8.3 Effects in the atmosphere

Low-Earth-orbiting satellites are gradually slowed by the atmospheric drag, which finally leads to the spacecraft re-entry. The atmospheric drag depends on the atmospheric neutral density, which at a given altitude is strongly modulated by the heating and increase of the scale height associated with geomagnetic activity. The increasing atmospheric density during only a single storm can cause a major loss of altitude of the spacecraft. On a longer time scale, changes in the ionization caused by the solar UV radiation cause a significant solar cycle modulation on the neutral density (Hastings and Garret, 1996).

In the middle atmosphere, precipitating energetic particles can significantly affect the chemical processes that are involved in the formation of odd nitrogen compounds. Recent simultaneous observations of a strong increase of solar protons and the global NOx concentration in the atmosphere have clearly established the causal relationship between the solar proton events and the enhancement of odd nitrogen compounds. If sufficient downward transport is available, these nitrogen compounds can reach the stratospheric altitudes and through catalytic reaction chains to strong decrease of the stratospheric ozone Seppälä et al. (2006). The effects caused by these short-lived solar proton events can last for months in the atmosphere. Similarly, relativistic electrons from the magnetosphere affect the nitrogen chemistry at somewhat higher altitudes (50 – 100 km).

As the amount of energy from the Sun reaching the surface of the Earth is variable, it is only natural to expect that its long-term variations would affect the large-scale climatology of the Earth. The “solar constant” that averages to about 1368 Wm2 has been shown to exhibit a distinct solar cycle variation. The total solar irradiance also varies at 27-day intervals, the variability being larger during high solar activity than during solar minimum conditions. The ultraviolet part of the spectrum is also a strong modulator of the production of ozone, which is clearly demonstrated in the annual variation of the polar ozone holes which intensify during the local spring when the amount of UV radiation increases.

Any change in the balance between incident and outgoing radiation can have an effect on the climate. Changes in the incoming energy may be associated with changes in cloudiness, amount of volcanic dust in the atmosphere, and amounts of either natural or anthropogenic origin aerosols. Changes in the amount of energy radiated away from Earth can be associated with the varying amounts of greenhouse gases in the atmosphere such as carbon dioxide, methane, CFCs, or ozone. The surface properties are also important for the reflected radiation, as the albedos for ice cover, vegetation, and soil are quite different.

There is experimental evidence that the global temperature variations are correlated with a number of space physic parameters: the long term temperature anomaly follows the filtered solar cycle length, which is a measure of the intensity of the solar cycle (Friis-Christensen and Lassen, 1991). The global cloud coverage anomalies are correlated with the mean values of the galactic cosmic ray flux, which is modulated by the solar cycle as the stronger solar wind pressure during solar maximum tends to decrease the amount of anomalous cosmic rays that can reach the inner solar system (Marsh and Svensmark, 2000).

While statistical studies have found strong correlations of solar activity parameters and climatological parameters, the reasons for these changes are not fully understood. Furthermore, it is probable that several different processes influence the observed correlations. The effects of the varying total solar irradiance are too small to be of major importance for the climate change. The correlation of cosmic rays on only low-altitude cloud cover suggests that the cosmic rays may influence the formation of aerosols and through that the abundance of condensation nuclei in the atmosphere which can form liquid water drops. At ground level, the cosmic ray particles cause ionization, and the associated vertical electric currents can influence the production of ice-forming nuclei and clouds in the upper troposphere. These effects might be related to local decreases in the amount of cloud cover associated with short-term changes in the cosmic rays due to increased solar activity.

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