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

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

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.

#### Figure 7.

(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].

However, the different time delays in different years could be due to variations in large-scale transport pattern (see [57] for details).
