Estimation of the westward auroral electrojet current using sparse magnetometer chain data

We investigate 1-D models of the westward substorm electrojet using magnetic field observations along a meridian chain of stations. We review two respective linear models from Kotikov et al. (1987) and Popov et al. (2001) with a large number of elementary currents at fixed positions. These models can be applied to a magnetometer chain with many magnetic stations. A new nonlinear method with one current element is designed for cases with a small number of stations. We illustrate the performance of these methods using data from the IMAGE (International Monitor for Auroral Geomagnetic Effects) and Yamal Peninsula stations. Several corrective measures are proposed to account for unphysical solutions or local extrema from the optimized functions. We also advertise a generic maximum likelihood approach to a problem that is feasible for any empiric model.

Estimation of westward auroral electrojet current with magnetometer chain data

We investigate one-dimensional models of westward substorm electrojet, using magnetic field observations along a meridian chain. We review two linear models of Kotikov et al. (1987) and Popov et al. (2001) with the large number of elementary currents at fixed positions. They can be applied to a magnetometer chain with many magnetic stations. A new nonlinear method with one current element is designed for the cases with small number of stations. We illustrate performance of these methods using data from IMAGE and Yamal Peninsula stations. Several corrective measures are proposed to account for unphysical solutions or local extrema of the optimized functions. We also advertize a generic maximum likelyhood approach to a problem, usable for any empiric model.

Characteristics of the electrojet during intense magnetic disturbances

Hall current variations in different time sectors during six magnetic storms from the summer seasons in 2003 and 2005 (Ritter, 2018) are examined, namely three storms in the day–night meridional sector and three storms in the dawn–dusk sector. The sequence of the phenomena, their structure and positions, and the strength of the polar (PE) and the auroral (AE) Hall electrojets were investigated using scalar magnetic field measurements obtained from the CHAllenging Minisatellite Payload (CHAMP) satellite in accordance with the study of Ritter et al. (2004a). We analyzed the correlations of the PE and AE as well as the obtained regression relations of the magnetic latitude MLat and the electrojet current intensity I with auroral and ring current activity, the interplanetary magnetic field, and the Newell et al. (2007) coupling function for the state of the solar wind. The following typical characteristics of the electrojets were revealed: The PE appears in the daytime sector at MLat ∼80∘–73∘, with a westward or an eastward direction depending on the interplanetary magnetic field (IMF) By component (By < 0 nT or By > 0 nT). Changes in the current flow direction in the PE can occur repeatedly during the storm, but only due to changes in the IMF By orientation. The PE increases with the intensity of the IMF By component from I∼0.4 A m−1 for By∼0 nT up to I∼1.0 A m−1 for By∼23 nT. The MLat position of the PE does not depend on the direction and intensity of the By component. There is no connection between MLat and I in the PE and the symmetric part of the magnetospheric ring current (index SymH). There is a correlation between I in the PE and the AsyH index, but only a very weak interconnection of this index with the MLat of the PE. Substorms occurring before the storm's main phase are accompanied by the appearance of an eastward electrojet (EE) at MLat ∼64∘ as well as that of a westward electrojet (WE). In the nighttime sector, a WE appears at MLat ∼64∘. During the main phase both electrojets persist. The daytime EE and the nighttime WE shift toward sub-auroral latitudes of MLat ∼56∘ and grow in intensity up to I∼1.5 A m−1. The WE is then located about 6∘ closer to the pole than the EE during evening hours and about 2∘–3∘ closer during daytime hours.

Global characteristics of auroral Hall currents derived from the Swarm constellation: dependences on season and IMF orientation

On the basis of field-aligned currents (FACs) and Hall currents derived from high-resolution magnetic field data of the Swarm constellation, the average characteristics of these two current systems in the auroral regions are comprehensively investigated by statistical methods. This is the first study considering both current types determined simultaneously by the same spacecraft in both hemispheres. The FAC distribution, derived from the novel Swarm dual-spacecraft approach, reveals the well-known features of Region 1 (R1) and Region 2 (R2) FACs. At high latitudes, Region 0 (R0) FACs appear on the dayside. Their flow direction, up or down, depends on the orientation of the interplanetary magnetic field (IMF) By component. Of particular interest is the distribution of auroral Hall currents. The prominent auroral electrojets are found to be closely controlled by the solar wind input, but we find no dependence of their intensity on the IMF By orientation. The eastward electrojet is about 1.5 times stronger in local summer than in winter. Conversely, the westward electrojet shows less dependence on season. As to higher latitudes, part of the electrojet current is closed over the polar cap. Here the seasonal variation of conductivity mainly controls the current density. During local summer of the Northern Hemisphere, there is a clear channeling of return currents over the polar cap. For positive (negative) IMF By a dominant eastward (westward) Hall current circuit is formed from the afternoon (morning) electrojet towards the dawn side (dusk side) polar cap return current. The direction of polar cap Hall currents in the noon sector depends directly on the orientation of the IMF By. This is true for both signs of the IMF Bz component. Comparable Hall current distributions can be observed in the Southern Hemisphere but for opposite IMF By signs. Around the midnight sector the westward substorm electrojet is dominating. As expected, it is highly dependent on magnetic activity, but it shows only little response to season and IMF By polarity. An important finding is that all the IMF By dependences of FACs and Hall currents practically disappear in the dark winter hemisphere.