bluuebunny/crossref_metadata_2025 huggingface dataset:
An observational test of the Tsyganenko (T89a) model of the magnetospheric field
Authors: Thomsen M. F. , McComas D. J. , Reeves G. D. , Weiss L. A.
Published: February 2004

The range of magnetic field tilt angles observed at several satellites in geosynchronous orbit is compared with the ranges predicted for the same locations by the different Kp parameterizations of the Tsyganenko 1989a magnetic field model as a function of local time and season. The data are examined separately for satellite locations near the magnetic equator and slightly off the equator. The model predicts reasonably well the observed basic variation in the tilt angle with location, and it permits a range of field inclinations adequate to encompass the majority of the observed angles for the dawn, dusk, and night quadrants. On the day side of the magnetosphere the model exhibits very little variation in tilt over the entire range of parameterizations and cannot reproduce the observed range of tilt angles. Near the magnetic equator the majority of observed tilt angles lie within the model range, with roughly equal numbers of cases that are overstretched or understretched with respect to the model range. Off the equator the models tend to be more stretched than is generally observed. With some modest season‐to‐season differences these results are valid for all four seasons.
Lower hybrid oscillations in multicomponent space plasmas subjected to ion cyclotron waves
Authors: Khazanov G. V. , Krivorutsky E. N. , Moore T. E. , Liemohn M. W. , Horwitz J. L.
Published: February 2004

It is found that in multicomponent plasmas subjected to Alfvén or fast magnetosonic waves, such as are observed in regions of the outer plasmasphere and ring current‐plasmapause overlap, lower hybrid oscillations are generated. The addition of a minor heavy ion component to a proton‐electron plasma significantly lowers the low‐frequency electric wave amplitude needed for lower hybrid wave excitation. It is found that the lower hybrid wave energy density level is determined by the nonlinear process of induced scattering by ions and electrons; hydrogen ions in the region of resonant velocities are accelerated; and nonresonant particles are weakly heated due to the induced scattering. For a given example, the light resonant ions have an energy gain factor of 20, leading to the development of a high‐energy tail in the H+ distribution function due to low‐frequency waves.
Role of turbulent dissipation and thermal convection in solar wind's temperature evolution
Authors: Verma Mahendra K.
Published: February 2004

In this brief report, we calculate the turbulent dissipation rates in the solar wind using the Kolmogorov‐like MHD turbulence phenomenology with Kolmogorov's constants calculated by Verma and Bhattacharjee [1995]. We find that the turbulent heating cannot account for the total heating of the non‐Alfvénic streams in the solar wind. Solar wind observations indicate that the thermal conduction could contribute significantly to the temperature evolution of the solar wind plasma. In this paper we theoretically estimate the contribution of turbulent thermal convection to the temperature evolution; the theoretical estimates are consistent with observations.
Observations and simulations of VLF harmonic generation with the high‐power auroral simulation array
Authors: Rowland H. L. , Keskinen M. J. , Villaseñor J. S. , Wong A. Y.
Published: February 2004

We report experimental results from the high‐power auroral simulation facility showing that the lower D region can be heated so as to produce either a single, narrowband ELF/VLF radiation signal at the modulation frequency or a signal containing the modulation frequency and many odd harmonics. The measurements are compared to a new 2 1/2‐dimensional simulation code that models the lower D region and the Earth ionospheric waveguide. The simulation spectra show good agreement with the observations. Rapidly switching the HF power off and on leads to perturbing currents in the D region that vary in time in a square wave type pattern. This generates an ELF/VLF wave with high, odd harmonics observed up to n = l3 (ƒ = nƒ0, where ƒ0 is the modulating frequency). A more smoothly changing power variation generates a narrowband signal at ƒ0. Simulations show how the D region affects the spectral shape. From these results we find that the heating should occur at a low altitude in the D region (approximately 75 km). Using the simulation results, we determine that the D region current needed to produce the magnetic field spectrum with the odd harmonics has a square wave time variation.
Chemistry of the nightside ionosphere of Mars
Authors: Haider S. A.
Published: February 2004

The chemistry of seven ions, O2+, NO+, CO2+, O+, N2+, CO+, and H+, is studied in the nightside ionosphere of Mars using the analytical yield spectrum approach and coupled continuity equations for the steady state condition. The source and sink processes of these ions are discussed in detail. We have found that the nightside Martian ionosphere produced due to precipitation of magnetotail electrons agrees with the Viking observations rather than other sources of ionization such as the precipitation of plasma sheet electron or horizontal plasma transport from dayside ionosphere to the nightside similar to that operating on Venus, The precipitation of magnetotail electron produces major ion CO2+ which is quickly removed by the reactions (1) CO2+ + O → O2+ + CO, (2) CO2+ + O → O+ + CO2 and (3) O+ + CO2 → O2+ + CO leading to O2+ as a dominant ion in the nightside ionosphere of Mars. The reactions of O+ with N and NO are the major sources of NO+, which is lost entirely by dissociative recombination reaction. The second reaction is the major source of O+ below 190 km. Above this altitude the magnetotail electron impact ionization process is an important source of O+. This process is also important for the production of CO+ and N2+. The ion O+ is destroyed by the third reaction, while CO+ and N2+ are destroyed by charge transfer reaction with CO2. The ion H+ is produced due to charge exchange reaction between O+ and H which is lost by CO2.
HF echoes from ionization potentially produced by high‐altitude discharges
Authors: Roussel‐Dupré R. A. , Blanc E.
Published: February 2004

The presence of ionization associated with high‐altitude discharges has been detected using an HF radar operating at 2.2, 2.5, and 2.8 MHz. On several occasions, oblique echoes lasting several hundred ms at night and 1 → 10s during the day were observed. The echoes turned on in several interpulse times of 70 ms and were generally correlated with strong lightning activity prior to onset. The angles of arrival of sferics detected at three goniometer stations were used to determine the distance to thunderstorms. The data are consistent with specular reflections from columns of ionization produced at 55–65 km altitude and having minimum electron densities of 6 × 104 – 105 cm−3. The source of the ionization is believed to be high‐altitude discharges.
Ulysses assessment of the Jovian planetary field
Authors: Dougherty M. K. , Balogh A. , Southwood D. J. , Smith E. J.
Published: February 2004

The magnetic field measurements made by the Ulysses dual fluxgate/vector helium magnetometer during the day of closest approach to Jupiter in February 1992 are reported. The data are compared to a model which describes the field in terms of an internal spherical harmonic component together with an external current sheet field. The sensitivity of the magnetic field model parameters is examined. It is concluded that the dipole term which best fits the Ulysses closest approach data is lower than that used in most recent models and is closer in value to that found for the Pioneer 11 epoch models. Our favored value of the dipole term is 4.059 G. The contribution of the external ring current is weaker than that observed during the Voyager epoch, although the sheet is in fact thicker. The lower dipole value is also found to produce a good fit to the Pioneer 11 and Voyager 1 closest approach data sets, and hence there is no evidence for any secular change in the planetary field at Jupiter.
On the generation of an electron beam by Io
Authors: Crary F. J.
Published: February 2004

The interaction between Io and the Jovian magnetosphere generates an Alfvénic disturbance which propagates away from Io. The effect of finite electron inertia on this Alfvén wave, which was not considered in previous models, is shown to produce a parallel electric field but to have no other, significant effects on the Io‐generated waves. The propagation of this disturbance is described analytically, treating it as the result of an impulsive disturbance rather than a continuous wave, and the solution does not require magnetic field strength to remain constant, as previous, similar models did. The wave is reflected by changing density and magnetic field strength as it propagates out of the Io plasma torus. Essentially none of power from the Io interaction reaches Jupiter as an Alfvén wave. However, the wave's parallel electric fields produce an electron beam, through a process of repeated Fermi acceleration. This beam has an estimated power of 0.5 to 1.5×1011 W and the average particle energy is of order 75 keV. This beam is consistent with many observations of Io‐related phenomena at high latitudes.
Study of the ionospheric variability within the Euro‐Asian sector during the Sundial/Atlas 1 mission
Authors: Pulinets S. A. , Yudakhin K. F. , Evans D. , Lester M.
Published: February 2004

In order to quantify, and to identify possible origins of, subauroral ionospheric variability during periods of moderate geomagnetic activity, ionospheric observations taken during the SUNDIAL/ATLAS ‐ 1 campaign (March 24 to April 2) from 10 stations were analyzed in conjunction with observations from EISCAT, geomagnetic observations from magnetometer networks in Scandinavia and the United Kingdom, and auroral particle energy input observations from the NOAA ‐ 12 satellite. The network of ionospheric stations spanned longitudes from 13°E to 90°E but were relatively confined in geomagnetic latitudes so that longitudinal and local time dependencies in ionospheric variability are more clearly exposed. The ionospheric observations were analyzed in terms of both ΔƒoF2, the difference between the hourly ƒoF2 at a given station and the monthly median ƒoF2 for that hour, and a new daily variability index AƒoF2. The analysis using both parameters demonstrated an apparent longitudinal variation in ionospheric variability with a reversal at about 55° E from a negative to a positive phase in the departure of ionospheric conditions from their median values. An analysis of these ionospheric data in conjunction with the NOAA/TIROS estimates of power deposition by auroral particles demonstrated a significant local time dependence in midlatitude ionospheric responses to auroral activity. This dependence may arise from the premidnight to postmidnight asymmetry in high‐latitude convection electric fields.
Solar cycle dependence of the solar wind dynamics: Pioneer Voyager, and Ulysses from 1 to 5 AU
Authors: González‐Esparza J. Américo , Smith Edward J.
Published: February 2004

Significant differences between Pioneer and Voyager observations were found in solar wind structure between 1 and 6 AU. These disagreements were attributed to temporal effects related to the solar cycle, but no unifying study of Pioneer‐Voyager observations was performed. On the basis of maps of large‐scale features we unify previous reports of Pioneers 10 and 11, Voyagers 1 and 2, and Ulysses observations from 1 to 5 AU. The five spacecraft traveled from Earth to Jupiter at different phases of the solar sunspot cycle. We compare the observations of solar wind streams, interplanetary shock waves, interaction regions, and magnetic sectors. We find that solar wind dynamics has a very irregular behavior throughout the solar cycle. There are continual transitions between periods of a few solar rotations dominated by slow solar wind, transient events, and irregular patterns of magnetic sectors; and periods of a few solar rotations dominated by interaction regions and well‐defined magnetic sectors. These transitions occur at temporal scales of the order of 2–4 solar rotations. It is suggested that these transitions are associated with changes in coronal holes and might be related to the yearly modulation of solar wind speeds. The five spacecraft observed, on average, about 3 to 4 interplanetary shocks per solar rotation period. The 90 percent of the total number of shocks detected by Pioneer 11 were caused by interaction regions. However, between 40 to 55 percent of the total number of shocks detected by the other spacecraft were transient forward shocks. The expansion rate of the CIRs between 1 to 5 AU is about 94 km/s, however, there is a significant diversity in the characteristics of the interaction regions.
Interaction of interplanetary shocks with the heliospheric termination shock: Two‐dimensional magnetohydrodynamic model
Authors: Baranov Vladimir B. , Barmin Aleksei A. , Pushkar' Eugene A.
Published: February 2004

The two‐dimensional problem of the oblique interaction between an interplanetary shock (IPS) and the termination shock (TS) with allowance for the effect of the interplanetary magnetic field is studied within the framework of the ideal magnetohydrodynamic model and a steady model of the solar wind interaction with the local interstellar medium. The self‐consistent axisymmetric model proposed by Baranov and Malama, which takes into account the processes of resonant charge exchange between H atoms and protons, is used. The electron number density and the solar wind velocity VSW ahead of TS depends on the parameters of this model. The postinteraction configuration consists of a new moving TS′, a transmitted IPS′, a set of Alfvén and slow magnetohydrodynamic waves, and a contact discontinuity which can change as functions of five dimensionless governing parameters and the specific heat ratio γ. The maximum electron number density ne is usually reached behind the new termination shock TS′ for an oblique (θ ≠ 0) impingement of IPS on TS rather than for the head‐on collision (θ = 0). Nevertheless, the maximum calculated values of ne are lower than those corresponding to plasma frequencies ωp ≅ 2 kHz observed by the Voyager spacecraft. It is found that all the quantities considered depends radically on the angle ψTS between the front of TS and the vector of the interplanetary magnetic field strength BSW.
Observations of slow‐mode shocks in Earth's distant magnetotail with the Geotail spacecraft
Authors: Seon J. , Frank L. A. , Paterson W. R. , Scudder J. D. , Coroniti F. V. , Kokubun S. , Yamamoto T.
Published: February 2004

Three crossings of the lobe‐plasma sheet boundaries encountered within Earth's distant magnetotail with the Geotail spacecraft are analyzed in terms of slow‐mode shocks. Quantitative comparisons with theoretical predictions for a steady state, one‐dimensional shock are made based upon measurements of the three‐dimensional velocity distributions of protons and electrons and of the magnetic fields. The observed plasma moments and magnetic field upstream from the shock are successfully used to predict these parameters in the downstream region. Slow‐mode Mach numbers in the upstream and downstream regions are also shown to be above and below unity, respectively, as required for the slow‐mode shock transition. However, a consideration of the relative uncertainties for the upstream and downstream slow‐mode Mach numbers allows convincing identifications for only two of the boundaries as slow‐mode shocks. Low number densities of plasmas for the other boundary, ∼0.1/cm3, are responsible for the large relative uncertainties for the Mach numbers. This analysis of slow‐mode shocks, together with a comprehensive accounting of statistical uncertainties and variances, allows us to provide firm evidence for the existence of slow‐mode shocks in Earth's distant magnetotail.
The Rayleigh‐Taylor instability is not damped by recombination in the F region
Authors: Huba J. D. , Bernhardt P. A. , Ossakow S. L. , Zalesak S. T.
Published: February 2004

The collisional Rayleigh‐Taylor instability plays a crucial role in explaining the onset and development of equatorial spread F. A number of linear and nonlinear analyses of the instability have been developed for the F region that include recombinative damping. It has been argued that recombination damps the unstable density perturbations and provides a mechanism to suppress the onset of instability or saturate the instability via mode coupling. We show that F region recombination, in fact, does not damp unstable density perturbations relative to the background ionosphere. Recombination acts to reduce the total electron density, both the ambient background and the perturbation, at a rate VR.
The evolution of Alfvénic perturbations in a three‐dimensional MHD model of the inner heliospheric current sheet region
Authors: Stribling T. , Roberts D. A. , Goldstein M. L.
Published: February 2004

We investigate an idealized model of the evolution of magnetohydrodynamic (MHD) scale velocity and magnetic field fluctuations in the equatorial region of the inner heliosphere at solar minimum. In this model a three‐dimensional current sheet is constructed from a sheared magnetic field embedded between two velocity shear layers. Simulations of the incompressible MHD equations are used to study the evolution of a variety of perturbations to this configuration. We also perform a linear stability analysis using the solutions of the Orr‐Sommerfeld and related equations for the current sheet model. From simulations in which the model is perturbed with small amplitude Alfvén waves, we find that the perturbing velocity and magnetic fields become decorrelated in the region of velocity and magnetic shear during the linear evolution of the fluid instability. This result and similar results for large amplitude perturbations agree qualitatively with previous two‐dimensional simulations and with observations made by the Helios spacecraft at current sheet crossings between 0.3 and 1 AU and gives further evidence that shear‐driven fluid instabilities provide a mechanism for the destruction of velocity‐magnetic field correlation. The simulations also show enhancement of the power in the radial components of the perturbing velocity and magnetic fields in regions where they are sheared, in agreement with observations. We show that the linear evolution of transverse perturbations to the sheared fields gives rise to these radial power enhancements. While these simulations elucidate the three‐dimensional evolution of shear flows, they must be extended further to include compression and expansion of the wind.
On the origins of the upward shift of elevated (bimodal) ion conics in velocity space
Authors: Miyake W. , Mukai T. , Kaya N.
Published: February 2004

We present a statistical study of elevated (bimodal) ion conics observed by the Exos D (Akebono) satellite from 0900 to 1500 magnetic local time (MLT). We especially focus on a comparative analysis of elevated conics with standard conics and ion beams. Elevated conics are observed mainly above 6,000 km, while standard conics begin to appear at lower altitudes. The energy of elevated conics is usually higher than that of standard ion conics. This is consistent with the idea that the elevated conics evolve from standard conics through some acceleration process while they flow up to high altitudes. Since the elevated conics studied here received additional energization as they evolve from standard conics, the velocity filter mechanism previously invoked to explain elevated conics is inconsistent with our results. Our observation suggests that two kinds of acceleration processes are responsible for the upward shift in the velocity distribution of elevated conics. The elevated conics with a large cone angle and small upward shift in velocity space are most likely to be caused by the perpendicular energization extended along the field line. The MLT occurrence of this subset of elevated conics is similar to that of standard conics. The ion conics with a cone angle near 60° are found to be the most energetic, which strongly suggests the extended energization. On the other hand, the elevated conics with a small cone angle and large upward shift are mainly due to the parallel acceleration acting in tandem with perpendicular energization. This subset of elevated conics is found more often in the afternoon sector than are standard conics, which evokes rather some similarity to ion beams.
Statistics of gravity waves seen in O2 nightglow over Bear Lake Observatory
Authors: Wiens R. H. , Wang D. Y. , Peterson R. N. , Shepherd G. G.
Published: February 2004

Fluctuations in the O2 nightglow column emission rate and rotational temperature were measured over Bear Lake Observatory on 63 nights, 11 of them nearly coinciding with a mesosphere‐lower thermosphere coupling study (MLTCS) campaign on January 20–30, 1993. The instrument allows for simultaneous measurement at 12 points in the sky every 2.4 min. From these the frequency, the horizontal wavelength, and the propagation direction can be measured directly, and from these the horizontal phase velocity and the vertical wavelength can be derived. Frequency spectra are shown to have slopes near to the widely accepted −5/3 law. Correction to intrinsic frequency was made by using the simultaneously acquired winds from the satellite‐borne wind imaging interferometer (WINDII). The relation between frequency and horizontal wavenumber was scattered rather than clearly linear, and there was no obvious boundary suggesting severe mesospheric filtering when the vertical wavenumber was plotted against the horizontal. Zonal phase speeds were favored during the January period, but on average the waves were isotropic at speeds above 30 m/s. Absorption of westward traveling waves at speeds less than 30 m/s was evident.
Reexamination of driven and unloading aspects of magnetospheric substorms
Authors: Baker D. N. , Klimas A. J. , Vassiliadis D. , Pulkkinen T. I. , McPherron R. L.
Published: February 2004

It is widely accepted that substorms consist of both directly driven and loading‐unloading processes. However, a recent study has presented results which suggested that over 90% of the auroral electrojet (AE) variation was directly predictable from the solar wind variations alone. This would imply that only a small residual in the AE variability is due to internal magnetospheric dynamics. The present paper considers nonlinear dynamical models of the global solar wind‐magnetosphere interaction and uses the observed, highly variable solar wind electric field (VBs) to drive the Faraday loop analogue model. It is found that it is critically important to include magnetotail unloading in the model in order to replicate the main features of geomagnetic activity: with just the driven response in the model, one does not obtain realistic time behavior of the model AL index. Thus these results show quite clearly that both driven and unloading processes must be included in a realistic model of geomagnetic activity.
Simultaneous ground‐satellite observations of structured Pc 1 pulsations
Authors: Erlandson R. E. , Mursula K. , Bösinger T.
Published: February 2004

Structured Pc 1 pulsations are investigated using simultaneous multipoint ground‐satellite observations recorded on September 10, 1986, during Viking orbit 1103. The multipoint Pc 1 observations were acquired using the Viking magnetic field experiment at 13,550 km altitude from L = 5.1 to 5.5 and three Finnish ground stations at Rovaniemi, Ivalo, and Kilpisjärvi. These stations are all located within 30 min magnetic local time and 1° in latitude of the ionospheric field line footprint of the Pc 1 source field line. Structured Pc 1 pulsations between 0.5 and 1.0 Hz were observed both on the ground and in space at similar frequencies, with similar frequency dispersion, and with a similar repetition period. The wave source region, based on Viking Langmuir probe observations, was just inside the plasmapause. The wave transit time between Viking and the ground was 12 ± 2 s, where Viking leads the ground. This implies that the waves propagate downward from Viking to the ground, consistent with previous downward Poynting flux estimates.
Ponderomotive force acceleration of ions in the auroral region
Authors: Shukla P. K. , Stenflo L. , Bingham R. , Dendy R. O.
Published: February 2004

A general description of ion acceleration by the ponderomotive forces of circularly polarized electromagnetic waves with arbitrary frequency in magnetized plasmas is presented. The time derivative parallel ponderomotive force turns out to play an important role for nonstationary ion acceleration. Our results can be useful in understanding the acceleration of escaping ions in the presence of intense electromagnetic waves in the auroral zone of the Earth's ionosphere.
Lower ionospheric cyclotron maser theory: A possible source of 2ƒce and 3ƒce auroral radio emissions
Authors: Yoon Peter H. , Weatherwax A. T. , Rosenberg T. J. , LaBelle J.
Published: February 2004

A lower ionospheric electron cyclotron maser has recently been suggested as a possible source of natural radio emissions that have been observed with ground‐based instruments in the auroral zone. According to this theory, the observed emissions are interpreted as X mode electromagnetic waves at 2 and/or 3 times the ionospheric electron cyclotron frequency (ƒce). In the present study we carry out a more thorough analysis of the cyclotron maser instability, using a more complete growth rate expression and more realistic energetic electron distribution functions. Growth rates of various electromagnetic modes and harmonics are calculated. It is shown that growth rates for 2ƒce and 3ƒce X modes can exceed collisional ionospheric damping rates, confirming the original estimations. However, the present analysis also reveals that O mode waves at the fundamental cyclotron frequency, ƒce, should also be excited at the source. The propagation of excited electromagnetic waves from the source region to the ground inside a model horizontal density cavity structure is also examined by means of ray tracing technique. It is found that the O mode wave is unable to reach the ground, an explanation of why the emissions at ƒce are not detected on the ground.
Heating and activity of the solar corona: 3. Dynamics of a low beta plasma with three‐dimensional null points
Authors: Galsgaard Klaus , Nordlund Åke
Published: February 2004

We investigate the self‐consistent nonlinear evolution of an initially force‐free three‐dimensional magnetic field subjected to stress on two boundaries. The results illustrate how complicated magnetic field structures, such as those found in the solar corona, evolve dynamically when forced by stress from boundaries and how the energy which is temporarily stored in the magnetic field may be converted into other forms of energy such as heat, flow energy, and fast particles. The initial model state is triple periodic and contains eight magnetic null points. During the time evolution, the current density concentrates near particular locations in space that can be identified with the singular field lines connecting pairs of null points of the initial state. Current sheets are found to grow out of the singular lines formed by the intersection of surfaces across which the magnetic connectivity is discontinuous. Jets of plasma shoot out from the edges of the currents sheets, driven by the “sling‐shot” Lorentz force created by reconnecting magnetic field lines. As a result of the reconnection, most of the magnetic connectivity between the two boundaries is lost, and the remaining magnetic field develops arcade‐like structures along the boundaries. These arcade structures are long‐lived, and the system enters a quasi‐stationary state, where small‐scale current sheets are continually appearing and disappearing. The distribution of size of these current sheets is limited only by the numerical resolution. The current sheets dissipate the energy supplied by the boundary driving and also slowly deplete the magnetic energy from the initial constant alpha magnetic field. The dissipation occurs in an increasing number of current sheets of decreasing size at higher numerical resolution, which keeps the overall reconnection rate nearly independent of the numerical resolution. This suggests that “fast reconnection” may occur through the collaborative effort of a large number of many small‐scale current sheets, rather than in the very large magnetic Reynolds number limit of single current sheets that has been so extensively discussed in the literature. This has important applications to both the problem of understanding coronal heating and the search for efficient flare energy release mechanisms.
Investigations of thermospheric‐ionospheric dynamics with 6300‐Å images from the Arecibo Observatory
Authors: Mendillo Michael , Baumgardner Jeffrey , Nottingham Daniel , Aarons Jules , Reinisch Bodo , Scali James , Kelley Michael
Published: February 2004

Pilot observations were conducted at Arecibo, Puerto Rico, using an all‐sky, image‐intensified CCD camera system in conjunction with radar, ionosonde, and Global Positioning System (GPS) diagnostic systems during the periods January 19–28, 1993, and February 21 to August 22, 1995. These represent the first use of campaign mode operations of an imager at Arecibo for extended periods of F region observations. The January 1993 period (the so‐called “10‐day run”) yielded a rich data set of gravity wave signatures, perhaps the first case of direct imaging of thermospheric wave train properties in the F region. The 6‐month 1995 campaign revealed two additional optical signatures of F region dynamics. A brightness wave in 6300 Å passing rapidly through the field of view (FOV) has been linked to meridional winds driven by the midnight temperature maximum (MTM) pressure bulge. On May 3, 1995, during a period of geomagnetic activity, a 6300‐Å airglow depletion pattern entered the Arecibo FOV. Such effects represent the optical signatures of equatorial spread F instabilities that rise above the equator to heights near 2500 km, thereby affecting Arecibo's L = 1.4 flux tube.
A note on the interchange stability criterion
Authors: Liu W. William
Published: February 2004

The correct stability criterion of interchange motions in the magnetosphere has been a question of some controversy. Many researchers hold the view first espoused by Gold [1959]. For a low‐β plasma in a dipole magnetic field, Gold argued that the gradient of pVγ, where p is the plasma pressure, V is the unit flux tube volume, and γ is the adiabatic polytropic index, determines the interchange stability. This notion was further developed by Chandrasekhar [1960]. This common belief was questioned by Cheng [1985], who was subsequently questioned by Rogers and Sonnerup [1986] and Southwood and Kivelson [1987] on the issue. Is Gold's criterion the necessary and sufficient condition for interchange stability? We shall demonstrate that the answer is “no” and provide a more accurate criterion for the dipole magnetic field. We shall also discuss the interchange instability in a plasma distribution with a longitudinal gradient.
Linear gradient drift instabilities in the daytime equatorial electrojet
Authors: Hu S. , Bhattacharjee A. , Harrold B. G.
Published: February 2004

Linear gradient drift instabilities, which are associated with type II irregularities in the equatorial electrojet, are obtained for daytime profiles using initial‐value and eigenmode methods in a nonlocal theory. The methods are shown to be in agreement, thus resolving a recent discrepancy between the theories of Ronchi et al. [1989] and Wang and Bhattacharjee [1994]. A set of equilibrium profiles that are consistent with observed daytime electric and current density profiles are given and analyzed for nonlocal stability. Instabilities with horizontal wavelengths of the order of 1 km are found to be dominant over most of the electrojet, with instabilities of shorter wavelengths (of the order of 100 m) localized in regions of weak velocity shear. The predictions of linear theory are in accord with the observed preponderance of kilometer‐scale irregularities in the daytime electrojet.
An updated model of the hot nitrogen atom kinetics and thermospheric nitric oxide
Authors: Gérard J.‐C. , Bisikalo D. V. , Shematovich V. I. , Duff J. W.
Published: February 2004

New observations and reanalysis of previous measurements suggest an upward revision of the measured thermospheric nitric oxide density. Our previous model of NO production by fast N(4S) atom collisions with O2 has been updated. It includes the effect of soft solar X rays, Auger electrons, and a detailed treatment of N2 dissociative ionization channels. In addition, new calculations indicate that the transfer of translational energy in N + N2 collisions is less efficient than in the hard sphere approximation. This result leads to reevaluation of the energy dependent relaxation cross section and to an upward revision of the reacting efficiency of collisions of N with O2 to form nitric oxide. The calculated peak NO density increases by a factor of ∼ 2 when the effect of superthermal nitrogen atoms is included. The model response of the N(4S) energy distribution function and NO density to solar cycle variations is presented. The NO density at 110 km changes from 5.4 × 107 to 1.3 × 108 cm−3 when the solar F10.7 index varies from 70 to 245, but its response depends on the magnitude of the soft X ray increase with solar activity.
Octupole model of Jupiter's magnetic field from Ulysses observations
Authors: Connerney J. E. P. , Acuña M. H. , Ness N. F.
Published: February 2004

A spherical harmonic model of the planetary magnetic field of Jupiter is obtained from the Ulysses encounter observations. The magnetic field in the Jovian magnetosphere is represented using a degree and order 3 spherical harmonic for the planetary (“internal”) field, and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. The model is obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, and a subset of the octupole spherical harmonic coefficients are determined and compared with those obtained using Voyager and Pioneer observations. The model requires a less intense magnetodisc current, in early 1992, compared with that observed during the Voyager 1 encounter in 1979 and the Pioneer encounters of 1973 and 1974.
An examination of the Tsyganenko (T89a) field model using a database of two‐satellite magnetic conjunctions
Authors: Weiss L. A. , Thomsen M. F. , Reeves G. D. , McComas D. J.
Published: February 2004

We use a database of magnetic conjunctions between Defense Meteorological Satellite Program (DMSP) and geosynchronous satellites to (1) evaluate the mapping of the Tsyganenko T89a magnetic field model between the ionosphere and geosynchronous orbit and (2) test the ability of different magnetospheric parameters to identify the T89a stretching level (parameterized by Kp = 0–5) which comes closest to reproducing the observed conjunctions. The database, covering a range of magnetospheric activity, local times, and season, was compiled using 4 months of plasma electron data from two geosynchronous satellites and three low‐altitude DMSP satellites. The conjunctions were determined using an automated spectral comparison and selection technique which identified intervals of close spectral matching between the distributions observed at the two satellites. Comparison of the observed conjunctions with the closest T89a mappings show that the observed range of field line stretch at geosynchronous orbit (in the dawn and dusk local time sectors) is generally greater than that allowed by T89a. In nearly half the cases, the observed mappings are less stretched than the least‐stretched (Kp = 0) version of T89a. We found that none of the magnetospheric parameters we evaluated (Kp, AE, Dst, the Auroral Boundary Index (ABI), or the inclination of the magnetic field at geosynchronous orbit) could provide a unique pointer to the most appropriate T89a stretching level for every conjunction. However, both the tilt of the field at geosynchronous orbit and the ABI show a strong correlation with the observed degree of field stretch and indices based on these parameters are thus promising candidates for incorporation into future models.
High‐voltage interactions in plasma wakes: Results from the charging hazards and wake studies (CHAWS) flight experiments
Authors: Enloe C. L. , Cooke D. L. , Pakula W. A. , Violet M. D. , Hardy D. A. , Chaplin C. B. , Kirkwood R. K. , Tautz M. F. , Bonito N. , Roth C. , Courtney G. , Davis V. A. , Mandell M. J. , Hastings D. E. , Shaw G. B. , Giffin G. , Sega R. M.
Published: February 2004

Data from the charging hazards and wake studies (CHAWS) flight experiments on board space shuttle missions STS‐60 and STS‐69, during which a negatively biased, high‐voltage (0–5 kV) probe was placed in a plasma wake in low Earth orbit, are presented. For these experiments the source of the wake was the 4‐m‐diameter Wake Shield Facility (WSF), which was operated both as a free‐flying spacecraft and attached to the shuttle orbiter's robot arm. Current collection by the biased probe is investigated as a function of the density and temperature of the ambient plasma and the probe's location in the plasma wake. Current collection behavior is determined by the expansion of the high‐voltage sheath into the ambient plasma stream. Consistent with preflight predictions, current collection on the probe is highly nonuniform, varying by more than 5 orders of magnitude across the surface of the probe. The onset of current collection, however, begins at voltages that are an order of magnitude lower than anticipated. This is likely due to the low‐energy, turbulent plasma (typically 2–5% of the ambient density and up to 40% on occasion) observed in the ambient environment. This important minority constituent of the plasma was observed in the vicinity of the shuttle Orbiter and observed while the WSF was free‐flying.
Dynamics of the H+ and O+ polar wind in the transition region as influenced by ionospheric convection and electron heating
Authors: Ho C. Wing , Horwitz J. L. , Wilson G. R.
Published: February 2004

We have conducted a set of systematic generalized semikinetic simulations to study the polar H+/O+ upflows in the ionosphere and transition region as influenced by varying convection. Effects of both frictional ion heating and centrifugal acceleration are included. We find that in regions where the convection electric field is strong (Ei ≥ 100 mV/m) the steady state polar wind may be characterized as primarily a centrifugally accelerated O+ outflow together with an ambipolar H+ outflow, as a minor component, up to 4 RE geocentric distance. Owing to the increase in the O+ upward flow speed, the increase in the O+ density, and the decrease in the H+ flow speed, H+–O+ collisions are important to extended altitudes during enhanced convection periods. The exobase (defined here as the altitude where the O+ scale height is equal to the mean free path of an H+ ion with a speed three thermal speeds larger than the H+ bulk speed) shifts from 1900 km for Ei = 0 mV/m to 3000 km for Ei = 100 mV/m. For the range of convection electric fields considered here (Ei = 0 mV/m to Ei = 100 mV/m), we identify an upper and a lower transition region which coincide roughly with the region of downward and upward H+ heat flux, respectively. A set of relationships between ion parallel speeds and normalized collisional mean free paths was found which are associated with the maximum upward and downward heat flux, regardless of the value of Ei, for steady state conditions. We find that the heated and centrifugally accelerated O+ ions can obtain upward bulk velocities of 5 km/s above 3 RE geocentric distance for Ei ≥ 80 mV/m. These ions exert a large downward drag on the H+ ions which stretches out the tail on the lower velocity side of the distribution creating large downward heat fluxes. These effects may explain features of the large downward heat fluxes observed in the H+ distributions to large altitude by the retarding ion mass spectrometer (RIMS) instrument on DE 1. We have also considered impulsive events, consisting of pulses of cleft associated enhanced convection and elevated electron temperatures, followed by convection across the polar cap. These result in O+ ions falling back into the ionosphere on the dayside and nightside [e.g., Horwitz and Lockwood, 1985]. Downward speeds of 1–2 km/s are seen up to several thousand kilometers altitude which is consistent with DE 1/RIMS observations as presented by Chandler[1995].
Modeling studies of the middle and upper atmosphere radar observations of the ionospheric F layer
Authors: Su Y. Z. , Fukao S. , Bailey G. J.
Published: February 2004

Observations of electron density and plasma drift velocity made during noon, June 25, 1990, to noon, June 27, 1990, by the middle and upper atmosphere (MU) radar at Shigaraki, Japan, are investigated by comparing them with values modeled by the Sheffield University plasmasphere‐ionosphere model. The model calculations, with inputs for the E×B drift and neutral wind velocities derived from the observed plasma drift velocities, give diurnal variations for the electron density profile in reasonable accord with the observations and show that at the magnetic latitude of the MU radar (25°N) the effects of the west‐east electric field on the electron density profile are comparable to the effects arising from the neutral wind. The observed feature that the F‐region electron density is lower at around noon than during the morning and presunset periods results mainly from the effect of the daytime poleward neutral wind. At night the observations show an increase in the F2‐layer peak electron density accompanied by a lowering of the F layer. This feature is shown to be caused by the postsunset eastward electric field which gives rise to a plasma flux from the plasmasphere to the ionosphere. It is found that the effects arising from the electric field are different from those arising from its replacement by an equivalent neutral wind derived from the E×B drift velocity.
Temporal and spectral characteristics of terrestrial gamma flashes
Authors: Nemiroff Robert J. , Bonnell Jerry T. , Norris Jay P.
Published: February 2004

We have analyzed the Burst and Transient Source Experiment (BATSE) high‐resolution timing data for 13 terrestrial gamma flashes (TGFs) to better characterize this newly identified phenomenon, which may be related to atmospheric lightning. We find that the minimum timescale for TGF variability is ∼25–250 μs, with 50 μs near typical. In general, TGFs are spectrally much harder than cosmic gamma ray bursts (GRBs). We additionally find that as with GRBs, individual pulses within a TGF tend to peak earlier at higher energies. This time‐asymmetry rules out models such as sweeping beams. We also find that different pulses can have different spectra, with spectra typically softening as a pulse progresses. Event‐averaged spectra for the TGFs were examined and found to be better fit in the 25–500 keV range by a power law than by a blackbody model. However, in general, even a power law is not a perfect fit. We find correlation between minimum TGF timescale and the power law spectral index, with rapidly varying TGFs appearing softer. From empirical comparisons of timescales and structures we speculate that if TGFs are somehow related to known high‐atmospheric lightning events, then they are more probably related to red sprites than to blue jets or transionospheric pulse pairs.
Effects of wave‐particle interactions on H+ and O+ outflow at high latitude: A comparative study
Authors: Barghouthi I. A.
Published: February 2004

A Monte Carlo simulation was used to study the effects of wave‐particle interactions (WPI) on ion outflow at high latitudes (the auroral region and the polar cap). As the ions drift upward along the geomagnetic field lines, they interact with the electromagnetic turbulence and, consequently, get heated in the direction perpendicular to the geomagnetic field. The mirror force converts some of the gained ion energy in the perpendicular direction into parallel kinetic energy. These effects combine to form an ion‐conic distribution. Previous studies of WPI in the auroral region neglected the body forces (i.e., gravitational and polarization electrostatic) and the altitude dependence of the spectral density. In contrast, this work includes the effect of body forces and an altitude‐dependent spectral density. The ion distribution function, the profiles of ion density, drift velocity, and parallel and perpendicular temperatures are presented for both H+ and O+ ions. These results are compared with the ones corresponding to polar wind conditions. The main conclusions are as follows: (1) the effect of body forces is more important in the polar wind case and for the O+ ions than it is for the auroral region and the H+ ions, respectively; (2) the O+ ions are preferentially energized in both regions; (3) both ions (H+ and O+) are more energetic in the auroral region at most altitudes; and (4) the results of the Monte Carlo simulations agree with the “analytical” results of the mean particle theory.
Characteristics of downward flowing ion energy dispersions observed in the low‐altitude central plasma sheet by Akebono and DMSP
Authors: Hirahara M. , Yamazaki A. , Seki K. , Mukai T. , Sagawa E. , Kaya N. , Hayakawa H.
Published: February 2004

We present characteristics of downward flowing ion (DFI) energy dispersions observed in the low‐altitude central plasma sheet by two polar‐orbiting satellites: Akebono and DMSP F8. In general, the typical energy decreases with decreasing latitude. Their main ion composition frequently consists of singly charged oxygen, suggesting that the energy dispersing ions are of ionospheric origin. We conclude that the energy‐dispersed signature represents a spatial structure, produced mainly by the global plasma convection driven by the E × B drift on the closed field lines. A probable source of these DFI energy dispersions is the upward flowing ion (UFI) beam accelerated by parallel electrostatic potential at low altitudes. Some case studies show that the energy ratios of the multiple (overlapped) energy dispersions and the pitch angle distributions have some important characteristics consistent with features expected from this model. Clear DFI energy dispersions are observed frequently in stable eastward convection in the postmidnight region, rather than in the premidnight sector. Although the occurrence frequency is different from the local time dependence of the UFIs as reported previously, this discrepancy is due probably to some differences of the convection patterns in the premidnight and postmidnight sectors. The clear energy dispersions might be smeared out by the distorted or complicated convection pattern, irregularities, and disturbances frequently seen in the premidnight sector.