bluuebunny/crossref_metadata_2025 huggingface dataset:
Photochemistry of SO2in the atmosphere of Io and implications on atmospheric escape
Authors: Kumar Shailendra
Published: February 2008

Photochemical models of Io's atmosphere are presented with the assumption that SO2is the major gas and that the SO2surface pressure is controlled by vapor‐pressure equilibrium at the surface. Photolysis of SO2leads to efficient production of SO, O2, S, and O. Of these products, O and S are likely to be the dominant constituents in the upper atmosphere, and the atmospheric escape is expected to be in atomic form. Nonthermal escape processes are necessary to populate the plasma torus. SO2surface densities greater than 1011cm−3are needed on the dayside to supply the required amounts of S and O to maintain the torus. Surface chemistry presents an interesting possibility, namely, the conversion of O to O2, in which case O2will be the dominant constituent throughout the nightside and over the polar caps with a surface O2density of 1010cm−3.
Energetic protons, alpha particles, and electrons in magnetic flux transfer events
Authors: Scholer M. , Hovestadt D. , Ipavich F. M. , Gloeckler G.
Published: February 2008

We have analyzed energetic proton, alpha particle, and electron measurements during two magnetopause crossings of ISEE 1 which show magnetic field signatures characteristic of flux transfer events. While the ions are streaming highly collimated along the magnetic field, the electrons show either isotropic distributions or distributions which are consistent with convected small‐scale structures. Fluxes and relative abundances of ions in flux transfer events are similar to the trapped particle values. We have also observed similar particle signatures when no change in the magnetic field normal component has been measured. During the occurrence of isotropic electron distributions, the absolute electron flux (above 75 keV) is greatly reduced when compared with the flux of the trapped electron population. Thus the electrons cannot be explained by simple leakage out of the magnetosphere.
Nonlinear equations of motion for Landau resonance interactions with a whistler mode wave
Authors: Inan U. S. , Tkalcevic S.
Published: February 2008

A simple set of equations are derived for describing the cyclotron averaged motion of Landau resonant particles in a whistler mode wave propagating at an angle to the static magnetic field. It is argued that for the parameters of the inner magnetosphere, the equations would be an accurate description of the near‐resonant particles' motion. The relative magnitudes of the wave magnetic and electric field forces are discussed and the parameter ranges for which the wave magnetic field effects can be neglected are specified.
The production efficiency of O+(²P) ions by auroral electron impact ionization
Authors: Rees M. H. , Abreu V. J. , Hays P. B.
Published: February 2008

A set of complementary observations obtained by experiments on the Atmosphere Explorer D satellite was used to deduce the production efficiency of O+(²P) by electron impact ionization. An efficiency of 18% was derived.
Large‐scale variations of the interplanetary magnetic field: Voyager 1 and 2 observations between 1–5 AU
Authors: Burlaga L. F. , Lepping R. P. , Behannon K. W. , Klein L. W. , Neubauer F. M.
Published: February 2008

Observations by the Voyager 1 and 2 spacecraft of the interplanetary magnetic field between 1 and 5 AU have been used to investigate the large‐scale structure of the IMF in the years 1977 to 1979, a period of increasing solar activity. This complements the Pioneer 10, 11 investigation between 1 and 8.5 AU during 1972–1976 when the sun was less active. In contrast to the good agreement of the Pioneer observations with the ideal field configuration of the Parker spiral model during near solar minimum conditions, the Voyager spacecraft found notable deviations from that configuration. We attribute these deviations both to temporal variations associated with increasing solar activity, and to the effects of fluctuations of the field in the radial direction. The amplitude of the latter fluctuations was found to be large relative to the magnitude of the radial field component itself beyond approximately 3 AU. The IMF sector structure was generally not well‐developed during the period of this study. Notable differences were found between Voyager 1 and 2 observations. Differences in the region 1–2 AU are attributed to the substantially different latitudes of the two spacecraft during much of the period. Later differences are most likely associated with the fact that the Voyagers moved through the region between 4 and 5 AU at different times. Both Voyager 1 and Voyager 2 observed decreases with increasing heliocentric distance in the amplitude of ‘transverse’ fluctuations in B that are consistent with the presence of predominantly undamped Alfvén waves in the solar wind, although not necessarily implying the presence of them. The presence of convective structures, compressive modes, and/or a saturated instability of Alfvén waves cannot be excluded by these Voyager results. Fluctuations in the strength of B (relative to mean field strength) were found to be small in amplitude, with a RMS that is approximately one third of that for the transverse fluctuations, and they are essentially independent of distance from the sun.
Computer simulations of basic processes of coherent whistler wave‐particle interactions in the magnetosphere
Authors: Omura Yoshiharu , Matsumoto Hiroshi
Published: February 2008

Basic processes of coherent whistler mode wave‐particle interactions in the magnetosphere are studied by self‐consistent computer simulations. The wave equations used in the simulation to determine the wave amplitude and frequency are discussed and physical interpretations of them are given. Nonlinear processes of the wave growth in a uniform magnetic field are examined in detail. Difference between uniform (periodic) and nonuniform (encounter) interactions is studied. Taking into account the inhomogeneity of the dipole geomagnetic filed, we found that untrapped resonant electrons as well as trapped electrons play significant roles in the wave evolution. The roles of trapped and untrapped electrons are different and exchange their roles depending on the sign of the geomagnetic gradient. In the hemisphere where the whistler wave propagates toward the equator, trapped electrons cause wave growth, while untrapped resonant electrons cause wave damping. In the opposite hemisphere, these roles are exchanged. Four simulations are carried out to demonstrate these differences separately.
Atmospheric tide: 2. The solar and lunar semidiurnal components
Authors: Forbes Jeffrey M.
Published: February 2008

Solar and lunar semidiurnal wind and temperature oscillations from the surface to 400 km are simulated for average solar activity conditions utilizing the numerical tidal model described in part 1. Hough mode decomposition of the solstitial solar semidiurnal tide excited by H2O and O3 insolation absorption below 80 km indicates that it is characterized by a predominance of (2, 2) below 60 km; about equal contributions of (2, 2), (2, 3), and (2, 4) between 70 and 90 km; mostly (2, 4) with some (2, 5) and (2, 2) between 90 and 120 km; and a predominance of (2, 2) above 140 km with secondary contributions from (2, 3) and (2, 5). The solstitial lunar semidiurnal tide is characterized by a predominance of (2, 2) below 70 km; (2, 4) with secondary contributions from (2, 2), (2, 3), and (2, 5) between 80 and 110 km; and by (2, 2) with a fairly strong secondary contribution from (2, 3) above 120 km. Similar descriptions for both the solar and lunar oscillations apply at equinox, except that the asymmetric (2, 3) and (2, 5) modes are absent. Relative to the solar component, lunar temperature and wind amplitudes are sufficiently large (∼20%) in the lower and upper thermosphere to account for some of the observed variability previously attributed to the solar tide. Amplitudes of the (2, 4) and (2, 5) modes above 100 km are significantly affected by mesospheric eddy diffusion coefficients in excess of 2×106 cm² s−1. The total solar semidiurnal exospheric temperature oscillation for average solar conditions ranges from ∼10–20°K poleward of 40°N to ∼40–60°K equatorward of 30°N and has its origin in three sources of excitation all of comparable importance: (1) EUV solar radiation absorption in the lower thermosphere (100–200 km), (2) in situ momentum coupling due to the interaction of diurnal winds and diurnally‐varying ion drag, and (3) tides propagating upwards from below 100 km. In addition, the calculations are compared with rocket, meteor radar, partial reflection, Thomson scatter, and satellite data to evaluate the consistency of the model with observations.
The SSC on July 29, 1977 and its propagation within the magnetosphere
Authors: Wilken B. , Goertz C. K. , Baker D. N. , Higbie P. R. , Fritz T. A.
Published: February 2008

At 0027 UT on July 29, 1977 an interplanetary shock wave arrived at the front side magnetosphere and triggered substantial geomagnetic activity throughout the day. The propagation of the resulting MHD wave within the magnetosphere has been studied with measurements from a total of six satellites in (or near) the geostationary orbit and the interplanetary space and groundbased magnetometers. At the time of the SSC the European spacecraft GEOS 1 was located at R = 6.7 RE and 1300 LT providing accurate reference measurements for the hydromagnetic impulse spreading out in the magnetosphere. The signal transmission from the front side magnetopause down to the equatorial ionosphere corresponded to an average wave speed of υ = 600 km/s. A propagation speed of υ = 910 km/s was found for the signal transmission in the outer magnetosphere in and beyond the geostationary altitude. The results compared reasonably well with model calculations.
Impulse‐excited pulsations during the July 29, 1977, event
Authors: Nopper R. W. , Hughes W. J. , Maclennan C. G. , McPherron R. L.
Published: February 2008

The propagation of a geomagnetic sudden impulse (si) and the magnetic field pulsations excited by it in the magnetosphere is traced from the bow shock in the solar wind, through the magnetosphere, to the ground. Within the magnetosphere the impulse appears as a compressive magnetohydrodynamic (MHD) impulse that travels rapidly (∼1500 km/s) tailward. A resonant oscillation observed both in space and on the ground is excited near geostationary orbit. The effect of the si is enhanced by a factor of at least 5 on the ground near the geomagnetic equator. We suggest that discontinuities in the solar wind may be a more important source of exciting dayside pulsations than has been commonly assumed.
Multiple‐spacecraft and correlated riometer study of magnetospheric substorm phenomena
Authors: Baker D. N. , Hones E. W. , Belian R. D. , Higbie P. R. , Lepping R. P. , Stauning P.
Published: February 2008

Substantial progress has been made in recent years toward the separation of spatial and temporal effects within the magnetosphere through the use of simultaneous multipoint measurements. From 1977 to the present we have had identical energetic particle instruments in geostationary orbit on the two spacecraft, and during 1977–1978 these satellites were separated by ∼4.5 hours in local time. During this same epoch, 13 variously located riometer stations provided broad latitudinal and longitudinal ground‐based coverage of substorm activity. Throughout this period of coordinated investigation we have found numerous examples of substorm growth phase features at widely separated local times. Simultaneous measurements of the magnetosheath and interplanetary magnetic field (north‐south) orientation during substorm growth phases suggest quite close correlations between energy input to the magnetosphere and the development of growth‐phase‐related stresses near geostationary orbit. Thus the results of this study demonstrate the broad local time region in which growth phase features are observable in spacecraft data, and they also support the concept of storage of energy prior to substorms, with the sudden release of this energy at substorm expansion onset.
The unstable discrete RF emission spectrum of rippled, magnetically focused, neutralized, cylindrical electron beams
Authors: Cuperman S. , Petran F.
Published: February 2008

A calculation of the electrostatic wave spectrum emitted by rippled, neutralized, magnetically focused cylindrical electron beams is presented. For this, the wave equation for the rippled beam (obtained in the local approximation) is solved by a resonant mode coupling procedure for the fast and slow waves coupling case. Both surface and volume space charge waves are considered. As a result we obtain the frequency bands in which unstable waves can occur as well as the corresponding growth rates. These results are relevant for laboratory experiments simulating the injection of electron beams under space like conditions as well as for active experiments with electron beams in space.
A quantitative model of geomagnetic activity
Authors: Holzer Robert E. , Slavin James A.
Published: February 2008

A quantitative model of geomagnetic activity is developed and utilized to investigate the causes of the diurnal, seasonal, and IMF sector variations in the AL index records. This auroral index was chosen for study because of its high sensitivity to the strength of the westward electrojet and, hence, magnetospheric substorm activity. After the introduction of corrections for processes not related to substorms, ability of the function Bs0.85V² to produce the observed variations in AL was examined. The annual variation of Bs was determined by superposing the contributions to Bs due to the inclination of the magnetic axis (Russell and McPherron, 1973) on an empirical mean distribution of Bz. V was assumed constant. The predicted values of ALc have been compared with observed averages for 9 years of solar cycle 20. The predicted annual variation of ALc for toward and away sectors are in good agreement with observation. While the predicted semi‐annual component of ALc is in phase with observation, it is less than half the observed amplitude. The predicted diurnal variation of ALc for June is in satisfactory agreement with observation.
Generation of nonthermal continuum radiation in the magnetosphere
Authors: Okuda H. , Ashour‐Abdalla M. , Chance M. S. , Kurth W. S.
Published: February 2008

Generation of electromagnetic continuum radiation from electrostatic fluctuations near the upper hybrid resonance frequency has been calculated by using cold plasma theory in an inhomogeneous plasma near the plasmapause. It is shown that both the polarization and the amplitude of electromagnetic radiation are in good quantitative agreement with spacecraft observations for nonthermal continuum radiation.
Thermal and suprathermal protons and alpha particles in the Earth's plasma sheet
Authors: Ipavich F. M. , Scholer M.
Published: February 2008

We have investigated thermal and suprathermal protons (from ∼13 keV to ∼1 MeV) and alpha particles (from ∼30 keV/Q to 130 keV/Q) in the quasi‐stable plasma sheet near local midnight at geocentric distances of ∼12–20 RE. The data, obtained during five different plasma sheet penetrations with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 1, represent the first measurements of plasma sheet ion composition in this energy range. Below ∼16 keV the proton spectra can be represented by a Maxwellian distribution. Above ∼16 keV we find a suprathermal tail which cannot be represented by a single power law. In some cases the suprathermal population consists of two distinct components: a low‐energy component falling off sharply at ∼100 keV and a high‐energy component extending up to ∼1 MeV. The high‐energy component often decays with time during expansion of the plasma sheet. Alpha particle to proton ratios average ∼3% and are observed to be constant at either equal energy per nucleon or energy per charge.
Pulsations in magnetic field and ion flux observed at L = 4.5 on August 5, 1972
Authors: Engebretson M. J. , Cahill L. J. , Williams D. J.
Published: February 2008

After the unusually strong compression of the earth's magnetosphere associated with the August 1972 geomagnetic storm, large amplitude ULF pulsations were observed for several hours in space by Explorer 45 magnetic field and particle instruments near L = 4.5 and at magnetic observatories on the ground over a large range of latitudes. Spectral analysis of Explorer 45 magnetometer data suggests that a compressional mode oscillation coupled to a transverse mode oscillation. Enhancements in amplitude of a 300‐s period wave near 0040 UT August 5 coincide with an intensification of 100‐ to 1000‐Hz magnetic and electric field oscillations and with the appearance of enhancements of fluxes of energetic ions. During this period the ion pitch angle distribution in each available energy channel (24–300 keV) followed a periodic sequence, apparently synchronized with the magnetic pulsations, from normal trapping (highest fluxes near 90° and lowest near 0° and 180°) to a nearly isotropic particle distribution. During the transitions the particle flux near 90° pitch angle was alternately larger earthward of the satellite (before isotropy) and larger radially outward from the satellite (after isotropy). Intense field‐aligned fluxes of lower energy ions (E ≤ 5 keV) were observed periodically throughout the interval. Possible configurations of the magnetosphere consistent with the wave and particle observations are discussed, the most likely candidate being the presence of a wavelike boundary near the satellite. It is possible that the satellite sensed the low‐latitude boundary layer at L = 4.5 during this period of extreme magnetospheric compression.
Magnetospheric convection effects at mid‐latitudes: 1. Saint‐Santin observations
Authors: Blanc Michel
Published: February 2008

We use E×B data from the Saint‐Santin incoherent scatter facility to study the behavior of mid‐latitude ionospheric electric fields during magnetic storms and substorms. To this end, we analyze the relation between departures of the Saint‐Santin field from its quiet day values and the simultaneous variations of parameters characterizing the state of magnetospheric convection: the IMF BƵ component, the auroral electrojet index, and the ring current inflation rate as deduced from time variations of the Dst index. Mid‐latitude E×B drift disturbances, when they occur during substorms, are generally associated with a large southward value of BƵ and with large values of the ring current inflation rate. They are also associated in several instances with an activation of the eastward electrojet in the afternoon and evening sectors. Large values of the drift disturbances are also observed during magnetic storms, but their relationship to the large‐scale convection parameters monitored does not seem to be reductible to the simple real‐time relationships found for the substorm cases. This suggests that an additional mechanism characterized by a longer time constant may be at work. We calculate the average disturbance drifts for Kp ≧ 30 and express them as a diurnal harmonic function of local time. There is no clear effect in the north‐south drifts (east‐west electric fields), but the east‐west drifts (north‐south electric fields) display large average disturbances. They are westward at all local times, maximize at 2200 LT with a 75‐m/s amplitude, but are very small in the 0612 LT quadrant. A 34‐m/s westward steady component and a 26‐m/s diurnal oscillation are the two dominant terms of its harmonic content. This pattern is interpreted in the third companion paper.
Supply of SO2 for the atmosphere of Io
Authors: Lanzerotti L. J. , Brown W. L.
Published: February 2008

We point out that in addition to a vapor pressure equilibrium source for SO2 in an atmosphere of Io, recent laboratory sputtering results, combined with Voyager‐measured particle fluxes, indicate that SO2 could also result from charged particle erosion of frost deposits on the satellite's surface. On the nightside, and for dayside frost patches where the temperature may be ≤ 100°K, such erosion will be a dominant mechanism.
Multiple transonic solutions with a new class of shock transitions in steady isothermal solar and stellar winds
Authors: Habbal Shadia R. , Tsinganos Kanaris
Published: February 2008

A new class of shock transitions are shown to arise in the transonic solutions of the steady isothermal solar wind equations when, for example, momentum deposition gives rise to multiple critical points in the flow. These shock transitions between critical solutions occur for a certain range of the parameters characterizing the momentum deposition function. In the presence of such shock transitions, the isothermal wind equations admit multiple transonic solutions, namely a continuous solution passing through an inner critical point and solutions involving a shock transition between critical solutions. These multiple transonic solutions have the same flow speed at the base but different supersonic flow speeds at infinity. An additional interesting feature of the isothermal equations is the equivalence of nonradial flow tube divergence and momentum addition in giving rise to multiple critical points and hence to multiple transonic solutions with shock transitions. The physical relevance of these properties for astrophysical systems such as the inner solar wind, flows in extragalactic jets and accretion discs are discussed.
Venus' nighttime horizontal plasma flow, ‘magnetic congestion’, and ionospheric hole production
Authors: Grebowsky J. M. , Mayr H. G. , Curtis S. A. , Taylor H. A.
Published: February 2008

A persistent feature of the nightside Venus ionosphere is the existence of localized magnetic field aligned columns of depleted ionization—i.e., ‘holes’—in which the radial component of the field is enhanced. By analogy to the earth's auroral regions it has been inferred that parallel electric fields can rapidly deplete the ionization. Another, complementary plasma loss mechanism at night is downward transport into the lower ionosphere where chemical depletion occurs. A simple rectilinear two dimensional MHD model is used to explore the effects of field aligned plasma loss and cooling on a dense plasma convecting across a weak magnetic field. By parameterizing field aligned variations and explicitly solving for cross
variations, it is shown that the abrupt horizontal enhancements of the vertical magnetic field as well as abrupt decreases of the plasma density to very low values—the two characteristics of a hole—can be produced in the presence of field aligned losses. The buildup of the magnetic field occurs in conjunction with a deceleration of the horizontal flow. The horizontal decrease in plasma density occurs abruptly when the magnetic field intensifies to such a level that the associated horizontal transport time becomes smaller than the field aligned loss time; this process is referred to as ‘magnetic congestion’.
Measurements of metastable N+(¹D) 6584 Å emission in the twilight thermosphere
Authors: Torr D. G. , Abdou Wedad A. , Torr M. R. , Burnside R. G. , Meriwether J. W.
Published: February 2008

Measurements of the 6584‐Å emission that arises from the transition N+ (³P − ¹D) were made at Sutherland, South Africa, in the summer of 1977. The twilight variations show a rapid decay at sunset from > 10 R to 1 R followed by a persistent emission at ∽1 R into the very late twilight. We identify the early twilight emission as being produced mainly by photodissociative ionization of N2 and estimate a branching ratio β ≃0.1 for the production rate of N+ in the (¹D) state. We conclude that electron quenching is the main loss process with an estimated value of ∽ 10−7 cm³ s−1 for its rate coefficient k. (These estimates are based on a measurement of the ratio k/β that we found to be ∽10−6.) Upper limits of ×10−12 cm³ s−1 and 1×10−11 cm³ s−1 are estimated for the quenching rate coefficients by both O and N2, respectively. The emission in the late twilight is ascribed to galactic contamination.
Theoretical modeling of low‐latitude Mg+
Authors: Fesen Cassandra Garay , Hays Paul B. , Anderson David N.
Published: February 2008

F region metal ion morphology is important because these long‐lived ions act as tracers of the dynamical processes. The behavior of Mg+ was inferred from the 2800 Å resonance emission observed by the Visible Airglow Experiment on the Atmosphere Explorer E satellite in equatorial orbit. Data were obtained over 5 months near winter solstice during solar maximum conditions. Theoretical modelling of the Mg+ densities was undertaken by solving the coupled continuity and momentum equations for one major and one minor ion. A uniform layer of 100 metal ions cm−3 at the lower boundary of 125 km was assumed. The numerical value of the density at the boundary is not critical, as all results are linearly scaled to the boundary density. The calculations reproduced the observed appearance of largest metal ion densities in the early afternoon in winter solstice conditions, and number density profiles that are nearly altitude independent near the equator near noon. Longitudinal effects are apparent mainly at high altitudes (≳350 km) near dusk. Correlations of high altitude densities with season and solar cycle are supported: larger densities at fixed altitudes are seen during solar maximum conditions, and at equinox near sunset. Neutral winds, particularly in the E region, were found to be very important in determining the metal ion distribution at high altitudes. The E region tidal oscillations especially seem to be the largest single factor controlling the F region metal ion behavior.
F region ion temperature enhancements resulting from Joule heating
Authors: Baron Murray J. , Wand Ronald H.
Published: February 2008

F region ion temperature measurements were made by the Chatanika and Millstone Hill incoherent scatter radars as part of the Magnetosphere‐Ionosphere‐Thermosphere Radar Studies program of coordinated high‐latitude observations. At both radars, periods of enhanced ion temperature associated with Joule heating events were detected. A regular feature of the observations was the existence of larger and longer lasting temperature enhancements in the morning sector as contrasted with the evening sector during periods of comparable electric field magnitudes. Because the ion temperature increases in proportion to the square of the vector difference between the ion and neutral velocities, the morning/evening temperature enhancement asymmetry implies a morning/evening neutral wind asymmetry. The neutral wind in the evening must be more closely aligned to the ion flow vector. This might arise as a consequence of the higher plasma density in the evening sector, enabling the ions to set the neutral air in motion. Comparison of the simultaneous plasma density and ion velocity measurements with the ion temperature data supports the foregoing explanation for the observed greater morning sector temperature enhancements.
Magnetic field line reconnection experiments: 5. Current disruptions and double layers
Authors: Stenzel R. L. , Gekelman W. , Wild N.
Published: February 2008

In a large laboratory plasma a current sheet is generated in the process of magnetic field line reconnection. The stability of the sheet with respect to local current increases is investigated. When the current density in the center of the sheet exceeds a critical value, spontaneous local current disruptions are observed. The current from the center of the sheet moves out to the sides. Magnetic flux variations in regions remote from the current sheet generate an inductive voltage in the current loop that drops off inside the plasma in the form of a potential double layer. This leads to particle acceleration with velocities much larger than those expected from the steady state electric fields in the plasma. The particle beams acquire their energy at the expense of the stored magnetic field energy of the current system. Beam‐plasma instabilities are generated that dissipate some of the directed kinetic energy and heat the background plasma. A model for the mechanism of the current disruptions is formulated. The potential structure leads to ion expulsion creating a localized density drop. The associated current drop in an inductive circuit drives the potential structure, thereby providing feedback for the disruptive instability. It saturates at a total current loss upon which the current system recovers, and the process repeats randomly. Similarities and differences to magnetospheric substorm phenomena are pointed out.
Latitudinal and magnetic flux tube extension of the equatorial spread F irregularities
Authors: Abdu M. A. , de Medeiros R. T. , Nakamura Y.
Published: February 2008

A comparative study has been carried out of the spread F occurrence characteristics at the magnetic equatorial station, Fortaleza (4°S, 38°W, dip latitude 1.8°S) and the low‐latitude station, Cachoeira Paulista (22°S, 45°W, dip latitude 14°S) located with a relatively small longitudinal difference, in Brazil. The results show strong correlation between the occurrences of spread F events at the two stations for most of the time. While all the spread F events over Cachoeira Paulista are accompained by such events over Fortaleza, the converse is not true. Also, the spread F onset times over the equator are systematically earlier than those over the low latitude. Another interesting result is that while the spread F durations at the two stations are approximately linearly related, an event over Cachoeira Paulista is observed only for those that have durations more than 4–5 hours over Fortaleza. Possible implications of these results on the plasma bubble rise velocities in the equatorial ionosphere are discussed. It seems possible also to predict the approximate duration of a spread F event over the equator by observations carried out at a low‐latitude station situated close to a common magnetic meridional plane.
Waves observed upstream of interplanetary shocks
Authors: Tsurutani Bruce T. , Smith Edward J. , Jones Douglas E.
Published: February 2008

The properties of waves with frequencies below 3 Hz observed upstream of low Mach number (2–3) interplanetary shocks are discussed. High‐frequency emissions (0.2–2 Hz in the spacecraft frame) are commonly detected immediately upstream (<2 RE) of the shocks, whereas lower frequency emissions (∼0.05 Hz) are found to extend upstream to much greater distances (typically 10 RE). Both emissions are right‐hand circularly or elliptically polarized and generally propagate within a 15° cone angle relative to the ambient magnetic field. The lack of a significant compressional component for either of these waves is in agreement with propagation parallel to the ambient magnetic field. Upstream waves are detected principally in association with quasi‐parallel shocks (θBn < 65°). Assuming the waves are propagating outward from the shock, an expression is derived for the wave frequency in the solar wind rest frame. The waves are found to have rest frame frequencies of 0.05–1 Hz and 10−3 to 10−2 Hz. Arguments are presented which exclude the possibility that the high‐frequency waves are standing whistler mode waves. The most likely source of these emissions is generation at the shock by 100 eV to 1 keV electrons and propagation of the whistler mode waves into the upstream region. The lower frequency 10−3 to 10−2 Hz waves propagate at speeds near the Alfvén velocity, and hence cannot outrun the super‐Alfvénic shocks. These waves must be locally generated by plasma instabilities in the upstream region. Generation by Landau electrons or ions can be ruled out due to the observation of parallel propagation of the waves. The most likely source is 1–10 keV cyclotron‐resonant ions propagating away from the shock. The upstream waves bear many similarities to those observed in the earth's foreshock. The frequencies, polarization, and typical upstream extent are nearly identical. It is also deduced that the lower frequency waves of both regions are generated by keV ions streaming away from the shock. There are some differences, however. Waves upstream of interplanetary shocks are found to propagate parallel to the magnetic field (<15°), are noncompressive (ΔB/B ≤ 0.25) and are generally lower in amplitude. Additionally, there are extraordinary interplanetary events, for which the scale of the upstream wave region is greatly extended (∼1300 RE or 0.04 AU) and the field takes on a more turbulent character. The latter events should be of interest in modeling Fermi acceleration of ions at collisionless shocks.
A note on the accuracy of the auroral electrojet indices
Authors: Akasofu S.‐I. , Ahn B.‐H. , Kamide Y. , Allen J. H.
Published: February 2008

The accuracy of the present AE(12) index is evaluated by using magnetometer data from the six IMS meridian chains and other high‐latitude stations. It is shown that the accuracy of AE(12) becomes progressively worse for lower values. There are at least a few causes for this inaccuracy, and three of them are identified. Although it is not possible to provide a single critical value, one should be cautious about the accuracy of the absolute value of AE(12) less than ∼250 nT. In particular, since 70% of AE(12) values are less than 250 nT, one must be cautious in correlative use of the AE(12) index with solar wind parameters.
Observations of optical emissions from precipitation of energetic neutral atoms and ions from the ring current
Authors: Rohrbaugh R. P. , Tinsley B. A. , Rassoul H. , Sahai Y. , Teixeira N. R. , Tull R. G. , Doss D. R. , Cochran A. L. , Cochran W. D. , Barker E. S.
Published: February 2008

Observations of N2+ 1 N, H Balmer β(HBaβ) and other emissions due to particle precipitation have been observed at two low‐latitude sites (Mt. Haleakala, Hawaii and Cachoeira Paulista, Brazil) and one mid‐latitude site (McDonald Observatory, Southwest Texas). Results are compared for magnetic storms of April 13, 1981 and July 14, 1982. The emissions have the characteristics appropriate to the precipitation into the thermosphere of energetic neutral atoms and/or ions originating in the ring current. These characteristics include high rotational/vibrational excitation of the N2+ 1 N bands and at times the occurrence of HBaβ emission with the same onset time as the N2+ 1 N emission and partial correlation with it afterward. The latitude variation shows a strong increase from low to mid latitudes. The strongest emissions occur in the evening to midnight local time period, and the storm time variations shows strongest emissions during main phases. The time variations of HBaβ and N2+ 1 N emissions indicate that there is more O/O+ precipitation than H/H+ precipitation in the latter part, and sometimes the whole durations of the precipitation events and the variations are consistent with H+ being lost from the inner ring current faster than other species, such as O+ and He+. Lower limits for the energy deposition rates for the strongest emissions at 40–45° dip latitude are 1–2 mWm−2 and for the strongest emissions at 12°S dip latitude, 0.05 mWm−2. Ionization production at its peak altitude somewhere above 110 km would be in the range from 10² cm−3 s−1 to a few times 10³ cm−3 s−1 for the events in Texas, and from 100 to 10² cm−3 s−1 for the stronger events in Hawaii and Brazil.
Occurrence properties of ducted whistler‐mode signals from the new VLF transmitter at Siple Station, Antarctica
Authors: Carpenter D. L. , Bao Z. T.
Published: February 2008

A new VLF transmitter called Jupiter was installed at Siple Station, Antarctica (L ∼4.3) in 1979. A study has been made of the occurrence properties of magnetospheric signals from the transmitter detected at the conjugate station, Roberval, Canada and in the two‐hop mode at Siple in 1980. The diurnal variation of the signals exhibited the previously noted major activity peak near dawn and a secondary peak near dusk. The former is believed to be due to a corresponding peak in resonant particle fluxes within the outer plasmasphere and the latter to destabilization of energetic particles that locally penetrated the evening bulge of the plasmasphere. The propagation paths followed by the better defined 2–4 kHz Siple signals had equatorial radii in the range 4 <L < 4.75. This region maps at 100 km altitude to a north–south range of approximately ±150 km, centered on the latitude of Siple. The east–west range is believed to have been of comparable size. This concentration of signal paths is attributed to the effects of an injected wave power threshold for fast temporal wave growth in the magnetosphere, combined with spatial variation in ionospheric illumination by the transmitter. Also important was a concentration of operator‐selected transmitter frequencies below 4 kHz, which permitted ducted propagation (i.e., f < ƒHeq/2, where ƒHeq is the equatorial gyrofrequency) on paths beginning nearly overhead at ∼ 100 km altitude. The data are consistent with the existence of a relatively narrow north–south separation of ∼ 100 km or less between the ionospheric point of wave injection and the field lines of the duct itself. The Siple transmitter has been able to induce magnetospheric wave growth and triggered emissions over the range 0.2 < ƒ/ƒHeq < 0.5. These results differ from the reported concentration at ƒ/ƒHeq ∼0.5 of discrete emission triggering by whistlers and by certain VLF transmitters. Enhanced Siple signal activity at ground stations has resulted from increases in transmitter power, a notable change from 1978 to 1980 being in the 2–3 kHz range, where Jupiter realized its largest (∼10 dB) power advantage over Zeus.
Standing wave patterns in VLF Hiss
Authors: Brittain R. , Kintner P. M. , Kelley M. C. , Siren J. C. , Carpenter D. L.
Published: February 2008

Observations have been made of systematic patterns in VLF hiss, which can be interpreted as a standing wave pattern formed by reflection in the lower ionosphere. Multicomponent VLF electric and magnetic field experiments were flown on three sounding rockets (Nike‐Tomahawk 18.203‐205) from Siple Station. Antarctica during December 1980 to January 1981. One feature of the natural emissions was observed in a very similar form during each flight. A band of hiss, typically from 1.5 to 3 kHz, was seen on the upleg to form a series of closely spaced stripes with whistler‐like dispersion. These first appeared at an altitude of 90–95 km and extended for as much as 40 km in altitude. On the downleg the stripes were observed at the same altitude with the pattern reversed in time. No such patterns were observed by the VLF receivers operating at the same time on the ground at Siple or at its conjugate point. It is suggested that the patterns are interference effects due to downcoming waves reflecting from a layer in the E region and forming a standing wave pattern. The observed stripes are then due to the rocket traversing the standing wave pattern in opposite directions on upleg and downleg. If this interpretation is correct, the fringe spacing should be related to the wavelength and permits a calculation of the refractive index. In one example we calculate n = 43.7 in good agreement with two independent determinations of the refractive index.
Observations of odd‐half cyclotron harmonic emissions in a shell‐Maxwellian laboratory plasma
Authors: Urrutia J. M. , Stenzel R. L.
Published: February 2008

Loss cone distributions have, until recently, been considered the prime cause of odd‐half cyclotron harmonic emissions. On the basis of new observations, it has been proposed that shell‐Maxwellian distributions are also unstable to these modes. To test this model, we have designed a laboratory experiment where a shell‐Maxwellian distribution is produced. Analysis of the plasma noise spectrum shows that the emissions fall within the cyclotron harmonic branches (Bernstein waves). Test wave dispersion analysis, spectral correlation measurements, and real time observations indicate that our plasma, which has the upper hybrid resonance in the first Bernstein wave branch (ωuh/ωce = 1.8), exhibits a nonconvective (absolute) instability at ω/ωce = 1.3, in contrast to the predictions of loss cone theory for the upper hybrid branch. In addition, the test wave dispersion relation of the plasma reveals that below the instability frequency the system behaves as if it were composed of shell electrons only. Present theories do not account for these findings.
Small‐scale auroral arc deformations
Authors: Wagner J. S. , Sydora R. D. , Tajima T. , Hallinan T. , Lee L. C. , Akasofu S.‐I.
Published: February 2008

An auroral arc undergoes various types of deformations, such as curls, folds, and spirals. In this paper we examine the formation mechanism of folds by a plasma simulation method. We suggest that folds, as well as curls, can arise from electrostatic shear‐induced instabilities. The differences between the two forms are controlled by the presence of ambient ions that must be present to shield the electric field of the precipitating electron charge sheet. It is shown that curls form when the ion sheath thickness is large in comparison with the electron sheet. Folds form when the ion sheath is thin and lags slightly behind the electron guiding centers, setting up an additional electric field that controls the later stages of instability growth.
Narrow spectral peaks in electrons precipitating from the slot region
Authors: Imhof W. L. , Reagan J. B. , Gaines E. E. , Anderson R. R.
Published: February 2008

Narrow L‐dependent peaks commonly occur in the energy spectra of electrons precipitating from the inner radiation belt at L ≈ 1.5–1.85, and the cause of the peaks has been attributed to cyclotron resonance interactions with waves generated by VLF transmitters. In the slot region, L‐dependent peaks have also been reported at L ≈ 2–3.5, but these have been predominately wider and consistent with their origin being cyclotron resonance interactions involving naturally occurring hiss. Here we investigate the infrequent occurrence of narrow peaks in electrons precipitating at L ≳ 2. From coordinated wave and plasma density measurements it is found that if the narrow peaks are formed by first‐order cyclotron resonant interactions occurring close to the magnetic equator between narrow band waves and the trapped electron population, then the equatorial plasma density gradients are unusually steep. This finding is consistent with the evidence previously obtained by other techniques for structured plasma density profiles in that L shell region of space.
Photometry and polarimetry of Saturn at 2640 and 7500 Å
Authors: West R. A. , Sato M. , Hart H. , Lane A. L. , Hord C. W. , Simmons K. E. , Esposito L. W. , Coffeen D. L. , Pomphrey R. B.
Published: February 2008

We have reduced and tabulated photometry and polarimetry data at 2640 and 7500 Å observed by the Voyager 2 photopolarimeter experiment. Spatially resolved limb‐to‐terminator scans across Saturn's Equatorial Zone from 12° to 68° phase angle provide information on the altitude distribution of UV absorbing hazes and the phase function and polarizing properties of stratospheric and tropospheric aerosols. Limb‐to‐terminator scans across the northern hemisphere at 10° phase angle are used to study altitude variations of the tropospheric cloud at several latitudes. For the Equatorial Zone we find (1) the UV photometry and polarimetry are best fit by Rayleigh's phase matrix. (2) A stratospheric haze of small particles is allowed as long as the optical depth is near unity or less and the center of the haze layer is in the 30‐ to 70‐mbar region. A diffuse haze fits better than a thin layer, and the aerosol/gas mixing ratio diminishes above 10 mbar. The vertical distribution and optical depth of the haze differ significantly from models proposed by others. To be in agreement with ground based and other spacecraft data, the haze optical depth is about 0.4 at 2640 Å and decreases by a factor of 10 or more at 6400 Å. If the haze aerosol scattering properties are similar to those for spheres with mean radius 0.1 μm, their imaginary refractive index is 0.4 or larger at 2640 Å and the total column density above the tropopause is 109 cm−2. (3) UV contrasts between belts and zones are interpreted as altitude variations in the top of the tropospheric cloud. The altitudes derived here for three latitudes agree with altitudes derived from ground‐based methane band studies and analyses of polarization from Pioneer 11. A high altitude absorber is abundant in the polar regions. (4) At 7500 Å, the phase function of tropospheric aerosols in the Equatorial Zone is described by a synthetic two‐term Henyey‐Greenstein function with g1 = 0.54 ± 0.11, g2 = −0.47 ± 0.08, ƒ = 0.87 ± 0.03, and ω = 0.986. The single scattering albedo in the North Equatorial Belt is ω = 0.967. The Equatorial Zone tropospheric aerosols are positively polarizing at all the phase angles of our observations.
Influence of solar sector boundaries on ionospheric variability
Authors: Mendillo Michael , Schatten Kenneth
Published: February 2008

A latitudinal network of total electron content (TEC) observations has been used to assess the contributions to day‐to‐day F region variability associated with a wide variety of solar‐terrestrial phenomena. Recent discussions of a proposed “sun‐weather coupling” effect related to solar wind sector boundary (SB) crossings prompted a systematic search to document any upper atmospheric components of such a linkage. TEC data from several North American sites close to the 70°W meridian (Narssarssuaq, Greenland (L = 5); Goose Bay, Labrador (L = 4); Sagamore Hill, Massachusetts (L = 3); and the Kennedy Space Center, Florida (L = 2) were examined during the years 1973–1975. Average daytime (0900–1700 LT) TEC values were compared with a 27‐day mean centered on the day of an SB crossing. The resultant percentage deviations were examined for sector polarity (plus to minus versus minus to plus) effects and for overall latitudinal patterns during a period of ±5 days from the SB crossings. Average results for the entire data set show peak excursions well within the ±20–25% deviations associated with day‐to‐day variability. There is, however, a remarkable consistency in the coherence of the small amplitude latitudinal patterns. At middle to high latitudes (L = 3–5), the TEC variations show an ordered transition from enhancements to depletions following an SB crossing. At lower latitudes (L = 2), the opposite pattern of depletions to enhancements occurs. An identical analysis for geomagnetic activity variations (using the planetary index Ap) reveals the well‐known pattern of a transition from relatively quiet to relatively disturbed activity as a sector is crossed. A merging of these two results suggests that the manifestation in the F region of SB crossings is nothing more than a small‐scale version of the average “geomagnetic/ionospheric storm effect” that is well documented over the L = 2–5 range. The latitudinal patterns for ΔTEC, and their seasonal variations, are consistent with the morphology of O/N2 variations during geomagnetic disturbances and southward IMF effects during equinoctial versus solstice periods.