Which types of particle precipitation matter in the middle atmosphere?

The project sought to determine which types of particle precipitation are significant in changing the atmospheric chemistry. Our approach has been based on long time series of ground-based observations as well as atmosphere and climate modelling. High-energy protons and electrons precipitating into the atmosphere from space can have a significant impact on the atmosphere's chemical composition and dynamics. They are known to facilitate ozone depletion in the middle atmosphere, which may ultimately affect the surface temperatures in the polar regions. In this project we have investigate which types of particle precipitation events are significant in driving these atmospheric effects, and which ones play only a minor role. Solar proton events, which account for most of the energetic proton input, are very strong but rare, occurring only a few times a year. Their effects on the atmosphere are rather well-known. Energetic electron precipitation, however, is more common and less intense, resulting from dynamical processes within the magnetosphere, which transfer relativistic electrons from the radiation belts into the atmosphere. The chemical changes due to electron precipitation have been demonstrated for detailed studies of individual events, but little is known about their relative importance globally and over decadal time scales. The Coupled Model Intercomparison Project (CMIP) has recommended that the Intergovernmental Panel on Climate Change (IPCC) reports would include a solar particle forcing. Several tools have been developed for detecting energetic particle precipitation and assessing its atmospheric effects. With the long-term ground-based observations we have validated the currently used proxy for the proton impact area. We have further assessed the electron impact area and its effect on mesospheric chemistry during specific type of electron aurora, the pulsating aurora, which is concluded to be one of the key electron components due to its long duration. We have prepared an improved solar particle forcing description for both protons and auroral electrons to be tested for use in the future climate modelling in decadal time scales.

The most important outcome of the project is one new doctorate on the topic of the solar particle events, and an enhanced collaboration between observational and modelling groups within the field of energetic particle precipitation (EPP). The new EPP forcing descriptions will be thoroughly tested on climate models for decadal simulations, and may end up leading to updates on the currently used descriptions for energetic particles.

High-energy protons and electrons precipitating into the Earth's atmosphere from space can have a significant impact on the chemical composition and dynamics of the atmosphere. These particles are known to facilitate ozone depletion in the middle atmosphere, which may ultimately cause changes in surface air temperatures in the polar regions. In this project we seek to determine which types of particle precipitation events are significant in driving these atmospheric effects, and which events play only a minor role. Solar proton events, which account for most of the energetic proton precipitation, are very strong but rather rare, occurring only a few times per year. Their effects on the atmosphere are relatively well-known. Energetic electron precipitation, however, is more common and less intense, resulting from dynamical processes within the magnetosphere, which transfer relativistic electrons from the radiation belts into the Earth's atmosphere. The chemical changes due to electron precipitation have been demonstrated for detailed studies of individual events, but little is known about their relative importance globally and over decadal time scales. The Coupled Model Intercomparison Project (CMIP) has for the first time recommended that the next Intergovernmental Panel on Climate Change (IPCC) would include a solar particle forcing in the climate reports. Several tools and proxies have been developed for detecting energetic particle precipitation and assessing its atmospheric effects. With the necessary long-term datasets it is now possible to validate the currently used estimates of particle forcing, and determine which types of particle precipitation have a significant global and long-term impact on the atmospheric system. This knowledge can be used to improve future recommendations of solar particle forcing as a part of natural climate variability.