bluuebunny/crossref_metadata_2025 huggingface dataset:
Space weather: Response of large‐scale geopotentials to an interplanetary magnetic cloud
Authors: Lanzerotti L. J. , Medford L. V. , Sayres D. S. , Maclennan C. G. , Lepping R. P. , Szabo A.
Published: February 2004

The interaction of solar wind disturbances with the Earth's magnetosphere can produce disturbances, and at times complete disruptions, of technological systems on the Earth and in the space around the Earth. This brief report shows the changes induced in the large‐scale geopotentials of the Earth (as provided from measurements across transoceanic cables) produced by a well‐documented interplanetary magnetic cloud event. The study of such a well‐measured event can be used to begin to make empirical space weather phenomena more quantitative. We show that geopotentials at low geomagnetic latitudes can be used to infer the time derivative of the near‐equatorial magnetic disturbance index, Dst. At low geomagnetic latitudes, a peak geopotential of about 4 mV/km is found to correspond to a time rate of change of this index of about 50 nT/hr. Further, we show that in this event increases in the near‐equatorial geopotential are linearly related to the energy input to the magnetosphere from the solar wind as given by the ε parameter [e.g., Akasofu, 1979]. We find that an increase in the geopotential of about 4 mV/km corresponds to an energy input of about 2.8 × 1011 W for the event analyzed here.
Self‐consistent simulation of the photoelectron‐driven polar wind from 120 km to 9 RE altitude
Authors: Su Yi‐Jiun , Horwitz J. L. , Wilson G. R. , Richards P. G. , Brown D. G. , Ho C. W.
Published: February 2004

It has long been recognized that photoelectrons can enhance the ambipolar electric fields affecting polar wind outflows [e.g., Axford, 1968; Lemaire, 1972]. Since ionospheric ions and electrons are produced in large part by photoionization of the neutral atmosphere at lower altitudes, and the maximum photoelectron production rate occurs in the 130–140 km altitude range, it is essential to model this photoelectron‐driven polar wind self‐consistently from the E region to an altitude of several Earth radii. Here we describe a new steady state coupled fluid‐semikinetic model to efficiently couple the source region to the high‐altitude regions. This model couples a fluid treatment for the 120–800 km altitude range, a generalized semikinetic (GSK) treatment for the altitude range 800 km to 2 RE, and a steady state collisionless semikinetic method for the altitude range 2–9 RE. We apply this model to investigate the photoelectron‐driven polar wind with ionospheric conditions ranging from solar minimum (F10.7 = 90) to solar maximum (F10.7 = 200). The O+ and H+ densities are found to increase by factors of approximately 5 and 2, respectively, from solar minimum to solar maximum below 3 RE altitude. However, the parallel bulk velocities display little variation with increased F10.7 for altitudes below 3 RE. An electric potential layer of the order of 40 V develops above 3 RE altitude, when the included downward magnetosheath electron fluxes (such as polar rain) are insufficient to balance the ionospheric photoelectron flux. Such potential layers accelerate the ionospheric ions to supersonic speeds at high altitudes, above 3 RE, but not at low altitudes. We also found that the potential layer decreases from 40 to 8.5 V for solar minimum conditions and from 46 to 12 V for solar maximum conditions when the magnetospheric electron density is increased from 0.05 to 2 cm−3.
The excitation of lower hybrid waves driven by the ∇B effect in the current sheet
Authors: Sundaram A. K. , Fairfield D. H. , Vassiliadis D.
Published: February 2004

We have investigated the excitation of electrostatic and electromagnetic lower hybrid waves in the magnetic field reversal region of the plasma sheet using a two‐fluid approach. We consider a two‐dimensional tail configuration that includes the Bx and Bz magnetic field components and examine the nonlocal properties of lower hybrid modes near the weak field region, where the field Bx(z) reverses sign. We have shown that an electrostatic lower hybrid mode is excited by the magnetic field gradient while electromagnetic modes are driven unstable by the combined effects of the magnetic field gradient and the electron fluid drift across the Bx field. We have shown that the electrostatic and electromagnetic lower hybrid modes grow in a few milliseconds and a few tens of milliseconds, respectively, the former for wavelengths shorter than the current sheet width (that is, for kyλ ≥ 1, where λ is the current sheet width) and the latter for long wavelengths (kyλ ≪ 1).
Super Dual Auroral Radar Network observations of convection during a period of small‐magnitude northward IMF
Authors: Bristow W. A. , Ruohoniemi J. M. , Greenwald R. A.
Published: February 2004

Observations of the high‐latitude northern hemisphere convection pattern are presented for varying small‐magnitude interplanetary magnetic field (IMF) conditions. A new data analysis technique is used which makes use of all available data from the SuperDARN network rather than only using data from the regions of overlap of radar fields of view. Using this technique, the area over which convection is determined is often greatly extended from the region of overlap. The IMF during this period was small magnitude, |B| < 4 nT. During one interval, IMF Bz was northward, while By was nearly zero. Convection during this interval evolved to a pattern with a large dusk cell with flow across the noon meridian from dusk toward dawn. In a later interval, IMF Bz and By were both nearly zero. During this interval, convection evolved to a four‐cell pattern. Comparison of the observations with conceptual models showed significant differences for both intervals. During the first interval, statistical models and the observations compared favorably. During the second interval, however, the observations showed a significantly different pattern from the first interval, while statistical models predict the same pattern for both intervals. The behavior of the convection pattern can be in part ascribed to the role played by Bx, which was the dominant IMF component throughout this interval.
Field and flow perturbations in the October 18–19, 1995, magnetic cloud
Authors: Janoo L. , Farrugia C. J. , Torbert R. B. , Quinn J. M. , Szabo A. , Lepping R. P. , Ogilvie K. W. , Lin R. P. , Larson D. , Scudder J. D. , Osherovich V. A. , Steinberg J. T.
Published: February 2004

We examine magnetic field and plasma perturbations in the October 18–19, 1995, magnetic cloud. Besides the front boundary, the 3‐s‐averaged magnetic field measurements made by the Magnetic Field Investigation on the Global Geospace Mission spacecraft Wind reveal a further 15 clear magnetic field directional discontinuities (DDs) with field rotations >15°, each lasting for ∼1 min. A number of these DDs are clustered near the trailing edge of the cloud. Using 3‐s resolution proton data from the Three‐Dimensional Plasma and Energetic Particle Experiment on Wind, we find that these DDs are accompanied by perturbations in the flow. We find that except for the front boundary, which is a clear tangential discontinuity, all the others are rotational. Across the DDs the bulk flow speed is sometimes enhanced and sometimes depressed. Changes in proton temperature across the DDs suggest a more elaborate structure, for example, a reconnection layer. In a search for large‐scale regularities, we apply minimum variance analysis to determine the normals to contiguous 1‐hour‐long stretches of the cloud data and find that there are large‐scale structures ordering the field and the flow for the first 21 hour of cloud data. Thus we identify three coherent segments of several hours' duration each with a well‐defined normal, but the normals to the individual segments are very different from each other. In particular, one segment in the cloudBz< 0 phase is almost orthogonal to another segment in theBz> 0 phase. The normals to the DDs within a given segment are closely aligned with the normal to that segment. For the last 6 hours of cloud data, no coherent structure was found since no reliable normals could be determined. Studying flow anisotropies of electrons in the energy range 0.1 – 100 keV and changes in intensity in the >1 keV electrons,Larson et al. [1997] inferred several instances of disconnection of cloud field lines from the Sun, which were attributed to reconnection between adjacent bundles of cloud field lines. Our results are supportive of this interpretation. Furthermore, our results suggest the presence of detailed substructure in, and/or large distortions of, the magnetic cloud which reached 1 AU on October 18–19, 1995.
Cosmic ray reentrant electron albedo: High‐Energy Antimatter Telescope balloon measurements from Fort Sumner, New Mexico
Authors: Barwick S. W. , Beatty J. J. , Bower C. R. , Chaput C. J. , Coutu S. , de Nolfo G. A. , DuVernois M. A. , Ficenec D. , Knapp J. , Lowder D. M. , McKee S. , Müller D. , Musser J. A. , Nutter S. L. , Schneider E. , Swordy S. P. , Tarlé G. , Tomasch A. D. , Torbet E.
Published: February 2004

The High‐Energy Antimatter Telescope (HEAT) balloon cosmic ray detector flew from Fort Sumner, New Mexico on May 3–5, 1994. The instrument measured electron and positron abundances and spectra from ∼1 to 100 GeV at a vertical geomagnetic cutoff rigidity that varied between 4.0 and 4.5 GV. The intensities of electrons and positrons have been measured as a function of atmospheric depth between 3.8 and 7.4 g cm−2 of overburden. At magnetic rigidities below cutoff, the intensity of downward moving e± consists of secondary (spallogenic) particles and the reentrant (or return) albedo. We determine the contribution of the reentrant electron albedo and compare it with earlier measurements and limits at similar geomagnetic cutoff levels. In the range of 1.0–2.4 GeV, the reentrant albedo component amounts to 40% of the total electron intensity observed.
Self‐consistent stochastic preacceleration of interstellar pickup ions in the solar wind including the effects of wave coupling and damping
Authors: le Roux J. A. , Ptuskin V. S.
Published: February 2004

A self‐consistent model was developed to study the stochastic acceleration (second‐order Fermi) of interstellar pickup ions in the quiet solar wind (SW) and inside corotating merged interaction regions (CMIRs) in the presence of the effects of wave coupling and the damping of magnetohydrodynamic waves by pickup ions. Our simulations of the solar wind wave spectrum show that (1) Pickup H+ is mainly responsible for wave damping at large wave numbers with pickup He+ contributing very little and pickup O+ contributing nothing. (2) Strong wave‐wave interactions wash out wave‐damping effects for radial distances r ≤ 10 AU from the Sun in the quiet solar wind and for r ≤ 23 AU inside corotating merged interaction regions. This is to be contrasted with previous work which calculated strong wave damping inside 10 AU from the Sun in the absence of wave coupling. For the simulated pickup ion spectra the main results are as follows: (1) The pickup He+ and O+ spectra are unaffected by the wave damping. (2) In contrast, the accelerated pickup H+ spectra have concave features for kinetic energies 1–5 KeV which become more pronounced with increasing radial distance. This suppression of the pickup H+ acceleration is attributed to the wave damping caused by the pickup H+. (3) Inside corotating merged interaction regions the preacceleration of pickup ions is stronger because of the enhanced heliospheric magnetic field. The suppression of pickup H+ acceleration is smaller and only starts to show for r > 23 AU. (4) Under the assumption that the seed population of anomalous cosmic rays should have speeds v > 400 km s−1 the H+/He+ density ratio in this population can be as low as ∼2.6 in the quiet solar wind and ∼13 inside corotating merged interaction regions. Compared with the observed ratio of ∼6 for anomalous cosmic rays (ACRs), our conclusion is that if CMIR and quiet SW regions are considered together, then the observed under abundance in ACR H+ could in principle be produced on average.
The phase speed of artificial field‐aligned irregularities observed by CUTLASS during HF modification of the auroral ionosphere
Authors: Eglitis P. , Robinson T. R. , Rietveld M. T. , Wright D. M. , Bond G. E.
Published: February 2004

The RF heater facility at Ramfjordmoen, Tromsø can generate field‐aligned plasma irregularities in the field of view of the Cooperative UK Twin‐Located Auroral Sounding System (CUTLASS) coherent backscatter radar at Hankasalmi, Finland. In a recent set of experiments conducted in April 1996, the phase speed of the generated plasma irregularities has been compared with independent measurements of the plasma drift velocity observed by the European Incoherent Scatter radar facility. The phase speed of the plasma irregularities is found to be equal to the component of the plasma drift velocity in the direction of propagation of the plasma wave. This result is a further verification of the high performance of the CUTLASS radar and also demonstrates how artificially generated plasma irregularities can be employed to detect small plasma drift velocities and contribute to geophysical research. The characteristics of the Doppler spectrum of the artificial plasma waves are also described and discussed.
Central axial field direction in magnetic clouds and its relation to southward interplanetary magnetic field events and dependence on disappearing solar filaments
Authors: Zhao X. P. , Hoeksema J. T.
Published: February 2004

Extended periods of strong southward interplanetary magnetic field Bs are the primary cause of intense geomagnetic storms. Most such intervals are associated with magnetic clouds in the solar wind. We define a magnetic cloud Bs event as the interval of southward interplanetary magnetic field observed as a magnetic cloud passes by the Earth. For the 26 well‐characterized events studied here, we find that (1) magnetic cloud central axial field directions are almost evenly distributed between −90° and 90° ecliptic latitude, the longitudinal distribution is slightly peaked around the east and west, (2) the duration and intensity of magnetic cloud Bs events correlate linearly with the direction of the cloud's central axial field, and (3) cloud central axial field directions are correlated with the central axial field directions of the associated disappearing filament on the Sun. These findings are useful in predicting from solar observations the duration and intensity of those magnetic cloud Bs events that hit Earth.
Spatial distribution of ionospheric plasma and field structures in the high‐latitude F region
Authors: Kivanç Ö. , Heelis R. A.
Published: February 2004

Ion density and velocity measurements from the Dynamics Explorer 2 (DE 2) spacecraft are used to obtain the average magnetic local time versus invariant latitude distribution of irregularities in the high‐latitude F region ionosphere. To study the small‐scale structure and its relationship to background conditions in the ionosphere, we have formed a reduced database using 2‐s (≈ 16 km) segments of the ion density and velocity data. The background gradients associated with each 2‐s segment and the spectral characteristics, such as power at 6 Hz (≈ 1.3 km) and spectral index, are among the reduced parameters used in this study. The relationship between the observed plasma structure and its motion is complex and dependent on the externally applied fields as well as locally generated plasma structure. The evolution of plasma structures also depends critically on the conductivity of the underlying ionosphere. Observations indicate an enhancement of irregularity amplitudes in two spatially isolated regions in both the ion density and the velocity. Convective properties seem to play a more important role in winter hemisphere where smaller‐scale structures axe maintained outside the source regions. ΔV irregularity amplitudes are enhanced in the cusp and the polar cap during northward interplanetary magnetic field regardless of season. The power in ΔV is usually higher than that associated with local polarization electric fields, suggesting that the observed structure in ΔN/N is strongly influenced by ΔV structure applied to large density gradients.
Mirror waves downstream of the quasi‐perpendicular bow shock
Authors: Czaykowska Anja , Bauer Thomas M. , Treumann Rudolf A. , Baumjohann Wolfgang
Published: February 2004

We analyze data of the AMPTE/IRM spacecraft downstream of a supercritical, quasi‐perpendicular bow shock with an upstream β (ratio of thermal to magnetic pressure) greater than unity. The observed ion temperature anisotropy (Tp⊥ > Tp‖) satisfies the mirror instability criterion, and the mirror growth rate is positive. We investigate the low‐frequency magnetic fluctuations and find that several minutes downstream of the shock ramp the compressive mode dominates in two frequency intervals below the local proton gyrofrequency. Band‐pass filtering of the two frequency ranges shows that the particle pressure and magnetic pressure vary in antiphase, as is typical for the mirror mode. In addition, we estimate the proton compressibility, that is, the ratio of the relative fluctuations of proton density and magnetic field, and the Alfvén ratio, that is, the ratio of fluctuations of the bulk velocity and the Alfvén velocity. Combining these results, we find strong indications for the existence of mirror waves in the region downstream of the bow shock. The observed spectrum of the mirror waves has a two‐banded structure.
A statistical study of the magnetic signatures of FTEs near the dayside magnetopause
Authors: Sanny J. , Beck C. , Sibeck D. G.
Published: February 2004

During magnetopause crossings, the AMPTE CCE satellite frequently observed flux transfer events (FTEs) characterized by fluctuations in the magnetic field strength (B) and bipolar signatures in the field component (BN) normal to the nominal magnetopause. In this study, we survey 110 events observed from October to December 1984 and during January 1986. Nearly all events exhibited increases in B, and although the majority of events exhibited a symmetric bipolar signature in BN, a significant number (31 of 110) had asymmetric bipolar signatures in which the trailing pulse was dominant. Most of the asymmetric events were observed near the magnetic equator. This is consistent with an explanation in which FTEs form via merging along a single subsolar X line with strongly asymmetric signatures but that these signatures evolve into the familiar symmetric bipolar form with distance from the merging line.
The thermal self‐focusing instability near the critical surface in the high‐latitude ionosphere
Authors: Guzdar P. N. , Chaturvedi P. K. , Papadopoulos K. , Ossakow S. L.
Published: February 2004

A fully nonlinear development of the thermal self‐focusing instability of high power radio waves in the ionosphere in the region near the critical surface is the subject of the present study. In the simulation model studied, a high‐powered radio wave in the frequency range 5 – 10 MHz, with a 1% amplitude modulation, is launched vertically. In the high latitude geometry this represents a direction antiparallel to the magnetic field which is almost vertically downwards. The modulated wave undergoes strong self‐focusing at the critical surface, where the group velocity of the wave goes to zero. The scale size of the structures transverse to the magnetic field is controlled by the wave intensity and the diffraction effects. The large parallel thermal conduction leads to the diffusion of these irregularities into the underdense and overdense plasma in narrow filaments. The depletion in the density in the overdense plasma allows propagation of the wave to higher altitude above the original critical surface and hence into the overdense plasma.
Two‐step development of geomagnetic storms
Authors: Kamide Y. , Yokoyama N. , Gonzalez W. , Tsurutani B. T. , Daglis I. A. , Brekke A. , Masuda S.
Published: February 2004

Using the Dst index, more than 1200 geomagnetic storms, from weak to intense, spanning over three solar cycles have been examined statistically. Interplanetary magnetic field (IMF) and solar wind data have also been used in the study. It is found that for more than 50% of intense magnetic storms, the main phase undergoes a two‐step growth in the ring current. That is, before the ring current has decayed significantly to the prestorm level, anew major particle injection occurs, leading to a further development of the ring current, and making Dst decrease a second time. Thus intense magnetic storms may often be the result of two closely spaced moderate storms. The corresponding signature in the interplanetary medium is the arrival of double‐structured southward IMF at the magnetosphere.
On the possible excitation of electromagnetic ion cyclotron waves in solar ejecta
Authors: Farrugia Charles J. , Gratton Fausto T. , Gnavi Graciela , Ogilvie Keith W.
Published: February 2004

We study the possibility of exciting electromagnetic ion cyclotron waves (EICWs) in solar ejecta (CMEs) by a kinetic instability driven by ion temperature anisotropies. Our approach is to vary key parameters about assumed baseline values. Since Tp,‖ > Tp,⊥ in most solar ejecta, the polarization of the unstable waves is right‐handed. If the average proton beta is low (βp ≤ 0.3), the activity is negligible for moderate temperature ratios, Tp,‖/Tp,⊥. Increasing βp increases both the frequency range and the instability growth rate. Increasing the temperature anisotropy brings about qualitatively similar effects as increasing βp, with comparable growth rates. Increasing the relative alpha‐to‐proton density ratio η has two effects: the active frequency range is shifted toward lower frequencies and the growth rate increases. Between η = 0 and η = 0.15, the maximum growth rate increases by a factor of ∼20, highlighting the importance of the alphas for generating this instability. A case that may represent some magnetic clouds with exceptional parameters, βp = 0.2, Tp,‖/Tp,⊥ = 10, and η = 0.08 − 0.15, is considered. The maximum growth rate is found to be twice the reference CME case, while the active frequency range is 3 times wider. We conclude that EICWs should be present in some ejecta and possibly also in those magnetic clouds with relatively weak magnetic field, high He++ content, and large Tp,‖/Tp,⊥ ratios, and whose βp is high, for example, through interaction with a succeeding fast stream. We also suggest that substantial changes with respect to normal conditions should occur in the power spectrum of EICWs in the terrestrial plasma depletion layer when a CME, or a magnetic cloud, with negative anisotropy passes Earth.
Stabilized electron beams: An auroral thermal wave
Authors: Vlasov V. G. , Zhizhko G. O.
Published: February 2004

The behaviour of the ionosperic plasma of the high‐latitude F2 region is investigated when it is in collective interaction with auroral electron beams. It is shown that the usual increase in plasma density from the electron beam formation region to the ionospheric F2 region maximum is the cause of the electron beam stabilization relative to the upper hybrid waves generation at the Cherenkov resonance. The collective dissipation of the beam energy at the Cherenkov resonance is possible only in the region of small plasma density gradients, which always takes place near the F2 region maximum. The auroral thermal wave is a region of high‐frequency plasma turbulence traveling upward to meet the electron beam. The plasma wave dissipation due to large collision frequencies with ions and neutrals must result in anomalously large electron temperatures and their transverse gradients, repeatedly registered in auroral discrete arcs. The plasma wave conversion (E ∼ 0.5 V/m are measured) into electromagnetic waves at 2fp in the auroral thermal wave and the plasma turbulence region upward motion must result in a fast decrease of the radiation frequency at 2fp, what was also registered. Results of a numerical simulation of the auroral thermal wave are in good agreement with experiment. The influence of the ion inertia, the altitude of the F2 region maximum, plasma density, energy, density, and the degree of energy homogeneity of the auroral electron beam on the formation and development dynamics of the thermal wave are investigated.
Observational constraints on the dynamics of the interplanetary magnetic field dissipation range
Authors: Leamon Robert J. , Smith Charles W. , Ness Norman F. , Matthaeus William H. , Wong Hung K.
Published: February 2004

The dissipation range for interplanetary magnetic field fluctuations is formed by those fluctuations with spatial scales comparable to the gyroradius or ion inertial length of a thermal ion. It is reasonable to assume that the dissipation range represents the final fate of magnetic energy that is transferred from the largest spatial scales via nonlinear processes until kinetic coupling with the background plasma removes the energy from the spectrum and heats the background distribution. Typically, the dissipation range at 1 AU sets in at spacecraft frame frequencies of a few tenths of a hertz. It is characterized by a steepening of the power spectrum and often demonstrates a bias of the polarization or magnetic helicity spectrum. We examine Wind observations of inertial and dissipation range spectra in an attempt to better understand the processes that form the dissipation range and how these processes depend on the ambient solar wind parameters (interplanetary magnetic field intensity, ambient proton density and temperature, etc.). We focus on stationary intervals with well‐defined inertial and dissipation range spectra. Our analysis shows that parallel‐propagating waves, such as Alfvén waves, are inconsistent with the data. MHD turbulence consisting of a partly slab and partly two‐dimensional (2‐D) composite geometry is consistent with the observations, while thermal paxticle interactions with the 2‐D component may be responsible for the formation of the dissipation range. Kinetic Alfvén waves propagating at large angles to the background magnetic field are also consistent with the observations and may form some portion of the 2‐D turbulence component.
Ion acoustic‐like waves observed by Ulysses near interplanetary shock waves in the three‐dimensional heliosphere
Authors: Hess R. A. , MacDowall R. J. , Goldstein B. , Neugebauer M. , Forsyth R. J.
Published: February 2004

The Ulysses spacecraft encountered 160 interplanetary shock waves between the time of its launch in October 1990 and the end of 1993 when it reached 48° S heliographic latitude. The onboard radio and plasma wave instrument observed many plasma waves associated with these shocks. Electrostatic waves at frequencies between the ion and electron plasma frequencies are observed intermittently throughout the solar wind but are often strongly enhanced near the shocks. These waves are usually interpreted as ion acoustic waves that have been Doppler shifted to the observed frequencies. They show very diverse characteristics in their amplitude and in their temporal behavior relative to the time of the shock passage at Ulysses. They are most likely to occur within a few minutes of shock passage, but on average they are enhanced above background levels for several hours before and after passage of the shock. The temporal behavior of the probability of occurrence of these waves is roughly symmetric relative to the time of shock passage and furthermore is quite similiar for quasi‐parallel and quasi‐perpendicular shocks and for forward and reverse shocks. These waves are correlated with several of the local plasma parameters measured by the Ulysses plasma instrument, most strongly with Te/Tp, the ratio of electron temperature to proton temperature, and with the electron heat flux. There is no significant correlation of the waves with the upstream plasma β or the shock strength. The similiarity of the quasi‐parallel and quasi‐perpendicular cases implies that the mechanism for wave generation is not sensitive to θBn, the angle between the upstream magnetic field and the shock normal. Examination of the particle distribution function at the shocks indicate that beams in the proton distribution function and enhanced electron heat flux may contribute to driving the waves unstable, particularly when Te/Tp is small.
Super Dual Auroral Radar Network observations of near‐noon plasma convection at small interplanetary magnetic field Bz and By
Authors: Kustov A. V. , Lyatsky W. B. , Sofko G. J.
Published: February 2004

Routine Super Dual Auroral Radar Network ionospheric plasma convection maps are used to study the structure of the near‐noon convection established at small interplanetary magnetic field (IMF) Bz and By. During such intervals, the influence upon convection of the quasi‐viscous processes at the magnetopause is expected to be at least as important as the effect of magnetic field line reconnection processes. It was found that, despite the small IMF Bz and By, there was a significant plasma flow across the noon meridian, so that the convection throat with the antisunward plasma flow was shifted toward earlier magnetic local times (to ∼1100 magnetic local time). It was also found that the size and shape of the morning convection cell was similar to the size and shape of the evening cell. It is argued that the shift of the convection throat toward morning hours is most likely caused by the effects of the ionospheric polar cap plasma disconnection from corotation with the ionospheric plasma on closed magnetic field lines. Other possible causes for the observed asymmetry are discussed.
Mathematical separation of directly driven and unloading components in the ionospheric equivalent currents during substorms
Authors: Sun W. , Xu W.‐Y. , Akasofu S.‐I.
Published: February 2004

This paper attempts to separate objectively the directly driven and unloading components in substorm processes by applying the method of natural orthogonal components (MNOC). A time series of the ionospheric equivalent current function with time resolution of 5 min during March 17–19, 1978 is calculated on the basis of six meridian chains magnetometer data during the International Magnetospheric Study in order to obtain the fundamental orthogonal basis set. The first and second natural components of the set thus obtained dominate over the rest of the natural components. The first natural component is found to have a two‐cell pattern, which is well known to be associated with global plasma convection in the magnetosphere. It is enhanced during the growth phase and expansion phase of substorms and decays during the recovery phase of substorms. Further, it is in fair correlation to the ϵ parameter with time lag of 20–25 min. This can be identified as the directly driven component. The second natural component reveals itself as an impulsive enhancement of the westward electrojet around midnight between 65° and 70° latitude during the expansion phase only. It is much less correlated with the ϵ parameter than the first one. Thus, as a first approximation, we identify it as the unloading component. It is shown that the directly driven component tends to dominate over the unloading component except for a brief period soon after substorm onset. This is the first clear determination of the time profile of the unloading component.
Unusual whistler with very large dispersion near the magnetopause: Geotail observation and ray‐tracing modeling
Authors: Nagano I. , Wu X.‐Y. , Yagitani S. , Miyamura K. , Matsumoto H.
Published: February 2004

A case of highly dispersed (∼860 s1/2) whistler‐like ELF (700−960 Hz) wave was detected with the waveform capture (WFC) receiver aboard the Geotail satellite during a dayside magnetopause skimming. Features of the single event distinguish it from the usual falling tone discrete emissions. By ray‐tracing and full‐wave calculation, the accessibility of the waves from a ground source of a distorted model magnetosphere were investigated. We propose that a lightning discharge at high latitudes is the most plausible source of the event via a special propagation effect revealed by the ray tracing. We demonstrate that the observed large wavenormal angle with respect to the geomagnetic field, the unusually large level of dispersion, and the lack of the expected nose frequency might be attributable to the frequency‐dependent nonducted paths to the satellite. The rare occurrence may partly be caused by the Landau damping by the convecting suprathermal electron beams of several hundred eV. It is also shown that the multiple‐hop magnetospherically reflected (MR) whistlers from middle latitudes, though being refracted to higher L shells by the plasmapause, are unable to reach the outer magnetosphere. Interhemisphere ducted propagation is possible for ELF waves along a narrow, modestly (15% or more) enhanced flux tube and shows an upper cutoff frequency governed by the high‐latitude minimum‐magnetic field “horns” regions and the duct enhancement and diameter. A full‐wave calculation also implies no lower‐frequency cutoff in the upward ionospheric transmission of lightning radiation down to 200 Hz. Because of the complexity of the various effects that influence the propagation from a ground source and the very low occurrence, effective whistler mode waves diagnostics of the highly variable outer magnetosphere are at present beyond our reach.
Comparison of empirical and theoretical polar cap convection patterns for the January 1992 GEM interval
Authors: Hill T. W. , Toffoletto F. R.
Published: February 2004

We compare empirical and theoretical models of polar cap boundary locations and ionospheric convection patterns for the Geospace Environment Modeling (GEM) Boundary Layer Campaign Interval during January 1992. The empirical model is described by Lyons [this issue], and the theoretical model is described by Toffoletto and Hill [1993]. The empirical and theoretical models agree on the large‐scale pattern of polar cap flow, including its dawn‐dusk deflection in response to the dawn‐dusk component of the interplanetary magnetic field (IMF). There is, however, a discrepancy with respect to the magnitude of the flow, as measured by the polar cap potential drop. This parameter is an input to the theoretical model, for given solar wind/IMF parameters, based on previous statistical studies, and the standard (empirical) input exceeds the empirical value obtained for these cases by a factor of order 2. There is also a discrepancy with respect to the size of the polar cap, which is smaller in the theoretical model than in the empirical one. The size discrepancy is partly, but not entirely, attributable to the potential‐drop discrepancy. The comparison provides clear directions for improvement of the theoretical model and perhaps also for refinement of the empirical modeling technique.
On the generation of auroral radio emissions at harmonics of the lower ionospheric electron cyclotron frequency: X, O and Z mode maser calculations
Authors: Yoon Peter H. , Weatherwax A. T. , Rosenberg T. J.
Published: February 2004

Electron cyclotron maser emission has been suggested as a possible explanation of ground‐based observations of auroral radio waves at twice and three times the lower ionospheric electron cyclotron frequency (2ƒce and 3ƒce). Preliminary theoretical work concluded that direct maser excitation at the first and second harmonics of the free space extraordinary mode (X2 and X3) could exceed collisional ionospheric damping rates, although growth rates were low. Recently, however, ground‐based observations indicate that these emissions are predominantly O mode polarized. Therefore a reassessment of the lower ionospheric cyclotron maser as an emission mechanism is necessary. In this paper, kinetic cyclotron maser instability calculations are extended to include the trapped Z mode for varying thermal energies and auroral potential drops within the context of a realistic ionospheric electron distribution model. It is found that when the local upper hybrid frequency is in the close vicinity of 2ƒce or 3ƒce, harmonic Z mode (Z2 and Z3) growth rate is shown to be exceedingly high. This finding suggests that an indirect mechanism that involves linear or nonlinear mode conversion of the excited harmonic Z mode into the ordinary (O) mode is a possible emission mechanism.
A statistical study of coronal mass ejection plasma flows
Authors: Goldstein R. , Neugebauer M. , Clay D.
Published: February 2004

This is a report on the results of a statistical study of plasma flow characteristics before, during, and after coronal mass ejections (CMEs) observed by the instruments on board the ISEE3 spacecraft located near Earth's L1 libration point during the period 1978 to 1980. We have used parametric relationships between plasma properties (such as flow speed and proton temperature) to help distinguish between transient flow resulting from CMEs and more quiescent or quasistatic solar wind flows. A superposed epoch analysis of 29 CMEs during the period studied shows that the transient plasma flow, as we have defined it, often begins earlier and extends later than the start and end times of the CMEs based on such criteria as bidirectional flow of suprathermal electrons or energetic protons. Since the bidirectional flow is tied to the magnetic topology of the CME, our results indicate that plasma having transient flow characteristics (as we have defined them) may extend both in front of as well as behind any magnetically closed region within the CME.
A comparison of global numerical simulation results to data for the January 27–28, 1992, Geospace Environment Modeling challenge event
Authors: Fedder J. A. , Slinker S. P. , Lyon J. G.
Published: February 2004

A magnetohydrodynamic (MHD) numerical simulation model is used to study the magnetospheric state during the Geospace Environment Modeling (GEM) challenge period on January 27–28, 1992. Direct comparisons are made between the simulation results and the GEM Program data as summarized in the synoptic maps. The features we concentrate on are the cross‐polar convection measurements and the ionospheric projection of magnetospheric boundaries. The results show good agreement at times and locations and poor agreement at other times. All in all, the results are rather mixed. We discuss the results and what they might mean for the accuracy of both the simulations and the synoptic maps. Moreover, we relate the results to other previous research on the cross‐polar potential and draw guidance for future work.
High‐latitude energetic particle boundaries and the polar cap: A statistical study
Authors: Kanekal S. G. , Baker D. N. , Blake J. B. , Klecker B. , Cummings J. R. , Mewaldt R. A. , Mason G. M. , Mazur J. E.
Published: February 2004

Instruments aboard SAMPEX have been used to monitor high latitude energetic particle boundaries. These boundaries are related to the polar cap areas threaded by open magnetic flux tubes. Variations of the area poleward of the energetic particle boundary and the boundary location itself are examined statistically for possible association with the interplanetary magnetic field (IMF) Bz, VBz, and Kp. We find the average location of the energetic particle boundary under weak IMF coupling conditions (i.e, |Bz| < 2.0 nT) to be ≈ 69°, (≈ 71° at noon and ≈ 67° at midnight) with the data showing considerable scatter. The boundary varies with IMF |Bz| for |Bz|> 2 nT and shifts equatorward at a rate of ≈ 0.4°/nT(≈ 0.1°/nT) for negative (positive) values of Bz. Varying the time offset of the hourly Bz values from 1 to 3 hours prior to the crossing had no significant effect on the correlation. The area poleward of the boundary is well represented by an offset circle fitted to the data. A weak dependence is also seen with VBz for VBz < −1.0 mV/m with the boundary location shifting equatorward at a rate of ≈ 0.9°/(mV/m). The equatorward motion of the boundary with increasing Kp is most pronounced about midnight and least pronounced at noon. In comparison with plasma boundaries the energetic particle boundaries show a lower degree of correlation with Bz, VBz, and Kp. This finding suggests a temporal and spatial decoupling between plasma and energetic particle populations.
Advantages of electric circuit models for treating the substorm breakup problem
Authors: Block L. P. , Fälthammar C.‐G. , Rothwell P. L. , Silevitch M. B.
Published: February 2004

It is shown, by using a circuit model for the magnetospheric current system, that the substorm breakup can be triggered either by some instability anywhere in the circuit or by a decrease in the generator emf, i.e., a northward turning of the interplanetary magnetic field.
Equilibrium conditions and magnetic field rotation at the tangential discontinuity magnetopause
Authors: De Keyser J. , Roth M.
Published: February 2004

De Keyser and Roth recently have developed a kinetic model of the tangential discontinuity magnetopause. This model predicts (1) that not all configurations of magnetic field vectors and magnetosheath velocity allow an equilibrium to exist and (2) that there is a preference for a particular magnetic field rotation sense across the magnetopause due to the different response of ions and electrons to the electric field in the current layer. In the present paper we extend the original model to allow for different magnetospheric and magnetosheath densities and temperatures, and we show that the conclusions remain essentially unchanged. Given the large‐scale magnetosheath flow pattern around the magnetosphere, we also compute which regions of the dayside magnetopause may be in tangential discontinuity equilibrium for a given magnetosheath field orientation.
Magnetic field rotation at the dayside magnetopause: AMPTE/IRM observations
Authors: De Keyser J. , Roth M.
Published: February 2004

Given the large‐scale magnetosheath flow pattern around the magnetosphere, the tangential discontinuity magnetopause model of De Keyser and Roth predicts, for a prescribed magnetic field rotation angle and rotation sense, where equilibrium is possible on the dayside magnetopause surface and where it is not. In this paper we verify these predictions using 5 s time resolution magnetic field and plasma observations of the low‐latitude dayside magnetospheric boundary acquired by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite. The model is confirmed by (1) the dominant presence of large positive magnetic field rotations among the dawnside crossings north of the equator, (2) the observation of positive and negative rotations near the stagnation point and at the duskside, and (3) the rare occurrence and questionable tangential discontinuity nature of low magnetic shear dawnside crossings. The absence of tangential discontinuity equilibrium in dawnside low shear crossings is consistent with the observation of increased dawnside low‐latitude boundary layer thickness for northward magnetosheath field reported in the literature.
New models of Jupiter's magnetic field constrained by the Io flux tube footprint
Authors: Connerney J. E. P. , Acuña M. H. , Ness N. F. , Satoh T.
Published: February 2004

Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Io's orbital radial distance in the equator plane (5.9 Jovian radii). The IFT footprint is a valuable constraint on magnetic field models, providing “ground truth” information in a region close to the planet and thus far not sampled by spacecraft. The magnetic field is represented using a spherical harmonic expansion of degree and order 4 for the planetary (“internal”) field and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. Models fitting Voyager 1 and Pioneer 11 magnetometer observations and the IFT footprint are obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, octupole, and a subset of higher‐degree and higher‐order spherical harmonic coefficients are determined and compared with earlier models.
The relationship between large‐scale solar magnetic field evolution and coronal mass ejections
Authors: Luhmann J. G. , Gosling J. T. , Hoeksema J. T. , Zhao X.
Published: February 2004

The idea that coronal mass ejections (CMEs) originate from the evolution of the large‐scale solar field and in particular arise from regions of freshly opening coronal flux is reexamined using a new approach. Potential field models constructed from Wilcox Solar Observatory magnetograms are applied to study both the rate of “opening” of coronal fields with time and the locations of observed CMEs compared to the inferred newly open field regions on the Sun. Case studies are drawn from SMM coronagraph data, Yohkoh soft X ray images, and counterstreaming electron events observed on the ICE spacecraft. The results suggest that the large‐scale field evolution paradigm of CMEs deserves further attention and has potential for applications to “space weather” forecasting.
Dynamical age of solar wind turbulence in the outer heliosphere
Authors: Matthaeus William H. , Smith Charles W. , Oughton Sean
Published: February 2004

In an evolving turbulent medium, a natural timescale can be defined in terms of the energy decay time. The time evolution may be complicated by other effects such as energy supply due to driving, and spatial inhomogeneity. In the solar wind the turbulence appears not to be simply engaging in free decay, but rather the energy level observed at a particular position in the heliosphere is affected by expansion, “mixing,” and driving by stream shear. Here we discuss a new approach for estimating the “age” of solar wind turbulence as a function of heliocentric distance, using the local turbulent decay rate as the natural clock, but taking into account expansion and driving effects. The simplified formalism presented here is appropriate to low cross helicity (non‐Alfvénic) turbulence in the outer heliosphere especially at low helio‐latitudes. We employ Voyager data to illustrate our method, which improves upon the familiar estimates in terms of local eddy turnover times.
The Southeast Indian Ridge between 88°E and 118°E: Variations in crustal accretion at constant spreading rate
Authors: Sempéré Jean‐Christophe , Cochran James R.
Published: February 2004

The temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid‐ocean ridges. To investigate the variability in crustal accretion that develops in response to variations in mantle temperature, we have conducted a geophysical investigation of the Southeast Indian Ridge (SEIR) between the Amsterdam hotspot and the Australian‐Antarctic Discordance (88°E–118°E). The spreading center deepens by 2100 m from west to east within the study area. Despite a uniform, intermediate spreading rate (69–75 mm yr−1), the SEIR exhibits the range in axial morphology displayed by the East Pacific Rise and the Mid‐Atlantic Ridge (MAR) and usually associated with variations in spreading rate. The spreading center is characterized by an axial high west of 102°45′E, whereas an axial valley is prevalent east of this longitude. Both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform. A region of intermediate morphology separates axial highs and MAR‐like rift valleys. Local transitions in axial morphology occur in three areas along the ridge axis. The increase in axial depth toward the Australian‐Antarctic Discordance may be explained by the thinning of the oceanic crust by ∼4 km and the change in axial topography. The long‐wavelength changes observed along the SEIR can be attributed to a gradient in mantle temperature between regions influenced by the Amsterdam and Kerguelen hot spots and the Australian‐Antarctic Discordance. However, local processes, perhaps associated with an heterogeneous mantle or along‐axis asthenospheric flow, may give rise to local transitions in axial topography and depth anomalies.