bluuebunny/crossref_metadata_2025 huggingface dataset:
ISEE 1 and 2 observations of ion distributions at the plasma sheet‐tail lobe boundary
Authors: Takahashi K. , Hones E. W.
Published: February 2008

Observations of plasma ions (0.075–29 keV) at or near the plasma sheet‐tail lobe boundary made with the ISEE 1 and 2 spacecraft are the principal topic of this paper. We describe crossings of this boundary that occurred during various levels of geomagnetic activity at distances XSM = −7 to −13 RE, near local midnight, and 2 RE or more above the estimated location of the tail current sheet. Fluxes of ions directed along the magnetic field lines (referred to as ion beams) with kinetic energies of 1‐ > 29 keV are commonly observed during crossings of this boundary. The ion beams nearest the edge of the plasma sheet are directed earthward, while deeper within the plasma sheet, additional ion beams, directed tailward, are encountered, usually resulting in bidirectional or counterstreaming ion distributions. The ion distribution functions for these clearly indicate that the tailward directed beams are reflected versions of the earthward directed beams, returning after mirroring in the stronger magnetic field near the Earth. From an intersatellite timing analysis the thickness dB of the region of solely earthward streaming ions is estimated to be a few tenths of an Earth radius. The fastest particles (plasma electrons and the highest‐energy ions) are the first to be seen as a satellite enters the plasma sheet from the lobe and last to be seen as it leaves the plasma sheet to enter the lobe. This suggests that the ion beams are components of a layer of plasma that is projected earthward along field lines from a more or less steady source tailward of the satellite and that is E × B drifting equatorward. The observations are consistent with theoretical predictions for an open model of the magnetosphere having a neutral line tailward of the satellite and having the plasma sheet‐lobe interface coincident with the separatrix between closed and open field lines. We therefore identify the beam‐containing plasma layer contiguous to, and extending equatorward from, this interface as the separatrix layer. The velocity, temperature, and kinetic energy flux of the earthward beams in the separatrix layer are found to correlate positively with the AE index. We rarely observe isolated tailward streaming ion beams near the boundary, although slow (<300 km/s) tailward flows there sometimes result from an imbalance of particle fluxes carried by the oppositely streaming beams. We show that such net tailward ion anisotropies, occasionally observed in our data set, differ distinctly from the tailward bulk flow event reported by Hones et al. (1986). That event had all ion components moving tailward with a net velocity of ∼500 km/s; it was interpreted to be the result of magnetic reconnection at a near‐Earth neutral line that formed at the onset of a substorm and tailward departure of the resulting plasmoid.
Transition region, corona, and solar wind in coronal holes: Some two‐fluid models
Authors: Hollweg Joseph V. , Johnson Walter
Published: February 2008

We consider the possibility that the heating of the corona and the heating and acceleration of the solar wind can be described by a single process, namely, the turbulent dissipation of solar‐generated Alfven waves at the Kolmogorov rate. The model assumes that Te = Tp in r ≤ 2 Rs but drops electron‐proton coupling in r > 2 Rs. The dissipated wave energy is assumed to heat only the protons. Classical heat conduction is used in r ≤ 10 Rs, and an electron polytrope is used in r > 10 Rs. The models have the right qualitative features: a steep temperature rise to T > 106 K and acceleration to supersonic speeds. But models with base pressures neT > 2×1014 (cgs) are too slow: υ(1 AU) < 280 km s−1. Models with υ(1 AU) > 400 km s−1 have lower base pressures: 1014 < neT < 2×1014 (cgs). A difficulty with the model is that line‐of‐sight proton random velocities (thermal plus wave) are larger than values deduced from Lyman α resonant scattering observations in 1.5 ≤ r/Rs ≤ 4, and they do not fall off with r to the extent observed. The large random velocities are due in part to a proton temperature peak at r ≈ 3–4 Rs. On the whole, this model seems unsatisfactory, but several possible resolutions are discussed.
Fine resolution measurements of the L‐dependent energy threshold for isotropy at the trapping boundary
Authors: Imhof W. L.
Published: February 2008

The fluxes of trapped and precipitating particles are often nearly equal at the trapping boundary in the radiation belts at low satellite altitudes. Near midnight this isotropy phenomenon was previously discovered frequently to occur only above an energy threshold which decreases rapidly with increasing L value. Now, much finer energy resolution measurements of the phenomenon for electrons have been performed with much lower backgrounds and higher sampling rates, and hence better statistical accuracies in the energy spectra. Here we present details on the strong dependence of the measured ratios of precipitating to trapped electrons on the gyroradius of the particles. Published theoretical calculations for the L dependences of the magnetic rigidity of the onset of isotropy for both electrons and protons near midnight are compared with the present measurements under both very quiet and very disturbed magnetic activity conditions. The measured curves are generally steeper than the theoretical ones except for a few at very disturbed times when reasonable agreement is achieved. The frequency of occurrence of an energy selective threshold for isotropy at the trapping boundary is presented as a function of magnetic local time; it was observed on 34% of the passes near midnight and seldom near noon.
A reexamination of the O I 6300‐Å nightglow
Authors: Link R. , Cogger L. L.
Published: February 2008

In an analysis of 6300‐Å nightglow measurements obtained at Arecibo on the night of April 25/26, 1976, Cogger et al. (1980) reported that model estimates exceeded the observed 6300‐Å intensity by a factor of 2. We have reanalyzed the data in light of newer laboratory and aeronomic determinations of the parameters used to calculate the 6300‐Å intensity and now obtain agreement with the measurements. The rate coefficient for the reaction O+ + O2 → O2+ + O required to fit the data is 27% smaller at 700 K than previously assumed, but is in agreement with the thermal drift tube measurements of Chen et al. (1978). A specific recombination rate of 1.2×107 cm³ s−1 (at 700 K) for production of O(¹D) from O2+ is consistent with the data. This value is somewhat smaller than the recent theoretical calculation of Guberman (1988) for recombination from O2+(υ = 0) and may imply that O2+ is not completely vibrationally relaxed in the nighttime thermosphere. The reaction N(²D) + O2 → NO + O(¹D) contributes <10% to nightglow 6300‐Å production, even for an assumed O(¹D) yield of unity. Quenching of O(¹D) by O(³P) and the larger transition probabilities suggested by Abreu et al. (1986) result in calculated 6300‐Å intensities virtually indistinguishable from those obtained using the rate coefficients of Link et al. (1981). This suggests that it will be difficult to verify O(¹D) quenching by atomic oxygen using nightglow measurements.
An investigation of the second negative system of O2+ by electron impact
Authors: James G. K. , Ajello J. M. , Shemansky D. E. , Franklin B. , Siskind D. , Slanger T. G.
Published: February 2008

We have measured the calibrated optically thin spectrum of O2 from 115 to 300 nm induced by electron impact at 200 eV. Spectra were obtained in a crossed beam configuration at 1.25‐nm resolution. We have observed the second negative system of O2+ (A ²Πu → X²Πg) in the range 186–300 nm. By comparison with O I (130.4 nm) and using the results of model calculations we estimate the emission cross section for the O2+ A state at 200 eV to be 2.57±0.57×10−18 cm². The excitation function from threshold to 400 eV has also been measured at 220 nm and compared to the total ionization cross section. We have used the results of our laboratory measurements to model the contribution made by the second negative system of O2+ to auroral spectra in the same wavelength range. Observations of the spectral region from 140 to 185 nm show no detectable O or O+ transitions with a cross section upper limit of 10−21 cm².
An upper E region quarterdiurnal tide at Arecibo?
Authors: Tong Yu , Mathews J. D. , Ying W. ‐P.
Published: February 2008

Studies of the time‐height trajectories of ionosphere E region layers at Arecibo have revealed not only the usual diurnal sporadic‐E and semidiurnal intermediate layer structures but also that on occasion, extra layers form and descend through the E region. In particular, during January 1981, four layers per day were observed to descend through the upper E region. This four‐per‐day layer system was remarkably coherent over the 3 days of observation and suggests the existence of a 6‐hour period or quarterdiurnal tidal wind system in the upper E region. Numerical simulations of ion trajectories in a realistic model atmosphere with diurnal, semidiurnal, and quarterdiurnal tidal wind systems of appropriate amplitude and phase structures confirm that the presence of a quarterdiurnal tide is sufficient to explain the observations.
Longitudinal differences in electron precipitation near L = 4
Authors: Bering E. A. , Benbrook J. R. , Leverenz H. , Roeder J. L. , Stansbery E. G. , Sheldon W. R.
Published: February 2008

The differences in the electron precipitation characteristics as seen above Siple Station, Antarctica, and the Kerguelen Islands have been studied. These two sites are both in the southern hemisphere at nearly the same magnetic latitude (L=4). The two stations are at longitudes that place them roughly equal distances east and west of the center of the South Atlantic magnetic anomaly at this L value. With respect to the direction of electron drift, Siple is upstream of the anomaly, and Kerguelen is downstream. The primary data used in the study were counting rates from rocket‐borne, parachute‐deployed, sodium iodide scintillation counters at altitudes of 80 to 30 km and VLF wave data from both ground‐based and rocket‐borne receivers. Nine rocket flights are involved, five from Kerguelen and four from Siple. The flights were made under a range of geomagnetic and VLF activity conditions. The data differ in two major respects. First, the precipitation background at Kerguelen is very low, with high levels of wave activity being required to produce any detectable precipitation, an observation which indicates that there is a range of longitude and/or magnetic local time in which the electron pitch angle diffusion rate is 2 orders of magnitude lower than the long‐term global average rate. Second, X ray microbursts were found to be essentially absent at Kerguelen and very common at Siple. This observation appears to support models of the microburst generation process which predict maximum pitch angle scatterings of only a few tenths of a degree.
The electromagnetic effect on the critical ionization velocity process
Authors: Machida S. , Goertz C. K.
Published: February 2008

Electromagnetic effects on the critical ionization velocity (CIV) process become important when the neutral gas velocity Vn exceeds the local Alfvén speed VA. The electron heating due to unstable lower hybrid waves necessary for CIV still occurs, but the efficiency of the electron heating is significantly reduced when the electromagnetic effect comes into play. This is verified by a series of simulation runs using two‐dimensional electromagnetic particle code combined with PANIC. The significance of the electromagnetic effects for the occurrence of CIV in the comet‐solar wind interaction and other space phenomena is briefly discussed. We find that the comet environment is marginal for the excitation of CIV.
Magnetospheric ULF waves observed during the major magnetospheric compression of November 1, 1984
Authors: Takahashi K. , Kistler L. M. , Potemra T. A. , McEntire R. W. , Zanetti L. J.
Published: February 2008

Magnetic field and medium‐energy ion (E > 4 keV) data acquired by the AMPTE CCE spacecraft have been used to study magnetospheric ULF waves observed during the unusually strong magnetospheric compression event of November 1 (day 306), 1984. Although the spacecraft does not carry an electric field experiment, a method has been developed to infer the electric field of ULF waves from the particle data, based on the gyration acceleration mechanism. We have found three types of waves: (1) a 10‐ to 15‐min wave with perturbations in the magnetic field intensity and in the flux of ions, (2) a 3‐ to 5‐min standing Alfvén wave with perturbations in the azimuthal magnetic field component and in the radial component of the electric field, and (3) a 2‐ to 5‐min irregular disturbance near the magnetopause which involves all components of the magnetic field and the intensity of the ion flux. The origin of the 10‐ to 15‐min compressional wave is not clear, although a global fast‐mode cavity resonance is one possibility among others. Since the frequency of the compressional wave is much lower than that of the Alfvén wave, the simultaneous presence of the two waves cannot be described by the coupling of a global cavity resonance to a standing Alfvén wave. The magnetopause disturbance, which has frequency components similar to the standing Alfvén wave, has some properties inconsistent with an azimuthally propagating standing wave. The Alfvén wave observed throughout the outer magnetosphere may be attributed to the transient response of the field lines to a series of externally applied pressure disturbances.
Boundary layer dynamics in the description of magnetospheric substorms
Authors: Eastman T. E. , Rostoker G. , Frank L. A. , Huang C. Y. , Mitchell D. G.
Published: February 2008

Magnetospheric substorm activity has been previously explained primarily as the effects of the episodic unloading of energy stored in the Earth's magnetotail. The physics of the unloading process has been associated with magnetic field line reconnection, with one dominant school of thought contending that reconnection occurs close to the Earth (i.e., inside of ∼15 RE). This near‐Earth neutral line model seeks to explain plasma and field characteristics observed in the magnetotail by the development and ultimate disappearance downtail of a plasmoid involving the population of the plasma sheet antiearthward of the near‐Earth neutral line. In this paper we present a study of a significant number of substorm expansive phase events where the phenomenology at ionospheric levels is clearly defined in terms of onset time and locale of the substorm disturbed region. Through the detailed presentation of a subset of these events it is demonstrated that the magnetotail particle and field observations associated with a substorm expansive phase can equally well be explained through the spatial movement of the boundary layers and central plasma sheet in the magnetotail. The sweeping of these regions past the satellite, even in the absence of temporal variations within the various regions, can lead to a set of plasma flow observations typical of what is observed in the magnetotail during substorm activity. We further demonstrate that filamentary currents in the plasma sheet boundary layer can produce the magnetic field signatures presently attributed to retreating plasmoids in the magnetotail. The major strength of this emphasis on boundary layer dynamics is that it provides a framework which naturally encompasses both the driven system and loading‐unloading system which are recognized to be operative simultaneously during magnetospheric substorm activity.
A multi‐instrument study of flux transfer event structure
Authors: Farrugia C. J. , Rijnbeek R. P. , Saunders M. A. , Southwood D. J. , Rodgers D. J. , Smith M. F. , Chaloner C. P. , Hall D. S. , Christiansen P. J. , Woolliscroft L. J. C.
Published: February 2008

We present simultaneous measurements at high temporal resolution of flux transfer events (FTEs) earthward of the magnetopause using four instruments on the Active Magnetospheric Particle Tracer Explorers (AMPTE) UKS spacecraft. The seven events in our study are distributed in local time from early dawn to midafternoon over a GSM latitude range of ∼40°, from 24°N to 17°S. A multilayered structure is revealed with systematic behavior in fields and charged particle populations. The plasma pressure varies in anticorrelation with the field pressure, the total pressure in general exceeding that of the surrounding medium. While it is not the prime concern of this paper to discuss the validity of models proposed for FTEs, the new, detailed experimental results it contains make it clear that the Russell‐Elphic [1978] model needs, at the very least, more detailed development.
Particle acceleration and wave emissions associated with the formation of auroral cavities and enhancements
Authors: Winglee R. M. , Pritchett P. L. , Dusenbery P. B. , Persoon A. M. , Waite J. H. , Moore T. E. , Burch J. L. , Collin H. L. , Slavin J. A. , Sugiura M.
Published: February 2008

Observations from DE 1 and electrostatic particle simulations are combined in an effort to provide a unified model for (nightside) auroral particle acceleration and wave emissions and their association with plasma cavities and enhancements. The observations show that enhanced electron precipitation during inverted‐V events is associated with broadband electrostatic bursts (BEB), increased upward field‐aligned currents, and density enhancements. These regions are flanked by return current regions where the density is depleted (i.e., by plasma cavities). Perpendicular acceleration of ambient plasma ions can occur in both upward and return current regions. It is shown through the simulations that these processes are integrally related and are not independent of each other. The free energy for the auroral particle acceleration can be provided by energetic ion beams in the plasma sheet boundary layer with nonzero perpendicular energy. The perpendicular energy allows charge separation between the beam ions and costreaming electrons to occur. The resultant space charge fields accelerate electrons on the same field lines as the costreaming electrons downward toward the ionosphere, without the beam ions actually propagating down to auroral altitudes. Ambient plasma electrons on adjacent field lines are accelerated upward, forming a return current. Because these currents are spatially separate, a perpendicular electrostatic shock develops which accelerates the plasma ions across the field lines in an effort to close the currents. This acceleration creates ion conics in velocity space and, in coordinate space, plasma cavities are formed in the return current regions and plasma enhancements in the enhanced positive current regions. Strong broadband electrostatic waves are generated with spectral maxima being generated near the local electron plasma frequency by the accelerated electrons and near the lower hybrid frequency by the accelerated ions, similar to that observed in BEB.
Low‐altitude trapped protons at the geomagnetic equator
Authors: Guzik T. G. , Miah M. A. , Mitchell J. W. , Wefel J. P.
Published: February 2008

Geomagnetically trapped protons in the 0.6‐ to 9‐MeV energy range were measured at latitudes near the geomagnetic equator by the Phoenix 1 experiment on board the S81‐1 mission from May to November 1982. The protons show a distribution in latitude along the line of minimum magnetic field strength with a full width at half maximum of ∼10° but with no appreciable longitudinal variation. Between 170 and 290 km the peak proton flux shows a fifth‐power altitude dependence, in contrast to previous measurements at higher altitudes, possibly demonstrating source attenuation. The efficiency of the telescope is calculated as a function of particle pitch angle and used to investigate the time dependence (1969‐1982) of the intensity.
Pickup ions in the unshocked solar wind at comet Halley
Authors: Kecskeméty K. , Cravens T. E. , Afonin V. V. , Erdös G. , Eroshenko E. G. , Gan Lu , Gombosi T. I. , Gringauz K. I. , Keppler E. , Klimenko I. N. , Marsden R. , Nagy A. F. , Remizov A. P. , Richter A. K. , Riedler W. , Schwingenschuh K. , Somogyi A. J. , Szegö K. , Tatrallyay M. , Varga A. , Verigin M. I. , Wenzel K.‐P.
Published: February 2008

The Tünde‐M experiment on board the VEGA 1 spacecraft detected energetic cometary ions as far as 10 million kilometers from comet Halley. The measured ion fluxes increased with decreasing radial distance from the nucleus, in agreement with theoretical expectations. Sharp enhancements spaced at approximately 4‐hour intervals were superimposed on the general increase of the ion fluxes on the inbound leg of VEGA 1. Periodic neutral structures originating from gas production by active areas on the nucleus are suggested to explain the recurrent intensity peaks. These neutral density enhancements would then produce the ion density enhancements seen by Tünde‐M as the spacecraft traversed the structures. This explanation requires a considerably larger neutral expansion velocity (≈6 km/s) than was actually measured near the nucleus in order to allow the rotation period of the nucleus to generate the observed period of the enhancements. No obvious correlations of the ion fluxes with the limiting energy for pickup were found. However, at a distance of about 107 km from the comet, a weak anticorrelation was found between the ion flux and the angle between the interplanetary magnetic field and solar wind direction. Cometary ion distribution functions in the solar wind reference frame were derived from measured counting rates. These distributions are approximately Maxwellian for energies between about 100 keV and 150 keV. The derived temperatures are between about 5 and 8 keV for most cometocentric distances, although in a region between 10 and 8 million km from the nucleus the ion spectra were harder, with temperatures ranging from 10 to 20 keV.
Magnetospheric cavity modes: Numerical model of a possible case
Authors: Allan W. , McDiarmid D. R.
Published: February 2008

Recently, a transient ULF wave observed using ground‐based magnetometers and an incoherent scatter radar was interpreted as a highly damped field line resonance driven by a compressional magnetospheric cavity mode. By applying a numerical model of MHD wave coupling in the magnetosphere, and choosing parameters to simulate the event, ground magnetic field time series are derived which are similar to the observed time series, provided the plasma mass density gradient in the equatorial plane is chosen to be relatively small. We consider this to be good supporting evidence for a cavity mode drive. However, our results are of direct interest in themselves, as they demonstrate for the first time the structure of a highly damped field line resonance driven by a compressional cavity mode. By analysis of the fields in the equatorial plane, it is shown firstly that the compressional magnetic field has a relatively large amplitude near the magnetopause, so that most of the cavity mode energy is stored within about 3 RE of the magnetopause; and secondly that, because of the high damping, the modeled field line resonance is poorly developed and acts to channel roughly one third of the cavity mode energy to the ionosphere. In the model, which has the same ionospheric boundary condition for both electric field components, the rest of the cavity mode energy is lost by direct Joule dissipation in the ionosphere. However, the size of the observed ionospheric electric field components suggests that a more reflecting boundary condition may be appropriate for the cavity mode electric field. If so, we speculate that only about 10% of the cavity mode energy was lost directly to the ionosphere; more than half was possibly lost by leakage from the dayside cavity down the magnetotail.
On the effect of a tangential discontinuity on ions specularly reflected at an oblique shock
Authors: Burgess D.
Published: February 2008

In seeking to explain the events observed close to the Earth's bow shock known as hot, diamagnetic cavities (HDC), or active current sheets (ACS), attention has focused on the microphysics of the interaction of a magnetic field directional discontinuity and a collisionless, supercritical shock. Here we investigate the case of a tangential discontinuity (TD) convecting into a shock at some arbitrary angle. As a first stage we adopt an approach in which test particles represent ions specularly reflected at the shock front. Widely different behavior is possible depending on the sense of ion gyration relative to the TD. Particles can be injected into the plane of the TD so that they travel upstream trapped close to the TD. This implies that ACS events, presumed to be the result of the interaction of the solar wind with a large density reflected component, are detached from the bow shock. For other geometries, ions interact with the TD but stay close to the shock, implying that ACS events are modifications of the shock. The TD can deprive a limited spatial region of a downstream reflected‐gyrating ion population (necessary for the quasi‐perpendicular supercritical shock to be steady), and so we can anticipate where the shock will not be in equilibrium, and consequently where strong reflection may occur. The detailed behavior of the shock in such a situation must be investigated with self‐consistent simulations.
Phase and amplitude perturbations on the NWC Signal at Dunedin from lightning‐induced electron precipitation
Authors: Dowden R. L. , Adams C. D. D.
Published: February 2008

Localized ionospheric depressions near the NWC‐Dunedin great circle path diffract echoes which interfere with the direct signal at the Dunedin receiver to produce perturbations in phase and amplitude. The statistics both of these perturbations and of the echo phasors (echo magnitude and echo phase) which can be deduced from them are studied here. From these statistics it is deduced that echo paths must be frequently more than a wavelength (14 km) longer than the direct path so that many of the diffracting centers (electron precipitation beams) must be laterally displaced up to 200 km from the direct path. Since echo signals from these must be diffracted through angles of ∼10°, ionization enhancements produced by electron precipitation must frequently have lateral (cross‐path) dimensions of less than 50 km, with some as narrow as 25km. The largest perturbation magnitudes seem to require ionization enhancements of longitudinal (parallel to path) dimensions of ∼300 km. Electron precipitation confined to thin L‐shells could produce such enhancements for the NWC‐Dunedin path.
Observation of OH Meinel (7,4) P( N″=13) transitions in the night airglow
Authors: Pendleton W. , Espy P. , Baker D. , Steed A. , Fetrow M. , Henriksen K.
Published: February 2008

Improved spectral measurements of the OH Meinel Δv = 3 night airglow emissions have revealed unexpectedly intense transitions from high rotational levels. The example selected for this communication involves the previously unreported P(N″ = 13) transitions in the OH M (7, 4) band. Under the extant conditions, the column emission rates associated with these new features exceeded by factors ∼104 those expected on the basis of the assumption of rotational‐kinetic equilibrium for the v′ = 7 rotational manifold. We present the key observations and discuss some of the implications.
The influence of anisotropic F region ion velocity distributions on ionospheric ion outflows into the magnetosphere
Authors: Suvanto K. , Lockwood M. , Fuller‐Rowell T. J.
Published: February 2008

The contribution to the field‐aligned ionospheric ion momentum equation, due to coupling between pressure anisotropy and the inhomogeneous geomagnetic field, is investigated. We term this contribution the “hydrodynamic mirror force” and investigate its dependence on the ion drift and the resulting deformations of the ion velocity distribution function from an isotropic form. It is shown that this extra upforce increases rapidly with ion drift relative to the neutral gas but is not highly dependent on the ion‐neutral collision model employed. An example of a burst of flow observed by EISCAT, thought to be the ionospheric signature of a flux transfer event at the magnetopause, is studied in detail and it is shown that the nonthermal plasma which results is subject to a hydrodynamic mirror force which is roughly 10% of the gravitational downforce. In addition, predictions by the coupled University College London‐Sheffield University model of the ionosphere and thermosphere show that the hydrodynamic mirror force in the auroral oval is up to 3% of the gravitational force for Kp of about 3, rising to 10% following a sudden increase in cross‐cap potential. The spatial distribution of the upforce shows peaks in the cusp region and in the post‐midnight auroral oval, similar to that of observed low‐energy heavy ion flows from the ionosphere into the magnetosphere. We suggest the hydrodynamic mirror force may modulate these outflows by controlling the supply of heavy ions to regions of ion acceleration and that future simulations of the effects of Joule heating on ion outflows should make allowance for it.
Flow and aspect angle dependence of 3‐m irregularities in the auroral E region
Authors: André Dieter , Sofko George J. , Koehler James A. , McNamara Allen G. , McIntosh Bruce A.
Published: February 2008

Measurements from the BARS radars have been used to derive the dependence of the relative backscatter cross section on flow angle, aspect angle and irregularity drift velocity. Aspect angles between 87° and 81° and drift velocities up to 700 m/s could be covered. The variation of the backscatter cross section shows general agreement with the growth rate for two‐stream instability. The flow angle dependence is approximately a cos2 variation, whose amplitude increases with velocity and decreases with aspect angle. The aspect angle dependence shows almost no variation with velocity; the cross section decreases steeply between 87° and 86° and stays constant between 86° and 81°.
Velocity and abundance of silicon ions in the solar wind
Authors: Bochsler P.
Published: February 2008

Using data from the ISEE‐3 ion composition instrument (ICI), velocities and abundances of silicon ions in the solar wind have been determined. The period of investigation covers the maximum of solar cycle 21, beginning with launch of ISEE‐3 in August 1978 and ending at the removal of the spacecraft from the Lagrangian Point L1 in June 1982. The results generally confirm previous ICI observations of iron, the other heavy element with a low first ionization potential measured with the ICI. Silicon ions (and other ions in the same M/Q range) tend to stream at the bulk velocity of 4He++ in low‐speed solar wind. At high‐speed solar wind, Si lags by up to or about 20 km/s after 4He++. By means of a minimum variance estimation technique, fluxes (and densities) of silicon in the solar wind have been obtained free of bias. An average Si/O flux ratio of 0.19±0.04 is derived. This value is larger by a factor of 3 or 4 than the Si/O abundance ratio at the solar surface.
A survey of electron cyclotron waves in the magnetosphere and the diffuse auroral electron precipitation
Authors: Roeder J. L. , Koons H. C.
Published: February 2008

Natural electron cyclotron harmonic emissions in the outer magnetosphere are often cited as the electron scattering mechanism which results in the diffuse auroral precipitation. A survey is presented of the characteristics of these waves using data from both the SCATHA and Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM plasma wave instruments. The emissions were observed most often in the 0300–0600 LT sector at L ∼ 4–8 and magnetic latitudes in the range ±10°. In this region, emissions exceeding 35 µV m−1 were detected only 25% of the time, and those exceeding 12 µV m−1 were detected 60% of the time. In agreement with Belmont et al. (1983), these amounts are grossly insufficient to account for the diffuse auroral electron precipitation by quasi‐linear pitch angle diffusion.
Quantitative patterns of large‐scale field‐aligned currents in the auroral ionosphere
Authors: Foster J. C. , Fuller‐Rowell T. , Evans D. S.
Published: February 2008

Quantitative patterns of the distribution of field‐aligned current (FAC) density have been derived from gradients of the average patterns of the Hall and Pedersen currents at high latitudes under the assumption that the total current is divergence‐free. The horizontal currents were calculated from empirical convection electric field models, derived from Millstone Hill radar observations, and the ionospheric Hall and Pedersen conductances, based on satellite observations of the precipitating particle energy flux and spectrum and including an average (equinox) solar contribution. These independent empirical models, and the resultant patterns of the field‐aligned currents, are keyed to an auroral precipitation index which quantifies the intensity and spatial extent of high‐latitude particle precipitation and which is determined from a single satellite crossing of the auroral precipitation pattern. The patterns detail the spatial distribution of the currents as a function of increasing disturbance level. The magnitudes of the total single‐hemisphere currents into or out of the ionosphere are closely balanced at each activity level and increase exponentially between 0.1 and 6 MA with increasing values of the precipitation index. The interplanetary magnetic field (IMF) sector dependence of the FAC patterns is investigated for disturbed conditions. A large portion of the FAC pattern is closed by local Pedersen currents (current into the ionosphere is balanced by an equal current out of the ionosphere at that local time). This locally balanced portion of the FAC system is enhanced in the prenoon (postnoon) sector for IMF By > +1 nT (By < −1 nT). In addition, there are net currents into the ionosphere postnoon and out of the ionosphere in the premidnight sector. Whereas the magnitude of these nighttime net FACs increases with increasing negative IMF Bz, the magnitude and distribution of the net currents appear to be independent of the IMF By orientation.
Distribution of auroral arcs during quiet geomagnetic conditions
Authors: Lassen K. , Danielsen C.
Published: February 2008

Auroral arcs observed from the Greenland all‐sky camera network during quiet intervals (AE<50 nT) have been ordered in corrected geomagnetic latitude/time mass plots for different values of AE. The arcs are distributed in a pattern which is shown to coincide with the precipitation belt of auroral electrons determined by the DMSP satellites. This belt is known to be composed of two parts: an equatorial part (average energy of >500 eV) and a poleward, low‐energy part. Previous studies have shown that the arc pattern is composed of two subpatterns, too, the “polar cap arc pattern” and the “oval arc pattern.” It is demonstrated that the “polar cap arc pattern” is situated in the poleward, low‐energy part of the precipitation belt, connected to the low‐latitude boundary layer, whereas the “oval arc pattern” is in the equatorial higher energy belt, connected to the plasma sheet. The dividing line between the two arc patterns is associated with the boundary of trapped ≥40‐keV electrons. The designation “polar cap arc pattern” is shown to be ambiguous, wherefore it is proposed to replace it by the term “high‐latitude arc pattern.”
Estimates of alpha particle heating in the solar wind inside 0.3 AU
Authors: Thieme K. M. , Marsch E. , Rosenbauer H.
Published: February 2008

Helios observations are used to determine and compare the average temperature gradients of protons and alpha particles in the solar wind between 0.3 and 1 AU. The He2+ ions are found to cool off more slowly than adiabatically. The radial profiles of their temperature are somewhat steeper in fast than in slow solar wind, a result which is in contrast to the proton results. These in situ measurements are extrapolated to inside 0.3 AU in order to estimate the radial gradients of Tα there and the required heating rates. Thereby, the temperature profiles are adjusted to match coronal boundary values. Clear evidence is found for continuous interplanetary alpha particle heating, which supports theories invoking extended ion heating by waves in the outer solar corona.
Interplanetary energetic ions and polar radio wave absorption
Authors: Armstrong T. P. , Laird C. M. , Venkatesan D. , Krishnaswamy S. , Rosenberg T. J.
Published: February 2008

This is a study of the ionization input of interplanetary (including solar flare) energetic protons and alpha particles into the south polar ionosphere over the interval 1982–1985. Energetic particle data are collected by the charged particle measurement experiment (CPME) of the Applied Physics Laboratory/Johns Hopkins University, onboard the IMP 8 satellite orbiting the Earth at 30–35 Earth radii. It is well known that interplanetary ions have full and prompt access to the polar ionosphere. The incremental ionization produced at 20–120 km. altitudes causes enhanced radio wave absorption which is observed by riometers operated by the University of Maryland, at South Pole, Antarctica. We compute the expected absorption from the vertical structure of the ionization deposited by these energetic particles and compare the computed values with the observations. The contribution of the alpha particles is found to be quite small as a percentage of the absorption except at the peak of the day 35, 1983, event, when their contribution to the absorption is about 0.6 dB out of a total of 3.4 dB. The dominant contribution to absorption at 30 MHz usually arises from protons below 10 MeV, specifically in the 2‐ to 4‐MeV interval. We have propagated the observed fluxes and energy spectra of protons and alpha particles through a seasonally adjusted slab model of the polar atmosphere. The atmospheric ionization resulting from the slowing and stopping of protons and alpha particles is used to estimate an equilibrium vertical ionization profile which is then convolved with an absorption efficiency profile to yield a calculated absorption. There is good agreement between the computed and observed absorption when the daily averaged absorption is above 0.1 dB; this shows that the interplanetary ions are the dominant contributors on those days. This set of observations constitutes the most complete and definitive illustration to date of the control by interplanetary particles of the shape and magnitude of the ionization profile in the lower ionosphere.
Consequences of using simple analytical functions for the high‐latitude convection electric field
Authors: Rich F. J. , Maynard N. C.
Published: February 2008

We have developed analytical functions to represent the convection electric field of Heppner and Maynard (1987), which are realistic representations of the field. The functions include an explicit variability with geomagnetic activity for southward interplanetary magnetic field (IMF). We have used a simple computer algorithm to model the ionosphere and investigate the consequences of using the Heppner‐Maynard analytical functions for the high‐latitude convection electric field. The Heelis et al. (1982) model, which has been widely used by investigators with computer simulations of the magnetosphere, ionosphere and/or thermosphere, is used here to provide a comparison. The Heppner‐Maynard functions improve on the simple Heelis function for southward IMF. The Heppner‐Maynard functions yield reasonable configurations and magnitude for Joule heating and field‐aligned currents for a southward IMF. The Heelis model may or may not yield reasonable results depending on the input parameters. The major differences between the Heppner‐Maynard functions and the Heelis function for southward IMF are for regions near (1) the dayside cusp, (2) the Harang discontinuity and (3) the subauroral latitudes. In general, the Heppner‐Maynard functions yield results that more closely compare with other investigators. For northward IMF, the functional representations of the Heppner‐Maynard do not reproduce the field‐aligned currents in the polar cap which have been referred to as the NBZ current system.
Ionospheric absorption, typical ionization, conductivity, and possible synoptic heating parameters in the upper atmosphere
Authors: Walker John K. , Bhatnagar V. P.
Published: February 2008

Relations for the average energetic particle heating and the typical Hall and Pedersen conductances, as functions of the ground‐based HF radio absorption, are determined. Collis and coworkers used the geosynchronous GEOS 2 particle data to relate or “calibrate” the auroral absorption on the same magnetic field lines with five levels of D region ionization. These ionospheric models are related to a Chapman layer that extends these models into the E region. The average energetic particle heating is calculated for each of these models using recent expressions for the effective recombination coefficient. The corresponding height‐integrated heating rates are determined and related to the absorption with a quadratic expression. The average Hall and Pedersen conductivities are calculated for each of the nominal absorption ionospheric models. The corresponding height‐integrated conductances for night‐time conditions are determined and related to the absorption. Expressions for these conductances during disturbed sunlit conditions are also determined. These relations can be used in conjunction with simultaneous ground‐based riometric and magnetic observations to determine the average Hall and Pedersen currents and the Joule heating. The typical daily rate of temperature increase in the mesosphere for storm conditions is several 10 K for both the energetic particle and the Joule heating. The increasing importance of these parameters of the upper and middle atmospheres is discussed. It is proposed that northern hemisphere ionospheric, current, and heating synoptic models and parameters be investigated for use on a regular basis.
Corotation lag of the Jovian atmosphere, ionosphere, and magnetosphere
Authors: Huang T. S. , Hill T. W.
Published: February 2008

We modify the Jovian ionosphere‐magnetosphere coupling model presented by Hill (1979) to include rotational slippage of the neutral atmosphere at ionospheric heights, relative to a frame of reference corotating rigidly with Jupiter. In the modified model, as altitude increases, the drift velocities of neutrals and ions relative to the corotating frame increase from zero at the bottom of the ionosphere to their respective maximum values at the top, and the corotation lag of the magnetosphere is enhanced for a given rate of mass loading of the magnetosphere. The height variations of the drift velocities of neutrals and ions in the ionosphere and the enhancement factor for the corotation lag of the magnetosphere are related to the atmospheric eddy diffusion coefficient at ionospheric heights. On the basis of ionospheric properties deduced from measurements of Pioneer 11, Voyager 1, and Voyager 2, we derive height profiles of ion and neutral drift speed for various possible values of the eddy diffusion coefficient. If we accept the ion injection rate from Io (1029 ions/s) and the ion transport rate through the Jovian magnetosphere (3×1028 ions/s), corresponding to the observed values of the corotation lag of the Io torus (δωI = 0.04ωJ) and the critical distance for magnetospheric corotation (Lo = 20), respectively, the effective ionospheric conductance is reduced by a factor of the order of 10, resulting in an enhanced corotation lag, for a given rate of mass loading, by the same factor compared to earlier models. The eddy diffusion coefficient in the high‐latitude ionosphere is inferred to be ∼1013 nn−1/2 m1/2/s, or about a factor of 20 larger than the value inferred from Voyager measurements at near‐equatorial latitudes.
A seven‐month solar cycle observed with the Langmuir probe on Pioneer Venus Orbiter
Authors: Hoegy W. R. , Wolff C. L.
Published: February 2008

An approximately 7‐month solar cycle has been observed in the photoelectron current measured by the Langmuir probe on Pioneer Venus Orbiter over the time period from 1979 through 1987. The probe photoelectron current, Ipe, is obtained when the spacecraft is outside the Venus ionosphere, and the measured current is due to photoelectron emission caused by EUV solar radiation in the wavelength range from about 30 nm to 121.6 nm. About one half of the Ipe is due to solar Lyman alpha emission. The Ipe data from mid‐1984 through 1987 are dominated by a 7‐month or 216‐day cycle. Spectral analysis of the 1980–1988 data shows that this cycle dominates the spectrum for periods less than 300 days; the second most dominant cycle is at 155 days, a 5‐month period. The Ipe data were spectrally analyzed with three different methods. The 2800‐MHz radio flux, observed from Earth over the same time range, exhibits a similar 7‐month cycle at about 234 days which is stronger than a 5‐month (158‐day) cycle. Solar Mesosphere Explorer (SME) Lyman alpha observations for the time period mid‐1981 through mid‐1988 also have cycles near 5 and 7 months. Since the 7‐month and 5‐month cycles are observed from Venus (Ipe) and from Earth (2800‐MHz radio flux and SME Lyman alpha), the observations suggest that they are intrinsic frequencies, independent of the orbital frequency of the observer. The 7‐ and 5‐month cycles are believed to be caused by long‐lived flux enhancements from nonlinear interactions of global oscillation modes in the Sun's convective envelope (r modes) and radiative interior (g modes).
Ion reflection and downstream thermalization at the quasi‐parallel bow shock
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Russell C. T.
Published: February 2008

It is well known that for sufficiently high Mach numbers (typically Mf>2) ion reflection is the primary process by which ion energy dissipation is initiated at quasi‐perpendicular, collisionless shocks in space. In this paper we present evidence from ISEE 2 measurements that coherent ion reflection is also often an important element of the ion energy dissipation process at high Mach number, quasi‐parallel shocks. Such coherent reflection is not evident in a number of published hybrid simulations of quasi‐parallel shocks wherein the dissipation process appears to be associated primarily with incoherent scattering of solar wind ions in the upstream and downstream wave fields. However, such reflection is present in other simulation results. The main evidence for coherent ion reflection as an important element in achieving dissipation at quasi‐parallel shocks is the following: (1) A cold secondary (to the solar wind) beam of ions is often observed within the ramp of the shock where the magnetic field intensity rises precipitously and where the major electron heating occurs. Typically this secondary beam of ions has a velocity space position roughly consistent with specular reflection of a portion of the solar wind ions incident on the shock. (2) Downstream from the shock, a relatively dense and cool core ion population is typically observed, accompanied by either multiple ion bunches or, more commonly, a suprathermal ion shell. (These populations are in addition to the extremely hot, tenuous diffuse ions which appear nearly ubiquitously both upstream and downstream from the shock.) The downstream core and shell (or bunches) are believed to represent directly transmitted solar wind ions and reflected and later scattered ions, respectively, with the motions of the latter population accounting for a large fraction of the ion energy dissipated at the shock.