bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of pressure pulses in the bow shock and magnetosheath driven by variations in interplanetary magnetic field direction
Authors: Lin Y. , Swift D. W. , Lee L. C.
Published: February 2004

Two‐dimensional (2‐D) hybrid simulations are carried out to study the effects of the variation in the interplanetary magnetic field (IMF) direction on the bow shock, magnetosheath, and magnetosphere. A curvilinear coordinate system is used in the simulation. The 2‐D simulation is also compared with our one‐dimensional simulation results. It is found that pressure pulses are generated as a result of the interaction between the bow shock (BS) and an interplanetary rotational discontinuity (RD). First, a structure consisting of a rotational discontinuity and two slow shocks are present downstream of the bow shock after the BS/RD interaction. The magnetic field and plasma density are anticorrelated in this structure. The dynamic pressure increases in the structure, leading to a pressure pulse in the magnetosheath. Second, a pressure pulse associated with reflected ions at the bow shock may be generated in the foreshock when the IMF changes its direction, especially when a local quasi‐parallel bow shock becomes a quasi‐perpendicular shock. The magnetic field, plasma density, and dynamic pressure are positively correlated in the upstream pressure pulse. This pressure pulse convects through and interacts with the bow shock, producing a pressure pulse in the downstream region. The downstream pressure pulses propagate to the magnetopause. The amplitude of the downstream pressure pulses can be up to 100% of the background magnetosheath value. It is suggested that the pressure pulses impinging on the magnetopause may lead to the magnetic impulse events observed in the high‐latitude ionosphere.
Plasma wave characteristics of the Jovian magnetopause boundary layer: Relationship to the Jovian aurora?
Authors: Tsurutani Bruce T. , Arballo John K. , Goldstein Bruce E. , Ho Christian M. , Lakhina Gurbax S. , Smith Edward J. , Cornilleau‐Wehrlin Nicole , Prangé Renée , Lin Naiguo , Kellogg Paul , Phillips John L. , Balogh Andre , Krupp Norbert , Kane Mark
Published: February 2004

The Jovian magnetopause boundary layer (BL) plasma wave spectra from 10−3 to 102 Hz have been measured for the first time. For one intense event the magnetic (B′) and electric (E′) spectra were 2 × 10−4 ƒ2.4 nT2/Hz and 4 × 10−9 ƒ2.4 V2/m2 Hz, respectively. Although no measurable wave amplitudes were detected above the electron gyrofrequency, ∼140 Hz, this finding may be due to the low signal strength characteristic of this region. The B′/E′ ratio is relatively frequency independent. It is possible that waves are obliquely propagating whistler mode waves. The B′ and E′ spectra are broadband with no obvious spectral peaks. The waves are sufficiently intense to cause cross‐field diffusion of magnetosheath plasma to create the BL itself. A Jovian BL thickness of 10,700 km is predicted, which is consistent with past in situ measurements. The Jovian boundary layer wave properties are quite similar to the BL waves at Earth (however, the Jovian waves are orders of magnitude less intense). It appears that the solar wind/magnetosphere dynamos at the two planets are similar enough to be consistent with a common wave generation mechanism. The predicted ionospheric latitudinal width of the BL of ∼100–200 km is quite similar to the Jovian auroral high‐latitude ring measured by Hubble. The location of the BL at and inside the foot point of the last closed field line may place the boundary layer and the aurora on approximately the same magnetic field lines. The Jovian BL waves are sufficiently intense to cause strong pitch angle diffusion for <5‐keV electrons and 1‐keV to 1‐MeV protons. The estimated energy precipitation rate from this interaction <1 erg cm−2 s−1, sufficient for a weak high‐latitude auroral ring. This intensity is 2 to 3 orders of magnitude too low to cause the main aurora ring, however. If it is found that this main aurora maps into the boundary layer, then other mechanisms such as (ionospheric) double layers must be responsible for the particle energization and precipitation.
Influence of negative ions on mesospheric turbulence traced by ionization: Implications for radar and in situ experiments
Authors: Hall C. M.
Published: February 2004

Turbulence in the middle atmosphere is often investigated by assuming that the ionized component of the air acts as a passive scalar tracer for the neutral gas. This is the case for rocket‐borne electrostatic probes in various configurations and for radar. It has long been accepted that the ions are not necessarily the faithful tracers one would like. Here we shall address the influence of the presence of negative ions on the turbulence spectra observed when positive ions represent the tracer. We shall see that departure of the spectral form from that expected for the neutral turbulence alone has considerable repercussions for observations, either in situ or remote, in the lower part of the mesosphere, and particularly in nighttime conditions when negative ion densities are greatest.
High‐latitude ground‐based response to sudden changes in solar wind dynamic pressure
Authors: Sitar R. J. , Clauer C. R. , Friis‐Christensen E.
Published: February 2004

Sudden changes in the solar wind dynamic pressure have been linked, both theoretically and observationally, to the formation of a number of transient ionospheric phenomena. Still, the precise role that these pressure changes play in the formation of these phenomena remains uncertain. By using a combination of ground‐based magnetic measurements from arrays of magnetometers located in Greenland and IMP 8 satellite measurements of the solar wind velocity and density, we have been able to better study the effect that pressure changes have on the high‐latitude ionosphere. Our study is based on approximately 2500 hours of solar wind plasma data collected during 1991 and 1992 by the IMP 8 satellite and focuses on step changes of |Δp | > 2 nPa occurring on a timescale of Δt < 15 min. We have found that the ground response does not consistently conform to existing theoretical models of field‐aligned currents generated by changes in dynamic pressure. We also do not find any explicit dependence on interplanetary magnetic field orientation. These results lead us to believe that the nature of the ground response is due to a more complicated combination of external controls.
On the self‐consistent description of dynamic magnetosphere‐ionosphere coupling phenomena with resolved ionosphere
Authors: Dreher Jürgen
Published: February 2004

A set of model equations is presented that allows for the self‐consistent description of dynamic magnetosphere‐ionosphere coupling phenomena with finite ionosphere. The model is very similar to magnetohydrodynamics (MHD), but the plasma‐neutral gas interaction in the lower ionosphere is taken into account by a frictional force between ions and neutrals and an ionization‐recombination term in the plasma transport equation. Further, the Hall term and the electron pressure term are retained in Ohm's law. It is shown that for the collision‐dominated E layer, the familiar Pedersen and Hall conductivities can be derived directly from the basic equations. The model is used to numerically simulate the dynamic formation of a magnetospheric‐ionospheric current system as the response to prescribed localized magnetospheric convection. Apart from a pair of Birkeland current sheets that are closed by Pedersen currents, associated Hall currents are generated. In addition, density irregularities in the E layer form as a direct consequence of the current closure. They are, however, not related to electron precipitation. For small length scales (≈ 10 km), these density perturbations result in considerably enhanced conductivities below the upward Birkeland currents which in turn lead to a spatial concentration of the latter compared to the downward currents. The timescale for the relaxation of the current system and the ionospheric convection toward a stationary state after the onset of magnetospheric convection is longer than the Alfvén travel time and depends on the height‐integrated Pedersen conductivity. This is in good agreement with earlier theoretical predictions [Southwood and Kivelson, 1991].
Artificial airglow due to modifications of the ionosphere by powerful radio waves
Authors: Gurevich A. V. , Milikh G. M.
Published: February 2004

A new model of the artificial airglow due to the ionospheric modification is proposed. The model is based on a recently developed nonlinear theory of stationary striations, which predicts a strong enhancement of the electron temperature inside the striations. According to our model the neutral species can be excited by the hot electrons inside the striations, which spread both upward and downward from the source region located between the radio wave reflection and the upper hybrid resonance point. The discussed model allows us to estimate the local electron temperature inside the striations, and to assess the intensity of red‐ and green‐line emissions due to the excitation of O(1D) and O(1S) levels by the electron impact. These results are checked by comparison with the observations.
Potential structure near a probe in a flowing magnetoplasma and current collection
Authors: Singh Nagendra , Leung W. C. , Vashi B. I.
Published: February 2004

The distributions of plasma and potential near an electric probe in a relative motion with respect to a magnetized plasma are studied by means of three‐dimensional (3‐D) numerical simulations. The relative motion is simulated by a plasma flowing past the probe across the ambient magnetic field. The plasma flow is imposed by a convection electric field
. A probe with a positive potential bias is considered. The prominent features of the potential distribution include (1) wings of positive potential perturbations extending along the magnetic field and swept forward in the direction of the plasma flow and (2) a “fan” shaped structure in planes transverse to the magnetic field in the region where the convection electric field
is opposed by space charge electric fields. The wing‐like structure can be interpreted in terms of electrostatic plasma waves belonging to the oblique resonance cone in a magnetized plasma. The relative flow causes the formation of a “bow shock” in front of the probe, where plasma density is enhanced due to the combined effect of the retardation of the flowing ions and the modification in the
drift of the electrons in the sheath of a positive probe. The electron collection by the probe is significantly enhanced above the theoretical upper bound current obtained from the conservation of energy and the canonical angular momentum for the case without the relative motion. The current in the plasma, contributing to the collection of electrons by the probe, flows in a magnetic field‐aligned channel in the vicinity of the probe where electric fields parallel to the magneitc field are relativley strong. Electron flux is fed into the channel all along its length by the
drift in the self‐consistent electric field, considerably enhancing the current collected by the probe. The field‐aligned current channel is localized near the probe where parallel electric fields dominate; it does not extend to infinity along the probe's magnetic shadow, unlike that for the case of a nonflowing plasma.
Ionospheric F region currents at middle and low latitudes estimated from Magsat data
Authors: Olsen Nils
Published: February 2004

Two methods for determining the F region current density at satellite heights by means of a decomposition of the magnetic field into toroidal and poloidal parts are presented. The first method allows for determination of the radial current component only by neglecting the height variation of the satellite orbit. Application to Magsat data yields the well‐known field‐aligned currents of polar latitudes (average current density Jr ≲ 200 nA/m2) and the meridional current system of the equatorial electrojet (Jr ≈ 10 – 20 nA/m2) during dusk. In addition, a midlatitude interhemispheric current system could be observed for the first time. Current direction is from the winter to the summer hemisphere during dusk and in opposite direction during dawn, as predicted by three‐dimensional models of the ionospheric dynamo. Presumably, for the first time a lunar contribution to the meridional current system of the equatorial electrojet was found, too. Amplitude at the dip equator is Jr ≈ 1.8 nA/m2; time of maximum is in agreement with magnetic ground observations, ionospheric electric field measurements, and ionospheric dynamo theory. The second method allows for estimation of both the radial and the horizontal current density by expanding the radial dependence of the magnetic field in Taylor series. First results confirm the current direction of the low‐latitude meridional current system as suggested by dynamo calculations: upward currents at the dip equator and field‐aligned downward currents at low latitudes.
Solar activity variations in the composition of the low‐latitude topside ionosphere
Authors: West K. H. , Heelis R. A. , Rich F. J.
Published: February 2004

Ion composition data from the retarding potential analyzer onboard the Defense Meteorological Satellite Program (DMSP) F10 have been analyzed for the months of June and September for the years 1991–1994 when the solar F10.7 flux changed from near 200 to less than 100. Low‐latitude composition data have been averaged by dip latitude and geographic longitude for morning and evening passes to investigate variations attributable to solar variability. In 1991 the dominant ion is O+ in both the morning and the evening, but by 1994, O+ is the dominant ion only at certain locations in the morning. The O+ −H+ transition height is well above the DMSP altitude (800 km) in 1991, but the transition height is near or below 800 km in 1994. Neutral wind induced longitude variations in the topside are present under all levels of solar activity, but the differing role of interhemispheric plasma transport at different solar activity levels dramatically changes the latitude distributions resulting from the winds. At high solar activity the O+ −H+ transition height is well above 800 km, and interhemispheric transport of O+ in the flux tubes connecting the northern and southern hemispheric topside reduces the latitude asymmetry in O+ while producing the minimum observable H+ asymmetries at night. At lower solar activity levels, H+ dominates the topside ionosphere above 800 km, and larger latitudinal asymmetries in the O+ concentration are observed, while the H+ latitude distribution remains quite symmetric at all local times.
In situ ionospheric observations of severe weather‐related gravity waves and associated small‐scale plasma structure
Authors: Kelley Michael C.
Published: February 2004

On July 27, 1988, two sounding rockets were launched over a small thunderstorm cell which constituted the remnants of a large frontal event which had lasted for several hours over the eastern seaboard. One of the rockets was instrumented for detection of the electromagnetic impulse from lightning strikes and its subsequent interaction with the ionospheric plasma [Kelley et al., 1990]. The second had on board an absolute electron density probe, the results from which we report here. We present evidence that a gravity wave was spawned by the front and propagated nearly to the F peak in the ionosphere, where it steepened and created structure in the medium at scales much less than the vertical wavenumber of the major disturbance. The fluctuation spectrum along the rocket path was elevated for scales from 25 km down to less than 10 m. At scales between 10 km and just under 100 m, characterization of the spectrum by a power law yields a spectral index less than that displayed by such well‐studied processes as bottomside spread F and barium cloud striations. Similar results have been reported for gravity wave induced intermediate scale structures at midlatitudes [Wernik et al., 1986]. The mixing theory described by Fridman [1990] may be relevant to these observations.
Response of the auroral electrojets to the solar wind modeled with neural networks
Authors: Gleisner H. , Lundstedt H.
Published: February 2004

The dissipative processes in the Earth's magnetosphere, such as the ring current and the auroral electrojets, depend on both the external solar wind forcing and factors internal to the magnetosphere. Previous studies have shown that artificial neural networks are able to compute the ring current index Dst very accurately from only solar wind data. In this study, we use neural networks to model the response of the auroral electrojets to the solar wind conditions. The solar wind input to the networks consist of 5‐min averaged data from the Earth‐orbiting spacecraft IMP 8, while the output is the auroral electrojet index AE. The relationships between the solar wind and the AE index, as modeled by the neural networks, are investigated in a parameter study. The relative importance of individual solar wind variables is studied, as well as the abilities of various coupling functions. It is shown that the use of individual solar wind variables as input to a neural network is superior to the use of corresponding coupling functions. The nonlinear neural networks are related to earlier linear techniques, and the abilities of linear networks (linear filters) are compared to those of nonlinear networks. It is found that a nonlinear network with n, V, By, and Bz as input during 100 min can account for 76% of the variance (r≈0.87) in the AE index. No influence of Bx is found. With the coupling function p1/2V2Bs as input to a nonlinear network, 71% of the AE index variance is predicted. These results are averaged over a large test set (∼ 330 hours) of data not used to train the networks. The test data are from 1973–1974 and include a diverse set of conditions, ranging from almost quiet to exceptionally disturbed.
Structures in ionospheric number density and velocity associated with polar cap ionization patches
Authors: Kivanç Ö. , Heelis R. A.
Published: February 2004

Spectral characteristics of polar cap F region irregularities on large density gradients associated with polar ionization patches are studied using in situ measurements made by the Dynamics Explorer 2 (DE 2) spacecraft. The 18 patches studied in this paper were identified by the algorithm introduced by Coley and Heelis [1995], and they were encountered during midnight‐noon passes of the spacecraft. Density and velocity spectra associated with these antisunward convecting patches are analyzed in detail. Observations indicate the presence of structure on most patches regardless of the distance between the patch and the cusp where they are believed to develop. Existence of structure on both leading and trailing edges is established when such edges exist. Results, which show no large dependence of ΔN/N power on the sign of the edge gradient ∇N, do not allow the identification of leading and trailing edges of the patch. The ΔN/N is an increasing function of ∇N regardless of the sign of the gradient. The correlation between ΔN/N and ΔV is generally poor, but for a given intensity in ΔV, ΔN/N maximizes in regions of large gradients in N. There is evidence for the presence of unstructured patches that seem to coexist with unstructured horizontal velocities. Slightly smaller spectral indices for trailing edges support the presence of the E × B drift instability. Although this instability is found to be operating in some cases, results suggest that stirring may be a significant contributor to kilometer‐size structures in the polar cap.
Characteristics of high‐latitude TIDs from different causative mechanisms deduced by theoretical modeling
Authors: Kirchengast Gottfried
Published: February 2004

The high‐latitude ionosphere is a physically complex regime where traveling ionospheric disturbance signatures (TIDs) are caused not only by atmospheric gravity waves (AGWs) but also by other processes. Among these, wavelike variations in E × B drift strength, particle precipitation intensity, and ionospheric return current density are the most relevant ones; they can produce TIDs similar to AGW/TIDs (typical periods around 1 hour), frequently seen, for example, in incoherent scatter (IS) data. This study is concerned with elucidating the characteristics of TIDs from these different mechanisms in the high‐latitude F region, aiming at three major aspects: (1) the causative mechanism‐TID relationship, (2) the interpretation of TIDs in ionospheric data, especially IS data, in terms of causative mechanisms, and (3) the reliable recognition of AGW‐induced TIDs, which are of special interest. To study these aspects tailored to IS data exploitation, theoretical simulations of TIDs were performed for representative cases of the causative mechanisms, utilizing a realistic ionospheric model (“Graz Ionospheric Flux Tube Simulation (GIFTS) model”) which self‐consistently yields the fundamental TID quantities, electron density, ion drift, ion and electron temperature, and further mechanism‐dependent quantities. The physical understanding of the non‐AGW/TIDs could be significantly improved. A general finding was that because of nonlinear processes (e.g., frictional heating) and its coupling to the thermosphere, the ionosphere is capable of generating complicated TID waveforms (e.g., a dominant second harmonic) even for a monochromatic source mechanism. TID information derivable from IS data was found sufficiently unique for each mechanism to allow its identification. In particular, the unique features of AGW/TIDs allow their reliable recognition among other TIDs. The results provide a good basis for future automation of the recognition and quantification of causative mechanisms based on IS data.
Simulations of the F region during the January 1993 10‐day campaign
Authors: Fesen C. G. , Emery B. A. , Buonsanto M. J. , Zhou Q. H. , Sulzer M. P.
Published: February 2004

The 10‐day World Day campaign during January 20–30, 1993, provided an opportunity to test the current capability of the National Center for Atmospheric Research general circulation models and to conduct simple numerical experiments to investigate possible causes of day‐to‐day variability. Detailed data sets from the Arecibo and Millstone Hill incoherent scatter radars provided information on the middle‐ and low‐latitude thermosphere and ionosphere during low solar activity which can be compared with model predictions. The theoretical model was used to examine the impact of varying two of the model inputs: the high‐latitude energy and momentum sources and the semidiurnal tidal waves from the lower atmosphere. These exercises indicated that varying the high latitude inputs affect the simulations even to relatively low latitudes. The neutral winds in the models were responsive to the level of auroral activity and also to the magnitude of the waves from the lower atmosphere, particularly the neutral zonal winds. The simulated hmax were only affected at night by varying the model inputs. Use of the assimilative mapping of ionospheric electrodynamics (AMIE) technique is necessary to produce realistic quiet‐time zonal ion drifts at low latitudes following local sunset. The ion and neutral temperatures proved nearly insensitive to the specifications of the auroral or the tidal inputs, particularly the temperatures at Arecibo. This is in contrast to the observations in which temperatures may vary by up to 100 K from day to day with more pronounced variability at night. In the models, only a large geomagnetic disturbance produced a perturbation in the temperatures but with magnitudes significantly smaller than those observed. The discrepancies may indicate an underestimate of the high‐latitude Joule heating due to small‐scale variability in magnetospheric electric fields, which would affect the neutral circulation and composition, and inadequate representation of the F region dynamo and conjugate effects in the models.
Photoelectron effects on the self‐consistent potential in the collisionless polar wind
Authors: Khazanov G. V. , Liemohn M. W. , Moore T. E.
Published: February 2004

The presence of unthermalized photoelectrons in the sunlit polar cap leads to an enhanced ambipolar potential drop and enhanced upward ion acceleration. Observations in the topside ionosphere have led to the conclusion that large‐scale electrostatic potential drops exist above the spacecraft along polar magnetic field lines connected to regions of photoelectron production. A kinetic approach is used for the O+, H+, and photoelectron (p) distributions, while a fluid approach is used to describe the thermal electrons (e) and self‐consistent electric field (E‖). Thermal electrons are allowed to carry a flux that compensates for photoelectron escape, a critical assumption. Collisional processes are excluded, leading to easier escape of polar wind particles and therefore to the formation of the largest potential drop consistent with this general approach. We compute the steady state electric field enhancement and net potential drop expected in the polar wind due to the presence of photoelectrons as a function of the fractional photoelectron content and the thermal plasma characteristics. For a set of low‐altitude boundary conditions typical of the polar wind ionosphere, including 0.1% photoelectron content, we found a potential drop from 500 km to 5 RE of 6.5 V and a maximum thermal electron temperature of 8800 K. The reasonable agreement of our results with the observed polar wind suggests that the assumptions of this approach are valid.
Incoherent scatter radar and Digisonde observations at tropical latitudes, including conjugate point studies
Authors: Scali J. L. , Reinisch B. W. , Kelley M. C. , Miller C. A. , Swartz W. E. , Zhou Q. H. , Radicella S.
Published: February 2004

In this paper we compare ionospheric measurements made during the January 1993 10‐day World Day Campaign with three separate instruments: the Arecibo incoherent scatter radar, the Ramey Digisonde (located 50 km west of Arecibo), and the Puerto Madryn, Argentina, Digisonde (located 400 km from the magnetic conjugate point of Arecibo). F layer peak heights from Ramey agree well with those from Arecibo throughout the entire 10‐day period, with a median difference of 4 km, and along with the F peak density show effects of a 4‐hour periodic temporal variation. We relate this 4‐hour variation to the familiar midday biteout in the electron density that is commonly observed at Arecibo in the winter months. However, this periodic temporal variation is not observed at Puerto Madryn, which is located at a higher geographic latitude. Only occasionally do the Arecibo incoherent scatter radar and the Ramey Digisonde velocity measurements agree. Of course, it must be remembered that while the Doppler shift in an incoherent scatter radar echo is a direct measure of the line‐of‐sight plasma velocity component, the Doppler shift of a Digisonde echo (or any ionosonde echo for that matter) is a measure of the time rate of change of the electrical distance to the reflection point. Differences between the velocity components obtained by the two techniques are indications of the importance of chemistry and divergent transport instead of simple motions. Over the ten‐day campaign and in the average of the 10 days, the Arecibo and Ramey horizontal velocities show good agreement only for a short period at night from 0000 to 0700 LT (0400 to 1100 UT). During this time the HF radio waves were reflected from relatively large zenith angles and the ionosphere was fairly high. An increased westward component of the horizontal velocity measured at Arecibo and Ramey around 0330 LT (0730 UT), is correlated with sunrise in the summer conjugate hemisphere, but the drift velocity was smaller by about a factor of 2 at the southern station. Since we doubt that the field lines are not equipotentials, the difference observed could be due to Puerto Madryn not being located at the exact conjugate point for Arecibo and Ramey.
Comparison of incoherent scatter radar and Digisonde measurements with field line interhemispheric plasma modeled results at middle and low latitudes
Authors: Scali J. L. , Reinisch B. W. , Richards P. G. , Zhou Qihou , Sulzer Mike , Swartz W. E.
Published: February 2004

Comparisons of modeled and measured ionospheric characteristics are made for data recorded during the January 1993 10‐day world day campaign. The peak of the F region “hmF2” measured by Digisondes at Ramey (located 50 km west of Arecibo) and at Puerto Madyrn, which is located close to the conjugate point, are used to drive the field line interhemispheric plasma (FLIP) model. In the summer hemisphere for times when the geomagnetic conditions are quiet, the modeled and measured NmF2 are in good agreement, displaying median differences of approximately 13%. In the winter hemisphere, good agreements in NmF2 are observed during midday and evening hours. However, FLIP underestimates NmF2 values at night and in the postsunrise sector. Comparison of the Arecibo ISR and FLIP parallel velocities also show good agreement during daytime, while large differences are observed at night. The discrepancies in the Arecibo/Ramey measurements and FLIP modeled results, at night, is related to the effect of a 4‐hour periodic variation in hmF2 and NmF2. Further analysis identifies, for the first time, this 4‐hour variation and discusses its characteristics as being consistent with the effect of a 4‐hour tide. The signature of the 4‐hour periodic temporal variation in the height of the ionosphere is only clearly observable at F region heights during the winter months and at low solar activity. The 4‐hour periodic temporal variation is only observed at the tropical latitude stations Arecibo/Ramey and is not observed at the midlatitude station Puerto Madryn. A 4‐hour tidal component is not present in the MSIS86 model which FLIP uses for its neutral concentration. Therefore, when tidal forcing is moving the ions and neutrals to lower altitudes, FLIP artificially calculates large recombination rates. The loss in ionization increases and to maintain the nighttime ionosphere the downward flow of ionization also increases.
Sprites produced by quasi‐electrostatic heating and ionization in the lower ionosphere
Authors: Pasko V. P. , Inan U. S. , Bell T. F. , Taranenko Y. N.
Published: February 2004

Quasi‐electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes lead to ambient electron heating (up to ∼5 eV average energy), ionization of neutrals, and excitation of optical emissions in the mesosphere/lower ionosphere. Model calculations predict the possibility of significant (several orders of magnitude) modification of the lower ionospheric conductivity in the form of depletions of electron density due to dissociative attachment to O2 molecules and/or in the form of enhancements of electron density due to breakdown ionization. Results indicate that the optical emission intensities of the 1st positive band of N2 corresponding to fast (∼ 1 ms) removal of 100–300 C of thundercloud charge from 10 km altitude are in good agreement with observations of the upper part (“head” and “hair” [Sentman et al., 1995]) of the sprites. The typical region of brightest optical emission has horizontal and vertical dimensions ∼10 km, centered at altitudes 70 km and is interpreted as the head of the sprite. The model also shows the formation of low intensity glow (“hair”) above this region due to the excitation of optical emissions at altitudes ∼ 85 km during ∼ 500 μs at the initial stage of the lightning discharge. Comparison of the optical emission intensities of the 1st and 2nd positive bands of N2, Meinel and 1st negative bands of
, and 1st negative band of
demonstrates that the 1st positive band of N2 is the dominating optical emission in the altitude range around ∼70 km, which accounts for the observed red color of sprites, in excellent agreement with recent spectroscopic observations of sprites. Results indicate that the optical emission levels are predominantly defined by the lightning discharge duration and the conductivity properties of the atmosphere/lower ionosphere (i.e., relaxation time of electric field in the conducting medium). The model demonstrates that for low ambient conductivities the lightning discharge duration can be significantly extended with no loss in production of optical emissions. The peak intensity of optical emissions is determined primarily by the value of the removed thundercloud charge and its altitude. The preexisting inhomogeneities in the mesospheric conductivity and the neutral density may contribute to the formation of a vertically striated fine structure of sprites and explain why sprites often repeatedly occur in the same place in the sky as well as their clustering. Comparison of the model results for different types of lightning discharges indicates that positive cloud to ground discharges lead to the largest electric fields and optical emissions at ionospheric altitudes since they are associated with the removal of larger amounts of charge from higher altitudes.
Radiation near 2ƒp and intensified emissions near ƒp in the dayside and nightside auroral region and polar cap
Authors: Cairns Iver H. , Menietti J. D.
Published: February 2004

This paper reports the discovery in the DE 1 data of propagating radiation near 2ƒp (the H component) and relatively intense electromagnetic waves near ƒp with fields typically ≲ 1 mV m−1 (the PF or plasma frequency component) on both the dayside and the nightside of Earth. These emissions are observed at auroral and polar cap latitudes for radial distances ranging from 2.5 to 4.5 RE. The H component is unique in that no other 2ƒp emissions are known to be generated where the electron gyrofrequency ƒg exceeds 2ƒp. Since existing theories for 2ƒp radiation assume ƒg/ƒp < 1, new theories will be required to explain the H component. The PF waves near ƒp are electromagnetic, but with large ratios E/cB ∼ 20. On the basis of cold plasma theory, the wave frequencies and the ratios E/cB, the PF component plausibly consists of z‐mode and/or whistler mode waves near ƒp, presumably driven by an electron instability. The H emissions have modest bandwidths of ∼ 50% at frequencies ranging from 5 to 20 kHz. Grounds for interpreting the H component as emissions generated near 2ƒp are provided by the very good frequency tracking of the PF and H components and typical frequency ratios near 2.0. Strong evidence exists that part of the H component is propagating, electromagnetic radiation, based on propagation effects and spin modulation patterns. However, no magnetic signals have yet been detected for the H component, so that it could be partly electrostatic. Cold plasma theory and the observed wave characteristics favor interpreting the H component as composed of o mode and/or z mode signals. Combining the DE 1 observations with rocket observations, it is predicted that the much more intense Langmuir‐like fields observed in the low altitude auroral zones should also generate observable 2ƒp radiation. This prediction should be testable using Polar and future rocket flights. Directions for future research are also described.
A hemispherical model of anisotropic interstellar pickup ions
Authors: Isenberg Philip A.
Published: February 2004

We present an analytical model for the distribution function of interstellar pickup ions in a radially directed heliospheric magnetic field which naturally results in anisotropic particle distributions. The model includes the effects of convection and spatial transport in the solar wind, adiabatic deceleration in the radial flow, adiabatic focusing in the radial field, and pitch angle scattering toward isotropy in the frame of the solar wind. The pitch angle scattering is approximated by the hemispherical assumption: we take the scattering to be very efficient within each pitch angle range μ ≷ 0 (where μ is the cosine of the pitch angle) but inhibited between the hemispheres separated by μ = 0. The analytical solution is obtained for the case where the scattering rate across μ = 0 scales as the particle speed divided by the radial position of the fluid parcel. The model distribution functions can be used to interpret recent observations of anisotropic pickup ions.
A comparative study of cosmic ray radial and latitudinal gradients in the inner and outer heliosphere
Authors: McDonald F. B. , Ferrando P. , Heber B. , Kunow H. , McGuire R. , Müller‐Mellin R. , Paizis C. , Raviart A. , Wibberenz G.
Published: February 2004

The radial and latitudinal intensity gradients of 145–255 MeV/nucleon He, 34–50 MeV/nucleon He and 30–69 MeV H are studied over an extensive range of heliocentric distances and latitudes for the 1993.0–1996.0 time period using data from cosmic ray experiments on the Ulysses, IMP 8, Voyager 1 and 2, and Pioneer 10 spacecraft. The radial gradients are found to decrease rapidly with increasing heliocentric distance and agree with those measured 20 years earlier at a similar phase of the heliomagnetic cycle. The latitudinal gradients measured in the inner and outer heliosphere are in reasonable agreement and positive albeit exceedingly small. In agreement with other Ulysses energetic particle experiments it is found that a shift of heliolatitude by −7° to −10° is necessary to get reasonable symmetry in the measurements at midlatitudes. From the Ulysses data it appears there is a significantly reduced latitudinal variation in the intensity of the three energetic particle components at (magnetic) heliolatitudes above about 50° at this phase of the modulation cycle. Such a reduced entry of cosmic rays over such an extensive area above the solar poles implies a strong modification of the previously assumed cosmic ray transport processes at high latitudes, most probably a considerably increased rate of scattering combined with reduced particle gradient and curvature drifts. A significant higher intensity is observed over the north solar pole than over the south pole for the low‐energy components after the corrections have been applied for the temporal changes at the 1‐AU baseline.
Model study of ground magnetic signatures of traveling convection vortices
Authors: Zhu L. , Gifford P. , Sojka J. J. , Schunk R. W.
Published: February 2004

We conducted a model study of ground magnetic signatures of traveling convection vortices (TCVs) that included both the ionospheric conductivity enhancement associated with the TCVs and the ground induction effect. We found that the localized conductivity enhancement can cause a significant distortion of the TCV current system and lead to a distortion of the ground magnetic disturbance patterns. The patterns of all three magnetic components are asymmetric, mainly in the E‐W direction, and the patterns of the Z component show the strongest asymmetry (20–30%). We also found that the effect of induction currents on ground magnetic signatures of the TCVs is insignificant (less than 5%). The results show that because of the presence of localized conductivity enhancements the polarity and speed of the TCVs can significantly influence the distortion features of ground magnetic patterns. The upward and downward current filaments of a TCV with a clockwise leading convection cell can wrap with each other, resulting in a rotation of the whole ground magnetic disturbance pattern. This rotation feature is most significant when the speed of the TCVs is high.
Kinetic model of the ring current‐atmosphere interactions
Authors: Jordanova V. K. , Kozyra J. U. , Nagy A. F. , Khazanov G. V.
Published: February 2004

Our numerical model of the ring current‐atmosphere coupling (RAM) is further developed in order to include wave‐particle interaction processes. The model calculates the time evolution of the phase space distribution function in the region from 2 RE to 6.5 RE, considering losses due to charge exchange, Coulomb collisions, and plasma wave scattering along ion drift paths. The spatial regions of ion cyclotron wave instability are determined by calculating the convective growth rates for electromagnetic ion cyclotron (EMIC) waves, integrating them along wave paths, and selecting regions of maximum wave amplification. The source regions are located on the duskside in agreement with the predominant occurrence of EMIC waves. A spectral power density of 1 nT2/Hz is adopted within the unstable regions. According to quasi‐linear theory, the fluctuating fields are regarded as imposed on the system, and the losses due to wave‐particle interactions are described with diffusive processes. The effects of the presence of heavy ion components on the quasi‐linear diffusion coefficients are also considered. Resonance with ion cyclotron waves reduce the anisotropy of the proton population and the unstable regions disappear with time. Global patterns of precipitating ion fluxes are obtained and compared with observations.
Accuracy of using 6300 Å auroral emission to identify the magnetic separatrix on the nightside of Earth
Authors: Blanchard G. T. , Lyons L. R. , Samson J. C.
Published: February 2004

Ground observations of 6300 Å auroral emission at the polar cap boundary are studied to determine the accuracy with which the latitudinal profile of emission intensity can be used to identify the separatrix. Meridian scanning photometers at Rankin Inlet and Gillam provide the observations of 6300 Å emission, and the separatrix determinations obtained from the photometer data are compared with those determined from DMSP F9 precipitating particle data obtained within 1.5 hours in MLT of the ground stations. We assume that the separatrix lies at the poleward edge of boundary plasma sheet precipitation. We find that the average intensity of 6300 Å in the polar cap is fairly uniform at 60 R. In the auroral zone the average emission is fairly uniform at 170 R. On the basis of the efficiency and accuracy of separatrix identification, choosing a threshold of 110 R provides the best identification of the polar cap boundary. The rms error in this identification is 1.2°, and the boundary is located in 54% of the cases. The latitudinal gradient of the emission intensity is also investigated as a possible identifier of the polar cap boundary. However, using the intensity gradient to identify the polar cap boundary is less accurate, with a minimum uncertainty of 2.4°. Finally, fitting the measurements of the 6300 Å emission to a latitudinal step function, which represents an idealized emission profile, reduces the rms error in the identification of the separatrix to 1.0° while still identifying the boundary in 54% of the cases.
Bow shock analysis at comets Halley and Grigg‐Skjellerup
Authors: Coates A. J. , Mazelle C. , Neubauer F. M.
Published: February 2004

The interaction of an active comet with the solar wind is effected by the pickup of heavy cometary ions over extended regions of space. It was predicted before any spacecraft encounters with comets that a bow shock would form at a critical point in the mass‐loaded flow. Here we present an analysis of the results from Giotto's encounters of comets Halley and Grigg‐Skjellerup on the plasma structure of the bow shock regions at the two comets. This is achieved in a joint analysis of data on solar wind and cometary ions, magnetic field, and electrons. We find that these mass‐loading shocks are among the most complex in the solar system with heavy cometary ions dominating the component pressures. Results of Mach number calculations at Halley support earlier interpretations that bow shocks were, indeed, seen with possible solar wind disturbance inbound. At Grigg‐Skjellerup, Mach number results support the interpretation of a “wave” inbound and a “shock” outbound; both features also occur near to the expected location for a bow shock. A weak correlation between magnetic field strength and electron density is noted at the Grigg‐Skjellerup outbound shock in agreement with quasi‐perpendicular hybrid simulations.
Classification and mean behavior of magnetic storms
Authors: Loewe C. A. , Prölss G. W.
Published: February 2004

The Dst index is used to identify more than 1000 storms which occurred in the time interval 1957 to 1993. Using the minimum Dst value as an indicator, we classify the storms as weak (482), moderate (346), strong (206), severe (45), and great (6). For each of these classes the mean time variation is determined. In contrast to the well‐known study of Sugiura and Chapman [1960], the Dst minimum is used as a common reference epoch. This leads to much better agreement between the average and the typical storm behavior. We also find that the maximum ap and AE activity precedes the Dst minimum by 1 to 2 hours. Finally, we demonstrate that both sudden commencement and gradual commencement storms are associated with a distinct decrease in the Bz component of the interplanetary magnetic field.
Melting of mantle peridotite at pressures approaching the spinel to garnet transition: Application to mid‐ocean ridge basalt petrogenesis
Authors: Kinzler Rosamond J.
Published: February 2004

This paper reports experiments carried out between 1.5 and 2.3 GPa using synthetic analogs of the molten peridotite system to investigate the effects of pressure (P), temperature (7), and variable bulk chemistry on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and spinel (garnet). The multidimensional surface defined by melts coexisting with the Iherzolite mineral assemblage produced in this study as well as from the literature is fit as a function of the variables: P, melt Mg number, and melt mole percent NaO0.5, KO0.5, Cr2O3, and TiO2, using multiple regression techniques. Forward models of polybaric, near‐fractional melting using the parameterization presented here demonstrate that small extent, pooled magmas produced at greater initial pressures of melting differ in composition from small extent, pooled magmas produced at lower initial pressures of melting. Furthermore, the small extent, pooled magmas generated at shallower initial pressures of melting fractionate at low P to magmas that are similar to the high‐Na2O, low‐FeO mid‐ocean ridge basalts of the global array of Klein and Langmuir [1987], while the small extent, pooled magmas generated at greater initial pressures of melting fractionate at low P to magmas that do not resemble any mid‐ocean ridge basalts present in the global data set. Preliminary data relevant for melting garnet‐lherzolite indicate that the systematics of the key major element indicators of depth of melting (FeO and SiO2 in the melt) observed in melts generated in the spinel stability field persist in melts generated in the lower P portion of the garnet stability field.