Latitude Variation and True Background. Although we have now determined a set of “rock bottom” counting rates for each of the main rate channels, the possibility remains that some of these (especially the lowest energies) have never been at background. This is likely to show up as peculiar spectra in low-level events or as the high latitude increase sets in.

We have also already noted that in most channels there is a steady increase in the lowest counting rates observed as the spacecraft moves to higher latitudes. To properly handle backgrounds we must quantify these variations and attempt to understand them.

To quantify this variation we first adjusted by eye the background levels from those of Table 5.7 to account as best we could for the presence of foreground in the lower energy background rates. The criterion used was that the spectrum of the background-subtracted fluxes should be nowhere inverted, and as far as possible it should be “sensible.” In the lowest channels this typically required a reduction of 15-30% in the background count-rate, while most of the higher-energy channels were unchanged. The count rates used are listed in Table 5.8.

Table 5.7 Minimum count rates in each channel for entire mission

Table 5.8 Background rates used for the latitude variation study

For each group of channels (DE1-4, E'1-4, P1-4, P'1-4, E1-4, P'5-8, FP'5-7, FP5-7, and P5-8) separately we selected a number of intervals when the flux was low (these will be referred to as "non-events"), typically 16-19 in the interval from day 280 of 1991 (just after conjunction) up to the end of the data set at 1994 day 105. It was necessary to select different sets of non-events for each group of channels as there is not an exact match between the timings of events in the electrons, low-energy ions, and high-energy ions.

For each of the non-events, the counting rates were determined in the same manner as described for the minimum rates in the previous section. We also extracted the spacecraft heliographic latitude and heliocentric distance for each non-event.

The variation of non-event counting rate with latitude is plotted for all the channels in Figure 5.1.

The most striking feature of these variations is the consistency between the different detectors in each energy band, especially above 30º latitude where Ulysses emerged from the immediate influence of the heliospheric current sheet. Below 30º there is more variation, especially in the lower energy channels, implying that the influence of low-level heliospheric events is important even in these non-event times. Thus it seems the most useful parameter to determine is the change in count-rate between 30º and 60º; these values are listed in Table 5.9. The uncertainties have not been determined properly as these values are eyeball fits, but are in the region of 20% for channels 1 and 2 and 10% for the remaining channels. Thus within the uncertainties of the determinations the change in counting rate is the same in all channels of the same energy (remember FP7 and FP'7 cover the energy range of P7-8 and P'7-8).

Table 5.9 Change in non-event rates from 30º to 60º latitude

There would appear to be the following possible explanations:

1. Low-energy galactic electrons-This cannot be the explanation as the increase is seen in the LEMS channels as well as LEFS and WART-B, and also the increase in WART-B and LEFS is constant in counting rate, not flux.
2. Low-energy solar ions and electrons escaping from the corona-This can be ruled out immediately as the WART channels do not show an increase while the high P-channels do, and also on the flux vs. counts argument.
3. Intrinsic background (RTG, etc.)-Unlikely as the correlation between the variations and known heliospheric features would not be expected for an effect intrinsic to the spacecraft.
4. Energetic galactic ions-Unlikely as the several 100 MeV ions needed to penetrate the instrument housing should show little variation with latitude.
5. Energetic galactic electrons-The most likely explanation of most of the increase, as these would show an increase with latitude and would show the same counting rate increase in all 1-parameter detectors but be rejected by the 2-parameter Wart channels.

The determination of a usable functional form for the variation is more of a problem as the obvious sin |L| or sin2 L forms give a poor fit; the difficulty being that at 60º the increase has slowed more than would be predicted by either of these forms. A reasonable fit seems to be given by a form proportional to (90 - |L|)2. However, confirmation of the real relation must await the availability of post polar-pass data to allow us to distinguish latitudinal, radial, and temporal effects. Preliminary fits for selected channels with a (90 - |L|)2 fit look promising (Figure 5.2).

Figures:

• Figure 5.1 Variation of the background excess over the values in Table 5.8 with latitude for all the main rate channels. The shaded area indicates the range of latitudes over which the low-speed solar wind disappeared.
• Figure 5.2 Preliminary attempts to fit a latitude-dependent background level to the P7, P2, and DE2 channels, the fits are of the form background rate = c - a × 10-5 (90 - |?|)2 with the parameters a and c as given on the plots. The arrows to the right of the plots indicate the approximate change of background level across the plot.
• Figure 5.3 Daily average P'3 and DE3 rates vs. heliolatitude
• Figure 5.4 Rates vs. heliolatitude for all HISCALE channels

5.1.3 Summary

1. The constant part of the background (intrinsic + ionic penetrators?) is lower than the previously accepted values of both GMS91 and SJT93. This is partly due to the fact that the low energy channels always contain some signal and partly due to the presence of a latitude-dependent component.
2. The latitude-dependent component is at each energy constant in counting rate from head to head, thus implying that it is a penetrating component. From the variation with latitude we believe this to be penetrating electrons with energies of a few MeV.
3. There is no evidence for a separate component of low-energy galactic electrons which would show a constant flux increase over the electron channels and be absent in the ion-only channels.

Next: 5.2 Cover Close Calibrations

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