First International Conference on MOLDAVIAN RISKS – FROM ...

<strong>First</strong> <strong>International</strong> <strong>Conference</strong> on MOLDAVIAN RISKS - FROM GLOBAL TO LOCAL SCALE
16-19 May 2012, Bacau, Romania
GEOMAGNETIC STORMS ─ BETWEEN BEAUTY AND RISK
Mioara Mandea
Centre National d'Etudes Spatiales, Directorate for Strategy and Programmes
Corresponding author: Mioara Mandea, E-mail address mioara.mandea@cnes.fr
Abstract: The main part of the geomagnetic field is generated by a convective motion in the
Earth’s iron-rich, electrically conducting, fluid outer core by a process known as the
geodynamo. This mechanism generates a magnetic field known as the core field or main field,
characterized by a temporal variation over time-scales from years to millennia, named secular
variation. The field produced in the core is more than 90% of the field measured at the Earth's
surface. Another internal contribution is the lithospheric (crustal) magnetic field, with its origin
in the remanent and induced magnetization parts of the crust and upper mantle, which is not
only weaker, but also of much smaller spatial scale, when compared to the large scale core
field. The geomagnetic external fields mainly stem from the interaction with the solar wind,
due to the Sun activity. The effect is to compress the main magnetic field lines on the sunward
side and stretches them into a long tail on the night side. Generally, solar wind particles do not
cross magnetic field lines and are thus primarily deflected around our planet. They may,
however, enter the magnetosphere when interplanetary and geomagnetic fields merge during
times of increased solar activity, or close to poles where the field-lines are nearly vertical.
Their interaction with the atmosphere then causes the well-known aurora, amazing the viewer
with the beauty of luminosity. The magnetic variations associated with these phenomena are
known as geomagnetic storms and during their main phase the electric current in the
magnetosphere create a magnetic force which pushes out the boundary between the
magnetosphere and the solar wind. The frequency of geomagnetic storms increases and
decreases with the sunspot cycle, some 11 years. There are several space-weather issues which
tend to be associated with large geomagnetic storms which cause radio and radar scintillation,
disruption of navigation and spacecraft operations, and even aurora displays at much lower
latitudes than normal. More interestingly, time-varying geomagnetic external fields induce
electric currents in the conducting ground. These currents create a secondary magnetic field,
and an electric field at the Earth’s surface is induced, associated with time variations of the
magnetic field. The surface electric field causes electrical currents, known as geomagnetically
induced currents (GIC), flowing in any conducting structure, for example, power or pipeline
grids. Since the largest magnetic field variations are observed at higher magnetic latitudes, GIC
have been regularly measured in some Northern countries power grids and pipelines since
some decades. However, GIC have also been recorded at mid-latitudes during major storms,
and there may even be a risk to low-latitude areas, especially during a storm commencing
suddenly because of the high, short-period rate of change of the field that occurs on the dayside
of the Earth. The GIC hazard to pipelines is that these currents cause swings in the pipe-to-soil
potential, increasing the rate of corrosion during major geomagnetic storms. In these
circumstances, the GIC risks are not of a catastrophic failure, but a reduced service life of the
pipeline grids. Considerations on the globally and regionally geomagnetic field variations, with
some particular space-weather effects (aurora borealis and risks) are given. A special attention
is paid to a few risks on power grids and pipelines due to geomagnetically induced currents,
and how this situation can be considering on a regional scale.
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