2004MNRAS.355.1066M

We have used Zeeman-Doppler maps of the surface magnetic field of the rapidly rotating (P_rot= 1.61 d) K2 star LQ Hya to extrapolate the coronal field, assuming it to be potential. Using the data sets from the observations in 2000 December and 2001 December, chosen for their excellent phase coverage, we show how the global structure of the magnetic field can change drastically in the space of one year. In 2000 December the large-scale field resembled a tilted dipole, with most of the open field emerging in two mid-latitude regions separated by 180° of longitude. One year later, most of the open field emerged at the pole, and the large-scale field most closely resembled an aligned dipole albeit with a significant contribution from many smaller scale east-west arcades of magnetic field-lines. This appears to be quite different from what is seen on the Sun, where the emergence of many east-west bipoles occurs towards cycle maximum, when the large-scale heliosphere field resembles a tilted, not an aligned, dipole. We have also modelled the X-ray emission (assuming an isothermal corona) and find that, despite the changes in the field structure, the magnitude and the rotational modulation of the emission measure (10^51.15/cm3) are largely unchanged. While the emission measure is close to observed values, the density (10^9.8/cm3) is somewhat lower.