Understanding geomagnetic reversals · Understanding geomagnetic reversals Nathana el Schae er1, A....

Understanding geomagnetic reversals

Nathanael Schaeffer1, A. Fournier2, T. Gastine2

1 ISTerre / CNRS / Universite Grenoble Alpes2 IPGP

GENCI, Joliot-Curie Grand Challenges,Bruyeres le Chatel, 24 June 2019

nathanael.schaeffer@univ-grenoble-alpes.fr understanding reversals GENCI 24 Jun 2019 1 / 27

Outline

1 Objectives of this Grand ChallengeThe core and magnetic field of the EarthNumerical simulations of the geodynamo

2 Results

3 About the Irene machine

4 Outlook for future machines

Structure of the Earth

http://www.xkcd.com/913/

Structure of the Earth

Cool facts about the Earth’s core

A broad range of time-scalesI from months (SV) to million years (reversals)

Viscosity of waterI Earth’s spin (Coriolis force) dominates the dynamics ⇔ E ∼ 10−15

I Momentum diffuses much slower than magnetic field⇔ Pm ∼ 10−5

Large scale motions at the top of the core have speeds around10 km/year (0.3 mm/sec, turnover time is about 200 years)

I Turbulent motion (very high Reynolds number Re & 108).I Magnetic Reynolds number Rm & 1000 (moderate compared to

astrophysical objects)I Earth’s spin (Coriolis force) dominates the dynamics ⇔ Ro ∼ 3× 10−6.

Cool facts about the Earth’s magnetic field

Magnetic field at the surface is dominated by a tilted dipole.

Magnetic energy dominates kinetic energy by a factor 104 (4 mTestimated in the core, 0.5 mT or 5 gauss at the surface).

I Earth’s spin (Coriolis force) still dominates the fast dynamics⇔ Le ∼ 10−4 (e.g. Jault 2008).

I The magnetic field and the Coriolis force influence long-term dynamics⇔ Λ & 10.

Heat flux extracted by the mantle (∼ 10TW, < 100mW/m2).I Strong convection (very high Rayleigh number Ra� 1020 ? Probably

many times critical – See also poster S5-P29 by Christensen).I A (thermo-chemical) convection-driven dynamo produces the

Earth’s magnetic field.

The magnetic north pole is moving

1590–2020 https://maps.ngdc.noaa.gov/viewers/historical_declination/

Polarity reversals

A superchron with the same polarity for almost 40 Millions years.

Frequently reversing periods, where a given polarity stays for 1Million year or less.

from Hulot et al., 2010

Paleointensity models: sint2000

Typical intensity variations from 0.5 to 1.5 times the mean – factor 3between min and max outside reversals.

from Valet et al., 2005.

20+ years of geodynamo simulations: toward the Earth

1995 : first geodynamo simulation by Glatzmaier & Roberts64 x 32 x 49

hyperviscosity

Earth-like, reversals

Non-reversing simulations

high resolution (up to 5 billionpoints, 16384 cores);

short time-scales (few thousandyears);

correct dynamical regime (forcebalance);

dominated by magnetic energy;

do not reverse.

Reversing simulations

low resolution (the lower, thelonger);

long time-scales (tens of millionsof years);

wrong dynamical regime (tooviscous!);

reversals observed.

20+ years of geodynamo simulations: toward the Earth

1995 : first geodynamo simulation by Glatzmaier & Roberts64 x 32 x 49

hyperviscosity

Earth-like, reversals

Non-reversing simulations

high resolution (up to 5 billionpoints, 16384 cores);

short time-scales (few thousandyears);

correct dynamical regime (forcebalance);

do not reverse.

Reversing simulations

low resolution (the lower, thelonger);

long time-scales (tens of millionsof years);

wrong dynamical regime (tooviscous!);

reversals observed.

Recent advances in non-reversing simulations

Schaeffer et al., 2017 highlights the very large spatial and temporalfluctuations of the magnetic field.

Intense polar vortices: thermal winds, magnetically shaped.

When do geodynamo models reverse?

Kutzner and Christensen, 2002

This regime diagram cannotrepresent Earth’s core. Because

Magnetic energy in Earth’s coreis 10000 times larger thankinetic energy.

The reversals are observed formore than 200 million years; itis unlikely the Ra/Racrit stayedstable for such a long time.

Earth-like reversals?

Glatzmaier & Coe, chapter 8.11 of the Treatise on geophysics (2015) :

[...] it is unlikely that the large-scale convection cells seen in [laminar] studiesproduce a reversal mechanism representative of that in the Earth’s turbulentouter core.

Objectives of this Grand Challenge

Produce geodynamo reversals which are closer to Earth’s core.

Strong magnetic field

Turbulent flow

metrics

Ratio of magnetic over kinetic energy as high as possible (104 in thecore);

Pm and E (viscosity) as low as possible (Pm = 10−5, E = 10−15 forthe core);

Re (Reynolds number) as large as possible (Re ' 108 in the core).

Many rather small but very long jobs.

Outline

2 Results

Examples of reversing dynamos

geodynamo run at Re = 217, Pm = 3, Emag/Ekin = 0.84

3 reversals within 2 Myr

same polarity for more than 17 Myr (superchron)

3.3M points, 3 KNL nodes (17k points/core).

Magnetically-driven reversals?

Re = 491, Pm = 2, Emag/Ekin = 2.8,50M points, 7 KNL (110k points/core).

Zooming on a reversal

Conclusion

This Grand Challenge (16M core.hours on KNL, 7M core.hours on SKL)allowed us:

to produce the most realistic reversing geodynamo simulations todate, including turbulence.

to get a reversing regime that resembles that of Earth’s core in manyaspects.

to invalidate the previous theory based on viscosity-dominatedsimulations.

Outline

2 Results

the XSHELLS code

A high performance simulation code for rotating incompressible flows andmagnetic fields in spherical shells.

written in C++

Free & Open-source softwarehttps://nschaeff.bitbucket.io/xshells

Compilers: gnu or intel (OpenMP 4 support needed)

Dependencies: FFTW (or MKL) and SHTnshttps://bitbucket.org/nschaeff/shtns/

Parallelization: domain decomposition with MPI + OpenMP.

The SHTns library is hand-vectorized for AVX and AVX-512.

Strong scaling on Irene: 50M points

100 101

number of nodes

xshells (NR=336, Lmax=213, 4 process/node)

Irene (KNL) in-shell OpenMP, 64 cores/nodeIrene (KNL) OpenMP tasks, 64 cores/nodeIrene (SkylakeX) in-shell OpenMP, 48 cores/nodeIrene (SkylakeX) OpenMP tasks, 48 cores/node

KNL scaling is not so good for this small case (50M points). Butstarts competitive with SKL !

OpenMP tasks were needed to get decent performance on KNL.

Strong scaling on Irene: 600M points

101 102 103 104

number of cores

Performance scaling of xshells (NR=512, Lmax=511)

Occigen2 (BroadWell) in-shell OpenMPIrene (SkylakeX) in-shell OpenMPIrene (KNL) in-shell OpenMPFroggy (SandyBridge) in-shell OpenMPTuring (Blue Gene/Q) in-shell OpenMP

Seemingly bad network performance on KNL...

Strong scaling on Irene: >4G points

100 101 102

number of nodes

xshells (NR=1152, Lmax=893, 4 process/node)

Occigen (Haswell) in-shell OMP, 24 cores/nodeIrene (SkylakeX) in-shell OpenMP, 48 cores/nodeIrene (KNL) in-shell OMP, 64 cores/nodeOuessant (Power8+Pascal) cuda (vs #nodes), 4 gpus/node [NR=1024]Ouessant (Power8+Pascal) cuda (vs #gpu), 4 gpus/node [NR=1024]

What worked for me

on SKL

for hand-vectorized code: gcc is best

icc for auto-vectorization, with -qopt-zmm-usage=high to useavx512 as much as possible.

remark: important speedup with avx512 only in computationintensive parts.

on KNL

gcc does not work well on KNL (too slow init ?).

avx512 vectorization is key (KNL fp latency is about 3x SKL;avx512 needed to leverage high-speed memory).

remark: higher fp latency means 12 independent operations areneeded to reach peak performance (8 on SKL).

remark: on Frioul, the ability to put everything in HBM helped.

Outline

2 Results

Outlook for future machines

To go further, we need to use techniques to accelerate the obtention ofreversals.

rare-events algorithms (our next ANR proposal).

turbulence models to avoid computing the small scales (not availableyet).

This implies:

Run MANY small to medium cases (parametric study) for possiblyvery long times (several months).

MPI needs to have high performance for small messages (low latency).

a few large cases, but probably not much larger than 4G points.

Understanding geomagnetic reversals · Understanding geomagnetic reversals Nathana el Schae er1, A....

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