bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionospheric disturbances during low‐latitude auroral events and their association with magnetospheric processes
Authors: Tanaka T. , Ohtaka K.
Published: February 2004

Ionospheric disturbances associated with low‐latitude auroral events that occurred during geomagnetic storms on October 21, 1989, and May 10, 1992, are investigated from measurements of the total electron content (TEC) by the U.S. Navy Navigation Satellite System (NNSS) and ionosonde data from the Japanese meridian chain. Features of the ionospheric disturbances are then associated with the progression of geomagnetic disturbances during the storms. After the onset of the main phase of each storm, anomalous TEC distributions characterized by depressed TEC distributions on the high‐latitude side (> 35° geographic latitude) and an enhanced equatorial ionization anomaly (EIA) on the low‐latitude side (<35° geographic latitude) were observed over Japan both by the NNSS and by the meridian chain of ionosonde stations. This enhancement of the EIA suggests the penetration of magnetospheric electric fields during the storms. Corresponding to the anomalous TEC decreases in the northern part of Japan, ionosonde stations in this region observed specific disturbances of ionospheric variation characterized by simultaneous decreases of ƒ0F2 and increases of h′F. These features of ionospheric variations on the high‐latitude side can be attributed to the upward escape of ionospheric plasma caused by the heating and evacuation mechanisms, which are induced by depletion of the plasmasphere and the resulting access of ring current particles to low latitudes. After the appearance of the characteristic ionospheric variations mentioned above, low‐latitude aurorae were observed at the maximum development stage of the Dst, associated with the recovery of midlatitude geomagnetic horizontal (H) components. Prior to the appearance of the low‐latitude aurorae, magnetic disturbances in the auroral region increased with the decrease of midlatitude H components, as a natural consequence of magnetic storms. However, they showed a recovery before the start of the low‐latitude aurorae, while the magnitude of the low‐latitude magnetic field continued to decrease. These observed features of the magnetic disturbances suggest that the position of the auroral oval shifted toward the equator before the appearance of the low‐latitude aurorae. Consequently, these observations are consistent with the plasmapause and auroral zone moving toward the equator before the onset of low‐latitude aurorae. We conclude that the convection electric field penetrating toward low latitudes causes the enhancement of the EIA, the shrinkage of the plasmasphere, and the penetration of high‐energy particles toward the low‐latitude region.
Effect of anisotropic thermal conductivity on the temperature structure of the ionosphere‐plasmasphere system
Authors: Khazanov G. V. , Moore T. E. , Horwitz J. L. , Richards P. G. , Konikov Y. V.
Published: February 2004

Anisotropic plasma hydrodynamics makes use of two heat fluxes representing the transfer of field‐aligned and transverse (with respect to the external magnetic field) energies to describe thermal energy transfer. This is a considerably more complex treatment than isotropic plasma hydrodynamics in which only one heat flux component is considered. The present work analyzes the heat flux structure within the framework of anisotropic hydrodynamics and examines the effect of the anisotropic heat fluxes on the thermal plasma distribution both in the ionosphere and in the magnetically connected plasmasphere. The dependence of heat conductivities on the anisotropy parameter a = T⊥/T‖ is studied (T⊥ and T‖ are transverse and field‐aligned temperatures, respectively). The heat conductivities increase with field‐aligned temperature (with decreasing value of a), thereby enhancing the thermal energy transfer. The analytical solution obtained for the heat conduction equation is used to examine the effect of the electron energy distribution anisotropy on the electron temperature in the plasmasphere. The dependence of heat conductivity on the parameter a has been shown to result in a variation of electron temperature compared with the isotropic case (a=l). The calculated plasmaspheric temperature values rise when a>1 and fall when a<1. The strongest effect of the temperature anisotropy on the derived value of the electron temperature occurs at high altitudes along a flux tube in the vicinity of the equatorial plane. Predictions of this simplified analytical model are compared to observations of temperature anisotropies in the ionosphere. Basic agreement in the magnitude and sense of the electron temperature anisotropy between the model and observations is found.
On the nonadiabatic precipitation of ions from the near‐Earth plasma sheet
Authors: Delcourt D. C. , Sauvaud J.‐A. , Martin R. F. , Moore T. E.
Published: February 2004

We examine the precipitation of ions which result from nonadiabatic pitch angle scattering in the near‐Earth magnetotail. We focus on dynamical situations between the adiabatic limit where the particle magnetic moment is conserved and the current sheet limit where particles experience meandering motion about the midplane. Defining the κ parameter as the square root of the minimum curvature‐radius‐to‐maximum Larmor radius ratio, the situations we consider correspond to κ between 1 and 3. We demonstrate that throughout this range of κ values, magnetic moment variations are systematically organized into three categories: (1) at small (a few degrees) equatorial pitch angles, large magnetic moment enhancements regardless of gyration phase; (2) at large equatorial pitch angles (typically, above 30°), negligible magnetic moment changes; and (3) in between, a prominent dependence upon gyration phase with either magnetic moment enhancement or damping. We show that these three distinct regimes can be understood in terms of relative magnitude of the centrifugal force experienced by the particles with regard to that of the Lorentz force. In agreement with previous studies, we show that detrapping of plasma sheet ions due to nonadiabatic pitch angle scattering is effective throughout the 1‐ to 3‐κ range. However, the degree of loss cone filling critically depends upon the angular characteristics of the incident population. It is significantly greater for initially isotropic populations than for initially field‐aligned ones, because only particles away from the magnetic field direction experience strong damping of magnetic moment.
Evidence for nongyrotropic alpha particle and proton distribution functions: TAUS solar wind measurements
Authors: Astudillo H. F. , Livi S. , Marsch E. , Rosenbauer H.
Published: February 2004

The goal of this paper is to present convincing evidence for the occurrence of nongyrotropic alpha particle and proton velocity distribution functions in the solar wind plasma. We analyze the three‐dimensional velocity distribution functions obtained by measurements with high time resolution (8 s for alpha particles and 2 s for protons) of the plasma experiment TAUS on the PHOBOS II mission to planet Mars. The measurements were made at heliocentric distances near 1.3 AU during an event which occurred after a discontinuity in the interplanetary magnetic field. It was characterized by a spike in the density and temperature ratios, with nα/np and Tα/Tα reaching values of about 0.3 and 12, respectively. In a detailed analysis of the distributions, we show that the nongyrotropic features are associated with sizable fractions of the alpha particle distribution functions. Furthermore, it is also shown that similar nongyrotropic features are present in solar wind proton distributions. The nongyrotropic particles of both species in the distributions can be characterized as particles being gyro‐phase‐bunched in velocity space. We indicate that there might be a close relation between the nongyrotropy of the distribution and the large temperature anisotropies (with T⊥ > T‖) observed in these events. We also point out some differences in the symmetry properties with respect to the local magnetic field direction between the nongyrotropic proton distribution functions found in the undisturbed solar wind and the nongyrotropic distributions originating at interplanetary shocks.
The shape and field of the magnetopause as determined from pressure balance
Authors: Sotirelis Thomas
Published: February 2004

This article presents a theoretical study of the dependence of the magnetopause shape and field on the dynamic pressure of the solar wind. The magnetopause surface (assumed to be axisymmetric) is determined by requiring the plasma pressure outside, estimated by the “Newtonian approximation,” to be balanced by the magnetic pressure inside. The magnetic field inside is given by the sum of (1) the Earth's internal field, (2) the field from various magnetospheric current systems (cross‐tail current, ring current, etc.) as given by an empirical model, and (3) the field due to magnetopause currents. The magnetopause field is a function of the magnetopause shape and is updated each time this shape is adjusted toward pressure balance. In previous studies of this kind, only dipole and magnetopause fields were incorporated. This is the first three‐dimensional solution of the problem to include the cross‐tail current and thus to obtain a realistic magnetotail configuration. The results of this study are twofold. First, the shape of the magnetopause surface is computed under a variety of conditions. The resulting shapes are compared with fits to observed magnetopause crossings, and the scaling of magnetopause position on solar wind dynamic pressure is determined. Second, the model that results can be viewed as an improved version of the empirical model that was used as input, in that the magnetic field in the vicinity of the magnetopause (where empirical models have less guidance from data) is made more realistic.
Parametric study of electromagnetic ion cyclotron instability in the Earth's magnetosphere
Authors: Xue Song , Thorne Richard M. , Summers Danny
Published: February 2004

The growth rate of field‐aligned electromagnetic ion cyclotron (EMIC) waves in the terrestrial magnetosphere is investigated using an anisotropic kappa particle distribution to model the energetic ring current ions. Under such conditions, the wave dispersion relationship can be expressed in terms of the recently introduced modified plasma dispersion function. This function is analogous to the plasma dispersion function defined by Fried and Conte which has previously been used extensively to investigate wave instability in a hot Maxwellian plasma. Calculations for kappa distributions under magnetospheric conditions indicate that the previous results, obtained with a Maxwellian distribution, tend to overestimate the peak convective growth rate of L mode waves. Nonetheless, the dominant spectral properties of EMIC waves, as reported from AMPTE observations, can be understood in terms of cyclotron resonant instability using realistic magnetospheric parameters. Significant convective amplification is mainly confined to the outer (L ≥ 5) magnetosphere. In the afternoon sector, where plasma densities can exceed 107 m−3, intense wave growth is possible in two bands, one above and one below the helium gyrofrequency ΩHe+. Conversely, on the nightside or for early morning conditions, lower ambient plasma density only allows instability above ΩHe+. High concentration of thermal helium can strongly suppress instability in the band above ΩHe+, but it has little effect on wave growth in the band below ΩHe+.
H2+ pickup ions in the solar wind: Outgassing of interplanetary dust
Authors: Gruntman Mike
Published: February 2004

Heliospheric neutral atoms and molecules can be ionized and picked up by the solar wind plasma flow. A new generation of space plasma instruments allows detailed study of the solar wind pickup ions with high sensitivity. Detection of H2+ pickup ions in the solar wind is an exceptionally sensitive tool for the search of molecular hydrogen in the heliosphere. Possible sources of neutral molecular hydrogen include contributions from the local interstellar medium release from cometary bodies, and outgassing of interplanetary dust. The flux of the solar wind H2+ pickup ions due to outgassing of interplanetary dust is theoretically established in this work. Solar wind ions bombard and penetrate dust grains where they are neutralized and subsequently released in the form of neutral atoms and molecules. The processes of molecular hydrogen interaction with the solar radiation and solar wind plasma are considered in detail and the process rates are determined. Solar wind H2+ pickup ion fluxes are obtained for heliocentric distances < 5 AU for different heliocentric latitudes. These heliocentric radial and latitudinal dependences would provide guidance for search of the solar wind H2+ pickup ions by Ulysses, WIND, and SOHO missions.
Relationship of the ionospheric convection reversal to the hard auroral precipitation boundary
Authors: Troshichev O. A. , Shishkina E. M. , Lu G. , Richmond A. D.
Published: February 2004

Plasma drift and particle measurements from the DMSP spacecraft for the three Geospace Environmental Modeling (GEM) campaign periods (January 27–29, March 28–29, and July 20–21, 1992) have been used to study the relationship between the plasma convection reversal and the poleward boundary of the diffuse auroral zone characterized by hard electron precipitation with energies greater than 0.45 keV. This boundary, named the hard auroral precipitation boundary (or the HAP boundary) in this paper, is often regarded as the ionospheric footprint of the boundary between the central plasma sheet (CPS) and the boundary plasma sheet (BPS). By examining simultaneous ion drift and particle measurements from about 500 satellite passes we find that the large‐scale plasma flow in the morning and evening sectors changes its direction within the auroral oval at the HAP boundary. However, exceptions are found in the early morning sector between 0300 and 0600 MLT, where the convection reversal is sometimes (in 30% of the DMSP crossings) displaced poleward relative to the HAP boundary. It is shown that the shape of the region bordered by the HAP boundary can be roughly represented by a circle, whose size is influenced by the IMF Bz component. There is roughly a linear correlation between the diameter of this circle and the cross‐polar‐cap potential drop, with the best correlation coefficient of 0.65 for winter season. Our study suggests that the HAP boundary corresponds to the magnetospheric boundary between the quasi‐dipolar region and the region with more stretched field lines, and the source of the region 1 field‐aligned current is located near the HAP boundary. A By‐dependent shift of the HAP boundary with respect to the noon‐midnight meridian is also found. In the northern hemisphere, the shift is dawnward for positive By and duskward for negative By in the southern hemisphere, the shift is opposite to that in the northern hemisphere.
Ring current development during storm main phase
Authors: Fok Mei‐Ching , Moore Thomas E. , Greenspan Marian E.
Published: February 2004

The development of the ring current ions in the inner magnetosphere during the main phase of a magnetic storm is studied. The temporal and spatial evolution of the ion phase space densities in a dipole field are calculated using a three dimensional ring current model, considering charge exchange and Coulomb losses along drift paths. The simulation starts with a quiet time distribution. The model is tested by comparing calculated ion fluxes with Active Magnetospheric Particle Tracer Explorers/CCE measurement during the storm main phase on May 2, 1986. Most of the calculated omnidirectional fluxes are in good agreement with the data except on the dayside inner edge (L < 2.5) of the ring current, where the ion fluxes are underestimated. The model also reproduces the measured pitch angle distributions of ions with energies below 10 keV. At higher energy, an additional diffusion in pitch angle is necessary in order to fit the data. The role of the induced electric field on the ring current dynamics is also examined by simulating a series of substorm activities represented by stretching and collapsing the magnetic field lines. In response to the impulsively changing fields, the calculated ion energy content fluctuates about a mean value that grows steadily with the enhanced quiescent field.
Solar total irradiance observations from spacecraft: 1992–1993
Authors: Mecherikunnel Ann T.
Published: February 2004

A brief review of the simultaneous observations of solar total irradiance performed by the UARS ACRIM II (upper atmosphere research satellite active cavity radiometer irradiance monitor), EURECA SOVA 1 and SOVA 2 (European retrievable carrier platform solar variability experiments), ERBS ERBE (Earth Radiation Budget Satellite Earth Radiation Budget Experiment), Nimbus 7 ERB, and Atlas 1 and 2 in 1992–1993 (a period characterized as the declining phase of solar cycle 22), is presented in this paper. Statistical comparison of the irradiance values and the linear relationships among the time series are examined for the overlap period August 1992 to May 1993. The mean irradiance at 1 astronomical unit (AU) observed by the UARS ACRIM II, EURECA SOVA 1, EURECA SOVA 2, and ERBS ERBE is within the range of 1365.5–1366.9 W m−2, and high correlation (r2 = 0.94) exists among the time series. Linear regression models based on the measurements are used in estimating the observed irradiance and predicting the missing values. The challenge in establishing a homogeneous long‐term database from the independent observations of shorter duration requires a clear understanding of the compatibility of the data sets and any instrument‐related drift or degradation.
Multiradar observations of medium‐scale acoustic gravity waves using the Super Dual Auroral Radar Network
Authors: Bristow W. A. , Greenwald R. A.
Published: February 2004

Observations of medium‐scale acoustic gravity waves in the thermosphere, using multiple HF radars, are presented. Examples of waves from a magnetically quiet interval and a more magnetically active interval are examined along with statistics of gravity wave occurrence. It is shown that during quiet intervals gravity wave phase fronts may extend longitudinally for more than 3400 km. During the more active interval, the level of wave activity is elevated to the point that it is difficult to distinguish individual wave packets. Also, power spectra from the active period showed evidence of a harmonic sequence, indicating the possibility of some nonlinear process acting as the gravity waves propagate. Statistics of occurrence showed that high‐latitude medium‐scale gravity waves are more likely in the winter than in the summer and that the probability of observing waves on multiple radars is essentially the same as observing waves on any one radar.
Evolution of the fire‐hose instability: Linear theory and wave‐wave coupling
Authors: Quest K. B. , Shapiro V. D.
Published: February 2004

Large ion thermal or kinetic pressure anisotropies have been inferred to exist in conjunction with supernova shocks as well as in the solar wind/cometary interaction region and upstream from planetary bow shocks. For sufficiently strong thermal or beam‐driven anisotropies, electromagnetic instability develops, isotropizing and scattering the ion populations. In particular, if the effective plasma β > 2 (where β is the ratio of plasma pressure to magnetic pressure), and if the anisotropy is such that the temperature parallel to the magnetic field exceeds the perpendicular, then fire‐hose instability can result, generating transverse magnetic field fluctuations. In high‐β interstellar plasmas with large anisotropies, the level of the excited fluctuations may be quite large, exceeding even the ambient magnetic field. After a period of inverse‐cascade to longer wavelengths, it may provide a potential source for the scattering of cosmic rays. In this study we simulate the evolution of the fire‐hose instability using a standard one‐dimensional hybrid code (macroparticle ions, massless fluid electrons). We find that the wave evolution proceeds in two stages. A rapid period of growth brings the plasma back to approximate marginal stability. There follows a second stage of slower evolution dominated by wave‐wave interaction. During the second stage, the wave energy spectrum clearly exhibits an inverse cascade. Implications for cosmic ray scattering will be discussed.
Upstream and downstream propagating waves generated by pickup ions near comet Halley
Authors: Johnstone A. D. , Krauklis I. , Huddleston D. E. , Söding A. , Glassmeier K‐H. , Neubauer F. M.
Published: February 2004

It is well known that most of the Alfvén waves generated in the solar wind by cometary ion pickup are propagating upstream in the solar wind reference frame. However, the magnitude of the small proportion propagating downstream is extremely important because it controls the amount of ion acceleration driven by the second order Fermi process. The velocity diffusion coefficient is approximately proportional to the intensity of the weaker component of the upstream or downstream propagating waves. In previous papers we developed a new technique to calculate the wave spectrum produced by pickup ions which successfully accounted for the total wave spectrum quantitatively. The theory also gives the spectrum of upstream and downstream waves separately. We set out to test the theory by obtaining the upstream and downstream proportions of the wave energy from the data. We have used analyses based on Elsässer variables and the use of the Poynting vector and find that while the downstream proportion is small, it is much larger than expected, and its spectral shape is different from our theoretical prediction. The observations reveal that the spectrum of the cometary ion generated downstream waves is similar to that of the upstream waves, that is, a power law spectrum with a spectral index of −2 above the wavenumber corresponding to the cometary ion gyrofrequency. Since the weaker component is stronger than expected over a wide range of frequencies (and thus resonant velocities), the acceleration process will be enhanced and may be sufficient to explain the discrepancies between theory and observation which still exist. The mechanism responsible for the generation of the increased amount of downstream waves could be nonlinear induced scattering of the upstream waves by the solar wind protons.
A low‐dimensional energy‐conserving state space model for substorm dynamics
Authors: Horton W. , Doxas I.
Published: February 2004

A low‐ (four‐) dimensional state space model for the basic energy components of the night‐side magnetosphere is developed based on truncated descriptions of the collisionless microscopic (Hamiltonian) energy transfer processes occurring in the quasi‐neutral layer. The substorm trigger, due to bifurcation of the system either from magnetic reconnection or from ballooning‐mirror modes, is modeled by a fast unloading above a critical current. For constant southward interplanetary magnetic field solar wind input, the system performs oscillations with recurrence times in the range of 60–70 min. For northward IMF, the system decays to a static MHD equilibrium. For the intermediate case of a weak solar wind input, there occur uncorrelated, isolated substorms. The theoretical basis for a low‐dimensional model may be supported within the framework of self‐organized criticality since the geotail is a highly stressed global system during the growth phase of substorms. The new energy‐conserving state space model eliminates the “dripping faucet” unloading role of the Klimas et al. [1992] model and includes the parallel streaming kinetic energy in the substorm dynamics. The new model shows a wide range of dynamical behavior according to the values of the system parameters and the form of the solar wind input signal.
Pc 1 induced electromagnetic lift of background plasma in the magnetosphere
Authors: Guglielmi A. , Kangas J. , Mursula K. , Pikkarainen T. , Pokhotelov O. , Potapov A.
Published: February 2004

We discuss the ponderormotive redistribution of ions along geomagnetic field lines due to the action of Pc 1 pulsations, using a simple diffusion equilibrium model. The field‐aligned component of the ponderomotive force is derived for Alfvén waves as well as for ion cyclotron waves propagating in a multicomponent plasma. Our theory gives a possibility for a better understanding of the problem and a convenient way to make rough estimates of the ponderomotive efficiency of Pc 1 waves under concrete magnetospheric conditions. Qualitative analysis of equations and satellite information on the intensity of wave fields show that the ponderomotive forces can have a pronounced effect on plasma. Ground‐based observations of Pc 1 pulsations also suggest a ponderomotive redistribution of the background plasma.
F region climatology during the SUNDIAL/ATLAS 1 campaign of March 1992: Model‐measurement comparisons and cause‐effect relationships
Authors: Szuszczewicz E. P. , Torr D. , Wilkinson P. , Richards P. , Roble R. , Emery B. , Lu G. , Abdu M. , Evans D. , Hanbaba R. , Igarashi K. , Jiao P. , Lester M. , Pulinets S. , Reddy B. M. , Blanchard P. , Miller K. , Joselyn J.
Published: February 2004

We present the first joint comparison of global measurements of F region characteristics with three models used widely in the specification of the ionospheric‐thermospheric system. The models, the International Reference Ionosphere (IRI), the field line interhemispheric plasma (FLIP) model, and the Thermospheric‐Ionospheric General Circulation Model (TIGCM), represent a unique set of capabilities with major differences in approaches to the prevailing physics and different levels of computational complexity. The database was developed by a global network of 53 ionosonde stations operating around‐the‐clock for the period March 22 through April 4, 1992 in collaboration with the ATLAS 1 mission. The emphasis is on the F region characteristics of peak heights (hmF2) and densities (NmF2), their climatological (i.e., average) behavior during the ATLAS 1 period, and associated cause‐effect relationships. We explore latitudinal and local time variations with attention to the influences of meridional winds and plasmaspheric fluxes in the maintenance of different domains in the ionospheric‐thermospheric system. We find that all three models tend to underestimate the values of hmF2 and NmF2 with the largest discrepancies in NmF2 resulting in the FLIP and TIGCM representations at night. These discrepancies can grow to levels as large as 110% near 0400 LT, a “rediscovery” of the old but unsettled issue of maintenance of the nighttime ionosphere. This nighttime discrepancy is traceable in first order to model underestimates of prevailing meridional winds. The contributions of plasmaspheric fluxes are also considered, with the conclusion that they are of secondary importance, but substantially more work is necessary to uniquely quantify their role. In contrast to their nighttime characteristics, the FLIP and TIGCM generally have excellent agreement (i.e., 6 ± 6%) with daytime observations of NmF2, and the IRI tends to underestimate the observed values of NmF2 by a nominally LT‐insensitive level of 28 ± 6%. Other campaign results are reviewed in this issue, with a focus on regional responses to the prevailing conditions and their characterization in terms of latitudinal distributions of F region heights and densities.
Plasmasphere electron temperature profiles and the effects of photoelectron trapping and an equatorial high‐altitude heat source
Authors: Balan N. , Oyama K.‐I. , Bailey G. J. , Abe T.
Published: February 2004

The electron temperature Te in the Earth's plasmasphere up to 10,000 km altitude, has been measured routinely by a thermal electron energy distribution instrument on board the EXOS D satellite. The measurements made during the years 1989–1995 have been analyzed to obtain altitude (1000–8000 km) profiles of Te for magnetic latitudes 0°–40°N at different times of the day. The profiles are compared with those computed by the Sheffield University plasmasphere‐ionosphere model, modified to include nonlocal heating due to trapped photoelectrons and an equatorial high‐altitude heat source. The photoelectrons are trapped at altitudes above 600 km, and the high‐altitude heat source is applied as energy input along the magnetic field lines with apex altitude greater than 600 km between ±10° magnetic latitude. The results for 8000 km altitude show that the modeled values of Te computed without photoelectron trapping and the high‐altitude heat source are much less than the mean measured values, 3700 K compared with 6500 K. Depending upon altitude and latitude, a photoelectron trapping of up to 100% is required to raise the modeled electron temperatures to the mean measured values. However, photoelectron trapping alone cannot account for the observed latitude variation of Te, which depends on altitude. Model calculations carried out with 50% photoelectron trapping and an altitude and local time‐dependent high‐altitude heat source reproduce the measurements, including the latitude variation; the heat source required for nighttime is about one fourth of that required for daytime. An equatorial high‐altitude heat source appears to be the only mechanism that can account for the measured values of Te; values in excess of 12,000 K have been measured. There are some quantitative differences between the measured and modeled temperatures at night in the lower plasmasphere (<2500 km), which could be caused, in whole or in part, by the inaccuracies of the nighttime measurements due to the low plasma densities.
Dynamic response of the cusp morphology to the solar wind: A case study during passage of the solar wind plasma cloud on February 21, 1994
Authors: Yamauchi M. , Nilsson H. , Eliasson L. , Norberg O. , Boehm M. , Clemmons J. H. , Lepping R. P. , Blomberg L. , Ohtani S.‐I. , Yamamoto T. , Mukai T. , Terasawa T. , Kokubun S.
Published: February 2004

On February 21, 1994, both Geotail and IMP 8 satellites detected an interplanetary plasma cloud with intense interplanetary magnetic field (IMF > 50 nT) and high dynamic pressure (> 50 nPa). During this interval the Freja satellite detected intense cusp‐like plasma injections in four out of six dayside traversais. The first two traversais are carefully studied. During the first traversal the overall morphology of the ion injection is characterized by a “multiple‐injection” signature over a wide magnetic local time (MLT) range, whereas it is characterized by a “single‐injection” signature with narrow injection region at 8 MLT in the second traversal. The solar wind conditions were also quite different between these two periods: while both dynamic and magnetic pressures stayed high during entire period, the dynamic beta was much higher during the first Freja traversal than during the second traversal. Between these two traversais, the cusp plasma injection is detected by the Søndre Strømfjord radar. The radar signature of the plasma injection is identified using the satellite particle data when the satellite and the radar were conjugate (the satellite's footprint was in the radar's field of view.) The cusp position and dynamics observed by the Søndre Strømfjord radar again show a very good correlation to the solar wind condition, especially to the dynamic pressure. The result indicates the following. (1) During southward IMF the cusp morphology differs for conditions of high or low solar wind dynamic pressure. High dynamic pressure widens the cusp (with multiple injections), whereas high magnetic pressure narrows it (with single injection). The effect of the IMF on the cusp locations and morphology becomes dominant only when the dynamic pressure is not very high. (2) Such a morphological difference reflects dynamic pressure more than dynamic beta during southward IMF at least during times of high solar wind dynamic pressure. (3) The cusp morphology responds very quickly to the changes in the solar wind conditions.
Diurnal oscillation of Dst: A manifestation of the Russell‐McPherron effect
Authors: Siscoe George , Crooker Nancy
Published: February 2004

During away sectors of the interplanetary magnetic field (IMF) around autumnal equinox and toward IMF sectors around vernal equinox, Dst underwent an average 4.0 nT peak‐to‐peak diurnal oscillation during a 13‐year analysis period. Such an oscillation is predicted by the Russell‐McPherron effect. The same effect is also found in a case study showing Dst diurnally oscillating through a period of sustained, recurrent geomagnetic activity. The time of minimum Dst lags the time of maximum energy coupling as given by the Russell‐McPherron effect by between 0.5 and 1.5 hours, which in the framework of the Russell‐McPherron coupling model gives a measure of the energy flow time from the solar wind to the ring current. This lag seems to be significantly shorter than expected for the Russell‐McPherron effect used as a driver to an energy input and decay model of ring current dynamics. The results suggest that a mechanism besides ring current decay might operate on the diurnal timescale, such as a flow of ring current energy into the ionosphere via the region 2 currents.
Ulysses observations of corotation lags in the dayside Jovian magnetosphere: An evaluation of the hinged magnetodisc and magnetic anomaly models
Authors: Hoogeveen Gary W. , Phillips John L. , Dougherty Michele K.
Published: February 2004

Ulysses observations during the inbound leg of the February 1992 encounter with Jupiter offer the opportunity to evaluate both the hinged magnetodisc and the magnetic anomaly models currently used to describe the Jovian magnetospheric corotation lag. Particle and field detectors on board Ulysses measure signatures of the current and plasma sheets. The locations of these sheets in radius and system III longitude are then used as inputs to the models. It is shown that the hinged magnetodisc model is able to explain some characteristics of the data but unable to explain others. The magnetic anomaly model appears to account for the overall nature of the magnetospheric corotational lag; however, it, too, cannot fully account for the Ulysses data. A modification of the magnetic anomaly model is offered which adequately explains the Ulysses data and, with the assumption of a sufficiently significant, time‐dependent Iogenic plasma source, explains data from previous Jupiter encounters as well.
Magnetospheric structure with a southward interplanetary magnetic field
Authors: Yeh Tyan
Published: February 2004

A magnetospheric magnetic field that has a topological structure with one pair of neutral points and one segmental neutral line is studied. Such a magnetic field forms when the interplanetary field is southward. There is a surface current on an open‐ended complete flux surface in this magnetospheric magnetic field. This current sheet on which the induced Chapman‐Ferraro current flows is the outer separatrix, which separates field lines attached to the Earth and field lines detached from the Earth. In addition, there is an inner separatrix, which separates closed and open field lines. The two neutral points are at the north and south apices of the common wall shared by the two separatrices, on which the surface current is intense. The surface current is weak or even insignificant on other parts of the outer separatrix. The neutral line is on the equatorial plane with two end points at the east and west apices of the common wall shared by the two separatrices. It is a separator at which closed, open, and detached field lines meet. The magnetic topology indicates that field line reconnection, which can take place only at a separator, must occur away from the subsolar part of the dayside magnetopause in order for an open magnetosphere to remain in a quasisteady state.
An investigation into solar wind plasma periodicities
Authors: Paularena Karolen I.
Published: February 2004

Plasma and magnetic field data from the IMP 8 Earth‐orbiting spacecraft and plasma data from the Voyager 2 spacecraft are analyzed to determine their frequency spectra. The resulting spectra are compared with solar oscillation mode frequencies which recent papers (particularly Thomson et al. [1995]) report are present in solar wind data. The frequency spectra of the plasma and magnetic field data sets are characterized by a diverse set of strong peaks, many of which are stronger than those occurring at frequencies associated with solar oscillatory modes. The frequencies of these peaks are not consistent between data sets, making it unlikely that they represent solar oscillation modes. Although the analysis technique used, the Lomb‐Scargle method, does not permit accurate assignment of statistical uncertainties, it is clear that solar oscillatory frequencies are not the major contributors to the frequency spectra of these data.
A magnetic cloud as a distended flux rope occlusion in the heliospheric current sheet
Authors: Crooker N. U. , Intriligator D. S.
Published: February 2004

A magnetic cloud was detected both near Earth by IMP 6–8 and 30° of longitude away from Earth by Pioneer 11 at 4.8 AU. The nearly identical magnetic signatures at the two spacecraft imply that its radial dimension increased little between them. The magnetic field within the cloud rotated from toward to away polarity, marking sector boundary passage at both locations. Interpreted in terms of a flux rope, its axis was highly inclined to the ecliptic plane, implying that its cylindrical cross section in the ecliptic plane was distended by at least 30° of longitude, forming an extensive occlusion in the heliospheric current sheet. A similar magnetic signature was observed at the same sector boundary by Pioneer 10 at 6.1 AU, at a longitude between Earth and Pioneer 11. Its radial dimension, however, was diminished by a factor of 3. We suggest that it was the same cloud, compressed by a second, oncoming transient that was observed behind the cloud at 1 AU.
Field line integration and localized modes in the equatorial spread F
Authors: Rappaport H. L.
Published: February 2004

The field line integrated Rayleigh‐Taylor and localized mode models of the equatorial spread F (ESF) are unified by a variational principle. Those modes which can be analyzed by taking local approximations in two dimensions are considered. It is shown that the field line integrated Rayleigh‐Taylor model can be interpreted as a one‐term Rayleigh‐Ritz approximation to the localized mode model of Basu and Coppi [1983]. In making this observation, the field line integration technique is generalized, so as to include the effects of the perturbation electric field parallel to the Earth's magnetic field. As additional terms are included into the Ritz sum, the growth rate found for ESF increases. At the smaller perpendicular wavelengths, localization of the electrostatic potential along the field line reduces the effects of E region loading.
Modifications of the electrodynamic interaction between Jupiter and Io due to mass loading effects
Authors: Kopp Andreas
Published: February 2004

The electrodynamic interaction between Jupiter and Io is investigated by means of three‐dimensional resistive MHD simulations with Jupiter being modeled as an aligned rotator situated in the origin of a cylindrical coordinate system. For numerical reasons, however, only qualitative statements can be made, which do not allow quantitative comparisons with observations. The main topic is the generation of electric currents parallel to the magnetic field. In consideration of the fact that some observational features cannot be qualitatively understood with present theories, the rôle of the plasma production from lo's atmosphere as well as by ionization in the plasma torus is investigated. If these two plasma sources are significantly cooler than the plasma environment, they can be shown to cause a torus corotational lag, in consequence of which a field‐aligned current system develops. If microinstabilities occur along this system, anomalous resistivity regions may be built up where the electric field attains a component parallel to the magnetic field. Thus ionization within the torus might explain the observed decametric radiation upstream of Io's current position. Furthermore, plasma production from Io's atmosphere shows a strong influence on the MHD modes excited by the Io/torus interaction. The pressure signatures generated by slow mode perturbations disclose the formation of a tail‐like structure in the wake behind Io. Thus previous conceptions about a new current systems in Io's wake can be confirmed by self‐consistent MHD simulations. Finally, the correlations between this current and gradients in angular velocity and density are studied in detail.
The topology of intrasector reversals of the interplanetary magnetic field
Authors: Kahler S. W. , Crocker N. U. , Gosling J. T.
Published: February 2004

A technique has been developed recently to determine the polarities of interplanetary magnetic fields relative to their origins at the Sun by comparing energetic electron flow directions with local magnetic field directions. Here we use heat flux electrons from the Los Alamos National Laboratory (LANL) plasma detector on the ISEE 3 spacecraft to determine the field polarities. We examine periods within well‐defined magnetic sectors when the field directions appear to be reversed from the normal spiral direction of the sector. About half of these intrasector field reversals (IFRs) are cases in which the polarities match those of the surrounding sectors, indicating that those fields have been folded back toward the Sun. The more interesting cases are those with polarity reversals. We find no clear cases of isolated reverse polarity fields, which suggests that islands of reverse polarity in the solar source dipole field probably do not exist. The IFRs with polarity reversals are strongly associated with periods of bidirectional electron flows, suggesting that those fields occur only in conjunction with closed fields. We propose that both those IFRs and the bidirectional flows are signatures of coronal mass ejections (CMEs). In that case, many interplanetary CMEs are larger and more complex than previously thought, consisting of both open and closed field components.
Heliospheric termination shock motion due to fluctuations in the solar wind upstream conditions: Spherically symmetric model
Authors: Ratkiewicz R. , Barnes A. , Molvik G. A. , Spreiter J. R. , Stahara S. S.
Published: February 2004

Large‐scale fluctuations in the solar wind plasma upstream of the heliospheric termination shock (TS) will cause inward and outward motions of the shock. Using numerical techniques, we extend an earlier, strictly one‐dimensional (planar) analytic gasdynamic model [Barnes, 1993] to spherical symmetry to investigate the features of global behavior of shock motion. Our starting point is to establish a steady numerical solution of the gasdynamic equations describing the interaction between the solar wind and the interstellar medium. We then introduce disturbances of the solar wind dynamic pressure at an inner boundary and follow the subsequent evolution of the system, especially the motion of the termination shock. Our model solves spherically symmetric gasdynamic equations as an initial‐boundary value problem. The equations in conservative form are solved using a fully implicit total variation diminishing (TVD) upwind scheme with Roe‐type Riemann solver. Boundary conditions are given by the solar wind parameters on an inner spherical boundary, where they are allowed to vary with time for unsteady calculations and by a constant pressure (roughly simulating the effect of the local interstellar medium) on an outer boundary. We find that immediately after the interaction, the shock moves with speeds given by the earlier analogous analytic models. However, as the termination shock propagates, it begins to slow down, seeking a new equilibrium position. In addition, the disturbance transmitted through the TS, either a shock or rarefaction wave, will encounter the outer boundary and be reflected back. The reflected signal will encounter the TS, causing it to oscillate. The phenomenon may be repeated for a number of reflections, resulting in a “ringing” of the outer heliosphere.
Post midnight VLF chorus events, a substorm signature observed at the ground near L = 4
Authors: Smith A. J. , Freeman M. P. , Reeves G. D.
Published: February 2004

Clouds of energetic electrons, injected sporadically into the nightside magnetosphere during substorm expansion phase onsets, can generate VLF whistler mode noise through the gyroresonance instability, which may then be observed on the ground or in space. Although these substorm‐related chorus events (SCEs) have been reported occasionally in the literature, there seems to have been no systematic study, probably because of the lack, until now, of a well‐adapted experimental technique. The VLF/ELF Logger Experiment (VELOX) instrument, located at Halley, Antarctica (76°S, 26°W, L = 4.3), is, however, particularly well suited to a systematic study of this aspect of the substorm phenomenon. The data exist almost continuously from January 1992 onward, at 1‐s time resolution in eight quasi‐logarithmically spaced frequency bands covering the range 0.25–10 kHz. For this paper, 327 days of continuous data from 1992 have been analyzed. The 243 SCEs identified were observed on about 50% of days, almost exclusively in the 2300–0600 MLT local time range, and were characterized by limited duration (typically, ∼10 min at 1.5 kHz) and upward frequency drift of a band of usually weak and relatively unstructured chorus at a rate of ∼200 Hz min−1 between 0.5 and 5 kHz (corresponding to parallel electron energies in the range ∼10–100 keV). This drift is consistent with the combined eastward and inward motion of the resonant electrons due to azimuthal gradient‐curvature drift and radial E × B drift under the action of substorm‐enhanced westward electric fields of order l mVm−1 near the equatorial plane. The limited MLT viewing window of the station implies an overall detection efficiency for SCEs of ∼20%. The inferred annual mean substorm rate, 1366 ± 188 year−1, and inter substorm interval, 5.5 ± 0.8 hours, are similar to the values derived using other techniques. However, the distribution of intervals between successive SCEs is different from that for substorm‐related particle injections at geostationary orbit; in particular, the models around 1 hour rather than 2–3 hours. The SCE as seen by a VELOX‐type VLF receiver with a wide field of view is an important alternative ground‐observable substorm signature, complementary to those (such as bays and Pi 2 pulsations) indicated by magnetometers.
Propagation of three‐dimensional Alfvén waves in a stratified, thermally conducting solar wind
Authors: Orlando S. , Lou Y.‐Q. , Rosner R. , Peres G.
Published: February 2004

We model the propagation of three‐dimensional, adiabatic, linear Alfvén waves in the solar atmosphere and wind, taking into account relevant physical effects, including gravity stratification, thermal conduction, radiative losses, and heating (via a phenomenological term). Our magnetohydrodynamic solar wind model also accounts for the momentum deposition by a spectrum of non‐WKB Alfvén waves. The transmission and reflection of such waves has been previously studied by a variety of techniques, including calculations based on the computation of the ratio between the wavelength and the scale length of the Alfvén speed change and based on a globally ‐ computed transmission coefficient. In this paper we discuss both techniques and show how they are related. We also discuss the physics underlying the reflection process and the possible role wave reflection might play in the acceleration of the solar wind and the winds from other stars.