bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of three‐dimensional anisotropies of 35‐ to 1000‐keV protons associated with interplanetary shocks
Authors: Sanderson T. R. , Reinhard R. , van Nes P. , Wenzel K.‐P.
Published: February 2008

We present results of a detailed analysis of three‐dimensional anisotropies of protons in the energy range 35–1000 keV observed in association with interplanetary shocks on ISEE 3. We compare observations of high time resolution anisotropies made close to the shock in seven energy channels with theoretical predictions for Fermi acceleration and shock drift acceleration, and we find good evidence for both types of acceleration. We find a small number (six) of events exhibiting the signature of Fermi acceleration, and a somewhat larger number (20) exhibiting the signature of shock drift acceleration, the “Fermi” events being associated with strong, fast quasi‐parallel shocks and the “shock drift” events being associated with weaker, slower quasi‐perpendicular events. In the solar wind frame of reference the Fermi events have moderate upstream anisotropies, with flow away from the shock persisting for periods of one to two hours, the anisotropy decreasing with increasing energy, whereas downstream these events are isotropic. These events exhibit slow quasi‐exponential intensity increases of 1–2 orders of magnitude, peaking at the shock, and slowly decaying after the shock, often rising to a secondary peak some hours later. The shock drift events have large upstream first‐order anisotropies close to the shock, with flow away from the shock, and moderate downstream first‐order anisotropies, with flow toward the shock. The most notable feature of the shock drift events is a large negative second harmonic immediately downstream of the shock, signifying protons gyrating around the magnetic field at pitch angles of around 90°. These events have shock spike intensity increases lasting for a few minutes or tens of minutes. At all energies the largest intensity increases are observed with the Fermi events. Since the Fermi events are associated with the largest fluxes of solar flare protons, this may be due to a combination of solar particle background and protons accelerated in the vicinity of the shock.
Gyrating ions and large‐amplitude monochromatic MHD waves upstream of the Earth's bow shock
Authors: Thomsen M. F. , Gosling J. T. , Bame S. J. , Russell C. T.
Published: February 2008

Episodes of nearly monochromatic, low‐frequency (∼0.03 hz) hydromagnetic waves are occasionally observed upstream of the earth's bow shock. These waves have previously been associated with suprathermal ions of the “intermediate” type of distribution and have been attributed to the early stage of disruption of a field‐aligned ion beam through the electromagnetic ion beam instability. However, high time resolution (3 s) measurements of two‐dimensional ion distributions during two nearly monochromatic wave events reveal that the ion distributions associated with these waves are, in fact, “gyrating ions.” Such distributions consist of suprathermal ions with parallel and perpendicular velocities confined to a fairly narrow range of (nonzero) values. The ions are also often confined to a fairly narrow range of gyrophase angle (“gyrophase bunched”). In one of the two cases, the observed frequency of the waves agrees quite well with the Doppler shifted resonance frequency of waves in right‐hand resonance with the observed gyrating ions. In the second case, the observed frequency is lower than the predicted frequency by a factor of ∼1.5–2.
Large‐amplitude MHD waves upstream of the Jovian bow shock: Reinterpretation
Authors: Goldstein Melvyn L. , Wong Hung K. , Viñas Adolfo F. , Smith Charles W.
Published: February 2008

Observations of large‐amplitude MHD waves upstream of the Jovian bow shock have previously been interpreted as arising from a resonant electromagnetic ion beam instability (Goldstein et al., 1983). That interpretation was based on the conclusion that the observed fluctuations were predominantly right elliptically polarized in the solar wind rest frame. Because it has been noted by the authors that the fluctuations are, in fact, left elliptically polarized (Smith et al., 1983; Goldstein et al., 1983), a reanalysis of the observations is necessary. In this paper we investigate several mechanisms for producing left‐hand‐polarized MHD waves in the observed frequency range. Instabilities excited by protons appear unlikely to account for the observations. We conclude that a resonant instability excited by relativistic electrons streaming along the magnetic field away from Jupiter is a likely source of free energy consistent with the observations. Evidence for the existence of such a population of electrons has been found in both the low energy charged particle experiments and cosmic ray experiments on Voyager 2. This new interpretation is reminiscent of observations reported by Smith et al. (1976) using Pioneer data of left‐polarized (solar wind frame) MHD fluctuations in association with relativistic electrons.
Plasma conditions inside Io's orbit: Voyager measurements
Authors: Bagenal Fran
Published: February 2008

The Voyager 1 ion data that were obtained inside the orbit of Io allow accurate determination of convective velocity, temperature, and density of the major ionic species (S+, O+, S2+ and O2+ ions). The irregular radial profiles of ion temperature and flux tube content are not consistent with simple models of radial transport of plasma from a source near Io. The evidence of a source of ions well inside Io's orbit is provided by the detection of molecular (SO2+) ions at 5.3 RJ, the prevalence of non‐Maxwellian tails to the ion distribution functions, the persistent presence of oxygen ions throughout the inner torus, and a 1–3% lag behind corotation outside 5.4 RJ.
On the anticorrelation of the electric field and peak electron energy within an auroral arc
Authors: Mallinckrodt A. J. , Carlson C. W.
Published: February 2008

Data from the second flight of the Aries Porcupine sounding rocket campaign are presented and analyzed. The data reveal auroral electrons with spectral peaks at energies up to 4 keV and the dc electric field shows a remarkable degree of anticorrelation with the peak electron energy. Electron precipitation data are used as input to a numerical simulation of the auroral ionosphere to calculate ion production rates. From these and the recombination rates, the equilibrium electron density is calculated, properly accounting for the effective drift velocity of ionization from its point of production. Using the classical formulas, the Pedersen and Hall conductivities in the neutral frame are then calculated. Finally, various combinations of the arc motion velocity and neutral wind velocity are used to compare the resulting electric field component normal to the arc with the observations. The results imply that the ionization produced by the auroral electrons does not move with the E × B drift velocity, perhaps the result of a larger collisional coupling to neutrals than expected. A relationship is obtained between the components of the neutral wind velocity parallel and perpendicular to the arc. The observed anticorrelation is attributed to the blockage of field‐aligned current and the resulting polarization of the ionosphere as well as the relatively linear relationship between peak electron energy and height integrated conductivity for peak energies below a few keV.
A numerical simulation of auroral ionospheric electrodynamics
Authors: Mallinckrodt A. J.
Published: February 2008

A computer simulation of auroral ionospheric electrodynamics in the altitude range 80 to 250 km has been developed. The routine will either simulate typical electron precipitation profiles or accept observed data. Using a model background ionosphere, ion production rates are calculated from which equilibrium electron densities and the Hall and Pedersen conductivities may be determined. With the specification of suitable boundary conditions, the entire three‐dimensional current system and electric field may be calculated within the simulation region. The results of the application of the routine to a typical inverted‐V precipitation profile are demonstrated. In a companion paper (Mallinckrodt and Carlson, this issue), the routine is used to explore the observed anticorrelation between electric field magnitude and peak energy in the precipitating electron spectrum of an auroral arc.
Fast magnetospheric echoes of energetic electron beams
Authors: Wilhelm Klaus , Bernstein William , Kellogg Paul J. , Whalen Brian A.
Published: February 2008

Electron beam experiments using rocketborne instrumentation have confirmed earlier observations of fast magnetospheric echoes of artificially injected energetic electrons. A total of 234 echoes have been observed in a pitch angle range from 9° to 110° at energies of 1.87 and 3.90 keV. Out of this number, 95 echoes could unambiguously be identified with known accelerator operations at 2‐, 4‐, or 8‐keV energy and highest current levels resulting in the determination of transit times of typically 300 to 400 ms. In most cases, when echoes were present in both energy channels, the higher‐energy electrons led the lower‐energy ones by 50 to 70 ms. Adiabatic theory applied to these observations yields a reflection height of 3000 to 4000 km. An alternative interpretation is briefly examined, and its relative merit in describing the observations is evaluated. The injection process is discussed in some detail as the strong beam‐plasma interaction that occurred near the electron accelerator appears to be instrumental in generating the source of heated electrons required for successful echo detection for both processes.
Short‐wavelength electrostatic waves in the Earth's magnetosheath
Authors: Gallagher D. L.
Published: February 2008

Recent observations with the ISEE 1 spacecraft have found electric field emissions in the dayside magnetosheath whose frequency spectrum is modulated at twice the spacecraft spin period. The upper frequency cutoff in the frequency‐time spectrum of the emission has a characteristic parabola shape or “festoon” shape. The low‐frequency cutoff ranges from 100 to 400 Hz, while the high‐frequency limit ranges from about 1 to 4 kHz. The bandwidth is found to minimize for antenna orientations parallel to the wave vectors. The wave vector does not appear to be related to the local magnetic field, the plasma flow velocity, or the spacecraft‐sun directions. The spacecraft observed frequency spectrum results from the spacecraft antenna response to the Doppler‐shifted wave vector spectrum which exists in the plasma. Imposed constraints on the plasma rest frame wave vectors and frequencies indicate that emissions occur within the frequency range from about 150 Hz to 1 kHz, with wavelengths between about 40 and 600 m. These constraints strongly suggest that the festoon‐shaped emissions are ion‐acoustic waves. The small group velocity and k direction of the ion‐acoustic mode are consistent with wave generation upstream at the bow shock and convection downstream to locations within the outer dayside magnetosheath.
Escape of suprathermal O+ ions in the polar cap
Authors: Waite J. H. , Nagai T. , Johnson J. F. E. , Chappell C. R. , Burch J. L. , Killeen T. L. , Hays P. B. , Carignan G. R. , Peterson W. K. , Shelley E. G.
Published: February 2008

Instruments on board the Dynamics Explorer (DE) 1 and 2 spacecraft have been used to investigate the characteristics of a very low‐energy (less than 10 eV) outflow of O+ ions at high altitudes over the polar cap. The measured O+ outflow has a relatively high Mach number (2–6) and a large flux (∼2×108 cm−2 s−1). A statistical study using 50 orbits of retarding ion mass spectrometer (RIMS) data indicates that the outflows occur during active magnetic conditions, lasting for several hours over large areas of the polar cap. The observations are then discussed and analyzed in a framework based on polar wind models with particular attention paid to the new information obtained by the DE 2 Fabry‐Perot interferometer (FPI), and the impact these flows have on the composition of the magnetosphere. The observed suprathermal outflow of O+ suggests a scenario requiring both significant compositional changes in the high latitude thermosphere and significant heating of the ions and electrons in the topside ionosphere.
Magnetic fields on the sun and the north‐south component of transient variations of the interplanetary magnetic field at 1 Au
Authors: Tang F. , Akasofu S.‐I. , Smith E. , Tsurutani B.
Published: February 2008

In order to study the relationship between solar magnetic fields and the transient variations of the north‐south component, Bz, of the interplanetary magnetic field, IMF, at 1 AU, we collect (1) flares from unusual north‐south oriented active regions; (2) large IMF Bz events; (3) large flares with comprehensive flare index >12 and then investigate the associated IMF Bz changes or the magnetic field of the initiating flares, whichever the case may be. For those cases where an association between a transient IMF Bz variation and an initiating flare is plausible, we find that, for a given flare field, the orientation of the corresponding transient variation of Bz may be in agreement with the flare field, opposite to it, or, more often, fluctuating in both magnitude and direction. Conversely, an IMF Bz event may originate in a flare field in the same magnetic orientation, opposite to it, or in the east‐west orientation (i.e., having no component in the north‐south direction). Our studies show that a simple relationship between the orientation of the IMF Bz component and the magnetic orientation of the associated flare region is not apparent. This in turn suggests that the disturbed solar wind does not retain the apparent orientation of the photospheric flare field during its passage to a distance of 1 AU. The implication of this conclusion is discussed in terms of the “magnetic tongue” model of the magnetic field configuration in the disturbed solar wind.
Evidence for long period Alfvén waves in the inner solar system
Authors: Bruno R. , Bavassano B. , Villante U.
Published: February 2008

Magnetic field and plasma data of Helios 1 and 2 between 0.3 and 1 AU have been used to investigate the Alfvénic character of the solar wind fluctuations with periods above 1 hour. Clear evidence for the existence of very long period Alfvén waves, up to ∼15 hours (in the spacecraft frame) close to the sun, has been obtained. The observations at different heliocentric distances suggest that the longest wavelengths are removed from the Alfvénic regime leaving the sun.
Large‐ and small‐scale dynamics of the polar cusp
Authors: Sandholt P. E. , Egeland A. , Holtet J. A. , Lybekk B. , Svenes K. , Åsheim S. , Deehr C. S.
Published: February 2008

Photometric observations from two stations on Svalbard, Norway, have been used to map the location and dynamics of polar cusp auroras. Cases showing the behavior of cusp auroras and the local magnetic field related to changes in the interplanetary magnetic field (IMF) and irregularities in the solar wind plasma are presented. Dynamical phenomena with different time scales are studied. Southward and northward expansions of the midday sector of the auroral oval are discussed in relation to southward and northward turnings of the magnetosheath magnetic field and geomagnetic substorm activity. Examples showing no direct relationship to substorm activity are presented. Intensifications and rapid motions of discrete auroral structures in the cusp region are shown to be associated with local Pi‐type magnetic pulsations, each event lasting a few minutes. These small‐scale dynamical phenomena are discussed in relation to the concept of impulsive penetration of plasma irregularities across the dayside magnetopause, from the magnetosheath to the polar cusp region of the magnetosphere.
Changes in the microturbulence spectrum of the solar wind during high‐speed streams
Authors: Coles W. A. , Filice J. P.
Published: February 2008

Interplanetary scintillation (IPS) observations are used to investigate the distribution of microscale density fluctuations in high‐speed solar wind streams. A new technique for estimating the electron density power spectrum which can be applied when the solar wind is grossly inhomogeneous is presented. This technique requires accurate knowledge of the radio source structure, in situ measurements of the solar wind velocity, and well‐calibrated IPS observations. The estimation algorithm is applied to data obtained during 1973 and 1974 when the solar wind was dominated by recurrent fast streams. It is found that the spectrum has a power law exponent α ≈ 3.2 in the decreasing and low‐velocity regions of streams. The spectrum steepens and its level increases in the regions of enhanced density which precede such streams, and it remains steep into the high‐velocity regions. The average exponent in these regions is α ≈ 4.1. The average density spectrum obtained from this analysis is consistent with estimates of the spectrum obtained via other techniques.
Morphology of thermospheric composition variations in the quiet polar thermosphere from Dynamics Explorer measurements
Authors: Hedin A. E. , Carignan G. R.
Published: February 2008

Composition data obtained by the Neutral Atmosphere Composition Spectrometer carried aboard the Dynamics Explorer 2 satellite have been examined for variations in the polar regions during magnetically quiet conditions. A persistent enhancement of N2 density and a depletion of He density are present in the vicinity of the magnetic poles with maximum density response in the morning hours (magnetic) on average. A universal time (UT) variation in average density levels is evident near both the geographic and magnetic poles. There are systematic morphology changes with UT and between summer and winter which are qualitatively consistent with the shifting and spreading of thermospheric heating effects by the global circulation systems driven by EUV and magnetospheric sources. The magnitude of the UT variations is larger in the southern hemisphere on average, and the UT variations are larger in local winter than summer.
The poleward leap of the auroral electrojet as seen in auroral images
Authors: Hones Edward W.
Published: February 2008

This paper discusses a recently published series of 12 successive images of the northern auroral oval obtained by the University of Iowa's auroral imager on DE 1 during a substorm on November 8, 1981. The images showed a sudden (within <12 min) thickening and poleward expansion of the midnight sector of the oval that occurred at the “maximum epoch” of the substorm, i.e., as auroral zone bays reached their peaks and began to subside about 90 min after the expansive phase onset. We propose that this was the auroral manifestation of the poleward leap of the auroral electrojet that has been reported on many occasions to coincide with thickening of the plasma sheet in the outer magnetotail (beyond ∼15 RE) late in a substorm. The plasma sheet thickening is thought to be associated with a tailward retreat of the substorm magnetic neutral line. We note evidence that the neutral line's tailward retreat influences auroral currents and particle precipitation even at low auroral latitudes. Modifications of the present auroral substorm model are suggested.
ISEE particle observations of surface waves at the magnetopause boundary layer
Authors: Couzens David , Parks G. K. , Anderson K. A. , Lin R. P. , Reme H.
Published: February 2008

The dual‐spacecraft ISEE mission provides a uique opportunity to study the motions of the magnetopause and adjacent boundary layer. By comparing high‐time‐resolution energetic particle data from ISEE 1 to those of ISEE 2, the velocity and orientation of the inner boundary of the boundary layer can be determined. Two cases are presented. In one, tailward propagating sinusoidally shaped surface waves with a wavelength in excess of 42,000 km and an amplitude of ∼5000 km are found. In the other, surface waves are indicated with a wavelength of ∼40,000 km and an amplitude of ∼11,000 km having steepened nonsinusoidal shapes. The existence of such large‐amplitude waves suggests that the particle dynamics near the magnetopause support nonlinear processes.
Quiet ionospheric currents of the northern hemisphere derived from geomagnetic field records
Authors: Campbell Wallace H. , Schiffmacher Edward R.
Published: February 2008

A special selection of extremely quiet 1965 geomagnetic daily variation records from North America, Europe, central Asia, and east Asia was used to determine monthly spherical harmonic analysis coefficients representing the Sq fields from these regions. The coefficients permitted a separation of the external and internal contributions to the daily variations. Month‐by‐month behavior of the equivalent external current system and contours of the daily range of this current showed regional differences in amplitude and location of the equivalent ionospheric currents with seasonal change. Current foci locations reached 35° to 42° geomagnetic latitude in summer and 18° to 35° in winter with a regional dependence. The largest daily range of external current was 14.6×104 A in North America, 15.3×104 A in Europe, 13.7×104 A in central Asia, and 12.3×104 A in east Asia. Continental differences in the behavior of the current seemed to be explained by the relative position of the geographic and geomagnetic poles with respect to these regions.
Spectral classification of medium‐scale high‐latitude F region plasma density irregularities
Authors: Singh M. , Rodriguez P. , Szuszczewicz E. P.
Published: February 2008

Plasma diagnostics aboard the S3‐4 satellite have provided direct measurements of high‐latitude F region ionospheric density irregularities in the wavelength domain of tens of meters to hundreds of kilometers. The irregularities are found to be ubiquitous, generally populating the auroral oval, the polar cap, and the dayside oval/polar cusp area with varying degrees of intensity and scale size distributions. To investigate the causal mechanisms for various irregularity distributions, including the primary input term from precipitating particles and possible plasma instability processes, we have spectrally analyzed 20 orbits of data during geomagnetically quiet and moderately disturbed conditions (Kp ≤ 3). In the lower F region (Z ∼ 200 km), where molecular ions dominate, power spectra of medium‐scale plasma density structures (1–50 km) can be characterized by a weak most probable spectral index of 2.8 for the nightside‐dayside ovals and the polar cap and an index of 1.4 for the dusk oval. At higher altitudes (Z ≥ 260 km), where O+ is the dominant ion, the analyses show a well‐defined most probable spectral index of 1.9 for all the high‐latitude regions. Our spectral analyses suggest that at the molecular‐ion‐dominant altitudes, fast chemical recombination inhibits plasma convection/diffusion and that the observed irregularities are primarily the consequence of locally structured particle precipitation patterns. On the other hand, in the O+‐dominant F region, particle‐produced irregularities are long lived. Such irregularities convect into and across the polar cap and can be a long‐lived input term for a number of plasma instability processes. As a result, the molecular‐dominant polar cap F region (Z ≤ 200 km) is generally free of plasma disturbances, while its atomic‐dominant counterpart (Z ≥ 260 km) is mostly structured with a broad range of intense plasma density irregularities.
Coulomb collisions in the solar wind
Authors: Klein L. W. , Ogilvie K. W. , Burlaga L. F.
Published: February 2008

The ion composition experiment on ISEE 3 obtains the velocity distributions for 4He++, completely separated from protons. Temperatures and velocities derived from these distributions have been compared with the Los Alamos National Laboratory proton temperatures and velocities obtained from the National Space Science Data Center. The results, which apply on the macroscale, confirm and extend previous observations and show the following: (1) The temperature ratio Tα/Tp drops with increasing transit time R/V This indicates that interactions among the ions take place, but it does not identify the collision mechanism. (2) At low τe/τc (the ratio solar wind expansion time/collision time) the temperatures of the individual species are proportional to their mass. As τe/τc increases, the He temperature drops, and the H temperature increases to a common intermediate value determined by equipartition of energy. (3) Tα/Tp is approximately proportional to (mpTp + mαTα)/(mα + mp), as suggested for Coulomb collisions. The temperature ratio is more sensitive to the prevailing value of T than to that of the density N. Thus the scattering depends more strongly on the thermal speed than on the interparticle separation. (4) The state of ion equilibrium is determined by the value of τe/τc, independent of the type of flow. (5) As τe/τc increases from 0.01 to unity and above, the temperature ratio first remains constant near 4, then drops uniformly to unity, and remains constant for τe/τc ≳ 1. The relative velocity difference |Vα ‐ Vp|/VA decreases monotonically to zero as a function of τe/τs, where τs is the slowing down tune due to collisions. This evidence establishes Coulomb collisions as the important ion‐ion equilibrating process in the solar wind when the temperature is low.
Observations of molecular ions in the Earth's magnetosphere
Authors: Craven P. D. , Olsen R. C. , Chappell C. R. , Kakani L.
Published: February 2008

The retarding ion mass spectrometer on Dynamics Explorer 1 operating over the polar cap during a large magnetic storm has measured fluxes of up to 106 ions cm−2 s−1 of the molecular ions N2+, NO+, and O2+. These ions were measured beginning low in the satellite orbit (1.1 RE) and extending to about 3 RE geocentric altitude. Near perigee, the ions have a rammed distribution indicating a cold Maxwellian plasma (1000°–2000°K). The molecular ions gradually shift to a field‐aligned distribution at the higher altitudes. An upward flow of 5–10 km/s is found in these field‐aligned measurements. The density of the molecular ions is on the order of 2 cm−3 at all altitudes, and the energy of the ions generally increases as the satellite moves sunward across the southern polar cap. Kinetic energies of at least 20 eV were found at 2.5 RE geocentric distance.
Flow of artificial plasma in a simulated magnetosphere: Evidence of direct interplanetary magnetic field control of the magnetosphere
Authors: Minami Shigeyuki , Takeya Yoshio
Published: February 2008

Using laboratory simulation experiments, we investigate plasma flow in the vicinity of the model magnetosphere, especially in the tail region. Luminous thermal plasma is injected by means of a newly designed plasma emitter called the powered double probe (PDP). Time exposure photographs show luminosity along the magnetic field lines which cross the position of the PDP. The results obtained from different positions of the PDP show four regions of illumination. The dependence of the illumination width of such artificial plasma on the PDP positions yields information on energetic particles and their diamagnetic effect. Artificial plasma injection illuminates trajectories of energetic particles produced in the nightside magnetosphere. In particular, when the PDP is located in the tail at a geocentric distance corresponding to about 15 to 18 RE, an especially bright luminous line was found to form. It is suggested that this phenomenon results from plasma intrusion into the magnetosphere through the dayside cusp region. In order to examine the effect of the intrusion of the interplanetary magnetic field (IMF) into the magnetosphere, an external quasi‐static magnetic field is applied that simulates the IMF. The result reveals the structural form of magnetic field lines which indicate characteristics of a stable “open” magnetosphere. In the presence of southward IMF, a neutral line was formed at about the location where the linear sum of the external vaccum magnetic field (IMF) and the distorted dipole field BM (without BZ) becomes null. This result seems to imply a direct IMF control of energy coupling between the solar wind and the magnetosphere.
Fundamental and harmonic electron cyclotron maser emission
Authors: Winglee R. M.
Published: February 2008

Auroral kilometric radiation has been attributed to an electron cyclotron maser instability and is known to have components at both the cyclotron frequency and its harmonics. Solar microwave spike bursts and similar bursts from flare stars are also thought to be maser emission at harmonics of the cyclotron frequency. Fundamental X mode growth is known to dominate when ωp/Ωe ≲ 0.3. At higher ωp/Ωe the dominant radiation from the maser depends on the prevailing plasma conditions. In particular, it is shown that the dominant radiation can be (1) fundamental Z or O mode radiation or second‐harmonic X mode radiation when 0.3 ≲ ωp/Ωe ≲ 1, (2) fundamental Z mode or second‐harmonic X mode radiation when
, (3) second‐harmonic O mode or third‐harmonic X mode radiation when
or (4) second‐harmonic Z mode radiation when
. In these ranges of ωp/Ωe X mode growth at the relevant harmonic tends to be suppressed if the distribution of the energetic electrons only has a loss cone anisotropy, whereas it tends to dominate if the distribution is peaked at large pitch angles.
CO2 component of daytime Earth limb emission at 2.7 micrometers
Authors: Sharma Ramesh D. , Wintersteiner Peter P.
Published: February 2008

A detailed study of the vibrational states of CO2 in the upper atmosphere was conducted in order to enable the calculation of emission near 2.7 µm expected for sunlit conditions. New line‐by‐line computer codes were used to evaluate the solar pumping of vibrational levels and to calculate the limb radiance. Excitation and quenching mechanisms are quantified and compared for several important vibrational states in the altitude range 40–110 km. The dominance of fluorescent CO2 hot band emission for altitudes below 75 km is confirmed. The spectral distribution, and to a lesser degree the total band radiance, are shown to be very sensitive to collisional interactions. Detailed results are presented and compared with experiment.
The global distribution of nitric oxide in the thermosphere as determined by the Atmosphere Explorer D Satellite
Authors: Cravens T. E. , Gérard J.‐C. , LeCompte M. , Stewart A. I. , Rusch D. W.
Published: February 2008

The ultraviolet nitric oxide spectrometer (UVNO) experiment on the Atmosphere Explorer D (AE‐D) satellite measured thermospheric nitric oxide during the winter of 1974–1975 using resonant fluorescence from the 1–0 gamma band of the molecule. Almost complete latitude coverage was obtained, but the observations were confined to morning local times close to 0900. The 1–0 gamma band intensity profiles measured by the instrument were inverted to provide vertical profiles of the NO number density between about 90 and 200 km. Typically, the measured NO concentrations reached a maximum between altitudes of 100 and 110 km, and more NO was observed at higher latitudes than at low latitudes, in agreement with previous observational studies. The shape of the NO profile was also found to be a function of latitude, with a plateau appearing in the profile near 130 km for low latitudes and mid‐latitudes in the winter hemisphere.
Evolution and interaction of interplanetary shocks
Authors: Whang Y. C. , Burlaga L. F.
Published: February 2008

The paper presents a computer simulation for the evolution and interaction of shocks resulting from large interplanetary streams based on multispacecraft observations and an unsteady, one‐dimensional, MHD model. We studied two events, each observed by two or more spacecraft separated by a distance of the order of 10 AU and consisting of a sequence of shocks in the outer heliosphere. The first simulation is based on the plasma and magnetic field observations made by radially aligned spacecraft: first by IMP 7 and 8 in November 1977 at 1 AU, and later by Pioneer 10 in January 1978 at 15 AU. The second simulation is based on an event observed first by Helios 1 in May 1980 near 0.6 AU and later by Voyager 1 in June 1980 at 8.1 AU. These simulations can explain a complex history for each sequence of the shocks observed in the outer heliosphere. They correctly identify the nature of the shock, facing forward or reverse. The calculated shock arrival time and the observed shock arrival time are within 10% accuracy of the total shock traveling time over a distance of the order of 10 AU. The examples show that the shock process, including the formation, collision and merging of shocks, dominates the dynamical evolution of large‐scale solar wind structures. In the outer heliosphere, the large‐scale solar wind and magnetic field evolve into a much simpler structure, and MHD shocks are present as a principal component of the solar wind. The simulation results shed new light on interpretation for the interaction and evolution of large interplanetary streams. Observations were made only along a few limited trajectories, but simulation results can supplement these by providing the detailed evolution process for large‐scale solar wind structures in the vast region not directly observed. The use of a quantitative nonlinear simulation model including the shock merging process is crucial in the interpretation of data obtained in the outer heliosphere.
Low‐frequency waves and associated energetic ions downstream of Saturn
Authors: Behannon K. W. , Goldstein M. L. , Lepping R. P. , Wong H. K. , Mauk B. H. , Krimigis S. M.
Published: February 2008

Coherent, low‐frequency waves were observed occasionally in the interplanetary magnetic field (IMF) by Voyager 2 during 5 months in 1981–1982 following its encounter with Saturn. These waves had periods ranging from 1.4 to 2.8 hours in the spacecraft reference frame (frequencies near 10−4 Hz) and were detected at distances ranging from 150 to 2800 RS (9.0×106 to 1.7×108 km) from Saturn. The waves were characterized by large‐amplitude fluctuations in both magnitude and direction of B, an ellipticity >0.5 on average, and in most cases have been interpreted to have right‐hand polarization in the plasma frame. Although there was at best only a weak signature of these waves in the low‐energy (<6 keV) plasma, large enhancements were observed in the count rate of energetic (28–80 keV) ions in association with wave occurrences out to a distance of ∼1300 RS (7.8×107 km) from Saturn. The distribution of the enhanced ions was highly anisotropic, with the highest count rates measured in the 45° sector most nearly facing Saturn. The observation of ion enhancements was found to depend on the latitude angle of the IMF. It is concluded that the waves were excited by energetic plasma ions propagating away from Saturn. They are similar to waves produced upstream of earth by ions reflected from the bow shock or escaping from the magnetosphere. Because of the unique combination of IMF orientation at Saturn's orbit and the Voyager 2 postencounter trajectory, these observations represent the most remote detection of such planet‐associated (foreshock) waves. It is shown that viable mechanisms for exciting such waves under the given conditions include two beam‐driven resonant cyclotron instabilities as well as the nonresonant firehose instability. The absence of any strong signature of these waves in the thermal plasma suggests that the magnetic field fluctuations may arise from the firehose instability.
Electromagnetic electron beam instabilities: Hot, isotropic beams
Authors: Gary S. Peter
Published: February 2008

This paper considers the linear theory of electromagnetic instabilities driven by an electron beam in a homogeneous, nonrelativistic, Vlasov plasma. The beam is relatively hot, isotropic in its own frame, and streams parallel or antiparallel to a magnetic field B. Numerical solutions of the full dispersion equation for propagation parallel or antiparallel to B are presented, and the linear properties of the whistler heat flux and electron beam firehose instabilities are exhibited and compared. Under a broad range of parameters the former mode has the lower beam speed threshold, and the larger maximum growth rate. In addition, it is demonstrated that, for a sufficiently large relative beam density, relative beam temperature, and plasma beta the whistler heat flux instability has a much lower beam speed threshold than the electrostatic electron beam instability. The application of these instabilities to first‐order Fermi acceleration of electrons at space plasma shocks is discussed.
An MHD simulation of By‐dependent magnetospheric convection and field‐aligned currents during northward IMF
Authors: Ogino T. , Walker R. J. , Ashour‐Abdalla M. , Dawson J. M.
Published: February 2008

We have used a high‐resolution and time‐dependent three‐dimensional magnetohydrodynamic (MHD) model to study the interaction between the solar wind and the earth's magnetosphere during intervals when the interplanetary magnetic field (IMF) has both a By component and northward Bz component. When the IMF is northward, the plasma sheet thickens near the noon‐midnight meridian and extends projections into the northern and southern lobes. When projected onto the polar cap, this appears as a narrow channel extending from midnight toward noon. This plasma sheet extension from the nightside toward the dayside shifts toward dawn for By < 0 and toward dusk for By > 0. Two types of upward field‐aligned currents were found: an arc‐shaped current which extends around the auroral zone and a tail lobe region 1 current running almost parallel to the sun‐earth line. Like the plasma sheet extension the sun‐aligned tail lobe region 1 current moves across the northern polar cap from dusk to dawn when the IMF orientation changes from duskward to dawnward during northward IMF. At latitudes north of the region 1 current in the polar cap the currents have the same direction as the region 2 currents (i.e., upward at dawn and downward at dusk). As the IMF orientation changes from northward to dawnward, the currents rotate such that the upward current joins with the upward region 1 current, while the earthward current expands in the polar region. These magnetospheric features can be well explained by high‐latitude merging cells and tail lobe convection cells resulting from antiparallel merging between the northward IMF and the tail field. They also are consistent with several phenomena of theta aurora dynamics.
Parabolic harmonics in magnetospheric modeling: The main dipole and the ring current
Authors: Stern David P.
Published: February 2008

A new representation is developed for that portion of the magnetospheric magnetic field which is produced by currents on the magnetopause or outside it. This field is represented by a scalar potential γ, expanded in parabolic harmonics: it may be obtained, in first approximation, by regarding the solar wind as a perfect conductor which excludes all fields of magnetospheric sources (Chapman‐Ferraro boundary conditions). For a magnetopause which is a paraboloid of rotation and which confines all field lines of magnetospheric sources, the fields due to magnetopause currents are derived for two sources, a dipole and a two‐parameter model of the ring current, each arbitrarily inclined to the earth‐sun line. Methods are also developed for accomodating changes in the size of the magnetospheric cavity and for using nonparaboloidal boundaries, open at the nightside. Neutral points and polar cusps are derived, and in agreement with observations it is found that a growth of the ring current shifts them equatorward. By extrapolation from these results, it is argued that the observation of such shifts during magnetic storms, together with Dst, may help resolve the relative importance of two processes proposed for magnetic storms.
A simulation study of forced reconnection processes and magnetospheric storms and substorms
Authors: Lee L. C. , Fu Z. F. , Akasofu S.‐I.
Published: February 2008

We simulate, on the basis of a two‐dimensional incompressible MHD code, the plasma dynamics of a long magnetotail (∼200 RE) under a constant and time‐varying driving force delivered from the solar wind. It is found that under a constant driving force the magnetic fields in the magnetotail tend to reconnect impulsively and the formation of X line and plasmoids occurs intermittently and repeatedly every 2–4 hours. Under a time‐varying driving force the post‐driven phase of a plasmoid formation and the spontaneous reconnection process are also studied. Our results show that magnetic reconnection in the magnetotail during magnetospheric substorms and storms is basically a driven process. A complete sequence of the event, going from the gradual pile‐up of magnetic flux to the formation and the antiearthward convection of the new X line and to the ejection of plasmoids from the antiearthward end of the magnetotail, requires the presence of the driving force. The simulation results may also be useful in unifying the different interpretations (1) of the plasmoid formation and plasma sheet thinning and (2) of the relative importance of magnetotail processes with respect to the direct solar wind effects during substorms.
X ray microbursts and VLF chorus
Authors: Roeder J. L. , Benbrook J. R. , Bering E. A. , Sheldon W. R.
Published: February 2008

On January 4, 1978, at 1140 UT, a SuperArcas sounding rocket was launched from Siple Station, Antarctica (L = 4.2, 76°S, 84°W), during a geomagnetically disturbed period (Kp = 6−) with intense X ray and VLF chorus activity. The parachuted payload observed an intense microburst precipitation event of 10‐minute duration. These data have been correlated with measurements of VLF chorus by receivers on the ground at both Siple and its magnetic conjugate point, Roberval, Quebec. Detailed one‐to‐one correspondence between the microbursts and the chorus was not a consistent feature of the data. Time series analysis of the data did indicate a significant correlation between the Siple X ray precipitation and the Roberval VLF waves with an arrival time delay of 0.1±0.3 seconds.
Ion partitioning in the hot Io torus: The influence of S2 outgassing
Authors: Moreno Miguel A. , Newman William I. , Kivelson Margaret G.
Published: February 2008

Calculations of ion partitioning in the hot outer Io torus have failed to account for certain observed features. Notably, the low concentration of OIII measured in the first half of 1981 has been difficult to reconcile with abundances of other ions inferred from Voyager 1 and 2 measurements in 1979. One possible explanation invokes a two‐state plasma torus and suggests time‐dependent changes of bulk composition. In this paper we propose an alternative time‐independent model (time‐independent in a mean‐value sense) but introduce the ratio of sulfur to oxygen in the neutral source as a free parameter. The recent evidence that there exist distinct sources of S2 and SO2 justifies this introduction of independent sources of S and O. In our calculations, we also varied the relative abundances of hot (1 keV) and thermal electrons, the neutral injection rate and characteristic radial diffusion time scale, and the characteristic ion temperatures. We integrated the rate equations over time until a steady state was achieved. For a pure SO2 source, we found no solutions qualitatively compatible with observations. A solution in qualitative agreement with observations emerges when the injection rate of neutral S2 is about 60% the rate of SO2. The deficiency of OIII observed in 1981 is obtained in a model consistent with other aspects of composition and dynamical features of the torus observed in 1979 at the time of the Voyager encounter by including of order 0.05% of 1 keV electrons and using a radial diffusion time of order 35 days. The intensity of EUV spectral lines attributed to OII and OIII can be produced by our sulfur‐rich torus model if collision strengths of lines near 833 Å are taken near the upper limit of their range of uncertainty.
Chemistry of vibrationally excited ozone in the upper atmosphere
Authors: Rawlins Wilson T.
Published: February 2008

Recent atmospheric and laboratory experiments have shown that significant nonthermal vibrational excitation of O3 can occur near the mesopause. These results can be interpreted in terms of a steady state model in which O3(υ) is formed by radiative and chemical excitation and is destroyed by radiative and collisional deactivation. The only atmospheric chemical source of O3(υ) which can be clearly identified is the three‐body recombination of O with O2. Little is known about the detailed, level‐dependent kinetics of O3(υ), and several key assumptions are required for kinetic interpretation of the data. Predicted vibrational distributions, infrared band radiances, and diurnal effects are compared to in situ data in various assumed limits, and critical processes are discussed. Possible auroral sources of O3(υ) are also examined.
Observations of E region irregularities generated at auroral latitudes by a high‐power radio wave
Authors: Djuth F. T. , Jost R. J. , Noble S. T. , Gordon W. E. , Stubbe P. , Kopka H. , Nielsen E. , Boström R. , Derblom H. , Hedberg Å. , Thidé B.
Published: February 2008

In September 1983 a series of ionospheric modification experiments was performed in Scandinavia using the high‐frequency (HF) heating facility located near Tromsø, Norway. The experiments were designed to study the production of geomagnetic field‐aligned irregularities in the auroral E region by a powerful HF radio wave. In this initial report, observations of 3.2‐m irregularities made with a mobile 46.9‐MHz field radar are presented. When ionospheric conditions are correct, irregularities having peak cross sections of about 104 m² are excited in the E region over Tromsø. The present results are consistent with theoretical studies which indicate that it is easier to generate short‐scale field‐aligned irregularities at locations where the geomagnetic dip angle is large. When the HF‐induced radar echo at 46.9 MHz is strong, the e‐folding growth times of the echo power are typically between 50 and 150 ms. However, when the E region echo over Tromsø is weak, the growth period can range from seconds to tens of seconds. The irregularities generated by the HF wave often exhibit dynamic structure both in time and in space. In general, the observations provide experimental support for instability theories that employ interactions near the upper hybrid resonance to drive the artificial irregularities. The results also indicate that naturally occurring irregularities may at times play a key role in the excitation of HF‐induced irregularities in the auroral E region. Additional processes are probably required to explain the slower irregularity growth rates often observed when radar echoes are relatively weak and occasionally observed even for relatively strong signals.