bluuebunny/crossref_metadata_2025 huggingface dataset:
Morphological characteristics and the polarization of plasmaspheric ELF hiss observed at Moshiri (L ∼ 1.6)
Authors: Hayakawa Masashi , Tanaka Yoshihito , Okada Toshimi
Published: February 2008

Morphological characteristics (local time and Kp dependences and emission frequency) of ground‐based plasmaspheric ELF hiss have been examined on the basis of the magnetic tape records observed at Moshiri (geomagnetic latitude 35°N; L = 1.6) during the four‐year period from January 1973 to December 1976. About 160 hiss events have been identified during the relevant period, and they are found to be observed during severely disturbed periods (Kp = 4–7) and during daytime. The ground‐based morphological properties including the above results are shown to support the previous in‐situ observational results. The most important contribution of the present paper is the information on the relative location of the ionospheric exit region of the hiss with respect to the observing station. About one fifth of the ELF hiss events are found to be cut off at the low frequency coincident with the cut off frequency of the first‐order mode of the earth‐ionosphere waveguide mode propagation, indicating that the ionospheric exit point of the hiss is located far away from the observing station and that the hiss has propagated over a considerable distance in the waveguide. One third of the emission events did not exhibit any effect of the waveguide propagation, suggesting that they have penetrated the ionosphere close to the station, and this is confirmed by the polarization measurement for such an event. The present observational results from a low‐latitude ground station are clearly complementary to the in‐situ observations and are used to elucidate the generation mechanism of plasmaspheric ELF hiss and its subseuequent propagation in the magnetosphere and ionosphere.
Electrostatic ion cyclotron, beam‐plasma, and lower hybrid waves excited by an electron beam
Authors: Singh Nagendra , Conrad J. R. , Schunk R. W.
Published: February 2008

The excitation of waves at frequencies ω < ωlh, where ωlh is the lower hybrid (LH) frequency, by a low‐energy electron beam is examined under the electrostatic approximation; the frequency range ω < ωlh includes the ion cyclotron frequencies (Ωi) and their harmonics in a multi‐ion plasma. Since the beam velocity Vb ≫ νte, the electron thermal velocity, the excited waves have phase velocities Vp ≃ Vb ≫ νte. Thus, the background electrons can be treated as cold, and the electron density fluctuations are small. In the limit of cold ion approximation, the excited waves are either the ion cyclotron resonance cone (ICRC) waves or the beam‐plasma (B‐P) mode. Both of these waves show stop bands near the gyrofrequencies of the constituent ions. The ICRC waves are limited to the highest gyrofrequency while the B‐P mode propagates even beyond it. A comparison of the characteristics of the electrostatic ICRC waves with those determined from a full electromagnetic treatment at frequencies ω < Ωi shows that the electrostatic approximation is valid when beam energy is low (∼100 eV) and for relatively more energetic beams the validity holds good only when the perpendicular wave number k⊥ is sufficiently large (k⊥ > 10³Ω(H+)/C, Ω being the H+ gyrofrequency, and C the velocity of light in vacuum). Warm ion effects on the waves are examined; when K⊥ = k⊥rl(H+) > 0.3, where rl(H+) is the H+ Larmor radius in a He+‐H+ plasma, the excited waves are the electrostatic‐ion cyclotron (EIC) waves. Because of their phase velocity Vp ≫ νte, these EIC waves are pure ion Bernstein (PIB) waves. The characteristics of the electron beam‐excited PIB waves are compared with their counterpart, the neutralized ion Bernstein (NIB) waves, which are EIC waves excited by drifting bulk electrons. The consideration of electrostatic waves in the frequency band Ω(H+) < ω < ωlh shows that the B‐P waves with k⊥rl(H+) ≪ 1 have considerably larger temporal growth rates than the EIC waves. The interconnections between LH and B‐P waves are found to be similar to those between EIC and B‐P waves; B‐P waves have long perpendicular wavelengths (λ⊥ while EIC and LH waves have comparatively much shorter ones. The relevance of ICRC and PIB waves to satellite observations of waves in the ELF band is discussed. The B‐P waves appear to be similar to the electrostatic broadband noise emissions in the frequency range from below the ion cyclotron frequency to the lower hybrid frequency observed from various satellites in the auroral and cusp regions.
Dynamical features and electric field strengths of double layers driven by currents
Authors: Singh N. , Thiemann H. , Schunk R. W.
Published: February 2008

Results from numerical simulations are presented showing the temporal evolution of the potential profiles of double layers in association with the slow temporal fluctuations in the electron current. These fluctuations in our simulations are caused by the electrostatic ion cyclotron waves driven by the current and also by the potential relaxation instability. When the time‐average current density Jo is smaller than the electron thermal current Jth, the formation of a potential dip at the low potential end of a double is a common occurrence even when the conditions for the excitation of ion‐acoustic double layers do not exist. On the other hand, when Jo > Jth the formation of potential dips are associated with interruptions in the current. When the current interruptions are deep causing Je ≃ Jth, where Jth might itself undergo a considerable temporal evolution, the dips are localized. When the current density is large, the double layers tend to form potential humps on their high potential side which are dynamic and may evolve into new double layers. The above features of double layers are seen in different types of numerical simulations ranging from one‐dimensional Vlasov type to two‐and one‐half‐dimensional particle‐in‐cell code. Also, these simulations differ in their initial and boundary conditions. There are some satellite observations on electric fields and on electron beams which indicate the possibility of the formation of potential dips in the parallel potential profiles of the auroral potential drops. Comparisons of the electric field amplitudes from satellite observations with those predicted by two‐and one‐half‐dimensional simulations indicate that the simulations predict electric field strengths of the right order of magnitude.
Mars atmospheric circulation: Aspects from Viking Landers
Authors: Ryan J. A.
Published: February 2008

Winds measured by the two Viking landers have been filtered and then compared with predictions from the general circulation model and to orbiter observations of clouds and surface phenomena that indicate wind direction. This was done to determine the degree to which filtered winds may represent aspects of the general circulation. Excellent agreement was found between wind direction data from Lander 1 and the model predictions and orbiter observations. For Lander 2, agreement was generally good, but there were periods of disagreement which indicate that the filtering did not remove other extraneous effects. It is concluded that Lander 1 gives a good representation of the general circulation at 22.5°N latitude but that Lander 2 is suspect. Most wind data from Lander 1 have yet to be analyzed. It appears that when analyzed these Lander 1 data (covering 3.5 Mars years) can provide information about interannual variations in the general circulation at the lander latitude.
Mid‐latitude Pi 2 polarization pattern and synchronous orbit magnetic activity
Authors: Gelpi C. , Hughes W. J. , Singer H. J. , Lester M.
Published: February 2008

Pi 2 pulsations are often used to indicate the start of the substorm expansion phase. In this report the relationship between the Pi 2 polarization pattern observed at a mid‐latitude longitudinal chain of stations and synchronous orbit magnetic activity is explored. Eighteen single‐ and multiple‐substorm‐onset intervals, consisting of 55 Pi 2's, are examined using data from the Air Force Geophysics Laboratory Magnetometer Network and the synchronous orbit satellites GOES 2 and 3. Magnetic fluctuations at synchronous orbit are found to be enhanced when the satellite is near the Pi 2 pattern center, defined as the longitude where the major axis of the polarization ellipse is along the H axis. The pattern center is also observed to separate regions of dipolarization and more taillike magnetic signatures. East of the pattern center the field inclination becomes larger, i.e., more dipolelike, while west of the center the field inclination becomes smaller, i.e., more taillike. Although regions of substorm activation, as determined by the Pi 2 pattern, move between onsets during multiple‐onset events, no consistent azimuthal motion was found.
Radiative contamination in rocket‐borne infrared photometric measurements
Authors: López‐Moreno J. J. , Rodrigo R. , Vidal S.
Published: February 2008

In a rocket experiment designed to measure the infrared atmospheric system of the O2 and several OH Meinel bands in the Δυ = 2 sequence, an unexpected enhancement of the emission was observed on the upleg when the rocket reached an altitude of 92 km. The glow was detected only in the channels with larger half‐power bandwidth, those devoted to the study of the OH emission. This enhancement is interpreted in terms of local contamination originating from interactions between the instrument and the atmosphere. On the basis of the spectral distribution of the radiative glow the contaminating agent is tentatively identified as vibrationally excited OH, and the proposed mechanisms giving rise to the glow are discussed.
The effect of background plasma density on the growth of ordinary and Z mode emissions in the auroral zone
Authors: Omidi N. , Wu C. S.
Published: February 2008

Recent spacecraft observations in the auroral zone have clearly demonstrated the presence of both ordinary mode auroral kilometric radiation and Z mode radiation in this region. Using an electron distribution function measured in the auroral zone, we have calculated the growth rates of both ordinary and Z mode radiation via the cyclotron maser mechanism. The growth rates of the ordinary mode auroral kilometric radiation show that these waves can be amplified at frequencies near the electron gyrofrequency and with wave normal angles at or near 90°. The magnitude of the growth rate at a given frequency is essentially constant over a wide range of the ratio of the background electron plasma frequency to the gyrofrequency. The growth rates of Z mode radiation, on the other hand, are much more sensitive to the background plasma density. It is shown that in very low density regions these waves are dominant over the ordinary mode. However, as the ratio of the electron plasma frequency over the gyrofrequency increases, the growth rates of Z mode are substantially reduced, leading to the eventual dominance of the ordinary mode over the Z mode. A comparison between the growth rates of Z mode due to the upgoing loss cone electrons and those due to the trapped and “hole,” as well as the loss cone, electrons shows that the presence of trapped and hole electrons can greatly enhance the growth rates. This study demonstrates that a loss cone alone may not be sufficient to provide substantial amplification for Z mode waves in the auroral zone.
Venus nightside ionospheric troughs: Implications for evidence of lightning and volcanism
Authors: Taylor H. A. , Grebowsky J. M. , Cloutier P. A.
Published: February 2008

Ionization troughs are frequently observed in the Venus nightside ionosphere by the orbiter ion mass spectrometer (OIMS) on the Pioneer Venus Orbiter (PVO). These events typically exhibit density depletions which may range from a factor of 2 to an order of magnitude or more, often feature sharp density gradients, and frequently are associated with superthermal ion fluxes. Although their full spatial extent cannot be determined, the ion trough dimensions as perceived along the PVO orbit track extend from 1° to more than 10° in latitude and longitude. Troughs have been encountered at altitudes ranging from more than 1000 to less than 250 kilometers. The nightside trough boundaries are similar to both the ionization gradients which mark the Venus dayside ionopause and the ionization gradients which define the high‐latitude ionization trough in the earth ionosphere. In each of these Venus and earth phenomena there is a close association between sharp gradients in thermal ion distributions and energetic and dynamic processes stimulated by the interaction between the solar wind/interplanetary magnetic field (IMF) and the thermal ionosphere. A close correlation is identified between ion trough events and prominent published examples of 100‐Hz plasma waves detected by the orbiter electric field detector (OEFD), which have been interpreted by Scarf and Russell (1983) and by Ksanfomaliti et al. (1983) as whistler mode propagation stimulated by lightning in the lower atmosphere and frequently occurring over mountains. While the occasional coincidence between the detection of whistler waves and the overflight of Venus mountain ranges is also observed for the ion troughs, neither phenomenon is seen to exhibit convincing evidence of an association with the mountains. Overall, these findings lead us to believe that many of the 100‐Hz noise bursts previously attributed to lightning are, like the ion troughs, actually the result of the interaction between the ionosphere and the combined effects of the solar wind and the draped IMF. If this interpretation is correct, there may be little evidence for lightning from the PVO measurements. Similarly, a supporting argument for active volcanism as a possible source for the deduced lightning events would also be discounted.
On damping of auroral Z mode waves
Authors: White S. M.
Published: February 2008

This paper discusses the constraints on auroral z mode waves implied by damping. Damping of the magnetoionic z mode in a thermal plasma with ωp ≪ Ω is calculated and shown to be strong near the gyrofrequency. Analytic formulae for the optical depth of the corresponding absorption layer are presented. Ray tracing calculations are used to investigate the effect of refraction during the propagation of a z mode ray: refraction is found to be particularly important in the vicinity of the gyrofrequency layer and tends to cause the wave vector of the ray to change so that it is strongly damped. The results are applied to models for the generation of auroral z mode waves. A point of particular interest concerns the possibility of explaining observed emissions whose bandwidths straddle the gyrofrequency by an emission mechanism which amplifies waves either only above or only below the gyrofrequency. For a dipolar magnetic field and radial plasma density gradient it is found that a z mode ray in the auroral zone is unlikely to be able to pass through the gyrofrequency layer without significant damping occurring.
On the cause of approximate pressure isotropy in the quiet near‐Earth plasma sheet
Authors: Nötzel A. , Schindler K. , Birn J.
Published: February 2008

In this paper we attempt to provide a theoretical explanation of the observed approximate pressure isotropy in the near‐earth geomagnetic tail We first give a new formulation of the self‐consistent theory of two‐dimensional anisotropic steady states by generalizing the Grad‐Shafranov equation. Then it is shown that anisotropy‐driven instabilities, active in the midplane of the plasma sheet, lead to highly isotropic pressure tensors for the entire plasma sheet. The key point is that due to the weakness of the magnetic field in the midplane the instabilities require only a small pressure anisotropy to be excited. The approach is based on the assumption that an originally unstable system evolves into a steady state. An order of magnitude analysis is supplemented and confirmed by a numerical example.
High‐resolution studies of the HF ionospheric modification interaction region
Authors: Duncan L. M. , Sheerin J. P.
Published: February 2008

Incoherent backscatter radar studies were conducted of the HF ionospheric modification interaction region using the facilities of the Arecibo Observatory. Very high relative spatial and temporal resolutions were achieved. These measurements resolve the miniovershoot (2–10 ms after HF on) and main overshoot (20–40 ms after HF on) plasma wave excitation features. The main overshoot phenomenon is observed to develop very reproducibly with enhancements of a factor of 10 greater than the steady state HF‐enhanced plasma line intensity. However, this main overshoot effect is strongly suppressed for HF off times of less than 100 ms. In general, the miniovershoot excitation is found to occur 800–1000 m below the main overshoot excitation height. In addition, in one case, a small plasma line enhancement is detected at very early times (<1 ms) distinct from the miniovershoot and originating at a greater height. The main overshoot and the steady state enhanced plasma line both develop at essentially the same altitude. However, the interaction layer is observed to broaden by several hundred meters during the main overshoot relaxation. Possible explanations for these effects are proposed using a combination of the theories of soliton formation and collapse, wave ducting, profile modification, parametric instabilities, and other interactive processes.
Short‐circuiting, ion‐viscous, and ion‐inertial effects in striation “freezing”
Authors: Sperling J. L. , Glassman A. J.
Published: February 2008

A linear dispersion relation is derived for the collisional gradient drift instability under the assumption that partial short‐circuiting of perpendicular ambipolar electric fields, ion inertia, and ion viscosity are the significant factors in the “freezing” of striations with perpendicular scale sizes of several hundred meters. Quantitative evaluation of this dispersion relation indicates that eigenmodes tend to be localized to low‐density portions of the plasma when short‐circuiting is active. However, the eigenmodes can extend in relatively uniform fashion across the density gradient when short‐circuiting is very small. This situation would appear to be conducive to the formation of striations. Hence the calculations suggest that short‐circuiting of perpendicular ambipolar electric fields is weak at the sharp perpendicular density gradients where striations begin to evolve. The observation of perpendicular density gradients which are persistent and sharp supports the idea that short‐circuiting is weak. With the assumption that short‐circuiting is zero, a dispersion relation is derived with full ion viscous contributions. Evaluation of this dispersion relation suggests that the finite ion gyroradius contributions are the dominant factor in striation “freezing.” Accordingly, an expression is derived for estimating the outer scale wave number appropriate to “frozen” striations, and an appropriate application is made to the “U‐shaped curve” formulation of striation dynamics.
The O2 atmospheric 0‐0 band and related emissions at night from Spacelab 1
Authors: Torr Marsha R. , Torr D. G. , Laher R. R.
Published: February 2008

The photochemistry of the c¹Σ, C³Δ (now renamed the A′³Δu), and A³Σ excited states of O2 is of relevance to the energetics of the lower thermosphere and upper mesosphere and to the atomic oxygen concentration in this region. A means of obtaining the atomic oxygen concentration profile by remote sensing is via the O(¹S) emission at 5577 Å provided the production and loss mechanisms of the O(¹S) are well understood. The O(¹S) is now believed to be produced in a two‐step process originating with one of the above three states of O2 (the Barth mechanism). Earlier, it was thought to be produced directly in three‐body recombination of atomic oxygen (the Chapman mechanism). While the empirical evidence in recent years has tipped toward the first of these alternatives, the identity of the intermediate state is still in question. The O2 atmospheric bands arise from the b¹Σ state, which is populated via a two‐step process similar to the Barth mechanism, again involving an intermediate state. In this paper we investigate these related issues using spectral measurements gathered on the nocturnal middle atmosphere with an array of imaging spectrometers flown on the Spacelab 1 shuttle mission. At this time the problem still contains too many uncertain parameters to be unambiguously resolved by the data. However, the data appear best supported by the O2(b¹Σ) arising from the O2(c¹Σ), although an additional source of O2(b¹Σ) is indicated. The O(¹S) is more closely represented by a process with the Chapmanlike characteristics of an [O]³ dependence, which seems to favor the O2(A′³Δ) as the intermediate state in this case if the previously accepted rate coefficients are used. (We do not have sufficient information in our data set to reject the (A³) state as the O(¹S) precursor, although this has been rejected in other studies.) However, the state of knowledge of the rate coefficients and dominant quenching species for the potential intermediate states and their vibrationally excited forms is not yet stable. If the latest laboratory information on the O2(b¹Σ) state is correct in the relative roles of quenching by O and O2 of the intermediate state but incorrect in the rate of production of the intermediate state, another solution is possible. In this case the O2 atmospheric bands and O(¹S) can arise from the same intermediate state but with O(¹S) being generated only from vibrationally excited states with a yield of 0.01 and the O2 atmospheric generated also from the low vibrational states with a yield of at least .80.
Evidence for ion heat flux in the light ion polar wind
Authors: Biddle A. P. , Moore T. E. , Chappell C. R.
Published: February 2008

Cold, flowing hydrogen and helium ions are observed with the retarding ion mass spectrometer on Dynamics Explorer 1 in the dayside magnetosphere at subauroral latitudes at 4500‐km altitude. These ions show a marked flux asymmetry with respect to the relative wind direction. The observed data can be effectively modeled as drifting Maxwellian distributions perturbed by a first‐order Spitzer‐Härm heat flux distribution function. At high values of L, ∼14, both species are supersonic, and the shape and direction of the asymmetry are consistent with the presence of an upward heat flux. As L decreases to ∼6, both species evolve smoothly into warmer, subsonic upward flows with downward heat fluxes. In the subsonic cases this implies a significant heat source at higher altitudes. These results are found to match theoretical predictions for the polar wind. However, in both cases, in order to match the observed departures from a simple drifting Maxwellian, the mean free path must be as much as one‐half the thermal gradient scale length. This strongly suggests that collisionless effects are stronger than predicted and dominate the heat transport throughout the entire transition from supersonic to subsonic flow. These observations demonstrate a capability to observe transport effects in plasmas as higher moments of the distribution function, making possible comparisons with hydrodynamic theories which have until now been experimentally unverified.
Electromagnetic hot ion beam instabilities: Quasi‐linear theory and simulation
Authors: Rogers Barrett , Gary S. Peter , Winske Dan
Published: February 2008

This paper considers the quasi‐linear theory of the right‐ and left‐hand resonant electromagnetic instabilities driven by a hot ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma. Using the single‐mode approximation, the time evolutions of important parameters are obtained to show that for the range of parameters considered, reduction of the beam speed and formation of temperature anisotropies are the most significant factors in the quasi‐linear stabilization process. Combining both instabilities in a quasi‐linear study is found to produce a roughly equal mixture of both polarizations and relatively isotropic conditions for tenuous beam densities and low initial beam drift speeds. Computer simulations are used to compare with the quasi‐linear results. The simulations justify the single‐mode assumption, verify that quasi‐linear changes are the means of saturation for the parameter range of concern, and check the nonlinear evolution of the system when both modes are present.
Magnetic field properties of the distant magnetotail magnetopause and boundary layer
Authors: Sibeck D. G. , Siscoe G. L. , Slavin J. A. , Smith E. J. , Tsurutani B. T. , Bame S. J.
Published: February 2008

Models incorporating merging between dayside magnetosheath and magnetosphere field lines predict the location of the magnetotail boundary layer and bending of magnetotail field lines in the direction of the IMF. We review previous observations supporting these models and reexamine a case of dense magnetotail boundary layer plasma and strongly bent field lines observed by ISEE 3. We discuss observations of boundary layer plasma within a tangential discontinuity magnetopause. Our model to explain this phenomenon requires that interconnected magnetotail and magnetosheath field lines leave the magnetotail via a narrow “window.” Field lines in the dense boundary layer plasma veer away from the aberrated x axis more than those in the void magnetotail lobes in order to pass through the windows. Magnetosheath plasma must enter the magnetotail lobes through the windows at all downstream distances in order to supply the observed tailward flux of lobe plasma.
Parametric excitation of shear Alfvén modes by electrostatic ion cyclotron waves in the magnetosphere
Authors: Patel V. L. , Tripathi V. K. , Sharma O. P.
Published: February 2008

A nonlinear process, involving the parametric decay of electrostatic ion cyclotron waves into a shear Alfvén wave and an electrostatic sideband, is investigated as a possible internal source mechanism of the low‐frequency magnetic pulsations in the magnetosphere. The process is applicable to the pulsations with frequency ω ≲ 0.03 ωci (ion cyclotron frequency) and may cover the periodicity range of Pc 2 to Pc 5. The threshold for the process is set by the gradients in plasma density and magnetic field. In magnetospheric plasma the threshold is exceeded when the energy density of the pump wave fluctuations is of the order of a few percent of thermal energy density.
Classification of auroral electrostatic shocks by their ion and electron associations
Authors: Redsun M. S. , Temerin M. , Mozer F. S.
Published: February 2008

We have examined ion data associated with 1073 electrostatic shocks (regions of large perpendicular electric field) below 8000 km altitude and have found that at least 85% of these shocks are associated with upflowing ions in the form of either ion beams or ion conics. Analysis of these data indicates that significant differences exist between the distributions of shocks associated with ion beams and shocks associated with ion conics. Shocks associated with ion beams are concentrated in the evening, while shocks associated with ion conics are observed with comparable frequency at all MLTs. Although both types of shocks are seen more frequently at higher altitudes than at lower altitudes, the probability of seeing a shock increases more rapidly with altitude for shocks associated with ion beams than for shocks associated with ion conics. Shocks associated with ion beams have not been seen below 3000 km. Shocks associated with ion beams have, on the average, larger electric fields than shocks associated with ion conics. Furthermore, the average electric field tends to increase with altitude for shocks associated with ion beams. Shocks are far more likely to occur with ion conics rather than with ion beams during periods of high Kp. Ion beams associated with electrostatic shocks typically have higher energies than ion conics associated with electrostatic shocks. Also, examination of the energy spectra of electrons associated with these events revealed that 86% of the electrostatic shocks associated with ion beams have peak electron energies above 360 eV, while 66% of the electrostatic shocks associated with ion conics have peak electron energies below 360 eV. The average peak electron energy is somewhat higher than the peak ion energy. Most shocks associated with ion beams are either S shocks or V shocks. Although the peak ion energy of the ion beam is typically larger than the potential through the shock, there is a fair correlation between the two. The direction of the electric field of the shock is strongly correlated with the location of the ion beam. Of the shocks associated with ion beams, V shocks have the largest and most coherent structures, followed by S shocks and embedded shocks. Shocks associated with ion conics have smaller and less coherent structures than shocks associated with ion beams. The invariant latitude magnetic local time distribution of electrostatic shocks approximates an offset circle with a radius that has a linear dependence on Kp and an offset that is inversely related to Kp.
A simulation study of kilometric radiation generation along an auroral field line
Authors: Pritchett P. L. , Strangeway R. J.
Published: February 2008

Relativistic modifications to wave dispersion near the electron gyrofrequency are known to be important in a low‐density, energetic plasma. Previous investigations based on simple model distribution functions have suggested that these relativistic dispersion effects may substantially alter the predictions of the cyclotron maser theory as applied to the generation of auroral kilometric radiation (AKR). The present work combines a model for the high‐altitude auroral zone with local particle simulations in order to assess the quantitative implications of relativistic dispersion for the generation of AKR. Electron distribution functions, which include a loss cone and a “hole” at lower energies produced by the parallel electric field, are determined along an auroral field line. Number densities of both energetic auroral electrons and low‐energy backscattered electrons are obtained from a fit to the observed density‐depleted region on an auroral field line. It is found that the hot magnetospheric component is the dominant species at altitudes greater than 1.7 RE geocentric. These auroral zone parameters and distributions are used as input to one‐ and two‐dimensional particle simulations. The simulations show that the electric field results in more free energy in the hot electron population at the lower altitudes. This is counteracted, however, by the increased density of backscattered and secondary electrons at these altitudes, which tends to quench the maser instability. The most intense radiation is found to occur between 1.75 and 2.0 RE, corresponding to AKR frequencies in the range 200‐300 kHz. The peak in the radiation is very close to 90° to the ambient magnetic field, with an offset of a few degrees in the direction of the loss cone. The dominant emission is in the extraordinary mode with a wave frequency ω/Ωe ≈ 0.99. A weak (∼1%) ordinary mode component is produced in association with the extraordinary mode due to a relativistic coupling between perpendicular wave electric fields and parallel currents.
Currents to the ionosphere from thunderstorm generators: A model study
Authors: Nisbet John S.
Published: February 2008

Relations between the generator currents in a thunderstorm cell and the currents to the ionosphere are examined. The effects of variations in the breakdown electric field below the lower charge center, heights of the upper and lower charge centers, generator currents and geometry, and the conductivity profile of the cloud, the surrounding air, and the ionosphere are examined. It is shown that for clouds transferring negative charge to ground in cloud‐ground flashes, the major factor controlling the ratio of the current to the ionosphere to the generator current is the fraction of the major discharges that are between the lower charge center and ground. This parameter serves to organize data for a wide variety of assumptions about the parameters studied. The precipitation current between the lower charge center and ground does, however, remain an important variable which must be considered. It is shown that for both storms that are very active and for those that do not produce lightning, the most important single variable controlling the ratio of the current to the ionosphere to the generator current is the height of the lower active center. Because low altitudes correspond to high currents to the ionosphere, high‐latitude storms and those in the winter hemisphere are relatively more important than had been thought. It is shown that though the ionospheric conductivity from 40 to 120 km is very important in controlling the electric field and current density waveforms above the cloud, it has a negligible effect on the total current to the ionosphere.
A component of nongyrotropic (phase‐bunched) electrons upstream from the Earth's bow shock
Authors: Anderson K. A. , Lin R. P. , Gurgiolo C. , Parks G. K. , Potter D. W. , Werden S. , Rème H.
Published: February 2008

The count rates of 1.5‐ and 6‐keV electrons measured on ISEE 1 and ISEE 2 are at times strongly spin modulated despite the fact that the axes of the detectors lie along the spin axis of the spacecraft. Spatial gradients in the guiding center density are ruled out as a cause of the spin modulation. The characteristics of the spin modulation can be explained if the electron velocity distribution is nongyrotropic. The procedure is to show that the direction of the plasma flow velocity during times of spin modulation does not lie along the direction of the local magnetic field when both are projected onto the ecliptic plane. While a gyrotropic distribution will cause spin modulation, the count rate maximum will correspond to the direction of the local magnetic field. Gyrophase‐bunched electrons are seen within a few tens of kilometers of the shock and as far upstream as 30,000 km from the shock, suggesting that a wave is associated with their propagation. The magnetosheath electrons in the same energy range are also nongyrotropic at times.
Observations of VLF emissions at the electron gyrofrequency
Authors: Koons H. C. , Edgar B. C.
Published: February 2008

Electrostatic emissions with four different characteristics have been observed at or very near the electron gyrofrequency in broadband VLF data from the SCATHA satellite. The most common emission is sporadic, has a very narrow bandwidth, and occurs at the electron gyrofrequency. A second type is broadband, typically centered above the electron gyrofrequency, but extends down through the gyrofrequency. A third type has an attenuation band at the gyrofrequency with narrow bands of emissions above and below the gyrofrequency. The fourth type is broadband with a sharp lower‐frequency cutoff at the electron gyrofrequency. These emissions tend to occur at high L values on the nightside of the magnetosphere. They also tend to occur away from the magnetic equator.
The influence of the heliospheric current sheet and angular separation on flare‐accelerated solar wind
Authors: Henning Harald M. , Scherrer Philip H. , Hoeksema J. Todd
Published: February 2008

A complete set of major flares has been used to investigate the effect of the heliospheric current sheet on the magnitude of the flare‐associated disturbance measured at the earth. It was found that disturbances associated with flares located on the same side of the current sheet as the earth were of larger magnitude than those associated with flares located such that the flare‐accelerated material would have to cross the current sheet locus. It was also found that the angular separation between the flare position and the earth has a strong effect on the magnitude of the disturbance. A larger angular separation tended to result in a smaller disturbance. Third, it was determined that flares tend to occur near the heliospheric current sheet.
Cosmic ray modulation and turbulent interaction regions near 11 AU
Authors: Burlaga L. F. , McDonald F. B. , Goldstein M. L. , Lazarus A. J.
Published: February 2008

When Voyager 2 was near 11 AU, the counting rate of nuclei ≳75 MeV/nucleon decreased during the interval from July 1982 to November 1982, and it increased thereafter until August 1983. The counting rate fluctuated within this “minicycle” with short‐term decreases lasting 1 to 4 days and recoveries lasting several days. A decrease in cosmic ray flux was generally associated with the passage of an “interaction region” in which the magnetic field strength B was higher than that predicted by the spiral field model, Bp. Several large enhancements in B/Bp were associated with “merged interaction regions” which probably resulted from the interaction of two or more distinct flows. During the passage of interaction regions the cosmic ray intensity decreased at a rate proportional to (B/Bp ‐ 1), and during the passage of rarefaction regions (where B/Bp < 1) the cosmic ray intensity increased at a constant rate. The general form of the cosmic ray intensity profile during this ∼13 month minicycle can be described by integrating these relations using the observed B(t), and it can be understood in terms of the sizes and separations of interaction regions. Latitudinal variations of the interaction regions and of the short‐term cosmic ray variations were identified by comparing Voyager 2 observations with Voyager 1 observations made at higher latitudes (14° to 20°). The interaction regions were turbulent, with an f−5/3 spectrum from at least 3×10−4 Hz to fc = (1 to 2) × 10−6 Hz. A break in the spectrum at fc corresponds to the characteristic width of the interaction regions, and it represents a “stirring scale” for the solar wind. The interaction regions, including merged interaction regions, may be viewed as “turbulent boundary layers” which grow in size with increasing distance from the sun. They act as barriers which impede the net flow of cosmic rays toward the sun.
The perturbed neutral circulation in the vicinity of a symmetric stable auroral arc
Authors: Walterscheid R. L. , Lyons L. R. , Taylor K. E.
Published: February 2008

The neutral response to changes in the energy source associated with the appearance of a symmetric stable auroral arc is simulated using a sophisticated two‐dimensional numerical model. The model is a time‐dependent one which describes nonlinear, nonhydrostatic viscous flow in a rotating atmosphere. The energy sources are the ion drag momentum source and the Joule and particle precipitation heating sources. The arc model is an idealization of rocket observations of a fairly wide intense arc. The distribution of ion production and electric potential were assumed to be symmetric about the center of the arc and constant along the arc. A significant feature of the forcing is the reduction and reversal of the electric field within the arc. The arc is embedded in a region of enhanced ionization associated with a diffuse aurora. The simulation was carried out for an interval of 1 hour after the onset of the changed forcing. The main findings of our study are as follows: (1) the zonal ion drag force drives strong (∼200 m s−1) counterstreaming zonal winds on opposite sides of the arc; (2) the zonal flow within the arc is accelerated to ∼40% of the electric field drift velocity in the region of high Pedersen conductivity; (3) the nonhydrostatic interplay of buoyancy and vertical pressure gradient forces sets up a large‐scale buoyancylike oscillation; (4) the adiabatic circulation driven by the meridional ion drag force contributes to a net cooling in the lower region of the arc during the first half of the simulation; (5) forced convection within the arc resembles convection due to particle heating alone, but particle heating contributed only ∼25% of the net warming; and (6) the lateral extent of the meridional circulation is much more extensive than the lateral extent of the arc and the diffuse aurora.
Numerical modeling of the 22‐year variation of the cosmic ray intensity and anisotropy
Authors: Kadokura Akira , Nishida Atsuhiro
Published: February 2008

We have solved the two‐dimensional time‐dependent diffusion‐convection equation numerically to obtain the distribution and anisotropy of cosmic rays in the heliosphere. We have assumed that the parallel and perpendicular mean free paths are proportional to the particle Larmor radius, and we have treated each proportionality constant (a, b) as a parameter. We have found that the set (a, b) = (4, 2) gives the steady state solution compatible with observations on the intensity and the solar diurnal anisotropy of cosmic rays in 0.5‐ to 10‐GeV range as obtained at the earth. This set of (a, b) corresponds to the ratio of the diffusion coefficients D∥/D⊥ = 10. In our solution the intensity for the (pre‐1980) interplanetary magnetic field (IMF) state where the solar magnetic dipole and the angular velocity vector are parallel is higher than for the (post‐1980) state where they are antiparallel, while the phase of the diurnal anisotropy is about 15 hours for the parallel state and about 18 hours for the antiparallel state. We have also reproduced the observed small radial gradient for each IMF state. We discuss the nature of the solution in order to understand the effect of the density gradient drift motion on the cosmic ray distribution.
Generation of ordinary mode auroral kilometric radiation from extraordinary mode waves
Authors: Hayes L. M. , Melrose D. B.
Published: February 2008

The ratio of x to o mode radiation produced when an x mode wave strikes a steep density gradient is determined for wave and plasma parameters typical of those observed in the auroral kilometric radiation source region. We find that the waves reflected from or transmitted through the density gradient generally have an o mode component 20 dB or more weaker than the x mode component. This agrees with the ratio of x to o mode intensity observed in auroral kilometric radiation signals near the auroral plasma cavity by both the DE 1 and ISIS 1 spacecraft.
Isee 3 observations during a plasma sheet encounter at 140 RE: Evidence for enhancement of reconnection at the distant neutral line
Authors: Scholer M. , Terasawa T. , Baker D. N. , Gloeckler G. , Hovestadt D. , Smith E. J. , Tsurutani B. T. , Zwickl R. D.
Published: February 2008

Detailed energetic particle, plasma and magnetic field data are examined for the case of a plasma sheet encounter by ISEE‐3 at ∼140 RE. The event onset is characterized by tailward energetic proton and electron beams. Subsequently the magnetic field polar angle reverses from strongly southward to strongly northward indicating the tailward passage of a neutral line. Earthward of the neutral line the energetic electrons exhibit an isotropic distribution and the energetic protons stream earthward. Since no major magnetic field disturbances are seen at ground magnetometer stations and no particle injection events are recorded at synchronous orbit, we conclude that temporarily enhanced reconnection at the distant neutral line leads to the energetic ion beams and to a tailward retreat of the neutral line. About 5 min after the first observations of particle beams in the deep tail, ISEE‐1 observes a plasma sheet thickening in the near‐earth tail concurrent with earthward ion beams in the plasma sheet boundary layer. The observation of both tailward directed ion and electron beams in the region tailward of the particle source exclude particle acceleration by a field aligned electric field as a process for beam formation. The distant plasma sheet at ∼140 RE remains quite disturbed for about 40 min, exhibiting large earthward and tailward plasma flows. Furthermore, strong north as well as south magnetic field excursions are indicative of the presence of closed magnetic field structures in the region earthward of the distant neutral line. When ISEE‐3 exits the plasma sheet into the northern lobe a low density, low heat flux boundary layer is observed. The absence of ion beams in the boundary layer at the end of the event indicates that reconnection has ceased.
Low‐frequency electromagnetic waves driven by gyrotropic gyrating ion beams
Authors: Sharma O. P. , Patel V. L.
Published: February 2008

The origin of left‐ and right‐hand‐polarized low‐frequency waves in space plasmas is analyzed. It has been shown that a gyrotropic gyrating ion beam, a ring in velocity space, can excite electromagnetic modes in the plasma near the beam gyrofrequency. It excites left‐hand‐polarized shear Alfvén waves and their harmonics via the coupling of Alfvén modes with the beam modes. It can also excite right‐hand‐polarized fast‐mode magnetosonic waves and their harmonics as well. The excitation is possible for beam ions heavier than the plasma ions. The growth rate varies as one‐third power of the beam density and decreases with the angle of wave propagation with respect to the ambient magnetic field. The nonlocality has a stabilizing effect on the instability. The predicted values of the wave frequencies compare reasonably well with those observed in satellite data.
Scaling of the beam plasma discharge for low magnetic fields
Authors: Papadopoulos K.
Published: February 2008

A theoretical analysis of the scaling law and the value of the threshold current for beam plasma discharge (BPD) is presented, based on the requirement for an absolute instability near the plasma frequency. It is shown that both the scaling law as well as the numerical values of Ic are consistent with the experimental data, in the low pressure regimes and for weak magnetic field experiments (ωe > Ωe) if the dominant particle loss mechanism is due to Bohm diffusion. The implications of the findings to electron injection in space are discussed.
Mesospheric observations using a 2.66‐MHz radar as an imaging Doppler interferometer: Description and first results
Authors: Adams Gene W. , Brosnahan John W. , Walden David C. , Nerney Steven F.
Published: February 2008

We have made measurements in the 60–100 km region using a 2.66‐MHz radar operated as an imaging Doppler interferometer. This allows us to view the entire region within 24° of zenith simultaneously, Doppler‐sorting and then echo‐locating the returns to achieve image resolution of ±3° in zenith angle and ±5 m/s in velocity. The approach is contrasted with conventional beam‐forming radar methods. We assume that the phenomena responsible for the observed weak radar scattering can be described as a distribution of discrete scattering points. Each scattering point is then characterized by its three spatial coordinates, its radial velocity, and the amplitudes and phases of its spectral components measured on a number of independent antennas. We present a series of apparent‐motion profiles from 61 to 87 km, showing the repeatability of the profiles and their evolution in time, followed by a two‐dimensional image of the scattering surface at 85 km, showing the motions of the individual scattering points and their distribution in space. Then we show images of the scattering surfaces from altitudes of 58 to 103 km, every 9 km, showing at each altitude the distribution in the horizontal plane of the locations of the scattering points, the specularity, or aspect sensitivity, of the scattering, and the derived apparent‐motion vectors. A sequence of eight 51‐s frames shows highly structured activity at 103 km.