Fundamental Technologies

Voyager LECP Pages

Saturn Corrections

MEMO: VOYAGER 2 SATURN ELECTRON RESPONSES (CONTAMINATION) OF THE PL04, PL05 CHANNELS IN THE PERIAPSIS ENVIRONMENT.

PROBLEM: THE LEMPA MAGNET IS CAPABLE OF DEFLECTING ELECTRONS AWAY FROM THE P-ALPHA DETECTOR ONLY UP TO ENERGIES OF ABOUT 140 KEV, DEPENDING UPON ANGLE OF ENTRY INTO THE APERTURE. ELECTRONS OF ENERGIES HIGHER THAN THIS MAY BE COUNTED WITH SOME EFFICIENCY, PROVIDING THAT THEY DEPOSIT SUFFICIENT ENERGY TO TRIGGER THE RELEVANT THRESHOLD.

SOLUTION:

  1. OBTAIN MORE COMPLETE LABORATORY CALIBRATION DATA (NOT FEASIBLE BECAUSE OF TIME AND COST CONSTRAINTS.)
  2. CALCULATE OR SIMULATE THE RESPONSES ON A COMPUTER (NOT FEASIBLE BECAUSE OF TIME AND UNCERTAINTIES ABOUT THE BASIC PHYSICS.)
  3. EVALUATE VOYAGER FLIGHT DATA IN OTHER CIRCUMSTANCES TO CHECK THE EFFICIENCIES IN FLIGHT SITUATIONS.

    NOVEMBER 22, 1977, SOLAR PARTICLE EVENT. VOYAGER 1 WAS SCANNING DURING THE ONSET OF THIS EVENT AND OBTAINED RELIABLE MEASUREMENTS OF THE ELECTRONS AS DISTINGUISHED FROM THE RELATIVISTIC PROTONS ALSO PRESENT AT THIS TIME. VOYAGER 2 WAS NOT SCANNING AND THEREFORE HAS NO ACCURATE APPRAISAL OF THE OMNIDIRECTIONAL BACKGROUND OF THE ALPHA AND BETA DETECTORS. THE FOLLOWING IS AN ANALYSIS OF THAT EVENT AT THE MOST SUITABLE TIME IN THE ONSET TO LOOK FOR ELECTRON RESPONSES OF THE ALPHA DETECTOR:

    Day 326 12:51-14:14

    EB05 Sec. 4= 1.33 c/sec
    EB05 Sec. 8= 0.65 c/sec
    EB05 F/G=  .75-.1 (sec. 8 source)
      = 0.65 c/sec
    PL04 Sec. 4= 1.98 c/sec
    PL04 Sec. 8=  .36 c/sec
    PL04 F/G= 1.62 c/sec
    PL05 Sec. 4= 1.16 c/sec
    PL05 Sec. 8=  .40 c/sec
    PL05 F/G=  .76 c/sec

    Therefore:
    PL04 5*EB05, including a factor of 2 for the larger Voyager 2 alpha detector area.
    PL05 2.2*EB05

    For V2 periapsis:

    EB05 F/G 100 c/sec (typical). This implies about 500 c/sec for PL04 and 220 c/sec for PL05, as compared to 1400 and 1000 c/sec respectively, observed. In fact, we can be certain that the low energy electron response of the alpha detector is never worse than about 30% of the counts and is probably zero--because EB05 has a very small foreGround response.

    Conclusion: Our problem with the electron response of the alpha detector is not with low energy electrons.

    CONSIDERING THE HIGH ENERGY ELECTRON RESPONSE AS MEASURED BY ESA0, WE MUST USE THE VOYAGER 1 DATA FROM INSIDE THE IO ORBIT TO CHECK FOR ELECTRON SENSITIVITY.

    Day 64 9:29 - 9:30

    PL04 (4-8)=1 3500-9455=4045 c/sec
    ESA0 (8)= 353000 c/sec
    PL04/ESA0= 0.011

    Day 64 10:29 - 10:31

    PL04 (4-8)= 21800-15500=6300 c/sec
    ESA0 (8)= 450000 c/sec
    PL04/ESA0= 0.014

    Day 64 11:56 - 11:57

    PL04 (4-8)= 28800-22000=6600 c/sec
    ESA0 (8)= 574000 c/sec
    PL04/ESA0= 0.011

    NOW FOR SATURN PERIAPSIS:

    ESA0 = 35000 C/SEC
    PL04=0.022*ESA0=802 C/SEC H.E. ELECTRON CONTAMINATION.
    (A SIMILAR CALCULATION YIELDS THE SAME NUMBER FOR PL05.) THE OBSERVED RATES ARE 2500 C/SEC AND 1000 C/SEC AS NET FOREGROUND RATES FOR PL04 AND 5 RESPECTIVELY. THE UPPER LIMITS ARE, THEREFORE, 33% AND 80% RESPECTIVELY.

SUMMARY AND CONCLUSION

WE SHOULD REMEMBER THAT THE NUMBERS GIVEN ABOVE ARE UPPER LIMITS ONLY AND THE ACTUAL SITUATION SHOULD BE NO WORSE, AND MAY BE A LOT BETTER. THERE ARE OTHER FACTORS TO KEEP IN MIND ABOUT THE HIGH ENERGY ELECTRONS:

  1. THE MAGNET IS NOT COMPLETELY INEFFECTIVE AGAINST THEM. THE FACT THAT ELECTRONS DO ARRIVE AT THE GAMMA DETECTOR SHOWS THAT THE DEFLECTING POWER OF THE MAGNET IS CONSIDERABLE, EVEN AT ENERGIES APPROACHING 1 MeV.
  2. A SUBSTANTIAL FRACTION, MAYBE APPROACHING 100%, OF THE HIGH ENERGY ELECTRONS MEASURED BY ESA0 PENETRATE THE SUNSHIELD AS EVIDENCED BY THE FACT THAT ESA0 IN SECTOR 4 IS NOT SIGNIFICANTLY DEPRESSED. HENCE, WHEN WE SUBTRACT OUT THE SECTOR 8 DATA, WE ARE INCLUDING A SUBSTANTIAL PART OF THE HIGH ENERGY ELECTRONS WHICH OVERWHELM THE MAGNET IN THE UNSHIELDED POSITIONS. IN OTHER WORDS, WE PROBABLY HAVE HIGH ENERGY ELECTRONS COMING IN THROUGH THE APERTURE AND MAGNETIC FIELD AT ALL POSITIONS, INCLUDING THE SHIELDED ONE. THE ONLY ELECTRONS WE NEED TO WORRY ABOUT, THEREFORE, ARE THOSE WITH SUFFICIENT ENERGY TO BEAT THE MAGNETIC FIELD AND INSUFFICIENT ENERGY TO PENETRATE THE SUNSHIELD.

BY MY ESTIMATES, THEN, IF WE HAVE A PROBLEM, IT WOULD BE FOR 0.5 TO 1 MEV ELECTRONS. EVENT FOR THOSE, THE JUPITER DATA INDICATES THAT WE ARE NOT OVERWHELMED. COMPARISON OF THE ESA0 AND ALPHA ANGULAR DISTRIBUTIONS AND VARIATIONS AS FUNCTIONS OF DISTANCE SHOULD, THEREFORE, PROVIDE VALID TESTS TO FURTHER REINFORCE THE IDENTIFICATION OF THE P-ALPHA RESPONSES.

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Last modified 5/23/05, Tizby Hunt-Ward
tizby@ftecs.com