bluuebunny/crossref_metadata_2025 huggingface dataset:
Helium resonance and dispersion effects on geostationary Alfven/ion cyclotron waves
Authors: Mauk B. H.
Published: February 2008

To understand observed structure and growth patterns of geostationary Alfvén/ion cyclotron waves, the properties of the hot proton cyclotron instability within a helium rich plasma are explored here. This exploration proceeds with an examination of the net linear wave amplifications that result as a wave propagates through the magnetic gradients of a realistic magnetic field model (linearity is discussed and justified). By taking care in generalizing a single pass model to a multiple pass system, the following conclusions have been reached: (1) The basic structure of the frequency gap that is observed close to the helium cyclotron frequency can be explained by the ‘stop‐gap’ dispersion effect; however, the helium cyclotron resonance effect contributes to the gap formation leading to He+ ion energization. (2) The presence of the magnetic gradients virtually insures that helium ion energization (by means of the low frequency wave branch only) is the inevitable consequence of the wave generation process. (3) The energized helium ions will sparsely populate a broad range of geomagnetic latitudes (±15 degrees) but will be concentrated strongly within several (2–4) degrees of the geomagnetic equator. (4) The presence of low percentages of helium ions is most likely to suppress the wave generation process (ATS 6 satellite observations support this conclusion for a portion of the wave spectrum).
A broadband VLF burst associated with ring current electrons
Authors: Maeda Kaichi , Anderson Roger R.
Published: February 2008

During the periods after the maximum phase of geomagnetic storms on 2 April 1972 a broadband VLF emission burst has been observed by the equatorially orbiting satellite S³‐A (Explorer 45) along its inbound plasmapause crossings. The frequency band broadening takes place just outside of the nighttime plasmasphere, where the density of cold plasma has been known to be very low during the later phase of a geomagnetic storm. Instead of the gradual broadening of several hours duration, a burst type broadening of VLF emission lasting less than ten minutes was observed in the similar location. The magnetic field component of this emission is very weak and the frequency spreads below the local half electron cyclotron frequency. Corresponding enhancement of the anisotropic ring‐current electrons is also very sudden and limited below the order of 10 keV without significant velocity dispersion, in contrast to the gradual broadening events. The cause of this type of emission band spreading can be attributed to the generation of the quasi‐electrostatic whistler mode emissions of short wavelength by hot bi‐maxwellian electrons surging into the domain of relatively low density magnetized cold plasma. The lack of energy dispersion in the enhanced electrons indicates that the inner edge of the plasma sheet, the source of these hot electrons, is not far from the location of this event.
Thermospheric response observed over Fritz Peak, Colorado, during two large geomagnetic storms near Solar Cycle Maximum
Authors: Hernandez G. , Roble R. G. , Ridley E. C. , Allen J. H.
Published: February 2008

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high resolution Fabry‐Perot spectrometer. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867K (630 nm) line emission. Measurements made during two large geomagnetic storm periods near solar cycle maximum reveal a thermospheric response to the heat and momentum sources associated with these storms that is more complex than the ones measured near solar cycle minimum. In the earlier measurements made during solar cycle minimum, the winds to the north of Fritz Peak Observatory had an enhanced equatorward component and the winds to the south were also equatorward, usually with smaller velocities. The winds measured to the east and west of the observatory both had an enhanced westward wind component. For the two large storms near the present solar cycle maximum period converging winds are observed in each of the cardinal directions from Fritz Peak Observatory. These converging winds with speeds of hundreds of meters per second last for several hours. The measured neutral gas temperature in each of the directions also increases several hundred degrees Kelvin. Numerical experiments done with the NCAR thermospheric general circulation model (TGCM) suggest that the winds to the east and north of the station are driven by high‐latitude heating and enhanced westward ion drag associated with magnetospheric convection. The cause of the enhanced poleward and eastward winds measured to the south and west of Fritz Peak Observatory, respectively, is not known. During geomagnetic quiet conditions the circulation is typically from the southwest toward the northeast in the evening hours. We speculate that the enhanced flow to the south and west of Fritz Peak Observatory may be due to either (1) a hemispheric difference in the thermospheric energy input rate, (2) equatorial heating by neutral hydrogen precipitation, or (3) an increased midnight equatorial temperature bulge or some combination of these sources. Since the measurements indicate that the winds in the 630‐nm airglow layer converge from all directions at the observatory, we speculate that Fritz Peak is at the boundary of two circulation patterns during part of the storm.
Pioneer Venus observations of plasma and field structure in the near wake of Venus
Authors: Luhmann J. G. , Russell C. T. , Brace L. H. , Taylor H. A. , Knudsen W. C. , Scarf F. L. , Colburn D. S. , Barnes A.
Published: February 2008

Ionospheric plasma density depletions or ‘holes’ are observed by the Pioneer Venus orbiter in association with radial magnetic fields in the near wake of Venus. This report presents examples of the collected observations of these unexpected features of the Venus nightside ionosphere obtained by the Langmuir probe, magnetometer, ion mass spectrometer, retarding potential analyzer, plasma analyzer, and electric field experiments. The connection between plasma density depletions and temperature changes, changes in ion composition, plasma wave emissions, and magnetic fields with a substantial radial component is illustrated. Mechanisms that may be responsible for the formation and maintenance of holes are suggested.
PC 4‐PC 1 magnetic pulsations at synchronous orbit and their relation to pulsations on the ground
Authors: Maltzeva N. , Troitskaya V. A. , Schepetnov R. , Pokhotelov O. , Gochberg M. , Pilipenko V. , McPherron R. , Barfield J.
Published: February 2008

Three types of Pc 4 pulsations with periods in the range 40–200 s have been identified in the ATS 6 satellite data. Two types are not observed on the ground (80–200 s). Both of these are accompanied by Pc 1–2 activity that is seen simultaneously both at the satellite and on the ground. One of these two types occurs during substorms and coincides with the development of the IPDP subtype of Pc 1–2. The other is observed in the magnetosphere during slow, smooth changes of the background magnetic field during the recovery phase of storms. Pulsation amplitude for these two types is from 5 to several tens of nanoTeslas. The third type of Pc 4 pulsation (T ∼ 40–80) has significantly smaller amplitudes in the magnetosphere (A ≤ 5 nT) and can be observed on the ground with amplitudes usually greater than in the magnetosphere. The properties of the first two types of long‐period pulsations suggest that the mechanism for their generation is the drift‐dissipative instability of the ring current plasma.
Particle simulations of electrostatic emissions near the lower hybrid frequency
Authors: Roth I. , Hudson M. K.
Published: February 2008

The linear instability and nonlinear saturation of electrostatic emission near the lower hybrid frequency is examined for model cold and warm ion ring distributions and auroral zone parameters. In the cold ring case, a single coherent mode near ωLH evolves, and saturates by ion trapping. In the warm ring case, a discrete spectrum of unstable modes separated by the ion gyrofrequency is generated near and above ωLH. The latter instability saturates by quasilinear diffusion.
Experimental evidence for the acceleration of thermal electrons by ion cyclotron waves in the magnetosphere
Authors: Norris A. J. , Johnson J. F. E. , Sojka J. J. , Wrenn G. L. , Cornilleau‐Wehrlin N. , Perraut S. , Roux A.
Published: February 2008

Ion cyclotron waves (ICWs) at frequencies just above ƒHe+, the helium gyrofrequency, are often observed by the ESA satellites GEOS 1 and GEOS 2. Young et al. (1981) have shown the close connection between these ICWs and the presence of thermal He+ ions in the outer magnetosphere. The purpose of this paper is to provide experimental evidence for the acceleration of thermal electrons by the large amplitude ICWs. The wave power in the ultra low frequency (ULF) range near ƒHe+ is compared with the distribution function of low energy electrons, measured by instruments aboard the GEOS satellites. It shows that, near the geomagnetic equator, electrons are accelerated along field lines at times when the ICW energy is large and the cold plasma density is below a threshold value. It is suggested that these accelerated electrons can account for the ELF emissions, modulated at the ICWs frequency, observed by Cornilleau‐Wehrlin (1981). Other ULF events having frequencies close to the proton gyrofrequency ƒ>H+ are also observed and result in a very efficient acceleration of thermal electrons along field lines. The evidence in this case is that medium energy protons having large temperature anisotropies (T⊥ ≫ T∥) in the 100‐ to 500‐eV range are responsible for the ICW wave generation.
Pc 1 pulsation activity at Ottawa
Authors: Gupta Jagdish Chandra
Published: February 2008

Pulsations in the Pc 1 period range recorded at Ottawa for three years during the IMS are analyzed to study some of their occurrence characteristics. The diurnal occurrence pattern depends on the seasons and on the prevailing magnetic activity conditions. In both cases the morning peak shifts to earlier hours and the evening peak shifts to later hours with increasing conduction (electron density) in the ionosphere. The largest number of events were recorded in the winter months and about as many in the equinoctial months. A large majority of the events are inferred to occur, at Ottawa, during the recovery phase of magnetic storms. The enhancements of Pc 1's noted at different levels of magnetic activity seem to be controlled by the location of the plasmapause and also by the propagation conditions in the ionosphere. The existence of a relationship between the occurrence of Pc 1 pulsations and the sector polarity of the interplanetary magnetic field remains to be established.
Observations of small‐scale auroral vortices by the S3‐2 satellite
Authors: Burke William J. , Silevitch Michael , Hardy David A.
Published: February 2008

We have studied two intense auroral events which were encountered by the S3‐2 satellite at ∼0550 and ∼1930 MLT, during a substorm, near the equatorward edge of the region 1 current system. The events are marked by large deflections in the east‐west magnetic field component. In the dawn (dusk) event the deflection was 100 (300) nT with a total duration of 4(7) s. In both cases the deflection corresponding to an upward current sheet was sharpest, occuring over 0.25 s. Upward current strengths of 45 and 135 μA/m² within latitude extents of <2 km are inferred. The principal region of return current is not contiguous with the upward current sheet and is more spatially extended, with maximum intensities in the range of 10–15 μA/m². The events are associated with electric fields whose magnitude can exceed 200 mV/m. In the region between the current sheets, adjacent to the principal upward current sheet, the electric field rotated by 180° then returned to its original orientation. Such electric field variations give rise to plasma vortex flow patterns similar to these observed in auroral folds and curls. Despite spatial aliasing, the measurements of an electron detector in the vicinity of the upward current sheets provide useful information concerning the density and temperature of the parent populations and the field‐aligned potential drop. With some reasonable assumptions, it is shown that observed values ofj∥ and Φ∥ are consistent with collisionless, single‐particle theories.
Energetic ion losses near Io's orbit
Authors: Cheng Andrew F. , Maclennan C. G. , Lanzerotti L. J. , Paonessa M. T. , Armstrong T. P.
Published: February 2008

We analyze energetic ion phase space densities obtained from the low energy charged particle experiment during the Voyager 1 inbound pass to Jupiter, near Io's orbit. We consider the radial transport of charged particles and the depletion of energetic ion fluxes there. Voyager 1 low energy ion data from the plasma experiment and ground‐based observations of the Io plasma torus are combined to determine both the absolute normalization and the functional form of the radial diffusion coefficient. This same diffusion coefficient is then used to infer the energetic ion loss rates from Voyager 1 low energy charged particle ion phase space densities. Energetic ion losses are observed in the region 5.9 ≤ L ≤ 7.1. The ion loss rates in this region are consistent with plasma wave scattering in the strong diffusion limit. The expected energy dependence in this limit is not found, however. There is evidence that the ion loss rate is not spatially uniform even within the main loss region. The rate of satellite sweeping is small compared to the observed loss rate, even in the Io sweeping corridor. The charge exchange loss rate is also small compared to the observed loss rate for L ≳ 5.9 and becomes important only for L < 5.5. Ion pitch angle distributions are presented, which support the idea that charge exchange losses of energetic ions become important within L < 5.5. This interpretation also implies that heavy ions contribute appreciably to energetic ion fluxes in that region.
O I (7990 Å) Emission and radiative entrapment of auroral EUV
Authors: Christensen A. B. , Sivjee G. G. , Hecht J. H.
Published: February 2008

Auroral observations of the O I (7990 Å) multiplet intensity distribution have been performed using a 2‐m grille spectrometer and a Fabry‐Perot interferometer in Alaska during 1977 and 1982, respectively. In addition, an extensive set of 1‐m and ½‐m Ebert‐Fastie spectrometer data obtained at Poker Flat, Alaska, and Longyearbyen, Svalbard, from 1977 to 1980 has been analyzed. The grille spectrometer results for the singlet/doublet ratio of the O I (7990 Å) multiplet indicate a nonstatistical distribution in the excited state, consistent with preferential scattering of the singlet component of the O I (989 Å) EUV emission and in accord with recent radiative transfer calculations (Meier, 1982). The striking departure of the Fabry‐Perot results from the model may be due to blending of the O2 (4, 4) band with the O I (7990 Å). Complete absorption of the EUV doublet by N2 is precluded by the clear presence of the O I (7990 Å) doublet component in the auroral spectrum. A comparison of O I (7990 Å) intensities measured on the nightside with those from dayside magnetospheric cleft aurora shows a cascade contribution to the O I (8446 Å) line and ultimately the O I (1304 Å) transition that depends on the altitude of the aurora. This is attributed to the greater radiative entrapment of O I (989 Å) at low altitude and corresponding enhanced probability for branching from the 3 s′ to the 3 p level in atomic oxygen.
On the N2 Lyman‐Birge‐Hopfield Band Nightglow
Authors: Meier R. R. , Conway R. R.
Published: February 2008

A nightglow emission has been seen by the Naval Research Laboratory experiment on the STP 72‐1 spacecraft which appears to have the same characteristics as the N2 Lyman‐Birge‐Hopfield band emission reported by Huffman et al. (1980a). A seasonal variation is observed, with maximum brightness reaching several hundred Rayleighs in northern summer, and with little or no emission in southern winter. Synthesis of the spectrum given by Huffman et al. shows that the upper electronic state population peaks at vibrational quantum number υ′ = 0, suggesting a cascade mechanism. Results from these experiments are reviewed, along with positive and negative observations from a number of other experiments. A variety of excitation mechanisms are examined. None are able to account for the various features of the Lyman‐Birge‐Hopfield nightglow.
Plasmapause diffusion
Authors: Horwitz J. L.
Published: February 2008

The possible smoothing effect that diffusion has on steep plasmapause density gradients is examined for diffusion estimated from (a) the Bohm diffusion coefficient and (b) scattering by observed electrostatic waves. For characteristic plasma temperatures and magnetic fields at an L = 4 plasmapause, the (equatorial) perpendicular Bohm diffusion coefficient is approximately DB = 0.12 × 10−4 RE²/h. The estimate for diffusion resulting from scattering by observed electrostatic waves is much lower:
. However, this diffusion rate may be too small an estimate if, as is probable, the wave amplitudes are significantly higher for steep plasmapauses. Thus the effects are negligible for most considerations of macroscopic plasmapause dynamics but could be significant in limiting drift wave instabilities and similar phenomena which are driven by the steepness of the plasmapause density gradient. If Bohm diffusion is operative, the minimum persistent plasmapause scale length should be of the order 10−2 RE, or ∼60 km.
Eddy diffusion coefficients in the lower thermosphere
Authors: Kirchhoff V. W. J. H. , Clemesha B. R.
Published: February 2008

The seasonal variation of the eddy diffusion coefficient is deduced for a height of 94 km at 23°S on the basis of two independent methods of analysis. One of these methods, already used in the past, is related to indirect measurements of atomic oxygen, and the other one is a new method related to measurements of the density of upper atmospheric sodium. The results are mutually consistent, giving a maximum eddy diffusion coefficient during summer.
Corotating pressure waves without fast streams in the solar wind
Authors: Burlaga L. F.
Published: February 2008

Voyager 1 and 2 magnetic field and plasma data are presented which demonstrate the existence of large scale, corotating, nonlinear pressure waves between 2 AU and 4 AU that are not accompanied by fast streams. The pressure waves are presumed to be generated by the interaction between fast corotating streams and slower flows near the sun. For two of the three pressure waves that are discussed, the absence of a relatively fast stream is probably a real, physical effect, viz, a consequence of deceleration of the stream and acceleration of the ambient flow by the associated compression wave. For the third pressure wave, the apparent absence of a stream may be a geometrical effect; it is likely that the stream was at latitudes just above those of the spacecraft, while the associated shocks and compression wave extended over a broader range of latitudes so that they could be observed by the spacecraft. It is suggested that the development of large‐scale, nonlinear pressure waves at the expense of part of the kinetic energy of streams produces a qualitative change in the solar wind in the outer heliosphere. Within a few astronomical units the quasi‐stationary solar wind structure is determined by corotating streams whose structure is determined by the boundary conditions near the sun. Beyond several astronomical units there is a zone in which the solar wind structure is determined by nonlinear pressure waves without streams, in which memory of the source conditions has largely been erased. Far from the sun (≳25 AU), these pressure waves should interact extensively with one another producing a zone which is more homogenous on a large scale yet more disordered on a smaller scale, where a statistical description may be more appropriate than deterministic models. This new view of heliospheric structure should provide a better foundation on which to interpret retrospectively prior observations and to analyze future data with respect to basic physical processes.
Magnetic drifts at Io: Depletion of 10‐MeV electrons at Voyager 1 encounter due to a forbidden zone
Authors: Goldstein Bruce E. , Ip W.‐H.
Published: February 2008

The possibility that the partial dropout of the 10‐MeV electrons observed at Voyager 1 encounter with Io might be explained by a forbidden zone is investigated. Because the convection velocity in the vicinity of Io decreases, grad B drifts of energetic electrons opposing the convection result in a forbidden region around Io. It is found that the location and magnitude of the 10‐MeV electron depletion calculated on this basis agrees well with the location found by mapping along draped field lines from the Voyager trajectory to the location of Io. The depletion of 10‐MeV electrons thus should not be construed as evidence for a local dipole magnetic field at Io. If the forbidden zone explanation for the 10‐MeV dropout observed by Voyager is correct, then no electrons in this energy range would be lost to the surface of Io and there should not be a wake extending behind Io in which electrons of energies 10 MeV are depleted. Such a depletion of electrons near 9 MeV was observed by Pioneer, an apparent discrepancy. If more of the Jovian convection electric field had penetrated onto Io connected field (higher ratio of Alfvén wave conductance to Io conductance) during the Pioneer period than during the Voyager encounter, particle losses observed by Pioneer could be explained by precipitation at Io. Losses for both energetic electrons and protons at Io are calculated for different amounts of electric field shielding at Io.
Particle and wave dynamics during plasma injections
Authors: Koons H. C. , Fennell J. F.
Published: February 2008

The SCATHA satellite measures particle and wave parameters as it moves outbound on the duskside from the plasmasphere into the plasma sheet. In many cases, plasma waves are not observed in the quiescent plasma sheet prior to a plasma injection. The electron distribution function prior to entry into the plasma sheet is a nearly isotropic soft spectrum, J(E) ∼ 1/E. Just inside the plasma sheet the spectrum begins to harden and becomes anisotropic, J⊥ > J∥. As the satellite penetrates deeper into the plasma sheet, the spectrum further hardens, especially near α0 ∼ 90°. At the injection the electron spectrum drastically hardens and often becomes peaked in the keV energy range. The pitch angle anisotropy is further enhanced in favor of J⊥. The plasma wave emissions onset occurred at the time of the injection. Whistler mode waves are observed below the electron cyclotron frequency. Electrostatic waves are detected in bands between the electron cyclotron frequency harmonics.
An ISEE 3 high time resolution study of interplanetary parameter correlations with magnetospheric activity
Authors: Baker D. N. , Zwickl R. D. , Bame S. J. , Hones E. W. , Tsurutani B. T. , Smith E. J. , Akasofu S.‐I.
Published: February 2008

From 1978 to 1982 the ISEE 3 spacecraft was in a ‘halo’ orbit around the sun‐earth Lagrangian point L1 approximately 240 RE upstream of the earth and was continuously in the upstream solar wind. In order to evaluate the efficacy of such a spacecraft as a real time interplanetary monitor to predict substorm activity and to measure solar wind energy input into the terrestrial magnetosphere, three periods in 1978 were selected in which to perform detailed correlation analyses between solar wind and interplanetary magnetic field (IMF) parameters and magnetospheric activity indices such as AE and Dst. High time resolution (1 min) averages of combined solar wind parameters and IMF parameters were used to perform linear cross‐correlation analyses with the similarly averaged magnetospheric activity indices. A significant dependence of these results on levels of activity is found. During relatively quiet conditions, all tested parameters (ε, VBs, V²Bs) produce high (0.7–0.8), nearly identical linear correlation coefficients with AE. During disturbed times, substantially lower correlations between all parameters and AE are found (r ≲ 0.6 for VBs; r ≲ 0.4 for ε). A measure of total magnetospheric energy dissipation (UT) involving both auroral and ring current terms is useful and can be meaningfully calculated during disturbed periods of geomagnetic storm (or stormlike) activity. UT is rather highly correlated (r > 0.6) with all tested parameters during disturbed times. The parameter ε is often superior to the Bs parameters in prediction of UT. Notably, we find that the typical intrinsic 25‐ to 40‐min delay between interplanetary parameters and magnetospheric response during quiet periods diminishes to ≲15 min during disturbed times.
Polarization electric fields in the nighttime F layer at Arecibo
Authors: Burnside R. G. , Walker J. C. G. , Behnke R. A. , Gonzales C. A.
Published: February 2008

Incoherent scatter radar measurements made in 1981 and 1982 are used to determine the electrical conductivity of the nighttime E and F regions at Arecibo, Puerto Rico. The height‐integrated Pedersen conductivity of the F region exceeds that of the E region by a factor of 10 on the average. We therefore conclude that, for the solar maximum conditions we have studied, the local E region has little influence on nighttime F region electrodynamics. Simultaneous measurements of ion and neutral velocities are often inconsistent with the hypothesis that a fully developed electric polarization field is generated by dynamo action in the local F region. A better explanation of our results is obtained by including the influence of polarization fields that originate in the F region of the conjugate hemisphere.
Development of substorm activity in multiple‐onset substorms at synchronous orbit
Authors: Nagai T. , Baker D. N. , Higbie P. R.
Published: February 2008

The development of substorm activity at synchronous orbit (6.6 RE) for multiple‐onset substorms was studied by using the energetic particle and magnetic field data in three local time groups (the afternoon region, the premidnight region, and the postmidnight region). The geostationary spacecraft 1976‐059 and 1977‐007 provided the measurements of 30‐ to 2000‐keV electrons and 145‐ to 560‐keV protons, GMS the measurements of > 2000 keV electrons, and GOES 1 and GOES 2 the measurements of the magnetic field. The ground magnetic variations in a wide area were examined to determine the general characteristics of substorm activity and the timing of various phenomena. The substorm evolutions showed a variety of forms at synchronous orbit, but they can be characterized by the following view: Substorm activity initially starts in a longitudinally limited sector that is situated in the premidnight region, and the effect of this activity is not seen in other regions. The activity then expands westward and eastward during a period of successive onsets. There is an indication that the westward propagation is faster than the eastward propagation.
A quantitative description of the spatial distribution and dynamics of the energy flux in the continuous aurora
Authors: Whalen J. A.
Published: February 2008

Energy flux in the continuous (or diffuse) aurora is found to be constant instantaneously along contours that are defined analytically as a function of magnetic latitude and UT, coordinates that are approximately circles in magnetic latitude‐magnetic local time centered at a pole offset from the magnetic pole. As a consequence, the latitudinal distribution in these coordinates is the same instantaneously at all magnetic local times (the observed extent of which is 10 hours); changes that take place in the distribution do so simultaneously (within 15 min of UT resolution) throughout this entire extent. These properties are observed in a case study of 8 hours duration by an array of ground and air based ionospheric sounders that span the 10 hours of magnetic local time at auroral latitudes. Energy flux in the continuous aurora is derived from the resulting ionized auroral E layer critical frequency that is measured simultaneously at 15 min intervals by all members of the array. Two latitudinal distributions are identified. The principal one is a Gaussian distribution at lower latitude; a separate distribution that is of lower intensity occurs at higher latitude. The two correspond respectively to the central plasma sheet and boundary plasma sheet regions of particle precipitation. The central plasma sheet distribution is found in all 32 determinations to be well described by a single Gaussian function, 3.2° full width at half maximum. Accordingly, the dynamics of the entire latitude‐local time distribution can be described quantitatively by the time history of two parameters, the Gaussian maximum energy flux (which varies between extremes of 0.25 and 7 ergs/cm²s) and the latitude of the maximum (which varies throughout a range of 5°). The determination of these parameters every 15 min resolves such rapidly varying features as substorm onset; the 8 hour duration encompasses such long term features as substorm lifetime and UT dependent latitude variation. A number of areas of agreement are found between results reported here and those reported by others. In particular, evidence that the continuous aurora is highly uniform exists not only in this case study but also in statistical studies that find iso‐energy flux contours to be offset circles.
Photochemistry of N2+ in the daytime F region
Authors: Breig E. L. , Hanson W. B. , Hoffman J. H. , Abdou W. A.
Published: February 2008

The photochemistry of N2+ in the daytime F region continues to be a source of concern due primarily to the uncertain roles of the metastable O+(²D) ion and possible vibrational excitation. This investigation adopts a unique subset of data from the Atmosphere Explorer C satellite that spans distinct regions near the low‐latitude F peak wherein either chemical reactions or electron processes alternatively control the N2+ abundances. Concentrations of N2+ calculated according to current theory and including recent laboratory data for O+(²D) losses via N2 are found to exceed ionospheric observations between 220 and 400 km by a factor near 2. Relevant characteristics of the basic molecular ion concentration measurements are described, along with an analysis for select orbits conducted according to current photochemical equilibrium theory. Possible simple modifications of this theory are discussed in terms of the constraints suggested by the nature of the observations.
Removal of velocity bias in the interpretation of measurements of the azimuthal component of the interplanetary magnetic field
Authors: Eyni M. , Steinitz R.
Published: February 2008

Parker's model for the interplanetary magnetic field predicts an r−1 dependence for the azimuthal component. Departures from this relation were indicated in an analysis of a subset of the Helios 1 data. As in a previous analysis of Pioneer 10 data, these departures can be traced to velocity bias. We examine the decrease in solar wind speed which occurred as Helios 1 approached the sun and thus simulated a steeper gradient of the azimuthal magnetic field component. A simple linear fit between flow speed and distance from the sun enables us to remove from the data most of the velocity bias present. This procedure substantially improves the agreement between Parke's model and the Helios data.
Comparison of model high‐latitude electron densities with Millstone Hill observations
Authors: Sojka J. J. , Schunk R. W. , Evans J. V. , Holt J. M. , Wand R. H.
Published: February 2008

Model predictions of the diurnal variations of plasma convection velocities and electron densities in the high‐latitude ionosphere were compared with Millstone Hill incoherent scatter radar observations for an equinox day on which there was moderate magnetic activity. On the observation day, three major morphological features were evident at 500 km, including a dayside high density region, a nightside mid‐latitude trough, and a region of slightly enhanced densities in the auroral zone. Although the dayside high density region was due to solar EUV radiation, it was not symmetrical about local noon (1000–1900 LT sector) owing to the effect of horizontal transport. The nightside mid‐latitude trough was the deepest, the widest, and reached its most equatorward latitude in the morning sector. The model was able to reproduce these two features quite accurately. In the dusk sector, the trough was filled in, and its latitudinal extent was restricted by a discrete auroral arc, a feature not included in the ionospheric model. Except for this arc region, the enhanced electron densities in the auroral zone were adequately described by the average precipitation fluxes used in the model. The observed plasma drift velocities were consistent with a two‐cell, asymmetric convection pattern with enhanced flow in the dusk sector. Outside the polar cap, the fall‐off of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. The convection pattern employed in the model included these features and had a 60 kV cross‐polar‐cap potential. Efforts to reproduce the observed behavior by using a larger cross‐polar‐cap potential (90 kV) or a symmetric pattern are also presented. These were generally less successful and demonstrate the sensitivity of the morphology of the F region at high latitudes to the convection process.
Saturn's magnetosphere: Observations of ion cyclotron waves near the Dione L shell
Authors: Smith Edward J. , Tsurutani Bruce T.
Published: February 2008

High time resolution (0.75 s) measurements obtained by the Pioneer 11 vector helium magnetometer inside Saturn's magnetosphere show quasi‐periodic waves to be present near the Dione L shell between L = 6.3 and 6.7. Although Dione was far from the spacecraft, the waves were observed when Pioneer was both inbound to, and outbound from, periapsis and are presumably associated indirectly with Dione. The waves have a characteristic period of 18 s and a typical amplitude of 5 nT. In the region in which the waves were observed, the Pioneer 11 plasma analyzer detected a peak plasma density associated with heavy ions presumably sputtered from Dione's surface and tentatively identified as O++. Subsequent Voyager observations in this inner torus appear to be consistent with 0+. The characteristic period of the waves is well below the proton gyroperiod. Because the heavy ions are hot (∼106 K), the Alfvén phase speed and the ion thermal speeds are nearly the same. Ion cyclotron resonance of the waves with the dominant ions appears capable of generating the waves. Theoretical arguments based on the growth rates of the waves suggest that O++ is more likely to be responsible than O+ but that a resonance involving H+ ions with energies of a few keV cannot be excluded. The existence of a pitch angle anisotropy associated with a loss cone is implied. For O++, a pitch angle anisotropy of ≃0.70, corresponding to T⊥/T∥ ≃ 1.7, is predicted. Pitch angle scattering of the heavy ions by the waves should cause precipitation with the possible production of an aurora near 67° latitude.
Simulation of the ion tearing instability in the presence of a background plasma
Authors: Ambrosiano John J. , Lee Lou C. , Swift Daniel W.
Published: February 2008

A comparison is made between two classes of Vlasov equilibria with respect to the ion tearing mode instability. The first class is that of a current sheet in equilibrium with a vacuum (Harris type). The second class is that of a current sheet in equilibrium with a magnetized background plasma. The comparison is made via a simple magnetoinductive simulation involving ions alone. We find that the presence of the background species has several dramatic effects: The linear growth rates are affected, the tearing mode can be stabilized or destabilized by the background plasma, and the exchange of energy between particles and fields is affected. In particular, the saturation wave energy level of the tearing instability can be either enhanced or reduced by a factor of 10 from the Harris type depending on the depth of the penetration of the background particles into the neutral sheet.
O+ ions in the polar wind
Authors: Barakat A. R. , Schunk R. W.
Published: February 2008

A semikinetic model was used to describe the steady state collisionless flow of the polar wind along diverging geomagnetic field lines at high latitudes. Emphasis was given to studying the behavior of O+ ions for a wide range of boundary conditions at the baropause (4500 km). The main result obtained was that for high electron temperatures (Te ∼ 10,000 K) O+ is not gravitationally bound and significant escape fluxes (∼107 cm−2 s−1) of suprathermal (∼2 eV) O+ ions occur. The O+ flow is supersonic over most of the altitude range considered, and O+ Mach numbers as large as 20 are predicted at 10 RE. Also, depending on the boundary conditions, the O+ density can be comparable to or greater than the H+ density at high altitudes above the baropause when the electron temperature is high.
Relation of compressional HM waves at Goes 2 to low‐latitude Pc 3 magnetic pulsations
Authors: Yumoto Kiyohumi , Saito Takao
Published: February 2008

Daytime magnetic pulsations observed at synchronous orbit by GOES 2 are analyzed to determine whether there is a candidate for the source of low‐latitude Pc 3's in the outer magnetosphere. Compressional, radially transverse and azimuthally transverse modes of magnetic pulsations in a wide frequency range exist simultaneously in the outer daytime magnetosphere. The compressional and the transverse modes of the daytime magnetic pulsations observed at GOES 2 are statistically dominant in the Pc 3 and in the Pc 4 frequency ranges, respectively. Some 70% of the compressional Pc 3 pulsations at GOES 2 have periods similar to the low‐latitude Pc 3's observed at San Gabriel Canyon, which is located ∼11° west of GOES 2's longitude at L = 1.8. Amplitudes of the compressional. Pc 3 pulsations at GOES 2 (L = 6.67) are relatively larger than those of the low‐latitude Pc 3's at SGC (L = 1.8). The transverse pulsations with larger amplitude (δB ≳ 1.0 nT) at GOES 2 can propagate only into the high‐latitude ionosphere. The compressional Pc 3 pulsations with smaller amplitude (δB∼ 0.5 nT) at GOES 2 are theoretically expected to propagate across the ambient magnetic field to very low latitudes to couple with one or more the following: (1) a surface wave at the plasmapause (Lpp); (2) a trapped oscillation of the fast magnetosonic wave in the plasmasphere (L = 1.7 ‐ Lpp); (3) a higher‐harmonic standing oscillation of a local field line at mid‐latitude (L = 2.0 ‐ Lpp); or (4) a fundamental standing oscillation at very low‐latitudes (L = 1.1 and 1.7–2.6). Although further research is needed to clarify the wave and propagation characteristics of the low‐latitude Pc 3 pulsations, we believe that the compressional Pc 3 magnetic pulsation in the outer magnetosphere is one candidate for the source of the low‐latitude Pc 3 magnetic pulsation.
Rare ground‐based observations of Siple VLF transmitter signals outside the plasmapause
Authors: Carpenter D. L. , Miller T. R.
Published: February 2008

Signals from the Siple, Antarctica (L ∼4.3), VLF transmitter observed at the conjugate ground station Roberval, Canada, have previously been found to propagate either within the outer plasmasphere or within the region of steep plasmapause density gradients, but on paths with equatorial electron density that is within a factor of 2 of nearby plasmaspheric levels. We report here two cases in which propagation occurred just outside the plasmapause and at plasma trough density levels. In one case a series of 1‐s pulses and frequency ramps was observed; many of the pulses triggered risers with slopes of ∼10 kHz/s, much steeper than those usually observed within the plasmasphere. In the other case, no evidence of triggered emissions was seen on the Siple pulses, but instead efficient echoing occurred and noise band precursors to large wave bursts were initiated. The wave bursts were of a type that has been previously identified as driving transient bursts of electron precipitation into the ionosphere. Both cases occurred under magnetic conditions more disturbed than those typical of strong Siple signal propagation in the outer plasmasphere.
On the coupling of test ions to magnetoplasma flows through turbulence
Authors: Brinca Armando L.
Published: February 2008

The evolution in velocity and configuration spaces of ions injected into a flowing turbulent magnetoplasma is studied from a test particle viewpoint. Numerical integration of the equations of motion for several types of fluctuations and injection velocities permits assessing the effectiveness of the turbulence in forcing the ensemble average velocity of the ions to approach the bulk flow velocity. The choice of parameters models the geomagnetosheath, and the downstream ions eventually decoupled from the solar wind flow while crossing the bow shock, but the envisaged conditions are relevant to several space plasma situations. The observed power spectra of the fluctuations in the sheath suggest that, though infrequently, the coupling of the injected ions to the bulk plasma motion may be attained within a few gyroperiods, strongly modifying the dynamics imposed by the average magnetic and convection electric fields.
Kinetic theory of current and density drift instabilities with weak charged‐neutral collisions
Authors: Gary S. Peter
Published: February 2008

This paper describes the linear kinetic theory of electrostatic instabilities driven by a density gradient drift and a magnetic‐field‐aligned current in a plasma with weak charged neutral collisions. The configuration is that of a uniform magnetic field
, a weak, uniform density gradient in the x direction and a weak, uniform electric field in the z direction. Collisions are represented by the BGK model. The transition from the (kinetic) universal density drift instability to the (fluidlike) current convective instability is studied in detail, and the short wavelength properties of the latter mode are investigated.
Latitudinal variations of Joule heating due to the auroral electrojets
Authors: Duboin M.‐L. , Kamide Y.
Published: February 2008

Empirical scaling factors between the height‐integrated joule heating rate deduced from measurements of the Chatanika incoherent scatter radar and the square of magnetic perturbations at College are derived. These relationships, calculated for different situations (the eastward and westward electrojets and daytime) are then used to estimate the joule heating rate along the latitudinal extent of the auroral oval on the basis of magnetometer data from the IMS Alaska meridian chain of observatories. The total heat input for a disturbed day (April 12, 1978) is estimated to be 2.8×1015 J.
Solstitial and hemispherical asymmetry in the response of geomagnetic field
Authors: Shah G. N. , Kaul R. K. , Kaul C. L. , Razdan H. , Merryfield W. J. , Wilcox J. M.
Published: February 2008

We show that the geomagnetic field is more prone to disturbances around the June solstice than around the December solstice, as evidenced from a larger enhancement in geomagnetic activity indices ap, an, and as, following the onset of transient solar disturbances occurring in the three‐month period around June solstice than in the interval around the December solstice. Further, an asymmetry between the northern and southern hemisphere geomagnetic activity is shown to exist, independent of the level of the activity. This asymmetry, represented by (an−as)/(ap+as)/2 shows a regular annual variation with a maximum of 60% around June solstice and is almost absent around the December solstice.
The geomagnetic quiet nighttime thermospheric wind pattern over Fritz Peak Observatory during Solar Cycle Minimum and Maximum
Authors: Hernandez G. , Roble R. G.
Published: February 2008

Nighttime thermospheric winds and temperatures at F region heights have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer from about November 1972 to the present. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867 K (630 nm) line emission. The data obtained during geomagnetic quiet conditions (Ap ≤ 20) have been separated into two groups representing solar cycle minimum conditions (1972–1977) and solar cycle maximum conditions (1978–1979). The monthly variation of the nighttime zonal and meridional winds are presented for both groups. The results show that during solar cycle minimum the zonal winds are predominantly eastward during the winter months at speeds of 50–75 m s−1 and westward during the summer months at speeds of 50–100 m s−1. The meridional winds measured to the north and south of Fritz Peak Observatory are equatorward throughout most of the night at nearly the same speed. The equatorward winds in the summer peak near midnight at about 100–150 m s−1. The equatorward winds in the winter also peak near midnight but with smaller speeds, 50–75 m s−1. During solar cycle maximum the zonal winds are again predominantly eastward during the winter months, reaching peak speeds of 60–75 m s−1 in the early evening hours and decreasing afterward. In the summer the winds are eastward during the early evening hours, shifting to westward near midnight and increasing to a maximum speed of 75–100 m s−1 in the early morning hours. The meridional winds are poleward at ∼20 m s−1 in the early evening hours, shifting to equatorward near 2100 LT and then increasing in magnitude with maximum speeds near or after midnight. There is a considerable difference in the speed of the winds measured to the north and south of Fritz Peak Observatory. During the summer the equatorward winds measured to the north are 150–175 m s−1, whereas the equatorward winds measured to the south are 50–75 m s−1. During the winter the speeds are smaller, 50–75 m s−1 and 25–30 m s−1 to the north and south of the station, respectively. Calculations made with the National Center for Atmospheric Research thermospheric general circulation model for different seasons are in reasonable agreement with the observations during both solar minimum and solar maximum. During solar cycle maximum the observed and calculated meridional wind differences to the north and south of Fritz Peak Observatory are attributed to enhanced auroral heating and ion drag associated with magnetospheric convection north of the observatory. The equatorward driven winds slow south of Fritz Peak Observatory owing to the diminishing influence of high‐latitude forcings.
Electrostatic ion‐cyclotron waves in magnetospheric plasmas: Nonlocal aspects
Authors: Ganguli G. , Bakshi P. , Palmadesso P.
Published: February 2008

The importance of the effect of the magnetic shear and the finite size of the current channel on the electrostatic ion‐cyclotron instability for the space plasmas is illustrated. A nonlocal treatment is used. When the channel width Lc is larger than the shear length Ls, there is a large reduction in the growth rate along with a noteworthy reduction of the band of the unstable perpendicular wavelengths. For Lc ⪷ Ls/10 the growth rate is not much altered from its local value; however, for Lc/ρi ≲ 10² the growth rate starts falling below the local value and vanishes for Lc ∼ ρi. The nonlocal effects lead to enhanced coherence in the ion cyclotron waves.