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Why was it a CME impact and not a CH HSS?

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We take a look one more time at the Coronal Mass Ejection impact from yesterday (2013/06/27) at 14:00 UTC. We have received a question why we thought it was a Coronal Mass Ejection impact and not the start of the Coronal Hole stream. With this picture we hope to explain why it was a Coronal Mass Ejection.

We begin with the graphic in the upper left corner. The EPAM plot. EPAM stands for the Electron, Proton and Alpha Monitor and is an instrument on the ACE satellite that measures the electrons and protons that are send out with the solar wind. It's a very useful instrument to know if the CME is earth directed and when it's going to arrive. When a CME arrives we see a sudden rise on the monitor, a spike (the impact) and a drop again. As you see, this was clearly the case yesterday.

Now look at the graphic in the upper right corner. This shows everything we need to know about the solar wind and the Interplanetary Magnetic Field (IMF). Coronal Mass Ejections are characterized by sudden jumps in solar wind speed, density, temperature and the Bt. Often the Bz will also take a swing northwards or southwards. Did this happen yesterday? Also yes.

Now the bottom graph: this is an example from last April and shows 7 days worth of ACE data. It shows the solar wind data as a Coronal Hole solar wind stream flows passed earth. As we clearly see, coronal holes do not have these sudden jumps in the solar wind values like Coronal Mass Ejections do. The values rise and fall very smoothly. A very clear signature for Coronal Hole wind stream arrivals are first a rise in density with speed following later with density dropping.

Hopefully this clears things up a bit on how CME impacts and Coronal Hole stream arrivals look like on data from ACE.

If you have more questions please send post them in this forum and we will be more then happy to answer them. We have a dedicated team of spaceweather experts that will gladly answer any questions you have. More spaceweather related help articles can be found on this page.

CME: Coronal Mass Ejection

CH HSS: Coronal Hole High Speed Stream.

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