bluuebunny/crossref_metadata_2025 huggingface dataset:
Cusp proton signatures and the interplanetary magnetic field
Authors: Reiff Patricia H. , Burch James L. , Spiro Robert W.
Published: February 2008

The variation of proton average energy with latitude in the cusp has been suggested as an indicator of the means of particle entry. If magnetic merging is the principal means of particle entry, the proton average energy should fall with increasing invariant latitude; if diffusion is the principal means of particle entry, the average energy should first fall and then briefly rise as a function of latitude, showing a ‘V’ signature. In addition, the cusp protons should occur on open field lines in the former and closed field lines in the latter case. Four hundred passes of the AE‐D satellite were scanned for appropriate cusp passes during periods when IMP‐J interplanetary or magnetosheath magnetic field data were available. Sixty passes fulfilled all criteria. Of these, roughly a third showed clear or likely merging‐type energy dispersions; a third showed clear or likely V‐type energy dispersions, and a third showed unclear or no energy dispersions. The results are strongly correlated with the IMF: the merging signatures were associated with southward IMF, and the V signatures were associated with northward IMF. Unclear or no dispersion cases were associated with unsteady or weakly northward IMF or with orbits unfavorable to observe the dispersion (perpendicular to the convection direction). If one accepts the electron anisotropy boundary as an (imperfect) indicator of the last closed field line, then the cusp protons are typically on open field lines for southward IMF and on closed field lines for northward IMF. In addition, the presence of ≥3‐keV electrons in the cusp is strongly correlated with V dispersions and therefore with northward IMF.
Solar polar coronal holes and galactic cosmic ray intensities
Authors: Agrawal S. P. , Lanzerotti L. J. , Venkatesan D. , Hansen R. T.
Published: February 2008

We present herein a comparison between the areas of solar polar coronal holes and north‐south (N‐S) cosmic ray asymmetries (as measured by polar region neutron monitors). The observed N‐S asymmetries over a 2‐year period (1973–1974) are found to be related to asymmetries in the sizes of selected solar polar holes; the hemisphere with the smaller hole has a larger cosmic ray flux. An analysis is also made of the N‐S asymmetries for intervals when there exists a relatively symmetric coronal region (±40°) about the solar equator; the N‐S asymmetry tends to be small for this situation of comparable‐sized polar coronal holes. We discuss various mechanisms which can contribute to a N‐S asymmetry. The radial gradient drift effect (▽n × B) alone probably cannot explain a N‐S asymmetry. The importance of the role of coronal holes in cosmic ray modulation processes as demonstrated in this work is stressed.
An empirical model of the Venusian outer environment 1. The shape of the dayside solar wind‐atmosphere interface
Authors: Vaisberg O. L. , Intriligator D. S. , Smirnov V. N.
Published: February 2008

Solar wind plasma and magnetic field data and ionospheric data obtained from the Pioneer Venus orbiter are considered. It is shown that the variation of the magnetic field pressure within the magnetic barrier is similar to that expected for the solar wind pressure variations along an obstacle's boundary when a more realistic approximation of the shape of the ionosphere is included. Simultaneous solar wind pressure, ionospheric pressure, and magnetic barrier pressure data show that the ionospheric pressure below the ionopause is approximately equal to the solar wind pressure. The magnetic barrier pressure is equal to approximately 2/3 to 3/4 of both the solar wind pressure and the ionopsheric pressure. Estimates of the hot plasma pressure contribution to the total pressure within the magnetic barrier vary from 1/4 to 1/3. Just below the ionopause the ionospheric pressure deviations from the mean ionospheric pressure are significant especially below ∼400 km and appear to be indicative of the adjustment of the ionospheric structure to changing solar wind conditions. A first‐order model of the ionopause pressure variations as a function of height and solar‐zenith angle is suggested. This model provides an estimate of an ‘instant’ ionopause profile for the given solar wind conditions. The mean and ‘instant’ shapes of the ionopause do not appear to correspond to the estimates obtained from ionospheric pressure equilibrium for a given H/r0.
Observation of the Venus mantle, the boundary region between solar wind and ionosphere
Authors: Spenner K. , Knudsen W. C. , Miller K. L. , Novak V. , Russell C. T. , Elphic R. C.
Published: February 2008

For three orbit paths of the Pioneer Venus orbiter the interaction between the solar wind and the Venusian ionosphere has been studied. Results of the retarding potential analyzer and the magnetometer are described for the boundary region between the solar wind and the planetary ionosphere. These are the first measurements that show that a transition region exists between the two plasmas of different origin. The observed magnetic field and current system producing it appear strong enough to stop the solar wind flow in front of the ionosphere and to separate the shocked solar wind from the ionosphere. The transition region between the ionosheath and the ionosphere is called the ‘mantle.’ The observed mantle electron energy spectra close to the ionopause show ionospheric character. With increasing height the number of electrons that have ionospheric energies decreases, and the number of electrons that have solar wind energies gradually increases toward the ionosheath boundary, where only solar wind energy spectra are observed. The mantle surrounds the frontside of the ionosphere and extends probably more than eight Venus radii downstream.
The distribution of ion beams and conics below 8000 km
Authors: Gorney D. J. , Clarke A. , Croley D. , Fennell J. , Luhmann J. , Mizera P.
Published: February 2008

Measurements from electrostatic analyzers aboard the polar‐orbiting S3‐3 satellite have been tabulated to form a synoptic picture of the occurrence of upgoing 90eV to 3.9keV auroral ions. In this survey, a distinction is made between ion distributions having peak fluxes along B (beams) and those exhibiting flux maxima that are not field‐aligned (conics). It is shown that both beams and conics are common auroral phenomena, whose frequencies of occurrence in latitude, local time, and altitude have a marked dependence on magnetic activity. During quiet conditions (Kp ≲3) conical ion distributions are observed with constant frequency in altitude above 1000 km and appear to be associated with the daytime polar cusp region. In contrast, quiet time ion beams have a maximum occurrence frequency in the premidnight sector. Ion beams are observed primarily above 5000 km, with a frequency increasing with altitude up to the satellite apogee at 8000 km. During disturbed times, ion beams are a persistent phenomenon, mainly confined to the dusk sector, while conical distributions are observed uniformly in local time with a frequency that increases steadily in altitude. The results of this study, which are shown to be consistent with previous surveys of upward flowing ions if no distinction is made between conical and field‐aligned distributions, provide information relating to auroral ion acceleration processes.
F region east‐west drifts at Jicamarca
Authors: Fejer B. G. , Farley D. T. , Gonzales C. A. , Woodman R. F. , Calderon C.
Published: February 2008

F region east‐west drifts have been measured at Jicamarca for almost 10 years, using incoherent scatter. The drifts are westward during the day and eastward at night. The daytime drift velocities are about 50 m/s and change very little with season or solar cycle. The evening reversal occurs at about 1600 local time throughout the solar cycle. The maximum nighttime eastward drifts are about 105 and 130 m/s during solar minimum and maximum, respectively. The daytime and nighttime drifts show very litle variation with magnetic activity. These Jicamarca incoherent scatter results (especially the reversal times) differ appreciably from results obtained using other techniques, but there appear to be fairly simple explanations for the apparent disagreements.
Leakage of magnetospheric ions into the magnetosheath along reconnected field lines at the dayside magnetopause
Authors: Scholer M. , Ipavich F. M. , Gloeckler G. , Hovestadt D. , Klecker B.
Published: February 2008

Strong evidence is presented for escape of magnetospheric particles along reconnected field lines into the magnetosheath, using observations of ∼30‐ to ∼120‐keV/charge protons and alpha particles made by the Max‐Planck‐Institut/University of Maryland instrument on Isee 1. During three magnetopause crossings, which have been identified from tangential stress balance analysis as reconnection events, the magnetospheric particle distribution extends well into the magnetosheath, and the particles in the magnetosheath show a strong anisotropy along the magnetic field. The proton to alpha particle ratio in this layer as well as in distinct bursts within the magnetosheath is the same as this ratio within the magnetosphere (at equal energy per charge). We conclude that the most likely explanation for these observations is that magnetospheric particles are escaping along reconnected field lines into the magnetosheath. It is argued that magnetospheric particles are seen in the magnetosheath up to the reconnection separatrix, and the magnetosheath bursts are interpreted as multiple encounters of this magnetosheath layer by the satellite due to boundary motions.
A new ULF‐modulated electrostatic wave detected in the extremely low frequency range onboard Geos
Authors: Cornilleau‐Wehrlin Nicole
Published: February 2008

A new type of magnetospheric electrostatic wave, discovered onboard the GEOS 1 satellite, is described. These waves have a wide spectrum, extending between the lower hybrid frequency and the ion plasma frequency. Their main characteristic is their modulation in amplitude at a ULF frequency which is of the order of the helium gyrofrequency. The corresponding electromagnetic ULF waves are detected simultaneously in all cases (26) but one. These ULF‐associated electrostatic waves (10 <E <200 µVm−1 Hz−1/2) are observed in the dayside region of the outer magnetosphere (L >6) at all geomagnetic latitudes sampled by GEOS 1 (0° <λm <+30°). At least two factors seem to control the existence of these waves: the ULF wave power has to be stronger than 0.02 γ² Hz−1, and the cold plasma density has to be lower than 25 cm−3. Some possible mechanisms for explaining their generation are discussed.
The O II(7320‐7330Å) airglow: A morphological study
Authors: Yee J. H. , Abreu V. J. , Hays P. B.
Published: February 2008

A statistical study of the 7320‐30‐Å (O+(²P) ‐ O+(²D)) airglow was conducted by analyzing the data taken from the visible airglow experiment (VAE) on the Atmosphere Explorer (AE) satellites C and E during the time periods between 1974 and 1979. Averaged column emission rate profiles as a function of solar zenith angle and solar activity variation are presented here. The galactic background has been carefully subtracted. The result shows that the rate of decreasing emission as a function of solar zenith angle agrees with the theoretical calculation based upon a neutral atmosphere model and the solar spectrum as measured by the EUV spectrometer on the Atmosphere Explorer satellite. Furthermore, an expected increase with solar activity also appeared in a plot of emission brightness versus solar 10.7‐cm flux.
A comparison of the radio data and model calculations of Jupiter's synchrotron radiation, 1. The high energy electron distribution in Jupiter's inner magnetosphere
Authors: de Pater Imke
Published: February 2008

An electron distribution in Jupiter's magnetosphere as a function of energy, pitch angle and spatial coordinates is derived from a comparison of model calculations of the planet's synchrotron radiation with the radio data; the distribution is consistent with the information available from the Pioneer data. The model calculations are based upon the magnetic field configurations as derived by Acuna and Ness [1976a, b] (the O4 model) and Davis, Jones and Smith [quoted in Smith and Gulkis, 1979] (the P11 (3,2) A model) from the Pioneer data. The electrons are assumed to diffuse radially inwards with a rate D=3 × 10−9 L³ s−1 (where L is McIlwain's value) and have a lifetime against local losses τ=4 × 106 L−0.6 s, in agreement with the values derived by Goertz et al. [1979] from the Pioneer data. The number density of electrons with E ≥20 MeV at L=3 is in agreement with the values measured by the spacecraft, but the energy distribution appears to be much flatter. Electrons are absorbed with survival probabilities of ∼0.5 diffusing inwardly past Amalthea's sweeping region (for 20‐MeV electrons) and ∼0.35 past the ring, respectively. An energy dependent pitch angle scattering is found to occur at Amalthea's orbit, and there is no pitch angle scattering present near the ring.
Whistler mode wave propagation in the solar wind near the bow shock
Authors: Greenstadt E. W. , Fredricks R. W. , Russell C. T. , Scarf F. L. , Anderson R. R. , Gurnett D. A.
Published: February 2008

The presence of whistler mode waves in, and upstream from, the bow shock has been well established by observation. Theoretical descriptions of the mode under solar wind conditions have been relatively meagre, however, and it may not be generally appreciated how readily whistler waves generated in the shock could occupy most of the region ahead of the shock most of the time. Graphic descriptions of phase and group velocities and group velocity directions for typical solar wind parameters are presented by using the cold plasma approximation over all appropriate frequencies and directions with respect to the IMF. The relations of whistler phase and group velocities to observations of a quasi‐perpendicular shock crossing by ISEE are illustrated.
Charged particle behavior in low‐frequency geomagnetic pulsations 1. Transverse waves
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

The behavior of charged particles in low‐frequency geomagnetic pulsations is examined with particular emphasis on what a spacecraft‐borne detector would observe. We concentrate on the effects of purely transverse electromagnetic signals. The time scale of a particle's motion relative to the wave period is shown to determine the nature of its response. For low‐energy particles, the acceleration in the last gyroperiod before detection is what matters. At higher energies, what has occurred over recent bounce and drift motions becomes increasingly important and convection of gradients by the wave E × B drift must be considered. Distinguishing features such as phase differences between signals in back‐to‐back detectors or between channels of different energy are catalogued. In particular, we assess the detectability of resonance effects in the light of detector characteristics and finite signal bandwidth. Recent observations are used to illustrate the ideas developed.
The electromagnetic ion beam instability upstream of the Earth's bow shock
Authors: Gary S. Peter , Gosling J. T. , Forslund D. W.
Published: February 2008

The linear theory of the electromagnetic ion beam instability for arbitrary angles of propagation has been studied. The parameters considered in the theory are typical of the solar wind upstream of the earth's bow shock when a ‘reflected’ proton beam is present. Maximum growth occurs for propagation parallel to the ambient magnetic field B, but this instability also displays significant growth at wavevectors oblique to B. Oblique, unstable modes seem to be the likely source of the compressive magnetic fluctuations recently observed in conjunction with the ‘diffuse’ ion population. An energetic ion beam does not directly give rise to linear growth of either ion acoustic or whistler mode instabilities.
Some characteristics of spread F at the magnetic equatorial station Fortaleza
Authors: Abdu M. A. , Batista I. S. , Bittencourt J. A.
Published: February 2008

Spread F in ionograms of both the range and frequency spread types is studied for the magnetic equatorial station Fortaleza (geographic coordinates 38°W, 4°S; dip latitude −1.7°) for a 1‐year period during 1978–1979). Satellite traces suggesting ionization structures with scale sizes of 100–200 km are found to be necessary precursors to the formation of the irregularities that give rise to range type spread F, whereas frequency type spread F was observed under different circumstances. Striking differences are noted in the spread F characteristics and related parameters over Fortaleza as compared with Huancayo (geographic coordinates 75.3°W, 12.05°S; dip latitude 0.6°) and Jicamarca (geographic coordinates 76.8°W, 11.95°S; dip latitude 0.9°), situated within only ∼38° in longitude west of Fortaleza. The postsunset maximum in h′F, in winter, is delayed by about 1½ hr with respect to other seasons over Fortaleza, as compared with Huancayo, where it occurs earlier in winter than in other seasons. Similar differences also exist in the prereversal peak near sunset in the vertical drift velocity (Vz) between Fortaleza and Jicamarca. The differences are attributed possibly to the large difference in the magnetic field declination and to other factors such as the difference in the magnetic field intensity and the relative separation between geographic and magnetic equators that exist between Fortaleza and the other stations.
The effect of the charge exchange source on the velocity and ‘temperature’ distributions and their anisotropies in the Earth's exosphere
Authors: Hodges R. R. , Rohrbaugh R. P. , Tinsley B. A.
Published: February 2008

The velocity distribution of atomic hydrogen in the earth's exosphere has been calculated as a function of altitude and direction using a Monte Carlo simulation which includes the classical exobase source and the higher‐altitude plasmaspheric charge exchange source. The charge exchange ‘hot’ hydrogen atom source produces the satellite component and ‘hot’ ballistic and escape components. The velocity distribution functions F(υ) have been plotted as log F(υ) against υ². For a Maxwellian velocity distribution the slope would be constant with velocity υ and inversely proportional to the defined temperature. For the classical exobase source alone the slope is constant only for the upward radial velocity component, and it increases dramatically as the altitude increases above one earth radius for both the incoming radial and the two transverse velocity components. This ‘temperature decrease’ is the same effect as indicated in the analytical work of Chamberlain (1963), due to the absence of the incoming‐hyperbolic and flyby components of the particle distributions. The effect of charge exchange is to enhance the wings of the velocity distributions, especially for the upward‐radial component. Charge exchange does not, however, succeed in overcoming the ‘temperature decreases’ in the incoming‐radial and transverse directions at altitudes above one earth radius. The gradients of the ‘temperatures’ and their anisotropies are great enough to engender some lack of confidence in the results of radiative transfer treatments for Lyman α, which have been carried out so far on the basis of isothermal exospheric hydrogen models.
Generation of ion‐conic distribution by upgoing ionospheric electrons
Authors: Dusenbery P. B. , Lyons L. R.
Published: February 2008

Downward currents in auroral regions are commonly measured with amplitudes of 1–5 µ A/m². Such currents are likely the result of upgoing thermal ionospheric electrons falling through a field‐aligned potential drop on the order of their thermal energy. Similar distributions of upgoing ionospheric electrons may also occur in regions of diffuse auroral electron precipitation to preserve current continuity in the presence of the loss of the precipitating electrons to the ionosphere. The drift velocity of the upgoing electrons is sufficient to excite electrostatic ion‐cyclotron waves. In addition to being in Landau resonance with the upgoing electrons, these waves cyclotron resonate with the upgoing thermal ions with a parallel energy of several eV. Calculated quasi‐linear diffusion rates using measured wave spectra indicate the resonant ions can be heated to perpendicular energies on the order of 100 times the initial ion thermal energy. Comparison with auroral particle observations, both at low altitude (∼1500 km) and higher altitude (∼6000 km) shows that ion‐conic distributions can readily be explained by such quasi‐linear diffusion. The results imply that conic distributions should not occur simultaneously with the keV electron precipitation associated with discrete arcs in regions of upward current but should occur in regions of upgoing ionospheric electrons which may include regions near the edges of auroral arcs and regions of diffuse auroral electron precipitation.
The magnetic field of Jupiter: A generalized inverse approach
Authors: Connerney J. E. P.
Published: February 2008

The estimation of planetary magnetic fields from observations of the magnetic field gathered along a spacecraft flyby trajectory is examined with the aid of generalized inverse techniques, with application to the internal magnetic field of Jupiter. Model non‐uniqueness resulting from the limited spatial extent of the observations and noise on the data is explored and quantitative estimates of the model parameter resolution are found. The presence of a substantial magnetic field of external origin due to the currents flowing in the Jovian magnetodisc is found to be an important source of error in estimates of the internal Jovian field, and new models explicitly incorporating these currents are proposed. New internal field models are derived using the vector helium magnetometer observations and the high field fluxgate observations of Pioneer 11, and knowledge of the external current system gained from the Pioneer 10 and Voyagers 1 and 2 encounters.
Modeling the Jovian current sheet and inner magnetosphere
Authors: Connerney J. E. P. , Acuña M. H. , Ness N. F.
Published: February 2008

Voyager 1 and 2 magnetic field observations confirm and extend the earlier Pioneer 10 detection of the Jovian magnetodisc, a region of enhanced charged particles and plasma and reduced magnetic field intensity located near the magnetic equatorial plane. Modeling of the azimuthal current sheet by a finite thickness annulus of inner radius 5 RJ, 5‐RJ thickness, and extending to ∼50 RJ provides detailed fits of the vector magnetic field perturbations observed in relation to the planetary field for distances less than 30 RJ. Field line geometry is also investigated, and better insight into the phenomena of charged particle absorption by the Galilean satellites is obtained which provides improved explanations of observed effects due to Ganymede.
The magnetic‐anomaly model of the Jovian magnetosphere: A post‐Voyager assessment
Authors: Vasyliunas V. M. , Dessler A. J.
Published: February 2008

We reexamine the three predictions that we previously put forth as tests for the magnetic‐anomaly model (in which the anomalously weak magnetic field region in the northern hemisphere of Jupiter influences the outer Jovian magnetosphere by one or more plasma interaction processes), taking into account the Voyager and other recent observations. Concerning the prediction of a restricted longitude range of enhanced interaction between Io and Jupiter's ionosphere, the longitudinal asymmetries seen in ground‐based observations of sulfur emissions from the Io torus as well as in Voyager observations of Jovian auroral emissions agree with the predicted asymmetries. The lowered estimates of the Alfvén speed in the Io torus as a result of Voyager composition determinations modify many earlier ideas on the magnetosphere/Io interaction, but the conclusion remains that longitudinal asymmetries with a 360° period can be plausibly explained only by reference to magnetic‐anomaly effects. Concerning the predictions of plasma, energetic particle, and magnetic field periodicities in the outer magnetosphere being related to either a current sheet or a magnetic‐anomaly effect, we conclude that the hitherto common interpretation of Voyager observations in terms of a current sheet with no longitudinal asymmetries is seriously deficient because a bending of the current sheet from 40 to 60 RJ outward is required whereas the Pioneer 10 observations demand that there be no such bending; attempts to reconcile Voyager and Pioneer by invoking time variations or solar wind effects are inconsistent with the observed constancy of propagation speeds for configurational changes and with the observed ratio of magnetic pressure to solar wind pressure. We construct a simple quantitative model with a sinusoidal longitudinal variation of inverse speed and show that the inclusion of such a variation of propagation speed (as implied by the magnetic‐anomaly model) into the description of the current sheet removes these inconsistencies and allows a unified interpretation of both Voyager and Pioneer observations. Concerning the prediction of a longitudinally restricted distribution of magnetopause crossings, the evidence for a clustering of magnetopause crossings near the active sector remains suggestive but statistically inconclusive. These results, combined with a number of other observed phenomena that cannot be accounted for by a pure magnetodisc model with no longitudinal asymmetries, provide evidence that magnetic‐anomaly effects play a significant role in determining the behavior of the Jovian magnetosphere.
Transient nonlinear pitch angle scattering of energetic electrons by coherent VLF wave packets in the magnetosphere
Authors: Bell T. F. , Inan U. S.
Published: February 2008

With the aid of adiabatic theory, a study is made of the transient nonlinear pitch angle scattering experienced by energetic electrons under the influence of coherent VLF wave packets in the magnetosphere. Finite wave packets of both fixed and variable frequency are considered, as are wide ranges of L shell, wave frequency, and wave amplitude. Our results indicate that large mean pitch angle changes can be induced in the portion of the energetic population that undergoes a nonlinear cyclotron resonance interaction with the wave packet. Two classes of nonlinearly interacting particles are involved, the nonlinearly resonant particles trapped in the wave and the nonlinearly resonant nontrapped particles. It is shown that the mean pitch angle change of these two classes of particles are of opposite sign, but that the mean pitch angle change of the ensemble can still be significant. In the case of fixed‐frequency wave packets on magnetic shells near L=4, trapped particles typically experience pitch angle changes of 5° or more for moderate wave amplitudes (∼10 mγ). In the case of variable frequency wave packets, similar changes can be produced with much lower wave amplitudes (∼2 mγ). We conclude that the utilization of transient nonlinear pitch angle scattering effects offers interesting options for energetic particle control during ground‐ or space‐based VLF wave‐injection experiments.
Diffusion of small‐scale density irregularities during equatorial spread F
Authors: Huba J. D. , Ossakow S. L.
Published: February 2008

The diffusion of small‐scale density irregularities (i.e., those with density gradient scale lengths less than several hundred meters) is investigated during the decay phase of equatorial spread F. Both classical and anomalous diffusion processes are considered. The anomalous diffusion coefficient is based upon the transport properties associated with the universal drift instability. It is found that anomalous diffusion can smooth out small‐scale density irregularities on time scales τ, consistent with observational results, i.e., few minutes is ≲τ≲ few hours. On the other hand, classical diffusion is much too slow a process to be important. Also, anomalous diffusion is unable to diffuse large‐scale irregularities, i.e., those with scale lengths greater than a kilometer, on time scales τ≲ few hours and another mechanism must occur to smooth out these irregularities, e.g., shorting out to the E region.
DMSP/F2 electron observations of equatorward auroral boundaries and their relationship to the solar wind velocity and the north‐south component of the interplanetary magnetic field
Authors: Hardy D. A. , Burke W. J. , Gussenhoven M. S. , Heinemann N. , Holeman E.
Published: February 2008

Approximately 2500 equatorward boundaries of auroral electron precipitation were determined for times when hourly averages of the interplanetary magnetic field and solar wind velocity V were available. The equatorward boundaries were determined from data returned by the SSJ/3 electron detector on board the DMSP/F2 satellite in magnetic local sectors between 0400 and 1100 hours on the morningside of the oval and between 1500 and 2300 hours on the eveningside of the oval. The boundary data were separated into 1‐hour zones in magnetic local time. Within each zone the boundary locations were studied as functions of Bz and Bz², VBz and VBz². Significant results were obtained when the boundaries were correlated with hourly average of Bz and VBz for the hour preceding the one in which the boundary was measured and when the linear regression was performed separately for data points where Bz ≤ 1 nT and Bz > 1 nT. For points where Bz ≤ 1 nT, the correlation with Bz and VBz gave slopes generally between 0.8° and 1.2°/nT and between 1.5° and 2.5°/mV/m, respectively, with correlation coefficients of ∼0.7. The latitudes of the intercepts tend to decrease with increasing local time in the evening sector and with decreasing local time in the morning sector. In the range above 1 nT, the slope of the best fit line for correlation with Bz, and VBz changes sign. Slopes fall in the range −0.1° to −0.4°/nT and 0.21° to 1°/mV/m. Correlation coefficients are typically worse than −0.4. Correlations of the boundary with Bz² and VBz² for both ranges of Bz are uniformly worse than those for Bz and VBz. The trend in slopes and intercepts with magnetic local time for Bz ≤ 1 nT is found to be similar to that previously found for the correlation of the boundaries with Kp (Gussenhoven et al., 1981). In this range of Bz, Kp and VBz are found to be related by the equation Kp=2.01–0.91 VBz (mV/m). From the work of Ejiri et al. (1978) an equation relating the magnetospheric electric potential to VBz is derived. The results are in agreement with the general features of the half‐wave rectifier model of the magnetosphere and measurements of polar cap potential.
Acceleration of electrons by interplanetary shocks
Authors: Potter Douglas W.
Published: February 2008

Although proton acceleration often is observed at interplanetary shocks, electron acceleration rarely has been reported. In contrast, many of the shocks identified so far by instruments on the ISEE 3 spacecraft show significant increases in the spin averaged electron flux at energies greater than 2 keV. At 2 keV, the spin averaged electron flux usually increased by a factor of 2 to 3 but on several occasions increased to more than 10 times its preshock level. Often, the behavior of the electrons is similar to higher energy (several MeV) proton spikes: the electrons are field aligned before the shock and are highly anisotropic with the flux maximum perpendicular to the magnetic field at the shock and immediately after the shock. This is the signature of the shock drift model of acceleration where the gradient in the magnetic field causes the particles to move in the direction of lower potential energy in the convection electric field. Given that a primary assumption of the shock drift model is that the gyroradius is much larger than the shock thickness, and that this assumption is invalid for electrons, it is initially surprising that electrons and ions can act similarly at shocks. A closer look shows that by conserving the first adiabatic invariant, electrons obey the same expressions for reflected and transmitted particles as ions.
The small‐scale structure of electrostatic shocks
Authors: Temerin M. , Cattell C. , Lysak R. , Hudson M. , Torbert R. B. , Mozer F. S. , Sharp R. D. , Kintner P. M.
Published: February 2008

Small‐scale regions of large electric fields have been observed above the auroral zone by the S3‐3 satellite. The data from five such electrostatic shocks have been examined in as great a detail as is available. The three higher altitude shocks (all above 5700‐km altitude) were associated with upward‐going ion beams, indicating that the potential associated with the shock closed below the satellite to give rise to the parallel electric field required for the acceleration of the ion beam. Electrostatic ion cyclotron waves were found in all these cases adjacent to the shock and extending throughout the upward‐going ion beam region. The lack of noticeable Doppler shift in the electrostatic ion cyclotron waves in association with large convective drift velocities indicates that the wavelength of the electrostatic ion cyclotron wave can be several kilometers and the potential difference within the wave can be the order of 100 V. Regions having anisotropic electron distributions with the signature of parallel electric field acceleration close to but above the satellite were found on the other side of the shock from the ion beam region in two of these examples. Both these regions were associated with V shaped VLF hiss. In contrast, lower altitude shocks were associated with VLF saucers and conical ion distributions. The association of the lower altitude shocks with VLF saucers indicates that the low‐altitude shock region is a permanent feature on the scale of tens of seconds and that the low‐altitude shock region extends along the magnetic field. Since they occur in different current regions, the low‐altitude shocks described here are not merely the low altitude extension of the kind of higher altitude shocks associated with ion beams. Electrostatic shocks are always embedded in broader regions of low frequency turbulence. The shock itself is a complex low‐frequency structure that, if interpreted as a Doppler shift effect, implies a complicated small scale spatial structure. The important theoretical question of whether the parallel electric field inside the electrostatic shock exists over a large region of space parallel to the magnetic field or is confined to a narrow double layer or similar structure is not answered. The data are consistent with both possibilities.
Magnetic field line reconnection experiments, 4. Resistivity, heating, and energy flow
Authors: Stenzel R. L. , Gekelman W. , Wild N.
Published: February 2008

Detailed spatial and temporal measurements of the total vector electric field E and current density J in a plasma with dynamic magnetic field line reconnection have been made. The resistivity calculated via the generalized Ohm's law is found to be spatially inhomogeneous with values exceeding the classical resistivity by 1 to 2 orders of magnitude. Resistivity and current density do not maximize in the same locations. The dissipation E · J is determined and analyzed in terms of particle heating and fluid acceleration. Electron heating is found to be the dominant dissipation process in the diffusion region. Independent measurements of the divergence of the Poynting vector ▽ · (E × H), the change in stored magnetic field energy ∂/∂t(B²/2µo), and the dissipation give a consistent, detailed picture of the energy flow. Efficient conversion of electromagnetic energy into particle heating is observed.
Io's hot plasma torus—A synoptic view from Voyager
Authors: Sandel B. R. , Broadfoot A. L.
Published: February 2008

A study of the morphology of Io's hot plasma torus has encompassed hundreds of Voyager UVS measurements of torus intensity. The long‐term average state of the torus can be characterized by an axial asymmetry in the brightness of the prominent S III 685‐Å feature manifested as an enhancement in brightness whose peak is fixed near 1900 local time. No long‐term correlation of brightness with magnetic longitude is present. On time scales of a few Jupiter rotations, the torus can differ markedly from its average axial asymmetry, and its brightness is correlated with magnetic longitude for short times.
The injection of energy into the Io plasma torus
Authors: Shemansky D. E. , Sandel B. R.
Published: February 2008

Voyager EUV observations of the Io plasma torus showing a Jupiter local time asymmetry have been interpreted as a 10‐hour periodicity in electron temperature in the corotating reference frame. The plasma shows two remarkable morphological characteristics. First, the intensity periodicity showed no tendency to change over the ≃0.5‐year period of reduced data, indicating it to be a permanent feature of the torus. Second, deviations from the mean behavior, such as short‐term magnetic longitude effects, all appear to be caused by electron temperature effects. Thus no local plasma mass variations have been detected during the observational period. These characteristics and the variability of the energy source required to drive the observed asymmetry lead to the conclusion that the dominant mechanism of energy transport to the plasma is electron‐electron heating. If the parameters applied in coming to this conclusion are correct, the role played by mass loading from Io and planetary rotation in delivering energy to the plasma becomes much more obscure than the straightforward electron‐ion transfer mechanism that has been considered to date.
Titan's ion exosphere observed from Voyager 1
Authors: Hartle R. E. , Sittler E. C. , Ogilvie K. W. , Scudder J. D. , Lazarus A. J. , Atreya S. K.
Published: February 2008

Electron and ion measurements made by the Voyager 1 plasma science instrument revealed a plasma wake surrounding Titan in Saturn's rotating magnetosphere. This wake is characterized by a plasma that is more dense and cooler than the surrounding subsonic magnetospheric plasma. The density enhancement is produced by the deflection of magnetospheric plasma around Titan and the addition of exospheric ions picked up by the rotating magnetosphere. By using simple models for ion pickup in the ion exosphere outside Titan's magnetic tail and ion flow within the boundaries of the tail, the interaction between Saturn's rotating magnetosphere and Titan is shown to resemble the interaction between the solar wind and Venus. Outside the magnetic tail of Titan, pickup of H+ formed by ionization of the H exosphere is indicated when synthetic and observed ion spectra are matched. Close to the boundary of the tail, a reduction in plasma flow speed is found, providing evidence for mass loading by the addition of N2+/H2CN+ and N+ to the flowing plasma. The boundary of the tail is indicated by a sharp reduction in the flux of high‐energy electrons, which are removed by inelastic scattering with the atmosphere and centrifugal drift produced when the electrons traverse the magnetic field draped around Titan. Within the tail the plasma is structured as the result of spatial and/or temporal variations. The ion mass cannot be determined uniquely in the tail; however, one measurement suggests the presence of a heavy ion with a mass of order 28 amu: One candidate is H2CN+, suggested as the dominant topside ion of the ionosphere, which may flow from the ionosphere into the tail.
A dispersion formula for analyzing ‘modal interference’ Among guided and free Gravity wave modes and other phenomena in a realistic atmosphere
Authors: Tuan T. F. , Tadic D.
Published: February 2008

Gravity waves are known to propagate in the continuous as well as the partially and fully guided modes. Through the development of a dispersion formula we will show that these three modes can ‘interfere’ among themselves and that one important consequence of such ‘interference’ is that the guidance of a large number of partially guided modes can be destroyed. The simple formula for the effective attenuation distance, −1/Im kxn for each complex partially guided mode kxn (where kx is the horizontal wave vector), is no longer valid when there is heavy interference or ‘modal overlap.’ A simple criterion will be developed to distinguish those modes which remain observable from those which exist physically only as part of the general background. We will also show that the presence of an explicit gravity wave source can only selectively or simultaneously excite those modes which are observable and that the other modes remain as part of the background, unaffected by the source. The dispersion formula also shows that a discrete mode should be described by two parameters. One is the usual modal position kxn, while the other is the ‘weight’ which varies with different modes. There is also the possible existence of yet another form of guided mode which we will call the virtually guided mode and which possesses characteristics quite different from the other guided modes.
Field‐aligned ion streams in the Earth's midnight region
Authors: Olsen Richard Christopher
Published: February 2008

Plasma measurements from the University of California at San Diego auroral particles experiment on the geosynchronous Applied Technology Satellite 6 in the midnight region show that the low‐energy ion fluxes (1–100 eV) are field aligned and are well characterized as thermal populations (1–10 eV) with a streaming velocity of 30–100 km/s along the magnetic field line. The lowest energies are found prior to injections, on quiet days, with an increase of the streaming velocity evident when an injection occurs near the satellite. Multiple peaks in the ion distribution functions are attributed to the presence of different ion species (H+, He+, O+, O++) streaming at similar velocities, both during quiet times and as the plasma velocity increases in response to an injection.
The possible production of lower hybrid parametric instabilities by VLF ground transmitters and by natural emissions
Authors: Riggin D. , Kelley M. C.
Published: February 2008

A parametric instability at the lower hybrid frequency which is known as the ion quasi‐mode instability may be excited in the ionosphere. The instability considered is a three‐wave interaction in which an incident whistler mode wave near the lower hybrid frequency decays into a lower hybrid wave and an ion acoustic type of oscillation. Threshold calculations are made at intervals along the L = 4 magnetic field line starting at the earth's surface (the geomagnetic position of Siple Station, Antarctica) and proceeding to the equatorial plane. A model is developed for the ionospheric plasma parameters needed to evaluate the expression for threshold field strength. The fields directly radiated by the Siple transmitter are not strong enough to meet the requirement for threshold. However, it is possible that the threshold can be met for Siple transmissions which are amplified by natural processes and also for natural emissions.
The role of the big flare syndrome in correlations of solar energetic proton fluxes and associated microwave burst parameters
Authors: Kahler S. W.
Published: February 2008

In the previous studies correlating E > 10 MeV proton fluxes and spectra with various associated microwave burst parameters, the resulting high correlations were assumed to reflect a common acceleration process for the protons and the microwave‐emitting electrons. We suggest and test an alternative explanation for these correlations, which we term the big flare syndrome (BFS), that states that, statistically, energetic flare phenomena are more intense in larger flares, regardless of the detailed physics. Peak 1–8 Å X ray fluxes, characteristic of the thermal flare, are correlated with peak proton fluxes to derive correlation coefficients characteristic of the BFS. Of all microwave parameters tested for the 1973–1979 period, only the time‐integrated flux densities at 8800 and 15,400 MHz may be significantly larger than expected from the BFS. We fail to confirm previous results associating peak proton spectra with peak microwave spectral characteristics, thus finding no evidence that peak microwave fluxes are indicative of proton acceleration. We extend this conclusion to peak hard X ray correlations. The strongly nonlinear relationship deduced between flare energy and proton production also appears invalid.
The gravito‐electrodynamics of charged dust in planetary magnetospheres
Authors: Mendis D. A. , Houpis Harry L. F. , Hill Jay Roderick
Published: February 2008

The dynamics of small electrically charged dust grains within the rigidly corotating regions of planetary magnetospheres such as those of Jupiter and Saturn is considered. Depending on whether one is inside or outside the synchronous orbit, it is possible to have different populations of both positively and negatively charged particles moving in equilibrium circular orbits either in the prograde or retrograde sense. Not all these are stable, however, to small perturbations, such as would be produced by the gravitational tug of a neighboring satellite. The stable perturbed grains will perform a motion that can be described as an elliptical gyration about a guiding center which is in uniform circular motion. For different values of the specific charge, the ratio of the semiaxes of this ‘epicyclic’ ellipse lies between 1/2 and 1, while the gyration frequency ω of the grain about the guiding center lies between the Kepler frequency ΩK and ω0 (which is the grain gyrofrequency in a nonrotating frame). In the environments of Jupiter and Saturn, where the grains are expected to be negatively charged both in the sunlit side and in the shadow and which move in the prograde sense, their guiding centers must have speeds intermediate to the Kepler speed and the corotation speed. Such particles with a unique specific charge (and therefore a specific size) could have a 1:1 magneto‐gravitational resonance with a neighboring satellite. A dispersion relation between ω and the wavelength λ of the perturbed orbits in the frame of the perturbed satellite has been derived. This result has been used to discuss the appearance and disappearance of the waves in the F ring of Saturn elsewhere. We merely point out here that, while the existence of a single well‐defined wavelength implies a dust size distribution sharply peaked at a diameter of about 1 µ, the present theory also anticipates this situation. The only collisionless (and therefore nonevolving) state of small electrically charged dust grains moving in the same orbit is when they have precisely the same specific charge and therefore the same size (assuming the same density), since the electrical potential is the same for all sizes.
Statistical study of precipitating electrons
Authors: Fontheim E. G. , Stasiewicz K. , Chandler M. O. , Ong R. S. B. , Gombosi E. , Hoffman R. A.
Published: February 2008

Energy spectra of precipitating electrons are fitted to the sum of three distributions: a power law, a Maxwellian, and a Gaussian. This fitting procedure determines seven parameters that characterize the essential features of each spectrum. These characteristic parameters are used to carry out various studies involving precipitating electrons. It is shown that the absence of the power law population from a particular spectrum is related to the softness of the precipitating primary flux, that the Maxwellian temperature and the Gaussian peak energy have a positive correlation, the strength of which varies with local time, that the upward moving Gaussian population has a loss cone distribution, and that the one‐dimensional velocity distribution parallel to the magnetic field occasionally displays a plateau or even a hump on the tail.