Chromospheric magnetic topology of cool active stars
P. Petit, S. Berdyugina, J.-F. Donati, S.K. Solanki, N. Afram, J. Arnaud, D. Fluri, J.D. Landstreet

Abstract: We present the first simultaneous photospheric and chromospheric magnetic mapping of an active low-mass star. We employ time-resolved spectropolarimetric observations of the chromospherically active subgiant of the RS CVn binary system II Peg, obtained with ESPaDOnS during six consecutive nights in the summer of 2005. From each circularly polarized spectrum, we extract high S-N mean photospheric and mean chromospheric line profiles, using respectively 9,100 photospheric spectral features available in the echelogram and all components of the CaII infrared triplet. We then use the technique of Zeeman-Doppler Imaging to reconstruct the magnetic geometry of the star both at photospheric and chromospheric levels. The resulting maps reveal that a toroidal component of the field is dominating the magnetic energy of the photosphere, while the chromospheric field geometry is closer to a purely potential magnetic field. This result suggests that chromospheric maps may provide more adequate boundary conditions than photospheric magnetograms for coronal extrapolation of the field geometry.