bluuebunny/crossref_metadata_2025 huggingface dataset:
Competing processes of plasma wave instabilities driven by an anisotropic electron beam: Linear results and two‐dimensional particle simulations
Authors: de Agua L. Borda , Omura Y. , Matsumoto H. , Brinca A. L.
Published: February 2004

A magnetoplasma made up of a background of isotropic electrons and protons and a beam of anisotropic electrons (T⊥/T‖ > 1), with drift velocity parallel to the ambient magnetic field, can feed different unstable modes. Numerical solution of the dispersion equation in the wavenumber plane (k‖, k⊥) reveals several unstable modes with islets of oblique growth unconnected to the unstable parallel modes. The mechanisms involved in the growth and saturation of the fastest instability do not necessarily exclude later developments of other instabilities. Competing processes among the coexisting unstable waves are expected. Linear analysis or one‐dimensional computer simulations cannot properly address this problem; there would not be interaction of modes propagating at different angles to the static magnetic field. Linear and nonlinear evolutions of the modes are studied via computer simulation with a two‐dimensional particle code (KEMPO2). Results show that several modes are excited with different timescales. Some modes are only temporarily excited, while others persist in the system. The distribution function is significantly changed by the first excited modes, the beam‐mode waves. In later stages of development, forward and backward oblique whistler mode waves are excited. A final stage is characterized by coexistence of several waves with different frequencies and wavelengths. The beam‐mode waves have frequencies close to the plasma frequency, whereas the frequencies of the whistler mode waves fall near a half of the cyclotron frequency. These results may be relevant to the interpretation of wave activities observed in the Earth's magnetosphere by the GEOS 1, GEOS 2, and GEOTAIL satellites or in the Uranian bow shock by Voyager 2.
Hot plasma parameters of Jupiter's inner magnetosphere
Authors: Mauk B. H. , Gary S. A. , Kane M. , Keath E. P. , Krimigis S. M. , Armstrong T. P.
Published: February 2004

The bulk parameters of the hot (>20 keV) plasmas of Jupiter's inner magnetosphere, including the vicinity of the Io plasma torus, are presented for the first time (L = 5 to 20 RJ). The low‐energy charged particle (LECP) instrument on Voyager 1 that obtained the data presented here was severely overdriven within the inner regions of Jupiter's magnetosphere. On the basis of laboratory calibrations using a flight spare instrument, a Monte Carlo computer algorithm has been constructed that simulates the response of the LECP instrument to very high particle intensities. This algorithm has allowed for the extraction of the hot plasma parameters in the Jovian regions of interest. The hot plasma components discussed here dominate over other components with respect to such high‐order moments as the plasma pressures and energy intensities. Our findings include the following items. (1) Radial pressure gradients change from positive (antiplanetward) to negative as one moves outward past about 7.3 RJ. While the observed hot plasma distributions will impede the radial transport, via centrifugal interchange, of iogenic plasmas throughout the Io plasma torus regions out to 8 RJ, the plasma impoundment concept of Siscoe et al. [1981] for explaining the so‐called “ramp” in the flux shell content profile of iogenic plasmas (7.4–7.8 RJ [Bagenal, 1994]) is not supported. (2) We predict a radial ordering for the generation of the aurora, which translates into a latitudinal structure for auroral emissions. Planetward of about 12 RJ, intense aurora (10 ergs/(cm2 s) precipitation) can only be caused by ion precipitation, whereas outside of about 12 RJ such intense aurora can only be caused by electron precipitation. Uncertainties concerning the causes of Jovian aurora may stem in part from failures of some observations to resolve the latitudinal structure that is anticipated here and possibly from changes in the auroral configuration and/or charged particle spectral properties since the Voyager epoch.
Charged particle phase space densities in the magnetospheres of Uranus and Neptune
Authors: Paranicas Chris , Cheng Andrew F. , Mauk Barry H.
Published: February 2004

Phase space densities are calculated from measurements of energetic electrons and protons made during the Voyager 2 encounters with Uranus and Neptune, and they are expressed in terms of the adiabatic invariants μ, K, and Φ. Low‐energy charged particle data are binned by adiabatic invariants using an offset, tilted dipole model of the planetary magnetic field. By this method, no interpolation is necessary in pitch angle or energy, and therefore a much larger fraction of the available data is analyzed here than has been done previously. On the basis of this new binned data set, several outstanding issues of the Uranian and Neptunian magnetospheres are addressed, for example, satellite interaction effects and the variation of spectral properties. Assuming a power law spectrum, the spectral index γ is calculated for data within narrow second‐adiabatic‐invariant bins. We find the spectral index generally increases with increasing L shell, with local peaks at L shells corresponding to maximum absorption locations of planetary satellites. We also find data indicate that γ varies strongly with K.
A search for ELF/VLF activity associated with earthquakes using ISIS satellite data
Authors: Rodger Craig J. , Thomson Neil R. , Dowden Richard L.
Published: February 2004

For more than 10 years, researchers using satellite observations have reported anomalous phenomena associated with seismic activity. Increases in ELF/VLF filter amplitudes have been associated with preshock and postshock events near satellites in a number of studies. However, an examination of the wideband ELF/VLF data from the low‐altitude ISIS satellites shows no significant changes in electromagnetic (EM) activity associated with earthquakes. In the analysis of wideband data from 164 satellite passes above 39 earthquakes, no unique or familiar (previously observed) “seismo‐active emission” can be linked to the occurrence of a nearby earthquake. The electromagnetic activity levels in ELF/VLF spectra observed by the ISIS 2 satellite before and after 37 shocks is examined and compared with a set of selected control orbits. Comparisons are made of EM activity changes between earthquake passes and control passes over the epicenter location. No increase in the activity for earthquake passes is found. These results do not change when various constraints are placed on the data in an attempt to test for dependencies on different criteria. No precursory, coseismic, or postseismic effects are found. No significant evidence is found for differences between the data sets which would suggest any “seismo‐active” activity.
Pc 1 waves in the ionosphere: A statistical study
Authors: Erlandson R. E. , Anderson B. J.
Published: February 2004

A statistical study of Pc 1 waves has been performed using electric field data recorded in the ionosphere by the Dynamics Explorer 2 satellite. The study was performed by applying an automated wave detection algorithm to over 900 hours of data recorded at invariant latitudes (INV) greater than 40° from December 6, 1981, to February 16, 1983. A total of 390 Pc 1 waves in the electric field were identified using a selection technique based on spectral peak detection in the frequency range from 0.2 to 6.0 Hz. Most events were observed at frequencies between 0.4 and 2.0 Hz and in the dawn (0400–0600 MLT) and noon (1000–1500 MLT) sectors from 50° to 62° INV. Detection of events at high latitudes (INV > 65°) was limited by noise associated with auroral zone electric fields. Significant differences in the properties of ionospheric Pc 1 waves were observed in the dawn and noon sectors. First, noon sector waves were clearly ordered by the equatorial gyrofrequencies (ƒ < ƒeqO+ and ƒeqO+ < ƒ < ƒeqHe+), whereas in the dawn sector the ordering was not as clear. Second, a magnetic field spectral peak was observed in 90% of the large‐amplitude events, i.e., Ex > 10 (mV/m)2/Hz, in the noon sector compared with just 23% in the dawn sector. Third, for events with a magnetic field component, the median value of ΔE/ΔB was 410 km/s in the dawn and 330 km/s in the noon sector, although the median value of the ratio between (ΔE/ΔB) and the Alfvén velocity was similar in both sectors. Fourth, more short‐duration events (2–8 s) were observed in the noon than in the dawn sector. We conclude that the source of noon sector ionospheric Pc 1 waves is electromagnetic ion cyclotron waves generated in the equatorial magnetosphere. The source of the dawn sector waves is less certain, as many waves were observed only in the electric field.
Glimpses of space physics in the 1960s and 1990s
Authors: Petschek Harry E.
Published: February 2004

The author describes his experiences and impressions of space physics in the 1960s and on his return to the field in the 1990s. The field has made significant progress and has changed its operating style to that of a more mature science. It has also become a more closed society with less interaction with outside groups. There is a consequent pessimistic mood including a feeling of lack of progress. The author suggests that the field should become more outwardly oriented. The opportunity presented by present interest in space weather forecasting should be taken seriously by the community as an area of applied science. Additionally, the numerous basic science questions in the field should be viewed in terms of the opportunity of sharing basic discoveries with other parts of the physics community. Examples are given of particular problems where such an outlook will broaden the horizons of space science and allow us to measure our contributions to basic and applied science.
Plasma velocity distributions in the near—Earth plasma sheet: A first look with the Geotail spacecraft
Authors: Frank L. A. , Paterson W. R. , Ackerson K. L. , Kokubun S. , Yamamoto T.
Published: February 2004

An initial examination of the three‐dimensional velocity distributions of positive ions and electrons as observed with the Geotail spacecraft in the near‐Earth plasma sheet at radial distances of ∼10 RE near the magnetic equator has revealed several fascinating features of this region. The velocity distributions of the hot ions in the energy range of about 1–25 keV are not isotropic as a function of pitch angle but exhibit complex features that appear to be the “memory” of their previous acceleration and transport in the more distant magnetotail. The composition of the plasma is primarily H+, with traces of He++, He+, and O+, and thus its sources are both the solar wind and the ionosphere. Field‐aligned beams of ionospheric ions with energies in the range of 3–5 keV are observed to be streaming into the central plasma sheet and are presumably due to a field‐aligned potential drop of similar magnitude which is positioned at lower altitudes. The pitch angle distributions of hot electrons are generally more isotropic than those of the positive ions with one major exception. This exception is the presence of counterstreaming electron beams, magnetically field‐aligned, with energies in the range or 100–300 eV. These electron beams suggest the presence of a second field‐aligned potential drop of 100–300 V at altitudes above the acceleration region for the ions. It is likely that the field‐aligned electron beams are associated with the Birkeland currents to the ionosphere that provide the closure of currents in the equatorial plane associated with the distributions of the hot equatorial plasmas.
Resonant Alfven wave excitation in two‐dimensional systems
Authors: Uberoi C.
Published: February 2004

The resonant excitation of shear Alfven waves due to coupling with pressure perturbations is studied for an ideal MHD plasma with density varying both along and perpendicular to the constant magnetic field direction. For the case of infinite azimuthal wavelength it is noted that the MHD equations split into two sets describing the ordinary Alfven wave and the Alfven wave modified by pressure waves. Studying the latter set of equations analytically, we find that the pressure waves can resonantly excite the Alfven waves propagating along the z‐ direction, parallel to the magnetic field, in the neighborhood of the resonant layer x = x0 with local Alfven wave frequency ω = ωA(x0), with intensity varying in the z‐ direction. The intensity of the wave becomes maximum near the cutoff layer across which the wave changes from oscillatory to decaying. It is concluded that if the Alfven wave propagates along the magnetic field for considerable distance before reaching the cutoff layer, the Alfven waves in a two‐dimensional system will retain the property of phase mixing and consequently resonant absorption similar to the case of a one‐dimensional system.
The aeronomical pilgrim's progress
Authors: Donahue T. M.
Published: February 2004

Herein are recounted one aeronomer's recollections of how some advances in our understanding of how the Earth's upper atmosphere works were made during the quarter century beginning in 1950. That period was selected because it spans about half of his aeronomical career and ends at a time when there was a discontinuity in that career. During that time, aeronomers achieved a first‐order understanding of Earth's thermosphere and exosphere. Afterward many of them, including the author, became preoccupied with other problems.
Cosmic ray solar semidiurnal anisotropy: 2. Heliospheric relationships of anisotropy parameters
Authors: Ahluwalia H. S. , Fikani M. M.
Published: February 2004

We have used the corrected experimental semidiurnal variation observations described at length in Paper 1, to compute the amplitude and direction of semidiurnal anisotropy in free space for the period 1965 to 1992. The time interval covers several epochs of solar activity maxima, minima, and polar field reversals. We find that the amplitude of anisotropy fluctuates between 0.05% and 0.10%, except for the period 1971 to 1977 when it acquires a value as large as 0.25% at high primary rigidities (Rm > 16 GV). The physical cause of this anomalous behavior is not identified yet. The asymptotic time stays close to 0300 LT, independent of the primary rigidity (R). This direction is perpendicular to the nominal direction of interplanetary magnetic field (IMF) at Earth's orbit. The polarity of IMF has no effect on the semidiurnal anisotropy parameters. The physical significance of our results is discussed in the context of the theoretical ideas available in the literature. The contributions from the symmetric transverse gradients to semidiurnal anisotropy appear to be minimal, but those from pitch angle scattering may be appreciable.
The seasonal behavior of high‐latitude thermospheric winds and ion velocities observed over one solar cycle
Authors: Aruliah A. L. , Farmer A. D. , Rees D. , Brändström U.
Published: February 2004

The seasonal variation of nighttime thermospheric winds observed at Kiruna has been found to be significantly dependent on solar activity. Of particular interest is the observation that there is a large difference between the March and September equinox winds, despite similar levels of solar insolation. The September equinox winds are more December solstice‐like. The average March equinox meridional winds are up to 70% larger than for September. The existence of an equinoctial asymmetry has not been predicted by either thermospheric or ionospheric model simulations, which assume that the equinoxes are fundamentally the same, and use forcing functions which are symmetric about the solstices. The average ion velocities measured at EISCAT are larger during the March equinox than the September at solar maximum, while the converse is true at solar minimum. In contrast, the March equinox nighttime thermospheric winds are larger for both solar maximum and solar minimum. Furthermore, the asymmetry is greater at solar maximum.
Ground‐satellite correlation of low‐latitude Pi 2 pulsations: A quasi‐periodic field line oscillation in the magnetosphere
Authors: Saka O. , Akaki H. , Watanabe O. , Baker D. N.
Published: February 2004

Magnetometer data at dip‐equator and at geosynchronous altitudes (GOES 5 and 6 satellites) and energetic particle data from geosynchronous spacecraft (1982‐019, 1984‐037, 1984‐129) are studied to investigate field line and particle signatures associated with ground Pi 2 onset. Pi 2 pulsations observed at the ground station are discussed as field disturbances that axe launched in the midnight sector of the magnetosphere in association with field reconfiguration at substorm onset. In the geosynchronous orbit, both quasi‐periodic field line oscillations and particle injections are often observed in the nightside sector in association with the onset of ground Pi 2 pulsations. Of 216 Pi 2 events selected in this report, 16% showed associated quasi‐periodic field line oscillations. The dominant periods of the oscillations in space are similar to but not equal to those of the corresponding ground Pi 2 pulsations. The oscillations in space are polarized primarily in the azimuthal direction with a dc offset that develops to form a field‐aligned current signature, namely, eastward (westward) directed in the premidnight (postmidnight) sector. Simulation of forced field line oscillations has been carried out by adding the sudden increase in plasma pressure in the equatorial midnight sector region that is expected to come from particle injection at substorm expansion onset. We discuss the relevance of the quasi‐periodic oscillations to plasma injections at geosynchronous altitudes with reference to the result of the model calculations.
Triton's ionospheric source: Electron precipitation or photoionization
Authors: Sittler E. C. , Hartle R. E.
Published: February 2004

We have performed an evaluation to determine whether or not Neptune's magnetospheric electrons can provide the ionization of Triton's ionosphere as previously suggested or whether photoionization is the dominant ionization mechanism. Our approach has been to determine the accessibility of magnetospheric electrons to Triton's ionosphere. Using scaling relationships based on Venus and Titan observations, we have developed estimates of the centrifugal, gradient B and E × B drifts. We have computed trajectories of magnetospheric electrons and studied their accessibility to the Triton ionosphere. The following conclusions can be reached from this study: (1) Centrifugal drift delivers electrons to the ionopause. If centrifugal drift is impaired, then electron precipitation is severely limited. (2) Low‐energy electrons (E < 5 keV) are lost through E×B drift around the ionopause. (3) At higher electron energy the probability of precipitation increases. If the electron gyroradius is small relative to the ionopause thickness, then at pitch angles ∼90° grad B drift dominates with trapping of electrons in the ionopause and subsequent exclusion from the ionosphere. At pitch angles 0° and 180° curvature drift dominates, and electrons will precipitate on entry into the ionopause. If the electron gyroradius is large compared to the ionopause thickness, then electrons will precipitate at any pitch angle. Mass loading is estimated to be unimportant at Triton, and this contributes to the importance of E × B drift and the exclusion of low‐energy electrons to Triton's ionosphere. Our calculations have intentionally overestimated the effects of centrifugal drift to present the best case for electron precipitation. Although collisions are more important for low‐energy electrons (E < 5 keV), we estimate that cross‐field diffusion is small for ionopause heights greater than 725 km. At higher electron energies where collisions are less important, the threshold energy above which electrons become untrapped is only dependent upon the ionopause thickness and not collisions. Pressure balance arguments show that the ionopause is thick with Δz > 200 km. A magnetized ionosphere would be equivalent to the high ram pressure case for the Venus interaction. A thick ionopause would contribute to prevention of precipitation of magnetospheric electrons into Triton's ionosphere when E < 50 keV. Although our calculations at the present level of development cannot rule out the importance of electron precipitation as the source of Triton's ionosphere, we suggest that photoionization be considered viable for the production of Triton's ionosphere.
Heating and activity of the solar corona: 1. Boundary shearing of an initially homogeneous magnetic field
Authors: Galsgaard Klaus , Nordlund Åke
Published: February 2004

To contribute to the understanding of heating and dynamic activity in boundary‐driven, low‐beta plasmas such as the solar corona, we investigate how an initially homogeneous magnetic field responds to random large‐scale shearing motions on two boundaries, by numerically solving the dissipative MHD equations, with resolutions ranging from 243 to 1363. We find that even a single application of large‐scale shear, in the form of orthogonal sinusoidal shear on two boundaries, leads to the formation of tangential discontinuities (current sheets). The formation time scales logarithmically with the resistivity and is of the order of a few times the inverse shearing rate for any reasonable resistivity, even though no mathematical discontinuity would form in a finite time in the limit of vanishing resistivity. The reason for the formation of the current sheets is the interlocking of two magnetic flux systems. Reconnection in the current sheets is necessary for the field lines to straighten out. The formation of current sheets causes a transition to a very dynamic plasma state, where reconnection drives supersonic and super‐Alfvénic jet flows and where these, in turn, cause the formation of smaller‐scale current sheets. A statistically steady state level for the average Poynting flux and the average Joule dissipation is reached after a few correlation times, but both boundary work and Joule dissipation are highly fluctuating in time and space and are only weakly correlated. Strong and bursty Joule dissipation events are favored when the volume has a large length/diameter ratio and is systematically driven for periods longer than the Alfvèn crossing time. The understanding of the reason for the current sheet formation allows a simple scaling law to be constructed for the average boundary work. Numerical experiments over a range of parameter values, covering over 3 orders of magnitude in average dissipation, obey the scaling law to within a factor of 2. The heating rate depends on the boundary velocity amplitude and correlation time, the Alfvén speed, and the initial magnetic field strength but appears to be independent of the resistivity because of the formation of a hierarchy of current sheets. Estimates of the photospheric boundary work on the solar coronal magnetic field using the scaling law are consistent with estimates of the required coronal heating rates. We therefore conclude that the work supplied to the solar corona as a consequence of the motion of the magnetic foot points in the solar photosphere and the emergence of new flux is a significant contributor to coronal heating and flaring and that it quite plausibly is the dominant one.
Equilibrium electric potential of spherical, cylindrical, and planar dust grains moving through a plasma
Authors: Northrop T. G. , Birmingham T. J.
Published: February 2004

The charge on a dust grain in a plasma depends on the plasma parameters and on the grain's velocity through the plasma. Intuition suggests that at high velocity the grain should sweep up equal numbers of both the positive and negative plasma species, and so it should come to zero potential with respect to the plasma. This is in fact not always the case. We have derived the equilibrium potential ϕ of a small conducting sphere moving at speed w through a plasma of temperature T composed of electrons with charge −e and ions with charge Ze and mass ratio M = mi/me and find that for large w the potential goes to the finite value eϕ/kT = −1/2[(M − 1)/(M + Z)]. For a conducting cylinder of infinite length in the same plasma, eϕ/kT is slightly different owing to the different potential variation outside the grain, but for large cylinder speed the potential goes to the same nonzero limit as the sphere. The case of an infinite, conducting plane is quite different, and values depend on whether there is plasma on one side of the plane or on both. In the latter case, which is most analogous to those of the sphere and cylinder, ϕ does go to zero for large plane speed. We show equilibrium e|ϕ|/kT versus w curves for the three elementary shapes, in each instance for an O+ − e−, H+ − e−, and “5me”+ − e− plasma, and comment on characteristics. The last example is a plasma of electrons and ions having the artificially small mass ratio M = 5, of interest because results can be compared with computer simulations of particle dynamics in the presence of a charged grain.
Plasma flow in the Jovian magnetosphere and related magnetic effects: Ulysses observations
Authors: Cowley S. W. H. , Balogh A. , Dougherty M. K. , Dunlop M. W. , Edwards T. M. , Forsyth R. J. , Hynds R. J. , Laxton N. F. , Staines K.
Published: February 2004

The plasma flow in Jupiter's magnetosphere observed during the Ulysses flyby is compared with previous Pioneer, Voyager and ground‐based observations. These data show that near‐rigid corotation is enforced in the inner magnetosphere, but that the azimuthal flow plateaus at 150–300 km s−1 in the middle magnetosphere plasma sheet beyond ∼20 RJ. Such flows extend through the prenoon plasma sheet to ∼45 RJ in the compressed magnetosphere observed by the Voyagers and to ∼70 RJ in the expanded system observed by Ulysses. Higher speeds of ∼500 km s−1 occur in the postmidnight plasma sheet at 75–125 RJ in Voyager data, while preliminary Ulysses evidence is presented for anticorotation in the dusk plasma sheet beyond ∼50 RJ. In the outer magnetosphere the dawn and dusk flank flows are antisunward at several hundred kilometres per second, while in the prenoon sector the flow appears to depend magnetospheric state, being corotational at 250–600 km s−1 when compressed and anticorotational (and radially in) at ∼250 km s−1 when expanded. These observations are compared with theories proposed by Hill and Vasyliunas, augmented to include the effects of solar wind coupling. This model accounts qualitatively for many features, but not for the anticorotation flows observed. We also compare the flows with the bending of the magnetic field out of meridian planes. Given the subcorotation‐anticorotation nature of the observed flow, a pervasive “lagging” configuration is expected. This accords with in situ field data, except for the dusk outer magnetosphere observed by Ulysses, where a “leading” configuration was observed in the presence of subcorotating (downtail) flow. We conclude that field bending due to the tail‐magnetopause current system dominates that due to ionospheric coupling on the dusk flank.
A large polar crown coronal mass ejection and a “problem” goemagnetic storm: April 14–23, 1994
Authors: McAllister A. H. , Dryer M. , Mclntosh P. , Singer H. , Weiss L.
Published: February 2004

In the 1960s and 1970s, systematic programs to associate geomagnetic storms with solar flares gave rise to a category of “problem storms” for which there were no clear associations or for which the associations were with solar activity that seemed too insignificant to properly account for the magnitude of the geomagnetic events. In the last 2 decades the role of filament eruptions and coronal mass ejections (CMEs) in driving geomagnetic activity has received increasing attention, and an answer to the problem storm question has been pieced together. Until recently the inability to view CMEs on the solar disk has impeded a precise demonstration of the inferred relationships. The data from the Yohkoh Soft X ray Telescope (SXT), however, allow the association of soft X ray coronal structures with the aftermath of CMEs. Since CMEs are now believed to be the main cause of aperiodic severe disturbances of Earth's magnetosphere, the ability to distinguish the aftermath of a CME on the disk improves our ability to predict geomagnetic storms. On April 14, 1994, a large scale coronal arcade formation was observed by SXT after the eruption of a long section of the southern polar crown neutral line. Data from the Mauna Loa coronagraph are consistent with this event being the result of a CME that formed in the helmet streamer underlying the heliospheric current sheet. The CME was followed by a severe geomagnetic storm, starting 68 hr later, and was also observed at 3.2 AU and high heliographic latitude by the Ulysses satellite. Since there was no significant flare associated with the eruption, and no significant filament disappearance, the subsequent geomagnetic events are an extreme example of a problem storm and of the great utility of soft X ray imagers for the prediction of space weather. In this paper we have assembled a wide range of data relating to the solar event, the evolution of the region that generated it, and the various observed consequences at Earth and in the interplanetary medium. The global magnetic field changes leading to the CME are outlined, a brief description of the coronal arcade formation is given, and the arcade's association with subsequent events at Earth and the interplanetary disturbance observed at Ulysses is described. We present this case as an example of the advantages to be derived from a comprehensive multidisciplinary approach to studying these potentially disruptive phenomena.
High‐latitude ionospheric electrodynamics as determined by the assimilative mapping of ionospheric electrodynamics procedure for the conjunctive SUNDIAL/ATLAS 1/GEM period of March 28–29, 1992
Authors: Lu G. , Emery B. A. , Rodger A. S. , Lester M. , Taylor J. R. , Evans D. S. , Ruohoniemi J. M. , Denig W. F. , de la Beaujardière O. , Frahm R. A. , Winningham J. D. , Chenette D. L.
Published: February 2004

During the conjunctive SUNDIAL/ATLAS 1/GEM campaign period of March 28–29, 1992, a set of comprehensive data has been collected both from space and from ground. The assimilative mapping of ionospheric electrodynamics (AMIE) procedure is used to derive the large‐scale high‐latitude ionospheric conductivity, convection, and other related quantities, by combining the various data sets. The period was characterized by several moderate substorm activities. Variations of different ionospheric electrodynamic fields are examined for one substorm interval. The cross‐polar‐cap potential drop, Joule heating, and field‐aligned current are all enhanced during the expansion phase of substorms. The most dramatic changes of these fields are found to be associated with the development of the substorm electrojet in the post midnight region. Variations of global electrodynamic quantities for this 2‐day period have revealed a good correlation with the auroral electrojet (AE) index. In this study we have calculated the AE index from ground magnetic perturbations observed by 63 stations located between 55° and 76° magnetic latitudes north and south, which is larger than the standard AE index by about 28% on the average over these 2 days. Different energy dissipation channels have also been estimated. On the average over the 2 days, the total globally integrated Joule heating rate is about 102 GW and the total globally integrated auroral energy precipitation rate is about 52 GW. Using an empirical formula, the ring current energy injection rate is estimated to be 125 GW for a decay time of 3.5 hours, and 85 GW for a decay time of 20 hours. We also find an energy‐coupling efficiency of 3% between the solar wind and the magnetosphere for a southward interplanetary magnetic field (IMF) condition.
Ulysses observations of whistler waves at interplanetary shocks and in the solar wind
Authors: Lengyel‐Frey D. , Hess R. A. , MacDowall R. J. , Stone R. G. , Lin N. , Balogh A. , Forsyth R.
Published: February 2004

This study of whistler wave emission observed by the Ulysses Unified Radio and Plasma Wave (URAP) experiment between 1 and 5 AU is a complement to previous studies of whistler waves observed by the Helios spacecraft between 0.3 and 1 AU. The Helios spacecraft continuously detected a background of whistlers close to the Sun, and this background was found to decrease in intensity with larger heliocentric distance. Ulysses plasma wave observations confirm this trend. Within a heliocentric distance of approximately 2 AU, whistler waves are routinely observed. Beyond about 3 AU, the waves are usually observed only downstream of interplanetary shocks. Moreover, whistler waves are routinely observed within about 2 AU at all heliographic latitudes of the Ulysses trajectory (−80° to +80°). The combined observations from the Helios and Ulysses spacecraft suggest that whistler emission is always present in the solar wind, although at larger heliocentric distances the wave amplitudes are often below the thresholds of the URAP instrument. Observations throughout the first 5 years of the Ulysses mission show a clear correlation of whistler emission intensity with magnetic field strength, or gyrofrequency, such that increases in wave intensities coincide with increases in gyrofrequency. This correlation is especially evident in observations of interplanetary shocks and high‐speed streams. A possible cause of this correlation is increased whistler wave growth due to enhanced electron temperature anisotropies in regions of compressed magnetic field. A shift of the background whistler spectrum as a function of gyrofrequency could account for the observed decrease in whistler amplitudes with increasing heliocentric distance.
Wave resonances and induced flow due to nonlinear Alfvén waves in a stratified atmosphere
Authors: Stark B. A.
Published: February 2004

This paper presents the results of a self‐consistent, nonlinear, time‐dependent, magnetohydrodynamic numerical code developed to investigate Alfvén wave behavior in a smoothly stratified, isothermal, plane‐parallel atmosphere with physical parameters approximating solar coronal holes. Emphasis is given to studies of resonant wave behavior and the origin of flow induced by both freely propagating and partially reflected monochromatic Alfvén waves. The onset and subsequent development of this induced flow is calculated directly in both the linear and nonlinear regimes.The results of these investigations show that a resonance behavior exists, primarily because of continuous partial reflection experienced by the Alfvén wave as it propagates through the atmosphere into a region where the wavelength approaches the scale height of the medium. Waves having periods corresponding to resonant peaks exert considerably more force on the medium than off‐peak period waves, resulting in enhanced flow. For the segment of atmosphere considered, the more reflection experienced by the wave, the greater the enhanced flow. If only off‐peak periods are considered, the relationship between the wave period and induced longitudinal velocity shows that short‐period WKB waves push more on the medium than longer, non‐WKB waves. However, the increase in flow because of resonance effects at longer wave periods is much greater in magnitude. This enhanced flow at resonant periods could contribute to the observationally required acceleration of the solar wind originating from coronal holes. The resulting wave energy transferred to the longitudinal mode may also provide a source of localized heating via longitudinal wave steepening and subsequent shock dissipation.
MHD simulations of magnetotail dynamics
Authors: Birn Joachim , Hesse Michael , Schindler Karl
Published: February 2004

Resistive MHD simulations of large‐scale magnetotail dynamics demonstrate that the same unstable mode causes plasmoid formation and ejection into the far tail and dipolarization and the formation of the substorm current wedge in the inner tail, consistent with the neutral line model of substorms. However, they have also modified some aspects of the model and added details that could not easily be inferred without the self‐consistent approach. We review recent results that include the externally driven formation of a thin current sheet in the near tail, which eases the onset of instability and leads to a faster dynamic evolution. In contrast to earlier expectations, the field‐aligned current generation and diversion takes place in the inner tail earthward of the reconnection site, resulting from shear and diversion of the earthward flow caused by reconnection farther out. Dipolarization starts most pronounced in the tail‐dipole transition region, propagating both tailward and flankward. Strong electric fields and plasma heating also are most prominent in the inner tail. Three‐dimensional simulations without mirror symmetries have generalized the picture of plasmoid formation and ejection, demonstrating a tangled geometry of helical flux ropes with different connections that change increasingly from the Earth to the magnetosheath. The interconnection with the magnetosheath may also play a role in generating plasmoid flux ropes with strong core fields. Mass, energy, and momentum gain of plasmoids results mainly from the accumulation of already accelerated plasma rather than from a sling shot effect acting on the entire plasmoid.
Dynamical auroral morphology in relation to ionospheric plasma convection and geomagnetic activity: Signatures of magnetopause X line dynamics and flux transfer events
Authors: Øieroset M. , Lühr H. , Moen J. , Moretto T. , Sandholt P. E.
Published: February 2004

Ground‐based observations of auroral, geomagnetic, and radar signatures of solar wind‐magnetosphere coupling modes are presented. Two different categories of ionospheric signatures are studied: (1) enhanced geomagnetic deflections and associated equatorward displacements of the cusp/cleft aurora and (2) a sequence of poleward moving auroral and geomagnetic events. It is suggested that signature 1 is mainly caused by directional discontinuities in the interplanetary magnetic field changing the solar wind‐magnetosphere coupling geometry and that signature 2 is caused by a recurring modulation of the reconnection rate at the dayside magnetopause (flux transfer events (FTEs)), during the period of strong coupling. The observed auroral and geomagnetic signatures are placed in the context of plasma convection, as derived from magnetometers in Svalbard, Greenland, and Canada, and the European incoherent scatter (EISGAT) radar. A suggestion for an ionospheric current system explaining the sequence of poleward propagating events, i.e., the FTE signatures, is presented.
Measurements of transverse cosmic ray particle density gradient at high rigidities in the Ulysses era
Authors: Ahluwalia H. S.
Published: February 2004

Transverse cosmic ray gradient at high rigidities can only be determined from diurnal anisotropy data obtained with the neutron monitors and muon detectors based on a unique spinning platform at 1AU, namely, the Earth. This fortunate circumstance has several distinct advantages. Use of high counting rate detectors ensures excellent statistics. The detector performance is checked periodically to ensure stability which makes it possible to obtain long data strings with 100% coverage and reliability, over a wide range of primary rigidities. Very surprisingly, there is an impressive qualitative agreement between measurements made on Earth in the inner heliosphere and those made on board the various spacecrafts in the outer heliosphere, at much lower rigidities, at different short time intervals. This argues for the existence of long lived, spatially extensive, electromagnetic structures in the heliosphere, which affect a very large rigidity range in the galactic cosmic ray spectrum. We describe the results of our analysis of the data obtained with detectors with median rigidity of response (Rm) spanning the range, 16 GV ≤ Rm < 134 GV, over nearly three decades encompassing three epochs each of solar activity maxima, minima, and polar field reversals, including the era of pole‐to‐pole scan of Sun by the spacecraft Ulysses.
Variations in total solar irradiance during solar cycle 22
Authors: Chapman G. A. , Cookson A. M. , Dobias J. J.
Published: February 2004

In this study, we have attempted to model the variations in total solar irradiance from two spacecraft. Specifically, we have modeled the Earth Radiation Budget on the Nimbus 7 spacecraft and the active cavity radiometer irradiance monitor (ACRIM‐I) on the Solar Maximum Mission (SMM) spacecraft using ground‐based photometry of sunspots and faculae from the San Fernando Observatory (SFO). Additionally, for some cases, solar backscatter ultraviolet/2 data on the Mg II core‐to‐wing ratio from the NOAA 9 spacecraft was used. We have found that most of the solar cycle variation in the total solar irradiance can be accounted for by sunspots and faculae/network. The unexplained variation is not greater than approximately 0.0022% (22 ppm) per year for most of solar cycle 22. Using Nimbus 7 data from March 2, 1985, to December 13, 1993 (1281 data points), as the dependent variable, with the SFO photometric sunspot index (PSI) and the NOAA 9 Mg II core‐to‐wing ratio for the faculae/network as independent variables (the best model for this interval), we obtained a multiple correlation coefficient squared (R2) of 0.848. The rms noise in the residuals is approximately 0.221 W m−2 (162 ppm). This rms noise appears to be dominated by noise in the spacecraft data. For the same model, but for the time interval from March 2, 1985, to July 14, 1989, we obtained an R2 of 0.838 for 718 data points. The same type of model for this same interval, substituting SMM/ACRIM‐I total irradiance for Nimbus 7, gave an R2 of 0.857 for 685 data points. Our best correlation, however, came from a three‐parameter model, fitting Nimbus 7 data to the SFO digital PSI, the SFO facular index PFIFA, and the NOAA 9 data for the interval May 30, 1988, to December 13, 1993, giving an R2 of 0.887 (745 data points). These strong correlations suggest that most of the variation in solar irradiance is associated with known solar magnetic features. Whether or not these magnetic features can explain all of the solar irradiance variability will require more stable and accurate long‐term measurements from space and the ground.
Acceleration and heating of heavy ions by circularly polarized Alfvén waves
Authors: Gomberoff L. , Gratton F. T. , Gnavi G.
Published: February 2004

We study the dispersion relation of left‐hand‐polarized Alfvén waves in multicomponent plasmas. If initially the plasma components are not drifting relative to each other, the Alfvén waves propagate until they meet the gyrofrequency of the species with the largest Ml = ml/zlmp value (ml is the ion mass, zl is the degree of ionization, and mp is the proton mass). As a result of resonance absorption, these ions are heated and accelerated by quasi‐linear resonant interaction. When these ions reach a given velocity, the dispersion relation of the Alfvén waves changes drastically. The Alfvén branch of the dispersion relations no longer goes to the gyrofrequency of the ion species with the largest Ml value, but it goes to the gyrofrequency of the species with the second largest Ml value. In this way, more and more channels open up. We apply these results to high‐speed solar wind streams, and we argue that Alfvén waves generated in coronal holes or in nearby regions can heat and accelerate heavy ions.
The structure of mass‐loading shocks: 3. Magnetohydrodynamics
Authors: Story T. R. , Zank G. P.
Published: February 2004

The dissipative, mass‐loading, multifluid model, developed by Zank et al. [1993, 1994] to describe the structurally complex bow shocks observed at comets, has been extended to include the effects of a magnetic field. After describing the model, the shock structure problem associated with the one‐dimensional, steady state conservation equations is solved. The solutions are compared to those obtained from previous cold and warm gasdynamic models. The characteristic speeds of the system are calculated, and it is found that the physically relevant solutions correspond to fast magnetosonic shocks. The extended multifluid model is used to explore a new type of wave, called an “MHD draping shock,” which was proposed by Neubauer et al. [1990] to explain the unusual behavior of the magnetic field across the outbound Halley bow shock. It is found that mass loading induces switch‐on behavior at quasi‐parallel shocks, even in a high plasma beta environment. Across quasi‐parallel shocks we find switch‐on rotations as large as 30° in the cold solar wind case. This value decreases with increasing solar wind plasma beta, to a value of 16° at a value of βsw = 1. We do not find rotations large enough to explain the 69° twist in the field observed by Giotto, nor do we find any cases where the perpendicular component of the field changes sign.
Plasmasphere electron temperature studies using satellite observations and a theoretical model
Authors: Balan N. , Oyama K. I. , Bailey G. J. , Abe T.
Published: February 2004

Electron temperature variations in the Earth's plasmasphere are studied using the Exos D satellite observations and the Sheffield University plasmasphere‐ionosphere model. The observations made during the years 1989–1994 are analyzed to investigate the local time and altitude (1000–8000 km) variations of the electron temperature at magnetic latitudes 0°–45°N. The observed electron temperature Te is almost constant during both day and night and is found to have large day‐to‐night differences that vary with altitude and latitude; the largest day‐to‐night ratio in Te (≈6500 K/2600 K) occurs at the highest altitude at equatorial latitudes and the smallest ratio (≈3300 K/2300 K) at the lowest altitude at midlatitudes. During daytime, Te increases rapidly with altitude in the lower plasmasphere (altitude <2500 km) and slowly in the upper plasmasphere with mean gradients of 1.33 and 0.22 K km−1, respectively. At night, on the other hand, the lower plasmasphere is in thermal equilibrium, and in the upper plasmasphere Te increases slowly with a mean gradient equal to the daytime value. The electron temperature shows maximum latitude variation at medium plasmaspheric altitudes (around 4000 km) and smaller variations at lower and higher altitudes, particularly during daytime. At the altitude of maximum latitude variation, Te increases by about 1200 K between the equator and 40°N during both day and night. The model reproduces the observations reasonably well and is used to explain the occurrence of a prominent morning peak in Te. The morning peak becomes prominent in the lower plasmasphere at low latitudes because of the vertical E × B drift in the equatorial F region, which increases Te during morning hours and decreases Te during daytime hours. The observations and model results also show the occurrence of an afternoon peak in Te, which becomes prominent with increasing altitude and latitude; the peak could be caused by the daytime poleward neutral wind.