bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐energy magnetospheric protons and their dependence on geomagnetic and interplanetary conditions
Authors: Baker D. N. , Belian R. D. , Higbie P. R. , Hones E. W.
Published: February 2008

High‐energy (E > 0.3 MeV) proton increases within the magnetosphere both at synchronous orbit (6.6 RE) and in the plasma sheet (∼18 RE) have been studied. Measurements at 6.6 RE reveal that most (>80%) substorms have no associated injections of >0.3‐MeV protons (above the typical ambient level of 5 × 10³ (cm² s sr MeV)−1). Those relatively rare substorms which produce large fluxes of high‐energy particles show behavior ranging from drift echo to nondrift echo types of enhancements. The drift echo events are characterized by brief, well‐defined pulses of protons which are observed to drift azimuthally about the earth several times before dispersing. The nondrift echo events exhibit clear flux enhancements, but they do not show very evident pulsed behavior. The relative occurrence probability of high‐energy proton enhancements at 6.6 RE shows a strong positive correlation both with solar wind speed and with Kp. A substantial correlation with southward interplanetary magnetic field (IMF) is also found. Very similar Kp, solar wind, and IMF dependences are found for plasma sheet proton (Ep > 0.5 MeV) enhancements above a threshold of ∼10 (cm² s sr MeV)−1 as measured at 18 RE. The close similarity of differential energy spectra, Kp, solar wind, and IMF dependences at synchronous orbit and in the plasma sheet suggests a common acceleration source. Bursts of energetic protons (Ep ∼ 1 MeV) are also often observed simultaneously in data obtained in the interplanetary medium when proton enhancements are seen at 6.6 RE. A model depicting the relationship between proton events in various magnetospheric regions is presented. It is suggested that large transient (<1 min) induced electric fields exist within the plasma sheet at the time of those substorm onsets that occur during periods of high solar wind speed and southward IMF. Such electric fields then produce both the proton pulses seen at synchronous orbit and the previously reported plasma sheet ‘impulsive’ tailward flowing proton bursts. We relate these initial proton populations directly to the substorm onset acceleration mechanism. The more frequently observed type of plasma sheet proton enhancements which is observed in the expanding plasma sheet is, in the present model, attributed to previously injected protons from the outer radiation zone filling the plasma sheet volume during the recovery phase of substorms.
Magnetotail plasma flow during substorms: A survey with Imp 6 and Imp 8 satellites
Authors: Hones E. W. , Schindler K.
Published: February 2008

This paper presents the results of a statistical analysis of substorm‐associated plasma flow in the sector of the magnetotail within (−18 RE >XSM > −35 RE;|YSM| ≤ 12 RE; |dZSM| ≤2.5 RE). This ‘substorm analysis’ used data from the Los Alamos Scientific Laboratory plasma instruments on the Imp 6 and Imp 8 spacecraft and was keyed to substorms identified primarily in magnetograms from individual auroral zone stations. We find that a sudden establishment or enhancement of flow in a persisting direction is observed at the onset of about one half of occurring substorms. About 80 percent of these sudden changes produce plasma flow in the tailward direction and only 20 percent produce earthward flow. These results seem more consistent with a neutral line model of substorms (e.g., the tearing theory) that predicts tailward plasma flow tailward of a near‐earth ‘substorm neutral line’ than with a rarefaction wave model that does not envision neutral line formation and that predicts solely earthward plasma flow in the plasma sheet during substorms. We also repeated the ‘thinning analysis’ previously used by other authors, that is keyed to occurrence of plasma sheet thinning during periods when the AE index is enhanced, and for which there is no restriction on dZSM. This analysis did not reveal the preponderance of earthward flow during plasma sheet thinning that the other authors reported. Rather, it showed nearly equal occurrence of earthward and tailward flows, with some preference for tailward flows within |dZSM| < 2.5 RE.
Low‐frequency drift measurements in the upper mesopause region: A useful approach to regular high‐atmosphere neutral wind observations
Authors: Schminder R. , Kürschner D.
Published: February 2008

Results obtained during autumn and winter of 1978–1979 are used to demonstrate that low‐frequency drift measurements provide a useful means of permanently monitoring the circulation and tidal wind systems in the upper mesopause region (90–100 km) and represent a valuable supplement to the ground‐based instrumental pool, particularly with regard to the forthcoming middle atmosphere program (MAP).
Measurements of inner zone electron precipitation
Authors: Imhof W.L. , Reagan J.B. , Gaines E.E.
Published: February 2008

Energetic electron (≳0.16 MeV) data taken from November 1972 to April 1973 from over 1000 passes of a low‐altitude (∼750 km) noon‐midnight polar‐orbiting satellite across the L=1.75 field line in both the northern and southern hemispheres have been analyzed for evidence of the longitude and local time distributions in electron precipitation on that field line. The precipitation processes were observed to be weak, at least near local noon and midnight, as evidenced from the negligible fluxes observed at all pitch angles when the conjugate point is below sea level and from the absence of any direct observation of electrons at L=1.75 in the bounce loss cone at all longitudes. By observing quasi‐trapped fluxes with a spectrometer having a large geometric factor, a high sensitivity was achieved for detecting electron precipitation and measuring the energy spectra with fine resolution. The data have been analyzed and found to show that prounounced precipitation events near local noon and midnight were a rare occurrence between ∼60°E and ∼100°E. In the ∼100°E to ∼180°E longitude region, significant fluxes of quasi‐trapped electrons were often observed with a very pronounced noon‐midnight asymmetry. The quasi‐trapped fluxes observed locally in this region were often large near midnight but seldom significant near noontime. At longitudes east of 180°E, to the western edge of the South Atlantic Anomaly, the local noontime fluxes of quasi‐trapped electrons displayed an abrupt increase and, on the average, were higher than the midnight intensities. In this region the energy spectra of quasi‐trapped electrons observed at midnight displayed L‐dependent peaks less often than those observed at noontime. These very pronounced longitude and local time variations in the quasi‐trapped population provide conclusive evidence that the effectiveness of the electron precipitation mechanism(s) is dependent upon local time, (although without further information, detailed local time distributions in the precipitation mechanisms were not derived). From the electron data it was found that within a given local time and longitude interval the fluxes were widely variable, indicating that the precipitation mechanism(s) were not steady in time. Also, during the time period of observation, the average intensities of the quasi‐trapped electrons were higher at times of high negative Dst.
Millstone hill incoherent scatter observations of auroral convection over 60° ≤Λ ≤75° 2. Initial results
Authors: Evans J.V. , Holt J.M. , Oliver W.L. , Wand R.H.
Published: February 2008

The Millstone Hill radar, Westford, Massachusetts (Λ=56°), has been upgraded for observations in support of the international magnetosphere study (IMS) by means of the addition of a 150‐ft diameter fully steerable antenna. This antenna allows measurements of F region ion drifts, from which F region electric fields can be derived, using the existing UHF (68‐cm wavelength) radar. The paper describes measurements of the electric field in the auroral zone over the interval 60° ≤Λ ≤75°. The electric field is obtained by means of a least mean squares fit between the drifts obtained in four beam directions and a smoothly varying model with 204 degrees of freedom. Results are presented for 11 moderately quiet days in 1978 and these are compared with the models published by Heppner (1977). All but one of the days exhibit a simple two‐cell convection pattern. Two days appear reasonably consistent with the pattern expected when the IMF is toward the sun (symmetric cells) and six when the IMF is away (dusk cell larger). The Harang discontinuity is identifiable in all these cases in extending from 22 to 01 LT. The agreement between the observations and the Heppner (1977) models can be improved by moving the center of the dawn cell into the nightside by perhaps as much as 3 hours in the case of the asymmetric pattern. Two of the 11 days fit neither pattern in that a feature corresponding to the Harang discontinuity cannot be identified. The size and speed of the flow in the convection cells appear to increase with Kp (and/or decreasing Dst) without any obvious change in the direction of the flow (except in one instance near noon). We find no evidence for a permanent narrow ‘throat’ region in the dayside cleft through which all the flux tubes convect, though it seems possible that this behavior may manifest itself during disturbed periods (Kp >3). At other times, the Heppner (1977) models seem to describe our observations on the dayside quite adequately.
The signature of large‐scale auroral structure in radio wave absorption
Authors: Berkey F.T. , Anger C.D. , Akasofu S.‐I. , Rieger E.P.
Published: February 2008

High latitude riometer data from the Alaska meridian chain of stations are presented in the form of contour maps and are compared with the distribution of auroras derived from all‐sky camera observations for intervals of time during 3 days of 1973. From this comparison it has been shown that the signature of the various forms of the diffuse and discrete aurora can be readily identified in riometer data. In the early evening sector, frequently strong radio wave absorption has been observed in association with ‘trough region auroras,’ which are believed to be caused by quasi‐trapped particle populations situated equatorward of the normal diffuse aurora. Weak absorption (previously described as ‘prebay absorption’) is associated with the normal diffuse aurora. Very intense precipitation, often lasting for only a short time, accompanies westward traveling surges in the premidnight sector and is usually observed on the poleward edge of the auroral oval, although enhanced absorption may also occur equatorward of the surge in association with an enhancement of the diffuse aurora. In the midnight sector, intense absorption is coincident with the poleward expanding bulge observed after the substorm onset. Absorption events of this nature can occur simultaneously over a range of latitudes. In the morning sector, strong absorption events tend to occur near the equatorward edge of the auroral oval. These events are associated with torchlike structure or patches in the diffuse aurora.
Energetic particle penetrations into the inner magnetosphere
Authors: Ejiri M. , Hoffman R.A. , Smith Paul H.
Published: February 2008

Data from Explorer 45 (S³‐A) instruments have revealed characteristics of magnetospheric storm or substorm time energetic particle enhancements in the inner magnetosphere (L ≲5). The properties of the ion ‘nose’ structure in the dusk hemisphere are examined in detail. A statistical study of the local time dependence of noses places the highest probability of occurrence around 2000 MLT, but they can be observed even near the noon meridian. It also appears that most noses are not isolated events but will appear on successive passes. A geoelectric field enhancement corresponding to a minimum value of AE of about 205 γ seems to be required to convect the particles within the apogee of Explorer 45. The dynamical behavior of the nose characteristics observed along successive orbits is then explained quantitatively by the time‐dependent convection theory in a Volland‐Stern type geoelectric field (γ=2). These calculations of adiabatic charged particle motions are also applied to explain the energy spectra and dispersion in penetration distances for both electrons and ions observed in the postmidnight to morning hours. Finally, useful descriptions are given of the dispersion properties of particles penetrating the inner magnetosphere at all local times as a function of time after a sudden enhancement of the geoelectric field.
Comparison of magnetic field perturbations and solar electron profiles in the polar cap
Authors: McDiarmid I.B. , Burrows J.R. , Wilson Margaret D.
Published: February 2008

Northern hemisphere geomagnetic field perturbations and solar electron profiles measured during the magnetic storm of August 4–5, 1972, are compared. Pitch angle distributions of the solar electrons are used to infer the locations of closed geomagnetic field lines that are compared with the locations of certain features of the magnetic perturbations. An example is given, when the interplanetary magnetic field (IMF) is southward, that shows a peak in the postnoon cleft region perturbation occurring near the location where the solar electron pitch angle distribution changes from anisotropic to isotropic, consistent with the suggestion that the peak in the cleft region perturbation occurs at or close to the dayside limit of closed field lines. The same pass shows isotropic pitch angle distributions throughout the polar cap where the magnetic perturbation is sunward (antisunward convection), consistent with open geomagnetic field lines in this region. Examples are also given for times when the IMF is northward, and one type of perturbation pattern that is observed shows antisunward perturbations (sunward convection) in the central polar cap, sunward perturbations (antisunward convection) in adjacent regions changing again to antisunward perturbations (sunward convection) at lower latitudes. In one of these examples, pitch angle distributions throughout the polar cap are isotropic, and it is probable that in most cases when this type of perturbation pattern is observed, the whole polar cap is open. However, two examples are given where antisunward perturbations (sunward convection) in the polar cap are associated with a closed field region surrounded by open field lines. This field configuration may be rather rare and is probably associated with unusual IMF conditions that were present at the time. The measurements are discussed in terms of a magnetic merging model, and in a qualitative way the model appears to account for the observations.
The ionospheric disturbance dynamo
Authors: Blanc M. , Richmond A.D.
Published: February 2008

A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid‐latitudes. The transport of angular momentum by these winds produces a subrotation of the mid‐latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid‐latitude current is found to close partly via lower latitudes, resulting in an ‘anti‐Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet‐day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet‐day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm‐time ionospheric electric fields at middle and low latitudes.
Cosmic noise ‘Absorption’ by E region plasma waves
Authors: Mehta Naresh C. , D'Angelo N.
Published: February 2008

Ionospheric electric field data from Chatanika, Alaska, and simultaneous riometer data from College, Alaska, are analyzed in the light of a recently proposed ‘absorption’ mechanism, whereby attenuation of the cosmic radio noise measured on riometers is at times the result of backscatter into space of cosmic radio waves by E region plasma waves.
Dynamics of plasma, energetic particles, and fields near synchronous orbit in the nighttime sector during magnetospheric substorms
Authors: Sauvaud J.‐A. , Winckler J.R.
Published: February 2008

We discuss two phases of the substorm‐associated magnetospheric dynamics in terms of the particles and fields at synchronous orbit. The first phase corresponds to the ‘decreases’ of energetic particle flux first identified by Erickson and Winckler (1973) and discussed by Walker et al. (1976) and Erickson et al. (1979). This phase begins one‐half hour to one hour before the substorm onset and is characterized by (1) a distortion of the magnetosphere to a more taillike configuration caused by (2) an intensification and/or motion toward the earth of the cross‐tail current and of its earthward part, the partial ring current, (3) a shift of trapped particle trajectories closer to the earth on the nightside following contours of constant B causing the particle ‘decreases,’ accompanied by a change in the pitch angle distributions from ‘pancake’ to ‘butterfly’ as observed at geostationary orbit, (4) an initiation of a response of the auroral electrojet (AE) index. The decreases of energetic particle flux can correspond to the substorm growth phase as defined initially by McPherron (1970) or the growth or precursor phase of Erickson et al. (1979). Plasma motions and currents during decreases tend to be variable, but the description above nevertheless characterizes the large‐scale trend. It is suggested that the electric field induced by the increasing tail current near the earth acts opposite to the cross‐tail convection field and can temporarily inhibit convection near the geostationary orbit. The second phase is the conventional expansion phase that begins with the ‘onset,’ characterized in our study by (1) a sudden decrease in the tail current and a return of the inflated magnetosphere to a dipolelike configuration, (2) a sudden shift of trapped high‐energy particles toward the tail again following contours of constant B, and at the same time (3) a surge of tail plasma toward the earth as the induced electric field now increases the total convection field. Separate effects thus result in the dramatic increases of both high energy and plasma particles seen at substorm expansion phase onset, (4) an AE index response and the appearance of bays at stations near midnight local time accompanied by very active aurora as well as the precipitation of high‐energy particles. The different appearance of the responses at ATS 1 (on the magnetic equator) and ATS 6 (off the magnetic equator) can be well explained by the above description. True diamagnetic effects of the particle population are clearly evident at the ATS 6 region and must be carefully distinguished from the effects of distant currents. The use of oppositely directed detectors on ATS 6 that permit the evaluation of the guiding center particle density gradients has been of great use in this analysis.
Variability of the solar flux in the far ultraviolet 1175–2100 Å
Authors: Cook J.W. , Brueckner G.E. , VanHoosier M.E.
Published: February 2008

We examine the expected range of variability over the solar cycle of the far UV flux (irradiance) in selected lines and continua within the wavelength range 1175–2100 Å which results from a simple two‐component model of the full‐disk flux. We determine contrast factors I(plage)/I(quiet region) for various lines and continua and estimate the fraction of the disk that is covered by plages at various times in the solar 11‐year cycle. We assume (1) that enhanced far UV emission arises from the same plage areas visible in Ca II emission, (2) that our contrast values are typical, and (3) that both the plage and quiet region intensities per unit surface area remain constant over the solar cycle, so that flux variability is caused only by a greater or lesser fraction of surface area covered by plages. We discuss available observational evidence for these assumptions. Our results are in good agreement with the Lα fluxes published by Vidal‐Madjar [1975] and the recent broadband observations by Hinteregger (private communication, 1979), but they are in disagreement with the flux variability reported by Heath and Thekaekara [1977] over the 1700–3000‐Å range.
Deceleration of the solar wind upstream from the Earth's bow shock and the origin of diffuse upstream ions
Authors: Bame S.J. , Asbridge J.R. , Feldman W.C. , Gosling J.T. , Paschmann G. , Sckopke N.
Published: February 2008

Observations with the Los Alamos Scientific Laboratory/Max‐Planck‐Institut crossed‐fan solar wind ion experiment on ISEE 1 reveal that the solar wind is decelerated and deflected away from the direction of the earth’s bow shock as it enters that portion of the upstream region populated by ‘diffuse’ bow shock ions and long‐period (10–60 s) waves. Typically, the average directed velocity vector changes by 7–10 km s−1 as it enters the wave region. At times, average speed changes as large as 25–40 km s−1 are observed. Superposed upon these changes in average flow speed are large amplitude (∼±15 km s−1) fluctuations in flow speed associated with the waves themselves. The observations suggest that the solar wind deceleration is the result of momentum transfer from ‘reflected’ bow shock ions to the wind via the long‐period waves as the reflected ion beams go unstable. The broad angular distributions of the diffuse ions thus appear to be produced as a consequence of the disruption of reflected ion beams.
Auroral changes during the eighteenth and nineteenth centuries and their implications for the solar wind and the long‐term variation of sunspot activity
Authors: Feynman J. , Silverman S. M.
Published: February 2008

Both auroral and geomagnetic activity provide information from which the behavior of the magnetosphere and the solar wind can be inferred. Swedish auroral sightings during the eighteenth and nineteenth centuries show a remarkable pattern of changes in the latitudes at which the auroras were observed. Auroral reports from New England confirm that these variations were hemisphere‐wide. The pattern of changes took place over an 106‐year period and is easily distinguished from the much smaller changes that are related to single sunspot number cycles. We infer that the pattern reflects corresponding changes in the solar wind and the resultant magnetospheric configuration and that these changes were much greater than those observed since in situ measurements began. Our results show that a minimum solar wind occurred at the beginning of the nineteenth century. It has been argued elsewhere that minimum solar winds also occurred around the beginnings of the eighteenth and twentieth centuries. Since all these periods are also the reported minimums of the ‘80‐ to 100’‐year variation in sunspot activity, we conclude that both the changes in the solar wind and in the strength of the cycle in sunspot number reflect underlying fundamental long‐term changes in the sun itself.
Azimuthal magnetic field at Jupiter
Authors: Parish J.L. , Goertz C.K. , Thomsen M.F.
Published: February 2008

In this paper we model the azimuthal component of the magnetic field at Jupiter. We start by writing a current distribution which is the sum of two currents, one flowing along the magnetic field lines (J=αB) and another injected into the current sheet at r0 (∼10 RJ). We look at two cases, one in which current flows along the field lines into the equatorial plane and then radially outward and a second case in which the only current is that injected into the current sheet at r0. Each of these two cases results in an azimuthal magnetic field which fits the magnetic field data.
On the evolution of the geomagnetospheric coherent cyclotron resonance in the midst of noise
Authors: Brinca Armando L.
Published: February 2008

Analysis of the effects of magnetospheric noise on the inhomogeneous interaction between coherent whistlers and energetic electrons and particularization to two basic mechanisms of particle precipitation confirm the influential role played by turbulence in coherent resonances involving small‐amplitude waves.
Effective O2 absorption cross sections for solar absorption data from Lyman α ion chambers
Authors: Ilyas Mohammad
Published: February 2008

Effective absorption cross sections applicable to total signal from Lyman α ion chambers at all altitudes are presented as a function of normalized signal as well as altitude (and solar zenith angle) for four solar H Lyman α flux conditions (2, 3, 4, and 6 ergs cm−2 s−1) using Carver et al.'s recent σO2> data (over Lyman α) at 195 K and 294 K. The total effective cross sections (which increase to about 2 orders of magnitude over Ly α at ∞) allow the analysis of (generally neglected) Lyman α ion chamber signal at the upper altitudes (greater than or equal to about 95 km where non‐Lyman α radiation contributes in the detector signal) in a straightforward manner. The effective cross sections for ‘Lyman α absorption only’ using a theoretically developed photoabsorption profile representing average mesosphere are also presented. O2 densities derived from Lyman α ion chamber data at upper altitudes from several rocket flights using the total effective cross sections were found to be in good agreement with the O2 density data derived from quartz‐triethylamine ion chambers flown in some of the flights and in general agreement with the Cira (1972) model.
A comparison of solar wind and ionospheric ion acoustic waves
Authors: Kintner Paul M. , Kelley Michael C.
Published: February 2008

Ion acoustic waves produced during the Trigger experiment are compared to ion acoustic waves observed in the solar wind. After normalizing to the Debye length the spectra are nearly identical, although the ionospheric wave relative energy density is 10² larger than the solar wind case.
Theory of scan plane flux anisotropies
Authors: Northrop Theodore G. , Thomsen M. F.
Published: February 2008

When a spacecraft detector measures particle flux as a function of look direction in a plane (the scan plane) anisotropy is often seen. This anisotropy is caused by spatial gradients, by E × B particle drift, and by various spectral and geometric effects. This paper treats all of these effects systematically, starting from the nonrelativistic Vlasov equation. The general analysis is applied to a simple model of an anisotropic distribution to give a relation between the E × B drift, the gradient and the experimentally observed first, second, and third harmonics of the flux as a function of angle in the scan plane. Even with an assumed model, anisotropy observations in one plane alone do not suffice to determine the E × B drift velocity and the spatial gradient independently. If one is assumed, the other follows. If the E × B velocity is assumed (e.g., the corotational velocity in a rotating planetary magnetosphere), the spatial gradient may be deduced, and from it the time rate of change of flux in a nonrotating frame of reference.
Motion of trapped electrons and protons in Saturn's inner magnetosphere
Authors: Thomsen M. F. , Van Allen J. A.
Published: February 2008

A summary is given of basic formulas for the guiding center motion of energetic charged particles trapped in a dipolar magnetic field. These formulas for longitudinal drift rates, latitudinal bounce periods, equatorial gyroradii, and equatorial gyroperiods are then stated in convenient numerical form for electrons and protons as functions of kinetic energy E, magnetic shell parameter L, and equatorial pitch angle α0 for a slightly simplified model of the observed magnetic field of Saturn. To aid in the study of the interaction of charged particles with the rings and inner satellites of Saturn, additional formulas are given for the time interval between successive encounters of charged particles with a satellite in a circular prograde orbit and for the energy of electrons whose longitudinal angular velocity is resonant, or synchronous, with the Keplerian angular velocity of such a satellite.
Interior structure of Saturn inferred from Pioneer 11 gravity data
Authors: Hubbard W. B. , MacFarlane J. J. , Anderson J. D. , Null G. W. , Biller E. D.
Published: February 2008

The structure of Saturn is studied via a fourth‐order theory for rotating planets and equations of state for the envelope which depend parametrically on the helium abundance, on the starting temperature for the adiabat, and on adopted forms of the pressure‐density curve in the region of transition from molecular to metallic hydrogen. Models are constrained by the values of J2 and J4 obtained from the Pioneer‐Saturn celestial mechanics experiment. Equations of state are tested by computing Jupiter models, which can now be subjected to a more stringent comparison with observed zonal harmonics. We find that Saturn has a low‐density hydrogen‐helium envelope with no evidence for enhancement of H2O, CH4, or other abundant compounds. Such compounds are presumably located near the core. The helium mass abundance for Saturn's envelope appears to be in the range of ∼0.12 to ∼0.19, but this result is very model‐dependent. The helium abundance in the envelope of Jupiter is apparently very similar to that of Saturn.
Some spectral characteristics of the hectometric Jovian emission
Authors: Lecacheux A. , Moller‐Pedersen B. , Riddle A. C. , Pearce J. B. , Boischot A. , Warwick J. W.
Published: February 2008

The study of the dynamic spectra of hectometric Jovian emission (f<1.3 MHz) for the period January to June 1978 from the planetary radio astronomy (PRA) experiment on the Voyager 1 and 2 spacecraft shows that its shape is stable with the rotation of the planet. However, there are noticeable differences between the spectra observed at one‐month intervals by one spacecraft, or simultaneously by the two spacecraft, which can be clearly related neither to an effect of Io nor to the Jovicentric declination of the observer.
Three‐dimensional structure of ionospheric currents 1. Currents caused by diurnal tidal winds
Authors: Takeda Masahiko , Maeda Hiroshi
Published: February 2008

Three‐dimensional current structure in the ionosphere (90–400 km) is calculated for (1,−2) tidal winds, assuming that the parallel conductivity (σ0) is infinite and therefore that the electrostatic potential is constant along magnetic field line. Calculated structure of height‐integrated currents is essentially the same as the observed Sq current system. It is also shown that a meridional current system always exists in the equatorial region and that the east‐west component of currents could be reversed only near the equator. Though electric currents flow mainly in the E region, currents in the F region contribute a little, especially to poleward currents at middle and high latitudes.
Solar zenith angle dependence of ionospheric ion and electron temperatures and density on Venus
Authors: Miller Kent L. , Knudsen William C. , Spenner Karl , Whitten Robert C. , Novak Vit
Published: February 2008

Data from the first year of operation of the Pioneer Venus orbiter retarding potential analyzer are presented, showing the variation with solar zenith angle of median values of ion and electron temperatures and density. Ion density is found to decrease by about an order of magnitude from dayside to nightside. Median ion temperatures above 300‐km altitude are essentially constant with solar zenith angle for solar zenith angles less than 150° and have values near 2300°K at the ionopause. Near the antisolar point, the ion temperature rises to 5000°K. At altitudes below 300 km the median ion temperature is nearly constant with solar zenith angle throughout the dayside but increases with solar zenith angle on the nightside. Ion temperatures on the dayside are nearly constant and are larger than the neutral temperature in the altitude range between 175 and 225 km. A strong heat source near 160 km is required to model the observations. Joule heating is suggested as the probable source. The median electron temperature at a given height is independent of both solar zenith angle and ionopause height. The electron temperatures suggest a constant heat flux into the electron gas at the ionopause. The flux may be provided by dissipation of energy by whistler mode plasma waves at the ionopause and/or conduction of heat from the ionosheath through the mantle.
Latitude dependence of solar wind velocity observed ≳1 AU
Authors: Mitchell D. G. , Roelof E. C. , Wolfe J. H.
Published: February 2008

The large‐scale solar wind velocity structure in the outer heliosphere has been systematically analyzed for Carrington rotations 1587–1541 (March 1972 to April 1976). Spacecraft data were taken from Imp 7/8 at earth, Pioneer 6, 8, and 9 near 1 AU, and Pioneer 10 and 11 between 1.6 and 5 AU. Using the constant radial velocity solar wind approximation to map all of the velocity data to its high coronal emission heliolongitude, we examined the velocity structure observed at different spacecraft for latitudinal dependence and compared it with coronal structure in soft X rays and Hα absorption features. The constant radial velocity approximation usually remains self‐consistent in decreasing or constant velocity solar wind out to 5 AU, enabling us to separate radial from latitudinal propagation effects. We found several examples of sharp nonmeridional stream boundaries in interplanetary space (∼5° latitude in width), often directly associated with features in coronal X rays and Hα. In one structure there is evidence for significant (up to 40°) nonradial flow of the plasma in the corona below the altitude of transition to super‐Alfvénic flow.
The effect of solar wind structure on magnetospheric energy supply during solar cycle 20
Authors: Holzer Robert E. , Slavin James A.
Published: February 2008

The energy supplied to the magnetosphere by the solar wind has been estimated by computing the incremental energy build up in a simple magnetotail model as a result of magnetic flux transfer from the dayside via reconnection. A method utilizing the auroral AL index as a quantitative measure of the flux transfer process and, hence, the magnetospheric energy supply is presented. In this way the available AL records are examined for the purpose of determining the variation in energy input from the solar wind and its relationship to interplanetary conditions during 9 years of solar cycle 20 (1966–1974). The principal feature discerned on shorter time scales was a large modulation of the input energy with sector structure, while in the annual averages enhancements were observed in 1968, 1973, and 1974 of 19%, 33%, and 60% over the 1966–1972 mean of 3 × 1025 ergs/yr. Interpreting these results as being due to variations in the time history of the dawn‐dusk component of the interplanetary electric field in magnetospheric coordinates, Bz−V, leads to the conclusion that while on shorter intervals the energy supply is dominated by variations in Bz−, associated with both the IMF and the orientation of the terrestrial dipole, on the time scale of years Bz− changes are small compared with the solar cycle variation in V which clearly produced the increase in 1973–1974 as has been concluded in other studies using techniques different from the one employed here.
Energetic ion composition of the plasma sheet
Authors: Peterson W. K. , Sharp R. D. , Shelley E. G. , Johnson R. G. , Balsiger H.
Published: February 2008

Data obtained from the energetic ion mass spectrometer experiment on Isee 1 in the distant plasma sheet are presented. These data show that (1) the plasma sheet has a significant and variable ionospheric component (H+ and O+) representing from 10% to more than 50% of the total number density and (2) there is more than one process responsible for the energization of solar wind plasma (H+ and He++) to plasma sheet energies.
Generation of Pc 5 pulsations by polar electrojet modulation: First experimental evidence
Authors: Stubbe P. , Kopka H.
Published: February 2008

The heating facility at Ramfjordmoen near Tromsø, Norway, has been used to modulate the polar electrojet at frequencies in the Pc 5 range in order to excite micropulsations. The first experimental results indicate that pulsations with amplitudes of the order of 10 γ were generated. Even stronger effects are expected in future experiments.
Theoretical predictions for ion composition in the high‐latitude winter F‐region for solar minimum and low magnetic activity
Authors: Sojka J. J. , Raitt W. J. , Schunk R. W.
Published: February 2008

We combined a simple plasma convection model with an ionospheric‐atmospheric density model in order to study the ion composition in the high‐latitude winter F‐region at solar minimum for low geomagnetic activity. Our numerical study produced time‐dependent, 3‐dimensional, ion density distributions for the ions NO+, O2+, N2+, O+, N+, and He+. We covered the high‐latitude ionosphere above 54°N magnetic latitude and at altitudes between 160 and 800 km for a time period of 1 complete day. From our study we found the following (1) The ion composition exhibits a significant variation with latitude, local time, altitude, and universal time. (2) The variations of the ion composition with latitude and local time are in good agreement with the Atmosphere Explorer measurements both quantitatively and qualitatively. (3) At times and at certain locations the molecular ion density can be comparable to the O+ density at 300 km, and at 200 km the O+ density can be comparable to the molecular ion density. These results have important implications for the interpretation of incoherent scatter radar spectra obtained at high‐latitudes. (4) Different ground‐based observation sites should measure different diurnal variations in ion composition even if these sites are approximately at the same magnetic latitude owing to the UT response of the high‐latitude ionosphere. (5) A satellite in a 300 km circular polar orbit should measure large orbit to orbit variations in both electron density and ion composition, again owing to the UT response of the polar ionosphere. (6) Erroneous conclusions can be drawn about ion density scale heights if the variations along the track of a satellite in a highly elliptical polar orbit are assumed to be only due to altitude variations.
Quantitative simulation of a magnetospheric substorm 1. Model logic and overview
Authors: Harel M. , Wolf R. A. , Reiff P. H. , Spiro R. W. , Burke W. J. , Rich F. J. , Smiddy M.
Published: February 2008

This and the following two papers report results of the first comprehensive computer simulation of the behavior of the earth's inner magnetosphere during a substorm‐type event. Our computer model self‐consistently computes electric fields, currents, and plasma distributions and velocities in the inner‐magnetosphere/ionosphere system (L ≲10); parallel electric fields and ionospheric neutral winds are not included. In this paper, we derive the basic equations of the model, describe the inputs, and present an overview of results. The first appendix presents derivations of general, useful laws of bounce‐averaged gradient, curvature, and
drifts in a plasma with isotropic pitch angle distributions. A second appendix describes the numerical method used in our computer simulation. The succeeding two papers present analyses of model results and comparisons with data. The model was applied to a substorm‐type event that occurred on September 19, 1976. Satellite data (primarily from the Air Force S3‐2 satellite) were used extensively both for boundary conditions and for comparisons with model predictions. Other data were also used as input for our time dependent magnetic field and conductivity models. The S3‐2 data for the event show some novel features, independent of the simulation. Dawn‐dusk electric fields show a general correlation with east‐west magnetic field perturbations. Unexpectedly, two of the passes display substantial regions of sunward plasma flow poleward of the main part of the region 1 Birkeland currents. The cross‐polar cap potential drops computed from the data represent the first effort at satellite monitoring of this important parameter during various phases of a substorm, and show an important enhancement during the substorm. Numerical results from these first‐try simulations are consistent with most of the established features of convection in the inner magnetosphere, such as generally sunward flow, shielding of the potential electric field for L <5, and the tendency for stronger electric fields on the duskside than on the dawnside. In addition, the model reproduces some typical substorm phenomena, such as energy‐dependent particle injection with a dawndusk asymmetry and establishment of a partial ring current.
Energetic magnetosheath ions and the interplanetary magnetic field orientation
Authors: Crooker N. U. , Eastman T. E. , Frank L. A. , Smith E. J. , Russell C. T.
Published: February 2008

Times when energetic ions are absent and present in ISEE 1 magnetosheath plasma spectrograms are correlated with ISEE 3 IMF orientation measurements. The study indicates that when the plasma at the spacecraft is traced along a streamline to the bow shock surface, the angle between the surface normal at that point and the IMF is greater than 60° when the energetic ions are absent and less than 60° when they are present. The pattern is consistent with the ions coming from the same regions of the bow shock where intermediate and diffuse ions are found on the upstream side. The 60° criterion is used to draw schematic patterns of the location of energetic ions in the magnetosheath as a function of IMF orientation. Some orientations result in layers adjacent to the magnetopause and other orientations give layers adjacent to the bow shock.
Fast plasma flows in the translunar magnetotail
Authors: Shull Peter
Published: February 2008

Unusual, high‐speed, antisunward bursts of plasma have been discovered to persistently occur in the translunar magnetotail. Faster, cooler, and less dense than the plasma flows usually found in the translunar magnetotail, these bursts suggest that small‐scale magnetic merging occurs regularly in the plasma sheet. Calculations which assume that the measured particle energy spectra are flowing hydrogen Maxwellians indicate that the bursts have bulk speeds u=250 to 700 km/s, ion temperatures kT=20 to 100 eV, and ion densities n =0.001 to 0.01 cm−3. Most bursts occur within one hour of changes in the tail magnetic field that suggest the passage of the Moon near magnetic bubbles. Bursts do not seem to be fundamentally caused by strong solar or geomagnetic activity. Rather, the bursts seem to result from the transfer of magnetic field energy to mantle plasma in the translunar magnetotail.
Incoherent scattering of radar waves in the auroral ionosphere
Authors: Raman R. S. Venkat , St‐Maurice J. P. , Ong R. S. B.
Published: February 2008

We have systematically studied the effects that non‐Maxwellian ion velocity distributions produce on the ionic part of the spectrum of radar waves incoherently scattered from the disturbed high‐latitude ionosphere in the upper E and lower F regions. For ion to electron temperature ratios smaller than or equal to unity and electric field strengths greater than about 70 mV/m the spectrum is seriously distorted from the shape that it would normally have if the ion velocity distribution were Maxwellian. In events where the magnetospheric convection electric field exceeds 70 mV/m the interpretation of the data is affected if the erroneous assumption of a Maxwellian distribution is used to analyze the data. The electron temperature is the property most seriously affected by the erroneous interpretation, as it can be underestimated by as much as a factor of 2. Finally, the spectra obtained under disturbed conditions are anisotropic, the least distortions from a Maxwellian‐type spectrum being found along the magnetic field direction. The apparent ion temperature seen along the line of sight of the radar as it scans from parallel to the magnetic field to nearly perpendicular to the magnetic field also increases as the angle with the magnetic field increases, even for dc electric field strengths as small as 20 mV/m.
Determination of artificial auroral streak widths using guiding center diffusion theory
Authors: Lin D. L. , Strickland D. J.
Published: February 2008

The theory of two‐dimensional random walk is applied to the problem of guiding center diffusion of kilovolt electrons caused by particle‐particle interactions. The mean radius of the guiding center distribution is shown to depend, in a simple way, on the scattering and energy loss properties of the interactions. A formula for the width of an artificial auroral streak is derived which is consistent with observations and Monte Carlo results.
Spatial aspects of electron energy degradation in atomic oxygen
Authors: Singhal R. P. , Green A. E. S.
Published: February 2008

Spatial (radial and longitudinal) yield spectra for electron energy degradation in atomic oxygen have been obtained using a Monte Carlo method for 25 eV to 10 keV incident electrons. Four‐dimensional yield spectra have been analytically represented in terms of a model containing three simple microplumes. We find that the scaled spatial yield spectra for O is approximately the same as for N2. This feature provides a basis for inferring yield spectra for any atmosphere gas or mixture of gases.
The effects of electron‐neutral collisions on the intensity of plasma lines
Authors: Newman Alice L. , Oran Elaine S.
Published: February 2008

In the earth's ionosphere the presence of nonthermal electrons causes enhancement of the electron plasma line resonance observed by incoherent backscatter radar. This paper extends previous calculations of the plasma intensity by including resonance broadening due to electron‐neutral collisions. A BGK collision term is included in the derivation of the electron spectral density function obtained from fluctuation dissipation theory. Assuming the nonthermal electron distribution is a small perturbation on a Maxwellian distribution of background electrons, the expression describing the signal intensity for a plasma line reduces to the form obtained previously when only electron‐ion collisions were included. Results of this extended model are compared to recent measurements made with the Chatanika radar.
Whistler mode signals: Spectrographic group delays
Authors: Thomson N. R.
Published: February 2008

VLF Doppler frequency shifts and group delays for individual whistler ducts can now be separated and displayed in a multi‐duct environment. These measurements of the simultaneous motions of the individual ducts show that there is quite commonly a significant and readily measurable spatial variation in the azimuthal electric fields in the magnetosphere. The experimental technique, which involves an improvement of the previously reported computer cross‐correlation method, also allows the use of MSK coded VLF transmissions as well as allowing the whistler mode signals to be received from any direction without a null in the direction of the subionospheric signal as before.