Magnetospheric electric convection field

The collision of solar wind with the Earth's magnetic field creates an electric field in the inner magnetosphere ( with a the earth's radius ), the magneto- spherical electric convection field. It is oriented substantially from dawn to dusk side. The co-rotating plasma of the inner magnetosphere drifts perpendicular to this field and perpendicular to the force lines of Earth's magnetic field Bo. The production process of this field is not yet fully understood. One possibility would be a friction process between the solar wind and the boundary layer of the magnetosphere - the magnetopause. Reconnection of magnetic field lines is another possibility. Finally, a hydro- magnetic dynamo process in the polar regions of the magnetosphere is conceivable. From satellite measurements today gives a pretty good picture of the structure of this field. There are a number of models of this field.

A frequently used model in the literature is the Volland - Stern model ( engl. Volland - Stern model).

Model Description

The model is based on two simplifying conditions:

The actual earth's magnetic field is replaced by a coaxial dipole field. Its magnetic field lines can through the shell parameters

Are shown. It is the equation of a field line, the distance from the Earth, the Earth's radius and the pole spacing. Is the pole pitch of the base point of the magnetic field due to the Earth's surface, and is the radial distance of the line in the equatorial plane ( θ = 90 °)

It is believed that the electric field may be derived from an electrostatic potential øC. Because of the high electrical conductivity in the magnetosphere, the electric fields can only be aligned perpendicular to the electric potential and perpendicular to the magnetic field. Therefore, the electric potential must be parallel to the magnetic field. The relationship

Satisfies this condition. It is here ch = 1/sin2θm the so-called separatrix that the field of magnetospheric middle and low latitudes ( θ ≥? M ) with closed magnetic field lines from the polar region ( θ ≥? M ) ( which have only one base point on the earth's surface ) with open magnetic field lines separates. τ is the local time? m ≈ 20 °, the polar boundary of the auroral zone and Φco the total potential difference between the morning and the evening side. q, Φco, and τco empische are parameters that are determined from the observations. Gl. ( 2) applies to a firm away from the sun -handed coordinate system. The geomagnetic equator in this model is identical to the geographic equator. Since the electric potential is symmetrical with respect to the equator, it is sufficient to restrict oneself to the northern hemisphere. For a transformation of a non-rotating in a rotating coordinate system, the local time τ must be replaced by the longitude λ.

Inner Magnetosphere

With the numerical values ​​of q ≈ 2, and Φco and τco, depending on the geomagnetic activity (eg Φco ≈ 17 and 65 kV, and τco ≈ 0 and 1 h during geomagnetically quiet and disturbed conditions ), Eq. ( 2) the Volland - Stern model, valid outside the polar regions ( θ >? M ) in the inner magnetosphere (r ≤ 10 a) (see Figure 1a).

The use of electrostatic potential implies that this model is (greater than about a half-day periods ) applicable only for very slow changes over time. Because of the assumption of a coaxial magnetic dipole field only global structures can be simulated.

The electric field components are determined from

As

From electrodynamics it is known that the following relationship between an electric field in a rotating system Ero to the field is in a non-rotating system Enr:

U × B is the Lorentz force with U = R Ω the rotational speed, R the distance axis and Ω the angular frequency of rotation. In the co-rotating plasma in the inner magnetosphere therefore acts in the non-rotating coordinate system, the Lorentz force

With Φro = 90 kV. This is, the potential of so-called co- rotation of the electric field. In the non-rotating coordinate system, therefore, the thermal plasma in the inner magnetosphere responds to the two potentials in Eq. (2) and equation ( 4).:

The potential? R decreases with the distance from the earth, while the potential of growing øC. The sum of both potentials is called a torus -like structure with closed potential-energy surfaces, plasma sphere (Fig. 1b), in which the thermal plasma is trapped. In fact, Whistler observations show that the plasma within the plasma sphere by several orders of magnitude larger than outside the plasmapause, the last closed potential-energy surface (Fig. 1b) ). From the configuration of the plasma break, the exponent q = 2 can in Eq. (2 ) can be derived, while the expansion of the plasma pause determines the size Φco.

Origin of Konvektionsfeldes

The interaction of the solar wind with the Earth's magnetic field causes the creation of an electric field in the magnetosphere. In the polar regions of the magnetosphere, the field lines of the interplanetary magnetic field can be linked with those of the earth's magnetic field, so that these lines have only one foot on the ground. In the polar regions of the inner magnetosphere, these are directed almost vertically. The current flowing through the polar areas solar wind induces an electric field ( hydro- magnetic dynamo effect ), which is oriented from dawn to dusk side. There is an electric charge separation at the boundary layer of the magnetosphere, the magnetopause, instead. Electrical discharge currents ( current Birkeland ) flow in the ionospheric dynamo layer along the last closed magnetic field lines (Lm ) as a field - parallel currents within this highly electrically conductive layer in two parallel streams on the day and on the night side ( polar electric jets or DP1 currents) to the dusk side and from there flow back to the magnetosphere again. The variations of the geomagnetic field at ground level is a measure of the variability of such electric currents in the ionosphere and magnetosphere.

Polar magnetosphere

The electric convection field in the near-Earth polar regions of the magnetosphere, by the exponent in Eq. (2) can be simulated. At the separatrix (Lm ), the potential phi.C closes continuously to the potential in middle and low latitudes to ( Eq. ( 2) ). It is there, however, a field reversal, coupled with the Birkeland currents mentioned above, instead. This is confirmed by the observations.

In a more accurate model is the auroral zone between about 15o and 20o pole spacing, again facilitated by a coaxial magnetic field, introduced as a transition layer. The ionospheric dynamo layer is between about 100 and 200 km altitude is an area in which ions and electrons have a different mobility. Due to the influence of the earth's magnetic field, there are two types of flows: the flow Pedersen, parallel to the electric field E and perpendicular to the Hall current I and B.

In the auroral zone, the electrical conductivity as a function of geomagnetic activity is significantly increased. This is the model τco by the parameters in Eq. ( 2) taken into account. The electric convection field also drives electric currents in the polar regions of the ionospheric dynamo layer ( DP2), which can also be simulated by the model.

The derived from the geomagnetic fluctuations on the ground electrical currents are only valid for horizontal surface currents in the ionosphere. However, the real flows are three-dimensional in general. The Birkeland currents hardly have a magnetic effect on the earth's surface. To uniquely determine the actual current configuration direct magnetic field measurements in the ionosphere and magnetosphere are therefore necessary.

The model allows the separation of Pedersen and Hall currents. DP2 is for example almost entirely of Hall currents. The polar electric Jets (DP1 ) have both current components. The Jets have power levels of several hundred kilo amperes. Dissipation of Pedersenströme ( Joule heating ) is passed to the neutral gas of the thermosphere. This thermospheric and ionospheric disturbances are generated. Durable disorders associated with strong geomagnetic variations are becoming a global thermospheric and ionospheric storms.