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1.
The magnitude of the equatorial electrojet signature, S, is a measure of its magnetic field at the location of the satellite recording the signature. The general features of the large quantity of the magnetic field data of the electrojet observed by the series of POGO satellites from 1967 to 1970 have been studied here. We have compared the position of the axis of the electrojet as indicated by the position of the minimum of the electrojet signature with the position of the dip equator on the Earth's surface, and we find no significant latitudinal shift of the electrojet axis from the dip equator on the Earth. Apart from the expected decrease of the magnetic field of the electrojet with altitude above the electrojet, we have found unexpected cases in which the field increases with altitude. More surprisingly, we have discovered that the magnitude of S oscillates with altitude having maxima at about 460km and 635km and minima at about 580km and 725km, with a mean wavelength of 160 ± 29 km. It is suggested that this could be caused by additional weak current layers flowing above the main electrojet at about 110 km altitude. It is also pointed out that Onwumechili's model based on a single current system of the equatorial electrojet predicts field oscillation with altitude. The model therefore shows that a field oscillating with altitude can also result from a single complicated system of current unaided by additional current layers.  相似文献   

2.
In the companion paper (Lam and Rostoker, 1978) we have shown that Pc 5 micropulsations are intimately related to the behaviour and character of the westward auroral electrojet in the morning sector. In this paper we show that Pc 5 micropulsations can be regarded as LC-oscillations of a three-dimensional current loop involving downward field-aligned current flow near noon, which diverges in part to form the ionospheric westward electrojet and returns back along magnetic field lines into the magnetosphere in the vicinity of the ionosphere conductivity discontinuity at the dawn meridian. The current system is driven through the extraction of energy from the magnetospheric plasma drifting sunwards past the flanks of the magnetosphere in a manner discussed by Rostoker and Boström (1976). The polarization characteristics of the pulsations on the ground can be understood in terms of the effects of displacement currents of significant intensity which flow near the F-region peak in the ionosphere and induced currents which flow in the earth. These currents significantly influence the magnetic perturbation pattern at the Earth's surface. Model current system calculations show that the relative phase of the pulsations along a constant meridian can be explained by the composite effect of oscillations of the borders of the electrojet and variations in the intensity of current flow in the electrojet.  相似文献   

3.
Data from a line of magnetometers stretching along a corrected geomagnetic meridian ～ 302°E through western Canada are used to study the relationship between the convection westward electrojet and Pc 5 micropulsations in the morning sector. It was found that the dominant spectral bands in the Pc 5 range occur within the same latitudinal range occupied by the electrojet. The intensity contours and the character of the polarization parameters clearly show that the Pc 5 activity tracks the westward convection electrojet. The Pc 5 activity is found to be enhanced in conjunction with rapid reconfigurations of the electrojet. Evidence of spatial oscillations of the borders of the electrojet and variations in the intensity of the electrojet is presented. It is concluded from our study that the Pc 5 activity in the morning sector is closely related to the convection westward electrojet and its associated three-dimensional current system.  相似文献   

4.
Variations in the scalar magnetic field (ΔB) from the polar orbiting OGO 2, 4 and 6 spacecraft are examined as a function of altitude for times when the interplanetary magnetic field is toward the Sun and for times when the interplanetary magnetic field is away from the Sun. In both cases, at all altitudes, the total field variations form a region of positive ΔB between about 22 hr and 10 hr MLT and a region of negative ΔB between about 10 hr and 22 hr MLT. This morphology is basically the same as that found when all data, irrespective of inter-planetary magnetic sector, are averaged together (Langel, 1974a, b). Differences in ΔB occur, both between sectors and between seasons, which are similar in nature to variations in the surface ΔZ found by Langel (1973). The altitude variation of ΔB at sunlit local times, together with variations in the vertical component ΔZ at the Earth's surface, demonstrates that the ΔZ and ΔB which varies with sector has an ionospheric source. Langel (1974b) showed that the positive ΔB region in the dark portion of the hemisphere is due to at least two sources, the westward electrojet and an unidentified non-ionospheric source (s). Comparison of magnetic variations between season/sector at the surface and at the satellite, in the dark portion of the hemisphere, indicates that these variations are caused by variations in the latitudinally narrow electrojet currents and not by variations in the non-ionospheric source of ΔB.  相似文献   

5.
A large Forbush-type decrease with an amplitude of 16–22% was observed by the world-wide network of cosmic-ray detectors during the period 13–14 July, 1982. Combined neutron-monitor measurements with interplanetary plasma and magnetic field data, auroral data, and Earth's magnetospheric data are used for the study of this event. It is suggested that this interesting event is probably a consequence of the dynamic interactions of the solar wind with the Earth's magnetosphere as it is obvious from the large magnetic storm which was recorded in the auroral electrojet indices.  相似文献   

6.
Many previous researchers have shown that convection in the magnetosphere is reflected in the ionosphere by an eastward electrojet in the evening sector and a westward electrojet in the post-midnight sector. In this paper we shall demonstrate the existence of eastward electrojet flow in the dawn sector in the latitude regime normally occupied by the westward convection electrojet. It will be shown that the convection westward electrojet near dawn may co-exist with the eastward electrojet while lying poleward of it. It is suggested that this eastward electrojet consists of Pedersen current flow driven by an eastward electric field and it is shown that the field lines which penetrate the eastward electrojet are populated by energetic electrons normally associated with the plasma sheet as well as high energy electrons normally associated with the trapped particle population. The high conductivity channel is generated by processes associated with the precipitation of high energy (E > 20 keV) electrons drifting eastwards from midnight in the trapping region. It is further shown that antiparallel current sheets may flow on the magnetic lines of force penetrating the electrojet, and that this flow is closed in the ionosphere by Hall currents flowing equatorward in the high conductivity channel.  相似文献   

7.
Atmospheric expansion through Joule heating by horizontal electric fields   总被引：1，自引：0，他引：1  
Incoherent scatter measurements made along a magnetic field line into aurora during a period of high electric field in the recovery phase of a substorm show (1) considerably increased electron densities well above the normal F-region maximum, and (2) field-aligned plasma drifts that increase with altitude. A model invoking atmospheric expansion through Joule heating by the horizontal electric field driving the auroral electrojet is used to explain the observations. From this study it is concluded that during magnetically disturbed periods (1) Joule heating by the auroral electrojet raises the neutral temperature and density in the auroral zone ionosphere at F-region heights, (2) ionization formed by the aurora is transported upward by the expanding atmosphere, at times producing an appreciable increase in lower exospheric plasma densities on the field lines containing the aurora, and (3) combined satellite, radar, and optical observations during periods of aurora and high electric field could provide measured F-region collision frequencies.  相似文献   

8.
Measurements of the properties of Pi 2 pulsations along a magnetic meridian at high latitudes during a number of substorms have been analyzed for their relationship to the auroral electrojet. It is found that the maximum Pi 2 pulsation amplitudes are closely associated with the instantaneous position of the electrojet. That is, the average pulsation amplitude in the Pi 2 band as well as the amplitudes of pulsations at specific frequencies in the band have maximum amplitudes at latitudes close to the instantaneous electrojet location. Stations equatorward of the electrojet tend to observe a classical Pi 2 waveform concurrent with the onset of the substorm electrojet. Stations near the electrojet observe a broad spectrum of pulsations indicating a multiplicity of sources. Stations poleward of the initial electrojet position see little pulsation activity until the electrojet moves overhead. The appearance of large amplitude Pi 2 pulsations at a station which was poleward of the electrojet at the onset of a substorm appears to be coincident with the arrival of the poleward border of the electrojet.  相似文献   

9.
Average high latitude magnetic field data from northern observatories are examined for three ranges of magnetic disturbance level, Kp = 1− to 1+,2− to 3+ and ≥ 4−. Except for 0–8h MLT, 55–78° invariant latitude, during away interplanetary magnetic field sectors, the variations between season and sector have the same characteristics at all Kp ranges. Because the amplitude of sector differences is much larger at sunlit local times than in the midnight sector, it is concluded that the current system of Svalgaard (1973) is not adequate to describe the sector variations in magnetic disturbance. Other current systems are discussed briefly. The disturbance morphology and seasonal variation at all Kp levels confirms the results of previous studies which indicate that latitudinally broad current systems, like Sqp and nonionospheric sources are present in addition to latitudinally narrow electrojet currents. Comparison of data between Kp levels indicates that the Harang discontinuity shifts toward earlier MLT with increasing Kp level.  相似文献   

10.
Substorm onsets and intensifications are accompanied on a one-to-one basis by a Pi 2 magnetic pulsation burst. The source region for these pulsations is generally thought to lie in the region of substorm disturbance in the auroral oval. In this paper we outline the characteristics of Pi 2 pulsations in regions near the substorm enhanced electrojet but removed from the locale of the westward travelling surge. We show that a resonance region for the pulsations lies at the equatorwad edge of the westward electrojet, which in the evening sector marks the locus of the Harang discontinuity. Finally we show examples where the maximum amplitude of the Pi 2 is located at or equatorward of the southern border of the eastward electrojet or at the southern border of the westward electrojet. This is clear evidence for the coupling of wave energy into the L-shells far distant from the source of the energy. Mechanisms for Pi 2 generation are discussed in the context of the results presented in this paper.  相似文献   

11.
Based on magnetic data from the IMS Alaska meridian chain of observatories, the total current of the westward auroral electrojet flowing across the meridian is estimated by using two independent methods. The first one is a simple integration of the north-south component of magnetic perturbations along the meridian, providing the quantity F in units of nT·km. The other is to use the forward method, providing the total current I in units of A. It is shown that F and I have nearly identical time variations. Thus, by normalizing the two quantities and taking the numerical value of F in units of nT·km, it is possible to estimate the total electrojet current flowing across a magnetic meridian, with an accuracy of 90%, by using the latitudinal profile of the H component, namely I (A) = 2.0 F (nT·km).  相似文献   

12.
We propose a model three-dimensional current system for the magnetospheric substorm, which can account for the new findings of the field-aligned and ionospheric currents obtained during the last few years by using new techniques. They include (1) the ionospheric currents at the auroral latitude deduced from the Chatanika incoherent scatter radar data, (2) the field-aligned currents inferred from the vector magnetic field observations by the TRIAD satellite and (3) the global distribution of auroras with respect to the auroral electrojets appearing in DMSP satellite photographs. The model current system is also tested by a computer model calculation of the ionospheric current pattern. It is shown that the auroral electrojets have a strong asymmetry with respect to the midnight meridian. The westward electrojet flows along the discrete aurora in the evening sector, as well as along the diffuse aurora in the morning sector. The eastward electrojet flows equatorward of the westward electrojet in the evening sector. It has a northward component and joins the westward electrojet by turning westward across the Harang discontinuity. Thus, the latitudinal width of the westward electrojet in the morning sector is much larger than that in the evening sector. The field-aligned currents, consisting of two pairs of upward and inward currents (one is located in the morning sector and the other in the evening sector), are closed neither simply by the east-west ionospheric currents nor by the north-south currents, but by a complicated combination of the north-south and east-west paths in the ionosphere. The magnetospheric extension of the current system is also briefly discussed.  相似文献   

13.
On moderately disturbed days when substorms occur frequently, the quiet day daily variation in the polar region (Sqp) is enhanced. On such days, however, the quiet day variation along the dip equator appears to be suppressed, as well as being superposed with ‘fluctuations’.It is suggested that the enhancement of Sqp is related to a partial suppression of the equatorial electrojet. The asymmetric ring current also causes an apparent suppression of the electrojet.On the other hand, the substorm-associated electric field which drives the eastward current in the auroral and subauroral zone (causing positive bays) in the afternoon sector appears to enhance the equatorial electrojet.Thus, magnetic variations along the dip equator are influenced by a number of processes in the magnetosphere.  相似文献   

14.
On 11 November 1976, after a magnetically quiet period with the interplanetary magnetic field (IMF) directed northward, a sudden southward turning of the IMF immediately led to a world-wide intensification of convection which was observed to start almost simultaneously at stations within the auroral zone and polar cap. The two-dimensional equivalent current system over the northern hemisphere had a typical two-cell convection pattern with a maximum disturbance of ΔH = ?300 nT observed on the morningside in the westward electrojet region. This enhancement of activity ended after 35 min in a localized substorm onset in the midnight sector over Scandinavia.The recordings made in this area indicate large fluctuations of various ionospheric parameters starting several minutes before the substorm onset. Two subsequent stages can be resolved: (1) high-energy particle precipitation recorded by balloon X-ray detectors and maximum ionospheric current density increase, while the electrojet halfwidth shrinks and the total electrojet current becomes weaker; (2) the maximum ionospheric current density stays constant and the high-energy particle precipitation decreases, while the auroral brightness increases and the total electrojet current and its half-width show a growing trend prior to the final breakup. A suggestion is made that the time interval of these two stages should be called “trigger phase”. A short discussion explains the trigger phase observations in a magnetospheric scale. The energy coupling between solar wind and magnetosphere during the pre-substorm phases is discussed by utilizing the energy coupling function ? defined by Perreault and Akasofu (Geophys. J. R. Astr. Soc.54, 547, 1978). The ? values appear to be on substorm level during the period of enhanced convection. A good correlation between ? and the growth of the Joule heating rate (estimated from the AE data) is found in the beginning, but during the last 20 min before substorm triggering ? is high while the Joule heating rate decreases. The behaviour of ? during the two stages of the trigger phase suggests that the start of the trigger phase is purely internally controlled while the length of the trigger phase and the final substorm onset may be influenced by the variation in ?.  相似文献   

15.
A simple analysis indicates that Type I irregularities which have a slight component of propagation along the magnetic field may be more unstable than those which propagate across the field. Since these waves have very large group velocities, detailed ray tracing would be required to establish their true convective amplification. Nevertheless, there remains the possibility that significant irregularity amplitudes may occur at the northern or southern extremities of the equatorial electrojet from those modes with large north-south group velocity, and furthermore, they could significantly change our understanding of nonlinear solutions of the electrojet instability.  相似文献   

16.
MAGDAS, the Magnetic Data Acquisition System, was successfully installed at the University of Ilorin, Nigeria—an equatorial station—in August 2006. MAGDAS is an important component of the International Heliophysical Year, IHY. We examined the preliminary results obtained from the analysis of the geomagnetic data obtained from the MAGDAS measurement. Diurnal variations of the solar quiet daily variation in the three geomagnetic elements, H, D, Z were studied. The obtained signatures of the magnetic field variations were discussed in the context of the literature. The day-to-day variability of the horizontal and vertical intensities as well as the declination of the geomagnetic field were examined. The study justified the need for a partner magnetic field observatory along same meridian as Ilorin, north or south of it, to facilitate synthesis of the equatorial electrojet effect.  相似文献   

17.
The time-sequence of polar magnetic substorms is discussed to clarify some controversies on the magnetospheric substorm model including the growth phase. The main purpose of the analyses is to examine magnetic variations in the polar cap and in low latitudes. The onset of the expansion phase is confirmed to be reasonably defined by a vector change of polar-cap magnetic disturbance, a sharp intensification of the auroral electrojet disturbance and the beginning of positive ΔH disturbance in midlatitudes near midnight. It is shown that the growth phase signatures so far proposed are consistent when the onset of the expansion phase is identified from the above mentioned features.  相似文献   

18.
Using hourly values of the magnetic elements H, D and Z for 1964, 1965, their variation during night-time hours is examined from both their monthly means and from a previously used harmonic analysis method. The data set used represents quiet magnetic conditions. Consistent changes during the night are often found. Seasonal changes are also examined and it seems necessary to modify the Malin—Isikara hypothesis of a moving ring current by including a seasonal modulation of ring current strength with equinoctial maximum and a local time-varying component or partial ring current. The night-time D component shows considerable asymmetry between North and South hemispheres and this might be due to field-aligned current structure. There is a small amount of evidence for a night-time westward equatorial electrojet enhancement.  相似文献   

19.
An isolated substorm occurred in Northern Scandinavia on 1 March, 1977 around magnetic midnight. The ionospheric phenomena associated with this substorm were studied by ground magnetometers, the Scandinavian Twin Auroral Radar Experiment (STARE), riometers and an all-sky camera. The physical properties of the auroral electrojet are determined from the ground magnetic field and the ionospheric electric field data. Mid and low latitude magnetic field data show evidence of field-aligned current flow. It is shown that the enhancement of the electrojet's current density is essentially determined by an increase in the ionospheric conductivity. The current system derived from the data of this study corresponds to a model of Yasuhara et al. (1975a).  相似文献   

20.
A comparison of the variations in the count of electrons E > 36 keV on the satellite Vela 4A, and in the Macquarie Island magnetometer H trace, shows for a time lag of 22-8 min a correlation, r = 0.95, over a 90 min period of the recovery phase of a magnetospheric substorm on 17 August 1968. All-sky camera data suggest that during the correlation period the auroral electrojet showed very little latitudinal movement. Each peak in electron count relates to a current surge in the electrojet as shown by a deepening of the negative bay at Macquarie Island.Using the Fairfield (1968) model of the location of auroral shells in the solar magnetic equatorial plane, and the known location of the satellite, an estimate of the velocity of tail to Earth plasma convection in the plasma sheet of about 0·33 Re/min is obtained for the recovery phase.The relationship is discussed between plasma sheet thinning and subsequent broadening, and the extension of the magnetic field lines into the tail region and their subsequent return. This discussion makes use of the estimated time lags between electron count at the satellite and the time of arrival of auroral particles at the antisolar meridian.From a somewhat speculative explanation, but one largely supported from the literature, of the magnetospheric processes involved in this auroral substorm, a plasma velocity estimate of 0·42 Re/min for the initial phase of the substorm is obtained. These velocities are of the same order as the 0·5 Re/min obtained by Lezniak and Winkler (1970) at 6·6 Re.  相似文献   

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