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1.
About the relationship between auroral electrojets and ring currents   总被引：1，自引：0，他引：1  
The relationship between the storm-time ring current and the auroral electrojets is investigated using IMAGE magnetometer data, DSt and H-SYM, and solar wind data. Statistical results as well as the investigation of single events show that the auroral electrojets occur also during nonstorm conditions without storm-time ring current development and even during the storm recovery phase of increasing DSt. A close correlation between electrojet intensity and ring current intensity was not found. Though the eastward electrojet moves equatorward during the storm main phase there is no unequivocal relationship between the movement of the westward electrojet and the ring current development. All these results suggest that the auroral electrojets and the ring current develop more or less independently of each other.  相似文献   

2.
The idea of two separate storm time ring currents, a symmetric and an asymmetric one has accepted since the 1960s. The existence of a symmetric equatorial ring current was concluded from Dst. However, the asymmetric development of the low-latitude geomagnetic disturbance field during storms lead to the assumption of the real existence of an asymmetric ring current. I think it is time to inquire whether this conception is correct. Thus, I have investigated the development of the low-latitude geomagnetic field during all the magnetic local times under disturbed and quiet conditions. The storm on February 6–9, 1986 and a statistical analysis of many storms has shown that the asymmetry does not vanish during the storm recovery phase. The ratio between the recovery phase asymmetry and the main phase asymmetry is low only for powerful storms. Storms of moderate intensity show the opposite. The global picture of the field evolution of the February storm shows clear differences at different local times. For instance the main phase and recovery phase start time does not coincide with Dst. Also the ring current decay is not the same at different local times. Therefore, Dst gives an incorrect picture of the field development. Moreover, asymmetry does not disappear during international quiet days as the investigation of the low-latitude geomagnetic field shows. Considering all these observations, I think we must revise our ideas about the ring current. In my opinion only one ring current exists and this is an asymmetric one. This asymmetry increases during storms and develops rather fast to more or less symmetric conditions. However, in no case is itjustified to conclude from Dst that a symmetric ring current exists.  相似文献   

3.
The hourly averaged Polar Cap (PC) index was used as the input parameter for the ring current index Dst variation forecasting. The PC index is known to describe well the principal features of the interplanetary magnetic field as well as the total energy input to the magnetosphere. This allowed us to design a neural network that was able to forecast the Dst variations 1 h ahead. 1995 PC and Dst data sets were used for training and testing and 1997 data sets were used for validation. From 15 moderate and strong geomagnetic storms observed during 1997, 10 were successfully forecasted. In 3 cases the observed minimum Dst value was less than the predicted value, and only in 3 cases the neural network was not able to reproduce the features of the geomagnetic storm.  相似文献   

4.
Using the minute data of the H component of geomagnetic field from the 20°E magnetic meridian chain and the 30°N magnetic latitudinal chain, the temporal evolution characteristics of the equatorial ring current during the storm on November 7-10, 2004 are studied. It is indicated that the UT-MLT and UT-MLAT graphics extremely exhibit the local time distribution, latitudinal variation and temporal evo- lution of the H component. The results show: (1) The UT-MLT contour clearly shows the increasing of the H component mostly around noon during the initial phase, representing the geomagnetic effect from the magnetopause current system. During the main phase, most negative values of the H com- ponent appear around the dusk-side, indicating the dawn-dusk asymmetric distribution of the ring cur- rent. (2) The contour of UT-MLAT suggests the latitudinal variation of the H component decreasing with the enhancement of the latitudes during geomagnetic storm, which is in good agreement with the Dst index. The latitudinal variations provide a new sight for describing the temporal characteristics of the intensity of the storm-time ring current. (3) Both the contours of UT-MLT and UT-MLAT are useful to monitor the space-time distribution of the equatorial ring current.  相似文献   

5.
A morphological analysis of vertical sounding data obtained in Irkutsk from 2003 to 2008 has been performed. The AE index was used to determine the geomagnetic activity level, and the storm main phase onset was registered based on the D st index. The ionospheric response to a magnetic storm was estimated based on the relative deviation of the critical frequency and altitude of the ionospheric F2 region from the median values. Superstrong magnetic storms and storms without positive initial phases were not considered when the data were selected. We found that positive ionospheric disturbances, which were accompanied by an increase in the F2 region maximum altitude, predominated between the storm initial phase and main phases during all considered magnetic storms. Between these storm phases, negative disturbances were only registered at night. Predominance of positive ionospheric disturbances over negative ones can be related to the selection of storms for studies.  相似文献   

6.
The magnetospheric storm on November 20, 2003 was one of two greatest events in 1957–2003. The D st* index reached −472 nT, the polar cap potential drop exceeded 200 kV, the polar cap boundary expanded up to Φ = 60°, the plasma layer density in the synchronous orbit reached 5 cm−3, and the inner edge of the plasma sheet penetrated up to L ∼ 1.5R E. The sequence of disturbance modes including some previously unknown is described. The distribution of the total power input into the magnetosphere between the ionosphere (power Q i) and the ring current (Q DR), as well as the relative roles of the spontaneous substorms and the driven disturbances in the creation of the DR current, is analyzed. The values of the parameter α = Q DR/Q i are calculated with a step of 5 min. It is shown that intervals with α ≪ 1 and with maximums α ≫ 1 were observed in the events under consideration. These results contradict the dominant opinion that the energy input into the magnetosphere during disturbances is primarily dissipated in the ionosphere. The two types of α maximums are observed: one in the mode of a prevailing spontaneous substorm and the other in the mixed mode of the substorm and driven disturbance. It is concluded that both types of the maximums and corresponding enhancements of the DR current appeared due to the plasma turbulization processes in the substorm current wedge. The parameter α appears to slowly increase from α ≪ 1 to α > 1 with increasing activity level; this trend supports the driven model of creating the DR current due to an increase in the electric field of the solar wind.  相似文献   

7.
A review of modern dynamic models of the Earth’s magnetosphere (the A2000 paraboloid model and Tsyganenko’s T01 model) is presented. For the magnetic storm of January 9–11, 1997, the results of joint calculations of the magnetospheric magnetic field are presented and contributions of the large-scale magnetospheric currents to the D st variations are analyzed. Both models were shown to be well consistent with measurement data; the contribution of the magnetotail current system to D st is comparable to the contribution of the ring current. At the same time, the relative dynamics of magnetospheric current systems are different in different models. The differences in the magnetic field variation profiles for various current systems calculated by the A2000 and T01 models are explained by model parameterizations.  相似文献   

8.
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9.
The contribution of resonant wave-particle interactions to the formation and decay of the magnetospheric ring current is analysed in the framework of a self-consistent set of equations which take into account azimuthal plasmasphere asymmetry. It is shown that the cyclotron interaction of westward drifting energetic protons with Alfven waves in the evening-side plasmaspheric bulge region leads to the formation of a ring current asymmetry located near 18:00 MLT. The time-scale of this asymmetry is determined by the proton drift time through the plasmaspheric bulge and is about 1 - 3 h. A symmetrical ring current decays mainly due to charge exchange processes. The theory is compared with known experimental data on ions and waves in the ring current and on low-latitude magnetic disturbances. New low-latitude magnetometer data on the magnetic storm of 24 - 26 July 1986 are also discussed. The model presented explains the observed localization of an asymmetrical ring current loop in the evening sector and the difference in relaxation time-scales of the asymmetry and the Dst index. It also explains measured wave turbulence levels in the evening-side plasmasphere and wave observation statistics.  相似文献   

10.
The relationship between the auroral electrojet indices (AE) and the ring current magnetic field (DR) was investigated by observations obtained during the magnetic storm on 1–3 April 1973. During the storm main phase the DR development is accompanied by a shift of the auroral electrojets toward the equator. As a result, the standard AE indices calculated on the basis of data from auroral observatories was substantially lower than the real values (AE’). To determine AE’ during the course of a storm main phase data from subauroral magnetic observatories should be used. It is shown that the intensity of the indices (AE’) which take into account the shift of the electrojets is increased substantially relative to the standard indices during the storm main phase. AE’ values are closely correlated with geoeffective solar wind parameters. A high correlation was obtained between AE’ and the energy flux into the ring current during the storm main phase. Analysis of magnetic field variations during intervals with intense southward IMF components demonstrates a decrease of the saturation effect of auroral electrojet currents if subauroral stations magnetic field variations are taken into account. This applies both to case studies and statistical data. The dynamics of the electrojets in connection with the development of the ring current and of magnetospheric substorms can be described by the presence (absence) of saturation for minimum (maximum) AE index values during a 1-h interval. The ring current magnetic field asymmetry (ASY) was calculated as the difference between the maximum and minimum field values along a parallel of latitude at low latitudes. The ASY value is closely correlated with geoeffective solar wind parameters and simultaneously is a more sensitive indicator of IMF Bz variations than the symmetric ring current. ASY increases (decreases) faster during the main phase (the recovery phase) than DR. The magnetic field decay at low latitudes in the recovery phase occurs faster in the afternoon sector than at dusk.  相似文献   

11.
Observations have shown that, prior to substorm explosions, thin current sheets are formed in the plasma sheet of the Earth’s magnetotail. This provokes the question, to what extent current-sheet thinning and substorm onsets are physically, maybe even causally, related. To answer this question, one has to understand the plasma stability of thin current sheets. Kinetic effects must be taken into account since particle scales are reached in the course of tail current-sheet thinning. We present the results of theoretical investigations of the stability of thin current sheets and about the most unstable mode of their decay. Our conclusions are based upon a non-local linear dispersion analysis of a cross-magnetic field instability of Harris-type current sheets. We found that a sausage-mode bulk current instability starts after a sheet has thinned down to the ion inertial length. We also present the results of three-dimensional electromagnetic PIC-code simulations carried out for mass ratios up to Mi/me = 64. They verify the linearly predicted properties of the sausage mode decay of thin current sheets in the parameter range of interest.  相似文献   

12.
Pulsating of the generalized ion and neutral polar winds   总被引：1，自引：1，他引：0  
A three-dimensional, time-dependent model of the ion and neutral polar winds was used to study their dynamic evolution during the May 4, 1998 magnetic storm. The simulation tracked the dynamics of five species (O+, H+, Hs, Os, and electrons) and covered a 9-h period. During the storm, Dst decreased to −210 nT, Ap reached 300, and Kp was elevated. The IMF Bz component was southward at the start of the storm and for several hours thereafter and then turned northward. However, the magnetospheric energy input to the ionosphere exhibited a 50-min oscillation, with the plasma convection and particle precipitation patterns expanding and contracting in a periodic manner. As a consequence, the ion and neutral polar winds pulsated with an approximate 50-min period. The H+ and O+ ions displayed cyclic upflows and downflows in the topside ionosphere as well as a highly structured spatial distribution that varied with time. The vertical flux of the neutral Hs atoms was upward at the top of the ionosphere, but the magnitude varied in a cyclic manner in response to the oscillating stormtime energy input. The vertical flux of neutral Os atoms was downward at the top of the ionosphere and varied significantly with the stormtime energy input. For H+, O+, and Hs, the maximum total (integrated) vertical flux during the storm was upward at the top of the ionosphere, with values of 8–9×1025 particles/s for H+, 2–4×1026 particles/s for O+, and 2–3×1027 particles/s for Hs. The corresponding total vertical Os flux was predominately downward, with only localized areas with positive fluxes.  相似文献   

13.
The polar geomagnetic activity resulting from solar wind–magnetosphere interactions can be characterized the Polar Cap (PC) indices, PCN and PCS. PC index values are derived from polar magnetic variations calibrated on a statistical basis such that the index approximate values in units of mV/m of the interplanetary “geo-effective” (or “merging”) electric field (EM) conveyed by the solar wind. The timing and amplitude relations of the PC index to solar wind plasma and magnetic field parameters are reported. The solar wind effects are parameterized in terms of the geo-effective electric field (EM) and the dynamical pressure (PDYN). The PC index has a delayed and damped response to EM variations and display saturation-like effects for EM values exceeding 10 mV/m. Steady or slowly varying levels of solar wind dynamical pressure have little or no impact on the PC index above the effects related to EM for which the solar wind velocity is also a factor. Sharp increases in the dynamical pressure generate impulsive variations in the PC index comprising a initial negative impulse of 5–10 min duration followed by a positive impulse lasting 10–20 min. Typical amplitudes of both the negative and the positive impulses are 0.2–0.5 units. A sharp decrease in the pressure produces the inverse sequence of pulses in the PC index. Auroral substorm activity represented by the AL index level has a marked influence on the average PC/EM level at the transition from very quiet (AL0 nT) to disturbed conditions while more or less disturbed conditions (AL<−100 nT) have no systematic effect on the average PC/EM values. At distinct substorm events the PC/EM ratio has a minimum (0.8) in the pre-onset phase at around 20 min before substorm onset. The average ratio gradually increases in the expansion phase to reach a maximum value (1.1) at around 40 min after substorm onset (or 20 min after the largest (negative) peak in AL). At substorm recovery during the next 2 h the PC/EM ratio decreases. Finally, we report on the application of polar magnetic variations to model the disturbance storm time (Dst) index development during magnetic storms by using the PC index as a source function to quantify the energy input to the ring current representing accumulated storm energy and characterized by the Dst index.  相似文献   

14.
A complex of geophysical phenomena (geomagnetic pulsations in different frequency ranges, VLF emissions, riometer absorption, and auroras) during the initial phase of a small recurrent magnetic storm that occurred on February 27–March 2, 2008, at a solar activity minimum has been analyzed. The difference between this storm and other typical magnetic storms consisted in that its initial phase developed under a prolonged period of negative IMF B z values, and the most intense wave-like disturbances during the storm initial phase were observed in the dusk and nighttime magnetospheric sectors rather than in the daytime sector as is observed in the majority of cases. The passage of a dense transient (with N p reaching 30 cm−3) in the solar wind under the southward IMF in the sheath region of the high-speed solar wind stream responsible for the discussed storm caused a great (the AE index is ∼1250 nT) magnetospheric substorm. The appearance of VLF chorus, accompanied by riometer absorption bursts and Pc5 pulsations, in a very long longitudinal interval of auroral latitudes (L ∼ 5) from premidnight to dawn MLT hours has been detected. It has been concluded that a sharp increase in the solar wind dynamic pressure under prolonged negative values of IMF B z resulted in the global (in longitude) development of electron cyclotron instability in the Earth’s magnetosphere.  相似文献   

15.
Using the auroral boundary index derived from DMSP electron precipitation data and the Dst index, changes in the size of the auroral belt during magnetic storms are studied. It is found that the equatorward boundary of the belt at midnight expands equatorward, reaching its lowest latitude about one hour before Dst peaks. This time lag depends very little on storm intensity. It is also shown that during magnetic storms, the energy of the ring current quantified with Dst increases in proportion to Le–3, where Le is the L-value corresponding to the equatorward boundary of the auroral belt designated by the auroral boundary index. This means that the ring current energy is proportional to the ion energy obtained from the earthward shift of the plasma sheet under the conservation of the first adiabatic invariant. The ring current energy is also pronortional to Emag, the total magnetic field energy contained in the spherical shell bounded by Le and Leq, where Leq corresponds to the quiet-time location of the auroral precipitation boundary. The ratio of the ring current energy ER to the dipole energy Emag is typically 10%. The ring current leads to magnetosphere inflation as a result of an increase in the equivalent dipole moment.  相似文献   

16.
The level of wave geomagnetic activity in the morning and daytime sectors of auroral latitudes during strong magnetic storms with Dst min varying from ?100 to ?150 nT in 1995–2002 have been studied using a new ULF index of wave activity proposed in [Kozyreva et al., 2007]. It has been detected that daytime Pc5 pulsations (2–6 mHz) are most intense during the main phase of a magnetic storm rather than during the recovery phase as was considered previously. It has been indicated that morning geomagnetic pulsations during the substorm recovery phase mainly contribute to daytime wave activity. The appearance of individual intervals with the southward IMF B z component during the magnetic storm recovery phase results in increases in the ULF index values.  相似文献   

17.
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   

18.
We report on the response of high-latitude ionospheric convection during the magnetic storm of March 20–21 1990. IMP-8 measurements of solar wind plasma and interplanetary magnetic field (IMF), ionospheric convection flow measurements from the Wick and Goose Bay coherent radars, EISCAT, Millstone Hill and Sondrestrom incoherent radars and three digisondes at Millstone Hill, Goose Bay and Qaanaaq are presented. Two intervals of particular interest have been identified. The first starts with a storm sudden commencement at 2243 UT on March 20 and includes the ionospheric activity in the following 7 h. The response time of the ionospheric convection to the southward turning of the IMF in the dusk to midnight local times is found to be approximately half that measured in a similar study at comparable local times during more normal solar wind conditions. Furthermore, this response time is the same as those previously measured on the dayside. An investigation of the expansion of the polar cap during a substorm growth phase based on Faraday’s law suggests that the expansion of the polar cap was nonuniform. A subsequent reconfiguration of the nightside convection pattern was also observed, although it was not possible to distinguish between effects due to possible changes in By and effects due to substorm activity. The second interval, 1200–2100 UT 21 March 1990, included a southward turning of the IMF which resulted in the Bz component becoming -10 nT. The response time on the dayside to this change in the IMF at the magnetopause was approximately 15 min to 30 min which is a factor of \sim2 greater than those previously measured at higher latitudes. A movement of the nightside flow reversal, possibly driven by current systems associated with the substorm expansion phases, was observed, implying that the nightside convection pattern can be dominated by substorm activity.  相似文献   

19.
The evolutions of severe geomagnetic storms (Dst<−200 nT) during solar cycle 23 were examined. For each storm, certain timing landmarks (starting of increases of interplanetary total field B, its Bz component, Dst changes, etc.) were noted and from these, various antecedence intervals were calculated. It was noticed that the various delays varied in a very wide range from storm to storm. Thus, some storms had a warning of only 4 h at the ACE location, while others had a warning of up to 30 h. These variations do not depend upon the Sun–Earth transit time. Also, faster interplanetary structures do not necessarily give quicker or stronger Dst evolutions, though larger negative Bz seems to give stronger negative Dst, but not necessarily earlier.  相似文献   

20.
An artificial feed-forward neural network with one hidden layer and error back-propagation learning is used to predict the geomagnetic activity index (Dst) one hour in advance. The Bz-component and Bz, the density, and the velocity of the solar wind are used as input to the network. The network is trained on data covering a total of 8700 h, extracted from the 25-year period from 1963 to 1987, taken from the NSSDC data base. The performance of the network is examined with test data, not included in the training set, which covers 386 h and includes four different storms. Whilst the network predicts the initial and main phase well, the recovery phase is not modelled correctly, implying that a single hidden layer error back-propagation network is not enough, if the measured Dst is not available instantaneously. The performance of the network is independent of whether the raw parameters are used, or the electric field and square root of the dynamical pressure.  相似文献   

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