D. Rees | Utah State University (original) (raw)
Papers by D. Rees
Journal of Atmospheric and Terrestrial Physics, 1979
Abatr&-During the period December 1975-mid-February 1976 of the Western European Winter Anomaly C... more Abatr&-During the period December 1975-mid-February 1976 of the Western European Winter Anomaly Campaign, the local dynamical and thermal structure of the stratosphere and mesosphere was determined by some 31 rocket launches at El Arenosillo, Spain, while the mesospheric thermal structure in the winter (northern) hemisphere was derived from Nimbus 6 satellite observations. An analysis of these data is presented here. As determined by the A3 technique and the parameter L,,[ = L&I + l)], ionospheric absorption is significantly correlated with many features of the dynamical and thermal structure of the atmosphere. The strongest correlations are, on this occasion, with local win&, at a level between 85 and 92 km altitude, the sense being that high absorption corresponds to northward and eastward wind tendencies, while low absorption comsponds to southward and w+ward wind tendencies. Additionally, there is a significant correlation with local temperature above 60 km, high absorption corresponding to high mesospheric temperatures. These local variations are corroborated by data obtained from the PMR 3000 channel of the Nimbus 6 satellite (centred about 80 km). A trivariate analysis of absorption (L,,), temperature (40" latitude) and meridional temperature gradient along the 0' longitude meridian, which corresponds to the pressure gradient driving a 'geostrophic' xonal wind, shows extremely high significance. High ionospheric absorption in Spain is associated with disruption of the normally strong winter vortex around a warm winter mesospheric pole. As the polar temperature decreases, temperatures at low-and mid-latitudes increase and, responding to a decrease of meridional temperature gradient, the mid-latitude zonal and meridonal wind regime changes. Disturbances of the winter mesospheric polar vortex are triggered by the upward propagation of earlier perturbations within the troposphere and/or stratosphere. Delays of 2-4 days are indicated between the onset of stratospheric circulation disturbances, perturbation of the mesospheric polar vortex, and the consequent dynamical and thermal effects in the mid-latitude mesosphere which are associated with anomalously high ionospheric absorption.
Advances in Space Research, 2009
During recent years, special attention has been paid to the background circulation of the middle ... more During recent years, special attention has been paid to the background circulation of the middle atmosphere and, including a variety of new measurements particularly using radar especially in the mesosphere/lower thermosphere (MLT) region, to the comparison of existing empirical middle atmosphere wind models as CIRA-86 and HWM with new data. This leads to the construction of empirical models of MLT winds as the Global Empirical Wind Model (GEWM). Further investigations aim at the construction of new empirical and semi-empirical wind models of the whole middle atmosphere including the new experimental results. Results of a new wind climatology (0-100 km) are presented based upon the GEWM, stratospheric reanalysis data, and a numerical model to fill the gap between stratospheric and MLT data. Recently direct wind observations from the wind imaging interferometer (WINDII, Wang et al., 1997) and the high-resolution Doppler imager (HRDI, Fleming et al., 1996) on board the Upper Atmosphere Research Satellite (UARS) have provided a new global wind data set for the upper mesosphere/lower thermosphere region. From these data, also a vertical wind model (Fauliot et al., 1997) has been constructed. Fleming et al. (1996) and Portnyagin et al. (1999) concluded that, in general, the space-based zonal wind models exhibited significant differences relative to the ground-based models. In contrast, however, Fauliot et al. (1997) have stated that the WINDII-based prevailing meridional model winds have cellular structure, similar to those from the ground-based Portnyagin et al.
Astronomy and Astrophysics, 2006
Aims. A tunable, high spectral resolution, high effective finesse, vacuum ultraviolet (VUV) Fabry... more Aims. A tunable, high spectral resolution, high effective finesse, vacuum ultraviolet (VUV) Fabry-Perot interferometer (FPI) is designed for obtaining narrow-passband images, magnetograms, and Dopplergrams of the transition region emission line of CIV (155 nm). Methods. The integral part of the CIV narrow passband filter package (with a 2-10 pm FWHM) consists of a multiple etalon system composed of a tunable interferometer that provides high-spectral resolution and a static low-spectral resolution interferometer that allows a large effective free spectral range. The prefilter for the interferometers is provided by a set of four mirrors with dielectric high-reflective coatings. A tunable VUV piezoelectric-control interferometer has undergone testing using the surrogate F 2 eximer laser line at 157 nm for the CIV line. We present the results of these tests with a description of the overall concept for a complete narrow-band CIV spectral filter. The static interferometer of the filter will be built using a set of fixed MgF 2 plates. The four-mirror prefilter is designed to have dielectric multilayer Π-stacks employing the concept used in the Ultraviolet Imager of NASA's Polar Spacecraft. A 10-pm dual etalon system allows the effective free spectral range to be commensurate with the prefilter profile. With an additional etalon, a triple etalon system would allow a spectrographic resolution of 2 pm. The basic strategy has been to combine the expertise of spaceflight etalon manufacturing with VUV coating technology to build a VUV FPI which combines the best attributes of imagers and spectrographs into a single compact instrument. Results. High spectral-resolution spectro-polarimetry observations of the transition region CIV emission can be performed to increase our understanding of the magnetic forces, mass motion, evolution, and energy release within the solar atmosphere at the base of the corona where most of the magnetic field is approximately force-free. The 2D imaging of the full vector magnetic field at the height of maximum magnetic influence (minimum plasma beta) can be accomplished, albeit difficult, by measuring the Zeeman splitting of the CIV resonance pair. Designs of multiple VUV FPIs can be developed for integration into future orbiting solar observatories to obtain rapid cadence, spectral imaging of the transition region.
Surveys in Geophysics, 1993
The Solar Terrestrial Energy Programme (STEP) aims to improve our general understanding of how ma... more The Solar Terrestrial Energy Programme (STEP) aims to improve our general understanding of how mass, energy and momentum are transferred between the: various regions that form the Solar Terrestrial environment. STEP began in 1990 and will continue until 1997, during which time a number of major new spacecraft and ground-based projects will become operational. Six Working Groups form the basis of STEP, covering topics such as the Sun as a source of energy and disturbance, energy and mass transfer through the interplanetary medium and the magnetosphere-ionosphere system, ionosphere-thermosphere coupling and response to energy and momentum inputs, middle atmosphere responses to forcing from above and below, solar variability effects in the human environment, and informatics. A Royal Astronomical Society Geophysics Discussion meeting took place in March 1992 to draw to the attention of UK scientists the diverse nature of STEP and the opportunities offered by STEP. This paper consists of individual sections prepared by the speakers at the meeting and which cover most of the STEP Working Group topics. The main aims of each section are to provide a "benchmark" for the present status of the research area and to look ahead to the possible contributions that UK scientists can make during STEP.
Journal of Geophysical Research, 1998
We present the Jovian Ionospheric Model (JIM), a time-dependent, three-dimensional model for the ... more We present the Jovian Ionospheric Model (JIM), a time-dependent, three-dimensional model for the thermosphere and ionosphere of Jupiter. We describe the physical inputs for the hydrodynamic, thermodynamic and chemical components of the model, which is based on the UCL Thermosphere Model of Fuller-Rowell and Rees [1980]. We then present the results of an illustrative simulation in which an initially neutral homogeneous planet evolves for approximately 4 Jovian rotations, under the influence of solar illumination and auroral (electron) precipitation at high latitudes. The model shows that solar zenith angle, auroral activity, ion recombination chemistry and, to a lesser degree, magnetic field orientation, all play a role in forming the dayside and nightside global ionization patterns. We compare auroral and nonauroral/equatorial ionospheric compositions and find the signature of ion transport by fast winds. We also include a localized "spot" of precipitation in our model and comment on the associated ionization signatures which develop in response to this Io-like aurora. The simulation also develops strong outflows with velocities up to •600 m s-1 from the auroral regions, driven mainly by pressure gradients. These pressure gradients, in turn, arise from the differences in chemical composition between the auroral and nonauroral upper thermospheres, as evolution proceeds. This preliminary study indicates a strong potential for JIM in analysis of two-dimensional image data and simulation of time-dependent global events. a model that employs solar fluorescence and photoelectron excitation without the need for an extra excitation
Planetary and Space Science, 1985
&ring the period October to December 198 I, the Dpr~unzic.s fixplorer-2 (DE-2) spacecraft success... more &ring the period October to December 198 I, the Dpr~unzic.s fixplorer-2 (DE-2) spacecraft successively observed the South polar and the North polar regions, and recorded the temperature, composition and dynamical structureofthe upper thermosphere. In October 1981, perigeewas about 310km altitude, in the vicinity of the South Pole, with the satellite orbit in the 09.0~21.00 L.T. plane. During late November and December, the perigee bad precessed to the region of the North Pole, with the spacecraft sampling the upper thermosphere in the 06.00 18.00 L.T. plane. DE-2 observed the meridional wind with a FabryyPerot interferometer (FPl), the zonal wind with the wind and temperature spcctrometcr (WATS), the neutral temperature with the FPI, and the neutral atmosphere composition and density with the neutral atmosphere composition spectrometer (NACS). A comparison between the South (summer) Pole and the North (winter) Pole data shows considerable seasonal differences in all neutral atmosphere parameters. The region of the summer pole, under similar geomagnetic and solar activity conditions, and at a level of about 300 km, is about 300 K warmer than that of the winter pole. and the density of atomic oxygen is strongly depleted (and nitrogen enhanced) around the summer pole (compared with the winter pole). Only part of the differences in temperature and composition structure can be related to the seasonal variation of solar insolation, however, and both polar regions display structural variations (with latitude and Universal Time) which are unmistakeable characteristics of strong magnetospheric forcing. The magnitude of the neutral atmosphere perturbations in winds. temperature, density and composition within both summer and winter polar regions all increase with increasing levels of geomagnetic activity. The UCL 3-dimensional time dependent global model has been used to simulate the diurnal, seasonal and geomagnetic response of the neutral thermosphere, attempting to follow the major features of the solar and geomagnetic inputs to the thermosphere which were present during the late 1981 period. In the UCL model, geomagnetic forcing is characterized by semi-empirical models of the polar electric field which show a dependence on the Y component of the Interplanetary Magnetic Field, due to Heppner and Maynard (1983). It is possible to obtain an overall agreement, in both summer and winter hemispheres, with the thermospheric wind structure at high latitudes, and to explain the geomagnetic control of the combined thermal and compositional structure both qualitatively and quantitatively. To obtain such agreement, however, it is essential to enhance the polar ionosphere as a consequence of magnetospheric particle precipitation, reflecting both widespread aurora1 (kilovolt) electrons, and "soft" cusp and polar cap sources. Geomagnetic forcing of the high latitude thermosphere cannot be explained purely by a polar convective electric field, and the thermal as well as ionising properties of these polar and aurora1 electron sources are crucial components of the total geomagnetic input.
Planetary and Space Science, 1986
Journal of Atmospheric and Terrestrial Physics, 1991
A Doppler Imaging System has been used to map the polar thermospheric circulation in the vicinity... more A Doppler Imaging System has been used to map the polar thermospheric circulation in the vicinity of the high-latitude station of Kiruna (67"N, 20"E), in conjunction with extended coverage of ionospheric plasma flow patterns obtained simultaneously by the EISCAT radar facility. Combined wind and radar results were obtained during the nights of 24125 and 25126 November 1987. On 24 November 1987, a strong anti-cyclonic vortex was generated within the region of the dusk auroral oval and polar cap. associated with sunward wind velocities in the dusk amoral oval, of up to 400 m s-'. These winds were generated by the combination of ion drag and the resonant response of the thermosphere. The observations of this sustained anti-cyclonic vortex on the night of 24/25 November confirmed a concept predicted earlier by theoretical modelling. Following the intense activity which led to their generation, such vortices may survive during several hours of decreasing geomagnetic activity. The decrease of viscosity with decreasing altitude and the decrease of plasma density in the ionosphere below 250 km limits the viscous and the electrodynamic dissipation of the thermospheric 'flywheel'.
Journal of Atmospheric and Terrestrial Physics, 1991
Long-tee averages of Fabry-Perot lnte~eromet~r (FPI) observations of the night-time UI ('0) emiss... more Long-tee averages of Fabry-Perot lnte~eromet~r (FPI) observations of the night-time UI ('0) emission at 630 nm from the F-region thern~ospbere at a high latitude site are presented here. The data base contains measurements of thermospher~c neutral winds for every winter period from November 1981 to April 1989 inclusive. This covers nearly one complete solar cycle in terms of the radio and UV/EUV &IX variation, from the last solar maximum to the present solar maximum. The instrument is located in Kiruna, Sweden, which is situated at the equatorward edge of the amoral oval at quiet to moderate levels of geomagnetic activity and beneath the oval at higher Levels of activity. From this location, the FPI has sampled the response of the upper the~osphere winds at night to a wide range of geamagneti~ and solar activity conditions. The data presented here show a significant correlation between the solar radio or EUV fluxes and the response of thermospheric neutral winds to geomagnetic conditions in the amoral oval. For K,, < 2 the amoral oval is polewards of Kiruna and neutral winds are similar at solar minimum and solar maximum. At higher levels of geomagnetic activity, for a given level of activity, the neutral winds are a factor of two greater at high levels of solar flux, than at low levels of solar flux. Alternately, the same winds are seen at Kp = 3 at high solar flux levels, as at K, = 5 for low flux levels. Comparing the situation for high and low solar activity, for a given ievel of geomagnetic activity, in the dusk aurora1 oval, the sunward or westward winds are a factor of two larger at solar maximum. In the midnight period, the equatorward wind is a factor of two larger at high solar activity, compared with low solar activity. In this paper, the observations will be discussed, with the implication that I$ is a rather poor indicator of momentum and energy coupling from the solar wind to the upper thermosphere. In a second paper, the salar-terrestrial processes which cause this phenomenon will be discussed and modelled.
Journal of Atmospheric and Terrestrial Physics, 1994
The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are ... more The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of aurora1 precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B,, regions of sunward thermospheric winds (up to 200 m s-') may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by lowlatitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF BP The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 24 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B, negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting Eand F-region plasma densities. For example, in northern midwinter , sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.
Journal of Physics E: Scientific Instruments
Fabry-Perot etalons are used for a very wide range of applications involving high-resolution spec... more Fabry-Perot etalons are used for a very wide range of applications involving high-resolution spectroscopy, measurement of temperature and velocities by Doppler broadening and Doppler shift analysis, and for a wide range of spectroscopic filtering requirements in scientific instruments [1, 2]. There are a number of different methods of using Fabry-Perot etalons, for example in "fixed gap" or in tuneable configurations [1, 2, 3]. Capacitance-Stabilisation [3] has been used as a method of obtaining extremely precise control of the parallelism and optical path difference of a Fabry-Perot etalon for many years. This paper describes the current state of development of the Electronic Control System for the Hovemere Capacitance-Stabilised Etalon (CSE). The CSE-16 system offers a substantial improvement in the thermal and long-term stability compared with the previous generations of CSE systems (for example the Hovemere CSE-12 and the Queensgate / ICOS systems). The new system also gives the user unprecedented control over the etalon plate parallelism and Optical Path Difference (OPD), with each piezoelectric transducer capable of being driven through more than 64,000 discrete steps. In addition the total power consumption has been reduced and there is a fourfold reduction of the size of the overall system. Special attention has also been paid to minimise the thermal dependence of the electronics itself, by appropriate proprietary design and careful optimisation of PCB topology. The new design provides flexibility by accommodating a USB2.0 compatible interface while maintaining the earlier RS232 interface for compatibility with previous generations. A pre-loaded table of fixed Etalon positions can be stored in a non-volatile memory. In addition, an external trigger input can be used to synchronise the tuning of the Fabry-Perot Interferometer to a user-controlled signal. The gain of each of the three spacers may be controlled independently, providing precise compensation for small but significant inherent differences of sensitivity of the three transducer and amplification chains.
Electromagnetic Coupling in the Polar Clefts and Caps, 1989
ABSTRACT
Journal of Physics E: …, 1982
A triple-etalon Fabry-Perot interferometer has been flown from a high altitude balloon platform a... more A triple-etalon Fabry-Perot interferometer has been flown from a high altitude balloon platform as part of a development programme to implement a suggestion that stratospheric winds could be measured from a suitable spaceborne high resolution interferometer by ...
Pure and Applied Geophysics - PURE APPL GEOPHYS, 1988
The University College London Global Thermospheric Model and the Sheffield University High-Latitu... more The University College London Global Thermospheric Model and the Sheffield University High-Latitude Ionospheric Convection Model have been integrated and improved to simulate the self-consistent interaction of the thermosphere and ionosphere at high latitudes. For mid- and low-latitudes, equatorward of 65 degrees geomagnetic, the neutral thermospheric code maintains the use of an empirical description of plasma densities. The neutral thermospheric wind velocity, composition, density, and energy budget are computed, including their full interactions with the high-latitude ion drift and the evolution of the plasma densities of O+, H+, NO+, N2+, and O2+. Two 24 hr Universal Time (UT) simulations have been performed at high solar activity, for a level of moderate geomagnetic activity, at the June and December solstices, to investigate the UT and seasonal response of the coupled system. During winter, the diurnal migration of the polar convection pattern into and out of sunlight, togethe...
Journal of Atmospheric and Terrestrial Physics, 1982
Abstract A model of the O+ and H+ distributions in the plasmasphere and high-latitude ionosphere ... more Abstract A model of the O+ and H+ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O+ and H+ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner ...
Planetary and space science, 1983
Seven polar passes of the NASA ~y~arnics Enpiorer 2 (DE-2) satellite during October and early Dec... more Seven polar passes of the NASA ~y~arnics Enpiorer 2 (DE-2) satellite during October and early December 1981 have been used to examine the high-latitude circulation in the upper thermosphere. Vector winds along the satellite track are derived by appropriate merging of thedata from the remote-sensing Fabry-Perotinterferometer(meridiona1 wind)and the insitu wind andtemperaturespectrometer(zona1 wind)andare compared with the predictions of a three-dimensional, time-dependent, global model of the thermosphere. Major features of the experimental winds, such as the mean day to night circulation caused by solar u.v. and e.u.v. heating, augmented by magnetospheric processes at high latitude and the sharp boundaries and flow reversals imposed on thermospheric winds by momentum transfer (ion drag) from the magnetosphere, are qualitatively explained by a version of the global model using a semi-empirical global model of polar electric fields (Volland Model 2 or Heppner Mode1 A) and a model ofglobal electron density which excludes the effects
Annales Geophysicae, 2000
Journal of Atmospheric and Terrestrial Physics, 1979
Abatr&-During the period December 1975-mid-February 1976 of the Western European Winter Anomaly C... more Abatr&-During the period December 1975-mid-February 1976 of the Western European Winter Anomaly Campaign, the local dynamical and thermal structure of the stratosphere and mesosphere was determined by some 31 rocket launches at El Arenosillo, Spain, while the mesospheric thermal structure in the winter (northern) hemisphere was derived from Nimbus 6 satellite observations. An analysis of these data is presented here. As determined by the A3 technique and the parameter L,,[ = L&I + l)], ionospheric absorption is significantly correlated with many features of the dynamical and thermal structure of the atmosphere. The strongest correlations are, on this occasion, with local win&, at a level between 85 and 92 km altitude, the sense being that high absorption corresponds to northward and eastward wind tendencies, while low absorption comsponds to southward and w+ward wind tendencies. Additionally, there is a significant correlation with local temperature above 60 km, high absorption corresponding to high mesospheric temperatures. These local variations are corroborated by data obtained from the PMR 3000 channel of the Nimbus 6 satellite (centred about 80 km). A trivariate analysis of absorption (L,,), temperature (40" latitude) and meridional temperature gradient along the 0' longitude meridian, which corresponds to the pressure gradient driving a 'geostrophic' xonal wind, shows extremely high significance. High ionospheric absorption in Spain is associated with disruption of the normally strong winter vortex around a warm winter mesospheric pole. As the polar temperature decreases, temperatures at low-and mid-latitudes increase and, responding to a decrease of meridional temperature gradient, the mid-latitude zonal and meridonal wind regime changes. Disturbances of the winter mesospheric polar vortex are triggered by the upward propagation of earlier perturbations within the troposphere and/or stratosphere. Delays of 2-4 days are indicated between the onset of stratospheric circulation disturbances, perturbation of the mesospheric polar vortex, and the consequent dynamical and thermal effects in the mid-latitude mesosphere which are associated with anomalously high ionospheric absorption.
Advances in Space Research, 2009
During recent years, special attention has been paid to the background circulation of the middle ... more During recent years, special attention has been paid to the background circulation of the middle atmosphere and, including a variety of new measurements particularly using radar especially in the mesosphere/lower thermosphere (MLT) region, to the comparison of existing empirical middle atmosphere wind models as CIRA-86 and HWM with new data. This leads to the construction of empirical models of MLT winds as the Global Empirical Wind Model (GEWM). Further investigations aim at the construction of new empirical and semi-empirical wind models of the whole middle atmosphere including the new experimental results. Results of a new wind climatology (0-100 km) are presented based upon the GEWM, stratospheric reanalysis data, and a numerical model to fill the gap between stratospheric and MLT data. Recently direct wind observations from the wind imaging interferometer (WINDII, Wang et al., 1997) and the high-resolution Doppler imager (HRDI, Fleming et al., 1996) on board the Upper Atmosphere Research Satellite (UARS) have provided a new global wind data set for the upper mesosphere/lower thermosphere region. From these data, also a vertical wind model (Fauliot et al., 1997) has been constructed. Fleming et al. (1996) and Portnyagin et al. (1999) concluded that, in general, the space-based zonal wind models exhibited significant differences relative to the ground-based models. In contrast, however, Fauliot et al. (1997) have stated that the WINDII-based prevailing meridional model winds have cellular structure, similar to those from the ground-based Portnyagin et al.
Astronomy and Astrophysics, 2006
Aims. A tunable, high spectral resolution, high effective finesse, vacuum ultraviolet (VUV) Fabry... more Aims. A tunable, high spectral resolution, high effective finesse, vacuum ultraviolet (VUV) Fabry-Perot interferometer (FPI) is designed for obtaining narrow-passband images, magnetograms, and Dopplergrams of the transition region emission line of CIV (155 nm). Methods. The integral part of the CIV narrow passband filter package (with a 2-10 pm FWHM) consists of a multiple etalon system composed of a tunable interferometer that provides high-spectral resolution and a static low-spectral resolution interferometer that allows a large effective free spectral range. The prefilter for the interferometers is provided by a set of four mirrors with dielectric high-reflective coatings. A tunable VUV piezoelectric-control interferometer has undergone testing using the surrogate F 2 eximer laser line at 157 nm for the CIV line. We present the results of these tests with a description of the overall concept for a complete narrow-band CIV spectral filter. The static interferometer of the filter will be built using a set of fixed MgF 2 plates. The four-mirror prefilter is designed to have dielectric multilayer Π-stacks employing the concept used in the Ultraviolet Imager of NASA's Polar Spacecraft. A 10-pm dual etalon system allows the effective free spectral range to be commensurate with the prefilter profile. With an additional etalon, a triple etalon system would allow a spectrographic resolution of 2 pm. The basic strategy has been to combine the expertise of spaceflight etalon manufacturing with VUV coating technology to build a VUV FPI which combines the best attributes of imagers and spectrographs into a single compact instrument. Results. High spectral-resolution spectro-polarimetry observations of the transition region CIV emission can be performed to increase our understanding of the magnetic forces, mass motion, evolution, and energy release within the solar atmosphere at the base of the corona where most of the magnetic field is approximately force-free. The 2D imaging of the full vector magnetic field at the height of maximum magnetic influence (minimum plasma beta) can be accomplished, albeit difficult, by measuring the Zeeman splitting of the CIV resonance pair. Designs of multiple VUV FPIs can be developed for integration into future orbiting solar observatories to obtain rapid cadence, spectral imaging of the transition region.
Surveys in Geophysics, 1993
The Solar Terrestrial Energy Programme (STEP) aims to improve our general understanding of how ma... more The Solar Terrestrial Energy Programme (STEP) aims to improve our general understanding of how mass, energy and momentum are transferred between the: various regions that form the Solar Terrestrial environment. STEP began in 1990 and will continue until 1997, during which time a number of major new spacecraft and ground-based projects will become operational. Six Working Groups form the basis of STEP, covering topics such as the Sun as a source of energy and disturbance, energy and mass transfer through the interplanetary medium and the magnetosphere-ionosphere system, ionosphere-thermosphere coupling and response to energy and momentum inputs, middle atmosphere responses to forcing from above and below, solar variability effects in the human environment, and informatics. A Royal Astronomical Society Geophysics Discussion meeting took place in March 1992 to draw to the attention of UK scientists the diverse nature of STEP and the opportunities offered by STEP. This paper consists of individual sections prepared by the speakers at the meeting and which cover most of the STEP Working Group topics. The main aims of each section are to provide a "benchmark" for the present status of the research area and to look ahead to the possible contributions that UK scientists can make during STEP.
Journal of Geophysical Research, 1998
We present the Jovian Ionospheric Model (JIM), a time-dependent, three-dimensional model for the ... more We present the Jovian Ionospheric Model (JIM), a time-dependent, three-dimensional model for the thermosphere and ionosphere of Jupiter. We describe the physical inputs for the hydrodynamic, thermodynamic and chemical components of the model, which is based on the UCL Thermosphere Model of Fuller-Rowell and Rees [1980]. We then present the results of an illustrative simulation in which an initially neutral homogeneous planet evolves for approximately 4 Jovian rotations, under the influence of solar illumination and auroral (electron) precipitation at high latitudes. The model shows that solar zenith angle, auroral activity, ion recombination chemistry and, to a lesser degree, magnetic field orientation, all play a role in forming the dayside and nightside global ionization patterns. We compare auroral and nonauroral/equatorial ionospheric compositions and find the signature of ion transport by fast winds. We also include a localized "spot" of precipitation in our model and comment on the associated ionization signatures which develop in response to this Io-like aurora. The simulation also develops strong outflows with velocities up to •600 m s-1 from the auroral regions, driven mainly by pressure gradients. These pressure gradients, in turn, arise from the differences in chemical composition between the auroral and nonauroral upper thermospheres, as evolution proceeds. This preliminary study indicates a strong potential for JIM in analysis of two-dimensional image data and simulation of time-dependent global events. a model that employs solar fluorescence and photoelectron excitation without the need for an extra excitation
Planetary and Space Science, 1985
&ring the period October to December 198 I, the Dpr~unzic.s fixplorer-2 (DE-2) spacecraft success... more &ring the period October to December 198 I, the Dpr~unzic.s fixplorer-2 (DE-2) spacecraft successively observed the South polar and the North polar regions, and recorded the temperature, composition and dynamical structureofthe upper thermosphere. In October 1981, perigeewas about 310km altitude, in the vicinity of the South Pole, with the satellite orbit in the 09.0~21.00 L.T. plane. During late November and December, the perigee bad precessed to the region of the North Pole, with the spacecraft sampling the upper thermosphere in the 06.00 18.00 L.T. plane. DE-2 observed the meridional wind with a FabryyPerot interferometer (FPl), the zonal wind with the wind and temperature spcctrometcr (WATS), the neutral temperature with the FPI, and the neutral atmosphere composition and density with the neutral atmosphere composition spectrometer (NACS). A comparison between the South (summer) Pole and the North (winter) Pole data shows considerable seasonal differences in all neutral atmosphere parameters. The region of the summer pole, under similar geomagnetic and solar activity conditions, and at a level of about 300 km, is about 300 K warmer than that of the winter pole. and the density of atomic oxygen is strongly depleted (and nitrogen enhanced) around the summer pole (compared with the winter pole). Only part of the differences in temperature and composition structure can be related to the seasonal variation of solar insolation, however, and both polar regions display structural variations (with latitude and Universal Time) which are unmistakeable characteristics of strong magnetospheric forcing. The magnitude of the neutral atmosphere perturbations in winds. temperature, density and composition within both summer and winter polar regions all increase with increasing levels of geomagnetic activity. The UCL 3-dimensional time dependent global model has been used to simulate the diurnal, seasonal and geomagnetic response of the neutral thermosphere, attempting to follow the major features of the solar and geomagnetic inputs to the thermosphere which were present during the late 1981 period. In the UCL model, geomagnetic forcing is characterized by semi-empirical models of the polar electric field which show a dependence on the Y component of the Interplanetary Magnetic Field, due to Heppner and Maynard (1983). It is possible to obtain an overall agreement, in both summer and winter hemispheres, with the thermospheric wind structure at high latitudes, and to explain the geomagnetic control of the combined thermal and compositional structure both qualitatively and quantitatively. To obtain such agreement, however, it is essential to enhance the polar ionosphere as a consequence of magnetospheric particle precipitation, reflecting both widespread aurora1 (kilovolt) electrons, and "soft" cusp and polar cap sources. Geomagnetic forcing of the high latitude thermosphere cannot be explained purely by a polar convective electric field, and the thermal as well as ionising properties of these polar and aurora1 electron sources are crucial components of the total geomagnetic input.
Planetary and Space Science, 1986
Journal of Atmospheric and Terrestrial Physics, 1991
A Doppler Imaging System has been used to map the polar thermospheric circulation in the vicinity... more A Doppler Imaging System has been used to map the polar thermospheric circulation in the vicinity of the high-latitude station of Kiruna (67"N, 20"E), in conjunction with extended coverage of ionospheric plasma flow patterns obtained simultaneously by the EISCAT radar facility. Combined wind and radar results were obtained during the nights of 24125 and 25126 November 1987. On 24 November 1987, a strong anti-cyclonic vortex was generated within the region of the dusk auroral oval and polar cap. associated with sunward wind velocities in the dusk amoral oval, of up to 400 m s-'. These winds were generated by the combination of ion drag and the resonant response of the thermosphere. The observations of this sustained anti-cyclonic vortex on the night of 24/25 November confirmed a concept predicted earlier by theoretical modelling. Following the intense activity which led to their generation, such vortices may survive during several hours of decreasing geomagnetic activity. The decrease of viscosity with decreasing altitude and the decrease of plasma density in the ionosphere below 250 km limits the viscous and the electrodynamic dissipation of the thermospheric 'flywheel'.
Journal of Atmospheric and Terrestrial Physics, 1991
Long-tee averages of Fabry-Perot lnte~eromet~r (FPI) observations of the night-time UI ('0) emiss... more Long-tee averages of Fabry-Perot lnte~eromet~r (FPI) observations of the night-time UI ('0) emission at 630 nm from the F-region thern~ospbere at a high latitude site are presented here. The data base contains measurements of thermospher~c neutral winds for every winter period from November 1981 to April 1989 inclusive. This covers nearly one complete solar cycle in terms of the radio and UV/EUV &IX variation, from the last solar maximum to the present solar maximum. The instrument is located in Kiruna, Sweden, which is situated at the equatorward edge of the amoral oval at quiet to moderate levels of geomagnetic activity and beneath the oval at higher Levels of activity. From this location, the FPI has sampled the response of the upper the~osphere winds at night to a wide range of geamagneti~ and solar activity conditions. The data presented here show a significant correlation between the solar radio or EUV fluxes and the response of thermospheric neutral winds to geomagnetic conditions in the amoral oval. For K,, < 2 the amoral oval is polewards of Kiruna and neutral winds are similar at solar minimum and solar maximum. At higher levels of geomagnetic activity, for a given level of activity, the neutral winds are a factor of two greater at high levels of solar flux, than at low levels of solar flux. Alternately, the same winds are seen at Kp = 3 at high solar flux levels, as at K, = 5 for low flux levels. Comparing the situation for high and low solar activity, for a given ievel of geomagnetic activity, in the dusk aurora1 oval, the sunward or westward winds are a factor of two larger at solar maximum. In the midnight period, the equatorward wind is a factor of two larger at high solar activity, compared with low solar activity. In this paper, the observations will be discussed, with the implication that I$ is a rather poor indicator of momentum and energy coupling from the solar wind to the upper thermosphere. In a second paper, the salar-terrestrial processes which cause this phenomenon will be discussed and modelled.
Journal of Atmospheric and Terrestrial Physics, 1994
The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are ... more The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of aurora1 precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B,, regions of sunward thermospheric winds (up to 200 m s-') may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by lowlatitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF BP The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 24 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B, negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting Eand F-region plasma densities. For example, in northern midwinter , sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.
Journal of Physics E: Scientific Instruments
Fabry-Perot etalons are used for a very wide range of applications involving high-resolution spec... more Fabry-Perot etalons are used for a very wide range of applications involving high-resolution spectroscopy, measurement of temperature and velocities by Doppler broadening and Doppler shift analysis, and for a wide range of spectroscopic filtering requirements in scientific instruments [1, 2]. There are a number of different methods of using Fabry-Perot etalons, for example in "fixed gap" or in tuneable configurations [1, 2, 3]. Capacitance-Stabilisation [3] has been used as a method of obtaining extremely precise control of the parallelism and optical path difference of a Fabry-Perot etalon for many years. This paper describes the current state of development of the Electronic Control System for the Hovemere Capacitance-Stabilised Etalon (CSE). The CSE-16 system offers a substantial improvement in the thermal and long-term stability compared with the previous generations of CSE systems (for example the Hovemere CSE-12 and the Queensgate / ICOS systems). The new system also gives the user unprecedented control over the etalon plate parallelism and Optical Path Difference (OPD), with each piezoelectric transducer capable of being driven through more than 64,000 discrete steps. In addition the total power consumption has been reduced and there is a fourfold reduction of the size of the overall system. Special attention has also been paid to minimise the thermal dependence of the electronics itself, by appropriate proprietary design and careful optimisation of PCB topology. The new design provides flexibility by accommodating a USB2.0 compatible interface while maintaining the earlier RS232 interface for compatibility with previous generations. A pre-loaded table of fixed Etalon positions can be stored in a non-volatile memory. In addition, an external trigger input can be used to synchronise the tuning of the Fabry-Perot Interferometer to a user-controlled signal. The gain of each of the three spacers may be controlled independently, providing precise compensation for small but significant inherent differences of sensitivity of the three transducer and amplification chains.
Electromagnetic Coupling in the Polar Clefts and Caps, 1989
ABSTRACT
Journal of Physics E: …, 1982
A triple-etalon Fabry-Perot interferometer has been flown from a high altitude balloon platform a... more A triple-etalon Fabry-Perot interferometer has been flown from a high altitude balloon platform as part of a development programme to implement a suggestion that stratospheric winds could be measured from a suitable spaceborne high resolution interferometer by ...
Pure and Applied Geophysics - PURE APPL GEOPHYS, 1988
The University College London Global Thermospheric Model and the Sheffield University High-Latitu... more The University College London Global Thermospheric Model and the Sheffield University High-Latitude Ionospheric Convection Model have been integrated and improved to simulate the self-consistent interaction of the thermosphere and ionosphere at high latitudes. For mid- and low-latitudes, equatorward of 65 degrees geomagnetic, the neutral thermospheric code maintains the use of an empirical description of plasma densities. The neutral thermospheric wind velocity, composition, density, and energy budget are computed, including their full interactions with the high-latitude ion drift and the evolution of the plasma densities of O+, H+, NO+, N2+, and O2+. Two 24 hr Universal Time (UT) simulations have been performed at high solar activity, for a level of moderate geomagnetic activity, at the June and December solstices, to investigate the UT and seasonal response of the coupled system. During winter, the diurnal migration of the polar convection pattern into and out of sunlight, togethe...
Journal of Atmospheric and Terrestrial Physics, 1982
Abstract A model of the O+ and H+ distributions in the plasmasphere and high-latitude ionosphere ... more Abstract A model of the O+ and H+ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O+ and H+ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner ...
Planetary and space science, 1983
Seven polar passes of the NASA ~y~arnics Enpiorer 2 (DE-2) satellite during October and early Dec... more Seven polar passes of the NASA ~y~arnics Enpiorer 2 (DE-2) satellite during October and early December 1981 have been used to examine the high-latitude circulation in the upper thermosphere. Vector winds along the satellite track are derived by appropriate merging of thedata from the remote-sensing Fabry-Perotinterferometer(meridiona1 wind)and the insitu wind andtemperaturespectrometer(zona1 wind)andare compared with the predictions of a three-dimensional, time-dependent, global model of the thermosphere. Major features of the experimental winds, such as the mean day to night circulation caused by solar u.v. and e.u.v. heating, augmented by magnetospheric processes at high latitude and the sharp boundaries and flow reversals imposed on thermospheric winds by momentum transfer (ion drag) from the magnetosphere, are qualitatively explained by a version of the global model using a semi-empirical global model of polar electric fields (Volland Model 2 or Heppner Mode1 A) and a model ofglobal electron density which excludes the effects
Annales Geophysicae, 2000