bluuebunny/crossref_metadata_2025 huggingface dataset:
Heating of light ions in the presence of a large‐amplitude heavy ion cyclotron wave
Authors: Nishikawa K.‐I. , Okuda H.
Published: February 2008

Heating of hydrogen ions has been investigated by plasma numerical simulations in the presence of an electrostatic oxygen cyclotron (EOC) wave which is observed on auroral field lines at low altitudes (400–600 km). Two types of instabilities have been found which can be driven by an EOC wave. One of them is an oscillating current‐driven electrostatic hydrogen cyclotron instability whose frequency is near the hydrogen cyclotron frequency. The other is an oscillating lower hybrid two‐stream instability whose frequency is near the lower hybrid, ω ≈ ωpH. Two‐dimensional numerical simulations confirmed the presence of both instabilities resulting in strong heating of hydrogen ions. High‐energy tails of the hydrogen ions are observed in the perpendicular distribution.
Martian atmospheric photochemistry and composition during periods of low obliquity
Authors: Lindner Bernhard L. , Jakosky Bruce M.
Published: February 2008

During periods of low obliquity, previous work has shown that martian CO2 partial pressures decreased to 0.1 mbar; CO2 partial pressures decreased to 0.02 mbar prior to the formation of the Tharsis bulge. The permanent polar caps act as a cold trap and projected global average water vapor abundances drop to possibly as low as 10−5 precipitable µm. As a result, the odd hydrogen catalytic cycle would not be effective at recombining CO and O back into CO2, and as much as 0.12 mbar of CO and 0.06 mbar of O2 could exist. These increased abundances would radically affect surface oxidation, change the lower atmospheric thermal structure, and completely alter the upper atmosphere.
Coherent radar Doppler measurements and their relationship to the ionospheric electron drift velocity
Authors: Nielsen E. , Schlegel K.
Published: February 2008

We have examined the relationship between the radial mean Doppler velocities observed with coherent radars, such as in the STARE (Scandinavian Twin Auroral Radar Experiment) system, and the electron drift velocities observed with an incoherent scatter facility. The two sets of measurements were made simultaneously and can with high accuracy be related to a common magnetic flux tube. We find that the Doppler velocities of electron density irregularities produced by the two‐stream instability in the E region are limited to a value near the ion acoustic velocity. Moreover, we find that this value is a strong function of the electron drift velocity, in agreement with theoretical predictions. An algorithm has been determined which enables us to use the STARE data to derive estimates of the electron drift velocity which are in excellent agreement with values deduced from EISCAT (European Incoherent Scatter Faciilty) data.
Determination of inverted‐V stability from Dynamics Explorer satellite data
Authors: Thieman James R. , Hoffman Robert A.
Published: February 2008

The longevity of a number of inverted‐V electron precipitation events was measured by comparing electron distributions acquired from the two Dynamics Explorer spacecraft during close proximity passes through the southern auroral zone. Common events were identified in the two data sets for time separations up to 18 min. The maximum energy attained within an inverted‐V sometimes varied between observations indicating growth or decay of the event. The number of events increasing in energy was roughly equal to those exhibiting decreasing energy, suggesting similar time scales for growth and decay. Observations closely spaced in time (minutes) had some events not in common, implying either a sudden cessation of the event or a limited spatial extent precluding observation by one of the two spacecraft due to its orbital displacement. A statistical study of 28 close proximity passes showed that the number of matched inverted‐V events exceeded those which were unmatched. This was not true for comparisons of random pairs of electron distribution plots for the two spacecraft. This implies that the close proximity events observed by both spacecraft were not just random coincidences.
Estimation of ionospheric electric fields and currents from a regional magnetometer array
Authors: Murison M. , Richmond A. D. , Matsushita S. , Baumjohann W.
Published: February 2008

We investigate the technique of calculating ionospheric electric fields and currents and field‐aligned currents using an equivalent current function obtained from a regional array of ground‐based magnetometers at high latitudes (Scandinavian Magnetometer Array). Like similar calculations previously done on a global scale, the derived electric fields and currents are found to depend on the model of ionospheric conductivity assumed. Unlike the global scale calculations, the regional calculations are also dependent on the assumed boundary conditions. Additional information on the electric fields, currents, and/or conductivities is required, besides the ground magnetic variations, to obtain unique estimations of the ionospheric electrodynamic features. In the present case, ionospheric drift measurements from one of the Scandinavian Twin Auroral Radar Experiment radars help constrain the calculations. Analysis of an equivalent current function at 1835 UT on October 7, 1976, representing Harang discontinuity conditions, suggests that upward field‐aligned current probably existed near the boundary between eastward and westward electrojets.
Numerical studies on magnetotail formation and driven reconnection
Authors: Min K. , Okuda H. , Sato T.
Published: February 2008

The formation of the magnetotail configuration under the influence of the solar wind plasma flow entering the magnetosphere and the associated driven reconnection in the magnetotail region are studied numerically by means of a two‐dimensional time‐dependent nonlinear resistive MHD code. The initial magnetic field is generated by a two‐dimensional dipole, and the initial plasma pressure is assumed uniform. As the solar wind plasmas move in the tailward direction, the magnetic field lines are stretched so that a two‐dimensional tail‐like configuration is formed. Plasma sheet thinning is observed during the further development of tail formation. When the magnetic field line is stretched and hence the neutral sheet current is enhanced enough, x‐type reconnection is triggered in the presence of resistivity. Plasmoid is formed in the downstream region and is accelerated along the sun‐earth line as a consequence of reconnection. A moderate earthward and strong tailward plasma jetting is also observed. Dependence of the simulation results on the resistivity and the solar wind energy flux is discussed in detail. The reconnection rate does not depend on resistivity, but the resistivity influences the onset time of reconnection. In contrast, the reconnection rate is very sensitive to the incoming solar wind speed. A broad current sheet is formed in the neutral sheet region in the present case rather than a pair of sharply peaked current sheets, which were observed in the previous simulation studies in which the initial configuration was one‐dimensional.
An apparent solar magnetic effect on atmospheric zonal index in the Australia/New Zealand region
Authors: Goulter S. W. , Ward G. F. A.
Published: February 2008

An apparent response of atmospheric zonal index to the passage past the earth of solar magnetic sector boundaries is detected in the Australia/New Zealand/Southest Pacific region during the period 1957–1978. The apparent response, revealed by the method of superposed epoch analysis, is highly nonrandom, the statistical significance of which is assessed in a number of different ways. All approaches concur in showing that it is extremely unlikely that the apparent response could be produced by a chance mechanism.
A nonlocal theory of the high‐latitude Farley‐Buneman instability
Authors: St.‐Maurice J.‐P.
Published: February 2008

The linear theory of E region irregularities predicts that the unstable plasma waves should be strongly field aligned. At high latitudes, however, the medium is strongly inhomogeneous in the magnetic field direction owing mainly to neutral density changes, while the thickness of the layer itself rarely exceeds 20 km. The net result is that the waves acquire large positive parallel group velocities and can quickly escape from the unstable layer. Using a linear nonlocal fluid formulation of the Farley‐Buneman theory of the irregularities, we have found that, even in the presence of a 50 mV/m field, waves between 10 m and 50 m or greater in size will reach a finite amplitude due to convective effects produced by the inhomogeneity of the medium. We have also found that at shorter wavelengths, while the instability retains an absolute character, nonlocal properties can produce new effects, for example, the appearance of a multiple layering of the unstable region with a concentration of large‐amplitude waves in layers 2 to 5 km wide.
Seasonal, diurnal, and solar cyclical variations of the limiting H+ flux in the Earth's topside ionosphere
Authors: Richards P. G. , Torr D. G.
Published: February 2008

Quantitative information on the flux of H+ ions flowing from the ionosphere to the plasmasphere is important for studying the refilling of emptied plasmaspheric flux tubes. A rigorous calculation of the flux is cumbersome and costly for some applications, so one purpose of this study is to evaluate quantitatively the usefulness of a new simple analytical expression for the H+ flux by comparison with results derived from a comprehensive calculation. We also present model calculations of the seasonal and solar cyclical variations of the daytime limiting upward H+ flux applicable to the period between 1974 and 1980. These calculations indicate a relatively small seasonal variation of a factor of 1.6. The calculated solar cycle variation depends strongly on the neutral atmosphere model. Using the 1977 version of the mass spectrometer/incoherent scatter (MSIS) model, we obtained a factor of 10 decrease in flux from solar minimum to solar maximum; however, the 1983 MSIS model produced only a factor of 3 variation, principally because of a factor of 3 less variation in the neutral hydrogen density. The various factors that affect the limiting H+ flux were examined by using the simple analytical expression. In agreement with previous work we have found the limiting flux to be most sensitive to the topside ionosphere neutral hydrogen density, which decreases with increasing solar activity. However, a combination of smaller increases in O+ density, the O+ scale height, and the O+ + H reaction rate acts to oppose the decrease in the limiting flux that would be expected from the decrease in neutral H density. The net result is that the limiting flux variation is only one third of the neutral hydrogen variation. The 1983 MSIS model also produces a greater solar cycle variation in the F2 peak electron density because of a factor of 2 less variation in the O2 density.
An improvement of the Kamide‐Richmond‐Matsushita Scheme for the estimation of the three‐dimensional current system
Authors: Sun W. , Lee L. C. , Kamide Y. , Akasofu S.‐I.
Published: February 2008

The algorithm developed by Kamide, Richmond and Matsushita (KRM) has been extensively used in estimating ionospheric electric fields, ionospheric currents, and field‐aligned currents over the entire polar region from ground magnetic chain data with a time resolution of about 5 min. However, in the KRM and some other similar methods, geomagnetic field lines are assumed to be effectively radial, so that it is important to estimate errors resulting from such an assumption. In this paper, an iteration scheme is developed in which realistic curved geomagnetic field lines are employed. We have reestimated both the ionospheric and field‐aligned currents and found that the correction to the ionospheric currents and the field‐aligned currents obtained from the KRM scheme is 5–20% in the high‐latitude (60°–80°) region and is 15–35% in the middle latitude (<60°) region.
A two‐dimensional, high‐resolution, nested‐grid model of the thermosphere: 2. Response of the thermosphere to narrow and broad electrodynamic features
Authors: Fuller‐Rowell T. J.
Published: February 2008

Using a two‐dimensional, nested‐grid model of the thermosphere, with high‐latitude resolution, the dynamic, energy, and composition response to a number of electrodynamic features has been considered. The forcing is characterized by a Gaussian latitude distribution of the convective electric field across the dawn or dusk auroral ovals with scale sizes ranging from 1° to 5° latitude full width at half maximum, together with an enhancement of E region ion density characteristic of the diffuse aurora. An extreme case for long‐lived features of the 100 mV m−1 electric field is considered, together with a more realistic magnitude of 50 mV m−1, coexisting with a 3×1011 m−3 ion density enhancement at low altitudes. The dynamic response is shown to be critically dependent on the magnitude of the winds across the source channel, either self‐induced or imposed from external sources. Self‐induced cross winds result from the action of the Coriolis force and, in addition, in the lower thermosphere, by the action of Hall drag. Crosswinds are shown to create plumes of momentum downwind and inhibit the peak neutral wind response that would otherwise exist. The combination of increased self‐induced cross wind and increasing width of the source creates a surprisingly similar peak in the zonal neutral wind response at high altitudes. The crosswinds induced by the Hall drag imply that a greater lower thermosphere zonal neutral wind is created by the 50 mV m−1 electric field compared with the 100 mV m−1 source. During the recovery a lower thermosphere jet is shown to persist for many hours with a slight preference for the smaller‐scale features. A stronger response is predicted in the dusk sector due to an inertial balance that exists, which implies that small‐scale features existing in the dusk sector are more likely to have an observable neutral response. The temperature perturbation is shown to be dependent on a self‐consistent treatment of composition and to increase in magnitude as the width of the source increases.
Sources of F region ionization enhancements in the nighttime auroral zone
Authors: Robinson R. M. , Tsunoda R. T. , Vickrey J. F. , Guerin L.
Published: February 2008

Data obtained during elevation scans with the Chatanika incoherent scatter radar have revealed the presence of ionization enhancements in the nightside auroral F region with scale sizes ranging from tens of kilometers (medium scale) to hundreds of kilometers (large scale). We have attempted to identify the sources of these enhancements using the radar data and two models. One model combines a prespecified ionization source region with a realistic high‐latitude convection model so that the plasma distribution produced by various source regions can be compared with that mapped out by the radar measurements. The second model computes the distortions of drifting plasma blobs produced by convection in the absence of ionization sources and losses. The data and modeling indicate that the Harang discontinuity is an important source region for large‐ and medium‐scale ionization enhancements and that distortion by convection of large‐scale ionization enhancements can produce smaller‐scale enhancements even in the absence of plasma instabilities or other structuring mechanisms.
Electromagnetic electron temperature anisotropy instabilities
Authors: Gary S. Peter , Madland Christian D.
Published: February 2008

This paper considers electromagnetic Vlasov instabilities driven by electron temperature anisotropies in a homogeneous, nonrelativistic magnetized plasma. Numerical solutions of the full linear dispersion equation for bi‐Maxwellian distribution functions and instabilities propagating parallel to the magnetic field are presented. Parametric dependences of the maximum growth rates of the electron fire hose and whistler anisotropy instabilities are given.
Particle and field stress balance within a planetary magnetosphere
Authors: Mauk B. H. , Krimigis S. M. , Lepping R. P.
Published: February 2008

A technique is developed for experimentally estimating the local tensor stresses within a planetary magnetic field configuration characterized by local spacecraft measurements. Key to the technique is the determination of the shapes of field lines using the symmetry properties of the system coupled with local and instantaneous measurements of the field line inclination angles. The technique is applied here to the inner and middle Saturnian magnetosphere (r ∼ 7 to 16 Rs) using data returned by the MAG (Magnetic Field) Experiment on the Voyager 1 spacecraft. We conclude that the ring current, first characterized by Connerney et al. (1983), has substantial radial structure, heretofore not shown. Outside ∼13 Rs the newly derived field stresses match remarkably well the functional variation of the centrifugal corotation stresses of the cool particle population measured previously by the Plasma Science Experiment. Inside ∼13 Rs the key structure in the derived field stresses, a prominent local maximum, matches the approximate position of an apparent strong pressure gradient in the energetic particles (>30 keV) characterized by the Low‐Energy Charged Particle detectors. These correspondences between the field and particle stresses support the validity of the new technique, and it is suggested that the technique will be useful for other magnetospheric environments.
Whistlers and whistler‐stimulated emissions in the outer magnetosphere
Authors: Koons Harry C.
Published: February 2008

Lightning‐generated VLF whistlers are rarely detected by the VLF receiver aboard the SCATHA satellite. This satellite has an inclination of 7.9° and covers a range of L shells from 5.5 to 9.0 near the magnetic equator. Whistlers are occasionally detected during local afternoon and evening at the lower end of the range of L shells covered by the satellite orbit. When they are detected, there is usually a well‐defined band of continuous hiss emission present. Events with whistler‐stimulated emissions have also been detected. The general absence of whistlers over most of the region of space covered by the SCATHA satellite suggests that there are few if any propagation paths from the earth‐ionosphere waveguide to the outer regions of the magnetosphere, especially to regions outside the plasmasphere. Since radiation at VLF from lightning is much stronger than power line harmonic radiation, it is unlikely that power line harmonic radiation is present outside the plasmasphere. We conclude that whistlers and power line harmonic radiation do not play a significant role in wave‐particle interactions in the outer magnetosphere.
Correlation between upper atmospheric temperature and solar wind conditions
Authors: Prölss G. W.
Published: February 2008

Using ESRO 4 measurements and interplanetary medium data compiled by King (1977), the direct correlation between perturbations of the upper atmospheric temperature in the polar region and changes in the solar wind properties was investigated. Here it was necessary to take the sluggish response of the upper atmosphere into account. Of the four solar wind parameters tested, the ε parameter (Perreault and Akasofu, 1978) yielded the highest correlation coefficients (≥0.7). It is suggested that this (or a similar) parameter should eventually replace geomagnetic indices in modeling atmospheric perturbation effects. Changes of the regression line coefficients with local time and season were observed but could not be analyzed in detail.
Direct contribution of oblique field‐aligned currents to ground magnetic fields
Authors: Tamao Tsutomu
Published: February 2008

An attempt has been made to evaluate the direct contribution of the horizontal component of oblique field‐aligned currents (FAC's) to the surface magnetic field variations in the auroral region associated with a localized electric potential distribution on the horizontal plane, in comparison with that of the ionospheric eddy current. A relative importance of this direct contribution of oblique FAC's to that from the ionospheric eddy current is proportional to a product of two ratios, the Pedersen to Hall conductivities and the horizontal to vertical components of the local dipole magnetic field intensity. In addition, the shorter the horizontal scale length in the east‐west direction compared with that in the north‐south one, the larger the direct contribution of FAC's. For a localized perturbation with an isotropic horizontal structure at 60° geomagnetic latitude the direct effect of oblique FAC's to the ground magnetic field attains about 80% of the magnetic contribution of the associated ionospheric Hall current at the earth's surface. It is also shown that only the vertical component of the oblique FAC's connects with the irrotational part of ionospheric currents and that the effective conductance in the magnetosphere for three‐dimensional current systems is reduced to a smaller value than that for the case of vertical field lines.
Problems with deducing ionospheric plasma convection patterns
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

It is well‐known that convection electric fields have an important effect on the ionosphere‐thermosphere system at high latitudes and that a quantitative understanding of their effect requires a knowledge of the plasma convection pattern. Consequently, convection electric fields have been measured by a variety of techniques, including satellite, rocket, and balloon‐borne probes, optical tracking of ionized barium clouds, incoherent scatter observations of drifting F region plasma, and coherent scatter observations of drifting E region irregularities. Since all of these measurement techniques provide information on only a limited spatial region at any time, the construction of the overall convection pattern requires a synthesis of data obtained at a variety of places and times. Some of the problems associated with this procedure are examined in this study using incoherent scatter radar data as an example. In this case the main problem stems from the great variability of the convection electric field. Even for very similar magnetic and IMF conditions, the synthesis of convection data obtained on different days produces a convection pattern which has a fairly large uncertainty. For the case considered, the uncertainty is sufficiently large so that the characteristic differences between the H. Volland [1978] and R. A. Heelis et al. [1982] convection models cannot be clearly identified. In comparison to this uncertainty, the uncertainties associated with assuming how corotation is driven and with transforming velocities between geographic and geomagnetic coordinate systems are negligible. The selection of the “correct” magnetic coordinate system is also less important in view of the convection variability.
Observations of the artificially injected Porcupine xenon ion beam in the ionosphere
Authors: Häusler B. , Treumann R. A. , Bauer O. H. , Haerendel G. , Bush R. , Carlson C. W. , Theile B. , Kelley M. C. , Dokukin V. S. , Ruzhin Yu. Ya.
Published: February 2008

A heavy (xenon) ion beam injected approximately perpendicular to the ambient magnetic field into the collisionless ionospheric plasma during the Porcupine campaign in March 1979 was observed over a range of distances from the ion source and of time by instruments on board the main payload and on a spatially separated probe. Plasma and field measurements from these instruments are presented for different phases of beam propagation. After an initial development of a diamagnetic cavity around the ion source, one finds a buildup of the beam current in the near zone of the beam for distances of less than R = 15 m. At larger distances the beam represents a current carrying about 90% of the initial beam current of 4 A. The signature of the transverse magnetic field is consistent with a strong current flowing in the vicinity of the ion source at R < 3 m along the magnetic field into the source. This current is carried by the escaping beam electrons. In the near zone the electric field is saturated (>0.1 V m−1), indicating that part of the electron population follows the beam by E Λ B drift motion. In the intermediate and far zones the electric field E⊥ is only about 10% of the expected maximum polarization field. Neutralization of the beam has to be provided by different processes and constitutes a major problem. From current conservation it is concluded that field‐aligned electron fluxes contribute to depolarization of the beam, thereby closing the beam current system. No significant deceleration or scattering of the beam ions is observed over distances up to half of the ion gyroradius, so there is no violent scattering of ions on self‐generated ion plasma waves on the time scale of observation. This is in agreement with one‐dimensional numerical simulations (Roth et al., 1983). The strong parallel electric fields observed in the beam are attributed either to density gradients or to anomalous resistivity. In future missions it will be most important to resolve the electron distribution function on a time scale short enough to obtain information about the field‐aligned electron fluxes contributing to the overall and local charge neutralization and current closure in the beam.
Effects of vibrational enhancement of N2 on the cooling rate of ionospheric thermal electrons
Authors: Richards P. G. , Torr D. G. , Abdou Wedad A.
Published: February 2008

This study shows that the cooling rate of ionospheric thermal electrons by molecular nitrogen may be reduced by more than a factor of 3 as a result of the enhanced vibrational excitation of N2 from a number of chemical sources. Moreover, under conditions of enhanced F region electron densities (Ne > 106 cm−3), N2 may act as a small net source rather than sink of electron thermal energy. If the most commonly used electron cooling rates are correct, the influence of vibrational enhancement on the electron temperature is small because cooling through fine structure excitation of O is dominant below the altitude of the F2 peak electron density. However, recent measurements indicate that the cross sections used to evaluate the N2 cooling rate may have been a factor of 3 too small and other measurements indicate that the O fine structure cooling rate may be too large by a similar factor. Under these circumstances, N2 vibrational cooling would be dominant, and the neglect of enhanced vibrational excitation of N2 would lead to an underestimate of about 100°K in the thermal electron temperatures throughout the F region ionosphere at solar minimum. The effect on the electron temperature at solar maximum is small and confined to the region below the F2 peak density because of the enhanced ion cooling. Two new sources of N2 vibrational excitation are evaluated. One of these, the photoelectron excitation and subsequent deactivation of the A, B, C, and W triplet states of N2 is found to be a major source of N2 vibrational quanta. This source is several times larger than the direct (2.5 eV) photoelectron source above 250 km. The second new source, charge exchange between N2+ and N2, is comparable to the other sources at high altitudes but is small below 200 km.
Is Jupiter's ionosphere a significant plasma source for its magnetosphere?
Authors: Nagy A. F. , Barakat A. R. , Schunk R. W.
Published: February 2008

A semikinetic model was used to study the steady state, collisionless, polar wind outflow from the Jovian polar caps. H+ escape fluxes and energies were calculated for a range of conditions, including several values of the ambient electron temperature, different hot electron populations, and both with and without the effects of the centrifugal force. The calculations indicate that if hot electron populations exist over the Jovian polar caps, as they do on earth, polar wind escape fluxes of the order of 108 cm−2 s−1 are possible. When integrated over the polar cap area, escape fluxes of this order of magnitude imply an ionospheric source strength of 2×1028 ions/s, which is comparable to the present estimate of the total magnetospheric plasma source population. Therefore, the ionosphere may play an important role in populating the Jovian magnetosphere, specifically the “hidden,” low energy, light ion component of the population.
AMPTE/UKS Ion Experiment observations of lithium releases in the solar wind
Authors: Coates A. J. , Johnstone A. D. , Smith M. F. , Rodgers D. J.
Published: February 2008

The two Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium releases outside the bow shock were made in quite different conditions in the solar wind. The first took place in a high speed stream in the presence of a diffuse distribution of upstream ions which disturbed the solar wind. Lithium ions were only detected for 15 seconds. During the second release the solar wind flow was much smoother and slower. The lithium ions were observed for 4 minutes. In that time the energy of the detected ions increased from 10 eV up to almost 3 keV. This is consistent with a shell‐like expansion of the neutral lithium cloud and acceleration of the ions in the convection electric field. The solar wind slowed down and was deflected as it passed through the ion cloud.
On Io's control of Jovian decametric radio emissions
Authors: Winglee R. M.
Published: February 2008

Io's control of Jovian decametric radio emission (DAM) has been attributed to Io distorting the electron distribution in the inner Jovian magnetosphere. Observations of Faraday rotation in DAM are used to determine the properties of the electron distribution before and after its interaction with Io. It is shown that there is an enhancement in the density of the energetic component in the Io plasma torus correlated with certain Jovian longitude. Io's interaction with this energetic component can produce heating of this component. The Io‐controlled emission is attributed to enhanced emission from the heated electrons moving down the field lines to Jupiter.
Relations between the interplanetary magnetic field Bz, AE index, and cusp latitude
Authors: Carbary J. F. , Meng C.‐I.
Published: February 2008

The connection between the interplanetary magnetic field (IMF) Bz, AE index, and magnetospheric cusp location is examined qualitatively. Seven months during 1979–1980 were searched for periods of low auroral activity and/or periods when the IMF Bz component and the AE index varied inconsistently (i.e., southward Bz and quiescent AE). During quiet auroral conditions, the IMF Bz, AE index, and equatorward cusp boundary generally tend to act coherently, although during very quiet times the cusp may exhibit considerable variability that is apparently unassociated with either Bz or AE. We found a number of instances in which the cusp location clearly followed Bz more closely than AE, especially when a time delay of ≤ 1 hour was assumed. However, we cannot conclude that either Bz or AE exerts a dominant influence on the latitudinal position of the polar cusp. The effects of Bz and AE variations on the cusp are quite complicated and the observations simply do not indicate clear cusp response to one or the other. The largest equatorward cusp motions seem to occur as a result of prolonged southward Bz lasting 3 hours or more, and cusp latitude appears to depend strongly on time‐accumulated Bz.
Variation of cosmic rays and solar wind properties with respect to the heliospheric current sheet: 2. Rigidity dependence of the latitudinal gradient of cosmic rays at 1 AU
Authors: Newkirk Gordon , Asbridge John , Lockwood John A. , Garcia‐Muñoz Moises , Simpson J. A.
Published: February 2008

We discuss the role which empirical determinations of the latitudinal variation of cosmic rays with respect to the current sheet may have in illuminating the importance of the cross‐field drift of particles in the large‐scale heliospheric magnetic field. Using K coronameter observations and measured solar wind speeds, we have determined the latitudinal gradients with respect to the current sheet for cosmic rays in four rigidity ranges. Gradients vary between approximately −2 and −50% per astronomical unit. The rigidity dependence of the decrease of cosmic ray flux with distance from the current sheet lies between P−0.72 and P−0.86, with the exact dependence being determined by the definition used for the median rigidity of each monitor.
The effect of the heliospheric current sheet on cosmic ray intensities at solar maximum: Two alternative hypotheses
Authors: Thomas Barry T. , Goldstein Bruce E. , Smith Edward J.
Published: February 2008

There is now a growing awareness that solar cycle related changes in the large‐scale structure of the heliospheric current sheet may play an important role in the modulation of galactic cosmic rays. To date, attention has been focused on the configuration of the current sheet at times near solar minimum when the current sheet structure is relatively simple. Previous analyses have explored the effect on cosmic ray intensities of a single current sheet which is tilted with respect to the heliographic equator under the assumption that the tilt of the current sheet is a minimum at solar minimum and increases as solar maximum approaches. This paper attempts to extend the previous analyses into the period near solar maximum. Two alternative hypotheses are explored: (1) that the tilt of the current sheet continues to increase as solar maximum approaches, finally becoming vertical and overturning, and (2) that the single sheet structure breaks down near solar maximum and the sun at this time sheds the poloidal flux of the previous cycle and develops a new field structure of the opposite polarity. It is found that both hypotheses lead to variations in cosmic ray intensity comparable to those actually observed over the solar cycle.
Auroral zone thermospheric dynamics: 1. Averages
Authors: Sica R. J. , Rees M. H. , Romick G. J. , Hernandez G. , Roble R. G.
Published: February 2008

Forty‐four nights of thermospheric neutral wind and temperature measurements were obtained with a ground‐based Fabry‐Perot spectrometer at College, Alaska (65° invariant latitude), mostly during high to moderate levels of solar activity. The spectrometer scanned the O I 15,867 K (6300 Å) emission line yielding line profile measurements that originate from an altitude range between 170 and 280 km. The observed average wind direction is to the northwest in the early evening, changing to the south near midnight and then to the southeast in the morning. The overall wind pattern generally persists, but the wind speed increases with increasing geomagnetic activity. A change in the zonal wind direction occurs earlier in magnetic local time with increasing magnetic activity. The meridional wind pattern also shifts equatorward with the auroral oval during increasing magnetic activity. Consequently, the average wind pattern in the north during low geomagnetic activity is rather similar to the moderate activity average in the south, and the moderate geomagnetic activity average wind pattern in the north is similar to the high geomagnetic activity average in the south. The observed average thermospheric temperature is found to be governed by geomagnetic activity and the previous day's 10.7‐cm solar flux.
Effect of spread F irregularities on whistler wave propagation at low latitudes
Authors: Singh Birbal , Jain S. K.
Published: February 2008

By employing small‐scale field‐aligned irregularity models, ray paths are computed for VLF whistler mode waves to examine whether in the absence of true VLF ducts in the low‐latitude ionosphere such small irregularities, which are responsible for spread F, may significantly influence the propagation of VLF waves so that the waves may reach the ground. The results show that an irregularity of 10 km thickness and amplitude 5% brings the wave normal direction of downcoming waves almost vertical at the base of the F region ionosphere such that the waves may penetrate the lower ionosphere and reach the ground.
Lunar effects in the F region of the ionosphere
Authors: Stening Robert J.
Published: February 2008

Diurnal and seasonal variations in the lunar tide of foF2 are determined by analyzing hourly values for 2 years from several stations mostly at American longitudes. A phase advance of 3 or 4 hours from day to night is found at Huancayo in agreement with electric field measurements. Seasonal changes in the lunar tide of foF2 are similar to those in ΔH at nearby stations. An unexplained solar cycle variation is found at Concepcion (25° S geomagnetic).
On the effect of ion viscosity on the motion of two‐dimensional plasma clouds
Authors: Sperling J. L.
Published: February 2008

The finite ion‐gyroradius contributions to the ion viscosity are demonstrated to impact the motion of plasma clouds in the two dimensions transverse to the magnetic field. The viscosity reduces the relative velocity between a plasma cloud and the ambient plasma, and it also alters the direction of the cloud velocity relative to the driving force, e.g., neutral wind drag. As the formation of striations manifests the tendency of plasma clouds to elongate in the direction of an applied force, it thus appears that the ion viscosity may contribute to striation “freezing” by altering the zero‐order plasma state upon which striations evolve.
Spatial structure of ion bounce wave in the magnetosphere
Authors: Lanzerotti L. J. , Medford L. V. , Meloni A.
Published: February 2008

Large amplitude (∼5–30 nT), high‐frequency (∼0.14 Hz) geomagnetic field oscillations were observed across eastern North America and in the conjugate region in Antarctica during a large magnetic storm. These are the largest amplitude oscillations reported to date in the Pc 2 frequency band (0.1–0.5 Hz). The waves are first seen on the east coast, with the onset delayed by about 4 min at the midcontinent stations. The oscillations are consistent with a wave excited by the bounce motion of protons with a peak in the energy distribution at ∼100 keV, with the energy source drifting from east to west with the ion gradient B drift.
A study of the ionospheric disturbance induced by an injected electron beam
Authors: Margot‐Chaker Joëlle , McNamara A. G.
Published: February 2008

We report increased ionization and electron heating induced by interaction of an injected electron beam with the ionospheric plasma. Just after the end of each beam injection pulse, the thermal electrons are found to be heated up to 2 eV. The variation of the temperature after a beam pulse is shown to be related to the gradient of temperature across magnetic field lines. The scale length of the gradient is independent of the beam energy but is a function of both the beam pitch angle and current. The radial extent of the disturbed region across the B‐field is also studied as a function of the beam parameters and of the altitude, and is shown to extend up to 50 m. Higher current beam injections are accompanied by a substantial amount of ionization. Resulting plasma densities can reach more than 5 times the background value.
Verification of quantitative relationship among Birkeland current sheet motion, intensity, and convective velocity, and also of criterion for expansion onset
Authors: Atkinson G.
Published: February 2008

In this paper, 14 substorms are used to verify the conclusions of an earlier paper on current sheet motions. These conclusions can be summarized as follows: (1) The quantitative relationship VN = VCN − kvI∥ is valid when applied to the upward current sheet in the premidnight quadrant except during the expansion and early recovery phases of substorms (VN and VCN are the components, normal to the sheet, of the sheet velocity and the convection velocity, kv is a constant, and I∥ is the current sheet intensity). (2) At the time of local northward expansion of the auroras there appears to be a decrease by 1 order of magnitude of the amount of plasma on flux tubes. (3) The time of initiation of local northward expansion occurs when VCN = kvI∥. A few additional points emerge from this study: The above relationships are not valid at stations east of the location of initial onset of expansion, and the amount of plasma on flux tubes tends to return to its preexpansion value after a substorm. Further conclusions are (1) that the physical mechanism responsible for the northward expansion in a westward traveling surge is the same as in the initial near‐midnight expansion, (2) that the triggering of expansion occurs just as the tail begins to recede away from the earth, and (3) that the apparent decrease in plasma content is consistent with the formation of a near‐earth neutral line; however, it could also result from the breakdown of the sheet approximation for the currents.
An empirical relationship between Kp and the tail lobe magnetic field
Authors: Rothwell Paul L.
Published: February 2008

In this paper a useful empirical relationship is established between the readily available magnetic activity index, Kp, and the tail lobe magnetic field strength. A simple (dipole plus tail field plus ring current) magnetic field model is combined with the results of Gussenhoven et al. (1981) that relate the location of the equatorward boundary of auroral oval with Kp. The results are tested by taking over 500 samples of Kp and converting them to the corresponding lobe field strength values which are then plotted as a frequency distribution. This distribution is compared with a similar distribution of over 700 hours of IMP 4 tail lobe magnetic field measurements taken during the same time period by Fairfield and Ness (1970). The agreement between the two distributions is excellent only when a model ring current is included. This implies that the ring current makes a significant contribution to the location of the equatorward boundary of the auroral oval (Siscoe, 1979), particularly during periods of high Kp. Comparisons are made between the present model and the zero‐energy Alfven layer calculations that have appeared in the literature.
Electric field measurements during the Condor Critical Velocity Experiment
Authors: Kelley M. C. , Pfaff R. F. , Haerendel G.
Published: February 2008

The barium explosion of the Condor critical velocity experiment created a complex electric field pulse detected in situ by a single‐axis electric field double probe on a separate spacecraft a few kilometers away. The measured component of the pulse had a peak amplitude which exceeded 320 mV/m. The sign of this component was consistent with an electric field pointed back toward the explosion point. The large electric field pulse arrived nearly simultaneously with the fastest minor ions associated with the explosion. Just ahead of the pulse a packet of nearly monochromatic waves (3460 Hz) was detected near the oxygen lower hybrid frequency, with a measured electric field component of 6 mV/m along the double‐probe direction. The bulk of the barium beam was accompanied by a quasi‐dc electric field whose amplitude was between 100 and 200 mV/m. The E × B drift associated with this latter field was less than the speed of the main beam estimated from the time delay of the arrival of the initial beam‐related waves and particles. This implies a counterstreaming between the neutrals and ions in excess of the critical ionization velocity for barium. This region of the beam was accompanied by a peaked distribution in the soft electron fluxes and by intense electric field fluctuations with peak (one component) amplitudes exceeding 375 mV/m. The wave frequencies were in the range 0.1fLH ≤ f ≤ fLH where fLH is the barium lower hybrid frequency ((ΩiΩe)1/2/2π). The most significant electron heating was associated with the most intense wave activity. The observations provide evidence of several important links which may be required in the critical velocity chain and are consistent with theories which appeal to either the modified two‐stream instability or to an ion beam process. For example, the data support the hypothesis that mechanical energy in the beam is converted to electrical energy associated with cross‐field currents and that these currents are unstable to lower hybrid wave generation which subsequently heats the electron gas. In principle, the electrons then ionize the neutral gas to complete the loop. The observed (one component) lower hybrid electric field wave intensity was less than that predicted for a fully developed modified two‐stream instability, which may be related to the fact that the fastest‐growing modes are inhibited by the finite dimension of the beam parallel to the magnetic field. In turn, the reduction in the observed lower hybrid wave amplitude may be related to the low efficiency of the Alfvén process in the Condor experiment geometry reported in the companion papers.
Near‐conjugate observations of polar cusp electron precipitation using DE 1 and DE 2
Authors: Lin C. S. , Burch J. L. , Winningham J. D.
Published: February 2008

Near‐conjugate plasma measurements from the DE 1 high‐altitude plasma instrument (HAPI) and the DE 2 low‐altitude plasma instrument (LAPI) instruments in the dayside polar cusp are examined to determine quantitatively the energization of precipitating cusp electrons. The plasma moments, including number flux, energy flux, number density, and average energy, are computed from the plasma measurements with energies between 15 and 13,000 eV. At the DE 1 altitudes (near 20,000 km) the typical polar cusp has a downward density around 1 cm−3, a peak value of the downward number flux around 109 cm−2 s−1, a peak value of the downward energy flux around 1 erg/cm² s, and an average energy around 150 eV. At the DE 2 altitudes (near 1000 km) the downward energy flux is 1–15 times that measured by DE 1. Using the DE 1 plasma data, the downward energy flux expected at the DE 2 altitudes is modeled according to the convergence of the dipole magnetic field lines and compared with the DE 2 measurements. Contributions to the total downward energy flux by secondary electrons and turbulent processes are neglected in the modeling. For two cusp passes, the DE 1 HAPI plasma data produce energy fluxes larger than the LAPI measurements after mapping to the DE 2 altitudes without downward electron acceleration. A downward acceleration is found to be needed to explain a more than 10 times gain of energy flux for the other four conjugate passes examined. Assuming adiabatic acceleration of electrons by an electrostatic field, the upper limit of the cusp potential drop is about 400 V. This estimate is consistent with the cusp potential drop inferred separately from the characteristics of distribution functions with small pitch angles. These results suggest that significant plasma acceleration sometimes occurs along field lines between the two DE satellites.