Neutral Density During the Recent Solar Minimum Contributions from Solar, Geomagnetic Activity, and Anthropogenic Rodney Viereck NOAA Space Weather Prediction Center Drag Data provided by John Emmert NRL MURI Workshop, Boulder October 2010

400 km Neutral Density Derived from Satellite Drag (thanks to John Emmert, NRL)

Drag Density at Solar Minima (log plot)

Thermospheric Drivers Solar EUV (F10), Geomagnetic Storms (Ap), Climate Change(CO 2 ) October 2010Viereck: MURI 20104

Drag Density vs MSIS Density (again thanks to John Emmert) Delta: Difference between MSIS and Observed Neutral Density MSIS only has Solar (F10) and Geomagnetic (Ap) inputs (no climate change)

Estimating the Anthropogenic Contribution Density offset (delta) vs CO2 data. (fitting a line to only the first three minima) Extrapolating the correction to the full extent if the time series Note: This is about twice the rate of Roble’s estimate

Subtracting the Anthropogenic Correction

Detailed Plot of the Observations vs MSIS (With and without the CO 2 correction) Note the excellent fit during the first three minima Note the improved fit during the last minimum Minimum Values

Solar Irradiance Observations vs Proxies F10 solar min to min variation is too small (2.3% of max-min) SEM 304 solar min to min variation is too large (14% of max-min) Mg II index solar min to min variation is about right (7% of max-min)

Solar Data and Proxies Scaled to F10 Fitting SEM 304 and Mg II to F10 at the last minimum

MSIS Results Using the Three Solar Inputs (Corrected for CO 2 )

MSIS Results Using the Three Solar Inputs (Details) Mg II Index provides the best fit to the observed density

Relative Magnitude of the Density Changes Modeled density changes for each input while holding the other two constant March 2008Viereck: SDO and Space Weather13 Relative Contributions of the change from (1996 to 2009) Solar 1.49E-15 = 48% Geomag 1.03E-15 = 33% Anthro 0.61E-15 = 19%

Conclusions Neutral density at 400 km shows a decrease during the last solar minimum –Significantly lower than previous three minima Both F10 and Ap are also significantly lower during this most recent solar minimum. –Still can’t account for the observed decrease (using MSIS). Adjusting for Anthropogenic (CO 2 ) forcing and climate change accounts for some of the remaining difference –Still some density drop that cannot be accounted for Using the SOHO SEM 304 data instead of F10 provides too much of a decrease in the neutral density Using the Mg II Index instead of F10 provides a more accurate estimate of the density during this last solar minimum. –Solar 1.49E-15 = 48% –Geomag 1.03E-15 = 33% –Anthro 0.61E-15 = 19% March 2008Viereck: SDO and Space Weather14