Hemispheric Geomagnetic Activity Dependence on IMF By: Evidence from the Satellite and Pre-Satellite Eras

Abstract The sign of the interplanetary magnetic field (IMF) \((B_y)\) is known toproduce hemispheric asymmetries in the coupled magnetosphere--ionospheresystem, but its long-term signature in hemispheric geomagnetic activityis less well established. We investigate whether IMF polarity organizesthe distribution of geomagnetic activity between the Northern andSouthern Hemispheres independently of the overall activity level.Homogeneous hemispheric \((aa)\) indices are analyzed using directlymeasured IMF \((B_y)\) during 1965--2010 and reconstructed IMF sectorpolarity extending back to 1868. Positive- and negative-\((B_y)\) days arefirst compared after matching them within 5-nT bins of overallgeomagnetic activity. As an independent control, the \((B_y)\) dependenceis estimated while accounting directly for southward IMF, solar-windspeed, and long-term variations. A systematic \((B_y)\)-dependent hemispheric asymmetry remains under bothcontrol approaches, with the most consistent positive differences between \((B_y>0)\) and \((B_y<0)\) conditions occurring in May, June, and October.The seasonal organization of this hemispheric effect differs markedlyfrom the corresponding \((B_y)\) dependence of overall geomagneticactivity. The latter is dominated by an annual component with aspring--autumn reversal, whereas the activity-matched hemisphericcontrast is dominated by a semiannual component with fitted maximanear May and November. Thus, the hemispheric dependence is not simplya scaled representation of the polarity-dependent seasonal modulationof total geomagnetic activity. Reconstructed IMF sector polarity shows that the principal seasonalfeatures of the activity-matched hemispheric contrast were alreadypresent during 1868--1964 and recur in the satellite-era reconstruction.The results suggest that IMF \((B_y)\) provides a persistent organizationof the hemispheric distribution of geomagnetic activity, superposed onthe better-known polarity-dependent modulation of overall geomagneticactivity.