Letter to the Editor

Net circular polarization of sunspots in high spatial resolution

¹ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Toyko, 181-8588, Japan e-mail: ichimoto@solar.mtk.nao.ac.jp
² Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan
³ High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000 Boulder, CO80307-3000, USA (The National Center for Atmospheric Research is sponsored by the National Science Foundation.)
⁴ Lockheed Martin Advanced Technology Center, 3251 Hanover Street, Palo Alto, CA 94304, USA
⁵ Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502, Japan

Received: 13 November 2007
Accepted: 14 January 2008

Abstract

Context.Net circular polarization (NCP) of spectral lines in sunspots has been most successfully explained by the presense of discontinuities in the magnetic field inclination and flow velocity along the line-of-sight in the geometry of the embedded flux tube model of penumbrae (-effect).

Aims.The fine scale structure of NCP in a sunspot is examined with special attention paid to spatial relations of the Evershed flow to confirm the validity of the present interpretation of the NCP of sunspots.

Methods.High resolution spectro-polarimetric data of a positive-polarity sunspot obtained by the Solar Optical Telescope aboard Hinode are analysed.

Results.A positive NCP is associated with the Evershed flow channels in both limb-side and disk center-side penumbrae and with upflows in the penumbra at disk center. The negative NCP in the disk center-side penumbra is generated in inter-Evershed flow channels.

Conclusions.The first result is apparently inconsistent with the current explanation of NCP with the -effect but rather suggests a positive correlation between the magnetic field strength and the flow velocity as the cause of the NCP. The second result serves as strong evidence for the presence of gas flows in inter-Evershed flow channels.