Solar Effects on the Atmosphere

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The best-known solar effects that lead to variability of atmospheric composition and temperature include the 11-year solar cycle, the 27-day solar cycle, and the solar energetic particle (SEP) events.

The solar 11-year cycle, characterized by the change in the frequency of sunspots, is the major periodic solar variation. Although the corresponding change in the total solar irradiance (TSI) is only about 0.1%, the change in the solar UV flux increases rapidly with decreasing wavelength (from a few percent at 200 nm to several tens of percent at 120nm, the Lyman α region) and greatly affects Earth's atmosphere.

The change of solar flux over the solar 27-day cycle, which originates from the rotation of the Sun, has a similar wavelength-dependence but a smaller magnitude. The observations of various atmospheric responses such as trace gases, temperature, humidity, and circulation to both solar cycles have been reported.

Solar variability over other time scales has also been reported but with much weaker signals. The corresponding impacts on atmospheric chemistry, dynamics, and the climate are not fully understood, e.g., given the discrepancies between the observed and modeled magnitudes of the atmospheric responses. MLS observations of the middle atmosphere, e.g., O3, H2O, OH, HO2, CO, temperature, and geopotential height, have made a significant contribution to our understanding of solar effects on Earth's atmosphere.

Signals of both the 11-year solar cycle and the 27-day solar cycle have been extracted from MLS observations from the mesosphere to the upper troposphere. Ongoing investigations should lead to a better understanding of the mechanisms by which solar activity influences the natural variability of Earth's atmosphere.

In particular, we are currently in the midst of a prolonged solar minimum between solar cycles 23 and 24. Compared to previously recorded solar minimums (1976, 1986, and 1996), the current solar minimum (2008-2009) is unusually prolonged, with a record number of sunspot-free days in the history of satellite measurements since the 1970s.

Anomalously low upper atmospheric temperature and low levels of O3, CO, and OH in MLS observations during the current solar minimum also point to exceptionally low solar output. Reduced solar UV irradiance and the corresponding low levels of O3 could affect the recovery from O3 depletion by anthropogenic CFCs. These changes due to solar effects should be factored in when studying the climate change related to accumulated greenhouse gases in the upper atmosphere.

During solar energetic particle (SEP) events, which are also known as the energetic particle precipitation (EPP), high-energy protons, electrons, and ions originating from strong solar activity (e.g. solar flares) reach Earth's atmosphere, leading to changes in atmospheric composition.

In particular, solar proton events (SPEs) deposit energetic protons at high geomagnetic latitudes and trigger short-term changes in the distribution of many chemical species such as O3, OH, HO2, HNO3, N2O, and potentially ClO and HOCl. For example, OH responds rapidly to the proton forcing due to its short chemical lifetime and the negligible impact from transport. MLS mesospheric observations have shown strong enhancement of nighttime OH in polar regions during SPEs and the corresponding rapid decrease of O3 due to catalytic O3 destruction reactions.