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| Titel |
Cowling channel formation through the ionosphere-magnetosphere coupling via Alfven waves |
| VerfasserIn |
Akimasa Yoshikawa, Olaf Amm, Ryoichi Fujii |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034228
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| Zusammenfassung |
| An inclusive formulation of Magnetosphere-Ionosphere (MI) coupling system, which enables
to describe the Cowling channel formation through the MI-coupling via the shear Alfven
wave, is proposed. By applying the Walen-relation [Walen, 1944] to the Alfvenic disturbance
near the ionosphere, arbitrary incompressible MHD fields (b,v) can be separated into the
incident component to the ionosphere and the reflected component from the ionosphere. The
separated incident component gives an electromotive force (emf) for the excitation of the
MI-coupled system. In addition, the thermospheric wind dynamo also becomes a
source of “emf” in the ionosphere, while the reflected components are generated
as a result of the MI-coupling process. Using this separation, we clarify that the
MI-coupling processes are composed of the “direct reflection” process for incident MHD
field, the “radiation” process for the thermospheric wind-driven dynamo, and the
“polarization” process for the charge separation originating from conductivity gradients.
Clearly, the “reflection” and the “radiation” processes are caused by the cancellation
process of the divergence of the primary Pedersen current (primary current means
directly driven current by the “emf” field). The “polarization” process is caused
by the both the divergent part of the primary Pedersen and of the primary Hall
current.
Since the direction of the primary Pedersen current and the direction of the primary Hall
current are perpendicular to each other, the reflected fields generated to cancel out these two
irrotational parts of the primary currents are also expected to be perpendicular to each other.
Thus, the MI-coupled ionospheric current system can be separated into two orthogonal
irrotational current systems, the first channel originating from the primary Pedersen
current divergence (generator-channel) and the second channel from the primary
Hall current divergence (loading-channel). A combination of these two channels
forms so-called the Cowling channel. The generator-channel is directly coupled to
the external generator of the system, which supplies the “emf” for generating the
MI-coupled system. In this channel, the Pedersen and the Hall currents flow in the same
direction, and the reflected Alfvenic potential ΦP is generated. On the other hand, the
loading-channel does not directly couple to the external generator, but couples to the internal
Cowling-generator, which mediates electromagnetic energy between the generator and the
loading channels. In the loading channel, the Pedersen and the Hall current flows in
the opposite direction to each other, and the reflected Alfvenic potential ΦH is
produced.
Our theoretical framework proposed in this paper uniquely describes how much of
field-aligned currents (FACs) are closed in the generator-channel and in the loading-channel,
respectively, and how these channels are coupled to each other in the context of energy
conservation. Simulation results show that significant rotation and elongation of the potential
structure is caused by the Hall polarization field ΦH which is generated in regions of
steep conductivity gradients. We will present the theory in detail and show some
numerical results of the formation process of the Cowling channel via Alfven waves. |
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