bluuebunny/crossref_metadata_2025 huggingface dataset:
Intense 2.3‐Hz electric field pulsations in the stratosphere at high auroral latitude
Authors: Bering E. A. , Benbrook J. R.
Published: February 2004

A survey has been performed of the power spectra of the electric field measured by stratospheric balloon‐borne instruments during the 1985–1986 South Pole Balloon Campaign. The survey reviewed all 468 hours of available data and covered the frequency range from 0 to 4 Hz. Three episodes of narrow‐band emissions were detected in this frequency range. These emissions occurred about 5.5% of the time, with intense emission occurring about 1% of the time. These narrow‐band emissions had frequencies in the horizontal component between 2 and 2.5 Hz. The frequencies observed in the vertical component were different, with several bands present. The fundamental frequency in the vertical component was around 0.8 to 1.4 Hz depending on event. The spectra of the events in the vertical component showed multiple emission bands, not all harmonically spaced. The vertical component showed much more pronounced time variations of the frequencies of the emission bands than the horizontal component showed. The amplitude of the emission in the vertical component was usually but not always larger than that in the horizontal components. The ratio EV/EH was typically ∼2–3. The amplitude of the horizontal component of the emissions was quite large in one of the three events, with individual bursts having amplitudes of 60 mV/m. The polarization was complicated. In the horizontal plane, there were two narrow bands of opposite‐handed circularly polarized emissions. Since the vertical axis of the emission had frequency components not present in the horizontal plane, the polarization in the zonal plane was linear, with a quasi‐Lissajou character. An extensive list of possible explanations for these emissions was considered. Since similar emissions with a similar occurrence rate have been seen by other workers, instrumental explanations were ruled out. Local external artifacts were also ruled out. Distant sources associated with tropospheric storms were considered in detail. During the most intense of the 2‐Hz emission events, the nearest bad weather was more than 1000 km away. It proved impossible to account for the amplitude and polarization of the horizontal component with any reasonable tropospheric source. Explaining the narrow‐band tuning was also a problem for any weather source model. An ionospheric model based on the concept of the ionospheric Alfvén resonator is capable of accounting for the tuning, amplitude, and polarization of the horizontal components. However, this model has moderate difficulty in accounting for the observed vertical component of the emissions. In the end, no available model proved fully satisfactory.
A new approach to the linear theory of single‐species tearing in two‐dimensional quasi‐neutral sheets
Authors: Brittnacher M. , Quest K. B. , Karimabadi H.
Published: February 2004

We have developed the linear theory of collisionless ion tearing in a two‐dimensional magnetotail equilibrium for a single resonant species. We have solved the normal mode problem for tearing instability by an algorithm that employs particle‐in‐cell simulation to calculate the orbit integrals in the Maxwell‐Vlasov eigenmode equation. The results of our single‐species tearing analysis can be applied to ion tearing where electron effects are not included. We have calculated the tearing growth rate as a function of the magnetic field component Bn normal to the current sheet for thick and thin current sheets, and we show that marginal stability occurs when the normal gyrofrequency Ωn is comparable to the Harris neutral sheet growth rate. A cross‐tail By component has little effect on the growth rate for By ≃ Bn. Even in the limit By ≫ Bn, the mode is strongly stabilized by Bn. We report that random pitch angle scattering can overcome the stabilizing effect of Bn and drive the growth rate up toward the Harris neutral sheet (Bn = 0) value when the pitch angle diffusion rate is comparable to Ωn.
Whistler emissions produced by modulated electron beams injected into the ionosphere
Authors: Lundin B. , Krafft C. , Matthieussent G.
Published: February 2004

The electric and magnetic fields emitted by semi‐infinite, modulated electron beams artificially injected into the Earth ionosphere are calculated. For whistler mode waves recorded by a remote VLF receiver the spectrum of the signal depends crucially on the time from the beginning of the injection. The amplitude of a transient signal from the beam front is controlled by the difference between the modulation frequency and the frequency determined from the stationary phase point in the corresponding integral (the latter varies with time due to the beam front motion). Far from the beam front, only the forced oscillations due to the modulation of the beam density will be registered. The intrinsic frequency width of the whistler signal δf connected with the ray representation of a wave field determines the optimal frequency bandwidth of the receiver in use. The value of δf ∼ (1/R)1/2 (R is the transverse distance between the beam axis and the receiver) also determines the beam domain, ∼ R1/2 along its axis which gives the main contribution to the registered signal (Fresnel zone). The interference of waves emitted by electrons with different parallel velocities u can reduce the amplitude of the registered wave field significantly. This problem is discussed in connection with the existence of a stationary phase point (SPP) over u. It is shown that this SPP condition can be satisfied only for the wave packets emitted in the near zone of the electron gun. The amplitudes of the coherent and incoherent beam emissions are compared. In the case of a dense plasma (where the plasma frequency ωp is much greater than the electron gyrofrequency ωc) the main results are obtained analytically.
Infrared parameters of atmospheric ozone and the great geomagnetic storm of 1953
Authors: Adel Arthur
Published: February 2004

Significant effects on the concentration and temperature of ozone in the upper atmosphere were observed during a major solar flare, and subsequent geomagnetic storm, which occurred during the interval August 6 to September 4, 1953. Ozone concentrations, as measured by the absorption of solar radiation by the 9.6 micron ozone band, decreased during the event but retuned to previous levels after the event. Temperature changes were also observed. The changes lag changes in Kp, the geomagnetic activity index, by approximately 5 to 7 days.
Pickup proton cyclotron turbulence at comet P/Halley
Authors: Lakhina G. S. , Verheest Frank
Published: February 2004

Low‐frequency electromagnetic turbulence near the proton gyrofrequency observed far upstream of comet P/Halley can be excited by a beam instability driven by relative streaming between cometary protons, solar wind protons, and water group ions. For a given solar wind velocity the growth rates peak at a certain optimum frequency shift (from the exact proton gyrofrequency), and the wavelengths involved can be deduced in a self‐consistent way from the dispersion law. Only under ideal conditions when all parameters remain constant would the mode corresponding to the optimum frequency shift grow fastest and might it be possible to observe a nearly constant frequency mode. However, if the solar wind parameters were not constant, then a mode that was in resonance earlier would no longer remain so, and some other mode with a slightly different frequency shift might start to grow fastest, leading to a mixing of many modes. Thus only rarely would one be able to observe a single mode near the proton gyrofrequency, exactly as happens in the observations. Our self‐consistent approach yields resonant instabilities with left‐handed polarizations in the spacecraft frame.
D/H and 18O/16O ratio in the hydronium ion and in neutral water from in situ ion measurements in comet Halley
Authors: Balsiger H. , Altwegg K. , Geiss J.
Published: February 2004

The D/H ratio and the 18O/16O ratio in hydronium ions in the gas coma of comet Halley have been determined from ion data obtained by the high intensity sensor (HIS) of the ion mass spectrometer (IMS) on board the spacecraft Giotto in 1986. The HIS instrument measured reliable data for the water group ions over a large distance range from the nucleus. We could therefore minimize the statistical errors. This large range also allowed us to study the systematic influence of formaldehyde on the D/H ratio in the hydronium ion and to deduce from the hydronium ion a D/H ratio for the water molecule. The measured 18O/16O ratio of (1.93±0.12) × 10−3 is compatible with the telluric value of 2.06 × 10−3. The D/H ratio for the hydronium ion measured is D/H = 3.08−0.53+0.38 × 10−4. We show that since the water chemistry is not significantly different for deuterated and nondeuterated water molecules and ions and since the contribution from other deuterated molecules to the deuterium abundance in the hydronium ion is small, the D/H value of the neutral water molecule is not significantly different from the value determined for the ion, although the uncertainty is somewhat larger. An upper limit for the deuterium abundance in formaldehyde in comet Halley can be derived from the D/H ratio in the hydronium ion as a function of distance from the nucleus. From our results we conclude that the D/H ratio in formaldehyde is certainly smaller than 2% and probably smaller than 0.5% and that the enrichment of the deuterium in the hydronium ion due to formaldehyde is lower than 5%. The D/H ratio in water in comet Halley is therefore significantly higher than the telluric sea water value, whereas the deuterium enrichment in formaldehyde is not as high as, for example, that observed in the Orion compact ridge.
On two‐component models of solar wind fluctuations
Authors: Lou Yu‐Qing
Published: February 2004

Matthaeus et al. first suggested a two‐component scenario for solar wind fluctuations which contains both parallel propagating Alfvén waves together with a population of quasi‐two‐dimensional turbulence. On the basis of these earlier ideas and further evidence, Tu and Marsch developed a two‐component model, in which small‐scale incompressible fluctuations are decomposed into Alfvén waves and convective structures, in an attempt to account for observed radial evolutions of energy density, normalized cross helicity σc, and Alfvén ratio rA of solar wind fluctuations. According to a recent analysis on two‐dimensional Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field, we discuss the theoretical basis of this important two‐component concept and further indicate necessary ingredients that a more self‐consistent two‐component turbulence model should contain. From the perspective of Alfvénic disturbances in the solar wind, the wave and convective components coexist in a naturally intermixed manner. Our analysis specifically describes behaviors of Alfvénic disturbances of all scales, including the intermediate ones. In a fully developed spiral magnetic field at large radii, the criterion for separating radially propagating Alfvén waves and radially convective Alfvénic structures depends on whether |ƒ + mƒS| is greater or less than the characteristic frequency ƒc ≡ (FMU³/FB²)1/2, where ƒ is the perturbation frequency, ƒS is the solar rotation frequency, m is an integer for the azimuthal wavenumber, FM is the solar wind mass flux, FB is the radial magnetic flux, and U is the asymptotic wind speed. For typical solar wind parameters, the timescale corresponding to ƒc is ≳ 15 hours.
Alpha particle heating at comet‐solar wind interaction regions
Authors: Sharma A. S. , Papadopoulos K.
Published: February 2004

The satellite observations at comet Halley have shown strong heating of solar wind alpha particles over an extended region dominated by high‐intensity, low‐frequency turbulence. These waves are excited by the water group pickup ions and can energize the solar wind plasma by different heating processes. The alpha particle heating by the Landau damping of kinetic Alfven waves and the transit time damping of low‐frequency hydromagnetic waves in this region of high plasma beta are studied in this paper. The Alfven wave heating was shown to be the dominant mechanism for the observed proton heating, but it is found to be insufficient to account for the observed alpha particle heating. The transit time damping due to the interaction of the ions with the electric fields associated with the magnetic field compressions of magnetohydrodynamic waves is found to heat the alpha particles preferentially over the protons. Comparison of the calculated heating times for the transit time damping with the observations from comet Halley shows good agreement. These processes contribute to the thermalization of the solar wind by the conversion of its directed energy into the thermal energy in the transition region at comet‐solar wind interaction.
A fully self‐consistent fluid theory of anomalous transport in Farley‐Buneman turbulence
Authors: Hamza A. M. , St.‐Maurice J.‐P.
Published: February 2004

Existing renormalized theories of E region Farley‐Buneman turbulence are reviewed, and their strengths and weaknesses briefly highlighted. A general framework for a fully self‐consistent fluid theory is presented next. The new theory includes nonlinear electron inertia, which has been neglected in previous fluid theories. The effects of the nonlinear electron inertia are studied carefully and linked directly to an anisotropic anomalous collision frequency as well as to an anomalous polarization effect. The new theory indicates that Farley‐Buneman wave energy is converted into parallel electron energy, owing to the presence of a wave‐induced damping rate given by a νkω∥ term. The theory offers a self‐consistent link between electron heating along the magnetic field lines and the anomalous collision frequency due to Farley‐Buneman turbulence. This paper focuses on the self‐consistent derivation of the anomalous effects and their physical interpretation, followed by a detailed discussion of the energy balance equation.
Cosmic ray modulation during solar cycle 22: Solar maximum loops at earth and global transients in the heliosphere
Authors: Popielawska Barbara
Published: February 2004

Neutron monitor data from two pairs of cosmic ray stations, Kiel/Tsumeb and Climax/Huancayo, are used to study the rigidity dependence of solar modulation during the solar activity cycle 22. The long‐term decrease of cosmic ray intensity during the ascending phase of cycle 22 is characterized by the same rigidity dependence as for the long‐term recovery during the descending phase of cycle 21. This is equivalent to a lack of any significant spectral hysteresis in cosmic ray modulation for both half cycles surrounding the 1987 cosmic ray maximum. The above is true at least for 27‐day averages of cosmic ray intensity and at neutron monitor rigidities. In December 1988 the first large Forbush decrease of cycle 22 marked the beginning of increased solar activity which resulted in several prominent interplanetary shocks in 1989‐1991. These strong heliospheric disturbances produced global merged interaction regions (GMIRs) which were responsible for large step decreases in cosmic ray intensity seen at all spacecraft throughout the heliosphere, including Voyager 2, Pioneer 10 and 11, as well as Voyager 1. It was found that in the neutron monitor rigidity range, such decreases were followed by rigidity‐dependent recoveries. The rigidity‐dependent recovery manifests itself by formation of hysteresis loops on correlation plots for low‐ versus high‐rigidity cosmic ray intensity changes. The loops, if completed, close approximately at the modulation level from which the corresponding step decrease started. On the other hand, the current long‐term cosmic ray recovery (starting from Bartels solar rotation 2162) follows the "normal" rigidity dependence characteristic of both the A>0 recoveries and A<0 decreases/recoveries (where A<0 and A>0 are two different phases of the solar magnetic cycle). All this together confirms the bimodal character of cosmic ray modulation for 1985‐1992, the same as found previously for cycles 19‐21; the transient modulation (T ≅ 0.5 to ∼1 year) related to GMIRs is accompanied on recovery by the phenomenon of hysteresis, while the long‐term modulation (T>1 year) is hysteresis free. The two explanations of the phenomenon of spectral hysteresis proposed in the literature, i.e., a time‐dependent modulation via convection, diffusion, and drift effects and an acceleration of a normal cosmic ray spectrum in multiple shocks in the outer heliosphere (as proposed by Stoker and Moraal (1986)), are discussed in the context of the presented cosmic ray data.
Cosmic ray decreases and solar wind disturbances during late October 1989
Authors: Cane H. V. , Richardson I. G.
Published: February 2004

We describe the interplanetary phenomena (energetic particles, solar wind plasma, and magnetic field) seen at IMP 8 and at ICE, located 65° west of IMP 8, during the period October 19‐31, 1989, when neutron monitors observed three ground level events originating in one active region when it was in the longitude range E09° to W57°. At least four shocks, associated with energetic particle enhancements, which can be attributed to a sequence of coronal mass ejections from the same active region, were seen at both spacecraft. An additional shock was observed only at ICE late in this period when the active region was behind the west limb. Considering all the data (which unfortunately suffer from large gaps), it appears that the ejecta associated with the shocks were detected only when the spacecraft and solar source longitude were separated by less than 50°. The shocks extended over a greater range of longitudes. The cosmic ray record at Earth is consistent with this picture such that only the first two shock‐associated cosmic ray decreases had the signature expected for intercepting ejecta material. This same time period was also examined by Bavassano et al. (1994). However, we do not agree with their conclusion that “magnetic clouds” extending at least 75° from the source longitude were present.
Magnetohydrodynamic Riemann problem and the structure of the magnetic reconnection layer
Authors: Wu C. C.
Published: February 2004

We present a complete solution for a set of magnetohydrodynamic (MHD) Riemann problems in which the upstream and downstream states have the same total pressure, and in which the normal component of the magnetic field is very small. These solutions are pertinent to subfast flows in the earth's magnetic tail and near the magnetopause. In a coplanar situation a family of solutions exists that depend on two parameters as well as on dissipation mechanisms. In the parallel case the transverse magnetic field either does not change direction or changes the direction twice by involving two intermediate shocks. In the antiparallel case an intermediate shock is always required, except when the solution consists of two switch‐off shocks. In a noncoplanar case the solution is not self‐similar as a function of x/t, but continues to evolve. At early times the evolution is similar to the coplanar case. In general two time‐dependent intermediate shocks are required to rotate the magnetic fields. The velocity shear has strong effect on the Riemann solution. In some cases no Riemann solution can exist because of the cavitation caused by the slow rarefaction waves. The calculated magnetopause structure resembles the observed structure for northward interplanetary magnetic field (IMF). However, for southward IMF, the MHD result shows the existence of a depletion layer, which is not supported by observations. We also show that on the magnetosheath side, the Walén relation, which is exact for a rotational discontinuity, can also be well satisfied by a slow shock, an intermediate shock, or the head of a slow rarefaction wave.
Geomagnetically trapped anomalous cosmic rays
Authors: Selesnick R. S. , Cummings A. C. , Cummings J. R. , Mewaldt R. A. , Stone E. C. , von Rosenvinge T. T.
Published: February 2004

Since its launch in July 1992, the polar‐orbiting satellite SAMPEX has been collecting data on geomagnetically trapped heavy ions, predominantly O, N, and Ne, at energies ≳ 15 MeV/nucleon and in a narrow L shell range near L = 2. Their location, elemental composition, energy spectra, pitch angle distribution, and time variations all support the theory that these particles originated as singly ionized interplanetary anomalous cosmic rays that were stripped of electrons in the Earth's upper atmosphere and subsequently trapped. The O are observed primarily at pitch angles outside the atmospheric loss cones, consistent with a trapped population, and their distribution there is nearly isotropic. The abundances relative to O of the N, possibly Ne, and especially C are lower than the corresponding interplanetary values, which may be indicative of the trapping efficiencies. The distributions of trapped N, O, and Ne in energy and L shell suggest that most of the ions observed at the SAMPEX altitude of ∼600 km are not fully stripped when initially trapped. A comparison of the trapped intensity with the much lower interplanetary intensity of anomalous cosmic rays provides model‐dependent estimates of the product of the trapping probability and the average trapped particle lifetime against ionization losses in the residual atmosphere for particles that mirror near the SAMPEX altitude.
Geomagnetic storm effects in the low‐ to middle‐latitude upper thermosphere
Authors: Burns A. G. , Killeen T. L. , Deng W. , Carignan G. R. , Roble R. G.
Published: February 2004

In this paper, we use data from the Dynamics Explorer 2 (DE 2) satellite and a theoretical simulation made by using the National Center for Atmospheric Research thermosphere/ionosphere general circulation model (NCAR‐TIGCM) to study storm‐induced changes in the structure of the upper thermosphere in the low‐ to middle‐latitude (20°‐40°N) region of the winter hemisphere. Our principal results are as follows: (1) The winds associated with the diurnal tide weaken during geomagnetic storms, causing primarily zonally oriented changes in the evening sector, few changes in the middle of the afternoon, a combination of zonal and meridional changes in the late morning region, and mainly meridional changes early in the morning. (2) Decreases in the magnitudes of the horizontal winds associated with the diurnal tide lead to a net downward tendency in the vertical winds blowing through a constant pressure surface. (3) Because of these changes in the vertical wind, there is an increase in compressional heating (or a decrease in cooling through expansion), and thus temperatures in the low‐ to middle‐latitudes of the winter hemisphere increase. (4) Densities of all neutral species increase on a constant height surface, but the pattern of changes in the O/N2 ratio is not well ordered on these surfaces. (5) The pattern of changes in the O/N2 ratio is better ordered on constant pressure surfaces. The increases in this ratio on constant pressure surfaces in the low‐ to middle‐latitude, winter hemisphere are caused by a more downward tendency in the vertical winds that blow through the constant pressure surfaces. Nitrogen‐poor air is then advected downward through the pressure surface, increasing the O/N2 ratio. (6) The daytime geographical distribution of the modeled increases in the O/N2 ratio on a constant pressure surface in the low‐ to middle‐latitudes of the winter hemisphere correspond very closely with those of increases in the modeled electron densities at the F2 peak.
Large enhancements in the O/N2 ratio in the evening sector of the winter hemisphere during geomagnetic storms
Authors: Burns A. G. , Killeen T. L. , Carignan G. R. , Roble R. G.
Published: February 2004

In this paper, we have looked for enhancements of the O/N2 ratio in data measured by the Dynamics Explorer 2 (DE 2) satellite in the middle latitudes of the winter hemisphere, based on a prediction that was made by the National Center for Atmospheric Research thermosphere/tonosphere general circulation model (NCAR‐TIGCM) that such increases occur. The NCAR‐TIGCM predicts that these enhancements should be seen throughout the low latitude region and in many middle latitude locations, but that the enhancements in O/N2 are particularly strong in the middle‐latitude, evening‐to‐midnight sector of the winter hemisphere. When this prediction was used to look for these effects in DE 2 NACS (neutral atmosphere composition spectrometer) data, large enhancements in the O/N2 ratio (∼50 to 90%) were seen. These enhancements were observed during the main phase of a storm that occurred on November 24, 1982, and were seen in the same region of the winter hemisphere predicted by the NCAR‐TIGCM. They are partially the result of the depletion of N2 and, as electron loss is dependent on dissociative recombination at F2 altitudes, they have implications for electron densities in this area. Parcel trajectories, which have been followed through the NCAR‐TIGCM history file for this event, show that large O/N2 enhancements occur in this limited region in the winter hemisphere for two reasons. First, these parcels of air are decelerated by the antisunward edge of the ion convection pattern; individual parcels converge and subsidence occurs. Thus molecular‐nitrogen‐poor air is brought from higher to lower heights. Because neutral parcels that are found a little poleward of the equatorial edge of the eveningside convection pattern are swept inward toward the center of the auroral oval, the enhancements occur only in a very limited range of latitudes. Second, nitrogen‐poor air is transported from regions close to the magnetic pole in the winter hemisphere. During geomagnetic storms, enhanced meridional winds are driven by the increased pressure‐gradient force that is associated with intensified Joule heating in the auroral oval. These pressure‐driven winds decrease rapidly on the dayside beyond the auroral oval where the parcels originate, limiting the region into which the parcels can be transported. Thus these two processes drive values of O/N2 in a limited region of the winter hemisphere, and reinforce only in the evening sector, causing large changes in this region.
Two‐dimensional simulations of magnetic pulsations upstream of the Earth's bow shock
Authors: Dubouloz N. , Scholer M.
Published: February 2004

The development of turbulence upstream of the quasi‐parallel portion of the Earth's bow shock is investigated using two‐dimensional hybrid simulations involving the injection of a very hot ion beam against a cold incident ion flow. Initially, waves are produced in the right‐hand resonant ion beam mode and with wave vectors mainly parallel to the ambient magnetic field B0 as expected in linear theory. The waves attempt to propagate upstream but are convected back toward the shock by the incident flow and strongly grow in amplitude as they encounter larger beam densities. Wave growth is associated with reduction of both incident and beam ion bulk velocities. Differential slowing of the incident flow between the different wave fronts and, when the angle θBn between B0 and the shock normal n is non zero, along the fronts themselves, leads to wave front refraction parallel to the shock and fragmentation into smaller structures. Simultaneously, strong scattering by the waves generates beam ion clumps containing a significant fraction of ions with velocities antiparallel to the beam bulk velocity, which enables the destabilization of the left‐hand resonant ion beam mode. The beam ions ultimately form a diffuse distribution whose density ratio relative to the solar wind is a few percents in average, but increases up to about 20% in the immediate vicinity of the shock. These processes result in the formation of magnetic pulsations whose extension parallel and perpendicular to n is typically 15 and a few tens of ion inertial lengths, respectively. These structures can reach total amplitudes δB of 3 to 4 B0, and exhibit most of the time a left‐handed polarization in the plasma rest frame, in agreement with spacecraft observations upstream of the Earth's bow shock. The simulations show that pulsations induce slowing and moderate heating of the incident flow and play an active role in the quasi‐parallel shock transition.
Simulation of magnetic cloud propagation in the inner heliosphere in two‐dimensions: 1. A loop perpendicular to the ecliptic plane
Authors: Vandas M. , Fischer S. , Dryer M. , Smith Z. , Detman T.
Published: February 2004

We present results of simulations of a magnetic cloud's evolution during its passage from the solar vicinity (18 solar radii) to approximately 1 AU using a two‐dimensional MHD code. The cloud is a cylinder perpendicular to the ecliptic plane. The external flow is explicitly considered self‐consistently. Our results show that the magnetic cloud retains its basic topology up to 1 AU, although it is distorted due to radially expanding solar wind and magnetic field lines bending. The magnetic cloud expands, faster near the Sun, and faster in the azimuthal direction than in the radial one; its extent is approximately 1.5–2× larger in the azimuthal direction. Magnetic clouds reach approximately the same asymptotic propagation velocity (higher than the background solar wind velocity) despite our assumptions of various initial conditions for their release. Recorded time profiles of the magnetic field magnitude, velocity, and temperature at one point, which would be measured by a hypothetical spacecraft, are qualitatively in agreement with observed profiles. The simulations qualitatively confirm the behavior of magnetic clouds derived from some observations, so they support the interpretations of some magnetic cloud phenomena as magnetically closed regions in the solar wind.
Tristatic EISCAT‐UHF measurements of the HF modified ionosphere for low background electron temperatures
Authors: Westman A. , Leyser T. B. , Wannberg G. , Rietveld M. T.
Published: February 2004

The effects of a powerful high‐frequency electromagnetic wave transmitted in the ordinary mode from the European Incoherent Scatter (EISCAT) ionospheric modification facility up to the daytime ionospheric F region have been studied with the EISCAT‐UHF radar system. This is one of the first tristatic EISCAT‐UHF incoherent scatter experiments, in which the excited Langmuir and ion acoustic fluctuations are measured simultaneously at scattering angles near parallel (Tromsø site), ∼22° (Kiruna site), and ∼31° (Sodankylä site) to the geomagnetic field. We present measurements of the spectral structure and temporal evolution with a maximum resolution of 13 ms, at the different scattering angles. For the first time, up to five cascade lines in addition to the decay line in the Tromsø HF‐enhanced plasma line and well‐developed cascade type plasma lines in Sodankylä were observed. In spite of low background electron temperature, and hence negligible electron Landau damping parallel to the geomagnetic field, the excited plasma turbulence varied considerably.
Surface wave excitation of local field line oscillations
Authors: Uberoi C.
Published: February 2004

The well‐behaved solution of the hydromagnetic wave equation for a general class of initial conditions is used to study the ringing, phase‐mixing, transient amplification of field line resonances. An expression for the phase‐mixing time in terms of the amplitude of initial impulse at the magnetopause and the gradient of Alfven speed in the magnetosphere is derived. The features of free oscillation of field lines studied here compare well with the previous numerical studies for particular types of impulse excitations of the magnetosphere.
Highly structured ionospheric convection for northward interplanetary magnetic field: A case study with DE 2 observations
Authors: Taguchi S. , Sugiura M. , Iyemori T. , Winningham J. D. , Slavin J. A.
Published: February 2004

Dynamics Explorer 2 observed a highly structured plasma convection in the dawn sector of the high‐latitude ionosphere on October 25, 1981. Analyzing the magnetic and electric field and plasma data for the event, we have clarified the relation between the electric field variations and the plasmas. There is a region of spiky electric field variations that create a highly structured convection. This region of spiky electric field variations generally coincides with a region having source electrons of low temperature (roughly 60–100 eV) and high density (roughly 1‐5 cm−3, sometimes much higher). Ions having energies of several hundred to several kilo electron volts precipitate in the region of the spiky electric field variations. Results of the analysis also suggest that the highly structured plasma drift is detached from the dayside reverse convection, that is, sunward flow at the highest latitudes and antisunward flow at lower latitudes. Thus our results do not agree with some of the previous studies which suggested that the structured plasma convection is a continuation of the dayside convection that has become irregular near the nightside edge and that its source region is the plasma sheet. We suggest that the source for the highly structured convection lies in the distant low‐latitude boundary layer, and propose a source electric field configuration.
Inference of atomic oxygen concentration from remote sensing of optical aurora
Authors: Shepherd M. G. , McConnell J. C. , Tobiska W. K. , Gladstone G. R. , Chakrabarti S. , Schmidtke G.
Published: February 2004

A remote sensing method has been developed for the determination of the [O]/[O‐MSIS] ratio in aurora, using ratios of the O I (557.7 nm) and N2+ (391.4 nm) emissions. It is shown that the method can be used for the analysis of measurements integrated along the line of sight, provided data only above the emission rate peak are used. The method is applied to the case of horizontal viewing from a vertically oriented rocket so that a large volume of space was sampled around the rocket. The method can potentially be applied to satellite limb images, provided some independent information about the location of the aurora is available, as it was for the rocket observations. Photometric measurements of the N2+ (391.4 nm) and O I (557.7 nm) emissions obtained during the Energy Budget Campaign 1980 on flight E‐2 with the instrument EF11 and its reflight in 1981 were used in the analysis presented. During the first flight the rocket horizontally viewed two distinct aurorae, a nearby diffuse patch, and a more distant pulsating aurora. Results obtained by the same EF11 instrument on a second flight through an auroral arc in 1981 are also presented. Two types of atomic oxygen variability were found in both of the flights. In the first type, [O] is increased above [O‐MSIS] by a factor of 1.5 at 180 km, is equal to the MSIS model at 160 km, and is less than MSIS below that; that is, the scale height of [O] was increased. The experimental I(557.7)/I(391.4) ratio was constant with altitude. In the second type, the [O] was depleted by about a factor of 2 over the altitude range of 120–180 km, while the I(557.7)/I(391.4) ratio decreased with altitude. The inferred atomic oxygen concentrations of 0.5 to 2 with respect to MSIS suggested different vertical flows on the two cases. Independent evidence is provided by atmospheric composition measurements made during the same campaign.
The VAN earthquake predictions
Authors: Rhoades D. A. , Evison F. F.
Published: February 2004

Assessment of the proposed VAN method for predicting earthquakes in Greece remains inconclusive. Authors who have attempted to evaluate the method have had to make their own subjective decisions about some features of the hypothesis, and to propose their own algorithms for testing against a null hypothesis. Different treatments of the inhomogeneity in space and time have lead to widely different conclusions. The binomial distribution has been misused in considering whether predictions may have been satisfied “by chance.” Objective tests on the performance of the method, using independent data, cannot begin until the VAN hypothesis and the proposed null hypothesis have been fully formulated.
Precursor candidacy and validation: The VAN Case so far
Authors: Mulargia Francesco , Gasperini Paolo
Published: February 2004

A reliable identification of seismic precursors requires a two‐step statistical approach: a retrospective “learning” step to establish the candidate precursors on the basis of ad hoc chosen laws of the game, and a “validation” step which evaluates candidate precursors on an independent data set according to the previously established laws of the game. No published work has so far provided a sound support to the candidacy of the VAN signals as earthquake precursors. The problem is complicated by a chronic fuzziness in the text of VAN's predictions, which often allows different interpretations, and by the absence of any clear statement by VAN of the laws of the game.
Modified geostrophy in the thermosphere
Authors: Larsen M. F. , Walterscheid R. L.
Published: February 2004

A number of modeling studies, as well as analyses of satellite data, have shown that the neutral winds in the F region are characterized by an approximate balance between the pressure gradient, the Coriolis and curvature acceleration, and viscous and ion drag in the direction perpendicular to the electric field momentum source. The ion drag in the F region is associated with the Pedersen conductivity terms and is often called the Pedersen drag. The dynamics associated with such a balanced flow are reasonably well understood. In the lower thermosphere, the balance of forces becomes more complicated and leads to interesting new flow configurations as the effect of the Hall drag becomes increasingly important. Specifically, the Hall drag coefficient has the effect of decreasing the magnitude of the effective Coriolis parameter, since the Coriolis parameter always appears in combination with the Hall drag term. We examine the balance of forces in the lower E region across the range of heights where the Hall drag terms dominate at the lower altitudes and the Pedersen drag terms dominate at the higher altitudes. The balanced flow solutions predict distinctly different flows in the regions in which the plasma accelerates the neutrals and in the regions in which the plasma induces a drag. The analogs of the thermal wind equations are also presented. The results show that the flow in the lower E region is enhanced and will be in a direction that is antiparallel to the F region plasma flow direction and that the largest shears should occur on the bottomside of the wind maximum. Large convergent/divergent flows are expected on the equatorward side of the auroral oval, and the maximum convergence/divergence is expected near a height of ∼110–120 km.
Ponderomotive effects on distributions of O+ ions in the auroral zone
Authors: Witt E. , Hudson M. K. , Li X. , Roth I. , Temerin M.
Published: February 2004

Test particle calculations are used to compute the effects of gravity and ponderomotive acceleration by shear Alfvén wave oscillations on the distribution function of O+ ions along auroral field lines, assuming an ionospheric Maxwellian source of the ions at 2000 km altitude with ∼0.5 eV of thermal energy in the parallel component of velocity. The electric field model corresponds to a standing wave oscillation with a frequency ∼1 Hz in the azimuthal direction superimposed on the background dipole field, in which the wave amplitude is either increasing or decreasing in time. The electric field is taken to be primarily in the perpendicular direction. The time varying wave produces broad distributions with widths of 2 to 10 times the initial 0.5‐eV thermal energy of the Maxwellian source, and the density and flux of upward going O+ ions at one Earth radius are both enhanced in this model. The oxygen ion distribution functions at 1 RE altitude resulting from interaction with waves whose amplitudes are increasing in time have a more gradual lower energy cutoff than do the distribution functions resulting from decaying waves. The high‐energy part of the distribution functions in growing waves reflects the temperature of the Maxwellian source, while the high‐energy part of the distributions resulting from decaying waves steepens with time, independent of the source temperature.
Midnight velocity shear zone and the concept of Harang discontinuity
Authors: Koskinen Hannu E. J. , Pulkkinen Tuija I.
Published: February 2004

We discuss the concept of Harang discontinuity based on ionospheric observations of electrodynamic parameters, magnetic field mapping from the ionosphere to the magnetosphere, and theoretical reasoning. We consider the Harang discontinuity as synonymous to the midnight‐premidnight sector velocity shear zone in the auroral ionosphere, arguing that its main importance is related to the dynamic development of magnetospheric conditions, especially substorms. We show that typical two‐dimensional ionospheric observations are not quite consistent with the statistical Heppner‐Maynard convection models and this inconsistency becomes even more apparent when mapped into the equatorial magnetotail. The upward field‐aligned current from the velocity shear zone is one of the major problems in performing a realistic mapping procedure. Because the auroral arcs close to the Harang discontinuity are often found to activate and break up at the time of the substorm onset, this same upward current is probably connected with the substorm initiation mechanism in the magnetotail. We discuss the electrodynamics of all parts of this current circuit, i.e., the ionosphere, the magnetosphere, and the connecting field lines. We conclude that surprisingly few observationally proven facts are known of this circuit outside the ionosphere, and even there the great variability of the physical parameter values makes this region very difficult to study in detail.
O+ escape through the plasma sheet of Mars and its causative mechanism
Authors: Haider S. A.
Published: February 2004

A model for the polar ion exosphere of Mars is developed to calculate the escape flux and density of oxygen ions and electrons through the plasma sheet of Mars at different exospheric electron temperatures (600K–2300K) along the magnetic field lines originating from the baropause at latitude 75 deg and longitude zero degree. The intensity of the magnetic field lines in the noon‐midnight meridian plane at all latitudes are calculated by assuming that Mars has weak magnetic field of dipolar nature. To calculate the electric potential in the region of magnetic field, quasi‐neutrality condition is satisfied in such a way that both the flux and density of oxygen ions are equal at every point to that of electrons. It has been found that above 2300K, escape flux and density of O+ do not increase. The total escape rate of O+ at this temperature is ∼3.5×1024 ions/s, which shows close agreement with the observations taken by Solar Wind Plasma Instrument (TAUS) and Automatic Space Plasma Experiment with Rotating Analyser (ASPERA) experiments in the plasma sheet of Mars. The present calculation concludes that O+ ions in the plasma sheet of Mars are mainly due to the escape of oxygen ions from the ionosphere in presence of charge separation electric field.
Magnetospheric filter effect for Pc 3 Alfvén mode waves
Authors: Zhang X. , Comfort R. H. , Gallagher D. L. , Green J. L. , Musielak Z. E. , Moore T. E.
Published: February 2004

We present a ray‐tracing study of the propagation of Pc 3 Alfvén mode waves originating at the dayside magnetopause. This study reveals interesting features of a magnetospheric filter effect for these waves. Pc 3 Alfvén mode waves cannot penetrate to low Earth altitudes unless the wave frequency is below approximately 30 mHz. Configurations of the dispersion curves and the refractive index show that the gyroresonance and pseudo‐cutoff introduced by the heavy ion O+ block the waves. When the O+ concentration is removed from the plasma composition, the barriers caused by the O+ no longer exist, and waves with much higher frequencies than 30 mHz can penetrate to low altitudes. The result that the 30‐mHz or lower frequency Alfvén waves can be guided to low altitudes agrees with ground‐based power spectrum observations at high latitudes.
The ARIA I Rocket Campaign
Authors: Anderson P. C. , Christensen A. B. , Sharber J. R. , Straus P. R. , Hecht J. H. , Larsen M. F. , Kayser D. C. , McEwen D. J. , Sharp W. E. , Koehler J. A. , Sivjee G. G. , Lyons L. R. , Gutierrez D. J. , Winningham J. D.
Published: February 2004

The Atmospheric Response in Aurora (ARIA) I rocket experiment was designed to measure the energy and momentum forcing of the atmosphere during auroral disturbances and the resultant compositional and dynamical changes. It consisted of one instrumented rocket, three trimethyl aluminum chemical release rockets, and various ground‐based optical instruments. The rockets were launched from Poker Flat Research Range, Alaska, in March 1992. The instrumented payload included a set of eight instruments for measuring various atmospheric and ionospheric quantities. This paper describes the contents of the program and the results of electrodynamic modeling and measurements. A substorm onset occurred approximately 4 hours before launch of the instrumented payload, giving rise to both particle and Joule heating in the vicinity of Poker Flat. By launch time, the substorm was well into recovery. We used optical measurements, electron density measurements from the Langmuir probe instrument, and model results from the Strickland electron transport code to specify latitudinal profiles of the height‐integrated Pedersen conductivity. Comparison with assimilated mapping of ionospheric electrodynamics (AMIE) calculations of the Pedersen conductivities for this event indicated that AMIE located the enhanced auroral conductivity region well. However, the magnitudes of the AMIE conductivities in the enhanced region were considerably less than the measurements due to localized substorm‐related particle precipitation enhancements not accounted for by AMIE. Our conductivity profiles were used in conjunction with electric field values produced by the AMIE routine to examine the atmospheric heating rates associated with the substorm. The latitudinally integrated Joule heating rate was initially less than the particle heating rate, but rapidly increased to its maximum value at the time of the substorm maximum while the particle heating rate peaked prior to substorm maximum. The particle and Joule heating were collocated during the expansion and maximum phase, but as the substorm recovered, the Joule heating moved to higher latitudes, so that by the time of launch, the two heating regions were completely separated by several degrees. The analysis indicates that the rocket was launched directly into the atmospheric region where the maximum heating had occurred.
Structure of mass‐loading shocks: 2. Comparison of theory and observation at comet Halley
Authors: Zank G. P. , Coates A. J. , Neubauer F. M. , Rème H. , Mazelle C. X.
Published: February 2004

The multifluid diffusive model of G. P. Zank et al. (1994), which describes the interaction of the solar wind with a cometary plasma in the outer coma, has been used to model the structure of the Halley bow shock. The theoretical results are compared to in situ observations made by Giotto. We compare the solar wind and cometary ion number densities and pressures upstream and through the quasi‐perpendicular and quasi‐parallel shocks (observed on the inbound and outbound legs of the encounter, respectively). In general, good agreement is found between theory and observations in terms of shock structure, strength, and location, especially for the quasi‐parallel shock. The comparison between the quasi‐perpendicular shock observations and theory is complicated by the apparently nonstationary behavior of the shock, a feature which has been remarked upon by other investigators. The cometary bow shock appears to be an excellent example of an energetic‐particle‐mediated shock where the energetic particles comprise less than 10% of the total number density.