Showing papers in "Geophysical Research Letters in 1981"

Observational, reference and model data on solar EUV, from measurements on AE-E

Abstract: Information on solar irradiance at wavelengths below 185 nm, observed by the EUVS experiment on the AE-E satellite over the entire development of the present sunspot cycle 21, is important to many investigations of planetary thermospheres and ionospheres. Strictly observational information is generally lacking in both the completeness and the spectral detail required by the more advanced study programs. Therefore, it has been necessary also to develop computer models in connection with fully detailed compilations of an appropriate reference spectrum. Our selection of appropriate forms of effective publication has been difficult for various reasons. Recognizing that full reproduction of our various lists of observational, reference, and model data in scientific journals would be impracticable, we started a more or less informal procedure of timely release of information to a limited number of particularly interested colleagues. With the present letter, we hope to mitigate at least some of the dissatisfactory aspects of this procedure.

A rapid method for magnetic granulometry with applications to environmental studies

Abstract: We have developed a rapid method of characterizing the fine-scale variation (sub-micron to hundreds of microns, µm) in the grain size of magnetite in long sediment cores. This method is a new rock magnetic tool and can be readily applied to the following disciplines: (1) environmental studies; (2) hydrology; (3) petrology; and (4) geomagnetism. The method consists of a comparative plot of the depth variation of two magnetic parameters. The first parameter, low field (less than 0.1 millitesla, mT, or 1 Oe) susceptibility in a steady field is plotted on the abscissa. The second parameter, anhysteretic remanent magnetization (ARM) imparted by a smoothly decreasing AF of peak value 100 mT (1,000 Oe) in the presence of a weak steady field (0.01-0.1 mT, or 0.1-1 Oe) is plotted on the ordinate. For a sediment column in which the magnetic mineral is magnetite, a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points (χ, ARM).

Peroxyacetyl nitrate (PAN) in the unpolluted atmosphere: An important reservoir for nitrogen oxides

Abstract: Based on the measured atmospheric distributions of ethane and propane (at midlatitudes in the northern hemisphere) and a simplified mechanism for their oxidation, it is predicted that acetaldehyde, acetone, and PAN (CH/sub 3/C(O)OONO/sub 2/) are ubiquitous components of the troposphere and the lower stratosphere. Average acetaldehyde concentrations (from ethane oxidation) of 22 parts per trillion (ppt), 3 ppt, and 7 ppt; average acetone concentrations (from propane oxidation) of 111 ppt, 15 ppt, and 3 ppt and average PAN concentrations of 17 to 34 ppt, 90 to 360 ppt, and 40 to 85 ppt are estimated for lower troposphere (approx.2 km), upper trposphere (approx.9 km), and the lower stratosphere (approx. 15 km), respectively. These calculations suggest that in the troposphere, nitrogen oxides (NQ/sub x/) contained in their organic form may be as much or more abundant as their inorganic form. This organic form of reactive nitrogen is in chemical equilibrium (CH/sub 3/C(O)OONO/sub 2/ arrow-right-left CH/sub 3/C(O)OO+NO/sub 2/) with inorganic NO/sub 2/ and acts as reservoir of inorganic NO/sub x/. Measurement methods for PAN are currenlty available to verify these predicted results.

Voyager measurement of the rotation period of Saturn's magnetic field

Abstract: Saturn's radio rotation period was determined using measurements made by the planetary radio astronomy experiment onboard the Voyager spacecraft. The sidereal period deduced, 10 hr 39 min 24 sec ? 7 sec, is within the 10 hr to 11 hr range of optical periods derived from a century of atmospheric spot and Doppler spectroscopy observations. The radio rotation period is presumably that of the planet's magnetic field. A provisional Saturn longitude convention is proposed and equations are provided to compute a longitude ephemeris and to transform between the proposed system and the (10 hr 14 min) system used for the Pioneer 11/Saturn encounter. The degree of longitude smearing which could result over the long term from the merging of data sets organized in this system is evaluated. No evidence of control of the radio emission by any of Saturn's satellites was found.

The auroral plasma cavity

Abstract: A region of diminished plasma density has been found to occur at the source of auroral kilometric radiation (AKR). The density within this auroral plasma cavity, determined from limited Hawkeye wave data, was less than 1/cu cm from 1.8 to 3 earth radii geocentric, at 70 deg + or - 3 deg invariant magnetic latitude. The altitude variation of the magnetic field produces a minimum in the ratio of plasma frequency to cyclotron frequency within the cavity which accounts for the observed spectrum of AKR.

Lower hybrid acceleration and ion evolution in the suprauroral region

Abstract: It is shown that ions can be accelerated perpendicularly to the magnetic field lines by resonant interactions with lower hybrid modes. Taking into account the effects of the magnetic field inhomogeneity, we demonstrate that the accelerated portion of the ions of ionospheric origin in the suprauroral region of the magnetosphere can evolve into conic distributions and propagate upwards along the field lines. Thus these ions can reach the region where they can be strongly energized along the field lines by the electrostatic shocks. We can argue that the ion distribution resulting from the combination of these accelerated ions with the background ions can depart significantly from a thermal distribution and lead to the excitation of electrostatic ion cyclotron modes.

Measurement of heavy ozone in the stratosphere

Abstract: The distribution of heavy ozone (molecular mass 50) has been measured for the first time in the stratosphere with a mass spectrometer system. During a balloon descent after midnight on September 4, 1980, a pronounced enhancement of heavy ozone was found. The maximum in the isotopic ratio, (O-18)(O-16)2/(O-16)3, occurred at 32 km where the enhancement of heavy ozone was over 40%. The ratio decreased toward higher and lower altitudes, and reached the standard value below 24 km. These measurements confirm an earlier prediction that there exists a preferred production mechanism for heavy ozone in the stratosphere.

A scattering model to explain seismic Q observations in the lithosphere between 1 and 30 Hz

Abstract: Observations of lithospheric Q for shear waves in the range 1--30 Hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres. For the single scattering approximation I have assumed 1/Q = 1/Q/sub i/+gv/..omega.., where Q/sub i/ is the intrinsic Q due to anelasticity, v is the velocity of shear waves, ..omega.. is angular frequency and g is the turbidity. g is taken as a constant in this paper, since g = ..integral.. n(a)sigmada, where n(a)da is the number of scattering spheres of radius a/unit volume and sigma is the scattering cross-section for a sphere, this suggests that geometrical scattering dominates. This will be true for ka>1, where k is wavenumber. Based on this criterion and the observation that Q is large (i.e. g small) at a frequency of 0.05 Hz, a is of the order of magnitude 1--10 km. Fits of observations of shear wave Q from Japan and Central Asia indicate that Q/sub i/ = 2000 +- 500 for the lithosphere of both regions; g = 0.01 km for the Japan observations and g = 0.005 km/sup -1/ for Central Asia.

Simulation of a perpendicular bow shock

Abstract: Simulations of a high Mach number shock with parameters typical of the earth's bow shock have been performed using a hybrid (particle ions, fluid electrons) code. The simulations reproduce the observed ion reflection and overshoots in the magnetic field and density. These features are shown to be closely associated with ion gyration.

Greenhouse effect of trace gases, 1970-1980

Abstract: Increased abundances were measured for several trace atmospheric gases in the decade 1970-1980. The equilibrium greenhouse warming for the measured increments of CH4, chlorofluorocarbons and N2O is between 50% and 100% of the equilibrium warming for the measured increase of atmospheric CO2 during the same 10 years. The combined warming of CO2 and trace gases should exceed natural global temperature variability in the 1980's and cause the global mean temperature to rise above the maximum of the late 1930's.

Hot oxygen atoms in the upper atmosphere of Venus

Abstract: Using data obtained by an ultraviolet spectrometer on the Pioneer-Venus Orbiter, energy and altitude distribution of nonthermal oxygen atoms in the Venus atmosphere for typical daytime and nighttime conditions are calculated using two different numerical methods. Agreement is found between results obtained by the two-stream transport method and the diffusion equation technique. Dissociative recombination of molecular oxygen ions and charge exchange of atomic oxygen ions with the neutral hydrogen and oxygen gas are the source terms, and calculations show that the dissociative recombination term is the dominant one. It is shown that measured hot atom concentrations exceed estimates of the thermal hydrogen atom concentrations derived from Brinton et al. (1980) over most of the daytime exosphere, and the calculated concentrations do so to an even greater extent. It is concluded that hot oxygen atoms are an important component of the dayside exosphere of Venus, their most important effect being the absorption of solar wind.

Cosmogenic 10Be/7Be as a probe of atmospheric transport processes

Abstract: Using accelerator spectrometry to measure 10Be, the ratio of the cosmogenic isotopes 10Be/7Be has been determined in 5 stratospheric and 2 South Pole ground level air filters. It is argued that this ratio can be a very sensitive probe of the origin, residence time, and mixing patterns of different atmospheric air masses, and in particular for identifying incursions of stratospheric air into the troposphere.

40Ar/39Ar dating of terrestrial minerals with a continuous laser

Abstract: A new method of fusing samples for 40Ar/39Ar dating using a continuous argon-ion laser is presented. Samples of hornblende, biotite, muscovite and slate were fused with a continuous laser and the results obtained agree with acres determined using our radio-frequency (RF) fusion system. By varying the laser beam power level, it was possible to perform a step-heating laser fusion of the slate which closely matched two RF step-heating runs on the same material. Thus, not only does the continuous laser give an age-microprobe capability, but it can routinely perform multi-step heating experiments which are crucial to 40Ar/39Ar age interpretation.

Interferometer studies of equatorial F region irregularities and drifts

Abstract: A radar interferometer technique developed at Jicamarca, Peru and first used to study electrojet irregularities has now been used successfully to study plasma turbulence in the equatorial F region. Our first results have shown that the most ‘turbulent’ echoes appear to come from a region that extends for tens of kilometers in altitude but for only a kilometer or less in the east-west direction. This slab may very well be the wall of a depleted region, a plasma ‘bubble’. Sometimes the irregularities can be tracked as they move eastward or westward. Velocity profiles for the evening period obtained in this way show a strong shear, with westward velocities at the lowest altitudes observed and eastward velocities above. A plausible explanation for this shear is that the westward drifts are driven by electric fields produced by westward E region winds and mapped up along magnetic field lines, while at higher heights, where the electron density is greater, the drifts are controlled by the F region dynamo driven by eastward winds.

Magma transport of heat on Io: A mechanism allowing a thick lithosphere

Abstract: Models indicate that the high heat flow from Io would result in a very thin (approximately 5 km) silicate lithosphere overlying a molten interior, if all heat was transported through the lithosphere via conduction. However, the presence of mountains with relief in excess of 10 km would seem to demand a thick lithosphere, at least locally. A significant fraction of Io's heat flow may be transported via advection through volcanoes. Advective heat transfer permits a thicker lithosphere than does pure conduction, possibly reconciling Io's high heat flow with the rugged topography.

The effect of microscopic turbulence on magnetosphere‐ionosphere coupling

Abstract: The effect of turbulence on the coupling of the magnetosphere and ionosphere has been investigated by including an effective collision frequency in the electron equation of motion. When this term is combined with the continuity equation, the ion equation of motion and Maxwell's equations, a dispersion relation for the kinetic Alfven wave including effective collisions is found. The wave-particle interaction leads to a strong damping of the wave. Inclusion of the effects of plasma sheet kinetics yields a scale size transverse to the magnetic field which corresponds to the size of visual auroral arcs.

Dynamic and static moduli

Abstract: Static and dynamic bulk moduli (Ks and Kd) are measured as continuous functions of pressure from zero to 2-3 kilobars for two sandstones, a tuff, limestone, granite, and oil shale. Results for the sandstones and granite are in good agreement with previously reported data with Ks/Kd varying from about 0.5 at atmospheric pressure to close to unity at pressures 2 kilobars and above. For rocks behaving elastically under static loading, the Ks/Kd ratio is inversely related to the microcrack density. For the limestone, time dependent deformation associated with pore collapse results in Ks/Kd ratios approaching 0.1 at high pressure. Upon unloading, while initially high ( ∼ 1.0) at high pressures, Ks/Kd becomes lower than values obtained during loading at low pressures ( < 1 kilobar) due to opening of microcracks generated during pore collapse. For the oil shale, in which few microcracks exist, Ks/Kd remains relatively constant with pressure at a value of about 0.7.

Electromagnetic active source sounding near the East Pacific Rise

Abstract: An active source electrical conductivity sounding was carried out near the East Pacific Rise at 21°N. The average electrical conductivity from 2 km beneath the seafloor into the upper mantle was found to be about 0.004 S/m. There is no evidence of fully molten magma at a distance of 7 km from the spreading axis. The inferred conductivity with depth of the crustal rock is consistent with Brace’s model of connected, seawater filled pores in rocks heated from beneath.

The origin of particulate organic carbon in the marine atmosphere as indicated by its stable carbon isotopic composition

Abstract: Organic carbon concentration and isotopic composition were determined in samples of atmospheric particulate matter collected in 1979 at remote marine locations (Enewetak atoll, Sargasso Sea) during the SEAREX (Sea-Air Exchange) program field experiments. Atmospheric Particulate Organic Carbon (POC) concentrations were found to be in the range of 0.3 to 1.2 mg. m/sup -3/, in agreement with previous literature data. The major mass of POC was found on the smallest particles (r<0.5 mm). The /sup 13/C//sup 12/C of the small particles is close to the one expected (d/sup 13/C = 26 +- 2/sup 0///sub infinity/) for atmospheric POC of continental origin. For all the samples analysed so far, it appears that more than 80% of atmospheric POC over remote marine areas is of continental origin. This can be explained either by long-range transport of small sized continental organic aserosols or by the production of POC in the marine atmosphere from a vapor phase organic carbon pool of continental origin. The POC in the large size fraction of marine aerosols (<20% of the total concentration) is likely to have a direct marine origin since its carbon isotopic composition is close to the expected value (d/sup 13/C = -21 +- 2/sup 0///sub 00/)more » for POC associated with sea-salt droplets transported to the marine atmosphere.« less

Dynamical cooling induced by dissipating internal gravity waves

Abstract: In this analysis we show that vertically propagating internal gravity waves induce a downward transfer of sensible heat from regions of wave dissipation, and that this transfer of heat may result in a net cooling of regions of the upper atmosphere.

Seasonal dependence of large-scale Birkeland currents

Abstract: Seasonal variations of large-scale Birkeland currents are examined in a study of the source mechanisms and the closure of the three-dimensional current systems in the ionosphere. Vector magnetic field data acquired by the TRIAD satellite in the Northern Hemisphere were analyzed for the statistics of single sheet and double sheet Birkeland currents during 555 passes during the summer and 408 passes during the winter. The single sheet currents are observed more frequently in the dayside of the auroral zone, and more often in summer than in winter. The intensities of both the single and double dayside currents are found to be greater in the summer than in the winter by a factor of two, while the intensities of the double sheet Birkeland currents on the nightside do not show a significant difference from summer to winter. Both the single and double sheet currents are found at higher latitudes in the summer than in the winter on the dayside. Results suggest that the Birkeland current intensities are controlled by the ionospheric conductivity in the polar region, and that the currents close via the polar cap when the conductivity there is sufficiently high. It is also concluded that an important source of these currents must be a voltage generator in the magnetosphere.

Polar cap arcs and the plasma sheet

Abstract: This letter suggests that most of the so-called 'polar cap arcs' with the sun-aligned orientation at the high geomagnetic latitude region occurring during the quiescent magnetosphere are actually a part of discrete arcs in the auroral oval as a consequence of the poleward widening of the quiet auroral oval. This suggestion is based on (1) a detailed examination of two months of DMSP electron precipitation data and simultaneous auroral observations, (2) the observed widening in the latitudinal extent of the auroral oval precipitation to the very high geomagnetic latitude (> or approx. =80 /sup 0/) and the simultaneous occurrence of the sun-aligned arcs, and (3) the conjugate widening of the quiet auroral oval over both the northern and southern polar regions. The new interpretation of the 'polar cap arcs' is discussed in terms of the present understanding of the solar wind-magnetospheric interaction.

Radio evidence for shock acceleration of electrons in the solar corona

Abstract: A new class of kilometer wavelength solar radio bursts has been observed with the ISEE-3 Radio Astronomy Experiment. These events resemble groups of ordinary type III bursts but have some unique properties. They are very intense and have durations considerably longer than groups of type III bursts. The new class of events do not necessarily occur at the times of reported meter wavelength type III activity and therefore do not appear to be the continuation of such activity to long wavelengths. Instead they occur at the reported times of type II events, which are indicative of a shock wave. An examination of records from the Culgoora Radio Observatory shows that the associated type II bursts have fast drift elements emanating from them i.e. herringbone structure. It is proposed that the new class of bursts are the long wavelength continuation of herringbone structure and it seems probable that the electrons producing the radio emission are accelerated by shocks. The new type of events will be referred to as shock accelerated (SA) events. The characteristics of SA events are discussed.

Positive cloud‐to‐ground lightning flashes in severe storms

Abstract: The occurrence of cloud-to-ground flashes that effectively lower positive charge to earth (+CG flash) over flat terrain has been documented in the mature stage of severe thunderstorms. Of the 31 documented +CG flashes, most had only one return stroke. Zero-to-peak rise times for the strokes averaged 7 microsec. The +CG flashes averaged 520 ms in duration, with 25 percent lasting more than 800 ms. Many of these had field changes suggestive of continuing current. Positive flashes have been observed to emanate from several regions of severe storms: high on the back of the main storm tower, through the wall cloud, and from the downshear anvil. Visually most of these positive flashes have emanated from high in the storm, and acoustic mapping of two shows thunder sources to a height of about 15 km.

Solar wind control of auroral zone geomagnetic activity

Abstract: Solar wind magnetosphere energy coupling functions are analyzed using linear prediction filtering with 25 minute data The relationship of auroral zone geomagnetic activity to solar wind power input functions are examined, and a least squares prediction filter, or impulse response function is designed from the data Computed impulse response functions are observed to have characteristics of a low pass filter with time delay The AL index is found well related to solar wind energy functions, although the AU index shows a poor relationship High frequency variations of auroral indices and substorm expansions are not predictable with solar wind information alone, suggesting influence by internal magnetospheric processes Finally, the epsilon parameter shows a poorer relationship with auroral geomagnetic activity than a power parameter, having a VBs solar wind dependency

X-ray enhancements detected during thunderstorm and lightning activities

Abstract: An X-ray detector has been flown on-board a NASA F-106 jet into thunderstorms. We have concluded that significant quantities of 3 to >12 keV X-rays are produced within thunderstorm clouds which support lightning discharge activity. It is inferred that the X-rays are the bremsstrahlung radiation emitted from energetic electrons which have been accelerated through run-away processes. It is suggested and supported that the structures responsible for the acceleration are strong double-layers which are supported by the very high current densities attributed to lightning discharges.

Evidence for the tailward retreat of a magnetic neutral line in the magnetotail during substorm recovery

Abstract: Plasma sheet observations made during a substorm recovery at 0400 to 0430 UT on March 1, 1978 with the ISEE satellites strongly suggest that the observed field-aligned flow of plasma in the earthward direction has mirrored in the vicinity of the earth and returned to the position of the satellites to produce the tailward flow seen at this time. The correlation between the estimated speeds of the earthward and tailward moving distribution indicates that the observed changes in velocity are spatial rather than temporal. It is concluded that the movement of the plasma sheet surface results from the propagation of an energetic particle source on to new magnetic field lines which progressively map deeper into the tail and also to higher polar latitudes at the earth. The movement of the source onto new magnetic field lines is clearly consistent with the tailward retreat of a magnetic neutral line in the plasma sheet during substorm recovery.

Hugoniot equation of state of periclase to 200 GPa

Abstract: New shock wave data on {100} oriented single crystal periclase covering the pressure range from 160 to 200 GPa suggest that MgO is described by a single Hugoniot up to 200 GPa, with no displacive phase transitions of volume change greater than 1-1.5 per cent. For a third order finite strain fit, with K_0 constrained to its ultrasonically determined value of 162.7 GPa, the implied K_0′ of 4.27 ± 0.24 is in agreement with ultrasonically determined value of 4.17 ± 0.14. The new data indicate a somewhat steeper Hugoniot than that suggested by previously published shock wave results under 120 GPa. A previously published result at 258 GPa shows more compression in the light of the present data than would be expected for MgO in the B1 structure, and may signal the onset of a phase transition, although we cannot confidently make this interpretation. If MgO forms an ideal solid solution with FeO, our data does not support the occurrence of a significant transition in magnesiowustite at lower mantle pressures.

Structure of the inner core inferred from observations of its spheroidal shear modes

Abstract: Spheroidal modes of elastic-gravitational oscillation that have an energy partition dominated by shear energy in the inner core are called core modes. Recordings from the IDA gravimeter network of an earthquake in the Tonga region on June 22, 1977 have provided the first unambiguous observations of core modes. They conclusively show that, at low frequencies, the inner core has a very high Q (about 3500). They also tightly constrain the ratio of the inner core radius to the mean shear velocity to be 345s with an uncertainty of 0.5%.

A high time resolution study of interplanetary parameter correlations with AE

Abstract: Using the earth-orbiting spacecraft IMP-8, we have performed a high time-resolution (2.5-min) study of the linear cross-correlations of a quantitative indicator of magnetospheric substorm activity, the auroral electrojet (AE) index, with several interplanetary (IP) parameters. The present study includes all available satellite data (from an eight-month period) rather than selecting a small number of substorm events, or a relatively small number of chosen days. Prior published studies have suggested that geomagnetic activity is related to a variety of solar wind parameters: solar wind speed (V); north-south magnetic field component (Bz); total interplanetary magnetic field (IMF) strength (B); and various combinations of these parameters. Very recently, a new parameter, ϵ(= VB² sin4(θ/2)lo²), has been suggested as a superior predictor of substorm activity. The other major preferred correlator with geomagnetic activity is VBs, where Bs is the southward (Bz < 0) component of the IMF. Substantial discussion has ensued as to which parameter is the "best" predictor of substorm activity. The present non-selective, high-resolution analysis reveals that VBs generally reaches a higher peak correlation than ϵ (0.6 vs 0.54). Both parameters reach a peak correlation at the same lag time (with AE lagging the IP parameters by ∼40 min.), but VBs also appears to have a preferable qualitative lag profile shape for AE prediction purposes.