Ironing out the mystery of Earth's magnetic field

June 01, 2016

The earth's magnetic field has been existing for at least 3.4 billion years thanks to the low heat conduction capability of iron in the planet's core. This is the result of the first direct measurement of the thermal conductivity of iron at pressures and temperatures corresponding to planetary core conditions. DESY scientist Zuzana Konôpková and her colleagues present their study in the scientific journal Nature. The results could resolve a recent debate about the so-called geodynamo paradox.

The geodynamo generating the earth's magnetic field is fed on convection in the iron-rich outer core of our planet that stirs the molten, electrically conducting material like boiling water in a pot. Combined with the rotation of the earth, a dynamo effect sets in, giving rise to the geomagnetic field. "The magnetic field shields us from harmful high-energy particles from space, the so-called cosmic radiation, and its existence is one of the things that make our planet habitable," explains Konôpková.

The strength of the convection in the outer core depends on the heat transferred from the core to the earth's mantle and on the thermal conductivity of iron in the outer core. If a lot of heat is transferred via conduction, there is not much energy left to drive convection - and with it the earths's dynamo. Low thermal conductivity implies stronger convection, making the geodynamo more likely to operate. "We measured the thermal conductivity of iron because we wanted to know what the energy budget of the core is to drive the dynamo," says Konôpková. "Generation and maintenance of our planet's magnetic field strongly depend on the thermal dynamics of the core."

Measurements of thermal conductivity at relevant conditions proved to be difficult in the past. Recent theoretical calculations postulated a quite high thermal conductivity of up to 150 Watts per meter per Kelvin (150 W/m/K) of iron in the earth's core. Such a high thermal conductivity would reduce the chances of the geodynamo starting up.

According to numerical models, a high thermal conductivity would have allowed the geodynamo effect to be supported only rather recently in the earth's history, about one billion years ago or so. However, the existence of the geomagnetic field can be traced back at least 3.4 billion years. This geodynamo paradox has puzzled scientists. "There's been a fierce debate among geophysicists because with such a large thermal conductivity, it becomes hard to explain the history of the geomagnetic field which is recorded in ancient rocks", says Konôpková.

The physicists used a specially designed pressure cell that allows to compress samples between two diamond anvils and to heat them simultaneously with infrared lasers, shining right through the diamonds. Konôpková teamed up with Stewart McWilliams and Natalia Gómez-Pérez from the University of Edinburgh and Alexander Goncharov from the Carnegie Institution in Washington DC to measure the thermal conductivity of iron at high pressure and high temperature conditions in Goncharov's lab.

"We compressed a thin foil of iron in the diamond anvil cell to up to 130 Giga-Pascals, which is more than a million times the atmospheric pressure and corresponds to approximately the pressure at the earth's core-mantle boundary," explains Konôpková. "Simultaneously we heated up the foil to up to 2700 degrees Celsius with two continuous infrared laser beams, shining through the diamonds. Finally, we used a third laser to send a low power pulse to one side of the foil to create a thermal perturbation and measured the temperature evolution from both sides of the foil with an optical streak camera." This way the scientists could watch the heat pulse travelling through the iron.

These measurements were conducted at several pressures and temperatures to cover different conditions of planetary interiors and to obtain a systematic investigation of the thermal conductivity as a function of pressure and temperature. "Our results strongly contradict the theoretical calculations," reports Konôpková. "We found very low values of thermal conductivity, about 18 to 44 Watts per meter per Kelvin, which can resolve the paradox and make the geodynamo operable since the early ages of the earth."
-end-
Deutsches Elektronen-Synchrotron DESY is the leading German accelerator centre and one of the leading in the world. DESY is a member of the Helmholtz Association and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 per cent) and the German federal states of Hamburg and Brandenburg (10 per cent). At its locations in Hamburg and Zeuthen near Berlin, DESY develops, builds and operates large particle accelerators, and uses them to investigate the structure of matter. DESY's combination of photon science and particle physics is unique in Europe.

Reference

Direct measurement of thermal conductivity in solid iron at planetary core conditions; Zuzana Konôpková, R. Stewart McWilliams, Natalia Gómez-Pérez, Alexander F. Goncharov
Nature, 2016; DOI: 10.1038/nature18009

Deutsches Elektronen-Synchrotron DESY

Related Magnetic Field Articles from Brightsurf:

Investigating optical activity under an external magnetic field
A new study published in EPJ B by Chengping Yin, Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China, aims to derive an analytical model of optical activity in black phosphorous under an external magnetic field.

Magnetic field and hydrogels could be used to grow new cartilage
Instead of using synthetic materials, Penn Medicine study shows magnets could be used to arrange cells to grow new tissues

Magnetic field with the edge!
This study overturns a dominant six-decade old notion that the giant magnetic field in a high intensity laser produced plasma evolves from the nanometre scale.

Global magnetic field of the solar corona measured for the first time
An international team led by Professor Tian Hui from Peking University has recently measured the global magnetic field of the solar corona for the first time.

Magnetic field of a spiral galaxy
A new image from the VLA dramatically reveals the extended magnetic field of a spiral galaxy seen edge-on from Earth.

How does Earth sustain its magnetic field?
Life as we know it could not exist without Earth's magnetic field and its ability to deflect dangerous ionizing particles.

Scholes finds novel magnetic field effect in diamagnetic molecules
The Princeton University Department of Chemistry publishes research this week proving that an applied magnetic field will interact with the electronic structure of weakly magnetic, or diamagnetic, molecules to induce a magnetic-field effect that, to their knowledge, has never before been documented.

Origins of Earth's magnetic field remain a mystery
The existence of a magnetic field beyond 3.5 billion years ago is still up for debate.

New research provides evidence of strong early magnetic field around Earth
New research from the University of Rochester provides evidence that the magnetic field that first formed around Earth was even stronger than scientists previously believed.

Massive photons in an artificial magnetic field
An international research collaboration from Poland, the UK and Russia has created a two-dimensional system -- a thin optical cavity filled with liquid crystal -- in which they trapped photons.

Read More: Magnetic Field News and Magnetic Field Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.