Showing papers in "Geophysical Research Letters in 1989"
TL;DR: In this paper, a global average model of the coupled mesosphere, thermosphere, and ionosphere is used to examine the effect of trace gas variations on the overall structure of these regions.
Abstract: A global average model of the coupled mesosphere, thermosphere, and ionosphere is used to examine the effect of trace gas variations on the overall structure of these regions. In particular, the variations caused by CO2 and CH4 doublings and halvings from present day mixing ratios are presented. The results indicate that the mesosphere and thermosphere temperatures will cool by about 10 K and 50 K, respectively, as the CO2 and CH4 mixing ratios are doubled. These regions are heated by similar amounts when the trace gas mixing ratios are halved. Compositional redistributions also occur in association with changes in the temperature profile. The results show that global change will occur in the upper atmosphere and ionosphere as well as in the lower atmosphere during the 21st century.
558 citations
TL;DR: In this article, it is suggested that the polar heliospheric magnetic field at large heliocentric distances may deviate considerably from the generally accepted Archimedean spiral, and that the large-scale field near the poles may be dominated by randomly oriented transverse magnetic fields with magnitude much larger than the average spiral.
Abstract: It is suggested that the polar heliospheric magnetic field, at large heliocentric distances, may deviate considerably from the generally accepted Archimedean spiral. Instead, it is suggested that the large-scale field near the poles may be dominated by randomly-oriented transverse magnetic fields with magnitude much larger than the average spiral. The average vector field is still the spiral, but the average magnitude may be much larger. In addition, the field direction is transverse to the radial direction most of the time instead of being nearly radial. This magnetic-field structure has important consequences for the transport of cosmic rays. Preliminary model calculations suggest changes in the radial gradient of galactic cosmic rays which may improve agreement with observations.
356 citations
TL;DR: In this article, the authors performed radiative transfer calculations for volcanic ash clouds assumed to consist of mixtures of ash particles, ice spheres, water droplets and ash particles coated with sulphuric acid.
Abstract: Radiative transfer calculations are performed for volcanic ash clouds assumed to consist of mixtures of ash particles, ice spheres, water droplets and ash particles coated with sulphuric acid The results show that for nascent volcanic eruption clouds a reverse absorption effect is noticeable in the infrared window between 10µm and 13µm, for which absorption of infrared radiation increases with increasing wavelength This effect may be of use in detecting hazardous volcanic clouds from space
322 citations
TL;DR: In this article, a partial melt undergoing large scale deformation is shown to be unstable with respect to small scale redistribution of melt, provided the shear viscosity of the matrix depends on melt fraction.
Abstract: A partial melt undergoing large scale deformation is shown to be unstable with respect to small scale redistribution of melt, provided the shear viscosity of the matrix depends on melt fraction. In the physically realistic case where melt “softens” the matrix, melt migrates along the direction parallel to the axis of minimum compressive stress and accumulates in “veins” (melt-rich lenses). The maximum growth rate of the instability is of order —⋵_od ln η/df (⋵_o ≡ largest component of the principal strain tensor, η = shear viscosity, f = melt fraction), and plausibly fast enough to invalidate the conventional applications of Darcy's law (e.g., mid-ocean ridges, subduction zones) since melt migrates into veins in preference to being pervasively flushed vertically. The preferred lengthscale of the instability is poorly determined but probably ∼ meters. Veins may eventually form an interconnected drainage network, allowing rapid vertical flushing of melt.
299 citations
TL;DR: In this article, the authors used calculated, as well as observed ozone profiles at the station Hohenpeisenberg in Bavaria, FRG, to show that decreases of UV-B radiation at the earth's surface are possible even when total ozone declines.
Abstract: Increases in tropospheric ozone due to photochemical production, mostly due to growing industrial and technological NOx- emissions, in the industrialized northern hemisphere can overcompensate for increased UV-B radiation resulting from ozone depletions due to chlorine-catalyzed reactions in the stratosphere. This is especially the case in the summer months, because during this period a large fraction of the UV, which reaches the surface as diffuse radiation, is scattered by air molecules, droplets and aerosol particles in the troposphere. This causes a relatively longer path through tropospheric than through stratospheric ozone. Analyses using calculated, as well as observed ozone profiles at the station Hohenpeisenberg in Bavaria, FRG, show, that decreases of UV-B radiation at the earth's surface are possible even when total ozone declines.
253 citations
TL;DR: In this paper, a Doppler Lidar is used to measure the wave-mean flow interaction in the middle atmosphere using Rayleigh scattering from air molecules, which is designed to cover the height range 25-60 km, where radars cannot operate.
Abstract: The possibility of measuring winds in the middle atmosphere with a Doppler lidar has just been demonstrated. It is aimed at studying the wave-mean flow interaction, when used is association with the Rayleigh lidar providing density and temperature profiles and their fluctuations. The new Doppler lidar relies on the Rayleigh scattering from air molecules is designed to cover the height range 25-60 km, a region where radars cannot operate. The Doppler shift to the backscattered echo is measured by inter-comparing the signal detected through each of the two high-resolution, narrow band-pass Fabry-Perot interferometers tuned on either side of the emitted laser line.
232 citations
TL;DR: In this paper, it is inferred that the observed northwestward progression of ruptures on cross-faults may continue, and the authors hypothesize that rupture of this crossfault may trigger rupture on either of these main faults by a mechanism similar to that which occurred in the Superstition Hills earthquake sequence.
Abstract: Two large strike-slip ruptures 11.4 hours apart occurred on intersecting, nearly orthogonal, vertical faults during the November 1987 Superstition Hills earthquake sequence in southern California. This sequence is the latest in a northwestward progression of earthquakes (1979, 1981, and 1987) rupturing a set of parallel left-lateral cross-faults that trend northeast between the Brawley seismic zone and Superstition Hills fault, a northwest trending main strand of the San Jacinto fault zone. The first large event (M{sub s} = 6.2) in the 1987 sequence ruptured the Elmore Ranch fault, a cross-fault that strikes northeasterly between the Brawley seismic zone and the Superstition Hills main fault. The second event (M{sub s} = 6.6) initiated its rupture at the intersection of the cross-fault and main fault and propagated towards the southeast along the main fault. The following hypotheses are advanced; (1) slip on the cross-fault locally decreased normal stress on the main fault, and triggered the main fault rupture after a delay; and (2) the delay was caused by fluid diffusion. It is inferred that the observed northwestward progression of ruptures on cross-faults may continue. The next cross-fault expected to rupture intersects both the San Andreas fault and the San Jacinto fault zone. Themore » authors hypothesize that rupture of this cross-fault may trigger rupture on either of these main faults by a mechanism similar to that which occurred in the Superstition Hills earthquake sequence.« less
203 citations
TL;DR: In this article, the temperature distribution across the transition zone has been determined on the high pressure phase equilibria in the system Mg2SiO4-Fe 2 SiO4 recently determined by Katsura and Ito [1988] and Itó and Takahashi [1988], assuming that spinel dissociation is responsible for the sharp seismic discontinuity implied by study of reflected waves.
Abstract: The temperature distribution across the transition zone has been determined on the high-pressure phase equilibria in the system Mg2SiO4-Fe2SiO4 recently determined by Katsura and Ito [1988] and Ito and Takahashi [1988]. The temperature at the base of the transition zone (655 km depth) is estimated to be about 1600°C, assuming that the spinel dissociation is responsible for the sharp seismic discontinuity implied by study of reflected waves. This estimation would be little modified for the chemically stratified mantle models, since the lower mantle is characterized by the stability of perovskite. The temperatures in the region from 350 km to 655 km depth are calculated for a peridotitic mantle model, assuming an adiabatic temperature gradient. The α-β and β-γ transformations in (Mg,Fe)2SiO4 make the temperature profile steep in the depth regions of 390–415 km and 500–530 km, respectively. The temperature would increase from about 1400°C to 1600°C over the depths interval 350 between 655 km.
203 citations
TL;DR: In this article, it was shown that (Mg,Fe)SiO3 perovskite reacts chemically with liquid iron at pressures greater than 70 GPa and temperatures above 3700 K X-ray diffraction analyses of quenched samples demonstrate that the reaction products include SiO2 stishovite and iron alloys (Fe,Mg)xO and FexSiy, at the interface between the Fe and silicate.
Abstract: Experiments with the laser-heated diamond cell show that (Mg,Fe)SiO3 perovskite reacts chemically with liquid iron at pressures greater than 70 GPa and temperatures above 3700 K X-ray diffraction analyses of quenched samples demonstrate that the reaction products include SiO2 stishovite and iron alloys (Fe,Mg)xO and FexSiy, at the interface between the Fe and silicate Our results suggest that similar chemical reactions occur at the Earth's core-mantle boundary, forming chemical heterogeneities composed of silicate-rich and iron alloy-rich regions in the D′ layer This scenario is consistent with seismological observations
200 citations
TL;DR: In this paper, large scale one-dimensional hybrid simulations with resistive electrons have been carried out of a quasi-parallel high-Mach-number collisionless shock, where the shock initially appears stable, but then exhibits cyclic behavior.
Abstract: Large scale one-dimensional hybrid simulations with resistive electrons have been carried out of a quasi-parallel high-Mach-number collisionless shock. The shock initially appears stable, but then exhibits cyclic behavior. For the magnetic field, the cycle consists of a period when the transition from upstream to downstream is steep and well defined, followed by a period when the shock transition is extended and perturbed. This cyclic shock solution results from upstream perturbations caused by backstreaming gyrating ions convecting into the shock. The cyclic reformation of a sharp shock transition can allow ions, at one time upstream because of reflection or leakage, to contribute to the shock thermalization.
184 citations
TL;DR: In this paper, it was shown that the thermal expansivity at high pressure may be described by a constant value of (∂1nα/∂ 1nV)T = 5.5 ± 0.5.
Abstract: New data on thermal expansivity, α, of iron to over 400 kbar in laser- and electrically-heated diamond anvil cells and on MgO and forsterite from spectroscopic data to over 200 kbar and previously measured adiabats [Boehler, 1982], imply that α at high pressure may be described by a constant value of (∂1nα/∂1nV)T = 5.5 ± 0.5. This behavior is supported by previous α measurements at lower pressures on a large variety of materials. Densities of magnesio-wustite and silicate perovskite in a pyrolitic ratio calculated at lower mantle conditions using a finite strain equation and α at high pressures, using the above systematics, match those in PREM through the entire lower mantle, making a strong case for a uniform mantle.
TL;DR: In this article, the authors present an internally consistent calculation which leads to a narrow "conduit" of rapid vertical advection and melting of mantle under a spreading center, where the viscosity of the mantle is considered to be a strong function of the amount of partial melt present.
Abstract: We present the first internally consistent calculation which leads to a narrow ‘conduit’ of rapid vertical advection and melting of mantle under a spreading center. In this model, mantle flow is driven by plate separation and compositional buoyancy. Melt segregation is described as flow through a permeable media. The major new feature is that the viscosity of the mantle is considered to be a strong function of the amount of partial melt present. Experiments show that the bulk viscosity of a partially molten rock is sharply reduced when the melt fraction exceeds a critical value. In the model, the viscosity is reduced as the critical melt fraction is approached. Whether or not a critical melt fraction can be reached under a spreading center depends on the mantle permeability for melt flow. The width of the upwelling area is controlled by the magnitude of the melt related viscosity reduction. Crust should be formed above the focused upwelling. Seismic observations show that the region of crustal accretion is only a few kilometers wide at fast spreading centers. With a viscosity reduction of three orders of magnitude the model predicts a zone of crustal accretion of this width.
TL;DR: In this paper, a sampling of the Devono-Carboniferous transition (375-320 my) at several global sites reveals a rapid change in carbonate isotopic compositions.
Abstract: Monomineralic, abiotic marine cements formed in low-latitude Phanerozoic reefs provide the direction and amplitude of secular variation of {delta}{sup 13}C and {delta}{sup 18}O in marine calcite and defines two end member compositions - 580 to 360 my ({minus}7 to {minus}5{per thousand}{delta}{sup 18}O{sub PDB}) and 360 to present ({minus}3 to 0{per thousand}{delta}{sup 18}O{sub PDB}). Sampling of the Devono-Carboniferous transition (375-320 my) at several global sites reveals a rapid change in carbonate isotopic compositions. Bracketed within Fammenian to Early Visean-aged strata, a 7 to 15 my time interval, this shift corresponds to a 2% offset in mean {delta}{sup 13}C and 3-4% offset in {delta}{sup 18}O. The abruptness of such change, and its overall correlation with variations in {sup 87}Sr/{sup 86}Sr, {delta}{sup 34}S, {delta}{sup 13}C, and Li/Al ratios in marine sediments suggests a primary offset in marine water composition.
TL;DR: In this article, the authors present simultaneous solar wind plasma observations for two previously reported transient ground events observed at dayside auroral latitudes, which were associated with the postulated signatures of patchy, sporadic, merging of magnetosheath and magnetospheric magnetic field lines at the dayside magnetospause.
Abstract: Contrary to the prevailing popular view, we find some transient ground events with bipolar north-south signatures are related to variations in solar wind dynamic pressure and not necessarily to magnetic merging. We present simultaneous solar wind plasma observations for two previously reported transient ground events observed at dayside auroral latitudes. During the first event, originally reported by Lanzerotti et al. (1987), conjugate ground magnetometers recorded north-south magetic field deflections in the east-west and vertical directions. The second event was reported by Todd et al. (1986), we noted ground rader observations indicating strong northward then southward ionospheric flows. The events were associated with the postulated signatures of patchy, sporadic, merging of magnetosheath and magnetospheric magnetic field lines at the dayside magnetospause, known as flux transfer events. Conversely, we demonstrate that the event reported by Lanzerotti et al. was accompanied by a sharp increase in solar wind dynamic pressure, a magnetospheric compression, and a consequent ringing of the magnetospheric magnetic field. The event reported by Todd et al. was associated with a brief but sharp increase in the solar wind dynamic pressure. copyright American Geophysical Union 1989
TL;DR: The grain size dependent behavior of pyrrhotite during a magnetic LT-transition at 34 K is suitable for granulometric purposes as discussed by the authors, and it is shown that cycling down to liquid Helium temperature suppresses soft MD Jrs more than hard SD Jrs, so that it may be effective in removing soft secondary NRM components.
Abstract: The grain-size dependent behavior of pyrrhotite during a magnetic LT-transition at 34 K is suitable for granulometric purposes. Cycling down to liquid Helium temperature suppresses soft MD Jrs more than hard SD Jrs, so that it may be effective in removing soft secondary NRM components. The domain structure of large pyrrhotite grains changes from MD at room temperature to SD or PSD below the LT-transition.
TL;DR: In this paper, the authors consider the consequences of the reconnection that occurs randomly on the dayside magnetopause and propose that the solar wind plasma streams into the magnetosphere along these field lines as long as they are open, and stays there after the field lines become closed again.
Abstract: The author considers consequences of the reconnection that occurs randomly on the dayside magnetopause. Open field lines produced by such reconnection events often intersect with each other, and they do not necessarily show some of the features that would be expected from the reconnection if this mutual interaction were absent. Some of the randomly produced open field lines would reconnect and result in closed field lines. The solar wind plasma streams into the magnetosphere along these field lines as long as they are open, however, and stays there after the field lines become closed again. He proposes that this is the mechanism of the plasma supply into the low-latitude boundary layer. He shows that many of the observed signatures of the LLBL are consistent with this model.
TL;DR: The dynamical effect of temporal variations in the zonal harmonics of the earth's gravitational potential due to tidal and meteorological mass redistribution has been observed using satellite laser ranging (SLR) measurements to Starlette as mentioned in this paper.
Abstract: The dynamical effect of temporal variations in the zonal harmonics of the earth's gravitational potential due to tidal and meteorological mass redistribution has been observed using satellite laser ranging (SLR) measurements to Starlette. The secular variations in the Earth's zonal harmonics and the year-to-year fluctuation of the even degree annual and semi-annual tides, S(a) and S(sa) have been determined from a continuous Starlette orbit spanning from 1983 through 1985. The mean values for S(a) and S(sa) tides are in good agreement with other tide solutions. The year-to-year fluctuations of the even degree S(a), deduced from Starlette node variations, are about 25 percent of the mean value obtained during the three-year Starlette orbit span.
TL;DR: In this paper, the magnetic properties of fine-grained magnetite produced by two newly isolated anaerobic bacteria, a magnetotactic bacterium (MV-1) and a dissimilatory iron-reducing (GS-15), were compared.
Abstract: We compare the magnetic properties of fine-grained magnetite produced by two newly isolated anaerobic bacteria, a magnetotactic bacterium (MV-1) and a dissimilatory iron-reducing bacterium (GS-15). Although room-temperature magnetic properties are generally different between the two microorganisms, MV-1 and GS-15 magnetites can be most easily distinguished by the temperature variation of saturation remanence obtained at liquid helium temperatures. Magnetite produced by MV-1 displays a sharp discontinuity in intensity at 100 K related to the Verwey transition. Magnetite produced by GS-15 displays a gradual decrease in intensity with temperature due to the progressive unblocking of magnetization. The differing behavior is due exclusively to different grain size distributions produced by these microorganisms. MV-1 produces magnetite with a narrow grain size distribution that is within the stable single domain size range at room temperature and below. GS-15 produces magnetite with a wide grain size distribution extending into the superparamagnetic (SPM) size range. Our results show that a substantial fraction of particles produced by GS-15 are SPM at room temperature.
TL;DR: The authors measured ultraviolet solar spectra from Palmer Station, Antarctica during the Austral spring of 1988 and showed that the noontime irradiance observed in the wavelength band 295-305 nm on 19 October, two months prior to summer solstice, exceeded any value measured through 21 December.
Abstract: Measurements of ultraviolet solar spectra from Palmer Station, Antarctica have defined the surface radiation environment of the region during the Austral spring of 1988. At wavelengths where absorption by ozone is negligible, 335–345 nm, the noontime irradiances show the expected gradual increase from the first day of measurements, 19 September, through 21 December. Large variations related to cloudiness are imposed on this background. At wavelengths less than 310 nm the influence of the 1988 ozone “hole” is apparent. The noontime irradiance observed in the wavelength band 295–305 nm on 19 October, two months prior to summer solstice, exceeded any value measured through 21 December.
TL;DR: In this article, the authors explored the effects of compositional and thermal buoyancy on mantle flow and crustal production and showed that buoyant upwelling can significantly enhance the crustal thickness relative to that which would be produced by plate spreading alone.
Abstract: Dynamical consequences of compositional buoyancy and the combined effects of compositional and thermal buoyancy on mantle flow and crustal production are explored. The results show that for a low enough mantle viscosity, buoyant upwelling can significantly enhance the crustal thickness relative to that which would be produced by plate spreading alone, while for a mantle viscosity of 10 to the 22nd Pa s, upwelling due to plate spreading is dominant and crustal thickness is predicted to be a function of spreading rate. The results indicate that thermal and compositional density variations result in opposing buoyancy forces that can cause time-dependent upwelling.
TL;DR: The melting temperature of (Mg,Fe)SiO3 perovskite, the dominant mineral phase of the Earth's lower mantle, has been measured to a pressure of 96 (±10) GPa as mentioned in this paper.
Abstract: The melting temperature of (Mg,Fe)SiO3 perovskite, the dominant mineral phase of the Earth's lower mantle, has been measured to a pressure of 96 (±10) GPa. Below 60 GPa we find good agreement with the previous measurement of Heinz and Jeanloz [1987a], and above 60 GPa the melting curve of silicate perovskite has a small positive slope of 19.5 (±5.5) K/GPa. The melting point of (Mg0.9Fe0.1)SiO3 perovskite is 3800 (±300) K at 96 GPa; by extrapolation to 136 GPa, it is 4500 (±500) K at the core-mantle boundary. These values provide an upper limit to the geotherm through the solid mantle, and they are compatible with recent estimates of the temperature in the core being high (∼4500 K at the core-mantle boundary). Our melting curve implies that the volume change on melting increases at 60 GPa from ΔVm = 0.0 (±0.2) cm³/mole to ΔVm ≅ 0.16 (±0.03) cm³/mole. We derive an equation of state for the melt from our data, and find that a highly coordinated structure is required to explain the molar volume of liquid (Mg,Fe)SiO3 at pressures above ∼ 20 GPa.
TL;DR: In this article, the horse-collar aurora pattern was reported to appear in one-third or more of the image sequences recorded during quiet times, and some evidence for interplanetary magnetic field B(y) influence on the temporal development of the pattern.
Abstract: The frequent appearance of the 'horse-collar aurora' pattern in quiet-time DE 1 images is reported, presenting a two-hour image sequence that displays the basic features and shows that it sometimes evolves toward the theta configuration. There is some evidence for interplanetary magnetic field B(y) influence on the temporal development of the pattern. A preliminary statistical analysis finds the pattern appearing in one-third or more of the image sequences recorded during quiet times.
TL;DR: In this paper, the top surface of the fluid is defined by a tectonic plate with horizontally uniform velocity and subduction, and the heat transport is as efficient as Rayleigh-Benard convection with constant viscosity.
Abstract: The heat transport by a viscous fluid with temperature dependent viscosity has been studied numerically. As opposed to previous models, the top surface of the fluid clearly defines a tectonic plate with horizontally uniform velocity and subduction. Past studies failed to incorporate plates resulting in inefficient heat transport. With tectonic plates, the heat transport is as efficient as Rayleigh-Benard convection with constant viscosity; there is a strong buffering between internal temperature and heat loss. Past studies of parameterized convection which incorporated parameters indicative of a strong buffering between internal temperature and total heat output still provide the most physically plausible representation of the Earth's thermal evolution.
TL;DR: In this article, the authors investigated the occurrence of dayside Pi 2 geomagnetic pulsations at low and mid latitudes and showed that Pi 2's occur simultaneously in both the night side and dayside hemispheres at low latitudes.
Abstract: In this paper the authors investigate the occurrence of dayside Pi 2 geomagnetic pulsations at low and mid latitudes. The technique of data adaptive filtering is used to identify Pi 2's concealed by the presence of typical daytime Pc type pulsations. Convincing new evidence is presented demonstrating that Pi 2 pulsations occur simultaneously in both the nightside and dayside hemispheres at low latitudes. Dayside Pi 2's are occasionally identified at mid latitudes. These results have implications with regard to the source mechanism for low latitude Pi 2 pulsations and allude to a global cavity mode.
TL;DR: In this paper, the authors used SEASAT-derived gravity data to investigate crustal deformation in the NE Indian Ocean and found that the undulations vary in trend similarly to the variation of the maximum compression directions predicted by a plate driving force model.
Abstract: We use SEASAT-derived gravity data to investigate crustal deformation in the NE Indian Ocean. Gravity highs reflecting crustal undulations vary in orientation from E-W in the Central Indian Basin to NE-SW in the Wharton Basin. The undulations vary in trend similarly to the variation of the maximum compression directions predicted by a plate driving force model [Cloetingh and Wortel, 1985], and are essentially restricted to the area of predicted compression for both principal horizontal stresses. This agreement implies that the stress model describes the basic features of the deformation observed in the gravity as well as the seismicity. The gravity data also provide insight into two enigmatic tectonic features. The transition in the morphology of the 90°E Ridge at 10°S from a continuous high to a complex blocky structure appears related to the deformation, since undulations in the Central Indian Basin can be projected eastward to blocks on the Ridge. The morphology, formerly interpreted as a fossil feature, may reflect the recent deformation. The discrepancy between the trends of the southernmost 85°E Ridge and the 90°E Ridge, previously thought to exclude similar hot spot track origins, appears to result from treating a crustal undulation as part of the 85°E Ridge. Earthquakes do not appear to be preferentially located with respect to the peaks and troughs of the undulations.
TL;DR: In this article, the authors performed annealing experiments on portions of a quartzite sample previously deformed at 700°C, 10−5 sec−1, and 1500 MPa which had a high dislocation density but few subgrain boundaries.
Abstract: In order to test the effect of ‘water‧ at high pressure on the rate of dislocation climb in quartz, we have performed annealing experiments on portions of a quartzite sample previously deformed at 700°C, 10−5 sec−1, and 1500 MPa which had a high dislocation density but few subgrain boundaries. Portions of the deformed sample were annealed at 1500 MPa and 800°C for 1 and 4 days; samples with 0.17 wt % water added have a lower average dislocation density and a greater number of subgrain boundaries than those without water added. Annealing at 800°C for 4 days at 1500 MPa after vacuum drying at 800°C, or at 550 MPa with 0.17 wt % water added, produces no significant change in the dislocation microstructure. These results indicate that: (1) the presence of a water-related defect in quartz accomplishes hydrolytic weakening by making the climb of dislocations easier, thus increasing the rate of recovery at a given imposed strain rate, and (2) for laboratory times this weakening requires a pressure >550 MPa.
TL;DR: In this paper, a statistical survey on the relation between ion density and pressure in the Earth's plasmasheet has been conducted using eight months of magnetotail plasma data, and the results show that the ion population behaves adiabatically both in the central plasma sheet and the plasma sheet boundary layer.
Abstract: Using eight months of magnetotail plasma data, we have done a statistical survey on the relation between ion density and pressure in the Earth's plasmasheet. More than 270,000 spin averaged (4.5s) samples of ion density and thermal pressure obtained in the central plasma sheet and the plasma sheet boundary layer were cross-correlated in order to obtain typical values of the polytropic index ..gamma.. for different tail regions and disturbance conditions. The plasma sheet ion population behaves, on average, adiabatically both in the central plasma sheet and the plasma sheet boundary layer. However, a polytropic index of about 1.4 for the quiet plamsa sheet indicates that the latter behaves like a poorly insulated vessel. Hence, there seems to be no quiet time magnetotail equilibrium (''ground state''), but rather continuous cooling until new enegy is entering from the outside. copyright American Geophysical Union 1989
TL;DR: In this article, the authors argue that the cusp region field-aligned (Birkeland) currents arise from the motion of reconnected open field lines shortening over the dayside magnetopause.
Abstract: The authors argues that the cusp-region field-aligned (Birkeland) currents, first reported by Iijima and Potemra (1976b), arise from the motion of reconnected open field lines shortening over the dayside magnetopause. Initially, field tension is dominant so the open field lines have a significant east/west component to their motion. Later, as the field lines shorten, the magnetosheath flow becomes increasingly important, and the field lines are pulled antisunward into the polar cap. Where the field lines change their direction of movement from east/west to antisunward, the magnetic field must tilt and this is the site of the cusp Birkeland current. All the major observational features of the cusp Birkeland currents fit qualitatively well with our model.
TL;DR: In this article, the authors used linear theory and numerical simulations to study instabilities caused by an electron beam in a thermally mixed plasma, and showed that two instabilities, the electron acoustic and electron-ion instabilities can combine to form a broadband wave spectrum that rapidly destroys the electron beam.
Abstract: There are observations of broadband electrostatic noise in the plasma-sheet boundary layer that are associated with field-aligned currents (electron beams), which often have an upper cutoff frequency above the electron plasma frequency. In this paper linear theory and numerical simulations are used to study instabilities caused by an electron beam in a thermally mixed plasma. It is shown that two instabilities, the electron acoustic and electron-ion instabilities, can combine to form a broadband wave spectrum that rapidly destroys the electron beam.
TL;DR: This paper showed that HCl is not easily incorporated into ice crystals, but is strongly partitioned towards the grain boundaries, and that the diffusion of HCl through ice crystals is slow.
Abstract: We have carried out experiments to study the incorporation and movement of HCl within the structure of ice. These involved freezing HCl solutions, and observing them in a scanning electron microscope fitted with an X-ray microanalysis system. We are able to show that HCl is not easily incorporated into ice crystals, but is strongly partitioned towards the grain boundaries. Furthermore, the diffusion of HCl through ice crystals is slow. These results contradict the interpretation of earlier experiments. They mean that if HCl is to be available for reaction on polar stratospheric cloud particles, as required by current theories of Antarctic ozone depletion, then it must be present in some form other than a solid solution.