Time--distance inversions for horizontal and vertical flows on supergranular scales applied to MDI and HMI data

TL;DR

This study compares helioseismic flow measurements from MDI and HMI instruments, finding high correlation but systematically larger flow magnitudes from HMI, possibly due to formation depth differences.

Contribution

It demonstrates a method to extend helioseismic campaigns by harmonizing data from MDI and HMI, enabling consistent flow measurements across instruments.

Findings

HMI-derived flows are about 20% larger than MDI.

Flow measurements from both instruments are highly correlated.

Differences may be due to formation depth of Doppler signals.

Abstract

We study the possibility of consistent extension of MDI full-disc helioseismic campaigns with the growing data set of HMI observations. To do so, we down-sample and filter the HMI Dopplegrams so that the resulting spatial power spectrum is similar to the spatial power spectrum of MDI full-disc Dopplergrams. The set of co-spatial and co-temporal datacube pairs from both instruments containing no missing and no bad frames were processed using the same codes and inverted independently for all three components of the plasma flow in the near surface layers. The results from the two instruments are highly correlated, however systematically larger (by ~20%) flow magnitudes are derived from HMI. We comment that this may be an effect of the different formation depth of the Doppler signal