bluuebunny/crossref_metadata_2025 huggingface dataset:
On the relationship between dynamics of the polar thermosphere and morphology of the aurora: Global‐scale observations from Dynamics Explorers 1 and 2
Authors: Killeen T. L. , Craven J. D. , Frank L. A. , Ponthieu J.‐J. , Spencer N. W. , Heelis R. A. , Brace L. H. , Roble R. G. , Hays P. B. , Carignan G. R.
Published: February 2008

Simultaneous measurements of global‐scale auroral luminosity distributions and neutral winds over the northern (winter) polar cap have been obtained using instrumentation on the Dynamics Explorers 1 and 2 spacecraft. Several examples of these simultaneous measurements are presented to illustrate the relationship between the circulation of neutral air in the high‐latitude F region and the spatial distribution of the aurora. The auroral images are obtained using the Spin‐Scan Auroral Imager on the high‐altitude spacecraft (DE 1), and the neutral wind vectors are obtained along the orbit of the low‐altitude spacecraft (DE 2), using the Fabry‐Perot interferometer and the Wind and Temperature Spectrometer. These measurements make it possible to appreciate more fully the pattern of neutral winds when placed in the context of global auroral activity. Supplementary DE 2 measurements of neutral composition, ion densities, and the cross‐track component of the ion drift enable the ion drag force on the neutral gas to be correlated with the aurora. Our initial study of these data indicates (1) that a definite correlation exists between boundaries in the neutral wind field and the location of the auroral oval, with large‐scale features of the neutral circulation tracking the substorm‐dependent expansion and contraction of the auroral oval, (2) that the influence of ion drag from sunward convecting ions can extend to latitudes much lower than those normally associated with the auroral oval, (3) that the sunward neutral flow associated with the auroral oval in the dusk sector is, in general, more pronounced than that associated with the dawn sector and that this asymmetry is ascribed to the different effect of the Coriolis force in the two sectors, (4) that the flow patterns for neutrals and ions within the geomagnetic polar cap are generally asymmetric with respect to the noon‐midnight meridian, an effect considered to be controlled by the orientation of the interplanetary magnetic field, (5) that the region of the polar cusp and the apparent “midday gap,” or reduction in luminosities observed in the VUV wavelength auroral images near local noon, is closely associated with a large, antisunward surge in the neutral wind, and (6) that the morphology of the ion drag force in the polar regions is considerably more complex, even for very quiet geomagnetic conditions, than that computed by the National Center for Atmospheric Research thermospheric general circulation model for the “steady state.” This last result is considered to be a consequence of the short‐term variability in the characteristics of high‐latitude ion convection and, in particular, the motion of convection boundaries with respect to boundaries in the less responsive neutral circulation pattern.
On the rocket‐induced wave disturbances in the daytime equatorial ionosphere
Authors: Abdu M. A. , Muralikrishna P. , Batista I. S. , Chaves A. H. P.
Published: February 2008

In situ measurements of daytime equatorial ionospheric E and F region electron densities were carried out using a high‐frequency capacitance probe on board a SONDA III rocket launched from Natal, Brazil, on July 26, 1984. Wave structures in the electron density profile having a scale length of ∼1.5 km to ∼10 km and amplitude of ∼10 percent of the ambient density were detected during the downleg trajectory. Possible sources of these waves are discussed, and their possible implication for irregularity formation under nighttime conditions is briefly mentioned.
Effects of eddy viscosity and thermal conduction and Coriolis force in the dynamics of gravity wave driven fluctuations in the OH nightglow
Authors: Hickey M. P.
Published: February 2008

Recently, Walterscheid et al. (1987) have described a dynamical‐chemical model of wave‐driven fluctuations in the OH nightglow which incorporated a five‐reaction photochemical scheme and the dynamics of linearized acoustic‐gravity waves in an isothermal, motionless atmosphere. The intensity oscillation (δI) about the time‐averaged intensity (I0) and the temperature oscillation (δT) about the time‐averaged temperature (T0) were related by means of the complex ratio η ≡ (δI/I0)/(δT/T0). One of the main conclusions of their work was that the inclusion of dynamical effects is absolutely essential for a valid assessment of η at any wave period. In this paper the model of Walterscheid et al. (1987) is modified to include in the gravity wave dynamics the effects of eddy viscosity, eddy thermal conduction, and Coriolis force (with the shallow atmosphere approximation), and calculations are performed for the same nominal case as used by these previous authors (i.e., λx = 100 km and atmospheric conditions pertinent to 83 km altitude), but only gravity wave periods are considered. It is found that for wave periods greater than some 2 or 3 hours the value of η is greatly modified by the inclusion of eddy thermal conduction. Although when acting alone the eddy viscosity is relatively unimportant, it significantly modifies the results when acting in conjunction with the eddy thermal conduction. The inclusion of the Coriolis force is found to be insignificant at any wave period. Although it is for the longest‐period waves that the values of η are most modified by the inclusion of dissipation, this dissipation may be severe enough to place an observational constraint on such waves. Results of Walterscheid et al. (1987) suggest that η is virtually independent of horizontal wavelength (λx), but it is indicated here that the inclusion of dissipation is likely to make η highly dependent on λx and to complicate comparisons which have been made between observation and theory. The effects of varying the Prandtl number are also discussed.
Aspect angle dependence of backscatter intensity of 1‐m auroral plasma waves
Authors: Nielsen Erling
Published: February 2008

The backscatter intensities of primary and secondary 1‐m unstable plasma waves in the auroral E region have been measured as a function of magnetic aspect angle (α) for electric field values between 25 and 30 mV/m. The variation of the absorption A, measured in decibels, is of the form A = −IaCo cos² α/(1 + Co cos² α). The best fits to the observed attenuation yield parameter values of (Ia, Co) = (49.6, 530) and (48.8, 900) for primary and secondary waves, respectively. The backscatter intensity from primary waves has the same aspect angle dependence as the mean Doppler velocity aspect angle dependence observed in previous auroral radar data sets (Nielsen, 1986). The observations suggest that the radar backscatter cross section of the primary waves maximizes a few kilometers higher in altitude than that of the secondary waves.
Ionospheric ion upwelling in the wake of flux transfer events at the dayside magnetopause
Authors: Lockwood M. , Smith M. F. , Farrugia C. J. , Siscoe G. L.
Published: February 2008

The effects of flux transfer events (FTE) on the dayside auroral ionosphere are studied, using a simple twin‐vortex model of induced ionospheric plasma flow. It is shown that the predicted and observed velocities of these flows are sufficient to drive nonthermal plasma in the F region, not only within the newly opened flux tube of the FTE, but also on the closed, or "old" open, field lines around it. In fact, with the expected poleward neutral wind, the plasma is more highly nonthermal on the flanks of, but outside, the open flux tube: EISCAT observations indicate that plasma is indeed driven into nonthermal distributions in these regions. The nonthermal plasma is thereby subject to additional upforce due to the resulting ion temperature anisotropy and transient expansion due to Joule heating and also to ion accelerations associated with the FTE field aligned current system. Any upflows produced on closed field lines in the vicinity of the FTE are effectively bunched‐up in the "wake" of the FTE. Observations from the AMPTE‐UKS satellite at the magnetopause reveal ion upflows of energy ∼100 eV flowing out from the ionosphere on closed field lines which are only found in the wake of the FTE. Such flows are also only found shortly after two, out of all the FTEs observed by AMPTE‐UKS. The outflow from the ionosphere is two orders of magnitude greater than predicted for the "classical" polar wind. It is shown that such ionospheric ion flows are only expected in association with FTEs on the magnetopause which are well removed from the sub‐solar point‐either towards dusk or, as in the UKS example discussed here, towards dawn. It is suggested that such ionospheric ions will only be observed if the center of the FTE open flux tube passes very close to the satellite. Consequently, we conclude the ion upflows presented here are probably driven by the second of two possible source FTEs and are observed at the satellite with a lag after the FTE which is less than their time‐of‐flight.
Detailed determination of the orientation and motion of the plasma sheet boundary layer using energetic protons on ISEE 1 and 2: Waves, curves, and flapping
Authors: Kettmann G. , Daly P. W.
Published: February 2008

During a geomagnetic substorm on April 12, 1979, the ISEE 1 and 2 spacecraft entered the plasma sheet, crossing the boundary layer several times. The spacecraft were located approximately 21 Earth radii from the Earth on the nightside. Using the time delay between the two satellites, one can determine the speed of the plasma sheet surface only by making assumptions about its orientation. By contrast, in this work, the plasma sheet boundary is investigated with the method of remote sensing using energetic proton data from the medium energy particle experiment (MEPE) to measure the speed and the orientation simultaneously. It is shown that, in this case, the plasma sheet boundary is oriented more than 45° to the GSM equator and has a speed normal to its surface of the order of tens of kilometers per second in the dawn direction. The ion flux gradient scale length at the boundary is estimated to be some hundred kilometers. The orientations of the subsequent exit and reentry are such as to indicate curvature of the plasma sheet surface on the order of several thousand kilometers, the whole surface structure moving in the dawn direction at speeds of about 70 km s−1. In addition, close analysis of the data suggests that the plasma sheet boundary may be locally aligned nearly perpendicular to the magnetic equator. These dawnward moving surface structures are possibly related to eastward moving auroral features observed on the ground.
Influence of the interplanetary magnetic field orientation on polar cap ion trajectories: Energy gain and drift effects
Authors: Delcourt D. C. , Horwitz J. L. , Swinney K. R.
Published: February 2008

The influence of the interplanetary magnetic field (IMF) orientation on the transport of low‐energy ions injected from the ionosphere is investigated using three‐dimensional particle codes. It is shown that, unlike the auroral zone outflow, the ions originating from the polar cap region exhibit drastically different drift paths during southward and northward IMF. During southward IMF orientation, a “two‐cell” convection pattern prevails in the ionosphere, and three‐dimensional simulations of ion trajectories indicate a preferential trapping of the light ions (H+) in the central plasma sheet, due to the wide azimuthal dispersion of the heavy ions (O+). In contrast, for northward IMF orientation, the “four‐cell” potential distribution predicted in the ionosphere imposes a temporary ion drift toward higher L shells in the central polar cap. In this case, while the light ions can escape into the magnetotail, the heavy ions can remain trapped, featuring more intense acceleration (from a few electron volts up to the keV range) followed by precipitation at high invariant latitudes, as a consequence of their further travel into the tail.
Effect of field‐aligned potential drop in a global magnetosphere‐ionosphere coupling model
Authors: Kan J. R. , Cao F.
Published: February 2008

Effects of field‐aligned potential drops on the magnetosphere‐ionosphere coupling in a steady state are studied on a global ionospheric scale. It is shown that a constant‐current generator can support a larger field‐aligned potential drop than a constant voltage generator under similar conditions. The magnetospheric convection pattern is distorted more in the constant current generator case than in the constant voltage generator case. The main difference between a constant current generator and a constant voltage generator is found to lie in their ability to adjust the vorticity of the magnetospheric convection. The results show that a constant current generator allows the vorticity of the magnetospheric convection to adjust so that the field‐aligned current can be kept constant under the loading influence of the field‐aligned potential. On the other hand, a constant voltage generator by definition cannot adjust the vorticity of the magnetospheric convection to maintain the field‐aligned current under the loading influence of the field‐aligned potential.
Sounding the magnetosphere at high frequency using ducted long‐delayed echoes
Authors: Ellis G. R. A. , Goldstone G. T.
Published: February 2008

Observations of ducted magnetospheric echoes have been made at 1.91 MHz using a ground‐based transmitter with peak power of 17 kW and pulse length 150 µs. The echo delays were between 250 and 260 ms and sometimes showed multiple fine structure on a scale of about 200 µs. Their direction of arrival was approximately 11.5° north of the zenith.
CO+ and N2+ in the Venus ionosphere
Authors: Paxton L. J.
Published: February 2008

The photochemistry of CO+ and N2+ in the Venus ionosphere is investigated. Analytic expressions for the distribution of O+ (²D), O+ (²P), CO+, and N2+ in the Venus thermosphere are reported. These expressions reproduce the results obtained from the solution of the coupled mass‐continuity equations to within a few percent from 110 km to 195 km. Predicted CO+ and N2+ densities are compared to Pioneer Venus Orbiter ion mass spectrometer (PVOIMS) data. Changes in model atmosphere or solar flux are not sufficient to bring the observations and theory into agreement using the photochemistry of Fox (1982a, b). Use of the proper CO+ dissociative recombination coefficient (Mitchell and Hus, 1985) together with metastable O+ chemistry (Fox, 1982b) removes much of the previous discrepancy (e.g., Nagy et al. 1980) between the mass 28 observations and the calculated CO+ + N2+ density. The Hedin et al. (1983) model atmosphere formulation is supported by this study, though it is apparent that in situ measurements of the mass 28 ion density are not entirely adequate to the task of ruling out specific model atmospheres. Calculated limb and disk intensities for the CO+ (B²Σ+−X²Σ+) first negative, CO+ (AΠ−X²Σ+) comet tail bands, and N2+ (B²Σu+−X²Σg+) first negative bands are reported. Rayleigh and cloud scattering precludes use of a nadir‐viewing geometry at low (Δλ/λ > 10−3) spectral resolution; however, limb imaging of these bright ( > 10 kR) bands from low (500–150 km) orbit would enable future emissions to derive the CO+ and N2+ density profiles as well as CO/CO2 and N2/CO2 mixing ratios in the thermosphere.
Effects of geomagnetic activity in the winter thermosphere: 2. Magnetically disturbed conditions
Authors: Hagan Maura E.
Published: February 2008

The effects of geomagnetic activity on the middle‐latitude ionosphere and thermosphere which were monitored by the Millstone Hill incoherent scatter radar during a series of large magnetic substorms during February 7–10, 1986, are presented and discussed. These data are compared with similar measurements from the geomagnetically undisturbed January 1986 period described in paper 1 (Hagan and Salah, this issue). The effects of ion‐neutral frictional heating associated with magnetic substorms are initially quantified by comparative analyses of the ion temperatures from the January and February 1986 experiments. Ion temperature enhancements of 900°–1500°K were observed between 30° and 50°N during the most intense substorm. Thermospheric temperature and wind determinations from the two periods are similarly compared. Exospheric temperature enhancements of 200°–500°K characterize the entire storm period, with larger enhancements during the most disturbed times. Thermospheric temperatures remained elevated well into the recovery period. The accuracy of the Joule heating correction to the temperature determinations for the February experiment is measured against three cases: no neutral motion, neutral motion equal in magnitude and direction to the ion flow, and neutral motion equal in magnitude but opposite in direction to the ion flow. Finally, thermospheric meridional wind fields are presented and discussed. Southward surges of 400–600 m/s occurred near 2400 UT on February 7 and 8. Additionally, residual southward winds of 200–500 m/s were strong enough to reverse the northward flow, driven by the diurnal pressure gradients which characterize magnetically undisturbed conditions, during the afternoon of February 8. The thermospheric meridional wind field recovered rapidly as soon as the geomagnetic activity subsided.
A comparison of the plasma wave spectra for the eight AMPTE chemical releases
Authors: Koons H. C. , Anderson R. R.
Published: February 2008

The Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM spacecraft performed a series of eight chemical releases in essentially comparable pairs. There were two lithium releases in the solar wind, two lithium and two barium releases in the near‐Earth magnetotail, and two barium releases to simulate artificial comets, one in the solar wind and the other in the magnetosheath. A variety of plasma waves were observed in conjunction with each of the releases. Although there were unique features associated with some of the individual releases, the comparable releases generally produced similar wave emissions. This paper discusses the similarities and differences among the releases.
A study of SCATHA eclipse charging
Authors: Li Wei‐Wei , Whipple Elden C.
Published: February 2008

We study eclipse charging events on the SCATHA spacecraft in a plasma environment which produced large vehicle potentials. We derive a simple relation which relates the spacecraft potential to spacecraft surface‐averaged and angle‐averaged fluxes and yields without assuming Maxwellian velocity distributions for the environment:
. The potential varies with the ion mean energy Ei and with the net high‐energy electron current Ies above a critical energy Ec. Below Ec the incoming electrons are balanced by their own secondaries. Above Ec the incoming electrons are balanced by incoming ions and both electron‐produced and ion‐produced secondaries. The potential also depends on the ion thermal current Iio and the average ion yield
. Data from five events show that the spacecraft potential correlates well with Ies when the ion properties stay relatively constant. Data from other events show that the potential varies with the ion mean energy. The magnitude of Ec is on the order of 20 to 30 keV. Quantitative prediction of the spacecraft potential depends sensitively on the secondary yield due to electrons.
A precursor to the Venus bow shock
Authors: Brace L. H. , Theis R. F. , Curtis S. A. , Parker L. W.
Published: February 2008

Electron and ion current measurements made by the Langmuir probe on the Pioneer Venus Orbiter exhibit significant enhancements just upstream from the Venus bow shock where one might have expected only solar wind plasma to be present. The region of enhancement, which we call the Venus precursor, is approximately centered on the subsolar point but extends in latitude up to solar zenith angles of about 80°; i.e., ahead of nearly the entire dayside shock. At higher latitudes, where its radial profile can be measured, the precursor extends about 1000 km ahead of the shock. The electron current is enhanced by a factor of about 1.6 at the nose, while the net ion current is enhanced by only a few percent because it is dominated by a large, but fixed, photoelectron emission current. There is little change in the amplitude and latitude of the precursor from orbit to orbit, a stability that suggests that this phenomenon is not very sensitive to the often variable solar wind and interplanetary magnetic field conditions. We suspect that the precursor signatures represent the effects of a small population of energetic ions in this region. Several possible planetary sources for these ions are discussed.
An analysis of the response of the thermospheric normal modes to temporally varying convection at high latitudes
Authors: Clark M. A. , Larsen M. F. , Mikkelsen I. S.
Published: February 2008

A normal modes model that includes realistic temperature and electron density profiles, and the effects of viscosity, heat conduction, and Lorentz forcing has been used to study the response of the high‐latitude thermosphere to temporal variations in the forcing due to the plasma convection pattern. In particular, the rotation rate of the two‐cell convection pattern was varied from the usual 2π in 24 hours to produce a crude simulation of the effects of temporal variations in the forcing. The model calculation proceeds from an initial steady state at 48 hours through four 6 hour integrations, where the rotation rate is alternately decreased and increased, corresponding to decelerated and accelerated flow. The results show that the rotational wind component is only slightly modified when the plasma rotation has been varied. The divergent wind component responds more strongly to variation of the rotation rate, an effect which is greatest in the E region. Variation of the rotation rate then results in greater magnitude and phase variation with height of the divergent wind component in the E region, indicating that vertically propagating gravity waves are of importance in that height range. To the extent to which varying the rotation rate can simulate variations in the configuration of the plasma convection, as are due to substorms or variations in the interplanetary magnetic field, the model indicates that detailed consideration of such geophysical effects is necessary if the transient response of the neutral wind is to be better understood.
Toward a comprehensive theory for the sweeping of trapped radiation by inert orbiting matter
Authors: Fillius Walker
Published: February 2008

There is a need to calculate loss rates when trapped Van Allen radiation encounters inert orbiting material such as planetary rings and satellites. An analytic expression for the probability of a hit in a bounce encounter is available for all cases where the absorber is spherical and the particles are gyrotropically distributed on a cylindrical flux tube. The hit probability is a function of the particle's pitch angle, the size of the absorber, and the distance between the flux tube and the absorber when distances are scaled to the gyroradius of a particle moving perpendicular to the magnetic field. Using this expression, we have computed hit probabilities in drift encounters for all regimes of particle energies and absorber sizes. This technique generalizes our approach to sweeping lifetimes and is particularly suitable for attacking the inverse problem, where one is given a sweeping signature and wants to deduce the properties of the absorber(s).
The tail lobe ion spectrometer: Theory and observations
Authors: Candidi M. , Orsini S. , Horwitz J. L.
Published: February 2008

In this study we present a comparison between the properties of the ionospheric ion streams in the Earth's magnetotail and the predictions of a two‐dimensional kinetic model for the transport of ionospheric ions from the cleft topside ionosphere into the tail lobes. General agreement is apparent between average data and model with appropriate assumptions for the ionospheric source. The same general features in the topology and similar numerical values of the stream density and velocity appear in the data and in the model predictions. However, variations associated with varying magnetic activity levels do not appear to be well modelled yet. In particular the assumption that magnetospheric activity as represented by the AE index, and the model numerical values of the cross‐tail electric field might be simply related to each other does not prove to be satisfactory. Model sophistication and data coverage and organization will have to be improved to study these effects in deeper detail.
Field‐aligned beams of energetic electrons (16 keV ≤ E ≤ 80 keV) observed at geosynchronous orbit at substorm onsets
Authors: Kremser G. , Korth A. , Ullaland S. L. , Perraut S. , Roux A. , Pedersen A. , Schmidt R. , Tanskanen P.
Published: February 2008

Magnetospheric substorms start with strong temporal and spatial variations of a large number of magnetospheric parameters. We report on the occurrence of bidirectional field‐aligned beams of energetic electrons (16 keV ≤ E ≤ 80 keV) that were observed on the geosynchronous satellite GEOS 2. The electron beams appear about 4 min after the onset of the substorm expansion phase and last about 1.5 min. They are associated with strong spatial gradients of the ion intensity and fluctuations of the geoelectric and geomagnetic fields. The beams are interpreted as the result of an acceleration of electrons in the parallel electric field of kinetic Alfvén waves. It is assumed that these waves are generated by mode conversion from a surface wave that is generated by a magnetohydrodynamic instability in the transition region between dipolelike and taillike geomagnetic field lines.
Duct propagation of hydromagnetic waves in the upper ionosphere, 1, Electromagnetic field disturbances in high latitudes associated with localized incidence of a shear Alfvén wave
Authors: Fujita Shigeru , Tamao Tsutomu
Published: February 2008

Mode conversion to the fast magnetosonic wave by anisotropic ionospheric currents from a localized incident shear Alfvén wave is studied in detail, based on a simple layered model inclusive of an anisotropic conducting sheet (the ionospheric E layer) and an upper ionospheric ducting layer under vertical, uniform ambient magnetic field lines. A horizontal profile of electromagnetic field intensity at the E layer shows a wider spread than that of the incident wave due to trapping of the fast magnetosonic wave in the ducting layer. Since electrostatic electric field intensity of the incident Alfvén wave is dominant in the central region, the eddy Hall current associated with the Alfvén wave generates the ground magnetic field variations. On the other hand, since electromagnetic electric field intensity of the trapped fast magnetosonic wave exceeds to that of the incident Alfvén wave in the region surrounding the central region, both the magnetic field of the fast magnetosonic wave and the secondary field due to the eddy Pedersen current contribute to the ground magnetic field variations.
Large‐scale fluctuations in the solar wind at 1 AU: 1978‐1982
Authors: Burlaga L. F. , Mish W. H. , Roberts D. A.
Published: February 2008

Large‐scale fluctuations (with spacecraft frame periods between several hours and the solar rotation period) in the magnetic field strength B(t) and bulk speed V(t) observed at 1 AU were analyzed using data from IMP 8 and ISEE 3. Most of the power is in the low‐frequency range (3 × 10−7 to 3 × 10−6 Hz). In the intermediate frequency range (3 × 10−6 to ∼3 × 10−5 Hz), the spectra of V(t) and B(t) computed for successive ∼6‐month intervals from 1978 to 1982 had the form f−b, where 〈bB〉 = 1.92 ± 0.06 for the magnetic field spectra and 〈bυ〉 = 1.92 ± 0.10 for the bulk speed spectra. There was no systematic variation of the spectral exponent with time during this interval. A detailed analysis of the bulk speed and magnetic field strength signals was carried out for the data from day 226 to day 365, 1978, which is representative of other such intervals. The spectra of the low‐frequency fluctuations during this interval are essentially the same as those computed from piecewise linear (jump‐ramp) approximations to the speed and magnetic field profiles. The spectra of the intermediate frequency fluctuations during that interval are the result of a superposition of two components: a component due to the jump‐ramp series, which falls off faster than f−2, and a component which falls off slower than f−2. The average width of the jumps in B is 2.2 hours. Twenty‐two percent of the jumps in B have the signature of a forward or reverse shock. The jumps in V are not all simultaneous with the jumps in B. The average width of the jumps in V is 2.1 hours. Thirteen percent of the speed jumps have the signature of a forward or reverse shock.
Neutron monitor latitude survey of cosmic ray intensity during the 1986/1987 solar minimum
Authors: Moraal H. , Potgieter M. S. , Stoker P. H. , van der Walt A. J.
Published: February 2008

A latitude survey of the cosmic ray intensity at sea level was conducted during the 1986/1987 solar minimum period on commercial vessels of the South African Marine Corporation (SAFMARINE). The results show that the differential response function for the 1986/1987 solar minimum agrees well with that measured in 1965. Both these response functions are significantly lower than those for 1976 and 1954. This result supports the 22‐year modulation cycle as predicted, for example, by models including drift effects of the charged cosmic ray particles in the large‐scale interplanetary magnetic field. A cross‐over of the spectra at rigidities of about 7 GV was also observed. Such a crossover is necessary to explain both the stationary neutron monitor counting rates and the lower‐energy balloon and space observations in consecutive solar cycles.
Lidar technique for measuring ionospheric barium release ion density
Authors: Goree J. , Neff J. S.
Published: February 2008

An evaluation of the resonant fluorescence lidar technique as a method of measuring ion density during ionospheric barium releases is presented. We find that it is technically feasible to measure ion densities of 103 cm−3 at an altitude of 200 km with a time resolution of 0.5 s by using a ground‐based laser and telescope. One application of the lidar technique would be observation of the critical ionization velocity effect at nighttime, without the complication of solar illumination. Criteria useful for evaluating the feasibility of other experiments are presented.
The effect of the ion acoustic decay instability on microwave‐plasma interactions
Authors: De Groot J. S. , Mizuno K.
Published: February 2008

Laboratory measurements are presented that show the ion acoustic decay instability strongly effects p‐polarized microwave‐plasma interactions. It is shown that the ion acoustic decay instability is only weakly excited if the unperturbed density profile is relatively steep. Only suprathermal (hot) electrons are heated due to resonance absorption. For a relatively shallow density profile, the ion acoustic decay instability is strongly excited in the underdense region near the critical surface. In this case, suprathermal electrons traveling both up and down the density profile are heated due to the electron waves driven by the ion acoustic decay instability. This additional heating results in a higher return current of thermal electrons. This current of thermal electrons must flow through the turbulent region so the thermal electrons are strongly heated due to anomalous dc resistivity. In addition, a strong dc electric field is driven in the turbulent region to maintain the required return current. These measurements can guide our understanding of the effect of strongly driven ion acoustic decay instability on microwave‐plasma interactions in the ionosphere.
ISIS 1 measurements of high‐frequency backscatter inside the ionosphere
Authors: James H. G.
Published: February 2008

A study has been made of high‐frequency backscatter observed by the ISIS 1 topside sounder at orbital heights from approximately 600 to 800 km. Signals are detected during high‐latitude passes at radar ranges x of up to about 400 km and at frequencies from about the X mode cutoff at the spacecraft (usually around 5 MHz) up to 10 MHz typically and 18 MHz occasionally. The signals are interpreted as coherent aspect‐sensitive scatter. The absolute power of return signals at high latitudes usually varies as x−2 for x⪙150 km. This implies that the scattering cross‐section density σ is constant for all azimuths about the magnetic field and for ranges out to 150 km or, in other words, that irregularity patches have a constant σ over cross‐field dimensions of 300 km. At larger ranges the signal often falls off more sharply with x, indicating azimuthal variations in σ. The cross section scaled from the data using the radar equation is found to have values centered near 10−8 to 10−7 m−1 for the most intense signals, referred to an assumed polar‐aspect angle sensitivity. However, the magnitude of σ drops by about 2 orders of magnitude in the frequency range 5–15 MHz. Previous statistical studies have established that strong backscatter is restricted to auroral lattitudes. Here the polar distribution of backscatter has been plotted for two sets of passes over the south pole, one collected during austral winter and the other during summer. The strongest backscatter is located at invariant latitudes around 80°. Backscatter is significantly stronger and distributed more extensively over the polar cap in winter than in summer.
Bounds on current collection from the far field by plasma clouds in the ionosphere
Authors: Hastings D. E. , Blandino J.
Published: February 2008

Plasma clouds can enhance the collection of current to a charged body in the ionosphere in two ways. The first is by providing a large collection area for ionospheric electrons to be collected. The second is by ionization of neutral gas in the vicinity of the plasma cloud. The collection of electrons across a magnetic field is examined, and it is shown that the effective collection area is limited to the region of the cloud where the magnetic field effects are overwhelmed by the electric field effects from the charged body. Upper and lower bounds are obtained for the radius of the core region where the electric field influence dominates. From these, upper and lower bounds are obtained for the current collection.
Computer simulations of cometary‐ion/ion instabilities and wave growth
Authors: Gary S. Peter , Akimoto Kazuhiro , Winske Dan
Published: February 2008

The growth of electromagnetic fluctuations driven by newborn cometary ions is studied by means of hybrid computer simulations of homogeneous plasmas. The simulations are one‐dimensional with the direction of spatial variation in the direction of the ambient magnetic field B0 Newborn ions are injected into the simulations at a constant rate, and are given a velocity relative to the solar wind which makes a nonzero angle α with respect to B0. For relatively strong free energy injection rates corresponding to oxygen ion injection at relatively small values of α, the fluctuating field energy density exhibits exponential temporal growth of the ion/ion right‐hand resonant instability. At the end of this growth regime there is a peak value of the fluctuating magnetic field energy density; at 0°≤α≲45° this quantity is approximately proportional to cos8α. For relatively weak free energy injection rates corresponding to oxygen ion injection at relatively large α and proton injection at all α values, there are two distinct regimes. At early times the fluctuating field energy density exhibits linear temporal growth, whereas at later times the fluctuating field energy approaches a constant value in an asymptotic regime. The exponential growth regime corresponds to fluctuating magnetic field energy density spectra with clear peaks at the oxygen ion cyclotron resonance wave number in agreement with observations. In contrast, spectra of the linear temporal growth regime exhibit less distinct peaks at shorter wavelengths which then show a transfer of wave energy to longer wavelengths (an apparent “inverse cascade”) as the asymptotic regime is approached. This absence of a clear spectral peak at the proton cyclotron resonance frequency is similar to cometary observations. The simulations and observations share two additional properties: a linear correlation between cometary proton energy density and proton resonant magnetic fluctuations, as well as a shift of magnetic fluctuation energy of the oxygen‐ion/proton modes to shorter wavelengths and weaker amplitudes as α increases toward 90°.
Magnetic aspect angle dependence of spectra from coherent radio aurora backscatter
Authors: Watermann J. , McDiarmid D. R. , Koehler J. A. , Sofko G. J. , McNamara A. G.
Published: February 2008

In 1981, a multiple path radar system was operated on the Canadian prairies with three 50‐MHz transmitters and two receivers all separately located such as to establish six backscatter links. The radar beams illuminated the same scatter volume in the ionospheric E layer. Four of these links produced coherent backscatter signals from 3‐m wavelength irregularities under different magnetic aspect angles ranging from 1.5° to 5.9° (reference height 110 km), but under almost identical drift flow angles. The radars were operated in the CW mode which yielded high time and spectral resolution but only moderate spatial resolution. In order to avoid confusion between spectral peaks from different locations within the backscatter volume, only single peaked spectra have been analyzed and compared. The Doppler shifts of both type I and type II spectra exhibit an aspect angle dependence which is much weaker than that predicted by existing linear and nonlinear theories.
The excitation of active whistler mode signal paths in the magnetosphere by lightning: Two case studies
Authors: Carpenter D. L. , Orville R. E.
Published: February 2008

In two ∼1 hr case study periods, the properties of whistlers propagating along multiple geomagnetic‐field aligned paths from points of origin in the northern hemisphere were compared to data on the location and intensity of lightning. The whistlers were recorded at the approximately conjugate stations Lake Mistissini, Canada and Siple Station, Antarctica, while the lightning data were acquired by the SUNY‐Albany lightning detection network operating in the eastern United States. In the two studies, which represented times near 0700–0800 LT and relatively quiet magnetospheric conditions, between one quarter and one half of the two‐hop whistlers observed at Lake Mistissini were found to have originated in ground flashes detected by the network. The uncorrelated whistlers are believed to have originated in lightning outside the network viewing area or in undetected ground flashes within the network. However, intracloud flashes within the network area cannot be ruled out as causes of some of the uncorrelated events. We confirm evidence from other workers that lightning can excite ducted whistler paths whose ionospheric endpoints are at ranges up to 2500 km or more from the lightning location. Once in the magnetosphere, the ducted waves were found to “spread” in L value by an interpath coupling process that was operative over the entire L range of detected whistler paths (∼1000 km in the north‐south direction at ionospheric heights in one case and ∼1500 km in the other). In both cases there was a single most active whistler path at L ≃ 4.7, at the high‐L limit of propagation paths within the plasmasphere. In one of the cases, lightning at L ≃ 1.6–1.8 illuminated this path through interpath coupling, after first exciting a path at L ≃ 2.2. In the other case, each of 15 flashes in a storm center at L ≃ 3.5 excited the path directly at a range of roughly 600 km. An approximately linear relation was found between the normalized first stroke peak magnetic field (which is proportional to the peak current) and whistler amplitude observed during a 10‐min period. The detection of whistler waves on a particular magnetospheric path and the relative intensity of the waves in successive whistlers appear to be strongly dependent upon the field strength of the impulsive radio signal from lightning at the point of ionospheric wave injection. Meanwhile, the distribution in space of the multiple paths, the absolute wave levels on the various paths, the duration of propagation in terms of higher order echoes, and the interactions of whistlers and VLF emissions propagating on the same path are found to depend primarily upon magnetospheric propagation conditions. For example, on both days the path transmitting the most whistler wave energy to a ground receiver was the one with entrance point the most distant from the lightning source.
Modeled F region response to auroral dynamics based upon Dynamics Explorer auroral observations
Authors: Sojka J. J. , Schunk R. W. , Craven J. D. , Frank L. A. , Sharber J. , Winningham J. D.
Published: February 2008

Auroral images from the Dynamics Explorer 1 (DE 1) scanning auroral imager have been combined with in situ auroral precipitation data from the DE 2 low altitude plasma instrument to form a time‐dependent global auroral energy flux model. This model has both good time (12 min) and spatial (100 km) resolution compared to that currently available for global scale ionospheric and thermospheric modeling. The development and comparison of this model with others are discussed. Data from an aurorally active period, November 25, 1981, are presented and used as a case study for this model. Using a global ionospheric model, the effect of the DE auroral model is contrasted with that of a conventional empirical auroral energy flux model. Major differences in the modeled F region ionosphere are predicted from this comparative study. Specifically, F region densities differ by factors of two to four, while density boundary locations differ by up to 5° in latitude. The results indicate that “pixel size” auroral fine‐structure must be included in the global ionosphere and thermosphere models when they are tested against specific ground‐based or satellite data sets if an unambiguous result is to be obtained. The longer time constants of the F region are not enough to smooth‐out the auroral (spatial and temporal) dynamics.
The POLAR Code Wake Model: Comparison with in situ observations
Authors: Murphy G. , Katz I.
Published: February 2008

Measurements of the ion and electron densities associated with the wake of the shuttle orbiter were made by the Plasma Diagnostics Package (PDP) during the 1985 Spacelab 2 mission. Cross sections of the wake at distances of 50–250 m downstream and measurements along the wake axis from 5 to 100 m were obtained. The POLAR wake model, developed for The Air Force Geophysics Laboratory to study charging of spacecraft in low‐altitude high‐inclination orbits, was used to perform a three‐dimensional simulation of the plasma wake evaluated at points along relative trajectory of the PDP. The POLAR code uses several simplifying assumptions to predict wake densities. These include neglecting the magnetic field and assuming that the plasma is quasi‐neutral. The code models plasma density ahead of the expansion front, using a neutral approximation, and models the plasma density behind the expansion front by using the self‐similar solution of the expansion of a plasma into a vacuum. For cases where Ti ≈ Te, the front is not sharp and thermal motion can account for most of the expansion. This approach is computationally very efficient. The results presented here are the first known comparison between such a model and actual in situ data obtained for objects of scale size ≈104 λd. Excellent qualitative and quantitative agreement are found at distances greater than ≈30 m, indicating that at least to first order, the model's approximations are justified. An intriguing disparity between the model and data suggests that the orbiter's near wake may be filled predominantly by a pick‐up ion population created from neutral contaminants and that these would have to be included if accurate wake models of large gas‐emitting objects are required.
Interpretation of electron cyclotron harmonic waves observed at an Ampte lithium release in the solar wind
Authors: Baumgärtel K. , Sauer K.
Published: February 2008

Recently reported observations of electrostatic emissions in the first five harmonic bands of the electron cyclotron frequency, following a lithium release in the solar wind, are interpreted in terms of quasi‐static waves with k ⊥ B (Bernstein waves), excited by photoionization of Li atoms in the magnetized, warm solar wind plasma. The observed distinct frequencies are selected from the excited broad band spectrum by the condition of zero group velocity in the satellite frame.
Entry of solar wind particles into Earth's magnetosphere
Authors: Lee L. C. , Akasofu S.‐I.
Published: February 2008

Entry of solar wind particles into Earth's magnetosphere is studied. Particles can move directly across an “open” magnetopause layer, where the magnetic field has a nonzero normal component (Bn ≠ 0). Under the assumption that the Walén relation holds near the open magnetopause, the solar wind particle influx can be expressed as
, where
is the average particle number density, ψB is the open magnetic flux at the magnetopause, μ0 is the permeability of free space, and mi is the ion mass. The total particle flux from the solar wind to the entire magnetosphere for a moderate negative value of IMF Bz is estimated to be ∼7.0×1028 s−1, although most of the particles leave from the antisolar end of the tail without being trapped. The peak flux through the dayside magnetopause is ∼1.1×1027 s−1, while the peak flux through the near‐Earth tail magnetopause (|x| ≤ 30RE) is ∼3.0×1027 s−1. The average particle flux through the dayside and tail magnetopause in the region with |x| ≤ 200RE is found to be ∼7.9×1027 s−1, which is comparable to the tailward particle flux (∼7.1×1027 s−1) observed at |x| ≃ 200 − 220RE in the distant tail. On the other hand, the total flux through the closed magnetopause (Bn = 0) into the boundary layer in the region with |x| ≤ 200RE is estimated to be ∼7×1027 s−1, which is also comparable to the observed flux. For comparison, the ionospheric source contributes a particle flux of only 3 − 5 × 1026 s−1. Thus, the ionosphere can be an important particle source for the inner magnetosphere (|x| ≤ 30RE), but not for the entire tail or entire magnetosphere.