bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of the IMF By‐dependent field‐aligned currents in the cleft region based on DE 2 observations
Authors: Taguchi S. , Sugiura M. , Winningham J. D. , Slavin J. A.
Published: February 2008

The interplanetary magnetic field (IMF) By‐dependent distribution of field‐aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF By and Bz components satisfy the criteria |By|≥5 nT and |Bz|≤5 nT during the satellite's crossing of the relevant field‐aligned current region. When By is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field‐aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These By‐dependent field‐aligned currents in the equatorward and the poleward sheets are referred to as the low‐latitude cleft current (LCC) and the high‐latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of By. For positive (negative) By, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field‐aligned current regime. The proposed pair of cleft field‐aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi‐steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath By is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward higher latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field‐aligned currents when By is positive; the pair of field‐aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the By negative case, the model explains the reversal of the field‐aligned current direction in the LCC and HCC regions.
High‐Z energetic particles at geosynchronous orbit during the Great Solar Proton Event Series of October 1989
Authors: Belian R. D. , Gisler G. R. , Cayton T. , Christensen R.
Published: February 2008

Comparatively high levels of 2‐ to 50‐MeV ions of carbon, nitrogen, oxygen, neon, magnesium, silicon, sulphur, and iron have been identified at geosynchronous orbit by the synchronous orbit particle analyzer, the “SOPA” detector, on board the satellite 1989‐046, which became operational in September 1989. This detector is described, and time histories of some of the above mentioned ions are given for the solar energetic particle event series of late October 1989.
Particle scattering and current sheet stability in the geomagnetic tail during the substorm growth phase
Authors: Pulkkinen T. I. , Baker D. N. , Pellinen R. J. , Büchner J. , Koskinen H. E. J. , Lopez R. E. , Dyson R. L. , Frank L. A.
Published: February 2008

The degree of pitch angle scattering and chaotization of various particle populations in the geomagnetic tail during the substorm growth phase is studied by utilizing the Tsyganenko 1989 magnetic field model. A temporally evolving magnetic field model for the growth phase is constructed by enhancing the near‐Earth currents and thinning the current sheet from the values given by the static Tsyganenko model. Changing the field geometry toward an increasingly taillike configuration leads to pitch angle scattering of particles whose Larmor radii become comparable to the field line radius of curvature. Several different cases representing substorms with varying levels of magnetic disturbance have been studied. In each case, the field development during the growth phase leads to considerable scattering of the thermal electrons relatively close to the Earth. The current sheet regions where the electron motion is chaotic are magnetically mapped to the ionosphere and compared with low‐altitude measurements of electron precipitation. The chaotization of the thermal electron population occurs within a few minutes of the substorm onset, and the ionospheric mappings of the chaotic regions in the equatorial plane compare well with the region of brightening auroras. Even though the temporal evolution of the complex plasma system cannot be self‐consistently described by the temporal evolution of the empirical field model, these models can provide the most accurate estimates of the field parameters for tail stability calculations.
Heat budget of the thermosphere and temperature variations during the recovery phase of a geomagnetic storm
Authors: Maeda S. , Fuller‐Rowell T. J. , Evans D. S.
Published: February 2008

Poststorm temperature variations have been simulated by using a two‐dimensional, time‐dependent model of coupled dynamics and composition. The numerical simulation results have been analyzed to obtain the characteristic time scale of the temperature relaxation as a function of pressure level and latitude. The relaxation time was defined by the e‐folding time of temperature variations in response to the cessation of geomagnetic activity, specified by a decrease in the TIROS/NOAA power index level. The time variation of the power index level was described by a step function. The causal mechanisms responsible for the recovery of the perturbed temperature have been investigated from a term analysis of the energy conservation equation. The relative importance of the different terms of the energy conservation equation has been evaluated from the solutions of model equations as a function of time. By using the calculated individual heating/cooling rates, a time constant is derived from the heat balance between the major heating/cooling rates at a particular simulation time as a function of time. The principal results include the following: (1) the relaxation time of the temperature is in the range from 12 to 15 hours in the upper thermosphere around 350 km and in the range from 15 to 21 hours in the lower region around 150 km; (2) the relaxation time of the temperature is longer within the source region at high latitudes than at middle and low latitudes; (3) the time constants derived from the heat balance at each simulation time increase with time, particularly at high latitudes; (4) the major cooling mechanism in the upper thermosphere is the downward molecular heat conduction; (5) at lower altitudes the radiative cooling from the 5.3‐μm nitric oxide (NO) band is dominant, but the vertical heat advection is also important because of the vertical winds associated with the meridional circulation; and (6) the vertical winds in the lower region also contribute to the adiabatic heating at high latitudes and to the adiabatic cooling at middle latitudes. The relaxation of the temperature during the recovery phase of a geomagnetic storm is not determined solely by the local heat balance but is also influenced by the global dynamical and thermal coupling between various regions of the thermosphere.
Comparison of magnetospheric magnetic field models with CRRES observations during the August 26, 1990, storm
Authors: Jordan C. E. , Bass J. N. , Gussenhoven M. S. , Singer H. J. , Hilmer R. V.
Published: February 2008

Four magnetospheric magnetic field models are compared on the basis of the current systems they represent, their agreement with observations, and their computational performance. These models are the Olson‐Pfitzer dynamic (1988), Tsyganenko 1989, Hilmer‐Voigt, and Olson‐Pfitzer tilt‐dependent (1977). The models are summarized in terms of their components: the main field, the ring currents, the tail currents, and the magnetopause. Differences among the models must be considered when determining which is best for a specific application. The models are evaluated using data from CRRES during both quiet and storm time conditions on August 26, 1990. CRRES is in a geosynchronous transfer orbit with perigee at 360 km and apogee at 33,000 km. Thus, the focus of our comparisons is on the inner magnetosphere out to about 6.3 RE (distant tail properties will not be discussed). Tsyganenko 1989 is a good average model of the magnetospheric magnetic field for various Kp levels. Olson‐Pfitzer dynamic and Hilmer‐Voigt both clearly show changes in the magnetic field due to the compression of the magnetopause as determined from the standoff distance. Hilmer‐Voigt is the most flexible model as it has a parameter to specify the position of the inner edge of the plasma sheet, but it is also the slowest to execute. Olson‐Pfitzer dynamic showed the best agreement with the data presented here.
Thinning and expansion of the substorm plasma sheet
Authors: Baumjohann W. , Paschmann G. , Nagai T.
Published: February 2008

More than 50,000 measurements with the AMPTE IRM satellite in the magnetotail are used for a superposed epoch analysis of the occurrence frequency of the near‐Earth central plasma sheet, which is directly related to its thickness, around 39 major substorm onsets. For all radial distances between 10 and 19 RE, the central plasma sheet gets thinner during the 30–45 min preceding substorm onset, but starts to expand immediately after the onset. The central plasma sheet regains its initial thickness around the beginning of the recovery phase. These observations fit into the framework of the near‐Earth neutral line model, if the neutral line is assumed to form beyond the IRM apogee.
The generation of magnetic fields by the polarization electric field in the ionosphere of Venus
Authors: Shinagawa H. , Cravens T. E. , Wu D.
Published: February 2008

Measurements by the magnetometer on the Pioneer Venus orbiter have established that during conditions of low solar wind dynamic pressure, large‐scale magnetic fields are not present in the ionosphere of Venus but that during conditions of high solar wind dynamic pressure the ionosphere of Venus is magnetized. The source of the magnetic field is thought to be currents induced in the ionosphere by the solar wind. We will show that the ionospheric polarization electric field can act as a source, or “battery,” producing a small magnetic field, even without any initial magnetic field. We have calculated this polarization source as a function of altitude and solar zenith angle. The magnetic field was then determined using a two‐dimensional kinematic dynamo model of the ionosphere of Venus. The magnetic field attains a maximum strength of about 5 nT at a solar zenith angle of about 120°. This magnetic field might act as a “seed” field for magnetic flux ropes and terminator waves.
A multiple‐mode three‐dimensional model of VLF propagation in the Earth‐ionosphere waveguide in the presence of localized D region disturbances
Authors: Poulsen William L. , Inan Umran S. , Bell Timothy F.
Published: February 2008

Transient localized D region disturbances, such as those associated with lightning discharges, affect the characteristics of VLF waves propagating in the Earth‐ionosphere waveguide. In particular, both phase and amplitude changes in the subionospheric signal can be observed at receiving sites as a result of the wave scattering that takes place in the disturbed region. In the present paper we present a multiple‐mode three‐dimensional model of VLF propagation in the Earth‐ionosphere waveguide in the presence of localized D region disturbances. The model takes into account great circle (GC) propagation paths with realistic ground and ionospheric conductivity changes that result in mode conversion along the path. It is assumed that conductivity changes transverse to the GC paths are negligible except in the vicinity of the D region disturbance and that mode coupling is negligible within the disturbed region. This new model is applied to experimental observations and is found to be in general agreement. The diagnostics potential of the model for characterizing energetic particle precipitation events is discussed.
A search for ELF/VLF emissions induced by earthquakes as observed in the ionosphere by the DE 2 satellite
Authors: Henderson T. R. , Sonwalkar V. S. , Helliwell R. A. , Inan U. S. , Fraser‐Smith A. C.
Published: February 2008

Satellite observations of ELF/VLF wave activity by groups from both the Soviet Union and France have indicated the possibility of ELF/VLF radio emissions generated by earthquakes. However, an examination of ELF/VLF wave data from the low‐altitude (apogee ∼ 1300 km, perigee ∼300 km, inclination ∼90°) Dynamics Explorer 2 (DE 2) satellite showed no clearly distinguishable ELF/VLF signatures associated with earthquakes. After an initial survey of approximately 5000 DE 2 orbits, ELF and VLF wave data were selected from 63 satellite orbits, called earthquake orbits, in which the ionospheric footprint of the DE 2 crossed the geographic latitude while passing within ±20° geographic longitude of the epicenters of imminent or recent earthquakes of magnitude ≥5.0. ELF/VLF noise measured near the epicenters was analyzed for occurrence rates and average spectra, as well as for peak and mean electric field intensities in three spectrometers covering a frequency range of 4 Hz ‐ 512 kHz in 20 channels. The same analysis was then repeated for 61 carefully matched control orbits when there were no imminent or recent earthquakes within ±20° geographic longitude or within ±10° geographic latitude of the satellite footprint. These control orbits resembled the earthquake orbits with respect to latitude, longitude, local time, and geomagnetic index Kp. Sixty‐three percent of the earthquake orbits showed an ELF or VLF emission above 10 µV/m in at least one of the 20 channels when the satellite passed near an epicenter. The same analysis performed on control orbit data yielded a 62% chance of observing similar emissions. Moreover, these results did not change when geomagnetic latitudes, instead of geographic latitudes, were considered. Further analyses failed to indicate any significant differences between the ELF/VLF noise measured on earthquake orbits and control orbits with regard to the general nature of the spectra, the frequency of occurrence of emissions, and peak and mean values of the electric field of the emissions.
Effects of ion demagnetization in the plasma sheet
Authors: Swift Daniel W.
Published: February 2008

A 2 1/2‐D nonradiative, electromagnetic particle code is used to investigate processes in the plasma sheet in the Earth's magnetotail that can generate the field‐aligned currents associated with auroral arcs. The setting of the simulation is the midnight meridian plane on closed magnetic field lines. The magnetic field is supported self‐consistently by a plasma sheet population. The system is driven by an external convection electric field that causes particles from the lobe region to drift onto the plasma sheet. Momentum exchange between the field and the lobe particles accelerated through the plasma sheet balances the sunward Maxwell stresses. The particle and field boundaries are spatially separated by a collisional layer representing the effect of the neutral atmosphere. During the simulation, new particles are continually injected into the system from the “ionospheric” regions. The simulations show the generation of currents circulating in the noon‐midnight plane due to the differing degrees of magnetization between ions and electrons. These currents give rise to magnetic fields perpendicular to the noon‐midnight meridian plane. North‐south asymmetries in the lobe plasma density enhance current circulation between the plasma sheet and the ionospheric boundaries. Necessary conditions for generation of auroral currents suggest a new interpretation of the growth and expansive phases of the auroral substorm.
Energetic ion anisotropies in the geomagnetic tail: 1. A statistical survey
Authors: Kettmann Georg , Fritz Theodore A. , Hones Edward W. , Daly Patrick W.
Published: February 2008

During 1978 and 1979, the medium energy particles experiment on board the ISEE 2 spacecraft (apogee 22.7 RE) collected almost 122 days of data in the central region of the Earth's magnetotail. We have surveyed 16‐s averages of the energetic ion (E > 25 keV) and plasma measurements in order to establish the statistical properties of high ion anisotropy events (|A| > 1) which are frequently observed in the near‐Earth plasma sheet. Our major findings from the analysis of 1944 high‐anisotropy samples (HAS) are as follows: (1) Strong energetic ion anisotropies are not maintained over long periods of time but occur in bursts lasting for fractions of a minute up to a few minutes. (2) For almost 80% of the HAS, the anisotropy vector points either earthward or tailward rather than across the tail (3) The ratio of HAS toward the Earth and HAS toward the tail is approximately 4.8:1. (4) The probability of observing high‐anisotropy events is well enhanced beyond ≈16 RE downtail distance, on the duskside, and close (within 2–3 RE) to the neutral sheet. (5) Strong tailward anisotropies can be observed anywhere in the near‐Earth plasma sheet. However, their occurrence frequency is considerably above average beyond xSE ≈ −19 RE and within 2 RE of the neutral sheet. (6) Both plasma bulk flow and ion anisotropy at higher energy (>115 keV) are generally in the same direction as the energetic ion anisotropy at E > 25 keV. (7) Streaming energetic electron populations in the same direction as the ions are significantly more often observed when the ions are streaming tailward than simultaneously with earthward streaming ions when the electrons are usually isotropic. We conclude that statistically the observation of strong tailward ion streaming indicates the existence of a particle source in the near‐Earth magnetotail between the spacecraft and the Earth. Certain features suggest that this source might be identical with a near‐Earth magnetic neutral line.
High‐latitude distributions of plasma waves and spatial irregularities from DE 2 alternating current electric field observations
Authors: Heppner J. P. , Liebrecht M. C. , Maynard N. C. , Pfaff R. F.
Published: February 2008

An 18‐month data base from the Dynamics Explorer 2 AC electric field spectrometers is used to obtain average high‐latitude magnetic local time (MLT) versus invariant latitude (INL) distributions of signal intensities in 12 frequency bands between 4 Hz and 512 kHz. Three distinctly different distributions are obtained, corresponding to (1) Doppler‐shifted signals from spatial structures in the electric field (i.e., irregularities) and Alfven waves between 4 and 512 Hz, (2) ELF waves between 256 Hz and 4.1 kHz, and (3) VLF waves between 4.1 and 64 kHz with extensions into the 128–512 kHz band. The ELF and VLF distributions closely resemble previously published results based on more limited sampling. Comparable distributions for the seven channels between 4 and 512 Hz, showing a prominent zone of maximum intensities at 72.5°–80° INL between 0500 and 1300 MLT, have not previously been reported. The power law frequency dependence of average power spectral densities (PSDs) between 4 and 512 Hz is also mapped in MLT‐INL coordinates. At all locations, two power law indices (slopes) are required to closely fit the PSDs with an inverted knee joining the two slopes in the 32–64 Hz band. This knee band corresponds to the range of O+ cyclotron frequencies encountered, and it lends credence to Gurnett et al.'s (1984) contention that Alfven waves are an essential ingredient in explaining the low‐frequency in situ satellite signals which were previously attributed to polarization fields accompanying spatial irregularities in plasma densities. However, other aspects of the 4–512 Hz observations, including seasonal variations, favor the earlier spatial irregularity interpretation. As discussed, the difficulties encountered in seeking interpretations exclusively in terms of either spatial irregularities or Alfven waves can be resolved with a synthesis approach requiring both types of signals. It is proposed that the averaged intensities and corresponding spectral characteristics in the 4–512 Hz band represent the consequence of intermittently superimposing shear Alfven waves on a spatially irregular medium. There are then three principal contributions: (1) an omnipresent 4–512 Hz signal from Doppler‐shifted responses to 2000–15 m spatial irregularities having an average power law spectral index near −1.9, (2) intermittent signals from locally generated shear Alfven waves having maximum power at frequencies of <4 Hz and average power law spectral indices of ≤(−2.8) extending only to fc(O+), and (3) spatial irregularity modulations of shear Alfven waves originating both locally and in the distant magnetosphere.
Characteristic features of electron temperature and density variations in field‐aligned current regions
Authors: Abe Takumi , Oyama Koh‐Ichiro , Watanabe Shigeto , Fukunishi Hiroshi
Published: February 2008

Observations by the thermal electron energy distribution (TED) instrument on board the Akebono (EXOS ‐ D) satellite show that variations in electron temperatures, at altitudes from 300 to 2300 km, are closely related to field‐aligned current (FAC) structures. At higher altitudes, electron temperatures increase in upward FAC regions and decrease in regions of downward FAC, while at the lower altitudes only temperature increases in upward FAC and adjacent regions are observed. Electron temperature variations were also found to be related to the number density of ambient electrons and the field‐aligned current density. A numerical simulation is carried out to examine energetic particle precipitation and Joule dissipation as a possible mechanism for the observed temperature variations using the electron energy equation. In the present study, we focus the simulations on the temperature increase. Thermal electrons are found to be efficiently heated by Joule dissipation at lower altitudes; however, for increasing altitudes, energetic particle precipitation becomes the major heating source. These conclusions are supported by TED observations at low altitudes (<500 km); however, an additional heat source is required to explain the observed higher‐altitude (>800 km) temperature increases.
A proposed production model of rapid subauroral ion drifts and their relationship to substorm evolution
Authors: Anderson P. C. , Hanson W. B. , Heelis R. A. , Craven J. D. , Baker D. N. , Frank L. A.
Published: February 2008

Multisatellite data are used to examine the temporal relationship between Subauroral Ion Drifts (SAID) and the phases of an auroral substorm. Utilizing images of auroral luminosities taken by the Dynamics Explorer 1 (DE 1) spacecraft and observations of particle injection at geosynchronous orbit, we identify the time of expansive phase onset and estimate the time at which recovery begins. Noting the times at which SAID are observed simultaneously by the DE 2 spacecraft, we find that SAID typically occur well after substorm onset (more than 30 min), during the substorm recovery phase. Substantial westward ion drifts and field‐aligned currents are observed well equatorward of the auroral oval during the expansion phase of a substorm, but the drifts lack the narrow spike signature associated with SAID. Prior to substorm onset and after substorm recovery, field‐aligned currents are absent equatorward of the auroral oval and the ionosphere is very nearly corotating. A phenomenological model of SAID production is proposed that qualitatively agrees with the observed ionospheric signatures and substorm temporal relationship. In this model, substorm‐generated, subauroral field‐aligned currents close via Pedersen currents with the outward flowing, region 1 currents at higher latitudes. These Pedersen currents flow in the region of low conductivity equatorward of the auroral oval and are associated with relatively large, poleward directed electric fields. The frictional heating of the ions caused by collisions with the corotating neutral atmosphere substantially increases the rate of ion‐atom interchange between O+ and N2. Subsequent fast recombination of NO+ with electrons further reduces the subauroral F region conductivities with a corresponding increase in the electric field and the frictional heating. This heating leads to thermal expansion, substantial field‐aligned plasma flow, and very large depletions in the F peak concentration, thus additionally reducing the height‐integrated Pedersen conductivity.
VLF wave growth from dispersive bursts of field‐aligned electron fluxes
Authors: Ergun R. E. , Delory G. T. , Klementis E. , Carlson C. W. , McFadden J. P. , Roth I. , Temerin M.
Published: February 2008

Large‐amplitude (>200 mV/m) electrostatic whistler waves near the lower hybrid frequency (∼5 kHz to ∼20 kHz) were observed by an auroral sounding rocket during substorm breakup. The measured wavelengths (10 m to 40 m) indicate that the emissions were electrostatic and resonant with electrons that had parallel energies of a few hundred electron volts. We propose that the intense emissions drew their energy from dispersive bursts of low‐energy (≲ 100 eV to ∼3 keV), field‐aligned electron fluxes. The dispersive bursts are known to cause a brief (∼100 ms), but intense instability that results in large‐amplitude Langmuir emissions (∼1 MHz). The high‐frequency emissions can rapidly form a plateau in the one‐dimensional electron distribution. We show, however, that these distributions remain unstable to electrostatic whistler waves near the lower hybrid frequency. The amplitude and wavelength of the observed emissions were sufficient to accelerate the hydrogen ions with energies between ∼50 eV and ∼200 eV.
Equatorward and poleward expansion of the auroras during auroral substorms
Authors: Nakamura Rumi , Oguti Takasi , Yamamoto Tatsundo , Kokubun Susumu
Published: February 2008

Using all‐sky TV auroral data with high spatial and temporal resolution, the formation of the auroral bulge is investigated. We show how the discrete auroral structures within the poleward expanding bulge develop systematically toward the west, the east, and also equatorward from a localized breakup region. Auroral structure at the western end of the bulge (a surge) develops with clockwise rotation as viewed along the magnetic field direction. At the eastern part of the bulge, thin auroral features propagate eastward from the breakup region. Around the central meridian of the bulge, auroral features expand equatorward and become north‐south aligned (the N‐S aurora). The N‐S aurora and the eastward propagating aurora develop into diffuse and pulsating aurora after the expansion. We suggest that these discrete auroral structures in the bulge develop along the plasma streamlines in a localized distorted two‐cell equipotential distribution; the negative potential peak is concentrated at the western part of the bulge and the positive potential region is spread poleward of the eastward propagating aurora. Equatorward expansion of auroral structures would be attributed in this scenario to the subsequent earthward transport of plasma from the onset region. Energetic particles would also be injected at the N‐S auroral region as well as to the east of the region and would trigger the pulsating auroral activity. Poleward expansion, on the other hand, would be produced by the tailward motion of the onset region in accordance with the tailward moving current disruption region.
Auroral kilometric radiation sources: In situ and remote observations from Viking
Authors: Roux A. , Hilgers A. , de Feraudy H. , le Queau D. , Louarn P. , Perraut S. , Bahnsen A. , Jespersen M. , Ungstrup E. , Andre M.
Published: February 2008

The present paper is a summary of studies carried out from Viking measurements on the propagation and the generation of the auroral kilometric radiation (AKR hereafter). Advantage is taken of the spin modulation of the AKR observed as Viking was rotating in the cartwheel mode. This, together with the study of the cutoff of the various spectral components, confirms that low‐amplitude Z and O modes are generated at the same time as a larger‐amplitude X mode. Hence Z, O and X mode AKR is all generated by the same sources. The spectrum of the dominant polarization, the X mode, usually contains several spectral peaks. An AKR source crossing is characterized by a minimum in the frequency of the lowest‐frequency peak (fpeak) and by a maximum of its amplitude. About 50 AKR source crossings are used to demonstrate that fpeak approaches fce, the electron gyrofrequency: (fpeak ‐ fce)/fce ≈ 0.025 in AKR sources. Similarly, the low‐frequency cutoff of the AKR is found, on average, to coincide with fce. The density inside AKR sources is determined from four sets of independent measurements, namely (1) the upper frequency cutoff of the hiss, (2) the relaxation sounder, (3) the Langmuir probe, and (4) particle measurements. It is shown that an AKR source coincides with a strong depletion in the density of the cold/cool electrons that becomes comparable to or less than the density of energetic electrons (E ≥ 1 keV). The total density inside AKR sources is of the order of 1 cm−3, typically a factor 5 to 10 below that of the surrounding regions. AKR sources are found to coincide also with an acceleration region characterized by a potential drop of ≥1 kV, both below and above the spacecraft. Evidence for this comes from the observation of electrons accelerated above the spacecraft and ions accelerated below it. In addition to a strong depletion in the density of the cool electrons, particle measurements on Viking give evidence of several possible free energy sources that could drive unstable the AKR, namely (1) a loss cone, (2) a hole for parallel velocities smaller than that of the observed downgoing electron beam, and (3) a trapped electron component for a pitch angle a ≈ 90°. The trapped electron component, bounded at low perpendicular energies (a few keVs) by an enhanced loss cone, is observed inside, and only inside, AKR sources. It is therefore concluded that the corresponding ∂f/∂v⊥ > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser.
A nonsingular model of the open magnetosphere
Authors: Toffoletto F. R. , Hill T. W.
Published: February 2008

We present a modified version of the Toffoletto and Hill (1989) open magnetosphere model that incorporates a tail‐like interconnection field with a discontinuity to represent the slow‐mode expansion fan that defines the high‐latitude tail magnetopause. (The interconnection field is defined as the perturbation on an initially closed magnetosphere model to make it open.) The expansion fan controls the open field line region in the tail, and the intersection of the fan with the tail current sheet is, by design, the x line. The new interconnection field allows greater control of the tail field structure; in particular, it enables us to eliminate the nightside mapping singularity that occurs in previous models when the interplanetary magnetic field is nonsouthward. Also, in contrast to earlier models, the far tail x line extends farther downstream on the flanks than in the center of the tail, consistent with observations.
Nonlinear evolution of electromagnetic ion beam instabilities
Authors: Akimoto K. , Winske D. , Gary S. P. , Thomsen M. F.
Published: February 2008

An ion beam streaming along a magnetic field is known to be mainly unstable to two field‐aligned, low‐frequency electromagnetic instabilities: the ion/ion right‐hand resonant instability and ion/ion nonresonant instability. A comparative study of the two instabilities by means of linear analysis and simulation is carried out to investigate and contrast their properties. Linear analysis demonstrates that the nonresonant instability becomes resonant if the density of the ion beam is sufficiently high. When the relative drift speed between the ion beam and the ambient ions is 10 υA, where υA is the Alfvén speed, the nonresonant instability undergoes a transition to a resonant interaction at nb/no≃0.065, nb(no) being the beam (total ion) density. Hybrid simulations carried out for the densities of nb/no = 0.02, 0.1, and 0.25 show both the resonant and nonresonant instabilities result in the formation of nonlinear pulses called pulsations but with distinct features. For example, the pulsations generated by the resonant instability have positive correlation with the ion density, while those generated by the nonresonant instability are likely to have relatively weak negative correlation. Furthermore, the waves generated by the nonresonant instability are subject to a parametric decay instability and tend to form a state of condensate where the turbulence becomes nearly monochromatic. These features can be utilized to differentiate between pulsations produced by the two instabilities in space and, in particular, short large amplitude magnetic structures.
Charge transfer and momentum exchange in exospheric D‐H+ and H‐D+ Collisions
Authors: Hodges R. R. , Breig E. L.
Published: February 2008

Mechanisms that control the escape of deuterium from planetary exospheres include the acceleration of D+ in the polar wind, and the production of suprathermal D atoms through nonthermal collisions. In this paper we examine the effects of neutral‐ion interactions involving deuterium and hydrogen on the velocity distribution of neutral D. A two‐center scattering approximation is used as the basis for calculations of the differential cross sections for charge transfer and elastic scatter in collision of H with D+ and of D with H+ for ionosphere‐exosphere collision energies below 10 eV. These data are used to derive temperature dependent rate coefficients for the charge transfer branches of these interactions, and to determine the effects of ion‐neutral temperature differences on the rate of generation of suprathermal D through charge transfer and elastic scatter.
Structure and length scales of rotational discontinuities
Authors: Krauss‐Varban D.
Published: February 2008

We have carried out a series of one‐dimensional hybrid (kinetic ions, fluid electrons) simulations, using a nondynamic method to form rotational discontinuities (RDs) at an angle of θBn = 60° between the normal direction and the upstream magnetic field. Ion kinetic effects are found to be important concerning the stability of the current layer, its thickness and scaling, and its dependence on initial conditions. When the ions are initialized with transverse velocities derived from Hall MHD, RDs with both senses of rotation α1 = ±180° can be stable down to widths of a few c/ωpi, (ion inertial lengths). This initialization gives reliable results except in the case of thin electron‐sense RDs with a rotation angle α1 = −180°, which have a pronounced depression of the transverse ion velocity, and develop more easily starting from a nonrotational transition of the ion flow. At a plasma beta of order unity, there is a minimum width of a few c/ωpi for transitions of both senses of rotation. Thin RDs show a significant upstream heat flux and temperature anisotropies. Their small‐scale structure is embedded in a larger envelope of the order of 100 c/ωpi. In a cold plasma, ion‐sense RDs with α1 = 180° have a minimum width which scales with the ion inertial length. In a warm plasma, the thickness increases with the square root of the ion beta due to the finite ion Larmor radius. There appear to be no stationary structures for rotation angles |α1| significantly larger than 180°. However, the associated breakup time increases with the initial thickness, eventually approaching the stable MHD limit for transitions very wide compared to the ion Larmor radius. The two senses of rotation α1 = ±270° show completely different mechanisms for disintegration. Electron‐sense rotations appear to be composed of a stable, large amplitude α1 = −360° solitary wave at the upstream edge and an ion sense α1 = 90° rotation; a slight difference in speed separates the α1 = −270° total rotation over time. Ion‐sense RDs with α1 = 270° are metastable: their current structure disintegrates explosively after an initial stable interval (many 100 Ωci−1 for wide transitions, where Ωci is the ion cyclotron frequency), when internal ion heating has increased the average gyroradius above the width of the transition.
2‐ to 3‐kHz continuum emissions as possible indications of global heliospheric “breathing”
Authors: Grzedzielski S. , Lazarus A. J.
Published: February 2008

The main features of 2‐ to 3‐kHz heliospheric emissions are analyzed in the context of a general heliospheric “breathing” as inferred from the Voyager 2 solar wind average ram pressure data. Following McNutt (1988) we suggest triggers for the three 3‐kHz emission events seen to date, and we obtain good agreement in timing and expected 2fnof;p (postshock) frequency for termination shock distances ∼90 AU. This determination seems to correspond to epochs when the heliosphere is larger than on the average. We suggest that the visibility of the individual 3‐kHz events and their observed upward frequency drift are enhanced when the postulated global heliospheric expansion results in formation of a transient, compressed external plasma barrier around the heliopause that prevents for several months the escape of the radiation. We suggest that the 2‐kHz emission band is due to typical solar wind density maxima interacting with the termination shock and that the disappearance of that emission after mid‐1987 is caused by the 2fnof;p (postshock) frequency moving into the 2.4‐kHz spacecraft interference band during a period of closer shock distance corresponding to years of lower solar wind ram pressure. From all the events considered, we estimate the average termination shock distance to be in the range 80‐90 AU.
Cosmic ray decreases and magnetic clouds
Authors: Cane H. V.
Published: February 2008

A study has been made of energetic particle data, obtained from IMP 8, in conjunction with solar wind field and plasma data at the times of reported magnetic clouds. It is shown that magnetic clouds can cause a depression of the cosmic ray flux but high fields are required. A depression of 3% in a neutron monitor requires a field of about 25 nT. Such high fields are found only in a subset of coronal ejecta. The principal cause for Forbush decreases associated with energetic shocks is probably turbulence in the postshock region, although some shocks will be followed by an ejecta with a high field. Each event is different. The lower‐energy particles can help in identifying the dominant processes in individual events.
Giant transient decreases of cosmic rays in the outer heliosphere in September 1991
Authors: Webber W. R. , Lockwood J. A.
Published: February 2008

Large transient decreases of > 20% were observed in the > 70 MeV cosmic ray intensity in September 1991, at the three spacecraft, V1, V2, and P10, in the outer heliosphere between 35 and 53 AU. These decreases appear to be related to the intense solar activity occurring in late May and early June, which was responsible for a series of rapid transient decreases at the Earth that probably reduced the > 70‐MeV intensity by more than 50% to the lowest level ever recorded. Average transit speeds of ∼600 to 800 km/s are deduced for the propagation of these transients between the Earth and the outer heliosphere. The overall picture of the propagation of these transients is consistent with a massive, almost spherical modulating region moving outward in the heliosphere. The smooth almost exponential recovery of intensities at all three spacecraft for over 160 days after the decrease, and the large total modulation beyond P10 at the time of the decrease, suggest that the modulation boundary is well beyond 53 AU and probably beyond 100 AU.
Magnetic reconnection with large separatrix angles
Authors: Yan M. , Lee L. C. , Priest E. R.
Published: February 2008

A two‐dimensional incompressible MHD simulation model is developed to study magnetic reconnection with large magnetic separatrix angles. It is found that the normal magnetic field on the inflow boundary plays an important role in determining the reconnection configuration. When the normal magnetic field on the inflow boundary is large, steady state reconnection with large magnetic separatrix angles is obtained. When the normal magnetic field is small, a localized resistivity is required to maintain the steady state Petschek‐like reconnection configuration, which has small magnetic separatrix angles. Field‐aligned plasma jets are observed when the normal magnetic field and magnetic Reynolds number Rm are large. The plasma jets are located slightly downstream of the magnetic separatrices and each plasma jet consists of two parts: (1) a slow‐mode shock and (2) a fast‐mode compressional structure. The length and width of the diffusion region depend on the values of Me, Rm, and the normal magnetic field on the inflow boundary. For a fixed normal inflow magnetic field the diffusion region becomes longer and thinner as Me becomes larger; the width of the diffusion region decreases with an increasing Rm, while the length of the diffusion region has a minimum value at about Rm = 500. The length of the diffusion region increases with a decreasing normal magnetic field, while the width does not change remarkably. It is also found that the multiple X line bursty reconnection tends to occur with the development of tearing‐mode instability in cases with a large Me, a large Rm, or a small normal magnetic field on the inflow boundary.
Interplanetary proton fluence model: JPL 1991
Authors: Feynman J. , Spitale G. , Wang J. , Gabriel S.
Published: February 2008

We describe an updated predictive engineering model for the interplanetary fluence of protons with energies >1, >4, >10, >30, and >60 MeV. This has been the first opportunity to derive a model from a data set that has been collected in space over a long enough period of time to produce a valid sample of solar proton events. The model provides a quantitative basis for estimating the exposures to solar protons of spacecraft during missions of varying length and of surfaces and atmospheres of solar system objects. It is derived from the set of data collected by the IMP and OGO spacecraft between 1963 and 1991. The >10 and >30 MeV data sets cover the period from 1963 to day 126 of 1991. The >1, >4, and >60 MeV data sets were collected between 1973 and 1991. Both data sets contain several major proton events (>10‐MeV fluences exceeding 3 or 4 × 109 protons/cm2) comparable to the 1972 event. The method of statistical analysis used in producing the model of the proton environment is the same as that used for earlier models. For the cases of the >10 and >30 MeV particles, the fluences are somewhat lower than in our earlier model (JPL 85). No >1, >4, and >60 MeV proton fluence models have been published in the literature previously. We present our results in a convenient graphical form which may be used to calculate the 1 AU fluence expected at a given confidence level as a function of the length of the exposure. A method of extending this estimate to other heliocentric distances is described.
A narrow region of electron beams at the poleward edge of the cusp
Authors: Yamauchi M. , Lundin R.
Published: February 2008

Viking electron energy spectrogram (ESP) and positive ion energy spectrogram (PISP) data show that there is a thin and faint but distinguishable region at the poleward edge of the cusp‐cleft region. This region is characterized by 100‐ to 300‐eV field‐aligned electrons with no striking ion feature except for coexisting low‐energy upward ions. These faint electron beams are almost always observed unless the interplanetary magnetic field (IMF) is southward, yet they are different from the polar arcs, boundary cusp electrons, and the merging type electron burst on merging field lines for the following reasons. First, the thickness of this region (very thin) and the electron beams' pitch angle distribution of this region are clearly different from those of the boundary cusp or the polar arc region. Second, these electron beams are observed during weak IMF when the ions do not show any merging‐associated energy dispersion. This region sometimes exists even when the merging line is located on the other (equatorward) side of the cusp, indicating that these electron beams are not the merging‐related electron burst. Third, this region is absent near local noon (1130‐1230 MLT). The electron beams are found rather on the morningside and the afternoonside. This special distribution is not expected for polar arcs or merging‐related electron bursts. The absence of the thin electron beam region near local noon indicates, together with its latitudinal location (always between the cusp and the polar cap or polar arcs), that the tail boundary layer can be a possible source of these electron beams. Since this newly found region is very thin and it is always located between the cusp and polar cap or polar arcs, we call it the “cusp poleward edge,” despite its absence near local noon.
A turbulent theoretical framework for the study of current‐driven E region irregularities at high latitudes: Basic derivation and application to gradient‐free situations
Authors: Hamza A. M. , St‐Maurice J. ‐P.
Published: February 2008

We have used a mode‐coupling hypothesis to study the nonlinear evolution of E region irregularities at high latitudes. Conservation of energy and the identification of two distinct time scales for the problem at hand has allowed us to obtain an expression in the fluid regime for the mean frequency and the spectral width of different types of echoes observed by coherent radars. In this particular paper we have applied our results to a few simple cases, namely, to situations that are free of large‐scale gradients and for which aspect angles are close to zero. Even though anomalous diffusion effects were also neglected, our theory nevertheless predicts that in the absence of density gradients, strongly driven Farley‐Buneman waves should normally saturate at a mean speed between 70% and 100% of the ion‐acoustic speed of the medium. The theory also predicts that zero‐frequency type 2 waves in strongly turbulent situations should have a frequency width, which when translated to a Doppler width, should be approximately equal to the ion acoustic speed of the medium. Those results are a direct consequence of assuming that mode coupling is responsible for the saturation of all linearly unstable waves in the E‐region plasma. A companion paper will consider the modifications introduced by large‐scale density gradients.