Trends in the Short-Term SSI Variability during the Declining Phase of SC23: Spectral
decomposition of over 100 Carrington rotations from the UV through the near IR
Lauren Bearden, Odele Coddington, Marty Snow, and Erik Richard; LASP, University of
Colorado, Boulder
The Sun's energy output varies on timescales ranging from minutes to decades. This
variability is due to the turbulent convection-induced dynamical motions and magnetic
phenomena within the Sun. The UV chromospheric irradiance, at wavelengths shorter than 300
nm, is closely related to the persistent chromospheric heating in plage and enhanced network
lasting on rotational timescales with little center-to-limb variation. In contrast, at wavelengths
longer than 300 nm (in the visible and near infrared) the irradiance variations are due to
photospheric features related to sunspots and faculae that evolve faster. While the integrated
spectral variability (TSI) varies by small amounts (typically <0.3% over days), the wavelength
dependent spectral variability from the UV through the infrared spans 5 orders. For modeling
Earth atmospheric response, the temporal and spectral irradiance variations across the full solar
spectrum must be considered.
The Spectral Irradiance Monitor (SIM) instrument has been measuring solar spectral
irradiance from 200 to 2400 nm since 2003. In this work, we compute the spectral contrast,
[(Imax – Imin )/ Imin]CR, in solar spectral irradiance over 109 Carrington rotations covering the
declining phase of SC 23 through the last solar minimum. The periods of maximum and
minimum solar activity are determined using known indices that reflect changes in
chromospheric activity: the Magnesium II index (Mg II- near 280 nm) and Calcium II index
(CaII- near 390 nm). We relate the rotational variability to the Solar disk location of magnetic
features on the surface of the Sun using Carrington synoptic map images. In addition, we
investigate case studies where the periods of maximum solar activity as defined by the MgII and
CaII indices were different.