Notes on the Archive Data of the Giotto Energetic Particle Experiment (EPA) during the P/Grigg-Skjellerup flyby Submitted by Patrick W. Daly on Febuary 1, 1995 Modified at SBN on November 17, 1995 Abstract This brief paper gives some information about the EPA (or EPONA) experiment and the selected data that have been put on to the ESA archive. It explains the energy levels, geometry factor, and look directions of the various sectors under the conditions prevailing during the Giotto encounter with the comet P/Grigg-Skjellup. 1 The Experiment ----------------- The EPA experiment is described in detail elsewhere (McKenna-Lawlor et al., 1986, 1987). Here only a brief description is given to explain something about the observing geometry during the Giotto flyby with comet P/Grigg-Skjellerup. 1.1 The Detectors ------------------ EPA detects energetic ionizing particles, such as electrons, protons, and heavier ions, by means of sets of solid state detectors so constructed that they effectively form telescopes pointing in distinct directions. The detection area of each telescope is 0.384 cm^2; the opening aperture is the same size, positioned 2.6 cm from the detection area. This makes a half opening angle of 15 degrees, or a reception solid angle of 0.214 sr. The geometry factor (GF) of 8.1 x 10^-2 cm^2 sr quoted by McKenna-Lawlor et al. (1986) is simply the product of the detection area and opening solid angle. A more realistic value can be derived by considering the detection area as a function of the input direction A(theta) to be integrated over all directions (theta, phi), where theta is the angle to the central axis of the collimation system, and phi is the azimuth about that axis. The system is symmetrical about phi. Thus G.F. = integral [A(theta)*sin(phi)*d(phi)] (1) = 2.1 x 10^-2 cm^2 sr (2) The conversion from count rates into flux J in units of particles s^-1*cm^-2*sr^-1 is simply J = rate/G.F. (3) Note that this is the integrated flux for the entire energy range. In principle, the differential flux j(E) is found by dividing by the energy range, but then there is a question of what value is to be assigned to the energy E. Complicated spectral analysis is necessary to determine this. 1.2 Species Determination -------------------------- EPA does not possess any of the usual methods of determining the species of particles detected: no magnetic fields to remove electrons, no time of flight. However, there is a method employed by Anderson et al. (1978) to yield some. information: telescope 2 is covered with an aluminized plastic foil which absorbs low energy protons with energies up to ~ 350 keV, while allowing electrons of energy > 20 keV to penetrate. Since the count rates in telescope 2 never exceed 10% those in telescope 3 (and are more often less than 1%), one concludes that EPA observed predomi- nantly positive ions throughout the entire flyby. For this reason, the count rates in telescope 2 are not included in the archived data. 1.3 Energy Thresholds ---------------------- The EPA count rates are sorted into 4 energy channels, the thresholds of which are dependent on species. Table 1 lists the upper and lower limits for these channels for protons and water ions, the two species that are most likely to be found in the vicinity of the comet. Table 1: Energy channels for protons and H20+ --------------------------------------------- Channel Protons H2O+ ------------------------------------ 1 29-46 keV 78-115 keV 2 44-77 97-145 3 78-215 144-270 4 217-3500 280-3500 ------------------------------------ Because of the species dependency on the energy thresholds, and because there is no direct species determination, the derivation of energy spectra j(E) is very complicated. For this reason, in the interest of simplicity, only count rates have been delivered to the archive data set. 1.4 Orientation ---------------- The three telescopes are positioned on Giotto as shown in EPAGEOM.TIF, at angles of 45 degrees and 135 degrees to the spacecraft +Z axis. Note that the actual spin axis is the -Z axis, the axis that goes through the bumper shield and points towards the direction of motion relative to the comet. The accumulated counts are sectorized during a spacecraft spin into 8 sectors. The combination of telescope and sector number determines the look direction of the detection system. In principle the two are independent, but for the mode in which EPA was measuring during the flyby, the odd-numbered sectors are assigned to telescope 1 and the even ones to telescope 3. Thus the sector number alone determines the look direction. Table 2: Look directions of the sectors, relative to the spacecraft spin axis ----------------------------------------------------------------------------- Sector (Tel.) Theta phi --------------------------------------------- 1 1 135 degrees -102.5 degrees 2 2 45 degrees -57.5 degrees 3 1 135 degrees -12.5 degrees 4 2 45 degrees 32.5 degrees 5 1 135 degrees 77.5 degrees 6 2 45 degrees 122.5 degrees 7 1 135 degrees 167.5 degrees 8 2 45 degrees -147.5 degrees --------------------------------------------- The look directions relative to the spacecraft spin axis are listed in Table 2. Note that theta here is the angle to the spin axis, or the spacecraft -Z axis. The azimuth angle phi is the rotation in the spin direction from the plane containing the sun and Z axis. 2 The Encounter Geometry ------------------------- During the encounter with comet P/Grigg-Skjellerup, the spacecraft spin axis (the -Z axis) was oriented almost in the ecliptic plane and nearly perpendic- ular to the spacecraft-sun line. The coordinate system normally used is called CSE for Comet Solar Ecliptic: the comet is the origin, the X axis points to the sun, and Y is parallel to the ecliptic such that the Z axis is towards the north. Measuring theta as the angle from the Z axis and phi, as the azimuth in the XY plane, with phi = 0 being the sun, then at the time of closest approach, the spin axis is oriented theta = 90.0 degrees; phi = 90.4 degrees (CSE) (4) It remains now to rotate the look directions in Table 2 into CSE coordinates. These are listed in Table 3. Also included is the angle alpha between the look direction and the coet-sun line. Table 3: CSE coordinates of the sector look directions at closest approach ------------------------------------------------------------------------------ 1 2 3 4 5 6 7 8 theta (in degrees) 46.3 53.4 81.2 112.3 133.7 126.6 98.8 67.7 phi (in degrees) -101.8 62.2 -45.3 50.3 -77.4 118.6 -133.9 130.5 ------------------------------------------------------------------------------ alpha (in degrees) 98.5 68.0 46.0 53.7 80.9 112.6 133.3 127.0 ------------------------------------------------------------------------------ alpha = angle to the comet-sun line ----------------------------------- 3 The Archive Data Set ----------------------- The EPA data provided to the archive data set are averages over one minute covering the time interval 1992 July 9 UT 1620 to July 11 UT 0314 For each minute, 32 data words are given, representing the counts per second in 8 directional sectors (look directions in Table 3) and 4 energy channels (Table 1). In the original submission, there were 5 files, each containing no more than 8 hours of data. These were EPA-1. DAT July 9, 1620-2400 EPA-2. DAT July 10, 0000-0800 EPA-3. DAT July 10, 0800-1600 EPA-4. DAT July 10, 1600-2400 EPA-5. DAT July 11, 0000-0314 Each file contained a 20-line header describing its contents and the format of its data. The modified files contain 1-minute average count rates in each of eight sectors organized according to channels 1,2,3,or 4 of the Energetic Particle Analyser (EPA) on Giotto during its extended mission to comet P/Grigg-Skjellerup. Start time for the average is given in each file; averages were obtained simultaneously. There is one record for each 1-minute interval in the time span covered by the file. Missing count rates are indicated by the value '-999.999'. 3.1 Missing Sectors -------------------- During the time interval 1992 July 9 UT 1810 to July l0 UT 0420 there were some telemetry problems that resulted in only half of the data from each spin being received. As a result, the even sectors are missing during this time. 3.2 Solar Contamination ------------------------ The count rates in energy channel 1 are contaminated by sunlight in sectors 2, 5, 6, 7, and 8. References ---------- Anderson K A, Lin R P, Potter D W, & Heetkerks H D 1978, An experiment to measure interplanetary and solar electrons, IEEE Trans., GE-16, 153-156. McKenna-Lawlor S, Thompson A, O'Sullivan D, Kirsch E, Melrose D, & Wenzel K P 1986, The Giotto Energetic Particle Experiment. In: The Giotto Mission: Its Scientific Investigations, ESA SP-1077, pp. 53-65, European Space Agency, Noordwijk, Netherlands. McKenna-Lawlor S, Kirsch E, Thompson A, O'Sullivan D, & Wenzel K P 1987, The lightweight energetic particle detector EPONA and its performance on Giotto, J. Phys. E: Sci. Instrum., 20, 732-740.