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Twenty Years of Radiation Measurements in Low-Earth Orbit - What Have We Learned Space Radiation Environment?The advent of the Space Shuttle program has made possible space radiation environment measurements spanning a wide range of altitudes and orbital inclinations over multiple solar cycles. These measurements range from routine integral dose measurements with thermoluminescent dosimeters to particle energy spectra measurements made with a charged particle telescope. This paper will review the new understanding about the space radiation environment gained from this diverse data set. Major findings from these measurements include: estimations of the westward drift rate of the South Atlantic Anomaly (SAA) of 0.28-0.49/y; evidence for a northward component to the SAA drift of 0.08-0.12/y; observation of the formation and decay of the pseudo-stable additional radiation belt following the Mar 1991 SPE and geomagnetic storm with an estimated decay e-folding time of 9-10 months; observation of a local geomagnetic east-west trapped proton exposure anisotropy with an estimated magnitude of 1.6-3.3; demonstration that the trapped proton exposure in low-Earth orbit (LEO) can be reasonably modeled as a power law function of atmospheric density in the SAA region, with best correlations obtained when the exospheric temperature saturates at 938-975 K; the actual solar cycle modulation of trapped proton exposure in LEO is less than predicted by the AP8 model; and the testing and validation of GCR flux models, radiation transport codes, and dynamic geomagnetic cutoff models. Long-term, time-resolved proportional counter measurements made aboard the Mir during the same period provides further demonstration of the solar cycle modulation of the trapped protons at low altitudes - the observed modulation is also well described as power law function of atmospheric density. These data and findings have helped to improve the overall accuracy of pre-mission crew exposure projections using various semi-empirical space environment models, radiation transport codes, and spacecraft radiation shielding models. During the rise phase of solar cycle 22 (1987-1991), the RMS error between preflight exposure projections and measured crew exposure was 73%. For the rise phase of cycle 23 (1997-2001), the preflight exposure projection RMS error has decreased to 23%. The launch and assembly of the Space Station has begun a new era of long-term LEO space environment monitoring. The radiation environment at the Space Station will be monitored with three external charged particle telescopes oriented in the velocity vector, anti-velocity vector, and zenith directions. Data from the telescopes will provide charge, mass, energy, and arrival direction for incident particles with energy to mass ratios of 13- 450 MeV/amu and Z of 1-24. The external environment data will be complimented by measurements from a portable charged particle telescope and proportional counter located inside the vehicle.
Document ID
20100029831
Acquisition Source
Johnson Space Center
Document Type
Conference Paper
Authors
Golightly, Michael J.
(NASA Johnson Space Center Houston, TX, United States)
Weyland, Mark D.
(Lockheed Martin Corp. Houston, TX, United States)
Johnson, A. S.
(Lockheed Martin Corp. Houston, TX, United States)
Semones, E.
(Lockheed Martin Corp. Houston, TX, United States)
Date Acquired
August 25, 2013
Publication Date
July 23, 2001
Subject Category
Space Radiation
Report/Patent Number
JSC-CN-6928
Meeting Information
Meeting: International Space Environment Conference 2001
Location: Queenstown
Country: New Zealand
Start Date: July 23, 2001
End Date: July 27, 2001
Sponsors: Rice Univ.
Funding Number(s)
OTHER: 575-81-06-24
Distribution Limits
Public
Copyright
Other

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