bluuebunny/crossref_metadata_2025 huggingface dataset:
HF radar observations of Pc 5 field line resonances in the midnight/early morning MLT sector
Authors: Ruohoniemi J. M. , Greenwald R. A. , Baker K. B. , Samson J. C.
Published: February 2008

On a number of occasions The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay, Labrador, has observed the effects of field line resonances on the drift velocities of irregularities in the F region of the high‐latitude ionosphere. These resonances are seen as quasi‐monochromatic oscillations in the Doppler velocities measured by the radar. One of the most interesting sets of resonances occurs near midnight MLT and may be associated with shear in the convective flow in the magnetotail. This paper discusses in detail a particularly clear example which shows field line resonances equatorward of a region of shear flow in the early morning sector. Velocity oscillations were recorded at the three discrete frequencies 1.3, 1.95, and 2.6 mHz over a 3 hour period. The motions were predominantly in the geomagnetic east‐west direction, indicating north‐south electric fields. As expected of field line resonance pulsations, these oscillations had pronounced peaks in their latitudinal power distribution. For the pulsation at 1.95 mHz, a latitudinal phase shift of 180° was observed across the peaks in all the look directions of the radar, and a longitudinal wavelength corresponding to an m value of about 3 was obtained. For the 2.6‐mHz pulsation, the phase shifts across the peaks had a variation with look direction that indicated a significant longitudinal as well as latitudinal variation; for this activity we estimate an m value of about 16. The pulsations could occur simultaneously but remained distinct, as the latitude of peak response was observed to vary inversely with the frequency of the pulsation. We interpret these features in terms of field line resonance theory and discuss the possible sources of the pulsation energy.
Structure of the dissipation region during magnetic reconnection in collisionless plasma
Authors: Burkhart G. R. , Drake J. F. , Chen J.
Published: February 2008

The results of an analytic and numerical investigation of the structure of the X line during steady state magnetic reconnection in collisionless plasma are presented. The structure of the X line essentially depends on a single dimensionless parameter F, which is a measure of the influx of plasma into the reconnection region. For small F the self‐consistent plasma current driven at the X line is small, and the magnetic fields are nearly unchanged from the initial vacuum state. With increasing F the current driven at the X line becomes large, and the dissipation region collapses in the direction of the inflow and elongates along the outflow. For sufficiently large F the velocity of the plasma ejected from the X line exceeds the local Alfvén velocity. In this regime a fast mode shock forms at the outflow end of the dissipation region which slows the high‐velocity outflow plasma to the subsonic flow characteristic of the broader outflow region. Finally, at a critical plasma flux Fc the dissipation region collapses to zero thickness; no steady solutions are found for F > Fc. By matching the energy dissipated at the neutral line with the change in global magnetic energy, a self‐consistent equation for F is derived which indicates that F always adjusts so that F ⪝ Fc. Predictions of reconnection rates and associated parameters for the geomagnetic tail are presented which are in reasonable agreement with observations.
Simulation of high‐speed (orbital) releases of electron attachment materials in the ionosphere
Authors: Scales W. A. , Bernhardt P. A.
Published: February 2008

The dynamics of ionospheric plasma irregularities produced by the release of electron attachment materials at orbital velocities across the geomagnetic field is studied. A two‐dimensional electrostatic fluid model which includes electron attachment and mutual neutralization chemistry, self‐consistent electric fields, and three‐species transport is developed. Numerical simulations are performed to study the behavior at early and at late times after the release. At early times, of the order of or less than the attachment material neutral collision time, the negative ion cloud produced by the release may structure owing to the shear in the E×B velocity within the cloud. At high altitudes the cloud may bifurcate and form vortices on the back. At lower altitudes where ion‐neutral collisional effects dominate, this structuring is suppressed. At late times, after a plasma depletion has formed due to neutralization chemistry, the cloud structures by the E×B interchange instability. Depending on the release altitude, the depletion structures by the collisional or inertial limit of this instability.
Is there a strange attractor in the magnetosphere?
Authors: Roberts D. Aaron
Published: February 2008

This paper reviews recent attempts to determine if some aspects of magnetospheric dynamics, and in particular substorms as measured by AE and AL geomagnetic indices, can be represented by a low‐dimensional dynamical system. If true, this result would imply that a small set of ordinary differential equations could describe important aspects of substorm dynamics, greatly simplifying modeling and prediction efforts and providing significant input to more detailed modeling. The “embedding” and “correlation dimension” methods used to investigate the dimensionality of a physical process from a single time series are considered in detail with an emphasis on what can go wrong and what can be done about it. Two main conclusions of this work, which includes some new results on the particular case of AL, are (1) that a low‐dimensional and probably strange attractor does exist in magnetospheric dynamics, and (2) that there is no reliable substitute for using large numbers of data points in performing analyses leading to this conclusion.
Interplanetary magnetic field control of the Mars bow shock: Evidence for Venuslike interaction
Authors: Zhang T. L. , Schwingenschuh K. , Lichtenegger H. , Riedler W. , Russell C. T. , Luhmann J. G.
Published: February 2008

The Mars bow shock location and shape have been determined by examining the PHOBOS spacecraft magnetometer data. Observations show that the position of the terminator bow shock varies with interplanetary magnetic field orientation in the same way as at Venus. The shock is farthest from Mars in the direction of the interplanetary electric field, consistent with the idea that mass loading plays an important role in the solar wind interaction with Mars. We also find that the shock cross section at the terminator plane is asymmetric and is controlled by the interplanetary magnetic field as expected from the asymmetric propagation velocity of the fast magnetosonic wave. Comparing with earlier mission data, we show that the Mars shock location varies with solar activity. The shock is farther from Mars during solar maximum. Thus the solar wind interaction with Mars appears to be Venuslike, with a magnetic moment too small to affect significantly the solar wind interaction.
Differential memory in the Earth's magnetotail
Authors: Burkhart G. R. , Chen J.
Published: February 2008

The process of differential memory is quantitatively studied in the modified Harris magnetotail geometry. This process arises as a consequence of nonlinear particle dynamics in the magnetotail which gives rise to partitioning of phase space into disjoint regions. Different regions are occupied by distinct classes of orbits and have widely separated time scales. This paper gives the first study of the time scales and potentially observable signatures in plasma distribution functions associated with the process of differential memory. It is found that the effective “trapping” time of stochastic orbits plays a critical role in differential memory and that in the magnetotail geometry, this time has resonances at certain values of the parameter Ĥ. A scaling law Ĥ1/4 has been found for this previously unknown resonance effect. This scaling is directly related to the phase space structures of this stochastic system and leads to signatures in the distribution functions, and their velocity moments (density, velocity components, and kinetic temperatures) are computed following a prescribed change in the boundary conditions. The relationships between the initial changes and the time‐asymptotic distribution functions are discussed. The results depend only on the large‐scale phase space structures and not on individual chaotic orbits.
A three‐dimensional simulation model for electron beam injection experiments in space
Authors: Pritchett P. L.
Published: February 2008

A three‐dimensional electrostatic particle simulation model is developed for studying the injection of an electron beam from an isolated source (the “spacecraft”) in the ionosphere. The dimensionless beam velocity in the simulations is determined by matching the beam energy and injection current to representative experimental values. It is found that the beam stagnation time ts is typically nearly an order of magnitude longer than in previous two‐dimensional simulations. As a result, under normal conditions ts is more than an order of magnitude larger than the plasma response time trp, and the beam is able to escape from the near environment of the spacecraft. The limit on the amount of current that can be injected is provided by the condition ts ≳ 2trp, and this limit can substantially exceed the thermal current to the spacecraft. After escape, the beam is subject to strong space‐charge oscillations produced by the beam‐plasma interaction. For injection parallel to the magnetic field this interaction produces a series of trapping vortices in phase space in which the peak potentials reach 10–15% of the beam voltage. For injection at 45° to the magnetic field, the beam initially forms a hollow cylinder whose radius is the beam gyroradius and whose width is about the beam Debye length. After two or three gyroperiods, however, the space‐charge oscillations lead to a loss of both the parallel and transverse coherence of the beam. Inside the beam cylinder the wave spectrum is dominated by a peak just below the upper hybrid frequency; outside the cylinder the most intense modes are at lower frequencies near the ion plasma frequency.
Pc 1 micropulsations at a high‐latitude station: A study over nearly four solar cycles
Authors: Mursula K. , Kangas J. , Pikkarainen T. , Kivinen M.
Published: February 2008

An old but rarely used resonance method is introduced and examined in order to study the very long term occurrence of Pc 1 micropulsations. By this method it is possible to detect Pc 1 pulsations with a frequency close to the eigenfrequency of the magnetometer system. We have analyzed the magnetograms of the LaCour‐type magnetometer used in a quick‐run operation at the high‐latitude station of Sodankylä, Finland. The eigenfrequencies of the three components covered the frequency range from about 0.3 to 0.5 Hz. Our analysis extends over the whole registration period from 1932 to 1944 and from 1953 to 1983, i.e., covering nearly four solar cycles. The results show a very strong negative correlation between the annual Pc 1 activity and the annual sunspot number which persists over the whole data period. According to the statistics obtained, it is, for example, very unlikely that the Pc 1 activity maximum at a high‐latitude station would occur during the declining phase of the sunspot cycle. Furthermore, as a result of the negative correlation, some of the differences between the various solar cycles can be seen in the corresponding cycles of annual Pc 1 activity. Our results also reveal some new features common for the long‐term variations of Pc 1 activity at high‐ and mid‐latitude stations and help in understanding the differences between them.
An improved algorithm for determining neutral winds from the height of the F2 peak electron density
Authors: Richards P. G.
Published: February 2008

One of the greatest uncertainties in modeling the ionospheric densities and temperatures lies in the neutral winds that affect the vertical ion drift. Wind measurements are difficult to make, and current wind models are not adequate. An alternative is to deduce an equivalent meridional neutral wind from measurements of the height of the F2 peak electron density. This method has been effective in allowing ionospheric modelers to reproduce the observed height with reasonable precision when the peak height does not vary rapidly. However, when there are rapid movements of the peak height, this method underestimates the magnitude of the necessary wind changes, and it is also complicated and computer intensive to do accurately. This paper introduces an improved algorithm for use in time dependent ionospheric models that reduces the complexity of the procedure while more accurately reproducing the observed heights. The improvement is achieved by estimating the wind that is needed to reproduce the observed height at the next time step using the calculated height and wind at the current time step. Thus the winds are continuously being adjusted to bring the calculated height into better agreement with the observed height during the time dependent simulation. The winds from the new algorithm agree well with both optical and radar measurements made at Arecibo on August 17–18, 1982.
Long‐term variations in pulsation activity and their relationship to solar wind velocity, geomagnetic activity, and F2 region electron density
Authors: Zieger B.
Published: February 2008

Significant 11‐year, annual, and semiannual variations of the total pulsation activity at the mid‐latitude station Nagycenk are reported and compared with solar wind velocity, geomagnetic activity, and F2 region electron density. Monthly average data from 1957 through 1989 were analyzed using the discrete convolution filtering technique. The 11‐year variation of the total pulsation activity is closely related to the 11‐year solar wind cycle supposedly through solar wind controlled pulsations. Its annual variation reflects the combination of two effects: one is the annual variation of the solar wind due to interstellar wind, and the other is some damping mechanism related to F2 region electron density. The nature of this ionospheric damping has not been clarified yet. The semiannual variation of the total pulsation activity, however, seems to be controlled mainly by the geomagnetic activity. Occurrence frequency and amplitude data for pulsations in 12 period bands ranging from 1 s up to 10 min were also involved in order to clarify the contribution of different types of pulsations to the total pulsation activity. It was found that the total pulsation activity refers mainly to Pc3 pulsations and, that the pulsations with a period of 20–25 s, which correspond to the eigenperiod of field line resonance at L ∼ 2, have the strongest correlation with solar wind velocity.
Steepening of parallel propagating hydromagnetic waves into magnetic pulsations: A simulation study
Authors: Akimoto K. , Winske D. , Onsager T. G. , Thomsen M. F. , Gary S. P.
Published: February 2008

The steepening mechanism of parallel propagating low‐frequency MHD‐like waves observed upstream of the Earth's quasi‐parallel bow shock has been investigated by means of electromagnetic hybrid simulations. It is shown that an ion beam through the resonant electromagnetic ion/ion instability excites large‐amplitude waves, which consequently pitch angle scatter, decelerate, and eventually magnetically trap beam ions in regions where the wave amplitudes are largest. As a result, the beam ions become bunched in both space and gyrophase. As these higher‐density, nongyrotropic beam segments are formed, the hydromagnetic waves rapidly steepen, resulting in magnetic pulsations, with properties generally in agreement with observations. This steepening process operates on the scale of the linear growth time of the resonant ion/ion instability. Many of the pulsations generated by this mechanism are left‐hand polarized in the spacecraft frame.
Ion kinetic processes and thermalization at quasi‐perpendicular low Mach number shocks
Authors: Wilkinson W. P.
Published: February 2008

Observations of some low Mach number collisionless shocks in space have revealed that their associated ion heating is greater than adiabatic, is very rapid, is primarily in the direction perpendicular to the magnetic field, and takes place in the bulk of the distribution. These features are very different from those encountered at crossings of supercritical shocks and not yet fully understood. In this paper we use a one‐dimensional hybrid kinetic simulation to study the details of the evolution of the incoming ions across a set of low Mach shocks. We find that the bulk of the ion velocity distribution becomes elongated along the direction perpendicular to the shock front as the ions traverse the shock ramp. The subsequent gyration of the ions in the steady field downstream gives rise to a temperature signature with features that correspond closely to those enumerated above. This suggests that these features may originate primarily from the kinetic behavior of the ions at the shock rather than from the operation of some instability. We also investigate the dependence of our results on a number of physical and numerical parameters in the simulation.
Pressure anisotropy and radial stress balance in the Jovian neutral sheet
Authors: Paranicas C. P. , Mauk B. H. , Krimigis S. M.
Published: February 2008

By examining particle and magnetic field data from the Voyager 1 and 2 spacecraft, we have found signatures indicating that the (≳28 keV) particle pressure parallel to the magnetic field is greater than the pressure perpendicular to the field within the nightside neutral sheet (three nightside neutral sheet crossings, with favorable experimental conditions, were used). By incorporating the pressure anisotropy into the calculation of radial forces within the nightside neutral sheet, we find that (1) force balance is approximately achieved and (2) the anisotropy force term provides the largest contribution of the other particle forces considered (pressure gradients and the corotation centrifugal force). With regard to the problem of understanding the balance of radial forces within the dayside neutral sheet (McNutt, 1984; Mauk and Krimigis, 1987), the nightside pressure anisotropy force is larger than the dayside pressure gradient forces at equivalent radial distances; however, a full accounting of the dayside regions remains to be achieved.
The X ray corona, the coronal hole, and the heliosphere
Authors: Parker E. N.
Published: February 2008

The X ray emission from the Sun arises primarily from the gas trapped in the bipolar magnetic fields of both small and large active regions. It appears that the trapped gas is heated by the intermittent dissipation of magnetic energy (nanoflares) at the current sheets that arise spontaneously in any magnetic field subject to continuous deformation. The solar wind issues from regions of weak field pushed open by the expanding corona. Most of the heat input is close to the Sun, in the first 1–2 RS, raising the gas slowly out through the gravitational field and gradually accelerating it through the speed of sound at a distance of perhaps 3–5 RS. The waves generated by photospheric convection dissipate only at distances of 5 RS and beyond, where their heat input and momentum accelerate the wind to the high velocities of 600–800 km/s sometimes observed. The only source for the principal heat input close to the Sun appears to be the network activity, as suggested by Martin, Porter and Moore. Thus the mass loss and the formation of the heliosphere are primarily a consequence of the smallest‐scale activity supplemented by occasional flares and coronal mass ejections. The X ray emission is largely a consequence of the smallest flares, the nanoflares, supplemented by occasional X ray bursts from large flares. It is presumed that the mass loss and associated circumstellar spheres and the X ray emission from most other stars arise in the same manner, although there is no possibility for detailed observation, of course.
Signature of burst particle precipitation on VLF signals propagating in the Antarctic Earth‐ionosphere waveguide
Authors: Cotton P. D. , Smith A. J.
Published: February 2008

The burst precipitation of energetic electrons (≥40 keV), induced by interactions with lightning‐generated whistler mode waves, has been observed to cause phase and amplitude perturbations on subionospheric VLF signals (“Trimpi” events). With a knowledge of the propagation characteristics of the subionospheric signal, analysis of the perturbation details can lead to estimates of the energy, extent, and location of the precipitation.

Trimpi events have been observed on VLF signals propagating at high latitudes (L ≥ 4) over Antarctica, on 3.79‐kHz signals transmitted from the horizontal dipole at Siple station. A mode theory computer model for propagation of VLF waves in the Antarctic Earth‐ionosphere waveguide is used to illustrate the characteristics of 3.79‐kHz signals as they propagate from Siple toward VLF receivers at Halley and South Pole stations. To simulate the effects of precipitation, localized depressions in the ionospheric reflection height are introduced over the great circle propagation paths in the model, and it is seen that, while the amplitude and phase perturbations of some specific events are accurately reproduced, large positive amplitude (up to 6 dB) Trimpi events at Halley cannot be reproduced. Calculations are presented which show that signals echoing from precipitation patches located away from the great circle path could be the cause of such signatures.
Numerical simulations of large‐scale plasma turbulence in the daytime equatorial electrojet
Authors: Ronchi Corrado , Sudan R. N. , Farley Donald T.
Published: February 2008

We present numerical simulations of the large‐scale electron density irregularities in the daytime equatorial electrojet driven by the gradient drift instability. The nonlocal nonlinear two‐fluid equations are integrated numerically with scales ranging from about 10 km to less than 100 m directly resolved on a 128×128 grid, while the effects of the smaller subgrid scales are included with the use of anomalous electron mobility and diffusion coefficients [Ronchi et al., 1990a]. The instability evolves to a state in which the perturbations propagate primarily in the east‐west direction with a typical horizontal wavelength of about 2 km. The output of the numerical simulations does not indicate the presence of a significant anisotropy in the power spectrum of the irregularities in the plane perpendicular to the ambient magnetic field, in contrast to the marked differences observed in physical space between the vertical and horizontal dynamics. The one‐dimensional integrated density and electric field power spectra have a power law dependence with a power index ranging between −2.5 and −1.2. The numerical results are compared with in situ rocket observations by probing the simulation region along different flight paths, following both eastward and westward trajectories. Electron vertical turbulent velocity distributions are computed from the code output and are contrasted with radar backscatter data. The features typical of the 3‐m type 2 echoes (such as the broadening and asymmetry in the frequency power spectra) are also present in the computed distributions, indicating that during weak electrojet conditions the small‐scale structures act as tracers of the large‐scale electric field variations. A conclusion of particular note is that a purely linear nonlocal analysis (valid for wavelengths λ ≈ 1 km) leads to the result that all perturbations are eventually damped, either by shear and then diffusion or by recombination. The inclusion of nonlinear effects, however, restores the instability. In the strongly turbulent regime a nonsteady saturated state is reached, whereby the linear convection of energy via shear to high wavenumbers is countered by the nonlinear modification of the equilibrium density and electric field gradients and by mode coupling of shorter wavelengths back to long.
Occurrence characteristics of VLF bursts in the nightside ionosphere of Venus
Authors: Ho C.‐M. , Strangeway R. J. , Russell C. T.
Published: February 2008

Many of the impulsive VLF signals observed by the Pioneer Venus orbiter electric field detector (OEFD) in the nightside ionosphere have been interpreted as arising from lightning in the Venus atmosphere. In order to determine the characteristics of the source, and to compare with terrestrial lightning, we determine the normalized occurrence rate for individual bursts at Venus, as opposed to previous studies of activity in 30‐s intervals. We establish burst identification criteria which take into account the decay constant of the instrument to calculate the burst rate. Under the assumption that all OEFD observations in the nightside ionosphere are due to impulsive signals, we find that the 100‐Hz channel has the highest burst rate, about 0.20/s in the altitude range 150–180 km and in the postmidnight local time sector. The burst occurrence rates for all four frequency channels decrease with increasing altitude. Burst rates at frequencies above 100 Hz are greater in the premidnight hours while 100‐Hz signals peak after midnight; however, these burst rates vary from orbit to orbit. The 100‐Hz low‐frequency burst rate has a stronger dependence on the magnetic field strength than the higher‐frequency rates. Most 100‐Hz bursts are closely spaced and occur independently of signals at higher frequencies. These dependences suggest that the low‐ and high‐frequency signals have different propagation mechanisms. The properties of the bursts are generally consistent with a lightning source, and the planet‐wide burst rate at Venus may be comparable to or larger than the terrestrial lightning rate.
On the position of the near‐Earth neutral sheet: A comparison of magnetic model predictions with empirical formulas
Authors: Peredo Mauricio , Stern David P.
Published: February 2008

A detailed comparison of the near‐Earth neutral sheet position according to several approximations has been made. In particular, two empirical formulas for the neutral sheet location, recently derived from AMPTE/CCE data, have been compared with the corresponding positions derived from Tsyganenko's magnetic field models. Cylindrical coordinates referenced to the geodipole have been used, and the position of the neutral sheet has been identified by the reversal of the radial component of the magnetic field. The analysis reveals that in the region −9 RE ≤ XGSM ≤ −5 RE, |YGSM| ≤ 5 RE, close agreement exists between the empirical formulas and the Tsyganenko models. Furthermore, the Tsyganenko models provide a representation for the neutral sheet position in the transition region between near‐Earth and distant tail models.
Simulation of whistler waves excited in the presence of a cold plasma cloud: Implications for the CRRES mission
Authors: Pritchett P. L. , Schriver D. , Ashour‐Abdalla M.
Published: February 2008

A one‐dimensional electromagnetic particle simulation model is constructed to study the excitation of whistler waves in the presence of a cold plasma cloud for conditions representative of those after the release of lithium in the inner plasma sheet during the CRRES mission. The results indicate that a standing‐wave pattern with discrete wave frequencies is formed within the cloud. The magnetic wave amplitude inside the cloud, which is limited by quasi‐linear diffusion, is of the order of several nanoteslas. Assuming a magnetospheric loss cone of 5°, the observed pitch angle diffusion produced by the whistler waves is sufficient to put the electrons on strong diffusion.
Origin of diffuse superthermal ions at quasi‐parallel supercritical collisionless shocks
Authors: Kucharek H. , Scholer M.
Published: February 2008

Large‐scale one‐dimensional hybrid simulations of quasi‐parallel shocks have been performed in order to study the acceleration of upstream energetic ions. In these self‐consistent simulations a certain part of the incident ions is accelerated and constitute diffuse upstream particles, which are subject to further scattering in upstream magnetosonic waves of their own making. The number of superthermal particles close to the shock reaches a steady state within ≲30 Ωci−1. The ratio of diffuse upstream particles to solar wind particles decreases slightly with increasing shock Mach number and increases with decreasing angle ΘBn between the upstream magnetic field and the shock normal. The acceleration of thermal particles to superthermal energies occurs by a more or less coherent process: thermal ions of the incident distribution stay for an extended time period at the shock. Because of the large noncoplanarity magnetic field component they grad B drift within the coplanarity plane and gain energy by the tangential electric field. They also gain energy due to wave‐particle scattering when they stay near the shock and when they finally leave the shock in the upstream direction during a shock re‐formation cycle. Since the backstreaming particles excite the upstream waves by an ion/ion beam instability, they feed energy to the wave field. Therefore, in the shock frame a few of the backstreaming ions have an energy below the initial energy. The particle distribution is diffuse in velocity space and exhibits a spherical hole, which is approximately centered at the phase velocity of the upstream waves. This indicates that the particles are pitch angle scattered in the upstream wave field. The results show that superthermal upstream particles are an integral part of quasi‐parallel collisionless shocks and that no particular seed particle population is necessary for shock acceleration. The shock structure and the first‐order Fermi acceleration problem have therefore to be considered simultaneously.
The cosmic radiation in the heliosphere at successive solar minima
Authors: McDonald Frank B. , Moraal Harm , Reinecke J. P. L. , Lal Nand , McGuire Robert E.
Published: February 2008

Comparative cosmic ray studies at the time of successive solar minima are of special importance in establishing the relative role of large‐scale drift effects in the modulation process and in estimating the size of the modulation region and the local interstellar spectra of low‐ and medium‐energy cosmic rays. In this paper the 1 AU cosmic ray observations are compared for the last three solar minimum periods along with the 1977/1978 and 1987 Pioneer 10 and Voyager 1 and 2 data from the outer heliosphere. There is good agreement between the 1965 and 1987 galactic cosmic ray H and He spectra at 1 AU. However, there are significant and complex differences between the 1977/1978 and 1987 measurements of the galactic and anomalous cosmic ray components at 1 and 15 AU. In the outer heliosphere there are large negative latitudinal gradients that reach their maximum magnitude when the inclination of the outer heliosphere current sheet is at a minimum. The radial gradients decrease with heliocentric distance as ∼1/r0.7 and do not differ significantly at the successive solar minima. While these effects, along with the shift in the intensity maximum of anomalous helium (He+), are in the direction expected from large‐scale gradient and curvature drifts in the heliosphere, they are about an order of magnitude smaller than those calculated using the standard Parker model of the interplanetary magnetic field. The measured radial and latitudinal gradients are used to estimate the particle transport parameters in the outer heliosphere. The slope of the H spectra measured at Pioneer 10 for both solar minimum periods is given by γH ≈ 0.5 down to energies of 40 MeV. Using the local interstellar He spectrum of Webber et al. (1987), it is estimated that the modulation boundary is of the order of 160 AU.
The effect of magnetic topography on high‐latitude radio emission at Neptune
Authors: Sawyer C. B. , Warwick James W. , Romig J. H.
Published: February 2008

Occultation by a local elevation on the surface of constant magnetic field is proposed as a new interpretation for the unusual properties of Neptune high‐latitude emission. Abrupt changes in intensity and polarization of this broadband smooth radio emission were observed as the Voyager 2 spacecraft passed near the north magnetic pole before closest approach. The observed sequence of cutoffs with polarization reversal would not occur during descent of the spacecraft through regular surfaces of increasing magnetic field. The sequence can be understood in terms of constant‐frequency (constant‐field) surfaces that are not only offset from the planet center but are locally highly distorted by an elevation that occults the outgoing extraordinary‐mode beam. The required occulter is similar to the field enhancement observed directly by the magnetometer team when Voyager reached lower altitude farther to the west. We present evidence that the sources of the high‐latitude emission are located near the longitude of the minimum‐B anomaly associated with the dipole offset and that the local elevation of constant‐B surfaces extends eastward from the longitude where it is directly measured by the magnetometer to the longitude where occultation of the remote radio source is observed. Together, the radio and magnetometer experiments indicate that the constant‐frequency surfaces are distorted by an elevation that extends 0.3 rad in the longitudinal direction. On the 462 kHz surface, about 1.1 RN from the dipole center, the local elevation must be at least 0.45 RN above the undisturbed (dipole model) surface.
Structure of the magnetopause current layer at the subsolar point
Authors: Okuda H.
Published: February 2008

A one‐dimensional electromagnetic particle simulation model developed for the magnetopause current layer between the shocked solar wind and the dipole magnetic field at the subsolar point has been extended to include the interplanetary magnetic field (IMF) in the solar wind. Interaction of the solar wind with the vacuum dipole field as well as the dipole field filled with a low‐density magnetospheric plasma are studied. It is found that the width and the structure of the magnetopause current layer differ markedly depending on the direction of the IMF. When the IMF is pointing southward, the current layer between the solar wind and the dipole field is narrow, and the magnetic field has a single ramp structure caused by the reflection of the solar wind at that point. The current layer becomes several times wider and the magnetic field develops a multiple ramp structure when the IMF is northward. This broadening of the current layer is caused by the multiple reflection of the solar wind by the magnetic field. For the northward IMF the magnetic field does not change its sign across the current layer, so that the E × B drift of the solar wind electrons remains the same direction, while for the southward IMF it reverses the sign. This results in a single reflection of the solar wind for the southward IMF and multiple reflections for the northward IMF. When a low‐density magnetospheric plasma is present in the dipole magnetic field, a small fraction of the solar wind ions are found to penetrate into the dipole magnetic field beyond the reflection point of the solar wind electrons. The width of the ion current layer is of the order of the solar wind ion gyroradius; however, the current associated with the ions remains much smaller than the electron current so long as the density of the magnetospheric plasma is much smaller than the density of the solar wind. Comparisons of our simulation results with the recent spacecraft data of the magnetopause crossing near the subsolar point are provided.
Observations of VHF emissions from 50‐mA electron beam injections in the ionosphere that are associated with beam‐induced discharges
Authors: Goerke R. T. , Kellogg P. J. , Monson S. J. , Franz R. C. , Nemzek R. J. , Anderson H. R. , Potter D. W. , Denig W. F. , Szuszczewicz E. P. , Earle G. D.
Published: February 2008

We have observed strong VHF plasma waves with amplitudes in excess of 0.1 mV/m (Hz)1/2 associated with 50 mA electron beam injections in the ionosphere. These observations were made up to 44 MHz, which is approximately 20 times the ambient plasma frequency. Data from three swept‐frequency receivers carried on two daughter payloads have been analyzed to determine the emission spectra of the electron beam for various energies and currents. These results were obtained from the rocket‐bome experiment SCEX 3 (Several Compatible EXperiments using a rocket‐borne accelerator), NASA flight 39.002 UE, launched February 1, 1990, from Poker Flat Research Range (65.1°N, 147.5°W). Such VHF emissions from electron beams are normally considered to be caused by a beam plasma discharge (BPD) (Llobet et al., 1985). The accelerator payload also carried photometers, which measured luminosity at wavelengths of 391.4 nm and 380.5 nm. Several times during electron gun activity the measured luminosity increased much faster than proportional to the beam current. This nonlinear increase is evidence that a discharge is occurring in the vicinity of the accelerator payload. The uniform distribution of the luminosity around the accelerator payload implies that the observed discharge extends significantly outside the beam cylinder. Because the photometers did not view the beam region directly, there is no direct evidence that a beam plasma discharge is occurring. Neutral density gauges carried on the accelerator payload and a daughter payload measured a neutral density enhanced by ∼3 orders of magnitude over model density profiles. This excessive neutral density (presumably caused by outgassing) may account for the discharge. A possible mechanism for igniting the beam plasma discharge in space is also presented.
Cross‐polar cap potential difference, auroral electrojet indices, and solar wind parameters
Authors: Ahn B. ‐H. , Kamide Y. , Kroehl H. W. , Gorney D. J.
Published: February 2008

The cross‐polar cap potential difference Φ (KRM) is estimated from ground magnetic perturbation data through the magnetometer inversion method of Kamide, Richmond, and Matsushita (KRM), combined with an “empirical” ionospheric conductance distribution estimated from the DMSP X ray image data. A significant correlation is found between Φ (KRM) and the AE(12) index; Φ (KRM, in kilovolts) = 36 + 0.082 AE(12, in nanoteslas) with the correlation coefficient being 0.80. Φ (KRM) is then compared with the potential difference estimated from a more direct method of the satellite electric field measurements [Weimer et al., 1990] and also with Φ (IMF) based on solar wind parameters [Reiff and Luhmann, 1986]. Φ (IMF) is found to be linearly correlated with Φ (KRM), as Φ (IMF) = 29.8 + 0.999 Φ (KRM), with the highest correlation obtained for a 40‐min lag in the interplanetary magnetic field (IMF). Note that Φ (IMF) is systematically larger than Φ (KRM) by 30 kV, suggesting the possibility that the theoretical method overestimates the cross‐polar cap potential difference. During steady southward IMF periods where steady Φ (IMF) variations are expected, significant fluctuations in calculated Φ (KRM) values are obtained. Since the decrease in Φ (KRM) is closely associated with enhancements in auroral particle precipitation during these periods, a highly correlative relation between Φ (IMF) and Φ (KRM) cannot be deduced unless the phases of substorms are taken into account. The overall high correlation between them, however, supports the view expressed by Wolf et al. [1986] that directly driven processes are more important than unloading processes during disturbed periods.
Excitation of electron cyclotron harmonic waves in Ionospheric Modification Experiments
Authors: Rao N. N. , Kaup D. J.
Published: February 2008

We investigate the resonance excitation of electrostatic as well as electromagnetic electron cyclotron harmonic waves by mode conversion during ionospheric F region modification experiments when the pump wave frequency is near harmonics of the electron cyclotron frequency. In the electrostatic case we calculate the bandwidth around each harmonic frequency within which excitation of electrostatic electron Bernstein waves is possible. For typical ionospheric parameters the bandwidth is found to be of the order of a few kilohertz which is in good agreement with the frequency dependence of the excitation and the quenching of the downshifted maximum (DM) feature in the recent modification experiments at Tromsø (Norway) and Arecibo (Puerto Rico). In the electromagnetic case there exist frequency ranges in which the fast branch of the X mode pump wave can be directly converted into cyclotron harmonic waves. Excitation of the upper hybrid wave in the ionospheric E region by means of the fast X mode pump wave seems to be possible for certain parameter ranges. When the pump frequency is near the second harmonic, mode conversion is found to be enhanced in both the cases. Relevance of these results to the various spectral features like the DM, the broad upshifted maximum, and the broad symmetrical structures found in the stimulated electromagnetic emission from the ionosphere is pointed out.
Radial expansion of the tail current disruption during substorms: A new approach to the substorm onset region
Authors: Ohtani S. , Kokubun S. , Russell C. T.
Published: February 2008

The substorm onset region and the radial development of the tail current disruption are examined from a new viewpoint. The reconfiguration of the magnetotail field at substorm onset can be understood in terms of a sudden decrease (disruption) in tail current intensity. The north‐south component (BZ) is very sensitive to whether the spacecraft position is earthward or tailward of the disruption region, while the change in the Sun‐Earth component (BX) is most sensitive to the change in tail current intensity near the spacecraft. If the current disruption starts in a localized range of radial distance and expands radially, a distinctive phase relationship between the changes in BX and BZ is expected to be observed. This phase relationship depends on whether the current disruption starts on the earthward side or the tailward side of the spacecraft. Thus it is possible to infer the direction of the radial expansion of the current disruption from magnetic field data of a single spacecraft. This method is applied to ISEE observations of a tail reconfiguration event that occurred on March 6, 1979. The phase relationship indicates that the disruption region expanded tailward from the earthward side of the spacecraft during the event. This model prediction is consistent with the time lag of magnetic signatures observed by the two ISEE spacecraft. The expansion velocity is estimated at 2 RE/min (∼200 km/s) for this event. Furthermore, it is found that the observed magnetic signatures can be reproduced to a good approximation by a simple geometrical model of the current disruption. The method is used statistically for 13 events selected from the ISEE magnetometer data. It is found that the current disruption usually starts in the near‐Earth magnetotail (|X| < 20RE and often within 15 RE of the Earth. The phase relationship between BX and BZ is discussed in terms of the near‐Earth neutral line model, and the result is compared with previous studies on the substorm onset region.
Effects of gravity waves on complex airglow chemistries: 1. O2(b1Σg+) emission
Authors: Tarasick David W. , Shepherd Gordon G.
Published: February 2008

The theory of Hines and Tarasick (1987) for the effects of gravity waves on airglow emissions is extended to consider more complex airglow chemistries, including multiple and multiple‐step production mechanisms, quenching, and other loss processes. Relations for the dependence of η, the ratio of brightness to temperature fluctuations, on emission chemistry are presented in a generalized form which is readily applicable to other emissions. The specific case of O2(b1Σg+) airglow is examined in detail for three different proposed chemical production mechanisms. Loss mechanisms are also considered; quenching is found to be important to predictions of η The results obtained are compared with the very limited set of published observations of gravity waves in O2(b1Σg+) airglow. It is shown that gravity wave observations may place new constraints on possible mechanisms for airglow production, constraints that are not obtainable by other means.
The dynamic cusp
Authors: Potemra T. A. , Erlandson R. E. , Zanetti L. J. , Arnoldy R. L. , Woch J. , Friis‐Christensen E.
Published: February 2008

A unique alignment of the Viking satellite with respect to a network of magnetometers in Greenland has provided the opportunity to study the relationship of pulsations and plasma characteristics in the dayside cusp. Observations in the interplanetary medium were not available during the event studied here, but particle data from the DMSP satellite and hot plasma observations from Viking provide strong evidence that the IMF had a strong northward component. The presence of Pc 1 bursts, Pc 4–5 pulsations, and a tailward traveling twin vortex pattern of ionospheric convection suggests that the magnetosphere may have been temporarily compressed. Magnetic field data acquired at synchronous altitude from GOES 5 and on the ground from Huancayo support this suggestion. Plasma with ion dispersion characteristics associated with a cusp during southward IMF was detected by Viking over a 3.5° range of latitude. The presence of standing Alfvén waves and ring current ions suggests that this “cusplike” plasma was observed on closed geomagnetic field lines. As Viking moved further poleward, it detected a different region of plasma with characteristics associated with a cusp during northward IMF. The presence of plasma on closed field lines with “southward IMF” ion dispersion characteristics can be explained with a poleward moving plasma source. We suggest that the magnetosphere, during a northward IMF, is temporarily compressed by a solar wind pressure enhancement that produces the Pc 1 bursts, Pc 4–5 pulsations, and ionospheric vortices. As the magnetosphere recovers to its “precompressed” shape, the source of cusp plasma will move poleward until it reaches an equilibrium position for northward IMF. The Viking satellite, following in the wake of this source, will detect plasma with “southward IMF” characteristics until it reaches the latitude of the actual “northward IMF” cusp. These observations support the view that the shape of the magnetosphere may rarely be static but is often changing as a result of the delicate and variable balance between the solar wind and geomagnetic field.
Asymptotic and time‐dependent solutions of magnetic pulsations in realistic magnetic field geometries
Authors: Wright Andrew N.
Published: February 2008

The coupling of compressional and Alfvén modes is important in solar, magnetospheric, and laboratory plasmas. We investigate such coupling within a general framework, but concentrate upon applying our results to the excitation of Alfvén waves in the magnetosphere. The Alfvénic response of an arbitrary (curl‐free) magnetic field and ideal cold plasma is considered, and the Alfvén wave equation is shown to be driven by perturbation magnetic pressure gradients aligned with the Alfvén fields. The efficiency with which any particular standing Alfvén wave is excited is governed by an overlap integral along the background field line of the Alfvén mode and the perturbation magnetic pressure gradient. Three types of compressional driving function are investigated which produce diverse behavior and may excite resonant and nonresonant Alfvén waves. These waves oscillate with either the natural Alfvén frequency or the frequency of the driver. Finally we turn our attention to the long‐term, or asymptotic, state of the fields. A nonlinear solution is derived which is a generalization of earlier work. We find that the standing Alfvén waves will be confined to thin layers.