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Multi-wavelength studies of solar fine structures.

Final Activity Report Summary - SPICULES (Multi-wavelength studies of solar fine structures)

The highly inhomogeneous appearance of the quiet Sun is characterised by a rich diversity of fine-scale dynamic structures which have profound effects on the mass and energy flow to the outer solar atmosphere. The principal inhomogeneities are related to roughly cellular patterns that constitute the 'network', which is best observed in chromospheric lines and persists throughout the chromosphere-corona transition region and the low corona. There is a clear distinction between the bright chromospheric patches, which make up the 'network boundaries', and the somewhat darker areas, which constitute the 'internetwork' (also called the 'cell interiors'). The most prominent features residing at the chromospheric network boundaries are mottles and spicules, which are short-lived dynamic events, originating at low heights, rising with an apparent speed of app. 25 km/s, reaching a maximum height of about 10 Mm, and then either fading or descending to the photosphere. It has also been shown that in the underlying photosphere, apart from sunspot and pores, the magnetic field is concentrated into small flux tubes with field strengths of 1-2 kG, which form patches of magnetic flux concentrations. A strong spatial coincidence exists between these magnetic flux concentrations and the overlying network boundaries. Therefore, it is not surprising that several studies are devoted to better understand the network phenomena and the structures associated with it.

Over the past decade, apart from the well-known spicules, many complicated and dynamic fine structures have been discovered in association with the network boundaries, like explosive events, blinkers, network flares, upflow events, Halpha-1A jets. However, their interpretation, inter-relationship and their relation to the underlying photospheric magnetic concentrations remain ambiguous, because the same feature has a different appearance when observed in different spectral lines. For most of the events mentioned above, magnetic reconnection has been suggested as the driving mechanism just like for spicules. The majority of them appear within the network boundaries where many new bipolar elements that emerge in the network cells drift by the supergranular flow and cancel there against magnetic elements of the opposite polarity. Magnetic reconnection, suggested as their driving mechanism, is probably the most suitable mechanism not only for releasing energy with important implications for the heating of the chromosphere and corona, but also for the transfer of cool gas from the chromosphere to the corona and the solar wind.

During the Marie Curie Reintegration Grant a lot of observational work has been performed with multi-instrument and multi-wavelength observations made both from the ground and space. The aim was the comprehension of the dynamical behaviour of mottles / spicules and other fine structures, their association with the magnetic field and their interrelationship. For the analysis of the data several statistical approaches and non-LTE inversion methods have been developed and used in order to gain insights on their physical properties (such as velocity, temperature and density) and their temporal evolution. Once these physical properties are determined, the diagnostics of their dynamics help at shedding light on the mechanism responsible for their formation. The so far performed research, under the auspices of this program, has shown that there is a dynamic interconnection of small scale phenomena from the lower to the higher levels of the solar atmosphere. This is of vital importance for understanding the transfer of mass and energy from the solar surface outwards and hence the interaction of the Sun-Earth system, an interaction which is the key element for space weather predictions and for the development of future space technology and telecommunications.