To: sisafe@head-cfa.harvard.edu Subject: proton alert levels Date: Tue, 26 Oct 1999 14:09:06 -0400 From: Rob Cameron I attach plots showing a year of ACE data. The first plot shows one-hour averaged electron and proton fluxes over the past year. The red datasets cover the recommended energy ranges. Mark Bautz' suggested electron flux limit (100 e/cm^2-s-sr-MeV) and proton flux limit (10^5 p/cm^2-s-sr-MeV) (each for one hour) are shown on the plots, together with Steve O'Dell's proton flux warning level of 3.10^4 p/cm^2-s-sr-MeV. As Shanil has noted, Mark's electron flux limit is exceeded many times in the past year. Mark's and Steve's proton flux limits are each exceeded about 10 times in the past year. The second plot shows proton fluence measures over the past year. Fluence has been calculated over timescales ranging from 1 hour to 256 hours (= 10.67 days). Steve recommended a fluence limit of 10^10 p/cm^2-sr-MeV per "event". I have converted Mark's one-hour proton flux limit to a fluence limit of 3.6.10^8 p/cm^2-sr-MeV. It can be seen that Mark's fluence limit is exceeded several times, on several different timescales, while Steve's limit is exceeded only once or twice. Comments: 1. I recommend against safing ACIS in response to ACE electrons at Mark's suggested level. There would be many, long interruptions to ACIS science - without convincing evidence that electrons damage ACIS. 2. Can we converge to a single proton flux limit and a single proton fluence limit? The recommended flux limits differ by a factor of 3, and (my inferred) fluence limits differ by a factor of 30. 3. Keeping in mind that there would have been 10 ACIS shutdowns in the past year, off the solar maximum; shutdowns of ACIS observing will keep the SOT and FOT quite busy in maintaining a flow of Chandra science observations as solar max approaches. - Rob ------------- To: sisafe Subject: re: proton alert levels Date: Thu, 28 Oct 1999 14:37:16 -0400 From: Shanil Virani Hi, for those who haven't yet figured out how to decomm Steve's last email yet (!), below was his text. The other two components were the ACE proton fluence pltos over various time intervals produced by Rob earlier. Shanil Rob - I have a somewhat revised recommendation, based upon more precise (not necesarily more accurate) values for the AP8MAX external proton spectral fluence, adjusted to the ACE EPAM P3 band centered near 0.17 MeV. AP8MAX spectral fluence per orbit Ik1 = 3.6*10^11 p / (cm^2 sr MeV) @ 0.17 MeV CTI degradation per exposed orbit dCTI1 = 2.5*10^-5 CTI sensitivity factor dCTI/dIk = 6.9*10^-17 (cm^2 sr MeV) / p @ 0.17 MeV If we want to limit further CTI degradation to < 1*10^-4, then the total 10-y-mission proton spectral fluence must satisfy Total mission spectral fluence IkM < 1.5*10^12 p / (cm^2 sr MeV) @ 0.17 MeV. Note that this is lower than my previous estimate because of the different spectral band and some sloppyness in converting from integrated omnidirectional flux to spectral intensity. The challenge is to come up with a way to allocate this allowed exposure so that we do not safe the ACIS more often than necessary to avoid significant further degradation. Because solar-proton events are so episodic, we probably need to manage the proton-fluence allocation by fluence per event, which we may have to update throughout the mission. To complicate this, I note that AP8MAX is probably an upper bound to the proton exposure, which means it's a lower bound to our sensitivity. We really need to use a better model --- such as the Magnetospheric Specification Model --- to give a better estimator of the 100-keV proton fluence during the radiation-belt passes when ACIS was exposed. The OFLS also should be using the MSM. In conclusion, we need to be conservative to start, because we may have underestimated the CTI sensitivity, by using AP8MAX. Thus I would propose an initial fluence threshold of 5*10^9 p / (cm^2 sr MeV) @ 0.17 MeV, using ACE EPAM P3 and P3' real-time data. If comparison of MSM and AP8MAX predictions do not differ substantially, we could raise this fluence threshold back up to 1*10^10 p / (cm^2 sr MeV) @ 0.17 MeV, or perhaps higher as we gain experience and confidence. - Steve ------------------------------------------------------------------------ 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 ------------------------------------------------------------------------