The evolving concept of a magnetospheric substorm

A magnetospheric substorm is an episode of energy transport and dissipation in the Earth|s ionosphere and mag- netosphere which takes place in response to a time limited increase in energy input from the solar wind to the magnetosphere. For the past few decades, scientists have tried to understand the physical processes which take place that are responsible for the substorm disturbances of the geospace environment. In this paper, The development of the substorm concept is reviewed from its origins at the beginning of the 20th century to the present time. The theoretical framework in which substorm physics is normally presented is then discussed, and an outline is given of how that framework has changed in recent times. This paper concludes by posing two questions which need to be answered if further progress is to be made in solving the substorm problem.     

Electric circuits and their generators in the earth’s magnetosphere: The concept of electric circuits as applied to the first phase of the April 6, 2000, superstorm

{1} The first phase of the superstorm on April 6, 2000 was studied based on the analogy between systems of magnetospheric currents and wire electric currents. The conventional dataset supplemented with maps of ionospheric equivalent currents (ECs) and field-aligned currents (FACs) was also used. The application of this analogy made it possible to introduce spatial R.N inhomogeneities into FAC distributions in the two-dimensional ionosphere and three types of meridional current systems, MCS-0, MCS-1, and MCS-2, providing electric coupling of three Iijima and Potemra FAC Regions. This basis was used to describe the formation and observed dynamics of ionospheric auroral electrojets and three-dimensional current systems in a disturbed magnetosphere-ionosphere system. The results the modify known paradigms of the substorm current wedge (SCW). A new important fact was noted: simultaneously with the beginning of the disturbance expansion phase due to the stepwise growth in the dynamic pressure of the solar wind (SW), the stepwise growth in the area of polar cap and in the electromagnetic energy flux coming to the magnetosphere from the SW were observed.     

From discovery to prediction of magnetospheric processes

Over the last 50 years magnetospheric research has transferred its focus from geomagnetism to space physics, or from inferring the intensity of extraterrestrial currents, through discoveries of the main plasma regions in the magnetosphere, to predicting the processes occurring in the entire solar wind–magnetosphere–ionosphere system. Relating advances in magnetospheric physics to the framework of substorm research, this review paper demonstrates that the “recent” space age since 1960s consisted of (1) an exploratory/discovery phase in which the magnetotail, the plasma sheet, and the acceleration region of auroral particles were identified, and (2) a phase of comprehensive understanding in which we have attempted to comprehend the nature and significance of the near-Earth space environment. This progress in solar-terrestrial physics has coincided with a number of new discoveries of solar and interplanetary phenomena such as magnetic clouds, coronal mass ejections and coronal holes. Computer simulation techniques have been developed to the degree that satellite observations from a very limited number of points can be used to trace and reproduce the main energy processes. We are now entering a new phase in which we hope to be able to predict the dynamic processes that take place in the solar-terrestrial environment.     

Weimer电场模式在地球磁层内的特征

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.     

Influence of Solar Wind Plasma Parameters on the Intensity of Isolated Magnetospheric Substorms

Parameters of the interplanetary magnetic field and solar wind plasma during periods of 163 isolated substorms have been studied. It is shown that the solar wind velocity V and plasma density N remain approximately constant for at least 3 h before substorm onset Т _o and 1 h after Т _o . On average, the velocity of the solar wind exhibits a stable trend toward anticorrelation with its density over the whole data array. However, the situation is different if the values of V and N are considered with respect to the intensity of substorms observed during that period. With the growth of substorm intensity, quantified as the maximum absolute value of AL index, an increase in both the solar wind plasma velocity and density, at which these substorms appear, is obsreved. It has been found that the magnitude of the solar wind dynamic pressure P is closely related to the magnetosphere energy load defined as averaged values of the Kan–Lee electric field E_KL and Newell parameter dΦ/dt averaged for 1 h interval before Т _o . The growth of the dynamic pressure is accompanied by an increase in the load energy necessary for substorm generation. This interrelation between P and values of EKL and dΦ/dt is absent in other, arbitrarily chosen periods. It is believed that the processes accompanying increasing dynamic pressure of the solar wind result in the formation of magnetosphere conditions that increasingly impede substorm generation. Thus, the larger is P, the more solar wind energy must enter the Earth’s magnetosphere during the period of the growth phase for substorm generation. This energy is later released during the period of the substorm expansion phase and creates even more intense magnetic bays.     

太阳风动量涨落激发磁层亚暴的机制

本文将太阳风涨落传输能量产生磁层亚暴的机制推广到无碰撞等离子体过程。太阳风的涨落在磁层顶激发压缩阿尔文波,并在磁尾的无碰撞等离子体中传播。尾瓣中满足条件β?1,而等离子体片中β≥1,其中β为等离子体压力与磁压之比。这样,快磁声波在尾瓣中几乎不衰减,而在等离子体片中很快衰减,将波动能量耗散在等离子体片中使等离子体加热或者粒子加速。这种机制还表明,磁尾等离子体片中的高能粒子可以由太阳风涨落动能耗散而被加速,不一定是直接源于太阳。     

Contribution of global Pc5 oscillations to magnetic disturbance during geomagnetic storms

The contribution of global magnetospheric oscillations to magnetic disturbance during magnetospheric storms is studied. The bases of magnetic data from the INTERMAGNET global network in combination with the interplanetary and intramagnetospheric measurements of the magnetic field and plasma and the sets of the Kp, Dst, and AE indices are used for this purpose. The most favorable conditions in the solar wind and magnetosphere for generation of global Pc5 have been revealed. The contribution of these oscillations to the variations in the magnetic disturbance level, characterized by the AE index, has been estimated. The findings confirm that magnetospheric MHD oscillations participate in the processes of energy income from the solar wind and energy dissipation in the magnetosphere.     

How can we solve the problem of substorms? (A review)

The works in the alternative direction of magnetospheric studies are reviewed. In contrast to the traditional approach, where the basis process is magnetic field line reconnection, transformation of kinetic energy into electromagnetic one at the bow shock front is the basis process in the proposed approach. It has been indicated that this new paradigm makes it possible to overcome the main difficulties that remained within the scope of the previous paradigm. It has been briefly demonstrated how several following processes and phenomena are explained within the scope of the new approach: (1) transformation of the solar wind kinetic energy into the electromagnetic energy; (2) electromagnetic energy transfer into the magnetosphere; (3) organization of the system of bulk currents, formation of field-aligned currents from the magnetosphere, and compatibility of these currents with the ionospheric current systems; (4) shape, value, and dynamics of the particle precipitation auroral regions; and (5) substorm expansion (auroral breakup). Other possibilities of the new approach and paradigm replacement consequences are briefly considered.     

Magnetic Pulsations: Their Sources and Relation to Solar Wind and Geomagnetic Activity

Ultra low frequency (ULF) waves incident on the Earth are produced by processes in the magnetosphere and solar wind. These processes produce a wide variety of ULF hydromagnetic wave types that are classified on the ground as either Pi or Pc pulsations (irregular or continuous). Waves of different frequencies and polarizations originate in different regions of the magnetosphere. The location of the projections of these regions onto the Earth depends on the solar wind dynamic pressure and magnetic field. The occurrence of various waves also depends on conditions in the solar wind and in the magnetosphere. Changes in orientation of the interplanetary magnetic field or an increase in solar wind velocity can have dramatic effects on the type of waves seen at a particular location on the Earth. Similarly, the occurrence of a magnetospheric substorm or magnetic storm will affect which waves are seen. The magnetosphere is a resonant cavity and waveguide for waves that either originate within or propagate through the system. These cavities respond to broadband sources by resonating at discrete frequencies. These cavity modes couple to field line resonances that drive currents in the ionosphere. These currents reradiate the energy as electromagnetic waves that propagate to the ground. Because these ionospheric currents are localized in latitude there are very rapid variations in wave phase at the Earth’s surface. Thus it is almost never correct to assume that plane ULF waves are incident on the Earth from outer space. The properties of ULF waves seen at the ground contain information about the processes that generate them and the regions through which they have propagated. The properties also depend on the conductivity of the Earth underneath the observer. Information about the state of the solar wind and the magnetosphere distributed by the NOAA Space Disturbance Forecast Center can be used to help predict when certain types and frequencies of waves will be observed. The study of ULF waves is a very active field of space research and much has yet to be learned about the processes that generate these waves.     

Low frequency 1/f-like fluctuations of the AE-index as a possible manifestation of self-organized criticality in the magnetosphere

Low frequency stochastic variations of the geomagnetic AE-index characterized by 1/f^b-like power spectrum (where f is a frequency) are studied. Based on the analysis of experimental data we show that the B_z-component of IMF, velocity of solar wind plasma, and the coupling function of Akasofu are insufficient factors to explain these behaviors of the AE-index together with the 1/f^b fluctuations of geomagnetic intensity. The effect of self-organized criticality (SOC) is proposed as an internal mechanism to generate 1/f^b fluctuations in the magnetosphere. It is suggested that localized spatially current instabilities, developing in the magnetospheric tail at the initial substorm phase can be considered as SOC avalanches or dynamic clusters, superposition of which leads to the 1/f^b fluctuations of macroscopic characteristics in the system. Using the sandpile model of SOC, we undertake numerical modeling of space-localized and global disturbances of magnetospheric current layer. Qualitative conformity between the disturbed dynamics of self-organized critical state of the model and the main phases of real magnetospheric substorm development is demonstrated. It is also shown that power spectrum of sandpile model fluctuations controlled by real solar wind parameters reproduces all distinctive spectral features of the AE fluctuations.     

Solar wind effects on ionospheric convection: a review

The solar wind, magnetosphere, and ionosphere are intrinsically coupled through magnetic field lines. The electrodynamic state of the high-latitude ionosphere is controlled by several geophysical processes, such as the location and rate of magnetic reconnection at the magnetopause and in the magnetotail, and the energisation and precipitation of solar wind and magnetospheric plasmas. Amongst the most observed ionospheric manifestation of solar wind/magnetospheric processes are the convection bursts associated with the so-called flux transfer events (FTEs), magnetic impulse events (MIEs), and travelling convection vortices (TCVs). Furthermore, the large-scale ionospheric convection configuration has also demonstrated a strong correspondence to variations in the interplanetary medium and substorm activity. This report briefly discusses the progress made over the past decade in studies of these transient convection phenomena and outlines some unsettled questions as well as future research directions.     

磁层系统的能量输入、输出与日地耦合

用１９７８年和１９８２年３６个磁暴期间的太阳风、行星际磁场（ＩＭＦ）和地磁资料，分析和检验已有的两类太阳风－磁层能量耦合函数．结果表明：Ａｋａｓｏｆｕ提出的耦合函数ε能大致地预报亚暴和磁暴的发生。ε开始起重要作用时即出现亚暴；电离层能耗达到饱和值是发生磁暴的标志。ε与磁层体系能耗之间有接近于对数量的线性关系．用１９７８－１９８６年的资料，分析环电流和极光区电离层能耗在１２１个太阳自转周内的分布表明，日面上可能存在相对持久的活动区域     

Injection of relativistic electrons into the internal magnetosphere during magnetic storms: Connection with substorms

The connection between rapid increases in the intensity of electrons with energies >0.3 MeV and magnetospheric substorms was studied for the first time by measurements of energetic electrons on the low-orbit SERVIS-1 satellite. In addition to the well-known process of radial diffusion detected at the recovery phase, the increases during a period of time no longer than 1.5 h at the main phase of six magnetic storms in a channel of 0.3–1.7 MeV (in three of them, in a channel of 1.7–3.4 MeV) were measured. An analysis of auroral zone magnetograms demonstrated that the increases occurred at the instant of magnetospheric substorm activation. A conclusion is made that the increases are caused by the radial injection of electrons by a pulse electric field induced during substorm activations. Pulse injections are shown to be one of the main mechanisms of electron radiation belt completion in the inner magnetosphere and, in combination with moderate radial diffusion, to be responsible for the appearance of large fluxes of energetic electrons (“killers”) in the magnetosphere after magnetic storms.     

Magnetospheric disturbances excited by solar wind density enhancements on April 11, 1997

Disturbances in the solar wind density, geomagnetic field, and magnetospheric plasma density and fluxes are analyzed. The disturbances have the same sign and are close to each other in time. They accompany the process of amplitude modulation of Pc1 geomagnetic pulsations during the recovery phase of the moderate magnetic storm of April 10–11, 1997. The magnetospheric disturbances were recorded by ground-based observatories and on spacecraft in all local time sectors with insignificant time delays. It is concluded that in this case variations in the geomagnetic field and magnetospheric plasma density are primary, whereas the amplitude modulation of Pc1, 2 is a secondary manifestation of fast magnetosonic (FMS) waves that are generated during the interaction between the magnetosphere and solar wind density irregularities.     

阿尔文涨落与地球磁层亚暴

本文讨论了一种地球磁层的亚暴机制。当行星际磁场有大的南向分量时,磁层的位形可由基本闭式转变为开式。磁鞘中的阿尔文波可以携带超过10~(18)尔格/秒的能流传入磁层尾部,并将能量耗散于等离子体片中。等离子体片中的粒子被加热和加速后,注入近地空间,产生环电流和极区亚暴。计算了剪切流场中阿尔文波的传播过程,以及磁层中阿尔文波的耗散。将本文的结算与[4]中的结果合在一起,可以说明当行星际磁场转向南时,容易发生地球磁层亚暴,但这两者并非一一对应的关系,行星际磁场没有南向分量时也可以发生地球磁层亚暴。     

Planetary features of aurorae: Results of the IGY (a review)

In the period of the International Geophysical Year (IGY), almost the entire planet was covered for the first time by ground-based geophysical observations. Their analysis led to two fundamental results: the existence of the auroral oval and auroral (magnetospheric) substorm. At the final stage of the IGY, satellite explorations of the near-Earth space began. The auroral luminosity appeared to be related to the plasma structure of the magnetosphere. That opened new possibilities for parameters diagnostics of the Earth’s magnetosphere on the basis of ground-based aurora observations. The concepts of auroral oval and magnetospheric substorm became paradigms of the new science of solar-terrestrial physics.     

High-latitude geomagnetic disturbances during the initial phase of a recurrent magnetic storm (from February 27 to March 2, 2008)

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.     

Planetary distribution of geomagnetic pulsations during a geomagnetic storm at solar minimum

We investigate the features of the planetary distribution of wave phenomena (geomagnetic pulsations) in the Earth’s magnetic shell (the magnetosphere) during a strong geomagnetic storm on December 14–15, 2006, which is untypical of the minimum phase of solar activity. The storm was caused by the approach of the interplanetary magnetic cloud towards the Earth’s magnetosphere. The study is based on the analysis of 1-min data of global digital geomagnetic observations at a few latitudinal profiles of the global network of ground-based magnetic stations. The analysis is focused on the Pc5 geomagnetic pulsations, whose frequencies fall in the band of 1.5–7 mHz (T ～ 2–10 min), on the fluctuations in the interplanetary magnetic field (IMF) and in the solar wind density in this frequency band. It is shown that during the initial phase of the storm with positive IMF Bz, most intense geomagnetic pulsations were recorded in the dayside polar regions. It was supposed that these pulsations could probably be caused by the injection of the fluctuating streams of solar wind into the Earth’s ionosphere in the dayside polar cusp region. The fluctuations arising in the ionospheric electric currents due to this process are recorded as the geomagnetic pulsations by the ground-based magnetometers. Under negative IMF Bz, substorms develop in the nightside magnetosphere, and the enhancement of geomagnetic pulsations was observed in this latitudinal region on the Earth’s surface. The generation of these pulsations is probably caused by the fluctuations in the field-aligned magnetospheric electric currents flowing along the geomagnetic field lines from the substorm source region. These geomagnetic pulsations are not related to the fluctuations in the interplanetary medium. During the main phase of the magnetic storm, when fluctuations in the interplanetary medium are almost absent, the most intense geomagnetic pulsations were observed in the dawn sector in the region corresponding to the closed magnetosphere. The generation of these pulsations is likely to be associated with the resonance of the geomagnetic field lines. Thus, it is shown that the Pc5 pulsations observed on the ground during the magnetic storm have a different origin and a different planetary distribution.     

Multiscale phenomena in the near-Earth magnetosphere

Several studies on the scaling properties of the near-Earth magnetosphere and auroral phenomena are reviewed. These studies employ modern analysis techniques that include fractal, multifractal, wavelet, wavelet bicoherence, and sign-singularity analyses as well as cellular automaton simulations of sandpile and avalanches. The results provide strong evidence for the multiscale, cross-scale coupling, and reorganization nature of auroral and magnetospheric phenomena, suggesting the possibility that the magnetosphere is in a forced and/or self organized critical state. Signatures of inverse cascade are found in magnetic fluctuations in current disruption events, which may indicate large-scale substorm features such as substorm current wedge and plasmoid may be evolved from small-scale plasma turbulence structures. Insights gained from these studies help to discriminate the existing competing substorm models. The multiscale properties of magnetospheric substorms are consistent with substorm models with intrinsic multiscale processes and not with substorm models with only a macroscopic process.     

Response of currents in Earth’s and Saturn’s dayside magnetopause to a sudden change in the solar wind density

When the effect of a solar wind dynamic pressure pulse on the magnetospheric and ionospheric dynamics is studied, it is usually difficult to detect the effect of a sudden change in the density against the background of the other varying solar wind parameters, which often play a most pronounced role. Cases in which the solar wind plasma density gradient dominated in the dynamics of the different parameters of an interplanetary medium and its magnetic field are considered in this work. Variations in the Earth’s dayside magnetopause current caused by a change in the solar wind ion density are presented for two such cases (February 11 and January 11, 1997) based on the method developed by us previously. Variations in the dayside magnetopause current for collisions of the magnetosphere with corotating interacting flows in January 2004, studied in detail by us previously, are also presented for Saturn. The estimates are comparable with the current values in the transitional three-dimensional current systems of Saturn that were previously calculated by us.