bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of auroral electron precipitation on the morningside
Authors: Lyons L. R. , Fennell J. F.
Published: February 2008

Quantitative information on auroral electron precipitation is needed to evaluate the effects of the precipitation on the ionosphere and upper atmosphere. In this paper we present such information for the morningside aurora. We use simultaneous observations of precipitating electrons and auroral images from the low‐altitude, polar‐orbiting DMSP‐F6 satellite, and we relate specific precipitation features to specific types of auroras. We find the equatorward region of diffuse aurora to have precipitating electron energy fluxes, εp, which typically reach 10 ergs/cm² s, and such fluxes can extend over several degrees in latitude. Energy spectra over the diffuse aurora are quite hard and show no evidence of acceleration by a field‐aligned potential drop, V∥. The poleward region of structured aurora includes a soft background of relatively weak, uniform precipitation. Arclike structured precipitation results from a variable V∥ acceleration of this background. Over the arcs, εp can exceed 1 erg/cm² s. Bursts of precipitation at energies above a few keV are identifiable in the data equatorward of the peak in the diffuse auroral precipitation. We believe these bursts are associated with pulsating auroral patches. Omega band waves are occasionally observed along the poleward boundary of the diffuse aurora. Electron spectra over the omega bands are unique for the morningside. The spectra show hard precipitation as observed within the diffuse region. In addition, the spectra have a peak at energies generally ≲1 keV indicative of acceleration by a V∥ ≲ 1 keV.
Modeling substorm current systems using conductivity distributions inferred from DE auroral images
Authors: Kamide Y. , Craven J. D. , Frank L. A. , Ahn B.‐H. , Akasofu S.‐I.
Published: February 2008

Images of auroral emissions observed with Dynamics Explorer 1 (DE 1) are used to calculate ionospheric conductivities over the polar region. The combination of ground‐based magnetometer data and the inferred conductivities enables us to estimate on a global scale the distributions of ionospheric electric fields and currents and field‐aligned currents, as well as the Joule heating production rate, with a time resolution of 12 min. Although our inference of the conductivity distribution requires further improvements, the following conclusions may be drawn from this work: (1) The westward electrojet is, in general, collocated with the region of high auroral luminosity, while the region of relatively low luminosity in the evening sector is collocated with the eastward electrojet. (2) The upward field‐aligned currents exist in the brightest auroral region on the poleward side of the evening auroral oval and on the equatorward side of the morning oval. However, the magnitudes of these upward currents are not in proportion to the auroral luminosities, suggesting that the characteristic energies of precipitating electrons carrying the upward currents and generating the auroral luminosities may be different. (3) A significant amount of ionospheric currents can flow in regions where there are no bright auroral emissions. An example of this effect is found over the poleward half of the westward electrojet in the morning sector, where the electric field is the main contributor to the ionospheric currents. (4) The distribution of calculated field‐aligned currents displays many localized structures which characterize the development of the substorm current system. Previous satellite observations may represent gross time averages of such complicated distributions.
On the dependence of the phase velocity of equatorial irregularities on the polarization electric field and theoretical implications
Authors: St.‐Maurice J.‐P. , Hanuise C. , Kudeki E.
Published: February 2008

We have studied the mean Doppler shift of equatorial irregularities in the E region, using data from a coherent radar situated in Ethiopia between 1976 and 1980. We found that even though the primary two‐stream (or type I) waves are drifting at a speed close to the expected ion acoustic speed, there is a small but nonnegligible increase in the phase velocity with increasing electron drift in the electrojet. We attribute this increase to a heating of the electron gas by the large‐amplitude, low‐frequency waves that are present in the region when conditions are favorable to the production of large‐scale gradient drift instabilities. On the other hand, for the single day when the production of large‐scale gradient drift instabilities was inhibited, the behavior of the irregularities was quite different. Namely, the Doppler shift of 10‐m waves (type O waves) was usually below the linear threshold value while the spectra remained narrow and type I‐like rather than type II‐like. On the other hand, the Doppler shift followed a dependence similar to that expected from type II waves. We have tentatively attributed this type O behavior to weak mode coupling, following similar inferences from recent Condor observations made near the top of the electrojet.
Parabolic heavy ion flow in the polar magnetosphere
Authors: Horwitz J. L.
Published: February 2008

Recent observations by the Dynamics Explorer 1 satellite over the dayside polar cap magnetosphere have indicated downward flows of heavy ions (O+, O++, N+, N++) with flow velocities of the order 1 km/s (Lockwood et al., 1985b). These downward flows were interpreted as the result of “parabolic” flow of these heavy ionospheric ions from a source region associated with the polar cleft topside ionosphere. Here we utilize a two‐dimensional kinetic model to elicit features of the transport of very low energy O+ ions from the cleft ionosphere. Bulk parameter (density, flux, thermal energies, etc.) distributions in the noon‐midnight meridian plane illustrate the effects of varying convection electric fields and source energies. The results illustrate that particularly under conditions of weak convection electric fields and weak ion heating in the cleft region, much of the intermediate altitude polar cap magnetosphere may be populated by downward flowing heavy ions. It is further shown how two‐dimensional transport effects may alter the characteristic vertical profiles of densities and fluxes from ordinary profiles computed in one‐dimensional steady state models.
Empirical models for the plasma convection at high latitudes from Millstone Hill observations
Authors: Holt J. M. , Wand R. H. , Evans J. V. , Oliver W. L.
Published: February 2008

Since 1978, radar observations of F region electric fields within the region 55° < Λ < 75° have been made from Millstone Hill (42.6°N, 71.5°W). Averge convection patterns have been calculated from the ion drift data gathered in 109 of these experiments conducted between January 1978 and August 1981. Most of the experiments lasted between 24 and 48 hours, and over 3,700,000 values of the line‐of‐sight velocity were determined and included in the averages. The observed velocities were sorted into “bins” of ½‐hour intervals of magnetic local time and 2° intervals of apex latitude. Each of these cells had been viewed by the radar over a wide range of aspect angles in the course of the 109 experiments, allowing the average vector velocity to be determined. The data were separated further into three levels of Kp, according to whether the interplanetary magnetic field (IMF) was “toward” or “away” from the sun, and by season. The average patterns are discussed and compared with earlier models based on satellite and incoherent scatter data. There is an expansion and intensification of the pattern with Kp for all seasons and IMF orientations. The polar cap entry and exit points of the plasma, the center of the cells, and the polar cap boundary all depend on the IMF. The Harang discontinuity, which can be seen clearly on individual days, is largely lost in the averages.
Pioneer 10 and 11 observations of waves upstream of interplanetary corotating shocks
Authors: Bavassano Bruno , Smith Edward J. , Tsurutani Bruce T.
Published: February 2008

An extended region of enhanced magnetic field fluctuations is found upstream of some of the corotating shocks observed by Pioneer 10 and Pioneer 11 between 1 and 5 AU. This perturbed region is present when the corotating shock, generally quasi‐perpendicular, becomes oblique or quasi‐parallel due to a temporary out‐of‐spiral direction of the upstream magnetic field. The observed waves are almost not compressional. Their amplitude is a large fraction of the ambient field, and their frequency is around 1 mHz in the spacecraft frame. A brief discussion of the possible mechanicms of generation is given.
Spiky parallel dc electric fields in the aurora
Authors: Rowland H. L. , Palmadesso P. J.
Published: February 2008

A series of computer simulations is reported on the detailed shape and distribution of the dc electric fields that result when an external potential drop is applied to a plasma with strong low‐frequency turbulence. The structures are shown to depend on the size of the potential drop and upon the shape of the ion turbulence. For the type of turbulence expected for current driven electrostatic ion cyclotron waves the dc fields seen in the simulations are in good agreement with those observed in the aurora.
The interchange instability in high‐latitude plasma blobs
Authors: Chaturvedi P. K. , Huba J. D.
Published: February 2008

The stability of high‐latitude plasma density enhancements (“blobs”) is analyzed with regard to the interchange mode (driven by neutral wind or equilibrium transverse electric field acting on the density gradient at the walls of the blobs). The effects arising from the finite parallel length of the blobs along the magnetic field lines are included in the analysis. Plasma regions of differing collisionalities, to which the blobs extend in altitude, are considered. We find that the finite parallel blob size results in a modest reduction in the growth rates of the small (≲1 km) and intermediate (1–10 km) scale sizes but severely reduces the growth rates for the large scale sizes (>10 km) for the observed parallel blob lengths (∼ 300–600 km). Further, it is found that the instability growth rates show a moderate reduction at higher altitudes (where ion‐inertial effects may be dominant over the ion‐neutral collisional effects). Thus the E × B instability is considered a plausible candidate for the scintillation‐causing irregularities (1–10 km) associated with the high‐latitude blobs.
Electron precipitation bursts in the nighttime slot region measured simultaneously from two satellites
Authors: Imhof W. L. , Voss H. D. , Mobilia J. , Gaines E. E. , Evans D. S.
Published: February 2008

Based on data acquired in 1982 with the Stimulated Emission of Energetic Particles payload on the low‐altitude (170‐280 km) S81‐1 spacecraft and the Space Environment Monitor instrumentation on the NOAA 6 satellite (800–830 km), a study has been made of short‐duration nighttime electron precipitation bursts at L = 2.0–3.5. From 54 passes of each satellite across the slot region simultaneously in time, 21 bursts were observed on the NOAA 6 spacecraft, and 76 on the S81‐1 satellite. Five events, probably associated with lightning, were observed simultaneously from the two spacecraft within 1.2 s, providing a measure of the spatial extent of the bursts. This limited sample indicates that the intensity of precipitation events falls off with width in longitude and L shell but individual events extend as much as 5° in invariant latitude and 43° in longitude. The number of events above a given flux observed in each satellite was found to be approximately inversely proportional to the flux. The time average energy input to the atmosphere over the longitude range 180°E to 360°E at a local time of 2230 directly from short‐duration bursts spanning a wide range of intensity enhancements was estimated to be about 6 × 10−6 ergs/cm² s in the northern hemisphere and about 1.5 × 10−5 ergs/cm² s in the southern hemisphere. In the south, this energy precipitation rate is lower than that from electrons in the drift loss cone by about 2 orders of magnitude. However, on the basis of these data alone we cannot discount weak bursts from being a major contributor to populating the drift loss cone with electrons which ultimately precipitate into the atmosphere.
Ion beams and the ion/ion acoustic instability upstream from the Earth's bow shock
Authors: Fuselier S. A. , Gary S. Peter , Thomsen M. F. , Bame S. J. , Gurnett D. A.
Published: February 2008

This paper considers the generation of enhanced ion acoustic fluctuations by field‐aligned ion beams upstream from the earth's bow shock. Steep slopes in the beam distribution parallel and possibly strongly oblique to the magnetic field are correlated with bursts of ion acoustic noise in the upstream region. Linear theory shows that it is the slope of the beam distribution at oblique angles to the magnetic field that determines the growth rate of the ion/ion acoustic instabilty. Because of instrument limitations we suggest but cannot confirm that enhanced ion acoustic fluctuations in the upstream region and in the presence of field‐aligned beams are driven by such steep‐sided distributions.
Magnetic field‐aligned electric field acceleration and the characteristics of the optical aurora
Authors: Christensen A. B. , Lyons L. R. , Hecht J. H. , Sivjee G. G. , Meier R. R. , Strickland D. G.
Published: February 2008

The long‐recognized association of brighter aurora with more deeply penetrating, and hence more energetic electrons is examined. Using the Knight (1973) relation between the magnetic field aligned current density j∥ and potential drop V∥, derived from the theory of single particle motion in the presence of a magnetic field‐aligned electric field, an approximate expression relating the energy flux Q of the precipitating electrons over discrete aurora and the mean particle energy EM is derived: Q(ergs/cm² s) = kEMγ, where 1.5 ≤ γ ≤ 2 and k depends on the temperature and density of the plasma in the source region. This expression is used in conjunction with an auroral optical excitation and emission model to specify the dependence of the red (O I[630.0])/blue (N2+(427.8)) ratio of auroral optical emissions on the brightness of the aurora. It is shown that the quantitative predictions of the discrete auroral theory are in accord with observations of the aurora. Thus the often observed anticorrelation of the red/blue ratio with auroral brightness is consistent with the single‐particle motion theory. It follows that in discrete auroral events where the theory is valid, the brightening of the aurora involves an increase in both the mean energy and the number flux of precipitating electrons.
A search for forerunner activity associated with coronal mass ejections
Authors: Karpen Judith T. , Howard Russell A.
Published: February 2008

The possible existence of energetic disturbances in the corona significantly before the associated surface events has profound implications for the location and mechanisms of preevent coronal energy storage. Precursor activity associated with coronal mass ejections (CMEs), if it exists, would reflect the evolution and magnitude of the energy release or failure of magnetic equilibrium characterizing the interval before and during the events. Jackson and Hildner (JH) studied 21 CMEs observed with the Skylab white‐light coronagraph, and found a low‐density plateau rimming each event for which good coverage was available. They concluded that this “forerunner” material could not be explained by mere translation of the overlying coronal plasma. Jackson further inferred from the Skylab data that the forerunner must start moving significantly before the onset of the associated CME, thus identifying this phenomenon as a form of precursor activity. We have performed a systematic search for forerunners using the white‐light coronagraph observations obtained with the Solwind instrument on board the P78‐1 satellite. Forty‐four bright, well‐observed events were selected and analyzed, employing selection criteria and analysis methods similar to those used by JH. Approximately half of these events either do not exhibit low‐density plateaus in front or are questionable (e.g., a frontal plateau only appears intermittently). Based on our analysis of the remaining CMEs, we conclude that identification of the forerunner as a distinct entity probably is not warranted, and that the low‐density plasma is an integral part of the CME itself.
A comparison of coronal rotation and interplanetary recurrence during 1964–1976
Authors: Parker G. D.
Published: February 2008

Contemporaneous coronal and interplanetary data sets which span the first full solar cycle of in situ space observations illuminate the solar origins of corotation and recurrence in the solar wind. Because solar rotation is differential, comparisons of solar rotation and interplanetary recurrence periods suggest latitudes of origin for recurrent space phenomena. The 27‐day recurrence period of geomagnetic activity and solar wind streams is associated with lower latitudes in the 1.5 RS corona than in the 1.1 RS corona. If we associate the recurrence period with coronal tracers at the lower height, we determine an observational upper limit of 20° on typical solar wind motions in latitude over 1 AU. However, if we suppose that magnetic field lines which close just beneath the source surface at the equator are anchored typically 20° away from the equator, stream recurrence periods approximate high‐altitude equatorial rotation periods, in which case, no meridional motion of solar wind over 1 AU is implied. Plausible coronal sources of IMF corotation periods can be found for most intervals, but we discuss one interval in which solar magnetic tracers may not reflect field line rotation at the base of the solar wind.
Kinetic parameters related to sources and sinks of vibrationally excited OH in the nightglow
Authors: McDade I. C. , Llewellyn E. J.
Published: February 2008

Kinetic parameters related to vibrational deactivation and chemical removal of vibrationally excited OH radicals in the mesosphere are deduced from ground‐based measurements of the mean vibrational distribution of the OH Meinel bands in the nightglow. The derived parameters, which rely on a laboratory measured rate coefficient for the removal of OH(υ = 9) by O2 (Finlayson‐Pitts and Kleindienst, 1981) and a set of relative Meinel band transition probabilities (Murphy, 1971), have been obtained for two limiting Meinel band excitation models that differ in the extent to which single‐quantum vibrational deactivation and “sudden death” collisional removal processes determine the OH vibrational distribution. It is shown that the OH Meinel band emission can be adequately explained with the deduced parameters and the H + O3→OH + O2 reaction as the only chemical source of vibrationally excited OH. Evidence is presented which suggests that the reaction HO2 + O ↔ HO + O2 may perhaps be involved as a sink of vibrationally excited OH rather than as a potential source. The deduced kinetic parameters should be particularly useful in future Meinel band studies as they have been obtained from an analysis for which there is no assumption about the very uncertain OH radiative lifetimes.
Enhanced current flow through a plasma cloud by induction of plasma turbulence
Authors: Hastings D. E.
Published: February 2008

Electrodynamic tethers have been proposed as a means of generating power in low earth orbit. One of the limitations on the power generated is the relatively low electron current that can be collected. It is proposed that the electron current can be significantly enhanced by means of current‐induced plasma turbulence in a plasma cloud around the collecting anode. This is examined for the specific case of ion acoustic turbulence. An important conclusion is that the use of plasma clouds in the ionosphere will entail a high‐impedance (no instability) and a low‐impedance (ion acoustic instability) mode of operation. The low‐impedance mode of operation will have two submodes, one steady state and one pulsed.
Ram ion scattering caused by space shuttle v × B induced differential charging
Authors: Katz I. , Davis V. A.
Published: February 2008

Observations of secondary, high‐inclination ions streams have been reported in the literature. The authors of these previous papers attributed the source of the secondary ions to a disturbed region in the plasma about 10 m from the space shuttle Orbiter. A new theory has been developed which shows how v × B induced differential charging on the plasma diagnostics package (PDP) can scatter the ram ion flux. Some of these ions are reflected back to the PDP and may be the source of the observed ion distributions. The effect is unique to large spacecraft; it occurs only when the magnitude of the induced v × B potentials are much larger than the electron thermal energy and of the order of the ion ram energy. That the ion streams observed at large angles must have been reflected from the PDP surface is demonstrated with three‐dimensional sheath and particle trajectory calculations using the low earth orbit version of the NASA Charging Analyzer Program (NASCAP/LEO).
Diurnal characteristics of field line resonances at College, Alaska
Authors: Olson John V.
Published: February 2008

Magnetic variations measured at College, Alaska (L = 5.6), contain a low‐amplitude, intermittent tone near 5 mHz. The tone shows a regular variation in frequency throughout the day with the shortest periods (highest frequencies) near local noon and the longest periods (lowest frequencies) in the midnight sector. The wave train is composed of a superposition of individual packets which are not phase coherent. The polarization states of the pulses tend to be left hand, elliptical in the prenoon sector, becoming nearly linear in the afternoon‐evening sector with the H component dominant. The short duration of individual packets and the lack of phase coherence imply that these pulsations are not the result of driven resonance but rather are the transient oscillations of the local field line which occur when there is an impulsive change in magnetospheric parameters. Since both field and plasma conditions change on the local field line throughout the day, the natural response frequency of the field line will change as well. The level of diurnal variation is consistent with that calculated by Singer et al. (1981) using realistic field geometries.
The configuration of slow‐mode shocks
Authors: Wolfson Richard
Published: February 2008

Interest in slow‐mode shocks in space physics applications has increased in recent years. These shocks, which can arise from the steepening of linear slow‐mode magnetohydrodynamic waves, might form ahead of objects moving through a magnetized, highly conducting gas at speeds greater than the slow‐mode speed. In particular, slow shocks are the only shocks possible for speeds between those of the slow and fast modes. This appears to be the speed range, for example, of many and perhaps most coronal mass ejections. Despite the interest in and detection of slow‐mode shocks in the heliosphere, there appear to date to be no calculations of flows involving slow shocks. Basic properties like shock configuration and standoff distance are therefore unknown, although several authors have pointed out that slow shocks should extend upstream of the obstacle giving rise to the shock. This paper presents a simple method for computing slow shock flows in an infinitely conducting plasma, based on the solution of a free boundary problem for the shock configuration that matches a post shock potential flow to a uniform preshock flow. Such potential‐flow methods have been used in ordinary gasdynamics to obtain crude approximations to blunt‐body flows containing shocks; arguments given here suggest that the potential‐flow approximation should be more accurate in the case of slow MHD shocks. The method is applied to shocks arising from flow about spherical and paraboloidal obstacles. A parametric study of shock formation as a function of sonic and Alfvén Mach numbers shows standoff distance increasing as either Mach number decreases. Streamlines and shock configurations for several slow shock flows are also presented.
Radial force balance within Jupiter's dayside magnetosphere
Authors: Mauk B. H. , Krimigis S. M.
Published: February 2008

A local field stress technique, developed previously in a study of the Saturnian magnetosphere, is introduced to the problem of determining the radial force balance characteristics of Jupiter's magnetosphere. We begin by estimating the near‐equatorial, radial magnetic force densities using the data obtained by Voyager 1 principally on the dayside (inbound) portion of its trajectory (7.4 ≤ R(RJ) ≤ 42). Using the low‐energy charged particle data (≳30 keV) and other published data we then explore ways in which the field forces might be balanced. Inside 22 RJ the magnetic radial forces match in both magnitude and radial variation the hot particle pressure gradient forces, assuming a mix of H+ and On+ ions. Outside 22 RJ we confirm qualitatively a previously reported force balance problem, and we suggest two possible solutions. The magnetic radial forces could be balanced in part either by the hot particle pressure anisotropy forces (with P∥/P⊥ ∼ 1.5 for On+) or by the centripetal acceleration of the cool particles that may be streaming in bulk (υs) parallel to the magnetic field within the small field line curvature region of the neutral sheet (with υs ∼ υcorot). Comparing present results with the results of our previous study, we note that contrary to common expectations, Saturn rather than Jupiter is unique in having the corotation centrifugal forces dominate over other sources of radial particle force in some regions of the middle (ring current) equatorial magnetosphere.
Circulation of the polar thermosphere during geomagnetically quiet and active times as observed by Dynamics Explorer 2
Authors: McCormac F. G. , Killeen T. L. , Thayer J. P. , Hernandez G. , Tschan C. R. , Ponthieu J. ‐J. , Spencer N. W.
Published: February 2008

Neutral wind measurements obtained by instruments on board the Dynamics Explorer 2 (DE 2) spacecraft have been used to study the effects of geomagnetic activity on the circulation of the high‐latitude neutral thermosphere for solar maximum conditions during the periods of November 1981 through January 1982 and November 1982 through January 1983. The data have been sorted and ordered according to the two geophysical indices Kp and (auroral electrojet) AE. Simple expressions have been derived which describe (1) the maximum antisunward wind speed in the geomagnetic polar cap, (2) the maximum sunward wind speeds in the dawn and dusk sectors of the auroral oval, and (3) the latitudinal extent of the polar cap antisunward neutral wind as functions of Kp and AE. The results show a positive correlation between the geomagnetic indices and the three characteristic features of the neutral circulation described above. Averaged vector wind fields in geomagnetic coordinates for Kp ≤ 2 and Kp ≥ 4 in both northern and southern hemispheres for the 6 months have been derived from the data. In doing this, a first‐order invariance of the neutral wind circulation in geomagnetic coordinates as a function of universal time (UT) was assumed. The results show a two‐cell circulation pattern in the northern winter hemisphere for both quiet and active geomagnetic periods. The cell sizes increase with increasing geomagnetic activity. The dusk cell is always dominant. The southern summer hemisphere averages show only the dusk circulation cell for both quiet and active geomagnetic periods. A diminution of this cell occurs for reduced levels of geomagnetic activity. Data from an individual DE 2 orbit are presented to illustrate how these average patterns tend to smooth out typically observed structure in the thermospheric wind.
Remote sensing of atomic oxygen: Some observational difficulties in the use of the forbidden O Iλ1173‐Å and O Iλ1641‐Å transitions
Authors: Erdman P. W. , Zipf E. C.
Published: February 2008

Recent sounding rocket and satellite studies suggest that simultaneous measurements of the O I λ989‐Å and λ 1304‐Å resonance lines and of the forbidden λ1172.6‐Å and λ1641.3‐Å transitions which also originate from the 3s'³D° and 3s ³S° states would form the basis of a useful remote sensing technique for measuring the O I density and optical opacity of a planetary or stellar atmosphere. Because the λ1172.6‐Å and λ1641.3‐Å emissions are weak lines and are emitted in a wavelength region rich in spectral features, it is important to determine whether typical flight instruments can make measurements with sufficient spectral purity so that the remote sensing observations will yield accurate results. We have made a detailed, high‐resolution study of the far ultraviolet emission features in the regions surrounding the atomic oxygen transitions at λ1172.6‐Å and λ1641.3‐Å. These spectra, which were excited by electron impact on O2 and N2, are presented in an attempt to display some potential sources of interference in aeronomical measurements of these O I lines. Both atomic and molecular emissions are found, and the spectral resolution necessary to make unambiguous measurements is discussed.
Quantitative analysis of thermal (∼1 eV) ion data from magnetospheric spacecraft
Authors: Young D. T. , Farrugia C. J.
Published: February 2008

Predictions of expected count rates from ion collectors with circular openings having half acceptance angles of 16° and 45° are compared for the two extreme cases of (1) a Laplace field and (2) one in which the potential drop between the spacecraft surface and the plasma occurs in a sheath of infinitesimal spatial extent. It is found that for the narrower field of view (16°) and typical values of the spacecraft potential in the plasmasphere, the predictions are nearly identical in both cases and agree with observed ion data. However, the two cases increasingly deviate as the instrument field of view is made wider. This conclusion, derived here for arbitrary ion Mach numbers and finite Mach angles, applies to particle analysis based on both the retarding potential method as well as on the spin modulation method. We also argue that the method of obtaining plasma parameters (N and T) by analysis of spin‐modulated fluxes has several advantages over retarding potential analysis. In the case of narrow aperture instruments, a simple formula for the integral ion flux is put forward, suitable for rapid processing of spin modulated data. It reduces in the appropriate limits to well‐known analytic formulas. Applications to the European Space Agency (ESA) ESA/GEOS 1 ion composition data are included.
Critical Mach numbers in classical magnetohydrodynamics
Authors: Kennel Charles F.
Published: February 2008

We use stationary point analysis to compute generalized critical Mach numbers for finite amplitude fast and slow shocks in classical MHD fluids. We pay particular attention to the case where the resistive and thermal conduction dissipation scale lengths are comparable and much larger than the viscous scale lengths. With both resistivity and thermal conduction, the critical Mach number at which viscosity must be invoked is determined by the condition that the downstream flow speed equal the isothermal sound speed. We also show that resistivity and thermal conduction can provide convergent stationary point solutions for nearly all slow shocks, except perhaps switch‐off shocks.
Increase of ion kinetic temperature across a collisionless shock: 2. A simulation study
Authors: Lee L. C. , Mandt M. E. , Wu C. S.
Published: February 2008

The present paper is a continuation of the preceding article (Lee et al., 1986) in which it is suggested that the nonadiabatic motion of the directly transmitted ions in a quasi‐perpendicular shock wave can result in an increase of the ion kinetic temperature transverse to the ambient magnetic field in the downstream. A series of computer simulations based on a hybrid code have been carried out to examine the dynamics of the transmitted ions in both the subcritical and supercritical shock waves. It is found that in both cases the directly transmitted ions can contribute to the heating process. In the case of a resistiveless supercritical shock the reflected and transmitted ions can be equally important; whereas for a subcritical shock the transmitted ions are primarily responsible for the ion heating.
The field‐aligned flow approximation for electrons within layers possessing a normal mass flux: A corollary to the deHoffmann‐Teller Theorem
Authors: Scudder J. D.
Published: February 2008

This paper establishes that the deHoffmann‐Teller frame electron bulk velocity VeHT(x) is nearly parallel to the magnetic field B(x) everywhere through those one‐dimensional, time stationary layers which possess a normal mass flux, even though electric fields perpendicular to the magnetic field exist throughout such layes. Examples of layers of this class are the fast and slow shocks as well as rotational discontinuities. The explicitly calculated corrections to are proportional to (1) the electron pressure anisotropy, (2) the electron inertia, and (3) the bulk resistivity. The near alignment of electron bulk velocity inside layers with a normal mass flux represents, for electrons only, a nearly complete extension of the 1950 deHoffmann‐Teller theorem concerning the exact alignment of electron and ion flows with the magnetic field in both asymptotic regions outside such layers. This alignment would be perfect within such layers if the electrons were approximated as a massless, dissipationless, isotropic gas. Without such approximations this alignment is better in more oblique shocks of either type; low electron β improves the approximation in the fast‐mode shock, and higher ion sound Mach numbers enhance the approximation for slow shock layers. This approximate symmetry provides a simple description of the causes of currents within the layers and an overview of the two‐fluid “mixing” which is caused by passage through them. It also permits a clarification of the coplanarity theorems often used in shock work. A clear comparison is also provided of the relative efficiencies of the electron pressure anisotropy, inertia, and resistivity in the slippage between the electron flow lines and the magnetic tubes of force known as “frozen flux violations.” The natural scale lengths for amplification and twists of the magnetic field in the fast and slow shocks are recovered, starting with the field‐aligned approximation. This calculation also correctly recovers the polarization of the fast‐ and slow‐mode shock layers, in addition to allowing a direct calculation of the initial scale length for such twists within the shock layer. Finally, in the regime of validity of the field‐aligned electron flow approximation it is demonstrated that testing with electric and magnetic field profiles for the existence of a deHoffmann‐Teller frame at magnetopause crossings is generally superior to ion bulk velocity‐magnetic field tests of tangential stress balance. Such tests can be performed whenever the scale lengths of the layer are much larger than the electron gyroradius, regardless of the size of the layer relative to the ion gyroradius.
Multiple frequency radar observations of high‐latitude E region Irregularities in the HF modified ionosphere
Authors: Noble S. T. , Djuth F. T. , Jost R. J. , Gordon W. E. , Hedberg Å. , Thidé B. , Derblom H. , Boström R. , Nielsen E. , Stubbe P. , Kopka H.
Published: February 2008

In September 1983, experiments were conducted in Scandinavia using the high‐power heating facility near Tromsø, Norway, The purpose of the HF ionospheric modification experiments was to investigate the behavior of artificially produced E region irregularities at auroral latitudes. The majority of observations were made with backscatter radars operating at 46.9 and 143.8 MHz, but limited observations were also made at 21.4 and 140.0 MHz. These radars are sensitive to irregularities having scale lengths of between 1 and 7 m across the geomagnetic field lines. The growth and decay of the irregularities are scale length dependent with the shorter lengths growing and dissipating more rapidly than the longer lengths (e‐folding growth times = 10¹–10² ms; decay times = 10²–10³ ms). During periods of full power ordinary mode heating, irregularities having peak cross sections of 104 m² at 46.9 MHz and 105 m² at 143.8 MHz are observed. However, the cross sections normally measured are 1 to 2 orders of magnitude smaller than the peak values. The cross sections are nonlinearly dependent on the HF power and begin to saturate at levels greater than 50–75 percent of full power. Past E and F region data from Arecibo are used in conjunction with the Tromsø measurements to ascertain the relative roles played by various mechanisms in exciting irregularities. In the E region, the results tend to favor those instability processes which operate at the upper hybrid resonance level (e.g., thermal parametric and resonance instabilities) over those that operate at the reflection level (e.g., parametric decay instability). However, it is likely that any of the mechanisms studied could at times contribute to irregularity production in the E region. At F region altitudes, the findings show support for the parametric decay, four‐wave thermal parametric, and resonance instabilities. The aspect sensitivity of the irregularities is measured using the 140.0 and 143.8 MHz radars. The results indicate a reduction in echo strength of 8–14 dB/deg for geometries that do not match those for specular reflection.
Effects of charged particles on the surfaces of the satellites of Uranus
Authors: Lanzerotti L. J. , Brown W. L. , Maclennan C. G. , Cheng A. F. , Krimigis S. M. , Johnson R. E.
Published: February 2008

Measurements of the ion and electron fluxes in the Uranian magnetosphere made by the low‐energy charged particle (LECP) instrument on the Voyager 2 spacecraft are used to discuss possible particle‐induced modifications of the moons and rings of Uranus. The energy spectra of the orbit‐integrated particle dosages expected on the major moons are derived from the LECP measurements of particle intensities and pitch angle distributions. Laboratory‐derived results on charged‐particle‐induced chemical and physical modifications of H2O, CH4, CO, and CO2 ions are used. The erosion rates of water ice and the darkening of organic ices on the surfaces of the moons are estimated from the orbit‐integrated fluxes. Significant darkening of fresh organic ices to depths of the order of a micron is expected to occur for times as short as a few thousand years. Darkening at deeper depths will occur at increasingly longer times. The implications of these results for the interpretation of remote sensing data from such objects are discussed.
The radiation belt origin of Uranus' Nightside radio emission
Authors: Curtis S. A. , Desch M. D. , Kaiser M. L.
Published: February 2008

From the location of the source field lines of the smooth nightside component of Uranus kilometric radiation (UKR) (Kaiser et al., this issue) the most likely free energy source is the outer radiation belts. As the terminator sweeps over the magnetic north polar region, precipitation of electrons produced by solar heating of the upper atmosphere and consequent submergence of the electron mirror points deeper in the atmosphere will create a backscattered electron distribution with an enhanced population at large pitch angles. This distribution will propagate to the conjugate magnetic south pole of Uranus, where the converging magnetic field lines will produce a mirroring electron distribution moving upward from the south pole, which will drive a cyclotron maser instability. The upper frequency limit to the UKR emissions corresponds closely to the electron gyrofrequency of the mirror point of the weaker northern pole and is consistent with a backscattered electron population originating from precipitation near the northern pole and hence mirroring at higher altitudes at the conjugate point in the southern magnetic hemisphere. The rise in emission intensity is shown to scale directly with the terminator's speed across the northern pole; the smooth emission result from sunrise over the northern pole and collision‐induced precipitation. Strong radio emissions are limited to the continually dark south pole, since only there is the electron plasma frequency to gyrofrequency ratio sufficiently small. The clocklike radio emission is shown to be a direct consequence of the terminator's control of the emission process. Hence solar‐driven upper atmospheric tides underlie the Uranian radio clock. Since the radiation belt population is most likely only weakly dependent on solar wind conditions, strong solar wind control of smooth UKR is not expected.
Whistler mode emissions in the Uranian radiation belts
Authors: Coroniti F. V. , Kurth W. S. , Scarf F. L. , Krimigis S. M. , Kennel C. F. , Gurnett D. A.
Published: February 2008

Voyager 2 detected intense whistler mode emissions and fluxes of energetic electrons during the outbound pass through the region of auroral L shells. The observed energetic (E > 22 keV) electron distribution, a model warm (E < 27.5 keV) electron distribution, and the cold plasma density profile deduced by Kurth et al. (this issue) are used to calculate the ray path‐integrated spatial amplification of whistlers which arrive at Voyager 2 from the magnetic equator. By matching the calculated amplification and the relative gains at different frequencies deduced from the observed whistler power spectrum, the pitch angle anisotropy parameters of the electron distributions are determined to within a fairly narrow range of values. The estimated bounce average pitch angle diffusion coefficient indicates that electrons are on strong diffusion over a wide range in energies. The electron precipitation energy flux is sufficient to produce the observed auroral light emissions.
The magnetotail of Uranus
Authors: Behannon K. W. , Lepping R. P. , Sittler E. C. , Ness N. F. , Mauk B. H. , Krimigis S. M. , McNutt R. L.
Published: February 2008

Voyager 2 observations have shown that Uranus possesses a well‐developed bipolar magnetotail similar in certain characteristics to that of Earth, in spite of an anomalously large tilt of the planetary magnetic dipole to the rotation axis at Uranus. The intensity of the magnetic field in the tail lobes decreases with increasing distance down the tail from the planet as |xSM|−0.59±0.03. This gradient is similar to that found in the Earth's tail but significantly less steep than that observed in the tails of Jupiter and Saturn. The thickness of the plasma sheet is a minimum (∼10 RU) near the tail center, increasing toward the flanks as at Earth. Pressure balance within the plasma sheet is maintained predominantly by protons and electrons with energies 10 eV to 6 keV. Except in transient events, the contribution of ≥28‐keV protons to pressure balance in the sheet is <5%. This is in contrast to the dominant role played by more energetic plasma ions in the Jovian magnetotail. An average value of β ∼ 7 was found in the plasma sheet at Uranus and ∼0.1 in the lobe plasma. The Uranian magnetic tail was observed to rotate 360° about its longitudinal axis, a result of the approximately sunward pointing planetary rotation axis at the time of encounter. This, together with the large tilt (60°) of the magnetic dipole, results in a small but measurable twist in the tail's magnetic lines of force, with a derived helical pitch of 5.5° ± 3.0°. The Byz(SM) component of the tail field can be modeled as the sum of the twist component, a radial (diverging or converging from the xSM axis) component, and a component parallel to the zSM axis that closes through the neutral sheet and is strongest there. The cross‐tail current density at the neutral sheet is estimated to be ≃3 × 10−11 A m−2. Large temporal variations observed in magnetic fields and plasmas during the Voyager 2 traverse of the magnetotail may have been produced by substorm activity.
Waves on the Uranian downstream magnetopause
Authors: Warwick James W. , Evans David R. , Dusenbery Paul B.
Published: February 2008

As Voyager passed outward through the magnetosphere of Uranus, the planetary radio astronomy (PRA) experiment recorded continuum radio emissions produced by sources near the Uranian nightside magnetic pole. Previously, we reported scintillations that occurred in this emission over a period of about 5 days as Voyager crossed the Uranian magnetosheath and that were observed by the magnetometer (MAG) and plasma (PLS) experiments. Early on January 29, 1986, the scintillations were extremely wideband, of order of magnitude 6∶1 in frequency, and deep, of order of magnitude 50% in power. We now show that these scintillations originated in propagating waves with wavelengths near 2 × 104 km, phase velocities near 575 km s−1, and minute plasma content. In effect, the kilometric emission caught and highlighted a diaphanous wave pattern drifting downwind along the magnetosheath between Voyager and the kilometric radio source regions near Uranus.
A model study of how electric field structures affect the polar cap F region
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

A three‐dimensional time‐dependent ionospheric model was used to study how electric field structures affect the polar F region. The electric field structures are represented by elongated Volland two‐cell models whose dimensions range from tens to 1000 km. These model structures are intended to represent the polar cap electric field for IMF Bz northward conditions. A statistical method is used to generate a set of these structures. Their electric field strength and polarity are varied in order to study the F region's dependence on this magnetospheric input. For electric field structures whose size and electric field strengths are consistent with Bz northward observations, several ionospheric dependencies were found. The NmF2 parameter, in general, decreased due to the presence of the higher electric fields associated with the structures. Decreases ranging from a few percent to a factor of 4 were obtained from the simulations. This decrease in NmF2 was frther complicated by the initial NmF2 conditions, i.e., past history of the flux tube, being quite different when the structure is present. HmF2 in the vicinity of a structure can be raised or lowered by up to 100 km, however, this change is not uniquely dependent upon the vertical induced drift. The role of enhanced reaction rates due to elevated ion temperatures is very important. The F region becomes spatially very structured. The modulation of this structure in NmF2 is up to an order of magnitude in this study.