bluuebunny/crossref_metadata_2025 huggingface dataset:
Direction finding of half‐gyrofrequency VLF emissions in the off‐equatorial region of the magnetosphere and their generation and propagation
Authors: Muto H. , Hayakawa M. , Parrot M. , Lefeuvre F.
Published: February 2008

The spectra of VLF/ELF emissions in the off‐equatorial region in the outer magnetosphere are, on occasion, found to consist of upper and lower bands with a frequency gap between them. The wave normal directions of the upper band VLF emissions have been determined by means of wave distribution function analyses based first on the measurement of three magnetic field components only, and then with the additional use of an electric field component. It is found that the wave distribution functions are composed of two peaks whose central wave normal angles θ with respect to the magnetic field at a geomagnetic latitude of ∼17° are close to the local oblique resonance cone θres but whose values of θ are always about 15°–20° less than θres at a higher latitude of ∼26°. This observed wave normal behavior is compared with the theoretical prediction from direct ray tracing studies using the hypothesis deduced from a previous investigation that the waves are generated at the equator with wave normals close to θres. A combination of this comparison and inverse ray tracing in which the initial wave normal directions are those determined from the wave distribution analysis has yielded the result that the θ values of the upper band VLF emissions are very close to θres at the equator and that the normalized frequency there, Λeq, is above 0.5. Hence the upper band VLF emissions are identified as being “half‐gyrofrequency” VLF emissions generated in the vicinity of the magnetic equator, with their wave normals close to θres. The characteristics of propagation between the equatorial source region and the observing positions have been investigated by direct ray tracing. The generation mechanism is also discussed.
Spin modulation of spectrally broadened VLF signals
Authors: James H. G. , Bell T. F.
Published: February 2008

The spectra of broadened VLF signals from ground transmitters received on the spinning ISIS spacecraft have a characteristic temporal dependence when the long receiving dipoles pass close to parallel to the earth's magnetic field direction. As the angle between the dipole and the field direction passes its minimum, the spectra evolve from having enhanced negative Doppler components to having enhanced positive Doppler components or vice versa. The relative orientation of the spin, spacecraft velocity, and magnetic field vectors determines whether the enhancement evolves from positive to negative Doppler or from negative to positive. Theoretical calculations of the Doppler spectra agree with observations from the ISIS spacecraft. These analyses show that wave vectors corresponding to broadened spectra are confined to a narrow range of azimuths about the magnetic field direction. The wave vectors are distributed through a range of polar angles with respect to the magnetic field direction, possibly as the result of scattering of the upcoming waves in the ionosphere below the spacecraft. The technique has been used to deduce the preferred azimuth in the case of one spectrum.
The statistical electron environment for Defense Meteorological Satellite Program eclipse charging
Authors: Yeh H. ‐C. , Gussenhoven M. S.
Published: February 2008

Precipitating high‐energy electrons can produce high‐level (≥ 100 V negative) vehicle to plasma potentials on spacecraft operating in high‐inclination, low‐altitude polar orbits. In order to precisely specify the charging level, both the incident electron population and the spacecraft materials properties must be accurately defined. This is because the effect of the total incoming electron flux is largely balanced by the production of backscattered and secondary electron fluxes escaping the satellite surface. In this paper we categorize and model the precipitating electron spectra during 11 Defense Meteorological Satellite Program (DMSP) charging events that occurred during the winter months of 1983. During the charging events the electron energy distribution functions have a large high‐energy component, are definitely not single Maxwellian in shape, and are often rapidly changing. The spectra can be classified into four types, three of which we model. The critical parameters in the models are temperature and density of the multiple Maxwellian components and the energy at transitions in the spectral shape. The average values of these parameters are calculated for two levels of DMSP charging. The model electron spectra are then used to calculate the net electron flux charging current for various satellite materials using two different formulations of electron secondary and backscatter yield coefficients. The electron charging currents differ significantly for the two cases.
Parallel current effects on two‐stream electrojet plasma instabilities
Authors: Chaturvedi P. K. , Huba J. D. , Ossakow S. L. , Satyanarayana P. , Fedder J. A.
Published: February 2008

We consider the effects of a field‐aligned electron drift on the development of two‐stream plasma instabilities which may have applications to the auroral electrojet. It is found that the inclusion of a parallel drift modifies the threshold criteria for the onset of the Farley‐Buneman instability and may cause excitation of oblique ion sound waves. However, the phase velocity of the linearly excited waves remains greater than or equal to the local ion acoustic velocity. We apply these results to an auroral E region and show that they may explain some observations of auroral irregularities, for example, the observation of irregularities which are not highly field aligned and the observation of ion acoustic wavelike irregularities for ambient electric fields below the critical value.
Energy diffusion of pickup ions upstream of comets
Authors: Isenberg Philip A.
Published: February 2008

We present a steady state model of pickup ion energization upstream of a cometary bow wave in order to investigate the effects of quasi‐linear energy diffusion in the turbulence there. The model assumes that the ions are immediately isotropized at pickup, and it includes the effects of adiabatic acceleration in the slowing solar wind and of continual pickup of ions as the comet is approached. By taking all physical quantities to fall off as power laws with distance from the comet, an analytical expression is obtained for the distribution function of pickup ions in the reference frame moving with the solar wind. To illustrate the application of this model, we compare the model results to the observations of pickup ions at comet Giacobini‐Zinner. At present, this is the only cometary encounter for which sufficient quantitative information is available. The model does not compare well with these observations, but it is not clear whether the differences are due to artificial problems in the model or the data analysis or to the action of other energization processes at this comet. Preliminary results from the Halley encounters appear to agree more closely with this model.
Volcanic Control of the Io atmosphere and neutral and plasma torus
Authors: Eviatar Aharon
Published: February 2008

Rate equations for the column density of Io's atmosphere and the number density of neutral atoms and ions in the torus have been formulated in terms of surface sublimation and venting of volcanic gases, sputtering efficiency, thermal escape, and the plasma processes that take place in the torus which included electron impact ionization, charge exchange, and transport. In the limit of low volcanic activity, estimates of the densities can be obtained, and the baseline configuration that would exist in the absence of volcanic activity is found. Solution of the coupled steady state rate equations shows that the densities are sensitive, to a varying degree, to the level of Io volcanism. The steady state densities predicted by the model are consistent with a volcanic flux of no more than 1012 cm−2 s−1. This is significantly lower than the values obtained on the basis of resurfacing rates and appears to imply that surface lava flows play a major role in the resurfacing process.
Transverse Kelvin‐Helmholtz instability with parallel electron dynamics and Coulomb collisions
Authors: Satyanarayana P. , Lee Y. C. , Huba J. D.
Published: February 2008

Electron parallel dynamics and Coulomb collisions are included in the analysis of the transverse Kelvin‐Helmholtz instability. The electrons are treated kinetically while the ions are treated in the fluid limit. It is shown that in the collisionless case, for an inhomogeneous velocity profile V(x) = V0 tanh (x/L) the Kelvin‐Helmholtz instability is stable for kz/ky > (V0/Lωlh)kyL [2(1 ‐ ky²L²)]1/2 in the limit ω ‐ kyV0 » kzυe. Here V0 is the flow velocity, L is the scale length of the velocity shear layer, kz and ky are the parallel and perpendicular wave numbers, respectively, ωlh = (ΩeΩi)1/2, and υe is the electron thermal velocity. The stabilization of the mode is shown to be caused by the compressional energy given to the electrons parallel to B. In the collisional limit, Coulomb collisions are shown to increase the unstable kz domain because they inhibit the electron motion parallel to B. Applications to the high‐latitude ionosphere are discussed.
On the distribution of sunspot cycle periods
Authors: Wilson Robert M.
Published: February 2008

A comparison is made between the observed distribution of sunspot cycle periods and distributions based on uniform, normal, and bimodal distributions. The bimodal distribution, composed of short‐ and long‐period cycles, each normally distributed about its respective mean, is found to best describe the observed distribution. Compared to the normal distribution for the most reliably determined cycles (cycles 8–20), the bimodal distribution has a residual (sum of squares of differences) that is about 86% smaller. Means for short‐ and long‐period cycles are estimated to be 122 ± 4 months and 140 ± 5 months, respectively.
Coordinated ground and space measurements of an auroral surge over South Pole
Authors: Rosenberg T. J. , Detrick D. L. , Mizera P. F. , Gorney D. J. , Berkey F. T. , Eather R. H. , Lanzerotti L. J.
Published: February 2008

Coincident ground‐based and satellite observations are presented of a premidnight auroral surge over Amundsen‐Scott South Pole station. The set of near‐simultaneous measurements provides an excellent opportunity to gain a more quantitative understanding of the nature of premidnight substorm activity at high geomagnetic latitudes. The surge produced a rapid onset of cosmic radio noise absorption at the station. On the polar‐orbiting DMSP F6 spacecraft, intense X ray emissions with E > 2 keV energy were imaged 1° to 2° magnetically equatorward of South Pole approximately 1 min prior to the peak of the absorption event. The spectrum of precipitating electrons determined from the X ray measurements could be characterized by an e‐folding energy of ∼11 keV and is found to be adequate to account for the cosmic noise absorption and maximum auroral luminosity recorded at South Pole. Photometer, all‐sky camera, riometer, and magnetometer data are used to estimate the velocity of motion and spatial extent of the auroral precipitation and the ionospheric currents associated with the surge. The electron precipitation region is deduced to have a latitudinal scale size of < 100 km and to move poleward with a speed of ∼1–2 km/s coincident with the movement of a westward electrojet. Northern hemisphere auroral station magnetometer measurements also indicated the movement of a westward electrojet from local midnight toward the conjugate point of South Pole station at the same time. The 427.8‐nm photometric data are used to infer that precipitating electron fluxes (E > 2 keV) exceeded 200 ergs/cm² s and contributed to a vertical current density of ∼0.017 A/cm. This current density is comparable, to within a factor of ∼2, with the horizontal ionospheric current density (∼0.028 A/cm) inferred from the ground‐based magnetometer measurements at South Pole station. A contribution of a large flux of electrons with E < 2 keV to the vertical current density is discounted as an explanation for the difference on the basis that the expected 630‐nm auroral luminosity would exceed by about an order of magnitude the less than 1 kR luminosity that was observed for this emission.
Simulation of January 1–7, 1978, events
Authors: Chao J. K. , Moldwin M. B. , Akasofu S. ‐I.
Published: February 2008

The solar wind disturbances during January 1–7, 1978, are reconstructed by a modeling method. First of all, the interplanetary magnetic field background pattern, including a corotating shock, is reproduced using the Stanford source surface map. Then, two solar flares with their onset times on January 1, 0717 at 17°S, 10°E and 2147 UT at 17°S, 32°E, respectively, are selected to generate two interplanetary transient shocks. It is shown that these two shocks interacted with the corotating shock, resulting in a series of interplanetary events observed by four spacecraft, Helios 1 and 2, IMP 8, and Voyager 2. In particular, our simulation results show that these three shock waves interact and coalesce in interplanetary space such that Helios 2 and Voyager 2 observed only one shock and Helios 1 and IMP 8 observed two shocks. All shocks observed by the four spacecraft, except the corotating shock at Helios 1, are either a transient shock or a shock which is formed from coalescing of the transient shocks with the corotating shock. Our method is useful in reconstructing a very complicated chain of interplanetary events observed by a number of spacecraft.
Pioneer 8 traversal of the geomagnetic tail at 1600 RE
Authors: Scarf Frederick L.
Published: February 2008

The Pioneer 8 spacecraft (launched on December 13, 1967) first crossed the geomagnetic tail at a downstream distance of 500–800 RE as it left the Earth‐moon system, and during a two‐week period centered on January 23, 1968, the magnetometer, plasma probe and plasma wave instrument all detected specific tail‐related phenomena. In April and May of 1985, Pioneer 8 traversed the expected position of the Earth's tail once again, at an average downstream distance of 1650 RE. For this second tail monitoring opportunity the available spacecraft power was so low that only the plasma wave instrument could be operated, but the electric field observations were sufficient to give unambiguous evidence of several distant tail crossings. This report contains a summary of the 1985 Pioneer 8 measurements, and a brief discussion of the limited information available on solar wind variations and terrestrial magnetic activity indicators.
Drifts of auroral structures and magnetospheric electric fields
Authors: Nakamura Rumi , Oguti Takasi
Published: February 2008

Drifts of pulsating auroral patches and discrete auroral arc fragments are analyzed on the basis of all‐sky TV observations of auroras. The drifts of auroral structures in this study correspond on a gross scale with other measurements of magnetospheric convection. The result strongly suggests that not only auroral patches but also arc fragments, when detached from the main body of the discrete aurora, drift owing to the magnetospheric electric fields. The measurement of the drifts of auroral structures could possibly provide us with a convenient and accurate method to estimate the magnetospheric electric fields.
Simultaneous measurement of aurora‐related, irregular magnetic pulsations at northern and southern high latitudes
Authors: Arnoldy R. L. , Rajashekar R. , Cahill L. J. , Engebretson M. J. , Rosenberg T. J. , Mende S. B.
Published: February 2008

A dominant feature of high‐latitude magnetic pulsations is large‐amplitude irregular pulsations (Pi) which are closely correlated with the movement of the observing station under particle precipitation, producing the dayside aurora and the high‐latitude expansion of nightside aurora. The dayside Pi‐1 pulsation maximum centered about local magnetic noon has no strong seasonal dependence, indicating that the dayside aurora illuminates both hemispheres independent of the latitude of the subsolar point. The summer noon pulsation maximum has, however, a greater longitudinal extent than the winter noon maximum, as measured at 74°–75° invariant latitude. The nightside magnetic pulsations are bursts of Pi (PiB) having an average duration of 15 min. From Defense Meteorological Satellite Program photos the auroral forms related to the high‐latitude PiB can be identified as the poleward discrete arc generally having a large longitudinal extent. If the auroral forms are very similar in both hemispheres, then the large longitudinal extent coupled with movement of the aurora could explain why 85% of the PiB events have onsets within 10 min at opposite hemisphere sites (South Pole, Antarctica, and Sondre Stromfjord, Greenland) separated in local magnetic time by about 1.5 hours. There is no seasonal dependence in the statistical occurrence of PiB, nor in its simultaneity in opposite hemispheres. Apparently, the seasonal distortion of the tail plasma sheet has little effect on the acceleration of high‐latitude auroral beams. The actual several minute time difference in opposite hemisphere onsets of PiB is probably due to the westward/poleward motion of the longitudinally extended aurora. Assuming that the nonsimultaneity (within 10 min) of opposite hemisphere PiB is because one of the sites is too far removed from magnetic midnight because of the nearly 2‐hour site separation, one can infer that the typical longitudinal extent of the poleward arc and/or the longitudinal extent of travel of the westward surge is from 2 hours after midnight to 4 hours before. The PiB having a sudden onset are likely the magnetic manifestation of the stepwise motion of the westward electrojet current wedge poleward and/or westward.
HF radar observations of E region plasma irregularities produced by oblique electron streaming
Authors: Villain J. P. , Greenwald R. A. , Baker K. B. , Ruohoniemi J. M.
Published: February 2008

Data obtained with the Applied Physics Laboratory HF radar located in Goose Bay, Labrador, have been used to determine the characteristic features of high‐latitude ionospheric irregularities at decameter wavelengths. In this paper, we describe a set of four events exhibiting particular characteristics. These observations took place in the postmidnight sector at E region altitudes. The scanning capabilities of the radar indicated that arclike regions of irregularities were moving approximately along L contours with a drift velocity of the order of 200 m/s or less. For periods of a few minutes to a few tens of minutes, localized regions of irregularities exhibiting high Doppler velocities (350 to 650 m/s) and large signal to noise ratios appeared within the radar arcs. Among the high Doppler velocity signals, two distinct types have been identified. Both types can be present simultaneously. One type is distributed between 320 and 550 m/s and has an average value of 445 m/s, while the other is distributed between 500 and 650 m/s and has an average value of 580 m/s. If one assumes the lower of the high‐velocity signals to be the ion acoustic velocity Cs, the higher velocity can be interpreted as electrostatic ion cyclotron (EIC) waves produced by NO+ ions. These EIC waves follow perfectly the dispersion relation established from the fluid approximation ωr = (Ωi² + k² Cs²)½. The radar echoes with low Doppler velocities are associated with irregularities produced by the gradient drift instability which presumably was operative at the top of the E layer. We suggest that magnetic field‐aligned drifts combined with the subcritical perpendicular electron drifts are responsible for the production of both the EIC waves and the ion acoustic waves that were observed.
Geocoronal structure: 3. Optically thin, Doppler‐broadened line profiles
Authors: Bishop James , Chamberlain Joseph W.
Published: February 2008

Theoretical line profiles, applicable to the analysis of geocoronal Hα profile measurements, are presented for illustrative cases. While retaining a number of simplifications (classical exobase and diffusive equilibrium plasmasphere conditions), distinctive spectral signatures of mechanisms governing the geocorona are isolated. Examining the consequences of solar radiation pressure dynamics is the main point here. In the prototype evaporative case, radiation pressure acts to form narrow profiles via the creation of an extensive quasi‐satellite component. Comparison with a simple extension of the earlier analytic theory discloses the influence of an exopause in this regard. The main modifications to evaporative spectral shapes in the geocoronal application, for shadow heights greater than 2 RE, are predicted to be (1) a blueward “shift” or bias near line center, for look directions parallel to the antisolar axis, generated by loss mechanisms acting over the time of flight of exospheric constituents (for example, solar ionization) and (2) an enhanced red ward wing at spectral displacements exceeding that defined by the shadow height escape speed, produced by plasmaspheric charge exchange collisions. Implications of these results for recent observations of geocoronal Hα line profiles are briefly discussed.
The eastward deflection of fast coronal mass ejecta in interplanetary space
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Zwickl R. D.
Published: February 2008

Previous work has shown that a bidirectional solar wind electron heat flux is one of the more prominent signatures of a coronal mass ejection event in the solar wind at 1 AU. Using ISEE 3 solar wind electron measurements obtained during 1978 and 1979, we have used this signature to identify the fast coronal mass ejecta driving 19 interplanetary shocks. In 17 of the 19 shock events an eastward deflection of the ejection plasma (apparent arrival from west of the sun) was observed. The average eastward deflection for all of the events was ∼3°, corresponding to a typical transverse velocity of ∼25 km/s. Usually an oppositely directed (i.e., westward) flow deflection of comparable magnitude was observed within the compressed ambient plasma ahead of the ejecta. The sense of these deflections—first westward within the compressed ambient plasma and then eastward within the ejecta—is the same as is observed near the leading edges of quasi‐stationary, corotating high speed streams. However, in contrast to the case of corotating streams, the west‐east flow reversal does not coincide with a local maximum in static pressure. No consistent flow deflection pattern perpendicular to the ecliptic has been observed for these events, and no similar deflections have been observed for ejecta (bidirectional electron heat flux events) traveling at the same speed as or slower than the ambient plasma ahead. We suggest that the preferred eastward deflection of these fast coronal mass ejecta is a consequence of solar rotation and the spiral geometry of the ambient solar wind.
Modeling of the O I 989‐Å to 1173‐Å ratio in the terrestrial dayglow
Authors: Gladstone G. R. , Link R. , Chakrabarti S. , McConnell J. C.
Published: February 2008

The O I 989‐Å and 1173‐Å intensities in the mid‐latitude dayglow are modeled for the conditions of a January 17, 1985, rocket flight. It is found that the branching ratio required to fit the 1173‐Å data is in agreement with the laboratory measurement of 1.3–1.5 × 10−4. The results also suggest that the electron impact excitation cross section for the 3s′ ³D° state in current use may be too large by a factor of ∼2–3, in agreement with the most recent laboratory results. If this is indeed the case, then the 1173‐Å triplet is by far the most important branch for the 989‐Å multiplet.
ELF oscillations associated with electron beam injections from the space shuttle
Authors: Cai D. , Neubert T. , Storey L. R. O. , Banks P. M. , Sasaki S. , Abe K. , Burch J. L.
Published: February 2008

ELF oscillations (f < 500 Hz) were observed during the electron beam emissions of the space experiments with particle accelerators (SEPAC) flown on the Spacelab 1 shuttle mission. The beams had energies up to 5 keV and currents up to 300 mA, and the oscillations were present in the data from a Langmuir probe, a floating probe, an electron energy analyzer, and a photometer. The VLF (1 kHz < f < 10 kHz) wave stimulation monitored by a wave receiver during one particular beam sequence has already been reported by Neubert et al. (1986). The amplitudes of the ELF and VLF oscillations observed during this sequence have almost identical variations with beam pitch angle, the strongest emissions being observed for parallel beams; the ELF power spectra for the strongest emissions have peaks about 10 dB above the broadband ELF noise at frequencies around 50–60 Hz. In another beam sequence the power spectra had a harmonic structure with the fundamental frequency around 200 Hz. The power density and frequency of the fundamental increased with the shuttle charge‐up potential. The emission level observed during the beam sequences increased with the charge‐up potential of the orbiter, which largely depended on the wake structure. We find it most likely that the ELF oscillations are expressions of fluctuations in the return current and the shuttle potential and that these fluctuations are caused by processes involving charge imbalances in the near environment of the shuttle, possibly in a comoving plasma cloud. The observations suggest that the plasma cloud has a particle lifetime at least of the order of 100 ms.
Further on the October 31, 1977 upstream event: A response to D. C. Ellison
Authors: Anagnostopoulos G. C. , Sarris E. T. , Krimigis S. M.
Published: February 2008

Over the past decade, and on the basis of simultaneous multi‐spacecraft observations, we have shown that energetic ions escape upstream from the earth's bow shock by drifting through the magnetospheric and magnetosheath fields from their source inside the plasma sheet [Sarris et al., 1978; Krimigis and Sarris, 1979]. Furthermore, Voyager 1 and 2 observations, during their encounters with Jupiter [Zwickl et al., 1980; Krimigis et al., 1985], and more recently with Uranus [Krimigis et al, 1986], have also shown that energetic ions of magnetospheric origin leak upstream from the planetary bow shocks.
Observations of polar rain at low and high altitudes
Authors: Baker D. N. , Bame S. J. , Gosling J. T. , Gussenhoven M. S.
Published: February 2008

Observations in the distant magnetotail lobes (r ≳ 100 RE) reveal that electrons in the ∼50‐ to 1000‐eV energy range often exhibit strong, field‐aligned bidirectional anisotropies. Because of large tail motions in the variable flow of the solar wind, ISEE 3 data provide extensive sampling of most tail plasma regions in rapid succession. It is found that bidirectional electron fluxes occur predominantly in the lobe connected directly to the sunward interplanetary magnetic field in the open magnetosphere model (north lobe for away sectors and south lobe for toward sectors). Using data from the low‐altitude DMSP spacecraft, we compare polar rain electron intensities, energy spectra, and time variations at low altitudes with concurrent bidirectional electron properties measured by ISEE 3. The field‐aligned lobe electron phase space densities above ∼200 eV agree well (i.e., to within a factor of ∼2–3) with DMSP‐measured polar rain phase space densities near the polar cap. Furthermore, the spectral slopes above 200 eV also are similar at low and high altitudes. Below 100–200 eV there is a thermal electron population in the distant tail, probably arising from local entry of plasma through the distant magnetopause, which is not present at DMSP altitudes. These data suggest that the suprathermal tail lobe electrons are essentially a test particle population which can move freely along field lines to form polar rain; in contrast, the thermal electrons are bound to the tailward flowing lobe ion population far down the tail and thus cannot reach the polar cap regions.
The Condor Equatorial Electrojet Campaign: Radar results
Authors: Kudeki Erhan , Fejer Bela G. , Farley Donald T. , Hanuise Christian
Published: February 2008

A review of the experimental and theoretical background to the Condor equatorial electrojet campaign is followed by the presentation and discussion of VHF radar interferometer and HF radar backscatter data taken concurrently with two rocket in situ experiments reported in companion papers (Pfaff et al., this issue (a, b). Both experiments were conducted in strongly driven periods with the on‐line radar interferometer displaying signatures of what has been interpreted in earlier radar work (Kudeki et al., 1982) as kilometer scale gradient drift waves. Low‐frequency density fluctuations detected by in situ rocket sensors confirm the earlier interpretation. VHF radar/rocket data comparisons also indicate the existence of a turbulent layer in the upper portion of the daytime electrojet at about 108 km altitude driven purely by the two‐stream instability. Nonlinear mode coupling of linearly growing two‐stream waves to linearly damped 3‐m vertical modes could account for the radar echoes scattered from this layer, which showed no indication of large‐scale gradient drift waves. Nonlinear mode coupling may therefore compete with the wave‐induced anomalous diffusion mechanism proposed recently by Sudan (1983) for the saturation of directly excited two‐stream waves. Nighttime radar data show a bifurcated layer with the two parts having comparable echo strength but oppositely directed zonal drift velocities. The lower layer shows narrow backscatter spectra; the upper layer is characterized by kilometer scale waves and vertically propagating type 1 waves. The charateristics of the topside large‐scale waves show clear consistency with the predictions of nonlocal gradient drift instability theories. The observed sheared flow could be due to neutral winds or a reversal of the vertical polarization fields. Lack of concurrent in situ polarization field or density profile measurements, unfortunately, prevents us from determining the actual cause unambiguously.
Electric field and plasma density measurements in the strongly driven daytime equatorial electrojet: 1. The unstable layer and gradient drift waves
Authors: Pfaff R. F. , Kelley M. C. , Kudeki E. , Fejer B. G. , Baker K. D.
Published: February 2008

Electric field and plasma density instrumentation on board a sounding rocket launched from Punta Lobos, Peru, detected intense electrostatic waves indicative of plasma instabilities in the daytime equatorial electrojet. Simultaneous measurements taken by the Jicamarca radar showed strong 3‐m type 1 electrojet echoes as well as evidence of kilometer scale horizontally propagating waves. The in situ electric field wave spectra displayed three markedly different height regions within the unstable layer: (1) a two‐stream region on the topside between 103 and 111 km where the electron current was considered to be strongest, (2) a gradient drift region between 90 and 106.5 km where the upward directed, zero‐order electron density gradient was unstable, and (3) an “interaction” region between 103 and 106.5 km where both of these instabilities were linearly unstable. The unstable altitudes and differentiation showed good agreement with the simultaneous 3‐m Jicamarca backscatter radar observations. In the region where the density gradient was unstable, large‐amplitude waves with large scale sizes (wavelengths of roughly 1–2 km) were observed. These kilometer scale waves dominated the observed in situ spectrum despite the fact that the peak in the linear gradient drift growth rate occurred at wavelengths of only a few hundred meters. Comparisons of the measured δE and δn/n components of the large‐scale waves verify the basic process inherent to the gradient drift instability: density enhancements were observed coincident with westward electric fields, and density depletions were associated with eastward fields. The amplitudes (10–15 mV/m) of these horizontal waves were strong enough to drive vertical two‐stream secondary waves. In the region where these waves existed and the electrojet current was strongest, evidence of wave steepening was seen, and the resulting waveforms of the large structures displayed a “flat‐topped” nature. In the lower region of the electrojet the irregularity power occurred over a broad range of wavelengths, estimated to be in the range of tens of meters to kilometers, and fell off rapidly for the shorter wavelengths. Throughout the gradient drift region, shorter‐scale waves often occurred in bursts which appeared controlled by the larger electric field structures.
Injection and propagation of a nonrelativistic electron beam and spacecraft charging
Authors: Okuda H. , Berchem J.
Published: February 2008

Two‐dimensional numerical simulations have been carried out in order to study the injection and propagation of a nonrelativistic electron beam from a spacecraft into a fully ionized plasma in a magnetic field. Contrary to the earlier results in one dimension, a high‐density electron beam whose density is comparable to the ambient density can propagate into a plasma. A strong radial electric field resulting from the net charges in the beam causes the beam electrons to spread radially, reducing the beam density. When the injection current exceeds the return current, significant charging of the spacecraft is observed along with the acceleration of the ambient electrons back to the spacecraft. As a result, a beam current much larger than the thermal return current can be injected into space. Recent data on the electron beam injection from the Spacelab 1 (SEPAC) are discussed.
Electromagnetic ion instabilities in a cometary environment
Authors: Gary S. Peter , Madland Christian D.
Published: February 2008

This paper considers the linear theory of electromagnetic ion beam and ion ring‐beam instabilities in a homogeneous Vlasov plasma. Propagation parallel or antiparallel to a uniform magnetic field and frequencies at or below the proton cyclotron frequency are considered. For parameters representative of the distant cometary environment, we show that instabilities with right‐hand polarization in the zero momentum frame have larger linear growth rates than left‐hand polarized instabilities at α values up to 90° where α is the angle between the solar wind velocity and the uniform interplanetary magnetic field. If both a proton beam and an oxygen beam are present with α = 0°, two right‐hand resonant instabilities may grow; these two modes are distinct and relatively independent of one another for a very wide range of proton/oxygen beam density ratios.
Temperature effects on the pickup process of water group and hydrogen ions: Extensions of “A theory for low‐frequency waves observed at comet Giacobini‐Zinner” by M. L. Goldstein and H. K. Wong
Authors: Brinca Armando L. , Tsurutani Bruce T.
Published: February 2008

Cometary heavy ions can resonantly excite hydromagnetic wave activity with spacecraft frequency spectra strongly deviating from the ion cyclotron frequency (Goldstein and Wong, 1987). We assess the influence of the newborn particle temperature on this effect, discuss its relevance to the interpretation of the observations, and suggest an alternative, more efficient mechanism to generate spacecraft frequencies of the order of the proton cyclotron frequency.
Evolution of interplanetary slow shocks
Authors: Whang Y. C.
Published: February 2008

The possible existence of traveling forward slow shocks, their global geometry and their transition to forward fast shocks have been discussed in a recent paper. The decrease in the Alfven speed at increasing heliocentric distance causes the evolution of a forward slow shock into a forward fast shock. During the transition the shock system consists of a slow shock, a fast shock, and a rotational discontinuity. This paper continues to discuss three aspects about the evolution of interplanetary slow shocks. We first present a survey of slow shock solutions in a three‐dimensional A,θ,β parameter space. Here A = Un/(a cos θ) is the shock Alfven number, Un the normal component of the relative shock speed, a the Alfven speed, θ the acute angle between the shock normal and the magnetic field, and β the ratio of the thermal pressure p to the magnetic pressure B²/8π. In a region where the plasma β value is on the order of 1 or greater, the jumps in thermodynamic properties across a slow shock are small but the directional changes for the magnetic field and the relative flow velocity are not necessarily small. On the other hand, in the region where the plasma β value is on the order of 0.1 or less, the jumps of all physical properties across a slow shock may vary over a wide range of magnitudes. Next, we discuss that at the onset of the transition process an interplanetary slow shock smoothly converts to a system consisting of a slow shock, a very weak rotational discontinuity (an Alfven wave), and a very weak fast shock (a fast MHD wave). We also show that during the transition, the amplitude of fluctuations in flow velocity and magnetic fields are large in the turbulent region behind the fast shock.
Plasma drifts associated with a system of Sun‐aligned arcs in the polar cap
Authors: Mende S. B. , Doolittle J. H. , Robinson R. M. , Vondrak R. R. , Rich F. J.
Published: February 2008

A series of four sun‐aligned arcs passed over Sondre Stromfjord, Greenland, on the night of the 17th and 18th of February, 1985. Observations of these arcs were made using the Sondrestrom incoherent scatter radar and an intensified all‐sky imaging TV system that was operated at the radar site. The first of the four arcs crossed the Sondre Stromfjord meridian just before local midnight moving westward, and the other three arcs followed at approximately half‐hour intervals. When we account for the earth's rotation, the arc drift in an inertial frame was eastward, or dusk to dawn. The half‐hour interval between meridian crossings of the arcs implies that the mean spacing between the arcs was 180 km. A Defense Meteorological Satellite Program (DMSP) F6 satellite pass at 0110 UT revealed the presence of highly structured electron and ion precipitation throughout the polar cap. The DMSP visible imager detected a single, sun‐aligned arc associated with the largest peak in precipitating electron flux. This arc was also observed at Thule, Greenland, with an intensified film camera. These observations suggest that at least one of the arcs that were observed at Sondre Stromfjord extended across a large part of the polar cap. The radar at Sondre Stromfjord measured electron density and ion drift velocities associated with the four arcs. The radar drift measurements were superimposed on the all‐sky video images to determine the location of the measurements relative to the arcs. Plasma drifts outside the arcs were found to be both sunward and antisunward, while within the arcs the drifts were predominantly antisunward. The variability of the drifts in the direction parallel to the arcs indicates that the electric fields were highly structured even though the configuration and motion of the arcs were well behaved. The motion of the auroral forms perpendicular to the arcs was generally in the same direction as the perpendicular component of the plasma velocity.
Excitations of low‐frequency hydromagnetic waves by freshly created ions in the solar wind
Authors: Price C. P. , Gaffey J. D. , Dong J. Q.
Published: February 2008

Low‐frequency hydromagnetic waves excited by newborn ions in the solar wind plasma are studied. The freshly created ions appear in the solar wind frame with a ring beam distribution. Both Alfvén and fast magnetosonic waves are made unstable by the presence of the newborn ions. The dependence of the growth rate of both waves on the newborn ion density, the angle between the interplanetary magnetic field (IMF) and solar wind flow, and the angle of wave propagation relative to the IMF is investigated. Analytic approximations for the growth rates are presented, and numerical solutions of the dispersion equation are shown. The approximations are quite close to the numerically determined growth rates. We find that the waves grow preferentially in the direction parallel to the IMF and that the growth rates increase with both newborn ion density and the angle between the IMF and the solar wind flow.
Electrodynamics of very high latitude arcs in the morning sector auroral zone
Authors: Robinson R. M. , Vondrak R. R. , Hardy D. , Gussenhoven M. S. , Potemra T. A. , Bythrow P. F.
Published: February 2008

In November 1985 an experimental campaign was conducted at Sondre Stromfjord, Greenland, to study the electrodynamics of auroral arcs at very high latitudes. The experiments involved coordinated observations by the Sondrestrom radar and two polar orbiting spacecraft. For several hours around local dawn on November 17 a system of auroral arcs was observed over Sondrestrom. These arcs varied in intensity and width but were all extended generally in the magnetic east‐west direction. A Defense Meteorological Satellite Program photograph obtained during the observations showed that two types of arcs were present. In the south the arcs were localized enhancements or striations within a broad diffuse aurora. Poleward of the diffuse aurora was a system of arc segments that extended over about 5° of latitude. The radar measured enhanced ionospheric electron densities associated with each of these arcs, although the altitudes of the enhancements were different for the two types. In the diffuse aurora the ionization peaked at altitudes near 115 km, whereas in the arc segments the ionization peaked at altitudes above 180 km. Examination of electric field data from the radar indicated that the boundary in ionization associated with the two types of aurora occurred where the southward electric field maximized. The HILAT satellite passed in the vicinity of Sondrestrom during the experiment and measured precipitating electron fluxes and field‐aligned currents. The electron flux data showed that the low‐altitude ionization observed by the radar was produced by electrons with energies near 5 keV. The higher‐altitude ionization was produced by precipitation associated with enhancements in the fluxes of low‐energy electrons. The magnetometer data indicated that the boundary between the region 1 (downward) and region 2 (upward) current sheets in the morning sector is just poleward of the boundary separating the hard and soft precipitation regions. The latitudinal variation in field‐aligned current is consistent with the measured variations in electric field and conductivity.
Population development of auroral molecular nitrogen species in a pulsed electric discharge
Authors: Morrill J. , Carragher B. A. , Benesch W.
Published: February 2008

A series of experiments has been performed with pulsed electric discharges in low‐pressure nitrogen in order to elucidate excitation processes which underlie auroral photon emission. Flowing nitrogen in a large‐volume glass container at 30 to 3000 mtorr is excited by 4 µs pulses which, typically, are at 12000 V and 900 A. The radiation issuing from the discharge tube is analyzed by means of time‐resolved spectroscopy with particular attention given to the nitrogen first positive system, B³Πg → A³Σu+. Dramatic changes have been noted in the relative band intensities of the 1PG spectra subject to the influence of the variation of the experimental conditions. In particular, the distribution of population among vibrational levels of the B³Πg state evolves in time both during and after the exciting electric pulse, with the trends strongly correlated with the numbers of collisions. Further, radiation from these excited vibrational levels continues at much higher emission rates than those commensurate with their radiative lifetimes. Both of these observations constitute evidence of the intersystem collisional transfer of excitation. The distinguishing feature of this array of experiments is the application of time‐resolved spectroscopy which permits examination of the development of level populations on a collision‐by‐collision basis.
A unidimensional model of comet ionosphere structure
Authors: Eviatar Aharon , Goldstein Bruce E.
Published: February 2008

We have considered the one‐dimensional continuity and momentum equations for the plasma created in the expanding coma of a comet near the Sun. For an inverse square dependence of neutral density we have solved the continuity equation in the presence of radial plasma flow. For distances large compared to the size of the nucleus, our distribution falls off as r−1 with a velocity dependent coefficient which reduces to the photochemical steady state case in the limit of vanishing radial plasma outflow. This density distribution is used in the treatment of the momentum equation from which an analytic expression for the magnetic field configuration is derived in the presence of outflow, photoionization, dissociative recombination, plasma fluid pressure, and friction between the ions and neutrals. The magnetic field thus derived can be shown to have a maximum value which we fix to be attained at the point corresponding to the point shown by the Giotto observations and find that for a Halley type comet there will be a region sunward of the nucleus from which the magnetic field is excluded, primarily by the effect of ion neutral collisions, which is consistent with the Giotto observations. We have performed the calculations for both Halley and Giacobini‐Zinner type comets. In the field‐free region we find that the dominant terms in the momentum equation balancing the magnetic pressure gradient are the ion neutral friction and the net mass loading momentum gain. For large distances the field reduces to a constant field. The scale size of field diffusion as a result of finite conductivity, brought about by electron neutral collisions, is found to be vanishingly small.
An experimental study of ELF/VLF hiss generation in the Earth's magnetosphere
Authors: Solomon Jacques , Cornilleau‐Wehrlin Nicole , Korth Axel , Kremser Gerhard
Published: February 2008

Simultaneous measurements of whistler mode wave spectra in the frequency range 200 Hz to 3 kHz and of electrons in the energy range 15‐300 keV obtained on board the satellite GEOS 1 or GEOS 2, allow us to study ELF hiss generation. This study is performed both inside and outside the plasmasphere by computing the wave growth rate from the data at different magnetic latitudes λm (0 ≤ λm ≤ 30°). Inside the plasmasphere it is found that the integrated spatial gain Γ > 100 dB, is large enough to account for the observed wave intensity (∼1 pT² Hz−1) in a single transit of the wave through the equatorial region (λm ≤ 20°), which appears as the preferential amplification region. Outside the plasmasphere the situation is more intricate. The total gain Γ ranges from 16 dB to 70 dB, and the observed wave intensities are often larger than inside the plasmasphere; they could be obtained by 2‐3 transits of the waves through the equatorial region with reflection at high latitudes. This does not exclude a contribution due to amplification of exohiss leaking out of the plasmasphere at high latitudes. Both inside and outside the plasmasphere, the patterns considered to explain hiss generation which imply small values of the angle θ between the wave normal and the static magnetic field (i.e. θ < 30°) within a limited latitude range around the magnetic equator (λm ≲ 10°), are well supported by recent studies of the propagation characteristics of ELF hiss observed on board GEOS 1.