# Meridional Motion and Reynolds Stress from Debrecen Photoheliographic Data

## TL;DR

This study analyzes sunspot group motions from Debrecen data, revealing meridional flows towards active zones and angular momentum transfer, with implications for solar dynamics.

## Contribution

It provides new insights into meridional motions and Reynolds stress using high-resolution sunspot data, highlighting differences from Doppler measurements.

## Key findings

- Meridional flow directed towards active zones.
- Angular momentum transfer towards the equator.
- Reynolds stress increases with latitude up to 40°.

## Abstract

The Debrecen Photoheliographic Data catalogue is a continuation of the Greenwich Photoheliographic Results providing daily positions of sunspots and sunspot groups. We analyse the data for sunspot groups focusing on meridional motions and transfer of angular momentum towards the solar equator. Velocities are calculated with a daily shift method including an automatic iterative process of removing the outliers. Apart from the standard differential rotation profile, we find meridional motion directed towards the zone of solar activity. The difference in measured meridional flow in comparison to Doppler measurements and some other tracer measurements is interpreted as a consequence of different flow patterns inside and outside of active regions. We also find a statistically significant dependence of meridional motion on rotation velocity residuals confirming the transfer of angular momentum towards the equator. Analysis of horizontal Reynolds stress reveals that the transfer of angular momentum is stronger with increasing latitude up to about 40$^{\circ}$ where there is a possible maximum in absolute value.