bluuebunny/crossref_metadata_2025 huggingface dataset:
Radial diffusion in magnetodiscs
Authors: Birmingham Thomas J.
Published: February 2008

We consider further the orbits of charged particles in magnetodiscs. The bounce motion is assumed adiabatic except for transits of a small equatorial region of weak magnetic field strength and high field curvature. Previous theory and modeling have shown that particles scatter randomly in pitch angle with each passage through the equator. A peaked distribution thus diffuses in pitch angle on the time scale of many bounces. We argue in this paper that spatial diffusion is a further consequence when the magnetodisc has a longitudinal asymmetry. A general expression for DLL, the diffusion of equatorial crossing radii, is derived. DLL is evaluated explicitly for ions in Jupiter's 20–35 RJ magnetodisc, assumed to be represented by Connerney et al.'s Voyager model plus a small image dipole asymmetry. Rates are energy, species, and space dependent but can average as much as a few tenths of a planetary radius per bounce period.
Simulations and observations of plasma depletion, ion composition, and airglow emissions in two auroral ionospheric depletion experiments
Authors: Yau A. W. , Whalen B. A. , Harris F. R. , Gattinger R. L. , Pongratz M. B. , Bernhardt P. A.
Published: February 2008

In an ionospheric depletion experiment where chemically reactive vapors such as H2O and CO2 are injected into the O+ dominant F region to accelerate the plasma recombination rate and to reduce the plasma density, the ion composition in the depleted region is modified, and photometric emissions are produced. We compare in situ ion composition, density, and photometric measurements from two ionospheric depletion experiments with predictions from chemical modeling. The two injections, Waterhole I and III, were part of an auroral perturbation experiment and occurred in different ambient conditions. In both injections a core region of greater than fivefold plasma depletion was observed over ≈5‐km diameter within seconds of the injection, surrounded by an outer region of less drastic and slower depletion. In Waterhole I the plasma density was depleted tenfold over a 30‐km diameter region after 2 min. The ambient O+ density was drastically reduced, and the molecular O2+ abundance was enhanced fivefold in the depletion region. OH airglow emission associated with the depletion was observed with a peak emission intensity of ≈1 kR. In Waterhole III the ambient density was a decade lower, and the plasma depletion was less drastic, being twofold over 30 km after 2 min. The airglow emissions were also much less intense and below measurement sensitivity (30 R for the OH 306.4‐nm emission; 50 R for the 630.0‐nm emission). In the chemical model the major ion chemical production and loss processes in the injection are considered, and the continuity equations for major ion and airglow‐producing species are solved numerically to simulate the temporal‐spatial profiles of plasma depletion, ion composition, and airglow emissions. The simulations indicate that the overall depletion rates in different parts of the depletion region are governed by different parameters.
Particle acceleration in Saturn's outer magnetosphere: In memoriam Alois Schardt
Authors: Schardt A. W. , Kurth W. S. , Lepping R. P. , Maclennan C. G.
Published: February 2008

Numerous events attributed to particle acceleration were observed in the Saturnian magnetosphere during the outbound pass of Voyager 2 between ∼20 RS and the magnetopause. For observations made between 25 and 40 RS a close correlation exists between impulsive injections of 0.35‐ to 2‐MeV electrons, increases in the hot ion flux (28‐ to 215‐keV), a dip in the magnetic field magnitude, and a signal in the 562‐Hz channel of the Voyager 2 plasma wave instrument. Our explanation for these observations involves the release of magnetic energy which heats the ion component of the plasma and accelerates electrons to energies of about 2 MeV. The field depressions are caused by the diamagnetic effects of the hot ions. The ∼562‐Hz noise is due to electron plasma oscillations at the local plasma frequency excited by the streaming electrons. The noise moved to higher‐frequency channels as the electron density rose when Voyager entered the dawnside boundary layer. Time dispersions seen between several of the effects indicate that the site of the acceleration event may have been at some distance from the point of observation. Interplanetary conditions during this time interval were highly unusual, since Saturn's magnetosphere was evidently embedded in the distant magnetotail of Jupiter.
Magnetic disturbances in the vicinity of synchronous orbit and the substorm current wedge: A case study
Authors: Singer Howard J. , Hughes W. J. , Gelpi C. , Ledley B. G.
Published: February 2008

Some features of Pi 2 pulsations and magnetic bays observed on the ground using the mid‐latitude Air Force Geophysics Laboratory (AFGL) Magnetometer Network previously have been shown to agree with ideas concerning the development of a substorm current wedge during isolated substorm onsets. In particular, the H and D component magnetic bay perturbations and the azimuths of the horizontal component of the Pi 2 polarization ellipse observed by the AFGL Network have been used to locate the approximate center and azimuthal extent of the substorm current wedge. In this paper we apply this technique to a well‐defined substorm to locate the substorm current wedge position relative to the GOES 2 and 3 synchronous satellites at L ≃ 6.6 and the nearby SCATHA satellite at L ≃ 8.1 to explain the magnetic features observed by those satellites. We find that the two satellites within the current wedge, at a position just after local midnight, simultaneously observe a dipolarization of the background field configuration beginning at the time of the substorm onset; however, the impulsive, irregular oscillations frequently observed at substorm onset are only observed by one of the satellites. The other satellite within the wedge observes a similar disturbance after a 3‐minute delay which may be attributed to an intensification or movement of currents in the downward current portion of the substorm current wedge. The satellite west of and outside the current wedge observes no significant change in the magnetic field during the event. These observations illustrate the difficulties of interpreting single‐satellite measurements of substorm effects. If magnetic data from only the satellite west of the wedge had been available, the substorm would have gone undetected. Furthermore, an incorrect substorm onset time could have been deduced from one of the satellites within the wedge.
The analysis of SPIRE measurements of atmospheric limb CO2(ν2) fluorescence
Authors: Caledonia G. E. , Green B. D. , Nadile R. M.
Published: February 2008

Spectrally resolved, infrared limb emission measurements of upper atmospheric CO2(ν2) band radiation have been analyzed to determine altitude profiles of excited state densities and rotational temperatures.
Analysis of O7+/O6+ observations in the solar wind
Authors: Ogilvie K. W.
Published: February 2008

The ratio of O7+/O6+ in the solar wind can be used to deduce the temperature at the point in the corona where the charge states become “frozen” in the flow, and its determination therefore depends both upon the characteristics of the instrument and those of the source. The apparent increase of the ratio O7+/O6+ observed in the solar wind by K. W. Ogilvie and C. Vogt (1980) at speeds above 450 km s−1 has been reevaluated using a greatly expanded data set. The ratio observed for solar wind speeds less than 450 km s−1 corresponds to an electron temperature of 1.7 ± 0.2×106°K in the region of “freezing in” of the flow, as was previously reported. At higher speeds, when the kinetic temperature of the solar wind often exceeds 2–3×105°K, instrumental effects become large and corrections difficult to determine.
Time‐dependent flows in the renovated model of the magnetosphere
Authors: Atkinson G.
Published: February 2008

A revised empirical model of the magnetosphere, based on observations of Birkeland currents, is presented with earthward flow in the plasma sheet confined to the duskside. Convection (E × B drift) carries tail‐associated features in the sunward direction. This competes with Birkeland‐current‐associated plasma depletion (divergence of ion drifts), which allows flux tubes to collapse to a more dipolar form, thereby moving tail‐associated features in the antisunward direction. In substorm intensifications, depletion dominates, and the taillike region recedes from the earth. The recession appears to be unstable. A quantitative relationship between Birkeland currents and convection is developed and is used on published results. It indicates that 4×1020 ions drift off a plasma sheet flux tube as it collapses from taillike to dipolar. Application of the quantitative relationship to a published set of presubstorm and expansion data raises the following points: (1) Consistency is high, suggesting that the assumptions of hot protons, cold electrons, and a pressure‐dominated equilibrium apply in the plasma sheet. (2) The breakup instability condition is that the taillike region is just beginning to recede. (3) The changes at breakup can be interpreted as either the breakdown of the Birkeland current sheet approximation or the formation of a near‐earth neutral line.
STARE and SABRE observations of a Pc 5 pulsation with large azimuthal wave number during a storm period
Authors: Misra K. D. , Nielsen E. , Lühr L.
Published: February 2008

The STARE (Scandinavian Twin Auroral Radar Experiment) and SABRE (Sweden and Britain Radar Experiment) systems have been used to estimate electric fields in the high‐latitude ionosphere during a Pc 5 geomagnetic pulsation. With each coherent radar system, measurements are made over a large area (∼200,000 km²) with good spatial resolution of 20×20 km². Combined, the systems cover a range in L value from 4.4 to 8.8. The observations were made during the local magnetic afternoon hours in the recovery phase of a magnetic storm. The period is essentially constant with latitude (geographic latitude 64.6°–70.2°), having a mean value of 365 s for both toroidal and poloidal components. The azimuthal wave number is large (∼24), and the phase velocity is directed equatorward and westward. When mapped in a dipole field into the equatorial plane, at L = 6.14, the wavelengths were found to be 9177–9585 km for both the STARE and SABRE field of view. The pulsation amplitude is <8 mV m−1. The pulsation amplitude maximizes at 69° geographic latitude for STARE and at 67.6° latitude for SABRE (corresponding geomagnetic latitude 66.4°–66°). The variation of azimuthal phase velocity with L resembles the gradient curvature drift of energetic hot protons. The observations are compared with the drift wave instability of compressional and shear Alfvén mode in a high β plasma.
CNA pulsations associated with quasi‐periodic VLF emissions
Authors: Sato Natsuo , Shibuya Senkichi , Maezawa Kiyoshi , Higuchi Yoshihiro , Tonegawa Yutaka
Published: February 2008

Quasi‐periodic variation of cosmic radio noise absorption (called CNA pulsation), measured by a fast‐response 30‐MHz riometer at Syowa Station in Antarctica (L ∼ 6), was examined. CNA pulsations with the period of 10–500 s are observed mostly during morning hours. Most of the CNA pulsations are associated with magnetic pulsations and quasi‐periodic (QP) VLF emissions. The examination of the relationships among CNA pulsation, QP emission, and magnetic pulsation, using the cross‐correlation analysis method, shows that some CNA pulsations with the period of 10–500 s are well related to magnetic pulsations and QP emissions. In these cases, CNA pulsations are more correlated with the D component of magnetic pulsations than with the H component. It is also shown that the phase of the CNA pulsation lags behind QP emissions by 60–120° independent of pulsation period.
Solar cycle variability of exospheric temperature at Millstone Hill between 1970 and 1980
Authors: Hagan M. E. , Oliver W. L.
Published: February 2008

Neutral exospheric temperatures from Thomson scatter measurements made between 1970 and 1980 at Millstone Hill (42.6°N) are examined for seasonal and solar cycle variations during geomagnetically undisturbed times. Analysis reveals unexpectedly large diurnal temperature amplitudes during solar cycle 21 at moderate to high levels of solar activity
pointing to the increased role of ion drag in the adiabatic compression of the thermosphere. A heuristic model assuming momentum balance between the pressure gradient and ion drag, and thermal balance between EUV heating, its vertical conduction and adiabatic compression serves to illustrate modulation of adiabatic cooling efficiency by enhanced ion drag effect.
Venus mesosphere and thermosphere: 1. Heat budget and thermal structure
Authors: Dickinson Robert E. , Bougher Stephen W.
Published: February 2008

The calculation of a reasonable global average temperature structure is the first step in the simulation of the dynamics and observed characteristics of Venus's upper mesosphere and thermosphere. Various physical processes that influence Venus temperatures are examined using updated inputs and new parameterizations. We demonstrate, using a one‐dimensional NLTE radiative transfer code, that temperatures observed during the Pioneer Venus mission can be reproduced using an O‐CO2 collisional excitation rate coefficient of 4×10−13 cm³ s−1 for strong 15‐µm cooling balanced by 9.5%‐efficient EUV heating. Cooling by eddy mixing is at best a minor contribution to the total cooling required. Exospheric temperatures are calculated to vary by 60 K or less over a solar cycle, in rough agreement with observations. We conclude that CO2 cooling effectively buffers against such solar perturbations, due to its nonlinear temperature dependence.
Observations of field‐aligned currents, waves, and electric fields at substorm onset
Authors: Smits D. P. , Hughes W. J. , Cattell C. A. , Russell C. T.
Published: February 2008

Substorm onsets, identified by Pi 2 pulsations observed on the Air Force Geophysics Laboratory Magnetometer Network, are studied using magnetometer and electric field data from ISEE 1 as well as magnetometer data from the geosynchronous satellites GOES 2 and 3. The mid‐latitude magnetometer data provides the means of both timing and locating the substorm onset so that the spacecraft locations with respect to the substorm current systems are known. During two intervals, each containing several onsets or intensifications, ISEE 1 observed field‐aligned current signatures beginning simultaneously with the mid‐latitude Pi 2 pulsation. Close to the earth broadband bursts of wave noise were observed in the electric field data whenever field‐aligned currents were detected. One onset occurred when ISEE 1 and GOES 2 were on the same field line but in opposite hemispheres. During this onset ISEE 1 and GOES 2 saw magnetic signatures which appear to be due to conjugate field‐aligned currents flowing out of the western end of the westward auroral electrojets. The ISEE 1 signature is of a line current moving westward past the spacecraft. During the other interval, ISEE 1 was in the near‐tail region near the midnight meridian. Plasma data confirms that the plasma sheet thinned and subsequently expanded at onset. Electric field data shows that the plasma moved in the opposite direction to the plasma sheet boundary as the boundary expanded which implies that there must have been an abundant source of hot plasma present. The plasma motion was towards the center of the plasma sheet and earthwards and consisted of a series of pulses rather than a steady flow.
Scintillations associated with bottomside sinusoidal irregularities in the equatorial F region
Authors: Basu Santimay , Basu Sunanda , Valladares C. E. , DasGupta A. , Whitney H. E.
Published: February 2008

A new category of equatorial F region plasma irregularities characterized by nearly sinusoidal waveforms in the ion number density Ni observed by the Atmosphere Explorer satellites has recently been discussed by Valladares et al. (1983) and Cragin et al. (1985). We present multisatellite scintillation observations made at Huancayo, Peru, and spaced‐receiver drift measurements made at Ancon, Peru, associated with such irregularities observed by AE‐E on a few nights in December 1979. Utilizing ray paths to various geostationary satellites, it is found that the irregularities grow and decay almost simultaneously in long‐lived patches extending at least 1000 km in the E‐W direction. The scintillations continue for a period of almost 6 hours, at a level that varies from moderate to fairly intense (S4 = 0.1–0.8 at 250 MHz), and these S4 fluctuations are quite well correlated, even over a distance of 1000 km. The irregularities constituting the large patch are found to drift eastward at a velocity of approximately 140 m/s. This and other such events are accompanied by the frequency spread signatures on Huancayo ionograms, as previously reported. The unique feature of the Fourier spectra associated with such bottomside sinusoidal (BSS) irregularities is the presence of Fresnel oscillations, which allow a determination of the velocity of the diffraction pattern perpendicular to the direction of the ray from the satellite to the ground station. The velocity so determined agrees well with the results of simultaneously performed spaced‐receiver drift measurements. The presence of Fresnel oscillations indicates that the BSS irregularities occur in a relatively thin layer. However, while the scintillation data indicate a high‐frequency roll‐off with a spectral index of the order of −3 to −4, the in situ data tend to indicate that the index is of the order of −5 to −6. Modeling studies are necessary to resolve this difference.
Cosmic rays near the heliospheric current sheet: 2. An ensemble approach to comparing theory and observation
Authors: Jokipii J. R. , Kota J.
Published: February 2008

In this paper we report on a quantitative comparison between theoretical predictions and observations of the intensity of galactic cosmic rays near the interplanetary current sheet. In the present work, comparison of our model calculations is made with a statistical analysis of observations of galactic cosmic rays at the earth and the simultaneous position of the current sheet. Since the observations were made over a period of several years, the inclination of the current sheet varied considerably, and comparison with any one model calculation referring to a given inclination would not be appropriate. In this paper we report the expected values from our model, using an ensemble of different current sheet inclinations, in order to make the analysis of the computations approximate the method used to analyze the data. We find agreement between theory and observation at energies of the order of a few GeV.
Latitudinal extent of the heliospheric current sheet and modulation of galactic cosmic rays
Authors: Smith Edward J. , Thomas Barry T.
Published: February 2008

The possible relation between changes in the maximum heliographic latitude of the heliospheric current sheet during solar maximum and cosmic ray modulation has been investigated. A causal relation between the intensity of galactic cosmic rays and the inclination or maximum latitude of the current sheet is an important feature of the gradient drift model. The maximum latitudinal extent in the northern and southern solar hemispheres during 1976–1982 was determined from current sheet contours at the solar source surface (2.5 solar radii) derived by Stanford University by extrapolating the photospheric magnetic fields into the corona using potential theory. The maxima in the north and south were found to be reasonably symmetrical about the solar equator except for an interval of about one year near sunspot maximum when the sun's polar caps were reversing. It was therefore possible to take the difference between the maximum latitudes in the north and south as a measure of current sheet inclination. The latitude difference so derived showed the evolution of the current sheet to high latitudes near sunspot maximum with a subsequent return to lower inclinations, but with reversed polarity, following maximum. In addition to this general behavior, a number of episodes occurred in which the latitude difference changed significantly over intervals of one year or less. When the changing latitude differences from 1976 through 1982 were compared with Deep River neutron monitor observations (corresponding to cosmic rays with E ≥ 1 GeV), a significant correlation was found. Changes in maximum latitude also correlate well with cosmic ray measurements being made at lower energies (E > 200 MeV) by Pioneer 10 between 10 and 30 AU. The correlation is improved by introducing a delay corresponding to the time required for the solar wind to propagate from the sun to the spacecraft. The sensitivity of the cosmic ray intensity to changes in the current sheet is significantly different before and after the reversal in the polar cap and heliospheric magnetic field, the sensitivity being enhanced after the reversal when the field is inward in the north. The existence of this correlation and its sensitivity to the polarity of the solar heliospheric field provide significant support for the gradient drift model of cosmic ray modulation.
Survey of 0.1‐ to 16‐keV/e plasma sheet ion composition
Authors: Lennartsson W. , Shelley E. G.
Published: February 2008

A large statistical survey of the 0.1‐ to 16‐keV/e plasma sheet ion composition has been carried out using data obtained by the Plasma Composition Experiment on ISEE 1 between 10 and 23 RE during 1978 and 1979. This survey includes more than 10 times the quantity of data used in earlier studies of the same topic and makes it possible to investigate in finer detail the relationship between the ion composition and the substorm activity. The larger data base also makes it possible for the first time to study the spatial distribution of the principal ion species. As found in previous studies, the ion composition has a large variance at any given value of the AE index, but a number of distinct trends emerge when the data are averaged at each activity level. During quiet conditions the plasma sheet is dominated by ions of solar origin (H+ and He++), as found in earlier studies, and these ions are most numerous during extended periods of very low activity (AE ≲ 30 γ). The quiet time density of these ions is particularly large in the flanks of the plasma sheet (GSM Y ∼ ± 10 RE), where it is about twice as large as it is near the central axis of the plasma sheet (Y = Z = 0). In contrast, the energy of these ions peaks near the central axis. When the AE index approaches zero for extended periods (several hours), the energy of the solar ions approach values that are similar to solar wind kinetic energies (∼1 keV/nucleon). Conversely, as the AE index increases, the solar ion energy increases. When a correction is made for the finite instrumental energy window, the data indicate that the solar H+ and He++, on the average, retain more nearly equal energy/nucleon than equal energy/charge. With increasing AE index the solar ion density decreases at all GSM Z, on the average, and the solar ions are partially replaced by ions of terrestrial origin. The most conspicuous of the terrestrial ions is the O+, which has an average energy of about 3–4 keV/ion at all activity levels. The increase in the O+ density is strongest around local midnight (GSM |Y| ≲ 5 RE), where the O+ often becomes the most numerous ion during strongly disturbed conditions (AE ∼ 1000 γ). At each level of substorm activity the average O+ density has a long‐term variability, increasing by a factor of 3 between early 1978 and early 1979, possibly in response to changing solar EUV radiation.
SSC‐excited pulsations recorded near noon on GEOS 2 and on the ground (CDAW 6)
Authors: Wedeken U. , Voelker H. , Knott K. , Lester M.
Published: February 2008

The SSC occurring on March 22, 1979, at 0826 UT had an unusually sharp onset in Scandinavia, in Middle Europe and in experiments on the geostationary satellite GEOS 2, which was near noon, local magnetic time. The ground magnetometer stations showed a small preimpulse which started ∼5 s before the main impulse. Both impulses needed ∼2 s to “propagate” from ground stations at L = 6.3–4.6. Search coil magnetometers indicate a very small precursor in northern Finland (L ∼ 4.4–6.0) which started ∼15–20 s before the main impulse. This small precursor also occurred close to the time of the SSC onset at GEOS 2. We interpret this precursor as an effect of precipitating electrons changing the ionospheric conductivity in a localized region. The main impulse triggered damped magnetic pulsations (Psc) with periods near 160 s and 50 s visible in northern Scandinavia and the electric field detector on GEOS 2. Furthermore, the magnetic field and the energetic ions at GEOS observed pulsations with periods near 80 s, but these could only be observed at the northernmost ground stations. There are several indications that the first three harmonics of standing hydromagnetic waves are detected. They may correspond to periodic oscillations of the subsolar point or eigenperiods of the SSC‐excited fast mode (compressional cavity resonance). The tentatively identified second harmonic wave (period ∼80 s) is indicative of a bounce resonance of ring current protons. Inside the plasmasphere the dominant period of a superimposed Psc 4 event increased with latitude for the H component indicating several toroidal eigenoscillations.
Medium‐energy electrons and heavy ions in Jupiter's magnetosphere: Effects of lower hybrid wave‐particle interactions
Authors: Barbosa D. D.
Published: February 2008

A theory of medium‐energy (∼ keV) electrons and heavy ions in Jupiter's magnetosphere is presented. Lower hybrid waves are generated by the combined effects of a ring instability of neutral wind pickup ions and the modified two‐stream instability associated with transport of cool Iogenic plasma. The quasi‐linear energy diffusion coefficient for lower hybrid wave‐particle interactions is evaluated, and several solutions to the diffusion equation are given. Calculations based on measured wave properties show that the noise modifies substantially the particle distribution functions. The effects are to accelerate superthermal ions and electrons to keV energies and to thermalize the pickup ions on time scales comparable to the particle residence time. The [S2+]/[S+] ratio at medium energies is a measure of the relative contribution from Iogenic thermal plasma and neutral wind ions, and this important quantity should be determined from future measurements. The theory also predicts a preferential acceleration of heavy ions with an acceleration time that scales inversely with the root of the ion mass. Electrons accelerated by the process contribute to further reionization of the neutral wind by electron impact, thus providing a possible confirmation of Alfvén's critical velocity effect in the Jovian magnetosphere.
Projections of plasma cloud structures and their spectra
Authors: Zabusky Norman J. , Hyman Ellis , Mulbrandon Margaret
Published: February 2008

We have established a relationship between the asymptotic (k → ∞) spectral index of idealized optically thin localized radiating sources, n(x, t) and their scans, f. For piecewise‐constant (pc) clouds, if the envelope of the one‐dimensional asymptotic spectrum of the power of n is proportional to k−2, then the asymptotic spectrum of the scan depends upon the character of the contour at point x0 = (x0, y0) where it is tangent at an extremal ray. If the contour at (x0) behaves like y ‐ y0 = γ(x ‐ x0)τ, then the asymptotic spectral envelope of the power spectral density (PSD) of the scan varies as k−2(τ+1). For a convex curve with finite curvature at x0, we have τ = 1/2 and obtain the well‐known result, k−3. In addition, if the radiating density varies spatially in a cloud flank with a power law μ, then the asymptotic spectral envelope of the scan's PSD varies as k−2(1 +τ+μ). Thus by examining projections of radiating clouds, one cannot distinguish between local gradients of density and the local shape of the boundary curve of the cloud. We also apply the result to the interpretation of optical scans from two PLACES events. These show the onset of steepening and striations. However, the accurately resolved wave number range is insufficient for determining the spectrum associated with the nonlinear dynamics of striation evolution.
Shock jump conditions modified by pressure anisotropy and heat flux for Earth's bowshock
Authors: Lyu L. H. , Kan J. R.
Published: February 2008

The effects of pressure anisotropy and backstreaming heat flux on the fast and slow shock jump conditions are studied parametrically. The fast shock jump condition can be brought into agreement with earth bow shock observations by (1) q1 < q2 in the quasi‐parallel shock region (θBN < 45°), and (2) q1 > q2 for Alfvén Mach number MA > 10 in the quasi‐perpendicular shock region (θBN > 45°) where q1 and q2 are the magnitude of the backstreaming heat flux on the upstream and downstream sides, respectively and θBN is the angle between the upstream magnetic field and the shock normal. The slow shock jump condition is insensitive to the heat flux. However, slow shocks cease to exist if p∥ = p⊥ upstream and p∥ > p⊥ downstream where p∥ and p⊥ are the pressure parallel and perpendicular to the local magnetic field. The minimum critical Mach number imposed by the entropy principle is shown to increase from unity as q1 becomes increasingly greater than q2.
ISEE 1 and 2 observations of Birkeland currents in the Earth's inner magnetosphere
Authors: Kelly T. J. , Russell C. T. , Walker R. J. , Parks G. K. , Gosling J. T.
Published: February 2008

Data from the ISEE 1 and 2 magnetometers were examined for evidence of Birkeland currents in the earth's inner magnetosphere, from 2.4‐ to 7‐RE radial distance. This survey reports on the two spacecraft measurements of the properties of the Birkeland currents. Both line and sheetlike currents were observed. Large‐scale Birkeland current signatures were observed throughout the 18 hours of local time in which the data coverage was sufficient. In local time this coverage was from local noon through dusk to local midnight to the dawn meridian. In approximately 27% of the spacecraft's outbound passes, large‐scale current structures were identifiable as parts of the region 1 or region 2 current systems. In approximately 19% of the outbound passes, no distinguishable region 1 or region 2 Birkeland currents were observed. These cases occurred during periods of low magnetic activity. In the remaining 54%, both multiple current structures and ambiguous magnetic signatures were observed. The multiple structures were generally of smaller scale than the structures identified as the region 1 and 2 current structures, and the ambiguous cases had no clear sources or causes. For these cases the spatial and temporal variations were not easily separated. Two passes through the dayside magnetosphere showed Birkeland current layers at the interface between the outer plasma sheet convection boundary and a plasma mantlelike boundary region, located inside the postnoon magnetopause. These two cases provided clear examples of region 1 Birkeland current flow in the dayside magnetosphere.
The effect of different resistivity models on magnetotail dynamics
Authors: Lyon J. G. , Fedder J. A. , Huba J. D.
Published: February 2008

Two‐dimensional, time‐dependent MHD simulations of the interaction between the solar wind and the earth's magnetosphere have been performed to study magnetotail dynamics with varying forms of anomalous resistivity. In general, the resulting models conform to the neutral line model proposed for substorms with near‐earth x points and high‐speed tailward flows occurring in the magnetotail. However, in the case where Joule heating is included in the physical description of the system, the near‐earth x point never moves far down tail, and high‐speed tailward flows never stop. Only in the case where there is no Joule heating does the x point move down tail. Simultaneously, the high‐speed tailward flows cease. These results indicate that the mechanism of energy dissipation can have an important effect on reconnection processes and the global magnetospheric dynamics.
Extreme and far ultraviolet emissions from the polar cap
Authors: Chakrabarti Supriya
Published: February 2008

Satellite observations of polar cap emissions in the 800 to 1400Å band are reported. The data were obtained along the noon‐midnight meridian on March 22, 1979, by the EUV spectrometer on board the STP78‐1 satellite. The time‐averaged spectrum shows no molecular band emissions. Several bright atomic oxygen lines are present in the spectrum. Weak signals from atomic nitrogen are also seen. We speculate that these emissions are caused by particles having similar characteristic energy to those producing emissions observed in the dayside cusp during the same pass.
Analyses of convective flows and spatial gradients in energetic ion observations
Authors: Mitchell D. G. , Lundin R. , Williams D. J.
Published: February 2008

We present two techniques for analyzing bulk flows, ϵ × B convection and pressure gradients in magnetospheric plasmas using the measured anisotropy of three‐dimensional energetic ion (>30 keV) distributions from the Medium Energy Particle Instrument experiment on the ISEE 1 spacecraft. The first technique uses a Lorentz (Compton ‐ Getting) transform into the appropriate rest frame (plasma or ϵ × B) in an iterative fashion, converging to the unique velocity that removes the first‐order anisotropy from the flux angular distribution. Gradients are distinguished from ϵ × B convection on the basis of the dependence on energy of the anisotropy. The gradients are described by assuming an exponential spatial dependence and obtaining a characteristic gradient scale length. The second technique involves a first moment of the distribution function, in which the shift in velocity space of the distribution function is obtained at two phase space density values. Based upon the energy (velocity) dependence of the shift, it can be determined whether convection or a gradient is responsible for the shift. By assuming an MHD generator condition (which may apply particularly in the magnetospheric boundary layer), the convection contribution to the velocity shift can be assumed to be perpendicular to the pressure gradient contribution, thus rendering the two shifts separable. Using this assumption, we obtain both a gradient scale and a lower limit to the convection electric field in one (noniterative) calculation.
Convective stabilization of ionospheric plasma clouds
Authors: Drake J. F. , Huba J. D.
Published: February 2008

We derive a stability criterion for the large‐scale structuring of ionospheric plasma clouds due to the E × B gradient drift instability. For the equilibrium we consider a cylindrical two‐dimensional water bag cloud aligned along a uniform magnetic field that is polarized by a uniform neutral wind. We perform a stability analysis that allows three‐dimensional perturbations (in r, θ, and z), and we consider both local and global modes. We find that when the parallel wave number kz exceeds a threshold value, exponentially growing poloidal eigenmodes form which are localized on the “backside” of the cloud. This is in contrast to the kz = 0 limit in which there are no exponential solutions. As kz is increased further, the unstable modes localize at a finite angle away from the backside, at a point where the diamagnetic propagation velocity Vd balances the convective flow velocity of the background plasma around the cloud, Vb. We find that the E × B gradient drift instability is stable when Vd > Vb so that the cloud is no longer susceptible to large‐scale structuring. We apply these results to ionospheric barium clouds and estimate that they will cease structuring when L⊥ < (cT/eB)(M + 2)/2Vn where L⊥ is the transverse size of the cloud, T = Te + Ti is the total temperature, M = nc/nb (nc is the cloud density, and nb is the background density), and Vn is the neutral wind velocity. For mid‐latitude barium releases at ∼180 km we estimate L⊥ ∼ 160–480 m, which is consistent with observations.
Near‐midnight observations of nitric oxide δ‐ and γ‐band chemiluminescence
Authors: Tennyson P. D. , Feldman P. D. , Hartig G. F. , Henry R. C.
Published: February 2008

Chemiluminescent nightglow emission of the nitric oxide delta and gamma bands was measured from a sounding rocket launched on April 27, 1981, near local midnight. The integrated band emission rates for this near zenith observation above 205 km were less than 10 Rayleighs. The solar zenith angle was 127°. The branching ratio from the C²Π state to the A²Σ+ state of NO was determined from comparison of the total emission rate of the delta band system to that of the gamma band system and found to be 0.30±0.06. The branching ratios within each of the band systems were found to be consistent with previous theoretical and experimental determinations. The vertical atomic nitrogen distribution, derived with the use of a model atmosphere, was found to have a peak density of 2.0×107 atoms cm−3 at an altitude of 205 km. The analysis of these data indicate the presence of residual NO emission above 270 km at local midnight on the order of 1 Rayleigh of total band emission.
Aspect angle dependence of mean Doppler velocities of 1‐m auroral plasma waves
Authors: Nielsen E.
Published: February 2008

The mean phase velocity of 1‐m unstable plasma waves, excited by the two‐stream instability in the auroral E region, has been measured as a function of aspect angle. The normalized function is consistent with the predicted variation of the phase velocity at threshold of the instability. To reconcile the observations with theoretical predictions of the functional relationship for excitation far beyond threshold, an effective electron collision frequency 6 times the electron‐neutral collision frequency is required.
Plasma and field observations of a Pc 5 wave event
Authors: Waite J. H. , Gallagher D. L. , Chandler M. O. , Olsen R. C. , Comfort R. H. , Johnson J. F. E. , Chappell C. R. , Peterson W. K. , Weimer D. , Shawhan S. D.
Published: February 2008

Micropulsation measurements of a Pc 5 wave event on July 14, 1982, in the afternoon magnetosphere are reported as observed by wave and particle instruments on board the Dynamics Explorer 1 (DE 1) spacecraft. The overall structure of the Pc 5 event as noted in the low‐energy particle and quasi‐static electric field data seems to order the event into two distinct halves. The appearance is reminiscent of a wave packet and suggests a temporal or spatial variation of the micropulsation, which has a scale of 20 min or 3000 km. The wave packet structure is also well correlated with a variation in the pitch angle distribution of the low‐energy plasma with single direction field‐aligned flow associated with the maximum amplitude of the wave packet structure and bidirectional field‐aligned flows associated with the nodal point of the wave packet structure. The field‐aligned velocities of the observed H+, He+, O+, N+, and O++ ions combined with the elliptical E × B wave oscillation in the first half of the event produce a helical motion of the plasma along the field line. In the second half of the event although all ion species are still present, strong magnetospheric convection seems to have a significant effect on the field‐aligned motions and Pc 5 oscillations of the low‐energy plasma. The event is characterized in the low‐energy plasma by a left‐hand to near‐linear polarized rotation of the plasma over the first half of the event. Variations of the eccentricity over the first half of the event which are indicated by both the quasi‐static electric field detector and the low‐energy particle detector suggest that the center of resonance lies just radially inside of the DE 1 orbit (L = 4.7) and just outside of the dayside plasmapause, which appears to be located at L = 4 as indicated by experiments on board the DE 2 spacecraft. Comparisons of the measured Pc 5 wave period to the theoretical period derived from in situ evaluation of the plasma mass loading factor yield values of 190 s and 192 s, respectively. This is consistent with the center of resonance being nearly coincident with the DE 1 orbit. The second half of the event occurs in a region of enhanced magnetospheric convection, which makes determination of the eccentricity of the wave difficult and harder to interpret. The wave is found to be right‐hand or linearly polarized during this portion of the event, which suggests DE 1 is just radially inward of the resonance center. However, disagreement between the measured period of 233 s and the derived period of 189 s does not necessarily support this interpretation. Density variations of plasma along the field line which are hemispherically asymmetric may explain the inconsistencies between the measured and derived period in this half of the event.
Origin of magnetic pulsations associated with regular period VLF pulsations (type 2 QP) observed on the ground at Syowa Station
Authors: Sato Natsuo , Matsudo Takashi
Published: February 2008

Type 2 quasi‐periodic (QP) VLF emissions with fairly regular‐period pulsations sometimes show good coherency with the H component of magnetic pulsations. From the relative‐phase analysis between QP emissions and magnetic pulsations, it is found that the relative phases are independent of the pulsation periods. It is suggested that the magnetic pulsations observed on the ground are generated in the ionosphere. The quasi‐periodic precipitating electron fluxes induced by quasi‐periodic VLF emissions modify the ionospheric conductivity. The quasi‐periodic enhancement of the conductivity in the ionospheric D and E regions would modify the Sqp current flows in the ionosphere, resulting in the magnetic variations observed on the ground.
Subcritical and supercritical interplanetary shocks: Magnetic field and energetic particle observations
Authors: Bavassano‐Cattaneo M. B. , Tsurutani B. T. , Smith E. J. , Lin R. P.
Published: February 2008

We have studied 34 forward interplanetary shocks observed by ISEE 3 during 1978 and 1979. Magnetic field and high‐energy particle data have been used, and for each shock the first critical Mach number has been determined. The first surprising result is that the majority of the observed shocks appear to be supercritical, and consistent with their supercritical character, many shocks have a foot and/or an overshoot in the magnetic field structure. Large‐amplitude low‐frequency waves (period ∼20 s in the spacecraft frame) are commonly observed upstream of all supercritical shocks (except for a few quasi‐perpendicular shocks) and also upstream of the few subcritical shocks. Intense particle events are frequently observed at many shocks: spikes at quasi‐perpendicular shocks and energetic storm particle events associated with quasi‐parallel shocks can be comparably intense. The correlation of the high‐energy particle peak flux with various shock parameters is in agreement with the acceleration mechanisms proposed by previous studies.
Generation of broadband noise in the magnetotail by the beam acoustic instability
Authors: Dusenbery P. B.
Published: February 2008

Analytic properties of the ion beam acoustic instability are evaluated for reasonable magnetotail conditions and are found to be consistent with observations of broadband turbulence not only in the plasma sheet boundary layer but also in the lobes and central plasma sheet regions. Numerical calculations confirm many of the analytic results. Specifically, the dependence of the ion beam acoustic mode on particle distribution models is investigated. Without cold ions present the acoustic mode is stable. With cold streaming ions present the acoustic mode is unstable, but the growth rate is significantly enhanced when warm streaming boundary layer ions are simultaneously present. These results suggest that the interaction between the warm and cold ion streams is responsible for the peak in the electrostatic wave intensities observed within the plasma sheet boundary layer. In addition, the beam acoustic instability, for magnetotail models, is found to be due to resonant, not nonresonant, interactions. Relative streaming effects and dependence on ion species are considered and found to play significant roles in the behavior of beam acoustic growth as a function of wave normal angle and wave number.
Feedback between neutral winds and auroral arc electrodynamics
Authors: Lyons L. R. , Walterscheid R. L.
Published: February 2008

We analyze the feedback between neutral atmospheric winds and the electrodynamics of a stable, discrete auroral arc. We solve simultaneously, as a function of time, the ionospheric current continuity equation and the equation for neutral gas acceleration by ion drag. Our results show that, in general, the electric field in the ionosphere adjusts to neutral wind acceleration so as to keep auroral field‐aligned currents and electron acceleration approximately independent of time. We thus conclude that the neutral winds that develop as a result of the electrodynamical forcing associated with an arc do not significantly affect the intensity of the arc.
Auroral E region plasma waves and elevated electron temperatures
Authors: Fejer B. G. , Providakes J. , Farley D. T. , Swartz W. E.
Published: February 2008

We have observed 3‐m auroral E region plasma waves with a high‐resolution 50‐MHz radar interferometer at Ithaca. During postmidnight periods of very strong magnetic activity at Ottawa (L = 3.5), the backscattered power and Doppler spectra vary rapidly with time and range and may have both mean Doppler shifts and half power widths in excess of 200 Hz (600 m/s). In addition, when the radar and magnetometer data imply that the current is approximately parallel to the radar line of sight, sharp spectral peaks corresponding to phase velocities approaching 1 km/s sometimes appear suddenly. These persist for at most several tens of seconds at any particular range and their amplitude seems to be more strongly dependent on the drift velocity than is the strength of the wide spectral component, which generally broadens substantially before the high‐velocity peak appears. The shape of the peaks and their time evolution lead us to believe that they represent a true two‐stream instability, implying an ion acoustic velocity in the medium of about 900 m/s in extreme cases, which, in turn, means an E region electron temperature of as much as 2000 K or more at latitudes as low as L = 3.5. This interpretation of the data is consistent with recent E region incoherent scatter measurements and helps to resolve some questions raised in connection with earlier coherent radar studies.
A laboratory study of collisional electrostatic ion cyclotron waves
Authors: Suszcynsky D. M. , Cartier S. L. , Merlino R. L. , D'Angelo N.
Published: February 2008

The effects of neutral‐particle collisions on current‐driven electrostatic ion cyclotron (EIC) waves are studied in a Q machine with a cesium (Cs+) plasma. We find that even when νin ≈ 0.3Ωci, EIC waves of substantial amplitude (Δn/n of several percent) can be excited.
Coherent radar observations of a storm sudden commencement having a preliminary reverse impulse
Authors: McDiarmid D. R. , Nielsen E.
Published: February 2008

Observations of the February 4, 1983, storm sudden commencement (ssc) by the STARE and SABRE radar systems are presented. The observing stations were in the dusk sector, where the ssc was accompanied by a preliminary reverse impulse (PRI). The radar data show the PRI to be a consequence of the ssc compression wave producing an initial antisunward flow. The polarization of the ssc electric field in the ionosphere is seen to have both longitudinal and latitudinal structure. The observations are discussed in terms of the propagation of the ssc disturbance throughout the magnetosphere.