A1S2WP5

Objectives of WP5 on Irradiance

The Sun continuously radiates in all wavelengths of the electromagnetic spectrum, from gamma rays to radio waves. The variability of the solar irradiance and the impact on the terrestrial environment are highly wavelength dependent.

Most of the solar irradiance is emitted in the visible and near-infrared parts of the spectrum, where the variability is also lowest (< 0.1% over a solar cycle). This part of the spectrum mostly causes direct heating of the Earth's surface and of the troposhere. In that sense, its impact is relatively well understood. The impact of the UV part of the spectrum is much more complex since it is mostly absorbed in the upper atmosphere (from the stratosphere onwards) and can vary by 1 up to 100% depending on the wavelength. For that reason, it has received much more interest recently. Unfortunately, there has been a severe lack of continuous measurements in the UV. Indeed, measurement have to be made outside of the atmosphere, and the lifetime of present radiometers and spectrometers is typically a few years. For that reason, various solar proxies (such as the sunspot number or the f10.7 index) are used today as substitutes for the solar UV flux.

WP 5 focuses on two aspects of downstream research on solar irradiance. One is the the Total Solar Irradiance (TSI, i.e. the solar spectrum integrated over all wavelengths), which is a key input for climate models. The second is the spectral irradiance in the ultraviolet (UV, from 10-300 nm), and its impact on the upper atmosphere. The overall objective is to improve the scientific understanding by a better exploitation of scattered observations from ground and space instruments.

To do so, we want to

• determine the physical connections by which the variability of the TSI and the UV irradiance are related
• develop a prototype for online nowcast of the solar UV spectrum from radiometric data.
• compute and publish a now-cast of the chemical composition of the terrestrial atmosphere influenced by the variations of the solar spectral irradiance

The outcomes of this WP are

• a better understanding of the physical mechanisms that relate the temporal variability of the TSI to the spectral varibility in the UV, both on short term (days to weeks) to long term (decades).
• two online models, one for a nowcast of the solar UV spectrum and one for a nowcast of the chemical composition of the mesosphere.

For a detailed description of WP5, see the dedicated web page : http://lpc2e.cnrs-orleans.fr/~ddwit/irradiance/