bluuebunny/crossref_metadata_2025 huggingface dataset:
Auroral N2 emissions and the effect of collisional processes on N2 triplet state vibrational populations
Authors: Morrill Jeff S. , Benesch William M.
Published: February 2004

Previous model results have shown that the N2 triplet vibrational level populations in the aurora are strongly affected by cascade and quenching by atomic and molecular oxygen. As the aurora penetrates to lower altitudes (less than 100 km) the role of quenching by atomic oxygen becomes less important and processes involving N2 collisions begin to play a more prominent part. We are developing a model which will yield steady state vibrational level populations for both the singlet and triplet valence states of N2. The model currently provides results for the seven low‐lying N2 triplet states (A 3Σu+, B 3IIg, W3Δu, B′ 3Σu−, C3IIu, D3Σu+, and E3Σg+). These states are responsible for auroral emissions from the UV (Vegard‐Kaplan (VK), second positive (2PG)) through the visible to the infrared (first positive (1PG), infrared afterglow (IRA), Wu‐Benesch (WB)). We have included two additional collisional processes in the current model which were not treated previously. These are the intersystem collisional transfer of excitation (ICT) between the B state and the A, W, and B′ states and vibrational redistribution within the A state vibrational manifold, both due to collisions with ground state N2. The present work compares our current model results with those of a previous model, as well as ground, airborne, and rocket observations. The comparison between N2(A) (VK) and N2(B) (1PG) vibrational level populations predicted by our model and a number of auroral observations indicate that the current model achieves a significant improvement in the fit between calculation and observation. In addition, the current model predicts a shift in the band intensity distribution of the 1PG Δν = 3 sequence from the infrared into the visible red at the lower altitudes (less than 90 km) as well as an overall enhancement in the entire 1PG system. Consequently, this provides a possible explanation of a dominate feature of type b aurora, the auroral red lower border.
Solar cycle effect on Pc 3 geomagnetic pulsations
Authors: Verő J.
Published: February 2004

Hypotheses about long‐period variations (i.e., seasonal, annual, and solar cycle) of geomagnetic pulsations have existed since the intensive research started. Nevertheless, insufficiently long data series prevented a conclusive and unambiguous detection of such variations. The Nagycenk observatory (Hungary) at an L value slightly below 2 has a pulsation time series including three solar cycles. Based on these data, period changes of Pc 3 pulsations are presented here, and an indirect connection with the yearly variation of the pulsation activity is supposed, both being due to changes of the magnetospheric plasma density. Longer periods of the pulsations around solar activity maxima should be due to higher particle concentrations at the respective shell of field lines, while 11‐year average periods are influenced by long‐term changes of the interplanetary magnetic field.
Simulation of upstream pressure pulse propagation through the bow shock
Authors: Thomas V. A. , Winske D. , Thomsen M. F.
Published: February 2004

The interaction of upstream pressure pulses with the Earth's bow shock and their propagation through the magnetosheath are studied by means of one‐dimensional hybrid (particle ions, massless fluid electrons) simulations. The pulses either are generated self‐consistently as a result of the presence of ions streaming away from the quasi‐parallel shock or are created artificially through density or velocity perturbations in the solar wind some distance upstream of the shock so that interaction of pressure pulses with both the quasi‐parallel and quasi‐perpendicular portion of the bow shock can be investigated. We demonstrate that the pulses tend to perturb the shock structure only for a limited time and then propagate in a coherent manner at the magnetosonic speed for large distances (> 5RE) downstream of the shock. Implications for the intersection of such pulses with the magnetopause are also addressed.
Gravity waves near 300 km over the polar caps
Authors: Johnson F. S. , Hanson W. B. , Hodges R. R. , Coley W. R. , Carignan G. R. , Spencer N. W.
Published: February 2004

Distinctive wave forms in the distributions of vertical velocity and temperature of both neutral particles and ions are frequently observed from Dynamics Explorer 2 at altitudes above 250 km over the polar caps. These are interpreted as being due to internal gravity waves propagating in the neutral atmosphere. The disturbances are characterized by vertical velocity perturbations of the order of 100 m/s and horizontal wave lengths along the satellite path of about 500 km. They often extend across the entire polar cap. The associated temperature perturbations indicate that the horizontal phase progression is from the nightside to the dayside. Vertical displacements are inferred to be of the order of 10 km and the periods to be of the order of 10³ s. The waves must propagate in the neutral atmosphere, but they usually are most clearly recognizable in the observations of ion vertical velocity and ion temperature. By combining the neutral pressure calculated from the observed neutral concentration and temperature with the vertical component of the neutral velocity, an upward energy flux of the order of 0.04 erg/cm²‐s at 250 km has been calculated, which is about equal to the maximum total solar ultraviolet heat input above that altitude. Upward energy fluxes calculated from observations on orbital passes at altitudes from 250 to 560 km indicate relatively little attenuation with altitude.
Interstellar pickup ions and quasi‐perpendicular shocks: Implications for the termination shock and interplanetary shocks
Authors: Zank G. P. , Pauls H. L. , Cairns I. H. , Webb G. M.
Published: February 2004

A new mechanism for the acceleration of pickup ions by repeated reflections from the electrostatic cross shock potential of a quasi‐perpendicular shock is presented. The acceleration mechanism, multiply reflected ion (MRI) acceleration, offers a resolution to the issue of injecting pickup ions into an efficient particle energization scheme, and the injection efficiency for pickup ions is found to be inversely proportional to ion mass and proportional to charge. By studying the particle energy gain in the motional electric field (where a steady shock frame is assumed) the energized pickup ion spectrum can be computed. Extremely hard power law spectra (E−1.5, for example) emerge from the upstream pickup ion distribution. The maximum energy that a reflected pickup ion can gain is found to be proportional to the square of the product of the Alfvén speed and (r − 1), where r is the shock compression ratio. For solar wind conditions at either interplanetary shocks or the termination shock the upper energy limit is typically in excess of 0.5 MeV. It is suggested here that MRI acceleration provides an efficient mechanism for injecting low‐energy pickup ions into a subsequent acceleration process such as diffusive Fermi acceleration. Such a two‐step acceleration scheme alleviates many of the difficulties which plague ion energization models at perpendicular shocks. The structure of a quasi‐perpendicular shock modified by shock reflection of pickup ions is discussed in general terms. By way of application we present a detailed study of the MRI acceleration mechanism at the termination shock for a wide range of parameters and discuss the implications for the anomalous cosmic ray component. The acceleration of pickup ions by an interplanetary traveling shock is also discussed, and the observations made by Ulysses [Gloeckler et al., 1994] are addressed. The puzzling aspects of the Gloeckler et al. [1994] observations appear to be explained quite naturally by shock energization based on repeated pickup ion reflections. Observational tests of MRI acceleration may be possible by using pickup He+ at either the terrestrial or Jovian bow shock or by using cometary ions at a cometary bow shock.
The alternative paradigm for magnetospheric physics
Authors: Parker E. N.
Published: February 2004

This paper emphasizes that the macrodynamics of the terrestrial magnetosphere is more effectively treated in terms of the primary variablesBandv(theB,vparadigm). The common practice of relating the dynamics toEandj(theE,jparadigm) provides direct answers in a variety of symmetric cases, but breaks down in even so simple a static problem as a flux bundle displaced (perhaps by reconnection with the magnetic field in the solar wind) from its normal equilibrium position in a static dipole field. The essential point is that a direct derivation from the equations of Maxwell and Newton leads to field equations written in terms of the continuum fieldsBandv. The equations can be recast in terms ofEandj, of course, but they are then unwieldy, being integrodifferential equations. Hence theE,jparadigm, when correctly applied, is seriously limited in its effectiveness in dynamical problems. Circumventing the limitations with the common declaration thatEis the prime mover, actively penetrating from the solar wind into the magnetosphere, provides dynamics that is unfortunately at variance with the results that follow directly from Maxwell and Newton. The paper outlines the standard derivation of the basic field equations and then goes on to treat a variety of circumstances to illustrate the effectiveness of the deductiveB,vparadigm in the continuum dynamics of the magnetic field and plasma. There is no attempt to develop a comprehensive model of magnetospheric activity. However we suggest that the ultimate task is more effectively attacked with theB, vparadigm.
Pc 5 ULF waves observed simultaneously by GOES 7 and the CANOPUS magnetometer array
Authors: Ziesolleck C. W. S. , Feng Q. , McDiarmid D. R.
Published: February 2004

Applying pure state filtering techniques to 1 month (March 1990) of GOES 7 and CANOPUS (Canadian Auroral Network for the OPEN Program Unified Study) magnetometer data, we have detected several dominantly compressional Pc 5 events in the satellite data which were well correlated with the pulsation signal recorded by the ground stations. On the ground, near the GOES 7 magnetic footprint, these pulsations were seen with polarization characteristics similar to those observed at geosynchronous orbit, i.e., without ionospheric rotation of the transverse polarization major axis. Using data from 13 stations of the CANOPUS array, we show that these compressional wave events seen by GOES 1 at L = 6.67 were associated with field line resonances (FLRs) at higher L values (L ≥ ∼8), propagating antisunward with small azimuthal wave numbers. Besides these compressional waves, we identified a number of azimuthally (east–west) polarized transverse Pc 5 events at geostationary orbit. Near the subsatellite point these appeared on the ground with the polarization major axis aligned in about the north–south direction, i.e., rotated through ∼90° between magnetosphere and ground. The spatial characteristics, as derived from the ground array data, show that these transverse events represent FLRs on or near the satellite's L shell. By means of selected events with clear FLR characteristics in the CANOPUS data, we determine the resonance position from the ground array data and examine the variation of the signal characteristics at geosynchronous orbit with increasing radial distance of GOES 7 from the resonant field line. We find that the transverse wave characteristics, dominant near the resonance, decay on relatively small‐scale lengths and are transformed into compressional wave characteristics within about 1–2 RE from the resonance position. The diurnal variation of the signal parameters for these events suggests that compressional Pc 5 waves at geosynchronous orbit associated with FLRs at L > 8 are preferentially generated near local noon. Finally, via consideration of solutions of the uncoupled toroidal mode wave equations based on the observed signal frequencies and resonance positions, we discuss the field‐aligned harmonic of these resonances.
The average ionospheric electrodynamics for the different substorm phases
Authors: Kamide Y. , Sun W. , Akasofu S.‐I.
Published: February 2004

The average patterns of the electrostatic potential, current vectors, and Joule heating in the polar ionosphere, as well as the associated field‐aligned currents, are determined for a quiet time, the growth phase, the expansion phase, the peak epoch, and the recovery phase of substorms. For this purpose, the Kamide‐Richmond‐Matsushita magnetogram‐inversion algorithm is applied to a data set (for March 17, 18, and 19, 1978) from the six meridian magnetometer chains (the total number of magnetometer stations being 71) which were operated during the period of the International Magnetospheric Study (IMS). This is the first attempt at obtaining, on the basis of individual substorms, the average pattern of substorm quantities in the polar ionosphere for the different epochs. The main results are as follows: (1) The substorm‐time current patterns over the entire polar region consist of two components. The first one is related to the two‐cell convection pattern, and the second one is the westward electrojet in the dark sector which is related to the wedge current. (2) Time variations of the two components for the four substorm epochs are shown to be considerably different. (3) The dependence of these differences on the ionospheric electric field and the conductivities (Hall and Pedersen) is identified. (4) It is shown that the large‐scale two‐cell pattern in the electric potential is dominant during the growth phase of substorms. (5) The expansion phase is characterized by the appearance of a strong westward electrojet, which is added to the two‐cell pattern. (6) The large‐scale potential pattern becomes complicated during the recovery phase of substorms, but the two‐cell pattern appears to be relatively dominant again during their late recovery as the wedge current subsides. These and many other earlier results are consistent with the present ones, which are more quantitatively and comprehensively demonstrated in our global study. Thus the two components are tentatively identified as the directly driven and the unloading components, respectively, although there is some indication that both components are actually coupled in the ionosphere. In the present paper we show that the directly driven component is present throughout the lifetime of substorms, becoming a dominant feature during the recovery phase of substorms as the unloading component wanes. The fact that the two components exist and that their time variations are different indicates that different processes are in progress even for the same value, say, −500 nT, of AL and that we must be cautious in using a single electrojet index, such as AL, in ordering satellite data.
C+ Pickup ions in the heliosphere and their origin
Authors: Geiss J. , Gloeckler G. , Fisk L. A. , von Steiger R.
Published: February 2004

C+ pickup ions were discovered with the solar wind ion composition spectrometer flying on Ulysses. Whereas the other nonlocally occurring pickup ions are produced from the interstellar gas penetrating deep into the heliosphere, C+ comes from an “inner source” which is located at a solar distance of a few AU and extends over all heliospheric latitudes investigated so far. The total production of C+, N+, and O+ by this inner source is of the order of 10−3 relative to the total production of O+ from the interstellar gas in the heliosphere. Thus the inner source does not significantly contribute to oxygen or nitrogen in the anomalous cosmic rays (ACR), but its contribution to ACR carbon may not be negligible. We propose that the inner source material is carbon compounds evaporating from grains. At this time, the evidence points to interstellar grains as the major source, but we do not want to exclude yet a contribution from grains of solar system origin.
Implications of the small aspect angles of equatorial spread F
Authors: Hysell D. L. , Farley D. T.
Published: February 2004

Small‐scale equatorial spread F irregularities are almost perfectly aligned with the geomagnetic field. We develop here an analytic plasma kinetic theory of small‐scale, quasi‐field‐aligned irregularities that includes ion viscosity and finite Larmor radius effects. We conclude, for one thing, that the measured aspect angles are too small to be consistent with a dissipative drift wave source of 3‐m irregularities. Nonlinearly driven flute modes appear to be the only available mechanism. We compare the relative influence of parallel and perpendicular dissipation and conclude that the aspect width depends only weakly on any single geophysical parameter, such as collision frequency, gradient length, temperature, etc. This finding is consistent with our observation that the measured aspect angles vary little with altitude and only weakly with instability level.
High‐latitude daytime absorption spike events
Authors: Stauning Peter , Rosenberg T. J.
Published: February 2004

Imaging riometer observations of cosmic noise absorption intensities have revealed a new class of ionospheric radiowave absorption events, the high‐latitude daytime absorption spike events. The occurrences have been examined on the basis of imaging riometer observations from Sondrestrom at an invariant latitude of 73.7°. Representative examples are presented in this report. The spike absorption events, typically, have durations of 1–2 min, and are limited in spatial extent to a region of typical dimensions of 50–100 km. They are rather weak, typically 0.2–0.3 dB, rarely up to l dB at the observing frequency of 38 MHz. Their local magnetic signatures are weak, there is no clear relation to the planetary magnetic activity level (Kp) or to specific interplanetary field variations. The bulk of these events occur at or within a few degrees equatorward of the convection reversal boundary at the afternoon convection cell for typical two‐cell polar ionospheric convection patterns. We suggest that the absorption events relate to the sudden precipitation of high‐energy (30–300 keV) magnetospheric electrons. It is further suggested that the precipitation mechanism for a major part of the afternoon cases is associated with dynamic inverted‐V potential structures developed through a magnetosphere‐ionosphere feedback process acting through near‐saturated upward region 1 field‐aligned currents at the afternoon convection reversal. The consequences derived from the suggested mechanism match very well the observed characteristics of absorption spike events.
Time response of O+ to a weak transverse ion heating event in the polar ionosphere
Authors: Singh Nagendra
Published: February 2004

Time response of O+ ions to transverse ion heating in the polar ionosphere is studied by means of a particle code. Here we deal with relatively low levels of heating over a few minutes yielding superthermal ions energized up to a few eV. Under the influence of the upward mirror and downward gravitational forces, most of the heated ions are trapped between the topside ionosphere and some high altitude depending on their perpendicular energies. A few ions with sufficiently large perpendicular energy escape the gravitational pull, and the flux of such ions increases with the heating level. The trapped ions bounce back and forth. During the first upward transit of the heated ions, the transient outflux from the topside ionosphere is as large as 107 ions Cm−2S−1 and it subsequently decays to quite small values in a few tens of minutes. A large transient outflux (∼ 3 × 108 ions Cm−2S−1) consisting of unheated ions develops even below the heating region, lasting over just a few minutes. As the ions having relatively large energies continue their upward journey, and the less energetic ones begin to fall down, the O+ drift velocity ranges from a relatively large negative value (downward) just above the dense O+ plasma in the topside ionosphere to a large positive value at high altitudes. Substantial flux of downward moving ions is also seen. During subsequent upward motions, the bouncing ions form a well‐defined, quite sharp expansion front. The bounce motion generates a succession of plateau formation over a period of several hours in response to a brief heating period of just a few minutes. This leads to nearly periodic oscillations in local densities, with the oscillation period increasing with altitude. In a time of about one day, the oscillations in the density cease due to the spatial dispersion of the heated ions having differing initial energies. Eventually the O+ density profile becomes nearly stable but remains extended with significantly enhanced O+ density at high altitudes. A consequence of trapping of the ions by the downward gravitational force is that the kinetic energy of the ions decreases with the increasing altitude; the coldest ions appear near the top of apex of the trapped ion trajectory, while the warmest ions appear near the bottom of the flux tube. Such cold ions are likely to contribute to the thermal O+ population observed at altitudes of several thousand kilometers.
Heliospheric plasma sheets as small‐scale transients
Authors: Crooker N. U. , Burton M. E. , Phillips J. L. , Smith E. J. , Balogh A.
Published: February 2004

Ulysses plasma and magnetic field data from the ecliptic plane near 1 AU signaling multiple heliospheric plasma sheets across a sector boundary are analyzed in order to determine their geometrical configuration. The plasma sheets coincided with a region of sporadic heat flux dropouts and planar magnetic structure. The plane of the structure was inclined to the ecliptic plane by 56° and nearly orthogonal to the Parker spiral. Pronounced flow deflections within the plasma sheets were closely aligned with the magnetic structure plane. Geometrical constraints and the lack of correlation between the highly variable magnetic fields in the planar structure and the flow deflections preclude a single, wavy plasma sheet interpretation. The flow deflections were nested in similar, larger‐scale deflections on the downslope of a pressure wave. The observations are consistent with spacecraft passage through the flanks of a transient disturbance which distorted the sector boundary plane. We interpret the planar magnetic structure with its embedded plasma sheets as a layer of small‐scale, intertwined flux ropes with varying degrees of helicity, flattened into sector‐boundary‐aligned sheets by kinematic distention and compression. The sporadic dropout of heat flux may have signaled flux‐rope detachment from the Sun or particle scattering, possibly owing to the high variability of plasma parameters across the plasma sheets.
A simple model of core field generation during plasmoid evolution
Authors: Hesse Michael , Birn Joachim , Kuznetsova Masha M. , Dreher Jürgen
Published: February 2004

Bipolar magnetic field signatures in the far magnetotail observed by the ISEE 3 spacecraft are commonly interpreted as signatures of a passing magnetic bubble, or plasmoid. A large number of such plasmoid‐type variations in the north–south component of the magnetic field are accompanied by large core magnetic fields which are directed primarily in the cross‐tail direction, indicating a flux rope like structure. Similar signatures are also found in a recent examination of GEOTAIL deep tail data. The fact that more of these flux ropelike plasmoids are encountered in the far tail than closer to the Earth raises the question whether they are the result of an evolution from no or low core fields to high core fields or whether plasmoids without core fields and flux ropes are entirely different entities. We present a model which explains the evolution of a looplike plasmoid in the near tail to a thinner flux rope in the far tail. The transition is accomplished by magnetic reconnection, which progressively connects the plasmoid magnetic field lines to the colder plasma in the low‐latitude boundary layer and magnetosheath. The connection leads to a draining of hot plasma from plasmoid field lines and a subsequent collapse due to the plasma pressure reduction. The collapse causes a strong enhancement of any preexisting cross‐tail magnetic field component, until a quasi‐force‐free state is reached. We also present MHD simulations to demonstrate the process. Last, we show that this mechanism can produce core field enhancements beyond the ambient lobe field strength.
Merging of slow‐mode compressive waves in low‐β plasma
Authors: Whang Y. C.
Published: February 2004

A unsteady one‐dimensional model can be used to study the interaction of two colliding streams for the formation of MHD shocks. If both β and θ are small, the interaction evolves to a shock system consisting of slow shocks only. At increasing β and θ, the interaction may evolve to a system consisting of both slow and fast shocks and eventually to a system consisting of fast shocks only. We study the merging rates of neighboring characteristic curves for compressive waves associated with the interaction region. We first study a linearized wave system produced by an initial small perturbation. The results show that at small β and θ, the merging rate of neighboring characteristic curves for the slow‐mode compressive waves is far greater than that for the fast‐mode compressive waves. Then we use a nonlinear example to demonstrate that at small β and θ, the slow‐mode characteristic curves merge rapidly to form a pair of slow shocks, while there is no merging of fast‐mode characteristic curves. The results explain why β and θ have an important effect on the nature of shocks.
Ground‐based evidence of latitude‐dependent cyclotron absorption of whistler mode signals originating from VLF transmitters
Authors: Clilverd Mark A. , Horne Richard B.
Published: February 2004

We report the results of the first multisite campaign using VLF Doppler receivers to detect whistler mode signals in the south Atlantic originating from VLF transmitters in the northern hemisphere. The signals at 24 and 21.4 kHz were recorded by a mobile receiver on board the RRS Bransfield during February–March 1993 and an identical system at Faraday, Antarctica. The data show that the output power of plasmaspheric ducts varies with latitude, decreasing with increasing L shell between 1.6 ≤ L ≤ 2.7. Using the HOTRAY ray tracing code, we find that the primary cause of the latitudinal variation in duct output power is absorption via cyclotron resonance with energetic electrons in the plasmasphere. The variation is due mainly to the reduction in resonant energy with increasing L shell which brings more particles into resonance with the wave. We demonstrate that ground‐based observations of the absorption of whistler mode signals originating from VLF transmitters can be used as a new way of estimating the energetic electron flux between 1 and 40 keV inside the plasmasphere between 1.6 ≤ L ≤ 2.7. Typically, estimates of electron flux of 1 to 7 times greater than that reported by Schield and Frank [1970] are required to model the experimental data recorded here during nondisturbed geomagnetic conditions.
Ionospheric F2 layer seasonal and semiannual variations
Authors: Millward G. H. , Rishbeth H. , Fuller‐Rowell T. J. , Aylward A. D. , Quegan S. , Moffett R. J.
Published: February 2004

An extensive series of computations, using the Coupled Thermosphere‐Ionosphere‐Plasmasphere model (CTIP), has been undertaken to investigate the semiannual variation in peak noontime electron density, a common feature of the F2‐layer, particularly at low latitudes and in the southern hemisphere at mid‐latitudes. Results from the model reveal such a variation, most prominently, at mid‐latitudes, in the South American sector. An analysis of this phenomenon shows that it is intimately related to the large offset of the geomagnetic axis from Earths spin axis in the southern hemisphere. Because of this offset, a given geographic latitude in the South American sector corresponds to a lower magnetic latitude than in other sectors and is thus farther from the energy inputs associated with the auroral regions. As a result, the composition changes are much smaller during the winter months than at other longitudes, the mean molecular mass being essentially constant for a 4‐month period centered on the winter solstice. This result is understood in terms of the global thermospheric circulation. In the absence of any composition changes, noon ionospheric density is influenced primarily by the solar zenith angle. This angle reaches a maximum at the winter solstice, leading to diminished ion production, a minimum in NmF2, and therefore a semiannual variation overall. On the basis of the model results, the semiannual variation is seen as a feature of the midlatitude ionosphere at geographic longitudes opposite to the location of the geomagnetic pole. This phenomenon is seen in both northern and southern hemispheres, though the effect is much larger in the southern hemisphere as a result of the greater magnetic offset.
Magnetic impulse events and associated Pc 1 bursts at dayside high latitudes
Authors: Arnoldy R. L. , Engebretson M. J. , Alford J. L. , Erlandson R. E. , Anderson B. J.
Published: February 2004

Ground measurements of bursts of Pc 1 pulsations (minutes duration) are a common dayside phenomenon at high latitudes. For the past several years, dayside ground measurements of similar‐duration bursts of Pc 5 period pulsations, magnetic impulse events (MIE), have attracted a great deal of interest concerning their role in dayside reconnection. From an analysis of several years of induction antenna data from southern and northern hemisphere sites in the Antarctic, Greenland, and Canada, it is found that about 70% of all isolated MIE events are associated with Pc 1 bursts, and about half of all events are conjugate between hemispheres. There appears to be no definitive phase relationship between the onset of the Pc 1 bursts and the MIE, suggesting that both might be produced by the same event on the dayside but have different source locations. Correlative data from ground and spacecraft measurements suggest this to be the case and also show that the Pc 1 are waves inside the magnetosphere propagating into the ionosphere. Finally, a timing study between ground sites separated by 2 hours local time shows that the isolated MIEs are measured first near the subsolar point and at later times down both flanks of the magnetosphere. This suggests that the source mechanism for these events has an E–W component of motion which when projected to the equatorial plane is of the order of the antisunward flow velocity in the magnetosheath. As this source moves down the magnetospheric flanks it creates well within the magnetosphere conditions (pressure pulses?) ripe for the ion cyclotron generation of the Pc 1 bursts associated with the MIE.
Hard X ray survey of energetic electrons from low‐Earth orbit
Authors: Feldman W. C. , Symbalisty E. M. D. , Roussel‐Dupré R. A.
Published: February 2004

Hard X ray and neutron emissions measured in low‐Earth orbit are surveyed to develop a global overview of lightning‐related energetic‐electron precipitation and acceleration processes. Comparison of geographic intensity maps shows the dominance of enhanced hard X ray intensities measured when the satellite was above the continental United States and above the southern Indian Ocean between Madagascar and Australia. This emission is most enhanced during the northern summer months. Lesser although significant enhancements are seen between the Middle East and the Tibetan Plateau, a stretch of ocean off the east Asian coast between the Philippines and Korea, a stretch of equatorial Africa from the Ivory Coast to Mozambique, a region of the eastern equatorial Pacific just west of Columbia, and a patch of the Indian Ocean stretching between the southern tip of India and Indonesia. Although emissions from many of these regions are generally enhanced during the northern summer and fall seasons, none show any regularity relative to local time of day. Many but not all of these enhancements support natural interpretations in terms of lightning‐induced energetic‐electron precipitation from the terrestrial trapped radiation belts. Electron scattering induced by radio waves from VLF transmitters most likely contributes to this precipitation.
Survey of discrete hard X ray enhancements observed from low‐Earth orbit
Authors: Feldman W. C. , Symbalisty E. M. D. , Roussel‐Dupré R. A.
Published: February 2004

Systematics of discrete hard X ray enhancements detected using the Army Background Experiment between April 1990 and October 1992 are presented. A total of 822 enhancement events were detected, most often within two bands of geomagnetic latitude that overlay (1) the inner trapped radiation belt and (2) the slot between inner and outer radiation belts. Local hour and seasonal variations are strongest for the innerzone events, which occur preferentially just after local dawn and during the northern summer months.
Wavelength and direction filtering by magnetic measurements at satellite arrays: Generalized minimum variance analysis
Authors: Motschmann U. , Woodward T. I. , Glassmeier K. H. , Southwood D. J. , Pinçon J. L.
Published: February 2004

The main goal of the Cluster mission, consisting of four identical spacecraft, is the spatial résolution of plasma structures. For the determination of the wave vectors of a wave field from four positions, classical Fourier analysis is inappropriate. We develop a generalized minimum variance technique which gives a high wave vector resolution though the spatial grid is restricted to only a few sampling positions. This technique uses the amplitude and phase information of the magnetic field from the four satellite positions and determines the optimum wave field corresponding to the measured data. The components of the magnetic field are assumed to be normally distributed. The divergence‐free nature of the magnetic field is used as a constraint. Using the magnetic data measured at four positions allows up to seven different wave vectors at one frequency to be uniquely resolved.
Breakdown of the neutral atmosphere in the D region due to lightning driven electromagnetic pulses
Authors: Rowland H. L. , Fernsler R. F. , Bernhardt P. A.
Published: February 2004

Electromagnetic pulses (EMP) driven by lightning can cause breakdown of the neutral atmosphere in the lower D‐region. Using a computer simulation model, we study the dependence of the breakdown on the pulse strength, the orientation of the lightning discharge, the ambient plasma density, the ionization model, and the neutral density. For a discharge along a straight line the EMP is strongest in the plane perpendicular to the current so that for a given current, horizontal discharges will radiate the D‐region more strongly than a vertical discharge. For horizontal currents, breakdown occurs for E100 > 20 V/m (I > 55 kA) in a low‐density, nighttime ionosphere, where E100 is the amplitude of the pulse normalized to 100 km from the discharge and I is the discharge current. Vertical strokes require E100 > 50 V/m (I > 140 kA). Discharges with higher currents and fields form ionization patches which are larger in volume, larger in degree of ionization, and lower in altitude. The ionization is most sensitive to the pulse strength, pulse orientation, ambient plasma density, and neutral gas density at breakdown threshold. Higher ambient plasma densities reduce the ionization, but for large EMP, breakdown can occur even with high daytime densities. The breakdown increases the plasma density which acts to limit the EMP and ionization. This feedback reduces the sensitivity of the breakdown to the ionization model. Neutral density variations, such as caused by atmospheric gravity waves, can cause spatial variations in the ionization density.
Present status of the recovery phase of cosmic ray 11‐year modulation
Authors: Ahluwalia H. S. , Wilson Margaret D.
Published: February 2004

The recovery from cosmic ray 11‐year modulation follows two distinct repetitive patterns [Ahluwalia, 1994]. When the magnetic polarity of the Sun in the northern hemisphere is negative (qA < 0), the recovery is completed in 5 to 8 years but for the positive magnetic polarity epochs (qA > 0) the recovery period is reduced to less than half as much. These repetitive patterns have been observed for the last five solar activity cycles (17 to 21). The recovery for cycle 22 is still in progress. By the end of 1994, the intensity monitored with the nuetron detectors at Deep River, Climax, and Huancayo continues to be significantly below the cosmic ray maxium observed in March 1987. The physical significance of our results is discussed.
Cross polar cap diameter and voltage as a function of PC index and interplanetary quantities
Authors: Troshichev O. , Hayakawa H. , Matsuoka A. , Mukai T. , Tsuruda K.
Published: February 2004

Measurements of precipitating particles and the electric field on board EXOS D spacecraft for the period January–June 1990 have been used for estimation of the diameter of the polar cap along dawn‐dusk meridian and the polar cap voltage. Identification of the polar cap boundaries has been made on the basis of specific features of precipitating ions. The data on the polar cap boundary location obtained for different geophysical conditions have been used to derive the statistical relationship between the polar cap diameter and PC index. The analysis has shown an approximately linear relationship between the polar cap diameter and the PC index for values PC < 3, the diameter tending to be asymptote when the PC index reaches large positive values. Cross polar cap voltage derived from EXOS D data is in good correlation with interplanetary quantities including the interplanetary magnetic field (IMF) southward component. The best correlation is obtained for the merging electric field υBT sin2 θ/2, with a coefficient of correlation higher than 0.82. Almost the same correlation is observed between polar cap voltage and PC index. The effect of “saturation” is not traced in the voltage dependencies on the PC index and interplanetary quantities up to values BT ≤ 10 nT.
Neutral line model of substorms: Past results and present view
Authors: Baker D. N. , Pulkkinen T. I. , Angelopoulos V. , Baumjohann W. , McPherron R. L.
Published: February 2004

The near‐Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere‐tail coupling, large‐scale mapping, onset triggering, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near‐tail, midtail, and deep tail features of substorms. Nonetheless, the NENL model still seems to provide the best available framework for ordering the complex, global manifestations of substorms.
Anisotropic three‐dimensional MHD turbulence
Authors: Matthaeus William H. , Ghosh Sanjoy , Oughton Sean , Roberts D. Aaron
Published: February 2004

Direct spectral method simulation of the three‐dimensional magnetohydrodynamics (MHD) equations is used to explore anisotropy that develops from initially isotropic fluctuations as a consequence of a uniform applied magnetic field. Spectral and variance anisotropies are investigated in both compressible and incompressible MHD. The nature of the spectral anisotropy is consistent with the model of Shebalin et al. [1983] in which the spectrum broadens in the perpendicular wavenumber direction, the anisotropy being greater for smaller wavenumbers. Here this effect is seen for both incompressible and polytropic compressible MHD. In contrast, the longitudinal (compressive) velocity fluctuations remain isotropic. Variance anisotropy is observed for low plasma beta compressible MHD but not for incompressible MHD. Solar wind observations are qualitatively consistent with both variance and spectral anisotropies of the type discussed here.
Behavior of wave packets and pulse compression in the ionosphere
Authors: Albright B. J. , Cowley S. C.
Published: February 2004

Many interesting effects, e.g., the generation of short‐wavelength plasma turbulence, can arise when ground‐based transmitters irradiate the ionosphere with radio frequency (RF) waves. The thresholds and growth rates of these effects typically depend on the intensities of the pump fields, sparking interest in finding an inexpensive way to increase the intensities of these fields. A method has been proposed [Cowley et al., 1993] for generating a high‐intensity RF pulse in the ionosphere by launching a frequency‐chirped pulse of much lower intensity from a ground‐based antenna. The dispersive properties of the ionosphere compress the pulse, generating a short, intense pulse. A one‐dimensional model with a linear density profile has shown that pulse compression can lead to significant intensity enhancements, up to a factor of 100 over steady state field intensities. Pulse compression is modeled in ionospheres with more realistic density profiles than have been considered previously. A novel asymptotic technique is developed for finding wave packet solutions in general, slowly varying density profiles. Various factors affecting compression, such as launch frequency and frequency bandwidth, are discussed. Large pulse compression requires a good knowledge of the plasma parameters, particularly the plasma density near the compression point. The sensitivity to incomplete knowledge of the density profile is investigated. Information about the density profile is contained in the reflected pulse. A simple algorithm for obtaining this information is derived.
Equatorial ionospheric vertical plasma drift model over the Brazilian region
Authors: Batista I. S. , de Medeiros R. T. , Abdu M. A. , de Souza J. R. , Bailey G. J. , de Paula E. R.
Published: February 2004

Comparison between equatorial inospheric F region vertical plasma drift from satellite measurements [Fejer et al., 1995], for the Brazilian longitude sector, and the drifts derived from ionosonde measurements around sunset shows significant differences on the prereversal peak behavior during solstices of high solar activity periods. Using ionosonde measurements around sunset and satellite measurements at other local times, we constructed an ionospheric vertical plasma drift model that is representative of the equatorial region over the Brazilian longitudes, where the magnetic declination is around −20°. The so derived drift model, here called IDM (ionosonde drift model), is used as an input to the Sheffield University plasmasphere‐ionosphere model (SUPIM). It is shown that the F layer heights given by SUPIM with IDM are in good agreement with ionosonde measurements over the Brazilian longitudes and that IDM better simulates the F layer heights than the averaged drifts given by the satellite drift model [Fejer et al., 1995].
Dynamics of the inner magnetosphere near times of substorm onsets
Authors: Maynard N. C. , Burke W. J. , Basinska E. M. , Erickson G. M. , Hughes W. J. , Singer H. J. , Yahnin A. G. , Hardy D. A. , Mozer F. S.
Published: February 2004

The electrodynamics of the inner magnetosphere near times of substorm onsets have been investigated using CRRES measurements of magnetic and electric fields, energetic electron fluxes, in conjunction with ground‐based observations. Six events were studied in detail, spanning the 2100 to 0000 MLT sector and L values from 5 to 7. In each case the dawn‐dusk electric field was enhanced over typical background electric fields, and significant, low‐frequency pulsation activity was observed. The amplitudes of the pulsations were larger than the background electric fields. Dusk‐dawn excursions of the cross‐tail electric field often correlated with changes in currents and particle energies at CRRES and with ULF wave activity observed on the ground. Variations of the electric field and Poynting vectors with periods in the Pi 2 range are consistent with bouncing AlfVén waves that provide electromagnetic communication between the ionosphere and plasma sheet. Magnetic signatures of field‐aligned current filaments directed away from the ionosphere, presumably associated with the substorm current wedge, were observed during three orbits. In all cases, ground signatures of substorm expansion were observed at least 5 min before the injection of electrons at CRRES. Field‐aligned fluxes of counter‐streaming, low‐energy electrons were detected after three of the injections. We develop an empirical scenario for substorm onset. The process grows from ripples at the inner edge of the plasma sheet associated with dusk‐dawn excursions of the electric field, prior to the beginning of dipolarization. Energy derived from the braking of the inward plasma convection flows into the ionosphere in the form of Poynting flux. Subsequently reflected Poynting flux plays a crucial role in the magnetosphere‐ionosphere coupling. Substorms develop when significant energy (positive feedback?) flows in both directions, with the second cycle stronger than the initial. Pseudobreakups occur when energy flow in both directions is weak (negative feedback?). “Explosive‐growth‐phase” signatures occur after onset, early in the substorm expansion phase. Heated electrons arrive at the spacecraft while convection is earthward, during or at the end of electromagnetic energy flow away from the ionosphere.
Midlatitude E region coherent backscatter observed simultaneously at two HF radar frequencies
Authors: Haldoupis C. , Bourdillon A. , Six M. , Delloue J.
Published: February 2004

We present results from a dual‐frequency ionospheric backscatter experiment carried out with the Valensole HF radar facility in the south of France. The radar was operated in a mode that allowed detailed Doppler spectrum recordings of coherent echoes from midlatitude E region irregularities simultaneously at 9.0 MHz and 12.4 MHz, corresponding to aspect‐sensitive backscatter from plasma waves with 16.7‐m and 12.1‐m wavelength, respectively. In this decameter wave band and in the presence of intense nighttime sporadic Es layers that contribute strong destabilizing density gradient components perpendicular to B, the E region is susceptible to direct plasma wave generation by the gradient drift instability even if electric fields are only a few mV/m. We observe that 9.0‐MHz echoes are stronger, more frequent, and more spatially extended than the 12.4‐MHz ones, which is in line with the threshold requirements of the gradient drift instability. Further, the data supports the notion of primary and secondary waves with spectral shapes similar to those seen, for example, at the equator. In one case, a situation analogous to the equatorial electrojet was clearly seen, in which the Doppler spectrum changed from a negatively shifted narrow peak (type I‐like) to a broad type II near zero shift and then to a positively shifted narrow type I‐like component, in the radar's azimuthal sector span from 22° east to 38° west of geomagnetic north. We have studied the ratio of the observed 12.4‐MHz versus 9.0‐MHz Doppler velocities in an attempt to test the widely used hypothesis that primary gradient drift waves have their phase velocity limited at instability threshold values. For simultaneous velocities in the range from 40 to 120 m/s, the V12.1 m/V16.7 m ratio was found on the average close to 1.1 and thus quite small compared to the anticipated value of 1.6 if the waves were to propagate at instability threshold values. Instead, the evidence is more supportive of the theory's dispersion relationship which requires that the wave phase velocity matches the relative electron‐ion drift along the direction of propagation.