bluuebunny/crossref_metadata_2025 huggingface dataset:
Pioneer Venus Orbiter magnetic field and plasma observations in the Venus magnetotail
Authors: Slavin J. A. , Intriligator D. S. , Smith E. J.
Published: February 2008

This study uses Pioneer Venus orbiter (PVO) magnetometer and plasma analyzer measurements to investigate the draped‐field tail of Venus with an emphasis on determining the magnetic field and plasma conditions within the various tail regions and their dependence upon interplanetary magnetic field (IMF) orientation. For this purpose PVO orbits during which the spacecraft's high inclination trajectory took it through the central magnetotail were identified. The criteria used to select these orbits were the existence of relatively complete magnetic field and plasma data during crossings of the magnetotail which lasted more than 3 hours. Examination of observations from four PVO tail seasons in 1981–1983 produced 12 orbits meeting these requirements. Analysis of the observations taken during those orbits indicates that the distribution of plasma within the magnetotail is highly asymmetric and controlled by the orientation of the IMF. In the plasma sheet and adjacent lobe regions downstream of the Venus hemisphere over which the solar wind motional electric field, Esw = −Vsw × Bsw, is directed away from the planet, PVO observed increasing fluxes of H+ and O+ as the spacecraft moves away from the tail axis toward the outer boundary of the tail. The greatest concentration of these O+ ions was found in the vicinity of the cross‐tail current layer downstream of this Venus hemisphere. No O+ ions were observed outside of the magnetotail based upon the magnetic field data and the definitions adopted in this study. However, only slow, <310 km/s, antisolar directed O+ is detectable by the PVO plasma analyzer and this factor may be the reason for the frequent observation of O+ in the cross‐tail current and the absence of measurable O+ outside of the magnetotail where the bulk flow speeds are significantly slower than in the distant ionosheath. Downstream of the Venus hemisphere over which the solar wind motional electric field is directed in toward the planet, PVO does not usually observe significant fluxes of E/Q = 0–8 kV ions, except sometimes directly adjacent to the outer boundary of the tail. The outer boundary of the Venus magnetotail downstream of this hemisphere is typically a well‐defined magnetopauselike current layer. The magnetotail‐ionosheath interface downstream of Venus hemisphere over which the solar wind motional electric field is outward, however, is very broad and resembles a slow mode expansion fan with field strength slowly decreasing and plasma density gradually increasing as PVO moves outward and enters the ionosheath. These results are interpreted as being due to the more efficient pick‐up of newly ionized atmospheric neutrals over the Venus hemisphere where the initial gyromotion takes the newly created ions away from the dense, lower atmosphere where they might be lost due to scattering (Cloutier et al., 1974). The implications of these findings for the formation and maintenance of the Venus magnetotail are discussed.
Some aspects of the relation between Pi 1‐2 magnetic pulsations observed at L = 1.3‐2.1 on the ground and substorm‐associated magnetic field variations in the near‐Earth magnetotail observed by AMPTE CCE
Authors: Yumoto K. , Takahashi K. , Saito T. , Menk F. W. , Fraser B. J. , Potemra T. A. , Zanetti L. J.
Published: February 2008

The relation between Pi 1–2 pulsations on the ground and substorm‐associated magnetic field variations in space has been studied using data obtained on the ground at low‐latitude conjugate stations (L = 1.3–2.1) and in the near‐Earth magnetotail by the AMPTE CCE spacecraft. The ground‐based data were acquired in a campaign period from July 20 to September 16, of 1986, during which the apogee of CCE (8.8 RE) was located between 2330 and 0230 hours magnetic local time. Of 16 clear magnetic field dipolarizations observed at CCE, all had a corresponding Pi 2 pulsation on the ground, with a time lag of +1 to −7 min. For most (13) of these cases, the time lag was equal to or shorter than 2 min. However, we also found Pi 2 pulsations that do not accompany a dipolarization at CCE. These results are consistent with previous observations, which showed that Pi 2 pulsations are a global indicator of the expansion phase onset of a substorm, whereas dipolarizations occur in a limited region in the near‐Earth magnetotail. One of the 16 events, which occurred on August 28, 1986, is studied in detail because a 13‐s Pi 1 pulsation was observed on the ground in addition to an ordinary Pi 2 pulsation. For this event, CCE also observed a ∼13‐s oscillation at ∼8.1 RE in the mid‐plane of the magnetotail near midnight (Takahashi et al., 1987). We suggest that field line resonance driven by a quasi‐monochromatic oscillation in the near‐Earth tail is the cause of the Pi 1 pulsation observed on the ground. The commonly observed Pi 2 pulsations could be attributed to other wave excitation mechanisms including transient response of the magnetospheric cavity to a substorm‐associated impulse.
Discrete wave packets upstream from the Earth and comets
Authors: Le G. , Russell C. T. , Smith E. J.
Published: February 2008

Discrete wave packets are nearly monochromatic waves lasting several wave cycles which are frequently found on the leading edge of steepened low‐frequency waves or shocklets upstream from the Earth's bow shock. They apparently are whistler mode waves which are generated as the shocklets steepen. These shocklets are blown back across the spacecraft, so that the structure is observed in reverse order. Similarly appearing waves are present at comet Giacobini‐Zinner. To determine if these waves are the same as those observed at Earth we have undertaken a detailed comparison of the properties of the two sets of waves. The waves have very similar amplitudes, durations, and directions of propagation. Thus we feel that the source mechanism for the two phenomena must be the same, i.e., wave steepening.
Derivation of ion‐neutral collision frequencies from a Combined Ion Line/Plasma Line Incoherent Scatter Experiment
Authors: Bjørnå N.
Published: February 2008

Data from a combined ion line/plasma line incoherent scatter experiment with the European Incoherent Scatter (EISCAT) UHF radar are used to derive the ion‐neutral collision frequency in the lower E region (101–114 km). The ion line measurement is based on a 5×14 µs multipulse scheme giving a range resolution of 3 km. A spectrum analyzer is used to measure plasma line signals from the same altitudes. A 1‐hour period at local noon where stable photoelectron enhanced plasma lines were detected is chosen for the analysis. The temperature/collision frequency ambiguity in the ion line autocorrelation function is resolved by incorporating the observed plasma line offset in the least squares fitting procedure. No assumption on the electron‐to‐ion temperature ratio is needed. The fitted collision frequencies compare well with a profile based on the mass spectrometer/incoherent scatter (MSIS) 1983 neutral atmosphere model.
The evolution of corotating stream fronts near the ecliptic plane in the inner solar system, 1. Two dimensional fronts
Authors: Pizzo V. J.
Published: February 2008

Numerical models of corotating solar wind flows have enjoyed considerable success in simulating the evolution of shocks and corotating interaction regions (CIRs) in the region beyond 1 AU, but their performance with respect to stream fronts located nearer the Sun has been somewhat disappointing. In particular, they tend to predict erroneously that corotating shock pairs should occur relatively frequently within 1 AU, given the sort of sharp boundaries between slow and fast flows observed at stream fronts near 0.3 AU by Helios. We use an existing two‐dimensional MHD numerical model for corotating flow in the supersonic, superalfvenic solar wind to show that the predictions of premature shock pair formation are due to improper specification of flow conditions on the initial surface (inner boundary) used as the starting point in such models. This faulty initialization leads to the generation of a physically extraneous strong compression along the stream interface just outside the initial surface, which results in the appearance of evolutionary artifacts (like spurious discontinuities) further on in the solution. We describe an initialization scheme incorporating flow conditions more appropriate to stream fronts near the Sun and demonstrate that it produces the smooth initial behavior expected on physical grounds. Thus free of the evolutionary artifacts, we see that the shear flow at the stream interface approximately balances the kinematic steepening near the Sun, which for typical input conditions keeps the corotating shock pair from forming before about 1.5 AU. We describe the criteria for shock formation in terms of the interface dynamics and show that the steepening process cannot be treated even approximately with conventional kinematic techniques. In a subsequent paper we investigate how the three‐dimensional geometry of the stream front affects the dynamical evolution and the resulting CIR structure.
Orientation, motion, and other properties of flux transfer event structures on September 4, 1984
Authors: Papamastorakis I. , Paschmann G. , Baumjohann W. , Sonnerup B. U. Ö , Lühr H.
Published: February 2008

Three flux transfer events (FTEs), observed by the AMPTE/IRM spacecraft in the southern hemisphere magnetosheath are studied by use of variance analysis of measured magnetic fields, B, and convection electric fields, Ec = −v × B, with the objective of determining the orientation and motion of the flux tube or magnetopause bulge causing the FTE signature. These FTEs preceded a series of magnetopause crossings during which the high plasma flow speeds, characteristic of quasi‐steady reconnection, were present. The main results are as follows: (1) For each FTE, a moving so‐called deHoffmann‐Teller (HT) frame of reference can be found, in which the local plasma velocities are nearly antiparallel to the local B vectors and have magnitudes in the range 70%–90% of the local nominal Alfvén speed (assuming all measured ions to be protons). The velocities of motion, vHT, of the HT frames for all three events, and for two subsequent magnetopause crossings, are sufficiently similar so that a single HT frame orders the data in this manner for one full hour. (2) In the first FTE, the spacecraft appears to have sampled fields and flow around a moving tube or elongated magnetopause bulge. The tube orientation and motion (given by the component of vHT perpendicular to the tube axis) could be determined along with impact parameter (ℓ ≃ 1.4a), tube diameter (2a ≃ 8000 km), and, with reasonable assumptions, tube length (L>20,000 km). The tube was found to move southward past the spacecraft, consistent with the observed negative‐positive signature in the component of B along the magnetopause normal. The ambient magnetosheath plasma moved in the opposite direction. (3) For the second and third FTEs, which were close encounters (ℓ/a<1), the tube orientation and therefore its motion could not be reliably determined. (4) On the whole, the observations are consistent with ongoing magnetopause reconnection with a time‐modulated reconnection rate that leads to repeated ejection of bulges in the magnetopause from the reconnection site.
Observations of low‐frequency radio emissions in the Earth's magnetosphere
Authors: Filbert Paul C. , Kellogg Paul J.
Published: February 2008

A study is made of electromagnetic radiation in the Earth's magnetosphere in the frequency range between 10 kHz and 80 kHz using data from the University of Minnesota Plasma Wave Experiment aboard the IMP 6 satellite. Two types of radio emissions are investigated. First is the nonthermal continuum radiation, and it is found that discrete enhancements above ambient levels are correlated with enhancements of the magnetic substorm index AE and appear to follow the onset of the negative bay feature of the AU index by about 20 min or so. The directions of these discrete source regions of continuum radiation are measured as a function of time, and movement of the source region in a dusk‐to‐dawn direction is directly observed. This drift motion is used to measure the energy of the generating electrons by a time‐of‐flight method, and a range between 10 keV and 50 keV is found in agreement with previous studies. A second type of radiation is also observed which correlates with auroral kilometric radiation (AKR) on a time scale of ∼1 min. This radiation lies between 10 and 60 kHz with a spectral peak near 30 kHz and is found to have a source direction very near that of the coincident AKR. The lower frequency of the spectral peak, in conjunction with the analysis of the spin‐modulated wave data, suggests a source location at a higher elevation than the higher‐frequency AKR indicating a source altitude of roughly 3 Earth radii.
An experimental study of electrostatic ion cyclotron waves in a two‐ion component plasma
Authors: Suszcynsky D. M. , D'Angelo N. , Merlino R. L.
Published: February 2008

Two‐ion component electrostatic ion cyclotron (EIC) waves are studied in a single‐ended Q‐machine containing variable concentrations of Cs+ and K+ ions. Measurements of the excitation and propagation characteristics of the waves are presented and compared with available theories. The waves are internally excited by drawing an electron current along the magnetic field to an 11‐mm exciter disk. Both Cs+ and K+ EIC waves can be simultaneously excited over a large range of relative ion concentrations. The EIC wave features are found to depend on the relative ion concentrations. As the light ion (K+) concentration is increased, the frequency of the heavy ion (Cs+) mode shifts toward its cyclotron frequency. Likewise, the frequency of the light ion mode approaches the light ion cyclotron frequency as the heavy ion concentration is increased. The critical drift velocity for the excitation of the heavy ion (Cs+) mode is more strongly dependent on the relative ion concentrations than that of the light ion (K+) mode. These results are of interest in the context of EIC wave excitation in the ionosphere.
A three‐dimensional time‐dependent model of the polar wind
Authors: Schunk R. W. , Sojka J. J.
Published: February 2008

A time‐dependent global model of the polar wind was used to study transient polar wind perturbations during changing magnetospheric conditions. The model calculates three‐dimensional distributions for the NO+, O2+, N2+, N+ and O+ densities and the ion and electron temperatures from diffusion and heat conduction equations at altitudes between 120 and 800 km. At altitudes above 500 km, the time‐dependent nonlinear hydrodynamic equations for O+ and H+ are solved self‐consistently with the ionospheric equations. The model takes account of supersonic ion outflow, shock formation, and ion energization during a plasma expansion event. During the simulation, the magnetic activity level changed from quiet to active and back to quiet again over a 4.5‐hour period. The study indicates the following: (1) Plasma pressure changes due to Te, Ti or electron density variations produce perturbations in the polar wind. In particular, plasma flux tube motion through the auroral oval and high electric field regions produces transient large‐scale ion upflows and downflows. At certain times and in certain regions both inside and outside the auroral oval, H+‐O+ counterstreaming can occur, (2) The density structure in the polar wind is considerably more complicated than in the ionosphere because of both horizontal plasma convection and changing vertical propagation speeds due to spatially varying ionospheric temperatures, (3) During increasing magnetic activity, there is an overall increase in Te, Ti and the electron density in the F‐region, but there is a time delay in the buildup of the electron density that is as long as five hours at high altitudes, (4) During increasing magnetic activity, there is an overall increase in the polar wind outflow from the ionosphere, while the reverse is true for declining activity, (5) In certain regions, however, localized ionospheric holes can develop during increasing magnetic activity, and in these regions the polar wind outflow rate is reduced, (6) During changing magnetic activity, the temporal evolution of the ion density morphology at high altitudes can be different from, and even opposite to, that at low altitudes.
Velocity space diffusion of pickup ions from the water group at comet Halley
Authors: Coates A. J. , Johnstone A. D. , Wilken B. , Jockers K. , Glassmeier K.‐H.
Published: February 2008

We have studied the diffusion in velocity space of cometary ions using the distributions of ions measured by the implanted ion spectrometer on Giotto during the inbound pass. The measurements were transformed into a frame comoving with the solar wind and oriented with the magnetic field. The observations show the evolution of the pitch angle distribution in the solar wind turbulence to form a shell from the initial ring. Diffusion in energy takes place simultaneously but on a longer time scale. Comparison with theory is inhibited by the lack of a suitable spatial model, but the simple arguments we can make indicate that pitch angle diffusion, and the process of parallel pickup, take place more slowly than theory suggests.
Generation models of electron conies
Authors: Roth I. , Hudson M. K. , Temerin M.
Published: February 2008

Electron distribution functions with a peak oblique to the magnetic field, adjacent to but distinct from loss cone features, have been observed by the DE 1, Viking and S3‐3 spacecraft in passes through the nightside auroral zone, polar cap, dayside cusp and extended dayside auroral oval. Using particle simulations, we investigate two types of wave excitation and particle acceleration mechanisms which may contribute to producing these electron conic distributions. The first involves excitation of upper hybrid waves by the electron loss cone, and the subsequent perpendicular heating of the background and thermal electrons. The second involves excitation of downward propagating parallel modes by an auroral electron beam which frequently accompanies the upflowing electron conics. These modes provide parallel acceleration, which modifies the electron distribution function. Those electrons which are not lost to the atmosphere and mirror back up the magnetic field line give rise to enhancements in the distribution function at the edge of the loss cone.
Diurnal and seasonal variation of the nighttime OH (8‐3) emission at low latitudes
Authors: Abreu Vincent J. , Yee J. H.
Published: February 2008

Nighttime morphological maps of the hydroxyl emission (8–3 band) measured by the Visible Airglow Experiment on board the Atmosphere Explorer E satellite are presented for solstice and equinox conditions. The volume emission rate profile of the emission was determined by inverting satellite perigee observations. This profile was later used to convert limb measurements into zenith intensities of the emission. A theoretical model has been used to correct twilight observations for the presence of the thermospheric O+(²P) emission at 7320–7330 Å. The derived morphology agrees well with ground based measurements and theoretical simulations.
Characteristic velocity of equatorial F region irregularities determined from spaced receiver scintillation data
Authors: Bhattacharyya A. , Franke S. J. , Yeh K. C.
Published: February 2008

Spaced receiver scintillation data for a 250‐MHz signal transmitted from a geosynchronous satellite and received at an equatorial station are subjected to correlation analysis to determine the average drift of equatorial F region irregularities transverse to the signal path, as well as the characteristic velocity which is a measure of the temporal variability of the irregularities. These parameters are computed for periods with significant amplitude scintillations during different seasons and magnetic activity levels. Three features of the characteristic velocity Vc have been observed in this study. Firstly, a steep decline in Vc following the onset of scintillation during the pre‐midnight period is reconfirmed as has been observed previously. Secondly, the characteristic velocity shows a gradual increase in the post‐midnight period which, when combined with the pre‐midnight decline, gives a minimum around 0100 LT. Thirdly, the pattern followed by the characteristic velocity in the course of a magnetically disturbed night is on some occasions substantially different from that of a quiet period. Theoretical calculations of the space‐time correlation function of moderate to strong intensity scintillations show that the steep decline of Vc, which is almost the same as the standard deviation of the velocity fluctuations, calls for a geophysical explanation while the minimum around 0100 LT may be attributed to the phenomenon of scintillation itself. The possibility that the velocity fluctuations are caused by electric field fluctuations associated with the generalized Rayleigh‐Taylor instability is explored in this paper. This conjecture provides a plausible explanation for the steep decline of the characteristic velocity observed in the pre‐midnight hours.
Evidence of local generation of Alfvénic turbulence in the solar wind
Authors: Bavassano B. , Bruno R.
Published: February 2008

Recent papers suggested that solar wind regions with low Alfvénic‐type correlation have to be related to local generation of Alfvén waves with opposite sense of propagation. Stream shears were proposed as local sources of fluctuations through the development of a Kelvin‐Helmholtz instability. We show that a decrease of the Alfvénic correlation can be accompanied by the appearance of strong compressive fluctuations. In these cases the low Alfvénic correlation should be related not to a superposition of Alfvén modes with mixed propagation but rather to a lack in Alfvénicity of the fluctuations themselves, due to the presence of other kinds of perturbations or structures in the solar wind. Conversely, when incompressibility is reasonably well satisfied, we are able to gain convincing evidence of an Alfvén wave generation in a velocity shear region.
Solar cycle variations of the electron densities near the ionospheric peak of Venus
Authors: Kim J. , Nagy A. F. , Cravens T. E. , Kliore A. J.
Published: February 2008

Photochemical equilibrium calculations of electron and ion densities, appropriate for altitudes below about 180 km, were carried out for the Venus dayside ionosphere corresponding to solar cycle maximum and minimum conditions. The results were compared with data from radio occultation measurements. The agreement between the calculations and measurements was in general quite good. These comparisons indicate that the most commonly used neutral atmosphere model of Venus (Hedin et al., 1983) predicts densities which are somewhat low near the electron density peak for solar cycle maximum, but provides surprisingly good predictions for solar cycle minimum conditions.
Quantitative magnetic field model including magnetospheric ring current
Authors: Kosik Jean Claude
Published: February 2008

A quantitative model of the magnetic field in the ring current region of Earth's magnetosphere is developed by using poloidal vector fields. The magnetic field is divergence‐free by construction. The main features are described by a two‐term model, but a third term is necessary to describe the tilt effects. Contour plots of constant ΔB are in agreement with magnetometer data. Analytical expressions for the current density are derived, and the contour plots of constant JΦ are in topological agreement with those obtained from self‐consistent models.
Equatorial F region vertical plasma drifts during solar maxima
Authors: Fejer B. G. , de Paula E. R. , Batista I. S. , Bonelli E. , Woodman R. F.
Published: February 2008

Incoherent scatter radar measurements at Jicamarca are used to study the effects of large solar fluxes and magnetic activity on the F region vertical plasma drifts. The average drifts from the two last solar maxima are almost identical except in the late afternoon‐early evening sector where their variations with solar flux and magnetic activity are strongly season dependent. The average evening winter (May‐August) drifts appear to remain almost constant after a certain solar flux level is reached but increase with magnetic activity. The equinoctial evening drifts increase systematically with solar‐flux but decrease with magnetic activity. Very large prereversal enhancement velocities, up to about 80 m/s, were often observed during the 1978–1981 equinoctial periods when the solar flux was very high. Comparison of incoherent scatter radar drifts with vertical velocities inferred from ionosonde observations indicate that the latter technique substantially underestimates the plasma drifts during periods of large solar fluxes except during winter.
Standing hydromagnetic waves in the Io plasma torus: Voyager 1 observations
Authors: Glassmeier K.‐H. , Ness N. F. , Acuña M. H. , Neubauer F. M.
Published: February 2008

Geomagnetic pulsations are one of the dominant features of the dynamics of the Earth's solar wind‐magnetosphere‐ionosphere coupling system. Whether such ultralow‐frequency waves are also excited within the Jovian magnetosphere has been the subject of a close inspection of Voyager 1 magnetic field observations during its close encounter with Jupiter. These observations clearly indicate the existence and an increase of ultralow‐frequency wave activity and indicate that the activity becomes more regular as soon as Voyager 1 entered the Io plasma torus at around 0700 spacecraft event time on March 5, 1979. In particular, periodic transverse and compressional magnetic field fluctuations with periods of about 1200 s and 800 s, respectively, are observed with the different periods pointing toward a decoupling between these two different types of oscillations. The coincidence between the increase in wave activity and the entry into the Io plasma torus is in support of treating the torus as a low Alfvén velocity region and thus as a hydromagnetic waveguide. A first theoretical treatment of hydromagnetic wave propagation within the torus suggests that decoupling of toroidal and poloidal type oscillations can occur under the condition of axisymmetry of the wave field. Numerical calculations of the fundamental mode toroidal and first harmonic poloidal eigenperiods for a model Jovian magnetosphere give values quite in accord with the observed periods. We thus conclude that nearly axisymmetric, decoupled toroidal and poloidal mode eigenoscillations of the Io plasma torus are observed, indicating a large‐scale source mechanism for the magnetic field fluctuations detected. The total electromagnetic energy of the observed torus oscillations is of the order of 1013 J and is thus significant with respect to the energetics of the Io plasma torus.
A study of geosynchronous magnetopause crossings
Authors: Rufenach C. L. , Martin R. F. , Sauer H. H.
Published: February 2008

Sixty‐four magnetopause crossings at geosynchronous orbit were detected by the GOES 2, 5, and 6 satellites between 1978 and 1986. The total accumulated crossing duration was 4 hours/year, or about 0.1% of the time. These rare events permit investigation of the interaction of the solar wind with the magnetosphere. This interaction has been examined through comparisons of solar wind and geophysical parameters during crossing episodes, and through superposed epoch event analysis of these periods. The study suggests that both high solar wind dynamic pressures and southward interplanetary Bz are required for the excursion of the magnetopause to geostationary orbit and beyond. Thus dynamic pressure alone is not sufficient to produce a magnetopause crossing; some erosion seems also to be required. An approximate relationship between the strength of the southward Bz and the subsolar distance to the magnetospheric boundary has been obtained: ΔR (RE) = 0.08 Bz (nT), where ΔR is the change in standoff distance attributed to erosion. Further, magnetopause crossings are associated with the more intense, sudden‐commencement geomagnetic storms. On average, storm sudden commencements occur 5 hours before a crossing, while an average Bz turns southward 7 hours before the crossing.
Average electric wave spectra across the plasma sheet and their relation to ion bulk speed
Authors: Baumjohann W. , Treumann R. A. , LaBelle J. , Anderson R. R.
Published: February 2008

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer of the wave experiment and the three‐dimensional plasma instrument on board the AMPTE/IRM satellite, we have done a statistical survey on the electric wave spectral density in the Earth's plasma sheet. More than 50,000 10‐s‐averaged electric wave spectra were analyzed with respect to differences between their values in the inner and outer central plasma sheet and the plasma sheet boundary layer as well as their dependence on radial distance and ion bulk speed. High‐speed flows are dominated by broadband electrostatic noise with highest spectral densities in the plasma sheet boundary layer, where broadband electrostatic noise also exists during periods of low‐speed flows. The broadband electrostatic noise has a typical spectral index of about −2. During low‐speed flows the spectra in the central plasma sheet show distinct emissions at the electron cyclotron odd half‐harmonic and and upper hybrid frequency. On average, such waves do not appear as distinct emissions in the plasma sheet boundary layer, where they are either absent or masked by the intense broadband electrostatic noise always observed there. Finally, wave intensities during episodes of fast perpendicular flows are higher than those associated with fast parallel flows.
Quasi‐linear wave‐particle interactions in the Earth's radiation belts
Authors: Villalón Elena , Burke William J. , Rothwell Paul L. , Silevitch Michael B.
Published: February 2008

This paper studies the theory of gyroresonant interactions of energetic trapped electrons and protons in the Earth's radiation zones with ducted electromagnetic cyclotron waves. Substorm injected electrons in the mid‐latitude regions interact with coherent VLF signals, such as whistler mode waves. Energetic protons may interact with narrow‐band hydromagnetic (Alfvén) waves. A set of equations is derived based on the Fokker‐Planck theory of pitch angle diffusion. They describe the evolution in time of the number of particles in the flux tube and the energy density of waves, for the interaction of Alfvén waves with protons and of whistler waves with electrons. The coupling coefficients are obtained based on a quasi‐linear analysis after averaging over the particle bounce motion. It is found that the equilibrium solutions for particle fluxes and wave amplitudes are stable under small local perturbations. The reflection of the waves in the ionosphere is discussed. To efficiently dump the energetic particles from the radiation belts, the reflection coefficient must be very close to unity so waves amplitudes can grow to high values. Then, the precipitating particle fluxes may act as a positive feedback to raise the height integrated conductivity of the ionosphere which in turn, enhances the reflection of the waves. In addition, by heating the foot of the flux tube with high intensity, RF energy the mirroring properties of the ionosphere are also enhanced. The stability analysis around the equilibrium solutions for precipitating particle fluxes and wave intensity show that an actively excited ionosphere can cause the development of explosive instabilities.
Dayside observations of thermal‐ion upwellings at 800‐km Altitude: An ionospheric signature of the cleft ion fountain
Authors: Tsunoda R. T. , Livingston R. C. , Vickrey J. F. , Heelis R. A. , Hanson W. B. , Rich F. J. , Bythrow P. F.
Published: February 2008

There is a growing body of evidence that energetic heavy ions observed at one or more Earth radii over the polar cap originate from the dayside ionosphere in the vicinity of the dayside cleft. The ions, consisting mostly of O+, are often characterized by conic pitch‐angle distributions, suggesting that they have undergone acceleration transverse to geomagnetic field lines. This process of ion injection from a latitudinally localized source region in the dayside auroral oval followed by dispersal throughout the entire polar cap has been called the “cleft ion fountain.” But, except for placement of the ion injection source somewhere in the vicinity of the dayside auroral oval, very little is known about the altitude regime in which the source operates and the nature of the process that transports ionospheric ions into the magnetosphere. Results are presented of upward thermal‐ion flows measured at 800‐km altitude in the dayside polar ionosphere by the Hilat satellite. The characteristics of these thermal‐ion upwellings (TIU) are described and shown to be closely associated with the cleft ion fountain. We show that TIU events are latitudinally confined and spatially collocated with cleft electron precipitation, upward field‐aligned currents, and velocity gradients in magnetospheric convection.
Electric field mapping and auroral Birkeland currents
Authors: Kaufmann Richard L. , Larson Douglas J.
Published: February 2008

Magnetic field lines, electric fields and equipotentials have been mapped throughout the magnetosphere in the vicinity of strong Birkeland currents. It was found that a uniform electric field at either the ionospheric or the equatorial end of a field line can map to a highly structured field at the other end if strong Birkeland currents are located nearby. The initiation of sheet currents of the region 1‐region 2 scale size and intensity resulted in magnetic field line displacements of about 1/2 hour in local time between equatorial and ionospheric end points. As a result, a uniform dawn to dusk electric field at the equator mapped to an ionospheric electric field with strong inward pointing components (a southward component at the northern current sheet and a northward component at the southern current sheet) in the dusk hemisphere. Similar distortions were produced by Birkeland currents associated with narrow east‐west‐aligned auroral arcs. A specific model for the auroral current system, based on ionospheric measurements during a large substorm, was used to study effects seen during disturbed periods. An iterative procedure was developed to generate a self‐consistent current system even in the presence of highly twisted field lines. The measured ionospheric electric field was projected to the equatorial plane in the presence of the model Birkeland current system. It was concluded that such projections are more reliable if it is possible to start with an approximate equipotential structure in the ionosphere rather than attempting to project individual electric field point measurements. Several physical processes were seen to influence ionospheric and equatorial electric fields, and the associated plasma convection, during a substorm. Electic fields well tailward of the current diversion region near the equator and well poleward of the auroral display in the ionosphere were controlled by processes taking place at high altitude. Measurements are consistent with the presence of a roughly dawn to dusk equatorial electric field in this region. The diversion of cross tail current to the ionosphere takes place at an apparent westward traveling surge (WTS) and at the eastward end of the substorm current system. Ionospheric polarization appears to control the electric field pattern within this region. Two processes are important in producing spiral patterns near the strongest Birkeland current densities, at the WTS. Parallel electric fields and an inward pointing perpendicular electric field above the WTS produce circular plasma drift in the topside ionosphere. In addition, magnetic field distortion produces spiral magnetic field lines and a characteristic winding/unwinding plasma motion in the same region.
Growth of obliquely propagating ion cyclotron waves in the magnetosphere
Authors: Ludlow G. R.
Published: February 2008

We present linear growth rate calculations of electromagnetic ion cyclotron waves driven by hot anisotropic protons in a magnetized plasma. The angle between the wave vector and the magnetic field is arbitrary. This introduces Landau damping by the thermal electrons, an effect not present for strictly parallel propagation. The model contains concentrations of O+ and He+ as well as H+, with densities and temperatures typical of ion cyclotron wave generation regions inside the plasmasphere around L = 3. The behavior of growth regions in k space is studied. Although the growth rates peak at θ = 0°, they do not depend strongly on θ for θ<30°. At large wave normal angles, weak peaks in wave growth are found with much smaller growth rates than at 0°. Electron Landau damping of ion cyclotron waves maximizes at very oblique wave normal angles where the growth rate can be reduced significantly, but has little or no effect in the region of k space (θ<45°) where the waves are growing the fastest.
Compressibility and cyclotron damping in the oblique Alfvén wave
Authors: Harmon J. K.
Published: February 2008

Compressibility, magnetic compressibility, and damping rate are calculated for the obliquely propagating Alfvén shear wave in high‐ and low‐beta Vlasov plasmas. There is an overall increase in compressibility as beta is reduced from β = 1 to β≪1. For high obliquity θ and low frequency (ω≪Ωp) the compressibility C follows a k² wave number dependence; for high θ and low β the approximation C(k) ≈ kn² ≡ (kVA/Ωp)² holds for wave numbers up to the proton cyclotron resonance, where Ωp is the proton cyclotron frequency and VA is the Alfvén velocity. Strong proton cyclotron damping sets in at kn of the order of unity; the precise kn position of the damping cutoff increases with decreasing β and increasing θ. Hence compressibility can exceed unity near the damping cutoff for high‐θ waves in a low‐β plasma. The magnetic compressibility of the oblique Alfvén wave also has a k² dependence and can reach a maximum value of the order of 10% at high wave number. It is shown that Alfvén compressibility could be the dominant contributor to the near‐Sun solar wind density fluctuation spectrum for k>10−2 km−1 and hence might cause some of the flattening at high wave number seen in radio scintillation measurements. This would also be consistent with the notion that the observed density spectrum inner scale is a signature of cyclotron damping.
Source region of the smooth high‐frequency nightside Uranus kilometric radiation: A ray‐tracing study
Authors: Menietti J. D. , Wong H. K. , Wah D. A. , Lin C. S.
Published: February 2008

Assuming the cyclotron maser instability is the source mechanism of the smooth high‐frequency, nightside component of Uranian kilometric radiation, a plasma density model is developed for the nightside at r < 4.2 ru based on radio wave observations of the Planetary Radio Astronomy instrument on board the Voyager 2 spacecraft. Using this plasma model and the Q3 magnetic field model (Connerney et al., 1987), a systematic ray tracing of right‐hand extraordinary (RX) emission is performed from a grid of nightside sources along magnetic field lines at frequencies just above the RX cutoff. By requiring that the rays satisfy several observational constraints, we are able to specify the source region with more accuracy than previously prescribed. Our results indicate a source region extended in longitude compared to those previously published. In addition, the emission at higher frequencies that exhibits a periodic “bite‐out” appears to be observed by the satellite at wave normal angles in the range 90° < Ψ < 140°. The emission observed near closest approach has a different source region and may have different radiation lobe characteristics. These results are consistent with the cyclotron maser instability for moderately low densities in the source region (fp/fce < 0.25, where fp and fce are the plasma and gyrofrequency, respectively).
Characteristics of wave‐particle interactions during sudden commencements: 1. Ground‐based observations
Authors: Gail W. B. , Inan U. S. , Helliwell R. A. , Carpenter D. L. , Krishnaswamy S. , Rosenberg T. J. , Lanzerotti L. J.
Published: February 2008

ELF‐VLF (0.3–30 kHz) wave data measured at ground‐based observatories for 250 sudden commencements were analyzed for amplitude and spectral modifications and correlated with magnetic field and precipitating particle observations. Changes in ELF‐VLF wave activity at high‐latitude stations were observed in 50–60% of the events studied and for approximately 80% of the events when the observing station was on the dayside. Characteristic, well‐defined modifications of both coherent and incoherent ELF‐VLF wave emissions were observed, including wave growth of the order of 20 dB, increases in the upper frequency limit of the waves, and enhanced triggering of discrete emissions. Wave growth generally occurred first at lower frequencies and with increasing delay at upper frequencies. The growth rate for the incoherent wave emissions (0.3–2.7 dB/s) was found to be at least 2 orders of magnitude less than known growth rates for coherent waves. Measurable particle precipitation inferred from cosmic noise signal absorption was observed to begin simultaneous to within 5 s of the wave growth onset in a significant number of cases, suggesting that at least a part of the observed precipitation results from wave‐induced scattering.
Effect of short‐scale turbulence on kilometer wavelength irregularities in the equatorial electrojet
Authors: Ronchi C. , Sudan R. N. , Similon P. L.
Published: February 2008

The kilometer scale irregularities in the daytime equatorial electrojet are studied within the framework of a two‐fluid, nonlocal theory of the gradient drift instability. A separation of scales is introduced into the equations in order to model the effects of the subgrid, short‐wavelength (λ < 100 m) modes. The presence of the short‐scale turbulence is included in the large‐scale equations through the average nonlinear flux due to the small‐scale nonlinear terms. With the use of the linear ion continuity equation the nonlinear flux is expressed in terms of the large‐scale quantities and of the small‐scale density fluctuation spectrum. It is shown that the small‐scale turbulence contributes to the large‐scale equations through turbulent mobility and diffusion coefficients. For a particular choice for the small‐scale density fluctuation spectrum (modeled after some of the available rocket data), the turbulent mobility is determined as a function of altitude, and its peak equals a few times the classical Pedersen mobility value. The equilibrium solutions of the large‐scale equations are also derived in the presence of the short‐wavelength turbulence. The localization of the current layer is seen to shift toward higher altitudes, and the current density profile conforms well with some of the available experimental data. Neglecting at this point the large‐scale nonlinearities, the local and nonlocal linear growth rates of the long‐wavelength modes are also obtained and discussed. The renormalized linear nonlocal equations for the large scales are integrated numerically, and the effects of the turbulent mobility and of velocity shear are observed and discussed. Nonlocal modes with horizontal wavelengths in the kilometer range dominate the linear stage of the instability, thus providing a possible explanation for the experimentally observed predominance of such wavelengths in the electrojet's wave spectrum. The dispersive nature of the large‐scale modes is also discussed and reconsidered in the presence of the turbulent mobility term.
Secular changes in the cosmic ray equator
Authors: van der Walt A. J. , Stoker P. H.
Published: February 2008

Since the cutoff rigidity at the cosmic ray equator varies with longitude, the location of the minimum cosmic ray intensity obtained during a latitude survey depends on the direction by which the geographic equator is crossed. A method is developed to compare the location of the recorded minimum cosmic ray intensity with that of the predicted minimum along a route as calculated from cutoff rigidities. The data of several surveys, covering the period 1954–1986/1987, are used. Instead of the accurate but computer intensive trajectory‐tracing method, cutoff rigidities for every location and instant are interpolated from 5°×15° world grids. It is found that the positions of the minima in cosmic ray intensities obtained from a survey across the equator and those derived from the interpolated cutoff rigidities along the survey route are mutually consistent. The rms difference between the experimental and calculated positions of the cosmic ray minima for all the surveys amounts to only 0.32°. Major secular changes in cutoff rigidities occur in the Atlantic Ocean region. Whereas a fast northerly shift of the cosmic ray equator is evident between 320° and 345° longitude, the data points from different epochs within this longitude range show a negligible difference between the simulated and experimental positions of the route‐dependent cosmic ray equator. This indicates that the secular variation in cutoff rigidity is well accounted for by the grid values derived from the International Geomagnetic Reference Fields for 1955.0, 1965.0, and 1980.0, even when the values are extrapolated linearly beyond 1980.0 through to 1987 in equatorial regions.
Whistler precursors on a VLF transmitter signal
Authors: Paschal E. W.
Published: February 2008

Whistler precursors are discrete emissions which are occasionally seen just before two‐hop whistlers. Most theories of precursors assume they are triggered emissions and focus on creating a triggering signal with the proper time delay from the causative sferic. Whistler precursors have now been seen on a signal from the Siple VLF transmitter. Phase analysis shows that these precursors are caused by a rapid increase in growth activity, and not by a triggering signal.
Response of Venus exospheric temperature measured by neutral mass spectrometer to solar EUV flux measured by Langmuir probe on the Pioneer Venus Orbiter
Authors: Mahajan K. K. , Kasprzak W. T. , Brace L. H. , Niemann H. B. , Hoegy W. R.
Published: February 2008

The photoelectron current from the Pioneer Venus Langmuir probe has provided measurements of the total flux of solar EUV photons at Venus since 1979. The neutral oxygen scale height measured by the orbiter neutral mass spectrometer has permitted the exospheric temperature to be derived during the same mission. In this paper the EUV observations are used to examine the response of exospheric temperature to changes in solar activity, primarily those related to solar rotation. It is found that the dayside exospheric temperature quite faithfully tracks variations in the EUV flux. Comparison is also made with the Earth‐based solar activity index F10.7 adjusted to the position of Venus. This index varied from 142 to 249 flux units (10−22 W m−2 Hz−1) during the period of measurements. The exospheric temperature is better correlated with EUV flux than with the 10.7‐cm solar radio flux.
Determining the strength of the ring and the magnetopause currents during the initial phase of a geomagnetic storm using cosmic ray data
Authors: Flückiger E. O. , Smart D. F. , Shea M. A.
Published: February 2008

During a geomagnetic storm the strength of the magnetospheric current systems is strongly increased. In the initial phase of most events, however, the magnetic field at the Earth's equator (as characterized by the Dst index) shows only a relatively small perturbation due to the opposite magnetic effects caused by the magnetopause currents compared to the ring current. Analysis of Dst and of the cosmic ray cutoff rigidity changes at about 55° geomagnetic latitude offers the unique possibility to estimate the intensity of these two current systems separately. The procedure is illustrated for the geomagnetic storm on December 17, 1971.