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Stereo Ahead CME passage 2013/07/25

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Some interesting ''belated'' news from the other side of the sun. Do you remember this farside Coronal Mass Ejection from July 22th?


The STEREO Ahead spacecraft was directly in the line of fire and the Coronal Mass Ejection arrived there around 06:00 UTC on July 25th.

Normally not very interesting, a Coronal Mass Ejection (CME) passage that doesn't affect earth I hear you think. True, but this Coronal Mass Ejection caused some very interesting and impressing readings at the spacecraft. The STEREO spacecrafts have the same kind of instruments as the ACE spacecraft near earth and thus we can see the solarwind and IMF parameters near STEREO Ahead.

Now to the data: solar wind speeds jumped to over 500km/s. This is not fast at all but it is the IMF that showed some mind blowing values. At the height of the storm, the Bt reached values of around 40nT and also the Bz went to -40nT. If this Coronal Mass Ejection was earth directed, we could have seen G4 (Kp8) geomagnetic storming and widespread auroras even for the low latitudes.

Source of the blast was Active Region 1794. This region is still on the farside and is about 5 days away from rotating onto the earth-facing solar disk. Otherwise, solar activity remains quiet on the earth-facing side of the sun. None of the regions currently on the disk are capable of producing a strong solar flare.

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Old Active Region 1794 is now rotating back on the visible solar disk. This region launched on July 22th, a Coronal Mass Ejection towards STEREO Ahead which caused very impressive IMF readings at the spacecraft's location. At the height of the storm, the Bt reached a value of more then 40nT and the Bz went down all the way to -40nT. Those are some very impressive stats and would have caused significant geomagnetic storming if that Coronal Mass Ejection was earth-directed. This region has received number 1809 today. In the coming days, 1809 will rotate more into earth's view and we will see what is left of this region and judge it's flaring potential.

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