Abstract
A data survey reveals multiple periodic variations in auroral hemispheric power (HP) and thermospheric composition (O/N2) in 2006. The periods include 27, 13 - 14, 9, and 6.7 days. These periods of 13 - 14, 9 and 6.7 days are essentially the harmonics of the 27-day solar rotation. Similar multi-periodicities were found in the dayside magnetic merging rate (MMR) (Newell et al. 2007) which depends upon solar wind speed (V), magnitude (BT) and clock angle (θc) of interplanetary magnetic field (IMF). A high correlation coefficient (0.93) between MMR and HP indicates MMR is the driver of the periodic variations. While high solar wind speed associated with coronal holes plays an important role in the HP variations, IMF BT is equally important. The term [BT 2/3 sin 8/3 (θc /2)] is even more important as its correlation coefficient with HP is higher than that for BT or solar wind speed. Nevertheless, MMR has the highest correlation with HP. Similar results were seen in the 2005 data where the 9-day variation is dominant. These results indicate that both solar wind speed and IMF conditions are required for accurate specification of periodic variations in aurora hemispheric power and thermosphere composition. Key word: High speed solar wind, Aurora, Thermosphere, Periodic variation
Highlights
Periodic re-occurrence of auroral and geomagnetic activities/storms during the declining phase of a solar cycle is well known (Tsurutani et al 2006 and references therein)
The correlation coefficient between MMR and hemispheric power (HP) coefficient is 0.93. All these data suggest that the multiple periodic variations observed in HP and O/N2 are driven by the magnetic merging rate
The 13 - 14, 9 and 6.7 day variations are the harmonics of the 27 day solar rotation
Summary
Periodic re-occurrence of auroral and geomagnetic activities/storms during the declining phase of a solar cycle is well known (Tsurutani et al 2006 and references therein). The strong 9-day variations in 2005 have attracted much attention recently and have been seen in thermospheric neutral density, composition (O/N2 column density ratio) and auroral hemispheric power (Crowley et al 2008; Lei et al 2008; Thayer et al 2008; Zhang et al 2010). The high speed streams increase the magnetic field merging rate on the dayside magnetopause (Newell et al 2007) and enhance the energy input into the magnetosphere. This results in enhanced auroral particle precipitation, heating in the polar ionosphere and thermosphere. Thermospheric O/N2 column density ratios based on GUVI data (Zhang et al 2004) are examined for their response to the auroral variations
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