bluuebunny/crossref_metadata_2025 huggingface dataset:
F region electron temperature signatures of the plasmapause based on Dynamics Explorer 1 and 2 measurements
Authors: Brace L. H. , Chappell C. R. , Chandler M. O. , Comfort R. H. , Horwitz J. L. , Hoegy W. R.
Published: February 2008

Early satellite measurements in the F region found a peak in electron temperature, Te, at night on L shells that thread the expected location of the equatorial plasmapause. This Te enhancement has been attributed to heating by magnetospheric sources. The Te peak typically occurs near 60° invariant latitude, moves equatorward during magnetically active periods, and recovers during quieter times in a fashion similar to observed plasmapause behavior. Recent Dynamics Explorer 1 and 2 measurements near 2100 LT have shown that the Te peak in that sector occurs on L shells that are either within the gradient of the plasmapause or just equatorward of that location. Dayside Te signatures in the F region near 60° invariant latitude also have been reported, but these are observed primarily above 600 km, and their relationship to day side plasmaspheric features, if any, has been unknown. In this paper we employ a more extensive set of DE‐1 and ‐2 measurements to examine this relationship at all local times. Statistically, the diurnal variation of the F region Te signature and the plasmapause at altitudes of the order of 104 km are essentially identical between 2200 and 1300 LT, but they diverge in the afternoon sector. The quiet time Te signature remains in the vicinity of 60° at all local times, while the plasmapause itself bulges to about 70° at 1500 LT. Simultaneous DE‐1 observations, however, show that the plasmasphere in the bulge region often exhibits an internal feature in the vicinity of 60°, which takes the form of a sharp gradient in H+. In each case examined in detail this gradient marked the L coordinate of an F region Te ledge which has higher temperatures on the equatorward side. Te signatures are not usually seen at the plasmapause itself which is at higher latitudes in this local time sector. The light ion gradient seems to represent a sharp boundary between an old plasmasphere and a new plasmasphere that has been created too recently for refilling to have washed out the sharp boundary between them. This boundary has sometimes been called the inner plasmapause. The latitudinal movement of the Te signature with magnetic activity is similar at all local times but is more sensitive to kp above L = 4 and less sensitive below L = 4. The Te signatures are seldom observed below L = 2.5. These characteristics of the Te signature are qualitatively consistent with plasmasphere depletion and refilling time constants. The Te enhancements at all local times appear to have their quiet time source in the photoelectron energy that is stored in the thermal plasmaspheric electrons. During magnetic substorms an additional magnetospheric heat source enhances the Te signatures, perhaps a result of penetration of the ring current into the plasmasphere.
Ionospheric convection inferred from interplanetary magnetic field‐dependent Birkeland currents
Authors: Rasmussen C. E. , Schunk R. W.
Published: February 2008

It is fairly well‐established that sunward convection of the ionosphere exists in the central portion of the polar cap when the interplanetary magnetic field (IMF) is northward. What is unclear, however, is whether the sunward convection is created by additional cells within the polar cap or by a distortion of the normal two‐cell pattern. Computer simulations of ionospheric convection were conducted by combining empirical models of Birkeland currents with a model of ionospheric conductivity to study IMF‐dependent convection characteristics. Birkeland currents for three IMF configurations were available: (1) Bz southward, (2) Bz weakly northward, and (3) Bz more strongly northward. In all cases the currents used represented conditions in the northern polar cap of a negative IMF By component. Two different conclusions are possible given the limited dayside measurements of Birkeland currents. The first is that as the IMF turns northward, the morning cell becomes slightly elongated in roughly the sunward direction, and the evening cell begins to wrap around the morning cell by extending into the morning sector. This process continues as the IMF becomes more strongly northward, until what was originally the evening cell becomes severely distorted, almost completely surrounding the morning cell. What was originally the morning cell shifts into the polar cap and it is this cell along with the distorted evening cell which provide for sunward flow in the polar cap. With a different distribution of Birkeland currents on the dayside, it is also possible that when the IMF is strongly northward, a three‐cell convection pattern exists. However, this is different from the expected four‐cell pattern since the low‐latitude cell in the morning sector is missing.
Hemispherical asymmetry in cusp precipitation near solstices
Authors: Newell Patrick T. , Meng Ching‐I.
Published: February 2008

A statistical comparison of the peak flux in electron and ion polar cusp precipitation in the summer and winter hemispheres as observed by the low‐altitude DMSP F7 satellite is performed. Data studied encompass four consecutive solstices from December 1983 to June 1985, comprising 77 days of data with a total of 292 individual cusp passes. On each day, observations were restricted to those few hours UT in which the interhemispherical MLT variation of DMSP F7 was smallest. After the remaining local time effect was averaged out, the summer hemisphere ion (electron) precipitating energy flux was larger, on the average, by 61 ± 11% (51 ± 5%) than that in the winter hemisphere. However, the average particle energy was always lower for both species in the summer hemisphere. These effects generally hold true for northward as well as southward interplanetary magnetic fields (IMF). It is argued that the observed asymmetry is very hard to explain if the most intense part of the cusp lies on closed field lines, but it is shown that the standard open field line model of the cusp virtually requires the observed differences to occur. The present results thus suggest that the most intense portion of the cusp lies on open field lines even for northward IMF.
Shock analysis: Three useful new relations
Authors: Smith Edward J. , Burton Marcia E.
Published: February 2008

Over the years a variety of methods for analyzing hydromagnetic shocks have been developed. The Rankine‐Hugoniot relations on which the analyses are based are sufficiently complex and involve sufficiently large numbers of variables that in specific instances a decision must be made on how to proceed. Our analyses have tended to emphasize the magnetic field (B) and velocity (V) as the starting point since they are typically the most accurately determined measurables. We have also tended to carry out the analysis in a reference frame aligned, and moving, with the shock. Three new relations have been derived which are generally useful. (1) The shock speed can be calculated from the upstream and downstream values of B and V independent of the shock normal or its direction relative to the upstream field. (2) An explicit relation is derived relating the angles between the jump in velocity, the shock normal, and the upstream field. (3) From the magnetic field and two‐dimensional velocity measurements, which are frequently all that are available, it is possible to infer the third component of the upstream‐downstream velocity jump.
A theory of substorms: Onset and subsidence
Authors: Kan J. R. , Zhu L. , Akasofu S. ‐I.
Published: February 2008

It is shown by a computer simulation study of the magnetosphere‐ionosphere (M‐I) coupling that an enhanced magnetospheric convection can lead to auroral substorm onset (e.g., a sudden brightening of an auroral arc) in about 40 min after a southward turning of the IMF. The simulation results show that the enhanced M‐I coupling can produce intense upward field‐aligned currents in the midnight sector near the poleward boundary of a high‐conductance belt associated with the diffuse auroral precipitation. Two necessary conditions for substorm onset are that (1) the polar cap potential must exceed a certain value (about 70 kV in our model) and (2) the convection reversal region must overlap with the poleward gradient of the diffuse auroral conductance in the ionosphere in the midnight sector. The matching ensures that the divergence of the Pedersen current colocates with the divergence of the Hall current to maximize the upward field‐aligned current near the poleward boundary of the diffuse aurora in the midnight sector for substorm onset. Without the matching, the auroral brightening would not occur, even if the electrojets are appreciable. Substorm subsides when one of the two necessary conditions is no longer satisfied. This can occur owing either to a northward turning of the IMF or weakening of Alfven shielding due to substorm enhancement of the ionospheric conductance during a prolonged southward IMF.
Production of auroral kilometric radiation by gyrophase‐bunched double‐layer‐emitted electrons: antennae in the magnetospheric current regions
Authors: Borovsky Joseph E.
Published: February 2008

When temporally modulated, the gyrophase‐bunched sheet beams of electrons that emanate from magnetized double layers may produce electromagnetic radiation with high efficiencies in narrow frequency bands δf ∼ 200 Hz, the electromagnetic waves being broadcast in the north‐south (constant longitude) direction. For the modulated‐beam mechanism, efficiencies, power levels, and polarizations are estimated. It is found that of the order of 160 auroral arcs that are each 10³ km long are necessary to produce 109 W of radiation. Beam modulation mechanisms, auroral zone measurement techniques, and electromagnetic wave reflection are discussed, and a laboratory experiment to study the modulated‐beam emission mechanism is outlined.
Striated spectral activity in Jovian and Saturnian radio emission
Authors: Thieman James R. , Alexander Joseph K. , Arias Tomas A. , Staelin David H.
Published: February 2008

Examination of high time resolution frequency‐time spectrograms of radio emission measured near the Voyager 1 and 2 encounters with Jupiter reveals occasional striation patterns within the normally diffuse hectometric radiation. The patterns are characterized by distinctive banded structures of enhanced intensity meandering in frequency over time scales of minutes to tens of minutes. This banded form of striated spectral activity (SSA) has an occurrence probability of the order of 5% during the three weeks before and after Jupiter encounters. Plots of single 6‐s frequency sweeps often exhibit a slow rise in intensity followed by a sharp drop‐off in each band as frequency decreases. Banded SSA is often preceded or followed by chaotic SSA in which banding of the emission becomes discontinuous or unrecognizable, although the intensity modulation is still evident. Although SSA normally occurs in the frequency range of roughly 0.2–1.0 MHz, similar but longer‐lasting patterns have been found occasionally in decametric emission above 10 MHz. Analogous modulation has also been observed in the Saturnian radio emission, suggesting that SSA may be a common feature intrinsic to the radio emission at both planets. Observed properties suggest a number of constraints on the possible source mechanism which make theoretical interpretations difficult.
A photochemical equilibrium model for ionospheric conductivity
Authors: Rasmussen C. E. , Schunk R. W. , Wickwar V. B.
Published: February 2008

A photochemical equilibrium model of the high‐latitude ionosphere has been developed. This model provides densities of the ionospheric constituents, N2+, O2+, O+, and NO+, from 85 km to approximately 220 km. These densities are then used to calculate Pedersen and Hall conductivities. A comparison of the model results with Arecibo and Chatanika radar observations was made, covering periods of solar minimum and solar maximum. The comparison showed the model to predict ionospheric densities to within 50% and conductivities to within 40% in the illuminated portion of the ionosphere. In regions of electron precipitation, the model showed good agreement with measurements. Results of this study indicate the following: (1) Ionospheric conductivity increases by a factor of ∼1.6 from solar minimum to solar maximum conditions; (2) the portion of the ionosphere above 170 km can contribute as much as 40% during daylight and 80% during nighttime to the total height‐integrated Pedersen conductivity; (3) the ratio of the height‐integrated Hall to Pedersen conductivities is approximately 1.1–1.3 for sunlit conditions; this is appreciably lower than the value of 2 found in previous studies; (4) these and other factors indicate that, under certain conditions, the height‐integrated Pedersen conductivity may be as much as 2–3 times larger than previously reported.
Brunt‐Doppler ducting of small‐period gravity waves
Authors: Wang D. Y. , Tuan T. F.
Published: February 2008

The variation of the Brunt period with height lends itself to a natural ducting and filtering mechanism for low‐altitude short‐period gravity waves. We investigate this mechanism in combination with Doppler ducting produced by the variation in horizontal winds. Both the frequency dispersion at fixed propagation direction and the direction dispersion at fixed frequency have been examined in a COSPAR background atmosphere with zonal and meridional winds. Our results show that not only are the low‐altitude short‐period gravity waves ducted, but unlike the usual ducting mechanisms due to uneven structure and dissipation which produce only partially guided modes, this mechanism produces primarily guided modes in the absence of winds and a mixture of fully and partially guided modes with winds. The wind effects are very large on the higher modes and less significant on the few lowest modes, including the Lamb mode. Investigations of viscous dissipation, nonlinearity, and instability have shown that viscosity is unimportant for most altitudes of interest and that nonlinearity and instability can play a role for all but the lowest guided modes. We propose that simultaneous continuous observation of airglow at mesospheric and ionospheric altitudes be made to verify not only the low‐altitude Brunt‐Doppler ducting for short‐period gravity waves, but also the vertical energy distribution of the medium‐ and large‐scale TIDs.
Pulsations observed during high‐speed flow in the ionosphere
Authors: Waldock J. A. , Southwood D. J. , Freeman M. P. , Lester M.
Published: February 2008

Pulsation signals detected in the E region ionospheric convection flow with the SABRE radar are described. Large amplitude oscillatory signals were detected, localized to a region of approximate dimension 300 km in the north‐south direction and 100 km in the east‐west direction. The spectral characteristics of the signals show the presence of linearly polarised waves propagating westward with an apparent horizontal phase speed of 8 km/s. The pulsations occur at a time when the background flow is changing and, in particular, are located next to a region where a sheet of high vertical vorticity develops. The oscillatory signal contains field‐aligned current colocated with the vorticity in the background flow. Vertical current density is of order 10−6 Am−2. We propose that the pulsations are system transients excited in response to flow reconfiguration in the magnetosphere. We conclude by showing spacecraft data indicating magnetopause flux transfer event activity was occurring in the same local time sector at the time of the events reported here.
Influence of instabilities on plasma flow around a comet
Authors: Kellogg Paul J.
Published: February 2008

The ion pickup process is examined in a multifluid hydrodynamic approximation which allows for relative motion, along the magnetic field, of the newly created ions and the original fluid. If this relative motion is appreciable, the cometary ion flow will not be exactly antisunward, and its reaction on the solar wind will deflect the solar wind in the opposite direction. Fluid equations are given, but they would necessitate calculating the flow of several fluid species.
Coherent Cerenkov radiation from the Spacelab 2 electron beam
Authors: Farrell W. M. , Gurnett D. A. , Goertz C. K.
Published: February 2008

During the Spacelab 2 mission, a spacecraft called the Plasma Diagnostics Package (PDP) was released from the space shuttle to investigate the surrounding plasma environment. During an interval when the shuttle and PDP were magnetically connected, a continuous 1‐keV/50‐mA electron beam was ejected along a field line from an electron generator on board the shuttle. As the PDP flew by the beam, the PDP plasma wave instrument detected intense whistler mode radiation originating from the beam. It is believed that coherent Cerenkov radiation from bunches of beam electrons is responsible for the whistler mode radiation, where an electrostatic beam‐plasma instability forms the coherently radiating bunches. In this paper a detailed model of the coherent Cerenkov emission process is presented. A one‐dimensional computer simulation of the beam is used to model the expected phase space structure of the electrons, and power emitted from Cerenkov radiation is computed using an analytical expression. The calculated power from the modeled 200‐m beam segment is ∼5 × 10−8 W/Hz, which can easily account for the measured whistler mode wave power. The inclusion of coherent effects in the beam increases the wave powers by nearly 90 dB above incoherent power levels. These calculations demonstrate that a spontaneous emission process, alone, can account for the observed whistler mode wave powers.
Spatial variation and evolution of heliospheric sector structure
Authors: Behannon K. W. , Burlaga L. F. , Hoeksema J. T. , Klein L. W.
Published: February 2008

The magnetic sector polarity at the Sun and in interplanetary space has been surveyed during approximately three quarters of sunspot cycle 21 using photospheric magnetic field observations from the Wilcox Solar Observatory at Stanford University and spacecraft observations by Voyagers 1 and 2, Pioneer Venus Orbiter, and, to a lesser extent, Helios A, IMP 8, and ISEE 3. During the more than 8‐year period of the study, late 1977–1985, the Voyagers measured the interplanetary magnetic field (IMF) over a heliocentric distance range of 1–25 AU and a heliolatitude range of −5°–27°. The photospheric data were used together with a potential field (PF) model to calculate the location of the heliospheric current sheet near the Sun throughout the same period, which included sunspot maximum and much of the declining phase. The large‐scale magnetic polarity structure was found to be in almost continuous evolution throughout the period of study. This included an alternation between two‐sector and four‐sector patterns, with a mean duration in each case of approximately seven solar rotations. Although the azimuth of the IMF was found by the Voyager spacecraft to be oriented more than 60° from the theoretical Parker spiral angle 20% of the time, application of smoothing techniques produced IMF polarity patterns which could be compared with the pattern derived at the Sun. Such comparisons revealed very good agreement (82%) between solar and IMF sector patterns observed by Pioneer Venus in the inner heliosphere and significantly poorer agreement (61–64%) between solar and outer heliosphere patterns observed by Voyagers 1 and 2. This may be the result of dynamical processes in the solar wind which alter the position and shape of the current sheet beyond a few astronomical units. Sector pattern stability was found to decrease with increasing heliocentric distance, also. Evidence was found for the more frequent occurrence of four‐sector structure near the Sun than in the outer heliosphere. In 1985, when Voyager 1 reached a heliolatitude of 25°, it observed the disappearance of the sector structure, consistent with the tilt of the current sheet deduced from photospheric magnetic field measurements.
The effect of reconnection upon the linkage and interior structure of magnetic flux tubes
Authors: Wright Andrew N. , Berger Mitchell A.
Published: February 2008

The topological properties of a magnetic field are interpreted in terms of magnetic helicity. The total helicity of a collection of flux tubes arises from the linking of flux tubes with one another (mutual helicity) and the internal magnetic structure of each flux tube (self‐helicity). Reconnection changes the topology and magnetic connectivity of flux tubes. This can also be viewed as a redistribution of self‐ and mutual helicities. If total magnetic helicity is approximately conserved, it is possible to put quantitative limits upon the changes in self‐ and mutual helicities. This can be interpreted as the change in magnetic flux tube linkage (due to reconnection) and amount of twist present in the reconnected flux tubes. The implications for reconnection in the terrestrial magnetosphere are also discussed.
Global mode ULF pulsations in a magnetosphere with a nonmonotonic Alfvén velocity profile
Authors: Zhu Xiaoming , Kivelson Margaret G.
Published: February 2008

For a nonmonotonic Alfvén velocity profile, the global mode of the magnetosphere can be shown to couple to multiple resonant field lines. The mode structure itself is greatly altered by introduction of nonmonotonicity. Because of the rapid decrease of density at the plasmapause, the variation of the Alfvén velocity naturally separates the magnetosphere into inner and outer parts. ULF wave perturbations are found to be trapped in the Alfvén velocity valleys and, under certain circumstances, ULF wave amplitudes in the inner magnetosphere are greatly enhanced. The results obtained may explain some ULF perturbations observed in the low‐latitude regions of the magnetosphere.
Nonlinear evolution of the auroral electron beam
Authors: Maggs James E.
Published: February 2008

The nonlinear spatial evolution of the auroral electron beam and beam‐generated electrostatic whistler noise is followed from source to atmosphere. Changes in beam parameters are determined by using the equations of conservation of total particle and wave energy and momentum flux density. Wave power fluxes are calculated by numerically integrating the wave kinetic equation. Levels of beam generated noise are quantified by using thermal levels of Cerenkov radiation as a source. Wave refraction and source geometry as well as beam recoil and thermalization influence beam generated power flux spectra. Beam parameters evolve on ionospheric scale lengths, and their positive slope feature in velocity space is maintained over thousands of kilometers of altitude even though they can generate wave energy density fluxes sufficient to modify the ionospheric density profile. For a beam source at 7000 km altitude, peak fluxes of electrostatic whistler noise as high as 10−10 W/m² Hz can be generated in the altitude region between 3000 and 5000 km. Beams with densities large enough to be unstable at the source altitude generate sufficient wave noise at the plasma frequency to disrupt the source structure.
On the stability of rotational discontinuities and intermediate shocks
Authors: Lee L. C. , Huang L. , Chao J. K.
Published: February 2008

The stability of rotational discontinuities and intermediate shocks is studied based on a hybrid simulation code. The simulation results show that rotational discontinuities are stable and intermediate shocks are not stationary. Intermediate shocks tend to evolve to rotational discontinuities and waves. We employ several different initial profiles for the magnetic field in the transition region and find that the final structure of the discontinuities or shocks is not sensitive to the initial magnetic field profile. The present results are different from those obtained from the resistive MHD simulations. Furthermore, our study indicates that the kinetic effect of particles plays an important role in the structure and stability of rotational discontinuities and intermediate shocks.
Structure and dynamics in the polar cleft: Coordinated satellite and ground‐based observations in the prenoon sector
Authors: Sandholt P. E. , Jacobsen B. , Lybekk B. , Egeland A. , Meng C.‐I. , Newell P. T. , Rich F. J. , Weber E. J.
Published: February 2008

Observations of electron and proton precipitation fluxes and Birkeland currents from Defence Meteorological Satellite Program spacecraft F7 and simultaneous ground‐based optical measurements from Svalbard (∼75° geomagnetic latitude) of dayside aurorae (∼0900 MLT) are presented. In these cases, the auroral emissions within the field of view can be separated into different latitudinal zones with corresponding structures in particle precipitation and Birkeland current. The temporal evolution of these structures is monitored by the ground‐based instruments, indicating the presence of both stationary cleft emissions produced by soft electron fluxes (energy of a few hundred eV) and multiple, discrete arcs and arc fragments of transient or more persistent nature, corresponding to the precipitation of keV electrons. The latter are associated with narrow sheets of upward flowing Birkeland currents and reduced proton precipitation, suggesting the presence of field‐aligned potential drops above the spacecraft. Assuming that these low‐altitude structures map to the low‐latitude boundary layer of the magnetosphere, they correspond to a strong filamentation of the boundary layer, in the prenoon sector. Field‐aligned current systems separated in latitude and associated with different particle populations, i.e., plasma‐sheet‐like and low‐latitude‐boundary‐layer‐like particles, respectively, are observed in one case, after a sharp interplanetary magnetic field Bz polarity change.
The role of Landau resonance in energetic electron precipitation from the inner radiation belt
Authors: Pinto O. , Gonzalez W. D.
Published: February 2008

In this report the role of Landau resonance between energetic electrons and plasmaspheric hiss waves at the inner radiation belt is investigated. These nearly structureless turbulent waves have been observed to exist at the inner belt over a large range of latitudes and to propagate with a wide distribution of wave normal angles relative to the geomagnetic field. For these waves, we show that at low L values, Landau resonance with electrons near their mirror points can give pitch angle diffusion coefficients larger than those expected for equivalent cyclotron resonances at the equator. Because of this fact, and because Landau resonance can occur at practically all electron energies that characterize this region (≤1 MeV), we suggest that this resonance may be regarded as the most important process to explain the enhancement in energetic electron precipitation at the inner belt, at least during the recovery phase of storms, when the plasmaspheric hiss population seems to be intensified (e.g., Tsurutani et al., 1975).
A comparison between the experimentally and theoretically determined equatorial electrojet electric field
Authors: de la Vega M. , Duhau S.
Published: February 2008

Laminar theories of electrojet dynamics predict that the primary eastward electric field is almost constant with height. Nevertheless, according to previous findings of the authors, the electrojet intensity predicted by these theories agrees with the measured one only if that component of the electric field is assumed to be almost half its measured F region value at E region heights. In the present paper this apparent contradiction is investigated further by contrasting the physical hypotheses on which theoretical models rely with empirical data and taking into account the turbulent contribution to the Pedersen conductivity as proposed by Rogister (1971). It is found that turbulence due to either gradient drift or two‐stream instabilities, or both, reduces the secondary electric field by increasing the Pedersen conductivity from 30% to 100% of its laminar value within almost all the E region, and that the geomagnetic field lines seem not to be equipotential, a result that cannot be explained within the framework of present theories.
Thermal He+ in the plasmasphere: Comparison of observations with numerical calculations
Authors: Newberry I. T. , Comfort R. H. , Richards P. G. , Chappell C. R.
Published: February 2008

Observations from the retarding ion mass spectrometer (RIMS) on the Dynamics Explorer 1 (DE‐1) satellite are used to examine He+ and its relation to H+. These observed values are also compared to theoretical values from the field line interhemispheric plasma (FLIP) model. It is found that the observed density ratio of He+/H+ may vary from less than 0.01 to over 1.0 in the plasmasphere. The average value of 0.2 is greater than has been previously found by either observation or theoretical calculation. In order to produce good agreement between the model and measured ion temperature, it is necessary to assume that approximately 55% of the energy of the ionospheric photoelectron escape flux is deposited in the plasmasphere. This proportion is much higher than found in previous theoretical estimates. When the plasmaspheric heating rate is adjusted to bring the model temperatures into agreement with the measured temperatures, generally good agreement is obtained between the modeled and observed ion density profiles, except at the highest altitudes. Typically, there is a region in the plasmasphere above ∼4000 km and L = 2.0 where the He+/H+ density ratio has been observed to maintain a nearly constant value of approximately 0.2. This characteristic is not reproduced by the model. It is also found that the H+ density varies primarily with the magnetic activity while the He+ density varies more with the solar activity. These effects combine in such a way that the ratio of these densities is found to be dependent on both indexes, with the solar activity having the greatest effect. This appears to account for the observation of large He+/H+ density ratios since the DE‐1/RIMS observations took place during a period of unusually high solar activity. The model also indicates that there is a factor of 3 diurnal variation in the high‐altitude He+ density, but little diurnal variation in the H+ density. The measured densities, which were limited to short local time periods in the morning and evening sectors, suggest more diurnal variation in the evening than the model.
Coupling of magnetohydrodynamic waves in inhomogeneous magnetic field configurations
Authors: Mond M. , Hameiri E. , Hu P. N.
Published: February 2008

In this paper we study the resonant coupling of fast magnetosonic waves to the Alfven or slow magnetosonic waves in general geometries. If the magnetic field lines are straight and the Alfven velocity does not vary along them, the linearized magnetohydrodynamic (MHD) equations exhibit a singularity at a given line which manifests the resonant coupling on that line. However, if the lines are not straight, or if the Alfven velocity does vary along them (or both) no singularity in the linearized MHD equations is apparent. Nonetheless, we show here that the solutions of the linearized MHD equations behave singularly in the neighborhood of an entire field line. This gives rise to the resonant coupling of the fast to the Alfven or slow waves along the entire line. We calculate the solutions near the resonant magnetic field line as well as the nonzero net energy flux across it. The method used is applicable to general geometries and plasma pressures.
Magnetic pulsations at the quasi‐parallel shock
Authors: Thomsen M. F. , Gosling J. T. , Bame S. J. , Russell C. T.
Published: February 2008

The plasma and field properties of large‐amplitude magnetic field pulsations upstream from the quasi‐parallel region of the Earth's bow shock are examined in high time resolution using data from ISEE 1 and 2. The relative timing of the magnetic field profiles observed at the two spacecraft shows that some of the pulsations are convecting antisunward across the spacecraft while others are brief out/in motions of the bow shock across the spacecraft. Pulsations with both timing signatures are the site of slowing and heating of the solar wind plasma. The ions tend to be only weakly heated in the convecting pulsations, while within the out/in pulsations the ion heating can be quite substantial but variable. This variation occurs not only from pulsation to pulsation but also from point to point within a given pulsation. In general, the hottest distributions within the out/in pulsations tend to occur in regions of lower density and field strength. Magnetic pulsations bear a number of similarities to previously identified hot diamagnetic cavity events as well as to more durable crossings of the quasi‐parallel shock itself. These various phenomena may be different manifestations of the same basic physical processes, in particular the coupling of coherently reflected ions to the solar wind beam.
Large‐scale characteristics of magnetospheric equatorial currents
Authors: Iijima T. , Potemra T. A. , Zanetti L. J.
Published: February 2008

We have analyzed the vector magnetic field data acquired with the AMPTE CCE satellite for prolonged periods of disturbed conditions (2 ≤ Kp ≤ 6, and −70 ≤ Dst ≤ −20 nT). A total of 300 orbits for 180 days during 1985 and 1986 were subjected to the present analysis. We have determined the characteristics of the intensities and flow directions of currents flowing parallel to and near the geomagnetic equatorial plane in the region between L = 4.0 and 8.8 RE. This analysis was done in a cylindrical magnetic coordinate system and by assuming north–south symmetry with respect to the geomagnetic equatorial plane. The characteristics determined from this study include the following: (1) The azimuthal currents flow westward and exhibit pronounced midnight‐noon asymmetries, with the nightside intensities greater than dayside intensities by a factor of 2 to 3. The enhanced nightside current intensities have localized relative minima between 2200 and 0200 MLT and L = 5.6 and 8.8 RE, resulting from the integrated effect of a number of substorm‐associated geomagnetic dipolarizations. (2) The inferred equatorial radial currents flow toward the Earth in the evening sector and away from the Earth in the morning sector throughout the region between L = 4.0 and 8.8 RE. (3) A net positive divergence (source) of equatorial current in the evening sector and a net negative divergence (sink) in the morning sector is inferred from the distribution of azimuthal and radial currents. The location, flow direction, and intensities of these divergent currents are almost identical with those of the region 2 Birkeland system. We conclude that the equatorial currents, primarily the westward ring current, and the region 2 Birkeland current system are closely connected.
Stimulated scattering of radio waves off acoustic waves in partially ionized plasmas
Authors: Tsintsadze N. L. , Buadze T. D. , Shukla P. K. , Stenflo L.
Published: February 2008

Stimulated scattering mechanisms involving electromagnetic and acoustic waves in a partially ionized plasma are considered. The growth rates and thresholds for three‐wave decay interactions as well as modulational instabilities are obtained. The relevance of our investigation to the nonlinear propagation of powerful radio waves through the upper atmosphere is pointed out.
Optical auroral conjugacy: Viking UV Imager—South Pole Station ground data
Authors: Burns G. B. , McEwen D. J. , Eather R. A. , Berkey F. T. , Murphree J. S.
Published: February 2008

High time resolution satellite UV images of the northern auroral oval are compared with South Pole station keogram camera and all‐sky camera observations to determine the displacement of the South Pole conjugate point from its location calculated using the international geomagnetic reference field magnetic field model. The results presented extend the study of optical conjugacy to high magnetic latitudes in a solstice period, and to sunlit auroral events. Three high‐latitude events were analyzed: a poleward expansion, an arc poleward of the midnight oval, and a substantial movement in the afternoon auroral oval. They were determined to occur on closed field lines. A range of displacement for the South Pole conjugate point of 5.3° in latitude and 1.9 hours in MLT was measured.
Auroral boundary dynamics observed simultaneously from the Viking spacecraft and from the ground
Authors: Shepherd G. G. , Steen Å. , Murphree J. S.
Published: February 2008

High spatial and temporal resolution auroral imaging from space and from the ground continues to improve our understanding of auroral dynamics. The ultraviolet imager on the Viking spacecraft gives global information on the activity of the auroral distribution. The ground‐based optical facility at Swedish Institute of Space Physics, in Kiruna, Sweden, gives information on the finer‐scale auroral structure. Simultaneous satellite‐ and ground‐based measurements using this instrumentation were obtained for three selected events during the spring of 1986. For some aspects we find a remarkable correspondence of particular features observed from space and from the ground. For others the different views are complementary in nature. Taken together, they provide a comprehensive picture of presubstorm, substorm, and postsubstorm dynamics. The major findings are (1) a narrow active rayed arc set within a background of softer precipitation that defines what is generally called the “diffuse” aurora, with repeated fading on the time scale of 5 min; (2) a period of rapid convection manifested by the disappearance of transpolar arcs and the onset of undulations of the poleward boundary arc observed well before substorm onset; and (3) two clearly separated latitudinal regions following substorm onset, the poleward one being more variable on a time scale of 10 min, but the equatorward one having at its equatorward boundary a coherent pulsating feature with periods in the range 1–3 min.
Plasma preconditioning and the role of elevated vibrational temperature in production of excited N2 vibrational distributions
Authors: Morrill Jeff , Benesch William
Published: February 2008

We have undertaken a series of investigations of excitation and decay processes in molecular nitrogen in pulsed electric discharges under conditions relevant to those of the aurora. Previously, we had observed the continuous evolution of the spectrum of the N2(B³Πg) → N2(A³Σu+) (1 PG) with time and under the influence of collisions after the initial direct electron excitation. In the present investigation, we find that there are progressive changes in the condition of the gaseous medium which occur during the time interval between pulses. At all but the very lowest repetition rates, the sample atmosphere is found to be “preconditioned” at the beginning the excitation pulse by virtue of its previous excitation‐deexcitation history. One possible component of a preconditioned medium is the presence of electronically excited metastables. Another is the retention of excitation in the ground state vibrational manifold. Our observations indicate N2 ground state vibrational temperatures of greater than 3000 K just prior to the beginning of the excitation pulse when conditions for preconditioning are favorable. In addition, our model results indicate that currently accepted N2(B) quenching coefficients are not adequate to explain our observations during the afterglow following the discharge. Ground‐based and rocket measurements of the auroral and airglow spectra have revealed alterations in the distribution of nitrogen molecular band emission intensities only very recently. We are led to believe that most of these phenomena being uncovered in the laboratory are likewise occurring in the aurora, to varying extents depending on conditions and will become evident as observational technology advances. We include a discussion of some conditions under which deviations in the distribution of excited N2 band intensities have been observed in the aurora.
Representations of currents and magnetic fields in anisotropic magnetohydrostatic plasma
Authors: Heinemann M.
Published: February 2008

We discuss the mathematical representation of the electric current density and the magnetic field in equilibrium anisotropic magnetized plasma for which the particle motions parallel to the magnetic field are adiabatic. The current density is the sum of contributions due to plasma‐pressure currents and force‐free currents. The plasma pressure contribution can be expressed in terms of a vector field that is a function of the equatorial parallel pressure, the equatorial pitch angle distribution, and the magnetic field geometry. In the absence of force‐free currents it is possible to introduce a magnetic scalar potential in the presence of anisotropic magnetized plasma. The second‐order partial differential equation governing the scalar potential expresses the full coupling between the plasma and the magnetic field.
On the radial evolution of MHD turbulence in the inner heliosphere
Authors: Marsch E. , Tu C.‐Y.
Published: February 2008

The radial evolution of MHD turbulence in the inner heliosphere, as observed by the Helios probes between 0.3 and 1 AU, is reanalyzed and newly evaluated by means of Elsässer variables. It is found that the spectra of the energy of the outward (e+) and inward (e−) oriented fluctuations, of the cross helicity and cross correlation and of related dimensionless ratios, like the ones named after Alfvén or Elsässer, all undergo considerable radial evolution with an overall trend for the spectra to steepen. These evolutionary tendencies are established both by individual typical case studies for a recurrent high‐speed stream and neighboring low‐speed flows as well as by a statistical analysis of many sample spectra pertaining to various heliocentric distances. In particular, the e+ spectrum considerably steepens in the low‐frequency part. Whereas the fluctuations in the Alfvénic domain are more transverse in the perihelion, they radially evolve toward an equipartition of the energy in the three spatial dimensions at larger heliocentric distances, the e− spectrum does not change a lot at low frequencies but strongly steepens beyond about 2×10−4Hz. The spectral results on e− suggest that isotropic turbulence with an index of −5/3 is probably the ultimate state toward which solar wind MHD fluctuations seem to evolve. The radial evolution of the cross correlation eR is also analyzed.
Ballooning instability of the Earth's plasma sheet region in the presence of parallel flow
Authors: Lakhina G. S. , Hameiri E. , Mond M.
Published: February 2008

Stability of the plasma sheet and plasma sheet boundary layer against the ballooning mode instability is investigated. The equilibrium state of a two‐dimensional plasma sheet configuration with parallel sheared flow is modeled. This equilibrium is shown to be ballooning unstable when δW is not positive definite, where δW is the potential energy. The eigenmode structure of the ballooning mode is found by imposing the boundary conditions that the waves are totally reflected from the ionosphere, and that no waves are coming in from infinity. The eigenmode structure of the unstable ballooning modes is highly oscillatory, extending beyond ∼100 RE. The ballooning modes are thus a possible candidate for explaining the MHD waves and other dynamical events observed in the magnetotail by ISEE 3 and other spacecraft.
Artificial ion beam instabilities: 2. Simulations
Authors: Scales W. A. , Kintner P. M.
Published: February 2008

Plasma instabilities that result when a heavy ion beam is injected into the F region ionosphere are studied by using electrostatic particle simulations. The background plasma is taken to be O+ dominated with a small concentration of H+. Particle heating is studied for various beam parameter regimes. In the beam drift velocity regime, υd/υH > 1, only perpendicular H+ heating is observed at low beam densities. Parallel electron and perpendicular O+ heating are also observed when the beam density is on the order of the background O+ density. Beam thermalization and slow down are observed to be small in the high beam temperature regime but become more severe for beams with temperatures on the order of those of the background particles.
Particle acceleration at shocks with surface ripples
Authors: Decker R. B.
Published: February 2008

We consider the acceleration of superthermal ions within ripples on the surface of a fast‐mode hydromagnetic shock. We confine attention to small‐amplitude surface ripples, characterized by width L and amplitude A, that are large compared to an energetic ion's gyroradius rg, i.e., rg ≪ A ≪ L. Furthermore, we focus upon shocks that are quasi‐perpendicular on average. To simplify the problem, we consider a perpendicular shock having a single surface ripple with a sinusoidal form. We investigate the effects of confinement, evolving geometry, and finite shock curvature that are associated with the ripple by integrating along the orbits of test particles (protons). We assume scatter‐free motion away from the shock and quasi‐static conditions. As an upstream magnetic field line convects through the surface ripple, it intersects the shock at two points, forming a temporary magnetic trap. A few particles injected into this trap undergo many reflections at the shock and are accelerated nonadiabatically (i.e., the first adiabatic invariant is not conserved) before being convected downstream. In some cases these particles form a high‐energy power law tail on the energy spectrum. Large‐amplitude, spike like flux enhancements of width ∼rg are coincident with the shock passage. These spikes are superposed upon broader, but less intense, flux increases with widths that are energy‐dependent but are typically ∼A. Flux‐time profiles and angular distributions in a given ripple vary markedly depending upon path through the ripple and distance from the shock. Angular distributions upstream range from unidirectional to bidirectional along the field, while those downstream are generally peaked nearly transverse to the field. The model‐predicted results reproduce many features of observed ion shock‐spike events, including their singly or multiply spiked impulsive structure, the variability of this structure with particle energy in a given event, and the large variety of structures observed from event to event.