Seasonal Variation of High-Latitude Geomagnetic Activity in Individual Years
Tutkimustuotos: Lehtiartikkeli › › vertaisarvioitu
|Julkaisu||Journal of geophysical research: Space physics|
|Tila||Julkaistu - 21 marraskuuta 2017|
|OKM-julkaisutyyppi||A1 Julkaistu artikkeli, soviteltu|
- University of Oulu
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.