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.     

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).     

Position variations of the polarization jet and injection boundary of energetic ions during substorms

The comparison of selected cases of polarization jet observation at ground stations and measurements of energetic ions at the AMPTE/CCE satellite shows that these phenomena occur simultaneously and on the same L shells. Polarization jet observations at DMSP satellites make it possible to statistically determine the dependence of its equatorial boundary position on the AE-index value. It is also shown that, in the case of isolated magnetic disturbances, the position of the inner boundary of injection of energetic ions measured at the AMPTE/CCE satellite depends on the AE index. It was found that the dependences of both boundaries on the AE index match over a wide range of AE variations. This is evidence that the equatorial boundary polarization jet band and the inner boundary of the injection of energetic ions are physically interconnected and are formed on the same L shells during substorms.     

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.     

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.     

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.     

Statistical Study of the Characteristics of Isolated Bursts of Midlatitude <Emphasis Type="Italic">Pi</Emphasis>2 Geomagnetic Pulsations

The characteristics and interplanetary excitation conditions of isolated bursts of Pi2 geomagnetic pulsations observed during the development of magnetospheric substorms (substorm Pi2) and in its absence (nonsubstorm Pi2) on the night side of the Earth are comparatively analyzed. It is shown that, regardless of the local time and season, the amplitude of isolated Pi2 substorm bursts is always higher than that of the nonsubstorm ones, and the periods and duration of the wave packets of substorm Pi2 bursts are less than those of nonsubstorms. Diurnal and seasonal variations in the characteristics of the two groups of Pi2 bursts differ in the form and position of maxima and minima. It is found that the start of excitation of isolated Pi2 bursts, during substorms and in its absence, is controlled by the preferred direction of the interplanetary magnetic field (IMF) vector perpendicular to the Sun–Earth line (angle θ_xB = arccos(B_x/B) → 90°). It is assumed that isolated Pi2 bursts of both groups are triggered by reorientation of the IMF vector in the ecliptic plane and the plane perpendicular to it ~15 min before their onset. The most likely source of midlatitude isolated Pi2 bursts during substorm development and in its absence are bursty bulk flows (BBFs) in the plasma sheet of the magnetospheric tail, the regularities of which coincide in many respects with the observed features of Pi2 bursts.     

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.     

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.     

Properties of daytime long-period pulsations during magnetospheric storm commencement

Long-period geomagnetic pulsations during the SSC of July 14, 2012, are studied. The prenoon longitudinal sector (09:20–11:30) MLT, from the boundaries of which pulsations propagate azimuthally onto the dawn and dusk sides with an opposite polarization direction and increased amplitude, has been distinguished. The position of this sector relative to noon (a shift to the dawn side) depends on the front azimuthal inclination. It has been found that the polarization direction reverses in going from low (<30°) to middle/subauroral (≥50°) latitudes on the entire dayside. The geomagnetic pulsations mainly fluctuate near the f₁ = 2.9 and f₂ = 4.4 mHz frequencies. Fluctuations with frequency f₁, which coincide with the fluctuation frequency of the IMF х component, predominate at the polar cap latitudes (the open field line region) in the form of rapidly attenuating impulses and at low latitudes with a much smaller amplitude. Fluctuations with frequency f₂ are globally registered at all latitudes in the dayside sector below the magnetopause projection as a train of several fluctuations. It is assumed that fluctuations with frequency f₁ penetrate from the solar wind, and fluctuations with frequency f₂ are radial magnetopause oscillations.     

Dependence of geomagnetic disturbances on extreme values of the solar wind <Emphasis Type="Italic">E</Emphasis><Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> component

The dependence of the zonal geomagnetic indices (AE, Ap, Kp, Kn, and Dst) on the solar wind parameters (the electric field E _y component, dynamic pressure P _d and IMF irregularity σB) has been studied for two types of events: magnetic clouds and high-speed streams. Based on the empirical relationships, it has been established that the AE, Ap, Kp, and Kn indices are directly proportional to the E _y value at E _y < 12 mV m^?1 and are inversely proportional to this value at E _y > 12 mV m^?1 for the first-type events. On the contrary, the dependence of Dst on E _y is monotonous nonlinear. A linear dependence of all geomagnetic indices on E _y is typical of the second-type events. It has been indicated that the specific features of geoeffectiveness of magnetic clouds and high-speed solar wind streams are caused by the dependence of the electric field potential across the polar cap on the electric field, solar wind dynamic pressure, and IMF fluctuations.     

Long-period irregular pulsations under the conditions of a quiet magnetosphere

Simultaneous observations of high-latitude long-period irregular pulsations at frequencies of 2.0–6.0 mHz (ipcl) and magnetic field disturbances in the solar wind plasma at low geomagnetic activity (Kp ~ 0) have been studied. The 1-s data on the magnetic field registration at Godhavn (GDH) high-latitude observatory and the 1-min data on the solar wind plasma and IMF parameters for 2011–2013 were used in an analysis. Ipcl (irregular pulsations continuous, long), which were observed against a background of the IMF Bz reorientation from northward to southward, have been analyzed. In this case other solar wind plasma and IMF parameters, such as velocity V, density n, solar wind dynamic pressure P = ρV² (ρ is plasma density), and strength magnitude B, were relatively stable. The effect of the IMF Bz variation rate on the ipcl spectral composition and intensity has been studied. It was established that the ipcl spectral density reaches its maximum (~10–20 min) after IMF Bz sign reversal in a predominant number of cases. It was detected that the ipcl average frequency (f) is linearly related to the IMF Bz variation rate (ΔBz/Δt). It was shown that the dependence of f on ΔBz/Δt is controlled by the α = arctan(By/Bx) angle value responsible for the MHD discontinuity type at the front boundary of magnetosphere. The results made it possible to assume that the formation of the observed ipcl spectrum, which is related to the IMF Bz reorientation, is caused by solar wind plasma turbulence, which promotes the development of current sheet instability and surface wave amplification at the magnetopause.     

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.     

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.     

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.     

The causal sequence investigation of the ring current ion-flux increasing and the magnetotail ion injection during a major storm

Comprehensive records are available in ENA data of ring current activity recorded by the NUADU instrument aboard TC-2 on 15 May, 2005 during a major magnetic storm(which incorporated a series of substorms). Ion fluxes at 4-min temporal resolution derived from ENA data in the energy ranges 50–81 and 81–158 ke V are compared with in situ particle fluxes measured by the LANL-SOPA instruments aboard LANL-01, LANL-02, LANL-97, and LANL-84(a series of geostationary satellites that encircle the equatorial plane at ~6.6 R_E). Also, magnetic fields measured simultaneously by the magetometers aboard GOES-10 and GOES-12(which are also geostationary satellites) are compared with the particle data. It is demonstrated that ion fluxes in the ring current were enhanced during geomagnetic field tailward stretching in the growth phases of substorms rather than after Earthward directed dipolarization events. This observation, which challenges the existing concept that ring current particles are injected Earthward from the magnetotail following dipolarization events, requires further investigation using a large number of magnetic storm events.     

Critical frequencies <Emphasis Type="Italic">foF</Emphasis>2 as an indicator of trends in thermospheric dynamics

The time variations in three parameters during the last decades are considered. R(foF2) is the correlation coefficient between the nighttime and daytime values of foF2 for the same day. Stable trends are found for the minimum (R(foF2)(max)) and maximum (R(foF2)(min)) values of R(foF2) during a year. The foF2(night)/foF2(day) ratio demonstrates both, negative and positive trends, and the trend sign depends on the inclination I and declination D of the magnetic field. The correlation coefficient r(h, fo) between foF2 and the 100 hP level in the stratosphere demonstrates a decrease (in the years of maximum and minimum solar activity) from the 1980s to the 1990s. The trends in all three groups of data are considered under the assumption of long-term changes in the circulation in the upper atmosphere.     

Diagnostics of Turbulent Dynamo from the Flux Emergence Rate in Solar Active Regions

Line-of-sight magnetograms acquired by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory (SDO) and by the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO) for 14 emerging ARs were used to study the derivative of the total unsigned flux–the flux emergence rate, R(t). We found that the emergence regime is not universal: each AR displays a unique emergence process. Nevertheless, two types of the emergence process can be identified. First type is a “regular” emergence with quasi-constant behavior of R(t) during a 1–3 day emergence interval with a rather low magnitude of the flux derivative, R_max = (0.57 ± 0.22) × 10²² Mx day^–1. The second type can be described as “accelerated” emergence with a long interval (>1 day) of the rapidly increasing flux derivative R(t) that result in a rather high magnitude of R_max= (0.92 ± 0.29) × 10²² Mx day^–1, which later changes to a very short (about a one third of day) interval of R(t) = const followed by a monotonous decrease of R(t). The first type events might be associated with emergence of a flux tube with a constant amount of flux that rises through the photosphere with a quasi-constant speed. Such events can be explained by the traditional largescale solar dynamo generating the toroidal flux deep in the convective zone. The second-type events can be interpreted as a signature of sub-surface turbulent dynamo action that generates additional magnetic flux (via turbulent motions) as the magnetic structure makes its way up to the solar surface.     

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.