bluuebunny/crossref_metadata_2025 huggingface dataset:
Oblique ring instability driven by nongyrotropic ions: Application to observations at comet Grigg‐Skjellerup
Authors: Cao J. B. , Mazelle C. , Belmont G. , Rème H.
Published: February 2004

We investigate the oblique behavior of low‐frequency left‐handed electromagnetic waves driven by a ring of nongyrotropic ions. We use the unperturbed orbit integral technique to obtain for the first time the dispersion equation of oblique modes for a nongyrotropic particle distribution in a magnetoplasma. When the oblique propagation angle θ with respect to the magnetic field increases, the polarization of the unstable wave mode rapidly loses the left hand circular nature of the parallel mode and tends to be linear. Electron density compression and magnetic field compression appear and both maximize at θ ∼22°. The difference between gyrotropic ring instability and nongyrotropic ring instability diminishes with the increase of obliquity. The nongyrotropy of the ring ions decreases the growth rate for large wave numbers and to a lesser extent at its maximum. In addition, the nongyrotropy increases slightly the electron density and magnetic field compressions. The growth rate of the mode with θ >0 (k• Vp<0, Vp being the perpendicular plasma flow velocity) is larger than that of the mode with θ<0 (k• Vp>0); this effect does not exist in the gyrotropic case. The plasma parameters used in these theoretical calculations are those observed during the Giotto encounter with comet Grigg‐Skjellerup, when nongyrotropic newborn heavy ion ring distributions as well as associated low frequency waves have been reported. The results on the electron density compression are in good quantitative agreement with the observations upstream of the cometary shock.
Identification of auroral oval boundaries from in situ magnetic field measurements
Authors: Gary J. B. , Zanetti L. J. , Anderson B. J. , Potemra T. A. , Clemmons J. H. , Winningham J. D. , Sharber J. R.
Published: February 2004

We have conducted an investigation into the relationship between magnetic field fluctuations below 100 Hz observed with the UARS and Freja magnetic field experiments and concluded that increases in this “ac” activity serve as an excellent indicator of the boundaries of large‐scale field‐aligned current systems. Magnetic field fluctuations in these regions generally correspond to increased electron fluxes at energies below roughly 2 keV. Using a single equatorward boundary crossing as a reference point, the statistical field‐aligned current pattern of Iijima and Potemra [1978] for disturbed magnetic conditions can be extrapolated to provide an estimate of the global position of the auroral oval. Real‐time in situ magnetic field measurements from high‐latitude spacecraft can be used to provide a quick, simple, locator of the equatorward boundaries of the large‐scale field‐aligned currents, accurate to within ± 2° in latitude. A proof‐of‐concept implementation of this technique has been performed using magnetic field data acquired by the Freja spacecraft. Development of the automated detection algorithm, essentially a measurement of the standard deviation of the low‐frequency magnetic field measurements, was aided by using UARS magnetic field data. The UARS data were binned by MLT and magnetic latitude for varying Kp conditions and demonstrated observable ac boundaries at all local times and all Kp levels. In a separate statistical study consisting of 96 UARS high‐latitude observations, we determined the differences between the observed and estimated equatorward boundaries of the large‐scale field‐aligned current system for each high‐latitude pass. This difference was within 2 deg of latitude in 53 of 96 cases, with 84 of 96 cases within 5 deg.
Multiscale current structures observed by Freja
Authors: Stasiewicz K. , Potemra T.
Published: February 2004

Multiscale structure of field‐aligned currents and plasma density records are analyzed with multiresolution wavelet technique. We show that field‐aligned currents measured by Freja in the auroral region have a spectrum of scales from ∼ 500 km down to about a hundred meters. The intensity of currents has a maximum of ∼100–300 μAm−2 at the subkilometer scales. The locations of small‐scale currents (corresponding presumably to discrete auroral structures) is not coincident neither with strong macroscopic density gradients, nor with the most intense large‐scale currents. These structures are observed to cluster inside medium scale density cavities and represent most likely interference patterns of short (λ⊥ ≥ c/ωpe) inertial Alfvén waves. At kilometer scales the energetic electrons (30 eV to 30 keV) carry less than ∼ 20% of the magnetometer current, indicating that thermal electrons are the major current carriers. We discuss also vector polarization of the magnetic perturbations at various scales which may be used as a tool for resolving polarization patterns and sense of rotation for Alfvén waves, vortices, and current filaments.
Flux rope structures in the magnetotail: Comparison between Wind/Geotail observations and global simulations
Authors: Winglee R. M. , Kokubun S. , Lin R. P. , Lepping R. P.
Published: February 2004

On December 11, 1994, Geotail observed several reconnection (negative Bz) events at a position 30–40 RE down the tail over a 24 hour period. This period is also interesting because the solar wind conditions were well monitored by Wind, which was only about 20 RE in front of the bow shock, and because IMP 8 was at nearly the same x value as Geotail, but at high latitudes. Global fluid simulations that incorporate higher‐order corrections to Ohm's law are used to produce a three dimensional picture of the reconnection site in conjunction with the Geotail data. It is shown that many of the negative Bz events have appreciable core magnetic fields, which in the modeling appear to form around 20 RE about 10 min earlier than observed by Geotail. The predicted time of the formation coincides approximately with increased ionospheric activity observed by the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer chain. The flux ropes also appear to be highly localized (2–4 RE), particularly in x and z when they are first created. In addition, they have a distinctive bipolar core magnetic field, but the probability that a spacecraft observes a unidirectional or bipolar signature is highly dependent on the position of the center of the current sheet relative to the spacecraft. Bipolar signatures are predicted to be most easily seen when the spacecraft is near the center of the current sheet, while unidirectional signatures are most likely to be observed at more distant encounters.
Magnetopause structure and the role of reconnection at the outer planets
Authors: Huddleston D. E. , Russell C. T. , Le G. , Szabo A.
Published: February 2004

In situ measurements have been obtained at the magnetopause boundary of all the giant outer planets. The Jovian magnetopause was probed by Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, and most recently by Galileo. Saturn was visited by Pioneer 11 and Voyagers 1 and 2, and Uranus and Neptune were visited by Voyager 2. The observations at Jupiter show evidence for flux transfer event (FTE) structures and rotational discontinuities associated with magnetic reconnection with the interplanetary magnetic field (IMF), but in accord with previous studies we find no FTE signatures beyond Jupiter in the data sets we studied. At Saturn, one magnetopause encounter shows evidence for reconnection in the form of a rotational discontinuity with a finite magnetic field component through the boundary and significant plasma acceleration. At Uranus and Neptune the flank magnetopause boundary observed by Voyager 2 exhibits a complex structure (similar to that seen at Earth during high‐β conditions) for which unsteady reconnection appears to occur. Closely spaced multiple magnetopause crossings at Jupiter and Saturn are consistent with a boundary surface disturbance or wave, occurring with unsteady reconnection, and coincident with a jump in upstream solar wind dynamic pressure in the Jupiter case. Because of the transient, bursty nature of the reconnection signatures and their low (∼0.1 to 0.4 mV/m) convective electric fields compared to those of corotation at Jupiter and Saturn, the role of reconnection in driving the dynamics of these magnetospheres is thought to be minimal in the general case. Nevertheless, the unsteady reconnection observed is important to the properties of the boundary layers. At Uranus and Neptune, observations are limited (Voyager 2 only) but suggest that bursty reconnection at the flank magnetopause can only remove plasma (in the antisunward direction), and infrequent reconnection on the dayside would provide little opportunity for solar wind entry to energize the ionospheric plasma and supply the magnetosphere; these factors may contribute to producing the relatively empty magnetospheres that were seen.
Forecasting ionospheric structure during the great geomagnetic storms
Authors: Cander L. R. , Mihajlovic S. J.
Published: February 2004

Characteristics of midlatitude ionospheric disturbances during several great geomagnetic storms have been investigated using data from the European geomagnetic observatories and ionospheric stations with the aim of developing the local forecasting models, as part of the prediction and retrospective ionospheric modeling over Europe project. Based on the analysis of the geomagnetic storms of February 6, 1986, and March 13, 1989, a detailed picture of the local H component of geomagnetic field and the ionospheric critical frequency ƒ0F2 variations is presented. The results show that ƒ0F2 was dramatically changed above or below the monthly median level in a relatively narrow band about 15° of latitude and 30° longitude during the different phases of the storms. These results support the view that day‐to‐day F region ionospheric variability is essentially altered in great storms. Consequences of those effects for short‐term modeling purposes are discussed.
Experimental constraints on pulsed and steady state models of the solar wind near the Sun
Authors: Feldman W. C. , Habbal S. R. , Hoogeveen G. , Wang Y.‐M.
Published: February 2004

Ulysses observations of the high‐latitude solar wind were combined with Spartan 201 observations of the corona to investigate the nature and extent of uncertainties in our knowledge of solar wind structure near the Sun. In addition to uncertainties stemming from the propagation of errors in density profiles inferred from coronagraph observations [see, e.g., Lallement et al., 1986], an assessment of the consequences of choosing different analysis assumptions reveals very large, fundamental uncertainties in our knowledge of even the basics of coronal structure near the Sun. In the spirit of demonstrating the nature and extent of these uncertainties we develop just one of a generic class of explicitly time‐dependent and filamentary models of the corona that is consistent with the Ulysses and Spartan 201 data. This model provides a natural explanation for the radial profiles of both the axial ratios and apparent radial speeds of density irregularities measured at radial distances less than 10 Rs using the interplanetary scintillation technique.
Pioneers of space physics: A career in the solar wind
Authors: Neugebauer Marcia
Published: February 2004

This paper is a personal history of the author's experiences, starting with the earliest direct measurements of the solar wind and continuing through later experiments to investigate the physics of the solar wind and its interaction with comets.
Influence of the interstellar magnetic field direction on the shape of the global heliopause
Authors: Pogorelov Nikolai V. , Matsuda Takuya
Published: February 2004

In this paper we study the influence of the angle between the local interstellar medium velocity and magnetic field vectors on the interaction of this medium with the solar wind. Both winds are supposed supersonic. Applicability of the two‐shock model of the interaction is discussed. Investigation is performed by numerical solution of the MHD equations for an ideal plasma in the one‐fluid approximation.
The effective polytropic index in a magnetized plasma
Authors: Pudovkin Mikhail I. , Meister Claudia‐Veronika , Besser Bruno P. , Biernat Helfried K.
Published: February 2004

The effective polytropic index γeƒƒ in a magnetized anisotropic plasma is obtained in the frame of one‐fluid, double‐adiabatic magnetohydrodynamics. The value of γeƒƒ is found to depend on the characteristics of the plasma flow, and on the temperature anisotropy. It is shown that the most probable value of γeƒƒ is 1.4–1.9 at the bow shock, and it may be less than unity in the magnetopause region.
Contribution of pressure gradients to the generation of dawnside region 1 and region 2 currents
Authors: Iijima Takesi , Potemra Thomas A. , Zanetti Laurence J.
Published: February 2004

We have examined 92 observations of magnetic fields and ion fluxes acquired with the DMSP F7 satellite in the dawn sector (0600 to 1000 eccentric dipole local time (EDLT)). The location and amplitude of large‐scale region 1 and region 2 field‐aligned currents have been identified in the traditional manner from gradients in the transverse magnetic field disturbances. Latitude profiles of ion pressure were computed from the ion flux measurements by assuming these fluxes to be isotropic. We have discovered that the locations of the ion pressure distributions are almost identical with those of the field‐aligned currents and they have peaks which are very close to the latitudes of the interface between the region 1 and region 2 currents. Consequently, positive and negative gradients in ion pressure occur in the region 2 and region 1 field‐aligned current regions, respectively, at the satellite's altitude of 840 km. Using these facts and a logic of stress strategy to generate the field‐aligned current system, we proposed that the dawnside regions 1 and 2 currents are derived from a common magnetospheric current which is driven by pressure‐gradient force and flows across these currents. This finding recognized the important role that field‐aligned currents have in transporting tangential stress (transverse momentum) along geomagnetic field lines.
DE observations of electric field oscillations associated with an electron conic
Authors: Menietti J. D. , Weimer D. R.
Published: February 2004

An electron conic has been reported from low‐altitude particle observations of Dynamics Explorer 2 (DE 2). It has been suggested that the source of the electron conic is an oscillating parallel electric field, perhaps associated with an inertial Alfvén wave. We report here on electric field observations made at midaltitude by Dynamics Explorer 1 and at low altitude by DE 2 during the same time as the electron conic observations of Burch [1995]. We find evidence for oscillations of E‖ with frequency ƒ ∼ 0.5 Hz, in agreement with the model of electron conics presented by Thompson and Lysak. However, there are discrepancies in the measured values of E‖/E⊥ and B/E compared with those expected for Alfvén waves. We believe these discrepancies may be due to nonlinear effects.
Experimental study of the collisional parallel velocity shear instability
Authors: Willig J. , Merlino R. L. , D'Angelo N.
Published: February 2004

Simultaneous density and electric field fluctuation spectra, associated with velocity shear (both in the transverse to the magnetic field ion flow velocity and in the parallel ion flow velocity), have been observed by the DE 2 spacecraft in the F region [Basu et al., 1984]. Basu and Coppi [1988, 1989] argued that the subkilometer scale irregularities are due to collisional electrostatic modes excited in a partially ionized plasma in the presence of shear in the parallel ion flow. In this paper we describe results of a laboratory investigation of the effects of neutral‐particle collisions on the parallel velocity shear instability, performed using a double‐ended Q machine. The critical value of the shear dv0/dx, the derivative of the field‐aligned ion flow with respect to the transverse (to B) coordinate, is determined and compared with theoretical predictions.
The numerical simulation of VLF chorus and discrete emissions observed on the Geotail satellite using a Vlasov code
Authors: Nunn D. , Omura Y. , Matsumoto H. , Nagano I. , Yagitani S.
Published: February 2004

The Geotail satellite skims the dayside magnetosphere in the equatorial region at about L = 10. During such passes the WFC (Waveform Capture Receiver) observes VLF chorus and discrete emissions in the band 200 ∼ 1200 Hz. The most common waveforms observed are rising tones and rising chorus, with hooks and fallers being seen occasionally. Analysis shows that the k vectors are closely parallel to the ambient magnetic field. The Geotail satellite has available comprehensive wave and particle data observed in the vicinity of the generation regions of such VLF emissions. A well‐established Vlasov simulation code has been used to simulate the observed emissions, using detailed data from Geotail. The code readily simulates rising frequency VLF emissions with a steep frequency gradient. With appropriate parameter values the code will produce fallers and hooks in good agreement with those observed on Geotail. These self‐consistent simulations suggest that nonlinear trapping of cyclotron resonant electrons is the underlying mechanism behind VLF chorus and VLF emissions.
Simulations of ring current proton pitch angle distributions
Authors: Chen Margaret W. , Roeder James L. , Fennell Joseph F. , Lyons Larry R. , Schulz Michael
Published: February 2004

We employ a three‐dimensional ring current model to trace the bounce‐averaged drift of singly charged ions during storm‐associated enhancements in the convection electric field. Using the simulation results, we map proton phase space density during the main and recovery phases of a storm in accordance with conservation of phase space density ƒ. We map from an initial quiescent phase space distribution that is obtained by solving the steady state transport equation (bounce‐averaged charge exchange balancing bounce‐averaged radial diffusion) with observed plasma sheet proton spectra as outer boundary conditions. We obtain proton pitch angle distributions at L ∼ 3–4.5 by evaluating ƒ at representative ring current energies (∼20–170 keV). We find that the prestorm and stormtime proton pitch angle anisotropy at any given L between 3 and 4.5 increases with particle energy in agreement with observations. The actual anisotropy at specific energies depends strongly on the shape of the plasma sheet source spectrum at ∼0.5–3 keV. Relatively large enhancements in the stormtime phase space density from the quiescent distribution occurs at all pitch angles for low energies (≲80 keV) except where the quiescent distribution lies on open drift shells. These increases result primarily from stormtime access to L ≲ 4.5 along open drift trajectories from the plasma sheet. For higher‐energy (≳150 keV) protons, which are transported via radial diffusion, there is little change in the anisotropy over a 3‐hour storm. At intermediate energies (E ∼ 80–150 keV) the stormtime enhancements in the phase space density can vary quite strongly with equatorial pitch angle. For such particles the stormtime transport is intermediate between directly convective and quasi‐diffusive, which can complicate the analysis of stormtime pitch angle anisotropies at intermediate energies. Decay lifetimes obtained from CRRES data during the recovery phase of a moderate storm are found to be considerably shorter than charge‐exchange lifetimes. It thus appears that charge exchange alone is not enough to explain the observed rapid decay of the proton ring current.
Type III radio source located by Ulysses/Wind triangulation
Authors: Reiner M. J. , Fainberg J. , Kaiser M. L. , Stone R. G.
Published: February 2004

Radio triangulation from the widely separated Ulysses and Wind spacecraft is used to reconstruct the trajectory of a type III radio burst in the three‐dimensional heliosphere. The derived radio trajectory follows a (Parker) spiral path corresponding to a solar wind speed of ∼200 km/s and progresses to the south of the ecliptic plane. These remote radio observations also measure the interplanetary plasma density along the path of the radio source. The derived average density‐distance scale is very similar to the previously derived RAE density scale, which was determined in a different way. The results of the radio triangulation combined with a drift rate analysis give an average electron exciter speed of ∼0.3c. The radio source size and the brightness temperature as viewed from Ulysses and Wind are determined and compared as a function of observing frequency.
Detection of localized structures from multispacecraft data: Adaptive correlation function
Authors: Rezeau L. , Belmont G. , Reberac F.
Published: February 2004

When several spacecraft pass at different distances from a localized structure like a field‐aligned current tube, the corresponding magnetic signatures recorded on each spacecraft generally show different timescales, due to the different “viewpoints” of the spacecraft. For this reason, it is impossible to identify the presence of a unique structure from these signatures by using the classical correlation functions. In this paper, we present an “adaptive correlation function” which is a new analysis tool allowing to look for such “correlations” whatever the dilation factor is between two signals.
The turbulent state of cometary ion pickup generated fluctuations near comet P/Halley
Authors: Krauklis I. , Johnstone A. D.
Published: February 2004

One of the principal effects of an outgassing comet is to introduce additional mass into the solar wind flow. This mass‐loading process results in the generation of Alfvén waves via ion‐cyclotron resonance. In the case of the Halley encounter this source of “new” fluctuations was introduced into a low speed solar wind 0.89 AU from the sun. Previous in situ observations in low speed flows at this distance have described the fluctuations as turbulently evolved or “aged.” The spectral characteristics and time dependent anisotropies of these new cometary fluctuations have been effectively explained by a conservation‐of‐energy description of the cometary ion pickup process. An alternative description has been proposed whereby turbulent cascading generates the observed spectrum, implying that the fluctuations evolve significantly in the time between their generation and their observation. Analysis of the rugged invariants and, in particular, the Alfvén ratio suggests that throughout the interaction region upwind from the comet the cometary ion pickup generated fluctuations became less evolved as the bow shock was approached. Hence turbulent cascading of the fluctuation energy is unlikely to be an effective description of the observations or to modify the spectral characteristics of these fluctuations significantly in the upwind interaction region.
High‐latitude ionospheric perturbations and gravity waves: 1. Observational results
Authors: Huang Chao‐Song , Andre Dieter A. , Sofko George J.
Published: February 2004

The Saskatoon HF radar has been used to observe ionospheric perturbations and gravity waves at high latitudes. The observations on December 29, 1993, taken during an extended period of northward interplanetary magnetic field, are analyzed in detail. Two distinct intervals of backscatter are received, a near‐range interval of ground scatter echoes from 900 to 1500 km and a far‐range interval of ionospheric scatter from 2000 to 2600 km. The ground scatter intensity shows strong quasi‐periodic enhancements caused by Earth‐reflected gravity waves. The ionospheric echoes, which form a circular band of radius about 600 km, occur in the auroral zone below 80° magnetic latitude, just equatorward of the convection reversal. They cover a magnetic longitude interval from −60° to 80°, corresponding to a magnetic local time period from about 1100 to about 1800 LT. From the shape of the Earth‐reflected gravity wave intensity bands on the latitude‐time plots, those gravity waves can be traced back to the ionospheric perturbation region, which lies in the high‐latitude echo zone, namely the auroral zone. This is confirmed by a two‐dimensional cross‐correlation between the time series for the intensity of the gravity‐wave modulated ground scatter and the auroral zone ionospheric scatter. That correlation shows evidence for a time delay corresponding to the Earth‐reflected gravity wave propagation time between the regions. Also, the correlation peaks for a range separation about 1100 km, which implies that the normal one‐and‐a‐half‐hop propagation mode usually used to explain the presence of the far‐range auroral echoes cannot be operative and that the Pedersen ray “trapped” ionospheric mode is responsible for the observations.
A magnetic cloud containing prominence material: January 1997
Authors: Burlaga L. , Fitzenreiter R. , Lepping R. , Ogilvie K. , Szabo A. , Lazarus A. , Steinberg J. , Gloeckler G. , Howard R. , Michels D. , Farrugia C. , Lin R. P. , Larson D. E.
Published: February 2004

This work discusses the relations among (1) an interplanetary force‐free magnetic cloud containing a plug of cold high‐density material with unusual composition, (2) a coronal mass ejection (CME), (3) an eruptive prominence, and (4) a model of prominence material supported by a force‐free magnetic flux rope in a coronal streamer. The magnetic cloud moved past the Wind spacecraft located in the solar wind upstream of Earth on January 10 and 11, 1997. The magnetic field configuration in the magnetic cloud was approximately a constant‐α, force‐free flux rope. The4He++/H+abundance in the most of the magnetic cloud was similar to that of the streamer belt material, suggesting an association between the magnetic cloud and a helmet streamer. A very cold region of exceptionally high density was detected at the rear of the magnetic cloud. This dense region had an unusual composition, including (1) a relatively high (10%)4He++/He+abundance (indicating a source near the photosphere), and (2)4He+, with an abundance relative to4He++of ∼1%, and the unusual charge states of O5+and Fe5+(indicating a freezing‐in temperature of (1.6–4.0) × 105°K, which is unusually low, but consistent with that expected for prominence material). Thus we suggest that the high‐density region might be prominence material. The CME was seen in the solar corona on January 6, 1997, by the large angle and spectrometric coronagraph (LASCO) instrument on SOHO shortly after an eruptive prominence. A helmet streamer was observed near the latitude of the eruptive prominence a quarter of a solar rotation before and after the eruptive prominence. These observations are consistent with recent models, including the conceptual model ofLow and Hundhausen[1995] for a quasi‐static helmet streamer containing a force‐free flux rope which supports prominence material and the dynamical model ofWu et al.[1997] for CMEs produced by the disruption of such a configuration.
Solar wind measurements with SOHO: The CELIAS/MTOF proton monitor
Authors: Ipavich F. M. , Galvin A. B. , Lasley S. E. , Paquette J. A. , Hefti S. , Reiche K.‐U. , Coplan M. A. , Gloeckler G. , Bochsler P. , Hovestadt D. , Grünwaldt H. , Hilchenbach M. , Gliem F. , Axford W. I. , Balsiger H. , Bürgi A. , Geiss J. , Hsieh K. C. , Kallenbach R. , Klecker B. , Lee M. A. , Managadze G. G. , Marsch E. , Möbius E. , Neugebauer M. , Scholer M. , Verigin M. I. , Wilken B. , Wurz P.
Published: February 2004

The proton monitor, a small subsensor in the Charge, Element, and Isotope Analysis System/Mass Time‐of‐Flight (CELIAS/MTOF) experiment on the SOHO spacecraft, was designed to assist in the interpretation of measurements from the high mass resolution main MTOF sensor. In this paper we demonstrate that the proton monitor data may be used to generate reasonably accurate values of the solar wind proton bulk speed, density, thermal speed, and north/south flow direction. Correlation coefficients based on comparison with the solar wind measurements from the SWE instrument on the Wind spacecraft range from 0.87 to 0.99. On the basis of the initial 12 months of observations, we find that the proton momentum flux is almost invariant with respect to the bulk speed, confirming a previously published result. We present observations of two interplanetary shock events, and of an unusual solar wind density depletion. This large density depletion, and the correspondingly large drop in the solar wind ram pressure, may have been the cause of a nearly simultaneous large increase in the flux of relativistic magnetospheric electrons observed at geosynchronous altitudes by the GOES 9 spacecraft. Extending our data set with a 10‐year time span from the OMNIWeb data set, we find an average frequency of about one large density depletion per year. The origin of these events is unclear; of the 10 events identified, 3 appear to be corotating and at least 2 are probably CME related. The rapidly available, comprehensive data coverage from SOHO allows the production of near‐real time solar wind parameters that are now accessible on the World Wide Web.
Magnetic field near Venus: A comparison between Pioneer Venus Orbiter magnetic field observations and an MHD simulation
Authors: Kallio E. , Luhmann J. G. , Lyon J. G.
Published: February 2004

Pioneer Venus Orbiter (PVO) measurements revealed the shape and the changing location of the Venus bow shock with solar cycle and provided a detailed picture of the magnetic field pileup in the dayside magnetosheath. Nevertheless, the reason for the increase of the terminator shock position to the observed distances has evaded our understanding, and the “magnetic barrier” region has been studied primarily by comparisons with gasdynamic models due to the difficulty of using more sophisticated treatments. In this study we investigate the extent to which a three‐dimensional magnetohydrodynamic (MHD) model of the Venus‐solar wind interaction, with and without “mass loading” by photoionization of the atomic oxygen upper atmosphere, can reproduce some of the basic features of the dayside magnetic field observed on PVO. The ideal MHD model uses a conducting sphere to represent the basic Venus ionospheric obstacle to the solar wind flow. We adopt the viewpoint that during solar maximum, a conducting obstacle with oxygen mass loading is appropriate, while a no‐mass loading case is a good first approximation to the solar minimum situation. The MHD simulations are found to give a realistic picture of both the shape of the bow shock and its observed elliptical cross section at the terminator. The introduction of the oxygen mass loading moves the shock position to that observed at solar maximum. The magnetic field strength on the dayside has a dependence on solar zenith angle similar to that found in statistical analyses of the PVO data, although the field is stronger than that measured. The mass loading creates a layer near the planet where the magnetic pressure is replaced with the thermal pressure much like observed. Our studies also raise the question of the role of nightside flow vortices in the formation of the effective obstacle boundary. Overall, our results illustrate that many features seen in the MHD model are consistent with the previously reported observations of the Pioneer Venus Orbiter.
Concerning the location of magnetopause merging as a function of the magnetopause current strength
Authors: Alexeev I. I. , Sibeck D. G. , Bobrovnikov S. Y.
Published: February 2004

We start from an assumption that merging occurs in regions of the magnetopause where current strengths are greater than some threshold value which corresponds to the total jump in the field across the magnetopause greater than 50 nT. Because time and cost constraints preclude running numerical simulations for a wide variety of interplanetary magnetic field (IMF) orientations to determine these locations, we adopt an analytical model based on previously derived formulations for magnetospheric and magnetosheath magnetic fields. The magnetospheric magnetic field is confined within a paraboloid. The magnetosheath magnetic field is derived from that in the solar wind and lies between the magnetopause and a paraboloid bow shock. We allow a slight diffusion of the magnetosheath magnetic field into the magnetosphere. The results of the model show that during periods of due southward IMF orientation, merging occurs (as expected) in a wide region centered on the subsolar magnetopause. During periods of northward IMF, connection continues near the subsolar point but also poleward of the cusps. Magnetic energy is only released to the plasma in the latter regions. During periods of strongly northward IMF (By = 0), reconnection ceases on the subsolar magnetopause but continues poleward of the cusp. If the IMF points northward but By is nonzero, reconnection continues near the subsolar point and poleward of the cusps. During periods of sunward IMF orientation, merging nearly ceases on the northern hemisphere (except in the vicinity of the subsolar point) but continues outside the southern lobes. Dawnward and duskward IMF orientations produce tilted patches of enhanced current densities in the subsolar region. We compare the results of our model with previous predictions of the “component” and “antiparallel” merging models.
MHD wave coupling in the geomagnetic tail with field‐aligned density variations
Authors: Wright Andrew N. , Garman Andrew R.
Published: February 2004

The normal MHD modes of the tail lobe are calculated for a simple model that is stratified in z. An important feature of our equilibrium field is that it may be tilted at an arbitrary angle (θ) to the antisunward direction, i.e., B = B(cosθ,0,sinθ). When θ = 0, the familiar singular second‐order equation of Southwood [1974] is recovered. When θ ≠ 0, the system is goverened by a nonsingular fourth‐order equation. Hansen and Harrold [1994] (hereafter HH) considered exactly this system and concluded that (for θ ≠ 0) energy was no longer absorbed by a singularity but rather over a thickened boundary layer across which the time‐averaged Poynting flux (〈Sz〉) changed. Our results are not in agreement with those of HH. We find (〈Sz〉) is independent of z and find no evidence of boundary layers, even for θ as small as 10−6 rad. Our solutions still demonstrate strong mode conversion from fast to Alfvén modes at the “resonant” position, but the small component of Alfvén speed in the ˆ direction permits the Alfvén waves to transport energy away from this location and prevents the continual accumulation of energy there. The implications for MHD wave coupling in realistic tail equilibria are discussed.
Intercomparison of physical models and observations of the ionosphere
Authors: Anderson D. N. , Buonsanto M. J. , Codrescu M. , Decker D. , Fesen C. G. , Fuller‐Rowell T. J. , Reinisch B. W. , Richards P. G. , Roble R. G. , Schunk R. W. , Sojka J. J.
Published: February 2004

Five physical models of the ionosphere were compared with each other and with data obtained at the Millstone Hill Observatory. Two of the models were self‐consistent ionosphere‐thermosphere models, while for the other ionospheric models the thermospheric parameters were provided by empirical inputs. The comparisons were restricted to midlatitudes and low geomagnetic activity, but four geophysical cases were considered that covered both the summer and winter solstices at solar maximum and minimum. The original motivation of the study was to determine why several physical models consistently underestimated the F region peak electron density, by up to a factor of 2, in the midlatitude, daytime ionosphere at solar maximum. This problem was resolved, but the resolution did not identify a lack of physics in any of the models. Instead, various chemical reaction rates, photoionization processes, and diffusion coefficients had to be adjusted, with the main one being the adoption of the Burnside factor of 1.7 for the diffusion coefficients. The subsequent comparisons of the models and data were for “standard” simulations in which uncertain inputs or processes were not adjusted to get better agreement with the data. For these comparisons, the five models displayed diurnal variations that, in general, agreed with the measurements. However, each one of the five models exhibited a clear deficiency in at least one of the four geophysical cases that was not common to the other models. Therefore, contrary to expectations, the coupled ionosphere‐thermosphere models were not found to be superior to the uncoupled ionospheric models for the cases considered. The spread in NmF2 calculated by the five models was typically less than a factor of 2 during the day but was as large as a factor of 10 at certain local times during the night. The latter problem was traced to insufficient nocturnal maintenance processes in two of the uncoupled ionospheric models. The general findings of this study have important implications for the National Space Weather Program.
Observation of a correspondence between poleward moving auroral forms and stepped cusp ion precipitation
Authors: Farrugia Charles J. , Sandholt Per Even , Denig William F. , Torbert Roy B.
Published: February 2004

In a case study, we document for the first time a one‐to‐one association between three auroral forms which are separated in latitude and moving poleward and three intervals of almost constant, but different, low‐energy cutoffs in the ion precipitation in the cusp region (“stepped cusp”) precipitation. The ground observations are from Ny Alesund at 75° magnetic latitude (MLAT), and the precipitation pattern is observed by the DMSP F13 satellite as it crosses the noon meridian into the morning sector. The energy cutoffs increase with decreasing latitude. The auroral forms brighten up at different times but at practically the same MLAT (∼73°) and start fading 5 min later at ∼76° MLAT. Interplanetary conditions, monitored by WIND, show a stable, slow, and cold solar wind with an interplanetary magnetic field which points south and west (By > 0). Staircase ion precipitation signatures such as observed in this example have been successfully interpreted in terms of a sequence of reconnection bursts at the low‐latitude, dayside magnetopause. The time‐history of the auroral activity observed from the ground confirms this interpretation. We exclude the possibility that the auroral forms are triggered by changes in the interplanetary medium because the latter is very steady. In particular, the (low) dynamic pressure varies little about a mean value of 1.6 nPa.
Spatially quasiperiodic plasma and electromagnetic signatures in the poleward boundary of the auroral oval: Observations on the morningside
Authors: Hirahara M. , Mukai T. , Shinohara I. , Kaya N. , Fukunishi H. , Hayakawa H. , Yamamoto T. , Kaneda E.
Published: February 2004

We present observational characteristics of the spatially quasiperiodic signatures of plasma and electric and magnetic fields observed in the poleward boundary of the morningside auroral oval by two polar‐orbiting satellites: Akebono and DMSP F9. The two satellites observed the proton discrete signatures at different altitudes (∼5000 km (Akebono) and ∼800 km (DMSP F9)) during nearly the same intervals. The data indicate that the ion (proton) signatures repetitively appeared and had a good correlation with occurrence of spiky electrons associated with upward components of small‐scale field‐aligned currents and variations of electric fields. We conclude that the repetitive (quasiperiodic) proton signatures had spatially energy‐dispersed signatures, which were not due to transient effect, on the basis of a comparison of the simultaneous observations at the different altitudes. Characteristic energy of each dispersed trace decreased with decreasing latitude, and the energy‐dispersed signatures frequently overlapped each other. A global plasma convection reversal often occurred during the quasiperiodic proton signatures. Sequential observations show that the region with these proton signatures was latitudinally wide and extended significantly to higher latitudes under northward interplanetary magnetic field conditions. It is plausible that instabilities in the tail‐flank boundary of the magnetosphere, represented by the Kelvin‐Helmholtz instability, are driven by the interaction between the solar wind and the central plasma sheet and then could produce the precipitating ions and the electrons accompanied by the variations of the electric and magnetic fields.
Theory, simulation, and observation of discrete eigenmodes associated with lower hybrid solitary structures
Authors: Schuck P. W. , Seyler C. E. , Pinçon J.‐L. , Bonnell J. W. , Kintner P. M.
Published: February 2004

A three‐dimensional fluid description of nonlinear lower hybrid waves is investigated in the context of plasma density depletions. The objective is a basic understanding of lower hybrid solitary structures associated with transverse ion acceleration in the topside auroral ionosphere. The equations are linearized about a parabolic density depletion and solved. The solution consists of potential structures (eigenfunctions) which rotate in angle about the center of the density depletion. The eigenfrequencies are discrete for |ω| < ωLH. These eigenfunctions rotate in a left‐handed sense about the geomagnetic field and the solutions fall off exponentially outside the density depletion. The eigenfrequencies are continuous for |ω| > ωLH and κz ≠ 0 but become discrete for kz = 0 in agreement with previous two‐dimensional results [Seyler, 1994]. Simulations of the full nonlinear system are performed, and rotating eigenmodes are extracted from the spectrum. The results agree with the analytic results obtained from the linearized equations. The spectral properties of a lower hybrid solitary structure from the TOPAZ III sounding rocket are reexamined and found to be consistent with theoretical predictions for lower hybrid waves trapped within a density depletion as presented herein. A local wavelet frequency‐wavenumber spectrum is constructed from data taken by moving an interferometer through a simulation at a typical rocket velocity. The results compare favorably with the local frequency‐wavenumber spectrum of a lower hybrid solitary structure observed by the TOPAZ III sounding rocket.
Characteristics of upstream energetic (E≥50 keV) ion events during intense geomagnetic activity
Authors: Anagnostopoulos G. C. , Rigas A. G. , Sarris E. T. , Krimigis S. M.
Published: February 2004

In this work we examine the statistical presence of some important features of upstream energetic (≥50 keV) ion events under some special conditions in the upstream region and the magnetosphere. The 125 ion events considered in the statistic were observed by the IMP 7 and IMP 8 spacecraft, at ∼35 RE from the Earth, during nine long time intervals of a total of 153 hours. The time intervals analyzed were selected under the following restrictions: existence of high proton flux (i.e., ≥900 p cm−2 s−1 sr−1) and of a great number of events (an occurrence frequency of ∼10 events per 12 hours in the whole statistics) in the energy range 50–220 keV. The most striking findings are the following: (1) The upstream events were observed during times with high values of the geomagnetic activity index Kp (≥3‐); (2) all of the upstream events (100%) have energy spectra extending up to energies E≥290keV; (3) 86% of these events are accompanied by relativistic (E ≥ 220 keV) electrons; and (4) the majority of the upstream ion events (82%) showed noninverse velocity dispersion during their onset phase (22% of the events showed forward velocity dispersion, and 60% showed no velocity dispersion at all when 5.5‐min averaged observations were analyzed). Further statistical analysis of this sample of upstream particle events shows that the 50‐ to 220‐keV proton flux shows a positive correlation with the following parameters: the Kp index of geomagnetic activity and the flux of the high‐energy (290–500 keV) protons and (≥220 keV) electrons. More specific findings are the following: (1) The spectral index γ for a power law distribution of ions detected by the National Oceanic and Atmospheric Administration Energetic Particle Experiment (EPE) instrument (50 ≤ E ≤ 220 keV) and The Johns Hopkins University Applied Physics Laboratory Charged Particle Measurement Experiment (CPME) instrument (290 ≤ E ≤ 500 keV) ranges between 2 and 6, with maximum probability between 4 and 5 and (2) the peak‐to‐background flux ratio of the 290‐ to 500‐keV protons and ≥220‐keV electrons increases with the time duration of upstream events. We infer that the vast majority of the upstream ion events considered in this study (under conditions of intense particle activity in the upstream region and enhanced geomagnetic activity within the magnetosphere) can be consistently explained in terms of particle leakage from the magnetosphere.
Pi2 pulsations observed from the Akebono satellite in the plasmasphere
Authors: Osaki Hiro , Takahashi Kazue , Fukunishi Hiroshi , Nagatsuma Tsutomu , Oya Hiroshi , Matsuoka Ayako , Milling David K.
Published: February 2004

Magnetic field, electric field, and electron density measurements from the Akebono satellite are used to study the properties of two Pi2 pulsations that occurred in succession on February 13, 1990, when the satellite was in the plasmasphere at L = 2.4–3.8, 24°–40° magnetic latitude, and 22.5 hours magnetic local time. Magnetic pulsations with a nearly identical waveform were observed in the same time interval at three ground stations (Aedey, L ≈ 6.84; York, L ≈ 2.55; and Hermanus, L ≈ 1.83), which were located near midnight, confirming that the pulsations propagated to the ground. At the satellite the pulsations had comparable radial and azimuthal components in both the magnetic and electric fields. In contrast to the observations near the magnetic equator by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer spacecraft [Takahashi et al., 1995], no compressional component was detected in the magnetic field. The orthogonal components of the electric and magnetic fields oscillated either in phase or 180° out of phase, a property of a propagating (rather than standing) wave. The Poynting flux of the Pi2 pulsations was parallel to the ambient magnetic field and directed toward the nearer ionosphere, with little indication of ionospheric reflection. This unidirectional flow of electromagnetic energy is consistent with the strong ionospheric damping of Alfvén waves estimated from a numerical calculation. It is significant that the measured Poynting flux could damp a cavity mode oscillation in ∼10 s, assuming that the previously reported equatorial compressional Pi2 pulsations represent the cavity mode. Since the transverse Pi2 pulsations at Akebono lasted ∼400 s, they cannot be due to a gradual energy leakage from a simple cavity mode oscillation. Consequently, if the observed energy flow is a general property of plasmaspheric Pi2 pulsations, a simple cavity mode oscillation excited by an impulsive source is not an appropriate model for Pi2 pulsations.