[Artist's Concept of AXAF]

CXO Project Science:
ACE/EPAM - Chandra/EPHIN Correlations


Energy Channels & Data Timelines

EPHIN and ACE/EPAM do not sample identical energy bands. The closest overlap is between EPHIN SCP4GM (4.3-7.8 MeV) and EPAM LEMS30 P8 (1.9-4.75 MeV). Ideally, one would like to use EPHIN data to help signal potential high fluxes of protons at around 100 keV. Though EPHIN does not have a suitable energy band, the EPAM LEMS30 P3 (0.112-0.187 MeV) channel is ideal. Thus, if a correlation can be made between the EPHIN and EPAM channels and a correlation can be made between EPAM P3 and P8 then the potential for EPHIN to predict low energy protons is improved. Here is the data available (many thanks to the Sources):

ACE/EPAM LEMS30 ACE/ASC p3 0.112-0.187 MeV 970814 - 990108
ACE/EPAM LEMS30 ACE/ASC p8 1.91-4.75 MeV 970814 - 990108
ACE/EPAM LEMS30 ACE Real-Time Solar Wind p3 0.112-0.187 MeV 970814 - current
ACE/EPAM LEMS30 Ed Hawkins III, JHU p8 1.91-4.75 MeV 990601 - 990915
Chandra/EPHIN SCP4GM Dick Edgar, CXC scp4gm 4.30-7.80 MeV 990727 - current
(NOTE: The Chandra/EPHIN SCP4GM bandwidth adopted here is from R. Mueller-Mellin, et al., Solar Physics 162,483,1995; but, according to Mueller-Mellin's "EPHIN Response to AXAF Radiation Environment, 25 Nov 97, it is 5.0 to 8.3 MeV.)

So, we can compare ACE/EPAM LEMS30 P3 to P8 over a 513 day baseline, but with no overlap to Chandra data (first two rows of the above table), and EPAM P8 and P3 to EPHIN SCP4GM over a portion of the Chandra mission, about 52 days to date. (Note: the NOAA real-time data, available at the SEC, includes P3 but does not include P8 and is not a level 2 product).


After scaling by the appropriate geometric factor (0.18 cm^2*str) and energy bandwidth and averaging the EPHIN data over 1 hour intervals to be consistent with the EPAM standard dataset (cts/cm2/s/str/MeV), the light curves look like this (red is EPAM P8 and green is EPHIN SCP4GM):

[EPAM P8 and EPHIN P4 light curve]

So there is a correlation there as expected (ignoring EPHIN data near perigee!) but this serves basically as a control since we are comparing similar energy bands. The interesting correlation, if it exists, is between EPHIN SCP4GM and EPAM P3: The point-by-point correlation, ignoring points in the radiation belts (defined here simply as those with SCP4GM greater than 10 cts/cm2/s/str/MeV) for these two datasets is:

[EPAM P3 against EPHIN P4]

which shows evidence of a trend, perhaps, but Chandra has not been in orbit long enough to obtain a good sample. On the other hand, the point-by-point correlation for EPAM P3 and EPAM P8, including all data with flux in p8.gt.1, over the period 970814-990108 is more promising, as shown below. The best fit is found by fitting a power law to the data and the approximate 1-sigma curves are estimated by fitting a normal distribution to the data (in log space) in three slices: p8 between 1.0 and 2.5, between 10 and 20, and between 50 and 100 cts/s/cm2/str/MeV and then plotting a straight line through the resulting +/- 1-sigma points. The power law fit to the data is p3 = 355.4*(p8)**0.8

[EPAM P3 against EPAM P8]

Now, since the correlation is strong between EPAM P8 and EPHIN SCP4GM and the long time-line comparison of EPAM P3 and P8 shows a correlation, it follows that EPHIN SCP4GM and EPAM P3 are, indeed correlated but we need a longer baseline to determine fit parameters with confidence. -----------------------------------------------------------------

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