Systematic errors in measuring solar magnetic fields with a FPI spectrometer and MDI

¹ Astrophysikalisches Institut Potsdam, Telegrafenberg A27, 14473 Potsdam, Germany
² W. W. Hansen Experimental Physics Laboratory, Center for Space Science and Astrophysics, Stanford University, CA 94305-4085, USA

Corresponding author: A. Settele, asettele@aip.de

Received: 10 April 2001
Accepted: 28 February 2002

Abstract

We will show the potential errors of observations using instruments, where a line spectrum is obtained not instantly but by scanning through the spectral line in time. We give two examples in simulations of a sunspot observation with the measuring procedure of a typical Fabry-Perot interferometer (FPI) spectrometer and the Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO). The effect described is inherent for observations with a low intensity input signal, where the accumulation time for one image and therefore the whole process of measuring a spectrum is relatively high compared to the timescale of the underlying physical processes. We show that our simulated data produce an error in magnetic field strength and – in a time series – result in an apparent oscillation of the magnetic field strength, while the input contained only velocity oscillations. As the FPI spectrometer is being used more and more widely, one has to be aware of the disadvantages and potential errors when interpreting its data. We encourage every observer to do the simple simulation as described for every specific setup used.