Large-scale photospheric motions determined from granule tracking and helioseismology from SDO/HMI data

TL;DR

This study combines granule tracking and helioseismology techniques to accurately measure large-scale solar surface flows, including differential rotation, meridional circulation, and torsional oscillations, using SDO/HMI data.

Contribution

It demonstrates that combining CST and helioseismology provides consistent measurements of solar surface flows, even with limited data duration.

Findings

CST and helioseismology agree on flow amplitudes and directions

Meridional circulation can be measured with just three days of data

The methods detect torsional waves at expected latitudes

Abstract

Large scale flows in the Sun play an important role in the dynamo process linked to the solar cycle. The important large scale flows are the differential rotation and the meridional circulation with an amplitude of km/s and few m/s , respectively. These flows also have a cycle related components, namely the torsional oscillations. Our attempt is to determine large-scale plasma flows on the solar surface by deriving horizontal flow velocities using the techniques of solar granule tracking, dopplergrams, and time distance helioseismology. Coherent structure tracking (CST) and time distance helioseismology were used to investigate the solar differential rotation and meridional circulation at the solar surface on a 30 day HMI SDO sequence. The influence of a large sunspot on these large scale flows with a specific 7 day HMI SDO sequence has been also studied. The large scale flows measured by the CST on the solar surface and the same flow determined from the same data with the helioseismology in the first 1 Mm below the surface are in good agreement in amplitude and direction. The torsional waves are also located at the same latitudes with amplitude of the same order. We are able to measure the meridional circulation correctly using the CST method with only three days of data and after averaging between + and -15 degrees in longitude. We conclude that the combination of CST and Doppler velocities allows us to detect properly the differential solar rotation and also smaller amplitude flows such as the meridional circulation and torsional waves. The results of our methods are in good agreement with helioseismic measurements.