bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetosheath quasi‐trapped distributions and ion flows associated with reconnection
Authors: Neff J. E. , Speiser T. W. , Williams D. J.
Published: February 2008

Using a sample of ISEE 1 and 2 magnetopause crossings previously identified as times of quasi‐steady reconnection, we identified flows of medium energy ions in the magnetosheath. We then investigated the particle pitch angle distribution immediately before and after each of these events for the signature of quasi‐trapped distributions of energetic ions. Several of the ion flows that we identified were observed simultaneously with previously identified flux transfer events (FTEs). While FTEs identified from the magnetometer tracings typically show evidence of ion flows, the converse is not necessarily true. However, all properties of the magnetosheath ion flows are the same regardless of whether an FTE can be identified from the magnetometer data. We find evidence for small‐scale reconnection processes (FTEs, ion flows) embedded within a larger region of interconnected field, which is traced out by the quasi‐trapped particles. Quasi‐trapped distributions of medium‐energy ions are seen to sandwich reconnection‐associated ion flows in the magnetosheath. The results of this survey have been used to suggest a morphology for reconnection events that incorporates both large‐ and small‐scale features.
IMF By and day‐night conductivity effects in the expanding polar cap convection model
Authors: Moses J. J. , Siscoe G. L. , Crooker N. U. , Gorney D. J.
Published: February 2008

During southward Bz periods the open field line region in the ionosphere (polar cap) expands due to increased dayside merging. Ionospheric plasma flow patterns result which can be classified by the sign of the interplanetary magnetic field (IMF) By component. In this paper we construct a time‐dependent ionospheric convection model to simulate these flows. The model consists of a spiral boundary with a gap in it. The sign of the IMF By component determines the geometry of the gap. A potential is applied across the gap and distributed around the boundary. A flow results which enters the polar cap through the gap and uniformly pushes the boundary outward. Results of the model show that By effects are greatest near the gap and virtually unnoticeable on the nightside of the polar cap. Adding a day‐night ionospheric conductivity gradient concentrates the polar cap electric field toward dawn. The resulting flow curvature gives a sunward component that is independent of By. These patterns are shown to be consistent with published observations.
Field‐aligned currents associated with substorms in the vicinity of synchronous orbit: 2. GOES 2 and GOES 3 observations
Authors: Nagai T.
Published: February 2008

Substorm‐associated field‐aligned currents have been studied using magnetic field observations at synchronous orbit and on the ground. The GOES 2 and GOES 3 satellite pair, separated in local time by about 2 hours, clearly demonstrates local time dependence of field‐aligned current signatures at synchronous orbit. We have made comparisons between magnetic disturbances at synchronous orbit and ground mid‐latitude magnetic disturbances. In the morning region and the evening region, D perturbations at two locations have the same sign, indicating that major field‐aligned currents are those flowing into (out of) the ionosphere located on L shells greater than the spacecraft L shell in the morning (evening) region. However, the D perturbations at synchronous orbit are more transient than those on the ground in the same local time meridian. Near midnight an opposite sense for the sign of D between synchronous orbit and the ground is frequently observed. In such a case the D perturbation is always negative at synchronous orbit, while the ground D perturbation is positive. We propose that the downward current sheet extending from the morningside overlaps the upward current sheet extending from the eveningside in the center of the current system. In this model the double field‐aligned current sheets are formed in association with the onset of a substorm expansion phase near the midnight meridian, and the meridian for the major field‐aligned currents flowing into the ionosphere shifts progressively eastward in the morning region and the meridian for the major field‐aligned currents flowing out of the ionosphere shifts progressively westward in the evening region as substorms progress. These field‐aligned currents should have the longitudinally limited extent, and they most likely correspond to small‐scale structure in the large‐scale current system derived on the basis of low‐altitude observations.
Energetic He+ precipitation in a mid‐latitude aurora
Authors: Ishimoto M. , Torr M. R.
Published: February 2008

Optical spectra of a mid‐latitude aurora on September 21–22 indicated that the observed emissions were caused by energetic ions (O+ or He+). In an earlier paper it was shown that the spectral features could be fairly well explained by a flux of precipitating energetic O+ with characteristics similar to O+ fluxes that have been observed in large magnetic storms. A comprehensive model is used to trace the energy degradation of the incoming O+ ions, the production of energetic neutrals, and the subsequent scattering of these neutrals in altitude and energy. In this study we have computed the atmospheric impact (in terms of heating, ionization, and excitation of certain emissions) if the incoming flux is He+ instead of O+. The results show that an energetic He+ flux is very efficient in producing both ionization and a backscattered flux out of the atmosphere. Of the total energy influx, approximately 50% goes into ionization (compared with ∼2% in the case of O+) and 40% is backscattered to the magnetosphere or escapes (compared with 16% for O+). By contrast the He+ flux is less efficient at heating the atmosphere (10% compared with 82%). The He+ flux is very efficient in producing N2+ 1 N (first negative) emission, but essentially does not excite the N2 2 P (second positive) system. From these results it is concluded that the aurora observed on September 22, 1982, is well explained by an influx of energetic O+ together with a 6% admixture of He+.
Comparison of solutions to bi‐Maxwellian and Maxwellian transport equations for subsonic flows
Authors: Demars H. G. , Schunk R. W.
Published: February 2008

A systematic comparison has been conducted of the solutions to the bi‐Maxwellian‐based 16‐moment and the Maxwellian‐based 13‐moment transport equations for conditions corresponding to the steady state subsonic flow of a fully ionized electron‐proton plasma in the terrestrial ionosphere. The bi‐Maxwellian‐based equations can account for large temperature anisotropies and the flow of both parallel and perpendicular thermal energy, while the Maxwellian‐based equations account for small temperature anisotropies and only a total heat flow. The comparison was conducted for a range of lower boundary temperatures (2000–10,000 K) and temperature gradients (1‐4 K km−1) over the altitude range 1500–13,000 km. These boundary parameters are the most important in the subsonic flow regime since they produce different temperature anisotropies and heat flows, which are handled differently in the two formulations. The systematic comparison has led to the following conclusions: (1) For “low” boundary temperatures (T ∼ 2000 K) and low boundary temperature gradients (▽T ≲ 2 K km−1), the differences between the bi‐Maxwellian‐based 16‐moment and Maxwellian‐based 13‐moment solutions are negligibly small, (2) For intermediate boundary temperatures and temperature gradients, both the 16‐moment and the 13‐moment formulations predict electron and proton temperature anisotropies with perpendicular temperatures (T⊥) greater than parallel temperatures (T∥). However, there are significant differences with regard to the magnitude of the anisotropies predicted by the two formulations, (3) For high boundary temperatures (T≳ 8000 K) and high boundary temperature gradients (▽T∼4 K km−1), the two formulations predict different thermal structures for both the electrons and the protons. Specifically, the bi‐Maxwellian‐based equations predict temperature anisotropies with T∥ > T⊥ at high altitudes, whereas the Maxwellian‐based equations predict that T⊥ > T∥ at all altitudes, (4) The electron total heat flow profiles predicted by the two formulations are virtually identical for all cases considered, and the proton total heat flow profiles are similar, (5) In the 16‐moment formulation, the flow of parallel energy is almost always greater than the flow of perpendicular energy for both the electrons and the protons, (6) The 16‐moment solutions are very sensitive to small changes in the boundary temperature and heat flow values.
Observations of wave‐driven fluctuations of OH nightglow emission from Sondre Stromfjord, Greenland
Authors: Hecht J. H. , Walterscheid R. L. , Sivjee G. G. , Christensen A. B. , Pranke J. B.
Published: February 2008

OH nightglow emissions from the (6, 2) band were observed during February 12–14, 1986, at Sondre Stromfjord, Greenland. The data were analyzed using time series analysis techniques to determine
, where ΔI and ΔT represent correlated fluctuations from the mean OH intensity (Ī) and temperature (
) in various frequency bands. For correlated fluctuations due to measurement error, |η| < 1, while for correlated fluctuations caused by atmospheric gravity waves (AGWs) that pass through the OH emission layer the measured |η| should be generally greater than 1.5. While some of the observed OH intensity‐temperature fluctuations were correlated and had an |η| < 1, two correlated intensity‐temperature waves with periods of 4 and 2 hours were seen with an |η| > 1.5. The 4‐hour (2‐hour) period wave had an |η| equal to 3.1 ± 1.1 (1.6 ± 0.5) and a phase angle, between the intensity and temperature components, of 5° ± 23° (40° ± 20°). These results are consistent with a recent theoretical model for the passage of AGWs through an OH emission layer. Agreement is closest for the 4‐hour (2‐hour) period wave when the layer is peaked at 87 km (83 km) and the O scale height is −2.0 (−4.0km).
Seasonal and diurnal variations of ELF emission occurrences at 750‐Hz band observed at geomagnetically conjugate stations
Authors: Suzuki Hiroyuki , Sato Natsuo
Published: February 2008

Statistical characteristics of emission occurrence are examined, using digital data of 750‐Hz intensity records obtained at the conjugate pair of stations, Syowa Station in Antarctica and Husafell in Iceland. The geographic local time at Syowa and Husafell is magnetic local time plus 3 hours and minus 1 hour, respectively. The following notable diurnal variations and seasonal variations were found: (1) The emissions were mostly observed during the daytime in the conjugate region. However, the magnetic local time when the emission occurrence rate reached maximum at Husafell was 2–3 hours later than that at Syowa. (2) The seasonal variations of emission occurrence showed the same tendency at the conjugate stations. The emission intensities showed a maximum during local summer and a minimum during local winter in both hemispheres. The ratio of average emission intensity in each season to the intensity in winter is approximately 2.1–2.2 for summer, 1.7–1.9 for autumn, and 1.7 for spring. From these statistical characteristics, ELF emission intensity strongly depends on not only magnetic local time but also geographic local time and seasons, suggesting that ELF emissions observed on the ground are strongly controlled by the sunlight effects. The sunlight may affect the asymmetry of wave duct enhancement and wave propagation from the magnetosphere to the ionosphere in both hemispheres. Other effects are also discussed in this paper.
Geomagnetic conjugacy of 30‐kHz band auroral hiss emissions observed at L = 6.1
Authors: Sato Natsuo , Kokubun Susumu , Saemundsson Thorstein
Published: February 2008

Statistical characteristics of auroral hiss emissions in the 30‐kHz band were examined using the data observed simultaneously at the Syowa‐Husafell conjugate pair of stations during the period of September‐November 1983. The occurrence probabilities of conjugate events observed simultaneously at the two stations were almost the same in September and October and decreased during November. The nonconjugate events observed only at Syowa Station were detected more often in September. The number of nonconjugate events decreased in October and November. These phenomena were mostly observed during 1700–2300 MLT around the geographic midnight at Syowa. On the other hand, the number of nonconjugate phenomena detected only at Husafell in Iceland increased from September to November. The occurrence peaks were around 2100–0200 MLT, before geographic midnight at Husafell. From these statistical characteristics it is found that auroral hiss emission occurrences strongly depend on not only the magnetic local time but also the geographic local time and seasons, suggesting that auroral hiss emissions observed on the ground are strongly controlled by the sunlit effects in the topside ionosphere.
Steady state flow/field model of solar wind interaction with Venus: Global implications of local effects
Authors: Cloutier P. A. , Taylor H. A. , McGary J. E.
Published: February 2008

The structure and variability of ion concentrations, velocities, and electric and magnetic fields in the ionosphere of Venus result from a combination of physical and chemical processes driven by the interaction of the solar wind with the atmosphere of Venus. In this paper we describe these processes and formulate a model describing their interrelationships in a self‐consistent manner. We show that the consequences of this model are in agreement with Pioneer Venus observations in the Venus ionosphere. It is shown that the magnetic field configuration of the Venus ionosphere is a steady state global structure which results from a combination of a large‐scale “open” current system connecting through the ionopause to the postshock solar wind and two small‐scale “closed” current systems entirely confined within the ionosphere itself and generated by ionospheric flows and shears. A previously detected ionospheric layer in O2+ near 170 km altitude is shown to be a new type of ionospheric layer resulting from transition of solar‐wind‐driven vertical flow to velocities approaching Mach 1 (sonic flow). Accordingly, this layer has been named the “sonic flow layer” or “S layer.” It is suggested that this model should apply to other nonmagnetic bodies in magnetized plasma flows, such as comets and Jovian satellites.
Upstream waves simultaneously observed by ISEE and UKS
Authors: Russell C. T. , Luhmann J. G. , Elphic R. C. , Southwood D. J. , Smith M. F. , Johnstone A. D.
Published: February 2008

Measurements obtained in the solar wind by ISEE‐ 2 and the United Kingdom Subsatellite (UKS) have been examined for observations of upstream waves. These data reveal that the waves in the foreshock region are enhanced at all frequencies from at least 0.003 Hz to 0.5 Hz. The wave spectra generally have a spectral peak, but this peak is usually broad and the peak frequency depends on the position of the spacecraft. Generally, the spectra seen at the two spacecraft are most similar at high frequencies and least similar at low frequencies. The geometry of the interaction is displayed in the plane containing the magnetic field, the solar wind velocity, and the spacecraft location. However, this coordinate system does not order all the observed wave properties. It does not clearly explain or order the handedness of the waves, or their direction of propagation. It is clear that the upstream region is inherently three‐ dimensional. The position‐dependent nature of the upstream waves indicates that comparisons between ground‐based measurements and in‐ situ observations must be undertaken with some caution.
Observations of electron cyclotron harmonic emissions associated with field‐aligned electron beams
Authors: Koons H. C. , Edgar B. C. , Fennell J. F. , Gorney D. J.
Published: February 2008

Several cases have been found in plasma wave and electron distribution function data from the SCATHA satellite in which strong gyrofrequency emissions near 1.5 times the local electron gyrofrequency ƒc correlate in time with field‐aligned electron beams. The energy of the electron beams ranges from 100 to 800 eV. Before the beam appears the emissions are observed at a lower frequency, typically 1.1ƒc to 1.2ƒc. As the beam develops, the emissions rise in frequency and grow in amplitude. Above 2 keV the electrons had a normal loss cone distribution. This loss cone deepened by a small amount when the beams appeared at lower energy. These observations were made near local midnight between L = 7.0 and 7.8 RE.
H3+ in the Jovian ionosphere
Authors: McConnell John C. , Majeed Tariq
Published: February 2008

Recent measurements and theoretical calculations suggest that H3+ ions in their ground vibrational level recombine much more slowly than in the vibrational levels υ ≥ 3. Using these results as a guide, we have investigated the impact of modified H3+ chemistry on the structure of the Jovian ionosphere for both a solar EUV ionization source and a high‐altitude ionization source, associated with the electroglow. The results are sensitive to the value of the H3+ recombination coefficient, k10, adopted for υ = 0. For the solar EUV source of ionization, and a value of k10 = 2 × 10−8 cm³ s−1 equal to the published experimental upper limit, we find that H+ is the major ion at the peak of the ionosphere, similar to previous results. However, given the current uncertainty in k10, an ionosphere dominated by H3+ cannot be ruled out. A value of k10 ∼ 10−10 cm³ s−1 will produce an ionosphere that consists mainly of H3+. By including a high‐altitude source of ionization we find that H3+ is the major component of the upper ionosphere, for all values of k10 less than or equal to the experimental upper limit. Thus, our calculations suggest that the topside plasma scale height measured by the radio science experiments may be indicative of electron temperatures ∼3000–5000 K or greater, consistent with current models for calculation of electron temperature. Our results may also help shed some light on the Voyager low‐energy charged particle observations of H3+ ions in the magnetosphere. We also reemphasize the fact that H2 molecules in the Jovian upper atmosphere must be vibrationally excited.
The emission of narrow‐band Jovian kilometric radiation
Authors: Fung S. F. , Papadopoulos K.
Published: February 2008

A model based on the nonlinear coupling of electrostatic plasma waves is proposed to explain the emission of the narrow‐band Jovian kilometric radiation (nKOM) observed by the Voyager spacecraft, it is shown that upper‐hybrid branch electrostatic waves propagating through the inhomogeneities in the outer periphery of the Io plasma torus can attain the proper geometry for localized upconversion interactions leading to pump depletion. Plasma waves propagating into a weak density gradient (increasing ωp/Ωe) and reflected at the critical layer (ωL = ωp), interact with the incident waves leading to the electromagnetic emission at 2ωUH, which is beamed at large angles with respect to the background magnetic field. In general, both L‐O and R‐X mode waves can be generated. The observed power and net polarization (L‐O) are consistent with pump depletion of electrostatic waves at a level of ∼10 mV/m. A possible excitation mechanism for the electrostatic waves is also discussed.
Birkeland currents and energetic particles associated with optical auroral signatures of a westward traveling surge
Authors: Bythrow P. F. , Potemra T. A.
Published: February 2008

The surflike auroral shape commonly associated with the westward traveling surge (WTS) is a remarkably repeatable feature of the polar auroral display. In this paper, we examine the details of one such form that is located on the poleward edge of the diffuse aurora. For this study, we have used the simultaneous imagery, high‐resolution magnetic field, and charged particle measurements from the DMSP F7 spacecraft, acquired in the northern hemisphere on December 31, 1983. F7 is the latest of the DMSP series and the first to include a magnetic field experiment. A large‐scale upward directed Birkeland current dominates across the entire form, colocated with precipitating electrons having spectra peaked from 3 to 12 keV. A pair of narrow (20 km) parallel arcs extends along the poleward edge of the auroral oval for a few hours in local time west of the surge. They appear to diverge to higher and lower latitudes because of an intrusion of aurora from lower latitudes and later local times. In the center of the intrusion, the Birkeland current is directed upward and electrons exhibit accelerated spectra with a monoenergetic peak at 12 keV similar to spectra observed at much lower latitudes. Each of the two narrow arcs poleward and equatorward of the diffuse region is characterized by intense upward directed Birkeland currents, “inverted V” electrons with spectra peaked from 1 to 3 keV, and enhanced ion fluxes. Electron spectra in both arcs suggest that these particles are streaming earthward from the plasma‐sheet boundary layer. Thus, the WTS appears to result from an expansion of the plasma sheet and an intrusion of the plasma‐sheet boundary layer into the high‐latitude tail lobe. These observations support the view that the WTS is related to a Kelvin‐Helmholtz instability in the distant magnetotail.
The characteristics of ionospheric heating‐produced ELF/VLF waves over 32 hours
Authors: Rietveld M. T. , Mauelshagen H. ‐P. , Stubbe P. , Kopka H. , Nielsen E.
Published: February 2008

In October 1981, ELF/VLF waves were produced in the ionosphere in an alternating sequence of approximately 1, 2, 3, and 5 kHz by modulated HF heating of the ionosphere near Tromsø, Norway, during a 32‐hour period of high geomagnetic disturbance. The apparent source heights, which are derived from the wave phases, show a diurnal variation from about 55 km during the day to about 74 km at night. Other wave parameters such as amplitude, direction, and ellipticity of the ELF polarization ellipse also show a diurnal variation as well as modulation by Pc 5 hydromagnetic waves. We compare the variation of the ELF wave parameters with electric fields measured by the Scandinavian Twin Auroral Radar Experiment (STARE) and with riometer and magnetometer data in an attempt to understand the factors controlling the ELF wave generation process. We are able to successfully model many of the measured wave characteristics. High electron densities at low altitudes were found necessary to explain the daytime measurements.
Radial and latitudinal variations of the interplanetary magnetic field
Authors: Klein Larry W. , Burlaga L. F. , Ness N. F.
Published: February 2008

This paper presents observations of the radial and latitudinal variations of the interplanetary magnetic field measured by the Voyager 1 (V1) and Voyager 2 (V2) spacecraft from mid‐1977 to mid‐1985. The data extend from 1 to 20 AU and from −5° to 26° in heliographic latitude. Data obtained at 1 AU are used to separate temporal variations from radial variations, and plasma measurements from V2 are used to consider the effect of temporal variations in the bulk speed. Observations of the radial variation of the large‐scale magnetic field strength in the ecliptic agree with the predictions of Parker's model when temporal variations in the magnetic field and bulk speed are taken into account. The latitudinal variation of the magnetic field observed by V1 is in agreement with the predictions of Parker's model to first approximation. The magnetic field strength at higher latitudes is somewhat lower than expected on the basis of observations made in the ecliptic, but this could be due to an increase in bulk speed and/or a decrease of solar magnetic field strength with latitude. Fluctuations in the strength of the magnetic field are small compared to the large‐scale field itself, and they decrease in amplitude with increasing distance approximately as R−1/4. Fluctuations in the components are relatively large, and they make a significant contribution to the mean field that is not described by Parker's model.
Strong electron heating at the Earth's bow shock
Authors: Thomsen M. F. , Mellott M. M. , Stansberry J. A. , Bame S. J. , Gosling J. T. , Russell C. T.
Published: February 2008

The electron heating at collisionless shocks in near‐Earth space is normally found to be relatively small. We report here on two sets of bow shock crossings observed by the ISEE 1 and ISEE 2 spacecraft in which very large electron temperature increases were found. The two sets of shocks are part of a larger set of 52 bow shock crossings compiled from the ISEE data set. When the shocks with the large electron heating are compared to the rest of the shocks in the compiled set, it is found that the only upstream parameter which is out of the ordinary is the upstream solar wind flow speed Vu. Indeed, for the entire shock set the only upstream parameters which correlate well with the electron heating are the solar wind speed and quantities derived from it. The best correlation among those tested is between the temperature increase (Ted ‐ Teu) and the difference in the square of the bulk flow speed (Vu² ‐ Vd²) across the shock, which is proportional to the total amount of bulk flow energy dissipated by the shock. A subsequent search for bow shock crossings which occur during intervals of high solar wind speed confirms that the electron heating is very large under high‐speed conditions. The first‐order dependence of the electron temperature increase on the available bulk flow energy is consistent with a heating process which is dominated by the macroscopic cross‐shock electrostatic potential jump. This study suggests that the temperature difference is the appropriate measure of electron heating at shocks (rather than the ratio Ted/Teu) and that the first‐order dependence on (Vu² ‐ Vd²) should be normalized out in future studies of electron heating at space shocks.
Daytime scintillations induced by high‐power HF waves at Tromsø, Norway
Authors: Basu Santimay , Basu Sunanda , Stubbe P. , Kopka H. , Waaramaa J.
Published: February 2008

During March 1984 the high‐power HF heating facility located at Ramfjordmoen (69.6°N, 19.2°E geographic) near Tromsø, Norway, was used to modify the ionospheric F region in the daytime. The intensity and phase scintillations of 250‐MHz transmissions from the quasi‐stationary polar beacon satellite were measured when the ray path from the observing site to the satellite intercepted the modified ionospheric volume. Narrow band spectral enhancements corresponding to an irregularity scale length of 750 m were detected in the intensity spectra when the radiated HF power developed an estimated power density of about 0.3 mW/m² at the height of reflection. Spectral enhancements at larger scales were not detected in the phase spectra. From the growth and decay of the intensity spectral enhancements during the successive 10‐min “on” and 10‐min “off” periods of the heater the e‐folding growth and decay times of ∼750 m irregularities were estimated to be on the order of 30 s and 2 min, respectively. The threshold power densities required for the generation of the observed irregularity scale sizes were calculated from the self‐focusing instability theory of Cragin et al. (1977) by the use of ionospheric background parameters measured by the EISCAT radar. The theoretical estimates were found to be within a factor of 2 of the HF power densities employed in the experiment. The presence of Fresnel oscillations in the intensity and phase spectra were attributed to a limited irregularity layer thickness less than 50 km. By using the formulations of the thin phase screen theory, it was found that the observed intensity scintillations at 250 MHz correspond to irregularity amplitudes (ΔN/N) of 3% with an outer scale of 1 km.
Do slow shocks precede some coronal mass ejections?
Authors: Hundhausen A. J. , Holzer T. E. , Low B. C.
Published: February 2008

The observed speeds of coronal mass ejections are often below the estimated Alfvén speed but above the sound speed for the background solar corona. This suggests that slow magnetohydrodynamic shocks may form as mass ejections sweep through the corona. We argue on the basis of the Rankine‐Hugoniot relations and the propagation of small‐amplitude slow mode waves that the shape of a slow shock front would be flattened (with respect to a sun‐centered sphere) or perhaps even concave outward (from the sun) and thus present a very different appearance from the fast coronal shock waves that have been commonly modeled as wrapping around a mass ejection. The region behind a slow shock front standing just off the top of a coronal mass ejection would extend well out beyond the visible flanks of the ejection. The deflections of coronal structures that are commonly observed well outside of these flanks (and which are inconsistent with a fast shock wrapped around the mass ejection) are consistent with the presence of the slow shock, whether they lie in the enlarged postshock region or in a region still further beyond. Although the flattening of the tops of some mass ejections suggests our proposed slow shock configuration, a true test of its existence awaits formulation of quantitative models and detailed comparison with observations.
HF Doppler observations of vertical plasma drifts in the evening F region at the equator
Authors: Jayachandran B. , Balan N. , Nampoothiri S. P. , Rao P. B.
Published: February 2008

Hf Doppler observations of vertical plasma drifts in the evening F region of the equatorial ionosphere at Trivandrum (8.5°N, 77°E; dip 0.9°S) are presented. The observations reveal a consistent pattern characterized by a post‐sunsent enhancement with peak drift velocity in the range 30–60 m s−1 for quiet conditions. Under moderately disturbed conditions, the enhancement is found to be down by as much as a factor of 2, while no such appreciable change is noted for highly disturbed conditions. The pattern of drift velocity dependence on mangnetic activity is seen to be consistent with that of spread F occurrence. A striking feature of the observations is the presence of a significant fluctuating component in the drift velocity with quasiperiods ranging from a few minutes to a few tens of minutes during both quiet and disturbed periods.
The space shuttle as a platform for observations of ground‐based transmitter signals and whistlers
Authors: Neubert T. , Bell T. F. , Storey L. R. O. , Gurnett D. A.
Published: February 2008

Recent experimental and theoretical studies indicate that coherent VLF waves, such as lightning‐generated whistlers and signals from ground‐based VLF transmitters, can often produce significant pitch angle scattering in energetic particles in the magnetosphere. However, the relative importance of these waves in controlling the lifetimes of energetic particles is only partially understood due to limited knowledge of the global distribution of the coherent waves throughout the magnetosphere. The present paper presents a preliminary global study of VLF transmitter signals and low‐latitude whistlers received at 245 km altitude on the space shuttle. The observations were made in a 5‐day period during the STS 3 mission of the space shuttle in March 1982. The threshold sensitivity of the wave receiver when mounted in the shuttle bay was 0.3 pT ± 10 dB (set by the shuttle electromagnetic interference), which was sufficient to detect the whistler mode signals in large regions of the ionosphere. We find that the direct signals from a 10‐kW transmitter located at 28°S magnetic latitude were received in a roughly circular region with a diameter of 6000 km centered around the transmitter. The signals propagating through the magnetosphere from a 500‐kW magnetically conjugate transmitter at 40°N magnetic latitude were received inside a region extending 5000 km in longitude and 2000 km in latitude. Finally, the direct signals from a 1 MW‐transmitter at 31°S magnetic latitude were received in a region extending 22,000 km in longitude, while the latitudinal extent (5000 km) was limited by the shuttle orbit and the day/night terminator. Except for one case, signals were received only during nighttime. Extremely small dispersion whistlers were detected on L‐shells between 1.04 and 1.11, suggesting that the lack of ducted whistlers on magnetic field lines in this range is not due to transmission loss across the D region boundary or to high ionospheric absorption loss, but most likely to a lack of ducts.
Dayside temperatures of the Martian upper atmosphere
Authors: Bittner Hermann , Fricke K. H.
Published: February 2008

One central problem in understanding the Martian upper atmosphere is the poor correlation between exospheric temperatures and the energy input from the Sun in the EUV and UV. Turbulence heats the atmosphere by dissipation of turbulent energy and cools it by downward heat transport. A time‐variable turbulence may introduce a stochastic component in addition to the solar‐driven, regular variation of the exospheric temperatures. To investigate the possible range of temperatures on the basis of this assumption, we developed a one‐dimensional mean‐dayside model of the energy balance of the Martian upper atmosphere. With plausible assumptions on the range of the eddy diffusion coefficient, we find a stochastic component of ±63 K for the exospheric temperatures. The comparison of observed data with the results of our model yields a best value for the efficiency of the heating by absorption of solar ultraviolet radiation of 0.145 ± 0.05.
A study of tearing instability in the presence of a pressure anisotropy
Authors: Shi Y. , Lee L. C. , Fu Z. F.
Published: February 2008

The effects of pressure anisotropy on the tearing mode in a current sheet are studied based on a linear compressible MHD code. It is found that in the presence of a small pressure anisotropy (P⊥ > P∥), tearing mode dominates the unstable modes except that the growth rates are enhanced, which is consistent with the recent results; for a large pressure anisotropy the growth rates of unstable modes become even larger, and a mixed mirror‐tearing configuration is formed for large kx (wave number along current sheet) implying that mirror instability is responsible for the enhancement of growth rate and modification of the tearing mode. In the presence of a large pressure anisotropy, a new oscillating tearing mode is observed when the wave number kx is small. Furthermore, a large y component of the magnetic field is found to be able to cancel the effect of pressure anisotropy, with the resultant mode being the same as that of isotropic tearing instability.
Interpretation of electron spectra in morningside pulsating aurorae
Authors: Evans D. S. , Davidson G. T. , Voss H. D. , Imhof W. L. , Mobilia J. , Chiu Y. T.
Published: February 2008

Energy spectra of precipitating electrons in morningside aurorae were obtained with instruments on the NOAA 6 satellite during several passes through the southern auroral zone in 1982. It is substantiated with the aid of coordinated data from particle detectors and a photometer in the Simulated Emission of Energetic Particles experiment payload on satellite S81‐1 that the precipitation observed by NOAA 6 was probably in a region of pulsating aurorae. The particle detectors and forward/downward looking quadrant photometer onboard S81‐1 all exhibited regular modulations at a period of about 5–15 s. The particle detectors on NOAA 6 showed fluctuating electron intensities over an energy range from several kiloelectronvolts to many tens of kiloelectronvolts, superimposed on a steady background component extending down to energies less than 1 keV. The NOAA 6 electron observations are in qualitative agreement with a time‐dependent precipitation model that includes the production of backscattered and secondary electrons from the atmosphere. In this model the time variations imposed on the precipitation are restricted to the energetic portion of the electron spectrum, while the integrating effects inherent in the production of backscattered and secondary electrons, together with phase mixing introduced as those electrons transit between conjugate hemispheres, insure the existence of a nearly time independent population of low‐energy electrons. A steady background precipitation is thereby created as a natural consequence of a time‐variable primary precipitation.
Ionospheric convection signatures and magnetic field topology
Authors: Coley W. R. , Heelis R. A. , Hanson W. B. , Reiff P. H. , Sharber J. R. , Winningham J. D.
Published: February 2008

We present here a statistical study of signatures of the high‐latitude ionospheric convection pattern and the simultaneously observed energetic electron precipitation. We most often find convection cells in which the sunward flowing region contains auroral particle precipitation but the antisunward flowing region does not. However, our observations also show the frequent occurrence of convection cells in which neither the antisunward nor the sunward flowing plasma region contains auroral particle precipitation. These findings may appear within the dawnside or duskside convection pattern and strongly suggest that such convection cells may be associated with open magnetic field lines that thread the magnetotail lobes. Examination of the interplanetary magnetic field (IMF) data shows that this “lobe cell” convection signature is most likely to be accompanied by the signature of dayside merging when the IMF has a significant y component but is directed southward. A lobe convection cell has a location and sense of circulation that are dependent on the sign of By. For the northern hemisphere, clockwise circulation displaced to the duskside appears roughly 35% of the time when By is positive, and anticlockwise circulation displaced to the dawnside appears when By is negative. The same circulation sense and location exist in the southern hemisphere for the opposite polarity of By. At times of northward IMF, the circulation within the polar cap can be at least partially on closed field lines and cannot be easily reconciled with merely a distortion of the standard “two‐cell” convection pattern. The significance of these results to several models of the solar wind/magnetosphere interaction is discussed.
Observations of large‐amplitude MHD waves in Jupiter's foreshock in connection with a quasi‐perpendicular shock structure
Authors: Bavassano‐Cattaneo M. B. , Moreno G. , Scotto M. T. , Acuña M.
Published: February 2008

Plasma and magnetic field observations performed on board the Voyager 2 spacecraft have been used to investigate Jupiter's foreshock. Large‐amplitude waves have been detected in association with the quasi‐perpendicular structure of the Jovian bow shock, thus proving that the upstream turbulence is not a characteristic signature of the quasi‐parallel shock.
A kinetic study of solar wind mass loading and cometary bow shocks
Authors: Omidi N. , Winske D.
Published: February 2008

A detailed numerical study is conducted to understand the kinetic processes associated with solar wind mass loading due to pickup of cometary ions and the formation of cometary bow shocks. Because in this study we are most interested in phenomena which take place over time and spatial scales associated with ion dynamics, electrons are treated as a massless fluid. On the other hand, solar wind protons and heavy cometary ions are treated kinetically. It is found that solar wind deceleration and pickup of cometary ions take place through both the macroscopic electromagnetic fields embedded in the solar wind, as well as the microscopic fields associated with low‐frequency electromagnetic waves that are generated by the unstable velocity distribution function of the cometary ions. These electromagnetic waves can grow to very large amplitudes and their nonlinear evolution is controlled by their wave normal angle. At parallel propagation where the waves are noncompressional, parametric instabilities seem to be operative, while the oblique, compressional waves are found to steepen and form shocklets. As for the nature of cometary bow shocks, three different regimes are found, based on the shock normal angle. For shock normal angles at or near 90° (quasi‐perpendicular), a typically thin transition region (shock) is formed through steepening of fast magnetosonic pulses. This shock is a pure proton shock in that no sharp change in density or velocity of the cometary ions is observed across the shock. At intermediate shock normal angles (oblique), a much wider transition region is seen where the solar wind is gradually heated and decelerated. This region, whose width is much larger than the gyroradius of cometary ions, is associated with a high level of turbulence due to ion pickup instabilities and is unlike a typical fast magnetosonic shock. Finally, at small shock normal angles (quasi‐parallel), a narrow transition region is found which is similar to a parallel terrestrial shock. The formation of this shock is not due to wave steepening but rather is the result of scattering and heating of the solar wind protons by the large‐amplitude electromagnetic waves that are generated through the relative drift between the protons and the cometary ions. These results are compared and shown to be in qualitative agreement with the recent observations at comets.
The NII 2143 Å emission in the dayglow
Authors: Cleary D. D. , Barth C. A.
Published: February 2008

Observations of the NII 2143 Å emission in the Earth's dayglow were obtained by a rocket‐borne spectrograph flown on November 9, 1981, and August 11, 1982. The ultraviolet spectra were analyzed using synthetic spectra to separate the contribution of the 2143 Å emission from the overlapping nitric oxide gamma band. The 2143 Å column emission rate profiles for both flights were quite similar with peak column emission intensities of approximately 500 Rayleighs at an altitude of 160 km. When the observed column emission rate profiles are compared with theoretical calculations, the production efficiency of this emission was found to be 0.10 ± 0.04. The 2143 Å emission varies by more than a factor of 3 over the solar cycle and therefore should be useful as an indicator of the solar EUV flux.
The Voyager 2 encounter with Uranus
Authors: Stone E. C.
Published: February 2008

The Voyager Uranus/Interstellar Mission is the continuation of the NASA program of exploration of the outer solar system. The first phase of the Voyager program included encounters with Jupiter and Saturn as summarized in the Journal of Geophysical Research (volume 86, pages 8123–8841, 1981, and volume 88, pages 8639–9018, 1983). With the successful completion of this first phase a second phase was undertaken with the objectives of exploring the Uranus system and investigating the interplanetary and interstellar media. Additional objectives included preserving the capability for extending the investigations to include an encounter with the Neptune system and a search for the heliopause.
Micron‐sized particle impacts detected near Uranus by the Voyager 2 Plasma Wave Instrument
Authors: Gurnett D. A. , Kurth W. S. , Scarf F. L. , Burns J. A. , Cuzzi J. N. , Grün E.
Published: February 2008

During the Voyager 2 flyby of Uranus the plasma wave and radio astronomy instruments detected a region of impulsive noise near the equatorial plane just inside the orbit of Miranda, at a radial distance of 4.51 RU. This noise is believed to be caused by micron‐sized particles hitting the spacecraft. Analysis of various coupling mechanisms shows that when a dust particle hits the spacecraft at a high velocity, the particle is instantly vaporized and ionized, thereby releasing a cloud of charged particles, some of which are collected by the antenna. The resulting voltage pulse is detected by the plasma wave instrument. Based on reasonable assumptions about the charge yield and collection efficiency of the antenna, the number density and mass of the particles can be estimated from the rate and amplitude of the voltage pulses. The analysis shows that the maximum number density of the particles is about 1.6 × 10−3 m−3, and the thickness of the impact region, based on a Gaussian fit, is 3480 km. The maximum number density occurs slightly after the ring plane crossing at a distance of about 280 km from the equatorial plane. The mass threshold for detecting the particles is estimated to be about 4.5 × 10−10 g, and the rms mass of the particles is about 2.6 × 10−9 g. For a density of a few grams per cubic centimeter the particles have radii of the order of a few microns. Possible sources for these particles include the rings, the small satellite 1985U1 discovered outside the ring system, or other unseen small bodies that lie between synchronous orbit (3.15 RU) and 4.51 RU. If the particles are charged, electromagnetic forces produced by the rotating tilted dipole of Uranus may play a role in their transport and diffusion.
Survey of electrons in the Uranian magnetosphere: Voyager 2 observations
Authors: Sittler Edward C. , Ogilvie Keith W. , Selesnick Richard
Published: February 2008

We present results of an analysis of the Voyager 2 plasma science experiment (PLS) electron measurements made during the Uranus encounter. The energy coverage is 10 eV ≤ E ≤ 5950 eV. The electron distribution functions are composed of a thermal (cold) and a non‐Maxwellian suprathermal (hot) component. Within the inner magnetosphere the electron density ne ranged between 0.02 cm−3 and 1.0 cm−3, while the electron temperatures Te ranged between ∼10 eV near the terminator and ∼100 eV within the nightside hemisphere. Thermal (suprathermal) electron temperatures Tc (Th) ranged from ∼7 to 10 eV (20–200 eV) near the terminator and from ∼10 to 30 eV (500 eV to 2 keV) within the nightside hemisphere. The observations revealed no significant electron fluxes above the detection limit at all observed energies in the dayside hemisphere, but electron fluxes orders of magnitude larger extended over the full PLS energy range in the nightside hemisphere. The large day‐night asymmetry together with the spin axis alignment with the solar direction and the large tilt ∼60° of the planetary magnetic dipole indicate that solar wind driven time dependent magnetospheric convection will be an important transport mechanism within the Uranian magnetosphere. The energy‐time evolution of the plasma boundary for ions and electrons near the terminator (L ∼ 6.7) at 1648 spacecraft event time (SCET) on January 24, 1986, is remarkably similar to that observed during substorms by spacecraft within the earth's magnetosphere at geosynchronous orbit in the dusk‐midnight quadrant. We show that Miranda cannot be the cause of the plasma sheet inner edge and that present “steady state” shielding models cannot account for the thinness of the “inner edge” (< 500 km) because of expected periodic merging (enhanced convection) every ∼17 hours on the dayside magnetopause. Resonant charge exchange collisions between protons and Uranus' hydrogen cloud are used to estimate an upper limit for the hydrogen corona density NH ∼200 cm−3 at L = 5. Using the local time asymmetry in the E ∼ 6 keV electron fluxes and the observation of E > 20 keV electron fluxes at all local times sampled by the spacecraft (Krimigis et al., 1986), we estimate that the steady state convection time τconv of the plasma is between 1 and 3 days. Finally, the local time asymmetry in the radio emissions from Uranus is consistent with the day‐night asymmetry in the keV electron fluxes observed by PLS which may provide the free energy for the radio emission.