bluuebunny/crossref_metadata_2025 huggingface dataset:
Wave dispersion and ray propagation in a weakly relativistic electron plasma: Implications for the generation of auroral kilometric radiation
Authors: Strangeway R. J.
Published: February 2008

Relativistic modifications to wave dispersion near the electron gyrofrequency are known to be important for sufficiently energetic electrons. In general, if the characteristic momentum of the energetic particles (po/moc) is greater than the ratio of the plasma frequency to the gyrofrequency (ωp/Ωe), wave propagation can be markedly different than the classical cold plasma limit. We investigate the wave dispersion in a weakly relativistic electron plasma, specifically in the context of the generation of auroral kilometric radiation (AKR). To do this, we have used a simple electron distribution consisting of a ring of energetic electrons at constant perpendicular momentum together with a stationary cold component. It is found that for such an electron distribution, where the two electron components have different gyrofrequencies due to relativistic effects, a new wave mode is introduced. This wave is trapped between the two electron gyrofrequencies. We find that the group velocity for such a wave is small; convective growth lengths for the wave are calculated to be of the order of 1 km. Consequently, ray propagation in a simple model for the density cavity present on auroral zone field lines is studied. We find that the ray paths tend to bring waves to altitudes where the growth rate maximizes; at these altitudes the waves propagate primarily across the ambient magnetic field. This could result in substantial amplification of the wave, since damping is minimized, and the waves may undergo multiple transitions across the auroral arc. However, the wave is trapped and must undergo mode conversion to escape from the auroral arc. Since the wave polarization is found to be very similar to the R‐X mode polarization, we deduce that coupling to the R‐X mode at the edge of the auroral arc may be reasonably efficient.
Some mass dependent features of energetic ion conics over the auroral regions
Authors: Collin H. L. , Johnson R. G.
Published: February 2008

The S3‐3 Lockheed ion mass spectrometer data set contains many examples of ion conics, a few of which have more than one species of ion measurable at energies between 0.5 and 16 keV. These events were examined, and some similarities and some differences were found between characteristics of the different ion components. In each event the ion fluxes peaked at about the same pitch angle for each species. If it can be assumed that the ions were energized primarily normal to the geomagnetic field, then the altitude of the energization process was the same for each species. However, there was no preferred altitude. In five of the events the conics were composed of H+ and O+, which were observed to have similar energy distribution functions. These were typically very soft but sometimes exhibited a high‐energy tail which extended to above 10 keV. In the event where He+ was observed, its distribution function was much harder, with substantial fluxes extending up to 16 keV, in marked contrast to the very soft H+ component of the conic.
An asymptotic state of the critical ionization velocity phenomenon
Authors: Goertz C. K. , Machida S. , Smith R. A.
Published: February 2008

We consider the problem of how the momentum of ions created by electron impact ionization of neutrals moving at a speed υ0 perpendicular to the magnetic field through a background plasma is coupled to this plasma. We find that the plasma accelerates and the relative velocity between neutrals and plasma decreases. If this decrease is rapid and large enough the critical ionization velocity (CIV) phenomenon may turn off. We derive equations for the evolution of plasma density, electron and ion thermal energy and plasma velocity. We find that the CIV process reaches an asymptotic quasi‐steady state in which the ionization rate reaches a constant value which depends on the properties of the surrounding medium and the value of υ0.
The dynamics of charged dust in the tail of comet Giacobini‐Zinner
Authors: Horanyi M. , Mendis D. A.
Published: February 2008

A numerical simulation has been performed to calculate the distributions of dust particles of various sizes down the tail of comet P/Giacobini‐Zinner. When the electrostatic charging of the grains in the plasma and radiative environment of the comet is taken into account, it is found that the distribution of the grains (particularly at the lowest end of the mass spectrum) in a plane normal to the orbital plane is entirely different from what is expected had the grains been uncharged. Although the NASA‐ICE spacecraft, which will fly through the tail of this comet almost normal to its orbital plane on September 11, 1985, has no dedicated dust experiments, it is expected that the plasma wave instrument will serve as an indirect detector, even of the smallest grains, via the plasma clouds created by the high‐velocity dust impacts. Knowledge of the spatial variation of the grain sizes encountered along the flight path of the spacecraft will provide us with the information necessary to calculate the electrostatic potential of the grains, which in turn will lead to an estimation of the role of the electromagnetic forces on the dynamics of such grains.
On the applicability of relativistic dispersion to auroral zone electron distributions
Authors: Strangeway R. J.
Published: February 2008

The effect of including relativistic corrections in the dispersion relation for a low‐density plasma consisting of moderately energetic electrons (energy ∼2.5 keV) is investigated. Such a low‐density plasma is presumed to exist at those altitudes on auroral zone field lines where auroral kilometric radiation is generated. Two different types of dispersion relation are employed for the purpose of studying the wave dispersion. The simpler of these is a “ring” distribution, in which both ambient and hot electrons are assumed to be cold (i.e., have no thermal spread associated with them). Using this dispersion relation, we find that for sufficiently low densities, the most unstable mode is a “trapped” mode that is decoupled from the freely propagating R‐X mode found in a cold plasma. Since the dispersion relation neglects the effects of temperature, we also study wave dispersion using a spherically symmetric Dory‐Guest‐Harris (DGH) distribution. This “shell” distribution shows that provided the peak momentum of the DGH distribution is larger than the thermal spread, the most unstable mode at low wave vectors is still the trapped mode. Our analysis indicates that the trapped mode is usually the most unstable mode and the freely propagating R‐X mode is not driven unstable, when the wave dispersion is assumed to be given by a weakly relativistic dispersion relation. We show that relativistic effects may be important for typical auroral electron distributions, and that there is insufficient evidence from particle data to justify the usual assumption that the cold plasma approximation is adequate when describing wave dispersion near the gyrofrequency on auroral zone field lines.
Ion thermal noise in the solar wind: Interpretation of the “excess” electric noise on ISEE 3
Authors: Meyer‐Vernet N. , Couturier P. , Hoang S. , Steinberg J. L. , Zwickl R. D.
Published: February 2008

The excess electric noise component with a ω−1 power spectrum, previously identified on ISEE 3 below the plasma frequency, is quantitatively interpreted. The theory is based on quasi‐thermal electron noise to which two effects are added: (1) ion quasi‐thermal noise Doppler‐shifted by the solar wind velocity Vsw, and (2) less important, particle impacts on and emission from the antennas. The first effect yields a component which is highly spin modulated with the electric field maximum when the antenna is perpendicular to Vsw. This result, which occurs in general when the parameter ωL/Vsw is large, contradicts a hand waving argument often found in the geophysical literature. This theory is in excellent agreement with the observations for the long S wire antenna (L = 45 m) on which the impact noise is negligible. For the Z antenna which has a larger surface area, the contributions of both effects are necessary to interpret the measurements. These results could be used to give an independent measurement of the solar wind velocity magnitude and direction whenever the plasma frequency exceeds the lowest frequency channel (30 kHz) of the receiver.
Adiabatic theory in regions of strong field gradients
Authors: Whipple E. C. , Northrop T. G. , Birmingham T. J.
Published: February 2008

We develop the theory for the generalized first invariant for adiabatic motion of charged particles in regions where there are large gradients in magnetic or electric fields. The general condition for an invariant to exist in such regions is that the potential well in which the particle oscillates change its shape slowly as the particle drifts. We show how the Kruskal (1962) procedure can be applied to obtain expressions for the invariant and for drift velocities that are asymptotic in a smallness parameter ε. We illustrate the procedure by obtaining the invariant and drift velocities for particles traversing a perpendicular shock, and we compare the generalized invariant with the magnetic moment, and the drift orbits with the actual orbits, for a particular case. In contrast to the magnetic moment, the generalized first invariant is better for large gyroradii (large kinetic energies) than for small gyroradii. We also give expressions for the invariant when an electrostatic potential jump is imposed across the perpendicular shock, and when the particle traverses a rotational shear layer with a small normal component of the magnetic field.
ISEE observations of the plasma sheet boundary, plasma sheet, and neutral sheet: 2. Waves
Authors: Cattell C. A. , Mozer F. S. , Anderson R. R. , Hones E. W. , Sharp R. D.
Published: February 2008

The companion paper describes the dc electric and magnetic fields, E × B velocities, plasma flow, composition and ground magnetic activity for two periods in the magnetotail near 20 RE. The relationship of the plasma waves to the dc field characteristics is described herein. In addition, for two subintervals which had strong ground magnetic activity, we report comparisons of plasma waves in the magnetotail to characteristics of the dc fields and fluid parameters such as the plasma flow, β, density, temperature, and fpe/fce. Upper hybrid waves at multiples of the electron cyclotron frequency near and above the electron plasma frequency were observed for the first time in this region of the magnetosphere and occurred in regions of low β and temperature and high density. This contrasts with electrostatic electron cyclotron waves which occurred in high‐temperature, high‐density regions, as previously reported by Gurnett et al. (1976), and were suppressed when β ≲ 1. Both types of waves were associated with small dc electric fields, nonturbulent dc electric and magnetic fields, and plasma flows of ≲100 km/s. Broadband electrostatic noise (BEN) and magnetic noise bursts were observed in regions where the dc electric fields and the plasma flows were large, and the dc electric and magnetic fields were turbulent. The dependence of the amplitude and frequencies of BEN on the magnitude of the magnetic field, plasma flow, density, and β was very complex. Enhancements at the upper hybrid frequency and the electron cyclotron frequency were common. BEN was usually suppressed near the neutral sheet but the amplitude reduction varied. The fact that the magnetic noise was most intense during tailward flow and often was not suppressed near the neutral sheet suggests that it may provide the dissipation necessary for reconnection to occur.
Magnetic field directional discontinuities: Characteristics between 0.46 and 1.0 AU
Authors: Lepping R. P. , Behannon K. W.
Published: February 2008

The characteristics of directional discontinuities (DDs) in the interplanetary magnetic field have been studied using data from the Mariner 10 primary mission. The entire data set was surveyed using an automated procedure to identify DDs as changes in field direction of at least 30° in a 42‐s interval. This study yielded an r−1.3±0.4 heliographic distance dependence for the daily average number of discontinuities per hour. In addition to this statistical survey, DDs were visually identified using 1.2‐s averages for three selected time intervals, and the corresponding 40‐ms data were studied in detail by means of the Sonnerup‐Cahill variance procedure. The three time intervals were November 4–17, 1973, February 5–12 and April 3–10, 1974, corresponding to the equally spaced heliocentric distances of 1.0, 0.72, and 0.46 AU. Editing the candidate DDs produced a total of 644 events. Two methods were used to estimate the ratio of the number of tangential discontinuities (TDs) to the number of rotational discontinuities (RDs). In the first approach those DDs with substantial normal components (Bn/〈B〉 ≥ 0.3) were interpreted as RDs, and the remainder were considered to be TDs, except that some RDs were eliminated on the basis of unacceptably large relative magnitude variances across their discontinuity zone. The second method considered the total number of RDs to be the sum of those DDs with substantial normal components (again excluding those with large variances) plus an estimated number of those DDs with small normal components. The estimate was based on the assumption that there is a uniform distribution of RDs per degree of discontinuity cone angle β (=cos−1 |Bn|/B, where B is the average magnitude of the field across the DD) for all β. Then all other DDs are assumed to be TDs. Both methods showed that the ratio of TDs to RDs decreased with decreasing radial distance, being 1.5, 1.2, and 0.86 for the distances 1.0, 0.72, and 0.46 AU, respectively, from the first method and 0.89, 0.66, and 0.43, respectively, from the second (preferred) method. A decrease in average discontinuity thickness of 41% was found between 1.0 and 0.72 AU and 56% between 1.0 and 0.46 AU, independent of type (TD or RD). This decrease in thickness for decreasing r is in qualitative agreement with Pioneer 10 observations between 1 and 5 AU. When the individual DD thicknesses were normalized with respect to the estimated local proton gyroradius (RL), the average thickness at the three locations given above was nearly constant, 36±5 RL. This held true for both RDs and TDs separately. Statistical distributions of these and other properties, such as normal directions and discontinuity plane angles (ω), are presented. No obvious relationship was found between ω and the thickness of either TDs or RDs when widely separated locations were examined.
Altitude variation of EUV emissions and evidence for proton precipitation at low latitudes in the Saturnian atmosphere
Authors: Yelle R. V. , Sandel B. R. , Shemansky D. E. , Kumar S.
Published: February 2008

Extreme ultraviolet observations of Saturn from Voyager 1 and 2 are analyzed. The Lyman alpha and H2 band emissions extend throughout the upper atmosphere, from the hydrocarbon homopause to well above the exobase. Analysis of the Lyman alpha emissions with a radiative transfer model indicates that the Lyman alpha source temperature is very high. This suggests that energetic protons or hydrogen atoms are responsible for a fraction of the emissions. Calculation of the solar‐scattered component of the emissions based on the neutral atmosphere of Smith et al. (1983) reveals that only 1–2 kR out of a total of 3.5 kR of the observed Lyman alpha intensity is due to solar scatter for the Voyager 2 disc observations; the remainder of the Lyman alpha emissions are collisionally excited. The Lyman alpha and H2 bands are constant in longitude but decrease in local time by a factor of 2 from dawn to dusk. This correlation of the Lyman alpha and H2 band intensities is further evidence that most of the Lyman alpha is collisionally excited.
The centrifugal flute instability and the generation of Saturnian kilometric radiation
Authors: Curtis S. A. , Lepping R. P. , Sittler E. C.
Published: February 2008

We show that the source region for Saturnian kilometric radiation (SKR) which originates in the high latitude near‐noon dayside ionospheric can be mapped via the Saturn magnetic field to the outer edge of the dayside equatorial plasmasheet. The plasmasheet is known to be unstable at this boundary due to the outward centrifugal forces generated by the heavily mass‐loaded equatorial Saturnian magnetosphere. Also previous work has shown that Voyager 1 observations of MHD waves near the plasmasheet are consistent with their being driven by the centrifugal flute instability. The resulting field‐aligned MHD waves at higher latitudes can trap and accelerate electrons. For the plasma environment measured by Voyager, field‐aligned accelerated electrons up to energies of several keV are expected to be deposited into the SKR ionospheric source region. A necessary condition for the acceleration of the electrons to several keV is a population of hot electrons inside of the plasmasheet, near the unstable boundary. The hot electrons are produced as a byproduct of the pick‐up of exospheric atoms from Saturn's moons. The solar wind dependence of the SKR is thus a consequence of the elongation of the Saturn nightside magnetosphere with greater solar wind pressure leading to greater centrifugal pressures on the nightside plasmasheet. Although the theory presented correctly gives the local time dependence of the emissions, we suggest that the strong emission peak at a fixed Saturnian longitude is a consequence of locally reduced electron plasma frequency to electron gyrofrequency ratio. The lack of a nightside source of SKR is consistent with the disruption of the plasmasheet due to loss of plasma down the Saturn magnetotail.
Magnetostrophic balance in planetary dynamos: Predictions for Neptune's magnetosphere
Authors: Curtis S. A. , Ness N. F.
Published: February 2008

In order to obtain an improved estimate of Neptune's magnetic field and hence the size and structure of the magnetosphere, a new scaling law for planetary magnetic fields has been developed. Starting from magnetostrophic balance between the coriolis force and the j × B ponderomotive force, we have derived a scaling relation which can be used to calculate magnetic field strengths using only the observable properties of a planet. Specifically, using the planet's mean density, radius, mass, rotation rate and internal heat source luminosity, we can obtain an estimate of the magnitude of the planet's magnetic field from the same parameters for the earth and earth's magnetic field. The estimated magnetic field is, however, an upper bound and good agreement with observations is expected only if the planet's dynamo is fully developed. This is apparently true for earth, Jupiter, Saturn, and Mercury for which good agreement is obtained between predicted and observed magnetic fields. In contrast, the moon, Venus, and Mars apparently lack currently active internal dynamos and force balance may not hold. From a comparison of theory and observations, we conclude that planetary dynamos are two state systems with either zero intrinsic magnetic field or a field near the upperbound determined from magnetostrophic balance. The calculated upper limit for the Uranus magnetic field is 0.3 G at the equator 1 bar level. The value for Uranus' magnetic field reported by Ness et al. based on Voyager observations is 0.23 G. Given this excellent agreement between calculation and observation and also noting that Neptune possesses a large internal heat source, we expect Neptune's magnetic field at the 1 bar level to be between 0.5–0.4 G. The expected bow shock stand‐off distances range from 20–40 RN (Neptune radii). Hence, Neptune's satellite Triton at 14.6 RN lies within the magnetosphere. Nonthermal radio emission generation may be possible in regions of sufficiently low polar ionospheric plasma densities. Finally, we show that agreement between observed and predicted magnetic fields derived from Blackett's “Bode's law” scaling relation are fortuitous and are owing to a weak sensitivity of predictions to some combinations of parameters appearing in our scaling law.
Plasma diagnostics package measurements of ionospheric ions and shuttle‐induced perturbations
Authors: Reasoner David L. , Shawhan Stanley D. , Murphy Gerald
Published: February 2008

The plasma diagnostics package (PDP) on the space shuttle STS‐3 mission in March 1982 carried among its instrument complement a retarding potential analyzer. This instrument measured both the ambient ion plasma density and temperature, and perturbations to the plasma produced by shuttle orbiter effects. Whenever the plasma flow streamline at the instrument was more than a distance of the order of thermal ion gyroradii away from any orbiter surface, the measurements were characteristic of the ambient ionosphere. In several situations, the PDP was positioned so as to scan the wake in the plasma flow produced by orbiter surfaces. The density profile of the major species O+ was consistent with a classic Mach cone. However, strong perturbations extended for several meters outside the Mach cone which resulted in failure of flowing Maxwellian distributions to represent the data. Configurations where the plasma flow impacted orbiter surfaces downstream of the PDP resulted in generation of a suprathermal ion component. The observations are discussed in terms of a recent model of the mechanism for generation of shuttle glow.
Effect of dissociative electron‐ion recombination on the propagation of critical ionization discharges
Authors: Murad Edmond , Lai Shu
Published: February 2008

Dissociative recombination of electron with molecular ions may be fast enough to interfere with the propagation of effects of critical ionization velocity in the shuttle environment. It may quench the plasma Discharge and impose limits on the conditions for the plasma discharge process in the shuttle glow. Nevertheless, ionization and dissociative recombination processes may still take place, albeit in a non‐sustained manner. We suggest that these competing processes may be detected and identified by spectroscopic measurements.
Modification of the ionospheric electron velocity distribution function due to resonant absorption of HF waves
Authors: Shoucri Merit , Morales G. J. , Maggs J. E.
Published: February 2008

The nonlinear modification of the electron distribution function caused by the interaction with a resonant electrostatic field is studied analytically using a perturbation analysis. This interaction produces tail heating and for a limited range of parameter values can form a bump in the distribution, which may lead to emission of secondary waves. These results are applied to the ionosphere's F and upper E regions in the context of ionospheric modification experiments using ground‐based powerful HF transmitters.
Resonance between coherent whistler mode waves and electrons in the topside ionosphere
Authors: Neubert T. , Bell T. F. , Storey L. R. O.
Published: February 2008

Landau resonance and cyclotron resonance of coherent whistler mode waves and energetic electrons are explored for magnetoplasmas with appreciable gradients in the plasma density and magnetic field strength. It is shown that in the topside ionosphere of the earth near the ion transition height the gradients in plasma density and magnetic field strength along a magnetic field line may match in a way which enhances both Landau and cyclotron interactions between waves and electrons at the loss cone pitch angle. The pitch angle scattering induced by a signal from a ground‐based VLF transmitter in the ionosphere above the transmitter has been estimated and compared to the pitch angle scattering induced by naturally occurring ELF hiss through cyclotron resonance. It is found that the expected scattering due to plasmaspheric hiss is an order of magnitude larger than that due to Landau resonance in the topside ionosphere. Pitch angle scattering due to cyclotron resonance in the topside ionosphere, however, may be larger by a factor of 2. We suggest that the “fast Trimpi” effect may be caused by a cyclotron resonance interaction in the topside ionosphere.
Upper limit on the intrinsic magnetic field of Venus
Authors: Phillips J. L. , Russell C. T.
Published: February 2008

A data set of 18,000 low‐altitude nightside vector measurements from the Pioneer Venus Orbiter (PVO) magnetometer is used to establish a new upper limit on the intrinsic dipolar magnetic moment of Venus. Various techniques are used to search for coherence in the magnetic observations which might be attributable to such an intrinsic field, with negative results. A similar search is conducted for surface magnetism, that is, geographically correlated magnetic features, also with negative results. The present study represents the best limits that PVO observations can provide and is limited by the amount of low‐altitude nightside data available. We do not expect to obtain a significant amount of new low‐altitude data later in the mission. Our new upper limit to the magnetic moment is 8.4 × 1010 T m³ or about 10−5 of the terrestrial magnetic moment. This is a factor of 4 lower than the previous upper limit.
Plasma observations at the Earth's magnetic equator
Authors: Olsen R. C. , Shawhan S. D. , Gallagher D. L. , Green J. L. , Chappell C. R. , Anderson R. R.
Published: February 2008

The magnetic equator provides a unique location for thermal plasma and plasma wave measurements. Plasma populations are found to be confined within a few degrees latitude of the equator, particularly the ions. The equatorially trapped ion population is found to be primarily hydrogen, and we find little evidence for preferential heating of heavier ions. Helium is occasionally found to be heated along with the protons, and forms about 10% of the equatorially trapped populations at such times, similar to the percentage of He+ in the cold, core plasma of the plasmasphere. One case of a heated O+ component was found; at the 0.1% level it generally comprises in the outer plasmasphere core plasma. The heated H+ ions can be characterized by a bi‐Maxwellian with kT∥ = 0.5–1.0 eV, and kT = 5–50 eV, with a density of 10–100 cm−3. The total plasma density, as inferred from the plasma wave instrument measurements of the upper hybrid resonance (UHR), is relatively constant with latitude, occasionally showing a local minimum at the magnetic equator, even though the ion flux has increased substantially. The first measurements of the equatorially trapped plasma and coincident UHR measurements show that the trapped plasma is a feature of the plasmapause region, found at total plasma densities of 20–200 cm−3. The warm, trapped plasma is found in conjunction with equatorial noise, a plasma wave feature found at frequencies near 100 Hz, with a broad spectrum generally found between the proton gyrofrequency at the low frequency edge and the geometric mean gyrofrequency at the high frequency edge. This latter frequency is generally the lower hybrid resonance (LHR) for a proton‐electron plasma. Sharp spatial boundaries are occasionally found with latitude, delimiting the equatorially trapped plasma. In such cases, the equator is a region with a relative minimum in density, and it appears that field‐aligned ions found at higher latitudes are “bounced” from these boundaries, indicating a positive plasma potential of a volt or two in the equatorial region.
Nearly simultaneous measurements of radar auroral heights and doppler velocities at 398 MHz
Authors: Moorcroft D. R. , Ruohoniemi J. M.
Published: February 2008

Nearly simultaneous measurements of radar auroral heights and Doppler velocities were obtained using the Homer, Alaska, 398‐MHz phased‐array radar over a total of 16 hours on four different days. The heights show a consistent variation with time, being highest near the time of electrojet current reversal, and lowest late in the morning. A variety of east‐west height asymmetries were observed, different from those previously reported, which can be explained in terms of favorable flow angles preferentially favoring high‐altitude primary two‐stream waves to one side of the field of view. Low‐velocity echoes, presumably due to secondary irregularities, are found to be more restricted in height range than echoes with ion acoustic velocities, which presumably come from primary two‐stream instabilities. Echo power was examined as a function of velocity and height. For the westward electrojet it was found that echoes with ion acoustic velocities are relatively constant in strength over most of their height range, but for low‐velocity echoes the power is a maximum between 100 and 105 km and falls off steadily at greater heights. Doppler speeds show a noticeable decrease at heights below 105 km, in agreement with the expected variation in ion acoustic velocity.
Resonant Alfvén waves on auroral field lines
Authors: Chiu Y. T.
Published: February 2008

It is shown that resonant Alfvén waves on dipole magnetic field geometry and plasma distributions suitable for auroral field lines can be conveniently treated in the theory of Mathieu functions. Resurgent interest in invoking large‐scale Alfvén waves to structure some elements of auroral electrodynamics calls for interpretation of measured perpendicular electric and magnetic disturbance fields in terms of Alfvén waves. The ability to express the resonant eigenmodes in closed form in terms of Mathieu functions allows for convenient tests of the Alfvén wave structuring hypothesis. Implications for current vector electric and magnetic disturbance measurements are discussed.
Electron precipitation in the vicinity of a VLF transmitter
Authors: Vampola A. L.
Published: February 2008

Using high‐resolution pitch angle measurements made by a magnetic focusing electron spectrometer on the S3‐3 satellite, angular distributions of 235‐keV electrons precipitated in the slot region of the magnetosphere by a ground‐based VLF transmitter are compared with the pitch angle distributions that would be produced by various patterns of longitudinal interaction regions. The observed electrons are in the drift loss cone, necessitating the use of a trace‐back‐to‐longitude‐of‐origin technique coupled with a two‐dimensional convolution program describing the response of the electron spectrometer. The data are well fit both with theoretical calculations of ionospheric field intensity patterns above a transmitter and with a similar pattern of received field intensities measured along a traverse in the conjugate region. The agreement between the data and field patterns implies a linear or quasi‐linear wave‐particle interaction. The energy‐frequency relationship between the electrons and the waves implies an interaction region low on the magnetic field line rather than near the equator, as has been determined for similar precipitations in the inner zone.
The role of ring current O+ in the formation of stable auroral red arcs
Authors: Kozyra J. U. , Shelley E. G. , Comfort R. H. , Brace L. H. , Cravens T. E. , Nagy A. F.
Published: February 2008

Coulomb collisions between ring current protons and thermal electrons were first proposed by Cole (1965) as the energy source for stable auroral red (SAR) arcs. Recent observations have shown that the ring current and magnetospheric plasma contain significant amounts of heavy ions (Johnson et al., 1977; Young et al., 1977; Geiss et al., 1978; and others). In fact, the ring current is often dominated by heavy ions at those energies (E ≤ 17 keV) important for Coulomb collisions on SAR arc field lines (Kozyra et al., 1986a). Observations (during four SAR arcs in 1981) of thermal and energetic ion populations by the Dynamics Explorer 1 satellite in the magnetospheric energy source region and nearly simultaneous Langmuir probe measurements of enhanced electron temperatures by Dynamics Explorer 2 within the SAR arc at F region heights have allowed us to examine the role of heavy ions in the formation of SAR arcs. We find that (1) sufficient energy is transferred to the electron gas at high altitudes via Coulomb collisions between the observed ring current ions and thermal electrons to support the enhanced (SAR arc) F region electron temperatures measured on these field lines, (2) the latitudinal variation in the electron heating rates calculated using observed ion populations is consistent with the observed variation in electron temperature across the SAR arc, and (3) in all cases, ring current O+ is the major source of energy for the SAR arcs. This implies a relationship between the heavy ion content of the magnetospheric plasma and the occurrence frequency and intensity of SAR arcs.
Solar activity and heliosphere‐wide cosmic ray modulation in mid‐1982
Authors: Cliver E. W. , Mihalov J. D. , Sheeley N. R. , Howard R. A. , Koomen M. J. , Schwenn R.
Published: February 2008

A major episode of flare activity in June and July of 1982 was accompanied by a pair of heliosphere‐wide cosmic ray modulation events. In each case, a large Forbush decrease (FD) at earth was followed in turn by apparently related decreases at Pioneer 11 (P11) and Pioneer 10 (P10). The Pioneer spacecraft were separated by ∼155° in ecliptic longitude. We reviewed white light coronagraph and near‐sun (≤1 AU) satellite data to identify plausible solar origins of these modulation events. The first widespread intensity decrease (FD 1) can be attributed to the combined effects of a backside flare on June 3 from solar active region 18382/18383, located 23° in ecliptic longitude from Pioneer 10, and a visible disk flare from 18405 on June 6, when this region was 9° from Pioneer 11. The second widespread modulation event during this period (FD 2) may be linked to flares from active region 18474 on July 12 and 22. The July 12 flare was located 34° in azimuth from Pioneer 11, and the July 22 flare was 24° from Pioneer 10. Since even fast shocks would take ∼1 month to propagate to Pioneer 11 (12 AU) and ∼2 months to reach Pioneer 10 (28 AU) in mid‐1982, these “one‐to‐one” associations must be regarded with caution. The processes of entrainment and coalescence can cause a given traveling interplanetary disturbance to lose its identity enroute to the outer heliosphere. The fact that we were able to identify plausible solar flare candidates for each of the four Forbushlike decreases observed at the Pioneer satellites (two each at P10 and P11), however, removes the need to invoke a shock from a single flare as the sole cause of either FD 1 (at both P10 and P11) or FD 2. Such single‐flare scenarios have recently been suggested by several investigators to account for the widespread intensity decreases in mid‐1982. Instead, the heliosphere‐wide modulation during this period appears to result primarily from a sustained episode of powerful flares from a relatively narrow range of active solar longitude. A significant fraction (1/2 to 3/4) of the major coronal mass ejections (CMEs) and near‐sun shocks observed during June and July 1982 originated in flares occurring in a 45° band of Carrington longitude. Because of solar rotation these flares occur over the full range of ecliptic longitude and can generate an outward propagating shell of CMEs and shocks that encompasses the sun to produce the observed azimuthal symmetry in the cosmic ray modulation. The prolonged high‐speed wind stream at P10 in the second half of 1982 may have resulted, at least in part, from the coalescence of a series of fast transient streams directed toward that distant spacecraft into an extended compound stream.
Analysis of Martian ionosphere and solar wind electron gas data from the planar retarding potential analyzer on the Viking spacecraft
Authors: Mantas G. P. , Hanson W. B.
Published: February 2008

Approximate expressions for the electron current collected by a planar retarding potential analyzer (RPA) mounted on a moving, conducting, charged spacecraft are derived. They are utilized for the analysis of electron current data obtained by the RPAs on the Viking spacecraft in the ionosphere of Mars and in the disturbed and undisturbed solar wind near this planet. It is shown that contamination currents by photoelectrons emitted from the spacecraft can be distinguished and removed from the signal. Parameters deduced from the analysis of RPA electron sampling data are the multicomponent electron temperatures, the number densities, and the spacecraft potential. This paper seeks only to establish the usefulness of the technique by application to a representative data set.
Observations of intense trapped electron fluxes at synchronous altitudes
Authors: Davidson G. T. , Filbert P. C. , Nightingale R. W. , Imhof W. L. , Reagan J. B. , Whipple E. C.
Published: February 2008

The Lockheed high‐energy particle spectrometer on the SCATHA satellite, P78‐2, occasionally measures energetic electron fluxes near synchronous altitude that exceed the commonly cited Kennel and Petschek trapping limit. Twelve cases have been analyzed in detail, including several from relatively quiet times and from times when the spectrum became very hard. These were compared with an energy‐dependent formulation of the trapping limit (Schulz and Davidson, this issue), which predicts a steeply descending spectrum above the minimum resonant energy for wave growth, and a 1/E spectrum at high energies. The spectrometer permits pitch angle resolution fine enough to resolve the loss cone and to derive the anisotropies that were needed to make the comparisons with the theory. In all the cases examined, over a wider range of conditions, there is a region of the spectrum, often less than a decade wide in energy, that matches the theoretical spectrum. The plasma densities derived from the fitting of ideal theoretical spectra to the measured spectra were compared with ion densities determined from the University of California, San Diego, ion spectrometer aboard the SCATHA satellite. The agreement between the two independent determinations of the plasma density supports an interpretation of the energetic electron spectra based on the occurrence of flux limiting during the events analyzed. It is concluded that flux limiting may be active much of the time over some portion of the electron spectrum, and that the overall spectrum exhibits greater variability than can be simply explained by steady state flux limiting alone. The results are consistent with a model in which the limiting process takes place in sporadic precipitation events, punctuated by intervals of weak diffusion. Numerical estimates of the energy‐integrated flux above the minimum resonant energy agree well with the Kennel and Petschek prediction. A new empirical energy‐integrated flux limit of I* ≈ 2 × 1011/L4 electrons cm−2 s−1 is suggested.
An observational study of MHD wave‐induced density fluctuations upstream of the Earth's bow shock
Authors: Spangler Steven , Fuselier Stephen , Fey Alan , Anderson Gregory
Published: February 2008

A study has been made of the characteristics of MHD waves and associated density fluctuations in the Earth's foreshock. Data from two periods were analyzed. In one period the waves were circularly polarized, of 20% fractional amplitude, and propagating in a plasma with β less than one. In the second period the waves were elliptically polarized, of 50% fractional amplitude, and the plasma β was in excess of unity. The following results emerge. (1) For both periods there is a prominent feature in the density power spectrum at the same frequency as that characterizing the dominant magnetic field fluctuations. We attribute this feature to oblique wave propagation. (2) Ponderomotive effects are responsible on both days for the generation of density fluctuations on scales comparable to the wave packet sizes. There is observational evidence for modulational instability of the MHD waves on the day with β in excess of unity. (3) Evidence for a parametric decay instability was not observed, but conditions were only marginally favorable for such an observation. (4) Generation of density fluctuations by linearly polarized waves does not seem to be an important contributor to the observed density fluctuations.
Upper hybrid and Langmuir turbulence in the auroral E region
Authors: Kelley Michael C. , Earle Gregory D.
Published: February 2008

Oscillations at a frequency between the local upper hybrid and plasma frequencies have been detected in the upper E region of the morning auroral oval. The emission occurs in a narrow band of frequencies when the dipole antenna is nearly perpendicular to B but broadens when the antenna has a component parallel to B. The waves have a low‐altitude cutoff at about 125–130 km, and their intensity increases in regions of low plasma density. No theoretical explanation yet exists for these waves, which may be the manifestation of a plasma instability process due to the suprathermal electron flux which accompanies collisional ionization in the auroral E region.
Isolated electron precipitation regions at high latitudes
Authors: Imhof W. L. , Voss H. D. , Datlowe D. W. , Mobilia J.
Published: February 2008

An investigation has been made at high latitudes of regions or patches of isolated electron precipitation with bremsstrahlung X ray images (>4 keV) and simultaneous particle and 391.4‐nm data acquired with the Stimulated Emission of Energetic Particles payload on the S81‐1 satellite. More extensive analyses of the phenomena than were originally reported (Imhof et al., 1985) have now been made. From this investigation it has been found that when some patch regions are processed with greater sensitivity, nearby weaker patches are found. This characteristic suggests that some of them may be parts of larger structures. There is considerable overlap in spectral shape between patches and larger structures, but the energy spactra of the X rays emitted from the patches are, on the average, softer, even at similar invariant latitudes. Although some patches may be associated with auroral substorms as indicated from comparisons with the AE index, the majority are not and therefore appear not be related to westward traveling surges. Most patches are significantly narrower in one direction, consistent with arc‐shaped precipitation. Fine structure is present in some patches, and study of the bremsstrahlung X ray measurements indicates that on a statistical basis many of the spikes associated with the fine structure are spatial and not temporal in nature, suggesting multiple arcs. Also consistent with arc structure, the local time variations have been found to display a similarity to those associated with inverted‐V events, but the measured electrons are at higher energies than are normally associated with such events. In summary, at least some of the small precipitation patches reported here seem to be parts of larger structures which may encompass a variety of forms, including those that are arc‐shaped in nature.
Observations of electron beam structure in space experiments
Authors: Banks Peter M. , Raitt W. John
Published: February 2008

Observations of a 100‐mA, 1‐keV electron beam were made with a low light level TV system and other instruments aboard the space shuttle STS 3 in March 1982. When fired at a moderate pitch angle with respect to the magnetic field, the beam electrons initially moved outward along a classical helical trajectory. Within a short distance, however, the beam electrons were deflected away from the helical trajectory into paths parallel to magnetic field lines threading the primary helix. This resulted in the formation of a thin‐walled, cylindrical electron beam structure having a radius equal to the primary beam gyroradius. This structure, which is consistent with earlier laboratory vacuum chamber observations and recent two‐dimensional computer electrostatic plasma simulations, has important implications for determining the extent to which pulsed electron beams can radiate electromagnetic energy in the ionosphere.
Large‐scale ionospheric structures associated with mid‐latitude spread F
Authors: Bowman G. G.
Published: February 2008

This paper contains a brief survey of some of the papers, dating back to the 1950s, which support the concept of large‐scale structures being associated with the phenomenon of ionospheric spread F. Then, three different recording techniques, which are effective in revealing evidence of large‐scale structures at times of spread F, are illustrated and discussed. These techniques involve (1) sweeping the gain of the receiver, (2) imposing phase fringes on ionogram traces, and (3) identifying interference minima resulting from overlapping traces.
Viking electron temperature measurements: Evidence for a magnetic field in the Martian ionosphere
Authors: Hanson W. B. , Mantas G. P.
Published: February 2008

Further analysis of the Viking RPA data has now provided measurements of the thermal electron temperature in the upper Martian ionosphere. It is found that Te is several thousand degrees K, i.e., only of the order of twice the ion temperature. The sum of all the measured partial plasma pressures, including ions and suprathermal electrons, has a minimum value of ∼5×10−10 dyn cm−2 near 350 km and is found to be insufficient to balance the measured electron pressure in the shocked solar wind near 1000 km altitude, by a factor of the order of 4. Thus there is no doubt that a magnetic field of at least 30 to 40 nT permeates the ionosphere. This conclusion is not inconsistent with previous assessments, but it now has a firm observational basis. These data do not uniquely establish whether the magnetic field is intrinsic or induced, but our assessment is that a significant intrinsic moment is not required.
Simultaneous observation of upward moving field‐aligned energetic electrons and ions on auroral zone field lines
Authors: Hultqvist B. , Lundin R. , Stasiewicz K. , Block L. , Lindqvist P.‐A. , Gustafsson G. , Koskinen H. , Bahnsen A. , Potemra T. A. , Zanetti L. J.
Published: February 2008

We present Viking observations of energetic electrons and ions which move upward along the magnetic field lines with energies of the same order of magnitude. The field‐aligned particles are associated with a strong broadband low‐frequency turbulence in the electric field and with downward Birkeland currents. We suggest that both ions and electrons are accelerated by a time‐varying field‐aligned electric field component. The downward field component accelerating the electrons upward is experienced as quasi‐static by the electrons while they remain in the height‐limited acceleration region. For the ions the fluctuating field is a wave field which has only a small translational effect on them.
Stationary auroral current oscillations resulting from the magnetospheric generator
Authors: Weimer D. R. , Maynard N. C. , Burke W. J. , Sugiura M.
Published: February 2008

An instance of steady state spatial current oscillations in auroral field‐aligned currents has been detected with the two Dynamics Explorer (DE) satellites. The oscillations are apparent in both the north–south electric fields and the east–west magnetic fields measured with DE 1 and DE 2 near a magnetic conjunction between the two satellites. Both satellites measured very similar large‐scale oscillations at the same location but separated in time by about 20 min. Three cycles with an average wavelength of approximately 266 km were detected. These waves, which consisted of alternating sheets of upward and downward currents, were imbedded within the region 2 current system near 13 hours local time, at 66° to 73° invariant latitude. An explanation for these oscillations has been developed in terms of a simple steady state model of the magnetosphere‐ionosphere current system. The wavelength of the oscillations is found to be dependent on the ionospheric height‐integrated Pedersen conductivity, the magnetic‐field‐aligned conductivity, and the conductivity of a generator at the magnetospheric equator. Stationary current oscillations may exist if the magnitude of the generator conductivity is less than the ionospheric Pedersen conductivity. There is good agreement between this model and the measurements from the DE satellites. The conductance of the magnetospheric generator is also found to be consistent with the magnetohydrodynamic equations.