Lower Extreme Ultraviolet Irradiance During This Current Solar Cycle Minimum
Tom Woods [Tom.Woods@lasp.colorado.edu], Laboratory for Atmospheric and Space
Physics (LASP), University of Colorado, Boulder
This current cycle minimum, between solar cycle 23 and 24, is clearly different than the
last few minima in that open magnetic flux is about 30-40% less, solar wind pressure is about
40% lower, and there are more low-latitude coronal holes, which in turn provide more highspeed solar wind streams towards Earth. With lower magnetic activity, there is expected to
be a reduction in total solar irradiance (TSI). With more low-latitude coronal holes as seen in
solar images, there is expected to be reduced solar coronal emissions that dominate in the
extreme ultraviolet (EUV) range. There are several concurrent solar EUV irradiance
measurements during this cycle minimum (2007-2010) from NASA’s Solar Radiation and
Climate Experiment (SORCE), SOHO Solar EUV Monitor (SEM), Solar EUV Experiment
(SEE) aboard the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics
(TIMED) satellite, and the underflight SEE calibration rocket payload that also included the
prototype EUV Variability Experiment (EVE) developed for the future Solar Dynamics
Observatory (SDO) mission. Unfortunately, there are fewer spectral irradiance
measurements during the previous minima, and only the SOHO SEM made measurements
during both the last minimum in 1996 and our current minimum. The SOHO SEM 26-34 nm
irradiance trend indicates that the solar EUV irradiance is 15% lower in 2008 than in 1996,
with uncertainty of about 6% (L. Didkovsky, 2009). Modeling of the additional low latitude
coronal holes in 2008, relative to 1996, suggest that the EUV irradiance in the 26-34 nm band
could be lower in 2008 by about 10%. The coronal holes only reduce the solar irradiance in
the EUV range as the spectral irradiance outside the EUV range (> 120 nm) has minor
contributions from coronal emissions.