bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermospheric nighttime neutral temperature and winds over Fritz Peak Observatory: Observed and calculated solar cycle variation
Authors: Hernandez G. , Roble R. G.
Published: February 2004

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer for nearly 17 years between 1968 and 1985. We use data for two specific November periods, (1) 1984, near solar cycle minimum, and (2) 1979, near the peak of solar cycle 21, to illustrate the measured solar cycle variation of nighttime neutral gas temperature and winds over the station. In particular, we present data for November 11, 1979, the day following the day when the solar F10.7 radio flux emission reached its greatest daily value of 367 J (1 J = 1.0×10−22 W m−2 Hz−1). The nighttime measurements, all selected for geomagnetic quiet conditions, show a considerable variation of the thermospheric temperature between solar minimum and solar maximum of nearly 500 K but a relatively minor variation in the thermospheric winds. The recently developed National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) is used to simulate the geomagnetic quiet time variation of global thermospheric properties for the two periods and also to perform a time‐dependent simulation to calculate the thermospheric variation during November 9–12, 1979, when the solar F10.7 flux varied from 314 J, 367 J, 325 J, and 294 J, respectively. The TIE‐GCM histories are used to construct the diurnal variation of thermospheric temperatures and winds as a function of altitude over Fritz Peak Observatory. The aeronomy calculated by the TIE‐GCM is also used to predict the diurnal variation and height of the 630‐nm volume emission rate. The station processor calculates the emission‐weighted height‐integrated Doppler line profiles and Doppler shift profiles as would be observed from a ground station. These profiles are reduced into winds and temperatures in a manner similar to the experimental measurements and can be compared with the actual observations. The results show that the TIE‐GCM calculated Doppler temperature and winds are in reasonable agreement with the observations for the two periods representing solar minimum and extreme maximum conditions, suggesting that the solar flux model and aeronomic processes in the TIE‐GCM can simulate the main thermospheric variations throughout the solar cycle. Furthermore, the time‐dependent calculated variations of Doppler temperature and winds also remain in good agreement with the observations of November 11, 1979, which is the day following the maximum solar EUV perturbation of the Earth's upper atmosphere during the entire solar cycle 21. These results show the need for time‐dependent calculations when geophysical parameters have large changes during the course of the period of simulation. The station processor results indicate that the altitude of the 630‐nm emission varies during the solar cycle by keeping at a near‐constant pressure surface, whose geometrical height changes with the solar cycle.
Observations of plasmas and magnetic fields in Earth's distant magnetotail: Comparison with a global MHD model
Authors: Frank L. A. , Ashour‐Abdalla M. , Berchem J. , Raeder J. , Paterson W. R. , Kokubun S. , Yamamoto T. , Lepping R. P. , Coroniti F. V. , Fairfield D. H. , Ackerson K. L.
Published: February 2004

We are reporting the first direct comparison of in situ observations of plasmas and magnetic fields in Earth's distant magnetotail with the results of a time‐dependent, global magnetohydrodynamic (MHD) simulation of the interaction of the solar wind with the magnetosphere. The magnetotail observations were taken with the Geotail spacecraft during the period 0300–0630 UT on October 27, 1992 at a position near the dawnside magnetopause at a downstream distance of about 81 RE. During this period a dense, cold ion stream similar in density and speed to that expected for the magnetosheath plasmas was intermittently observed. When the cold ion stream was not present, the spacecraft was located in the northern magnetotail lobe. The dense, cold ion stream differed from that expected for the magnetosheath in the Y and Z components of ion bulk flow and in the Y component of the magnetic field. These cold ion streams are associated with a magnetopause accommodation region positioned just outside the classical magnetopause, as identified by a well‐defined transition from magnetic fields typical of those found in the lobe to the lesser and more fluctuating fields in the magnetosheath. This accommodation region exhibits perturbations in plasma flows and magnetic fields that appear to be related to the complex topology of the magnetopause at these large downstream positions. Simultaneous observations of the solar wind ions and the interplanetary magnetic field (IMF) with the IMP 8 spacecraft upstream from Earth provided the driving input for a global MHD model. The solar wind ion flow was steady during this period, and the IMF exhibited a series of rotations from northward to duskward. The dynamics of the magnetotail were controlled by the Y and Z components of the IMF. When this By was strongly positive, the magnetotail lobe appeared at the downstream Geotail position. Examination of the modeled plasma parameters in the Y‐Z plane through the spacecraft position shows that this By provides a torque on the magnetotail about its central axis. The MHD model also accurately positions the spacecraft alternately in the magnetopause accommodation region and the magnetotail lobe as the IMF clock angle varied from northward to duskward, respectively. The temporal variations of modeled parameters, i.e., ion densities, temperatures, and bulk flow velocities and the magnetic field components, are directly compared with the Geotail measurements. This first comparison of the Geotail observations with the modeled plasma parameters and magnetic fields provides substantial encouragement that a global MHD model can provide a valid description of important aspects of the large‐scale topology and dynamics of the magnetotail.
Satellite remote sensing of thermospheric O/N2 and solar EUV: 1. Theory
Authors: Strickland D. J. , Evans J. S. , Paxton L. J.
Published: February 2004

Simultaneous measurements of disk‐viewing OI 135.6 nm and N2 Lyman‐Birge‐Hopfield (LBH) dayglow can be used to monitor the solar EUV flux QEUV and the column abundance of thermospheric O relative to N2 (O/N2). We report on a study that quantifies the relationships between these emissions and the above parameters. Emission is considered from 134.5 to 139.0 nm (designated 135.6 nm) and from 155.0 to 170.0 nm (designated as LBH) at a resolution of 3.6 nm. The intervals and resolution were chosen for analysis of satellite dayglow data to be reported in the companion paper by Evans et al. (this issue). The first interval is dominated by OI 135.6 nm with minor contributions from LBH 135.4 and 138.3 nm. The second interval contains only LBH. An important finding is that 135.6/LBH is essentially independent of the solar EUV spectrum from low to high activity based on using the Hinteregger formulation for characterizing spectral changes with solar activity. Given this behavior in 135.6/LBH, one can then unambiguously interpret changes in this ratio in terms of changes in O/N2. Model results show that the relationship between O/N2 and 135.6/LBH is essentially independent of model atmosphere. Given either 135.6/LBH or O/N2, QEUV can then be obtained directly from the absolute intensity of either 135.6 nm or LBH.
Synergistic effects of hot plasma‐driven potentials and wave‐driven ion heating on auroral ionospheric plasma transport
Authors: Brown D. G. , Horwitz J. L. , Wilson G. R.
Published: February 2004

Transverse acceleration by waves and parallel acceleration by field‐aligned electric fields are important processes in the transport of ionospheric ions along auroral field lines. In order to study the transport of ionospheric plasma in this environment we have developed a generalized semikinetic model which combines the tracking of ionospheric ion gyrocenters with a generalized fluid treatment of ionospheric electrons. Large‐scale upward and downward directed electric fields are generated within the model by introducing magnetospheric plasma whose components have differing temperature anisotropies. We study the effects of such potentials when combined with the effect of ion heating by a distribution of waves along the flux tube. We find that the combination of wave heating and an upward electric field results in an order of magnitude increase in O+ outflow (compared to a case with an upward electric field and no wave heating). Under these conditions we observe the formation of bimodal conics. When a downward electric field is added to a case with wave heating, the energy gained by the ions from the waves increases by a factor of 2 or 3 (over the scenario with wave heating and no hot plasma‐driven electric field) owing to their slower transit of the heating region. Typically, the velocity distributions under these conditions are toroids and counterstreaming conics. We also find that the upflowing, dense, heated ionospheric plasma acts to reduce the potential set up by the anisotropies in the magnetospheric components.
Using Fe X 6374 Å and Fe XIV 5303 Å Spectral line intensities to study the effect of line of sight integration on coronal temperature inferences
Authors: Esser R. , Brickhouse N. S. , Habbal S. R. , Altrock R. C. , Hudson H. S.
Published: February 2004

Polar coronal holes are relatively stable structures persisting over many solar rotations. The appearance of coronal holes in remote observations, however, can change on a daily basis due to variations of the denser and hotter plasma surrounding them. We explore the effect of these denser and hotter surrounding regions on coronal hole observations, using daily intensity measurements at 1.15 RS of the green Fe XIV 5303 Å and red Fe X 6374 Å spectral lines. The observations, which were carried out at the National Solar Observatory at Sacramento Peak, New Mexico, cover a time period of about four solar rotations. We show that the “coronal hole” temperatures derived using the line ratio technique, vary by more than 0.8×106 K over the time interval considered here. We also provide a short discussion of the expected accuracy of the atomic data for these two iron spectral lines. Using intensity measurements as a function of distance from the Sun, we briefly discuss how the regions surrounding the coronal holes might influence the inference of the temperature gradient in the coronal holes. The line of sight effect on the temperature gradient should be explored in more detail in the future using daily observations of the line intensities as a function of distance. These observations could be provided by ground‐based coronagraphs and by instruments on board SOHO.
Effect of local interstellar medium hydrogen fractional ionization on the distant solar wind and interface region
Authors: Baranov V. B. , Malama Y. G.
Published: February 2004

The paper investigates the influence of the local interstellar medium (LISM) proton number density on the gasdynamics of LISM / solar wind interaction. It uses the self‐consistent axisymmetric model of Baranov and Malama (1993) for a constant value of the LISM neutral hydrogen number density (nH∞ = 0.14 cm−3). It is shown that the flow structure is asymmetric relative to the upwind and downwind directions. Numerical results are presented for four values of the proton number density in the LISM: np∞ = 0.005; 0.025; 0.1 and 0.28 cm−3. The results are given along the axis of symmetry only (in upwind and downwind directions). Velocity, number density, and temperature distributions of the plasma component as well as of the H atoms of the LISM are presented. It is shown that the influence of H atoms, penetrating from the LISM into the solar system on the solar wind may be strong enough to be observed by the “Voyager” spacecraft. The results show that the number density of the LISM's plasma component as well as of the solar wind has very large gradients in the interface region (between the bow and termination shocks). This effect may be very important for the interpretation of the kHz radiation detected by “Voyager”. Moreover, the asymmetric (in upwind and downwind directions) distributions of H atom number density, bulk velocity, and temperature may be observed on the basis of the scattered solar Lyman alpha radiation data.
Ionospheric electron densities calculated using different EUV flux models and cross sections: Comparison with radar data
Authors: Buonsanto M. J. , Richards P. G. , Tobiska W. K. , Solomon S. C. , Tung Y.‐K. , Fennelly J. A.
Published: February 2004

The recent availability of the new EUVAC (Richards et al., 1994) and EUV94X (Tobiska, 1993b, 1994) solar flux models and new wavelength bin averaged photoionization and photoabsorption cross section sets led us to investigate how these new flux models and cross sections compare with each other and how well electron densities (Ne) calculated using them compare with actual measurements collected by the incoherent scatter radar at Millstone Hill (42.6°N, 288.5°E). In this study we use the Millstone Hill semiempirical ionospheric model, which has been developed from the photochemical model of Buonsanto et al. (1992). For the F2 region, this model uses determinations of the motion term in the Ne continuity equation obtained from nine‐position radar data. We also include two simulations from the field line interhemispheric plasma (FLIP) model. All the model results underestimate the measured Ne in the E region, except that the EUV94X model produces reasonable agreement with the data at the E region peak because of a large Lyman β (1026 Å) flux, but gives an unrealistically deep E‐F1 valley. The ionospheric models predict that the O2+ density is larger than the NO+ density in the E region, while numerous rocket measurements show a larger NO+ density. Thus the discrepancy between the ionospheric models and the radar data in the E region is most likely due to an incomplete understanding of the NO+ chemistry. In the F2 region, the photoionization rate given by EUV94X is significantly larger than that given by the EUVAC and earlier models. This is due to larger EUV fluxes in EUV94X compared to EUVAC over the entire 300‐1050 Å wavelength range, apart from some individual spectral lines. In the case of EUVAC, this is partly compensated for by larger photoelectron impact ionization due to the larger EUV fluxes below 250 Å. The differences between ionospheric model results for the different cross‐section sets are generally much smaller than the differences with the data.
Comparison of satellite electron density and temperature measurements at low latitudes with a plasmasphere‐ionosphere model
Authors: Su Y. Z. , Oyama K.‐I. , Bailey G. J. , Takahashi T. , Watanabe S.
Published: February 2004

Observations made by the Hinotori satellite of the latitude and diurnal variations of electron density and temperature near 600 km altitude in the low‐latitude region are studied by comparison with values from the Sheffield University plasmasphere‐ionosphere model (SUPIM). The model results show that the observed features of higher electron density in the summer hemisphere and higher electron temperature in the winter hemisphere are caused principally by the difference in the summer and winter hemisphere values of the meridional neutral wind. Closer agreement between the modeled and observed values is obtained when the interhemisphere difference in the meridional wind, as given by the horizontal wind model (HWM) 90, is reduced and when the peak value of the daytime poleward wind is moved to the afternoon sector in the winter hemisphere and to the morning sector in the summer hemisphere. The model results also show that the altitude variation of the vertical E×B drift velocity plays an important role in the development of the ionospheric equatorial anomaly. The latitude and diurnal variations of the modeled electron density and temperature are in good agreement with the observations when the E×B drift velocity used by the model is in accord with the observations made by the AE‐E satellite for magnetic field lines with apex altitude less than 400 km and at Arecibo for magnetic field lines with apex altitude greater than 2000 km; linear interpolation of the observed values is used for the intermediate magnetic field lines.
Rotational anisotropy of the Jovian magnetosphere at high latitudes
Authors: Hawkins S. E. , Cheng A. F. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2004

The Ulysses spacecraft passed through the Jovian system in February 1992. Its trajectory, determined by its interplanetary mission, directed the spacecraft to the previously unexplored high‐latitude regions of the Jovian magnetosphere. Like Pioneers 10 and 11 and the two Voyagers, Ulysses entered the magnetosphere in the near equatorial morning sector, but unlike the previous spacecraft, Ulysses exited at high latitude near local dusk. This report considerably extends the work of Lanzerotti et al. (1993) by analyzing the extent of rotational anisotropy at high southern Jovigraphic latitudes attained by Ulysses during its outbound pass. Using data acquired by the Ulysses Heliosphere Instrument for Spectral, Composition, and Anisotropy at Low Energies (HI‐SCALE), we report some evidence of corotational flow in the high‐latitude duskside magnetosphere. On the inbound pass the Ulysses HI‐SCALE instrument observed anisotropies indicating partial corotation and a corotation lag. These data are consistent with Voyager plasma and energetic particle results. This is the first report of flow in the corotational direction from 44–57 RJ in the duskside, high‐latitude Jovian magnetosphere. There is strong evidence for a substantially reduced corotational anisotropy in this duskside region relative to a comparable radial distance range, 47–59 RJ, on the dayside.
Particle interactions with obliquely propagating magnetosonic waves
Authors: Srivastava Krishna M. , Tsurutani Bruce T. , Sauer Konrad , Sharma V.
Published: February 2004

Four cases of nonlinear obliquely propagating magnetosonic (MS) waves are considered using a test particle approach for particles interaction with (1) monochromatic waves propagating both sunward and antisunward, (2) monochromatic waves propagating unidirectionally toward the Sun, (3) a broad band spectrum, propagating both sunward and antisunward, and (4) a broadband spectrum, propagating sunward only. As the solar wind decelerates rapidly inside the bow shock, the interplanetary magnetic field (IMF) increases. Calculations have been performed taking into account such a spatially dependent IMF (based on observations). We find that significant particle acceleration is achieved for both the monochromatic wave and the “turbulent” MS waves, for oblique propagation (angles between the interplanetary magnetic field and the propagation vector larger than 30°). This is due to the fact that at oblique angles resonance width increases. We have compared the relative acceleration of particles for the four cases. Pitch angle scattering and acceleration of particles are found to be much larger in the case of sunward and antisunward propagating waves than those for the sunward waves only. This is due to larger relative phase velocities in the two cases. Also, acceleration of particles is less in the case of a uniform magnetic field than that in the case of spatially increasing magnetic field. The particles get more pitch angle scattered and accelerated in the latter case.
Kinetic theory of the Farley‐Buneman instability in the E region of the ionosphere
Authors: Dimant Y. S. , Sudan R. N.
Published: February 2004

The linear stage of the Farley‐Buneman instability in the ionospheric E region is reexamined. Unlike previous theories, the present analysis is based on a consistent kinetic theory for electrons developed earlier which takes into account such important factors as the non‐Maxwellian nature of the perturbed electron distribution, different rates for electron momentum and energy relaxation, and the velocity dependence of the electron‐neutral collision frequency. The dispersion relation obtained using this theory is applicable for wave frequencies small compared to the ion‐neutral collision frequency and for sufficiently low altitudes where the electron‐electron collisions are negligible. The threshold conditions prove to be strongly modified compared to those resulting from the earlier simplified theories. For strong electric fields which can occur in the auroral regions, unstable long‐wave waves excited along the bisector between the directions of the electric field E0 and E0×B drift velocity are predicted at low atitudes. The excitation of such waves becomes possible due to a new kinetic mechanism associated with the perturbed electron current caused by the Pedersen conductivity.
Solar cycle changes in the level of compressive microturbulence near the Sun
Authors: Coles W. A. , Grall R. R. , Klinglesmith M. T. , Bourgois Gabriel
Published: February 2004

Observations of the coherence bandwidth of interplanetary scintillations (IPS) made at the Nançay Radio Observatory between 1982 and 1992 have been used to estimate the variance of electron density δNe² at solar distances of 5 to 15 Rs. These IPS observations are sensitive to scales of the order of 10 km; i.e., they measure the “microscale variance,” which is a small fraction of the total density variance. Measurements in the equatorial region are invariant over the solar activity cycle and show the same behavior with distance as the mean density, i.e., δNe² ∝ Ne². They do not show a statistically significant change in the “transonic region” as reported by other observers (Lotova et al., 1985). Measurements over the polar region show a clear variation with the solar cycle. At solar maximum, δNe² is invariant with latitude, as is Ne (Withbroe, 1988). However, the apparent δNe² measured by IPS is about 10 times lower in the polar streams which form during the declining and minimum phases of solar activity. Compensation for the line of sight integration inherent in the IPS observations indicates that the local value of δNe² is actually about 15 times lower in the polar streams. We cannot estimate the latitude variation of the “development” of the compressive turbulence δNe/Ne between 5 and 15 Rs because measurements of Ne in the polar hole are lacking.
Observations and modeling of correlated Pi B magnetic and auroral luminosity pulsations
Authors: Grant I. F. , Burns G. B.
Published: February 2004

A comparison of observations of Pi B magnetic pulsations at the ground with wide‐angle photometer measurements of the accompanying auroral luminosity fluctuations has found several cases of well‐correlated magnetic and optical pulsations, with periods in the range 15–50 s. This finding strongly suggests that precipitation fluctuations drive at least some of those components of the Pi B signal with periods in this range. We found no luminosity fluctuations associated with the 3‐s enhancement of Pi B pulsations. We modeled the magnetic perturbation under a patch of enhanced E region conductivity as it drifts or as its conductivity changes, for circular and strip patch shapes. These idealized models can reproduce some features of the Pi B signature. Most notably, a monotonic variation in the model parameters can produce a reversal in the sense of polarization rotation. A moderate enhancement in the conductivity of a 1‐km‐wide strip can produce magnetic perturbations with amplitudes typical of Pi B pulsations.
Auroral oxygen precipitation at Jupiter
Authors: Cravens T. E. , Howell E. , Waite J. H. , Gladstone G. R.
Published: February 2004

The Jovian aurora is much more powerful than the terrestrial aurora, and some of the auroral energy deposition is thought to come from energetic oxygen and sulfur precipitation. Auroral oxygen and sulfur ions deposit their energy in the atmosphere by means of ionization, dissociation, and heating processes. Energetic oxygen precipitation at Jupiter was previously modeled with the lowest four charge states (O, O+, O2+, and O3+) (Horanyi et al., 1988). In this paper, the model will be extended to include all the charge states of oxygen from neutral oxygen up to fully stripped oxygen. The energy deposition by incident ions with energies in excess of about 200 keV/amu is largely determined by the higher charge states. The model extension is also needed to study auroral X ray emission by energetic highly charged ions. In this paper we estimate that a total auroral X ray power of about 108 W is produced by highly charged excited oxygen ions, which is in agreement with the X ray power observed at Jupiter by the Röntgen satellite (ROSAT; Waite et al., 1994). We also construct a “predicted” X ray energy spectrum for sulfur and oxygen ions which agrees rather well with the spectrum measured by ROSAT. Our calculations support the assertion made by Waite et al. (1994) that the Jovian X ray emission observed by ROSAT can be explained by energetic ion precipitation.
Structure of the subsolar magnetopause on November 1, 1978, 1510–1530 UT: An effect of reconnection?
Authors: Pudovkin M. I. , Semenov V. S. , Biernat H. K. , Bachmaier G. A. , Kiendl M. T.
Published: February 2004

The terrestrial magnetopause region shows a big variety of different structures and properties depending on different situations, mainly in the free solar wind. In a recent paper, Song et al. (1993) reported a multi‐instrument study on magnetopause parameters on November 1, 1978, 1510–1530 UT. We complement this descriptive study by examining this event in terms of a reconnection model. For doing this we use a shock‐type reconnection model which allows for magnetic fields of arbitrary strength and skew and for arbitrary plasma flows on either side of the reconnection layer as well as for different densities. Thus we are able to model the magnetopause structure within a big parameter regime. We show that the entire sequence of regions and discontinuities detected on November 1, 1978, 1510–1530 UT, and reported by Song et al. (1993) agrees with the magnetopause structure predicted by our reconnection model. However, we point out that some features of the data need additional assumptions, which make the problem open for future study.
Reconnection‐associated auroral activity stimulated by two types of upstream dynamic pressure variations: Interplanetary magnetic field Bz ∼ 0, By ≪ 0 case
Authors: Farrugia C. J. , Sandholt P. E. , Cowley S. W. H. , Southwood D. J. , Egeland A. , Stauning P. , Lepping R. P. , Lazarus A. J. , Hansen T. , Friis‐Christensen E.
Published: February 2004

We present observations of transient auroral activity in the 1300–1600 MLT range at geomagnetic latitudes of 76° and interpret them in terms of modulations in the reconnection rate at the magnetopause which are, in turn, stimulated by the arrival at Earth of upstream dynamic pressure pulses. The period studied is 1000 ‐ 1200 UT on December 17, 1992. The interplanetary magnetic field (IMF), monitored by IMP 8, was aligned in a Parker spiral direction with a strong westward component (IMF By ≪ 0) and a small or zero north‐south component. The upstream dynamic pressure variations were of two types, reflecting two distinct origins: the unperturbed solar wind and the bow shock. The underlying reconnection interpretation, with the necessary magnetic shear across the magnetopause being provided by the strong westward component of the IMF, is substantiated by a study of ionospheric flows. A twin‐cell pattern of standard polarity is evident in which strong asymmetries about noon are seen, as expected from the predominantly westward pointing IMF. Flow perturbations are superimposed on this general pattern. The auroral observations consist of a latitudinally narrow zone of persistent auroral emission, at the poleward edge of which auroral forms are seen moving eastward (tailward) at speeds of 1–2 km s−1. This intermittent activity, which is often detached from the persistent aurora, has a red line intensity (line of sight) > 1.5 kR, a broad intensity‐versus‐zenith angle profiles, and a weak green line intensity (< 1 kR). The brightenings are also associated with the activation of discrete forms at the cleft equatorward boundary. A comparison with results obtained from mapping the magnetopause and its boundary layers to the ionosphere by means of particle precipitation characteristics indicates that the persistent aurora consists mainly of low‐ latitude boundary layer precipitation on closed field lines. The transients poleward of the persistent aurora are coming from the cusp or near cusp. We find that the auroral transients are correlated with a sequence of magnetic impulses of 50 nT or less amplitude seen at ground stations around the optical site. We also find that there is a further, good correlation between the transients and the arrival at Earth of rapid and large (> 20% increase relative to background levels) upstream dynamic pressure pulses. Smaller dynamic pressure changes do not seem to affect the aurora noticeably. Our interpretation is in line with some recent studies, namely, that an increase of dynamic pressure on the magnetopause enhances the reconnection rate there. Clearly, theoretical work is needed to understand this important, if indirect, effect of dynamic pressure on the magnetosphere.
Plasma temperature profiles in the magnetosphere of Neptune
Authors: Eviatar Aharon , Vasyliūnas Vytenis M. , Richardson John D.
Published: February 2004

It is generally agreed that plasma in the magnetosphere of Neptune is produced primarily in the vicinity of the orbit of Triton, near and beyond 14 RN, and is then transported inwards toward Neptune. The observed temperature profiles of protons and nitrogen ions, however, show an initial decrease with decreasing distance from Neptune, the expected increase from adiabatic compression not setting in until distances below 8 RN have been reached. We propose that cooling produced by collisions with atoms of a neutral hydrogen cloud that extends inward to 8 RN may explain this behavior, at least for protons. Inclusion of inelastic scattering in addition to the more commonly invoked charge exchange makes cooling sufficiently effective so that the observed temperatures may be accounted for with plausible values of neutral hydrogen density.
Structure within Jovian hectometric radiation
Authors: Higgins Charles A. , Green James L. , Thieman James R. , Fung Shing F. , Candey Robert M.
Published: February 2004

Observations of Jovian hectometric radio emission (HOM) by the Voyager planetary radio astronomy (PRA) experiment at frequencies from 300 kHz to 1.3 MHz indicate persistent dynamic spectral features that had not been previously studied. The features of interest appear as “lanes” of decreased emission intensity within the otherwise persistent HOM. The lanes are apparent in intensity and occurrence probability spectrograms of frequency versus Jovian System III (1965) longitude. In the investigation of the morphology of these features, we use inbound and outbound Voyager 2 data at Jupiter to show that the lane occurrence and characteristics do not depend on local time over the range sampled. Occurrence probability spectrograms of frequency versus magnetic latitude are created from the portion of the data when the spacecraft was between 0° and +10° magnetic latitude. These spectrograms represent both the inbound and outbound passes and are quite similar despite the different longitude ranges. A simple extension of decametric (DAM) arc features into the HOM wavelength does not account for all the lane features, giving further evidence that HOM is an independent emission component. Polarization signatures for the data show that the polarization is predominantly right‐hand circular and that it does not reverse across the lanes, suggesting the emission is from the same hemisphere. In addition, we investigate possible effects due to solar wind variations and find that the occurrence of the lanes appears to be independent of times of low and high solar wind densities. The intensity of the HOM emission on either side of the lanes is comparable, implying that the lane is probably not a result of a gap between fundamental and second harmonic emission regions. We present these data and analyses as a morphological study to establish that the lane features are an important part of the HOM emission and should be considered in HOM emission models. At this time, no theory of the source of the lanes explains all the observed features.
A comparison of the Voyager 1 ultraviolet spectrometer and plasma science measurements of the Io plasma torus
Authors: Taylor M. H. , Schneider N. M. , Bagenal F. , Sandel B. R. , Shemansky D. E. , Matheson P. L. , Hall D. T.
Published: February 2004

We have developed a sophisticated package (Colorado Io Torus Emissions Package, or CITEP) to simulate emissions from the Io plasma torus, and have used it to examine the consistency of Voyager in situ and remote observations. CITEP merges ion composition derived from the ultraviolet spectrometer line ratios with measurements of electron densities, electron temperatures, and ion temperatures obtained by the plasma science instrument. The program then predicts the brightness and morphology of torus EUV emissions. We find that the measured brightness is approximately 2 times larger than the model predicts. When scaled up by this factor, the morphology of the model radial emission profile is consistent with the data, but somewhat less sharply peaked at the ribbon location. We examine several possible causes for the brightness discrepancy such as calibration errors, measurement accuracy, inaccuracies of the model, and torus variability, but find no definitive cause.
Wave ducting in different wave modes
Authors: Calvert W.
Published: February 2004

Ducting by magnetic‐field‐aligned density irregularities is found to occur in seven regions of parameter space defined by plasma resonance (P = 0), unity refractive index (no = 1), longitudinal refractive‐index curvature reversal (no² = −P), and the boundary between ordinary and extraordinary propagation (no² = +P), where no² = R or L is the squared refractive index in the direction of the magnetic field and P, R, and L are the Stix parameters for wave propagation in a plasma. Ducting in the ordinary and extraordinary modes is found to occur in troughs, whistler ducting is found to occur in troughs and crests respectively above and below its curvature reversal at approximately half the cyclotron frequency, and ducting in the Z mode is found to occur in troughs from the Z cutoff to a curvature reversal about halfway between cutoff and the plasma frequency, in crests up to the plasma frequency, and then also in crests between the plasma and cyclotron frequencies whenever the latter is greater than the former. The strength of ducting, which is characterized by the square of the ducted wave angle divided by the fractional density deviation of the duct, is found to be proportional to the refractive‐index radius of curvature and the rate of increase of refractive index with density. Strongest ducting is thus found to occur in the Z mode below curvature reversal, and the weakest at low densities near the plasma frequency.
Radiation from electromagnetically driven Langmuir turbulence
Authors: Mjølhus Einar , Hanssen Alfred , DuBois Donald F.
Published: February 2004

A two‐level model for the interaction between the electromagnetic pump wave and the electrostatic turbulence is formulated for ionospheric radio modification experiments. On the local level, the Zakharov equations, or similar models, apply. The interaction with the global electromagnetic level is represented by a second‐order current density averaged over the local spatial variable. The energy exchange between the local and global level is represented by a Joulean product involving this second‐order current density. The global generation problem is solved in the simplest cases. The escaping energy flux in the sideband ω is shown to be represented as a folding between the power spectrum of the local source and a squared Airy function. This power spectrum has been calculated from numerical simulations of electromagnetically driven Langmuir turbulence, using a one‐dimensional model of the Zakharov type, for varying values of the parameters. For parameters in the cascade range, narrow line structured spectra were obtained, while for parameters in the cavitation range, very broad featureless spectra were obtained. Comparison with recent experimental stimulated electromagnetic emissions data did not confirm a signature for the existence of cavitation in the experiments.
Special features of a substorm during high solar wind dynamic pressure
Authors: Lui A. T. Y. , Elphinstone R. D. , Murphree J. S. , Henderson M. G. , Vo H. B. , Cogger L. L. , Lühr H. , Ohtani S. , Newell P. T. , Reeves G. D.
Published: February 2004

A substorm on July 24, 1986, exhibiting a rather unusual auroral morphology is analyzed with data from 10 spacecraft (Viking; DMSP F6 and F7; GOES 5 and 6; three LANL geosynchronous satellites; CCE; and IMP 8). This substorm occurred during high solar wind dynamic pressure (>5 nPa). Several notable features for this substorm are (1) the substorm onset activity was preceded by prominent auroral activations in the morning sector with spatial separations between adjacent bright regions ranging from ∼160 to 640 km, and their intensity was modulated at ∼3.2‐min intervals; (2) the initial substorm activity was concentrated in the morning sector, followed by a sudden activation in the dusk sector, leaving the midnight sector relatively undisturbed, in sharp contrast to the traditional substorm development in which the major activity occurs in the midnight sector; (3) while a substorm injection was observed at a geocentric distance of ∼8.4 RE by CCE in association with the substorm onset, particle injections (detectable with three LANL geosynchronous satellites) and dipolarization signatures (detectable by the two GOES satellites) were not observed until subsequent intensifications; (4) timing subsequent substorm intensifications from injections at the geosynchronous altitude differed from timing intensifications based on Viking auroral images by as much as ∼3 min even when the auroral activity occurred in the same magnetic local time sector as the geosynchronous satellite; (5) the polar cap boundary was at a significantly higher latitude than the poleward boundary delineated by detectable auroral luminosity in the auroral oval, supporting the previous result that auroral activity at substorm onset occurs substantially equatorward of the boundary between open and closed magnetic field lines. Detailed timing analysis suggests the substorm onset to be associated with southward interplanetary magnetic field (IMF), possibly with the crossing of an IMF sector boundary (interplanetary current sheet). The dimming of auroral luminosity in the midnight region was associated with a sudden northward turning of the IMF during high solar wind dynamic pressure condition.
Super dual auroral radar network radar imaging of dayside high‐latitude convection under northward interplanetary magnetic field: Toward resolving the distorted two‐cell versus multicell controversy
Authors: Greenwald R. A. , Bristow W. A. , Sofko G. J. , Senior C. , Cerisier J.‐C. , Szabo A.
Published: February 2004

Data from the Kapuskasing and Saskatoon radars of the evolving Super Dual Auroral Radar Network (SuperDARN) HF radar network have been analyzed to study the two‐dimensional structure and dynamics of dayside high‐latitude ionospheric convection under northward interplanetary magnetic field (IMF) conditions. A period extending from 1600 to 2030 UT (∼0900‐1330 MLT) on January 10, 1994, was examined. During this interval, magnetic field data were available from the IMP 8 satellite and indicated moderately stable northward IMF conditions. For the first few hours of observation the By component of the IMF was positive, reasonably steady, and approximately twice the magnitude of Bz. During this interval, the high‐latitude convection images obtained with the SuperDARN radars were very similar to the distorted two‐cell convection maps for positive By as presented by Heppner and Maynard (1987). At ∼1840 UT, a decrease in By in association with an increase in Bz, led to an extended period with By ≈ Bz. During this second interval the convection patterns were highly variable and even chaotic. Finally, a sharp decrease in the By component at 1914 UT, probably in association with a rotational discontinuity in the solar wind, led to an extended period with By ≪ Bz. During this third interval, the high latitude convection pattern was again stable and exhibited a single counterclockwise rotating vortex consistent with one of the polar cap merging cells proposed by Dungey (1963), Burke et al. (1979), and Reiff and Heelis (1994). The transition to the counterclockwise rotating vortex occurred over a large spatial area within two radar scans (200 s) and appeared to proceed via an equatorward incursion of the vortex into the previous convection configuration. Once this polar cap merging cell was formed, it remained stable for the remainder of the period (∼40 min) that By remained small. Sunward convection was also observed in the polar cap of the conjugate hemisphere at 1932 UT by the DMSP F11 satellite, confirming that both magnetopause and internal reconnection sites were active at that time.
Beam‐driven acoustic solitary waves in the auroral acceleration region
Authors: Marchenko Victor A. , Hudson Mary K.
Published: February 2004

The formation of ion acoustic solitary structures driven by electron and ion beams in the auroral acceleration region is studied using two‐dimensional electrostatic particle simulations. The beams are consistently present in regions of moderate potential drop (≤ 1 keV) where weak double layers have been observed on both the S3‐3 and Viking spacecraft. The presence of more than one ion species introduces the ion two‐stream instability besides the ion acoustic one into the system and modifies previous analysis and simulation results of solitary wave formation. Solitary structures form as a result of the microinstability development. The numerical simulation results show that positively peaked (ϕ > 0) localized structures are formed in the system driven by a dense (nib ≈ nic ≈ ne/2) ion beam. The solitary waves move in the direction of the ion beam velocity. By contrast, negative potential solitary structures form when the ion beam density is reduced to 10 % (nib ≈ 0.1 ne) and electron drift relative to background ions is sustained by an applied electric field. In this case, solitary waves drift downward at subsonic speeds relative to the background ions, which may carry the localized pulses upward. Evolving solitary waves do not carry any significant net potential drop and therefore cannot contribute much to the auroral particle acceleration. They are found to be a consequence of the larger‐scale V‐shaped potential distribution in the auroral region.
Electron and ion thermal structures above 1000 km: A multi‐ion generalized transport model for subsonic flows
Authors: Demars H. G. , Schunk R. W.
Published: February 2004

A multispecies (electrons, H+, He+, O+) generalized transport model has been developed that is based on 16‐moment transport theory. This model provides a simultaneous and self‐consistent solution for the densities, drift velocities, parallel and perpendicular temperatures, and parallel and perpendicular heat flows of the electrons and each of the three ion species. Solutions have been obtained for a wide range of ionospheric boundary conditions, but this study concentrates on the relations between temperature gradients applied at a lower boundary (1000 km) and the total temperatures and temperature anisotropies that exist at higher altitudes, for electrons, H+ ions, and He+ ions. It is seen that applying even a modest temperature gradient for a particular species has a major effect on the thermal structure of that species at higher altitudes. Further, while the thermal structures of the various species are coupled due to Coulomb collisions, this coupling is far more significant for certain pairs of species than for others. For a particular species, the presence of a large temperature gradient at the lower boundary may or may not result in highly non‐Maxwellian distribution functions at higher altitudes, depending on the relative magnitudes of the species temperatures and heat flows at those altitudes. Also, in comparing the total electron heat flows predicted by 16‐moment transport theory with those predicted by the standard, collision‐dominated transport theory, close agreement is found. For the ions, however, the differences between the 16‐moment and standard models can be very large, depending on the boundary conditions.
Diurnal trends in geomagnetic noise power in the Pc 2 through Pc 5 bands at low geomagnetic latitudes
Authors: Bloom Ronald M. , Singer Howard J.
Published: February 2004

Mean, integrated geomagnetic noise power spectra in six octaves between 0.002 and 0.128 Hz from the Air Force Geophysics Laboratory (AFGL) magnetometer network were grouped into seasonal ensembles. We found that the local time variation of the mean logarithm of this ensemble, with respect to both 3‐hr and 8.5‐min time blocks, consistently experienced a 5–10 dB maximum centered around local noon at higher frequencies and near dawn and dusk at lower frequencies, a factor of 3–10 in power. The shape of this trend differed with geomagnetic field component, geomagnetic latitude, and frequency octave. The high‐frequency Pc 3 and 4 waves have a broad maximum in power centered on local noon, but the longer‐period Pc 4 and 5 wave power maximizes away from noon toward dawn for the north‐south geomagnetic field component and toward dusk for the east‐west component. Two years of data did not provide a large enough sample to resolve a significant seasonal variation in the shape of the diurnal trend. These trends changed consistently during periods of enhanced global activity, as measured by the index Ap. These are among the first systematic observations of Pc 5 wave power at low geomagnetic latitudes (40°–55°), indicating that wave energy from the outer magnetosphere is coupling or propagating to low‐latitude locations. This statistical study extends the few previous event studies by providing an explicit parametrization of local time trends in wave power as a function of frequency subband, geographic location, and magnetic activity.
Magnetic field signatures of unevolved turbulence in solar polar flows
Authors: Horbury T. S. , Balogh A. , Forsyth R. J. , Smith E. J.
Published: February 2004

Between late 1993 and the middle of 1994, Ulysses stayed almost constantly within flows from the Sun's southern polar coronal hole, traveling from 44°S to 69°S heliolatitude and between 4.0 and 2.9 AU from the Sun. Magnetic field observations within this region reveal a very different medium from that at similar heliocentric distances near the ecliptic. Nearly 160 days of magnetic field data taken within this region are studied using structure functions. We demonstrate that the polar plasma flow is turbulent, with different populations of fluctuations on different scales. On spacecraft scales of 102–103 s, the fluctuations are indicative of inertial range turbulence and are intermittent, with a spectral slope near 1.7. On larger scales, the fluctuations are not intermittent, and the spectral slope is near 1. The similarities and differences between polar and near‐ecliptic fluctuations are discussed.