First Middle East and Africa IAU‐regional Meeting Proceedings MEARIM No. 1, 2008 © 2008 International Astronomical Union A.Hady @ M.I. Wanas, eds. DOI: 10.1017/97740330200173 Sun Earth’s System : The transient variations of the Earth’s Magnetic Field C. Amory‐Mazaudier1
1 LPP (UMR 7648), CNRS, UPMC, 4 Avenue de Neptune, 94107 Saint‐Maur‐des‐Fossés, France (e.mail : christine.amory@lpp.polytechnique.fr) Abstract. In this paper we review some sources and physical links in the Sun‐Earth system at the origin of the transient variations of the Earth’s magnetic field. We underline the key role of the magnetic fields in the Sun‐Earth system (toroïdal and dipolar components of the solar magnetic field, interplanetary and Earth’s magnetic fields). We privilege the historical approach. Keywords. Solar magnetic field, solar wind magnetospheric dynamo, ionospheric dynamo, Earth’s magnetic field ______________________________________________________________________ 1.Introduction This work is part of the effort developed in the frame of the International Heliopshysical Year (IHY) project. ‘The objectives of IHY 2007 are to discover the physical mechanisms that drive the coupling of Earth’s atmosphere to solar and heliospheric phenomena’. This paper presents the transient variations of the Earth’s magnetic field in relation with different large scale (time and space) dynamos of the Sun Earth’s system. The first section is devoted to generalities concerning dynamo mechanism. The second and third sections are related to the Sun and the Earth’s internal dynamo. Sections 4 and 5 present respectively the solar wind/magnetosphere dynamo and the ionospheric dynamo and their associated transient variations of the Earth’s magnetic field. We follow the historical approach to introduce the discoveries and the significant papers useful for this lecture. 1. Dynamo mechanism: generalities The Sun and the Earth are both magnetized bodies. The generation of both solar and terrestrial magnetic fields are explained by internal dynamo process: a) for the Sun : it is the interaction between differential rotation and convection which wraps an initially north‐south (poloidal) magnetic field b) for the Earth : it is an electrically conducting fluid in the core of the Earth which leads to the generation of a dipolar magnetic field. Figure 1 from Paterno 2006 (adapted from Priest 1984) recalls the dynamo mechanism . Table 1 summarizes the main large scale dynamo in the Sun Earth’s System 236 264
C. Amory Mazaudier Figure 1 : Schematic representation between plasma motion and magnetic field.‘A motion v across a magnetic field B induces an electric field vxB, which produces an electric current j=σ (E + v × B) via Ohm’s law where σ is the electric conductivity and E an electric field. This current produces in turn a magnetic field ∇ XB = μj, where μ is the permeability. The magnetic field creates both electric field E through Faraday’s law ∇ × E =‐δB/δt and Lorentz force j × B which reacts on the motion v.’text from Paterno, 2006. Dynamo Sun dynamo Solar wind/ Magnetosphere Dynamo Ionospheric dynamo Earth’s Dynamo inside the Earth Motions – V Sun Rotation and convection Solar wind Atmosphere Metallic core Magnetic field B Sun : 2 components Dipolar Toroïdal ~ sunspots Interplanetary medium ‐> Bi Earth’s ‐> Bt Earth’s ‐> Bt Order of Magnitude rotation speed : ~ 7280km/h at the equator Dipolar component : ~10 G speed ~ [ 400km/s to 800km/s] Bi ~ qq 10 nT speed ~ 100m/s Bt ~ qq 10 000 nT Change with latitude and longitude secular variation Velocity ~ qq km/year Bt ~ qq 10 000 nT Change with latitude and longitude Table 1: Dynamo mechanisms in the Sun Earth’s system 2. Sun dynamo processes The discovery of the sunspot is attributed to Galileo Galilei and Thomas Hariot around 1610. Later Christophe Scheiner and David Fabricius started the first observations in March 1611. The fisrt publication is from Johannes Fabricius in Autumn 1611. During several centuries, since 1610, the sunspots were observed and collected by scientists. On figure 2 is drawn the yearly average number of sunspot from 1610 to 2000. This diagram exhibits the well known sunspot cycle of ~11 years and also a solar cycle of ~80 years. We also observed the disappearance of the sunspots after 1645 until 1700. This period is called the Maunder minimum. Schawbe in 1851 discovered the sunspot cycle. The regular and continuous observation of the Sun during centuries revealed many morphological features: the migration of the sunspots from the middle latitudes to the equator, the reversal of the polar magnetic field etc… see the NASA website. 265
Earthʹs sun system Figure 2 : Solar sunspot cycle. The observations of the sun highlighted the 2 components of the solar magnetic field, the dipolar and the toroïdal ones. Figure 3 illustrates the model of the dipolar component proposed by Pneuman and Kopp in 1971. By analyzing the Aa magnetic index series from Mayaud (1971), Legrand and Simon (1989) and Simon and Legrand (1989) found that the solar dipolar magnetic field which regulates the solar wind is responsible of 91,5% of the geomagnetic activity(transient variations of the Earth’s magnetic field), the other 8.5% (shock event / CME ‐ Coronal Mass Ejection ) are related to the sunspot solar cycle. Figure 3 : from Pneuman and Kopp, 1971 : Model of the dipolar component of the Solar magnetic field The Scientists has not yet understood all the morphological characteristics of the large scale components of the solar magnetic field. In particular they do not understand how the solar magnetic field change from a dipolar to a multi‐polar configuration (spots). Recently, Paterno (2006) reviewed the solar dynamo phenomena and proposed a schematic view (see figure 7 of Paterno 2006 ) to understand the solar dynamo machine in term of the coupling of 2 dynamos the α dynamo/poloidal component (Differential rotation, change in rotation as a function of latitude and radius within the Sun) and the ω dynamo/toroidal component (twisting of the magnetic field lines is caused by the effects of the Sun’s rotation). 3. Earth’s magnetic field / internal Dynamo The Earth’s magnetic field is known since more than 2 millenaries. The magnetized needle was used since several centuries for boat navigation, and the first measurement of the Earth’s magnetic field declination were made by navigators. It is Gilbert in 1600 who proposed the concept of the magnet for the Earth’s magnetic field. 266
C. Amory Mazaudier Figure 4 : Left : the magnet from Gilbert 1600; Right : Model from Friedman, 1987. Figure 4 left panel presents the magnet drawn by Gilbert and Figure 4 right panel the model of generation of magnetic field by dynamo action in the Earth (Friedman, 1987). The magnetic field measured at the ground level of the Earth’s is expressed as follow: B = Bp + Ba + Be + Bin Bp = main field; Ba = magnetization field (due to the magnetization of the rocks in the Lithosphere); Be = external field (due to ionospheric and magnetospheric sources); Bin = induced field generated by the external field Be; Bp changes very slowly ‐> annual and secular variations, Ba is constant and its magnitude is ~ qq 10nT, Be changes at different time scales (second, minutes, hours, diurnal, seasonal, annual, solar cycle), Bin changes as Be. The transient variations of the Earth’s magnetic field : ΔB (ΔB ~ ΔΒe + ΔΒin) are essentially due to the external sources (ionospheric and magnetospheric) of the Earth’s environment presented in the sections 4 and 5. 4. Ionospheric dynamo ‐> daily regular external source It is Stewart in 1882 who proposed the right hypothesis to explain the regular variation of the Earth’s magnetic field observed all the days on the sunlight side of the Earth. In the original paper B. Stewart proposed 3 mechanisms to explain the regular variations of the Earth’s magnetic field. One of this mechanism, the dynamo, is the right one [ Text of Balfour Stewart : “ It has been imagined that convection currents established by the sun’s heating influence in the upper regions of the atmosphere are to be regarded as conductors moving across lines of the magnetic forces.” ]. The Earth’s magnetic field observations were used to determine indirectly the ionospheric electric currents at their origin. The first maps of equivalent electric currents were established by Schuster (1889). During more than seven decades the magnetic field measurements were used to approach the ionospheric electric currents (Chapman and Bartels, 1940; Sislbee, 1942; Mayaud, 1965). The daily regular variation of the Earth’s magnetic field observed at ground level is called Sq (Chapman and Bartels 1940) or SR (Mayaud, 1965) [ Sq is the averaged variation of the Earth’s magnetic field on the five quietest days of the month / SR is defined for each day].Later, at the end of the seventies, with the measurements of rockets, meteoric radars and Incoherent scatter sounders, measurements in situ of neutral atmosphere motions and ionospheric electric currents (Evans 1978; Brekke et al., 1974; Mazaudier, 1982), confirmed the hypothesis of B. Stewart. The measurements of the in situ ionospheric electric currents are seldom and very expensive, therefore the Earth’s magnetic observations are still used to approach the real ionospheric currents. Now it is well established, by data and 267
Earthʹs sun system models(See the review on Ionospheric wind dynamo theory of Richmond 1979), that the electriccurrents in the ionosphere at the origin of the daily variation of the Earth’s magnetic field are mainly generated by the motion of the atmosphere which drags preferentially the ions through the Earth’s magnetic field lines in the E dynamo region (100 km <h<150km) and caused a differential drift between ions and electrons. On the right panel of figure 5 from Evans (1977) is shown the absorption of the solar radiations in the ozone layer at the origin of the atmospheric tides which propagate in the upper atmosphere and are the main source of the atmospheric motions in the E dynamo layer. On the left panel of figure 5, is shown the equivalent current systems related to the ionospheric dynamo the Sq/ SR current system and the equatorial electrojet. The Sq/ SR is composed of two cells of current, one by hemisphere. The current circulate anti clockwise in the northern hemisphere and clockwise in the southern hemisphere. The equatorial electrojet (EEJ) is an electric current circulating along the magnetic equator. The equivalent current systems deduced from ground magnetic variations are 2D (Sq/ SR ) and 1D (EEJ). The real currents are more complex and 3D with field aligned currents connecting the 2 hemispheres. Figure 5 : The ionospheric dynamo The expression of the ionospheric electric current : J = Ne x e (Vi – Ve) , where Vi is the ion drift (m/s), Ve the electron drift (m/s), Ne the electronic density (number of particule/m3), e is the charge of the electron ( Coulomb), and J in amp/m2 .The ionospheric electric current can also expressed by the Ohm’s law : J = σ (E + VnxB), where σ is the conductivity (Ω/m) , Vn the neutral wind (m/s), B (T) the Earth’s magnetic field, VnxB the dynamo electric field , E the electrostatic electric field due to the space charge (V/m). The Sq/ SR current system and the equatorial electrojet are transient variations of the Earth’s magnetic field related to solar radiation phenomena. These transient variations of the Earth’s magnetic field (nT) measured at the ground level can be compared to the ionospheric electric current density integrated over the E‐region dynamo (Amp/km) by using the Ampere’s law ( Brekke at al., 1974 and Mazaudier, 1982.) 5. Solar Wind/magnetosphere Dynamo ‐> irregular external source Since the first mention of the Earth’s magnetic field in the encyclopaedist Shon‐Kua (China 1030‐93 A.D) and the first determination of the declination in 1450, many works were made on the Earth’s magnetic field as it is easy and not expensive to measure the Earth’s magnetic field at the ground level. Systematic observations of the Earth’s magnetic field are made in many observatories over the world since more than one century and a half. It was observed that on the regular variation of the Earth’s magnetic field (explained in section 4), an irregular variation is superimposed on specific days called disturbed magnetic days (Chapman and Bartels, 1940). During these disturbed days large disturbances of the Earth’s magnetic field were observed in auroral zone associated with the phenomena of aurora. Different authors ntroduced theory to explain the physical 268
C. Amory Mazaudier phenomena observed during the magnetic disturbed days. Birkeland (1908) postulated the existence of electric currents aligned along the Earth’s magnetic lines and made in his laboratory the experiment with a magnetized sphere. His proposition was at the origin of many controversies but finally in 1978 Ijima and Potemra measured the large scale field aligned currents (also named Birkeland’s currents). The field‐aligned currents connect the electric current systems of the magnetosphere and the ionosphere. Störmer (1907‐1913) introduced the existence of an electric current circulating in the equatorial plane around the Earth’s, known now as the ring current. As Bikerland he reproduced this current experimentally. Chapman and Ferraro (1931) proposed a theory to explain the magnetic storms. The first model of magnetic storm was designed by a no magnetized conductor plan interacting with the Earth’s dipole. Electric currents are associated to this magnetic discontinuity, these currents flowing on the nose of the magnetosphere are known now as the Chapman Ferraro’s currents. See Cole (1966), Akasofu and Chapman (1972), Fukushima and Kamide (1973). At the end of the sixties the satellites allowed many progress in the understanding of the interplanetary medium and the Magnetosphere. With the satellites it became possible to measure the interplanetary magnetic field as well as various parameters of the solar wind as its speed, its temperature and density. The era of magnetospheric studies started. In 1959, Gold defined the magnetosphere. In the abstract of his paper ‘Motions in the Magnetosphere of the Earth he wrote:“The conditions determining the dynamical behaviour of the ionized gas in the outer atmosphere of the earth’s are discussed. It is proposed to call this region in which the magnetic field of the earth dominates the ‘magnetosphere’”. The magnetospheric electric current systems as they are known today are presented on figure 6 : at the nose of the magnetosphere the magnetopause currents (Chapman Ferraro, 1931), in the equatorial plane, the ring current (Störmer 1907‐1913), the field‐aligned currents (Birkeland 1908) connecting the ionospheric and the magnetospheric electric currents, the tail currents (Akasofu and Chapman, 1972) . In 1961 Axford and Hines proposed a new theory to explain the geomagnetic disturbances observed in the auroral zone. He proposed the concept of the solar wind‐dynamo. The solar wind flow continuously around the magnetosphere and though the process of viscous interaction transforms its dynamics energy in an electric field applied to the magnetosphere [Ec = ‐ VsxBi, Ec electric field called convection electric field, Vs solar wind speed and Bi : interplanetary magnetic field]. Under the effect of the convection electric field the particles of the magnetosphere move from the tail to the Earth. Figure 6 : Magnetospheric electric currents . 269
Earthʹs sun system This theory was able to explain the 2 cells of equivalent current observed in auroral zone by Nishida in 1968 and called DP2. . Vasilyunas (1970) with a simple mathematical model reproduced also these observations. Figure 7: Equivalent current systems deduced from the ground magnetic variations during magnetic quiet days (left) and magnetic disturbed day (right). Figure 7 presents the equivalent current system (view from the pole) deduced from the ground magnetic variations by Nagata and Kokubun (left panel), in 1962 during magnetic quiet days (Sqp) and by Nishida (right panel) in 1968, during magnetic disturbed days (DP2). These current system are composed of two cells of current. They are confined on polar cap and auroral zone during magnetic quiet days and extend toward middle low latitudes during magnetic disturbed days. This transient variation of the Earth’s magnetic field called Sqp or DP2 (depending on the level of magnetic activity) is related to the convection electric field produced by the viscous interaction between the solar wind and the magnetosphere. Another mechanism was proposed by Dungey (1961) to explain the disturbances of the Earth’s magnetic field. This mechanism is called reconnection. In the model of Dungey the magnetic field lines carried by the solar wind link up with the terrestrial magnetic field where the magnetic field lines are anti parallel. Conclusion In this paper we presented the main large scale dynamos operating in the sun, the solar wind, the atmosphere and the Earth. We presented the physical mechanisms at the origin of various electric current systems in the Sun Earth’s environment and illustrated two main transient variations of the Earth’s magnetic field (Sq/ SR) and ((Sqp. DP2). The Earths’ magnetic field can be split in several components: 1) the main nucleus field due to the internal Earth’s dynamo (Bp), 2) the aimantation field related to the crust of the Earth’s (Ba) and 3) the transient variations of the Earth’s magnetic field related to the external dynamos (Be) with its induced telluric component (Bin). Two main large scale external dynamos in the Earth’s environment are presented: ‐ the ionospheric dynamo which converts the mechanic energy of the atmospheric motions in electric energy in the ionosphere and is at the origin of the daily regular variation of the Earth’s magnetic field (Sq/ SR) composed of one cell of current in each hemisphere and an electrojet along the magnetic equator (named equatorial electrojet) ‐ the solar wind/magnetosphere dynamo which converts the mechanic energy of the solar wind in electric energy transferred to the magnetosphere and is at the origin of irregular variations of the Earth’s magnetic field ((Sqp. DP2) composed of two cells of current near the pole and the 2 auroral electrojets. References
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