摘要:

A statistical study has been made of the high‐latitude impulsive events that were observed during the 1985–1986 South Pole Balloon Campaign. The events were selected by searching for unipolar pulses ≥ 10 nT above background in the vertical component of the magnetic field on the ground and/or pedestal or “W” shaped horizontal electric field perturbations ≥ 10 mV/m in amplitude and accompanied by perturbations in the vertical electric field at balloon altitude. A main event list comprising 112 events was compiled from the 468 hours of data available. Three aspects of the events were examined: the solar wind conditions prior to the event, local time of observation, and intrinsic properties of the events. The local time distribution was obtained from the 112 entry main event list and was found to be nearly uniform across the dayside, with no midday gap. The event rate found using our low‐amplitude selection criteria was 0.7 event/hr, comparable to expectations based on in situ studies of the magnetopause. A total of 42 events were found for which data were available from Interplanetary Monitoring Platform (IMP) 8. Of these events, 12 occurred when theZGSMcomponent (BZ) of the interplanetary magnetic field (IMF) was northward and 30 occurred whenBZwas southward or fluctuating. Only three of theBZnorthward cases and only five of theBZsouthward cases were preceded by pressure pulses greater than 0.4 nPa in amplitude. Ten of the events were studied in detail by means of a model‐fitting method discussed elsewhere. This method infers values of several parameters, including the total current flowing in a coaxial or monopole system and a two‐dimensional dipole system. The intrinsic properties of the events showed that only ∼ 10% of the total current contributed to momentum transfer to the high‐latitude ionosphere, that the direction of the motion depended more on local time of observation than IMFBy, and that events were usually several hundred kilometers in size. The observedBzcontrol found in the 42 event list and the prevalence of coaxial current dominated events are inconsistent with the predictions of the

ISSN:0148-0227    

DOI:10.1029/94JA01655

年代:1995

数据来源: WILEY

摘要:

Observations from August 2 and 3, 1991, of poleward progressing, dayside convection disturbances accompanied by geomagnetic perturbations and ionospheric radio wave absorption have been analyzed and compared to variations in the solar wind parameters as observed from the IMP 8 satellite. The convection disturbances appear to start at dayside cusp latitudes from where they progress antisunward to high latitudes. The reported observations have enabled calculations of the progression directions and velocities and precise estimates of the delays between solar wind variations as measured by the IMP 8 satellite and ionospheric convection changes as observed from an array of polar magnetic observatories. The progressing ionospheric disturbance events occur during intervals of southward interplanetary magnetic fields (negative interplanetary magnetic field (IMF)BZcomponent); they are found to be closely related to variations of the east‐west componentBYof the IMF. The close coupling between the solar wind and the polar ionosphere(s) is explained in an open magnetospheric model in which the geomagnetic field extending from a localized region of the dayside polar cap merges with the southward interplanetary field. Variations in the IMFBYcomponent are reproduced in corresponding modulations of the east‐west component of the plasma flow at the ionospheric foot points of the connecting “open” field lines. The perturbations of the plasma flow persist while the open field lines are convected with the ionospheric plasma across part of the dayside polar cap. The observed geomagnetic perturbations result from the combined effects of field‐aligned currents and horizontal ionospheric currents, notably the convection‐related Hall currents. The associated radio wave absorption events are explained as the result ofEregion electron heating by the horizontal electric fields associated with the convection e

ISSN:0148-0227    

DOI:10.1029/94JA01825

年代:1995

数据来源: WILEY

摘要:

Viking midaltitude observations of ions and electrons in the postnoon auroral region show that field‐aligned acceleration of electrons and ions with energies up to a few kiloelectron volts takes place. The characteristics of the upgoing ion beams and the local transverse electric field observed by Viking indicate that parallel ion acceleration is primarily due to a quasi‐electrostatic field‐aligned acceleration process below Viking altitudes, i.e., below 10,000–13,500 km. A good correlation is found between the maximum upgoing ion beam energy and the depth of the local potential well determined by the Viking electric field experiment within dayside “ion inverted Vs.” The total transverse potential throughout the entire region near the ion inverted Vs is generally much higher than the field‐aligned potential and may reach well above 10 kV. However, the detailed mapping of the transverse potential out to the boundary layer, a fundamental issue which remains controversial, was not attempted here. An important finding in this study is the strong correlation between the maximum upgoing ion beam energy of dayside ion inverted Vs and the solar wind velocity. This suggests a direct coupling of the solar wind plasma dynamo/voltage generator to the region of field‐aligned particle acceleration. The fact that the center of dayside ion inverted Vs coincide with convection reversals/flow stagnation and upward Birkeland currents on what appears to be closed field lines (Woch et al., 1993), suggests that field‐aligned potential structures connect to the inner part of an MHD dynamo in the low‐latitude boundary layer. Thus the Viking observations substantiate the idea of a solar wind induced boundary layer polarization where negatively charged perturbations in the postnoon sector persistently develops along the magnetic field lines, establishing accelerating potential drops along the geomagnetic field lines in

ISSN:0148-0227    

DOI:10.1029/94JA02523

年代:1995

数据来源: WILEY

摘要:

The “radial” transport of energy by internal ULF waves, stimulated by dayside magnetospheric boundary oscillations, is analyzed in the framework of one‐fluid magnetohydrodynamics. (The term radial is used here to denote the direction orthogonal to geomagnetic flux surfaces.) The model for the inhomogeneous magnetospheric plasma and background magnetic field is axisymmetric and includes radial and parallel variations in the magnetic field, magnetic curvature, plasma density, and low but finite plasma pressure. The radial mode structure of the coupled fast and intermediate MHD waves is determined by numerical solution of the inhomogeneous wave equation; the parallel mode structure is characterized by a WKB approximation. Ionospheric dissipation is modeled by allowing the parallel wave number to be complex. For boundary oscillations with frequencies in the range from 10 to 48 mHz, and using a dipole model for the background magnetic field, the combined effects of magnetic curvature and finite plasma pressure are shown to (1) enhance the amplitude of field line resonances by as much as a factor of 2 relative to values obtained in a cold plasma or box‐model approximation for the dayside magnetosphere; (2) increase the energy flux delivered to a given resonance by a factor of 2–4; and (3) broaden the spectral width of the resonance by a factor of 2–3. The effects are attributed to the existence of an “Alfvén buoyancy oscillation,” which approaches the usual shear mode Alfvén wave at resonance, but unlike the shear Alfvén mode, it is dispersive at short perpendicular wavelengths. The form of dispersion is analogous to that of an internal atmospheric gravity wave, with the magnetic tension of the curved background field providing the restoring force and allowing radial propagation of the mode. For nominal dayside parameters, the propagation band of the Alfvén buoyancy wave occurs between the location of its (field line) resonance and that of the fast mode cutoff that exists at larg

ISSN:0148-0227    

DOI:10.1029/94JA02293

年代:1995

数据来源: WILEY

摘要:

Longitudinal flow bursts observed by the European Incoherent Scatter (EISCAT) radar, in association with dayside auroral transients observed from Svalbard, have been interpreted as resulting from pulses of enhanced reconnection at the dayside magnetopause. However, an alternative model has recently been proposed for a steady rate of magnetopause reconnection, in which the bursts of longitudinal flow are due to increases in the field line curvature force, associated with theBycomponent of the magnetosheath field. We here evaluate these two models, using observations on January 20, 1990, by EISCAT and a 630‐nm all‐sky camera at Ny Ålesund. For both models, we predict the behavior of both the dayside flows and the 630‐nm emissions on newly opened field lines. It is shown that the signatures of steady reconnection and magnetosheathBychanges could possibly resemble the observed 630‐nm auroral events, but only for certain locations of the observing site, relative to the ionospheric projection of the reconnection X line: however, in such cases, the flow bursts would be seen between the 630‐nm transients and not within them. On the other hand, the model of reconnection rate pulses predicts that the flows will be enhanced within each 630‐nm transient auroral event. The observations on January 20, 1990, are shown to be consistent with the model of enhanced reconnection rate pulses over a background level and inconsistent with the effects of periodic enhancements of the magnitude of the magnetosheathBycomponent. We estimate that the reconnection rate within the pulses would have to be at least an order of magnitude larger than the background level betwee

ISSN:0148-0227    

DOI:10.1029/94JA02264

年代:1995

数据来源: WILEY

摘要:

We discuss the characteristics of magnetosheath plasma precipitation in the “cusp” ionosphere for when the reconnection at the dayside magnetopause takes place only in a series of pulses. It is shown that even in this special case, the low‐altitude cusp precipitation is continuous, unless the intervals between the pulses are longer than observed intervals between magnetopause flux transfer event (FTE) signatures. We use FTE observation statistics to predict, for this case of entirely pulsed reconnection, the occurrence frequency, the distribution of latitudinal widths, and the number of ion dispersion steps of the cusp precipitation for a variety of locations of the reconnection site and a range of values of the local de‐Hoffman Teller velocity. It is found that the cusp occurrence frequency is comparable with observed values for virtually all possible locations of the reconnection site. The distribution of cusp width is also comparable with observations and is shown to be largely dependent on the distribution of the mean reconnection rate, but pulsing the reconnection does very slightly increase the width of that distribution compared with the steady state case. We conclude that neither cusp occurrence probability nor width can be used to evaluate the relative occurrence of reconnection behaviors that are entirely pulsed, pulsed but continuous and quasi‐steady. We show that the best test of the relative frequency of these three types of reconnection is to survey the distribution of steps in the cusp ion dispersion charac

ISSN:0148-0227    

DOI:10.1029/94JA02197

年代:1995

数据来源: WILEY

摘要:

Cusp currents that arise from ionospheric vorticity generated by the combined merging outflow and gasdynamic flow fields at the magnetopause, as proposed by Saunders (1989), are quantified and compared with those calculated by Mei et al, (1994) from vorticity generated by mapping the solar wind electric field into a limited cusp region of the polar cap, as proposed in the synthesis view of Banks et al. (1984). The results are essentially identical for strong interplanetary magnetic field (IMF)By, thus demonstrating equivalence between mechanical and electrical descriptions of reconnect ion‐driven convection. For southward IMF, however, the mechanical description yields weak cusp currents with dawn‐dusk bipolarity, as deduced from early observations by Iijima and Potemra (1976), whereas the electrical description yields none. The bipolar currents arise from the diverging pattern of gasdynamic flow. The currents become unipolar asByincreases and the asymmetry of the merging outflow dominates. Additional cusp currents in both models arise at kinks in the flow contours (additional ionospheric vorticity) around the border of the cusp region, owing to limiting the area of mapping from the magnetopause. The border currents form a bipolar pair that rotates around the circumference of the cusp as the IMF rotates in clock angle. They dominate the currents arising from vorticity within the c

ISSN:0148-0227    

DOI:10.1029/94JA03370

年代:1995

数据来源: WILEY

摘要:

The extended flight of the Airborne Ionospheric Observatory during the Geospace Environment Modeling (GEM) Pilot program on January 16, 1990, allowed continuous all‐sky monitoring of the two‐dimensional ionospheric footprint of the northward interplanetary magnetic field (IMF) cusp in several wavelengths. Especially important in determining the locus of magnetosheath electron precipitation was the 630.0‐nm red line emission. The most striking morphological change in the images was the transient appearance of zonally elongated regions of enhanced 630.0‐nm emission which resembled “rays” emanating from the centroid of the precipitation. The appearance of these rays was strongly correlated with theYcomponent of the IMF: when the magnitude ofBywas large compared toBz, the rays appeared; otherwise, the distribution was relatively unstructured. Late in the flight the field of view of the imager included the field of view of flow measurements from the European incoherent scatter radar (EISCAT). The rays visible in 630.0‐nm emission exactly aligned with the position of strong flow jets observed by EISCAT. We attribute this correspondence to the requirement of quasi‐neutrality; namely, the soft electrons have their largest precipitating fluxes where the bulk of the ions precipitate. The ions, in regions of strong convective flow, are spread out farther along the flow path than in regions of weaker flow. The occurrence and direction of these flow bursts are controlled by the IMF in a manner consistent with newly opened flux tubes; i.e., when |By|>|Bz|, tension in the reconnected field lines produce east‐west flow regions downstream of the ionospheric projection of the x line. We interpret the optical rays (flow bursts), which typically last between 5 and 15 min, as evidence of periods of enhanced dayside (or lobe) reconnection when |By|>|Bz|. The length of the reconnection pulse is difficult to determine, however, since strong zonal flows would be expected to persist until the tension force in the field line has decayed, even if the duration of the enhanced reconnection was

ISSN:0148-0227    

DOI:10.1029/94JA03360

年代:1995

数据来源: WILEY

摘要:

We present a unique Viking cusp observation during a period when the interplanetary magnetic field (IMF) observed by IMP 8 made a steplike change from southward to steady northward and subsequently back to steady southward again. The solar wind density and velocity were fairly steady during the entire period. Viking detected two independent plasma signatures. The first (equatorward part) is associated with southward IMF, and the second (poleward part) is associated with northward IMF. In the transition from the first signature to the second one, the ion data show an “overlapping injection” signature, i.e., two independent plasma populations exist on the same field lines and are well separated in the energy domain at any time and at any pitch angle. The second plasma population is found on the higher‐energy side of the first population. A good relation between keV ion morphology and the field‐aligned current pattern exists even though the particle cusp is undergoing a dynamic change. We discuss two possible scenarios for the present observation which involve the dynamic reaction of the cusp morphology to the stepwise change in the IMF co

ISSN:0148-0227    

DOI:10.1029/95JA00333

年代:1995

数据来源: WILEY

摘要:

Continuous ground‐based observations of ionospheric and magnetospheric regions are critical to the Geospace Environment Modeling (GEM) program. It is therefore important to establish clear intercalibrations between different ground‐based instruments and satellites in order to clearly place the ground‐based observations in context with the corresponding in situ satellite measurements. HF‐radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis. In this paper we report on an intercalibration study made using the Polar Anglo‐American Conjugate Radar Experiment radars located at Goose Bay, Labrador, and Halley Station, Antarctica, and the Defense Meteorological Satellite Program (DMSP) satellites. The DMSP satellite data are used to provide clear identifications of the ionospheric cusp and the low‐latitude boundary layer (LLBL). The radar data for eight cusp events and eight LLBL events have been examined in order to determine a radar signature of these ionospheric regions. This intercalibration indicates that the cusp is always characterized by wide, complex Doppler power spectra, whereas the LLBL is usually found to have spectra dominated by a single component. The distribution of spectral widths in the cusp is of a generally Gaussian form with a peak at about 220 m/s. The distribution of spectral widths in the LLBL is more like an exponential distribution, with the peak of the distribution occurring at about 50 m/s. There are a few cases in the LLBL where the Doppler power spectra are strikingly similar to those observed

ISSN:0148-0227    

DOI:10.1029/94JA01481

年代:1995

数据来源: WILEY