12 February


Solar wind parameters, as observed at the ACE spacecraft, reflected what
appears to be the recovery from a coronal hole high speed stream (CH
HSS) transitioning to the influence of weak transient. The beginning of
the period saw solar wind speeds steadily dropping from approximately
443 km/s to a minimum of 396 km/s. At the same time, IMF total filed
strength was fairly steady between 4 nT and 6 nT, IMF Bz was positive,
and Phi angle was in a negative (towards) sector. Around 11/1800 UTC, a
solar sector boundary change (SSBC) was observed, transitioning Phi
angle to a positive (away) orientation, dropping Bz to approximately -5
nT, and a slight increase in solar wind speed, back to approximately 440
km/s. Right around 11/2300 UTC, Phi angle again switched back to a
negative (towards) orientation, IMF Bt increased to approximately 6 nT,
Bz switched back to positive, and wind speeds began a steady climb. By
12/0330 UTC, wind speeds were back above 460 km/s, Bt was a steady 5 nT,
Bz dropped back southward to -3 nT, and Phi was firmly back into a
negative (towards) orientation. This is believed to be the influence of
a weak transient passing close enough to affect the Earth’s
magnetosphere.

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