bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical simulations of tearing mode instabilities
Authors: Swift Daniel W.
Published: February 2008

The results of a series of two‐dimensional numerical particle simulations and a linear analysis of the collisionless tearing mode instability are presented. Comparisons are made between codes which are run with (1) ion dynamics only and magnetic field confined to the plane of the simulation, (2) ion and electron dynamics, including electrostatic interaction, and magnetic field in the plane of the simulation, and (3) ions and electrons and all three components of the magnetic field, as well as the electrostatic field. It is found that the addition of electrons significantly reduces the linear growth and decreases the wavelength of the most linearly unstable mode. The growth rate and wavelength of the most linearly unstable mode are found to be consistent with linear analysis. The addition of the magnetic field component perpendicular to the plane of the simulation has a negligible effect on the linear growth rates or on subsequent nonlinear behavior. Significant electrostatic fields are generated in the nonlinear explosive phase. Somewhat larger potentials are generated in run 3 because of the finite component of the magnetic field normal to the plane of the simulation.
In situ magnetic field measurements during AMPTE solar wind Li+ releases
Authors: Lühr H. , Southwood D. J. , Klöcker N. , Acuña M. , Häusler B. , Dunlop M. W. , Mier‐Jedrzejowicz W. A. C. , Rijnbeek R. P. , Six M.
Published: February 2008

Data recorded by the magnetometers on the German (IRM) and British (UKS) spacecraft of the Active Magnetospheric Particle Tracer Explorers (AMPTE) spacecraft mission are described during the immediate period following the two releases of lithium from the IRM during September. Ions created in the first seconds of the release form a coherent obstacle to solar wind flow. A cavity from which the interplanetary magnetic field is excluded is detected. Outside the cavity the field is compressed, and subsequently the cavity is convected downstream. We compare what is observed with other relevant natural interactions but also emphasize the unique features of this experiment.
AMPTE/UKS electron measurements during the lithium releases of September 11 and 20, 1984
Authors: Hall D. S. , Bryant D. A. , Chaloner C. P. , Bingham R. , Lepine D. R.
Published: February 2008

Following releases of lithium ions into the solar wind by the Active Magnetospheric Particle Tracer Explorers (AMPTE)‐IRM spacecraft on September 11 and 20, 1984, electron intensities measured at the AMPTE/UKS spacecraft, some 33 km away from the IRM, increased significantly for tens of seconds. The causes of the increases are considered and found to be the result of energization of solar wind electrons. The degree of energy dependence of the energization is established, and possible causative processes are considered. No positive indications of adiabatic compression or acceleration through a potential difference are found. There is, however, evidence for a resonant wave‐particle interaction.
Near‐tail reconnection as the cause of cometary tail disconnections
Authors: Russell C. T. , Saunders M. A. , Phillips J. L. , Fedder J. A.
Published: February 2008

In a cometary tail disconnection event the plasma tail appears to separate from the coma and to accelerate away from it. As this occurs a new tail begins to form. We propose that these disconnections arise in a manner analogous to geomagnetic substorms, i.e., by the formation of a strongly reconnecting region in the near tail that forms a magnetic island in the coma and ejects the plasma tail by strengthening the magnetic “slingshot” within the tail. This reconnection process may be triggered by several different processes, such as interplanetary shocks or variations in the Alfven Mach number.
Ion acceleration by lower hybrid waves in the suprauroral region
Authors: Retterer John M. , Chang Tom , Jasperse J. R.
Published: February 2008

In an effort to describe ion‐conic acceleration, particle plasma simulation is used to study the generation of VLF waves below field‐aligned potential drops and the effect of the resulting turbulence on the ion population in the suprauroral region. To model the situation, a weak, energetic electron beam traveling along the magnetic field is allowed to destabilize a cool electron‐ion plasma. To describe the ion acceleration observed in the simulation, a theoretical model is developed using mode‐mode coupling processes to generate the low phase velocity VLF waves with which the ions first interact. By scaling the simulation results to suprauroral conditions, we demonstrate that the VLF turbulence generated below field‐aligned potential drops can account for the acceleration of ions observed in energetic ion conic distributions.
The dependence of solar modulation on the sign of the cosmic ray particle charge
Authors: Garcia‐Munoz M. , Meyer P. , Pyle K. R. , Simpson J. A. , Evenson P.
Published: February 2008

We compare the solar modulation of galactic cosmic ray helium and electrons at 1 AU, within the 600‐ to 1000‐MV magnetic rigidity interval, during a period covering 20 years, from the solar minimum of 1965 through 1984. Most of the data were obtained by University of Chicago cosmic ray telescopes on board balloons and the IMP series and ISEE 3 satellites. Comparison of the time‐intensity variations during the two solar maxima around 1970 and 1981 shows that after 1970 the intensity of helium recovers earlier than that of the electrons, whereas after 1981 the intensity of electrons recovers earlier than that of helium. Within the measured magnetic rigidity interval the flux ratio of helium to electrons (He/e) undergoes a major increase during 1969–1971 and a major decrease during 1979–1983, periods that include the times of reversal of the polarity of the solar polar magnetic fields. A conventional model of solar modulation can describe the modulation of protons and helium from 1965 through 1984, but its predictions depart significantly from the observed modulation of electrons around and after the 1981 solar maximum. The difference in particle velocity between nuclei and electrons in the rigidity interval under study cannot alone account for the observations. The above experimental findings are consistent with a dependence of the solar modulation of galactic cosmic rays on the sign of the particle charge.
Accelerated plasma flows at the near‐tail magnetopause
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Russell C. T.
Published: February 2008

The Los Alamos/Garching fast plasma experiments on ISEE 1 and 2 detected a large number of accelerated flow events at the near‐tail dusk magnetopause separating the plasma sheet from the magnetosheath. Such events are common when the local magnetosheath and plasma sheet magnetic fields are nearly antiparallel, and they can persist for hours at a time. Accelerated plasma flows at the near‐tail magnetopause are observed only within the current layer where the field rotates from its magnetosheath to its magnetotail orientation and are characterized by (1) tailward directed flow at speeds up to ∼2 times faster than observed in the adjacent magnetosheath, (2) densities comparable to but slightly lower than those in the magnetosheath, and (3) ion and electron temperatures intermediate between magnetosheath and plasma sheet values. The highest flow speeds and the lowest densities within the current layer typically occur near the inside edge. A thin region of density‐depleted field lines often exists immediately earthward of the accelerated flow region. The observed velocity changes at the magnetopause are in rough agreement with tangential stress balance, and we interpret these flows as direct evidence for field line merging near and perhaps tailward of the dawn‐dusk terminator. Accelerated flows are much more rare at the dawn tail magnetopause in the ISEE data. This dawn‐dusk asymmetry may be an artifact associated with a seasonally dependent warping of the midtail neutral sheet. A more likely explanation is that the asymmetry is caused by the preferred draping of the interplanetary magnetic field about the dusk magnetopause.
Relativistic electron and energetic ion precipitation spikes near the plasmapause
Authors: Imhof W. L. , Voss H. D. , Reagan J. B. , Datlowe D. W. , Gaines E. E. , Mobilia J. , Evans D. S.
Published: February 2008

An investigation has been made of electron and associated ion precipitation spikes near the plasmapause that are narrow in L shell and in which relativistic electrons are favored. The electron energy spectra during the spikes sometimes had equivalent e fold energies in excess of 500 keV. In approximately 31% of these spike events observed from the low‐altitude polar orbiting satellites P72‐1, P78‐1, and S81‐1, nearly simultaneous precipitation was measured in energetic ions above ∼30 keV at about the same L value. Several of the precipitation spikes occurred primarily in the drift loss cone, but in some cases, significant precipitation (∼10−2 ergs/cm² s) was also observed in the bounce loss cone. The electron spikes occurred preferentially in the evening sector, and all of the associated narrow ion spikes were in that local time interval. Narrow relativistic electron spikes were observed on less than 1% of the crossings of the plasmapause. From the set of S81‐1 events a search was made for those also observed on the NOAA 6 spacecraft. On rare occasions, nearly simultaneous (<2000 s) narrow spikes with hard electron spectra were found at approximately the same L value from both spacecraft and at longitudes differing by 8°–47°. These findings suggest a patchy profile, sometimes with an arc structure which may extend over longitude intervals as great as 25° and time intervals as long as 2000 s. From consideration of the AE index for 17 events, 12 were found to occur close to the times of substorms. The spike precipitation is interpreted in terms of cyclotron resonance wave‐particle interactions involving radiation belt particles, the narrow widths being associated with fine structure in the cold plasma density profiles near the plasmapause and the energy selectivity associated with an upper frequency cutoff in the waves.
Corotation of the Kronian magnetosphere
Authors: Eviatar Aharon , Richardson John D.
Published: February 2008

Observations of the radial variation of azimuthal plasma velocity in the inner part of the magnetosphere of Saturn show deviations from corotation occur as far in as L = 4. Major deviations from rigid corotation occur near the orbits of Rhea and Dione. We use Voyager‐derived neutral and plasma density and temperature vs. altitude profiles in the upper atmosphere to verify that the atmospheric torque needed to drive corotation through neutral‐ion coupling is available. We find that the observed azimuthal velocities and calculated Pedersen conductance are consistent with one another, but that the conductance is much less than previous values estimated for Saturn and Jupiter. We attribute the lower than anticipated conductivity to the large heliocentric distance, to the absence of an active satellite (such as Jovian Io) and to a possible role of ring material in depleting the ionosphere.
Evidence of high densities and ion outflows in the polar cap during the recovery phase
Authors: Gallagher D. L. , Menietti J. D. , Burch J. L. , Persoon A. M. , Waite J. H. , Chappell C. R.
Published: February 2008

During the recovery phase of a large storm on October 14, 1981, instruments on board Dynamics Explorer 1 (DE 1) the Retarding Ion Mass Spectrometer (RIMS), the High Altitude Plasma Instrument (HAPI), and the Plasma Wave Instrument (PWI) detected unusually high plasma densities and ion flows in the polar cap. At the time of detection, DE 1 was located at a radial distance of about 3.5 earth radii, a magnetic local time near midnight, and between 70° and 80° invariant latitude. Total plasma density was found to be about 50 cm−3, an order of magnitude above median polar cap densities at the altitude of observation. In addition, highly collimated flows of hydrogen and oxygen are found flowing through a background hydrogen plasma. The O+ component of the plasma discussed is not directly identified but is inferred to be O+ through the combined analysis of data from three instruments. Results of the combined instrument analysis indicate that the detected plasma was composed of outflowing H+ with a density of 6–10 cm−3 with a temperature of about 0.15 eV; isotropic H+ with a density of about 15–20 cm−3; and outflowing and strongly convecting O+ with an average density of about 20 cm−3 and a temperature of about 0.26 eV. The flux of outflowing H+ and O+ are both about 107 cm−2 s−1. The data indicate that the O+ detected by HAPI seems to originate in the dayside ionosphere, while the H+ detected by RIMS has a source in the nightside polar cap.
Generation of band‐limited VLF noise using the Siple transmitter: A model for magnetospheric hiss
Authors: Helliwell R. A. , Carpenter D. L. , Inan U. S. , Katsufrakis J. P.
Published: February 2008

Band‐limited VLF noise generated by the experimental transmitter at Siple Station, Antarctica, is used to simulate the interaction of natural magnetospheric hiss with energetic radiation belt particles. While the observed spectrum of the generated noise at times closely resembles that of natural VLF hiss, at other times discrete emissions are found to be generated from the incoherent noise signal. Results imply that magnetospheric chorus can be triggered by hiss signals, indicating the similarity of generation mechanisms for coherent and incoherent VLF emissions. A model based on second‐order cyclotron resonance can explain the conversion of the relatively short duration wavelets of a hiss spectrum into the longer, semicoherent discrete emissions that are typical of chorus.
Three‐dimensional observations of Birkeland currents
Authors: Barfield J. N. , Saflekos N. A. , Sheehan R. E. , Carovillano R. L. , Potemra T. A. , Knecht D.
Published: February 2008

The dominant coupling between the ionosphere and magnetosphere is now understood to be Birkeland currents. During substorms the Birkeland current system in the midnight sector is generally interpreted as a cross‐tail current interruption and diversion along magnetic field lines to the conducting ionosphere, which carries the intensified auroral electrojet. In the present study, the equatorial current diversion, the Birkeland current intensification, and the auroral electrojet increase in activity were observed to be initiated simultaneously at the onset of substorms. Our principal finding is that the field‐aligned currents of the components of the current wedge have been observed simultaneously on the ground, at low‐altitude satellite orbits, and at geosynchronous satellite orbits. Recent dynamic modeling work by Chen et al. (1982) appears to support our findings. These observations contribute significantly toward the confirmation of the substorm current wedge model and complement and extend the results obtained by Nagai (1982).
Pitch angle diffusion in morningside aurorae: 2. The formation of repetitive auroral pulsations
Authors: Davidson G. T.
Published: February 2008

Filling of the loss cone by pitch angle diffusion will quench the growth of VLF waves that interact with substorm‐injected electrons in the morningside auroral region. This occurs for any wave mode whose growth rate is sensitive to the anisotropy of the electron distribution near the loss cone. A pair of coupled nonlinear differential equations is developed to describe the coupling between the growth of waves and the pitch angle diffusion of electrons. The equations differ from those previously used to model repetitive precipitation pulsations (Davidson, 1979) in that the wave growth in the earlier model was assumed to be sensitive mainly to the total electron flux. The earlier model therefore was limited by the strong diffusion lifetime. The pulsation periods in the earlier model were too long and strongly damped. The new model predicts cyclic or repetitive precipitation pulsations for a wide range of conditions, with periods of 3–30 s, and negligible damping. The model agrees well with an extensive body of observational data.
Hemispheric asymmetries of the thermospheric semiannual oscillation
Authors: Maeda K. , Hedin A. E. , Mayr H. G.
Published: February 2008

The semiannual oscillation (SAO) of the terrestrial thermosphere, which has the equinoctial maxima both in the polar and equatorial regions, shows a striking hemispheric asymmetry. The maximum amplitudes of the thermal SAO at 300 km are 25.7°K and 15.8°K for the northern and southern polar regions, respectively. The corresponding SAO amplitudes of the neutral N2 density are 10.5% and 3.7%, respectively. Furthermore, the phase of the southern hemispheric SAO lags behind the phase of the northern SAO; i.e., the maximum of SAO at 90°S is delayed from that at 90°N by approximately 55 days in the temperature and 25 days in the N2 density. These values are obtained from mass spectrometer (17 satellites) and incoherent scattering observations (4‐radar stations), and from several rocket‐soundings (Heding, 1983). The larger thermospheric SAO amplitude in the northern hemisphere over that in the southern hemisphere, as well as the phase‐difference between the two hemispheres can be ascribed to dynamical heating by upward propagating small amplitude internal gravity waves (IGW), in addition to the well‐established sources for the thermospheric SAO, i.e., solar EUV heating at low latitudes and auroral heating at high latitudes. The upward energy flux transferred by propagating IGW increases during the transition periods between summer easterlies and winter westerlies, which occur twice a year around the equinoxes. Due to the orography, the atmospheric wave activities are stronger and the zonal winds are weaker in the northern hemisphere than in the southern hemisphere. Correspondingly, the contribution from dynamical heating by IGW to the thermospheric SAO is larger in the northern hemisphere than in the southern counterpart, and the zonal wind reversal is delayed in the southern hemisphere relative to the northern hemisphere. We propose that this may explain the larger amplitude and the phase‐advance of the thermospheric SAO in the northern hemisphere relative to the southern hemisphere.
The O2 atmospheric dayglow in the thermosphere
Authors: Torr Marsha R. , Welsh Barry Y. , Torr Douglas G.
Published: February 2008

In this paper we report spectral measurements (Δλ = 8Å) from Spacelab 1 of the O2 atmospheric bands in the dayglow at thermospheric altitudes for a tangent ray height of 150 km. Vibrational levels up to ν′ = 4 are found in the data, requiring a source in addition to the energy transfer from O(¹D) to O2. We suggest that this source is the collisional deactivation of N(²D) by O2.
On the use of a sunward libration‐point‐orbiting spacecraft as an interplanetary magnetic field monitor for magnetospheric studies
Authors: Kelly T. J. , Crooker N. U. , Siscoe G. L. , Russell C. T. , Smith E. J.
Published: February 2008

Magnetospheric studies often require knowledge of the orientation of the interplanetary magnetic field (IMF). In order to test the accuracy of using magnetometer data from a spacecraft orbiting the sunward libration point for this purpose, the angle between the IMF at ISEE 3, when it was positioned around the libration point, and at ISEE 1, orbiting the earth, has been calculated for a data set of 2‐hour periods covering 4 months. For each period, a 10‐minute average of ISEE 1 data is compared with 10‐minute averages of ISEE 3 data at successively lagged intervals. At the lag time equal to the time required for the solar wind to convect from ISEE 3 to ISEE 1, the median angle between the IMF orientation at the two spacecraft is 20°, and 80% of the angles are less than 38°. The results for the angles projected on the y‐z plane are essentially the same. The median minimum angle between the IMF orientation at the two spacecraft attained during each 2‐hour segment is 11°, with 80% of the angles less than 19°. These low values indicate relatively little temporal or small scale variation between the spacecraft. The minimum angle generally occurs at a lag time different from the convection time. The sign of the difference depends on IMF orientation in the sense that magnetic features tend to arrive sooner when the IMF is directed along the line between the spacecraft. However, the difference between a lag time appropriate to this corotation geometry and the convection lag time is not large enough to produce a significant decrease in the angles between the IMF vectors at the two spacecraft. We conclude that the IMF orientation at a libration‐point‐orbiting spacecraft, lagged by the time required for the solar wind to convect to the earth, is a convenient predictor of IMF orientation near the earth, to within ∼20° accuracy.
A theoretical model of convection with distributed Birkeland currents
Authors: Atkinson G.
Published: February 2008

Analytic solutions are presented for magnetospheric convection which include the effects of energetic ions and a radial gradient of the plasma content of flux tubes. The latter assumption leads to results that differ from previous studies. These include (1) distributed, rather than sheet, Birkeland currents, (2) simple expressions for the oval, its thickness, its flow, its currents, and (3) Harang discontinuities and their properties, including location, length, and the angle they make with the oval. The effect on the above of differing nightside boundary conditions is presented. Some numerical solutions of cases with a weak azimuthal gradient of plasma content were also studied. The best semblance to observational models is for a maximum of the nightside earthward convection in the premidnight quadrant and a weak component of the gradient of plasma content along the oval from day to night. The calculated plasma energy distribution is reasonably consistent with the observed temperature of the plasma sheet in the energy range of importance, suggesting that the approach used to introduce self‐consistency is successful.
The effect of parallel inhomogeneity on magnetospheric hydromagnetic wave coupling
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

In a cold homogeneous plasma, fast mode and transverse mode MHD waves can be independently excited. Inhomogeneities couple the modes and allow the excitation of field line resonances by compressional (fast mode) perturbations of the magnetosphere. Past discussions have focused on inhomogeneities in the radial direction (or its equivalent in simpler geometries), ignoring gradients of the Alfvén velocity along the unperturbed field direction. The nature of the coupling and the effectiveness of excitation are significantly modified by the presence of such gradients. One essential change is that the compressional and transverse field perturbations differ in their structure along the field. This reduces the effectiveness of the coupling between the fast (global) mode and standing (localized) field line resonances for identical harmonics of the parallel structure but allows different harmonics to couple. A fast mode wave propagating in from the outer boundary can, therefore, drive field line resonances in the region exterior to its turning point, or effective reflection point, where its amplitude has not yet decayed.
Upwelling O+ ion source characteristics
Authors: Moore T. E. , Lockwood M. , Chandler M. O. , Waite J. H. , Chappell C. R. , Persoon A. , Sugiura M.
Published: February 2008

Recent observations from the Dynamics Explorer 1 (DE 1) spacecraft have shown that the dayside auroral zone is an important source of very low‐energy superthermal O+ ions for the polar magnetosphere. When observed at 2000‐ to 5000‐km altitude, the core of the O+ distribution exhibits transverse heating to energies on the order of 10 eV, significant upward heat flux, and subsonic upward flow at significant flux levels (exceeding 108 cm−2 s−1). The term “upwelling ions” has been adopted to label these flows, which stand out in sharp contrast to the light ion polar wind flows observed in the same altitude range in the polar cap and subauroral magnetosphere. We have chosen a typical upwelling ion event for detailed study, correlating retarding ion mass spectrometer observations of the low‐energy plasma with energetic ion observations and local electromagnetic field observations. The upwelling ion signature is colocated with the magnetospheric cleft as marked by precipitating energetic magnetosheath ions. The apparent ionospheric heating is clearly linked with the magnetic field signatures of strong field‐aligned currents in the vicinity of the dayside polar cap boundary. Electric field and ion plasma measurements indicate that a very strong and localized convection channel or jet exists coincident with the other signatures of this event. These observations indicate that transverse ion heating to temperatures on the order of 105 K in the 2000‐ to 5000‐km ionosphere is an important factor in producing heavy ion outflows into the polar magnetosphere. This result contrasts with recent suggestions that electron heating (to temperatures of order 104 K) is the most important parameter with regard to O+ outflow.
The field swelling and mirror modes: Connection of the two instabilities
Authors: Migliuolo Stefano
Published: February 2008

A theory of zero frequency and low growth rate (lower than the ion cyclotron frequency) modes in homogeneous plasmas with temperature anisotropies is presented. These modes are considered to propagate at large angles to the ambient magnetic field and are electromagnetic in nature. From the analysis of a general dispersion relation, with arbitrary electron to ion temperature ratio Te/Ti, it is found that the mirror (Te→0) and field swelling (Ti→0) modes are smoothly connected. Thus an anisotropic homogeneous plasma will always relax via one of these instabilities (as well as through the field‐aligned ion cyclotron instabilities) for any value of Te/Ti, provided electron and/or ion temperature anisotropies exceed the required thresholds (determined in the Te = 0 and Ti = 0 limits). This broadens the applicability of this mechanism for producing low‐frequency waves in space plasmas, since a cold species is no longer essential for plasmas with little or no spatial (density and/or temperature) gradients.
Excitation of oxygen permitted line emissions by SF6 injection into the F region
Authors: Bernhardt P. A. , Weber E. J. , Moore J. G. , Baumgardner J. , Mendillo M.
Published: February 2008

The use of SF6 releases to excite the airglow from atomic oxygen in the ionosphere is demonstrated by experiment and by theory. Enhanced 777.4‐nm emissions from O(5P) states were measured during the AFGL‐sponsored Ionospheric Modification Studies campaign at Wallops Island, Virginia. For this rocket campaign, 8×1025 molecules of SF6 were released at 350‐km altitude into the midnight ionosphere on November 9, 1983. The 777.4‐nm airglow increased by over 150 R. The temporal evolution of the airglow is simulated using an ion chemistry model involving six positive and negative ion species and 16 reactions. The model indicates that substantial enhancements in 844.6 nm and 135.6 nm will be triggered by SF6 injections into the ionosphere. The model also demonstrates that the intensity of the 777.4‐nm line is strongly dependent on the temperature of the neutral atmosphere.
Magnetic activity and solar wind velocity at and preceding solar minimum for the past 500 years
Authors: Silverman S. M.
Published: February 2008

The magnetic activity and solar wind speed at and preceding sunspot minimum has been estimated for the past 5 centuries. Empirical equations relating aa, solar wind speed, auroras, or the southward component of the interplanetary field to sunspot numbers at solar activity maxima and/or minima were used to derive aa and solar wind speed estimates. Observed and estimated sunspot numbers for the period 1500–1976 were used in the calculations. The estimated magnetic activity was much more sensitive than the solar wind speeds to the prolonged solar activity minima around the beginnings of the eighteenth and nineteenth centuries. The aa varied from about 3–5 during these solar activity minima to 27 for the period around 1954. The solar wind speed, on the other hand, varied only between 300 and 600 km/s over the 5‐century period. The lower speed was approximately the same for the two prolonged solar activity minima at the beginnings of the eighteenth and nineteenth centuries. This result implies that the coronal holes continued to send out strong solar wind streams even during periods of extended low sunspot production.
Field line twist and field‐aligned currents in an axially symmetric equilibrium magnetosphere
Authors: Voigt Gerd‐Hannes
Published: February 2008

Field‐aligned Birkeland currents and the twist of magnetic tail field lines are calculated for an axially symmetric pole‐on magnetosphere. The magnetic field components in the magnetospheric tail result from linear solutions to the MHD equilibrium problem. The field line twist angle, ε = arc tan (Bϕ/Bz), is shown to be dependent on the radial distance measured from the tail axis, on the amount of thermal plasma confined in closed magnetotail flux tubes, and on the ionospheric Pedersen conductivity. The field lines are maximally twisted in the force‐free tail configurations; they are minimally twisted if the magnetotail reaches the state of maximum plasma pressure, the so‐called Harris sheet limit. The field line twist results from the planetary rotation, which leads to the development of a toroidal magnetic Bϕ component and to differentially rotating magnetic field lines. If centrifugal forces become important, the thermal plasma pressure P(A) in the Grad‐Shafranov equation assumes the form P(A, r) which allows centrifugal forces to be included in the MHD equilibrium formalism. The equilibrium model reveals that magnetopause field lines do not rotate because the magnetopause is held in place by the solar wind stream. For a given ionospheric conductivity and a given thermal plasma content within the tail, the field line helix is twisted most near the center of the circular tail plasma sheet. The field‐aligned Birkeland currents reach their maximum strength on tail field lines that confine the plasma sheet. Thus polar aurorae are expected to appear around both the dayside and nightside magnetic poles at latitudes where the plasma sheet field lines map into the planet's ionosphere. This theoretical model was originally developed for the anticipated pole‐on magnetosphere of Uranus. Thus references to the Uranian magnetosphere that appear in this study and in the quoted literature apply to hypotheses that had been developed before the closest approach of Voyager 2 at Uranus on January 24, 1986. However, the results of this study are sufficiently general to allow their application to other rotating axially symmetric plasma‐magnetic field configurations in space.
Interplanetary shocks preceded by solar filament eruptions
Authors: Cane H. V. , Kahler S. W. , Sheeley N. R.
Published: February 2008

We discuss the solar and interplanetary characteristics of six interplanetary shock and energetic particle events associated with the eruptions of solar filaments lying outside active regions. The events are characterized by the familiar double‐ribbon Hα brightenings observed with large flares, but only very weak soft X ray and microwave bursts. Both impulsive phases and metric type II bursts are absent in all six events. The energetic particles observed near the earth appear to be accelerated predominantly in the interplanetary shocks. The interplanetary shock speeds are lower and the longitudinal extents considerably less than those of flare‐associated shocks. Three of the events were associated with unusual enhancements of singly ionized helium in the solar wind following the shocks. These enhancements appear to be direct detections of the cool filament material expelled from the corona. We suggest that these events are part of a spectrum of solar eruptive events which include both weaker events and large flares. Despite their unimpressive and unreported solar signatures, the quiescent filament eruptions can result in substantial space and geophysical disturbances.
The coalescence of two merged interaction regions between 6.2 and 9.5 AU: September 1979 event
Authors: Whang Y. C. , Burlaga L. F.
Published: February 2008

In September 1979 an unusual large‐scale interplanetary compound stream was observed in data from the plasma and magnetic fields experiments on Voyager 1 at a heliocentric distance of 6.2 AU. Two interaction regions were observed by Voyager 1, each lasting more than 4 days, in which the total pressure was nearly 100 times the ambient pressure. Speeds of greater than 450 km/s were observed for more than 27 days. A strong forward shock FD with a shock speed of 960 km/s and a density ratio of 3.85 was present at the front of the second interaction region, and two reverse shocks, R1 and R2, were at the end of the first interaction region. A major restructuring of the flow took place over the distance of 3.3 AU between Voyager 1 and Pioneer 11, which was nearly radially aligned with Voyager 1. During the restructuring the two interaction regions observed at 6.2 AU coalesced to form a new single structure (a “merged interaction region”) at 9.5 AU. This paper presents a simulation study of the evolution and interaction of solar wind structures to explain the observed restructuring process. The computer simulation is based on an unsteady, one‐dimensional, one‐fluid, MHD model and uses the plasma and magnetic field data from Voyager 1 at 6.2 AU as the input function. The model shows that the forward shock FD passed through the reverse shocks R1 and R2 and into the first interaction region, becoming weaker in each of these interactions. The reverse shocks coalesced to form a stronger reverse shock R; thus the shock signature changed from R1‐R2‐FD to FD‐R between 6.2 and 9.5 AU. The major stream structures at 9.5 AU predicted by the simulation model agree well with those directly observed from Pioneer 11.
Electromagnetic ion beam instabilities: Growth at cyclotron harmonic wave numbers
Authors: Smith Charles W. , Gary S. Peter
Published: February 2008

The linear theory of electromagnetic ion beam instabilities for arbitrary angles of propagation is studied, with emphasis on the conditions necessary to generate unstable modes at low harmonics of the ion cyclotron resonance condition. The present results extend the analysis of Smith et al. (1985). That paper considered only the plasma parameters at a time during which harmonic wave modes were observed in the earth's foreshock. The parameters of that paper are used as the basis of parametric variations here to establish the range of beam properties which may give rise to observable harmonic spectra. We show that the growth rates of both left‐ and right‐hand cyclotron harmonic instabilities are enhanced by an increase in the beam temperature anisotropy and/or the beam speed. Decreases in the beam density and/or the core ion beta reduce the overall growth of the cyclotron harmonic instabilities but favor the growth of these modes over the growth of the nonresonant instability and thereby enhance the observability of the harmonics.
The current‐voltage relationship in auroral current sheets
Authors: Weimer D. R. , Gurnett D. A. , Goertz C. K. , Menietti J. D. , Burch J. L. , Sugiura M.
Published: February 2008

The current‐voltage relation within narrow auroral current sheets is examined through the use of high‐resolution data from the high‐altitude Dynamics Explorer 1 satellite. The north‐south perpendicular electric field and the east‐west magnetic field are shown for three cases in which there are large amplitude, oppositely directed paired electric fields which are confined to a region less than 20 km wide. The magnetic field variations are found to be proportional to the second integral of the high‐altitude perpendicular electric field. It is shown that at the small‐scale limit, this relationship between ΔB and E is consistent with a linear “Ohm's law” relationship between the current density and the parallel potential drop along the magnetic field line. This linear relationship had previously been verified for large‐scale auroral formations greater than 20 km wide at the ionosphere. The evidence shown here extends our knowledge down to the scale size of discrete auroral arcs.
The interpretation of space shuttle measurements of ionic species
Authors: Caledonia George E. , Person James C. , Hastings D. E.
Published: February 2008

Measurements of ionic species made in the space shuttle bay area during low‐earth orbit are interpreted within the framework of a kinetic analysis that incorporates shuttle cloud/ambient interactions, flow, and electromagnetic phenomena. This analysis indicates that a pressure of water vapor near the shuttle surface of 10−6 torr is consistent with [H2O+]/[O+] and [H3O+]/[H2O+] ratios of 0.1 and 0.2, respectively, and the latter ratio is within the range of measurements. For CO2 the analysis shows that a [CO2+]/[O+] ratio of 0.01 is consistent with a CO2 pressure near the shuttle of 8 × 10−8. For these conditions the dominant formation of ions is by charge exchange from ambient O+ ions, and the dominant loss of ions is by parallel flow along the magnetic field lines. In addition, Monte Carlo calculations of the slowing of the incident O+ ions indicate that charge exchange with H2 molecules occurs before the ions have lost a significant amount of energy when the H2O pressure near the shuttle surface is less than 10−6 to 10−5 torr.
The coronal mass ejection of July 6, 1980: A candidate for interpretation as a coronal shock wave
Authors: Sime D. G. , Hundhausen A. J.
Published: February 2008

Sime et al. (1984, 1985) have argued that two common characteristics of looplike coronal mass ejections are inconsistent with interpretation of such loops as shock waves moving through the corona. These are the cessation of lateral motions of the loop sides to form nearly radial, stationary bright columns or “legs” in the late stages of many mass ejections and the pushing aside or bending of pre‐existing coronal features well outside the loop sides. Both are contrary to the notion of a propagating wave with a shock at its front and in disagreement with quantitative models of shock propagation in the corona. We report here Solar Maximum Mission (SMM) coronagraph observations of a looplike coronal mass ejection that occurred on July 6, 1980, and that was atypical in the two characteristics mentioned above. The sides of the loop moved laterally while they were visible and did not stop to form stationary “legs.” A group of raylike features initially outside the mass ejection loop were not pushed aside or bent until the laterally moving loop sides passed over them. We thus propose the July 6, 1980, mass ejection as a candidate for identification as a shock wave. Quantitative analysis of the motion of the bright loop is also consistent with such an identification. The loop top moves outward with an apparent radial speed of ∼1000 km s−1, faster than all but one other mass ejection observed by SMM in 1980 and probably faster than the Alfvén speed in the corona. The loop sides move laterally at a speed of ∼800 km s−1 and show no evidence for a significant deceleration. The excess mass in the bright loop is estimated at ≤2 × 1014 g, comparable to the expected error in its measurement and well below the value found to be typical of coronal mass ejections.
Variations of electron distribution functions in the solar wind
Authors: Pilipp W. G. , Miggenrieder H. , Mühlhäuser K. ‐H. , Rosenbauer H. , Schwenn R. , Neubauer F. M.
Published: February 2008

Variations of electron distribution functions in the solar wind have been investigated using the electron data observed aboard Helios 2 during the first four months of its mission in 1976 in the distance range between 0.3 and 1 AU. In particular, variations across the sector structure of the interplanetary magnetic field and across the plasma stream structures have been studied. It has been found that there is a strong correlation between the electron properties and the sector structure of the magnetic field. Within the interior of magnetic sectors the electron distribution functions are extremely anisotropic and skewed with respect to the magnetic field direction at high particle energies (above 100 eV); i.e., the distribution functions have a narrow strahl (beam) directed along the magnetic field away from the sun. Often a slight indication of a second strahl directed along the magnetic field toward the sun has been found. Toward sector boundaries the electron distribution functions become less anisotropic and less skewed. Right at sector boundaries the electrons are relatively cool, and their distributions are nearly isotropic, often showing a slight bidirectional anisotropy. These observations have been interpreted to indicate that scattering of electrons with energies above 100 eV is weak within the interior of magnetic sectors but anomalous scattering increases drastically toward sector boundaries for all energies in the halo regime up to several hundred eV. There is evidence for closed magnetic field structures, probably occurring frequently in the solar wind, where the magnetic field lines should usually be connected to the sun outside sector boundaries but may or may not be disconnected from the sun at sector boundaries.
On the reversal of the dipolar field of the Sun and its possible implication for the reversal of the Earth's field
Authors: Saito T. , Akasofu S. ‐I.
Published: February 2008

Changes of the neutral line on the source surface (analogous to the magnetic dip equator of the earth) during the period between 1976 and 1983 are examined on the basis of the Stanford solar magnetic field data. Instead of the standard Mercator‐like projection, the neutral line is shown on a spherial surface for 16 selected Carrington rotations. In spite of great complexity of the field variations, this presentation depicts clearly a fairly systematic rotational reversal of the dipolar field on the source surface during the sunspot maximum years. It is suggested that this solar situation is somewhat analogous to the planet earth in the sense that the core surface and the earth's surface may correspond to the photosphere and the source surface, respectively.
Large‐scale fluctuations in the interplanetary medium
Authors: Burlaga L. F. , Mish W. H.
Published: February 2008

Power spectra of measurements of the magnetic field strength in the heliosphere obtained by Voyager 1 between 1 AU and 9 AU have the form of a power law ƒ−a from periods of several hours to at least 6 days. The exponent was a = 2.0 ±0.05 for all of the spectra considered, which is the exponent for a series of steps and for Burgers' turbulence. Spectra of large‐scale speed fluctuations also have the form ƒ−b from a period of a few hours to periods greater than 13 days in the region from 1 AU to 8.9 AU. The exponent b is generally somewhat larger than b = 2, implying some “persistence” of the speed fluctuations. The low‐frequency cutoff (outer cutoff) of the power law increases from a period of 6.5 days at 1 AU to 26 days at (6.1–8.9) AU, which can be attributed to (1) the coalescence of interaction regions and (2) a transfer of energy from the spectrum of large‐scale speed fluctuations. The outer cutoff of the spectrum of speed fluctuations increases from a period of 13 days at 1 AU to 26 days between a few AU and 8.9 AU. Both the magnetic field strength fluctuations and the speed fluctuations have fractal behavior, suggesting that they are self affine rather than dominated by a few large discontinuities.
On the noncoplanarity of the magnetic field within a fast collisionless shock
Authors: Thomsen M. F. , Gosling J. T. , Bame S. J. , Quest K. B. , Winske D. , Livesey W. A. , Russell C. T.
Published: February 2008

Within the magnetic ramp of fast collisionless plasma shocks observed with spacecraft instruments and simulated numerically, the magnetic field undergoes an excursion out of the plane of coplanarity (the plane determined by the upstream magnetic field and the shock normal). This rotation is consistently in the direction such that the electrostatic potential jump across the shock, as measured in the de Hoffman‐Teller frame of reference (HTF), is ∼2–6 times smaller than the electrostatic potential jump measured in the normal incidence frame. The preferred direction is consistent with a basic whistler mode transition between the upstream and downstream orientations. The potential jump in the HTF is considerably smaller than the change in bulk flow energy across the shock, confirming the recent suggestion that magnetic forces contribute importantly to the slowing of the plasma in that frame. A further consequence is that suprathermal particles leaking back into the upstream region across the shock do not gain much energy from the cross‐shock electric field.
Unsteady Petschek reconnection
Authors: Biernat Helfried K. , Heyn Martin F. , Semenov Valdimir S.
Published: February 2008

A model of unsteady Petschek reconnection with propagating shock waves is presented. The results depend uniquely on a prescribed, time‐dependent reconnection rate. An application to flux transfer events at the dayside magnetopause is discussed.
Periodicities in the interplanetary magnetic field polarity
Authors: Gonzalez A. L. C. , Gonzalez W. D.
Published: February 2008

The daily data of the polarity of the interplanetary magnetic field for the years 1926–1982 were studied by means of a power spectrum analysis in order to determine prevailing structures of this polarity as well as to search for recurrences in those structures. On a global scale, the presence of significant peaks at about 27.5, 13.5, 9.1 and 6.8 days was observed, with amplitudes decreasing in that same order. However, for shorter intervals of time (of the order of 1 year) the resulting spectral distribution does not necessarily present the same characteristics, particularly concerning the relative power of the peaks at about 27.5 and 13.5 days. These results are interpreted, through a simulation with rectangular waves, as cases that correspond either to a two‐sector or to a four‐sector structure, when the dominant peak is that of about 27.5 or 13.5 days, respectively. Thus, by performing an annual survey of the data, it was found that there is a continuous change in the type of dominant structure, with intervals having a well‐defined two‐sector structure alternating with those having either a less defined structure or a well‐defined four‐sector structure. A further application of the power spectrum technique to this type of alternating variability in the dominant structures led to the conclusion that significant periodicities of about 3.7 and 1.5 years seem to exist in such a variability. These periodicities become important when compared to similar ones reported in studies of solar and geomagnetic activity. Other less significant periodicities in this alternated variability are also reported and discussed. Finally, a study of variations in the period corresponding to the peak of about 27.5 days was also accomplished, showing that it changes from about 28 days at the beginning of the solar cycle to about 27 days at the end of the cycle, as also found by other authors.