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1.
Using the method of characteristics to invert ground-based data of the ground magnetic field disturbance and of the ionospheric electric field, we obtain spatial distributions of ionospheric conductances, currents, and field-aligned currents (FACs) associated with a north-south auroral form that drifts westwards over northern Scandinavia around 2200 UT on December 2, 1977. This auroral form is one in a sequence of such north-south structures observed by all-sky cameras, and appears 14 min after the last of several breakups during that extremely disturbed night. Our analysis shows that the ionospheric Hall conductance reaches values above 200 S in the center of the form, and upward flowing FACs of up to 25 μA/m2 are concentrated near its westward and equatorward edge. The strong upward flowing FACs are fed by an area of more distributed, but still very strong downward-flowing FACs northeastward of the auroral form. In contrast to the conductances, the electric field is only slightly affected by the passage of the form. We point out similarities and differences of our observations and results to previously reported observations and models of ‘auroral fingers’, ‘north-south aurora’, and ‘auroral streamers’ which are suggested to be ionospheric manifestations of bursty bulk flows in the plasma sheet.  相似文献   

2.
The synchronous observations of strong electric fields and large-scale undulations observed on December 12, 2004, in the evening sector of the diffuse auroral zone 0900-1000 UT (~1700-1800 MLT) have been analyzed. The appearance of strong northward electric field at ~0900 UT was almost simultaneously registered at Tixie Bay ionospheric station (71.6° N, 128.9° E, L =, 5.6) and on the DMSP F15 satellite. At 0910-1000 UT, the all-sky TV camera at Tixie Bay and the DMSP satellites (F13, F14, and F15) registered eight undulations propagating westward at a velocity of 0.7—0.8 km/s. The undulation parameters registered during the TV observations agree with the satellite measurements. The distinctive feature of the analyzed event consists in that an intense electric field and undulations were localized within the diffuse zone in the region of increased precipitation of keV electrons. A comparison of the ground-based and satellite measurements made it possible to draw the conclusion on the necessary conditions for formation of diffuse undulations.  相似文献   

3.
The effect of the interplanetary magnetic field (IMF) By component on the dayside auroral oval from Viking UV measurements for March–November 1986 is studied. Observations of dayside auroras from Viking UV images for large positive (15 cases) and negative (22 cases) IMF By (∣By∣>4 nT), suggest that: (1) the intensity of dayside auroras tends to increase for negative IMF By and to decrease for positive By, so that negative IMF By conditions seem preferable for observations of dayside auroras; (2) for negative IMF By, the auroral oval tends to be narrow and continuous throughout the noon meridian without any noon gap or any strong undulation in the auroral distribution. For positive IMF By, a sharp decrease and spreading of auroral activity is frequently observed in the post-noon sector, a strong undulation in the poleward boundary of the auroral oval around noon, and the formation of auroral forms poleward of the oval; and (3) the observed features of dayside auroras are in reasonable agreement with the expected distribution of upward field-aligned currents associated with the IMF By in the noon sector.  相似文献   

4.
Auroral phenomena are controlled by the geomagnetic field.Since the terrestrial field lines connect the auroral oval to the equatorial region at large distances, the collisionless plasma in this remote space environment can act as a power supply for the high-latitude upper atmosphere where auroral emissions take place. The coupling process is intimately linked to currents which flow across the local magnetic field direction both in the equatorial part and at the atmospheric end of the auroral field lines. These two auroral key regions are connected through currents flowing along the terrestrial field lines, thereby completing the auroral current circuit. Such field-aligned currents are carried by Alfvén waves, that is, magnetohydrodynamic shear waves, which are thus a means to exchange momentum and energybetween rather remote parts of the geomagnetically controlledspace environment. Auroral dynamics is further affected by a third key region in the auroral current circuit, namely the auroral acceleration region, where parallel electric fields accelerate particle to keV energies. This review focuses on key region coupling through Alfvén waves. Continuity requirements for currents and electric fields provide a convenient means to describe the interaction of Alfvén waves with different plasma regimes. Basic coupling aspects can be demonstrated with the help of a simplified model. Inhomogeneities and nonlinear feedback can lead to resonance effects and instabilities.  相似文献   

5.
A case is described of multiple current sheets crossed by the MAGION-2 satellite in the near-midnight quieting auroral oval. The data were obtained by the magnetometer experiment onboard. Results show during a quieting period after a preceding substorm, or during an early growth phase of the next substorm, two double-sheet current bands, POLE and EQUB, located at respectively the polar and equatorial borders of the auroral oval separated by about 500 km in latitude. This is consistent with the double-oval structure during recovery introduced by Elphinstone et al. (1995). Within the POLE, the magnetic field data show simultaneous existence of several narrow parallel bipolar current sheets within the upward current branch (at 69.5–70.3° invariant latitude) with an adjacent downward current branch at its polar side at (70.5–71.3°). The EQUB was similarly stratified and located at 61.2–63.5° invariant latitude. The narrow current sheets were separated on average by about 35 km and 15 km, respectively, within the POLE and EQUB. A similar case of double-oval current bands with small-scale structuring of their upward current branches during a quieting period is found in the data from the MAGION-3 satellite. These observations contribute to the double-oval structure of the late recovery phase, and add a small-scale structuring of the upward currents producing the auroral arcs in the double- oval pattern, at least for the cases presented here. Other observations of multiple auroral current sheets and theories of auroral arc multiplicity are briefly discussed. It is suggested that multiple X-lines in the distant tail, and/or leakage of energetic particles and FA currents from a series of plasmoids formed during preceding magnetic activity, could be one cause of highly stratified upward FA currents at the polar edge of the quieting double auroral oval.  相似文献   

6.
We present a statistical study of Polar electric field observations using auroral oval passes over Scandinavia above the acceleration region. We are especially interested in seeing whether we can find large perpendicular electric fields associated with an upward extended classical U-shaped potential drop for these passes, during which Polar is in the northern hemisphere usually at about 4 RE altitude. We also use Polar magnetic field data to infer the existence of a field-aligned current (FAC) and conjugate ground-based magnetometers (the IMAGE magnetometer network) to check whether the event is substorm-related or not. We find several events with a FAC but only weak perpendicular electric fields at Polar. In those rare cases where the Polar electric field was large, its direction was mostly found to be incompatible with the U-shaped potential model, or it was associated with disturbed conditions (substorms), where one cannot easily distinguish between inductive and static perpendicular electric fields. We found only two cases which are compatible with the upward extended U-shaped potential picture, and even in those cases the potential value is quite small (1–kV). To check the validity of the analysis method we also estimate the perpendicular electric field on the southern hemisphere, where Polar flies within or below the acceleration region, and we found a large number of inverted-V-type signatures as expected from previous studies. To explain the lack of perpendicular electric fields at high altitudes we suggest an O-shaped potential model instead of the U-shaped one.  相似文献   

7.
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8.
We have investigated ion outflows observed by the Akebono satellite and the EISCAT radar in the nightside auroral region on February 16, 1993. The Akebono satellite at about 7000 km altitude observed the region of suprathermal ion outflows and inverted-V type electron precipitation alternately with a horizontal separation of 70–150 km at the ionospheric level. These two regions corresponded to the upward and downward field-aligned current region, respectively, and intense ELF waves were observed in the ion outflow region. From the EISCAT VHF radar observation (Common Program 7 mode), it has been suggested that the ion outflow region and the enhanced electron temperature region were aligned along geomagnetic field lines with vertical and horizontal separations of 200–400 and 70–80 km, respectively and these two regions convected equatorward across the EISCAT radar at Tromsø site. Based on these results, we propose a model for this ion outflow as follows. In the nightside auroral region, downward FAC regions exist near the edge of the inverted-V type electron precipitation regions. ELF waves are excited probably by a plasma instability due to the upward thermal electron beam carrying the downward FACs, and these ELF waves cause transverse ion heating at the top of the ionosphere. The produced ion conics contribute significantly to ion outflow.  相似文献   

9.
The distribution of the electric potential, generated by the magnetospheric field-aligned currents flowing along the auroral oval and in the dayside cusp region at the upper atmospheric boundary in the polar ionosphere, is calculated. The obtained electric potential distributions are used to calculate the electric field strength near the Earth’s surface. The results of the model calculations are in good agreement with the electric field measurements at Vostok Antarctic station. It has been indicated that large-scale magnetospheric fieldaligned currents, related to IMF variations, can affect variations in the electric field strength in the polar regions via changes in the electric potential in the polar ionosphere, associated with these currents.  相似文献   

10.
A stable evening sector are is studied using observations from the FAST satellite at 1250 km altitude and the MIRACLE ground-based network, which contains all-sky cameras, coherent radars (STARE), and magnetometers. Both FAST and STARE observe a northward electric field region of about 200 km width and a field magnitude of about 50 mV/m southward of the arc, which is a typical signature for an evening-sector arc. The field-aligned current determined from FAST electron and magnetometer data are in rather good agreement within the arcs. Outside the arcs, the electron data misses the current carriers of the downward FAC probably because it is mainly carried by electrons of smaller energy than the instrument threshold. Studying the westward propagation speed of small undulations associated with the arc using the all-sky cameras gives a velocity of about 2 km//s. This speed is higher than the background ionospheric plasma speed (about 1 km//s), but it agrees rather well with the idea originally proposed by Davis that the undulations reflect an E × B motion in the acceleration region. The ground magnetograms indicate that the main current flows slightly south of the arc. Computing the ionospheric conductivity from FAST electron data and using the ground magnetograms to estimate the current yields an ionospheric electric field pattern, in rather good agreement with FAST results.  相似文献   

11.
The sensitive method for detecting and measuring the velocity of a weak luminosity wave, traveling from bottom to top along an arc or isolated auroral beams, has been developed. This wave is caused by dispersion of precipitating electrons over velocities and by a differential atmospheric penetration of different-energy electrons, and the wave velocity gives information about the location of the electron acceleration region in the magnetosphere. The method was tested using different model signals and was used to study pulsating auroras and auroral breakup. A luminosity wave has been detected in pulsating auroras, and it has been estimated that the injection region is located at a distance of 5–6 R e . The application of the method to intensification of auroras during breakup indicated that such a wave is absent; i.e., breakup electrons being accelerated near the ionosphere at altitudes of 2000–8000 km. It has been assumed that the regions of anomalous resistance, generated in the ionosphere by field-aligned currents during the breakup phase, cause intense local field-aligned electric fields. These fields accelerate thermal electrons and form the auroral breakup pattern.  相似文献   

12.
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13.
对流电场、场向电流和极光区电集流是磁层一电离层耦合的主要物理过程．它们的演化发展时间分别为几分钟至半小时的量级．本文用１００°Ｅ和３００°Ｅ的两个地磁经度链附近各１１个台站的１ｍｉｎ均值地磁Ｈ和Ｚ分量资料,分析了１９９４年４月１６-１７日磁暴期间磁层耦合过程对极光区和中低纬区电离层扰动的地磁特征．强磁暴开始时,台站所处的地方时位置不同,则观测到的电离层和地磁响应也完全不同．这是磁层对流和一、二区场向电流共同作用的结果．一般说,扰时极光区的西向电集流变化更为强烈．随着耦合的发展,极光区范围会向南北扩展,电集流中心带则向低纬侧移动．在中低纬区,二区场向电流的建立能屏蔽一区场向电流所产生的扰动,并引起反向的电流及地磁变化．由此,中低纬区夜间有可能出现短时间的东向电场,又可通过ＥＸＢ的垂直向上漂移作用抬升Ｆ层等离子体,并发生同一经度链附近的多站电离层ｈ＇Ｆ同时突增现象．另一方面,磁赤道附近的台站则更多地受内磁层赤道环电流和电离层赤道电集流的影响．  相似文献   

14.
In this study, we investigate the relation of auroral substorm onset to the sharp decrease in the local AL index (IL) during substorms. With a database of over 4200 onsets determined from auroral images, we have statistically examined the timing between the auroral substorm onset and the sharp decrease in the IL index, as determined with data from the IMAGE magnetometer network. From the database of onsets, 54 substorms were determined to be within 6° of the central meridian of the IMAGE ground array. Our superposed epoch median curve shows that the IL index begins to sharply decrease 3 min before the auroral onset, which is twice the 2 min resolution of the auroral imager. However, the mean difference determined by measuring the time between the start of the IL decrease and the auroral substorm onset is about 1.1±0.6 min. An analysis of the superposed epoch median curves of the SOPA particle data for the LANL spacecraft closest to the auroral onset meridian indicates that both the electron and proton injections begin about 3 min before the auroral onset. However, the mean time of the difference between the minimum of the particle dispersionless injection and the auroral onset is simultaneous within the uncertainty of the auroral onset and the error of the mean for the injection. The location of the electron injection relative to the IMAGE ground array seems to be 01–04 MLT, while the proton injection appears to be in the 22–01 MLT sector. These statistical results support the idea that the field aligned and ionospheric currents of the substorm current wedge begin to flow before the auroral onset.  相似文献   

15.
Polar regionSq     
Geomagnetically quiet day variations in the polar region are reviewed with respect to geomagnetic field variation, ionospheric plasma convection, electric field and current. Persistently existing field-aligned currents are the main source of the polar regionSq. Consequently, the morphology and variability of the polar regionSq largely depend upon both field-aligned currents and ionospheric conductivity. Since field-aligned currents are the major linkage between the ionosphere and the magnetosphere, the latter is controlled by solar wind state, in particular, the interplanetary magnetic field, the polar regionSq exhibits remarkable IMF dependence.  相似文献   

16.
Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E × B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Polar Geophysical Institute, Apatity, Russia  相似文献   

17.
Flow bursts within the ionosphere are the ionospheric signatures of flow bursts in the plasma sheet and have been associated with poleward boundary intensifications (PBIs). Some PBIs extend equatorward from the polar cap boundary, where they can be roughly divided into north–south-aligned and east–west-aligned structures. In this paper, we present two flow burst events observed by the new Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) in the pre-midnight auroral zone on 28 April 2007, one towards the west and the other towards the east. In both cases, enhanced flows lasted for about 8–10 min with peak velocities exceeding 1500 m/s. The concurrently measured electron density showed that the flow bursts occurred in low conductivity regions. However, near the poleward (equatorward) edge of the westward (eastward) flow burst, strong electron density enhancements were observed in the E region, indicating the presence of discrete auroral arcs. Auroral images from the Polar spacecraft were available at the time of the eastward flow burst and they indicate that this burst was associated with an east–west-aligned auroral structure that connected at later MLT to a north–south structure. In addition, simultaneous precipitating particle energy spectrum measured by the the Defense Meteorological Satellites Program (DMSP) F13 satellite reveals that this auroral structure resulted from mono-energetic electron precipitation associated with a significant field-aligned potential drop. These observations show direct evidence of the relationship between flow bursts, field-aligned currents and auroral intensifications, and suggest that eastward/westward flow bursts are associated with east–west-oriented PBI structures that have extended well within the plasma sheet. This is in contrast to the equatorward-directed flow that has been previously inferred for PBIs near the polar cap boundary and for north–south auroral structures. This paper illustrates the use of the PFISR radar for studying the magnetosphere–ionosphere coupling of flow bursts.  相似文献   

18.
Numerical calculations of the thermospheric and ionospheric parameters above EISCAT are presented for quiet geomagnetic conditions in summer. The Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) was used. The numerical results were obtained both with a self-consistent calculation of the electric fields of magnetospheric and dynamo-action origin and with the magnetospheric electric fields only. It was found that the dynamo-electric field has some effect on the ionospheric convection pattern during quiet geomagnetic conditions. It has a marked effect mainly on the zonal neutral wind component above EISCAT (±20m/s at 140 km altitude). We have studied the effects of various field-aligned current (FAC) distributions on thermosphere/ionosphere parameters and we show that a qualitative agreement can be obtained with region-I and -II FAC zones at 75° and 65° geomagnetic latitude, respectively. The maximum FAC intensities have been assumed at 03–21 MLT for both regions with peak values of 2.5 × 10–7 Am–2 (region I) and 1.25 × 10–7 A m–2 (region II). These results are in agreement with statistical potential distribution and FAC models constructed by use of EISCAT data. The lack of decreased electron density in the night-time sector as observed by the EISCAT radar was found to be due to the spatial distribution of ionospheric convection resulting from electric fields of magnetospheric origin.  相似文献   

19.
We present the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes from the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-known features such as the Harang discontinuity, westward travelling surge and poleward leap, but also discrete auroral forms, known as auroral streamers, appeared propagating westward along the centre of the electrojet. Besides the magnetic field measurements, there were auroral observations and plasma flow and conductivity measurements obtained by EISCAT. The data of all three sets of instruments are consistent with the notion of upward field-aligned currents associated with the moving auroral patches. A detailed analysis of the electro-dynamic parameters in the ionosphere, however, reveals that they do not agree with the expectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated with field-aligned current filaments, are not collocated with the centres of equivalent current vortices. Furthermore, there is a clear discrepancy between the measured plasma flow direction and the obtained equivalent current direction. All this suggests that steep conductivity gradients are associated with the transient auroral forms. Also self-induction effects in the ionosphere may play a role for the orientation of the plasma flows. This study stresses the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.  相似文献   

20.
Three SuperDARN coherent HF radars are employed to investigate the excitation of convection in the dayside high-latitude ionosphere in response to transient reconnection occurring in the cusp region. This study demonstrates the existence of transient antisunward-propagating backscatter features at the expected location of the ionospheric footprint of the cusp region, which have a repetition rate near 10 min. These are interpreted as the ionospheric signature of flux transfer events. Moreover, transient sunward-propagating regions of backscatter are observed in the convection return flow regions of both the pre- and post-noon sectors. These patches are observed to propagate towards the noon sector from at least as far around the auroral zone as 07 MLT in the pre-noon sector and 17 MLT in the post-noon sector, travelling with a veloCity of approximately 1.5 to 2 km s−1. These return flow patches have a repetition rate similar to that of the transient features observed at local noon. While providing supporting evidence for the impulsive nature of convection flow, the observation of sunward-propagating features in the return flow region is not consistent with current conceptual models of the excitation of convection.  相似文献   

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