bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for chorus associated electrostatic bursts
Authors: Grabbe Crockett L.
Published: February 2008

The linear theory of the generation of electrostatic bursts of noise by electrons trapped in chorus wave packets is developed for a finite temperature electron beam and a Maxwellian electron and ion background. The growth rates determined are qualitatively in good agreement with those obtained by previous authors from a more idealized model. Two connected instability mechanisms seem to be occurring: a beam plasma (electron‐ion two‐stream) instability commonly associated with intensification of the chorus power levels, and a transitional or borderline resistive medium instability commonly associated with chorus hooks. The physical reasons for the two mechanisms is discussed. In the second case, electron beams are difficult to identify in the particle data. An expression is obtained for the maximum growth rate in terms of the ratios of the beam and electron thermal velocities to the beam velocity and of the beam density to plasma density. It is anticipated that this may allow the observed peak in the electrostatic noise spectrum to be used as a diagnostic for the beam characteristics.
Measurements of the optical emission produced during the laboratory beam plasma discharge
Authors: Hallinan T. J. , Leinbach H. , Mantjoukis G. , Bernstein W.
Published: February 2008

Optical observations of a beam‐plasma discharge (BPD) in the laboratory showed that the discharge remained confined to a diameter little more than double that of the beam for injection parallel to the magnetic field and approximately equal to that of the beam for injection at large pitch angles. The diameter was independent of beam current but varied linearly with beam velocity and inversely with magnetic field strength. The ionization rate inferred from the total emission of 3914 Å, integrated over the radial extent of the beam, was proportional to the excess beam current above that required for BPD ignition. The proportionality constant (12±2) × 1014 ions/cm s A was valid over a wide range of pressure and of magnetic field strength. Power loss to ionization in a 20 m path was estimated at up to 4% of the beam power. Evidence is presented for effective confinement of suprathermal electrons (parallel to B) by some unidentified process other than electrostatic confinement.
Magnetic field line reconnection experiments: 6. Magnetic turbulence
Authors: Gekelman W. , Stenzel R. L.
Published: February 2008

Extensive statistical analysis of the vector components of fluctuating magnetic fields have been performed in a time dependent neutral magnetic sheet. Cross spectral analysis indicates a variety of wave numbers present for each frequency investigated. Comparison of Fourier components of the cross spectral function with dispersion surfaces in k space demonstrates the waves are large amplitude whistlers; this is verified by polarization analysis, which shows the random waves' magnetic fields to be right‐hand circular. Ion acoustic and Langmuir turbulence are also observed along with bursts of microwave radiation. Measurements of the electron distribution function f(v, r, t) and its fluctuations in velocity space relate wave and particle activity.
Direct multiple path magnetospheric propagation: A fundamental property of nonducted VLF waves
Authors: Sonwalkar Vikas S. , Bell T. F. , Helliwell R. A. , Inan U. S.
Published: February 2008

ISEE 1 satellite observations of nonducted whistler mode signals from the Siple Station VLF transmitter show that all well‐defined pulses (S/N ≈ 20 dB) are elongated by 20 ms to 200 ms. This elongation is attributed to closely spaced multiple paths of propagation between the ground and the satellite. The presence of multiple paths is further confirmed by the observed amplitude fading pattern where fading occurs not only at half the spin period (1.52 s) of the rotating antenna but also at time scales of the order of 1 s. Electric field measurements show a 2 to 10‐dB amplitude variation, which is again consistent with direct multiple path propagation. We illustrate the results with two cases: one on October 29, 1977, inside the plasmapause and the other on May 7, 1979, outside the plasmapause. Our results establish that, in general, at any point in the magnetosphere the direct signals transmitted from the ground arrive almost simultaneously along two or more closely spaced direct ray paths. It is shown that multiple paths can be explained by assuming field‐aligned irregularities of 1 to 10‐km horizontal scale in the ionosphere with a few percent enhancement or depletion in the plasma density. We discuss the implications of our results for nonducted wave‐particle interaction in the earth's magnetosphere. We show that for reasonable parameters of nonducted multiple path propagation, a cyclotron resonant electron will experience a wave doppler broadening of a few tens of hertz to a few hundreds of hertz.
Relations between polarization and the structure of ULF waves in the magnetosphere
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

Large‐scale low‐frequency ULF signals in the terrestrial magnetosphere are expected to have the structure of standing waves along the ambient magnetic field and to propagate across the field. By making simple assumptions regarding the wave form, we show that wave polarization in a plane perpendicular to the magnetic field B may be interpreted in terms of the signal structure across B. Two cases are distinguished, corresponding to a pure Alfvén mode (transverse) wave and a purely compressional wave. Examining published ULF data, we find that many compressional signals appear to have transverse polarization more appropriate to the Alfvén mode. We conclude that the compressional signals are commonly coupled to Alfvénic disturbances, but there is evidence of purely Alfvénic signals at times.
Ionospheric and Birkeland current distributions for northward interplanetary magnetic field: Inferred polar convection
Authors: Zanetti L. J. , Potemra T. A. , Iijima T. , Baumjohann W. , Bythrow P. F.
Published: February 2008

A new, stable system of Birkeland and ionospheric currents has been added to the existing Region 1‐Region 2‐auroral electrojet system when the interplanetary magnetic field (IMF) is directed northward. These currents exist poleward of the Region 1 system, across the polar region, and have been called the northward IMF Bz (NBZ) current system. A companion paper [Iijima et al., this issue] describes in detail the Birkeland currents' magnetic signature, a W‐shaped pattern, in the sunward‐antisunward direction and the NBZ system' dependence on the various components of the IMF. The companion study used MAG‐SAT magnetometer data from 146 orbits over the southern polar regions from November 1979 to January 1980. Data from two days, November 11, 1979, and January 8, 1980, have been analyzed in great detail with regard to the NBZ W signature of Birkeland currents. In addition, a Fourier technique has been used in an analysis of the disturbance component parallel to the main field to derive the ionospheric current system during those two days. The NBZ ionospheric current system has shown a similar W‐shaped pattern with antisunward current (implying sunward convection) over the magnetic pole and with return currents on either side. Multicell patterns emerge from the NBZ ionospheric patterns, and an asymmetry appears to be caused by the IMF By component as well. Suggestions are given for multicell patterns over the polar regions for northward IMF during an IMF By sign change.
Scale lengths in quasi‐parallel shocks
Authors: Scudder J. D. , Burlaga L. F. , Greenstadt E. W.
Published: February 2008

Examples of an interplanetary shock and the earth's bow shock are presented to illustrate the small‐scale size Lp of the fluid deceleration relative to the scale of the magnetic fluctuations, LM, at quasi‐parallel shocks. The increase in electron and ion random energies is also illustrated to occur on the short “inner” scale of Lp. The selected interplanetary and bow shock examples are both supercritical high‐β shocks but have different Alfvén Mach numbers. The thickness Lp in absolute and convected Larmor radii units of the lower Alfvén Mach number interplanetary shock is larger (∼10 U′/Ωci) than that at the bow shock (∼(1–2) U′/Ωci), where U′ is the plasma flow speed viewed in the normally incident shock rest frame. In both examples the scale of the fluid deceleration is much smaller than that of the up‐ or downstream magnetic fluctuations. The existence of steady state quasi‐parallel shocks requires that the Lorentz deceleration force be much larger than the electrostatic deceleration force along the shock normal.
A fast Fermi process: Energetic electrons accelerated by a nearly perpendicular bow shock
Authors: Wu C. S.
Published: February 2008

The highly localized acceleration of electrons at the foreshock reported by Anderson et al. (1979) is explained in terms of a fast Fermi process. The basic notion is that in the solar wind frame the nearly perpendicular bow shock at the point of tangency of the interplanetary magnetic field acts as a fast‐moving magnetic mirror which can reflect electrons with sufficiently large pitch angles. The reflection process can effectively energize electrons and drive them upstream. If the seed electrons have energies of several hundred electron volts, they can attain energies of several keV through the acceleration process.
A model for the propagation of the westward traveling surge
Authors: Rothwell Paul L. , Silevitch Michael B. , Block Lars P.
Published: February 2008

A unified model is developed for the propagation of the westward traveling surge (WTS) that can explain the diversity in the observed surge characteristics. We start with the Inhester‐Baumjohann model for the surge region, which implicitly includes both the Hall and Pedersen currents. It is found that precipitating electrons at the conductivity gradient modify the gradient, causing it to propagate as a wave front. The velocity of propagation is directly dependent on the ionization efficiency of the precipitating electrons and therefore increases dramatically when they become more energetic during substorm onsets. For example, we predict that when the incident electron energy changes from 1 keV to 10 keV the surge velocity should increase from 2 km/s to 34 km/s. The direction of the surge motion depends on the presence of polarization charges on the poleward surge boundary. This is related to the efficiency with which the poleward ionospheric currents are closed off into the magnetosphere by the field‐aligned currents. Inclusion of the electron‐ion recombination rate modifies the surge propagation velocity and leads to explicit expressions for the conductivity profile. Sufficient precipitation current is required to overcome electron‐ion recombination in order for the surge to expand. When the precipitating current is less than this threshold the WTS retreats. Therefore, the model describes the ionospheric response to both the expansion and recovery phases of the magnetic substorm.
Morphology of the polar rain
Authors: Gussenhoven M. S. , Hardy D. A. , Heinemann N. , Burkhardt R. K.
Published: February 2008

Results of a statistical study of the occurrence and characteristics of the polar rain (Winningham and Heikkila, 1974) are reported. Precipitating electron data in the energy range from 50 eV to 20 keV from the SSJ/3 sensor on the polar‐orbiting DMSP/F2 satellite were used for the study. Intervals of clear polar rain were identified in all orbits in the 1‐year interval September 1977 to August 1978. The spectra from the intervals were binned in a two‐dimensional spatial array in geomagnetic latitude and magnetic local time. Separate arrays were maintained for different levels of magnetic activity, values of the components of the interplanetary magnetic field (IMF), and season. The average spectra were determined in each bin and were used to calculate the integral flux and average energy. Two‐dimensional maps of average energy and integral flux show significant large‐scale spatial variations that are roughly symmetric about an axis running prenoon to premidnight. Integral flux (average energy) decreases (increases) from the dayside to the nightside along this axis by a factor of almost 15 (3). This basic variation was maintained in all separations by magnetic activity, season, intensity of the polar rain, and sector structure, although small rotations in magnetic local time of the symmetry axis could be seen in some cases. The second most prominent variation is one that has been noted previously (Yeager and Frank, 1976; Meng and Kroehl, 1977); polar rain occurs preferentially in the northern (southern) cap for away (toward) IMF sectors. In the preferred cap the precipitation is stronger in the morning. Approximately 70% of the spectra occurred for Bz negative. The overall intensity of the polar rain increases and cools with increasing magnetic activity. The occurrence of polar rain falls within a circular region whose center is offset toward premidnight and whose radius increases with magnetic activity.
Simultaneous observations of Pc 5 pulsations by VHF radars spaced 10 hours in MLT
Authors: Keys J. G.
Published: February 2008

Long‐period Pc 5 pulsations have been recorded simultaneously by VHF radars operating in Scandinavia and New Zealand. The radars are in opposite hemispheres and are spaced about 10 hours in local time. Those pulsations analyzed are either of the “quasi‐regular” variety or take the form of damped trains that often follow magnetic sudden commencements. An estimation of equatorial plane ion mass densities shows azimuthal variations attributable to several possible causes, including the presence of localized density gradients or a significant proportion of heavy ions. A transient‐type pulsation shows the effect of the two radars being directed inside and outside the plasmapause, and a longer‐lasting event suggests a monochromatic drive of different period on the dawn and dusk flanks of the magnetosphere.
Path‐integrated growth of auroral kilometric radiation
Authors: Omidi N. , Gurnett D. A.
Published: February 2008

Using Poeverlein's graphical method, three‐dimensional ray path calculations are performed to evaluate the path‐integrated growth of auroral kilometric radiation (AKR). The ray tracing results indicate that waves whose initial wave vector lie in the local meridian plane continue to propagate in that plane and that among these waves, those with frequencies near the cutoff frequency (fR = 0) refract substantially, whereas those with frequencies well above the cutoff frequency suffer little refraction. It is also shown that waves whose initial wave vector lies outside of the local meridian plane propagate in the longitudinal as well as the radial and the latitudinal directions. The refraction of these waves is also highly dependent upon the wave frequency, i.e., waves with frequencies near fR = 0 refract substantially, whereas waves with frequencies much above fR = 0 undergo little refraction. In order to test the electron cyclotron maser mechanism as a method for generation of AKR, a typical electron distribution function measured in the auroral zone by the S3‐3 satellite is used to calculate path‐integrated growth of representative rays. The results of this study indicate that electron distribution functions like those measured by the S3‐3 satellite are not capable of amplifying cosmic noise background to the observed intensities of auroral kilometric radiation and that much steeper slopes (∂F/∂υ⊥) at the edges of the loss cone are required. The presence of such distribution functions in the auroral zone is plausible if one assumes that backscattered electrons in this region have energies less than a few hundred eV.
The L shell region of importance for waves emitted at ground level as a loss mechanism for trapped electrons > 68 keV
Authors: Imhof W. L. , Reagan J. B. , Gaines E. E. , Datlowe D. W.
Published: February 2008

The objective of this paper is to identify the L shell region(s) where VLF waves emitted at ground level may play a significant role as a loss mechanism for radiation belt electrons with energies above 68 keV. This assessment is made possible by studying the day‐night differences in inner belt electron precipitation and comparing with the much stronger transmission of waves at frequencies of 15–25 kHz through the ionosphere at night and the known higher daytime intensities for waves of natural origin other than lightning. Intensities of electrons trapped on drift shells which dip below sea level in the anomaly are used to study the rates of loss from the radiation belts. In 1979 the inner radiation belt was shown to have had the following unique features for electrons >68 keV: (1) the rates of precipitation from the radiation belts at longitudes west of 140°E were significantly greater near midnight than near noontime for L ≈ (1.6–2.2); (2) at L ≈ 1.75 the nighttime rates of injection showed little correlation with the precipitation on other L shells, in contrast with the daytime precipitation in which a significant correlation existed with the precipitation rates throughout the range L = 1.2–5.0. From these empirical findings one concludes that in the heart of the inner radiation belt at longitudes west of 140°E, lightning and/or transmitter‐generated VLF waves emitted from ground level probably played a significant role in precipitating electrons >68 keV at nighttime. Below the region L = 1.6 and above L = 2.2 the observed diurnal variations were much less pronounced, indicating that VLF waves emitted at ground level in this frequency range were not the major source of precipitation.
Earth's magnetic field perturbations as the possible environmental impact of the conceptualized solar power satellite
Authors: Lee M. C. , Kuo S. P.
Published: February 2008

It is predicted that the earth's magnetic field can be significantly perturbed locally by the microwave beam transmitted from the conceptualized solar power satellite (SPS) at a frequency of 2.45 GHz with incident power density of 230 W/m² at the center of the beam. The simultaneous excitation of earth's magnetic field fluctuations and ionospheric density irregularities is caused by the thermal filamentation instability of microwaves with scale lengths greater than a few hundred meters. Earth's magnetic field perturbations with magnitudes (∼ a few tens of gammas) comparable to those in magnetospheric substorms can be expected. Particle precipitation and airglow enhancement are the possible, concomitant ionospheric effects associated with the microwave‐induced geomagnetic field fluctuations. Our present work adds earth's magnetic field perturbations as an additional effect to those such as ionospheric density irregularities, plasma heating, etc., that should be assessed as the possible environmental impacts of the conceptualized solar power satellite program.
Acceleration of >47 keV Ions and >2 keV electrons by interplanetary shocks at 1 AU
Authors: Tsurutani B. T. , Lin R. P.
Published: February 2008

We present initial results from a survey of the effects of interplanetary shocks on energetic ≥2 keV electrons and ≥47 keV ions, as observed by the field, plasma, and energetic particle experiments on the ISEE 3 spacecraft. Shock normals, velocities, Mach numbers, and upstream and downstream plasma parameters were determined for 37 forward shocks out of a total of 55 shocks observed between August 1978 and November 1979. We find that a minimum shock velocity along the upstream magnetic field of ∼250 km/s is required for an interplanetary shock to have a significant effect on acceleration of ≥2 keV electrons or ≥47 keV ions. Shocks with no effect on the energetic particle populations also had relatively small ratios of downstream to upstream magnetic field magnitudes. These results suggest that magnetostatic reflection off the shock itself is a significant mechanism in the acceleration process. Both energetic electron and ion flux variations associated with shocks can be classified into four general types: (1) no significant variation at all, (2) a spike of a few minutes duration at or near the shock, (3) a steplike postshock increase, and (4) a slow rise beginning several hours before the shock (energetic storm particle event). Essentially, every quasi‐perpendicular shock with θBn ≳ 70° produced a shock spike in the proton fluxes, while every quasi‐parallel shock (θBn ≲ 50°) produced a proton energetic storm particles event, provided the shock velocity was greater than the above stated threshold. Electron spikes were also observed for most, but not all, shocks with θBn ≳ 70°. The most common effect observed in the electron fluxes was a steplike postshock increase of a factor of ∼2. These had no obvious dependence on θBn, but were found for every shock with speed greater than ∼175 km/s. Shock effects in the electron fluxes were about as common as for protons, but were limited to the 2–10 keV energy range except for 3 events which extended up to ∼50 keV. We find that significant ambient populations of both ≥2 keV electrons and ≳47 keV ions are present in the interplanetary medium prior to every shock. These particles could be the “seed” particles for the shock acceleration.
Electron plasma waves upstream of the Earth's bow shock
Authors: Lacombe Catherine , Mangeney André , Harvey Christopher C. , Scudder Jack D.
Published: February 2008

Electrostatic waves are observed around the plasma frequency fpe in the electron foreshock, together with electrons backstreaming from the bow shock. Using data from the sounder aboard ISEE 1, we show that this noise, previously understood as narrow band Langmuir waves more or less widened by Doppler shift or nonlinear effects, is in fact composed of two distinct parts: One is a narrow band noise, emitted just above fpe, and observed at the upstream boundary of the electron foreshock. This component has been interpreted as Langmuir waves emitted by a beam‐plasma instability. We suggest that it is of sufficiently large amplitude and monochromatic enough to trap resonant electrons. The other is a broad band noise, more impulsive than the narrow band noise, observed well above and/or well below fpe, deeper in the electron foreshock. The broad band noise has an average spectrum with a typical bi‐exponential shape; its peak frequency is not exactly equal to fpe and depends on the Debye length. This peak frequency also depends on the velocity for which the electron distribution has maximum skew. An experimental determination of the dispersion relation of the broad band noise shows that this noise, as well as the narrow band noise, may be due to the instability of a hot beam in a plasma.
Long duration lion roars associated with quasi‐perpendicular bow shocks
Authors: Rodriguez P.
Published: February 2008

A statistical study of the plasma waves known as lion roars shows that long duration lion roars (≳5 min) are often detected immediately behind or in the nearby downstream region of subsolar quasi‐perpendicular bow shocks. The long duration lion roars (LDLR) are associated with relatively ordered magnetosheath conditions as indicated by lower‐than‐normal magnetic field variances and higher‐than‐normal magnetic field intensities. These statistical results suggest that the relatively ordered downstream conditions associated with quasi‐perpendicular bow shocks are conducive to nearly continuous lion roar emissions. The LDLR exhibit field‐aligned propagation, as indicated by wave fluctuation amplitudes that minimize in directions parallel to the magnetic field. The distribution of LDLR frequencies f with respect to the electron gyrofrequency fge peaks at f/fge ≃ 0.25. The propagation direction and frequency ratio is consistent with previous identification of lion roars as whistler waves. By using the whistler dispersion relation and assuming field‐aligned propagation, magnetosheath plasma densities during the occurrence of LDLR can be calculated. The distribution of densities thus determined is consistent with the enhanced plasma densities of the flow downstream of quasi‐perpendicular bow shocks. The source of LDLR has not been determined; however, instabilities related to temperature anisotropy in thermal or beam electrons are possible candidates.
Long‐delayed radio echoes: Mechanisms and observations
Authors: Vidmar R. J. , Crawford F. W.
Published: February 2008

Experiments carried out in Alaska with the object of observing long‐delayed radio echoes (LDE) are described. The results are subjected to critical sonogram analysis, together with data from previous experiments carried out at Stanford and elsewhere. It is concluded that, although strong supporting evidence for sporadic LDE phenomena has accumulated over the last 50 years, and a variety of plausible mechanisms have been proposed, stringent analysis of records such as those discussed in the paper almost invariably reveal alternative possible explanations for them. Further detailed experimentation is required on mechanisms, for which the phenomenon seems best established, e.g., round‐the‐world echoes and other forms of guided propagation.
Observations of a possible ground signature of flux transfer events
Authors: Goertz C. K. , Nielsen E. , Korth A. , Glassmeier K. H. , Haldoupis C. , Hoeg P. , Hayward D.
Published: February 2008

Two periods during which the convection boundary was in the Scandinavian Twin Auroral Radar Experiment field of view a few hours before and after magnetic noon are analyzed. We find that sporadic and spatially isolated flow across the convection boundary occurs on time scales of a few minutes and in regions which in the ionosphere range in size from 50 km to 300 km. We suggest that the antisunward flow immediately north of the convection boundary maps into the low‐latitude boundary layer. Our observations then indicate that the magnetic flux in the boundary layer is transported outward. We estimate a typical flux transfer rate of 2.5×104 Wb s−1 for one flux transfer event. On one occasion, GEOS 2 observes a rapid simultaneous dropout of energetic ions and electrons inside the magnetopause. This dropout is associated with a ground E × B signature of strong northwesterly flow. We believe that this constitutes good evidence for the occurrence of flux transfer from closed to open field lines.
A statistical model of auroral electron precipitation
Authors: Hardy D. A. , Gussenhoven M. S. , Holeman E.
Published: February 2008

A statistical study has been completed using data from the Defense Meteorological Satellite Program F2 and F4 and the Satellite Test Program P78‐1 satellites to determine the average characteristics of auroral electron precipitation as a function of magnetic local time, magnetic latitude, and geomagnetic activity as measured by Kp. The characteristics were determined for each whole number value of Kp from 0 to 5 and for Kp ≥ 6‐. At each level of Kp, the high‐latitude region was gridded, and the average electron spectrum in the energy range from 50 eV to 20 keV was determined in each grid element. The results show that the high‐latitude precipitation region separates into two parts based on the electron average energy. There is a region of relatively hot electrons (EAVE ≥ 600 eV). In this region, the electron average energies are highest on the morningside of the oval. There are two average energy maxima at each Kp level: one postmidnight and the other prenoon. The hot electron region is generally not continuous in MLT but shows a gap between 1200 and 1800 MLT. The hotter electrons carry most of the energy flux into the oval. The energy flux on the nightside increases with Kp, while the level at noon increases with Kp when Kp is small but decreases at higher Kp. The average energy of the hot electrons increases from Kp = 0 to Kp = 3 but is approximately constant for higher Kp. In the second region, average energies are low (EAVE < 600 eV). This region extends from the poleward edge of the hot electron region to the pole. The precipitating electrons in this region carry the majority of the number flux at high latitudes. The largest number fluxes are found on the dayside. The highest fluxes are confined to a crescent‐shaped region centered slightly prenoon and extending in MLT over most of dayside and, in some cases, into the nightside. There is a prenoon maximum in the number flux that shows little variability in MLT or in intensity with Kp. The average energy shows a minimum typically between 1100 and 1200 MLT and located toward the poleward edge of the crescent‐shaped region of highest integral number flux. We identify the cusp as the region near the average energy minimum and the cleft as the crescent‐shaped region.
Geocoronal structure: The effects of solar radiation pressure and the plasmasphere interaction
Authors: Bishop James
Published: February 2008

The theory of planetary exospheres is extended to incorporate solar radiation pressure in a rigorous manner, and an evaporative geocoronal prototype (classical, motionless exobase) is constructed using Liouville's theorem. Model calculations for density and kinetic temperature at points along the earth‐sun axis (solar and antisolar directions) reveal an extensive satellite component, comprising ∼⅔ of the total hydrogen density near 10 earth radii, and a temperature profile suggestive of an isotropic quasi‐Maxwellian velocity distribution for the bound component. A geotail is also evident as an enhancement of the density at local midnight compared to local noon that increases outward (from ∼25% at 10 earth radii to over 60% at 20 earth radii). Additional mechanisms acting upon the geocorona alter the basic evaporative case in notable ways. Solar ionization has been included in a simple fashion; the effect is to partially deplete the density without otherwise altering the structure. Interaction with a simple plasmasphere via the Boltzmann equation results in “heating” the geocorona and enhancing the escape flux at the expense of the density of the bound component, an effect not appreciated in earlier studies; the geotail survives this interaction.
The ultraviolet dayglow at solar maximum: 1. Far UV spectroscopy at 3.5 Å resolution
Authors: Eastes R. W. , Feldman P. D. , Gentieu E. P. , Christensen A. B.
Published: February 2008

The earth's far ultraviolet dayglow (1080–1515 Å) was observed at ∼3.5 Å resolution during a period of high solar activity near solar maximum on June 27, 1980. The observations were made at local noon by rocket‐borne spectrometers viewing toward the earth's northern limb at 90° zenith angle (ZA) at altitudes between 100 and 245 km, and at 98° ZA between 245 and 260 km. The solar zenith angle was 8.9°. These spectra are compared with earlier lower‐resolution dayglow data obtained during a period of lower solar activity and with auroral spectra. The brightness ratio of O I λ1356 to the N2 Lyman‐Birge‐Hopfield (LBH) system, an indicator of the O to N2 density ratio, is lower than that previously measured at mid‐latitudes and closer to the value found in aurorae. In the LBH system a depletion of the bands originating on the υ′ = 3 vibrational level of the excited state is found. Some weak N2 Birge‐Hopfield bands and N I lines only marginally detected previously in the dayglow are confirmed.
The quadrupole magnetopause
Authors: Leubner M. P. , Zollner K.
Published: February 2008

The magnetopause of a symmetric quadrupole embedded in a parallel streaming plasma is calculated by means of the method of the Chapman‐Ferraro problem. The results are discussed with special attention being given to the position of the neutral points for the two symmetric cases of interaction.
Simulation of the electron acoustic instability in the polar cusp
Authors: Lin C. S. , Winske D. , Tokar R. L.
Published: February 2008

Simulations of the electron acoustic instability are conducted with plasma parameters based on the DE 1 observations near the polar cusp. The cusp plasma is assumed to consist of hot electrons, cold electrons, a warm electron beam, and a neutralizing background. The electron acoustic mode, a relatively unknown electrostatic mode, is found to be driven unstable by an electron beam in the presence of a two‐temperature electron plasma. We use a particle‐in‐cell code to examine the temporal behavior of this instability over a range of parameters, including the beam speed and the cold electron density. The simulation results are shown to agree with the linear theory with regard to the wave frequency, the growth rate, and the electrostatic nature of the instability. It is found that the heating rate for each electron component determined from the simulations is in agreement with second‐order theory and that the saturation level of the fluctuations is consistent with nonlinear theory. The simulations demonstrate that the instability saturates by either trapping the beam, trapping the cold electrons, or forming a plateau on the beam distribution. Under some conditions the instability can strongly heat the cold electrons. By comparing the simulation results with the DE 1 plasma and wave observations, we conclude that the observed low‐energy electron beam can generate broadband electrostatic emissions in the frequency range of cusp hiss by means of the electron acoustic instability. However, the simulations also indicate that a more detailed examination of the plasma observations is needed to explain the origin of the electromagnetic component of cusp auroral hiss.
Drift motions of very high latitude F region irregularities: Azimuthal Doppler analysis
Authors: Hanuise C. , Greenwald R. A. , Baker K. B.
Published: February 2008

Since October 1983, a new HF radar facility has been operated from Goose Bay, Labrador, for the purpose of studying high‐latitude ionospheric irregularities. In this paper, we present autocorrelation functions of the backscattered signals from these irregularities and show how these are processed to yield information on the drift velocities of the irregularities. Since the radar is typically operated in an azimuth scan mode, the Doppler data derived from the autocorrelation functions may be used to study the two‐dimensional structure of the irregularity drift. A procedure is presented for this analysis, and it is shown that the results are reasonably accurate during periods in which the irregularity drift is approximately uniform. However, if the flow is nonuniform, as it is in the vicinity of the cusp and in the midnight local time sector, then the procedure leads to erroneous results. We consider the implication that these results have on azimuth scan measurements of plasma drift with incoherent scatter radars, and we present possible ways in which measurements might be made under nonuniform flow conditions.
Helios images of solar mass ejections
Authors: Jackson B. V. , Leinert C.
Published: February 2008

The zodiacal light photometers on board the Helios B spacecraft are used to form images of solar mass ejection transients in the interplanetary medium. Several aspects of these data are unique: (1) They trace shapes of solar mass ejections observed near the solar surface in coronagraph images to distances at least as great as 0.5 AU using electron Thomson scattering. (2) When compared with earth‐based observations, the Helios data allow determination of the three‐dimensional shapes of the ejections. (3) The global viewpoint of the Helios spacecraft photometers allows imaging of mass ejections heading toward the spacecraft and then observations from within the ejection. Excess masses (above the ambient level) are determined for several ejections, and we find that they are generally comparable to or somewhat larger than masses obtained by coronagraph observations of ejections when they are closer to the sun.
Precipitating electron interaction with the atmosphere: 2. The dayside cusp region
Authors: Prasad Sheo S. , Strickland Douglas J. , Chiu Yam T.
Published: February 2008

Interaction of precipitating low‐energy magnetosheath electrons with the atmosphere in the dayside cusp region has been studied. Both pitch angle and energy distributions of the fluxes as well as excitation functions for selected N2 and O UV emission were obtained by numerically solving the multiangle equations of electron transport. The angular dependence of the elastic scattering was treated more precisely—in contrast to its treatment in previous studies via limiting approximations involving either sharply forward peaked or isotropic shapes. Pitch angle distributions with broad peaks around 90° were encountered, albeit in a very narrow energy, band, in the upper atmosphere above the collision‐dominated lower atmosphere where isotropic pitch angle distributions predominate. Pitch angle distributions in the upper atmosphere at energies below this narrow band are approximately mirror images of the distributions at higher energy in the sense that the high and low values of the two distributions are located in diametrically opposite pitch angle directions. In our energy regime, it is difficult to accurately infer the spectral characteristics of the incident flux from the spectral characteristics of the flux measured at 350–250 km because the densities at even these altitudes are sufficient to substantially alter the spectral characteristics of low‐energy incident electrons. In contrast, the incident flux in the keV region travel through this altitude region almost unaffected. For low precipitating fluxes on the order of (1–2) × 108 el cm−2 s−1, photoelectrons may constitute a significant fraction of the measured flux for solar zenith angles as large as 100°. Influence of the photoelectrons decreases if either the precipitating flux increases or the solar zenith angle increases. There is some possibility that atmospheric emissions may be used for remote measurements of incident soft energy flux, because the ratios of molecular to atomic emission line intensities in the low‐energy region are quite different from those in the high‐energy region.
Ion‐acoustic‐like waves excited by the reflected ions at the Earth's bow shock
Authors: Akimoto K. , Winske D.
Published: February 2008

Some model distributions based on recent observations and simulations of the plasma at the earth's bow shock are found to be unstable to obliquely propagating electrostatic instabilities. The model distributions consist of either an ion beam or an ion velocity ring accompanied by a bi‐Maxwellian background ion distribution and a flattop electron distribution. Ion anisotropies and nonthermal electrons are capable of significantly lowering the threshold of the ion beam instability. The generated waves share many properties in common with the ion acoustic waves that have been observed at the earth's bow shock. The results also indicate the importance of an anisotropy in the background ion velocity distribution in identifying sources of the ion‐acoustic‐like waves observed in the ion foreshock and the solar wind.
Field‐aligned currents in the Earth plasma sheet
Authors: Hruška A.
Published: February 2008

Steep gradients, directed across the lines of force, of quantities characterizing the “plasma sheet boundary layer” plasma cause a diversion of the perpendicular electric current j⊥ into the direction of B, provided that j⊥ has a component parallel to grad⊥ B, i.e., whenever the angle α between j⊥ and grad⊥ B is different from 90°. The global variation of α is inferred from particle observations on board polar‐orbiting satellites; the global pattern of the field‐aligned currents (FACs) corresponding to this variation of α is in agreement with the features of FACs indicated by their “region 1” distribution; small‐scale, irregular FACs are related to the transverse structure of the boundary layer.
A parametric study of slow shock Rankine‐Hugoniot solutions and critical Mach numbers
Authors: Edmiston J. P. , Kennel C. F.
Published: February 2008

We present a parametric survey of the Rankine‐Hugoniot solutions for slow MHD shock waves. We also calculate the parametric dependences of the critical Mach numbers above which resistivity or thermal conduction alone can no longer provide all the shock dissipation, as well as the Mach number above which the flux of shock‐catching ions can be significant. Slow shocks rival fast shocks in the degree to which their properties might depend upon upstream parameters.
Thermal ions at Saturn: Plasma parameters and implications
Authors: Richardson John D.
Published: February 2008

Plasma data from the PLS experiment on Voyagers 1 and 2 are analyzed at the Saturn encounters. Measured ion currents are simulated using the full response function of the PLS instrument assuming plasma distributions are isotropic Maxwellians. The plasma velocity, densities, and temperatures which best simulate the observations are determined. Saturn's magnetosphere is found to subcorotate outside of L = 5.5 for Voyager 1 and L = 7 for Voyager 2. Large radial and vertical flows are observed, especially in the outer magnetosphere and on the nightside. Ion temperatures increase with L to a maximum at L = 10, outside of which they vary rapidly. A temperature anisotropy with T⊥ > T∥ is necessary to account for the observed temperatures and densities. The ions present are H+ and O+; there is no evidence of heavier ions or higher charge states. Calculations of NL² show an O+ peak at L = 10 for both encounters, indicating the major O+ source is at L = 10.
Monte Carlo simulation of charged particle impact on the satellites of Jupiter and Saturn
Authors: Bell E. V. , Armstrong T. P.
Published: February 2008

Ensemble‐averaged lifetimes of Jovian and Saturnian trapped charged particles against loss by impact on natural satellites are presented. Precise values of these loss rates can be obtained from the known geometry of the satellite orbits, magnetic fields, and particle trajectory. These loss rates are needed as an ingredient of transport theory which is usually formulated in terms of the first and second adiabatic invariants and radial distance. The effects of satellite orbital inclination and eccentricity, ion charge to mass ratio and energy, equatorial pitch, and distance from the planet on the particle lifetime are discussed utilizing a tilted, but not offset, dipolar field geometry resulting in lifetime values expressed as functions of the first two adiabatic invariants and the equatorial radial distance from the planet (τlife(μ, K, L)).
Differential excitation cross section of atomic oxygen by electron impact: (³P‐¹D transition)
Authors: Shyn T. W. , Sharp W. E.
Published: February 2008

Absolute differential excitation cross sections of atomic oxygen (³P‐¹D transition) by electron impact have been measured by a crossed‐beam method. Atomic oxygen was generated by a thermal dissociation method. The energy and angular range covered were from 7 to 30 eV and from 30° to 150°, respectively. A few theoretical results were compared with the present measurements and they are in good agreement with the experimental results within the accuracy of the measurement.
Kelvin‐Helmholtz instability at the magnetopause boundary
Authors: Wu C. C.
Published: February 2008

At the magnetopause boundary, the Kelvin‐Helmholtz instability should be convective; the excitation of the instability should move tailward with a finite velocity. However previous studies of the Kelvin‐Helmholtz instability have been limited to periodic system with system length equal to the initial perturbation wavelength. In this paper the convective Kelvin‐Helmholtz instability is modelled by solving the MHD equations numerically as an initial value problem. It is found that the convective Kelvin‐Helmholtz instability has the same linear growth rate as the periodic Kelvin‐Helmholtz instability. But at the nonlinear stage, the periodic conditions in the periodic system stabilize the nonlinear growth of the instability. In contrast, the convective Kelvin‐Helmholtz instability can grow to a much larger amplitude and, thus, introduce many interesting phenomena. For instance, in the perpendicular configuration (B0 ⊥ v0), it can create a very large vortex flow and generate shocks off the vortex boundary. In the parallel configuration (B0 ∥ v0), expulsion of the magnetic field is observed when B0 is small, and a boundary layer is formed when B0 is large. Our conclusion is that the Kelvin‐Helmholtz instability may be more important than indicated by previous studies. However, it is suggested that its effects on the magnetosphere can be estimated only when realistic geometry and flow conditions are used.
Highly relativistic electrons in the Earth';s outer magnetosphere: 1. Lifetimes and temporal history 1979–1984
Authors: Baker D. N. , Blake J. B. , Klebesadel R. W. , Higbie P. R.
Published: February 2008

Highly relativistic electrons (3–10 MeV) at times are observed to populate the earth's magnetosphere near the geostationary orbit (r ∼ 6.6 RE). Electron fluxes and energy spectra are shown which were measured by two high‐energy electron sensor systems at 6.6 RE from 1979 to the present. Large, persistent increases in this electron population were found to be relatively infrequent and sporadic in 1979–1981 around solar maximum. This situation contrasts markedly with the frequent occurrence of very large flux increases from late 1981 to the present. Furthermore, in this latter period, which constitutes the approach to solar minimum, it is observed that the highly relativistic electrons occur with a relatively regular 27‐day periodicity and are well associated with the reestablished solar wind stream structures. On the basis of an examination of long‐term flux variations as a function of solar rotation we find strongly periodic behavior beginning with Bartels rotation ∼2035. Through a superposed epoch analysis technique we study empirical particle lifetimes as a function of particle energy. An energetic electron enhancement typically rises on a 2‐ to 3‐day time scale and decays on a 3‐ to 4‐day time scale at essentially all energies above ∼3 MeV. The present analysis suggests either that a regular pulsing by high‐speed solar wind streams makes the magnetosphere an efficient relativistic electron accelerator or else that the Jovian magnetosphere is a recurrent source of a significant electron population in the outer terrestrial magnetosphere.
Theory and simulations of broadband electrostatic noise in the geomagnetic tail
Authors: Ashour‐Abdalla M. , Okuda H.
Published: February 2008

Linear theory analysis and particle simulation studies were carried out in an attempt to further our understanding of the mechanisms generating broadband electrostatic noise and the effects of BEN on particles in the geomagnetic tail region. The plasma sheet particle population was modeled by counter‐streaming ion beams as well as by warm ions and electrons. When the beam temperature, Tb, is much smaller than that of the warm plasma sheet electrons, Te, both beam‐ion acoustics and ion‐ion two‐stream instabilities grow. The frequency of the ion acoustic instability is given by ω = ±(k(nb/ne)1/2 cs − k∥U) ≃ ± k∥U ≤ ωpe where cs = (Te/mi)1/2 is the ion sound speed and U is the ion beam speed relative to the warm plasma sheet electrons. When the beam ion temperature is comparable to the temperature of the warm plasma sheet particles, electrostatic ion cyclotron instabilities become unstable, giving rise to low frequency noise at ω ≲ nΩi where Ωi is the ion gyrofrequency. The simulation results confirm the presence of broadband electrostatic noise extending from ω = 0 to ω = k · U ≲ ωpe, with an amplitude of E ∼ mV/m, which is in good agreement with the satellite data. The observed wave intensity also peaks near perpendicular propagation, which is also in agreement with space observations.