| Abstract : | The Radial Velocity (RV) method is an indirect one for detecting extra-solar planets via observations of Doppler shifts in the spectrum of the planet’s host star. Stellar magnetic fields suppress granular convective motions in the photospheric layers leading to much reduced convective blueshift of spectral lines, thus translating into RV variation – commonly referred to as stellar jitter or noise. Dark spots, bright faculae, and magnetic network regions each have differing impact on stellar RV owing to their flux imbalances. Estimating the relative coverage of these magnetic regions is hence necessary to accurately retrieve the RV due to planets. The Sun is the only test case that allows us to investigate directly such effects of stellar heterogeneities. Here, we derive fill-factors of different magnetic regions on the Sun from photospheric magnetic, velocity and intensity data from Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory and study modulations that they cause in the RV data available from the HARPS-North Solar Telescope for the period of 2015 – 17. We have extended this analysis to include magnetic proxies from chromospheric UV emissions at 1600 Å, observed by Atmospheric Imaging Assembly (AIA) onboard SDO to better estimate and separate the quiet-network, enhanced-network, and brighter facular/plage regions. While confirming previous studies on the correlation of area fill-factors of faculae and active-regions (sunspots) with RV fluctuations, through a careful separation of weak inter-network magnetic features from the faculae and active regions we find surprising opposite influences of these two components: the former distributed weak magnetic features enhance while the latter reduce the contributions from convective blueshift. This hints at fundamental differences between weak and strong magnetic fields in modifying the granular convection, through which the RV fluctuations emerge on the Sun. We discuss the implications of our results for exoplanet RV studies. |
