Long-Term Oscillations of Sunspots and a Special Class of Artifacts in SOHO(MDI) and SDO(HMI) Data

TL;DR

This paper investigates artifacts affecting long-term sunspot oscillation data from SOHO and SDO, proposes methods to mitigate these artifacts, and confirms the physical nature of sunspot oscillations through multi-parameter observations.

Contribution

It introduces criteria for identifying and reducing image displacement artifacts in sunspot data and validates the physical reality of long-term sunspot oscillations using multiple observational parameters.

Findings

Artifacts can distort long-term oscillation measurements.

Integral parameters effectively reduce artifact influence.

Sunspot magnetic field and ultraviolet intensity oscillate with consistent periods.

Abstract

A specific type of artifacts, that originate due to displacement of the image of a moving object along the digital (pixel) matrix of receiver are analyzed in detail. The criteria of appearance and the influence of these artifacts on the study of long-term oscillations of sunspots are deduced. The obtained criteria suggest us methods for reduction or even elimination of these artifacts. It is shown that the use of integral parameters can be very effective against the artifact distortions. The simultaneous observations of sunspot magnetic field and ultraviolet intensity of the umbra have given the same periods for the long-term oscillations. In this way the real physical nature of the oscillatory process, which is independent of the artifacts have been confirmed again. A number of examples considered here confirm the dependence between the periods of main mode of the sunspot magnetic field long-term oscillations and its strength. The dependence was derived earlier from both the observations and the theoretical model of the shallow sunspot. The anti-phase behavior of time variations of sunspot umbra area and magnetic field of the sunspot demonstrates that the umbra of sunspot moves in long-term oscillations as a whole: all its points oscillate with the same phase.