Ionospheric dynamo region

The ionospheric dynamo layer, a region in the earth's atmosphere, is located between about 85 and 200 km altitude and is characterized by an electrically conducting ionospheric plasma, which is moved by solar and lunar atmospheric tide against the force lines of the geomagnetic field and thereby induced electric fields and currents, as well. like in a technical dynamo, the moving coil, which moves against a magnetic field The magnetic field of such currents is measured on the surface of the earth as the earth magnetic field variation. These variations are Sq - variations (S = solar; q = quiet) ( = l lunar ) called and L variations. The tides themselves are caused by different solar radiation in the atmosphere or by the gravitational influence of the moon

A magneto -varying spherical electric convection field produces additional electric currents within the ionospheric dynamo layer, the DP1 ( the polar electric jets) and the DP2 currents. Furthermore, there is a polar ring current, which depends on the interplanetary magnetic field. Such geomagnetic variations are part of the earth's magnetic field outside, reach the amplitudes rare 1% of the internal main field Bo.

• 3.1 Sq - current 3.1.1 morphology
• 3.1.2 theory

Atmospheric electrical conductivity

Radioactive gas from the earth and galactic cosmic rays ionize a small part of the air within the lower and middle atmosphere and make the neutral gas electrically conductive. Electrons combine very rapidly with neutral gas particles and form negative ions. The ions are monatomic in the rule. The electrical conductivity depends on the mobility of the ions. This mobility is proportional to the reciprocal of air density and therefore increases exponentially with the height. The ions move with the neutral gas, so that the electrical conductivity is isotropic, but is extremely small.

In the altitude range between about 85 and 200 km - the dynamo layer - the solar X-ray and extreme ultraviolet the radiation ( XUV ) are almost completely absorbed and thereby partially ionizes the air. There arise the different ionospheric layers. In this altitude range, the electrons are already bound to the earth's magnetic field and gyrate around the magnetic field lines several times before they collide with neutral gas particles. The ions should move essentially with the neutral gas. The result is an anisotropic electrical conductivity. The conductivity parallel to the electric field E is said Pedersen conductivity. Pedersen currents have ohmic losses and thus generate Joule heating. The conductivity perpendicular to E and to the Earth's magnetic field Bo is the Hall conductivity. The component parallel to Bo (parallel conductance) continues to grow with height. Near the geomagnetic equator, a West - Eastern electric field generates a vertical Hall current which is not closed. Wherein a vertical polarization field is established, which generates a horizontal Hall current. This additional current intensifies the Hall Pedersen stream. Such gain is described by the cowling conductivity. Pedersen and Hall conductivity reached a maximum at about 120 to 140 km. On the day they have numerical values ​​of about 1 mS / m. At night, these values ​​can go back to one tenth. The values ​​of the conductivity depends on the time of day, the latitude, the time of year and the Elfjahreszyklus the sun. The height of the integrated conductivity are of the order of 50 s, and have a resistance of about 0.02 ohms.

In the auroral zones, which are located around 70 ° to 75 ° north and south geomagnetic latitude, falling high-energy particles from the magnetosphere, which ionize the air in about 110 to 120 km altitude in addition, thus increasing Pedersen and Hall conductivity. This conductivity increases during strong geomagnetic disturbances.

Above about 200 km the collisions between neutral gas and plasma are becoming increasingly rare, so both positive ions and electrons only gyrate around the magnetic field lines, or can drift to E and Bo vertically. The parallel conductance is such that the geomagnetic field lines are electrical equipotential lines. It can therefore only electric fields exist orthogonal to Bo ( see magnetosphere ).

Atmospheric tides

Atmospheric tides are large-scale atmospheric waves that travel through regular differential insolation (solar tides ) or by the gravitational influence of the moon ( lunar tides ) are excited. The atmosphere acts like a giant waveguide closed down ( on the ground ) and is open upward. In such a waveguide can an infinite number of internal waves ( wave modes ) are generated. However, the waveguide is not perfect, so that only waves with large horizontal and vertical dimensions develop enough to be filtered out of the meteorological noise can. These waves are solutions to Laplace's equation. They are called Hough functions and can be approximated by spherical harmonics.

There are two types of modes ( also referred to as gravitational waves ) waves of the class I and class II of the shafts ( rotary shafts ). Class II waves exist only due to the Coriolis force and disappear less than 12 hours for periods. The eigenmodes are either internal waves with a finite large vertical wavelengths, the wave energy can be transported upwards or external waves with infinitely large vertical wavelengths whose phases are constant with height. The amplitudes of internal waves grow exponentially with height. External waves, however, can not transport wave energy, and their amplitudes take away from their source region decreases exponentially with height. Each wave mode is characterized by four numbers: the zonal wave number n, the meridional wave number m ( the meridional structure of the waves with increasing m and more complex ), due to their intrinsic value ( named in reference to ocean tides also equivalent depth ), and through her ​​period in the case of tide 12 hours ( half-day waves) and 24 hours ( full-day waves ), etc. The modes are characterized by the Zahlenduo (n, m). Even numbers of n valid for symmetric waves with respect to the equator, odd numbers of n for antisymmetric waves. Waves of class II are characterized by negative values ​​of n.

In the altitude region above 150 km, the waves develop to external waves, and the Hough functions degenerate into spherical harmonics. For example, the wave mode (1, -2) is the spherical function P11 ( θ ), mode ( 2, 2) P22 ( θ ), etc. with θ the polar pitch, etc..

Wandering solar tides

The fundamental all-day tidal wave that best fits your Meridionalstruktur the sunlight and is therefore excited the most, is the mode ( 1, -2). He is an external wave of class II and emigrated westward with the sun. Its maximum pressure amplitude at the ground is 60 Pa. However, this wave is the dominant mode in the thermosphere and exosphere temperature reached in the amplitudes of the order of 100 K and wind speeds of 100 m / s and more.

The most powerful half-day wave has the ID ( 2, 2). It is an internal wave of class I and has a maximum pressure amplitude of 120 Pa on the ground. This amplitude increases with the height. Although their solar excitation energy is only half as large as that of the full-day wave (1, -2), its amplitude on the ground is twice as large. This identifies the suppression of external shaft relative to an internal shaft.

Half-day lunar tide

The dominant lunar tidal wave of fashion (2, 2). It depends on the local lunar day. Its maximum pressure amplitude at the ground is 6 Pa. Such tiny amplitude can only be laboriously remove from the meteorological noise. This mode is an internal wave whose amplitude increases exponentially with altitude and 100 km altitude two orders of magnitude greater than on the ground.

Electric currents

Sq - current

Morphology

More than 100 geomagnetic stations on Earth regularly measure the variations of the geomagnetic field. Daily variations during selected quiet geomagnetic activity are used to form a monthly agent. For the horizontal component of this average value DELTA.H an equivalent electric current J can be derived in the ionospheric dynamo layer. Its strength is

Where J is the electric current in an infinitely thin layer at about 120 km altitude, AH ( nano Tesla) the observed horizontal component of the geomagnetic variation and μ are the permeability of free space ( in milliamps per meter). The direction of the magnetic field with respect to the electric current can be determined using the right- hand rule. When the right thumb points in the direction of the current, then the magnetic field is oriented in the direction of the crooked finger.

One has to consider that this relationship is not unique here. In general, the electric currents are three-dimensional in the ionosphere and magneto sphere and an infinite number of flow configurations fits to the measured magnetic field at ground level. Magnetic field measurements far above the earth's surface is therefore necessary to obtain a clear picture.

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Figure 1a shows the electrical equivalent of the power lines Sq stream, viewed from the sun. In any given day eddy current a total current of 140 kA flows.

The rotating Sq - current as well as the electrically conductive interior of the earth behave like a giant transformer with the dynamo layer as the primary winding and the Earth as a secondary winding. Since the Sq - current moves with a period of one day, in the earth, a secondary current is induced, its magnetic field is superimposed on the primary field. Figure 1b shows that the secondary current, as seen from the sun. The magnetic field amplitude of this secondary power system approximately one third of the primary field. This means that actually the ratio between the primary current and magnetic field

Is. The strength of the current depends on Sq of the season. The summer vortex intensified compared with the winter vortex. A length dependence exists due to the Earth's magnetic field inclined with respect to the Earth's axis. But nichtwandernde internal tidal waves that penetrate the dynamo layer may additionally produce variations of the Sq - current. During the 11- year cycle of solar activity, the amplitude of the Sq - current can change by a factor of two and more. Two-thirds of this variation can be explained by the change in conductivity over the fluctuating solar activity. The residue may depend on the fluctuation of the solar wind. During the night, the electron density of the ionospheric E- layer is reduced. Therefore, the center of the dynamo layer moves upwards ..

The main producer of the Sq - current is the external all-day tidal wave (1, -2). Since its phase is constant with height, their height- independent coherent wind system is particularly effective, while the winds of internal waves interfere destructively. A Fourier analysis shows that there is a half-day component, which has an amplitude of approximately 1/ 2 the amplitude of full-time component, phase-shifted by 180 °. This seems to be the result of a non-linear coupling between varying wind all day and all-day varying conductivity. The center of the vortex flow is a variation of the day. This is the consequence of the action of internal waves and tides of the meteorological conditions, but also the effect of solar activity.

A high beam current by a factor of about four is larger than the SQ stream in middle latitudes, is observed within the range of about ± 150 km distance from the geomagnetic equator. This is caused by the influence of the Cowling conductivity near the equator.

During a solar flare increased solar radiation reaches the iononosphärische D- and E - layer on the day side. Thus, the electrical conductivity is increased, which makes noticeable as small bulging of the geomagnetic variation ( geomagnetic Solar Flare effect, also called Crochet ). During a solar eclipse occurs in the shadow region the opposite. The conductivity is decreased and a slight decrease of the geomagnetic variation observed ( eclipse effect of the geomagnetic field ). Both effects can be observed only in geomagnetically quiet conditions.

In the course of the decay of a strong geomagnetic storm is briefly developed called a kind of anti - Sq - current, Ddyn. It is generated by Joule heating in the polar ionospheric dynamo layer.

Theory

To quantitatively calculate the dynamo effect of the tidal wind, going from the horizontal components of the momentum equation ( Laplace equation ) together with an equation for the divergence of the wind out. In the Laplace equation, the inertial force, the Coriolis force, the horizontal pressure gradient and the ampere force jx Bo are in equilibrium. The power amp couples the electric current density j at the wind and printing system. The electric current j obeys Ohm's law. An electric polarization field E caused by charge separation and provides the freedom of divergence of the power system. The feedback between wind and electricity via the Lorentz force Ux Bo. In general, the electrical conductivity tensor σ are the height- integrated conductivity tensor Σ and the current density j is replaced by a vertically integrated surface current J.

In conventional dynamo theories, the ampere force is neglected. This means that the gate is open B in Figure 2. This is called a kinematic dynamo. Models with closed gate B are called hydro- magnetic dynamo. The influence of the mutual coupling between wind and electricity can be seen immediately if one assumes an infinite electrical conductivity. In the kinematic model of the electric power is infinite, however, remain the wind unaffected. In the hydro- magnetic model, however, the electric current reaches a maximum, similar to a technical dynamo case of short circuit, while the wind goes back to a minimum. Charge separation acts as a self- impedance, which prevents the electric current increases to infinity.

L- current

The lunar (L) current is by a factor of about 20 less than the SQ stream. It behaves similarly to the Sq - current with the difference that there are four instead of two current vortices. In each eddy current flows on average a total current of about 4 kA. His years term variations are also those of the Sq - current similar. During the day the L- current is amplified. At night it is very small. There is thus a modulation that depends on the lunar phase. The geomagnetic effect of L- current is especially visible in the region of the geomagnetic equator where the Cowling conductivity can increase this current significantly.

DP1 - current

The influence of solar wind on the magnetosphere generates a large-scale magneto- spherical electric convection field, which is oriented by the morning side to the night side. The maximum electrical potential difference is about 15 kV at low geomagnetic activity and significantly more in disturbed conditions. Such field forces a charge separation on both sides of the magnetopause. An electric discharge current flows in the morning side along the last open field lines of the geomagnetic field in the auroral zones of the ionospheric dynamo layer, there in two narrow bands on the dusk side and back to the western side of the magnetopause. The currents flowing in the dynamo layer current hot bands DP1 - polar electric currents or jets. Even at geomagnetically quiet conditions, they can reach power levels of several mega amps. The thus generated ohmic losses and Joule heating are comparable with the solar XUV radiation in middle and low latitudes and significantly greater in disturbed conditions. This heat source is responsible for the formation of strong thermospheric and ionospheric storms.

DP2 current

A magneto- spherical electric convection field drives a two- cell electric power system, which is located on the East and West side in the polar regions of the dynamo layer. It's called DP2 current. This current system already exists in extremely quiet geomagnetic conditions and is then called SQP. It consists essentially of Hall current.

Polar ring current

When the earth is in an interplanetary magnetic field sector, which is directed away from the sun, the magnetospheric plasma is decelerated in the north polar cap and accelerated in the southern polar cap. In the reverse case, the Northern cap is accelerated and decelerated the Südkappe. This deviation from the co- rotation vanishes outside the polar caps. The magnetic effect on the ground corresponding to a polar Hall current, which encircles the pole at a distance of about 10 ° pole pitch in the clockwise direction to an observer on the earth in the case of inter- polar sector structure, which is directed away from the sun, in the counterclockwise case directed to the sun sector structure.