Equivalent ionospheric current systems representing IMF sector effects on the polar geomagnetic field

Equivalent ionospheric current systems representing IMF sector effects on the geomagnetic field in high latitudes are examined for each of the twelve calendar months by spherical harmonic analyses of geomagnetic hourly data at 13 northern polar stations for seven years. The main feature of obtained equivalent current systems includes circular currents at about 80° invariant latitude mostly in the daytime in summer and reversed circular currents at about 70° invariant latitude mainly at night in winter. Field-aligned current distributions responsible for equivalent currents, as well as vector distributions of electric fields and ionospheric currents, are approximated numerically from current functions of equivalent current systems by taking assumed distributions of the ionospheric conductivity. Two sets of upward and downward field-aligned current pairs in the auroral region, and also a field-aligned current region near the pole show seasonal variations. Also, ionospheric electric-field propagation along geomagnetic field lines from the summer hemisphere to the winter hemisphere with auroral Hall-conductivity effects may provide an explanation for the winter reversal of sector effects.     

Numerical analysis of equatorial enhancement of geomagnetic sudden commencement

Equatorial behaviour of a polar-originating ionospheric current is examined by solving numerically the continuity equation on a two-dimensional spherical shell with appropriate assumptions for the ionospheric conductivity and the field-aligned source currents. The results show a clear daytime equatorial enhancement of the ionospheric currents in spite of much reduced electric field due to shielding effects of the enhanced Cowling conductivity there. The results are used for interpretation of the preliminary impulse of the geomagnetic sudden commencement.     

Indication of anomalous conductivity at the nigerian dip equator

The geomagnetic daily variations at the Nigerian dip equator have been analyzed with the methodology introduced in a previous paper. It has been found that the height integrated current presents a notoriously higher amplification in Nigeria than in Peru. It has also been found that there exists a strong and inhomogeneous anomaly in the Earth's conductivity in Nigeria. And contrary to what is usually accepted, it is shown that its latitudinal distribution can not be precisely determined until the distribution and magnitude of the ionospheric currents at F-region heights is more accurately known.     

Space weather risk in power systems and pipelines

Geomagnetically induced currents (GIC) in technological systems, such as electric power transmission grids, oil and gas pipelines, telecommunication cables and railway equipment, are a harmful space weather effect at the earth's surface. In power systems GIC cause saturation of transformers, which may lead to serious problems and even to a collapse of the whole system, as occurred in Quebec in March 1989, or to permanent damage of transformers. In buried pipelines GIC give rise to corrosion problems. GIC are driven by the geoelectric field induced by a geomagnetic disturbance. The electric and magnetic fields primarily depend on ionospheric currents and secondarily on currents induced in the earth. GIC risk in a technological system can be decreased by help of forecasting methods. This requires predictions of ionospheric currents to be used as an input for the calculation of the geoelectric field and GIC. Recent developments in the calculation techniques based on the Complex Image Method (CIM) permit fast and accurate computations suitable for a time-critical application like GIC forecasting.     

Dynamo currents representing geomagnetic L variation demonstrated by a multi-layer ionospheric model

A multi-layer ionospheric model and lunar (2,2) tidal mode have been used to calculate dynamo current systems representing lunar geomagnetic semidiurnal variations. Since both the height variation of the ionospheric conductivities and latitudinal dependence of the height of the conductivity peaks have been taken into account, the dynamo current systems agree with equivalent ones (estimated from geomagnetic data) better than those for a thin shell model of the ionospheric conductivity, especially in the polar region.     

On the effects of geomagnetic storms and pre storm phenomena on low and middle latitude ionospheric F2

This paper presents some features of the ionospheric response observed in equatorial and mid-latitudes region to two strong geomagnetic storms, occurring during Oct. 19–23, 2001 and May 13–17, 2005 and to understand the phenomena of pre-storm that lead to very intense geomagnetic storms. The result point to the fact that pre-storm phenomena that leads to intense ionospheric storm are; large southward turning of interplanetary magnetic field Bz, high electric field, increase in flow speed stream, increase in proton number density, high pressure ram and high plasma beta. The magnitude of Bz turning into southward direction from northward highly depends upon the severity of the storm and the variation in F2 layer parameter at the time of geomagnetic storm are strongly dependent upon the storm intensity. A detailed analysis of the responses of the ionosphere shows that during the storm periods, foF2 values depleted simultaneously both in the equatorial and mid latitude. Observation also shows that low to moderate variations in ionospheric F2 at the pre-storm period may signal the upcoming of large ionospheric disturbances at the main phase. The ionospheric F2response for low and mid latitude does not show any significant differences during the storm main phase and the pre-storm period. The ionospheric response during the pre-storm period is thought very puzzling. The period is observed to be depleted throughout with low-moderate effect across all the stations in the low and mid latitude.     

A study of the relationship between geo-magnetic storms and ionospheric disturbances at mid-latitudes

The problem of the ionospheric disturbances associated with geomagnetic storms is examined with the goal of searching for a relationship between the time-developments of the two phenomena. Faraday rotation measurements of total electron content (N_T) are used to monitor the ionospheric F-region at a mid-latitude site, while a variety of geomagnetic parameters are examined as possible ways of following the geomagnetic variations. The ionospheric and geomagnetic data taken during 28 individual storms from 1967 to 1969 are used to search for a predictive scheme which can be tested using data from 17 storms in 1970. The specific aim is to find the geomagnetic parameter whose time-development can best forecast whether or not the ionospheric response will include an initial positive phase prior to the normally extended period of F-region depletions. Correlations between N_T and the geomagnetic indices K_p, and equatorial Dst(H) prove to be wholly inadequate. The local times of main-phase-onset (MPO) determined from the equatorial Dst(H) indices as well as from local horizontal component data, also prove to be unsatisfactory. The best correlations are obtained using local measurements of the total geomagnetic field (F). These results show that a storm commencement (SC) will produce an enhancement in n_t during the afternoon period following the SC unless there is an intervening post-midnight period with a strong depression of the geomagnetic field. Operationally this is taken to be a depression in F of at least 100γ near 03:00 LT     

Meridional ionospheric electric field caused by curvature current in the magnetosphere

The bending of geomagnetic field lines towards the geotail produces a curvature drift of charged particles parallel to the geomagnetic axis. The divergence of the current so produced forms Birkeland current to the ionosphere where a meridional electric field is created. This field would drive ionospheric currents to form a negative magnetic bay in the dawn sector of the auroral zone and a positive one in the dusk sector. Also it would cause a dawn-dusk field across the polar cap.     

A hydromagnetic theory of geomagnetic storms and auroras

A theory of geomagnetic storms, auroras and associated effects is further developed. It depends on motions in the Earth's exosphere or magnetosphere initiated by a combination of pressure and frictional drag of the solar wind and modified and extended by electric fields and currents in the ionosphere. Motion may be non-divergent, streamline flow opposed only by Lorentz forces in the ionosphere and not propagating to Earth, or divergent, non-streamline motion opposed by Lorentz forces in the Earth. The two types of motion are coupled in the E region where the former is identified with free flow of Hall current and the generation of non-streamline motion. The latter is identified with blockage of Hall current, the creation of a polarization field and hence the generation of streamline motion.

A theory of all components of a geomagnetic storm is given in terms of combinations of these motions, and their distant, ionospheric and earth currents. This includes a new theory of the preliminary reverse part of the DS field and the transition from the sudden commencement to the main phase of the DS field. It is extended to introduce briefly a theory of auroras based mainly on ionospheric drifts caused by the magnetospheric motions.     

The role of the Hermanus Magnetic Observatory in geomagnetic field research

Geomagnetic field research carried out at the Hermanus Magnetic Observatory over the past decade is reviewed. An important aspect of this research has been the study of geomagnetic field variations, with particular emphasis on ULF geomagnetic pulsations. Features of geomagnetic pulsations which are unique to low latitude locations have been investigated, such as the cavity mode nature of low latitude Pi 2 pulsations and the role played by ionosphericO ⁺ ions in the field line resonances responsible for Pc 3 pulsations. A theoretical model has been developed which is able to account for the observed relationships between geomagnetic pulsations and oscillations in the frequency of HF radio waves traversing ionospheric paths. Other facets of the research have been geomagnetic field modelling, aimed at improving the accuracy and resolution of regional geomagnetic field models, and the development of improved geomagnetic activity indices.     

Ionospheric currents obtained from the Chatanika radar and ground magnetic perturbations at the Auroral latitude

The relationship between substorm ionospheric currents and the corresponding ground magnetic perturbations is examined, by using the height-integrated ionospheric current density deduced from the Chatanika incoherent scatter radar and the simultaneous magnetic variations along the Alaska meridian chain of stations. Although time variations of the H component near the radar site on the Earth's surface are in good agreement with those of the east-west ionospheric current, there is a substantial disagreement between the current deduced from the D perturbations and the observed north-south current in the evening sector. It is shown that the disagreement can be removed by introducing a new finding by Yasuhara et al. (1975) that the upward field-aligned current on the poleward side of the auroral oval in the evening sector is more intense than its counterpart fieldaligned current and that it contributes greatly to the ground D perturbations.     

Latitudinal characteristics of the geomagnetic field during the storm of 15–16 July 2000

In this paper geomagnetic disturbances at middle and low latitudes are discussed by using geomagnetic data of the magnetic storm of 15–16 July 2000. This storm is a response to the solar Bastille Day flare on 14 July. Generally, the geomagnetic disturbances at middle and low latitudes during a storm are mainly caused by three magnetospheric–ionospheric current systems, such as the ring current system (RC), the partial ring current and its associated region II field-aligned currents (PR), and the region I field-aligned currents (FA). Our results show that: (1) The northward turning of IMF-Bz started the sudden commencement of the storm, and its southward turning caused the main phase of the storm. (2) The PR- and FA-currents varied violently in the main phase. In general, the field of the FA-current was stronger than that of the PR-current. (3) In the first stage of the recovery phase, the RC-field gradually turned anti-parallel to the geomagnetic axis from a 15° deviation, and the local time (Λ) pointed by the RC-field stayed at 16:00. After that, Λ rotated with the stations, and the RC-field was not anti-parallel to the geomagnetic axis, but 5°–10° deviated. These facts suggest that the warped tailward part of the ring current decays faster than the symmetric ring current.     

Ionospheric dispersion and Pc 1 pulsations

Dispersion measurements were performed on geomagnetic pulsation data recorded over an Australasian network in a search for evidence of ionospheric dispersion of Pc 1 signals. A method of analysis was adopted in which the slope of emission elements of a selected Pc 1 event are examined individually. It has been found that there are no significant ionospheric dispersion effects for propagation between middle and low latitudes. Magnetospheric propagation paths calculated from dispersion measurements show large variations and are not considered generally reliable.     

中低纬度变化磁场的分析(英文)

地球变化磁场呈现复杂时空特点,这是由引起该磁场的磁层一电离层电流以及地球内部感应电流的特性决定的。为了研究变化磁场的物理成因及其在日地物理事件中的特性。首先必须将组成变化磁场的各种成分分离开来,然后逐一加以研究。 我们采用自然正交分量法对我国八个地磁台站的时均值序列进行了分析,这些台站展布在27°12′48″到49°36′的中低纬度带内,正是Sq电流体系焦点所在的纬度带。分析结果表明,由发电机过程产生的Sq电流体系是这一纬度带主要的电流体系,与磁暴环电流有关的扰动电流体系也是十分重要的电流体系,在冬季月份,它往往超过Sq程度。此外与UT有关的磁扰变化也被明显地分离出来,它的成因可能与地球磁场的偏心结构有关。这些成份的相对大小随季节变化,而且有确定的纬度分布。 我们提出了一套单台分析和多台分析的方法。考虑到自然正交分量法收效快,稳定性好,所需资料列序列短的特点,这种方法可以推广到台站使用。自然正交分量法可以从成因上分离不同成因,使它在理论研究中具有优于一般付氏分析、时序迭加等方法,可为中低纬电流系成因研究提供有用的结果。     

Birkeland currents in the Earth's magnetosphere

As a result of his polar expeditions at the beginning of this century, Kristian Birkeland determined that intense ionospheric currents were associated with the aurora. Birkeland suggested that these currents originated far from the Earth and that they flowed ointo and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned or Birkeland currents was disputed because it was not possible to unambiguously identify current systems that are field-aligned (as suggested by Alfvén, 1939, 1940) and those which are completely contained in the ionosphere (as developed by Vestine and Chapman, 1938) with surface magnetic field observations. The presence of Birkeland currents has been absolutely confirmed with satellite-borne particle and magnetic field experiments conducted over the past two decades. These satellite observations have determined the large-scale patterns, flow directions, and intensities of Birkeland currents in the auroral and polar regions, and their relationship to the orientation and magnitude of the interplanetary magnetic field. The Birkeland currents are directly associated with visible and UV auroral forms observed with satellites. The results obtained from a variety of recently launched satellites are discussed here. These include Sweden's first satellite, VIKING, which has provided evidence for resonant Alfvén waves on the same geomagnetic field lines that guide stationary Birkeland currents. These observations demonstrate the important role that these currents play in the coupling of energy between the interplanetary medium and the lower ionosphere and atmosphere.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Description of the coastal effect at equatorial latitudes with applications to the Peruvian and Nigerian zones

The problem of the electromagnetic induction produced by a localized and an extended ionospheric current near an ocean coast, over a mantle of infinite conductivity, has been reduced to the solution of an integral equation where the induced current density appears in an implicit form. This formalism is applied to calculate the field induced by the geomagnetic daily variation due to the presence of the ocean at the Peruvian and Nigerian equatorial zones.     

Short-term critical frequency variations and their predictions in the midlatitude ionospheric F2 region

Short-term variations δf₀F₂ in the values of the critical frequency of the ionospheric F2 region in middle latitudes due to solar and geomagnetic activities have been investigated. Diurnal and seasonal features of the energy flow from the auroral into midlatitude ionosphere are revealed. It is shown that they could be taken into account if instead of the 3-hour geomagnetic indices or their daily averages a new index is employed which characterizes the average level of geomagnetic activity over intervals of time no less than nine hours usually during the evening and night hours. A technique for short-term predicting δf₀F₂ in the midlatitude ionosphere is developed which employs the indices of solar and geomagnetic activities, and errors in the predictions are estimated.     

Latitude variation of the spectral components of auroral zone Pi2

Evidence is presented from spectral analysis of Pi2 pulsations detected during a substorm by the University of Alberta meridian chain of magnetometers to support the conclusion that at auroral latitudes there is no apparent correlation between the principal spectral components of Pi2 pulsations and the latitude of the observations. From these data we infer that the Pi2 magnetic variations observed at the Earth's surface are not generated by simple MHD eigenoscillations of magnetospheric field. As well, the data show clear contributions to the Pi2 pulsation spectrum by ionospheric currents. These observations lead to the suggestion that Pi2 pulsation spectra are produced by the sudden changes in magnetospheric and ionospheric current systems which take place at the beginning of a substorm.     

Ionospheric response to an isotropic spectrum of internal gravity waves

This paper presents computer-produced tonal-value plots in which darkness or intensity are used to display the ionospheric response to an isotropic spectrum of internal gravity waves. Each plot shows at a glance those portions of the wave spectrum, i.e. those wave periods and azimuths of propagation, producing the ionospheric irregularities of greatest magnitude. Arrays of the plots illustrate the variations of this response with time of day, season, latitude, geomagnetic dip, and height.     

Ionospheric foF2 storm forecasting using neural networks

The ionosphere shows a large degree of variability on time scales from hours to the solar cycle length. This variation is associated with magnetospheric storms, the Earth's rotation, the season, and the level of solar activity. To make accurate predictions of key ionospheric parameters all these variations must be considered. Neural networks, which are data driven non-linear models, are very useful for such tasks. In this work we examine if the F2 layer plasma frequency, foF2, at a single ionospheric station can be predicted 1 to 24 hours in advance by using information of past foF2 observations, magnetospheric activity, and time as inputs to neural networks. Particular attention has been paid to periods when great geomagnetic storms were in progress with the aim to develop a successful ionospheric storm forecasting tool.