bluuebunny/crossref_metadata_2025 huggingface dataset:
Alfvén solitons in the solar wind
Authors: Ovenden C. R. , Shah H. A. , Schwartz S. J.
Published: February 2008

The interaction of circularly‐polarized Alfvén waves with the surrounding plasma in high speed solar wind streams is investigated. Alfvén wave modulational instability is discussed and nonlinear envelope soliton solutions of the magnetohydrodynamic equations are introduced. The characteristics of these Alfvén solitons are compared with observational results obtained from Helios I and II. A model of the expected turbulent spectrum due to a collection of such solitons is constructed and its radial dependence is investigated, again along with comparison to Helios data.
Evolution of ion distributions across the nearly perpendicular bow shock: Specularly and non‐specularly reflected‐gyrating ions
Authors: Sckopke N. , Paschmann G. , Bame S. J. , Gosling J. T. , Russell C. T.
Published: February 2008

Ion velocity space distributions measured with Los Alamos/Garching instruments on ISEE 1 and 2 across the earth's bow shock have revealed that the nearly perpendicular shock reflects some fraction of the incident solar wind ions over a wide range of Mach numbers (MA ≃ 2.0–12.4). The relative density of the reflected ions varies from 1–3% of the solar wind density at MA ≃ 2 to 15–25% at MA = 8–12, in qualitative agreement with computer simulations and with predictions that indicate that such ions are needed to provide essential dissipation at the shock. These ions gyrate about B in the plasma rest frame with a speed of ∼2 υin, where υin is the normal component of the solar wind flow, and their density increases by ∼2 orders of magnitude at the forward end of the ’foot‘ of the shock profile ∼0.7 ion gyroradii upstream from the shock ramp. This is substantial evidence that the bulk of these ions have been reflected at the shock in a nearly specular fashion. Depending on the shock geometry, some low numbers of reflected‐gyrating ions are sometimes observed a few gyroradii ahead of the outer edge of the foot. Model calculations suggest that these additional ions are produced by non‐specular reflection of a much smaller fraction of the incident solar wind. Downstream of the shock ramp, the secondary ions gyrate with a slightly higher speed, υg ≃ 2.15 υin, in the downstream rest frame. Individual groups of gyrating ions rapidly disperse in velocity space to form a high‐energy torus centered at the bulk velocity of the distributions. Downstream ion toruses are most clearly seen, sometimes persisting for many minutes, when the shock Mach number is low. Downstream of highly supercritical shocks (M ≫ 3), the distributions are more turbulent. In either case, the distributions retain some non‐Maxwellian features for a long time downstream from the ramp, in accordance with earlier observations. Reflected‐gyrating ions have also been identified in a number of crossings with more oblique field orientations (θnB ≳ 45°, i.e., for quasi‐perpendicular shocks). Specularly reflected ions play an important role in producing down‐stream ion thermalization for all θnB ≳ 45°, being relatively more important the higher is the Mach number.
Electromagnetic lower hybrid waves in the solar wind
Authors: Marsch Eckart , Chang Tom
Published: February 2008

An analysis of electromagnetic lower hybrid waves or hybrid whistlers that propagate nearly perpendicular to the magnetic field lines in the solar wind is described. Doppler‐shifted, these waves have a broadband nature when observed in the spacecraft frame depending on the direction of propagation and the supersonic flow velocity of the wind. Lower hybrid waves can energize ions normal to the interpletary magnetic field. They may occasionally be responsible for the observed temperature anisotropy in high speed streams and possibly for the acceleration of heavier ions such as the oxygen ions, alpha particles, etc. These modes are probably driven unstable by the anisotropic halos of the solar wind electron distributions exhibiting ‘heat flux’ profiles.
The localization of Pi 2 pulsations: Ground‐satellite observations
Authors: Singer Howard J. , Hughes W. Jeffrey , Fougere Paul F. , Knecht David J.
Published: February 2008

Much of the attention given to Pi 2 pulsations has resulted from their close relationship with substorm onsets. Most of the observations have been from ground‐based observatories, with few observations in space. Consequently, there are problems in determining the extent of the pulsation region in the magnetosphere and in clarifying the substorm process from information contained in the Pi 2 signal. To further our understanding of Pi 2's and substorms, simultaneous Pi 2 observations from the ground, using the AFGL magnetometer network, and from space, using GOES 2 and 3 satellites, are presented. The observations indicate that the Pi 2 signal in space is often localized such that synchronous satellites separated by 30° see quite different field signatures. In contrast to this, on the ground at mid‐latitudes, similar features are often observed across the 60° width of the AFGL magnetometer network. In addition, when Pi 2's occur, as indicated by ground observations, their observation at synchronous orbit near midnight is more likely during times of large rather than small Kp. These results are interpreted in terms of the plasma sheet model which has previously been used to explain some of the observed properties of Pi 2 pulsations.
Quasi‐thermal noise corrections due to particle impacts or emission
Authors: Meyer‐Vernet N.
Published: February 2008

Recent measurements of the “quasi‐thermal” noise in a stable plasma have exhibited an additionnal local and diffuse noise below the electron plasma frequency; this has been suggested to be due to impacts or emission of charged particles on the antenna surface. To investigate this problem beyond the previous semi‐heuristic evaluations, we extend the quasi‐thermal noise theory to antennas that are not transparent to particles. We apply the results for small radius spherical and cylindrical antennas in an isotropic equilibrium plasma at frequencies of the order of magnitude of the plasma frequency. For antennas near the plasma dc potential, the theoretical noise consists of the usual thermal noise on a transparent antenna plus a component that has the following properties: (1) For small double‐spheres antennas, it has an f−2 spectrum around the plasma frequency fp; just below fp, it is of the same order of magnitude of the thermal noise if the antenna length L and plasma Debye length LD satisfy L/LD ≪ 1. It is much larger if L/LD≪1. (2) For fine cylindrical antennas, it is only broadly f−2 below fp; just below fp, it is much smaller than the quasi‐thermal noise, except if the antenna surface is larger than L4/L²D. Practical consequences are given for recent experiments in the solar wind and terrestrial magnetosphere.
Low‐energy plasma ion observations in Saturn's magnetosphere
Authors: Lazarus Alan J. , McNutt Ralph L.
Published: February 2008

We summarize observations of positive ions in Saturn's magnetosphere made by the Voyager plasma science experiments during the two Saturn encounters. In the outer magnetosphere, the number density and temperature of the plasma are quite variable. When cold, two ionic species are clearly resolved: the lighter ion is probably H+ and the heavier ion has a mass/charge ratio consistent with it being O+ or N+. In the vicinity of 15 Saturn radii (Rs), there is a region in which the composition or charge state of the ions apparently changes, but it then returns to the typical composition. The two ionic components do not have the same thermal speeds or temperatures. In the outer magnetosphere, the two components partially corotate with the planet; their azimuthal speeds are approximately 20% below that expected for rigid corotation. There is some evidence for inward and outward radial flow in addition to the azimuthal motion in approximately one‐third of the Voyager 1 cold ion spectra. The sources of the plasma are still undetermined. The plasma sheet becomes well established within 16 Rs from the planet in the Voyager 1 data and within 10 Rs in the Voyager 2 data. Near the ring plane crossings close to Saturn, O+ is the dominant ion within 1 Rs of the ring plane, whereas H+ dominates outside of that distance from the ring plane. Two scale heights characterize the O+ ion number density distribution in the vicinity of the ring plane: within 0.5 Rs of the plane, the scale height is ∼0.3 Rs; it is almost twice as great outside of 0.5 Rs. The directly observed temperature of the O+ ions is too high to be consistent with such scale heights. A temperature anisotropy of T⊥/T∥ approximately equal to 6 would explain the observations. The cause of the anisotropy is not certain. In general, the plasma and magnetic field appear to be in dynamical equilibrium. In comparing Saturn's magnetosphere to Jupiter's, there appears to be much less acceleration of low‐energy plasma to higher energies at Saturn.
The flux and source of energetic protons in Saturn's inner magnetosphere
Authors: Schardt A. W. , McDonald F. B.
Published: February 2008

The flux of energetic protons in Saturn's inner magnetosphere was observed in two channels from 48–63 and 63–160 MeV. Absorption features caused by the G ring and the satellites Enceladus and Mimas were easily identifiable. The flux observed in the absorption slot of Mimas can be maintained by the decay of a cosmic ray albedo neutron flux of 7×10−3 cm−2 s−1 sr−1. This flux is entirely consistent with calculations of the neutron flux produced by galactic cosmic ray interactions with the rings of Saturn. The omnidirectional proton flux of 8.2×10³ cm−2 s−1 at 2.734 Rs requires a residence time of 30 years. Both the residence time and the energy spectrum are comparable to those found in the inner radiation belt of the earth. The angular distribution is nearly isotropic in the Mimas slot and beyond 4 Rs. Otherwise the pitch angle distribution is pancake and has been approximated by sinn θ with n in the range 2–7. This distribution is consistent with an isotropic neutron source in the ring plane.
Source localization of Saturn kilometric radio emission
Authors: Lecacheux A. , Genova F.
Published: February 2008

From Voyager 1 and 2 near‐ and far‐encounter data, a very precise localization of the sources of Saturn radio emission is found with no a priori hypothesis on the emission directivity, by assuming only that there are two sources of emission, each emitting in one circular polarization, and that the global variations of the polarization are due to occultations of the sources by the planet. This and the previous localization results do not allow one to decide between various models of the emission source.
Impulsive particle precipitation and concurrent magnetic field changes observed in conjugate areas near L = 4
Authors: Lanzerotti L. J. , Rosenberg T. J.
Published: February 2008

Using sensitive riometers (30 MHz) and magnetometers at Siple Station, Antarctica, and at four locations in the conjugate area in the northeastern United States and Southern Quebec, we have studied the temporal and spatial extent of the impulsive losses of electrons from the radiation belts and the concurrent accompanying magnetic field variations produced by the changes in ionospheric current systems. The impulsive variations have a short (∼2–4 s) onset time and a ∼5–10 s decay time. The observed decay times in the riometer and magnetometer signals are consistent with the recombination times in the ionosphere at ∼90–100 km. Often the magnetic field fluctuations are observed to occur primarily in the east‐west direction and are out of phase between the southern and northern hemispheres. These observations suggest that predominantly north‐south ionospheric currents are modified and/or produced by such impulsive precipitation. Time delays in the occurrence of the events between the northern and southern hemispheres, as well as time and spatial differences observed between the northern hemisphere stations, suggest that irregularly shaped, gradient B drifting patches of precipitating electrons could be the source of these impulsive events.
Spread F plasma bubble vertical rise velocities determined from spaced ionosonde observations
Authors: Abdu M. A. , de Medeiros R. T. , Sobral J. H. A. , Bittencourt J. A.
Published: February 2008

Systematic time differences in the onsets of spread F events in the ionograms are observed between the magnetic equatorial station Fortaleza (4°S, 38°W, dip latitude 1.8°S) and the low‐latitude station Cachoeira Paulista (23°S, 45°W, dip latitude 14°S), two stations in Brazil, located at close‐by magnetic meridional planes (actually some 12° of magnetic longitude apart). On the assumption, justified from different experimental observations, that the spread F irregularities occur in strongly field‐aligned plasma bubbles that extend several degrees on either side of the magnetic equator, and rise up in vertically elongated columns over the magnetic equator, we have related the observed time differences in the onsets of spread F events at the two stations to the plasma bubble vertical rise velocities over the magnetic equator. The vertical rise velocities of the plasma bubble so determined are found to be well within the values measured by VHF radar and satellite techniques, and further show, at times, good correlations with the amplitude of the prereversal peak in the vertical drift velocities and the heights of the evening equatorial F layer. Possible implications of these results are discussed.
The effects of Coulomb collisions on solar wind ion velocity distributions
Authors: Marsch E. , Goldstein H.
Published: February 2008

The effects of coulomb collisions on solar wind ion velocity distributions are investigated by using Helios data obtained between 0.3 and 1 AU. The mean free path of ions moving at thermal speed (in the solar wind frame) varies by three orders of magnitude in dependence on the plasma parameters. The number of collisions (roughly defined as collision frequency times the solar wind expansion time) can easily exceed one in low‐speed wind near the heliospheric current sheet. In these regions, almost Maxwellian distributions are observed. In solar wind with intermediate speeds (ranging from 400–600 km/s) one usually finds distributions with a nearly isotropic core which again can be understood by the action of coulomb collisions. On the other hand, the major part of high‐speed solar wind ion distributions can, by good reason, be called collisionless on local scales.
High time resolution observations of corotating interaction region proton events by Pioneer 11
Authors: Pesses M. E. , Van Allen J. A. , Tsurutani B. T. , Smith E. J.
Published: February 2008

Pioneer 11 high time resolution, ∼1‐minute observations within ±3 hours of corotating interaction region shock waves of proton intensities, pitch angle distributions, and crude differential energy spectra of the range of 0.6 ≤ Ep ≤ 3.4 MeV are presented. The principal result is the evidence for the persistent flow of particles away from the shocks. The observations are found to be in good agreement with the hypothesis of local interplanetary shock acceleration.
Chorus‐related electrostatic bursts at Jupiter and Saturn
Authors: Reinleitner L. A. , Kurth W. S. , Gurnett D. A.
Published: February 2008

Analyses of the wide band plasma wave data obtained by Voyager 1 and 2 at Jupiter and Saturn have revealed electrostatic bursts similar to those recently discovered at earth in association with whistler mode chorus. In all three magnetospheres the bursts are characterized by sporadic emissions near or slightly below the electron plasma frequency with bandwidths ranging from 10% to more than 50% of the center frequency. The events found at Jupiter occur in the middle magnetosphere during both the dayside as well as the early morning passes. At Saturn the bursts occurred in the outer regions of the magnetosphere during the dayside pass. In each of the events analyzed, evidence exists for modulation of the electrostatic bursts by a low‐frequency wave, presumably chorus. One of the observations gained at Jupiter includes the detection of a low‐frequency band at the proper frequency for chorus. Detailed waveform analysis confirms that this band does, indeed, modulate the electrostatic bursts. On the basis of the present understanding of the terrestrial observations, it is believed that the electrostatic bursts are generated by an electron beam trapped in Landau resonance with the chorus.
The thermosphere of Titan
Authors: Friedson A. James , Yung Yuk L.
Published: February 2008

The diurnal variation of the vertical structure of Titan's thermosphere is calculated through simultaneous solution of the equations of heat transfer and hydrostatic equilibrium. The temperature and density profiles are found above the mesopause. The dynamical response of the thermosphere to heating is for the most part neglected. Nevertheless, we are able to draw some interesting qualitative and quantitative conclusions regarding the vertical structure. Heating of the upper thermosphere occurs primarily through absorption of solar Lyman α radiation by methane, with an additional amount of heating (≲20%) due to low‐energy magnetospheric electron precipitation. The heat is conducted downward to the mesopause, where it is removed by IR cooling due principally to acetylene. The mesopause is found to occur where the density is 2.2×1012 cm−3 (736 km), and has a temperature of ∼110 K. The exospheric temperature is unlikely to exceed 225 K in the course of a Titan day. The diurnally averaged exospheric temperature is in the range 187–197 K depending on the amount of magnetospheric electron heating that is included in the model. The amplitude of the diurnal variation is found to be ≲28 K. We find that the vertical extent of the hydrogen cloud is too large to be explained in terms of simple thermal escape of hydrogen from a ∼225‐K exosphere, and we conclude that other processes must be important for populating or heating the neutral torus.
Nonlinear evolution of magnetopause tearing modes
Authors: Coroniti F. V. , Quest K. B.
Published: February 2008

Since the magnetosheath plasma is highly turbulent, reconnection at the dayside magnetopause is likely to be temporally unsteady. The tearing mode can be viewed as a model for the unsteady development of a reconnecting magnetic topology. Magnetopause tearing occurs in the guide‐field limit and has a wave packet spatial structure in the east‐west direction. We solve for the nonlinear evolution of a single wavelength guide‐field tearing mode including the effects of finite transit time on the Landau resonant electrons. Short wavelength modes evolve algebraically in time with perturbation amplitudes proportional to t². Long wavelength modes are fully nonlinear, and the amplitude grows linearly in time.
Incoherent scatter observations of an artificially modified ionosphere
Authors: Wand Ronald H. , Mendillo Michael
Published: February 2008

The launch of NASA's HEAO‐C satellite by an Atlas/Centaur rocket on September 20, 1979, provided the first “experiment of opportunity” to test incoherent scatter radar techniques for the diagnostic study of a chemically‐induced modification of the ionosphere. The cause of the disturbance was the rocket's exhaust cloud of H2 and H2O molecules that cause a rapid recombination of the F region plasma at heights above 250 km. The launch from the Kennedy Space Center (28°N, 81°W) was monitored by the incoherent scatter radar at Millstone Hill (42.6°N, 71.5°W) using low elevation angle observations over ranges exceeding 2000 km. The experiment employed various pulse lengths, integration times and scanning modes to explore optimization methods for an upcoming series of Space Shuttle induced ionospheric hole experiments. The results from the HEAO‐hole campaign include the first unambiguous observations of a gas expansion “snowplow effect,” the derivation of local and height‐integrated plasma recombination rates, and the full spatial, temporal, and dynamical morphologies of a large‐scale ionospheric hole. The campaign succeeded in demonstrating that incoherent scatter radars can play a primary role in space plasma physics “active experiments” during the next decade.
Dependence of auroral power spectra variations upon electron drift velocity in the eastward electrojet
Authors: Nielsen E. , Haldoupis C. I. , Fejer B. G. , Ierkic H. M.
Published: February 2008

Power spectral observations of auroral electron density fluctuations with a scale length of nearly 1 m have been made with the STARE system (Scandinavian twin auroral radar experiment). Simultaneous measurements of the mean radial Doppler velocities were used to derive estimates of the ionospheric electron drift velocity. The data were analyzed to determine the spectral dependence on the magnitude and direction of the electron drift velocity. The two spectral types, characterized as “narrow” and “broad,” were observed simultaneously from the same scattering volume. The width of the broad spectrum (up to about 1200 Hz) can be at least 3 times that of the narrow spectrum. Narrow spectra were observed for both small and large flow angles, while broad spectra were observed only for larger flow angles (typically larger than 70°). Narrow spectra have a width which is insensitive to variations of the flow angle and to variations in the magnitude of the electron drift velocity. The width of the broad spectra, on the other hand, increases rapidly with increasing flow angle and with increasing electron drift velocity. The spectra tend to the asymmetric when the mean line‐of‐sight Doppler velocity exceeds 350 to 400 m s−1. These observations are discussed in terms of the combined effects of the two‐stream and gradient drift instabilities.
Time scales for the decay of induced large‐scale magnetic fields in the Venus ionosphere
Authors: Luhmann J. G. , Russell C. T. , Elphic R. C.
Published: February 2008

Observations from the Pioneer Venus Orbiter magnetometer suggest that a large‐scale magnetic field is induced in the dayside ionosphere when the solar wind dynamic pressure is high. The persistence of the large‐scale field structure is investigated by using realistic models of the neutral atmosphere and ionosphere. Under the assumptions that the magnetic field is horizontal with vertical gradients, the magnetic field evolution is described by a diffusion‐convection equation where the diffusion coefficient depends on the sum of the electron‐neutral and electron‐ion collision frequencies. In this one‐dimensional model the diffusion coefficient and the vertical plasma drift velocity together determine the time scale for the field disappearance. Numerical solutions of the field equation suggest that a large‐scale field in the dayside Venus ionosphere disappears with a time scale of minutes for a vertical velocity of ∼10 m/s, but that the lifetime increases to several hours if the vertical velocity is small. Above ∼200 km altitude, the observed antisolar convection of the ionospheric plasma would cause the field to diminish more rapidly than these diffusion time scales, but at lower altitudes the diffusion process determines the rate of field decay. This result may explain some of the observations of quasi‐steady, large‐scale magnetic fields in the Venus ionosphere during steady solar wind conditions as remnants of fields previously induced by the solar wind interaction.
Ring coupling model: Implications for the ground state of the magnetosphere
Authors: Crooker N. U. , Siscoe G. L. , Doyle M. A. , Burke W. J.
Published: February 2008

The ring coupling model predicts that the radius of the polar cap remains appreciable down to very small values of the transpolar potential Φ and then decreases rapidly to zero as Φ goes to zero. Electric field data from the S3‐2 spacecraft are shown to be consistent with the prediction in that both the theory and the data show a weak dependence of polar cap radius on potential in the observed range. This behavior leads to theoretical insights about the ground state of the magnetosphere. Because the theoretical ground state (Φ = 0) has no polar cap, the model implies that the finite polar cap observed after extended periods of northward interplanetary magnetic field (IMF) is the result of a small potential being generated continually by the solar wind. If it is assumed that this potential is generated on closed magnetic field lines by some viscouslike interaction, and the bulk of potential is generated sporadically by magnetic merging, then the model implies that the polar cap retains open flux during extended periods of northward IMF and that it does so because of the presence of the potential generated by the viscouslike interaction.
On the stability of the ionopause of Venus
Authors: Elphic R. C. , Ershkovich A. I.
Published: February 2008

We examine the stability of the Venus ionopause in light of the importance of gravitation and curvature. Using a one fluid approximation for the equation of motion of the plasma, and ignoring the effects of neutrals, we obtain a dispersion relation that includes the effects of magnetic field, sheared plasma flow, buoyancy, centrifugal force and magnetic tension due to boundary curvature. We find that buoyancy acts to neutralize the flute instability. As expected, the Kelvin‐Helmholtz mode is the dominant instability over most of the dayside ionopause. The expected growth times of this mode are short in comparison with the wave convection time over the boundary; the waves can grow and saturate quickly, producing a turbulent boundary that may affect electrodynamic coupling between the solar wind and ionospheric plasmas.
Aperiodic ion temperature variations in the Io plasma torus
Authors: Eviatar Aharon , Mekler Yuri
Published: February 2008

We consider the observed aperiodic variations of S III temperature reported by ground‐based observers, which take place on time scales short in comparison with the radiation and electron collision relaxation times. We suggest that ion‐ion charge exchange processes are responsible for the heating and ion‐atom collisions are the source of the cooling. We attribute the fluctuations to random strong variations in the output of neutral matter from the volcanoes and surface of Io. Freshly ionized sulfur that will have near full corotation gyroenergy will undergo charge exchange with thermal doubly ionized sulfur and oxygen, thus producing hot S III. Newly injected neutral atoms can cool the ambient hot plasma by collisions on a time scale comparable to their lifetime against ionization processes. Analytic solutions of the temperature rate equations, including the time variation of the neutral and ionized matter density are found to reproduce the observed fluctuations for reasonable values of initial densities.
Kp dependence of auroral zone field‐aligned current intensity
Authors: Robinson R. M.
Published: February 2008

More than 200 hours of Chatanika radar elevation scan data have been used to show that the Pedersen current in the ionosphere is limited to a value that is proportional to the planetary magnetic index Kp. By assuming a simple model for the closure of field‐aligned currents in the ionosphere, we argue that this limit is equal to the latitudinally integrated field‐aligned current intensity. A least‐squares fit to the data gives the limiting current intensity J0 by the following expression: J0 (A/km) = 97 + 71 × Kp. As an extension of this result we use a previously derived relation between polar cap potential drop and Kp to establish a current‐voltage relationship for the magnetospheric generator. We also show how these results can be used to put an upper limit on the Joule heating rate for a given polar cap potential drop.
Saturn's ionosphere: Inferred electron densities
Authors: Kaiser M. L. , Desch M. D. , Connerney J. E. P.
Published: February 2008

During the two Voyager encounters with Saturn, radio bursts were detected that appear to have originated from atmospheric lightning storms. Although these bursts generally extended over frequencies from as low as 100 kHz to the upper detection limit of the instrument, 40 MHz, they often exhibited a sharp but variable low‐frequency cutoff below which bursts were not detected. We interpret the variable low‐frequency extent of these bursts to be due to the refraction and reflection of the radio waves as they propagate through an ionosphere that varies with local time. We obtain estimates of electron densities at a variety of latitude and local time locations. These compare well with the dawn and dusk densities measured by the Pioneer 11 and Voyager radio science investigations and with model predictions for dayside densities. However, we infer a two order of magnitude diurnal variation of electron density, which had not been anticipated by theoretical models of Saturn's ionosphere, and an equally dramatic extinction of ionospheric electron density by Saturn's rings.
On the theoretical/observational comparison of field‐aligned ion beams in the Earth's foreshock
Authors: Schwartz Steven J. , Burgess David
Published: February 2008

We demonstrate that the de Hoffman‐Teller frame, the transformation for which removes the motional V × B electric field while keeping a planar shock at rest, is a logical choice for data analysis. Previous comparisons between local planar theories and observations of backstreaming ion beam velocities are streamlined and related directly to the one physically meaningful angle, between the magnetic field and shock normal, by transforming the data into this frame. Our limited data set suggests that both reflection processes and leakage ones may be operating at different times. A larger survey employing the techniques described here could provide confirmation of our tentative conclusions.
Pitch angle diffusion in the Jovian magnetodisc
Authors: Birmingham Thomas J.
Published: February 2008

Currents in the Jovian middle magnetosphere draw out magnetic field lines radially into a configuration which has high equatorial curvature and weak field strength. Charged particles spiral along field lines into this equatorial region, where, depending on their energies, they may have gyroradii comparable to or larger than the scale length of the field. Magnetic moment violation resulting in a stochastic change in pitch angle may thus occur locally at the equator. Using a linear approximation to the field variation and a mathematical technique for distorting the orbit integration into the complex gyrophase plane, we derive a formula for the nonadiabatic change Δµ in magnetic moment. Δµ varies as cos Ψr where Ψr is particle gyrophase at the equator, so that if a particle traverses the equator several times with different values of Ψr, it suffers randomly correlated values of Δµ. The algebraic formula for Δµ is in excellent agreement with previous numerical computations. The phase space density, averaged over several bounce periods, satisfies a diffusion equation in pitch angle. The diffusion coefficient depends on particle energy, pitch angle, and the field line along which the particles are moving. Energetic sulfur ions (hundreds of keV and higher) are diffused in a few tens of bounce periods beyond 20 RJ, and the behavior of protons is not too dissimilar. Electrons, which at the same energy have much smaller gyroradii, become nonadiabatic and diffusive only near 28 RJ, where the magnetic model predicts a combination of especially weak field and large spatial variation. Pioneer and Voyager particle results are summarized in the light of this diffusion mechanism; while there is evidence for magnetic moment violation, an utter absence of pitch angle structure in the outer magnetodisc region (which should thus follow from the magnetic model) has not been confirmed (or looked for, to a large degree). Implications for other magnetospheric mechanisms are discussed. Finally, the possibility of a similar diffusion in the Saturnian ring current region is addressed, and it is found that only at much higher energies (>100‐MeV protons) is this type of nonadiabatic behavior to be expected.
Adiabatic theory in rapidly rotating magnetospheres
Authors: Cheng Andrew F.
Published: February 2008

The Jovian magnetosphere is a rapid rotator in the sense that the corotation velocity exceeds the gyrovelocity for most of the low‐energy plasma particles. Ordinarily, an electric drift velocity greater than the gyrovelocity will destroy the near periodicity of the bounce motion, so the parallel invariant J will no longer be valid. However, this paper shows that J invariance actually remains valid in rapidly rotating magnetospheres so long as departures from rigid rotation are not too great. Northrop and Birmingham have already proved J invariance for the case of rapid but exactly rigid rotation, and this paper extends their results to nonrigid, rapid rotation with corotation lag and/or convection. These results allow the magnetic field to be arbitrarily asymmetric. The guiding‐center equations in the rotating frame are also used to obtain conditions for plasma corotation in arbitrarily asymmetric but rigidly rotating magnetic fields. The plasma bulk velocity equals the corotation velocity for any trapped particle population that is steady state in the corotating frame provided E + (Ω × r) × B/c = 0 and all first‐order drifts are negligible. These conditions are satisfied, and corotation is expected, for a steady‐state cold plasma on equipotential field lines of any geometry threading a rigidly rotating central conductor.
Electron energization in the geomagnetic tail current sheet
Authors: Lyons L. R.
Published: February 2008

Electron motion in the distant tail current sheet is evaluated and found to violate the guiding center approximation at energies ≳ 100 eV. Most electrons within the energy range ˜10−1 ‐ 10² keV that enter the current sheet become trapped within the magnetic field reversal region. These electrons then convect earthward and gain energy from the cross‐tail electric field. If the energy spectrum of electrons entering the current sheet is similar to that of electrons from the boundary layer surrounding the magneto tail, the energy gain from the electric field produces electron energy spectra comparable to those observed in the earth's plasma sheet. Thus current sheet interactions can be a significant source of particles and energy for plasma sheet electrons as well as for plasma sheet ions. A small fraction of electrons within the current sheet has its pitch angles scattered so as to be ejected from the current sheet within the atmospheric loss cone. These electrons can account for the electron precipitation near the high‐latitude boundary of energetic electrons, which is approximately isotropic in pitch angle up to at least several hundred keV. Current sheet interaction should cause approximately isotropic auroral precipitation up to several hundred keV energies, which extends to significantly lower latitudes for ions than for electrons in agreement with low‐altitude satellite observations. Electron precipitation associated with diffuse aurora generally has a transition at 1–10 keV to anisotropic pitch angle distributions. Such electron precipitation cannot be explained by current sheet interactions, but it can be explained by pitch angle diffusion driven by plasma turbulence.
Remote sensing, eigenanalysis, and information content of cosmic variations
Authors: El‐Raey M. , Sabbah I. S.
Published: February 2008

Remote sensing of the primary component of cosmic 11‐year and diurnal variations over a wide rigidity range is carried out by nonlinear inversion of the Fredholm integral equation relating secondary variations to the primary spectra. Data from three and four stations are simultaneously analyzed, using recently developed coupling functions based on Feynman scaling hypothesis. The range of rigidity over which the retrieval of the primary spectra is possible is investigated. Information content of measured intensities is studied by eigenanalysis of the covariance matrix of kernels of the integral equation. It is concluded that a redundancy of existing stations in the low rigidity range and a lack of stations in the high rigidity range already exist. Implications, suggestions, errors, and possible extensions are discussed.
Temporal evolution of whistler growth in a cold plasma injection experiment
Authors: Ganguli G. , Palmadesso P. , Fedder J.
Published: February 2008

Using a simple time dependent cold plasma density model and assuming a typical ambient radiation belt environment, we study the evolution of the whistler mode turbulence and particle precipitation in a cold plasma release experiment similar to one that may be conducted as part of the Active Magnetospheric Particle Tracer Explorers (AMPTE) program. It is known from earlier work that the release of cold lithium ions can significantly lower the critical energy Ec above which the resonant radiation belt electrons can pitch angle scatter. We study the time evolution of the one pass gain factor for a whistler wave packet and find that for parameters accessible to AMPTE type experiments the gain factor is large enough to ensure strong whistler turbulence and strong pitch angle diffusion of radiation belt particles with energies in the range between the ambient value of Ec and the reduced value of Ec. Estimates for the total power input to the ionospheric footprint of the release are of the order of an erg/cm²/s. This precipitated energy should produce a patch of visible aurora. This effect should also persist for many hours.
High time resolution studies of upstream ions
Authors: Anderson Kinsey A. , Levedahl W. K. , Lin R. P. , Parks G. K.
Published: February 2008

Diffuse upstream ions at low energy (∼1.6 keV) fluctuate rapidly compared to higher energy ions and vary in intensity in a systematic way with ϕ, the angle between the interplanetary magnetic field BI and the earth‐sun direction. The 1.6 keV ion intensity is highest when ϕ is in the range 60° to 90°, and these ions may nearly disappear for values of ϕ near 0°. There is a rather narrow energy range in which the flux remains remarkably constant. At times this energy interval may be centered at an energy as high as 30 keV and at other times as low as 6 keV. This energy appears to vary systematically with the angle ϕ. We have also found that few ions scatter out of the foreshock onto less disturbed interplanetary field lines upstream from the foreshock boundary. Finally, we have found several events in which diffuse ions with energies at least as high as 60 keV are present on field lines that make angles up to 60° with respect to the earth‐sun direction. In these cases a first‐order Fermi process would have only about 3 minutes to increase the energy of the upstream ions. Energization at the bow shock may provide the required additional acceleration. Intermediate upstream ions of 60 keV energy appear even at ϕ angles up to 80°.
Sources for energetic ions at the plasma sheet boundary: Time varying or steady state?
Authors: Williams D. J. , Speiser T. W.
Published: February 2008

Analyses of energetic ions observed at the plasma sheet boundary usually have assumed or inferred the existence of either a time‐varying or a steady state source. Because these assumptions are both important and conflicting, we have investigated the uniqueness with which they can be applied to the energetic ion observations. We have applied the results of assuming either a time‐varying or a steady state source to analytic distribution function mappings and to a computer simulation of the ISEE 1 medium energy particles instrument (MEPI). The predicted MEPI ion signatures then are compared with observations. We find that in general it is not possible to determine the time‐varying or steady state nature of the source with single‐point observations of the ion distribution function. In specific instances, however, the time sequence of the ion distribution may allow such a determination. The likelihood of time variations in various parameters of the models (electric field, magnetic field, boundary velocity, etc.), plus the existence of complex boundary features (e.g., transverse and parallel waves), further complicates the issue, making it very difficult for even multipoint observations to determine the time‐varying or steady state nature of the source. Nevertheless, independent of these assumptions, energetic particle events at the plasma sheet boundary analyzed to date show that the observations are consistent with single‐particle motion from a source several tens of earth radii down the tail which is characterized by an E−γ spectrum. Whether the source is time varying or steady state cannot be uniquely inferred from the data.
The measured motions inside equatorial plasma bubbles
Authors: Hanson W. B. , Bamgboye D. K.
Published: February 2008

A preliminary study of the vertical and north‐south horizontal ion motions in plasma bubbles in the near‐equatorial ionosphere utilizing drift meter data from Atmosphere Explorer E is presented. High‐resolution data show that the vertical ion velocity in some bubbles increases approximately linearly with (No‐N)/N, where No is the background ion concentration and N is the bubble ion concentration. At sufficiently large No/N the vertical ion velocity saturates, but often at a value substantially larger than the ratio of the gravitational acceleration to the ion neutral collision frequency (g/νin), which is the nominal collision‐dominated velocity limit for cylindrical bubbles. These larger than nominal velocities may result from background eastward electric fields and/or from a vertically elongated bubble cross section. The unanticipated observation that large poleward horizontal drifts accompany these vertical drifts seems to follow naturally from a redistribution of plasma along flux tubes as the plasma convects from the bottomside of the F region to high altitudes.
Linear analysis of ion cyclotron interaction in a multicomponent plasma
Authors: Gendrin Roger , Ashour‐Abdalla Maha , Omura Yoshiharu , Quest Kevin
Published: February 2008

The mechanism by which hot anisotropic protons generate electromagnetic ion cyclotron waves in a plasma containing cold H+ and He+ ions is quantitatively studied. Linear growth rates (both temporal and spatial) are computed for different plasma parameters: concentration, temperature, and anisotropy of cold He+ ions and of hot protons. It is shown that (1) for parameters typical of the geostationary altitude the maximum growth rates are not drastically changed when a small proportion (∼1–20%) of cold He+ ions is present and (2) because of the important cyclotron absorption by thermal He+ ions in the vicinity of the He+ gyrofrequency, waves which could resonate with the bulk of the He+ distribution cannot be generated. Therefore quasi‐linear effects, in a homogeneous medium at least, cannot be responsible for the heating of He+ ions which is often observed in conjunction with ion cyclotron waves. The variation of growth rate versus wave number is also studied for its importance in selecting suitable parameters in numerical simulation experiments.
By‐dependent convection patterns during northward interplanetary magnetic field
Authors: Potemra T. A. , Zanetti L. J. , Bythrow P. F. , Lui A. T. Y. , Iijima T.
Published: February 2008

Observations of Birkeland currents, electric fields, and auroral forms in the dayside polar regions during periods of northward interplanetary magnetic field (IMF) can be unified to fit into consistent convection patterns ordered principally by the IMF By component. During periods of weak IMF By, two polar convection cells are symmetrically located on either side of the noon‐midnight meridian, producing sunward convection over the pole, as previously reported. As the IMF By becomes significantly positive (negative), the dawn (dusk) convection cell expands across the northern polar cap, whereas the other cell shrinks. In the southern polar cap it is the dusk (dawn) cell which expands. During strong By this cell expansion gives the appearance of a single convection cell in the polar region. At the “collapsed” cell a large convective flow gradient is developed where reversal of the northward Bz (NBZ) Birkeland current system, antisunward polar ionospheric current, and the most intense polar cap electric fields are statistically observed. We suggest that this convection gradient region is also associated with sun‐aligned arcs and the transpolar arc of the theta aurora, which are observed in the polar regions during northward IMF. The convection patterns proposed here are consistent with the antiparallel merging model in which the IMF merges with the geomagnetic field lines in the tail lobe during northward IMF intervals. The resulting convection in the tail brings plasma from the plasma sheet to the tail. This process divides the open field lines of the lobes and produces an area of closed field lines in the polar cap associated with the transpolar arc of the theta aurora.
Driven magnetic reconnection in three dimensions: Energy conversion and field‐aligned current generation
Authors: Sato Tetsuya , Walker Raymond J. , Ashour‐Abdalla Maha
Published: February 2008

We have analyzed the energy conversion processes occurring in three‐dimensional driven reconnection. In particular, we studied the energy conversion processes during localized reconnection in a taillike magnetic configuration. We find that three‐dimensional driven reconnection is a powerful energy converter which transforms magnetic energy into plasma bulk flow and thermal energy. Three‐dimensional driven reconnection is an even more powerful energy converter than two‐dimensional reconnection, because in the three‐dimensional case, plasmas were drawn into the reconnection region from the sides as well as from the top and bottom. Field‐aligned currents are generated by three‐dimensional driven reconnection. We have identified the physical mechanism responsible for these currents which flow from the tail toward the ionosphere on the dawnside of the reconnection region and from the ionosphere toward the tail on the duskside. The field‐aligned currents form as the neutral sheet current is diverted through the slow shocks which form on the outer edge of the reconnected field lines (outer edge of the plasma sheet).
Correlations between thermospheric density and temperature, solar EUV flux, and 10.7‐cm flux variations
Authors: Hedin A. E.
Published: February 2008

Simultaneously measured thermospheric N2 densities and solar EUV fluxes obtained by the AE‐E satellite are compared with ground‐based solar 10.7‐cm fluxes and calcium plage measurements. Short‐wavelength (coronal) EUV emissions correlate better with thermospheric density than the 10.7‐cm flux, although the reduction in density residuals is small. Correlation of density with a calcium plage index was somewhat worse than with 10.7. Although the overall correlation of 10.7‐cm flux with EUV fluxes is high, the best fit slopes are different for short‐ and long‐term variations, and there are instances where the short‐term behavior of various EUV emissions and that of the 10.7‐cm flux are distinctly different. For both density and EUV, a two‐factor formula based on daily 10.7‐cm flux and a running mean of the 10.7‐cm flux provides a better fit than using the daily 10.7‐cm flux alone.