bluuebunny/crossref_metadata_2025 huggingface dataset:
A new theory for magnetospheric substorms
Authors: Lyons L. R.
Published: February 2004

It is proposed here that the expansion phase of substorms results from a reduction in the large‐scale electric field imparted to the magnetosphere from the solar wind, following a ≳ 30‐min growth phase due to an enhancement in this electric field. The reduction in the electric field is assumed to propagate antisunward within the magnetosphere. Triggering by a reduction in the electric field is suggested by the observation that substorms are often triggered by northward turnings of the interplanetary magnetic field (IMF). However, under the theory presented here, substorms may be triggered by anything that causes an electric field reduction such as a reduction in the magnitude of the y component of the IMF. A reduction in the large‐scale electric field disrupts both the inward motion and energization of plasma sheet particles that occurs during the growth phase. It is suggested here that this can lead to formation of the expansion phase current wedge and active aurora. The current wedge results from the magnetic drift of ions, which has a speed proportional to particle energy, and a large azimuthal gradient in mean particle energy that is expected to develop in the vicinity of magnetic midnight during the growth phase. Current wedge formation will most likely be initiated near the radial distance (∼6‐10 RE) of the peak in the growth phase plasma pressure distribution, and then propagate tailward from that region. Order‐of‐magnitude calculations show that the above proposal can account for the rapid development of the expansion phase relative to the growth phase, the magnitude of the reduction in the cross‐tail current within the current wedge, the speeds of tailward and westward expansion of the current reduction region, the speeds of poleward and westward motion of active aurora in the ionosphere, and the magnitude of wedge field‐aligned currents that connect the ionospheric region of active auroral to the divergent cross‐tail current within the magnetosphere.
Steady state magnetic reconnection at high magnetic Reynolds number: A boundary layer analysis
Authors: Jamitzky Ferdinand , Scholer Manfred
Published: February 2004

A boundary layer analysis of two‐dimensional current sheets appearing in magnetic reconnection is presented in the framework of resistive incompressible two‐dimensional magnetohydrodynamics. The coordinate normal to the current sheet is rescaled by a power of the resistivity in order to obtain inner limit equations which satisfy the symmetry conditions and represent magnetic annihilation to lowest order. They can be matched to the outer expansion of the incompressible resistive MHD equations with a domain of validity smaller than the current sheet length. The magnetic vector potential and the stream function are expanded in powers of the inverse magnetic Reynolds number Rm. For the inner limit equations, four cases are found depending on the boundary conditions in the overlap region. These cases correspond to different scalings of the velocity and the magnetic field which are equivalent to different prescribed boundary conditions of the magnetic field and plasma velocity. For the case of the magnetic field dominating over the flow, inner and outer solutions are obtained up to the second order in the expansion parameter and are matched in the intermediate region. Scaling laws for the width of the current sheet are derived: It decreases for increasing magnetic Reynolds number. The characteristic length for growth of the magnetic field component normal to the current sheet increases with magnetic Reynolds number. A Sweet‐Parker scaling is found for the external reconnection rate.
Preliminary nonlocal analysis of cross‐field current instability for substorm expansion onset
Authors: Lui Anthony T. Y. , Chang Chia‐Lie , Yoon Peter H.
Published: February 2004

The modified‐two‐stream instability (MTSI) is one of the modes of the cross‐field current instability which has recently been considered as one plausible mechanism for substorm expansion onset and intensification. This paper extends the previous analysis of this instability to nonlocal treatment under the simplifying assumptions of electrostatic perturbations and neglect of thermal and kinetic effects. The result enables us to gain new insights on the onset threshold of the instability. We find that a current sheet with a constant velocity associated with the current (exemplified by the Harris equilibrium current sheet) is stable with respect to MTSI, whereas one with a prominent velocity peak at its center is unstable when it is sufficiently thin. This result therefore indicates that the onset threshold of the instability is very sensitive to the global characteristics of the current sheet, and its value can be determined only through nonlocal analysis. The higher onset threshold than that predicted by the local theory allows one to better understand why a substantial cross‐tail current enhancement can develop and a thin current sheet can remain stable for an extended period before the onset of substorm expansion and current disruption.
Ion density, temperature, and composition of the Venus nightside ionosphere during a period of moderate solar activity: Implications for maintaining the central nightside
Authors: Spenner K. , Knudsen W. C. , Lotze W.
Published: February 2004

Ion density, temperature, and the partial densities of the main constituents of the Venusian nightside ionosphere during moderate solar activity are presented. The data were measured by the retarding potential analyzer of the Pioneer Venus Orbiter during the entry phase of the mission in 1992. The highly variable plasma density is, on average, clearly reduced relative to that measured at high solar activity. A significant dawn‐dusk asymmetry, an excess of H+ at dawn and O+ at dusk, occurs in the postterminator sector. In the central nightside sector, beyond 150° solar zenith angle, the ionosphere is strongly depleted. The ion temperature distribution is similar to that measured during high solar activity, but the values are slightly smaller. The measurements indicate that the ion flux across the terminator, which is the dominant maintenance source for the nightside ionosphere at high solar activity, decreases with decreasing solar activity. At moderate solar activity we find that plasma transport and particle precipitation contribute approximately equally to the ionization of the central sector.
In situ observations of magnetotail reconnection prior to the onset of a small substorm
Authors: Sergeev V. A. , Angelopoulos V. , Mitchell D. G. , Russell C. T.
Published: February 2004

On April 15, 1979 at 0645 UT, several minutes prior to the expansion phase onset of a weak substorm, the ISEE 1 and 2 spacecraft were located 16 RE downtail and close to local midnight. The two spacecraft were on opposite sides of the current sheet, allowing us to better constrain the possible interpretations of the available plasma, magnetic field, and energetic particle data. Plasma sheet acceleration occurring earthward of the spacecraft was evident in the energetic particle data 2–3 min before the first ground‐based signatures of substorm onset. The ensuing magnetic field and particle changes are consistent with an X‐type neutral line moving tailward of ISEE 1 and 2 with an apparent speed of ∼30 km/s, while the plasma sheet thickness decreased to a minimum of less than 0.2 RE. Although the substorm effects at geosynchronous altitude and on the ground were weak, the reconnection rate was locally quite intense (>1 mV/m) and produced earthward bursty bulk flows of peak velocity >600 km/s. Ion velocity‐space distributions confirm that the flows are indeed convective flows of a single‐ion population and not counterstreaming beams, as is often the case at the plasma sheet boundary. The fast tailward flows observed at the initial stage of reconnection were localized to a thin layer surrounding the neutral sheet. The magnitude of the current disruption and the region 1 sense field‐aligned currents associated with reconnection were sufficiently large to explain the intensity of the substorm current wedge, as inferred from ground magnetometers.
Acceleration of electrons by a nearly perpendicular curved shock wave: 1. Zero shock thickness
Authors: Vandas M.
Published: February 2004

From the Earth's bow shock observations and theoretical modeling it is known that electrons are accelerated by a nearly perpendicular shock wave. The theory of acceleration is well established for the case of a plane shock wave. In this paper we revisit the role of a shock curvature. We describe our model of a curved shock wave with zero thickness in two and three dimensions (cylindrical and spherical) and compare the results of acceleration by plane and curved shock waves. We present a method for the calculation of electron acceleration by a curved shock wave of a general shape. We show that the acceleration by the spherical shock can be reduced to the acceleration by the cylindrical shock, but this is not true for a general shock (like the Earth's bow shock). We verified the approximation of a nearly perpendicular shock by a tangent plane with variable magnetic field normal component. Even though the zero shock thickness is not very realistic for electron acceleration, it enables us to elaborate methods and test ideas that will be applied in future work on the problem of nonzero shock thickness.
Computer experiments of spacecraft‐plasma interactions in a dilute and high‐βe plasma with a fast plasma flow
Authors: Okada M. , Omura Y. , Matsumoto H.
Published: February 2004

We study an electromagnetic environment around a spacecraft in the solar wind plasma, where the Debye length λD is much larger than the scale size of the spacecraft Ro. We perform computer experiments using a two‐dimensional electromagnetic particle code (KEMPO) with a half open boundary model whose internal boundary corresponds to the spacecraft. We show formation of an ion acoustic shock around the spacecraft and clarify its parametric dependence on the background plasma parameters. Scale size of the shock is about 3 ∼ 4λD. The shock formation is observed when the solar wind proton temperature is lower than the electron temperature (Ti/Te < 1). We also investigate the effects of electron secondaries from the surface of the spacecraft, such as photoelectrons and/or secondary electrons emitted by the solar wind proton impact. Our results show that the electron secondaries play an important role in the shock formation. The shock disappears when the temperature of the electron secondaries is comparable with the solar wind electron temperature Ts ∼ Te.
Equatorial plasma fountain and its effects: Possibility of an additional layer
Authors: Balan N. , Bailey G. J.
Published: February 2004

The importance of diffusion, perpendicular electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University plasmasphere‐ionosphere model for the ionosphere above Jicamarca under magnetically quiet equinoctial conditions at medium solar activity. The effects of the fountain, which include the equatorial anomaly, are also investigated. As expected, the F region electrodynamic (E×B) drift generates the plasma fountain and the anomaly, which are symmetric with respect to the magnetic equator. The neutral wind introduces asymmetries, with larger plasma flow (toward the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. During daytime, when the drift is upward, the fountain rises to about 800 km altitude at the equator and covers about ±30° magnetic latitude; outside the reach of the fountain, plasma flows toward the equator from both hemispheres. This convergence of plasma leads to the formation of an additional layer (called the G layer) within ±10° of the magnetic equator during the prenoon hours when the drift is large. At the magnetic equator, the maximum plasma concentration of the G layer can be greater than that of the F layer for a short period of time just before noon and when the drift starts to decrease. In the evening, soon after the drift turns downward, the fountain becomes a reverse fountain with supply of ionization from both hemispheres from regions outside the fountain. The reverse fountain acts as the main source for the nighttime increase in ionization at equatorial anomaly latitudes, with some contribution from the prereversal strengthening of the forward fountain. The importance of the prereversal strengthening of the forward fountain, which rises to about 1000 km altitude at the equator, and the following reverse fountain on the generation and propagation of plasma bubbles and spread F irregularities is discussed. It is also shown that the equatorial anomaly in the vertical ionospheric electron content need not be as pronounced as the interpretation of the observations suggests.
Magnetic fields and cosmic rays in the distant heliosphere at solar maximum: Voyager 2 observations near 32 AU during 1990
Authors: Burlaga L. F. , Ness N. F. , McDonald F. B.
Published: February 2004

The intensity of >70 MeV cosmic ray protons observed during solar cycle 22 by Voyager 2 (V2) in the distant heliosphere reached a minimum in 1990, corresponding to the maximum of solar activity in mid‐1989 (allowing for the propagation time of the solar wind from the sun to Voyager 2 near 32 AU). A step decrease in the cosmic ray intensity observed within the first ≈ 125 days of 1990 was caused by a global merged interaction region (GMIR), consisting of a cluster of large‐amplitude magnetic field strength fluctuations, in which the field was predominantly stronger than average. The step decrease consists of four events in each of which the decrease in cosmic ray intensity is relatively large, and the recovery is either brief or absent. The relation between the cosmic ray intensity and the magnetic field strength observed near 32 AU during this period at the maximum of solar activity in 1989/1990 is similar to that which Voyager 2 observed near 11 AU in 1982/1983 following the maximum of solar activity in the previous solar cycle in 1980. The local changes in the cosmic ray intensity profile are related to the magnetic field strength. Near solar maximum the emission rate of ejecta, which contain magnetic fields that deviate significantly from the spiral direction, is a maximum. Nevertheless, the distribution of elevation angles of the magnetic field observed by Voyager 2 during 1990 is essentially the same as that observed during earlier parts of the solar cycle from 1986 through 1989, including solar minimum in 1986 and 1987. During 1990, the width of the distribution of elevation angles at ≈ 2 AU was the same as that observed at 1 AU. Sectors were observed, but no sector structure was present during 1990, just as in 1988, 1989, and 1991. The distribution of daily averages of the magnetic field strength is lognormal for 0.05 nT < B < 0.25 nT. However, there are more strong magnetic fields than the lognormal distribution predicts for 0.25 nT < B < 0.4 nT, owing to the large MIRs. The large‐scale fluctuations of the magnetic field strength have a multifractal structure in the low‐frequency range from 2.7 days to at least 21.3 days that influences the cosmic ray intensity profile.
Ideal MHD flow behind interplanetary shocks driven by magnetic clouds
Authors: Erkaev N. V. , Farrugia C. J. , Biernat H. K. , Burlaga L. F. , Bachmaier G. A.
Published: February 2004

We present an ideal MHD theory to describe for the first time the “magnetic barrier” (or “depletion layer”) of that class of interplanetary ejecta called magnetic clouds. By “magnetic barrier” we mean that region of the sheath where the magnetic pressure is comparable to, or larger than, the gas pressure and where, therefore, the effects of the magnetic field on the flow are substantial. We model magnetic clouds as cylindrical flux ropes. We consider three cases: one steady state and two nonsteady situations. The two nonsteady situations correspond to (1) a self‐similarly expanding magnetic cloud, and (2) to a nonexpanding magnetic cloud which has a net bulk motion with respect to the medium at infinity. In all cases the cloud drives an interplanetary shock ahead of it. We describe an algorithm to integrate the MHD equations in which the behavior of the sum of the magnetic and plasma pressure is prescribed. We assume here that the sum of the magnetic and plasma pressure is constant along any line normal to the magnetic cloud boundary. We find that in steady state the cloud boundary cannot be a tangential discontinuity, that is, a finite magnetic barrier thickness can only be obtained with a reconnecting cloud boundary. In general, the magnetic barriers of magnetic clouds are thick, that is, they are a substantial fraction of the cloud's sheath. In steady state and the nonsteady case (situation 2, above), their width depends inversely on the Alfvén Mach number. The non‐steady state (situation 1) has similarities with the problem of solar wind flow around the terrestrial magnetosphere. In particular, the barrier thickness in this case is proportional to the inverse square of the Alfvén Mach number. This work should be useful in the interpretation of data from the sheath region ahead of magnetic clouds driving interplanetary shocks.
Statistical properties of particle precipitation in the polar cap during intervals of northward interplanetary magnetic field
Authors: Shinohara Iku , Kokubun Susumu
Published: February 2004

The characteristics of particle precipitation in the polar cap during intervals of northward interplanetary magnetic field (IMF) are investigated. When the IMF is directed northward, localized and structured electron fluxes, called polar showers, appear in the polar cap and interrupt uniform and soft electron precipitation. We classify polar showers into two types, taking into account the association of ion precipitation: (1) polar showers accompanied by ion fluxes (type A) and (2) polar showers without ion fluxes (type B). In order to investigate the characteristics of each type, we examine the DMSP F8 plasma data obtained for one‐year period. We found that type A and B polar showers respond to the direction of the IMF in quite different ways. Type A polar showers do not significantly depend on either the IMF Bx or By component. Most type A ion fluxes are more intense and energetic than those of the solar wind, and their intensity is similar to that in the boundary plasma sheet. This intensity implies that type A polar showers occur on closed field lines. On the other hand, the statistical properties of the type B polar shower are similar to those of polar rain, which is believed to originate in the solar wind. We therefore suggest that the plasma in type B polar showers still originates in the solar wind and that some magnetic field lines in the polar cap are open even when the IMF is directed northward.
Whistler wave interaction with a density striation: A laboratory investigation of an auroral process
Authors: Bamber J. F. , Maggs J. E. , Gekelman W.
Published: February 2004

Whistler waves are launched toward a field‐aligned density striation in a laboratory plasma. Characteristic scale length and frequency ratios are scaled to closely reproduce situations found in the auroral ionosphere. Detailed measurements show that at the striation edge nearest the wave‐launching antenna, besides a reflected and a transmitted whistler wave, lower hybrid waves are also stimulated on both sides of the striation boundary in a manner consistent with the linear mode‐conversion model. We find that the energy density of the mode‐converted lower hybrid waves is typically 10% of the incident whistler wave energy density, reaching a maximum of 30% in one region. Lower hybrid waves are confined to within 2–3 perpendicular wavelengths in the interaction zone. Our results show that the interaction of electromagnetic whistler mode waves with density striations can cause significant amounts of energy to be deposited in the largely electrostatic lower hybrid mode and that it may therefore be a significant generation mechanism for these waves in certain regions of the ionosphere.
Nonlinear resonant absorption of Alfvén waves in three dimensions, scaling laws, and coronal heating
Authors: Ofman L. , Davila J. M.
Published: February 2004

The nonlinear evolution of the resonant absorption of standing and propagating Alfvén waves in an inhomogeneous plasma is studied via solution of the time‐dependent, three‐dimensional, low‐β, resistive MHD equations over a wide parameter range. When the nonlinear effects become important, the velocities at the dissipation layer were found to be lower than the linear scaling of S1/3 would predict, where S is the Lundquist number. Highly sheared velocities that are subject to the Kelvin‐Helmholtz‐like instability were found at the narrow dissipation layers. Three‐dimensional Kelvin‐Helmholtz‐like vortices appear at and near the dissipation layers and propagate along the slab of plasma when traveling Alfvén wave solution are considered. The narrow resonant heating layers are deformed by the self‐consistent shear flow. In the solar active regions where the resonant absorption of Alfvén waves is believed to occur, the instability may lead to turbulent enhancement of the dissipation parameters and account for the observed turbulent velocities inferred from the nonthermal broadening of x‐ ray and EUV emission lines. The self consistent J×B force changes significantly the density structure of the loop that leads to a shift in the global mode frequency response of the loop and a subsequent drop in the heating rate. In the solar corona the density evolution of the loop is likely to be dominated by evaporation of material from the transition region.
Measurements of the nonthermal helium escape from Mars
Authors: Barabash S. , Kallio E. , Lundin R. , Koskinen H.
Published: February 2004

The automatic space plasma experiment with a rotating analyzer (ASPERA) onboard the Phobos 2 spacecraft has recently revealed the presence of planetary He+ ions at Mars (Barabash and Norberg, 1994). In the present work the analysis is continued in order to estimate the total outflow of the He+ ions which are swept away by the solar wind. For the Phobos epoch the total He+ outflow rate was found to be (1.2±0.6)×1024 ions/s. The escape occurs mainly near the Martian magnetopause. Considering extreme errors in the measurements, the maximum helium outflow could range up to 2.4×1024s−1. From a scaling of the helium profile suggested by Moroz et al. (1990) to obtain the measured loss rate, one can deduce the helium abundance in the Martian upper atmosphere. It turns out that helium is a dominant gas in the Martian exosphere at altitudes between 500 and 1250 km. However, recently reported observations of the weak EUV emissions (108 photons) from the Martian He I suggest an abundance that is 18.5 times lower (Krasnopolsky et al., 1994). Possible reasons for this disagreement are discussed. The helium production rate near Mars can, in turn, be roughly estimated from the production rate for the Earth by using a scaling argument, since the only source of helium in the atmospheres of the terrestrial planets is radioactive decay of certain isotopes of uranium and thorium. Present estimates suggest a degassing rate of 8×1022 atoms/s only (Krasnopolsky et al., 1993). However, under steady state conditions one would expect the production and loss rates to be equal. The discrepancy leads us to the conclusion that either the helium degassing rate should be corrected (or the amount of uranium is higher on Mars than anticipated) or helium may also be delivered on Mars by other sources, for example, as solar wind α particles. The observed high total outflow of ions which are 4 times heavier than protons may result in an effective mass loading. Thus helium may play an important role in the solar wind‐Mars interaction.
Interaction of an expanding plasma cloud with a simple antenna: Application to anomalous voltage signals observed by Voyager 1, Voyager 2, ICE and Vega spacecraft
Authors: Sheehan D. P. , Casey C. A. , Volz L. T.
Published: February 2004

High‐velocity impacts of interplanetary dust grains with spacecraft can give rise to transient plasma clouds from the spacecraft bodies. It is believed these plasma clouds can affect spacecraft instruments. Laboratory results are presented demonstrating the interaction of small expanding plasma clouds with a simple antenna. Results corroborate the hypothesized origin of anomalous impulsive voltage signals recorded by Voyager 1 and 2 spacecraft during flybys of Saturn, Uranus, and Neptune, the International Cometary Explorer (ICE) during its flyby of comet Giacobini‐Zinner, and Vega during its flyby of comet Halley. Results suggest that preflight calibration of antenna‐plasma interactions may extend the range of spacecraft diagnostics.
Substorms: Fundamental observational features, distinction from other disturbances, and external triggering
Authors: Lyons L. R.
Published: February 2004

Fundamental observational signatures of substorms and other auroral zone disturbances are summarized in this review. It is shown that at least three distinct types of major geomagnetic disturbance have been identified in the auroral zones, and it is suggested that care should be taken in distinguishing substorms from the other types of disturbance when reaching observational conclusions concerning substorms. Substorms can be uniquely identified by the formation of the current wedge and by the associated westward electrojet and region of active aurora that move poleward within the ionosphere during the expansion phase. Periods of enhanced convection have been observed to continue without substorms for as long as the interplanetary magnetic field (IMF) remains stably southward. They can have AL signatures that look very much like intense substorms, but these signatures result from strong DP 2 ionospheric currents driven by enhanced convection rather than from a nightside, westward electrojet. A third type of disturbance is characterized by short intervals of enhanced equatorward flow within the ionosphere and enhanced auroral emissions. These enhancements move equatorward from near the polar cap boundary, and they may be associated with several‐minute‐long enhancements of earthward flow within the plasma sheet. Studies that have examined the triggering of substorm expansions by IMF changes are also reviewed. When considered together, and substorms are distinguished from the other disturbances, these studies imply that most, and perhaps all, expansions are triggered by IMF changes. These changes include both northward turnings and reductions in the magnitude of the y component. If most substorms are indeed externally triggered as suggested here, then substorms must generally not be the result of an internal magnetospheric instability.
Periodicities in the Io plasma torus
Authors: Brown Michael E.
Published: February 2004

We present a 6‐month baseline of spatially resolved measurements of the Io plasma torus intensity and perpendicular ion temperature which we use to determine the periodicities of the torus during this time. We find large anticorrelated variations in the intensity and ion temperature which are periodic with the Jovian rotation rate (System III). The intensity variations are found to be a simple manifestation of the temperature variations, though no explanation for the temperature variations is apparent. Periodogram analysis shows an additional intensity periodicity which rotates 2.91±0.06% more slowly than System III. This period is found only in the intensity and not in the ion temperature. We conclude from these observations that the torus has a sinusoidal ion temperature variation locked into the rotation of Jupiter and that superimposed on this is a long‐lived density pattern which rotates 2.91% more slowly than Jupiter. Based on the spatial structure and physical properties, we rule out all currently proposed mechanisms for the creation of these periodicities within the torus.
Collisional losses of ring current ions
Authors: Jordanova V. K. , Kistler L. M. , Kozyra J. U. , Khazanov G. V. , Nagy A. F.
Published: February 2004

The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce‐averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce‐averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time‐dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low‐energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low‐energy ion precipitation inside the plasmasphere.
ULF high‐ and low‐m field line resonances observed with the Super Dual Auroral Radar Network
Authors: Fenrich F. R. , Samson J. C. , Sofko G. , Greenwald R. A.
Published: February 2004

Numerous field line resonance events have been observed with three HF radars (Saskatoon, Kapuskasing, and Goose Bay) of the Super Dual Auroral Radar Network (SuperDARN). The field line resonances cause oscillations in the F region plasma flows which are detected in the measured line of sight Doppler velocities. After analysis, it was found that the resonances were of two types: those with low azimuthal wave number, low‐m, and those with high azimuthal wave number, high‐m. The high‐m events showed many similarities with high‐m pulsations of previous reports including local time of most occurrences (noon‐dusk), pulsation frequencies, westward propagation, increase in phase with latitude, and north‐south polarization. The low‐m events exhibited typical field line resonance characteristics and were found near dusk and dawn with anti‐Sunward propagation. The most notable result was the fact that the high‐ and low‐m events shared many common features. They both were found to occur at the same discrete and stable frequencies. The most common frequencies were 1.3, 1.9, and 2.5‐2.6 mHz, which have previously been associated with magnetospheric waveguide modes. They also occurred at other less common frequencies, such as 1.5–1.6 mHz. Both types of events were localized in latitude with an inverse relation between frequency and latitude. Both were characterized by a wave packet structure with a duration of approximately 1 hour. The numerous features shared by the high‐ and low‐m resonances strongly suggest that they are caused by the same source mechanism. A dispersive waveguide model as a source for the field line resonances is discussed.
A ring beam mechanism for radio wave emission in the interplanetary medium
Authors: Kainer S. , MacDowall R. J.
Published: February 2004

Radio wave emission sources in the interplanetary medium (IPM) include type II and type III radio bursts and emission from planetary bow shock regions. The presence of electron beams in a range of energies suggests the possibility of a ring beam‐plasma interaction as a mechanism for the generation of radio waves at both the fundamental and the second harmonic of the background electron plasma frequency. In this paper, ring beam‐plasma interactions are simulated for the conditions of the IPM, ωe/Ωe = 100 to 200. In these simulations, the ring beam‐plasma interaction couples the electrostatic Langmuir waves at the fundamental plasma frequency to electromagnetic waves; harmonic radiation is produced by an electrostatic trapped electron instability. Mode conversion processes permit the Z mode radiation generated through these instabilities to be converted to the X mode. Examination of both theory and observations suggests the ring beam‐plasma interaction as a candidate for many of the radio phenomena in the IPM.
Current‐voltage characteristics of the TSS 1 satellite
Authors: Dobrowolny M. , Guidoni U. , Melchioni E. , Vannaroni G. , Lebreton J. P.
Published: February 2004

One of the primary science objectives of the TSS 1 project was that of studying the current‐voltage (I‐V) characteristics of a highly charged body in the ionosphere. In this paper, using data from experiments of the TSS 1 mission for an event of high satellite charging, we derive points of the I‐V characteristics of the TSS satellite. These are then compared with different theoretical models. The comparison shows that the measured currents, for given potentials, are well above the predictions of the Parker and Murphy model. The results obtained appear, in fact, to support a model which computes an enhancement of the collected current due to the rapid motion of the collector across the geomagnetic field.
The effects of heavy ions on the slow shocks in the geomagnetotail
Authors: Nakamura M. S. , Fujimoto M. , Maezawa K.
Published: February 2004

The effects of heavy ions on the slow shocks, which are considered to accompany the reconnection in the geomagnetotail, are investigated using one‐dimensional hybrid code simulation (ion particle, charge neutralizing massless electron fluid). First, when all the ions are protons, the slow shock is found to have an ion cyclotron wave train in its downstream region in agreement with the two‐fluid theory. This wave train, however, has not been observed by the spacecraft excursions in the geomagnetotail. In a case where we include oxygen ions in the lobe population with the density fraction as small as 2% of the total, the structure of slow shock deviates from the proton 100% case on the following two points. One is the disappearance of the wave train used to phase stand in the downstream region. The other is generation of the Alfvénic precursor, which is induced ahead of the slow shock and propagates upstream. This result suggests that the effect of a few percent of oxygen ions is one of the possible mechanisms which may explain the discrepancy between the two‐fluid theory and observations. We propose the Alfvénic precursor generation mechanism and discuss its possible effects on the wave disruption.
Comparison between EISCAT UHF and VHF backscattering cross section
Authors: Cabrit B. , Opgenoorth H. , Kofman W.
Published: February 2004

We present a comparison between the backscattering cross sections at 224 and 933 MHz measured with European Incoherent Scatter (EISCAT) radars during the passage of a discrete arc. It shows a difference of 2 orders of magnitude which cannot simply be explained by normal thermal ionospheric density fluctuations. We claim that the observed difference in the scattering cross sections is due to so‐called anomalous echoes which typically occur in the vicinity of high‐altitude ionospheric perturbations, caused by low‐energy particle precipitation. Two possible explanations for the observed differences are discussed: spatial localization and k dependence. Using UHF tristatic observations, we show that there, indeed, is a possibility that the observed echoes are very localized in space. Analyzing the possible k dependence, we show that the experimental geometry used at least in principle will allow us to rule out one of the earlier proposed production mechanisms.
Kinetic theory of E0×B0 instabilities in the ionosphere
Authors: Basu B.
Published: February 2004

A self‐consistent kinetic theory of the E0×B0 instabilities in partially ionized, inhomogeneous plasmas with arbitrary values of να/Ωα, where να represents the electron (ion)‐neutral collision frequency and Ωα is the electron (ion) gyrofrequency, is presented. The theory is based on the kinetic equation with a particle number conserving collision term, which allows the particle distribution function to relax toward a local Maxwellian distribution at rest. The method consists of first solving the zero‐order kinetic equation to determine the self‐consistent equilibrium distribution function. The distribution function is shown to accurately represent the plasma equilibrium state, with appropriate Hall and Pedersen drifts that occur in a collisional plasma in the presence of crossed ambient electric and magnetic fields. The linear dispersion relation is then derived from the first‐order kinetic equation, and it can be used to study the E0×B0 instabilities in all altitude regions of the ionosphere in a unified manner, without the need for any a priori knowledge of the different types of particle drifts (Hall and Pedersen drifts) that are responsible for the instabilities in different altitude regions. The present theory therefore provides a more rigorous kinetic description of the E0×B0 instabilities than that obtained from the previously studied kinetic model in which the zero‐order particle drifts are not determined self‐consistently but have to be specified extraneously.
The propagation characteristics and Landau damping of Jovian whistlers in the Io torus
Authors: Wang Kaiti , Thorne Richard M. , Horne Richard B.
Published: February 2004

The propagation characteristics and path‐integrated gain of Jovian lightning‐generated whistlers are explored using the HOTRAY code. All waves are launched into the magnetosphere from just above the density peak in the ionosphere and followed using a realistic analytical density model based on Voyager and Pioneer data. Over a broader range of input L shells (3.5 ≤ L ≤ 6.5) these unducted waves are strongly guided into the equatorial region near 5.3 ≤ L ≤ 6 by the density maxima in the Io plasma torus. This is consistent with the limited spatial extent of the region where whistlers were detected by Voyager. The path‐integrated attenuation of whistlers is relatively weak in the cool inner torus but becomes strong for waves that propagate into the outer warm torus region due to Landau damping by suprathermal electrons (E ≥ 200 eV). This can explain the absence of whistlers observations by Voyager at L > 6. To account for the observed upper frequency cutoff (ƒ ≤ 9 kHz ), our calculations indicate that the lightning source locations should be confined to lower L shells (L ≤ 4). The alternative explanation based on the plasma frequency cutoff requires an unreasonable plasma density model.
Nonlinear evolution of equatorial spread F: 3. Plasma bubbles generated by structured electric fields
Authors: Huang Chao‐Song , Kelley Michael C.
Published: February 2004

Observations have shown that in some cases equatorial spread F (ESF) irregularities, including plasma bubbles, are related to temporal and spatial structures of electric fields. It has been conjectured that the electric field structures may be responsible to production of ESF bubbles. We present here numerical results of electric field effects in nonlinear evolution of ESF. In our simulations we have modeled electric field structures in both time and space. It is found that the temporal structures, including the prereversal enhancement, of the eastward electric field cause motion of the F layer in the vertical direction, but they cannot produce spatial structures of F region plasma. In contrast, plasma bubbles are produced when spatial structures of electric field are used as input in the simulations. The spatially irregular electric fields initiate plasma perturbations in the bottomside F region, then the perturbations are amplified by the Rayleigh‐Taylor instability and evolve into topside plasma bubbles. It is concluded that spatial structures of electric field are needed for seeding of ESF bubbles. It is shown also that electric fields generated by E region gravity waves can result in production of F region bubbles.
Direct determination of the local ionospheric Hall conductance distribution from Two‐dimensional electric and magnetic field data: Application of the method using models of typical ionospheric electrodynamic situations
Authors: Amm Olaf
Published: February 2004

The applicability of a method to directly deduce the ionospheric Hall conductance distribution ΣH from ground magnetic and ionospheric electric field observations, here called “method of characteristics”, is tested by using input data from models of some typical ionospheric electrodynamic situations. We shall show that the method in the form of a fully automatical computer algorithm is able to reproduce well the ΣH distributions of all modeled situations, i.e. a two‐dimensional eastward electrojet, a Harang discontinuity, an omega band, and a westward traveling surge. Furthermore, we will show quantitatively that the ambiguity implied by the necessary assumption of the distribution of the Hall to Pedersen conductance ratio has only a small effect on the results obtained. We also prove quantitatively that the assumption of vertical, straight geomagnetic field lines made in the derivation of the method leads only to very small errors if the case of oblique, but straight geomagnetic field lines is taken to be realistic. Moreover, we review the general theory of the method and discuss some additional theoretical aspects. In particular, we will show that isolated points with
, but nonvanishing
are associated with extrema or saddle points of the ΣH distribution, and calculate the magnetic field disturbance of an ionospheric current system with oblique, but straight field‐aligned currents directly below the ionospheric plane. The method does not require an electrostatic situation; i.e.,
is allowed.
Alfvén wave heating of coronal holes and the relation to the high‐speed solar wind
Authors: Ofman L. , Davila J. M.
Published: February 2004

Coronal hole regions are well‐known sources of high‐speed solar wind; however, to account for the observed properties of the solar wind, a source of energy must be included in addition to heat conduction. Alfvén waves were suggested as the possible source of heating that accelerates the solar wind. We investigate the heating and propagation of the fast and shear Alfvén waves in coronal holes via numerical solution of the time‐dependent, linearized, resistive, low‐β, two‐dimensional MHD equations in slab geometry. The waves are driven at the lower boundary of the coronal hole and propagate into the corona. We find that fast waves are partially reflected at the coronal hole boundary and significant part of the wave energy leaks out of the coronal hole. We compare the calculated wavelengths and the attenuation rate of the fast waves in the leaky waveguide formed by the coronal hole with the analytical ideal MHD solutions for ky = 0, where ky is the perpendicular wavenumber, and find an excellent agreement. When ky ≠ 0 the fast waves couple to the shear Alfvén waves and transfer energy across field lines. Resonance heating layers are found to occur when shear Alfvén waves are driven and a continuous density profile is assumed for the coronal hole. When resonance absorption is considered, the leakage is small compared to the heating rate. The heating is enhanced by phase mixing when coronal hole inhomogeneities (i.e., plumes) are included. We investigate the dependence of the heating rate on the driver frequency and the Lundquist number S and find a good agreement with the analytical S1/3 scaling of the dissipation length. We find that when S = 104 the low‐frequency Alfvén waves can be a significant source of heating of coronal holes at several solar radii. At larger values of S, nonlinear effects might reduce the effective dissipation length. We discuss the relation of our results to the observed properties of high‐speed solar wind and coronal holes.
Effects of a constant cross‐tail magnetic field on the particle dynamics in the magnetotail
Authors: Holland Daniel L. , Chen James , Agranov Alex
Published: February 2004

The effects of a constant cross‐tail magnetic field By on the phase space structures and the observational signatures of nonlinear particle dynamics in the quiet time magnetotail are examined. The separation of phase space into dynamically distinct regions (transient, stochastic and integrable) is found to persist albeit with considerable complications. In addition, it is shown that if |By| is less than the Bz component, the resonant coherent chaotic scattering of particles and the resulting distribution function features persist. If |By| ≳ Bz, on the other hand, the majority of the particles are forward scattered through the current sheet, and there is no discernible signature of the resonance in the distribution function. This is significant in that the presence or absence of the resonance effect in observed distribution functions can constrain the magnitude of By.
Three‐dimensional electromagnetic Monte Carlo particle‐in‐cell simulations of critical ionization velocity experiments in space
Authors: Wang J. , Biasca R. , Liewer P. C.
Published: February 2004

Although the existence of the critical ionization velocity (CIV) is known from laboratory experiments, no agreement has been reached as to whether CIV exists in the natural space environment. In this paper we move towards more realistic models of CIV and present the first fully three‐dimensional, electromagnetic particle‐in‐cell Monte‐Carlo collision (PIC‐MCC) simulations of typical space‐based CIV experiments. In our model, the released neutral gas is taken to be a spherical cloud traveling across a magnetized ambient plasma. Simulations are performed for neutral clouds with various sizes and densities. The effects of the cloud parameters on ionization yield, wave energy growth, electron heating, momentum coupling, and the three‐dimensional structure of the newly ionized plasma are discussed. The simulations suggest that the quantitative characteristics of momentum transfers among the ion beam, neutral cloud, and plasma waves is the key indicator of whether CIV can occur in space. The missing factors in space‐based CIV experiments may be the conditions necessary for a continuous enhancement of the beam ion momentum. For a typical shaped charge release experiment, favorable CIV conditions may exist only in a very narrow, intermediate spatial region some distance from the release point due to the effects of the cloud density and size. When CIV does occur, the newly ionized plasma from the cloud forms a very complex structure due to the combined forces from the geomagnetic field, the motion induced emf, and the polarization. Hence the detection of CIV also critically depends on the sensor location.