bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrodynamics of the westward traveling surge
Authors: Kan J. R. , Kamide Y.
Published: February 2008

It is shown that the global convection pattern, the ionospheric current, and the field‐aligned current associated with the westward traveling surge in the asymptotic state can be modeled quantitatively as consequences of a blockage of the Hall current from closure in the magnetosphere via field‐aligned currents. The conductivity is allowed to increase self‐consistently with increasing upward field‐aligned current in the model. This inclusion of the self‐consistent enhanced ionospheric conductivity due to discrete auroral precipitations is found to generate a localized intense westward electrojet on the poleward side of the Harang discontinuity. The westward electrojet is also found to rotate counterclockwise, merging into the eastward electrojet around the leading edge of the surge. Thus the major features of the westward traveling surge can be reproduced reasonably well in our model.
Atomic nitrogen densities near the polar cusp
Authors: Engebretson M. J. , Nelson J. T.
Published: February 2008

The neutral atmospheric composition spectrometer on board the Dynamics Explorer 2 spacecraft sampled several major and minor thermospheric gases including atomic nitrogen. We present here a selection of passes over the polar cusp that provide a quantitative measure of N densities in this region and provide evidence of localized density increases due to soft particle precipitation. Increases in N densities are frequently observed but are smaller than accompanying increases in N2 densities. Our observations support earlier studies suggesting that (1) N densities increase more rapidly than O densities during periods of high solar EUV flux and (2) N densities are larger in the summer hemisphere than in the winter hemisphere. A series of passes in February 1983, late in the lifetime of DE 2, indicated N densities at 200 km altitude were a factor of 2 larger near the southern cusp than near the northern cusp.
A Safari‐Eiscat comparison between the velocity of F region small‐scale irregularities and the ion drift
Authors: Villain J. P. , Caudal G. , Hanuise C.
Published: February 2008

Simultaneous data were obtained between the SAFARI HF coherent radars and the EISCAT incoherent scatter experiment in December 1983. The SAFARI experiment (Scandinavian And French Auroral Radar Investigation) is a set of two HF coherent radars located at Lycksele (Sweden) and Oulu (Finland). These radars are able to detect λ = 10 m F region ionospheric irregularities in the polar and auroral zone. The phase velocity of the irregularities is deduced from the Doppler spectra. During the joint measurements, EISCAT was operated in a meridian scan mode. In this mode, the ion drift velocity is measured at 325‐km altitude for each different position of the radar beam, and after completion of the scan, a two‐dimensional altitude ‐ latitude map of electron density is obtained. This electron density map is used together with a ray tracing program to accurately localize the HF radar echoes. Then, the phase velocity of small‐scale F region irregularities is compared with plasma drift velocity from simultaneous observations of the same volume of the ionosphere. For the set of measurements these drifts are found to be approximately equal. Implications in the existing theories for the generation of F region plasma irregularities at high‐latitude are presented, as well as the potentialities of HF radars to measure the plasma convection at very high latitude.
Self‐consistent analysis of electron drift velocity measurements with the STARE/SABRE System
Authors: Reinleitner L. A. , Nielsen E.
Published: February 2008

Recent work with the STARE (Scandinavian Twin Auroral Radar Experiment) and SABRE (Sweden and Britain Radar Experiment) data sets has shown some inconsistency with existing theories (fluid and kinetic theory) of radar aurora. A new technique, which we call the superimposed grid point method, has proven to be a very valuable tool in the study of this inconsistency. This technique can be used under conditions where the electron drift velocity is approximately constant over an entire STARE (or SABRE) latitude interval and consists of superimposing all of the observed radial Doppler velocity components for the entire latitude interval together into one grid point. This yields, for a given electron drift velocity, a set of Doppler velocity vectors for typically more than 30 different flow angles. Thus this technique allows the study of the angular variation of the Doppler velocities observed with the radars, for a given electron drift velocity. One of the main results of using this technique has been strong evidence that very often the radar Doppler velocities are limited to near the ion acoustic velocity in a manner very similar to the equatorial electrojet. Doppler velocities at flow angles greater than some critical flow angle appear to follow a cosine law, as is usually assumed in the fluid and kinetic theory, while velocities at flow angles less than this critical flow angle seem to deviate greatly from the cosine law and are often limited to the ion acoustic velocity. We have reinterpreted the radar Doppler velocity measurements in the ion acoustic approach (Nielsen and Schlegel, 1985) and find consistency between the results of the superimposed grid point analysis and this new approach.
Heavy ion beam‐ionosphere interactions: Electron acceleration
Authors: Kaufmann R. L. , Arnoldy R. L. , Moore T. E. , Kintner P. M. , Cahill L. J. , Walker D. N.
Published: February 2008

Operation of a divergent 25‐eV Ar+ gun within an auroral arc produced dramatic changes in the flux of electrons with energies between 1 keV and the 88‐eV detector cutoff. The observations suggest that intense return currents flowed parallel to
to neutralize the Ar+ beam, particularly within a few meters of the rocket. These neutralization currents were carried above and below the rocket by the few‐eV electrons which were emitted by the gun and by colder ionospheric electrons. Such low‐energy electrons could not be measured directly by detectors on the rocket. We concluded that generation of strong field‐aligned return currents was the most important effect of ion gun operation, and that these field aligned currents were responsible for many other observable effects. Downgoing hectovolt electrons appear to have been accelerated because of interactions with waves or quasi‐stationary electric fields that were generated by the field‐aligned current. This acceleration took place throughout a cylinder centered on the rocket, with a radius of at least several meters. Acceleration of hectovolt electrons depended surprisingly little on the direction in which the Ar+ gun was pointing. All downgoing electrons were accelerated, whether or not they passed through or near the heavy ion beam. The most likely energy source for acceleration of downgoing electrons involves an interruption of the auroral current system. Strong low‐frequency waves or quasi‐stationary electric fields impede the flow of the principal current carrying electrons. Energy is transferred to the more energetic observed electrons, which can pass through the strong low‐frequency electric fields. These downgoing energetic electrons are then pitch angle scattered through Doppler resonance with the low‐frequency waves. This process produces an approximately isotropic Maxwellian distribution of downgoing electrons. Those electrons with sufficiently large gyroradii appear to scatter past 90° through a perpendicular resonance, or a finite gyroradius effect. Electrons which scatter through 90° below the rocket then are detected as upgoing electrons. The resulting upgoing electron distribution has a depression at low energies because electrons with small gyroradii cannot scatter efficiently through 90°. Stability calculations identify waves that are expected to be produced directly by the heavy ion beam and by the depression in the electron distribution function, and suggest features to look for in future flights.
Jovian ionospheric conductivity and magnetospheric plasma outflow: Voyager 1
Authors: Hairston M. R. , Hill T. W.
Published: February 2008

Using magnetic field and plasma data from the Voyager 1 inbound encounter, we have derived local values for the mass outflow rate and the radial bulk velocity in the dayside Jovian magnetosphere, and for the height‐integrated Pedersen conductivity of the Jovian ionosphere. These values, accurate within a factor of 2, are compatible with previous order‐of‐magnitude estimates of these quantities. Our results suggest that the shape of the plasma sheet is more complicated than previously suspected. A search for evidence of enhanced plasma outflow in the active sector was inconclusive.
Magnetospheric interchange instability
Authors: Cheng Andrew F.
Published: February 2008

It is shown that the conventional derivation of the MHD interchange instability criterion using an energy approach yields incorrect results. A special case involving straight, parallel flux tubes is considered in which the conventional energy argument should apply but disagrees with the result of an elementary dynamical argument. It is shown that the disagreement arises because the conventional derivation neglects the self‐consistent changes in the magnetic field that result from the flux‐tube interchange. When these self‐consistent changes are included in the energy approach, a new interchange instability criterion is derived that agrees with the dynamical criterion. The new instability criterion is qualitatively different from the conventional one and suggests that the pressure gradient of energetic ions at Jupiter may not stabilize the Io torus against interchanges. The inner magnetospheres of Earth, Jupiter, and Saturn may all be interchange unstable.
Longitudinal phase and polarization characteristics in mid‐latitude Pi 2 pulsations
Authors: Gelpi C. , Hughes W. J. , Singer H. J.
Published: February 2008

Polarization and phase characteristics of mid‐latitude Pi 2 pulsations are studied using the Air Force Geophysics Laboratory Magnetometer Network. From the analysis of 55 Pi 2 events it is concluded that the sense of apparent phase velocity and the sense of polarization are related as follows: Westward phase velocity is associated with left‐handed polarization, while eastward phase velocity coincides with right‐handed polarization. Phase changes per degree longitude as determined by the D component are larger than those computed from the H component. These results are compared to a previous analysis of nighttime pulsations. The differences in apparent phase velocities are discussed with regard to a recent model for the Pi 2 longitudinal polarization pattern.
Kinetic temperatures of heavy ions in the solar wind
Authors: Bochsler P. , Geiss J. , Joos R.
Published: February 2008

From a refined analysis of 4 years of data of the ion composition instrument on board ISEE 3 we find that in the overwhelming majority of observations kinetic temperatures of ions are approximately proportional to their masses. The logarithmic average for T(4He++) is 5.363; for T(O6+) and T(O7+) it is 5.978 and 6.000, respectively, corresponding to T(O)/T(He) = 4.2. For 〈log T(Fe)〉 we find 6.52, corresponding to T(Fe)/T(He) = 14. The correlation coefficients between oxygen and helium kinetic temperatures are high (∼0.78 or above), whereas for the correlation log T(Fe) versus log T(He) we find a value of only 0.44 due to the large measurement uncertainties of T(Fe). Significant deviations from the mass/temperature proportionality are found at occasions of cool and dense solar wind flow when Coulomb collisions succeed to equilibrate kinetic temperatures of different ion species.
Simulation of the westward traveling surge and Pi 2 pulsations during substorms
Authors: Kan J. R. , Sun W.
Published: February 2008

The westward traveling surge and the Pi 2 pulsations are simulated as a consequence of an enhanced magnetospheric convection in a model of magnetosphere‐ionosphere coupling. The coupling is characterized by the bouncing of Alfvén waves launched by the enhanced convection. The reflection of Alfvén waves from the ionosphere is treated in which the height‐integrated conductivity is allowed to be highly nonuniform and fully anisotropic. The reflection of Alfvén waves from the magnetosphere is characterized by the coefficient Rm, depending on whether the field lines are open or closed. On open field lines the reflection coefficient from the magnetopause is given by Rm ≃ −1. On closed field lines the reflection coefficient is given by (1) Rm ≃ +1 in the near‐earth plasma sheet and (2) −1 ≲ Rm ≲ +1 in the low‐latitude boundary layer and the distant plasma sheet. The conductivity in the model is self‐consistently enhanced with increasing upward field‐aligned current density. The results of the simulation, including the convection pattern, the electrojets, the field‐aligned current, the conductivity enhancement, the oscillation of the westward electrojet, and the average speed of the westward surge are in reasonable agreement with the features of the westward traveling surge and the Pi 2 pulsations observed during substorms.
The generation of electrostatic noise in the plasma sheet boundary layer
Authors: Dusenbery P. B. , Lyons L. R.
Published: February 2008

The one and two ion beam instability is considered as a possible explanation for the observations of broadband electrostatic noise in the plasma sheet region of the geomagnetic tail. When only hot streaming plasma sheet boundary layer ions are present, no broadband waves are excited. Cold, streaming ionospheric ions can generate electrostatic broadband waves propagating in the slow beam acoustic mode, but the growth rates of the waves are significantly enhanced when hot boundary layer ions are present. (Both the slow and fast beam acoustic modes can be excited, depending on the relative ion drift.) This model predicts that the wave intensity of the broad band noise should peak in the plasma sheet boundary layer. Observations of less intense electrostatic waves in the lobes and plasma sheet are likely a result of the absence of hot ion beams or large ion temperatures, respectively, which result in smaller growth rates. The ion beam instability may play an important role in the formation of the central plasma sheet.
Evidence for 50‐MHz bistatic radio observations of electrostatic ion cyclotron waves in the auroral plasma
Authors: Haldoupis C. , Prikryl P. , Sofko G. J. , Koehler J. A.
Published: February 2008

Crossed‐beam spectral measurements, made with two bistatic CW Doppler systems operating at 50 MHz, often reveal a type of radio auroral spectrum characterized by narrow peaks near 30 and 55 Hz. The experiment was designed so that spectra could be measured simultaneously from a common scattering region along two directions which differed in azimuth by 65° and which were at magnetic aspect angles of ∼5° and ∼12° from perpendicularity with the earth's field at 110 km altitude. During two magnetically active periods of continuous backscatter, this spectral type was found to occur regularly, especially in the few hours prior to magnetic midnight. Usually, the echoes were strong and lasted several minutes. The resonant peak in the spectrum occurs at either positive or negative shifts, or sometimes both. Mostly, the power lies in a narrow Doppler frequency band centered at either 55 or 30 Hz, but sometimes the power is in both bands, particularly for the path of larger magnetic aspect angle. The spectra can be almost identical in both observing directions but also can show systematic differences. From the relatively fixed locations of the spectral peaks, and from the occurrence of the echoes at angles well away from perpendicularity with the magnetic field, one infers that the echoes are due to primary 3‐m plasma waves having a finite wave vector component k∥ along the magnetic field lines and with ratio k∥ /k⊥ possibly as large as 1/3.5. The spectral evidence suggests the existence of electrostatic ion cyclotron waves near the gyrofrequencies of the main E region ions (i.e., NO+, O2+, O+). The existing kinetic and fluid theories of the ion cyclotron instability explain some, but not all, of the observed properties. Kinetic theory cannot explain the direct excitation of short (3 m) waves in the E region at such large magnetic aspect angles. Yet the frequent observation of these waves shows that the theories must be modified so as to relax the conditions under which the waves occur and to allow for Doppler shifts at almost exactly the gyrofrequencies of the main ions.
The heating of the solar wind by the interstellar neutral gas
Authors: Isenberg P. A. , Chih P. P. , Fisk L. A.
Published: February 2008

We use a time‐dependent, one‐dimensional, spherically symmetric, one‐fluid solar wind model to investigate the heating of the solar wind by an interaction with the interstellar neutral hydrogen. The interaction is modeled through particle, momentum, and energy source terms equivalent to those of T. E. Holzer (1972). The equations are solved using a flux‐corrected transport algorithm which incorporates stream‐stream heating directly into the model. We input solar wind data at 1 AU for the time between October 12, 1978, and February 25, 1980, and generate 52‐day averages of solar wind temperature as functions of radius out to 10 AU for values of the interstellar hydrogen density NH = 0.0, 0.03, and 0.1 cm−3. The radial dependence of temperature, averaged over the entire time period, yields power law exponents α = 0.714, 0.597, and 0.428 for the three densities, respectively. A more detailed comparison between the temperature measurements of Voyager 1 and Pioneer 11, and the model solar wind temperatures at the positions of these spacecraft indicates that the observations are consistent with solar wind heating by the interstellar neutral hydrogen, at least within the context of the model. We close with a suggestion for further data analysis to determine the radial gradient of proton temperature beyond 4 AU.
On the dynamo generation of flux ropes in the Venus ionosphere
Authors: Luhmann J. G. , Elphic R. C.
Published: February 2008

Small scale (∼10 km) magnetic field structures or “flux ropes” observed in the ionosphere of Venus can be interpreted as the result of a kinematic dynamo process acting on weak seed fields. The seed fields result from the prevailing downward convection of magnetic flux from the vicinity of the ionopause, while small scale fluctuations in the velocity of the ionospheric plasma, which can be caused by collisional coupling to gravity waves in the neutral atmosphere, provide the mechanism by which the field is twisted and redistributed into features of similar scale. This mechanism naturally explains some of the average properties of flux ropes such as the variation of their characteristics with altitude and solar zenith angle. It also elucidates the relationship between the large scale and small scale ionospheric magnetic fields.
Observations of the right‐hand resonant ion beam instability in the distant plasma sheet boundary layer
Authors: Tsurutani B. T. , Richardson I. G. , Thorne R. M. , Butler W. , Smith E. J. , Cowley S. W. H. , Gary S. P. , Akasofu S.‐I. , Zwickl R. D.
Published: February 2008

From the ISEE 3 distant tail prime intervals of study (days 24–43 and 169–193, 1983), 66 large amplitude wave events at the plasma sheet boundary layer were identified, analyzed and tabulated. It is found that (1) 83% of the wave events are clearly right‐hand polarized in the spacecraft frame (arguments are given as to why they must also be right‐hand polarized in the plasma frame), (2) the waves are propagating nearly parallel to the ambient magnetic field (θkB < 15°), (3) they have a significant compressional‐to‐transverse amplitude ratio (typically 0.2–0.5), of which only part can be ascribed as being due to cross‐field propagation, (4) the waves have a maximum amplitude of 6–7 nT peak to peak (presumably caused by a presently unexplained saturation effect), (5) the waves have rest frame frequencies within the range 0.3 to 2.0 Ω+, and (6) the waves are strongly substorm dependent. Several case studies involving simultaneous energetic (>35 keV) ion measurements have been analyzed and are presented. Highly anistropic, tailward streaming, relatively cold ion beams (1000–100∶1 anisotropy ratio) are found to occur in near time coincidence with the waves. The ion beam properties are calculated and are compared with theoretical predictions of right‐hand resonant and nonresonant ion beam instabilities. It is demonstrated that for the cases examined, the measured beam and wave parameters are in agreement with the presence of the resonant instability. It is believed that this is the first simultaneous observation of the waves and the unscattered cold ion beam of the right‐hand resonant ion beam instability in either space plasma or laboratory measurements. Because of the absence of strong convective effects separating the waves and the generating ion beam, the plasma sheet boundary layer is an excellent region of space to study the details and development of this very important plasma instability.
Broadband electrostatic noise produced by ion beams in the Earth's magnetotail
Authors: Omidi N.
Published: February 2008

Spacecraft observations in the earth's magnetotail at distances of 30 to 40 RE have revealed the presence of broadband electrostatic waves. These waves are generally most intense in the regions just outside of the plasma sheet and are correlated with the observations of relatively cold and energetic ion beams traveling in either the earthward or the tailward direction. These waves are observed to propagate obliquely to the geomagnetic field with wave normal angles around 70°. Because the broadband electrostatic noise is the most intense of the waves observed in the magnetotail, it is important to understand the generation mechanism of these waves. The purpose of this study is to provide for the first time a correct solution to the dispersion equation for ion beams observed in the magnetotail. By numerically solving this equation, it is shown that obliquely propagating waves have growth rates that can be an order of magnitude larger than those of parallel propagating waves, in agreement with observations. In addition, the effect of beam temperature on the ion beam instability is studied, and it is shown that this instability can be a viable generation mechanism only when the ion beam has a relatively small thermal spread.
An interplanetary magnetic field ensemble at 1 AU
Authors: Matthaeus W. H. , Goldstein M. L. , King J. H.
Published: February 2008

A method for calculating ensemble averages from magnetic field data is described. A data set comprising approximately sixteen months of nearly continuous ISEE‐3 magnetic field data is used in this study. Individual subintervals of this data, ranging from 15 hours to 15.6 days comprise the ensemble. The sole condition for including each subinterval in the averages is the degree to which it represents a weakly time‐stationary process. Averages obtained by this method are appropriate for a turbulence description of the interplanetary medium. The ensemble average correlation length obtained from all subintervals is found to be 4.9×1011 cm. The average value of the variances of the magnetic field components are in the approximate ratio 8∶9∶10, where the third component is the local mean field direction. The correlation lengths and variances are found to have a systematic variation with subinterval duration, reflecting the important role of low‐frequency fluctuations in the interplanetary medium.
Simulation of the collisionless tearing instability in an anisotropic neutral sheet
Authors: Ambrosiano John , Lee Lou C. , Fu Z. F.
Published: February 2008

A two‐dimensional magnetoinductive particle‐in‐cell code is used to compare the stability of isotropic and anisotropic current layers in a collisionless plasma. We find, in agreement with recent theories, that temperature anisotropy with T⊥ > T∥ significantly enhances the instability. The simulation growth rates agree well with analytical predictions of growth rates in the linear phase. Nonlinear growth rates are significantly larger than linear growth rates and are apparently the result of coalescence instability. For a fixed system length the influence of coalescence on the growth rates increases with anisotropy. This is evidently because of a shift in the most unstable mode to shorter wavelength with increasing anisotropy. Thus anisotropic current layers tend to produce large amplitude small wavelength islands which rapidly merge.
Beat excitation of whistler mode sidebands using the Siple VLF transmitter
Authors: Helliwell R. A. , Inan U. S. , Katsufrakis J. P. , Carpenter D. L.
Published: February 2008

The process of whistler mode sideband generation in the magnetosphere was studied by transmitting two monochromatic signals closely spaced in frequency (Δf = 5–45 Hz) from the experimental VLF transmitter at Siple Station, Antarctica. The signals were observed following ducted magnetospheric propagation to the conjugate station at Roberval, Quebec. Sidebands up to seventh order were generated extending to ±100 Hz with respect to the average frequency of the carriers. New frequencies were observed both under conditions of little or no growth of the input signals and when one of the input signals did grow significantly. At times the sideband amplitudes exceeded the intensity of either input signal. A small signal mechanism is proposed in which emissions are triggered by each beat between the input waves but are then suppressed by the following beat. The energy from the phase bunched particles is believed to feed preferentially into the sidebands rather than causing growth of the input waves themselves. In this model, phase trapping of resonant electrons by the wave is not required. The observed process of sideband generation provides a mechanism to break down the coherence of relatively narrowband waves in the magnetosphere and may thus account for existing evidence of the transformation of relatively coherent wave packets into broader more variable bands of noiselike signals.
On plasma disturbances caused by the motion of the space shuttle and small satellites: A comparison of in situ observations
Authors: Samir U. , Stone N. H. , Wright K. H.
Published: February 2008

Recent results regarding the interactions between a body and its environmental space plasma, made by charged particle probes mounted in the bay of the space shuttle Orbiter Columbia (STS 3 mission), are compared with earlier results, obtained from small ionospheric satellites, in an attempt to widen our scope of knowledge and understanding regarding such interactions. The objective is to work toward a unified model of body‐space plasma interactions in the solar system covering a variety of plasma and body conditions. The comparisons focus mainly on (1) the (wake/ram) current ratio; (2) the generation of charged particle density fluctuations (indicative of plasma turbulence) around the body; and (3) the increase in electron temperature ahead and in the wake of the satellite. The main results of the comparison are that (1) the (wake/ram) current ratio (or current depletion in the wake) for the Orbiter is 1 to 2 orders of magnitude larger than the ratio for small ionospheric satellites; and (2) fluctuations in density (or turbulence) are observed for both “large body” (shuttle Orbiter) and “smaller body” (standard ionospheric satellites). However, the cause of the turbulence may not be the same for both cases; (3) the results for the electronic temperature enhancement due to the shuttle Orbiter are in contrast with measurements from smaller ionospheric satellites. We suggest a path to follow in future shuttle experiments and caution that care be taken in interpreting local particle and field measurements.
The role of atmospheric planetary‐scale waves in the D region winter anomaly
Authors: Muraoka Y. , Petzoldt K. , Labitzke K.
Published: February 2008

Characteristics of the D region winter anomaly are examined, by using ionospheric data along a meridian chain of stations near Japan. The data were obtained by means of long‐distance VLF radio wave propagation and HF radio wave vertical sounding. Interesting features are the equatorward extension of the anomaly and the variability of its duration. These features are further discussed in comparison with radiance data measured by pressure modulator radiometer channel 3000 (maximum weight near 80‐km altitude in the mesosphere) on board Nimbus 6 during two winters, 1975/1976 and 1976/1977. Meteorological conditions during these winters were quite different, a major stratospheric sudden warming taking place during the latter winter but not during the former winter. The key result of this study is that the occurrence of the winter anomaly is basically associated with the amplification of a planetary‐scale wave with zonal wave number 1 in the mesosphere. At the same time we find that the local appearance of the anomaly near Japan is controlled by the movement and latitudinal structure of the excited wave 1. It is further indicated that the increase in mesospheric local temperature is not always associated with the occurrence of the anomaly, suggesting that another dynamical effect of planetary‐scale waves is important for the occurrence of the anomaly.
Early time interaction of lithium ions with the solar wind in the AMPTE Mission
Authors: Lui A. T. Y. , Goodrich C. C. , Mankofsky A. , Papadopoulos K.
Published: February 2008

The early time interaction of an artificially injected lithium cloud with the solar wind is simulated with a one‐dimensional hybrid code. Simulation results indicate that the lithium cloud presents an obstacle to the solar wind flow, forming a shock‐like interaction region. Several notable features are found: (1) The magnetic field is enhanced up to a factor of about 6, followed by a magnetic cavity downstream. (2) Solar wind ions are slowed down inside the lithium cloud, with substantial upstream reflection. (3) Most of the lithium ions gradually pick up the velocity of the solar wind and move downstream. (4) Intense and short‐wavelength electric fields exist ahead of the interaction region. (5) Strong electron heating occurs within the lithium cloud. (6) The convection electric field in the solar wind is modulated in the interaction region. The simulation results are in remarkable agreement with in situ spacecraft measurements made during lithium releases in the solar wind by the AMPTE (Active Magnetospheric Particle Tracer Explorers) Program.
New models of Saturn's magnetic field using Pioneer 11 vector helium magnetometer data
Authors: Davis Leverett , Smith Edward J.
Published: February 2008

In a reanalysis of the Vector Helium Magnetometer data taken by Pioneer 11 during its Saturn encounter in 1979, using improvements in the data set and in the procedures, studies are made of a variety of models. The best is the P1184 model, an axisymmetric spherical harmonic model of Saturn's magnetic field within 8 Saturn radii of the planet. Its coefficients for the internal sources are g10 = 0.2114, g20 = 0.0160, g30 = 0.0226 gauss and for the external sources G10 = −7.1 nT while Pioneer 11 was approaching Saturn and −12.6 nT while it was leaving. The appropriately weighted root mean square average of the difference between the observed and the modeled field is 1.13%. For the Voyager‐based Z3 model of Connerney, Acuña, and Ness, this average difference from the Pioneer 11 data is 1.81%. The external source currents in the magnetopause, tail, bow shock, and perhaps ring currents vary with time and can only be crudely modeled. An algebraic formula is derived for calculating the L shells on which energetic charged particles drift in axisymmetric fields.
An auroral arc in the late evening plasma flow reversal
Authors: Cahill Laurence J. , Arnoldy R. L. , Behm D. , McCready M. A.
Published: February 2008

A sounding rocket equipped to measure the electric field, the charged particle distribution, and plasma waves was launched on January 27, 1980, into the late evening plasma flow reversal. Chatanika radar provided plasma flow velocities and plasma densities from overhead at the launch site to 500 km north of the launch site. The radar observations before, during, and after the rocket flight together with the rocket measurements give an unusually detailed view of the plasma flow reversal near midnight and of an auroral arc in the flow reversal. A small substorm started about an hour before launch, causing a southward movement of the flow pattern. One half hour before launch the flow reversal line was 300‐km north of the launch site, within range of the rocket trajectory. Shortly before launch the flow pattern abruptly moved north but returned briefly as the rocket was passing through peak altitude. Just after the rocket passed through the reversal, near peak altitude, an auroral arc was encountered. Associated with the auroral arc was an inverted V electron energy spectrum and, on the equatorward edge, an intense burst of auroral hiss and a very thin sheet of field‐aligned electrons. The flow reversal moved north after this brief encounter and returned south again only after the rocket flight.
Variation of auroral electrojet spatial location as a function of the level of magnetospheric activity
Authors: Rostoker Gordon , Duc Phan Tai
Published: February 2008

The auroral oval is the site of ionospheric electric currents, some of which diverge into and out of the ionosphere to flow as Birkeland currents along the magnetic lines of force. The currents flowing parallel to the contours of the oval are known as the auroral electrojets, and their strength reflects the combined effect of ionospheric conductivity and the electric field. In this paper we show quantitatively the variation of the poleward and equatorward borders of the auroral electrojets as a function of the level of magnetospheric activity as quantified by the AE index. We find the latitudinal width of the electrojets to be relatively constant over levels of magnetic activity ranging over AE values of up to ∼500 nT. The eastward and westward electrojets, while retaining a constant width of ∼6°, both expand equatorward with increasing magnetospheric activity; however, the equatorward shift is much more pronounced for the eastward jet than for the westward one, particularly in the dusk sector.
Hot, diamagnetic cavities upstream from the Earth's bow shock
Authors: Thomsen M. F. , Gosling J. T. , Fuselier S. A. , Bame S. J. , Russell C. T.
Published: February 2008

An unusual type of plasma structure was observed by ISEE‐1 and ‐2 upstream from the earth's bow shock on eight occasions during the first 2 years of the mission. These events typically last ∼1–2 minutes and have very striking magnetic field and plasma signatures. The magnitude of the field is strongly reduced within the events, with the low‐field region (the magnetic cavity) flanked on one or both sides by strong enhancements. Within the cavity the plasma density is reduced compared to the ambient solar wind, and the flow velocity is much smaller and strongly deflected, with the direction of deflection correlated with the spacecraft location relative to the sun‐earth line. In addition, the ion and electron temperatures are very high (Ti ∼2×107 °K and Te ∼106 °K). Within the low‐field region the total pressure is an order of magnitude higher than in the ambient solar wind, and this pressure imbalance appears to be only partially offset by magnetic tension. These structures are interpreted as hot, probably expanding, diamagnetic cavities with a scale size on the order of a few RE, essentially convecting with the solar wind. A number of possible origins for the cavities have been explored, but no clear and completely satisfying explanation has been found. The most promising possibility considered is that the observed structures represent the solar wind signature of magnetopause flux transfer events.
Intense sheared flow as the origin of large‐scale undulations of the edge of the diffuse aurora
Authors: Kelley M. C.
Published: February 2008

Using photographs from the DMSP satellite Lui et al. (1982) have found several examples of large scale undulations of the edge of the diffuse aurora. Several satellite experimenters have previously reported intense sheared flow near the edge of the diffuse aurora as evidenced either by detection of large localized meridional electric fields or by narrow regions of high azimuthal flow velocity. It is suggested here that the diffuse auroral undulations are caused by an instability of this sheared flow. In order to test this idea, a search for in situ satellite flow data associated with the events presented by Lui et al. was made. In addition the shear flow events reported by Rich et al. (1980) were used as a basis to search for near simultaneous DMSP photos. The cross‐section of these data sets was not large. The closest example was a satellite pass indicating shear 20 minutes after a DMSP photo showing strong undulations which had been developing for at least an hour. A theory recently published by Viñas and Madden (1986) is in good agreement with the observations in that, (1) several events studied by Rich et al. satisfied the magnetic Richardson number criterion for instability and (2) the observed wavelengths are in good agreement with the theory. A recent numerical simulation by Pritchett (1985) also seems to support the concept that the edge of the diffuse aurora may be unstable to a shear driven process. An interesting feature of the analysis presented here is that when scaled from ionospheric heights to the equatorial plane, the shear frequency is only reduced by 30–40% from ionospheric values.
A theoretical study of the production and decay of localized electron density enhancements in the polar ionosphere
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

The origins, transport, and decay of large‐scale (≳10 km) F region density irregularities were theoretically studied using a high‐latitude time dependent ionospheric model. Such density irregularities (blobs) have been found both in the polar cap and the auroral zone. The model study, which focuses on blobs being produced by auroral precipitation, shows that the observed energy fluxes can readily account for the blob densities if a plasma flux tube is exposed to the precipitation for 5–10 min. Once the flux tube is transported away from the source, the F region density profile recovers its shape on a time scale of 10–20 min. Hence after this time period it is not possible to determine whether hard or soft precipitation was the source of the blob. Once created a blob will maintain its relative size for many hours, since it decays at the same rate as the background plasma. Blobs are removed by being transported into a region of high production, where the new background density exceeds the old blob density. The high hmF2's observed with some blobs need not necessarily imply soft precipitation; induced upward plasma drifts from neural winds or E×B convection can readily account for them. Since the high latitude ionosphere contains discrete auroral forms in the oval and polar cap and since plasma is continually convecting through these regions, blobs are the rule rather than the exception.
The motion of ions specularly reflected off a quasi‐parallel shock in the presence of large‐amplitude, monochromatic MHD waves
Authors: Fuselier S. A. , Gosling J. T. , Thomsen M. F.
Published: February 2008

A test particle model is used to study the motion of ions specularly reflected off a shock in the presence of large‐amplitude, monochromatic, transverse MHD waves. The characteristics of the motion depend on the frequency, wavelength, phase, and amplitude of the wave that is being convected into the shock. For low wave frequencies and long wavelengths (ω′ ≪ ion gyrofrequency, λ ≫ ion gyroradius), the ion motion depends only upon θBn(ϕ0), the instantaneous angle between the total magnetic field at the shock (ambient + wave) and the shock normal. For high wave frequencies and short wavelengths (ω′ ≫ ion gyrofrequency, λ ≪ ion gyroradius), the ion motion depends only upon θBn0, the angle between the ambient magnetic field and the shock normal. For intermediate frequencies and wavelengths, including those of interest in the region upstream from the earth';s bow shock (ω′ ∼ ion gyrofrequency, λ ∼ ion gyroradius), no simple θBn0 or θBn(ϕ0)criterion for the ion motion is found. For example, at intermediate frequencies, the motion depends both on θBn(ϕ0) and θBn0 as well as upon b/B, the ratio of the wave amplitude to ambient magnetic field strength. In general, the presence of upstream waves inhibits the escape of specularly reflected ions from the shock, the effect being greatest when the wave amplitude is large. Other properties of ion motion that are affected by the presence of an MHD wave include the propagation time of an ion from the shock to a given upstream distance, the time variation of the gyrospeed and guiding center speed, and the energy upon return to the shock.
Computer simulations of electromagnetic cool ion beam instabilities
Authors: Gary S. Peter , Madland Christian D. , Schriver David , Winske Dan
Published: February 2008

Electromagnetic instabilities driven by a relatively cool ion beam are studied by one‐dimensional hybrid computer simulations. Both the beam and the instabilities propagate parallel or antiparallel to a uniform magnetic field. At relatively small beam‐core relative drift speeds, similar to conditions predicted for ion beams upstream of slow shocks in the magnetotail, the right‐hand resonant instability exhibits quasi‐linear behavior. In particular, instability saturation is due to a reduction in the beam‐core relative drift speed as well as to an increase in the perpendicular‐to‐parallel beam temperature. For tenuous beams in this quasi‐linear regime the fluctuating magnetic field amplitude at saturation scales as the beam drift kinetic energy. At higher drift speeds, including parameters similar to those upstream of the quasi‐perpendicular bow shock, the right‐hand resonant and nonresonant instabilities both lead to phase bunching of the ion beam. The relative phase between the fluctuating velocity vector of the beam and the fluctuating magnetic field is examined; computed values agree with both linear theory during the early growth phase and nonlinear arguments at saturation. In this nonlinear regime both instabilities saturate when approximately half the initial beam drift kinetic energy density is converted to fluctuating magnetic field energy density.
The magnetic field of the equatorial magnetotail from 10 to 40 RE
Authors: Fairfield D. H.
Published: February 2008

A statistical study of IMP 6, 7, and 8 magnetotail magnetic field measurements near the equatorial plane reveals new information about various aspects of magnetospheric structure. More magnetic flux crosses the equatorial plane on the dawn and dusk flanks of the tail than near midnight, but no evidence is found for a dependence on the interplanetary magnetic field sector polarity. Field magnitudes within 3 RE of the equatorial plane near dawn are more than twice as large as those near dusk for −20 < Xsm < −10 RE. The frequency of occurrence of southward fields is greatest near midnight, and such fields are seen almost twice as often for −20 < Xsm < −10 RE as for Xsm beyond −20 Re. This latter result supports the idea that the midnight region of the tail between 10 and 20 RE is a special location where neutral lines are particularly apt to form. Such a neutral line will approach nearest the earth in the midnight and premidnight region, where substorms are thought to have their onset.
Observation of reconnection phenomena at synchronous orbit
Authors: Croley D. R. , Fennell J. F. , Ledley B. G.
Published: February 2008

On October 6, 1979, the solar wind pressure increased suddenly causing earthward motion of the magnetopause in past 6.6 RE. Plasma and magnetic field measurements were made in the magnetosheath and boundary layer close to the magnetopause in the noon sector. Large‐amplitude variations in the magnetosheath magnetic field intensity with a quasi‐period of 10 s were observed. Nearly static structures of high‐β plasma associated with the magnetic field minima and low‐β plasma associated with the magnetic field maxima were observed convecting toward the earth. Flux transfer events were observed at the magnetopause with the same time scale and amplitude as the magnetosheath magnetic field variations in the southward magnetosheath magnetic field. No flux transfer events were observed during an extended period of northward magnetosheath magnetic field. The morphology of charged particle data for the transition between the magnetosphere and the magnetosheath for a northward magnetosheath magnetic field is presented. A convection electric field of ∼9 mV/m was inferred from the spectral difference between solar and antisolar drifting ions in the magnetosheath near a magnetopause rotational discontinuity. A trapped component was observed in the magnetosheath plasma during both northward and southward magnetosheath magnetic fields. It is concluded that flux transfer events result from the interaction of the earthward convecting plasma filaments with magnetospheric magnetic field at the magnetopause. The estimated flux erosion from the dayside magnetosphere is ∼3×1011 Mx/s.
Digital ionosonde observations during equatorial spread F
Authors: Argo P. E. , Kelley M. C.
Published: February 2008

In this paper we present and discuss equatorial spread F data taken with a digital ionosonde/HF radar located at Huancayo, Peru. A modified phenomenology is developed which uses the system's ability to do echo location. The onset of irregularities is seen to occur in the east and to move westward, while inside this large‐scale structure the plasma is found to drift eastward. A very curious difference has been identified between spread F observations with the ionosonde and with the VHF radar at Jicamarca. At VHF, spread F onset often occurs when the ionosphere is rising, whereas in all five examples presented here, the digital ionosonde detected onset when the apparent ionosphere motion was downward. The result even held on the one night of common data taking. The effect could be instrumental but may be related to the considerable orographic differences in the two sites. Isolated scattering patches are observed and are tentatively identified as detached or “fossil” plumes. At frequencies above the nominal f0F2 the system (and other ionosondes) may in fact function as a coherent radar. During one night, data were obtained simultaneously with the HF radar, a rocket, and the Jicamarca VHF radar. Comparisons of these data are discussed in detail. Finally, additional evidence is presented that acoustic gravity waves play a role in the development of equatorial spread F and in the formation of detached plumes. To be self‐consistent, the gravity waves must come from nearby sources such as the tropical rain forest to the east of Jicamarca.