bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristic recovery times of Forbush‐type decreases in the cosmic radiation: 1. Observations at Earth at different energies
Authors: Lockwood J. A. , Webber W. R. , Jokipii J. R.
Published: February 2008

About 30 Forbush‐type decreases occurring from 1972 to 1984 at 1 AU for which data were available from the IMP spacecraft (P median of ∼1.7 GV) and the Mt. Washington neutron monitor (P median of ∼5 GV) were examined to determine the characteristic recovery times of the events. The characteristic recovery time t0 was found to be the same for rigidities of ∼1.7 GV and ∼5.0 GV. Characteristic recovery times for additional Forbush decreases, including some in 1957–1963, were determined, using the Mt. Washington neutron monitor and monitors at cutoff rigidities of ∼12 GV, and were also found to be the same. Hence the characteristic recovery time of Forbush decreases at 1 AU is independent of energy (or rigidity) over a factor of at least 10. The average recovery time t0 at 1 AU is ∼5 days but varies from ∼3 to ∼10 days, and the recovery in most cases is well represented by an exponential. No significant difference in the average recovery time is observed when the solar magnetic field reversed in 1980 nor during the decrease and recovery portion of the solar modulation cycle. A physical model is proposed to explain these results.
New waves at multiples of the plasma frequency upstream of the Earth's bow shock
Authors: Cairns I. H.
Published: February 2008

The first observations of waves at harmonics higher than the second of the electron plasma frequency are reported. The observations were made by the ISEE 1 spacecraft upstream of the earth's bow shock. The waves are interpreted as electromagnetic radiation at the fundamental and up to the fifth harmonic of the plasma frequency, with effective temperatures decreasing from 5×1017 K to 1010 K over this range. Two models for the emission of the waves are sketched.
A gas dynamic magnetosheath field model for unsteady interplanetary fields: Application to the solar wind interaction with Venus
Authors: Luhmann J. G. , Warniers R. J. , Russell C. T. , Spreiter J. R. , Stahara S. S.
Published: February 2008

The steady state gas dynamic model of magnetosheath magnetic fields previously developed by Spreiter and Stahara (1980) is generalized for the common situation of a temporally varying interplanetary field orientation. Examples for the particular case of Venus in the solar wind illustrate the application of the model to the passage of rotational discontinuities and MHD waves through planetary magnetosheaths. The results of this model illustrate how the field structure near a planetary magnetopause or ionopause can be affected by interplanetary field variations rather than by local processes because of the “pile up” of magnetosheath fields from a sequence of upstream fields. Changes in the spectrum of interplanetary waves on their transmission to the magnetopause are also indicated.
Perpendicular ion beam‐driven instability in a multicomponent plasma: Effects of varying ion composition on linear flute mode oscillations
Authors: Walker David N.
Published: February 2008

Numerous observations of low frequency electrostatic waves associated with the injection of heavy ion beams from sounding rockets have prompted an investigation of the collisionless linear flute mode (k∥ = 0) dispersion relation (Ichimaru, 1973; Seiler et al., 1976; Hamelin and Beghin, 1976) for a multi‐component plasma. Assuming a two‐ion component background plasma whose masses differ by at most a factor of 2 (e.g., O+, O2+; O+, NO+), solutions are obtained as a function of varying ion composition both in the absence of a beam term (i.e., multicomponent ion Bernstein modes) and in the presence of an unmagnetized Ar+ ion beam assuming small thermal spreading of the beam. Numerical solutions for resonantly driven oscillations using small beam densities and perpendicular beam velocities yield increasing growth rates for increasing wave number, k, for the first three modes studied. An analytical approximation to the growth rates, following a procedure similar to one used by Roth et al. (1983), is consistent with this conclusion. The results compare favorably to Langmuir probe electrostatic wave observations made during the injection of an Argon ion beam from the ARCS‐I sounding rocket into the auroral ionosphere. During the injection process the rocket descended through a region in which the ratio of light to heavy ions can vary considerably.
Automatic determination of the electron density measured by the relaxation sounder on board ISEE 1
Authors: Trotignon J. G. , Etcheto J. , Thouvenin J. P.
Published: February 2008

Relaxation sounders have proved to be successful in measuring the electron density in dilute space plasmas. The resonances excited at the characteristic frequencies of the medium depend very much on the local plasma. We present here methods to determine automatically, by a computer, the plasma frequency in the various regions encountered. In the solar wind and in the magnetosheath, only one strong, long‐lasting resonance appears close to the plasma frequency fpe, yielding directly the electron density. We use a pattern recognition method based on Sebestyen';s discrimination approach, and each identified resonance is given with a quality criterion. From one year of data there is no drift of the resonance features, and the method is used with a success rate of 95% as determined by manual means. In the magnetosphere various types of resonances are excited at the characteristic frequencies of the medium (cyclotron harmonic frequencies nfce and maximum frequencies of the Bernstein';s modes fqn). We compare the dispersion relation of electrostatic waves, which contains fce and fpe as parameters, to the observed frequencies of the resonances. First fce is extracted using the harmonicity of the nfce resonances, and then the fqn series is compared to the dispersion relation for various values of fpe. In 93% of 250 cases the error for fpe is less than 10%. In the magnetotail where the resonances are closely spaced in frequency, the frequency resolution of the experiment does not allow us to use this method and we determine fpe by the enhancement of the resonances power.
Simultaneous Hα line profile and radar measurements at Arecibo
Authors: Kerr R. B. , Atreya S. K. , Meriwether J. W. , Tepley C. A. , Burnside R. G.
Published: February 2008

High resolution measurements of the geocoronal Balmer alpha (Hα) emission were made with a Fabry‐Perot interferometer at Arecibo, Puerto Rico, in six observing campaigns between March 1980 and November 1984. Four of these included simultaneous radar measurements of the topside F region. The nighttime variation of the zenith Hα intensity is highly symmetric about solar midnight. However, effective hydrogen temperatures near the morning exobase are generally hotter than near the evening exobase, implying a depletion of exobase hydrogen density in the early morning hours. The exospheric temperature is derived from the Doppler line profiles and is compared to the topside F region O+ temperature. The cooling of H relative to O+ due to the escape of hot, energetic H is not obvious in our data, and it appears that a downward flux of hot protons often causes heating of the bound neutral H population by charge exchange during low to moderate solar activity conditions. This downward flux is usually strongest between 0200 and 0600 LT, and occurs throughout the night in the late fall and early winter. We have detected a deficiency of inbound high‐energy hydrogen relative to the high‐energy outbound population in radial velocity Hα profiles. We have also detected large distortions of the Hα profile wings during low to moderate solar activity periods and during periods of large downward ion speeds. We attribute these wing distortions to efficient charge exchange of H with H+. A narrowing of zenith profile widths near midnight demonstrates the gravitational cooling of H due to the inverse relationship between geocentric distance and the kinetic energy required for escape. We have also searched for evidence that satellite hydrogen populations with small orbital eccentricities may be contributing to narrow line widths.
Velocity filter mechanism for ion bowl distributions (bimodal conics)
Authors: Horwitz J. L.
Published: February 2008

Klumpar et al. (1984) have recently observed ion distributions in the high‐altitude auroral region which are conical at high energies and field‐aligned at lower energies. In velocity space these distributions appear “bowl‐shaped.” These observed distributions were interpreted as being due to two‐stage acceleration involving transverse ion heating and upward field‐aligned electric field acceleration. Here we show that qualitatively similar bowl‐shaped distributions can also be formed by transverse heating in a region of finite horizontal extent, followed by essentially adiabatic convective flow to the observation location. The latter stage contributes a velocity‐filtering effect which produces some characteristics of the distributions observed. For the published distributions of Klumpar et al. the evidence favors the parallel electric field acceleration as the dominant mechanism for the second stage. However, in general, it may be possible to use multiple ion species observations to distinguish these two mechanisms for ion bowl distributions.
Jovian VLF chorus and Io torus aurora
Authors: Inan U. S.
Published: February 2008

A test particle model of the cyclotron resonance interaction of waves and trapped radiation belt particles is used to estimate the energetic electron fluxes precipitated by Jovian VLF chorus waves observed on the Voyager 1 and 2 spacecraft near the Io torus. The precipitation fluxes induced by 1‐s‐long chorus wave packets at L ≃ 7.6 and 8.6 are estimated to be bursts of ∼5s duration with a peak of 0.3–3 and 0.7–7 ergs/cm² s that consist of electrons of ∼5‐100 keV energy and that arrives at the ionosphere ∼15 s after the generation of the chorus wave at the equatorial plane. The effects in the Jovian ionosphere of the chorus‐induced precipitation are estimated using existing ionospheric models. A possible experiment for measuring Jovian chorus‐induced aurora is proposed and discussed.
Dayside equatorial‐plane convection and IMF sector structure
Authors: Baumjohann W. , Nakamura R. , Haerendel G.
Published: February 2008

Using 1290 hours of GEOS‐2 electron gun measurements between 0800 LT and 1900 LT we have analyzed the dependence of dayside magnetospheric convection in the equatorial plane at L = 6.6 RE on the IMF sector structure, i.e. IMF By ≷ 0 and IMF Bx ≷ 0. For all IMF sector orientations the plasma flow at noon is directed predominantly sunward with a slight dawnward component. For positive IMF By the plasma convection around 0900 LT has a duskward component which turns into a dawnward component for negative IMF By values. The ratio between the electric field amplitudes at 1500 LT (dusk sector) and those at 0900 LT (dawn sector) increases from about 2 for IMF By positive to 3–4 for negative IMF By components. The convection pattern does neither show a dependence on the Bx component of the IMF nor is it significantly different for gardenhose (Bx · By < 0) and nongardenhose (Bx · By > 0) orientations of the IMF. These results support the idea of component merging rather than the antiparallel merging hypothesis.
Energetic electron precipitation during a magnetospheric substorm and its relationship to wave particle interaction
Authors: Kremser G. , Korth A. , Ullaland S. , Stadsnes J. , Baumjohann W. , Block L. , Torkar K. M. , Riedler W. , Aparicio B. , Tanskanen P. , Iversen I. B. , Cornilleau‐Wehrlin N. , Solomon J. , Amata E.
Published: February 2008

Energetic electron precipitation was observed via X ray measurements from three simultaneously flown balloons in a region conjugate to the location of the geosynchronous satellite GEOS 2. Data from the particle, wave, and field experiments on this satellite were used to investigate the relationship between magnetospheric processes in the equatorial plane and the electron precipitation. A large part of the electron precipitation during a substorm on July 3, 1979, was accompanied by VLF waves at whistler frequencies and had a temporal structure grossly similar to the intensity variations of the equatorial electron fluxes. Some precipitation also occurred in the absence of VLF waves. During the wave associated precipitation the equatorial electrons were anisotropically distributed with maximum fluxes perpendicular to the geomagnetic field. The anisotropy was larger than the critical anisotropy and thus enabled wave growth. The growth rate itself was determined numerically from the observations and turned out to have a frequency dependence similar to that of the observed waves. The interaction of the electrons with the waves led to strong pitch angle diffusion. The temporal variations of the electron flux in the loss cone were similar to those of the electron precipitation observed with a balloon near the conjugate point to GEOS 2. It is concluded that most of the electron precipitation during this substorm was caused by wave particle interaction.
Latitudinal plasma distribution in the dusk plasmaspheric bulge: Refilling phase and quasi‐equilibrium state
Authors: Décréau P. M. E. , Carpenter D. , Chappell C. R. , Comfort R. H. , Green J. , Olsen R. C. , Waite J. H.
Published: February 2008

This paper analyzes the plasma characteristics measured in the dusk sector of the plasmasphere during the period May–June 1982, mainly by the satellites DE 1 and GEOS 2, in order to examine the hypothesis of a lack of connection between the equatorial plasmaspheric bulge and the underlying ionosphere. The orbits of the spacecraft allow a study of the dusk bulge from two different perspectives: GEOS 2, geostationary, yields the local time variations of the plasma parameters, whereas DE 1 stays roughly in a meridian plane and provides their latitudinal variations. Different techniques (waves and ion spectrometer in the very low energy range) are used and compared. A period of a few days (May 22–26) is analyzed in detail. Large density irregularities are commonly observed, as well as significant plasma drifts, but the bulge does not seem to be limited in latitude, at least between +40 and −40 degrees magnetic latitude. The analysis of the plasma distribution along a field line shows the existence of different plasma regimes, depending on the geophysical conditions encountered. In particular, we have observed (1) during the replenishment of depleted flux tubes, an apparent accumulation of plasma at about 3 earth radii and 40° latitude, and a double field‐aligned regime in the equatorial region, and (2) under conditions of moderate magnetic activity, the appearance of a high density region, with origin presumably in the inner plasmasphere, extending to geostationary orbit. In this region of L = 4.6 to 6, the density, normalized to the volume of the flux tubes, is constant over the latitudinal range explored (from minus to plus 40°), as expected from a diffusive equilibrium distribution.
Relationship of the interplanetary electric field to the high‐latitude ionospheric electric field and currents: Observations and model simulation
Authors: Clauer C. Robert , Banks Peter M.
Published: February 2008

F ‐ region ion convection in the vicinity of the dayside polar cleft is strongly controlled by the interplanetary magnetic field, suggesting a direct current‐driven coupling between the solar wind and dayside, high‐ latitude ionosphere. This coupling has been explored through observations of high‐latitude ion convection measured by the Sondre Stromfjord, Greenland incoherent scatter radar and subsequent computer simulations. The computer simulation calculates the ionospheric electric potential distribution for a given configuration of field‐aligned currents and conductivity distribution. Using a simple model of the field‐aligned currents linking the solar wind with the dayside, high‐latitude ionosphere, we can explore the consequences of variations of the currents on ionospheric plasma convection. The direction and strength of the currents are set according to measurements in the upstream solar wind and the time varying ionospheric electric field and resulting plasma convection are simulated. Comparisons of the simulated plasma convection with the ion velocity measurements at Sondrestrom show very good agreement. Thus, a very simple model which provides a direct current‐driven connection between the solar wind and the high‐latitude ionosphere via field‐aligned currents in the polar cleft is able to explain a large portion of the ionospheric electric field variations observed in the vicinity of the polar cleft.
Dynamics of a discrete auroral arc
Authors: Brüning K. , Goertz C. K.
Published: February 2008

Porcupine Flight 4 data were used to determine the field‐aligned currents associated with a southward moving discrete auroral arc in the postmidnight sector. We use three different methods for determining j∥ which should give identical results if the arcs are quasi‐stationary and no parallel electric field exists between the payload and the dynamo region of the ionosphere. As long as the rocket is above the arc, the three methods agree. The integral of precipitating electron flux, the local magnetic field perturbations, and the divergence of the horizontal Pedersen current all indicate an upward current of 5±3 µA/m². Immediately north of the arc a strong downward current of about 10–20 µA/m² is detected. The magnitude, however, is not well known because the rocket's velocity relative to the arc cannot be clearly established. Further north of the southward moving arc the two methods that can be applied (magnetic field perturbations and divergence of the horizontal Pedersen current) yield contradictory results not only about the magnitude of the current but also about the direction of the current. We suggest that this discrepancy is due to time‐dependent electric fields.
Heating by Alfven wave dissipation in the Jovian thermosphere
Authors: Kar Jayanta , Mahajan K. K.
Published: February 2008

An estimate of heating rates by the dissipation of Io‐generated Alfven waves by ion‐neutral collisions in the Jovian thermosphere is made. Heating rates at the foot of the Io flux tube are found to be significant.
Interruption of field‐aligned current due to electrostatic turbulence
Authors: Yamamoto Takashi , Kan J. R.
Published: February 2008

By one‐dimensional particle simulations it is shown that wave‐particle interactions can significantly reduce upward field‐aligned currents when a potential drop exists along the field lines and the electrons of current carriers are sufficiently accelerated by the parallel electric field. In our simulation model, a potential difference across the simulation domain is fixed, and constant fluxes of electrons and ions are provided from the plasma sheet boundary. The ionospheric boundary provides plasmas of ionospheric origin. The ionospheric boundary also reflects the accelerated electrons outside the loss cone, which represents mirroring by the converging geomagnetic fields. The simulation results show that the field‐aligned current density is about 30% reduced from the adiabatic field‐aligned current density when the turbulent region is assumed to be located at an altitude of 8000 km on the auroral field lines. The dominant mechanism for reducing the field‐aligned current is as follows: The electrons accelerated by the potential drop lose the parallel kinetic energy through the Landau interaction with electrostatic waves excited by the beam instability. Most of the decelerated electrons will not be mirrored back to the plasma sheet even when they are outside the loss cone. This effect as well as precipitation into the ionosphere would produce an asymmetric velocity distribution of electrons with more downward (toward the ionosphere) flux than upward flux in the region of the current source, i.e., in the plasma sheet. This asymmetric distribution tends to relax toward the more symmetric thermal distribution through interactions with excited plasma oscillations. Therefore a fraction of the downard flux of plasma sheet electrons is converted to the upward flux, which causes a decrease in the field‐aligned current.
Simulations of electron beam excited modes in the high‐altitude magnetosphere
Authors: Roth I. , Hudson M. K.
Published: February 2008

Excitation of waves by electron distributions consisting of hot, beam, and cold populations is investigated theoretically and with the help of particle simulations. The main modes excited include the upper hybrid oscillation for nearly perpendicular propagation; the whistler mode at oblique angles, which becomes the plasma two‐stream oscillation for parallel propagation; and the electron acoustic mode for nearly parallel propagation. The whistler mode, excited by thermal fluctuation enhancement, has a broad range of wave numbers and quasi‐linearly decreases the beam slope, while the electron acoustic mode, which is linearly unstable, has a narrow spread in phase velocities and traps the beam and the warm electrons forming a double humped distribution. Both modes contribute to forming a tail in the cold electron distribution. The resulting wave spectrum is discussed in the context of DE 1 observations.
The application of the ring current correction model to Magsat passes
Authors: Zaaiman Hermien , Kühn Gideon J.
Published: February 2008

The application of the ring current correction model to the Magsat data over the southern African region is considered. We found that by using dip latitude computed at 3 RE above the surface of the earth (representing the ring current altitude) to derive the E, I coefficients and apply the correction, the effectiveness of the generally used ring current model is improved. These results are of particular importance to work in the southern African region. However, the physical reasoning should be generally applicable in the use of the ring current correction.
Possible effects of solar flares on the ionosphere of Venus from Pioneer Venus Orbiter measurements
Authors: Kar J. , Mahajan K. K. , Srilakshmi M. V. , Kohli R.
Published: February 2008

The sharp enhancements in ionization seen in the outbound profiles around 200 km, for three orbits, during the second and the third sets of dayside periapsis passes, have been identified as possible ionospheric effects of three class 1 solar flares, simultaneously monitored on the earth. Two of these flares were accompanied by sudden ionospheric disturbances on the earth also. The effects of these flares could be observed by the orbiter, which subsequently sampled the ionosphere after a time comparable to the relaxation time of the ionosphere at these heights.
Some molecular nitrogen emission from Titan‐solar EUV interaction
Authors: Haider S. A.
Published: February 2008

Voyager 1 explored the satellite Titan at a solar zenith angle of 73°. Photoelectron fluxes in the atmosphere of Titan for this same angle (73°) have been calculated for an atmospheric model with molecular nitrogen. The Photoelectron impact excitation rates of a′πg and b′πu states of N2 are also calculated.
A proposed experiment to study the critical ionization velocity theory in space
Authors: Murad Edmond , Lai S. T. , Stair A. T.
Published: February 2008

A simple, novel experiment is suggested to verify the critical ionization hypothesis in space. The experiment involves the isentropic expansion of gas through a carefully chosen nozzle and skimmer so that the formation of dimers and trimers is enhanced. Because of the reduced ionization potentials of these species and because of the increased mass, it is expected that the critical ionization velocity will be lowered considerably even for simple gases such as N2. The release of the gas aboard the space shuttle, whose orbiting velocity is about 7.3 km s−1, will provide the necessary velocity relative to the ambient ions to ionize the dimers and trimers. The advantages of this method over previously tried or proposed methods are briefly discussed.
Pioneer 11 observations of effects of Ganymede and Callisto on Jupiter's trapped radiation
Authors: McKibben R. B. , Connerney J. E. P.
Published: February 2008

Beyond about 10 RJ from Jupiter the configuration of magnetic field lines in Jupiter's magnetosphere is strongly affected by the presence of an equatorial current sheet. Using a Voyager 1 magnetic field model that explicitly includes the effects of this current sheet to describe the magnetic field line geometry, we have reexamined charged particle observations from Pioneer 11's high‐latitude flyby of Jupiter in 1974. We find that if the trajectory of Pioneer 11 is mapped to the equatorial plane along the model magnetic field lines, significant features in the time‐intensity profiles of trapped protons and electrons, including one microsignaturelike feature, are found to correspond to shells of closed field lines crossed by the orbits of the satellites Ganymede and Callisto. Although many of the characteristics of the features, including their magnitude, persistence, and similarity of appearance for protons and electrons, are difficult to explain, we suggest that these features are signatures of interaction of the trapped particles with Ganymede and Callisto. If this interpretation is correct, it offers explanations for several large features in the time‐intensity profile of trapped particles observed during Pioneer 11's flyby that could not be explained using earlier magnetic field models which did not include effects of the current sheet, and it leads to reinterpretation of some features previously associated with Ganymede as signatures of Callisto. It also implies that the configuration of the current sheet during the Pioneer 11 flyby in 1974 was very similar to that observed by Voyager 1 in 1979 and provides confirmation of the essential accuracy of current sheet magnetic field models for describing the magnetic field configuration out to radii of about 30 RJ from Jupiter. Finally, the characteristics of the signatures described suggest interesting problems for future research on the dynamics and interactions of trapped particles with satellites in the outer regions of the Jovian trapped radiation zone.
Energetic proton and helium fluxes associated with interplanetary shocks and their relation to the solar wind composition
Authors: Tan L. C. , Mason G. M. , Ipavich F. M. , Gloeckler G. , Zwickl R. D. , Bame S. J.
Published: February 2008

Observations from sensors on the IMP 8 and ISEE 3 spacecraft have been used to study the characteristics of interplanetary shock‐associated energetic particle events. In the energy range above 200 keV, we find that large ambient proton flux levels from the parent solar flare are associated with hard spectra downstream from the shock. At ∼40 keV/nucl, immediately behind the shock fronts the He/H flux ratio of shock‐accelerated particles is strongly correlated with the upstream He/H density ratio in the solar wind, but not with the flare He/H ratio. The correlation between the shock accelerated He/H and the solar wind He/H ratios is linear. Immediately downstream from the shock front, the energy spectral indices of protons and alpha particles are equal. Measured at the same energy/nucleon, the (46 keV/nucl) energetic particle He/H ratio is the same as the solar wind He/H ratio upstream from the shock, while at the same energy/charge, the energetic particle He/H ratio shows an enhancement by a factor of 4 to 5 compared with the solar wind. Many of these findings are consistent with a first‐order Fermi acceleration origin of the energetic particles. Suprathermal particles from the shock‐heated solar wind appear to be the primary seed population of the shock‐associated energetic particles near ∼40 keV/nucl. However, for shock accelerated energetic particles of energies in the range above 200 keV, a more energetic seed population appears to be important.
Simultaneous polar cap and magnetotail observations of intense polar rain
Authors: Greenspan M. E. , Meng C.‐I. , Fairfield D. H.
Published: February 2008

Six prolonged episodes of intense polar rain with average or above average temperatures occured during the four‐month period from mid‐February to mid‐June 1978. Electrons with energies up to 1 keV are observed throughout these events, and 9‐keV electrons occasionally are detected. To investigate whether this intense polar rain is accelerated within the magnetosphere, we compare DMSP F2 electron measurements made at low altitudes over the polar cap with near‐simultaneous ISEE 1 electron measurements made in the northern tail lobe at distances between 10 and 22.6 RE. In some cases, the electron spectra measured at low altitude are nearly identical to those measured in the tail lobe. In other cases, intensities are similar, but the phase space densities measured by DMSP exceed those measured by ISEE at energies above 500 eV. The episodes with extremely similar spectra show that intense polar rain can pass through the tail lobes without undergoing acceleration between 22.6 RE and 800 km altitude. Three‐dimensional ISEE measurements of electron distribution functions suggest that the field‐aligned pitch angle anisotropy of the tail lobe electron population may account for most if not all of the differences when the spectra do not match.
Heater‐induced 1‐meter irregularities
Authors: Høeg Per , Nielsen E. , Stubbe P. , Kopka H.
Published: February 2008

Series of ionospheric modification experiments have shown the possibility of artificially exciting short‐scale field‐aligned irregularities in the E region at high latitudes. During magnetically quiet conditions the heating facility situated close to Tromø, Norway, produced 1‐m irregularities. The observations were done by the two radars constituting the STARE system operating at 140 MHz and 144 MHz. A statistical analysis of the data shows a preference for overdense (f0,Heat < f0E) O‐mode heating. In 5% of the experiments, X‐mode polarization of the heater wave was exciting irregularities. A smaller portion of the data confirmed underdense heating. The e‐folding growth times varied from less than 2 s to 15 s with the shortest growth times as the typical values. The estimated average phase velocity of artificial irregularities amounted to 175 m s−1. Due to features in the backscatter pattern observed by STARE we can conclude that not only one type of instability was operating. Nonlinear saturation processes may be the cause of the observations. But also two‐step processes, where an instability type with low threshold conditions modifies the plasma of the ionosphere for a more efficient instability mechanism, may explain the data.
Excitation of electrostatic fluctuations by thermal modulation of whistlers
Authors: Stenflo L. , Shukla P. K. , Yu M. Y.
Published: February 2008

It is shown that a large‐amplitude field‐aligned whistler wave can excite nonducted whistler sidebands and nonresonant low‐frequency electrostatic collisional gradient drift modes. The effects of the ponderomotive force as well as the heating nonlinearities are included in the study of the modulational instabilities. For this case it is found that the heating nonlinearity dominates over the ponderomotive force effects, provided that the doppler‐shifted frequency of the collisional modes is smaller than the electron collision frequency. Our investigation can be relevant in understanding why enhanced density irregularities appear in the presence of electromagnetic heater waves in the lower part of the ionosphere.
On the location of the magnetopause neutral points
Authors: Wu C. C.
Published: February 2008

We have studied the configuration of the polar cusps in a static MHD equilibrium model. By introducing two quasi‐linear equilibrium models, we show that one can obtain magnetospheric configurations with the magnetopause neutral points located tailward of the earth and at the same time with reasonable tail magnetic field geometry. We also conclude that by the pressure balance principle across the magnetopause, if the magnetopause is a smooth surface then the neutral points cannot be on the dayside boundary.
Systematic errors in determining the propagation direction of interplanetary Alfvenic fluctuations
Authors: Goldstein M. L. , Roberts D. A. , Matthaeus W. H.
Published: February 2008

Analyses of Alfvenic fluctuations in the solar wind contain a systematic effect in which outward propagation is favored. The effect is generally small at and beyond 1 AU in a Mach 10 flow but can become important where the Mach number is lower, e.g. inside 1 AU. If the propagation speeds of the inward and outward propagating Alfvenic fluctuations are properly included, this systematic effect can be removed. A technique for doing this is illustrated using both artificial and spacecraft data. The results suggest that time intervals originally thought to be outward propagating sometimes contain wave number bands in the cross helicity spectrum of inward propagating Alfvenic fluctuations.
A statistical study of auroral radar spectra obtained with SABRE
Authors: Schlegel K. , Thomas E. C. , Ridge D.
Published: February 2008

About 77,000 spectra obtained with the auroral radar SABRE have been analyzed on a statistical basis. They have been characterized by their first four moments: power, mean velocity, width, and skewness. Histograms of the frequency of occurrence for these moments are presented and discussed. It is striking that only very few broad spectra are found, contrary to the results of other studies of auroral radar spectra. Different methods to determine the phase velocities of the auroral irregularities are also discussed. Due to a very frequent skewness of the spectra, the often applied double‐pulse technique seems least suited to determine the Doppler velocity accurately. Strong differences are also found in the characteristics of the spectra for positive and negative Doppler velocities. Possible reasons for these differences are discussed.
Recurrence of periods of enhanced cosmic ray solar diurnal variation associated with a two‐sector structure
Authors: Swinson Derek B. , Saito Takao
Published: February 2008

Swinson et al. (1981) showed that in 1974 the two‐sector structure of the interplanetary magnetic field gave rise to conditions in which there were extended trains of enhanced diurnal variation in cosmic ray intensity which were organized according to the sector structure. In this paper we show that the heliomagnetosphere, in its continuing evolution, has again produced a distinct two‐sector structure in 1984. We demonstrate, using neutron monitor data and data from underground muon telescopes, that these conditions once more give rise to extended periods of enhanced cosmic ray solar diurnal variation, again organized by the sector structure. These field‐dependent anisotropies are consistent with the observations and expectations described in the earlier paper.
The spatial component of ionospheric electron flow derived from two‐dimensional drift measurements
Authors: Shen C. ‐S. , Nielsen E.
Published: February 2008

The electron drift velocities in the ionosphere can be measured as a function of both latitude and longitude using coherent radar systems like STARE and SABRE. The temporal resolution of the observations is high compared to the time it takes for the field of view to corotate with the earth over a longitude interval equal to its longitudinal width. An analysis technique is developed that allows us to determine whether an observed time variation is caused by spatial effects or by real time changes. Three events, during which the flow was dominated by spatial variations, have been analyzed and are found to show good agreement with model predictions.
Bremsstrahlung X ray mappings of an intense, widespread, and pulsating electron precipitation event following a sudden commencement
Authors: Imhof W. L. , Voss H. D. , Mobilia J. , Datlowe D. W.
Published: February 2008

For the first time, remote bremsstrahlung X ray spatial, temporal, and spectral mappings from a satellite are presented for a strong energetic electron precipitation event following a sudden commencement. After the sudden commencement of July 13, 1982, and at the beginning of a magnetospheric substorm, X rays were observed over a period of more than 4 min from the P78‐1 spacecraft at 600‐km altitude. The X rays above 21 keV were emitted over a magnetic local time (MLT) interval spanning 6 hours. Five precipitating temporal spikes were observed with coherency over wide longitude intervals of 3 hours or more in MLT near midnight. The energy spectra over broad longitude intervals tended to be harder simultaneously at the times of greatest X ray intensity. The power input to the atmosphere from precipitating electrons that produced the X rays emitted within 2200–2230 MLT is estimated to have reached values of 5 × 1016 ergs/s over an area of about 3 × 104 km², leading to an average energy deposition rate of about 160 ergs/cm² s. The precipitation inputs at other magnetic local times displayed nearly simultaneous sharp maxima in time but with lower power inputs.
The electron distribution function upstream from the Earth's bow shock
Authors: Cairns Iver H.
Published: February 2008

A general analytic theory for the distribution function of particles backstreaming from an arbitrary shock in a magnetized plasma (neglecting wave‐particle scattering) is presented. The theory explicitly shows that abrupt cutoffs in the parallel velocity, where ∂f/∂|v∥| > 0, arise naturally in the upstream region, irrespective of the acceleration processes active at the shock. These “cutoff” distributions may provide free energy for wave growth. Two classes of cutoff arise, escape cutoffs due to the particles requiring a characteristic speed to outrun the shock, and edge cutoffs due to the shock having an edge. Analytic expressions for the particle distribution function upstream of finite planar shocks and shocks whose two‐dimensional sections are parabolic are derived. A theory for the electron distribution function upstream from the earth's bow shock and the source of free energy for the observed Langmuir waves is presented. The electron distribution has an escape cutoff in the parallel velocity. This cutoff distribution is the source of free energy for Langmuir wave growth. Distribution functions and the spatial variation of the cutoff velocity are illustrated graphically. The theory is consistent with the spatial variation in the electron beam velocity inferred from indirect observations, assuming the observed electron beams to be the wave‐particle scattered remnants of cutoff distributions. An application of this theory to type II solar radio bursts is sketched.
On the correlation between asymptotic directions of cosmic ray particles and cutoff rigidities in the evolving geomagnetic field
Authors: Flückiger E. O. , Smart D. F. , Shea M. A. , Gentile L. C.
Published: February 2008

The concept of geomagnetic optics, as described by the asymptotic directions of approach, is extremely useful in the analysis of cosmic radiation data. However, when changes in cutoff rigidities occur as a result of evolution in the geomagnetic field, there are corresponding changes in the asymptotic cones of acceptance. It has been demonstrated that the effect of geomagnetic disturbances on asymptotic directions of approach can be estimated for nonpolar locations from the corresponding change in the cutoff rigidity. In this paper we show that the same procedure is applicable with respect to the secular evolution in the geomagnetic field, and that with this procedure it is possible to adjust reference published asymptotic directions to other epochs if the change in cutoff rigidity is known.
Daytime F layer trough observed on a macroscopic scale
Authors: Whalen J. A.
Published: February 2008

This paper reports a new technique of observing the high‐latitude ionospheric trough which defines its continuity in space and time on a macroscopic scale not previously possible. Vertical ionospheric soundings provide the observations in the midday to evening sector where the signature of the trough is found to be a “bite‐out” of a factor of 3–10 in the diurnal distribution of the solar produced electron density at the F layer maximum. The data base employed is that produced by the extensive array of ionospheric sounders operating at high latitudes in the northern hemisphere during the International Geophysical Year period of December 1958, hence at solar maximum. The worldwide extent, high density, simultaneity, and continuity of these measurements make possible the description of the trough for the first time as an integral, spatially continuous entity in its entire daytime extent and UT duration. The technique is demonstrated in studies of 4 days which span the range of activity for the month. On the most disturbed day the daytime trough is continuous in magnetic latitude, magnetic local time between (75°, 1030) and (51°, 1630) and remains nearly stationary in this frame of reference for a duration of 14.5 hours UT. A “snapshot” constructed from multiple simultaneous observations on this day shows the trough to have a continuous length of 6 × 10³ km and an area of >4 × 106 km². The trough, which is also stable for similar durations on the other 3 days, is found to rotate clockwise to earlier magnetic local time by 5.5 hours and to expand equatorward in magnetic latitude by 20° as activity increases from quiet to disturbed. Activity dependence and morphology are consistent with other observations that the trough locates rapid westward convection within the dusk convection cell. These properties of the trough appear on each of the 31 days studied but only in a sector which lies primarily in the eastern magnetic longitudes. Elsewhere, the daytime trough can occur principally in the morning to midday sector.
Observations of the Jovian UV aurora by Voyager
Authors: Herbert Floyd , Sandel B. R. , Broadfoot A. L.
Published: February 2008

Voyager ultraviolet spectrometer (UVS) observations of the Jovian aurora during 1979 are analyzed. From the relationship between observing geometry variations and corresponding fluctuations of detected intensity are derived constraints on variations in source intensity and on mean optical depth over the auroral arcs. The UVS observations are fit most accurately when the northern auroral oval is assumed to lie near the computed Io torus footprint or the locus found earlier by the UVS, with the latter position yielding better fits. Moreover, the derived intensity maxima consistently occur at the positions of the maximum value of −dB/dλIII (B is magnetic field strength, and λIII is System III longitude) rather than at minimum B (“windshield wiper” effect). These positions are also consistent with observations made by the International Ultraviolet Explorer satellite. If maximum auroral excitation occurs at longitudes where the mirror point altitude is decreasing with time as the precipitating particles drift in longitude (i.e., (dB/dλIII)(dλIII/dt) < 0), then these intensity maximum positions suggest that at least the particles exciting the spatially variable portion of the aurora are drifting west (dλIII/dt > 0). This implies either that these particles are electrons or that they are positive ions drifting east more slowly than the corotation lag of the Io torus region carries them to the west. The former possibility is difficult to justify on grounds of energetics, while the latter case requires ion energies less than or equal to 1 MeV (for a 5% corotation lag and a purely dipolar field). The latter case seems most consistent with high‐energy charged particle measurements, and the presence of the windshield wiper effect suggests that pitch angle scattering of the dominant aurora‐exciting ions is slower than the strong limit. The possibility of a local time variation in the auroral intensity profile was investigated, and no evidence for a local time dependence was found. This absence is expected on theoretical grounds but is opposite to the situation at earth. The model fits indicate a total UV emission (at depth) in H Ly α and the (λ < 1216 Å) H2 bands of about 3 to 8 × 1012 W during the Voyager 1 flyby and about 3.8 × 1012 W for the northern aurora alone during the departing leg of the Voyager 2 flyby.
Energetic particle absorption at Io
Authors: Wright Andrew N.
Published: February 2008

The absorption of test particles is studied analytically for the Jovian satellite Io. Io, being a conductor, perturbs the ambient E and B fields in and near the magnetic flux tube that is connected to it. The work is a development of some very early work by Schulz and Eviatar (1977). This paper extends their analysis not only by including more completely the effects of the dipole geometry of the background fields and by treating all particle pitch angles, but also by not being restricted to the infinite satellite conductivity limit. We also take into account the effect of the perturbation varying in amplitude as it propagates through the Io torus. We show that the total energy sufaces of test particles can be used to understand how the different interaction regimes derived by Schulz and Eviatar arise. The theory also enables estimates of the energy range for each type of interaction to be made and can be applied to interpret particle distributions from Io's L shell. As an example, the electron distribution near Io's flux tube has been examined and features expected from the theory are found to be present.
Numerical modeling of proton and electron bursts in a realistic magnetotail
Authors: Kutchko Frank J. , Armstrong Thomas P. , Yu Bernard
Published: February 2008

A numerical method to reconstruct and interpret magnetotail energetic ion (>0.3 MeV) and electron (>0.2 MeV) bursts is presented in this paper. Numerical calculation of particle trajectories for selected times in burst events are used to investigate possible sources and mechanisms of particle energization. Actual observations of burst events supplied the initial conditions for numerically calculated solutions of the Lorentz force equation. A three‐dimensional model of the magnetosphere produced by dipole, magnetopause, and magnetotail currents was used (D. B. Beard et al., 1982). Particle trajectories were computed both in the absence of an electric field, and with electric field introduced locally perpendicular to the magnetic field. Several noteworthy results were obtained from the simulation. While the motions of energetic particles near the nominal neutral sheet are complicated and obviously nonadiabatic, they can, nevertheless, generally be divided into two types, parallel and perpendicular. Proton and electron parallel trajectories had most of their trajectory motion parallel to the magnetic field lines and traveled long distances along field lines even to the near‐earth region. Perpendicular proton and electron trajectories traveled much shorter distances with much of their trajectory motion perpendicular to the magnetic field, with proton trajectories having significant interaction with the neutral sheet region. Both protons and electrons mirrored adiabatically at short distances above and below the neutral sheet region, while protons showed a nonadiabatic mirroring in the neutral sheet region. Another important result is that the preferential occurrence of earthward energetic proton burst anisotropies in the premidnight sector is explained by the fact that only the premidnight trajectories connect to reasonable source locations.