bluuebunny/crossref_metadata_2025 huggingface dataset:
Field‐aligned currents as a diagnostic tool: Result, a renovated model of the magnetosphere
Authors: Atkinson G.
Published: February 2008

In the outer magnetosphere, currents across magnetic field lines are carried by protons, because of their greater energy, whereas field‐aligned currents are carried by electrons. Consequently, there is plasma enhancement or depletion on flux tubes where the two types of current connect. Thus field‐aligned currents observed at low altitudes can be used to diagnose plasma redistribution in the outer magnetosphere. A stationary current sheet accompanies convection through a stationary plasma boundary. The observations of the location and strength of field‐aligned currents are used to create a model of the currents, flows, magnetic topology and plasma distribution in the outer magnetosphere. Differences from previous models include the following: (1) A steady state flow may exist at subauroral latitudes. (2) The upward current sheet (Region 1 dusk, Harang, region II dawn) separates dipolar flux tubes from inflated taillike flux tubes. Flux tubes convecting earthward from the tail collapse to a dipolar shape as they convect through this sheet. (3) There is a significant intrusion of solar wind protons from the dawn flank into the plasma sheet between the region I and II currents.
Seasonal variations of L equivalent current systems
Authors: Matsushita S. , Xu Wen‐Yao
Published: February 2008

Equivalent ionospheric current systems of the lunar daily geomagnetic variations during a solar active period are estimated from geomagnetic data for three seasons and the yearly average. The external current intensity ratio in summer over winter is 1.4–6.0 for the diurnal current, 1.3–3.7 for the semidiurnal current, and 1.3–4.1 for the combined current of the first four harmonics. Also, the phase of the current in local winter frequently shifts, to a large extent, relative to the yearly average current system. This anomalous behavior during solstitial months, particularly in winter, is caused by lunar tidal wind variations. It is emphasized that the L current flows mainly at the ionospheric E region which is same as for the Sq current.
A survey of dayside flux transfer events observed by ISEE 1 and 2 magnetometers
Authors: Rijnbeek R. P. , Cowley S. W. H. , Southwood D. J. , Russell C. T.
Published: February 2008

A survey is made of magnetic flux transfer events (FTE's) observed both exterior and interior to the dayside magnetopause region by the ISEE 1 and 2 spacecraft during the first two years of operation. FTE's are found to be a common feature of the magnetopause region covered by the spacecraft when the magnetic field in the magnetosheath has a southward component but not when it is northward. The most commonly observed signature of an FTE in the field component normal to the magnetopause is an outward (positive) followed by an inward (negative) bipolar perturbation and is usually seen at northern latitudes. A reversed normal component signature is seen most frequently at low or southerly latitudes. Only very rarely are both signatures seen on the same pass. On occasion, FTE's are observed with a more complex normal field signature. During periods of southward magnetosheath field the average number (˜5) and recurrence time (˜8 min) of FTE signatures observed per magnetopause crossing in the magnetosheath is similar to the average number (˜4) and recurrence time (˜7 min) of FTE signatures observed interior to the magnetopause. We conclude that the magnetosheath and magnetosphere FTE's are part of the same physical phenomenon. The dimension of FTE flux tubes normal to and along the magnetopause is estimated as ˜1 RE and ˜2 RE, respectively. We derive a lower limit of ˜6 kV for the voltage associated with FTE magnetic flux removal during southward field conditions. However we argue that this value actually may be up to an order of magnitude larger. Thus FTE's could provide the dominant means of flux transfer required to drive geomagnetic disturbances.
The structure of oblique subcritical bow shocks: ISEE 1 and 2 observations
Authors: Mellott M. M. , Greenstadt E. W.
Published: February 2008

We have studied the structural elements, including shock ramps and precursor wave trains, of a series of oblique low‐Mach number terrestrial bow shocks. We used magnetic field data from the dual ISEE 1 and 2 spacecraft to determine the scale lengths of various elements of shock structure as well as wavelengths and wave polarizations. Bow shock structure under these conditions is essentially that of a large‐amplitude damped whistler mode wave which extends upstream in the form of a precursor wave train. Shock thicknesses, which are determined by the dispersive properties of the ambient plasma, are too broad to support current‐driven electrostatic waves, ruling out such turbulence as the source of dissipation in these shocks. Dissipative processes are reflected in the damping of the precursors, and dissipative scale lengths are ∼200–800 km (several times greater than shock thicknesses). Precursor damping is not related to shock normal angle or Mach number, but is correlated with Te/Ti. The source of the dissipation in the shocks does not appear to be wave‐wave decay of the whistlers, for which no evidence is found. We cannot rule out the possibility of contributions to the dissipation from ion acoustic and/or lower‐hybrid mode turbulence, but interaction of the whistler itself with upstream electrons offers a simpler and more self‐consistent explanation for the observed wave train damping.
Coronal mass ejections observed during the Solar Maximum Mission: Latitude distribution and rate of occurrence
Authors: Hundhausen A. J. , Sawyer C. B. , House L. , Illing R. M. E. , Wagner W. J.
Published: February 2008

Sixty‐five coronal mass ejections have been identified in a systematic examination of white‐light coronal images obtained between March and September 1980 by the coronagraph/polarimeter flown on the solar maximum mission spacecraft. These ejections were more uniformly distributed in position angle (or “projected” solar latitude) than the similar events observed during the Skylab mission in 1973–1974; 27% of the solar maximum mission mass ejections were centered at positions more than 45° from the solar equator. The average rate of occurrence of the observed mass ejections for the entire solar maximum mission epoch, based on the assumption that one coronagraph image per spacecraft orbit is sufficient for detection, was 0.9±0.15 per 24‐hour day. Application of the same sampling assumption to the Skylab data set leads to a rate of 0.75 per 24‐hour day and thus a change in this rate from the Skylab era (on the declining phase of sunspot cycle 20) to solar maximum mission (near the maximum of sunspot cycle 21) of only ∼20%.
Electromagnetic ion cyclotron mode (ELF) waves generated by auroral electron precipitation
Authors: Temerin M. , Lysak R. L.
Published: February 2008

Narrow‐banded ELF waves with frequencies between the local hydrogen and the singly‐charged helium gyrofrequencies have been seen in the S3‐3 satellite electric field wave data at altitudes between 800 and 8000 km. The waves are generated in or just below the auroral acceleration region by the accelerated electron beam and propagate in the Alfvén‐ion cyclotron branch of the cold electromagnetic dispersion relation as modified by the presence of a two ion H+‐He+ plasma.
Eighty‐eight year periodicity in solar‐terrestrial phenomena confirmed
Authors: Feynman J. , Fougere P. F.
Published: February 2008

The existence of a 60–100 year periodic variation in solar and/or solar‐terrestrial phenomena has been a matter of dispute for many years. A wide variety of data sets previously have been analyzed, and the results of the analyses have been interpreted as showing evidence either for or against such a variation. However, all data sets that are proxy for solar wind in the ecliptic at 1 AU show variations consistent with a period of about 88 years. Here we report that a maximum entropy spectral analysis of the number of aurora reported per decade in Europe and the Orient from 450 A.D. to 1450 A.D. shows a strong stable line at a 88.4±0.7 years. Since the data set contains 11 cycles, this analysis establishes the reality of the “long cycle” for 1000 years. The mean amplitude and phase are then estimated from a superposed epoch analysis. The mean amplitude was 2.2 auroral reports per decade and the last minimum phase in these data occurred between 1403 A.D. and 1413 A.D.
F region electron density irregularity spectra near auroral acceleration and shear regions
Authors: Basu Sunanda , Basu Santimay , MacKenzie E. , Coley W. R. , Hanson W. B. , Lin C. S.
Published: February 2008

Spectral characteristics of auroral F region irregularities were studied by the use of high‐resolution (∼35 m) density measurements made by the retarding potential analyzer (RPA) on board the Atmosphere Explorer D (AE‐D) satellite during two orbits when the satellite was traversing the high‐latitude ionosphere in the evening sector. Coordinated DMSP passes provided synoptic coverage of auroral activity. The auroral energy input was estimated by intergrating the low‐energy electron (LEE) data on AE‐D. It was found that the one‐dimensional in situ spectral index (p1) of the irregularities at scale lengths of <1 km showed considerable steepening in regions of large auroral acceleration events with p1 values of ∼ −3. This is interpreted as resulting from the effects of E region conductivity on the F region irregularity structure. The regions in between the precipitation structures, where presumably the E region conductivity was small, were generally associated with large shears in the horizontal E‐W drifts and large velocities, as measured by the ion drift meter on board AE‐D. The maximum drifts measured were ∼2 km s−1, corresponding to an electric field of 100 mV m−1. The large‐velocity regions were also associated with substantial ion heating and electron density depletions. The largest shear magnitudes observed were ∼80 m s−1 km−1, and the shear gradient scale lengths were ∼10 km, which was approximately the resolution of the ion drift meter data set used. The spectral characteristics of irregularities in the large, variable flow regions were very different, with p1 being ∼−1. Since these regions were also associated with the largest irregularity amplitudes, it seemed probable that either the velocity shears or, alternatively, the large velocities provided a source of irregularities in the auroral F region ionosphere. Current work on plasma instabilities related to velocity shears and two‐dimensional fluid turbulence is briefly summarized and critically compared to the findings of the present study.
Particle and field characteristics of the high‐latitude plasma sheet boundary layer
Authors: Parks G. K. , McCarthy M. , Fitzenreiter R. J. , Etcheto J. , Anderson K. A. , Anderson R. R. , Eastman T. E. , Frank L. A. , Gurnett D. A. , Huang C. , Lin R. P. , Lui A. T. Y. , Ogilvie K. W. , Pedersen A. , Reme H. , Williams D. J.
Published: February 2008

Particle and field data obtained by eight ISEE spacecraft experiments are used to define more precisely the characteristics of the high‐latitude boundary region of the plasma sheet. We find that there is a region immediately adjacent to the high‐latitude plasma sheet boundary whose particle and field characteristics are distinctly different from its neighbors, the lobe and the plasma sheet. This region supports intense ion flows, field‐aligned currents, large amplitude electric fields, and enhanced broad band electrostatic noise. A detailed analysis of events detected on April 19, 1978, shows that the plasma distributions in the region are unstable. For instance, bidirectional field‐aligned electron distributions are observed at nearly all energies (a few electron volts to a few hundred keV). Both the differential energy spectra and the reduced distribution function F(υ∥) = 2π∫υ⊥f(υ∥, υ⊥) dυ⊥ show significant peaks at 100‐400 eV. These peaks comprise electrons coming from the earthward direction. Ions below a few keV are convected in the direction perpendicular to B, whereas the higher energy ions (>30 keV) are field‐aligned and travelling mainly toward the earth. The electric field in the region is intense, spiky, and possibly time varying. The magnetic By component increases dramatically from a few nanoteslas to more than 20 nT, indicating presence of a field‐aligned current. These features are in contrast with the plasma characteristics outside the region. In the adjacent regions (closer to the lobe and the inner plasma sheet regions), the electron distributions are nearly isotropic. Ion flow is virtually at a standstill. The high‐energy ions (>30 keV) are anisotropic and field‐aligned but bidirectional.
Earthward directed high‐density Birkeland currents observed by HILAT
Authors: Bythrow P. F. , Potemra T. A. , Hanson W. B. , Zanetti L. J. , Meng C.‐I. , Huffman R. E. , Rich F. J. , Hardy D. A.
Published: February 2008

An intense (94 µA/m²) earthward directed Birkeland current was detected by the HILAT satellite on July 23, 1983, less than one month after launch. It was located at the equatorward edge of a large‐scale earthward flowing current in the late evening sector. This current is the most intense ever to be reported that is consistent with upward flowing thermal electrons. Simultaneous vacuum ultraviolet images from the auroral imager mapper show that the large‐scale current is embedded within the diffuse aurora. Energetic electron measurements in the range 20 eV to 20 keV indicate an increase in downward flux from ∼5×107 to ∼7×109 (cm² sec sr)−1 in ∼0.75 s. This increase in flux results in a gradient of ∼2 mhos/km in the height‐integrated Pedersen conductivity (Σp). In the presence of a typical dc electric field (∼40 mV/m), this sharp gradient in the ionospheric conductivity is consistent with the inferred high‐density current. Measurement of horizontal ion drift from the ion drift meter (IDM) reveals strong turbulence present for ∼10 km on both sides of the region of intense Birkeland current. The IDM also measured an ion density (ni) of ∼2×104 cm−3; thus, if ne ≃ ni, the parallel electron drift velocity derived from J = ne Vd q is ∼30 km/s, resulting in a net electron flux of ∼6×1010 cm−2 s−1. A flux of electrons of this magnitude is sufficient to destabilize O+ ion cyclotron waves. The velocity of 30 km/s coupled with an F region ion density gradient scale length of ∼3 km is also comparable to criteria required to drive the current convective instability.
Longitudinal variability of annual changes in activity of equatorial spread F and plasma bubbles
Authors: Maruyama Takashi , Matuura Nobuo
Published: February 2008

Global distribution maps of equatorial spread F (ESF) activity for various annual periods were derived from the topside soundings by the Ionosphere Sounding Satellite b (ISS‐b) in 1978–1980. The ESF activity during the northern winter period reveals maximum enhancement at the Atlantic longitudes of large westward geomagnetic declination, and during the northern summer at the Pacific longitudes of large eastward declination. On several orbits passing over the region of the ESF activity enhancement, abrupt depletions of the electron density or equatorial plasma bubbles were observed at the satellite altitude of about 1100 km. It has also been found that the background electron density distributions tend to be symmetric with respect to the magnetic equator in the region of the enhanced ESF activity and asymmetric in the region of the suppressed ESF activity. These observations are taken account of by the influence of a transequatorial thermospheric wind upon the suppression of the Rayleigh‐Taylor type plasma instability.
Polarization of geomagnetic sudden commencements observed by geostationary satellites
Authors: Nagano H. , Araki T.
Published: February 2008

Using magnetic data from GOES 2 and 3 geostationary satellites for the interval from August 1978 to August 1980, local time dependence of polarization of geomagnetic sudden commencements (SC's) was statistically analyzed. The polarization in the plane perpendicular to the geomagnetic field is counter clockwise on the morning side and clockwise on the afternoon side, when viewed along the field direction. This is consistent with the statistical results from ground‐based magnetic data by Wilson and Sugiura (1961). A latitudinal reversal of the polarization of SC observed on the ground does not seem to occur at geostationary orbit.
DP 1 and DP 2 current systems for the March 22, 1979 substorms
Authors: Clauer C. R. , Kamide Y.
Published: February 2008

The March 22, 1979 substorm interval selected for analysis by the CDAW 6 has been investigated using a ground magnetic data inversion scheme which computes equivalent current patterns as well as real ionospheric and field aligned current distributions. These have been computed at 5‐minute increments during the two substorm intervals on this day. The computed results have been analyzed using a differential technique which permits us to isolate the current development which occurs during selected time intervals. We find that both DP 1 and DP 2 currents develop during the course of the substorm activity. The first substorm is characterized by the gradual enhancement of a DP 2 equivalent current system following the southward turning of the interplanetary magnetic field. The major magnetic disturbance associated with the expansion phase of the substorm is dominated by the development of a DP 1 current system. During the second interval which consists of several hours of magnetic disturbance, the DP 2 system grows to be much stronger and is the dominant contribution to much of the auroral magnetic activity. The peak of the activity during the second substorm interval appears to be composed of both strong DP 2 and DP 1 currents coexisting. Following the decay of the DP 1 system a strong DP 2 system continues to exist during the enhanced convection phase of the substorm.
Modeling the equatorial electrojet
Authors: Stening Robert J.
Published: February 2008

The equivalent circuit method has been modified to give greater accuracy and greater detail near the equator in order to model the equatorial electrojet. Electron collision frequencies used in the conductivity model are consistent with laboratory measurements. Variations with longitude are allowed, and the electrojet in the model is driven by suitable emf's generated by a global thermotidal wind system. The height of maximum current density in the Indian electrojet provided by the model at 104 km is consistent with some observations. The model gives the same height in Peru when an electron density profile typical of that region is used. The form of the electron density profile is shown to have a considerable affect on the current profile. The calculated variation with latitude of height‐integrated current density gives good agreement. The two‐layer equivalent circuit model is more successful than the single‐layer model in modeling the latitude profile of the jet, but the observed depression in ΔH near 4° dip latitude requires much larger changes in currents with latitude than either model can provide. The theory that currents are limited by the two‐stream instability does not agree with measured altitude profiles of the jet. Before latitude variations of ΔH and ΔZ on the ground can satisfactorily be explained, greater understanding of the contribution of conductivity anomalies to internal components will be required, but with suitable assumptions, a good fit with observed results is obtained. The effects produced by a simple local F region wind system are also investigated. A discrepancy with the observed relationship between integrated current densities and ΔH still awaits explanation.
An explanation for the H Ly α longitudinal asymmetry in the equatorial spectrum of Jupiter: An outcrop of paradoxical energy deposition in the exosphere
Authors: Shemansky D. E.
Published: February 2008

An analysis of the Voyager EUV spectra of the Jupiter sunlit equatorial emissions shows no evidence for a substantial dependence of atomic hydrogen abundance on magnetic longitude, required by earlier theories of the H Ly α longitudinal asymmetry. An explanation for the H Ly α bulge phenomenon is advanced in this work that conforms to the observations and does not require a strong asymmetry in atomic hydrogen abundance. It is proposed that the H Ly α bulge is caused by a combination of proton collisional transfer of H(2s) atoms into the H(2p) state, and production through recombination of H2+ and H3+, in an asymmetric ionosphere. The asymmetry in the ionosphere is presumably caused by Jupiter's magnetic anomaly. The processes producing the H Ly α bulge require a negligible amount of directly applied energy. However, the phenomenon is a symptom of a process of substantial deposition of energy in the exosphere, forming the major source for ionospheric particles and contributing substantially to the upper atmospheric temperature. According to the present model a large fraction of the observed H Ly α emission from the equatorial region is electron excited, at least at times of solar maximum. Other related phenomena and quantities required to maintain the H Ly α asymmetry are discussed in the text.
Saint‐Santin radar observations of lower thermospheric storms
Authors: Mazaudier C. , Bernard R.
Published: February 2008

Incoherent scatter measurements of ion drift velocities obtained by Saint‐Santin ‐ Nancay radar have been used to study the lower thermospheric wind disturbances during geomagnetic storms. A storm‐time meridional circulation with equatorward wind velocities at higher altitudes (≥135 km) is revealed. The transition altitude below which the circulation reverses direction is also traceable. The strength of the circulation is in good agreement with Blanc and Richmond's (1980) model simulation for intense storms. Some evidence is also found for phase variations in the semidiurnal tide during storm periods, but its interpretation is difficult because of the restriction of the data to daytime hours.
On the propagation of daytime whistlers at low latitudes
Authors: Tanaka Yoshihito , Hayakawa Masashi
Published: February 2008

For the daytime whistlers frequently observed in the late afternoon at low latitudes, ducted propagation has been experimentally evidenced, and association with the equatorial anomaly has been suggested. It is suggested from the present computations of ray tracing and wave absorption that the ground‐based daytime whistlers may be attributed to propagation trapped in field‐aligned structures of electron density (ducts) with a small dimension of 10–50 km width and a large enhancement factor of 100–300% either present within or superimposed on the high‐latitude flank of the equatorial anomaly. We predict a low‐latitude cutoff of occurrence of the daytime whistlers at a geomagnetic latitude of ∼20°, as experimentally established.
Local time distribution of HF Doppler frequency deviations associated with storm sudden commencements
Authors: Kikuchi Takashi , Ishimine Tsuyoshi , Sugiuchi Hidetoshi
Published: February 2008

Local time features of frequency deviations of the HF standard signals associated with storm sudden commencements (ssc), SCF's, are examined by using data obtained at Kokubunji and Okinawa, Japan, during the period of March 1981 to August 1983. It is shown that of 30 SCF's 17 cases are type SCF(+ −) and 7 cases type SCF(− +). It is found that SCF(+ −) appears in the daytime and evening sectors (0600–2100 LT), while SCF(− +) appears in the nighttime sector (2100–0600 LT). This result indicates that the preliminary frequency deviations (PFD's) ((+) of SCF(+ −) and (−) of SCF(− +)) are caused by an electric field directed from dusk to dawn, while the main frequency deviations ((−) of SCF(+ −) and (+) of SCF(− +)) by a dawn‐to‐dusk electric field. It is suggested that the nighttime PFD is caused by a polar‐originating electric field associated with ionospheric currents responsible for the preliminary reverse impulse of ssc(− +) (sc*), while the daytime PFD could be caused by an induction electric field associated with the abrupt magnetic increase of ssc.
Energetic Particle Observations in the Low‐Latitude Boundary Layer
Authors: Williams D. J. , Mitchell D. G. , Eastman T. E. , Frank L. A.
Published: February 2008

We present three‐dimensional energetic (≥ 24 keV) particle distribution observations for two traversals of the magnetospheric low‐latitude boundary layer (LLBL). One traversal occurred during local morning hours (∼0515 LT; December 19, 1977, 0800–1600 UT), and the second during local evening hours (∼1845 LT; July 7, 1978, 0400–1200 UT). Both cases were for a northward magnetosheath field. In the dawnside LLBL the data show trapped electron distributions and tailward convecting ion distributions that display a small but observable bidirectional field‐aligned component. In the duskside LLBL the data reveal tailward convecting, trapped ion distributions and no measurable electron fluxes. LLBL electric fields inferred from the convecting energetic ion distributions are ∼ 3–5 mV/m. Earthward of the dusk LLBL is a region of enhanced fluxes in which the convective anisotropies of the energetic ion distributions become too small to measure. Earthward of this region is the plasma sheet where nearly isotropic ion fluxes are seen with a small but steady sunward convection anisotropy. From an analysis of the data we conclude that for these two cases of northward magnetosheath magnetic field, the flank LLBL is contained on closed geomagnetic field lines, the plasma sheet is the main source of energetic particles in the flank LLBL, and it is possible that two large energetic particle circulation cells (electrons in the dawnside and protons in the duskside) exist in the geomagnetic tail.
Magnetosheath whistler turbulence
Authors: Rodriguez Paul
Published: February 2008

Magnetic field short‐period fluctuations δB are observed throughout the earth's magnetosheath. We report a study of these fluctuations with frequencies 20 < f < 1000 Hz in which the turbulence has a broadband spectrum δB²(f) varying approximately as f−α, where α ∼ 4±0.5, or consists of a narrowband intermediate frequency peak superimposed on the broadband spectrum. The spectrum is interpreted as due to whistler turbulence in both cases. The broadband fluctuations may arise from the cascade of spectral power from lower‐frequency (∼0.01 Hz) magnetohydrodynamic modes, while the narrowband peak corresponds to short‐period lion roar bursts. The usual whistler cutoff as f approaches fge, the electron gyrofrequency, is observed, as well as the polarization of the wave amplitudes perpendicular to the static magnetic field. The frequency of occurrence of whistler turbulence maximizes in the inner regions of the sunward magnetosheath, apparently associated with compressional modes in MHD fluctuations which tend to occur preferentially in the same region. The distribution of fluctuation amplitudes at 200 Hz suggests a tendency for the greatest whistler mode intensities to occur near the magnetopause, a result which contrasts with the intensity distribution of 1‐kHz electrostatic waves, which tends to maximize at the bow shock. These two intensity distributions are thus indicative of the different wave‐particle interactions occurring at the inner and outer boundaries of the magnetosheath. The mapping of the magnetosheath in space by the occurrence of whistler turbulence indicates whistlers are at least quasi‐trapped in the magnetosheath.
Magnetospheric convection observed between 0600 and 2100 LT: Solar wind and IMF dependence
Authors: Baumjohann Wolfgang , Haerendel Gerhard
Published: February 2008

Using data from the GEOS 2 electron gun experiment, we have analyzed the dependence of the dayside convection electric field at L = 6.6 (averaged into 3‐hour LT bins) on solar wind and interplanetary magnetic field conditions. The convection electric field does not correlate at all with the solar wind momentum flux density (correlation coefficients of <0.25). Hence viscous interaction plays only a minor role for equatorial magnetospheric convection at L = 6.6. The correlation coefficients for convection electric field versus merging electric field are of the order of 0.5–0.6, thus indicating that dayside convection is predominantly driven by dayside merging. The regression coefficients, describing the (LT‐dependent) transfer of the merging electric field to synchronous orbit, are of the order of 0.1–0.2. Their LT dependence follows approximately that described by a Volland‐Stern model with γ = 2, except that the regression analysis yields a dawn‐dusk asymmetry with the dusk sector electric fields being about twice as strong as those in the dawn sector. For the above shielding factor (γ = 2) the regression coefficients are consistent with a merging efficiency of about 20%. The regression constants (of the order of 0.05 mV/m) apparently describe an electric field stemming from the ionospheric wind dynamo. A comparison of the electric field described by the regression constants and two empirical models of quiet day electric fields implies that these models still include a substantial dawn‐to‐dusk electric field, thus indicating the presence of solar wind dynamo action even during quiet intervals.
Theoretical study of anomalously high F region peak altitudes in the polar ionosphere
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

During the last solar maximum period several observations of anomalously high F region peak altitudes have been made by the high latitude incoherent scatter radars. The observations indicate that there are several distinctive features associated with these high hmF2 ionospheric profiles: (1) they are observed near midnight with the plasma flowing out of the polar cap, (2) NmF2 ranges from 105 to 106 cm−3, (3) hmF2 ranges from 400 to 500 km, (4) below 300 km the profile is devoid of ionization, and (5) the observations are for solar maximum conditions. In an effort to explain these radar observations, a time‐dependent high latitude ionospheric model was used to study transport effects for a wide range of solar cycle, seasonal, magnetic activity, and neutral wind conditions. The model results indicate that high hmF2 values in the midnight sector of the polar region can be generated without the need for ionization due to auroral precipitation. For solar maximum, all of the observed features of the high hmF2 density profiles are reproduced by the model if the neutral wind across the polar cap is greater than 400 m/s. Such wind speeds have been frequently measured during the last solar maximum period. The study also shows general results for the influence of transport in the polar cap for different seasonal and solar cycle conditions. NmF2 and hmF2 are lower for solar minimum than solar maximum. However, the seasonal dependences are strongly coupled with both the strength of the convection and the neutral wind speed.
Coronal mass ejections: 1979–1981
Authors: Howard R. A. , Sheeley N. R. , Koomen M. J. , Michels D. J.
Published: February 2008

In an examination of the Solwind coronagraph images obtained during the interval March 28, 1979, to December 31, 1981, we have identified 998 coronal mass ejections and recorded their structural classes, central latitudes, latitudinal spans, speeds, excess brightnesses, and relative importances. A statistical analysis revealed the following general results. (1) The properties of coronal mass ejections (CMEs) depended strongly on their structure. Curved front, halo, and complex CMEs were the most energetic, and single spike, streamer blowout, and diffuse fan CMEs were the least energetic. CMEs occurred over a wide range of position angles, broadly centered on the equator, and had an average angular span of 45°. The leading edge moved at an average of approximately 470 km/s, and the average ejected mass and kinetic energy were 4.1×1015 g and 3.5×1030 erg, respectively. The average CME proton flux at the equator at 1 AU was 2.2×107 cm−2 s−1 or approximately 5% of the measured in situ flux during 1971–1976. (2) During 1979–1981, the average occurrence rate was 1.8/day for all CMEs, 0.9/day for “major” CMEs, and 0.15/day for all CMEs that crossed the equator and had an angular span of at least 45°. (3) The temporal variations in the CME occurrence rate did not show an obvious persistent relation to the variations in the sunspot number on time scales ranging from 7 to 180 days. During 1979–1981 the maximum in the 180‐day average CME rate peaked in the second half of 1980, whereas the 180‐day average sunspot number peaked during the first half of 1980. The 180‐day average rate of fast CMEs (speeds of at least 800 km/s) had a monotonic increase that seemed to be more closely associated with the occurrence rate of large solar flares than with the variation of the sunspot number.
Electrical measurements in the atmosphere and the ionosphere over an active thunderstorm: 1. Campaign overview and initial ionospheric results
Authors: Kelley M. C. , Siefring C. L. , Pfaff R. F. , Kintner P. M. , Larsen M. , Green R. , Holzworth R. H. , Hale L. C. , Mitchell J. D. , Le Vine D.
Published: February 2008

The first simultaneous electric field observations performed in the ionosphere and atmosphere over an active nighttime thunderstorm are reported here. In the stratosphere, typical storm‐related dc electric fields were detected from a horizontal distance of ∼100 km, and transient electric fields due to lightning were measured at several different altitudes. In the ionosphere and mesosphere, lightning‐induced transient electric fields in the range of tens of millivolts per meter were detected with rise times at least as fast as 0.2 ms and typical duration of 10–20 ms. The transients had significant components parallel to the magnetic field at 150 km altitude. This implies that either considerable Joule heating occurs or a collective instability is present because of the high drift velocities induced by the transient electric fields. Copious numbers of whistlers were genrated by the storm and were detected above but not below the base of the ionosphere. We present here the outline of a new model for direct whistler wave generation over an active thunderstorm based on these observations. The intensity of the observed two‐hop whistlers implies that they were amplified along their propagation path and suggests that particles were precipitated in both hemispheres.
Energetic neutral atoms (E ∼ 50 keV) from the ring current: IMP 7/8 and ISEE 1
Authors: Roelof E. C. , Mitchell D. G. , Williams D. J.
Published: February 2008

Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid‐state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange. ENA observations during the recovery phase of two moderate geomagnetic storms are analyzed in detail: November 22–23, 1973, from IMP 8 at 33 RE, and December 17, 1977, from ISEE 1 at 20 RE. The three‐dimensional viewing capability of the Medium Energy Particle Instrument (MEPI) on ISEE 1 provides coarse spatial information; i.e., the ENA emission fell off significantly beyond L ≃ 5 in the midnight and morning sectors of magnetic local time. The spectral resolution of the MEPI allowed us to deduce that oxygen atoms dominated the ENA fluxes at energies of ∼100 keV because the decay time constant (7.5 hours) for the ENA fluxes was identical in the two lowest energy channels over 10 hours. Combining the spatial and spectral information from ISEE 1, we estimate an ENA unidirectional differential flux jO ≃ 30 (cm² s sr keV)−1 for 100‐keV oxygen atoms with an e‐folding energy of 20 keV at the beginning of the recovery phase. This ENA flux implies a singly charged oxygen ion flux jO+ ≃ 3.5×104 (cm² s sr keV)−1 near the inner edge of the ring current region, and we estimate a corresponding decay time for geocorona H atoms ≃ 9 days at that time and location. Using a newly derived relation between the recovery rate of the ring current disturbance (dDst/dt) and the measured ENA flux, we show self‐consistently that charge‐exchange loss of O+ ions was the dominant mechanism in the recovery of Dst in both of the storms observed in ENA by IMP 8 and ISEE 1. Even though oxygen dominated the ENA flux and the ring current energy loss rate at ∼100 keV, H+ intensities at these energies could still have been comparable to those of O+ in the ring current (because the charge‐exchange cross section for H+ is much smaller than that for O+ at 100 keV).
Flux transfer events at Mercury
Authors: Russell C. T. , Walker R. J.
Published: February 2008

An examination of high‐resolution Mariner 10 magnetic measurements in the vicinity of the Mercury magnetopause for the three available crossings at high resolution reveals the signatures of what have been called flux transfer events (FTE). These events occur both in the magnetosheath and in the magnetosphere. They last about 1 s and hence have a dimension of about 400 km, or about 5% of the width of the Mercury magnetosphere. This relative dimension is similar to that observed at earth, but the repetition rate is about an order of magnitude faster at Mercury. The net amount of flux transfer is much less than that at the earth. We estimate that less than 1% of the available solar wind potential drop is applied by FTE's to the magnetosphere of Mercury. Evidence for “steady state” reconnection is also observed which may apply a potential drop from 5 to 25 kV across the Mercury magnetopause. The magnetopause itself appears to be about 500 km thick. The shape of the magnetopause at the crossing locations can be approximated with a simple rotationally symmetric conic section with its focus at the center of the planet and an eccentricity of 0.8.
Electron impact excitation of atomic oxygen: Revised cross sections
Authors: Zipf E. C. , Erdman P. W.
Published: February 2008

Revised cross‐section values for the excitation of the O I(3s ³S°‐2p ³P; λ1304 Å), O I(3d ³D°‐2p ³P; λ1027 Å), and O I(3s′ ³D°‐2p ³P; λ989 Å) resonance transitions by electron impact on atomic oxygen are presented from threshold to 300 eV. These results are smaller than the excitation cross sections used in some airglow models by a factor of ∼2.8. The revised values are in good agreement with recent quantum‐scattering calculations. The downward revision is required by new laboratory studies in which the direct and dissociative cross sections for λ1304 Å excitation were normalized with small probable error to the O and O2 ionization cross sections. The results also reflect new advances in VUV optical calibration techniques. A number of outstanding airglow problems are simplified by these revisions.
Characteristic time constants and velocities of high‐latitude Pi 2's
Authors: Samson J. C. , Harrold B. G.
Published: February 2008

Ground‐based measurements of the phase velocities, group velocities, and rise times of the magnetic fields of high‐latitude (60°–68°N corrected geomagnetic latitude), substorm associated Pi 2's indicate that these pulsations may be caused by the transient reflection of an Alfven wave from the auroral ionosphere. Individual pulsation wave packets can be very short, typically one cycle, with a duration of 100–150 s. The phase velocities are eastward to the east of the region of the onset of the field‐aligned currents, and westward to the west of the region of the onset. The group velocities follow a pattern similar to that of the phase velocities but tend to have values somewhat lower than the phase velocities. The longitudinal phase velocities are very high, typically 20 km/s or greater, suggesting that the Pi 2's are not connected with auroral forms such as the westward traveling surge but are more likely connected with the initial brightening of the arc associated with the breakup. Consequently, the longitudinal expansion of the field‐aligned currents and Alfvén waves causing the Pi 2's may be a direct result of the mechanism leading to the onset of the substorm expansive phase in the magnetotail.
Coupled hydromagnetic wave excitation and ion acceleration upstream of the Jovian bow shock
Authors: Smith Charles W. , Lee Martin A.
Published: February 2008

We extend the Lee (1982) self‐consistent theory of upstream wave excitation and particle energization to address observations by Voyager 2 of sunward propagating MHD waves and diffuse suprathermal particle populations upstream of the Jovian bow shock. Two new ideas are incorporated into the theory. First, the interplanetary seed wave population is taken to be an equal admixture of waves propagating both toward and away from the shock parallel to the interplanetary magnetic field. Second, finite connection times are incorporated approximately into the theory in an effort to understand whether the particle spectra at high energy are limited by particle escape or finite connection time. It is found that finite connection times dominate the particle distribution at energies above 40 keV. In this manner the suprathermal proton distributions can be accounted for by a multiple reflection, shock acceleration theory. We find that the theory can also account for the low‐frequency waves observed upstream of the shock in conjunction with the suprathermal ions.
Fractal structure of the interplanetary magnetic field
Authors: Burlaga L. F. , Klein L. W.
Published: February 2008

Under some conditions, time series of the interplanetary magnetic field strength and components have the properties of fractal curves. Magnetic field measurements made near 8.5 AU by Voyager 2 from June 5 to August 24, 1981, were self‐similar over time scales from ∼20 s to ∼3×105 s, and the fractal dimension of the time series of the strength and components of the magnetic field was D = 5/3, corresponding to a power spectrum P(f) ∼ f−5/3. Since the Kolmogorov spectrum for homogeneous, isotropic, stationary turbulence is also f−5/3, the Voyager 2 measurements are consistent with the observation of an inertial range of turbulence extending over approximately four decades in frequency. Interaction regions probably contributed most of the power in this interval. As an example, one interaction region is discussed in which the magnetic field had a fractal dimension D = 5/3.
SCATHA survey of high‐level spacecraft charging in sunlight
Authors: Mullen E. G. , Gussenhoven M. S. , Hardy D. A. , Aggson T. A. , Ledley B. G. , Whipple E.
Published: February 2008

High‐time‐resolution (0.5 s) measurements of the vehicle frame to ambient plasma potential were made with a 50‐m antenna experiment on the SCATHA satellite. Accurate measurements were limited to sunlight conditions or to low‐level eclipse charging periods. Significant variations in the spacecraft potential in sunlight occurred which depended on the orientation of the satellite with respect to the sun. Using the SCATHA data, statistical occurrence of charging at near‐geosynchronous orbit in daylight is studied. Charging greater than −10 V (in the negative sense) occurs only between 1900 LT and 0900 LT but at all altitudes and latitudes of the SCATHA orbit. High‐level (> −100 V) charging occurs only for magnetic activity, as measured by Kp, of 2+ or greater. Three “worst case” daylight charging event periods (−340 V to −740 V) show that the electron population that directly drives the vehicle potential on the SCATHA satellite has an energy typically greater than 30 keV. The vehicle potential (in the negative sense) is directly proportional to the electron flux carried by the population above 30 keV, although the linear regression coefficients change from case to case. The vehicle potential is insensitive to changes in the electron flux below 30 keV, even though the fluxes in this range are considerably more intense. To explain the data, we suggest that the low‐energy electron fluxes are essentially self‐balanced by the combination of their own secondary and backscattered emissions and that a substantial portion of the photoelectron emissions are returned to the satellite by the action of the surrounding magnetic field.
Average characteristics of the polar rain and their relationship to the solar wind and the interplanetary magnetic field
Authors: Riehl Kevin B. , Hardy David A.
Published: February 2008

A study of the characteristics of the polar rain has been completed using data from the SSJ/3 sensor on the Defense Meteorological Satellite Program F2 satellite. Approximately 262 polar passes of data were analyzed. Orbits were chosen which exhibited an extended region of polar rain and for which solar wind and interplanetary magnetic field (IMF) data were available. For each pass, 45‐s‐averaged spectra of the polar rain were calculated. In approximately 70% of the cases the resulting spectra are well fit by a single simple Maxwellian distribution with an average temperature of 80 ± 13 eV and an average density of 5.5×10−2 ± 3.8×10−2 cm−3. In the remaining 30% of the cases an additional hot component was fit by a second Maxwellian. The distribution of temperatures for the hot component shows a prominent peak at ∼525 eV but extends over a range from 300 to 2800 eV. The high‐energy component has an average density of 6.0×10−4 ± 3.5×10−4 cm−3. The distribution of the ratios of the high‐energy to low‐energy temperatures has a prominent peak between 5 and 8. No clear relationship was found between the temperatures of the solar wind and the polar rain or their number densities. The occurrence of the events studied is strongly dependent on the Bz component of the IMF being negative. The intensity of the polar rain is dependent on both the orientation and the magnitude of the IMF Bx and By components. For By positive (negative) in the north (south) pole the intensity of the polar rain increases exponentially with the magnitude of By. For By negative (positive) in the north (south) pole the level of the polar rain is constant as a function of By. Essentially the same behavior is seen if comparable separations are made using the IMF Bx component. Differentiation of whether the behavior is primarily dependent on Bx or on By was not possible. Positive gradients in the intensity of the polar rain were found primarily along the satellite trajectory in the dusk to dawn direction. For By positive (negative) in the north (south) pole these gradients are most prominent. For the opposite case of By negative (positive) in the north (south) pole the gradient remains dusk to dawn, or the polar rain level is flat. Only in a small number of cases in either cap does the gradient reverse and appear in the dawn to dusk direction.
Mid‐winter intensities of the night airglow O2 (0–1) atmospheric band emission at high latitudes
Authors: Myrabø H. K. , Deehr C. S. , Romick G. J. , Henriksen K.
Published: February 2008

Absolute intensities of the O2 (0–1) atmospheric band night airglow emission have been observed from near Longyearbyen on West Spitsbergen (78.4°N latitude, 15°E longitude, geographic) during a 2‐month period around winter solstice (1982–1983). Intensities ranging from 110 R to 1590 R with a mean of 570 R ± 60 R are observed. There is no clear maximum or minimum around solstice. A semidiurnal tide component giving rise to a 25–30% intensity variation of the O2 (0–1) atmospheric emission is present in the data. The maximum and minimum are found to occur at about 0400 and 1000 local time, respectively. No diurnal tide component larger than ±3–4% is present in the data. The day‐to‐day and short time variations both show a quasi‐regular wave pattern which may be associated with gravity waves.