Thermosphere/Ionosphere Response to
the Recent Solar Minimum
Stan Solomon and Liying Qian
High Altitude Observatory
National Center for Atmospheric Research
Tom Woods
Laboratory for Atmospheric and Space Physics
University of Colorado
2010 SORCE Science Meeting
•
20 May 2010
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Much Speculation Concerning Solar Minimum
• The 2007-2009 solar minimum was unusually long — was it also unusually “quiet?”
—The solar wind and interplanetary magnetic field were at times very weak
—Terrestrial Geomagnetic activity was very low for extended periods
—But were solar ultraviolet and X-ray irradiance also lower than “normal?”
• Also speculation about terrestrial effects
—Unusually low ionospheric altitudes/densities
—Some evidence of unusually low thermospheric densities
...and conjecture concerning climate
• This raises a fundamental question for solar physics:
—Are irradiance levels at solar minimum generally similar, or not?
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Long-Term Satellite Drag Data
Emmert et al., 2010; Solomon et al., 2010
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If the Thermosphere is Really Colder and Less Dense...
Then Why?
• Because it’s solar minimum.
This is part of it — empirical models overestimate thermospheric density at
solar minimum, and we have to remind ourselves of this every 11 years.
• Because of long-term secular change caused by increase of CO2 and other GHG?
This is part of it — but probably a small part of it according to models and
observations.
• Because the Sun is actually dimmer in the EUV and X-ray regions?
Maybe...
• Because of something else:
— Geomagnetic Activity?
...probably a small part
— Duration of solar minimum? ...largely ruled out by model studies
— Weather?
...under examination...
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Solar EUV Measurements from SOHO SEM
•
•
SOHO SEM band (26-34 nm) indicates 15% less irradiance in 2008 than in 1996
[Leonid Didkovsky, SOHO-23 Workshop Proceedings, 2009]
– Quoted uncertainty is 6%
LASP rocket and TIMED SEE results are consistent with the SEM measurements
(uncertainty of ~20%)
Long-Term Satellite Drag Data
Emmert et al., 2010; Solomon et al., 2010
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Carrington Rotational Analysis of Low-Latitude Coronal Holes
1996: Cycle 22/23 minimum
2008: Cycle 23/24 minimum
Polar coronal holes are smaller in 2008 by ~20% than in 1996
But large, mid- and low-latitude coronal holes persisted during this cycle minimum
SOHO EIT analysis by Giuliana de Toma
How Much Have the Coronal Holes Changed?
• Polar coronal hole has decreased by 10-20%, but these are on limb
e.g., Kirk et al., Solar Physics, 2009
• Total amount of low-latitude coronal holes has increased
~10% from Liang Zhao (University of Michigan)
• CHIANTI spectral model is used to estimate the ratio of 2008 irradiance to 1996
irradiance (Tom Woods, SOHO-23 Workshop Proceedings, 2009)
• Predict -15% change for SOHO SEM if coronal hole area increases by 18%
Derived from using CHIANTI Quiet Sun (QS) and Coronal Hole (CH) DEMs
• TIMED SEE observations are consistent with CHIANTI model estimates
Solar Minimum Spectra
T. N. Woods spectrum ratio analysis applied EUVAC model [Richards, et al. 1994]
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Simulations using the NCAR TIE-GCM
Temperature and Density simulations at 400 km
2008 simulation includes combined effect of solar EUV decrease and CO2 increase
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Altitude Dependence of Simulated Changes
Temperature and Density change as a function of altitude.
Blue lines — solar EUV change only
Black lines — additional effect of CO2 increase
Points — Altitude dependence of density change estimated by Emmert et al. [2010]
Conclusions
• The thermosphere/ionosphere system was indeed cooler, less dense, and lower,
during the minimum of solar cycle 23/24 than during a “typical” solar minimum.
• The primary cause of this was lower than “usual” solar irradiance, particularly
coronal emissions in the soft X-ray region of the spectrum (1~30 nm).
• The cause of the lower than “usual” coronal emissions may be the prevalence of
coronal holes and additional supergranulation-scale open magnetic field in the
solar equatorial region.
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