Long-term variations in geomagnetic and solar activities and secular variations of the geomagnetic field components

Summary After the removal of the eleven-year periodicity, long-term patterns of the aa indices of geomagnetic activity and of Wolf's sunspot numbers are defined. The positions of maxima and minima exhibit the same regularities as the secular variations of the geomagnetic filed components. This result is associated with the motion of the Sun round the barycentre of the solar system.Presented at symposium Planet 88, Tihany, September 1988.     

Regional features of the geomagnetic field secular variations

The secular variations in the geomagnetic field have been studied based on the satellite vector magnetic survey. A high data accuracy made it possible to estimate the spatial and temporal characteristics of different variation types on the interval 1980–2007. The growth and decomposition of midlatitude foci have been qualitatively estimated, and the structure and velocity of a drifting equatorial anomaly have been determined.     

Results of the monitoring of geomagnetic field variations in the Kamchatka region

The transfer function between the vertical and horizontal components of geomagnetic field variations is studied and the frequency responses of its parameters are presented. The relation to geoelectric heterogeneities of the medium is analyzed. The coast effect is considered and a deep curve of the apparent electrical resistivity constructed on its basis is used for estimating the depth to the asthenospheric conducting layer. The behavior of the induction vectors in the frequency-time domain is studied. Specific features of the behavior of the real and imaginary parts of the induction vector related to geoelectric heterogeneities of the medium are determined. Monitoring results are compared with time moments of earthquakes of K = 13?14 at epicentral distances of up to 150 km.     

Analysis of diurnal dynamics of the geomagnetic field variation spectrum

A statistical analysis of the variations of three components of the Earth’s magnetic field obtained at Almaty during 2008 is performed. It is found that in the daytime and nighttime periods, a distinct difference in the spectra of variations with periods corresponding to acoustic-gravity waves is observed. An explanation of this difference is proposed. It is based on the mechanism of change in the acoustic-gravity wave spectrum caused by the interaction of atmospheric gas with solar radiation.     

Plumes of the plasmasphere and variations in the geomagnetic field horizontal component

The effects of the dayside and dusk plumes of the plasmasphere during the ring current recovery phase on the disturbance level of the ground geomagnetic field horizontal component have been considered. It has been indicated that the geomagnetic field horizontal component changes specifically and synchronously in the region corresponding to the plasmaspheric dayside plume. Outside the plume the time variations in the geomagnetic field horizontal component pronouncedly differs. A spectral analysis of disturbances in the geomagnetic field horizontal components in the range of geomagnetic pulsations indicated that the intensity in the range of Pc4 pulsations increases at magnetic stations located on field lines corresponding to the dayside and dusk plumes of the plasmasphere. These pulsations detected in the dynamic spectrum of the geomagnetic field horizontal components in the dayside plume region of the plasmasphere, probably reflect the resonance oscillations of magnetic field lines in the region of field-aligned currents at comparatively low altitudes. We assume that this is caused by the instability of field-aligned currents originating as a result of the interaction between the ring current energetic ions and electromagnetic waves in the region with a relatively dense background plasma of the dayside plume.     

Lunar and luni-solar variations of the geomagnetic field in the Indian region

Summary Lunar and luni-solar geomagnetic components have been computed upto four harmonics for low latitude station Alibag, outside equatorial electrojet belt, and the equatorial electrojet stations Annamalainagar, Kodaikanal and Trivandrum in the south Indian region. The computations are confined to data of very high solar activity period 1958–61. Amplitudes of lunar semidiurnal component (L ₂), in the horizontal intensity (H), undergo an equatorial enhancement. Phase difference of 2 hrs is noticed inL ₂ (H) between nonelectrojet and electrojet stations. In the vertical intensity (Z), L ₂ is maximum ine andj-seasons at Trivandrum, close to the magnetic equator. Ind-season, however, maximumL ₂ (Z) occurs at Annamalainagar (dip 5°.4N). The phase difference between the electrojet and nonelectrojet stations observed inL ₂ (H) is not noticed inL ₂ (Z). The differential vertical upward drift motion of charged particles may explain the observed phase difference inL ₂ (H). Seasonal variations in amplitudes and times of maxima are noticed at all the stations inL ₂ (H) andL ₂ (Z). Similar variation is also noticed at Alibag inL ₂ of declination (D).     

Low-latitude variations in the geomagnetic field caused by solar wind disturbances

In view of the actual question regarding the effect of a solar-wind pressure jump on disturbances in the Earth’s magnetosphere, events with high velocity and density gradients are of special interest. In this work, we consider the response of the current at the dayside magnetopause to these events and the corresponding strengthening of the geomagnetic field in the low-latitude magnetosphere. A transient process is studied that accompanies reconfiguration of the magnetosphere under the effect of disturbances of solar wind parameters. An analytical equation is received for estimation of an increase in the northern component of low-latitude magnetic field of the magnetosphere in a transient current system (transient ring current) versus initial values of the solar-wind velocity and density and their disturbances.     

Preliminary study on the source field mode of geomagnetic six-month-period variations

The monthly means of north component X of geomagnetic field from 16 observatories during 1984–1988 were analyzed using the Sompi spectral analysis technique Most of these observatories are located in China. The analysis of the semiannual variations indicates that the latitude has no apparent effect on the X component. This clearly implies that the source field mode of semiannual variations cannot simply be described by using the p ₁ ⁰ mode. Using the p ₁ ⁰ mode to estimate the inductive scale lengthC in the semiannual period, the value ofC at each observatory would be biased significantly. The purpose of this study is to find which kind of modes is optimal for estimating the values ofC corresponding to the semiannual variations. The results show that a composite mode, involving five terms P _n ⁰ (n = 1,, 5), might be a reasonable and acceptable one     

Lunar and solar daily variations of the geomagnetic field at Toolangi

Summary Mean hourly values of magnetic declination, horizontal intensity and vertical intensity observed at Toolangi during two ten year periods (1924–1933 and 1949–1958) have been analysed to determine their solar and luni-solar diurnal components. The results, showing the variations of the first four harmonic components with season, degree of magnetic activity and annual sunspot number, are tabulated and discussed. It is shown that there are marked differences in the dependence ofS andL on the various parameters and a tentative explanation of this phenomenon is given.     

Secular variations in the main geomagnetic field within the scope of the dynamic model of field sources

The effect of different-level sources on the spatial structure of the secular variations has been considered based on the dynamic model of sources of the main geomagnetic field developed by us. It has been obtained that the development of 13 most powerful dipoles only roughly characterizes global anomalies of the secular variations, and each anomaly results from the superposition of the dynamics of several sources. The model secular variations have been compared with the data from the observatories. It has been obtained that it is impossible to describe local anomalies of the observed secular variations ignoring sources of the third order of smallness as compared to the main dipole. It has been assumed that topographic vortices, originating around inhomogeneities of the core-mantle boundary, can be physical sources responsible for dipoles of the third order.     

Synchronism of variations in the main geomagnetic field and earth rotation velocity fluctuations

Variations in the value of the source magnetic moment, responsible for the dipole component of the main geomagnetic field, and Earth rotation velocity fluctuations have been studied comparatively. Three quasi-harmonic components with close periods, for which their own phase shifts were obtained, have been detected in time series of the analyzed processes. The mantle conductivity effect on these shifts has been studied in the scope of a one-layer model; for this purpose, the amplitude-phase characteristic of the mantle as a filter has been constructed. The mantle conductivity has been estimated (σ = 270 ± 20 S m^?1) on the assumption that the phase shift mainly depends on the mantle effect. It has been assumed that the variations in the time series of the analyzed processes are almost cophased and can be caused by one external factor, e.g., tides.     

Long period variations of the geomagnetic field and their spatial distribution in Europe

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地球主磁场空间功率谱的变化特征

根据第8代IGRF模型，计算分析了1900－2000年地球磁场和分量X，Y，Z的空间功率谱的变化特征。结果表明1900－2000年主磁场的空间功率谱呈减小趋势，由功率谱随谐波阶次n的变化关系得出的非偶极子场的等效磁源位于核幔边界附近，而且其位置随时间变化，用n=1,2两阶谐波的谱得到的等效磁源位于地球内核边缘，1900年以来离开地心一直向外增大。     

南极中山站与北极地磁活动相关性研究

本文利用南极中山站和新奥尔松（NAL）等5个北极站2000年9月到2003年3月共719天的地磁观测数据,以20 min间隔内地磁北向水平分量的互相关系数大于0.9为判据,对南北极地磁活动相关的发生率进行了统计,并与T89地磁模型得到的中山站共轭位置的分布进行了比较. 结果表明,南北极地磁活动相关的发生率与台站位置、季节、地方时和地磁活动性有着密切的关系. 在北极的5个台站中朗伊尔站（LYR）与中山站地磁相关的时间最多（占17%）;在两分季发生相关的情况比冬季和夏季多;在傍晚发生相关的情况最多,中午发生相关的情况最少;地磁活动较强时,相关的情况比地磁宁静时更多一些.     

Changes in cosmic ray cut-off rigidities due to secular variations of the geomagnetic field

An analytical expression is derived for the cutoff rigidity of cosmic rays arriving at a point in an arbitrary direction, when the main geomagnetic field is approximated by that of an eccentric dipole. This expression is used to determine changes in geomagnetic cutoffs due to secular variation of the geomagnetic field since 1835. Effects of westward drift of the quadrupole field and decrease in the effective dipole moment are seen in the isorigidity contours. On account of the immense computer time required to determine the cutoff rigidities more accurately using the particle trajectory tracing technique, the present formulation may be useful in estimating the transmission factor of the geomagnetic field in cosmic ray studies, modulation of cosmogenic isotope production by geomagnetic secular variation, and the contribution of geomagnetic field variation to long term changes in climate through cosmic ray related modulation of the current flow in the global electric circuit.     

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.     

基于XGBoost机器学习的地磁日变重构方法研究

为了重构或恢复存在严重干扰或数据缺失的台站观测数据,本文基于周边已有台站的高质量观测数据采用XGBoost机器学习方法重构地磁日变数据.仿真试验结果显示,无论是磁静日还是磁扰日,地磁场分量的绝对残差均值均低于0.1 nT.试验统计数据及重构结果残差曲线的对比分析表明,地磁日变重构精度与地磁活动性和待重构信号的时变剧烈程...     

Horizontal geomagnetic field variations at low latitude in South Atlantic Magnetic Anomaly

Summary Horizontal geomagnetic field variations on international quiet days at a low latitude station situated in the South Atlantic Magnetic Anomaly are studied with reference to the problem whether these variations are caused by the daytime increases or by the night time depressions of the field. It is found that both the mechanisms are operative significantly. The correction for noncyclic change decreases the magnitude of the night time depression. However, it does not account fully for the same. Possible causes for the night depressions are also discussed.