Seasonal variation of high‐latitude geomagnetic activity in individual years

Abstract We study the seasonal variation of high‐latitude geomagnetic activity in individual years in 1966–2014 (solar cycles 20–24) by identifying the most active and the second most active season based on westward electrojet indices AL (1966–2014) and IL (1995–2014). The annual maximum is found at either equinox in two thirds and at either solstice in one third of the years examined. The traditional two‐equinox maximum pattern is found in roughly one fourth of the years. We found that the seasonal variation of high‐latitude geomagnetic activity closely follows the solar wind speed. While the mechanisms leading to the two‐equinox maxima pattern are in operation, the long‐term change of solar wind speed tends to mask the effect of these mechanisms for individual years. Large cycle‐to‐cycle variation is found in the seasonal pattern: equinox maxima are more common during cycles 21 and 22 than in cycles 23 or 24. Exceptionally long winter dominance in high‐latitude activity and solar wind speed is seen in the declining phase of cycle 23, after the appearance of the long‐lasting low‐latitude coronal hole