CME Zeiten (von Cary Oler) (sehr lang)

[Ulrich Rieth (z.Z. Würzburg)]

Hallo !

Dieser Beitrag dürfte für alle "Spekulanten" unter uns interessant sein.
Cary Oler hat ein paar Antworten auf die Frage nach der Beschleunigung / Abbremsung von CME´s gegeben.
Ich kopiere den langen Text mal hier rein.
Gruß

Ulrich

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Name: Cary
Date: Mon, 24 Sep 2001 20:51:21 -0600
Subject: (Jan) Re: questions on shock front (part 1 of 2?)

Some good questions, Jan. I'll try to answer them.

1. High velocity CMEs may experience somewhat stronger deceleration than other smaller amplitude shock fronts. It depends on
solar wind conditions ahead of the shock front. For example, if a shock front encounters an area where the plasma is rarefied, the
shock front can actually accelerate rather than decelerate. This is why, in some of the CME analyses you read, you'll notice they
sometimes say a CME accelerated through the field of view, or decelerated. It depends on conditions ahead of the shock front.

2. We estimate there may be an error bar in our prediction of the arrival time by as much as +/- 10 hours, centered on about 18:00 UTC
on 25 Sept. We expect the CME will arrive later rather than earlier. But there have been instances where disturbances have crossed
the distance from the Sun to the Earth very rapidly indeed (as I'll elaborate on below). Our models did not converge as much as we
would have liked in assessing this particular CME. Hence the rather large error bar.

3. A CME with this velocity will tend to compress the solar wind ahead of the main plasma cloud. I would be very surprised if we don't
see fairly considerable RAM pressure (magnetospheric pressure exerted by the disturbance on the Earth's magnetosphere). We're fully
expecting geosynchronous spacecraft to experience potentially several hours outside of the magnetosphere - within the zone known as
the magnetosheath.

4. As for "funny" or "unexpected" happenings such as a storm occuring before the shock arrives - no, we will see the arrival of the
shock front before a storm develops. Irregular shaped shock fronts do occur and have been modelled. For example, a shock front that
propagates along the heliospheric current sheet can cause a small cavity or dip to develop along the HCS where the CME has been
slowed more than elsewhere.

5. See the next message....

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Name: Cary
Date: Mon, 24 Sep 2001 20:27:48 -0600
Subject: Our Estimated time of arrival of the CME

An analysis of available data suggests the CME from todays major flare may arrive centered in time around 18:00 UTC on 25 September
(which translates to 2 pm EDT on 25 September), give or take several hours.

The disturbance should last at least 24 to 36 hours, which should provide most of the world's middle latitude zones a chance to witness
activity.

Name: Cary
Date: Mon, 24 Sep 2001 21:08:34 -0600
Subject: (Jan): Re: questions on shock front (part 2 of 3)

5. There are a few historical CMEs that impacted the Earth with high velocities. Here are a few of the more notable ones, known as
Fast Transit Events:

1. 01 Sep 1859 (if anyone can identify the significance of this date, you deserve a tap on the back - I'll explain at the bottom).
Location: N20 W12, Size of Region: 2300, Time to impact: 17.5 hours

2. 16 Jan 1938. Location: probably N17 E31. Associated with proton event. Region area: est. more than 2500, produced a mag storm
with an Ap of 130 (ranked 80th most powerful mag storm). Time from flare to impact at Earth: 21.8 hours.

3. 15 Apr 1938 (N28 W12). Proton event: yes, Spot area: 1098. Ap=136. Rank: 70. Time to impact: 21.2 hours.

4. 28 Feb 1941 (N12 W14). Proton event: yes. Spot area: 683. Ap=212. Ranked: 18. Time to impact: 18.4 hours.

5. 17 Sep 1941 (N07 W09). Proton event: yes. Spot area: 1896. Ap=312. Ranked: 1. Time to impact: 19.6 hours. (This is still the most
powerful geomagnetic storm in recorded history).

6. 28 Feb 1942 (N07 E05). Proton event: yes (ground level event - GLE). Spot area: 1865. Ap=136. Rank: 71. Time to impact: 20.5
hours.

7. 06 Feb 1946 (N27 W19). Proton event: yes. Spot area: 4799. Ap=199. Rank: 22. Time to impact: 17.8 hours.

8. 16 Jul 1959 (N16 W31). Proton event: GLE. Spot area: 1430. Ap=183. Rank: 29. Time to impact: 19.5 hours.

9. 12 Nov 1960 (N27 W04). Proton event: GLE. Spot area: 1740. Ap=Unknown. Rank: Unknown. Time to impact: 21.2 hours.

10. 04 Aug 1972 (N14 E08). Proton event: GLE. Spot area: 1140. Ap=223. Rank: 12. Time to impact: 14.6 hours.

11. 19 Oct 1989 (S26 E10). Proton event: GLE. Spot area: 1080. Ap=162. Rank: 40. Time to impact: 20.8 hours.

That should suffice. The list was produced by E.W. Cliver a few years ago while at the Geophysics Laboratory at Hanscom AFB.

If you haven't figured out why the first one in the list above is significant, read on...

Name: Cary
Date: Mon, 24 Sep 2001 21:21:10 -0600
Subject: (Jan): Re: questions on shockfront (part 3 of 3)

The reason why the first item (dated 01 Sep 1859) is significant is because, as rare as fast transit events are, this particular event
happened to be the very first solar flare ever detected. This is the famous date that Richard Carrington detected a white-light flare and
successfully reasoned that the cause of the great geomagnetic storm 17.5 hours later was caused by what he then saw on the sun.

You will notice the seemingly abnormally short delay of 14.6 hours for the arrival of the CME associated with the flare activity of 04
August 1972 (item #10 in the list I presented). There was considerable debate about that particular event, which - if you work it out,
suggests the CME had an average transit velocity of 2850 km/sec. Some have argued that the disturbance was caused by an event
that occurred two days earlier (on 02 August 1972) which would imply an average CME velocity of 850 km/sec (much more
reasonable). And still others claimed that the geomagnetic storm was produced by a coronal hole (not considered that probable now).
Many of the items I've listed have been questioned by researchers as to their authenticity. That is, it is possible many of the events
might have been produced by flare activity that occurred hours earlier than the one they have been officially associated with - resulting
in inappropriately high transit velocities. But several of the events do appear to indicate that on rare occasions, the sun has produced
very fast moving ejections of mass.

By the way, a CME having an average transit velocity of 2000 km/sec would impact the Earth in approximately 20 hours (you can use
this to help digest the table of events in the last message).

I hope this answers your questions sufficiently, Jan.