepted: 23 June 2009

Abstract

Context. We present an investigation of the effects of solar magnetic field on the elemental abundance ratios of heavy ions in the solar wind.

Aims. We compare the elemental abundance ratios Fe/O, Si/O, Mg/O, Ne/O, and C/O over the 23rd solar cycle, attention being given to the varying fraction of open magnetic flux on the visible side of the solar disc.

Methods. We use ACE solar wind data, the current sheet source surface (CSSS) model of the corona, and SoHO/MDI data to relate solar wind compositional properties with the fraction of open magnetic flux on the Sun. We include both a quasi-stationary and intermittent solar wind.

Results. For the six investigated solar wind speed ranges, the elemental composition shows a similar FIP-bias of the quasi-stationary solar wind composition with respect to the photosphere but with three plateaus separating low-, mid-, and high-FIP elements. The average enrichment factor for the low-FIP element group (Mg, Fe, Si) with respect to the photospheric values is between 1.67 and 2.47 when the fraction of open magnetic flux on the visible side of the solar disc is above 14%, and it ranges from 2.78 to 4.15 when this fraction is less than 7%. They are close to the enrichment of low-FIP elements with respect to the photospheric values in quasi-stationary fast- and slow- solar wind, which implies similar fractionation mechanisms for the stationary solar wind and the intermittent solar wind. In the low alpha (<7%) case (alpha is the fraction of open magnetic flux on the visible side of the solar disc), the enrichment of heavy elements for the fast solar wind is well correlated with the high fraction of CMEs. In addition, both the charge state ratios and the elemental abundance ratios exhibit a lower dependence on alpha in a slow solar wind than in a fast solar wind. The dependence of the charge state ratios on alpha is correlated with FIP, which implies that solar magnetic effects play a non-negligible role in the elemental fractionation of the solar wind. We suggest that lower hybrid waves play an important role in the possible linkage between solar magnetic field and elemental fractionation.