# Geomagnetism and Aeronomy

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### Results of Observations of Ionospheric Plasma Drift in the Polarization Jet Region

Sun, 09/01/2019 - 00:00
Abstract

Doppler measurement data from the Yakutsk subauroral ionospheric station have been used to reveal for the first time new and previously unknown dynamic phenomena during the development of the polarization jet. This study of horizontal and vertical velocities of ionospheric drifts reveals that the peaks of vertical and horizontal velocity components do not always coincide during the observation of a polarization jet. The horizontal plasma drift velocities are ~300–600 m/s on average (there are events with velocities of 900–1000 m/s). The vertical drift velocities are 30–50 m/s on average (there are events with velocities of 100–150 m/s). Analysis of ground-based ionospheric data has revealed that all events with the development of the polarization jet over Yakutsk were uniformly divided into four cases. In the first case, the vertical velocities in the polarization jet band reached a maximum simultaneously with horizontal velocities. In the other two cases, the times of the vertical velocity peaks are shifted to both sides of the reference time by ±1 h. In the latter case, the vertical velocities have similar peaks on both sides of horizontal velocity peaks.

### Estimate of Variations in the Parameters of the Midlatitude Lower Ionosphere Caused by the Solar Flare of September 10, 2017

Sun, 09/01/2019 - 00:00
Abstract

Changes in the state of the D and E ionospheric regions lead to variations in the amplitude-phase characteristics of VLF radio signals. The existing theoretical and empirical models of the propagation of low-frequency electromagnetic waves qualitatively describe the relative variations in the parameters of the lower ionosphere associated with strong heliogeophysical disturbances; however, these models do not allow estimation of the absolute value and distribution of the electron concentration. We used the measurement data for the amplitude-phase characteristics of VLF radio signals with different frequencies propagating along two closely spaced paths. This made it possible not only to quantify the parameters of the D region of the ionosphere on a spatial scale of thousands of kilometers during the powerful solar flare of September 10, 2017 but also to restore the electron concentration profile before the onset of X-ray radiation.

### Determination of Specific Time Variations in the Energy of the Earth’s Magnetic Potential Field from the IGRF Model

Sun, 09/01/2019 - 00:00
Abstract

A method is proposed for the analysis of time series based on a logarithmic derivative or a specific variation. As compared to conventional analyses based on the Fourier transform, the advantage of the analysis of such variations is that it allows a rough estimate of the characteristic times exceeding the length of the series. The main disadvantages are the incomplete determination of the statistical weight of each considered variation, its initial discreteness, the absence of periodicity, and the impossibility of transformation into orthogonal functions. As a simple statistical hypothesis, it is assumed that each specific variation provides an equally probable contribution to its manifestation within a considered series, and the possibility for its manifestation outward is the same as within. The proposed method of analysis is determined and tested on the series of values of the Earth’s magnetic potential field energy from the IGRF model of 1900–2020. The median value is 1/(1995 years) for all specific variations, 1/(1942 years) for the positive values, and –1/(1628 years) for the dominant negative ones. These values are in good agreement with the geodynamo theory, observations, and generally accepted observation estimates of the western drift. The least probable values extreme in moduli –1/(128 years) and –1/(19 013 years) are related to MHD advection and magnetic diffusion, which make it possible to estimate the average flow velocity and the value of the Earth’s core conductivity, respectively. The large ratio of extreme specific variations indicates that the geodynamo has a strongly nonlinear and turbulent geodynamo character . The arithmetic mean and mean square specific variations are also analyzed with respect to the geodynamo theory and observations.

### Influence of the Solar Wind and Geomagnetic Activity Parameters on Variations in the Cosmic Ray Cutoff Rigidity during Strong Magnetic Storms

Sun, 09/01/2019 - 00:00
Abstract

The correlation between the variations of geomagnetic cutoff rigidity ΔR and interplanetary parameters and the Dst index of geomagnetic activity during one moderate and six strong storms of solar cycles 23 and 24 has been calculated. The ΔR values have been obtained with two methods: (1) the spectrographic global survey method (SGS), in which the determination of the cutoff rigidity RSGS is based on observational data from the neutron monitor network, and (2) a method in which the particle trajectories are calculated numerically in a model magnetic field of the magnetosphere to determine the cutoff rigidity Reff. In general, the results obtained by the two methods are in close agreement. The Dst index of geomagnetic activity has the greatest effect on ΔR, and the correlation increases with storm intensity. The sensitivity of ΔR to interplanetary parameters vary greatly for different storms. The most geoeffective interplanetary parameter is the solar-wind speed V. A significant anticorrelation of ΔR and V can be traced for almost all storms. The correlation of ΔRSGS with the Bz component of the interplanetary magnetic field is observed only for two storms, on November 7–14, 2003, and November 7–8, 2004, for which the absolute Bz value was very high (≈−50 nT). At the same time, there is a rather high correlation of ΔReff with Bz for most storms. The azimuthal component of the interplanetary field By and the solar-wind dynamic pressure P show almost no connection with ΔR.

### Numerical Simulation of Vertical Current Sheets in Solar Chromospheric Plasma with the Hall Effect

Sun, 09/01/2019 - 00:00
Abstract

The consequences of the entry of the magnetic field from underlying layers into the Sun’s chromosphere are studied via numerical solution of the 2D MHD problem (with constant physical quantities along the straight horizontal magnetic lines of force). Chromospheric plasma is considered to be collisional; its Joule dissipation and the Hall effect are taken into account. The initial magnetic field is characterized by a value of β = 1.5 and corresponds to an upward current sheet of finite thickness. There are cases in which the coevolution of the magnetic field and plasma leads to the formation of a very thin, vertical current sheet, from which a plasma stream flows downward vertically. The process turned out to be typical of rather large heights, where the influence of the Hall effect on the magnetic field evolution begins to exceed the field-drift effect due to the (partial) freezing of the force lines. The formed thin current sheet then ceases to exist under the action of pinch (sausage and/or kink) instabilities. The reconnection of force lines is not seen under the conditions of this numerical experiment due to the assumed constancy of physical quantities (including velocity) along a force line. Accordingly, the described processes in nature will act as alternative or additional processes with respect to the typically considered scenarios of reconnection.

### Interpretation of Measurement Results of the Potential Difference in Lake Baikal

Sun, 09/01/2019 - 00:00
Abstract

A model is proposed in which the electrical voltage measured between the electrodes in Lake Baikal is a consequence of two effects: electrochemical processes near the electrodes and a positive charge current flowing through the lake. The electrochemical component of the voltage in the case of lead electrodes arises due to the difference in concentrations of carbonate anion— $${\text{CO}}_{3}^{{2 - }}$$ at different depths. In the case of the use of chlorine-silver electrodes, only the effect of the positive charge current flowing through the lake is measured. We proposed an interpretation of an increase of electrical voltage registered in Lake Baikal during an earthquake in August 2008. The reason for the increase in voltage is the release of positively charged hydrogen-containing gases from the Earth’s interior.

### Large-Scale Variations in Ion Flows in the Central Part and the Boundary of the Plasma Sheet

Sun, 09/01/2019 - 00:00
Abstract

The results of multisatellite observations in a magnetotail at short distances ( $${\sim }10{{R}_{E}}$$ ) in two close regions inside the plasma sheet of the magnetotail and at the sheet boundary indicate the kinetic nature of processes in the magnetotail plasma. Disturbances simultaneously observed in ion flows and in their anisotropy occur due to the presence of a thin, kinetic current sheet in the tail. It is located at greater distances than the observation regions in such an experiment. It was shown earlier that the thin current sheet nested in the thicker plasma sheet is the most important component of the magneto-plasma structure, corresponding to the process of magnetic reconnection in the magnetotail. Therefore, the kinetic structures observed in such an experiment are manifestations of a typical disturbance that corresponds to magnetic reconnection at larger distances.

### Ionospheric Precursors of Geomagnetic Storms. 1. A Review of the Problem

Sun, 09/01/2019 - 00:00
Abstract

The problem of the occurrence of disturbances of the ionospheric F2-layer parameters prior to the beginning of a geomagnetic storm is discussed. It is shown that disturbances (substantial deviations from quiet conditions) of both the critical frequency foF2 and the total electron content (TEC) in an ionospheric column are found in many studies a few hours and sometimes even two days before the SC (sudden storm commencement) moment. The amplitudes of the aforementioned disturbances are on average of 30–60%, however, they can exceed 100% in some cases. Deviations from the quiet conditions of both signs are possible; however, positive prestorm disturbances of foF2 and TEC are more common.

### Diurnal Variations in the Statistical Characteristics of the Variability of the Midlatitude NmF 2 during Quiet Geomagnetic Conditions at Low Solar Activity

Sun, 09/01/2019 - 00:00
Abstract

The work examines diurnal variations in the statistical characteristics of the variability of the electron number density NmF2 of the maximum of the ionspheric F2 layer during quiet geomagnetic conditions at low solar activity based on hourly ionosonde measurements of the critical frequency of the ionspheric F2 layer from 1957 to 2017 over Moscow. The statistical parameters of NmF2 are calculated for each month M of each year at the universal time UT = 0, 1, … 23 h: the mathematical expectation NmF2E; the most probable value NmF2MP; the average monthly median NmF2MED; the arithmetic mean NmF2A; the standard deviations of NmF2 from NmF2E, NmF2MP, and NmF2MED; and the coefficients of variation CVE, CVMP, and CVMED of NmF2 relative to NmF2E, NmF2MP, and NmF2MED, respectively. It follows from the calculations that the CVE, CVMED, and CVMP values vary within the intervals of 12–43%, 12–60%, and 13–75%, respectively, and, in the vast majority of cases, CVE(UT, M) < CVMED(UT, M) and CVE(UT, M) < CVMP(UT, M). If the coefficient CVE in each month of each year is compared for different time points, the lowest CVE value varies from 12% (July) to 19% (December) and occurs during daytime, while the highest CVE value lies in the interval from 26% (June) to 43% (December). For each UT, the lowest and highest values of this coefficient in the autumn, winter, and spring months are greater than those for the summer months. It was found that the difference of NmF2A(UT, M) from NmF2E(UT, M) does not exceed 0.2%.

### Ion Pressure at the Auroral Precipitation Boundaries and Its Relationship with the Solar Wind Dynamic Pressure

Sun, 09/01/2019 - 00:00
Abstract

The data from the DMSP F7 satellite for 1986 are used to study the behavior of ion pressure at the boundaries of auroral precipitation. The study is based on 7489 satellite passages in the nightside sector of the auroral zone, including over 5000 passages in the 2100–2400 MLT sector. Ion pressure is determined as the average for 5 s of observations, which corresponds to a distance of ~40 km in the satellite trajectory portions adjacent to the precipitation boundaries. It is shown that the plasma pressure at the boundaries of auroral precipitation almost linearly increases at all levels of magnetic activity with an increase in the solar wind dynamic pressure (Psw). The pressure distribution as a function of MLT indicates that the precipitation boundaries, including the isotropy boundary, are not isobars, even at a low level of geomagnetic activity. The plasma pressure is maximal in the 22–24 MLT sector and decreases both in the morning and evening sides. The latitudinal position of the precipitation boundaries and the plasma pressure at the boundaries are found during all the phases of a typical substorm with an intensity of AL = –410 nT at its maximum. The latitudinal profile of ion pressure is constructed with respect to the isotropy boundary (IB) at the beginning of substorm. It is shown that, with an increase in dynamic pressure, there is not only a substantial increase in plasma pressure at the auroral precipitation boundaries but also a change in the latitudinal position of the boundaries themselves. As Psw grows, the latitude of the polarward boundary of the oval increases, while that of the equatorward boundary decreases. Despite the considerable expansion of the precipitation region, the latitudinal pressure gradient between the oval boundaries increases, even during quiet periods (average AL = –18 nT, IMF Bz = +1.4 nT) without any disturbances in the auroral zone, by approximately a factor of 2, from 0.06 to 0.12 nPa/deg.

### Effects in the Ionosphere after the Chilean Earthquake on February 27, 2010, According to Data of Ground-based Ionosondes

Sun, 09/01/2019 - 00:00
Abstract

The paper analyzes the measurement data of the critical frequency (foF2) of the regular ionospheric F2-layer at a number of ground based stations of the vertical sounding of the ionosphere in order to detect disturbances in the ionosphere that followed the earthquake with a magnitude of M = 8.8. The earthquake occurred in Chile on February 27, 2010 at 0634 UT at an epicentral distance of 335 km from its capital city of Santiago. It was found that at significant distances (over 5000 km) from the earthquake epicenter under a quiet geomagnetic conditions, wave disturbances are observed in the behavior of foF2, exceeding approximately two standard deviations from the background level for three hours, with the maximum relative deviation of 20%. The analysis shows that the characteristics of disturbances in the ionosphere correspond in time to the arrival of a packet of acoustic-gravity waves excited in the atmosphere at the time of the seismic shock.

### Modeling of the Auroral Hiss Propagation from the Source Region to the Ground

Sun, 09/01/2019 - 00:00
Abstract

A numerical model of auroral hiss propagation from the region of its generation to the ground surface is developed for the interpretation of results from ground-based high-latitudinal VLF observations. The model includes modules describing the statistical properties of electrostatic whistler waves generated due to Cerenkov resonance at the heights of 6000–20 000 km, the propagation of these waves in the magnetosphere to the region of upper ionosphere (under 5000 km), which is filled with small-scale irregularities of electron concentration, the scattering of electrostatic waves from these irregularities into the transition cone, and further propagation of the waves through the lower ionosphere down to the ground surface. The modeling results agree with the observations.

### Geomagnetic Variations during the Fall of Meteorites

Sun, 09/01/2019 - 00:00
Abstract

The results of instrumental observations of variations in the Earth’s magnetic field conducted at a number of observatories of the INTERMAGNET network and the Mikhnevo geophysical observatory of Institute of Geosphere Dynamics of Russian Academy of Sciences, during the fall of meteorites have been analyzed. The Vitim (September 24, 2002), Chelyabinsk (February 15, 2013), Romania (January 7, 2015), Buryatia (October 25, 2016), Khakassia (December 6, 2016), St. Petersburg (September 11, 2017), and Lipetsk (June 21, 2018) events have been used to show the geomagnetic effect of falling cosmic bodies. The effect has a nonlocal character, occurs simultaneously, and is observed at distances up to 7000 km from the location of falling cosmic bodies. The amplitude of induced geomagnetic variations has been found to depend weakly on the distance to the event location. The resulting data can be used to verify theoretical and computational models of the geophysical processes accompanying the fall of meteorites.

### Specific Features of the Distributions of the Hall Currents and Magnetic Field of Pс 4 Pulsations

Sun, 09/01/2019 - 00:00
Abstract

The distributions of the intensities and directions of the Hall currents in the ionosphere have been determined by the given coordinates and intensities of magnrtospheric field-aligned currents (FAC). The examples of distributions calculated for three FAC show that the coincidence of specific features of the field of the Hall currents and the magnetic field on the ground level near the points of their projections is explained by the induction character of the formation of the Hall current. This makes possible to estimate the number, location, and intensity of field-aligned currents with experimentally constructed magnetic pulsation field distributions under the condition that the distances between the sources are larger than the ionospheric height. The presented solution of the equations for the field-lines and isolines of the Hall current intensity shows that the arcs of these curves are locally generated by a pair of equivalent field-aligned currents.

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