bluuebunny/crossref_metadata_2025 huggingface dataset:
Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity
Authors: Tsunoda Ronald T.
Published: February 2008

An enigma of equatorial research has been the observed seasonal and longitudinal occurrence patterns of equatorial scintillations (and range‐type spread F). We resolve this problem by showing that the seasonal maxima in scintillation activity coincide with the times of year when the solar terminator is most nearly aligned with the geomagnetic flux tubes. That is, occurrence of plasma density irregularities responsible for scintillations is most likely when the integrated E‐region Pedersen conductivity is changing most rapidly. Hence the hitherto puzzling seasonal pattern of scintillation activity, at a given longitude, becomes a simple deterministic function of the magnetic declination and geographic latitude of the magnetic dip equator. This demonstrated relationship is consistent with equatorial irregularity generation by the collisional Rayleigh‐Taylor instability and irregularity growth enhancement by the current convective and (wind‐driven) gradient drift instabilities. Some discrepancies in this relationship, however, have been found in scintillation data obtained at lower radio frequencies (below, say, 300 MHz) that suggest the presence of other irregularity‐influencing processes. The role of field‐aligned currents, associated with the longitudinal gradient in integrated E‐region Pedersen conductivity produced at the solar terminator, in equatorial irregularity generation via the current convective instability has not been discussed previously.
Retrieval of non‐LTE vertical structure from a spectrally resolved infrared limb radiance profile
Authors: Zachor A. S. , Sharma R. D.
Published: February 2008

An algorithm is described for retrieving vertical profiles of a non‐LTE radiator (lower and upper state densities) and temperature from a spectrally resolved profile of infrared limb radiance. The limb radiance profile integrated over an optically thin line or region of the spectrum can be expressed as an Abel integral equation that may be solved for the vertical profile of volume emission rate. The volume emission rate profiles thus obtained from two or more thin spectral lines or regions representing a single species are used to solve for the vertical profiles of translational temperature and excited state molecular density. These two solutions and the limb radiance profile for nonthin spectral lines or regions are used in a nonlinear recursion scheme to obtain the species vibrational temperature profile. Results obtained in a numerical simulation using the υ2 band of CO2 are presented to demonstrate the method and, in particular, the convergence properties of the relaxation technique for recovery of vibrational temperature.
Electric fields in the plasma sheet and plasma sheet boundary layer
Authors: Pedersen A. , Cattell C. A. , Fälthammar C.‐G. , Knott K. , Lindqvist P.‐A. , Manka R. H. , Mozer F. S.
Published: February 2008

Data from the spherical double probe electric field experiment on ISEE 1 have been used to study a number of plasma sheet/lobe boundary crossings during intervals selected for the Coordinated Data Analysis Workshop 6 on March 22 and 31, 1979. These crossings took place during periods of substorm occurrence. They could be identified by keV plasma measurements and by using the electric field probes as a reference for measurements of the spacecraft potential. Typical spacecraft potentials in the plasma sheet are +10 V to +20 V. Because of decreased plasma density in the lobes the satellite potential increases here to +20 V to +30 V. Strong electric fields, with a dominant dawn‐to‐dusk component, are observed throughout the boundary layer outside the plasma sheet both for contracting and expanding motions of the plasma sheet and for different magnetic field directions. Characteristic amplitudes and durations are 5–10 mV m−1 and 5–15 min. The corresponding E × B vectors are always toward the plasma sheet. The spacecraft were situated both north and south of the plasma sheet edge, in such boundary layers, and in two or three cases the spacecraft were most likely on the earthward side of an X line. These latter cases coincided with the maximum of the expansion phase of substorms. Prior to these substorms, during plasma sheet thinning, onsets of sunward convection were observed by an electric field experiment on GEOS 2 in the geostationary orbit.
Mid‐latitude Pi 2 pulsations, geosynchronous substorm onset signatures and auroral zone currents on March 22, 1979: CDAW 6
Authors: Hughes W. J. , Singer H. J.
Published: February 2008

We relate the mid‐latitude Pi 2 pulsation signatures observed using the Air Force Geophysics Laboratory Magnetometer Network during the March 22, 1979, Sixth Coordinated Data Analysis Workshop interval to the magnetic variations observed at geosynchronous orbit and to ionospheric current distributions derived from high latitude magnetometer data and an ionospheric conductivity model. We are able to show that at least three separate substorm onsets or intensifications occurred around 1100 UT, specifically at 1053:50, 1104:10 and 1110:30 UT. We contrast an earlier onset which occurred at 0614 UT during a geomagnetically quiet interval to these more disturbed events. In both cases the Pi 2 polarization pattern locates the region of strong westward electrojet. Changes in the Pi 2 polarization during a single event appear to accompany small shifts in the electrojet position. These events do not obey the Pi 2 period‐electrojet latitude relationship obtained by Kuwashima and Saito (1981).
IMF By‐dependent plasma flow and Birkeland currents in the dayside magnetosphere: 2. A global model for northward and southward IMF
Authors: Reiff Patricia H. , Burch J. L.
Published: February 2008

This paper extends to all interplanetary magnetic field (IMF) orientations the qualitative convection pattern presented by Burch et al. (this issue), containing viscous, merging, and lobe cells driven, respectively, by diffusion or other quasi‐viscous processes, merging of interplanetary fields with closed dayside field lines, and merging of interplanetary fields with open tail lobe field lines. The model is based on the antiparallel merging hypothesis of Crooker (1979a) with the addition of small but finite cells driven by quasi‐viscous processes on the dawn and dusk edges of the polar cap. The data and model presented by Burch et al. pertained to southward IMF conditions. This paper generalizes that model and proposes a qualitative dependence of the three types of convection cells on the x, y, and z components of the IMF. For example, the lobe cell should be enhanced in the northern hemisphere if Bx < 0 and in the southern for Bx > 0, and, for a given Bx and Bz, should be larger as |By| increases. For northward IMF, the merging cell disappears, leaving only the lobe cell in a smaller polar cap. If the y component of the IMF (By) is small, we infer the four‐cell pattern of Burke et al. (1979), with two counter‐rotating lobe cells having sunward flow in the central polar cap and tailward flow on the flanks. If By is large, the antiparallel merging model predicts a single lobe cell filling the polar cap, whose direction of rotation depends on the sign of By and is opposite in opposite hemispheres. We argue that this vortex is unstable to reconnection in the magnetotail, leading to two (or more) vortices in each polar cap, each with the same sense of rotation, but again differing between the hemispheres. Each vortex has a region of closed field lines in the sunward‐flowing section and open field lines in the antisunward‐flowing section. This model encompasses several features of the theta arc phenomenon, and makes several predictions with respect to symmetries and antisymmetries between the two polar caps.
Ion cyclotron waves below the proton gyrofrequency in the magnetosphere
Authors: Gomberoff L. , Molina M.
Published: February 2008

A numerical comparison between the linear theory of ion‐cyclotron waves below the proton gyrofrequency and the data recorded on board the GEOS satellites is made. It is shown that the experimental data are in good agreement with the theory.
Pioneer Venus suprathermal electron flux measurements in the Venus umbra
Authors: Knudsen William C. , Miller Kent L.
Published: February 2008

Statistical properties of the two Maxwellian electron densities and temperatures characterizing the differential suprathermal electron fluxes measured by the Pioneer‐Venus Lockheed‐IPW retarding potential analyzer in the Venus umbra between approximately 1000 and 2000 km altitude and nightward of 130° solar zenith angle are reported. We find that the median density n2 and temperature T2 of the principal Maxwellian component are 3 cm−3 and 14 eV, respectively, and do not exhibit an altitude or solar zenith angle dependence within the umbra region studied. Two thirds of the measured values of n2 and T2 typically fall within plus or minus a factor of 2 of the median values. The second suprathermal component density n3 is less well defined but has a median density of 0.06 cm−3 and similarly exhibits no clear altitude or solar zenith angle dependence. Two thirds of the n3 values fall within plus or minus a factor of 2 of the median value. The temperature T3 of the second component is assumed constant and equal to 200 eV. The median integral flux measured in this study between 15 and 45 eV energy is consistent with that reported earlier for the Venus nightside ionosphere by Spenner et al. (1981) at altitudes below 1000 km. The integral flux measured by the Pioneer‐Venus retarding potential analyzer appears to be larger than the Venera measured median flux by a factor of approximately 2 in the energy range of the Pioneer‐Venus retarding potential analyzer.
Relation between energetic electrons, protons, and the thermal plasma sheet population: Plasma sheet recovery events
Authors: Scholer M. , Sckopke N. , Ipavich F. M. , Hovestadt D.
Published: February 2008

We have investigated thermal, low‐energy suprathermal, and high‐energy protons and electrons in the plasma sheet near local midnight at geocentric distances of 14–20 RE during five different plasma sheet penetrations. The data have been obtained with the Los Alamos National Laboratory/Max‐Planck‐Institut fast plasma analyzer and the Max‐Planck‐Institut/University of Maryland energetic particle sensor system on ISEE 1. It is found that the intensity time profile of suprathermal protons up to ∼150 keV follows closely the intensity profile of the thermal proton population. The protons with energies ≳200 keV exhibit a typical fast rise‐slow decay profile; the decay time constant decreases with increasing energy. When entering the plasma sheet from the lobe the high‐energy protons are delayed in relation to the thermal and low‐energy suprathermal protons. Energetic electrons do not follow the energetic proton profile. They reach their maximum about 30 min after the appearance of the plasma sheet as determined by the thermal electrons. After the energetic electrons have reached their maximum intensity, their intensity time profile follows closely any changes in the electron temperature of the thermal population. The observations are discussed in terms of various models. In particular, acceleration of the ions at a tailward retreating neutral line during recovery can consistently explain the ion observations. The acceleration mechanism for energetic electrons is unclear, and it is argued that they may be accelerated by a stochastic process out of the thermal population.
High‐amplitude VLF transmitter signals and associated sidebands observed near the magnetic equatorial plane on the ISEE 1 satellite
Authors: Bell T. F.
Published: February 2008

On February 16, 1983, during a period of strong magnetic disturbance (Kp ∼ 5), as the ISEE 1 satellite passed near the magnetic meridian of the Omega navigation transmitter (98°W, 46°N geographic, L ∼ 3.4), the Stanford University VLF wave receiver detected the presence of Omega transmitter signals over the range 2 ≤ L ≤ 3.7 at magnetic latitudes (λm) within 10° of the magnetic equator. Over a 500‐km orbital segment near L ∼ 3.4 and λm ∼ 5°S, the electric field amplitude of the Omega signals reached ∼ 0.6 mV/m, a value > 40dB higher than the signal amplitude both preceding (L ∼ 3.7–3.5) and following (L ≤ 3.2) the high‐amplitude period. Over the range 3.3 ≤ L ≤ 3.7 the transmitter pulses at 13.1 and 13.6 kHz were associated with sideband signals spaced in frequency roughly symmetrically about the carrier and generally reduced in amplitude ∼5 dB with respect to the carrier. The strongest sidebands were generally offset in frequency from the carrier by ∼ ±55 Hz. Simultaneous data from the University of Iowa Plasma Wave experiment indicated that the region of high wave amplitude was located just beyond the outer edge of the plasmapause where low values of cold plasma density (∼40 el/cm³) prevailed. High amplitudes were observed only during the time in which the signal frequency lay within the range of a natural noise band of rising center frequency in which bursts of VLF emissions were occasionally triggered by “knee‐trace” whistlers. On the basis of the group time delay and amplitude distribution of the Omega signals and the presence of knee‐trace whistlers, it is concluded that the high amplitude signals originally propagated along the base of the plasmapause surface into the southern hemisphere and subsequently reflected or scattered back up to the satellite. The high‐wave amplitude was presumably produced through the coherent whistler mode instability as the input waves interacted with gyroresonant energetic electrons near the magnetic equatorial plane, and the presence of sideband signals indicates that this interaction had reached a nonlinear stage. The wave magnetic field in the interaction region is estimated to be approximately 20 mγ. The high amplitudes reached by the signals indicates that particle trapping effects could be responsible for the sideband generation.
Experimental results from the HERO Project: In situ measurements of ionospheric modifications using sounding rockets
Authors: Rose G. , Grandal B. , Neske E. , Ott W. , Spenner K. , Holtet J. , Måseide K. , Trøim J.
Published: February 2008

The HEating ROcket project HERO comprised the first in situ experiments to measure artificial ionospheric modifications at F layer heights set up by radio waves transmitted from the Heating facility at Ramfjord near Tromsø in Northern Norway. Four instrumented payloads were launched on sounding rockets from Andøya Rocket Range during the autumn of 1982 into a sunlit ionosphere with the sun close to the horizon. The payloads recorded modifications, in particular, the presence of electron plasma waves near the reflection level of the heating wave. The amplitude and phase of the three components of the electric and magnetic fields of the heating wave were measured simultaneously as a function of altitude. Coherent spectra of the three electric field components of the locally generated electron plasma waves were obtained in a 50‐kHz‐wide band. At the same time quasi‐continuous measurements were made on several fixed frequencies from 4 kHz to 16 kHz below the heating frequency and in the VLF‐range using linear dipole antennas. Moreover, measurements were made of electron temperature, suprathermal electrons and local electron density along the rocket trajectory. The experimental results will be presented and discussed.
HIRIS rocketborne spectra of infrared fluorescence in the O3(ν3) band near 100 km
Authors: Rawlins W. T. , Caledonia G. E. , Gibson J. J. , Stair A. T.
Published: February 2008

High‐resolution O3(ν3) spectral emission data, obtained between 80 and 125 km during an aurora by the rocketborne HIRIS cryogenic interferometer spectrometer, have been analyzed by a spectral simulation/least squares technique. Data obtained below 110 km exhibit behavior independent of local auroral strength and thus appear to represent quiescent nightglow. The derived vibrational state population distributions for levels υ3′ ≥ 1 can be expressed as vibrational “temperatures” of 400–600 K. This result is interpreted in terms of formation of O3(υ3′) by three‐body recombination of O with O2 as well as other radiative and collisional processes. Additional data, obtained near 120 km and sampling intense auroral arcs, indicate a hitherto unexpected auroral enhancement of O3(ν3) fluorescence intensity and vibrational state distribution.
An incoherent scatter radar measurement of the average ion mass and temperature of a nighttime sporadic layer
Authors: Tepley C. A. , Mathews J. D.
Published: February 2008

We report the results of incoherent scatter radar, total power, and ion line observations of a nighttime sporadic layer centered at 92‐km altitude. The height variation of the absolute widths of the ion line yields a layer temperature of 210°±5°K and a mean ion mass of 50±10 amu. This ion mass, when compared with average meteor composition, indicates that the layer is composed totally of metallic ions.
Magnetic field change across the Earth's bow shock: Comparison between observations and theory
Authors: Winterhalter Daniel , Kivelson Margaret G. , Walker Raymond J. , Russell Christopher T.
Published: February 2008

We have examined 204 bow shock crossings observed in the magnetic field and plasma data on ISEE 1. Using the measured upstream field and plasma data, we calculated the downstream field magnitudes by using the single‐fluid MHD Rankine‐Hugoniot jump conditions, and we compared the results with the observed downstream field magnitudes. We find that the simple MHD model approach can predict the downstream field magnitude only on the average, but does so quite well if (1) a polytropic index of approximately 5/3 is used in the calculations; (2) the angle between the upstream field and the shock normal is greater than 45°; and (3) the Alfvén Mach (Ma) number is less than about 10. For angles less than 45° the predictions overestimate the observed field strength. For Ma > 10 the predictions underestimate the observations.
The downshift of electron plasma oscillations in the electron foreshock region
Authors: Fuselier S. A. , Gurnett D. A. , Fitzenreiter R. J.
Published: February 2008

Electron plasma oscillations in the earth's electron foreshock region are observed to shift above and below the local electron plasma frequency. As plasma oscillations shift downward from the plasma frequency, their bandwidth increases and their wavelength decreases. Observations of plasma oscillations well below the plasma frequency are correlated with times when ISEE 1 is far downstream of the electron foreshock boundary. Although wavelengths of plasma oscillations below the plasma frequency satisfy kλDe ≈ 1, the Doppler shift due to the motion of the solar wind is not sufficient to produce the observed frequency shifts. A beam‐plasma interaction with beam velocities on the order of the electron thermal velocity is suggested as an explanation for plasma oscillations above and below the plasma frequency. Frequency, bandwidth, and wavelength changes predicted from the beam‐plasma interaction are in good agreement with the observed characteristics of plasma oscillations in the foreshock region.
Observations of 35‐ to 1600‐keV protons and low‐frequency waves upstream of interplanetary shocks
Authors: Sanderson T. R. , Reinhard R. , van Nes P. , Wenzel K.‐P. , Smith E. J. , Tsurutani B. T.
Published: February 2008

We present results of a detailed analysis of ISEE 3 observations of low‐energy protons (35–1600 keV) and low‐frequency waves (period greater than 6 s) associated with the passage of the large oblique interplanetary shock of April 5, 1979. We observe a foreshock region for several hours prior to the arrival of the shock. Within this region we find (1) a peak in the proton energy spectrum at about 200 keV, (2) the proton energy density (in the range 35–1600 keV) comparable with the total energy density of the magnetic field, (3) a good correlation between the amplitude of the low‐frequency waves and the 200‐keV proton intensity, and (4) a good agreement between the peak in the wave power spectrum and the resonant frequency corresponding to the peak in the proton spectrum, only if large pitch angles are considered, suggesting that the protons are responsible for the waves by virtue of a resonant interaction. Our observations suggest that the high energy density of the high‐energy solar flare protons preceding the shock could be responsible for “seed” waves which provide the scattering centers necessary for the acceleration of the lower‐energy protons via a first‐order Fermi acceleration mechanism.
Electrodynamic interaction of Ganymede with the Jovian magnetosphere and the radial spread of wake‐associated disturbances
Authors: Tariq G. F. , Armstrong T. P. , Lowry J. W.
Published: February 2008

To understand and explain the Voyager 2 observations of Ganymede's wake‐associated disturbances, an investigation of the electrodynamic interaction of the Jovian magnetospheric plasma with the satellite has been carried out. Two different aspects of the interaction are studied: (1) To see plasma kinematic effects during the interaction of the Jovian plasma and the expansion of the plasma into the cavity, particle‐in‐cell solutions of the electron and ion Vlasov equations are obtained. The disturbances of ions, potentials, and ion and electron thermal and drift energies in the wake region are obtained. (2) To provide explanations of the observed large radial spread of the disturbances, linear Kelvin‐Helmholtz instability theory is applied to the plasma flowing past the boundary of the cavity. Theoretical calculations show that the ambient plasma flow deviates from corotational flow, in both magnitude and direction. Theoretical conditions for the existence of magnetosonic waves and transverse Alfvén waves are also examined. The effect of the distortion of the Jovian magnetic field lines due to the annular current sheet on the charged particle absorption by Ganymede is also examined.
Poynting vector as a diagnostic of hydromagnetic wave structure
Authors: Junginger Hans
Published: February 2008

The Poynting vector of resonant shear Alfvén waves which are generated by surface waves in a cold, inhomogeneous plasma is investigated in a simple model of the magnetosphere with ionospheres of finite conductivity. The results are applied to solar‐wind‐driven magnetospheric pulsations with periods between 100 and 600 s. In the equatorial plane the Poynting vector has only perpendicular components (i.e., azimuthal and radial components) if the conductivities of both ionospheres and the mass distributions of both sides of the equatorial plane are equal and if the energy source is symmetric around the equator. The magnitude of the radial component, which is pointing inward everywhere, depends on the ionospheric resistance. If the latter is considerably less than the wave resistance of the shear wave (as typical in the dayside magnetosphere), the radial component of the Poynting vector is smaller than the azimuthal component near the magnetopause and in the resonance region. The azimuthal component is parallel to the phase velocity vector of the surface wave between the magnetopause and the resonant field line and is antiparallel on the earthward side of the resonance region for typical magnetospheric mass density distributions and magnetic field gradients. The Poynting vector component parallel to the background magnetic field in the resonance region near the ionospheres is smaller than the perpendicular components in the equatorial plane for typical ionospheric conductivities, indicating that most of the energy is converted by the surface waves into the geomagnetic tail and into the nose of the magnetosphere. A realistic range for the energy flux is 1015 to 1016 erg/s and 10−4 to 10−3 erg/(s cm²) for the Poynting vector of typical solar‐wind‐driven surface waves on the magnetopause and the low‐latitude boundary layer of the dawn and dusk flanks. As the ratio between the wave electric and magnetic fields near geostationary orbit generally should increase with increasing frequency for fundamental modes, it should be possible to measure Poynting fluxes there for long‐period pulsations with periods between 400 and 600 s.
Lower hybrid heating of ionospheric ions due to ion ring distributions in the cusp
Authors: Roth I. , Hudson M. K.
Published: February 2008

The stability of H+ and He++ ring distributions which have been observed downflowing into the cusp on the DE and S3‐3 satellites is examined in the context of the feedback of those instabilities on plasma of ionospheric origin (H+ and O+). Lower hybrid waves are excited by the ring distributions in three distinct phases of wave‐particle interaction: linear growth, trapping, and quasi‐linear diffusion. The latter phase accounts for most particle heating. Including background O+ and/or a He++ ring introduces new modes not present in a pure H+ plasma which play an important role in heating heavier ions. O+ is heated significantly more by a He++ ring than a H+ ring of comparable energy density. It is suggested that lower hybrid waves generated by downflowing ion ring distributions play a role in energizing ion conies in the cusp.
Simulation study of type 2 counterstreaming electrons along auroral field lines
Authors: Wagner J. S. , Lin C. S. , Tajima T.
Published: February 2008

We examine the production of counterstreaming electrons associated with parallel fields along auroral field lines through the use of computer simulation. We use a 2½‐dimensional (two spatial and three velocity dimensions) electrostatic particle algorithm and auroral boundary conditions to set up a self‐consistent V potential structure. The simulation produces signatures of counterstreaming electrons resembling those observed by the Dynamics Explorer 1 (DE 1) satellite. The main signatures are as follows: (1) the phase space contours of the electron distribution function are elongated along the V∥ axis, and (2) the energy of electrons streaming in the upward direction is comparable to the energy of the accelerated electron beam. The simulation indicates that a portion of the accelerated electron beam is trapped by large amplitude electrostatic waves produced through the two‐stream instability. Strong wave‐particle interactions then thermalize the trapped electrons to produce suprathermal electrons streaming in the direction opposite to that of the accelerated electron beam. These results suggest a possible mechanism of producing counterstreaming electron fluxes through nonlinear processes of the two‐stream instability.
Evidence of mutual induction between the ionosphere and the Earth at equatorial latitudes
Authors: Duhau S. , Osella A. M.
Published: February 2008

The mutual induction between the ionosphere and the earth has been suggested as one of the possible causes of the longitudinal inhomogeneities in the phase velocity of type II irregularities in the equatorial electrojet. From the analysis of the geomagnetic daily variations at equatorial latitudes in Peru, Nigeria, and central Africa, latitudinal profiles of the integrated current density and clear evidence of latitudinal discontinuities in the distribution of the earth conductivity were found in previous work. In the present paper a very high correlation is found between tectonic features and inhomogeneities both in the phase velocity and in the integrated current density. This shows that ionospheric inhomogeneities are related to specific lateral discontinuities in the conductivity of the upper layers of the earth. This result gives enough insight into that effect to allow its inclusion in future theoretical models.
Beam‐driven electron acoustic waves upstream of the Earth's bow shock
Authors: Marsch E.
Published: February 2008

Intense electrostatic waves of short wavelength are frequently observed upstream of the earth's bow shock. These waves are very impulsive, and their peak amplitudes correlate strongly with the temporal flux profiles of simultaneously observed energetic ions and electrons which are reflected from the shock. Our paper suggests an explanation of this broadband electrostatic noise in terms of electron acoustic waves driven unstable by the ion and/or electron beams. These waves do not exist in thermodynamic equilibrium plasma and therefore require nonthermal distributions for their occurrence. A simple model is developed for finite temperature beam and background particles with Maxwellian or Lorentzian distributions. The beam‐associated instability can be classified as being due to Landau resonance or negative wave energy. Depending on the plasma parameters the mode discussed here naturally extends and links to the ion acoustic wave or electron plasma oscillation. A numerical parameter study is provided for various beam plasma situations. The frequency theoretically obtained ranges between the ion and the electron plasma frequency, and the wavelength amounts to several Debye lengths for maximum growth. Since the beam mode has a linear dispersion relation, the Doppler‐shifted frequency in the spacecraft frame should depend linearly on the beam velocity and on the wave vector as well. It is anticipated that these characteristics may allow parts of the observed broadband high‐frequency noise to be identified as beam‐driven electron acoustic waves.
Coordinated measurements of auroral zone plasma enhancements
Authors: Weber E. J. , Tsunoda R. T. , Buchau J. , Sheehan R. E. , Strickland D. J. , Whiting W. , Moore J. G.
Published: February 2008

The Air Force Geophysics Laboratory's Airborne Ionospheric Observatory (AIO) performed radio, optical, and scintillation measurements on a series of north‐south flight legs along the Chatanika radar magnetic meridian. The incoherent scatter radar was operated in a north‐south magnetic meridian scan mode to measure ionospheric parameters (electron density, temperature, and ion drift) from 600 km north to 600 km south of the radar and from ∼80 to 700 km altitude. Ionospheric structure measured by the radar is compared with remote optical and ionosonde measurements from the AIO and with precipitating electron characteristics measured by a Defence Meteorological Satellite Program satellite. Long‐lived F region plasma enhancements (plasma blobs) were observed during this experiment. From the simultaneous measurements it is shown that these enhancements were not locally produced by precipitating particle fluxes. F region electron concentrations, calculated from simultaneously observed precipitating electron fluxes, are significantly less than those observed in the plasma enhancements. These premidnight features (plasma blobs) were in fact observed to be convecting sunward (to the west) at a few hundred meters per second and are thus presumed to have been produced well upstream in the convection flow within a region of significantly greater production rate. Intense scintillation of satellite signals due to ionospheric irregularities is generally confined to these regions of enhanced F region density. Simultaneous measurements of independent parameters identifying the auroral E layer (1–20 keV precipitating electrons, 4278‐Å N2+ emission, f0E, and radar electron densities) all agree well as to location and the latitude profile of the auroral E layer and associated diffuse aurora.
Weak ion acoustic double layers and solitary waves on the auroral field lines
Authors: Bujarbarua S. , Goswami K. S.
Published: February 2008

In this paper we have investigated the time‐stationary solutions of weak double layers and solitary waves moving near the ion acoustic speed. It has been found that these nonlinear waves exist when the electric potential is negative and the reflected (trapped) ion density is very much depleted. Both cases of isothermal and nonisothermal electrons have been considered. It has been shown that the double layers and solitary waves discussed in this paper may explain the recent observations on the auroral field lines.
Viscosity in a magnetized plasma: Physical interpretation
Authors: Hollweg Joseph V.
Published: February 2008

We provide a physically motivated alternate derivation of the η0 terms in the viscous stress tensor given by Braginskii (1965) and others. We show that the η0 terms are fully accounted for by a gyrotropic diagonal pressure tensor; off‐diagonal terms do not contribute to the η0 part of the viscous stress. The η0 part of the viscous stress results from the plasma's tendency to develop small thermal anisotropies as it evolves. Collisions oppose the production of anisotropy and ultimately lead to irreversible heating.
A statistical study of wave poynting vectors measured during long‐period magnetospheric pulsations at geostationary orbit
Authors: Junginger H. , Haerendel G. , Melzner F.
Published: February 2008

Electric and magnetic field data measured by the electron beam experiment and the magnetometer on board the geostationary satellite GEOS 2 were used to investigate wave Poynting vectors associated with long‐period (150–600 s) magnetospheric pulsations. A total of 3580 vectors were calculated for pulsations occurring during 186 days in the dayside magnetosphere. The ratio between the electric and magnetic field wave amplitudes was in general well above the local Alfvén speed and was found to increase with increasing wave frequency. The fraction of electric field pulsations for which magnetic wave components could also be identified was therefore larger for the low‐ and smaller for the high‐frequency events in the range 1.67–6.67 mHz. Poynting fluxes were found to have values between 10−10 and 10−5 W/m². For most pulsations with periods between 400 and 600 s the part of the vectors perpendicular to the ambient magnetic field had an inward component and was directed toward the nose of the magnetosphere in the prenoon and afternoon sectors. The behavior of the corresponding components of the pulsations with a 150‐ to 300‐s period was not as clear. For all events observed in the winter season (between November 1, 1978, and February 28, 1979), 59% of the vectors had a field‐aligned component directed into the northern hemisphere, whereas 41% of the vectors were directed southward. The frequency dependence of the E/B ratio, the magnitudes of the Poynting vectors, and their directional distributions are consistent for most events with the picture of standing shear Alfvén waves caused by solar wind driven surface waves on the outer boundaries of the magnetosphere.
Pc 3–4 ULF magnetic variations measured in the ionosphere by Eiscat
Authors: Glangeaud F. , Lathuillere C. , Lambert M. , Zhao Z. Y.
Published: February 2008

ULF pulsations of Pc 3–4 type were recorded in the ionosphere at several longitudes with the European Incoherent Scatter facility. The analysis of these pulsations needs sophisticated signal processing. We found that a detailed description of these pulsations must be made burst by burst. The characteristic parameters are time, frequency, and space extent. Comparison with ground recordings indicates that only the H component is well related to the ionospheric ion velocity. The ionosphere‐ground relation is discussed in terms of variability of the auroral conductivity and spatial extent.
Waterhole auroral arc modification experiments: Electrodynamic response
Authors: Whalen B. A. , Yau A. W. , Creutzberg F. , Wallis D. D. , McNamara A. G. , Harris F. R. , Pongratz M. B. , Bernhardt P. A. , Kintner P. M. , Labelle J. , Sheldon W. R. , Benbrook J. R. , Bering E. A. , Forsyth P. A. , Koehler R. A.
Published: February 2008

A series of perturbation experiments (Waterhole I, II, and III), in which “holes” were created in the F region ionosphere by explosive releases of large quantities of water vapor, has been conducted to test theories of the electrodynamic structure of auroral arcs. The water vapor releases created large (≈50‐km diameter) holes in the ionosphere in and near structured premidnight auroral arcs. It was anticipated that these holes would interrupt or perturb the ionospheric current systems associated with the arcs and that this perturbation would in turn affect the acceleration mechanism responsible for the aurora. Results from the two successful rocket flights (Waterhole I and III) are presented, and it is shown that significant modifications of the energetic electron precipitation patterns were induced by both releases. The first release was made 10 km poleward of a discrete early evening auroral arc, and the perturbation caused a cessation of electron precipitation through the hole and a significant modification of the arc. The second release, which occurred on magnetic field lines connected to the center of a series of arcs, produced a much smaller hole mainly due to the preexisting low electron density and also induced a less striking energetic electron response. In this case the flux was enhanced. Various models and theories of the perturbation mechanism are discussed, and it is shown that both responses are consistent with current theories of the electrodynamic structure of auroral arcs.
Strange currents over Saint‐Santin
Authors: Mazaudier C. , Venkateswaran S. V.
Published: February 2008

Analysis of 29 days of ionospheric current data obtained by Saint‐Santin radar, including both magnetically quiet and disturbed periods, has confirmed the existence of northward currents at all daytime hours. The postulate of field‐aligned Van Sabben currents driven by interhemispherical asymmetries seems to be able to account for the anomaly. The desirability of quantitative modeling of the Van Sabben currents on a seasonal or day‐to‐day basis is highlighted by the Saint‐Santin measurements.
Inversion of a spectrally resolved limb radiance profile for the NO fundamental band
Authors: Zachor A. S. , Sharma R. D. , Nadile R. M. , Stair A. T.
Published: February 2008

A recursive algorithm was developed to invert spectrally resolved earth limb radiance profiles representing the non‐local thermodynamic equilibrium altitude regime and species/tangent heights that are optically thin. It was used to infer vertical distributions of temperature and excited NO density from data measured by the Spectral Infrared Rocket Experiment in the NO fundamental band near 5.3 µm. The solution for excited NO density was used to compute atmospheric cooling rates for the 5.3‐µm band and the de‐excitation rate constant ko for the reaction NO* + O → NO + O.
Atomic nitrogen in aurora: Production, chemistry, and optical emissions
Authors: Rees M. H. , Romick G. J.
Published: February 2008

The production of ground configuration nitrogen atoms in aurora is investigated. The two principal mechanisms are electron impact dissociation of N2 and chemical‐ionic reactions, with the latter dominating the production of N(²D). Between 60 and 70% of the N atoms are produced in the excited ²P and ²D states over a large altitude range. Simultaneous field‐aligned measurements of the 5200‐Å and 3466‐Å lines of N I and the 4278‐Å first negative group band of N2+ were made in the northern nightside auroral oval under varying auroral conditions. These data, taken over a continuous 4‐hour period on the night of February 5, 1978, yield an average emission rate ratio of the 5200‐Å/3466‐Å lines near 0.5. Model calculations yield ratios of 0.5 and 1.4 for particle characteristic energies (alpha) of 2 keV and 0.1 keV in an assumed Maxwellian electron spectrum in good agreement with the observations reported here and consistent with those reported in the literature for very soft precipitation in the dayside cusp region. The production rate of energetic (hot) N(4S) atoms in the aurora is evaluated from considerations of the excess translational energy associated with N atom production. It may be a significant factor in the production rate of NO and the odd nitrogen density in general in the auroral atmosphere.
Equatorial ionospheric plasma bubble irregularity occurrence and zonal velocities under quiet and disturbed conditions, from Polarimeter observations
Authors: Abdu M. A. , Batista I. S. , Sobral J. H. A. , de Paula E. R. , Kantor I. J.
Published: February 2008

Amplitude scintillation and Faraday rotation angle fluctuations of satellite VHF beacons registered by electronic polarimeters operated at two stations spaced 110 km apart in the magnetic east‐west direction namely, Cachoeira Paulista, 22°41′S, 45°W, dip angle of −26°, and São José dos Campos, 23°12′S, 45°51′W, dip angle of −26°, in Brazil are analyzed for equinoctial and summer months of 1982–1983 to determine the irregularity patch occurrence characteristics and their zonal velocities under varying levels of magnetic disturbances. The scintillation patch occurrences and total electron content fluctuations representing plasma depletion structures decrease during premidnight hours and are enhanced in the postmidnight period. The occurrences often extend to morning hours, with increasing magnetic activity index. While the night events are associated with eastward irregularity patch motion, the morning events have westward velocities (the reversal) occurring around 0400 LT, reflecting the morning velocity reversal in the F region bulk plasma motion reported earlier from Jicamarca radar measurements. The westward plasma bubble motions reported here represent the first observation of this kind. Another important finding from the present investigation is a clear tendency for the zonal plasma motion, namely, the equatorial meridional electric field, to become enhanced mainly in the premidnight hours immediately following strong magnetic activity.
Amplitude distribution of small‐scale magnetic fluctuations over the polar ionosphere observed by Magsat
Authors: Iyemori T. , Ikeda T. , Nakagawa A.
Published: February 2008

Small‐scale (1<L<35 km) or short time scale (0.12<T<5 s) magnetic fluctuations observed by Magsat over the polar ionosphere were analyzed statistically. On the dayside, the maximum of the amplitude distribution appears near the poleward edge of the large‐scale Birkeland current region 1 or at a latitude poleward of it. The amplitude strongly depends on the ionospheric conductivity and varies about factor 3 or more even in the same conditions of geomagnetic activity, suggesting that the fluctuations are generated mainly by small‐scale Birkeland currents rather than Alfvén waves. The IMF By causes the dawn‐dusk asymmetry of the amplitude and the IMF Bz controls the latitudinal distribution of the fluctuations. On the nightside, the amplitude depends on geomagnetic activity, and the most active region roughly corresponds to the middle of the large‐scale Birkeland current region. On both sides, the latitude of the maximum of the amplitude distribution roughly corresponds to that of the large‐scale magnetospheric convection reversal, suggesting that the small‐scale fluctuations are generated at the strong convection shear.
The ionosphere‐magnetosphere structure during a geomagnetic storm based on measurements in the morning auroral zone
Authors: Denig W. F. , Rich F. J.
Published: February 2008

Measurements of field‐aligned currents (FAC's), convection electric fields, and precipitating electrons were made by the S3‐2 satellite in the midmorning time sectors of both the northern and southern high‐latitude ionospheres before and during a geomagnetic storm on June 24–25, 1976. The patterns of electric fields and FAC's did not match the standard configuration of two‐cell convection and the region 1/region 2 FAC's. In particular, near the beginning of the main phase of the storm an intense (242 mV/m) convection electric field and an extra FAC were observed just poleward of the region 1 current in the southern hemisphere. The observations, made over a period of at least 3 hours, were consistent with a quasi steady state three‐cell convection pattern with stresses from a strong By component of the interplanetary magnetic field (IMF). An analysis of the particle and field data from within the region of the intense convection electric field is in sharp disagreement with the single‐particle motion model of Lyons (1980) and implies that a significant portion of the measured FAC was from ionospheric ions. Finally, large‐amplitude AC electric waves from broadband electrostatic noise (BEN) were present in the high‐altitude southern (or winter) passes but not in the northern passes at lower altitudes. In general, BEN has been observed occasionally by S3‐2 within the high‐altitude (1000–1500 km) winter auroral regions but has never been detected during other seasons or at lower altitudes. The implication of these measurements is that the low‐altitude limit for the region of field‐aligned potentials is determined by the local plasma density.
Diffusive acceleration of auroral primaries
Authors: Lotko William
Published: February 2008

The acceleration of electrons in a parallel electric field containing randomly distributed, micro double layers is investigated. The study is motivated by recent electric field observations in the auroral acceleration region. Its objective is to determine what effects stochastic versus static parallel electric fields may have on the spectral features of precipitating electrons. The average electron response as characterized by friction and diffusion coefficients is first analyzed. These coefficients are then used in a Fokker‐Planck equation for the altitudinal variation in the electron energy distribution. Electron sources at the top of the acceleration region (plasma sheet electrons) and at its latitudinal edges (ionospheric electrons) are included. It is shown that (1) the plasma sheet source gives rise to “inverted V” precipitation, as in quasi‐static models, and (2) the edge source is responsible for highly collimated, suprathermal precipitation. The spectral features of the former depend primarily on the average electric field and only weakly on its fluctuations. The properties of collimated precipitation are very sensitive to the microfield, however, owing to trapping phenomena. This analysis represents the first quantitative treatment of collimated electron precipitation; its predictions are in agreement with the observed intensity, pitch angle distribution, energy spectrum, and latitudinal width of edge region precipitation.
A simple kinetic theory of auroral arc scales
Authors: Chiu Y. T.
Published: February 2008

A kinetic theory of the origins of the auroral arc scale spectrum is presented in this paper. The conceptual basis of the theory is current conservation in a turbulent plasma at the magnetospheric equatorial region in which a field‐aligned current is generated and the local electrostatic potential structure is forced to adjust to the presence of the field‐aligned current. This simple model uses an ad hoc Ohm's law relationship between the perpendicular current and the perpendicular electric field, but with a negative conductance in the generator region so that J⊥·E⊥ < 0. An exact solution of a simple model of the concept yields a bistatic auroral generator for which multiple‐arc formation is predicted if the field‐aligned current exceeds a critical value. The predicted scale spectrum is inversely proportional to the square root of the field‐aligned current strength spectrum.