(BEING CONTINUED FROM 2/06/16)
IB)GLOBAL CHANGES IN EARTH’S GEOMAGNETIC FIELD
Earth’s magnetic field, which reflects the complex energy processes in its inner and outer cores, is an essential physical characteristic of our planet. It is believed that Earth’s magnetic field is formed mostly due to a flow of huge masses of liquid iron, which constitute Earth’s outer core, around its inner solid core.
It was William Gilbert, an English physician and natural philosopher, who first assumed the existence of Earth’s magnetic field in his book “De Magnete” in 1600. Observations by the English astronomer Henry Gellibrand proved that the geomagnetic field is not constant, but slowly changes. Carl Friedrich Gauss put forward a theory about the origin of Earth’s magnetic field and proved in 1839 that most of it originates from within Earth and that the cause of minor short deviations of its rate should be sought in the external environment. Let us have a brief look at the structure of Earth’s magnetosphere. At the distance of approximately three radii from Earth, magnetic lines of force have a dipolar orientation. This region is called the plasmasphere.
Fig. 39. Structure of Earth’s magnetosphere
The solar wind’s strength grows with the distance from Earth’s surface, with the geomagnetic field shrinking on the sun side and stretching out in a long trail on the opposite side. Currents in the ionosphere have a significant impact on the magnetic field at Earth’s surface.
Fig.40. Earth magnetic field
The upper region of the atmosphere (plasmasphere), about 100 km and higher, contains plenty of ions. The condition of plasma retained by Earth’s magnetic field is determined by the interaction of Earth’s magnetic field with the solar wind, which explains the relationship between terrestrial magnetic storms and solar flares (K. P. Belov, N. G. Bochkarev, 1983).
The magnetic field intensity at Earth’s surface highly depends on the geographical location, being about 0.5 Oe (50 microT ) on average, about 0.34 Oe at the magnetic equator, and 0.66 Oe at the magnetic poles.
This intensity rises sharply near magnetic anomalies, reaching, for example, 2 Oe inside the Kursk Magnetic Anomaly. Periodically, Earth’s magnetic field experiences disturbances called magnetic pulsations, resulting from the excitation of hydromagnetic waves in Earth’s magnetosphere. Pulsation frequency ranges from millihertz to one kilohertz (V. A. Troitskaya, A. V. Guglielmi, 1969).
The geomagnetic field is not so constant and varies from time to time. For instance, some 2500 years ago the strength of the magnetic field was 50% higher than it is today.
The so-called inversions of the geomagnetic field, or geomagnetic reversals, when the positions of the north and south magnetic poles become interchanged, have occurred over and over throughout Earth’s history. Along with inversions of the geomagnetic poles, there are less dramatic shifts of the geomagnetic field, the so-called “excursions,” when the geomagnetic poles migrate rapidly to rather great distances but no geomagnetic reversal takes place. Earth’s history has seen repeated occurrences of “excursions” of the geomagnetic poles when the North geomagnetic pole traveled towards the equator and reversed upon reaching it, returning to its former location.
It is hard to overestimate the importance of the geomagnetic field for the existence and evolution of life on Earth, for the lines of force of the magnetic field create a kind of a magnetic shield around the planet that protects Earth’s surface from cosmic rays pernicious to all living things, and from the influx of charged particles of high energies.
The North geomagnetic pole is now located in the Canadian Arctic and continues to drift northwestwards, while the South geomagnetic pole is located off the coast of Antarctica, south of Australia.
Mandea and Dormy (2003), summarizing their ground observations and discussing the movement of the North geomagnetic pole, stated that its velocity “has more than doubled in the last 30 years, reaching the huge velocity of about 40 km per year in 2001”. A subsequent model of time change of Earth’s magnetic field (Olsen, et al., 2006) showed that the North Magnetic Pole’s movement accelerated further, reaching 50 kilometers per year in 2000 and 60 kilometers per year in 2003. However, the North Magnetic Pole has decelerated slightly since 2003 and currently moves with a velocity slightly exceeding 50 km per year. Meanwhile, during the same time period, the South geomagnetic pole was moving with a constant speed of about 5-10 kilometers per year. The positions of the North and South geomagnetic poles are shown in the updated version of the CHAOS model (Olsen, et al., 2006), which includes more recent satellite data with ground observations (Newitt, et al., 2002).
According to a forecast by N. Olsen and M. Mandea (2007), the North geomagnetic pole will be closest to the North Geographic Pole (at a distance of 400 kilometers) in 2018, and will continue to move towards Siberia.
Studying the geomagnetic reversals and sea level fluctuations in the Phanerozoic Era has enabled a number of researchers to conclude that there is a certain correlation between those processes (E. E. Milanovskiy, A. G. Gamburtsev, 1998). The intensity of Earth’s magnetic field in the past has also been subject to significant fluctuations. For instance, a study by G. N. Petrova and A. G. Gamburtsev established the existence of rhythms in the paleointensity of the geomagnetic field, predominated by rhythms with periods of 20-25 ka, 70 ka, 160-170 ka and other, though less distinct, periods (G. N. Petrova, A. G. Gamburtsev, 1998).
Fig. 41. Graph of velocity of North Geomagnetic Pole movement
(N. Olsen and M. Mandea, 2007)
Fig. 41 contains a graph showing the movement of the North geomagnetic pole. As can be seen from the graph, the North geomagnetic pole’s drift rate had increased almost fivefold by the late 1990s as compared to 1980. This fact might point to a substantial change in energy processes within Earth’s core, which form the geomagnetic field of our planet. No doubt the observed phenomenon may be indicative of the beginning of another cycle of surge in Earth’s endogenous activity.
To what further consequences may the vastly accelerated displacement of the North Magnetic Pole lead? Given that a decrease in Earth’s magnetic field intensity accompanies this process, it can be assumed that global climate change will be influenced as well. There are so-called “cusps” in the polar ice cap areas – polar gaps that have increased in size in recent years. Radiation particles from the solar wind and interplanetary space enter Earth’s atmosphere and hit its surface through those cusps, which means that huge amounts of extra matter and energy get into the polar areas resulting in “heating” of polar caps. Naturally, changing of the positions of the geomagnetic poles also causes shifting of the cusps and, consequently, displacement of the areas of high flux of solar energy into Earth’s atmosphere and towards its surface. This process is followed by a redistribution of cyclones and anticyclones across the planet, leading to serious global climate change (V. E. Khain, E. N. Khalilov, 2008, 2009).
5.2. VARIATIONS OF ANGULAR VELOCITY OF EARTH’S ROTATION
Irregularity of Earth’s diurnal rotation rate was found as early as in the beginning of the twentieth century. According to V. M. Kiselev (1980), these variations are mostly expressed in three ways: 1. the rotation axis changes its spatial orientation; 2. the rotation axis changes its position relative to Earth’s surface; 3. the angular velocity of Earth’s rotation is variable relative to the instantaneous axis.
Changes in the spatial position of Earth’s axis are mainly caused by the gravitational influence of the Moon, Sun and Solar system’s planets on Earth. This value can be calculated quite accurately. Much more difficult is the case with the second and third aspects, which manifest themselves in the form of, respectively, movement of the poles relative to Earth’s surface and variations of Earth’s angular velocity (Fig 42). All movements of the poles can be classified into three categories: a motion with a period of 14 months and variable amplitude of 0.1”, discovered by Chandler; a motion with a period of one year and amplitude of 0.08” which corresponds to2.5 m at Earth’s surface; and the third one, a very slow and irregular secular motion of about0.003”, or 10 cm, per year on average (A. A. Mikhailov, 1984).
Fig. 42. Earth’s precession and nutation diagram
The Chandler motion reflects the free movement of the poles. Today, there is no definite answer explaining the causes of such fluctuations; however, there are various hypotheses including those connecting these fluctuations to large earthquakes and volcanic eruptions. Annual fluctuations are associated with meteorological phenomena: deposition and melting of snow, winter clustering of air masses over Northeast Asia, when the atmospheric pressure becomes above normal. A pole’s secular motion does not follow strict patterns and has, to date, no unequivocal explanation (A. A. Mikhailov, 1984).
However, these movement types are not dealt with in this paper; therefore, attention will be focused on the irregularity of Earth’s diurnal rotation rate. There are three main aspects usually singled out as to variations of the length of the 24-hour day: 1) Secular changes of 1-2 ms per 100 years, 2) Seasonal variations with an amplitude of about 0.5 ms, and 3) Irregular yearly changes whose magnitude exceeds secular changes by more than a factor of ten.
Secular changes in the day length are mostly associated with the effect of tide-raising forces resulting from Earth’s gravitational interaction with the Moon and the Sun. Seasonal variations of Earth’s angular velocity are due to the changes in zonal atmospheric circulation during the year and partly due to lunar tides.
Isaac Newton first noticed irregular variations of Earth’s rotation rate in 1875 when he was studying the motion of the Moon. The existence of the irregular changes of Earth’s rotation became evident after the works of de Sitter and Spencer Jones, who found simultaneous changes in the mean motion of the Moon, Sun, Mercury, Venus, Mars, and the satellites of Jupiter, proportional to their mean motions. However, to date there is no general consensus as to what causes the irregular changes of Earth’s angular velocity (V.M. Kiselev, 1980).
Fig. 43 contains a graph of irregular variations of Earth’s day length from 1850 to 2000, smoothed out via 5-year running averages. There have been attempts by various researchers to put forward some concepts to explain the mechanism of irregular changes of Earth’s diurnal rotation. W. Munk (1964) and S. Chapman (1960) reviewed studies on the interaction between the geomagnetic field and the interplanetary medium, and examined the possibility for this interaction to influence the variations of Earth’s angular velocity. As Y. A. Bilde showed in his work (1976), noticeable changes of Earth’s rotation speed can occur when the variation rate of an external magnetic field (for example, of ionospheric origin) is as close as possible to Earth’s rotation rate. A work by J. Ginsberg (1972) provides some estimates for the torque resulting from the solar wind’s interaction with the geomagnetic field, showing at the same time that this torque is not enough to explain the observed changes in Earth’s day length. According to a hypothesis proposed in 1965, impulsive changes of Earth’s diurnal rotation rate can be caused by electromagnetic interaction between Earth and solar plasma streams having force-free configuration of magnetic fields, called M-elements (V. I. Afanasiev, 1965). The concept was later elaborated upon in the paper of N. P. Benkov (1976), where he demonstrated that if the solar wind contains plasma formations with M-element features, then they can explain the sudden changes in Earth’s diurnal rotation rate.
P. N. Kropotkin, N. N. Pariysky and other researchers attribute the observed variations of Earth’s diurnal rotation rate to possible changes in its radius and shape: P. N. Kropotkin (1984), N. N. Pariysky (1984), V. E. Khain, Sh. F. Mehdiyev, E. N. Khalilov (1984, 1986, 1987, 1988, 1989)
Fig. 43. Graph for day length variations between 1850 and 2000,
according to data by V. M. Kiselev (1980)
Y is day length variations graph;
γ (ms) axis is changes in day length.
As P. N. Kropotkin pointed out in his work (1984), the periodic changes in Earth’s radius are the original cause of both the cyclicity of tectonic processes’ manifestation and the variations of Earth’s angular velocity (Kropotkin, 1984). The same idea was simultaneously proposed by V. E. Khain, Sh. F. Mehdiyev and E. N. Khalilov (1984) who, similar to P. N. Kropotkin in 1984, drew a conclusion about the periodic changes in Earth’s radius, which is reduced due to intensification of the subduction process and slowing down of the spreading process during the times when Earth is getting compressed, with the opposite process taking place during the periods of Earth’s expansion.
It is noteworthy that P. N. Kropotkin in his work (1984) established a good correlation between the Chandler motions, Earth’s angular velocity, and seismic activity that makes it possible to integrate all these processes into a single and logically valid system.
The theoretical calculations of Earth’s elastic deformation and of respective changes in its moment of inertia, rotation, and surface gravity were made by N. N. Pariysky as early as 1954. Based on his calculations, N. N. Pariysky concluded that neither solar activity effects nor atmospheric phenomena could cause the observed changes in Earth’s angular velocity. In his view, those variations might be the result of Earth’s global deformation processes leading not only to the periodic changes of its radius, but also to the complex change of its shape. Judging from his description of this process, it must be quadrupole in nature, that is, Earth must “change its shape, expanding in the middle and polar regions and shrinking ten times more in the equatorial areas” (N. N. Pariysky, 1984).
Research findings on irregular changes of gravity, cited in a work by D. D. Ivanenko (1984), refer to the situation where the shrinking of Earth at the measuring point would be in line with the overall increase in Earth’s moment of inertia, which is only possible if another part of the globe is expanding. According to V. M. Fedorov, there are some specifics regarding the distribution of catastrophic earthquakes in the diurnal cycle of Earth’s rotation. Those specifics are explained by the cause-and-effect relationship between the distribution of earthquakes and the dynamics of constituent tide-rising forces of the Moon and Sun in connection with Earth’s diurnal rotation.
While studying the correlation between Earth’s global seismic activity and its rotation speed, a group of scientists (Friedmann, Klimenko, Polyachenko, 2005) came to interesting conclusions: 1) the correlation between the frequency of near-surface earthquakes and Earth’s angular acceleration grows monotonically with increasing magnitude, and 2) correlations between the seismic activity and variations of Earth’s angular velocity in subduction zones drawn along the latitude and the meridian are qualitatively different. At the end of their research, the authors conclude: “It is the processes of crustal compression and extension in the direction transverse to the rotation axis that are responsible for the changes in the annual seismic activity and angular velocity of Earth’s rotation.”
The most recent works by N. S. Sidorenkov, a well-known researcher of Earth’s rotation irregularity, contain some interesting conclusions about the relation of Earth’s rotation instability to hydrometeorological processes. Those studies formed the basis for the method of forecasting hydrometeorological characteristics, patented by scientists (N. S. Sidorenkov, P. N. Sidorenkov, 2002). N. S. Sidorenkov mentions the existence of a statistically significant correspondence between the tidal fluctuations of Earth’s rotation speed and changes of weather processes in the atmosphere. Natural synoptic periods coincide with Earth’s rotation modes. Lunar-solar zonal tides cause tidal fluctuations of Earth’s rotation rate. According to those researchers, the evolution of synoptic processes in the atmosphere occurs not only because of the climate system’s internal dynamics, but also under the control of the lunar-solar zonal tides (Sidorenkov, 2004).
The research conducted by a number of scientists (Zharkov, Pasynok, 2004) allowed them to conclude that the variations of Earth’s angular velocity are very complex in nature, with completely different harmonics. When superimposed on each other, those harmonics create a very complex pattern of variation in Earth’s day length. Based on that, V. N. Zharkov and S. L. Pasynok attempted to develop a theory of Earth’s rotation, calling it a new theory of nutation. According to that theory, nutation of Earth’s rotation is conceived as a quite complex though harmonious system that has a specific hierarchy of many superimposing nutational movements of the rotation axis of different degrees.
In our view, the variations of Earth’s diurnal rotation are undoubtedly connected to the deformation processes and mass changes in the core-lithosphere – hydrosphere – atmosphere system. The aforesaid can be confirmed by the changes in the angular velocity of Earth’s rotation and displacement of its axis following the catastrophic earthquakes in Indonesia (Sumatra, 26 December 2004) and Chile (27 February 2010), to name a few. The Indonesian earthquake of Dec. 26, 2004 shifted the position of the Geographic North Pole by 2.5 centimeters in the direction of 145 degrees east longitude. The change in the planet’s rotation speed brought about a 2.68 microsecond increase in the day length, and the movement of the masses caused the planet’s shape to alter. As a result of the earthquake, the planet’s proportions changed by one ten-billionth, that is, Earth has become less flattened and more compact.
(TO BE CONTINUED)