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.     

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.     

Dynamics of the ionospheric electric currents and auroral luminosity boundaries during strong magnetic storms

The regularities in the southward drift of the ionospheric current centers and luminosity boundaries during strong magnetic storms of November 2003 and 2004 (with Dst ≈ ?400 and ?470 nT, respectively) are studied based on the global geomagnetic observations and TV measurements of auroras. It has been indicated that the eastward and westward electrojets in the dayside and nightside sectors simultaneously shift equatorward to minimal latitudes of Φ _min ^° ～53°–55°. It has been obtained that the Φ _min ^° latitude decreases with increasing negative values of Dst, IMF B _z component, and westward electric field strength in the solar wind. The dependence of the electrojet equatorward shift velocity (V _av) on the rate of IMF B _z variations (ΔB _z /Δt) has been determined. It is assumed that the electrojet dynamics along the meridian is caused by a change in the structure of the magnetosphere and electric fields in the solar wind and the Earth’s magnetosphere.     

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.     

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.     

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.     

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).     

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.     

Daytime quasiperiodic geomagnetic pulsations during the recovery phase of the strong magnetic storm of May 15, 2005

Intense quasimonchromatic geomagnetic pulsations with a period of ～15 min, observed on the Earth’s surface in the near-noon sector at the beginning of the recovery phase of a very strong (Dst _min = ?260 nT) magnetic storm of May 15, 2005, are analyzed. The variations were registered at auroral latitudes only in the X field component, and wave activity shifted into the postnoon sector of the polar cap an hour later; in this case pulsations were observed in the X and Y field components. Within the magnetosphere the source of magnetic pulsations could be the surface waves on the magnetopause caused by the pulse of the solar wind magnetic pressure. Geomagnetic pulsations in the polar cap, observed in phase at different latitudes, could apparently reflect quasiperiodic variations in the NBZ system of field-aligned currents. Such variations can originate due to the series of pulsed reconnections in the postnoon outer cusp at large (～20 nT) positive B _z values and large (about ?40 nT) negative values of IMF B _x .     

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.     

Characteristics of Pc4–5 pulsations obtained using the method of bistatic backscatter of HF radio waves,the EISCAT HF heating facility,and ground-based magnetometers

The results of studying the Pc4–5 pulsation parameters based on the method of bistatic backscatter of radio waves, using the EISCAT/Heating HF facility (Tromsø, Norway) and IMAGE ground-based magnetometers (Scandinavia), are presented. The observations were performed during the morning hours on October 3, 2006, when a substorm developed on the nightside. An analysis of the observational data obtained from 1000 to 1020 UT indicated that wave-like disturbances with periods corresponding to Pc4–5 pulsations (80–240 s) existed at that time. The variations in the full vector of the ionospheric irregularity motion and the electric field strength in an artificially disturbed high-latitude ionospheric F region has been reconstructed based on simultaneous Doppler observations on two paths. A general conformity is observed among the time variations in Pc4–5 pulsations in the magnetic and ionospheric data: between the velocity amplitude (|V|) and the X component of the Earth’s magnetic field and between the irregularity motion azimuth and the Y component. Large-scale waves, corresponding to the natural resonances of magnetic field lines (small values of the azimuthal number |m| ～ 2–4), and small-scale waves (large values |m| ～ 17–20) were simultaneously registered during the experiment based on magnetic data. It has been indicated that the periods of wave-like processes registered using the method of bistatic backscatter and ground-based magnetometers were in agreement with one another. The formation of wave-like processes is explained by the nonstationary impact of the solar wind and IMF on the Earth’s magnetosphere. The variations in the IMF, according to the ACE satellite measurements, were characterized by a sharp increase in the solar wind plasma dynamic pressure that occurred at about 09 UT on October 3, 2006, and was accompanied by rapid polarity reversals of the north-ward-southward (B _z) and transverse (B _y) IMF components.     

Analysis of high-latitude current systems during the BEAR experiment based on the IZMEM model

Based on the model of large-scale high-latitude current systems developed at IZMIRAN (IZMEM model), it has been indicated that auroral electrojets and current systems concentrated in the polar cap were the generators of long-period geomagnetic variations during the BEAR experiment on the electromagnetic field registration at the Scandinavian test site on June 1–July 15, 1998. Precisely circumpolar current systems, prevailing in the high-latitude ionosphere during the periods of a quiet magnetospheric state, which is characterized by the presence of the northern vertical (B _z >0) component of the IMF vector in the solar wind, are responsible for the magnetotelluric fields.     

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.     

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.     

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.     

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.     

Role of solar activity in formation of the anomalous El Nin’o current

Monthly indices of Southern Atmospheric Oscillation (SOI) and corresponding Wolf numbers, geoeffective solar flares, magnetic AE indices as well as daily average values of the southward component of the interplanetary magnetic field (IMF B _z) and data on the wind characteristics at Antarctic stations Vostok, Leningradskaya, and Russkaya are analyzed. It is shown that a sharp decrease in the SOI indices, which corresponds to the beginning of El Nin’o (ENSO), is preceded one or two months before by a 20% increase in the monthly average Wolf numbers. In warm years of Southern Atmospheric Oscillation a linear relationship is observed between the SOI indices and the number of geoeffective solar flares with correlation coefficients p < ?0.5. It is shown that in warm years a change in the general direction of the surface wind to anomalous at the above stations is preceded one or two days before by an increase in the daily average values of IMF B _z. An increase in the SOI indices is preceded one or two months before by a considerable increase in the monthly average values of the magnetic AE indices.     

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.     

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.     

Statistical relations between the probability of occurrence of Pc3-4 pulsations at high latitudes in the antarctic regions and the solar wind and IMF parameters

Geomagnetic pulsation in the Pc3-4 bands have been studied at high Antarctic latitudes during the local summer. The statistical relation between the occurrence probability of Pc3 and Pc4 pulsations and the solar wind (SW) and IMF parameters has been revealed by verifying the hypothesis that an indication is identical in two distributions. Different dependences of the occurrence probability of high-latitude Pc3 and Pc4 pulsations on the IMF value and orientation and SW density and velocity have been found out. It has been indicated that these dependences remain unchanged in the range of geomagnetic latitudes from 66° to 87°. It has been established that the Pc3 observation probability at small (20°–50°) IMF cone angles (θ = cos^?1(B _x/|B|)) is a factor of 1.5 higher than the average statistical probability and depends on the IMF value, which confirms the hypothesis that the Pc3 source is the turbulent region upstream of the magnetospheric quasiparallel low shock. On the contrary, the probability of occurrence of Pc4 weakly depends on the IMF cone angle and is maximal at θ ～ 0° and ～90°. With increasing negative B _z values, the generation probability increases in the Pc4 band and tends to decrease in the Pc3 band. It has been found out for the first time that the dependence of the Pc4 occurrence probability on the IMF clock angle (? = tan^?2 (B/B _z) is identical in the regions of projections of closed and open field lines, whereas this dependence is different for Pc3. In the region of projections of closed field lines, the Pc3 occurrence probability increases at B _z < 0 and B _y > 0 (the condition under which the cusp shifts on the dawn side) and at B _y < 0 and B _z > 0 (which is typical of the formation of the low-latitude boundary plasma sheet). In the region of projections of open field lines such a probability increases at B _y < 0 and B _z < 0 (which results in the formation of the high-latitude boundary plasma sheet). Based on the discovered regularities, the conclusion has been made that the sources of generation of high-latitude Pc3 and Pc4 pulsations are different.