bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrons in the ionosphere of Titan
Authors: Gan L. , Keller C. N. , Cravens T. E.
Published: February 2008

A theoretical model of the spatial and energy distribution of electrons in the ionosphere of Titan has been constructed using the two‐stream electron transport method and the electron energy equation. The calculated electron spectra show abrupt decreases that can contribute to the “bite‐out” signature observed by the Voyager 1 plasma science instrument. Energy deposition rates in the exosphere of Titan by photoelectrons and magnetospheric electrons are calculated. Calculated N2 EUV airglow emission rates lead to the conclusion that airglow emission due to photoelectron impact is much more important than airglow emission due to magnetospheric electron interactions. Thermal electron temperatures at Titan are calculated for the first time. The electron gas remains well thermalized with the neutral atmosphere for radial distances from the center of Titan less than 3500 km. For radial distances beyond about 4000 km, energy transport dominates the energetics, and the electrons are almost isothermal along magnetic field lines.
Analysis of the ionosphere‐plasmasphere transport of superthermal electrons, 1. Transport in the plasmasphere
Authors: Khazanov George V. , Gombosi Tamas I. , Nagy Andrew F. , Koen Michael A.
Published: February 2008

Analytic solutions to the kinetic equation which describes the transport of superthermal electrons in the plasmasphere are presented. Relations which allow the calculation of the photoelectron energy spectra and the plasmaspheric transparency are obtained. An expression for the heating rate of the thermal electron population by the superthermal electrons is also given.
Intensity variations in the interplanetary magnetic field measured by Voyager 2 and the 11‐year solar cycle modulation of galactic cosmic rays
Authors: Perko J. S. , Burlaga L. F.
Published: February 2008

We present new evidence to support the hypothesis that the 11‐year solar cycle modulation of galactic cosmic rays is caused by strong particle diffusion inside long‐lived, merged interaction regions. These regions are represented by local enhancements in the heliospheric magnetic field strength. To test this hypothesis, we solved the one‐dimensional, force field approximation of the cosmic ray modulation equation. The only variables were the strength of the local magnetic field and the position of the spacecraft, both taken directly from Voyager 2 data. We assume that a constant solar wind speed convects magnetic field compressions and rarefactions unchanged through a model heliosphere. The result is a reasonable simulation of the integrated, high‐energy cosmic ray intensity profile from about 1982 to mid‐1989. This period encompasses both the full recovery portion of the last 11‐year cosmic ray cycle and the first year and a half of the new cycle. In particular, this model responds to the Voyager 2 magnetic field data by correctly timing the beginning of the new modulation cycle in late 1987. We conclude that our hypothesis is consistent with the results of this simulation.
Advances in magnetospheric storm and substorm research: 1989–1991
Authors: Fairfield D. H.
Published: February 2008

Geomagnetic storms represent the magnetospheric response to fast solar wind and unusually large southward interplanetary magnetic fields that are caused by solar processes and resulting dynamics in the interplanetary medium. The solar wind/magnetosphere interaction is, however, more commonly studied via smaller, more common, magnetospheric substorms. Accumulating evidence suggests that two separate magnetospheric current systems are important during magnetospheric substorms. Currents directly driven by the solar wind/magnetosphere interaction produce magnetic field variations that make important contributions to the AE index but have little relation to the many effects traditionally associated with sudden substorm onsets. Currents driven by energy unloaded from the magnetotail form the nightside current wedge and are associated with onset effects such as auroral breakup, field dipolarization, and particle acceleration. Observations are gradually leading to a coherent picture of the interrelations among these various onset phenomena, but their cause remains a controversial question. The abrupt nature of substorm onsets suggests a magnetospheric instability, but doubt remains as to its nature and place of origin. Measurements increasingly suggest the region of 7–10 RE near midnight as the likely point of origin, but it is not clear that the long‐popular tearing mode can go unstable this close to the Earth, where it may be stabilized by a small northward field component. Also the tailward flows that would be expected tailward of a near‐Earth neutral line are seldom seen inside of 19 RE. The changing magnetic field configuration during substorms means that existing static models cannot be used to map phenomena between the magnetosphere and the ground at these interesting times. Although first steps are being made toward creating time dependent models, the lack of such models remains an important limitation to further progress.
Heat flux dropouts in the solar wind and Coulomb scattering effects
Authors: Fitzenreiter R. J. , Ogilvie K. W.
Published: February 2008

Measurements of solar wind electrons at ISEE 3 located 0.01 AU upstream from the Earth (McComas et al., 1989) indicate periods of time when the flux of antisunward suprathermal electrons decreases suddenly, leaving the velocity distribution nearly isotropic and causing the solar wind heat flux to drop. These heat flux dropouts (HFDs) are usually found in regions of increased plasma density and decreased electron temperature, and they are associated with sector boundaries. It has been suggested that HFDs may be due either to disconnection from the Sun of the magnetic flux tube in which they are found, or to enhanced Coulomb scattering of halo electrons in transit from the Sun to the Earth. Using the vector electron spectrometer on ISEE 1, we have found eight intervals of greatly reduced heat flux which appear to be associated with HFDs at ISEE 3. Five of the eight events were delayed by an appropriate convection time and had approximately the same duration as the corresponding ISEE 3 event. The onset times for the other three intervals of reduced heat flux could not be determined in the ISEE 1 data. Other events from the ISEE 3 set were not observed because ISEE 1 either was not in the solar wind or was inside the foreshock, which obscured the HFD identification. Velocity distributions during HFDs at ISEE 1 show that the depletion of halo electrons traveling away from the Sun is most pronounced in the 100‐eV range, while there is essentially no depletion in the 1‐keV range, and that in four cases the magnitude of the halo depletion and its upper velocity limit both depend on the density increase in the HFD. These results are shown to be in agreement with the υ−3 dependence of the Coulomb collision frequency. Thus we conclude that Coulomb scattering effects play a substantial role in at least some heat flux dropout events.
Periodic magnetopause oscillations observed with the GOES satellites on March 24, 1991
Authors: Cahill L. J. , Winckler J. R.
Published: February 2008

The GOES 6 and 7 satellites were in the dayside magnetosphere late on March 24, 1991, when the magnetopause moved in to geosynchronous orbit. Observations on GOES 6 near 1030 local time (LT) indicated six inward and outward periodic movements of the magnetopause past the satellite over a 30‐min interval. Later the magnetopause moved farther in, placing GOES 6 (1100 LT) in the magnetosheath and then moving in past GOES 7, near 1245 LT. The periodic oscillations of the magnetopause at GOES 6 suggest surface waves propagating toward the dawn flank of the magnetopause.
Resolution of temporal and spatial ambiguities of intensity variations within pulsating aurorae
Authors: Kosch M. J. , Scourfield M. W. J.
Published: February 2008

Systems for imaging optical emissions all have a detection sensitivity which, be it uniform or not, can make image interpretation very difficult. A time differential technique involving pixel by pixel subtraction of successive images has been developed to overcome this problem. The technique has been applied to images of pulsating aurorae recorded with a low‐light‐level television system. Individual auroral forms exhibit within a short interval of time a diversity of pulsation modes not evident by direct visual inspection of the television images. The results are important for possible source mechanisms or any meaningful classification scheme for pulsating aurorae.
Properties of mass‐loading shocks, 2. Magnetohydrodynamics
Authors: Zank G. P. , Oughton S. , Neubauer F. M. , Webb G. M.
Published: February 2008

The one‐dimensional magnetohydrodynamics of shocked flows subjected to significant mass loading are considered. Recent observations at comets Giacobini‐Zinner and Halley suggest that simple nonreacting MHD is an inappropriate description for active cometary bow shocks. The thickness of the observed cometary shock implies that mass loading represents an important dynamical process within the shock itself, thereby requiring that the Rankine‐Hugoniot condition for the mass flux possess a source term. In a formal sense, this renders mass‐loading shocks qualitatively similar to combustion shocks, except that mass loading induces the shocked flow to shear. Nevertheless, a large class of stable shocks exist, identified by means of the Lax conditions appropriate to MHD. Thus mass‐loading shocks represent a new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upsteam of weakly magnetized and nonmagnetized planets, has not been discussed in any detail. Owing to the shearing of the flow, mass‐loading shocks can behave like switch‐on shocks regardless of the magnitude of the plasma beta. Thus the behavior of the magnetic field in mass‐loading shocks is significantly different from that occurring in nonreacting classical MHD shocks. It is demonstrated that there exist two types of mass‐loading fronts for which no classical MHD analogue exists, these being the fast and slow compound mass‐loading shocks. These shocks are characterized by an initial deceleration of the fluid flow to either the fast or the slow magnetosonic speed followed by an isentropic expansion to the final decelerated downstream state. Thus these transitions take the flow from a supersonic to a supersonic, although decelerated, downstream state, unlike shocks which occur in classical MHD or gasdynamics. It is possible that such structures have been observed during the Giotto‐Halley encounter, and a brief discussion of the appropriate Halley parameters is therefore given, together with a short discussion of the determination of the shock normal from observations. A further interesting new form of mass‐loading shock is the “slow‐intermediate” shock, a stable shock which possesses many of the properties of intermediate MHD shocks yet which propagates like a slow mode MHD shock. An important property of mass‐loading shocks is the large parameter regime (compared with classical MHD) which does not admit simple or stable transitions from a given upstream to a downstream state. This suggests that it is often necessary to construct compound structures consisting of shocks, slip waves, rarefactions, and fast and slow compound waves in order to connect given upstream and downstream states. Thus the Riemann problem is significantly different from that of classical MHD.
On the evolution of ion conics along the field line from EXOS D observations
Authors: Miyake W. , Mukai T. , Kaya N.
Published: February 2008

The altitude dependence of ion conics is investigated by using EXOS D observations on the dayside below 10,000 km altitude. The cone angle of ion conics tends to decrease with increasing altitude, but not so much as expected from a simple adiabatic model. The conic temperature, on the other hand, tends to increase with increasing altitude. The occurrence frequency of ion conics increases with altitude below 6000 km but is approximately constant above 6000 km. The appearance of newly born conics and the extinction of old conics in the statistics at any altitude could make some contribution, if the appearance and the extinction are large enough, to the observation results: less significant change in cone angle and increasing temperature with altitude, but this effect alone hardly provides a full explanation for the differences in the conic characteristics between the observations and the simple adiabatic model. The results rather seem to reflect the real evolution of an ion conic as ions flow up along the field line, suggesting nonconservation of the adiabatic invariant and the height‐integrated transverse acceleration of ion conics over a wide range of altitude.
Electromagnetic ion/ion cyclotron instability: Theory and simulations
Authors: Winske D. , Omidi N.
Published: February 2008

The linear and nonlinear properties of the electromagnetic ion/ion cyclotron (EMIIC) instability are investigated using linear theory and hybrid simulations. The instability is driven by the relative streaming of two field‐aligned ion beams. In the electrostatic limit it reduces to the well‐known ion beam driven electrostatic ion cyclotron instability, but for finite beta the instability is enhanced and propagates over a broader range of angles with respect to the magnetic field. Linear theory and one‐dimensional hybrid simulations are used to study the characteristics of the instability as a function of beam density, angle of propagation (θ), ion beta, ratio of the streaming velocity to the Alfven speed, electron to ion temperature ratio, and other parameters. Generally, the one‐dimensional calculations show that the instability behaves like its electrostatic counterpart when θ is near 90°, saturating at low levels by heating the ions in the perpendicular direction. At smaller angles the instability is dominated by electromagnetic effects, large‐amplitude waves, and stronger beam coupling. Two‐dimensional hybrid simulations show some evidence for coherent effects due to a narrowing of the wave spectrum during the linear growth stage and a more quasi‐linearlike heating process in the nonlinear phase, eventually yielding similar asymptotic values for plasma parameters. Applications of the EMIIC instability to upstream and downstream waves and ion dissipation at slow shocks in the magnetotail and to ion heating in the plasma sheet boundary layer are briefly discussed.
Theory of magnetohydrodynamic waves: The WKB Approximation revisited
Authors: Barnes Aaron
Published: February 2008

Past treatments of the eikonal or WKB theory of the propagation of magnetohydrodynamic waves have assumed a strictly isentropic background. If in fact there is a gradient in the background entropy, then in second order in the WKB ordering, adiabatic fluctuations (in the Lagrangian sense) are not strictly isentropic in the Eulerian sense. This means that in the second order of the WKB expansion, which determines the variation of wave amplitude along rays, the violation of isentropy must be accounted for. The present paper revisits the derivation of the WKB approximation for small‐amplitude magnetohydrodynamic waves, allowing for possible spatial variation of the background entropy. The equation of variation of wave amplitude is rederived; it is a bilinear equation which, it turns out, can be recast in the action conservation form. It is shown that this action conservation equation is in fact equivalent to the action conservation law obtained from Lagrangian treatments (Dewar, 1970; Jacques, 1977a, b).
Digisonde measurements of polar cap convection for northward interplanetary magnetic field
Authors: Cannon Paul S. , Crowley Geoffrey , Reinisch Bodo W. , Buchau Jurgen
Published: February 2008

Controversy still exists regarding even the average convection pattern when the interplanetary magnetic field (IMF) has a northward component. Using two years of convection data from a Digisonde located at Qaanaaq only 3° from the corrected geomagnetic pole we have examined the diurnal convection flow direction variation in the central polar cap when the IMF is particularly stable. We find that when Bz is positive, and when By positive and By negative data are treated independently, each exhibits a clear diurnal pattern. The patterns are most nearly consistent with a multicell convection model, e.g. Potemra et al. (1984); there are, however, two anomalies. Our synthesized polar cap convection patterns exhibit a polar cap cell centered on 10 corrected geomagnetic local time (CGLT) when By > 1 nT and 13 CGLT when By < −1 nT in contrast to 06 and 18 CGLT predicted by the multicell models. Furthermore, in contrast to the simple multicell models the convection flow patterns for opposite By polarities are not simple mirror images of each other. When By < −1 nT the convection is directed across the central polar cap toward 02 CGLT for much of the day but when By > 1 nT the flow is tangential to the Qaanaaq geomagnetic latitude for much of the day.
On the flow around the diamagnetic cavity of comet Halley
Authors: Israelevich P. L. , Ershkovich A. I. , Ioffe Z. M.
Published: February 2008

The ion‐neutral friction (including mass loading) happens to be insufficient to support the magnetic barrier at comet Halley under conventional parameters of ion‐neutral interaction. It is shown that a centrifugal force resulting from the transverse plasma motion should be added to the momentum balance in order to explain the observed magnetic field structure. The radial profile of the transverse plasma velocity is derived from the best fit with magnetic field data. The transverse velocity reaches ∼8 km s−1 at 10,000 km from the nucleus. This value depends little on both the ion‐neutral momentum exchange rate and the comet production rate, whence one may conclude that it is centrifugal force which dominates in the momentum balance at this distance.
The effects of electrostatic forces and random velocities on dust ring thickness profiles in rings with a dust size distribution
Authors: Aslaksen Torsten K. , Havnes Ove
Published: February 2008

We have considered vertical density profiles of dust rings with a dust size distribution. The density profiles are determined by random vertical velocities of the dust, the vertical component of gravity, and the electrostatic lifting force on the charged dust due to the self‐screening electric field of the dust ring. We reconsider the case of electrostatically supported dust rings with monosized dust and no random vertical velocity (Havnes and Morfill, 1984). We thereafter look at a pure electrostatically supported dust ring with dust size distribution, which we find will be perfectly separated according to dust size with the largest dust particles in the ring plane and the smallest and lightest furthest above the plane. When the dust particles are given a random velocity, this leads to an increasing mixing of the different dust sizes as the random velocity is increased. The electrostatic effect will still lead to separation of dust particles according to size, and we find that a general trend in dust rings will be the existence of a halo of small dust particles with a degree of mixing of dust sizes near the ring plane which is dependent on the dust random kinetic energy and on the dust and plasma parameters.
The Venus transterminator ion flux at solar maximum
Authors: Knudsen William C. , Miller Kent L.
Published: February 2008

The median Venus transterminator ion flux nv(h) derived from Pioneer Venus retarding potential analyzer measurements of the ion velocity and ion density during a period of solar maximum is presented as a function of altitude h from 150 km to the solar maximum median ionopause altitude of 800 km. This flux is integrated with altitude to provide the integrated total ion flux S(h) crossing the terminator plane as a function of altitude. S(h) is nearly a linear function of h, reflecting the nearly constant value of nv with altitude. The value of S at 800 km altitude, called F, is 5×1026 s−1, a value equal to the total nightside hemispheric molecular ion recombination rate L calculated previously. The loss of ions down the wake of Venus is smaller than F or L by at least a factor of 5. Thus the equality of F and L is evidence that the dominant Venus nightside ionization source at solar maximum is transterminator ion transport.
Correlation dimension of underground muon time series
Authors: Bergamasco L. , Serio M. , Osborne A. R.
Published: February 2008

This paper presents results obtained for the correlation dimension of cosmic ray muon time series detected at a depth underground of 570 hg/cm² over a period of 8 years. The corresponding galactic primaries have rigidities of the order of a few teravolts and are therefore at least partially under the influence of solar modulation effects. The principal indication is that the physical mechanisms which shape the muon time series in the frequency range below 2 × 10−5 Hz are entirely stochastic in behavior and have a monofractal structure. The correlation dimension is finite and shows an inverse dependence on the level of solar activity, oscillating between 4.5 ± 0.4 at solar maximum and 6.2 ± 0.6 at solar minimum.
Wave properties near the subsolar magnetopause: Pc 3–4 energy coupling for northward interplanetary magnetic field
Authors: Song P. , Russell C. T. , Strangeway R. J. , Wygant J. R. , Cattell C. A. , Fitzenreiter R. J. , Anderson R. R.
Published: February 2008

Strong slow mode waves in the Pc 3–4 frequency range are found in the magnetosheath close to the magnetopause. We have studied these waves at one of the ISEE subsolar magnetopause crossings using the magnetic field, electric field, and plasma measurements. We use the pressure balance at the magnetopause to calibrate the Fast Plasma Experiment data versus the magnetometer data. When we perform such a calibration and renonnalization, we find that the slow mode structures are not in pressure balance and small scale fluctuations in the total pressure still remain in the Pc 3–4 range. Energy in the total pressure fluctuations can be transmitted through the magnetopause by boundary motions. The Poynting flux calculated from the electric and magnetic field measurements suggests that a net Poynting flux is transmitted into the magnetopause. The two independent measurements show a similar energy transmission coefficient. The transmitted energy flux is about 18% of the magnetic energy flux of the waves in the magnetosheath. Part of this transmitted energy is lost in the sheath transition layer before it enters the closed field line region. The waves reaching the boundary layer decay rapidly. Little wave power is transmitted into the magnetosphere.
An investigation into the nonlinear frequency shift in magnetospherically propagated VLF pulses
Authors: Shklyar D. R. , Nunn D. , Smith A. J. , Sazhin S. S.
Published: February 2008

When ducted narrow‐band VLF pulses pass through the Earth's magnetosphere, small frequency shifts of ∼1 Hz may be observed at the receiving site on the ground. Such shifts may be due to Doppler effects, but nonlinear electron cyclotron resonance near the magnetospheric equator may also play a part. This paper is a study of nonlinear frequency shifts induced by wave‐particle interaction effects. The development assumes strong nonlinearity and the dominance of particle trapping. The VLF pulses are considered to be much shorter than the scale length of the interaction region; however, for long pulses the results remain valid for the front part of the pulse as seen at the receiving site. Numerical simulations for the NAA and Siple transmitters are performed using a Vlasov code. The results are in general agreement with the theoretical analysis. Experimental observations are reviewed, and future active experiments are suggested.
Radial profiles of quiet time magnetospheric parameters
Authors: Lui A. T. Y. , Hamilton D. C.
Published: February 2008

Magnetospheric plasma parameters during geomagnetically quiet conditions are studied with ion measurements covering energies from ∼1 keV to ∼4 MeV from the Charge Composition Explorer. It is found that the observed quiet time plasma pressure in the midnight sector is comparable (within a factor of 2) to that deduced by Spence et al. (1989) and Kan et al. (1992) from inverting the magnetic field models of Tsyganenko and Usmanov (1982) and Tsyganenko (1987). The radial profile of the total plasma pressure shows a peak generally at L ≈ 3 to 4 and decreases from L ≈ 4 to L ≈ 9 rather monotonically. No large‐scale earthward decrease in plasma pressure occurs in these outer L shells, contrary to some theoretical expectation for the magnetospheric closure of the region 2 field‐aligned current system. The plasma pressure within L ∼ 9 is generally anisotropic with a larger pressure component perpendicular to the magnetic field than parallel to the field. This anisotropy tends to increase with smaller L. While the anisotropy is always below the fire hose instability threshold during all quiet time passes studied, it is near the mirror instability threshold in the midnight sector at large L shells due mostly to the large value of plasma beta (>1) there. The radial profile of the azimuthal volume current density is also computed, with the result suggesting that the current density associated with the quiet time ring current population is distributed broadly over the L shells between L ≈ 3.5 and L ≈ 7 with average values in the range of ∼1 to 4 nA/m² and peak values in the range of ∼4 to 8 nA/m². Another current region situated beyond L ≈ 7, which may be related to the inner portion of the quiet time cross‐tail current, is also apparent and has average and peak current density values similar to those of the ring current.
A scenario for solar wind penetration of Earth's magnetic tail based on ion composition data from the ISEE 1 spacecraft
Authors: Lennartsson W.
Published: February 2008

Energetic (0.1‐16 keV/e) ion data from the Plasma Composition Experiment on the ISEE 1 spacecraft show that Earth's plasma sheet (inside of 23 RE) always has a large population of H+ and He++ ions. This population is the largest, both in absolute numbers and relative to the terrestrial ions, during periods of extremely weak geomagnetic activity and is then also the most “solar wind like”, in the sense that the He++/H+ density ratio is at its peak (about 3% on average in 1978 and 79), and the H+ and He++ ions have mean (thermal) energies that are proportional to ionic mass and barely exceed the typical bulk flow energies in the solar wind. These quiet time conditions occur when the IMF is persistently northward and not excessively strong. At more active times the H+ and He++ ions are heated in the central plasma sheet, but they can still be found with lower solar wind like energies closer to the tail lobes, at least during plasma sheet thinning. In a number of cases examined here, such low‐energy H+ and He++ ions are found flowing antisunward close to the lobes, flowing roughly parallel (or antiparallel) to the tail magnetic field at speeds ranging from a few tens to a few hundred kilometers per second, as though they have recently entered the tail earthward of the ISEE 1. In some cases the flow vector appears to contain a significant E × B component directed away from the nearest tail flank. In order to explain these flows, as well as the solar wind like appearance of the plasma sheet as a whole during northward interplanetary magnetic field, it is argued that the solar plasma enters along slots between the tail lobes and the plasma sheet, even quite close to Earth, convected inward along the plasma sheet boundary layer, or adjacent to it, by the electric fringe field of the ever present low‐latitude magnetopause boundary layer (LLBL). The main point of the argument is that the required E × B drifts are produced by closing LLBL equipotential surfaces through the plasma sheet.
Remote sensing of auroral E region plasma structures by radio, radar, and UV techniques at solar minimum
Authors: Basu Sunanda , Basu S. , Eastes R. , Huffman R. E. , Daniell R. E. , Chaturvedi P. K. , Valladares C. E. , Livingston R. C.
Published: February 2008

The unique capability of the Polar BEAR satellite to simultaneously image auroral luminosities at multiple ultraviolet (UV) wavelengths and to remote sense large‐scale (hundreds to tens of kilometers) and small‐scale (kilometers to hundreds of meters) plasma density structures with its multifrequency beacon package is utilized to probe the auroral E region in the vicinity of the incoherent scatter radar (ISR) facility near Sondrestrom. In particular, we present coordinated observations on two nights obtained during the sunspot minimum (sunspot number <10) January–February 1987 period when good spatial and temporal conjunction was obtained between Polar BEAR overflights and Sondrestrom ISR measurements. With careful coordinated observations we were able to confirm that the energetic particle precipitation responsible for the UV emissions causes the electron density increases in the E region. These E region electron density enhancements were measured by the ISR at Sondrestrom. The integrations up to the topside of these ISR electron density profiles were consistent with the total electron content (TEC) measured by the Polar BEAR satellite. An electron transport model was utilized to determine quantitatively the electron density profiles which could be produced by the particle precipitation, which also produced multiple UV emissions measured by the imager; these profiles were found to be in good agreement with the observed ISR profiles in the E region. Surprisingly large magnitudes of phase and amplitude scintillations were measured at 137 and 413 MHz in the regions of TEC enhancements associated with the particle precipitation. Steep phase spectral slopes with spectral index of 4 were found in these regions. Strength‐of‐turbulence computations utilizing the ISR electron density profiles and observed characteristics of phase and amplitude scintillations are interpreted in terms of an irregularity amplitude varying between 10 and 20% at a several‐kilometer outer scale size in the E region extending approximately 50 km in altitude. This outer scale size is also consistent with the measured phase to amplitude scintillation ratio. An estimate of the linear growth rate of the gradient‐drift instability in the E region shows that these plasma density irregularities could have been generated by this process. The mutual consistency of these different sets of measurements provides confidence in the ability of the different techniques to remote sense large‐ and small‐scale plasma density structures in the E region at least during sunspot minimum when the convection‐dominated high‐latitude F region is fairly weak.
The dependence of the Martian magnetopause and bow shock on solar wind ram pressure according to Phobos 2 TAUS ion spectrometer measurements
Authors: Verigin M. I. , Gringauz K. I. , Kotova G. A. , Remizov A. P. , Shutte N. M. , Rosenbauer H. , Livi S. , Richter A. , Riedler W. , Schwingenschuh K. , Szegő K. , Apáthy I. , Tátrallyay M.
Published: February 2008

The location of the Martian magnetopause and that of the bow shock are studied on the basis of three‐dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits. The clear and strong dependence of the areomagnetopause position on solar wind ram pressure was revealed, while the position of the bow shock was practically independent of this parameter. In the power law expression telling the dependence of the Martian magnetotail thickness D on the solar wind ram pressure: D∼(ϱυ²)−1/k, the power index turned out to be k∼5.9±0.5. The close coincidence of this index with k = 6 for a dipole geomagnetic field, and the large areomagnetotail thickness compared with the planetary diameter, suggest that an intrinsic dipole magnetic field is likely to be an important factor in the solar wind interaction with Mars. On the other hand, the relatively stable position of the subsolar point of the Martian magnetopause and unambiguous induction effects observed by the Phobos 2 MAGMA magnetic experiment in the magnetotail indicate the essential role of an induced magnetic field, too. The weak dependence of the terminator bow shock position on the solar wind ram pressure may be related to the relatively stable position of the subsolar magnetopause.
Electric field fluctuations and charged particle precipitation in the cusp
Authors: Matsuoka A. , Tsuruda K. , Hayakawa H. , Mukai T. , Nishida A. , Okada T. , Kaya N. , Fukunishi H.
Published: February 2008

In the cusp region, irregular fluctuations of the electric field are often observed by EXOS D at altitudes of several thousands of kilometers. Amplitudes of the fluctuations sometimes reach 100 mV/m and their spectra are broad. The electric to magnetic field ratios in the frequency range of 0.5‐3 Hz agree well with the Alfvén velocity at observation point. Hence the electric fluctuations are considered to be Alfvén waves. The flux of precipitating ions at 500 eV to 10 keV and that of the electrons at 70‐500 eV are enhanced in the cusp and they are well correlated to each other. On the other hand, correlation coefficients between the power spectral density (PSD) of the electric field at 1 Hz and the precipitating particle flux vary from case to case. When the latitude of the cusp is low and IMF is expected to be southward the coefficient is high. This suggests that the waves are generated in association with the injection of particles into the magnetosphere when reconnection occurs. On the other hand, when the latitude of the cusp is high and IMF is expected to be northward, the coefficient is low and the PSD of the electric field is smaller for the same flux of particles than when IMF is southward. In these cases the intensity of the electric fluctuations in the region of the particle injection is possibly not so great as that when reconnection occurs.
The role of magnetic loops in particle acceleration at nearly perpendicular shocks
Authors: Decker R. B.
Published: February 2008

We consider a system in which a planar shock propagates into a plasma where the magnetic field consists of a uniform background component that is nearly parallel to the shock surface (i.e., the shock is nearly perpendicular) plus a random component that is transverse to the background field. The random component is a superposition of Alfvén waves with a wide range of wavelengths and with amplitudes determined from a power spectral density function that is flat at long wavelengths and a power law at short wavelengths. We investigate the characteristics of superthermal ion (proton) distributions accelerated in this system by integrating along particle orbits. The main difference between results reported herein and those reported previously by us is the major role played by magnetic loops in mediating multiple shock interactions. The transverse magnetic fluctuations are composed of short wavelength components that scatter the ions in pitch angle, and long wavelength components that produce a large‐scale spatial meandering of magnetic field lines. At nearly perpendicular shocks, this meandering results in a distribution of transient magnetic loops of various sizes at the shock. Charged particles are guided by the large‐scale spatial variations and can interact with the shock at many points, becoming temporarily trapped along several such loops, and also being scattered by resonant waves during and between shock interactions. A few particles can acquire large energy gains before the field lines along which they propagate convect through the shock. As a specific application of the model, we investigate the acceleration of a monoenergetic seed proton population for both impulsive and continuous time injection at a shock that is on average perpendicular. In the model the typical energy density of the magnetic fluctuations is 10–20% that of the background field. Characteristic features of the accelerated proton distributions are (1) energy spectra with power law tails extending to at least 1000 times the injection energy, with spectral slopes somewhat steeper than those predicted, for example, by diffusive shock acceleration theory; (2) upstream intensity precursors that grow exponentially towards the shock, with e‐folding scales that increase with energy up to a maximum scale of the order of the spatial scale of the meandering field lines; (3) upstream angular distributions with large field‐aligned bidirectional anisotropies throughout the intensity precursor; (4) downstream angular distributions with weaker anisotropies peaked transverse to the magnetic field within an intensity plateau extending from the shock to the convection boundary, and field‐aligned bidirectional distributions beyond the convection boundary. We develop a simplified model that explains the salient features of the upstream precursors in terms of the statistical properties of the long‐wavelength components that comprise the random field and suggest how electron acceleration can be accommodated in our model.
On the stability of nongyrotropic ion populations: A first (analytic and simulation) assessment
Authors: Brinca A. L. , de Água L. Borda , Winske D.
Published: February 2008

The wave and dispersion equations for perturbations propagating parallel to an ambient magnetic field in magnetoplasmas with nongyrotropic ion populations show, in general, the occurrence of coupling between the parallel (left‐ and right‐hand circularly polarized electromagnetic and longitudinal electrostatic) eigenmodes of the associated gyrotropic medium. These interactions provide a means to driving linearly one mode with free‐energy sources of other modes in homogeneous media. Different types of nongyrotropy bring about distinct classes of coupling. The stability of a hydrogen magnetoplasma with anisotropic, nongyrotropic protons that only couple the electromagnetic modes to each other is investigated analytically (via solution of the derived dispersion equation) and numerically (via simulation with a hybrid code). Nongyrotropy enhances growth and enlarges the unstable spectral range relative to the corresponding gyrotropic situation. The relevance of the properties of nongyrotropic populations to space plasma environments is also discussed.
Geomagnetic substorm association of plasmoids
Authors: Moldwin Mark B. , Hughes W. Jeffrey
Published: February 2008

The relationship of geomagnetic substorms and plasmoids is examined by determining the correlation of the 366 plasmoids identified by Moldwin and Hughes (1992) with ground auroral zone magnetograms and geosynchronous particle data signatures of substorm onsets. Over 84% of the plasmoid events occurred between 5 and 60 min after a substorm onset. We also find near one‐to‐one correlation between large isolated substorm signatures in the near‐Earth region and signatures consistent with a passing plasmoid in the distant tail (i.e., a traveling compression region, or an actual plasmoid observation). However, there does not appear to be an absolute correspondence of every substorm onset to a plasmoid signature in the deep tail especially for periods of prolonged disturbance that have multiple substorm onsets. A correlation of interplanetary magnetic field Bz south with plasmoid observations was also found. The locations of the near‐ and far‐Earth reconnection sites are estimated using the time of flight of the plasmoids from substorm onset to their observation at ISEE 3. The estimates of the near‐ and far‐Earth reconnection sites are highly variable and range from 10 to 140 RE.
Mass density and pressure changes across the dayside magnetopause
Authors: Fuselier S. A. , Shelley E. G. , Klumpar D. M.
Published: February 2008

Plasma composition measurements at ∼2‐min time resolution from 27 magnetopause crossings are used to determine if mass density ρ, modified by the anisotropic pressure term (1‐α), is constant across the magnetopause. In the process of testing this relation across the magnetopause using the low time resolution measurements, several new properties of the discontinuity and its layers are presented. Pressure and mass density in the magnetosheath and low‐latitude boundary layer were found to be dominated by H+ except on one occasion (out of the 27 crossings) when O+ was the dominant contributor to the total mass density in the low‐latitude boundary layer. Using these measurements, it is found that ρ(1‐α) is not constant for almost all magnetopause crossings even when there is independent evidence for magnetic reconnection and an open magnetopause. These results, when combined with previous observations at much higher time resolution, indicate that the reason why ρ(1‐α) is not constant across the magnetopause is that the H+ density decrease from the magnetosheath to the low‐latitude boundary layer is not compensated for by either a change in the pressure anisotropy or a change in the total mass density and suggest that the magnetopause can never be described as a time‐stationary one‐dimensional rotational discontinuity.
Effect of AC electric field on the whistler mode instability in the magnetosphere
Authors: Misra K. D. , Haile Tigistu
Published: February 2008

The dispersion relation for the whistler mode instability in an infinite bi‐Maxwellian magnetoplasma in the presence of a parallel ac electric field has been derived using the method of characteristic solutions. The perturbed and unperturbed particle trajectories have been used to determine the perturbed distribution function. The growth rate has been determined. The earlier known results are recovered under suitable approximations, when the ac field amplitude and frequency are set to zero. It is found that the ac field parameters modify the growth rate and the resonance condition of the charged particles. The growth rate for various frequencies and magnitudes of the ac field, suitable to magnetospheric conditions, has been evaluated. It has been found that the growth rate depends significantly on the variation of the frequency rather than on the variation of the magnitude.
A search for geomagnetic storm evidence of the reversal of the solar dipole magnetic field and interplanetary Bz
Authors: Kahler S.
Published: February 2008

The axis of the solar dipole magnetic field is aligned to within 30° of the solar rotational axis for up to 2 years during solar minima. Coronal mass ejections (CMEs) during those periods arise from the equatorial streamer belts and should share the magnetic orientation of the dipole field. If those field orientations are maintained in interplanetary space, CMEs producing geomagnetic storms should be characterized by southward Bz during minima when the fields point outward in the northern solar hemisphere and by northward Bz at alternate minima when the solar dipole is reversed. Since southward Bz is an important factor in producing geomagnetic storms, we should expect that storms during minima characterized by southward Bz are significantly larger than those during the alternate minima. Storm data from 10 solar minima are used to test this hypothesis. The test yields a null result.
Parameterized study of the ionospheric modification associated with sun‐aligned polar cap arcs
Authors: Crain D. J. , Sojka J. J. , Schunk R. W. , Zhu L.
Published: February 2008

Sun‐aligned (SA) arcs are a prevalent feature of the polar cap ionosphere during northward interplanetary magnetic field conditions. These arcs are, like the auroral arc, a complex electrodynamic system coupling the ionosphere and magnetosphere. The electron precipitation and convection electric field associated with this system modify the polar cap ionospheric plasma distribution. In the past decade, a wealth of observational information has indicated the complexity of these ionospheric modifications, but only a limited number of model studies have been carried out to elucidate the changes associated with SA arcs. In this investigation, an extensive parametric study has been conducted to identify and order these ionospheric modifications. Specifically, we are interested in modeling the local evolution and decay of ionospheric modification due to a source consisting of an idealized, though representative, SA arc structure. The Utah State University time‐dependent ionospheric model (TDIM) has been used to define the ionospheric physics and its dependencies on both solar cycle and season. An SA arc model has been developed. This model enables one to systematically vary the key arc parameters; electric field strength, width, electron precipitation energy flux, and electron precipitation characteristic energy. Our findings indicate that the changes associated with the E and F regions are quite different. F region structures have relatively long lifetimes and are significantly affected by transport. This leads to a significant role for cross‐arc convection. The E region is much less affected by transport due to short chemical recombination times and thus, E region modifications are confined to the arc structure itself where precipitation and enhanced electric fields are present. Furthermore, the response of the F region to changes in the arc parameters is not linear, and the largest ionospheric modifications occur for intermediate arc widths and electric field strengths. An important indicator of F region modification is the total electron content (TEC) for flux tubes convecting through SA arc structures. The magnitude and variation of the TEC enhancement with various arc parameters can be an important indicator of real processes which are occurring in the arc structure. In addition to TEC we examine the variation of Te, Ti, Ne, ΣP, and ΣH for various arc parameters.
Energy storage and dissipation in the magnetotail during substorms: 1. Particle simulations
Authors: Winglee R. M. , Steinolfson R. S.
Published: February 2008

The dynamics of the magnetotail during substorms is controlled by two external influences: the pressure in the magnetospheric boundary layer and the strength of the dawn‐to‐dusk electric field. Pressure pulses are often used in MHD models of the tail to drive enhanced reconnection while particle models often invoke the dawn‐to‐dusk electric field to produce the observed particle acceleration. Multiscale length electromagnetic particle simulations are presented which self‐consistently investigate the influences on the tail of these processes and their coupling. It is shown that, while both processes can lead to the compression of the current sheet, they do so in different ways. Pressure pulses can produce local pinching of the magnetic field, which can in turn lead to reconnection and the tailward ejection of a plasmoid, similar to MHD models. However, there is no net energy stored in the magnetic field nor any strong earthward flows induced during the growth phase, contrary to observations. On the other hand, increases or surges in the electric field can produce significant storage of energy in the magnetic field as well as strong earthward flows. However, plasmoid formation is unlikely for typical values of the dawn‐to‐dusk electric field. It is argued that these two processes are not independent but are rather coupled through the conductivity of the system. This conductivity is shown to decrease with increasing magnetic field strength as the current sheet is compressed. Thus when the pressure increases in the magnetospheric boundary layer, the dawn‐to‐dusk electric field must increase if the current system is to be sustained. When this coupling is incorporated, the model is able to more comprehensively describe the development of substorms, including earthward flows, enhanced energy storage in the magnetic field during the growth phase and the formation and tailward ejection at high speeds of plasmoids in association with the production of high‐energy particles and intense field‐aligned currents.