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Laboratory for Atmospheric and Space Physics

Energetic particle precipitation

Energetic particle precipitation (EPP) refers to highly energetic electrons, protons, neutrons, and ions that are accelerated into the atmosphere through various heliophysical and geomagnetic processes. They enter the atmosphere mainly in the geomagnetic polar regions. When energetic particles enter the atmosphere they ionize and dissociate atmospheric constituents, resulting in the formation of reactive odd nitrogen (NOx = NO + NO2). EPP has been shown to contribute up to 10% of the stratospheric NOx budget and up to 40% of the polar stratospheric NOx budget. Once in the stratosphere, NOx produced by EPP (EPP-NOx) interferes with catalytic cycles involving ozone (O3). Theoretically, changes in O3 can lead to changes in temperature and winds, which means that EPP has the potential to impact climate as well. There are two main mechanisms by which EPP affects the stratosphere: the direct effect (EPP DE) and indirect effect (EPP IE). Our research focuses primarily on the EPP IE.

Direct Effect

The EPP DE occurs when highly energetic particles produce NOx directly in the stratosphere. It is associated with protons with energies greater than 30 MeV and electrons with energies greater than 300 keV. The EPP DE happens sporadically throughout the solar cycle and occurs more often near solar maximum when coronal mass ejections (CMEs) are more common. CMEs accelerate solar energetic particles (SEPs) away from the Sun at energies up to 1000 MeV. The Earth’s polar regions are particularly susceptible to SEPs because of the open magnetic field lines, and the energetic particles can deposit their energy down to 20 km in altitude. It is estimated that the EPP DE contributes up to 10% of the global annual source of stratospheric odd nitrogen (NOy = N + NO + NO2 + NO3 + HNO3 + ClONO2 + 2N2O5 + HO2NO2 + BrONO2).

Indirect Effect

The EPP IE occurs when low and medium energy particles produce NOx in the mesosphere and lower thermosphere. In the absence of sunlight in the polar night, this NOx can be transported to the stratosphere through a combination of diffusion and advection by the residual circulation. The first satellite evidence of the EPP IE was obtained by the Limb Infrared Monitor of the Stratosphere (LIMS).


Plentiful observational evidence of the EPP IE has been obtained since LIMS, along with observational evidence for the destruction of O3 by EPP-NOx.