bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption coefficients of ice from 250 to 400 nm
Authors: Perovich Donald K. , Govoni John W.
Published: February 2008

Absorption coefficients for pure bubble‐free ice are a critical element in theoretical efforts to determine levels of ultraviolet radiation reaching marine biota in and under a sea ice cover. A 3‐m block of ice was used to measure these coefficients from 250 to 400 nm. Absorption coefficients were found to increase from 0.092m−1 at 400 nm to 0.665m−1 at 250 nm. Values in the ultraviolet were shown to be comparable to visible results from 580 to 720 nm. This suggests that existing data on the interaction of visible light with snow and sea ice can be used as a first‐order estimate of ultraviolet optical properties.
An estimation of the electric field in the magnetotail current sheet using the observed energetic ion bulk flow
Authors: Li Xiaolin , Speiser Theodore W.
Published: February 2008

It is important to know the electric field in the tail current sheet in order to understand how particles behave and how much energy is being dissipated. The electric field is also a measurement of the reconnection rate during substorms. For the CDAW‐6 substorm period of March 22, 1979, we used the ion data from the medium energy particles experiment (MEPE) on the ISEE‐1 satellite, and studied nine measurements of the 3D distribution function centered on the center of the current sheet. The measured distribution function was then integrated to obtain the average of bulk flow velocity in the geocentric solar ecliptic (GSE) frame. This bulk flow velocity was then broken up into its components perpendicular and parallel to the magnetic field for the nine cases. It was further assumed that the perpendicular component was due, in part, to an energy dependent drift and to an energy independent electric field drift. Using the bulk flow velocities from any two energy channels we can separate out the electric and energy dependent drifts and thus obtain electric field and energy dependent components. The two lowest energy channels (34.3 keV and 54.9 keV) give the main results, and the 80.4 keV and 118.8 keV channels are used as a cross check. We find that Ex fluctuates approximately ±5 mV/m, and Ey ± 10 mV/m, in reasonable agreement with measurements by the electric field instrument [Pedersen et al., 1985], with most of the fluctuation presumably due to the motion of the current sheet. Using current sheet oscillation theory and the central current sheet data points, we can estimate Ey in the frame of the current sheet and find a positive average Ey with a magnitude of ≈ 0.1 mV/m, which is also consistent with that expected for reconnection in this substorm time period. The Ez component has a remarkable linear correlation with Bx, with a correlation coefficient of 0.91. Assuming Bx varies linearly with z, a positive ion density of 3.14×10−20/d(Re) Coulombs/m³ is implied. Such a positive space charge near the current sheet center is expected theoretically.
Four‐beam measurements of ionospheric structure with the MU radar during the Low‐latitude auroral event of 20–23 October 1989
Authors: Oliver William L. , Fukao Shoichiro , Takami Tomoyuki , Tsuda Toshitaka , Kato Susumu
Published: February 2008

The MU radar was used to observe the ionospheric F‐region electron density simultaneously in four oblique beams during the geomagnetic storm of 20–23 October 1989, when the first significant auroral display over Japan since 1960 was observed. The four beams, separated by about 250 km horizontally in the F region, observed drastically different behavior, with independent and extreme changes occurring on time scales of one minute during the period of peak activity, indicating a strongly structured ionosphere streaming over the radar. We observed cases in which, simultaneously, a deep trough was seen in one beam, densities exceeding 4×106 cm−3 were seen in another, and a normal ionosphere was seen in a third. During the most disturbed periods the F‐layer peak height appeared to rise to 800 km altitude in one beam while it remained near 500 km in another.
The flywheel effect: Ionospheric currents after a geomagnetic storm
Authors: Deng W. , Killeen T. L. , Burns A. G. , Roble R. G.
Published: February 2008

In the period following a geomagnetic storm the high‐latitude, magnetospheric‐driven convection pattern is normally weak. However, the neutral circulation, set up by ion‐neutral momentum coupling during the main phase of the storm, may continue for several hours after the storm has ended. This persistent neutral circulation has the potential to drive Hall currents for some hours. In this paper we investigate these “flywheel” currents by simulating a storm which occurred on the 23rd of November 1982 using the National Center for Atmospheric Research Thermosphere Ionosphere General Circulation Model (NCAR‐TIGCM). The resulting high‐latitude, height‐integrated Hall currents are dominated by the neutral‐wind‐driven component for several hours after the end of main phase of the storm. The direction of these currents is reversed from normal. Analysis of the neutral and ion components of this current system indicates that the neutral component may drive as much as 80% of the high‐latitude current system immediately after the storm has ended, and may continue to dominate this system for 4 to 5 hours.
Diffuse alpha particles upstream of simulated quasi‐parallel supercritical collisionless shocks
Authors: Trattner K. J. , Scholer M.
Published: February 2008

One‐dimensional hybrid simulations of quasi‐parallel collisonless shocks have been performed. Alpha particles are included in the simulations self‐consistently in such a way that sufficient simulation particles are present to give reasonable statistics. It is found that a fraction of the incoming solar wind protons and alpha particles is accelerated near the shock and escapes upstream. These particles constitute a diffuse upstream particle population. The density ratio bias factor, i.e., the density ratio of diffuse alpha particles to diffuse protons normalized to the solar wind He++ to H+ density ratio varies between 0.5 and 4.0, and depends on the solar wind He++ to H+ density ratio and on the shock Mach number. The density ratio bias factor for a low Mach number shock (MA = 4.6) is f ∼ 0.5 when the alpha particles are treated as test particles and increases to ∼1 when the solar wind ratio of alpha to protons is 0.25. For a high Mach number shock of MA = 8.9 with a very small alpha particle contribution to the solar wind we obtained a density ratio bias factor of f ∼ 4.
Inference of surface power spectra from inversion of multifrequency polarimetric radar data
Authors: van Zyl Jakob J. , Burnette Charles F. , Farr Tom G.
Published: February 2008

During the summer of 1988 an intensive field experiment was conducted in the vicinity of Pisgah lava field in the Mojave Desert of southern California. As part of the experiment, physical properties such as microtopography, composition, soil moisture and dielectric constant at five different sites representing surfaces with r.m.s. heights varying from less than one centimeter to tens of centimeters, were measured. In addition, polarimetric radar images at P‐band (68 cm wavelength), L‐band (24 cm) and C‐band (5.7 cm) were acquired at three different incidence angles with the NASA/JPL airborne imaging radar polarimeter. Using trihedral corner reflectors deployed in the area prior to imaging, the radar images were calibrated to provide σ0 values for each resolution element in each scene. This paper reports on the derivation of the power spectrum of surface microtopography by solution of the small perturbation model for multiple incidence angle and multiple frequency radar data. Power‐law fits to the power spectra have exponents (slope in log‐log plots) that are nearly the same for all surfaces. These values are close to those from measured microtopography profiles. The offset in log‐log plots shows very good correlation with measured power spectrum offsets, however the image‐derived offsets are consistently lower than measured values. This may be the result of calibration errors, using the wrong dielectric constants in the inversion, or the fact that not all observed energy was scattered by the surface interface alone.
Possible measurements of small‐amplitude tid's using parallel, unpowered telecommunications cables
Authors: Lanzerotti L. J. , Medford L. V. , Kraus J. S. , Maclennan C. G. , Hunsucker R. D.
Published: February 2008

We report the observation of ∼40 min quasi‐periodic variations during local daytime conditions in the differences between the voltages measured on two parallel (separation distance ∼100 km) unpowered telecommunications cables between California and Hawaii (length of each ∼4000 km). The events are inferred to be produced by small changes in magnitude and/or position of the Sq ionosphere current system, the focus of which passes on average daily over the cables. The changes in the current system are probably produced by medium‐scale traveling ionospheric disturbances (TID's). This dual cable system appears to provide a very sensitive technique for measuring small amplitude, medium‐scale mid‐latitude changes in ionospheric currents, especially those produced by TID's.
Electromagnetic ion cyclotron waves observed in the plasma depletion layer
Authors: Anderson B. J. , Fuselier S. A. , Murr D.
Published: February 2008

Observations from AMPTE/CCE in the Earth's magnetosheath on October 5, 1984 are presented to illustrate 0.1–4.0 Hz magnetic field pulsations in the subsolar plasma depletion layer (PDL) for northward sheath field during a magnetospheric compression. The PDL is unambiguously identified by comparing CCE data with data from IRM in the upstream solar wind. Pulsations in the PDL are dominated by transverse waves with F/FH+ ≤ 1.0 and a slot in spectral power at F/FH+ = 0.5. The upper branch is left hand polarized while the lower branch is linearly polarized. In the sheath the proton temperature anisotropy, A = T⟂/T∥ − 1, is ≈ 0.6 but it is ≈ 1.7 in the PDL during wave occurrence. The properties and correlation of waves with increased anisotropy indicate that they are electromagnetic ion cyclotron waves.
‘Self‐consistent’ production of ion conics on return current region auroral field lines: A time‐dependent, semi‐kinetic model
Authors: Brown David G. , Wilson Gordon R. , Horwitz James L. , Gallagher Dennis L.
Published: February 2008

We describe initial results from a time‐dependent, semi‐kinetic model of plasma outflow incorporating wave‐particle interactions along current‐carrying auroral field lines. Electrostatic waves are generated by the current driven ion cyclotron instability (CDICI), causing perpendicular velocity diffusion of ions plus electron heating via anomalous resistivity when and where the relative drift between electrons and ions exceeds certain critical velocities. Using the local bulk parameters we calculate these critical velocities, and so are able to self‐consistently switch on and off the heating of the various particle species. Due to the dependence of these critical velocities on the bulk parameters of the species the heating effects exhibit quite complex spatial and temporal variations. A wide range of ion distribution functions are observed in these simulations, including conics with energies of a few electron volts and ‘ring’ distributions. The rings are seen to be a natural result of transverse heating and velocity filter effects and do not require coherent acceleration processes. We also observe the formation of a density depletion in hydrogen and enhanced oxygen densities at high altitudes.
Variability of the equatorial electrojet: Its relations to the Sq current system and semidiurnal tides
Authors: Stening R. J.
Published: February 2008

Average diurnal and seasonal variations of the horizontal geomagnetic element ΔH are examined. The semiannual variation in equatorial electrojet strength is found to disappear in afternoon hours. This may be related to changes in the latitude of the Sq current system focus during the day. This in turn may be caused by a superposed semidiurnal tide and may also explain ΔH variations at stations near the focus.
Far‐field detection of slow precursors to fast seismic ruptures
Authors: Jordan Thomas H.
Published: February 2008

An earthquake is said to have a short‐term slow precursor when there is significant moment released in an interval immediately before its high‐frequency origin time t0. A method is formulated for detecting slow precursors from observations of the earth's free oscillations. It assumes that the far‐field radiation at low frequencies (≲ 10 mHz) is due to a compact source of constant orientation having a moment‐release function which is zero prior to some start time t* and nondecreasing for t ≥ t*. Any moment‐release function for which t0 = t* (no precursor) must then satisfy the inequality is the centroid time shift, τc is the characteristic duration, and α is a dimensionless measure of the temporal skewness. A detector based on this inequality is applied to two events. Data from the shallow‐focus Loma Prieta earthquake of 18 Oct 89 are consistent with no moment release prior to the high‐frequency origin time, whereas the intermediate‐focus Peru‐Ecuador earthquake of 12 Apr 83 appears to have a significant slow precursor. The systematic analysis of large earthquakes using this detector may elucidate earthquake nucleation processes and help to assess, on a global scale, the feasibility of short‐term earthquake prediction from near‐field measurements.
Application of the genetic algorithm to residual statics estimation
Authors: Wilson William G. , Vasudevan Kris
Published: February 2008

We present an application of a genetics‐based optimization algorithm in an attempt to compute residual statics in seismic data processing. Genetic algorithms have a long history, but have only recently been applied to complex optimization problems. In this paper, optimization results are shown for a synthetic data set to test whether the genetic algorithm can obtain the correct qualitative and/or quantitative features.
Evolution of lunar topography by impact processes
Authors: Sugita Seiji , Matsui Takafumi
Published: February 2008

Using a simple model of lunar topographic evolution resulting from the formation of impact craters, we try to constrain the total number of impactors striking the Moon after the solidification of the lunar crust. The numerical results indicate that the evolution of the Moon's hypsometric curve is strongly influenced by the total number of craters and is less affected by their size distribution. It is suggested that the total number of craters necessary to explain the standard deviation of the present hypsometric curve is roughly equivalent to the number density of craters on the oldest lunar crust, corresponding to about 10−4 lunar mass of impactors. This may be the total mass of impactors striking after solidification of the lunar crust, more specifically, after about 4.4 to 4.5 Ga ago when the viscous degradation of craters became ineffective.
The hybrid polynya at the northern end of Nares Strait
Authors: Kozo Thomas L.
Published: February 2008

NOAA satellite images (November through June 1984 to 1989) show sea ice arches accompanied by unique polynya patterns immediately north of the Nares Strait. Typically the arches formed by late November, but the polynya patterns did not stabilize until January, the month when sea ice transport through the Strait usually ceases. The polynyas created to the lee of the arches appear to be of an unusual “hybrid” mix, switching from a combined latent and sensible heat type in fall to a “pure” sensible heat type in winter. Early in the polynya formation, sea ice is removed through the temporarily open Nares Strait, while the ice arch to the north blocks ice from drifting into the polynya. Once the Strait freezes solid, newly formed ice cannot be transported downstream. The polynya then begins to freeze but ice growth is still moderated apparently due to upward vertical mixing of warmer, more saline water generated by tidal currents flowing over the shallow seabed northwest of Robeson Channel.
Bullialdus: Strengthening the case for lunar plutons
Authors: Pieters Carlé M.
Published: February 2008

Although many craters expose material of a composition different from that of the local surroundings, Bullialdus has excavated material representing three distinct stratigraphic zones that occur in the upper 6 km of crust, the top two of which are gabbroic and the deepest of which is noritic. This three‐component stratigraphy at Bullialdus provides strong evidence that the lunar crust includes pockets of compositionally layered material reminiscent of mafic layered plutons. When combined with previous information on the compositional diversity at other large craters, these remote analyses obtained in a geologic context substantially strengthen the hypothesis suggested from lunar samples that plutons play an integral role in lunar crustal evolution.
Simulation of the Pinatubo aerosol cloud in general circulation model
Authors: Boville Byron A. , Holton James R. , Mote Philip W.
Published: February 2008

A high resolution stratospheric version of the NCAR Community Climate Model (CCM2) with an annual cycle was used to simulate the global transport and dispersion of the Pinatubo aerosol cloud. A passive tracer was injected into the model stratosphere over the Philippine Islands on model day June 15, and the transport was simulated for 180 days using an accurate semi‐Lagrangian advection scheme.

The simulated volcanic aerosol cloud initially drifted westward and expanded in longitude and latitude. The bulk of the aerosol cloud dispersed zonally to form a continuous belt in longitude, and remained confined to the tropics (30°N–25°S) centered near the 20 mb level for the entire 180 day model run, although a small amount was transported episodically into the upper troposphere in association with convective disturbances. Aerosol transported to the troposphere was dispersed within a few weeks into the Northern Hemisphere extratropics. In the Southern Hemisphere the aerosol was mixed into the region equatorward of the core of the polar night jet during the first 50 days, but penetration into Southern Polar latitudes was delayed until the final warming in November.

These results, which are generally consistent with observed behavior of the El Chichon aerosol, will be compared with observations of the Pinatubo cloud in the course of the next several months.
Balloonborne measurements of the Pinatubo aerosol size distribution and volatility at Laramie, Wyoming during the summer of 1991
Authors: Deshler T. , Hofmann D. J. , Johnson B. J. , Rozier W. R.
Published: February 2008

Measurements using balloonborne optical particle counters at Laramie, Wyoming during the summer of 1991 are used to study the particle size distribution and volatility of the aerosol which formed in the stratosphere following the mid‐June eruptions of Mt. Pinatubo. Enhanced aerosol layers were observed below 20 km as early as 16 July, about 1 month after the eruption. During late July, a transient though substantial particle layer was observed in the 23 km region. High concentrations of large particles in this high altitude layer resulted in aerosol mass mixing ratios as large as 0.5 ppm, considerably larger than observed following the eruption of El Chichón. Aerosol volatility tests indicated that well over 90% of the particles were composed of an H2SO4/H2O solution in all layers observed, indicating rapid conversion of SO2 to H2SO4 and subsequent droplet growth. High concentrations of droplets suggest homogeneous or ion nucleation as the most likely aerosol production mechanism.
SAGE II measurements of early Pinatubo aerosols
Authors: McCormick M. P. , Veiga R. E.
Published: February 2008

SAGE II satellite measurements of the Mt. Pinatubo eruption cloud in the stratosphere during June, July, and early August 1991 show that aerosols in the tropics reached as high as 29 km altitude with most of the cloud between 20 and 25 km. The most optically thick portions of the cloud covered latitudes from 10°S to 30°N during the early part of this period. By late July, high stratospheric optical depths were observed to at least 70°N, with the high values north of about 30°N from layers below 20 km. High pressure systems in both hemispheres were observed to be correlated with the movement of volcanic material at 21 km into the westerly jet stream at high southern latitudes and similarly to high northern latitudes at 16 km. By August, the entire Southern Hemisphere had experienced a 10‐fold increase in optical depth relative to early July due to layers above 20 km. Initial mass calculations using SAGE II data place the aerosol produced from this eruption at 20 to 30 megatonnes, well above the 12 megatonnes produced by El Chichon.
Scattering behavior of Lunar Lake Playa determined from parabola bidirectional reflectance data
Authors: Shepard Michael K. , Arvidson Raymond E. , Guinness Edward A. , Deering Donald W.
Published: February 2008

Bidirectional reflectance data obtained with the PARABOLA instrument at Lunar Lake, Nevada, were analyzed to determine the scattering properties of playas. The data are approximated by a Lambertian function, except at high phase angles in the solar principal plane, where Fresnel reflectance appears to dominate. The data also depart slightly (15%) from a Lambertian function at high emission angles in all azimuthal planes. No published photometric model accounts for the observed data. It is hypothesized that the observed scattering behavior is the superposition of volume and surface (Fresnel) scattering mechanisms.
Meteorological transport of continental soot to Antarctica?
Authors: Murphey B. B. , Hogan A. W.
Published: February 2008

An impactor/concentrator/microdensitometer (ICM) instrument system has been constructed and calibrated. This system is sufficiently sensitive to measure the black (carbon soot) component of Antarctic aerosol with a sampling time of four hours. The impactor concentrator was exposed to Antarctic air at Ross Island in September 1987. Microdensitometer analysis of the collected specimens indicates that the maximum black aerosol concentration was observed concurrently with the arrival of the warmest air accompanying a cyclonic storm. This is similar to the concurrence of continental radon and lead isotopes with warm advection, measured on the Antarctic coast by Polian et al. (1986). It is possible that continental soot can be transported to the Antarctic coast several times each year by this mechanism.
Airborne observations of SO2, HCl, and O3 in the stratospheric plume of the Pinatubo Volcano in July 1991
Authors: Mankin William G. , Coffey M. T. , Goldman Aaron
Published: February 2008

We have used a high resolution infrared spectrometer aboard the NASA Wallops Flight Facility Electra aircraft to measure the total column amount of SO2, O3, and HCl above the aircraft while flying over the Caribbean three weeks after the June 15 eruption of Mt. Pinatubo in the Philippines, South of 20°N latitude we observed columns of SO2 ranging from 2.0 × 1016 to 3.7 × 1016 molecules‐cm−2. In addition, the column amount of HCl averaged 1.5 × 1015 molecules‐cm−2 in the region of the plume. This may represent a small increase in HCl above the amount, estimated from our previous measurements, that would have been present had there been no volcanic eruption, but the increase is substantially less than that seen following the 1982 eruptions of El Chichón [Mankin and Coffey, 1984].
Monitoring the Mt. Pinatubo aerosol layer with NOAA/11 AVHRR data
Authors: Stowe L. L. , Carey R. M. , Pellegrino P. P.
Published: February 2008

The NOAA/NESDIS operational aerosol optical thickness product has provided an exceptional view of the development of the Mt. Pinatubo stratospheric aerosol layer. The product is derived from reflected solar radiation measurements of the Advanced Very High Resolution Radiometer onboard the NOAA/11 polar orbiting environmental satellite. The greater the optical thickness, the greater the amount of reflected solar radiation. Daily and weekly composites of aerosol optical thickness (AOT) at a wavelength of 0.5 micrometers have been analyzed to monitor the spatial and temporal variability of the aerosol layer and its optical thickness since the major eruption of Mt. Pinatubo on June 15, 1991. These analyses show that: the volcanic aerosol layer circled the Earth in 21 days; there are inhomogeneities in the layer that seem to remain after over two months of circling the Earth; using an AOT of 0.1 to define the layer, it covered about 42% of the Earth's surface area after two months, over twice the area covered by the El Chichon aerosol layer two months after its eruption; the layer is confined to the latitude zone 20S to 30N, with occasional patches seen at somewhat higher latitudes; the largest mean optical thickness of the layer was 0.31, occurring on August 23rd; the mass of SO2 required to produce this aerosol optical thickness is 13.6 megatons; and, the globally averaged net radiation at the top of the atmosphere may be reduced by about 2.5 Wm−2 (cooling effect of at least 0.5°C) once the aerosol is distributed globally over the next two to four years.
MHD flow past an obstacle: Large‐scale flow in the magnetosheath
Authors: Wu C. C.
Published: February 2008

As a step to the study of the large‐scale flow in the magnetosheath, the MHD flow past an obstacle is investigated. A time asymptotic method is used to obtain 3D steady‐state solutions. The results indicate the formation of a depletion layer near the obstacle due to the increase of the magnetic field. Along the earth‐sun line the plasma density increases first and then decreases from the post bow shock to the magnetopause. The local density maximum in front of the magnetopause may correspond to what was recently observed. When the interplanetary magnetic field direction is tilted from the solar wind flow, the IMF influences the shape of the bow shock as well as the location of the stagnation point at the magnetopause in a way consistent with observation. In addition, the results show the existence of a magnetosheath current in the post parallel‐shock region.
Non‐methane hydrocarbons in the Arctic atmosphere at Barrow, Alaska
Authors: Doskey Paul V. , Gaffney Jeffrey S.
Published: February 2008

C2‐C8 non‐methane hydrocarbons (NMHCs) were detected in the arctic atmosphere at Barrow, Alaska, during seven sampling periods in March 1989. The most prominent NMHCs were ethene, acetylene, ethane, propane, and benzene. The concentrations and distribution of these species were similar to those observed at the same location in 1982 and in the Norwegian Arctic in 1983. The distribution of NMHCs in background arctic air during this period was similar to that of aged urban air. These findings provide additional evidence that NMHCs in the arctic atmosphere during spring may originate from industrialized mid‐latitude regions and contribute to the arctic haze phenomenon.
Revised global model of thermosphere winds using satellite and ground‐based observations
Authors: Hedin A. E. , Biondi M. A. , Burnside R. G. , Hernandez G. , Johnson R. M. , Killeen T. L. , Mazaudier C. , Meriwether J. W. , Salah J. E. , Sica R. J. , Smith R. W. , Spencer N. W. , Wickwar V. B. , Virdi T. S.
Published: February 2008

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground‐based incoherent scatter radar and Fabry‐Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground‐based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry‐Perot observations.
The solar wind interaction with Mars: Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations of bow shock position and shape
Authors: Slavin J. A. , Schwingenschuh K. , Riedler W. , Yeroshenko Ye.
Published: February 2008

Observations taken by Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 are used to model the shape, position, and variability of the Martian bow shock for the purpose of better understanding the interaction of this planet with the solar wind. Emphasis is placed upon comparisons with the results of similar analyses at Venus, the only planet known to have no significant intrinsic magnetic field. Excellent agreement is found between Mars bow shock models derived from the earlier Mariner‐Mars data set (24 crossings in 1964–1974) and the far more extensive observations recently returned by Phobos 2 (94 crossings in 1989). The best fit model to the aggregate data set locates the subsolar bow shock at a planetocentric distance of 1.56±0.04 RM. Mapped into the terminator plane, the average distance to the Martian bow shock is 2.66±0.05 RM. Compared with Venus, the bow wave at Mars is significantly more distant in the terminator plane, 2.7 RM versus 2.4 RV, and over twice as variable in location with a standard deviation of 0.49 RM versus 0.21 RV at Venus. The Mars 2, 3, and 5 and Phobos 2 data also contain a small number of very distant dayside shock crossings with inferred subsolar obstacle radii derived from gasdynamic modeling of 2000 to 4000 km. Such distant bow shock occurrences do not appear to take place at Venus and may be associated with the expansion of a small Martian magnetosphere under the influence of unusually low solar wind pressure. Finally, the altitude of the Venus bow shock has a strong solar cycle dependence believed to be due to the effect of solar EUV on the neutral atmosphere and mass loading. Comparison of the Phobos 2 shock observations near solar maximum (Rz = 141) with the Mariner‐Mars measurements taken much farther from solar maximum (Rz = 59) indicates that the Martian bow shock location is independent of solar cycle phase and, hence, solar EUV flux. These results are interpreted in terms of a hybrid solar wind interaction model in which this planet possesses a weak, but significant, intrinsic magnetic field.
Gravity wave‐driven fluctuations in OH nightglow from an extended, dissipative emission region
Authors: Schubert G. , Walterscheid R. L. , Hickey M. P.
Published: February 2008

The theory of gravity wave‐driven fluctuations in the OH nightglow from an extended source region is generalized to account for effects of eddy kinematic viscosity ν and eddy thermal diffusivity κ. In the nondiffusive case, the amplitudes and phases of vertically integrated normalized intensity 〈δI〉/〈Ī〉 and temperature perturbations and vertically integrated Krassovsky's ratio 〈η〉 as functions of period are influenced by the upper limit of vertical integration of the extended source, especially at long periods when vertical wavelengths λυ are small. The effects, which include oscillations in 〈δT〉/〈Ī〉, , and 〈η〉, particularly at long periods, are due to constructive and destructive interference of nightglow signals from vertically separated levels of the OH emitting region that occur when λυ is comparable to or smaller than the thickness of the main emission region. The sensitivity of these ratios to the upper limit of vertical integration occurs because of the relatively small rate of decay of the intensity of OH emission with height above the peak emission level and the exponential growth with altitude of nondissipative gravity waves. Because eddy diffusion increases λυ, especially at long periods, and reduces wave growth with height compared with the case ν = κ = 0, inclusion of eddy diffusion removes the sensitivity of 〈η〉 and the other ratios to the maximum height of vertical integration. It is essential to account for both eddy diffusion and emission from the entire vertically extended emission region to correctly predict 〈η〉, 〈δI〉/〈Ī〉, and at long gravity wave periods. Observations of Krassovsky's ratio are consistent with theoretical predictions of 〈η〉 for horizontal wavelengths larger than about 500 km, provided diffusion and emission from an extended region are both taken into account.
Shuffling foot points and magnetohydrodynamic discontinuities in the solar wind
Authors: Neugebauer M. , Alexander C. J.
Published: February 2008

ISEE 3 field and plasma data are used to investigate the frequency of occurrence of isolated, large‐amplitude rotational (RD) and tangential (TD) discontinuities in different types of solar wind flow. It is found that there are relatively more TDs in solar wind that originates in closed field regions and is ejected into interplanetary space by coronal transients than in the solar wind that originates in open field regions. An exception is the plasma in bidirectional electron streaming events which has very few discontinuities of any type. The speed of the wind from open field regions is approximately linearly related to the number of RDs per hour; such a relation does not exist for the wind associated with coronal mass ejections. These results are consistent with the hypothesis that the convection‐driven shuffling of magnetic foot points at the solar surface leads to TDs, magnetic reconnection, and heating of the corona on closed field lines as suggested by Parker, while in the open field regions the disturbances created by the shuffling are carried off by waves which contribute to the acceleration of the solar wind. It is also demonstrated that when the proton β is low, the plasma cannot support either type of discontinuity.
Superthermal >36‐eV ions observed in the near‐tail region of Venus by the Pioneer Venus Orbiter Neutral Mass Spectrometer
Authors: Kasprzak W. T. , Grebowsky J. M. , Niemann H. B. , Brace L. H.
Published: February 2008

The Pioneer Venus orbiter neutral mass spectrometer (ONMS) has made measurements of >36‐eV ions in the altitude range 1300–3700 km for solar zenith angles greater than 120°. The observations cover a time period from late 1982 to 1989. The superthermal ions form part of the near‐tail region of Venus termed the ionotail. The ONMS superthermal ions represent only a small fraction (about 0.3%) of the plasma density in the ionotail region. The composition is mainly O+, but He+, N+, (CO+ + N2+), NO+, and O2+ have been identified. CO2+ is very rarely observed in this region, and H+ is not measured. The average O+ flux is about 105 cm−2 s−1, but higher fluxes from 106 to 108 cm−2 s−1 have been observed about 10% of the time. The directions of the apparent O+ ion flow in the ecliptic plane show predominantly tailward components with a smaller number of nontailward components. Since the energy of the superthermal ions is sufficient for planetary escape, the >36‐eV O+ escape flux in the ionotail is estimated to be about 105 cm−2 s−1. Other species observed also have enough energy to escape. The O+ flux data show a factor of 2.5 increase from solar minimum to solar maximum implying a photoionization source for these ions. Neither the origin of the >36‐eV ions nor the acceleration mechanism is precisely known. The O+ flux observations do not appear to be correlated with the direction of the “cross‐tail” magnetic field as might be expected if the ions were due to the asymmetric pickup of newly ionized atmospheric neutrals above the ionopause. The composition of the superthermal ions in the ionotail suggests that their source is most likely the high‐altitude nightside ionosphere where O+ and not O2+ is the dominant ion. Transport of superthermal O+ across the terminator to the nightside has been observed, and measurements in the ionotail region at solar minimum near 2000 km show that O+ is mainly superthermal. Much further down the tail, very energetic (0–8 keV) O+ has been observed as it escapes Venus. Conditions in the lower nightside ionosphere have been shown to be consistent with an upward ion flow. Parallel electric fields or J × B forces associated with the convection of the interplanetary magnetic field through or above the ionosphere have been suggested as acceleration mechanisms in the ionotail. Similar mechanisms have been proposed for the tail region of Mars where energetic molecular ions such as O+ have also been observed and may also be appropriate for Titan if the 28‐amu species observed is due to H2CN+.
Seasonal variations in the subauroral electron temperature enhancement
Authors: Fok M.‐C. , Kozyra J. U. , Warren M. F. , Brace L. H.
Published: February 2008

The subauroral nightside electron temperature peak is one of the phenomena showing the response of the subauroral ionosphere to the influx of magnetic storm energy in the vicinity of the plasmapause. A statistical study of the seasonal variations of the subauroral electron temperature enhancement was undertaken using data from the Langmuir probe experiment on the DE 2 satellite throughout most of the mission (1981–1982). In the winter hemisphere the nighttime background electron temperature is the highest and the magnitude of the peak Te responds most weakly to the geomagnetic activity. This behavior can be explained by seasonal trends in the nighttime downward heat flux due to conjugate photoelectrons. Moreover, model results indicate that a factor of ∼3 increase in heat inflow during equinox relative to solstice is required to raise the electron temperature to a given level. This is a consequence of the higher electron densities at the Te peak near equinox. The Te peak occurs on field lines which thread the outer plasmasphere in the vicinity of the plasmapause and thus can be used as a tracer of the plasmapause position. Correlating the position of the Te peak with Kp′ (the highest value of 3‐hour Kp in the preceding 12 hours) indicates a trend toward a more expanded quiet time plasmasphere and one which is more easily compressed by magnetic storms at equinox than during the solstice period.
Helios spacecraft photometer observation of elongated corotating structures in the interplanetary medium
Authors: Jackson B. V.
Published: February 2008

The Helios spacecraft zodiacal light photometers can be used to measure persistent elongated structures at interplanetary distances from the Sun. These data give the heliospheric positions of these structures from their east to west motion with time. An outward motion of the material within each structure can be determined from its curvature. This technique has been used to measure over 40 of these structures from solar minimum to solar maximum (1976 through 1979) using both Helios A and Helios B observations. The positions of the structures measured range from low latitude to as high as 50°. The speeds determined have a mean value of 301±47 km s−1. The modeling, which determines densities for these structures from 0.15 to over 1.0 AU, indicates that these heliospheric structures can have densities many times above the ambient and that the density in excess of an r−2 ambient can differ greatly along their curved extents at any given time. Most of the structures measured in this sample either remain approximately the same density or decrease in density with height above the Sun. This implies that these structures are generally remnants of features maintained by the physical processes which formed them near the surface of the Sun (as streamers) and that they are not caused by interplanetary interactions.
A cross‐field current instability for substorm expansions
Authors: Lui A. T. Y. , Chang C.‐L. , Mankofsky A. , Wong H.‐K. , Winske D.
Published: February 2008

We investigate a cross‐field current instability (CFCI) as a candidate for current disruption during substorm expansions. The numerical solution of the linear dispersion equation indicates that (1) the proposed instability can occur at the inner edge or the midsection of the neutral sheet just prior to the substorm expansion onset although the former environment is found more favorable at the same drift speed scaled to the ion thermal speed, (2) the computed growth time is comparable to the substorm onset time, and (3) the excited waves have a mixed polarization with frequencies near the ion gyrofrequency at the inner edge and near the lower hybrid frequency in the midtail region. On the basis of this analysis we propose a substorm development scenario in which plasma sheet thinning during the substorm growth phase leads to an enhancement in the relative drift between ions and electrons. This results in the neutral sheet being susceptible to the CFCI and initiates the diversion of the cross‐tail current through the ionosphere. Whether or not a substorm current wedge is ultimately formed is regulated by the ionospheric condition. A large number of substorm features can be readily understood with the proposed scheme. These include (1) precursory activities (pseudobreakups) prior to substorm onset, (2) substorm initiation region to be spatially localized, (3) three different solar wind conditions for substorm occurrence, (4) skew towards evening local times for substorm onset locations, (5) different acceleration characteristics between ions and electrons, (6) tailward spreading of current disruption region after substorm onset, and (7) local time expansion of substorm current wedge with possible discrete westward jump for the evening expansion.
Sondrestrom radar measurements of the reconnection electric field
Authors: de la Beaujardiere O. , Lyons L. R. , Friis‐Christensen E.
Published: February 2008

Solar wind energy is transferred to and from the closed field line region of the magnetosphere by transport across the boundary between open and closed field lines (the separatrix). The rate of this transfer is measured by the reconnection electric field, which is tangential to the separatrix. Although it is not possible to measure directly the reconnection electric field in the magnetosphere, the electric field in the ionosphere can readily be used to calculate the magnetic reconnection rate. This paper describes a technique for using the Sondrestrom incoherent scatter radar in Greenland to estimate this rate from measurements of the plasma velocity in the separatrix reference frame. The ionospheric plasma drift and the separatrix location and velocity are determined from the radar observations, and the separatrix orientation is inferred from all‐sky images. This technique has been applied to obtain measurements of the reconnection electric field in the midnight sector, with 3‐ to 5‐min time resolution during the night of January 14–15, 1989. The reconnection electric field was found to be less than 15 mV/m during periods of local polar cap expansion, which corresponded to substorm recovery phases, and to be 30 to 40 mV/m during times of polar cap contraction, which corresponded to substorm expansive phases. During a short interval, the measurements showed the separatrix moving equatorward faster than the plasma, which implies that there was plasma transfer from closed to open field lines, rather than the usual nightside transfer from open to closed field lines.
Ion injection simulations of quasi‐parallel shock re‐formation
Authors: Onsager T. G. , Winske D. , Thomsen M. F.
Published: February 2008

The process of quasi‐parallel shock re‐formation is investigated using one‐dimensional hybrid simulations. These simulations are used to study the coupling of a beam of ions reflected at the shock to the incoming solar wind. We have constructed a simple simulation configuration that allows us to control the properties of the background plasma and of the reflected ions. The simulations initially do not contain a shock, but rather include a background plasma, corresponding to the solar wind, into which we inject a counterstreaming ion beam, corresponding to shock‐reflected ions. We investigate the length and time scales for the coupling of the reflected ions to the background plasma as functions of θBn, beam density, and beam temperature. This coupling can lead to the formation of shocklike structures upstream from the injection location which we associate with quasi‐parallel shock re‐formation. In addition, we have perturbed the upstream plasma in a controlled manner to study the effect of upstream waves on the coupling process. We find that the coupling length and time scales vary systematically with the upstream magnetic field direction, θBn. The coupling occurs at roughly the time and location where the injected ions become deflected transverse to the shock normal direction. Also, as seen in previous quasi‐parallel shock simulations, the reflected (injected) ions become trapped downstream of the newly forming shock by the growing perturbations. In this way, shock‐reflected ions may contribute directly to the downstream ion temperature.
He2+ and H+ dynamics in the subsolar magnetosheath and plasma depletion layer
Authors: Fuselier S. A. , Klumpar D. M. , Shelley E. G. , Anderson B. J. , Coates A. J.
Published: February 2008

Active Magnetospheric Particle Tracer Explorer (AMPTE)/Charge Composition Explorer observations downstream from the quasi‐perpendicular bow shock are combined with upstream observations from the AMPTE/UKS and IRM spacecraft to distinguish intrinsic changes in the solar wind H+ and He2+ plasma and magnetic field from changes inherent in the magnetosheath and near magnetopause region. Using these spacecraft, a plasma depletion layer and an external accelerated flow layer (on the magnetosheath side, sunward of the magnetopause) are identified in the magnetosheath near the magnetopause. Compared to the upstream region, the magnetosheath plasma has higher densities and temperatures and the same He2+/H+ density and average temperature ratios. Compared to the magnetosheath, the plasma depletion layer has decreased solar wind ion densities, increased magnetic field magnitude, lower and more anisotropic ion temperatures, a higher He2+/H+ density and temperature ratio, and an energetic H+ population leaked from the magnetosphere. Compared to the depletion layer, plasma the accelerated flow layer on the magnetosheath side of the magnetopause has an increased flux of energetic electrons and the addition of magnetosheath ions reflected off the magnetopause. The density, temperature, and magnetic field changes and the presence of leaked magnetospheric protons in the plasma depletion layer are consistent with the draping and pileup of the magnetic field sunward of but near the dayside magnetopause. Changes in the ion distributions in the accelerated flow layer are consistent with dayside magnetic reconnection. These results indicate that at times the plasma adjacent to the dayside magnetopause has significantly different characteristics from the magnetosheath or upstream solar wind plasma.
Nonadiabatic particle motion and corotation lag in the Jovian magnetodisk
Authors: Cheng Andrew F. , Decker Robert B.
Published: February 2008

In the sharply curved magnetic field lines of Jupiter's magnetodisk, energetic ions follow nonadiabatic orbits violating the first and second adiabatic invariants. Even in the corotating reference frame, the energetic ion distribution function is in general anisotropic, as is shown by numerically computed orbits in the Goertz et al. (1976) magnetodisk model. Pitch angle scattering that can be induced by magnetic field irregularities or plasma waves is included in the calculations. This numerically calculated anisotropy of nonadiabatic ions can account for the anisotropy of energetic ions measured by Voyager in the Jovian magnetodisk, depending strongly on the radial distribution of particles injected into the current sheet but not strongly on the scattering rate. The measured anisotropy has previously been interpreted as indicating approximate corotation in the outer Jovian magnetodisk, but the present calculations show that the measured anisotropy can be consistent with a substantial corotation lag.