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M. M. Lam, G. Chisham and M. P. Freeman, The interplanetary magnetic field influences mid-latitude surface atmospheric pressure, Environ. Res. Lett., 8, 045001, 2013

 

The paper is available from http://iopscience.iop.org/1748-9326/8/4/045001/

 

Abstract

 

The existence of a meteorological response in the polar regions to fluctuations in the interplanetary magnetic field (IMF) component By is well established. More controversially, there is evidence to suggest that this Sun–weather coupling occurs via the global atmospheric electric circuit. Consequently, it has been assumed that the effect is maximized at high latitudes and is negligible at low and mid-latitudes, because the perturbation by the IMF is concentrated in the polar regions. We demonstrate a previously unrecognized influence of the IMF By on mid-latitude surface pressure. The difference between the mean surface pressures during times of high positive and high negative IMF By possesses a statistically significant mid-latitude wave structure similar to atmospheric Rossby waves. Our results show that a mechanism that is known to produce atmospheric responses to the IMF in the polar regions is also able to modulate pre-existing weather patterns at mid-latitudes. We suggest the mechanism for this from conventional meteorology. The amplitude of the effect is comparable to typical initial analysis uncertainties in ensemble numerical weather prediction. Thus, a relatively localized small-amplitude solar influence on the upper atmosphere could have an important effect, via the nonlinear evolution of atmospheric dynamics, on critical atmospheric processes.

 

Background to this paper - The Mansurov Effect

 

The existence of a correlation between the dawn-to-dusk potential difference in the ionosphere and surface pressure was first identified over 35 years ago and has since been well-documented (Mansurov et al., 1974; Tinsley and Heelis, 1993; Burns et al., 2007, 2008). In all these cases the IMF was used as a proxy for the ionospheric potential. The correlation was termed the Mansurov effect, describing the surface pressure response to a change in sign of the y-component (east-west) of the IMF (By), with the pressure response being opposite for high magnetic latitude stations in the Northern hemisphere when compared to the Southern hemisphere. The need to use an IMF proxy was due to the unavailability of electric potential measurements in the polar ionosphere. Hence, this process has only been indirectly studied and is one of the most underexplored possible mechanisms for the solar forcing of weather and climate.

 

In the most detailed study in this area to date, Burns et al. (2008) showed that fluctuations in surface pressure varied systematically with variations in IMF By. This variation comprised ~1-2 hPa of pressure for a change of ~8 nT in By, which they described as a small but significant effect. By using pressure variations at a range of Arctic and Antarctic stations they also showed that the size of this effect varied with latitude, being larger closer to the geomagnetic poles, and also that the effect was hemispherically asymmetric.  Earlier studies (Frank-Kamenetsky et al., 2001; Corney et al., 2003; Burns et al., 2007) suggested that the net diurnal effect of changes in the z-component (north-south) of the IMF (Bz) were small, even though it is well known that Bz has a much greater influence on the strength of the ionospheric potential than By, which controls the dawn-dusk asymmetry of the potential pattern (see Figure 5).

 

IMF Bz broadly strengthens the dawn-to-dusk electric field applied across the polar caps. It increases the solar-wind-imposed-potential (and thus Jz) on the dawn-side of the polar cap and decreases it on the dusk-side. Except at very high magnetic latitudes, a station averages as much time under a decreased solar-wind-imposed potential as an increased one. At high magnetic latitudes (> 80 degrees) the station generally remains under either a positive or negative solar-wind-imposed-potential influence as the Earth rotates. IMF By has a slower variation than IMF Bz, broadly changing twice or four times in a solar rotation, suggesting that it is the cumulative influence (at least a day, maybe a few) that eventually modifies pressure.

 

Since IMF Bz has a dominant influence on the strength of the solar wind-Earth interaction, the dominance of the IMF By influence on surface pressure provides evidence of the GEC-weather linkage.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5: Statistical ionospheric electric potential patterns for eight different IMF directions (see centre dial) (Ruohoniemi and Greenwald, 2005). In each panel the co-ordinates are geomagnetic, ranging from 60° latitude to the geomagnetic pole, with magnetic local noon at the top, and dawn to the right. Equipotentials are shown by the solid and dashed lines and the X and + symbols represent the locations of the extreme potential values. The contours are at 6 kV intervals.

 

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