bluuebunny/crossref_metadata_2025 huggingface dataset:
Whistler induced suppression of VLF noise
Authors: Gail W. B. , Carpenter D. L.
Published: February 2008

New evidence has been found connecting whistlers with transient reductions in the amplitude of magnetospheric VLF noise bands. Until now, the only reported examples of such effects were observed during a several hour period at Siple Station, Antarctica, when bursts of particle precipitation were correlated with whistlers that suppressed a background hiss band. Data acquired at South Pole Station during 1981 showed similar suppression events on 20% of all winter days. A review of data from Siple, Byrd, Eights, and Palmer Stations in Antarctica and Roberval in Quebec, Canada, has since identified many more events. The ground signature of an event is characterized by the attenuation of an existing VLF noise band following the arrival of a whistler. The attenuation typically reaches a maximum of 3–5 dB in 5–10 s, and the noise band recovers to the pre‐event amplitude 15–30 s after the whistler. The noise bands are generally mid‐latitude hiss, but suppression associated with polar chorus has been seen. Regularly observed features of suppression events include multi‐hop echoing of the driving whistler; confinement of the whistler echoes to the frequency band occupied by the suppressed noise; suppression during the first pass of the whistler; continuing suppression by the whistler echoes as the suppressed noise amplitude reaches a minimum and then recovers; and recovery of the noise band to the pre‐event level. Event duration was found to increase with the duration of the whistler echo train. Some data suggest that this phenomenon may be a natural analog of the previously reported quiet band effect in which signals from the VLF transmitter at Siple Station were observed to attenuate portions of noise bands.
The plasma wave environment of an auroral arc: Electrostatic ion cyclotron waves in the diffuse aurora
Authors: Bering E. A.
Published: February 2008

Emissions that appear to have been electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. The rocket was launched from Poker Flat, Alaska, at ∼2030 MLT. The flight successively traversed ∼70 km of the diffuse aurora, a dark region, and a quiet 40 kR auroral arc. In the diffuse aurora, peaks were observed in the power spectrum of the electric field at frequencies near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave spectrum parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those expected for H+ and O+ EIC waves. Taken together, the two analyses indicated that both emission bands were due to waves propagating both up and down the field line and eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting large Landau damping require that the source be local. Free energy for the waves was apparently available in the 5 µA/m² downward parallel current density which was inferred from the magnetometer data. The presence of the waves indicates that this current was being carried by less than 2% of the plasma, presumably in the form of a field aligned beam of electrons with energies of a few eV.
Prediction of photon yields for proton aurorae in an N2 atmosphere
Authors: Van Zyl B. , Gealy M. W. , Neumann H.
Published: February 2008

Absolute photon yields for band emissions of the N2+ first‐negative and N2 second‐positive systems are predicted for proton aurorae in an N2 atmosphere. The predictions are made by using a proton auroral model based largely on collision cross‐section information, including the production of these emissions in the atmosphere by secondary electrons. The photon yields for Balmer‐alpha and Balmer‐beta emissions and the total secondary electron production during proton aurorae are also determined. The results are compared with other predictions and with a limited number of proton auroral observations.
A close look at the solar wind‐magnetosphere interaction
Authors: Rossberg L.
Published: February 2008

A study of the short‐term (12‐hour) variation of the linear correlation between the solar wind‐magnetosphere energy coupling function ε and the electrojet index AE revealed unexpectedly large variations of the correlation coefficient R and of the ratio between ε and AE. About half of all R values fell below the 95% confidence level, and the ratio between ε and AE varied by more than 100% of the average value, even when R was large. This result reveals that the ε function is not a very good predictor of substorms and their intensities. Several possible causes for the variations are discussed. It is shown that some of the variations of the correlation coefficient occur because of a variable temporal relationship between ε and AE.
Effects of energetic heavy ions on electromagnetic ion cyclotron wave generation in the plasmapause region
Authors: Kozyra J. U. , Cravens T. E. , Nagy A. F. , Fontheim E. G. , Ong R. S. B.
Published: February 2008

An expression for the linear electromagnetic ion cyclotron convective growth rate has been derived, considering multiple ions in the energetic anisotropic component of the plasma (which provides the free energy for the instability) as well as in the cold component of the plasma. This represents a modification of recent treatments investigating electromagnetic ion cyclotron growth rates which have considered only hydrogen ions in the energetic component. Four major effects on the growth and propagation characteristics result from inclusion of heavy ions in the energetic component. Some wave growth occurs at low frequencies below the corresponding marginally unstable wave mode for each heavy ion. Enhanced quasi‐monochronomatic peaks in the convective growth rate appear just below the O+ and He+ gyrofrequency and can be quite pronounced for certain plasma conditions. Stop bands, decreased group velocity and other effects normally attributed to cold heavy ions can be produced or enhanced by heavy ions in the energetic plasma component. Partial or complete suppression of wave growth at frequencies above the marginally unstable wave mode for a particular energetic heavy ion can greatly alter the growth rates that would occur in the absence of this energetic heavy ion. The expression for the linear electromagnetic ion cyclotron convective growth rate along with appropriate plasma parameters was used to investigate the nature of linear wave growth in the plasmapause region. The frequencies of peaks in the convective growth rate given by this model compare favorably with wave measurements in this region. It is conceivable that through wave‐particle interactions, electromagnetic ion cyclotron waves could supply the energy source for various plasmapause region phenomena such as the O+ torus, the plasma cloak, and stable auroral red arcs.
Turbulent heating of heavy ions on auroral field lines
Authors: Ashour‐Abdalla M. , Okuda H.
Published: February 2008

Electrostatic ion cyclotron turbulence and the associated acceleration of ions on auroral field lines are investigated analytically and by plasma simulations. The auroral plasma is assumed to consist of drifting electrons and stationary hydrogen and oxygen ions. It is found that for a given critical drift, the maximum ion perpendicular heating is generally larger for oxygen ions than for hydrogen ions. Simulation results show that unless the oxygen ions are a minority species, oxygen transverse heating generally exceeds that of hydrogen ions. Theory and numerical simulations are in good agreement.
Composite equatorial spread F wave number spectra from medium to short wavelengths
Authors: Singh M. , Szuszczewicz E. P.
Published: February 2008

In an effort to provide a full‐spectral classification of equatorial spread F irregularities from large‐scale behavior (≈100 km) to the meter‐size domain, we have analyzed a combination of rocket and satellite data that in concert cover the complete six decade irregularity distribution. Our results provide a composite perspective in the hierarchy of processes that contribute to the total phenomenon called equatorial spread F and provide “in situ” meter‐size irregularity measurements that help explain the relationship of spectral power at short wavelengths to that in the transitional and intermediate domain. In a synoptic perspective we find that medium and intermediate domains (≈ 50 km → 200 m) display k−1.5±0.4 and k−2.4±0.2 behavior, respectively, in both vertical and horizontal distributions. The transitional wavelengths (≈200 m → 20 m) are dominated by a steep one‐dimensional k−4.8±0.2 universal drift wave spectral behavior but break at kri ≳ 1 (λ ≲ 20 m) to much higher power levels, a result that is consistent with ground‐based radar results in the meter‐scale domain. Our shortest wavelength rocket results (1 m ≲ λ ≲ 20 m) are qualitatively consistent with a lower hybrid drift wave interpretation, but the presence of resonant structure in the observed spectral distribution strongly suggests a resonant wave‐particle interaction process. While our analyses point to specific spectral indices in each of the wavelength domains, we caution against the unique identification of any spectral index with a specific geoplasma mechanism. Clearly, time in the instability growth and decay process can modify the spectral index, as can changing ionospheric conditions, transport, and diffusion. Our satellite results provide for a large statistical sampling of spectral behavior in the various wavelength domains, point to the most probable spectral indices, and highlight their variability.
Solar wind ionization temperatures inferred from the charge state composition of diffuse particle events
Authors: Galvin A. B. , Ipavich F. M. , Gloeckler G. , Hovestadt D. , Klecker B. , Scholer M.
Published: February 2008

The charge state composition of heavy ions, primarily the carbon‐nitrogen‐oxygen (CNO) group, has been determined for 12 diffuse particle events in the energy‐per‐charge range ∼33 to 132 keV/e observed upstream of the earth's bow shock. The measurements were made with the University of Maryland/Max‐Planck‐Institut ultra low energy charge analyzer (ULECA) sensor on ISEE 1. The relative charge state distributions found for Q ≥ 3 ions at ∼33, 66, and 132 keV/e are consistent with a charge state composition that is constant as a function of energy per charge. It is suggested that the charge state composition of these locally accelerated heavy ions can be used to estimate the charge state composition in the solar wind and hence to estimate the equilibrium coronal temperatures associated with a variety of solar wind flows. In particular, we obtain average CNO ionization temperatures of (1.5±0.2) ×106 K for diffuse particle events occurring during interstream solar wind conditions, (1.3± 0.2) × 106 K for events occurring during coronal‐hole‐associated high‐speed streams, and (2.2±0.3) × 106 K for events occurring during transient‐related flows. The diffuse particle event ionization temperatures corresponding to coronal‐hole‐associated high‐speed streams are particularly interesting, since reported solar wind charge state measurements for high‐speed streams are presently very limited. The temperature values we infer for coronal‐hole‐associated solar wind are approximately constant over the range of solar wind speeds covered (420–680 km s−1) and are consistent with equilibrium temperatures predicted for coronal holes.
Electron precipitation zones around major ground‐based VLF signal sources
Authors: Inan U. S. , Chang H. C. , Helliwell R. A.
Published: February 2008

The spatial distribution of electron precipitation induced by VLF signals from ground‐based transmitters is determined by using a test particle computer model of the gyroresonant wave‐particle interaction (Inan et al., 1982). The results are presented as contours of energy flux on a map of the region around each transmitter. It is shown that the size of the precipitation zones is a strong function of the geographic location of the transmitter, as well as its radiated power and operating frequency. In general, the precipitation zones are much wider in longitude than in latitude and are oriented along lines of constant geomagnetic latitude. Assuming backscatter and/or wave echoing, precipitation zones around the points that are magnetically conjugate to the sources are also estimated. The results presented can be used to interpret satellite‐ or ground‐based measurements of the precipitation induced by ground‐based VLF transmitters.
Solar‐flare‐induced Forbush decreases: Dependence on shock wave geometry
Authors: Thomas Barry T. , Gall Ruth
Published: February 2008

This paper presents an analysis of the effect of flare‐induced shock waves on the intensity of relativistic cosmic rays at 1 AU. We find that the intensity of cosmic ray protons falls abruptly as the shock wave passes the earth and recovers as the compression region moves into the outer heliosphere. The intensity versus time profile we obtain is consistent with observed flare‐induced Forbush decreases. The principal mechanism is found to be simply prolonged containment of the cosmic ray particles in the region between the flare shock wave and the sun. This leads to additional adiabatic cooling of the particles and a corresponding decrease in particle intensity. Acceleration mechanisms at the shocks are found to play little role at relativistic energies. We investigate the dependence of both the maximum intensity decrease and recovery time on such parameters as particle diffusion coefficient, particle rigidity, and flare geometry. It is found that flares located to the east of the earth‐sun line produce larger effects than those located to the west. The magnitude of the intensity reduction decreases as a function of increasing diffusion coefficient and increasing particle rigidity. Throughout most of our analysis we assume the flare compression region to have a longitudinal extent of 180°, but as an extreme case we investigate the effect of a completely spherical blast wave. This, as expected, has a substantially increased effect on both the magnitude of the decrease and, more significantly, on the recovery profile.
Plasma in Saturn's magnetosphere
Authors: Eviatar Aharon
Published: February 2008

The spatial and compositional distribution of the thermal plasma in the magnetosphere of Saturn is described in the light of the Voyager encounters. Theoretical considerations are applied to the elucidation of the structure, including two external and two internal boundaries. The outer boundary is a magnetohydrodynamic entity while the inner boundary of locally created thermal plasma is a result of the dissociative recombination of corotating molecular ions. The internal boundaries which we have reason to conclude separate plasmas of different composition are explained as a charge exchange quasi‐resonance phenomenon.
Electrostatic charging of the rings of Saturn: A parameter study
Authors: Ip W.‐H.
Published: February 2008

Diurnal variation of the electrostatic potential of the rings of Saturn is considered by examining a wide range of possible combinations of plasma sources and sinks. It is shown that because of the presence of the ring absorption effect, the ambient thermal plasma density is kept very low
. If no trapping or thermalization of the photoelectrons, ionospheric plasma, and impact plasma generated by meteoroid impact occurs, the ring surface potential could vary between 4–8 V (dayside and nightside) and −7 V (shadow cone). The operation of trapping the field‐aligned plasma flow changes the time variation of the ring surface potential only slightly. It is the variations of the peak ionospheric plasma flux and of the impact plasma which have more significant effect. The impact plasma, in particular, can change the ring surface potential from a value of −7.3 V to a small but positive value if its flux reaches ∼106 ions cm−2 s−1.
A two‐dimensional model of the nightside ionosphere of Venus: Ion energetics
Authors: Bougher S. W. , Cravens T. E.
Published: February 2008

The energetics of the nightside ionosphere of Venus are still not well understood. Rather large ion temperatures have been observed on the nightside. We have constructed a two‐dimensional theoretical model of the energetics of the nightside ionosphere which takes into account the effects of horizontal and vertical bulk transport of heat. We demonstrate that the observed maintenance of the ion temperature can be explained in terms of these transport terms for solar zenith angles less than 150°. Simple heat transport cannot explain the observed high temperatures for angles greater than 150°.
Correlated dynamical changes in the near‐Earth and distant magnetotail regions: ISEE 3
Authors: Baker D. N. , Bame S. J. , Belian R. D. , Feldman W. C. , Gosling J. T. , Higbie P. R. , Hones E. W. , McComas D. J. , Zwickl R. D.
Published: February 2008

During October 1982 and January–March 1983, ISEE 3 made its first traversals of the distant (r = 60–220 RE) geomagnetic tail. Throughout this period the Los Alamos ISEE 3 plasma electron instrument detected the tailward magnetosheath, magnetopause, plasma sheet, and tail lobes. For the entire tail traversal period, nearly continuous concurrent data were available from Los Alamos charged‐particle analyzer instruments on board the spacecraft 1977‐007, 1981‐025, and 1982‐019 at geostationary orbit (6.6 RE). Using these geostationary orbit data in the local midnight sector, numerous substorm particle injection events were detected, allowing substorm onset determinations to accuracies of a few minutes. Remarkably high degrees of correlation between near‐earth substorm events and ISEE 3 transitions from one magnetotail plasma regime to another were often found throughout the tail crossing. Particularly notable were periods of “taillike” magnetic field stretching at 6.6 RE (substorm growth phases) and diametrical expansions of the distant tail seen by ISEE 3. Similarly, substorm expansion onsets seen at 6.6 RE were followed by rapid apparent contractions of the translunar tail which took ISEE 3 into magnetosheath and/or boundary layer plasmas. At ISEE 3 distances beyond ∼200 RE it is commonly observed that substorm particle injection events at 6.6 RE precede the occurrence of strong tailward plasma flow by ∼25±5 min. These results suggest a close relationship between processes in the near‐earth plasma sheet and the distant tail during substorms, with particular support for the flaring tail model of the substorm growth phase and for substorm initiation in the near‐earth magnetotail region.
Measurements of E region ionization and conductivity produced by solar illumination at high latitudes
Authors: Robinson R. M. , Vondrak R. R.
Published: February 2008

Electron density measurements made by the Chatanika radar during times when auroral particle precipitation was absent have been used to determine the variation of E region ionization and height‐integrated ionospheric conductivity as a function of solar zenith angle and solar flux. From the Chatanika radar data taken over an entire solar cycle we have derived the electron density as a function of altitude between 90 and 250 km for five solar zenith angles between 45° and 85° and for four different levels of solar flux. From 16 to 40 profiles were averaged together to determine the mean values; typically, the standard deviations were less than 25% of the mean. The height‐integrated conductivities computed from these profiles increase by about a factor of 2 between the lowest and highest levels of solar flux. The solar contribution to the Hall and Pedersen conductances ΣH and Σp is well represented by ΣH = 1.5 (Sa cox χ)0.5 = 1.7 Σp, where χ is the solar zenith angle and Sa is the 10.7‐cm solar flux. In order to determine the total conductance when other ionization sources are present the altitude‐dependent production rate is required. This was computed from the data by using an altitude‐dependent model for the effective recombination coefficient.
Plasma and energetic particle structure upstream of a quasi‐parallel interplanetary shock
Authors: Kennel C. F. , Scarf F. L. , Coroniti F. V. , Russell C. T. , Wenzel K.‐P. , Sanderson T. R. , Van Nes P. , Feldman W. C. , Parks G. K. , Smith E. J. , Tsurutani B. T. , Mozer F. S. , Temerin M. , Anderson R. R. , Scudder J. D. , Scholer M.
Published: February 2008

This paper assembles ISEE 1, 2, and 3 observations of the interplanetary magnetic and electric fields, plasma, magnetohydrodynamic waves, electromagnetic and electrostatic plasma waves, 1‐ to 6‐keV protons and electrons, and >30‐keV/Q ions for the interplanetary shock of November 12, 1978. The shock was high speed (640 km s−1), supercritical, quasi‐parallel, and an efficient accelerator of energetic protons. The flux of >35‐keV protons increased by a factor of 15 in the last 45 min and 270 RE before shock encounter. The >10‐keV proton energy density approached that of the magnetic field and thermal plasma upstream of the shock. The shock was inside a closed magnetic structure that was connected at both ends to the shock. The intensity of ion acoustic and low‐frequency MHD waves increased inside the closed magnetic bubble.
Distribution of upflowing ionospheric ions in the high‐altitude polar cap and auroral ionosphere
Authors: Yau A. W. , Whalen B. A. , Peterson W. K. , Shelley E. G.
Published: February 2008

A statistical study is presented of the occurrence frequency distribution of upflowing ionospheric ions (UFI) in the high‐altitude (8000–23,000 km) auroral and polar cap ionosphere, using measurements from the energetic ion composition spectrometer aboard Dynamics Explorer 1. In this survey, distinction is made between UFI of different masses (H+ and O+), energies (0.01–1, 1–4, and 4–17 keV/e), and pitch angle characteristics (within 20° from
, between 20° and 80° from
, and transverse to
). The increased statistical accuracy of the data and the added dimensions in this study relative to earlier work at lower altitudes provide a more detailed picture of UFI events. Occurrence frequencies are determined for both H+ and O+ UFI at the different energies as a function of altitude, invariant latitude, magnetic local time, and magnetic activity (Kp index). It is shown that the occurrence frequency and intensity distribution of O+ UFI have a marked magnetic dependence, whereas the H+ UFI occurrence displays little variation with magnetic activity. The occurrence frequency of O+ conics decreases with altitude, in relation to that of H+ conics. Also, ≈90° H+ conics occur 2–3 times more frequently than O+. The invariant latitude‐local time distribution of the occurrence frequency for both H+ and O+ is auroral‐oval‐like in that the frequency peaks at a more equatorward latitude and has a broader latitudinal extent in the nightside than in the dayside. However, this association with the auroral oval is energy‐dependent, the oval for low‐energy UFI being distinctly more poleward than that for energetic UFI. The low‐energy O+ UFI occurrence is relatively uniform in local time; the H+ UFI occurrence frequency distribution shows a broad distribution peaking in the noon sector. Both the H+ and O+ conics distributions display a dawn‐dusk asymmetry in favor of the dusk sector. In the polar cap (poleward of the statistical auroral oval), the H+ and O+ ion outflows (≈2×1024 s−1 and 3×1024 s−1, respectively, at quiet times, and ≈3×1024 s−1 and 1×1025 s−1 at disturbed times) represent a significant ionospheric source to the magnetotail. The results of this study are shown to be consistent with previous, less detailed surveys of UFI at lower altitudes and provide a basis with which to examine possible acceleration mechanisms.
energetic oxygen in a mid‐latitude aurora
Authors: Torr Marsha R. , Torr D. G.
Published: February 2008

On the night of September 21/22, 1982, ground‐based observations were made of an aurora which extended as far equatorward as Logan, Utah (41°N, 111°W). An imaging spectrometer was used to obtain spectral images over the wavelength range 3300–5300 Å at a spectral resolution of 2.7 Å. This segment of the spectrum was found to be rich in atomic and molecular features. The vibrational and rotational structures of the molecular nitrogen bands provide extremely valuable information on the velocity of the precipitating particles. From a comparison of the N2+ vibrational and rotational structures, which are strongly populated at the higher levels, and similar data for the N2 bands we conclude that the events indicate strong precipitation of ring current energetic heavy atoms. Because of the absence of observable Hβ emission and because of the characteristic energy inferred from the velocity the energetic atoms are probably oxygen.
Observations of the dynamics of the polar thermosphere
Authors: Hays P. B. , Killeen T. L. , Spencer N. W. , Wharton L. E. , Roble R. G. , Emery B. A. , Fuller‐Rowell T. J. , Rees D. , Frank L. A. , Craven J. D.
Published: February 2008

The dynamics of the polar thermosphere are examined by using observations made from the Dynamics Explorer 2 satellite. The results used in this study were obtained primarily from the Fabry Perot interferometer (FPI) and the wind and temperature spectrometer (WATS) during the time period from September 1981 through January 1982. Two primary geophysical conditions were examined: these were the southern summer and the northern winter polar regions. The results support the conclusion that above 60 degrees of latitude the neutral winds are strongly controlled by ion/neutral frictional momentum transfer resulting from magnetospheric convection. This implies that the natural coordinate system within which to display the neutral winds in the high polar thermosphere is magnetic. The collected observations of this study were used to assess the validity of two of the large thermospheric general circulation models. The result of this assessment was that the models reasonably represent the vector winds at high altitudes but do not, at present, accurately simulate the thermodynamics of that regime.
A note on Southwood's instability criterion
Authors: Uberoi C.
Published: February 2008

It is shown that Southwood's instability criterion for the onset of the Kelvin‐Helmholtz instability at the magnetopause can be directly obtained from the marginal instability condition for the pure Alfven surface waves propagating along the interface between two incompressible media in the limit when the wave propagation direction is nearly perpendicular to the direction of the largest magnetic field. The phase velocity of the surface waves first excited at the onset of the instability depends on the angle between the interplanetary magnetic field and flow velocity in the solar wind in front of the bow shock.
Flux transfer events on the magnetopause: Spatial distribution and controlling factors
Authors: Berchem Jean , Russell C. T.
Published: February 2008

Magnetic field profiles across the dayside magnetopause, obtained during the first five years (1977–1981) of the ISEE mission have been examined in boundary normal coordinates to determine the spatial distribution and controlling factors of flux transfer events (FTE’s). Owing to the natural evolution of the orbital plane of the spacecraft, the dayside magnetopause region is eventually sampled both above and below the magnetic equator. Nevertheless, at the end of the period surveyed, the satellites still had not yet reached latitudes as far south of the equator as they were initially north of the equator. Two types of magnetic signature in the component normal to the magnetopause are regularly observed when an FTE passes the spacecraft. In the northern dawn quadrant the perturbation along the normal direction is usually first outward and then inward, whereas in the southern dusk quadrant the reverse behavior occurs. The dividing line between the two signatures is tilted somewhat with respect to the equator along a south dawn‐north dusk direction. However deviations from this simple pattern are observed in both hemispheres. Examination of simultaneous IMP 8 and ISEE 3 interplanetary data reveals that FTE’s occur almost exclusively during southward interplanetary magnetic field (IMF) conditions. FTE’s occur about 45% of the time when the IMF has a southward component, with the highest rates of occurrence for due southward IMF. The sign of the east‐west component of the IMF does not exert control on the rate of FTE occurrence.
Origin of the plasma in a cross‐polar cap auroral feature (theta aurora)
Authors: Peterson W. K. , Shelley E. G.
Published: February 2008

Ion composition data obtained from a particularly well observed cross‐polar cap auroral feature (theta aurora) are presented. Two components of the hot plasma are identified, one from the ionosphere below and the second from the distant plasma sheet. These observations provide strong support for the conclusion that the cross‐polar cap auroral feature occurs on field lines closing through the distant plasma sheet or plasma sheet boundary layer. Taken together with the previously published observations, these new results establish substantial constraints on models for the overall polar cap topology. However, they do not unambiguously favor either of the two principal topologies discussed in the literature.
Direct evidence for solar wind control of Jupiter’s hectometer‐wavelength radio emission
Authors: Desch M. D. , Barrow C. H.
Published: February 2008

Observations of the solar wind close to Jupiter by the Voyager 1 and Voyager 2 spacecraft in 1978 and 1979 are compared with the hectometer‐wavelength radio emission from the planet. A significant positive correlation is found between variations in the solar wind plasma density at Jupiter and the level of Jovian radio emission output. During the 173‐day interval studied for the Voyager 2 data the radio emission displayed a long‐term periodicity of about 13 days, identical to that shown by the solar wind density at Jupiter and consistent with the magnetic sector structure association already proposed for ground‐based observations of the decameter‐wavelength emission.
A comparison of specularly reflected gyrating ion orbits with observed shock foot thicknesses
Authors: Livesey W. A. , Russell C. T. , Kennel C. F.
Published: February 2008

ISEE 1 and 2 flux gate magnetometer data has been used to measure the length or thickness of the “foot” of 60 quasi‐perpendicular supercritical bow shocks along the shock normal. These bow shock crossings are contrasted with 10 quasi‐perpendicular subcritical shocks which do not exhibit the precursor foot. For the supercritical shocks θBn, the angle between the shock normal and the upstream magnetic field, ranges from 43° to 88°. The range of θBn for the laminar shocks is 56° to 76°. The extent of the foot structure is compared to the orbit of an ion having solar wind velocity and undergoing simple specular reflection about the shock normal. Good quantitative agreement between the measured value of the foot and the predicted value of ∼0.7 ion gyroradii [Woods, 1971] is evident. The foot thickness normalized to this predicted value is compared to M/MC, the ratio of fast Mach number to critical fast Mach number. There is no dependence of the normalized foot thickness on Mach number other than at the transition point between subcritical and supercritical regimes, M/MC = 1. No dependence of the normalized foot thickness on upstream ion β is noted. The sole dependence of the normalized foot thickness on θBn appears to be one of simple geometry, i.e., the determination of the component of the normal solar wind perpendicular to the upstream magnetic field.
Stimulated electromagnetic emission: A new technique to study the parametric decay instability in the ionosphere
Authors: Stubbe P. , Kopka H. , Thidé B. , Derblom H.
Published: February 2008

A powerful HF wave, transmitted in the O mode with a frequency not exceeding the critical F region frequency, gives rise to secondary electromagnetic radiation, filling a frequency band of several 10 kHz around the frequency of the primary wave. The spectrum of these secondary waves is richly structured. The systematically occurring spectral features are identified and described. The majority of these features can be understood by scatter processes involving Langmuir waves and low‐frequency density perturbations excited by the parametric decay instability. Other features, including a broad spectral maximum at 20 to 40 kHz on the upshifted side, are not as yet fully understood.
Relativistic dispersion, the cyclotron maser instability, and auroral kilometric radiation
Authors: Pritchett P. L.
Published: February 2008

The most successful explanation proposed for the generation of auroral kilometric radiation (AKR) is the direct cyclotron mechanism in which radiation excited near the local electron cyclotron frequency is amplified through a gyroresonant interaction. Previous work has shown that inclusion of the relativistic mass dependence of the cyclotron frequency is essential to determine the resonant contours in velocity space. In the present work it is demonstrated that relativistic effects can also significantly modify the wave dispersion, even for only mildly relativistic electrons, when ωpe²/Ωe² ≪ 1, which is frequently the case in the AKR source region. For a relativistic Maxwellian distribution the R‐X mode cutoff is found to be shifted below Ωe for (υte/c)² > ⅔ (ωpe/Ωe)². Linear analysis of a delta function ring distribution in p⊥ indicates that the R‐X mode is unstable for k∥ = 0 (i.e., propagation perpendicular to the magnetic field) with Re ω ≲ Ωe when υ/c > ωpe/Ωe. The effect of finite velocity spread on the maximum growth rate (which occurs for kc/Ωe ∼ 1) is minor for k∥ = 0, while with finite k∥ the maximum growth rate is reduced considerably. The relativistic dispersion effects are found to be significant at propagation angles up to 10° away from normal. The addition of a cold electron component produces a new relativistic mode with extraordinary polarization and Re ω ≲ Ωe which is unstable for normal and near‐normal propagation. Computer simulations using a relativistic electromagnetic particle code indicate that for ring and shell distributions the k∥ = 0 modes with Re ω ≲ Ωe dominate the radiation emission resulting from the cyclotron maser instability. The relativistic dispersion effects should lead to larger growth rates for the generation of AKR than were estimated by cold‐plasma calculations either by allowing ω < Ωe waves to sample all the energy growth regions simultaneously or by allowing Ωe < ω < ωx (ωx is the R‐X mode cutoff) waves in the upgoing loss cone to sample regions of velocity space closer to the origin.
Nighttime variation of thermospheric winds and temperatures over fritz peak observatory during the geomagnetic storm of March 2, 1983
Authors: Hernandez G. , Roble R. G.
Published: February 2008

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer during the March 2, 1983, geomagnetic storm. The winds and temperatures are obtained from the Doppler shifts and line profiles of the [O I] 15,867 K (630 nm) line emission. The planetary geomagnetic index, Kp, increased abruptly from 4 to 7 during the beginning of the measuring period near 0000 UT on March 2 and remained at nearly that level throughout the night. During the early evening hours near 1900 LT (0200 UT) the zonal winds measured in both the east and west directions from Fritz Peak Observatory were westward at a speed of about 100 m s−1. The westward winds increased in magnitude during the next 2 hours, with the wind speed east of the station reaching 400 m s−1 and the winds to the west of the station reaching 300 m s−1. The westward winds remained at roughly these speeds until about local midnight (0700 UT), when they gradually decreased to about 50 m s−1 in the east, and the winds to the west of the station shifted to eastward at about 50 m s−1. The meridional winds measured in the early evening hours both to the north and south of the station were poleward at speeds ranging between 50 and 10 m s−1. Near local midnight the wind measured north of the station gradually shifted to equatorward, reaching a maximum speed of about 300 m s−1 near 0200 LT (0900 UT) and then decreasing to about 100 m s−1 near dawn. The wind measured south of the station remained poleward throughout the night at speeds ranging between 25 and 100 m s−1. The neutral gas temperature over Fritz Peak Observatory increased throughout the night from about 1300° to 1600°K. Calculations made with the National Center for Atmospheric Research thermospheric general circulation model suggest that the observed winds over Fritz Peak Observatory (L ≅ 3) are under the influence of an expanded magnetospheric convection pattern during the storm. Enhanced magnetospheric convection at mid‐latitudes characteristically produces westward winds during the early evening hours and a strong equatorward surge in the postmidnight winds.
Bragg backscatter from plasma inhomogeneities due to a powerful ionospherically reflected radio wave
Authors: Fejer J. A. , Djuth F. T. , Gonzales C. A.
Published: February 2008

Backscatter from quasi‐periodic inhomogeneities in the electron density, generated by a powerful ionospherically reflected wave, has been observed at Arecibo. The backscatter was more intense by about 40 dB than that observed earlier by other workers at higher latitudes. It is suggested that the anomalous ionospheric absorption was greater in the earlier experiments.
O2 (b¹Σg+ – X³Σg−) atmospheric band night airglow measurements in the northern polar cap region
Authors: Myrabø H. K. , Henriksen K. , Deehr C. S. , Romick G. J.
Published: February 2008

The O2 atmospheric (0–1) band at 8645 Å has been observed in the airglow during the 1982/1983 winter solstice from the ground at Longyearbyen, West Spitsbergen (78.4°N latitude, 15°E longitude, geographic). The average (0–1) band intensity for the continuous 18‐hour period of the observations is found to be 445 R ± 40 R. The mean temperature deduced from the O2 (0–1) atmospheric band rotational structure is found to be 254 K±3 K compared to a mean of 249 K±2 K for the OH (8–3) band rotational temperature. This indicates a shallow mesopause with an upper temperature gradient of ∼1 K/km. Comparison with previous observations shows little or no latitude dependence, although there is considerable scatter in the data indicating that the O2 airglow is highly variable in time of the order of hours or less.
Perturbations of subionospheric LF and MF signals due to whistler‐induced electron precipitation bursts
Authors: Carpenter D. L. , Inan U. S. , Trimpi M. L. , Helliwell R. A. , Katsufrakis J. P.
Published: February 2008

First evidence of whistler‐induced burst precipitation effects on subionospherically propagating signals in the LF and MF ranges have been observed at Palmer (L ∼ 2.4) and Siple stations (L ∼ 4.3), Antarctica. The occurrence rate on a 37.2‐kHz LF signal originating in California was usually comparable to that on the more disturbed VLF paths. At Palmer, examples at 37.2 kHz were seen on 70% of the nights during a March‐April 1983 observing period. Perturbations on a ∼1800 km‐long 780‐kHz MF path to Palmer occurred on nights of high activity on VLF paths, but fewer than 10 MF events were usually detected as compared to >50 on the active VLF paths. The MF perturbations were of order 50% in amplitude and were not in general followed by a ∼30‐s decay toward a pre‐event level, as is usually the case for the VLF signals. Test particle modeling of the whistler‐particle interaction supports the idea that the MF signals are affected by ionization extending above the nominal ∼85‐km reflection height for VLF signals and also are consistent with the observed time delay between the 780‐kHz signal perturbation and the whistler‐generating lightning stroke. Fast phase advances correlated with the whistlers were regularly observed on a 12.9‐kHz Omega signal. The data provide a means of inferring the existence of multiple precipitation regions and offer new evidence that whistlers originating in the northern hemisphere can cause significant precipitation in the southern hemisphere, at least in the vicinity of the South Atlantic magnetic anomaly.
Transmission of magnetohydrodynamic waves through the rotational discontinuity at the Earth's magnetopause
Authors: Kwok Y. C. , Lee L. C.
Published: February 2008

Reflection and transmission of small amplitude magnetohydrodynamic (MHD) waves of solar‐wind origin through the earth's open magnetopause are studied. The open magnetopause with a nonzero normal component of magnetic field is assumed to be a rotational discontinuity. For any of the incident waves, there is one reflected wave (fast magnetosonic wave) and five transmitted waves (one fast magnetosonic wave, two slow magnetosonic waves, one Alfvén wave and one entropy wave). In this paper, cases with different incident waves, i.e., Alfvén wave, fast magnetosonic wave, forward slow magnetosonic wave and entropy wave are studied. It is found that depending on the type of incident wave, the incident angle and the orientation of the ambient magnetic field, the amplitudes of the emanating waves can be amplified. The research findings also suggest that the transmission of MHD waves across the rotational discontinuity at the earth's magnetopause can be an important mechanism for the energy transfer from the solar wind into the magnetosphere.
Observations of electron beams in the low‐latitude boundary layer
Authors: Ogilvie K. W. , Fitzenreiter R. J. , Scudder J. D.
Published: February 2008

Observations made with the electron spectrometer on the ISEE 1 spacecraft have been used to perform a study of the electron distribution function in the low‐latitude boundary layer of the magnetosphere (LLBL) in the sunward direction. The LLBL is defined to be a layer of plasma of thickness ∼1 RE inside and adjacent to the magnetopause, having a density intermediate between that of the magnetosheath and the magnetosphere. The study included 43 examples of especially well defined layers having densities, nBL, such that (nmagnetosphere/nBL) ∼0.1 and (nBL/nmagnetosheath) ∼0.3, although both ratios were occasionally as low as 0.01. Recent observations (e.g., Burch et al., 1983) have indicated that electrons with energies in the 100 eV range are frequently exchanged between the ionosphere and the magnetosphere, traveling along the magnetic field lines near to the cusps and giving rise to Birkeland currents. Using three‐dimensional model‐independent distribution functions of electrons observed in the boundary layer, we have identified beams of field aligned electrons with energy maximum typically 100 eV, distributed over the energy range 50 eV to 200 eV, streaming in either one or both directions. The phase densities in the LLBL and at lower altitudes are similar, and assuming no further acceleration to have occurred, we identify the beams we observe in the LLBL with those observed at lower altitudes. They form an important component of the plasma electrons in the LLBL, which otherwise resembles a mixture of magnetosheathlike and magnetospheric electrons. The bidirectional streaming also suggests that the field lines threading the LLBL are often closed. The reduced distribution functions of electrons in the LLBL do not show positive slopes in the velocity range of the beams, suggesting that the plasma is stable to the growth of longitudinal electrostatic waves.
The second‐order theory of electromagnetic hot ion beam instabilities
Authors: Gary S. Peter , Tokar Robert L.
Published: February 2008

The wave‐particle interactions of a hot ion beam streaming along a magnetic field B are studied. A second‐order theory of electromagnetic instabilities in a homogeneous, collisionless plasma at propagation parallel to B is used. The two instabilities most likely to be driven by a hot beam are the right‐hand and left‐hand resonant ion beam instabilities. If the conditions necessary for the validity of the theory are met, the two modes are found to reinforce one another. That is, each mode acts to produce a beam anisotropy which reduces its own growth rate but enhances the growth rate of the other. Thus this theory predicts that whenever sufficiently hot or “diffuse” ions are found at a collisionless shock and the plasma is sufficiently homogeneous that significant wave growth is possible, ion beam instabilities will act to produce a mixture of both right‐ and left‐hand polarized MHD‐like waves.
On the distribution of θBn for shocks in the solar wind
Authors: Chao Jih K. , Chen Yih H.
Published: February 2008

The distribution of the shock normal angles, θBn, can be calculated for all the heliocentric distances when the distributions of
and
are available, where θBn is the angle between the upstream magnetic field
and the shock normal
. The average shock normal is assumed to propagate approximately in the radial direction. However, the distribution of the angles, θnr′ between the shock normals and radial direction is assumed to follow a Rayleigh distribution. The distribution of
is obtained by observations by Helios 1 and 2, Voyager 1, and Pioneer 10 spacecraft. The distribution of each component of
is assumed to follow a Gaussian. Our results show that even very close to the sun, the probability of observation of parallel and quasi‐parallel shocks is still smaller compared to the observation of quasi‐perpendicular and perpendicular shocks. Therefore it is concluded that the parallel and quasi‐parallel shocks do not easily form in the solar wind because of the fluctuating character of
and
and not because of any other physical mechanism. The observed distributions of θBn using Helios 1 and 2 at heliocentric distances between 0.3 to 0.5 AU, 0.5 to 0.75 AU, and 0.75 to 1.0 AU are compared with our calculated distributions. The agreement is good, with the agreement at 0.75 to 1.0 AU being the best. The comparison is also made with the results from ISEE 3 observations at 1 AU. Our calculated distribution agrees very well with these observations.
Doppler scintillation observations of interplanetary shocks within 0.3 AU
Authors: Woo Richard , Armstrong J. W. , Sheeley N. R. , Howard R. A. , Koomen M. J. , Michels D. J. , Schwenn R.
Published: February 2008

Near‐sun spacecraft Doppler scintillation observations have been combined with Solwind coronagraph and Helios 1 plasma measurements to provide more definitive measurements of the evolution and propagation of interplanetary shock waves between the sun and earth orbit than have been available from previous observations. This study shows that substantial deceleration of fast shocks (shock speeds exceeding 1000 km s−1) takes place near the sun and that the amount of deceleration increases with shock speed. This is consistent with the significantly lower and rather narrow range of shock velocities observed by direct spacecraft near earth orbit. When coronal mass ejection (CME) speeds are available for the fast shocks, they are considerably lower than the speeds measured farther out but near the sun. This implies that either the fast shocks first accelerate before decelerating on their way out from the sun (assuming the CME front is identified with the shock) or the CME speeds do not represent and substantially underestimate the shock speeds in the outer corona. If the CME speeds underestimate the shock speeds of the fast shocks, they do not appear to do so for the slow shocks. If the shocks are being driven over distances indicated by the acceleration region or to the point where deceleration begins, then their velocity profiles imply that the slower shocks are being driven farther out than the faster shocks. The analysis of one piston‐driven shock shows that the velocity of the contact surface is about 0.58 that of the shock front velocity.