bluuebunny/crossref_metadata_2025 huggingface dataset:
Differential scattering cross sections for collisions of 0.5−, 1.5−, and 5.0‐keV helium atoms with He, H2, N2, and O2
Authors: Newman J. H. , Smith K. A. , Stebbings R. F. , Chen Y. S.
Published: February 2008

This paper reports the first results of an experimental program established to provide cross section data for use in modeling various atmospheric processes. Absolute cross sections, differential in the scattering angle, have been measured for collisions of 0.5−, 1.5−, and 5.0‐keV helium atoms with He, H2, N2, and O2 at laboratory scattering angles between 0.1° and 5°. The results are the sums of cross sections for elastic and inelastic scattering of helium atoms; charged collision products are not detected. Integration of the differential cross section data yields integral cross sections consistent with measurements by other workers. The apparatus employs a position‐sensitive detector for both primary and scattered particles and uses a short target cell with a large exit aperture to ensure a simple and well‐defined apparatus geometry.
A new type of VLF emission triggered at low altitude in the subauroral region by Siple Station VLF transmitter signals
Authors: Bell T. F. , Katsufrakis J. P. , James H. G.
Published: February 2008

VLF wave data from the ISIS 2 satellite has revealed the existence of a new phenomenon in which coherent VLF signals from the Siple Station, Antarctica, VLF transmitter are observed to trigger a new type of VLF emission as these signals propagate upward to the satellite at 1400 km altitude through the ionosphere and low‐altitude magnetosphere. The emissions have the form of band‐limited impulses of approximately 20–30 ms duration. The bandwidth of the emissions is as much as 1 kHz and their amplitude is as much as 20 dB above that of the triggering signal. The emissions are thought to be the result of a rapidly evolving quasi electrostatic plasma instability triggered by the transmitter signals. The effect occurs generally when the transmitter signals lie just below a band of impulsive VLF hiss which is commonly observed to occur in the subauroral region poleward of the plasmapause. The impulsive VLF hiss band has a lower cutoff frequency in the range 3–4 kHz and appears to be the subauroral extension of the well‐known auroral VLF impulsive hiss Band. This phenomenon is possibly the transient analog of the recently reported spectral broadening effect [Bell et al., 1983b] but the connection is not clear. In the present paper we show examples of this new phenomenon and delineate the conditions under which it occurs. The role of precipitating energetic particles in producing the phenomenon is discussed.
Preliminary positive impulse of geomagnetic sudden commencement observed at dayside middle and low latitudes
Authors: Kikuchi Takashi , Araki Tohru
Published: February 2008

It is found that an impulsive geomagnetic increase, named the preliminary positive impulse (PPI), often precedes the main impulse (MI) of geomagnetic sudden commencement at the dayside middle latitude (Memambetsu). The duration time of the PPI is about 1 min, and the average magnitude is 16 nT. The PPI also appears near the geomagnetic equator (Guam), although the occurrence is less frequent. No equatorial enhancements are present in the PPI, while the MI is enhanced appreciably at Guam. Most of PPI's accompany an electric field with a westward component in the ionospheric F region, as observed with the HF Doppler technique. These observational results lead to a conclusion that the PPI is transmitted from above the ionosphere by a compressional hydromagnetic wave, rather than produced by ionospheric electric currents generated in the polar ionosphere.
Theory of the current‐driven ion cyclotron instability in the bottomside ionosphere
Authors: Satyanarayana P. , Chaturvedi P. K. , Keskinen M. J. , Huba J. D. , Ossakow S. L.
Published: February 2008

A theory of the current‐driven electrostatic ion cyclotron (EIC) instability in the collisional bottomside ionosphere is presented. It is found that electron collisions are destabilizing and are crucial for the excitation of the EIC instability in the collisional bottomside ionosphere. Furthermore, the growth rates of the ion cyclotron instability in the bottomside ionosphere maximize for k⊥ρ i ≥ 1, where 2π/k⊥ is the mode scale size perpendicular to the magnetic field and ρi the ion gyroradius. Realistic plasma density and temperature profiles typical of the high‐latitude ionosphere are used to compute the altitude dependence of the linear growth rate of the maximally growing modes and critical drift velocity of the EIC instability. The maximally growing modes correspond to observed tens of meter size irregularities, and the threshold drift velocity required for the excitation of EIC instability is lower for heavier ions (NO+, O+) than that for the lighter ions (H+). Dupree's resonance‐broadening theory is used to estimate nonlinear saturated amplitudes for the ion cyclotron instability in the high‐latitude ionosphere. Comparison with experimental observations is also made. It is conjectured that the EIC instability in the bottomside ionosphere could be a source of transversely accelerated heavier ions and energetic heavy‐ion conic distributions at higher altitudes.
Equatorial F region zonal plasma drifts
Authors: Fejer B. G. , Kudeki E. , Farley D. T.
Published: February 2008

We have examined in detail the F region plasma drifts measured at Jicamarca, Peru, during 1978–1981, a period of high solar activity, and compared these drifts with Jicamarca data taken during periods of lower activity, as well as with other equatorial zonal neutral wind and plasma drift measurements. The increase in solar activity causes larger nighttime eastward plasma drifts at Jicamarca and delays the morning reversal time from eastward nighttime to westward daytime drifts. The radar data seem to be in good agreement with nighttime neutral wind measurements made by the DE‐2 satellite, but are systematically smaller than spaced receiver drifts measured with both the scintillation and the VHF polarimeter techniques. For integration times of 20–30 min (but perhaps not for shorter integrations), the Jicamarca zonal plasma drifts, measured with both the incoherent scatter and the radar interferometer technique, change little with altitude near the F region electron density peak and above. However, at nighttime, below the F peak, there is a clear shear in the zonal plasma drifts, with decreasing eastward drifts below the F peak reversing to westward drifts at lower altitudes. The nighttime profiles of the equatorial plasma drifts are in good agreement with the results from recent numerical models of the equatorial ionosphere.
F2 region ion densities from analysis of O+ 834‐Å airglow: A parametric study and comparisons with satellite data
Authors: McCoy R. P. , Anderson D. E. , Chakrabarti S.
Published: February 2008

A multiple‐scattering radiative transfer model is used to infer F2 region ion densities from measurements of the O+ 834‐Å dayglow obtained from a spinning satellite. A parametric study is performed to demonstrate the sensitivity of the 834‐Å intensity profiles to variations in the solar ionizing flux, neutral atmospheric composition, and ionospheric density distribution. Results show that the solar flux and neutral densities govern the magnitude of the O+ 834‐Å intensity, but the shape of the intensity profiles as a function of look angle depends on the distribution of ions. Selected comparisons of the model results with the STP 78‐1 EUV airglow data show that the height of the F2 peak, the peak ion density, and scale height of the ionosphere can be determined from the satellite data. Near the magnetic dip equator the data show increased intensity in the zenith look direction and increased limb brightening due to the effects of the upward plasma drifts associated with the equatorial anomaly. The parametric study shows that the behavior of the optical data is consistent with an upward movement of the height of the F2 peak in the anomaly.
TID observations at spaced geographic locations
Authors: Tedd B. L. , Morgan M. G.
Published: February 2008

Observations of traveling ionospheric disturbances (TID's) have been performed with a three‐station network of rapid‐run ionosondes located in northern New Hampshire and Vermont, together with an additional ionosonde in Bermuda 1600 km to the southeast in the predominant “downstream” direction of travel. The height‐dependent horizontal and vertical TID phase trace speeds in the time domain have been obtained for individual spectral components at the three‐station network, and the height‐dependent spectral composition and vertical phase trace speeds have been obtained at the Bermuda location. The broad features of the wave response at the spaced locations, that is, at the network and in Bermuda, may be interpreted in terms of isothermal dissipationless gravity wave theory. However, the finer details of the response, such as the increase in horizontal and vertical phase trace speeds with height, can only be interpreted using more realistic theories of gravity‐wave propagation. The cause‐effect relationship between the energetics and dynamics of the source region and the wave response is one of the outstanding problems remaining in TID‐related studies. Since source information obtained from our TID observations is of uncertain accuracy, we cannot purport to address this problem directly; nevertheless, our results will be of significant value in the planning of future experiments.
Dependence of auroral middle UV emissions on the incident electron spectrum and neutral atmosphere
Authors: Daniell R. E. , Strickland D. J.
Published: February 2008

In this paper we examine the relationship between three important middle UV auroral emissions, the electron energy spectrum, and the neutral atmosphere. We display calculated emission rates for the Vegard‐Kaplan and second positive band systems of N2 and the O I (¹S → ³P) 2972 Å line. All three features are proportional to the incoming energy flux, but they have different dependences on the characteristic energy of the incident electron spectrum and on the concentration of atomic oxygen. Therefore, in principle, observations of the intensities of these three emission features could be used to characterize the incident electron spectrum. Unfortunately, uncertainties in the chemistry of the ¹S state of atomic oxygen and the A³Σu+ state of nitrogen make it difficult to use such observations quantitatively. We discuss these uncertainties and possible alternative far ultraviolet features such as O I 1356 Å and 1304 Å and the N2 Lyman‐Birge‐Hopfield bands.
Plasma observations on AMPTE/IRM during the lithium releases in the solar wind
Authors: Paschmann G. , Carlson C. W. , Baumjohann W. , Loidl H. , Curtis D. W. , Sckopke N. , Haerendel G.
Published: February 2008

This paper investigates the lithium releases in the solar wind as they appear in the measurements of ions and electrons above 20 eV/q and 15 eV, respectively, made with the Max‐Planck‐Institut für extraterrestrische Physik/University of California, Berkeley, fast three‐dimensional plasma instrument on AMPTE/IRM. The dominant effects are the pick up of the newly created Li ions by the solar wind electric field and the heating of electrons. When first observed, the beam of Li ions is substantially broadened, probably as a result of its interaction with the cross‐streaming solar wind. Later on, when the Li density has decreased, the Li beam becomes narrower than can be resolved by the instrument. The observed increase of beam energies with time to more than ∼1 keV can be directly attributed to the expansion of the lithium atoms with a maximum speed of ∼3.8 km/s. Throughout the observations the Li ions move nearly along the electric field direction. This is a result of their large gyroradius. Shortly after the release, while the spacecraft is still within the magnetic cavity, hot (E > 70 eV) electrons are observed. Their density and temperature is further enhanced when entering the magnetic field compression region ahead of the cavity, consistent with adiabatic heating. There is no evidence in our data of a shock in the upstream region, nor of reflected solar wind ions.
Resonance absorption of Alfven oscillations in a nonaxisymmetric magnetosphere
Authors: Inhester Bernd
Published: February 2008

For a certain class of magnetic field line geometries and for an arbitrary distribution of the plasma density used as a model for the earth's magnetosphere, we study the conditions under which resonantly enhanced Alfvén oscillations may occur. By calculating the field‐aligned integral of the energy density flux, it is demonstrated that these oscillations are associated with localized energy absorption and are driven by gradients of the field‐aligned component of the magnetic wave field. The results are generalized to account for a finite conductivity of the ionosphere. Finally, it is shown that a lack of symmetry of the magnetosphere may cause the polarization of the resonant oscillation to conflict with the requirement for the wave field to be almost incompressible and thereby prevent the resonance absorption. We discuss probable consequences on the occurrence probability of ULF pulsations with magnetic local time.
Mean neutral circulation in the winter polar F region
Authors: Killeen T. L. , Roble R. G. , Smith R. W. , Spencer N. W. , Meriwether J. W. , Rees D. , Hernandez G. , Hays P. B. , Cogger L. L. , Sipler D. P. , Biondi M. A. , Tepley C. A.
Published: February 2008

Direct measurements of the F region neutral wind field in the northern (winter) hemisphere during late November and December 1981 were obtained by using the Dynamics Explorer (DE 2) satellite and the network of ground‐based Fabry‐Perot interferometers sited in the North American and Scandinavian sectors. All available data for the study period have been collated and averaged according to universal time (UT) to provide a global‐scale experimental determination of the mean or average circulation in the northern hemisphere F region. The data are organized into UT “bins” and are presented as a sequence of 12 “climate maps” depicting the neutral wind system at 2‐hour UT intervals during a “mean” day in December 1981. The data set provides the most comprehensive experimental measure of the global‐scale high‐latitude thermospheric circulation yet reported. The averaged measurements from the different instruments exhibit a satisfactory degree of internal consistency when viewed in terms of the global‐scale neutral flow. A detailed comparison with the predictions of the NCAR thermospheric general circulation model is discussed. The model predictions are in good general agreement with the UT‐dependent mean circulation, reproducing the basic control of the high‐latitude neutral wind system by ion drag forces modulated in UT as a result of the offset between the geomagnetic and geographic poles. Differences in flow directions and velocities between model predictions and experimental results are ascribed to natural variability in the thermosphere as well as to the use in the model of a less than fully realistic, symmetric ion convection geometry and density distribution. The two basic conclusions of this study are that (1) instrumentation deployed around the globe and on satellites can provide powerful, composite data sets that can be used to monitor global‐scale mean thermospheric “climatology” and that (2) the mean thermospheric (F region) neutral circulation for the given geophysical situation (solar maximum, near solstice) can be modeled reasonably well using a three‐dimensional, time‐dependent model with appropriate parameterizations for the energy and momentum inputs.
On a difficulty with accelerating particles at slow‐mode shocks
Authors: Isenberg Philip A.
Published: February 2008

We point out that, although slow‐mode shocks produce upstream particles and waves in a manner similar to fast‐mode shocks, they are not capable of accelerating these particles as fast‐mode shocks are known to do. The crucial difference is that the self‐generated fast waves upstream of a slow shock are able to escape from the shock and therefore do not compress the energetic particle distribution.
Radial variation of interplanetary Alfvénic fluctuations: Pioneer 10 and 11 observations between 1 and 5 AU
Authors: Bavassano Bruno , Smith Edward J.
Published: February 2008

The radial evolution of the power spectra of Alfvénic fluctuations observed within the trailing edge of high speed streams in the solar wind has been investigated using magnetic field measurements by Pioneer 10 and Pioneer 11 at heliocentric distances between 1 and 5 AU. In the analyzed frequency range (2.8×10−4 to 8.3×10−3 Hz) the spectral index of the computed spectra does not depend on frequency and is close to the value predicted for hydrodynamic turbulence. The radial gradient of the power density is independent of frequency and only slightly steeper than that expected for undamped propagation of Alfvénic fluctuations in a rarefaction region. The relation between the present results and those previously obtained by Helios 1 and 2 inside 1 AU is discussed.
Particle acceleration at a termination shock: 1. Application to the solar wind and the anomalous component
Authors: Jokipii J. R.
Published: February 2008

The results of a numerical study of the diffusive acceleration of charged particles at the termination shock of the solar wind are reported. In the model a realistic magnetic field structure is employed which is similar to that observed in the solar wind. In addition to causing spatial variation of the diffusion tensor, a major effect of the magnetic field is to cause guiding‐center drifts of the accelerated particles, both in the solar wind and at the shock. It is demonstrated that the inclusion of the drifts has a large effect on the acceleration. It is concluded, furthermore, that acceleration at the termination shock, in conjunction with drifts, can explain several observed features of the anomalous component.
Evidence of transmission of polar electric fields to the low latitude at times of geomagnetic sudden commencements
Authors: Kikuchi Takashi
Published: February 2008

The HF Doppler frequency deviation associated with the geomagnetic sudden commencement (sc), called SCF, consists of the negative preliminary frequency deviation (PFD) and the succeeding positive main frequency deviation (MFD) when observed during the night at geomagnetic low latitude (geomagnetic latitude = 25°). This type of HF Doppler frequency deviation, SCF(∓), corresponds to the sc(+) at low latitude (Kakioka, Japan) and the sc(∓) (=sc*) at high latitude (Husafell, Iceland). The SCF(∓) starts earlier than the low latitude sc by 50 s, while it occurs simultaneously with the high latitude sc* within a time accuracy of 10 s. This result implies that the negative PFD of the nighttime SCF(∓) is caused by a dusk‐to‐dawn polar electric field responsible for the preliminary reverse impulse (PRI) of sc*, the occurrence of which is earlier than the occurrence of the world‐wide geomagnetic increase. Consequently, the magnetospheric electric field generated at the onset of sc is transmitted to the low‐latitude ionosphere by way of the polar ionosphere, earlier than the direct transmission through the magnetosphere by compressional hydromagnetic waves. It is also shown that the daytime SCF(∓) starts simultaneously with the low‐latitude sc(+), suggesting that the positive PFD of the SCF(±) is caused by the westward electric field transmitted directly from the magnetosphere by the compressional hydromagnetic wave.
Resonant generation of ion waves on auroral field lines by positive slopes in ion velocity space
Authors: André M. , Temerin M. , Gorney D.
Published: February 2008

Simultaneous observations of wave spectra and particle velocity distributions obtained by the S3‐3 satellite on auroral field lines are studied. Emissions below the lower hybrid frequency occur just below multiples of the proton gyrofrequency. At higher frequencies the observed spectra show peaks just above such multiples. The ion and electron distributions are modeled, and numerical methods are used to calculate the corresponding spatial wave growth rates. This theoretically obtained growth agrees rather well with the observed emissions. The major source of free energy is a positive slope perpendicular to the ambient magnetic field in the ion velocity distribution.
Collisional tearing instability in the current sheet with a low magnetic Lundquist number
Authors: Lee L. C. , Fu Z. F.
Published: February 2008

The growth rate of the collisional tearing instability is calculated for a current sheet with a low magnetic Lundquist number S. The maximum growth rate is found to be γm ≃ 0.075tA−1 tanh (8/S0.5) for 3 ≦ S ≦ 105, where tA is the Alfvén transit time in the current sheet. For S > 300, the growth rate can be written approximately as γm ≃ 0.6tA−1 S−0.5; for S < 50, the growth rate approaches an asymptotic value of ∼0.075tA−1. The present result is applicable to the current sheet associated with the diffusion region of a magnetic reconnection configuration, in which the Lundquist number, based on the thickness of the diffusion region, is typically 2 ≦ S ≦ 100.
Characteristics of large Forbush‐type decreases in the cosmic radiation: 2. Observations at different heliocentric radial distances
Authors: Webber W. R. , Lockwood J. A. , Jokipii J. R.
Published: February 2008

Cosmic ray data from IMP 8, Voyager 1 and 2, Pioneer 10 are used to investigate the heliocentric radial dependence of the characteristics of about 20 Forbush‐type transient decreases which occurred from 1978 to 1984. These characteristics include (1) the recovery time, (2) the amplitude, and (3) the time to decrease to minimum. It is found that the average recovery time is about 5 times longer at R = 30 AU than at 1 AU. The magnitudes of the transient decreases are observed to decrease about 1.5%/AU on average so that the magnitude of the decrease is half as great at R ∼ 30 AU as at 1 AU. The time for the cosmic ray intensity to decrease to the minimum in the transient decrease is found to be greater at larger distances and is about 5 times longer at R = 30 AU than at 1 AU. The behavior of these effects as a function of radius is obviously related to the evolution of the disturbances causing the transient decreases as they propagate outward. A model of the Forbush‐type decrease is proposed to explain the observed radial dependence of the recovery time and time to minimum of the decrease. The implications of these results for understanding the relationship between Forbush‐type decreases and the 11‐year variation are discussed.
Time scales of expansion and contraction of the auroral oval
Authors: Nakai H. , Kamide Y. , Hardy D. A. , Gussenhoven M. S.
Published: February 2008

The auroral boundary index calculated by Gussenhoven et al. [1983] on the basis of Defense Meteorological Satellite Program precipitating electron data is used to examine the dynamics of the auroral oval in the midnight sector. The equatorward boundary of the oval expands equatorward and contracts poleward corresponding to southward and northward turnings, respectively, of the interplanetary magnetic field (IMF). The time scales are found to be, on the average, 45 min for the expansion and 8 hours for the contraction. A general empirical formula to predict the oval location for any IMF variation is devised. Physical processes in the magnetosphere consistent with the “rapid” expansion and “slow” contraction of the auroral oval are discussed.
Three‐dimensional axisymmetric magnetosphere in pressure balance with the solar wind
Authors: Yates G. Kenneth , Heinemann M.
Published: February 2008

A computer model of a magnetosphere in which the magnetic dipole axis points into the solar wind has been developed. The axial symmetry implied by this physical situation admits a two‐dimensional treatment. The model features (1) a magnetopause in pressure balance with the solar wind, (2) an analytic asymptotic tail model to account for the distant magnetotail, and (3) a full accounting of the magnetohydrodynamic pressure in the magnetosphere. The plasma pressure is a function of the Euler potential, α (=rAϕ). The principal physical result is that with an increase in the plasma pressure from zero the magnetic field configuration changes slowly at first. As the pressure increases further, the field configuration changes more rapidly until X and O lines are formed.
A three‐dimensional MHD simulation of the interaction of the solar wind with the Earth's magnetosphere: The generation of field‐aligned currents
Authors: Ogino Tatsuki
Published: February 2008

A global computer simulation of the interaction of the solar wind with the earth's magnetosphere was executed by using a three‐dimensional magnetohydrodynamic model. As a result, we were able to reproduce quasi‐steady‐state magnetospheric configurations and a Birkeland field‐aligned current system which depend on the polarity of the z component of the interplanetary magnetic field (IMF). Twin convection cells and a dawn to dusk electric potential of 30–100 kV appeared at the equator in the magnetosphere. Four types of field‐aligned currents were observed. Region 1 and 2 field‐aligned currents generated for all IMF conditions were 0.6–1.0×106 A and 0.15–0.61×106 A, respectively, in the total current. Region 1 currents at high latitudes are generated from the field‐aligned vorticity at the flanks through a viscous interaction and are strengthened by a twisting of open magnetic field lines in the tail region for southward IMF. On the other hand, the low‐latitude region 2 currents probably are generated mainly from the inner pressure gradient of the plasma sheet. The region 1 current obtained from the simulation was in good agreement with an estimate from our theoretical analysis of the localized Alfvén mode. The other two types of field‐aligned currents are the dayside magnetopause currents in the dayside cusp region, which increase for northward IMF, and the dayside cusp currents for southward IMF. The cusp currents are associated with a twisting of open magnetic field lines in the magnetopause region.
Modulation instability of circularly polarized Alfvén waves
Authors: Longtin M. , Sonnerup B. U. Ö.
Published: February 2008

The stability of a finite amplitude circularly polarized Alfvén wave of wave number k0 is studied by using the two‐fluid isentropic equations. Linear perturbation analysis, involving two sideband transverse waves having wave numbers k0 ± k and a longitudinal wave with wave number k, is used to find the exact sixth‐order dispersion relation. The analysis is then limited to the case where k ≪ k0. The resulting fourth‐order dispersion relation is examined analytically and numerically, and a surface is found that separates stable and unstable regions in parameter space. This surface describes the boundary between stable and unstable regions not only for k ≪ k0 but for the entire branch of the dispersion relation which extends to k = 0. We refer to this branch as the modulation branch and the corresponding instability as a modulation instability. A sufficient condition for modulation stability is found to be υϕ0 < cs for left‐hand polarized waves and υϕ0 > cs for right‐hand polarized waves, where υϕ0 and cs are the phase velocity of the unperturbed wave and the unperturbed sound speed, respectively. Modulation wave amplitudes and growth rates are given.
A statistical study of plasma sheet dynamics using Isee 1 and 2 energetic particle flux data
Authors: Dandouras J. , Rème H. , Saint‐Marc A. , Sauvaud J. A. , Parks G. K. , Anderson K. A. , Lin R. P.
Published: February 2008

During magnetospheric substorms, satellites embedded in the plasma sheet often detect transient dropouts of plasma and energetic particle fluxes, a phenomenon generally interpreted as indicating the exit of the satellite into the magnetospheric lobe due to a plasma sheet thinning. In order to determine the large‐scale dynamics of the near‐earth plasma sheet during substorms, three satellite years of ISEE 1 and 2 energetic particle flux data (1.5 and 6 keV), corresponding to 461 particle flux dropouts, have been analyzed. The principal results show that flux dropouts can be observed anywhere in the nightside plasma sheet, independent of the satellite's, geocentric distance (for R > 12 RE), magnetic local time (except near the magnetospheric flanks) and estimated distance to the neutral sheet. Furthermore, flux dropouts can be observed for any combination of the AE index value and the satellite's distance to the neutral sheet, which shows that the plasma sheet is dynamic even during weak magnetospheric disturbances. Substorms during which the satellites, though situated in the plasma sheet, did not detect any flux dropout, have also been examined, and it is found that the plasma sheet thickness can locally remain unaffected by substorm development for AE index values up to at least 1000 nT. The predictions of the two major plasma sheet thinning models, i.e., the near‐tail X‐type magnetic neutral line formation model and the MHD rarefaction wave propagation model, are compared to the experimental results, and it is concluded that neither model can account for all of the observations; plasma sheet dynamics are more complex. Phenomenologically, this study suggests that multiple pinching of the plasma sheet and/or large‐amplitude three‐dimensional plasma sheet oscillations are important in plasma sheet dynamics.
Wave and plasma observations during a compressional Pc 5 wave event August 10, 1982
Authors: Engebretson M. J. , Cahill L. J. , Waite J. H. , Gallagher D. L. , Chandler M. O. , Sugiura M. , Weimer D. R.
Published: February 2008

Magnetometer and thermal plasma instruments on the polar‐orbiting Dynamics Explorer 1 satellite observed a small‐amplitude ultralow frequency (ULF) pulsation event at the outer edge of the plasmapause near the geomagnetic equator in the midafternoon sector on August 10, 1982, during the recovery phase of a magnetic storm. Transverse pulsations of 30–50 s period were observed throughout the event, and a 270‐s period, purely compressional Pc 5 pulsation with several shifts in phase occurred within ±5° of the geomagnetic equator. Electric fields and the motion of thermal ions appeared to be in quadrature with pulsations in magnetic field magnitude throughout the event. This suggests that the net Poynting flux for the compressional waves was zero, consistent with their being standing waves. Large fluxes of trapped 90° pitch angle 10‐eV protons, also symmetric about the geomagnetic equator, were observed in conjunction with the waves. These may serve as a source of free energy for the pulsations. Our observations lend support to recent studies suggesting that many dayside compressional wave events are related to localized field line resonance near plasmapauselike boundaries, but also include features that cannot be explained by existing theories.
How are the VLF quasi‐periodic emissions controlled by harmonics of field line oscillations? The results of a comparison between ground and Geos satellites measurements
Authors: Tixier Michel , Cornilleau‐Wehrlin Nicole
Published: February 2008

The characteristics of quasi‐periodic (QP) VLF emissions observed near the equatorial plane by the GEOS satellites are first compared with simultaneous ground data obtained at L ≃ 6. The frequency structure of QP elements is shown to be the same in the magnetosphere and at ground, whereas their amplitude measurement gives evidence of a ≃20‐dB attenuation between GEOS and the ground. Different events are described as a function of their more or less complex relationships with concomitant Pc 3–4 magnetic pulsations. The spectral analysis of the simultaneous QP envelopes and ULF waves permits us to deduce that QP emissions are due to the modulation of a VLF hiss by a compressional component of ULF waves. This component is a harmonic of an ULF standing wave, showing that the classification between type I and type II is not so obvious in space. When QP envelope and ULF periodicities are different, they are shown to correspond to different harmonics of a same fundamental oscillation of the field line. This set of new results is discussed in the light of the quasi‐linear theory of wave‐particle interactions. The modulation rate of nondispersive QP type can be roughly well explained, and some ideas are given about a relaxation mechanism which is able to interpret the frequency modulation of the QP type rising tone.
ELF generation in the lower ionosphere via collisional parametric decay
Authors: Ko K. , Menyuk C. R. , Reiman A. , Tripathi V. , Palmadesso P. , Papadopoulos K.
Published: February 2008

Generation of ELF waves by stimulated parametric coupling of two HF waves in the lower ionosphere is considered. In this region the nonlinear force is dominated by the thermal rather than the ponderomotive nonlinearity. It is shown that this results in lowering the pump threshold for complete decay by more than an order of magnitude while achieving efficiencies in excess of those expected on the basis of the Manley‐Rowe relations. The lower‐frequency mode excited for E region altitudes is the helicon mode, which continues to frequencies below the ion cyclotron frequency because ion‐neutral collisions freeze the ion motion. The application of these results to ELF generation in the lower ionosphere, including power estimates for a proof of principle experiment, is discussed.
On the spatial relationship between auroral emissions and magnetic signatures of plasma convection in the midday polar cusp and cap ionospheres during negative and positive IMF Bz: A case study
Authors: Sandholt P. E. , Egeland A. , Lybekk B.
Published: February 2008

The dynamics of midday auroras, including polar cusp and cap emissions, and their relation to the interplanetary magnetic field (IMF) have been investigated with optical ground‐based observations from Svalbard (75° geomagnetic latitude), Norway, and IMF data from spacecraft ISEE 2. One case is presented, showing the spatial relationship—along the magnetic meridian in the midday sector—between the cusp aurora and IMF By‐related convection currents (the DPY signature) for negative and positive values of IMF Bz.
Modeling of ion acceleration through drift and diffusion at interplanetary shocks
Authors: Decker R. B. , Vlahos L.
Published: February 2008

We describe a test particle simulation designed to study energetic charged particle acceleration at oblique fast‐mode shocks when magnetic fluctuations exist upstream and downstream of the shock. The technique consists of integrating along exact particle orbits in a system where the angle θ1 between the shock normal and mean upstream magnetic field, the level of magnetic fluctuations, and the energy of injected particles can assume a range of values. This allows us to study time‐dependent shock acceleration under conditions not amenable to analytical techniques. To illustrate the capability of the numerical model, we consider proton acceleration under conditions appropriate for interplanetary shocks near 1 AU, including large‐amplitude transverse magnetic fluctuations derived from power spectra of both ambient and shock‐associated MHD waves. With all other parameters held fixed, protons injected at θ1 = 0° and θ1 = 60° shocks are accelerated from 10 keV to ∼100 keV and ∼ 1 MeV, respectively, within 300 gyroperiods by a combination of the shock drift and first‐order Fermi processes. The energy spectrum downstream of the 60° shock can be fit with two power laws, with spectral exponent γ = 1.7 from 10 keV to ∼80 keV and γ = 2.6 from ∼ 80 keV to 800 keV. Results are also presented for the situation where the variance and upstream extent of the shock‐associated waves at the 60° shock are reduced relative to the values used at the 0° shock.
Quantitative modeling of the “convection surge” mechanism of ion acceleration
Authors: Mauk B. H.
Published: February 2008

To explain the presence of the so‐called “bounce‐phase‐bunched” ion distributions (eV range to ∼20 keV) observed in the earth's geosynchronous magnetosphere, Quinn and Southwood (1982) examined the general properties of what they called the “convection surge” mechanism. This mechanism is associated with a sudden earthward displacement of curved field lines resulting from the short‐lived application of an intense, east‐west electric field. To explore its properties a quantitative computer model of the mechanism has been constructed that incorporates the dipolarization of the field line shapes, and recent transient electric field measurements. It is confirmed that the mechanism easily generates the bounce‐phase‐bunched ion distributions in question and that for some particles it can give rise to dramatic ion energization in the field parallel direction associated with the violation of the second adiabatic invariant. It is shown additionally that depending on initial conditions the mechanism can generate dramatically field aligned distributions. It is hypothesized that the convection surge mechanism is fundamentally associated with the processes responsible for transporting tail populations to the middle (geosynchronous) regions of the earth's magnetosphere, and it is proposed that a key general characteristic of geosynchronous ion pitch angle distributions, field‐aligned low energies and field‐perpendicular high energies, is a signature of this energization mechanism. The presence of field aligned ions cannot be presumed to be a signature of recent ionospheric extraction.
The generation of low‐frequency electrostatic waves in association with auroral kilometric radiation
Authors: Winglee R. M. , Pritchett P. L.
Published: February 2008

Observations of the electron distribution in the source region of auroral kilometric radiation (AKR) indicate that it can have positive gradients with respect to both υ⊥ and υ∥. It is shown that this type of distribution is unstable to both (1) the electron cyclotron maser instability, which is responsible for AKR, and (2) the bump‐in‐tail instability which generates electron acoustic waves in the hiss band. Simulations indicate that the two instabilities do not develop independently but compete for the available free energy due to the reduction in the positive gradients of the distribution caused by the quasi‐linear diffusion associated with each instability. The dominant instability depends on the ratio of the density of the energetic electrons nE to that of the background electrons nb. For nE/nb ≳ 1 the maser instability dominates and the bump‐in‐tail instability is suppressed, whereas the reverse is true if nE/nb ≲ 1. Comparison with observations indicates that probably nE/nb ≳ 1 in the source region of AKR.
The influence of charge exchange on the velocity distribution of hydrogen in the Venus exosphere
Authors: Hodges R. Richard , Tinsley Brian A.
Published: February 2008

The simulation of the exosphere of Venus by a Monte Carlo technique has been extended to provide velocity distribution profiles that can be used with radiative transfer simulation to model the Lyman α emission seen by spacecraft. The line profiles show a narrow core due to the exobase thermal source of hydrogen superimposed on the broad profile of hot hydrogen from charge exchange with hot ions in the nighttime ionosphere. Nightside radial profiles show long tails of upward flowing, escaping atoms. There are some downward and lateral superescape signatures because the charge exchange source extends well above the exobase. At higher altitudes, flattening and even shallow central valleys appear in the transverse profiles. The planetary average escape rate due to charge exchange was found to be 2.8×107 cm−2 s−1, which is several times larger than escape rates for other candidate mechanisms.
Absolute experimental differential and integral electron excitation cross sections for atomic oxygen: 2. The (³P → ³S°) transition (1304 Å) from 16.5 to 200 eV with comparison to atomic hydrogen
Authors: Vaughan S. O. , Doering J. P.
Published: February 2008

Previously reported measurements of the absolute experimental electron excitation cross section for the O (³P → ³S°) transition (1304 Å) at 100 eV have been extended to a wide range of incident energies from 16.5 to 200 eV. Both the O I and H I (n = 2) cross sections were measured simultaneously, and the H I (n = 2) cross sections were compared with previous results as a test of experimental reliability. The O I differential cross sections vary smoothly with incident energy and are flatter with scattering angle at the lower incident energies, as expected. The absolute integral cross section is found to agree with the theoretical shape and magnitude at higher energies. Comparison is made between fluorescence cross sections of Zipf and Erdman and present direct electron excitation cross sections. At low energies the direct excitation cross section is roughly 50% of the fluorescence cross section, suggesting that the cascade contribution to 1304‐Å fluorescence from excitation of the 2³P and other states is about the same magnitude as direct excitation. Generalized oscillator plots of the 100‐, 150‐, and 200‐eV data demonstrate the applicability of the Born approximation. Present results for the H I (n = 2) integral cross sections agree with previous results within 15%.
Average plasma and magnetic field variations in the distant magnetotail associated with near‐Earth substorm effects
Authors: Baker D. N. , Anderson R. C. , Zwickl R. D. , Slavin J. A.
Published: February 2008

Examination of many individual event periods in the ISEE 3 deep tail data set collected in 1982–1983 has suggested that magnetospheric substorms produce a characteristic pattern of effects in the distant magnetotail. ISEE 3 data are appropriate for the study of these effects, since the spacecraft remained relatively fixed within the nominal tail location for long periods. Using auroral electrojet indices (AE and AL) and geostationary orbit particle data to time substorm onsets at earth, we have performed superposed epoch analyses of ISEE 3 and near‐earth data prior to and following substorm expansive phase onsets. Individual events illustrate the wide range of variations possible throughout the magnetosphere, while epoch‐averaged results illustrate the general properties of the deep tail pattern of response to global substorm‐induced dynamical effects. During the growth, or tail energy storage, phase of substorms the distant magnetotail appears to grow diametrically in size, often by many earth radii. Subsequently, about 20–40 min after the substorm expansive phase onset at earth the distant tail undergoes a repeatable sequence of plasma, field, and energetic particle variations as large‐scale plasmoids move rapidly down the tail following their disconnection from the near‐earth plasma sheet.
On the generation of field‐aligned plasma flow at the boundary of the plasma sheet
Authors: Schindler K. , Birn J.
Published: February 2008

This paper deals with a possible cause of the large plasma flow velocities parallel to the magnetic field observed near the boundary of the plasma sheet in the earth's magnetotail. For a large class of steady state configurations with typical cases involving magnetic reconnection we first show qualitatively that high parallel flow velocities can be expected to exist on field lines connecting to a region of weak magnetic field. For reconnection configurations this weak field region contains the diffusion region. The maximum value of the parallel flow velocity is sensitive to the lowest magnetic field magnitude just outside the diffusion region. The physical mechanism causing large values of the parallel velocity component υ∥ can be visualized as a strong imbalance of perpendicular mass flux into and out of magnetic flux tubes passing through regions where the magnetic field is weak and inhomogeneous. In a steady state the unbalanced perpendicular flow requires large parallel flow to establish mass conservation. The presence of a parallel velocity is not unusual in MHD systems. The new (generic) aspect is that near the separatrix, parallel flow is the dominant form of plasma transport. For a quantitative evaluation we consider an MHD model specialized for the domains where the inertia force can be neglected. By a self‐consistent treatment we evaluate υ∥ and find that |υ∥| can substantially exceed the perpendicular velocity υ⊥ = E/B; in a typical example with stretched magnetic field lines we obtain |υ∥| ≈ 40υ⊥. We apply our results to the earth's magnetotail and conclude that this mechanism is able to explain the parallel flow velocities near the boundary of the plasma sheet in the range of several hundreds of kilometers per second.
Deuterium in the daytime thermosphere
Authors: Breig E. L. , Sanatani S. , Hanson W. B.
Published: February 2008

Ion concentration measurements for H+ and D+ from the magnetic ion mass spectrometer on Atmosphere Explorer C satellite are used, in conjunction with other atmospheric data, to determine the concentrations of H and D in the nonpolar daytime thermosphere. The ratio of the observed D+ to H+ concentrations has essentially the same height dependence in the 300‐ to 800‐km region as expected for their neutral counterparts, ever in the presence of ion temperature gradients and probable large vertical ion fluxes. Rapid charge exchange with atomic oxygen ensures that [D]/[H] ≃ [D+]/[H+] at the lower altitudes where the derived D to H concentration ratio is a factor of ∼6 larger than its sea level value, for an exospheric temperature of 930°K. This relative enhancement of deuterium arises from the fact that hydrogen more readily escapes the earth, and a large vertical gradient in the H concentration relative to its diffusive equilibrium value is necessary to drive this upward flux through the lower thermosphere. If these planetary losses of hydrogen are much greater than those associated with evaporative escape, as is the current view, then correspondingly larger deuterium loss rates are also likely in order that the thermospheric [D]/[H] ratio not increase well above the observed value. The absolute winter daytime concentration of deuterium at 300 km is found to be 210 ± 50 atoms/cm³.
The effect of mass loading outside cometary bow shock for the plasma and wave measurements in the coming cometary missions
Authors: Sagdeev R. Z. , Shapiro V. D. , Shevchenko V. I. , Szegő K.
Published: February 2008

The neutral gas emitted by comets is partly photoionized along its path. The interaction of the ions with the solar wind leads to observable particle and wave effects in the ambient plasma. These are described in the present paper.