Astronomy & Astrophysics

Free access article

A&A 416, 749-758 (2004)
DOI: 10.1051/0004-6361:20031650

Spectroscopic measurement of the plasma electron density and outflow velocity in a polar coronal hole

E. Antonucci¹, M. A. Dodero², S. Giordano¹, V. Krishnakumar³ and G. Noci⁴

¹ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
² University of Torino, via P. Giuria 1, 10125 Torino, Italy
³ National Solar Observatory, 3083, Corona Loop, PO Box 4, Sunspot, NM 88349, USA
⁴ Università di Firenze, Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 5, 50125 Firenze, Italy

(Received 30 January 2001 / Accepted 16 October 2003)

Abstract
A new spectroscopic method, aimed to derive the plasma electron density and outflow velocity in expanding solar coronal regions, is discussed in this paper. The method is based on the analysis of a pair of coronal lines emitted via collisional and radiative excitation by the same ion, such as the O VI 1032, 1037 Å doublet. The merit of this technique consists in allowing us to derive at the same time electron density and outflow velocity of the coronal plasma from nearby lines detectable with the same instrument, provided the constraint on mass flux conservation along the flow tube connecting solar corona and heliosphere is taken into account. The results obtained from the analysis of the OVI emission imply that the physical conditions of a polar coronal hole plasma, observed during minimum activity, are the following. The electron density decreases from $4\times 10^5$ cm ^-3 at 1.7 $R_{\odot}$ to $2{-}4\times 10^4$ cm ^-3 at 3.1 $R_{\odot}$ , whereas the outflow velocity of the oxygen ions is monotonically increasing to reach 350-500 km s ^-1 at 3.1 $R_{\odot}$ , depending on the assumptions on the degree of anisotropy of the velocity distribution of the ions. These results of the velocity of expansion of the fast wind confirm those obtained with Doppler dimming techniques when assuming the lowest observed density values for the coronal hole plasma. This implies that, for a rarified corona, the outflow velocity of the fast solar wind in polar holes can be traced by the motion of the O VI ions at least up to 2.4 $R_{\odot}$ . The analysis also shows that the degree of anisotropy of the oxygen ions, due to the acceleration of the ions across the magnetic field in a coronal hole, exhibits a steep increase and that the geometry of the flow tube diverges very rapidly low down in the inner corona/transition region. The observations of the extended corona analysed in this paper are obtained with the Ultraviolet Coronagraph Spectrometer of the SOHO space mission.

Key words: Sun: corona -- Sun: UV radiation -- Sun: solar wind

Offprint request: E. Antonucci, antonucci@to.astro.it

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