Forbush Decrease

This is a great place to post questions and answers about the Sun.
Post Reply
Phobos
Posts: 34
Joined: Thu Oct 19, 2017 2:54 pm

Forbush Decrease

Post by Phobos » Fri Mar 16, 2018 11:11 am

I've been reading about the 'Forbush Decrease', which occurs before and during a CME. I just wondered if this phenomenon could, (or is being) be used to predict the occurrence of a large CME?

Again, thanks for all your useful info. :-)

User avatar
lisa
Site Admin
Posts: 28
Joined: Tue Oct 10, 2017 6:28 pm

Re: Forbush Decrease

Post by lisa » Fri Mar 23, 2018 5:02 pm

Hi Phobos,

Unfortunately, I've not heard of this phenomena. However, I'll check with some of my colleagues and look into it to see if I can get you an answer.

Cheers,
-Lisa

Phobos
Posts: 34
Joined: Thu Oct 19, 2017 2:54 pm

Re: Forbush Decrease

Post by Phobos » Sat Mar 24, 2018 10:05 pm

No problem, LIsa.

User avatar
lisa
Site Admin
Posts: 28
Joined: Tue Oct 10, 2017 6:28 pm

Re: Forbush Decrease

Post by lisa » Tue Mar 27, 2018 4:25 pm

Hi again Phobos!

I spoke to one of my colleagues, Joan Burkpile, who tells me that the 'Forebush Decrease' refers to a drop in the cosmic ray background when a CME passes the Earth. She went on to tell me that:
This is now known to occur during the passage of a shock. The decreases do not provide any kind of long term forecast. The shock arrival is sort of a 'nowcast' for a storm. There can be strong southward fields at and just behind the shock. The CME (i.e. the driver gas) is usually hours behind. It varies but rough numbers are ~3-12 hours. The storm can begin with the shock arrival especially if there is southward Bz at and behind the shock. If the CME contains southward Bz you'll get an additional 'punch' when it arrives.
So it looks the phenomena won't be much help as a predictor (as the CME as already left the Sun by this point), but it will certainly let you know that one is arriving. I hope this helps!

Cheers,
-Lisa

Phobos
Posts: 34
Joined: Thu Oct 19, 2017 2:54 pm

Re: Forbush Decrease

Post by Phobos » Thu Mar 29, 2018 9:22 am

Thank you for having taken the trouble to find an answer to my question!

Post Reply