Specific features of the characteristics of midlatitude substorm and nonsubstorm Pi2 geomagnetic pulsations and their generation conditions

The specific features of the diurnal and seasonal variations in different characteristics of two Pi2 geomagnetic pulsation groups (observed during magnetospheric substorms and when these substorms are absent) and the pulsation generation geophysical conditions have been experimentally studied based on observations at the Borok midlatitude observatory. It has been indicated that the dynamics of the occurrence frequency of Pi2 substorm and nonsubstorm bursts and their amplitude, duration, and intervals between peaks depending on the local time and season is identical in many respects. It has been found that substorm Pi2 bursts are mostly observed when the IMF is sunward and the solar wind electric field (Ey) is positive, whereas nonsubstorm bursts are observed when the IMF is antisunward and Ey is negative. The fundamental differences in the diurnal and seasonal variations in index α, which characterizes the slope of the distribution function of the two-group Pi2 burst amplitudes, have been revealed. It has been found that the index α value substantially depends on Ey and the IMF longitude (ψ). It has been assumed that the plasma sheet turbulence of the metastable magnetotail is responsible for reconnection and the generation of substorm and nonsub-storm Pi2 pulsation bursts.     

Effect of solar dynamics parameters on the formation of substorm activity

An algorithm for retrieving the AL index dynamics from the parameters of solar-wind plasma and the interplanetary magnetic field (IMF) has been developed. Along with other geoeffective parameters of the solar wind, an integral parameter in the form of the cumulative sum Σ[N*V ²] is used to determine the process of substorm formation. The algorithm is incorporated into a framework developed to retrieve the AL index of an Elman-type artificial neural network (ANN) containing an additional layer of neurons that provides an “internal memory” of the prehistory of the dynamical process to be retrieved. The ANN is trained on data of 70 eight-hour-long time intervals, including the periods of isolated magnetospheric substorms. The efficiency of this approach is demonstrated by numerical neural-network experiments on retrieving the dynamics of the AL index from the of solar wind and IMF parameters during substorms.     

Investigation of isolated substorms: Generation conditions and characteristics of different phases

Characteristics of isolated substorms selected by variations in the 1-min values of the AL index are analyzed. The substorms were divided into several types with respect to the behavior of the Bz component of the interplanetary magnetic field (IMF) during the expansion phase. The probability of observations of substorms associated with the northward turn of the Bz component of IMF was ~19%, while the substorms taking place at Bz < 0 were observed in 53% of cases. A substantial number of events in which no substorm magnetic activity was observed in the auroral zone after a long (>30 min) period of the southward IMF and a following sharp turn of the Bz component of IMF before the north was detected. The data suggest that a northward IMF turn is neither a necessary nor sufficient condition for generating substorms. It has been shown for substorms of the both types that the average duration of the southward IMF to moment T ₀ and the average intensity of the magnetic perturbation in the maximum are approximately the same and amount to ~80 min and–650 nT, respectively. However, for substorms at Bz < 0, their mean duration, including the expansive and recovery phases, is on average 30 min longer than that at a northward turn of IMF. Correlations between the loading–unloading processes in the magnetosphere in the periods of magnetospheric substorms were investigated with different functions that determine the efficiency of the energy transfer from the solar wind to the magnetosphere. It has been shown that the highest correlation coefficient (r = 0.84) is observed when the function suggested by Newell et al. (2007) is used. It has been detected that a simple function VB _S yields a high correlation coefficient (r = 0.75).     

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.     

Large Magnetic Storm on September 7–8, 2017: High-Latitude Geomagnetic Variations and Geomagnetic <Emphasis Type="Italic">Pc</Emphasis>5 Pulsations

The features of daytime high-latitude geomagnetic variations and geomagnetic pulsations in the Рс5 range during the recent, large, two-stage magnetic storm of September 7–8, 2017 are studied. The discussed disturbances were observed at the recovery phase of the first stage of the storm after the interplanetary magnetic field (IMF) turned northward. It is shown that the large sign-alternating variations in Ву and Bz components of the IMF caused intense geomagnetic disturbances up to 300–400 nT with a quasi-period of ~20 min in the daytime sector of polar latitudes, probably in the region of the daytime polar cusp. These disturbances may have reflected quasi-period motions of the daytime magnetopause and may have resulted from nonlinear transformation of the variations in the interplanaterary magnetic field in the magnetosheath or in the magnetospheric entry layers. The appearance of high-latitude long-period variations was accompanied by the excitation of bursts (wave packets) of geomagnetic Pc5 pulsations. The onset of Pc5 pulsation bursts often coincided with a sudden northward turn of the IMF. It was discovered for the first time that the development of a “daytime polar substorm,” i.e., a negative magnetic bay in the daytime sector of polar latitudes, led to a sudden termination of the generation of geomagnetic Pc5 pulsations over the entire latitude range in which these oscillations were recorded before the appearance of the daytime bay.     

Increase in the critical frequency of the ionospheric <Emphasis Type="Italic">F</Emphasis> region prior to the substorm expansion phase

Specific variations in the critical frequency of the ionospheric F ₂ layer during magnetospheric substorms have been found based on the data of vertical sounding stations in Europe and North America. Maximal attention has been paid to the positive peaks of ΔfoF2 with a duration of 6–8 h before the beginning of the substorm expansion phase (T ₀). The possible physical mechanisms by which these peaks are formed (related to the impact of fast particles in the foreshock region of the solar wind on the Earth’s magnetosphere and different for middle and high latitudes) have been considered. The positive peaks of ΔfoF2 can be used in a short-term prediction of the ionospheric disturbance onset and space weather on the whole.     

High Latitude Magnetic Impulse Events (MIEs): Polarization Peculiarities and Excitation Conditions

Polarization characteristics (polarization type, ellipticity ε, tilt angle τ of the polarization ellipse’s major axis) of high-latitude magnetic impulse events (MIEs) observed at the latitude of the dayside polar cusp are studied. It is established that all impulses are elliptically polarized, being right-polarized in 43% of cases (R-type) and left-polarized in 57% of cases (L-type). The right-polarized MIEs on the ground are more pronounced in the azimuthal direction, whereas the left-polarized events are more clearly marked in the meridional direction. The MIEs of both polarization types have the properties of intermittent processes. It is shown that diurnal and seasonal variations in the occurrence frequency and amplitudes of the events depend significantly on the type of their polarization. The R- and L-type impulse events are predominantly observed during the descending and ascending phase of the solar cycle, respectively. Solar wind high-speed streams (HSSs) are more favorable for exciting right-polarized impulses, whereas left-polarized impulse events are more efficiently excited by coronal mass ejection (CME). It is established that R-type impulses emerge in the conditions when the orientation of the interplanetary magnetic field vector is close to the radial direction against the development of moderate magnetospheric substorms whereas the L-type impulses appear when IMF is perpendicular to the Sun–Earth line in the absence of substorms. The behavior of the characteristics of impulse events significantly depends on the value of the IMF Bz-component and on the angle θ_xB = arccos(Bx/B). It is conjectured that excitation of the two groups of impulses is caused by the IMF structures in the solar wind stream with the characteristic configuration in the ecliptic plane, which determine the polarization type and properties of MIEs.     

Effects of geomagnetic disturbances in daytime variations of the atmospheric electric field in polar regions

We have studied the influence of daytime polar substorms (geomagnetic bays under the IMF Bz > 0) on variations of the vertical gradient of the atmospheric electric field potential (Ez) observed at the Polish Hornsund Station (Svalbard, Norway). Only the observations of Ez under “fair weather” conditions were used, i.e. in the absence of strong wind, precipitations, low cloud cover, etc. We studied more than 20 events of daytime polar substorms registered by the Scandinavian chain of IMAGE magnetometers in 2010–2014 during the “fair weather” periods at the Hornsund Station. Analysis of the observations showed that Ez significantly deviates from the its background variations during daytime, as a rule, when the Hornsund Station is in the region of projection of the daytime auroral oval, the position of which was determined from OVATION data. It was shown that the development of a daytime polar substorm leads to fluctuating enhance of Ez values. It was found that Ez surges are accompanied by intensification of field-aligned electric currents outflowing from the ionosphere, which were calculated from the data of low-orbit communication satellites of the AMPERE project.     

Effect of the IMF azimuthal component on the longitudinal position of the westward electrojet intensity maximum during strong geomagnetic storms

The distribution of the ionospheric currents during the geomagnetic storms of November 20–21, 2003, November 7–8, 2004, and November 9–10, 2004, depending on the IMF B _y component, has been studied based on the data from the global network of magnetic stations. It has been indicated that, during geomagnetic disturbances, the westward electrojet intensity maximum is localized in the evening sector at IMF B _y < 0 and in the morning sector at IMF B _y > 0. The region of the westward electrojet intensity maximum shifts to morning hours with increasing positive B _y values. Thus, the IMF azimuthal component forms not only the magnetospheric convection pattern during magnetic storms but is also responsible for the longitudinal position of ionospheric structures.     

Magnetospheric-ionospheric convection in an open model of the magnetosphere

Magnetospheric-ionospheric convection has been calculated for an open model of the magnetosphere with an ellipsoidal magnetopause in an approximation of uniform IMF. It is assumed that only 0.1 part of IMF falls in the magnetosphere as a result of the effect of IMF shielding by the magnetopause. The modeling of convection has been performed for the cases when the IMF B _z component is directed southward and the B _y component is westward or eastward. A Tsyganenko 96 model has been used as a magnetospheric model. The model calculations are compared with the data on the ion drift in the ionosphere. A certain disagreement between the experimental and calculated data has been found in the pattern of convection on the dayside of the ionosphere for the case of B _y >0, which manifested itself in the dimensions of a convection “tongue” and in the position of the convection throat on the dayside. It has been indicated that the convection pattern agrees with the results of observations if the azimuthally inhomogeneous magnetospheric conductivity is taken into account.     

Variations in the IMF vertical component in isolated solar wind streams

The regularities of the variations in the IMF B _z component have been studied based on the data on the solar wind streams and their solar sources. Isolated solar wind streams such as magnetic clouds and shock layers before them, undisturbed heliospheric current sheets (HCSs), leading edges and bodies of high-speed streams from coronal holes (HSSs from CHs) have been considered. It has been revealed that each type of isolated streams in the interplanetary medium has it own features in the variations in the value and direction of the B _z component related to the stream immanent properties and conditions of propagation in the interplanetary plasma. The appearance of the southward B _z component is obligatory for all these streams which are, therefore, geoeffective.     

Estimation of storm-time level of day-side wave geomagnetic activity using a new <Emphasis Type="Italic">ULF</Emphasis> index

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.     

Shape and position of Earth’s bow shock near-lunar orbit based on ARTEMIS data

Earth’s bow shock is the result of interaction between the supersonic solar wind and Earth’s magnetopause. However, data limitations mean the model of the shape and position of the bow shock are based largely on near-Earth satellite data. The model of the bow shock in the distant magnetotail and other factors that affect the bow shock, such as the interplanetary magnetic field (IMF) B_y, remain unclear. Here, based on the bow shock crossings of ARTEMIS from January 2011 to January 2015, new coefficients of the tail-flaring angle α of the Chao model (one of the most accurate models currently available) were obtained by fitting data from the middle-distance magnetotail (near-lunar orbit, geocentric distance -20R_E>X>-50R_E). In addition, the effects of the IMF B_y on the flaring angle α were analyzed. Our results showed that: (1) the new fitting coefficients of the Chao model in the middle-distance magnetotail are more consistent with the observed results; (2) the tail-flaring angle α of the bow shock increases as the absolute value of the IMF B_y increases. Moreover, positive IMF B_y has a greater effect than negative IMF B_y on flaring angle. These results provide a reference for bow shock modeling that includes the IMF B_y.     

Distribution of the field−aligned currents in the ionosphere: dawn–dusk asymmetry and its relation to the asymmetry between the two hemispheres

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 I_R0/I_R2 ≥ 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.     

Studying the Relationship between High-Latitude Geomagnetic Activity and Parameters of Interplanetary Magnetic Clouds with the Use of Artificial Neural Networks

The cause-and-effect relations of the dynamics of high-latitude geomagnetic activity (in terms of the AL index) and the type of the magnetic cloud of the solar wind are studied with the use of artificial neural networks. A recurrent neural network model has been created based on the search for the optimal physically coupled input and output parameters characterizing the action of a plasma flux belonging to a certain magnetic cloud type on the magnetosphere. It has been shown that, with IMF components as input parameters of neural networks with allowance for a 90-min prehistory, it is possible to retrieve the AL sequence with an accuracy to ~80%. The successful retrieval of the AL dynamics by the used data indicates the presence of a close nonlinear connection of the AL index with cloud parameters. The created neural network models can be applied with high efficiency to retrieve the AL index, both in periods of isolated magnetospheric substorms and in periods of the interaction between the Earth’s magnetosphere and magnetic clouds of different types. The developed model of AL index retrieval can be used to detect magnetic clouds.     

Poleward expansion of the westward electrojet depending on the solar wind and IMF parameters

The effect of the interplanetary parameters on the latitudinal position of the substorm westward electrojet is studied in the work. The data from the IMAGE chain of magnetic stations and POLAR and WIND satellites for the period close to the solar activity minimum (1995–1996) and for the period of the solar activity maximum (2000) have been used for this purpose. It has been indicated that the electrojet poleward edge reaches, on average, higher latitudes at a higher solar wind velocity and at a larger (B _s ) IMF southward component. It has been indicated that the average latitude of the westward electrojet center increases with increasing solar wind velocity and decreases with increasing IMF southward component, as a result of which the electrojet center is, specifically, not observed at high geomagnetic latitudes at large values of the IMF southward component.     

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.     

Spontaneous substorms and ordered type of magnetospheric disturbances during the superstorm of November 20, 2003

The interval 0000-1400 UT of the superstorm of November 20, 2003, has been studies based on the ACE/WIND data and the MIT2 magnetogram inversion technique. The distributions of the electric potential and currents, field-aligned currents, and Joule heat in the ionosphere have been calculated. The variable magnetotail length and powers coming into the magnetosphere, ionosphere and ring current have been estimated. The selected superstorm intervals, when it became possible to identify the disturbance mode produced by the interaction between the variable solar wind dynamic pressure and IMF effects, have been described. Spontaneous substorms, two types of driven responses to changes in IMF or in the solar wind dynamic pressure (P _d ), zero events at simultaneous jumps of IMF and P _d , and a previously unknown mode of saturation of the ionospheric electric field at a redistribution of the energy coming into the magnetosphere between the ionosphere and ring current are among the selected modes.     

Geomagnetic disturbances and earthquakes in the Vrancea zone

Based on the example of the Vrancea zone of concentrated seismicity, it is shown how the stress-strain state of the medium responds to a disturbance of the geomagnetic field. Geomagnetic conditions are examined in relation to earthquakes in the Vrancea zone in the period 1988–1996. It is established that the seismic energy release in the Vrancea zone is associated with differences (“gradients”) in the H component of the geomagnetic field. Such a gradient preceding earthquakes is shown to be the midnight polar substorm and the degree of its mid-latitude effect. The time interval from the maximum of the substorm development to a shock (τ, h) is directly related to the focal depth. The seismic characteristics K _en and h (km) are demonstrated to be related to morphological features of the substorm development, namely, its duration T (min), intensity, and background. Differences in the duration of polar substorms before crustal (shallow) and deep earthquakes are revealed. Morphological features of the spectrum of geomagnetic variations preceding the seismic energy release are established.     

Isolated bursts of irregular geomagnetic pulsations in the region of the dayside cusp

In this work, the results of comparative analysis of morphological regularities of right-polarized (R type) and left-polarized (L type) isolated bursts of ipcl pulsations (irregular pulsations continuous long period) with an anomalously large amplitude in the region of the daytime polar cusp, as well as conditions of their excitation, are presented. It has been found that R and L bursts are similar in the maximum amplitude level, wave packet duration, spectral composition, magnitude of ellipticity, diurnal variation shape, and other characteristics. At the same time, bursts of the R and L type are excited at different degrees of plasma turbulence in the generation region, at different IMF orientations in the plane of ecliptic, as well as in the plane perpendicular to it, and at different dynamics of the parameter β (characterizing the ratio of the thermal pressure to the magnetic pressure) and Alfvén Mach number Ma. It is supposed that the generation of isolated bursts of the R and L types can be related to the amplification of the plasma turbulence level due to the development of wind instability at the front boundary of the magnetosphere, and features of their polarization can be interpreted in the scope of the model of nonlinear propagation of Alfvén waves.