High Speed Stream Properties and Related Geomagnetic Activity During the Whole Heliosphere Interval (WHI): 20 March to 16 April 2008

Abstract

We study the interplanetary features and concomitant geomagnetic activity of the two high-speed streams (HSSs) selected by the Whole Heliosphere Interval (WHI) campaign participants: 20 March to 16 April 2008 in Carrington rotation (CR) 2068. This interval was chosen to perform a comprehensive study of HSSs and their geoeffectiveness during this “deep” solar minimum. The two HSSs within the interval were characterized by fast solar-wind speeds (peak values > 600 km s−1) containing large-amplitude Alfvenic fluctuations, as is typical of HSSs during normal solar minima. However, the interplanetary magnetic field (IMF) magnitude [Bo] was exceptionally low (≈3 – 5 nT) during these HSSs, leading to lower than usual IMF Bz values. The first HSS (HSS1) had favorable IMF polarity for geomagnetic activity (negative during northern Spring). The average AE and Dst for the HSS1 proper (HSS1P) were + 258 nT and − 21 nT, respectively. The second HSS (HSS2) had a positive sector IMF polarity, one that is less favorable for geomagnetic activity. The AE and Dst index averages were + 188 nT and − 7 nT, both lower than corresponding numbers for the first event, as expected. The HSS1P geomagnetic activity is comparable to, and the HSS2P geomagnetic activity lower than, corresponding observations for the previous minimum (1996). Both events’ geomagnetic activities are lower than HSS events previously studied in the declining phase (in 2003). In general, Vsw was faster for the HSSs in 2008 compared to 1996. The southward IMF Bz was lower in the former. The product of these two parameters [Vsw and IMF Bz] comprises the solar-wind electric field, which is most directly associated with the energy input into the magnetosphere during the HSS intervals. Thus the combined effects led to the solar wind energy input in 2008 being slightly less than that in 1996. A detailed analysis of magnetic-field variances and Alfvenicity is performed to explore the characteristics of Alfven waves (a central element in the geoeffectiveness of HSSs) during the WHI. The Bz variances in the proto-CIR (PCIR) were ≈ 30 nT2 and < 10 nT2 in the high speed streams proper.