Data compression for local correlation tracking of solar granulation

TL;DR

This study evaluates how various lossy data compression techniques impact the accuracy of local correlation tracking measurements of solar differential rotation, finding quantization to be particularly robust.

Contribution

It provides an assessment of the effects of different data compression methods on LCT measurements of solar rotation, highlighting quantization's robustness.

Findings

Quantization preserves LCT accuracy despite compression.

JPEG compression introduces significant errors.

Systematic errors dominate measurement uncertainties.

Abstract

Context. Several upcoming and proposed space missions, such as Solar Orbiter, will be limited in telemetry and thus require data compression. Aims. We test the impact of data compression on local correlation tracking (LCT) of time-series of continuum intensity images. We evaluate the effect of several lossy compression methods (quantization, JPEG compression, and a reduced number of continuum images) on measurements of solar differential rotation with LCT. Methods. We apply the different compression methods to tracked and remapped continuum intensity maps obtained by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. We derive 2D vector velocities using the local correlation tracking code FLCT and determine the additional bias and noise introduced by compression to differential rotation. Results. We find that probing differential rotation with LCT is very robust to lossy data compression when using quantization. Our results are severely affected by systematic errors of the LCT method and the HMI instrument. The sensitivity of LCT to systematic errors is a concern for Solar Orbiter.