bluuebunny/crossref_metadata_2025 huggingface dataset:
Periodic auroral events at the high‐latitude convection reversal in the 16 MLT region
Authors: Sandholt Per Even , Lockwood Mike
Published: February 2008

Combined optical and radar observations of two breakup‐like auroral events near the polar cap boundary, within 74–76° MLAT and 1210 – 1240 UT (roughly 1540 – 1610 MLT) on 9 Jan. 1989 are reported. A two‐component structure of the auroral phenomenon is indicated, with a local intensification of the pre‐existing arc as well as a separate, tailward moving discrete auroral event on the poleward side of the background aurora, close to the reversal between well‐defined zones of sunward and tailward ion flows. The all‐sky TV observations do not indicate a connection between the two components, which also show different optical spectral composition. The 16 MLT background arc is located on sunward convecting field lines, as opposed to the 12–14 MLT auroral emission observed on this day. Although the magnetospheric plasma source (s) of the 16 MLT events are not easily identified from these ground‐based data alone, it is suggested that the lower and higher latitude components, may map to the plasma sheet boundary layer and along open field lines to the magnetopause boundary, respectively. The events occur at the time of enhancements of westward ionospheric ion flow and corresponding eastward electrojet current south of 74° MLAT. Thus, they seem to be very significant events, involving periodic (10 min period), tailward moving filaments of field‐aligned current/discrete auroral emission at the 16 MLT polar cap boundary.
High resolution observations of HF‐induced plasma waves in the ionosphere
Authors: Djuth F. T. , Sulzer M. P. , Elder J. H.
Published: February 2008

Langmuir oscillations excited by a powerful, high‐frequency (HF) radio wave reflecting in the F‐region have been studied in detail with the 430 MHz radar at Arecibo Observatory, Puerto Rico. Exceptionally good altitude and temporal resolution made it possible to closely examine transient plasma phenomena and allowed very weak plasma features to be seen in the ionosphere. The observations reveal an unexpected richness of complex plasma structure. Langmuir turbulence detected with the 430 MHz radar often occupies an extended altitude interval, particularly during the “plasma line overshoot” and at late times following HF turn‐on. In the late time plasma environment, multiple layers of Langmuir turbulence are often observed. In general, important distinctions can be made between the excitation of Langmuir oscillations in a “cold” background ionosphere and in an ionosphere “preconditioned” by prior HF transmissions. The greatest differences are evident in the altitude and range extent of the backscatter measured immediately after HF turn‐on.
Airborne gravity measurement over sea‐ice: The Western Weddell Sea
Authors: Brozena John , LaBrecque John , Peters Mary , Bell Robin , Raymond Carol
Published: February 2008

An airborne gravity study of the western Weddell sea, east of the Antarctic Peninsula, has shown that floating pack‐ice provides a useful radar altimetric reference surface for altitude and vertical acceleration correction to airborne gravimetry. Airborne gravimetry provides an important alternative to satellite altimetry for the sea‐ice covered regions of the world since satellite altimeters are not designed or intended to provide accurate geoidal heights in areas where significant sea‐ice is present within the radar footprint. Errors in radar corrected airborne gravimetry are primarily sensitive to the variations in the second derivative of the sea‐ice reference surface in the frequency pass‐band of interest. With the exception of imbedded icebergs the second derivative of the pack‐ice surface closely approximates that of the mean sea‐level surface at wavelengths > 10–20 km. With the airborne method the percentage of ice coverage, the mixture of first and multi‐year ice and the existence of leads and pressure ridges prove to be unimportant in determining gravity anomalies at scales of geophysical and geodetic interest, provided that the ice is floating and not grounded. In the Weddell study an analysis of 85 crosstrack miss‐ties distributed over 25 data tracks yields an rms error of 2.2 mGals. Significant structural anomalies including the continental shelf and offsets and lineations interpreted as fracture zones recording the early spreading directions within the Weddell Sea are observed in the gravity map.
Paleomagnetic transition records of the Cobb Mountain Event from sediments of the Celebes and Sulu Seas
Authors: Hsu Vindell , Merrill Dean L. , Shibuya Hidetoshi
Published: February 2008

The Cobb Mountain (CM) event is a short normal polarity interval within the Matuyama Chron just before the Jaramillo Subchron. This event was found in the sediments of Site 767 in the Celebes Sea and Site 768 in the Sulu Sea during ODP Leg 124. Reversal depth and age relation analysis gave ages of 1.11 and 1.10 m.y. ago for the onset and termination of this event. We used discrete samples and high‐density pass‐through measurements to study the behavior of the geomagnetic field crossing this event. Although the two sites are very close to each other, the records of the transitional fields were found to be quite different. They are also very different from another CM event record from the North Atlantic Ocean. Based on the transition data we conclude that the transitional fields for the CM event were non‐axisymmetric. One brief reverse in direction, called rebound, was found within the onset reversal at Site 767. The absence of this feature at Site 768 raises an interesting question concerning the source for rebounds.
Room‐temperature amorphlzation of fayalite and high‐pressure properties of Fe2SiO4 liquid
Authors: Richard Gaël , Richet Pascal
Published: February 2008

High‐pressure X‐ray diffraction and electron‐microscopy experiments show that fayalite (Fe2SiO4) transforms at around 350 kbar to an amorphous phase that can be quenched down to 1 bar. This room‐ternperature vitrification represents a thermodynamically driven fusion to a glass, as implied by the existence of a maximum above 100 kbar in the metastable extension of the melting curve of fayalite. The diffraction measurements, a thermodynamic analysis of the melting curve of fayalite, and the difference in bulk modulus between fayalite and Fe2SiO4 liquid are consistent with a reversible switch of silicon in the amorphous phase from a high coordination number at pressure down to 4‐fold coordination at 1 bar.
Attenuation structure beneath the Lau Back Arc Spreading Center from teleseismic S phases
Authors: Flanagan Megan P. , Wiens Douglas A.
Published: February 2008

Teleseismic sS and sScS waveforms provide constraints on the attenuation structure above deep seismic rones. We estimate the average Qβ along sS and sScS paths beneath the Lau back arc by comparison with S and ScS waveforms. Q is determined using both time and frequency domain analyses of transverse long period records assuming Q is frequency independent. In the frequency domain technique, spectra from reflected (sS or sScS) and direct (S or sScS) waveforms are analyzed using least squares. The time domain method uses a grid search to determine the attenuation operator that, when convolved with the direct phase, produces the best fit to the reflected phase. In both methods the direct phase is convolved with a crustal response function to simulate the crustal interaction of the reflected phase. Large events subject to source directivity effects and source‐station combinations involving ray propagation down the subducting slab are not used. The resulting attenuation estimates show low Q values which increase with focal depth, suggesting high attenuation in the upper mantle and a rapid decrease in attenuation below 200–300 km. Lateral variations are also indicated with very high attenuation (Q ∼ 20–35) in the upper mantle near the active spreading centers and lower attenuation west of the Lau Ridge. Q‐temperature relationships extrapolated from laboratory experiments suggest that the low Q regions represent zones of asthenospheric partial melting.
Ion cyclotron absorption at the second harmonic of the oxygen gyrofrequency
Authors: Horne R. B. , Thorne R. M.
Published: February 2008

The ray paths of unstable electromagnetic waves generated inside the plasmapause during magnetic storm conditions are followed using the HOTRAY code. Unducted waves which are preferentially generated in the field aligned direction are rapidly refracted to large wave normal angles. At frequencies between the Oxygen and Helium gyrofrequencies the waves reflect once their frequency falls below the bi‐ion frequency and the waves subsequently bounce back and forth across the equator. The wave energy is ultimately absorbed by Landau resonance with plasmaspheric electrons or by second harmonic cyclotron resonance with thermal Oxygen; the latter being an important effect which has not previously been considered. Both processes may provide a significant heating mechanism for either population during storms.
The AMPTE lithium releases in the solar wind: A possible trigger for geomagnetic pulsations
Authors: Lühr Hermann , Baumjohann Wolfgang , Potemra Thomas A.
Published: February 2008

Shortly after the AMPTE/IRM satellite had released Lithium ions into the solar wind, magnetometers on the ground and in the frontside magnetosphere observed deflections of a type similar to those associated with solar wind pressure pulses. Since the natural solar wind was very stable during this event and did not exhibit pressure changes, the magnetic deflections have likely been caused by the artificially injected Lithium cloud when it hit the magnetosphere. Order of magnitude estimates show that 300 g of Lithium vapor released in the solar wind are indeed able to produce a 300 nT magnetic deflection on the ground under favourable conditions.
Tail‐like reconfiguration of the plasma sheet during the substorm growth phase
Authors: Kan J. R.
Published: February 2008

Tail‐like reconfiguration observed in the near‐earth plasma sheet during the substorm growth phase is surrounded by enhanced convection except inside and tailward of the reconfiguration region itself. The required distribution of enhanced convection can be driven by an enhanced dayside reconnection. The enhanced dayside convection can be transmitted across open field lines by the tangential discontinuity to reach the last open field line in the tail lobe in about 20 minutes. This can cause Bx to increase in the plasma sheet. On the other hand, the enhanced dayside convection can also be transmitted across closed field lines by the fast rarefaction wave to arrive at the near‐earth plasma sheet (x ≤ −10 RE) in about 15 minutes, causing Bz to decrease in the plasma sheet. Thus, a tail‐like reconfiguration can start within about 15 to 20 minutes after a southward turning of the IMF (interplanetary magnetic field). The duration of the growth phase can be identified with the M‐I coupling time scale of ∼30 to ∼40 minutes. The region‐2 field‐aligned current is shown to increase to 0.2 μA/m2 at the ionospheric level, resulting from the tail‐like reconfiguration.
Chaotic scattering and acceleration of ions in the Earth's magnetotail
Authors: Ashour‐Abdalla M. , Berchem J. , Büchner J. , Zelenyi L. M.
Published: February 2008

In this paper the steady‐state proton distribution in the Earth's magnetotail is derived both in configuration and velocity space, taking into account the proton's energization in the dawn‐dusk electric field and its motion due to deterministic chaos. This stochastic motion effectively leads to pitch angle scattering and a redistribution of protons between the central plasma sheet and its boundary, the plasma sheet boundary layer. Knowledge of these effects allows us to explain the previously observed features of the ion distribution and to predict others for future experimental work.
Urban heat islands in China
Authors: Wang Wei‐Chyung , Zeng Zhaomei , Karl Thomas R.
Published: February 2008

We used 1954–1983 surface temperature from 42 Chinese urban (average population 1.7*106) and rural (average population 1.5*105) station pairs to study the urban heat island effects. Despite the fact that the rural stations are not true rural stations, the magnitude of the heat islands was calculated to average 0.23 °C over the thirty‐year period with a minimum value during the 1964–1973 decade and maximum during the most recent decade. The urban heat islands were found to have seasonal dependence which varied considerably across the country. The urban heat islands also had a strong regional dependence with the Northern Plains dominating the magnitude of the heat islands. The changes in heat island intensity over three decades studied suggest a general increase in heat island intensity of about 0.1°C, but this has not been constant in time. These results suggest that caution must be exercised when attributing causes to observed trends when stations are located in the vicinity of metropolitan areas.
A quasi‐linear WKB kinetic theory for nonplanar waves in a nonhomogeneous warm plasma, 1. Transverse waves propagating along axisymmetric B0
Authors: Hollweg Joseph V.
Published: February 2008

A new set of quasi‐linear kinetic equations is presented for transverse waves propagating along an axisymmetric magnetic field configuration. A WKB expansion is used to include effects of nonhomogeneity and nonplanarity of the waves. The equations allow simultaneous calculation of the spatial (and temporal) evolution of wave power spectra and the spatial (and temporal) evolution of particle distribution functions, including for the first time important wave‐particle interactions which depend explicitly on the nonplanarity of the waves and the nonhomogeneity of the plasma and fields. The usefulness of the equations is demonstrated for a cold plasma, where a number of new results for wave propagation and acceleration of the plasma have been obtained. Along the way, a new quasi‐linear separation of the average distribution function has been introduced, the usefulness of which is demonstrated by comparison with fluid theory. The principal new results are briefly summarized in section 6 of the paper.
Multiple‐satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection‐driven negative bays
Authors: Pytte T. , McPherron R. L. , Hones E. W. , West H. I.
Published: February 2008

It is generally believed that long intervals of intense (500–1000 γ) negative auroral zone bays are caused by magnetospheric substorm occurrences at a high repetition rate. Since individual substorm phases may then overlap in time and occur within limited local time sectors, it may be difficult to identify their characteristic signatures. In this paper we have examined ground and multiple‐satellite recordings during an ∼5‐hour interval of persistent auroral zone activity in a systematic search for substorm expansion signatures. It appears that this enduring activity started as a rather weak substorm which thereafter developed into strong magnetospheric activity driven by a continuously southward interplanetary magnetic field (IMF). During this period there were no clear mid‐latitude positive bays, and pulsation recordings from different local time sectors showed only one instance of weak Pi 2‐like variations. The associated high‐latitude equivalent ionospheric current pattern is consistent with a convection system, and we refer to this activity as the convection bay. Following the plasma sheet and ground signatures of substorm recovery early in the convection bay, there was no definite indication of substorm expansions other than this weak magnetic pulsation in any recordings during the next ∼3½ hours until a clear substorm emerged. This substorm onset, which was associated with a not particularly strong auroral zone bay, was accompanied by the full complement of substorm expansion signatures. At this time a minimum plasma sheet thickness of only a few tenths of an earth radius was observed in the region where a neutral line appears to form during substorms. Prior to this onset, the plasma sheet was relatively thick, and it seems that the neutral line was located well tailward of the Vela orbit (r ∼ 18 RE). This event and other similar observations suggest that continuous auroral zone activity during southward IMF, such as a geomagnetic storm period, may be driven by a steady state magnetospheric convection. The long absence of clear substorm onset signatures may indicate that the nonsteady enhancements of convection associated with localized substorm expansions then make only small contributions to the total convection pattern. Similarly, the ground magnetic disturbances may then be caused mainly by Sqp currents and enhanced currents due to strong particle precipitation rather than by substorm‐associated current wedges. During this quasi‐steady state the tail reconnection rate appears nearly to balance the dayside merging rate, allowing most energy extracted from the solar wind to be released continuously rather than to be first stored in the tail. It seems that a large‐scale substorm expansion may be triggered if this convection mode is perturbed, for instance, by a significant reduction of the imposed convection electric field.
Plasma fluctuations in the solar wind
Authors: Neugebauer M. , Wu C. S. , Huba J. D.
Published: February 2008

Ogo 5 plasma and magnetic field data are used to compute power spectra of solar wind fluctuations over the frequency interval 10−3‐10−1 Hz. We confirm the validity of the assumption made in earlier papers that the power spectra calculated from total flux measurements are approximately equal to the power spectra of density fluctuations times the square of the average solar wind speed. The relative density power spectrum Pn/no² is usually of the same order of magnitude as the power spectrum of speed fluctuations relative to the Alfvén speed, Pv/vA². All cases studied show evidence of the presence of Alfvén waves in this frequency range. In some data sets the density and field fluctuations are consistent with magnetosonic waves. In other sets the ratio of the power in field magnitude fluctuations to that in density fluctuations is inconsistent with magnetosonic waves; for these cases we postulate static inhomogeneities with a balance between electron thermal and magnetic pressures. Finally, we suggest that the power enhancements near 1 Hz reported in earlier papers may be caused by a resonant proton cyclotron instability driven by the proton thermal anisotropy in the solar wind.
A theory of quiet auroral arcs
Authors: Sato Tetsuya
Published: February 2008

Being fed by the large‐scale magnetospheric convection, the coupled ionosphere‐magnetosphere system is subject to a feedback instability for a conjugate perturbation elongating in the east‐west direction, having a width of several tens of kilometers at the ionospheric level, standing along the magnetic field line. The growth time is as short as several minutes even when no hot electrons are involved in the field‐aligned current. Nonlinear development of the feedback instability is numerically investigated, taking hot electrons into account. The following conclusions are obtained. (1) An auroral arc can develop within a few tens of seconds once hot electrons take part. (2) The induced potential associated with the arc can reach several hundred volts. This predicts that the upward field‐aligned current may be provided by electrons with energies of the order of several hundred electron volts, provided anomalous resistivity or double layers develop. (3) The electron density enhancement is directly connected with the upward field‐aligned current and hence the auroral arc. (4) The downward current is localized equatorward of the upward current, when the background electric field is westward. (5) The electrojet current can grow to a few thousand amperes, which can either be westward or eastward, depending on the direction of the electric field. (6) The induced electric field inside the arc is almost constant and different from outside. These results are consistent with the observational results.
Geomagnetic pulsations observed simultaneously on three geostationary satellites
Authors: Hughes W. J. , McPherron R. L. , Barfield J. N.
Published: February 2008

Simultaneous observations of magnetic pulsations have been made by three geostationary satellites carrying similar magnetometers and so acting as an azimuthal array. Autospectral and cross‐spectral analysis yields coherence and phase differences between the pulsations at the satellite positions. The majority of the data fit the Kelvin‐Helmholtz generation mechanism. The azimuthal wave number changes sign near noon and corresponds to propagation away from noon. Usually, the wave number is <10° per degree of longitude. Later in the afternoon, however, strong pulsations are observed with low coherence, implying large wave numbers. This suggests an instability driven by a gradient in the intensity of energetic protons, which may be expected at this local time. The data also suggest that some of the observed phase difference between the azimuthal components is due to small separations in magnetic shell, whereas this is not the case for the radial components. This implies a localized field line resonance. We obtain estimates of 1/4 and 1/2 RE for the equatorial extent of the resonant region.
A comprehensive model of the Venus ionosphere
Authors: Chen R. H. , Nagy A. F.
Published: February 2008

The coupled time‐dependent continuity‐momentum and energy balance equations were simultaneously solved for CO2+, O2+, O+, He+, and H+ densities and electron and ion temperatures for an altitude range of 120–500 km. The solar zenith angle was varied from 0° at the subsolar point to 90° at the terminator. The horizontal bulk transport of ions by neutral winds was included in these calculations, although horizontal diffusion was not. The two‐stream photoelectron transport method was used to find the heating rates for the ambient electrons. The effects of different boundary conditions were investigated, and a nightside ionosphere was calculated with electron precipitation as the source of ionization. The results of these model calculations agree well with measurements in the region of maximum electron density. The topside densities and temperatures are shown to be very strongly dependent upon the energy input at high altitudes. For solar zenith angles greater than 90°, densities and temperatures decrease rapidly, which implies that the nightside ionosphere cannot be maintained by neutral wind driven short‐lived ions carried across the terminator. However, the electron densities on the nightside can be explained in terms of electron precipitation.
Relations between transverse electric fields and field‐aligned currents
Authors: Mallinckrodt A. J. , Carlson C. W.
Published: February 2008

A model for the field‐aligned propagation of transverse electric fields and associated field‐aligned sheet currents is presented which takes into account the wave nature of the process. The model is applied to the separate cases of ionospheric and magnetospheric sources, and the resulting ionospheric electric field to field‐aligned sheet current ratios are determined for comparison with experimental observations. It is found that the magnetospheric wave ‘conductivity’ for shear mode Alfvén waves is small with respect to typical values of the height‐integrated ionospheric Pedersen conductivity. For plasma convecting across a stationary disturbance a dynamic equilibrium is achieved in which field‐aligned currents flow continuously away from the source on convecting field lines. Consistency with typical ionospheric electric fields requires that the field‐aligned sheet currents are limited to around 0.1 A/m for ionospheric polarization sources, while magnetospheric sources are easily capable of 1 A/m or more.
Shock‐associated energetic proton events at large heliocentric distances
Authors: Palmer I. D. , Gosling J. T.
Published: February 2008

Enhancements of energetic protons (≳0.5 MeV) in association with forward‐reverse shock pairs have been observed at large heliocentric distances. An interpretation of the time profiles of these events is offered in terms of a model of solar wind stream structure. Persistent sweeping of energetic particles by each shock front and their banking‐up by reflection lead to the formation of a gradient along field lines upstream of the shocks. The rise (fall) of intensity before (after) the forward (reverse) shock is explained by a gradient in particle density directed along the upstream field lines toward the shock. A marked intensity decrease between the two shocks is interpreted in the same fashion as being due to parallel density gradients directed toward the shocks. First‐order Fermi acceleration upon reflection at each highly oblique shock front offsets loss by leakage through the shock front and sustains the particle intensity in the upstream region. Other acceleration mechanisms may not be required. A simple model is explored in which the gradient profile is primarily determined by a balance between shock sweeping and diffusive flow away from the shock (it is argued that acceleration by reflection and leakage through the shock cause a secondary modification of the profile). Profiles outside the compression region in 15 observations of corotating events suggest a radial mean free path λr = 0.021 → 0.64 AU (the mean is
over r = 2.6–6 AU. The model predicts an anisotropy which upstream of each shock is lower in magnitude than observations by a factor of ≈ 2. The predicted direction in the upstream region is in the direction of corotation, i.e., perpendicular to the radius vector and counterclockwise in the ecliptic plane. Although net flows in this direction are observed, observations have not generally sought to distinguish upstream from downstream regions where net flows are expected to be approximately reversed.
On the modulation of the Jovian decametric radiation by Io, 1. Acceleration of charged particles
Authors: Smith Robert A. , Goertz C. K.
Published: February 2008

We perform a steady state analysis of the current circuit between Io and the Jovian ionosphere, assuming that the current is carried by electrons accelerated through potential double layers in the Io flux tube. The circuit analysis indicates that electrons may be accelerated up to energies of several hundred keV. We also discuss several problems associated with the formation of double layers. The parallel potential drops decouple the flux tube from the satellite's orbital motion.
The coupling of Alfvén and compressional waves
Authors: Lin C. S. , Parks G. K.
Published: February 2008

The article studies the hydromagnetic wave propagation characteristics in a mixture of cold and hot plasma in the presence of an inhomogeneous magnetic field. Electron and ion distribution functions with a temperature anisotropy and a density gradient are used to obtain the dispersion equation by solving the Vlasov equation and Maxwell equations. From the solutions of the dispersion equation we find that the Alfvén waves can couple to unstable drift mirror waves under certain conditions. The polarization of the coupled waves is studied for varying parameters of temperature anisotropy and the cold to hot density ratio. From detailed comparison of the theoretical results with the low‐frequency wave properties observed in the magnetosphere we propose that the storm‐associated magnetic field oscillations with periods of 100–600 s might be caused by the coupling of Alfvén waves and the unstable drift mirror waves.
Ion temperature troughs and interhemispheric transport observed in the equatorial ionosphere
Authors: Heelis R. A. , Bailey G. J. , Hanson W. B.
Published: February 2008

Ion temperature and ion drift velocity data from Atmosphere Explorer D have verified the existence of interhemispheric plasma transport and ion temperature troughs in the topside equatorial ionosphere. The data were taken during solar minimum conditions at night where the exospheric temperature was typically 700°K and the O+‐H+ transition height was 520 km. Large field‐aligned ion velocities were observed above about 700 km, where the H+/O+ number density ratio was about 3. Model calculations have shown that the ion temperature decrease is produced by quasi‐adiabatic expansion of the plasma, but the expansion cooling mechanism is less efficient at solar minimum than at solar maximum owing to the lower O+‐H+ transition height. Ion temperature troughs greater than 400°K were not observed even when the field‐aligned ion velocity was greater than 700 m s−1. Most of the expansion occurs below the transition height, and during sunspot minimum, thermal coupling to the neutral atmosphere is much more effective in quenching the cooling which the field‐aligned transport tends to produce.
The radial gradients and collisional properties of solar wind electrons
Authors: Ogilvie K. W. , Scudder J. D.
Published: February 2008

The plasma instrument on Mariner 10 carried out measurements of electron density and temperature in the interplanetary medium between heliocentric distances of 0.85 and 0.45 AU. Owing to the stable coronal configuration and low solar activity during the period of observation (January 9 to March 30, 1974), the radial variations of these quantities could be obtained. The power law exponent of the core temperature was determined to be ‐0.3 ± 0.04, and the halo temperature was found to be almost independent of heliocentric distance. The exponent of the power law for the density variation was 2.5 ± 0.2, and the extrapolated value at 1 AU was consistent with measured values during the same period. Calculations of the core electron Coulomb collision time and the core‐halo equipartition time were made as a function of radial distance. These measurements indicate a macroscale picture of a Coulomb collisional core and a collisionless isothermal halo. Extrapolating back to the sun, core and halo temperatures become equal at a radial distance of ∼2–15 RS. Temperature variations were observed at the time of passage of high‐speed streams, and at the time of observation of stream interfaces, where the density falls before the temperature, Coulomb collisions in the core become much less important, and the halo density is observed to decrease by more than an order of magnitude.
The heliocentric radial variation of plasma oscillations associated with Type III radio bursts
Authors: Gurnett D. A. , Anderson R. R. , Scarf F. L. , Kurth W. S.
Published: February 2008

A survey is presented of all of the electron plasma oscillation events found to date in association with low‐frequency type III solar radio bursts using approximately 9 years of observations from the Imp 6 and 8, Helios 1 and 2, and Voyager 1 and 2 spacecraft. Plasma oscillation events associated with type III radio bursts show a pronounced increase in both the intensity and the frequency of occurrence with decreasing heliocentric radial distance. This radial dependence explains why intense electron plasma oscillations are seldom observed in association with type III radio bursts at the orbit of the earth. Possible interpretations of the observed radial variation in the plasma oscillation intensity are considered.
Sources of magnetic fields in recurrent interplanetary streams
Authors: Burlaga L. F. , Behannon K. W. , Hansen S. F. , Pneuman G. W. , Feldman W. C.
Published: February 2008

We examined the sources of magnetic fields in recurrent streams observed by the Imp 8 and Heos spacecraft at 1 AU and by Mariner 10 en route to Mercury between October 31, 1973, and February 9, 1974, during Carrington rotations 1607–1610. Most fields and plasmas at 1 AU were related to coronal holes, and the magnetic field lines were open in those holes. However, some of the magnetic fields and plasmas at 1 AU were related to open field line regions on the sun which were not associated with known coronal holes, indicating that open field lines may be more basic than coronal holes as sources of the solar wind. Magnetic field intensities in five equatorial coronal holes, estimated by projecting the measured interplanetary magnetic fields back to the sun by using the principle of flux conservation, ranged from 2 to 18 G with an average of 9 G. Average measured photospheric magnetic fields along the footprints of the corresponding unipolar fields on circular equatorial arcs at 2.5 RS had a similar range and average, but in two cases the intensities were approximately 3 times higher than the projected intensities. The coronal footprints of the sector boundaries on the source surface at 2.5 RS, determined by a potential field extrapolation of the measured photospheric fields, meandered between −45° and +45° latitude, and their inclination with respect to the solar equator ranged from near 0° at some longitudes to near 90° at others. It is possible that sector boundaries are related to convergence surfaces of the flow near the sun. The high densities observed near sector boundaries between streams might be due in part to the convergence of flows from adjacent coronal holes.
NO2 continuum in aurora
Authors: Sharp W. E.
Published: February 2008

Rocket measurements of the vertical profile of the NO2 continuum in low‐ to medium‐intensity aurorae give intensities at 5200 Å of the order of 1.0 R/Å. By using atomic oxygen densities deduced from the quenching rate of the N2 (A³Σu+) state [NO] densities of the order of 1 to 4 times 109 cm−3 are found at 110 km. [NO] densities deduced from the measured [NO+] / [O2+] ratio tend to confirm these abundances.
High‐resolution energetic particle measurements at 6.6 RE, 1. Electron micropulsations
Authors: Higbie P. R. , Belian R. D. , Baker D. N.
Published: February 2008

The three papers dealing with data from satellites 1976‐059A and 1977‐007A which we present in this issue represent the first publication of data from the new series of charged particle analyzer (CPA) instruments designed to measure energetic particle fluxes at geosynchronous altitudes. This first report presents new results on electron micropulsation phenomena and includes a concise description of the instrument. We often observe highly periodic modulations which persist for times as long as 2 hours in the spin‐averaged counting rate data. These flux oscillations occur most frequently in the 30‐ to 300‐keV electron data but are occasionally seen in higher‐energy electron or low‐energy proton data. The pitch angle distributions of the observed modulated fluxes may be either ‘cigar‐shaped’ or ‘pancake‐shaped.’ Oscillations at different energies are in phase, although the gross counting rate may be changing in an energy‐time dispersive manner. The occurrence distribution of these modulations in local time suggests that they are related to Pc 5 geomagnetic micropulsations observed at ground stations.
High‐resolution energetic particle measurements at 6.6 RE 3. Low‐energy electron anisotropies and short‐term substorm predictions
Authors: Baker D. N. , Higbie P. R. , Hones E. W. , Belian R. D.
Published: February 2008

Multiple detectors giving nearly complete 4π coverage of particle pitch angle distributions have provided high resolution measurements (in energy and time) of 30‐ to 300‐keV electrons. Data from a spacecraft (1976‐059A) in geostationary orbit show a remarkably consistent sequence of variations of the electron anisotropy before and during magnetospheric substorms. For periods typically 1–2 hours prior to the onset of substorms, electron distributions, peaked along the direction of the local magnetic field, are observed in the premidnight sector. These cigarlike anisotropies are accompanied by a local taillike magnetic field which may develop further during the event. At substorm onset an abrupt transition usually occurs from the cigar‐shaped distributions to pancake‐shaped distributions. This anisotropy sequence may be due to the buildup and subsequent release of stresses in the magnetotail; the cigar phase may also be due to associated processes at the dayside magnetopause causing a loss of 90° pitch angle particles. The present observations, based on ∼100 events, appear to provide a predictive tool for assessing the probability of occurrence of a substorm.
Interplanetary magnetic holes: Theory
Authors: Burlaga L. F. , Lemaire J. F.
Published: February 2008

Magnetic holes in the interplanetary medium are explained as stationary nonpropagating equilibrium structures in which there are field‐aligned enhancements of the plasma density and/or temperature. Magnetic antiholes are considered to be associated with depressions in the plasma pressure. In this model the observed changes in the magnetic field intensity and direction are due to diamagnetic currents that are carried by ions which drift in a sheath as the result of gradients in the magnetic field and in the plasma pressure within the sheath. The thickness of the sheaths that we consider is approximately a few ion Larmor radii. An electric field is normal to the magnetic field in the sheath. Solutions of Vlasov's equation and Maxwell's equations are presented which account for several types of magnetic holes, including ‘null sheets,’ that have been observed.
Theory of the interaction of gravity waves with O2(¹Σ) airglow
Authors: Weinstock J.
Published: February 2008

A theory is developed which determines the effect of gravity waves upon chemical reactions in the atmosphere. This theory is then applied in detail to O2(¹Σ) airglow measurements made by Noxon. A theoretical result for OH is given without details. It is shown that fluctuations of O2(¹Σ) airglow can be used to study gravity waves and enhanced diffusion in the vicinity of the atomic oxygen layer (roughly 100 km). This study requires a two‐fluid consideration of gravity waves because the fluctuations of atomic oxygen density are not necessarily in phase with the fluctuations of the major constituent. Neither are the amplitudes equal. It is found that the temperature dependence of the reaction rate constant for O2(¹Σ) and the shape of the atomic oxygen layer both have a major influence on fluctuations of O2(¹Σ) airglow. Inversely, both can be estimated from observations of O2(¹Σ) airglow fluctuations. Eddy diffusion coefficients are estimated from the rate at which the observed airglow intensity increases with time under the assumption that the airglow is horizontally uniform. For OH a separate theoretical calculation is made of the ratio of brightness fluctuations to temperature fluctuations. An important conclusion to be emphasized is that the observed temperature provides a much more reliable diagnostic for studying dynamics than does the observed brightness of O2(¹Σ).
The energy balance of the nighttime thermosphere
Authors: Glenar David A. , Bleuler Ernst , Nisbet John S.
Published: February 2008

Energy balance considerations for the thermosphere are affected by the uncertainty of the solar EUV flux. The problem is simplified by investigating only the nighttime thermosphere. Vapor trail measurements provide wind data at the sunset and sunrise terminators and near the polar cap boundary. Temperature and density profiles were obtained from incoherent scatter and Ogo 6 measurements. Based on the available data the energy losses are estimated to be 8 × 1010 W due to downward conduction at 120 km and 3 × 1010 W due to infrared radiation. The energy carried by the measured winds from the polar cap is found to be 4.6 × 1010 W and that transported across the mid‐ and low‐latitude terminators, 7.6 to 11 × 1010 W. The greatest uncertainty in this analysis was in the estimates of the differences in the densities and temperatures in the lower thermosphere at the dawn and dusk terminators. Because of the large rotational velocity of the earth, small differences in these parameters correspond to large amounts of energy transported to the night hemisphere.
Radar observations of two‐dimensional turbulence in the equatorial electrojet, 3. Nighttime observations of Type 1 waves
Authors: Farley D. T. , Fejer B. G. , Balsley B. B.
Published: February 2008

The large vertically directed 50‐MHz radar at Jicamarca has sufficient sensitivity to study nighttime echoes from the equatorial electrojet in detail with good resolution. Here we concentrate on type 1 (‘two‐stream’) echoes. We find that (1) these echoes sometimes dominate the spectrum (which is never the case in daytime for a vertically directed radar), (2) they are observed over a much wider range of altitudes than they are during daytime, (3) they show an asymmetry which reverses from day to night (downgoing waves are more common at night), (4) the magnitude of the mean Doppler shift appears to increase somewhat with altitude, and (5) there is evidence of large‐scale (kilometers) structure as well as intermediate (tens or hundreds of meters) and small (the wavelengths of meters or less which are directly responsible for radar echoes). We describe a model which qualitatively accounts for all the important characteristics of the observations. Long and intermediate waves with roughly horizontal k are initially unstable, and a complicated two‐dimensional energy cascade process develops, with energy transfers probably going in both directions in k space. The asymmetry in the echoes can be explained if either the density depletions at the intermediate wavelengths are stronger than the enhancements or the positive density gradients are steeper than the negative.
Centrifugal distortion of the Jovian magnetosphere by an equatorially confined current sheet
Authors: Hill T. W. , Carbary J. F.
Published: February 2008

Ions from the Galilean satellites form a thin equatorial current sheet as they diffuse outward in the Jovian magnetosphere. We present here a self‐consistent quantitative model of the magnetic field perturbation produced by such ions, assuming loss‐free radial transport, strong equatorial confinement, and a static balance between magnetic tension and the corotational centrifugal force. The resulting field configuration exhibits a sharp cusplike distortion near the equatorial plane, resembling qualitatively the ‘magnetodisc’ configuration inferred from Pioneer encounter data. The model contains one adjustable parameter that is related to the (unknown) mass injection rate at the satellite(s); the value of this parameter determines the distance at which the outward distortion becomes important. Comparison of observed and model field configurations may provide a means of estimating the rate of mass injection from the satellites.
The role of O‐type neutral lines in magnetic merging during substorms and solar flares
Authors: Stern David P.
Published: February 2008

Observational evidence suggests that magnetospheric substorms may be associated with the formation of a pair of neutral points or lines in the geomagnetic plasma sheet, containing an X‐type point (or line) and an O‐type one. While magnetic merging theory has concentrated almost entirely on X‐type neutral configurations (points, lines, or sheets), here the role of O‐type configurations is examined, with special attention to three points: (1) How does the X‐O configuration extend in three dimensions? To this end, an analytical model of the configuration was derived, useful for visualizing the geometry and for numerical treatment of plasma flows in it. (2) What modifications are needed in the MHD condition E = v × B near the O‐type line, where it tends to make v grow without limit? By analyzing equations of motion for charged particles near an O‐type neutral line and their solutions in limiting cases, it was found that at a certain distance from the neutral line the mean particle motion became decoupled from that of magnetic field lines (which obeys the MHD condition). The decoupling distance depended on initial conditions in momentum space, suggesting that the MHD approximation which averages out such conditions may not suffice for describing plasma dynamics near the neutral line. Similar problems arise with merging flows near X‐type neutral lines, and although the treatment there is more difficult and requires more approximations, it appears that the same qualitative conclusions apply there as well. (3) What is the role of O‐type neutral lines in particle acceleration? It was found that after inflowing particles are decoupled from the field line motion, they go over to a mode of runaway acceleration along the neutral line. This process is much more efficient along an O‐type line than along an X‐type line, and it is concluded that if merging occurs at an X‐O pair, two particle populations may be expected: low‐energy particles accelerated adiabatically by earthward convection past the X‐type line, dependent mainly on the total amount of flux which has been merged, and high‐energy particles converted toward the O‐type line and undergoing there runaway acceleration. The second acceleration process depends critically on the rapidity of merging and is therefore expected to vary considerably from event to event. All this agrees with observations, and similar processes may also be important in solar flares, where a ‘Y‐type neutral point’ has been proposed, which actually represents the limiting form of an X‐O configuration.
Prediction of solar flare proton spectral slope from radio burst data
Authors: Bakshi Pradip , Barron William
Published: February 2008

We have studied the correlations between the width of the U‐shaped peak flux density spectra of solar radio bursts and the slope of the associated proton energy spectra observed by satellites. We find the wider radio spectra U's lead to shallow proton energy slopes (harder spectrum), while narrower U's lead to steeper slopes (softer spectrum). Out of the straight line, power law, and exponential forms used to study the correlations, the power law form yields the best correlation (r ≈ 0.77). This leads to a practical prediction scheme which can be used in real time for forecasting the spectral character of the protons that can be expected to arrive in the vicinity of the earth on the basis of the continuously monitored radio burst data.
Photoelectron fluxes in the Martian ionosphere
Authors: Mantas G. P. , Hanson W. B.
Published: February 2008

The equilibrium photoelectron distribution in the upper atmosphere of Mars is calculated and compared with a typical theoretical model of the photoelectron distribution in the upper atmosphere of the earth. The neutral upper atmosphere and ionosphere models used in the calculations for Mars are based on the Viking 1 neutral mass spectrometer and retarding potential analyzer data. The corresponding models in the case of the earth are based on Arecibo incoherent scatter data. While the primary photoelectron spectra in Mars' and earth's ionospheres are remarkably similar in spectral shape and finer structure features, the Martian equilibrium photoelectron spectrum is more structured and much softer than the earth's. The equilibrium photoelectron spectrum is used to investigate the loss of the photoelectron kinetic energy to the various constituents of the upper atmosphere of Mars, including the heating of the ambient electron gas. The ionization rates by photoelectron impact are calculated and are found to contribute about 30% to the total ionization rate on Mars. Photoelectron impact excitation rates of the A²Πu and B²Σu+ states of CO2+ and the a³Π and A¹Π states of CO are calculated and compared with the corresponding excitation rates by other mechanisms. Photoelectron impact excitation is found to contribute 20–30% to the CO2+ and CO airglow of Mars.