Abstract
Abstract. Energetic cosmic rays are the main source of ionization of the low-middle atmosphere, leading to associated changes in atmospheric properties. Via the hypothetical influence of ionization on aerosol growth and facilitated formation of clouds, this may be an important indirect link relating solar variability to climate. This effect is highly debated, however, since the proposed theoretical mechanisms still remain illusive and qualitative, and observational evidence is inconclusive and controversial. Therefore, important questions regarding the existence and magnitude of the effect, and particularly the fraction of aerosol particles that can form and grow, are still open. Here we present empirical evidence of the possible effect caused by cosmic rays upon polar stratospheric aerosols, based on a case study of an extreme solar energetic particle (SEP) event of 20 January 2005. Using aerosol data obtained over polar regions from different satellites with optical instruments that were operating during January 2005, such as the Stratospheric Aerosol and Gas Experiment III (SAGE III), and Optical Spectrograph and Infrared Imaging System (OSIRIS), we found a significant simultaneous change in aerosol properties in both the Southern and Northern Polar regions in temporal association with the SEP event. We speculate that ionization of the atmosphere, which was abnormally high in the lower stratosphere during the extreme SEP event, might have led to formation of new particles and/or growth of preexisting ultrafine particles in the polar stratospheric region. However, a detailed interpretation of the effect is left for subsequent studies. This is the first time high vertical resolution measurements have been used to discuss possible production of stratospheric aerosols under the influence of cosmic ray induced ionization. The observed effect is marginally detectable for the analyzed severe SEP event and can be undetectable for the majority of weak-moderate events. The present interpretation serves as a conservative upper limit of solar energetic particle effect upon polar stratospheric aerosols.
Highlights
Cosmic rays are energetic particles of extra-terrestrial origin impinging upon the Earth’s atmosphere. They are categorized according to their origin as either galactic cosmic rays (GCRs) or solar cosmic rays, the latter more conventionally referred to as solar energetic particles (SEPs)
It is common to study the relation between aerosols or different types of clouds and short-term suppressions of GCR flux during Forbush decreases caused by strong interplanetary transients. Such studies are numerous (e.g. Kniveton, 2004; Kristjansson et al, 2008; Pierce and Adams, 2009; Svensmark et al, 2009; Laken et al, 2009; Calogovic et al, 2010) and highly controversial and inconclusive. They are looking for a possible disappearance of existing aerosols or cloud condensation nuclei (CCN) associated with a reduction in the GCR flux, which may be quite different from production of new particles by enhanced flux
The effect was observed for larger particles, but it was most likely related to an independent cooling event
Summary
Cosmic rays are energetic particles (mostly protons and α-particles) of extra-terrestrial origin impinging upon the Earth’s atmosphere. They are looking for a possible disappearance of existing aerosols or cloud condensation nuclei (CCN) associated with a reduction in the GCR flux, which may be quite different from production of new particles by enhanced flux In this sense, it is more promising to perform a case study of the potential atmospheric response to a strong GLE event. We look for a possible influence of a sudden dramatic increase in atmospheric ionization due to the extreme SEP/GLE event of 20 January 2005 on the behavior of aerosol particles in the polar lower stratosphere. We have checked other data sets potentially providing data for that period and found that only the data sets discussed here can be used for our study
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