bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of non‐drifting radio emissions associated with the intense solar activity in March 1991
Authors: Reiner M. J. , Stone R. G. , Fainberg J.
Published: February 2008

Three very unusual radio bursts, separated in time by 4 hours and 35 minute‐intervals were observed on March 28, 1991 by the radio and plasma wave (URAP) experiment on the Ulysses spacecraft. Although they are preceded by drifting type III bursts at high frequencies, they show no frequency drift at frequencies from 52 kHz down to 16 kHz. In addition, they have extraordinary sawtooth‐like profiles; they have a very rapid exponential rise of about 20 minutes independent of frequency and a much longer nonexponential decay which increases with decreasing frequency. The bursts are interpreted as due to synchrotron emission, which seems to fit many of their characteristics. This radio emission appears to be associated with an expanding source region. However, the observed intensity may be somewhat higher than expected and the possible role of the precursor type III radio bursts in triggering the low frequency non‐drifting emission remains unanswered.
The equatorial neutral thermospheric response to geomagnetic
Authors: Burns A. G. , Killee T. L.
Published: February 2008

Thermospheric temperature and neutral density measurements from Dynamics Explorer 2 (DE 2) have been used to study the time‐dependent response of the equatorial thermosphere to geomagnetic forcing. Data taken following the onsets of several different isolated geomagnetic storms have been used in a superposed‐epoch scheme to investigate the characteristic response of the density and temperature structure of the equatorial thermosphère at ∼350 lcm altitude in the rooming and evening local‐time sectors. The principal post‐storm‐onset features that have been observed are as follows: 1) Two travelling waves, one generated at northern high latitudes and one at southern high latitudes, pass through the equatorial thermosphere, with wave crests crossing the equator at a time lag of 3–6 hours after the storm onset. The maximum neutral density perturbations associated with these waves are ∼15–25% for atomic oxygen and ∼30–40% for molecular nitrogen. 2) Following the passage of these travelling waves, the densities and temperatures in the evening sector return relatively rapidly to their undisturbed state, whereas those in the morning sector have significant variations at longer lag times. In particular, significant increases in neutral density and temperature occur in the morning sector about 12 hours after the onset of the storm. The N2 density enhancements are seen earliest in the summer hemisphere, while the O density enhancements are seen earliest in the winter hemisphere.
The impact of snow cover on diurnal temperature range
Authors: Cerveny Randall S. , Balling Robert C.
Published: February 2008

Many parts of the world are presently experiencing a decrease in the daily temperature range, and a number of investigators in the past have suggested that this change in climate may be related to the observed buildup of greenhouse gases. However, other geophysical variables, themselves related to increases in greenhouse gases, may be influencing the daily temperature range. In this investigation, we demonstrate a statistically significant, inverse correlation between the area of North American snow cover and the United States diurnal temperature range. Because global warming is expected to reduce the areal extent of snow cover, the results of this study reveal snow cover should act to increase the diurnal temperature range in the years to come. This climate variable appears therefore to be influencing the daily temperature range counter to other feedbacks discussed in previous studies. Ultimately, our work demonstrates that the direct linkage between the diurnal temperature range and greenhouse gases may be complicated by feedbacks associated with other geophysical variables.
Permeability of thermally cracked granite
Authors: Darot Michel , Gueguen Yves , Baratin Marie‐Laure
Published: February 2008

Permeability of granite specimens heated up to 650°C were measured in the laboratory with respect to both confining and pore pressure. An unexpected behavior (permeability decrease) was found in the low temperature range (20–125°C). Permeability variations are interpreted with the help of a theoretical model of porosity. Complementary measurements of porosity and absolute surface area have also been obtained. A discussion in terms of microstructural controlling parameters concludes to a decrease of crack aperture at low temperatures (20–125°C), followed by an increase at higher temperatures (T >125°C).
The relationship between shear and compressional velocities at high pressures: Reconciliation of seismic tomography and mineral physics
Authors: Isaak Donald G. , Anderson Orson L. , Cohen Ronald E.
Published: February 2008

The value of the parameter ν from low pressure laboratory measurements disagrees with the value from seismic observations. The parameter ν relates the isobaric change in shear velocity Vs to the change in compressional velocity Vp. Seismic evidence indicates ν exceeds 2.0 in the lower mantle, whereas data on a variety of minerals at high temperature T and ambient pressure P result in lower values. We reconcile these differences. Our ab initio model calculations on MgO show that ν increases with P and is 2.0–2.5 at lower mantle pressures. There is no need to assume partial melting to explain the seismic data. These calculations also provide insight into the P and T dependence of the dimensionless parameter Γ ≡ −(1/αG)(∂G/∂T)p, where G is the isotropic shear modulus and α is the volume thermal expansivity. Using measured values of thermoelastic parameters coupled with thermodynamic identities, we seek constraints on δs ≡ −(1/αKs)(∂Ks/∂T)p, where Ks is the adiabatic bulk modulus, and confirm that P causes δs to decrease. We find Poisson's ratio increases with P and T. Altogether these results show that for MgO, ν increases from 1.3 at ambient conditions to over 2 at lower mantle conditions. We expect other mantle minerals to behave similarly. Therefore we find that reconciliation of the mineral physics approach with that of seismic tomography concerning ν does not require special assumptions about the state of the lower mantle.
Global coordinates with centimeter accuracy in the International Terrestrial Reference Frame using GPS
Authors: Blewitt Geoffrey , Heflin Michael B. , Webb Frank H. , Lindqwister Ulf J. , Malla Rajendra P.
Published: February 2008

Using 21 days of Global Positioning System (GPS) data from 21 globally distributed receivers operating during early 1991, we solve for a 7‐parameter transformation between a GPS free‐network solution and coordinates of 12 stations listed in the International Terrestrial Reference Frame (ITRF). Standard errors of GPS coordinates are derived by applying an orthogonal projection operator to the free‐network covariance. The weighted RMS difference between 33 transformed GPS and ITRF coordinates is 12 mm in the northern hemisphere. Best results are obtained by mapping ITRF coordinates to the epoch of this experiment assuming no vertical site motions. Fixing selected sites in the GPS solution to ITRF'90 does not improve the agreement. We conclude the use of fiducial constraints is unnecessary for global networks.
Positive europium anomaly at the Permo‐Triassic Boundary, Spiti, India
Authors: Bhandari N. , Shukla P. N. , Azmi R. J.
Published: February 2008

A limonitic layer at the base of theOtoceras‐Ophiceraszone in the Lalung section in Spiti valley shows a significant positive europium anomaly with maximum Eu/Eu*= 1.9. This layer defines the contact between the earliest Griesbachian and the PermianProductusshale sequence. The layer shows a small cerium anomaly having the maximum value of (Ce/La)N= 1.4. The iridium concentration in this layer, at 73 pg/g, is only marginally above the levels found in the adjacent shales although higher than that found in the overlying limestones. Another horizon having 114 pg/g of Ir occurs 70 cm below this limonitic layer in the shale sequence. These observations have important implications, both for the terrestrial (mainly volcanic) and extra‐terrestrial impact hypotheses for the Permo‐Triassic transition which are discussed.
Revisions to the age of the Brunhes ‐ Matuyama Boundary and the Pleistocene geomagnetic polarity timescale
Authors: Spell Terry L. , McDougall Ian
Published: February 2008

The currently referenced age of 730 ka for the Brunhes‐Matuyama (B‐M) geomagnetic polarity boundary [Mankinen and Dalrymple, 1979] is based largely on K‐Ar and paleomagnetic data from postcollapse rhyolites in the Valles Caldera, New Mexico [Doell and Dalrymple, 1966; Doell et al., 1968]. Dating of sanidine from Valles rhyolites by the 40A/39Ar laser fusion technique indicates that samples critical to defining the B‐M boundary are up to 90 ka older than previously thought. The new data combined with earlier results yield an age for the B‐M boundary of 780 ± 10 ka. 40Ar/39Ar ages from other samples suggest the Jaramillo normal polarity subchron extended from 1010 to 915 ka.
Results of ULF magnetic field measurements near the epicenters of the Spitak (Ms = 6.9) and Loma Prieta (Ms = 7.1) earthquakes: Comparative analysis
Authors: Molchanov O. A. , Kopytenko Yu. A. , Voronov P. M. , Kopytenko E. A. , Matiashvili T. G. , Fraser‐Smith A. C. , Bernardi A.
Published: February 2008

The characteristics of the ULF magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the Ms = 6.9 earthquake that occurred near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the Ms = 7.1 earthquake that occurred near Loma Prieta, California, on October 17, 1989. The main features of the Spitak measurements, according to observations made at the Dusheti station (128 km to the Spitak epicenter), as compared with the Loma Prieta measurements, which were made at Corralitos, California (7 km to the Loma Prieta epicenter), are the following: (1) The intensity of ULF background activity started growing 3 to 5 days before the Spitak earthquake, whereas the corresponding increase in activity began 12 days before the Loma Prieta earthquake; (2) a substantial ULF emission burst was recorded at Dusheti starting 4 hours prior to the main shock; a similar large burst of ULF activity commenced 3 hours before the Loma Prieta event, and continued until the occurrence of the main shock; (3) ULF activity remained high for about two weeks after the Spitak earthquake, and for several months after the Loma Prieta earthquake; (4) ULF noise bursts were observed 1 to 6 hours before powerful aftershocks at Spitak during the period of enhanced activity, but there was no conclusive link between the ULF noise at Corralitos and the after‐shocks. A major difference in the ULF activity preceding the two earthquakes is a difference in amplitude (0.2 nT at Spitak and 5 nT at Loma Prieta), but this is easily explained as being caused by the different distances of the observation stations from the epicenters.
Magnetospheric plasma flows associated with boundary waves and flux transfer events
Authors: Sibeck D. G. , Smith M. F.
Published: February 2008

We describe plasma flow perturbations in the outer magnetosphere during the passage of magnetopause boundary waves and cylindrically shaped flux transfer events (FTEs). Spacecraft which remain within the magnetosphere observe bipolar flows normal to the nominal magnetopause accompanied by flows nearly opposite to the direction of boundary wave and/or FTE motion. Flows are generally discontinuous across FTE and magnetopause boundaries, with reversals in the component of flow tangential to the nominal magnetopause expected during all magnetopause crossings and some entries into FTEs. No bipolar flows normal to the nominal magnetopause occur within FTEs unless they rotate and none occur in the magnetosheath unless the boundary waves propagate relative to the magnetosheath flow. IRM satellite observations on October 28, 1984 may be interpreted either in terms of FTEs or boundary waves.
Climate forcing by stratospheric aerosols
Authors: Lacis Andrew , Hansen James , Sato Makiko
Published: February 2008

We illustrate how climate forcing by stratospheric aerosols depends on aerosol properties. The climate forcing is a function of aerosol size distribution, but the size dependence can be described well by a single parameter: the area‐weighted mean radius, reff.If reff is greater than about 2 μm, the global average greenhouse effect of the aerosols exceeds the albedo effect, causing a surface heating. The aerosol climate forcing is less sensitive to other characteristics of the size distribution, the aerosol composition, and the altitude of the aerosols. Thus stratospheric aerosol forcing can be defined accurately from measurements of aerosol, extinction over a broad wavelength range.
A major circulation anomaly in the western Bering Sea
Authors: Stabeno P. J. , Reed R. K.
Published: February 2008

The Alaskan Stream ceased to flow northward into the western Bering Sea, starting in summer 1990. This unexpected lack of inflow, caused by a southward shift in the path of the Alaskan Stream, persisted through at least fall 1991. If the anomalous flow continues, substantial changes in the physical environment will occur, possibly leading to impacts on the rich fisheries stocks in the region.
Thermal expansivity in the lower mantle
Authors: Chopelas A. , Boehler R.
Published: February 2008

The pressure dependence of the thermal expansion coefficient, α, previously reported as (∂lnα/∂lnV)T = 5.5 ± 0.5 by Chopelas and Boehler [1989] is refined, using systematics in the volume dependence of (∂T/∂P)s measured for a large number of materials at high pressures and high temperatures. Since (∂ln (∂T/∂P)s/∂(V/V0))T is found to be constant and material independent over a very large compression range, (∂lnα/∂lnV)T is proportional to the compression, V/V0. We find α decreases by a factor of 5 for MgO throughout the mantle, reaching a value of 1.0 · 10−5 K−1 at its bottom. Densities of perovskite (PV) and magnesiowüstite (MW) are calculated for lower mantle conditions using our new α(P, T), a room temperature finite strain equation, and recent data on the Mg‐Fe partitioning in the PV‐MW system. Both minerals have nearly identical densities to those of PREM throughout the entire lower mantle, which allows variable PV:MW ratios. A lower mantle made entirely of PV with a molar ratio of Mg:Fe of 88:12 would be about 0.11 g/cm3 or 2.5% denser than this mixture, but this density would just be within the uncertainty in PREM. A change in chemistry at 660 km depth to a PV mantle requires a thermal boundary which would improve the match in the densities between PV and PREM. These density agreements therefore preclude evaluation of a mineralogical model for the lower mantle using density comparisons. Recent measurements on melting of Fe, FeO, and FeS, however, suggest temperatures at the core‐mantle boundary below 3500 K, which tends to favor a geotherm without a large thermal boundary at 660 km depth.
Subcrustal earthquakes in the northern Apennines (Italy): Evidence for a still active subduction?
Authors: Selvaggi Giulio , Amato Alessandro
Published: February 2008

Previously unreported subcrustal earthquakes have been located up to 90 km depth beneath the Northern Apennines (Italy). These earthquakes occur beneath a zone of abundant upper crustal seismicity, mostly confined to the upper 20 km of the crust. Although there are relatively few well located subcrustal earthquakes, there appears to be a general southwestward deepening of the hypocenters, from the Adriatic to the Tyrrhenian Sea, consistent with other geophysical data which suggest that the Adriatic lithosphere is presently subducting beneath the Northern Apennines.
Heat capacity of MgSiO3 perovskite
Authors: Akaogi Masaki , Ito Eiji
Published: February 2008

Heat capacity of MgSiO3 perovskite has been measured in the temperature range 140–295 K by differential scanning calorimetry. The Kieffer model approach has been used to calculate the heat capacity beyond the measured temperature range. The estimated entropy of MgSiO3 perovskite is 57.2±1.0 J/mol.K at 298 K which is higher than that of ilmenite, in accord with the negative pressure‐temperature slope for the ilmenite‐perovskite transition. From the heat capacity thermodynamic Debye temperature of MgSiO3 perovskite is evaluated to be 980±15 K at 298 K and 1030±20 K at temperatures above about 700 K. Thermodynamic Gruneisen parameter of MgSiO3 perovskite at 30 GPa and above 1000 K is also estimated as 1.45±0.15. An adiabatic temperature profile in the pyrolitic lower mantle is calculated with the average gradient of 0.32±0.04 K/km. The temperature at the top of the D″ layer is estimated to be 2500±100 K.
On the thermal viability of dikes leaving magma chambers
Authors: Rubin Allan M.
Published: February 2008

Dikes beginning to propagate away from a magma source are thin and grow slowly, and thus are susceptible to freezing. A self‐similar solution is obtained for a dike propagating down a temperature gradient when the wallrock and magma temperatures are equal at the chamber wall. The solution applies only to the special case of a single‐component magma intruded at the solidus temperature, but provides insight into the case in which the magma temperature is above the solidus. The dike propagation velocity, relative to that in the absence of freezing, is determined by a single dimensionless parameter β. For β sufficiently large, magma freezing chokes the dike faster than propagation can elastically widen it, and the dike cannot grow. If the magma intrudes at a temperature above the solidus, there is some value of β above which the dike will propagate a certain distance and freeze, and below which it will propagate until depletion of the source or three‐dimensional elastic effects promote freezing. For reasonable parameters this process can explain why basaltic dikes are found far from obvious source regions, while granitic dikes are scarce except in the immediate vicinity of granitic plutons. Even if a rhyolite is 5 orders of magnitude more viscous than a basalt, however, this distinction can be offset if the rhyolite has an excess source pressure only 1 order of magnitude greater than that of the basalt.
Fault Mechanism of the 40 Seconds Subevent of the 1980 Irpinia (Southern Italy) Earthquake from Levelling Data
Authors: Pingue F. , De Natale G.
Published: February 2008

A model for the 40 s subevent of the 1980 Irpinia earthquake is obtained from levelling data. An inverse method has been used to compute fault parameters, based on the computation of the probability density on strike and dip angles and midpoint fault location. The highest probability model consists of a fault almost parallel to the one associated to the first subevent. This fault dips about 80°SW, forming a small graben like structure with the main fault. Fault strike and dip parameters appear well constrained by levelling data, to within 10° (at 95% confidence level). It is noteworthy that seismological and accelerometric data were not able to well constrain the fracture mechanism of this subevent, which was the most controversial one in literature (Crosson et al., 1986; Westaway, 1987; Crosson et al., 1987; Westaway and Jackson, 1987; Bernard and Zollo, 1989).
Tropospheric rivers? – A pilot study
Authors: Newell Reginald E. , Newell Nicholas E. , Zhu Yong , Scott Courtney
Published: February 2008

Computations of daily global tropospheric water vapor flux values show the presence of a filamentary structure. The filaments, here called rivers, have lengths many times their widths and persist for many days while being translated through the atmosphere. They are present in data analysed for both 1984 and 1991. The water vapor flux maxima coincide quite closely to reflectivity features (averaged from wavelengths of 380 and 360 nm) as revealed by the Total Ozone Mapping Spectrometer (TOMS). It is suggested that the filamentary structure may also be present in other trace constituents.
Observed currents on the Earth's high‐latitude magnetopause
Authors: Van Allen J. A. , Adnan J.
Published: February 2008

This paper reports a survey of electrical currents on the Earth's magnetopause, principally at high latitudes, as inferred from magnetic vector data acquired by the University of Iowa/Langley Research Center/NASA satellite Hawkeye 1. The very eccentric orbit of this satellite had an inclination to the equator of approximately 90°, an initial apogee at a radial distance of about 21 RE (Earth radii) over the north pole, a relatively low altitude perigee, an essentially constant semi‐major axis of 11.0 RE, and a corresponding period of 51.3 hours. A total of 536 candidate crossings of the magnetopause were examined. Many of these were in bundles of multiple crossings and were considered essentially redundant. Others were poorly defined. A reduced data set of 139 selected cases was analyzed in detail though solar wind dynamic pressure data were available for only 117 of these cases. Some 17 crossings of the cusp/cleft region were included. Inferred values of the lineal current densities
on the magnetopause are in the range 5.5 to 157.5 mA m−1 over a wide range of solar wind dynamic pressure P from 1.17 to 16.1 nPa. On the high‐latitude magnetopause near the noon‐midnight meridian plane, a suggested dependence of
on P is
among widely scattered data. The sense of the current system at high latitudes is counterclockwise as viewed from the Sun, in qualitative agreement with the Chapman‐Ferraro theory. The apparent normal thickness of the magnetopause current sheet ranges from 30 to 850 km with mean and median values of 185 and 158 km, respectively. It is argued provisionally that the radial rate of motion of the magnetopause is of the order of 2 km s−1 and hence that its true thickness is of similar magnitude, being of the order of one gyroradius for a 500‐eV proton moving perpendicular to the mean magnetic field in the current sheet or several gyroradii if moving more nearly parallel to the field. The relationship of our results to models of the geomagnetic field and to other related work is discussed briefly.
Thermospheric dynamics above Saint‐Santin: Statistical study of the data set
Authors: Duboin M. ‐L. , Lafeuille M.
Published: February 2008

The meridional neutral winds at 300 km altitude have been calculated from the data obtained during 15 years of measurements by the incoherent scatter facility at Saint‐Santin (France). The analysis is based on the classical technique: the meridional neutral wind is given by the difference between the measured ion velocity and the computed diffusion velocity. This relies on knowledge of the O+‐O collision frequency, which is still the subject of some controversy. A simple but reasonable expression is given and has been agreed upon with workers at Millstone Hill, so that comparisons may be made. This expression gives values consistent with the equations used elsewhere, which are more complicated but no more reliable. The whole set of data gives 223 twenty‐four‐hour periods of measurements from which the average meridional winds are computed for a range of conditions. A statistical analysis shows how the diurnal variations depend on season, solar flux, and geomagnetic activity. It is shown that these variations are well represented by four harmonic components, of which amplitudes and phases are given.
The mirror and ion cyclotron anisotropy instabilities
Authors: Gary S. Peter
Published: February 2008

The linear dispersion equation for fully electromagnetic waves and instabilities at arbitrary directions of propagation relative to a background magnetic field Bo in a homogeneous Vlasov plasma is solved numerically for bi‐Maxwellian particle distributions. For isotropic plasmas the dispersion and damping of the three modes below the proton cyclotron frequency are studied as functions of βi and Te/Ti. The transport ratios of helicity, cross‐helicity, Alfvén ratio, compressibility, and parallel compressibility are defined. Under the condition that the proton temperature perpendicular to Bo is greater than the parallel temperature, the growth rates and transport ratios of the mirror instability and the ion cyclotron anisotropy instability are examined and compared. Both the proton parallel compressibility and the proton Alfvén ratio are significantly different for the two growing modes.
Counterstreaming solar wind halo electron events: Solar cycle variations
Authors: Gosling J. T. , McComas D. J. , Phillips J. L. , Bame S. J.
Published: February 2008

Previous work has suggested that counterstreaming solar wind halo electron events axe a reliable signature of coronal mass ejections, CMEs, in the solar wind at 1 AU. Using observations of counterstreaming events made by the Los Alamos plasma experiment on ISEE 3 / ICE, we have determined the percentage of time that counterstreaming was observed each year during an ∼12.4‐year interval extending from August 1978 through December 1990. These annual percentages vary roughly in phase and amplitude both with the 11‐year sunspot cycle and with annual CME rates derived from coronagraph observations. The total variation in annual percentages over the solar cycle was approximately a factor of 21. Near solar activity maximum, counterstreaming events accounted for ∼14.7% of all ISEE 3 / ICE solar wind measurements, while near solar minimum they accounted for ∼0.7% of all the measurements. Inferred rates of counterstreaming events at a single point in the ecliptic plane ranged from ∼72/yr near solar maximum to less than 12/yr near solar minimum. These inferred rates are in reasonable quantitative accord with predicted rates for CME events in the ecliptic plane derived from SMM coronagraph observations, further supporting the thesis that most CMEs in the solar wind at 1 AU can be reliably identified by the counterstreaming electron signature.
Slow mode transition in the frontside magnetosheath
Authors: Song P. , Russell C. T. , Thomsen M. F.
Published: February 2008

Three magnetosheath passes with density enhancements in front of the magnetopause are studied with data from ISEE 1,2, and 3. The density structure appears to be locally generated and slow mode in nature. In one pass when ISEE 1 and 2 were well separated, the motion of the density structure can be determined. The density structure appears to stand in the magnetosheath flow. Thus it propagates upstream in the rest frame of the flow. The flow in and near the density structure appears to be closer to isothermal than adiabatic. The flow velocity decreases from super‐slow to being close to the intermediate and slow mode velocities at the outer edge of the density structure. This study provides additional evidence that the density structure in front of the magnetopause is a slow mode transition in which the flow velocity decreases to the MHD slow mode velocity. The slow mode transition may consist of two waves fronts and a region with strong slow mode waves. This slow mode transition may play an important role in establishing the flow and field pattern near the magnetopause.
Acceleration, heating, and compositional mixing of the thermosphere due to upward propagating tides
Authors: Forbes Jeffrey M. , Roble Raymond G. , Fesen Cassandra G.
Published: February 2008

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) is utilized to evaluate the role of upward propagating diurnal and semidiurnal tides in determining the zonal mean states of the thermosphere and ionosphere above 100 km. Differences between various zonal mean fields from TIGCM simulations are examined with and without tidal forcing at the lower boundary. The following effects are observed: (1) A westward jet of the order of 10–30 m s−1 in the equatorial lower thermosphere and weaker eastward flanking mid‐latitude jets are induced by eddy and molecular dissipation of the tidal motions. (2) Salient characteristics of the equatorial jet are independent of tidal phase. (3) Increases in the zonal mean N2 and O2 number densities of the order of 10–30% are produced above 110 km and between ± 30° latitude, mostly by virtue of the net heating due to tidal dissipation; these are accompanied by electron density depletions of the order of 10–15% above about 160 km due to the enhanced chemical loss. (4) A low‐latitude (< 30°) depletion (∼ 30–50%) of atomic oxygen occurs due to the increase in effective recombination rate within a tidally displaced air parcel. The latter may explain the low‐latitude depression in the OI (5577 Å) airglow intensity from satellite‐based observations. In addition, variations in the zonal mean densities of NO, N(4S), and N(2D) of the order of 30–60% are induced by reactions with the tidally modified distributions of O, O2, and temperature. The occurrence frequency of these effects is influenced by the amplitude of diurnal propagating tide assumed at the lower boundary of the TIGCM, which according to radar observations is exceeded about 20–40% of the time, depending on season.
By‐controlled field‐aligned currents near midnight auroral oval during northward interplanetary magnetic field
Authors: Taguchi Satoshi
Published: February 2008

Using the magnetic field data from southern passes of Magsat for which the interplanetary magnetic field (IMF) Bz component was positive and the By component was particularly stable, we present a new By‐controlled field‐aligned current system which develops in the midnight sector. This current system is composed of three sheets of the field‐aligned current approximately parallel to the auroral oval. This is referred to as the midnight sector triple‐sheet (MTS) field‐aligned current system. The MTS current system appears in the premidnight sector (postmidnight sector) of the southern hemisphere when By is negative (positive). When By is negative (positive), the current system consists of the most equatorward and the most poleward sheets with currents flowing away from (into) the ionosphere and a sheet in between with currents flowing into (away from) the ionosphere. The MTS current system develops at higher latitudes than the usual region 1 and region 2 field‐aligned current system. When IMF Bz takes negative values or values close to zero, the region 1 and region 2 current system develops, and the MTS current system itself disappears. The MTS current system is defined in 70 passes among 100 passes for which IMF Bz was positive and By was stable. In most of the remaining 30 passes, we can see the region 1 and region 2 field‐aligned current system that is similar to what is observed for southward IMF. Statistical results from the 70 cases indicate that the main controlling factors of the intensity of the MTS current system are magnitude of By and the magnetic local time. The intensity of the current system increases with |By|, and the current system having a strong intensity often appears approximately at 0200 or at 2200 MLT, depending on the sign of By.
A nonlinear dynamical analogue model of geomagnetic activity
Authors: Klimas A. J. , Baker D. N. , Roberts D. A. , Fairfield D. H. , Büchner J.
Published: February 2008

The solar wind‐magnetosphere interaction is discussed within the framework of deterministic nonlinear dynamics. Linear prediction filter studies have shown that the magnetospheric response to energy transfer from the solar wind contains both directly driven and unloading components. These studies have also shown that the response is significantly nonlinear and, thus, the filter technique and other correlative techniques cannot give a complete description of that response. Phase space reconstruction studies have shown that the evolution of the nonlinear solar wind‐magnetosphere system is dominated by only a few degrees of freedom; the system approaches a low‐dimensional attractor on which its behavior can be described using a relatively simple nonlinear dynamical model. An earlier dripping faucet analogue model of the low‐dimensional solar wind‐magnetosphere system is briefly reviewed, and then a plasma physical counterpart to that model is constructed. A Faraday loop in the magnetotail is considered, and the relationship of electric potentials on the loop to changes in the magnetic flux threading the loop is developed. This approach leads to a model of geomagnetic activity which is similar to the earlier mechanical model but described in terms of the geometry and plasma contents of the magnetotail. The model is best characterized as an elementary time‐dependent global convection model. The convection evolves within a magnetotail shape that varies in a prescribed manner in response to the dynamical evolution of the convection. The result is a nonlinear model capable of exhibiting a transition from regular to chaotic loading and unloading. The behavior of the model under steady loading and also some elementary forms of time‐dependent loading is discussed. The model appears to properly account for all macrophysical aspects of magnetotail geomagnetic activity, it incorporates both the directly driven and the unloading components of geomagnetic activity, and it includes, in a fundamental way, the inherent nonlinearity of the solar wind‐magnetosphere interaction.
Diodelike response of high‐latitude plasma in magnetosphere‐ionosphere coupling in the presence of field‐aligned currents
Authors: Mitchell H. G. , Ganguli Supriya B. , Palmadesso P. J.
Published: February 2008

The dynamic processes in the plasma along high‐latitude field lines plays an important role in ionosphere‐magnetosphere coupling process. We have created a time‐dependent, large‐scale simulation of these dynamics parallel to the geomagnetic field lines from the ionosphere well into the magnetosphere. The plasma consists of hot e− and H+ of magnetospheric origin and low‐energy e−, H+, and O+ of ionospheric origin. Including multiple electron species, a major improvement to the model, has allowed us for the first time to simulate the upward current region properly and to dynamically simulate the diodelike response of the field‐line plasma to the parallel currents coupling the ionosphere and magnetosphere. It is shown that return currents flow with small resistance, while upward currents produce kilovolt‐sized potential drops along the field, as concluded from satellite observations. The kilovolt potential drops are due to the effect of the converging magnetic field on the high‐energy magnetospheric electrons.
Do the sunspot numbers form a “chaotic” set?
Authors: Price C. P. , Prichard D. , Hogenson E. A.
Published: February 2008

The problem of predicting future cycles of the sunspot number is physically significant. Recently, a number of authors have made complex systems analyses (“chaos”) of the set of monthly Wolf sunspot numbers. Each of these analyses revealed the presence of low‐dimensional deterministic behavior, and some of the papers applied the techniques of nonlinear prediction to predict future sunspot numbers. All of these papers analyzed filtered or smoothed sunspot number sets derived from the monthly Wolf sunspot numbers. Here, we have performed the same type of analysis on the raw (e.g., unfiltered) monthly sunspot number data set and on data sets derived from it, in particular, an “unrectified” set with a 22‐year period. We find no evidence that the sunspot numbers are generated by a low‐dimensional deterministic nonlinear process; further, by considering suitably constructed surrogate data sets (Theiler et al., 1991), we show that filtering techniques can give some spurious evidence for the presence of deterministic nonlinear behavior. Consequently, any predictions based on the assumption of such a process are not significantly better than those from linear stochastic models (Casdagli, 1991).
Quiet time upper thermospheric winds over Millstone Hill between 1984 and 1990
Authors: Hagan M. E.
Published: February 2008

More than 60 multiday incoherent scatter radar experiments conducted at Millstone Hill, Massachusetts, between 1984 and 1990 have been analyzed to determine neutral winds along the magnetic meridian. The wind determinations from geomagnetically undisturbed periods have been sorted into seasonal, solar cycle, and local time bins, and climatological averages have been calculated and compared with complementary empirical wind model predictions (Hedin et al., 1991). The climatological wind averages have also been harmonically decomposed. Strong seasonal and solar cycle dependences characterize both the mean winds and diurnal wind amplitudes. This analysis suggests that the aurorally driven high‐latitude circulation cell is important to the interpretation of variability in thermospheric circulation over Millstone Hill even during geomagnetically undisturbed periods.
Transient magnetic field signatures at high latitudes
Authors: Sibeck D. G.
Published: February 2008

We survey GOES 2/5/6 geosynchronous and Huancayo, Peru, ground magnetometer observations at the times of 70 transient (2–10 min) events recorded at South Pole Station, Antarctica. The simultaneous observations indicate that most South Pole events correspond to sudden sharp variations in the equatorial magnetospheric and low‐latitude ground magnetic field. The exceptions occur when the South Pole events have weak amplitudes and/or Huancayo and GOES 2/5/6 are far from local noon. The corresponding features observed at GOES 5 and GOES 6 are generally similar, with a lag indicating antisunward motion. A similar antisunward motion may be inferred from the ground observations themselves. On a case‐by‐case and statistical basis, the characteristics of the events observed in South Pole ground magnetograms resemble those previously interpreted as sudden impulse and sudden storm commencement signatures at other high‐latitude stations. These observations suggest that the transient events at South Pole form part of the magnetospheric and ionospheric response to a sudden change in the fraction of the solar wind dynamic pressure applied to the magnetosphere.
Waves and turbulence in the solar wind
Authors: Zank G. P. , Matthaeus W. H.
Published: February 2008

A new and unified model which draws together the effects of both compressibility and incompressibility is presented for the investigation of waves and turbulence in the solar wind.
EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind
Authors: Abe T. , Whalen B. A. , Yau A. W. , Horita R. E. , Watanabe S. , Sagawa E.
Published: February 2008

We report observations of the H+, He+, and O+ polar wind ions in the polar cap (>80° invariant latitude, ILAT) above the collision‐dominated altitudes (>2000 km), from the suprathermal mass spectrometer (SMS) on EXOS D (Akebono). SMS regularly observes low‐energy (a few eV) upward ion flows in the high‐altitude polar cap, poleward of the auroral oval. The flows are typically characteristic of the polar wind, in that they are field‐aligned and cold (Ti < 104 °K), and the parallel (field‐aligned) velocities of the different ion species vary inversely with the respective ion masses. A statistical study of the altitude, invariant latitude, and magnetic local time distributions of the parallel velocities of the respective ion species is described, and preliminary estimates of ion temperatures and densities, uncorrected for perpendicular drifts and spacecraft potential effects, are also presented. For all three ion species, the parallel ion velocity increased with altitude. In the high‐latitude polar cap (>80° ILAT), the average H+ velocity reached 1 km/s near 2000 km, as did the He+ velocity near 3000 km and the O+ velocity near 6000 km. At Akebono apogee (10,000 km), the averaged H+, He+, and O+ velocities were near 12,7, and 4 km/s, respectively. Both the ion velocity and temperature distributions exhibited a day‐to‐night asymmetry, with higher average values on the dayside than on the nightside.
Observations of nightside auroral cavities
Authors: Doe Richard A. , Mendillo Michael , Vickrey James F. , Zanetti Lawrence J. , Eastes Richard W.
Published: February 2008

Radar observations of field‐aligned auroral F region density depletions (cavities) have been identified in a portion of the Sondre Stromfjord incoherent scatter radar (ISR) data base covering the period February 1986 to January 1988. These “auroral cavities” are nightside phenomena with localized, field‐aligned F region density depletions of 20 to 70 percent below surrounding values. They occur during moderate to quiet geomagnetic conditions when the poleward edge of the auroral oval is within view of Sondre Stromfjord. Seasonally, they are a wintertime phenomena occurring just poleward of the statistical auroral oval. Unlike the previously reported “polar hole,” the average width of the cavities is less than 100 km. Case studies show that the cavities closely track the poleward edge of the most poleward auroral arc. Sequential radar scans show that cavities appear on time scales as short as several minutes, suggestive of local electrodynamic formation or rapid transport. Data from January 24, 1987, collected during coordinated optical, radar, and satellite observations spanning an hour of local time, were examined for possible cavity formation mechanisms. The cavity formation processes examined herein include locally enhanced chemical loss, vertical diffusion, drifting horizontal gradients in the background plasma, and evacuation as a result of field‐aligned currents. The formation time scales, calculated evacuation fluxes, close proximity to E region aurora, and field‐aligned current signatures seen in magnetometer and radar observations suggest a strong association of the cavities with upward flowing electrons carrying region 1 downward field‐aligned currents.
Relationship between Birkeland current regions, particle precipitation, and electric fields
Authors: de la Beaujardiere O. , Watermann J. , Newell P. , Rich F.
Published: February 2008

Data from eight DMSP F7 satellite passes coincident with Sondrestrom radar observations have been examined to determine how the large‐scale dayside Birkeland currents are related to the particle precipitation regions and to the convection pattern. The classification schemes recently developed from DMSP particle data were adopted. The observations were limited to the prenoon local time hours and led to the following conclusions: (1) The local time of the mantle currents (which were traditionally called cusp currents) is not limited to the longitude of the cusp proper, but covers a larger local time extent. (2) The mantle currents flow entirely on open field lines (where “open field lines” is defined as a region where the ion precipitation and electron precipitation have the characteristics of plasma mantle, cusp, or polar rain.) This confirms and extends to all local times similar results obtained from other observations. (3) About half of region 1 currents flow on open field lines. This is consistent with the assumption that the region 1 currents are generated by the solar wind dynamo and flow within the surface that separates open and closed field lines. (4) More than 80% of the Birkeland current boundaries do not correspond to particle precipitation boundaries. Region 2 currents extend beyond the plasma sheet poleward boundary; region 1 currents flow in part on open field lines; mantle currents and mantle particles are not coincident. (5) On most passes when a triple current sheet is observed (region 2, region 1, and mantle currents), the convection reversal is located on closed field lines. When only two current sheets are observed (either region 2/region 1, or region 1/mantle currents), the convection reversal is on open field lines. (6) The data appear to be more consistent with a topology such that mantle currents are an extension of region 1 currents, rather than a separate system located poleward of the region 1 current system.
Storm time plasma transport at middle and high latitudes
Authors: Foster John C.
Published: February 2008

Associated with the large‐scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar‐produced F region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector. As a result of the offset between the geomagnetic and geographic poles, the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 UT and 24 UT. A snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm‐enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large‐scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid‐ or low‐latitude source region. For local times away from noon, the latitude of most probable SED occurrence moves equatorward by 6° for an increase of 2 in the Kp index. During strong disturbances the topside SED is observed to be convecting sunward at ∼750 m s−1 with a flux of 1014 m−2 s−1. This feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms (called the dusk effect) and constitutes a source for the enhanced F region plasma observed in the polar cap during disturbed conditions.
Solar Cycle 21 effects on the interplanetary magnetic field and related parameters at 0.7 and 1.0 AU
Authors: Luhmann J. G. , Zhang T. ‐L. , Petrinec S. M. , Russell C. T. , Gazis P. , Barnes A.
Published: February 2008

Magnetometer data obtained over the course of the previous solar cycle by the Pioneer Venus orbiter (PVO) at ∼ 0.7 AU and IMP 8 at 1.0 AU are used to compare the long‐term behavior of the interplanetary magnetic field (IMF) at these two heliocentric distances. Similarities include an enhancement in the typical or median field magnitude during the declining phase of the solar cycle as compared to solar maximum or minimum, slight decreases in the Parker spiral angle from the declining phase through solar minimum, similar trends in the Alfvénic and magnetosonic Mach numbers, and the remarkably consistent sector structure noted previously. Differences include the temporal behavior of the high‐field tail of the field distribution, showing that high fields are most frequently observed during solar maximum at the Earth but during the declining phase of activity at Venus. This latter feature suggests that the perceived occurrence history of large fields from transient disturbances such as coronal mass ejections is a sensitive function of position within the heliosphere.
Auroral arc thicknesses as predicted by various theories
Authors: Borovsky Joseph E.
Published: February 2008

Twenty‐two theoretical mechanisms for auroral arcs are examined (12 electron‐acceleration mechanisms and 10 generator mechanisms) and a characteristic auroral‐arc thickness is worked out for each mechanism except one. The arc thicknesses are then mapped down to the ionosphere along the terrestrial magnetic‐field lines: near the Earth a dipole magnetic‐field model is used and farther from the Earth the mapping includes the effects of magnetic‐field‐line draping. The predicted thicknesses are compared with published ground‐based measurements of the optical thicknesses of auroral arcs in the ionosphere, which typically find arcs to be 100 m wide. The 21 theoretical models all predict auroral‐arc thicknesses that are at least an order of magnitude wider than the optically observed arcs. As an alternative explanation of the optical observations of narrow auroral arcs, the acceleration of ionospheric electrons to produce airglow in electrical‐discharge mechanisms is explored: these electrical‐discharge mechanisms are found to be improbable. Also explored is the possibility that the observed narrow auroral arcs are caused by interference effects when Alfven waves reflect off the ionosphere: this is found to be an unlikely explanation. Suggestions are made for future ground‐based auroral‐arc measurements.