The influence of solar wind on extratropical cyclones – Part 1: Wilcox effect revisited

A sun-weather correlation, namely the link between solar magnetic sector boundary passage (SBP) by the Earth and upper-level tropospheric vorticity area index (VAI), that was found by Wilcox et al. (1974) and shown to be statistically significant by Hines and Halevy (1977) is revisited. A minimum in the VAI one day after SBP followed by an increase a few days later was observed. Using the ECMWF ERA-40 re-analysis dataset for the original period from 1963 to 1973 and extending it to 2002, we have verified what has become known as the "Wilcox effect" for the Northern as well as the Southern Hemisphere winters. The effect persists through years of high and low volcanic aerosol loading except for the Northern Hemisphere at 500 mb, when the VAI minimum is weak during the low aerosol years after 1973, particularly for sector boundaries associated with south-to-north reversals of the interplanetary magnetic field (IMF) <I>B_Z</I> component. The "disappearance" of the Wilcox effect was found previously by Tinsley et al. (1994) who suggested that enhanced stratospheric volcanic aerosols and changes in air-earth current density are necessary conditions for the effect. The present results indicate that the Wilcox effect does not require high aerosol loading to be detected. The results are corroborated by a correlation with coronal holes where the fast solar wind originates. Ground-based measurements of the green coronal emission line (Fe XIV, 530.3 nm) are used in the superposed epoch analysis keyed by the times of sector boundary passage to show a one-to-one correspondence between the mean VAI variations and coronal holes. The VAI is modulated by high-speed solar wind streams with a delay of 1–2 days. The Fourier spectra of VAI time series show peaks at periods similar to those found in the solar corona and solar wind time series. In the modulation of VAI by solar wind the IMF <I>B_Z</I> seems to control the phase of the Wilcox effect and the depth of the VAI minimum. The mean VAI response to SBP associated with the north-to-south reversal of <I>B_Z</I> is leading by up to 2 days the mean VAI response to SBP associated with the south-to-north reversal of <I>B_Z</I>. For the latter, less geoeffective events, the VAI minimum deepens (with the above exception of the Northern Hemisphere low-aerosol 500-mb VAI) and the VAI maximum is delayed. The phase shift between the mean VAI responses obtained for these two subsets of SBP events may explain the reduced amplitude of the overall Wilcox effect. <br><br> In a companion paper, Prikryl et al. (2009) propose a new mechanism to explain the Wilcox effect, namely that solar-wind-generated auroral atmospheric gravity waves (AGWs) influence the growth of extratropical cyclones. It is also observed that severe extratropical storms, explosive cyclogenesis and significant sea level pressure deepenings of extratropical storms tend to occur within a few days of the arrival of high-speed solar wind. These observations are discussed in the context of the proposed AGW mechanism as well as the previously suggested atmospheric electrical current (AEC) model (Tinsley et al., 1994), which requires the presence of stratospheric aerosols for a significant (Wilcox) effect

North-south asymmetries in the 530.3 nm coronal line from 1958 to 1980

The north-south asymmetry in the intensity of the 530.3 nm emission coronal line is examined in the time period 1958 to 1980 using data at all latitudes. The results show that overall the northern hemisphere of the Sun is more active in this line. The general pattern has changed after the beginning of the 21st cycle with the southern polar region being more active after the last solar minimum (1975). These anisotropies may be related to phase differences between the activity of northern and southern hemispheres. North-south asymmetries may have implications in the structure and evolution of the heliosphere, its current sheet and even in the cosmic ray propagation. © 1983 D. Reidel Publishing Co

North-south asymmetries in the 530.3 nm coronal line from 1958 to 1980

The north-south asymmetry in the intensity of the 530.3 nm emission coronal line is examined in the time period 1958 to 1980 using data at all latitudes. The results show that overall the northern hemisphere of the Sun is more active in this line. The general pattern has changed after the beginning of the 21st cycle with the southern polar region being more active after the last solar minimum (1975). These anisotropies may be related to phase differences between the activity of northern and southern hemispheres. North-south asymmetries may have implications in the structure and evolution of the heliosphere, its current sheet and even in the cosmic ray propagation. © 1983 D. Reidel Publishing Co