bluuebunny/crossref_metadata_2025 huggingface dataset:
On the effect of finite, field‐aligned plasma length on a loss‐cone instability
Authors: Sperling J. L.
Published: February 2008

The finite extent of plasma clouds in the ionosphere imposes a nonzero parallel wave number on plasma modes. The consequent parallel modal currents and modal electric fields have a profound effect on the development of eigenmodes and the possible interaction of the eigenmodes with both the ionosphere and the magnetosphere. Specific consideration is given to the kinetic loss cone instability, which has been postulated to cause the initial growth of prompt striations. It is shown that an enhancement, in the effective electron collision frequency above the value expected from purely classical considerations, produces an increase in the parallel wave number of the fundamental eigenfunction of this instability. This provides a plausible explanation for the observation of prompt striations in the Buaro barium release. The development of eigenmodes implies the transfer of free energy originally residing in the loss cone ions into the parallel electron and ion motion. In this way, the eigenmodes can directly contribute to field‐aligned plasma currents and auroral precipitation of particles.
Solar irradiance variations: 1. Analysis of modeling techniques and intercomparison of ground‐based data
Authors: Oster Ludwig
Published: February 2008

We compute the full disc irradiance at 2800 MHz (‘F10.7’) on the basis of individual active regions (ARs) on the sun as observed in the light of the Ca II K line at 393 nm. The solar radio emission is used as a stand‐in for UV measurements from above the atmosphere, since it does not have significant calibration problems, and since day by day records are available for many years. We illustrate in detail the techniques for parameterization of the model calculations which, with few changes, are applicable to UV measurements. In addition to the excellent overall agreement and the impressively close correspondence of day to day variations, we find no indication of a systematic discrepancy between model and observations over the entire 10‐year period. Nor do we find a contribution to the 2800 MHz irradiance that is not directly related to individual ARs on the sun at any given day. We believe that our analysis, by comparison with previous work, is much more detailed without being much more complex, and that it offers a serious chance for an independent evaluation of short‐term as well as long‐term fluctuations of the solar observed irradiance.
Plasma rest frame frequencies and polarizations of the low‐frequency upstream waves: ISEE 1 and 2 Observations
Authors: Hoppe M. M. , Russell C. T.
Published: February 2008

Using magnetic field data from the dual ISEE 1 and 2 spacecraft we have determined the plasma rest frame frequencies and polarizations of the large amplitude low frequency (0.03 Hz) upstream waves. The monochromatic sinusoidal waves associated with ‘intermediate’ ion fluxes are magnetosonic waves with rest frame frequencies ∼0.1 ΩP and wavelengths ∼1 RE. The obliquely propagating ‘shocklet’ form of the waves is also predominantly in the magnetosonic mode, but occasionally also appears in the Alfven mode with similar rest frame frequency and wavelength. The generation of the magnetosonic mode has been explained by the well established cyclotron resonance mechanism driven by narrow reflected ion beams but the concurrent observation of Alfven mode waves appears to require wave generation by the more isotropic diffuse ion distributions as well.
Observations of large magnetospheric electric fields during the onset phase of a substorm
Authors: Aggson T. L. , Heppner J. P. , Maynard N. C.
Published: February 2008

A large (30‐mV/m peak) impulsive westward electric field was observed in the midnight, low latitude, dipole L = 7.5 region of the earth's magnetospheric tail at the onset of a large substorm. The measurements were made with the long, 179‐m baseline, cylindrical electric field probes carried by ISEE 1. The electric field impulse was coincident with a sharp 60 nT steplike change in the x component of the magnetic field at the satellite and the onset of a sharp 60 nT decrease in the H component of the field at a magnetic observatory near the subconjugate point. The 2‐min envelope of the westward Ey field correlates with the time derivative ∂Bx/∂t of the collapsing magnetic field attributed to the decrease in the cross‐tail current. Associated with this inductive impulse, large electric field variations are also observed on time scales of tens of seconds to tens of milliseconds. The low‐frequency (10 s) wave variations show a coherent phase relationship between the electric and magnetic field that changes from correlative to anti‐correlative during the event. Higher frequency (t < 1 s) turbulent electrostatic fields of similar magnitude are present throughout the event. These large time‐dependent electric fields seem to have the proper amplitude, duration, and timing relative to the auroral substorm sequence to explain the energetic proton enhancements frequently observed near midnight both at synchronous orbit and in the magnetotail at times of substorm onsets. The azimuthal direction of Ey (westward) for this event appears to conflict with the IMP 6 observations of an eastward Ey at higher latitudes under similar conditions. However, these observations are in agreement when magnetotail collapse is viewed in terms of a sharp reduction in the anti‐solar tension on auroral field lines as opposed to interpretations based solely on the effects of a sudden decrease in the cross‐tail current sheet.
Io's atmosphere: Pressure control by regolith cold trapping and surface venting
Authors: Matson Dennis L. , Nash Douglas B.
Published: February 2008

A new model for the basal pressure of Io's atmosphere is developed. This model takes into account the previously ignored fact that much of Io's surface has very high porosity, typically ∼90%. Such porosity allows efficient subsurface cold trapping of atmospheric gases which tends to keep ambient surface pressures very low. SO2 is the only gas identified on Io, and the basal pressures for atmospheric models are usually pegged to local surface temperature via the SO2 vapor pressure equilibrium curve. Near Io's subsolar point the pressure in equilibrium with a surface SO2 frost deposit is ∼10−7 bar. Porous surface models of the type we develop invoke equilibrium with the colder, subsurface permafrost (at ∼3‐cm depth) and yield pressures of ∼10−12 bar. The subsurface cold trapping model explains many but not all observations relevant to Io's atmosphere. The new subsurface cold trapping model and the earlier surface frost equilibrium model, when taken together, provide lower and upper limits, respectively, on the basal SO2 pressure of Io's atmosphere.
Depth of the nonconducting layer at the Nigerian Dip Equator
Authors: Duhau S. , Osella A. M.
Published: February 2008

The geomagnetic daily variations at the Nigerian dip equator are analyzed to find the thickness of the nonconducting layer in this zone. To do so, the external current system was determined from the external part of the geomagnetic daily variations by using a method discussed in a previous work. The field induced by this system was calculated assuming a two‐layer model for the earth conductivity. Fitting this induced field to the internal part of the geomagnetic daily variations yielded a thickness of 500 km for the nonconducting layer in the interval of 100 km south to 500 km north of the dip equator. However, there is evidence showing a latitudinal gradient in the thickness south of this interval.
Equivalent ionospheric current systems representing lunar daily variations of the polar geomagnetic field
Authors: Matsushita S. , Xu Wen‐Yao
Published: February 2008

Equivalent ionospheric current systems of the lunar daily geomagnetic variations at both high and mid‐low latitudes during a solar active period are estimated from geomagnetic data for three seasons and the yearly average. Clear convective curents at high latitudes are found in addition to the well‐known mid‐low latitude current system. Field‐aligned electric currents as well as the wind dynamo effects which produce the obtained electric current systems for the semidiurnal variation are examined. It is concluded that the wind dynamo plays a basic role in producing the current system at high latitudes, since lunar effects on the field‐aligned current distribution are difficult to explain.
Analytical yield spectrum approach to electron energy degradation in Earth's atmosphere
Authors: Haider S. A. , Singhal R. P.
Published: February 2008

Analytical spatial yield spectrum approach has been applied to calculate the electron flux, volume excitation, and ionization rates for monoenergetic electrons incident on the earth's atmosphere. The results have been compared with the previous model calculations of Mantas and Walker (1976) and Banks et al. (1974).
Comparison of height‐integrated current densities derived from ground‐based magnetometer and rocket‐borne observations during the Porcupine F3 and F4 flights
Authors: Ziesolleck Ch. , Baumjohann W. , Brüning K. , Carlson C. W. , Bush R. I.
Published: February 2008

On March 19, 1979, the Porcupine F3 rocket was launched from Esrange (Sweden) during a period of low magnetospheric activity while the Porcupine F4 rocket launched on March 31, 1979, flew during a rather disturbed interval. Besides other experiments the rockets took measurements of ionospheric electric fields and energetic particle spectra. The electron spectra measured by the rocket were used to calculate height‐integrated ionospheric conductivities by using Rees' method. Together with the electric field measurements, height‐integrated current densities could be calculated from the rocket measurements. During both flights the magnetic perturbations recorded on the ground by the Scandinavian Magnetometer Array indicated a westward electrojet. Since during both flights the electric field was nearly southward directed and parallel to the ground magnetic disturbance vectors the westward electrojet could be attributed to a nearly pure Hall current in both cases. From the magnetic data height‐integrated ionospheric current densities have been computed at the magnetic footprints of the rockets in the ionosphere by means of field separation and upward continuation. A comparison of the current densities derived from ground‐based magnetometer and rocket‐borne observations shows a very good agreement in the quiet period during the Porcupine F3 flight, while there are differences during stronger magnetospheric activity (Porcupine F4 flight) due to the different spatial and temporal resolution of rocket and ground observations.
The non‐Maxwellian energy distribution of ions in the warm Io torus
Authors: Richardson John D. , Siscoe George L.
Published: February 2008

Observations of Io's torus indicate that the majority of ions have energies of 55–75 eV, with a high‐energy tail extending up to the corotation energy. We have found that such a distribution can be established via the Coulomb cooling of ions heated at the corotation energy onto the cold 5‐eV electrons. The energy E* of the main body of the ions and the shape of the energy distribution are functions of the transport loss time. Matching E* with the data (E* = 55–75 eV) requires transport loss times in the range 25–100 days.
The radial evolution of a single solar wind plasma parcel
Authors: Schwartz Steven J. , Marsch Eckart
Published: February 2008

We have studied one of the rare occasions when the same piece of solar wind plasma passed both Helios 1 (at 0.507 AU) and Helios 2 (at 0.72 AU). Our particular example occurred in the middle of a fairly smooth and typical high‐speed stream. We find clear indications that the ion behavior during this transit did not conserve the particles' adiabatic invariants and that some heating is required for the proton and alpha perpendicular temperature. The situation regarding the parallel temperature is less clear. The decreasing alpha‐proton differential speed does not appear to release very much free energy owing to a natural “adiabatic” cooling process. A very strong radial dependence of the electromagnetic energy flux, along with other factors, suggests the possibility of strong dissipation of Alfvenic fluctuations. Our conclusions are weakened somewhat by the many sources of inaccuracy that we have found and that seem unavoidable in a study of this kind.
Electromagnetic theory of collisional interchange instabilities
Authors: Glassman A. J. , Sperling J. L.
Published: February 2008

Finite parallel wavelength is shown to have a significant effect on the linear growth of instabilities of the Rayleigh‐Taylor or E×B of gradient drift types. The flute approximation, which explicitly neglects electric fields parallel to the ambient magnetic field, is asymptotically correct for long perpendicular wavelengths, if the parallel component of the electric field is primarily inductive in character. However, when the parallel electric field is primarily electrostatic, there is a profound reduction in the growth rate for long perpendicular wavelengths. This reduction in growth rate arises because parallel electron currents short‐circuit the perpendicular electric field driving the instability. Parallel electric fields are more likely to be electrostatic in lower‐density plasmas than in higher‐density plasmas. Consequently, the extrapolation of structuring from low‐density plasmas, such as those found in the spread‐F environment, to higher‐density ionospheric plasmas may not be reasonable.
Measurements of the solar flux at 220 nm
Authors: Hanser Frederick A. , Sellers Bach
Published: February 2008

The solar flux at 220±5 nm has been measured from a balloon near 40 km in September of 1975, 1976, and 1977. The average value of the unattenuated flux corrected to 1 AU is 4.43×1012 photons/(cm² s nm) for near solar minimum conditions. Comparison with other measurements suggests that the solar flux at 220±5 nm may vary by as much as 25% over a solar sunspot cycle. This conclusion is strongly dependent on a few points and is not inconsistent with a variation below 10%.
Calculated stormtime variations in plasmaspheric thermal ion composition
Authors: Miller N. J. , Mayr H. G. , Harris I.
Published: February 2008

Model calculations describing stormtime variations in the earth's dayside plasmasphere are used to examine variations in ion composition. The model storm is initiated by high‐latitude thermospheric heating that generates meridional winds that carry neutral species, momentum, and energy equatorward. The thermosphere acts on the plasmasphere through collisional transfer of momentum and through chemical reactions between neutral species and ions. Over latitudes near the region of thermospheric heating, the thermosphere‐plasmasphere coupling processes cause enhancement in the density of oxygen ions while protons are being lost. Meanwhile, densities of oxygen ions and protons near the equator are increasing together, almost in phase. The largest enhancements in ion density develop at latitudes near 45° invariant for both oxygen and hydrogen.
Hemispherical Joule heating and the AE indices
Authors: Baumjohann W. , Kamide Y.
Published: February 2008

A linear regression analysis of Joule energy deposition rates integrated over the northern hemisphere as a function of the standard auroral electrojet indices yields a correlation coefficient of r = 0.7–0.9. Except for very disturbed times, when the AE(12) index tends to underestimate the electrojet current, the hemispherical Joule heating rate can be calculated by substituting 1 nT in the AE index by approximately 0.3 GW. This scale factor is appreciably larger than those employed in earlier energy coupling studies. A higher‐scale factor is found for the regression between Joule heating caused by eastward current versus AU than for that caused by the westward electrojet versus AL. This is consistent with typically lower ionospheric conductivity values in the eastward electrojet region which require higher electric fields and thus more Joule heating for a given eastward current or AU value than for the same intensity of the westward electrojet.
Generation of auroral kilometric and Z mode radiation by the cyclotron maser mechanism
Authors: Omidi N. , Wu C. S. , Gurnett D. A.
Published: February 2008

The relativistic Doppler‐shifted cyclotron resonance condition for electromagnetic wave interactions with a plasma is studied. The results indicate that the resonance condition defines an ellipse in velocity space when the product of the index of refraction and cosine of the wave normal angle is less than or equal to 1 and that it defines a partial ellipse when the product is larger than 1. It is also shown that waves with frequencies greater than the gyrofrequency (or its harmonic for higher harmonic resonances) can only resonate with particles moving in the same direction along the magnetic field, while waves with frequencies less than the gyrofrequency (or its harmonic) resonate with particles moving in both directions along the magnetic field. Growth rates for the terrestrial auroral kilometric and Z mode radiation have been calculated by using the relativistic resonance condition and the electron pitch angle distribution measured by the S3‐3 satellite in the auroral zone region. In the case of auroral kilometric radiation, it is shown that both the upgoing and the downgoing electrons are unstable and can give rise to the growth of auroral kilometric radiation, although growth due to the downgoing electrons is limited to a much narrower frequency band. The magnitudes of the growth rates for both the upgoing and the downgoing auroral kilometric radiation are comparable and they indicate that the path lengths required to account for the observed intensities of auroral kilometric radiation are of the order of a few hundred kilometers, which is probably too large. Therefore in order for cyclotron maser instability to remain a viable mechanism for the generation of auroral kilometric radiation, it is essential to either have steeper slopes (∂F/∂υ⊥) in the loss cone or reflections to increase the path length. The growth rate calculations for the Z mode radiation show that for wave frequencies just below the gyrofrequency and wave normal angles at or near 90°, the electron distribution is unstable and the growth rates are large enough to account for the observed intensities. The fact that Z mode waves with frequencies smaller than the gyrofrequency are amplified can explain the DE 1 observations of Z mode radiation which indicate no radiation exists above the local electron gyrofrequency.
Electrostatic hydrogen‐cyclotron wave emission below the hydrogen‐cyclotron frequency in the auroral acceleration region
Authors: Singh Nagendra , Schunk R. W. , Conrad J. R.
Published: February 2008

A mechanism is suggested for the excitation of electrostatic ion‐cyclotron waves at frequencies below the ion‐cyclotron frequency in the midst of the auroral acceleration region, which is assumed to consist of downward moving double layers. The mechanism involves upward flowing ions interacting with a downward flowing background plasma. The upward flowing ions are the ion beams accelerated by the double layer. The downward motion of the background plasma corresponds to a plasma expansion into the density cavity that develops in connection with double layer formation in the acceleration region. The ion‐cyclotron waves excited by the counterstreaming flows are doppler shifted to frequencies below the harmonics of the ion cyclotron frequency. It is suggested that such wave emissions could account for some very narrow‐banded waves at frequencies below the hydrogen cyclotron frequency that were observed by the S3‐3 satellite.
Dayside auroral dynamics
Authors: Eather R. H.
Published: February 2008

Keogram cameras operated at South Pole during 1980 and 1981 have monitored the position, intensity, color, and dynamic behavior of dayside aurora. The position of the midday aurora for very quiet magnetic conditions is some 5° further poleward than was previously recognized. The average energy of dayside electron precipitation (∼100 eV) does not change as magnetic activity increases. Rapid latitudinal movements of discrete dayside aurora structures and pulsating structures with Pc 5 periodicities are regularly observed and have no obvious explanation in terms of interplanetary field conditions.
A study of particle trapping by whistler mode waves in the geomagnetic field: The early development of the VLF quiet band
Authors: Matthews J. P. , Omura Y. , Matsumoto H.
Published: February 2008

The interaction between a coherent whistler mode signal and the energetic electron population near the equator at L = 4 has been studied by using a computer simulation technique. Our results show that trapping of particles by the wave at locations removed from the equator can cause a distortion in the equatorial distribution function and have an important influence on the subsequent growth or damping of broad band hiss. The size and form of the distortion depends on whether a positive or negative gradient is present in the incident electron flux with respect to parallel velocity. This gradient is defined between the parallel velocity limits of the interaction. In the positive gradient case there is an intensification of growth above the frequency of the coherent signal. Below the signal, frequency hiss is strongly damped over a range of frequencies, and we interpret this quenching as representing a simulated VLF quiet band. In this model the frequency width of the quiet band varies as the square root of the amplitude of the causative coherent signal.
Short‐period magnetic pulsations associated with periodic VLF emissions (T ∼ 5.6 s)
Authors: Sato Natsuo
Published: February 2008

One‐to‐one correlation between periodic VLF emissions (T ∼ 5.6 s) and short‐period magnetic pulsations was found at Syowa Station (L ∼ 6) in Antarctica on August 8, 1981. The periodic VLF emissions were observed under extremely quiet geomagnetic conditions (Kp = 0–1). Fortunately, the periodic VLF emissions were recorded simultaneously on the ground at Syowa and on the ISIS 2 satellite. At ISIS 2 satellite level (∼1400 km altitude), the periodic emissions showed one‐to‐one correspondence with the periodic emissions observed at Syowa in the wide latitude range from L ∼ 3.5 to L ∼ 14.0, and also in the region about 2 hours away from Syowa in magnetic local time. From the ground data, the intensity of the H component of magnetic pulsations associated with the periodic VLF emissions is about 0.02 nT, approximately 5 times larger than that of the D component of magnetic pulsations. Magnetic pulsations associated with the periodic emissions have constant frequency and show a “linelike” frequency‐time spectrum. The amplitude of magnetic pulsations observed at Mizuho Station, about 270 km poleward from Syowa along the geomagnetic meridian plane, is approximately 4 times less than that at Syowa. For the origin of the short‐period magnetic pulsations it is suggested that the periodic precipitating electron fluxes induced by periodic VLF waves would modify the ionospheric conductivity. The periodic enhancements of conductivity in the ionospheric D and E regions would modify the Sqp current flows in the ionosphere, resulting in the magnetic variations observed on the ground.
Kinetic model of auroral plasma formation by magnetospheric convection and injection: 1. Electrons
Authors: Chiu Y. T. , Kishi A. M.
Published: February 2008

We show that Green's function solutions to the collisionless time‐dependent Boltzmann equation can be economically used to construct a time‐dependent model of auroral plasma formation by global time‐dependent covection electric fields in dipolar magnetic geometry. We show that recently observed features, such as formation of field‐aligned distributions, formation of injection fronts and formation of inverse dispersion signatures, can be accounted for in a global time‐dependent convection model. This paper also sets forth the theoretical and presentation framework for subsequent data analytic studies of convected electron and ion distribution functions.
Thermospheric superrotation revisited
Authors: Mayr H. G. , Harris I. , Hedin A. E. , Spencer N. W. , Wharton L. E.
Published: February 2008

A theoretical spectral model is presented to describe the mean (zonally averaged) circulation and latitudinal distributions in the temperature and composition. The results show that solar differential heating by radiation at low latitudes and Joule dissipation at auroral latitudes are primarily responsible for the observed temperature and density variations. Due to energy advection by the mean meridional circulation, the (latitudinal) variations in the total pressure field are relatively small, but significant temperature and composition gradients are maintained through wind induced diffusion, consistent with the empirical model of Hedin (1983). In the dissipative medium of the upper thermosphere, pressure gradients are not very effective in driving the mean circulation; thus the contribution to superrotation is negligible. At low and mid‐latitudes, the largest contribution to superrotation comes from correlations between the horizontal wind field of the diurnal tide and the diurnal variations in the ion density (drag) which give rise to a momentum source (see the review of Rishbeth (1972)). This mechanism, and to a lesser extent the correlations associated with the viscous shears and pressure gradients of the diurnal tide, produce a superrotation rate with a maximum of ΔΩ/Ω ∼ 0.03 at 200 km, falling off to 0.01 at exospheric heights. At lower altitudes, less than 200 km, our model is in reasonable agreement with the satellite drag data but departs drastically at higher altitudes where King‐Hele and Walker (1983) are quoting a value of 0.22 (350 km). At these altitudes, our theoretical results are consistent with the wind measurements on the Dynamics Explorer 2 by Spencer et al. (1982) and Wharton et al. (1984), who deduce a small average superrotation rate of <0.01.
Microstructure of magnetic reconnection in Earth’s magnetotail
Authors: Bieber J. W. , Stone E. C. , Hones E. W. , Baker D. N. , Bame S. J. , Lepping R. P.
Published: February 2008

A recent study of magnetic reconnection in the earth’s magnetotail identified five events in which a pulse of heated electrons appeared near the end of an interval of fast tailward plasma flow. Further investigation of these events has revealed the following information on their microstructure: (1) Measurements of the electron velocity distribution show that the temperature rise can be characterized as a true heating process, in that the gross evolution of the distribution is from a narrower to a broader Maxwellian. (2) During the heating pulse a bump is often apparent in the tail of the electron distribution function at an energy of 2–5 keV. This bump could be the remnant of a beam produced by a 2‐ to 5‐kV parallel potential drop. Thus a possible mechanism for the heating of electrons is the beam plasma instability. (3) The magnetic field during these events is highly variable in all three components. Part of this variability may result from magnetic turbulence generated by the tearing mode instability, but minimum variance analysis suggests that some of the more rapid variations (up to 20 nT/s) are caused by rotational or tangential discontinuities. (4) Both the Y and Z components of the magnetic field are quite large (10–20 nT) at times. The strongest southward fields are usually observed near the beginning of the heating pulse, while the strongest Y components generally occur during the pre‐heating interval of tailward plasma flow. Transient large‐scale changes in the normal magnetotail current system are indicated by these observations.
Quasi steady state multiple‐plasma‐cloud configurations in the F region ionosphere
Authors: Chen J. , Satyanarayana P. , Ossakow S. L.
Published: February 2008

The morphology of a finite array of plasma density enhancements (multiple plasma clouds) in the ionosphere is studied in order to determine the condition under which the system undergoes a quasi steady state E × B drift. In such a system the polarization‐induced self‐electric field inside all the plasma clouds must be essentially equal and uniform. The method of image dipoles previously developed for multiple equatorial spread F bubbles is used to calculate the instantaneous electric field exactly. It is found that even for small separation distances for which the intercloud interaction is significant, the electric field remains nearly uniform both internally and from cloud to cloud for typical ionospheric plasma cloud parameters, so that the multiple plasma clouds drift in unison without changing their geometrical shapes. This constitutes a quasi steady state configuration. The implications to the behavior of late time plasma cloud striations are discussed.
The forward‐reverse shock pair at large heliocentric distances
Authors: Whang Y. C.
Published: February 2008

An unsteady one‐dimensional MHD model is developed to study (1) the essential physical processes involved in the development of the forward‐reverse shock pair at large heliocentric distances, (2) the interaction of the shock pair with the rarefaction regions of the stream structure, and (3) the merging of two forward or reverse shocks. We use a method of solution which is quite different from the finite difference methods: MHD shocks (or contact surfaces) are treated as boundary surfaces. The presence of moving shocks divides the domain of interest in the r‐t plane into several flow regions. The jump conditions of MHD shocks describe the flow conditions across the moving boundaries between flow regions. The method of characteristics describes the variation of flow conditions in each region. The solutions explain the formation of the shock pair in the leading edge region as resulting from the merging of fast waves. The strong MHD disturbances generated in the corotating interaction region (CIR) propagate at a fast speed relative to the moving material. The wave propagation speed is greater in CIR than in its surroundings. This causes the disturbances in CIR to pile up to form a shock pair. During the formation process the shocks continuously grow into a fully developed state. The newly formed shock pair will then propagate outward from the leading edge to interact with the ambient rarefaction regions. The double‐sawtooth configuration of the velocity profile is a result of this interaction. We also obtain solutions to demonstrate that the merging of two shocks produces a stronger shock and a contact surface on its backside.
The charge state composition of 0.4‐MeV helium ions in Earth's outer radiation belts during quiet times
Authors: Chenette D. L. , Blake J. B. , Fennell J. F.
Published: February 2008

We present measurements of the charge state of 0.40‐ to 0.55‐MeV helium ions in earth's outer magnetosphere (L ∼ 6) using a novel technique. The radial gradient of the helium ion flux and the east‐west anisotropy in the pitch angle distribution are both measured. These are combined with measurements of the magnetic field to calculate the flux‐weighted average ion gyroradius. Since the magnitude of the magnetic field and the energies of the ions are known, the gyroradius determines the ion's charge state. During two pairs of geomagnetically quiet days in 1979 we find that most of these ions are singly charged. This result is in accord with theoretical calculations which find that charge exchange reactions between these ions and the neutral hydrogen geocorona are the dominant factor in controlling the charge state of these ions at times longer than a day after their injection. Our results also suggest that the mean charge state of these helium ions may vary with position in the magnetosphere and with time.
Interaction between an interplanetary shock and the Earth's magnetosphere on August 27, 1978: ISEE 1 electric field and ISEE 2 plasma observations
Authors: Alfsen K. H. , Bonifazi C. , Pedersen A. , Lindqvist P.‐A.
Published: February 2008

An interplanetary shock caused a rapid compression of the magnetosphere on August 27, 1978. The ISEE 1/ISEE 2 pair of satellites were in the frontside magnetosphere before the shock, and subsequently a magnetosonic wave front, the magnetopause, and the bow shock passed the ISEE satellites in a very short time. Previous papers determined boundary velocities from magnetic time delay measurements. These papers form a starting point for the study of electric field data during these events. By a combination of electric and magnetic field data it is possible to determine both the magnetosonic velocity and the magnetopause velocity, in good agreement with the magnetic time delay measurements. Furthermore, it is possible to establish with good confidence that during this compressional motion the E × B velocity was toward the magnetopause from both sides, ∼75 km s−1 from the earth side and ∼50 km s−1 from the sun side. A power conversion flux close to +400 W km−2 could be determined for the magnetopause. A sunward potential drop of 135–280 V was detected near the bow shock, and in this case the power conversion flux was negative, and in the range −(80–160 W km−2), or ∼10% of the solar wind kinetic energy flux.
Ion cyclotron instability in the auroral F region
Authors: Lakhina G. S. , Bhatia K. G.
Published: February 2008

An ion cyclotron instability, propagating nearly transverse to the ambient magnetic field and having frequencies near the harmonics of the ion cyclotron frequency, can be excited by the distorted ion velocity distribution functions present in the F region of the high‐latitude ionosphere. The instability occurs for D > 1.256 (D being the ratio of E × B0 drift speed to the ion thermal speed, corresponding to the temperature of the neutral species), and its growth rate increases by the increase of D. The instability can generate irregularities with the wavelengths ranging from 3 m to 15 m. At an altitude of about 250 km the instability‐induced diffusion dominates over the ion‐neutral collisions for wave electric field amplitude levels of about 5% of the ambient electric field value. At higher altitudes the diffusion effects are expected to become even more important.
Global excitation of wave phenomena in a dissipative multiconstituent medium: 1. Transfer function of the Earth's thermosphere
Authors: Mayr H. G. , Harris I. , Varosi F. , Herrero F. A.
Published: February 2008

A spectral model is presented to describe the acoustic‐gravity wave response in a multiconstituent thermosphere on a spherical earth with realistic vertical temperature distribution. The waves are treated as perturbations in a globally uniform atmosphere without background winds. For periods much less than one day, the effects of planetary rotation (Coriolis force) are relatively small, and Legendre polynomials (zonal spherical harmonics) are then the appropriate eigenfunctions to describe the horizontal variations. Through numerical integration in height, the transfer function is constructed for some vertical source distribution in the thermosphere. In the horizontal wave number space (order of polynomials), the transfer‐function reveals four components: (1) The trapped component which is confined to the source region. (2) The quasi‐horizontally propagating wave which is represented by the lower cut‐off and the first resonance maximum in the transfer function. In the thermosphere it is also the dominant maximum. The horizontal wave length and propagation velocity (about 700 m/s) are large. (3) The obliquely propagating waves, generated primarily through partial reflection from the base of the thermosphere or total reflection from the earth's surface. These waves appear as broad secondary maxima in the transfer function; their horizontal wave lengths and propagation velocities are relatively small; they are important near the source but cannot propagate very far horizontally. (4) The ducted waves which are produced in the lower atmosphere by total reflection from the earth's surface and partial reflection from the mesopause temperature minimum. Leaking back into the thermosphere where they originate, these waves have relatively short wavelengths but can travel large horizontal distances away from the source (pole to equator). The important first and second harmonic modes have propagation velocities of 250 and 170 m/s respectively and appear in the transfer function as narrow resonance maxima. Such waves may explain the medium scale travelling ionospheric disturbances at low latitudes which are associated with magnetic activity.
Global excitation of wave phenomena in a dissipative multiconstituent medium: 2. Impulsive perturbations in the Earth's thermosphere
Authors: Mayr H. G. , Harris I. , Varosi F. , Herrero F. A.
Published: February 2008

The transfer function approach in terms of vector spherical harmonics is used to describe the thermospheric response to time dependent sources on the globe. The computations are carried out for frequencies from 0 to 140 cycles per day (c/d), corresponding to periods of almost 10 min; the horizontal wavelength resolution is about 200 km. In the transfer function, we can distinguish basically two kinds of waves, trapped and propagating ones. The trapped waves have the character of a low pass filter, i.e., the large scale and long term variations are preferentially excited. The propagating waves comprise the fundamental, quasi‐horizontally propagating wave and the reflected and ducted wave modes. These waves become important at frequencies above 3 c/d. Using the Fourier integral representation, impulsive perturbations are simulated for source distributions on the globe. The following features characterize the thermospheric response: (1) The dynamic processes are most important during the energization phase. Winds then redistribute energy and mass globally. (2) The temperature and density perturbations remain for some time after the source has been turned off, heat conduction and diffusion accounting primarily for the slow decay. With decreasing source altitude the relaxation time‐constant increases. (3) Propagating gravity waves are launched mainly when the source is abruptly turned on or off. The source frequency spectrum is then very broad and the waves are generated by ringing. Near the source, their pulse durations are typically on the order of 20 min and are determined, through the dispersion relation for gravity waves, by the horizontal dimension of the source (about 700 km). As the waves propagate away from the source, the longer periods (and wave lengths) become more important, consistent with classical gravity wave theory. (4) After the source is turned off, the atmospheric circulation reverses, exhibiting a global scale oscillation which is determined by the duration of the disturbance. (5) Outside the source region, the onset of the disturbance is delayed corresponding to horizontal signal velocities of about 800 m/s, near the local speed of sound. Due to reflection from the base of the thermosphere, a second signal is launched which propagates with a velocity of about 380 m/s. This wave, however, is rapidly attenuated. A weaker third signal is launched which propagates with a velocity of only 230 m/s and therefore dominates the response near the source region (high latitudes) some time after the onset of the disturbance. This is the ducted wave from the lower atmosphere, which is weakly attenuated and can travel large distances away from the source. (6) The time delays between the maxima in the temperature and densities are controlled by the time constants for heat conduction and diffusion (which can be relatively long), but are limited by the duration of the disturbance.
Numerical study of the upstream wave excitation mechanism: 1. Nonlinear phase bunching of beam ions
Authors: Hoshino Masahiro , Terasawa Toshio
Published: February 2008

By means of particle simulation, we study the excitation mechanism of low‐freqeuncy (0.01–0.05Hz) upstream hydromagnetic waves. Initially, we observe excitation of the right‐hand polarized waves propagating parallel to the field‐aligned ion beam, which is given as the free energy source. In the nonlinear stage we observed the phase space bunching of beam ions by the excited waves. We apply this bunching effect for the explanation of “gyrophased bunched” ions observed in the foreshock region.
Ion and electron heating at collisionless shocks near the critical Mach number
Authors: Thomsen M. F. , Gosling J. T. , Bame S. J. , Mellott M. M.
Published: February 2008

The evolution of the ion and electron distribution functions across a set of 10 low‐Mach number, nominally subcritical, quasi‐perpendicular shocks is examined in high time resolution (full distribution every 3 s) using data from the ISEE 1 and 2 spacecraft. Both ions and electrons sometimes show slight preheating upstream of the shock, but otherwise the ion and electron temperatures rise together in the magnetic ramp and show no further increase downstream. Contrary to the usual assumption based on early laboratory and theoretical work that at subcritical shocks the bulk of the energy dissipation occurs as resistive heating of the electrons, it is found that the ion temperature increase exceeds that of the electrons. This difference is attributed to the distinction between dispersive shocks, such as those studied here, and resistive shocks, such as those observed in most laboratory studies. The increase in ion temperature is predominantly in the perpendicular direction and is due to heating of the entire distribution rather than to the formation of a high‐energy tail. The perpendicular temperature increase is typically a factor of 10–20, much greater than the usual assumption of adiabatic heating. The downstream to upstream ratio of perpendicular electron temperature is equal to the magnetic field ratio (∼2–2.5). The electrons also show significant heating in the parallel direction, with the downstream T∥/T⊥ ∼1–1.2. The downstream electron distribution exhibits the characteristic flattop seen downstream of supercritical shocks, and there is evidence for the field‐aligned electron beam identified previously within those shocks. As previously reported, the downstream ion and electron total temperatures are nearly equal. These observations are interpreted as evidence for the simultaneous operation of several plasma instabilities, including the modified two‐stream instability, driven by the cross‐field current within the shock, and the ion acoustic instability, driven by the field‐aligned electron beam.
Magnetospheric convection observed between 0600 and 2100 LT: Variations with Kp
Authors: Baumjohann W. , Haerendel G. , Melzner F.
Published: February 2008

Using data from the GEOS 2 electron gun experiment, we have constructed average patterns of the convective flow of plasma in the equatorial plane at L = 6.6 between 0600 and 2100 LT and for Kp ranging between 0 and 4. During very quiet conditions (Kp = 0–1) the plasma flows predominantly westward with an outward component before noon and an inward component thereafter. The most likely dominant agent for this kind of convection is the atmospheric dynamo electric field. For moderate activity (Kp = 3–4) the plasma flows as predicted by a Volland‐Stern‐type dawn‐to‐dusk cross‐tail electric field model (if the atmospheric dynamo effect is subtracted) with a shaping factor close to 3 and the axis of symmetry rotated toward earlier local times. The shielding of the cross‐tail electric field from the inner magnetosphere seems to be somewhat weaker on the dayside than on the eveningside. During times of Kp = 2 sunward convection and atmospheric dynamo effect seem to compete, i.e., sunward convection prevails, but our average pattern apparently is still substantially influenced by the atmospheric dynamo field.
Heating of heavy ions on auroral field lines in the presence of a large‐amplitude hydrogen cyclotron wave
Authors: Nishikawa K.‐I. , Okuda H. , Hasegawa Akira
Published: February 2008

Heating of heavy ions is studied analytically and by means of numerical simulations in the presence of a large‐amplitude hydrogen cyclotron wave which is commonly observed on auroral field lines. Both one‐ and two‐dimensional numerical simulations showed a decay instability resulting in strong heating of both the oxygen ions and the hydrogen ions. In particular, detailed results of simulations in two dimensions where E × B drift plays an important role indicate a much stronger heating of the oxygen ions than that in the one‐dimensional simulation. A high‐energy tail of the oxygen ions is commonly observed in the perpendicular distribution.
Collisionless tearing in a field‐reversed sheet pinch assuming nonparallel propagation
Authors: Quest Kevin B. , Coroniti Ferdinand V.
Published: February 2008

We examine the linear stability properties of the collisionless tearing in a reversed‐field sheet pinch assuming that k × B ≠ 0, where B is the equilibrium magnetic field. We show that currents are driven in the direction perpendicular to the sheet pinch, a consequence of the differing drift response of the protons and electrons. This current is closed by a component parallel to B, resulting in strong field‐aligned currents concentrated near the null line. Momentum balance is used to infer the functional shape of the current system at the null line, and it is demonstrated that the external current system cannot be carried by those particles with orbits crossing the null line. The relevance of this analysis to reconnection at the dayside magnetopause is discussed.
Dayside magnetopause reconnection
Authors: Heyn Martin F. , Biernat Helfried K. , Semenov Vladimir S. , Kubyshkin Igor V.
Published: February 2008

The Petschek model of magnetic field line reconnection is generalized for magnetopause‐like conditions to enable a comparison of data and theory. The two‐dimensional, compressible MHD model considered reveals four discontinuities which are necessary to match the different magnetic fields and plasma parameters inside and outside the magnetopause. The magnetosheath plasma is accelerated and deflected by a large‐amplitude Alfvén (rotational) wave and either a slow shock or a slow rarefaction wave (slow expansion fan). Another slow shock focuses the plasma flow from the magnetospheric side. A contact discontinuity inside the boundary layer separates the plasma flows from magnetosheath and magnetosphere. Using realistic magnetopause parameters, the results are illustrated by numerical examples and discussed in connection with geophysical observations.
Generation of Z mode radiation by diffuse auroral electron precipitation
Authors: Dusenbery P. B. , Lyons L. R.
Published: February 2008

It has been previously demonstrated that an upgoing loss cone in the auroral electron population can be unstable to Z mode radiation. In this paper we propose that the specific electrons responsible for high‐altitude (∼4 RE, geocentric) Z mode radiation are diffuse auroral electrons which form an upgoing loss cone free energy region. These waves have frequencies ω greater than the local electron cyclotron frequency Ωe and, based on previous growth rate calculations, have maximum growth rates γ ∼ 10−3 Ωe. We employ a model of diffuse auroral electrons with a number density of 0.1 cm−3 and thermal energy of 1 keV. Assuming that the upgoing free energy region in velocity space is limited to electrons with energies of < 10 keV, we show that the fastest growing Z mode waves have Ωe < ω < 1.02Ωe and are generated in regions where 0.2 < ωpe/Ωe < 1.4. In addition, we estimate the amplification length of the fastest growing Z mode waves to be ∼100 km, which is sufficient for Z mode radiation to grow to the intensities observed by satellites at 3–5 RE geocentric altitude in the auroral regions.