Modulation of Solar Irradiance on Geomagnetic Activity: An Idealized Experiment with WACCM5.

,


BACKGROUND
The Sun affects all the layer of the Earth atmosphere, but the main influence is on troposphere and stratosphere that contain most of the atmosphere mass.Although solar irradiance and its change year by year is supposed to modify the stratosphere, another forcing, the geomagnetic activity, which drives energetic electron precipitation, can affect the stratosphere.
For example, increasing irradiance leads to more ozone creation, but an higher geomagnetic activity should reduce ozone, at least in the upper stratosphere.However, this relationship is not that straightforward and the region of ozone production can depend on the strength of the geomagnetic activity.
This study study investigated the interaction of the irradiance, in terms of maximum and minimum solar activity,with the geomagnetic activity using the atmospheric version of WACCM5.

MODEL SET UP
4 experiments were carried out with WACCM5 using different values of irradiance (SSI) and geomagnetic activity (GMA).
All the experiments were run for 40 years with the first 5 years used as a spin-up.

SCORE DEFINITION for STATISTICAL SIGNIFICANCE
To evaluate the effect of the forcing terms on a variable, for example the 500 hPa temperature in a specific area, we first compute the mean value of the difference between two experiments (e.g.H7 and L3) on all the grid points, and we compute how many grid points in a specific domain, for example the polar region, are statistically significant.Then we perform a permutation test combining the years of a pair of experiments and rearranging these years to find the number of significant points in each rearrangement (Fig. 2).Once we have a distribution of number of significant point we compute the rank of the original number of significant points.The rank represents a score measuring the impact on large scale.When the rank is larger than 94, the effect on the region is considered statistically significant.
The choice to investigate the impact on a domain was made because when we deal with many grid points, a few of them can be statistically significant by chance.

Influence of solar irradiance under low geomagnetic activity (H3-L3)
With low geomagnetic activity, the observed changes on zonal velocity and temperature are driven by the change in solar irradiance.Large differences of zonal velocity and temperature are visible mainly only in the Southern Hemisphere (Figure 3), where during the wintertime and the following spring season we observe an intensification of the polar vortex, while the temperature shows a temperature dipole, with colder air in H3 at lower levels of the stratosphere and warmer air in the upper stratosphere.
Northern Hemisphere does not show statistically significant differences in the wind field.All these differences seem to related to dynamical effect rather than radiative as ozone difference is very limited.The wave activity, shown here as difference of Eliassen-Palm Flux divergence is in agreement with the changes in temperature and zonal velocity.

Influence of solar irradiance under high geomagnetic activity (H7-L7)
With high geomagnetic activity the features observed in the experiments with low geomagnetic activity are no longer present in the Southern Hemisphere, although the temperature features, associated with warmer air in H7 than in L7, in the upper stratosphere are still present along almost all the latitudes (Figure 4).
Ozone in general is more abundant in H7, since the more NOx produced in the mesosphere and descending in the stratosphere because of high geomagnetic activity (present in both the experiments) depletes ozone in the upper stratosphere.However, as H7 has an higher value of irradiance more ozone in the lower stratosphere stratosphere.
The Northern Hemisphere shows larger variations in these experiments than in the experiments shown above.
However, the zonal wind is much weaker in H7 than L7 and the dipole temperature is reversed.

Geomagnetic activity under solar minimum condition (L7-L3)
Under solar minimum condition (Figure 5), the effect of the geomagnetic activity is very limited to the Southern Hemisphere during the winter and spring.It well visible a dipole in the temperature field in the Southern Hemisphere.There, the polar vortex is stronger in L7 than in L3.A temperature dipole is present in the austral winter and spring.However, these features are less intense than that observed in H3-L3 difference fields, with warmer air in the upper stratosphere in L7 than in L3 and a colder air in lower levels.
However, the reduction of ozone in the upper stratosphere is not matched by an increasing of ozone in the lower stratosphere.The wave activity, as seen by the divergence of EP Flux would explain the temperature and zonal wind patterns that have a dynamical cause rather than radiative.

IMPACT ON TROPOSPHERIC TEMPERATURE IN POLAR REGIONS
Under solar minimum condition the score of the impact of the geomagnetic activity in the troposphere is very low (Figure 6).Low scores mean that there are more statisticaly significant points in differences obtained by chance than the experiment differences L7-L3.

Geomagnetic activity under solar maximum condition (H7-H3)
The maximum solar condition change completely the stratosphere response that is opposite to that observed in L7-L3.Moreover there are in the Northern Hemisphere during the wintertime.
In the Souther Hemisphere, the combination of geomagnetic activity with the maximum solar condition is responsible for the ozone dipole, as the geomagnetic activity produces more NOx and reduces ozone in the upper stratosphere.This allows to UV radiation to produce more ozone in the lower stratosphere.However the temperature dipoles have the opposite sign compared with the L7-L3 difference fields.
The zonal wind is weaker in H7 than H3, during the winter in both the hemisphere and this would suggest that in combination with high radiative forcing the geomagnetic activity would tend to reduce the polar vortex.
Again we observed temperature dipoles that are reversed compared with H3-L3 in a way like the L7-L3 differences.However the ozone distribution shows some dipoles with higher values of ozone in the lower stratosphere.Again this is caused by the geomagnetic activity that deplets ozone in the upper stratosphere allowing the production of ozone in the lower stratosphere.

IMPACT ON TROPOSPHERIC TEMPERATURE IN POLAR REGIONS
In contrast with the features observed in L7-L3 that are not significant, the 500 hPa differences in H7-H3 show higher scores with the boreal autumn that has a score of 96, indicating the possibility that 500 hPa temperature is actually affected by geomagnetic activity.As this result appears in solar maximum condition, then solar irradiance modulates effects of the geomagnetic activity.

CONCLUSIONS
Although energetic electron precipitation occurs at all times during a solar cycle, the flux of particles into the atmosphere is not constant.
The effect of these particles is controversial and although evidence suggests that they produce NOx that depletes ozone in the upper stratosphere, with a possible influence on tropospheric weather and climate, how it interacts with solar radiation is still under investigation.
In this work we compare four experiments, with solar max and solar min conditions and high and low geomagnetic activity.
We found the following results: With low geomagnetic activity the solar irradiance plays a key role (H3-L3), as the stratospheric temperature and zonal ind are affected by changes in solar irradiance.
However, when the geomagnetic activity (H7-L7) is high the patterns of zonal wind and temperature are reversed compared with the equivalent experiments having low geomagnetic activity (H3-L3).
Under solar minimum conditions (L7-L3) the geomagnetic activity hardly affects the stratospheric variables.The 500 hPa temperature has very low scores in both the hemispheres indicating that no influence of geomagnetic activity exists in the model.
When there is a solar max condition (H7-L3) the role of the geomagnetic activity is instead stronger with changes of zonal wind, temperature and ozone, but with an opposite sign to the contribute of solar forcing term (cf H7-H3 with H3-L3).
The joint effect of solar irradiance and geomagnetic activity (H7-L3) suggests that there is a combination of the effects that we have seen in the other experiments.For example in the Southern Hemisphere the difference patterns resemble those observed in H3-L3, whereas the features in the Northern Hemisphere resemble those observed in H7-L7.Some of results presented here come from the following paper:

Figure 2 .
Figure 2. Algorithm used to define the distribution of number of statistically significant points within a specific region.The number of significant points of the original difference (H7-L3 in the example of the figure).

Figure 8 .
Figure 8. 500 hPa temperature H7-H3 differences.The dots indicate statistically significant points.The purple line surrounds positive values; the green line surrounds negative values.The number over each panel indicates the significance of the statistical test over the polar region.

Figure 10 .
Figure 10.500 hPa temperature H7-L3 differences.The dots indicate statistically significant points.The purple line surrounds positive values; the green line surrounds negative values.The number over each panel indicates the significance of the statistical test over the polar region.
This work is funded by the project SOLENA -Solar effects on natural climate variability in the North Atlantic and Arctic -Research Council of Norway, Program for Space Research Project: 255276/E10.