O. Amm

At substorm onset, 40% of AL comes from underground

Magnetic variations observed at the Earths surface are caused by external and internal sources. External variations arise from currents in the ionosphere and magnetosphere, and internal variations arise from currents induced in the solid Earth. In this paper we examine how large the internal contribution is to magnetic variations measured at the Earths surface. We use IMAGE magnetometer measurements to analyze 77 substorms during 1997. For each event we evaluate the internal and external parts of a locally derived auroral electrojet index (IL index). The magnetic field separation is performed using the Siebert-Kertz equations. A superposed epoch analysis of all events clearly shows that the internal contribution peaks strongly at substorm onset, when the internal contribution is ∼ 40% of the total field. After the substorm peak intensity, the internal contribution decreases almost linearly to the quiet time value of 10-20%. The induction effects are largest during the times of rapid changes and at stations located over the Arctic Ocean.

Direct determination of the local ionospheric Hall conductance distribution from Two-dimensional electric and magnetic field data: Application of the method using models of typical ionospheric electrodynamic situations

The applicability of a method to directly deduce the ionospheric Hall conductance distribution Σ H from ground magnetic and ionospheric electric field observations, here called method of characteristics, is tested by using input data from models of some typical ionospheric electrodynamic situations. We shall show that the method in the form of a fully automatical computer algorithm is able to reproduce well the Σ H distributions of all modeled situations, i.e. a two-dimensional eastward electrojet, a Harang discontinuity, an omega band, and a westward traveling surge. Furthermore, we will show quantitatively that the ambiguity implied by the necessary assumption of the distribution of the Hall to Pedersen conductance ratio has only a small effect on the results obtained. We also prove quantitatively that the assumption of vertical, straight geomagnetic field lines made in the derivation of the method leads only to very small errors if the case of oblique, but straight geomagnetic field lines is taken to be realistic. Moreover, we review the general theory of the method and discuss some additional theoretical aspects. In particular, we will show that isolated points with E=0, but nonvanishing ⊇ h .E are associated with extrema or saddle points of the Σ H distribution, and calculate the magnetic field disturbance of an ionospheric current system with oblique, but straight field-aligned currents directly below the ionospheric plane. The method does not require an electrostatic situation ; i.e., ⊇ h ×E¬=0 is allowed.

Modeling geomagnetically induced currents during different ionospheric situations

We use several realistic three-dimensional models of ionospheric currents to calculate geomagnetically induced currents (GIC) in the Finnish high-voltage power system. Of special interest are events during which the magnetic field changes rapidly and GIC are large. The geoelectric field driving GIC is determined with the complex image method, which is a fast and accurate tool for taking into account induction effects in the Earth. A detailed investigation is made applying a model of a westward traveling surge (WTS). It is capable of producing magnetic field variations and GIC which are of the same magnitude as the observed values. However, the WTS model yields too large time derivatives of the magnetic field. A much simpler line current model produces very realistic magnitudes of both the magnetic field and GIC. However, in contrast to WTS, it lacks the realistic spatial structure of the ground magnetic field. The requirement of accurate models of the Earths conductivity is demonstrated by comparing a resistive and conductive structure in connection with a very rapid change of the magnetic field. Consideration of some other typical ionospheric events (Harangs discontinuity, omega band, pulsation) indicates that these phenomena probably cannot produce extremely large GIC.

Observations of an active thin current sheet

We analyze observations of magnetotail current sheet dynamics during a substorm between 2330 and 2400 UT on 28 August 2005 when Cluster was in the plasma sheet at [-17.2, -4.49, 0.03] R-E (GSM) wit ...

One-dimensional upward continuation of the ground magnetic field disturbance using spherical elementary current systems

Ionospheric equivalent currents are defined as spherical sheet currents, which reproduce the observed magnetic disturbances below the ionosphere. One way of determining these currents is to place several so called spherical elementary current systems (SECS) in the ionospheric height and to solve an inversion problem for the amplitudes of these systems. In previous studies this method has been applied to two-dimensional data sets, having both latitudinal and longitudinal spatial coverage (2D SECS method). In this paper a one-dimensional variant of this method (1D SECS) is developed. The 1D SECS method can be used even in those situations where the data set is one dimensional, e.g. with one meridionally aligned magnetometer chain. The applicability of the 1D SECS method is tested using both synthetic and real data. It is found that in real situations the errors in the 1D SECS results are 5—10% in current density profiles and ~5% in integrated currents, when compared to the results of the more accurate 2D SECS method.

Development and validation of inversion technique for substorm current wedge using ground magnetic field data

The classic substorm current wedge model represents ground and space magnetic perturbations measured during substorms. We have developed an inversion technique to calculate parameters determining the intensity and geometry of the current system using magnetic field data at midlatitudes. The current wedge consists of four segments: a sheet-like field-aligned current downward to the ionosphere postmidnight, a westward current across the auroral bulge, an upward sheet-like current from the westward surge premidnight, and an eastward current in the equatorial plane. The model has five parameters including the current strength, the locations, and breadths of the two field-aligned current sheets. Simultaneous changes in the ring current are represented by the superposition of a symmetric ring current and a partial ring current characterized by three additional parameters. Parameters of the model are determined as a function of time based on midlatitude ground magnetometers, using realistic field lines and accounting for Earths induction. The model is validated by a variety of techniques. First, the model predicts more than 80% of the variance in the observations. Second, the intensity of the current wedge and the ring current follows the same trends of the westward electrojet and the ring current indices. Third, the intensity of the westward electrojet agrees extremely well with the intensity of the current wedge. Finally, spacecraft observations of the aurora correspond with the evolution deduced from the model. This model of the substorm current wedge provides a valuable tool for the study of substorm development and its relation to phenomena in space.

Prediction of geomagnetically induced currents in power transmission systems

Abstract Geomagnetically induced currents (GIC) in technological systems are a manifestation of space weather at the Earths surface. In power systems GIC cause saturation of transformers, which may even result in a collapse of the whole system and in damage of transformers. Predictions of GIC could help power systems operate smoothly through large geomagnetic disturbances. In physical modelling the key quantity is the geoelectric field at the Earths surface. Efforts aiming at a reliable physical GIC forecasting should thus concentrate on the determination of the electric field, in particular in extreme storm conditions containing rapidly varying ionospheric currents. The method of calculation of the electric field should not simplify the geophysical situation too much but it should permit fast computation. Recently, new techniques based on the complex image method (CIM) have been developed, which are accurate and fast. This paper describes the applicability of CIM to GIC forecasting. A westward travelling surge (WTS) event is considered in detail, and the GIC produced in the Finnish high-voltage power system are determined.

Separation of the geomagnetic variation field on the ground into external and internal parts using the spherical elementary current system method

Traditionally the separation of the ground geomagnetic field variations into external and internal parts is carried out by applying methods using harmonic functions. However, these methods may require a separate field interpolation and extrapolation, can be computationally slow, and require a minimum wavelength to be specified to which the spatial resolution is limited globally. A novel method that utilizes elementary current systems can overcome these shortcomings. The basis is the fact that inside a domain free of current flow, the magnetic field can be continued to any selected plane in terms of equivalent currents. Two layers of equivalent currents, each composed of superposition of spherical elementary systems, are placed to reproduce the ground magnetic field: One above the surface of the Earth representing the field of ionospheric origin, and one below it representing the field caused by induced currents in the Earth. The method can be applied for single time steps and the solution of the associated underdetermined linear system is found to be fast and reliable when using singular value decomposition. The applicability of the method is evaluated using synthetic magnetic data computed from different ionospheric current models and associated image currents placed below the surface of the Earth. Following these tests, the method is applied to the measurements of Baltic Electromagnetic Array Research (BEAR) (June–July 1998). External and internal components of geomagnetic variations were computed for the entire measurement period. Also the adequacy of the sparser IMAGE magnetometer network for the full field separation was tested.

Improved electrodynamic modeling of an omega band and analysis of its current system

We improve a previous electrodynamic model of an omega band by applying a nonuniform spatial distribution of the Hall to Pedersen conductance ratio instead of using a constant value. The analysis of the current system due to our model reveals that after separating the background westward electrojet off, the omega band currents mainly consist of Hall currents circulating around concentrated upward field-aligned currents in the auroral tongue. However, the upward field-aligned currents exhibit a nearly circular configuration around the center of the tongue and almost vanish directly in the center. Equally strong downward field-aligned currents are missing. Additionally, at the western flank of the tongue we notice an about northwest–southeast aligned Cowling channel. Its currents are mainly responsible for the spike-like D peak in magnetograms and for the general asymmetry in D and Z longitudinal profiles on the ground. An additional enhancement of the Hall conductance in this channel further improves the agreement between calculated and measured ground magnetic disturbance profiles.

Extreme solar‐terrestrial events of October 2003: High‐latitude and Cluster observations of the large geomagnetic disturbances on 30 October

Extreme solar-terrestrial events of October 2003: high latitude and Cluster observations of the large geomagnetic disturbance on October 30