Artifacts of SDO/HMI data and long-period oscillations of sunspots

¹ University of Turku, 20014 Turku, Finland
e-mail: vvsvid.smirnova@yandex.ru
² Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, Pulkovskoe shausse 65, 196140 St. Petersburg, Russia
³ Sobolev Astronomical Institute, Saint Petersburg State University, Universitetsky pr.28, 198504 St. Petergof, Russia

Received: 30 November 2012
Accepted: 3 May 2013

Abstract

Aims. The artifacts of SDO/HMI magnetograms that may affect the low-frequency power spectrum of sunspot oscillations are analyzed.

Methods. Several examples are given that present false (artificial) harmonics, which are produced by Doppler shifts in the power spectra of long-period oscillations of sunspots. This arises from peculiarities in the orbital movements of SDO.

Results. It was found that those artifacts with periods of 12 and 24 h, as revealed even in variations of weak background magnetic fields, are actually present in SDO/HMI magnetograms. However, the quantitative impact of artifacts remains quite weak and does not change the picture of sunspot oscillations dramatically for as long as the magnetic field in the spot is less than about of 2000 Gauss. When the magnetic field strength is greater than 2000 G, the influence of these artifacts increases sharply to become the dominant factor. One can suggest that the amplification of noise components of these artifacts has a highly nonlinear character with the growth of the magnetic field, and the field strength of about 2000 G then takes on meaning of a threshold value.