bluuebunny/crossref_metadata_2025 huggingface dataset:
Response of dayside Pc 5 pulsations to substorm activity in the nighttime magnetosphere
Authors: Samson J. C. , Rostoker Gordon
Published: February 2008

The possibility of using ULF (1–20 mHz) waves to diagnose the structure of the magnetosphere has recently given new impetus to the study of Pc 4,5 magnetic pulsations. In this paper we demonstrate that the frequency spectrum of dayside Pc 4,5 pulsations near noon may be significantly altered in association with the onset of a magnetospheric substorm near midnight. The response time for the dayside Pc pulsations to a substorm onset can be as short as 2–3 min, suggesting information transfer across the magnetosphere at velocities of the order of the Alfvén speed. The characteristic response of the dayside pulsations is a marked increase in the dominant frequency at stations inside the dayside auroral oval. Our results taken together with the observations of dayside auroras by Eather et al. [1979] suggest that substorm onsets are accompanied by a sudden inward motion of the center of the partial ring current. We propose that this ring current motion causes changes in the magnetic field in the equatorial plane of the magnetosphere that result in changes in the Alfvén velocity on field lines where the magnetic pulsations are observed. Possible mechanisms for the generation of Pc 4,5 pulsations are discussed in the light of the observations reported in this paper.
EUV flux variations during solar cycle 21 from AE‐E He+ abundances
Authors: Oppenheimer M. , Babeu S. , Brinton H. C.
Published: February 2008

The ionization rate of helium in the thermosphere is inferred from AE‐E ion and neutral particle composition measurements, made on several days during solar cycle 21, in the context of a steady state photochemical model for helium ions. The inferred ionization rate varies by a factor of 4 between July 1976 and January 1979, a value consistent with the variation in the flux measured during the same period by the EUV spectrometer on board the AE‐E satellite.
Anomalous heating of the polar E region by unstable plasma waves 1. Observations
Authors: Schlegel Kristian , St.‐Maurice J. P.
Published: February 2008

With the incoherent scatter facility at Chatanika, Alaska, we have measured electron temperatures of up to 1200°K near 110‐km altitude in the polar E region in the presence of strong electric fields. No classical heat process can account for these high temperatures, which are clearly correlated with the dc electric field strength. We show that the enhancement is most probably caused by the unstable plasma waves known to exist in the disturbed polar E region.
Neutral winds and electric fields in the dusk auroral oval 1. Measurements
Authors: Mikkelsen I. S. , Jørgensen T. S. , Kelley M. C. , Larsen M. F. , Pereira E. , Vickrey J.
Published: February 2008

In February and March 1978 two rockets were launched from Poker Flat, Alaska. Each rocket carried both an upleg and a downleg trimethyl aluminum (TMA) chemical trail release as well as three barium cannisters. The TMA trails provided neutral wind measurements between 80 and 180 km. The horizontal separation between the two trails was approximately 250 km so that the meridional gradient in the neutral winds could also be measured. The barium releases occurred at 200 km and 280 km near apogee. Strontium impurities in the barium provided measurements of the neutral winds at these heights, and the barium drifts provided electric field measurements. The Chatanika radar was operated prior to and during the rocket flights on both days, giving plasma densities and electric fields. On both launch dates the geomagnetic activity was high, with Kp values ranging from 3 to 5. On the first day, February 28, the launch at 0420 UT was preceded by a substorm at 0240–0330 UT, the magnetic signature of which was present over a broad local time interval. On the second day, March 2, a substorm also occurred but at a later universal time (0340). This substorm was confined to local midnight in the magnetograms. The wind measurements on both days showed very similar vertical profiles with the winds at the lower altitudes in the direction of the E×B drift, turning northward with increasing altitude. The westward neutral winds were in the direction opposite to that expected if the driving force was the solar heating pressure gradient. Thus the Lorentz force and Joule heating must have had a strong influence on the observed neutral winds. However, the differing geophysical conditions on the two days indicate that the neutral flow cannot be explained completely by the local magnetic activity. This is explored further in a companion paper, wherein a two dimensional numerical model is applied to the observations. The barium cloud drifts on both days showed intense poleward electric fields in the subauroral region. The drifts were nearly independent of latitude and were equivalent to electric fields of 60 mV/m on February 28 and 40 mV/m on March 2.
The absolute scattering cross section at 50 MHz of equatorial electrojet irregularities
Authors: Farley D. T. , Ierkic H. M. , Fejer B. G.
Published: February 2008

We have made carefully calibrated radar scattering measurements by using the large 50‐MHz Jicamarca antenna. Typical results from the altitudes of maximum echo power for the vertically directed beam are σradar ∼1–2 × 10−10 m−1 for strong daytime electrojet conditions with type 1 irregularities present, with values a factor of 10 or so smaller during moderate conditions when only type 2 are observed. These cross sections, which are very large in comparison with those for incoherent scatter (σradar ≃5 × 10−18 m−1 for an electron density of 1011 m−3), are not nearly large enough, however, to cause pseudo‐absorption events on riometer records, as was suggested by D'Angelo in 1976. If a model for the k dependence of the irregularities is assumed, it is possible to relate the radar data, which correspond to a single point in k‐space, to in situ measurements of δn/no, which correspond to a single point in physical space (an integral over the k‐spectrum). The comparisons are reasonable, but depend significantly upon the assumed large scale cutoff for type 2 irregularities and the small scale cutoff for type 1.
Temporal variation in the geomagnetic response to IMF sector boundary passage
Authors: Arora B. R. , Rangarajan G. K.
Published: February 2008

The geomagnetic response, as characterized by the change in daily Ap, in relation to the passage of solar magnetic sector boundaries is studied for different epochs using data for the years 1947–1978. The results derived from the whole year data indicate that the response of geomagnetic activity to a +/− boundary has remained unaltered, whereas that for a −/+ boundary has shown large variability in both form and magnitude. In the post‐1962 period, when the geomagnetic response for either type of boundary is similar in form, the activity associated with a +/− boundary is found to be larger than that associated with a −/+ boundary up to about 1972; from then onward this dominance is reversed. This feature is considered to be the consequence of the modulation of sector structure by north‐south asymmetry in solar activity. The temporal evolution of response during equinoxes for boundaries considered favorable in the model of Russell and McPherron exhibits a 22‐year modulation. This is believed to result from the modulation through the dominant polarity influence (Rosenberg and Coleman, 1969) of the basic effect related to the Russell‐McPherron hypothesis.
Frequency band broadening of magnetospheric VLF emissions near the equator
Authors: Maeda Kaichi , Lin C. S.
Published: February 2008

The broadening of the whistler mode VLF emission band has frequently been observed by the equatorially orbiting S³‐A (Explorer 45) satellite outside the midnight sector of the plasmasphere, during periods of geomagnetic disturbance. Prior to the broadening, the band of this emission is narrow with a sharp gap at the half electron gyrofrequency fH/2. The gradual broadening of the emission band on the low‐frequency side is associated with the simultaneously observed spreading of the anisotropy of the ring current electrons to higher and wider energy ranges. The observed electron distributions are modeled according to the form E−n sinm(E) α with time dependent parameters m and n, where m depends also on energy E. Using the modeled distribution function, the linear growth rates of the cyclotron instability are calculated numerically. The results suggest that broadening of the VLF emission band near the plasmasphere can be caused by spreading of the ring current electron anisotropy toward higher energies.
Upstream particles observed in the Earth's foreshock region
Authors: Eastman T. E. , Anderson R. R. , Frank L. A. , Parks G. K.
Published: February 2008

On the basis of primarily an extensive study of fully three‐dimensional plasma data, we describe the interrelationships of the upstream particles and plasma waves observed in the earth’s foreshock region. The University of Iowa LEPEDEAS detect ions and electrons from 1 eV to 45 keV over all except ∼2% of the unit sphere. Comparisons are made with high time resolution particle data obtained by the University of California (Berkeley) instruments and plasma wave data collected by the University of Iowa plasma wave instruments on the two ISEE spacecraft. The presence of ion beams or dispersed ion distributions is found to be a sufficient condition for the presence of electrostatic and electromagnetic wave emissions. Detailed correlations of ions with plasma waves down to a tenth of an ion gyroperiod indicate that ion acoustic emission is enhanced when increased anisotropies and gyrophase organization are observed. Time aliasing effects limit the interpretation of velocity distributions taken within the foreshock region. High time resolution correlations between the different instruments, however, demonstrate that time variations of a single isotropic or anisotropic distribution cannot produce the dispersed ion distributions. Detailed analysis of high time resolution data reveals that the upstream particles undergo significant spatial and temporal variations including gyrophase organization. Gyrophase organization comprises groups of ions or ion clusters each one of which includes ions with similar pitch angles that gyrate together about a common guiding center. On the basis of our high time resolution analysis of three‐dimensional plasma data combined with magnetic field and plasma wave data, we conclude that (1) ions observed in the foreshock region display gyrophase organization produced by ion clusters with a spatial scale <1 Rg, and (2) dispersed ion distributions are produced primarily by direct sources at or near the bow shock.
Reflected and diffuse ions backstreaming from the Earth's bow shock 1. Basic properties
Authors: Bonifazi C. , Moreno G.
Published: February 2008

Plasma data supplied by the ISEE 2 solar wind experiment are used to perform the first extended statistical analysis of the basic moments of the ions backstreaming from the earth's bow shock. The analysis is based on 3253 ion spectra, corresponding to a total observation time of ≃87 hours. It turns out that the density and total energy density of the backstreaming ions are, on the average, equal to ≃1% and ≃10% of those of the solar wind, respectively. The distinction between the ‘reflected’ and ‘diffuse’ populations has been confirmed and put on a quantitative basis using the ratio A =VBP/WBP between the bulk velocity and the rms thermal speed of the ions. The reflected ions are characterized by a bulk velocity VBP of the order of 2 times the solar wind velocity and by a temperature of ∼7 × 106 K. In contrast, the diffuse ions have, on the average, a bulk velocity 1.2 times the solar wind velocity and a temperature of 40 × 106 K. Therefore the total energy density of the diffuse ions is ≃30% larger than that of the reflected ions. Finally, the kinetic and thermal energy densities are distributed quite differently in the two ion populations: in fact, ≃70% of the total energy density is kinetic for the reflected ions, while this percentage decreases to ≃20% for the diffuse ions.
Correlated wave and particle observations upstream of the Earth's bow shock
Authors: Harvey C. C. , Bavassano‐Cattaneo M. B. , Dobrowolny M. , Orsini S. , Mangeney A. , Russell C. T.
Published: February 2008

Data from three ISEE experiments has been analyzed during several periods of turbulence observed in the solar wind upstream of the earth's quasi‐parallel bow shock. Radio observations are used to validate a shock model, which is subsequently used to compute various geometrical parameters during all the periods studied. One typical 9‐hour period on November 4, 1977, is discussed in some detail to illustrate the parameters studied and the correlations found. It is shown that during this period, the radio noise spectrum has two components, one centered around the local electron plasma frequency and the other at somewhat lower frequencies; the latter component has a shorter wavelength and correlates with the level of MHD turbulence. A multivariate canonical statistical analysis of particle and MHD data during a 2‐week period shows that the proton anisotropy and turbulence level correlate well with the minimum backstreaming proton parallel velocity pmin which, as defined here, is a purely geometrical parameter. Trivariate analysis shows that the correlation of particles and turbulence with the angle between the magnetic field and the shock normal have their sense reversed when allowance is made for the strong correlations with pmin. A very good correlation has been found between power and compressibility in magnetic fluctuations.
Coronal streamers in the solar wind at 1 AU
Authors: Gosling J. T. , Borrini G. , Asbridge J. R. , Bame S. J. , Feldman W. C. , Hansen R. T.
Published: February 2008

Examination of solar wind plasma data obtained by the Los Alamos experiments on Imp 6, 7, and 8 during the 1971–1978 interval has revealed a frequent association between minimums in helium abundance and maximums in proton density. These events occur at low flow speeds and are strongly correlated with polarity reversals in the interplanetary magnetic field. A large fraction of these high proton density‐low helium abundance events are examples of noncompressive density enhancements (NCDE), i.e., large positive density signals not readily associated with stream‐stream interactions. The cleanest examples of these events often occur at well defined sector boundaries; complex, multiple events with 2 or more peaks in proton density and lasting ∼3–7 days are, however, common, and are associated with multiple field polarity reversals. When mapped back to the sun, assuming constant speed along a stream tube, these high proton density ‐ low helium abundance events associated with magnetic field reversals usually correspond to intersections of the spacecraft trajectory with the mid line of a coronal streamer belt that encircles the sun. The duration or multiplicity of these 1 AU events is generally correlated with the local tilt of the middle of the streamer belt to the solar equator. These events thus appear to be the 1 AU extensions of coronal streamers. A fine scale warping of the streamer belt on a longitudinal scale of ∼10° and varying thickness and density of the streamer belt at 1 AU adequately account for the variety of low helium abundance ‐ high proton density events observed. It is not yet understood why the helium abundance is low within coronal streamers at 1 AU.
Observational evidence for a boundary layer source of dayside region 1 field‐aligned currents
Authors: Bythrow P. F. , Heelis R. A. , Hanson W. B. , Power R. A. , Hoffman R. A.
Published: February 2008

The addition of magnetic field measurements to the Atmosphere Explorer C data base has enabled us to examine the relations between field‐aligned currents (FAC), energetic particles, and plasma convection in the high‐latitude ionosphere. We observe that in general, magnetic field perturbations extend throughout the regions of gradients in the plasma convection, particularly equatorward of the plasma convection reversal. A study of 13 dayside polar passes shows that the region 1 FAC's (Iijima and Potemra, 1976a) are observed to span the convection reversal and extend an average of 0.5° invariant latitude poleward of the reversal. Region 2 currents are observed entirely on sunward convecting plasma. Simultaneous observations of low‐energy electron data (200 ev → 25 keV) indicate that the poleward limit of the region 1 FAC's is nearly coincident with the high‐latitude limit of 1‐keV plasma sheet electrons. These observations are consistent with a model in which the dayside region 1 FAC's have a source that is at least partially due to a ‘viscouslike” interaction on closed field lines in the low‐latitude magnetospheric boundary layer.
Three‐dimensional interaction of interplanetary shock waves with the bow shock and magnetopause: A comparison of theory with ISEE observations
Authors: Zhuang H. C. , Russell C. T. , Smith E. J. , Gosling J. T.
Published: February 2008

The propagation of shock waves in the solar wind and their three‐dimensional interaction with the terrestrial bow shock, magnetosheath, and magnetopause is examined theoretically and compared with observations made by the ISEE spacecraft. First the formulae underlying the new position and orientation of the bow shock front after the interaction are given. Then magnetic field and plasma data from the ISEE 1, 2, and 3 spacecraft are compared with the theoretical estimates of the propagation time of the shock waves between ISEE 1 and 3. Examination of data obtained on August 18, 1978, when both ISEE 1 and 3 were in the solar wind, show good agreement with theory. Using a simple model of the magnetosheath, we can extend our predictions to include the propagation time through the magnetosheath. Two examples of the interplanetary shock interaction with the magnetosphere while ISEE 1 and 2 were in the outer magnetosphere on August 27 and October 4, 1978, show quite satisfactory agreement with the predictions of this simple model. On August 27, 1978, the magnetopause is blown well past ISEE 1 and 2, and the two spacecraft pass completely through the magnetosheath in 2.5 min. Rapid as this may appear, we show that this behavior is as predicted by the theoretical treatment.
Scale sizes and lifetimes of F region plasma cloud striations as determined by the condition of marginal stability
Authors: McDonald B. E. , Ossakow S. L. , Zalesak S. T. , Zabusky N. J.
Published: February 2008

The two‐dimensional integrated Pedersen conductivity model for F region plasma cloud striation development may be cast in dimensionless form. The result implies that a structure's stability against bifurcation depends upon whether a diffusion parameter R exceeds a critical value. The critical R for striations is determined by high resolution computer simulation for conductivity ratios M from 2 to 30. Our results combined with electron diffusivity of order 1 m²/s agree with observed minimum scale sizes of 15 m. For turbulent diffusivity of order 100 m²/s, our results agree with frequent observations of kilometer scale structures which ‘freeze up’ and last for times of order 104 s. This lifetime is also predicted by our results.
Auroral X ray images
Authors: Mauk B. H. , Chin J. , Parks G.
Published: February 2008

A new X ray ‘pin‐hole’ camera has been developed to study the spatial and temporal characteristics of the energetic component of auroral electron precipitation. The camera obtains spectral information on bremsstrahlung X rays with energies between 22 and 110 keV, and it details spatial structure with scale lengths between 10 and 80 km (9°–62° cone angles). The camera has been deployed by a balloon into several auroral break‐up associated events. In this report preliminary images are shown from these events, and several types of observed phenomena are discussed. These phenomena include the observation of a spectrally soft, 20 km spot which appeared to initiate a major break‐up event and which appeared together with a component which was spectrally hard and spatially uniform. The timing of this event, the multiple components, and the discrepant spatial scales are reminiscent of geostationary observations of dispersionless substorm injections. Also discussed is the slow spatial evolution of X ray burst generating regions, as well as the spatial modulation of precipitation by hydromagnetic oscillation (Pc 5) within the magnetosphere. It appears that small scale spatial structure (≳15 km) is a prominent feature of energetic electron precipitation.
Light ion velocities in the polar ionosphere
Authors: Murphy J. A. , Heelis R. A.
Published: February 2008

High‐speed flow of light ions along magnetic field lines is not simply a consequence of those field lines having an open topology. Here we point out that the magnitude of the H+ flow velocity in the polar wind is not predicted by theoretical models but rather it is imposed as a boundary condition. Thermal diffusion, the electron temperature profile, and a diverging magnetic field can be shown to have important effects on the H+ flow velocity which need not be supersonic even when the pressure at large distances from the earth is low.
Further observational support for the limited‐latitude magnetodisc model of the outer Jovian magnetosphere
Authors: Thomsen M. F. , Goertz C. K.
Published: February 2008

The distinction between the magnetic anomaly model of the outer Jovian magnetosphere (Dessler and Vasyliunas, 1979; Vasyliunas and Dessler, 1981) and the magnetodisc model (as, for example, discussed most recently by Goertz (1981)) is reviewed, and a further observational comparison of the two models is pointed out. The comparison involves the latitudinal variation of the intensity of energetic charged particles confined near the magnetic equatorial plane. Because of this confinement, there should be a direct relationship between the maximum latitude excursion (relative to the magnetic equatorial plane) made by a spacecraft during one 10‐hour planetary rotation period and the ratio of the maximum intensity observed in the magnetic equatorial plane to the minimum intensity observed off the equator. Thus if the maximum latitude excursion is large, the minimum between two maxima should be quite deep. If the maximum latitude excursion is known, a scale height for the energetic particle confinement can be estimated from observed maximum‐to‐minimum ratios. The two magnetospheric models (magnetic anomaly and magnetodisc) in general predict different maximum latitude excursions. For the outbound passes of Voyagers 1 and 2, these predictions are combined with observations of energetic particle intensities (Krimigis et al., 1979a, b) to calculate the confinement scale heights for the energetic particles. For the magnetodisc model, the scale height is generally less than 2 RJ and is a weakly decreasing function of r, in excellent quantitative agreement with the scale height derived from a combination of simple MHD theory and a magnetic field model based on Pioneer 10 outbound magnetic field measurements (Goertz, 1976). The magnetic anomaly model, on the other hand, yields a scale height which apparently increases to large values at large radial distances. Furthermore, when the calculation is reversed for the magnetodisc model, so that the observed max/min ratio is combined with the theoretically predicted scale height to yield a maximum latitude excursion for each of the current sheet encounters, it is found that the resultant current sheet latitude agrees very well with the current sheet latitude estimated on other grounds, namely, the timing of the intensity maxima. Moreover, as shown by Goertz (1981), excursions of this current sheet latitude away from the nominal value of 9.6° are reasonably well associated with periods of enhanced solar wind dynamic pressure.
Dependence of polar cap potential drop on interplanetary parameters
Authors: Reiff P. H. , Spiro R. W. , Hill T. W.
Published: February 2008

We have computed the convection potential drop across the polar cap from data obtained on high‐inclination low‐altitude satellites (AE‐C, AE‐D, S3‐3) and correlated these potential measurements with various combinations of parameters measured simultaneously in the upstream solar wind. These combinations of solar wind parameters consist of predictions based on magnetic merging theory and suggestions based on earlier empirical work. We find that the bulk of the potential drop, and its variation with interplanetary magnetic field (IMF) parameters, are successfully predicted by merging theory (to the accuracy with which they can presently be measured), but that a significant ‘background’ potential drop (∼35 kV) does not depend on IMF parameters and may thus be attributed to an unknown process other than merging. Our results indicate that small values of the IMF are amplified by a factor of 5–10 at the dayside magnetopause as a combined effect of bow shock compression and the Zwan‐Wolf depletion layer effect; correlations between IMF parameters and the polar cap potential drop are dramatically improved when this amplification is taken into account. The potential drop is better correlated with IMF parameters than with geomagnetic activity indices, presumably because the latter are affected by nonlinear reponses of the magnetosphere to the polar cap input.
Broadband electrostatic noise and field‐aligned currents in Jupiter's middle magnetosphere
Authors: Barbosa D. D , Scarf F. L. , Kurth W. S. , Gurnett D. A.
Published: February 2008

Voyager 1 plasma wave observations have revealed the presence of an impulsive electrostatic emission localized to the Jovian middle magnetosphere 10 <R <30 RJ that appears on the edges of the plasma sheet. This plasma mode has the same spectral and morphological characteristics of an emission that has been extensively studied in the earth's magnetosphere and has been associated with the presence of field‐aligned currents. We present the results of a detailed study of the properties of this Jovian emission by using comparisons with terrestrial observations as a basis for mode identification. The occurrence regions of the waves are compared with the measured magnetic field configuration to establish a correspondence with the plasma sheet. We then argue for a quasi‐permanent global system of field‐aligned currents linking the ionosphere of Jupiter to the middle magnetosphere, which powers energetic plasma heating processes occurring there. On the basis of knowledge of the consequences of field‐aligned currents in the terrestrial magnetosphere, we suggest a scenario for acceleration/precipitation of inverted V electrons concomitant aurorae, and energetic (∼10 keV) proton deposition into the middle magnetosphere resulting from field‐aligned potential drops associated with this current system.
Colors on Jupiter
Authors: Owen Tobias , Terrile Richard J.
Published: February 2008

The colors present in the clouds of Jupiter at the time of the Voyager encounters are described as they appeared in high resolution images. Although many different tints are present in these pictures, some cloud units can be characterized by very discrete colors. We can show that latitude, altitude, and dwell‐time are all critical factors in determining which colors appear where, but the identities of the responsible chromophores remain unestablished. Simultaneous ground‐based 5 µm observations permit the determination of the relative altitudes of these cloud systems. In descending order we have the white clouds (presumably ammonia cirrus), the light brown or tawny clouds (ammonium hydrosulfide and/or monosulfide(?)), a dark brown cloud belt present only at some latitudes, and the blue‐gray ‘hot spots’ of the equatorial region. The top of the Great Red Spot may exceed the altitude of the white clouds on occasion. Despite the turbulence of the Jovian atmosphere, correlations between cloud color and certain latitudes have been maintained for decades, suggesting the importance of the internal energy source and the deep circulation in generating some of the observed chromophores.
Minor ions in the low corona
Authors: Hollweg Joseph V.
Published: February 2008

The ability of Coulomb friction to drag minor ions out of the subsonic region of the low corona is examined analytically. With some assumptions, we obtain new analytical expressions for the ‘minimum proton flux’ that is required to drag minor ions out of the corona and for the velocity, relative to the protons, at which the minor ions are dragged out. We use these new results to suggest that the positive nα/np versus υp correlation and the negative nα/np versus npυp correlation observed for solar wind helium at 1 AU are due to variations in the temperature of the low corona: hotter coronal regions produce solar wind flows that are slower but that have a greater proton flux density relative to cooler coronal regions, but because of the temperature dependence of the Coulomb friction the hotter coronal regions result in smaller values of nα/np at 1 AU. We also suggest that the charge dependence of the Coulomb friction may lead to errors in determining the coronal temperature from measurements of the ionization state at 1 AU.
On the injection and scattering of protons in Jupiter's magnetosphere
Authors: Barbosa D. D.
Published: February 2008

We investigate a model by which 10 keV protons accelerated on Jovian auroral field lines are scattered and localized to the magnetic equator. The criterion that the beam density be sufficiently large to generate magnetosonic waves leads to the evaluation of proton beam fluxes consistent with recent Voyager observations of a hot high β plasma sheet in the magnetosphere. A model of the Alfven speed in the plasma sheet for both inbound and outbound legs of the Voyager 1 encounter is presented. This model confirms Piddington's limit of departure from rigid corotation beyond the point where the rigid body corotation speed equals the Alfven speed. Finally, we explore the possibility of nonadiabatic scattering of super‐Alfvenic ions by magnetohydrodynamic waves throughout the plasma sheet as a basis for high‐energy tail formation of ambient particle distributions.
Classification of hydromagnetic emissions based on frequency‐time spectra
Authors: Fukunishi Hiroshi , Toya Takeshi , Koike Kazuhara , Kuwashima Masayuki , Kawamura Makoto
Published: February 2008

By using 3035 hydromagnetic emission events observed in the frequency range of 0.1–2.0 Hz at Syowa (L ∼6), HM emissions have been classified into eight subtypes based on their spectral structures, i.e., HM whistler, periodic HM emission, HM chorus, HM emission burst, IPDP, morning IPDP, Pc 1–2 band, and irregular HM emission. It is seen that each subtype has a preferential magnetic local time interval and also a frequency range for its occurrence. Morning IPDP events and irregular HM emissions occur in the magnetic morning hours, while dispersive periodic HM emissions and HM emission bursts occur around magnetic local noon, then HM chorus emissions occur in the afternoon hours and IPDP events occur in the evening hours. Furthermore, it is noticed that the midfrequencies of these emissions vary from high frequencies in the morning hours to low frequencies in the afternoon hours. On the basis of these results, the generation mechanisms of each subtype are discussed.
Geomagnetic control of the winter anomaly in absorption of radio waves at mid‐latitudes
Authors: Sato Teruo
Published: February 2008

Morphological features of the winter anomaly in radio wave absorption, i.e. anomalous and large excessive absorptions of MF and HF waves occuring on groups of winter days at mid‐latitudes, are examined based on absorption data sets of MF waves at three ionospheric stations and fmin data sets at 35 stations in the northern and southern hemispheres. It is found that the winter anomaly occurs simultaneously in an extensive daytime area, at least scatteringly, mostly several hours to 1–3 days following the onset of a sc (or si) geomagnetic storm or storms. The magnitude, duration and occurrence area of the winter anomaly seem to depend on geomagnetic activity. It is also shown that fmin enhancements occur simultaneously in the daytime at geomagnetically conjugate stations in both hemispheres in association with a storm, irrespective of season, and they are closely correlated to cosmic noise absorption enhancements. These results suggest that the winter anomaly observed at a point in one hemisphere is linked to an excessive absorption event at the conjugate point in the other (summer) hemisphere and that they would be caused by ionization enhancements due to precipitating electrons associated with a sc storm. It is therefore proposed that the winter anomaly is not the peculiar event implying that anomalously large excessive absorptions of radio wave arise solely in winter, but the phenomenon indicating that excessive wave absorptions appearing throughout the year associated with sc storms are abnormally enhanced in winter. An explanation for the large seasonal variation of excessive wave absorption is also presented.
The case of the noisy derivatives—Evidence for a spacecraft‐plasma interaction
Authors: Hanson W. B. , Cragin B. L.
Published: February 2008

Data from the retarding potential analyzers on Atmosphere Explorers C and D indicate the existence of 2‐m irregularities in the ion flux to the satellite. These irregularities are seen only occasionally, and usually very near the magnetic dip equator. They occur both in sunlight and eclipse and span at least two decades of ion concentration. They tend to be present when the spacecraft is moving nearly parallel to B and are not seen at all from the low latitude AE‐E orbit. We believe the irregularities, whose amplitude is of the order of 1% of the background ion flux, are the result of a spacecraft interaction with the ionospheric plasma. A mechanism for producing this interaction that utilities a fast beam of ambient ions reflected upstream from the satellite to produce a two‐stream instability is examined in some detail.
Magnetostatic atmospheres in a spherical geometry and their application to the solar corona
Authors: Hundhausen J. Rohrer , Hundhausen A. J. , Zweibel Ellen G.
Published: February 2008

The formalism for deriving ‘two‐dimensional’ magnetostatic equilibria is extended to spherical coordinates and applied to magnetic fields that are functions of radius and polar angle. A family of analytic solutions is readily found. The basic properties of these solutions are displayed for a dipole magnetic field at the base of the atmosphere and for physical parameters appropriate to the solar corona. Variation of the concentration of plasma at the ‘magnetic equator’ illustrates the distortion of a simple dipole magnetic field by the electric currents required to maintain force balance in the presence of the imposed pressure gradients in the polar direction. The deviation of the magnetostatic field lines from the simple dipole configuration depends on the parameter (Peq–Ppole)/(B0²/8π), where Peq and Ppole are the equatorial and polar plasma pressures and B0 is the dipole field strength at the base of the corona. Reasonable choices of these physical quantities give a value for this parameter of about 1/2, implying deviations in the large‐scale coronal magnetic field geometry from the commonly used potential field that are not negligible. These deviations lead to field lines that are more nearly vertical at the base of the corona and to more magnetic flux on open field lines than in potential field models with the same magnetic boundary conditions.
Several features of the earthward and tailward streaming of energetic protons (0.29–0.5 MeV) in the Earth's plasma sheet
Authors: Lui A. T. Y. , Krimigis S. M.
Published: February 2008

The characteristics of earthward and tailward streaming of energetic protons (0.29–0.50 MeV) in the magnetotail at downstream distances of 20 to 40 RE are examined with ∼5.5‐min averaged data from the APL/JHU Charged Particle Measurements Experiment on board the IMP 7 and IMP 8 spacecraft. On the basis of observations from September 1972 to May 1978 it is found that the occurrence frequency of energetic magnetospheric protons streaming either tailward or earthward with a front‐to‐back flux ratio of >2 is at least 23%. Tailward streaming is found to be prevalent in the postmidnight plasma sheet, while earthward streaming is more frequent in the premidnight sector. The particle spectrum is progressively harder from the dawn flank to the dusk flank of the plasma sheet and is generally harder for tailward streaming than for earthward streaming. It is suggested that the dawn‐dusk reversal in the dominant streaming direction results from an underlying circulation pattern of energetic protons in the magnetotail, tailward in the postmidnight region and earthward in the premidnight region.
Multiple crossings of a very thin plasma sheet in the Earth's magnetotail
Authors: Fairfield D. H. , Hones E. W. , Meng C.‐I.
Published: February 2008

High‐resolution magnetic field, plasma, and energetic particle data from the IMP 8 spacecraft have been used to study multiple crossings of the earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. IMP 8 is found to traverse the entire thickness of the β > 1 plasma sheet in times that are as short as 10 s. Such traversals recur on a time scale of several minutes, and they are associated with high velocity plasma flows that are usually directed tailward but are occasionally directed earthward for brief intervals. We argue that these observations can be explained by rapid oscillations of a plasma sheet that is only a few thousand kilometers thick, a dimension that is comparable to the gyroradius of energetic protons. When ions of 50 keV and 290 keV are detected at a location on tail lobe field lines near the plasma sheet, they are often seen to be moving in the dawn to dusk direction and may be undergoing serpentine cross tail motion. Differences in the angular distributions of the two energies indicate that the higher energy ions are preferentially located on field lines deeper in the tail lobe. This result is consistent with ion acceleration at a neutral line with subsequent
drift in a cross‐tail electric field that moves the lower energy particles closer to the equatorial plane. A neutral line acceleration model is also supported by our observation that tailward streaming energetic electrons are occasionally present at the lobe‐plasma sheet interface.
Severe ionospheric disturbances caused by the sudden response of evening subequatorial ionospheres to geomagnetic storms
Authors: Tanaka T.
Published: February 2008

By monitoring C band beacon signals from geostationary satellites in Japan, we have observed anomalously strong ionospheric scintillations several times during three years from 1978 to 1980. These severe scintillations occur associated with geomagnetic storms and accompany sudden and intense ionospheric perturbations in the low‐latitude region. Through the analysis of these phenomena we have identified a new type of ionospheric disturbances characterized by intensifications of equatorial anomalies and successive severe ionospheric scintillations that extend to the C band range. The events occur only during a limited local time interval after the sunset, when storm time decreases of midlatitude geomagnetic fields in the same meridian take place during the same time interval. From the viewpoint of ionospheric storms, these disturbances precede the occurrence of midlatitude negative phases and storm time depressions of equatorial anomalies to indicate that the cause of the events is different from disturbed thermospheric circulations. The timing and magnitude of substorms at high‐latitudes not always correlate with the events. We have concluded that the phenomena are closely related with penetrations toward low‐latitudes of electric fields owing to the partial closure of asymmetrical ring currents.
Numerical simulation of semidiurnal atmospheric tides
Authors: Aso T. , Nonoyama T. , Kato S.
Published: February 2008

Numerical modeling of the solar and lunar semidiurnal atmospheric tides has been performed by invoking a comprehensive approach that includes both algebraic manipulation and numerical solution of the primitive equation system. The effect of latitudinally varying temperature and associated mean zonal wind in a modeled atmosphere with realistic forcing are successfully incorporated to reproduce tidal variabilities at meteor heights delineated by varieties of radar observations.
The Venus ionopause current sheet: Thickness length scale and controlling factors
Authors: Elphic R. C. , Russell C. T. , Luhmann J. G. , Scarf F. L. , Brace L. H.
Published: February 2008

Data from the Pioneer Venus orbiter magnetometer, plasma wave detector and Langmuir probe are used to investigate the scale thickness and controlling factors associated with the current sheet at the Venus ionopause, the boundary between the ionosphere and the solar wind. The current sheet is found to have two principal scale sizes: (1) a few ion gyroradii when the external compressed solar wind field is less than 80 nT and the ionopause altitude is above ∼300 km; (2) several tens of ion gyroradii when high external fields occur and ionopause altitudes are below ∼300 km. The current sheet thickness may be significantly affected by collisional diffusion at low altitudes, and wave turbulence may also play a role in controlling thickness. The high‐altitude boundaries are consistent with the expected thickness of the thinnest charge‐neutral, collisionless current sheet.
Energetic electrons in the magnetosheath and upstream of the bow shock
Authors: Bieber J. W. , Stone E. C.
Published: February 2008

The characteristics and transport of >200 keV electrons upstream of Earth's bow shock and in the outer magnetosheath are studied using nearly 4 years of data from the California Institute of Technology Electron/Isotope Spectrometer aboard IMP 8. In the upstream region, elevated electron intensities ranging from near the background level ∼0.1 (cm² s sr)−1 up to ∼100 (cm² s sr)−1 are observed about 2% of the time and typically persist for less than 5 min. These burst electrons stream intensely (ξ ∼ 0.5 to 1.5) sunward along the interplanetary magnetic field line. With the aid of a new coordinate system, it is demonstrated that the upstream bursts are most frequently seen on interplanetary field lines that trace to the inner magnetosheath, and that the total energy transported sunward by >200 keV upstream electrons averages ∼1.6 × 1014 ergs/s, a figure comparable to the tailward energy flow observed in the permanent layer of energetic electrons adjacent to the magnetopause. Bursts of energetic electrons in the outer magnetosheath have time scales similar to those observed upstream, but they are typically ∼10 times more intense and much less anisotropic (ξ ≲ 0.4). The net flow of these electrons is toward the shock and very nearly equals the net flow away from the shock in the adjacent upstream region. The magnetosheath data suggest that energetic electron transport in this region may be described by a leaky box model with a scattering mean free path ∼0.6 RE. It is concluded that the energetic (>200 keV) component of upstream electrons originates downstream of the bow shock. Energetic electrons in the upstream region, in the outer magnetosheath, and in the magnetopause electron layer appear to be closely related and probably have a common origin in the inner magnetosheath or possibly the magnetosphere.
Interplanetary magnetic clouds At 1 AU
Authors: Klein L. W. , Burlaga L. F.
Published: February 2008

Magnetic clouds are defined as regions with a radial dimension ≈ 0.25 AU (at 1 AU) in which the magnetic field strength is high and the magnetic field direction changes appreciably by means of rotation of one component of
nearly parallel to a plane. The magnetic field geometry in such a magnetic cloud is consistent with that of a magnetic loop, but it cannot be determined uniquely. Forty‐five clouds were identified in interplanetary data obtained near earth between 1967 and 1978; at least one cloud passed the earth every 3 months. Three classes of clouds were identified, corresponding to the association of a cloud with a shock, a stream interface, or a CME. There are approximately equal numbers of clouds in each class, and the field and plasma parameters in each class are similar, suggesting that the three types of clouds might be different manifestations of a single phenomenon (e.g., a coronal transient). Interface‐associated clouds may have been swept up by corotating streams. Shock‐associated clouds move faster than the other two types, which are basically slow flows. The magnetic pressure inside the clouds is higher than the ion pressure, and the sum is higher than the pressure of the material outside of the cloud. This implies that the magnetic clouds were expanding even at 1 AU, and the average expansion speed is estimated to be of the order of half the ambient Alfvén speed.
Self‐absorption of the N2 Lyman‐Birge‐Hopfield bands in the far ultraviolet dayglow
Authors: Conway Robert R.
Published: February 2008

Absorption by N2 of the Lyman‐Birge‐Hopfield (LBH) bands in the far ultraviolet dayglow has been calculated using a line‐by‐line synthesis of individual bands. The effects of overlapping rotational lines and the combined magnetic dipole and electric quadrupole nature of the electronic transition are included in the calculation. A band transmission function which can be used in the radiative transfer theory of any band in the system is presented for rotational and kinetic temperatures from 200 to 1000 K. Analysis of rocket observations of the LBH emissions in the 1250–1400 Å region of the dayglow indicates that pure absorption by O2 dominates self‐absorption except for the (6, 0) band at 1275 Å where the horizontal slant intensity is reduced by 20%. The effect is much less than given in a previous analysis based on a band optical depth approximation.
The bidirectional particle event of October 12, 1977, possibly associated with a magnetic loop
Authors: Kutchko F. J. , Briggs P. R. , Armstrong T. P.
Published: February 2008

A fortuitous set of interplanetary and solar events which occured on October 12, 1977, allowed the observations by several interplanetary spacecraft, IMP 7 and 8, of features in charged particle angular distributions which suggest the presence of a closed magnetic loop. This loop must have extended beyond the orbit of earth and have been connected to particle sources and/or mirrors at both ends. This magnetic topology is similar to a suggestion made by Gold (1960). The salient feature of the anisotropies which calls attention to the possible presence of a loop is the occurrence of a trapping‐type electron distribution peaked at 90° to the magnetic field, while the ions are strongly field aligned and bidirectional.
Theory and computer simulations of magnetospheric very low frequency emissions
Authors: Vomvoridis J. L. , Crystal T. L. , Denavit J.
Published: February 2008

A theory of magnetospheric VLF emissions must account for the following features: (a) the triggering of monochromatic emissions by signals of sufficient strength and duration, while the background noise and weak short signals are not amplified, and (b) the occurrence of frequency changes after the emissions have reached a sufficiently large amplitude. A nonlinear mechanism exhibiting these features, with fixed and varying frequencies, is examined analytically and by computer simulation techniques. This mechanism depends on a simultaneous propagation and amplification of wave packets along geomagnetic lines to maintain the nonuniformity ratio R ∝ ▽B0/Bw in the regime |R| ≈ 0.5, corresponding to maximum amplitication. (B0 is the geomagnetic field and Bw is the wave magnetic field.) For a constant frequency, this condition yields triggering thresholds which are related to the properties of the magnetosphere. For a varying frequency ω(t), it yields the condition ∂ω/∂t ∝ ωt² for the large‐amplitude portion of the risers, where ωt ∝ Bw1/2 denotes the trapping frequency of the wave.