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First auroral ionospheric electric currents and conductances from Swarm: a new method tested

First auroral ionospheric electric currents and conductances from Swarm: a new method tested

A new method developed at the Finnish Meteorological Institute enables procession of satellite measurements into important data products for auroral research.

Complicated structures of electric currents, conductances and fields are associated with various auroral forms. These structures and their development are not well understood, because key measurements, especially ionospheric conductances, have not been sufficiently available in the past. The European Space Agency's Swarm mission, launched in 2013, has improved the situation. Swarm consists of three identical satellites on near-polar orbits carrying magnetometers and electric field instruments: Swarm A and C fly side-by-side at a lower altitude of 450 km, while Swarm B flies alone at a higher orbit of 530 km. Simultaneous measurements from several satellites provide valuable information on the structure of the fields. A method developed earlier at the Finnish Meteorological Institute [1] allows computation of ionospheric electric currents and conductances from the measurements.

In the first part of this investigation [2], the ionospheric electric currents computed from Swarm and ground-based magnetic field measurements were compared. The comparison yielded information on the sizes of the current structures that can be studied using Swarm. Moreover, it was noticed that when currents are calculated from magnetic field measurements, especially those from ground-based instruments, it is important to take into account that rapid changes of the ionospheric currents induce electric currents into the conducting ground and sea water. These so-called telluric currents add their own contribution to the measured magnetic field. The telluric current effects should be accounted for when ionospheric currents are calculated, otherwise the results will not be correct.

In the second part of the investigation [3], the above mentioned method was applied for the first time to real Swarm magnetic and electric field measurements. Previously the method had only been tested with artificial data. A simple auroral arc was selected for the demonstration. The results yielded the structure of the electric currents, fields and conductances associated with the arc. The biggest surprise was the strong intensity of the arc-associated electric field. However, as observations indicating similar field strengths have been reported previously, the strength of the field was judged to be a real feature and not a measurement error. The results demonstrate that the method developed at the Finnish Meteorological Institute can be used to compute ionospheric conductances and currents from Swarm magnetic and electric field measurements. These data are important material for auroral research.

Solar wind buffets the Earth's magnetosphere, the protective magnetic bubble surrounding our planet. Occasionally this buffeting launches a series of events that leads to particles plummeting into the Earth's upper atmosphere. These particles collide with the atmospheric atoms and molecules, exciting and ionizing them. The excited atoms and molecules soon return to ground state, emitting auroral light. The ionized population, on the other hand, increases the electric conductivity of the atmosphere. This part of the upper atmosphere around 100 km altitude is called the ionosphere.

As the precipitating magnetospheric particles are electrically charged, they carry electric current between the magnetosphere and the ionosphere. When the vertical current reaches the electrically conducting ionospheric layer, it changes direction and proceeds horizontally along the conducting channels until it can return back to the magnetosphere. Such a current circuit produces a weak but measurable disturbance to the Earth's strong internal magnetic field.

The research was funded by the Academy of Finland EISCATTomo project.

More information:

Researcher Liisa Juusola, tel. +358 29 539 4687, liisa.juusola@fmi.fi T
Researcher Kirsti Kauristie, tel. +358 50 597 8874, kirsti.kauristie@fmi.fi

[1] http://en.ilmatieteenlaitos.fi/science-news-archive/-/asset_publisher/5UiQ/content/a-new-method-for-the-research-of-the-electrical-current-systems-of-the-northern-lights?redirect=http%3A%2F%2Fen.ilmatieteenlaitos.fi%2Fscience-news-archive%3Fp_p_id%3D101_INSTANCE_5UiQ%26p_p_lifecycle%3D0%26p_p_state%3Dnormal%26p_p_mode%3Dview%26p_p_col_id%3Dcolumn-2%26p_p_col_count%3D1

[2] Juusola L., K. Kauristie, H. Vanhamäki, A.T. Aikio, and M. van de Kamp (2016), Comparison of Auroral Ionospheric and Field-Aligned Currents Derived From Swarm and Ground Magnetic Field Measurements, J. Geophys. Res., 121,  doi:10.1002/2016JA022961. http://dx.doi.org/10.1002/2016JA022961

[3] Juusola L., W.E. Archer, K. Kauristie, J.K. Burchill, H. Vanhamäki, and A.T. Aikio (2016), Ionospheric Conductances and Currents of a Morning-Sector Auroral Arc From Swarm-A Electric and Magnetic Field Measurements, Geophys. Res. Lett., 43, doi:10.1002/2016GL070248. http://dx.doi.org/10.1002/2016GL070248


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