The relation between stellar magnetic field geometry and chromospheric activity cycles I: The highly variable field of Epsilon Eridani at activity minimum

TL;DR

This study investigates the evolving large-scale magnetic field of Epsilon Eridani during its activity minimum, revealing complex structures and rapid field regeneration, which challenge solar-based dynamo models.

Contribution

It provides high-cadence magnetic field maps of Epsilon Eridani at activity minimum, showing complex geometry and rapid field regeneration, advancing understanding of stellar magnetic cycles.

Findings

Epsilon Eridani's magnetic field is more complex than the Sun's at minimum.

Rapid regeneration of a strong axisymmetric toroidal field observed.

Magnetic field geometry varies significantly among stars, unlike the Sun.

Abstract

The young and magnetically active K dwarf Epsilon Eridani exhibits a chromospheric activity cycle of about 3 years. Previous reconstructions of its large-scale magnetic field show strong variations at yearly epochs. To understand how Epsilon Eridani's large-scale magnetic field geometry evolves over its activity cycle we focus on high cadence observations spanning 5 months at its activity minimum. Over this timespan we reconstruct 3 maps of Epsilon Eridani's large-scale magnetic field using the tomographic technique of Zeeman Doppler Imaging. The results show that at the minimum of its cycle, Epsilon Eridani's large-scale field is more complex than the simple dipolar structure of the Sun and 61 Cyg A at minimum. Additionally we observe a surprisingly rapid regeneration of a strong axisymmetric toroidal field as Epsilon Eridani emerges from its S-index activity minimum. Our results show that all stars do not exhibit the same field geometry as the Sun and this will be an important constraint for the dynamo models of active solar-type stars.