摘要:

Two‐dimensional distributions of ground magnetic and ionospheric electric fields in the evening sector auroral oval have been simultaneously observed by the Scandinavian Magnetometer Array and the Scandinavian Twin Auroral Radar Experiment (Stare) radars, respectively, on February 15, 1977. They were associated with varying, substorm‐intensified, eastward electrojet current flows. Using both data sets, we have derived three‐dimensional models for the current systems of the western, middle, and eastern segment of the eastward electrojet. We conclude that the substorm‐intensified eastward electrojet was a nearly pure Hall current driven by northward electric fields. The observed eastward increase of the current in the western segment of the electrojet was due to a gradual enhancement of the Hall conductivity. Here, the electrojet was fed by a broad sheet of net downward field‐aligned current. During one period, the eastern‐terminating part of the eastward electrojet diverged up the field lines in a rather local area because of a strong longitudinal decrease in the northward‐directed electric field. On another occasion, it diverged northward within the ionosphere and joined the westward‐flowing current because of a rotation of the northward electric field with increasing latitude through west‐ to southward. These two observed mechanisms of current divergence in the region where eastward and westward electrojets coexist may shed some new light on the controversy over the existence of upward field‐aligned current flow in the H

ISSN:0148-0227    

DOI:10.1029/JA085iA05p01963

年代:1980

数据来源: WILEY

摘要:

Starting with a depleted protonosphere, the behavior of O+and H+atL=.2 is examined theoretically to determine the influence of a quiet time diurnal electromagnetic drift with a 24‐hour period. The conditions are appropriate to sunspot maximum. Large changes in the H+tube content atL=3.2 occur as the protonosphere is replenished. In the absence of the electromagnetic drift, such changes would imply field‐aligned proton fluxes up to the order of 1010cm−2s−1; the actual field‐aligned proton flux is of the order of 108cm−2s−1. The combined effect of diurnal drifts and frequency of magnetic storms can produce multiple peaks in the diurnal behavior of the H+tube content atL=3.2. As the drift lowers the altitude of the O+layer during daytime, the values ofNmF2decrease owing to the change in the linear loss coefficient. At night, cross‐Lgradients inNmF2are significant for the conditions considered, and the outward drift increasesNmF2values atL=3.2. When the protonosphere is sufficiently replenished, the inward drift causes downward proton

ISSN:0148-0227    

DOI:10.1029/JA085iA05p01979

年代:1980

数据来源: WILEY

摘要:

Vector magnetic field observations made with the three‐axes magnetometer on the Triad satellite have been used to study the orientation of magnetic disturbances in the dayside polar region. These measurements were all made over the southern polar region and recorded at McMurdo, Antarctica. These disturbances are transverse to the main geomagnetic field and may be interpreted as being caused by field‐aligned Birkeland current sheets consistent with Maxwell's equations. The current sheets in the regions usually associated with the morning and afternoon auroral regions are most often aligned in the geomagnetic east‐west direction. The amplitudes of these ‘south auroral’ currents are larger in the morning than in the afternoon when the interplanetary magnetic field (IMF) is directed toward the sun (By0,Bx<0). This is the systematically reversed relationship between Birkeland current intensities and IMF found in the northern auroral zone by McDiarmid et al. (1977). The Birkeland current sheets in the region associated with the southern cusp are also aligned in the geomagnetic east‐west direction during periods of negativeBz. During periods ofBy0 andBx<0. From a search of the Triad data set, some rare examples of magnetic disturbances with a large north‐south (noon‐midnight) component have been discovered in the polar cap near noon. These always occurred during periods of northward IMF. These disturbances have been interpreted as being caused by sun‐aligned Birkeland current sheets. The intensities of these currents in a few cases studied here are correlated with the positive amplitude ofBzand are comparable to the cusp and auroral currents studied earlier (0.1–0.8 A/m). The fluxes of electrons associated with sun‐aligned polar arcs measured with the Isis 2 satellite (Ismail et al., 1977) are sufficiently intense to produce upward flowing sun‐aligned Birkeland current sheets, but the origin of the downward flowing currents is not clear. These sun‐aligned current sheets may be associated with the rare occurrence of multiple‐cell convective flow patterns in the polar cap (Burke et al.

ISSN:0148-0227    

DOI:10.1029/JA085iA05p01987

年代:1980

数据来源: WILEY

摘要:

A large data base of Atmosphere Explorer C measurements of the bulk ion velocity vector in the high‐latitudeFregion has been examined. The concept of shear and rotational flow reversals is expanded upon to determine the extent to which the boundary between antisunward and sunward convection (i.e., a polar cap boundary) is an electrostatic equipotential. A most important factor in characterizing the nature of the large‐scale geometrical configuration of the nighttimeFregion convection is the location and extent of quasi‐equipotential regions of the convection reversal bou

ISSN:0148-0227    

DOI:10.1029/JA085iA05p01995

年代:1980

数据来源: WILEY

摘要:

Observations obtained from instruments on the Isis 2 spacecraft in polar orbit at 1400 km have been used to investigate the identity of the charge carriers in the large‐scale field‐aligned current systems present in the high‐latitude region. The vertical current is inferred from observed perturbations to the east‐west component of the local geomagnetic field obtained by the magnetometer. Simultaneous observations of the charged particle flux were obtained by the soft particle spectrometer (in the 0.005‐ to 15‐kev range) and by the retarding potential analyzer in the suprathermal energy range (≧1 eV). The flux of electrons in the suprathermal energy range has been studied in an attempt to identify and obtain consistency between the velocity distribution function of the charge carriers and the inferred currents. The observed flow of upwelling suprathermal electrons increased substantially when currents were inferred of either direction. The mean net current density due to suprathermal electrons was 0.4±0.2 µA/m² when the expected current was upward, compared to a mean expected value of 0.2±0.1 µA/m² inferred from the magnetometer. This mean net current density was derived by combining the observed much larger downward flux of electrons, primarily energetic, with an observed upward flux of suprathermal electrons. The partial cancellation of the charge transfer due to the precipitating particles by a return current of suprathermals results in reasonable agreement between the observed net particle flow and the magnetic field perturbation. When the expected field‐aligned currents were downward, the mean current density due to suprathermal electrons was −0.8±0.2 µA/m², compared to a magnetometer inferred value of −0.4±0.1 µA/m². The suprathermal electrons therefore contribute substantially to field‐aligned currents, producing most, i

ISSN:0148-0227    

DOI:10.1029/JA085iA05p02003

年代:1980

数据来源: WILEY

摘要:

Motivated by recent observations of highly variable hot plasma composition in the magnetosphere, control of the ionospheric escape flux composition by low‐altitude particle dynamics and ion chemistry has been investigated for ane−, H+, O+ionosphere. Theoretical polar wind results of Banks and Holzer and observations of highly nonthermal ion escape have been used as guides. It is found that the fraction of the steady state escape flux which is O+can be controlled very sensitively by the occurrence of parallel or transverse ion acceleration at altitudes below the altitude where the neutral oxygen density falls rapidly below the neutral hydrogen density and the ionospheric source of O+tends to be rapidly converted by charge exchange to H+. The acceleration is required both to overcome the gravitational confinement of O+and to violate charge exchange equilibrium so that the neutral hydrogen atmosphere appears ‘optically’ thin to escaping O+. Constraints are placed on the acceleration processes, and it is shown that O+escape is facilitated by observed ionospheric responses to magnetic a

ISSN:0148-0227    

DOI:10.1029/JA085iA05p02011

年代:1980

数据来源: WILEY

摘要:

A one‐dimensional steady state fluid mechanical model is developed of the low‐latitude plasma boundary layer inside the dawn and dusk magnetopause. Momentum transfer in the layer is produced by viscosity and/or mass diffusion. Coupling to the ionosphere is achieved via field‐aligned currents, the magnitude of which is limited by parallel potential drops. These currents flow into and out of the ionosphere in the manner described by Iijima and Potemra. The higher‐latitude (region 1) currents are associated with the boundary layer proper, while the lower‐latitude (region 2) ones are associated with a region of sunward return flow adjacent to the boundary layer. The parallel potential drops have a magnitude of typically 2–3 kV and a north‐south extent of 100–200 km. The calculated potential profile corresponds reasonably well to observed inverted V preci

ISSN:0148-0227    

DOI:10.1029/JA085iA05p02017

年代:1980

数据来源: WILEY

摘要:

Whistler precursors are discrete emissions that precede two‐hop whistlers, starting shortly after the one‐hop delay. More evidence is presented that the precursor and associated whistler propagate through the same duct, and observations of a new type of precursor are presented. This new precursor has a monochromatic precursor start (MPS) which may or may not trigger a riser. Although MPS's may be emissions entrained by power line radiation (PLR), phase analysis of the starting frequencies shows that they are not simply related to harmonics of the power line frequencies in the two conjugate regions (50 Hz in New Zealand, 60 Hz in Alaska). If the MPS is due to entrainment by PLR, then previous theories of precursor generation need not be discarded. Forward triggering of a precursor at a power line harmonic by a hybrid whistler may oc

ISSN:0148-0227    

DOI:10.1029/JA085iA05p02027

年代:1980

数据来源: WILEY

摘要:

On November 20, 1977, at 0230–0300 UT, Isee 1 encountered unusual charged particle distributions within the magnetosphere. The three‐dimensional distribution observations for energetic (>24 keV) ions and plasma show the development of field‐aligned asymmetries in the energetic ion distributions simultaneously with a marked change in plasma flow. We conclude that the most likely explanation for these observations is that Isee 1 encountered open magnetospheric field lines at its position within the magnetosphere (∼1030 LT and ∼1200 ±300 km from the magnetopause). Field lines were open near the geomagnetic equator, and the geometry was spatially or temporally variable. Other features of the field line topology are

ISSN:0148-0227    

DOI:10.1029/JA085iA05p02037

年代:1980

数据来源: WILEY

摘要:

We discuss two phases of the substorm‐associated magnetospheric dynamics in terms of the particles and fields at synchronous orbit. The first phase corresponds to the ‘decreases’ of energetic particle flux first identified by Erickson and Winckler (1973) and discussed by Walker et al. (1976) and Erickson et al. (1979). This phase begins one‐half hour to one hour before the substorm onset and is characterized by (1) a distortion of the magnetosphere to a more taillike configuration caused by (2) an intensification and/or motion toward the earth of the cross‐tail current and of its earthward part, the partial ring current, (3) a shift of trapped particle trajectories closer to the earth on the nightside following contours of constantBcausing the particle ‘decreases,’ accompanied by a change in the pitch angle distributions from ‘pancake’ to ‘butterfly’ as observed at geostationary orbit, (4) an initiation of a response of the auroral electrojet (AE) index. The decreases of energetic particle flux can correspond to the substorm growth phase as defined initially by McPherron (1970) or the growth or precursor phase of Erickson et al. (1979). Plasma motions and currents during decreases tend to be variable, but the description above nevertheless characterizes the large‐scale trend. It is suggested that the electric field induced by the increasing tail current near the earth acts opposite to the cross‐tail convection field and can temporarily inhibit convection near the geostationary orbit. The second phase is the conventional expansion phase that begins with the ‘onset,’ characterized in our study by (1) a sudden decrease in the tail current and a return of the inflated magnetosphere to a dipolelike configuration, (2) a sudden shift of trapped high‐energy particles toward the tail again following contours of constantB, and at the same time (3) a surge of tail plasma toward the earth as the induced electric field now increases the total convection field. Separate effects thus result in the dramatic increases of both high energy and plasma particles seen at substorm expansion phase onset, (4) anAEindex response and the appearance of bays at stations near midnight local time accompanied by very active aurora as well as the precipitation of high‐energy particles. The different appearance of the responses at ATS 1 (on the magnetic equator) and ATS 6 (off the magnetic equator) can be well explained by the above description. True diamagnetic effects of the particle population are clearly evident at the ATS 6 region and must be carefully distinguished from the effects of distant currents. The use of oppositely directed detectors on ATS 6 that permit the evaluation of the guiding center particle density gradients has b

ISSN:0148-0227    

DOI:10.1029/JA085iA05p02043

年代:1980

数据来源: WILEY