bluuebunny/crossref_metadata_2025 huggingface dataset:
Deep seismic structure and tectonics of northern Alaska: Crustal‐scale duplexing with deformation extending into the upper mantle
Authors: Fuis Gary S. , Murphy Janice M. , Lutter William J. , Moore Thomas E. , Bird Kenneth J. , Christensen Nikolas I.
Published: February 2004

Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10‐ to 15‐km‐thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub‐Moho reflector.
Global retrieval of bidirectional reflectance and albedo over land from EOS MODIS and MISR data: Theory and algorithm
Authors: Wanner W. , Strahler A. H. , Hu B. , Lewis P. , Muller J.‐P. , Li X. , Schaaf C. L. Barker , Barnsley M. J.
Published: February 2004

This paper describes the theory and the algorithm to be used in producing a global bidirectional reflectance distribution function (BRDF) and albedo product from data to be acquired by the moderate resolution imaging spectroradiometer (MODIS) and the multiangle imaging spectroradiometer (MISR), both to be launched in 1998 on the AM‐1 satellite platform as part of NASA's Earth Observing System (EOS). The product will be derived using the kernel‐driven semiempirical Ambrals BRDF model, utilizing five variants of kernel functions characterizing isotropic, volume and surface scattering. The BRDF and the albedo of each pixel of the land surface will be modeled at a spatial resolution of 1 km and once every 16 days in seven spectral bands spanning the visible and the near infrared. The BRDF parameters retrieved and recorded in the MODIS BRDF/albedo product will be intrinsic surface properties decoupled from the prevailing atmospheric state and hence suited for a wide range of applications requiring characterization of the directional anisotropy of Earth surface reflectance. A set of quality control flags accompanies the product. An initial validation of the Ambrals model is demonstrated using a variety of field‐measured data sets for several different land cover types.
Three spacecraft observations of the geomagnetic tail during moderately disturbed conditions: Structure and evolution of the current sheet
Authors: Zhou X.‐Y. , Russell C. T. , Gosling J. T. , Mitchell D. G.
Published: February 2004

On April 22, 1979, from 0840 to 1018 UT, ISEE 1 and ISEE 2 stayed in the tail current sheet at 17 RE, crossing the current sheet center several times while the initially northward interplanetary magnetic field Bz turned southward under conditions of constant solar wind velocity and dynamic pressure. An overview of the period from 0840 to 1050 UT, discussing the solar wind conditions, the tail magnetic field response at ISEE 1, ISEE 2 and IMP 8, and the ground electrojet activities has been presented in an accompanying paper. Herein under the assumption that the magnetic field is antisymmetric about the current sheet center, the hyperbolic tangent current sheet model of Harris has been used to calculate the current sheet thickness from these two close spacecraft measurements of the tail magnetic field. The thickness varied from 2.5 to 0.5 RE during this period. The current sheet thinned just prior to a rapid poleward expansion of the auroral electrojet and just after the arrival of a southward turning of the IMF Bz and apparent tail field reconnection at ISEE 1 and 2. The plasma density and temperature distribution in the vertical direction appears to be close to adiabatic at this time and the total pressure is roughly constant through the lobe. In this case study, the polytropic index of the tail plasma is found to be 5/3 on the average. This implies that the plasma convection at 17 RE in tail was steadily adiabatic over the period of about 100 min.
Changes in the distant tail configuration during geomagnetic storms
Authors: Nakamura R. , Kokubun S. , Mukai T. , Yamamoto T.
Published: February 2004

Changes in the structure of the distant tail associated with geomagnetic storms are studied by using plasma and magnetic field data obtained from Geotail. Thirteen storm intervals between October 1993 and October 1994 are examined when the satellite was located in the distant tail between X = −83 RE and X = −210 RE. Geotail observed the magnetosheath during all storms including those when the satellite was located near the nominal tail axis. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail axis, we estimated the dimension and the flux of the tail for the magnetopause crossing events. Systematic changes of the distant tail structure are found in association with the development of the storms. Before the main phase onset of the storms, when the Dst shows positive excursion, the enhanced solar wind pressure reduces the average radius of the distant tail to ∼23 RE as compared with the quiet time value, ∼31 RE. During the storm main phase the dimension of the tail is comparable to the quiet time value in spite of the high solar wind pressure. This is attributed to the enhanced magnetic flux in the tail in association with the southward interplanetary magnetic field (IMF) Bz. An average energy of ∼5 × 1015 J is stored also in the distant tail during the storm main phase, which is a comparable value to that stored in the midtail during an intense substorm growth phase reported in the previous studies. During storm time, when the Bz component of the magnetosheath field was larger compared with the By component, the average tail cross section has a north‐south elongated elliptical shape. The average tail for quiet time, however, was elongated toward the dawn‐dusk direction. By component was larger than Bz component in our quiet time data set. These observations imply that the anisotropy in the magnetosheath magnetic field pressure for different IMF orientation changes the shape of the tail, which was also shown in global MHD models. Changes in the average magnetosheath parameters (density, speed, direction) observed during the storms are found to be consistent with solar wind fluctuations expected from corotating interaction regions.
Precipitation of magnetospheric electrons caused by relativistic effect‐enhanced chaotic motion in the whistler wave fields
Authors: Faith James , Kuo Spencer , Huang Joe , Schmidt G.
Published: February 2004

In the magnetosphere, energetic electrons in the radiation belts are trapped by the Earth's dipole magnetic field and undergo bounce motion about the geomagnetic equator. It is shown that the trajectories of some of the trapped particles can in the presence of a whistler wave become chaotic and wander into the loss cone. Comparing the surface of section plots obtained from the both relativistic equations of motion and from the nonrelativistic ones, the effect of the relativistic correction to the electron motion are shown. The threshold field for the commencement of chaos in the trajectories of electrons with energies of a few hundred keV is found to be lowered by the inclusion of relativistic effects by about an order of magnitude. Waves with these smaller magnetic field amplitudes (about 1% of the geomagnetic field) have been observed propagating between hemispheres. Since this chaotic scattering process does not have a directional preference, it offers a plausible explanation for the simultaneous observation of electron precipitation into the upper atmosphere at geomagnetically conjugate regions because of a single lightning flash [Burgess and Inan, 1990].
Spatial variations of 0.2 to 5 MeV protons in the 1–5 AU in‐ecliptic region from Ulysses, Voyager 1 and 2, and IMP 8 gradient studies
Authors: Boufaida Moncef , Armstrong Thomas P.
Published: February 2004

Protons with energies ranging from 0.3 to 2 MeV are nearly always present in the environment of Earth. Solar flare events and interplanetary shock waves can be identified as producing or enhancing these fluxes. However, interplanetary particles are observed even in the absence of solar flares. Explaining the presence of these proton fluxes in the interplanetary medium and accounting for their variations is a major problem in space physics. Observations of interplanetary proton fluxes have been made continuously at 1 AU with the Interplanetary Monitoring Platform (IMP) 8 from 1973 to the present and in the 1–5 AU range by Voyagers 1 and 2 (1977–1978) and Ulysses (1990–1991). Daily averaged proton fluxes of IMP 8, Voyagers 1 and 2, and Ulysses have been carefully interpolated to matching energy passbands so that fluxes in the same passbands at two radial distances could be compared. These daily averaged fluxes were then compared as ratios and were autocorrelated and cross‐correlated as functions of time delay. The radial gradient, the energy spectra, and the distribution of these proton fluxes were also examined. The results showed that protons in the 0.3 to 5 MeV energy range using the Voyager 1/IMP 8, Voyager 2/IMP 8, and Ulysses/IMP 8 paired observations in the 1 to 5 AU in‐ecliptic region tend to “decorrelate” with increasing radial separation and become uncorrelated by about 4 or 5 AU. Higher‐energy fluxes decorrelate less rapidly, and lower‐energy proton fluxes have more dominantly positive radial gradients than those of higher energy. The radial gradient of 0.3 to 0.5 MeV proton fluxes is dominantly positive for 1–2 AU, while that of 2 to 4 MeV proton fluxes is negative. In contrast, from 2 to 4 AU the radial gradient of 0.3 to 0.5 MeV proton fluxes is weakly negative, and that of 2 to 4 MeV protons is weakly positive. This finding suggests that 0.3 to 0.5 MeV protons are more subject to interplanetary acceleration than are 2 to 4 MeV protons in the 1–2 AU range. The results also showed that radial gradients are robust and persist, evert if all heliolongitude coherence is purposely removed by shuffling the time order within 120‐day time intervals.
Effects of interplanetary magnetic field z component and the solar wind dynamic pressure on the geosynchronous magnetic field
Authors: Wing Simon , Sibeck David G.
Published: February 2004

A study of the correlation of the geosynchronous magnetic field with interplanetary magnetic field (IMF) Bz and the solar wind dynamic pressure (Pd) is presented. Hourly averages of 5 years of GOES 6 and 6 years of GOES 7 observations are correlated with IMF Bz and Pd. As previously reported, increases in Pd enhance geosynchronous Bz on the dayside, most strongly around noon, but depress it on the nightside, most strongly around midnight. This has been interpreted in terms of increases in the cross‐tail and magnetopause currents. Our study shows that the dayside geosynchronous magnetic field decreases with IMF Bz, particularly during periods of southward IMF. During periods of northward IMF, this trend continues but at much slower rate. The results of a multiple regression analysis of GOES Bz as a function of IMF Bz and Pd during periods of northward IMF show that roughly 30% of the IMF Bz uniformly “penetrates” the geosynchronous Bz. In contrast, during periods of southward IMF, the effect of IMF Bz on geosynchronous Bz is nonuniform and much larger at all local times, especially near dawn and dusk, apparently because of enhanced cross‐tail current. During periods of southward IMF, geosynchronous Bx depends most strongly on IMF Bz near dawn, dusk, and midnight. The dawn and dusk correlations can be attributed to the enhanced region 1 Birkeland currents, and the midnight correlation can be attributed to enhanced cross‐tail current. Geosynchronous By has the best correlations with IMF Bz near 2000 LT and 0400 LT, which can be attributed to enhanced region l Birkeland and/or cross‐tail currents. Also, we show that these magnetic field perturbations can be interpreted in terms of fast rarefaction waves and merging at equatorial regions.
Statistical relationships between high‐latitude ionospheric F region/topside upflows and their drivers: DE 2 observations
Authors: Seo Y. , Horwitz J. L. , Caton R.
Published: February 2004

A statistical analysis is conducted on the relationships among high‐latitude topside (850 – 950 km altitude) ionospheric plasma parameters and precipitating soft (≤1 keV) electron characteristics based on DE 2 satellite measurements from seven auroral zone passes. The parameters examined statistically for these relationships are 1137 independent samples of the field‐aligned ion flow velocities, fluxes, Mach numbers, densities, ion and electron temperatures, and soft electron energy fluxes and average or characteristic energies. We find that both ion upward velocities and upward fluxes are well correlated with electron and ion temperatures. Least squares fits to the data averaged in restricted bins show the following correlation coefficients: Ion upward velocity with Te, correlation coefficient r= 0.97; with Ti, r= 0.94; for ion upflux with Te, r= 0.97; with Ti, r= 0.91. The somewhat higher correlations with Te than Ti of both upflow velocities and upfluxes suggest the important role of enhanced ambipolar electric fields associated with enhanced Te, as heated by both direct collisions with the precipitating electrons as well as downward magnetospheric heat fluxes. The largest (≥1010 ions cm−2 s−1) ion upfluxes are associated with “ultrasoft” electron precipitation having average energies of ≤80 eV. Significant anticorrelations of electron (r= −0.90) and ion (r= −0.89) temperatures with the average energies of the precipitating soft electrons suggest that for the same precipitation energy flux, the lowest‐energy precipitating electrons are most effective in heating the topside thermal electrons. Finally, analysis of ion field‐aligned flow Mach numbers shows that these Mach numbers were almost always less than 0.4 and are typically less than 0.2. Such Mach number measurements suggest that low‐speed approximations in fluid transport models are usually valid for ≤1000 km altitude, even at high latitudes.
Spectroscopic evidence for suprathermal electrons in enhanced auroras
Authors: Hallinan T. J. , Kimball J. , Stenbaek‐Nielsen H. C. , Deehr C. S.
Published: February 2004

Auroral arcs frequently exhibit bands of enhanced intensity along the bottom edge. These bands have sharp upper borders and are referred to as enhanced lower borders or auroral hems. They are thought to be related to thin layers of enhanced luminosity observed both in auroral arcs and in pulsating auroras. These anomalous distributions of intensity as a function of altitude are referred to collectively as enhanced aurora (EA) and are of interest because they suggest a more complex process for producing auroral ionization and luminosity than the collisional degradation of precipitating electrons. Spectra of eight auroral hems were obtained with an imaging spectrograph and showed consistently that within the hem, there is an enhanced ratio of N2 1st positive emissions (red) to N2+ 1st negative emissions (blue). The enhancement in the red/blue ratio ranged from 10% to 100%. There was a generally smaller and less consistent enhancement in the ratio of the O2+ 1st negative band at 5275 Å to the N2+ 1st negative. Observations of the far red spectra of several hems showed no enhancement in the ratio of N2 1st positive to N2+ Meinel emissions. There were no simultaneous measurements of the Meinel and the N2+ 1st negatives. It is argued that the enhanced spectral ratios are indicative of a population of suprathermal electrons with a sharp energy cutoff near the ionization potential of N2 and that the electrons are energized in situ through wave‐particle interactions or possibly by dc electric fields. It is also noted that some but not all of the hems are type b red lower borders.
Global ionosphere‐polar wind system during changing magnetic activity
Authors: Schunk R. W. , Sojka J. J.
Published: February 2004

A time‐dependent, three‐dimensional, multi‐ion model of the global ionosphere‐polar wind system was used to study the system's response to an idealized geomagnetic storm for different seasonal and solar cycle conditions. The model covered the altitude range from 90 to 9000 km for latitudes greater than 50° magnetic in the northern hemisphere. The geomagnetic storm contained a 1‐hour growth phase, a 1‐hour main phase, and a 4‐hour decay phase. Four storm simulations were conducted, corresponding to winter and summer solstices at both solar maximum and minimum. The simulations indicated the following: (1) O+ upflows typically occur in the cusp and auroral zone at all local times, and downflows occur in the polar cap. However, during increasing magnetic activity, O+ upflows can occur in the polar cap, (2) The O+ upflows are typically the strongest where both Te and Ti are elevated, which generally occurs in the cusp at the location of the dayside convection throat, (3) The upward H+ and O+ velocities increase with Te, which results in both seasonal and day‐night asymmetries in the ion velocities, (4) During “increasing” magnetic activity, O+ is the dominant ion at all altitudes throughout the polar region, (5) For solar minimum winter, there is an H+ “blowout” throughout the polar region shortly after the storm commences (negative storm effect), and then the H+ density slowly recovers. The O+ behavior is opposite to this. There is an increase in the O+ density above 1000 km during the storm's peak (positive storm effect), and then it decreases as the storm subsides, and (6) For solar maximum summer, the O+ and H+ temporal morphologies are in phase; but the ion density variations at high altitudes are opposite to those at low altitudes. During the storm's peak, the H+ and O+ densities increase at high altitudes (positive storm effect) and decrease at low altitudes (negative storm effect).
A statistical study of giant pulsation latitudinal polarization and amplitude variation
Authors: Chisham G. , Mann I. R. , Orr D.
Published: February 2004

Using data from 34 Pg events observed by the European incoherent scatter (EISCAT) magnetometer cross, we complete a statistical study of the horizontal polarization and amplitude variation of Pgs with latitude. The polarization statistics consistently display the same latitudinal variation, being exclusively anticlockwise equatorward of linear polarization and exclusively clockwise poleward of linear polarization. The polarization azimuth (or ellipse orientation) changes from an approximately northeast to southwest orientation equatorward of linear polarization to an approximately northwest to southeast orientation poleward of linear polarization. However, the oscillations are not polarized exactly east–west at linear polarization but some distance poleward of linear polarization. The amplitude statistics for the D component display the distinct latitudinal variation characteristic of a resonance, whilst the H component is less well defined and has a tendency to dip at the position of the D component maximum. We employ an MHD model to describe the evolution of Pg‐like waves and compare the numerical results to the observational statistics. We suggest that a mechanism such as drift bounce resonance drives a radially (poloidally) polarized wave of limited radial width on L shells where the instability is operative. This mechanism favors large azimuthal wave numbers and drives dominantly Alfvénic waves. Because of their incompressible nature, these waves also possess an azimuthal (toroidal) component with a radial amplitude variation proportional to the radial gradient of the poloidal component. Hence the toroidal amplitude variation will be double peaked with a node at the position of the poloidal peak. This is in good agreement with the ground‐based observations as the toroidal (poloidal) oscillations map to H (D) component oscillations on the ground due to the 90° rotation of Alfvén waves by the ionosphere. Once driven these waves evolve due to the presence of the radial plasma inhomogeneity, which causes the wave polarization to rotate from a poloidal to a toroidal configuration in time. We show how this can explain why the position of the east–west oriented polarization ellipse lies poleward of linear polarization. We conclude that Pgs are likely to be guided poloidal Alfvén waves which are being continually driven by a population of westward drifting energetic protons and also speculate about the evolution of poloidal Alfvén waves in the afternoon sector of the magnetosphere.
Shuttle solar backscatter UV observations of nitric oxide in the upper stratosphere, mesosphere, and thermosphere: Comparisons with the Halogen Occultation Experiment
Authors: Stevens Michael H. , Siskind David E. , Hilsenrath Ernest , Cebula Richard P. , Leitch James W. , Russell James M. , Gordley Larry L.
Published: February 2004

The shuttle solar backscatter ultraviolet (SSBUV) spectrometer measured nitric oxide (NO) γ(1,0) and γ(0,2) band solar fluorescence at 215 and 247 nm, respectively, over a total of four orbits in April 1993 and eight orbits in March 1994. From nadir observations the apparent NO column abundance down to 1 mbar was inferred as a function of latitude and longitude using modeled γ band emission rate factors. SSBUV 5 results show a factor of 2 increase in NO from 20°S–20°N to 40°–60°S and a large variation of NO with a strong dependence on geomagnetic latitude near 52°S. Nimbus 7 solar backscatter ultraviolet (SBUV) spectrometer NO γ band fluorescence observations made between 1979 and 1986 were compared with SSBUV observations, and results from the two experiments are consistent for the latitudes observed by SSBUV. Coincident with some of the SSBUV solar fluorescence observations were absorption observations in the 5.3 μm band of NO by the Halogen Occultation Experiment (HALOE). A one‐dimensional model was used to adjust the HALOE density profiles inferred from sunrise occultations to the local solar time of the SSBUV observations. The emission rate factor model was used to convert these density profiles to nadir intensities for direct comparison with SSBUV observations. Results show that HALOE inferred intensities are generally within 25% of SSBUV observations on the same day.
Equilibrium conditions for the tangential discontinuity magnetopause
Authors: De Keyser J. , Roth M.
Published: February 2004

Early satellite observations of the dayside magnetopause have suggested that the magnetic field typically rotates clockwise above the solar‐magnetospheric equatorial plane and counterclockwise below it, in agreement with the predictions of first‐order orbit theory for magnetopause crossings of the rotational discontinuity type. The present paper treats the tangential discontinuity (TD) case. The influence of magnetosheath magnetic field and plasma flow on the magnetopause equilibrium structure is analyzed by means of a Vlasov model. The nature of the current layer plays a major role; the analysis is carried out for ion‐dominated, electron‐dominated, and mixed layers. Necessary and sufficient conditions for the existence of an equilibrium magnetopause are derived. It is found that (1) the magnetopause is best modeled as a transition layer of mixed type; (2) for high magnetic shear the magnetic field at the dayside magnetopause preferentially rotates clockwise above the equatorial plane and counterclockwise below it, while it rotates counterclockwise above and clockwise below the equatorial plane at the tail flanks; this effect becomes more manifest as the magnetosheath flow is faster and as the difference in proton and electron transition lengths is more pronounced; (3) for low magnetic shear, TD equilibrium is expected to be lost more easily at the dawnside than at the duskside; and (4) the model provides a magnetopause thickness estimate; in particular, the low magnetic shear dawn magnetopause is predicted to be thinner than the dusk magnetopause.
Solitary waves and weak double layers in a two‐electron temperature auroral plasma
Authors: Berthomier M. , Pottelette R. , Malingre M.
Published: February 2004

We show that the main characteristics of ion acoustic solitary waves and weak double layers observed by the Swedish satellite Viking can be well reproduced assuming the presence of two electron components in the auroral plasma. The characteristics of the ion acoustic solitons excited in such a plasma are derived with the help of the Sagdeev potential. The results show that the interactions between the hot and the cold electron component in the presence of a finite ion temperature produce rarefactions of the localized density. Such nonlinear structures exist in a more extended range of plasma parameters than the one previously studied in the small amplitude limit case using the Korteweg‐de‐Vries equation. We find that the density of the cold population must be always smaller than the hot one, while the hot to cold temperature ratio must be greater than ∼ 10. The characteristics of these structures are quite different from those obtained in the small amplitude limit case and better reproduce the Viking observations in terms of their velocity, width, and amplitude scales.
Coupling of shear flow and pressure gradient instabilities
Authors: Voronkov I. , Rankin R. , Frycz P. , Tikhonchuk V. T. , Samson J. C.
Published: February 2004

The nonlinear dynamics of a shear flow and its subsequent evolution in the equatorial plane of the inner plasma sheet is studied. A linear analysis of the ideal MHD equations reveals a hybrid vortex instability which appears because of the coupling of Kelvin‐Helmholtz (KH) and Rayleigh‐Taylor instabilities. The hybrid vortex mode grows faster than a KH mode, extracts ambient potential energy, and leads to vortex cells that have a larger spatial extent than a simple KH vortex. In the nonlinear stage, vortices become surge‐like and may destroy the shear flow region. The relevance of this model to vortex generation and auroral arc intensifications at the inner edge of the plasma sheet is discussed.
On the feasibility of imaging coronal mass ejections at radio wavelengths
Authors: Bastian T. S. , Gary D. E.
Published: February 2004

Coronal mass ejections (CMEs) can have a profound impact on the interplanetary medium and the near‐Earth environment. We discuss the feasibility of detecting coronal mass ejections at radio wavelengths with a ground‐based instrument. In particular, we explore the possibility that a radio telescope employing Fourier synthesis imaging techniques can detect thermal bremsstrahlung emission from CMEs. Using a simulated database from such a telescope, we explore three detection schemes: direct detection, an approximate differential detection scheme, and an “exact” differential detection scheme. We conclude that thermal bremsstrahlung emission from CMEs can be detected by such a telescope provided differential techniques are employed. While the approximate differential detection scheme may be sufficient for CMEs viewed near the solar limb, detection of CMEs against the solar disk may require the more sensitive exact differential scheme. The detection and imaging of nonthermal radio emissions from CMEs is also discussed.
Three‐dimensional nature of interaction regions: Pioneer, Voyager, and Ulysses solar cycle variations from 1 to 5 AU
Authors: González‐Esparza J. Américo , Smith Edward J.
Published: February 2004

We combined in‐ecliptic observations of interaction regions from 1 to 5 AU by four spacecraft that traveled to Jupiter at different phases of the solar cycle: Pioneer 11 (declining phase of cycle 20), Voyager 1 and 2 (ascending phase of cycle 21), and Ulysses (just after solar maximum 22). We used this set of 97 interaction regions to study two different aspects of these events. (1) We analyzed the geometry of 38 stream interfaces, and found that the interaction regions detected by the three missions have different orientations. Some of these results might be associated with solar cycle variations such as the low‐latitudinal inclination of the interaction regions detected by Ulysses and the high‐latitudinal tilts of some interaction regions detected by Pioneer 11. Other results, such as the deviations with respect to the Parker spiral orientation, seem to be related to the persistent variations in the characteristics of the solar winds streams. (2) We studied the bulk speeds and the dynamic, magnetic, and thermal pressures of the fast and slow streams associated with 75 of these interaction regions. As a consequence of solar cycle variations, the bulk speed distributions of the fast and slow streams were different in the three missions. We found that the pressure ratios (dynamic, thermal, and magnetic) between fast and slow streams vary continuously. In about half of the interaction regions the dynamic pressure of the slow stream was higher than the dynamic pressure of the fast stream. We found similar variations in magnetic and thermal pressures. This implies that in many interaction regions the slow stream transfers momentum to the fast stream. The results presented here show that the solar wind streams associated with interaction regions vary at all the phases of the solar cycle, leading to interaction regions with different characteristics.
Pi 2‐associated particle flux and magnetic field modulations in geosynchronous altitudes
Authors: Saka O. , Okada K. , Watanabe O. , Baker D. N. , Reeves G. D. , Belian R. D.
Published: February 2004

A Pi 2 pulsation event was observed at the dip‐equator (Huancayo, Peru; 75°W, 0306 LT) during the period from 0806 to 0815 UT on March 25, 1986. Geosynchronous satellite data from GOES 6 (0106‐0115 LT) and 1984–129 (2146–2155 LT) in the midnight sector during the above interval were examined to investigate the field and particle (electron, 30–300 keV; proton, 72–573 keV) signatures in space that correlated with the Pi 2 pulsation at the dip‐equator. The Pi 2 onset was seen to occur in association with the flux enhancement both for electrons and ions, and concurrent modulation of the flux and magnetic fields lasted thereafter for 10 min. In spite of a local time separation of 5.3 hours between the ground and satellite locations, the waveform in space resembled that of the Pi 2 on the ground. To account for such coherent oscillations between space and ground locations, we invoke a transient response of the nightside magnetosphere to the injected particles at the substorm expansion onset, rather than hydromagnetic instabilities arising from the interactions with the injected hot particles.
Freja studies of the current‐voltage relation in substorm‐related events
Authors: Olsson A. , Andersson L. , Eriksson A. I. , Clemmons J. , Erlandsson R. E. , Reeves G. , Hughes T. , Murphee J. S.
Published: February 2004

Field‐aligned currents and electrostatic potentials play important roles in the coupling between the magnetosphere and the ionosphere. If one assumes that the ionosphere‐magnetosphere potential difference is mainly due to the mirror force, one can use the single particle adiabatic kinetic theory to describe the system. From this theory, a linear relationship j‖=KV between field‐aligned current density j‖ and potential drop V along the same field line can be derived, provided that the potential drop is not too large and not too small. With rare exceptions, observational tests of this relation have mainly concentrated on quiet magnetospheric situations, with acceleration voltages V ≲ 5 kV. Here we use observations from the Freja satellite of precipitating auroral electrons at 1.700 km altitude to study substorm related events, with acceleration voltages up to 20 keV. The observations are found to be consistent with a linear current‐volt age relation even in these conditions, although with values of the field aligned K lower than previously reported (1–5 × 10−11 S/m2). This can be explained by lower densities and higher characteristic electron energies in the magnetospheric source region of the precipitating electrons. We analyze the data by three different methods, which are all found to be in general agreement. The results are in agreement with a previous study [Olsson et al., 1996 b], where the spectra of precipitating electrons were indirectly infered by inversion of data from the EISCAT incoherent scatter radar, thereby validating the use of radar data for studies of auroral electrons. Comparisons with previous studies are made, emphasizing the dependence of the results on the type of auroral structure and magnetospheric conditions.
Premidnight plasmaspheric “plumes”
Authors: Ober Daniel M. , Horwitz J. L. , Thomsen M. F. , Elphic R. C. , McComas D. J. , Belian R. D. , Moldwin M. B.
Published: February 2004

To explain observations of brief intervals of cold, dense plasma by geosynchronous satellites in the midnight sector prior to or during substorm onset, it has recently been proposed that dense plasmaspheric plasma is drawn out to geosynchronous orbit in the premidnight region by inductive electric fields during the growth phase of a geomagnetic substorm. We present here the results of a statistical study of such intervals observed with the Los Alamos magnetospheric plasma analyzer (MPA) on geosynchronous satellite 1989‐046 between March and December 1993. We find that these premidnight cold plasma intervals occur only after extended periods of low magnetospheric activity identified by Kp and the midnight boundary index (MBI). We also find that the satellite typically enters the cold plasma region from the trough region and exits it into the plasma sheet. Finally, while measurements of the flow velocity of the cold plasma are rendered uncertain by the asymmetric spacecraft charge or photoelectron sheath, such measurements show no evidence of the outward flow that would be expected from the extrusion hypothesis. Rather, there are some indications that these cold plasma regions flow sunward in the (corotation) satellite frame. These results suggest an alternative explanation for the premidnight cold plasma: corotation‐dominated transport of day side plasmaspheric structures into the premidnight sector. The implications of our observations for the extrusion hypothesis and for the alternative explanation are discussed.
Euler potential models of Jupiter's magnetospheric field
Authors: Khurana Krishan K.
Published: February 2004

We present a new model of the Jovian magnetospheric field which couples the internal field spherical harmonic coefficients from the Goddard Space Flight Center O6 model with an Euler potential formulation of the external field. The effects of the hinging and the delay of the Jovian current sheet and the sweep‐back of the field lines resulting from the subcorotation of plasma in the magnetosphere are incorporated self‐consistently into the Euler potential equations. Because Jovian magnetosphere undergoes substantial temporal changes in its configuration, it was found necessary to obtain different fit parameters for each of the outbound passes of Pioneer 10, Voyager 1, and Voyager 2. A detailed comparison of the model with the observations obtained in the nightside Jovian magnetosphere reveals that the Euler potential approach can be used successfully to represent the field of external origin in the Jovian magnetosphere. The proposed models are applicable to the inner magnetosphere at all local times and to the middle and the outer magnetosphere on the nightside. The models should provide reliable magnetic field predicts along the trajectory of Galileo in all 11 of the initially planned orbits. We also discuss further extensions of the model which would extend their applicability to all local times and radial distances.
Auroral electrojets during geomagnetic storms
Authors: Feldstein Y. I. , Grafe A. , Gromova L. I. , Popov V. A.
Published: February 2004

On the basis of digital magnetometers from the International monitor auroral geomagnetic effects (IMAGE) and European incoherent scatter (EISCAT) meridional chains in Scandinavia dynamics of the eastward and westward electrojets during the main phase of magnetic storms are considered. For the intense magnetic storm on May 10–11, 1992, with Dst = −300 nT, magnetograms of subauroral and midlatitudinal stations Leningrad, Borok, and Moscow were examined. It is found that the eastward electrojet center during the storm main phase shifts equatorward as |Dst| increases. The electrojet center is located at the corrected geomagnetic latitude Φ ∼ 59°–60° when Dst ∼ −100 nT and at Φ ∼ 54°–55° when Dst ∼ −300 nT. Data from meridional chains of magnetometers support earlier results pertaining to the relationship between the westward electrojet center position and the ring current intensity for intervals between substorms. During substorms expansive phases the westward electrojet expands poleward covering auroral latitudes Φ ∼ 65°. The electrojets location during the storm main phase and their dynamics in connection with substorms allow for interpretations of effects described in the literature: the AE indices saturation during the main phase of magnetic storms; approximately equal values of AU and AL indices during the storm initial phase and AL ≫ AU during the storm main phase.
Exterior and interior polar cusps: Observations from Hawkeye
Authors: Chen S.‐H. , Boardsen S. A. , Fung S. F. , Green J. L. , Kessel R. L. , Tan L. C. , Eastman T. E. , Craven J. D.
Published: February 2004

Hawkeye plasma, magnetic field, and plasma wave instruments directly sampled the throat of the northern polar cusp as the orientation of the interplanetary magnetic field (IMF) changed from southward to northward on July 3, 1974. Two distinct regions in the polar cusp were identified based on magnetic field, plasma flow and magnetic and electric noise: the interior and exterior cusps. The observations show highly variable flows in the exterior portion of the cusp and constantly strong dawn‐dusk flows in the interior portion during periods of strong IMF By component. Results of a minimum variance analysis of the magnetic field at each cusp interface crossing provides evidence that the magnetopause surface normal deviated highly from empirical models. During intervals of relatively steady solar wind dynamic pressure, the motion of the cusp relative to the slow moving spacecraft was modulated by the varying IMF clock angle as observed by IMP 8 in the upstream solar wind. The motion did not show a correlation with internal processes monitored by the AE index. We propose that observed plasma flow patterns and cusp motion are results of reconnection between the IMF and the magnetospheric magnetic field. Flow velocity observed in the interior cusp is consistent with stress balance for a reconnection process. This unique interval provides an opportunity for detailed studies of the plasma, magnetic field, and plasma wave properties in both the exterior and interior cusp.
Disruption of thin current sheets: A two‐fluid theory
Authors: Liu W. William
Published: February 2004

A growing list of observations and arguments has located the source region of magnetospheric substorms at the near‐Earth plasma sheet within 10 RE. A popular descriptive paradigm conceptualizes the substorm initiation as a sudden disruption of a severely stretched magnetic field in the active region. The physical mechanism(s) responsible for the disruption have evoked intense theoretical and computational interests. In this paper we emphasize the macroscopic aspect of substorm onsets and propose to tackle the problem as a two‐fluid instability of a plasma significantly polarized by an intense current. It is found that, for a center‐maximized current distribution, a drift‐wave instability with westward phase speed can grow on a timescale ∼ 10 s when the current sheet thins to 1500−3000 km. The phase speed, when mapped to the ionosphere, amounts to 3–5 km/s and is directed westward. The longitudinal size (wavelength) of the fastest growing mode, when similarly mapped, is about 250–500 km. These numbers agree with the morphology of current disruptions and westward traveling surges.
Wave heating of He+ by electromagnetic ion cyclotron waves in the magnetosphere: Heating near the H+‐He+ bi‐ion resonance frequency
Authors: Horne Richard B. , Thorne Richard M.
Published: February 2004

We investigate the absorption of electromagnetic ion cyclotron (EMIC) wave energy by He+ in the outer magnetosphere and the subsequent perpendicular‐heating of He+. Plasma models, based on satellite data, are constructed to represent conditions at dawn and noon MLT for L > 7. Ray tracing using the HOTRAY program shows that EMIC waves generated by an anisotropic H+ distribution have path integrated gains in excess of 60 dB for the dawn model. The waves undergo strong cyclotron resonant absorption near the bi‐ion resonance frequency when the He+ concentration is sufficiently small, typically < 1%. Growth and absorption occur on one pass across the equator and do not require reflection. The amount of wave absorption decreases with decreasing density, but the amount of energy absorbed per particle remains comparable over a range of densities > 2 × 106 m−3. The energy absorbed per particle is smaller for very low densities ∼ 5 × 105 m−3 due to a smaller path integrated wave gain. In a dipole magnetic field the absorption occurs in a localized region of space at latitudes of |λm| = 20 − 30°. At noon MLT strong wave growth occurs at lower frequencies consistent with observations. Wave absorption at the bi‐ion frequency is also stronger at lower frequencies. When a nondipole magnetic field is included to represent magnetospheric compression, it is found that wave refraction is reduced which results in a substantial increase in the path integrated gain (> 90 dB) and wave absorption takes place at higher latitudes. By considering refraction effects, we find that the spectral peak shifts to lower frequencies above the equator and that this effect is most pronounced in a dipole field. We suggest that absorption at the bi‐ion resonance frequency is responsible for most of the X‐type pitch angle distributions of He+ detected by the Active Magnetospheric Particle Tracer Explorer (AMPTE) spacecraft.
On the nature of the λIII brightness asymmetry in the Io torus
Authors: Schneider Nicholas M. , Taylor Martin H. , Crary Frank J. , Trauger John T.
Published: February 2004

The optical brightening of the Io torus around λIII∼200° has been known for 20 years, but observations have only recently revealed the nature of the asymmetry. We show that the optical asymmetry is due to changes in ion temperature, with little or no change in flux tube content. These results have significant ramifications for magnetospheric transport and torus energy flows. We examine causes of ion temperature variations and demonstrate that existing zero‐dimensional energy balance models are inadequate. We develop a more complete model of energy flow in the torus and find that small changes in energy sources result in large changes in ion and electron temperatures and densities. We extend the theory to predict brightness asymmetries at other wavelengths and show that a search for EUV asymmetries at the 30% level can help resolve the torus “energy crisis.” “Neutral Cloud Theory” predicts an EUV brightness asymmetry of exactly opposite phase to the optical asymmetry, while an unknown electron energy source around λIII∼200° could cancel or reverse this EUV asymmetry.
Heating and low‐frequency modulation of electrons observed during electron beam operations on TSS 1
Authors: Gough M. P. , Hardy D. A. , Burke W. J. , Oberhardt M. R. , Gentile L. C. , Huang C. Y. , Cooke D. L. , Raitt W. J. , Thompson D. C. , McNeil W. , Bounar K.
Published: February 2004

We have studied electron responses measured by two electrostatic analyzers (ESA A and B) that comprise the Shuttle Potential and Return Electron Experiment (SPREE) to 60 prolonged beam emissions by the fast pulsed electron generator (FPEG) during the first flight of the Tethered Satellite System (TSS 1). When the beam cleanly escaped into space, responses depended on whether the pitch angle of the beam, αB, was less than or greater than 90°. Beam‐like structures were detected by SPREE when αB < 90°, but not when αB > 90°. Secondary electron fluxes measured by SPREE peaked at pitch angles a between 65° and 75° when αB < 90°, and at α ≈ 90° when αB > 90°. At other pitch angles the distributions of electrons returning to the shuttle had repeatable thermal and power law shapes. The distinctive distribution functions are attributed qualitatively to the different regions in and near the beam traversed by electrons reaching SPREE under the two αB conditions. A large fraction of the trajectories of electrons reaching SPREE ESA A with αB < (>)90° lie inside (outside) beam flux tubes. Measurements by a particle correlator in the SPREE data processor show that in 25 cases some of the returning‐electron distributions fe were modulated at frequencies in the low kilohertz range. The modulations appeared in portions of the distributions where ∂ƒe/∂υ < 0 and at frequencies that correspond to none of the plasma's normal modes. In light of previously reported wave measurements taken near the shuttle during electron beam emissions, we suggest that the modulated electrons were bunched by large‐amplitude, ion acoustic waves propagating nearly perpendicular to the Earth's magnetic field. The waves were generated as plasma responses to negative space charges in the electron‐beam flux tubes moving at orbital speed across the ionosphere.
Guided plasmaspheric hiss interactions with superthermal electrons: 1. Resonance curves and timescales
Authors: Liemohn M. W. , Khazanov G. V. , Kozyra J. U.
Published: February 2004

Under the proper conditions, guided plasmaspheric hiss is shown to be more efficient than Coulomb collisions at scattering electrons in the superthermal energy range of 50 to 500 eV. Broadband, whistler mode hiss becomes guided by plasma density gradients, intensifying the wave energy densities and focusing the wave normal angles. These waves are shown to interact through Cherenkov (Landau) resonance with electrons below 500 eV, and the presented equatorial plane timescales for pitch angle, energy, and mixed diffusion are shown to be faster than Coulomb collision timescales for typical values at the inner edge of the plasmapause and in detached plasma regions. In the latter case, energy diffusion timescales of less than 100 s for small pitch angle electrons between 250 and 500 eV indicate that these waves have the potential to dramatically change the distribution function.