bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical properties of low‐frequency magnetic field fluctuations in the solar wind from 0.29 to 1.0 AU during solar minimum conditions: HELIOS 1 and HELIOS 2
Authors: Denskat K. U. , Neubauer F. M.
Published: February 2008

To obtain information on the temporal and spatial evolution of MHD waves and discontinuities in the solar wind, we studied by means of statistical methods magnetic field fluctuations measured by the two HELIOS spacecraft in the frequency range between 2.4×10−5 Hz and 1.3×10−2 Hz at distances from the sun between 0.29 AU and 1.0 AU. Some statistical properties as magnetic field variances show the fluctuations remaining very similar on their way out from the sun. A different picture of the fluctuations emerges from analyzing the spectral properties of the magnetic field: The slope of the power spectral densities (both the vector components and the magnitude of the magnetic field) as a function of frequency becomes steeper with increasing heliographic distance—the solar wind seems to act as a low‐pass filter. The major changes occur within a heliographic distance up to about 0.4 AU. Field magnitude fluctuations fall off less rapidly with increasing radius than do fluctuations in the vector components. Cross spectral analysis between magnetic field components reveal the fluctuations as generally being field magnitude conserving.
Discovery of an Io‐correlated energy source for Io's hot plasma torus
Authors: Sandel B. R. , Broadfoot A. L.
Published: February 2008

Energy flowing into Io's hot plasma torus in the vicinity of the satellite causes an azimuthal modulation of the brightness of the torus that is correlated with the position of Io. Voyager ultraviolet spectrometer observations show that the plasma downstream from Io is brighter in S III 685‐Å emission, reflecting a higher electron temperature there. The electron temperature is raised by a mechanism operating within 45° of the azimuthal position of Io in its orbit; the interaction region may extend over a much smaller range of azimuth. Differences in electron temperature inferred from spectral analysis account for all the observed difference in brightness, implying that no change in the composition or density of the hot plasma occurs. The Io‐correlated source provides a time‐averaged power of 8×10−14 erg cm−3 s−1 (4×1011 W) or about 20% of the power radiated by the torus. The time constant for radiative cooling of the torus is such that most of the remainder of the power must be supplied by the local‐time source, which is distinct from the Io source reported here. The specific mechanism by which the electrons are energized is unknown.
Large amplitude undulations on the equatorward boundary of the diffuse aurora
Authors: Lui A. T. Y. , Meng C.‐I. , Ismail S.
Published: February 2008

Global auroral pictures from the Defense Meterological Satellite Program (DMSP) satellites are presented to show, for the first time, occurrences of large amplitude undulations on the equatorward boundary of the diffuse aurora in the afternoon‐evening sector. The crest‐to‐trough amplitude of these waveforms ranges from about 40 to 400 km and the wavelength varies from about 200 to 900 km. The undulations are seen in one case to extend over 3000 km along the equatorward boundary. Auroral images from successive DMSP passes suggest that this phenomenon lasts for about 0.5 to 3.5 hours. In each of the four cases observed, the undulations occur during a geomagnetic storm interval near the peak development of the storm time ring current. In all instances, auroral pictures displaying the undulations show simultaneous substorm patterns of discrete auroras. In one instance when simultaneous electron (50 eV to 20 keV) measurements from DMSP satellite are available, the electron spectra near the diffuse auroral equatorward boundary resemble power law spectra, and the scale length for the density gradient at the boundary is determined to be about 12 km. Weak electron precipitation is also found equatorward of the diffuse aurora and the associated electron spectra frequently show a secondary population with peak fluxes at 1–5 keV. The observed undulations are interpreted as surface waves propagating on the inner edge of the plasma sheet, and possible plasma instabilities responsible for it are briefly discussed.
A nonlocal theory of an electrostatic sinusoidal density drift instability
Authors: Thomsen M. F. , Gary S. Peter
Published: February 2008

The stability of space plasmas in which the macroscopic variables vary in the direction
perpendicular to the magnetic field is usually studied by means of the local approximation, in which the fluctuating potential is assumed to be independent of x. To remove this approximation, we study a nonlocal problem in which the background density, and hence the fluctuating potentials, are periodic in x. From the linear Vlasov/Poisson equations we derive a set of coupled, linear, homogeneous, algebraic equations relating the Fourier amplitudes of the eigensolution. We solve the equations numerically and find a hierarchy of exact eigenmodes characterized by different growth rates and spatial structures. At points where the density gradient is locally zero, the mode amplitude is generally several orders of magnitude lower than the peak amplitude. We study the dependence of these solutions on the parameters which define the background density perturbation and demonstrate a correspondence between these nonlocal results and those obtained with the local approximation. This allows us to illuminate the conditions under which the local theory is a valid approximation and permits us to extend its range of useful application.
Solar irradiance variations and solar activity
Authors: Willson Richard C.
Published: February 2008

A mean value for the 1 AU total solar irradiance of 1368.2 W/m2 and a downward trend of 0.05% per year were derived from measurements by the Active Cavity Radiometer Irradiance Monitor (ACRIM) experiment on the Solar Maximum Mission during 1980. Distinct temporary solar irradiance decreases associated with solar activity maxima were observed with a series of nine dips from April to October recurring at fairly regular intervals averaging 24 days. The decreases correlate inversely with sunspot area, 2800‐MHz flux, and Zurich sunspot number. Dominant periods common to the irradiance and sunspot area power spectra link the irradiance decreases to sunspot flux deficit in solar active regions. Evidence of significant total irradiance modulation by facular flux excess is cited. A persistent radiative cycle of active regions consistent with the ACRIM irradiance results and the morphology of solar active regions was found. The pattern of regularly recurrent active region maxima between April and October suggests an asymmetry in solar activity generation during this period.
Generation of the auroral kilometric radiation
Authors: Wu C. S. , Wong H. K. , Gorney D. J. , Lee L. C.
Published: February 2008

In two preceding papers (Wu and Lee, 1979, Lee et al., 1980) an effective loss‐cone distribution function is used in the stability analysis relevant to the theory of auroral kilometric radiation. Furthermore, in these discussions it is assumed that the population of low‐energy background electrons prevails over that of the energetic electrons. Obviously, this assumption may not be justifiable at high altitudes. The present work considers a loss‐cone distribution close to that observed and, moreover, includes the effect of a parallel electric field on the loss‐cone distribution function. It is found that this effect tends to enhance the growth rate. We have also removed the assumption concerning the high population of background electrons. We find that at high altitudes, where the density of energetic electrons may dominate and the usual cold electron dispersion relation is invalid, the cutoff frequency of the X mode can be significantly affected by the energetic electrons and thereby the growth rate is indirectly altered. In the present discussion, the emission of the auroral kilometric radiation is studied over a range of altitudes. We have considered three density models and two electric field models. In addition to the temporal growth rates, the corresponding e folding distances of spatial amplification are also computed and discussed.
Charged particle sputtering of ice surfaces in Saturn's magnetosphere
Authors: Cheng Andrew F. , Lanzerotti L. J. , Pirronello V.
Published: February 2008

Following recent investigations of the Saturn system in situ by Pioneer 11 and Voyager 1 and 2 and remotely by the International Ultraviolet Explorer, we have reconsidered the sputtering of water ice surfaces by magnetospheric ions as a possible source of hydrogen atoms and heavy ions. We use results from laboratory measurements of water ice sputtering and dissociation rates as a function of ice temperature. We also use Voyager charged particle data from the low energy charged particle experiment to obtain more accurate assessments of energetic ion fluxes and loss rates. Ion phase space densities show that, if charged particle sputtering is an important physical process, it will occur predominantly in the vicinity of Saturn's E ring and the moons Dione and Tethys, not at the outer edge of the A ring as previously suggested prior to the availability of in situ data. Charged particle sputtering of ice surfaces between 4.5 and 8 RS, followed by the ionization of H2O and dissociation fragments, may be an important local source of the heavy ion plasma reported in the magnetosphere by Pioneer 11. We review several existing theories for the ring atmosphere and conclude that difficulties remain with all of them.
Thermospheric extensions of the classical expansion functions for semidiurnal tides
Authors: Forbes Jeffrey M. , Hagan Maura E.
Published: February 2008

Solutions to the thermospheric tidal equations are described that define the extensions into the thermosphere of normalized wind and temperature structures associated with the (2, 2), (2, 3), (2, 4), and (2, 5) semidiurnal propagating tidal modes. Sample figures are presented that illustrate the degree of alteration of vertical structures with latitude and the change in horizontal shapes with height, corresponding to semidiurnal oscillations in northerly, westerly, and vertical velocity and temperature in the thermosphere. Additional figures and tables covering the 80‐ to 400‐km altitude region for five levels of solar activity at 6° latitude increments are available on microfiche for potential users. The structures can be used to extend meteor wind (80–100 km) and partial reflection drift (60–100 km) measurements to heights above 100 km for consistency checks with tidal winds and temperatures from Thomson scatter measurements at possibly different latitudes, or to simultaneously fit data covering these height regions for modeling purposes. Supplementary figures and tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20002. Document J82‐003; $1.00 Payment must accompany order.
Spatial irregularities in Jupiter's upper ionosphere observed by Voyager radio occultations
Authors: Hinson David P. , Tyler G. Leonard
Published: February 2008

Dual frequency radio occultation experiments carried out with Voyagers 1 and 2 provided data on the spatial irregularities in Jupiter's ionosphere at four different locations. Sample spectra of weak fluctuations in amplitude and phase of the 3.6‐cm and 13‐cm wavelength radio signals can be interpreted by using the theory for scattering from an anisotropic power law phase screen. Least squares solutions for ionospheric parameters derived from the observed fluctuation spectra yielded estimates of (1) the axial ratio, (2) angular orientation of the anisotropic irregularities, (3) the power law exponent of the spatial spectrum of irregularities, and (4) the magnitude of the spatial variations in electron density. Equipment limitations and the method of analysis constrain the observations to irregularities of approximate size 1–200 km. No evidence of the inner or outer scale of the irregularities was found. For length scales in the range given, the three‐dimensional spatial spectrum obeys a power law with exponent varying from −3.0 to −3.7, and the root mean square fractional variations in electron density are 1–15%. All observed irregularities appear to be anisotropic with axial ratios between 2∶1 and 10∶1. Ionospheric parameters vary with altitude and latitude. We conclude that the measured angular orientation of the anisotropic irregularities indicates magnetic field direction and may provide a basis for refining Jovian magnetic field models.
On the complex state of the interplanetary medium of July 28–29, 1977
Authors: King J. H. , Lepping R. P. , Sullivan J. D.
Published: February 2008

Plasma and magnetic field variations observed on July 28–30, 1977, in the near‐earth solar wind are presented and discussed. Both a corotating stream and a driven shock are present. The driver gas seems to be enveloped in the rising speed phase of this stream; this appearance is attributed to a transverse‐motion‐induced convoluted surface separating the two plasma domains. The magnetic field in the post shock flow (0030–1230 UT of July 29) has a large and geoeffective southward component at times; the Akasofu energy coupling coefficient ‘ε’ reaches ∼5.4×1019 ergs/s. In the driver gas (1230 UT of July 29 to 0110 UT of July 30) the magnetic field is dominantly northward and has a profile resembling a magnetic cloud. Significant plasma parameter variations within the driver gas are attributed to structure in the parent solar mass ejection event and to interplanetary kinematics. The density and dynamic pressure decrease by almost 2 orders of magnitude (≳100 to ≲2 cm−3) from just behind the interplanetary shock to ∼3 hours into the driver gas flow.
Whistler mode turbulence in the disturbed solar wind
Authors: Coroniti F. V. , Kennel C. F. , Scarf F. L. , Smith E. J.
Published: February 2008

Magnetic field fluctuations with frequencies f between the ion (fci) and electron (fce) cyclotron frequencies are enhanced downstream of interplanetary shocks and in fast streams. These f > fci fluctuations are related to those below fci that also accompany solar wind activity. The spectra over the range 10−2 fci ≤ f ≲ fce synthesized from ISEE 3 magnetometer and plasma wave instrument data can generally be described by one power law below f ≈ 1 Hz ≈ fci and a different one above. However, behind four shocks the spectral index above fci was measured to be about twice that below fci, whereas no clear relationship is apparent in the weaker fast stream events. Behind one shock the interference in the low‐frequency electric field channels of the ISEE 3 plasma wave instrument was small enough to permit a statistical study of the B/E ratios or, equivalently, the indices of refraction, of the fci ≤ f ≤ fce waves. Although the data base is limited, the B/E ratios confirm that the waves are whistler mode emissions, as their frequency range already suggested. Various indirect lines of evidence indicate that these whistler waves are generated propagating at large angles to the local interplanetary field, a fact that helps identify possible free energy sources for their growth.
Stability of negatively charged dust grains in Saturn's ring plane
Authors: Northrop T. G. , Hill Jay Roderick
Published: February 2008

The boundary between the inner and outer parts of Saturn's B ring is located at the theoretical limit of stability of dust grains with large negative charge to mass ratio. A grain inside of this stability limit will move along (pseudo) magnetic field lines and strike Saturn if given a slight velocity component normal to the ring plane. Outside of this marginal stability radius, a perturbed grain merely oscillates back and forth through the ring plane. The theoretical location of the marginal stability radius is at 1.625 RS. Observations by Pioneer 11 and Voyager 2 in the infared see the boundary as a prominent change in ring brightness at this radius (within the spatial resolution). The occultation of δ‐Scorpii by the rings in the ultraviolet seen by Voyager 2 shows about a factor of 2 change in optical depth beginning very close to this radius. This close agreement is conceivably a numerical coincidence. Whether this is the case, or whether the instability is actually the physical reason for the boundary's existence, requires further study of the implications.
Detrapping by an additional wave of wave‐trapped electrons
Authors: Dowden Richard L.
Published: February 2008

Electrons spiralling along the geomagnetic field can be trapped by a whistler mode wave travelling in the opposite direction if the electrons are near‐resonant and if the wave amplitude is sufficient to overcome the plasma inhomogeneity (e.g., geomagnetic field gradient), if any. An additional wave at a slightly different frequency can detrap these electrons even if the additional wave is 20 dB weaker than the trapping wave if the frequency difference is appropriate. The sensitivity of such electrons to detrapping versus frequency is determined for both homogeneous and inhomogeneous plasmas. Such detrapping may play a part in the control or entrainment of VLF emissions by a transmitter wave and in whistler quenching and echo‐induced suppression of entrained emissions.
He+ and other unusual ions in the solar wind: A systematic search covering 1972–1980
Authors: Zwickl R. D. , Asbridge J. R. , Bame S. J. , Feldman W. C. , Gosling J. T.
Published: February 2008

Following the recent discovery of large fluxes of He+ in the solar wind, a systematic search of solar wind energy/charge (E/Q) spectra was initiated, using plasma data from IMP 7, IMP 8, ISEE 1, and ISEE 3 that covered the time period from October 1972 to February 1980. A technique of vertically stacking successive E/Q spectra made it possible to detect systematic trends in the spectra that were indicative of ion species beyond the He++ peak down to the one count level. A search of nearly 8 years of data yielded only three distinct events with identifiable levels of He+. These events occurred on July 29, December 1, and December 4, 1977, all within 1 year and during the rising phase of solar cycle 21. We believe more events would have been detected if the instruments had been more sensitive and tracking more complete. Two of the events occurred within driver gas, following interplanetary shocks, while the third event occurred in a noncompressive density enhancement (NCDE). For all three the abundances of various minor ions, including He++, were enhanced in comparison to normal solar wind abundances. One unusual feature found in these three He+ events, in contrast to normal solar wind, was the appearance of ionization states corresponding to both warm and very cold coronal freezing‐in conditions within the same plasma. In addition to the unusual appearances of large fluxes of He+ ions, iron ions ranging from Fe+10 to Fe+5 were observed. Evidence was found for the appearance of C+4 ions, and in one event the data can be interpreted as containing O+2 and O+3 at detectable levels. One of the phenomenological results following from these observations is that certain unusual transient events eject both hot and unusually cold coronal plasma mixed and coexisting on the same magnetic field lines.
Magnetotail dynamics excited by the streaming tearing mode
Authors: Sato Tetsuya , Walker Raymond J.
Published: February 2008

We have modeled magnetotail reconnection in the presence of plasma streaming parallel to the neutral sheet. The tearing mode is excited much more violently in the case with parallel plasma flow in the plasma sheet than in the case with no flow. The flow patterns for the nonlinear resistive tearing mode and the streaming tearing mode are much more complex than those for the linear tearing mode. Flow vortices are observed in both cases.
Ion frictional heating at high latitudes and its possible use for an in situ determination of neutral thermospheric winds and temperatures
Authors: St.‐Maurice J.‐P. , Hanson W. B.
Published: February 2008

We have used the Atmosphere Explorer C data base of ion temperature and ion drift velocities to study the long recognized principle that large scale convection electric fields are responsible for substantial enhancements in the ion temperature at high latitudes. As had been indicated by various authors previously, our study shows that there is a clear relationship between large ion velocities and ion temperature enhancements. Furthermore, with the help of a more quantitative study, we find that, since ion heating is sensitive to the relative motion between the ion and neutral gases we can use the ion temperature and drift measurements to extract neutral wind and neutral temperature information along the satellite track. From a study of several examples we conclude that the present approach can be used with varying degrees of accuracy for the diagnostic of thermospheric dynamical properties above 300 km when those properties are not measured directly. When neutral dynamical properties are measured our approach could be used for a detailed study of the ion energy balance at high latitudes.
Airglow characteristics of equatorial plasma depletions
Authors: Mendillo Michael , Baumgardner Jeffrey
Published: February 2008

A new low‐light‐level, all‐sky, 6300 Å airglow imaging system has been developed for ground‐based studies of the optical signatures of equatorial plasma depletions. The system was designed for narrowband interference filters (6 Å full width at half power) with interchangeable all‐sky (180°) and narrow‐field (60°) lenses and records images photographically by using a standard 35 mm camera. A field test of the apparatus was conducted on Ascension Island (8.0°S, 14.4°W) from January 24 to February 10, 1981. The photographs were analyzed for broad morphological characteristics by using simple overlay techniques augmented by digital image processing methods to extract more quantitative detail. The initial results to come from these observations show that airglow depletions occur most often during the 2030–2330 LT period, often covering one‐fourth of the visible sky; they extend from the magnetic equator to beyond the crest of the equatorial anomaly, with east‐west cross sections of ∼100 km to several hundred kilometers; they are usually not aligned with magnetic meridians, showing a progressive skewness to the west at further distances from the equator; several cases of apparently twisting, overlapping, and bifurcating depletions were observed; the depletions drifted to the east with speeds that decreased from ∼190 m/s at 2100 LT to ∼80 m/s at 0100 LT; the airglow intensities within depletions were typically 40–50% of the background levels, and the western walls of the depletions usually had a steeper gradient than the corresponding eastern walls.
Atomic carbon in the atmosphere of Venus
Authors: Fox J. L.
Published: February 2008

The densities of atomic carbon in the Venusian thermosphere are computed for a model which includes both chemistry and transport. The maximum density of C is 2.8×107 cm−3 near 150 km for an assumed O2 mixing ratio of 1×10−4. Photoionization of atomic carbon is found to be the major source of C+ above 200 km, and resonance scattering of sunlight by atomic carbon may be the major source of the C I emissions at 1561 Å, 1657 Å, and 1931 Å. The computed C+ densities are found to be in substantial agreement with those measured by Pioneer Venus.
Quadrupole distortions of the heliospheric current sheet in 1976 and 1977
Authors: Bruno R. , Burlaga L. F. , Hundhausen A. J.
Published: February 2008

Latitudinal variations in the position of the heliospheric current sheet were investigated using interplanetary measurements from Helios A, Helios B, IMP 8, and white light corona measurements in the period May 1976 to May 1977. The latitude of the heliospheric current sheet between 0.3 AU and 1 AU is consistent with that of the maximum brightness curve of white light polarization brightness at 1.75 solar radii within approximately ±7°. The combined data sets show that the four‐sector magnetic sector pattern observed in the ecliptic plane for most of this epoch could be attributed to a warped current sheet whose shape near the sun resembles a saddle surface that is nearly symmetrical about the equatorial plane. The latitudinal extent of the current sheet is on the average 8° and the maximum extent is 15° (except on one rotation, when it is 20°). These results can be interpreted as a consequence of the solar magnetic field, the dominant components of the field being due to a dipole nearly aligned with the sun's spin axis and a quadrupole component whose strength is ∼17% of the dipole.
Relationship between field‐aligned currents, diffuse auroral precipitation and the westward electrojet in the early morning sector
Authors: Senior Catherine , Robinson Robert M. , Potemra Thomas A.
Published: February 2008

On the basis of Chatanika radar data and simultaneous Triad satellite data, the relationships between field‐aligned currents, diffuse aurora, electric fields, and currents in the early morning sector are examined. The diffuse aurora is coincident with the upward region 2 field‐aligned current. Within the upward region 2 field‐aligned current, small‐scale current structures are often present. These structures seem to coincide with small‐scale electron density enhancements in the E region, which are probably produced by energetic electron precipitation. The westward electrojet extends across the region 2/region 1 boundary in the morning sector. The southward electric field is large at and poleward of this boundary. It is most probable that the region 2 field‐aligned currents are closed by southward Pedersen currents within the ionosphere.
A two‐dimensional simulation of the interaction of the plasma sheet with the lobes of the Earth's magnetotail
Authors: Swift Daniel W.
Published: February 2008

A code involving two spatial dimensions and three velocity dimensions is used to model the tail of earth's magnetosphere in a continuation of earlier work using a one‐dimensional code. The model assumes the existence of an externally imposed convection electric field and the presence of cold particles in the lobes of the magnetotail. The model reproduces the results of the previous one‐dimensional model. In addition, the two‐dimensional model exhibits the development of turbulence which persists so long as cold lobe particles are being convected into the neutral sheet. The model also exhibits the earthward jetting of plasma, which becomes particularly intense after the tail field begins to collapse toward a more dipolar configuration. Magnetic energy is converted into energy of bulk motion. The model is used to identify the physical processes that may occur during the growth and recovery phases of substorms. It is also shown that if the convection electric field is strong enough, the magnetic energy of the tail will increase, resulting in a more extended magnetotail.
A threshold effect for spacecraft charging
Authors: Olsen Richard Christopher
Published: February 2008

The existence of a threshold energy dependence is shown for the charging of geostationary spacecraft in eclipse. Applied Technology Satellite 6 and P78‐2 (SCATHA) data show that plasma sheet fluxes must extend above 10 keV for these satellites to charge in eclipse. The existence of a threshold energy is significant because magnetospheric convection boundaries produce relatively sharp phase space boundaries, with electron fluxes falling rapidly above the energy associated with the local Alfven layer. The threshold effect is due to the shape of the normal secondary yield curve, in particular the high energy crossover, where the secondary yield drops below 1. A large portion of the ambient electron flux must exceed this energy for a negative current to exist.
On plasma transport in the vicinity of the rings of Saturn: A Siphon Flow Mechanism
Authors: Ip W.‐H.
Published: February 2008

The unique combination of a rapidly rotating ionosphere and meteoroid impact ionization at the rings of Saturn provides the elements of many interesting plasma transport phenomena. One of the most significant processes might be the upward field‐aligned flow of the impact plasma at the equatorial region inside 1.6252 Rs. Such a siphoning mechanism limits the efficiency of ring plane matter recycling to a minimum and could lead to appreciable loss of ring mass in this region. At the same time, channeling of the heavy ions into the mid‐altitude ionosphere could also cause a large reduction in the ionospheric electron content as observed by the Pioneer 11 and the Voyager radio science experiments. The resulting electrodynamical coupling of the Saturnian rings with the ionosphere thus represents a completely new kind of ionospheric process than studied before.
Kelvin‐Helmholtz Instability at the magnetopause: Energy flux into the magnetosphere
Authors: Pu Zu‐Yin , Kivelson Margaret G.
Published: February 2008

A magnetohydrodynamic approach is used to investigate how compressional linearly unstable Kelvin‐Helmholtz surface waves on the magnetopause extract energy from the velocity shear and radiate that energy away from the boundary in the separate rest frames of the plasmas on the two sides. On the magnetospheric side, the energy transport velocity normal to the magnetopause may become comparable with the Alfvén speed. The energy flux, for typical conditions on the dayside magnetopause, is found to be ≃ 10−3 ergs/cm² s, and the total energy flux over the unstable region on the dayside magnetopause is estimated to be ≃ 1017 ergs/s in magnetic quiet times, or ≳ 1018 ergs/s in disturbed times. The discussion of energy input into the magnetosphere illuminates how surface waves on the magnetopause provide the power to drive resonant regions within the magnetosphere at large distances from the boundary.
A global model of the neutral thermosphere in magnetic coordinates based on AE‐C data
Authors: Stehle Carl G. , Nisbet John S. , Bleuler Ernst
Published: February 2008

Molecular nitrogen, atomic oxygen, and helium densities obtained from the AE‐C satellite are analyzed in magnetic latitude and magnetic local time coordinates and compared to OGO 6 data for various seasons and magnetic activity levels. A depletion region for atomic oxygen and helium with respect to molecular nitrogen at high magnetic latitudes in the postmidnight magnetic local time sector persists under both high and low solar activity conditions. A global model in magnetic coordinates, patterned after the mass spectrometer and incoherent scatter (MSIS) model, is developed for molecular nitrogen, atomic oxygen, and helium. It is shown to represent the data well, without any residual UT dependence, and with an accuracy comparable to that provided by the more complex MSIS model with longitude terms. The advantage of using magnetic coordinates is that they are more directly related to the major energy inputs and momentum sources in the polar regions than are geographic coordinates and are more convenient to use in studies of high latitude energy deposition processes. This is important for comparison with theoretical models where the number of coordinates is limited.
The oblique whistler instability in the Earth's foreshock
Authors: Sentman D. D. , Thomsen M. F. , Gary S. Peter , Feldman W. C. , Hoppe M. M.
Published: February 2008

The linear Vlasov stability properties of electron velocity distributions, similar to those observed in the upstream foreshock region in association with obliquely propagating whistler waves at approximately 1 Hz, are studied. These distributions are modeled by a sum of bi‐Maxwellians with drift speeds parallel to the magnetic field B. We find such distributions to be stable to modes with wavevectors k parallel to B but unstable to whistler waves propagating obliquely to the magnetic field. The frequencies and wavelengths of these unstable modes agree well with those of whistlers observed upstream of the earth's bow shock. The free energy source driving the instability is a region of positive parallel slope ∂fe/∂υ∥ > 0 at large pitch angles (about 85°) and intermediate energies (about 20 eV), probably corresponding to solar wind electrons magnetostatically reflected from the magnetic ramp of the bow shock. The whistlers grow via electromagnetic Landau resonance with this free energy source.
Convective growth rate of ion cyclotron waves in a H+−He+ and H+−He+−O+ plasma
Authors: Gomberoff L. , Neira R.
Published: February 2008

The behavior of the convective growth rate of the electromagnetic proton‐cyclotron instability is investigated in detail in a H+, He+ plasma and when a third (minority) cold ion component such as O+ is taken into account. It is shown that amplification is the result of an interplay between the cold species and the thermal anisotropy of the energetic protons. The results seem to be in good agreement with the observations performed on board GEOS 1 and 2 concerning the generation of ULF waves, below the proton gyrofrequency. It is also shown that a small amount of O+ can strongly affect the behavior of the convective growth rate of the instability below the He+ gyrofrequency, even far away from the O+ stop band.
Initial tests of an index based on AL values for modeling magnetic storm related perturbations of the thermosphere
Authors: Nisbet John S. , Stehle Carl , Bleuler Ernst
Published: February 2008

Very large perturbations in the thermosphere and ionosphere are induced by motions driven by heating and by electric fields in the auroral electrojet regions. Winds carry these effects to all latitudes. Because of this, thermospheric models have terms associated with magnetic activity. The Kp and Ap indices have usually been employed; however, these are poorly correlated with the temporal variations. Initial experiments are described with new indices based on the auroral electrojet indices of magnetic activity that are more directly related to the energy driving functions. The new indices are based on the time‐weighted average of the auroral electrojet indices over a period rather than on instantaneous values. Comparisons are made with the 3‐hour ap index for high‐latitude atomic oxygen densities under disturbed conditions. Conclusions are drawn on the problems and promise of the approach.
On the preferential acceleration and heating of solar wind heavy ions
Authors: Isenberg Philip A. , Hollweg Joseph V.
Published: February 2008

We investigate the preferential heating and acceleration of solar wind heavy ions by the resonant cyclotron interaction with parallel‐propagating left‐polarized hydromagnetic waves. We set up a scenario whereby the energy for this interaction is taken from saturated low‐frequency Alfven waves via a cascade to the higher, resonant frequencies. In order to utilize the existing theoretical work, the particles are taken to be thermally isotropic, and the waves are taken to be dispersionless. This scenario is incorporated into a numerical solar wind code describing the flow from an inner radius (taken to be 10 solar radii) to 1 AU. Thus we present the first model of a wave‐driven, three‐fluid, supersonic solar wind. By varying the model parameters we test the ability of the resonant interaction in this model to produce the excess speeds and temperatures of heavy ions that are observed. We find that unrealistically steep wave spectra are required to produce differential speeds of the order of the Alfven speed at 0.3 AU. In the case of He2+, the power spectrum of waves must be steeper than γ ≃ 4.7, where γ is the power law spectral index. Ions of oxygen or iron, with larger mass‐per‐charge ratios, are accelerated more readily than helium, but still require steeper spectra than are observed. This model is also unable to produce the mass‐proportional heavy ion temperatures that are observed. We show that the production of mass‐proportional temperatures is inconsistent with preferential acceleration of heavy ions by this mechanism. The model also produces a heavy ion‐to‐proton temperature ratio at 1 AU which is anticorrelated with solar wind speed, in contradiction to the observed behavior.
Observations on the GEOS 1 satellite of whistler mode signals transmitted by the Omega Navigation System Transmitter in northern Norway
Authors: Neubert T. , Ungstrup E. , Bahnsen A.
Published: February 2008

Whistler mode signals of 0.9–1.1 s duration at 10.2, 11.33, and 13.6 kHz launched by the Omega Navigation System transmitter located in northern Norway have been observed on GEOS 1. The signals were observed both inside and outside the plasmasphere in two regions of the magnetosphere, in December 1977 at 10–20° north of the equator on the morningside and in May 1978 10° south of the equator on the nightside. For a sequence of particularly strong pulses in the December period, the ‘instantaneous’ wave normal directions were calculated and found to have angles of 120–140° to the magnetic field of the earth pointing 0–40° eastward. These pulses have amplitudes of the order of 10 μV/m and refractive indices above 30. At times the amplitude envelope of the signals is unmodulated, but often the amplitude and the wave normal direction are strongly modulated on a time scale of 0.2 s. We attribute these features to ray path mixing. The signals are often prolonged up to 1 s, which is interpreted as an overlap of the primary pulse with the echo of the pulse reflected from the topside ionosphere in the southern hemisphere. While the signals do not trigger strong emissions, we do observe ∼0.1‐s long bursts at the tail of some direct pulses.
Where does the O(¹D) energy go?
Authors: Harris R. D. , Adams G. W.
Published: February 2008

The excitation energy of O(¹D) is a potentially significant heating source in the region 60–150 km. Some of the O(¹D) energy is thermalized directly by collisional deactivation, some of it is radiated at 6300 and 7619Å and thus lost to the atmosphere, and the rest of the O(¹D) energy (about 25%) enters the near‐resonant N2(υ = 1) −CO2(001) system. Since the atmosphere is optically thick to 4.26‐μm radiation below about 80 km, the O(¹D) energy that enters this N2‐CO2 system and is not locally quenched is multiply scattered to other altitudes where it can be thermalized or lost out of the top of the atmosphere. Our calculations show the relative proportions of each of the O(¹D) energy loss options.
The aeronomic dissociation of water vapor by solar H Lyman α radiation
Authors: Lewis B. R. , Vardavas I. M. , Carver J. H.
Published: February 2008

The photoabsorption cross sections of molecular oxygen and water vapor have been measured as a function of temperature at 0.1 Å intervals between 1214 and 1219 Å. Over the width of the solar Lyman α line there is no fine structure apparent in the cross sections which were measured with 0.05 Å resolution. A large wavelength and temperature dependence is seen in the molecular oxygen cross sections, but the variations are much smaller in the case of water vapor. The measured cross sections are used in calculations of the rate of photodissociation of atmospheric water vapor by solar Lyman α radiation, taking full account of both the temperature and wavelength dependences. It is found that the photodissociation rates may be accurately reproduced at altitudes above those where Lyman α ceases to dominate the dissociation by using molecular oxygen and water vapor cross sections corresponding to fixed temperatures near 200 K. Analytical models for these cross sections are presented for use in photochemical calculations.
Observations of charging dynamics
Authors: Olsen R. C. , Purvis C. K.
Published: February 2008

Plasma data from the University of California, San Diego, Auroral Particles experiments on Applied Technology satellites 5 and 6 are used to investigate the dynamics of natural charging events. Both eclipse and daylight charging events are considered, and typical responses are illustrated by data from specific events. Two different physical processes are found to be involved in the charging process. One of these is straightforward: the spacecraft structure potential responds rapidly to changes in the environment, typically changing by hundreds of volts in a few seconds. The other process is more subtle: differential charging and potential barrier formation precede structural charging and determine the time scale; typically, tens of minutes are required for the potential to change by several hundreds of volts. The latter process is found to be predominately responsible for daylight charging on both spacecraft.
Systematics of the equatorward diffuse auroral boundary
Authors: Gussenhoven M. S. , Hardy D. A. , Heinemann N.
Published: February 2008

DMSP/F2 and DMSP/F4 precipitating electron data are used to determine statistically the systematic variations of the equatorward diffuse auroral boundary with Kp as a function of local time. This work extends a previous study of dawn and dusk boundaries to the noon and midnight regions. The boundaries are well‐ordered by Kp in the night sector but show increasingly greater scatter from dawn to postnoon. In the noon sector it is often the case that no diffuse aurora is discernible within detector sensitivity. The equatorward diffuse auroral boundary is well fit by a circle at each activity level. The center of the circle is offset from the geomagnetic pole, and the radius of the circle increases with increasing magnetic activity. The circular fits are projected to the ecliptic plane by using the Mead‐Fairfield magnetic field model where they are identified with the inner edge of the plasma sheet in order to make comparison with Volland‐Stern type convection electric field predictions. Previously found variations of the electric field with Kp are confirmed, including a rotation of the axis of symmetry, away from the dawn‐dusk meridian. All equatorward auroral boundaries from DMSP for 1978 were compiled. Using the equations for boundary variations with Kp, each evening sector boundary was projected to a midnight boundary. The projected midnight boundary serves as an index of auroral activity and an indicator of the strength of the large‐scale magnetospheric convection electric field. The auroral boundary index is given in monthly plots for 1978.
Variability of the Lyman alpha flux with solar activity
Authors: Lean J. L. , Skumanich A.
Published: February 2008

A three‐component model of the solar chromosphere, developed from ground based observations of the Ca II K chromospheric emission, is used to calculate the variability of the Lyman alpha flux between 1969 and 1980. The Lyman alpha flux at solar minimum is required in the model and is taken as 2.32×1011 photons/cm²/s. This value occurred during 1975 as well as in 1976 near the commencement of solar cycle 21. The model predicts that the Lyman alpha flux increases to as much as 5×1011 photons/cm²/s at the maximum of the solar cycle. The ratio of the average fluxes for December 1979 (cycle maximum) and July 1976 (cycle minimum) is 1.9. During solar maximum the 27‐day solar rotation is shown to cause the Lyman alpha flux to vary by as much as 40% or as little as 5%. The model also shows that the Lyman alpha flux varies over intermediate time periods of 2 to 3 years, as well as over the 11‐year sunspot cycle. We conclude that, unlike the sunspot number and the 10.7‐cm radio flux, the Lyman alpha flux had a variability that was approximately the same during each of the past three cycles. Lyman alpha fluxes calculated by the model are consistent with measurements of the Lyman alpha flux made by 11 of a total of 14 rocket experiments conducted during the period 1969–1980. The model explains satisfactorily the absolute magnitude, long‐term trends, and the cycle variability seen in the Lyman alpha irradiances by the OSO 5 satellite experiment. The 27‐day variability observed by the AE‐E satellite experiment is well reproduced. However, the magnitude of the AE‐E Lyman alpha irradiances are higher than the model calculations by between 40% and 80%. We suggest that the assumed calibration of the AE‐E irradiances is in error.
A double‐resonance Pc 5 pulsation
Authors: Nielsen E. , Allan W.
Published: February 2008

Observations of a monochromatic Pc 5 ULF pulsation event with the simultaneous occurrence of resonance at two latitudes are presented. The event indicates the presence of a large inward directed positive gradient in the equatorial cold plasma density. Model calculations are used to deduce the radial variation of the plasma density. The results support the idea that resonant Pc 5 pulsations are driven by an inward propagating wave, which may be generated at the magnetopause by the Kelvin‐Helmholtz instability.
Absorption of energetic protons by Saturn's ring G
Authors: Van Allen James A.
Published: February 2008

A restudy has been made of Pioneer 11 data on the distribution of energetic protons Ep > 80 MeV in Saturn's inner magnetosphere. An improved value of the ratio of the cosmic ray albedo neutron decay source strength $ to the radial diffusion coefficient D is 6.9 × 10−24 cm−5 at r ∼ 2.67 Rs (1 Rs = Saturn's equatorial radius = 60,000 km). Using the recently calculated lower limit on $ by Blake et al., one finds an upper limit on the mean residence time T against diffusion of 4.6 × 108 s (15 years) in the major peak of the distribution, whereas using our earlier estimate of $, one finds T ∼ 2.2 × 108 s (7.0 years). The two corresponding determinations of D are >1.3 × 10−11 and ∼ 2.8 × 10−11 Rs² s−1, respectively. A generous upper limit on D is 2 × 10−10 Rs² s−1 as found from study of the slot region associated with the co‐orbiting satellites 1979 S2 (1980 S3) and 1980 S1. The mean lifetime τ of energetic protons against absorption by particulate matter in Ring G is < 1.1 × 108 s (3.5 years) and ∼ 5.1 × 107 s (1.6 years) corresponding to the two choices of $. Then, using the Voyager data on normal optical opacity η ∼ 3 × 10−5 and on radial width Δr ∼ 500 km, one finds that the particulates in Ring G have an effective radius R ≳ 0.035 cm, an areal mass density σ > 1.4 × 10−6 g cm−2, and an areal number density n < 8 × 10−3 cm−2. It is unlikely that R exceeds 0.1 cm. The foregoing values of R (for assumed spherical particulates of water ice, all of the same size) are in effect the ratio 〈R³〉/〈R²〉 for a distribution of sizes, where the angle brackets denote the mean value over the distribution. It is reasonably certain that there are no satellites having radii of the order of a kilometer or larger associated with Ring G and that objects having radii of ≳10 cm contribute less than 10−2 of its opacity. The mean radius of Ring G is 170,200±900 km.
Numerical simulation of the dayside reconnection
Authors: Hoshino M. , Nishida A.
Published: February 2008

To examine the basic characteristics of reconnection on the dayside magnetopause, we have numerically studied the reconnection process at an interface where the total pressure is in balance but where the thermal pressure is higher on one side than on the other. Reconnection is caused by anomalous resistivity that is assumed to operate only in a localized region at the interface. Boundaries are assumed to be free boundaries, but reflection of the perturbations originating from inside the simulation region is suppressed by placement of an absorbing region. Results can be expressed as follows, denoting the high‐pressure and low‐pressure regions as magnetosheath and magnetosphere, respectively. (1) a slow shock is formed in magnetosheath and a slow expansion fan is formed in magnetosphere; (2) at the slow shock the Lorentz force and pressure gradient are of comparable importance in accelerating plasma, but at the expansion fan the pressure gradient has the dominant effect, and (3) the acceleration depends on the direction of the interplanetary magnetic field (IMF) and when the reconnection line is assumed to be directed at the half angle (θ/2) between IMF and geomagnetic field, the velocity of the accelerated plasma is roughly proportional to sin² (θ/2) + sin³ (θ/2).