Cosmic Rays and Space Weather Lev I. Dorman

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Fig. 3.3. Monitoring and preliminary alert for starting of great FEP events on the web-site of ICRC and ESO:

  1. 4.4. Monitoring and preliminary Alert for starting of great FEP events on the web-site of ICRC and ESO

  2. Figure 4.3 shows an example of the image in the web-site of ICRC and ESO for the moment 22.13 UT at 12 May 2001 (it was obtained 3 seconds after registration by NM in ESO, at 22.13.03 UT). On this image are shown on-line one-minute data of total neutron intensity for the last 6 hours (360 circles in the upper part) and one-hour data during the last 6 days (144 circles in the down part). One minute data are upgraded each minute in ASCII form in the, and one hour data – in . In the bottom of image is given information on variations in the A and B channels on the basis of one-, two-, and three-minute data (A1, A2, A3. B1, B2 and B3 in units of standard deviations). In the last line is shown the Alert for starting of great FEP (based on data of both channels and checked by situation in the previous minute). It can be seen that in the considered moment of time there is negative Alert: FEP ALERT: No.

4.5. The probability of false alarms

Because the probability function , that the probability of an accidental increase with amplitude more than 2.5? in one channel will be , that means one in 161.3 minutes (in one day we expect 8.93 accidental increases in one channel). The probability of accidental increases simultaneously in both channels will be that means one in 26007 minutes ? 18 days. The probability that the increases of 2.5? will be accidental in both channels in two successive minutes is equal to that means one in 6.76minutes ?1286 years. If this false alarm (one in about 1300 years) is sent it is not dangerous, because the first alarm is preliminary and can be cancelled if in the third successive minute is no increase in both channels bigger than 2.5? (it is not excluded that in the third minute there will be also an accidental increase, but the probability of this false alarm is negligible: that means one in years). Let us note that the false alarm can be sent in the case of solar neutron event (which really is not dangerous for electronics in spacecrafts or for astronauts health), but this event usually is very short (only few minutes) and this alarm will be automatically canceled in the successive minute after the end of a solar neutron event.

4.6. The probability of missed triggers

The probability of missed triggers depends very strongly on the amplitude of the increase. Let us suppose for example that we have a real increase of 7? (that for ESO corresponds to an increase of about 9.8 %). The trigger will be missed if in any of both channels and in any of both successive minutes if as a result of statistical fluctuations the increase of intensity is less than 2.5?. For this the statistical fluctuation must be negative with amplitude more than 4.5?. The probability of this negative fluctuation in one channel in one minute is equal , and the probability of missed trigger for two successive minutes of observation simultaneously in two channels is 4 times larger: . It means that missed trigger is expected only one per about 70000 events. In Table 1 are listed probabilities of missed triggers for ESO (where standard deviation for one channel for one minute ?=1.4%) as a function of the amplitude of increase A.

Table 4.1. Probabilities of missed triggers as a function of amplitude of increase A (in % and in ?)

A, %










A, ?










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