![Hier klicken, um den Treffer aus der Auswahl zu entfernen](images/unchecked.gif) |
Titel |
Torsional waves operating in geodynamo and magnetoconvection simulations |
VerfasserIn |
Robert Teed, Chris Jones, Steve Tobias |
Konferenz |
EGU General Assembly 2015
|
Medientyp |
Artikel
|
Sprache |
Englisch
|
Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 17 (2015) |
Datensatznummer |
250104689
|
Publikation (Nr.) |
EGU/EGU2015-4120.pdf |
|
|
|
Zusammenfassung |
Torsional waves are a principal feature of the dynamics of the fluid outer core where the
Earth’s magnetic field is generated. These oscillations are Alfvén waves operating about
an equilibrium known as a Taylor state (Taylor, 1963) and they propagate in the
cylindrical radial direction. The change in core angular momentum inferred from
geomagnetic observations has a measurable impact on the length of the day, and the small
decadal variations in the length-of-day signal confirm the existence of torsional
oscillations (Holme & de Viron, 2013). Many questions remain unanswered about the
exact nature of these waves and this presentation will attempt to address some of
these.
In order to gain insight we perform three-dimensional spherical dynamo and
magnetoconvection simulations in parameter regimes where Earth-like magnetic fields are
produced. Many of our simulations produce the desired torsional oscillations, identified by
their movement at the correct Alfvén speed, and several show Earth-like core travel times of
around 4 years.
Our dynamo simulations (Teed et al., 2014) show torsional waves within the tangent
cylinder region that also have the ability to pass through this theoretical cylinder. By
calculating the driving terms for these waves we find that both the Reynolds force and
ageostrophic convection acting through the Lorentz force can be important in driving
torsional oscillations.
Driven by a desire to reach smaller Ekman numbers and larger magnetic field strengths,
which are computationally unattainable in dynamo simulations, we perform, in our follow up
work, magnetoconvection simulations (Teed et al., 2015) by imposing a dipolar field on the
core-mantle boundary. Under this configuration we find a transition, at low Ekman numbers,
to regimes where excitation is found only at the tangent cylinder, is delivered by the Lorentz
force and gives rise to a periodic Earth-like wave pattern. This pattern is approximately
operating on a 4 to 5 year timescale, similar to the 6 year period expected of the waves in the
Earth.
References
Holme, R. & de Viron, O., 2013. Characterization and implications of intradecadal
variations in length of day. Nature 499 (7457), 202–204.
Taylor, J., 1963. The magneto-hydrodynamics of a rotating fluid and the Earth’s dynamo
problem. Proc. Roy. Soc. A 274, 274–283.
Teed, R., Jones, C., Tobias, S., 2014. The dynamics and excitation of torsional waves in
geodynamo simulations. Geophys. J. Int. 196 (2), 724–735.
Teed, R., Jones, C., Tobias, S., 2015. The transition to Earth-like torsional oscillations in
magnetoconvection simulations. Submitted |
|
|
|
|
|