Dependence of time derivative of horizontal geomagnetic field on sunspot number and aa index

This work investigated an interrelationship between the monthly means of time derivatives of horizontal geomagnetic field, dH/dt, sunspot number, R _z , and aa index for the period of substorms (from ?90 to ?1800 nT) during the years 1990–2009. A total of 232 substorms were identified during the period of study. The time derivative of horizontal geomagnetic field, dH/dt, used as a proxy for geomagnetically induced current (GIC) exhibited high positive correlation with sunspot number (0.86) and aa index (0.8998). The obtained geomagnetic activity is in 92.665% explicable by the combined effect of sunspot number and aa index. The distribution of substorms as a function of years gives a strong support for the existence of geomagnetic activity increases, which implies that as the sunspot number increases the base level of geomagnetic activity increases too.     

Effect of zonal E × B plasma drift on electron density in the low-latitude ionospheric <Emphasis Type="Italic">F</Emphasis> region at a solar activity maximum near vernal equinox

The variations in the density of the ionospheric F2 layer maximum (NmF2) under the action of the zonal plasma drift perpendicularly to the magnetic (B) and electric (E) fields in the direction geomagnetic west-geomagnetic east have been studied using the three-dimensional nonstationary theoretical model of electron and ion densities (N _e and N _i ) and temperatures (T _e and T _i ) in the low-latitude and midlatitude ionospheric F region and plasmasphere. The method of numerical calculations of N _e , N _i , T _e , and T _i , including the advantages of the Lagrangian and Eulerian methods, is used in the model. A dipole approximation of the geomagnetic field (B), taking into account the non-coincidence of the geographic and geomagnetic poles and differences between the positions of the Earth’s and geomagnetic dipole centers, is accepted in the calculations. The calculated NmF2 and altitudes of the F2 layer maximum (hmF2) have been compared with these quantities measured at 16 low-latitude ionospheric sounding stations during the geomagnetically quiet period October 11–12, 1958. This comparison made it possible to correct the input model parameters: the NRLMSISE-00 model [O], the meridional component of the neutral wind velocity according to the HWW90 model, and the meridional component of the equatorial plasma drift due to the electric field specified by the empirical model. It has been indicated that the effect of the zonal E × B plasma drift on NmF2 can be neglected under daytime conditions and changes in NmF2 and hmF2 under the action of this drift are insignificant under nighttime conditions north of 25° and south of ?26° geomagnetic latitude. The effect of the zonal E × B plasma drift on NmF2 and hmF2 is most substantial in the nightside ionosphere approximately from ?20° to 20° geomagnetic latitude, and the neglect of this drift results in an up to 2.4-fold underestimation of NmF2. The found dependence of the effect of the zonal E × B plasma drift on NmF2 and hmF2 on geomagnetic latitude is related to the longitudinal asymmetry of B, asymmetry of the neutral wind about the geomagnetic equator, and changes in the meridional E × B plasma drift at a change in geomagnetic longitude.     

Determination of the paleointensity in the early proterozoic from granitoids of the Shumikhinskii complex of the Siberian craton

The extreme scarcity of data on the behavior of the paleointensity H _an in the geological past from rocks older than 400 Ma significantly hinders the development of our ideas of the geomagnetic field evolution and the geological history of the Earth as a planet. This work presents H _an determinations for the Early Proterozoic using the Thellier method and meeting modern requirements for their reliability. The data are obtained from 1850-Ma rocks of granite intrusions sampled in the south of the Siberian platform. The rocks are virtually unaltered granites and granitoids. The paleointensity was determined on 15 samples; results from 11 samples were found to be suitable for the calculation of H _an, which is good for experiments of this type. The common feature in the behavior of the natural remanent magnetization (NRM) is a very narrow interval of blocking temperatures: destruction of (60–90)% NRM often took place between 500 and 550°C. Because of the large thickness of the sampled magmatic body, the paleointensity estimates were corrected for its slow cooling rate. With regard for this correction, the probable value of the virtual dipole moment (VDM) from the given collection amounts to 5 × 10²² A m². Analysis of all published data obtained by the Thellier method for the Precambrian and satisfying the well-known minimal criteria of reliability showed that the average VDM value is about 2 × 10²² A m², which is four times smaller than the VDM value of the last million years. This phenomenon can be interpreted in terms of the hypothesis that the solid inner core formed only in Proterozoic and, in its absence, the generation of the geomagnetic field was relatively weak, which yielded a small intensity value of the geomagnetic field at early stages of the Earth’s evolution.     

Scaling characteristics of SEGMA magnetic field data around the Mw 6.3 Aquila earthquake

We apply detrended fluctuation analysis (DFA) on fluxgate and search-coil data in ULF range (scales 10–90 s or 0.1–0.011 Hz) for the months January–April 2009 available from the South European GeoMagnetic Array stations: Castello Tesino (CST), Ranchio (RNC), and L’Aquila (AQU) in Italy; Nagycenk (NCK) in Hungary; and Panagyuriste (PAG) in Bulgaria. DFA is a data processing method that allows for the detection of scaling behaviors in observational time series even in the presence of non-stationarities. The H and Z magnetic field components at night hours (00-03 UT, 01–04 LT) and their variations at the stations CST, AQU, NCK, and PAG have been examined and their scaling characteristics are analyzed depending on geomagnetic and local conditions. As expected, the scaling exponents are found to increase when the K _p index increases, indicating a good correlation with geomagnetic activity. The scaling exponent reveals also local changes (at L’Aquila), which include an increase for the Z (vertical) component, followed by a considerable decrease for the X (horizontal) component in the midst of February 2009. Attempts are made to explain this unique feature with artificial and/or natural sources including the enhanced earthquake activity in the months January–April 2009 at the L’Aquila district.     

The qualitative character of the global electrical conductivity of the earth

A spectral stacking and smoothing procedure has been applied to unbroken hourly values of H and Z, for 1964 and 1965, from 17 observatories, in order to estimate the magnitude and phase of the P₁⁰ response of the Earth to long-period geomagnetic fluctuations. Exploratory techniques have been used to gauge when sufficient smoothing has been applied, and to identify the qualitative character of the global electrical conductivity of the Earth.     

A stratified core motion inferred from geomagnetic secular variations

An examination of the westward drift of the geomagnetic field indicates that the drift velocity is almost independent of latitude, suggesting a uniform rigid rotation of spherical shape. When the geomagnetic field is separated into standing and drifting components and expressed in a spherical harmonic series, a lack of sectorial terms is noted in the standing field. It is shown that these features are well explained by a stratified core model.The core is supposed to be stratified near the surface where toroidal fluid motions are predominant. In the deeper part, the fluid motion is two-dimensional, forming Taylor columns. A simplified core model is assumed to represent these features, in which the core is divided into two parts, an outer spherical shell that rotates westwards at a uniform rate of 0.3° y^?1 and a central sphere in which the two-dimensional columnar motions reside. The toroidal motions in the outer spherical interact with the dipole field to induce the drifting field, whereas the columnar motions generate the standing field through interaction with a toroidal field. It follows that a small velocity as 5 × 10^?3 cm s^?1 for the stratified motion is sufficient to create the observed drifting field.     

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.     

Manifestation of the Lunar–Solar Tide and Free Oscillations of the Earth in the Variations of the Magnetic Field

The results of processing and analyzing the instrumental observations of the Earth’s magnetic field at the Geophysical Observatory Mikhnevo of the Institute of Geosphere Dynamics of the Russian Academy of Sciences (IGD RAS) for 2010–2015 are presented. Quasi-harmonic components with the periods close to the lunar–solar tidal waves are revealed in the spectra of geomagnetic variations over a period of 0.4 to 30 days. The elliptical S₁ tidal wave which is detected in the geomagnetic variations has modulations with periods of 1/3, 1/2, and 1 year. The spectra of the geomagnetic variations contain peaks corresponding to the free oscillations of the Earth. The analysis of the time series of the magnetic field for the period of the strong earthquakes in the absence of geomagnetic disturbances revealed the fine structure of the Earth’s fundamental spheroidal mode ₀S₂, which splits into five singlets. The established features of the spectrum of geomagnetic variations are helping the development of the new method for studying the deep structure of the Earth and the properties of the inner geospheres for estimating the viscosity of the Earth’s outer core and dynamics of the current systems in the outer (liquid) core, as well as for exploring, with the use of empirical data, the general regularities governing the regimes of energy exchange processes in the geospheres.     

The influence of geomagnetic and solar variabilities on lower thermosphere density

Atmospheric density measurements near 200 km from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment are analyzed for geomagnetic and solar flux variability effects. Data from the SETA experiment, onboard two satellites, are available for the periods of May to November 1982, and July 1983 to March 1984. The data utilized the span ±79.5° latitude, and are available for both day (1030 LT) and night (2230 LT). Annual and semiannual density variations are removed and regression analyses are performed on the residuals using a series of lagged 3 h K_p indices to determine and remove geomagnetic fluctuations. Densities are found to increase by as much as 134% in response to an increase in the K_p index from 1 to 6. Monthly curves are generated for the K_p regression coefficients to delineate seasonal-latitudinal and day/night dependences, which reflect the effects of mean meridional advection of disturbances from high to low latitudes. Further analyses are performed comparing measured densities with MSISE-90 predictions. Results show that the model is able to capture many of the prominent features, but does not fully predict the level of variability for the individual disturbance periods analyzed. After the geomagnetic effects are removed, the residual densities are interpreted in terms of solar flux variability. The daily-averaged SETA density residuals are strongly correlated with long-term solar flux variability, and exhibit a much greater dependence on the 27-day solar rotation period than MSISE-90 predictions. Variations in residual density of the order of 10–20% occur in association with day-to-day and 27-day solar flux variations. The MSIS model does not accurately predict the magnitude of these short-term density variations in response to solar activity.     

Spherical cap harmonic analysis of regional magnetic anomalies based on CHAMP satellite data

We used CHAMP satellite vector data and the latest IGRF12 model to investigate the regional magnetic anomalies over mainland China. We assumed satellite points on the same surface (307.69 km) and constructed a spherical cap harmonic model of the satellite magnetic anomalies for elements X, Y, Z, and F over Chinese mainland for 2010.0 (SCH2010) based on selected 498 points. We removed the external field by using the CM4 model. The pole of the spherical cap is 36N° and 104°E, and its half-angle is 30°. After checking and comparing the root mean square (RMS) error of ΔX, ΔY, and ΔZ and X, Y, and Z, we established the truncation level at K ^max = 9. The results suggest that the created China Geomagnetic Referenced Field at the satellite level (CGRF2010) is consistent with the CM4 model. We compared the SCH2010 with other models and found that the intensities and distributions are consistent. In view of the variation of F at different altitudes, the SCH2010 model results obey the basics of the geomagnetic field. Moreover, the change rate of X, Y, and Z for SCH2010 and CM4 are consistent. The proposed model can successfully reproduce the geomagnetic data, as other data-fitting models, but the inherent sources of error have to be considered as well.     

Absolute18O content of standard mean ocean water

The absolute values of the¹⁸O/¹⁶O ratio (R_s) and the relative¹⁸O content (X_s) in SMOW have been determined by comparing SMOW mass spectrometrically with well-defined synthetic mixtures of pure D₂¹⁸O and H₂¹⁶O. The results are:R_S = (2005.20 ± 0.45) × 10^?6, X_S = (2000.45 ± 0.45) × 10^?6     

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

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

A possible crust-mantle configuration near Trivandrum deduced from Indian geomagnetic data

Anomalies in H and Z, primarily attributable to subsurface features, were obtained from the annual mean values at five Indian magnetic stations for the period 1965–1973, after correcting for the secular and normal variations. When the average anomaly at each station over the nine years chosen, after projecting on to a reference longitude (78°E), was plotted against its geographic latitude, the resulting ΔZ and ΔH latitudinal profiles were found to reach their maximum values near Trivandrum (TRV). This rise in anomaly near TRV has been interpreted in terms of a probable plutonic dyke-like structure, superposed on the general upwarp of the mantle known to be associated with a land-to-ocean transition. A Bouguer gravity profile along the same reference longitude, supporting this inference, is also presented. A sample calculation has been done with assumed parameters for the model and the theoretical and observed anomalies are compared.     

The effect of solar activity on the secular variation of the geomagnetic field in Romania

The 11-year solar cycle effect in the geomagnetic components H and Z is made clear for Surlari Observatory and 19 repeat stations for the interval 1952–1974. The correlation with Wolf number and its time derivative is discussed in terms of the effects of the external and induced current systems.The $\text{H}\text{?}$ data available for solar cycle 20 (1964–1976) were processed to give the geographical distribution of the secular variation impulse for epoch 1969.5 in Romania. It is suggested that this distribution might reflect the deep internal structure of the area considered.A qualitative correlation is noted between long-period solar activity and variation of the horizontal component of the geomagnetic field at some repeat stations.     

Features of the latitudinal distribution of the annual variation of the geomagnetic field level and circumpolar current system in 1964

The results of investigations of the latitudinal distribution of annual variation of the geomagnetic field level in the seasons of 1964 (International Quiet Sun Year) using the data from 95 world magnetic observatories located at various longitudes in the latitude range 83° N–90° S are reported. The latitudinal features of the X, Y, and Z components of the geomagnetic field have been analyzed. It has been shown that the amplitudes (summer-winter differences) of the annual variation are maximal in the polar regions, decrease continuously to zero towards the equator, and are identical for both the Northern and Southern hemispheres with a half-year delay (local summer). The amplitudes of the equinox-winter difference in the equinox periods are smaller than those in summer, but are manifested simultaneously in the Northern and Southern hemispheres. An equivalent circumpolar current system has been constructed, which is responsible for the annual variation of the geomagnetic field level. Its parameters have been determined.     

Geomagnetic solar flare effect on February 4 and 6, 1986 at the China Antarctic Great Wall Station

Taking the sampled every minute values of the horizontal, declination and vertical componentsH, D, Z and the intensity of total field F calculated fromH andZ on the magnetograms at ten geomagnetic observatories in China in the same periods, and at the China Antarctic Great Wall Station (CAGWS), the authors conducted the maximum entropy analysis and band-pass filtration of these data and obtained the following results: (1) At the periodT=10 ? 90 min geomagnetic solar flare effect (sfe) is evident on the sunlit hemisphere. It is more pronounced at periods 15, 20, 25 and 30 min, and most prominent at 30 – 35 min. The solar X-ray spectra at the same time showed their peaks at 10 and 15 min; (2) The periodT=10 ? 70 min of sfe at the CAGWS in the western Hemisphere was also recognizable after spectral analysis and filtration, but the corresponding period of the maximum amplitude was different from that in the sunlit hemisphere. The results further proved that the geomagnetic effect of solar flares could also be observed in the dark hemisphere; (3) The subsolar points of two solar flares were found around Lanzhou, and the associated current density in the ionosphere was about 24 A/km. The transitional zone from positive to negative sfe was found around the geographic latitude?=22° ? 24°N, where the sfe inH-crochet was almost illegible.     

On the periodic variations of geomagnetic activity indices Ap and ap

Yearly averages of geomagnetic activity indices Ap for the years 1967–1984 are compared to the respective averages of v² · B_s, where v is the solar wind velocity and B_s is the southward interplanetary magnetic field (IMF) component. The correlation of both quantities is known to be rather good. Comparing the averages of Ap with v² and B_s separately we find that, during the declining phase of the solar cycle, v² and during the ascending phase B_s have more influence on Ap. According to this observation (using Fourier spectral analysis) the semiannual and 27 days, Ap variations for the years 1932–1993 were analysed separately for years before and after sunspot minima. Only those time-intervals before sunspot minima with a significant 27-day recurrent period of the IMF sector structure and those intervals after sunspot minima with a significant 28–28.5-day recurrent period of the sector structure were used. The averaged spectra of the two Ap data sets clearly show a period of 27 days before and a period of 28–29 days after sunspot minimum. Moreover, the phase of the average semiannual wave of Ap is significantly different for the two groups of data: the Ap variation maximizes near the equinoxes during the declining phase of the sunspot cycle and near the beginning of April and October during the ascending phase of the sunspot cycle, as predicted by the Russell-McPherron (R-M) mechanism. Analysing the daily variation of ap in an analogue manner, the same equinoctial and R-M mechanisms are seen, suggesting that during phases of the solar cycle, when ap depends more on the IMF-B_s component, the R-M mechanism is predominant, whereas during phases when ap increases as v increases the equinoctial mechanism is more likely to be effective.     

On spatial and temporal separation of geomagnetic anomalies at the southern tip of the Indian peninsula

In this communication, the applicability of differential geomagnetic deep sounding at the southern tip of the Indian peninsula has been investigated by analysing the nature of anomalies at Annamalainagar (ANN) and Trivandrum (TRD). Hyderabad (HYB) has been chosen as a reference station for this purpose. The results indicate that, despite a strong current channelling in this region, the anomalies do not permit separation into the spatial and temporal components which is possible in other areas with strong current channelling, like the Pyrenees and the Rhine Graben. This non-relationship between ΔH_A and ΔZ_A at ANN and TRD has been attributed to contributions from both coast effect (local induction) and channelling (regional effect) being simultaneously present in the area. During the day, the geometry of the source field produces further differences.     

Solar dynamo and geomagnetic activity

The correlation between geomagnetic activity and the sunspot number in the 11-year solar cycle exhibits long-term variations due to the varying time lag between the sunspot-related and non-sunspot related geomagnetic activity, and the varying relative amplitude of the respective geomagnetic activity peaks. As the sunspot-related and non-sunspot related geomagnetic activity peaks are caused by different solar agents, related to the solar toroidal and poloidal fields, respectively, we use their variations to derive the parameters of the solar dynamo transforming the poloidal field into toroidal field and back. We find that in the last 12 cycles the solar surface meridional circulation varied between 5 and 20 m/s (averaged over latitude and over the sunspot cycle), the deep circulation varied between 2.5 and 5.5 m/s, and the diffusivity in the whole of the convection zone was ～10⁸ m²/s. In the last 12 cycles solar dynamo has been operating in moderately diffusion dominated regime in the bulk of the convection zone. This means that a part of the poloidal field generated at the surface is advected by the meridional circulation all the way to the poles, down to the tachocline and equatorward to sunspot latitudes, while another part is diffused directly to the tachocline at midlatitudes, “short-circuiting” the meridional circulation. The sunspot maximum is the superposition of the two surges of toroidal field generated by these two parts of the poloidal field, which is the explanation of the double peaks and the Gnevyshev gap in sunspot maximum. Near the tachocline, dynamo has been operating in diffusion dominated regime in which diffusion is more important than advection, so with increasing speed of the deep circulation the time for diffusive decay of the poloidal field decreases, and more toroidal field is generated leading to a higher sunspot maximum. During the Maunder minimum the dynamo was operating in advection dominated regime near the tachocline, with the transition from diffusion dominated to advection dominated regime caused by a sharp drop in the surface meridional circulation which is in general the most important factor modulating the amplitude of the sunspot cycle.     

The equatorial electrojet

The typical quiet day variations of the equatorial electrojet (EEJ) current intensity with time of the day, season, sunspot number, and geomagnetic latitude are presented in terms of the corresponding variations of H which is the deviation of the horizontal component (H) of the geomagnetic field from its steady nighttime level. The observed height structure of the current density in the EEJ as measured in rocket flights is presented, along with the theoretically computed structure. Theoretical model results on the polarization electric fields and east-west currents as generated by the local interactions of height-varying winds in the EEJ show large height gradients and reversals for both currents and electric fields; experimental evidence for the reality of such height structures is also shown. The characteristics of the counter-electrojet events are presented and the possible causative mechanisms are discussed critically.Some typical experimental results are presented on the electric field changes in the EEJ which result from its sensitive response to electrodynamic disturbances in the magnetosphere and the auroral-polar latitude ionosphere during geomagnetic substorms and storms; and their implications are discussed. Possibilities for utilizing the EEJ as a very useful medium for important scientific studies on the larger space domain of ionosphere-magnetosphere system, on plasma waves, and on the earth's conductivity are emphasized.