Solar Influences on Global Change (1994) / Chapter Skim
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4 SOLAR VARIATIONS AND THE UPPER ATMOSPHERE
4 SOLAR VARIATIONS AND THE UPPER ATMOSPHERE
Pages 73-88
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From page 73... ...
While solar variability exerts a dominating influence on the Earth's upper atmosphere, any direct effect on the biosphere appears to be more subtle than that exerted by solar forcing of the middle and lower atmospheres. The fact that the highly variable upper atmosphere is coupled to the middle atmosphere through chemical, radiative, and dynamical 73
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From page 74... ...
and for driving most of the bulk motions of the gases within the entire region. Large variability in the basic properties of both the thermosphere and ionosphere is the direct result of the variability in the solar EUV and UV input (as illustrated in Figure 1.2 by the change in the temperature profile from minimum to maximum solar activity)
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From page 75... ...
This contradicts current understanding of the origin of the EUV irradiance variations, which predicts that solar activity causes an increase in the EUV radiation at all wavelengths. The discrepancy is most likely the result of instrumental effects (see Lean, 1990 for details)
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From page 76... ...
Trradiance Variability Parameterizations The absence of continuous, reliable observations of the solar EUV spectral irradiance has forced reliance on empirical variability models based on solar activity surrogates to estimate EUV spectral irradiances for use in upper atmosphere research and in operational applications. The AK-E solar irradiance data have been used to construct parameterizations of the solar EUV flux variations as a function of primarily the solar 10.7 cm radio emission that can be measured from the ground (Hinteregger et al., 1981; Tobiska, 1991~.
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From page 77... ...
. 60 65 70 77 75 80 85 90 YEAR FIGURE 4.1 Comparison of measured solar EUV irradiance variations with the empirical model calculations of Hinteregger et al.
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From page 78... ...
High uncertainty surrounded the launch of the Hubble Space Telescope because of insufficient knowledge of the atmospheric drag that it would experience when launched at a time near maximum solar activity (Withbroe, 1989~. AURORAL PARTICLE AND ELECTRIC FIELD INPUTS The Earth's thermosphere and ionosphere system responds not only to changes in the solar EUV and UV radiative input but also to particulate inputs of solar energy and momentum at high latitudes associated with auroral processes.
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From page 79... ...
, determined as the change per day in the altitude, is strongly influenced by variations in solar energy input, as indicated by the daily F~07 solar activity proxy (linearly transformed to an equivalent decay rate, dashed line)
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From page 80... ...
At times, such as In brief periods during intense geomagnetic storms, energized electrons bombard the upper atmosphere, colliding with atmospheric constituent and transferring their energy, resulting in visual displays of auroral phenomena. The energy deposited at high latitudes in the aurora can increase by as much as two orders of magnitude relative to geomagnetically quiet conditions, locally exceeding the energy deposited from solar EUV radiation.
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From page 81... ...
The atmospheric region above about 60 km is generally considered the upper conducting boundary of the global electric circuit, which includes electrical interactions between all atmospheric regions. This upper conducting boundary is formed by solar ionization of atmospheric constituents.
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From page 82... ...
Solar and auroral variability alter thermospheric winds and ionospheric electrical conductivities and thus influence the currents and fields in the dynamo region. The flow of solar wind around and partly into the magnetosphere produces the solar wind/magnetosphere dynamo, which sets up plasma motion in the magnetosphere as well as producing electric fields and currents.
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From page 83... ...
Electrical conductivity enhancements in polar regions associated with energetic particle precipitation, solar proton events, and Forbush decreases in cosmic ray fluxes following solar eruptions all influence the properties of the global circuit in ways that are not well understood at present. Electrical Couplings Between the Upper and Lower Atmospheres Large horizontal electric fields (100 to 1000 km)
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From page 84... ...
Furthermore, human activities are slowly changing the atmospheric structure, atmospheric and ionospheric composition, aerosol loading, land surface properties, and other tropospheric variables, all of which will probably have some influence on the properties of the global electric circuit (Price and Rind, 19941. SOLAR FORCING AND GLOBAL CHANGE liVITHIN THE UPPER ATMOSPHERE That the Earth's upper atmosphere is forced directly by variable solar energy inputs on all time scales is well established.
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From page 85... ...
COUPLINGS OF Thy: UPPER ATMOSP~:RE TO THE LOUVER ATMOSPHERE The most direct coupling between the upper and lower atmospheres is electrical. As discussed previously, large horizontal electric fields map almost unattenuated to the Earth's surface, where they perturb the vertical electric field maintained by global thunderstorm activity.
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From page 86... ...
Also suspected are chemical and dynamical couplings between the upper and lower atmospheres. Nitric oxide is produced by solar soft X-ray and EUV radiation and auroral particle dissociation of molecular nitrogen into excited and ground states of atomic nitrogen that react with molecular oxygen.
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From page 87... ...
Two-dimensional modeling studies, as well as spacecraft observations of middle atmospheric ozone abundances, suggest that this may indeed be a viable coupling mechanism (Garcia et al., 1984; Solomon and Garcia, 1984~. Changes in nitrate content of antarctic snow associated with solar activity may reflect these processes (Dreschhoff and Zeller, 1990)
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