The POES SEM-2 MEPED detectors are intended to measure the flux of both energetic protons and electrons separately with two different sets of telescope detectors. However, numerical simulations as well as experimental tests of the instrument performed by placing the detectors in particle beams show that the instrument does not always correctly separate the two species. Some protons entering the electron telescopes are mistakenly counted as electrons and vice versa. Here we focus on correcting the electron flux measurements by subtracting the contaminating proton flux. Fortunately, a correction is at least possible because the SEM-2 proton telescopes measure the flux of protons within the energy range that will also contaminate the electron telescopes. However, the energy resolution of the proton detectors is coarse making an accurate correction challenging. The goal of the method described here is to define the proton energy spectrum and thus the electron correction as accurately as possible with the limited information available.
 Simulations and tests show that the POES measurements of >30 keV, >100 keV, and >300 keV electrons are contaminated by 210–2700 keV protons, 280–2700 keV protons, and 440–2700 keV protons, respectively. Three of the six energy channels sampled by the MEPED proton telescope span these ranges. The p2, p3 and p4 channels measure the flux of protons with energies from 80–240 keV, 240–800 keV, 800–2500 keV, respectively. Unfortunately, these channels do not perfectly bracket the contaminating proton ranges so the correction is not just a simple subtraction of these counts. For example, the >30 keV electron channel is contaminated by protons with energy above 210 keV but the p2 channel begins measuring protons at 80 keV. The counts measured by the p2 channel from only those protons with energy >210 keV will depend heavily on how the flux varies with energy within the 80–240 keV range. Likewise, only some fraction of the counts measured in the p3 channel will contribute to the contamination of the >100 and >300 keV electron measurements. Since the energy spectrum within a given energy channel is not known, it must be assumed.
 We estimate the proton energy spectrum by assuming that it is a series of piecewise exponential functions across each measured proton energy channel range. We define the initial exponential functions using a bow tie method and iterate to improve our knowledge of that spectrum. The bow tie method is defined by Selesnick and Blake  and repeated here for completeness.
 The total counts measured within an energy band can be defined by
where C is the number of counts recorded by the detector during the integration time interval (1 s for the MEPED detectors), J(E, Ω, A, t) is the differential flux (in cm−2 s−1 sr−1 keV−1) of particles entering the detector, E is the energy of the particle, Ω is solid angle, A is the silicon detector area, and t is time. Assuming that the flux hitting the detector does not vary with the angle of incidence area, or time then integrating gives
where G is the geometric factor. G for the MEPED instrument is .01 cm−2 s−1 str−1. Now assume that the flux decreases exponentially with energy such that,
 Approximate the integral as the rectangular area with height J() and width ΔE such that,
 Using the approximation from (9), the counts measured within a given instrument energy band are converted to differential flux by
 To improve our knowledge of the proton flux and energy spectrum we iterate. We calculate new values for E0 using neighboring flux values. Keeping fixed, we use equation (A6) and the new values of E0 to calculate ΔE for each energy channel. We use the new value of ΔE in equation (A5) to define new flux values and repeat the process until the values of E0 do not change significantly. Lastly, using the piecewise exponential description of the proton energy spectrum we integrate the proton flux from 210–2700 keV, 280–2700 keV, and 440–2700 keV and subtract from the >30, >100 keV and >300 keV electron flux, respectively.
 As a final check, Figure 8 shows the integrated precipitating proton flux in the p2 and p3 channels for three levels of geomagnetic activity corresponding to Figure 1. We note that the flux has a different MLT distribution to the electron precipitation shown in Figure 1 and even, for active conditions, the peak average proton precipitation flux is up to 2 orders of magnitude less than the peak average electron precipitation flux. We therefore conclude that after subtracting the proton contamination the error introduced into the electron flux in Figure 1 is small.
Figure 8. The average flux of precipitating protons for (left) 80–240 keV and (right) 240–800 keV for (a and d) quiet (AE < 100 nT), (b and e) moderate (100 < AE < 300 nT), and (c and f) active (AE > 300 nT) conditions.
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