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Papers by Brian Gilchrist
Measurements of thermospheric horizontal neutral winds were made with a Fabry-Perot Interferomete... more Measurements of thermospheric horizontal neutral winds were made with a Fabry-Perot Interferometer (FPI) in Carmen Alto, Chile (23.1° S, 69.4° W, -10.2° dip), during July-October 1997. During the same period, in Antofagasta, Chile (23.43° S, 70.6° W, -10.29° dip), UHF scintillation and zonal irregularity drifts were obtained using two geostationary satellites. The proximity of these two ground sites, located on
Geophysical Monograph Series, 1998
Many scientific questions concerning the distribution of electromagnetic fields and plasma struct... more Many scientific questions concerning the distribution of electromagnetic fields and plasma structures in the ionosphere require measurements over relatively small temporal and spatial scales with as little ambiguity as possible. It is also often necessary to differentiate several geophysical parameters between horizontal and vertical gradients unambiguously. The availability of multiple tethered satellites or sensors, so-called "pearls-on-a-string," may make the necessary measurements practical. In this report we provide two examples of scientific questions which could benefit from such measurements (1) high-latitude magnetospheric-ionospheric coupling; and, (2) plasma structure impact on large and small-scale electrodynamics. Space tether state-of-the-art and special technical considerations addressing mission lifetime, sensor pointing, and multi-stream telemetry are reviewed.
Abstract. Measurements of the current collected by the Tethered Satellite System (TSS)satellite a... more Abstract. Measurements of the current collected by the Tethered Satellite System (TSS)satellite as a function of voltage and ambient plasma parameters are presented. The satellite current is found to vary approximately with the square root of the potential from below 10 to nearly 1200 V. The collected current exceeded premission expectations, based on the Parker and Murphy [1967] collection model, by factors of two to three. Possible reasons for discrepancies between the measurements and model are briefly discussed.
Two methods of passive remote sensing of mildly (E 5 MeV) relativistic electron beams as they pro... more Two methods of passive remote sensing of mildly (E 5 MeV) relativistic electron beams as they prop-agate through the Earth’s upper and middle atmo-sphere are presented. Utilization of bremsstrahlung emissions as a diagnostic indicator of beam charac-teristic energy and particle flux is compared and con-trasted with that of the optical emission technique. A new MeV aurora1 electron model has been developed to compute line emission rates of O(lD)-+ O(3P) (X = 630.0-636.4 nm doublet), O(rS) + O(iD) (X = 557.7 nm), Nz(B3C,f) 3 N~(X2C~) (A = 391.4 nm and 427.8 nm from the N.$(lN) band), and Ns(C311,) 4 Nz(B3111,) (X = 337.1 nm from the Nz(2P) band). The 427.8 nm,.391.4 nm, and 337.1 nm lines are strong in intensity,.with production rates several or-ders of magnitude greater in than those of the 0 lines examined here. It is shown that the production of 337.1 nm is insensitive to compositional change and has a quenching height lower in altitude than the propagation depth of a 5 MeV electro...
Journal of Geophysical Research: Space Physics
Vacuum Nanoelectronics International Conference, 2004
A gated structure of arrays of micron-sized holes has been developed at the University of Michiga... more A gated structure of arrays of micron-sized holes has been developed at the University of Michigan. The structure can be positioned atop any uniformly structured planar emitting surface and biased to effect electron emission. The structure is designed to be compatible with a variety of emission surface technologies such as thin films (e.g., boron nitride), carbon nanotubes, and self-assembled nanostructures.
ICOPS 2000. IEEE Conference Record - Abstracts. 27th IEEE International Conference on Plasma Science (Cat. No.00CH37087), 2000
Summary form only given. In controlled high vacuum environments, field emitter array have shown s... more Summary form only given. In controlled high vacuum environments, field emitter array have shown substantial capability, but have failed in harsher environments more typical of space applications. We argue that a combination of localized arc suppression coupled with robust, low work function coatings such as zirconium, carbide can provide the needed raggedness to withstand energetic ions, oxygen fluxes, and adsorbates
The use of sampling for broadband frequency synthes is presented. This approach offers several si... more The use of sampling for broadband frequency synthes is presented. This approach offers several significant advantages over present techniques including reduced size, power consumption, switching speed, and circuit complexity while exhibiting improved synthesis reliability.
Geophysical Research Letters, 1998
First ever simultaneous, dual-point, in situ measurements of natural ionospheric structures using... more First ever simultaneous, dual-point, in situ measurements of natural ionospheric structures using widely spaced tethered sensors, flying in formation, were made during the reflight of the Tethered Satellite System (TSS-1R) mission. A “target-of-opportunity” observation provided a direct comparison of structured ionospheric irregularity features at two altitudes near the South American geomagnetic equator at approximately 2000 hours local time and at an altitude of ≈300 km. With the TSS-1R satellite and space shuttle separated by a vertical distance of 10 km, correlated plasma signatures detected by plasma instruments at each end indicated a strong eastward displacement in the irregularity features and possible growth of steepened features at the higher altitude. Observations made by a SUNDIAL ground station ionosonde located north of the flight path also indicated considerable spread-F activity at the time. A SUNDIAL corrected ionospheric model indicated that the shuttle was flying near the F-peak. Therefore, the strongly correlated in situ observations were most likely associated with irregularities in their early development or modulations near the F-peak due to equatorial spread-F (ESF). While the TSS-1R system was not optimized for dual-point in situ ionospheric measurements, and the tether break eliminated additional equatorial zone observations planned for later in the mission, the results reported here nevertheless indicate that vertically correlated plasma features can exist and can also have strong structural variations as a function of altitude. Such variations need closer examination in order to understand their effects on radiowave scintillation. These observations also demonstrated the feasibility of tethered sensors in the study of ionospheric irregularities using controlled vertical sampling.
Geophysical Research Letters, 1998
Geophysical Research Letters, 1998
Geophysical Research Letters, 1998
Geophysical Research Letters, 1998
We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the... more We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the tether to shuttle ground. Ion spectral peaks detected by the Shuttle Potential and Return Electron Experiment indicate that the shuttle consistently charged negatively with respect to the local plasma. With the 15 Ω shunt in the circuit, shuttle potential, Φs, decreased
Advances in Space Research, 1999
Although the deployment distance of the TSS-1 tethercd satellite was only about 1 % of nominal, e... more Although the deployment distance of the TSS-1 tethercd satellite was only about 1 % of nominal, experiments to study the current collection and vehicle charging effects at low voltages were performed. In this paper we present measurements of Orbiter charging resulting from electron beam emission from the Orbiter, currents in die TSS system with and without electron bcarn emissions, and the effects of Orbiter thrusters on charging and currents. Generally, charging induced by beam emission was limited to a few volts, though during times with low ambient plasma density the Orbiter was charged up IO 80V. Thrusters are seen to enhance Orbiter charging during beam emission, and reduce ion current collection at other times. The first mission of the Tethered Satellite System (TSS-1) was an experiment to demonstrate the feasibility of deploying an instrumented current collecting satellite attached to a 20km length of conducting, insulated wire from the cargo bay of the Space Shuttle. Upward ...
Journal of Atmospheric and Solar-Terrestrial Physics
Measurements of thermospheric horizontal neutral winds were made with a Fabry-Perot Interferomete... more Measurements of thermospheric horizontal neutral winds were made with a Fabry-Perot Interferometer (FPI) in Carmen Alto, Chile (23.1° S, 69.4° W, -10.2° dip), during July-October 1997. During the same period, in Antofagasta, Chile (23.43° S, 70.6° W, -10.29° dip), UHF scintillation and zonal irregularity drifts were obtained using two geostationary satellites. The proximity of these two ground sites, located on
Geophysical Monograph Series, 1998
Many scientific questions concerning the distribution of electromagnetic fields and plasma struct... more Many scientific questions concerning the distribution of electromagnetic fields and plasma structures in the ionosphere require measurements over relatively small temporal and spatial scales with as little ambiguity as possible. It is also often necessary to differentiate several geophysical parameters between horizontal and vertical gradients unambiguously. The availability of multiple tethered satellites or sensors, so-called "pearls-on-a-string," may make the necessary measurements practical. In this report we provide two examples of scientific questions which could benefit from such measurements (1) high-latitude magnetospheric-ionospheric coupling; and, (2) plasma structure impact on large and small-scale electrodynamics. Space tether state-of-the-art and special technical considerations addressing mission lifetime, sensor pointing, and multi-stream telemetry are reviewed.
Abstract. Measurements of the current collected by the Tethered Satellite System (TSS)satellite a... more Abstract. Measurements of the current collected by the Tethered Satellite System (TSS)satellite as a function of voltage and ambient plasma parameters are presented. The satellite current is found to vary approximately with the square root of the potential from below 10 to nearly 1200 V. The collected current exceeded premission expectations, based on the Parker and Murphy [1967] collection model, by factors of two to three. Possible reasons for discrepancies between the measurements and model are briefly discussed.
Two methods of passive remote sensing of mildly (E 5 MeV) relativistic electron beams as they pro... more Two methods of passive remote sensing of mildly (E 5 MeV) relativistic electron beams as they prop-agate through the Earth’s upper and middle atmo-sphere are presented. Utilization of bremsstrahlung emissions as a diagnostic indicator of beam charac-teristic energy and particle flux is compared and con-trasted with that of the optical emission technique. A new MeV aurora1 electron model has been developed to compute line emission rates of O(lD)-+ O(3P) (X = 630.0-636.4 nm doublet), O(rS) + O(iD) (X = 557.7 nm), Nz(B3C,f) 3 N~(X2C~) (A = 391.4 nm and 427.8 nm from the N.$(lN) band), and Ns(C311,) 4 Nz(B3111,) (X = 337.1 nm from the Nz(2P) band). The 427.8 nm,.391.4 nm, and 337.1 nm lines are strong in intensity,.with production rates several or-ders of magnitude greater in than those of the 0 lines examined here. It is shown that the production of 337.1 nm is insensitive to compositional change and has a quenching height lower in altitude than the propagation depth of a 5 MeV electro...
Journal of Geophysical Research: Space Physics
Vacuum Nanoelectronics International Conference, 2004
A gated structure of arrays of micron-sized holes has been developed at the University of Michiga... more A gated structure of arrays of micron-sized holes has been developed at the University of Michigan. The structure can be positioned atop any uniformly structured planar emitting surface and biased to effect electron emission. The structure is designed to be compatible with a variety of emission surface technologies such as thin films (e.g., boron nitride), carbon nanotubes, and self-assembled nanostructures.
ICOPS 2000. IEEE Conference Record - Abstracts. 27th IEEE International Conference on Plasma Science (Cat. No.00CH37087), 2000
Summary form only given. In controlled high vacuum environments, field emitter array have shown s... more Summary form only given. In controlled high vacuum environments, field emitter array have shown substantial capability, but have failed in harsher environments more typical of space applications. We argue that a combination of localized arc suppression coupled with robust, low work function coatings such as zirconium, carbide can provide the needed raggedness to withstand energetic ions, oxygen fluxes, and adsorbates
The use of sampling for broadband frequency synthes is presented. This approach offers several si... more The use of sampling for broadband frequency synthes is presented. This approach offers several significant advantages over present techniques including reduced size, power consumption, switching speed, and circuit complexity while exhibiting improved synthesis reliability.
Geophysical Research Letters, 1998
First ever simultaneous, dual-point, in situ measurements of natural ionospheric structures using... more First ever simultaneous, dual-point, in situ measurements of natural ionospheric structures using widely spaced tethered sensors, flying in formation, were made during the reflight of the Tethered Satellite System (TSS-1R) mission. A “target-of-opportunity” observation provided a direct comparison of structured ionospheric irregularity features at two altitudes near the South American geomagnetic equator at approximately 2000 hours local time and at an altitude of ≈300 km. With the TSS-1R satellite and space shuttle separated by a vertical distance of 10 km, correlated plasma signatures detected by plasma instruments at each end indicated a strong eastward displacement in the irregularity features and possible growth of steepened features at the higher altitude. Observations made by a SUNDIAL ground station ionosonde located north of the flight path also indicated considerable spread-F activity at the time. A SUNDIAL corrected ionospheric model indicated that the shuttle was flying near the F-peak. Therefore, the strongly correlated in situ observations were most likely associated with irregularities in their early development or modulations near the F-peak due to equatorial spread-F (ESF). While the TSS-1R system was not optimized for dual-point in situ ionospheric measurements, and the tether break eliminated additional equatorial zone observations planned for later in the mission, the results reported here nevertheless indicate that vertically correlated plasma features can exist and can also have strong structural variations as a function of altitude. Such variations need closer examination in order to understand their effects on radiowave scintillation. These observations also demonstrated the feasibility of tethered sensors in the study of ionospheric irregularities using controlled vertical sampling.
Geophysical Research Letters, 1998
Geophysical Research Letters, 1998
Geophysical Research Letters, 1998
Geophysical Research Letters, 1998
We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the... more We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the tether to shuttle ground. Ion spectral peaks detected by the Shuttle Potential and Return Electron Experiment indicate that the shuttle consistently charged negatively with respect to the local plasma. With the 15 Ω shunt in the circuit, shuttle potential, Φs, decreased
Advances in Space Research, 1999
Although the deployment distance of the TSS-1 tethercd satellite was only about 1 % of nominal, e... more Although the deployment distance of the TSS-1 tethercd satellite was only about 1 % of nominal, experiments to study the current collection and vehicle charging effects at low voltages were performed. In this paper we present measurements of Orbiter charging resulting from electron beam emission from the Orbiter, currents in die TSS system with and without electron bcarn emissions, and the effects of Orbiter thrusters on charging and currents. Generally, charging induced by beam emission was limited to a few volts, though during times with low ambient plasma density the Orbiter was charged up IO 80V. Thrusters are seen to enhance Orbiter charging during beam emission, and reduce ion current collection at other times. The first mission of the Tethered Satellite System (TSS-1) was an experiment to demonstrate the feasibility of deploying an instrumented current collecting satellite attached to a 20km length of conducting, insulated wire from the cargo bay of the Space Shuttle. Upward ...
Journal of Atmospheric and Solar-Terrestrial Physics