bluuebunny/crossref_metadata_2025 huggingface dataset:
A dynamic model for the auroral field line plasma in the presence of field‐aligned current
Authors: Mitchell H. G. , Palmadesso P. J.
Published: February 2008

We have developed a dynamic numerical model of the plasma along an auroral field line in order to provide a vehicle for studying ionosphere‐magnetosphere coupling processes. The model is a multimoment, multifluid approximation of a gyrotropic plasma consisting of three species (electrons, hydrogen ions, oxygen ions) along a segment of auroral magnetic field line extending from an altitude of 800 km to 10 earth radii. We have performed simulations for the case of a current‐free polar wind equilibrium of the field line plasma and the case in which a large upward field‐aligned current is applied to the field line. In the former case, the agreement between our model and previous static results is reasonable given the differing boundary conditions inherent in the two cases. In the case of a field‐aligned current, we note that the flux tube plasma responds to the current on several time scales. After an initial rapid heating of the electrons due to precipitation in a converging magnetic field, electric field coupling of the electrons to the ions causes thermal oscillations of the flux tube plasma to persist on time scales of the order of 1 hour, illustrating the complicated nature of the response of a collisionless plasma when heat flow transport is treated in a dynamic manner.
Characterization of geostationary particle signatures based on the ‘Injection Boundary’ Model
Authors: Mauk B. H. , Meng C.‐I.
Published: February 2008

To lend further support to the ‘injection boundary’ concept, this paper characterizes the details of geostationary particle signatures using a very simple‐minded analysis procedure. The signatures are generated using the time of flight effects which evolve from an initial sharply defined, double‐spiraled boundary configuration. By using only the most fundamental characteristics of standard convection configurations, the very complex and highly variable dispersion patterns frequently observed by geostationary satellites are successfully reproduced. In particular, seven distinctly different ion‐electron paired dispersion patterns on energy versus time spectrograms (1 eV to 100 KeV) are predicted, and all seven of these are observed on a regular basis by both the SCATHA satellite (in the near geostationary orbit) and the ATS‐6 satellite. Many of the details of the patterns have not been previously presented. It is concluded that most dynamical dispersion features (including energetic ion and electron echoes) can be mapped to the double‐spiral boundary without further ad hoc assumptions. It is shown further that the predicted and observed dispersion patterns have symmetries which are distinct from the symmetries generally associated with the quasistationary particle convection patterns.
Generation of lower hybrid waves by inhomogeneous electron streams
Authors: Keskinen M. J. , Huba J. D.
Published: February 2008

A linear, electrostatic, kinetic theory of velocity‐sheared inhomogeneous electron streams flowing parallel to a magnetic field is presented. Our development, which includes the effects of a density gradient, background plasma, and electron collisions, is valid for arbitrary Tes/Ti and Teb/Ti, where Tes, Teb, and Ti are the temperatures of the sheared electrons, background electrons, and background ions, respectively. For hot electron streams Tes/Ti ≫ 1 and Tes ≫ Teb, we find a new resonant instability driven by velocity shear with frequency near the lower hybrid frequency. This mode is characterized by wave numbers k such that krLes ≲ 1, where rLes is the sheared electron gyroradius and is damped by collisional effects. Application is made to wave turbulence in the auroral zone and solar corona.
Three‐dimensional current flow and particle precipitation in a westward travelling surge (observed during the Barium‐Geos Rocket Experiment)
Authors: Opgenoorth Hermann J. , Pellinen Risto J. , Baumjohann Wolfgang , Nielsen Erling , Marklund Göran , Eliasson Lars
Published: February 2008

During the Barium‐GEOS rocket experiment on September 24, 1979 the passage of a westward travelling surge (WTS) was observed over Scandinavia. Extended instrument networks in that area, viz., the Scandinavian Magnetometer Array, the STARE radars, all‐sky cameras, and riometers, allowed simultaneous observation of the two‐dimensional character of magnetic and electric field disturbances and particle precipitation associated with the auroral feature. By combination of the different datasets and additional information from particle and electric field measurements aboard the Barium‐GEOS rocket it was possible to derive the two‐dimensional distribution of ionospheric electric fields and conductivities and to model the three‐dimensional current flow in the vicinity of the westward travelling surge. The main feature of the resulting model current system is the presence of a localized upward field‐aligned current directed out of the head of the surge and fed by a westward electrojet, which is composed of both Hall and Pedersen currents. Secondary ionospheric currents, e.g. a counterclockwise loop of mainly Hall currents around the leading edge of the surge, are found to be responsible for most of the transient effects observed by ground‐based magnetometers as the WTS passed overhead. The most energetic particle precipitation as inferred from cosmic noise absorption measurements and triangulation of auroral arc altitudes is found to be confined to the leading part and central regions of the surge and to travel westward with the visual auroral form.
The plasmaspheric electric field as measured by ISEE 1
Authors: Maynard N. C. , Aggson T. L. , Heppner J. P.
Published: February 2008

Data from the double cylindrical probe electric field instrument on ISEE 1 has been analyzed to determine the average characteristics of the plasmaspheric electric field from L of 2–6. One year's data has been sorted and averaged covering all local times. The unaveraged electric field is highly variable, especially as magnetic activity increases. Typical magnitudes in the equatorial plane are between 0.2 and 0.8 mV/m in a frame corotating with the earth. During intense activity the electric fields become larger occasionally exceeding 2 mV/m. The average quiet electric fields are qualitatively in agreement with previous whistler and ionospheric radar measurements and also with representative ionospheric dynamo models. Under moderate and disturbed conditions the magnetospheric convection electric field penetrates inside the plasmasphere in the late evening and early morning sectors. A region of significant flow toward the prenoon magnetopause was detected postnoon, extending around toward dusk. This may be a substorm related effect. The electric field in the prenoon quadrant was small under most all conditions. Conversion of the electric field to flow velocities in the inertial frame shows that the plasma is moving in the direction of corotation at all local times in the inner plasmasphere. In the evening sector the flow velocity is reduced, and in the morning sector it is enhanced as convection penetrates the outer L shells. A mixed behavior is seen in the afternoon hours.
Preferential perpendicular acceleration of heavy ionospheric ions by interactions with electrostatic hydrogen cyclotron waves
Authors: Singh Nagendra , Schunk R. W. , Sojka J. J.
Published: February 2008

The energization of heavy ionospheric ions by electrostatic hydrogen cyclotron (EHC) waves is examined. In particular, the problem of preferential perpendicular acceleration of heavy ions above 5000 km along auroral field lines is examined critically. Observations have indicated that above this altitude the occurrences of ion beams and EHC waves are highly correlated and that the occurrence rates are high. The EHC waves are found to be coherent. The energization process is examined in the light of interactions of a single coherent wave with the ions. The interactions can be stochastic or nonstochastic, depending on the parameters of the wave and of the background H+ ions supporting the wave. With electric field wave amplitudes of a few tens of mV/m, it is shown that the interaction can give perpendicular energies of the order of keV for O+ ions. He+ ions are shown to gain perpendicular energies smaller than O+ ions. It is indicated that upward flowing H+ ions cannot gain any appreciable amount of perpendicular energy. For a given electric field wave amplitude Eo, the extent of energization critically depends on the average perpendicular energy (temperature) W⊥B of the background H+ ion plasma. Examples are worked out for values of W⊥B ranging from 1 to 200 eV. The resulting energizations and pitch angles for O+ and He+ ions are compared with observations from satellites. The heating times for the ions are found to be of the order of 2 s at an altitude of 5000 km.
A theory of coherent radar spectra in the auroral E region
Authors: St.‐Maurice J.‐P. , Schlegel Kristian
Published: February 2008

We have developed a theory of auroral radar spectra for conditions under which Farley‐Buneman waves should be strongly excited in the high‐latitude E region. The theory assumes that the unstable waves spend the larger part of their lifetime in a linear growth phase. It also uses a previously unexplored property of the waves, namely, that waves with different wave vector components along the magnetic field have markedly different phase velocities. We have constructed synthetic auroral radar spectra for 1‐m size irregularities. We have calculated the width of the spectra, their mean Doppler shift, and anisotropy. We have studied how the properties of spectra produced by Farley‐Buneman waves are affected by the presence of ion drifts and neutral winds, by the electron temperatures and densities, by the neutral atmospheric profile, and by the strength of the dc electric fields. The results seem to be in good qualitative agreement with existing observations.
Correlated irregular magnetic pulsations and optical emissions observed at Siple Station, Antarctica
Authors: Engebretson M. J. , Cahill L. J. , Arnoldy R. L. , Mende S. B. , Rosenberg T. J.
Published: February 2008

Pi 1 magnetic pulsations observed at Siple Station, Antarctica, contain an unusual class of fluctuations. They are large amplitude, 0.2–1 nT/s, one‐sided pulsations in dB/dt with variable direction. The pulsations are from 2 to 10 s wide and occur in an irregular train of pulses that lasts, sporadically, up to an hour. An earlier study (Arnoldy et al., 1982) reported that in 1979 data these pulses were often closely associated with riometer and auroral pulsations. The present report extends this work by using 1980 data and describes the variety in observation of these pulsations. The pulsations are observed from 2200 local time until noon with most frequent occurrence near dawn. They usually occur during or shortly after a burst of high amplitude pulsation activity within a long duration (several hours) interval of large or moderate amplitude Pi. Although the direction of the pulsations is typically west or southwest other directions are also observed, with occasionally a shift of direction (for example, from north to south) occurring in a few minutes. We interpret these Pi 1 pulsations to be the result of local enhancements in ionospheric and field‐aligned currents associated with precipitating electrons responsible for the associated pulsations in auroral luminosity. No evidence was found for a sinusoidal component of these magnetic pulsations, travelling as a wave in the magnetosphere.
On the generation and growth of equatorial backscatter plumes: 2. Structuring of the west walls of upwellings
Authors: Tsunoda Roland T.
Published: February 2008

The west wall of large‐scale ‘upwellings’ that develop in the bottomside of the nighttime equatorial F layer becomes structured by the wind‐driven gradient drift instability, the same process that leads to the formation of striations in barium ion clouds. Upwellings are initiated by wavelike perturbations with long spatial wavelengths (∼400‐km) and are amplified by the collisional Rayleigh‐Taylor instability (and sometimes assisted by the gradient drift instability in the case of an upward moving F layer). The west wall structuring process is driven by an eastward neutral wind enhanced by reduced drag during the postsunset hours and a velocity shear in east‐west bulk plasma drift in the bottomside F layer. West wall structures evolve in a manner analogous to primary plasma bubbles, i.e., secondary plasma bubbles grow from the west wall. Their characteristics are compared with those of the primary bubbles and discussed in the light of existing theories.
Ionospheric and Birkeland current distributions inferred from the MAGSAT magnetometer data
Authors: Zanetti L. J. , Baumjohann W. , Potemra T. A.
Published: February 2008

The MAGSAT vector magnetometer data, with the aid of an accurate magnetic field model, have been used to infer ionospheric and field‐aligned sheet current density distributions. The MAGSAT spacecraft is uniquely suited for this study because it provides, for the first time from a satellite, simultaneous observations of effects due to both ionospheric and Birkeland current systems. The method emphasized here consists of a Fourier transform technique developed for use on surface magnetometer data and recently used by Baumjohann et al. (1979). Data acquired by MAGSAT on two days in 1979 were analyzed in detail. The first was December 21, 1979, a reasonably quiet (Σ Kp = 8−) time, when data acquired in the sunlit hemisphere (with presumably uniform conductivity) were analyzed. The second example deals with a rather active time, November 13, 1979, (Σ Kp = 35+) in the northern hemisphere, with more complicated conductivity and current distributions. By comparing Hall current densities inferred from the MAGSAT data and those inferred from simultaneously recorded ground‐based data acquired by the Scandinavian magnetometer array, it is found during two other events that the former are underestimated due to the considerably high damping of magnetic variations with high spatial wave numbers between the ionosphere and the MAGSAT orbit. Important results of this study include the fact that the Birkeland and electrojet current systems are colocated. In addition, more realistic ionospheric current distributions have been developed. The analyses have shown a tendency for triangular rather than constant electrojet current distributions as a function of latitude, which is also consistent with the statistical, uniform region 1 and region 2 Birkeland current patterns.
Ray‐tracing studies and path‐integrated gains of ELF unducted whistler mode waves in the Earth's magnetosphere
Authors: Huang C. Y. , Goertz C. K.
Published: February 2008

Gyroresonance and Landau resonance interactions between unducted low‐frequency whistler waves and trapped electrons in the earth's plasmasphere have been studied. Ray paths for waves launched near the plasmapause have been traced. In agreement with recent findings by Thorne et al. (1979), we have found waves which return through the equatorial zone with field‐aligned wave normal angles. However, when the growth along the ray path is calculated for such waves, assuming an electron distribution function of the form E−n sinm α, we find that for all the waves considered, the local growth rate becomes negative before plasmapause reflection, limiting the total gain to small values. Most waves reach zero gain before reflection. This is the result of Landau damping at oblique propagation angles, which necessarily occurs before reflection can take place. We conclude that the concept of cyclic ray paths does not provide an explanation for the generation of unguided plasmaspheric hiss.
Observational evidence of Z and L‐O mode waves as the origin of auroral kilometric radiation from the Jikiken (EXOS‐B) satellite
Authors: Oya Hiroshi , Morioka Akira
Published: February 2008

The Jikiken satellite, which had an initial perigee and apogee of 250 km and 30,050 km, respectively, and an orbit inclination of −31°, passed through critical regions where auroral kilometric radiation (AKR) spectra were obtained on at least five occasions during a period from January 31, 1979, to June 21, 1980. On all of these occasions the AKR spectra revealed cutoff phenomena at the local Z cutoff frequency, indicating a continuation of the spectra from the high‐frequency side down to the Z mode wave frequency range and crossing over the local X cutoff frequency rather than being cut off at that frequency. Thus when observations are made near the low‐latitude portion of the source region, the AKR waves generate spectra that continuously cover the frequency range corresponding to Z mode and L‐O mode waves. We argue that the most plausible mechanism that gives a consistent interpretation of this spectral characteristic is the mode conversion theory. In other words, the plasma waves that are generated in the hybrid mode in the source regions are converted into Z mode waves that propagate toward dense plasma regions where the wave frequency coincides with the local plasma frequency and where a part of their energy is transferred to waves of the L‐O mode that can escape toward outer space. By using the propagation characteristics of Z mode waves, this conversion mechanism also gives a self‐consistent interpretation of previously presented evidence that was reported as the cutoff of AKR near the local electron cyclotron frequency. Although there is a possible ambiguity in the location of the source region, there is no conflict between the conversion mechanism for AKR generation and previous polarization observations carried out by Voyager spacecraft.
Type IPDP magnetic pulsations and the development of their sources
Authors: Pikkarainen T. , Kangas J. , Kiselev B. , Maltseva N. , Rakhmatulin R. , Solovjev S.
Published: February 2008

Intervals of pulsations of diminishing periods (IPDP) magnetic pulsation data have been analyzed from several recording campaigns and from stations that are widely spaced in both meridional and azimuthal directions. On the basis of their local time occurrence it seems useful to divide the events into at least two main categories: 1400 < LT < 1800 and 1800 < LT < 2400. Those events which are only observed at auroral latitudes typically develop slowly and extend at the most to 1 Hz, whereas those events that are observed at a low‐latitude station may extend to several Hz. Simultaneous observations at several meridians extending over 100° show that the IPDP source region moves from east to west at a speed of 2–5° min−1. There are necessarily no remarkable changes in frequency‐time characteristics of IPDP pulsations. In the radial direction, IPDP wave frequency seems to be related to the He+ gyrofrequency in the equatorial plane. Observations are shown to be in general agreement with the evolution of the ion cyclotron instability in the plasma convection system of the magnetosphere. Several mechanisms seem to contribute simultaneously to the frequency shift of pulsations during IPDP events.
Electron energy deposition in the middle atmosphere
Authors: Vampola A. L. , Gorney D. J.
Published: February 2008

Spectra of locally precipitating 36‐ to 317‐keV electrons obtained by instrumentation on the S3‐2 satellite are used to calculate energy deposition profiles as a function of latitude, longitude, and altitude. In the 70‐ to 90‐km altitude, mid‐latitude ionization due to these precipitating energetic electrons can be comparable to that due to direct solar H Lyman α. At night, the electrons produce ionization more than an order of magnitude greater than that expected from scattered H Lyman α. Maximum precipitation rates in the region of the South Atlantic Anomaly are of the order of 10−2 erg/cm² s with a spectrum of the form j(E) = 1.34 × 105 E−2.27 (keV). Southern hemisphere precipitation dominates that in the north for 1.1 < L < 6 except for regions of low local surface field in the northern hemisphere. Above L = 6, local time effects dominate; i.e., longitudinal effects due to the asymmetric magnetic field which are strong features below L = 6 disappear and are replaced by high‐latitude precipitation events which are local time features.
Production of auroral zone E region irregularities by powerful HF heating
Authors: Hibberd F. H. , Nielsen E. , Stubbe P. , Kopka H. , Rietveld M. T.
Published: February 2008

Production of ionospheric 1‐m irregularities in the auroral zone E region by high power ionospheric heating has been observed by means of the STARE auroral radar. Irregularities were produced at night, on eight occasions by using an O mode heating wave and on two occasions by using an X mode heating wave. The growth time of the irregularities is found to be approximately 30 s. Several physical possibilities to explain these findings are discussed, and a mechanism is suggested that is closely related to Sato's mechanism of quiet auroral arc formation.
Self‐consistent theory of three‐dimensional convection in the geomagnetic tail
Authors: Birn J. , Schindler K.
Published: February 2008

The self‐consistent theory of time‐dependent convection in the earth's magnetotail of Schindler and Birn (1982) is extended to three dimensions to include more realistic tail geometry and three‐dimensional flow. We confirm that a steady state solution implies unrealistic tail geometry or large particle or energy losses that are unrealistic during quiet times and conclude therefore that as in the 2‐dimensional case the magnetotail becomes time‐dependent for typical convection electric fields. Explicit solutions are derived, even analytically, for the three‐dimensional flow and the electric and magnetic field in a realistic tail geometry, and quantitative examples are presented. Consequences of time‐dependent convection are demonstrated considering two idealized cases of magnetosphere response to solar wind changes: (1) uniform compression as the likely consequence of increasing (static, dynamic or magnetic) solar wind pressure; and (2) compression only in the z direction perpendicular to the plasma sheet as the probable consequence of a dawn to dusk external electric field (Ey > 0), corresponding to a southward interplanetary magnetic field component (Bz < 0). It is found that in case 1 the stability properties do not change very much in time, while case 2 leads to an evolution toward instability, consistent with the observed correlation of Bz < 0 or Ey > 0 with geomagnetic activity. Several other features, already present in the 2‐dimensional theory, are confirmed.
In situ measurements of transionospheric VLF wave injection
Authors: Kintner P. M. , Brittain R. , Kelley M. C. , Carpenter D. L. , Rycroft M. J.
Published: February 2008

VLF waves at 3.85 kHz from the Siple, Antarctica, transmitter were continuously measured by electric and magnetic receivers on a sounding rocket as the waves propagated through the neutral atmosphere and into the ionosphere. The change from linear to circular polarization was clearly observed along with severe attenuation of the VLF signal in the D region. Interferometric measurements established the index of refraction and provided an independent calibration of the rocket‐borne receivers. An upper bound on the VLF power radiated is estimated to be 1.7×10³ W during a period when the antenna was driven at 115×10³ W. The power estimated to be entering the ionosphere is between 3 W and 12 W and depends on assumptions concerning the geometrical distribution of wave power.
Spatial relationship of field‐aligned currents, electron precipitation, and plasma convection in the auroral oval
Authors: Coley W. R.
Published: February 2008

A technique has been developed that allows the analysis of magnetometer data from Atmosphere Explorer C during those periods when the spacecraft is spinning and eclipsed (not sunlit). The utilization of these magnetometer measurements to determine field‐aligned current structure in conjunction with energetic particle and plasma convection measurements from AE‐C enables us to determine relationships between these phenomena in the nighttime auroral zone. From a study of some 18 auroral oval crossings in the sectors 1800–2200 MLT and 0100–0600 MLT it is observed that, in general, the regions of energetic electron precipitation (>1 keV) span both region 1 and region 2 currents with the poleward boundary of the region 1 currents approximately coincident with the poleward kilovolt electron boundary. Simultaneous observations of plasma convection indicate that the reversal from sunward to antisunward convection is also, in general, collocated to within a degree of invariant latitude to the poleward field‐aligned current boundary in both the evening and morning sectors. It is seen that the low conductivity in the winter polar cap implies the absence of field‐aligned currents in that region. The data indicate that the region 1 currents in the late evening and early morning sectors lie on closed field lines extending earthward from the inner edge of the magnetospheric boundary layer.
Observational evidence on the origin of ions upstream of the Earth's bow shock
Authors: Thomsen M. F. , Schwartz S. J. , Gosling J. T.
Published: February 2008

The energies predicted by four different source hypotheses for ions observed upstream of the earth's bow shock are compared with observations of upstream field‐aligned beams and “gyrating ion” events. A kinematic formalism in a frame of reference in which the motional electric field vanishes is used. Specular reflection of a fraction of the incident solar wind is found to be the most credible explanation of gyrating ion events observed upstream of shocks ranging from quasi‐parallel to nearly perpendicular. The recent hypothesis that field‐aligned beams are the result of leakage from the magnetosheath of ions which were originally specularly reflected at quasi‐perpendicular portions of the shock provides good agreement with observed energies of many field‐aligned beams. Of the several source hypotheses examined, only magnetic moment conserving reflection of solar wind ions is capable of accounting for two very energetic beam events. Although the magnetic moment of an ion should not be conserved during a single encounter with the shock, it is possible that multiple encounters of ions which are originally specularly reflected at the shock can produce an upstream distribution of particles which appear to have conserved their magnetic moment.
Numerical simulations of counterstreaming plasmas and their relevance to interhemispheric flows
Authors: Singh Nagendra , Schunk R. W.
Published: February 2008

The collisionless expansion of counterstreaming plasmas has been studied by solving the self‐consistent set of Vlasov and Poisson equations in one dimension. The motivation for the study is to elucidate some of the basic physical processes which may occur during the initial refilling of depleted flux tubes after a magnetic storm. The simulation geometry consisted of two high‐density H+‐O+‐electron plasmas (conjugate ionospheres) separated by a low density H+‐electron plasma (equatorial plasmasphere). The temporal evolution of the expanding plasmas and the electrostatic potential in the region between the two sources has the following characteristics. The initially minor H+ions rapidly flow out of the source regions, creating counterstreaming density shock fronts which propagate at the Sagdeev Mach number for ion acoustic shocks (M ∼1.6). However, the shocks are preceded by suprathermal forerunner ions, which are the first to fill the “equatorial” region. When the counterstreaming ion acoustic shocks collide, the density in the equatorial region becomes nearly a constant, twice the value of the density in the individual shocks. The electrostatic potential distribution from the source plasmas to the midpoint of the expansion region displays an interesting feature. A potential hill forms near the midpoint after the arrival of the main density shock fronts. This localized potential hill plays an important role in the thermalization of the ion streams and may occur in the equatorial plasmasphere after magnetic storms. The numerical simulations indicate that the ion beams in the counterstreaming plasmas are remarkably stable with respect to the ion acoustic instability, which is in agreement with the linear instability theory. However, in the presence of a magnetic field, the linear instability theory predicts that the counterstreaming suprathermal forerunner ions can excite ion cyclotron waves, which in turn can thermalize and trap the suprathermal forerunners. Such a mechanism may be operating in the equatorial plasmasphere shortly after magnetic storms.
Characteristics of low‐energy plasma in the plasmasphere and plasma trough
Authors: Reasoner D. L. , Craven P. D. , Chappell C. R.
Published: February 2008

Data from the light ion mass spectrometer (LIMS) on the SCATHA satellite is used to study the low‐energy (<100 eV) plasma populations at a near‐geosynchronous orbit. The characteristics of the plasma in the noon to midnight sector during quiet to moderately active times are emphasized. The observed plasma populations tended to group into three distinct types. These were a warm trapped distribution, a warm field‐aligned distribution, and a distribution with kTi < 1 eV. The cold plasma was seen after 1800 LT during periods of magnetic quiet and was observed to be flowing sunward during active periods. The cold plasma is identified as encounters with the plasmasphere, while the warm plasma populations are assumed to lie outside of the plasmasphere in the plasma trough. The presence of these warm plasma populations may be responsible for the difference in the shape of the bulge‐region plasmasphere deduced from whistler measurements (Carpenter, 1966) and from in situ measurements made on OGO 5 (Chappell et al., 1970). The effects of the spacecraft potential upon the low‐energy plasma measurements is studied. A threshold effect may be operative whereby when the cold plasma density decreases to less than about 2 ions/cm³, the spacecraft potential rises to a positive value such that cold plasma fluxes are effectively excluded from detection.
Energetic particle microsignatures of Saturn's satellites
Authors: Carbary J. F. , Krimigis S. M. , Ip W.‐H.
Published: February 2008

The low‐energy charged particle (LECP) experiment observed satellite microsignatures in ions (28 keV–4 MeV) and electrons (22 keV–20 MeV) during the Voyager encounters with Saturn in 1980 and 1981. Each of the five major satellites within the orbit of Titan had associated with it at least one absorption feature in the high time resolution (400 ms) data. Generally, microsignatures involve abrupt rate decreases of factors up to 10 or more, lasting on time scales less than 2 min. Occasionally, substructure within the microsignature is observed on time scales of 5–10 s at a single pitch angle. Pitch angle distributions and spectral behavior of low‐energy ions (28–500 keV) imply that particles of low energy with pitch angles near 90° are absorbed most efficiently. Microsignatures are usually observed in the corotational wake region within ∼25° in azimuth of a satellite. The peculiar microsignature observed in the vicinity of Enceladus may suggest clumping of E ring material similar to clumping known to exist in the F ring. A microsignature in both ions and electrons (∼450 km wide) observed at the expected crossing of Mimas' L shell outbound exhibits characteristics that are not fully consistent with it being due to Mimas. The microsignature could be interpreted as due to material co‐orbiting with Mimas ∼147° ahead of the satellite. We use the microsignature of Dione to deduce the diffusion coefficient of ∼25 to ∼100 keV electrons and find D ≃ 10−8 Rs²/s at L ≃ 6.3, i.e., much faster (∼10³) than diffusion rates estimated at the orbit of Mimas (L ≃ 3) for MeV electrons (Van Allen et al., 1980c). Microsignatures do not generally occur at times predicted by a centered, aligned dipole; a better predictive model seems to be that of an aligned, axisymmetric field with significant quadrupole and octupole terms.
Dynamic spectra of Saturn kilometric radiation
Authors: Genova F. , Pedersen B. M. , Lecacheux A.
Published: February 2008

Dynamic spectra of Saturn kilometric radiation (SKR) obtained by the Voyager planetary radio astronomy experiment have been searched for variations at a time scale longer than that of the arcs (about 1 hour). Two kinds of episodic changes of the shape of the dynamic spectrum can be distinguished: one is an overall long‐term change, lasting for one or several Saturn rotations; the other is a modification of the dynamic spectrum from one rotation to another, limited to the sub‐solar longitude range from about 270° through 0° to about 60°. Both correspond to a change in the shape of the intensity spectrum of the emission. The long‐term variation probably contains the so‐called ‘Dione gaps’, together with similar and sometimes much longer events. The already discovered influence of strong solar wind activity corresponds to increases of the average intensity level of the emission, but not to changes in the shape of its intensity spectrum. Both kinds of episodic spectral variations are usually observed in magnetically quiet conditions, in particular when Saturn enters Jupiter's magnetic tail. These changes of the SKR spectrum are compared with those already observed in the terrestrial kilometric radiation. They could be due to complex propagation effects, temporary modifications of the emission beam or changes in the source altitude.
Upstream gyrophase bunched ions: A mechanism for creation at the bow shook and the growth of velocity space structure through gyrophase mixing
Authors: Gurgiolo C. , Parks G. K. , Mauk B. H.
Published: February 2008

The conditions necessary for the production of gyrophase bunched ions at the bow shock are developed. The conditions are applied to the reflection mechanism presented by Paschmann et al. (1980), showing that when in their model a portion of the incident parallel velocity is converted into reflected perpendicular velocity, the reflected particles are gyrophase bunched. The growth of velocity space structure in the gyrophase bunched distribution through gyrophase mixing is also explored. The structure is found to be similar to that reported in diffuse and dispersed ion events. This together with the close correlation of the observation of gyrophase bunched ions with diffuse and dispersed ions has led us to speculate that these two populations may be closely related.
Distribution of MHD wave activity in the foreshock region and properties of backstreaming protons
Authors: Bavassano‐Cattaneo M. B. , Bonifazi C. , Dobrowolny M. , Moreno G. , Russell C. T.
Published: February 2008

Magnetic field and plasma data obtained by ISEE 2 experiments in the earth's foreshock region have been used, in conjunction with a shock model, to correlate with geometrical parameters the intensity of MHD turbulence and some moments of the distributions of back‐streaming protons. The geometrical parameters are the distance of the observation point from the proton foreshock boundary and the distance from the shock surface. The results obtained allow a preliminary discussion of two basic problem of the physics of the foreshock. These are (1) the relative role of shock structure and/or wave particle interaction in determining the form of the backstreaming ion distributions and (2) the physical mechanisms determining the observed saturation of the MHD turbulence.
Differences in the temporal variations of solar UV flux, 10.7‐cm solar radio flux, sunspot number, and Ca‐K plage data caused by solar rotation and active region evolution
Authors: Donnelly R. F. , Heath D. F. , Lean J. L. , Rottman G. J.
Published: February 2008

Two types of temporal variations in the solar UV spectral irradiance, caused by solar rotation and active region evolution, are presented and discussed. These particular UV variations differ markedly from the concurrent variations in the 10.7‐cm radio flux and sunspot number. The temporal variations of the modeled UV flux based on Ca‐K plage data are similar to the observed UV flux. The first type of dissimilar temporal behavior occurs when concentrations of solar active regions evolve at solar longitudes nearly 180° apart. Both the UV observations and modeled UV fluxes based on Ca‐K plage data then show strong 13‐day periodicity, while the 10.7‐cm solar radio flux and sunspot number exhibit quite dissimilar temporal variations. This type of dissimilarity is related to the modeled UV flux, having a dependence on the solar central meridian distance that is narrower than that for the 10.7‐cm radio flux or for sunspot numbers. A second case of marked dissimilarity occurs when major new solar active regions arise and dominate the full‐disk fluxes for several rotations. The strongest peaks in 10.7 cm and sunspot numbers tend to occur on their first rotation, for example, during major dips in the total solar irradiance, while the Ca‐K plages and UV enhancements peak on the next rotation and then decay more slowly on subsequent rotations. This type of dissimilarity is related to major active regions having a more rapid growth, peak and decay of sunspots, their strong magnetic fields and related coronal radio emission at centimeter wavelengths than for the Ca‐K plages and their related UV enhancements.
Multiple spacecraft observations of interplanetary shocks: ISEE three‐dimensional plasma measurements
Authors: Russell C. T. , Gosling J. T. , Zwickl R. D. , Smith E. J.
Published: February 2008

ISEE 1 and ISEE 3 three‐dimensional solar wind plasma measurements are used together with magnetic field measurements across five previously studied interplanetary shocks to test the accuracy of the mixed‐mode shock‐normal determination technique and to test whether the shock properties are best approximated with a ratio of specific heats of 5/3 or 2. In the shocks examined, the assumption that the velocity jump was along the normal provided an estimate of the shock normal within 15° of our best fit normal 50% of the time and within 50°, 90% of the time. The mixed‐mode normals lay within 12° of our best fit normal 50% of the time and within 36°, 90% of the time. Part of this deviation may be due to differences in the orientation of the local normal from that of the average normal. Finally, the jump in plasma and field across the shock is better predicted from the Rankine‐Hugoniot equations using a ratio of specific heats (γ) of 5/3 rather than 2.
Comparison of the characteristics of potential drop and current‐driven double layers
Authors: Singh Nagendra , Schunk R. W.
Published: February 2008

The characteristics of double layers driven by an applied potential drop and by an injected current into a plasma are compared. In the latter case the potential drop across the double layer appears because of the formation of a virtual cathode. The double layers formed by the two mechanisms show striking similarities with regard to their structure, temporal evolution, and dynamics. However, in the case of current‐driven double layers a large energization of ions trapped in the virtual cathode region is observed. Such a large energization of trapped ions is not seen in the case of the potential‐drop driven double layers. The interrelation between the field‐aligned currents and potential drops for the auroral plasma is discussed. For current‐driven double layers, it is found that the current density Je α фo1/2, where фo is the potential drop.
Polar cap electron densities from DE 1 plasma wave observations
Authors: Persoon Ann M. , Gurnett Donald A. , Shawhan Stanley D.
Published: February 2008

Electric field spectrum measurements from the plasma wave instrument on the Dynamics Explorer 1 spacecraft are used to study the local electron density at high altitudes in the northern polar cap region. The electron density is determined from the upper cutoff of whistler mode radiation at the electron plasma frequency. Median density values over the polar cap at L greater than 10 are found to vary from 35.2±8.5 cm−3 at 2.1 RE to 0.99±0.51 cm−3 at 4.66 RE. The steady state radial outflow model is examined for consistency with the observed density profile. A power law fit to the radial variation of the electron density yields an exponent of −3.85±0.32, which for the radial outflow model implies a flow velocity increasing nearly linearly with increasing radial distance. Comparison of the observed electron densities with theoretical polar wind densities yields consistent results up to 2.8 RE. A comparison of the observed electron densities with low‐altitude density profiles from the Alouette II and ISIS 1 spacecraft illustrate transitions in the slope of the profile at 1.16 RE and between 1.55 and 2.0 RE. The changes in the density profile suggest that changes occur in the basic radial transport processes at these altitudes.
Equatorial anomaly in ionospheric electron content and its relation to dynamo currents
Authors: Balan N. , Iyer K. N.
Published: February 2008

Using the ATS 6 satellite radio beacon data collected over a chain of stations in the Indian zone, a study has been made of the equatorial anomaly in total electron content (TEC) and its relation to the E region currents. Two parameters, ΔHd, the difference in the magnetic field perturbation between the equator and a low‐latitude station, and Vd, the backscatter radar measurement of the draft velocity of the electrojet irregularities, are used to represent the equatorial current strength. The results, presented in the form of regression plots relating the strength of the anomaly and that of the current, indicate that the anomaly dependence on the current strength is about the same for all the three seasons. It is also noted that, regardless of the season, the anomaly crest moves toward a farther latitude from the equator with increasing current strength. The total ionization inventory, Σ TEC, of the anomaly belt is found to increase linearly with the strength of the equatorial current and this result seems to bear some significance for the correlation observed between TECmax and ΔHdmax at the equator.
Lagrangian least‐squares prediction of solar flux
Authors: Holland Robert L. , Vaughan William W.
Published: February 2008

The result of a study on the application of an improved statistical prediction method for estimating the intermediate‐term (months) and long‐term (years) behavior of solar flux
is discussed. The study indicates that better predictions, in a chi square sense, are possible by selecting sets of the solar flux data such that each set (cycle) starts and ends at the maxima (or minima) for the data base and initialization point of the procedure. Then one applies a Lagrangian least‐squares statistical technique. Evidence is also presented to support the existence of an aperiodic variation in the periods as well as the amplitudes.
Magnetic field orientations in Saturn's upper ionosphere inferred from Voyager radio occultations
Authors: Hinson David P.
Published: February 2008

Dual‐wavelength radio observations during the occultations of Voyagers 1 and 2 provided 3.6‐cm and 13‐cm data on Saturn's ionospheric irregularities at two locations. Spectra of the weak fluctuations in amplitude and phase can be interpreted using the theory for scattering from a power law spectrum of anisotropic irregularities embedded in a thin screen. Comparisons of measured scintillation spectra with the theory produced least squares solutions for four parameters which characterize the ionospheric irregularities: (1) the axial ratio, (2) the orientation, (3) the power law exponent of the spatial spectrum, and (4) the magnitude of the spatial variations in electron density. Scattering was observed from irregularities of approximate size 2–60 km; these lower and upper limits result from noise limitations and the method of analysis, respectively, and do not represent either an inner or outer scale. Within this range of length scales, the three‐dimensional spatial spectrum obeys an inverse power law with exponent near 3.5, and the fractional variations in electron density are about 10–20%. All observed irregularities appear to be anisotropic with axial ratios greater than 4∶1. Magnetic field orientations in Saturn's ionosphere were inferred from the direction of alignment of the anisotropic irregularities. When used as a test of Saturnian magnetic field models, the occultation measurements generally confirm the accuracy of the model predictions but also suggest that a small adjustment of model parameters might improve the field representation.
Whistler mode turbulence generated by electron beams in Earth's bow shock
Authors: Tokar R. L. , Gurnett D. A. , Feldman W. C.
Published: February 2008

In this study, the Landau and cyclotron growth rates of whistler mode waves in earth's bow shock are calculated by using electron distribution functions obtained with the fast plasma experiment on ISEE 2. Three electron distribution functions measured within the transition region of the shock are analyzed. An important feature of these electron distribution functions is the presence of a field‐aligned beam with a T⊥ > T∥ anisotropy. The beam velocity vector is directed toward the magnetosheath. The calculations show that the electron distribution functions spontaneously generate whistler mode waves with plasma rest frame frequencies between about 0.1 and 100 Hz. The wave normal angles of the generated whistlers range from 0° to the resonance cone angle. Electromagnetic Landau resonance and/or cyclotron resonance contribute to wave growth over the range of velocity distributions observed. The waves that are generated by the normal cyclotron resonance have wave vectors directed toward the solar wind, while those that are generated by the Landau and the anomalous cyclotron resonances have wave vectors directed toward the magnetosheath. The predictions of the study are in qualitative agreement with observations of whistler mode waves near earth's bow shock.
The magnetopause as a tangential discontinuity for large field rotation angles
Authors: Papamastorakis I. , Paschmann G. , Sckopke N. , Bame S. J. , Berchem J.
Published: February 2008

Three passes of the ISEE 1 and 2 satellites through the dayside terrestrial magnetopause are discussed where the magnetopause is identified as a tangential discontinuity. This identification is based primarily on the failure of the plasma and magnetic field data to satisfy the conditions for a rotational discontinuity. In all these cases the interplanetary magnetic field was directed strongly southward and the angles between the fields on the two sides of the magnetopause ranged between 136° and 170°. As this is precisely the field geometry thought to be most conducive for reconnection, one would have expected the magnetopause to be a rotational discontinuity. The simplest explanation of this result would appear to be that the magnetic field orientation is not the only factor controlling the onset of reconnection. However, as our identification of the discontinuity applies only locally, it cannot be excluded that for the magnetic field conditions investigated here, different portions of the magnetopause can be described as tangential and rotational discontinuities simultaneously.