bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrostatic model of a quiet auroral arc
Authors: Chiu Y.T. , Cornwall J.M.
Published: February 2008

Given auroral electron and ion distribution functions as observed with satellites in inverted V events, we construct a self‐consistent electrostatic field distribution (both parallel and perpendicular to the earth's magnetic field). This field distribution is determined by (1) magnetic mirror forces which cause charge separation for species with different pitch angle distributions; (2) Poisson's equation, which gives the electric field in terms of the charge separation; (3) the ionospheric physics of charge and current conservation, coupled with precipitation sources and recombination losses; and (4) particle distribution functions specified at the equator (and for thermal and backscattered plasma, at the ionosphere). We assume that equatorial particle distribution functions depend on L only through the dependence of the electrostatic potential on L; an assumed factorized form for the potential allows this L dependence to be simply parameterized. These ingredients combine to yield a self‐consistent latitudinal scale length of some tens of kilometers, typical for quiet arcs. A variety of particle and field data at and below S3‐3 altitude are fit semiquantitatively with the model. We do not consider in this paper physical processes which act in the equatorial magnetosphere, although they could well be connected to the processes we do consider through some sort of feedback.
The formation of ducts and spread F and the initiation of bubbles by field‐aligned currents
Authors: Muldrew D.B.
Published: February 2008

Magnetospheric ducts for medium frequency (MF) radio waves are formed, for L ≳ 1.2, between about 20 and 22 hours local time. Their occurrence frequency remains constant for the remainder of the night and begins to decrease after sunrise. This decrease continues until near sunset. The seasonal and longitudinal variation of the ducts and of low‐latitude spread F implies that both are related to hemispheric asymmetry of the winds in the dynamo region and hence are probably generated by field‐aligned currents as suggested by K. D. Cole. These currents generate significant irregularities in the E region of the ionosphere at night when E region densities are low; these irregularities in turn produce perpendicular electric fields due to winds. The small‐scale electric fields (≲few kilometers) responsible for ducts can only be transferred to the F region and magnetosphere along magnetic field lines when the parallel conductivity is fairly large between the E and F layers. These electric fields interchange tubes of flux and hence can change tube densities. The transfer of electric fields occurs before the formation of the deep E‐F density valley. The valley forms about 22 hours, which explains why ducts with L ≳ 1.2 can only form after the E region has low density (after 19 hours) and before the E‐F valley forms. For L ≲ 1.2 the percentage occurrence of MF ducts increases from about 20 to roughly 02 hours local time and decreases from roughly 02 to about 10 hours. It is thought that these ducts are associated with ionospheric ‘bubbles.’ The frequency of occurrence of MF ducts with L ≲ 1.2 continues to increase for roughly 3 hours after the frequency of occurrence of ducts with L ≳ 1.2 stops increasing. Since the estimated rise time for bubbles from the bottom of the F layer to the topside is about 3 hours, these ducts may also have been initiated before about 22 hours.
Microscale instabilities in stream interaction regions
Authors: Eviatar Aharon , Goldstein Melvyn L.
Published: February 2008

A theoretical investigation of the microstructure of solar wind stream interaction regions is presented. We discuss the role of several electrostatic kinetic instabilities which may be important within the stream interface and the compression region. Inside of 1 AU the interface is likely to be stable against the electrostatic streaming instabilities considered. Between 1 and 2 AU we argue that the interface will excite the magnetized ion‐ion instability. The compression region is also found to be unstable beyond 1 AU, where the modified two‐stream instability, beam‐cyclotron instability, and ion‐acoustic instability will be important in determining the structure of the compressive pulses as they evolve into forward and reverse shocks. We conclude that the modified two‐stream instability and beam‐cyclotron instability predominately play a role in heating the electrons to the threshold for the ion‐acoustic instability. Various electrostatic plasma waves, ranging in frequency from the lower‐hybrid to harmonics of the electron cyclotron frequency, would be produced by these instabilities. Their signature should also be seen by high time resolution measurements of the temperature of the various plasma species.
Prolonged radar observations of an auroral arc
Authors: Stiles G.S. , Foster J.C. , Doupnik J.R.
Published: February 2008

On January 29, 1977, an auroral arc remained in the field of view of the Chatanika incoherent scatter radar for about 1 hour. Prolonged measurements were made of the line‐of‐sight velocities and densities from roughly 80‐ to 250‐km altitude within and on both sides of the arc. From these basic measurements electric fields, conductivities, currents, and heating rates were calculated. In spite of the fact that the arc changed its appearance considerably over the hour, the measured and calculated parameters remained qualitatively very much the same. The northward electric field was very strong and showed a latitudinal profile similar to that frequently observed on satellites. One hundred (100) km equatorward of the arc the field was ∼30 mV/m; from that point it increased to over 100 mV/m at the equatorward edge of the arc. The field then dropped sharply to ∼30 mV/m within the arc. The northward field did not change across the poleward edge of the arc, indicating that there was no polarization effect. The latitudinal profile of the calculated horizontal currents suggests that the major downward field aligned currents are located in the region equatorward of the arc and the upward currents at the equatorward edge of the arc. The integrated joule heating rate peaked just equatorward of the arc, reaching a value of over 150 ergs cm−2 s−1; inside the arc it dropped to 20–40 ergs cm−2 s−1. The particle heating rate was negligible outside and was 10–30 ergs cm−2 s−1 inside the arc. The joule heating always exceeded the particle heating.
Satellite studies of N(²D) emission and ion chemistry in aurorae
Authors: Rusch D.W. , Gérard J‐C.
Published: February 2008

The incident particle flux ion and neutral composition data taken on the AE‐D satellite have been used to investigate the quantal emission of N2+ at 4278 Å, N(²D) at 5200 Å, and the ion chemistry in aurorae. The results of a time dependent auroral model have been compared to the data. The calculated 4278 Å emission of N2+, the 5200 Å emission of N(²D), the densities of O2+, NO+, N2+, O+, and the electron density are generally in agreement with the measured values. These results are consistent with the branching ratios and quenching rates deduced from previous studies of the N(²D) densities in the daytime, mid‐latitude ionosphere. It is found that in an auroral arc, the measured atomic oxygen density is lower than predicted by the MSIS model.
Measurement of a structured profile of atomic oxygen in the mesosphere and lower thermosphere
Authors: Howlett L.C. , Baker K.D. , Megill L.R. , Shaw A.W. , Pendleton W.R. , Ulwick J.C.
Published: February 2008

The altitude distribution of O(³P) above White Sands, New Mexico (33°N), was investigated in situ at 1900 hours MST (0200 UT) on December 2, 1975. A rocket‐borne OI 1304‐Å resonance‐scattering system provided data over the altitude range 75‐126 km on ascent and descent. The oxygen profiles inferred from the data exhibit considerable structure, with more structure evident on ascent than descent. A double peak (≃1011 cm−3 at 90 and 98 km) occurs in both profiles. Ground‐based photometric measurements of the [OI] 5577‐Å airglow at the time of the flight agree with the [O] measurements to within 50% when interpreted within the framework of either the Chapman or Barth mechanisms. Preliminary consideration of several processes leads to the suggestion that wind shears provide the best possibility for explaining the double‐peaked concentration profiles.
A comparative study of cosmic ray intensity variations during 1972–1977 using spacecraft and ground‐based observations
Authors: Venkatesan D. , Agrawal S.P. , Van Allen J.A.
Published: February 2008

Cosmic ray intensity variations over the period 1972–1977 are studied using data registered by Detector C (E ≥80 MeV) on Pioneer 10 and the Sulphur Mountain neutron monitor (E ≳ 400 MeV). The spacecraft data have been corrected for temperature, Radioisotope Thermoelectric Generators (RTG) background, and contamination by energetic solar particle events. The data during the ‘encounter’ with Jupiter have also been deleted. A consistent long‐term solar cycle variation in intensity is observed in both data; however, in the neighbourhood of 5.1 AU an additional contribution to the interplanetary particle intensity is seen. This excess is attributed to energetic electrons (Ee >21 MeV) of Jovian origin in support of the more detailed results of Conlon on Jovian emitted electrons (3< Ee <6 MeV). The spectral variation in long‐term changes of the cosmic ray intensity is studied by a comparison of the low‐energy (satellite) and high‐energy (neutron monitor) data. An average value of 3.5±0.2 for this ratio obtained for the interval 1972–1977 agrees well with the same value obtained for an earlier study, for the interval 1965–1972. However, there is indication that the ratio for short‐term periods of a few rotations decreases for ∼5.0 to ∼1.0 during 1972–1977. Possible causes for this variation are suggested.
On possible causes of apparent longitudinal variations on Ogo 3 observations of VLF chorus
Authors: Russell C.T.
Published: February 2008

Apparent longitudinal variations of Ogo 3 observations of VLF chorus activity have been interpreted in terms of excitation by power transmission lines. An alternate possibility is that the observed longitudinal distribution is a result of the convolution of the latitudinal and local time distribution of chorus in space with the spatial coverage of the Ogo 3 spacecraft. The orbital period of Ogo 3 was such as to beat with the earth's rotation every 89 days and thus would produce four geographical peaks from a survey of 1 year of data.
Atmospheric pressure and velocity fluctuations near the auroral electrojet
Authors: Luhmann J.G.
Published: February 2008

The low‐frequency pressure and velocity perturbations caused by the temporally varying Lorentz force associated with auroral electrojet activity are modeled by calculating the disturbances generated by a two‐dimensional, time‐dependent current system in a gravitationally stratified, isothermal, windless atmosphere. These calculations provide information about the pattern of gravity waves around the hypothetical electrojet and give estimates of the magnitudes of near‐field auroral disturbances in the middle atmosphere. It is suggested that the near‐field vertical wind shears may be large enough to affect the development of air turbulence in the auroral zone.
Metastable N(²P) atoms in the aurora
Authors: Gérard J.‐C. , Harang O.E.
Published: February 2008

Auroral spectra have been obtained in the 3380‐ to 3560‐Å region to study the [NI] 4S‐2P doublet at 3466 Å. The [NI] 3466 Å/N2 Vegard‐Kaplan ratio is found to remain constant. This is in agreement with the results of a model calculation showing that the dominant source of N(2P) atoms is dissociative excitation of N2 by fast electrons, and the major quenching agent is atomic oxygen. A value of about 5 × 109 cm−3 s is deduced for the ratio of the N(²P) quantum yield to the atomic oxygen deactivation coefficient.
Comparison of the N2+ photochemistry at different phases of the solar cycle
Authors: Torr Marsha R. , Torr D.G. , Ong R.A.
Published: February 2008

Over the increasing phase of solar activity from 1974 to 1978 the F10.7 index increased from ∼70 to ∼180. Earlier studies of the N2+ ion based on the Atmosphere Explorer satellite measurements used data gathered in 1974. A study of N2+ using 1978 measurements made during significantly higher solar EUV fluxes reveals a change in the relative importance of the major chemical processes. Photoionization of N2 is found to be the major source of N2+ in the F2 layer, by contrast with the charge exchange reaction of O+(2D) with N2, which was dominant for the lower solar activity. The increased electron concentrations enhance the importance of reactions involving electrons, thus permitting good determinations of the relevant reaction rate coefficients, which could not be done in the earlier studies. As such, the 1978 data provide a useful opportunity to confirm the photochemical scheme established earlier and to improve the accuracy of the determination of the rate coefficients for certain processes. Specifically, we determine the rate coefficient for dissociative recombination of N2+ with electrons, confirming laboratory measurements; we refine an earlier determination of the charge exchange rate coefficient for O+(2D) with N2 to (7±3) × 10−11 cm3 s−1; and we determine the rate coefficient for quenching of O+(2D) by electrons, confirming a theoretical calculation.
Nighttime temperatures and ion chemistry from Ogo 6 Plasma measurements
Authors: Sanatani S. , Breig E.L.
Published: February 2008

An analysis is presented of ion temperature and concentration data from the retarding potential analyzer on board the Ogo 6 satellite. The investigation is restricted to plasma measurements in the undisturbed nighttime nonpolar ionosphere between 400 and 450 km spanning several months near solstice; under these conditions, observed ion temperatures are assumed to approximate the neutral gas temperature closely. Emphasis is placed upon statistical analyses of all data acquired within small cells of finite width in latitude and day of observation. Physical effects induced by changing solar and geophysical conditions are considered by comparing these average temperatures with the mass spectrometer and incoherent scatter (MSIS) atmospheric model. Measured nighttime temperatures between low summer and middle winter latitudes are, on the average, about 50°K lower in magnitude than those calculated from the model. The MSIS model does, however, provide an excellent representation of the observed long‐term temperature variations. Measured total concentrations of the molecular ions near the equator are larger in magnitude than those calculated according to equilibrium ion chemistry; this difference exceeds current stated uncertainties in either the reaction rates or the supporting neutral composition and suggests that important problems remain in our quantitative understanding of the ion chemistry of the nighttime F region. On the other hand, changes in the equatorial molecular ion concentrations are directly related to similar variations in the observed ion temperatures and in the MSIS neutral atmosphere.
Stellar wind theory
Authors: Summers D.
Published: February 2008

The theory of stellar winds as given by the equations of classical fluid dynamics is considered. The equations of momentum and energy describing a steady, spherically symmetric, heat‐conducting, viscous stellar wind are cast in a dimensionless form which involves a thermal conduction parameter E and a viscosity parameter γ. An asymptotic analysis is carried out, for fixed γ, in the cases E → 0 and E → ∞ (corresponding to small and large thermal conductivity, respectively), and it is found that it is possible to construct critical solutions for the wind velocity and temperature over the entire flow. The E → 0 solution represents a wind which emanates from the star at low, subsonic speeds, accelerates through a sonic point, and then approaches a constant asymptotic speed, with its temperature varying as r−4/3 at large distances r from the star; the E → ∞ solution represents a wind which, after reaching an approximately constant speed, with temperature varying as r−2/7, decelerates through a diffuse shock and approaches a finite pressure at infinity. A categorization is made of all critical stellar wind solutions for given values of γ and E, and actual numerical examples are given. Numerical solutions are obtained by integrating upstream ‘from infinity’ from initial values of the flow parameters given by appropriate asymptotic expansions. The role of viscosity in stellar wind theory is discussed, viscous and inviscid stellar wind solutions are compared, and it is suggested that with certain limitations, the theory presented may be useful in analyzing winds from solar‐type stars.
The longitudinal galactic cosmic ray intensity modulation in a diffusive and a scatter‐free model of the inner heliosphere
Authors: Morfill G. , Scholer M. , Van Hollebeke M.A.I.
Published: February 2008

Measurements of energetic solar flare electrons, Jovian electrons, and low‐energy solar protons have led to the suggestion that the inner heliosphere is essentially scatter free. We investigate a model in which scattering is only important within the corotating interaction regions (CIR's), while the cavity formed by the CIR's in the inner heliosphere is scatter free, as compared to an interplanetary diffusion model wherein scattering is determined by the solar wind and magnetic field microstructure everywhere in the heliosphere. The two models are compared in terms of the solar wind stream associated Forbush decreases at 1 AU and the longitudinal cosmic ray nucleonic intensity variations at larger heliocentric distances (Pioneer 11 measurements at ∼3 AU). It is shown that the diffusion model is able to explain consistently all longitudinal galactic cosmic ray intensity modulation measurements.
Electric fields in the dayside auroral oval
Authors: Jørgensen T.S. , Mikkelsen I.S. , Lassen K. , Haerendel G. , Rieger E. , Valenzuela A. , Mozer F.S. , Temerin M. , Holback B. , Björn L.
Published: February 2008

The results from four independent electric field experiments flown on three Black Brant 4 rockets in the forenoon dayside auroral oval in December 1974 and January 1975 are correlated with ground‐based observations and rocket particle data. The electric field varied from zero to 150 mV/m. The predominant plasma convection was toward noon along the auroral oval with a smaller component directed toward the polar cap. In one case, however, a reversal occurred within the oval with plasma convection away from noon. Comparisons with magnetometer data indicate that in the dayside auroral oval, Hall currents sometimes are responsible for magnetic fluctuations observed on the ground. Comparisons with particle data show that the magnitude of the electric fields is inversely correlated with the electron energy flux.
Observations of ULF electric field fluctuations in the dayside auroral oval
Authors: Temerin M. , Parady B.
Published: February 2008

Electric field oscillations at frequencies centered below 10 Hz were measured by a payload equipped with double‐probe electric field detectors which was launched into the dayside auroral oval during the Greenland rocket campaign on January 11, 1975. These oscillations exceeded an amplitude of 3 mV/m broadband and occurred over a period of about 90 s. These oscillations occurred during a proton injection event when the quasi‐static electric field was less than 20 mV/m on the average. However, during a brief 2‐s interval the electric field exceeded 50 mV/m. Analysis of the amplitude as a function of the spin frequency showed that the electric field was confined to a plane perpendicular to the magnetic field. The power spectrum was of the form f−6.5±1.5, where f is the frequency over the range from 8 to 18 Hz. The existence of a large electric field fluctuation is a spatially or temporally confined region of low‐amplitude turbulence suggests that this may be an observation of the low‐altitude projection of a turbulent region associated with electrostatic shocks which have recently been observed at higher altitudes.
Variability of plasma sheet dynamics
Authors: Coroniti F.V. , Frank L.A. , Williams D.J. , Lepping R. P. , Scarf F.L. , Krimigis S.M. , Gloeckler G.
Published: February 2008

From 1200 UT on October 27 to 1200 UT on October 28, 1972, Imp 7 moved across the tail plasma sheet at 35 RE and remained within 2 RE of the expected position at the neutral sheet. During this 24‐hour interval, at least five substorms occurred on the ground. We present Imp 7 observations by using high‐time resolution data that illustrate the rapid variability and multiplicity of plasma sheet phenomena both during and between substorms. Tailward plasma flows, lasting for intervals as short as 2 min and as long as 40 min, were associated with four, and possibly five, of the substorm onsets. Prolonged earthward flow was detected during the expansion phases of two substorms. Between substorms, moderate but variable plasma flows were observed, including two intervals of dawn‐directed flow and large BY field components. Hence the tail field can be significantly distorted out of the noon‐midnight meridian, possibly by the associated plasma flow stresses. During a rapid 90‐s neutral sheet crossing the magnetic field rotated smoothly through 180° with BY dominating the field strength at the center. The smooth field rotation suggests that the structure of the neutral sheet may sometimes resemble the hydromagnetic rotational discontinuity. Highly turbulent magnetic fields were detected during an earthward plasma flow interval in which the north‐south component changed from +8 γ to −8 γ in 1 s; the field exhibited large fluctuations in all three components on time scales of 1–10 s. The high level of magnetic turbulence may significantly contribute to plasma sheet dissipation.
The mean free path of low‐rigidity cosmic rays
Authors: Goldstein Melvyn L.
Published: February 2008

A simple heuristic argument is presented which suggests that the large rigidity‐independent mean free paths observed for low‐rigidity solar and galactic cosmic rays can be understood in terms of weak turbulence diffusion theory if one assumes that the interplanetary magnetic turbulence consists of a combination of Alfvén waves propagating with constant field magnitude and a small (∼5–10%) admixture of compressive fluctuations.
The relationships between high‐latitude convection reversals and the energetic particle morphology observed by Atmosphere Explorer
Authors: Heelis R.A. , Winningham J.D. , Hanson W.B. , Burch J.L.
Published: February 2008

Simultaneous measurements of the auroral zone particle precipitation and the ion convection velocity by Atmosphere Explorer show a consistent difference between the location of the poleward boundary of the auroral particle precipitation and the ion convection reversal. The difference, of about 1.5° of invariant latitude, is such that some part of the antisunward convection lies wholly within the auroral particle precipitation region. The nature of the convection reversals within the precipitation region is consistent with a region in which the convection electric field is generated on closed field lines that connect in the magnetosphere to the low‐latitude boundary layer.
Generation of low‐frequency magnetic noise on auroral field lines
Authors: Lakhina G.S.
Published: February 2008

A mechanism for the generation of a low‐frequency (1–562 Hz) magnetic noise, occurring on the auroral field lines (Gurnett and Frank, 1977), is proposed in terms of an electromagnetic lower hybrid instability. The instability can be driven either by electron temperature anisotropies or by field‐aligned currents. Several observed properties of the noise, such as (1) its occurrence from the same region where broadband electric field noise is observed, (2) its frequency range, (3) its tentative identification as a whistler type mode, and (4) its burstlike nature, can be explained by the theory.
Satellite X ray mappings of sporadic auroral zone electron precipitation events in the local dusk sector
Authors: Imhof W.L. , Kilner J.R. , Nakano G.H. , Reagan J.B.
Published: February 2008

Simultaneous mappings of bremsstrahlung X ray events from the atmosphere resulting from electron precipitation have been performed from a satellite with an array of collimated cadmium telluride spectrometers. The X ray intensities and energy spectra (≳21 keV) were measured over contiguous view directions. The satellite, P78‐1, was launched into a sun‐synchronous noon‐midnight orbit at ∼600‐km altitude on February 24, 1979, and through its spinning motion with a ∼5.5‐s period provides continual scans of the X ray sources below. Data are presented from three sporadic events in which the X ray intensities varied strongly with time. Within each of these events, bursts of ∼(30–100) second duration were observed. Some of the bursts were recorded in three or four sensors, thus setting a lower limit to their spatial extent of several hundred kilometers. Other bursts appeared in only one sensor, thereby restricting the area over which they could have been generated. These bursts of X rays emanated primarily from the dusk sector, a local time region where few balloon X ray measurements have been reported. The X ray fluxes were asymmetric about local dusk, the intensities at earlier times being quite low. During one of the X ray bursts in which the precipitation was confined to a relatively small spatial region it is estimated that the total number of electrons (>21 keV) precipitating into the atmosphere was ∼1023 per second.
Correlations between λ4278 optical emissions and VLF wave events observed at L ∼4 in the Antarctic
Authors: Helliwell R.A. , Mende S.B. , Doolittle J.H. , Armstrong W.C. , Carpenter D.L.
Published: February 2008

One‐to‐one correlations have been observed at L∼4 between bursts of ducted VLF noise in the ∼2 to 4‐kHz range and optical emissions at λ4278. The optical observations were made at Siple Station, Antarctica, in the austral winter of 1977; the wave activity was recorded both at Siple and its conjugate station Roberval, Canada. The one‐to‐one correlations were observed during 6 of 32 observing sessions. Most of the events occurred near dawn (prior to interruption of the observations by twilight interference) during or shortly following substorm events (Kp =2.5; Dst=−10–40). In most of the cases the plasmapause was equatorward of Siple, and the equatorial electron densities were low (∼10 cm−3). In one case, July 24, 1977, the equatorial density had recovered to a level of 100 cm−3 at the time of observation. The estimated precipitated energy fluxes ranged from 0.04 to 0.1 ergs cm−2 s−1. The correlated VLF activity usually consisted of clusters of discrete rising tones or chorus ranging in duration from ∼1 s to ∼10 s. In three of the six cases some or all of the chorus bursts were triggered by whistlers. Photometer peaks lagged the wave peaks by 1–2 s. In the case of July 24, 1977, a model of the interaction process was constructed using the lag time and the whistler‐derived cold plasma density. The data were found to be consistent with scattering of electrons into the loss cone over Siple by emissions that were triggered by waves propagating away from the equator after reflection in the ionosphere over Siple. This interpretation differs from that of a previous study of X‐ray bursts induced by similar VLF emissions. This difference is attributed to the occurrence of whistler echoing during the reported X‐ray event. During the same period, VLF hiss was observed in the range 2.5–5.0 kHz. Immediately following each discrete event the hiss and the λ4278 intensities were reduced below their pre‐event levels for about 10 s. Both reductions are tentatively attributed to a temporary depletion of particles near the loss cone. Recovery to pre‐event levels could be explained by refilling of the duct through east‐west gradient drift of energetic electrons.
Reply [to “Comment on ‘Magnetosheath burst of predominantly medium nuclei observed with Imp 8 on February 16, 1974’ by T.P. Armstrong, S.M. Krimigis, and R.P. Lepping”]
Authors: Armstrong T.P. , Krimigis S.M.
Published: February 2008

This is a reply to criticism by D. Hovestadt of our paper on unusual bursts observed with detectors aboard Imp 8. We conclude that our original analysis was valid and that it is supported by the observations of three other detectors.
Magnetospheric and ionospheric currents in the polar cusp and their dependence on the By component of the interplanetary magnetic field
Authors: Rostoker Gordon
Published: February 2008

Previous studies of magnetic field perturbations at very high latitude have indicated the presence of an equivalent current system which is strongly modulated by the azimuthal (By) component of the interplanetary magnetic field. This current system is now thought to flow in the portion of the ionosphere to which the polar cusp (or cleft) maps. In this paper we shall show that, while the By component of the interplanetary magnetic field does not significantly change the total current flowing in the magnetosphere, it does redistribute the ionospheric portion of the current flow. This redistribution results in eastward ionospheric current being observed in the prenoon quadrant on the poleward side of the normally present westward electrojet when By >0, whereas only westward current flows in the prenoon quadrant when By <0.
Density and temperature structure of helium ions in the topside polar ionosphere for subsonic outflows
Authors: Ottley J. Alan , Schunk Robert W.
Published: February 2008

We have obtained steady state solutions of the coupled continuity, momentum, and energy equations for He+, O+, and electrons for conditions appropriate to the daytime high‐latitude topside ionosphere. Our study was limited to subsonic He+ outflows, which result in He+ being either a major ion or an important minor ion over the altitude range from 500 to 2500 km. This study therefore complements the previous He+ studies, which were limited to situations where He+ was a minor ion at all altitudes. In addition to He+ outflow velocity, we studied the effect on the ion densities and temperatures of convection electric fields, different electron temperature distributions, and different assumed He+ and O+ heat fluxes at high altitudes. The latter effect was not considered in previous studies of the high‐latitude topside ionosphere. Some of the more important results of our study are the following: (1) the absence of topside He+ and O+ heat fluxes results in closely coupled He+ and O+ temperatures at all altitudes for low to moderate He+ outflow speeds; (2) a downward topside He+ heat flux results in elevated He+ temperatures at all altitudes above about 800 km and a large He+ temperature gradient over a significant altitude range; (3) the thermal diffusion associated with the steep He+ temperature gradient has a significant effect on the He+ and O+ density profiles for low to moderate He+ outflow speeds but a much smaller effect on these profiles for more substantial He+ outflow speeds; thermal diffusion acts to drive the He+ ions downward toward cooler regions and the O+ ions upward toward hotter regions; (4) because of thermal diffusion the change in the ‘diffusive equilibrium’ He+ density profile with an increase in the topside He+ heat flux is very similar to the change obtained for ‘dynamic equilibrium’ with an increase in the topside He+ escape flux; (5) in contrast with previous results, the He+‐O+ frictional heating that occurs when He+ is in a state of outflow can in some cases raise the He+ temperature above the O+ temperature at high altitudes; this new result is due in part to the use of different ionospheric parameters and in part to the fact that the diffusion‐thermal heat flow opposes ordinary He+ thermal conduction, thereby increasing the relative importance of He+‐O+ frictional heating; and (6) basically, a converting ionosphere and a nonconvecting ionosphere exhibit a similar variation with regard to changes in the topside He+ escape flux and in the topside He+ and O+ heat fluxes. The main differences stem from the additional heating that occurs at low altitudes owing to the frictional interaction between ions and neutrals.
F region above Kauai: Measurement, model, modification
Authors: Johnson Charles Y. , Sjolander Gary W. , Oran Elaine S. , Young Theodore R. , Bernhardt Paul A. , da Rosa Aldo V.
Published: February 2008

A detailed description and analysis is presented of the Lagopedo II results. The rocket was launched on September 11, 1977. Prior to an explosive chemical release a rocket‐borne ion mass spectrometer and dual‐frequency beacon measured the ion composition and electron content of the undisturbed F region above Kauai, Hawaii. These results are compared to a detailed calculation of ionospheric ion densities. Considerable H2O outgassing produced measurable 18+(H2O+) and 19+(H3O+) currents which together with O+ current were used to determine the H3O+/H2O+ dissociative recombination rate ratio. The explosive event at 283 km swept clean a 1‐km‐radius region. The boundary of the ionic void was characterized by a steep gradient in the ion density. Results for the first 60 s after the event are presented which show changes in ambient ion species 14+, 16+, 30+, and 32+, new reactant species 15+, 17+, 18+, 19+, 27+, and 46+, and evidence of ionic depletion by sweeping.
Improved Millstone Hill exospheric temperature measurements: Evidence for a seasonal variation of the magnetic activity effect
Authors: Oliver W.L.
Published: February 2008

An analysis has been performed of a new collection of exospheric temperature data, gathered by using the ‘two‐pulse’ incoherent scatter radar experiment at Millstone Hill. The results are compared with previously reported ‘one‐pulse’ data from Millstone Hill and related to differences previously noted between these one‐pulse and Saint Santin results. Implications of these differences for current thermospheric models also are discussed. The two‐pulse data appear to be of improved accuracy in relation to the one‐pulse data. Both the two‐pulse data and a recent revision of the older one‐pulse results imply that the previously reported one‐pulse exospheric temperatures were too low by about 50 K, on average. Otherwise, the two‐pulse data provide generally improved agreement with the seasonal and diurnal variations observed at Saint Santin. The primary exception to this rule is that the two‐pulse data confirm a larger seasonal variation magnitude at Millstone than at Saint Santin. It is shown that this difference might be accounted for by a strong seasonal dependence in the magnetic activity effect at the Millstone location. The two‐pulse data indicate that the temperature response to magnetic activity is four times stronger in summer than in winter at the Millstone location. This type of variation is not included in current thermospheric models.
Generation and propagation of an electromagnetic pulse in the trigger experiment and its possible role in electron acceleration
Authors: Kelley M.C. , Fahleson U.V. , Holmgren G. , Bostrom R. , Kintner P.M. , Kudeki E.
Published: February 2008

The dc electric field detector onboard the instrumented portion of the Trigger payload detected a large‐amplitude (∼200 mV/m) electric field pulse which was observed with a time delay consistent only with an electromagnetic wave. A model for this perturbation is constructed in this paper and the associated field‐aligned current calculated as a function of altitude. Near the explosion point, downward currents of 2500 µA/m² and upward currents of 90 µA/m² are indicated by the model. The lower upward current has the same direction as the field‐aligned current in discrete auroral arcs. Taking into account the attenuation of the wave and the changes in Alfven speed and plasma density, the associated differential drift velocity between electrons and ions is found to peak at 750‐km altitude at a value one‐third of the electron thermal speed. This is well above the threshold for ion cyclotron wave turbulence and approaches the value necessary for production of electrostatic shocks or double layers. The differential velocity associated with the downward current well exceeds the electron thermal speed. The time delay for observation of accelerated electrons is consistent with the Alfven propagation time to 750‐km altitude. The initial explosion pulse had internal temporal variations just above the oxygen ion cyclotron frequency in agreement with other experiments of this type. The Poynting flux in the electromagnetic pulse was sufficiently intense to create the observed particle precipitation.
Time‐averaged fluxes of heavy ions at the geostationary orbit
Authors: Konradi A. , Fritz T.A. , Su S.‐Y.
Published: February 2008

Measurements of average proton, helium, carbon, and oxygen fluxes at 6.6 RE are reported. The data represent averages obtained on ATS 6 between June 15 and October 3, 1974. The energy range covered was 0.36–1.1 MeV for protons and approximately 1–4 MeV for the heavier ions. The results indicate that above about 1 MeV the heavier ion fluxes dominate over the proton flux on the energy/ion scale. Using two different spectral dependencies to fit our data, we find the carbon to oxygen concentration ratio for energies above 1 MeV to be 0.43 for the power law spectrum and 0.44 for the exponential spectrum. Thus in either case we find the abundance ratio consistent with the solar origin of the particles. Similarly, the helium to oxygen concentration ratio is found to be 0.17 for the power law spectrum and 0.28 for the exponential spectrum. This is inconsistent with the solar wind ratio, which is about 10. The results quoted above are based on a very small portion of the distribution function above 1 MeV.
Preliminary results of polarimetry and photometry of Titan at large phase angles from Pioneer 11
Authors: Tomasko M. G.
Published: February 2008

The imaging photopolarimeter (IPP) aboard the Pioneer 11 spacecraft measured the linear polarization of the integrated disk of Titan in red and blue light at a variety of phase angles from 15° to 97°. The large polarization (54%) measured in blue light at 90° phase constrains the size of the aerosols near the top of Titan's atmosphere to have radii smaller than about 0.09 µm if they have a refractive index of 2.0. The polarization at 90° phase in red light is smaller (41%) and implies that the optical thickness of the layer of small aerosols is about 0.6 above an effectively depolarizing surface. The shape of the polarization versus phase curve in blue light suggests increasing particle size with increasing depth into the atmosphere. The limb darkening of Titan was measured at 28° phase by using the imaging mode of the IPP. The limb darkening observed in both colors is reasonably consistent with that given by the scattering models derived from the polarization observations.
Observations of the mean ionization states of energetic particles in the vicinity of the Earth's magnetosphere
Authors: Ma Sung L. S. , Gloeckler G. , Fan C. Y. , Hovestadt D.
Published: February 2008

The mean ionization states of 44–260 keV per charge ions observed as bursts in and near the earth's magnetosphere have been determined by using the particle data collected by the University of Maryland experiment on Imp 8. We find that during the period from October 1973 to December 1976 (1) the abundance ratio of heavy ions (Z >2) to alphas ranges from 0.04 to 0.10, with a mean value of 0.08±0.02; (2) the energy spectra of alphas and Z >2 ions in these bursts are adequately represented as exponentials in energy per charge with e‐folding energies of 30–50 keV/Q; (3) the e‐folding energies of both alpha particles and heavier ions are generally harder upstream from the bow shock than in the magnetotail and magnetosheath; and (4) the elemental abundances and ionization state distribution of the heavy ions are consistent with those of the corona at an equilibrium coronal temperature of 1–2 × 106 °K, which tends to support a solar wind origin for these particles.
An anomalous low‐latitude phenomenon observed by the Ogo 6 UV photometer between 400 and 800 km
Authors: Thomas Gary E.
Published: February 2008

The polar‐orbiting Ogo 6 UV photometer was designed to measure the high‐altitude airglow in the zenith at heights between 400 and 1100 km. However, at low latitudes and below 800 km the short‐wavelength channel (sensitive to Lyman alpha 1216 Å) showed localized increases in apparent intensity above the normal Lyman alpha airglow signal. Their large intensities (up to 8 kR) and their high altitude of occurrence make it unlikely that the instrument was recording UV emission. The morphology of the occurrence of these mainly quiet time events points to the possibility of direct charged particle interference. This interference is not caused by trapped inner Van Allen belt particles. It appears that the responsible particles are quasi‐trapped soft (<1 MeV) protons which are confined to low latitudes and low altitudes, at least during those local times for which they have been observed. Such a soft proton belt was discovered in 1972 by Moritz; however, no information on the geographical or local time dependence was known. The Ogo 6 observations have a distinct and unusual morphology, which may reflect the geographical and local time behavior of the protons, as well as their pitch angle distribution.
Phase skipping and packet structure in geomagnetic pulsation signals
Authors: Mier‐Jedrzejowicz W. A. C. , Hughes W. J.
Published: February 2008

Observations, made both in space and on the ground show that phase jumps occur between wave packets in dayside pulsation signals. During the course of a single wave packet, the signals from two stations may drift apart in phase. The action of the phase jump is to bring the signals back into phase. This suggests an impulsive source for such dayside pulsations, each wave packet being excited by a new impulse. We speculate about a nightside source connected with changes in magnetospheric convection.
The Pioneer Venus Program
Authors: Colin Lawrence
Published: February 2008

The nation's most extensive and intensive study of the planet Venus and its environment occurred during and subsequent to December 1978: Pioneer Venus. This introductory paper to a special issue of the Journal of Geophysical Research devoted to the scientific findings of both the orbiter and multiprobe missions documents the major aspects of the missions and provides the reader with necessary background information to cope intelligently with the myriad of scientific findings found elsewhere in the issue. Specific topics covered include program history, key scientific questions addressed by Pioneer Venus, scientific payloads, science management, spacecraft descriptions, launch and interplanetary cruise features, encounter features, nominal mission descriptions, instrument anomalies, and the orbiter extended mission.
Dynamic properties of the thermosphere inferred from Pioneer Venus Mass Spectrometer measurements
Authors: Mayr H. G. , Harris I. , Niemann H. B. , Brinton H. C. , Spencer N. W. , Taylor H. A. , Hartle R. E. , Hoegy W. R. , Hunten D. M.
Published: February 2008

In situ measurements of neutral thermospheric composition derived from orbiter neutral mass spectrometer (ONMS) and ion mass spectrometer (OIMS) experiments on Pioneer Venus are interpreted. Observed day to night density variations with asymmetries between dawn and dusk contain pronounced signatures of various transport processes due to winds, exospheric flow, and vertical diffusion. The relative magnitudes of these processes depend significantly on the rotation rate of the thermosphere and its turbulent properties. On the basis of a theoretical three‐dimensional multiconstituent model describing solar diurnal tides in a rotating atmosphere, the analysis leads to the following conclusions:(1) The day‐night temperature contrast on Venus is associated with wind velocities of about 200 m/s (for the lowest order harmonic) which transport 0, He, and H toward the nightside. (2) Mass exchange with the mesosphere, commensurate with an eddy diffusion coefficient of K=3 × 107 is required to buffer that horizontal advection such as to reproduce the observed daytime bulge in 0 as well as the comparatively small diurnal variations in He. (3) Exospheric flow significantly affects the H distribution. (4) The observed time response and magnitude of the day‐night density variations require that transport processes are only effective over time periods between 5 to 10 days, which, in comparison with the 243‐day rotation rate of the solid body, implies a super rotation rate or prevailing winds in excess of 50 m/s at the equator. (5) Nonlinear mass transport results in wave steepening and contributes to amplify the density extrema in H and He; owing to the long time constant for He transport, higher order tidal modes overtake the fundamental harmonic and shift the density maximum toward dawn.
The signature of auroral kilometric radiation on Isis 1 ionograms
Authors: Calvert W.
Published: February 2008

Auroral kilometric radiation (AKR) appears on the Isis 1 topside sounder ionograms as intense noise bands between the electron cyclotron frequency and 700 kHz. A variable gap occurs between the cyclotron frequency and the lowest AKR frequency. As Isis 1 traverses the source region, the gap narrows, and the AKR signals at higher frequencies weaken. This signature suggests that the AKR waves are generated directly in the extraordinary mode at frequencies just above the local cutoff frequency and that the radiation is initially perpendicular to the magnetic field.
A model for the anisotropic reentry of albedo
Authors: König P. J.
Published: February 2008

The trajectory‐tracing technique was used to obtain the angles of incidence, and hence ‘intensities,’ of negatively charged 0.88‐GV particles reentrant at Palestine, Texas. Splash albedo trajectories were traced from the conjugate point, and also from Palestine itself, for those trajectories that were unable to complete a full gyration before reentry into the shadow cone at Palestine. Both isotropic and anisotropic ejection configurations were used at these two locations. These simulations predict a north‐south anisotropy (hence also a zenithal anisotropy) for reentrant albedo, with a dearth of trajectories incident from the south. The anisotropy is large enough to explain experimentally determined north‐south anisotropies for lower‐energy particles, as observed by other groups in the Northern Hemisphere. The results are in agreement with measurements and simulations previously obtained in the Southern Hemisphere.
Nonlinear evolution of hydromagnetic waves in the solar wind
Authors: Steinolfson R. S.
Published: February 2008

The nonlinear evolution of linearly polarized, hydromagnetic waves propagating along the initial radial magnetic field in the solar wind is considered. Numerical solutions of the dissipationless, time‐dependent equations of motion are used to study the steepening of the waves into hydromagnetic shocks and the subsequent propagation of the shocks. If the shocks propagate into a region in which the plasma beta (ratio of thermal to magnetic pressure) is greater than one, each shock splits into a second shock and a rotational discontinuity. It is shown that these secondary hydromagnetic shocks readily propagate through the rotational discontinuities without altering their structure. With increasing time the secondary hydromagnetic shocks propagate out ahead of the rotational discontinuities and become hydrodynamic shocks. Hence an initial packet of linearly polarized hydromagnetic waves ultimately evolves into a packet of rotational discontinuities preceeded by a packet of hydrodynamic shocks. The conditions for which the waves steepen into shocks, the time required for shock formation, and further implications of the results in the solar wind are discussed.