Solar Influences on Global Change (1994) / Chapter Skim
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3 SOLAR VARIATIONS, OZONE, AND THE MIDDLE ATMOSPHERE
3 SOLAR VARIATIONS, OZONE, AND THE MIDDLE ATMOSPHERE
Pages 49-72
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From page 49... ...
radiation reaching the biosphere and are thus of direct concern for life on Earth. A decrease in column ozone of ~ percent causes the W dose in the spectral region damaging to deoxyribonucleic acid (DNA)
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From page 50... ...
, but also by longer wavelength visible solar radiation. Ozone's strong absorption in the UV region of the spectrum serves the dual role of heating the middle atmosphere and protecting the surface of the Earth from damaging doses of ultraviolet radiation.
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From page 51... ...
Superimposed on the long term downward ozone trend deduced from TOMS data during the past ~ ~ years is a solar cycle variation whose amplitude is estimated to be i.8 percent + 0.3 percent (Hood and McCormack, 1992~. These solar-related changes in ozone exacerbated the downward anthropogenic trend from 1982 to 1986 (the descending phase of the solar activity cycle)
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From page 52... ...
_` +10 +0.5 O -0.5 z o o -1.0 ~ -15 O FIGURE 3.1 In the upper panel are compared changes in global column ozone concentrations in a vertical column above the Earth's surface during the 11-year solar cycle and the long term decrease derived from statistical analysis of the JAMS ozone measurements (Hood and McCormack, 1992~. During decreasing solar activity from 1982 to 1986, solar forcing of ozone was approximately equivalent to the anthropogenic forcing, causing ozone depletion double that of the anthropogenic rate determined from the IOMS data, whereas from 1986 to 1991 the solar-induced changes approximately canceled the downward trend.
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From page 53... ...
The reason is that, compared with the visible radiation that contributes most to the total solar irradiance, the ultraviolet radiation is formed higher in the Sun's atmosphere and is more susceptible to the impact of magnetic fields that erupt into the solar atmosphere during times of enhanced solar activity. Accurate knowledge of the amplitude and temporal structure of solar UV spectral irradiance variations is clearly critical for studies of solar forcing of the middle atmosphere.
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From page 54... ...
, launched in September 1991 near the maximum of activity in solar cycle 22 (Rottman et al., 1993; Brueckner et al., 1993~. Other solar UV spectroradiometers have been flown intermittently on rockets and balloons, with half a dozen recent measurements from the Space Shuttle.
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From page 55... ...
Lean. Only the two UARS solar UV radiometers have the capability for end-toend, in-flight sensitivity monitoring using, in the case of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)
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From page 56... ...
Trradiance Variability Parameterizations In lieu of an adequate experimental data base, information about the Sun's UV spectral irradiance variations has been acquired by critically analyzing the available data in conjunction with knowlecige of solar activity derived independently, often from ground based observations (Lean et al., 1992b; DeLand and Cebula, 1993~. In concert with rising solar activity during the [l-year cycle is an overall increase in the solar ultraviolet spectral irradiance because of increaser!
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From page 57... ...
Calculations of the UV spectral irradiance directly from ground based, spatially resolved Ca IT K observations of the areas, locations, and spectral brightness of active regions have also been attempted with the purpose of elucidating the origins of the UV irradiance variations in terms of solar magnetic activity (Cook et al., 1980; Lean et al., 19821. Inferences about the amplitude of the ~ i-year solar cycle variation in the UV spectral irradiance from 120 to 300 nm derived from measurements of the rotational modulation (Cebula et al., 1992; Lean et al., 1992b)
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From page 58... ...
and greater variability than is estimated from parameterizations based on rotational modulation or SME observations Pioneer Venus Orbiter Langmuir probe observations of the integrated solar EUV and Lyman ax fluxes also provide indirect evidence for a Lyman cat ~ {-year irradiance circle in excess of 50 percent and poorly correlated with the Ca IT and He ~ indices near the maximum of solar cycle 22 (Hoegy et al., 1993~. Persistent inconsistencies among the measurements themselves and with parameterizations derived from solar activity surrogates caution against supposing that all is known even about the Sun's photon output variation at H ~ Lyman or or at any other UV wavelength.
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From page 59... ...
. Events with significant fluxes above this latter threshold are rare, but ground level neutron monitors, which detect thermal neutrons generated as secondary particles by the interaction of very energetic solar protons (several hundred MeV)
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Whereas the peak of solar cycle 20 (1969) showed substantial flaring and many moderate events, only a few of the particle disturbances (e.g., during August 1972)
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From page 61... ...
When all events are considered, solar cycle 22 emerged as one of the most active and most intensely disturbed periods known, with concomitant impact on the middle atmosphere. Relativistic Electrons Energetic electrons, like energetic protons, are capable of penetrating to the middle atmosphere.
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From page 62... ...
- /1T "Ordinary flares" - \1 T 1 10 Energy (MeV) 100 FIGURE 3.4 The integrated proton flux of high energy solar protons typical of ordinary flares is compared win two of He largest disturbances seen in He spacecraft era, in August 1972 and September 1989.
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From page 63... ...
indicate that the fraction of the high altitude flux of relativistic electrons that reaches the middle atmosphere near noon (when daytime REP fluxes peak) may actually be significantly larger, and penetrate deeper, than the nighttime REP events measured by earlier rockets (Goldberg et al., 1984~.
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From page 64... ...
In the lower figure, energy deposition due to relativistic electrons in June 1980 and September 1976 are compared with energy deposition from extreme ultraviolet radiation and galactic cosmic rays during minima and maxima of the 11-year activity cycle. Courtsey of Jackman (1991)
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From page 65... ...
Carbon, nitrogen, oxygen, and other medium-mass nuclei are also present, together with a significant heavy ion population. Like solar energetic protons and relativistic electrons, galactic cosmic rays are strongly shielded from reaching the equatorial regions of the Earth's surface by the terrestrial magnetic field and by the thick neutral atmosphere.
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From page 66... ...
and the sensitivity of ozone to longer period forcing is greater (Brasseur, 1993~. Trend studies of middle atmosphere ozone and temperature observations provide evidence for long term changes associated with solar cycle variations in UV radiation, albeit with large uncertainties in the magnitude of the effect, which has thus far been measured for only one ~ I-year solar cycle.
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From page 67... ...
l, where the solar cycle in global total ozone is compared with the long terror trend deduced from the TOMS observational record. Time series statistical analyses of the TOMS data (corrected for instrument degradation)
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From page 68... ...
Because the prime space based global data sets extend over only one Il-year solar activity cycle, more definite studies of, for example, the role that might be played by dynamics and by the QBO in the observed solar cycle variation of ozone, await longer observational records. Effects from Solar Proton Events During a solar proton event, the primary energetic protons produce secondary electrons by impact on atmospheric molecules, and these electrons can efficiently dissociate molecular nitrogen, producing a range of odd-nitrogen species by subsequent chemical reactions.
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From page 69... ...
Also, over the longer time scales of the il-year solar cycle, variations in the downward transport of nitrogen oxides produced by auroral particle precipitation at altitudes in the thermosphere (Garcia et al., 1984) and other solar particle effects could further affect stratospheric temperature and ozone.
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From page 70... ...
However, as discussed earlier, there remain many uncertainties associated with the amount of nitrogen oxides produced and the resulting effects on lower stratospheric ozone. ULTRAVIOLET RADIATION REACHING THE BIOSPHERE The amount of solar ultraviolet radiation reaching the biosphere is extremely sensitive to the amount of overhead ozone, especially at the UV-B wavelengths that coincide with a sharp decrease in the ozone (03)
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From page 71... ...
The spectral intensity of solar radiation received at Earth drops by eight orders of magnitude in the spectral band from 300 to 280 nm. The data from the existing ground based network probably lack adequate calibration stability, limiting their suitability for trend analyses.
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