Subject:
RSGA Reports & Solar-Event-List
Resent-From:
sec@noaa.gov
Date:
Wed, 10 Oct 2001 18:44:47 +0200
From:
"Lutz Schenk" lutz@lutz-schenk.de>
To:
"Viola. J. Raben" sec@sec.noaa.gov>
Hello Miss Raben,
I'am a hobby-astronomer and very interested in solar data, to get a glance of Aurora's.
Since a few weeks, your measurments of the radio-sweep-speeds seems to
be a little low in my opinion. The Shock-Speed at arrival at the
ACE-Satelite an the Mean-Traveltime of the CME's didn't correspond to your data.
Also, they didn't correspond to the "plane-of-sky" Speed, calculated by the Lasco-Team. Even other teams have other data. Is there something wrong in your calculation, or is there an fault in my
interpretation? May be, you have changed your equipment or your
calculation? I would please you, to give me some explanation.
(Sorry for bad english)
Regards,
Lutz Schenk
Und hier die Antwort:
Dear Mr. Schenk,
Your interesting letter has been referred to me. I am pleased to
give you a reply and encourage you to follow up these things in some of
the journals such as J. Geophysical Research, Solar Physics, and Space
Science Reviews. You will find papers by my colleagues and me as well
as by other people, of course. Now, to your letter.
Your calculations are not wrong. But you must (in the real time
operational context of these comments) separate the radio sweep
observations from the white light coronagraph observations. The former provide estimates of the shock speed....or, more exactly, some part (unknown) of the 3D shock. The latter provides (as you properly noted) the speed of one part of the coronal mass ejection (CME) that generally is behind the shock. As a caveat on this last remark, it is believed by some people (including me) that the leading edge of very fast CMEs that are currently observed only the plane-of-sky might be the shock itself....at least the part of it seen in the p-o-s.
We do not use the CME speed to forecast shock arrival at L1 or
Earth despite the publicity hype from some quarters. The published
efforts to do this (not in real time, by the way) are empirical methods that have very little physical support.....namely the use of the basic kinematic equation for a single particle that is in all basic physics courses...that quickly gets "adjusted" by many parameters including some guess as to deceleration actually takes place. In my opinion, these CME speeds will, some day, get combined with the radio data.
The radio data refer only to the shock and, at present, relies on a
very simple coronal density model to provide the shock speed estimate.
The model is based on a "quiet sun", spherically-symmetrnic model that is multiplied by an arbitrary factor. Our Air Force field sites use one factor; our Austrialian friends use another factor. Thus, these simplifying assumptions render the reported shock speeds to an
uncertainty of, perhaps, +/- 500 km/sec.....and, as hinted above, we do not know which part of the shock the radio emission is coming from.
Thus we have to assume that (1) the coronal density model is reasonable, and (2) the estimated speed represents the speed of the shock (while it is coming out of the low corona) at its fastest part....and that this part is, we assume, radially aligned with the flare site and the Sun's center. We then use physically-based models (three, in fact) to make our Shock Time of Arrival (SAT) predictions. We do not want to make any changes in shock speed estimates or anything else until we have compiled a good set of statistics with all of the present equipment and computer algorithms. We have published some statistics already in J. Atm. Solar-Terr. Physics (Z. Smith et al., vol. 62, page1265, 2000) and, just recently, in J. Geophys. Research (G. Fry et al., vol. 106, p. 20,985, 2001).
We have also advocated development of a 3D coronal density model
that is a function of time, so that the density profile above any
potential flare site can be used to make the shock speed estimates.
We have also found, for a limited set of statistics, that the reported (from the sites) shock speeds that are quite high (say, >1000 km/sec) should be increased by about 30%; action for the lower speeds is not clear.
Thus, the picture that emerges is not completely in focus for
us....from either an observational point of view or from a modeling
point of view. Some of us, under a DoD contract, are distributing an
informal (hence, unoffical) prediction of SAT's that uses the radio data (with the present speed approach) as well as the LASCO data....the latter only for complementary use along the lines mentioned above.
Please let me know if you would like to be added to our distribution
list.
I hope that these limited comments will help you to some degree.
Best Regards,
Murray Dryer, PhD
Guest Worker Emeritus
NOAA Space Environment
Boulder, CO 80305
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Gruß Lutz