bluuebunny/crossref_metadata_2025 huggingface dataset:
Relative contribution of ionospheric conductivity and electric field to the auroral electrojets
Authors: Kamide Y. , Vickrey J. F.
Published: February 2008

Data from continuous scans of the Chatanika radar beam along the magnetic meridian plane are used to determine the latitudinal profile of height‐integrated ionospheric conductivities and horizontal electric fields, from which the latitudinal distribution of ionospheric currents is deduced. The observations cover invariant latitudes between 62° and 68°, where the IMS Alaska meridian chain of magnetometers was also in operation. Although the conductivities and the electric fields are interrelated, the relative importance of the two in driving the eastward and westward auroral electrojet currents can be assessed. It is found that for moderate and large current densities (i.e., ≳0.2 A/m), the northward electric field strength increases as the magnitude of the eastward electrojet in the evening sector increases. The height‐integrated Hall conductivity stays generally at the level of 10 mhos even when the current density becomes as large as 1 A/m. However, when the eastward electrojet is small, substantial electric fields of 10–20 mV/m may still exist as if the magnetosphere has a persistent voltage source. There appear to be two distinct components to the westward electrojet. In the midnight and early morning sestors (<0300 MLT) intensity is characterized by a weak southward electric field and a high Hall conductivity, whereas its late morning portion ( >0300 MLT) is dominated by a strong southward electric field.
The dependence of the relative backscatter cross section of 1‐m density fluctuations in the auroral electrojet on the angle between electron drift and radar wave vector
Authors: André Dieter
Published: February 2008

With the STARE radar system it is possible to measure, with high spatial and temporal resolution, the electron drift velocity VD and the relative amplitude of electron density fluctuations of 1‐m wavelength in the auroral electrojet. These density fluctuations are generated by the combined effects of the two‐stream and the gradient drift instabilities. We have determined the angular dependence of the backscatter intensity (which is proportional to the square of the amplitude of the density fluctuations) on the angle θ between the electron drift direction and the direction from the scattering volume to the radar in the plane perpendicular to the magnetic field. We find a fluctuation minimum for θ = 90° and an increase towards θ = 0° over the whole velocity range up to 1000 m/s. This increase is small for velocities below the ion acoustic velocity Cs but reaches over 20 dB gain in the backscatter intensity (corresponding to a density fluctuation more than 10 times as great) for higher velocities. We explain that the backscatter is caused mainly by two‐stream instability in the range cos θ > Cs/Vd and by secondary gradient drift instability elsewhere.
The visual aurora as a predictor of solar activity
Authors: Silverman S. M.
Published: February 2008

Auroral occurrence frequency may be used as proxy data for solar activity. In recent years magnetic data and auroral data during the declining phase of one sunspot cycle have been used to predict the maximum sunspot number of the following cycle. This paper discusses appropriate use of auroral data for this purpose, and tests the validity of such a procedure for the period 1721–1943. The results show that the use of auroral sums for sunspot minimum and the preceding three years can predict the subsequent sunspot number maximum, but that the predictor involves two branches. One, fitting the majority of observations, may be considered the normal branch. The other, called here the deficit branch, gives fewer auroral occurrences than would be expected. Treatment of magnetic activity data in the same fashion gives similar, but not identical results.
CO2 on Titan
Authors: Samuelson R. E. , Maguire W. C. , Hanel R. A. , Kunde V. G. , Jennings D. E. , Yung Y. L. , Aikin A. C.
Published: February 2008

A sharp stratospheric emission feature at 667 cm−1 in the Voyager infrared spectra of Titan is associated with the ν2 Q branch of CO2. A coupling of photochemical and radiative transfer theory yields an average mole fraction above the 110 mbar level of
, with most of the uncertainty being due to imprecise knowledge of the vertical distribution. CO2 is found to be in a steady state, with its abundance being regulated principally by the ∼72 K cold trap near the tropopause and secondarily by the rate at which water‐bearing meteoritic material enters the top of the atmosphere. An influx of water about 0.4 times that at the top of the terrestrial atmosphere is consistent with a combination of the observed CO2 abundance and a steady state CO mole fraction of 1.1×10−4; the theoretical value for CO is close to the value observed by Lutz et al. (1983), although there are large margins for error in both numbers. If steady state conditions for CO prevail, little information is available regarding the evolution of Titan's atmosphere.
Phoebe: Voyager 2 observations
Authors: Thomas P. , Veverka J. , Morrison D. , Davies M. , Johnson T. V.
Published: February 2008

Voyager 2 images of Phoebe obtained over a period of 24 hours provide information on the size, rotation rate, surface markings, and photometric properties of this mysterious object. Phoebe is approximately equidimensional: its longest diameter, ∼230 km, is only 10% greater than the shortest diameter (∼210 km). A prograde rotation period of 9.4±0.2 hours was determined from both the disk‐integrated light curve and by tracking individual markings. Because of the limited resolution of the images (11 pixels across the disk) crater counts cannot be made. The geometric albedo of Phoebe is longitudinally variable from 0.046 to 0.060 (clear filter, λ = 0.47 μm). The most prominent surface markings are brighter patches at high northern and southern latitudes that have reflectances as much as 50% greater than the dark, bland areas. These patches are scattered and do not appear to constitute polar caps. The Voyager color data agree with earlier ground‐based spectra that show that Phoebe has a flatter spectrum than does the dark side of Iapetus; this observation is not consistent with simple contamination of Iapetus by debris from Phoebe.
Statistical study of Saturn electrostatic discharges
Authors: Zarka P. , Pedersen B. M.
Published: February 2008

During the Voyager Saturn mission the Planetary Radio Astronomy (PRA) experiment observed sporadic broadband emissions (20 KHz to at least 40 MHz) during a few days around the Voyager 1 and 2 closest approaches to the planet. These emissions are interpreted to arise from short‐lived electrostatic discharges in the vicinity of Saturn. We have made a statistical study of their characteristics from the Voyager 1 and 2 high‐band data of the PRA experiment (1.2 MHz to 40.2 MHz). In this paper, we compare the results obtained for the two encounters and describe in detail the appearance, intensity, and spectrum of the Saturn electrostatic discharges (SED). We show that the SED spectrum is likely smooth in the frequency range 1–40 MHz and extends down to 20 KHz. We have determined the period of the SED occurrence, which seems to be slightly different for Voyager 1 and 2 encounters (10 hours 9 min for the Voyager 1 encounter and 10 hours 00 min for the Voyager 2 encounter). An attempt to interpret the statistical results is presented.
Coordinated study of subkilometer and 3‐m irregularities in the F region generated by high‐power HF heating at Arecibo
Authors: Basu Santimay , Basu Sunanda , Ganguly S. , Gordon W. E.
Published: February 2008

High‐power high‐frequency transmitters near Arecibo were used to generate artificial ionospheric irregularities in the F region. Radio star scintillation observations at 430 MHz were performed at Arecibo Ionospheric Observatory with the 305‐m antenna, and radar backscatter measurements at 50 MHz were simultaneously made from Guadeloupe Island to probe the subkilometer and 3‐m irregularities in the heated volume. Scintillation studies indicate a low‐frequency modulation of the faster intensity fluctuation structure. By the use of plasma drift data this low‐frequency temporal structure translates to spatial dimensions of 1–2 km. The frequency of the modulation envelope is found to be controlled by the heater power and is related to the dominant irregularity wavelength generated by the self‐focusing instability. Scintillation spectra imply a steep power law index of −5 in the scale length range of about 300 m to 150 m and a shallow index of −2 at less than 150 m. The steep power law index may arise from an aperture averaging effect of the large 305‐m antenna at Arecibo. Simultaneous measurements of 430‐MHz scintillations and 50‐MHz radar backscatter from field‐aligned striations were performed to show that subkilometer irregularities can be generated by both O and X mode heating whereas the 3‐m irregularities are excited only by the O mode heating, as is predicted by the theories of self‐focusing and parametric instability. The width of the 50‐MHz echo Doppler spectra is observed to be very narrow, only ∼2–3 Hz, and independent of the background plasma drift, implying that the frequency bandwidth of the scattered signal is probably controlled by the instability process.
The O+ 834‐Å dayglow: Satellite observations and interpretation with a radiation transfer model
Authors: Kumar Shailendra , Chakrabarti Supriya , Paresce Francesco , Bowyer Stuart
Published: February 2008

The O+ 834‐Å dayglow observations made with the EUV spectrometer on the Air Force satellite STP‐78‐1 are interpreted with the use of a radiation transfer model. At 600‐km altitude the dayglow intensity was observed to vary with magnetic latitude from 30 R to 300 R in the near zenith direction (θ = 40°) and from 400 R to 500 R in the near nadir direction (θ = 140°); the correspondingly large near zenith to near nadir intensity ratio of 0.3–0.65 over much of the dayside indicates that the O+ ions in the topside ionosphere constitute an optically thick medium for resonance scattering of 834‐Å airglow emission even at this high altitude. Simultaneous measurements of the O+ density from the AE‐E spacecraft at 460‐km altitude near the equator are used to normalize the STP 78‐1 airglow data. A g value of 1.1×10−8 s−1 at zero optical depth is required for the ionization excitation of atomic oxygen leading to the production of O+ atoms in 4P state. The latitudinal distribution of O+ density derived from the 834‐Å airglow data shows a double peak (at ±13° latitude) surrounding an equatorial trough, characteristic of the equatorial anomaly, at altitudes below 1000 km and a single peak at the magnetic equator above 1000 km. At high latitudes (>20°) the O+ densities in the summer hemisphere are substantially lower than those in the winter hemisphere.
Velocity shear instabilities in the anisotropic solar wind and the heating of ions perpendicular to the magnetic field
Authors: Migliuolo Stefano
Published: February 2008

The linear and quasi‐linear theory of perturbations in finite‐β, collisionless plasmas that incorporate a sheared velocity flow is developed. A simple, one‐dimensional magnetic field geometry is assumed to adequately represent solar wind conditions near the sun, i.e., at R ≈ 0.3 AU. Two modes are examined in detail: an ion‐acoustic mode (finite‐β stabilized) and a compressional Alfvén mode (finite‐β threshold, high‐β stabilization). The role played by equilibrium temperature anisotropies, in the linear stability of these modes, is also presented. From the quasi‐linear theory, the effects of waves on the state of temperature components (parallel and perpendicular to the magnetic field) of the solar wind is determined: qualitative agreement with the observed anisotropies of electrons and ions is obtained. The quasi‐linear temperature changes are also shown to result in saturation of these instabilities.
Electric field and plasma density measurements in the auroral electrojet
Authors: Pfaff R. F. , Kelley M. C. , Fejer B. G. , Kudeki E. , Carlson C. W. , Pedersen A. , Hausler B.
Published: February 2008

Intense electrostatic waves in the auroral E region have been detected simultaneously on two payloads launched in a mother‐daughter configuration from Kiruna, Sweden. The data sets comprise electric field and density measurements from the ambient (dc) conditions to fluctuations as high as 50 kHz. The dc electric field measured by both payloads was 54 mV/m northwest, which corresponded to an electron drift velocity of 1080 m/s. This electric field drove two‐stream waves perpendicular to both B and E observed by both spacecraft throughout an altitude region which agrees quite well with the range predicted by linear two‐stream theory. The power in the waves depended on the electron density gradient, diminishing near 107 km where the gradient changed direction for a few kilometers. This observation is consistent with a gradient drift wave contribution to the instability process, since the auroral zone geometry does permit a component of the electric field perpendicular to B to be parallel to the vertical electron density gradient. Electric field spectra corroborate these results, as a strong component was detected at longer wavelengths (several hundred meters), the spectral regime associated with this instability. The spectra measured on both payloads also reveal an enhanced nearly coherent wave of a few meters wavelength at the topside of the layer (120 km), which also appears as electrostatic and parallel to the current. This spectral feature may be the consequence of a narrow range of wavelengths excited near the two‐stream instability threshold. Descending through the layer, the fluctuations fill out to a broadband, turbulent spectrum, extending to scale sizes down to the order of centimeters. Long wavelength waves form the strongest spectral component below 105 km. Power observed perpendicular to the direction of the current may indicate the presence of secondary plasma waves.
Auroral photometry from the Atmosphere Explorer Satellite
Authors: Rees M. H. , Abreu V. J.
Published: February 2008

Two passes of the Atmosphere Explorer C satellite over the Poker Flat Optical Observatory and the Chatanika Radar Facility provided the best auroral measurements from space and from the ground available to date for demonstrating the capability of remote sensing from space to yield quantitative auroral and ionospheric parameters. It is shown that the emission rate of the N2+ (1NG) 4278 Å band computed from intensity measurements of energetic auroral electrons tracks the same spectral feature measured remotely from the satellite over 2 decades of intensity. This provides a stringent test for the procedure adopted to correct the optical measurements for atmospheric scattering effects and verifies the absolute intensity with respect to the ground‐based photometric measurements, referenced to a laboratory calibration. In situ satellite measurements of ion densities and ground‐based radar measurements of electron density profiles provide a consistent picture of the ionospheric response to the auroral input that our model is capable of predicting while at the same time also predicting the observed optical emission rate. Modeling therefore is the link between global remote sensing optical measurements and the ionospheric response to auroral energetic particle bombardment.
Comparison of the solar cosmic ray events on May 7, 1978, and November 22, 1977
Authors: Debrunner H. , Flückiger E. , Lockwood J. A. , McGuire R. E.
Published: February 2008

An analysis has been made of the solar cosmic ray events on May 7, 1978, and November 22, 1977, to determine the energy spectra of the solar protons from 50 MeV to 10 GeV, the pitch angle distributions, the intensity‐time profiles, and the anisotropy‐time profiles. Data from neutron monitors and IMP 7 and 8 satellites were used. It is found that the energy spectra of the solar protons obtained from neutron monitors and satellite observations are in good agreement in the overlapping energy range when the pitch angle distribution is taken into account. The energy spectra covering three decades of energy and eight decades of intensity and their time variations were remarkably similar for the two events. The difference in the neutron monitor responses for the events was mainly due to the different pitch angle distributions. For the prompt particle event on May 7, 1978, the shapes of the intensity‐time profiles for various energies are the result of the energy dependence of the time‐extended injection of particles into the interplanetary medium. This event is especially important for the study of the acceleration of solar particles and their subsequent injection into the interplanetary medium. For the November 22, 1977, event, even though the propagation of the solar protons in the IMF was essentially diffusive, the solar particle injection was also observed to be time extended and energy dependent.
Geos 2 identification of rapidly moving current structures in the equatorial outer magnetosphere during substorms
Authors: Robert Patrick , Gendrin Roger , Perraut Sylvaine , Roux Alain , Pedersen Arne
Published: February 2008

The ULF magnetic and the dc electric antennas operating onboard GEOS have been used to study the origin and characteristics of short irregular pulsations (SIP's). Strong SIP's are always observed during substorm “onsets”, which are characterized by an abrupt change of the GEOS dc magnetic field from a taillike configuration to a more dipolar one and which is known to be associated with rapid poleward displacement of aurorals forms in the vicinity of the GEOS field line. By applying to the three‐component ULF signal a complex processing it is possible to demonstrate that most of SIP's are in fact the magnetic signatures of localized current structures passing by the spacecraft at a high velocity. Very intense spikes in the electric field (E ≃ 3–25mV/m) are observed in connection with the passing over of such structures at the satellite location, but these E field spikes are generally observed 10–20 s earlier than the SIP's. On the average the E field direction is earthward with a smaller component towards dusk. Provided that the duration of the magnetic signature of the signal (2τ) is short enough (less than 2 s) it is possible to show that they correspond to field aligned current tubes (FACT's) passing by the spacecraft. When 2τ > 2 s, the signature of the SIP is still consistent with that of a field‐aligned current tube, but this cannot be assessed without ambiguity from the ULF experiment alone. Nevertheless, the direction of the velocity of the moving structure can still be deduced from the magnetic signature. For structures detected during substorm onsets this direction is consistent (within 40°) with the drift velocity direction as determined by E × B. This situation occurs for 28 events out of a total of 42, which were analyzed in detail. For these events, and assuming that the amplitude of the velocity is given by E × B/B², it is possible to compute the characteristic parameters of the structure: current density J, radius R (by assuming a cyclindrical structure for simplification), and velocity v. The parameters that have thus been obtained are 6.10−9 < J < 3.10−7 A/m², 20 < R < 900 km, 15 < v < 170 km/s, with the following average values: 8×10−8 A/m², 215 km, and 70 km/s. When transposed to ionospheric altitudes these values are consistent with those found for localized field‐aligned current structures by ground or low orbiting spacecraft observations. The origin and nature of these localized current structures are discussed. A simple theory based on the MHD stability of a field‐aligned current structure is proposed to explain why the current density can hardly exceed in the equatorial plane a critical value that is of the order of 10−7 A/m².
The structure of the plasma sheet‐lobe boundary in the Earth's magnetotail
Authors: Orsini S. , Candidi M. , Formisano V. , Balsiger H. , Ghielmetti A. , Ogilvie K. W.
Published: February 2008

The structure of the magnetotail plasma sheet‐plasma lobe boundary is studied by observing the properties of tailward flowing O+ ion streams. These have been detected by the ISEE 2 plasma experiment and the ISEE 1 ion composition experiment inside the boundary during three time periods in April 1978; data from the ISEE 1 electron spectrometer are used to define the location of the boundary. The density of the O+ ion streams has a relative maximum, and their velocity has a relative minimum at the boundary. The xy drift velocity component across the local magnetic field reverses at the boundary, indicating a reversal of the Ez electric field component. These changes are consistent with the topology of the electric field lines in the tail as mapped from the ionosphere.
On the relationship between morning sector irregular magnetic pulsations and field aligned currents
Authors: Engebretson M. J. , Cahill L. J. , Potemra T. A. , Zanetti L. J. , Arnoldy R. L. , Mende S. B. , Rosenberg T. J.
Published: February 2008

Data from search coil magnetometer, riometer, and photometer instrumentation at Siple, Antarctica, are compared with vector magnetometer data from south polar passes of the MAGSAT satellite for three magnetically disturbed days in early 1980. Two kinds of association were found between irregular pulsation activity in the 0.5‐ to 40‐s frequency band (Pi 1) observed on the ground and magnetospheric‐ionospheric currents measured from space. (1) During each MAGSAT polar pass in the morning sector the level of Pi 1 activity correlates well with the intensities of three‐dimensional current systems. Fine structure was often observed in the field‐aligned currents during periods of intense Pi activity. Birkeland currents include 2‐ to 10‐s (16–80 km) structured perturbations evident in the transverse components of the field, most likely indicating filamentary currents. (2) The pulsations observed at Siple in the morning sector include an asymmetric class of fluctuations in dB/dt whenever the region 2 field‐aligned currents extend to or equatorward of the invariant latitude of Siple and do not have such components when the currents do not extend to that latitude. These observations support models suggesting that Pi 1 pulsations represent fluctuations of a three‐dimensional current system in the magnetosphere and are consistent with recent conductivity and current enhancement models for the ionospheric source of Pi 1 activity.
Global ionospheric current distributions during substorms
Authors: Ahn B.‐H. , Kamide Y. , Akasofu S.‐I.
Published: February 2008

The growth and decay of global ionospheric currents during magnetospheric substorms on March 17, 18, and 19, 1978 are examined on the basis of magnetic records from the six IMS meridian chains of observatories and others (the total number being 71). The computer code developed by Kamide et al. (1981) and the conductivity model developed by Ahn et al. (1983) are used. Several substorms centered around 1000–1200 UT are chosen in this presentation, since the simultaneous all‐sky and riometer records are essential in timing the substorm epochs. Several global features that are common to most substorms during the three‐day interval include the following: (1) During a quiet period, currents are often present in the cusp and/or polar cap regions. The cusp current consists of a pair of east‐west currents and the polar cap current consists of several vortices. (2) When the interplanetary magnetic field (IMF) Bz component is positive, but decreases in magnitude, a well‐defined westward electrojet develops in the midnight sector. However, this development is not evident in the AE index. (3) A gradual, but distinct growth (often followed by a rapid and large increase) in the AE index is identified as the intensification of a weak substorm current system, which was mentioned in (2), accompanied by typical auroral substorm features, including riometer absorption. (4) The subsequent sharp increase of the AE index arises primarily from a deep intrusion of the westward electrojet into the pre‐midnight sector and its equatorward shift. (5) The overall increase of the global current can be significantly different from what a sharp increase of the AE index indicates. (6) During the recovery phase, the intruded westward electrojet recedes towards the dawn sector. The electrojets recede also poleward.
The shape and position of the plasma sheet in Earth's magnetotail
Authors: Voigt Gerd‐Hannes
Published: February 2008

The configuration of the tail plasma sheet in earth's magnetotail has been calculated in connection with a three‐dimensional magnetospheric B field model. This model is based on the idea that thermal plasma, tail currents, and magnetic field be in magnetohydrostatic equilibrium during time periods of magnetically quiet conditions. The tail configuration is generated by a separation method assuming a cylindrical magnetotail boundary with constant radius. The separation method restricts self‐consistency to planes perpendicular to the tail axis. The computed tail plasma sheet is flexible and reacts to changes of the earth's dipole tilt angle and changes of the solar wind pressure. Consequences for the plasma sheet configuration with respect to the assumed tail magnetopause shape and the separation method are the following: (1) the plasma sheet thickness increases in YGSM direction toward the flanks of the tail; (2) the plasma sheet becomes thicker and more diffuse with increasing distance from the earth; (3) during the northern hemisphere summer, the neutral sheet is raised above the magnetospheric equatorial plane around local midnight but crosses this plane and is depressed below it near the flanks of the tail. The latter result agrees qualitatively with Fairfield's empirical neutral sheet model which he derived from spacecraft measurements of the tail field polarity. This agreement between theory and observational material provides a further piece of evidence that the magnetohydrostatic theory is an appropriate level for describing quantitatively the quiet state of the magnetosphere.
Multiple correlation between auroral and magnetic pulsations
Authors: Oguti Takasi , Meek Jack H. , Hayashi Kanji
Published: February 2008

Multiple‐correlation analysis between ground magnetic pulsations observed at a control point and auroral pulsating patches in various parts of the sky observed by an all‐sky TV camera shows that the magnetic variations (time derivatives) at the control point can be well represented by linear combinations of luminosity fluctuation in various parts of the sky and that the linear coefficients, which relate the luminosity fluctuations to the magnetic fluctuations, can be stable for a fairly long time (longer than 30 min). Further, time‐shifted multiple correlation shows that the lag time between magnetic and auroral pulsations is less than 1 s, which is much shorter than the propagation time of hydromagnetic waves from the magnetospheric equatorial plane to the auroral ionosphere. No evidence is found that the magnetic fluctuations, concurrent with auroral pulsations, are due to the effects of hydromagnetic waves propagating down from the magnetospheric equatorial plane. As a result it is concluded that the magnetic fluctuations observed on the ground concurrent with auroral pulsations are mostly due to fluctuations of ionospheric electric currents caused by spatial and temporal variations in ionospheric electric conductivity produced by the pulsating precipitation of auroral electrons into pulsating auroral patches, and that the erratic relationship between magnetic and auroral pulsations reported in the past is mostly due to poor spatial coverage of auroral observations.
A theoretical F region study of ion compositional and temperature variations in response to magnetospheric storm inputs
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

The response of the polar ionosphere to magnetospheric storm inputs was modeled. During the “storm,” the spatial extent of the auroral oval, the intensity of the precipitating auroral electron energy flux, and the plasma convection pattern were varied with time. The convection pattern changed from a symmetric two‐cell pattern with a 20‐kV cross‐tail potential to an asymmetric two‐cell pattern with enhanced plasma flow in the dusk sector and a total cross‐tail potential of 90 kV. During the storm there were significant changes in the ion temperature, ion composition, and molecular/atomic ion transition height. The storm time asymmetric convection pattern produced an ion temperature hot spot at the location of the dusk convection cell owing to increased ion‐neutral frictional heating. In this hot spot there were significantly enhanced NO+ densities and hence molecular/atomic ion transition heights. During the storm recovery phase, the decay of the enhanced NO+ densities closely followed the decrease in the plasma convection speed. During the storm, elevated ion temperatures also appeared at high altitudes in the midnight‐dawn auroral oval region. These elevated ion temperatures were a consequence of the storm‐enhanced topside O+ densities, which provided better thermal coupling to the hot electrons. This region also contained reduced molecular/atomic ion transition heights. These elevated ion temperatures and reduced transition heights persisted for several hours after the storm main phase ended.
Resonant Alfvén waves excited by a sudden impulse
Authors: Baumjohann W. , Junginger H. , Haerendel G. , Bauer O. H.
Published: February 2008

On March 6, 1979, long‐period hydromagnetic waves were excited in the forenoon magnetosphere by a sudden impulse (SI). The plasma drift velocity and magnetic field oscillations associated with these waves were observed by the electron gun experiment and the magnetometer, respectively, onboard the GEOS 2 satellite in the equatorial plane. The waves had both compressional and transverse components and had their probable source in a single, tailward traveling, large‐scale magnetopause surface “wavelet” (i.e., a single rarefaction/compression pulse) caused by the passage of an interplanetary shock front. This surface wavelet apparently coupled into the inner magnetosphere via the field line resonance mechanism. The satellite observations and the Poynting vectors calculated from these are consistent with a location of the resonance region earthward of GEOS 2 during the first 5 min after the SI while the satellite was located just inside the resonance region at later times. This shift in the relative location of the resonance region was probably caused by an increase of the Alfvén velocity as a result of the compression of the magnetosphere associated with the sudden impulse. The stronger‐than‐usual compressional magnetic field component most likely had its origin in the relatively larger wavelength of the SI‐excited surface wavelet (compared with that of Kelvin‐Helmholtz‐excited surface waves).
Average electron precipitation patterns and visual aurora characteristics during geomagnetic quiescence
Authors: Makita K. , Meng C.‐I.
Published: February 2008

The pattern of polar region electron precipitation during geomagnetically quiet periods (i.e., mostly under the positive IMF Bz condition) is examined on the basis of measurements from the Defense Meteorological Satellite Program (DMSP) F2, F3, and F4 satellites. The auroral electron precipitation during quiet times consists of two distinct types that differ from those found in active times. The high‐latitude part of the precipitation region has average electron energies lower than 500 eV and has bursty spatial structures, but the average energies of the low‐latitude part are generally greater than 500 eV with smoother spatial structures. The boundary between the hard and soft regions is called the transition boundary. It is found that during quiet times the average location of the poleward boundary of electron precipitation is at particularly high geomagnetic latitudes: about 82° to 84° in the morning, noon, and evening sectors and at about 81° to 82° in the midnight sector. The equatorward edge is at about 70° in the morning, noon, and evening sectors and about 69° in the midnight sector. The latitudinal width of the region of auroral electron precipitation is unusually large, indicating a widening of the auroral oval in periods of geomagnetic quiescence. DMSP auroral imaging data further reveal that there are no bright, discrete optical auroras in the region of low average energy electron precipitation. Faint, stable auroral arcs are sometimes seen in the equatorial region of the high average energy electron precipitation. The location of the poleward boundary is affected by the magnitude of the northward interplanetary magnetic field (IMF) component; however, the location of the equatorward and transition boundaries of precipitation are not related to the positive IMF Bz magnitude. These results indicate that the unusually high latitude of the poleward boundary of precipitation is the consequence of a very small number of geomagnetic flux lines interconnected with the IMF during a northward IMF condition. The results also imply that the size of the polar cap is controlled inversely by the magnitude of the northward Bz component. The insensitivity of the transition and equatorward precipitation boundaries to the northward IMF Bz can be used to imply that the boundary dynamics are dominated by internal magnetospheric processes and not by direct solar‐magnetospheric interactions.
The magnetic field of Mars: Implications from gas dynamic modeling
Authors: Russell C. T. , Luhmann J. G. , Spreiter J. R. , Stahara S. S.
Published: February 2008

On January 21, 1972, the Mars 3 spacecraft observed a variation in the magnetic field during its periapsis passage over the dayside of Mars that was suggestive of entry into a Martian magnetosphere. We have obtained the original data and trajectory (Sh. Sh. Dolginov, personal communication, 1983) and have attempted to simulate the observed variation of the magnetic field by using a gas dynamic simulation. In the gas dynamic model we first generate a flow field and then use this flow field to carry the interplanetary magnetic field through the Martian magnetosheath. The independence of the flow field and magnetic field calculation allows us to converge rapidly on an IMF orientation that would result in a magnetic variation similar to that observed by Mars 3. There appears to be no need to invoke an entry into a Martian magnetosphere to explain these observations.
Energetic particle transport in the upstream region of Jupiter: Voyager results
Authors: Baker D. N. , Zwickl R. D. , Krimigis S. M. , Carbary J. F. , Acuña M. H.
Published: February 2008

Using Voyager 1 and 2 energetic (≳ 30 keV) ion measurements near the magnetopause, in the magnetosheath, and immediately upstream of the Jovian bow shock, the available ion compositional patterns have been examined together with typical energy spectra in each of these regions. Field‐aligned, highly anisotropic ion bursts occurring early in most upstream events have relatively low fluxes of electrons and heavy ions associated with them. Characteristic spectral changes are found late in long‐lived ion events at the same time that heavy ion and energetic electron fluxes are enhanced. The interplanetary magnetic field is always connected toward the dawn side of the planet late in this subset of upstream events at the time that relativistic electrons and heavy ions are high. A model emphasizing energetic particle escape in the prenoon part of the Jovian magnetosphere late in events is presented to explain many of the upstream region features.
Suprathermal ions upstream from interplanetary shocks
Authors: Gosling J. T. , Bame S. J. , Feldman W. C. , Paschmann G. , Sckopke N. , Russell C. T.
Published: February 2008

Suprathermal ions with energies between solar wind thermal energies and ∼29 keV are occasionally observed ahead of outward propagating interplanetary shocks with the Los Alamos/Garching fast plasma experiment on ISEE 1 and 2. Compared with suprathermal ion velocity distributions observed upstream from the earth's bow shock, the upstream interplanetary shock ion velocity distributions are relatively structureless, and the particle fluxes are less intense. Typically the suprathermal ion distribution emerges smoothly from the solar wind thermal distribution and is nearly isotropic in the solar wind frame. Such distributions are observed with the fast plasma experiment only in association with interplanetary shocks. Field‐aligned beams, kidney‐bean‐shaped distributions, shells of ions in velocity space, and bunches of gyrating ions—all common to the upstream region of the earth's bow shock—have not been observed ahead of interplanetary shocks. Highly structured ion velocity distributions observed upstream of the earth's bow shock apparently are caused directly or indirectly by the nearly specular reflection of solar wind ions at the shock, a consequence of the generally high Mach number of the solar wind flow at 1 AU. By way of contrast, most interplanetary shocks at 1 AU have low, subcritical Mach numbers, and solar wind ion reflection at these shocks does not appear to play a role in producing upstream suprathermal ion distributions at 1 AU. Nevertheless, solar wind ions are accelerated to high energies at interplanetary shocks. Leakage of shocked thermal plasma across these low‐Mach‐number shocks from the downstream region may play an important role in producing upstream suprathermal ion populations and may therefore constitute the first step in the acceleration of solar wind ions to high energies at these shocks.
Nighttime auroral energy deposition in the middle atmosphere
Authors: Goldberg R. A. , Jackman C. H. , Barcus J. R. , Søraas F.
Published: February 2008

From 1976 through 1982, eight distinct nighttime auroral events have been probed with rocket payloads in a series of high latitude studies at Poker Flat Research Range, Alaska, and Andøya, Norway. The instrument packages have contained X ray and energetic electron detectors, permitting a measure of these energy forms and their absorption in the middle atmosphere. Although the specifics of each event show a wide range of values for energy flux and spectral hardness, certain general characteristics persist in all cases. The primary energy sources for the middle atmosphere under the active conditions examined here are mainly relativistic electrons and bremsstrahlung X rays. Each of these sources is found to dominate ionization in a separate altitude region, with the electrons usually controlling ionization in the upper heights above 55–60 km and the X rays below to about 40 km, where cosmic rays take over. The relativistic electron ionizing radiation source has not usually been considered in modeling studies for the ion chemistry of the lower mesosphere (55–70 km), yet this radiation is capable of increasing ion concentration and electrical conductivity in excess of a factor of 10. Moreover, the persistence of relativistic electrons as an important source in all events studied here strongly suggests that this deficiency be corrected.
HF‐enhanced ion and plasma line spectra with two pumps
Authors: Sulzer M. , Ierkic H. M. , Fejer J. A. , Showen R. L.
Published: February 2008

Spectra of the HF‐enhanced ion line and upshifted and downshifted plasma lines were obtained by a multiple‐pulse technique (9 pulses) and by a technique using pulse‐to‐pulse correlation (512 pulses) with a short (about 1 ms) interpulse period for high frequency resolution. Observations using the multiple‐pulse technique suggest the presence of two different types of spectral features. One type has a spectral width typically greater than 1 kHz, and another type a width much less than 400 Hz, the frequency resolution of the multiple‐pulse technique used. Observations using pulses with a short interpulse period show the width of the narrow spectral features to be in the 20‐ to 60‐Hz range. The results are in good qualitative agreement with predictions of the linear theory of parametric instabilities excited by two pumps. Observations of the narrow spectral features can be used to measure the line‐of‐sight component of ionospheric drift velocities of the electron gas with an accuracy of about 2 m/s.
Radial variations of large‐scale magnetohydrodynamic fluctuations in the solar wind
Authors: Burlaga L. F. , Goldstein M. L.
Published: February 2008

Two time periods are studied for which a relatively complete set of magnetic field and plasma measurements are available at both 1 AU (from IMP 8 and ISEE 3) and near 4 to 5 AU (from Voyager 1). One of these periods is characterized by the predominance of corotating streams, whereas many transient flows were observed in the second period. The evolution of these two flow systems with heliocentric distance is studied by using power spectral techniques. The power spectra for the system of transient flows are consistent with the hypothesis of turbulent evolution, including an inverse cascade of magnetic helicity to large scales. The power spectra for the system of corotating flows cannot be described so simply.
Collective capture of released lithium ions in the solar wind
Authors: Winske D. , Wu C. S. , Li Y. Y. , Zhou G. C.
Published: February 2008

The capture of newly ionized lithium ions in the solar wind by means of electromagnetic instabilities is investigated through linear analysis and computer simulation. Three instabilities, driven by a lithium velocity ring perpendicular to and drifting along the magnetic field, are considered. The capture time of the lithium by the solar wind is roughly 10 linear growth times, regardless of whether resonant or nonresonant modes dominate initially. Possible implications of the results for the Active Magnetosphere Particle Tracer Explorer (AMPTE) mission are discussed.
Thick current sheets in the renovated model of the magnetosphere
Authors: Atkinson G.
Published: February 2008

Earlier work using the observed location and direction of field‐aligned currents to diagnose plasma distributions and magnetic field configurations in the outer magnetosphere is expanded to include thick adjacent current sheets mapping to the whole plasma sheet. Conclusions are as follows: (1) In the tail the total plasma content of flux tubes in the plasma sheet increases from its value on the midnight meridian in going toward both the dawn and the dusk flank. (2) There is a dusk to dawn component of convective flow across the tail with a value of the order of 10 kV. The field‐aligned currents in the tail map to the nonsymmetric current sheets observed at ionospheric levels due to the distortion of field lines consistent with the current sheets. (3) In the near‐earth plasma sheet the dusk to dawn convective flow component reverses to a westward flow before midnight, producing the Harang discontinuity. (4) Some of our basic ideas on magnetotail stability require rethinking.
Comparison of mesospheric VHF radar echoes and rocket probe electron concentration measurements
Authors: Røyrvik O. , Smith L. G.
Published: February 2008

Refractive index irregularities in the equatorial mesosphere have been investigated using both the Jicamarca VHF radar and a rocket‐borne Langmuir probe launched from Punta Lobos, Peru. On February 27, 1983, a single layer of turbulence was observed in the upper mesosphere by both experiments. There is very good agreement between the observed radar echo power and the radar scattering cross section calculated from the rocket data when these are interpreted in the context of isotropic turbulence. The inner and outer scales of turbulence have been calculated from both the radar and the rocket data, and good agreement is found. The radar data show indications of large‐scale vortices in the layer of irregularities. Rocket data show that the inner scale of turbulence in the upper mesosphere is a few tens of meters and that the Jicamarca radar Bragg wavelength (3 m) is well within the viscous subrange of turbulence in this altitude range. The spectral index in the inertial subrange is close to −5/3, changing to about −7 at higher wave numbers. Energy dissipation rate in the layer was calculated to be 0.05 W kg−1, in good agreement with previous estimates.
Drift compressional instability in the magnetosphere
Authors: Ng P. H. , Patel V. L. , Chen S.
Published: February 2008

The properties of the drift compressional instability in the magnetosphere with a two‐component plasma are examined by using gyrokinetic theory. A dipole field is used to approximate the equilibrium geomagnetic field. In contrast to the slab model it is found that electrons of the hot component have a stabilizing effect on the mode excited by the protons. With appropriate values of various parameters, the wave periods in the range of 30–200 s are obtained from numerical calculations of the model. The drift compressional instability may provide possible explanation of the compressional waves observed in the magnetosphere.
Nonlinear pitch angle scattering and trapping of energetic particles during Landau resonance interactions with whistler mode waves
Authors: Tkalcevic S. , Inan U. S. , Helliwell R. A.
Published: February 2008

The time averaged equations of motion derived by Inan and Tkalcevic (1982) are used in a test particle study of Landau resonant interactions of whistler mode waves and energetic particles in the magnetopshere. By computing individual particle trajectories it is shown that the pitch angle scattering and energy exchange is significantly different for the two classes of particles, trapped and untrapped. The trapped particles are characterized by a bounded phase variation, whereas the nontrapped particles exhibit unbounded phase variation. The threshold wave electric field intensities necessary for trapping are determined. Full distribution test particle simulation is carried out to determine the precipitated electron flux that would be induced as a result of these interactions. It is shown that for typical parameters the resulting precipitation fluxes for Landau resonance interactions are much smaller than those induced in gyroresonance interactions, even for wave field intensities that are much higher than the trapping threshold.
Production of lower hybrid waves and field‐aligned plasma density striations by whistlers
Authors: Lee M. C. , Kuo S. P.
Published: February 2008

Injected VLF waves and lightning‐induced whistlers, if intense enough, are able to excite lower hybrid waves and field‐aligned density striations in the ionosphere within a few seconds. This instability can also be triggered in the magnetosphere by the injected VLF waves if the transmitter is operated continuously for several minutes. This instability can be excited in a broad whistler frequency range in two domains, wherein either the nonoscillatory beating current or the thermal pressure force is the dominant nonlinear effect. The excitation of this instability may be responsible for the airglow effects associated with the VLF transmitter cycle and may possibly be the cause of the lower hybrid waves correlated with the occurrence of lightning storms.
Modification to shock fitting program
Authors: Acuña M. H. , Lepping R. P.
Published: February 2008

A modified form of the Lepping‐Argentiero single spacecraft, shock normal determination procedure is presented. The modified method incorporates a simple predictor‐corrector algorithm which allows a faster convergence rate and the use of average values of the parameters for the starting vector.
The importance of vertical ion currents on the nighttime ionization in the equatorial electrojet
Authors: Raghavarao R. , Sridharan R. , Suhasini R.
Published: February 2008

Plasma density profiles in the equatorial electrojet region reveal that the ionization density at 100 km increases by a factor of 2 to 10 from the time of sunset to midnight. In addition, a valley in plasma density centered around 120 km altitude deepens by a factor of 2 to 5 during the same period. These two aspects are explained on the basis of the dynamical features peculiar to the electrojet region. It is demonstrated that the vertically downward motion of positive ions due to Pedersen currents is very important in the redistribution of ionization. The valley is shown to be formed at 120 km, where the vertical ion velocity maximizes.