During the US2 operations shift (the second eight-hour NASA shift) the IUE passes through the outer Van Allen radiation belts. Cerenkov radiation from high-energy electrons entering the ultraviolet converter section of the camera produces phosphorescence there, but only while the camera is actually exposing. This increases the background level on the entire image and usually limits the length of exposures that can be obtained. There is a Flux Particle Monitor (FPM) on the IUE which measured the particle radiation level in volts. Unfortunately this instrument has malfunctioned and is presently not in use. Presently estimates of the radiation level are made from exposures taken on previous days plus a rough calibration has been determined for data from the GOES-7 and GOES-6 satellites which can be accessed on-line. The fogging effect of the radiation can be quantified in terms of DN per hour. The rate at which the background level accumulates on the most sensitive part of the detector is an exponential function of FPM, and is given by:
This rate should be used to estimate the accumulated background in high dispersion images. For low-dispersion spectra, Table 4.2 can be used to estimate the background accumulation rate as a function of wavelength.
DN/hour = 1.35 x 10^FPM for the LWP camera,
DN/hour = 0.73 x 10^FPM for the LWR camera,
DN/hour = 1.00 x 10^FPM for the SWP camera.
In addition to the radiation-induced background, there is a 5 to 10 DN/hour accumulation of background caused by phosphorescence in the detectors (see Section 4.6). An exposure level of 210 DN is regarded as an optimum exposure. The telemetry saturates for any DN exceeding 254.
The average intensity of particle radiation varies with solar activity. A survey of the peak radiation levels (i.e. daily maxima) from 1981 through 1990 is given in Table 4.3. The data are taken from Walter and Imhoff (1983), Broude and Imhoff (1984), Taylor and Imhoff (1986), Loomis and Arquilla (1990) and Groebner and Arquilla (1991).
|Peak Radiation Level||Percentage of Days Affected|
|FPM < 1.0||< 10||19.2||8.8||1.4||2.7||4.4|
|1.0 < FPM < 1.7||10 to 50||39.2||22.7||19.7||26.0||21.6|
|1.7 < FPM < 2.0||50 to 500||21.4||26.6||15.9||12.3||11.0|
|2.0 < FPM < 2.4||100 to 250||14.2||19.7||26.3||19.9||25.5|
|2.4 < FPM < 2.8||250 to 500||4.7||18.9||26.6||22.4||26.6|
|2.8 < FPM < 3.0||500 to 1000||1.1||1.6||6.8||6.6||7.1|
|3.0 < FPM < 4.0||> 1000||0.3||1.9||3.2||10.1||3.8|
|FPM < 1.0||< 10||4.6||21.1||8.5||16.7||24.6|
|1.0 < FPM < 1.7||10 to 50||32.9||28.8||26.0||44.4||42.2|
|1.7 < FPM < 2.0||50 to 100||12.9||10.9||19.7||16.1||16.2|
|2.0 < FPM < 2.4||100 to 250||18.9||19.5||33.3||10.7||13.1|
|2.4 < FPM < 2.8||250 to 500||21.6||11.5||9.0||11.5||3.6|
|2.9 < FPM < 3.0||500 to 1000||6.6||4.6||2.7||0.3||0.3|
|3.0 < FPM < 4.0||> 1000||2.6||3.6||0.8||0.3||0.0|