Explicit IMF By-effect in geomagnetic activity Lauri Holappa and Kalevi Mursula ReSoLVE Centre of Excellence University of Oulu, Finland
Solar wind-magnetosphere coupling β’ IMF B z -component is the main driver of magnetic reconnection at the magnetopause β’ IMF B y is included in coupling functions, but its effect does not depend on its sign 8/3 πΞ¦ ππ π 2/3 sin = π€ 4/3 πΆ π , ππ’ 2 2 + πΆ π§ 2 πΆ π = πΆ π¨ π = arctan πΆ π§ πΆ π¨ Does the sign of B y matter? β’ β’ Stronger auroral electrojets for B y >0 than for B y < 0 in winter! [Friis-Christensen et al., 2017; Smith et al., 2017]
Russell-McPherron effect Spring Fall [Zhao et al., 2012] β’ Negative IMF B z in GSM coordinate system for IMF B y > 0 in fall and for B y < 0 in spring.
Russell-McPherron effect => Solar wind driving of the magnetosphere is enhanced for IMF B y > 0 in fall, and for B y < 0 in spring. Superposed monthly means and standard errors of the Newell function d Ξ¦ MP /dt in 1966-2015
Seasonal variation of AL index β’ Deep minimum in AL index during winter for B y < 0 β’ Cannot be explained by the Russell-McPherron effect. => Explicit B y -effect All data in 1966-2015
Explicit B y -effect in AL index |AL|, summer solstice Β± 15 days |AL|, winter solstice Β± 15 days β’ In NH winter, for the same value of β’ Opposite B y -dependence d Ξ¦ MP /dt, B y > 0 produces a in NH summer stronger AL-index than B y < 0.
Explicit B y -effect in AL index |AL|, spring equinox Β± 15 days |AL|, fall equinox Β± 15 days β’ Explicit B y -dependence is very weak around spring and fall equinoxes.
Removing Russell-McPherron effect a) We calculate measured and predicted polarity ratios includes the RMP-effect b) The ratio of these two ratios quantifies the explicit B y -effect AL index is about 40-50% stronger for B y >0 than for B y <0 around the winter solstice. [Holappa and Mursula, JGR, 2018]
Latitudinal dependence Ratio R +/- for local β’ geomagnetic indices (Ah-indices) at different latitudes for all seasons in 1995-2017 [Holappa et al., JGR, 2019] β’ For strong driving (B z < -5 nT), the By-effectis about 40% at subauroral latitudes (55Β°-60Β°) even when averaging over all seasons. => Substorms are stronger and latitudinally more extensive for B y > 0 than for B y < 0 .
Latitudinal dependence YOU ARE HERE Ratio R +/- for local β’ geomagnetic indices (Ah-indices) at different latitudes for all seasons in 1995-2017 [Holappa et al., JGR, 2019] β’ For strong driving (B z < -5 nT), the By-effectis about 40% at subauroral latitudes (55Β°-60Β°) even when averaging over all seasons. => Substorms are stronger and latitudinally more extensive for B y > 0 than for B y < 0 .
Summary β’ IMF B y is an explicit driver of high-latitude geomagnetic activity β’ Geomagnetic activity is significantly stronger for B y > 0 than for B y < 0 in winter β’ The B y -effect is strongest at subauroral latitudes at about 55 Β° -60 Β° geomagnetic latitude β’ IMF B y is important for space weather predictions β’ No physical explanation yet! References: β’ L. Holappa and K. Mursula, J. Geophys. Res . (Space) , 123, 2018. β’ L. Holappa, N. Gopalswamy and K. Mursula, J. Geophys. Res . (Space) , 124, 2019.
B y -effect in Southern Hemisphere β’ K-index of Syowa station in Antarctica β’ During SH winter, for the same value of d Ξ¦ MP /dt, B y < 0 produces stronger K-index than B y > 0. β’ B y -dependence in SH is opposite to that in NH
B y -effect in Southern Hemisphere β’ During SH summer, for the same value of d Ξ¦ MP /dt, B y > 0 produces stronger K-index than B y < 0.
No explicit B y -effect in AU index β’ B y -dependence in the AU index (eastward electrojet) is solely due to Russell- McPherron effect
UT-variation β’ The explicit B y -effect (in NH) maximizes around 5 UT, i.e., when the Earthβs dipole axis points away from the Sun ο The explicit B y -effect maximizes when the auroral region is maximally in darkness ο B y -effect is efficient under low ionospheric +/β (AL) for different UT hours and Left: Ratio π ππ¦ππ conductivity? months. +/β (AL) averaged over months. Right: π ππ¦ππ
No explicit B x effect β’ There is a correlation between B y and B x . Which of the two components is the driver? β’ Limiting the amplitude of B x has almost no effect to the results => B x has only little, if any, explicit effect on high latitude geomagnetic activity
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