Relationships between the solar wind magnetic field and ground-level longwave irradiance at high northern latitudes

Abstract

Longwave irradiances measured from two sites at different geomagnetic latitudes show different responses to changes in the east-west component of the interplanetary magnetic field (IMF By). At Barrow, Alaska, geomagnetic latitude 69-70oN, neither downward longwave irradiance from the atmosphere nor upward longwave irradiance from the ground show a significant correlation with By. However, at Alert, Canada, geomagnetic latitude near 87oN, a negative correlation that is marginally significant at the 95% level of confidence exists between By and downward longwave irradiance measured 3 days later. On average, a +3.5 nT increase in By is followed by a daily-mean downward longwave irradiance that is smaller by −0.60 ± 0.60% than would exist for a constant By. Similarly, daily-mean upward irradiance at a lag of 4 days is −0.51 ± 0.30% smaller than would exist otherwise, where error bars denote the 95% confidence range. The difference in upward irradiance corresponds to a surface cooling at Alert of approximately 0.33 ± 0.19 K. These results are qualitatively consistent with a previously proposed mechanism in which the interplanetary magnetic field perturbs the ionosphere-to-ground potential difference and the downward atmospheric current density over limited regions near the geomagnetic poles, altering local cloud properties. We find that the atmospheric longwave emission is altered on a time scale of 3 days, with a change in surface temperature appearing one day later, attributable to the thermal inertia of the surface. When one moves from the geomagnetic latitude of Alert (3° from the north geomagnetic pole) to the latitude of Barrow (∼20° from that pole), any connection between By and longwave irradiance becomes too small to isolate from the natural background variability.