To: mwb@space.mit.edu Subject: EPHIN Analysis During Day 280-290 cc: plucinsk, das, ybutt, rac, ht, rjb, martin@smoker.msfc.nasa.gov, odell@cosmos.msfc.nasa.gov, garmire@astro.psu.edu Date: Thu, 04 Nov 1999 21:12:36 -0500 From: Shanil Virani Hi Mark, Firstly, Mark, I apologise for not getting these plots to you earlier; it turned out to be a little bit more involved and I wanted to verify that my plots were correct. At any rate, at http://asc.harvard.edu/acis/radbelt/ you will find several new ps plots that I've made that you will want to print out before proceeding further (they're all quite small). They are: eph-flux-d280tod290.ps p4vsalt-day280-290.ps p4_3d.ps I will start with the last plot first. p4_3d.ps is a 3D plot of DOY, geocentric radius (in km), and the P4 counting rate. It is a neat picture but a little difficult to discern P4 altitude information from this plot. (Recall P4 is the proton channel with 5 MeV < E < 8.3 MeV.) The second file has more utility; it is a simple 2D plot of P4 counts/int_time (integration time=65.6s) as a function of geocentric radius. The vertical line is the 60,000 km barrier. As this plot clearly demonstrates, it is NOT sufficient to simply define rad belt transit duration in terms of a single altitude. From these plots, you can see that the entrance p4 counting rate increases at roughly 40-50,000km, whereas the P4 counting rate on the exit leg doesn't drop to the background level until roughly 70-80,000km. So the last two plots provide you with the EPHIN P4 data as function of altitude during the day 280-290 period that you requested at the ACIS ops meeting this morning. Now, the first file, eph-flux-d280tod290.ps, I think will shed some light on when the recent CTI change occurred and why it did. The first page of plots you should focus on are the P4 plots. Firstly, let me define the horizontal bars. The first one (ie the lowest one) represents the SIM translation time span. So the start represents the moment we translated the SIM to the HRC-S position -- during perigee crossing, ACIS was in the NIL position of course. The horizontal bar above it represents the rad belt duration as predicted by the OFLS model. What this plot illustrates is that of the 4 perigee crossings plotted, there was *one* crossing where ACIS was NOT protected -- perigee crossing on DAY 287. Now, if you look at the lowest energy electron plots (the E150 plots), you will see that this mis-timing was even worse from the low energy electron perspective. In fact, the SCE150 plots suggest we also mis-timed the belts on Day 284. However, what is important is that the E150 flux is roughly 3 orders of magnitude greater on Day 287 than on Day 284. So, if one believes that low energy protons were responsible for the small ACIS CTI degradation that was observed during this timeframe, then this analysis suggests that we may have a "smoking gun" as it were. In fact, since we do CTI measurements before and after each belt passage, if I am right then we should see the ACIS CTI change on Day 287 during the measurements that preceded the belt passage on that day. Of course, the one question that comes to mind is that the low energy protons implicated in the ACIS CTI degradation are ~100 kev and the P4 channel are protons of considerably higher energies. However, an argument has been suggested that the low energy electron channel, SCE 150, traces the ~100 kev protons. If that is the case, then as the SCE150 plots demonstrate, my conclusion doesn't change -- the ACIS CTI change occurred on Day 287 as a result of not properly protecting ACIS because of a mis-timing of the radiation belts. Question is, do you buy it?! The other observation you want to note in the SCE150 plots and consider is that you can clearly see the effect of the CME once again. Although the P4 channel does not show it, there were protons produced in that CME but the increase in flux was very small. Analysis I've done with the ACE data, as well as Rob's analysis, shows that the CME in all likelihood was not the cause of the ACIS CTI degradation -- it is a red herring. At any rate, those were the plots I made and the conclusions I drew. Perhaps your interpretation will be entirely different. Lastly, this analysis has implications for SI-safing as well. As I understand it, the E1300 and P4 thresholds I provided (10 and 10^3 counts/s/cm**2/sr, respectively) were designed such that if the daily load for some reason does not translate the SIM to put ACIS in the NIL position before rad belt entry, the spacecraft will do so if these limits are exceeded. These plots then also demonstrate that if the above scenario were to have taken place during perigee crossing on Day 287, we would have damaged the instrument (if my analysis above is correct of course!!). What this means is that I should look at my threshold analysis again and see if I can lower the P4 threshold some more. The above thresholds were chosen such that Chandra would have never safed the SIs outside of the radbelts (with an 8ks pad on each side as well). Perhaps I can lower it a little more. At any rate, I just bring that up as a topic of discussion for tomorrow's SI telecon and as something for everyone to consider. At any rate, I apologise for getting this analysis to you a little later than I anticipated, as well as for this lengthy email, but I hope it is of use. I would be interested in hearing your's or anyone else's comments regarding the above. 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 ------------------------------------------------------------------------