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Solar fibrils and spicules at high resolution

Final Activity Report Summary - MULTIDOT (Solar fibrils and spicules at high resolution)

The project 'Solar fibrils and spicules at high resolution' (MULTIDOT) focused on nature and dynamics of fine structures seen in the solar chromosphere. The project original scientific objectives involved:
1. motions of magnetic structures in active regions and their manifestation in chromospheric fibrils; and
2. nature, anchoring and dynamics of spicules.
The project objectives were later expanded on the relation of transition-region jet-like features and chromospheric fibrils.

The project realisation consisted of acquisition and analysis of time sequences of images obtained by the Dutch open telescope (DOT) and an analysis of brief image sequence obtained in 2002 with the very high angular resolution ultraviolet telescope (VAULT). The fellow actively took part as a co-proposer and observer in the setup and execution of two joint observing programmes, entitles 'Spectroscopy and imaging of the solar fibrils: photospheric drivers and coronal consequences accomplished by the DOT and the Swedish 1 m solar telescope (SST)'.

The first campaign was accomplished soon after launch of the project in October 2005, but due to very bad observing conditions only very limited amount of the acquired data was of good quality. Therefore, the second campaign was executed in April 2006, with satisfactory results. However, due to low solar activity the observations involved quiet areas without sunspots. Between the observing campaigns the fellow analysed the stratifications of the physical parameters derived through the inversion of the spectrograms which were obtained by the vacuum tower telescope even before the project beginning. The results were presented at various scientific gatherings and published in three papers. The fellow subsequently analysed the image sequence obtained by the DOT during the second campaign. The analysis focused on dynamic fibrils dark jet-like features exhibiting striking elongation and retraction on timescales of a few seconds. The fellow developed software for measuring the precise positions, orientation and lengths of fibrils which were believed to be on-disc counterparts of spicules.

The project brought the following new results towards the first objective:
1. fibrils exhibited transverse drift motions suggesting systematic turning from one limit position to another;
2. average angular velocity of turning was of the order of 1 deg/min;
3. shorter fibrils tended to drift faster than the longer ones.

Moreover, the project brought the following results towards the second objective:
1. tops of fibrils followed parabolic paths with sub-ballistic decelerations (i.e. lower than solar gravity) and had mostly supersonic (i.e. higher than local sound speed) onset velocities;
2. fibrils with larger initial velocities underwent larger deceleration.

The VAULT images had the highest spatial resolution ever achieved in imaging of the transition region. They were thus ready for analysis at the project beginning. The precise measurements of transition-region (TR) spicules seen in VAULT images brought the following new results towards the third objective:
1. TR spicules' tops followed paths that were consistent with incomplete parabolas;
2. TR spicules exhibited mostly supersonic onset velocities;
3. the population of TR spicules consisted of sub-ballistic and super-ballistic samples with decelerations higher than solar gravity;
4. the similarity of kinematic properties implied that sub-ballistic TR spicules demonstrated hot transition-region interfaces around relatively cool chromospheric fibrils.

The observational evidences on transverse drift of chromospheric fibrils and on coupling between chromospheric fibrils and transition-region jets were the most important project outcomes, materialised in numerous publications. All of them could be accessed at URL http://adsabs.harvard.edu/default_service.html by entering the fellow name.