bluuebunny/crossref_metadata_2025 huggingface dataset:
Coronal mass ejection and stream interaction region characteristics and their potential geomagnetic effectiveness
Authors: Lindsay G. M. , Russell C. T. , Luhmann J. G.
Published: February 2004

Previous studies have indicated that the largest geomagnetic storms are caused by extraordinary increases in the solar wind velocity and/or southward interplanetary magnetic field (IMF) produced by coronal mass ejections (CMEs) and their associated interplanetary shocks. However, much more frequent small to moderate increases in solar wind velocity and compressions in the IMF can be caused by either coronal mass ejections or fast/slow stream interactions. This study examines the relative statistics of the magnitudes of disturbances associated with the passage of both interplanetary coronal mass ejections and stream interaction regions, using an exceptionally continuous interplanetary database from the Pioneer Venus Orbiter at 0.7 AU throughout most of solar cycle 21 (1979–1988). It is found that both stream interactions and CMEs produce magnetic fields significantly larger than the nominal IMF. Increases in field magnitude that are up to 2 and 3 times higher than the ambient field are observed for stream interaction regions and CMEs, respectively. Both stream interactions and CMEs produce large positive and negative Bz components at 0.7 AU (∼ 14 nT on average), but only CMEs produce Bz magnitudes greater than 35 nT. CMEs are often associated with sustained periods of positive or negative Bz whereas stream interaction regions are more often associated with fluctuating Bz. CMEs tend to produce larger solar wind electric fields than stream interactions. Yet stream interactions tend to produce larger dynamic pressures than CMEs. Dst predictions based on the solar wind duskward electric field and dynamic pressure indicate that CMEs produce the largest geomagnetic disturbances while the low‐speed portion of stream interaction regions are least geomagnetically effective. Both stream interaction regions and CMEs contribute to low and moderate levels of activity with relative importance determined by their solar‐cycle‐dependent occurrence rates.
Comparison of UV optical signatures with the substorm current wedge as predicted by an inversion algorithm
Authors: Sergeev V. A. , Vagina L. I. , Elphinstone R. D. , Murphree J. S. , Hearn D. J. , Cogger L. L. , Johnson M. L.
Published: February 2004

Optical images of the auroral bulge as seen by the Viking UV imager were compared in several cases with the substorm current wedge (SCW) upward and downward field‐aligned currents (FAC) whose positions were determined using the inversion algorithm based on the substorm‐related magnetic variations observed at midlatitudes. With reasonable accuracy (better than 0.5 hours MLT) the estimated longitudes of the upward FAC generally pointed to the surge or to the brightest luminousity region in the western half of the bulge. The latter feature may imply a more complicated structure of the net FACs than can be described by the simple substorm current wedge scheme. Similarly, the estimated positions of the downward FAC pointed close to the eastern termination of the bulge. The associated optical signatures of this current system ranged from the well‐defined emission depletion regions to new auroral intensifications. The downward current appears to correspond in some cases at least with the division between the morning sector portion of the double oval and the nightside portion connected more directly to the substorm bulge. The results in general confirm the expected association between the auroral bulge and the SCW, as well as showing a reasonably good results from the inversion algorithm based on midlatitude magnetic observations. Our results, however, also indicate that one must be careful in interpreting the apparent motion of SCW‐related field‐aligned currents inferred from midlatitude observations in terms of a true westward or eastward expansion of the SCW or of the auroral bulge. The observed changes may instead sometimes be related to the redistribution of the net FACs within than a shift or expansion of the simple current system.
Satellite remote sensing of thermospheric O/N2 and Solar EUV: 2. Data analysis
Authors: Evans J. S. , Strickland D. J. , Huffman R. E.
Published: February 2004

The companion paper by Strickland et al. (this issue) describes a technique for deriving QEUV and O/N2 from disk observations of OI 135.6‐nm and N2 LBH dayglow emission. QEUV refers to the integrated solar EUV energy flux below 45 nm and is derived from knowledge of the 135.6/LBH ratio or O/N2 in conjunction with either the 135.6 nm or LBH intensity. O/N2 refers to the ratio of the atomic oxygen (O) column density to the molecular nitrogen (N2) column density at a given value of the N2 column density. Strickland et al. show that the least uncertainty in derived values O/N2 occurs in the vicinity of an N2 depth of 1017 cm−2. The O/N2 values presented in this paper are referenced to this depth. While QEUV is obtained from the intensity of either 135.6 nm or LBH, O/N2 is obtained from the intensity ratio designated by 135.6/LBH. The technique has been used to derive O/N2 and QEUV values from nadir‐viewing far ultraviolet dayglow data obtained by the auroral and ionospheric remote sensor instrument on board the Polar BEAR satellite. Data are considered from single passes on July 15, 16, and 21, 1987, spanning a latitude range from 25° to 55°N. The 3‐hour ap index was between 20 and 30 for the first two passes and only about 5 for the third pass. The 135.6‐nm and LBH signals were obtained from spectra recorded at a resolution of 3.6 nm from which a background was subtracted followed by integration over the intervals from 134.5 to 139.0 nm and 155.0 to 170.0 nm. Uncertainties were assigned to the signals and their ratios that took into account statistical uncertainties in the true signal and in the subtracted background signal. Profiles of derived O/N2 with uncertainties reflecting the data uncertainties are shown along with mass spectrometer/incoherent scatter (MSIS) O/N2 profiles over 25°–55°N. More structure is seen in the profiles on July 15 and 16, which are more disturbed days than July 21. In all cases, O/N2 increases from high to low latitudes. MSIS also shows such an increase but is much less structured. The results are shown to agree qualitatively with O/N2 values obtained from a general circulation model. Derived QEUV values are in the range from 1.1 to 1.4 ergs cm−2 s−1 and are in good agreement with values derived from the Hinteregger spectrum for the F10.7 values appropriate to the observations. The results illustrate the potential of the technique for monitoring thermospheric dynamics through latitudinal and temporal variations in O/N2 which are signatures of ascending and descending motion in the thermosphere.
Linear transport of solar wind fluctuations
Authors: Oughton Sean , Matthaeus William H.
Published: February 2004

Numerical solutions for the linear transport of solar wind fluctuations are presented. The model used takes into account the effects of advection, expansion, and wave propagation, as well as the recently illuminated effects of (non‐WKB) “mixing” terms. The radial evolution of the fluctuating kinetic and magnetic energies and of the cross helicity is computed, and it is demonstrated that in appropriate limits the solutions converge to the WKB forms. In more general cases, however, where the fluctuations consist of a superposition of various types of turbulence, mixing leads to solutions which differ substantially from those predicted by WKB theory. The degree of mixing shows considerable dependence on the nature of the turbulence, giving rise to varying levels, at 1 ∼ AU, of the ratio of “inward” and “outward” fluctuation energies and the ratio of kinetic and magnetic fluctuation energies. The transport properties described here may provide a partial explanation for the observed decrease of cross helicity with increasing heliocentric distance in the solar wind.
Gyrophase effects in the centrifugal impulse model of particle motion in the magnetotail
Authors: Delcourt D. C. , Sauvaud J. A. , Martin R. F. , Moore T. E.
Published: February 2004

We investigate the dynamics of charged particles in a magnetic field reversal in the particular case where the Larmor radius is comparable to the magnetic field line curvature radius. In the current interpretation framework based upon the parameter κ (defined as the square root of the minimum curvature radius‐to‐maximum Larmor radius ratio), this situation corresponds to κ of the order of 1. We show that this nonadiabatic regime, which lies at the transition between adiabatic motion (κ ≫ 1) and the nonadiabatic one characterized by oscillations about the field minimum (κ < 1), results from prominent centrifugal effects in the field reversal. To model these effects, we develop a simple analytical description which is based upon an impulsive centrifugal force perturbing the particle gyromotion on the time scale of the cyclotron turn. Comparisons with numerical calculations of ion trajectories demonstrate that this centrifugal impulse model adequately describes both the magnetic moment variations and the gyrophase variations experienced by the particles. As for the magnetic moment, three distinct behaviors (negligible change, strong phase dependence with possible damping, and systematic enhancement) are identified depending upon pitch angle. As for the gyration phase, important bunching effects are obtained which are interpreted as being due to the action of the impulsive centrifugal force. These phase bunching effects are enhanced as the κ parameter approaches unity. In the limit κ ∼ 1, a strong imbalance is obtained between the two phase sectors corresponding to duskward and dawnward motions. This imbalance, which extends over a few tenths of an Earth radius in the Z direction, leads to the formation of a thin current sheet in the vicinity of the magnetotail midplane.
Parametric decays of electromagnetic ion cyclotron waves in a H+ ‐He+ ‐O+ magnetosphericlike plasma
Authors: Gomberoff L. , Gnavi G. , Gratton F. T.
Published: February 2004

Parametric decays of large‐amplitude electromagnetic ion cyclotron waves (EICW) due to a minor O+ ion component in the magnetosphere are studied. It is shown that the presence of O+ ions leads to a number of new wave couplings which in turn lead to new instabilities. Some couplings involve sound waves carried mainly by the O+ ions, and a sideband EICW which has a resonance at the O+ ion gyrofrequency. These are decay instabilities which can lead to O+ heating through Landau damping and/or resonance absorption. There is also a modulational instability involving two sideband EICW, one propagating forward and the other propagating backward relative to the external magnetic field. These waves can also transfer energy to the O+ ions through resonance absorption. The other branches of the dispersion relation, namely, the He+ and proton branch, have additional decay instabilities (Gomberoff et al., 1995) due to the presence of a minor O+ ion component. It is also shown that in the fluid description, the decays to sound waves associated with the minority heavy ion species have growth rates comparable to, or even larger than, the decays to the acoustic branch corresponding to the majority proton species.
Ionospheric observations of magnetospheric low‐latitude boundary layer waves on August 4, 1991
Authors: Clauer C. R. , Ridley A. J.
Published: February 2004

We report on a coordinated set of observations from Greenland magnetometers, Sondrestrom incoherent scatter radar, DMSP F9 and F10 satellite data, and IMP 8 measurements of the interplanetary magnetic field (IMF) and solar wind plasma. These data were acquired during an interval (August 4, 1991, 1100 ‐ 1400 UT) which is characterized by magnetic ULF pulsations observed on the ground which occur in a limited band between 73° and 78° magnetic latitude. The spatial characteristics of the ground ULF magnetic pulsation disturbances are consistent with a continuous train of ionospheric convection vortices moving westward across local magnetic noon sector with a phase velocity of 1.3 km/s. The vortices occur at the ionospheric convection reversal boundary. Defense Meterological Satellite Program (DMSP) drift meter data are consistent with the incoherent scatter convection measurements, and DMSP particle data indicate that the ionospheric reversal boundary maps to the low‐latitude boundary. Using magnetometer stations across Greenland from the east coast, central Greenland, and the west coast, we can determine that individual vortices are long lived, lasting over 15 min. Cross correlations of the pulsation wave forms at stations separated longitudinally by about 500 km produce coefficients between 0.65 and 0.75. Cross correlations between stations separated longitudinally by 1000 km produce coefficients of only 0.39. This indicates that while individual vortices are long lived, they are evolving significantly in shape, latitudinal location, and intensity, as they move distances of 1000 km across the ionosphere. Using a field‐aligned current filament model to fit the data, we find the strength (50 to 100 kA) of these currents to be larger than typical currents previously reported. We find solar wind pressure variations an unlikely source for the generation of these pulsations and suggest that they are the ionospheric signature of either a Kelvin‐Helmholtz mechanism or perhaps magnetic merging during an interval of IMF oriented predominantly in the By direction.
Akebono observations of electron temperature anisotropy in the polar wind
Authors: Yau A. W. , Whalen B. A. , Abe T. , Mukai T. , Oyama K. I. , Chang T.
Published: February 2004

We report Akebono observations of anisotropic electron velocity distributions in the sunlit polar wind, in which the averaged energy (temperature) of the thermal energy electrons is higher in the upward magnetic field direction than in the downward and perpendicular directions. In the 1500–4000 km altitude region, the observed upward to downward temperature ratio, Teu/Ted, was in the range of 1.5 to 2. The observed downward and perpendicular temperatures were similar; Ted/Te⊥ ≃ 1. The heat flux associated with the observed thermal energy electron velocity distribution was upward and on the order of ∼10−2erg cm−2s−1, and was a factor of 5 larger than that of the atmospheric photoelectrons above 10 eV. In this altitude region, the higher‐energy (> a few eV) photoelectrons are essentially collisionless. In contrast, the ambient and lower‐energy photoelectrons remain collisional because of their larger Coulomb collision cross sections. In the steady state, an ambipolar electric field is required to maintain quasi‐neutrality along the field line. It is suggested that the ambipolar electric field and the Coulomb collisions modify the velocity distributions of the thermal energy electrons and the photoelectrons, resulting in the observed temperature anisotropy. The upward heat flux associated with the observed temperature anisotropy dominates any downward heat flux due to electron temperature gradients that may be present in the polar wind plasma and demonstrates the important role of the photoelectrons in the dynamics of the polar wind.
Idealized Saturn magnetosphere shape and field
Authors: Maurice S. , Engle Irene M.
Published: February 2004

We construct an idealized three‐dimensional model of Saturn's magnetosphere. The total magnetospheric field is a sum of three contributions; the first two contributions are internal, in the form of a planetary dipole and an equatorial ring current deduced from Voyager observations. We concentrate on the third contribution, resulting from the interaction of the solar wind with the Kronian magnetosphere. From the balance between the solar wind ram pressure and the magnetospheric pressure, we provide simultaneously models of the magnetopause and of the magnetic field due to the surface currents on the magnetopause. The resulting magnetopause has characteristics intermediate between those of analogous Earth and Jupiter magnetospheric models. A natural day‐night asymmetry arises in the total magnetospheric field. Its intensity is tested with Voyager 1 data: the third contribution improves the agreement between data and models in the outer magnetosphere. This work extends the range of a physically reliable model of Saturn's magnetosphere in latitudinal and longitudinal directions from regions where there have been observations to regions where there have not. We discuss several possible applications, particularly some relevant to the future Cassini mission.
Suprathermal He2+ in the Earth's foreshock region
Authors: Fuselier S. A. , Thomsen M. F. , Ipavich F. M. , Schmidt W. K. H.
Published: February 2004

ISEE 1 and 2 H+ and He2+ observations upstream from the Earth's bow shock are used to investigate the origin of energetic (or diffuse) ion distributions. Diffuse ion distributions have energies from a few keV/e to >100 keV/e and have near solar wind concentrations (i.e., an average of about 4% He2+). These distributions may evolve from suprathermal ion distributions that have energies between 1 and a few keV/e. Upstream intervals were selected from the ISEE data to determine which suprathermal distributions have He2+ concentrations similar to those of diffuse ion distributions. The type of distribution and the location in the foreshock were similar in all events studied. Two intervals that represent the results from this study are discussed in detail. The results suggest that diffuse ion distributions evolve from suprathermal distributions in the region upstream from the quasi‐parallel bow shock. For He2+, the suprathermal distribution is a nongyrotropic partial ring beam and has characteristics consistent with specular reflection off the quasi‐parallel bow shock. The suprathermal proton distributions associated with these He2+ distributions are nongyrotropic partial ring beams or nearly gyrotropic ring beams also approximately consistent with specular reflection. The location in the quasi‐parallel foreshock and the similarity of the suprathermal He2+ and H+ distributions suggest that these are the seed population for diffuse distributions in the foreshock region.
Magnetic effect due to hypervelocity dust particle impact on the Giotto spacecraft
Authors: Israelevich P. L. , Ershkovich A. I. , Samir U. , Neubauer F. M.
Published: February 2004

Magnetic field disturbance accompanying dust particle impacts is explained by means of the creation of a secondary cloud around the Giotto spacecraft. The enhancement of the magnetic field is seen as a result of the deceleration of electrons in the cometary plasma flow due to collisions with neutral atoms of the cloud surrounding the spacecraft. As the magnetic field lines are frozen into the electron component of the plasma flow, this deceleration leads to an increase of the magnetic field strength. The electric current responsible for the magnetic perturbation is the electron drift current under the action of the friction force between electrons and neutral atoms. Impact‐produced cloud parameters were estimated by using the magnetic disturbance and the angular distribution of the scattered cometary ions.
Particle energization near an X line in the magnetotail based on global MHD fields
Authors: Joyce G. , Chen J. , Slinker S. , Holland D. L. , Harold J. B.
Published: February 2004

Particle acceleration near an X line in the magnetotail and the resulting ion distribution functions in the central plasma sheet are studied by using 2½‐dimensional test particle simulations. The electric and magnetic fields are taken from a three‐dimensional global magnetohydrodynamic (MHD) simulation model describing the solar wind‐magnetosphere interaction. A southward interplanetary magnetic field is chosen as the input solar wind condition. Test particles are generated from a model ion source distribution consistent with the macroscopic MHD parameters in the lobe region. The particles are calculated earthward and tailward of the X line. It is found that ion distributions consist of two main components: a low‐energy population similar to the input distribution and a high‐energy population. The former corresponds to particles that do not cross the region in which the Bχ field reverses direction, while the latter consists of particles which cross the midplane, gaining energies on the order of keV. The spatial dependence of the relative population of low‐energy versus high‐energy components is discussed. The details of the acceleration process are determined from the motion of test particles in the simulation.
Fokker‐Planck modeling of auroral wave‐particle interactions
Authors: Dendy R. O. , Harvey B. M. , O'Brien M. , Bingham R.
Published: February 2004

Observations of electron velocity distributions in the auroral zone, carried out by spacecraft and by rockets, have found high‐energy, magnetic field‐aligned, suprathermal tails in regions where strong wave activity in the lower hybrid range of frequencies is also detected. A causal link between these observations could in principle arise from quasi‐linear diffusion of the electrons, driven by Landau resonant lower hybrid waves. To investigate this, a two‐dimensional finite difference Fokker‐Planck code has been written. The results of the numerical investigations carried out with this code indicate that the observed lower hybrid wave intensities are sufficient to produce strongly nonthermal electron distributions, which are similar to those observed.
Thermospheric nitric oxide from the ATLAS 1 and Spacelab 1 missions
Authors: Torr Marsha R. , Torr D. G. , Chang T. , Richards P. , Swift W. , Li N.
Published: February 2004

Spectral and spatial images obtained with the Imaging Spectrometric Observatory on the ATLAS 1 and Spacelab 1 missions are used to study the ultraviolet emissions of nitric oxide in the thermosphere. By synthetically fitting the measured NO gamma bands, intensities are derived as a function of altitude and latitude. We find that the NO concentrations inferred from the ATLAS 1 measurements are higher than predicted by our thermospheric airglow model and tend to lie to the high side of a number of earlier measurements. By comparison with synthetic spectral fits, the shape of the NO γ bands is used to derive temperature as a function of altitude. Using the simultaneous spectral and spatial imaging capability of the instrument, we present the first simultaneously acquired altitude images of NO γ band temperature and intensity in the thermosphere. The lower thermospheric temperature images show structure as a function of altitude. The spatial imaging technique appears to be a viable means of obtaining temperatures in the middle and lower thermosphere, provided that good information is also obtained at the higher altitudes, as the contribution of the overlying, hotter NO is nonnegligible. By fitting both self‐absorbed and nonabsorbed bands of the NO gamma system, we show that the self absorption effects are observable up to 200 km, although small above 150 km. The spectral resolution of the instrument (1.6 Å) allows separation of the N+(5S) doublet, and we show the contribution of this feature to the combination of the NO γ (1,0) band and the N+(5S) doublet as a function of altitude (less than 10% below 200 km). Spectral images including the NO δ bands support previous findings that the fluorescence efficiency is much higher than that determined from laboratory measurements. The Spacelab 1 data indicate the presence of a significant population of hot NO in the vehicle environment of that early shuttle mission.
High‐frequency Doppler radar observations of magnetic aspect sensitive irregularities in the midlatitude E region ionosphere
Authors: Bourdillon A. , Haldoupis C. , Delloue J.
Published: February 2004

In this paper, an experiment designed for dual‐frequency azimuthal Doppler spectrum studies of decameter‐scale field‐aligned irregularities in the midlatitude E region ionosphere is introduced, and the first results are presented. The observations were made in July 1993 with a large HF radar facility near Valensole (43.8°N, 6.1°E), in the south of France. The radar system is an oblique ionospheric sounder that employes long antenna arrays of vertical monopole pairs and covers the HF frequency band from 4 to 30 MHz in 1‐kHz steps. A scheme of broad beam width transmission and narrow‐beam, phased‐array, multireceiver coverage was used to scan with 2° step an azimuthal sector from about 24° east to 60° west of geomagnetic north. The 15‐gate viewing region was confined in range between 100 and 370 km in order to cover an area of E region magnetic aspect sensitive backscatter near 37° invariant magnetic latitude (L ≃ 1.7, magnetic dip, ∼60°). In this configuration, each azimuthal scan was completed in 80 s over a 15×42 spatial grid with the full Doppler spectrum at each grid point recorded in real time. The experiment provided observations simultaneously at two frequencies, 9.0 MHz and 14.8 MHz, that correspond to backscatter from plasma waves with wavelengths of 16.7 and 10.1 m, respectively. Here, we present an overview of the observations that include azimuthal and range‐time echo characteristics as well as mean Doppler shift and spectrum width properties. The first results show aspect sensitive decameter‐wavelength irregularities having mean phase velocities at least as large as 120 m/s to act as tracers of wavelike dynamic structures that drift westward with speeds in the 40‐ to 80‐m/s range and have characteristic times between 10 and 30 min and typical scale lenghts between 40 and 90 km. In our interpretation, we consider these structures to be sporadic Es ionization patches, possibly affected by the passage of atmospheric gravity waves, which are accompanied by intense horizontal electron density gradients and enhanced electric fields. Although secondary generation cannot be excluded entirely, the evidence supports the possibility that the gradient drift instability is the basic physical mechanism for direct generation of decameter aspect sensitive plasma waves in the midlatitude E region ionosphere.
Density fluctuations in different types of solar wind flow at 1 AU and comparison with results from Doppler scintillation measurements near the Sun
Authors: Huddleston D. E. , Woo R. , Neugebauer M.
Published: February 2004

Density fluctuations with periods in the range 10 min to 1 hour are investigated using ISEE 3 plasma measurements of solar wind flows at 1 AU. Coronal hole, interstream, plasma sheet, coronal mass ejection, and interaction region flow types are considered. The ISEE 3 results are consistent with the interpretation of large‐scale variations in density fluctuations observed by Doppler scintillation measurements inside 0.2 AU. The highest absolute and fractional density fluctuations are found ahead of and within the plasma from coronal mass ejections, with the maximum values occurring between the associated interplanetary shocks and the driver gas. For the quasi‐stationary solar wind, density and fractional density fluctuations are highest and most variable in the plasma sheet in which the heliospheric current sheet is embedded, and lowest and least variable in the high‐speed coronal hole flow. A superposed epoch analysis of heliospheric current sheet crossings shows that the regions of enhanced density fluctuations previously identified in Doppler scintillation measurements near the Sun persist to 1 AU and are broader there. The results of this study confirm the advantages of using density fluctuations rather than density as a tracer of solar wind flows of differing origins at the Sun and as a detector of propagating interplanetary disturbances.
BX control of polar cap potential for northward interplanetary magnetic field
Authors: Taguchi S. , Hoffman R. A.
Published: February 2004

We have examined how the magnitude of the polar cap potential for northward interplanetary magnetic field (IMF) changes with the angle of the field in the BX/BZ plane. From the dawn‐dusk passes (November–December 1981, May–June 1982, and November–December 1982) of the Dynamics Explorer 2 satellite, we selected 363 passes by requiring the IMF hourly value BZ ≥ 1 nT for the preceding 2‐hour interval. Of these, 72 cases displayed a reverse convection pattern in the electric field data, i.e., sunward at the highest latitudes and antisunward at lower latitudes. The potential difference across the region of sunward convection correlates well with various functions that consist of all or some of the IMF parameters, BZ, BY, the BY/BZ angle, and the solar wind velocity. We “normalize” the potential, using these functions. To test for a BX dependence while eliminating dipole tilt effects, we introduce ϕh*, the angle between the IMF vector projected to the X‐Z meridian and the Earth dipole axis. The normalized potential F for the northern hemisphere is the largest when ϕh*, measured from the northern dipole axis toward the Sun, is negative and large (∼ −100°), and decreases as ϕh* approaches zero and turns positive. An approximate equation for this relation is F(ϕh*) = a + b / (ϕh* + 125°), where the values for a and b lie between 0.0086 and 0.015 and between 1.6 and 1.9, respectively. This relation can be interpreted in terms of a model of magnetosheath field line motion which predicts that the reconnection belt on the magnetopause poleward of the cusp becomes longer as the magnetosheath field lines tilt toward the tail lobe.
Locations of the termination shock and the heliopause
Authors: Whang Y. C. , Burlaga L. F. , Ness N. F.
Published: February 2004

The locations of the termination shock and the heliopause are restudied by taking into account the effects of pickup protons. The study uses available plasma and magnetic field data from Voyagers over a 14‐year period (1978–1991) and Voyager observations of the 1992–1993 radio emission event, outside 30 AU, pickup protons have a significant influence on dynamical structures of the outer heliosphere. The solar wind is treated as a mixture of electrons, solar wind protons, and interstellar pickup protons. If the magnitude of the interstellar magnetic field Bint is given, one can quantitatively study the motion and location of the termination shock. We find that the location is anticorrelated with the sunspot number. The absolute mean of the shock speed is 19 km/s, and the quadratic mean of the shock speed is 24 km/s. Because Bint is poorly known, additional information is needed in studying the termination shock. Cummings et al. (1994) have used observations of anomalous cosmic rays to estimate the location of the shock. The observations of the 1991 global merged interaction region (GMIR) and GMIR shock and the 1992–1993 radio emission event provide another handle for the study of the termination shock and the heliopause. After its penetration through the termination shock, the GMIR shock continued to propagate in the subsonic region of the solar wind and eventually interacted with the heliopause. This interaction produced a transmitted shock propagating outward in the interstellar medium and a reflected shock propagating inward toward the Sun in the subsonic solar wind. The plasma frequencies behind the reflected and the transmitted shocks can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. We assume that the impingement of the GMIR shock at the heliopause occurred at the time when Voyagers started receiving the radio emissions. Taking into account the effects of pickup protons, we find that the locations of the termination shock and the heliopause in 1991–1992 are at approximately 66 AU and 150 AU, respectively.
Excitation of VLF waves by an electron beam injected into the ionosphere
Authors: Pivovarov V. , Burke A. , Ride S. K. , Shapiro V. D.
Published: February 2004

We propose a scenario for radiation of electromagnetic whistlers as a result of the injection of a modulated electron beam into the ionosphere. We show that for plasma parameters typical of the ionosphere and electron beam parameters typical of electron guns carried into space aboard rockets or spacecraft, the interaction can result in the instability of electrostatic waves driven by Cherenkov resonance between the waves and the beam. A linear stability analysis was carried out for two limiting cases: a wide and a narrow electron beam; the wave excitation is shown to develop at typical distances of the order of 1 km. Nonlinear evolution of the instability was investigated by a combination of analytical and numerical methods. Electron beam energy is efficiently transformed into the electrostatic mode. The mechanism of electromagnetic mode generation is then dipole radiation from the electron beam, which is strongly bunched as a result of the electrostatic instability. The efficiency of radiation is a strong function of the ratio of the electron beam velocity to electron Alfven velocity (Vae = cωce/ωpe); the possibility of increasing the efficiency of radiation is discussed.
Global variations of thermospheric winds and temperatures caused by substorm energy injection
Authors: Fujiwara H. , Maeda S. , Fukunishi H. , Fuller‐Rowell T. J. , Evans D. S.
Published: February 2004

Two numerical simulations of the thermospheric response to magnetospheric energy injection have been performed using a zonally averaged, time‐dependent model of neutral composition, dynamics, and energy budget. The simulations are distinguished by the duration of the source. The first simulation has an energy injection of 1 hour, representative of substorm type forcing, and the second one has a 12‐hour energy injection, representative of main storm type forcing. They were performed under the condition of equinox at solar minimum. In the first simulation, large‐scale atmospheric gravity waves (AGWs), generated by the substorm energy via Joule heating of ionospheric currents, are clearly identified in the wind‐field in a meridional plane as well as in the temporal and spacial variations of the total energy density of air above about 130 km height. These waves reach the equator after about 3 hours and propagate into the opposite hemisphere. The horizontal propagation speed is close to the speed of sound (for example, roughly 440 m/s at about 150 km altitude and 670 m/s at about 260 km altitude). Snapshots of the wind system affected by the substorm energy injection show a “four‐cell” pattern between the poles. Above 260 km, the cells have the opposite rotational direction to those below. These small‐scale features in the wind system are indicative of the internal atmospheric gravity waves with the vertical phase propagation. From a term analysis of the energy conservation equation, it is identified that the dominant energy process associated with the propagation of AGWs is adiabatic compressional heating and/or expansive cooling process. It can be concluded that the energy oscillations at middle and low latitude are mainly produced by AGWs propagating from high latitude during the substorm. The second simulation indicated that horizontal and vertical advections, vertical heat conduction, and infrared radiative cooling by nitric oxide are important in addition to adiabatic compressional heating and/or expansive cooling. It is suggested that short‐duration energy injection preferentially generates AGWs which dominate the energy oscillations at low latitudes through adiabatic heating and cooling. Long‐duration energy injection is more effective in generating a meridional circulation which transfers energy by both advective and adiabatic processes.
Nonadiabatic precipitation of ions at the cusp equatorward edge
Authors: Alem F. , Delcourt D. C.
Published: February 2004

The transport of magnetospheric particles in the outer cusp region is examined by means of three‐dimensional single particle codes. It is shown that, at high altitudes, particles can experience nonadiabatic motions leading to significant magnetic moment changes. It is demonstrated that this nonadiabatic behavior can be interpreted, at least partly, by the action of an impulsive centrifugal force perturbing the particle gyromotion. Systematic trajectory calculations reveal that plasma sheet ions which E×B convect toward the magnetopause from low L‐shells in the dayside sector are subjected to such nonadiabatic motions. As a result of magnetic moment damping, a number of them are injected into loss cone, leading to a narrow (a few tenths of a degree) latitudinal structure of precipitating ions at the equatorward edge of the dayside cusp.
Laboratory facility for magnetospheric simulation
Authors: Yur G , Rahman H. U. , Birn J. , Wessel F. J. , Minami S.
Published: February 2004

The magnetospheric simulation facility at the University of California, Riverside (UCR) has been recently modified to improve scalings based on magnetohydrodynamic (MHD) theory. In this facility a magnetized plasma stream interacts with a downstream dipole magnetic field simulating the Earth's magnetosphere. Specific improvements include: higher stream‐flow velocity, increased plasma density, more intense background and dipole magnetic fields, and improved diagnostic capability. This paper briefly discusses MHD scaling requirements and measurements that are relevant to the laboratory simulation; including a set of time‐resolved, visible‐light images showing the evolution of the model magnetosphere.
Energy distributions of thruster pickup ions detected by the Shuttle Potential and Return Electron Experiment during TSS 1
Authors: Burke W. J. , Gentile L. C. , Machuzak J. S. , Hardy D. A. , Hunton D. E.
Published: February 2004

Instrumentation to monitor the shuttle environment during the Tethered Satellite System (TSS 1) mission included the Shuttle Potential and Return Electron Experiment (SPREE) and the Quadrupole Ion‐Neutral Mass Spectrometer (QINMS). SPREE measured fluxes of electrons and ions with energies between 10 eV and 10 keV; QINMS monitored neutral and ion species close to the shuttle. We report on energy distributions of pickup ions detected during and after thruster emissions while the shuttle's velocity vector was nearly perpendicular to the Earth's magnetic field. With SPREE looking in the ram direction and almost perpendicular to the magnetic field, fluxes >1011 ions cm−2 s−1 sr−1 were detected in the 10‐to‐60‐eV energy range. Both prompt and prolonged ion flux enhancements were recorded. The prolonged fluxes lasted many seconds after the initiating thruster turned off. Only when thrusters fired close to the magnetic field direction were no pickup‐ion flux enhancements detected. The SPREE measurements are compared with the predictions of a simple two‐dimensional model of collisionless pickup‐ion trajectories. Using the distribution of ion species recorded by QINMS, the model explains the main features of the ion energy spectra measured by SPREE. Comparison of the model with data also indicates that strong scattering of ejected materials must occur almost immediately after thruster emission and later between the time of pickup‐ion creation and detection of the increased ion flux by SPREE.
Dissipation of pickup‐induced waves: A solar wind temperature increase in the outer heliosphere?
Authors: Williams L. L. , Zank G. P. , Matthaeus W. H.
Published: February 2004

The deposition of energy into the solar wind by the pickup of interstellar neutrals is due to both the creation of hot, nonthermal ions and the associated generation of low frequency magnetohydrodynamic (MHD) waves. Dissipation of some fraction of the free wave energy released by ion pickup and isotropization is possible through nonlinear turbulent processes which may lead to heating of the core thermal solar wind proton distribution. Simple energy budget arguments are utilized to show that pickup ion generated wave dissipation may play a significant role in determining the solar wind radial temperature profile in the outer heliosphere. In particular, depending on the density of interstellar hydrogen in the heliosphere, there will be some radial distance beyond which the thermal solar wind core temperature increases steadily until the termination shock. Existing Pioneer and Voyager temperature profiles are consistent with this interpretation.
Analytic MHD theory for Earth's bow shock at low Mach numbers
Authors: Grabbe Crockett L. , Cairns Iver H.
Published: February 2004

A previous MHD theory for the density jump at the Earth's bow shock, which assumed the Alfven (MA) and sonic (Ms) Mach numbers are both ≫ 1, is reanalyzed and generalized. It is shown that the MHD jump equation can be analytically solved much more directly using perturbation theory, with the ordering determined by MA and Ms, and that the first‐order perturbation solution is identical to the solution found in the earlier theory. The second‐order perturbation solution is calculated, whereas the earlier approach cannot be used to obtain it. The second‐order terms generally are important over most of the range of MA and Ms in the solar wind when the angle θ between the normal to the bow shock and magnetic field is not close to 0° or 180° (the solutions are symmetric about 90°). This new perturbation solution is generally accurate under most solar wind conditions at 1 AU, with the exception of low Mach numbers when θ is close to 90°. In this exceptional case the new solution does not improve on the first‐order solutions obtained earlier, and the predicted density ratio can vary by 10–20% from the exact numerical MHD solutions. For θ ∼ 90° another perturbation solution is derived that predicts the density ratio much more accurately. This second solution is typically accurate for quasi‐perpendicular conditions. Taken together, these two analytical solutions are generally accurate for the Earth's bow shock, except in the rare circumstance that MA ≤ 2. MHD and gasdynamic simulations have produced empirical models in which the shock's standoff distance as, is linearly related to the density jump ratio X at the subsolar point. Using an empirical relationship between as and X obtained from MHD simulations, as values predicted using the MHD solutions for X are compared with the predictions of phenomenological models commonly used for modeling observational data, and with the predictions of a modified phenomenological model proposed recently. The similarities and differences between these results are illustrated using plots of X and as predicted for the Earth's bow shock. The plots show that the new analytic solutions agree very well with the exact numerical MHD solutions and that these MHD solutions should replace the the corresponding phenomenological relations in comparisons with data. Furthermore, significant differences exist between the standoff distances predicted at low MA using the MHD models versus those predicted by the new modified phenomenological model. These differences should be amenable to observational testing.
The character of drift spreading of artificial plasma clouds in the middle‐latitude ionosphere
Authors: Blaunstein N.
Published: February 2004

Nonlinear equations describing the evolution of plasma clouds with real initial sizes, along and across the geomagnetic field B, which drift in the ionosphere in the presence of an ambient electric field and a neutral wind have been solved and analysed. An ionospheric model close to the real conditions of the middle‐latitude ionosphere is introduced, taking into account the altitude dependence of the transport coefficients and background ionospheric plasma. The striation of the initial plasma cloud into a cluster of plasmoids, stretched along the field B, is obtained. The process of dispersive splitting of the initial plasma cloud can be understood in terms of gradient drift instability (GDI) as a most probable striation mechanism. The dependence of the characteristic time of dispersive splitting on the value of the ambient electric field, the initial plasma disturbance in the cloud and its initial sizes was investigated. The stretching criterion, necessary for the plasma cloud's striation is obtained. The possibility of the drift stabilization effect arising from azimuthal drift velocity shear, obtained by Drake et al. [1988], is examined for various parameters of the barium cloud and the background ionospheric conditions. A comparison with experimental data on the evolution of barium clouds in rocket experiments at the height of the lower ionosphere is made.
The polar onset and development of the November 8 and 9, 1991, global red aurora
Authors: McEwen D. J. , Huang K.
Published: February 2004

On November 8, 1991, a major magnetic storm began with a sudden commencement at 0647 UT; it continued until about 1800 UT on November 9. Associated with this storm there was a great aurora which continued through November 8 and until at least 1200 UT on November 9. The first optical indication of this event was the appearance of bright sun‐aligned arcs over Eureka, North West Territories (89° corrected geomagnetic latitude (CGM)), Canada from 0750 UT until about 1230 UT on November 8. By that time, there was a gradual transformation to dominant 6300 Å emission over the central polar cap. Gradually, the major soft electron precipitation and associated red aurora moved equatorward. The aurora reached a low‐latitude limit of 40° CGM at 0600 UT on November 9. The polar arcs reached 22 kR intensity (5577 Å), while the red aurora seen later at low latitudes was at least 100 kR (6300 Å). The analysis of Defense Meteorological Satellite Program (DMSP) particle data from 83 passes during November 8 and 9 showed the extent and movement of precipitation from the polar region to midlatitudes. A comparison of this event was made with the earlier great auroral event of March 13–14, 1989. While the extent in low latitude and the duration of the two disturbances were similar, the November 8–9 event had approximately one half of the total energy input into the ionosphere. Its striking and unusual feature was the auroral onset and activity in the central polar region, a feature possibly related to the solar wind interplanetary magnetic field orientation.
Analysis of the linear response function relating AL to VBs for individual substorms
Authors: Blanchard Gerard T. , McPherron Robert L.
Published: February 2004

Time series of the AL index and rectified solar wind electric field are used to study solar wind coupling to auroral ionospheric currents during individual substorms. Calculation of the individual filters is accomplished by representing them as the product of two simple filters: a low‐pass filter that accounts for the response of the ionospheric electric field to the reconnection electric field, and a filter composed of delta functions. The delta functions each have the effect of delaying and scaling the response of AL relative to VBs without distorting the waveform of VBs. Nearly all significant correlation between AL and VBs can be accounted for when the second filter contains only two components. This result suggests that the westward electrojet indexed by AL is controlled by two distinct processes, both proportional to VBs. For an ensemble of 117 isolated substorms the average prediction efficiency of a filter with two delays is 71%. If we were able to determine the exact values of the two delays and the two gain factors prior to an event, then we would be able to estimate the instantaneous magnetospheric response function. We examined season, universal time, and prior activity as possible factors controlling the magnitude of the parameters but found no apparent relationships. Until such relations are found the average parameters provide the best estimate but account for only 47% of the variance in the AL index.