bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear evolution of a large‐amplitude circularly polarized Alfvén wave: Low beta
Authors: Ghosh S. , Goldstein M. L.
Published: February 2004

The nature of turbulent cascades arising from the parametric instabilities of a monochromatic field‐aligned large‐amplitude circularly polarized Alfvén wave is investigated via direct numerical simulation for the case of low plasma β and no wave dispersion. The magnetohydrodynamic code permits nonlinear couplings in the parallel direction to the ambient magnetic field and one perpendicular direction. Compressibility is included in the form of a polytropic equation of state. Anisotropic turbulent cascades, similar to those found in early incompressible two‐dimensional simulations, occur after nonlinear saturation of the parallel propagating decay instability. The turbulent spectrum can be divided into three regimes: the lowest wave numbers are dominated by lower sideband remnants of the parametric process, intermediate wave numbers display nearly incompressible dynamics, and the highest wave numbers are dominated by acoustic turbulence.
Properties of AE data and bicolored noise
Authors: Takalo J. , Timonen J. , Koskinen H.
Published: February 2004

A detailed analysis has been made of 13 periods of 20 days of the AE data. The average correlation dimension is found to be 3.4, and the calculated dimension is found to depend on the magnetospheric activity such that more active periods have smaller dimensions. This apparent correlation dimension does not, however, imply that the magnetospheric system behind the AE data is low‐dimensional. On the contrary, bicolored noise is shown to share many properties with the AE data. The AE time series is also found to be self‐affine such that the scaling exponent changes at the time scale of about 2 hours. This same time scale is suggested to appear as a spectral break at about 1/(5 hours) in the power spectrum of the AE data.
Modeling the formation of intermediate layers at Arecibo latitudes
Authors: Osterman G. B. , Heelis R. A. , Bailey G. J.
Published: February 2004

By using a modified version of the ionospheric model described by Bailey and Sellek (1990), we model the formation of intermediate ionization layers due to meridional neutral winds in the valley region between the E and F regions. The calculations are performed on a single field line with L = 1.4, in order to compare the results with observations of intermediate layers from the incoherent scatter radar at Arecibo. The winds are given a sinusoidal variation in altitude, and the effects of the wind amplitude and wavelength on layer formation are examined. Nighttime ionization rates are artificially specified to correspond with observed rates in the region. Our results show that tidal like meridional wind profiles act to deplete the apparent background ionospheric number density as well as enhance the number density in the intermediate layer. Thus the layer appears as an enhancement above a background that is much smaller than the background that would be present in the absence of a wind. In this way, layers with relatively high molecular ion concentrations can exist. Intermediate layers are seen to form at nulls in the neutral wind altitude profile, but in the altitude region above roughly 160 km this null need not coincide with a zero in the field‐aligned ion velocity.
A search for upstream pressure pulses associated with flux transfer events: An AMPTE/ISEE case study
Authors: Elphic R. C. , Baumjohann W. , Cattell C. A. , Lühr H. , Smith M. F.
Published: February 2004

On September 19, 1984, the Active Magnetospheric Particle Tracers Explorers (AMPTE) United Kingdom Satellite (UKS) and Ion Release Module (IRM) and ISEE 1 and 2 spacecraft passed outbound through the dayside magnetopause at about the same time. The AMPTE spacecraft pair crossed first and were in the near‐subsolar magnetosheath for more than an hour. Meanwhile the ISEE pair, about 5 RE to the south, observed flux transfer event (FTE) signatures. We use the AMPTE UKS and IRM plasma and field observations of magnetosheath conditions directly upstream of the subsolar magnetopause to check whether pressure pulses are responsible for the FTE signatures seen at ISEE. Pulses in both the ion thermal pressure and the dynamic pressure are observed in the magnetosheath early on when IRM and UKS are close to the magnetopause, but not later. These large pulses appear to be related to reconnection going on at the magnetopause nearby. AMPTE magnetosheath data far from the magnetopause do not show a pressure pulse correlation with FTEs at ISEE. Moreover, the magnetic pressure and tension effects seen in the ISEE FTEs are much larger than any pressure effects seen in the magnetosheath. A superposed epoch analysis based on small‐amplitude peaks in the AMPTE magnetosheath total static pressure (nkT + B²/2µo) hint at some boundary effects, <5 nT peak‐to‐peak variations in the ISEE 1 and 2 BN signature starting about 1 min after the pressure peak epoch. However, these variations are much smaller than the standard deviations of the BN field component. Thus the evidence from this case study suggests that upstream magnetosheath pressure pulses do not give rise to FTEs, but may produce very small amplitude signatures in the magnetic field at the magnetopause.
Sporadic Fe and E layers at polar, middle, and low latitudes
Authors: Alpers M. , Höffner J. , von Zahn U.
Published: February 2004

First observations of sporadic iron layers (FeS) at low and middle southern latitudes are reported. These and new measurements from northern polar latitudes are combined to study the latitudinal dependence of FeS layer properties and their correlation with simultaneously observed sporadic E layers (ES). We also compare the individual properties of FeS and sporadic sodium layers (NaS): (1) The overall appearance rate for FeS is much higher than for NaS. (2) FeS layers do not show the marked minimum in appearance. (3) Both the normal and the sporadic Fe layers are considerably more dynamic than in the case of Na layers. (4) Concerning the shape, FeS layers are typically broader, slower in growing and longer lasting than NaS layers. (5) The dependence of appearance rate of sporadic metal layers versus, local time and latitude shows a complex pattern. (6) The correlation with ES appears to be weaker for FeS than for NaS.
A Giotto/cometary dust interaction event near the diamagnetic cavity boundary
Authors: Israelevich P. L. , Ershkovich A. I.
Published: February 2004

A new scenario is suggested in order to explain the burst of particles observed by Giotto near the Halley ionopause. According to this scenario, an impact of a cometary dust particle against the Giotto spacecraft (identified by Neubauer et al. (1990)) creates a gas/plasma cloud. Cold cometary ions with a velocity of 70 km/s that impinge on the cloud around the spacecraft are scattered by the secondary ions/neutral species in the cloud. This scattering is shown to allow the cometary ions to enter the angular detectors. This mechanism casts some doubt on alternative explanations of the observed phenomenon and thereby may call into question the very existence of suprathermal particles near the Halley ionopause.
VLF emission produced by a rarefied electron beam during active experiments in the Earth ionosphere: Conditions of observation of the emission
Authors: Lundin B. , Chmyrev V. , Krafft C. , Matthieussent G.
Published: February 2004

The peculiarities of whistler waves spontaneously radiated by an electron beam artificially injected into the Earth ionosphere are discussed. The conditions of registration of the whistler wave packets by a remote on board VLF receiver have been analyzed; they depend on mutual disposition of the electron gun and the VLF receiver as well as on the pitch angle of the injected electrons. It is shown that the internal frequency width of the whistler signal, connected with ray representation of a wave field, depends on the distance between the emitting region of the beam and the receiver and determines the lowest bandwidth of the VLF receiver in use. In the case of a dense plasma (when the plasma frequency ωp is much greater than the electron gyrofrequency ωc) the main results are obtained analytically. The effective value of the electron collision frequency for the background plasma can be estimated through measurements of the maximum value of the injection pitch angle of the beam's electrons, which leads to a steep drop in the level of the whistler signal. It is shown that double‐pole singularity in the single electron Cherenkov energy loss near the Gendrin's velocity appears only at a single frequency depending on the beam and the background plasma parameters. Thus the total growth of the energy loss in this case reveals an increase by a factor of the order of ωp/ωc compared to the usual single‐pole contribution. The Cherenkov radiation from the sharp leading front of the beam does not reveal any Mach cone structure. The peculiarities of the spectral parameters of the whistler emission at the Doppler‐shifted resonances are also discussed; the Mach cone exists in these cases. Thus this feature can be responsible for the crucial difference between the registration pattern of high‐frequency and whistler frequency emission in active experiments similar to ARAKS.
Repetitive patterns in the recovery phase of cosmic ray 11‐year modulation
Authors: Ahluwalia H. S.
Published: February 2004

Emerging solar polar magnetic field regime has a significant influence on the profile of cosmic ray 11‐year modulation. Our conclusion is derived from an analysis of the shielded ion chamber data obtained at Cheltenham‐Fredericksburg (1937‐1974) and Yakutsk (1957‐1990) and of neutron monitor data obtained at Huancayo (1953‐1992), Climax (1951‐1992), Ottawa (1954‐1959), and Deep River (1960‐1992). The recovery of cosmic ray intensity follows two distinct patterns. During odd solar activity cycles (17, 19, 21) when the magnetic polarity of the Sun in the northern hemisphere is negative (qA <0) the recovery is completed in 5 to 8 years. For even cycles (18, 20) during epochs of the positive magnetic polarity in the northern hemisphere of the Sun when qA >0 the recovery period is reduced to less than half as much. The rapid recovery seems to set in following the reversal of solar polar field during even cycles (18, 20) unless interrupted by solar activity (as for cycle 22). The delayed rapid recovery for cycle 22 is in progress. At present, it is not clear whether the next cosmic ray maximum will follow the “mesa” pattern.
Neptune's inner magnetosphere and aurora: Energetic particle constraints
Authors: Mauk B. H. , Krimigis S. M. , Acuña M. H.
Published: February 2004

A dramatic and peculiar dropout of >500‐keV ions (but not electrons) was observed within Neptune's inner magnetosphere near 2 RN as the Voyager 2 spacecraft approached the planet. Unlike a number of other energetic particle features this feature could not be accounted for by known material bodies in the context of the most utilized magnetic field models (neither the offset tilted dipole models nor the spherical harmonic model “O8”). However, the configuration of Neptune's inner magnetosphere is highly uncertain. By applying a novel technique, utilizing energetic particle measurements, to constrain the magnetic field configuration of the inner regions, we show that appeals to unobserved materials within Neptune's system are unnecessary, and that the ion dropout feature was, in all likelihood, the result of ion interactions with the maximum L excursions of the ring 1989N1R. The constraints also favor the use of the M2 magnetic field model (Selesnick, 1992) over previous models. An electron feature was probably absent because the electron interactions with the ring occurred substantially before the ion interactions (about 2 hours for the electrons versus a few minutes for the ions). Pitch‐angle scattering apparently eliminated the electron signature. Minimum scattering rates determined based on this premise yield enough electron precipitation power to explain the brightest component of Neptune's aurora. We propose that this bright component is analogous to the Earth's diffuse aurora.
Two ion fluid numerical investigations of solar wind gas releases
Authors: Harold J. B. , Hassam A. B.
Published: February 2004

A two ion species system of fluid equations is used to investigate mechanisms responsible for transverse accelerations of plasma releases into high Mach number flows, such as that of the December 1984 Active Magnetospheric Particle Tracer Experiment (AMPTE) release of barium ions into the solar wind. We extend previous work in single ion species systems by developing a set of modified MHD equations which incorporates both the unmagnetized nature of the released ions and the interactions of the two ion fluids. An analysis of this system of equations suggests two mechanisms that could cause transverse shifts of the plasma release. One of these is based on a magnetic drift wave, the other on a hybrid cyclotron mode. Numerical simulations produce clearly observable shifts in the case of each mechanism. The modified two‐fluid MHD equations, simulation results, and physical mechanisms are discussed, and their relevance to the AMPTE barium release is examined.
Electric potential patterns in the northern and southern polar regions parameterized by the interplanetary magnetic field
Authors: Papitashvili V. O. , Belov B. A. , Faermark D. S. , Feldstein Ya. I. , Golyshev S. A. , Gromova L. I. , Levitin A. E.
Published: February 2004

Electric potential patterns have been obtained from the IZMIRAN electrodynamic model (IZMEM) for the northern and southern polar regions during summer, winter, and equinox. The model is derived from a large quantity of high‐latitude ground‐based geomagnetic data (above ± 57° corrected geomagnetic latitude) at all magnetic local time hours. A linear regression analysis technique has been used to obtain the quantitative response of each magnetic observatory to changes of interplanetary magnetic field (IMF) components. Since no ionospheric conductivity model exists specifically for the southern polar region, the statistical model of Wallis and Budzinski (1981) has been applied in both hemispheres. A cross‐polar “background” potential of ∼35 kV, derived by Reiff et al. (1981), is used to calibrate IZMEM's potential patterns. The model's responses to changes in the IMF By and Bz components are analyzed to obtain a set of “elementary” convection patterns in both polar regions for each season of the year. Asymmetry in the potential pattern geometry in both hemispheres can be attributed either to the influence of the “northern” ionospheric conductivity model which was applied to the southern polar region, or to some natural phenomena. The modeled background cross‐polar potential for the condition when Bz = By = 0 is found to be ∼37 kV. Average values of the modeled potential drop caused by each nanotesla of the IMF are the following: ∼14 kV for southward Bz; ∼ −4 kV for northward Bz; and ∼ ±4.5 kV for By components. The latter is not applicable to the “dawn‐dusk” potential drop; it may be applied across the cusp region only. Nevertheless, a combination of the background and elementary potential patterns in the case studies gives a certain estimation of the cross‐polar potential drop, which may be strongly distorted during time of large By. It is concluded that IZMEM provides realistic convection patterns parameterized by the IMF component directions and magnitudes and may be used to provide routine estimates of convection patterns and electric potentials if IMF data are available.
Temporal study of the current collected by a Langmuir probe biased to high voltages in the wake behind a conducting body
Authors: Meassick S. , Chan C.
Published: February 2004

The temporal evolution of the voltage‐current characteristic of a spherical Langmuir probe in the near‐wake region of a conducting body is investigated in a pulsed plasma device. Measurements of the current collected by a sphere placed on axis behind a conducting disk are presented as a function of the bias voltage and time. The bias voltage on the collecting sphere is large enough, ranging from +500 to −3000 V, to attract ions or electrons across the wake region. In addition, plasma potential profiles as a function of sphere bias voltage and time throughout the nearwake and midwake region are presented. It is found that for a negative sphere bias a threshold voltage exists for current collection. This threshold voltage decreases during the formation of the wake behind the conducting disk. Potential measurements indicate that the sheath region around the negatively biased sphere contracts as the wake region establishes itself. This contraction of the sheath region increases the electric field attracting ions to the sphere and decreases the threshold voltage for current collection. For positive sphere bias voltage the current collected by the sphere does not exhibit a threshold voltage but is monotonically increasing.
Inverse correlations between the ion temperature anisotropy and plasma beta in the Earth's quasi‐parallel magnetosheath
Authors: Fuselier Stephen A. , Anderson Brian J. , Gary S. Peter , Denton Richard E.
Published: February 2004

Average proton parameters in the magnetosheath downstream from the quasi‐perpendicular shock (the quasi‐perpendicular magnetosheath) for high solar wind dynamic pressure conditions are observed to vary continuously from high‐beta, low‐temperature anisotropy to low‐beta, high‐temperature anisotropy. Observations and theory have shown that this inverse correlation is a direct consequence of pitch angle scattering by electromagnetic ion cyclotron (EMIC) waves, which regulate the anisotropy, restoring the plasma toward marginal stability. Although the previously documented spectral characteristics of EMIC waves are not evident downstream of the quasi‐parallel bow shock, the inverse anisotropy‐beta relation found in the quasi‐perpendicular magnetosheath also holds in the quasi‐parallel magnetosheath. This indicates that the EMIC instability regulates the ion anisotropy regardless of the shock geometry.
Fine structure of Langmuir waves observed upstream of the bow shock at Venus
Authors: Hospodarsky G. B. , Gurnett D. A. , Kurth W. S. , Kivelson M. G. , Strangeway R. J. , Bolton S. J.
Published: February 2004

Highly structured Langmuir waves, also known as electron plasma oscillations, have been observed in the foreshock of Venus using the plasma wave experiment on the Galileo spacecraft during the gravity assist flyby on February 10, 1990. The Galileo wideband sampling system provides digital electric field waveform measurements at sampling rates up to 201,600 samples per second, much higher than any previous instrument of this type. The main Langmuir wave emission band occurs near the local electron plasma frequency, which was approximately 43 kHz. The Langmuir waves are observed to shift above and below the plasma frequency, sometimes by as much as 20 kHz. The shifts in frequency are closely correlated with the downstream distance from the tangent field line, implying that the shifts are controlled by the electron beam velocity. Considerable fine structure is also evident, with timescales as short as 0.15 ms, corresponding to spatial scales of a few tens of Debye lengths. The frequency spectrum often consists of beat‐type waveforms, with beat frequencies ranging from 0.2 to 7 kHz, and in a few cases, isolated wave packets. The peak electric field strengths are approximately 1 mV/m. These field strengths are too small for strongly nonlinear processes to be important. The beat‐type waveforms are suggestive of a parametric decay process.
The relationship between the magnetic field in the Martian magnetotail and upstream solar wind parameters
Authors: Rosenbauer H. , Verigin M. I. , Kotova G. A. , Livi S. , Remizov A. P. , Riedler W. , Schwingenschuh K. , Shutte N. M. , Slavin J. A. , Szegő K. , Tátrallyay M. , Zhang T. ‐L.
Published: February 2004

Magnetic field data measured by the MAGMA instrument in the Martian magnetotail lobes are compared with the ram pressure of the upstream solar wind observed by the TAUS instrument in the circular orbits of the Phobos 2 spacecraft. High correlation was found between the magnetic field intensity in the Martian magnetotail lobes and the solar wind ram pressure. From this relationship the average flaring angle of the Martian magnetotail was determined as ∼13°, and the average magnetosonic Mach number was estimated as ∼5. The observed relationship between the Martian magnetotail magnetic field intensity and the solar wind magnetic field reflects the correlation of the solar wind magnetic field to the ram pressure providing a value of ∼7 for the average Alfvenic Mach number. The flaring angle obtained for the Martian magnetotail was found to be an intermediate value between the flaring angle of the magnetotail of the Earth and that of Venus at comparable distances.
Plasma observations of magnetopause crossings at geosynchronous orbit
Authors: McComas D. J. , Elphic R. C. , Moldwin M. B. , Thomsen M. F.
Published: February 2004

The entire three‐spacecraft magnetospheric plasma analyzer (MPA) data set has been examined for intervals of magnetosheath plasma at geosynchronous orbit. Over the 6 1/2 spacecraft years of data reviewed, 916 5‐min intervals of magnetosheath plasma observations were identified and cataloged. Unlike previous studies that suggested that magnetopause encounters were far more likely on the prenoon than postnoon side at geosynchronous orbit, this study examines three independent lines of evidence and finds that only a small asymmetry (in this same sense) exists. On a statistical basis, a local time of ∼1130 is both the median and mean for our magnetosheath observations; this local time is simply consistent with the effects of an aberrated solar wind direction due to the motion of the Earth around the Sun. Simultaneous multipoint observations across both sides of local noon are also consistent with a small offset due to aberration of the magnetopause. Finally, we examined the locations of the dawn‐dusk flow reversals for a number of cases where one of the geosynchronous spacecraft was outside the magnetopause in the magnetosheath flow. These flow reversals tended to occur near local noon, again indicating that in contrast to previous findings, no large asymmetry exists.
Radio emissions and the heliospheric termination shock
Authors: Zank G. P. , Cairns I. H. , Donohue D. J. , Matthaeus W. H.
Published: February 2004

With the Voyager spacecrafts' discovery of low‐frequency radio emissions from the depths of the outer heliosphere has come the realization that the boundaries between our heliosphere and the local interstellar medium may have been detected. A model is presented here that can account for the observed radio emissions, based upon a termination shock modified by the dynamical effects of galactic and anomalous cosmic rays. Frequency and time domain properties of both continuum and transient radio events are explained, and new estimates for the distance to the termination shock (∼60‐70 astronomical units) and the heliopause (≳90 AU) are given.
Stochastic charging of dust grains in planetary rings: Diffusion rates and their effects on Lorentz resonances
Authors: Schaffer L. , Burns J. A.
Published: February 2004

Dust grains in planetary rings acquire stochastically fluctuating electrical charges as they orbit through any corotating magnetospheric plasma. Here we investigate the nature of this stochastic charging and calculate its effect on the Lorentz resonance (LR). First we model grain charging as a Markov process, where the the transition probabilities are identified as the ensemble‐averaged charging fluxes due to plasma pickup and photoemission. We determine the distribution function P(t;N), giving the probability that a grain has N excess charges at time t. The autocorrelation function τq for the stochastic charge process can be approximated by a Fokker‐Planck treatment of the evolution equations for P(t;N). For a typical plasma, τq is approximately the charging time constant for the grain. Linear perturbation theory shows that the orbital variations of weakly charged dust grains satisfy forced harmonic oscillator equations. The forcing terms take the form Q(t)cos[ωt + ϕ], where Q(t) is linear in the charge q, the forcing frequency ω is related to the rate at which a grain samples spatial periodicities of the magnetic field, and ϕ is a phase shift. Large orbital evolution effects take place at LR's, radial locations where ω is close to a grain's orbital frequency. Since the charge q(t) is piecewise constant over the time interval between the arrivals of plasma particles or solar photon, we can iterate solutions to the perturbation equations over these intervals. For grains near a resonance the ensemble average of the oscillation amplitudes undergoes a long‐period sinusoidal cycle of growth and decay. Charge fluctuations cause slow transport in the phase space of the oscillator. This diffusion is driven by a random walk process; as plasma density n decreases, τq, and hence the size of the typical random step, increases. We calculate the mean square response 〈β²〉 to the stochastic fluctuations in the Lorentz force; for times longer than τq we find 〈β²〉 ∝ σ²τqt, where σ is the deviation of the charge from its mean value
. Even when
, so long as n > 0.1 cm−3, we find that transport in phase space is very small compared to the resonant increase in amplitudes due to the mean charge, over the timescale that the oscillator is resonantly pumped up. Therefore the stochastic charge variations cannot break the resonant interaction; locally, the Lorentz resonance is a robust mechanism for the shaping of ethereal dust ring systems. Slightly stronger bounds on plasma parameters are required when we consider the longer transit times between Lorentz resonances.
Kinetic modeling of O+ upflows resulting from E × B convection heating in the high‐latitude F region ionosphere
Authors: Wilson G. R.
Published: February 2004

We report here the results of modeling work aimed at understanding the development of ionospheric O+ field‐aligned upflows that develop in response to high‐latitude E × B drift induced frictional heating. The model used is a collisional semikinetic model which includes ion‐neutral resonant charge exchange and polarization collisions as well as Coulomb self‐collisions. It also includes the process of chemical removal of O+ as well as all of the macroscopic forces: ambipolar electric, gravity, magnetic mirror, and centripetal. Model results show the development of several types of non‐Maxwellian velocity distributions including toroids at low altitude, distributions with large heat flow in the perpendicular component at intermediate altitudes, and distributions with a separate upflowing population or upward superthermal tail at high altitudes. Whenever the convection electric field increases from a small value (< 25 mV/m) to a large value (100‐200 mV/m) in 6 min or less large upflows develop with parallel drift speeds which peak (below 1000 km) at values between 500 m/s and 2 km/s, parallel fluxes which peak between 6.0 × 108 and 3.2 × 109 cm−2 s−1, and parallel per particle heat flows which peak between 8.0 × 10−9 and 8.0 × 10−8 ergs cm/s. The higher values in these ranges occur for a cooler neutral atmosphere, with a larger convection electric field that is turned on quickly. The model produces field‐aligned O+ flow speeds that are larger than those produced by a 20‐moment generalized transport model but smaller then those produced by an isotropic hydrodynamic model for comparable values of the convection electric field and convection turn on times. The model results compare favorably with some topside satellite and radar data.
Charged dust in planetary magnetospheres: Hamiltonian dynamics and numerical simulations for highly charged grains
Authors: Schaffer L. , Burns J. A.
Published: February 2004

We use a combination of analytical and numerical methods to investigate the dynamics of charged dust grains in planetary magnetospheres. Our emphasis is on obtaining results valid for particles that are not necessarily dominated either by gravitational or electromagnetic forces. A Hamiltonian formulation of the problem yields exact results, for all values of charge‐to‐mass ratio, when we introduce two constraints: particles remain in the equatorial plane and the magnetic field is taken as axially symmetric. In particular, we obtain locations of equilibrium points, the frequencies of stable periodic orbits, the topology of separatrices in phase space, and the rate of longitudinal drift. These results are significant for specific applications: motion in the nearly aligned dipolar field of Saturn, and the trajectories of arbitrarily charged particles in complex magnetic fields for limited periods of time after ejection from parent bodies. Since the model is restrictive, we also use numerical integrations of the full three‐dimensional equations of motion and illustrate under what conditions the constrained problem yields reasonable results. We show that a large fraction of the intermediately charged and highly charged (gyrating) particles will always be lost to a planet’s atmosphere within a few hundred hours, for motion through tilted‐dipole magnetic fields. We find that grains must have a very high charge‐to‐mass ratio in order to be mirrored back to the ring plane. Thus, except perhaps at Saturn where the dipole tilt is very small, the likely inhabitants of the dusty ring systems are those particles that are either nearly Keplerian (weakly charged) grains or grains whose charges place them in the lower end of the intermediate charge zone. Finally, we demonstrate the effect of plasma drag on the orbits of gyrating particles to be a rapid decrease in gyroradius followed by a slow radial evolution of the guiding center.
Effect of sudden impulses on currents in the auroral ionosphere under northward interplanetary magnetic field conditions: A case study
Authors: Russell C. T. , Ginskey M. , Angelopoulos V.
Published: February 2004

We examine the response of auroral magnetic records to the passage of an interplanetary shock at a time when the interplanetary magnetic field was northward. We restrict our attention solely to the sector within 3 hours of local magnetic midnight for a single case selected when a bursty bulk flow event was recorded in the near tail by ISEE 2. Over most of the nightside at high latitudes only a weak disturbance if any is seen. At lower latitudes a plateau is seen in the H component, coincident with the bursty bulk flow event. At 65° latitude from about midnight to 3:00 LT a weak pair of negative bays is observed, also coincident with the bursty bulk flow event. We conclude that the tail and the auroral ionosphere were closely coupled during this sudden impulse, but the auroral zone disturbance appears to be mainly the brief activation of a section of the auroral electrojet rather than a classic substorm. No expansion or motion of the electrojet was observed, and the activation was no longer than that of the bursty bulk flow in the tail.
Excitation of the low lying vibrational levels of H2O by O(³P) as measured on Spacelab 2
Authors: Meyerott R. E. , Swenson G. R. , Schweitzer E. L. , Koch D. G.
Published: February 2004

The data from the infrared telescope (IRT), which was flown on space shuttle Challenger Spacelab 2 mission (July 1985), were originally reported by Koch et al. (1987) as originating from near orbital emissions, primarily H2O. In this study, analysis of this data was extended to determine the collisional cross sections for the excitation of the low lying vibrational levels of H2O, present in the orbiter cloud, by atmospheric O(³P). The evaluation of the contribution to the measured signal from solar excitation and ram O excitation of outgassing H2O permits the determination of the H2O column density and the excitation cross section of the (010) level at an O(³P) velocity of ∼7.75 km/s. Contributions to the radiation in the 1.7‐3.0 µm band by transitions from the (100), (001), and multiquantum excited levels are discussed. The findings of the study are (1) the IRT data for the 4.5‐9.5 µm and the nighttime data for the 1.7‐3.0 µm sensors are consistent with being explained by collision excitation of H2O by O(³P), (2) diurnal variations of 4.5‐9.5 µm intensities follow the model predicted O density for a full orbit, (3) daytime increases in the H2O cloud density were not evident, (4) the cross sections for the collisional excitation process are derived and compared to values computated by Johnson (1986) and Redmon et al. (1986), (5) theoretical investigation suggests >60% of the radiation from H2O is a result of multiphoton emission resulting from collisional multiquanta excitation, and (6) the large daytime increase in the 1.7‐3.0 µm intensity data suggests that O+ may likely be instrumental in producing excited H2O+ through charge exchange.
Motion of the heliospheric termination shock: 4. MHD effects
Authors: Naidu Kamcilla , Barnes Aaron
Published: February 2004

In this paper we generalize earlier gasdynamic analyses of the motion of the heliospheric termination shock in response to upstream disturbances (Barnes, 1993, 1994; Naidu and Barnes, 1994), to include magnetohydrodynamic (MHD) phenomena. We assume that the termination shock is a strong, perpendicular shock and that the initial upstream disturbance is a tangential discontinuity. The resulting configuration after the interaction is very similar to that in the gasdynamic models after an interaction with a contact discontinuity or interplanetary shock, and for an increase (decrease) in dynamic pressure consists of an outward (inward) propagating termination shock and an outward propagating shock (MHD rarefaction wave) that carries the signal of the disturbance into the far downstream plasma. The plasma immediately behind the new termination shock is separated from the downstream signal by a tangential discontinuity. The results of the model show that the speed of the new termination shock depends mainly on the magnitude of the change in dynamic pressure and are typically of order ∼ 100 km/s, comparable to the results of the gasdynamic models.
Relationship between the observed and modeled modulation of the dayside ionospheric convection by the IMF By component
Authors: Papitashvili V. O. , Clauer C. R. , Levitin A. E. , Belov B. A.
Published: February 2004

The Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN) electrodynamic model (IZMEM) provides global patterns of polar ionospheric potential and is parameterized by the interplanetary magnetic field (IMF). Given IMF conditions measured by an upstream satellite, the model yields a good global approximation to the polar ionospheric convection patterns assuming the proper time delay. While the model assumes static patterns and is based upon statistical regression analysis of high‐latitude magnetometer data, it can furnish an appropriate global context within which to examine time‐varying phenomena. We use the IZMEM model to further develop our understanding of the coordinated analysis of Greenland radar, riometer, and magnetometer data on August 2, 1991, which is one of the geospace environment modeling program intervals. The event is characterized by geomagnetic pulsations observed near local magnetic noon, having a 25‐min period and poleward phase propagation. A modulation of the intensity and orientation of the convection electric field is observed by the Sondrestrom incoherent scatter radar. Modeled global convection patterns show striking agreement with observations in the area covered by the radar field of view. We interpret observed phenomena as a direct ground‐based evidence of the IMF By component reconnection at the dayside magnetopause.
Saturation of frequency spectra for mesospheric wind velocity observed with the middle and upper atmosphere radar
Authors: Muraoka Yoshikazu , Fukao Shoichiro , Tsuda Toshitaka , Sugiyama Takuya
Published: February 2004

A spectral analysis was made of the radial (line of sight) wind velocity observed in the mesosphere by the middle and upper atmosphere radar at Shigaraki (34.9°N, 136.1°E), Japan, during the daytime of September 5 and October 6, 1988. The radial velocity was measured every 1 min at 300‐m range resolution along the three beam directions of 0°, 10°, and 20° from the zenith. In this paper we first give an overview of the analytical results, in particular, on the height dependence of frequency spectra obtained in a wide range of altitudes in the mesosphere. The frequency spectrum of the vertical velocity shows an almost flat response at frequencies less than the Brunt‐Väisälä frequency, and the spectral shape hardly changes over the entire height range of the analysis. On the other hand, the frequency spectra of the oblique velocities considerably change their shape and magnitude with height. Although the oblique spectra show a rather flat response at low frequencies when they are taken at the lowest altitudes in this analysis, the overall shape asymptotically approaches a −2 power law with increasing height. It is further shown that the height dependence of the oblique spectra is closely associated with the vertical propagation of an inertio‐gravity wave with a period of ∼12 hours. In addition, the overall f−2 dependence observed at around 76 km indicates a saturation of the oblique spectra as the upper limit. Finally, it is suggested that the frequency spectra for the vertical and oblique velocities can be well interpreted in terms of gravity wave theory.
Recovery signatures of lightning‐associated VLF perturbations as a measure of the lower ionosphere
Authors: Pasko Victor P. , Inan Umran S.
Published: February 2004

A new model of the physical processes associated with subionospheric VLF signal perturbations caused by lightning‐induced electron precipitation (LEP) bursts is developed to diagnose the state of the lower ionosphere (e.g., electron number density and rate coefficients for various chemical reactions) on the basis of measurements of VLF recovery signatures. The model accounts for the energy spectrum of the electron bursts precipitated by lightning‐generated whistlers, the chemical relaxation of enhanced secondary ionization in the nighttime D region due to LEP bursts, and quantitatively treats the resultant effects on propagation of the VLF signal in the Earth‐ionosphere waveguide. Application of the model to experimental data obtained for the VLF propagation path from NPM station (Hawaii) to Palmer station (Antarctica) indicates that effective electron detachment rate γ, enhanced secondary ionization profile (e.g., energy content of LEP bursts), as well as the ambient electron density distribution, may be estimated using observed subionospheric VLF recovery signatures. The effective detachment rate was identified as ∼10−18 N s−1, where N is total number density of neutrals. Model indicates in particular that the attachment‐detachment processes play the dominant role in recovery of subionospheric VLF signal perturbations on timescales ∼ 100 s, and that the observed perturbations of the NPM‐Palmer signal correspond to the LEP bursts consisting of relatively soft (< 250 keV) electrons.
Prognoz 10 energetic particle data: Leakage from the magnetosphere versus bow shock acceleration
Authors: Kudela K. , Sibeck D. G. , Slivka M.
Published: February 2004

We present a case study and a statistical survey of Prognoz 10 energetic particle and magnetic field observations from the solar wind to the magnetosphere. The results provide evidence for both Fermi acceleration and leakage from the magnetosphere. Energetic ion (E > 15 keV) flux levels upstream of the bow shock and in the outer magnetosphere generally exceed those in the magnetosheath. Energetic electrons rarely accompany ion fluxes in the outer magnetosheath and upstream of the bow shock, but commonly accompany those in the inner magnetosheath. Upstream flux levels increase as the interplanetary magnetic field (IMF) rotates toward the bow shock normal, consistent with Fermi acceleration. Although the north/south magnetosheath magnetic field orientation has no effect upon energetic particle flux levels in the outer magnetosheath, flux levels in the inner magnetosheath increase during periods of southward IMF. We interpret the results as indicating that Fermi acceleration supplies most of the ions observed in the outer dayside magnetosheath but that leakage from the magnetosphere supplies most of the ions observed in the inner magnetosheath.
Modeling of pickup ion distributions in the Halley cometosheath: Empirical limits on rates of ionization, diffusion, loss and creation of fast neutral atoms
Authors: Huddleston D. E. , Neugebauer M. , Goldstein B. E.
Published: February 2004

The shape of the velocity distribution of water group ions observed by the Giotto ion mass spectrometer on its approach to comet Halley is modeled to derive empirical values for the rates of ionization, energy diffusion, and loss in the midcometosheath. The model includes the effect of rapid pitch angle scattering into a bispherical shell distribution as well as the effect of the magnetization of the plasma on the charge exchange loss rate. It is found that the average rate of ionization of cometary neutrals in this region of the cometosheath appears to be of the order of a factor 3 faster than the “standard” rates ∼1×10−6 s−1 that are generally assumed to model the observations in most regions of the comet environment. For the region of the coma studied in the present work (∼1−2×105 km from the nucleus), the inferred energy diffusion coefficient is D0 ≈ 0.002 to 0.005 km²s−3, which is generally lower than values used in other models. The empirically obtained loss rate appears to be about an order of magnitude greater than can be explained by charge exchange with the “standard” cross section of ∼2×10−15 cm². However such cross sections are not well known and for water group ion / water group neutral interactions, rates as high as 8×10−15 cm² have previously been suggested in the literature. Assuming the entire loss rate is due to charge exchange yields a rate of creation of fast neutral atoms of the order of ∼ 10−4 s−1 or higher, depending on the level of velocity diffusion. The fast neutrals may, in turn, be partly responsible for the higher‐than‐expected ionization rate.
Merged interaction regions and large‐scale magnetic field fluctuations during 1991: Voyager 2 observations
Authors: Burlaga L. F. , Ness N. F.
Published: February 2004

This paper analyzes Voyager 2 observations of the magnetic field between 33.6 AU and 36.2 AU during 1991 when extraordinary events were observed on the Sun and in the heliosphere. The magnetic field strength signal B(t) has the unusual form of two large transient merged interaction regions (MIRs) on a fluctuating background. The two MIRs moved past the spacecraft in 32 days and 18 days, respectively. The mean field strength in each transient MIR was ≈2.6 times the mean field during the remaining part of the year (0.11 nT). Each of the MIRs is related to a fast stream. The magnetic field is strong throughout each stream, suggesting that the strong fields are carried by the streams as well as produced by shock and stream compression. The fluctuations in B(t) during 1991 are not multifractal, and the MIRs cannot be approximated as multifractal clusters of intense magnetic fields. The distribution of the hour‐averaged magnetic field strengths is approximately lognormal over 90% of its intermediate range, and it has an exponential tail for B greater than the average magnetic field strength. The elevation angles of B have a normal distribution with a standard deviation of 16° ± 4°. The distributions of the azimuthal angles of B in the ranges 0°‐180° and 180°‐360° are approximately normal over a more limited range, and non‐Gaussian tails associated with nearly radial magnetic fields; the standard deviations are ≈40°. Individual sectors are present throughout most of the interval, even in the MIRs, but there is no recurrent sector pattern. A model of the large‐scale fluctuations in 1991 will have to include both deterministic and statistical factors.
Cosmic ray decreases and shock structure: A multispacecraft study
Authors: Cane H. V. , Richardson I. G. , von Rosenvinge T. T. , Wibberenz G.
Published: February 2004

We examine >60‐MeV/amu ion data from three spacecraft (IMP 8 and Helios 1 and 2) at the time of a number of short term (<20‐day duration) cosmic ray decreases (>1 GeV) detected by ground‐based neutron monitors in the years 1976 to 1979. The multispacecraft data allow us to investigate the structure of the modulation region and in particular the relative importance, as a function of location, of the shock and shock driver (ejecta) in causing the reduction in particle densities. Although the shocks contributing to cosmic ray decreases often have particle enhancements associated with them in the >60‐MeV/amu data, this is not the case for three of the events discussed in this paper where a shock‐associated decrease is also evident. Whereas the shock can cause an increase or decrease at low (i.e., less than neutron monitor) energies, the reduction of particle densities in the driver, if it is intercepted, is usually evident at all energies. Thus the overall shape of a decrease at >60 MeV/amu depends primarily on whether the ejecta is intercepted. We find that the particle density inside ejecta increases with increasing radial distance from the Sun. In many of the events in this study, entry and exit of ejecta are accompanied by abrupt changes in the decrease and recovery rates which indicate that the effect of the ejecta is local. In contrast, the effect of the shock lasts many days after the shock has passed by and is evident at large angular distances from the longitude of the solar source, i.e., the effect of the shock is nonlocal. Within 1 AU there seems to be no radial dependence of the shock effect. One cosmic ray decrease seen at Earth, which had an unusual profile, can be understood if the median plane of the ejecta was inclined to the ecliptic.
IR excitation of contaminant water by oxygen for the space shuttle at low Earth orbit altitude
Authors: Zhou D. K. , Pendleton W. R. , Bingham G. E. , Thompson D. C. , Raitt W. J. , Nadile R. M.
Published: February 2004

As the water outgas cloud of a space shuttle passes through the rarefied atmosphere at orbital altitude, collisions occur between the gases with sufficient energy to excite infrared‐active water molecules to various vibrational and rotational states. An infrared contaminant model (IR model) has been developed to study the shuttle‐induced excitation and emission of water molecules outgassed from the space shuttle. The focus of the first application of the model is translation‐to‐vibration (T‐V) energy transfer since estimates suggest that this process should dominate the production of vibrationally excited H2O under typical low Earth orbit conditions. Using the velocity and position distribution functions of interacting neutral gases obtained from a neutral gases interaction model, the spatial distributions of excitation and IR radiation from contaminant water are computed, and typical results are presented. Infrared spectral data (450‐2500 cm−1), measured by the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS‐1A) sensor on STS‐39 (April 28 to May 6, 1991) at an altitude near 265 km, are used to test model predictions. The dependence of the radiant emission structure and brightness on outgassing rates and altitudes is discussed. The time history of the contaminant water outgassing rate is inferred for STS‐39, and it is compared with the mass‐spectrometer‐based results for STS‐4 (June 26 to July 4, 1982). Also, estimates of H2O column density at mission elapsed time (MET) 50 hours are compared for missions STS‐2, STS‐3, STS‐4, and STS‐39.