Geomagnetic excursion

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A geomagnetic excursion, like a geomagnetic reversal , is a significant change in the Earth's magnetic field. Unlike reversals, an excursion is not a "permanent" re-orientation of the large-scale field, but rather represents a dramatic, typically a (geologically) short-lived change in field intensity, with a variation in pole orientation of up to 45° from the previous position. [1]

Contents

Excursion events typically only last a few thousand to a few tens of thousands of years, and often involve declines in field strength to between 0 and 20% of normal. Unlike full reversals, excursions are generally not recorded around the entire globe. This is certainly due in part to them not registering well in the sedimentary record, but it also seems likely that excursions may not typically extend through the entire global geomagnetic field. [1] There are significant exceptions, however. [lower-alpha 1]

Occurrence

Except for recent periods of the geologic past, it is not well known how frequently geomagnetic excursions occur. Unlike geomagnetic reversals, which are easily detected by the change in field direction, the relatively short-lived excursions can be easily overlooked in long duration, coarsely resolved, records of past geomagnetic field intensity. Present knowledge suggests that they are around ten times more abundant than reversals, with up to 12 excursions documented within the current reversal period Brunhes–Matuyama reversal.

Geomagnetic excursions for the Brunhes geomagnetic chron are relatively well described. [4]

Geomagnetic excursions in the Matuyama, Gauss and Gilbert chrons are also reported and new possible excursions are suggested for these chrons based on analysis of the deep drilling cores from Lake Baikal and their comparison with the oceanic core (ODP) and Chinese loess records. [5]

Possible causes

Scientific opinion is divided on what causes geomagnetic excursions. The dominant hypothesis is that they are an inherent instability of the dynamo processes that generates the magnetic field. [3] Others suggest that excursions occur when the magnetic field is reversed only within the liquid outer core, and complete reversals would occur when the outer and inner core are both affected. [1]

Disorganized dynamo hypothesis

The most popular hypothesis is that they are an inherent aspect of the dynamo processes that maintain the Earth's magnetic field. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core. In such cases, this spontaneous disorganization can cause decreases in the magnetic field as perceived at the Earth's surface. [lower-alpha 2]

This scenario is supported by observed tangling and spontaneous disorganization in the solar magnetic field (the 22 or 11 year solar cycle). However, the equivalent process in the sun invariably leads to a reversal of the solar magnetic field: It has never been observed to recover without a full-scale change in its orientation.

Outer-core inner-core opposition hypothesis

The work of David Gubbins suggests that excursions occur when the magnetic field is reversed only within the liquid outer core; reversals occur when the inner core is also affected. [1] This fits well with observations of events within the current chron of reversals taking 3,000–7,000 years to complete, while excursions typically last 500–3,000 years. However, this timescale does not hold true for all events, and the need for separate generation of fields has been contested, since the changes can be spontaneously generated in mathematical models.

External driver hypothesis

Plate tectonic-driven

A minority opinion, held by such figures as Richard A. Muller, is that geomagnetic excursions are not spontaneous processes but rather triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core–mantle boundary, and possibly mantle-core shear forces and displacements resulting from very large impact events. Supporters of this theory hold that any of these events lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field for a period of time necessary for it to recover.[ citation needed ]

Substantial cosmic impact

Richard A. Muller and Donald E. Morris suggest geomagnetic reversal due to very large impact event and following rapid climate change. The impact triggered a little ice age and change of water redistribution more to poles alters the rotation rate of crust and mantle. If the sea-level change is sufficiently large (>10 meters) and rapid (in a few hundred years), then the velocity shear in the liquid core disrupts the convective cells that drive the Earth's dynamo. [6]


Effects

Due to the weakening of the magnetic field, particularly during the transition period, greater amounts of radiation would be able to reach the Earth, increasing production of beryllium 10 and levels of carbon 14. [7] However, it is likely that nothing serious would occur, as the human species has certainly lived through at least one such event; Homo erectus and possibly Homo heidelbergensis lived through the Brunhes–Matuyama reversal with no known ill effect, and excursions are shorter-lived and do not result in permanent changes to the magnetic field.

The major hazard to modern society is likely to be similar to those associated with geomagnetic storms, where satellites and power supplies may be damaged, although compass navigation would also be affected. Some forms of life that are thought to navigate based on magnetic fields may be disrupted, but again it is suggested that these species have survived excursions in the past. Since excursion periods are not always global, any effect might well only be experienced in certain places, with others relatively unaffected. The time period involved could be as little as a century, or as much as 10000 years.

Relationship to climate

There is evidence that geomagnetic excursions are associated with episodes of rapid short-term climatic cooling during periods of continental glaciation (ice ages). [8]

Recent analysis of the geomagnetic reversal frequency, oxygen isotope record, and tectonic plate subduction rate, which are indicators of the changes in the heat flux at the core mantle boundary, climate and plate tectonic activity, shows that all these changes indicate similar rhythms on million years' timescale in the Cenozoic Era occurring with the common fundamental periodicity of ~13 Myr during most of the time. [9]

See also

Notes

  1. ↑ One of the first excursions studied was the Laschamp event, dated at around 40000 years ago. Although it is thought that many excursions only affect the field over a part of the globe, the Laschamp event did in fact involve a few hundred years when the magnetic poles were completely reversed; later discoveries showed that the reversed field was only 5% of its "normal" strength. [2] Since the Laschamp event has also been seen in sites around the Earth, it is suggested as one of the few examples of a truly global excursion. [3]
  2. ↑ Under the "disorganized dynamo" scenario, the Earth's internal magnetic field intensity does not significantly change within the core itself, but rather, its energy is transferred from the ordinary dipole configuration to higher order multipole configurations. The field external to a multipole decays more rapidly with the distance from the source – in this case the Earth's core. The magnetic field then expressed at the surface of the Earth would be considerably less intense, even without significant changes in its field strength deep in the core.

Related Research Articles

Geophysics Physics of the Earth and its vicinity

Geophysics is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term geophysics sometimes refers only to solid earth applications: Earth's shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations and pure scientists use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial physics; and analogous problems associated with the Moon and other planets.

Earths magnetic field Magnetic field that extends from the Earths outer and inner core to where it meets the solar wind

Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in Earth's outer core: these convection currents are caused by heat escaping from the core, a natural process called a geodynamo. The magnitude of Earth's magnetic field at its surface ranges from 25 to 65 μT. As an approximation, it is represented by a field of a magnetic dipole currently tilted at an angle of about 11° with respect to Earth's rotational axis, as if there were an enormous bar magnet placed at that angle through the center of Earth. The North geomagnetic pole actually represents the South pole of Earth's magnetic field, and conversely the South geomagnetic pole corresponds to the north pole of Earth's magnetic field. As of 2015, the North geomagnetic pole was located on Ellesmere Island, Nunavut, Canada.

Dynamo theory Mechanism by which a celestial body generates a magnetic field

In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales. A dynamo is thought to be the source of the Earth's magnetic field and the magnetic fields of Mercury and the Jovian planets.

Paleomagnetism Study of Earths magnetic field in past

Paleomagnetism, or palaeomagnetism, is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. Magnetic minerals in rocks can lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Geophysicists who specialize in paleomagnetism are called paleomagnetists.

Planetary core Innermost layer(s) of a planet

A planetary core consists of the innermost layers of a planet. Cores may be entirely solid or entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% to 85% of a planet's radius (Mercury).

The Brunhes–Matuyama reversal, named after Bernard Brunhes and Motonori Matuyama, was a geologic event, approximately 781,000 years ago, when the Earth's magnetic field last underwent reversal. Estimations vary as to the abruptness of the reversal. A 2004 paper estimated that it took over several thousand years; a 2010 paper estimated that it occurred more quickly, perhaps within a human lifetime; a 2019 paper estimated that the reversal lasted 22,000 years.

A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons.

Earths inner core Innermost part of Earth, a solid ball of iron-nickel alloy

Earth's inner core is the innermost geologic layer of planet Earth. It is primarily a solid ball with a radius of about 1,220 km (760 mi), which is about 20% of Earth's radius or 70% of the Moon's radius.

Australasian strewnfield

The Australasian strewnfield is the youngest and largest of the tektite strewnfields, with recent estimates suggesting it may cover 10%–30% of the Earth's surface. Research indicates that the impact forming the tektites occurred around 790,000 years ago, probably in Southeast Asia.

The Gauss–Matuyama Reversal was a geologic event approximately 2.58 Ma when the Earth's magnetic field underwent a geomagnetic reversal from normal polarity to reverse polarity. The reversal is named after German physicist Johann Carl Friedrich Gauss and Japanese geophysicist Motonori Matuyama.

Bernard Brunhes French geophysicist

Antoine Joseph Bernard Brunhes was a French geophysicist known for his pioneering work in paleomagnetism, in particular, his 1906 discovery of geomagnetic reversal. The current period of normal polarity, Brunhes Chron, and the Brunhes–Matuyama reversal are named for him.

Motonori Matuyama Japanese geophysicist

Motonori Matuyama was a Japanese geophysicist who was the first to provide systematic evidence that the Earth's magnetic field had been reversed in the early Pleistocene and to suggest that long periods existed in the past in which the polarity was reversed. He remarked that the Earth's field had later changed to the present polarity. The era of reversed polarity preceding the current Brunhes Chron of normal polarity is now called the Matuyama Reversed Chron; and the transition between them is called the Brunhes–Matuyama or Matuyama-Brunhes reversal.

The Jaramillo reversal was a reversal and excursion of the Earth's magnetic field that occurred approximately one million years ago. In the geological time scale it was a "short-term" positive reversal in the then-dominant Matuyama reversed magnetic chronozone; its beginning is widely dated to 990,000 years before the present (BP), and its end to 950,000 BP.

Future of Earth Long-term extrapolated geological and biological changes of Planet Earth

The biological and geological future of Earth can be extrapolated based on the estimated effects of several long-term influences. These include the chemistry at Earth's surface, the cooling rate of the planet's interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity. However, an uncertain factor is the continuous influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet. For example, the current Holocene extinction is being caused by technology. The effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.

Geomagnetic secular variation refers to changes in the Earth's magnetic field on time scales of about a year or more. These changes mostly reflect changes in the Earth's interior, while more rapid changes mostly originate in the ionosphere or magnetosphere.

The Laschamp or Laschamps event was a geomagnetic excursion. It occurred between 42,200 and 41,500 years ago, during the end of the Last Glacial Period. It was discovered from geomagnetic anomalies found in the Laschamps lava flows in Clermont-Ferrand, France in the 1960s.

Magnetic field reversal may refer to:

Magnetic field of Mars

The magnetic field of Mars is the magnetic field generated from Mars' interior. Today, Mars does not have a global magnetic field. However, Mars did power an early dynamo that produced a strong magnetic field 4 billion years ago, comparable to Earth's present surface field. After the early dynamo ceased, a weak late dynamo was reactivated ~3.8 billion years ago. The distribution of Martian crustal magnetization is similar to the Martian dichotomy. Whereas the Martian northern lowlands are largely unmagnetized, the southern hemisphere possesses strong remanent magnetization, showing alternating stripes. Our understanding of the evolution of the magnetic field of Mars is based on the combination of satellite measurements, Martian ground-based magnetic data, paleomagnetic analysis of meteorites, planetary thermal evolution modeling, and magnetohydrodynamic simulations.

References

  1. 1 2 3 4 Gubbins, David (1999). "The distinction between geomagnetic excursions and reversals" (PDF). Geophysical Journal International . 137 (1): F1–F4. Bibcode:1999GeoJI.137....1C. doi: 10.1046/j.1365-246X.1999.00810.x . Archived from the original (PDF) on 3 March 2012. Retrieved 19 April 2012.
  2. ↑ "Ice age polarity reversal was global event: Extremely brief reversal of geomagnetic field, climate variability, and super volcano". Sciencedaily.com. Science Daily. 2012-10-16. Retrieved 2013-07-28.
  3. 1 2 Roperch, P.; Bonhommet, N.; Levi, S. (1988). "Paleointensity of the Earth's magnetic field during the Laschamp excursion and its geomagnetic implications". Earth and Planetary Science Letters. 88 (1–2): 209–219. Bibcode:1988E&PSL..88..209R. doi:10.1016/0012-821X(88)90058-1.
  4. ↑ Roberts, A.P. (2008). "Geomagnetic excursions: Knowns and unknowns". Geophysical Research Letters. 35 (17). doi: 10.1029/2008GL034719 .
  5. ↑ Kravchinsky, V.A. (2017). "Magnetostratigraphy of the Lake Baikal sediments: A unique record of 8.4 Ma of continuous sedimentation in the continental environment". Global and Planetary Change. 152: 209–226. doi:10.1016/j.gloplacha.2017.04.002.
  6. ↑ Muller, Richard A.; Morris, Donald E. (November 1986). "Geomagnetic Reversals from Impacts on the Earth". Geophysical Research Letters. 13 (1): 1177–1180. doi:10.1029/gl013i011p01177.
  7. ↑ Helmholtz Association of German Research Centres (16 October 2012). "An extremely brief reversal of the geomagnetic field, climate variability and a super volcano" . Retrieved 2 November 2014.
  8. ↑ Rampino, Michael R. (1979). "Possible relationships between changes in global ice volume, geomagnetic excursions, and the eccentricity of the Earth's orbit". Geology. 7 (12): 584–587. Bibcode:1979Geo.....7..584R. doi:10.1130/0091-7613(1979)7<584:PRBCIG>2.0.CO;2.
  9. ↑ Chen, J.; Kravchinsky, V.A.; Liu, X. (2015). "The 13 million year Cenozoic pulse of the Earth". Earth and Planetary Science Letters. 431: 256–263. Bibcode:2015E&PSL.431..256C. doi:10.1016/j.epsl.2015.09.033.
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