RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-7. Astrophysics and Astro-Glaciology

Variation in chemical composition induced by solar energetic
particle events in the middle atmosphere
Y. Nakai,*1 Y. Motizuki,*1 M. Maruyama,*1 H. Akiyoshi,*1,2 and T. Imamura2
Recently, the influences of super solar flares on the earth
environment have been extensively investigated. One of theareas of focus in such investigations is the change ofchemical composition in the terrestrial atmosphere inducedby energetic particles generated in the super solar flares.
The bombardment of solar energetic particles (SEPs) to theatmosphere is called an SEP event, where a large amount ofX-rays, gamma-rays, protons, and heavier ions from thesurface of the sun intrude the terrestrial atmosphere. In
particular, high-energy protons come down into thestratosphere and induce dissociation of nitrogen molecules
followed by an increase of odd nitrogen oxides (NOx) andreactive odd nitrogen species (NOy). Consequently, theincrease of NOx and NOy affects the ozone concentration
over a period longer than SEP events.
We have performed simulations to investigate variation inchemical composition in SEP events by solving a large
number of rate equations for the concentrations of chemicalspecies in air.1) We adopt 77 chemical species including
ions and about 480 chemical reactions (ionic reaction
processes, neutral reactions, photolysis, etc.) in the present
simulation. A large number of ionic processes, including
recombination in the stratosphere, were treated for the firsttime, to the best of our knowledge. No transport processes
are taken into account. This model is referred to as theBox-model hereafter. We carried out calculations using
commercial software for solving complex chemical reaction
systems (FACSIMILE, MCPA Software Ltd).
In order to estimate the yields of prompt products by theSEP protons, we used the G-values (amount of products per
absorbed energy of 100 eV) investigated by radiolysis
studies2,3), based on the assumption that the prompt productyields are determined only by the energy deposit of the SEP
protons in air. The prompt products are assumed to be
charged products (positive atomic ions, positive molecularions, and electrons) and neutral products (ground-state andthe metastable atoms) generated from nitrogen and oxygen
molecules in air. The energy deposit was estimated using
the calculations of ion-pair creation by the SEP protons4).
During an SEP event, both the photochemical reactions
induced by the UV and visible radiation from the sun andthe reactions induced by the SEP protons are considered in
the simulation. The variation in chemical composition in anSEP event is estimated as the difference between the result
obtained through simulation including the radiolysis
processes due to the SEP protons and that obtained underconsideration of only photochemical reactions using thesame initial condition.
1
2RIKEN Nishina Center
National Institute for Environmental Studies

Figure 1 shows a tentative result of the variation in theozone concentration at the 50 km altitude in the northern
polar region for the SEP event in October-November 2003.
The energy deposit in the SEP event continued for six days.
A sharp decrease of the ozone concentration is observed
immediately after the SEP event starts. On the third day ofthe event, when the energy deposit also reaches themaximum, the decrease of the concentration reaches themaximum and increases back gradually after that. Theozone concentration does not completely recover andremains depleted for a few weeks or more. This depletion ofthe ozone presumably corresponds to a very slow recoveryof the NOx concentrations. The NOx concentration begins
to increase when an SEP event starts. It continues toincrease for four days and is almost maintained constantafter the SEP event. The depletion of ozone is thought tocontinue after the SEP event since the ozone is consumed in
the catalytic reaction cycle involving NOx, e.g., NO+O3!
NO2+O2.
In the near future, we will investigate the global andlong-term influence of SEP events on the atmosphericchemical composition. In our approach, we will estimate
short-term variations during an SEP event using ourBox-model, and the estimated variations will then be input
into a three-dimensional (3D) chemical climate model(CCM)5) as instantaneous perturbation of the chemicalcomposition. This is because the transport of chemical
species treated in 3D is essential for investigating the globaland long-term influence.

Figure 1. Tentative result for the variation in the ozone
concentration at the 50km altitude caused by SEP (see text).
References

1) For the early-stage model: K. Sekiguchi et al., RIKEN Accel.
Prog. Rep. 46, 124 (2013).
2) C. Willis and A. W. Boyd, Int. J. Radiat. Phys. Chem. 8, 71(1976).
3) H. Mätzing, Adv. Chem. Phys. LXXX, 315 (1991).
4) C.H. Jackman et al., Atmos. Chem. Phys. 8, 765 (2008).
5) H. Akiyoshi et al., J. Geophys. Res. 114, D3, 16 (2009).

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