bluuebunny/crossref_metadata_2025 huggingface dataset:
How regularly do earthquakes recur? A synthetic seismicity model for the San Andreas Fault
Authors: Ward Steven N. , Goes Saskia D. B.
Published: February 2008

We have been attempting to improve estimates of long‐term earthquake recurrence probabilities along the San Andreas Fault by means of synthetic seismicity calculations. The calculations are based on the concept of fault segmentation and incorporate the physics of static dislocation theory. Forecasts constructed from synthetic seismicity are robust in that: they embody regional seismicity information over several units of magnitude; they tie together in a physical manner, a spectrum of fault segment features such as length, strength, characteristic magnitude, mean repeat time and slip rate; they can reasonably account for fault segment interactions; and they are formulated from a catalog which can be extended as long as needed to be statistically significant. We contend earthquake recurrence is more aperiodic than previously thought and, as a result, probabilities of major San Andreas earthquakes should be revised downward.
Sea level rise at Key West, Florida, 1846‐1992: America's longest instrument record?
Authors: Maul George A. , Martin Douglas M.
Published: February 2008

The continuous series of sea level at Key West, Florida commenced in 1913, but we have discovered sporadic measurements that date back to 1846. From records at the U.S. Army Corps of Engineers and the U.S. Coast and Geodetic Survey, the sea level series has been connected to a Summary (common) Datum. Thus, a gappy record of monthly and annual mean heights (H[t]), perhaps the United States' longest series over San Francisco (ca. 1854) or New York ( ca. 1856), can be tested to ascertain if the rise in relative sea level at this site is stationary. Applying first and second order least squares and two‐phase regression analyses, we find that dH/dt is 0.19±0.01 cm/yr, and that d²H/dt²= [9.6±8.6]·10−3 cm/yr²; the two‐phase regression shows H[t] rising 0.15±0.03 cm/yr before ca. 1925 and 0.23±0.01 cm/yr afterwards. Neither the second‐order regression coefficient nor d²H/dt² nor the two‐phase calculation are significant above the 75% confidence level, but all three are weakly consistent with accelerated rise. For the epoch 1951–1987, Key West sea level, corrected for post‐glacial rebound, is best explained by concurrent measurements of 0–1,000 db dynamic height anomaly change.
Absence of magnetic trapping on closed field lines at Neptune
Authors: Paranicas C. , Cheng A. F.
Published: February 2008

The large offset of Neptune's magnetic field in an offset tilted dipole (OTD) approximation allows some fraction of magnetospheric field lines to have monotonically increasing field magnitude over their entire length in the magnetosphere. The usual magnetic trapping is impossible on such field lines, meaning that all charged particles on these field lines will precipitate after mirroring at most once. These field lines can be thought of as the part of the global magnetic anomaly where no particles on the corresponding drift shells remain trapped for an entire rotation of the planet. In this paper we examine both the morphology and size of the affected surface area in the OTD2 and O8 magnetic field models. For regions where there are indications that part of the aurora could be due to monotonic field line precipitation we analyze whether these field lines could be sufficiently populated to produce auroral signatures by considering photoelectron production rates and radial diffusion. We conclude that monotonic field line precipitation may contribute to the observed Neptune aurora but does not explain the global phenomenon.
An Early Solar Dynamo Prediction: Cycle 23 ∼ Cycle 22
Authors: Schatten Kenneth H. , Pesnell W. Dean
Published: February 2008

In this paper, we briefly review the “dynamo” and “geomagnetic precursor” methods of long‐term solar activity forecasting. These methods depend upon the most basic aspect of dynamo theory to predict future activity, future magnetic field arises directly from the magnification of pre‐existing magnetic field. We then generalize the dynamo technique, allowing the method to be used at any phase of the solar cycle, through the development of the “Solar Dynamo Amplitude” (SODA) index. This index is sensitive to the magnetic flux trapped within the Sun's convection zone but insensitive to the phase of the solar cycle. Since magnetic fields inside the Sun can become buoyant, one may think of the acronym SODA as describing the amount of buoyant flux. Using the present value of the SODA index, we estimate that the next cycle's smoothed peak activity will be about 210 ± 30 solar flux units for the 10.7 cm radio flux and a sunspot number of 170 ± 25. This suggests that solar cycle #23 will be large, comparable to cycle #22. The estimated peak is expected to occur near 1999.7 ± 1 year. Since the current approach is novel (using data prior to solar minimum), these estimates may improve when the upcoming solar minimum is reached.
Northern hemisphere mid‐stratosphere vortex processes diagnosed from H2O, N2O and potential vorticity
Authors: Lahoz W. A. , Carr E. S. , Froidevaux L. , Harwood R. S. , Kumer J. B. , Mergenthaler J. L. , Peckham G. E. , Read W. G. , Ricaud P. D. , Roche A. E. , Waters J. W.
Published: February 2008

MLS H2O, CLAES N2O and potential vorticity calculated from UK Meteorological Office data are used to study mid‐stratospheric vortex processes in the northern hemisphere winter of 1991–92. Areas of moist air (at ∼ 20 hPa) and N2O‐poor air (at ∼ 10 hPa) are well‐correlated with high values of potential vorticity and there is little or no large scale mixing across the vortex edge. We find evidence for the descent of relatively dry mesospheric air to the 840 K (∼ 10 hPa) level, as well as descent of moist air from the upper stratosphere to the 655 K (∼ 20 hPa) level. A reduction in the areas of the vortex and both the moist and N2O‐poor regions is observed and there is evidence of moist and N2O‐poor air parcels being extruded from the vortex.
Solar cycle variations of electron density and temperature in the Venusian nightside ionosphere
Authors: Theis Robert F. , Brace Larry H.
Published: February 2008

The return of periapsis to low altitudes during the Summer and Fall of 1992 provided a unique opportunity for the Pioneer Venus Orbiter (PVO) to make in situ measurements in the Venusian ionosphere at much lower levels of solar activity (F10.7 ≈120) than existed when periapsis was at low altitudes in 1979 and 1980 (F10.7≈220). We present the observations of electron density (Ne) and temperature (Te) made by the Orbiter Electron Temperature Probe (OETP) during the Entry Period. Empirical models of the Ne and Te height variations are presented and compared with similar models based on OETP measurements made at solar maximum. The median Ne at the ionospheric peak (∼140 km) was essentially unchanged from its solar maximum value, but the ionosphere was increasingly depleted at higher altitudes, reaching a factor of 7 lower densities at 200 km. Te was lower by almost a factor of 2 at 140 km but was rather significantly enhanced at higher altitudes; exceeding its solar maximum values by a factor of 1.3 at 200 km and a factor of 2 at 500 km. In general these results support the earlier conclusions that the nightside upper ionosphere is depleted at lower levels of solar activity by a reduction of the nightward ion flow. The lack of Ne variation near the peak (between solar maximum and entry) suggests that nightward ion transport does not play as large a role in the peak formation as does local ion production by energetic particles. The decrease in Te at low altitudes suggests that the low densities of the upper ionosphere at the time of PVO entry could no longer support the conduction of heat from the dayside ionosphere, thus allowing the lower nightside ionosphere to cool by collisions with ions and neutrals, and by heat conduction to the cooler regions below.
VLF signatures of lightning‐induced heating and ionization of the nighttime D‐region
Authors: Inan U. S. , Rodriguez J. V. , Idone V. P.
Published: February 2008

48.5 kHz signals from a transmitter in Silver Creek, Nebraska, propagating to Huntsville (HU), Alabama over a ∼1200 km Great Circle Path (GCP) exhibit characteristic amplitude changes which appear within 20 ms of cloud‐to‐ground (CG) flashes located within 50 km of the path. Data are consistent with the heating of ionospheric electrons by the electromagnetic (EM) pulse from lightning producing ionization changes in the D‐region over the thunderstorm.
Electric field measurements in the vicinity of noctilucent clouds and PMSE
Authors: Zadorozhny Alexander M. , Tyutin Alexander A. , Witt Georg , Wilhelm Nathan , Wälchli Urs , Cho John Y. N. , Swartz Wesley E.
Published: February 2008

We report mesospheric electric field structure in the vicinity of noctilucent clouds (NLC) and polar mesospheric summer echoes (PMSEs) measured on the DECIMALS‐B rocket payload launched during the international rocket‐radar campaign NLC‐91 from Esrange, Sweden on August 10, 1991. Unusually large vertical E‐ fields, Ez, about 100–300 mV/m on ascent and greater than 1 V/m on descent were detected at 82.5–84.5 km. The region of the large Ez was clearly limited by the NLC layer on the bottom and by the distinctly separated PMSE layer on the top. A narrow negative peak in the Ez height profile observed on ascent in the lower part of the NLC layer was apparently caused by the interaction of the field mill with impacting NLC particles possibly carrying negative charge. If the impact signature is due to single particles, their size is estimated to at least 0.5 µm and their concentration about 10−4 cm−3 locally. Based on the light‐scattering properties of NLC such massive particles can only be a minor part of the NLC population.
First height comparison of noctilucent clouds and simultaneous PMSE
Authors: Wälchli Urs , Stegman Jacek , Witt Georg , Cho John Y. N. , Miller Clark A. , Kelley Michael C. , Swartz Wesley E.
Published: February 2008

On the night of August 9–10, 1991, two rocket payloads were launched into simultaneously occurring noctilucent clouds (NLC) and polar mesospheric summer echoes (PMSE) above Esrange, a third rocket payload was launched into a NLC where a PMSE was detected 5 minutes later above Esrange, in Sweden as part of the NLC‐91 campaign. An aim of this experiment was to compare the vertical structures and locations of the NLC and PMSE events. To this end, in situ optical photometers and particle impact sensors were used to measure the altitude and vertical structure of the NLC layer, while the Cornell University portable radar interferometer (CUPRI) was used to probe the PMSE. Although this comparison is complicated by the horizontal separations between the in situ measurements and the radar volume, and low electron densities which reduced the overall radar reflectivity, we conclude that the PMSE layer in the CUPRI radar volume remained above the NLC layer detected by the in situ instruments by 300 to 2000 m throughout the experiment. We interpret this result as supporting the view that PMSE are more likely to result from the presence of aerosols smaller than the ones optically detectable as NLCs.
Wrinkle ridges, stress domains, and kinematics of Venusian plains
Authors: McGill George E.
Published: February 2008

Wrinkle ridges are nearly ubiquitous landforms on the plains of Venus. By analogy with similar structures on other planets, venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust, an inference that is verified by geometrical relationships with positive and negative relief features on Venus. Because plains are the dominant terrain on Venus, wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet. In most places there are two or more sets of wrinkle ridges, and commonly one of these persists over a very large area, defining a regional stress domain. Intersection relationships indicate that these domains differ in age.
Speleothems as proxy for the carbon isotope composition of atmospheric CO2
Authors: Baskaran M. , Krishnamurthy R. V.
Published: February 2008

We have measured the stable isotope ratios of carbon in a suite of recent cave deposits (<200 years) from the San Saba County, Texas, USA. The methodology for dating these deposits using excess 210Pb was recently established [Baskaran and Iliffe, 1993]. The carbon isotope ratios of these samples, spanning the time period ∼1800–1990 AD, reflect the carbon isotope ratio of atmospheric CO2 for the same period. The pathways by which the δ13C of atmospheric CO2 is imprinted on these speleothems can be explained using a model developed by Cerling (1984). The results suggest that the carbon isotope ratios of speleothems can be used to develop long‐term, high‐resolution chronologies of the δ13C of atmospheric CO2 and, by implication, the concentration of the atmospheric CO2.
Temporal variation of cloud fraction: Effects on a simulated sea‐ice cover
Authors: Maslanik J. A. , Silcox R. A.
Published: February 2008

The Arctic sea‐ice cover is typically simulated using monthly‐mean cloud fractions. To examine the sensitivity of sea‐ice thickness and distribution to the more realistic case of cloud fractions that vary over synoptic time scales, we simulate the ice cover with a two‐dimensional ice model and simple radiation parameterizations containing linear and non‐linear cloudiness terms. Using a common non‐linear parameterization with daily‐varying, clear‐sky/overcast cloud fractions that sum to the monthly mean rather than daily cloud fractions equal to the monthly mean, the simulated ice volume in the Arctic and peripheral seas increases by 38% overall with a net decrease in downwelling shortwave radiation of 26%. Mid‐summer ice extent increases by 17%, while ice thickness doubles in the Kara and Barents seas. Since the ice model is shown to respond nearly linearly to day‐to‐day variations in downwelling radiation, the need to consider synoptic variability of cloud fraction depends on the non‐linearity of the relationship between cloud fraction and radiation.
Episodic denudation in east Africa: A legacy of intracontinental tectonism
Authors: Foster David A. , Gleadow Andrew J. W.
Published: February 2008

Apatite fission‐track data from samples collected over an altitude range of >2.5 km in western and central Kenya reveal three elevation intervals where the apparent apatite age varies linearly with elevation. The mean ages of these intervals are 64 ± 1 Ma, 112 ± 2 Ma, and 180 ± 6 Ma and they result from cooling of rocks below 110° ± 10°C by erosional denudation. The form of the apatite age ‐ elevation profile and forward modeling of the fission‐track length distributions suggests that discrete periods of exhumation commenced at 70–75 Ma, 120–130 Ma, and ≥220 Ma. These times correspond with periods of intracontinental deformation of the African lithosphere, including: the separation of Madagascar from East Africa, opening of the South Atlantic Ocean, and changes in the geometry of plate spreading in the South Atlantic.
Temperature‐dependent Newtonian and non‐Newtonian convection: Implications for lithospheric processes
Authors: Larsen Tine B. , Malevsky Andrei V. , Yuen David A. , Smedsmo Jamie L.
Published: February 2008

Convection studies of temperature‐dependent Newtonian and non‐Newtonian rheology in aspect‐ratio seven boxes have been carried out for volume averaged Rayleigh numbers between O(105) and O(107). Large lateral viscosity variations, O(104) are found for non‐Newtonian cases with total temperature‐dependent viscosity variations up to 250. Flow structures for both rheologies are distinguished by large‐scale circulations with relatively stable descending limbs. Much larger hot thermal anomalies are found near the surface for non‐Newtonian rheology. Lithospheric thinning is facilitated by non‐Newtonian rheology because of stress‐softening and the lubrication of the descending limbs by hot diapirs.
An analysis of nearfield normal mode amplitude anomalies of the Landers Earthquake
Authors: Watada Shingo , Kanamori Hiroo , Anderson Don L.
Published: February 2008

The 1992 Landers earthquake (Mw=7.3) occurred in the middle of the TERRAscope network. Long‐period Rayleigh waves recorded at the TERRAscope stations (Δ≤3°) after traveling around the Earth show large amplitude anomalies, one order of magnitude larger than spherical Earth predictions up to a period of about 600 s. The ground motions over the epicentral region at and after the arrival of R4–5 are in phase at all stations. These observations are inconsistent with the nearly vertical strike slip mechanism of the Landers earthquake. Synthetic seismograms for a rotating, elliptic and laterally heterogeneous Earth model calculated by the variational method agree well with the observed waveforms. Calculations for various 3D Earth models demonstrate that the amplitudes are very sensitive to the large scale aspherical structure in the crust and the mantle. The anomalies for modes shorter than 300 s period can be explained by lateral heterogeneity shallower than the upper mantle. Rotation of the Earth and lower mantle heterogeneity are required to explain mode amplitudes at longer periods. Current whole mantle seismic tomographic models can fully explain the observed amplitudes longer than 300 s. To assess the effect of the high order lateral heterogeneity in the mantle more precise estimate of the crustal correction is required.
MLS CLO observations and Arctic polar vortex temperatures
Authors: Schoeberl M. R. , Stolarski R. S. , Douglass A. R. , Newman P. A. , Lait L. R. , Waters J. W. , Froidevaux L. , Ready W. G.
Published: February 2008

Analysis of UARS MLS observations in early January 1992 shows a clear relationship between predicted polar stratospheric cloud formation along the back trajectory and elevated ClO amounts. These findings are in good agreement with aircraft observations. The MLS observed variation of ClO amounts within the vortex also fits the pattern of ClO change as a result of air parcel solar exposure and nitric acid photolysis. Outside the polar vortex, the occasional highly elevated ClO appear statistically consistent with MLS measurement noise.
Contribution to the ozone trend of heterogeneous reactions of ClONO2 on the sulfate aerosol layer
Authors: Pitari Giovanni
Published: February 2008

A photochemical two‐dimensional model including a grid‐point temperature distribution calculated from long‐term circulation statistics has been used to assess the relative role of heterogeneous reactions of chlorine nitrate on background sulfate aerosols. The sulfuric acid weight percentage is calculated explicitly as a function of temperature and water vapor content. It is shown that the contribution of these reactions to the ozone trend in the 1980‧s accounts for about only 10% the ozone loss due to N2O5 + H2O during the Northern Hemisphere winter months at mid‐latitude and between 50–200 mb. The weight of these reactions, however, may substantially increase in the presence of larger aerosol loading. The reaction ClONO2 + HCl on sulfate aerosols is found to be of comparable importance with respect to ClONO2 + H2O, and it is found to represent a significant source of active chlorine in the Antarctic spring.
Observations of the quasi 2‐day wave from the High Resolution Doppler Imager on Uars
Authors: Wu D. L. , Hays P. B. , Skinner W. R. , Marshall A. R. , Burrage M. D. , Lieberman R. S. , Ortland D. A.
Published: February 2008

A strong westward traveling oscillation, with a period of 2 days and zonal wave number 3, is observed in the mesospheric and lower thermospheric winds from the High Resolution Doppler Imager on the Upper Atmosphere Research Satellite. The important events happen in January, July, and September/October, of which the occurrence in January is the strongest with an amplitude over 60ms−1. Detailed analyses for the periods of January 1992 and January 1993 reveal a cause‐and‐effect relationship in the wave developing process at 95km. The global structures of the wave amplitude and phase are also presented.
Higher order statistics of planetary gravities and topographies
Authors: Kaula William M.
Published: February 2008

The statistical properties of Earth, Venus, Mars, Moon, and a 3‐D mantle convection model are compared. The higher order properties are expressed by third and fourth moments: i.e., as mean products over equilateral triangles (defined as coskewance) and equilateral quadrangles (defined as coexance). For point values, all the fields of real planets have positive skewness, ranging from slightly above zero for Lunar gravity to 2.6σ³ for Martian gravity (σ is rms magnitude). Six of the eight excesses are greater than Gaussian (3σ4), ranging from 2.0σ4 for Earth topography to 18.6σ4, for Martian topography. The coskewances and coexances drop off to zero within 20° arc in most cases. The mantle convective model has zero skewness and excess slightly less than Gaussian, probably arising from viscosity variations being only radial.
Evidence for Langmuir oscillations and a low density cavity in the Venus magnetotail
Authors: Ho C. M. , Strangeway R. J. , Russell C. T.
Published: February 2008

We report the discovery of Langmuir oscillations in a very low plasma density region in the Venus magnetotail. These waves are observed more often at 30 kHz, but also at 5.4 kHz indicating densities as low as 0.3 cm−3 in the central tail lobe. The Langmuir probe on board the Pioneer Venus Orbiter cannot resolve such a low plasma density. We use the magnetic field strength and the assumption of total pressure balance to infer the electron temperature as a test of the Langmuir wave interpretation. By investigating the spatial distribution of this wave activity we find that the plasma cavity is ordered in a coordinate system defined by the interplanetary magnetic field and is found at either side of the central tail current sheet.
Solar‐flare‐implanted 4He/3;He and solar‐proton‐produced Ne and Ar concentration profiles preserved in lunar rock 61016
Authors: Rao M. N. , Garrison D. H. , Bogard D. D. , Reedy R. C.
Published: February 2008

Documented near‐surface samples from oriented lunar rock 61016 have yielded the first measurements of depth profiles for 21Ne, 22Ne, and 38Ar isotopes produced in situ from nuclear interactions of energetic solar flare protons. Further, the cosmogenic 21Ne/22Ne ratios are distinct for different proton energy ranges and increase systematically with increasing shielding. This variation in cosmogenic 21Ne/22Ne represents a progression from a mixed component produced from solar flare protons (SCR) and galactic cosmic rays (GCR) to a pure GCR component. Using the 2 m.y. exposure age for 61016, the calculated production rates for SCR 21Ne and 38Ar from solar energetic protons in near‐surface samples agree within 5% and 25%, respectively, with theoretical estimates. Calculated GCR production rates for 21Ne and 38Ar in the deepest (26 mm) samples agree within 25% and 10%, respectively, with theoretical estimates. Combined SCR and GCR concentration profiles for 21Ne, 22Ne, and 38Ar match theoretical calculations based on an omnidirectional flux above 10 MeV of 100 protons cm−2 s−1 with an exponential rigidity between 70 and 85 MV and a surface erosion rate of 1 mm m.y.−1. Selectively etched, near‐surface samples suggest a long‐term average 4He/3He ratio of ∼770 for implanted solar flare nuclei with E > ∼4 MeV amu−1, and a 4He/³He ratio of ∼3450 for solar flare nuclei with E > ∼1 MeV amu−1. Averaged over long term, the solar flare 4He/3He ratio appears energy dependent.
Magnetic activity dependence of high‐latitude thermospheric winds and densities below 200 km
Authors: Forbes Jeffrey M. , Roble Raymond G. , Marcos Frank A.
Published: February 2008

Satellite‐based measurements are utilized to elucidate the latitudinal, local time, and magnetic activity dependence of winds and densities in the scantily observed atmospheric region between 170 and 220 km above 45° magnetic latitude. One data set consists of atmospheric densities from high‐accuracy (time resolution ≈ 5‐6 hours) orbital analyses of three Doppler Beacon satellites in orbit during 1973. The perigees of these satellites are generally restricted to 160‐180 km, 1200‐1400 LST, and geographic latitudes greater than about 45°. Statistical relationships are derived between the density changes and the planetary magnetic index, Kp, and the 5‐hour mean of the auroral electro jet index, . The former relationships are compared with those derived from the MSISE90 empirical model (Hedin, 1991), which is found to overestimate the rate of increase of density with respect to Kp. Another data set consists of densities and cross‐track winds from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment for the March 21 to April 9, 1979, period, which includes several intervals of elevated magnetic activity. Besides comparing various time series, the data are also binned according to 10° latitude increments and unit increments of Kp to derive trends. Some typical results include the following, corresponding to average changes in the 45° to 65° magnetic latitude band as Kp is increased from 1 to 6: (1) for the nightside (≈ 2230 LT), a change in cross‐track (nearly zonal) wind from 25 ± 25 m s−1 (eastward) to −125 ± 25 m s−1 (westward), and an increase of about 20% in density; and (2) for dayside (≈ 1030 LT), a change in cross‐track wind from 25 ± 25 m s−1 (eastward) to 125 ± 25 m s−1 (eastward), and a density increase of 25%. For some individual sudden enhancements in magnetic activity, changes in winds and densities can be more than double the above average values. Comparisons are also made with the NCAR TIGCM (National Center for Atmospheric Research thermosphere‐ionosphere general circulation model) simulation for the complete 20‐day interval, and with recent empirical models of densities (Hedin, 1991) and winds (Hedin et al., 1991), with data points in each case derived for the satellite paths, instrument orientations, and sampling rates identical to that of the SETA experiment.
A method of measuring the temperature of the lower thermosphere
Authors: Barth C. A. , Eparvier F. G.
Published: February 2008

The temperature of the lower thermosphere between 110 and 140 km has been measured from the analysis of the rotational structure of the (1,1) nitric oxide gamma band in the ultraviolet dayglow. The ultraviolet spectra were obtained on a rocket experiment at Poker Flat, Alaska, at local noon on March 7, 1989. The temperatures were determined by generating synthetic spectra of the nitric oxide gamma bands and performing a multiple linear regression analysis of the rocket and synthetic spectra.
Vortex motion in the ionosphere and nonlinear transport
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

The relation between vorticity and ionospheric flow patterns is investigated by using a fluid mechanics approach in place of the more customary electromagnetic approach. The focus on the fluid features is justified by the observation that in the incompressible limit appropriate to the ionosphere, vorticity can be regarded as the source of the flow field. We show how vorticity can be introduced into the flow by local ionospheric conditions. However, in the cases of greatest interest, the vorticity is imposed by external sources, which can be in the magnetosphere or in the solar wind. As an important application, we consider traveling ionospheric vortices propagating around the polar cap boundary. We show that such traveling disturbances transport both momentum and magnetic flux in the direction of their phase velocity, typically antisunward. Like other intermittent disturbances of small scale, such as flux transfer events, individual traveling ionospheric vortices transport relatively little flux, but multiple disturbances could conceivably transport an important fraction of the polar cap magnetic flux from the dayside to the tail.
Vorticity equation for MHD fast waves in geospace environment
Authors: Yamauchi M. , Lundin R. , Lui A. T. Y.
Published: February 2008

The magnetohydrodynamic (MHD) vorticity equation is modified in order to apply it to nonlinear MHD fast waves or shocks when their extent along the magnetic field is limited. Field‐aligned current (FAC) generation is also discussed on the basis of this modified vorticity equation. When the wave normal is not aligned to the finite velocity convection and the source region is spatially limited, a longitudinal polarization (u⊥ · J⊥) causes a pair of plus and minus charges inside the compressional plane waves or shocks, generating a pair of FACs. This polarization is not related to the separation between the electrons and ions caused by their difference in mass (i.e., Langmuir mode), a separation which is inherent to compressional waves. The resultant double field‐aligned current structure exists both with and without the contributions from curvature drift, which is questionable in terms of its contribution to vorticity change from the viewpoint of single‐particle motion.
Observed Barium Emission Rates
Authors: Stenbaek‐Nielsen H. C. , Wescott E. M. , Hallinan T. J.
Published: February 2008

The barium releases from the CRRES satellite have provided an opportunity for verifying theoretically calculated barium ion and neutral emission rates. Spectra of the five Caribbean releases in the summer of 1991 were taken with a spectrograph on board a U.S. Air Force jet aircraft. Because the line of sight release densities are not known, only relative rates could be obtained. The observed relative rates agree well with the theoretically calculated rates and, together with other observations, confirm the earlier detailed theoretical emission rates. The calculated emission rates can thus with good accuracy be used with photometric observations. It has been postulated that charge exchange between neutral barium and oxygen ions represents a significant source for ionization. If so, it should be associated with emissions at 4957.15 Å and 5013.00 Å, but these emissions were not detected.
Ray‐tracing studies of the arrival direction and the ionospheric penetration of whistlers at low latitudes
Authors: Nakamura Yoshikatsu
Published: February 2008

Direction‐finding measurements of whistlers on the ground at low latitudes reveal that whistlers arrive from narrow regions of the zenith of only a few tens of kilometers in diameter. Using an appropriate geomagnetic field model and an ionospheric model including a field‐aligned duct, the characteristics of the arrival direction of low‐latitude whistlers are studied. Wave normal directions at the duct exit are calculated for whistler waves initially injected vertically upward at the conjugate duct entrance. Vertically downward wave normals appear at the duct exit quasi‐periodically for the injection positions at the duct entrance. Thus the whistler duct operates to direct the wave normal of whistlers at the duct exit vertically downward and make ionospheric penetration easier. The mechanism of this penetration is investigated comparing the observed characteristics of the arrival direction with the ray‐tracing calculations. The whistler energy emitted from the duct exit reaches the bottom of the ionosphere without spreading and then is radiated into free space from there. The heights of the conjugate duct ends at low latitudes are concluded to extend downward in the bottomside ionosphere for most ground whistlers. These results may be significant also for the study of ground whistlers at higher latitudes.
Large‐amplitude auroral electric fields measured with DE 1
Authors: Weimer D. R. , Gurnett D. A.
Published: February 2008

A large fraction of the available electric field data from the plasma wave instrument (PWI) on the Dynamics Explorer (DE) 1 satellite has recently been searched for events with large‐amplitude (over 100 mV m−1) electric fields. The magnitude and distribution of these peak events as functions of altitude have been determined. The largest amplitudes were found between 1.4 and 2.5 RE and the probability of finding large electric fields was greatest in the range of 1.5 to 1.7 RE. However, when the measured electric field values are “mapped” to the Earth's surface in order to account for the geometry of the geomagnetic field lines, then the mapped values always increase with increasing altitude. This radial dependence is considered to be evidence for magnetic field‐aligned electric fields. The largest electric field that was detected with the DE 1 instrument had a magnitude exceeding 840 mV m−1 and was found at 1.45 RE. This field appears to be associated with a low‐frequency wave, whereas the more commonly observed large‐amplitude events have longer durations, are generally found at higher altitudes, and appear to be due to static electric fields. It appears that two types of phenomena are being observed. A study of all electric field measurements, rather than just those with large amplitudes, was also conducted. It was found that at subauroral latitudes the average mapped electric field is nearly constant with altitude, as it should be where there are no magnetic field‐aligned potential drops. Small variations can be accounted for by long‐term changes in geomagnetic activity. However, within auroral latitudes the average value of the mapped electric field increases as altitude increases. The largest gradients are found between 1.3 and 2 RE. There is evidence for a “split” potential distribution, with an additional potential drop located above 2.7 RE.
Cosmic ray decreases and particle acceleration in 1978‐1982 and the associated solar wind structures
Authors: Cane H. V. , Richardson I. G. , von Rosenvinge T. T.
Published: February 2008

A study has been made of the time histories of particles in the energy range 1 MeV to 1 GeV at the times of all >3% cosmic ray decreases in the years 1978 to 1982. Essentially all of the 59 decreases are associated with passages of interplanetary shocks, the majority of which accelerate particles. We use the intensity‐time profiles of the particles to separate the cosmic ray decreases into four classes which we subsequently associate with three types of solar wind structures. Decreases in class 1 (15 events) and class 2 (26 events) are associated with shocks driven by energetic coronal mass ejections. For class 1 events, the ejecta is detected at 1 AU, whereas this is not usually the case for class 2 events. The shock must therefore play a dominant role in producing the cosmic ray depression in class 2 events. In all class 1 and class 2 events (which comprise 69% of the total) the departure time of the ejection from the Sun (and hence the location) can be determined from the rapid onset of energetic particles several days before the associated shock passes the Earth. Class 1 events originate from within 50° of central meridian while class 2 are predominantly from sources beyond 20° from central meridian. Class 3 events (10 decreases) can be attributed to less energetic ejections which are directed towards the Earth. In these events the ejecta is more important than the shock in causing the cosmic ray intensity depression. The remaining events (14% of the total) can be attributed to solar wind periods with multiple disturbances. All include ejecta material and at least half also include corotating high‐speed solar wind streams. It is argued that since energetic particles (from MeV to GeV energies) seen at Earth may respond to solar wind structures which are not detected at Earth, consideration of particle observations over a wide range of energies is necessary for a full understanding of cosmic ray decreases.
Quasi‐periodic transverse plasma flow associated with an evolving MHD vortex street in the outer heliosphere
Authors: Siregar Edouard , Roberts D. Aaron , Goldstein Melvyn L.
Published: February 2008

We study a transverse (“meridional” in heliocentric coordinates) plasma flow induced by the evolution of a Karman vortex street using a Chebyshev‐Fourier spectral algorithm to solve both the compressible Navier‐Stokes and magnetohydrodynamic (MHD) equations. The evolving vortex street is formed by the nonlinear interaction of two vortex sheets initially in equilibrium, such as are naturally found either side of the heliospheric current sheet at solar minimum. We study spatial profiles of the total plasma velocity, the density, the meridional flow angle and the location of sector boundaries and find generally good agreement with Voyager 2 measurements of quasi‐periodic transverse flow in the outer heliosphere. The pressure pulses associated with the meridional flows in the simulation are too small, although they are correctly located, and this may be due to the lack of any “warp” in the current sheet in this model. A strong flow‐aligned magnetic field, such as would occur in the inner heliosphere, is shown to lead to weak effects that would be masked by the background interplanetary turbulence. We also study the plasma and magnetic transport resulting from the meridional flow, and find that deficits of magnetic quantities do occur near the ecliptic and that while the effect is relatively small, it is in general agreement with the most recent analysis of ‘flux deficit’ in the outer heliosphere.
Modeling the ion loss effect on the generation of region 2 field‐aligned currents via equivalent magnetospheric conductances
Authors: Peymirat Christophe , Richmond Arthur D.
Published: February 2008

Vasyliunas (1972) showed that, for steady state lossless ion convection in the inner magnetosphere, the effects of region 2 field‐aligned currents generated by the pressure gradients of the magnetospheric plasma could be represented by an equivalent magnetospheric Hall conductance Ci. We examine in the framework of a linearized fluid theory how ion losses modify the formalism introduced by Vasyliunas. The equivalent magnetospheric Hall conductance Ci is decreased and a new term Cr is added to the Σφφ component of the ionospheric conductance tensor Σ. An evaluation of this new term shows that the eastward rotation of the convection potential pattern induced by Ci is reduced and that Cr contributes to a nonnegligible part of the shielding effect. This linear formalism is compared with the nonlinear model of C. Peymirat and D. Fontaine (Numerical simulation of the magnetospheric convection including the effect of field‐aligned currents and electrons precipitation, submitted to Journal of Geophysical Research, 1993), showing that the effect of the field‐aligned currents on the magnetospheric convection is reproduced up to first order in the linear formalism. The linear formalism presents a simple way to include the effects of region 2 field‐aligned currents into models of ionospheric electrodynamics for steady state conditions.
Variation of type I plasma wave phase velocity with electron drift velocity in the equatorial electrojet
Authors: Ravindran Sudha , Reddy C. A.
Published: February 2008

Using a 54.95‐MHz coherent backscatter radar at Thumba (8.5°N, 77°E; 0.5°N dip angle), the phase velocity variations of type I plasma waves generated by the modified two‐stream instability process in the equatorial electrojet have been measured on several occasions of large and rapid electric field fluctuations associated with geomagnetic substorms and storms. The measurements show a clear linear variation of type I wave phase velocity (VpI) over a considerable range with the simultaneously measured phase velocity (VpII) of gradient drift instability‐generated type II plasma waves. Since VpII is proportional to the drift velocity of electrons, the observed variations of VpI with VpII mean that VpI varies in step with the variations of the electron drift velocity Vey. The observed variation of VpI with Vey is consistent with the current theoretical understanding that the plasma wave‐associated electric fields cause anomalous electron diffusion and heating which result in the stabilization of the type I wave phase velocity near the increased value of the ion‐acoustic velocity. The enhanced electron temperatures estimated from the largest values of the observed VpI are found to be 2.0‐2.4 times the ambient value in the absence of waves.
Model of the solar wind interaction with the local interstellar medium: Numerical solution of self‐consistent problem
Authors: Baranov V. B. , Malama Yu. G.
Published: February 2008

A self‐consistent gasdynamic model of the solar wind interaction with the local interstellar medium (LISM), which took into account the mutual influence of the plasma component (electrons and protons) of the LISM and the LISM H atoms that penetrate into the heliosphere was constructed by Baranov et al. (1981) in the approximation of axial symmetry. This model, however, had a number of defects. In particular, the motion of the H atoms was described by hydrodynamical equations, although the mean free path of the H atoms and the characteristic length of the problem were comparable. An iterative method, that used a Monte Carlo simulation of H atom motion in the field of the plasma component hydrodynamic parameters, was suggested by Baranov et al. (1991) and only the first step of the iteration was realized (non‐self‐consistent problem solution). In this paper the results of the self‐consistent problem solution for a single set of the undisturbed solar wind and LISM parameters are presented. The structure of the upwind as well as wake regions of the flow is calculated. The geometrical pattern of the flow (bow shock, heliopause, termination shock, Mach disc, etc), the bulk velocity and the number densities of H atoms and plasma component are obtained and analyzed as a function of the distance from the Sun for different values of the polar angle. The effects of resonance charge exchange of the LISM H atoms as well as energetic H atoms “born” in the solar wind are taken into account. It is interesting to note that the effect of H atoms penetrating the solar wind results in the disappearance of the complicated flow structure as well as the supersonic regions between the heliopause and termination shock in the downwind region. In future we are going to compare our theoretical results with the results of Voyager 1/2, Pioneer 10/11, Ulysses spacecraft, and other experiments.
Estimate of the distance to the solar wind termination shock from gradients of anomalous cosmic ray oxygen
Authors: Cummings A. C. , Stone E. C. , Webber W. R.
Published: February 2008

The radial gradient of anomalous cosmic ray oxygen measured instantaneously between Voyager 2 and Pioneer 10 during 1985‐1988 is correlated with the inferred tilt of the heliospheric neutral sheet. This is consistent with a simple model in which the radial gradient is related to the length of the neutral sheet between the two spacecraft. With this model we show that the radial gradient and the tilt of the neutral sheet near the solar wind termination shock can be inferred from the Voyager and Pioneer observations. By comparing the time history of the inferred tilt with that derived from solar observations, we estimate that the termination shock was at AU at solar minimum in 1987. At solar maximum the shock should be located at ∼90 AU due to the increased pressure of the solar wind.
Ion dynamics in high‐mach‐number quasi‐parallel shocks
Authors: Lyu L. H. , Kan J. R.
Published: February 2008

Ion heating in high‐Mach‐number (MA ≥ 3) quasi‐parallel shocks is accomplished by ion reflections at the shock ramp and nonadiabatic pitch angle scattering through nonlinear waves in the shock transition region. Two types of ion reflection events are observed in high‐Mach‐number shock simulations: (1) the reflecting‐reentering event and (2) the backstreaming‐gyrating event. The behavior of ion reflection depends on the orientation of the ramp field relative to the upstream field. A gyroreflection model is proposed to provide a physical explanation for the two types of ion reflection events observed in the high‐Mach‐number shocks. The rotation of the ramp field with time leads to alternative occurrence of the two types of ion reflection events, which in turn leads to shock front reformation in the high‐Mach‐number quasi‐parallel shock. The time scale for the shock front reformation is estimated to be about 1 to 2 upstream ion gyroperiods, which is about 10 ion gyroperiods for ions experiencing overshot magnetic field in the shock ramp.
Cross‐tail current: Resonant orbits
Authors: Kaufmann Richard L. , Lu Chen
Published: February 2008

A technique to generate self‐consistent one‐dimensional current sheets is described. Groups of monoenergetic protons were followed in a modified Harris magnetic field. The field parameters were adjusted to model a magnetotail region in which ion orbits cannot be described by the guiding center equations. This sample current sheet is characterized by resonant quasi‐adiabatic orbits. The magnetic moment of a quasi‐adiabatic ion which is injected from outside a current sheet changes substantially during the orbit but returns to almost its initial value by the time the ion leaves. The resonant nature of the interaction is associated with a strong energy dependence. Several ion and electron groups were combined to produce a plasma sheet in which the charged particles carry the currents needed to generate the magnetic field in which the orbits were traced. An electric field also is required to maintain charge neutrality. Three distinct orbit types, one involving untrapped ions and two composed of trapped ions, were identified. Each class of ions carried a qualitatively different current distribution. Contributions from all three groups are needed in order to generate a typical quiet‐time self‐consistent current sheet. Distribution functions were evaluated so that the expected count rates and several fluid parameters could be plotted. Limitations associated with the use of a one‐dimensional model also were investigated. It was found that the model can provide a good physical picture of an important component of the cross‐tail current. However, we concluded that one‐dimensional models cannot adequately describe any region of the magnetotail in which the principal current sheet is separated from the plasma sheet boundary layer by a nearly isotropic outer portion of the central plasma sheet.
Magnetospheric convection pattern and its implications
Authors: Zhu Xiaoming
Published: February 2008

When we use 14 months of the Fast Plasma Experiment ion velocity measurements, the mean magnetospheric circulation pattern is constructed. It is shown that the magnetospheric convection velocity is of the order tens of kilometers per second. The convection is largely restricted to the outer magnetosphere. During magnetically active periods the convection velocity increases and the convection boundary extends to the region closer to the Earth, indicating more magnetic field flux is being transported to the dayside magnetosphere. It is also shown that the convective flows tend to follow contours of constant unit flux volume as they move around the Earth, especially on the duskside of the magnetosphere. This helps to avoid the pressure balance inconsistency often found in two‐dimensional magnetotail models.
Field‐aligned drifts in subauroral ion drift events
Authors: Heelis R. A. , Bailey G. J. , Sellek R. , Moffett R. J. , Jenkins B.
Published: February 2008

Calculations of the response of F region plasma to rapid subauroral ion drifts (SAID) show that the maximum field‐aligned drift is a strong function of the time scale over which the SAID is applied. For SAID velocities of 2 km s−1 the peak field‐aligned drifts can vary by almost a factor of 10 for differences in the onset time scale from a few seconds to a few minutes. Reasonable considerations of the convective configuration in a SAID event suggest that acceleration times between 10 and 20 min might typically be expected. In such cases, much smaller field‐aligned velocities should be observed than would be predicted assuming an essentially instantaneous turn on of the SAID event.