bluuebunny/crossref_metadata_2025 huggingface dataset:
Ground magnetic signatures of moving elongated plasma clouds
Authors: Wei C. Q. , Lee L. C.
Published: February 2008

Recent magnetic and optical observations of the cusp ionosphere indicate the presence of elongated plasma clouds (EPCs) in the magnetosphere. The formation of EPCs may originate from either flux transfer events or plasmas injected into the magnetosphere. Based on observations, a self‐consistent current system in the cusp ionosphere is modeled for elongated plasma clouds, which are allowed to move in any given direction. The current system consists of the Pedersen current, the Hall current, and the field‐aligned current. The field‐aligned current flows upward on one‐half of an elongated plasma cloud and downward on the other half. It is found that the distribution of the field‐aligned current of EPCs is quite different from the case with a circular cross‐section, which has been studied previously. The plasma convection pattern and the convection electric field are also obtained. The current and convection patterns are highly dependent on the orientation of the convection velocity. The magnetic field on the ground produced by this current system is calculated. The total field‐aligned current and profiles of the ground magnetic field are obtained. By assuming that the current system moves with a velocity V0 ∼ 1–4 km/s in the cusp ionosphere, the variation of the perturbed ground magnetic field as a function of time is obtained. It is found that, for a moving plasma cloud with a circular cross‐section, only unipolar profiles of the vertical magnetic field can be observed by a ground observer. However, for a moving plasma cloud with an elongated cross‐section, both unipolar and bipolar profiles can be observed.
Ionospheric signatures of cusp latitude Pc 3 pulsations
Authors: Engebretson M. J. , Anderson B. J. , Cahill L. J. , Arnoldy R. L. , Rosenberg T. J. , Carpenter D. L. , Gail W. B. , Eather R. H.
Published: February 2008

We have compared search coil magnetometer, riometer, photometer, and ELF‐VLF receiver data obtained at South Pole Station and McMurdo, Antarctica (at invariant latitudes of −74° and −79°, respectively), during selected days in March and April 1986. Narrow‐band magnetic pulsations in the Pc 3 period range are observed simultaneously at both stations in the dayside sector during times of low interplanetary magnetic field (IMF) cone angle, but are considerably stronger at South Pole, which is located at a latitude near the nominal foot point of the dayside cusp/cleft region. Pulsations in auroral light at 427.8 nm wavelength are often observed with magnetic pulsations at South Pole, but such optical pulsations are not observed at McMurdo. We have occasionally observed Pc 3 period modulations of the ELF‐VLF signal at South Pole in the 0.5‐ to 1.0‐kHz and 1.0‐ to 2.0‐kHz frequency ranges as well. When Pc 3 pulsations are present, they exhibit nearly identical frequencies, proportional to the magnitude of the IMF, in magnetometer, photometer, and ELF‐VLF receiver signals at South Pole Station and in magnetometer signals at McMurdo. Signals from the 30‐MHz riometer at South Pole are modulated in concert with the magnetic and optical variations during periods of broadband pulsation activity, but no riometer variations are noted during periods of narrow‐band activity. Because riometers are sensitive to electrons of auroral energies (several keV and above), while the 427.8‐nm photometer is sensitive to precipitation with much lower energies, we interpret these observations as showing that precipitating magnetosheathlike electrons (with energies ≤ 1 keV) at nominal dayside cleft latitudes are at times modulated with frequencies similar to those of upstream waves. We suggest that these particles may play an important role, via modification of ionospheric currents and conductivities, in the transmission of upstream wave signals into the magnetosphere and in the generation of dayside high‐latitude Pc 3 pulsations.
VLF group delay of lightning‐induced electron precipitation echoes from measurement of phase and amplitude perturbations at two frequencies
Authors: Adams C. D. D. , Dowden R. L.
Published: February 2008

Measurement of phase and amplitude perturbations (trimpis) of the NWC signal at Dunedin at both the NWC frequencies, 22,250 Hz and 22,350 Hz, enables measurement of the received phase of the echo signal (phasor difference of the perturbed and unperturbed signals) at each frequency and so the rate of decrease of phase with frequency. This, of course, is the group delay. The 100‐Hz difference implies that measurement of echo group delays of up to 5 ms could be made without ambiguity, though other factors limit this to about 2.5 ms. Some 38 “difference trimpis” during May and June 1988 showed echo delays up to 2 ms corresponding to reflection from points displaced more than 1000 km from the NWC‐Dunedin great circle path. The echo amplitudes observed at such large displacements are much greater than expected from smooth circular depressions of the ionosphere modifying the waveguide phase velocity and so imply sharper discontinuities in the waveguide.
Intensity modulation of HF heater‐induced plasma lines
Authors: Kuo S. P. , Lee M. C.
Published: February 2008

The Arecibo HF heater is normally composed of two separate sets of antenna array, transmitting waves vertically at the same frequency and polarization. However, when these two sets of antenna array radiate at slightly different frequencies, the intensities of HF heater‐induced plasma lines (HFPLs) can be drastically modulated. In recent Duncan et al.'s (1989) experiments the 100% intensity modulation of HFPLs was seen to persist even when the secondary set of antenna array radiated at a few percents of the power transmitted by the primary set of antenna array. We offer an explanation and show that there exists a minimum power Pmin, which can be roughly estimated from Pth[(E0/Eth)−1]² where Eth and Pth are the threshold field and threshold power of the parametric decay instability, respectively, E0 is the field intensity of heater wave. If the secondary set of antenna array radiates at a power lower than Pmin, the 100% intensity modulation of HFPLs will not be observed. The functional dependence of Pmin on the difference frequency of the two sets of antenna array is also predicted for future experiments to corroborate.
The Swedish Satellite Project Viking
Authors: Hultqvist Bengt
Published: February 2008

The Swedish satellite project Viking is described and related to earlier missions. Some new operational characteristics are discussed, including the real‐time data analysis campaigns that were an important part of the project. Some areas of important scientific impact of the project are also described.
Energy source and generation mechanism for auroral kilometric radiation
Authors: Ungstrup E. , Bahnsen A. , Wong H. K. , André M. , Matson L.
Published: February 2008

The orbit of the Swedish satellite Viking was optimized for observations of plasma phenomena in the auroral acceleration regions, including auroral kilometric radiation. The auroral kilometric radiation (AKR) is the strongest electromagnetic radiation produced on auroral field lines, and the Viking satellite passed through the source region of this radiation on many orbits, as evidenced by the very strong signals observed at the electron cyclotron frequency. For the first time, electron distribution functions were observed inside the AKR source region and immediately adjacent to the source region. The particle data show the presence of energetic electrons in and around the source region and also of upgoing ion beams of several keV energy in and only in the source region. Thus the particle data show clear signatures of parallel electric potentials both above and below the AKR source. The effect of a parallel potential below the source is to decrease the density of low‐energy electrons and expand the upward loss cone, particularly at small parallel velocities. The reduction in the cold plasma density is clearly observed. Adjacent to the AKR source a clear loss cone is observed in the upgoing electron population. However, inside the source region the loss cone is filled or nearly filled. Our observations are in agreement with the cyclotron maser instability theory including the effect of a parallel electric field as discussed by Wu et al. (1982) and Pritchett and Strangeway (1985).
Density measurements in key regions of the Earth's magnetosphere: Cusp and auroral region
Authors: Perraut S. , de Feraudy H. , Roux A. , Décréau P. M. E. , Paris J. , Matson L.
Published: February 2008

Two active experiments, a relaxation sounder and a mutual impedance probe, have been implemented on board Viking. These active measurements, together with measurements from the electron spectrometers, are used to determine the plasma density in the cusp region, up to 13,500 km, the apogee of Viking, as well as in the sources of the auroral kilometric radiation (AKR). When active experiments are switched on in the cusp region, several plasma resonances are detected; they correspond to zero group velocity waves, indicative of a characteristic frequency of the plasma, namely the plasma frequency, and the fqn (n = 1, 2, 3,…n) series, above fuh, the upper hybrid frequency. The fqn series allow an independent estimate of the plasma density. Plasma density in the cusp proper is found to be much larger (> 100 cm−3 in some cases) than in the adjacent regions. An attempt is also made to estimate the respective densities and temperatures of the various components of the plasma. In contrast, auroral regions are low‐density regions (fpe/fce ≪ 1). The active sounding of the plasma by a relaxation sounder gives a resonance at fuh, which allows an estimate of the plasma density. Low‐frequency whistler mode emissions are commonly observed in the night sector. Their upper cutoff frequency has often been used for estimating the plasma frequency. Active experiments are used to test this method, which is shown to be valid most of the time in the high‐altitude auroral region, yet also sometimes misleading in other regions. Once the estimate of the density through the upper cutoff frequency of the hiss is validated, it can be used to follow with a good time resolution the sharp density variations experienced as Viking crossed AKR sources. It is found that the density within AKR sources is of the order of even less than 1 cm−3.
EISCAT observations on plasma drifts connected with the Aureld‐VIP rocket and the Viking satellite
Authors: Pellinen‐Wannberg A. , Sandahl I. , Wannberg G. , Opgenoorth H. , SΦraas F. , Murphree J. S.
Published: February 2008

Coordinated simultaneous measurements with the EISCAT incoherent scatter radar, Aureld‐VIP sounding rocket, and Viking satellite are described. Background measurements from EISCAT provide us with the development of global plasma convection during the rocket night. The observed convection pattern is very distorted, with the eveningside reversal occurring at unusually low latitudes. On the morningside it withdraws back poleward from the measurement area. Viking particle measurements over the oval indicate a very complicated auroral topology with two sectors of boundary plasma sheet (BPS) and central plasma sheet (CPS) particles. The situation is interpreted as an intrusion of the evening side BPS into the morningside, which is also consistent with the convection pattern measured by EISCAT. Local measurements with the sounding rocket and radar indicate that the rocket flew in the northern part of the evening BPS area, approaching the “inner” transition region from BPS to CPS in its northward motion, thus confirming the existence of such a boundary.
Electron energetics in the inner coma of comet Halley
Authors: Gan Lu , Cravens T. E.
Published: February 2008

A quasi‐two‐dimensional model of the spatial and energy distribution of electrons in the inner coma of comet Halley has been constructed from a spherically symmetric ion density profile based on Giotto measurements, using the two‐stream electron transport method and the time‐dependent electron energy equation. A variety of heating and cooling processes were included, such as cooling via rotational, vibrational, and electronic excitation of water vapor due to electron impact. A sharp jump in the electron temperature was found to be present at a cometocentric distance of about 15,000 km. This thermal boundary separates an inner region where cooling processes are dominant from an outer region where heat transport is more important. Both thermal (Te less than about 104 K) and suprathermal electron populations exist inside the thermal boundary with comparable kinetic pressures. Outside the thermal boundary, a cold electron population does not exist, and the electrons are almost isothermal along the magnetic field lines.
Conjugate echoes of artificially injected electron beams detected optically by means of new image processing
Authors: Hallinan Thomas J. , Winckler John , Malcolm Perry , Stenbaek‐Nielsen Hans C. , Baldridge James
Published: February 2008

Following two upward injections of energetic electrons (38 keV and 26 keV) from the Echo 4 rocket‐borne electron accelerator (January 31, 1976, Poker Flat, Alaska), artificial auroral streaks were detected by ground‐based low‐light‐level television. The streaks were recognized recently by using newly developed image‐processing techniques. They were delayed relative to the injections by 2.06 s and 2.42 s, respectively. These measurements of the bounce times of electrons of known energy constitute two accurate determinations of a high‐latitude field line length (106±1×106 m). The delays are only 4–5% longer than calculated using a dynamic model of the geomagnetic field (Olson and Pfitzer, 1982). Other field models yielded shorter bounce times. Since the delays were in the inverse ratio of the relativistic velocities calculated for the nominal beam energies, it is concluded that the potential of the payload remained below 1 kV during 45 mA injections at an altitude of 210 km. The echo streaks showed little dispersion in either time or space, indicating that the portion of the beam returning to the northern hemisphere loss cone remained collimated and nearly monoenergetic. But there was a 70% loss in the return flux. A diligent search failed to locate similar echoes from the more powerful injections employed in the Echo 5 and Echo 7 rocket experiments, suggesting flux losses of at least 98% and 92%, respectively. The losses are thought to be due to pitch angle scattering out of the loss cone as the electrons traverse the equatorial region but could also be due to collective beam plasma interactions. The lower loss rate in Echo 4 is likely related to the presence of only weak diffuse aurora during that flight.
Injection of ionospheric ions into the plasma sheet
Authors: Orsini S. , Candidi M. , Stokholm M. , Balsiger H.
Published: February 2008

The ISEE 1 and ISEE 2 observations of the lobe/mantle and of the plasma sheet region, at distances between 10 and 20 RE downtail during 1978 and 1979, have been examined. Cold, tailward flowing ionospheric O+ streams (at energies between 50 eV/q and 5 keV/q), observed during geomagnetically disturbed periods, have been statistically analyzed. At the crossing between the lobe/mantle region and the plasma sheet the characteristics of the streams change significantly. These changes suggest the action of energization and isotropization processes which accelerate the ionospheric ions and make them a part of the plasma sheet plasma. The region where these processes are observed is assumed here to be the plasma sheet boundary layer. It is shown that the stream flow pattern in the lobe/mantle region is in good agreement with the tail lobe ion spectrometer model for the thermal speed distribution as well as for the flow velocity and density distributions. This agreement also holds in a qualitative sense in the plasma sheet boundary layer; assuming the appropriate electric field configuration; the E×B drift direction along the YGSE axis appears to reverse with respect to what is observed in the lobe/mantle region, in agreement with the assumed reversal of the Z component of the dc electric field at the plasma sheet boundary.
A multisatellite case study of the expansion of a substorm current wedge in the near‐Earth magnetotail
Authors: Lopez R. E. , Lui A. T. Y.
Published: February 2008

This study presents observations made by four spacecraft (AMPTE CCE, AMPTE IRM, GOES 5, and GOES 6) and two ground stations (San Juan and Tucson) during a substorm that occurred at ∼0830 UT on April 19,1985. The spacecraft were arrayed in a configuration that allows for the examination of the spatial evolution of the substorm current wedge. CCE was located between the GOES spacecraft in longitude, but at a radial distance of 8.0 RE. IRM was located west of the other three spacecraft in the same sector as Tucson, but at a radial distance of 11.6 RE. The relative times at which the signature of the substorm current wedge was first observed at the GOES spacecraft and the ground stations are consistent with a simple longitudinally expanding current wedge. However, the times at which IRM and CCE observed the current wedge are not consistent with a current wedge that expanded only longitudinally. IRM first observed the signature of the current wedge at about the same time the signature was observed by GOES 6 and Tucson, and CCE observed the current wedge only after both GOES satellites and the ground stations had done so. Moreover, both GOES spacecraft observed signatures consistent with entry into the central plasma sheet before CCE and IRM did, even though we estimate that CCE was slightly closer to the neutral sheet than the geosynchronous spacecraft. The sequence of events suggests that during this substorm the disruption of the cross‐tail current sheet, the formation of the substorm current wedge, and the expansion of the plasma sheet began in the near‐Earth region, and subsequently spread tailward as well as longitudinally.
The interior structure of reconnected flux tubes in a sheared plasma flow
Authors: Wright Andrew N. , Berger Mitchell A.
Published: February 2008

Two basic processes for producing twisted magnetic flux tubes are reconnection between skewed magnetic fields, and plasma motion causing twining or braiding of flux tubes/flux elements. The properties of the latter mechanism are studied for flux that is reconnected across a neutral sheet. If the neutral sheet is formed by skewed magnetic fields with a flow shear across the sheet, the direction of the twist may be predicted in terms of background magnetoplasma quantities. If the results are applied to flux transfer events (FTEs) we expect that the vast majority of FTEs have a twist and axial By component determined by IMF By. However, we also expect there to be a few cases where the flow shear will reverse the sense of both of these quantities.
Simulation studies of plasma waves in the electron foreshock: The generation of downshifted oscillations
Authors: Dum C. T.
Published: February 2008

The generation of waves with frequencies downshifted from the plasma frequency, as observed in the electron foreshock, is analyzed by particle simulation. Wave excitation differs fundamentally from the familiar excitation of the plasma eigenmodes by a gentle bump‐on‐tail electron distribution. Beam modes are destabilized by resonant interaction with bulk electrons, provided the beam velocity spread is very small. These modes are stabilized, starting with the higher frequencies, as the beam is broadened and slowed down by the interaction with the wave spectrum. Initially, a very cold beam is also capable of exciting frequencies considerably above the plasma frequency, but such oscillations are quickly stabilized. Low‐frequency modes persist for a long time, until the bump in the electron distribution is completely “ironed” out. This diffusion process also is quite different from the familiar case of well‐separated beam and bulk electrons. A quantitative analysis of these processes is carried out.
Solar wind and coronal structure near sunspot minimum: Pioneer and SMM observations from 1985–1987
Authors: Mihalov J. D. , Barnes A. , Hundhausen A. J. , Smith E. J.
Published: February 2008

The solar wind speeds observed in the outer heliosphere (20 to 40 AU heliocentric distance, approximately) by Pioneers 10 and 11, and at a heliocentric distance of 0.7 AU by the Pioneer Venus spacecraft, reveal a complex set of changes in the years near the recent sunspot minimum, 1985–1987. The pattern of recurrent solar wind streams, the long‐term average speed, and the sector polarity of the interplanetary magnetic field all changed in a manner suggesting both a temporal variation, and a changing dependence on heliographic latitude. Coronal observations made from the Solar Maximum Mission spacecraft during this same epoch show a systematic variation in coronal structure and (by implication) the magnetic structure imposed on the expanding solar wind. These observations suggest interpretation of the solar wind speed variations in terms of the familiar model where the speed increases with distance from a nearly flat interplanetary current sheet (or with “heliomagnetic latitude”), and where this current sheet becomes aligned with the solar equatorial plane as sunspot minimum approaches, but deviates rapidly from that orientation after minimum. We confirm here that this basic organization of the solar wind speed persists in the outer heliosphere with an orientation of the neutral sheet consistent with that inferred at a heliocentric distance of a few solar radii, from the coronal observations.
Latitudinal gradient in the plasma bubble zonal velocities as observed by scanning 630‐nm airglow measurements
Authors: Sobral J. H. A. , Abdu M. A.
Published: February 2008

Equatorial ionospheric plasma bubble zonal velocities were measured using two 630‐nm scanning photometers. Simultaneous scanning measurements were made in two east‐west planes that were tilted 30° northward and southward of the vertical. The results of measurements carried out during 5 days covering the summer equinoctial period of 1988, presented here, show evidence of marked latitudinal gradient in the bubble zonal velocities, the velocities decreasing with increasing latitude over our low‐latitude observing site, Cachoeira Paulista (−28°dip). We suggest that this velocity gradient is consistent with expectations at solar minimum, when our measurements may be associated with the region above the maximum in the eastward plasma velocity height profile.
Monitoring the polar rain over a solar cycle: A polar rain index
Authors: Gussenhoven M. S. , Madden D.
Published: February 2008

A polar rain index is constructed using Defense Meteorological Satellite Program particle data from 1977 to the present. It is determined from the average polar rain flux precipitating in a confined region (in corrected geomagnetic and local time coordinates) near the magnetic pole. The index is computed separately for each hemisphere on a daily basis. Descriptions are given of (1) the algorithm used to automatically select intervals of polar rain, (2) the in‐flight cross normalization of the various satellite detectors, and (3) the spectral corrections for background and noise. Using the index, three long‐period (tens of days to years) variations are found in the polar rain: (1) variation with interplanetary magnetic field (IMF) sector, (2) variation with season, and (3) variation with solar cycle. After removing the seasonal variation, a comparison of the adjusted polar rain flux in each hemisphere can be used to monitor IMF sector variations on a daily basis. These data show the evolution from a stable, double sector structure, lasting almost 2 years prior to solar minimum, to a nearly unipolar structure during solar minimum. There are two aspects of the polar rain variation that are most prominent over the solar cycle. The first is a variation in the yearly averaged polar rain flux that correlates best with variations in the yearly averaged magnitude of the interplanetary magnetic field, or with combinations of the average magnetic field to some power divided by the average solar wind density (e.g., the Alfvén speed). The second is that the hemispheric difference in polar rain number flux decreases markedly during solar minimum.
The excitation of plasma waves by a current source moving in a magnetized plasma: Two‐dimensional propagation
Authors: Rasmussen Craig E. , Banks P. M. , Harker K. J.
Published: February 2008

The steady motion of a large conducting body in a magnetized plasma has been shown to create a spatial structure of charges and currents in the plasma which propagate without attenuation along the magnetic field direction (the label “Alfvén wings” has been given to this structure because of its resemblance to the wings of a biplane). In order to show this, it has previously been assumed that the conducting body must be large enough so that frequencies excited in the plasma are significantly below the ion cyclotron frequency Ωi. In this paper, this restriction is relaxed in order to examine the wave pattern emitted at frequencies ω in the entire band, ω < Ωi. It has been found that the Alfvén wing structure breaks up into individual Fourier components which propagate away from the source at an angle slightly greater with respect to the magnetic field than the alignment of the original structure. This process depends on wave frequency. A given frequency is lost from the Alfvén wing structure at a distance which varies as 1/ω³. The amplitude of these traveling waves decreases at a rate which is inversely proportional to the square root of the distance from the source.
Generation of vortex‐induced tearing mode instability at the magnetopause
Authors: Pu Zu Yin , Yei Ming , Liu Zhen Xin
Published: February 2008

A two‐dimensional magnetohydrodynamic simulation is performed to study the generation of the vortex‐induced tearing mode instability at the magnetopause. When a sheared magnetic field exists along with the velocity shear, the tearing mode will couple with the Kelvin‐Helmholtz (K‐H) instability. The shear Alfvén Mach number MA plays an essential role in determining the linear properties and nonlinear evolution of the coupled instability. When the magnetic field is fixed, if MA < 0.4, the spontaneous tearing mode is dominated; when 0.4 ≤ MA ≤ 1, the tearing mode is apparently modified by the K‐H instability; as 1 < MA, the coupled instability, called the vortex‐induced tearing mode (VITM) instability, appears to be intrinsically different from the conventional tearing mode instability. The long time asymptotic quasi‐static state for the VITM instability is characterized by a large‐scale fluid vortex together with a concentric magnetic island. The linear properties and nonlinear evolution of the VITM instability are not significantly changed with different Reynolds and magnetic Reynolds number used in the simulation.
Flux transfer event footprint patterns and implications for convection
Authors: Crooker N. U.
Published: February 2008

The spatial relation between an open polar cap and the footprints of open magnetopause patches is calculated for a variety of IMF orientations. Patches mapped from an otherwise closed magnetopause elongate and fan out from the cusp point, located at noon on the polar cap boundary. The fan extends into the closed field region equatorward of the polar cap. The footprints of patches well down the flanks elongate the most and align with the polar cap boundary. As the patches convect with the magnetosheath flow, their footprints pivot antisunward about the cusp until they coalesce with the polar cap. In the course of pivoting, the footprints act like paddles and push adjacent closed flux antisunward, forming a low latitude boundary layer on open and closed field lines. If the patches are regions of enhanced flux imbedded in an open magnetopause, the footprints shorten, detach from the cusp, and extrude from positions increasingly displaced from noon as the field component normal to the magnetopause strengthens. Only if the normal component is large will the footprints degenerate to bulges on the polar cap boundary and move directly poleward, as previously supposed.
Double‐peaked electrostatic ion cyclotron harmonic waves
Authors: Boardsen S. A. , Gurnett D. A. , Peterson W. K.
Published: February 2008

Electrostatic H+ cyclotron harmonic waves are often observed along the auroral field lines at altitudes of 1–3.5 RE by the Dynamics Explorer 1 satellite. A small fraction of these waves are found to have two peaks associated with each harmonic instead of one peak. The waves occur below the lower hybrid frequency and are usually relatively weak, about a factor of 4 smaller than typical electric field amplitudes of other H+ cyclotron harmonic wave events. For two events the separation between the spectral peaks is found to be proportional to the harmonic number. The double‐peaked spectral signature is believed to be produced by Doppler shifts arising from the satellite velocity relative to the plasma rest frame. The wavelength and phase velocity of the harmonics can be determined by measuring the separation between the peaks. The waves were found to have wavelengths of the order of 300 m and phase velocities of the order of 150 km/s. The proportionality between the peak separation and the harmonic number indicates that the phase velocity is approximately constant, independent of harmonic number. Assuming that the phase velocity is constant, it is shown that as Δk/k increases, the double peaks merge to form a single spectral peak. For the wave events presented in this paper, Δk/k is estimated to be less than 0.1.
Numerical simulation of a catastrophe model for coronal mass ejections
Authors: Forbes T. G.
Published: February 2008

In 1978, W. Van Tend and M. Kuperus proposed a simple catastrophe model for magnetically driving coronal mass ejections, prominence eruptions, and two‐ribbon flares. Their model, which is based on simple circuit concepts, suggests that a stable configuration containing a current filament will lose equilibrium when the filament current exceeds a critical value. Here we use a two‐dimensional numerical simulation to test how the Van Tend‐Kuperus model works in an ideal MHD fluid. The simulation exhibits the expected loss of mechanical equilibrium near the predicted critical value, but the current filament moves only a short distance upward before coming to rest at a new equilibrium. However, this new equilibrium contains a current sheet which is resistively unstable to magnetic reconnection, and if magnetic reconnection occurs rapidly, the filament can continue to move upward at Alfvénic speeds.
Simulation of the ejection of plasma from the polar ionosphere
Authors: Swift Daniel W.
Published: February 2008

A numerical model calculates densities, fluxes and temperatures of H+ and O+ ions and the temperature of electrons on a convecting magnetic field line. The one‐dimensional fluid equations are solved along a field line path extending from 100 km altitude to many tens of Earth radii out in the magnetotail. Effects of photochemical reactions and charge exchange are included in the low altitude regions while dynamical effects due to field line convection, such as centrifugal acceleration and flux tube compression/dilation, are important at the higher altitudes. The model employs a variable spatial step to accommodate the disparate size scales. The evolution of O+ and H+ densities is calculated on field lines convecting over the polar cap. The simulation starts on a closed field line configuration on the dayside of the polar cap, the field line opens out into the magnetotail over the polar cap and eventually closes again on the nightside aurora zone. The results show that (1) centrifugal force is effective in accelerating ionospheric H+ plasma outward into the magnetotail, (2) the smothering effect of the H+ plasma on top of the O+ plasma prevents any ejection of O+ plasma, so long as the scale height of the O+ plasma is smaller than that of the H+ plasma (this indicates that heating processes specific to O+ are necessary for the appearance of O+ in the magnetotail) and (3) the upward component of the E×B drift on the dayside of the polar cap lifts the F region ionosphere to altitudes where charge exchange with neutrals and recombination is very slow so that ionization created on the dayside is transported across the polar cap with little loss. An enhanced F region peak in ionization density is created on the nightside of the polar cap because of compression of the O+ plasma by the subsequent downward component of the E×B drift.
Solar cycle variation of thermospheric nitric oxide at solstice
Authors: Gérard J.‐C. , Fesen C. G. , Rusch D. W.
Published: February 2008

A coupled two‐dimensional chemical‐diffusive model of the thermosphere is used to study the role of solar activity in the global distribution of nitric oxide. The model calculates self‐consistently the zonally averaged temperature, circulation, and composition for solstice under solar maximum (F10.7 = 200) and solar minimum (F10.7 = 80) conditions. A decrease of the NO density by a factor of 3 to 4 in the E region is predicted from solar maximum to solar minimum. These results are compared with the global NO distribution observed with the Solar Mesosphere Explorer (SME) satellite in 1982 and 1985. It is found that the main features of the overall morphology and the changes induced by the solar cycle are well reproduced in the model, although some details are not satisfactorily predicted. The observed increase of the E region concentrations with solar activity by a factor of 2.5 is slightly overpredicted by the model, but the SME data show a smaller solar cycle variation above 120 km than does the model. The sensitivity of the NO distribution to eddy transport and to the quenching of metastable N(²D) atoms by atomic oxygen is also described.
Remarks on transport theories of interplanetary fluctuations
Authors: Zhou Ye , Matthaeus William H.
Published: February 2008

The structure of approximate transport theories for the radial behavior of interplanetary fluctuations is reconsidered. The emphasis is on theories derived under the assumption of scale separation; i.e., the correlation length of the fluctuations is much less than the scale of large inhomogeneities. In these cases the zero‐wavelength limit provides a first approximation to the spectral evolution equations for the radial dependence of interplanetary fluctuation spectra. The goal here is to investigate the structure of a recently presented (Zhou and Matthaeus, 1989) transport theory, in which coupling of “inward”‐ and “outward”‐type fluctuations appears in the leading order, an effect we call “mixing.” In linear theory, mixing‐type couplings of inward‐type and outward‐type waves are formally a nonresonant effect. However, leading order mixing terms do not vanish at zero wavelength for fluctuations that vary nearly perpendicular to the local magnetic field, or when the mean magnetic field is weak. Leading order mixing terms also survive when the dispersion relation fails and there is a nonunique relationship between frequency and wave number. The former case corresponds to nearly two‐dimensional structures; these are included, for example, in isotropic models of turbulence. The latter instance occurs when wave‐wave couplings are sufficiently strong. Thus there are a variety of situations in which leading order mixing effects are expected to be present.
Trapping saturation of the bump‐on‐tail instability and electrostatic harmonic excitation in Earth's foreshock
Authors: Klimas Alexander J.
Published: February 2008

Trapping saturation of the bump‐on‐tail instability is discussed using electron plasma Vlasov simulation results. The role of electrostatic harmonic excitation is considered in detail and shown to play a decisive role in the saturation of the instability. An extensive discussion of the simulation results is given to show that the results are not significantly limited by the finite number of Fourier modes used nor by the discrete distribution of those modes in wave number. It is argued that in the leading edge of Earth's electron foreshock a narrow wave number band of unstable field modes leads to trapping saturation of the bump‐on‐tail instability while simultaneously exciting electrostatic plasma waves at harmonics of the plasma frequency in similar narrow bands of shorter wavelengths. The argument is based (1) on the observations of Lacombe et al. (1985), who found intense plasma waves at the leading edge of the foreshock with a spectral distribution sufficiently narrow to trap particles in resonance with the waves, and (2) on numerical simulations of the foreshock electron plasma which indicate that trapping saturation of the bump‐on‐tail instability leads to phase space vortex formation with consequent excitation of electrostatic harmonics. Thus it is suggested that observations of electrostatic harmonics in the leading edge of the foreshock would strongly implicate trapping as the saturation mechanism for the bump‐on‐tail instability in that region.
A model of Saturn's magnetic field based on all available data
Authors: Davis Leverett , Smith Edward J.
Published: February 2008

The SPV (Saturn Pioneer Voyager) model of Saturn's magnetic field presented here uses the most accurate magnetometer data transmitted from Pioneer 11, Voyager 1, and Voyager 2 during their Saturn encounters. It fits the data observed between 1.3 and 8.0 Saturn radii from the planet's center with a 1.13% weighted root‐mean‐square average of the percent differences between the observed and modeled fields. The coefficients for the interior source terms for the SPV model are g10 = 21,160. nT, g20 = 1560. nT, and g30 = 2320. nT for a Saturn radius of 60,330 km. They are the same for all three encounters and should be valid for a decade and perhaps very much longer. The distance unit (the radius of Saturn) affects the values of the Gaussian coefficients. We have adjusted the coefficients of all the models to correspond to 60,330 km so that they can be compared. On the basis of our weights and the use of percent residuals, rather than absolute residuals, the SPV model fits the observed fields better than does the previously published P1184 model or the Z3 model. The part of the field due to currents in the outer regions is not axisymmetric and varies by approximately 5 nT from one encounter to another and from the inbound to outbound legs. The best available, but crude, models for the part of the field due to these external sources are six uniform fields, two for each encounter.
Field‐aligned current signatures in the near‐tail region: 2. Coupling between the region 1 and the region 2 systems
Authors: Ohtani S. , Kokubun S. , Nakamura R. , Elphic R. C. , Russell C. T. , Baker D. N.
Published: February 2008

The development of the substorm‐associated current system in the near‐tail region is examined in the light of both experiment and theory. First, the March 28, 1979, event is examined intensively by using ground magnetometer data and satellite magnetic field and energetic particle data. The comparison of field‐aligned current signatures at geosynchronous altitude and in the near‐tail region indicates that the development of the region 1 system is not merely the enhancement in current intensity of the pre‐existing system. This finding is consistent with the so‐called current wedge model, in which the tail current is converted into a pair of field‐aligned currents with the region 1 polarity at substorm onsets. Detailed inspection, however, suggests that the region 2 system is as important as the region 1 system. Statistical properties of the azimuthal magnetic component at synchronous orbit indicate that the deviation during disturbed periods is larger than expected from the enhancement of the region 1 current. These individual and statistical studies suggest that the region 2 system tends to develop in the synchronous region and that the coupling between the region 1 and the region 2 systems is important. The current closure in the magnetosphere is discussed from a viewpoint of the macroscopic behavior of plasma. It is suggested that the dawnside and the duskside region 2 currents are closed in the magnetosphere by the curvature current during the growth phase and by the magnetic gradient current during the expansion phase. The field‐aligned currents of the region 1 and the region 2 systems are closed in the ionosphere by the Pedersen current. Consequently, the energy is dissipated as the Joule heating and this energy must be supplied from the magnetosphere. Therefore, the counter part of the Pedersen current, which closes the region 1 and the region 2 currents in the magnetosphere, must be the dynamo current. The diamagnetic current and the inertia current are possible candidates for this dynamo current. These currents flow radially outward in the morning sector and inward in the evening sector in association with the redistributions of the electric field and the plasma pressure during the expansion phase. In the dynamo process, the combination between the two currents may be important.
Spatial relationships between region 2 field‐aligned currents and electron and ion precipitation in the evening sector
Authors: Fujii R. , Hoffman R. A. , Sugiura M.
Published: February 2008

The equatorward cutoff of ion and electron precipitation in relation to the evening region 2 field‐aligned current during isolated substorms has been investigated using the magnetic field and plasma data obtained from the Dynamics Explorer 2 satellite. The equatorward boundaries of the region 2 currents relative to those of central plasma sheet (CPS) electron precipitation are determined predominantly by magnetic local time and subsequently change with substorm phases. With approaching midnight the equatorward boundary of CPS electron precipitation extends toward and eventually equatorward of that of the region 2 current. On the other hand, the equatorward boundary of the region 2 current coincides well with that of 10–20 keV ion precipitation during the whole course of substorms. We propose that these ions originate in the so‐called Alfven layer and that the location of this inner boundary determines the lower latitude boundary of the region 2 current.
Use of an array of satellites as a wave telescope
Authors: Neubauer F. M. , Glassmeier K.‐H.
Published: February 2008

The four‐satellite array of the Cluster mission will allow the separation of spatial and temporal variations in the Earth's magnetosphere and magnetosheath and in the solar wind. We are using a general linear filter formulation to construct a mathematical formalism to determine not only the frequency spectra but also directional distribution and mode separation in a general wave field. This formalism is completely model independent and requires knowledge of the basic dispersion and polarization properties of the participating waves only. The four‐satellite array combined with a suitable formalism is called a wave telescope. We show that at a given frequency the transmission properties expressed by a transmission function can be shaped by 72 independent parameters for a three‐component vector output of the wave telescope filter. Considering the periodicity of the transmission function in wave vector k space, we are led to the concept of an elementary cell in k space given by the geometric properties of the array such that outside the elementary cell the k ‐ vectors are subject to spatial aliasing analogously to aliasing in time at frequencies above the Nyquist frequency. We show that for MHD waves the fast mode is particularly benign as far as spatial aliasing is concerned. The aliasing properties of other wave modes are also discussed. As an example a filter is constructed which suppresses waves propagating in the magnetic field direction but allows passage of waves propagating antiparallel to the magnetic field.
Geodesy by radio interferometry: Determination of a 1.24‐km base line vector with ∼5‐mm repeatability
Authors: Rogers A. E. E. , Knight C. A. , Hinteregger H. F. , Whitney A. R. , Counselman C. C. , Shapiro I. I. , Gourevitch S. A. , Clark T. A.
Published: February 2008

The 1.24‐km base line vector between the two antennas of the Haystack Observatory was determined from X band radio interferometric observations of extragalactic sources via a new method that utilizes the precision inherent in fringe phase measurements. This method was employed in 11 separate experiments distributed between October 1974 and January 1976, each being between about 5 and 20 hours in duration. The rms scatters about the means for the vertical and the two horizontal components of the base line obtained from the 11 independent determinations were 7, 5, and 3 mm, respectively. The corresponding scatter for the base line length was 3 mm; the mean differed from the result obtained in a conventional survey by 8 mm, well within the 20‐mm uncertainty of the survey. (The determination of the direction from the survey was too crude to be useful.) Another external check on our data was possible, since the azimuth and elevation axes of one of the antennas do not intersect but are separated by 318 mm. We estimated this horizontal offset from the radio interferometry data and found a difference of 10 ± 9 mm from the directly measured value, the relatively large rms scatter being due to the ∼0.96 correlation between the estimate of this offset and that of the vertical component of the base line. Use of a newly completed calibration system in future experiments should allow the scatter to be reduced to the millimeter level in all coordinates for short base lines. For long base lines, such repeatability should be degraded only to about the centimeter level if calibrated observations with sufficient sensitivity are made simultaneously at two frequency bands. An assessment of the accuracy of either our present or future base line results awaits the availability of an accepted, more accurate, standard for comparison. Nonetheless, base line changes can be determined reliably at any established level of repeatability.
A survey of cadmium isotopic abundances
Authors: Rosman K. J. R. , De Laeter J. R.
Published: February 2008

A survey has been made of cadmium isotope abundances in 28 meteorites and three terrestrial samples. Within the limits of experimental error the isotopic composition of Cd was found to be uniform in all samples except two meteorites, Brownfield and Tieschitz, where it was isotopically fractionated. The origin of this anomaly has not yet been established, but some possible mechanisms are proposed. No evidence of thermal neutron irradiation or inhomogeneous mixing of r, s, and p process produced nuclides could be detected.