bluuebunny/crossref_metadata_2025 huggingface dataset:
On the beam speed and wavenumber of intense electron plasma waves near the foreshock edge
Authors: Bale S. D. , Larson D. E. , Lin R. P. , Kellogg P. J. , Goetz K. , Monson S. J.
Published: February 2004

Using high time resolution particle and wave data from the Wind spacecraft, we examine several crossings of the electron foreshock‐solar wind boundary. We show that the most intense electron plasma waves, observed near the foreshock boundary, often occur coincident with a flux of electrons with energies between 1 keV and 27 keV. This corresponds to electron beam speeds of 9vth ≤ vb ≤ 50vth, rather than vb ≈ 5vth, as is inferred from reduced distribution functions obtained by other instruments. Assuming Landau coupling, the resonant index of refraction is then 3 ≤ N0 ≤ 15, which implies that the waves are susceptible to strong scattering by ambient density fluctuations. The most intense electric fields are not well correlated with beam speed, and the distribution of electric field occurrence is broadly aligned with the interplanetary magnetic field direction. We compare the estimated maturity and bandwidth of the instability with those expected of the electrostatic decay instability and nonlinear Landau damping and find that our observations show a narrower bandwidth than expected. We suggest that the observations are consistent with scattering of electrostatic Langmuir waves to small wavenumber. The observed transverse polarization may be explained by the electromagnetic nature of the small‐wavenumber z‐mode, or as evidence of incident, reflected, and transmitted electrostatic components. The generation of electromagnetic emission at fpe and 2fpe is discussed in the context of the observations.
Hall effects on the Walén relation in rotational discontinuities and Alfvén waves
Authors: Wu B. H. , Lee L. C.
Published: February 2004

For Alfvénic fluctuations in magnetohydrodynamics (MHD) the perturbed transverse velocity Vt and magnetic field Bt can be related by the Walén relation, Vt = ±Bt/(μ0ρ)1/2 ≡;±VAt, where ρ is the plasma density, VAt is the transverse Alfvén velocity, and the plus (minus) sign is for antiparallel (parallel) propagation. However, observations of Vt and Bt for Alfvén waves and rotational discontinuities in the solar wind and at the magnetopause showed an obvious deviation from the relation. In this paper, modifications of the Walén relation for linear and nonlinear Alfvén waves and rotational discontinuities (RDs) are examined in the Hall‐MHD formulation. Let Vit (≈ Vt) be the transverse ion velocity and Vet be the transverse electron velocity. It is found that Vit = ±Bt(z)/(μ0ρ1)1/2 = ±(ρ(z)/ρ1)1/2 VAt(z) and Vet = ±(ρ1/μ0)1/2Bt(z)/ρ(z) = ±(ρ1/ρ(z))1/2 VAt(z)for RDs in Hall‐MHD, where ρ1 is the upstream plasma density. The ion and electron Walén ratios are defined as Ai = Vit/VAt and Ae = Vet/VAt, respectively. It is found in Hall‐MHD that

, AiAe = 1 and Ai < 1 (Ai > 1) for Alfvén waves and RDs with right‐hand (left‐hand) polarization. The Hall dispersive effect may modify the ion Walén ratio by ΔAi≈±0.14 for the magnetopause RDs and by ΔAi≈±0.07 for the interplanetary RDs.
“Broadband” plasma waves in the boundary layers
Authors: Lakhina G. S. , Tsurutani B. T. , Kojima H. , Matsumoto H.
Published: February 2004

Boundary layers are commonly encountered in space and astrophysical plasmas. For example, interaction of solar wind plasma with the planets and comets produces magnetopause and cometopause boundary layers, respectively. Generally, the boundary layers are formed when plasmas with different characteristics interact with each other. The plasma sheet boundary layer in the Earth's magnetotail is formed owing to the interaction of hot dense plasma in the plasma sheet region with the rarefied plasma of the lobe region. Boundary layers are the site where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The purpose of this review is to describe the main characteristics and the possible generation mechanisms of the broadband plasma waves (with frequencies of > 1 Hz) observed in the Earth's magnetopause boundary layer, the Jovian magnetopause boundary layer, the plasma sheet boundary layer, and the Earth's polar cap boundary layer. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufficient precipitated energy flux to the ionosphere to create the dayside aurora at Earth and a weak high‐latitude auroral ring at Jupiter. In general, the broadband plasma waves may play an important part in the processes of local heating/acceleration of the boundary layer plasma. Recent exciting high time resolution results on the broadband plasma waves coming from Geotail, Polar, and FAST will be discussed.
Current‐driven electromagnetic ion cyclotron instability at substorm onset
Authors: Perraut S. , Le Contel O. , Roux A. , Pedersen A.
Published: February 2004

ULF waves at frequencies of the order of the proton gyrofrequency are systematically detected at the early development of substorm breakups. The observed characteristics of these ULF waves, namely their polarization and δE/δB ratio are consistent with being electromagnetic waves driven unstable by a parallel current. In order to take into account properly wave particle interactions, a kinetic approach is used. We show that a parallel drift between electrons and ions leads to a strong instability, resulting from a coupling between the shear Alfvén (SA) mode and the fast magnetosonic mode via this drift. We call it current‐driven Alfvén instability (CDA). We have carried out a parametric study of this current‐driven electromagnetic instability in a parameter range adapted to conditions prevailing at the geostationary orbit before and during breakup. We conclude that even a modest parallel drift between electrons and ions (Vd), caused by a parallel current, can destabilize CDA waves. When the ratio between Vd/VA (VA being the Alfvén velocity) increases, the CDA mode couples with SA mode. These two modes have a substantial parallel electric field that leads to a fast parallel diffusion of the electrons. We suggest that this parallel diffusion leads to an interruption of the parallel current.
Simulation of the outer radiation belt electron flux decrease during the March 26, 1995, magnetic storm
Authors: Desorgher L. , Bühler P. , Zehnder A. , Flückiger E. O.
Published: February 2004

In this paper we study the variation of the relativistic electron fluxes in the Earth's outer radiation belt during the March 26, 1995, magnetic storm. Using observations by the radiation environment monitor (REM) on board the space technology research vehicle (STRV‐Ib), we discuss the flux decrease and possible loss of relativistic electrons during the storm main phase. In order to explain the observations we have performed fully adiabatic and guiding center simulations for relativistic equatorial electrons in the nonstationary Tsygarienko96 magnetospheric magnetic field model. In our simulations the drift of electrons through the magnetopause was considered as a loss process. We present our model results and discuss their dependence on the magnetospheric magnetic and electric field model, as well as on the prestorm fluxes used in the simulations.
Distortion of the ULF wave spatial structure upon transmission through the ionosphere
Authors: Pilipenko V. A. , Vellante M. , Fedorov E. N.
Published: February 2004

Distortion of the ground‐based spatial structure of ULF waves is described in detail using analytical and numerical models of the resonant Alfven wave transmission through the “thin” ionosphere. The numerical calculations have been done for geophysical conditions corresponding to the midlatitude Borok Observatory and the low‐latitude L'Aquila Observatory. As compared with a resonant field‐aligned oscillation above the ionosphere, the ground spatial structure has the following additional features which have not been described by existing models: the amplitude peak is shifted northward, the width of it is changed, and amplitude and phase are modified. The effect is most evident when the resonant frequency and ionospheric conductivity are high. From a physical point of view the effect is caused by two reasons: (1) excitation of a diffusive‐like surface‐type mode in the ionosphere and (2) finite Earth's conductivity. The possible influence of the predicted distortion of the ULF field on various ground‐based methods for the determination of the magnetospheric resonant frequency is discussed.
Identification of the low‐altitude cusp by Super Dual Auroral Radar Network radars: A physical explanation for the empirically derived signature
Authors: André R. , Pinnock M. , Rodger A. S.
Published: February 2004

The Super Dual Auroral Radar Network (SuperDARN) radars are proving to be a very powerful experimental tool for exploring solar wind‐magnetosphere‐ionosphere interactions. They measure the autocorrelation function (ACF) of the signal backscattered from ionospheric irregularities, and they derive parameters such as the Doppler velocity and the spectral width. The associated spectra have a specific behavior inside the cusp, a strong temporal and spatial evolution of the velocity and spectral width, and a high value of the spectral width. Until now, no studies have explained these characteristics, but they are routinely used to detect the cusp in the radar data, for example, to estimate the location of the open/closed field line boundary. Both satellite and ground‐based magnetometer data from the cusp region show broadband wave activity in the Pc1 and Pc2 frequency band. In this study we evaluate how such wave activity modifies the radar's ACF, and we conclude that it explains the spectra seen in the cusp. More specifically, we find that (1) even a monochromatic electric field variation can cause apparently turbulent behavior, including wide spectral widths and apparent multiple components, (2) even low‐amplitude waves are capable of causing large spectral widths, if the frequency is sufficiently high, (3) for a fixed low‐amplitude electric field variation the measured spectral width increases with wave frequency, displaying a sharp transition from low to high spectral width above an onset frequency, and (4) the determination of the background velocity field is not strongly affected by such conditions. While the wave activity is shown to have a major impact on the spectral width, it is found that the radar does accurately represent the large‐scale plasma velocity.
Low‐frequency Hall current instability in a dusty plasma
Authors: Rosenberg M. , Shukla P. K.
Published: February 2004

A Hall current instability in the dust frequency regime in a collisional dusty plasma is considered. The instability is driven by an electron drift perpendicular to an external magnetic field. Using linear kinetic theory, it is shown that for certain dust and plasma parameters the critical drift for the instability can be lower than the ion thermal speed. Possible application to dusty plasmas in the Earth's upper mesosphere or E region, such as meteor trails, is discussed.
Interplanetary magnetic field control of dayside ion conics
Authors: Miyake W. , Mukai T. , Kaya N.
Published: February 2004

Ion conics are frequently observed around the dayside cusp/cleft region and are supposed to be energized by an input of energy from the dayside magnetospheric boundary region. We examined possible influence of the interplanetary magnetic field (IMF) on dayside ion conics observed by the polar‐orbiting Exos D (Akebono) satellite. We found the effect of By and Bz components on the energy flux and occurrence frequency of ion conics. Ion conics are more energetic and more frequently observed when Bz is negative and the magnitude of By is large. The Bz effect is likely to be attributed to the change in energy input to the ion energization region. The By effect may be also due to the increased energy input to the cusp/cleft region during large By period, suggesting that a reconnection‐related energy input is responsible for dayside ion energization. An alternative interpretation of the By effect is that the convection pattern may affect the residence time in the ion energization region and therefore the efficiency of ion energization. We found that the polarity of By controls asymmetrical magnetic local time distribution of ion conics around the local noon, which is attributed to a possible effect of convection pattern on dayside ion energization.
The high‐altitude cusps: HEOS 2
Authors: Dunlop Malcolm W. , Cargill Peter J. , Stubbs Timothy J. , Woolliams Peter
Published: February 2004

Magnetic field data from the HEOS 2 spacecraft are presented for magnetopause crossings during the summers of 1972–1974. The orbit of HEOS 2 at this time was optimal for studying the high‐altitude cusp. Magnetopause crossings were categorized as being either tailward or on the dayside of the cusp, or through the cusp itself. The tailward crossings form a relatively distinct set, but the dayside and cusp crossings are mixed, indicating either considerable motion or distension of the cusp. The cusp crossings extend from 5.5 to 7.5 RE in the XGSM direction and from 4 to 7 RE in the ZGSM direction (30% of all crossings). A more detailed survey of five crossings representing each category is presented. Most of these illustrate that the cusp is a very dynamic region, with magnetic field fluctuations occurring on short timescales. Although useful for the above classification, in general there is little agreement with the current generation of magnetospheric field models. The gross character and location relative to the model field geometry, however, can be identified, given the prevailing magnetospheric activity.
A semiempirical magnetosheath model to analyze the solar wind‐magnetosphere interaction
Authors: Kallio Esa J. , Koskinen Hannu E. J.
Published: February 2004

We have developed a new semiempirical three‐dimensional (3‐D) magnetosheath model to study the solar wind‐magnetosphere interaction. Our goal has been to develop a model which takes into account the observed features of the interaction while keeping the model computationally simple. The model is semiempirical in the sense than the features obtained from global fluid models are used to guide the selection of the functional forms of the parameters in the magnetosheath. The model depends on a few free, empirically determined constants, and it includes separate magnetosheath and magnetosphere models. In the present study the basic features of the model are described, especially the morphology of the 3‐D magnetic field in the magnetosheath. The applicability of the model is analyzed by studying the magnetic shear at the magnetopause and the interconnectivity of the magnetosheath and magnetospheric fields. Preliminary analysis indicates that already a simple phenomenological model can be a useful tool for analysis of solar wind‐magnetosphere interaction.
An increase in Pc1 wave activity prior to magnetic sudden impulses
Authors: Guglielmi A. , Kangas J. , Kultima J. , Potapov A.
Published: February 2004

The enhancement of Pc1 wave activity after magnetic sudden impulses (SIs) is well established by ground‐based and satellite measurements. This phenomenon is explained by compressions of the magnetosphere under the action of interplanetary shocks. In this paper we use long‐term ground‐based observations of Pc1 to study the variation of the Pc1 wave activity before SIs. We conclude that there is a tendency for an increase of the rate of Pc1 occurrence as the time gets closer to the SI. We interpret this effect as an impact of the interplanetary foreshocks on the Earth's magnetosphere.
Field‐aligned currents between conjugate hemispheres
Authors: Benkevich L. , Lyatsky W. , Cogger L. L.
Published: February 2004

Interhemispheric field‐aligned currents flowing between dayside conjugate ionospheres are calculated for different seasons. The currents are a result of redistribution of original three‐dimensional currents because of different ionospheric conductivity in dark and sunlit conjugate hemispheres. Interhemispheric currents and related patterns for the electric potential distribution are computed for a reasonable model of the ionospheric conductivity. The interhemispheric field‐aligned currents are attached to the terminator position, and they close a part of ionospheric currents, flowing in the summer high‐latitude ionosphere, through the conjugate ionosphere of the opposite hemisphere. Although the interhemispheric currents are more significant for summer/winter conditions, they can be observed for equinoctial conditions as well because of a diurnal variation in the terminator position with respect to the geomagnetic poles. Interhemispheric currents in the winter ionosphere are generated mainly by sources in the summer hemisphere. An interesting consequence of the existence of interhemispheric currents is that auroral events can be observed not in the hemisphere with incident field‐aligned currents from a magnetospheric source but in the opposite hemisphere.
Wideband bursts of auroral kilometric radiation and their association with UV auroral bulges
Authors: Hanasz J. , de Feraudy H. , Schreiber R. , Parks G. , Brittnacher M. , Mogilevsky M. M. , Romantsova T. V.
Published: February 2004

Impulsive wideband bursts of auroral kilometric radiation (AKR) observed from the Interball 2 spacecraft are characterized by a rapid rise of intensity (< l min) over a wide frequency range from 30 to 900 kHz, followed by a decay over a few minutes. Their source regions expand rapidly both upward and downward along the auroral field lines. The expansion velocity is determined from the rapid broadening of the frequency range of their leading edge (sometimes more than 100 kHz s−1). The drifts can be negative or positive. Above 500 kHz the inferred downward source expansion is sometimes as small as 2 km s−1 (at altitudes below 3000 km), much smaller than the velocity of the ion sound waves. In the medium frequency range 200 – 500 kHz (3000 – 6000 km) the expansion velocities are usually large, sometimes more than 3000 km s−1, comparable to the velocity of the Alfvén waves. In the lower frequency range, below 200 kHz (> 6000 km), the bursts show an upward source expansion, often simultaneous with the downward expansion of the same source below 3000 km. We observe a close association of the AKR bursts with the fast expansion of the auroral bulges recorded with the UV imager on Polar. It is concluded that the bursts are triggered during the rapid development of auroral substorms. The bursts show a microstructure, which consists of multiple microbursts lasting sometimes less than 6 s. They indicate that the source regions are filamented. This filamentation may be the counterpart of a filamentary nature of the currents closing the current wedge, flowing parallel to the geomagnetic field.
Comprehensive imaging observations of midlatitude ionospheric disturbances during storm time substorms
Authors: Shiokawa K. , Otsuka Y. , Ogawa T. , Igarashi K. , Miyazaki S. , Rich F. J. , Saito A. , Yumoto Y.
Published: February 2004

This paper reports comprehensive imaging observations of 630‐nm airglow enhancements and related phenomena in the nighttime F region over Japan. We found two events during which the airglow intensity increases up to 300 R at the lower‐latitude side of Japan associated with storm time substorms of August 26, 1998, and February 18, 1999. Total electron content (TEC) observations by more than 900 GPS receivers over Japan show a clear TEC enhancement of ∼10–40 % at latitudes below 40°, which coincides with the airglow enhancements and the substorms. On the other hand, 2‐MHz virtual heights obtained by bottomside sounding at four ionosoride stations show equatorward moving large‐scale structures, which do not coincide with the airglow enhancements. Vertical ion drift data at an altitude of ∼800 km obtained by the Defense Meteorological Satellite Program satellites show upward ion motion in midlatitudes just after the substorm onset for the February event. These observations suggest that the midlatitude airglow and TEC enhancements are caused by upward electron motion (fountain) in the low latitudes clue to penetration of eastward electric field associated with the storm time substorms.
Auroral electron acceleration by Alfvén waves and electrostatic fields
Authors: Rönnmark Kjell , Hamrin Maria
Published: February 2004

We present a two‐dimensional numerical model for the formation of discrete auroral arcs. This model describes the evolution of shear Alfvén waves generated by a growing force near the equatorial plane, and the transition to electrostatic fields when the force becomes stationary. The parallel electric fields on auroral field lines may be regarded as shear Alfvén waves driven by a magnetospheric generator at zero frequency. In our collisionless model, precipitating auroral electrons are accelerated to an energy of 350 eV when the upward current is 3.1 μAm−2. We also find that the electrostatic potential drop is proportional to the square of the current density.
Velocity fluctuations in the interplanetary scintillation pattern
Authors: Chashei I. V. , Shishov V. I. , Kojima M. , Misawa H.
Published: February 2004

The skewness of the temporal cross‐correlation function of radio scintillation observed simultaneously at spaced antennas is considered as characteristic of random motions in the turbulent solar wind. The results of the skewness measurements using radio telescopes of Solar‐Terrestrial Environment Laboratory at 327 MHZ and European Incoherent Scatter facilities at 932 MHZ in the range of heliocentric distances 0.1–0.3 AU were presented by Misawa and Kojima [1996]. The measured skewness was sufficiently higher for the fast solar wind than for the slow wind inside 0.2 AU. We considered the skewness theoretically, taking into account the possible physical reasons of velocity fluctuations in drifting scintillation patterns. It is shown that the skewness is proportional to typical relative variance of velocities parallel to regular drift as in the case of frozen‐in irregularities and in the case of wave‐like irregularities. The skewness parameter is not sensitive to the random velocities transversal to the average pattern velocity. The theoretical estimates are compared to the observational data. It is shown that the observed skewness is caused mainly by the bulk velocity spread in the region outside heliocentric distance 0.2 AU. The enhanced skewness between 0.1 and 0.2 AU can be explained by the additional effects of fast magnetosonic waves. The attributes of the assumed fast magnetosonic waves are pronounced more strongly in the high‐latitude observations of the fast solar wind streams than in low‐latitude observations of the slow solar wind.
Structure of magnetosonic eigenoscillations of an axisymmetric magnetosphere
Authors: Leonovich A. S. , Mazur V. A.
Published: February 2004

The spatial structure of fast magnetosonic eigenoscillations of an axisymmetric magnetosphere is investigated theoretically. It is shown that transparent regions of the eigenmodes where the bulk of the fast magnetosonic oscillation energy is concentrated lie near the equatorial plane. The transparent regions have a narrow channel from the magnetosphere to the solar wind near the equatorial plane. This enables the magnetosonic waves to penetrate from the solar wind deep into the magnetosphere, as well as some of the energy of intramagnetospheric magnetosonic oscillations to escape to the solar wind. Some of the eigenmodes have an isolated portion of the transparent region above the plasmapause that forms a well‐known magnetospheric cavity for fast magnetosonic waves.
Anisotropy of magnetosonic turbulence in the solar wind between 0.1 and 0.4 AU
Authors: Chashei Igor V. , Kojima Masayoshi , Tokumaru Munetoshi
Published: February 2004

The anisotropy of the density fluctuations in the solar wind is studied with the assumption that density fluctuations are connected to magnetosonic turbulence. It is shown that the anisotropy of density fluctuations does not coincide with that of the turbulence energy in the case of the fast magnetosonic waves while they are equal for the slow magnetosonic waves. The radial evolution of the two‐dimensional (2‐D) anisotropy ratio in the interplanetary scintillation (IPS) pattern plane due to the change of the ambient magnetic field direction is investigated. In the case of the Parker's spiral magnetic field the radial dependence of the 2‐D anisotropy ratio is shown to be considerably stronger for the fast magnetosonic waves than for the slow magnetosonic waves. The comparison between theoretical estimations and the results of the Kashima high‐frequency IPS measurements shows that the geometric effects are not sufficient to account for the observed radial dependence of the axial ratio, and that the angular distribution of turbulent energy evolves in the region of heliocentric distances between 0.1 and 0.4 AU toward the state enriched by the magnetosonic waves propagating at smaller angles relative to the ambient magnetic field. The nonlinear wave‐wave interactions seem to be responsible for this evolution of the density turbulence spatial power spectrum.
Characteristics of the postnoon auroras inferred from EISCAT radar measurements
Authors: Kozlovsky A. , Kangas J.
Published: February 2004

Svalbard European Incoherent Scatter (EISCAT) radar and Polar satellite ultraviolet imager data have been used to investigate structure and dynamics of the postnoon auroral oval (in the so‐called “14 MLT” region). Southward turnings of the interplanetary magnetic field were associated with bright afternoon and evening arcs which drifted equatorward. Increase in the equatorward arc velocity occurred about 27 ± 7 min after the IMF Bz turned southward at the subsolar point. The observed equatorward motion may occur independently of the convection: In one of the cases, the velocity of the drifting arc was observed to be about 100 – 200 m/s southward whereas the ionospheric plasma drifted northward at a velocity of 160 m/s. The arcs were located in the region of convection shear that was shown by the ground magnetometers and the EISCAT data on ion velocity and temperature. In the cases when electric field data were available the meridional component of the electric field varied from about −60 mV/m (southward) equatorward of the arc to +30 mV/m (northward) poleward of the arc. Ionospheric plasma parameters were measured during the passes of the arcs through the radar beam. Downward field‐aligned ion flow at the 300 – 550 km altitude was observed poleward of the arc, whereas upward flows were observed equatorward of the arc. This variation is explained by the difference in the vertical gradient of plasma density. Convection flows associated with arc, and differences in electron precipitation and sunlight conditions create the vertical gradients of plasma density that determine the ambipolar diffusion and observed ion motion.
Model for the mass fractionation in the January 6, 1997, coronal mass ejection
Authors: Wurz Peter , Bochsler Peter , Lee Martin A.
Published: February 2004

For the coronal mass ejection (CME) of January 6, 1997, strong element fractionation of the heavy ions was observed at 1 AU with the Mass Time‐of‐Flight (MTOF) sensor of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO). During the passage of the CME plasma and the passage of the erupted filament, which followed the CME, a mass‐dependent element fractionation was found with an enhancement of heavy elements, increasing monotonically with atomic mass. Si/O and Fe/O ratios around 0.5 were observed, which corresponds to an increase of about a factor of 4 compared to regular slow solar wind. We present a theoretical model with which we can reproduce the observed element fractionation. The model assumes hot coronal loops with non‐Maxwellian electron distributions as the precursor structure of the CME on the solar surface. Diffusion perpendicular to the magnetic field results in the preferential loss of lighter ions from the loop, leading to mass fractionation. To quantitatively reproduce the fractionation process, the loops must have existed for ∼ 28 hours before they became part of the CME plasma, a time that is commensurate with optical observations of loops in the active region from which the CME was launched.
November 17–18, 1975, event: A clue to an internal structure of magnetic clouds?
Authors: Vandas M. , Geranios A.
Published: February 2004

During November 17–18, 1975, a magnetic cloud was observed by the IMP 8 satellite. The cloud was analyzed in several papers. It draws attention because it is the most clear example where the magnetic field components behave differently from the current single flux rope model. Various models and fits have been presented to explain the magnetic field measurements for this particular event: single‐polarity cylindrical flux rope, spheromak, toroidal flux rope, and two subsequent flux ropes (flux rope twins). We critically examine these models and fits and stress that not only magnetic field data but also plasma data must be taken into account. There is a remarkably sharp drop in the density inside the magnetic cloud. The most consistent explanation of the behavior of magnetic field and plasma data for this event is that the magnetic cloud consists of a dual‐polarity flux rope with a low density and strong magnetic field core surrounded by an annular region of the same chirality but opposite polarity. An implication of this possibility to explain other magnetic cloud observations is discussed.
A comparison of nighttime TID characteristics between equatorial‐ionospheric‐anomaly crest and midlatitude regions, related to spread F occurrence
Authors: Bowman G. G.
Published: February 2004

The occurrence characteristics of TIDs with enhanced wave amplitudes, as detected by spread traces on ionograms, has been investigated for an equatorial‐ionospheric‐anomaly crest station (Chung‐Li) for comparison with results from midlatitudes. Two distinct types of distributions are found. One set (type 1) is directly related to sunspot number, while the other (type 2) is indirectly related. Their semiannual variations are also inversely distributed. The type 2 variations are the same as those found for midlatitudes. Type 1 events occur mainly before midnight, being influenced by the significant postsunset height rises, while type 2 events predominate after midnight. The influence of large‐scale traveling ionospheric disturbances (LS‐TIDs) at both latitude regions has been considered, particularly relating to the spread F associated with their passage. The results presented for Chung‐Li have revealed characteristics similar to those found in midlatitudes. Thus an alternative explanation is available to explain, at least in terms of ionogram‐recorded spread F events, the direct relationship which exists between equatorial spread F and geomagnetic activity in the presunrise hours.
Storm‐time Pc1 activity at high and middle latitudes
Authors: Kerttula R. , Mursula K. , Pikkarainen T. , Kangas J.
Published: February 2004

We study structured and unstructured Pc1 pulsations observed at a high‐latitude station (Sodankylä; L = 5.1) and a midlatitude station (Nurmijärvi; L = 3.3) during 18 storms occurring in low solar activity years (1976–1978 and 1984–1988). Pc1 activity was studied from the day of storm sudden commencement (denoted by day 0) onward during six consecutive days. While unstructured pulsations are only weakly affected, structured pulsations are greatly dependent on storm evolution. During the storm main phase they nearly vanish on the ground, despite strong wave activity in space. Structured Pc1 activity increases from day 0 to day 4 by a factor of about 4–5, reaching maximum occurrence on day 4 at both stations. Also, the average daily frequency of structured Pc1s increases from day 0 to a maximum on day 3 at Nurmijärvi or day 4 at Sodankylä. The diurnal distribution of structured Pc1s suffers a dramatic change during the storm. On days 1 and 2, structured pulsations are strongly concentrated in the evening sector, but during the later recovery (days 3–5) the activity shifts to the morning sector. The latitudinal similarity of structured Pc1 occurrence and the daily evolution of wave frequency argue against the model according to which the outward expansion of plasmapause causes the maximum wave occurrence on the ground on day 4. We also note that the strong maximum of structured Pels during the late storm recovery phase is not supported by the model calculations of the magnetospheric wave source or by direct observations of waves in space. Instead, we argue that the ionospheric resonator and propagation conditions which strongly affect wave observations on the ground are deteriorated during the storm main and early recovery phases, impeding wave propagation to the ground. The subsequent recovery of the ionospheric conditions leads to the maximum occurrence of structured Pc1s on the ground during the late storm recovery phase.
Wave power studies of cusp crossings with the Polar satellite
Authors: Angelopoulos V. , Mozer F. S. , Bonnell J. , Temerin M. , Somoza M. , Peterson W. K. , Collin H. L. , Giles B.
Published: February 2004

Burst data from the electric field instrument (EFI) on the Polar satellite are used to classify cusp wave properties and the association of the observed waves with particle populations. The most intense wave type (most commonly triggering burst mode data collection) are the broadbanded waves which are then studied in detail using the three‐dimensional resolution of the EFI. Finite wavelength effects are removed using two techniques: time‐shifting of the sphere potentials to a spacecraft body reference time and spin angle selection. The corrected electric field signal exhibits a uniform spectrum in two dimensions. The wave properties are consistent with two‐dimensional turbulence, with phase velocities smaller than the spacecraft velocity and frequency as low as (or lower than) the oxygen gyrofrequency. Owing to their low frequency, we conclude that these waves cannot resonate with and thus heat the ions. The broad k‐spectrum and very low frequency in the plasma frame points toward drift or shear instabilities for wave excitation.
At substorm onset, 40% of AL comes from underground
Authors: Tanskanen E. I. , Viljanen A. , Pulkkinen T. I. , Pirjola R. , Häkkinen L. , Pulkkinen A. , Amm O.
Published: February 2004

Magnetic variations observed at the Earth's surface are caused by external and internal sources. External variations arise from currents in the ionosphere and magnetosphere, and internal variations arise from currents induced in the solid Earth. In this paper we examine how large the internal contribution is to magnetic variations measured at the Earth's surface. We use IMAGE magnetometer measurements to analyze 77 substorms during 1997. For each event we evaluate the internal and external parts of a locally derived auroral electrojet index (IL index). The magnetic field separation is performed using the Siebert‐Kertz equations. A superposed epoch analysis of all events clearly shows that the internal contribution peaks strongly at substorm onset, when the internal contribution is ∼ 40% of the total field. After the substorm peak intensity, the internal contribution decreases almost linearly to the quiet time value of 10–20%. The induction effects are largest during the times of rapid changes and at stations located over the Arctic Ocean.
Rapid flux transport in the central plasma sheet
Authors: Schödel R. , Baumjohann W. , Nakamura R. , Sergeev V. A. , Mukai T.
Published: February 2004

On the basis of several years of Geotail data we performed a comprehensive statistical analysis of rapid convective transport in the neartail and midtail central plasma sheet. We chose a new approach by using flux transport and not ion bulk velocity as the threshold parameter for the identification of rapid flows. This criterion for rapid convection is independent of the radial distance from the Earth. We found that the occurrence rate of earthward rapid flux transport events was constant at radial distances >15 RE and that it started to drop only earthward of 15 RE. Tailward rapid flux transport events with Bz < 0 were extremely rare inside of 20 RE. Their occurrence rate increased strongly beyond that distance. Tailward directed events with Bz > 0 could be seen at all distances. Their occurrence rate has a minimum at radial distances between 20 and 30 RE and increases earthward and tailward of that region. They are likely to have a different nature and different origins in the near‐Earth region and in the midtail beyond about 25–30 RE. In close analogy to bursty bulk flows we defined rapid convection events by using the flux transport criterion instead of a velocity criterion. We found that rapid convection events transport about the same amount of mass, energy, and magnetic flux and have about the same duration at all radial distances between 10 and 50 RE. We found that rapid convection was responsible for 30–50% of the observed total transport of mass, energy, and magnetic flux past Geotail at all observed distances in the central plasma sheet.
On the quasistatic development of thin current sheets in magnetotail‐like magnetic fields
Authors: Becker U. , Neukirch T. , Schindler K.
Published: February 2004

The mechanism for the formation of thin current sheets in magnetotail‐like magnetic fields is investigated by numerical experiments using topology conserving equilibrium sequences. Motivated by the magnetotail magnetic field in the midnight meridian plane, a two‐dimensional model field is computed including a line dipole‐like (inner) part and a tail‐like part. The plasma is modeled as a polytropic gas and the total amount of plasma in each flux tube is fixed during an equilibrium sequence. Topology conservation is ensured by the use of inverse coordinates. The resulting set of nonlinear partial integrodiiferential equations is solved numerically using a continuation method. Starting from a current‐free field, stretching and compressing deformations are applied at the boundaries to mimic the analogue of a quasistatic substorm growth phase in our model. To investigate whether the process of thin current sheet formation is robust and to understand better the basic properties of that process, the boundary deformations are chosen such that there is no preferred location of compression of the field which would predetermine the site of thin current sheet formation. Current sheets are only found to form if a transition region between dipolar and tail‐like field exists in the equilibrium, and the current density exhibits two different cross‐tail length scales showing the importance of the transition region between the dipolar and the tail‐like field. However, the thin current component is much smaller than in other models of thin current sheet formation in the magnetotail. This result can be attributed to the chosen type of boundary deformations. The results are consistent with the gradient‐of‐flux‐volume mechanism which has been proposed as an explanation for the formation of thin current sheets in the Earth's magnetotail.
Pitch angle diffusion coefficients and precipitating electron fluxes inferred from EISCAT radar measurements at auroral latitudes
Authors: Kirkwood S. , Osepian A.
Published: February 2004

Theoretical predictions of the generation of precipitating fluxes of energetic electrons by pitch angle scattering by whistler waves are made using the quasi‐linear theory of wave‐electron interaction from Kennel and Petschek [1966] and trapped electron spectra obtained from geosynchronous satellites during nine auroral absorption events. Cyclotron instability spatial growth rates are found to exceed the critical value needed for spontaneous generation of whistler mode waves in the morning and noon local time sectors in all events. Where available, conjugate observations (by the European Incoherent Scatter (EISCAT) UHF radar) of electron density disturbances produced in the ionosphere are used to determine the fluxes of precipitating electrons with E ≥ 30 keV and to estimate the required magnitudes of the pitch angle scattering. These are found to lie in the transition region between weak and strong diffusion regimes. Further, it becomes clear that changes in precipitation during the course of individual absorption events are primarily due to changes in the strength of the diffusion process occurring.
Dayside auroral bifurcation sequence during By‐dominated interplanetary magnetic field: Relationship with merging and lobe convection cells
Authors: Sandholt P. E. , Farrugia C. J. , Cowley S. W. H. , Lester M.
Published: February 2004

We report observations in the cusp region of synchronous activations of latitudinally separated auroral forms (“bifurcations”) occurring in a sequence and interpret the observations in terms of an intermittent magnetic reconnection process at the dayside magnetopause proceeding in a wave‐like manner from lower to higher latitudes. The observations refer to a By‐dominated interplanetary magnetic field (IMF) orientation. Optical auroral observations are combined with radar observations of ionospheric ion drift to illustrate the association between the aurora and plasma convection in two cases representing positive and negative IMF By conditions. In the meridian photometer scans, each individual event in the sequence appeared as an initial brightening in the south (type 1), followed by a second brightening/expansion farther to the north (type 2). The events occurring during an eastward pointing (By>0) IMF are observed typically to expand westward across the 1200 magnetic local time meridian, from the postnoon to the prenoon side. The higher‐latitude, type 2 activity is associated with strong westward convection, which we identify to be part of a lobe cell, while the equatorward boundary intensifications (type 1) occur in the region of a merging cell, distorted by the prevailing IMF By condition. The auroral sequence consists of several paired activation events, typically recurring at ∼ 5‐to 10‐min intervals, and each individual event lasting ∼10 min. The ion drift observations are found to be consistent with recent MHD modeling results on IMF magnetosphere interconnection geometry and the associated composite pattern of merging and lobe convection cells in the cusp region. The ground observations reveal the intermittent nature of two components of cusp region particle precipitation and the association between the corresponding type 1 and 2 auroras and merging and lobe convection cells, respectively.
Geomagnetic field response along the Polar orbit to rapid changes in the solar wind dynamic pressure
Authors: Fowler G. J. , Russell C. T.
Published: February 2004

Magnetometer observations on board the Polar spacecraft have been used to measure the magnetospheric response to rapid changes in the dynamic pressure of the solar wind. Over much of the magnetosphere the magnetic field increases when the solar wind dynamic pressure increases, roughly in proportion to the square root of the change of the solar wind dynamic pressure with a proportionality similar to that seen in ground‐based data. Nevertheless, dynamic pressure increases can lead to reductions in the magnitude of the magnetic field. These reductions occur on the dayside polar field lines, where the magnetospheric field points southward while the perturbation field due to the magnetopause current is northward. Throughout most of the magnetosphere these changes occur slowly over ∼ 5–10 min as the interplanetary shock envelopes the dayside magnetosphere and near tail. Compressions of the magnetosphere generally lead to increases in the intensity of the ULF waves along the field lines, but in one case the wave intensity decreased upon compression of the field. This decrease occurred in the predawn sector while the increases occurred in the noon to postdusk sector.
Multispacecraft modeling of the flux rope structure of interplanetary coronal mass ejections: Cylindrically symmetric versus nonsymmetric topologies
Authors: Mulligan T. , Russell C. T.
Published: February 2004

Data taken shortly after the launch of the Pioneer Venus spacecraft (PVO) has allowed us to compare an interplanetary coronal mass ejection (ICME) event observed nearly simultaneously on August 27, 1978, by the PVO and ISEE 3 spacecraft. During this period the spacecraft are in close proximity, separated by 0.02 AU radially and 12° (0.21 AU) in azimuth. Both PVO and ISEE 3 observe similar magnetic flux rope signatures within the ICME. Cylindrically symmetric flux rope fits to the observations indicate the handedness of the magnetic flux rope seen at PVO is the same as at ISEE 3, but the peak fields are not. Moreover, symmetric modeling of the magnetic rope observations returns flux rope diameters that are less than the azimuthal separation distance between the spacecraft. Thus, if the cylindrically symmetric model were correct, the two observations could not occur within the same rope, meaning that at least two separate flux ropes would be associated with the same ICME event. The axial orientations of these two ropes differ slightly, with the PVO rope having a clock angle of 238° and a cone angle of 82° and the ISEE 3 rope having a clock angle of 196° and a cone angle of 56° in solar equatorial coordinates. Applying instead a noncylindrically symmetric flux rope model and simultaneously inverting both time series results in a single stretched rope having a cross‐flow diameter of 0.86 AU, ∼4 times its radial thickness, a clock angle of 220°, and a cone angle of 74°. The stretched rope contains ∼50 TWb of magnetic flux, nearly twice the amount given for the cylindrically symmetric fit at PVO and a factor of 5 greater than the symmetric fit at ISEE 3. These values are consistent with the amount of magnetic flux observed in active regions on the Sun. While the individual cylindrically symmetric models adequately fit the single spacecraft data, the stretched rope model provides the simplest explanation of the multipoint observations and is consistent with a CME expanding in azimuthal diameter as it attempts to subtend a constant angle at the Sun of 45° as it convects outward. Although the inversion technique used does not require the field to be force‐free, there are at most only slight deviations from a force‐free configuration within the symmetric ropes and significant plasma forces only at the east and west ends of the nonsymmetric flux rope.
Magnetic cloud evolution in a two‐speed solar wind
Authors: Schmidt Joachim M. , Cargill Peter J.
Published: February 2004

The Ulysses spacecraft has observed interplanetary coronal mass ejections in pure high‐speed solar wind, in the shear layer between high‐speed and low‐speed, and, in conjunction with other satellites, the same coronal mass ejection (CME) has been seen in both high‐ and low‐speed wind. If CMEs originate in the equatorial streamer belt, they must be able to somehow penetrate into high‐speed solar wind flows. This paper presents numerical simulations of the interaction of the subset of CMEs known as magnetic clouds (i.e., a flux rope geometry) with a velocity shear layer. A spherical, two‐speed solar wind model represents the stream interface. It is shown that provided the CME has a large enough poleward velocity at the Sun, it can readily penetrate the stream interface and enter the high‐speed wind. It is also shown that under some initial conditions, internal magnetic reconnection can take place within the flux rope. As the flux rope field lines are sheared, oppositely directed fields are generated, which press together and reconnect.