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Fire activity has varied globally and continuously since the last glacial maximum (LGM) in response to long-term changes in global climate and shorter-term regional changes in climate, vegetation, and human land use. We have synthesized sedimentary charcoal records of biomass burning since the LGM and present global maps showing changes in fire activity for time slices during the past 21,000 years (as differences in charcoal accumulation values compared to pre-industrial). There is strong broad-scale coherence in fire activity after the LGM, but spatial heterogeneity in the signals increases thereafter. In North America, Europe and southern South America, charcoal records indicate less-than-present fire activity during the deglacial period, from 21,000 to ∼11,000 cal yr BP. In contrast, the tropical latitudes of South America and Africa show greater-than-present fire activity from ∼19,000 to ∼17,000 cal yr BP and most sites from Indochina and Australia show greater-than-present fire activity from 16,000 to ∼13,000 cal yr BP. Many sites indicate greater-than-present or near-present activity during the Holocene with the exception of eastern North America and eastern Asia from 8,000 to ∼3,000 cal yr BP, Indonesia and Australia from 11,000 to 4,000 cal yr BP, and southern South America from 6,000 to 3,000 cal yr BP where fire activity was less than present. Regional coherence in the patterns of change in fire activity was evident throughout the post-glacial period. These complex patterns can largely be explained in terms of large-scale climate controls modulated by local changes in vegetation and fuel load.

An autonomous system to detect both absolute and differential spacecraft charging aboard high altitude satellites, and to reduce those potentials before hazardous arcing levels are reached, is now being developed. Operation of the system is based on the empirical results of the Space Test Program SCATHA (p78-2) and NASA ATS-6 satellites, both of which successfully demonstrated the principle of safely reducing spacecraft charging levels by the emission of a low energy neutral plasma--effectively shorting the spacecraft and charged dielectric surfaces to the ambient space plasma. The Charge Control System will utilize a xenon-based plasma source capable of igniting within one second, and capable of emitting a quasi-neutral plasma containing more than 1 MA of ions. Satellite frame potential (relative to the ambient space plasma) will be determined by an electrostatic analyzer capable of detecting both ions and electrons in the energy range 50eV-20 kev. Automatic operation of the system will accomplished by microprocessor controller which will interpret the sensor data and activate the plasma source when predetermined threshold levels are exceeded. With a gas supply for more than 2000 hours of operation in space, the system may be expected to provide on-orbit spacecraft protection for up to 10 years., Prepared in collaboration with Hughes Research Labs., Malibu, CA.

SCATHA as well as the ATS-5 and -6 spacecraft have prov ided some answers and insights to the problem of spacecraft charging at geosynchronous altitudes. In particular they have indicated the approach to reducing the levels of both absolute and differential charging -- namely, by the emission of low energy neutral plasma. It is now appropriate to complete the transition from experimental results to the development of a system that will sense the state-of-charge of a spacecraft, and, when a predetermined threshold is reached, will respond automatically to reduce it. The Air Force is embarking on such a development program, utilizing sensors comparable to the proton electrostatic analyzer, the surface potential monitor, and the transient pulse monitor that flew in SCATHA, and combining these outputs through a microprocessor and controller to operate a rapid-start, low energy plasma source. A flight-ready system should be available for spacecraft integration by 1988., Presented at the Space Environmental Interactions Technology Conference, Colorado Springs, CO, 4-6 Oct 1983.

An autonomous system to detect both absolute and differential spacecraft charging, and to reduce those potentials before hazardous arcing levels are reached, is being developed. Operation of the system is based on empirical results of the Space Test Program SCATHA and NASA ATS-6 satellites, both of which successfully demonstrated the principle of safely reducing spacecraft charging levels by emission of a low energy neutral plasma--effectively shorting the spacecraft and charged dielectric surfaces t the ambient space plasma. The charge control system now being designed and built will use a xenon-based plasma source capable of igniting within one second, adn capable of emitting a quasi-neutral plasma containing more than 1 mA of ions. The spacecraft charging level will be detected by sensors similar to those aboard SCATHA. Satellite frame potential (relative to ambient space plasma) will be determined by an electrostatic analyzer capable of detecting both ions and electrons in the energy range 50eV-20 keV. Differential charging (relative to spacecraft from ground) will be determined by a surface potential monitor using two different dielectric surfaces, and covering a range of + or - 20 kV with a response time of one second. A transient pulse monitor will detect the presence of exterior spacecraft arcing by measuring its near-field electromagnetic radiation.