bluuebunny/crossref_metadata_2025 huggingface dataset:
The solar origin of long‐term variations of the interplanetary magnetic field strength
Authors: Wang Y.‐M. , Sheeley N. R.
Published: February 2008

Spacecraft measurements over the past two sunspot cycles have shown that the average strength of the interplanetary magnetic field (IMF) undergoes surprisingly modest long‐term variation, unlike the total magnetic flux observed on the Sun. Our attempt to model the IMF during sunspot cycle 21, based on a current‐free extrapolation of the observed photospheric field out to a fixed source surface where the field lines become radial, yields calculated IMF intensities which vary by an order of magnitude and which are far too low near sunspot minimum. We obtain much better agreement with a model containing both heliospheric sheet currents, which deflect polar flux toward the ecliptic, and volume currents, which maintain a residual latitudinal gradient in the IMF intensity. In order to match the observed IMF intensity levels, however, the measured photospheric fields had to be scaled up by approximately a factor of 2. Our composite model has the following main consequences: (1) The source of the radial component of the IMF may be represented to a first approximation by the dipole component of the photospheric field. (2) The radial IMF intensity is strongest in the direction of the dipole axis, which is aligned with the Sun's rotation axis near sunspot minimum but tilts toward the ecliptic near sunspot maximum. (3) The average strength of the photospheric field above latitude 55° is of order 10 G around sunspot minimum.
On the wavelengths of (n+1/2)fce gyroharmonic emissions in the Earth's magnetosphere
Authors: Filbert Paul C. , Kellogg Paul J.
Published: February 2008

Using data from the University of Minnesota Plasma Wave Experiment aboard the IMP 6 (Explorer 43) satellite, the wavelengths of certain electrostatic waves in Earth's magnetosphere are measured. The antenna response to electrostatic waves having several k vector distributions in space is calculated using the fact that the X and Y dipole antennas on IMP 6 are of unequal length. These results are used to determine the wavelengths of (n+1/2)fce gyroharmonic emissions observed just beyond the plasmapause. These wavelengths are then used used in conjunction with theoretical models to determine the energy of the electrons driving the waves, and a range of energies between several tens to several hundreds of electron volts is found. This technique is also applied to Langmuir waves observed upstream of Earth's bow shock, and the results are found to be in good agreement with theory.
Solitary vortices in the Io plasma torus
Authors: Horton Wendell , Smith Robert A.
Published: February 2008

Coupling of radial plasma gradients to the centrifugal and Coriolis forces are shown to give rise to localized, nonlinear vortex structures in rotating plasmas. The vortex density and potential structures are derived along with the nonlinear dispersion relation that relates the scale size of the vortex to its speed of propagation. The hypothesis is advanced that the vortex structures may be associated with certain localized emission features observed in the EUV and the narrow band kilometric radio emission observed in the Io plasma torus. The parameters for a vortex lagging corotation by 3% are derived for comparison with the features of a region of localized emission as given by this idealized model.
Coherent HF radar backscatter from small‐scale irregularities in the dusk sector of the subauroral ionosphere
Authors: Ruohoniemi J. M. , Greenwald R. A. , Villain J. ‐P. , Baker K. B. , Newell P. T. , Meng C.‐I.
Published: February 2008

The Applied Physics Laboratory high‐frequency (HF) radar located at Goose Bay, Labrador, observes coherent backscatter from decameter‐scale ionospheric plasma density irregularities. Over a period of 5 months centered on winter solstice, the daily activity includes a distinctive feature that appears at dusk and moves in a westerly direction through the radar field of view. The feature typically spans 1 hour in longitude and is observed for 1‐1½ hours. In this paper, we give examples and describe the characteristics of this dusk scatter under geomagnetically quiet conditions. We show that it can be readily distinguished from other types of late afternoon/early evening scatter by the extreme equatorward position of its source region and by the low values of its associated radar Doppler velocities (≤ 200 m/s) and spectral widths (≤ 200 m/s). Comparison of the HF radar observations with nearly simultaneous particle precipitation data obtained with the DMSP F6 satellite demonstrates that the source region lies equatorward of the main auroral zone. We argue that the occurrence of dusk scatter is related to the passage through the radar field of view of the area of intersection of the dusk terminator with the mid‐latitude trough. It is shown that the plasma drifts in the source region are slow (≤ 150 m/s) and sunward in the inertial reference frame, as described for the poleward portion of the trough in the Spiro et al. [1978] model of trough formation. The narrowness of the Doppler spectra is also consistent with the picture of the trough as a region of slow, uniform plasma drift. The small‐scale irregularities responsible for the radar backscatter may be generated by a cascade process from larger‐scale (> 100 m) irregularities that are unstable to the action of the gradient drift instability within the evening trough.
AMPTE CCE plasma wave measurements during magnetospheric compressions
Authors: Strangeway R. J. , Scarf F. L. , Zanetti L. J. , Klumpar D. M.
Published: February 2008

The near‐equatorial orbit of the Charge Composition Explorer (CCE) has an apogee of 9 RE, and the spacecraft frequently crosses the magnetopause when it is at its maximum range and within a few hours of local noon. On rare occasions the disturbed solar wind compresses the magnetosphere so much that CCE passes through the bow shock into the interplanetary medium. For these events, CCE is near apogee with a speed that is very low with respect to that of the nominal standing bow shock surface, and the spacecraft instruments are very well positioned to investigate details of bow shock structure and motion. Here we summarize plasma wave observations for the CCE bow shock crossings of November 1, 1984, and February 8, 1986, and then focus attention on the 1984 multiple shock crossing events. For this case we demonstrate that the characteristic features in the wave measurements provide unique signatures for the magnetosheath, bow shock, and foreshock. The most striking signature in the wave data is the presence of a relatively constant signal of 10−6 V m−1 Hz−1/2 observed in the 30‐kHz channel when the spacecraft is in the magnetosheath. Using this signature, we are able to determine that multiple quasi‐periodic bow shock crossings (20‐s period) detected near 0900 UT, November 1, 1984, were associated with wavelike motion of the shock surface rather than with structure of the shock itself. Properties of the 30‐kHz magnetosheath signal are consistent with those expected for thermal plasma fluctuations, and we find that this signal is associated with an increase in density for electrons of energies greater than 50 eV.
Test particle acceleration in turbulent reconnecting magnetic fields
Authors: Ambrosiano John , Matthaeus William H. , Goldstein Melvyn L. , Plante Daniel
Published: February 2008

The induced electric field produced by magnetohydrodynamic processes can accelerate test particles very efficiently under certain circumstances. Numerical experiments indicate that particles are accelerated to very high energy by the magnetohydrodynamic fields accompanying reconnection that is triggered by finite amplitude turbulent fluctuations. This turbulent neutral point mechanism includes both coherent and stochastic components of acceleration. Turbulence appears to influence the acceleration in two ways: It enhances the reconnection electric field while producing a stochastic electric field that gives rise to momentum diffusion; it also produces magnetic “bubbles” and other irregularities that can temporarily trap test particles in the strong reconnection electric field for times comparable to the magnetofluid characteristic time. We propose a scaling of the acceleration process to parameter regimes of interest which indicates that this type of acceleration may be an important factor in space physics and astrophysics.
Coherent mesoscale convection patterns during northward interplanetary magnetic field
Authors: Carlson H. C. , Heelis R. A. , Weber E. J. , Sharber J. R.
Published: February 2008

All‐sky imaging photometer (ASIP) and coincident DE 2 satellite plasma drift and particle data have been combined to study polar ionospheric convection in the presence of subvisual intensity, soft‐particle excited (F region), 6300‐Å, Sun‐aligned polar cap arcs. Coincident DE‐B drift meter data identify these arcs electrodynamically as lines of negative electric field divergence. Based on conductivities, derived from the DE‐B (low‐altitude plasma instrument) measured particle fluxes, and the measured electric field gradients, the divergence of horizontal current across these particle impact excited arcs is in good quantitative agreement with upward Birkeland currents carried by the measured particle fluxes. Although velocity structure can be found without arcs, given the ASIP identified condition of stable weak (hundreds of rayleighs) 6300‐Å Sun‐aligned arcs in the polar cap, electric field negative divergence is consistently found. These arcs (of the order of 100 km in width) are found by ASIPs in the polar cap about half the time under Bz ≥ 0 interplanetary magnetic field conditions. They are regularly seen by ASIP data to extend 1000 to over 2000 km in the sunward direction, and to persist in time often for over an hour. We are thus led to conclude that velocity gradients of this noon‐midnight elongated scale are typical of Bz ≥ 0 conditions. We further conclude that combined polar ASIP images and electrostatic potentials calculated along transpolar satellite tracks offer a valuable diagnostic for polar ionospheric convection studies under Bz ≥ 0 conditions. The ASIP time continuous two dimensionality and the satellite equipotential scaling allow individual “snapshots” of these polar convection boundaries. Application here demonstrates highly anisotropic temporally stable convection with greater order than has previously been suspected.
The cusp and the cleft/boundary layer: Low‐altitude identification and statistical local time variation
Authors: Newell Patrick T. , Meng Ching‐I.
Published: February 2008

Particles of roughly magnetosheath energies precipitate at low altitudes throughout the dayside, in a band referred to as the cusp or cleft. Recently it has been suggested that the cusp proper is a more limited region of the cleft localized near noon, although the criteria for distinguishing between the two regions have been unclear. An investigation into the distinction between the low‐altitude cusp and the cleft (with the latter herein identified as the ionospheric signature of the low‐latitude boundary layer (LLBL)) was performed on both a statistical and a case study basis. One year of DMSP F7 electron and ion data, comprising in all 5609 individual dayside passes, was employed. It was found that the average energy of precipitating particles allows for a clear morphological distinction between the cusp proper and the cleft/LLBL. Often both regions are observed on a given pass at the same MLT, each with its own characteristic properties. The probability of observing the cusp was found to be sharply peaked at 1200 MLT, while the probability of observing the cleft/LLBL was near unity away from noon and had a minimum at noon. The cusp was found to be 0.8°–1.1° magnetic latitude (MLAT) thick essentially independent of MLT, whereas the cleft was thinnest at noon and widened rapidly at local times away from noon. The ion number flux in the cusp was statistically 3.6 times higher than in the cleft. The peak flux within the cusp was located on average closer to the equatorward than to the poleward boundary. Yearly average composite spectrograms of precipitation in the two regions as a function of local time show that the properties of the cusp change comparatively little with local time, but that the peak ion energy flux in the cleft increases smoothly from roughly magnetosheath values close to noon to about plasma sheet boundary layer values near 0600 MLT.
Spectral broadening of VLF radio signals traversing the ionosphere
Authors: Groves K. M. , Lee M. C. , Kuo S. P.
Published: February 2008

Nearly monochromatic signals at 13.6 kHz±1 Hz injected from a ground‐based VLF transmitter can experience a bandwidth expansion as high as 1% (∼ 100 Hz) of the incident wave frequency as they traverse the ionosphere and reach satellite altitudes in the range of 600–3800 km (Bell et al., 1983). The off‐carrier components, having electrostatic nature, are believed to be induced lower hybrid wave modes. We investigate two different source mechanisms that can potentially result in the observed spectral broadening of injected monochromatic VLF waves. One is the nonlinear scattering of VLF signals by ionospheric density fluctuations that renders the nonlinear mode conversion of VLF waves into lower hybrid waves. These electrostatic modes result when the injected VLF waves are scattered by ionospheric density fluctuations with scale lengths less than
in the upper ionosphere, where c, ωpe, Ωe, and ωo are the speed of light in vacuum, the plasma frequency, the electron cyclotron frequency, and the VLF wave frequency, respectively. In the absence of ionospheric irregularities, a second mechanism that involves a parametric instability can excite the lower hybrid waves (Lee and Kuo, 1984). This process tends to produce a spectrally broadened transmitted pulse with peaks at a discrete set of frequencies on both sides of the nominal carrier frequency. By contrast, the nonlinear scattering mechanism generates single‐peaked spectra centered at the carrier frequency. Both types of spectra were observed in the experiments. Therefore, the two suggested source mechanisms contribute additively to the observed spectral broadening of injected VLF waves.
Correlated plasma wave, magnetic field, and energetic ion observations in the ion pickup region of comet Giacobini‐Zinner
Authors: Richardson I. G. , Wenzel K.‐P. , Cowley S. W. H. , Scarf F. L. , Smith E. J. , Tsurutani B. T. , Sanderson T. R. , Hynds R. J.
Published: February 2008

Relationships between simultaneous plasma wave, magnetic field, and energetic heavy ion data obtained by the International Cometary Explorer (ICE) spacecraft in the large‐scale solar wind particle pickup region surrounding comet Giacobini‐Zinner are examined. In particular, we investigate the conditions under which electrostatic emissions at frequencies of a few kilohertz and electromagnetic waves at a few tens of hertz are observed. It is shown that the data are consistent with the view that the kilohertz electrostatic emissions result from a beam‐type instability excited by the pickup photoelectron population when the angle α between the magnetic field and the plasma velocity vectors is not too large (α < 60°). The data also suggest that the few tens of hertz electromagnetic waves may be excited by a ring‐type instability associated with the pickup ion population, which occurs when the magnetic field is near to orthogonality with the flow (α > 60°).
Simulations relevant to the beam instability in the foreshock
Authors: Cairns I. H. , Nishikawa K.‐I.
Published: February 2008

Electrons backstreaming into Earth's foreshock generate waves near the plasma frequency fp by the beam instability. Two versions of the beam instability exist: the “reactive” version, in which narrow‐band waves grow by bunching the electrons in space, and the “kinetic” version, in which broadband growth occurs by a maser mechanism. Recently, it has been suggested that (1) the backstreaming electrons have steep‐sided “cutoff” distributions which are initially unstable to the reactive instability, (2) the back reaction to the wave growth causes the instability to pass into its kinetic phase, and (3) the kinetic instability saturates by quasi‐linear relaxation. In this paper we present two‐dimensional simulations of the reactive instability for Maxwellian beams and cutoff distributions. The results of the simulations are consistent with suggestions 1 and 2 above; suggestion 3 is not addressed here. We demonstrate that the reactive instability is a bunching instability and that the reactive instability saturates and passes over into the kinetic phase by particle trapping. A reactive/kinetic transition is shown to most likely occur within 1 km and 50 km of the bow shock. We suggest that the frequency of the intense narrow‐band waves decrease from above fp to perhaps 0.9 fp (dependent on the beam density) with increasing penetration into the high beam speed region (υb/Ve > 10) of the foreshock, before the wave frequency rises again as the waves become broadband deeper in the foreshock. Both the simulation results and numerical solutions of the dispersion equation indicate that for the observed beam parameters the center frequency of the waves near the foreshock boundary should be between 0.9 fp and 0.98 fp, rather than above fp as previously believed. The simulation results indicate that the effects of spatial inhomogeneity are vital for a quantitative understanding of the foreshock waves.
On the bulk motion of the ion clouds formed by the AMPTE solar wind/magnetosheath releases
Authors: Chapman Sandra C.
Published: February 2008

A principal objective of the Active Magnetospheric Particle Tracer Explorers mission was to release lithium and barium ion clouds that were initially sufficiently mass dense to strongly perturb the ambient solar wind or magnetosheath flow. A key property of these release clouds was that their spatial and temporal scales were smaller than, or of the order of, the Larmor scales of all the ion species involved. A one‐dimensional hybrid simulation study conducted by Chapman and Schwartz (1987) showed that momentum could be transferred locally from the oncoming protons to the majority of the release ions, which are associated with the diamagnetic cavity produced by the release, via the quasi‐steady boundary layer that forms at the cavity edge. The direction of motion of these “snowploughed” release ions and the field structure that accelerates them is just that of the oncoming protons. The remainder of the release cloud is photoionized outside of the diamagnetic cavity, and as a consequence of the “pickup” interaction of these ions in the oncoming proton flow the protons are deflected from their ambient flow direction in the release cloud vicinity. This deflected direction of the oncoming proton flow, just upstream of the snowplough fields, then defines the direction of motion of the snowploughed release ions, which eventually comprise the release ion cloud that is observed from the ground. This transverse component to the direction of the release ion cloud motion suggested by the above simple picture is qualitatively consistent with ground‐based observations of the barium releases. Here this simple model for the snowplough dynamics is discussed quantitatively for the particular conditions of the various lithium and barium releases. In particular, the model is found to accurately predict the time taken for the snowplough to return to the Ion Release Module (IRM) (the release spacecraft) location after the initial expansion of the cloud, an event which, as suggested from both the Chapman and Schwartz simulation results and the IRM in situ data, corresponds to the return of the magnetic field at the IRM. The magnitude of the transverse displacement of the December 27, 1984 barium release ion cloud that was seen from ground‐based optical observations is also predicted by results from the simple model. Finally, the model suggests that for the barium releases, ∼10‐15% of the total number of ions released were photoionized upstream of the snowplough field structure and were therefore available to balance the transverse momentum of the snowploughed release ion cloud, via the deflected proton flow. Simple considerations suggest that this fraction should be adequate to balance the observed transverse momentum of the release ion cloud.
Filamentary structure of a three‐dimensional plasmoid
Authors: Birn J. , Hesse M. , Schindler K.
Published: February 2008

We have examined the changes of the magnetic field topology and the field line connections of a simple explicit magnetic field model of a plasmoid in different stages from its formation to its complete disconnection from the Earth. Particular emphasis was on the effects of a net cross‐tail magnetic field component that breaks the symmetry around the neutral sheet. We find, consistent with the qualitative conclusions of Hughes and Sibeck (1987), that the initial stage of the plasmoid formation is characterized by the formation of helical field lines which cross the neutral sheet typically more than once, but are still connected with the Earth, so that no topology change occurs. In that case a nontopological notion of reconnection is required (Schindler et al., 1988). When reconnection proceeds to lobe field lines, the central plasmoid flux rope becomes enveloped by a sheath of open field lines that pull the plasmoid flux rope back toward the tail. The period of gradual separation of the plasmoid is characterized by a conversion through magnetic reconnection, of helical field lines connected with the Earth to open ones going tailward into interplanetary space. The detailed tracing of magnetic field lines reveals that the structure of the plasmoid region is more complicated than was earlier envisioned. We find that plasmoid field lines no longer form a separate class of field lines as in the symmetric case. During the stage of gradual separation from the Earth the plasmoid is characterized by the central flux rope connected with the Earth which is wrapped by strands of open, partially open, and closed field lines in a filamentary way forming a layer of intermingled flux tubes of different types inside the earlier mentioned sheath of open field lines. For decreasing values of |By| the field connection varies on smaller and smaller scales such that the connection becomes stochastic. This process can possibly cause or increase dissipation.
Three‐dimensional equilibria for the extended magnetotail and the generation of field‐aligned current sheets
Authors: Birn J.
Published: February 2008

Using the magnetotail equilibrium theory and a solution method outlined by Birn (1987), we have constructed self‐consistent three‐dimensional models for the quiet average magnetotail beyond about 20 RE distance but earthward of a potential distant neutral line, which take into account the decrease of the tail flaring with distance. We find that this effect is coupled with the presence of magnetic shear and thus with field‐aligned electric currents. These currents have the signature of region 1 currents, toward the Earth on the dawnside and away on the duskside, and contribute about 5 × 105 A to the total Birkeland current. They are strongly concentrated near the plasma sheet‐lobe boundary and increase toward the flanks of the tail. Associated with the field‐aligned currents and the corresponding magnetic field shear there is a bulging effect that tends to deform a circular cross section of the tail near the Earth into one that has bulges in the low‐latitude boundary region. We argue that this effect may be the cause for increased interaction with the solar wind in these regions, producing interconnected fields and tailward flowing plasma on magnetospheric‐like fields in the low‐latitude boundary layer, and deforming this boundary region into the observed dog bone shape of the plasma sheet cross section.
A calculation of ionospheric currents due to semidiurnal antisymmetric tides
Authors: Stening Robert J.
Published: February 2008

The fully developed equivalent circuit method is used to calculate the ionospheric electric current flow due to the (2,3) and (2,5) tidal modes. The (2,5) mode currents are of negligibly small amplitude. The (2,3) current pattern has meridional currents in the same direction at a given local time in both north and south hemispheres. There is also sometimes a minimum in the zonal current around 10° dip latitude and the equatorial electrojet pattern may be different in different longitude zones. These features may be possible explanations of (1) the meridional current pattern observed at Saint‐Santin, (2) current flows on abnormal quiet days (AQDs), (3) the reverse equatorial electrojet, (4) the seasonal phase anomaly in the lunar geomagnetic tide, and (5) the “invasion” of one hemisphere's current pattern by that from the opposite hemisphere. The recent detection of antisymmetric semidiurnal winds just below the dynamo region further demonstrates the importance of these modes.
The launch of coronal mass ejections: White light and X ray observations in the low corona
Authors: Harrison R. A. , Sime D. G.
Published: February 2008

As part of an ongoing program to determine the processes which lead to the launch of solar coronal mass ejections (CMEs) we present a thorough examination of periods where there are observations from the hard X ray imaging spectrometer on the Solar Maximum Mission which are complementary to observations made by the MkIII K‐coronameter on Hawaii. At the time of the observations no attempt was made to produce compatible data sets so the analysis is hampered by unfortunate timing and pointing. Nevertheless, with the analysis of six events we are able to draw some useful conclusions which agree with the findings of our previous work but dispute the scenarios presented by most theoretical models of the CME launch and the CME/flare relationship. To summarize briefly our results: (1) None of the CME launches occur in coincidence with a flare onset, (2) flares which do occur in the vicinity of a CME launch site occur both after the CME onset and in one foot of the CME structure, (3) there is weak X ray activity also associated with the footpoints of the CME structures and these indicate the presence of large coronal structures which underlie the CME structures, (4) there are X ray enhancements after all of the CME onsets; in some cases these are bright enough to be classed as flares, and in others they are lesser brightenings which we refer to as subflares.
Reconstruction of wave distribution functions in warm plasmas
Authors: Oscarsson T. E. , Rönnmark K. G.
Published: February 2008

The purpose of the present study is to present a new approach to the problem of reconstructing the wave distribution function from observed frequency spectra. A method which does not rely on cold plasma theory is developed. We use numerical methods to solve the dispersion relation for waves in a warm, homogeneous, anisotropic, and multicomponent plasma. This allows us to treat almost any linear wave mode and also to handle a large variety of plasma models, including warm plasma. Doppler effects are taken into account, and reconstruction is performed over a wide frequency range simultaneously. To define a solution to the inverse underdetermined problem occurring in the reconstruction, we use the method of maximum‐entropy. As a test of our method, we solve a model problem with simulated data.
Ion diffusion coefficients from resonant interactions with broadband turbulence in the magnetotail
Authors: Dusenbery P. B. , Lyons L. R.
Published: February 2008

A potentially important source of hot ions within the central plasma sheet (CPS) is warm plasma sheet boundary layer (PSBL) ions which interact with broadband electrostatic waves and electric field drift toward the CPS. In the PSBL, the local interaction between the waves and particles can be assumed to be unmagnetized. The effect of the magnetic field is to organize wave and particle distributions in velocity space. Under these conditions, particle diffusion is, in general, two‐dimensional and is similar to magnetized diffusion. The general quasi‐linear equations describing such diffusion are presented assuming that a spectrum of waves is excited similar to the waves observed in the boundary layer. In order to apply the general quasi‐linear diffusion coefficients, two particle distribution models are assumed based on PSBL observations. One is for the outer edge and one is for the inner edge of the boundary. An expression for the unmagnetized dielectric function is given and evaluated for wave frequency and growth rate for the assumed particle distribution models. It is found that slow and fast mode ion‐sound waves can be unstable for the range of plasma parameters considered. The diffusion coefficients are then evaluated for resonant warm PSBL ion interactions with ion‐sound waves. The results illustrate how resonant ion diffusion rates vary with pitch angle and speed, and how the diffusion rates depend upon the distribution of wave energy in k space. For the model of the outer edge of the PSBL, pitch angle diffusion is found to dominate energy diffusion, whereas both types of diffusion can be important for the model of the inner edge of the PSBL. It is found that the characteristic time for resonant warm boundary layer ions to diffuse in velocity space is ∼10 min (for a wave electric field of 1 mV/m), which is approximately an ion bounce period. In addition, significant pitch angle and energy diffusion should occur resulting in isotropization of the warm PSBL ion beams to form the hot isotropic ion component in the CPS.
Effects of quiet time plasmaspheric electric fields on electron and proton precipitation
Authors: Pinto O. , Gonzalez W. D.
Published: February 2008

The effect of the corotation and the quiet time ionospheric dynamo electric fields on the dynamics of energetic electrons and protons (with energies between 50 keV and 1 MeV), inside the inner magnetosphere, is investigated, using the concept of drift paths of constant effective potential. The quiet time ionospheric dynamo electric field is given by an analytic empirical model based on scatter radar observations. The corotation electric field and the Earth's magnetic field are given by analytic expressions corresponding to a centered and an eccentric dipole model. The main conclusion is that the quiet time electric fields cause a diurnal modulation of the electron and proton precipitation. At low L values the locus in longitude of the maximum precipitation is shown to be energy dependent.
Ionospheric electron acceleration by electromagnetic waves near regions of plasma resonances
Authors: Villalón Elena
Published: February 2008

Electron acceleration by electromagnetic fields propagating in the inhomogeneous ionospheric plasma is investigated. It is found that high‐amplitude short wavelength electrostatic waves are generated by the incident electromagnetic fields that penetrate the radio window. These waves can very efficiently transfer their energy to the electrons if the incident frequency is near the second harmonic of the cyclotron frequency.
Eastward propagation of transient field‐aligned currents and Pi 2 pulsations at auroral latitudes
Authors: Webster D. J. , Samson J. C. , Rostoker G.
Published: February 2008

Transient field‐aligned currents associated with the substorm expansive phase produce the damped, quasisinusoidal, geomagnetic pulsations called Pi 2's. Mid‐latitude and auroral latitude measurements of the fields of these pulsations have indicated that they are produced, for the most part, by westward propagating field‐aligned currents at auroral latitudes, although some studies have shown hints of eastward propagation, particularly far to the east of the longitude of the onset of the expansive phase. This study uses data from the University of Alberta magnetometer array to identify the characteristics of the eastward propagating mode at high latitudes by measuring polarizations and apparent group and phase velocities in regions far to the east of the expansive phase. Our results show that the eastward propagating Pi 2's have very high phase velocities (typically 40 km/s) which are comparable with those of the westward propagating mode. The polarizations and field configurations are compatible with a field‐aligned and ionospheric current model which is very similar to that for the westward propagating component. These current models suggest that Pi 2's may be caused by shear Alfvén waves produced by an azimuthally expanding, fast mode wave. The shear Alfvén waves are reflected from the auroral ionosphere to produce the latitudinally localized field‐aligned and ionospheric currents in the eastward (and westward) propagating modes.
Mean free path of low‐energy protons upstream of selected interplanetary shocks
Authors: Beeck J. , Sanderson T. R.
Published: February 2008

There are two fundamentally different approaches in calculating the mean free path of energetic charged particles. The first approach is concerned with the particle data. Here measured time intensity profiles are fitted to theoretical models and a best fit value for the mean free path is obtained. For undisturbed propagation of solar particles this theoretical model usually is a numerical solution of a Fokker‐Planck equation. In the case of quasi‐parallel interplanetary shocks the mean free path can be determined from the exponential increase of the particles intensity in the upstream region of the shock. The second approach starts with the magnetic field properties. One often‐applied model assumes that the scattering of the particles is due to Alfvén waves that propagate along the guiding field. Here the particles are scattered resonantly by the components of the field fluctuations perpendicular to the guiding field. A second model assumes that a different type of magnetohydrodynamic wave, the magnetosonic wave, in addition to Alfvén waves plays an important role in particle scattering. The scattering by magnetosonic waves is a nonresonant interaction. We discuss the time intensity profiles of low‐energy protons (35–1600 keV) associated with interplanetary shocks observed by ISEE 3 during the period August 1978 to December 1981. For five shock‐associated particle intensity increases, we calculate the amount of scattering upstream of the shock by fitting the intensity increase as an exponential in time. The anisotropy in the solar wind frame is analyzed. We hereby test the validity of our diffusion model. We compare the mean free paths obtained from the particle data with mean free paths derived from magnetic field data. Here we first calculate the amount of resonance scattering from the power density spectrum of the magnetic field fluctuations and then include the effect of magnetosonic waves. The main difference between these two models lies in the predicted rigidity dependence of the mean free path. For all five events, pure resonance scattering predicts mean free paths that increase with particle rigidity; this disagrees with the observed particle behavior. Quite good agreement is obtained by the model of a mixture of Alfvén waves and magnetosonic waves; here a more complicated rigidity dependence is possible. Therefore we conclude that magnetosonic waves together with Alfvén waves are responsible for scattering of particles, at least in the upstream region of interplanetary shocks.
Suprathermal electrons at Earth's bow shock
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Russell C. T.
Published: February 2008

A hot, suprathermal population of electrons is often present near the Earth's bow shock. The overall morphology of this hot population as observed with the Los Alamos/Garching fast plasma experiments on ISEE 1 and 2 in the energy range from 1 to 20 keV is much as described previously by others working primarily with more energetic particle measurements. In particular, at energies below 20 keV the flux of suprathermal electrons is most intense immediately downstream from the shock and decreases in intensity with increasing penetration into the magnetosheath. Upstream fluxes of electrons in this energy range are generally considerably weaker than are the downstream fluxes. The major new results from our ISEE measurements are as follows: (1) The suprathermal electrons are commonly found downstream from perpendicular and quasi‐perpendicular portions of the shock but not downstream from quasi‐parallel portions. (2) The suprathermal electron spectrum extends smoothly out of the shocked solar wind spectrum generally as a power law in energy with an exponent in the range from −3 to −4. (3) Angular distributions for the suprathermal electrons are generally isotropic immediately downstream of the shock ramp, but an anisotropy perpendicular to the magnetic field usually develops with increasing penetration into the magnetosheath. (4) Suprathermal electrons are often first observed within the shock layer itself as a field‐aligned beam escaping upstream. We interpret these observations in terms of a qualitative model wherein the suprathermal electrons are accelerated out of the solar wind thermal and halo populations as the solar wind convects across perpendicular portions of the shock. Subsequently, the suprathermal electrons leak back upstream along the magnetic field to form the backstreaming fluxes of energetic electrons commonly observed in the Earth's foreshock region. Although the mechanism of acceleration is presently uncertain, we believe it is unlikely that these electrons are accelerated by magnetic mirroring of upstream electrons at the shock or that they result from a simple adiabatic mapping of electron distributions from upstream to downstream.
Viscous flow interpretation of comet Halley's mystery transition
Authors: Perez‐de‐Tejada H.
Published: February 2008

A study of the solar wind within comet Halley's ionosheath is presented. It is shown that the plasma changes seen across the intermediate (“mystery”) transition, located approximately half way between the bow shock and the cometopause along the Giotto trajectory, are similar to those occurring across an equivalent transition present within the Venus ionosheath. As in Venus, the observed plasma changes are consistent with those expected from the onset of friction phenomena between the shocked solar wind and the main body of ionospheric plasma. It is suggested that the intermediate transition in comet Halley's ionosheath represents the outer boundary of a thick viscous boundary layer that develops from the nose of the cometopause and extends along the flanks of the ionosheath. On the basis of this interpretation it is concluded that the subsolar position of the cometopause may have reached ∼3.4 105 km upstream from the nucleus at the time of the Giotto measurements.
Simultaneous observations of a theta aurora and associated magnetotail plasmas
Authors: Huang C. Y. , Craven J. D. , Frank L. A.
Published: February 2008

Observations of a transpolar arc and simultaneous measurements of associated plasmas in the magnetotail lobe on March 25, 1982, are presented. The auroral imager on board Dynamics Explorer 1 observes a theta aurora in the northern polar cap for more than 2 hours, between 0502 and 0720 UT. ISEE 1 is located in the southern lobe of the geomagnetic tail at a distance of 22.2 RE during this time. The plasma and particle detectors measure intermittent bursts of particle fluxes between 0530 and 0705 UT. The observations suggest that these particle fluxes represent the high‐altitude signature of a theta aurora in the southern polar cap. The relatively dense and energetic plasmas are organized into several filamentary structures. Magnetic mapping between the two polar regions indicates that the theta aurora in the southern hemisphere is a mirror reflection about the noon‐midnight meridional plane of the theta aurora in the northern hemisphere.
Thin equatorial low‐speed region in the solar wind observed during the recent solar minimum
Authors: Miyake W. , Mukai T. , Oyama K.‐I. , Terasawa T. , Hirao K. , Lazarus A. J.
Published: February 2008

We make an attempt to quantify directly the latitudinal profile of the solar wind velocity around equatorial low‐speed region, based on observations made by Sakigake, Suisei, and IMP 8 between March 1986 and April 1987, without using the assumption that the location of the minimum velocity just coincides with that of the neutral sheet. We use the formula V(km/s) = A sin² (ϕ–ϕA) + B, to parametarize the latitudinal profile. A and B are found to be 1500–11,000 km/s and 340–400 km/s, respectively, which means that the angular width of the low‐speed region below 500 km/s is about 17°. These values also indicate the latitudinal velocity gradient of 30 km/s/deg at the portion of velocity of 500 km/s. The latitude of the minimum velocity, ϕA, oscillated sinusoidally around the solar equator as a function of longitude. The amplitude of the sinusoidal oscillation was 4°–9° in 1986 and 1°–2° in early 1987. These amplitudes are smaller than the tilt angle of the neutral line calculated from the photospheric magnetic field.
Electromagnetic instabilities in the plasma sheet boundary layer
Authors: Angelopoulos Vassilis , Elphic Richard C. , Gary S. Peter , Huang Cheryl Y.
Published: February 2008

Plasma and magnetic field data observed during two ISEE crossings of the plasma sheet boundary layer are studied in conjunction with a linear stability theory analysis. The plasma distribution functions exhibit three distinct potential sources of free energy: ion/ion relative drifts, electron/ion relative drifts corresponding to a nonzero field‐aligned current, and large ion temperature anisotropies. These observed distributions are used as source functions in a linear Vlasov theory of electromagnetic instabilities at propagation parallel or antiparallel to the average magnetic field. The theory shows that the field‐aligned current gives rise to the low frequency kink‐like instability, and anisotropies of the flowing ion components give rise to two distinct branches of the ion cyclotron anisotropy instability with spacecraft frame frequencies both below and above the proton cyclotron frequency Ωp. Analysis of the magnetic field data demonstrates that the appearance of two fast, counterstreaming, anisotropic ion components corresponds to a significant increase in the magnetic fluctuation energy density both below and above Ωp, in full agreement with the theoretical predictions.
Electrostatic noise in non‐Maxwellian plasmas: “Flat‐top” distribution function
Authors: Chateau Yves F. , Meyer‐Vernet Nicole
Published: February 2008

We calculate the electrostatic noise spectrum that should be measured by an antenna immersed in a plasma with a “flat‐top” electron velocity distribution function, for frequencies of the order of the plasma frequency fp. We find that the main part of the spectrum is nearly the same as the thermal noise in a Maxwellian plasma with the same density and equivalent temperature, while the peak near fp depends on the high‐energy tail of the distribution. We consider both wire dipole and double‐sphere antennae, and deduce some practical consequences for plasma wave measurements in the Earth magnetosheath.
Characteristics of wave‐particle interactions during sudden commencements: 2. Spacecraft observations
Authors: Gail W. B. , Inan U. S.
Published: February 2008

Wave data from the DE 1 spacecraft for 50 sudden commencements were analyzed for amplitude and spectral modifications and correlated with data from ground‐based observatories. Changes in wave activity were identified in 14 of the events studied. The changes, which were commonly observed at frequencies below the local electron gyrofrequency, exhibited a complex structure both in frequency and in time and varied considerably among events. The typical event lasted 1–10 min with a maximum narrowband amplitude increase or decrease of 10–30 dB. The onset time of the wave growth was correlated with the arrival of the magnetic disturbance at the spacecraft. Changes were most commonly observed at L shells in the range 3 < L < 6. Growth was observed on the nightside as well as the dayside, and no clear local time dependence was found. In several cases, growth at frequencies above the local gyrofrequency was noted and identified as auroral kilometric radiation.
Generation of low‐frequency waves at comet Halley
Authors: Goldstein M. L. , Wong H. K. , Glassmeier K. H.
Published: February 2008

In this paper we investigate the extent to which the properties of the waves observed at comet Halley during the Giotto encounter can be understood within the context of linearized Vlasov theory. In the region that is magnetically connected to the comet, fluctuations in the plasma frame of reference are detected near 4 and 10 mHz, close to the water cyclotron frequency, as well as at 20–60 mHz, which is well above the water cyclotron frequency. Using a variety of approximations for the ion distribution function, we show that waves having properties similar to those observed can then be generated with appropriate choices of plasma parameters. In the region that is magnetically disconnected from the comet, distinct peaks are observed in the magnetic power spectrum at 7, 21, 29, and 35 mHz, with a hint of a peak at 14 mHz. We show that an anisotropic distribution of pickup water group ions is able to generate unstable waves at as many as 6 harmonics in wave number in the observed range of frequencies. When Doppler shifted into the frame of the Giotto spacecraft, these harmonics in wave number will be observed as harmonics of the water ion cyclotron frequency.
Pc 5 pulsations in the outer dawn magnetosphere seen by ISEE 1 and 2
Authors: Mitchell D. G. , Engebretson M. J. , Williams D. J. , Cattell C. A. , Lundin R.
Published: February 2008

A long‐lasting Pc 5 pulsation at the dawn flank of the magnetosphere is studied using particle and field instrumentation from the ISEE 1 and 2 satellites. Electric field and particle modulation signatures were clearer than magnetic field variations, consistent with the satellites' position in latitude near the equatorial node of a fundamental resonance. Pulsation flow velocities along the ISEE 1 trajectory were calculated from particle characteristics using data from several instruments and from electric and magnetic field data. These flow velocities were all consistent with each other, but the velocities derived from plasma and energetic particle observations were a factor of 2.5 larger than velocities derived from the fields data. We have not been able to find the source of this discrepancy; one possibility is that the field near the spacecraft differs from the large‐scale field. In contrast to observations of pulsations during magnetic storms, which often involve resonant or gyrating particle behavior, particles at all energies sampled (10 eV to 200 keV) appeared to respond passively to the pulsation throughout most of the period of interest. Comparison of data from the two spacecraft, which traveled from ∼15 RE to ∼7 RE with a time separation of ∼1 hour, suggests the propagation of relatively broadband pulsation energy from the magnetopause/low latitude boundary layer and subsequent resonance of independent L shells in the fundamental toroidal mode after the cessation of power input.