bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized equations for the pressure transition and the stand‐off distance of the solar wind terminating shock
Authors: Petelski E. F.
Published: February 2008

It is commonly assumed that the supersonic solar wind terminates in a strong shock at positions where its pressure is balanced by the pressure of the interstellar medium. Presupposing that the solar wind is a fluid dynamic phenomenon, Parker has formulated equations for the stand‐off distance as well as for the pressure on the subsonic side of such a shock. These relations were derived from the simplified Rankine‐Hugoniot relations for highly supersonic (M ≫ 1) shocks, and from the Bernoulli equation as applied to the postshock flow. According to recent findings, however, the distant solar wind is more likely to have small Mach numbers (M ≳1) because it is heated and decelerated by new ions continuously created from interstellar neutral gas. Therefore, Parker's equations are extended to accommodate arbitrary Mach numbers using the unabridged Rankine‐Hugoniot relations, plus the Bernoulli equation for the polytropic postshock flow, and taking into account the influence of ionizing and charge exchange reactions. The equations found are applicable to the solar wind as well as to other stellar winds of any M >1, irrespective of whether or not these winds are interacting with interstellar neutral gas.
The current convective instability as applied to the auroral ionosphere
Authors: Chaturvedi P. K. , Ossakow S. L.
Published: February 2008

We extend the previous analysis of the current convective instability as applied to the diffuse auroral situation (Ossakow and Chaturvedi, 1979) to include ion inertial effects, important at high altitudes; and to the case in which ions are highly collisional (nonmagnetized), a situation that is realized at E region altitudes. In the inertial domain the instability growht rates are comparable to those found in the collision dominated domain. This extends the applicability of the instability process to high altitudes. The relevance of the instability to the E region is discussed. Finally, electromagnetic effects, which can be important for long wavelengths, are considered and are found to be small in ionospheric situations.
Polarization characteristics of hydromagnetic waves at low geomagnetic latitudes
Authors: Lanzerotti, L. J. , Medford L. V. , Maclennan C. G. , Hasegawa T. , Acuna M. H. , Dolce S. R.
Published: February 2008

A field campaign, using three magnetometer stations spaced in latitude around the equatorial magnetic field distance L ∼ 1.9, was conducted in early 1979 to investigate the polarization characteristics of hydromagnetic waves at low geomagnetic latitudes. The magnetic pulsations are observed to have periods in the range ∼20‐25 s and to occur primarily in the local morning hours. Statistically, the polarizations at all three stations were predominantly left handed in the local morning hours and right handed in the local afternoon. At the highest‐latitude station (L ∼ 2) the orientation of the major axis of the polarization ellipse changed from a predominantly NW‐SE direction in the local morning to a mixed NW‐SE/NE‐SW direction in the afternoon. These two statistical results are consistent with the excitation of the waves by the Kelvin‐Helmholtz instability at the magnetopause. However, frequent changes in phase are often observed in the magnetic variations, which result in polarization variations on the time scale of minutes, a situation not readily reconcilable with the Kelvin‐Helmholtz instability. The existence of the waves at very low latitudes, furthermore, places constraints on the damping rate of externally excited surface waves inside the magnetosphere. We conclude that present theories for hydromagnetic waves in the geomagnetosphere cannot readily incorporate all of these low‐latitude results.
Field‐aligned currents and electric fields observed in the region of the dayside cusp
Authors: Doyle M. A. , Rich F. J. , Burke W. J. , Smiddy M.
Published: February 2008

Thirteen passes of the S3‐2 satellite, approximately along the noon‐midnight meridian, during January 1976 through the region of the dayside cusp have been studied: eight in the northern hemisphere and five in the southern hemisphere. For all passes the IMF By < 0, and for all but three passes, Kp ≤3−, indicating relatively quiet times. Examination of the east‐west component of the magnetic field deflections indicates an extra current sheet, located poleward of region 1 and carrying current in the opposite sense, on the postnoonside (prenoonside) in the northern (southern) hemisphere but no additional current prenoon (postnoon) in the northern (southern) hemisphere. The component of the convection electric field along the satellite trajectory (north‐south) shows that convection reversals occur 2°–3° equatorward of the boundary between this extra current sheet and the region 1 field‐aligned current sheet. A boundary marking the transition from quasi‐trapped to untrapped 100‐keV protons was determined. The extra field‐aligned current sheet lies almost entirely poleward of this transition boundary, suggesting that it may lie in a region of open magnetic field lines. Fine scale structure of the magnetic deflections indicates numerous field‐aligned sheet and line currents embedded in the large‐scale field‐aligned current sheets.
The penetration of convection electric fields to the latitude of Millstone Hill (Λ=56°)
Authors: Wand R. H. , Evans J. V.
Published: February 2008

Incoherent scatter observations of E and F region drifts over Millstone Hill (Λ=56°) were made routinely from May 1976 to November 1977 at monthly intervals. Some 667 hours of data gathered over this 19‐month period has been analyzed to yield the F region polarization electric field in the magnetic south and magnetic east directions. In an effort to determine what part of the observed fields can be attributed to the penetration of auroral convection fields to the latitude of Millstone, these data have been used to construct an analytical model of these two field components having a time of day and Kp dependence (Wand, this issue). To guide the interpretation, mean convection fields also were constructed for Λ=60°, 65°, 70° and 75°, using data gathered between May 1978 and December 1978 from a separate series of experiments (Evans et al., 1979, 1980). The results suggest that, under quiet magnetic conditions, there is no significant penetration of the magnetospheric electric field to the latitude of Millstone at any time of day except possibly for the southward component of the field at night. During magnetically disturbed periods the incremental fields observed over Millstone exhibit all the gross characteristics associated with convection fields seen at higher latitudes. The results also indicate an eastward rotation of the convection pattern of about 4 hours in local time in going from quiet or moderate conditions (Kp ∼ 2+) at Λ=65° to disturbed conditions (Kp=5) at Λ=56°.
Magnetic loop behind an interplanetary shock: Voyager, Helios, and IMP 8 observations
Authors: Burlaga L. , Sittler E. , Mariani F. , Schwenn R.
Published: February 2008

Magnetic field and plasma data from five spacecraft (Voyager 1 and 2, Helios 1 and 2, and IMP 8) were used to analyze the flow behind an interplanetary shock. The shock was followed by a turbulent sheath in which there were large fluctuations in both the strength and the direction of the magnetic field. This in turn was followed by a region (magnetic cloud) in which the magnetic field vectors were observed to change by rotating nearly parallel to a plane, consistent with the passage of a magnetic loop. This loop extended at least 30° in longitude between 1 and 2 AU, and its radial dimension was approximately 0.5 AU. In the cloud the field strength was high, and the density and temperature were relatively low. Thus the dominant pressure in the cloud was that of the magnetic field. The total pressure inside the cloud was higher than outside, implying that the cloud was expanding as it moved outward, even at the distance of 2 AU. The momentum flux of the cloud at 2 AU was not higher than that of the preshock plasma, indicating that the cloud was not driving the shock at this distance. It is possible, however, that the shock was driven by the cloud closer to the sun where the cloud may have moved faster. An extraordinary filament was observed at the rear of the cloud. It was bounded by current sheets whose orientations were preserved over at least 0.12 AU and which were related to the plane of maximum variance of the magnetic field in the cloud.
Mechanism of parallel electric fields inferred from observations
Authors: Yeh H.‐C. , Hill T. W.
Published: February 2008

An analysis of satellite data from regions of upward Birkeland (magnetic‐field‐aligned) current shows that the typical magnetic‐field‐aligned potential drop in the auroral zone is no larger than required to provide direct acceleration of magnetospheric electrons by the field‐aligned electric field against the upward magnetic mirror force to produce the observed upward Birkeland current. A model of simple electrostatic acceleration without anomalous resistivity predicts observable relations between parallel current and parallel potential drop and between energy deposition and parallel potential drop. The temperature, density, and species of the unaccelerated charge carriers are the relevant parameters of the model.

Simultaneous measurements of electron precipitation and ion drift velocities on the satellites Atmosphere Explorer C and D were used to test these relations. In a steady state the divergence of ionospheric currents must be compensated by Birkeland currents. The model current‐voltage relation was applied to predict the densities of the primary charge carriers (i.e., plasma sheet electrons above the acceleration region for upward currents). In cases involving thin arc structures, where the reliable estimation of the divergence of ionospheric current is difficult and the steady‐state assumption may not apply, the precipitating energy flux versus voltage relation was used instead to predict the densities of the unaccelerated plasma sheet electrons. Within the experimental uncertainties, reasonable agreement is found between these predicted densities and those inferred directly from the simultaneous data of the Low‐Energy Electron Experiment. These results are interpreted as indicating that anomalous resistivity is not important in determining the magnitude of the field‐aligned potential drop in the auroral zone.
Goniometric direction finding for low‐latitude whistlers and their propagation mechanism
Authors: Hayakawa Masashi , Tanaka Yoshihito , Okada Toshimi , Iwai Akira
Published: February 2008

In order to study the propagation mechanism of low‐latitude whistlers, their ionospheric exit points have been determined by a triangulation of the goniometer‐polarimeter systems at the two stations of Kagoshima (geomagnetic coordinates 20°24′N, 197°53′E; L=1.22) and Ohgata (22°06′N, 199°51′E; L=1.25), supplemented by the routine observation at Sakushima (24°08′N, 203°19′E; L=1.28). Detailed analyses were made for January 23, 1978, when the whistler activity was most enhanced during one month’s measurement. The difference in temporal variations of the occurrence rate at the three stations and the temporal variation of the single‐valued dispersion enabled us to subdivide the daytime whistlers into three characteristic time intervals and the nighttime ones into two intervals. The characteristics during the whole intervals were found to be satisfactorily accounted for in terms of ducted propagation. The azimuth measurements have indicated that there is present a discontinuity in the characteristics between each two intervals, indicative of the different duct group for different time intervals; in other words, indicative of the identity and persistency of ducts for each period. Then we obtained the ‘apparent lifetime’ of ducts of 1–2 hours. During the last interval for the daytime whistlers, a few ducts were found to be aligned on a certain latitude, which showed a temporal movement, or first increased and then decreased. The simultaneous presence of a few ducts at the same latitude is also seen at night.
Sputter ejection of matter from Io
Authors: Haff P. K. , Watson C. C. , Yung Yuk L.
Published: February 2008

The direct collisional interaction of magnetospheric particles with Io will lead to sputtering of atoms and molecules from the satellite into circum‐Jovian space. The ∼520‐eV S (and ∼260‐eV O) ions composing the Io torus are the most effective agents for net sputter removal of matter from the satellite. An incident flux of ∼1010 cm−2 s−1 is estimated to provide ∼5 × 1010 S atoms cm−2 s−1 from sputtering of a (dayside) atmosphere with an exobase at a few hundred kilometers and up to ∼1012 S atoms cm−2 s−1 from an atmosphere at 1500°K with an exobase at ∼2.2 RIo. The supply of S (and O) required to stabilize the torus has been estimated by others to be from 1010 to 1012 cm−2 s−1. If Na and K are present in the atmosphere at a concentration level of 1%, the corresponding sputtering rates are calculated to be a few times 108 cm−2 s−1 for an exobase at several hundred kilometers. These numbers are large enough to supply the 107 cm−2 s−1 fluxes required to maintain the Na and K clouds. Sputtering can also remove heavy molecules, like Sn, from the atmosphere. At night, direct S sputtering of the unprotected surface is calculated to eject S and Na (1% concentration) at rates given approximately by ∼1010 and ∼108 cm−2 s−1, respectively. All atomic species residing on the surface must be ejected into circum‐Jovian space at a rate approximately proportional to their (surface) abundances, if direct surface sputtering occurs, so that the particle content of the inner Jovian magnetosphere should map rather faithfully all species present on Io's surface. The processes of plume sputtering, avalanche cascading, and ionic saltation may lead to spatial and temporal variations in the number of ejected particles.
ULF geomagnetic power near L=4, 6. Relationship to upstream solar wind quantities
Authors: Wolfe A. , Meloni A.
Published: February 2008

Geomagnetic energy densities, computed at hourly intervals in three frequency bands from magnetometer data at Pittsburg, New Hampshire, (L ∼3.5) for 28 days during July and August 1975, are correlated with upstream solar wind parameters obtained from instruments on the Imp J spacecraft. The diurnal ground magnetic energy variations generally show higher power levels for the lower frequency fluctuations. Enhancements in the power levels may appear at different local times on individual days. Dayside and nightside ground hourly magnetic energies are generally found to correlate best with the solar wind kinetic energy flux density fk=1/2npmpV³ from amongst the various upstream interplanetary quantities examined. The highest linear correlation coefficients found between power versus fk and power versus VSW are generally obtained during local afternoon hours, with a sharp reduction in correlation occurring at local noon. We conclude that different interplanetary source mechanisms contribute to the dayside ground power in different frequency bands. Improved correlations are expected for nightside power levels with geomagnetic tail quantities rather than with upstream solar wind quantities.
Temperature anisotropy instabilities driven by Whang's ion model
Authors: Leubner M. P.
Published: February 2008

Four plasma instabilities driven by proton temperature anisotropies are investigated theoretically by using Whang's ion model, i.e., heat flux modified non‐Maxwellian velocity distribution functions. The linear dispersion properties are studied in an infinite Vlasov plasma where the proton distribution is assumed to consist of one warm slow‐speed component and the electron background is isotropic. The parameters are chosen according to Whang's solar wind ion model. It is shown that the fire hose, mirror, and Harris instability do not occur under these conditions. The only unstable mode is the ion cyclotron mode. Growth rates are found to have values between 2.1 · 10−2 (T∥/T⊥=2) and 1.1 · 10−4 (T∥/T⊥=0.15).
Energetic particle events (≥30 keV) of Jovian origin observed by Voyager 1 and 2 in interplanetary space
Authors: Zwickl R. D. , Krimigis S. M. , Carbary J. F , Keath E. P. , Armstrong T. P. , Hamilton D. C. , Gloeckler G.
Published: February 2008

Short‐lived and long‐lived ion flux increases (E ≥30 keV) of Jovian origin have been observed by the low energy charged particle (LECP) instrument on the Voyager 1 and 2 spacecraft. The short‐lived events are observed more than 860 RJ upstream and more than 1500 RJ downstream of Jupiter. Observations of long‐lived events appear to be confined to ≲200 RJ upstream of Jupiter. The short‐lived events last from a few minutes to a couple of hours, while the long‐lived events last from 8 to 21 hours. Both types of events have sharp onsets and decays, are usually confined to energies below 1 MeV total energy, and show a large general enrichment of Z ≥6 particles relative to proton and helium particles when compared with energetic particle events of solar or interplanetary origin. Many of the events have a noticeable peak in the energy spectrum above 100 keV after the main portion of the event. In addition, the short‐lived events (upstream and downstream) are (1) extremely anisotropic with a flow direction consistent with flow away from Jupiter, (2) display no noticeable velocity dispersion, and (3) display initially steep energy spectra that flatten with time. The peak flux level at the lowest energies in the magnetosheath is similar to that observed during long‐lived events. We conclude that a significant fraction of the particles observed during Jovian ion events originate from within the magnetosphere of Jupiter and simply leak out into the magnetosheath. If the interplanetary magnetic field favorably connects to the bow shock, particles can leak out into the interplanetary medium. Together with the necessary leakage model, the observations presented here cannot rule out the existence of a wave particle acceleration region located immediately upstream of the bow shock.
The Jovian magnetotail and its current sheet
Authors: Behannon K. W. , Burlaga L. F. , Ness N. F.
Published: February 2008

Analyses of Voyager magnetic field measurements have extended our understanding of the structural and temporal characteristics of Jupiter's magnetic tail. The magnitude of the magnetic field in the lobes of the tail is found to decrease with Jovicentric distance approximately as r−1.4, compared with the power law exponent of −1.7 found for the rate of decrease along the Pioneer 10 outbound trajectory. Voyager observations of magnetic field component variations with distance from the magnetic dipole axis are consistent with the variations expected from the geometry of a magnetotail field and hence do not unambiguously support the uniform radial plasma outflow model derived from Pioneer 10 data. Voyager 2 has shown that the azimuthal current sheet which surrounds Jupiter in the inner and middle magnetosphere extends ‘tailward’ (in the antisun direction) to a distance of at least 100 RJ. In the tail this current sheet consists of a plasma sheet and embedded ‘neutral’ sheet. In the region of the tail where the sheet is observed, the variation of the magnetic field as a result of the sheet structure and its 10‐hour periodic motion is the dominant variation seen. Studies of both the large‐scale configuration of the current sheet viewed as a surface and of the internal structure of the sheet and its orientation indicate that (1) at distances ≳30 RJ in the tail the sheet is oriented within ±10° of the Jovian equatorial plane, most likely as a result of the solar wind interaction with the Jovian magnetosphere, (2) the surface moves north and south with an amplitude of several RJ with respect to that plane, and (3) at large distances this motion is primarily due to a rocking of the current sheet about the Jupiter‐sun line. A mathematical model that takes the tail geometry into account provides a simpler description of sheet motion in the deep tail than do models based on axial symmetry. The plasma sheet in the tail is estimated to have an average thickness of ≤5 RJ.
Plasma observations of the Alfvén wave generated by Io
Authors: Belcher J. W. , Goertz C. K. , Sullivan J. D. , Acuña M. H.
Published: February 2008

Measurements of positive ions near the Io flux tube are consistent with the detection of velocity perturbations in the magnetospheric flow due to the southward propagating Alfvén wave generated by Io.
Models for Jupiter's decametric arcs
Authors: Warwick James W.
Published: February 2008

Several papers of the present issue attempt to explain the remarkable arc‐shaped structures that dominate Jupiter's decametric emissions as recorded by Voyagers 1 and 2. Since these papers approach this subject from very different points of view, the introduction is designed to place these perspectives in context. The present paper understands these arcs in terms of a magnetic fine structure undetected by Pioneer 11 on account of its rather large closest approach distance (50 or 60 times the smallest scales in the structures involved). The nested sequences of arcs manifest the occurrence of widespread fine structures, similar in size and shape to the white ovals so ubiquitous over Jupiter's visible surface. An arc concave toward increasing time (vertex early) occurs at the east limb passage of such a structure, and the arc convex (vertex late) occurs at the west limb passage. This is consistent with the early source producing vertex early arcs and the late (and main) source producing vertex late arcs. Since arcs seem naturally to classify themselves into two large families—greater arcs (reaching frequencies ≥20 MHz) and lesser arcs (below 20 MHz)—polarized invariably right hand, or right hand or left hand symmetrically in longitude, respectively, we identify the right‐hand greater arcs with north polar cap sources, the lesser right‐hand (left hand) arcs with north (south) temperate sources, and call for the absence of fine magnetic structure in Jupiter’s southern polar cap.
The helium abundance of Jupiter from Voyager
Authors: Gautier D. , Conrath B. , Flasar M. , Hanel R. , Kunde V. , Chedin A. , Scott N.
Published: February 2008

The helium abundance in the Jovian atmosphere is derived from Voyager 1 data by two methods. The first method uses only infrared spectra from selected locations on the planet while the second method uses a thermal profile independently derived from radio occultation measurements and infrared spectra recorded near the occultation point. A hydrogen mole fraction of 0.897±0.030 is obtained from the first method, while the second method gives 0.880±0.036, corresponding to helium mass fractions of 0.19±0.05 and 0.21±0.06, respectively. The estimated errors for the first method are primarily due to systematic uncertainties in the H2 and He absorption coefficients, while those for the second method result mainly from errors in the radio occultation profile and are less well known. Random errors in the measured infrared spectra are found to be negligible in both cases. The results are consistent with a uniform mix of hydrogen and helium within Jupiter's interior, but a modest amount of helium depletion (ΔY ≤0.05) cannot be excluded.
The atmosphere of Jupiter: An analysis of the Voyager radio occultation measurements
Authors: Lindal G. F. , Wood G. E. , Levy G. S. , Anderson J. D. , Sweetnam D. N. , Hotz H. B. , Buckles B. J. , Holmes D. P. , Doms P. E. , Eshleman V. R. , Tyler G. L. , Croft T. A.
Published: February 2008

Coherently related S (2.3 GHz) and X band (8.4 GHz) signals transmitted from Voyager 1 and 2 have been used to probe the Jovian atmosphere during occultations of the spacecraft by Jupiter. The observations have yielded profiles in height of the gas refractivity, molecular number density, pressure, temperature, and microwave absorption in the troposphere and stratosphere of Jupiter at latitudes ranging from 0° to about 70°S. The data cover a pressure range from 1000 to 1 mbar over a height interval of 160 km. At the 1000‐mbar level, the temperature was 165±5 K, and the lapse rate was equal to the adiabatic value of 2.1 K/km, within the resolution of the measurements. The ammonia abundance in this region of the atmosphere was about 0.022±0.008%, in approximate agreement with the value derived from cosmic abundance considerations. The tropopause, which was detected near the 140‐mbar level, had a temperature of 110 K. Above the tropopause, the temperature increased with increasing altitude, reaching 160±20 K in the 10‐ to 1‐mbar region of the stratosphere. Significant horizontal density variations were detected in the stratosphere. This may imply a nonuniform temperature and aerosol distribution across the Jovian disk or high‐ and low‐pressure regions due to local atmospheric dynamics. The zenoid or gravity equipotential surface which best fits the 100‐mbar isobaric surface has an equatorial radius of 71,541±4 km and a polar radius of 66,896±4 km.
Thermal structure and dynamics of the Jovian atmosphere 2. Visible cloud features
Authors: Conrath B. J. , Flasar F. M. , Pirraglia J. A. , Gierasch P. J. , Hunt G. E.
Published: February 2008

Investigation of the thermal structure above selected cloud features in the Jovian atmosphere, making use of Voyager IRIS data, reveals strong similarities among a broad range of features which differ considerably in visual appearance. The atmosphere above anticyclonic features, including the major white ovals, the Great Red Spot, and a zone, are cold relative to the immediate surroundings in the upper troposphere and tropopause region. These results are consistent with upwelling and divergence in this part of the atmosphere. In contrast, a ‘hot spot’ and a ‘barge,’ which are localized cyclonic features, are found to be warm relative to their surroundings, implying subsidence with accompanying convergence. In all cases, the thermal wind shear associated with the features indicates a decay of the vorticity with height in the upper troposphere and lower stratosphere. Vertical velocities inferred from the observed temperature perturbations imply an upper limit of vertical mixing times near the tropopause of ∼20 years. Temperatures in the upper stratosphere above the anticyclonic features show considerable variation, but in most cases are found to be relatively warm. At the present time, no satisfactory explanation for this behavior appears to exist.
Processes influencing the diurnal variation of the AL index and its reliability
Authors: Holzer Robert E. , Slavin James A.
Published: February 2008

A study of the average daily variation of the AL index by months for the years 1966–1974, inclusive, shows that near the winter solstice there is a diurnal oscillation with an amplitude of 45% of the mean. The amplitude diminishes to about 10% of the mean near the summer solstice. The maximum and minimum of the winter oscillation occur at the hours when the stations contributing to the AL index have their maximum and minimum latitudinal coverage, respectively. In an attempt to interpret these results, a physical model based upon observational data concerning auroral electron precipitation and ionospheric conductivity has been developed. It is postulated that electron precipitation is the dominant cause of ionization responsible for the conduction of the westward electrojet throughout the year. In the fall and winter months the electrojet is confined to the band of precipitation, while in the spring and summer months the contributions of solar ionization permit a latitudinal spreading of the currents. The model predictions are consistent with the observed pattern of change of the average value of AL for the individual AE stations between the winter and summer solstices. It is also consistent with the change in the frequency with which the stations contribute to the determinations of the AL index between the solstices. The results indicate that AL is best suited for estimates of the westward electrojet on a continuous basis during the spring and summer months. An observed increase in the average value of AL with decreasing corrected magnetic latitude is interpreted as being due to larger electrojet currents associated with larger magnetic disturbances that produce an equatorward displacement of the polar cap boundary.
Post substorm convection and auroral arc currents determined from multiple ionospheric, magnetic, and electric field observations
Authors: Walker John K. , Koehler J. A. , Creutzberg F. , McNamara A. G. , Jones A. Vallance , Whalen B. A.
Published: February 2008

Field, particle, and optical observations from rocket‐borne probes, magnetic data from a meridian chain of stations, and data from three scanning photometers were used for analyses of auroral current systems during the recovery phase of a substorm. The rockets were launched southeastward over several discrete arcs from Ft. Churchill, Canada, on January 15, 1972. Current models were determined independently from the magnetic field observations and from the electric field and ionospheric observations. The electric fields were measured by a rocket‐borne electrostatic analyzer while the ionospheric model was determined from plasma probe measurements and also calculated from the energetic particle observations. Broad westward current systems were encountered on the initial and final segments of the trajectory and were determined from Ohm’s law and compared with those calculated from magnetic observations obtained along the rocket trajectory and from the meridian chain of stations. Auroral arc currents under the apogee segment of the trajectory were calculated from the electric field observations and a model ionosphere and were eastward. Field‐aligned currents were estimated from the divergence of the meridian current and added to these electrojets. The model was iterated to obtain a reasonable fit to both the ground‐based and the rocket‐borne observations. A very active region poleward of the arcs was much more structured than that indicated from the analysis of the ground‐based magnetic data. It consisted of both eastward and westward currents, which may have been remnants of the substorm. Intense field‐aligned currents flowed inward on the poleward edge of the electrojets and outward near the arcs, at the Harang discontinuity, and between the electrojets. These field‐aligned currents appeared to be located at the boundaries of cells that contained the convection, active remnant region, and auroral arc current systems.
The significance of VLF transmitters in the precipitation of inner belt electrons
Authors: Imhof W. L. , Anderson R. R. , Reagan J. B. , Gaines E. E.
Published: February 2008

From the low‐altitude satellite P78‐1, narrow and sometimes multiple peaks have been observed in the energy spectra of electrons from 68 to 1120 keV precipitating in the drift loss cone from the inner radiation belt. Simultaneously, measurements of the plasma wave environment at higher altitudes on the same field lines have been made with the ISEE plasma wave experiment in many cases. The peaks in electron energy are sometimes extremely narrow, being of the order of 20‐keV wide after the instrument resolution is unfolded. The plasma wave observations in these cases reveal the frequent presence of single and sometimes multiple monochromatic frequencies in the 10 kHz to 25‐kHz range that are attributed to ground‐based VLF transmitters. The plasma wave experiment also provides electron plasma density profiles. The variations of the peak energies with L value and the nearly simultaneous and spatially coordinated narrow band wave frequencies strongly support the conclusion that the narrow peaks in the electron spectra result from cyclotron resonance interactions with the waves produced by ground‐based VLF transmitters and that the effects of two or more transmitters are sometimes observed simultaneously.
Numerical simulation of a radially injected barium cloud
Authors: Swift Daniel W. , Wescott Eugene M.
Published: February 2008

Electrostatic two‐dimensional numerical simulations of a radially symmetric barium injection experiment show that ions created by solar UV irradiation are electrostatically bound to the electrons which remain tied to the field lines on which they are created. Two possible instabilities are identified, but neither of them causes the barium plasma cloud to polarize in a way that would allow the plasma to keep up with the neutrals. In a second model the velocity of the neutrals was allowed to be a function of the azimuthal angle. In this case a portion of the cloud did polarize in a way that allowed a portion of the plasma to detach and move outward at the approximate speed of the neutrals. No rapid detachment was found when only the density of the neutrals was given an azimuthal asymmetry.
Structure of the magnetopause rotational discontinuity
Authors: Lee L. C. , Kan J. R.
Published: February 2008

Rotational discontinuities are associated with the reconnection configuration on the dayside magnetopause and the energy transfer process in the tail magnetopause. In the present paper we develop a model of rotational discontinuity in which the ion dynamics is described by the fluid equations and electrons are assumed to move adiabatically along the magnetic field lines. This model is valid for a rotational discontinuity with a thickness greater than a few ion gyroradii. By including trapped electrons self‐consistently in our model, we show that (1) the trapped electron density profile is uniquely related to the rate of angular rotation of the magnetic field and (2) the sense of magnetic field rotation is determined by the ratio of the ion thermal and ion streaming energies and by the ratio of the normal and tangential components of the magnetic field. It is found that magnetopause rotational discontinuities should have the electron polarization in agreement with satellite observations.
Statistical characteristics of plasma flow in the magnetotail
Authors: Hayakawa H. , Nishida A. , Hones E. W. , Bame S. J.
Published: February 2008

The characteristics of plasma flow are studied statistically by using the IMP 6 plasma and magnetic field data. The analysis was performed by dividing the data set according to flow direction and flow velocity. Fast tailward flows (those with a tailward component of velocity exceeding 300 km/s) are observed only within a few RE of the estimated neutral sheet position and are associated with southward polarity of the magnetic field. They can therefore be interpreted as a product of reconnection operating at a location earthward of the spacecraft. On the other hand, slow tailward flows (those with a tailward component less than 300 km/s) are not associated with southward field polarity and are very similar to slow earthward flows (those with an anti‐tailward component less than 300 km/s) in such characteristics as association to northward inclination of the field and occurrence over a wide range of distance (± several RE) from the neutral sheet. These results suggest that there exist at least two source mechanisms of plasma flow in the magnetotail; X type neutral line in the near‐earth plasma sheet is likely to be the source of the fast tailward flows but other driving mechanisms are responsible for slow flows.
Structure of current sheets in the sector boundaries: Helios 2 observations during early 1976
Authors: Villante U. , Bruno R.
Published: February 2008

We examined the orientation and structure of 14 current sheets that were observed in sector boundary regions during the primary mission of Helios 2 (January through April 1976). We found that the field rotation through the current sheets (which are not null sheets) tends to occur at large angles with respect to the ecliptic plane. Most of the observed structures (which had typical thickness ranging between 1 × 104 and 6 × 104 km) can be interpreted as tangential discontinuities oriented at large angles with respect to the ecliptic plane. We also discuss the implications of a comparison between present results and the global configuration of the sector boundary surface. In particular, if we assume that the sheet normals are the same as the local normals to the global surface, our results do not support any solar cycle modulation for the orientation of boundaries.
Relationships between the Birkeland currents, ionospheric currents, and electric fields
Authors: Bleuler E. , Li C. H. , Nisbet J. S.
Published: February 2008

Calculations are made of the currents and electric fields in the ionosphere by using a global model of the electron densities including conjugate coupling along field lines. Incoherent scatter and rocket measurements of high‐latitude electron densities have been used to derive realistic variations of the polar conductivities as a function of magnetic activity. The Birkeland currents have been specified in terms of three indices, the total current into and out of the hemisphere, the ratio of the magnitudes of the currents in the AM and PM sectors, Rap, and R12, the ratio of the magnitudes of the currents in region 1 and 2. The relationship between these parameters of the Birkeland current systems and the auroral electrojet indices AE, AL, and AU is examined as well as the polar cap potential and the electric field at lower latitudes. The cusp currents have been modeled in relation to the interplanetary magnetic field and calculations are given of their effect on electric field and current patterns. One aim of this study is to produce a mathematical model of the currents, electric fields and energy inputs produced by field aligned currents that is consistent with, and specifiable in terms of, measured geophysical indices.
Annual and semiannual changes of the quiet daily variations (Sq) in the geomagnetic field at North American locations
Authors: Campbell Wallace H.
Published: February 2008

An analytic representation of the quiet daily variation of the geomagnetic field (Sq) was obtained for 1965 in the North American region, with a mean error of less than 10% of the daily variation range. The month‐by‐month 24‐, 12‐, 8‐, and 6‐hour Fourier components of the field in H, D, and Z had significant annual and semiannual changes. At the equator the Sq(H) semiannual change was dominant for all Fourier components. From 15° to 50° latitude the 24‐hour component annual change of Sq(D) was the principal seasonal variation in this region of the ionospheric current vortex. Great enhancements of the 24‐hour component of the daily field occurred at high latitude, with strong summertime maxima in the annual variation of H, D,, and Z above 70° latitude. The results, in general, agreed with the expectation for ionospheric current sources in all regions.
Long cloud observations on Mars and implications for boundary layer characteristics over slopes
Authors: Kahn Ralph , Gierasch Peter
Published: February 2008

Viking orbiter images of Mars show several instances of long continuous cloud formations on the slopes of Arsia and Pavonis Mons. We have searched all the images of the planet for occurrences of such formations. Only in the Tharsis region were long clouds unambiguously identified. We have measured the times and locations of occurrence, the wavelengths and when possible, the apparent velocity of motion of these clouds. We have also tabulated the wavelengths of patches of ripple clouds that are often found with the long formations. The long clouds are observed only in the early morning hours, suggesting that they are associated with drainage winds due to a cold planetary boundary layer. We develop simple mathematical models to examine various aspects of such boundary layer winds; these allow us to construct a complete and self‐consistent explanation of all the observed features of the cloud formations. We use the results to characterize some physical properties of the Mars boundary layer. There is strong downslope flow in the boundary layer on the high slopes of both volcanos. In the saddle region between the peaks the flow slows and undergoes a hydraulic jump, producing the long clouds. Downstream of the jump, standing internal gravity waves can exist and are excited by flow over surface irregularities. Small variations in their observed wavelengths behind the jump may be attributed to variation in the flow speed and depth. Finally, we can account for the location of the jumps by the variation of relative strength of the boundary layer flows on the two volcanos.
The electron pitch angle distribution at geosynchronous orbit associated with absorption spikes during the substorm expansion phase
Authors: Nielsen E. , Korth A. , Kremser G. , Mariani F.
Published: February 2008

Magnetospheric electron (16–300 keV) and ion (27–400 keV) fluxes associated with auroral abosrption events named ‘absorption spikes', have been investigated. The particle flux observations together with magnetic field measurements were made on board the geosynchronous satellite GEOS 2. Data from three consecutive substorm activations were analysed. The events were associated with strong variations of the electron fluxes, but only weak variations of the ion fluxes. We find strong indications that the electron pitch angle distributions associated with the absorption spike were characterized by maxima near 0°, 180°, and 90°, and dominated by a strong field aligned bi‐directional anisotropy at energies below 100 keV. At lower magnetic latitudes the field‐aligned anisotropy is decreased in magnitude and the flux maximum at 90° pitch angle.
Inverted V's and/or discrete arcs: A three‐dimensional phenomenon at boundaries between magnetic flux tubes
Authors: Atkinson G.
Published: February 2008

If discrete arcs and inverted V's are associated with current sheets and the U shaped electric potential structure, then existing two‐dimensional models are probably inadequate. The rapid east‐west electric‐field associated flow in the arms of the U shaped potential structure requires that there must be a substantial inflow to and outflow from each arm somewhere along the system since arcs and inverted V's have a limited east‐west extent. Thus strong north‐south polarization currents occur as the plasma enters and leaves the arms of the U. It is hypothesized that these currents determine the north‐south thickness. Three representative three‐dimensional models are considered in which the current sheets are either tangential or rotational discontinuities modified by the U shaped potential structure. Thicknesses of the order of a few tens of kilometers are obtained. The occurrence and type of discontinuity expected at various locations in the magnetosphere are considered. Discontinuities and hence inverted V's and/or arcs are expected at the interface between sunward and antisunward flows on the eveining side, at interfaces in the polar cap, possibly between open and closed field lines, which explains quiet time polar cap sun‐aligned arcs, and at interfaces between plasmas which have merged or been injected on the dayside or reconnected on the nightside in different impulsive events. The last two account for arcs occurring near the throat at active times and for parallel arcs within the oval. The occurrence of long parallel arcs within the oval is encouraged by the convective flow pattern and by the differences in precipitation from flux tubes with different histories.
Global circulation and temperature structure of thermosphere with high‐latitude plasma convection
Authors: Roble R. G. , Dickinson R. E. , Ridley E. C.
Published: February 2008

This paper examines the effect of magnetospheric convection in modifying the diurnal neutral gas temperature distribution and circulation of the thermosphere for equinox conditions, using NCAR's thermospheric general circulation model. Numerical experiments are presented to illustrate the differences in temperature structure and circulation due to (1) solar heating alone, (2) solar heating plus plasma convection with coincident geographic and geomagnetic poles, and (3) solar heating plus plasma convection with displaced poles. The high‐latitude plasma convection has an important influence on the global thermospheric structure and circulation. Plasma convection with displaced poles introduces a universal time dependence to the circulation and temperature structure; similar patterns occur in the northern and southern hemisphere, with a 12‐hour time difference. Magnetospheric convection drives a largely rotational, nondivergent, double‐vortex wind system at F region altitudes that can attain velocities greater than 500 m s−1 during moderate levels of geomagnetic activity. These vortices extend downward into the lower thermosphere. However, the cold low‐pressure cyclonic circulation near the dawn terminator is much more pronounced than the warm high‐pressure anticyclonic circulation in the evening sector.
Charged particle behavior in low‐frequency geomagnetic pulsations, 2. Graphical approach
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

The behavior of charged particles in an ultralow frequency magnetospheric pulsation with standing wave structure along the magnetic field is interpreted by using a graphical approach. Attention is directed principally to the way in which particles are accelerated as they bounce and drift through a wave with which they are in resonance, but nonresonant particles are also discussed. Under appropriate conditions, passage through the wave leads to modulation of particle energy. Applications emphasize the necessity of considering the large‐scale structure of a wave in interpreting the particle behavior seen at a particular point in space.
DMSP optical and electron measurements in the vicinity of polar cap arcs
Authors: Hardy D. A. , Burke W. J. , Gussenhoven M. S.
Published: February 2008

We have completed an extensive analysis of the electron and optical data from the DMSP satellites for an extended period of polar cap arc occurrences on December 12, 1977. The polar cap arcs are observed in three distinct intervals in a period of quieting after a time of intense substorm activity. Each interval occurs when the IMF Bx is less than 0 and follows a northward turning of the IMF Bz. The intervals are separated by periods of southward Bz and weak substorm activity during which no enhanced precipitation is seen in the polar cap. The polar cap arcs persist for periods of more than an hour with only gradual variation in position and intensity. At the time of our study the highest‐energy flux and average energy for electrons precipitating into the polar cap are seen in the northern polar cap. The observation of polar cap arcs is associated with the admittance of large and variable fluxes of low‐energy electrons into a major portion of both the northern and southern hemisphere polar caps. These fluxes fall into the following categories: First, nearly Maxwellian distributions of electrons with temperatures between 50 eV and 200 eV and number densities varying from 0.03/cm³ to 4/cm³. The highest densities are found at the poleward boundary of the diffuse aurorae and near the visible polar cap arcs. The lowest densities are associated with the polar rain. Second, distributions of electrons peaked between 50 eV and 200 eV. These distributions result from acceleration of the cold Maxwellian distribution through a potential of 50 to 200 V without any heating of the electrons. Third, distributions of electrons displaying two populations; an intense low‐energy component with a temperature of ∼20 eV and a much weaker high‐energy component with a temperature of 180 eV. We interpret such distributions as evidence of direct admittance of magnetosheath electrons into the polar cap. Fourth, distributions of electrons peaked at ∼1 keV. These distributions produce the visible arcs. They result from the acceleration of a two‐component electron population with temperatures of 100 and 350 eV through a potential drop of ∼750 V.