bluuebunny/crossref_metadata_2025 huggingface dataset:
Longitude variations in ion composition in the morning and evening topside equatorial ionosphere near solar minimum
Authors: West K. H. , Heelis R. A.
Published: February 2004

Ion composition data from the Defense Meteorological Satellite Program (DMSP) F10 have been averaged by geographic longitude and dip latitude for the months of June, September, and December 1993. The data were taken under near solar minimum conditions. Near 800 km at two fixed local times near 0920 hours and 2120 hours, and at all longitudes, significant variation in local time and season are found. Longitude variations are consistent with modulation of the F peak height by meridional and zonal neutral winds. The components of these winds parallel to the magnetic field lines act to raise and lower the height of the F peak and, additionally, at night, to modulate the plasma decay rate. Zonal winds were found to have significant effects in the longitude regions 150°E to 270°E and 300°E to 360°E, where the magnetic declination is significant. Under solstice conditions, the summer to winter meridional winds play a dominant role in regulating the F peak height, with the zonal winds enhancing or opposing the effects of the meridional winds at longitudes with significant magnetic declination. Zonal winds dominate the regulation of the F peak height near equinox, when the meridional winds are fairly symmetric about the dip equator. The longitude variations are most clearly seen in the O+ and H+ concentrations when O+ is the dominant ion and is in equilibrium with H+. These conditions were found during the daytime during all seasons. H+ is frequently the dominant ion near 800 km, and at night, the longitudinal variations clearly seen in the O+ concentrations were not as easily seen in the H+ concentrations due to the larger scale height of H+.
Boundary oscillations at Geotail: Windsock, breathing, and wrenching
Authors: Shodhan S. , Siscoe G. L. , Frank L. A. , Ackerson K. L. , Paterson W. R.
Published: February 2004

On March 18–19, 1993, when Geotail was beyond 150 RE down the tail, for about 30 hours it frequently transited between magnetosheath‐like plasma and lobe‐like plasma. The transitions take the form of smooth variation by about a factor of 10 in both density and speed. Some of the transitions are distinctly asymmetric with a fast rise to sheath‐like values and a slow decline to lobe‐like values. We have suggested that these transitional regions are the plasma mantle, but this raises the question of why the mantle crossed Geotail many times in 30 hours. Three possible causes are flapping of the tail in response to solar wind flow changes (the windsock mechanism), intrinsic expansions and contractions of the tail boundary, perhaps in response to substorm phases (the breathing mechanism), and an IMF‐squeezed elliptical tail cross section changing its orientation to follow the IMF (the wrenching mechanism). We test these possibilities here by examining simultaneous solar wind velocity and magnetic field data from IMP 8. For the cases studied, both the windsock and the breathing mechanisms appear to contribute to the motions that cause the transitions, whereas the wrenching mechanism seems less effective. Breathing dominates on a timescale of tens of minutes, and windsock dominates on a scale of hours. Since the windsock mechanism is unavoidable, the important finding here is that the breathing mechanism appears also to operate. We use the windsock variation to estimate the thickness of the mantle at Geotail's position and find that the density drops by a factor of 10 in 9 RE. We model the mantle as a one‐dimensional slow‐mode expansion fan and use the model to predict the change in plasma parameters that occurs from the outer edge of the mantle to its inner edge. A comparison of the predicted profile with the observed profile shows that the model simulates the observed changes reasonably well.
Time dependence of substorm recurrence: An information‐theoretic analysis
Authors: Prichard Dean , Borovsky Joseph E. , Lemons Patricia M. , Price Channon P.
Published: February 2004

Using information‐theoretic statistics, two series of substorm recurrence times are examined for evidence of dynamical structure. One series is from 1 year of energetic‐electron data collected by geosynchronous satellites and the other series is from 8 years of theALindex. It is found that there is evidence for some predictive structure in these series; that is, the length of the time interval between two subsequent substorms depends on the previous intersubstorm interval length and that a large amount of the predictability can be attributed to roughly periodic substorms trains.
Lower thermosphere mean temperatures, densities, and winds measured by EISCAT: Seasonal and solar cycle effects
Authors: Kirkwood S.
Published: February 2004

The European Incoherent Scatter (EISCAT) UHF radar in northern Scandinavia has now been making regular measurements of lower thermosphère temperatures, densities, and meridonal winds for 10 years, covering almost a complete solar cycle. Daily averages from a total of 110 days of operations have been used to study the seasonal, solar flux and magnetic activity dependence of these parameters. In the case of densities, lower values occur in summer than in winter, but no significant solar flux or magnetic activity dependence can be resolved. The daily mean meridional winds are only a few meters per second and show large day‐to‐day variability. Between 100‐ and 120‐km altitudes, there is a suggestion of a reversal from equatorward to poleward prevailing wind from winter to summer, particularly at solar minimum. As regards temperature, the seasonal variations are as expected, with lower values in summer than in winter below 98‐km altitude but with a summer maximum at higher altitudes. A clear dependence on magnetic activity is seen above 110‐km altitude in all seasons and amounts to about 1 K/Ap unit between 110‐ and 120‐km altitudes. In winter, magnetic activity effects seem to penetrate to lower altitudes. No clear dependence on solar flux is seen below 100‐km altitude. At higher altitudes, a positive dependence on solar flux is seen during summer, increasing with altitude and reaching 0.25 K/10.7 cm flux unit. In winter, however, at altitudes 100–115 km, temperatures are significantly lower (by about 10 K) at solar maximum than at solar minimum.
Modeling study of equatorial ionospheric height and spread F occurrence
Authors: Maruyama Takashi
Published: February 2004

In the ionospheric F region at equatorial latitudes, the strength of the zonal electric field in the evening hours is closely connected with the generation of equatorial spread F and plasma bubbles. Many researchers discuss the electric fields and dynamics of the ionosphere in terms of the time derivative of F layer virtual heights (dh′F/dt) scaled on the ionograms, and this paper examines the accuracy of zonal electric fields derived by such a method. Although the effect of transequatorial thermospheric wind had been thought to be negligible, model calculations of ion concentration show that this wind significantly changes ionospheric height in the evening hours. Further, the electric field strength is estimated based on observed dh′F/dt, considering the apparent vertical drift of the ionosphere due to the thermospheric wind effect. Rayleigh‐Taylor growth rates calculated for those electric fields agree quantitatively with the spread F occurrence.
The wave distribution function in a hot magnetospheric plasma: The direct problem
Authors: Santolík O. , Parrot M.
Published: February 2004

Studying electromagnetic waves in a magnetospheric plasma, it is often important to find their directions of propagation. When the assumption of a plane wave is not valid, the determination of the wave distribution function (WDF) is required. The WDF specifies the distribution of wave‐energy density with respect to the frequency and the wave‐normal direction. An alternative approach to the estimation of the experimentally accessible data for a given WDF is suggested with the hot‐plasma and/or Doppler effects taken into account. The method is based on an explicit frequency dependence of the WDF. Examples of solutions, including the determination of the wave‐refractive index, the wave‐growth rate, the group velocity, and theoretical predictions of experimental data at predefined frequencies, are given in different cases. First, whistler‐mode waves in a hot plasma near the geostationary orbit were studied. Second, the low‐frequency waves observed by the low‐altitude satellite Freja in the auroral zone were subjected to theoretical analysis. A complex, multimodal structure of the wave characteristics was found, mainly around the resonance angle. In the case of extremely short‐wavelength waves, the influence of the Doppler effect produced qualitative changes of the wave‐normal dependence of wave parameters.
Morphology of nightside precipitation
Authors: Newell Patrick T. , Feldstein Yasha I. , Galperin Yuri I. , Meng Ching‐I.
Published: February 2004

Considerable information on the state of the magnetosphere is embedded in the structure of nightside charged particle precipitation. To reduce ambiguity and maximize the geophysically significant information extracted, a detailed scheme for quantitatively classifying nightside precipitation is introduced. The proposed system, which includes operational definitions and which has been automated, consists of boundary 1, the “zero‐energy” convection boundary (often the plasmapause); boundary 2e, the point where the large‐scale gradient dEe/dλ switches from positive to ≤0 (the start of the main plasma sheet); boundary 2i, the ion high‐energy precipitation cutoff (the ion isotropy boundary or the start of the tail current sheet); boundaries 3a,b, the most equatorward and poleward electron acceleration events (spectra with “monoenergetic peaks”) above 0.25 erg/cm2 s; boundary 4s, the transition of electron precipitation from unstructured on a ≥10‐km spatial scale (spectra have 0.6–0.95 correlation coefficients with neighbors) to structured (correlation coefficient usually 0.4 and below); boundary 5, the poleward edge of the main auroral oval, marked by a spatially sharp drop in energy fluxes by a factor of at least 4 to levels below those typical of the auroral oval; and boundary 6, the poleward edge of the subvisual drizzle often observed poleward of the auroral oval.
Electric field structure and ion precipitation in the polar region associated with northward interplanetary magnetic field
Authors: Matsuoka Ayako , Tsuruda Koichiro , Hayakawa Hajime , Mukai Toshifumi , Nishida Atsuhiro
Published: February 2004

We investigated electric field and charged particle data of the EXOS D (Akebono) satellite and studied magnetospheric plasma flow and its relation with particle precipitation in the dayside polar region during northward interplanetary magnetic field. Sunward flow characterizes the convection in this region and suggests that reconnection takes place at the high‐latitude magnetopause. Sheath ion precipitation is often observed associated with the sunward flow. When it shows energy dispersion caused by the velocity filter effect, it has noteworthy characteristics: (1) the ion precipitation region has a long scale length (>200 km at the 120‐km altitude) along the flow, and (2) the upstream edge of this region is located at latitudes of >80°. We consider that ion precipitation with energy dispersion is a characteristic phenomenon when the Alfven Mach number MaA. is high at the reconnection site, and we propose a new interpretation for the transportation of sheath plasma into the magnetosphere during high MaA. In this model the injection of the sheath ion occurs on the field lines that have penetrated into the magnetosphere after the reconnection. When the sheath ions associated with the sunward flow are observed at latitudes of <80°, on the other hand, the length of the ion precipitation region along the flow is generally short (100–150 km). The signature of these ions indicates that they have entered through the entry layer.
Three‐dimensional magnetic reconnection without null points: 2. Application to twisted flux tubes
Authors: Démoulin P. , Priest E. R. , Lonie D. P.
Published: February 2004

Magnetic reconnection has traditionally been associated exclusively with the presence of magnetic null points or field lines tangential to a boundary. However, in many cases introducing a three‐dimensional perturbation in a two‐and‐half‐dimensional magnetic configuration implies the disappearance of separatrices. Faced with this structural instability of separatrices when going from two‐and‐half to three‐dimensional configurations, several approaches have been investigated to replace the topological ideas familiar in two‐dimensional, but no unanimity has yet emerged on the way reconnection should be defined. While it is true that the field line linkage is continous in three‐dimensional, we show here that extremely thin layers (called quasi‐separatrix layers (QSLs)) are present. In these layers the gradient of the mapping of field lines from one part of a boundary to another is very much larger than normal (by many orders of magnitude). Even for highly conductive media these extremely thin layers behave physically like separatrices. Thus reconnection without null points can occur in QSLs with a breakdown of ideal MHD and a change in connectivity of plasma elements. We have analyzed several twisted flux tube configurations, going progressively from two‐and‐half to three‐dimensional, showing that QSLs are structurally stable features (in contrast to separatrices). The relative thickness w of QSLs depends mainly on the maximum twist; typically, with two turns, w ≈ 10−6, while with four turns, w ≈ 10−12. In these twisted configurations the shape of the QSLs, at the intersection with the lower planar boundary, is typical of the two ribbons observed in two‐ribbon solar flares, confirming that the accompanying prominence eruption involves the reconnection of twisted magnetic structures. We conclude that reconnection occurs in three‐dimensional in thin layers or QSLs, which generalise the traditional separatrices (related only to magnetic null points or field lines tangential to the boundary).
Effect of coronal mass ejections on the structure of the heliospheric current sheet
Authors: Zhao Xuepu , Hoeksema J. Todd
Published: February 2004

The existence of a relatively stable large‐scale heliospheric current sheet (HCS) structure near sunspot maximum has recently been questioned [Hundhausen, 1992]. We consider this question here by determining the effect of coronal mass ejections (CMEs) on the spiral characteristics of the interplanetary magnetic field (IMF) and on the HCS. In general, CMEs do not have long‐term effects on the location of the HCS. The evidence shows that (1) the coronal streamer belt locally disrupted or blown out by CMEs reforms in a time interval shorter than the lifetime of the HCS structure; (2) the internal structure of IMF sector boundaries is temporarily changed during the passage of the interplanetary counterpart of CMEs; (3) even in the Carrington rotation just 1 month after the sunspot maximum of solar cycle 21 the IMF spiral characteristics are maintained, and the calculated sector pattern agrees very well with that observed at 1 AU; and (4) the fact that the calculated closed field regions correspond to the helmet streamers observed in the February 16, 1980, solar eclipse confirms the validity of the three‐dimensional model even at high activity, giving additional confidence in the predicted HCS location. The rapid reformation of disrupted helmet structures may explain the existence of a structured HCS during intervals when CMEs occur frequently and several coronal helmet streamers along the base of the HCS are disrupted or blown out. Ulysses observations at the next sunspot maximum may finally answer the question.
Mesospheric ozone concentration at an equatorial location from the 1.27‐μ m O2 airglow emission
Authors: Batista P. P. , Takahashi H. , Clemesha B. R. , Llewellyn E. J.
Published: February 2004

The vertical emission profile of the O2 (a1Δg) airglow at 1.27 μm has been measured during the evening twilight near the equator (Alcântara, Brazil; 2.5°S, 44.2°W) using a rocket‐borne infrared photometer. The profile is used to derive the vertical distribution of ozone between 60 and 86 km. The ozone profile shows a minimum concentration at 77 km and a secondary maximum of 1.34 × 108 cm−3 at 82 km. The height of the secondary maximum is lower than observed at midlatitudes, and the 77‐km minimum is sharper than that which has been observed at low latitudes by satellite measurements.
Whistler absorption and electron heating near the plasmapause
Authors: Thorne Richard M. , Horne Richard B.
Published: February 2004

Using the HOTRAY code, we demonstrate that lightning‐generated whistlers which enter the magnetosphere over a broad range of latitudes (Δλ ≈ 5°) just inside the plasmapause are strongly focused by the steep plasma density gradient into a narrow range of L shells near the equatorial region. The wave normal angle also remains closely aligned (±20°) with the magnetic field direction along the entire ray path. Under such conditions, Landau resonance is relatively unimportant, and the wave amplitude is controlled by cyclotron resonant interactions with energetic electrons. All waves with frequencies comparable to or larger than one third of the equatorial electron gyrofrequency can be strongly absorbed by resonant electrons, leading to electron heating perpendicular to the ambient magnetic field at energies above 100 eV. Consequently, in the presence of this strongly focused source of wave energy, the electron distribution should evolve toward a marginally stable anisotropic equilibrium distribution with T⊥>T‖. In order to simulate this perpendicular heating, we allow the anisotropy of the electron distribution to evolve so that damping is minimized at a frequency of 5 kHz, corresponding to the peak in the power spectrum of spherics above the ionosphere. When the plasmapause is located at Lp = 4.5, whistlers above 4 kHz experience more than 20 dB attenuation owing mainly to cyclotron resonance with 0.1 to 1 keV electrons near the equator. It is unlikely that these waves would be detectable on the ground. This attenuation will produce an upper cutoff in the whistler frequency considerably below one half the equatorial electron gyrofrequency for waves that are guided along the plasmapause. In contrast, lower‐frequency whistlers (ƒ ≈ 1–3 kHz) should be amplified by the anisotropic electron population; such waves are able to propagate to the conjugate ionosphere and thus be detected on the ground. This energy transfer between whistlers and cyclotron resonant electrons is relatively unimportant when Lp ≤ 3.0, but it should become significant for Lp ≥ 4.5.
Magnetospheric convection in the presence of interplanetary magnetic field By: A conceptual model and simulations
Authors: Khurana Krishan K. , Walker Raymond J. , Ogino Tatsuki
Published: February 2004

Existing observations from ISEE 3 and new observations from Galileo show that when the interplanetary magnetic field (IMF) has a By component, a By component also develops in the Earth's magnetotail, but only in those quadrants in which the addition of the newly opened magnetic flux tubes occurs. The presence of By on the closed field lines (which is in the same direction as the IMF By) is also seen. We suggest that for a positive IMF By, the magnetopause is open only in the north dawn and south dusk quadrants of the magnetotail. The continuity of magnetic field across the open boundary then requires that a By component be present in the magnetosphere in these quadrants but not in the other two quadrants. We present a model of the magnetospheric convection that postulates cross‐tail flows in the mantle/lobe system and the plasma sheet. We suggest that shear flows between the northern and the southern halves of the plasma sheet generate a B component on the closed magnetic field lines. The model is consistent with the two‐cell ionospheric convection models constructed from electric field observations by Heppner and Maynard [1987]. Results from global MHD simulations confirm the main features of the proposed model.
On the kinetic dispersion relation for shear Alfvén waves
Authors: Lysak Robert L. , Lotko William
Published: February 2004

Kinetic Alfvén waves have been invoked in association with auroral currents and particle acceleration since the pioneering work of Hasegawa [1976]. However, to date, no work has considered the dispersion relation including the full kinetic effects for both electrons and ions. Results from such a calculation are presented, with emphasis on the role of Landau damping in dissipating Alfvén waves which propagate from the warm plasma of the outer magnetosphere to the cold plasma present in the ionosphere. It is found that the Landau damping is not important when the perpendicular wavelength is larger than the ion acoustic gyroradius and the electron inertial length. In addition, ion gyroradius effects lead to a reduction in the Landau damping by raising the parallel phase velocity of the wave above the electron thermal speed in the short perpendicular wavelength regime. These results indicate that low‐frequency Alfvén waves with perpendicular wavelengths greater than the order of 10 km when mapped to the ionosphere will not be significantly affected by Landau damping. While these results, based on the local dispersion relation, are strictly valid only for short parallel wavelength Alfvén waves, they do give an indication of the importance of Landau damping for longer parallel wavelength waves such as field line resonances.
Low‐latitude dayside magnetopause and boundary layer for high magnetic shear: 1. Structure and motion
Authors: Phan Tai D. , Paschmann Götz
Published: February 2004

A large number (128) of Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) crossings of the low‐latitude (<30°) dayside (0800–1600 LT) high‐shear (>45°) magnetopause (MP) and the low‐latitude boundary layer (LLBL) have been analyzed in detail in order to study the structure and motion of the MP and LLBL regions. From a superposed epoch analysis of 59 of these events, we found the average high‐shear MP to be characterized by large changes in the magnetic field magnitude, together with small increases of the parallel ion and electron temperatures as one crosses the boundary from the magnetosheath. Accelerated plasma flows, indicative of magnetic reconnection, are often seen in the MP/LLBL region, and in conjunction with these flows, one often observes an enhancement of the parallel ion temperature. Large changes in the plasma thermal properties occur at the inner edge of the LLBL, where the bulk of the magnetospheric particles is depleted. Using measured plasma flow velocity normal to the MP, we have estimated the MP speed. Comparing the results of the analysis of the 128 high‐shear cases with results on 22 low‐shear crossings reported in an earlier paper, we conclude that the high‐shear MP moves on average twice as fast as the low‐shear one, while the duration of the crossings of the high‐shear MP/LLBL is half that of the low‐shear crossings, so that the average thickness of the MP/LLBL is nearly independent of the local magnetic shear. In the range covered, the average MP/LLBL width is also found to be independent of the distance from the subsolar point. The normal speed (thickness) of the MP/LLBL is correlated (anticorrelated) with the magnetosheath plasma β. Furthermore, the normal speed of the MP tends to be anticorrelated with the crossing duration of the LLBL, implying that the width of the region does not vary as much as crossing durations imply. The (geometric) average widths of the high‐shear and low‐shear MP/LLBL are 1380 km (31 ri,sheath 15 ri,sphere) and 1100 km (27 ri,sheath, 12 ri,sphere), respectively, where ri,sheath and ri,sphere are the average gyroradii of magnetosheath and magnetospheric protons. The average width of the entire region of current flow at the magnetopause is 630 km (14 ri,sheath, 7 ri,sphere). The width of the current flow region is also found to be anticorrelated with the magnetosheath β, while it is independent of ri,sheath and ri,sphere.
Effects of the strength of the geomagnetic main field strength on the dynamo action in the ionosphere
Authors: Takeda Masahiko
Published: February 2004

Effects of the strength of the geomagnetic main field on the electrodynamics of the ionosphere were studied. Electric conductivities of the ionosphere were estimated for the cases of three times, normal, and 1/10 strength of the main field. It was shown that when the field strength decreases, altitudes of Pedersen conductivity maxima shift upward, and height‐integrated Pedersen and Hall conductivities are enhanced. The electric potential difference and currents of the ionospheric dynamo were also simulated for the above three cases. Weakening of the geomagnetic main field decreases the potential difference because of the smaller induction field. However, the reduction is not as large as that of the main field because the dynamo region is lifted and effective neutral wind velocity becomes large. On the contrary, height‐integrated ionospheric currents are enhanced because the current intensity is essentially determined by the product of the electron density and the neutral wind velocity in the dynamo region. Lower gyrofrequencies of electrons and ions due to the weak main field shift the dynamo region upward to the altitudes where the electron density and the wind velocity are larger.
Interferometric determination of broadband ELF wave phase velocity within a region of transverse auroral ion acceleration
Authors: Bonnell J. , Kintner P. , Wahlund J.‐E. , Lynch K. , Arnoldy R.
Published: February 2004

Broadband electric field fluctuations with typical amplitudes of 10–20 mV/m peak‐to‐peak and frequencies from 0 Hz to 3 kHz (BB‐ELF) were observed coincident with a region of ≤200 eV transverse H+ acceleration (TAI) near the poleward edge of the pre‐midnight aurora. The coherence and phase velocity of the electric fields were measured using a interferometric antenna array over the frequency range of ≈ 100 Hz to 3 kHz. These electric field fluctuations were found to have the following characteristics: 1) incoherence perpendicular to the geomagnetic field, 2) coherence parallel to the the geomagnetic field, 3) parallel phase velocity (ω/k∥) of 30–35 km/s upwards, 4) 0 < |k∥/k⟂| < 0.22. We show that these properties are compatible with the emission being electrostatic H+ cyclotron (EHC) waves. We also discuss possible generation mechanisms for the waves, and their relationship to the TAI.
Computational results of the effect of localized ionospheric perturbations on subionospheric VLF propagation
Authors: Baba K. , Hayakawa M.
Published: February 2004

The purpose of this paper is to investigate the effect of localized perturbations in the lower ionosphere on subionospheric VLF propagation by means of the finite element methos. Owing to the complexity of the method, we have analyzed only a two‐dimensional case where a localized perturbation lies on the great circle path between the transmitter and receiver. The first‐order mode is assumed to be incident into the waveguide, and we have found significant oscillations in the changes of amplitude and phase in the vicinity of the perturbation (distance less than 600 km), which are due to mode conversion (i.e., conversion to higher‐order modes) and subsequent multiple‐mode propagation. In this paper, we present many computational results of the spatial distribution of scattered fields, frequency dependence, effects of enhancement factor, horizontal and vertical scale, and altitude of the perturbation, etc. Some important findings are summarized as follows: (1) The study of scattered fields has suggested that a single‐mode propagation model does not hold and that the geometrical structure of the perturbation, especially in the horizontal direction, plays an important role in the scattering. (2) The magnitude of the perturbation, that is, the enhancement factor, effects mainly the magnitude of the scattered field.
Earth's atmosphere in dynamic coupling envisaged through atmospheric tides and atmospheric gravity waves: A view on the past‐present‐future research
Authors: Kato Susumu
Published: February 2004

In 1982–1985 an international scientific endeavor was attempted as the Middle Atmosphere Program, contributing successfully to exploring the last unknown part of the atmosphere on the planet Earth, the middle atmosphere. Now it seems timely for us to consider the whole atmosphere, which extends from the ground to the upper atmosphere and further into space as an entity. Each part of the atmosphere couples not only horizontally (globally) but also vertically. While the horizontal coupling is mainly important for conventional meteorology, the vertical one is significant, say, for space physics and, probably, future meteorology and will be the present topic. Atmospheric waves play an important role for the dynamic coupling of the atmosphere, transporting energy, momentum, and material mainly from below and releasing on the way in their propagation upward. Already in the last century, the existence of the tidal dynamo for the geomagnetic Sq variation in the upper atmosphere was known as a manifestation of the dynamic coupling by atmospheric tides. Atmospheric internal gravity waves are now found to play an important role, among many others, in the general circulation of the middle atmosphere which is another manifestation of the dynamic coupling of the atmosphere. Reviewing remarkable progress in the last 40 years in the research of this subject, the author has suggested that frontiers of the future research would be to develop a GCM (General Circulation Model) to include the upper atmosphere and to establish an equatorial atmosphere observation system.
Two‐component auroral electrojet: Importance for substorm studies
Authors: Kamide Y. , Kokubun S.
Published: February 2004

This paper attempts to synthesize the diverse number of observations of electric fields and currents in the high‐latitude ionosphere during substorms. By demonstrating that there are often spatial shifts among regions of high ionospheric conductivity, large electric fields and intense currents in the auroral electrojet, it is shown that substorm time variations of the current patterns over the entire polar region consist of two basic components. The first is related to the two‐cell convection pattern and the second to the westward electrojet in the dark sector, which is in turn related to the three‐dimensional wedge current system. These two components result from the relative strength of electric fields and conductivities in the intensification of the auroral electrojet and are identified as the signatures for directly driven and the unloading components in solar wind‐magnetosphere interactions. We contend that disturbed intervals do not necessitate the presence of substorm expansion‐phase activity and that the vast number of earlier complex results concerning the auroral electrojet can be ascertained from the high degree of variability of the two components, depending on substorm events, substorm phases, and their own spatial/temporal scale sizes. It is demonstrated that several major issues that have remained controversial are now accounted for reasonably well in terms of this two‐component electrojet model. We also predict specific features of the substorm auroral electrojet that have not yet been observed.
Observations of magnetospheric substorms occurring with no apparent solar wind/IMF trigger
Authors: Henderson M. G. , Reeves G. D. , Belian R. D. , Murphree J. S.
Published: February 2004

An outstanding topic in magnetospheric physics is whether substorms are always externally triggered by disturbances in either the interplanetary magnetic field (IMF) or solar wind, or whether they can also occur solely as the result of an internal magnetospheric instability. Over the past decade, arguments have been made on both sides of this issue. Horwitz [1985] and McPherron et al. [1986] have shown examples of substorm onsets which they claimed were not externally triggered. However, as pointed out by Lyons [1995, 1996], there are several problems associated with these studies that make their results somewhat inconclusive. In particular, in the McPherron et al. study, fluctuations in the By component were not considered as possible triggers. Furthermore, Lyons suggests that the sharp decreases in the AL index during intervals of steady IMF/solar wind are not substorms at all but rather that they are just enhancements of the convection driven DP 2 current system that are often observed to occur during steady magnetospheric convection events. In the present study, we utilize a much more comprehensive data set (consisting of particle data from the Los Alamos energetic particle detectors at geosynchronous orbit, IMP 8 magnetometer and plasma data, Viking UV auroral imager data, midlatitude Pi 2 pulsation data, ground magnetometer data, and ISEE 1 magnetic field and energetic particle data) to show as unambiguously as possible that typical substorms can indeed occur in the absence of an identifiable trigger in the solar wind/IMF.
Occurrence patterns for transient magnetic field signatures at high latitudes
Authors: Sibeck D. G. , Korotova G. I.
Published: February 2004

Impulsive events marked by durations ranging from 5 to 15 min and peak‐to‐peak variations in the H component exceeding 40 nT are common in observations by high‐latitude day side ground magnetometers. We survey observations by a global network of high‐latitude ground magnetometers to distinguish between their proposed causes. Peak event amplitudes and occurrence rates occur prior to local noon in the range of geomagnetic latitudes from 70° to 76°. Although most events can be associated with abrupt changes in the interplanetary magnetic field (IMF) orientation, few can be associated with abrupt changes in the solar wind dynamic pressure. Many events can be observed during intervals of steadily northward IMF orientation, but only a few can be observed during intervals of steadily southward IMF orientation. We suggest an interpretation of the events in terms of a model in which changes in the IMF orientation modulate the fraction of the solar wind pressure applied to the magnetosphere.
A dispersive analysis of bispherical pickup ion distributions
Authors: Isenberg Philip A. , Lee Martin A.
Published: February 2004

We investigate the properties of a bispherical shell distribution of pickup ions when dispersion of the resonant waves is taken into account. We also extend the method of Johnstone et al. [1991] and Huddleston and Johnstone [1992] for determining the spectra of the quasi‐linear self‐generated waves to include dispersion. Our specific results assume that all waves propagate along the average magnetic field, and that the waves satisfy the cold electron‐proton dispersion relation. The major effect of including dispersion in the analysis is that the particles in a bispherical distribution retain more of their initial energy than in the nondispersive treatment, so the resulting wave spectra are less intense than predicted when dispersion is neglected. The dispersive analysis also encounters the well‐known gap in the cyclotron resonance with fast‐mode waves which is absent in the nondispersive picture. We present detailed results for pickup protons in the two extreme configurations of magnetic field perpendicular to, and parallel to, the solar wind flow for a range of values of VA/VSW. We find that when pickup protons are scattered to a bispherical distribution with VA/VSW = 0.1, typical of conditions in the solar wind, the discrepancy in the total energy density of the self‐generated waves between the dispersive and nondispersive results amounts to 18% in the perpendicular configuration. This discrepancy falls between 9% and 73% in the parallel configuration, depending on the extent of the particle transport through the resonance gap. These discrepancies are substantially larger for larger values of VA/VSW. These results indicate that dispersive effects can be important in the correct modeling of the quasi‐linear resonant wave‐particle interaction between pickup ions and self‐generated waves in the solar wind.
Resonant absorption of downward propagating electromagnetic hiss
Authors: Chust T. , Le Quéau D.
Published: February 2004

Electromagnetic hiss is regularly detected by electric and magnetic wave receiver on board low‐altitude satellites at high magnetic latitudes. We solve the full wave equation for electromagnetic magnetosonic waves (MSWs) initially launched earthward at high altitudes above the satellite. Owing to both the inhomogeneity of the medium (variation along the geomagnetic field) and the bi‐ionic composition of the plasma (H+, O+), the wave field undergoes a singularity at the altitude where its frequency matches the local gyrofrequency of H+. This phenomenon is accompanied by strong modifications of the wave properties (for example, a polarization reversal) and by a resonant absorption that contributes to the heating of the protons. It is shown that a WKB point of view still provides a qualitative description of the full wave.propagation. The resonant absorption can then be viewed as a three‐step process as follows: (1) downward propagation of MSWs followed by (2) a reflection and by (3) a mode conversion into electromagnetic ion cyclotron waves, which are appropriate to heat the protons. During this process wave polarization reverses near the crossover point, where the mode conversion mainly occurs. The amplitude of the component B⊥ is found to undergo a moderate increase at the point where the wave frequency matches the proton gyrofrequency. The analysis is extended to include kinetic effects resulting from the longitudinal motions of the electrons Te. This aims at explaining the strong enhancement of the parallel electric field recorded onboard AUREOL 3. If Te ≫ Ti, it is shown that this E‖. can be interpreted as being due to ion acoustic waves excited by upward propagating electromagnetic ion cyclotron waves. The estimate of the size of E‖ (and of B⊥) allows a precise diagnosis of the mode conversion of the incident electromagnetic hiss and therefore helps understanding how its energy is used to heat the protons.
Perpendicular transport in shock acceleration
Authors: Giacalone J. , Jokipii J. R.
Published: February 2004

We consider the foundations of charged‐particle acceleration at collisionless shocks in which the average magnetic field is perpendicular to the shock‐normal direction. Our shock model, which is a numerical integration of the trajectories of an ensemble of test particles in specified shock fields, includes an isotropic and homogeneous, Kolmogorov‐like, fluctuating magnetic field superimposed on a background field. The field is fully three‐dimensional so that transverse diffusion is possible. Moreover, the scattering is a natural consequence of the particle interaction with the fields and is not treated in an ad hoc manner. We consider two distinctly different regimes for the fluctuating component of the field: (1) one in which the field line mixing (or field line random walk), due to large‐scale fluctuations in the random component of the field, dominates the perpendicular diffusion coefficient and (2) one in which the the large‐scale variations in the field are removed and the perpendicular diffusion coefficient is smaller. In the first case, the transverse diffusion coefficient is larger than in the second case owing to the field line mixing which leads to field line loops which intersect the shock in several places and particles can become temporarily trapped as the field line convects through the shock. In the latter case, the loops are on a spatial scale that is shorter than the particle gyroradii, and the only means for particles to remain near the shock is by “resonant” transverse diffusion. Comparisons between previous numerical models and the diffusive theory will be discussed as well as the importance of these ideas to astrophysical applications such as particle injection to anomalous cosmic ray energies at the solar wind termination shock. The results from these simulations suggest that simple scattering off of convected magnetic fluctuations cannot readily account for the acceleration of nonaccelerated pickup ions.
Sudden‐commencement‐triggered pulsations at high latitudes and their sources in the magnetosphere
Authors: Shumilov Oleg , Kasatkina Elena , Raspopov Oleg , Hansen Truls , Frank‐Kamenetsky Alexandr
Published: February 2004

Sudden‐commencement‐associated geomagnetic pulsations have been studied using the magnetometer records from Mirny (Ф′=−77.2°) and Heiss Island (Ф′=74.1°) high‐latitude observatories, where Ф′ is the corrected geomagnetic latitude. These pulsations have time delays from the onsets ranging between 4 and 22 min. Dominant periods of the pulsations observed lie in the range 20‐50 s. The pulsations were accompanied by absorption events that are quite different from sudden commencement absorption and auroral absorption events observed earlier at high latitudes. The results of multifactoral analysis demonstrated that the solar wind speed is the most important parameter controlling the geomagnetic pulsation occurrence. It was assumed that a Kelvin‐Helmholtz instability arising at some magnetospheric boundaries seems to be a more probable cause of the pulsations observed. The work gives further evidence to the results of Rostoker et al. [1984], and Connors and Rostoker [1993], who considered auroral and magnetic disturbances in the light of KHI development at some magnetospheric interfaces.
The distance to the solar wind termination shock in 1993 and 1994 from observations of anomalous cosmic rays
Authors: Stone E. C. , Cummings A. C. , Webber W. R.
Published: February 2004

Using data from the Voyager and Pioneer spacecraft, we examine the energy spectra of anomalous cosmic rays (ACRs) during 1992–1994, a period of rapidly increasing intensities of these particles in the outer heliosphere. The 1992 period appears to be in nonequilibrium, marked by a rapid decrease in the tilt of the current sheet from ∼63° to ∼35° and a complex, evolving magnetic topology that affects interplanetary drifts and drift acceleration along the solar wind termination shock. In addition, large propagating interplanetary disturbances in 1991 may have weakened the shock temporarily, reducing the intensity of accelerated ions. The 1993–1994 period appears to be in quasi‐equilibrium, marked by a stable tilt of the current sheet and a steady, large‐scale magnetic topology that provides stable conditions for propagation and acceleration. The ACR intensities of higher‐rigidity particles rise slowly during this period, suggesting that the shock intensity has stabilized. At lower rigidities the ACR intensities are increasing rapidly, which we attribute to decreasing modulation. During 1994 days 157–209, we estimate that the shock was at 85 ± 5 AU. The shock strength s is estimated to be 2.63 ± 0.14, significantly weaker than a strong shock (s = 4) and consistent with a termination shock modified by the pressure of galactic cosmic rays. By comparing these 1994 observations with those made in 1987, we estimate that the latitudinal gradient of ACR He with energies 31–57 MeV nucleon−1 is ∼1.9%/deg along the shock from the equator to the pole. We estimate that the ratio of the efficiency of injection of He+ to H+ pickup ions into the acceleration process is 7.0 ± 1.6.
Ionospheric signatures of patchy‐intermittent reconnection at dayside magnetopause
Authors: Kan J. R. , Deehr C. S. , Lyu L. H. , Newell P. T.
Published: February 2004

Ionospheric signatures predicted by the patchy‐intermittent (P‐I) dayside reconnection model during southward interplanetary magnetic field are discussed. The P‐I reconnection can lead to spiky convection electric fields. The spiky convection electric fields in turn drives enhanced convection channels on closed field lines in the dayside ionosphere. The observed convection speed in the enhanced convection channel ranges from ∼0.5 to ∼2 km/s. The P‐I reconnection model also predicts that the dayside auroral forms should move noonward along enhanced convection channels. Poleward shift of dayside auroral forms occurs as the enhanced convection channels shift poleward due to reconfiguration of magnetic field following enhanced dayside reconnection. Multiple brightenings of dayside auroral forms should occur on a timescale of ∼2 to 4 min, equal to twice the Alfvén bounce period between the ionosphere and the equatorial plane. Patchy reconnection is intrinsically intermittent due to the rereconnection of elbow‐shaped open flux tubes. The resulting reclosed flux tubes should contain a mixture of magnetosheath‐magnetospheric plasmas as observed in the low‐latitude boundary layer.
Kinetic theory of geomagnetic pulsations: 3. Global analysis of drift Alfvén‐ballooning modes
Authors: Vetoulis Georgios , Chen Liu
Published: February 2004

The radial localization of the drift Alfvén ballooning modes (DABM), proposed by Chen and Hasegawa [1991] to explain Pc 4–5 pulsations excited by highly energetic ring current protons, is examined by using WKB approximations in the radial direction. The problem is reduced to two nested one‐dimensional ones, one along the equilibrium magnetic field lines and the other in the radial direction. By ignoring kinetic effects to lowest order it is found that a localization potential well can exist for sufficiently strong earthward pressure gradient (the outer edge of the ring current). However, mode energy tunnels through a finite barrier and gets absorbed at the field line resonance layer, thereby causing damping of the global mode. This damping process imposes a minimum azimuthal mode number for the mode to be localized. The bounce‐drift resonance of highly energetic protons is treated perturbatively. The results further support the theoretical picture that antisymmetric high‐azimuthal mode‐number drift Alfvén‐ballooning modes are a good candidate instability mechanism for internally excited geomagnetic pulsations.
Evidence for reverse draping of magnetosheath field around the magnetosphere in IMP 8 observations for northward interplanetary magnetic field
Authors: Kaymaz Z. , Luhmann J. G. , Fedder J. A. , Lyon J. G. , Spreiter J. R. , Stahara S. S.
Published: February 2004

Fedder and Lyon [1995] recently showed the results from a global three‐dimensional magnetohydrodynamic (MHD) model of the solar wind interaction with the Earth's magnetic field for northward interplanetary magnetic field (IMF) that exhibited a “tadpole”‐shaped closed magnetosphere. One consequence of this configuration is that the inner magnetosheath or boundary layer fields exhibit reverse draping at the flanks compared to what is expected from the classical gas dynamic picture. This reverse draping, which occurs as a result of nearcusp merging in the MHD model, appears to be present in the statistical field pattern observed on IMP 8 at x ≈ −25–31 RE. The implication is that boundary layer field configurations for northward IMF produce very different J × B forces on the local boundary layer plasma than would be inferred from classical draping and thus may be responsible for some of the observed IMF dependence of the low‐latitude boundary layer properties.
Noncoplanar magnetic field in the collisionless shock front
Authors: Gedalin M.
Published: February 2004

General expression for the noncoplanar component of the magnetic field in the front of the quasi‐perpendicular fast collisionless shock is derived from the two‐fluid hydrodynamics in the assumptions of (1) stationarity, (2) one‐dimensionality, and (3) quasi‐neutrality of the shock. The tensorial structure of ion and electron pressure is taken into account. Limits of validity of previously proposed phenomenological expressions are found.