bluuebunny/crossref_metadata_2025 huggingface dataset:
Doppler radar observations of harmonics of electrostatic ion cyclotron waves in the auroral ionosphere
Authors: Prikryl P. , André D. , Sofko G. J. , Koehler J. A.
Published: February 2008

Observations of short‐lived scattering events (0.5 s to 1 min) have been made with cross‐beamed 50‐MHz CW Doppler radars. Doppler spectra with high resolution in both frequency and time are obtained by detailed processing of time series digitized at high analogue to digital conversion rates. The spectra are highly structured, showing bursts of power at groups of discrete frequencies that are very short lived (usually ˜50 ms). Discrete spectral peaks can be found at Doppler velocities as high as 2500 m/s. The frequency separation between the successive discrete components of a group is often approximately the cyclotron frequency of one of the principal ions in the E region (NO+, O2+, N2+, and O+), so the components are postulated to be harmonics of electrostatic ion cyclotron (EIC) waves at elevated beam electron temperatures (more than 1000 K) in the upper E region. These transient events are closely correlated with intense auroral arc brightenings and/or the onset of an auroral breakup, when high temperatures, strong field‐aligned particle drifts, and field‐aligned currents can be anticipated. Numerical solutions of the kinetic dispersion relation for a range of ionospheric parameters, especially the parallel electron drift velocity and beam temperature, reveal that both the higher and lower harmonics of EIC waves can become unstable. Thus some of the rapid time variation of the observed spectral peaks in the transient events can be explained by an appropriate accompanying change in ionospheric parameters.
Generation, saturation, and convection of electrostatic waves in Jupiter's shock foot
Authors: Moses S. L. , Coroniti F. V. , Kennel C. F. , Scarf F. L.
Published: February 2008

In this paper we develop a model to analyze the electrostatic waves produced in the shock foot at Jupiter. We show that an ion beam instability involving the ions reflected at the shock ramp and the incoming solar wind electrons produces waves at the observed frequencies and that saturation via orbit diffusion limits the waves to amplitudes near to what is observed. Results from two‐dimensional model of the reflected beam in the foot indicates that the waves propagate against the solar wind away from the shock ramp and are amplified up to their saturation amplitudes. The saturation results combined with the electron temperature profile due to wave‐particle interactions predicted by quasi‐linear theory reproduce a wave amplitude profile for the shock foot that is in reasonable agreement with the observations.
A model of the westward traveling surge and the generation of Pi 2 pulsations
Authors: Rothwell P. L. , Silevitch M. B. , Block L. P. , Tanskanen P.
Published: February 2008

A model of the westward traveling surge (WTS) and the generation of Pi 2 pulsations is presented here. Previous work concentrated on the motion of the WTS as a function of the precipitating electron energy and the concurrent generation of Pi 2 pulsations via a feedback instability. Now we look in more detail at the physical assumptions used in deriving the present model and the relations between the zero‐order and the first‐order solutions. Constraints are placed on the electron temperature asymmetry in the plasma sheet by requiring the Pi 2 pulsations to be bounded. It is found that the electron temperature anisotropy in the plasma sheet plays a major role in determining the direction in which the surge will propagate. Narrower surges require greater electron heating parallel to the magnetic field for poleward motion. More energetic electron precipitation is predicted to produce higher‐frequency Pi 2 pulsations. Pulsations occur in multiple bursts with the time interval between bursts being shorter for shorter field lines. Initial amplitude and phase conditions are crucial in determining the pulse shape. The dominant period of the Pi 2 pulsation is found to be equal to twice the north–south dimension of the surge divided by a term which is proportional to the poleward velocity of the boundary. Finally, we show that the poleward surge velocities and Pi 2 pulsation periods as measured during the magnetospheric substorm of June 23, 1979, are consistent with our model. By noting the direction of the surge motion, one can use the model to estimate the magnitude of the polarization electric field. We find that it is consistent with zero for the onsets considered.
Nonlinear mixing and excitation of sidebands in the ionosphere
Authors: Lalita  , Tripathi V. K.
Published: February 2008

Two radio waves of frequencies ω1 and ω2 couple nonlinearly in the lower ionosphere to create a temperature perturbation TΔω, 2k1 (Δω = ω1 − ω2; 2k1 ≈ k1 + k2) and a density perturbation n Δω by the process of ambipolar diffusion. This perturbation couples with the ω1 frequency wave to generate electromagnetic sidebands of frequencies ω1 ± nΔω (n = 1, 2, ⋯ ) as observed in a recent experiment. The field of the sideband wave near the turning point has an Airy function dependence while away from the turning point it has a WKB sinusoidal solution. The power of the sideband decreases with increasing Δω. It also falls almost exponentially with the order of the sideband. The existence of a turning point for the pump waves is essential for efficient nonlinear coupling.
Observations of a gradual transition between Ps 6 activity with auroral torches and surgelike pulsations during strong geomagnetic disturbances
Authors: Steen Å , Collis P. N. , Evans D. , Kremser G. , Capelle S. , Rees D. , Tsurutani B. T.
Published: February 2008

A long‐lasting large‐amplitude pulsation event was observed on January 10, 1983, 0200–0600 UT (0411–0745 MLT) in the ionosphere and in the magnetosphere. In the ionosphere the characteristics of the pulsations changed from being Ps 6/auroral torches toward substorms and back to Ps 6 over the 4‐hour period. At the geostationary orbit (6.6 Re) the corresponding characteristics were a modulation of the high‐energy (≥20 keV) particle intensity and plasma dropouts. Following the work by Rostoker and Samson (1984), we propose an interpretation of the event in which the pulsations are caused by the Kelvin‐Helmholtz instability, during an interval of strong magnetospheric convection. The gradual transition between Ps 6 pulsations and substorm structures is interpreted as being different results of the Kelvin‐Helmholtz instability, caused by different states of the magnetospheric convection. The proposed explanation forms the basis for a discussion on a simplified scheme of the substorm sequence.
Filamentation instability of large‐amplitude Alfvén waves
Authors: Kuo S. P. , Whang M. H. , Lee M. C.
Published: February 2008

An instability that leads to the filamentation of large‐amplitude Alfvén waves and gives rise to purely growing density and magnetic field fluctuations is studied. The dispersion relation of the instability is derived, from which the threshold conditions and the growth rates of the instability are analyzed quantitatively for applications to the solar wind plasma. We have examined their dependence on the filamentation spectrum, the plasma β, and the pump frequency and intensity for both right‐hand and left‐hand circularly polarized Alfvén waves. The excitation of filamentation instability for certain cases of interest is discussed and compared with that of the parametric decay and modulation instability. The relevance of the proposed instability with some observations is discussed.
Birkeland currents and charged particles in the high‐latitude prenoon region: A new interpretation
Authors: Bythrow P. F. , Potemra T. A. , Erlandson R. E. , Zanetti L. J. , Klumpar D. M.
Published: February 2008

Simultaneous, conjugate measurements of magnetic fields and charged particles at low altitude in the high‐latitude prenoon sector and the magnetosheath were made with the DMSP F7, HILAT, and Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE satellites on November 1, 1984. These data show that the low‐latitude portion of the traditional “cusp” particle signature is coincident with the prenoon region 1 current system in both hemispheres and for both northward and southward interplanetary magnetic fields (IMF). The traditional “cusp” Birkeland currents are associated with the dispersion region of cusp ions when the IMF is directed southward and with electron fluxes that are slightly enhanced over polar rain intensities. Finally, electron spectra measured by AMPTE CCE in the magnetosheath near 1000 MLT are similar in shape and energy to those acquired at low altitude by both DMSP F7 and HILAT. These observations indicate that for both northward and southward IMF, the traditional “cusp particle” signature is coincident with the region 1 Birkeland current system and maps to low altitude along field lines that thread the dayside boundary layer. The traditional “cusp” Birkeland current system flows along field lines that lie poleward of the region of cusp particles and the region 1 current system. These field lines thread the plasma mantle. Thus, we suggest that the traditional “cusp” current system might be appropriately renamed the “mantle” Birkeland current system.
Geographic distribution of lightning‐induced electron precipitation observed as VLF/LF perturbation events
Authors: Inan U. S. , Wolf T. G. , Carpenter D. L.
Published: February 2008

Expected occurrence characteristics of lightning‐induced electron precipitation (LEP) events at longitudes of the western (110° W) versus eastern (71° W) United States are considered from the point of view of available trapped particle flux at the edge of the loss cone. Considering published data on nighttime fluxes of >68 keV electrons observed at L ≃ 2.5, and for “direct” precipitation into the northern hemisphere induced by northern hemisphere lightning, the occurrence rate and flux levels are expected to be a factor of 20–200 higher in the west than in the east, assuming no significant variation in lightning source activity with longitude. Again assuming lightning sources in the north, it is predicted that at 71° W, “mirrored” precipitation into the southern hemisphere would involve precipitation fluxes 30–300 times higher than “direct” precipitation into the northern hemisphere. However, at 110° W and again assuming lightning in the north, southern hemisphere precipitation would tend to be limited to that small fraction of particles that were initially scattered into the northern loss cone and that were then backscattered from the northern atmosphere so as to reach the south. Preliminary experimental investigation of these predictions is based on observations of lightning‐associated perturbations of two geographically separate subionospheric VLF/LF signal paths, one (48.5 kHz) originating at Silver Creek, Nebraska, and observed at Stanford, California, and the other (28.5 kHz) originating at Aguadilla, Peurto Rico, and observed at Lake Mistissini, Quebec. The association of the characteristic VLF signal perturbations with lightning is generally evidenced by simultaneous (within ∼1 s) observation of single or multiple radio atmospherics. In most cases, high‐resolution measurements of event signatures reveal a ∼0.5–1 s delay between the atmospheric and event onset, as well as an ∼1‐s onset duration, consistent with theoretical predictions of a test particle model of the gyroresonant whistler‐particle interaction. The data, considered in the light of previous observations in the southern hemisphere, provide qualitative support for several of the predictions based on considerations of the trapped flux level near the loss cone, in particular the prediction of comparable rates in the north at 110° W and the south at 71° W, and the prediction of substantially larger rates in the south than in the north near 71° W.
Effects of geomagnetic activity in the winter thermosphere: 1. Magnetically undisturbed conditions
Authors: Hagan Maura E. , Salah Joseph E.
Published: February 2008

A two‐part study of the influence of magnetic variability on the structure and dynamics of the winter thermosphere as observed by incoherent scatter radars has been conducted. Here, in paper 1, we present a thermospheric case study for magnetically undisturbed geophysical conditions, which is used as a reference in studying the magnetic storm effects in paper 2. Neutral exospheric temperatures derived from incoherent scatter radar measurements made at Millstone Hill (43°N), Arecibo (18°N), and Sondrestrom (67°N) between January 14 and 17, 1986, serve as the control case. Mean temperatures and diurnal temperature variations resulting from Fourier decomposition of these data are compared with the predictions of the MSIS‐86 model and recent theoretical calculations. These comparisons confirm the largely reliable predictive capabilities of the magnetically undisturbed winter thermosphere models, but point to some inconsistencies in the horizontal structure of diurnal temperature amplitudes between modeled and measured results. In particular, exospheric temperatures determined from Arecibo measurements appear to be suppressed with respect to both middle‐latitude temperature determinations and the predictions of MSIS. A tidal analysis of neutral wind determinations from Millstone Hill measurements, however, reveals that middle‐latitude meridional mean and diurnal flow was consistent with previous analogous results.
Latitudinal variation of thermospheric hydrogen near solstice from AE‐D observations
Authors: Sanatani S. , Breig E. L.
Published: February 2008

We describe an investigation of the variation of neutral atomic hydrogen with latitude in the thermosphere during a solstice season near solar minimum. Neutral hydrogen concentrations were derived with the charge exchange method applied to in‐situ ion and neutral composition measurements in the F region. The primary basic data were acquired with the polar‐orbiting AE‐D satellite which provided excellent latitudinal coverage within small increments of local time around the December 1975 solstice. Hydrogen concentrations at low latitude are comparable to those found under similar conditions from observations with AE‐E satellite but are slightly larger than concentrations given by the MSIS‐83 atmospheric model. Results, normalized to 300 km, for a morning‐daytime period confirm the general summer‐to‐winter density increase, the large latitudinal gradients in the summer hemisphere, and the winter enhancement of hydrogen observed in various AE‐C nighttime measurements. The AE‐D data, however, provide evidence for a small polar depression in hydrogen concentration at high winter latitudes similar to but of lesser latitudinal extent than that inferred from OGO 6 optical observations in the exosphere. This behavior of hydrogen in the winter polar region bears a resemblance to that found in AE‐D composition measurements of helium during the same period and attributed to atmospheric dynamics following auroral heating. The observed density gradients in the summer hemisphere are in sharp contrast to the more constant horizontal daytime profiles reported from OGO 6 and previous AE‐C measurements and might be indicative of a special importance of local time effects to the latitudinal variability of hydrogen during the summer season.
Switch‐on shocks
Authors: Kennel C. F. , Edmiston J. P.
Published: February 2008

We argue that parallel propagating Alfven waves steepen to second order in the wave amplitude, rather than first order, as is the case for oblique MHD waves. We derive an analytical solution for the structure of weak switch‐on shocks. The Rankine‐Hugoniot relations for finite amplitude switch‐on shocks are obtained in a compact form. The differential equation for resistive switch‐on shocks is solved, and the resistive critical Mach number is obtained. The differential equation for resistive switch‐on shocks with finite ion inertia is identical to the one obtained in the pure resistive case. We argue that efficient electron heat conduction can significantly extend the range of upstream parameters for which switch‐on shocks are expected.
Enhanced antisunward convection and F region scintillations at mid‐latitudes during storm onset
Authors: Foster John C. , Aarons Jules
Published: February 2008

Millstone Hill radar observations over a wide span of latitudes detail the onset of 300 m/s antisunward (westward) convection at mid and low latitudes in the morning sector as a region of storm‐enhanced sunward convection retreats poleward. Ring current observations reported by Lui et al. (1987) suggest that the magnetospheric shielding layer was coincident with the observed reversal between sunward and antisunward convection. A strong southward component of the F region neutral wind is observed at latitudes equatorward of the convection reversal. These observations are in agreement with the model of Spiro et al. (1988), who find that storm‐enhanced neutral winds at latitudes equatorward of the shielding layer can generate a long‐lived perturbation electric field in the inner magnetosphere. Our observations show the growth of the subauroral electric field as the shielding boundary moves poleward. We observe 136‐MHz scintillations in both the auroral sunward convection region and the region of subauroral antisunward convection when the convection electric fields exceed 5 mV/m.
Electron heating and the potential jump across fast mode shocks
Authors: Schwartz Steven J. , Thomsen Michelle F. , Bame S. J. , Stansberry John
Published: February 2008

The electron heating and the electrostatic potential jump across collisionless shocks play an important, if not dominant, role in the electron momentum balance. We present here a survey of these two quantities over a large sample of fast mode collisionless shocks. Results for estimates of the electrostatic potential (as measured in the de Hoffmann‐Teller reference frame) based on an estimate of the jump in electron enthalpy and on Liouville's theorem correlate well with each other, although the latter are consistently higher, perhaps due to irreversible processes affecting the shock electron dynamics. The size of the potential does not appear to be strongly controlled by any of the various upstream parameters (shock geometry, Mach number, etc.) and represents approximately 12% of the incident ion ram kinetic energy, this figure showing some tendency to decrease with increasing Mach number. The electron contribution to the total (ion plus electron) heating across the shock varies systematically from 50% or more at subcritical Mach numbers to less than 10% at the highest Mach number shocks in the sample. The electron heating shows only modest, but systematic, departure from that which would result by preserving the ratio of the perpendicular temperature to the magnetic field strength.
Density and white light brightness in looplike coronal mass ejections: Importance of the preevent atmosphere
Authors: Steinolfson R. S.
Published: February 2008

A recent study by Sime et al. (1984) suggests that numerical simulations of coronal mass ejections in which the energy source is an input of thermal energy may not reproduce several general features of looplike coronal mass ejections observed in white light coronagraphs. In this and the following paper (Steinolfson and Hundhausen, this issue) we examine why the models used in the comparison by Sime et al. were inadequate and show how modifications of such models to include a more realistic preevent atmosphere can improve agreement with observations. For models utilizing a localized energy source, the ambient atmosphere becomes very important in determining the white light signature of a mass ejection. The significance of the initial magnetic field configuration and the coronal outflow have been illustrated in previous publications. We emphasize the role of the thermal structure as it defines the fast‐mode speed in the initial corona. In this paper, we set the stage for detailed calculations in the following paper by considering three different ambient atmospheres. Two of these have already been used as initial states for results which have appeared in the literature. The third is new in that it contains an atmospheric heating term. Through comparison of all three atmospheres, we show that the important difference in the heated atmosphere is the larger fast‐mode speed near the equator.
Ring current development during the great geomagnetic storm of February 1986
Authors: Hamilton D. C. , Gloeckler G. , Ipavich F. M. , Stüdemann W. , Wilken B. , Kremser G.
Published: February 2008

The largest geomagnetic storm in recent decades began with a sudden commencement on February 6, 1986, developed slowly over the next two days, and, with a rapid intensification late on February 8, reached a minimum Dst of −312 nT during the first hour of February 9. Initial recovery was rapid, but full recovery took more than a month. In this paper we follow the ring current development during the storm using particle measurements from the charge‐energy‐mass (CHEM) instrument on the Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE spacecraft. We compare the energy content of the ring current ions with that expected from observed Dst values utilizing for the first time composition coverage over nearly the complete ring current energy range (1–310 keV/e). The ring current composition is followed for five days from prestorm quiet time to early recovery phase. Ions of both solar wind and ionospheric origin are important constituents of the storm time ring current. Although H+ carries the majority of the energy during most of the storm, O+ dominates near the storm's maximum phase, with 47% of the energy density compared with 36% in H+. This is in contrast with all of the more moderate storms which occurred during 1984–1985 in which H+ ions contained most of the energy density near storm maximum. The very rapid initial Dst recovery (τ ∼ 9.3 hours) in this storm results largely from the rapid loss of 75‐ to 100‐keV O+ via charge exchange in the inner portion of the ring current (L = 2.5–3.0). Since it has been long observed that initial Dst recovery is much more rapid in great storms than in moderate storms, we suggest that a major (>50%) O+ + N+ ring current component generally exists near the maximum phase of great storms.
Waveform and polarization of compressional Pc 5 waves at geosynchronous orbit
Authors: Higuchi Tomoyuki , Kokubun Susumu
Published: February 2008

The statistical characteristics of compressional Pc 5 waves are studied using magnetic field data obtained by GOES 2 and 3 geosynchronous satellites during 2‐year period of August 1978 to August 1980. Depending on the second‐harmonic component in the compressional component of the magnetic field oscillation, the compressional Pc 5 waves are classified into the three types: the harmonic, transitional and normal types. It is found that the harmonic and transitional types have significant azimuthal perturbations and show right‐handed polarization with respect to the local magnetic field. The occurrence of these types is limited to the region close to the effective equator in local time sector of 1500 ∼ 1700 LT, associated with a depression in the local field magnitude during geomagnetically active periods. Most of the normal type have small amplitude in the azimuthal component. However, when the normal type (corresponding to compressional Pc 5 waves examined in previous works) has a large azimuthal perturbation, it shows the left‐handed polarization. We try to find factors relating results to the excitation mechanisms developed in the past few years. Especially, we focus on the polarization properties of transverse perturbation which may reflect the spatial inhomogeneity of the media through the finite amplitude effect. It is suggested that the polarization of the transverse perturbation of compressional Pc 5 waves arises from that of the Alfvén mode.
DE 1 observations of return current regions in the nightside auroral oval
Authors: Marshall J. A. , Burch J. L. , Kan J. R. , Slavin J. A.
Published: February 2008

Data from the Dynamics Explorer 1 high altitude plasma instrument (HAPI) and magnetometer (MAG‐A) are used to examine particle acceleration and Birkeland current phenomena in the mid‐altitude (10,000–20,000 km) nightside auroral region between 2000 and 2200 hours MLT. Field‐aligned current densities as derived from the two instruments are compared, generally resulting in good agreement. The latitudinal distribution of upward and downward field‐aligned currents is examined, both on a large scale with respect to regional boundaries and on a local scale with respect to individual inverted‐V events. The charge carriers of the downward currents are found to be cold ionospheric electrons accelerated into upward beams by potential drops of a few tens of volts located at altitudes near 1 Earth radius. In several instances, large potential differences below the satellite are indicated by energetic upward ion beams with no (or very small) potential drops above the satellite. In these cases, very small Birkeland currents (≲10−8 A/m²) are observed above the inverted V, suggesting the possibility that the field‐aligned current systems associated with individual inverted‐V events may close at relatively low altitudes above the auroral oval, or that field‐aligned potential differences may be generated without significant Birkeland currents.
A reanalysis of rocket measurements of the ultraviolet dayglow
Authors: Link R. , Gladstone G. R. , Chakrabarti S. , McConnell J. C.
Published: February 2008

We have reexamined rocket measurements of O I 989‐, 1304‐, 1356‐Å and N2 Lyman‐Birge‐Hopfield band emissions in the mid‐latitude dayglow reported by Gentieu et al. (1979) and Eastes et al. (1985). MSIS‐83 atomic oxygen densities, the 1304‐ and 1356‐Å excitation cross sections of Zipf and Erdman (1985), and Solar Mesosphere Explorer solar 1304‐Å irradiance measurements are consistent with the observed O I and N2 emission intensities. Atmosphere Explorer E measurements of the solar EUV irradiance near solar maximum are consistent with the 1980 rocket airglow data (F10.7 = 213). However, the solar EUV irradiance required to explain the 1978 airglow data (F10.7 = 106) is a factor of 1.5 larger than indicated by AE‐E. Enhancement of the 1304‐Å excitation cross section due to radiative entrapment of cascade‐feeding photons is much less than the factor of 2 predicted by the cascade model of Julienne and Davis (1976). The 3s′³D° direct excitation cross section of Vaughan and Doering (1987) is consistent with the 989‐Å airglow data and leads to an inferred total branching ratio of 1–2 × 10−4 for transitions other than 989 Å from 3s′³D°, in agreement with the value for the 1173‐Å branch measured in laboratory experiments.
Multifrequency analysis of a decametric storm observed at Voyager 1 and ground‐based observatories
Authors: Maeda K. , Carr T. D.
Published: February 2008

Observations of a Jovian decametric non‐Io‐A noise storm made from Voyager 1, the University of Florida Radio Observatory, the University of Texas Radio Astronomy Observatory, and the Jupiter station at Goddard Space Flight Center at frequencies of 26.3, 22.2, 20.0, and 18.0 MHz were found to be correlated. The activity observed at the ground stations occurred 68 min after the corresponding activity at Voyager 1. After correction is made for propagation time differences, this delay is reduced to 34 min. It is demonstrated that at each frequency the envelope of the individual‐event beams occurring during the storm (some or all of which are associated with dynamic spectral arcs) is a quasi‐constant structure that corotates with the inner Jovian magnetosphere, and that the width of this envelope beam is frequency dependent. The width increases as frequency is decreased, mainly because of the change in position of the trailing‐edge beam boundary. Evidence for a relatively slow temporal change in beam geometry is also presented.
Mass loading in the solar wind interaction with Venus and Mars
Authors: Breus T. K. , Bauer S. J. , Krymskii A. M. , Mitnitskii V. Ya.
Published: February 2008

An analysis of available experimental data and theoretical concepts indicates that the interaction of the solar wind (SW) on the subsolar side with Venus, which has no intrinsic magnetic field, and with Mars, which has a small intrinsic magnetic field, is determined by the solar wind dynamic pressure with a contribution from the neutral planetary atmosphere to this interaction. The pattern of the SW interaction with these planets is different in principle for high and low dynamic pressures of the SW and is related to the varying intensity of ion formation processes (the SW Mass loading effect) in the vicinity of the SW obstacle boundary, which moves for different SW dynamic pressures into regions of different neutral atmosphere density. For moderate or high SW dynamic pressures, the subsolar Martian magnetosphere is also affected by this process. Results of numerical simulations of the SW‐Mars interaction for a magnetospheric obstacle boundary at an altitude of 300 km are presented. To estimate the relative role of photoionization and charge exchange processes and their effect on the shock front position, different versions of the mass loading effect were separately calculated.
Ground‐based and satellite observations of traveling magnetospheric convection twin vortices
Authors: Glassmeier Karl‐Heinz , Hönisch Michael , Untiedt Jürgen
Published: February 2008

Close inspection of observations from the International Magnetospheric Study Scandinavian Magnetometer Array revealed the existence of a peculiar type of short‐period magnetic variations. In typical cases it can be described as a positive deflection of the geomagnetic D component from the quiet time level with the disturbance of the H component approximately representing the negative time derivative of the D component. Thus these events are similar to magnetic variations recently reported by Lanzerotti et al. (1986) and discussed in terms of possible signatures of flux transfer events as well as to observations by Friis‐Christensen et al. (1988) in connection with readjustments of the magnetopause to a sudden change in solar wind dynamic pressure. The typical duration of such a transient magnetic perturbation is about 10 min, with amplitudes of about 20 nT. Between 1975 and 1979 about 80 of these events have been detected, with a sharp occurrence maximum at magnetic forenoon. Most events were observed during moderately quiet times (KP≃2). A more detailed analysis of two such events exhibits a two‐cell ground‐equivalent current vortex structure. Simultaneous magnetic field observations from GEOS 2 for one of these events support our interpretation of these events as representing pairs of downward (in the west) and upward (in the east) field‐aligned currents in the magnetosphere, moving over the observer from the dayside to the nightside. The physical mechanism generating these traveling magnetospheric convection twin vortices and the associated field‐aligned currents is unclear. Flux transfer events, however, are not necessarily associated with the observed short‐period transient magnetic field variations.
Wave phenomena preceding and during a beam‐plasma discharge
Authors: Boswell R. W. , Morey I. J. , Porteous R. K.
Published: February 2008

The waves and associated phenomena which are generated prior to and during the beam‐plasma discharge (BPD) in the waves on magnetized beams and turbulence (WOMBAT) apparatus are described. These range in frequency from below the ion gyrofrequency of a few kilohertz to strong harmonics of the electron plasma frequency above 1 GHz. The low‐frequency ion waves appear to be driven by E × B forces arising from the nonneutrality of the electron beam‐plasma column at low beam currents (Ib) or low gas pressures (P). As Ib increases, the motion becomes chaotic after a sequence of wave period doublings resulting in the generation of subharmonies. Windows of coherent motion are observed within the chaotic regions. Further increases in Ib lead to increases in ionizing collisions and when the background plasma density becomes sufficiently high, waves around the electron plasma frequency can be generated. Initially, these waves are modulated by the gross density perturbations of the large‐amplitude ion waves, but subsequently they also increase to large amplitudes: eϕ/kT ∼ 1. This is sufficient to accelerate the background electrons to ionizing energies and produce the BPD. At low pressures this occurs for fpe/fce ∼ 0.5 (BPD1) with the process occurring in short bursts (≲100τpe). Increasing Ib produces a sudden jump in the frequency to ∼1.5fce and a further increase in the plasma density and luminosity, which we term BPD2. A sequence of BPDs then occurs as Ib is further increased with characteristic frequencies between gyroharmonics up to fpe ≤ 6fce.
Solar wind‐magnetosphere coupling during intense magnetic storms (1978‐1979)
Authors: Gonzalez Walter D. , Tsurutani Bruce T. , Gonzalez Alicia L. C. , Smith Edward J. , Tang Frances , Akasofu Syun‐I.
Published: February 2008

The solar wind‐magnetosphere coupling problem is investigated for the ten intense magnetic storms (Dst<−100 nT) that occurred during the 500 days (August 16, 1978 to December 28, 1979) studied by Gonzalez and Tsurutani [1987]. This investigation concentrates on the ring current energization in terms of solar wind parameters, in order to explain the |−Dst| growth observed during these storms. Thus several coupling functions are tested as energy input and several sets of the ring current decay time‐constant τ are searched to find best correlations with the Dst response. From the fairly large correlation coefficients found in this study, there is strong evidence that large scale magnetopause reconnection operates during such intense storm events and that the solar wind ram pressure plays an important role in the ring current energization. Thus a ram pressure correction factor is suggested for expressions concerning the reconnection power during time intervals with large ram pressure variations. The best set of values found from the present study is in accord with recent similar and independent suggestions. With respect to the ring current energy injection rates during intense storm events, typical values of 150±50 nT/hour are obtained. These are considerably larger than values extrapolated from previous studies restricted to moderate storms. Such rates of energy injection are observed to get transmitted from the magnetopause to the inner magnetosphere with an average time delay of about 1 hour, although this delay can become shorter as the storm events get more intense. It is also found that AE does not respond as well as Dst to the coupling functions, except for the events that have Dst values only >−100 nT (less intense energization). This point suggests that there is a decoupling between the ring current and auroral processes during very intense storms with respect to their dependence on solar wind energization. Finally, a discussion is presented on associations of the coupling functions with the solar wind features described by Tsurutani et al. [1988] for the causes of these storm events.
Collisional and inertial dynamics of the ionospheric interchange instability
Authors: Zargham S. , Seyler C. E.
Published: February 2008

The dynamical properties and spectral characteristics of the ionospheric interchange instability in the collisional and inertial regimes are studied numerically and analytically. The study is based on a two‐dimensional fluid model that includes the ion inertia and ion‐neutral collisions. Using a pseudospectral method, we follow the evolution of rising plasma bubbles, unstable plasma strata, and driven plasma irregularities in both regimes. We find that the collisional and inertial dynamics are qualitatively similar in the linear stage but markedly different in the nonlinear stage. For both regimes, the fastest growing modes select a characteristic horizontal wavelength and dominate the linear dynamics. In the collisional regime, the vertical and the horizontal directions are not strongly coupled and the nonlinear effects generate steepened structures which propagate in the vertical direction. In this regime, the density and the energy power spectra are anisotropic and both have the same form. The vertical density power spectrum is a power law with an index of about −2.5, while the horizontal density power spectrum is knee‐like with a peak at about the wavelength of the most unstable modes. In the inertial regime, the nonlinear advection of the plasma generates an isotropic rotational flow that resembles the flow of a neutral fluid governed by the two‐dimensional Navier‐Stokes equations. The density power spectra are isotropic and have a power law dependence with the power index ranging between −2.0 and −1.5. The energy power spectra are also isotropic with a power law dependence with a break due to an injection band that corresponds to the characteristic horizontal wavelength of the most unstable modes. To the left of the injection band the power index is −1.6 and to the right it is −3, which appears to correspond to an inverse cascade and a forward cascade in k space, respectively.
Airglow enhancements associated with plasma cavities formed during Ionospheric Heating Experiments
Authors: Bernhardt P. A. , Tepley C. A. , Duncan L. M.
Published: February 2008

Optical measurements made at the Arecibo Observatory during the 1987 heating campaign showed large temporal and spatial variations in 630.0‐nm airglow enhancements during times of continuous power transmissions of high‐power radio waves. Photometric data displayed fluctuations of 60 R or more in the red‐line (630.0 nm) emission from atomic oxygen. These fluctuations were associated with heater‐induced cavities which drifted and evolved in the modified ionosphere. Data from the Arecibo incoherent scatter radar were used in conjunction with airglow images to provide a physical interpretation of the modification process. Electrons were accelerated by large amplitude Langmuir waves excited by parametric decay instabilities occurring near the wave reflection points inside the density cavities. Inelastic collisions with oxygen atoms produced excited states which yielded enhanced 630.0‐nm and 557.7‐nm emissions. A numerical model has been used to relate the enhanced airglow intensities to the energy spectrum of the accelerated electrons. The measured airglow could have been produced by an isotropic source at 340 km altitude that accelerated 0.01% of the ambient electrons into a suprathermal Maxwellian distribution with a temperature of 2.05 eV. Experimental and theoretical studies suggest that airglow clouds were directly coupled to plasma density cavities because (1) these cavities trapped the HF radio beam and (2) electrons accelerated out into regions of reduced plasma concentration were less effectively thermalized and, consequently, were more effective for collisional excitation of neutral species.
The absolute solar soft X ray flux in the 20‐ to 100‐Å region
Authors: Feng W. , Ogawa H. S. , Judge D. L.
Published: February 2008

In the present work we have used the best available solar spectral data together with our absolute neon ionization cell data to estimate the absolute solar soft X ray flux and its variation with solar activity in the 20‐ to 100‐Å region. Since the neon chamber data cover the integral spectral region from 20 to 575 Å we have constructed a composite spectrum covering that region so that the fraction of solar flux in the 20‐ to 100‐Å region could be determined. Both sounding rocket and satellite observations obtained over the past two decades have been used in the present analysis. Most of these prior data were acquired during low and moderate solar activity levels in contrast with the neon ionization cell data which were obtained during moderate and maximum solar activity levels. Based on these combined data sets we have shown the dependence of the integrated solar soft X ray flux between 20 and 100 Å on the daily 10.7 cm radio flux observed at 1 AU and have obtained an estimate of the maximum flux that can be expected over the solar cycle.
A new magnetic coordinate system for conjugate studies at high latitudes
Authors: Baker K. B. , Wing S.
Published: February 2008

Studying magnetically conjugate phenomena at very high latitudes requires a magnetic coordinate system that is smooth and well defined at the geographic poles. In addition, it should provide for accurate comparisons at different altitudes. In this report we present a variation on the corrected geomagnetic coordinate system that is well defined and smooth over the entire globe. It provides an analytic expression relating geographic coordinates, including altitude, to the magnetic coordinates. The coordinate system is produced by tracing magnetic field lines using the IGRF85 reference magnetic field model with time derivatives updating the model to 1988. An expansion of the relationship in terms of spherical harmonics has been determined, which then provides the required well‐defined and smooth relationship over the entire globe. Independent expansions for different altitudes show a smooth functional relationship of the coefficients of the expansion with altitude, and therefore simple interpolation schemes can be used to provide an appropriate expansion at any altitude between 0 km and approximately 600 km. By reversing the process, the inverse expansions relating the magnetic coordinates to geographic coordinates have also been determined. The effects of the seasonal variation in the Sun's declination along with the variation in the Sun's apparent position due to the eccentricity of the Earth's orbit result in a variation of nearly 1 hour of magnetic local time for a fixed UT over the course of a year. In many applications this variation may be important and should be included when presenting data in terms of magnetic latitude and MLT.
The Kelvin‐Helmholtz instability at the magnetopause and inner boundary layer surface
Authors: Ogilvie Keith W. , Fitzenreiter Richard J.
Published: February 2008

In this paper observations by the vector electron spectrometer on the ISEE 1 spacecraft are used to directly check the incompressible hydromagnetic stability condition given by Hasegawa (1975), which indicates how stability is influenced by the velocity shear, density ratio, magnetic field, and the direction of the wave vector of the unstable wave mode. Since the plasma quantities can be measured, the stability can be examined by means of a contour plot of the stability condition in θ, ϕ space, where θ is the polar angle and ϕ the azimuthal angle specifying the wave direction assumed. The magnetopause is generally found to be stable, by a large margin. In contrast, many cases of marginal stability or instability are found across density transitions in, and at the inner edge of, the boundary layer for wave vectors in plausible directions. It thus appears that the Kelvin‐Helmholtz instability is of importance at the inner edge of the magnetospheric boundary layer, as predicted by magnetohydrodynamic theory.
"Substorms, plasmoids, flux ropes, and magnetotail flux loss on March 25, 1983: CDAW 8""
Authors: Fairfield D. H. , Baker D. N. , Craven J. D. , Elphic R. C. , Fennell J. F. , Frank L. A. , Richardson I. G. , Singer H. J. , Slavin J. A. , Tsurutani B. T. , Zwickl R. D.
Published: February 2008

During a 9‐hour period following a storm sudden commencement on March 25, 1983, six spacecraft near geosynchronous orbit, one over the pole, and three in the magnetotail monitored a complex sequence of magnetospheric variations. Magnetic field compressions associated with the sudden commencement were seen first by the near‐Earth spacecraft and subsequently by the three downtail spacecraft with increasing time delays that were consistent with the tailward movement of an interplanetary‐shock‐associated pressure enhancement. Ground magnetograms and synchronous orbit data are used to identify seven substorm intensifications during this geomagnetically active period. Six of these intensifications are clearly associated with tail lobe field decreases ∼18 RE behind the Earth. Four of these intensifications are followed by both Bz field increases in the tail lobes at ∼18 and ∼30 RE and by the subsequent observation of rapidly flowing plasma sheet plasma at ISEE 3 ∼110 RE down the tail. During two substorms where DE 1 was optically observing the auroral oval, the area of the polar cap was observed to decrease as the tail lobe field decreased at 18 RE. All these observations are consistent with the substorm‐associated release of a plasmoid at a neutral line near 20 RE; however, the classical north‐south variation of the plasma sheet magnetic field thought to be characteristic of the passage of a plasmoid in the deep tail was not seen in every case.
A transient auroral event on the dayside
Authors: Heikkila Walter J. , Jorgensen T. Stockflet , Lanzerotti Louis J. , Maclennan Carol G.
Published: February 2008

On December 5, 1986, high‐latitude magnetometer stations in Greenland, as well as Iqaluit and the South Pole, showed a strong perturbation lasting for about 10 min beginning at 0930 UT in an otherwise quiet period. A pair of field aligned currents separated in the east‐west sense and moving westward (tailward) at 4–5 km/s is consistent with the data, producing a twin vortex pattern of Hall currents. Similar perturbations, but with reduced intensity, were also recorded on the afternoon side at Svalbard, Heiss Island, and several locations in northern Siberia. The perturbation was also observed with the incoherent scatter radar at Sondrestrom, these data agreeing with the twin vortex pattern. The perturbation was accompanied by auroral forms overhead at Sondrestrom that also traveled westward. Meridian scanning photometer recordings at the radar site showed the cleft, located about 3° to 5° poleward in latitude; the cleft did not move from the far northern sky for several hours, even while the disturbance was observed. Viking and Polar Bear satellites passed just before the disturbance over Greenland and DMSP encountered the disturbance near Baffin Island a few minutes later; these spacecraft observations increased our confidence in the interpretation of the data. ISEE 1/2 and IMP 8 recorded a magnetic disturbance in the solar wind, the likely cause of this event. Similar observations by others have been associated with flux transfer events. However, since the observed event occurred on closed field lines, our interpretation is quite different, it is that an impulsive penetration of solar wind plasma on an interplanetary magnetic flux tube took place through the magnetopause, ending up in the low latitude boundary layer. Some efficient mechanism is required to feed the boundary layer with the total amount observed. Other events reported in the literature may have a similar explanation.