To facilitate discussion, figure 1 shows a Mercator projection of the Earth, with some standard orbits traced and labeled. The satellite moves from west to east. The standard orbits are shown as terminating at the equator during the northbound crossing, whereas in the reality the satellite moves into a new standard orbit, which is 25.3° west of the previous orbit at the northbound crossing (due to the rotation of the earth). By interpolation between marked orbits, the data presented below can be identified with Earth latitude and longitude. The tick marks are inserted on standard orbit 1 to represent each 10 minutes (civil time) after the crossing of the node for a particular orbit (thus approximately representing the standard time, TSN, for each standard orbit). The contours of the magnetic field for B ~ 0.5 gauss at 700 km above the surface mark the positions of the North and South Magnetic Poles. The dashed lines enclosing most of South America represent the general region of the South Atlantic Anomaly (SAA) and, specifically, include the regions of exclusion used in scheduling Copernicus observations. The inner border, referred to as "Anomaly 2" is used for stars brighter than V = 3.0, while the outer border ("Anomaly 1") is used in scheduling stars fainter than V = 3.0 or for scheduling observations in regions of poor instrument sensitivity (e.g. Lambda < 1000 Å, 1400 Å < Lambda < 2200 Å) on bright stars.
A wide range of count rates on any given tube may be encountered in a given orbit. The extreme example of this is shown in figure 2 for standard orbit 2, which passes through the heart of the South Atlantic Anomaly, just before becoming standard orbit 69. Figure 2a shows the results for tubes U1, U2, and U3, where count rates for the respective tubes are plotted against the standard minute. For most of the orbit the U1 counts are less than 35/14 sec, the U2 counts are less than 25 counts/14 sec and the U3 counts are less than 17 counts/14 sec. However, when passing through the anomaly (right hand side) the count rates on all three tubes are in excess of 500/14 sec, and in fact, exceed 50,000/14 sec in the most intense part of the anomaly. Figure 2b shows similar results for V1, V2, and V3. The ordinate here is compressed by a factor of 100 to accommodate the much higher count rates observed on these tubes. For V1 and V2, the count rates outside the anomaly are below 7000/14 sec and 4000/14 sec, respectively. However, the curve for V3 has a different shape than the curves for the other tubes, tending to show a longer tail following passage of the satellite through the SAA in the previous orbit (in this case, standard orbit 7, recalling the precession of -25.3° per satellite orbit mentioned earlier). In the anomaly, the counts on all tubes are again quite high.
A more eastward orbit, (number 33), crossing though a different part of the SAA, and now showing the effects of the magnetic poles, is presented in figure 2c (far UV tubes) and 2b (near UV tubes). The polar approaches (standard minutes 30 and 77) have the least effect on V3, relative to the minimum counts in the orbit, and the tailing off of the V3 count rate following passage through the SAA is apparent.
A less drastic, but more complicated change of background levels is shown for standard orbit 38 in figures 2e and 2f. The data for U1, U2, and U3 ("far UV" - figure 2c) show an increase in count rate by a factor of 4-5 as the satellite reaches the closest approach to the North and South Magnetic poles (standard times of 26 minutes and 73 minutes, respectively), but a much larger increase at the center of the orbit due to the crossing of the satellite through the eastern edge of the anomaly. Figure 3b shows similar behavior for V1, V2, and V3, but the increase due to proximity to the poles is only a factor of about 2.7 for V1 and V2 and only a factor 1.7 for V3. It is also noted that the relative increase due to the SAA is about the same on the near UV tubes and the far UV tubes, although the absolute increase in counts is much greater for near-UV tubes. It may be noted that the relative heights of the two peaks within the anomaly are different for V3 and U3.
A sequence of standard orbits which demonstrates the gradual fading away of the feature in orbit 38 (figure 2) due to the anomaly, with the persistence of the polar features, is shown in figure 3. The last orbit (44) skirts the eastern region of the SAA but never enters the inner dashed region of figure 1.
Figures 4 and 5 include plots for each standard orbit of U2 and V3 as an aid in choosing the best orbits for low noise observations and in deciding when the use of V3 as a monitor is most reliable. The most obvious similarity in the behavior of the two tubes from standard orbit to standard orbit is the similar response to the SAA, while the most striking differences in behavior are the slower trailing off of the count rates on V3 following passing through the SAA and the lower enhancement on V3 in the approaches to the magnetic poles (compare, particularly, the data for the two tubes for standard orbits 30 through 50).
[Sect. I] [Sect. II] [Sect. III] [Sect. IV]