High-precision density measurements in the solar corona

I. Analysis methods and results for Fe XII and Fe XIII

¹ Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK e-mail: peter.young.ctr.uk@nrl.navy.mil
² George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
³ Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
⁴ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Mitaka, Tokyo 181-8588, Japan

Received: 6 May 2008
Accepted: 26 September 2008

Abstract

Aims. The EUV Imaging Spectrometer (EIS) instrument on board the Hinode satellite has access to some of the best coronal density diagnostics, and the high sensitivity of the instrument now allows electron number density, N_e, measurements to an unprecedented precision of up to ±5% in active regions. This paper gives a thorough overview of data analysis issues for the best diagnostics of  and  and assesses the accuracy of the measurements.

Methods. Two density diagnostics each from (λ186.88/λ195.12 and λ196.64/λ195.12) and   (λ196.54/λ202.04 and λ203.82/λ202.04) are analysed in two active region datasets from 2007 May 3 and 6 that yield densities in the range . The densities are derived using v5.2 of the CHIANTI atomic database. Blending, line fitting, and instrumental issues are discussed, and line fit parameters presented.

Results. The  and  diagnostics show broadly the same trend in density across the active region, consistent with their similar temperatures of formation. However, the high precision of the EIS measurements demonstrates significant discrepancies of up to 0.5 dex in derived values, with  always giving higher densities than . The discrepancies may partly be due to real physical differences between the emitting regions of the two plasmas, but the dominant factor lies in the atomic models of the two ions. Two specific problems are identified for  λ196.64 and  λ203.82: the former is found to be underestimated in strength by the CHIANTI atomic model, while the high-density limit of the λ203.82/λ202.04 ratio appears to be inaccurate in the CHIANTI atomic model. The small grating tilt of the EIS instrument is found to be very significant when deriving densities from emission lines separated by more than a few angstroms. Revised wavelengths of  Å and  Å are suggested for the  λ196.54 and  λ196.64 lines, respectively.