bluuebunny/crossref_metadata_2025 huggingface dataset:
On the abundance of metal ions in the lower ionosphere
Authors: Kopp Ernest
Published: February 2004

Height profiles of the metal ions Fe+, Mg+, Ca+, and Na+ in the lower ionosphere measured in situ in five rocket flights with a magnetic mass spectrometer are presented. The mean total column density of the ionized metals is within 4.4 ± 1.2 × 109 cm−2 in periods without special meteor shower activity. The column density increased by 1 order of magnitude during the Perseid meteor shower of August 12, 1976. The dominant metal ion in all five flights is Fe+. Its column density in the five flights is, in average, a factor 2.0 ± 0.6 larger than Mg+. A comparison of the column densities of Ni+, Al+, Ca+, Mg+, K+, and Na+, which were normalized to Fe+, to the corresponding ratios in carbonaceous chondrites type I (CI) showed a depletion in the abundance of aluminium (80%), calcium (65%), magnesium (55%), and nickel (35%). At heights above 95 km all metals under consideration should predominantly occur as ions, and no distinct mass fractionation between the different ionized metals is expected from the main loss process, the downward movement of the metal ion layers. The reduced abundance of Al, Ca, Mg, and Ni is therefore most likely an effect of a fractionation originating from partially melted micrometeorites and an associated partial evaporation retaining elements having a relatively high condensation temperature as compared to Fe, which is the case for Ni, Al, Ca, and Mg.
Neutral winds and temperature in the tropical mesosphere and lower thermosphere during January 1993: Observation and comparison with TIME‐GCM results
Authors: Zhou Q. H. , Sulzer M. P. , Tepley C. A. , Fesen C. G. , Roble R. G. , Kelley M. C.
Published: February 2004

We present the E region neutral winds and temperature observed by the Arecibo incoherent scatter radar (ISR) and a collocated spectrometer during the January 20–30, 1993, period and compare them with the thermosphere/ionosphere/mesosphere electrodynamics global circulation model (TIME‐GCM) predictions. The model is most successful in predicting the meridional wind phase and magnitude throughout the altitude range of 95–145 km. Although the model can also predict the overall pattern of zonal wind and temperature, its daytime eastward wind exceeds the observations by about 35 m/s at 105 km and its temperature deviation from mean is typically 3 times smaller than the observations. The overestimation of the eastward meanflow in the model is likely associated with an overestimation in gravity wave forcing which prevents the propagation of the diurnal tide to 100 km. The diurnal tide, which is quite substantial at 100 km from the radar measurement, deposits its momentum to enhance the westward meanflow [Miyahara and Wu, 1989]. One reason for the smaller simulated temperature fluctuation is that the current model does not include any semidiurnal forcing at the lower boundary (30 km). According to a recent study by Hagan [1996], water vapor absorption of solar near‐infrared radiation in the troposphere can have a very large effect on the tides in the lower thermosphere. We further note that the observed semidiurnal tide experiences a fast growth rate at 105 km and saturates at slightly above 110 km, while the model prediction shows a much smaller growth rate in this region despite a much longer vertical wavelength. This suggests that the model may overestimate the molecular/eddy viscosity in the turbopause region. During the January 1993 period, the observed mean mesopause temperature, 160 K, is about 30 K colder than the model prediction and current prevailing beliefs. Although such a cold mesopause at a tropical latitude needs to be verified by more experiments, it cannot be precluded theoretically due to large uncertainties in radiative and dynamical heating/cooling rates, most particularly the radiative cooling rate from CO2.
On the spectrum of thermospheric gravity waves observed by the Super Dual Auroral Radar Network
Authors: Bristow W. A. , Greenwald R. A.
Published: February 2004

The spectrum of acoustic gravity waves in the thermosphere observed using the Super Dual Auroral Radar Netowrk HF radar network is presented and various features are discussed. HF radars are sensitive to medium‐scale gravity waves in the thermosphere that interact with the F‐region ionization. The interaction of waves in the neutral atmosphere with the ionization is discussed to determine the relationship of the observed spectrum to the gravity wave spectrum. The observations showed that (1) the observed spectrum shows peaks at frequencies that correspond to quasi‐monochromatic wave packets observed in the time series; (2) the spectrum often shows multiple peaks at harmonic frequencies; and (3) the spectrum nearly always shows a background level with a power law decrease having a slope of −5/3. It was also found that the overall level of the spectrum was elevated when gravity waves were present, possibly indicating an energy cascade process from the quasi‐monochromatic wave to the power law background.
Geomagnetic storm effects on zonal and meridional equatorial electrojet currents
Authors: Chandra H. , Rastogi R. G.
Published: February 2004

The sudden commencements of geomagnetic storms at the equatorial electrojet station Kondaikanal are characterized by simultaneous increases of both the horizontal field H and the westerly declination D(W) or the decrease of eastward field, Y. The amplitudes of the SSC in H or Y fields (ΔH and ΔY) show a maximum deviation around midday hours. The ratio of ΔY to ΔH is about 0.23 for any hour of the day or night. During the main phase of the storm, the fluctuations in the H and Y components at Kodaikanal vary in unison with each other. Occasional large decreases of the H field at Kodaikanal at daytime hours during the main phase of the storm do not have corresponding decrease of H at Alibag or in the equatorial Dst index. These events are shown to be associated with the sudden disappearance of the equatorial type of sporadic E(Esq) at Kodaikanal indicating a temporary reversal of the ionospheric electric field over the magnetic equator to westward direction. The observed storm time variations of the magnetic fields at an equatorial electrojet station are suggested to be due to the combined effects of the equatorial ring current and of the electric field changes at the ionospheric region transmitted from high latitudes.
Effects of intense storms and substorms on the equatorial ionosphere/thermosphere system in the American sector from ground‐based and satellite data
Authors: Sobral J. H. A. , Abdu M. A. , González W. D. , Tsurutani B. T. , Batista I. S. , de González A. L. Clua
Published: February 2004

Equatorial ionospheric responses to magnetospheric storm/substorm‐associated electric fields are investigated for a few intense events of the equinoctial months of solar maximum years 1978–1979. All the magnetic storms considered here are the result of the transit at Earth of interplanetary magnetic clouds. The interplanetary magnetic field data Bz from the ISEE 3 satellite, the auroral electrojet activity index AE, and the ring current index Dst are used as indicators of the disturbed magnetospheric conditions, and the ionospheric response features are analyzed using the F layer critical parameters h′F, h′F3, hpF2, and ƒ0F2. Focus is given to identify, when a large number of sequential substorms occurs, (1) the responses to prompt penetration electric field (from individual substorm events) as different from the delayed effect from the disturbance dynamo electric field and (2) the verification of local time dependences of the disturbance electric field polarity as predicted from the existing theoretical models. We have found evidence of near‐midnight polarity reversal of prompt penetration disturbance electric field during the course of a developing substorm. Evidence is provided also on the near‐midnight polarity reversal for the disturbance dynamo electric field. The prereversal enhancement electric field at sunset, produced by the F layer dynamo, is found to undergo drastic day‐to‐day variations in the course of a disturbed interval. However, the competing influences of the prompt versus delayed electric fields after a series of substorms could result at times in partial, or even complete, cancellation of the effects, so that the prereversal enhancement in the vertical drift could appear unaffected by the disturbances. There are indications that the disturbance dynamo electric field effects on the equatorial ionosphere last for one more day past the end of the substorm recovery.
Reappearance of recurrent low‐energy particle events at Ulysses/HI‐SCALE in the northern heliosphere
Authors: Roelof E. C. , Simnett G. M. , Decker R. B. , Lanzerotti L. J. , Maclennan C. G. , Armstrong T. P. , Gold R. E.
Published: February 2004

In the high‐latitude northern hemisphere of the heliosphere, the Heliosphere Instrument for Spectra Composition and Anisotropy at Low Energies (HI‐SCALE) energetic particle detectors on Ulysses have measured quasi‐recurrent ∼26‐day increases of 40–65 keV electrons during 11 solar rotations from October 1995 through July 1996. They do not appear on all rotations, but when they do, they are associated with increases in 0.5–1.0 MeV protons (preceding the electron increases by several days) and sometimes with decreases in galactic cosmic rays. The northern recurrences form two series shifted half a solar rotation with respect to each other, unlike the very regular and more intense series of 21 recurrences observed by the same instrument throughout the middle‐to‐high‐latitude southern hemisphere from mid‐1993 to the beginning of 1995. Correlated energetic particle measurements from IMP 8 at Earth and Voyagers 1 and 2 at 42–62 AU establish that recurrent events during this period were intrinsically stronger in the southern heliosphere than in the north. The variability of the northern recurrences is attributed, using a generalization of the model of Fisk [1996], to temporal changes during 1966 in the near‐Sun polar magnetic field configuration. These changes would affect the connection of Ulysses via magnetic field lines to the corotating interaction regions at lower latitudes >10 AU beyond the spacecraft, where the low‐energy particles are accelerated and the galactic cosmic rays are modulated. The observed evolution of the northern polar coronal structure, as revealed in FeXIV (5303Å) synoptic maps and confirmed by FeXII (195Å) images from the Extreme Ultra‐Violet Imaging Telescope on the SOHO spacecraft, is just that which is required, according to the model, to explain the evolution of the low‐energy particle recurrences as observed by Ulysses/HI‐SCALE in the northern heliosphere.
Solar wind He2+ ring‐beam distributions downstream from the Earth's bow shock
Authors: Fuselier S. A. , Schmidt W. K. H.
Published: February 2004

The electrostatic potential at the shock causes incident solar wind He2+ and H+ distributions to slow differentially across the boundary. This differential slowing produces a He2+ ring‐beam distribution immediately downstream from the shock. The velocity (with respect to the downstream H+ distribution) and angle (with respect to the downstream magnetic field) of the ring‐beam distribution are determined from a simple model that uses the change in velocity experienced by the protons across the shock, the shock normal, and the downstream magnetic field direction. This simple model is tested with data from the ISEE 1 and 2 spacecraft. Model and observations are found to agree reasonably well for quasi‐perpendicular shocks over a wide range of plasma β and Mach number. The predicted angle that the ring beam makes with the magnetic field can be significantly different from the observed angle for quasi‐parallel shocks. This difference is probably due to the fluctuating magnetic field direction downstream from the shock.
A multievent study of broadband electrons observed by the DMSP satellites and their relation to red aurora observed at midlatitude stations
Authors: Shiokawa K. , Meng C.‐I , Reeves G. D. , Rich F. J. , Yumoto K.
Published: February 2004

Broadband electrons during magnetic storms are characterized by an unusually intense flux of precipitating electrons in the broadband energy range from 30 eV to 30 keV near the equatorward edge of the auroral oval (47°–66° magnetic latitude). Broadband electrons were first reported by Shiokawa et al. [1996]. In this paper, we report a multievent study of broadband electrons, using particle data obtained by the Defense Meteorological Satellite Program (DMSP) satellites during 23 magnetic storms from January 1989 through May 1992. Twelve broadband electron events are identified. Most of them are observed in the night sector, but some are observed in the morning sector. Particle data for successive polar passes of the DMSP multisatellites are used to show that broadband electrons generally last for less than 30 min and that for some events, they precipitate over a wide range of local times simultaneously. On the basis of a quantitative calculation of optical emissions from electrons in the neutral atmosphere, we conclude that broadband electrons are a possible cause of red auroras observed at midlatitude ground stations. We suggest that broadband electrons are associated with certain substorms during the main phase of magnetic storms. This conjecture comes from observations of H component positive bays and Pi 2 pulsations observed at low‐latitude magnetic stations and from magnetic field variations observed at geosynchronous satellites. We conclude that the magnetospheric source of broadband electrons lies within the inner part of the plasma sheet. This conclusion is based on the facts that broadband electrons appear in latitudes where plasma sheet particles were observed before the event and that broadband electrons are observed poleward of the subauroral ion drifts, a position that corresponds to the inner edge of the injected particle layer during storms. High‐energy particle data obtained at geosynchronous satellites show that both strong magnetopause compressions in the dayside and decrease of the particle fluxes in the nightside occur in association with the broadband electron events. Possible mechanisms of broadband electron production are discussed.
Mapping of the ionospheric field‐aligned currents to the equatorial magnetosphere
Authors: Lu G. , Siscoe G. L. , Richmond A. D. , Pulkkinen T. I. , Tsyganenko N. A. , Singer H. J. , Emery B. A.
Published: February 2004

The assimilative mapping of ionospheric electrodynamics (AMIE)‐derived ionospheric field‐aligned current patterns during the ninth Coordinated Data Analysis Workshop substorm event have been mapped to the magnetospheric equatorial plane, using the data‐based magnetospheric magnetic field model [Tsyganenko, 1995]. We find that at the onset of the substorm expansion phase, a pair of field‐aligned currents emerges about 8 RE tailward of the Earth, near the x axis. This pair of field‐aligned currents is consistent with the substorm current wedge model; it lines up in the east–west direction across the near–Earth tail current and has the sense of the nightside region 1 current, i.e., flowing into the ionosphere on the east side and out of the ionosphere on the west side. The ionospheric counterpart of the substorm wedge currents in the midnight sector, on the other hand, has a northeast‐to‐southwest alignment. During the expansion phase, the substorm wedge currents intensify and expand in spatial size; however, their peaks remain nearly at the same location in the equatorial magnetosphere, about 8 RE tailward from the Earth. We also find that the intensification of the substorm wedge currents during the expansion phase is accompanied by the intensification of the region 2‐sense currents located closer to the Earth, and the boundary between the region 2 and substorm wedge currents lies around x = −6 RE.
Global modeling of Pi 2 pulsations
Authors: Pekrides H. , Walker A. D. M. , Sutcliffe P. R.
Published: February 2004

A numerical model of Pi 2 disturbances in the magnetosphere is developed. The model of the magnetosphere is the same as one previously used for the study of Pc 5 pulsations. It is semicylindrical with field lines being semicircular arcs and longitude measured parallel to the axis. The model magnetosphere is excited by a source on the boundary which is limited both in time and in longitude. The disturbance is then a superposition of modes corresponding to all the azimuthal wave numbers excited by the source. The computations show that a global oscillation is excited and that there can be a significant signal on the dayside at low latitudes. The role of the plasmasphere in producing a low‐latitude response is important. Because of the simplifications of the model the most important aspect of the calculations is the qualitative understanding that they supply. Nevertheless, quite good quantitative agreement with observation is obtained. The calculations support earlier suggestions about the response of the magnetosphere to the initiation of the substorm current wedge.
Thermospheric meridional wind at low latitude from measurements of F layer peak height
Authors: de Medeiros R. T. , Abdu M. A. , Batista I. S.
Published: February 2004

Thermospheric meridional winds are calculated in this paper for the low‐latitude station, Cachoeira Paulista, in Brazil using ionospheric F layer peak height hmax as the primary database for different seasons and solar activity conditions. A servo model is used that expresses the ionospheric F layer peak height displacements as a function of chemical loss, diffusion, thermospheric meridional wind and vertical plasma drift. The method used is similar to, and an extension of, the one used in recent years for midlatitudes, where the effect of vertical plasma drift is considered negligible. We have included in our analysis the effect of vertical plasma drift on hmax over Cachoeira Paulista by using a vertical drift model which is a field line extension of an equatorial electric field model developed for our longitude, for which the radar data from Jicamarca, Peru, are complemented, in the sunset sector, by vertical drift from ionosonde data over Fortaleza. A numerical model on the electrodynamic coupling of the E and F regions is used to obtain the equatorial height dependence of the vertical drift needed for its field line mapping to low latitude. Meridional winds were calculated using the servo equations in which the vertical plasma drifts, and hmax values deduced from Cachoeira Paulista ionograms, were the main inputs. The magnetic meridional winds calculated for the summer equinoctial and winter months of high and low solar activity epochs are compared with the HWM‐90 and with the measurements by Fabri‐Perot technique available for Cachoeira Paulista. The results show varying agreements. The paper presents details of the method of analysis and the comparison of the results.
Daytime thermosphere above Millstone Hill during severe geomagnetic storms
Authors: Mikhailov A. V. , Foster J. G.
Published: February 2004

A self‐consistent method for daytime F region modeling was applied to Millstone Hill radar observations during the severe geomagnetic storm of April 6–12, 1990, when the F2 layer maximum practically disappeared at usual heights and the F1 layer constituted the ionospheric maximum during daytime hours. Neutral composition and temperature changes are shown to be the main reason for the observed tenfold decrease of electron concentration at the F2 layer heights. The decrease in [O] at 300 km is more than a factor of 6, and the [N2] and [O2] increases are 3 and 16 times, respectively, with respect to the prestorm quiet time level. Such changes of neutral composition lead to a strong decrease in O+ production rate and increase in O+ loss rate resulting in a complete disappearance of the F2 layer maximum. Our calculations for 300 km altitude indicate a nineteenfold O/N2 ratio decrease for April 10 with respect to quiet day of April 7. Horizontal plasma transfer due to the observed E–W drift is insufficient to account for the observed density changes during daytime hours. The calculated exospheric temperatures Tex are close to both the mass spectrometer incoherent scatter (MSIS) 83 model predictions and the Millstone Hill estimates for all days except for April 10, when Tex is higher than 2000°K, which may be attributed to Joule heating due to strong electric fields. The calculated meridional thermospheric wind Vnx is more equatorward on disturbed days. The strong Vnx (∼90 m/s northward) on April 11 can explain the fast recovery of the local thermospheric parameters to the April 9 level. Calculations indicate that the ionosphere was molecular‐ion‐dominated up to 350 km on April 10, requiring a correction to the routinely derived Te(h), Ti(h), and Ne(h) radar profiles.
On the amplitude of intense Langmuir waves in the terrestrial electron foreshock
Authors: Bale S. D. , Burgess D. , Kellogg P. J. , Goetz K. , Monson S. J.
Published: February 2004

Waveforms of large‐amplitude Langmuir oscillations were recorded by the Wind spacecraft in the Earth's upstream electron foreshock region. We present some statistics of the waveforms and discuss them in the context of various saturation mechanisms. In particular, it is found that the value of Epeak/Erms is not large, as previously suggested, and that the largest‐amplitude Langmuir waveforms are generally somewhat sinusoidal and lack structure on small spatial scales. The measured probability distribution of electric field amplitude and dimensionless energy suggest that some stochastic process may play a role in wave generation. The values of dimensionless energy needed to arrest Langmuir wave collapse occur with very small probability and the value of Epeak/Erms for large fields suggests that, statistically, Langmuir wave collapse is not an important process in the terrestrial foreshock.
Modeling of microburst electron precipitation using pitch angle diffusion theory
Authors: Datta S. , Skoug R. M. , McCarthy M. P. , Parks G. K.
Published: February 2004

Microburst electron precipitation is characterized by short bursty (duration ∼0.2–0.3 s) quasiperiodic precipitation of electrons in the dayside auroral zone. Pitch angle diffusion of electrons due to the interaction with whistler waves has been suggested previously as a possible mechanism for scattering electrons into the loss cone to cause microburst precipitation. In this paper we investigate the viability of the above mechanism through modeling. We assume the scattering to occur near the equatorial plane and solve the pitch angle diffusion equation numerically to find the time‐dependent form of the electron distribution function F, which results from a given time‐dependent diffusion coefficient D. Different aspects of our results (burst size, pitch angle dependence, risetime, burst width) are compared with observations from a recent rocket experiment on microburst launched from Poker Flat, Alaska. The comparison shows very good agreement, further supporting the idea that the pitch angle diffusion process is the driving mechanism for microbursts.
A fast solar wind model with anisotropic proton temperature
Authors: Hu Y. Q. , Esser R. , Habbal S. R.
Published: February 2004

We explore the energy requirements for the fast solar wind when the anisotropy in the proton temperature is taken into account. Using a one‐dimensional, two‐fluid model with anisotropic proton temperature, we present high‐speed solar wind solutions which meet most of the empirical constraints currently available from in situ measurements in interplanetary space and very recent remote sensing observations of the inner corona. Included in the model is the momentum exerted on the flow by Alfvén waves, as well as heating due to their damping. However, to produce solutions consistent with these empirical constraints, additional heat input to both electrons and protons, as well as momentum addition to the protons, are found to be needed. These are described by ad hoc functions with adjustable parameters. While classical thermal conduction is adopted for both electrons and protons in the inner corona in the model computations, the corresponding heat fluxes in the outer corona are limited to values comparable to current observations. The fast solar wind solutions thus obtained differ from each other mainly in their thermal properties within 0.3 AU from the Sun, a region that is still poorly probed by in situ and remote sensing measurements. To satisfy observational constraints, we find that the inclusion of a proton temperature anisotropy in the modeling of the solar wind requires that either the protons be highly anisotropic in the inner corona or that there exist a mechanism, in addition to adiabatic expansion, to cool them in the direction parallel to the magnetic field. Given these observational constraints and in the absence of knowledge of an efficient cooling mechanism, our model computations imply that the maximum temperature of the protons in the parallel direction has to be limited to 106 K in the corona. Furthermore, because of the strong coupling between electrons and protons, and between the parallel and perpendicular motions, at the coronal base, the electron temperature as well as the perpendicular proton temperature cannot be much higher than 106 K there. Although thermal anisotropy of the protons is found to have little influence on the dynamics of the fast solar wind, its inclusion imposes new requirements on the unknown coronal heating mechanisms.
Energetic gamma ray experiment telescope high‐energy gamma ray observations of the Moon and quiet Sun
Authors: Thompson D. J. , Bertsch D. L. , Morris D. J. , Mukherjee R.
Published: February 2004

High‐energy gamma radiation has been seen from the Moon as it passed through the large field of view of the energetic gamma ray experiment telescope (EGRET) on the Compton Gamma Ray Observatory a number of times between mid‐1991 and the end of 1994. The average lunar flux is (4.7 ± 0.7) × 10−7 photons (E > 100 MeV) cm−2 s−1. The observed energy spectrum is consistent with a model of gamma ray production by cosmic ray interactions with the lunar surface, and the flux varies as expected with the solar cycle. Although the same types of cosmic ray interactions that produce the lunar gamma rays may occur on the Sun, EGRET has not detected the quiet Sun. The 95% confidence upper limit on the flux above 100 MeV, 2.0 × 10−7 photons cm−2 s−1, lies well below the lunar gamma ray flux, as expected.
F region gravity waves in the central polar cap
Authors: MacDougall J. W. , Hall G. E. , Hayashi K.
Published: February 2004

This study identifies polar cap gravity waves from the perturbations that they produce in ionospheric vertical velocity. In the December 1994 to February 1995 interval, there were 15 wave groups that were defined well enough for us to study their properties. Most of the properties of these waves are similar to the gravity waves that were measured by the DE 2 satellite [Johnson et al., 1995]. Our waves had typical gravity wave speeds (75–225 m s−1) and produced vertical velocity fluctuations >20 m s−1. The typical wave period was 30 min. Source locations were mostly west of Eureka at distances ranging from 300 to > 1500 km. Source times and wave propagation directions were such that the gravity waves would not have been excluded by critical speed effects. Most of the wave groups showed large spatial resonance effects, the average vertical ionospheric displacement being 61 km pp. Spatial resonance occurs when the plasma convection velocity matches the wave speed. Our polar cap gravity waves were also associated with relatively large perturbations in ionospheric convective velocity and magnetic perturbations having typical magnitudes of 300 m s−1 and 20 nT (both peak to peak), respectively. The magnetic perturbations appeared to be due to E region currents driven by the electric fields associated with the convection perturbations.
Neural network modeling of solar wind‐magnetosphere interaction
Authors: Wu Jian‐Guo , Lundstedt Henrik
Published: February 2004

We study solar wind‐magnetosphere interaction using partially recurrent neural networks to find the best coupling functions. Hourly data of the solar wind and Dst are selected, from the period 1963 to 1992, covering 10,554 hours. The best coupling functions are found from the out‐of‐sample performance of trained neural networks, which are then compared with those obtained from cross‐correlation analyses. In most cases, good agreement is found. This indicates that cross‐correlation analysis can be used to optimize input parameters for neural networks, albeit at a limitation due to the assumption of linearity. However, there exist exceptions where coupling functions strongly correlated with Dst give poor predictions. Based on neural network modeling, if we take the coupling function in the form of
, then the conditions, 0 ≤ λ ≤ 1, 0.5 < μ ≤ 2, and 0.5 ≤ ν ≤ 2, will result in solar wind input functions well suited for predicting geomagnetic storms, e.g., P1/4V Bs, P1/6V Bs, P1/5V Bs, P1/3V Bs, P1/2V Bs, V2Bs, and V Bs. In contrast, ε and the polar cap potential are less good candidates for storm predictions. Therefore the best coupling functions do not necessarily have the dimension of power, energy or electric field. Furthermore, we predict geomagnetic storms up to five hours in advance with good accuracy from the best coupling functions. The confidence limits on the prediction accuracy are computed to show the statistical significance of the predictions and to confirm the best coupling functions. Finally, we discuss how the partially recurrent network models solar wind‐magnetosphere interaction.
Multisatellite observations of the outer zone electron variation during the November 3–4, 1993, magnetic storm
Authors: Li Xinlin , Baker D. N. , Temerin M. , Cayton T. E. , Reeves E. G. D. , Christensen R. A. , Blake J. B. , Looper M. D. , Nakamura R. , Kanekal S. G.
Published: February 2004

The disappearance and reappearance of outer zone energetic electrons during the November 3–4, 1993, magnetic storm is examined utilizing data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), the Global Positioning System (GPS) series, and the Los Alamos National Laboratory (LANL) sensors onboard geosynchronous satellites. The relativistic electron flux drops during the main phase of the magnetic storm in association with the large negative interplanetary Bz and rapid solar wind pressure increase late on November 3. Outer zone electrons with E > 3 MeV measured by SAMPEX disappear for over 12 hours at the beginning of November 4. This represents a 3 orders of magnitude decrease down to the cosmic ray background of the detector. GPS and LANL sensors show similar effects, confirming that the flux drop of the energetic electrons occurs near the magnetic equator and at all pitch angles. Enhanced electron precipitation was measured by SAMPEX at L ≥ 3.5. The outer zone electron fluxes then recover and exceed prestorm levels within one day of the storm onset and the inner boundary of the outer zone moves inward to smaller L (<3). These multiple‐satellite measurements provide a data set which is examined in detail and used to determine the mechanisms contributing to the loss and recovery of the outer zone electron flux. The loss of the inner part of the outer zone electrons is partly due to the adiabatic effects associated with the decrease of Dst, while the loss of most of the outer part (those electrons initially at L ≥ 4.0) are due to either precipitation into the atmosphere or drift to the magnetopause because of the strong compression of the magnetosphere by the solar wind. The recovery of the energetic electron flux is due to the adiabatic effects associated with the increase in Dst, and at lower energies (<0.5 MeV) due to rapid radial diffusion driven by the strong magnetic activity during the recovery phase of the storm. Heating of the electrons by waves may contribute to the energization of the more energetic part (>1.0 MeV) of the outer zone electrons.
Magnetohydrodynamic processes in the inner coma of comet Halley
Authors: Lindgren C. J. , Cravens T. E. , Ledvina S. A.
Published: February 2004

A two‐fluid, two‐dimensional, magnetohydrodynamical (MHD) model is used to study the dynamics of the inner coma of comet Halley. Ion production, electron‐ion recombination, charge transfer collisions, and ion‐neutral friction are included. The model adopts a cylindrical grid with nonuniform radial spacing extending from a cometocentric distance of 1000 km out to 107 km. A magnetic barrier and a diamagnetic cavity both appear in the model results, and the plasma flow is directed around the cavity. Electron temperature profiles have recently been suggested for the inner coma in which large temperature enhancements (Te ≈ 25,000 K) are present at a radial distance of about 10,000 km and have been used to explain, via reduced recombination, the ion density enhancement observed by the Giotto ion and neutral mass spectrometers at this location. Using our MHD model as well as analytical methods we find that large electron temperature enhancements in the inner coma result in large reductions in the magnetic field strength which were not observed by the Giotto magnetometer. We suggest that perhaps the electron temperature is only enhanced to a value of about 10,000 K rather than to 25,000 K at the ion pileup.
Atmospheric gamma rays at geomagnetic latitudes of −29° and +43°
Authors: Akyüz A. , Bhattacharya D. , Chuang K. W. , Dixon D. D. , O'Neill T. J. , Tümer O. T. , White R. S. , Zych A. D.
Published: February 2004

We present results of atmospheric gamma ray measurements obtained during two balloon flights from Alice Springs, Australia (λ = −29°), and Fort Sumner, New Mexico, United States of America (λ = 43°) at geomagnetic cutoff rigidities of 8.5 GV and 4.3 GV, respectively. The fluxes, in the energy range of 1–15 MeV, are derived as functions of zenith angle, residual depth, and latitudinal rigidity. We find while the downward moving gamma ray flux at the float level (4.8 g cm−2) is not a strong function of rigidity the upward flux at λ = −29° is, on average, by factors of 2 to 4 lower than at λ = 43°. The energy spectra of the downward moving gamma rays at various altitudes are harder than the upward moving gamma rays. The spectral indices for both upward and downward fluxes at λ = −29° are lower than at λ = 43°.
Monoenergetic ion drop‐off in the inner magnetosphere
Authors: Shirai H. , Maezawa K. , Fujimoto M. , Mukai T. , Saito Y. , Kaya N.
Published: February 2004

A monoenergetic drop‐off of ions around 10 keV, which we term as “ion drop‐off band” (IDB) in this paper, has been observed by Akebono. The IDB is identified as a sharp and deep dip at about 10 keV in ion spectra, which is usually observed at latitudes below the discrete auroral region over several degrees or more. As the ion motion of this energy in this inner part of the magnetosphere is basically described by the adiabatic theory, we have numerically traced the ion drift trajectories. From the results, it is proposed that the lower‐energy boundary of the drop‐off demarcates the open/closed character of the drift orbits, only below which continuous supply from the magnetotail is present. This model explains the energy, local time, and latitudinal extent of IDB as well as the formation of its poleward edge very well.
A statistical study of the magnetic field structure in the inner magnetosphere
Authors: Nakabe S. , Iyemori T. , Sugiura M. , Slavin J. A.
Published: February 2004

Three‐dimensional magnetic field structure in the inner magnetosphere is statistically analyzed using data obtained by the DE‐1 magnetometer. The main results are as follows: (1) The magnetic field structure of the rotational pattern associated with an eastward current is clearly seen in the dayside equatorial region at geocentric distances around 2.5 Re but not on the nightside. The current density is approximately 0.05μ A/m2, and does not show any clear dependence on geomagnetic disturbance, but shows a solar activity dependence which possibly suggests that a substantial part of the closure of the eastward current is through the dayside mid‐latitude ionosphere. (2) The azimuthal component of the magnetic field associated with field‐aligned currents is seen in the low‐latitude magnetosphere. The intensity of this component is about 5 to 10 nT, and has a dependence on geomagnetic activity. (3) Compared with the currently available magnetospheric models, the observed inner magnetospheric magnetic fields have much more complex structures than the models indicate. The necessity of including the field‐aligned current system and an eastward current in the models is evident. (4) A noon‐midnight asymmetry of the magnetic structure is clearly seen, particularly in the southward component associated with the ring current. The asymmetry is much larger in this meridian than that in the dawn‐dusk meridian. This is in contrast with the asymmetry observed on the ground or by low‐altitude satellites over the ionosphere.
Distributions of wave normals at the duct exit of a whistler and the morphology of ionospheric exits of low‐latitude whistlers observed by direction finding
Authors: Nakamura Yoshikatsu
Published: February 2004

Direction‐finding observations at low latitudes reveal that whistlers arrive from narrow regions of the zenith of tens of kilometers in diameter and that their ionospheric exits have a variety of shapes. The distribution of the wave normals of whistlers at the duct exit was studied using a field‐aligned duct model by ray tracing in order to examine the cause of the morphology of the ionospheric exits. The characteristics of the distribution and its relation to the trapping cone have been clarified. When the distribution is represented by plotting the wave normal dip angle against the escape position at the duct exit, two kinds of wave normal distributions are evident at the duct exit: One is a flat distribution of downward wave normals and has a region of almost vertically downward wave normals at the main area of the duct exit, and the other is a flat distribution of more upward wave normals. They can be explained, respectively, as the innermost direction and the outermost direction of a trapping cone. The region of vertical downward wave normals in the downward wave normal distribution occurs because of the vertical downward electron density gradient in the topside ionosphere and also because of the effects of the trapping cone shrinking on the whistler's passage through the ionospheric maximum electron density region and of the expansion of this cone in the lower ionosphere. This region of vertical downward wave normals is responsible for the whistler's penetration to the ground. Because the downgoing whistler's wave normals are oriented to the vertical downward direction uniformly at any point of the duct exit due to the downward wave normal distribution, whistler rays emitted from each point of the duct exit propagate in parallel and reach the bottom of the lower ionosphere. Thus the observed morphology of ionospheric exits can be interpreted as the result of the projection of the section of the duct exit onto the ionospheric lower boundary.
Self‐consistent structure of a thin anisotropic current sheet
Authors: Kropotkin A. P. , Malova H. V. , Sitnov M. I.
Published: February 2004

An approximate solution of the system of Vlasov‐Maxwell equations for a collisionless current sheet with strongly anisotropic ion species (the bulk velocity outside the sheet is much greater than the thermal one) and sharply curved magnetic field lines (the curvature radius is much less than the ion gyroradius) is obtained in the form of a universal function with two asymptotics and an iterative procedure in between. The magnetic field spatial scaling suggested by Francfort and Pellat [1976] is confirmed analytically.
Observation of broadband wave bursts with power law spectra above the plasma frequency in the underdense auroral plasma
Authors: Malingre Michel , Pottelette Raymond , Treumann Rudolf A. , Berthomier Mathieu
Published: February 2004

Viking spacecraft spectral observations of high‐frequency waves with frequencies fe<f<fce between the electron plasma (fe) and cyclotron (fce) frequencies in the strongly magnetized nightside auroral plasma occasionally exhibit impulsive emissions with featureless power law spectra extending sometimes up to frequencies close to fce. Most of these bursts are observed when the electric antenna is at large angles with respect to the ambient magnetic field. The index of the frequency electric spectra is in the range ∼3.5–4. Much flatter spectra are found for the transverse magnetic fluctuations. The electric to magnetic field ratio is large. Hence either the waves are of predominantly electrostatic nature, or the radiation propagates transverse to the ambient magnetic field, so that the Viking loop antenna underestimates their magnetic components. The bursts typically last several hundred milliseconds. Simultaneous electron observations exhibit the presence of upward suprathermal electrons associated with energetic downgoing electron fluxes having power law energy distribution with index close to 4 in an energy range extending from ∼2 keV up to 70–80 keV. Considering the wave bursts to be electromagnetic, propagating in the Z mode, and locally generated, we suggest that the observations might be explained in terms of a free electron maser mechanism whose free energy source lies in the observed power law energetic electron fluxes.
Influence of heavy ions on the high‐speed solar wind
Authors: Li Xing , Esser Ruth , Habbal Shadia R. , Hu You‐Qiu
Published: February 2004

We present the results of a parameter study of the influence of heavy ions on the background solar wind, choosing doubly ionized helium, or alpha particles, and O+6, as examples. Using a three‐fluid solar wind model, we keep the input parameters to the electrons and protons unchanged and investigate the effects of changing the input energy flux to the heavy ions and their coronal abundance, i.e., their abundance at 1 Rs, on the background electron‐proton solar wind. Our results confirm earlier studies that alpha particles can have a dramatic effect on the thermodynamic and flow properties of the protons in the solar wind. The maximum coronal abundance for which the changes in the energy input to the heavy ions has no effect on the protons is 5 × 10−4 for the alphas and 5 × 10−5 for the oxygen ions, which are well below the photospheric values. For larger coronal abundances, the sensitivity of the changes of the flow speed and proton mass flux to changes in the energy input to the heavy ions increases sharply with increasing abundance. When the heavy ions are not heated, the increase in the coronal abundance leads to an increase in flow speed, a decrease in proton mass flux, and an increase in proton temperature at 1 AU. However, as the heat input to the heavy ions increases, the dependence of these parameters on the abundance goes through a transition and starts to follow the opposite pattern, namely a decrease in flow speed and proton temperature at 1 AU, and an increase in proton mass flux. This study shows that, for currently known photospheric elemental abundances, the flow properties of heavy ions cannot be investigated independently of those of the bulk proton‐electron solar wind. The effect of heavy ions on the electron‐proton bulk solar wind is determined primarily by the collisions occurring very close to the coronal base. Hence including physical processes responsible for the preferential heating of heavy ions to temperatures exceeding those of protons in the inner corona cannot be done without considering the subsequent implications for the protons and electrons in a self‐consistent manner.
Limits on the asymmetry of solar wind temperature in the outer heliosphere
Authors: Gazis P. R.
Published: February 2004

Interstellar pickup ions, if they are thermalized, could produce a longitudinal asymmetry in solar wind temperature in the outer heliosphere. Between 1978 and mid‐1992, the Pioneer 10 and Voyager 2 spacecraft were well situated to search for such an asymmetry. Pioneer 10 was headed downstream with respect to the local interstellar medium while Voyager 2 was headed upstream. Both spacecraft were at large heliocentric distances; by mid‐1992, Pioneer 10 was at 55 AU, while Voyager 2 was at 37 AU. Measurements from these spacecraft and the Pioneer Venus Orbiter are complicated by solar cycle effects but suggest an upper limit of 5000 K for any asymmetry that might be present. These measurements are also consistent with no asymmetry. There is also some suggestion that the radial profile of solar wind temperature in the vicinity of the solar equator was steeper during the declining phase of the last solar cycle. The solar wind temperature has increased at Voyager 2 after mid‐1992, but Voyager 2 has also been moving to higher Heliographic latitudes, where such an increase should be expected from the well‐known latitudinal gradient in solar wind temperature that has been observed by Pioneer 11 and Ulysses.
Observations of long‐term field‐aligned flow of O+ ions near the equator during summer and winter
Authors: Cladis J. B. , Collin H. L.
Published: February 2004

In the equatorial region of the outer magnetosphere (R > 4 RE), highly asymmetric storm time pitch angle distributions (0° versus 180°) of O+ ions (<20 keV) have been observed near dawn and dusk during winter and summer seasons with ion spectrometers on board the CRRES and AMPTE/CCE satellites. These ion asymmetries, although variable in magnitude and ion energy, persist in the same sense for periods of up to 3 hours. The observed O+ pitch angle distributions suggest a net summer‐to‐winter ion flow in the dawn sector and a net winter‐to‐summer ion flow in the dusk sector. This is consistent with the direction of an induced parallel (to magnetic field B) electric field that accompanies the decay of the tail current or the growth of the Birkeland currents when the Earth's magnetic dipole is tilted toward or away from the Sun. A net magnetic flux due to the tail current (mainly along the ZGSM axis) threads the areas enclosed by these field lines only when the areas are inclined to the (X, Y)GSM plane. The net flux due to the Birkeland current arises because this current system is asymmetric during summer and winter, being stronger in the hemisphere of the sunlit polar ionosphere. To date, one of the 17 observed events has been examined in detail. IMP 8 data in the solar wind and onboard magnetic field measurements were used to estimate the field‐aligned electromotive force (EMF) induced by each of these current systems. The EMFs were found to be appropriate to account for the asymmetry of the ions. Furthermore, the general features of the observed pitch angle distributions were confirmed with a computer program that simulated the response of initially trapped ions to a parallel electric field.