bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron scattering loss in Earth's inner magnetosphere: 1. Dominant physical processes
Authors: Abel Bob , Thorne Richard M.
Published: February 2004

Pitch angle diffusion rates due to Coulomb collisions and resonant interactions with plasmaspheric hiss, lightning‐induced whistlers and anthropogenic VLF transmissions are computed for inner magnetospheric electrons. The bounce‐averaged, quasi‐linear pitch angle diffusion coefficients are input into a pure pitch angle diffusion equation to obtain L and energy dependent equilibrium distribution functions and precipitation lifetimes. The relative effects of each scattering mechanism are considered as a function of electron energy and L shell. Model calculations accurately describe the enhanced loss rates in the slot region, as well as reduced scattering in the heavily populated inner radiation belt. Predicted electron distribution function calculations in the slot region display a characteristic “top hat” distribution which is supported by observations. Inner zone electron lifetimes based on observed decay rates of the Starfish electron population are in approximate agreement with model predictions.
Properties and sources of low and very low latitude Pi2 pulsations
Authors: Li Y. , Fraser B. J. , Menk F. W. , Webster D. J. , Yumoto K.
Published: February 2004

Data from a low‐latitude ground array of 10 fluxgate magnetometer stations spanning a geomagnetic latitude range from 38°N through the equator to 47°S, with a geomagnetic longitudinal extent from 185°E to 227°E covering ∼3 hours in magnetic local time, and an additional high‐latitude station (65°N) are used to study the spatial and conjugate properties of Pi2 geomagnetic pulsation wave frequency, amplitude, phase, and polarization. Using fast fourier transform and pure state vector analysis techniques, multistation observations show that individual frequency elements of the Pi2 waves are constant with latitude and longitude, but the dominant frequency may vary. In fact, the higher frequencies often dominate at low latitudes, and the lower frequencies often dominate at higher latitudes. The D component amplitude shows a clearly decreasing trend to a near‐zero minimum at the equator, while the H component amplitude does not show a simple behavior. The H component phase is nearly constant at low latitudes, while the high‐latitude station may be out of phase or in phase with the low‐latitude stations for different events. The D component phase is constant in each hemisphere with a 180° phase difference between hemispheres. Azimuthal wave numbers m obtained from interstation phase differences are small, typically |m| < 3, and show a dominance of westward propagation at most longitudes. It is concluded that the main features of low‐latitude Pi2 properties are most easily explained by magnetospheric cavity or wave guide mode resonances, and field line resonances are most likely responsible for the high‐latitude Pi2s. This supports the idea that midlatitude and low‐latitude Pi2s are two‐source phenomena.
The distant tail at 200 RE: Comparison between Geotail observations and the results from a global magnetohydrodynamic simulation
Authors: Berchem J. , Raeder J. , Ashour‐Abdalla M. , Frank L. A. , Paterson W. R. , Ackerson K. L. , Kokubun S. , Yamamoto T. , Lepping R. P.
Published: February 2004

This paper reports a comparison between Geotail observations of plasmas and magnetic fields at 200 RE in the Earth's magnetotail with results from a time‐dependent, global magnetohydrodynamic (MHD) simulation of the interaction of the solar wind with the magnetosphere. The study focuses on observations from July 7, 1993, during which the Geotail spacecraft crossed the distant tail magnetospheric boundary several times while the interplanetary magnetic field (IMF) was predominantly northward and was marked by slow rotations of its clock angle. Simultaneous IMP 8 observations of solar wind ions and the IMF were used as driving input for the MHD simulation, and the resulting time series were compared directly with those from the Geotail spacecraft. The very good agreement found provided the basis for an investigation of the response of the distant tail associated with the clock angle of the IMF. Results from the simulation show that the stresses imposed by the draping of magnetosheath field lines and the asymmetric removal of magnetic flux tailward of the cusps altered considerably the shape of the distant tail as the solar wind discontinuities convected downstream of Earth. As a result, the cross section of the distant tail was considerably flattened along the direction perpendicular to the IMF clock angle, the direction of the neutral sheet following that of the IMF. The simulation also revealed that the combined action of magnetic reconnection and the slow rotation of the IMF clock angle led to a braiding of the distant tail's magnetic field lines along the axis of the tail, with the plane of the braid lying in the direction of the IMF.
The driving of the plasma sheet by the solar wind
Authors: Borovsky Joseph E. , Thomsen Michelle F. , Elphic Richard C.
Published: February 2004

The coupling of the plasma sheet to the solar wind is studied statistically using measurements from various satellite pairs: one satellite in the solar wind and one in either the magnetotail central plasma sheet or the near‐Earth plasma sheet. It is found that the properties of the plasma sheet are highly correlated with the properties of the solar wind: specifically that (1) the density of the plasma sheet is strongly correlated with the density of the solar wind, (2) the temperature of the plasma sheet is strongly correlated with the velocity of the solar wind, (3) the particle pressure and total pressure of the plasma sheet are strongly correlated with the ram pressure of the solar wind, (4) By in the plasma sheet is strongly correlated with the By in the solar wind, (5) Bz in the plasma sheet is weakly correlated with the Bz in the solar wind, (6) Ey in the plasma sheet is weakly correlated with the Ey in the solar wind, and (7) plasma sheet earthward‐tailward flow velocity is weakly anticorrelated with the solar wind velocity. After removing these solar wind dependencies, the dependencies of the properties of the plasma sheet on geomagnetic activity are reduced and changed. The time lags between the solar wind density and the plasma sheet density are investigated statistically and on a case‐by‐case basis; it is found that solar wind material reaches the midtail plasma sheet in ∼2 hours, reaches the near‐Earth nightside plasma sheet in ∼4 hours, and reaches the dayside plasma sheet in ∼15 hours. The pathway for mass transport into the plasma sheet is explored; it is suggested that solar wind material is transported rapidly up the tail to the near‐Earth region via the plasma sheet boundary layer, followed by poloidal transport via convection in the near‐Earth (low β) plasma sheet or followed by eddy diffusion in the not‐too‐distant (high β) plasma sheet. Analogies are drawn between the high Reynolds number plasma sheet and a high Reynolds number fluid wake.
Temperatures in the upper ionosphere and plasmasphere
Authors: Titheridge J. E.
Published: February 2004

Modeling of the topside ionosphere requires a knowledge of the electron and ion temperatures (Te and Ti) as a function of height. Data in the International Reference Ionosphere (IRI) model are sometimes conflicting, and extend only up to 3000 km. An exact, analytic solution is found for the variation of Te along a magnetic field line, making full allowance for changes in the cross section and inclination of the tube of force. The downward heat flux is assumed to decrease smoothly with height, becoming zero at the top of the field line. Changes in the Coulomb cross section increase plasmaspheric temperatures by 6–8%. For a given field line, the temperature profile is defined by two parameters, taken as the temperature To and gradient Go at a reference height of 400 km. Results are fitted to satellite data, for mean day and night conditions, at intervals of 5° in latitude and heights from 400 to 8000 km. Diurnal changes are reproduced using sunrise and sunset transitions that are matched to observed values. Seasonal changes in Te are generally less than experimental errors. A solar activity variation of 30–40% is required, at low heights, to match the change in neutral temperature. Exospheric temperatures must also increase by 10–20%, near solar maximum. A first‐order correction is derived for high‐latitude heating during periods of magnetic activity. Values of Ti are calculated from the Te profile and the neutral temperature Tn, giving results that agree well with mean observations. The use of physically acceptable temperature profiles, fitted to the often conflicting satellite data, should give more consistent results for most purposes.
Electron beams and radio waves of solar Type III bursts
Authors: Dulk George A. , Leblanc Yolande , Robinson Peter A. , Bougeret Jean‐Louis , Lin Robert P.
Published: February 2004

Using the radio wave and energetic particle experiments on the Wind spacecraft, we examine how the radio flux density of interplanetary type III bursts depends on the flux and energy of the energetic electrons. We derive the relationship between them, first by giving detailed radio and electron characteristics of one type III burst, and then using the results of similar analyses of 10 bursts. The times of commencement of the radio waves from decametric to kilometric wavelengths, in relation to the onset time of Langmuir waves, demonstrate forcibly that the initial type III radiation is at the fundamental and not the harmonic. Near and after the time of peak flux density the radiation could be either at the fundamental or the harmonic. In our theoretical analysis we examine this point, i.e., how the emissivity of the fundamental and harmonic at the time of peak flux density depends on the beam properties. The data of the 10 events are in good accord (rc ≈ 0.9) with the theoretical relation for fundamental emission, but in disaccord with the theoretical relation for harmonic radiation. For the 10 bursts we find poor correlation between the radio flux density and the electron flux N(E‖) at the energy E‖ estimated to be that of the two‐stream instability. However there is excellent correlation when N(E‖) is weighted by E‖ to a high power. From the best fit, we find rc ≈ 0.96 when N(E‖) is replaced by . Finally, we infer the efficiency of energy conversion from the kinetic energy of the electron beam to fundamental emission, and examine the attenuation of the peak emission within the source.
Field‐aligned current systems in the magnetospheric ground state
Authors: Watanabe Masakazu , Iijima Takesi , Nakagawa Masayuki , Potemra Thomas A. , Zanetti Laurence J. , Ohtani Shin‐ichi , Newell Patrick T.
Published: February 2004

Using magnetic field or particle data obtained by the Magsat, DMSP‐F6, and DMSP‐F7 satellites, we have investigated characteristics of the magnetospheric plasma environment and field‐aligned currents (FACs) during prolonged geomagnetic quiescence (Kp = 0) associated with extremely low values of the interplanetary magnetic field (|By| ≤ 1.5 nT, −0.5 nT ≤ Bz ≤ 1.5 nT). We refer to this as the “ground state” of the magnetosphere. The large‐scale FACs exhibit the traditional region 1/region 2 current pattern and the dayside region 0 (traditional cusp) current system. The median region 1 current intensity at 835‐km altitude is 135 nT (with half of the values contained in the range 126–156 nT) in the 0800–1600 magnetic local time (MLT) region excluding the cusp. These are much smaller than the region 1 peak intensities of 740 nT for the |AL| ≥ 100 nT case and 350 nT for the |AL| < 100 nT case as statistically determined by Iijima and Potemra [1978]. In addition, the region 1/region 2 FAC systems tend to degenerate into smaller‐scale FACs at local times separated from the dayside toward morning and evening hours. The median region 1 intensity is 70 nT (with one‐half the values between 52 and 96 nT) in the 2200–0200 MLT sector. In the prenoon and postnoon sectors, region O currents with intensities comparable to region 1 continue to be paired with region 1 currents. The region 0 and region 1 currents on the dayside extend in the azimuthal direction, are balanced in current intensity with a typical value of 140 nT in the 0915–1000 MLT sector, and are collocated with boundary layer particles. These observations have provided the first opportunity to study the basic flow patterns of field‐aligned currents during the magnetospheric ground state.
Multiple inverted‐V structures and hot plasma pressure gradient mechanism of plasma stratification
Authors: Antonova E. E. , Stepanova M. V. , Teltzov M. V. , Tverskoy B. A.
Published: February 2004

A mechanism of plasma stratification within the inner plasma sheet is discussed. It is suggested that plasma pressure is isoptropic and particle motion is unmagnetized for the spatial scales in which the structures are formed. The solution of the magnetosphere‐ionosphere interaction problem shows that upward field‐aligned current may split into separate structures. The number of structures formed is determined by a parameter which depends on upward field‐aligned current density, field‐aligned current band width, hot magnetospheric ion temperature, and height‐integrated Pedersen ionospheric conductivity. The Intercosmos‐Bulgaria‐1300 satellite base data has been used to identify events of multiple inverted‐V structures. It is shown that the hot stratification plasma mechanism offers a good description of the number of observed structures.
A statistical study of auroral electromagnetic ion cyclotron waves
Authors: Erlandson R. E. , Zanetti L. J.
Published: February 2004

This paper discusses results from a statistical study of narrowband auroral electromagnetic ion cyclotron (EMIC) waves recorded by the Freja magnetic field experiment from December 1, 1993, to November 30, 1994. The study involves data collected at all magnetic local times (MLT), invariant latitudes between 40° and 75°, and altitudes between 1200 and 1750 km. Wave events have been identified using a two‐step process. First, magnetic field spectra in the frequency range between 5 and 256 Hz were surveyed using an automated procedure which detects spectral peaks. Second, the results of this automated procedure were confirmed by visually inspecting wave spectrograms which contained these spectral peaks. A total of 316 auroral EMIC wave events were identified. The occurrence of these waves peak at auroral latitudes in the premidnight sector (1800–0100 MLT). The events have a strong seasonal dependence with peak occurrence in the winter. These properties are the same as those of auroral electron precipitation, which is suggested to be the free energy source of these waves. The spectral properties of the waves were also analyzed. The wave usually contained either one or two spectral peaks. In the cases where two spectral peaks were observed one peak was below the local O+ gyrofrequency (ƒO+) and one was above the He+ gyrofrequency (ƒHe+). The observations suggest that auroral EMIC waves can be generated below ƒO+. The wave spectra were consistent with wave generation at altitudes near or above Freja's altitude. Analysis of the spectra suggest that the auroral EMIC waves originated from a source altitude between 1500 and 5500 km.
Dayside auroral dynamics: South Pole ‐ AMPTE/CCE observations
Authors: Mende S. B. , Klumpar D. M. , Fuselier S. A. , Anderson B. J.
Published: February 2004

Coordinated observations were made between the South Pole Station 630‐nm all‐sky imager and the Active Magnetospheric Particle Tracer Explorer (AMPTE) CCE satellite when it was at 9 RE apogee in the midday sector in an effort to understand the variety of dynamical changes at the magnetopause that map to the dayside aurora. On June 18, 1988, three events were observed. The first, at 1150 MLT, was a local event showing a rotation of the magnetic field and the sudden appearance of low‐energy electrons indicating the satellite entering the magnetosheath or low‐altitude boundary layer. This event was preceded by a type of poleward expansion of the aurora which had been previously interpreted as a possible signature/footprint of a flux transfer event (FTE). Although the magnetic field signatures at the satellite are not characteristic of an FTE, the event could be a magnetic field change produced by an FTE, which after some delay propagated to the vicinity of the satellite. The foot of the field line containing the satellite was at some distance from the south pole meridian. The second event, starting at 1610 UT (1240 MLT), was a clear case of field erosion during which the satellite entered the magnetosheath characterized by a reduction in energetic electrons, an increase in soft electrons, and a decrease in the magnetic field strength accompanied by an equatorward motion of the aurora. The third event, in the afternoon at about 1900 UT (1530 MLT), showed similar changes in the electron fluxes and similar equatorward motion of the aurora. However, this event was associated with an increase in the magnetic field at the satellite. Such a field increase could be characteristic of a pressure pulse. However, there was a corresponding equatorward motion of the aurora, which suggests that the event was also a flux erosion event in which the fringing fields of the region 1 current system, which normally cause a field depression at the subsolar region, created ah increase in the field at the CCE satellite near the late afternoon side. The latitudinal (equatorward) movements of the aurora for all three events were strongly correlated with the global AE index which was dominated by stations in the dawn/dusk sectors.
AMPTE/CCE‐SCATHA simultaneous observations of substorm‐associated magnetic fluctuations
Authors: Ohtani S. , Takahashi K. , Higuchi T. , Lui A. T. Y. , Spence H. E. , Fennell J. F.
Published: February 2004

This study examines substorm‐associated magnetic field fluctuations observed by the AMPTE/CCE and SCATHA satellites in the near‐Earth tail. Three tail reconfiguration events are selected, one event on August 28, 1986, and two consecutive events on August 30, 1986. The fractal analysis was applied to magnetic field measurements of each satellite. The result indicates that (1) the amplitude of the fluctuation of the north‐south magnetic component is larger, though not overwhelmingly, than the amplitudes of the other two components and (2) the magnetic fluctuations do have a characteristic timescale, which is several times the proton gyroperiod. In the examined events the satellite separation was less than 10 times the proton gyroradius. Nevertheless, the comparison between the AMPTE/CCE and SCATHA observations indicates that (3) there was a noticeable time delay between the onsets of the magnetic fluctuations at the two satellite positions, which is too long to ascribe to the propagation of a fast magnetosonic wave, and (4) the coherence of the magnetic fluctuations was low in the August 28, 1986, event and the fluctuations had different characteristic timescales in the first event of August 30, 1986, whereas some similarities can be found for the second event of August 30, 1986. Result 1 indicates that perturbation electric currents associated with the magnetic fluctuations tend to flow parallel to the tail current sheet and are presumably related to the reduction of the tail current intensity. Results 2 and 3 suggest that the excitation of the magnetic fluctuations and therefore the trigger of the tail current disruption is a kinetic process in which ions play an important role. It is inferred from results 3 and 4 that the characteristic spatial scale of the associated instability is of the order of the proton gyroradius or even shorter, and therefore the tail current disruption is described as a system of chaotic filamentary electric currents. However, result 4 suggests that the nature of the tail current disruption can vary from event to event.
Pseudobreakup and substorm onset: Observations and MHD simulations compared
Authors: Pulkkinen T. I. , Baker D. N. , Wiltberger M. , Goodrich C. , Lopez R. E. , Lyon J. G.
Published: February 2004

The global conditions during a moderate geomagnetic disturbance event on May 15, 1996, are examined by comparing data from several ground‐based instruments and inner tail satellites with global MHD simulations of the same event. The ground‐based data show two substorm intensifications about 40 min apart, the first one being small and localized (a pseudobreakup) and the second leading to a major rearrangement of both the ionospheric auroral distribution and the magnetotail configuration. The simulation shows that during the pseudobreakup, open field lines were reconnecting in the midtail, but the flows were mainly tailward and very few effects were observable in the inner magnetosphere. The result that pseudobreakups can be associated with activity producing topological changes in the tail is an important new aspect that has not been discussed in earlier studies. Both the observations and the simulation show two distinct regions of activity: a thin current sheet in the inner tail magnetically connected with the auroral bulge and a reconnection region in the midtail associated with the most intense electrojet currents.
Simultaneous electric and magnetic field observations of Pc1–2, and Pc3 pulsations
Authors: Bering E. A. , Benbrook J. R. , Engebretson M. J. , Arnoldy R. L.
Published: February 2004

This paper presents results from a combined analysis of ULF fluctuations in the geoelectric and geomagnetic fields. Data were obtained at South Pole station during the 1985–1986 South Pole Balloon Campaign. Magnetic field perturbations were measured by the two‐axis induction or “search coil” magnetometer at South Pole station. Vector electric field data were obtained by a standard double‐probe experiment on the balloon payloads. The objective of this study is to investigate the relationship between the electric and the magnetic components of geomagnetic micropulsations in the Pc1–3 frequency bands (0.01 to 0.5 Hz). The entire 468‐hour database of the balloon campaign has been searched for examples of strong discrete magnetic micropulsation emissions in these bands. The two best days were found to be January 7 and 8, 1986. Two emissions were seen on both days, a Pc3 emission between 30 and 40 mHz that extended for about 3 hours on either side of magnetic noon and a Pc1–2 emission that started around magnetic noon and lasted for several hours. The amplitude histories of the emissions were found to be roughly comparable when account was taken of the effect of varying precipitation on the ionospheric conductivity. The two components of the magnetic field did not show significant coherence. Consequently, little coherence was expected or found between the electric and magnetic components. An ionospheric transmission model has been used to interpret the observed B/E ratio in terms of the horizontal wave number of the source. The wave number that was inferred for the Pc3 emissions implies a phase speed roughly equal to the phase speed of simultaneous traveling convection vortex events.
Spatial structure in the thermospheric horizontal wind above Poker Flat, Alaska, during solar minimum
Authors: Conde M. , Smith R. W.
Published: February 2004

A new all‐sky imaging, wavelength scanning Fabry‐Perot spectrometer was used to record high‐resolution (R ≃ 200,000) spectra of the λ630 nm thermospheric optical emission above Poker Flat, Alaska. These spectra were used to derive spatially resolved maps of the horizontal wind vector at approximately 250 km altitude. We describe the procedure used to infer vector winds from line‐of‐sight Doppler shifts, along with its limitations. We present the time evolution of the vector wind fields obtained from this method for 6 nights of observation. Five of the 6 nights contained periods when we inferred the existence of significant curvature, divergence or shear in the thermospheric wind across our instrument's ∼1000 km diameter field of view. The sixth night exhibited little spatial structure and is shown for comparison. We compare these results with a “generic” solar minimum winter time run of the National Center for Atmospheric Research's Thermosphere, Ionosphere, and Electrodynamics General Circulation Model. While agreement was good at the start and end of the night, considerable differences were found in the late evening and midnight sectors. Some possible origins for these discrepancies are proposed. In particular, we suggest that the F region horizontal wind may be deflected by upwelling vertical winds, which are in turn driven by E region heating in the auroral zone. We note that both the instrument used and our high‐latitude implementation of the analysis procedure are new experimental techniques. Thus the data presented here should be regarded as preliminary and, if possible, be validated by comparison with results from other techniques.
Comparison of kinetic and hydrodynamic descriptions of the proton polar wind in the transition to collisionless flow
Authors: Lie‐Svendsen Øystein , Olsen Espen Lyngdal
Published: February 2004

We have compared kinetic and fluid model descriptions of the proton polar wind outflow, from the collision‐dominated, subsonic regime at lower altitudes, through the transition to supersonic flow, and well into the supersonic flow regime. The kinetic model is based on the Fokker‐Planck collision operator, and the two fluid models employed are based on the 8‐moment expansion and the 16‐moment bi‐Maxwellian expansions, respectively. We find excellent agreement between the kinetic description and the fluid models for the proton density and flux, even in the transsonic and supersonic flow regimes. The models are also in qualitative agreement for the temperature and heat flux moments, although neither fluid model reproduces the negative (downward) kinetic heat fluxes found at high altitudes. The 16‐moment fluid model gives a temperature anisotropy similar to the anisotropy derived from the kinetic solution. The assumed forms for the velocity distribution, on which the fluid expansions are based, do not agree with the kinetic velocity distribution, except in the subsonic region where the departure from a Maxwellian distribution is small. Near the fluid critical point the kinetic model develops a double‐hump distribution, with an isotropic, low‐energy core and an anisotropic, high‐energy tail, and at higher altitudes the distribution function develops a “kidney bean” shape.
Equatorial density irregularity structures at intermediate scales and their temporal evolution
Authors: Kil Hyosub , Heelis R. A.
Published: February 2004

We examine high resolution measurements of ion density in the equatorial ionosphere from the AE‐E satellite during the years 1977–1981. Structure over spatial scales from 18 km to 200 m is characterized by the spectrum of irregularities at larger and smaller scales and at altitudes above 350 km and below 300 km. In the low‐altitude region, only small amplitude large‐scale (λ>5 km) density modulations are often observed, and thus the power spectrum of these density structures exhibits a steep spectral slope at kilometer scales. In the high‐altitude region, sinusoidal density fluctuations, characterized by enhanced power near 1‐km scale, are frequently observed during 2000–0200 LT. However, such fluctuations are confined to regions at the edges of larger bubble structures where the average background density is high. Small amplitude irregularity structures, observed at early local time hours, grow rapidly to high‐intensity structures in about 90 min. Fully developed structures, which are observed at late local time hours, decay very slowly producing only small differences in spectral characteristics even 4 hours later. The local time evolution of irregularity structure is investigated by using average statistics for low‐(1%<σ<5%) and high‐intensity (σ>10%) structures. At lower altitudes, little change in the spectral slope is seen as a function of local time, while at higher altitudes the growth and maintenance of structures near 1 km scales dramatically affects the spectral slope.
Structure and occurrence of polar ionization patches
Authors: Coley W. R. , Heelis R. A.
Published: February 2004

We use measurements from the retarding potential analyzer and ion drift meter on the DE 2 spacecraft to examine the density and velocity structure of 225 northern hemisphere polar ionization patches. An ionization patch is loosely defined as a large‐scale (≧ 100 km) region where the F region plasma density is significantly (≧ 100%) above the background level. We examine the occurrence frequency of ionization patches as a function of interplanetary magnetic field (IMF), season, and UT. While for all values of Bz patches tend to occur mostly in the northern hemisphere winter, when Bz <0, the greatest frequency of patch occurrence is in the 1000–2000 UT range, and when Bz >0, the UT distribution is much more uniform. We also examine the plasma velocity structure inside and conjoint with patches as a function of IMF in order to determine their stability with time. A distinct IMF variation in the velocity structure of patches was found with a much lower level of velocity structure during southward IMF conditions. As patches convect across the polar cap, intermediate‐scale (∼15 km) irregularities are found both on the edges of and inside a patch. The irregularity level is found to be only slightly higher on the trailing edge of a patch than the leading edge. Examination of the average density gradient on the leading and trailing edges of convecting patches shows that the leading edge tends to be steeper. This information is related to current theoretical mechanisms on the generation and evolution of patches.
MHD model merging with IMF By: Lobe cells, sunward polar cap convection, and overdraped lobes
Authors: Crooker N. U. , Lyon J. G. , Fedder J. A.
Published: February 2004

Sunward convection in the polar cap has long been given as evidence of convection cells that close within the polar cap. These are called “lobe cells” because they are confined to the open field lines that form the tail lobes. Whether lobe‐cell convection is physically possible, however, has been seriously questioned. In support of lobe cells, we demonstrate their existence in an MHD model and illustrate the geometry of their circulating magnetic field lines for merging with a purely azimuthal (+By) interplanetary magnetic field (IMF). We find that the ratio of flux circulating in lobe cells compared with merging cells increases with decreasing Alfven Mach number and that sunward flow in the polar cap occurs both in lobe cells and on newly opened field lines in normal or “merging” cells. Although at any given time the sunward moving fields are configured in an overdraped pattern similar to that associated with northward IMF, the concept of sunward motion driven by electric field mapping or, equivalently, tugging on these overturned field lines does not apply, at least for the azimuthal IMF case. The overdraped fields and the null points to which they converge are immersed in a diffusion region at the magnetopause and support field‐aligned potential drops so that in the course of time, the field lines lose their identity by changing partners. Thus the sunward flowing fields in the ionosphere are decoupled from their magnetopause counterparts and move in response to the ionospheric potential, which has been modified from the applied solar wind potential by diffusion effects. These MHD model results both support and elucidate concepts deduced from primitive superposition and phenomenological models.
Numerical simulation of VLF risers, fallers, and hooks observed in Antarctica
Authors: Smith A. J. , Nunn D.
Published: February 2004

The VLF database from Halley station, Antarctica, has been searched, and prominent examples of discrete emissions are presented. Risers, fallers, quasiconstant tones, and upward and downward hooks are all common. Also observed are risers and fallers triggered from the tops of hiss bands. A one‐dimensional Vlasov VHS code has been used to simulate the main types of events observed. Using realistic plasma parameters, the code successfully reproduces the main kinds of events observed, in good qualitative agreement with observations. Distinct generation region structures are found to be associated with risers and fallers. Hook formation is interpreted as being a spontaneous transition between these two quasi‐stable generation region structures.
On the improvement of analytical calculations of collisional auroral ion velocity distributions using recent Monte Carlo results
Authors: Gaimard P. , St.‐Maurice J.‐P. , Lathuillere C. , Hubert D.
Published: February 2004

The combined effect of ion‐neutral collisions and strong dc electric fields is known to drive marked departures from a simple Maxwellian shape in the supersonic ions of the auroral ionosphere. With the current strong interest in investigating highly disturbed periods, an accurate knowledge of the ion distribution function is gaining in importance. It is known that the shape of the ion velocity distribution below 500 km can have a pronounced toroidal character in the case of O+ ions while looking more like an ellipsoid of revolution in the NO+ case. Calculations of these effects have been made using analytical expansion techniques as well as numerical Monte Carlo simulations. While the two approaches yield results that are in qualitative agreement with each other, we show that there can at times be some important quantitative differences between the two techniques. This should not be surprising in view of the fact that analytic expansions are based on collision models that are independent of the relative energy between colliding particles, while the more recent Monte Carlo calculations do not suffer from this limitation. This drawback notwithstanding, the advantage of analytical techniques lies with the speed of the computations; these techniques are also free of the statistical noise that can clutter the Monte Carlo results. The latter can, however, be used to improve the accuracy of the analytical calculations and therefore their usefulness. This is done here by obtaining a new model of collision cross section integrals which is extracted from the more recent Monte Carlo results. For strong dc electric field conditions, the new model can, in all cases, greatly enhance the accuracy of the analytical temperature calculations. The new model also produces overall shapes that are in better agreement with the numerical simulations in the NO+ case. However, even with the new generalized cross section model, the analytical description of O+ ions remains problematic above 100 mV/m in an atmosphere with a strong atomic oxygen content.
Structure of the dissipation region during collisionless magnetic reconnection
Authors: Shay M. A. , Drake J. F. , Denton R. E. , Biskamp D.
Published: February 2004

Collisionless magnetic reconnection is studied using a 2 1/2‐dimensional hybrid code including Hall dynamics and electron inertia. The simulations reveal that the dissipation region develops a two‐scale structure: an inner electron region and an outer ion region. Close to the X line is a region with a scale of c/ωpe, the electron collisionless skin depth, where the electron flows completely dominate those of the ions and the frozen‐in magnetic flux constraint is broken. Outside of this region and encompassing the rest of the dissipation region, which scales like c/ωpi, the ion inertial length, is the Hall region where the electrons are frozen‐in to the magnetic field but the ions are not, allowing the two species to flow at different velocities. The decoupling of electron and ion motion in the dissipation region has important implications for the rate of magnetic reconnection in collisionless plasma: the ions are not constrained to flow through the very narrow region where the frozen‐in constraint is broken so that ion flux into the dissipation region is large. For the simulations which have been completed to date, the resulting rate of reconnection is a substantial fraction of the Alfvén velocity and is controlled by the ions, not the electrons. The dynamics of the ions is found to be inherently nonfluid‐like, with multiple ion beams present both at the X line and at the downstream boundary between the inflow and outflow plasma. The reconnection rate is only slightly affected by the temperature of the inflowing ions and in particular the structure of the dissipation region is controlled by the ion inertial length c/ωpi and not the ion Larmor radius based on the incoming ion temperature.
Characterizing the state of the magnetosphere: Testing the ion precipitation maxima latitude (b2i) and the ion isotropy boundary
Authors: Newell Patrick T. , Sergeev V. A. , Bikkuzina G. R. , Wing S.
Published: February 2004

Recently, efforts to characterize and monitor the state of the magnetosphere have intensified, along with the rising interest in space weather. The latitude of the ion energy flux precipitation maxima (“b2i”), which almost invariably occurs near the equatorward edge of the nightside main auroral oval, has been suggested as one such parameterization. It has been suggested that b2i corresponds to the ion isotropy boundary (IB), which has been independently researched as a measure of the extent to which the magnetotail is stretched. By comparing simultaneous observations by the Defense Meteorological Satellite Program (DMSP) and NOAA spacecraft, we confirm a close association between b2i and the isotropy boundary of 30 keV protons. Using 2.5 years of simultaneous data from DMSP and GOES spacecraft, we verified that magnetic field inclination (the extent to which the magnetotail is stretched) strongly controls the b2i/IB latitude. Based on use of the b2i latitude, corrected for local time variation, as an index of magnetic stretching in the tail to show a considerable dawn‐dusk asymmetry, we find that the magnetic field is more depressed and stretched at dusk than at dawn, and asymmetry increases with increasing magnetotail stretching. This asymmetry is consistent with the rotation of the symmetry line of the b2i(MLT) curve toward premidnight hours and suggests the growth of a so‐called “partial ring current” system with increasing activity. Finally, the utility of the b2i/IB boundary as a characterization of the state of the magnetosphere is shown by demonstrating that the average pressure in the magnetotail is better specified by b2i than by Kp.
On the distribution function of H Atoms in the problem of the solar wind interaction with the local interstellar medium
Authors: Baranov Vladimir B. , Izmodenov Vlad V. , Malama Yury G.
Published: February 2004

This paper is devoted to the analysis of the models of the solar wind interaction with the local interstellar medium (LISM) self‐consistently taking into account mutual influence of plasma (electrons and protons) and neutral (H atoms) components of the LISM. The axisymmetric interaction model of Baranov and Malama [1993], that used the kinetic description of H atoms motion together with the hydrodynamic approximation for the plasma component, showed that the distribution functions of the all H atom populations are essentially not Maxwellian. The hydrodynamic model by Zank et al. [1996] is based on the Maxwellian distribution functions of the LISM and solar wind hydrogen atoms. The comparisons of the results obtained on the basis of the kinetic model by Baranov and Malama [1993] and multifluid model by Zank et al. [1996] show that the distributions of H atom parameters have large quantitative as well as qualitative distinctions. A number of theoretical predictions and relevant, experimental data are analyzed.
Double discontinuity: A compound structure of slow shock and rotational discontinuity
Authors: Whang Y. C. , Zhou J. , Lepping R. P. , Szabo A. , Fairfield D. , Kokubun S. , Ogilvie K. W. , Fitzenreiter R.
Published: February 2004

This paper reports the observations of a compound structure in space plasma composed of a slow shock layer and an adjoining rotational discontinuity layer on the postshock side. The compound structure looks like a new kind of MHD discontinuity; it may be called a double discontinuity. Two dual‐spacecraft observations of double discontinuities using high‐resolution magnetic field data were made in interplanetary space in 1995. The first one observed from Wind and IMP 8 is composed of a reversed slow shock and a rotational discontinuity. The second one observed from Wind and Geotail is composed of a forward slow shock and a rotational discontinuity. A double discontinuity is a stable, large‐scale solar wind structure. The thickness of the slow shock layer is of a few ion inertial lengths. The flow in the preshock region has a low βi value and a large shock angle.
Evolution of mirror structures in the magnetosheath of Saturn from the bow shock to the magnetopause
Authors: Cattaneo M. B. Bavassano , Basile C. , Moreno G. , Richardson J. D.
Published: February 2004

Mirror modes have been systematically observed by Voyagers 1 and 2 in wide portions of Jupiter's and Saturn's magnetosheaths. In particular, in one crossing of Saturn's subsolar magnetosheath, mirror waves are present almost continuously from the bow shock to the magnetopause. Therefore in this crossing, taking advantage also of relatively steady interplanetary conditions, we can track the evolution of mirror structures from a quasi‐perpendicular bow shock to a low‐shear magnetopause. We find that these structures evolve from quasi‐sinusoidal waves to nonperiodic structures, consisting of both magnetic field enhancements and wells, and, finally, to dips in the plasma depletion layer (PDL) close to the magnetopause. Both the amplitude and wavelength of the fluctuations tend to increase with increasing distance from the bow shock, except in the PDL, where they decrease toward the magnetopause. The waves are always compressional, and the direction of maximum variance forms an angle of ∼30° with B in the outer magnetosheath and a smaller angle in the inner magnetosheath. A comparison with the predictions of a nonlinear theory of the mirror instability shows some discrepancies, indicating that further theoretical studies are necessary.
The effect of magnetosheath plasma flow on flux transfer events produced by the onset of merging at a single X line
Authors: Ku Hwar C. , Sibeck David G.
Published: February 2004

We present predictions on the signatures of flux transfer events (FTEs) at the dayside magnetopause by the single X line MHD model based on the subsolar environment: initially in the magnetosheath with background flow northward parallel to the dayside magnetopause. In conformity with our previous investigation [Ku and Sibeck, 1997] we choose realistic parameteres for the simulation with the ratio of ρmsh/ρsph =10, Bmsh/Bsph = 0.5, and Tmsh/Tsph = 0.175 along the magnetosheath and magnetospheric sides. The localized resistivity turns on near the low northern latitude of dayside magnetopause, and the initial magnitude of background magnetosheath plasma flow is set
, or equivalent to 30% of magnetosheath Alfvén velocity. Simulation results produce signatures of FTEs similar to the previous case without the background magnetosheath flow [Ku and Sibeck, 1997] but different in some respects. Events moving opposite to this flow slow down but intensify, whereas events moving in the direction of the magnetosheath flow accelerate but weaken. Similarly, the satellite observes no significant signatures of FTEs in the magnetosphere. All the FTEs exhibit asymmetric bipolar magnetic field signatures normal to the magnetopause. Scatterplots versus the plasma density reveal that steep changes for the temperature kT, the magnetic field Bz, and the Alfvén velocity occur in the magnetospheric side immediately adjacent to the magnetopause.
Jupiter's ionosphere: New results from Voyager 2 radio occultation measurements
Authors: Hinson David P. , Twicken Joseph D. , Karayel E. Tuna
Published: February 2004

High‐quality radio occultation data were acquired as the Voyager 2 spacecraft flew behind Jupiter on July 10, 1979. We have conducted a thorough analysis of dual‐frequency data from the ionosphere using a new method of data processing based on scalar diffraction theory. The method is capable of deciphering the effects of multipath propagation, which had not previously been possible for these data. This allows retrieved vertical profiles of electron number density to be extended downward throughout the lower ionosphere. The electron density profile at occultation entry (66°S, 258°W) has a topside scale height of 1020 km, consistent with previous results. New results include a peak density of 3.5 × 105 cm−3 at a height of 640 km above the 1‐bar pressure level. In addition, the lower side of the main peak contains quasi‐sinusoidal density oscillations with a vertical spacing of 25–40 km at altitudes of 450–600 km. These appear to signal the presence of upward propagating gravity waves. The electron density profile at occultation exit (51°S, 148°W) exhibits a peak density of 2.3 × 105 cm−3 at an altitude of 1900 km, and the topside has a uniform scale height of 850 km. These characteristics of the upper ionosphere, where the principal ion is believed to be H+, are consistent with earlier results. New results include two secondary peaks at lower altitudes. An intermediate layer, where the principal ion may be H3+, is centered near 1000 km with a peak density of 4.6 × 104 cm−3. A lower layer, possibly composed of metallic ions, is situated between 200 and 550 km with densities of 2–12 × 104 cm−3. The latter contains multiple, thin layers of ionization at altitudes of 300–450 km, which may be the result of vertical shear in the zonal wind or plasma instabilities. The height‐integrated Pedersen conductivity is about 0.5 and 0.4 mho at entry and exit, respectively.
Correlative study of solar activity and cosmic ray intensity
Authors: Usoskin I. G. , Kananen H. , Mursula K. , Tanskanen P. , Kovaltsov G. A.
Published: February 2004

We perform a correlative study of solar activity (sunspot numbers) and cosmic ray intensity (neutron monitor count rates) for the last four solar cycles. Analysis of the running cross correlation between the two series shows that the behavior of cosmic ray modulation is similar, in general, for particles with different energy. However, a strong rigidity dependence as well as an unusual behavior of the cross correlation function is found for the descending phase of cycle 20. We study the evolution of cosmic ray and solar activity cycles in a three‐dimensional phase space by means of the delayed component method. While all solar activity cycles and most cosmic ray cycles are planar, cosmic ray cycle 20 is significantly three‐dimensional. A concept of the momentary phase of a cycle is introduced, and the phase evolution of cosmic ray and solar activity cycles is studied. We also discuss the heliospheric conditions responsible for the unusual behavior of cosmic ray modulation in the descending phase of cycle 20.