bluuebunny/crossref_metadata_2025 huggingface dataset:
A coordinated satellite and ground‐based study of an intense electron precipitation spike over the southern polar cap
Authors: Imhof W. L. , Rosenberg T. J. , Lanzerotti L. J. , Reagan J. B. , Voss H. D. , Datlowe D. W. , Kilner J. R. , Gaines E. E. , Mobilia J. , Joiner R. G.
Published: February 2008

An electron precipitation event has been investigated with bremsstrahlung X ray mapping data taken from two satellites and with ground‐based riometer and magnetometer data. The event occurred near 2300 UT on June 27, 1982, in the vicinity of South Pole Station, which was in the dusk‐midnight local time sector. The main precipitation was associated with a poleward‐moving westward electrojet and produced the largest riometer absorption (exceeding 10 dB at 30 MHz) recorded during 1982 at that station. The feature examined in detail here is an intense spike of ∼10 s duration and limited spatial extent that occurred as a short‐lived eastward ionospheric current developed equatorward of the westward electrojet. At the time of the spike, two sensors on the P78‐1 satellite essentially simultaneously measured X rays from the region viewed by the ground‐based instruments. Movement of the spike precipitation region toward the pole was indicated by the measurements. From ratios of the riometer absorption at different frequencies it is concluded that the spike precipitation region is consistent with a strip of width ∼25 km and length greater than 100 km. The total flux of the precipitating electrons in the spike was ∼4×1023 el/s (∼2×1016 ergs/s) with an e‐fold energy of ∼40 keV. The fluxes and energy spectra of X rays emitted from a large area, as measured by the spacecraft, were about the same before and after the spike. However, following the spike there was a pronounced decrease in riometer absorption, which suggests a movement of the main precipitation region away from the south pole at that time.
Concerning the structure of Pi 2 pulsations
Authors: Southwood D. J. , Hughes W. J.
Published: February 2008

We look at some simple theoretical considerations relevant to the origin of Pi 2 geomagnetic pulsations observed on the ground with the onset of geomagnetic activity. Using a basic model, we examine how subsidiary current flows are created by auroral ionospheric conductivity enhancements in the electrojet regions. These current flows, which include field‐aligned currents, are associated with reflected shear Alfvén waves localized to the field lines which thread ionospheric conductivity gradients. Thus the ionosphere can create distinct latitudinal structure in the signal. We discuss the origin of the observed polarization pattern and predominantly westward phase propagation of Pi 2 pulsations in the light of our results. We conclude that a superposition of two travelling waves of differing amplitude is required to produce these characteristics.
Magnetotail energy storage and release during the CDAW 6 substorm analysis intervals
Authors: Baker D. N. , Fritz T. A. , McPherron R. L. , Fairfield D. H. , Kamide Y. , Baumjohann W.
Published: February 2008

A primary goal of the Coordinated Data Analysis Workshop (CDAW) 6 has been to study the flow of energy from the solar wind through the magnetosphere and in so doing to examine the role of magnetotail storage and rapid dissipation of this energy during magnetospheric substorms. The chosen intervals for CDAW 6 analysis (March 22 and March 31 to April 1, 1979) are well suited for such studies since spacecraft were situated upstream (IMP 8 and ISEE 3) to measure magnetospheric energy input from the solar wind and spacecraft were also located in the magnetotail (ISEE 1, 2) and the midnight sector at geostationary orbit (76–059, 77–007, GEOS 2, GOES 3) during several of the substorm periods. Using solar wind, magnetotail, and geostationary particle and field data, as well as ground‐based information, we present the evidence for enhanced solar wind‐magnetosphere coupling and concomitant increases of stored magnetotail energy. Clear examples of energy storage are found prior to the 1055 and 1435 UT substorms of March 22, as well as the 0250 UT and 2250 UT substorms on April 1 and March 31, respectively. In these cases we estimate the total energy increase in the tail prior to the substorm onsets, and we estimate the dissipation rate of this energy during the substorms themselves. Typical total energy increases in the tail (≳5×1022 ergs) appear to be more than adequate to supply the substorm energy dissipation inferred from ground‐based and spacecraft observations.
A comparison of ionospheric electric fields inferred from Scandinavian Twin Auroral Radar Experiment drift data and from global international magnetospheric study magnetometer data
Authors: Baker K. B. , Kamide Y.
Published: February 2008

For the two Coordinated Data Analysis Workshop 6 periods, March 22 and March 31 to April 1, 1979, Kamide et al. have used global International Magnetospheric Study (IMS) magnetometer data along with the Rice ionospheric conductivity model to estimate the electric fields and currents at high latitudes. This procedure can be checked by comparing these results with the electric fields inferred from the drift velocities measured by the Scandinavian Twin Auroral Radar Experiment radars. We find that, in general, the two methods agree to within 20%, confirming the utility of the IMS magnetometer data in estimating the global electric field distribution. However, systematic differences in the two methods suggest that modifications must be made in the statistical conductivity model when applying it to dynamic processes.
Superthermal ion signatures of auroral acceleration processes
Authors: Moore T. E. , Chappell C. R. , Lockwood M. , Waite J. H.
Published: February 2008

The retarding ion mass spectrometer on the Dynamics Explorer 1 spacecraft has generated a unique data set which documents, among other things, the occurrence of non‐Maxwellian superthermal features in the auroral topside ionosphere distribution functions. In this paper, we provide a representative sampling of the observed features and their spatial morphology as observed at altitudes in the range from a few thousand kilometers to a few earth radii. At lower altitudes, these features appear at auroral latitudes separating regions of polar cap and subauroral light ion polar wind. The most common signature is the appearance of an upgoing energetic tail having conical lobes representing significant ion heat and number flux in all species, including O+. Transverse ion heating below the observation point at several thousand kilometers is clearly associated with O+ outflows. In some events observed, transverse acceleration apparently involves nearly the entire thermal plasma, the distribution function becomes highly anisotropic with T⊥ > T∥, and may actually develop a minimum at zero velocity, i.e., become a torus having as its axis the local magnetic field direction. At higher altitudes, the localized dayside source region appears as a field aligned flow which is dispersed tailward across the polar cap according to parallel velocity by antisunward convective flow, so that upflowing low energy O+ ions appear well within the polar cap region. While this flow can appear beamlike in a given location, the energy dispersion observed implies a very broad energy distribution at the source, extending from a few tenths of an eV to in excess of 50 eV. On the nightside, upgoing ion beams are found to be latitudinally bounded by regions of ion conics whose half angles increase with increasing separation from the beam region, indicating low altitude transverse acceleration in immediate proximity to, and below, the parallel acceleration region. These observations reveal a clear distinction between classical polar wind ion outflow and O+ enhanced superthermal flows, and confirm the importance of low altitude transverse acceleration in ionospheric plasma transport, as suggested by previous observations.
Idealized model of polar cap currents, fields, and auroras
Authors: Cornwall John M.
Published: February 2008

During periods of northward Bz, the electric field applied to the magnetosphere is generally opposite to that occurring during southward Bz, and complicated patterns of convection result, showing some features reversed in comparison with the southward Bz case. We study a simple generalization of early work on idealized convection models, which allows for the coexistence of sunward convection over the central polar cap and antisunward convection elsewhere in the cap. The present model, valid for By ≃ 0, has a four‐cell convection pattern and is based on the combination of ionospheric current conservation with a relation between parallel auroral currents and parallel potential drops. We do not consider global magnetospheric issues involving, e.g., reconnection. The central result of this paper is an expression giving the parallel potential drop for polar cap auroras (with By ≃ 0) in terms of the polar cap convection field profile.
A theoretical model of polar cap auroral arcs
Authors: Kan J. R. , Burke W. J.
Published: February 2008

A theory of the polar cap auroral arcs is proposed under the assumption that the magnetic field reconnection occurs in the cusp region on tail field lines during northward interplanetary magnetic field (IMF) conditions. Requirements of a convection model during northward IMF are enumerated based on observations and fundamental theoretical considerations. The theta aurora can be expected to occur on the closed field lines convecting sunward in the central polar cap, while the less intense regular polar cap arcs can occur either on closed or open field lines. The dynamo region for the polar cap arcs is required to be on closed field lines convecting tailward in the plasma sheet which is magnetically connected to the sunward convection in the central polar cap.
Sunspot cycle and thermal structure of equatorial middle atmosphere
Authors: Devanarayanan S. , Mohanakumar K.
Published: February 2008

Correlation and linear regression analyses of the variations in the mean stratopheric (15–50 km) and the mean mesospheric (50–80 km) temperatures with sunspot number variations during a complete solar cycle (1971–1982) have been made over the equatorial rocket launching station, Thumba (8° N, 77°E), India. The results were treated with the Student's t test. The study revealed that practically no correlation seems to be present between temperature and sunspot number variations in the stratosphere, but strong, positive, and highly significant correlation and regression coefficients are found in the mesosphere. A change in mesospheric mean temperature of 14 K was seen during the period when the solar cycle passes from the minimum to the maximum. A vertical profile study indicated that solar‐related temperature variations are strong in the middle mesosphere. Higher correlation and regression coefficients values were reported during the equinox season, and lower during the summer season.
Alignment of velocity and field changes across tangential discontinuities in the solar wind
Authors: Neugebauer M.
Published: February 2008

Three‐dimensional IMP 8 and Voyager 2 plasma and field data are used to investigate the relative changes in direction of the velocity and magnetic field vectors across tangential discontinuities in the solar wind. It is found that Δv and ΔB/ρ1/2 are closely aligned either parallel or antiparallel to each other in the sense associated with the propagation of Alfvén waves or rotational discontinuities outward from the sun. This alignment is observed at all solar distances between 1 and 2.2 AU and is independent of the solar wind stream structure. Several possible causes of the effect are briefly discussed, including interplanetary turbulence, the propagation of large‐amplitude Alfvénic fluctuations traveling independently through the solar wind on both sides of the discontinuities, and the propagation of surface waves on tangential discontinuities.
On the heliolatitude distribution of galactic cosmic rays
Authors: Antonucci E. , Attolini M. R. , Cecchini S. , Galli M.
Published: February 2008

An analysis of the annual and semiannual variation of the galactic cosmic ray intensity has been performed for the period 1953–1979 by using the data from the Climax and Dourbes neutron monitors. This analysis, based on a method developed for searching periodicities and recurrences in the cosmic ray intensity, has confirmed the existence of such variations and their phase changes associated with the reversals of the solar magnetic dipole. Hence the importance in the cosmic ray transport of transverse diffusion arising from drift effects due to the curvature and gradient of the interplanetary magnetic field is confirmed, since this is the mechanism which can explain the dependence on the solar magnetic cycle. Such a mechanism is effective when the polarity configuration of the interplanetary magnetic field is well defined and stable. A phase advance of the semiannual variation is observed, which can be explained through the modulation of the heliolatitude distribution of cosmic rays by the activity of the solar magnetic regions migrating in both hemispheres toward the equator, during the 11‐year cycle of solar activity. A residual annual variation, detectable when averaging out the effects of the magnetic cycle or when the polarity configuration of the interplanetary magnetic field is not well defined, probably indicates the existence of a preferential azimuthal direction for the access of low‐energy galactic cosmic rays into the heliosphere, along the galactic magnetic field.
The boundary layers as the primary transport regions of the Earth's magnetotail
Authors: Eastman T. E. , Frank L. A. , Huang C. Y.
Published: February 2008

A comprehensive survey of ISEE and IMP LEPEDEA plasma measurements in the earth's magnetotail reveals that the magnetospheric boundary layer and the plasma sheet boundary layer are the primary transport regions there. These plasma measurements also distinguish various components of the plama sheet, including the central plasma sheet and plasma sheet boundary layer. A significant new result reported here is the existence of cold‐and hot‐plasma components that are spatially copresent within the central plasma sheet. Such plasma components cannot be explained merely by temporal variations in spectra involving the entire plasma sheet. Contributions to a low‐temperature component of the plasma sheet enter directly from the boundary layer located along the magnetotail flanks. Field‐aligned flows predominate within the plasma sheet boundary layer, which is almost always present and is located near the northern and southern border of the plasma sheet. The plasma sheet boundary layer comprises highly anisotropic ion distributions, including counterstreaming ion beams, that evolve into the hot, isotropic component of the plasma sheet. Tailward acceleration regions generate these ion beams with plasma input from the magnetospheric boundary layer. Antisunward flowing ion beams, at E/q < 1 kV and of ionospheric composition, are frequently observed in the plasma sheet boundary layer and in the tail lobes. These ion beams are likely accelerated at low altitude over the polar cap and especially along auroral field lines. An ionospheric component of the plasma sheet is supplied from this source via the plasma sheet boundary layer. On the basis of a survey of ∼2000 three‐dimensional velocity distribution functions sampled by the ISEE LEPEDEA in the magnetotail we find that interpretations based solely on moments parameters are inevitably inadequate for inferring plasma sources and acceleration processes. We find that earthward flowing ions or counterstreaming beams are often observed near expansive phase onset under conditions for which antisunward ion flow is predicted by the near‐earth neutral line model of magnetospheric substorms. Magnetotail plasma flows sampled out to 38 RE and reported as typical of onset and recovery of magnetospheric substorms can only be adequately understood by considering spatial and temporal variations of the boundary layers described above.
Temporal evolution and propagation of a plasmoid associated with asymmetric fast magnetic reconnection
Authors: Ugai M.
Published: February 2008

The temporal dynamics of a plasmoid is numerically studied on the basis of the plasmoid that is in due course formed when magnetic reconnection is coupled to anomalous resistivity. No external agency is imposed, so that the temporal behavior of the plasmoid, defined by three neutral points, X1, O, and X2 (X1<O<X2), is completely determined by how magnetic reconnection proceeds at the neutral points. It is shown that the plasmoid cannot stand stationarily but is likely to propagate through between the oppositely directed magnetic fields. When the plasmoid moves in the positive x direction, anomalous resistivity tends to be more enhanced near the X1 point than near the X2 point so that the reconnection rate at X1 exceeds that at X2; the magnetic body force resulting from the asymmetric fast reconnection process further accelerates the plasmoid. The plasmoid finally attains a propagation speed almost comparable with the Alfvén velocity.
The cold solar wind
Authors: Freeman John W. , Lopez Ramon E.
Published: February 2008

From 3 to 10% of the time, the solar wind has a radial proton temperature between 15,000°K and 3500°K (the lowest recorded temperature). This “cold” component shows some very important features that make it worthy of special study. It shows a temperature dependence on heliocentric distance that is nearly adiabatic. When extrapolated back to 3 RS, assuming adiabatic cooling, the resulting temperature for this cold component is in good agreement with coronal temperatures near the sonic point. Most importantly, the proton velocities and densities associated with this cold component match those predicted by solar wind models that do not incorporate significant wave heating. We conclude that this is one solar wind component that fits well with appropriate models.
Electrons and cosmic ray produced protons in Saturn's inner magnetosphere
Authors: Northrop T. G. , Fillius Walker
Published: February 2008

The University of California Cerenkov detector on Pioneer 11 previously observed cosmic ray albedo neutron decay (CRAND) protons above 600 MeV in Saturn's inner magnetosphere, mixed with a poorly understood background of energetic electrons (Fillius and McIlwain, 1980). Here we separate the electron from the proton counts and establish the first‐order angular distributions for each species. To do this, we use the theoretical relationships among the harmonic coefficents of the count rate as a function of spacecraft roll angle derived by Northrop (1985). The majority of the counts were electrons with energy above several MeV, i.e., with drift periods shorter than the satellite orbital resonance. The electrons have isotropic pitch angle distributions, and the protons pancake over most of the region between Mimas and the rings, although there is a small region of dumbbell proton distributions in the vicinity of Janus and Epimetheus.
Plasmoid‐associated energetic ion bursts in the deep geomagnetic tail: Properties of the boundary layer
Authors: Richardson I. G. , Cowley S. W. H.
Published: February 2008

Energetic (>35 keV) ion bursts in the deep geomagnetic tail associated with the passage of 37 plasmoids are examined using data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3. These bursts can usually be divided into four distinct phases: (1) strongly tailward streaming ions observed in the lobe for a few minutes prior to plasmoid entry, commencing ∼25 min after geomagnetic substorm onset; (2) the plasmoid interval, when the energetic ions have a broader tailward angular distribution arising from convection with the plasmoid; (3) the “post‐plasmoid” plasma sheet, where more strongly tailward streaming ions are observed in the plasma sheet on field lines disconnected from the earth at the substorm neutral line; and (4) a strongly tailward streaming ion population extending into the lobe for a few minutes after exit from the plasma sheet. We concentrate here on the streaming ion “boundary layers” observed in the lobe at the leading and trailing edges of these bursts. In a majority of these layers, a clear dawn‐dusk gradient anisotropy and energy dispersion are evident at the leading edge, and a similar gradient anisotropy with “reverse” dispersion is evident at the trailing edge. It is shown however that the dispersion at onset is not consistent with simple time of flight from a near‐earth neutral line or from a neutral line retreating tailward during substorm recovery. Instead, observations of 90° pitch angle ions with a time resolution of 16 s are used to infer that the ion onset is due to a layer of energetic ions expanding outward from the tail center plane and engulfing the spacecraft. At the trailing edge of the burst, this layer contracts back across the spacecraft toward the center plane. Mean expansion and contraction speeds are 94±74 km s−1 and 99±100 km s−1 respectively, with boundary layer thicknesses of ∼3 RE. From these observations, it is concluded that the expansion of the ion layer is caused predominantly by the ion layer being swept across the spacecraft by the arrival of the plasmoid in the deep tail, contributing ∼60 km s−1 to the expansion speed, rather than by a thickening of the region of lobe field lines disconnected at the substorm neutral line which expands at ∼35 km s−1. The energy dispersion at the leading edge can be reconciled with a near‐earth neutral line in this case. Using this dispersion and the measured expansion speed of the layer, the electric field along the near‐earth substorm neutral line is deduced. Values derived from layer expansions and contractions are both ∼0.4 mV m−1, equivalent to ∼110 kV across a ∼40‐RE tail width.
Transient tropospheric electric fields resulting from sudden changes in ionospheric conductivity
Authors: Dejnakarintra M. , Inan U. S. , Carpenter D. L.
Published: February 2008

The transient electric fields that would be generated by sudden changes in the atmospheric conductivity profile are estimated using a theoretical model. The model assumes the existence of a horizontal electric field with a spatial extent of ≥20‐km at 150‐km altitude and involves the solution of the boundary value problem in two dimensions. The geomagnetic field lines below 150‐km altitude are assumed to be vertical, but the anisotropy of the medium above ∼70‐km altitude is fully accounted for. The computed electric field strength depends sensitively on the size of the source field as well as the conductivitity profile of the atmosphere. In the steady state, a small‐scale (∼20‐km) source field produces relatively weak dc electric fields in the troposphere, whereas a large scale (∼100‐km) source produces stronger observed fields. On the other hand, during a transient the peak electric fields produced in the case of a small‐scale source field can be considerably larger than those due to large‐scale source distributions. The more rapid the increase in the slope of the conductivity profile with altitude, the more efficient should be the downward mapping of both transient and steady state fields. Since the transient electric fields are oscillatory and die out at a rate depending on the local conductivity, the ac electric fields at the ground level are found to last longer than those at higher altitudes.
The dynamic ionosphere over Arecibo: A theoretical investigation
Authors: Crary David J. , Forbes Jeffrey M.
Published: February 2008

Numerical simulations of the F region ionosphere over Arecibo, Puerto Rico, are presented. We deduce from these simulations that the dynamic behavior of the Arecibo ionosphere, particularly the so‐called “midnight collapse” phenomenon, is strongly influenced by the upward propagating semidiurnal tidal component of the neutral meridional wind excited below thermospheric levels. Presence of a terdiurnal component in the wind field improves the agreement with experiment and could result from nonlinear interactive coupling between the neutral wind and ion drag. The steep underside density gradients occurring in conjunction with the collapse are due to vertical shear in the meridional wind field associated with the propagating semidiurnal tide. These gradients are capable of triggering the gradient drift instability, thus accounting for plasma irregularity formation and observations of VHF scintillations associated with the collapse phenomenon.
Plasma waves observed by the IRM and UKS spacecraft during the AMPTE solar wind lithium releases: Overview
Authors: Häusler B. , Woolliscroft L. J. , Anderson R. R. , Gurnett D. A. , Holzworth R. H. , Koons H. C. , Bauer O. H. , Haerendel G. , Treumann R. A. , Christiansen P. J. , Darbyshire A. G. , Gough M. P. , Jones S. R. , Norris A. J. , Lühr H. , Klöcker N.
Published: February 2008

The two September 1984 solar wind lithium releases produced a rich variety of plasma waves which have been measured in situ by the plasma wave instrumentation on board the Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM and UKS spacecraft. Reflection of the natural galactic and terrestrial electromagnetic radiation from the dense Li plasma caused a cutoff in the high‐frequency electric field intensities from which the temporal and spatial variation of the plasma density can be determined. Inside the diamagnetic cavity the electron plasma frequency and also temporarily the Li plasma frequency have been excited. The emission at the electron plasma frequency is near the thermal fluctuation level. In addition, weak low‐frequency ion acoustic waves were observed. The boundary between the diamagnetic cavity and the external magnetic field was found to be surprisingly stable and contained extremely low levels of wave activity. In the transition region from the diamagnetic cavity to the solar wind, high wave activity at the medium and very low frequencies propagating mainly in the ion acoustic and electrostatic cyclotron harmonic modes was encountered. No wave magnetic fields were detected in this region. The upstream edge of the transition region was characterized by a steep decay in magnetic field strength and density and by a sudden increase in the quasi‐static electric field. At this time the ELF/MF rms wave amplitude explosively increased to a value of 50 mV/m and remained at an enhanced level for more than 1 min. The spectrum of this wave activity is similar to the electrostatic noise observed in collisionless shocks. Data from UKS indicate that during the releases, UKS was in the magnetic transition zone. The wave activity at UKS was distinctly different from that encountered by IRM. The intense emission at the electrostatic shocklike transition was weaker than that on IRM and for the second release appeared at a different time. This can be related to the different positions of the two spacecraft with respect to the interaction regions. Despite the high wave intensities the estimated wave energy densities are, however, too low by orders of magnitude to drive significant magnetic field diffusion during the in situ observation times. Some differences in the wave excitations for the two releases can be traced back to the different solar wind conditions.
Plasma waves in magnetotail flux ropes
Authors: Kennel C. F. , Coroniti F. V. , Scarf F. L.
Published: February 2008

We have studied the plasma waves associated with the magnetotail flux ropes of December 28, 1982, December 30, 1982, and March 25, 1983, originally identified by Sibeck et al. (1984). Broadband electrostatic noise was found in the sheaths and cores of all three flux ropes. The frequency range extended from about 100 Hz to the local electron plasma frequency. The electric field vector tended to be aligned either parallel or antiparallel to the local magnetic field direction throughout the complex flux rope magnetic field configuration. The March 25, 1983, flux rope also contained an intense band of whistler mode noise extending up to one half the local electron cyclotron frequency. The superthermal electrons generating the observed whistler mode noise may have had highly anisotropic pitch angle distributions.
Solar wind proton temperature‐velocity relationship
Authors: Lopez Ramon E. , Freeman John W.
Published: February 2008

Data from Helios 1 are examined to find an experimental fit for the T(V) relationship at 1 AU. The data show a distinct change in the proton temperature‐velocity slope at V ∼ 500 km s−1. For V > 500 km s−1, T = (0.77±0.021)V ‐ (265±12.5), while for V < 500 km s−1, T1/2 = (0.031 ± 0.002)V ‐ (4.39±0.08) or T1/3 = (0.0106±0.001)V ‐ (0.278±0.03). These results are compared to the results of Burlaga and Ogilvie (1973) and show excellent agreement in the V < 500 km s−1 range. In addition, the secular variation of T(V) is examined, and it is found that the above functional forms still provide a good fit for a 2‐year interval approaching solar minimum and a 2‐year interval approaching solar maximum.
Phase propagation of the solar modulation of galactic cosmic rays
Authors: Forman Miriam A. , Jones Frank C. , Perko John S.
Published: February 2008

The phase of the 11‐year galactic cosmic ray variation, due to a varying rate of emission of long‐lived, propagating regions of enhanced scattering, travels faster than the scattering regions themselves. The radial speed of the 11‐year phase in the quasi‐steady force field approximation is exactly twice the speed of the individual episodic decreases. A time‐dependent numerical solution for 1‐GeV protons at 1 and 30 AU gives a phase speed which is 1.85 times the propagation speed of the individual decreases.
Coupling of global magnetospheric MHD eigenmodes to field line resonances
Authors: Kivelson Margaret G. , Southwood David J.
Published: February 2008

Many features of the magnetospheric hydromagnetic wave spectrum are fully understood, but a remaining puzzle is why selected field line resonances appear to be excited by a broadband source. In this paper, we propose that the preferential resonances correspond to global eigenmodes of the magnetospheric cavity. In order to understand the behavior of ultralow frequency waves in the terrestrial magnetosphere (geomagnetic pulsations), we describe the structure of global eigenmodes in the hydromagnetic box model of an inhomogeneous plasma. Global modes are large‐scale modes whose frequencies match field line resonance frequencies somewhere in the system. The coupling between localized field line resonance effects and the large‐scale mode leads to damping of the latter. In the box model the eigenmode equation yields a spectrum of discrete eigenfrequencies. For each eigenfrequency, the equation contains singular points where the field line resonance occurs. We base much of our discussion on earlier works which have used the same equation in other contexts. We describe solutions and then use the results to outline features of the modes that may be observable.
On ion acoustic plasma waves at large magnetic aspect angles in the high‐latitude E region of the ionosphere
Authors: Haldoupis C. , Sofko G. J. , Koehler J. A.
Published: February 2008

A series of 50 MHz Doppler experiments has been performed during 3 consecutive summers, to investigate auroral backscatter properties at different magnetic aspect angles ranging from about 1° to 14°. In this paper the large data base is used to study a particular class of echoes, namely the ion acoustic‐type echoes. In the present understanding of radio aurora, this category of scatter, with characteristics reminiscent of equatorial type 1 echoes, is attributed to electrostatic plasma waves generated by the Farley‐Buneman instability. Linear theory predicts that the propagation vectors of such waves are limited to a range of angles within 0 to 1.5 degrees of perpendicularity to the magnetic field. Our observations, however, show that type 1 auroral echoes at 50 MHz occur regularly in the entire aspect angle range from about 0° to 14°. Therefore, the corresponding 3‐m irregularities could have finite wavevector components along the magnetic field, with ratios k∥/k⊥ as large as 0.25. On the average, the echoes appear to be weakly aspect sensitive at a rate near −1.5 dB/degree. The observed phase velocities decrease slowly with aspect angle at a rate of about 7 m/s per degree and, in general, have values in the nominal ion acoustic speed range of the plasma. The observations might be explained within the framework of the two‐stream instability, by invoking a very high anomalous electron collision frequency, since that would have a strong destabilizing effect at the larger aspect angles. At present, however, there is no theoretical justification for this interpretation and the aspect sensitivity of radio aurora remains an open and challenging problem.
Pi 2 pulsations and the westward traveling surge
Authors: Rothwell Paul L. , Silevitch Michael B. , Block Lars P.
Published: February 2008

A model is developed that relates the poleward leaps of the westward traveling surge (WTS) and the generation of Pi 2 pulsations. The feedback instability developed by Sato and coworkers is combined with the dynamic surge model of Rothwell et al. (1984). We find that our previous results on the motion of the WTS are related to the zero‐order terms in the Sato formulation. The linearized first‐order terms give rise to a dispersion relation with solutions in the Pi 2 frequency range. The Ith frequency, fI, is found to be turned on in integer multiples of one‐half the Alfvén bounce frequency between the ionosphere and the plasma sheet. Most important, however, is that the Ith frequency is turned on only when the precipitating electron energy exceeds a certain value. As previously shown, the velocity of the poleward surge boundary also increases when the energy of the precipitating electrons is enhanced. Therefore the poleward leap of the surge during substorm onsets is accompanied by the generation of higher Pi 2 frequency components. The time evolution of the composite Pi 2 pulse is obtained using the calculated decay rates, and agreement with data is shown.
Comparison of simultaneous Chatanika and Millstone Hill observations with ionospheric model predictions
Authors: Rasmussen C. E. , Schunk R. W. , Sojka J. J. , Wickwar V. B. , de la Beaujardiere O. , Foster J. , Holt J. , Evans D. S. , Nielsen E.
Published: February 2008

As part of the MITHRAS program, the Chatanika and Millstone Hill incoherent‐scatter radars made coordinated observations of the polar ionosphere on June 27 and 28, 1981. We compare these data with predictions made by a high‐latitude ionospheric model. Qualitatively, the same features are evident in both the model and the radar data: fairly constant densities on the dayside with a mid‐latitude trough forming poleward of 65 degrees around 1900 MLT (magnetic local time). This trough is seen to extend equatorward with increasing MLT, such that the minimum densities occurring in the trough appear just after midnight around 60 degrees dipole latitude. These features are primarily understood in terms of different regions of convection, further influenced by photoionization and vertical transport. The only areas of major disagreement between the measurements and model are noted in the auroral oval and at a portion of the times during which substorms occurred. Quantitatively, equally good agreement is obtained between the model predictions and the radar data. The densities predicted by the model are usually within 25% of those measured by the radars, although appreciable differences occur in some regions of the ionosphere at certain times.
Effects of different convection models upon the high‐latitude ionosphere
Authors: Rasmussen C. E. , Schunk R. W. , Sojka J. J.
Published: February 2008

It is well known that convection electric fields have an important effect on the ionosphere at high latitudes and that a quantitative understanding of their effect requires a knowledge of plasma convection over the entire high‐latitude region. Two empirical models of plasma convection that have been proposed for use in studying the ionosphere are the Volland and Heelis models. Both of these models provide a similar description of two‐celled ionospheric convection, but they differ in several ways, in particular, in the manner in which plasma flows over the central polar cap and near the polar cap boundary. In order to obtain a better understanding of the way in which these two models affect the ionosphere, two separate runs of our high‐latitude, time‐dependent ionospheric model were made, with only the convection models distinguishing the two runs. It was found that the two models lead to differences in the ionosphere but often the differences are subtle and are swamped by universal time effects. The most notable differences are in predictions of the height of the F2 peak and in the ion temperature, particularly along the evening polar cap boundary and in the cusp region. For these two parameters, the differences caused by the two different convection models dominate the universal time effects. One question that arises is whether one could examine measurements of plasma density and temperature and determine which of the two convection models most accurately represents actual ionospheric convection. Unfortunately, it is expected that when the effects of other ionospheric inputs are considered, such as the neutral wind, the uncertainties are sufficiently large that the characteristic differences between the Volland and Heelis convection models cannot be clearly identified in an examination of plasma density and temperature measurements.
Coupling of the dynamic ionosphere to auroral flux tubes
Authors: Lysak Robert L.
Published: February 2008

Ionospheric currents must close by means of field‐aligned currents into the magnetosphere if they are not divergence free. Particle precipitation associated with the field‐aligned current can change the ionospheric conductivity and therefore the current pattern, leading to a feedback process which can result in traveling enhancements of the ionospheric conductivity. These processes are studied by means of a two‐dimensional model of magnetosphere‐ionosphere coupling which includes enhancements in the Pedersen conductivity but assumes that the Hall current is divergence free. It is shown that under appropriate conditions this feedback instability can take place with the efficiency of the coupling dependent on the ratio of the height‐integrated Pedersen conductivity to the effective admittance of Alfvén waves on the flux tube. Since the Alfvén admittance changes along the field line, internal reflections of Alfvén waves from gradients in this admittance can lead to the generation of pulsations of the current in the Pi 1 range, with the frequency being dependent on the ionospheric scale height.
Absolute solar flux measurement shortward of 575 Å
Authors: Ogawa H. S. , Judge D. L.
Published: February 2008

A rare‐gas ionization chamber was used to obtain an accurate measurement of the absolute solar EUV flux in the 50‐ to 575‐Å region. The instrument, operating in total and near‐total absorption, was flown on a solar‐pointing sounding rocket on August 16, 1983. For the day of the flight the solar activity indices were F10.7 = 132.1 and RI = 80, and the integrated solar irradiance at the earth, corrected for atmospheric absorption, was found to be 4.31 ± 0.31×1010 photons cm−2 s−1. Almost exactly a year earlier (August 10, 1982) the same instrument measured an integrated solar flux of 5.71 ± 0.42×1010 photons cm−2 s−1 during a time of enhanced solar activity (F10.7 = 209.5 and RI = 155).
The effect of finite “blob” size on the current convective instability in the auroral ionosphere
Authors: Huba J. D. , Chaturvedi P. K.
Published: February 2008

It has been suggested that the current convective instability may be responsible for the structuring, i.e., generation of density irregularities, of density enhancements (known as “blobs”) in the auroral ionosphere. However, previous theories have neglected the finite extent of the “blob” along the geomagnetic field. In this paper we develop a nonlocal theory of the current convective instability which considers the finite extent of an ionospheric “blob” parallel to the geomagnetic field. We find that the growth rate of the instability can be substantially reduced in the finite‐sized “blob” case from the value obtained in the local approximation for an infinitely long blob. For auroral ionosphere parameters, the reduction in the growth rate for medium‐scale irregularities (1–10 km) can be 1 to 2 orders of magnitude for the typical observed values of “blob” sizes (approximately a few hundred kilometers). Thus it appears that the current convective instability is not a viable mechanism to generate scintillation causing irregularities, i.e., 1‐ to 10‐km irregularities.
Magnetic configuration of the Venus magnetosheath
Authors: Phillips J. L. , Luhmann J. G. , Russell C. T.
Published: February 2008

A data set consisting of nearly six Venus years of Pioneer Venus Orbiter magnetometer data is analyzed in order to describe the configuration of the magnetosheath. A coordinate system is developed which isolates the effects of the solar wind flow and the interplanetary magnetic field (IMF). The data indicate that the magnetosheath field magnitude is responsive to solar wind dynamic pressure and that the compression of the upstream field is controlled by magnetosonic Mach number. The draping of the field is similar to that predicted by gasdynamic modeling; specific draping features include distinct dependence on the radial and the transverse components of the IMF, and a tendency of the field to encircle the planet at low altitude. Certain features of the processes of mass loading by magnetosheath fluctuations and by the motional electric field are examined. Magnetic fluctuations can dominate the magnetosheath field for periods of low interplanetary cone angle and in general for the magnetosheath regions near the quasi‐parallel part of the bow shock. The magnetosheath magnetic field magnitude displays a hemispherical asymmetry; the sense of this asymmetry is controlled by the cross‐flow component of the IMF in a manner which indicates that ion pickup by the convective electric field occurs preferentially in one hemisphere. The results of this survey indicate that mass loading is a significant process which should be incorporated into computational models but that a fluid treatment of the planetary ion component may not be appropriate.
Distribution of plasma and magnetic fluxes in the Venus near wake
Authors: PÉrez‐de‐Tejada H.
Published: February 2008

The orientation of the interplanetary magnetic field lines in the Venus near wake is examined on the basis of the observed distribution of nightside ionopause crossings of the Pioneer Venus orbiter. It is argued that as the field lines slip over the (magnetic) polar regions of the terminator they bend around the planet and sink into the umbra at an angle larger than that expected from magnetic tension effects. A flow configuration in which the shocked solar wind plasma streaming by the terminator expands and forces the magnetic field lines against the nightside ionosphere appears to be consistent with the observed distribution of nightside ionopause crossings.
Chaotic particle dynamics near a two‐dimensional magnetic neutral point with application to the geomagnetic tail
Authors: Martin R. F.
Published: February 2008

The dynamical system composed of a charged particle moving in a two‐dimensional magnetic field containing a neutral point, with and without a perpendicular electric field, is studied. The system is found to be nonintegrable and chaotic. Lyapunov characteristic exponents are calculated for the various types of chaotic orbits. When parameters relevant to the magnetotail are used it is found that the stochastic nature of the motion is more important for a thick current sheet (20,000 km) than for a thin sheet (1000 km). Some consequences of this stochasticity are discussed, including the suggestion of a collisionless, “chaotic conductivity”; this represents the collisionlike randomizing effect of the chaotic dynamics.
Observations of waves generated by electron beams in the ionosphere
Authors: Kellogg Paul J. , Monson Steven J. , Bernstein William , Whalen Brian A.
Published: February 2008

Observations are reported of waves generated by 2‐, 4‐ and 8‐keV electron beams with currents up to 100 mA, emitted by an accelerator carried in a rocket to ionospheric heights. Observations were made from a separated payload, and attempted from the ground. The strongest signals, which are observed near or in the beam, are low‐frequency signals in the range 100 Hz to 1 kHz, which reach peaks of 1 V/m. There is relatively strong emission in the upper whistler mode range (100 kHz‐fce) for the 100‐mA electron pulses, and about half of the 10‐mA electron pulses. These whistler mode signals appear identical to those observed to indicate beam plasma discharge (BPD) in laboratory work. If this indicates that BPD is occurring in these experiments, then BPD is ignited at about 20 times lower current than in a large plasma chamber. Harmonics of the plasma frequency with amplitudes comparable to the fundamental are observed in and near the beam. Field strengths of all these modes are reported. For the whistler mode and plasma frequency emissions, “strong” is used above in a relative sense. The high‐frequency signals, i.e., those above 100 kHz, are too weak to have much effect on the beam, or to appreciably accelerate ambient electrons. Natural low‐frequency emissions were also observed in flight. No signals generated by the beam were observed on the ground.
Modification of type II irregularities by finite amplitude type I waves in equatorial electrojet
Authors: Sharma S. K. , Kaw P. K.
Published: February 2008

It is demonstrated that the presence of a finite‐amplitude type I mode can significantly alter the dispersion relation for type II irregularities in the equatorial electrojet region of the ionosphere. This may have interesting implications for models describing the nonlinear saturation of type II modes.
On the limits of energy transfer through dayside merging
Authors: Crooker N. U. , Siscoe G. L.
Published: February 2008

Empirical studies indicate that the rate of energy transfer to the magnetosphere increases as solar wind coupling parameters increase, up to a certain limit, and then the rate remains constant. The split separator line merging model undergoes the same behavior. When the model interplanetary magnetic field (IMF) magnitude exceeds a critical value, the field configuration in the Chapman‐Ferraro current plane ceases to expose closed field lines to the solar wind. The critical value depends upon IMF orientation, stagnation pressure, and earth's dipole tilt angle. The absence of exposed closed field lines prevents closed‐to‐open flux transfer and, consequently, energy transfer. Further, the limit on energy transfer governed by the model dipole tilt angle affords the first explanation of both the semiannual and UT variations of geomagnetic activity completely in terms of merging.