Anita Kullen

Introduction to the PhD-thesis:
Polar auroral arcs - when and why do they appear ?

The magnetosphere

The sun ejects constantly a thin, highly ionized gas (plasma) which spreads with supersonic speed deep into space (solar wind). The solar wind is a nearly perfect conductor such that it carries the interplanetary magnetic field (IMF) with it (principle of frozen field lines). The magnetosphere is defined as the cavity around the Earth which is to a first approximation shielded from the solar wind and thus, dominated by the Earth magnetic field. Its shape evolves due to the interaction between the solar wind, the IMF and the Earth dipole field. On the dayside the magnetosphere is compressed by the solar wind, on the nightside it forms a long tail (magnetotail). At the boundary between solar wind and magnetosphere (magnetopause) the principle of frozen field lines breaks down, the IMF and the Earth dipole field diffuse locally through the magnetopause, i.e., the field lines of the IMF reconnect with the field lines of the Earth dipole field. The tailward stretching of the Earth magnetic field lines inside the magnetotail is connected to a dawn-duskward flowing current in the equatorial plane (current sheet). The current flows inside a region called plasma sheet, a region with hot, high pressure plasma and a weak magnetic field. Due to the deformation of the geomagnetic field, high latitude field lines connect only with one end to the Earth (open field lines), via tail reconnection their tailward end is connected to the IMF. The regions in the magnetotail containing open field lines are called the lobes. The lobes are situated north and south of the plasma sheet which contains closed field lines only.

The aurora

The magnetosphere acts like a huge current generator which is driven by the solar wind. As electrons are free to move parallel to the magnetic field in a high conductivity plasma, currents having their source region in the magnetotail flow along the magnetic field lines into the high-latitude ionosphere. The ionosphere is the layer above the Earth atmosphere at around 100 to 1000 km from the Earth. As the ionosphere is only partly ionized, its plasma has a high resistivity. The ionosphere acts as a load in the solar wind-magnetosphere coupling system of which the polar lights (aurora) are the visible pattern. The aurora consists of emitted photons which are caused by the collision of accelerated electrons with gas particles of the atmosphere. The auroral emissions appear at around 70 degrees latitude forming auroral ovals around the magnetic north and south poles. Luminosity and size of the auroral oval depend strongly on the direction of the IMF. During a southward orientation of the IMF, the auroral oval is usually very bright and active and substorms occur periodically (a substorm describes a strong expansion of the entire tail followed by a collapse which causes a sudden brightening and a large expansion of the nightside auroral oval until it recovers to its original state). During northward IMF the auroral oval is strongly contracted, the auroral emissions are very weak and often auroral arcs occur poleward of the auroral oval inside the so called polar cap. They are called polar auroral arcs or polar arcs. Polar arcs are usually sun-aligned and have often with a dawnward or duskward motion. The more spectacular ones reach over the entire polar cap connecting the nightside with the dayside part of the auroral oval and are thus called transpolar arcs or theta aurora.

PhD thesis results

To investigate the influence of the solar wind and its magnetic field on large-scale polar arcs a statistical study has been performed (Kullen et al., 2002). It is based on global images of the auroral oval provided by the Polar UV imager and solar wind data from ACE, a satellite being located 220 Re sunward of the Earth. The study confirms previous observational results: the occurrence and location of polar arcs is strongly dependent on the direction of the IMF. It is shown that different types of large-scale polar arcs can be connected each to a characteristic combination of solar wind parameters. Changes of IMF direction in a plane orthogonal to the Sun-Earth line seem to be mainly responsible for which type of polar arc occurs. The most favorable conditions for large-scale polar arcs to occur are a northward direction of the IMF (this is well-known) combined with a high energy flux in the solar wind (this is new).
In a second part of the thesis, the large-scale topology of the magnetosphere during the occurrence of transpolar arcs is examined. In Kullen (2000) a model is presented which may explain the appearance of a polewardly moving transpolar arc (this type of arc is defined as 'moving arc' in Kullen et al. (2002)). It is known from observations that a moving arc occurs typically after a sign change of the east-west direction of the IMF (IMF By). Further on, it is known that the IMF By component causes the magnetotail to twist around the Sun-Earth axis. In the model proposed by Kullen (2000) it is assumed that a change of IMF By causes the magnetotail to change its twist successively, first in the near-Earth tail, and then in the far tail such that in an intermediate state, near-Earth and far tail regions are oppositely twisted. This tailward propagating rotation of the tail twisting leads to a complicated structure of the closed field line region. Field-lines originating in the high-latitude far tail flank bifurcate the near Earth tail lobes and map further and further poleward of the main oval. As transpolar arcs appear on closed field lines, the poleward moving bar of closed field lines is supposed to represent a moving transpolar arc.
It is shown in Kullen (2000) that in case the magnetotail has oppositely twisted regions, a region of closed field lines appears inside the polar cap with help of a modification of a standard magnetosphere model (T89). The details of the T89 modifications which lead to a twisted tail can be found in Kullen and Blomberg (1996). In Kullen and Janhunen (2004) the ideas presented in Kullen (2000) are checked with a fully developed magnetohydrodynamic model of the magnetosphere. The influence of the IMF direction on the polar cap boundary and closed field line region in the magnetotail is examined there in detail. The simulation results confirm the Kullen (2000) model. The IMF By sign reversal affects first the magnetotail flanks and the near-Earth tail before the inner tail and far tail regions are reorganized such that in an intermediate state, the plasma sheet resembles a horizontal S-shaped structure. In Kullen and Janhunen (2004) it is furthermore shown how the length of the magnetotail and the degree of tail twisting vary with the direction of the IMF. For an auroral arc to occur poleward of the main auroral oval the magnetotail is required to be long and highly twisted. This is the case for (constantly or transitionally) northward IMF with a small IMF By component. Hence, the simulation results explain why northward IMF is necessary for most types of polar arcs to occur.