bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluid signatures of rotational discontinuities at the Earth's magnetopause
Authors: Scudder Jack D.
Published: February 2008

Fluid signatures in the MHD approximation at rotational discontinuities (RD) of finite width called rotational shear layers (RSL) are examined for general flow and magnetic geometries. Analytical and geometrical arguments illustrate that the fluid speed can either go up or down across an RSL for a fixed normal mass flux. The speed profile may or may not be monotonic with the phase of the magnetic rotation depending on the boundary conditions. The flow velocity may or may not be field aligned or “jetting”
as a result of traversing the RSL. In general, significant “convection” is expected in the layer. The observable signatures of (MHD) RSL's depend on seven (boundary condition) parameters: (1) the mass density, (2)–(5) the incident normal and transverse components of the magnetic field and fluid velocity, (6) the angle ε between the incident tangential flow velocity and tangential magnetic field, and (7) the size of the magnetic angular rotation implemented by the layer Δϕ. This general survey illustrates the singular character of the Petschek geometry and that the model predictions of jetting and monotonic speed increases through the layer may not be used as general signatures of RSL's. This is especially true along the Levy, Petschek, and Siscoe curved magnetopause boundary, where the fluid flow, Δϕ, and ε are generally all different from the Petschek regime. Accordingly, the lack of observed speed increases across the magnetopause can no longer be used by itself to infer that the magnetopause is locally closed. Of the spectrum of MHD RSL's that are possible, the “accelerating” ones require special boundary conditions most generally occurring near the nose of the magnetopause; the “decelerating” RSL's become increasingly more probable at magnetopause crossings removed from the subsolar point. Documented RSL's have been located within the spectrum of possible signatures; the number of RSL's reported to date implies only a lower bound for their frequency of occurrence, since surveys conducted to date have excluded the decelerating ones.
Multiple correlation between auroral and magnetic pulsations: 2. Determination of electric currents and electric fields around a pulsating auroral patch
Authors: Oguti Takasi , Hayashi Kanji
Published: February 2008

The multiple correlation method has been applied to the study of concurrent auroral and magnetic pulsations on the basis of videotapes of all‐sky television camera auroral records and induction magnetometer records. The results are (1) the ground magnetic pulsations below pulsating auroras are well represented by a regression model where the magnetic change is expressed as a linear combination of auroral luminosity fluctuations at various parts of the sky, (2) the magnetic variation observed at a single station can be broken down into the magnetic contributions from various auroral patches, and subsequently, (3) the magnetic deflections at various ground points below a pulsating auroral form can be estimated even when many pulsating patches appear at the same time. It is also shown that (4) the horizontal magnetic deflections deduced from observations by use of this technique are fully explained by the combination of ionospheric and field‐aligned electric currents which are theoretically expected to be induced in and near a local enhancement of conductivity under the existence of a uniform, ambient electric field. This study thus gives a definite proof that the ground magnetic pulsation which occurs concurrently with auroral pulsations in the auroral oval is the result of successive growth and decay processes of the localized electric currents in and in the vicinity of pulsating auroral forms, which are caused by a conductivity pulsation due to the pulsating precipitation of auroral electrons in conjunction with the convection electric field. The model calculation also predicts the distribution pattern and intensity of the fluctuating electric fields below pulsating auroral patches.
Thermospheric response to the June 11, 1983, solar eclipse
Authors: Ridley E. C. , Dickinson R. E. , Roble R. G. , Rees M. H.
Published: February 2008

The NCAR thermospheric general circulation model (TGCM) that includes coupling of dynamics and composition is used to calculate the time‐dependent thermospheric response to the June 11, 1983 total solar eclipse. The path of totality originated at sunrise in the Indian Ocean near 35°S and 60°E. The path moved equatorward passing through Indonesia near 5°S latitude 125°E longitude and ending near 20°S latitude and 165°E longitude in the vicinity of the Solomon Islands. Although the path of totality is relatively small, the area of partial shadow is relatively large, and the total flux incident on the dayside earth decreases by about 7% during the total eclipse. The TGCM calculates the time‐dependent response of the winds, temperature, and mass mixing ratios of the major constituents throughout the thermosphere. Perturbations follow the path of totality, with maximum deviations occurring near 0530 UT at about 350 km. The winds converge from all directions toward the shadow at speeds reaching 60 m s−1 in the upper thermosphere. The maximum temperature anomaly (−70°K) and vertical wind anomaly (−8 m s−1) occur near the center of the shadow. The mass mixing ratio at N2 along a constant pressure surface decreases by up to 2%, and that of O increases by up to 2% in response to the sudden thermospheric cooling caused by the shadow passing over the dayside of the earth. At a constant altitude of 300 km the N2 and O number densities both decrease by about 6%. The time‐dependent calculation shows that disturbances are generated during the onset of the eclipse, with the largest perturbations following the path of totality.
Characteristics of plasmoidlike structures in the distant magnetotail
Authors: Scholer M. , Gloeckler G. , Hovestadt D. , Klecker B. , Ipavich F. M.
Published: February 2008

The average characteristics of 20 energetic particle burst events in the distant geomagnetic tail have been determined. The measurements were made with two particle sensors of the Max‐Planck‐Institut/University of Maryland experiment on ISEE 3 during the recent Geotail Mission. The events are characterized initally by beamlike energetic electron distributions which are followed by isotropic distributions. Assuming that the energetic protons (≳30 keV) are convected with the local plasma flow (which is generally tailward), we can determine the flow speed from the measured angular distributions and the differential energy spectral shape. We find that the average velocities range from ∼300 to ∼1000 km/s. The size of these structures, most probably plasmoids, along the convective flow direction is observed to be between ∼50 and ∼100 RE in most of the cases, with a mean value of ∼80 RE. The energy spectra (in the plasma frame) are consistently steeper than power law, being closer to exponentials in energy. The differential proton intensity in these events, on the average ∼500 protons cm−2 s−1 sr−1 keV−1 at 30 keV, is an order of magnitude smaller than in the near‐earth plasma sheet.
Plasma convection in the vicinity of the dayside cleft
Authors: Foster John C. , Doupnik Joe R.
Published: February 2008

Experiments conducted with the Chatanika, Alaska, incoherent scatter radar observed the ionospheric convection pattern equatorward of the noontime cleft. There plasma transport is characterized by low‐speed convergence toward a meridian centered somewhat before noon and rotation of the flow into a poleward direction at higher latitudes. For undisturbed conditions there is only weak convection at noon at these latitudes, but the eastward electric field reaches a maximum of 15 mV/m 2–4 hours before and after noon at latitudes between 70°Λ and 74°Λ. During active conditions at the peak of the solar cycle the ionospheric cleft was located within the Chatanika field of view on several occasions. A longitudinally narrow region of rapid poleward convection near noon was not observed; rather, poleward flow occurred over a 3–4 hour spread of local time. For disturbed conditions, the prenoon and postnoon regions of plasma entry into the polar cap were enhanced, and an eastward electric field of 25 mV/m was seen across the 2 hours of local time around noon. High‐density F region plasma was observed convecting poleward through the cleft from a source at lower latitudes in the afternoon sector. Such plasma, seen at very high latitudes within the polar cap, serves as a tracer of the convection pattern away from the cleft.
Night airglow OH (8–3) band rotational temperatures at Poker Flat, Alaska
Authors: Myrabø H. K. , Romick G. J. , Sivjee G. G. , Deehr C. S.
Published: February 2008

Temperatures of the mesopause region (85–90 km) have been deduced from OH (8–3) molecular band night airglow emission measurements made at Poker Flat (65°N), Alaska. The data cover the first 4 months of each of the years 1976, 1977, and 1978. Mean monthly temperatures of 229, 224, 213 and 193 K were obtained with no significant yearly differences. The mean temperature for each month is about 15 K higher than the respective monthly temperatures in the 85 and 90 km, 60° and 70°N, CIRA (1972) model, and it follows the general decreasing trend shown by the model.
Pitch angle properties of magnetospheric thermal protons and satellite sheath interference in their observation
Authors: Sojka J. J. , Wrenn G. L. , Johnson J. F. E.
Published: February 2008

Cold proton observations from the geosynchronous GEOS 2 satellite are presented to show the pitch angle distribution of the refilling cold ion population found beyond the plasmapause. This refilling cold ion population flows into the depleted equatorial regions of the flux tubes from the topside ionosphere. This refilling is observed both day and night. The ions have energies between 0.5 and 3 eV. These energy characteristics are consistent with other recent observations. Although the observed angular distribution of these ions is highly complex, when allowance is made for the satellite sheath several conclusions can be made concerning the ion pitch angle distribution. The angular distribution is very field aligned with a characteristic source cone angle ranging from 10° to 20°. This range of angles is consistent with ions originating below 20,000 km in the exosphere rather than in the heavy‐ion ionosphere. As the flux tube density increases, the degree of field alignment decreases; however, based on the limited pitch angle coverage it is not possible to determine if the field‐aligned component is simply being swamped by the local isotropic component or if the field‐aligned component changes its pitch angle characteristics. A crucial aspect of the study was the availability of a unique data base of sheath‐plasma observations. The observations were obtained by using the boom mounting and voltage biasing capabilities of the detector package in relation to the satellite. These data enabled us to describe how the satellite sheath affects the observation of cold (<5 eV) protons. This qualitative understanding enabled the highly anisotropic data to be interpreted in terms of magnetospheric pitch angle distributions. The overall implications of this sheath‐plasma coupling is that a technique to make satellite sheath observations has been demonstrated.
Survey of energetic (E > 35 keV) ion anisotropies in the deep geomagnetic tail
Authors: Daly P. W. , Sanderson T. R. , Wenzel K.‐P.
Published: February 2008

An analysis of the direction and magnitude of the anisotropy of energetic ions (E > 35 keV) in the plasma sheet of the deep geomagnetic tail has been undertaken using data from the energetic particle anisotropy spectrometer on board ISEE 3 during its geotail mission. An ensemble of samples, consisting of 10‐min averages of the data, has been chosen using selection criteria of magnetic field direction and particle intensity such that the samples are predominantly in the plasma sheet or energetic ion boundary layer. No distinction between quiet and disturbed times is made in this study. Tailward and earthward flows are seen with nearly equal probability out to a distance of 100 RE from the earth, beyond which tailward flow dominates. Beyond 150 RE, tailward flow occurs in 95% of the samples. The average derived ion flow velocity in the tailward direction increases steadily with distance to 650 km s−1 at 230 RE. The most common direction of ion anisotropy is 10° duskward of the GSM −X axis. The data are better ordered about this direction than about the direction of the magnetic field vector. This is explained by the presence of a transverse component of ion anisotropy in the duskward direction, the average value of which is independent of the strength of the tailward/earthward flow. It is argued that this transverse anisotropy is caused by a density gradient in the plasma sheet of scale length 2–3 RE.
Direct evidence for two‐stage (bimodal) acceleration of ionospheric ions
Authors: Klumpar D. M. , Peterson W. K. , Shelley E. G.
Published: February 2008

New measurements from the energetic ion composition spectrometer on the Dynamics Explorer 1 satellite reveal the existence of hybrid conical distributions which are interpreted as the first direct observation of a two‐stage acceleration of ionospheric ions. The ions display characteristics of having experienced both transverse and parallel acceleration in the process of being injected into the magnetosphere. This signature consists of field alignment of the ions at energies just above a lower energy cutoff below which no fluxes are measured. At increasing energy the field‐aligned distribution gradually becomes a conical one whose apex angle widens with increasing energy. The lower energy cutoff gradually increases and then decreases as the satellite moves in latitude in a manner like that found in ion inverted V's. In one case analyzed in detail transversely accelerated oxygen ions having energies to 5 keV were observed while the lower energy ions were primarily field‐aligned between 430 and 870 eV. It is concluded that ions were accelerated from the ionospheric plasma by a mechanism acting primarily transverse to the geomagnetic field at an altitude near 18,000 km and additionally accelerated through a parallel potential that exceeded 300 volts in the core of the inverted V. The heating mechanism produced a bi‐Maxwellian distribution of oxygen ions with transverse and parallel temperatures of ∼1.2 keV and ∼260 eV, respectively. The deduced O+ density was on the order of 0.5 cm−3 in the source.
Plasma heating at collisionless shocks due to the kinetic cross‐field streaming instability
Authors: Winske D. , Tanaka Motohiko , Wu C. S. , Quest K. B.
Published: February 2008

Heating at collisionless shocks due to the kinetic cross‐field streaming instability, which is the finite beta (ratio of plasma to magnetic pressure) extension of the modified two stream instability, is studied. Heating rates are derived from quasi‐linear theory and compared with results from particle simulations to show that electron heating relative to ion heating and heating parallel to the magnetic field relative to perpendicular heating for both the electrons and ions increase with beta. The simulations suggest that electron dynamics determine the saturation level of the instability, which is manifested by the formation of a flattop electron distribution parallel to the magnetic field. As a result, both the saturation levels of the fluctuations and the heating rates decrease sharply with beta. Applications of these results to plasma heating in simulations of shocks and the earth's bow shock are described.
The evolution of interplanetary shocks
Authors: Cane H. V.
Published: February 2008

Unambiguous identification of the solar events associated with 48 interplanetary (IP) shocks has been facilitated by the observation of radio emission generated by the shocks. This identification makes possible an investigation of the way in which these shocks evolve and of the relationship between solar event “strength” and shock velocity. All but two of the 48 solar events associated with IP type II radio emission included long‐duration soft X‐ray events (LDE's). In addition, it is shown that all intense LDE's are associated with IP shocks. A weak correlation between the integral flux of soft X rays and the average velocity of the associated shock is suggested. However, for two events all the usual flare signatures were absent, and the events are associated with the disappearance of solar filaments. It is shown that the shocks propagate isotropically over about 50° from the source site. Beyond 50° the shocks weaken particularly on their eastern flanks. As a consequence, shocks originating on the west limb cannot reach the earth. IP type II radio emission is dependent on shock velocity with faster shocks producing more intense emission. The study shows that radio intensities provide information about the evolution of IP shocks.
Coalescence of two pressure waves associated with stream interactions
Authors: Whang Y. C. , Burlaga L. F.
Published: February 2008

An MHD unsteady one‐dimensional model is used to simulate the interaction and coalescence of two pressure waves in the outer heliosphere. Each of the two pressure waves was a compression region bounded by a shock pair. Computer simulation using Voyager data as input demonstrates the interaction and coalescence process involving one pressure wave associated with a fast stream and the other pressure wave without a fast stream. The process predicts a significant change in the magnetic field and plasma signatures: The propagation of the forward and reverse shocks first widens the radial dimension of the shock compression region with increasing heliocentric distances. The shocks belonging to two neighboring compression regions eventially collide, and the two compression regions begin to overlap. Both shocks continue to propagate after the collision, but they are weakened. As a result of the collision, a contact surface is formed in the overlapping compression region. The magnetic field, plasma density, and temperature in the new compression region are enhanced. If this type of interaction is a dominant dynamical process in the outer heliosphere, it can significantly and irreversibly alter the structure of the medium.
Slow‐mode shocks: A semipermanent feature of the distant geomagnetic tail
Authors: Feldman W. C. , Baker D. N. , Bame S. J. , Birn J. , Gosling J. T. , Hones E. W. , Schwartz S. J.
Published: February 2008

A survey is made of the relative frequency of encounters with slow‐mode shocks observed by ISEE 3 in the distant geomagnetic tail. The association of these shocks with the phase of substorm activity as evidenced by enhanced currents in the auroral ionosphere and enhanced energetic‐particle populations at geostationary orbit is also documented. During the 25 days between January 18 and February 11, 1983, 26 unambiguous examples of slow shocks were observed. Although a very strong association with the level of geomagnetic activity is found, shocks were observed during all phases of substorm activity including one during quiet conditions. Slow‐mode shocks must therefore be a semipermanent feature of the bounding surfaces which separate lobe and plasma sheet particle populations in the distant geomagnetic tail.
Intensity variations of geocoronal Balmer alpha emission: 1. Observational results
Authors: Shih P. , Roesler F. L. , Scherb F.
Published: February 2008

Ground‐based observations of geocoronal Balmer α (Hα) emission were performed with a large‐aperture, dual‐etalon Fabry‐Perot spectrometer at a resolving power of about 25,000, which clearly isolates the geocoronal emission from galactic and zodiacal light backgrounds. During each observing night, observations were conducted alternately between two directions, one at 60° zenith distance in the sun's azimuth and the other in the antisolar direction, to compare the emission primarily from single scattering of solar Lyman β with that primarily from multiple scattering, and to evaluate the intensity variations over various time scales. Accurate intensity calibration was obtained by observing astronomical photometric standard sources on each night of observation. The morning enhancement of the observed Hα intensities in both directions is 10%. Assuming single scattering dominates, the observed intensity variation seems difficult to reconcile with the standard diurnal variation model which implies an increase at mid‐latitudes of about 30% in the exobase hydrogen density between the evening period and the post‐midnight period. In contrast to the standard diurnal variation model, some satellite measurements indicate a morning hydrogen trough in northern mid‐latitudes, a result which appears to be consistent with the present measurements. Detailed radiative transfer calculations and model refinements are required to clarify the situation. Comparisons of Hα intensities observed on different nights with the same scattering geometry reveal a clear inverse relationship between the geocoronal Hα emission rate and the exospheric temperature based on the Jacchia 1977 model. Preliminary calculation shows this Hα intensity variation agrees with the results of several satellite experiments which found that the variation of the exobase hydrogen density with temperature is less rapid than that predicted by consideration of Jeans escape as the only hydrogen escape mechanism.
The role of heavy ionospheric ions in the localization of substorm disturbances on March 22, 1979: CDAW 6
Authors: Baker D. N. , Fritz T. A. , Lennartsson W. , Wilken B. , Kroehl H. W. , Birn J.
Published: February 2008

Extensive ground‐based arrays of magnetometers and numerous satellite platforms in the outer magnetosphere have established that two separate substorm expansion onsets occurred on March 22, 1979. The first of these occurred at 1055 UT and is demonstrated to be localized in the 0200–0300 LT sector. Concurrent plasma sheet ion composition measurements are used to show that the growth and expansion phase of this substorm occurred while the outer magnetosphere was composed dominantly of solar wind (H+ and He++) plasmas. The 1055 UT substorm greatly perturbed and altered the ion composition of the plasma in the outer magnetosphere such that the second substorm expansion onset (1436 UT) occurred while the outer magnetospheric plasmas were dominantly of ionospheric (O+) origin. The 1436 UT substorm is shown to have a component of the westward electrojet localized further westward in local time relative to the first substorm. These results are a consistent, well‐documented example of the possible important role of heavy ions in the localization and initiation of plasma sheet instabilities during substorms.
Interplanetary Cosmic Ray Intensity: 1972ndash;1984 and Out to 32 AU
Authors: Van Allen J. A. , Randall B. A.
Published: February 2008

Using counting rates of a number of Geiger‐Mueller tubes in the University of Iowa instruments on Pioneer 10 (P10) and Pioneer 11 (P11), we have measured the interplanetary intensity of cosmic rays (Ep > 80 MeV) over a complete solar activity cycle and out to a heliocentric distance of over 32 AU. Reliable free‐space values of the background corrections due to the radioisotope thermal generators were determined during passage of Pioneer 11 under the rings of Saturn and in to 1.34 Rs (planetary radii) during the September 1, 1979 encounter. These improved corrections and ones inferred therefrom for Pioneer 10 have supplanted earlier estimates and have been applied to all data. During the period 1972–1984, intensity vs. time curves have a flat, more‐or‐less constant maximum during 1975, 1976, and 1977; a brief (∼8 months) minimum in mid‐1981; and another brief minimum centered around early January 1983. The ratio of maximum to minimum intensity (normalized to the same radial distance) for the solar activity cycle is ≈ 2.7. The temporal correlation of intensity at P10 with that at P11 is good, after allowance for propagation delay, as is that with data from the Deep River Neutron Monitor. The max/min intensity ratio for the latter is ≈ 1.25. As noted by us and others in previous studies, the decline in intensity after the prolonged maximum occurred primarily by an irregular succession of stepwise decreases with subsequent slow recovery such that successive decreases overlapped cumulatively. The logarithms of the ratios of the counting rates of a GM tube on P10 to that of a matched GM tube on P11 (after allowance for propagation delay) were plotted against the differences in heliocentric radial distance Δr. Three such independent plots were made, one for each of a matched pair of tubes. The three least‐squares‐fitted trend lines through the entire body of data for each pair of detectors yield integral radial gradients G of +2.00 (±0.07), +2.07 (±0.05), and +2.12 (±0.06) percent per AU. Our adopted mean value with estimated uncertainty for all known causes is G = +2.06 (±0.20)%/AU for the radial range 1 < r < 32 AU. The reciprocal of G, namely 50 AU, is the scale length of the heliosphere. By direct observation, the modulating influence of the sun is still clearly present at 32 AU. Hence the radius of the heliospheric modulation region rB > 32 AU. Various arguments, no one of which is convincing, suggest that rB lies between 50 and 100 AU.
Beam‐driven ion cyclotron harmonic resonances in the terrestrial foreshock
Authors: Smith Charles W. , Goldstein Melvyn L. , Gary S. Peter , Russell C. T.
Published: February 2008

We report an observation of low‐frequency terrestrial upstream waves which exhibit enhancements in the magnetic field power and polarization spectra at harmonics of the proton cyclotron resonance. These magnetic fluctuations exist concurrent with a “diffuse” suprathermal proton population, but a field rotation immediately following the wave observation reveals a “reflected” suprathermal distribution. We also compute instability growth rates due to this highly anisotropic “reflected” component and find relative maxima at frequencies corresponding to the enhancements in the observed spectra. We thus infer that the multiple spectral peaks are due to resonant, electromagnetic ion beam instabilities and conclude that the observed ion beam is the source of the observed upstream waves. Spectral analysis of the polarization, magnetic helicity, and minimum variance directions supports this interpretation. We find observational and theoretical evidence for both right‐hand polarized waves propagating parallel to the mean magnetic field and left‐hand polarized waves propagating oblique to the mean field.
Neutral wind spectra at the auroral zone mesopause: Geomagnetic effect?
Authors: Johnson R. M. , Luhmann J. G.
Published: February 2008

Measurements of auroral zone neutral winds at the mesospause obtained by the MST radar at Poker Flat, Alaska (65°N), in June and July of 1980–1982 have been spectrally analyzed for evidence of an effect of enhanced electrojet activity upon the wind spectrum. The horizontal component of the College magnetometer was used to indicate the level of geomagnetic activity. The high‐frequency portion of the gravity wave regime (periods shorter than about 6 hours) appears quite constant with no apparent correlation with local electrojet activity. The average intensity of lower‐frequency waves is quite variable from year to year. On a yearly basis, the data suggest alterations in the intensity of some low‐frequency waves in conjunction with enhanced geomagnetic activity. However, zonal and meridional wind fluctuations did not vary with the level of geomagnetic activity in a consistent manner for all 3 years of data studied at any specific frequency. This may indicate a dependence of any relationship between the intensity of low‐frequency neutral wind oscillations at the mesopause and geomagnetic activity on the average background state of the neutral wind spectrum for that period.
The polar ionosphere as a source of the storm time ring current
Authors: Cladis J. B. , Francis W. E.
Published: February 2008

The transport of ions from the polar ionosphere to the inner magnetosphere during storm time conditions has been computed using a Monte Carlo diffusion code. The effect of the electrostatic turbulence assumed to be present during the substorm expansion phase was simulated by a process that accelerated the ions stochastically perpendicular to the magnetic field with a diffusion coefficient proportional to the rate of energization of the ions by the induced electric field. This diffusion process was continued as the ions were convected from the plasma sheet boundary layer to the double‐spiral injection boundary. Inward of the injection boundary the ions were convected adiabatically. By using as input an O+ flux of 2.8×108 cm−2 s−1 (w>10 eV) and an H+ flux of 5.5×108 cm−2 s−1 (w>.63 eV) the computed distribution functions of the ions in the ring current were found to be in good agreement, over a wide range in L (4–8), with measurements made with the ISEE 1 satellite during a storm. This O+ flux and a large part of the H+ flux appear to be consistent with the DE‐1 and DE‐2 satellite measurements of the polar ionospheric outflow during disturbed times.
Magnetic field draping against the dayside magnetopause
Authors: Crooker N. U. , Luhmann J. G. , Russell C. T. , Smith E. J. , Spreiter J. R. , Stahara S. S.
Published: February 2008

Interplanetary magnetic fields observed upstream of Earth's magnetosphere at ISEE 3 form input for a gas dynamic model of magnetic field draping in the dayside magnetosheath. Model results near the magnetopause are compared with appropriately lagged observations at ISEE 1. In 16 of 24 cases the angle between the transverse component of the model and observed fields is less than 20°. The agreement is surprisingly good in view of the uncertainty introduced by the large distances between ISEE 1 and ISEE 3. The results indicate that magnetohydrodynamic and energy transfer processes at the magnetopause do not cause large distortions of the magnetosheath magnetic field. In addition, a comparison between observed and model field magnitudes indicates that immediately outside the magnetopause the observed field behaves like the model field at a distance of ∼0.5 RE from the magnetopause, outside the region where magnetohydrodynamic effects make the gas dynamic model inapplicable. Patterns of model magnetic field orientation at the magnetopause are presented for practical application.
Energetic ions upstream of Jupiter's bow shock
Authors: Krimigis S. M. , Zwickl R. D. , Baker D. N.
Published: February 2008

Comprehensive measurements of ion spectra, composition, and anisotropies over the energy range ∼30 keV to ∼5 MeV in Jupiter's foreshock, magnetosheath, and magnetopause were obtained by the low‐energy charged particle instrument during the Voyager 1 and 2 encounters with the planet in 1979. Detailed analyses of the spectral and compositional signatures show the following: (1) The ion spectral form at event onset is described well by a power law in energy (dj/dE = KE−γ) over the range ≲100 keV to ∼2 MeV; the spectrum becomes generally (but not always) harder at the lower (≲200 keV) energies as the event progresses. (2) The ion spectrum is depleted of protons at E ≳ 300 keV and is dominated by ions with Z ≥ 6 (probably oxygen and sulfur) at higher energies; this fact, taken together with the continuity of the spectrum over a factor of ∼107 in intensity, suggests that heavy ions (Z ≥ 6) also dominate at low (<300 keV) energies. (3) Spectra of similar form and ion composition are observed inside the magnetosheath and magnetopause. (4) Large (up to 100∶1) variable anisotropies are present throughout all events. (5) Events are seen whenever the projection of the interplanetary magnetic field connects the spacecraft to the bow shock. (6) The overall morphology of ion intensity enhancements resembles that observed upstream of the earth's bow shock in similar energy ranges, including inverse velocity dispersion. These observations lead to the conclusion that ions appearing upstream from Jupiter originate from within Jupiter's magnetosphere. The spectral and compositional features are shown to be consistent with ion escape from the magnetosheath followed by E × E drift in the solar wind electric field, as suggested by Anderson [1981] for the case of earth. The compositional signatures, together with the large anisotropies and the presence of MeV ions at event onset, preclude the possibility of diffusive, first‐order Fermi acceleration upstream of the shock, as has been proposed for the ions observed upstream of earth.
Experimental study of the relationship between energetic electrons and ELF waves observed on board GEOS: A support to quasi‐linear theory
Authors: Cornilleau‐Wehrlin Nicole , Solomon Jacques , Korth Axel , Kremser Gerhard
Published: February 2008

The interaction between energetic electrons (E > 15 keV) and whistler mode ELF/VLF waves (0.2–2 kHz) is studied experimentally in the light of the quasi‐linear pitch angle diffusion theory. This work is carried out with data from the European magnetospheric satellites GEOS 1 and GEOS 2. A method is given to calculate from the data in a complete manner the particle anisotropy A and the temporal growth rate of the waves, γ, as defined by Kennel and Petschek (1966), A and γ being frequency dependent. The data are averaged over 5 min, which is slightly less than the minimum lifetime of the electrons. Therefore the temporal evolution of weak diffusion cases can be studied. The calculated anisotropy is compared to the critical anisotropy AC, which is the minimum anisotropy needed for wave growth, and to the measured wave spectrum. In the presence of waves, A is greater than AC and roughly follows AC in the frequency range of wave emission. The difference [A‐AC] is smallest during strong diffusion but remains positive, as expected from pitch angle diffusion in a steady state situation. The behavior of A does not change considerably between the geomagnetic equator (GEOS 2) and latitudes of about 20° (GEOS 1). The normalized wave growth rate γ/ωce varies between ∼10−4 and 10−2 at the equator, leading to the generation of more or less intense waves and thus to strong or weak electron pitch angle diffusion. Conversely, the constant level of γ/ωce (∼10−5) at a latitude of 20° indicates that the diffusion is most efficient at lower latitudes. A very detailed discussion in terms of the quasi‐linear theory of wave‐particle interactions shows that this theory is applicable for the explanation of the observations. The discussion includes a consideration of the conditions for the existence of a steady state emission in terms of a minimum level of the integrated spatial growth rate needed to compensate for the wave losses at the ionosphere.
North‐south and dawn‐dusk plasma asymmetries in the distant tail lobes: ISEE 3
Authors: Gosling J. T. , Baker D. N. , Bame S. J. , Feldman W. C. , Zwickl R. D. , Smith E. J.
Published: February 2008

ISEE 3 plasma measurements made in the distant geomagnetic tail during rapid traversals of the current sheet from one lobe to the other show that the lobe densities on opposite sides of the current sheet often differ by a factor of ∼3–10 or more. On the dawnside of the tail the north lobe plasma has the higher density when the interplanetary magnetic field (IMF) has a +By component, and the south lobe plasma has the higher density when the IMF has a −By component. The sense of this asymmetry is reversed on the duskside of the tail. Such IMF‐controlled asymmetries in lobe plasma density are consistent with earlier observations made much closer to earth and can be interpreted most simply as a consequence of an “open” geomagnetic tail.
Long‐term (solar cycle) and seasonal variations of upflowing ionospheric ion events at DE 1 altitudes
Authors: Yau A. W. , Beckwith P. H. , Peterson W. K. , Shelley E. G.
Published: February 2008

A statistical analysis is presented of the long‐term variations of upflowing ionospheric ion (UFI) occurrence morphology in the high‐altitude (8000–23,300 km) auroral and polar cap ionosphere, using data from the Dynamics Explorer 1 Energetic Ion Composition Spectrometer from 1981 to 1984. The study is based on over 100,000 data samples, each consisting of pitch angle distributions of H+ and O+ intensities at 0.01–1, 1–4, and 4–17 keV/e. The data samples were grouped into 91‐day intervals (seasons). Within each interval they were sorted into altitude, invariant latitude, magnetic local time, and magnetic activity (Kp index) bins, and the occurrence frequency and intensity characteristics of H+ and O+ UFI in the respective bins were determined. The September 1981 to May 1984 period coincides with the declining phase of solar cycle 21, when the solar radio flux at 10.7 cm,
, decreased from a high of 222 (× 10−22 W m−2 Hz−1) in September 1981 to a low of 93 in November 1983. The observed occurrence frequency, intensity, and angular characteristics of O+ UFI exhibited marked variations which correlate with variations in the solar radio flux (and which envelop those of magnetic activity and seasonal origins). In contrast, the H+ UFI morphology did not display any observable long‐term variations. At both quiet (Kp ≤ 2+) and active times (3‐ ≤ Kp ≤ 5+) the solar maximum O+ data displayed (1) a factor of 2–3 increase in the occurrence frequency of intense UFI; (2) a similar increase in the occurrence frequency of transversely accelerated ions above 8000 km; and (3) an increase in conic abundance (fraction of ion conics in UFI). Together, items (1) and (3) imply about a twofold increase of energetic O+ outflow to the magnetosphere at solar maximum; items (2) and (3) indicate an upward shift in the perpendicular ion acceleration region. The correlation of O+ UFI abundance and occurrence with variations of the solar radio flux, in the absence of magnetic and other geophysical variations, suggests solar activity variations related to the solar flux to be the causative factor in the long‐term variations in observed UFI morphology. Seasonal variations in UFI occurrence were present in O+ and absent in H+. The O+ frequencies in the summer periods were found to be significantly higher than the values in winter periods of comparable solar activity. The observed solar activity and seasonal variations are interpreted in terms of increased ionospheric and atmospheric scale heights at solar maximum and summer solstice, which effectively shift the O+‐H charge exchange altitude upward and facilitate O+ injection to higher altitudes. The observed variations are compared with solar cycle variations in the ring current density, and their magnetospheric implications are discussed.
Test particle modeling of wave‐induced energetic electron precipitation
Authors: Chang H. C. , Inan U. S.
Published: February 2008

A test particle computer model of the precipitation of radiation belt electrons is extended to compute the dynamic energy spectrum of transient electron fluxes induced by short‐duration VLF wave packets traveling along the geomagnetic field lines. The model is adapted to estimate the count rate and associated spectrum of precipitated electrons that would be observed by satellite‐based particle detectors with given geometric factor and orientation with respect to the magnetic field. A constant‐frequency wave pulse and a lightning‐induced whistler wave packet are used as examples of the stimulating wave signals. The effects of asymmetry of particle mirror heights in the two hemispheres and the atmospheric backscatter of loss cone particles on the computed precipitated fluxes are discussed.
Propagation mechanism of daytime Pc 3–4 pulsations observed at synchronous orbit and multiple ground‐based stations
Authors: Yumoto Kiyohumi , Saito Takao , Akasofu Syun‐Ichi , Tsurutani Bruce T. , Smith Edward J.
Published: February 2008

In order to understand propagation mechanisms of daytime Pc 3–4 magnetic pulsations with 10–70 s periods in the magnetosphere, we studied relationships among the interplanetary magnetic field (IMF) observed at ISEE 3, Pc 3–4 pulsations detected at the GOES 2 geostationary satellite (L = 6.67, λ = 106.7°W), and those observed at the ground stations at College (5.6, 147.9°W), San Gabriel Canyon (1.8, 118.0°W), Ewa Beach (1.15, 158.1°W), and Onagawa (1.3, 141.5°E). Activities of low‐latitude Pc 3–4 pulsations at the ground stations and compressional Pc 3–4 waves at GOES 2 are found to be weakly correlated with the cone angle of the IMF [θXB = cos−1 (BX/BIMF)]. The frequency of both the daytime Pc 3–4 pulsations at the three low‐latitude stations and the compressional Pc 3‐4 waves in the magnetosphere are functions of the IMF magnitude, i.e., f(mHz) ∼5.5 × BIMF(nT). High‐latitude Pc 3–4 pulsations detected at College are interpreted as consisting of various pulsations which are sometimes associated with the compressional Pc 3–4 waves in the magnetosphere and upstream waves in the earth's foreshock. It is also found that although there are latitudinal and longitudinal dependences of the above relationships, the compressional Pc 3–4 waves in space are well correlated with the low‐latitude Pc 3–4 pulsations recorded at the ground near the satellite's meridian. From these correlation analyses, we infer that there are two different external sources for the daytime Pc 3–4 pulsations. One is surface waves (i.e., evanescent modes) in the Pc 3–4 frequency range excited near the magnetopause, and the other source is the upstream wave in the earth's foreshock. The surface waves are the major source of pulsations in the outer magnetosphere (L ≳ 10), whereas the upstream wave transmitted through the magnetosheath and the magnetopause is the major source of the compressional Pc 3–4 waves at synchronous orbit (L = 6.67). The compressional Pc 3–4 waves, which can propagate farther into the inner magnetosphere (L < 6.6), can couple with various hydromagnetic oscillations in the plasmasphere and, finally, can be observed as the low‐latitude Pc 3–4 pulsations on the ground.
Deep depletions of total electron content associated with severe mid‐latitude gigahertz scintillations during geomagnetic storms
Authors: Ogawa Tadahiko , Kumagai Hiroshi
Published: February 2008

Using 136‐MHz Faraday rotation data obtained at three closely spaced stations, we present evidence that severe nighttime gigahertz scintillations, which appear rarely at mid‐latitudes around Japan only during geomagnetic storm conditions, are closely associated with deep depletions of total electron content (TEC). The TEC depletions amount to 2–8×1016 el/m² (10–30% of the background TEC), and their durations range from 10 min to 1 hour. These depletions move northeastward or eastward with velocities between 60 and 260 m/s. The depletions are probably not counterparts of the equatorial bubbles but seem to be formed in localized regions around Japan under complicated and peculiar ionospheric conditions. There is an indication that the oscillation of the F region caused by large‐scale TID's propagating from north to south (∼600 m/s) may initiate the generation of the depletion.
Explosive tail reconnection: The growth and expansion phases of magnetospheric substorms
Authors: Coroniti F. V.
Published: February 2008

In the current “conceptual” model of magnetospheric substorms, the growth phase terminates and the expansion phase commences with the onset of rapid reconnection at a new, near‐earth X‐type neutral line. Physical concepts developed in the analysis of the collisionless tearing mode and the flow of collisionless plasma in weakly magnetized, thin current sheets are combined to construct a model of purely collisionless, time‐dependent, ion‐dominated reconnection. Formulated in the context of time‐dependent magnetospheric convection, the model describes the reconnection collapse of the initially thick plasma sheet. In the nonlinear phase the reconnection rate grows explosively in time and saturates into a steady collisionless reconnection flow when the initial magnetic flux in the current sheet has reconnected; at saturation the reconnection rate is comparable to the maximum Petschek rate. The time scale and dynamics of the explosive reconnection model are broadly consistent with observations of substorm growth phase and expansion phase onset. For typical plasma sheet parameters the explosive reconnection electromotive force across the tail approaches 1 MV at saturation.
Magnetic field properties of Jupiter's tail at distances from 80 to 7500 Jovian radii
Authors: Goldstein M. L. , Lepping R. P. , Sittler E. C.
Published: February 2008

A detailed study of the magnetic field data from both Voyager 1 and 2 has revealed interesting properties of the near and distant Jovian magnetotail. During the first encounter, when Voyager 1 passed between 80 and 140 RJ from Jupiter in the near tail and was almost entirely in the northern lobe magnetic field, the field lines were slightly twisted into a right‐handed helix with pitch angle of about 2°–3°. The frequency spectrum of magnetic fluctuations in this region does not appear to be turbulent. A slight tendency of the magnetic field in the distant magnetotail to twist in a right‐hand sense in the northern lobe and left‐hand in the southern lobe is found as far as 7500 RJ downstream in Voyager 2 data. At nearly 7000 RJ the spacecraft was apparently in the magnetosheath near the tail magnetopause, and a 5.3‐hour periodicity in the magnetic field was seen. The magnetic spectra of the distant tail encounters from 6000 to 7500 RJ have power law indices off−5/3, identical to that of solar wind magnetic field spectra at these distances. It is possible that the distant tail magnetic fields have become turbulent, or that solar wind fields from the magnetosheath have diffused into the tail. During one event an interplanetary sector crossing was observed in Voyager 1 solar wind data, as predicted by Kurth et al. (1982). This supports their conjecture that the tail had disconnected from Jupiter. The distant tail spectra from Voyager 2 are also compared with similar spectra obtained from Voyager 1 when it was in near radial alignment with Voyager 2. Limitations of the use of the spectral analysis algorithm in the very low density regions of the Jovian tail also are discussed.
Auroral zone conductivities within the field‐aligned current sheets
Authors: Robinson R. M. , Vondrak R. R. , Potemra T. A.
Published: February 2008

We have used field‐aligned current data obtained by the Triad satellite during 21 passes over Alaska to order conductivity measurements made simultaneously by the Chatanika radar. Three distinct field‐aligned current regions were sampled during these experiments: the region 2 current sheet in the morning and evening sectors and the region 1 current sheet in the evening sector. The conductances in the region 2 evening sector current sheet increase uniformly with latitude, and the total increase in conductance across the sheet is fairly constant. The latitudinal profiles of conductance across the region 1 evening sector current sheet show that auroral arcs tend to occur in the poleward portion of the sheet. In the morning sector region 2 current sheet the Hall conductance is larger and changes much more rapidly with latitude than the Pedersen conductance. The Hall and Pedersen conductance can be interpreted in terms of the number density and temperature of a Maxwellian distribution of electrons that would produce the same conductance values. We call this electron population the equivalent Maxwellian source. The temperatures and densities thus deduced are similar to those of the electrons in the plasma sheet. Conductances in the region 2 evening sector current sheet indicate a source with a temperature between 0.5 and 2 keV and density between 0.1 and 1 cm−3. The morning sector diffuse aurora within the region 2 current sheet originates from a population with temperatures between 1 and 4 keV and density between 0.6 and 2 cm−3. In auroral arcs the conductances are consistent with those expected from plasma sheet electrons that have been accelerated along magnetic field lines. The concept of equivalent Maxwellian source is also used to study the extent to which the precipitating electrons can carry the field‐aligned current in the various regions.