Daily differential emission measure ( DEM ) distributions of the solar corona are derived from spectra obtained by the Extreme-ultraviolet Variability Experiment ( EVE ) over a 4-year period starting in 2010 near solar minimum and continuing through the maximum of solar cycle 24 .
The DEMs are calculated using six strong emission features dominated by Fe lines of charge states \ion 8 , \ion 9 , \ion 11 , \ion 12 , \ion 14 , and \ion 16 that sample the non-flaring coronal temperature range 0.3–5 MK .
A proxy for the non- \ion Fe18 emission in the wavelength band around the 93.9 Å line is demonstrated .
There is little variability in the cool component of the corona ( T < 1.3 MK ) over the four years , suggesting that the quiet-Sun corona does not respond strongly to the solar cycle , whereas the hotter component ( T > 2.0 MK ) varies by more than an order of magnitude .
A discontinuity in the behavior of coronal diagnostics in 2011 February–March , around the time of the first X-class flare of cycle 24 , suggests fundamentally different behavior in the corona under solar minimum and maximum conditions .
This global state transition occurs over a period of several months .
The DEMs are used to estimate the thermal energy of the visible solar corona ( of order 10 ^ { 31 } erg ) , its radiative energy loss rate ( 2.5–8 \times 10 ^ { 27 } erg s ^ { -1 } ) , and the corresponding energy turnover timescale ( about an hour ) .
The uncertainties associated with the DEMs and these derived values are mostly due to the coronal Fe abundance and density and the CHIANTI atomic line database .