Solar influences on the atmosphere and climate

Sun, our home star, is a variable star. The variations in the appearance and energy output from the Sun are denoted as “solar activity”.  These variations range from milliseconds to decades, centuries, millennia, and beyond. The variations with time scales of up to a few solar rotation periods (~27 days) are referred to as “space weather”, and have significant impacts on the space-borne and ground-based technological systems. Longer-term solar activity variations are defined as “space climate”, and are supposed to be related to the terrestrial (and maybe other planets’) global climate variations.

A number of solar activity manifestations can affect the Earth’s atmosphere. The total solar irradiance (TSI) is the main energy source for the terrestrial system. TSI is dominated by visible light whose relative variations are only a fraction of a percent, but with the greatest absolute magnitude of change. With decreasing wavelength, the relative variations increase. Radiation in different spectral ranges affects different parts of the Earth’s atmosphere.

The Earth is also constantly exposed to the Sun’s corpuscular radiation as it together with the whole Solar System is inside the ever expanding solar atmosphere known as the “solar wind” – a flow of charged particles with embedded magnetic fields from the solar corona. On top of it, transient structures like Coronal Mass Ejections (CMEs) and High Speed Solar Wind Streams (HSSs) ride whose interactions with the terrestrial magnetic field lead to geomagnetic storms.

High energy particles from CME or HSS associated shocks, or (mostly) from their interactions with the Earth’s magnetosphere, precipitate in the high latitude atmosphere leading to increased ionization, enhanced production of compounds which affect the ozone balance, radiative heating and cooling, and finally changes in atmospheric dynamics and large-scale circulation modes like the North Atlantic Oscillation governing the weather over most of the Northern hemisphere.

The solar wind magnetic field modulates the flux of galactic cosmic rays, high energy particles coming from outside the Solar system. Due to their high energy, they penetrate deep into the atmosphere. Their variations are found to be strongly correlated with the atmospheric global electric circuit, cloud cover, albedo, and infrared opacity, determining the Earth’s energy balance.

In this review, after a short introduction to solar activity, I will describe its above mentioned agents, and explain the suggested mechanisms by which they influence the atmosphere and climate. I will present observational evidences of such influences, highlight the recent advances, and the still unsolved questions and uncertainties.