Precipitation of energetic magnetospheric electrons and accompanying solar wind characteristics

From 1957 up to the present time, the Lebedev Physical Institute (LPI) has performed regular monitoring of ionizing radiation in the Earth’s atmosphere. There are cases when the X-ray radiation generated by energetic magnetospheric electrons penetrates the atmosphere and is observed at polar latitudes. The vast majority of these events occurs against the background of high-velocity solar wind streams, while magnetospheric perturbations related to interplanetary coronal mass ejections (ICMEs) are noneffective for precipitation. It is shown in the paper that ICMEs do not cause acceleration of a sufficient amount of electrons in the magnetosphere. Favorable conditions for acceleration and subsequent scattering of electrons into the loss cone are created by magnetic storms with an extended recovery phase and with sufficiently frequent periods of negative Bz component of the interplanetary magnetic field (IMF). Such geomagnetic perturbations are typical for storms associated with high-velocity solar wind streams.     

On the relationship between magnetic clouds and the great geomagnetic storms associated with the period 1995–2006

The fact that magnetic clouds are one of the main sources causing geomagnetic storms is a well-established fact. One of the issues is to establish those features of magnetic clouds determinant in the intensity of the Dst corresponding to geomagnetic storms. We examine measurements of geoeffective magnetic clouds during the period 1995–2006 providing geomagnetic storms with Dst indexes lower than ?100 nT. These involve 46 geomagnetic storm events. After establishing the different characteristics of the magnetic clouds (plasma velocity, maximum magnetic intensity, etc.) we show some results about the correlations found among them and the storms intensity, finding that maximum magnetic field magnitude is a determinant factor to establish the importance of magnetic clouds in generating geomagnetic storms, having a correlation as good as the electric convective field.     

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.     

Geomagnetic activity and wind velocity during the Maunder minimum

Following a given classification of geomagnetic activity, we obtained aa index values for the Maunder minimum (1645–1715). It is found that the recurrent and fluctuating activities were not appreciable and that the shock activity levels were very low. The aa index level was due almost entirely to the quiet days. Calculated average solar-wind velocities were 194.3 km s^–1 from 1657 to 1700 and 218.7 km s^–1 from 1700 onwards. Also, the coronal magnetic field magnitude and southward interplanetary magnetic field component Bz were lower. It is concluded that the nearly absent levels of geomagnetic activity during this period were due to lower coronal and Bz magnetic field magnitudes as well as to the continuous impinging on the Earth of a slow wind.     

Interplanetary phenomena associated with very intense geomagnetic storms

The dominant interplanetary phenomena that are frequently associated with intense magnetic storms are the interplanetary manifestations of fast coronal mass ejections (CMEs). Two such interplanetary structures, involving an intense and long duration B_s component of the IMF are: the sheath region behind a fast forward interplanetary shock, and the CME ejecta itself. Frequently, these structures lead to the development of intense storms with two-step growth in their main phases.These structures, when combined, lead sometimes to the development of very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary structure accompanying another stream. The second stream can also compress the primary cloud, intensifying the B_s field, and bringing with it an additional B_s structure. Thus, at times very intense storms are associated with three or more B_s structures.Another aspect that can contribute to the development of very intense storms refers to the recent finding that magnetic clouds with very intense core magnetic fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense magnetospheric energization.     

Geophysical effects of the interplanetary magnetic cloud on October 18–19, 1995, as deduced from observations at Irkutsk

During an interaction of the Earth’s magnetosphere with the interplanetary magnetic cloud on October 18–19, 1995, a great magnetic storm took place. Extremely intense disturbances of the geomagnetic field and ionosphere were recorded at the midlatitude observatory at Irkutsk (Φ′≈45°, Λ′≈177°, L≈2) in the course of the storm. The most important storm features in the ionosphere and magnetic field are: a significant decrease in the geomagnetic field Z component during the storm main phase; unusually large amplitudes of geomagnetic pulsations in the Pi1 frequency band; extremely low values of critical frequencies of the ionospheric F2-layer; an appearance of intense E_s-layers similar to auroral sporadic layers at the end of the recovery phase. These magnetic storm manifestations are typical for auroral and subauroral latitudes but are extremely rare in middle latitudes. We analyze the storm-time midlatitude phenomena and attempt to explore the magnetospheric storm processes using the data of ground observations of geomagnetic pulsations. It is concluded that the dominant mechanism responsible for the development of the October 18–19, 1995 storm is the quasi-stationary transport of plasma sheet particles up to L≈2 shells rather than multiple substorm injections of plasma clouds into the inner magnetosphere.     

Quantitative estimation of local geomagnetic activity relative to the level of the magnetically quiet period in 2009

A new method for determining geomagnetic activity based on calculation of the hourly amplitudes of geomagnetic field variations at ground-based observatories has been developed. Observations performed in 2009, when unusually low solar and geomagnetic activity was registered, were used as a reference level. The described method was used to estimate the energy of local geomagnetic activity; such energy is estimated for observatories in the Earth’s Northern and Southern hemispheres, and a total estimation is made for both hemispheres and for the entire Earth’s surface during large magnetic storms. These are used to compare characteristics of magnetic storm intensity based on the classical Kp and Dst indices and calculated energy estimate.     

地球附近第23太阳活动周磁云和非磁云ICME的对比统计

行星际日冕物质抛射（ICME）,作为影响地球空间天气的重要源头之一,根据其磁场结构特点可分为磁云（MC）和非磁云ICME两个子集.本文对第23周的磁云和非磁云ICME结构及其地磁效应进行对比统计研究.第23周ICME事件总数为317个,其中磁云占ICME比例为33.75%,非磁云ICME占66.25%.统计结果表明,非磁云ICME数与太阳黑子数呈现出非常好的正相关性,而磁云与太阳黑子数的这种相关性并不明显.相反,磁云占ICME的比率与太阳黑子数呈现出一定的反相关性.对磁云与非磁云ICME引起的地磁暴的比较研究表明：磁云及其鞘区引发的地磁暴平均水平要高于非磁云ICME及其鞘区.磁云和非磁云ICME的磁场强度、南向磁场强度和传播速度整体上都随地磁暴水平提升而增加.对磁云与非磁云ICME参数的进一步对比分析表明,磁云及其鞘区的平均磁场强度和南向磁场分量平均值都明显要比非磁云ICME的大;而二者的等离子体温度、密度和速度平均值相差并不明显.     

Daytime geomagnetic disturbances at high latitudes during a strong magnetic storm of June 21–23, 2015: The storm initial phase

The high-latitude geomagnetic effects of an unusually long initial phase of the largest magnetic storm (SymH ~–220 nT) in cycle 24 of the solar activity are considered. Three interplanetary shocks characterized by considerable solar wind density jumps (up to 50–60 cm^–3) at a low solar wind velocity (350–400 km/s) approached the Earth’s magnetosphere during the storm initial phase. The first two dynamic impacts did not result in the development of a magnetic storm, since the IMF Bz remained positive for a long time after these shocks, but they caused daytime polar substorms (magnetic bays) near the boundary between the closed and open magnetosphere. The magnetic field vector diagrams at high latitudes and the behaviour of high-latitude long-period geomagnetic pulsations (ipcl and vlp) made it possible to specify the dynamics of this boundary position. The spatiotemporal features of daytime polar substorms (the dayside polar electrojet, PE) caused by sudden changes in the solar wind dynamic pressure are discussed in detail, and the singularities of ionospheric convection in the polar cap are considered. It has been shown that the main phase of this two-stage storm started rapidly developing only when the third most intense shock approached the Earth against a background of large negative IMF Bz values (to–39 nT). It was concluded that the dynamics of convective vortices and the related restructing of the field-aligned currents can result in spatiotemporal fluctuations in the closing ionospheric currents that are registered on the Earth’s surface as bay-like magnetic disturbances.     

Correlation between the near-Earth solar wind parameters and the source surface magnetic field

The solar magnetic field B _s at the Earth’s projection onto the solar-wind source surface has been calculated for each day over a long time interval (1976–2004). These data have been compared with the daily mean solar wind (SW) velocities and various components of the interplanetary magnetic field (IMF) near the Earth. The statistical analysis has revealed a rather close relationship between the solar-wind parameters near the Sun and near the Earth in the periods without significant sporadic solar and interplanetary disturbances. Empirical numerical models have been proposed for calculating the solar-wind velocity, IMF intensity, and IMF longitudinal and B _z components from the solar magnetic data. In all these models, the B _s value plays the main role. It is shown that, under quiet or weakly disturbed conditions, the variations in the geomagnetic activity index Ap can be forecasted for 3–5 days ahead on the basis of solar magnetic observations. Such a forecast proves to be more reliable than the forecasts based on the traditional methods.     

Energy balance during intense and super-intense magnetic storms using an Akasofu ε parameter corrected by the solar wind dynamic pressure

Geomagnetic storms are large disturbances in the Earth's magnetosphere caused by enhanced solar wind–magnetosphere energy transfer. One of the main manifestations of a geomagnetic storm is the ring current enhancement. It is responsible for the decrease in the geomagnetic field observed at ground stations. In this work, we study the ring current dynamics during two different levels of magnetic storms. Thirty-three events are selected during the period 1981–2004. Eighteen out of 33 events are very intense (or super-intense) magnetic storms (Dst ⩽−250 nT) and the remaining are intense magnetic storms (−250<Dst ⩽−100 nT). Interplanetary data from spacecraft in the solar wind near Earth's orbit (ACE, IMP-8, ISEE-3) and geomagnetic indices (Dst and Sym-H) are analyzed. Our aim is to evaluate the interplanetary characteristics (interplanetary dawn–dusk electric field, interplanetary magnetic field component B_S), the ε parameter, and the total energy input into the magnetosphere (Wε) for these two classes of magnetic storms. Two corrections on the ε energy coupling function are made: the first one is an already known correction in the magnetopause radius to take into account the variation in the solar wind pressure. The second correction on the Akasofu parameter, first proposed in this work, accounts for the reconnection efficiency as a function of the solar wind ram pressure. Geomagnetic data/indices are also employed to study the ring current dynamics and to search for the differences in the storm evolution during these events. Our corrected ε parameter is shown to be more adequate to explain storm energy balance because the energy input and the energy dissipated in the ring current are in better agreement with modern estimates as compared with previous works. For super-intense storms, the correction of the Akasofu ε is on average a scaling factor of 3.7, whilst for intense events, this scaling factor is on average 3.4. The injected energy during the main phase using corrected ε can be considered a criterion to separate intense from very intense storms. Other possibilities of cutoff values based on the energy input are also investigated. A threshold value for the input energy is much more clear when a new classification on Dst=−165 nT is considered. It was found that the energy input during storms with Dst<−165 nT is double of the energy for storms with Dst>−165 nT.     

Understanding CMEs and their source regions

CMEs are an important aspect of coronal and interplanetary dynamics. They can eject large amounts of mass and magnetic fields into the heliosphere which can drive large geomagnetic storms and interplanetary shocks, a key source of solar energetic particles. However, our knowledge of the origins and early development of CMEs at the Sun is limited. CMEs are most frequently associated with erupting prominences and long-enduring X-ray arcades, but sometimes with weak or no observed surface activity. I review some of the well-determined coronal properties of CMEs and what we know about their source regions, including recent studies using Yohkoh, SOHO and radio data. One exciting, new type of observation is of halo-like CMEs which suggest the launch of a geoeffective disturbance toward Earth. Besides their utility for forecasting the arrival at Earth of magnetic clouds and geomagnetic storms, halo CMEs are important for understanding the development and internal structure of CMEs since we can view their source regions near Sun center and can measure their in-situ characteristics along their central axes.     

2007-2012年Dstmin≤-50nT的中等以上地磁暴的行星际源统计

地磁暴的行星际源研究是了解及预报地磁暴的关键因素之一.本文研究了2007-2012年间的所有Dst_min ≤-50 nT的中等以上地磁暴,建立了这些地磁暴及其行星际源的列表.在这6年中,共发生了51次Dst_min≤-50 nT的中等以上地磁暴,其中9次为Dst_min≤-100 nT的强地磁暴事件.对比上一活动周相同时间段发现,在这段太阳活动极低的时间,地磁暴的数目显著减少.对这些地磁暴行星际源的分析表明,65%的中等以上地磁暴由与日冕物质抛射相关的行星际结构引起,31%的地磁暴由共转相互作用区引起,这与以前的结果一致.特别的,在这个太阳活动极低时期内,共转相互作用区没有引起Dst_min≤-100 nT的强地磁暴,同时,日冕物质抛射相关结构也没有引起Dst_min≤-200 nT的超强地磁暴.以上结果表明极低太阳活动同时导致了共转相互作用区和日冕物质抛射地磁效应的减弱.进一步,分析不同太阳活动期间地磁暴的行星际源发现:在太阳活动低年(2007-2009年),共转相互作用区是引起地磁暴的主要原因; 而在太阳活动上升期和高年(2010-2013年),大部分(75%,30/40)的中等以上地磁暴均由日冕物质抛射相关结构引起.     

The relation between solar activity and orientation of the solar wind electric field relative to the Earth’s magnetic moment

The relation of the Kp index of geomagnetic activity to the solar wind electric field (E _SW) and the projection of this field onto the geomagnetic dipole has been estimated. An analysis indicated that the southward component of the IMF vector (B _z < 0) is the main geoeffective parameter, as was repeatedly indicated by many researchers. The presence of this component in any combinations of the interplanetary medium parameters is responsible for a high correlation between such combinations and geomagnetic activity referred to by the authors of different studies. Precisely this field component also plays the main role in the relation between the Kp index and the relative orientation of E _SW and the Earth’ magnetic moment.     

磁暴主相多阶发展行星际起因的初步分析

本文利用1998～2006年与磁云有关的80起中强磁暴(Dst*≤-50 nT),对其主相期间不同发展阶数磁暴的行星际起因进行了统计分析.重点研究了鞘区磁场单独作用、磁云本体单独作用、鞘区与磁云共同作用以及其他复杂行星际结构在磁暴主相多阶发展中的相对重要性,并对导致磁暴主相增加一阶的行星际起因做了初步分析.统计结果表明...     

Effect of magnetic storms in variations in the atmospheric electric field at midlatitudes

The observations of the variations in the vertical component of the atmospheric electric field (E _z ) at Swider midlatitude Poland observatory (geomagnetic latitude 47.8°) under the conditions of fair weather during 14 magnetic storms have been analyzed. The effect of the magnetic storm main phase in the daytime midlatitude variations in E _z in the absence of local geomagnetic disturbances has been detected for the first time. Considerable (～100–300 V m^?1) decreases in the electric field strength (E _z ) at Swider observatory were observed in daytime simultaneously with the substorm onset in the nighttime sector of auroral latitudes (College observatory). The detected effects indicate that an intensification of the interplanetary electric field during the magnetic storm main phase, the development of magnetospheric substorms, and precipitation of energetic electrons into the nighttime auroral ionosphere can result in considerable disturbances in the midlatitude atmospheric electric field.     

Dynamics of magnetospheric current systems during magnetic storms of different intensity

The dynamics of the magnetospheric magnetic field during the magnetic storms of different intensity has been studied. The magnetic field variations on the Earth’s surface were calculated using the paraboloid model of the magnetosphere, taking into account the induction currents flowing in the diamagnetically conductive Earth. Dst and its components have been calculated for ten magnetic storms. It has been indicated that relative contributions of magnetospheric sources to Dst change depending on the storm power. For weak and moderate storms, the contribution of the magnetotail current sheet can reach values comparable with the ring current contribution and, sometimes, can even exceed this contribution. For strong storms, the ring current field dominates over the tail current field, the absolute value of which does not exceed 150 nT (also achieved during less intense storms). For storms with minimum Dst exceeding-200 nT, the tail current field is saturated, whereas the ring current can continue developing.     

Polar and high latitude substorms and solar wind conditions

All substorm disturbances observed in polar latitudes can be divided into two types: polar, which are observable at geomagnetic latitudes higher than 70° in the absence of substorms below 70°, and high latitude substorms, which travel from auroral (<70°) to polar (>70°) geomagnetic latitudes. The aim of this study is to compare conditions in the IMF and solar wind, under which these two types of substorms are observable on the basis of data from meridional chain of magnetometers IMAGE and OMNI database for 1995, 2000, and 2006–2011. In total, 105 polar and 55 high latitude substorms were studied. It is shown that polar substorms are observable at a low velocity of solar wind after propagation of a high-speed recurrent stream during the late recovery phase of a magnetic storm. High latitude substorms, in contrast, are observable with a high velocity of solar wind, increased values of the Bz component of the IMF, the Ey component of the electric field, and solar wind temperature and pressure, when a high-speed recurrent stream passes by the Earth.     

Phase shift (time) between storm-time maximum negative excursions of geomagnetic disturbance index Dst and interplanetary Bz

The evolutions of severe geomagnetic storms (D_st<−200 nT) during solar cycle 23 were examined. For each storm, certain timing landmarks (starting of increases of interplanetary total field B, its B_z component, D_st 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 D_st evolutions, though larger negative B_z seems to give stronger negative D_st, but not necessarily earlier.     

Chaotic behaviour of interplanetary magnetic field under various geomagnetic conditions

In the present study, the deterministic chaotic behaviour of interplanetary magnetic field (IMF) under various geomagnetic conditions of low and high solar active periods was analyzed, using the time series of IMF |B| and B_z, by employing chaotic quantifiers like, Lyapunov exponent, Tsallis entropy, correlation dimension, and non-linear prediction error. We have investigated whether the chaotic behaviour of interplanetary magnetic field would modify, when it produces major geomagnetic storms, and how it depends on the phase of solar activity. The yearly average values of Lyapunov exponent for the time series of IMF |B| and B_z, show solar flux dependence, whereas those values of entropy, correlation dimension and non-linear prediction error had no significant solar flux dependence. The yearly average values of entropy for quiet periods are higher compared to those values for major storm periods belonging to low/high solar active conditions, for both the time series |B| and B_z.