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Demonstration of Ozone Photochemistry with Eclipse

To increase the precision of the chemical transport model used for predicting the recovery of the stratospheric ozone, generally we will furthermore deal with the upper layer mesosphere (50~80km). Although ozone concentration in mesosphere is considered to stay certain since the mechanism is more simple than the layers below, it has been pointed out that the observation for years is inconsistent (*2) with the chemical transport model. It is of course because the model is incomplete, but we still do not understand whether it is caused by chemical reactions or phenomena we do not know or the ground experiment results such as the chemical reaction coefficient are not taken by the model correctly. Therefore, there has been more and more necessity to confirm the factors determining ozone concentration.

On the 15th January 2010, during the observation of SMILES, there occurred the longest annular solar eclipse of this millennium. During the eclipse, SMILES happened to observe the shadow field of the moon (Figure 3). As you know the eclipse is a natural phenomenon that the solar irradiance temporally changes, but the research team and I considered it to be a perturbative experiment on the whole earth and started to analyze the observation data of SMILES so as to inspect the existing theories on the ozone photochemistry.

Figure 3.

The mesospheric ozone concentrations observed by SMILES during the eclipse. [Click for larger image]
The white dotted contour lines are showing the magnitude of eclipse (ratio of the apparent size of the Moon to the apparent size of the Sun during an eclipse) by every 10% and the circles on the white line are the observation points of SMLIES. Additionally, the colors of these points indicates the mixing ratio (the number of moles of gas X included in 1 mole of air, or the volume of gas X in per unit volume of ideal gas) of ozone at 64km (mesosphere).

Figure 4 is the result from the comparison between the theoretical formula and the observation of SMILES. Generally, it shows the increase of ozone (y-axis direction) with the dimming (x-axis direction) during eclipse. The relational expression derived from the theory of the top and bottom of the mesosphere will differ depending on the difference of the main chemical reaction, but it consistent with the SMILES observations. This means that our consideration on ozone photochemistry is reasonable.

Figure 4.

Changes of ozone with dimming during eclipse. [Click for larger image]
The left is the result of 58km (bottom of mesosphere) while the right is that of 67~70km (top of mesosphere). The vertical axis is the ratio of ozone concentration during eclipse to daytime (that is the change of ozone concentration) and the horizontal axis is the dimming rate, and both are given with logarithms. The dotted and solid lines are the relationships from the theories of top and bottom mesosphere. (Imai et al., 2015)

This research is a demonstration of ozone photochemistry with the rare phenomenon eclipse, as well as a cross-field combined study of astronomy and atmospheric science. The results have been published on the geophysical journal “Geophysical Research LettersE(GRL) and the three institutes (JAXA, NIES and RISH of Kyoto University) made a press release (*3) in June 2015.

Our team has succeeded to detect the changes of the trace gasses besides ozone during the eclipse and is analyzing the data.

(*2) The inconsistence of observation with the chemical transport model: The chain reactions with HO_X (general name of H, OH and HO₂) are believed to be a dominant driver of ozone loss and it has been pointed out the discrepancies between observed and modeled HO_X concentrations (HO_X dilemma). Therefore, the inconsistence between observed and modeled ozone is considered to be because of the chemical reaction process with HO_X but it remains unsolved.

(*3) Press release: Study on the impact of sunlight on ozone in the atmosphere using solar eclipses. (JAXA Press Release Jun. 12, 2015)

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