bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence for solar wind control of saturn radio emission
Authors: Desch Michael D.
Published: February 2008

Using data collected by the Voyager 1 and 2 spacecraft in 1980 and 1981, strong evidence is presented for a direct correlation between variations in the solar wind at Saturn and the level of activity of Saturn's nonthermal radio emission. Correlation coefficients of 57–58% are reached at lag times of 0–1 days between the arrival at Saturn of high pressure solar wind streams and the onset of increased radio emission. During both 160‐day analysis intervals studied, the radio emission exhibits a long‐term periodicity of 25 days, identical to the periodicity seen in the solar wind at this time and consistent with the solar rotation period. The energy coupling efficiency between the solar wind and the Saturn radio emission is estimated and compared with that for the earth.
Coupled hydromagnetic wave excitation and ion acceleration upstream of the Earth's bow shock
Authors: Lee Martin A.
Published: February 2008

A self‐consistent theory is presented for the excitation of hydromagnetic waves and the acceleration of ‘diffuse’ ions upstream of the earth's bow shock in the quasi‐equilibrium that results when the solar wind velocity and the interplanetary magnetic field are nearly parallel. For the waves the quasi‐equilibrium results from a balance between excitation by the ions, which stream relative to the solar wind plasma, and convective loss to the magnetosheath. For the diffuse ions the quasi‐equilibrium results from a balance between injection at the shock front, confinement to the foreshock by pitch angle scattering on the waves, acceleration by compression at the shock front, loss to the magnetosheath, loss due to escape upstream of the foreshock, and loss via diffusion perpendicular to the average magnetic field onto field lines that do not connect to the shock front. Diffusion equations describing the ion transport and wave kinetic equations describing the hydromagnetic wave transport are solved self‐consistently to yield analytical expressions for the differential wave intensity spectrum as a function of frequency and distance from the bow shock z and for the ion omnidirectional distribution functions and anisotropies as functions of energy and z. In quantitative agreement with observations, the theory predicts (1) exponential ion spectra at the bow shock in energy per charge, (2) a decrease in intensity and hardening of the ion spectra with increasing z, (3) a 30‐keV proton anisotropy parallel to z increasing from −0.28 at the bow shock to +0.51 as z → ∞ (4) a linearly polarized wave intensity spectrum with a minimum at ∼6 × 10−3 Hz and a maximum at ∼2–3 × 10−2 Hz, (5) a decrease in the wave intensity spectrum with increasing z, (6) a total energy density in protons with energies >15 keV about eight times that in the hydromagnetic waves.
Latitudinal reversal of polarization of the geomagnetic sudden commencement
Authors: Araki Tohru , Allen Joe H.
Published: February 2008

Latitudinal variation of polarization of geomagnetic sudden commencements (sc) observed at geomagnetic latitude range 63°N–79°N is studied by the use of North American IMS magnetometer data from August 1978 to May 1979. Fourteen sc's out of 18 examined show clear polarization reversal between latitudes 64°N and 72°N. Among these, nine change their polarization direction from counterclockwise to clockwise with increasing latitude and the remaining 5 change it in the opposite sense. Clear local time dependence as shown statistically by Wilson and Sugiura (1961) could not be confirmed from this small data set. Their statistical results (Wilson and Sugiura, 1963), however, can be more reasonably interpreted by taking into account the existence of latitudinal polarization reversal described in this paper.
The dynamic response of the thermosphere to the energy influx resulting from energetic O+ ions
Authors: Torr Marsha R. , Torr D. G. , Roble R. G. , Ridley E. C.
Published: February 2008

Precipitating fluxes of energetic O+ ions during magnetic storms constitute a significant source of energy to the nightside thermosphere at altitudes above 200 km. The resulting perturbations of temperature and winds in the thermosphere due to energetic O+ precipitation are examined by computing the heating rate profile from such a precipitation event and using this as input to the NCAR thermospheric general circulation model. The results of a time dependent calculation show that heating due to energetic O+ ion precipitation produces a significant perturbation to the thermospheric structure above about 300 km. Energetic O+ precipitation events should produce considerable variability in the upper thermosphere during geomagnetic storms.
July 29, 1977, magnetospheric studies: Impulsive waves, global dynamics and geomagnetic indices
Authors: Kivelson Margaret G.
Published: February 2008

This paper concludes the initial series of reports on the CDAW (Coordinated Data Analysis Workshop) study of the events of July 29, 1977. The primary purpose of this presentation is to point out interrelations among other papers in the series and to show how they increase our knowledge of how the magnetosphere operates. The day in question included intervals of intense geomagnetic activity, with an initial impulsive response to an interplanetary shock followed by numerous substorms. The latter half of the day was of special interest because the activity was limited to high latitudes in the northern polar cap, where it was substantial. Problems of magnetospheric physics addressed with the CDAW study are herein grouped into several general areas. The immediate response of the magnetosphere to changes in the solar wind (shock or other) was considered in studies of impulsive waves, which were traced through the magnetosphere to the ground and interpreted in terms of fast‐mode wave propagation. The geometry and microstructure of the magnetopause, following the arrival of an interplanetary shock, were analyzed with data from near‐geostationary satellites. The global magnetic geometry was modeled for the entire day. Plasma sheet convection was monitored at low and high altitudes. A model of particle convection during the first quarter of the day gave important evidence that the convection electric field penetrated to small radial distances for extended intervals. A large substorm at midday was unusually well documented with data from geostationary spacecraft and ground observatories. In reviewing those data this paper addresses the question of control of the local time of substorm onset. A model is presented that relates the local time of substorm onset to the sector of the tail earliest stressed following the onset of dayside reconnection. The late‐day data provided evidence of lobe magnetic field reconnection when the solar wind magnetic field was pointing north. This paper marshals the evidence for different reconnection patterns and introduces an alternative pattern not previously considered. Remarks on the way in which the day's observations bear on several indices of geomagnetic activity end the summary.
A model of the high‐latitude ionospheric convection pattern
Authors: Heelis R. A. , Lowell J. K. , Spiro R. W.
Published: February 2008

Mathematical expressions have been constructed that allow the large‐scale global convection characteristics of the high‐latitude ionosphere to be reproduced. The model contains no discontinuities in the ion convection velocity and as such should be useful in F region chemical models. The number of variables in the model allow such features as the dayside throat and the Harang discontinuity to be modeled. The applicability of the model to magnetospheric physics is limited by the exclusion of large‐magnitude small‐scale flow features associated with discrete arcs and by the inability of the model to produce separate flow cells at the same local time.
Nonlocal stability analysis of the MHD Kelvin‐Helmholtz instability in a compressible plasma
Authors: Miura Akira , Pritchett P. L.
Published: February 2008

A general stability analysis is performed for the Kelvin‐Helmholtz instability in sheared magnetohydrodynamic flow of finite thickness in a compressible plasma. The analysis allows for arbitrary orientation of the magnetic field B0, velocity flow v0, and wave vector k in the plane perpendicular to the velocity gradient, and no restrictions are imposed on the sound or Alfvén Mach numbers. The stability problem is reduced to the solution of a single second‐order differential equation, which includes a gravitational term to represent coupling between the Kelvin‐Helmholtz mode and the interchange mode. In the incompressible limit it is shown that the Kelvin‐Helmholtz mode is completely stabilized for any velocity profile as long as the condition
is satisfied, where V0 is the total velocity jump across the shear layer. Numerical results are obtained for a hyperbolic tangent velocity profile for the transverse (B0 ⊥ v0) and parallel (B0∥v0) flow configurations. Only modes with kΔ < 2 are unstable, where Δ is the scale length of the shear layer. The fastest growing modes occur for kΔ ∼ 0.5‐1.0. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects. For the transverse case, only the fast magnetosonic mode is destabilized, but if k · B0 ≠ 0, the instability contains Alfvén‐mode and slow‐mode components as well. The Alfvén component gives rise to a field‐aligned current inside the shear layer. In the parallel case, both Alfvén and slow magnetosonic components are present, with the Alfvén mode confined inside the shear layer. The results of the analysis are used to discuss the stability of sheared plasma flow at the magnetopause boundary and in the solar wind. At the magnetopause boundary, the fastest growing Kelvin‐Helmholtz mode has a frequency of 0 (V0/2Δ), which overlaps with the frequency range of geomagnetic pulsations (Pc 3‐5). It is suggested that the MHD Kelvin‐Helmholtz instability could serve as a dynamo process driving small‐scale field‐aligned currents in the presence of the sheared plasma flow in the magnetosphere.
The narrow‐channel approximation for magnetospheric flows
Authors: Atkinson G.
Published: February 2008

The flux tube and electric current formulations of magnetospheric convection are developed in parallel. The magnetosphere is modeled as consisting of three types of flux tubes: polar cap, auroral oval (plasma‐rich flux tubes), and lower latitude. In each region the velocity can be expressed as the gradient of a potential which obeys the LaPlace equation. Boundary conditions between the three regions are that normal velocity and magnetic plus plasma pressure be continuous. The oval is much longer than it is wide (latitudinal extent) and approximately east‐west oriented. This allows a very useful approximation, the narrow‐channel approximation, to be used which is readily solvable and can be used to obtain flows in all three regions. The validity of this approximation is enhanced by the high conductivity of the oval. Application of this approximation to the oval results in the velocity along the oval being the gradient of a potential which is a function of the location of the oval boundaries. Thus flow in the oval can be solved from its boundary location or vice versa. The resulting solution provides boundary conditions for the other regions. The approximation is applied to a number of problems. The no‐flow solution consists of zero thickness for the oval, and the polar cap is circular, centered nightward of the pole. It is surmised that a minimum‐energy criterion applies to the magnetosphere. That is, it contains the minimum amount of auroral oval flux permitted for a given cross‐polar cap potential. This implies zero thickness for the oval at noon. Converting magnetospheres are considered next. An isostasy principle holds in that a bulge of the oval into the polar cap must be balanced by a bulge to lower latitudes at the same longitude. The flow equations were applied to the observed Feldstein ovals. The results showed velocities up to 1 km/s, cross‐polar cap potentials up to 110 kV, and intense inflow in a narrow region near midnight. These all vary systematically with the Q magnetic‐activity index and local time. The application of the equations to particles with significant field gradient drift indicated three differences: (1) a mid‐ and low‐latitude flow of several kilovolts occurs in the steady state, (2) a dawn‐dusk asymmetry is produced in the thickness of the oval, and (3) the form and slope of the Harang discontinuity are predicted. The introduction of ionospheric conductivity variation effects results in the following conclusions: (1) average velocities along the oval are unaffected, (2) the presence of the westward (eastward) electrojet displaces the oval to higher (lower) latitudes by distances of several hundred kilometers, and (3) the electrojets close by both partial ring currents and magnetotail currents.
Observations and photochemistry of O++ in the daytime thermosphere
Authors: Breig E. L. , Torr Marsha R. , Kayser D. C.
Published: February 2008

A new analysis has been performed of Atmosphere Explorer C observations of O++ in the daytime thermosphere. The revised chemical model includes the rapid losses of O++ via N2 as reported from laboratory measurements and its production from direct double photoionization of O as an additional source. An improved procedure has been introduced for the analysis of the basic ion currents recorded by the magnetic ion‐mass spectrometer and has special application for the very low concentrations near and below 200 km. Large‐scale features in the low‐altitude data are best represented with an additive term proportional to the N2 (or O2) density, with the preferred interpretation as a contaminant signal induced by neutral particles impinging on the instrument or spacecraft surfaces. This low altitude effect also accounts for otherwise excessive observed amounts of O++ previously reported near 200 km. The satellite data between 200 and 400 km are consistent with a rate coefficient of 6.6×10−11 cm³/s for charge‐exchange losses of O++ to O and with an observational estimate of 2.6×10−9 s−1 for the production of O++ from direct double photoionization for conditions near solar minimum. The latter ionospheric result also provides a reference value for the associated basic atomic photoionization cross section.
A scale analysis of the D region winter anomaly
Authors: Offermann D. , Brückelmann H. G. K. , Barnett J. J. , Labitzke K. , Torkar K. M. , Widdel H. U.
Published: February 2008

A scale analysis, i.e., an estimation of typical values of important atmospheric and ionospheric parameters and time scales is performed for the meteorological type of winter anomaly at medium latitudes (40°N). An interpretative model of the winter anomaly is developed by a correlation analysis on the basis of data obtained from the Western European Winter Anomaly Campaign 1975/76 at El Arenosillo, Spain, and a simultaneous winter anomaly campaign at Wallops Island, U.S.A large number of rocket experiments performed in six previous winters in Spain are also analyzed and found to support the model. The model explains the buildup and decay of the winter anomaly by combined action of temperature, turbulent downward transport of nitric oxide, and horizontal transport of nitric oxide by winds. The horizontal wind direction is not important for the picture. Mechanisms involved are fast enough to explain the rapid variations of the winter anomaly. Large‐scale winter anomaly structure is linked to planetary wave number 2. The model picture is found to be compatible with quite a number of experimentally known features of the winter anomaly. There are hints to a more general validity of the model.
Mg+ morphology from visual airglow experiment observations
Authors: Fesen Cassandra Garay , Hays Paul B.
Published: February 2008

Observations of low‐latitude Mg+ by the Visual Airglow Experiment on Atmosphere Explorer E were analyzed by a two‐dimensional inversion technique. Number density profiles as functions of geographic position were obtained in the F2 region. The profiles represent daytime northern winter, solar maximum conditions and reveal frequent occurrences of large number densities (50–100 cm−3) at altitudes up to 475 km. Maximum densities are usually found near the equator at 1200–1400 LT. The equatorial density then decreases as the peak concentrations shift to higher latitudes. Longitudinal differences in the time development of the metal ion density appear in the afternoon. These seem to be due to the increasing importance of zonal winds and/or to longitudinal differences in the vertical ion drift velocity.
The Saturn spectrum in the EUV‐electron excited hydrogen
Authors: Shemansky D. E. , Ajello J. M.
Published: February 2008

Recent laboratory observations of electron excited H2 in the EUV have brought about the realization that higher Rydberg series band systems make a significant contribution to the emission spectrum. Theoretical cross section estimates for the excitation of the D, D′, B′, B″ states agree with these results. Model calculations for particle excitation of the Saturn atmosphere, including the higher states, now show excellent agreement with Voyager auroral and dayside equatorial spectra. The model data also confirm the relative spectral response calibration of the Voyager instruments, providing a basis for accurate analysis of the excitation processes on both Jupiter and Saturn.
Nonlinear evolution of convecting plasma enhancements in the auroral ionosphere: 2. Small scale irregularities
Authors: Keskinen M. J. , Ossakow S. L.
Published: February 2008

Using analytical and numerical simulation techniques, the linear stability and nonlinear evolution of small scale (∼0.1–1 km) density irregularities in local unstable regions of large scale convecting auroral plasma enhancements have been studied. Our results show that these small scale size irregularities are driven unstable primarily by the effects of convection through the E×B gradient drift instability. We find that the density irregularities, in the nonlinear regime, in a plane nearly perpendicular to the magnetic field resemble steepened striation‐like structures (elongated in the north‐south direction for equatorward convection) that can form and cascade from kilometer to tens of meter scale sizes on the order of half an hour. The one‐dimensional spatial power spectra of the density irregularities in the north‐south P(ky) ∝ ky−n and east‐west P(kx) ∝ kx−n can be described by inverse power laws with n ≃ 2–3. Finally, we propose and demonstrate, using a crude model, that a two‐step process, in which small scale irregularities can grow on longer wavelength nonlinear structures, can account for the experimentally observed L shell aligned nature of the irregularities.
Solar wind control of the low‐latitude asymmetric magnetic disturbance field
Authors: Clauer C. Robert , McPherron Robert L. , Searls Craig
Published: February 2008

The technique of empirical linear prediction filters is used to investigate the extent to which the low latitude dawn‐dusk magnetic asymmetry is controlled by the dawn‐dusk solar wind motional electric field VBs and/or by substorm processes measured by the westward auroral electrojet index AL. The dawn‐dusk asymmetry is measured by a new index defined as the difference between dawn and dusk deviations in the X (geomagnetic Northward) magnetic field component. The empirically determined filters obtained from this analysis provide quantitative information which characterizes the coupling processes. For example, the VBs to AL filter is a delayed pulse beginning after a delay of about 15 minutes, peaking at 60 minutes and returning to zero at 120 min. The filter has the characteristics of a low pass filter with a cutoff frequency at 10−4 Hz. The VBs to ASYM filter is also a delayed pulse with similar constants. The VBs to ASYM filter, however, also has a long tail which gradually decays reaching zero near 5 or 6 hours lag time. The AL to ASYM filter is acausal, being nonzero at future lag times. The peak of the filter occurs at a delay of about 10 min. and decays exponentially becoming nearly zero after 4 hours lag. Our results indicate that some currents are directly driven by the solar wind‐magnetosphere interaction and that their magnetic perturbations contribute to both the AL and ASYM indices. A portion of the AL index that is uncorrelated with VBs is, however, correlated with ASYM suggesting that internal magnetospheric processes contribute to AL and ASYM as well.
Meridional neutral winds in the thermosphere at Arecibo: Simultaneous incoherent scatter and airglow observations
Authors: Burnside R. G. , Behnke R. A. , Walker J. C. G.
Published: February 2008

Simultaneous measurements of ion and neutral wind velocities have been obtained at Arecibo, Puerto Rico, on several nights in both the summer and winter seasons. The diffusion velocity of O+, calculated in an analysis by using the accepted value for the ambipolar diffusion coefficient and the MSIS model atmosphere, is found to be in fairly close agreement with the observations. Incoherent scatter radar data have also been used to derive height profiles of the meridional neutral wind velocity. In the spring and summer months, the meridional wind is strongest near 2100 AST, with a southward velocity ranging from 50–100 m s−1. At about 0100 AST the ‘midnight descent’ of the F layer occurs, in association with a slackening, or sometimes a reversal, of the meridional wind. In general, the meridional wind is fairly constant with altitude. However, prior to midnight in the spring and summer months, contours of constant velocity are often observed to slope downward with increasing time. Some degree of vertical wind shear is often observed between 0200 and 0500 AST as well. In the winter months, meridional wind velocities are lower, and the nocturnal variation of the wind field is less pronounced than it is at other seasons.
Multiple spacecraft observations of interplanetary shocks: Four spacecraft determination of shock normals
Authors: Russell C. T. , Mellott M. M. , Smith E. J. , King J. H.
Published: February 2008

ISEE 1, 2, 3, IMP 8, and Prognoz 7 observations of interplanetary shocks in 1978 and 1979 provide five instances where a single shock is observed by four spacecraft. These observations are used to determine best‐fit normals for these five shocks. In addition to providing well‐documented shocks for future investigations these data allow us to evaluate the accuracy of several shock normal determination techniques. When the angle between upstream and downstream magnetic field is greater than 20°, magnetic coplanarity can be an accurate single spacecraft method. However, no technique based solely on the magnetic measurements at one or multiple sites was universally accurate. Thus, we recommend using overdetermined shock normal solutions whenever possible, utilizing plasma measurements, separation vectors, and time delays together with magnetic constraints.
ISEE/IMP observations of simultaneous upstream ion events
Authors: Mitchell D. G. , Roelof E. C. , Sanderson T. R. , Reinhard R. , Wenzel K.‐P.
Published: February 2008

Propagation of upstream energetic (50–200 keV) ions is analyzed in sixteen events observed simultaneously by solid state detectors on ISEE 3 at ∼200 RE and on IMP 8 at ∼35 RE from the earth. Conclusions are based on comparisons of the pitch angle distributions observed at the two spacecraft and transformed into the solar wind frame. They are beamlike at ISEE 3 and are confined to the outward hemisphere. When IMP 8 is furthest from the bow shock, they are also usually beamlike, or hemispheric. However, when IMP 8 is closer to the bow shock, pancakelike distributions are observed. This systematic variation in the IMP 8 pitch angle distributions delimits a scattering region ℓ ≲ 15 RE upstream of the earth's bow shock (ℓ measured along the interplanetary magnetic field) that dominates ion propagation, influences the global distribution of fluxes in the foreshock, and may play a role in acceleration of the ions. When IMP 8 is beyond ℓ ∼ 15 RE, the propagation appears to be essentially scatter‐free between IMP 8 and ISEE 3; this is deduced from the absence of earthward fluxes at IMP 8 as well as the tendency for the spin‐averaged fluxes to be comparable at the two spacecraft.
Energetic ions upstream of the Earth's bow shock during an energetic storm particle event
Authors: Scholer M. , Ipavich F. M.
Published: February 2008

We present simultaneous observations of low‐energy protons and alpha particles from ISEE 3 far upstream and from ISEE 1 close to the earth's bow shock during the passage of an interplanetary shock wave with its associated energetic storm particles. Intensities, spectra, and anisotropies of the energetic storm particles are modified due to the interaction of these particles with the earth's bow shock. An intensity spike observed at ISEE 1 during the passage of the interplanetary shock is interpreted as being due to postacceleration of energetic storm particles at the bow shock by the first‐order Fermi mechanism. The spikes observed at ISEE 1 after the passage of the interplanetary shock are most probably due to reflection of the energetic storm particles at the bow shock.
The inner edge of Saturn's B ring
Authors: Northrop T. G. , Hill Jay Roderick
Published: February 2008

The sharp, 90‐km wide transition from an optical depth of 0.2 in the C ring to 1 in the B ring begins at 91,970 km from Saturn's center. This radius is found to be almost exactly at the inward stability limit of charged particles launched in the ring plane at the local Kepler velocity, provided these particles have large charge to mass ratio. The zonal harmonic models of Saturn's magnetic field from the Voyager data and the gravitational field model from Pioneer data are essential to get the very close agreement between theory and observation. The theoretical stability limits are 91,973±145 km from Voyager 1 magnetic field data and 91,991±145 km from Voyager 2 magnetic data. The zonal harmonic magnetic field lines are not perpendicular to the ring plane. Therefore, in addition to the magnetic mirror, gravitational, and centrigual forces, an unknown force must be postulated to produce equilibrium in the ring plane and make the stability calculation meaningful.
The Joule heat production rate and the particle energy injection rate as a function of the geomagnetic indices AE and AL
Authors: Ahn B.‐H. , Akasofu S.‐I. , Kamide Y.
Published: February 2008

As a part of the joint efforts of operating six meridian chains of magnetometers during the IMS, magnetic records from 71 stations are used to deduce the distribution of electric fields and currents in the polar ionosphere for March 17, 18, and 19, 1978. As a continuation of this project, we have constructed hourly distribution maps of the Joule heat production rate, the particle energy injection rate and their sum over the entire polar region on the three days. For this purpose the conductivity distribution is inferred at each instant partially on the basis of an empirical method devised by Ahn et al. (1982). The particle energy injection rate is estimated similarly by using an empirical method. The data set thus obtained allows us to estimate also the global Joule heat production rate UJ, the global particle energy injection rate UA and the sum UI of the two quantitites. It is found that the three global quantities (watt) are related almost linearly to the AE(nT) and AL(nT) indices. Our present estimates give the following relationships: UJ = 2.3 times 108 · AE, UA = 0.6 times 108 · AE and UI = 2.9 times 108 · AE; UJ = 3.0 times 108 · AL, UA = 0.8 times 108 · AL, and UI = 3.8 times 108 · AL.
Neutral and ion gas heating by auroral electron precipitation
Authors: Rees M. H. , Emery B. A. , Roble R. G. , Stamnes K.
Published: February 2008

The physical and chemical processes responsible for neutral gas heating resulting from energetic electron bombardment of the atmosphere are quantitatively described. The neutral heating efficiency, defined as the local heating per unit energy deposition, is evaluated for a range of typical auroral electron spectra, and it is shown that the altitude profile of the efficiency is independent of the electron spectrum. The efficiency is sensitive to the neutral atmosphere (composition, density, temperature) but differences can be minimized by adopting a pressure coordinate in place of the altitude. Over half the energy input rate associated with energetic particle precipitation goes into neutral heating, while less than 5% leads to ion heating. The coupling between particle precipitation and joule heating is discussed, and a parameterized Pedersen conductance curve is presented.
Power spectral analysis of auroral occurrence frequency
Authors: Silverman S. M. , Shapiro Ralph
Published: February 2008

Power spectral analyses have been carried out on a data set of Swedish visual auroras for the period 1721 to 1943. Besides the anticipated 11‐year, annual, and semiannual cycles, an unexpected peak near 1.4 years is found. The amplitude of this latter peak, while smaller than that of the 6‐month and 12‐month peaks, is statistically significant at between the 5% and 1% level and is found consistently in a sequence of spectra covering consecutive, overlapping, approximately 22‐year data segments. The semiannual variation becomes extremely weak during the prolonged solar activity minimum at the beginning of the nineteenth century, and also shows some lessening during the prolonged solar activity minimum near the beginning of the twentieth century. The 1.4‐year peak shows a strong modulation of about 65 to 68 years and an additional long‐term trend of the order of centuries. The origin of this peak is unknown.
Satellite observations of Pi 2 activity at synchronous orbit
Authors: Sakurai T. , McPherron R. L.
Published: February 2008

Pi 2 magnetic pulsations are frequently observed at synchronous orbit by the UCLA fluxgate magnetometer on ATS 6. Events that occurred in September 1974 have been studied by using digital power spectra and coherency analysis to determine wave characteristics. From examination of wave form and application of spectral analysis, these Pi 2 events can be divided into three types. The first is a superposition of a 100‐s oscillation and a large‐amplitude, higher‐frequency Pi 1 activity. The second is a 100‐s wave unaccompanied by Pi 1. Both types have a significant compressional component. The third is a pure transverse wave in the azimuthal component. The pure transverse waves are quite rare, while the compressional waves occur during almost every substorm. In general, if the satellite is in the local time sector 1900–0300 LT, a Pi 2 burst accompanies every onset. In addition, onsets at the satellite are associated with ground Pi 2 bursts. The peak occurrence time of the satellite Pi 2 is 2100 LT. An examination of Pi 2 polarization at the satellite suggests that a polarization reversal occurs around midnight during quiet magnetic conditions (Kp ≤ 3+), left‐handed premidnight and right‐handed postmidnight. This result is similar to that obtained from ground‐based studies at stations equatorward of the auroral electrojet. The initial perturbation in the azimuthal component of a Pi 2 event is in the same sense as the perturbations caused by substorm‐associated, field‐aligned currents, positive (eastward) premidnight, negative (westward) postmidnight. This observation indicates that there may be a very close association between their causative mechanisms.
A theoretical approach to the morphology and the dynamics of diffuse auroral zones
Authors: Fontaine D. , Blanc M.
Published: February 2008

Direct observations of the high‐latitude ionosphere have established the continuous presence of large‐scale emissions, referred to as diffuse auroras. Neglecting localized structures, such as discrete arcs, we focus upon a quantitative description of the effects of large‐scale convection on diffuse precipitation. The transport of 100 eV‐10 keV electrons from the geomagnetic tail earthward by the convection electric field, and their pitch angle diffusion into the loss cone by wave‐particle interactions are believed to be the main cause of diffuse auroral electron precipitation. A self‐consistent treatment balancing wave generation and particle diffusion appears to be still beyond the present state of the art. For the main purpose of diffuse precipitation modeling, we propose a simplified approach to this problem and test its validity against direct observations of the location and dynamics of diffuse auroral emissions. Using basically the strong pitch angle diffusion limit in the way proposed by Kennel [1969], we derive a set of fluid equations describing the three‐dimensional transport of plasma‐sheet electrons. Their integration provides the latitude and local time distribution of precipitation fluxes and characteristic energies at the top of the ionosphere as a function of the large‐scale dawn to dusk electrostatic potential drop. The calculated expansion of the auroral oval with magnetic activity, deduced from our model, approaches the experimental results. So this simplified theoretical study permits us to reproduce and explain the main characteristics of the diffuse auroral zone. However, for high values of magnetic activity, the theoretical results are found to be shifted poleward relative to the observations. This suggests that the assumption of strong pitch angle diffusion overestimates the efficiency of wave‐particle interactions.
The detection of X rays from Jupiter
Authors: Metzger Albert E. , Gilman David A. , Luthey Joe L. , Hurley Kevin C. , Schnopper Herbert W. , Seward Frederick D. , Sullivan James D.
Published: February 2008

X rays in the energy band 0.2–3.0 keV have been detected coming from both polar regions of Jupiter. The observations were made in 1979 and 1981 by using the imaging proportional counter and high resolution imaging detectors on the Einstein X ray astronomy satellite. The measured flux density of ∼6×10−4cm−2s−1 at earth corresponds to an X ray luminosity of ∼4×109 W in the 0.2‐ to 3.0‐keV energy band. The energy spectrum of the X rays is extremely soft and can be characterized by a power law with an exponent of ∼2.3. Detector energy resolution is insufficient to distinguish a soft line spectrum from a continuum. However, the shape of the response and the observed X ray power indicate that the source of this auroral emission is not electron bremsstrahlung as on the earth, but is most probably line emission from O and S ions with energies between 0.03 and 4.0 MeV/nucleon precipitating from the outer boundary of the Io plasma torus at L ∼ 8.
Energetic atomic and molecular ions in Saturn's magnetosphere
Authors: Hamilton D. C. , Brown D. C. , Gloeckler G. , Axford W. I.
Published: February 2008

We present observations and analysis of the composition, energy spectra and spatial distribution of energetic ions (≳0.2 MeV/nucleon) in Saturn's magnetosphere outside of ∼4 Rs. Our results are based on data from the low‐energy particle telescope (LEPT), one of the two sensors of the low‐energy charged particle (LECP) experiment on Voyager 1 and 2. The major species, in order of abundance, are H, H2+, He, H3+, C, and O. The energy spectra of the magnetospheric ions are typically very soft (γ ∼ 6–7), with an apparent cutoff in energy/charge at 1–2 MeV/e. From the abundance ratios we conclude that the energetic He, C, and O ions have a solar wind origin. The molecular hydrogen ions H2+ and H3+ probably originate from Saturn's upper ionosphere. The protons can originate from the solar wind, the ionosphere or the hydrogen atom torus in Saturn's outer magnetosphere. The local H+ sources apparently were dominant at the time of Voyager 1 encounter, but the solar wind may also have been an important source at the time of Voyager 2. To explain the fact that the energetic heavy ions (Z ≥ 6, E ≳ 0.2 MeV/nucleon) are apparently of solar wind origin, an acceleration process that favors ions with small mass/charge ratios seems to be required. The energetic heavy ions at a given energy/nucleon are mainly highly ionized solar wind particles even though singly or doubly charged local ions (e.g., O+, O++, N+, etc.) have been observed to be much more abundant in the low‐energy magnetospheric plasma. Energization may involve a two‐step process of electrostatic acceleration in the tail region combined with inward radial diffusion. The fluxes of all species decreased inside the orbit of Dione and were nearly wiped out in the ‘slot’ region within the orbit of Tethys. Both satellite absorption and precipitation due to pitch angle scattering may be important loss processes in that region. In comparison with the atomic ions, the decline in the flux of H2+ ions began at a larger L value (L ∼ 8) and proceeded more rapidly with decreasing L. The cause is apparently dissociation due to impact with the electrons and ions in the inner plasma torus. In the outer magnetosphere (L ≳ 9), photodissociation rapidly destroys a large fraction of the H2+ ions (those that are vibrationally excited), but dissociation by impact with neutral H atoms is faster for H2+ ions in the lowest vibrational state. The ground state lifetime (∼23 days) places a limit of ∼10–100 days on the mean overall residence time for energetic ions in Saturn's magnetosphere.
The relationship between Saturn kilometric radiation and the solar wind
Authors: Desch M. D. , Rucker H. O.
Published: February 2008

Voyager spacecraft radio, interplanetary plasma, and interplanetary magnetic field data are used to show that large‐amplitude fluctuations in the power generated by the Saturn kilometric radio emission are best correlated with solar wind ram pressure variations. In all, 13 solar wind quantities previously found important in driving terrestrial magnetospheric substorms and other auroral processes were examined for evidence of correlations with the Saturn radio emission. The results are consistent with hydromagnetic wave or eddy diffusion processes driven by large‐scale solar wind pressure changes at Saturn's dayside magnetopause.
Modulational instability of finite amplitude dispersive Alfvén waves
Authors: Sakai Jun‐Ichi , Sonnerup B. U. Ö.
Published: February 2008

The stability of a finite amplitude plane monochromatic circularly polarized Alfvén wave is studied by using an approximate two‐fluid model obtained by performing an amplitude and dual time scale expansion in which temporal changes, observed when moving with the wave, are assumed slow. The lowest order dispersive effects and coupling terms to sound waves are of the same order. A large‐amplitude Alfvén wave obeying an approximate dispersion relation is an exact solution of the resulting model equations. Small perturbations of this solution consisting of two sideband Alfvén waves and a sound wave are then introduced. The resulting dispersion relation, which is of fourth order, is examined analytically for small wave amplitudes of the parent wave. It reveals different regions of stability and instability in a plot of wave number k0 of the unperturbed wave versus β0, the ratio of sound speed to Alfvén speed. For long wave lengths, the left‐hand polarized mode is found to be stable for β0 > 1 and the right‐hand polarized mode for β0 < 1. Modulational instability of the former mode occurs for β0 < 1 and of the latter for β0 > 1. Decay instability is predicted near β0 = 0 and β0 = 1. Formulas for growth rates and unstable wave number bands are given.
Are equatorial electron cyclotron waves responsible for diffuse auroral electron precipitation?
Authors: Belmont Gérard , Fontaine Dominique , Canu Patrick
Published: February 2008

On the basis of theoretical calculations of electron diffusion coefficients and of OGO 5 data, Lyons [1974] suggested that electrostatic electron cyclotron harmonic waves had amplitudes large enough to cause the strong pitch angle diffusion of plasma sheet keV electrons and to be responsible for diffuse auroral precipitation. However, recent measurements of the wave location and amplitude performed aboard the GEOS spacecraft have brought new pieces of information challenging these conclusions. Our calculations are based on the theoretical tool developed by Lyons [1974] but take into account the recently observed wave confinement within a few degrees from the magnetic equator. Under these conditions, we avoid the numerical averaging of the pitch angle diffusion coefficient over the whole line of force, and we can derive an analytical expression of the minimum wave amplitude required to cause strong pitch angle diffusion for plasma sheet electrons. This expression has the advantage to be easily tractable in further calculations, and permits us to evaluate the dependence of the results on parameters that are not reported by observations, such as the wave number spectrum. Our results are found to differ from Lyons's [1974] predictions by a factor of about 2.5, and typically, a wave amplitude of more than 2 mV m−1 is required to put 1‐keV electrons on strong diffusion. On the other hand, on the basis of a statistical analysis of electron cyclotron wave amplitudes measured in the nightside plasmasheet by the GEOS 2 spacecraft, we show that most of the time (>91 %), this typical value of 2 mV m−1 is not reached. This result appears inconsistent with the hypothesis that diffuse auroras, which are a permanent feature of the auroral zones, are due solely to electrostatic electron cyclotron waves. This leads us to the conclusion that these waves are not the only cause of diffuse electron precipitation. It is suggested that other mechanisms involving for instance the dynamics of the ions (such as field‐aligned currents) could play an important role.
Radiation transport effects on the OI 1356‐Å limb intensity profile in the dayglow
Authors: Strickland D. J. , Anderson D. E.
Published: February 2008

The 1356‐Å dayglow limb intensity is calculated for a variety of conditions. Contributions are included from both the OI 1356‐Å and N2 LBH 1354‐Å features. Photoelectron excitation rates are calculated with a recently developed computer code by Strickland and Meier (1982). OI 1356‐Å multiple scattering solutions are compared with those based on the single scattering approximation and are found to be as much as a factor of 1.6 larger with the actual value changing along the profile. Multiple scattering can therefore not be ignored in analyses dealing with 1356‐Å limb data. Solutions are generated for four sets of conditions defined in terms of solar activity and the model atmosphere. Noticeable variations occur as these conditions are altered. Excellent agreement is obtained between the theoretical intensities and limb data from the DMSP‐F4 satellite UV spectrometer experiment.
Dynamical injections as the source of near geostationary quiet time particle spatial boundaries
Authors: Mauk B. H. , Meng C. I.
Published: February 2008

To test our understanding of quasi‐stationary magnetospheric particle convection, we address here a particular class of particle feature (plasma dropouts at 0 eV to 5 keV) observed regularly by near geostationary satellites in the noon to dusk quadrant, often during the apparent absence of recent (hours) substorm activity. At first consideration the feature appears to result from the passage of the satellites toward and into the so‐called “forbidden zone” of the quasi‐stationary particle convection patterns. It is demonstrated here that the energy dispersion of the feature cannot be explained by simple stationary convection models even when loss processes are imposed on those particles that penetrate most closely to the earth. Also, the radial position of the feature does not vary with geomagnetic activity as expected from steady convection models. It is concluded that dynamical processes are responsible. However, models based on the modification of the so‐called cross‐tail field configuration against initial stationary convection patterns are rejected here because these models preserve the qualitative sense of the energy dispersions of the initial patterns. It is proposed that the spatial structures of past (24 hours), dynamical, nightside particle injections determine to a great extent the character of the feature. It is shown that detailed simulations based on the double‐spiraled “injection boundary” concept (used previously to reproduce the fast changing nighttime features) reproduce very well the character and dispersion senses of the noon‐to‐dusk feature by allowing the distributions to evolve for many hours. It is emphasized that the portion of the original injection boundary which gives rise to this feature of interest is the decidely “non‐Alfvénic” portion. Because the ion‐electron energy dispersions expected from quasi‐steady convection are not observed, it is concluded that the dynamical injection with characteristics distinct from those of quasi‐stationary convection is at all times an inseparable component of the processes that populate the inner magnetospheric regions (r < 8 RE at all local times).
Estimating the variability of the solar flux between 200 and 300 nm
Authors: Lean J. L.
Published: February 2008

Observations of the solar ultraviolet irradiance during 1979 (solar maximum) by the Solar Backscatter Ultraviolet Experiment on the Nimbus 7 satellite have established that the irradiance at wavelengths between 200 and 300 nm is modulated by solar rotation. This short‐term variability, which is of the order of 3% at 200 nm and 1% at 250 nm, is associated with the evolution and rotation on the solar disc of plage areas which are enhanced in ultraviolet emission. By using ground‐based Ca II K observations of the fraction of the disc covered by plage during 1979, the intensity ratios for plage to quiet sun emission at these wavelengths have been derived. During the 11‐year solar cycle the number of plage regions on the solar disc increases. There is also an accumulation of active network arising from the remnants of decayed magnetic features. A three‐component model of the solar irradiance is used to estimate the temporal behavior of the ultraviolet emission between 200 and 300 nm, resulting from changes throughout the solar cycle in the active features on the disc. These calculations suggest that for solar cycle 21 the flux variability at 200 nm is 25%, decreasing to 10% at wavelengths between 210 and 250 nm, and to 2% at 300 nm. Ultraviolet flux variability of this magnitude corresponds to an average increase in the total solar irradiance from 1976 to 1979 of about 0.05%. For comparison, the average reduction in the total solar irradiance from 1976 to 1979 due to sunspot blocking is 0.07%. Because of the enhanced ultraviolet emission from solar active features, the variability of the total solar irradiance during cycle 21 is predicted to be less than is calculated by models which consider only the effects of sunspot blocking. This result is consistent with the need for an added faculae brightening term in total solar irradiance models.
Observations of the dynamics of the cosmic ray modulation
Authors: Lockwood J. A. , Webber W. R.
Published: February 2008

Measurements of the galactic cosmic ray intensity for energies E > 2 GeV and E > 60 MeV near earth and for E > 60 MeV at 5 < R < 30 AU from the earth reveal the relationship of interplanetary shocks and Forbush decreases to the 11‐year cosmic ray modulation. We find that (1) during the decreasing portion of the cosmic ray intensity cycle from 1977 to 1982 the modulation is propagated outward radially from the sun with a speed of 350 < Vp < 450 km/s, (2) large rapid, transient decreases of ∼20% are seen at R ∼ 15 AU and R ∼ 28 AU in 1982, (3) the long‐term variation for E > 2 GeV during 1955–1965 and 1965–1977 can be simulated by the accumulative effects of Forbush decreases, and (4) there is a difference in the energy dependence of decrease and recovery phases of the 11‐year variation. These results clearly indicate that a time‐dependent solution to the cosmic ray transport equation is required to describe the solar cosmic ray modulation.