Showing papers in "Geophysical Research Letters in 1991"

Total ozone trends deduced from Nimbus 7 TOMS data

Abstract: The Total Ozone Mapping Spectrometer (TOMS) on the Nimbus 7 satellite has been measuring the total column amount of ozone over the globe for more than 11 years. Recent improvements in the data analysis have led to a technique for determining and removing drift in the calibration such that the data at the end of the record are precise to + or - 1.3 percent (2-sigma) relative to the data at the beginning of the record. A statistical model, including terms for seasonal variation, linear trend, quasi-biennial oscillation, solar cycle and second-order autoregressive noise has been fit to the TOMS time series of total ozone data. The linear trend obtained when this statistical model is fit to the TOMS data averaged between 65 N and 65 S latitudes is -0.26 + or - 0.14 percent/year or -3 percent over the 11.6 year time period from November 1978 to May 1990. The trend is near zero (0.0002 + or - 0.2 percent/year) at the equator and increases toward both poles.

Global warming : evidence for asymmetric diurnal temperature change.

Abstract: Analyses of the year-month mean maximum and minimum surface thermometric record have now been updated and expanded to cover three large countries in the Northern Hemisphere (the contiguous United States, the Soviet Union, and the People's Republic of China). They indicate that most of the warming which has occurred in these regions over the past four decades can be attributed to an increase of mean minimum (mostly nighttime) temperatures. Mean maximum (mostly daytime) temperatures display little or no warming. In the USA and the USSR (no access to data in China) similar characteristics are also reflected in the changes of extreme seasonal temperatures, e.g., increase of extreme minimum temperatures and little or no change in extreme maximum temperatures. The continuation of increasing minimum temperatures and little overall change of the maximum leads to a decrease of the mean (and extreme) temperature range, an important measure of climate variability. The cause(s) of the asymmetric diurnal changes are uncertain, but there is some evidence to suggest that changes in cloud cover plays a direct role (where increases in cloudiness result in reduced maximum and higher minimum temperatures). Regardless of the exact cause(s), these results imply that either: (1) climate model projections considering the expected change in the diurnal temperature range with increased levels of the greenhouse gases are underestimating (overestimating) the rise of the daily minimum (maximum) relative to the maximum (minimum), or (2) the observed warming in a considerable portion of the Northern Hemisphere landmass is significantly affected by factors unrelated to an enhanced anthropogenically-induced greenhouse effect.

No-net-rotation model of current plate velocities incorporating plate motion model NUVEL-1

Abstract: NNR-NUVEL1 is presented which is a model of plate velocities relative to the unique reference frame defined by requiring no-net-rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model NUVEL-1 (DeMets et al., 1990). In NNR-NUVEL1, the Pacific plate rotates in a right-handed sense relative to the no-net-rotation reference frame at 0.67 deg/m.y. about 63 deg S, 107 deg E. At Hawaii the Pacific plate moves relative to the no-net-rotation reference frame at 70 mm/yr, which is 25 mm/yr slower than the Pacific plate moves relative to the hotspots. Differences between NNR-NUVEL1 and HS2-NUVEL1 are described. The no-net-rotation reference frame differs significantly from the hotspot reference frame. If the difference between reference frames is caused by motion of the hotspots relative to a mean-mantle reference frame, then hotspots beneath the Pacific plate move with coherent motion towards the east-southeast. Alternatively, the difference between reference frames can show that the uniform drag, no-net-torque reference frame, which is kinematically equivalent to the no-net-rotation reference frame, is based on a dynamically incorrect premise.

Fault stability inferred from granite sliding experiments at hydrothermal conditions

Abstract: Seismicity on crustal faults is concentrated in the depth interval 1–3 to 12–15km. Tse and Rice (1986) suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening (destabilizing) to velocity strengthening (stabilizing) friction. They inferred this transition from friction data for dry granite; however, pore fluids exist at elevated temperatures throughout the crust, and may strongly influence strength and sliding behavior. We present new data from sliding experiments on granite at elevated T (23° to 600°) plus elevated PH2O (100 MPa), Our results show velocity strengthening at room temperature, but velocity weakening from 100° to 350°C (except at 250°). From 350° to 600° there are systematic trends from velocity weakening to strong velocity strengthening, and from high to low friction; neither trend was seen in tests on dry granite. The velocity dependence data imply the potential for unstable slip in the interval 100° to 350°. Using a geotherm to map temperature to depth, this interval closely matches the observed earthquake distribution.

Distribution anisotropy: The cause of AMS in igneous rocks?

Abstract: Understanding of AMS in pristine basic igneous rocks in terms of flow-orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains (1) typically are irregularly equant or skeletal rather than distinctly ellipsoidal, and (2) usually crystallize interstitially and late, after magma flow has ceased. However, the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly-oriented (by flow) silicate “template”. This constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth, but in addition, we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of AMS. Experiments involving the casting of magnetite-epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected AMS symmetry, i.e. K3 perpendicular to glass plates or K1 parallel to glass rods. Slicing of near isotropic synthetic blank- and natural rock-cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from ∼0.4% to 2%, and the natural rock from ∼0.9% to ∼2.9%. In both the K3 axis migrates to be perpendicular to the glass slides.

Is the extent of glaciation limited by marine gas-hydrates

Abstract: Methane may have been released to the atmosphere during the Quaternary from Arctic shelf gas-hydrates as a result of thermal decomposition caused by climatic warming and rising sea-level; this release of methane (a greenhouse gas) may represent a positive feedback on global warming. The authors consider the response to sea-level changes by the immense amount of gas-hydrate that exists in continental rise sediments, and suggest that the reverse situation may apply - that release of methane trapped in the deep-sea sediments as gas-hydrates may provide a negative feedback to advancing glaciation. Methane is likely to be released from deep-sea gas-hydrates as sea-level falls because methane gas-hydrates decompose with pressure decrease. Methane would be released to sediment pore space at shallow sub-bottom depths (100's of meters beneath the seafloor, commonly at water depths of 500 to 4,000 m) producing zones of markedly decreased sediment strength, leading to slumping and abrupt release of the gas. Methane is likely to be released to the atmosphere in spikes that become larger and more frequent as glaciation progresses. Because addition of methane to the atmosphere warms the planet, this process provides a negative feedback to glaciation, and could trigger deglaciation.

does ion tearing exist

Abstract: Theoretical models of reconnection in the geomagnetic tail and its relationship to the collisionless tearing mode are examined analytically. The focus is on attempts to remove the stabilizing effects of electron compressibility in the quasi-neutral sheet. It is shown that this goal can be achieved by introducing spatial diffusion of electrons across the magnetic-flux surfaces, but not by wave-turbulent pitch-angle scattering (Coroniti, 1980) or nonadiabatic stochastic first-invariant diffusion (Buechner and Zelenyi, 1988). Hence the ion tearing mode does not exist, but the electron tearing mode does.

Generation of broadband electrostatic noise by electron acoustic solitons

Abstract: Broadband electrostatic noise (BEN) bursts whose amplitude sometimes reaches about 100 mV m{sup {minus}1} have been observed by the Viking satellite in the dayside auroral zone. These emissions have been shown to be greatly influenced by nonlinear effects and to occur simultaneously with the observation of particle distributions favouring the destabilization of the electron acoustic mode. It is shown that electron acoustic solitons passing by the satellite would generate spectra that can explain the high-frequency part of BEN, above the electron plasma frequency.

Fault steps and the dynamic rupture process: 2-D numerical simulations of a spontaneously propagating shear fracture

Abstract: Fault steps may have controlled the sizes of the 1966 Parkfield, 1968 Borrego Mountain, 1979 Imperial Valley, 1979 Coyote Lake and the 1987 Superstition Hills earthquakes. This project investigates the effect of fault steps of various geometries on the dynamic rupture process. We have used a finite difference code to simulate spontaneous rupture propagation in two dimensions. We employ a slip-weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes. The geometry of the two parallel fault planes allows for stepover widths of 0.5 to 10.0 km and overlaps of −5 to 5 km. Our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps. A major difference between the compressional and dilational cases is, that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment. Therefore our dynamic study implies that a compressional step will be jumped quickly, whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity. We also find that the rupture is capable of jumping a wider dilational step than compressional step.

Quasi-parallel shocks: A patchwork of three-dimensional structures

Abstract: Collisionless shocks at quasi-parallel geometries, i.e., for which the average magnetic field direction upstream of the shock is close to the shock normal, reveal temporally varying quantities, a variety of boundary crossing and kinetic signatures, and magnetic structures, often convecting, of finite extent. These results can be put together by a framework in which the shock can be viewed as an extended region containing three-dimensional Short Large Amplitude Magnetic Structures (SLAMS) which represent individual semi-cycles of the ambient upstream low frequency waves associated with diffuse ions in the ULF foreshock. As SLAMS convect with the flow they grow to large amplitudes and entrain inter-SLAMS regions to form an inhomogeneous downstream state. Their finite transverse extent is probably related to, and interacts with, ion beams, to produce a patchy transition zone which accounts for the variety of spacecraft signatures observed.

Heating and ionization of the lower ionosphere by lightning

Abstract: Nighttime ionospheric electrons at 90–95 km altitude are found to be heated by a factor of 100–500 during the upward passage of short (< 100 μs) pulses of intense (5–20 V/m at 100 km distance) electromagnetic radiation from lightning. Heated electrons with average energy of 4–20 eV in turn produce secondary ionization, of up to 400 cm−3 at ∼95 km altitude in a single ionization cycle (∼3 μs). With the time constant of heating being 5–10 μs, a number of such ionization cycles can occur during a 50 μs, radiation pulse, leading to even higher density enhancements. This effect can account for previously reported observations of ‘early’ or ‘fast’ subionospheric VLF perturbations.

The relationship between erythemal UV and ozone, derived from spectral irradiance measurements

Abstract: Spectral measurements of solar ultraviolet (UV) radiation received at the ground at Lauder, New Zealand (45°S) during 1990 are used in conjunction with ozone total column measurements to investigate the relative importance to erythemally active UV radiation of variations in solar zenith angle, ozone, and cloud cover. At this site solar zenith angle variations are the dominant factor, but clouds frequently attenuate the clear sky irradiances by more than 50%. Ozone reductions of 1% typically cause an increase in erythemally active UV irradiance of 1.25±0.20%.

Gravity wave modulation of gradient drift instabilities in mid-latitude sporadic E irregularities

Abstract: Recent E region VHF backscatter echoes observed by the MU radar at mid-latitudes show quasi-periodic striations with a fairly constant range vs. time tilt in a RTI display. These features are explained in terms of gravity waves with frequencies close to the Brunt-Vaisala frequency which modulate the shape of sporadic E layers. The conditions of instability, when the magnetic field has a significant dip angle, is revised. Differing from previous work, we argue that conditions of local gradient drift instability are not sufficient and one has to consider the integrated properties of each magnetic filed tube. Stratified sporadic E layers are stable using this new criteria, unless they are distorted to produce unstable integrated gradients. Gravity waves with phase fronts parallel to the magnetic dip angle are capable of producing such distortion, imposing its own temporal and spatial periodicity on the echoes.

Ion Reflection and transmission during reconnection at the Earth's subsolar magnetopause

Abstract: Composition measurements in an accelerated flow event at the earth's dayside magnetopause show evidence for reflection and transmission of magnetospheric and magnetosheath ion species. Furthermore, a single velocity transformation approximately tangent to the magnetopause surface orders the individual transmitted and reflected ion distributions on both sides of the magnetopause into field-aligned flow at V(A), the local Alfven speed. These observations provide strong evidence for a kinetic description of magnetic reconnection at the magnetopause.

The transition from hydrostatic to greater than hydrostatic fluid pressure in presently active continental hydrothermal systems in crystalline rock

Abstract: Fluid flow at hydrostatic pressure (P{sub h}) is relatively common through fractures in silicic and in mafic crystalline rocks where temperatures are less than about 350-370C. In contrast, pore-fluid pressure (P{sub f}) > P{sub h} has been encountered at the bottom of 3 geothermal exploration wells that attained temperatures >370C (at Larderello, Italy, at Nesjavellir, Iceland, and at The Geysers, California). Chemical sealing by deposition of minerals in veins appears to have allowed the development of the high P{sub f} encountered in the above wells. The upper limit for the magnitude of P{sub f} that can be attained is controlled by either the onset of shear fracturing (where differential stress is relatively high) that reopens clogged veins, or the hydraulic opening of new or old fractures (at relatively low values of differential stress). The brittle-plastic transition for silicic rocks can occur at temperatures as high as 370-400C in tectonically active regions. In regions where high-temperature geothermal systems develop and persist, it appears that either strain rates commonly are in the range 10{sup {minus}12} to 10{sup {minus}13}, or that silicic rocks in the shallow crust generally behave rheologically more like wet quartz diorite than wet Westerly granite.

Location and propagation of the magnetotail current disruption during substorm expansion: Analysis and simulation of an ISEE multi-onset event

Abstract: Magnetic field variations measured onboard the ISEE-1 spacecraft in the tail northern lobe (∼22 RE geocentric distance) during a substorm with multiple onsets are analyzed and simulated using a model current sheet This remote sensing study of the cross-tail current sheet shows that substorm onsets are associated with a partial disruption of the cross-tail current that begins in the near-earth plasma sheet (∼7 RE) and that expands tailward during substorm expansion with a velocity of the order of 300 km/s The two consecutive substorm onsets, which occur during a very active period (AE > 500 nT), correspond to a similar partial disruption of the near-earth current sheet that expands tailwards over a distance of the order of 80 RE

The thickness of the magnetosheath: Constraints on the polytropic index

Abstract: A statistical analysis of 351 independent bow shock crossings and 233 independent magnetopause crossings by the ISEE-1 spacecraft from 1977 to 1980 was performed to determine the average positions and shapes of the bow shock and magnetopause. The standoff distance between the magnetopause and the bow shock depends on the compressibility of the plasma which in the polytropic' approximation is related to the ratio of specific heats, {gamma}. Standoff distances for the bow shock and magnetopause were found to be 13.7 R{sub E} ({plus minus} 0.2 R{sub E}) and 10.3 R{sub E} ({plus minus} 0.3 R{sub E}), respectively. These distances are smaller than those observed during earlier epochs. The observed thickness of the magnetosheath is that expected for the compression of a gas whose polytropic index, {gamma}, is 1.76 {plus minus} 0.15. This value, representative of the entire magnetosheath, is consistent with the value of 1.67 deduced from the behavior of the plasma across individual shock transitions. A value of 1.67 is expected for an adiabatic process in a collisional, monatomic gas with three degrees of freedom with lower values for non-adiabatic processes and higher values for anisotropic heating at the shock. The observed value of 1.76 indicates that heatmore » flux does not much affect the position of the shock while the downstream anisotropy may have a small effect.« less

The resonance structure of low latitude Pc3 geomagnetic pulsations

Abstract: The spectral difference in ULF wave amplitude between closely spaced meridional ground stations may be used to measure the eigenfrequency of magnetospheric field lines (Baransky et al., 1985). A more reliable technique based on the crossphase spectrum has been used to identify eigenfrequencies and study the temporal evolution of local field line resonances. Pc3 (22-100 mHz) pulsations recorded with two pairs of low latitude ground stations have been specifically examined

Large scale lunar horizon glow and a high altitude lunar dust exosphere

Abstract: All three Apollo 17 astronauts sketched a `lunar horizon glow', seen from orbit above the moon. It is shown that the shape of the glow is compatible with scattering of sunlight off of gas or dust at high altitudes above the moon. The present mathematical modeling best simulates the glow with submicron dust grains whose spatial density varies with altitude above the moon as exp(-H/H(s)), where H(s) is in the range of 5 to 20 km. These dust grains are probably electrically charged and ejected above the lunar surface by local electric fields.

Effect of anthropogenic aerosols on biologically active ultraviolet radiation

Abstract: Aerosols from anthropogenic sources contribute significantly to the scattering of solar radiation in the atmosphere over most populated areas. By using observed values of visual range we estimate that in non-urban areas of the industrialized countries the amount of biologically active solar radiation (UVB, 280 to 315 nm) reaching the surface has decreased by a range of 5 to 18% since the industrial revolution, primarily due to aerosols formed from emissions of sulfur dioxide (SO2). The UVB reduction in the industrialized countries may have offset partly or fully the UVB increases associated with current stratospheric ozone depletion at NH continental mid-latitudes. However, this offset is not expected to continue because the SO2 emissions are leveling off in the industrialized countries.

A simplified spring-block model of earthquakes

Abstract: The time interval between earthquakes is much larger than the actual time involved during slip in an individual event. The authors have used this fact to construct a cellular automaton model of earthquakes. This model describes the time evolution of a 2-D system of coupled masses and springs sliding on a frictional surface. The model exhibits power law frequency-size relations and can exhibit large earthquakes with the same scatter in the recurrence time observed for actual earthquakes.

Sulfur isotope measurements of submicrometer sulfate aerosol particles over the Pacific Ocean

Abstract: Stable isotopes were used to analyze the submicron-size sulfate aerosol particles in the atmosphere over the Pacific Ocean, together with the air-mass back trajectories, in order to test the hypothesis of Charlson et al. (1987) who suggested that, over the remote ocean areas, the primary source of atmospheric nonseasalt (NSS) sulfate is marine emissions of dimethylsulfide (DMS). The observed results of isotopic fractionation between the seawater sulfate and NSS sulfate fractions was found to be consistent with the isotopic fractionation predicted for the transformation of the seawater sulfate to the atmospheric NSS sulfate via a DMS path way, supporting the hypothesis of Charlson et al.

The frictional behavior of serpentinite: Implications for aseismic creep on shallow crustal faults

Abstract: Serpentine is common in many active faults and may be responsible for aseismic creep along segments of these faults. To test this, we have conducted a series of velocity stepping experiments to determine the frictional velocity dependence of serpentinite. We slid initially bare, rough surfaces of antigorite serpentinite at room temperature, with velocities from 0.0032 to 10.0 μm/s (1.0×102 to 3.2×105 mm/yr) and normal stresses from 25 to 125 MPa. We find that the velocity dependence of serpentinite undergoes a transition from velocity weakening at fast loading velocities to velocity strengthening at slow velocities and that this change is accompanied by other changes in the constitutive behavior. These results suggest that serpentinite should not be the site of instability initiation during sliding at plate velocities, but may permit propagation of unstable slip initiated elsewhere.

Post-glacial rebound and asthenosphere viscosity in Iceland

Abstract: During the Weichselian glaciation Iceland was covered with an ice cap which caused downward flexure of the Earth's surface. The post-glacial rebound in Iceland was very rapid, being completed in about 1,000 years. The length of this time interval constrains the maximum value of asthenosphere viscosity in Iceland to be 1 {times} 10{sup 19} Pa s or less. Further clarification of the ice retreat and uplift history may reveal lower viscosity. Current changes in the mass balance of Icelandic glaciers must lead to measurable elevation changes considering this low viscosity. Expected current elevation changes around the Vatnajoekull ice cap are of the order of 1 cm per year, due to mass balance change in this century.

Dynamics of the equatorial mesosphere: First results with a new generation partial reflection radar

Abstract: The first observations of mesospheric winds made between January–August 1990 with an MF partial reflection radar located on Christmas Island (2°N, 157°W) in the central Pacific are described. The mean zonal winds are in general westward, but show clear evidence for a wave-driven circulation. Power spectral studies indicate that waves are present over a wide range of periods. Ultra-fast Kelvin waves are especially evident in January–March, with peak amplitudes ∼20 ms−1, and intrinsic phase speeds of ∼150 ms−1 indicated. The Kelvin waves are estimated to contribute an eastward acceleration of up to 10 ms−1 day−1. Gravity wave amplitudes are also found to be almost as large as those observed at mid-latitude sites, which suggests that convection is a major source of gravity wave activity.

Geomagnetic field control of 14C production over the last 80 Ky: Implications for the radiocarbon time-scale

Abstract: We have used a recent estimate of the geomagnetic dipole field strength over the last 80 ky to examine the extent to which changes in geomagnetic field intensity have affected the radiocarbon time scale. The magnetic record was converted into a 14C production signal which was then used to produce a synthetic atmospheric Δ14C record using a two box model for the carbon cycle. This calibration suggests that the geomagnetic field has been the main factor governing the production of cosmogenic 14C atoms over the period of time considered, and that Δ14C changes due to other causes, such as climatic changes, have been of lesser importance. The results indicate that the radiocarbon ages are 2 to 3 ky younger than true age in the interval 18–40 ky b.p., in good agreement with recent U-Th and 14C age comparisons. Moreover the results predict that a negligible difference between true and 14C ages should be observed around 45–50 ky.

Isotopic exchange between carbon dioxide and ozone via O(1D) in the stratosphere.

Abstract: We propose a novel mechanism for isotopic exchange between CO_2 and O_3 via O(^1D) + CO_2 → CO_3^* followed by CO_3^* → CO_2 + O(^3P). A one-dimensional model calculation shows that this mechanism can account for the enrichment in ^(18)O in the stratospheric CO_2 observed by Gamo et al. [1989], using the heavy O_3 profile observed by Mauersberger [1981]. The implications of this mechanism for other stratospheric species and as a source of isotopically heavy CO_2 in the troposphere are briefly discussed.

In situ measurements of carbon dioxide in the winter Arctic vortex and at midlatitudes: An indicator of the ‘age’ of stratopheric air

Abstract: We report measurements of the vertical distribution of the carbon dioxide mixing ratio in the lower and middle stratosphere. They were performed during a series of balloon flights over midlatitudes (44 °N) during different seasons and over high latitudes (68 °N) during winter in the time period from 1982 until 1990. CO2 was gaschromato-graphically determined in air samples collected by means of cryogenic whole-air samplers. The annual increase of the CO2 content of the mid-stratospheric air is comparable to the temporal CO2 trend in the troposphere. However, the stratospheric concentrations are lagging the tropospheric ones by a mean delay time of 5.6 ± 1.1 years. This delay time may be interpreted as the ‘age’ of stratospheric air masses. It is determined by the efficiency of the transport processes that mix tropospheric air into the stratosphere. Our CO2 observations during the last decade suggest that these processes have varied systematically with a time period of 3 to 5 years. A rather low ‘age’ of 2.3 ± 0.3 years was derived from the most recent observations over midlatitudes in summer 1989 as well as in the Arctic region during the CHEOPS III Campaign in winter 1989/90.

The Mid-Cretaceous Super Plume, carbon dioxide, and global warming

Abstract: Carbon-dioxide releases associated with a mid-Cretaceous super plume and the emplacement of the Ontong-Java Plateau have been suggested as a principal cause of the mid-Cretaceous global warming. A carbonate-silicate cycle model is developed to quantify the possible climatic effects of these CO2 releases, utilizing four different formulations for the rate of silicate-rock weathering as a function of atmospheric CO2. CO2 emissions resulting from super-plume tectonics could have produced atmospheric CO2 levels from 3.7 to 14.7 times the modern preindustrial value of 285 ppm. Based on the temperature sensitivity to CO2 increases used in the weathering-rate formulations, this would cause a global warming of from 2.8 to 7.7 C over today's glogal mean temperature. Altered continental positions and higher sea level may have been contributed about 4.8 C to mid-Cretaceous warming. Thus, the combined effects of paleogeographic changes and super-plume related CO2 emissions could be in the range of 7.6 to 12.5 C, within the 6 to 14 C range previously estimated for mid-Cretaceous warming. CO2 releases from oceanic plateaus alone are unlikely to have been directly responsible for more than 20 percent of the mid-Cretaceous increase in atmospheric CO2.

Modeling the growth phase of a substorm using the Tsyganenko Model and multi-spacecraft observations: CDAW-9

Abstract: The CDAW-9 Event C focused upon the early part of 3 May 1986 when a large substorm onset occurred at 0111 UT. By modifying the Tsyganenko 1989 magnetic field model, a model is constructed in which the near-earth current systems are enhanced with time to describe the observed development of the tail magnetic field during the growth phase. The cross-tail current intensity and the thickness of the current sheet are determined by comparison with three spacecraft in the near-earth tail. The location of the auroral bulge as recorded by the Viking imager is mapped to the equatorial current sheet. The degree of chaotization of the thermal electrons is estimated, and the consequences to the tail stability towards on tearing are discussed. It is concluded that the mapping of the brightening region in the auroral oval corresponds to the regions in the tail where the current sheet may be unstable towards ion tearing.