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1.
Enhancements in the auroral electrojets associated with magnetospheric substorms result from those in either the electric field or the ionospheric conductivities, or both. Their relative importance varies significantly, even during a single substorm, depending on the location as well as on the substorm phases. It is predicted that different parts of the electrojets tend to respond in different ways to substorm activity. The unprecedented, unique opportunity for CLUSTER spacecraft observations of electric/magnetic fields and precipitating particles, combined with radar measurements of ionospheric quantities and with ground magnetometers, will provide us with crucial information regarding the physical nature of the separation between the “electric field-dominant” and “conductivity-dominant” auroral electrojets. This study also discusses the implications of these two auroral-electrojet components in terms of solar wind-magnetosphere-ionosphere interactions.  相似文献   

2.
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3.
On 7 December 1992, a moderate substorm was observed by a variety of satellites and ground-based instruments. Ionospheric flows were monitored near dusk by the Goose Bay HF radar and near midnight by the EISCAT radar. The observed flows are compared here with magnetometer observations by the IMAGE array in Scandinavia and the two Greenland chains, the auroral distribution observed by Freja and the substorm cycle observations by the SABRE radar, the SAMNET magnetometer array and LANL geosynchronous satellites. Data from Galileo Earth-encounter II are used to estimate the IMF Bz component. The data presented show that the substorm onset electrojet at midnight was confined to closed field lines equatorward of the pre-existing convection reversal boundaries observed in the dusk and midnight regions. No evidence of substantial closure of open flux was detected following this substorm onset. Indeed the convection reversal boundary on the duskside continued to expand equatorward after onset due to the continued presence of strong southward IMF, such that growth and expansion phase features were simultaneously present. Clear indications of closure of open flux were not observed until a subsequent substorm intensification 25 min after the initial onset. After this time, the substorm auroral bulge in the nightside hours propagated well poleward of the pre-existing convection reversal boundary, and strong flow perturbations were observed by the Goose Bay radar, indicative of flows driven by reconnection in the tail.  相似文献   

4.
HF radar data during equinoctial, small IMF By conditions have enabled the ionospheric convection during the substorm growth phase and substorm pseudobreakup to be studied in both hemispheres. This has revealed both conjugate and non-conjugate convection behaviour during the substorm growth phase before and after the pseudobreakup onset. The nightside convection pattern is found to respond promptly to the southward turning of the interplanetary magnetic field (IMF) which impacts on the dusk flank of the magnetosphere due to an inclined phase front in the IMF in the case study presented. The subsequent interhemispheric observations of nightside convection are controlled by the IMF By polarity. The time scale for the response to changes in the IMF By component is found to be a little longer than for Bz, and the full impact of the IMF By is not apparent in the nightside convection until after substorm pseudobreakup has occurred. The pseudobreakup itself is found to result in a transitory suppression in the ionospheric electric field in both hemispheres. This flow suppression is very similar to that observed in HF radar observations of full substorm onset, with the exception of a lack of subsequent poleward expansion.  相似文献   

5.
High time resolution data from the CUTLASS Finland radar during the interval 01:30–03:30 UT on 11 May, 1998, are employed to characterise the ionospheric electric field due to a series of omega bands extending 5° in latitude at a resolution of 45 km in the meridional direction and 50 km in the azimuthal direction. E-region observations from the STARE Norway VHF radar operating at a resolution of 15 km over a comparable region are also incorporated. These data are combined with ground magnetometer observations from several stations. This allows the study of the ionospheric equivalent current signatures and height integrated ionospheric conductances associated with omega bands as they propagate through the field-of-view of the CUTLASS and STARE radars. The high-time resolution and multi-point nature of the observations leads to a refinement of the previous models of omega band structure. The omega bands observed during this interval have scale sizes 500 km and an eastward propagation velocity 0.75 km s–1. They occur in the morning sector (05 MLT), simultaneously with the onset/intensification of a substorm to the west during the recovery phase of a previous substorm in the Scandinavian sector. A possible mechanism for omega band formation and their relationship to the substorm phase is discussed.  相似文献   

6.
The DOPE (Doppler Pulsation Experiment) HF Doppler sounder located near Tromsø, Norway (geographic: 69.6°N 19.2°E; L = 6.3) is deployed to observe signatures, in the high-latitude ionosphere, of magnetospheric ULF waves. A type of wave has been identified which exhibits no simultaneous ground magnetic signature. They can be subdivided into two classes which occur in the dawn and dusk local time sectors respectively. They generally have frequencies greater than the resonance fundamentals of local field lines. It is suggested that these may be the signatures of high-m ULF waves where the ground magnetic signature has been strongly attenuated as a result of the scale size of the waves. The dawn population demonstrate similarities to a type of magnetospheric wave known as giant (Pg) pulsations which tend to be resonant at higher harmonics on magnetic field lines. In contrast, the waves occurring in the dusk sector are believed to be related to the storm-time Pc5s previously reported in VHF radar data. Dst measurements support these observations by indicating that the dawn and dusk classes of waves occur respectively during geomagnetically quiet and more active intervals.  相似文献   

7.
The expression for the increment of instability and decrement of diffusion damping of gradient drift waves for ionospheric altitudes above the F 2 layer maximum is obtained. The gradient drift instability is used to interpret the observations of spread F in the region of large-scale horizontal irregularities of the electron density. Two types of such irregularities observed on board the Intercosmos-19 (IC-19) satellite in the region of low latitudes (a peak of the density in the dusk ionosphere and a trough of the density in the dawn ionosphere) are considered. It is shown that the observed gradients of the density and electric field values in the dawn and dusk ionospheric sectors are quite sufficient for the instability development criterion to be satisfied in both considered cases.  相似文献   

8.
A discussion is given of plasma flows in the dawn and nightside high-latitude ionospheric regions during substorms occurring on a contracted auroral oval, as observed using the EISCAT CP-4-A experiment. Supporting data from the PACE radar, Greenland magnetometer chain, SAMNET magnetometers and geostationary satellites are compared to the EISCAT observations. On 4 October 1989 a weak substorm with initial expansion phase onset signatures at 0030 UT, resulted in the convection reversal boundary observed by EISCAT (at \sim0415 MLT) contracting rapidly poleward, causing a band of elevated ionospheric ion temperatures and a localised plasma density depletion. This polar cap contraction event is shown to be associated with various substorm signatures; Pi2 pulsations at mid-latitudes, magnetic bays in the midnight sector and particle injections at geosynchronous orbit. A similar event was observed on the following day around 0230 UT (\sim0515 MLT) with the unusual and significant difference that two convection reversals were observed, both contracting poleward. We show that this feature is not an ionospheric signature of two active reconnection neutral lines as predicted by the near-Earth neutral model before the plasmoid is “pinched off”, and present two alternative explanations in terms of (1) viscous and lobe circulation cells and (2) polar cap contraction during northward IMF. The voltage associated with the anti-sunward flow between the reversals reaches a maximum of 13 kV during the substorm expansion phase. This suggests it to be associated with the polar cap contraction and caused by the reconnection of open flux in the geomagnetic tail which has mimicked “viscous-like” momentum transfer across the magnetopause.  相似文献   

9.
Ionosphere-magnetosphere disturbances at high latitudes, e.g. magnetic substorms, are accompanied by energetic particle precipitation and strong variations of the ionospheric electric fields and currents. These might reasonably be expected to modify the local atmospheric electric circuit. We have analysed air-earth vertical currents (AECs) measured by a long wire antenna at Esrange, northern Sweden during 35 geomagnetic substorms. Using superposed epoch analysis we compare the air-earth current variations during the 3 h before and after the time of the magnetic X-component minimum with those for corresponding local times on 35 days without substorms. After elimination of the average daily variation we can conclude that the effect of substorms on AEC is small but distinguishable. It is speculated that the AEC increases observed during about 2 h prior to the geomagnetic X-component minimum, are due to enhancement of the ionospheric electric field. During the subsequent 2 h of the substorm recovery phase, the difference between substorm and quiet atmospheric currents decreases. The amplitude of this substorm variation of AEC is estimated to be less than 50% of the amplitude of the diurnal variation in AEC during the same time interval. The statistical significance of this result was confirmed using the Van der Waerden X-test. This method was further used to show that the average air-earth current and its fluctuations increase during late expansion and early recovery phases of substorms.  相似文献   

10.
用Ｋａｍｉｄｅ－Ｍａｔｓｕｓｈｉｔａ方法，在行星际磁场具有较小的北向分量，且｜Ｂｙ｜＞＞｜Ｂｋ｜时，对磁语和磁扰状态以及Ｂｒ＞０和Ｂｙ＜０等不同情况，分别计算了场向电流引起的电离层电势、电场和电流体系．结果表明，极隙区场向电流的存在使高纬向日面区域的电势发生畸变，当Ｂｙ＞０时，无论是磁扰还是磁静日，极隙区电场具有显著的北向分量；等离子体对流有较大的西向分量；电离层电流为东向电流．当Ｂｙ＜０时，电场和等离子体对流的方向与Ｂｙ＞０时相反；电离层电流在磁抗日有西向分量，但在磁静日没有西向分量．电导率对电场和电流体系的影响十分明显，磁扰极光带电导率增强使电流涡从背阳面向向阳而漂移，与静日相比，磁扰时极隙区场向电流引起的电场畸变更为明显，但电场和电流强度的大小却基本保持不变．  相似文献   

11.
During the 6th August 1995, the CUTLASS Finland HF radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14 s. Data from such scans, during the substorm expansion phase, revealed pulses of equatorward flow exceeding 600 m s–1 with a duration of 5 min and a repetition period of 8 min. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. These transient features, which propagate eastwards away from local midnight, have been interpreted as ionospheric current vortices associated with fieldaligned current pairs. The present study reveals that these ionospheric convection features appear to have an accompanying signature in the magnetosphere, comprising a dawnward perturbation and dipolarisation of the magnetic field and dawnward plasma flow, measured in the geomagnetic tail by the Geotail spacecraft, located at L = 10 and some four hours to the east, in the postmidnight sector. These signatures are suggested to be the consequence of the observation of the same field aligned currents in the magnetosphere. Their possible relationship with bursty Earthward plasma flow and magnetotail reconnection is discussed.  相似文献   

12.
We have analyzed the response of azimuthal component of the ionospheric electric field to auroral arc activity. We have chosen for analysis three intervals of coordinated EISCAT and TV observations on 18 February, 1993. These intervals include three kinds of arc activity: the appearance of a new auroral arc, the gradual brightening of the existing arc and variations of the arc luminosity. The arcs were mostly east-west aligned. In all cases, the enhancement of arc luminosity is accompanied by a decrease in the westward component of the ionospheric electric field. In contrast, an increase of that component seems to be connected with arc fading. The observed response is assumed to have the same nature as the short circuit of an external electric field by the conductor. The possible consequence of this phenomenon is discussed.  相似文献   

13.
Central polar cap convection changes associated with southward turnings of the Interplanetary Magnetic Field (IMF) are studied using a chain of Canadian Advanced Digital Ionosondes (CADI) in the northern polar cap. A study of 32 short duration (1 h) southward IMF transition events found a three stage response: (1) initial response to a southward transition is near simultaneous for the entire polar cap; (2) the peak of the convection speed (attributed to the maximum merging electric field) propagates poleward from the ionospheric footprint of the merging region; and (3) if the change in IMF is rapid enough, then a step in convection appears to start at the cusp and then propagates antisunward over the polar cap with the velocity of the maximum convection. On the nightside, a substorm onset is observed at about the time when the step increase in convection (associated with the rapid transition of IMF) arrives at the polar cap boundary.  相似文献   

14.
The dynamics of the system of field-aligned currents (FACs) and closing ionospheric Pedersen currents is considered with the use of original processing methods and the data from four substorms of different types. The total current system comprises of two parts. One is the well-known substorm current wedge (SCW) system, in which the zonal (westward ) current closes FACs in the R1 zone (region). The component 2 consists of two pairs of meridional currents flowing equatorward and poleward in the R1 region and creating regions R0 and R2 (according to the classification of Iijima and Potemra). It is shown that the total FAC of the disturbed magnetosphere–ionosphere system is dominated by the contribution of component 2, which contradicts the original version of the SCW model but is consistent with new data. The quantitative characteristics of the dawn–dusk asymmetry are determined for the FAC distribution in the ionosphere for each substorm. It is shown that the ratio of the average intensities of FACs in the regions R0 and R2 was IR0/IR2 ≥ 0.4, which contradicts the popular opinion that there are no FACs in the polar cap. In addition, a relatively rare event of the simultaneous start of the substorm explosive phase and the SSC caused by the dynamic impact of the solar wind when the polar cap expands (rather than compresses as usual) is considered.  相似文献   

15.
During substorms, large-scale changes of the topology of the Earths magnetosphere following the variation of the characteristics of the interplanetary medium are accompanied by the induction of the electric field. In this study a model of a time-dependent magnetosphere is constructed and the large-scale features of the induced electric field are described. Local-time sectors with upward or downward field-aligned component and with intense perpendicular component of the electric field are distinguished. The electric-field structure implies the existence of outflow regions particularly effective in ion energization. With the vector potential adopted in the study, the region from which the most energized ions originate is defined by the local-time sector near 2100 MLT and latitude zone near 71° MLAT. The motion of ionospheric oxygen ions of energy 0.3–3 keV is investigated during a 5-min reconfiguration event when the tail-like magneto-spheric field relaxes to the dipole-like field. As the characteristics of plasma in the regions near the equatorial plane affect the substorm evolution, the energy, pitch angle, and the magnetic moment of ions in these regions are analyzed. These quantities depend on the initial energy and pitch angle of the ion and on the magnetic and electric field it encounters on its way. With the vector potential adopted, the energy attained in the equatorial regions can reach hundreds of keV. Three regimes of magnetic-moment changes are identified: adiabatic, oscillating, and monotonous, depending on the ion initial energy and pitch angle and on the magnetic- and electric-field spatial and temporal scales. The implications for the global substorm dynamics are discussed.  相似文献   

16.
Radar observations of auroral zone flows during a multiple-onset substorm   总被引：1，自引：0，他引：1  
We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every \sim12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the preexisting extended current system driven by the large-scale flow. These filaments are \sim1 h MLT wide (\sim600 km), and initially expand poleward to a width of \sim300 km before contracting equatorward and coalescing with the preexisting current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s−1 or less during the first few minutes, before surging equatorward at 0.5-1.0 km s−1 during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed \sim100 mho, before decaying over a few minutes. This value compares with Hall conductivities of \sim20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.  相似文献   

17.
We study the influence of the interplanetary magnetic field (IMF) and convection electric field on the rate and destination of polar wind and other thermal (low-energy) ion outflows, and its resulting effects on magnetosphere–ionosphere coupling, using single-particle trajectory simulations in conjunction with ion velocity distribution measurements on Akebono and IMF and ionospheric convection data. We find that the ions preferentially feed the dusk sector of the plasma sheet when the IMF is duskward (By>0), and are more evenly distributed in the plasma sheet when the IMF is dawnward. The flow of oxygen ions originating from the noon or dusk sectors of the polar cap has a higher probability of reaching the magnetosphere and beyond compared with that from the dawn or midnight sectors, due to the increased centrifugal acceleration associated with the larger magnetic field curvature near noon and the increased convection electric field in the dusk sector. The flow is enhanced and confined to lower L-shells at times of strongly southward IMF, compared with that at times of northward IMF. The outflow rate to both the plasma sheet and the magnetotail correlates strongly with the ion temperature. As a result, the IMF and the convection electric fields affect both the overall magnitude and the detailed distribution of mass transfer from the ionosphere to the magnetosphere in magnetosphere–ionosphere coupling.  相似文献   

18.
地球磁层中的电场是研究磁层物理的重要参数,目前常用的对流电场有均匀晨昏电场和投影电场.电离层电场可以看做磁层电场沿磁力线在电离层的投影,本文选取的电离层电场模型为Weimer(2001模式)电场.利用T96磁场模式,沿磁力线将电离层电场投影到磁层空间,得到一个新的磁层电场模式,并讨论了磁暴、行星际磁场(IMF)、太阳风参数和亚暴等对磁层电场的影响.利用该模型计算的电场结果与卫星探测结果相符.  相似文献   

19.
王慧  虞蕾  郑志超 《地球物理学报》2020,63(4):1294-1307
本文利用Swarm卫星2015—2016年高精度的磁场矢量数据,将晨昏地方时扇区高纬场向电流(Field-Aligned Currents,FACs)事件按极性和电流密度分为四类,并首次比较研究了四类FACs事件的时空分布特征及其影响因素,研究发现:极性正常事件(晨侧靠极侧电流元向下流入电离层,靠赤道侧电流元向上流出电离层,昏侧电流极性相反,即传统意义上的R1和R2FACs)发生率约为70%,其中R1FACs强于R2事件的发生率为R1FACs弱于R2的3~5倍;极性异常事件(与传统的R1和R2FACs流向相反,两片电流元定义为R1*和R2*FACs)发生率约占30%,其中R1*R2*的1.5~2.5倍.进一步分析发现极性正常事件主要发生在南向IMF Bz期间,与重联电场相关性较好,净电流密度随着重联电场和电离层电导率的增加而增加.其中R1R2*事件通常发生在IMF By<0期间,昏侧事件主要发生在IMF By>0期间,而R1*相似文献   

20.
Advances in our understanding of the large-scale electric and magnetic fields in the coupled magnetosphere-ionosphere system are reviewed. The literature appearing in the period January 1991–June 1993 is sorted into 8 general areas of study. The phenomenon of substorms receives the most attention in this literature, with the location of onset being the single most discussed issue. However, if the magnetic topology in substorm phases was widely debated, less attention was paid to the relationship of convection to the substorm cycle. A significantly new consensus view of substorm expansion and recovery phases emerged, which was termed the Kiruna Conjecture after the conference at which it gained widespread acceptance. The second largest area of interest was dayside transient events, both near the magnetopause and the ionosphere. It became apparent that these phenomena include at least two classes of events, probably due to transient reconnection bursts and sudden solar wind dynamic pressure changes. The contribution of both types of event to convection is controversial. The realisation that induction effects decouple electric fields in the magnetosphere and ionosphere, on time scales shorter than several substorm cycles, calls for broadening of the range of measurement techniques in both the ionosphere and at the magnetopause. Several new techniques were introduced including ionospheric observations which yield reconnection rate as a function of time. The magnetospheric and ionospheric behaviour due to various quasi-steady interplanetary conditions was studied using magnetic cloud events. For northward IMF conditions, reverse convection in the polar cap was found to be predominantly a summer hemisphere phenomenon and even for extremely rare prolonged southward IMF conditions, the magnetosphere was observed to oscillate through various substorm cycles rather than forming a steady-state convection bay.Reporter view, presented to Commission III of the International Association of Geomagnetism and Aeronomy at 7th IAGA Scientific Assembly, Buenos Aires, Argentina, August 1993.  相似文献   

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