The place of the Sun among the Sun-like stars^⋆,⋆⋆

¹ Physikalisch-Meteorologishes Observatorium Davos, World Radiation Center, 7260 Davos Dorf, Switzerland
e-mail: alexander.shapiro@pmodwrc.ch
² Institute for Atmospheric and Climate science ETH, Zurich, Switzerland

Received: 7 October 2012
Accepted: 14 February 2013

Abstract

Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship.

Aims. We aim to identify possible reasons for the Sun to be currently outside of this relationship.

Methods. We employed different scenarios of solar HK emission and irradiance variability and compared them with available time series of Sun-like stars.

Results. We show that the position of the Sun on the diagram of photometric variability versus chromospheric activity changes with time. The present solar position is different from its temporal mean position as the satellite era of continuous solar irradiance measurements has accidentally coincided with a period of unusually high and stable solar activity. Our analysis suggests that although present solar variability is significantly smaller than indicated by the stellar data, the temporal mean solar variability might be in agreement with the stellar data. We propose that the continuation of the photometric program and its expansion to a larger stellar sample will ultimately allow us to constrain the historical solar variability.