bluuebunny/crossref_metadata_2025 huggingface dataset:
Observational evidence for marginal stability of solar wind ion beams
Authors: Marsch E. , Livi S.
Published: February 2008

Solar wind proton double streams and proton‐alpha differential streaming may provide sufficient free energy to drive field‐aligned magnetosonic waves unstable. We carry out a statistical analysis of this electromagnetic beam instability based on measured Helios ion distributions. Observational evidence is indeed found for unstable distributions, whereby the largest growth rates are obtained in the collisionless high‐speed wind. Alpha particles by themselves are not able to excite these waves but mostly tend to stabilize an existing proton double stream configuration by enhancing the cyclotron damping of the main proton population.
Frequency functions of Venus nightside ion densities
Authors: Knudsen William C.
Published: February 2008

The frequency with which the total ion density Ni and constituent densities O+, H+, and molecular ion density O2++M29+ (= [O2+] + [NO+] + [CO+] + [N2+]) fall between equal increments of the logarithm of the density is derived. The individual measurements are first normalized by dividing them by their median value at the altitude of measurement. This process removes the known altitude variation from the sets. The resulting frequency functions are assumed to result from temporal variations alone. The data used in deriving the functions are restricted to 120°–180° solar zenith angle (SZA) and altitudes between 140 and 900 km. The frequency function derived for Ni is found to be insensitive to division of the parent set into two approximately equal subsets representing division into dawn and dusk hemispheres, an upper and lower altitude interval, or two SZA intervals. The Ni function is found to be reasonably well peaked near the median value, and 78% of the normalized values fall within a factor of 2.8 of the median value. The frequency of occurrence decreases more rapidly on the high side of the median than on the low side. The frequency functions for the constituent ions are similar to that for Ni, with the percentage of normalized densities falling within a factor of 2.8 of the median value ranging from 82% for O2+ + M29+ to 71% for H+. The similarities and dissimilarities between the frequency function for O+ and those for H+ and O2+ + M29+ are discussed in relation to the role of O+ in producing the H+ and O2+ + M29+ constituents.
DE 1 observations of theta aurora plasma source regions and Birkeland current charge carriers
Authors: Menietti J. D. , Burch J. L.
Published: February 2008

We have performed detailed analyses of the DE 1 high‐altitude plasma instrument (HAPI) electron and ion data for four passes during which theta auroras were observed. The data indicate that the theta auroras occur on what appear to be closed field lines with particle signatures and plasma parameters that are quite similar to those of the magnetospheric boundary plasma sheet. The field‐aligned currents computed from particle fluxes in the energy range 18 eV < E < 13 keV above the theta auroras are observed to be generally downward on the dawnside of the arcs with a narrower region of larger (higher density) upward currents on the duskside of the arcs. These currents are carried predominantly by field‐aligned beams of accelerated cold electrons. Of particular interest in regions of upward field‐aligned current are downward electron beams at energies less than the inferred potential drop above the spacecraft. These beams may be due to atmospheric secondaries or to ionospheric electrons that have convected into a region of field‐aligned electric field.
Effects of atmospheric disturbances on polar mesopause airglow OH emissions
Authors: Sivjee G. G. , Walterscheid R. L. , Hecht J. H. , Hamwey R. M. , Schubert G. , Christensen A. B.
Published: February 2008

Temporal variations in the absolute intensity (I) and the rotational temperature (T) of the polar winter airglow OH(8, 3) bands were observed over Longyearbyen, Spitsbergen (78.2°N), in December 1984. The high latitude of the observatory places it in a continuous polar night around the winter solstice period, thereby permitting 24‐hour optical measurements. Spectral analyses of the I and T time series reveal a dominant semidiurnal component and a wave with a period of 3.7 hours. Other waves with shorter periods may be present for part of the total observing period, but the signal‐to‐noise ratios of their amplitudes averaged over a 24‐hour interval are too low to permit definitive identification. The ratio η of the correlated Fourier components of (I − Ī)/Ī to that of
(an overbar signifies time average) has amplitude 3.2 ± 0.9 and 4.3 ± 1.3 for the 12‐ and 3.7‐hour waves, respectively, while the phase ϕ of η for these periods is 6° ± 16° and −12° ± 12°. The value of η for the 3.7‐hour wave can be compared with the theoretical prediction of η for gravity wave modulation of the OH emission. For an emission height of 87 km and an atomic oxygen scale height ℋ(O) of −2 km, theory gives |η| = 4.2 and ϕ = 0° for the 3.7‐hour wave, in good agreement with the observation. The theoretical interpretation of η at the 12‐hour period is complicated by the likelihood that the observed semidiurnal oscillation is not a simple tide. For tidal modulation of the OH nightglow, theory gives |η| between 2.6 and 0.9 and ϕ between 0° and 10° for high‐order semidiurnal modes and for the same values of emission height and ℋ(O) as above.
Electric fields and electron precipitation during a sharp negative H perturbation in the early evening sector
Authors: Fujii Ryoichi , Ogawa Toshio , Kodama Masahiro
Published: February 2008

The relationship between electric fields and energetic electron precipitation for an intense substorm during the very early evening (1700–1800 MLT) was examined using balloon‐ and ground‐based data at Syowa Station, Antarctica. The electron precipitation (>25 KeV) was implied by the Bremsstrahlung X rays measured at the balloon altitudes. A large and prompt increase in X rays occurred simultaneously with the onset of a sharp H perturbation of −1000 nT, when the azimuthal direction of the electric field changed from southwest (poleward) to northwest (equatorward). This intense precipitation occurred during the developing stage of the negative H perturbation. Comparative studies of a time sequence of the electric field, electron precipitation, and auroral electrojet currents indicate that the precipitation region of intense energetic electrons (>25 KeV) in the very early evening sector was confined latitudinally to the vicinity of the region, where the enhanced westward electrojet pushes away the eastward electrojet.
Bidirectional solar wind electron heat flux events
Authors: Gosling J. T. , Baker D. N. , Bame S. J. , Feldman W. C. , Zwickl R. D. , Smith E. J.
Published: February 2008

Normally the ≳ 80‐eV electrons which carry the solar wind electron heat flux are collimated along the interplanetary magnetic field (IMF) in the direction pointing outward away from the sun. Occasionally, however, collimated fluxes of ≳ 80‐eV electrons are observed traveling both parallel and antiparallel to the IMF. Here we present the results of a survey of such bidirectional electron heat flux events as observed with the plasma and magnetic field experiments aboard ISEE 3 at times when the spacecraft was not magnetically connected to the earth's bow shock. The onset of a bidirectional electron heat flux at ISEE 3 usually signals spacecraft entry into a distinct solar wind plasma and field entity, most often characterized by anomalously low proton and electron temperatures, a strong, smoothly varying magnetic field, a low plasma beta, and a high total pressure. Significant field rotations often occur at the beginning and/or end of bidirectional heat flux events, and, at times, the large field rotations characteristic of “magnetic clouds” are present. Approximately half of all bidirectional heat flux events are associated with and follow interplanetary shocks, while the other events have no obvious shock associations. When shock associated, the delay from shock passage typically is ∼13 hours, corresponding to a radial separation of ∼0.16 AU. Independent of any shock association, bidirectional heat flux events typically are ∼0.13 AU thick in the radial direction, although considerable variability is evident from one event to another. Near solar activity maximum, bidirectional heat flux events occurred at a rate of ∼3 per month, and the solar wind electron heat flux was bidirectional ∼5% of the time. Bidirectional heat flux events often contain strong out‐of‐the‐ecliptic field components and thus can be effective in producing geomagnetic disturbances. This is particularly true for shock‐associated events where the intrinsically strong fields in the leading portions of the events are amplified by compression in transit from the sun and where strong out‐of‐the‐ecliptic field components resulting from compression and draping of the ambient field are often present within the shocked plasma immediately ahead. Consistent with previous work we interpret the bidirectional heat flux as evidence for a closed field topology in interplanetary space. Further, we suggest that these events are one of the more prominent signatures of coronal mass ejection events in the solar wind at 1 AU.
The Uranian bow shock: Voyager 2 inbound observations of a high Mach number shock
Authors: Bagenal Fran , Belcher John W. , Sittler Edward C. , Lepping Ronald P.
Published: February 2008

The Voyager 2 magnetometer and plasma detector measured a high Mach number (fast magnetosonic number MMS> 15), high β (∼ 3) bow shock on the dayside of the Uranian magnetosphere. Although the average conditions on either side of the shock are consistent with the Rankine‐Hugoniot (MHD) relations for a stationary, quasi‐perpendicular shock, the data revealed both detailed structure in the transition region as well as considerable variability in the downstream magnetosheath plasma. The bulk plasma parameters and the magnetic field exhibited some of the characteristics of a supercritical shock: an overshoot followed by damped oscillations downstream, consistent with recent theoretical models of high Mach number quasi‐perpendicular shocks.
On the stress balance in flux transfer events
Authors: Sonnerup Bengt U. O.
Published: February 2008

A theoretical discussion is presented of certain aspects of the structure and dynamics of holes in the magnetopause through which magnetic flux connects the magnetosphere with the magnetosheath in the manner described in the Russell‐Elphic model of flux transfer events (FTEs). It is shown that a net current with direction parallel to B for By > 0 and antiparallel to B for By < 0 must flow through the hole. This current may provide a natural explanation for the observed helical field observed in FTE flux tubes. If the hole radius, a, is less than a certain critical value, acrit, it is found that the hole will convect with the ambient plasma flow speed; if a > acrit, as is expected for most FTEs, the hole may under certain reasonable assumptions move away from the subsolar region (the assumed region of formation of a hole pair) at a speed that exceeds the ambient flow speed and approaches the value expected in steady state reconnection for a » acrit. For a > acrit, reconnection is assumed to occur between flux tube field and the field By contained in the magnetopause layer, the result being a flux tube structure consisting of an inner core surrounded by a mantle region which contains reconnected flux and energized plasma.
Competing processes of whistler and electrostatic instabilities in the magnetosphere
Authors: Omura Yoshiharu , Matsumoto Hiroshi
Published: February 2008

Competing processes of whistler mode and electrostatic mode instabilities induced by an electron beam are studied by a linear growth rate analysis and by an electromagnetic particle simulation. In addition to a background cold plasma we assumed an electron beam drifting along a static magnetic field. We studied excitation of whistler and electrostatic mode waves in the direction of the static magnetic field. We first calculated linear growth rates for the whistler mode and electrostatic mode instabilities, assuming various possible parameters in the equatorial magnetosphere. We found that the growth rate for the electrostatic instability is always larger than that of the whistler mode instability. A short simulation run with a monoenergetic electron beam demonstrates that a monoenergetic beam can hardly give energy to whistler mode waves as a result of competition with faster growing electrostatic waves, because the beam electrons are trapped and diffused by the electrostatic waves, and hence the growth rates for whistler mode waves become very small. A long simulation run starting with a warm electron beam demonstrates that whistler mode waves are excited in spite of the small growth rates and the coexisting quasi‐linear electrostatic diffusion process. The diffusion of the warm electron beam is explained by three different processes with different time scales. An electromagnetic diffusion follows the electrostatic diffusion, causing pitch angle scattering of the beam electrons to lower pitch angles. At the third stage the diffused electrons at high parallel velocities are further diffused down to a lower parallel velocity range because of the enhanced electrostatic fluctuations. The present simulation demonstrates that many grid points as well as an enormous number of time steps are required to simulate two coexisting instabilities with different time scales. Fewer grid numbers and fewer time steps lead to misunderstanding of the physics involved.
Detection of a hot plasma component within the core regions of Jupiter's distant magnetotail
Authors: Sittler Edward C. , Lepping Ronald P. , Mauk B. H. , Krimigis S. M.
Published: February 2008

We have combined the Voyager 2 low‐energy plasma data from the Plasma Science Experiment (PLS) and the magnetic field data from the Magnetometer Experiment (MAG) with the Voyager 2 Low Energy Charged Particle Experiment (LECP) ion data (E > 28 keV) for the previously described distant magnetotail observations (5000 < R < 9000 RJ). We show for the first time a definite enhancement of LECP fluxes within the core regions where the PLS densities and magnetic field pressure are lower than surrounding regions, indicating that this hot tenuous plasma is present within the core regions. In general there is a strong anticorrelation between PLS density and LECP fluxes, while a less pronounced anticorrelation between magnetic field pressure and LECP fluxes is observed. Estimates of LECP pressures suggest that this hot plasma will provide the previously described missing pressure in the core if heavy ions dominate the ion composition. The angular dependence of the LECP data indicates a flow of this hot plasma in the anti‐Jupiter direction. This outflowing plasma could be the remnant of the magnetospheric wind observed near Jupiter by LECP. On the basis of this preliminary study we have identified the core regions as having similarities to a plasma sheet.
Ion/ion and electron/ion cross‐field instabilities near the lower hybrid frequency
Authors: Gary S. Peter , Tokar Robert L. , Winske Dan
Published: February 2008

Instabilities near the lower hybrid frequency are investigated by numerically solving the linear Vlasov electromagnetic dispersion equation. The configuration is that of two unmagnetized ion components, a less dense beam and a more dense core, streaming across a uniform magnetic field with magnetized electrons in the zero momentum frame. All three components are considered to have similar temperatures. This paper studies the parametric dependences of maximum growth rate for three distinct modes: the electron/ion beam modified two‐stream instability, the electron/ion core modified two‐stream instability, and the ion/ion lower hybrid instability. In the electrostatic limit the electron/beam instability has a lower threshold and larger growth rate than its electron/core counterpart. Except near equal beam and core densities, our results also suggest that the electron/beam instability has a maximum growth rate that is equal to or greater than that of the ion/ion instability for all plasma beta values. Thus the electron/beam modified two‐stream instability is a likely candidate to account for the presence of lower hybrid fluctuations in the foot of the earth's bow shock at low beta.
Estimating the anomalously large fluences of solar flare protons with tritium data from 1900 to 1952
Authors: Nutley Hugh , Voth Mitchell D.
Published: February 2008

An analysis is presented for estimating the “anomalously large” fluences from solar flare protons by measuring tritium deposited between 1900 and 1952 in a high‐latitude ice core. Most of the tritium from cosmic rays and solar flare protons comes by way of indirect (p, n) and (n, t) reactions rather than from direct (p, t) reactions. Therefore the neutron spectrum produced by protons of the August 1972 flare is estimated first. (The August 1972 flare is the most recent “anomalous” fluence, and it was accurately measured by spacecraft detectors.) Then the calculated neutron spectrum is compared with that known from galactic cosmic rays. Finally, the tritium produced by (n, t) reactions is compared for the solar protons and cosmic rays. We thus calculate that the August 1972 flare‐produced tritium was 67 times larger than the cosmic ray‐produced tritium in all of 1972. Range tables of energetic protons in nitrogen and oxygen gases show that tritium from solar protons is produced in the stratosphere, while that from galactic cosmic rays is produced in both the stratosphere and troposphere. Other studies estimate that the stratosphere residence half‐life time for tritium is 16–20 months and the troposphere residence time is short (on the order of 10 days.) In any event, the tritium will scavange out of the atmosphere, and if it is locked in snow that does not melt in summer months, such as that in glaciers at very high latitudes and elevations, it will remain for many years. Therefore tritium frozen in a high‐latitude glacier would give a good measure of the fluences from the very largest solar flares.
Solar cycle invariance in solar wind proton temperature relationships
Authors: Lopez R. E.
Published: February 2008

The relationships between solar wind proton temperature and velocity and between temperature and momentum flux density at 1 AU are examined using National Space Science Data Center IMP 8 solar wind data obtained from late 1984 to early 1985. These relationships are compared with similar ones obtained from a variety of solar wind data spanning 14 years, from 1966 to 1980. It is found that these relationships, particularly the one between temperature and velocity, are very stable over the solar cycles from which these data were drawn. This suggests that the basic physical processes which accelerate and heat the solar wind have remained unchanged for the last 20 years. Over this same period, the energy, momentum, and mass fluxes produced by the Sun to generate the solar wind varied by about 60%. A qualitative explanation of these results is given in the context of a model for solar wind generation that includes significant postsonic momentum and energy deposition and assumes that solar wind protons are heated by magnetohydrodynamic waves of solar origin.
Effect of possible passage through Halley's magnetic tail on geomagnetic activity
Authors: Russell C. T. , Phillips J. L. , Fedder J. A. , Allen J. H. , Morris L. , Craig Roger A.
Published: February 2008

The geomagnetic aa index shows that a nonrecurrent magnetic disturbance occurred in May 1910 near the time of the passage of comet Halley through inferior conjunction. Examination of ground‐based magnetograms from May 18 to 20, 1910, shows that this disturbance consisted, in part, of a negative “bay” of the depth expected if the magnetopause currents had ceased. The duration of this bay could have been caused by the Earth's entry into a cometary tail lobe width of about 106 km. Other features during this period suggest that the Earth may have also earlier grazed the other lobe of the cometary tail and passed through the plasma sheet. If this interpretation is true, the entire region of interaction was about 6×106 km wide 24×106 km downstream from the nucleus. MHD computer simulations of the solar wind interaction with Halley to 7×106 km downstream are consistent with the inferred tail properties. The fact that the signature of the interaction was observed earlier than expected suggests that momentum transfer occurred from the comet to the solar wind over a very extended region of space around the comet. If this momentum transfer deflected the solar wind flow with the 45‐km/s transverse velocity of the comet, the observed timing would be consistent with an initially radially flowing solar wind of 750 km/s or a solar wind flow 4° from the radial of 500 km/s.
Noncoplanar magnetic fields, shock potentials, and ion deflection
Authors: Jones Frank C. , Ellison Donald C.
Published: February 2008

A puzzling aspect of the structure of collisionless shock waves concerns the existence of a component of magnetic field that lies out of the coplanarity plane in the transition layer. We show that the existence of this component may be understood and its value calculated from a simple expression derived from first principles. The value of this field component may be derived without reference to the transformation properties of the fields or the changes in energy that the ions and electrons undergo on traversing the electrostatic potential of the shock interface. In fact, the derivation does not depend on the particular value that this potential jump may be observed to have in any particular reference frame and relies solely on Maxwell's equations and the Rankine‐Hugoniot jump conditions. Our results, however, are consistent with previously published work that links these two notions and hence may be considered as an independent verification of this work.
Voids in Jovian magnetosphere revisited: Evidence of spacecraft charging
Authors: Khurana K. K. , Kivelson M. G. , Armstrong T. P. , Walker R. J.
Published: February 2008

As Voyager 2 moved inward toward Jupiter in the vicinity of Ganymede's orbit, the low‐energy ion and electron fluxes recorded by the Plasma Science Experiment (PLS) intermittently fell to very low levels. The low fluxes, which lasted for times of the order of 10 min and recurred over an 8‐hour interval, have been called “voids.” Early interpretations associated the voids with Ganymede wake effects. Further analysis has demonstrated that the location and extent of the voids are inconsistent with such an interpretation. In this study, we have reexamined the PLS data in conjunction with data from the magnetic field experiment and the low‐energy charged particle experiment (LECP) in the relevant interval. The LECP data showed that the PLS voids were accompanied by large enhancements of the flux of energetic electrons and ions. No systematic signatures were found in the magnetic data. We suggest that increased energetic electron fluxes in the void regions intermittently charged the spacecraft negatively to values between a few kV and tens of kV. For a realistic phase space distribution of cold and hot particles in the Jovian magnetosphere, spacecraft charging can produce dropouts in the measured cold ion and electron fluxes and enhancements in the measured fluxes of hot particles consistent with the observations. Spacecraft charging would not produce a signature detectable by the flux gate magnetometer. We propose that the plasma density in the nominal voids is not exceptionally low and that the near‐Ganymede anomalies arose from an unusual combination of enhanced fluxes of ∼10 to ∼100 keV electrons in a low‐density plasma environment. Further work is needed to explain why this phenomenon occurred intermittently and only over a limited portion of the Voyager trajectory.
Shock overshoots revisited
Authors: Mellott M. M. , Livesey W. A.
Published: February 2008

We investigated the role played by low‐frequency turbulence in the determination of magnetic field overshoots in collisionless shock waves. The data set used in this study included magnetometer and solar wind data from the ISEE 1 and 2 spacecraft for ∼65 quasi‐perpendicular bow shocks. Overshoots were calculated from both high‐resolution data and from data averaged to eliminate the effects of turbulence with frequencies greater than the ion cyclotron frequency. Overshoots determined by the two methods exhibited generally similar behavior, although those calculated from the high‐resolution data were generally larger by a factor of ∼2. Overshoot size correlated well with shock Mach number and electron beta in both cases. The size of overshoots calculated from the high‐resolution data increased strongly with Mach number and beta, while those calculated from the averaged data showed less dramatic increases. The behavior of overshoots calculated from the average data was generally consistent with hybrid simulation results. The difference between overshoots measured using averaged and unaveraged data was generally consistent with the presence of a component of overshoot magnitude due to low‐frequency turbulence. Measurable overshoots were observed for all shocks in the data set, although those associated with the weakest shocks were small. Neither set of overshoots showed any particular change in behavior at the first critical Mach number.
Uranus satellites: Densities and composition
Authors: Johnson Torrence V. , Brown Robert H. , Pollack James B.
Published: February 2008

We have analyzed the densities of the Uranian satellites derived from Voyager data to place constraints on their uncompressed densities and probable silicate mass fractions. Two bounding models were used: (1) a completely differentiated satellite with the current temperature/depth profile set by conduction of heat from the silicate core, and (2) a homogeneous model with the temperature profile also set by conduction. Results from such models for the largest satellites, Titania and Oberon, show that within the errors the silicate mass fraction of these two bodies should lie within the range 0.42–0.65, with the most probable values being 0.50–0.58. These values are significantly higher than similar model results for Saturn's small satellites (Rhea's silicate fraction is ∼0.35–0.42) but similar to the system averages for Jupiter and Saturn (including Titan). The derived silicate fractions are also higher than predicted for solar nebular models having CH4 as the dominant carbon‐bearing species. Examining current models of solar nebular chemistry and the three systems visited by Voyager, we suggest that the satellites accreted from material in the outer envelopes of their respective primaries, with the abundance of CO and solid organic material in the solar nebula and satellite‐forming nebulae being important determinants of the rock/water ice ratios in these systems, and thus the satellite mean densities. Preferential dissolution of the water component of planetesimals in the envelopes of the giant planets may have also contributed to determining the water/rock fractions incorporated into the planets' satellites.
The dependence of electroglow on the solar flux
Authors: Yelle Roger V. , McConnell J. C. , Sandel B. R. , Broadfoot A. L.
Published: February 2008

A comparison is made of the observed properties of electroglow on Jupiter, Saturn, and Uranus. It is found that the subsolar electroglow intensity measured by the Voyager Ultraviolet Spectrometer in the spectral region below 1110 Å scales as the inverse square of heliocentric distance to within 10%. There are large differences between the spectra from Uranus and the Jovian and Saturnian spectra at longer wavelengths, but this is a consequence of the different scattering properties of the Uranian atmosphere and is not related to electroglow. Excellent fits are obtained between model calculations of the spectrum of Rayleigh‐Raman scattered sunlight and the observed spectra from Uranus. Our interpretation differs from previous analyses where the differences between the Uranian and Saturnian spectra were interpreted to be a consequence of different electron temperatures. It is also found that the local time variations of electroglow on all three planets are consistent with an excitation rate proportional to the solar flux. These correlations with solar flux suggest that electroglow is a direct consequence of the absorption of solar radiation in the upper atmospheres of the outer planets. The most promising production mechanism for electroglow appears to be fluorescence of solar radiation in the H2 Lyman and Werner bands. We calculate that fluorescence of solar ultraviolet radiation in the Lyman and Werner band systems of H2 may be responsible for a large fraction of the observed intensity on Jupiter, Saturn, and Uranus; however, it is not clear why this source should scale as the inverse square of heliocentric distance. We also suggest that chemical reactions related to H− photolysis may be important in the upper atmospheres of the outer planets. Both fluorescence and H− photolysis merit further investigation.
Voyager 2 plasma ion observations in the magnetosphere of Uranus
Authors: Selesnick Richard S. , McNutt Ralph L.
Published: February 2008

Positive ion measurements in the magnetosphere of Uranus have been made by the Voyager 2 plasma science experiment. We present an overview of the entire data set and a detailed analysis of the observations from the inner magnetosphere which complements and extends results reported elsewhere. Densities and temperatures are obtained from an analysis which incorporates details of the instrumental response. These results are then used to calculate flux tube particle and energy content to support the hypothesis that the plasma transport is controlled by a solar wind‐driven magnetospheric convection system. Variations in the flux tube content suggest both a local source of plasma, produced from the neutral hydrogen corona of Uranus, and a nonlocal source, convected inward and heated by adiabatic compression. In each case a proton composition is inferred. Sharp boundaries in the high‐energy (approximately 1 keV) plasma population are interpreted in terms of the spatial extent of the magnetospheric convection, with significant shielding of the convection electric field. The convection theory is also used in a simulation of the low‐energy (approximately 10 eV) ion component using the neutral hydrogen source, resulting in distribution functions which qualitatively agree with the observations.
Energy branching in the Io plasma torus: The failure of neutral cloud theory
Authors: Shemansky D. E.
Published: February 2008

The energy branching of the hot Io plasma torus using model calculations which include all of the significant physical chemistry that affects the system has been examined in order to study energy source characteristics. Most theoretical discussions of the energetics of the torus assume that the system is maintained against radiative and other losses by the interaction of the plasma with neutral atomic clouds. The energy in this theory is derived from the kinetic energy acquired by ions created in the rotating planetary magnetic field. Coulomb collisions with the electron gas control the flow of energy to the ionizing and radiative processes. The energetics of this theoretical system is defined by fixing the electron density, the diffusive loss time, and the relative volumetric rates of injection of the major neutral constituents, oxygen and sulfur. On the basis of calculations of this kind in comparison with the characteristics of the observed system, the conclusion has been drawn that neutral cloud theory is qualitatively inadequate. At the measured electron densities in the plasma torus neutral cloud theory can only support a dominantly singly ionized system, whereas observation fixes the ratio [S II]/[S III] < 1.0. The theory also fails to predict observed plasma properties relative to variations in number density. A large fraction of the energy required by the observed system based on ion partitioning alone must be derived from some other interaction. Two possibilities for energy sources involving a particular interaction with an Io atmosphere and a heterogeneous source of energetic electrons are discussed.
On the spectrum of the secondary auroral electrons
Authors: Fung S. F. , Hoffman R. A.
Published: February 2008

We present the results from a statistical study of the spectral characteristics of the (power law portion) secondary electrons associated with inverted‐V electron precipitation events. A total of 106 inverted‐V events observed at low altitudes (<1000 km) by the DE 2 spacecraft are included in our data base. Spectral parameters for the electrons between 20 eV and 100 eV are plotted against the different parameters pertaining to the locations of the inverted‐V events and the primary beams to determine their empirical dependences. We have found that the differential spectrum of the secondary electrons can be modeled by a power law J α E−γ with an averaged γ = 1.85. This spectral parameter is relatively insensitive to the variation of locations of the inverted‐V events, but it depends on the primary beam parameters. We compare our results with the predictions and assess the applicabilities of the current theories on the generation of the low‐energy auroral power law secondary electrons. Our analysis shows that collisional processes are the dominant source of the secondary electrons.
High‐latitude dayside electric fields and currents during strong northward interplanetary magnetic field: Observations and model simulation
Authors: Clauer C. Robert , Friis‐Christensen Eigil
Published: February 2008

On July 23, 1983, the Interplanetary Magnetic Field turned strongly northward, becoming about 22 nT for several hours. Using a combined data set of ionospheric convection measurements made by the Sondre Stromfjord incoherent scatter radar and convection inferred from Greenland magnetometer measurements, we observe the onset of the reconfiguration of the high‐latitude ionospheric currents to occur about 3 min following the northward IMF encountering the magnetopause. The large‐scale reconfiguration of currents, however, appears to evolve over a period of about 22 min. Using a computer model in which the distribution of field‐aligned current in the polar cleft is directly determined by the strength and orientation of the interplanetary electric field, we are able to simulate the time‐varying pattern of ionospheric convection, including the onset of high‐latitude “reversed convection” cells observed to form during the interval of strong northward IMF. These observations and the simulation results indicate that the dayside polar cap electric field observed during strong northward IMF is produced by a direct electrical current coupling with the solar wind.
Excitation of whistlers and waves with mixed polarization by newborn cometary ions
Authors: Wu C. S. , He X. T. , Price C. P.
Published: February 2008

The present study has been motivated by the ICE wave measurements. It is found that the newborn cometary ions, particularly the protons, can excite whistlers and waves with frequencies much higher than the proton gyrofrequency but with mixed electrostatic and electromagnetic polarization. For the case of oblique propagation the newborn ions are treated as if they are unmagnetized. This is justified not only because the wave frequencies are high but also because the growth rates are large. On the other hand in the case of parallel propagation the growth rate is much smaller, and the excitation process seems to be unimportant.
Observational test of shock drift and fermi acceleration on a seed particle population upstream of Earth's bow shock
Authors: Anagnostopoulos G. C. , Sarris E. T. , Krimigis S. M.
Published: February 2008

Simultaneous observations of energetic protons of solar origin by the IMP 7 and 8 spacecraft in the vicinity of the dawn and dusk bow shock, respectively, are examined to determine the conditions for acceleration of an ambient energetic particle population at the bow shock. The data show that acceleration effects are observed in the intensities of the energetic protons with energies as high as E ∼ 4 MeV only at the quasi‐perpendicular region of the bow shock. Furthermore, the ion acceleration is observed for high values of the solar wind velocity and the interplanetary magnetic field (IMF) and for an expanded bow shock. The energy spectra and angular distributions of the intensities of the accelerated ions both upstream and downstream from the bow shock are consistent with the predictions of the shock drift acceleration (SDA) mechanism operating efficiently at the quasi‐perpendicular (dusk) side of the bow shock while the IMF is in the nominal direction. At the same time no evidence for any particle acceleration at E ≳ 50 keV was detected by the spacecraft located at the quasi‐parallel (dawn) side of the bow shock, under conditions which favor the Fermi mechanism. These results, together with an observation of a magnetospheric burst during a period of radial IMF suggest the following: (1) acceleration of a seed particle population only occurs at the quasi‐perpendicular bow shock through SDA and (2) leakage of magnetospheric ions at E ≳ 50 keV rather than in situ acceleration is the principal source of observed upstream ion populations.
Mapping electrodynamic features of the high‐latitude ionosphere from localized observations: Technique
Authors: Richmond A. D. , Kamide Y.
Published: February 2008

We present a new technique for mapping high‐latitude electric fields and currents and their associated magnetic variations from sets of localized observational data. The technique generalizes earlier ones that were designed to deduce these electrodynamic features from ground‐based magnetometer data alone. In the new technique, many different types of measurements can potentially be used: electric fields from radars and satellites; electric currents from radars; and magnetic perturbations at the ground and at satellite heights. The technique also makes use of available statistical information about averages and variances of the electrodynamic fields. One of its advantages over earlier techniques is that it quantifies the errors inherent in the mapped fields, taking into account the distribution of available data, their errors, and the statistical variances of the fields. A related application of the procedure is used for estimating the distributions of high‐latitude ionospheric conductances, using available direct and indirect measurements. The new technique is illustrated by application to an example of a substorm that was previously analyzed by Kamide et al. (1982a) with an earlier technique. The new technique tends to yield much simpler patterns of high‐latitude ionospheric convection in regions of low ionospheric conductance. When magnetometer data alone are used, as in this example, the statistical uncertainty in the derived electric fields is largest in regions of low conductance, because the electric fields in these regions have little influence on the magnetic perturbations. A companion paper (Richmond et al., this issue) presents a detailed application of the technique using multiple data sets.
Solar cycle changes in the morphology of the Venus ionosphere
Authors: Knudsen William C.
Published: February 2008

I present two median plasma density curves for the central nightside Venus ionosphere, one for solar cycle maximum (SCmax) conditions and one for solar cycle minimum (SCmin) conditions. The curves are constructed from Pioneer Venus retarding potential analyzer total ion density data. Although the two curves do not overlap in altitude because of orbit evolution, they constitute strong evidence that the nightside ionospheric density during SCmin is typically a factor of 10 smaller between 200 and 2000 km altitude than it is at SCmax. From these data, data presented previously (Knudsen et al., 1987), and Venera radio occultation data, I conclude that the Venus SCmin ionosphere is typically confined to an altitude below approximately 250 km at all solar zenith angles (SZA) with the possible exception of an SZA interval near 90°. The layer is composed primarily of molecular ions, O2+ being the most abundant. At SCmax an extensive O+ layer exists on top of the O2+ layer with the typical ionopause altitude changing from 300 km in the subsolar region to 900 km at the terminator and to over 1800 km altitude in the antisolar region.
Simulations of active ion injection experiments on ARCS 3
Authors: Hudson M. K. , Roth I.
Published: February 2008

The ARCS 3 sounding rocket experiment used two argon guns oriented parallel and perpendicular to the geomagnetic field. The parameters of this experiment have been used in a series of particle simulations in order to interpret ion beam interaction with the multispecies ionospheric plasma. Along with the lower hybrid mode between the electron and the hydrogen gyrofrequencies, ion‐ion hybrid modes between the hydrogen and helium gyrofrequencies and between the helium and oxygen gyrofrequencies are identified in the simulations. Besides these cold plasma modes, warm Bernstein modes of the respective ion species are identified. The parallel argon beam injections excite lower hybrid waves perpendicular to the magnetic field because of the finite thermal spread of the parallel beam. Perpendicular ion heating of background hydrogen, helium, and oxygen is comparable in the simulations. Comparisons are made with the wave and particle data from the ARCS 3 rocket experiment.
Field‐aligned current signatures in the near‐tail region: 1. ISEE observations in the plasma sheet boundary layer
Authors: Ohtani S. , Kokubun S. , Elphic R. C. , Russell C. T.
Published: February 2008

Signatures of field‐aligned currents in the near‐tail region are examined by using ISEE magnetometer data. We focus our attention on steplike changes of magnetic field direction, which are interpreted as the spacecraft crossing of field‐aligned current sheets. Individual case studies of two substorms on March 22, 1979 (CDAW 6 events), indicate that the field‐aligned current signatures in the near‐tail region are consistent with observations at lower altitudes and that the currents in the plasma sheet boundary layer can be identified as parts of the large‐scale current system, the region 1 system. Statistical properties of the field‐aligned currents are also studied. Earthward and tailward currents tend to be observed in postmidnight and in premidnight, respectively, as expected from the distribution of current direction in the region 1 system. The present result seems to be inconsistent with that of Elphic et al. (1985). This apparent discrepancy is due to the difference in identification of field‐aligned current signatures. They have analyzed field‐aligned currents at the edge of the plasma sheet boundary layer, while we examine large‐scale currents in the present analysis. It is thus suggested that small‐scale field‐aligned currents with opposite polarities are imbedded poleward of the region 1 system. The results of these two studies are considered to be complementary to each other. The dependence of the current intensity on the geomagnetic activity and the radial distance from the Earth indicates that there are two field‐aligned current sources in the magnetotail. One is in the region RXY = (X²GSM +X²GSM)1/2 ≥ 15 Re, and the other is nearer to the Earth, RXY < 15 Re. The distant current source persistently exists and the current intensity is almost independent of the activity of the magnetosphere. On the other hand, the near‐earth current source plays an active role during substorms.
The zonal neutral wind in the equatorial thermosphere
Authors: Louro A. A. , Duhau S.
Published: February 2008

A theoretical prediction of zonal neutral winds at the equator at F region heights is presented, using values for the ion velocity calculated with a model also due to the authors. The results, and the dynamic properties of the thermosphere that may be inferred from them, are viewed in light of information obtained from recent measurements performed with the satellite Dynamics Explorer 2. The calculated values of the zonal neutral wind agree in general with the measured values in phase; the predicted amplitude is smaller during the early nighttime, and to a lesser extent during the daytime period. The model predicts practically constant values of the zonal neutral wind with height, and a superrotation less than 5%, in agreement with the experimental findings.
A prediction for the maximum phase and duration of sunspot cycle 22
Authors: Wilson Robert M.
Published: February 2008

Based on the modern era sunspot cycles (cycles 10 to present), maximum smoothed sunspot number (EM) has followed minimum smoothed sunspot number (Rm) by approximately 48±6 months, being true for 8 out of the 12 cycles; for 12 out of 12 cycles, RM has followed Rm by 48±12 months. Thus for the most reliably known sunspot cycles, ascent (ASC) has always been between 36 and 60 months, displaying a strong clustering between 42 and 54 months. Because of inferred correlative (preferential) behavior between ASC and RM, it may be that ASC for cycle 22 can be better predicted (through the use of an appropriate projected value of RM). Similarly, because of inferred preferential behavior between maximum amplitude first‐difference and bimodal period class and between even‐ and odd‐cycle numberedness and bimodal period class, it may be that the bimodal period class and length of cycle 22 can also be better predicted. Assuming that it is not a statistical fluke, present indications are that cycle 22, an even‐numbered cycle, will have an RM below that observed for cycle 21. If true, then cycle 22 is a “negative‐valued” maximum amplitude first‐difference cycle, implying that it will have an ASC longer than or equal to 4 years (the median value), although it could be shorter, and that it is a long‐period cycle (having a length longer than 11 years, probably in the range 138±6 months, based on the average length of long‐period cycles during the modern era of sunspot observations). This suggests that cycle 22 will peak sometime in the latter half of 1990 to the first half of 1991 and that it will not end until early to mid 1998.
A mirror instability associated with newly created ions in a moving plasma
Authors: Wu C. S. , Krauss ‐Varban D. , Huo T. S.
Published: February 2008

An instability that resembles the usual hydromagnetic mirror instability is demonstrated. It is caused by freshly created ions in a moving plasma. The source of free energy is associated with the ring distribution of the newborn ions which results in a high “ion kinetic temperature” in the direction transverse to the ambient magnetic field. The stability analysis is carried out based on the linearized Vlasov equations. The effect of electron Landau damping is also discussed. This instability can lead to the amplification of magnetosonic waves.