To: plucinsk, svirani, rac, mwb@space.mit.edu, ybutt, martin@smoker.msfc.nasa.gov, ht@head-cfa.harvard.edu, das@cfa.harvard.edu, "Kolodziejczak, Jeffery" , "O'Dell, Steve" cc: mkarovska Subject: Re: ACE levels to watch Date: Sun, 24 Oct 1999 21:09:22 -0400 From: Shanil Virani Hi all, > Chandra personnel should continuously monitor two ACE channels > ACE Channel Energy "Danger" Level > EPAM Electrons 175 keV - 315 kev 100 electrons > /cm**2/s/ster/MeV > EPAM Protons 112 keV - 187 keV 1e5 protons/cm**2/s/ster/MeV > > If either channel exceeds its danger level for a period of more than > about 1 hour, then Chandra should consider taking action to protect ACIS. Just out of pure curiousity, I retrieved several ACE EPAM files yesterday that corresponded to the CME time-frame (ie, Day 287 - Day 293) just to see what ACE saw. To that end, I've made two postscript files that can be found in /home/svirani/CME on the HEAD LAN or at http://asc.harvard.edu/acis/radbelt/. "ace-electrons.ps" is a plot of the EPAM ELECTRON channel (175 keV < E < 315 keV) vs time; "ace-protons.ps" is a similar plot for the EPAM proton channel with 130 keV < E < 214 keV. Note that in the ACE EPAM files, there is NO proton data with 112 keV < E < 187 keV (although you can get a real-time plot for that energy range). Before proceeding further, another ps file you may want to print out is the "ephin-cme-cut.ps" file available at the above web site ("ephin-cme.ps" has a ymax of 10^5). Also plotted in the "ace-electrons.ps" file, is the provisional "danger" level specified by Mark (the horizontal line at 100). Looking at the ACE electron plot, it is clear that even though we did not reach the level and duration specified in Mark's email, we did flirt with those levels. Another point to note is that the peak electron value in the ACE data occurs at roughly the same time as the peak electron value (for the CME) in the EPHIN SCE150 channel (250 keV < E < 700 keV), as one would intuitively expect. The 'breaks' in the data are probably a combination of telemetry drop-outs and loss of ground-tracking. Based on these observations then, one could interpret these results in (at least) two ways. If this "danger" level is sufficient, then we ought to look at the back-history of ACE data to see if there was (were) any instance(s) where the "danger" conditions would have been satisfied and we would have safed the SIs as a result. At gopher://sec.noaa.gov:70/11/lists/ace/, the earliest data file they have posted is Sept 25; so they have roughly a month's data available and that's it. I will download the remaining files and redo my ACE plots. Now, if the increase in electron flux due to the CME is not significant, then the 'danger' level should clearly be set higher. I don't know which interpretation is correct but it obviously needs to be discussed. The "ace-protons.ps" plot doesn't show anything terribly exciting. The proton flux is about 3 orders of magnitude lower than the "danger" level specified by Mark. However, the rise in proton flux occurs well after the rise in electron flux (~ 3 days). Perhaps solar protons have a longer travelling time than electrons? At any rate, the proton flux is not "critical" by any means given its current constraints. However, since we are OR'ing the electron and proton danger levels, we need to investigate this electron level question further so that we are NOT safing the SIs needlessly. The last suggestion I wanted to put out to the group, and I know this will probably meet with a lot of opposition, is that we *could* use this ACE CME data to cross-calibrate the EPHIN E150 data to establish _upper_ threshold limits for this channel. The reason why there is utility in doing this is so that Chandra is protected in those instances where ACE does not have data and is in a high radiation environment; we should not be dependent solely on ACE to protect our observatory. Of course, this "upper limit" depends on which of the above interpretations is correct. The thresholds for the SCE1300 and SCP4GM channel I've been working on are radiation belt thresholds. The thresholds I am talking about now would apply *between* radiation belt transits and work in conjunction with ACE monitoring. Anyway, it is just a suggestion and we can discuss the merit of it. Regardless, have a look at these plots and read this email again (as well as Steve O'dell's) and let's discuss some more. Cheers, Shanil ------------------------------------------------------------------------ Shanil N. Virani svirani@head-cfa.harvard.edu Chandra X-ray Observatory Center Harvard-Smithsonian Center for Astrophysics http://hea-www.harvard.edu/~svirani 60 Garden Street, MS-70 FAX: 617-495-7356 Cambridge, MA 02138 USA PHONE: 617-496-7855 "It is not in the stars to hold our destiny but in ourselves." -- William Shakespeare ------------------------------------------------------------------------