5. Summary and conclusions

leading to a vertical current J<sup>z</sup> = σE<sup>z</sup> from the ionosphere to the ground through the

4. The observed timescales of the response of the atmosphere to SW

Timescales are important in order to discriminate the involved physical processes in the SW-atmosphere coupling. In their pioneering work, Francia et al. [15] found that, during 2007 and 2008, the ULF activity, in both Pc1-2 and Pc5 frequency ranges, is correlated with the surface air temperature with different delays. Their results, shown in Figure 7a, indicate that the temperature is significantly correlated with the Pc1-2 power at a time lag of 1 day. Although lower, the correlation with the Pc5 power is also significant, reaching the maximum value when the

In the meanwhile, Regi et al. [57] computed the cross-correlation between high

cloud cover (HCC, h > 6 km) over Antarctica and logPc1-2 time series at TNB during 2003–2010, band-pass filtered at 27 days. They found several Antarctic regions characterized by significantly high correlation values. Examples of positive cross-correlations are shown in Figure 7b. Their experimental results suggest an average time delay of approximately 1 day of the response of the atmospheric cloud cover to the SW-driven Pc1-2 ULF power intensification. Such delay is consistent with the timescales of electrodynamics/microphysical-related processes here proposed. In [57] a possible relationship with large-scale atmospheric transport was investigated by means of a correlation analysis between Pc1-2 power and CH4 (27 day band-pass filtered) concentrations from MACC dataset. CH4 is a long life chemical tracer, commonly used to characterize the middle atmosphere transport, also analyzing it in the framework of quasi-Lagrangian or conservative coordinate systems [60] and as a proxy to validate satellite measurements [61]. The results of the correlation analysis between Pc1-2 power and CH4, during 2003–2010 (not shown here), indicate that the correlation coefficients are generally lower and not significant and superimposable to the other tropospheric parameters. They suggest that the transport is not modulated by the 27 day periodicity of the ULF activity.

(a) The cross-correlation between the Pc5 and the Pc1-2 power and the surface air temperature, at TNB, at different time lags τ. A delay τ < 0 (τ > 0) indicates that Pc1-2/Pc5 power precedes (follows) surface air temperature at TNB. The dashed green lines represent the 90% confidence level. Figure adapted from [15]. (b) Examples of positive cross-correlation analysis as a function of the delay of the HCC with respect to Pc1-2 power, observed during winter months of 2003–2010. The green lines mark the 95% confidence level. A delay τ

< 0 (τ > 0) indicates that Pc1-2 power precedes (follows) HCC. Figure adapted from [57].

temperature is delayed by 3 days with respect to the Pc5 power.

stratospheric and tropospheric layers.

Antarctica - A Key to Global Change

variations

Figure 7.

14

In this work we presented a short review of our experimental results regarding the possible relationship between SW and atmospheric parameters at high latitude, in Antarctica, at Terra Nova Bay. Examining possible relationship between geomagnetic activity in the Pc1-2 and Pc5 frequency range and the polar cap potential difference with stratospheric and tropospheric parameters, the results provided in [54] can be summarized as follows:


Further investigations [57] at tropospheric heights at high latitudes indicate that SW-driven electrodynamic processes and energetic particle precipitation related with enhancement of Pc1-2 activity can affect tropospheric temperature, specific humidity, and cloud cover. The response is quick (within 1 day) at ground and in the troposphere. These results suggest that the electrodynamics modulate the physical properties of clouds, probably through electron scavenging microphysical mechanism. It is a matter of fact that electron scavenging is strongly dependent on vertical tropospheric-stratospheric conductivity variations, due to energetic particle precipitation driven by ULF waves, and on the vertical electric current, modulated by polar cap potential, associated to SW-magnetosphere reconnection processes. More recently, evidence of an SW signature in the mesosphere [62] has been published, indicating that the SW can affect the atmosphere through the whole atmospheric column. As discussed by [63], the processes involved in each atmospheric layer are almost certainly different, and transport phenomena could be important.

Our conclusions are supported by the observed short (<1–2 days) delay response in the atmospheric parameters at troposphere altitudes and at ground with respect to the much longer delay expected for chemical mechanism [15, 57]. However, this matter should be further investigated as underlined by [63] in particular through the examination of the time delays at stratospheric and mesospheric altitudes and at lower latitudes; the study of the dependence on different interplanetary conditions might be also useful for a more deep understanding of the atmosphere response to the SW.
