Core dynamics and rapid geomagnetic field variations H. Amit, L

Core dynamics and rapid
geomagnetic field variations
H. Amit, L. Huguet, B. Langlais
L'analyse de la variation séculaire géomagnétique révèle des aspects de la dynamique
rapide du noyau. Il s'agit notamment de l'amplitude de circulation à petite échelle dans le
noyau, de la cinématique de taches de flux intenses, du transfert d'énergie, d'hypothèses
pour l'inversion de la variation séculaire et la stratification au sommet du noyau.
First we presented a method to estimate
the flow magnitude near the coremantle boundary (CMB) based on the
geomagnetic field and its secular
variation (SV) together with information
about
field-flow
alignment
from
numerical dynamos [1]. An expression
linking the core flow magnitude to field
and SV spectra was derived from the
magnetic induction equation. This
involves the angle between the flow and
the field gradient. In numerical
dynamos, horizontal flow approximately
follows radial field contours close to
high-latitude flux patches, while at lower
latitudes zonal flows are often
perpendicular to these contours (Fig. 1).
Application to a geomagnetic field
model leads to a core flow magnitude of
11-14 km/yr. When extrapolating the
spectra beyond observed scales, the
flow magnitude is less than 50 km/yr.
Next we applied an algorithm to detect
and track in time centers of intense
archeomagnetic flux patches [2]. Most
patches appear near the edge of the
tangent
cylinder.
Quasi-stationary
periods occur more than drifts. This
could explain the roughly coincident
locations of high-latitude patches in the
historical field with that of the
paleomagnetic field together with the
much weaker patches intensity in the
latter.
Alternating
eastward
and
westward drifts are also observed. The
drifts are more westward than eastward,
especially in the southern hemisphere,
indicating that the time-average zonal
core flow may be driven by core-mantle
thermal coupling. Average patch lifetime
of ~300 years may indicate vortex
lifetime in the core.
Fig. 1: Regions of high (top) and low
(bottom) field-flow alignment in a numerical
dynamo model. From [1].
Fig. 1: Régions de fort (en haut) et faible (en
bas) alignement entre champ et écoulement
dans un modèle de dynamo numérique.
D'après [1].
Next we introduced a formalism to track
magnetic energy transfer between
spherical harmonic degrees due to the
interaction of fluid flow and radial
magnetic field at the top of Earth's core
[3]. The azimuthal phase relation
between the field and flow plays a major
role
in
the
energy
transfer.
Geomagnetic energy transfer induced
by core flow models exhibits a striking
transfer spectrum pattern of alternating
extrema suggestive of energy cascade,
but the transfer matrix reveals both local
and non-local transfers. The transfer
spectrum reverses from even maxima
odd minima between 1840-1910 to odd
maxima even minima between 19551990 (Fig. 2). The matrix shows forward
cascade and non-local transfer from the
dipole directly to higher degrees,
explaining the simultaneous dipole
decrease and non-dipole increase.
Fig. 2: Time-average geomagnetic energy
transfer for the period 1955-1990. Left:
Transfer matrix; Right: Observed (red), total
transfer within the observed spectrum
(dotted black), and including beyond (solid
black). From [3].
Fig. 2: Transfert d'énergie géomagnétique
moyenne temporelle pour la période 19551990. Gauche: Matrice de transfert; Droite:
Spectre du transfert d'énergie. D'après [3].
Next we compared two core flow
assumptions: Tangential geostrophy
(TG) and columnar flow (CF) [4]. CF is
consistent with quasi-geostrophy theory
an incompressiblity, whereas TG is not.
The non-uniqueness associated with
both assumptions is comparable. TG
flows exhibit a strong Atlantic/Pacific
dichotomy and an eccentric gyre,
whereas in CF flows these features are
less sharp. Both upwelling patterns are
localised in the equatorial region.
Upwelling/downwelling is correlated with
equatorward/poleward flow respectively.
CF upwelling is stronger but the
magnitude ratio is smaller than the
factor 2 distinguishing their analytical
expressions due to the dominance of
magnetic advection in the SV. Robust
upwelling below India/Indonesia may be
evidence for whole core convection.
Finally
we
analyzed
persistent
geomagnetic SV features on the CMB
to examine whether a kinematic
signature of core fluid downwelling can
be detected [5]. The radial field and its
SV were stacked in an intense flux
patch moving reference frame. Stacked
images were compared with forward
solutions to the radial induction equation
based on idealized field-flow models.
Clear advective SV below North
America indicates that these intense
flux patches may exhibit significant
mobility. Stretching signature seen in
persistent positive SV of the intense flux
patch below the Southern Indian Ocean
is considered as regional geomagnetic
evidence for whole core convection.
Collaborations
Chris Finlay (DTS, Denmark),
Alexandra Pais (Coimbra, Portugal),
Julien Aubert and Gauthier Hulot
(IPGP), Monika Korte (GFZ, Germany),
Cathy Constable (UCSD, US).
Références associées
1 - Finlay, C.C., Amit, H., 2011. On flow
magnitude and field-flow alignment at
Earth's core surface. Geophys. J. Int.,
186, 175-192.
2 - Amit, H., Korte, M., Aubert, J.,
Constable, C., Hulot, G., 2011b. The
time-dependence
of
intense
archeomagnetic flux patches. J. Geohys.
Res.,
116,
B12106,
doi:10.1029/2011JB008538.
3 - Huguet, L., Amit, H., 2012. Magnetic
energy transfer at the top of Earth's
core. Geophys. J. Int., 190, 856-870.
4 - Amit, H., Pais, M.A., 2013.
Differences
between
tangential
geostrophy
and
columnar
flow.
Geophys. J. Int., 194, 145-157.
5 - Amit, H., 2014. Can downwlling at
the top of the Earth's core be detected
in the geomagnetic secular variation?
Phys. Earth Planet. Inter., in press.