bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal variations of solar EUV, UV, and 10,830‐Å radiations
Authors: Donnelly R. F. , Hinteregger H. E. , Heath D. F.
Published: February 2008

The temporal characteristics of the full‐disk chromospheric EUV fluxes agree well with those of the ground‐based measurements of the chromospheric He I absorption line at 10,830 Å and differ systematically from those of the coronal EUV and 10.7‐cm flux. The ratio of the flux increase during the rise of solar cycle 21 to that during solar rotation variations is uniformly high for the chromospheric EUV and corroborating 10,830‐Å fluxes, highest for the transition region and “cool” coronal EUV fluxes (T < 2×106 °K), and lowest for the “hot” coronal EUV and 10.7‐cm flux. The rise and decay rates of episodes of major activity progress from those for the hot coronal EUV lines and the 10.7‐cm flux to slower values for the chromospheric H Lyman alpha line, 10,830‐Å line, and photospheric 2050‐Å UV flux. We suggest that active region remnants contribute significantly to the solar cycle increase and during the decay of episodes of major activity. The ratio of power in 13‐day periodicity to that for 27 days is high (1/3) for the photospheric UV flux, medium (1/6) for the chromospheric EUV and 10,830‐Å fluxes, and small to negligible for the hot coronal EUV fluxes. These ratios are used to estimate the dependence of active region emission on the solar central meridian distance for chromospheric and coronal EUV fluxes.
New models for fast steady state magnetic reconnection
Authors: Priest E. R. , Forbes T. G.
Published: February 2008

A new unified family of models for incompressible, steady state magnetic reconnection in a finite region is presented. They are obtained by expanding in powers of the Alfvén Mach number and may be used to elucidate some of the puzzling properties of numerical experiments on reconnection which are not present in the classical models. The conditions imposed on the inflow boundary of the finite region determine which member of the family occurs. Petscheklike and Sonneruplike solutions are particular members. The Sonneruplike regime is a special case of a weak slow mode expansion in the inflow region, and it separates two classes of members with reversed currents. These are the hybrid regime with a mixture of strong fast mode and slow mode expansions and the flux pileup regime with a strong slow mode expansion, in which the magnetic field strength increases as it approaches the diffusion region and the flow diverges. The Petscheklike regime is a singular case of a weak fast mode expansion, and it separates the hybrid regime from a regime of slow mode compressions. The hybrid expansions are fast mode in character in the center of the inflow and slow mode near the edges of the region, while the flux pileup expansions possess long thin diffusion regions and no maximum reconnection rate. The maximum rate is calculated for the other solutions as a function of the magnetic Reynolds number and compared with the classical Sweet‐Parker and Petschek rates. For the flux pileup and hybrid regimes, reconnection can be much faster than the maximum Petschek rate. Care should be taken in deciding which type of reconnection is operating in a numerical experiment. Indeed, no experiment to date has used boundary conditions appropriate for demonstrating steady state Petschek reconnection.
Parametric instabilities of circularly polarized Alfvén waves including dispersion
Authors: Wong H. K. , Goldstein M. L.
Published: February 2008

A class of parametric instabilities of large‐amplitude, circularly polarized Alfvén waves is considered in which finite frequency (dispersive) effects are included. The dispersion equation governing the instabilities is a sixth‐order polynomial which is solved numerically. As a function of K ≡ k/k0 (k0 and k are the wave number of the “pump” wave and unstable sound wave, respectively), there are three regions of instability: A modulation instability at K < 1, a decay instability at K > 1, and a relatively weak and narrow instability at K ≃ 1. As a function of β ≡ cs²/υA² (where cs and υA are the sound and Alfvén speeds, respectively), the modulational instability occurs when β < 1 (> 1) for left‐hand (right‐hand) pump waves, in agreement with the previous results of Sakai and Sonnerup (1983). For β ≤ 1, dispersive effects tend to enhance the decay instability for right‐hand waves and to reduce it for left‐hand waves. For β ≥ 1 this tendency is reversed. For right‐hand waves the decay instability is dominant for β ≤ 1, while the modulational instability dominates for sufficiently high β. The growth rate of the decay instability of left‐hand waves is greater than the modulational instability at all values of β. Applications to large‐amplitude waves observed in the solar wind, in computer simulations, and in the vicinity of planetary and interplanetary collisionless shocks are discussed.
Ionospheric convection signatures observed by De 2 during northward interplanetary magnetic field
Authors: Heelis R. A. , Reiff P. H. , Winningham J. D. , Hanson W. B.
Published: February 2008

Observations of the ionospheric convection signature at high latitudes are examined during periods of prolonged northward interplanetary magnetic field (IMF). The data from Dynamics Explorer 2 show that a four‐cell convection pattern can frequently be observed in a region that is displaced to the sunward side of the dawn‐dusk meridian regardless of season. In the eclipsed ionosphere, extremely structured or turbulent flow exists with no identifiable connection to a more coherent pattern that may simultaneously exist in the dayside region. The two highest‐latitude convection cells that form part of the coherent dayside pattern show a dependence on the y component of the IMF. This dependence is such that a clockwise circulating cell displaced toward dawn dominates the high‐latitude region when By is positive. Anticlockwise circulation displaced toward dusk dominates the highest latitudes when By is negative. Examination of the simultaneously observed energetic particle environment suggests that both open and closed field lines may be associated with the high‐latitude convection cells. On occasions these entire cells can exist on open field lines. The existence of closed field lines in regions of sunward flow is also apparent in the data.
The wave normals of ELF hiss emissions observed onboard GEOS 1 at the equatorial and off‐equatorial regions of the plasmasphere
Authors: Hayakawa M. , Parrot M. , Lefeuvre F.
Published: February 2008

The wave normal directions of plasmaspheric ELF hiss have been determined at the equatorial and off‐equatorial regions of the plasmasphere by using the data observed by GEOS 1 satellite. Two different methods of direction findings have been utilized: (1) the maximum entropy method for estimating the wave distribution functions and (2) the maximum likelihood method to estimate the propagation peaks. Three equatorial events have been analyzed, and it is found that just inside the plasmapause, the wave normal θ of ELF hiss is nearly aligned with the magnetic field, and when the observing position is 0.3–0.5 RE inside from the plasmapause, there are two different groups of wave normal angles; one is a medium wave normal angle ranging from 20° to 60° and the other is a large wave normal in a range from 70° to 80°, slightly smaller the Gendrin angle. Then, off‐equatorial observations have been made at geomagnetic latitudes 30° to 24° for L = 4.66 to 2.90 inside the plasmasphere. The direction findings have yielded that the waves are always composed of two plane waves; both main and secondary peaks take very large wave normal angles, θ = 80°–85°. The present direction finding results of equatorial ELF hiss have been extensively compared with the previous theoretical model to discuss the generation mechanism of plasmaspheric ELF hiss. Then, it is concluded that some of the plasmaspheric hiss emissions are generated by the cyclotron instability with θ ≅ 0°, initially proposed by Thorne et al. (1973), but majority of ELF hiss emissions are generated at large wave normals.
Ordering of momentum transfer along VѵB in the AMPTE solar wind releases
Authors: Chapman S. C. , Dunlop M. W.
Published: February 2008

As part of the active phase of the AMPTE mission, clouds of neutral lithium and barium were released into the solar wind from the German Ion Release Module (IRM). In all cases the clouds ionize significantly in the first few seconds following the release, such that the solar wind flow is deflected. A magnetic cavity (field zero) is formed, but this is not associated with a total exclusion of solar wind protons. The close (~35 km for lithium, ~100–200 km for barium) proximity of another spacecraft, the United Kingdom Subsatellite (UKS), allowed good data coverage of the local environment. In the events the minimum Larmor radii of the solar wind protons is of the order of the size of the interaction region of the release; that of "picked up" release ions being one (lithium) or two (barium) orders of magnitude larger, which indicate the need for a hybrid description in which the release ions respond as particles to the magnetic field structures, supported by a charge‐neutralizing electron fluid. We highlight important results from the data which, while invalidating a description using MHD theory, are consistent with an ordering of the momentum transfer approximately along the local vѵB direction. We suggest that the existence of high proton flows in the interaction region requires a description which features at least two ion populations, and we show that this ordering may be predicted by treating the ions as a multifluid.
Focusing anisotropy of solar cosmic rays
Authors: Bieber J. W. , Evenson P. A. , Pomerantz M. A.
Published: February 2008

Cosmic ray transport theories that incorporate the effect of adiabatic focusing into the usual pitch angle scattering formalism suggest that the commonly assumed first‐order (cosine of pitch angle) anisotropy should be replaced by an exponential function in which the scattering mean free path λ and the index q, which characterizes the dependence of the scattering rate upon pitch angle, appear as parameters. The solar cosmic ray event of February 16, 1984, which exhibited an unusually large, persistent anisotropy, provides an ideal observational basis for testing this theoretical prediction. Analysis of energetic particle data from the International Cometary Explorer and from eight high‐latitude neutron monitors indicates that an exponential with λ > 2 AU and with q in the range 1.3–1.7 does indeed provide a significantly better description of particle anisotropies than the first‐order form. Because the local interplanetary mean free path is determined purely from the form of the angular distribution, that is, without reference to the shape of the temporal density profile, the technique utilized here allows the relative influences of coronal versus interplanetary transport processes to be more cleanly separated. For the February 16 event the time profile was shaped primarily by coronal processes. Specifically, coronal diffusion with a rigidity‐independent diffusion coefficient ∼1018 cm² s−1 and with velocity‐dependent escape provides a good fit to both the ICE and the neutron monitor data.
Interaction of upgoing auroral H+ and O+ beams
Authors: Kaufmann R. L. , Ludlow G. R. , Collin H. L. , Peterson W. K. , Burch J. L.
Published: February 2008

Data from the S3‐3 and DE 1 satellites are analyzed to study the interaction between H+ and O+ ions in upgoing auroral beams. Every data set analyzed showed some evidence of an interaction. The measured plasma was found to be unstable to a low‐frequency electrostatic wave that propagates at an oblique angle to
. A second wave, which can propagate parallel to
, is weakly damped in the plasma studied in most detail. It is likely that the upgoing ion beams generate this parallel wave at lower altitudes. The resulting wave‐particle interactions qualitatively can explain most of the features observed in ion distribution functions. H+ ions in the low‐velocity tail of the H+ beam are decelerated and O+ ions in the high‐velocity tail of the O+ beam are accelerated through this process. This may explain the occasional observation of O+ beams with substantially higher streaming energies than the simultaneously observed H+ beams. The wave‐particle interactions also can explain the asymmetrical features that regularly are seen in H+ and O+ distribution functions. The wave which was found to be unstable in the measured plasma primarily accelerates O+ perpendicular to Bo. This process can produce weakly conical O+ distributions which are characterized by T⊥ > T∥ rather than by having a peak at 90° pitch angle within the acceleration region. In general, the interaction between different ion species in multicomponent beams appears to be a dominant factor in determining the structure of upgoing auroral ions. The same process is likely to be important in a number of other beam‐containing space plasmas.
First direct ground‐based measurements of electron drift in the ionospheric F region
Authors: Behnke Richard A. , Ganguly S.
Published: February 2008

For the first time, vertical electron drift velocities in the F region of the ionosphere have been directly determined using the Doppler shift of the electron component of the incoherent‐scatter spectra. Simultaneous measurements of ion drift velocities were also made. A large difference between the electron and ion velocities of the order of 100 ms−1 with corresponding currents of 10−5 Am−2 was observed at sunrise. The most likely source of these currents appears to be the action of the local thermospheric wind on the large conductivity gradients associated with the sunrise line.
Bidirectional solar wind electron heat flux and hemispherically symmetric polar rain
Authors: Gosling J. T. , Baker D. N. , Bame S. J. , Zwickl R. D.
Published: February 2008

The nearly continuous precipitation of low‐energy electrons into the earth's polar caps is known as polar rain. Normally polar rain is much more intense in one polar cap than in the other. Recently Makita and Meng have shown that polar rain is occasionally (approximately once a month for a period of ∼10 hours at a time) observed at high intensity levels simultaneously in both polar caps. Such events are known as hemispherically symmetric polar rain. We have examined ISEE 3 solar wind electron data obtained concurrent with reported symmetric polar rain events and have found that a bidirectional solar wind electron heat flux is present whenever such polar cap events occur. We interpret this result as a demonstration that the solar wind heat flux is the prime source of polar rain. A bidirectional solar wind heat flux is evidence that the local interplanetary magnetic field is part of either a magnetic bottle rooted at both ends in the sun or a closed magnetic loop entirely disconnected from the sun. Thus, we infer that, in contrast to the normal situation when only one of the earth's polar caps is magnetically connected to the sun, during hemispherically symmetric polar rain events either both of the earth's polar caps are magnetically connected to the sun, or else both are connected to a magnetic loop which is entirely disconnected from the sun. The relative timing between bidirectional solar wind heat flux and symmetrical polar rain events can be utilized to determine certain magnetospheric quantities such as the cross‐tail convection speed.
Ultraviolet nightglow production near the magnetic equator by neutral particle precipitation
Authors: Abreu V. J. , Eastes R. W. , Yee J. H. , Solomon S. C. , Chakrabarti S.
Published: February 2008

The near‐midnight latitudinal (±45) distribution of the O I 911‐Å, 1304‐Å, and 1356‐Å emissions, observed by the extreme ultraviolet (EUV) spectrometer on the STP 78‐1 satellite, have been analyzed and compared as a function of geomagnetic activity. The dominant source of these emissions is radiative recombination of atomic oxygen ions; however, for geomagnetically disturbed periods, excess O I 1304‐Å and 1356‐Å emission is observed within ±5° of the dip equator. Neutral particle precipitation from the ring current is discussed as a possible source of the excess O I 1304‐Å and 1356‐Å emission.
Formation of a compound stream between 0.85 AU and 6.2 AU and its effects on solar energetic particles and galactic cosmic rays
Authors: Burlaga L. F. , McDonald F. B. , Schwenn R.
Published: February 2008

During September 1979, Helios B (HB) and Voyager 1 (V1) were near 0.85 AU and 6.2 AU, respectively, and they were radially aligned with respect to the sun within 20°. A slow transient stream, two corotating streams, and two unusually fast transient streams passed HB during a 17‐day interval in that order. (A stream is defined here as a flow in which the speed is always greater than 475 km/s.) As the streams moved outward from HB to V1, they coalesced to form a single large “compound stream” which moved past V1 in 20 days. The compound stream was produced by motion of fast plasma away from slower plasma behind and by the interaction of the fast streams with slower streams ahead. The maximum speed observed was 1270 km/s at HB and 860 km/s at V1. Closely associated with this compound stream was one of the largest solar energetic particle events (protons 0.4–69 MeV) observed beyond 5 AU. The maximum intensity was in the region of increasing speed. The intensity‐time profile of the solar energetic particles was closely related to the speed profile. There is evidence of channeling and trapping of particles in a region following the highest speeds, where the magnetic field was strong and the magnetic field lines were nearly radial. The high‐energy protons (>150 MeV) and MeV electrons are believed to be accelerated in solar flare events. Interplanetary acceleration appears to play a major role in the low‐energy particle population. The galactic cosmic ray intensity remained low throughout the passage of the compound stream, recovering slowly as the speed decreased. The long “recovery time” is thus related to the large extent of the flow.
Multiple X line reconnection: 3. A particle simulation of flux transfer events
Authors: Ding D. Q. , Lee L. C. , Fu Z. F.
Published: February 2008

The multiple X line reconnection process, which was proposed by Lee and Fu to explain the flux transfer events observed at the dayside magnetopause, is studied by computer simulations based on a two‐dimensional magnetostatic particle code. A driven boundary condition is applied at the two sides of the simulation domain with an incoming plasma flow while a free boundary condition is used at the other two sides with an outflow. Repeated formation and convection of magnetic islands are observed in the simulation. Magnetic islands with various sizes are observed. Superthermal ions of energy 5–50 keV, with an energy spectrum f(E)∼E−1.9, are generated during the multiple X line reconnection process. It is found that the power spectrum of the normal magnetic field is given by a power law PB ∼ k−0.6, where k is the wave number. The convection speed of magnetic islands may exceed the Alfvén speed. The recurrence time of magnetic islands is found to be τ ≃ 100/ΩRo, where Ω is the ion gyrofrequency, Ro = V1/VA, V1 is the imposed inflow speed, and VA is the Alfvén speed. Our results are consistent with satellite observations of flux transfer events.
Ray tracing survey of Z mode emissions from source regions in the high‐altitude auroral zone
Authors: Menietti J. D. , Lin C. S.
Published: February 2008

Three‐dimensional ray tracing of Z mode radiation from sources in the auroral zone has been performed. A plasma model which includes a background ambient plasma, plasmasphere, and an optional region of field‐aligned current is used. In the source region the waves are assumed to be excited by the cyclotron maser mechanism for frequencies f ≲ fg (where fg is the electron gyrofrequency) and for wave normal angles in the range 83° < ψ < 90°; we have also considered cases when the source region was located near the center of a region of field‐aligned current. A number of different altitudes and a range of different field lines are used. In addition, we examine cases unrestricted by the cyclotron maser mechanism. Our results indicate that propagation is primarily perpendicular to the magnetic field line (horizontal propagation) for almost all sources considered, except when the wave source is assumed to be unrestricted by the cyclotron maser mechanism (f/fg < 0.9; and ψ < 80°). While the propagation is initially downward, refraction rapidly increases the wave normal angle, and only propagation at near‐constant altitudes occurs. While smaller wave normal angles (ψ ∼ 84°) produce more downward propagation, the frequency bandwidth estimated from the ray paths is found to be less than that observed by DE 1. Large horizontal magnetic field gradients associated with field‐aligned currents produce downward propagation, but still not sufficient to produce the observed bandwidths. These results indicate that wave growth may be due to mechanisms in addition to cyclotron maser resonance. In order to produce the observed bandwidth (Δf/fg ∼ 0.5), it is necessary that f/fg be as low as 0.7 and ψ be as low as 75°.
Heliospheric current sheet and its solar cycle variations
Authors: Akasofu Syun‐Ichi , Fry Craig D.
Published: February 2008

The three‐dimensional structure of the heliospheric current sheet is constructed for Carrington rotations 1647, 1654, 1661, 1667, 1693, 1699, and 1719. These rotations are chosen from different epochs during the sunspot cycle 21, so that one can infer geometrical changes of the current sheet during a sunspot cycle. Only for relatively simple neutral lines on the source surface, do the structures of the current sheet approximate to those envisaged by Svalgaard and Wilcox [1976] and Thomas and Smith [1981].
Absence of hydromagnetic waves in the magnetospheric equatorial region conjugate with pulsating auroras
Authors: Oguti Takasi , Hayashi Kanji , Yamamoto Tatsundo , Ishida Juro , Higuchi Tomoyuki , Nishitani Nozomu
Published: February 2008

Magnetic field variations measured by the GOES 2 and 3 satellites are examined at the time when auroras pulsate in their conjugate ionospheric regions. No hydromagnetic waves are found to be related to the pulsating auroras by the satellites. The result indicates that the mechanism leading to pulsating precipitation of electrons, which causes pulsating auroras, does not require the cooperation of hydromagnetic waves in the magnetospheric equatorial region.
Differential excitation cross section of atomic oxygen by electron impact: (³P‐¹S transition)
Authors: Shyn T. W. , Cho S. Y. , Sharp W. E.
Published: February 2008

Absolute differential excitation cross sections of atomic oxygen (³P‐¹S transition) by electron impact have been measured by a crossed‐beam method. A thermal dissociation method was used for generating atomic oxygen. The energy and angular range measured were from 10 to 30 eV and from 30° to 150°, respectively. A several theoretical results were compared with the present measurements. It is found that the present results do not agree with the theoretical predictions in angular distribution as well as magnitude of total cross sections. The theoretical values are generally smaller than the measurements by a factor of two.
Ray tracing of Jovian decametric radiation from southern and northern hemisphere sources: Comparison with Voyager observations
Authors: Menietti J. Douglas , Green James L. , Six N. Frank , Gulkis S.
Published: February 2008

Because of a lack of readily usable information pertaining to the polarization of the Voyager 1 and 2 high‐frequency band data, a technique has been developed that aids the identification of Io‐dependent decametric radiation originating from the southern hemisphere of Jupiter. This technique compares the results of model ray tracing calculations with the Planetary Radio Astronomy (PRA) observations. A large portion of the Voyager 1 and 2 PRA observations are sorted into bins (±3° wide) centered on a specific Io central meridian longitude. When the data are plotted (as a frequency‐longitude spectrogram) in this coordinate system, Io‐dependent features can be identified and compared with ray tracing calculations performed in a model Jovian magnetosphere where it is assumed that the decametric emissions are generated in the RX mode from low‐altitude source regions along the instantaneous Io flux tube. Two different magnetic field models are used, and the results are contrasted. In this study, we compare the observations for constant sub‐Io longitudes of 260° and 300° with the corresponding model ray tracings. The results permit the identification of decametric spectral features from source locations in both the northern and southern hemispheres: (1) The emission traditionally designated “Io‐B” originates at the Io flux tube footprint in the northern hemisphere when the sub‐Io system III longitude λIII is equal to 260°. (2) The component traditionally designated “Io‐C” is a combination of emissions emanating from the Io flux tube footprints in both northern and southern hemispheres when Io is located at longitudes 260° and 300°. (3) The traditional “non‐Io‐A” emission is, in fact, Io related at both Io configurations studied. When Io is located at λIII = 260°, this emission originates in the southern hemisphere flux tube footprint. When Io is at λIII = 300°, this component (“non‐Io‐A”) originates from the flux tube footprint in the northern hemisphere.
Gyroresonant pitch angle scattering by coherent and incoherent whistler mode waves in the magnetosphere
Authors: Inan Umran S.
Published: February 2008

A test particle approach is used to compare gyroresonant pitch angle scattering of energetic electrons by coherent versus incoherent whistler mode waves, for the case in which the coherent wave amplitude is below the nonlinear phase trapping threshold. Wave packets of 400 ms duration propagating along the magnetic field at L = 4 within the plasmasphere are considered, and the wave‐induced pitch angle scattering along the propagation path from one hemisphere to the other and the resulting precipitation flux are computed. An incoherent wave spectrum is simulated by random modulation of the wave frequency at intervals of 1 ms, thereby generating signals with nearly constant power spectral density over a bandwidth of 2 kHz centered at 5.5 kHz. The associated pitch angle scattering is compared with that of a monochromatic 5.5‐kHz signal of 400 ms duration. Results of the test particle analysis are compared with those expected on the basis of a classical diffusion treatment, and an expression is derived for an effective “diffusion” coefficient for pitch angle scattering by coherent waves. The trajectory followed by a particle when interacting with incoherent waves essentially represents a random walk in velocity space, while for coherent waves the pitch angle of the particle varies in a well‐defined manner. In spite of the fact that individual particle scatterings are typically larger for coherent waves, the peak precipitation fluxes induced by incoherent waves are found to be approximately the same as those for coherent waves having the same total power. This results from the fact that incoherent waves interact with particles over a wider range of energies. As a consequence, the energy spectrum and the temporal extent of transient precipitation pulses due to incoherent wave packets are broader than those for equivalent coherent ones.
On the role of IMF By in generating the electric field responsible for the flow across the polar cap
Authors: Vennerstrøm Susanne , Friis‐Christensen Eigil
Published: February 2008

During periods of southward interplanetary magnetic field (IMF) we have examined the relationship between magnetic variations in the central polar cap and the IMF By and Bz components. The geomagnetic polar cap index PC that can be used as a measure of the flow across the polar cap has been derived using data from Thule in the IMS period. The results have been compared with IMP 8 measurements of the IMF and the solar wind velocity. The statistical analysis shows that the absolute value of the azimuthal component |By| contributes to the cross‐polar cap flow in the same manner as the southward component Bs. The relative contributions of |By| and Bz have been examined and compared with the theoretical expression υBT sin² θ/2 for the merging electric field. It is found that the contribution of |By| compared to Bz is only half as big in the observations as in the theoretical expression. The By effect on PC is compared to an earlier reported effect of By on the geomagnetic index AL (Murayama et al., 1980) and found to be quite different from this. This is discussed in relation to interpretations in terms of merging site asymmetry.
Electron dynamics and potential jump across slow mode shocks
Authors: Schwartz Steven J. , Thomsen Michelle F. , Feldman William C. , Douglas Fraser T.
Published: February 2008

In the de Hoffmann‐Teller reference frame, defined so that the bulk velocities are aligned with the magnetic field in the uniform media on either side of a shock transition, the cross‐shock electric field is simply the thermoelectric field responsible for preserving charge neutrality. As such, it gives information regarding the heating and dissipation occurring within the shock. The total cross‐shock potential can be determined by integrating a weighted electron pressure gradient through the shock, but this requires knowledge of the density and temperature profiles. Here we exploit a recently proposed alternative approach relying on particle dynamics to provide an independent estimate of this potential. We apply both determinations to slow mode shocks which form the plasma sheet boundary in the deep geomagnetic tail as observed by ISEE 3. The two methods correlate well. There is no indication of the expected transition from resistive to viscous shocks, although the highest Mach number shocks show the highest potentials. We have not found any strong dependence on the various shock parameters, but this may be due in part to the relatively uniform sample of shocks, all of which lie close to the switch‐off limit, and in part to the lack of definitive thermal ion data. We discuss the implications of these results for the electron dissipation mechanisms and turbulence at the shock.
Weak electrostatic waves near the upper hybrid frequency: A comparison between theory and experiment
Authors: Horne R. B. , Christiansen P. J. , Gough M. P.
Published: February 2008

Two types of generating mechanisms have been proposed to explain the observations of weak electrostatic waves near the local upper hybrid frequency fuh. In this paper data from the wave and plasma diagnostic experiments on GEOS 1 are presented with simultaneously observed particle data in an attempt to identify the generating mechanism. The electron data contain two sources of free energy: a highly anisotropic "pancake" distribution in the warm (20 eV to ∼1 keV) plasma and a loss cone type distribution in the hot (>few keV) plasma. This leads us to consider a coherent generating mechanism. A model of the plasma distribution, with cold, warm and hot components is constructed and linear instability calculations are presented. It is shown that the relatively large resonant perpendicular velocities in the loss cone distribution generate instabilities at small perpendicular wave numbers preferentially in the upper hybrid band since the wave frequency can lie midway between the gyroharmonics where cyclotron damping is reduced. The calculations are able to reproduce the spectral characteristics observed by previous spacecraft over a range of cold plasma densities. The temporal growth rates are comparable to those obtained previously for strong electrostatic emissions near fuh, but it is shown that the unstable wave number region is an order of magnitude smaller. Thus it is argued that the waves remain weak since they are quickly refracted out of resonance. It is demonstrated that when there is a significant amount of warm plasma present the strongest natural emission can be used to derive the total plasma density, cold plus warm plus hot, and not the cold plasma density, and that the warm plasma introduces new regions of small group Velocities which result in fine structure in the wave spectrum. The effects of the warm plasma and particularly the pancake distribution are discussed in connection With the ray propagation of these and other classes of much stronger emissions.
Evaluation of auroral 5577‐Å excitation processes using Intercosmos Bulgaria 1300 satellite measurements
Authors: Gerdjikova Mariann G. , Shepherd Gordon G.
Published: February 2008

Energy transfer from N2(A³Σu+) to O has been extensively studied as a mechanism for the production of 5577‐Å emission in the aurora, but it has yet to be fully accepted as the dominant process. This reluctance arises from the three requirements that must be met. First, the correct absolute emission rate must be produced by this mechanism. Second, the lifetime of N2(A) at auroral altitudes must be compatible with the observed time delays between the N2+ and 5577‐Å emissions. Finally, the generally accepted high degree of covariance between the N2+ and 5577‐Å emissions must be explained. This last aspect is the subject of this paper, in which we investigate this covariance in terms of spectral ratios. Measurements made from the Intercosmos Bulgaria 1300 satellite include the emission rate of the O I 5577‐Å and N2+ 4278‐Å emissions, and the flux of precipitating auroral electrons in the energy range 200 eV to 15 keV. Using the measured fluxes and atmospheric composition models, the emission rate ratio I(5577)/I(4278) for the N2(A) energy transfer mechanism is calculated and compared with the observed values. The observed ratios vary mostly from 2 to 5, with a few values outside this range. We show that this variation arises from variations in the atomic oxygen concentration, partly as an altitude effect associated with changes in primary electron energy, but more dominantly as day‐to‐day variations that are larger than those indicated by the mass spectrometer/incoherent scatter 1983 (MSIS‐83) model. Independent evidence that atomic oxygen is the source of the ratio variations is provided by correlated variations in the I(7320)/I(4278) ratio. The aspects of absolute emission rate and time delay are also reviewed, and it is concluded that the evidence is conclusively in favor of N2(A) energy transfer as the dominant auroral mechanism.
Polar convection and Birkeland Currents during strongly positive IMF By
Authors: Min‐Yun Zi , Nielsen E. , Hanson W. B. , Potemra T. A.
Published: February 2008

The TRIAD satellite passed directly over the field of view of the Scandinavian Twin Auroral Radar Experiment (STARE) radar on May 1, 1978, at approximately 0618 UT when the By component of the interplanetary magnetic field (IMF) was unusually large in comparison to the other two components. The average IMF values in the interval from 0600 to 0700 UT on this day were Bx = −1.8 nT, By = 8.7 nT, and Bz = −0.2 nT (King, 1979). The TRIAD magnetic field perturbations revealed the presence of a very intense (ΔB ∼ 500 nT) region 1 downward‐flowing Birkeland current near 0930 MLT and poleward of a much weaker upward‐flowing region 2 current. This “net” (unbalanced) Birkeland current was observed at a 67° magnetic latitude in this morning sector (near 0930 MLT). The net Birkeland current was located equatorward of a region of intense westward (antisunward) ionospheric convection flow detected by the STARE radar. We conclude that the polar cap convection flow is distorted and displaced to low latitudes (< 67° magnetic latitude) in the morning sector during this period of strongly positive By and almost negligible Bx and Bz. The large net region 1 Birkeland current is associated with the convection flow reversal. Convection velocity data acquired by the AE‐C satellite on October 29, 1978, near 1723 UT when Bx = −0.5 nT, By = 8.3 nT, and Bz = −2.7 nT (average values during the 1700 to 1800 UT interval) show the convection reversal near 67° invariant latitude and 0800 MLT (King, 1979). These two examples support previous suggestions for the important influence that the By component of IMF has on the intensity and location of high‐latitude convection and Birkeland currents.
Optical and radar characterization of a short‐lived auroral event at high latitude
Authors: Jones A. Vallance , Gattinger R. L. , Shih P. , Meriwether J. W. , Wickwar V. B. , Kelly J.
Published: February 2008

Observations of optical emission intensities and incoherent scatter radar returns in the magnetic zenith were compared in a study carried out at Sondre Stromfjord (Λ = 76.1°) in Greenland. The results were used to test the consistency of a theoretical model of ion chemistry and optical emissions in aurora and to explore the accuracy of relations between optical measurements and the average energy of the incident electrons. The incident primary electron spectrum and its temporal variation were inferred from zenith electron density profiles from the radar. The inferred primary energy spectrum at the peak intensity of the event approximated a Maxwellian distribution of characteristic energy 1.3 keV accelerated by an energy increment between 2 and 5 keV. Average energies inferred from the radar electron density profiles, from the N2+ rotational temperature and the I(6300)/I(4278) ratio were in good agreement. The variation of the I(8446)/I(4278) ratio was studied and was found to be promising as an index of average incident electron energy. An empirical relation between this ratio and average energy was derived from the data. The observed values of I(4278) exceeded the theoretical values derived from the ionization rate profiles deduced from the radar data by a factor near 2.0. Observed electron density profiles and theoretical profiles calculated from optical data were in good agreement provided that the optically inferred ion production rates were reduced by the same factor of 2. This discrepancy is probably the cumulative result of small errors in instrument calibrations, viewing geometry, recombination coefficients and the excitation and ionization cross sections used in the model.
Temperature and chemistry of the polar mesopause OH
Authors: Sivjee G. G. , Hamwey R. M.
Published: February 2008

Intensities (I) and rotational temperatures (T) of the OH(8, 3) and (6, 2) bands were derived from spectrophotometric observations of airglow emissions, over Longyearbyen in Spitsbergen, made in December 1984. The high latitude of the Spitsbergen Observatory (78°15′N) permitted 24‐hour coverage of the wintertime polar airglow. These measurements yielded the following results: (1) T derived from P1 rotational lines of OH depend on the choice of A values (T (Honl‐London, A) > T (Mies, A) > T (Espy, A)); (2) P
=,213= 2072 K 207 implying a 3‐ to 5‐K/km temperature gradient in the atmosphere around 85 km height; (3) Ī(8, 3) = 314 ± 30 R and Ī (6, 2) = 1025 ± 110 R; the corresponding OH(υ′) columnar abundances are (5.5 ± 0.6) × 108 cm−2 and (8.0 ± 0.8) × 108 cm−2 for υ′ = 8 and υ′ = 6 vibrational levels. These results are compared with the predictions of a one‐dimensional oxygen‐hydrogen model which shows that (1) the reaction rate of OH
H + O 2 may be much higher than the laboratory‐measured k6 value for OH(υ′ = 0); (2) the higher k6 value accounts for the separation of OH layers for different υ′ levels observed in various rocket measurements of OH*(υ′) profiles and implies a positive temperature gradient in the atmosphere around 85 km; (3) Llewellyn et al.’s (1978) rate for quenching of OH* by M(N2 and O2) is required in the model to match the calculated column abundance of OH* in υ′ = 8 to the value derived from the observed intensities of the airglow OH(8, 3) band; and (4) a perhydroxyl source must be invoked for the model to yield a column abundance of OH* in υ′ = 6 consistent with the intensity of the airglow OH(6, 2) band observed in Spitsbergen.
Effect of a localized minimum in equatorial field strength on resistive tearing instability in the geomagnetotail
Authors: Hau L. ‐N. , Wolf R. A.
Published: February 2008

A two‐dimensional, resistive‐MHD computer code is used to investigate the spontaneous reconnection of magnetotaillike configurations. The initial conditions adopted in our simulations are of two types: (1) in which the equatorial normal magnetic field component Bze declines monotonically down the tail, and (2) in which Bze exhibits a deep minimum in the near‐earth plasma sheet. Configurations of the second type have been suggested by Erickson (1984, 1985) to be the inevitable result of adiabatic, earthward convection of the plasma sheet. To represent the case where the earthward convection stops before the X line forms, i.e., the case where the interplanetary magnetic field turns northward after a period of southward orientation, we impose zero‐flow boundary conditions at the edges of the computational box. The initial configurations are in equilibrium and stable within ideal MHD. The dynamic evolution of the system starts after the resistivity is turned on. The main results of our simulations basically support the neutral‐line model of substorms and confirm Birn's (1980) computer studies (which used a different numerical scheme and type 1 initial configurations). Specifically, we find spontaneous formation of an X‐type neutral point and a single O‐type plasmoid with strong tailward flow on the tailward side of the X point. In addition, the results show that the formation of the X point for the configurations of type 2 is clearly associated with the assumed initial Bz minimum. Furthermore, the time interval from the turning on of the resistivity to the formation of a plasmoid is much shorter in the case where there is an initial deep minimum. A simple analytic calculation is also carried out to demonstrate why the configuration with a deep minimum is more susceptible to the development of the neutral point.
A one‐dimensional time‐dependent model of the magnetized ionosphere of Venus
Authors: Shinagawa H. , Cravens T. E. , Nagy A. F.
Published: February 2008

The behavior and time evolution of the large‐scale magnetic fields and ionospheric plasma of the dayside Venus ionosphere were studied using a one‐dimensional model. The coupled continuity, momentum, and Maxwell's equations were solved simultaneously for three ions (O+, O2+, H+) and the magnetic field. The CO2+ ion was included photochemically. The calculated magnetic field profiles are in good agreement with observations made by the Pioneer Venus orbiter magnetometer. The magnetic field structure is found to be quasi‐steady for slow changes of the solar wind dynamic pressure. The peak of the magnetic field at 165 km is maintained by the downward convection from higher altitudes. The time scale for the decay of magnetic field by the pure one‐dimensional vertical diffusion/convection process is several hours unless the magnetic field flux is resupplied from the top of the ionosphere. Good agreement was obtained between the calculated and observed plasma densities for altitudes below 250 km including the electron density “ledge” near 190 km in magnetized ionospheres. However, the significant disagreement between the calculated and observed plasma densities at high altitudes suggests that under magnetized conditions, horizontal transport processes become important in removing the plasma and/or the magnetic field even in the subsolar region.
Different phases of a magnetospheric substorm on June 23, 1979
Authors: Tanskanen P. , Kangas J. , Block L. , Kremser G. , Korth A. , Woch J. , Iversen I. B. , Torkar K. M. , Riedler W. , Ullaland S. , Stadnes J. , Glassmeier K. ‐H.
Published: February 2008

A detailed case study was carried out of a magnetospheric substorm on June 23, 1979. Satellite, balloon and ground based particle and field data were used to correlate observations covering an extended area both in longitude and in latitude. The loading phase of the substorm was characterized by an equatorward convex, southward moving precipitation boundary. During the equatorward motion a hardening of the electron spectrum is seen. We show that betatron acceleration is stronger than Fermi acceleration for earthward converting particles in a dipolelike field, and that the reverse is true for a taillike field. It is argued that the observed precipitation front represents an ionospheric mapping of an injection boundary. Energy dependent increases of the ion phase space density during the loading phase can be explained in terms of an earthward displacement of the inner edge of the neutral sheet current. Pi 2 pulsations on the satellite and on the ground are present both at substorm onset and at subsequent intensifications. Evidence is given for plausible correlation between changes in the IMF and variations in ionospheric electric fields as observed on balloons. It is suggested that changes of the IMF from south to north may have triggered the substorm and expansion phase intensifications.
Hydrogen Balmer alpha intensity distributions and line profiles from multiple scattering theory using realistic geocoronal models
Authors: Anderson D. E. , Meier R. R. , Hodges R. R. , Tinsley B. A.
Published: February 2008

The H Balmer α nightglow is investigated by using Monte Carlo models of asymmetric geocoronal atomic hydrogen distributions as input to a radiative transfer model of solar Lyman β radiation in the thermosphere and exosphere. The radiative transfer model includes all orders of scattering, temperature variation with altitude and solar zenith angle, and anisotropic velocity distributions. The influences of multiple scattering of Lyman β radiation and of observing geometry on the H Balmer α intensity and effective temperature are evaluated in detail. Morning and evening hydrogen distributions for minimum, medium, and maximum solar activity are used in calculations of nightglow emission rates and line profiles. For each of the hydrogen models the H Balmer α intensity and effective temperature are displayed as a function of solar depression angle, observation zenith angle, and azimuth relative to the sun, to make possible detailed comparison to observations. Except for rather unique observing conditions, multiple scattering effects cannot be ignored for the determination of either intensity or temperature. Observed effective temperatures which are significantly less than the exobase temperature may be due either to low‐altitude thermospheric hydrogen or to high‐altitude exospheric hydrogen and depend on the observing conditions and the level of solar activity. Previous (selected) observations of the H Balmer α nightglow and effective temperatures are compared to models which most closely represent the solar conditions of the observations. Results of the theoretical investigation of H Balmer α emissions and of model‐data comparisons indicate that predicted morning‐evening concentration variations yield small intensity variations for solar depression angles greater than 20°. The variations with solar depression angle of observed intensities and temperatures both show discrepancies in comparison with model results. A substantial disagreement occurs in absolute intensity: the long‐standing result that the observations are a factor of 2 higher than model results is reaffirmed with recent observational data and improved estimates of the solar H Lyman β flux. Observing schemes are discussed which lead to first approximations to hydrogen content independent of solar flux and to the high‐altitude atomic hydrogen distribution.