A1S2WP2

The solar activity is a series of events and observable features of solar magnetic fields. The basic process of the activity cycle, the solar dynamo, takes place well within the solar interior. The lowest level in the solar atmosphere where the emerging magnetic fields are detectable is the visible solar surface, the photosphere. Three different features can be observed at this level: photospheric magnetic fields, sunspots and photospheric faculae. The magnetic fields are mapped in magnetograms. The sunspots are usually members of sunspot groups, in other terms the active regions, they have strong magnetic fields, they are darker than their surrounding and they can be observed in continuum light. The photospheric faculae have weak magnetic fields and they are brighter than their surroundings, they are also observable in continuum light in the outer regions of the solar disc.

The goals of WP "Photosphere" are on the one hand to achieve a substantial advancement in data production about the above features, on the other hand to analyze the distributions and variations of the magnetic fields with special regard to geoeffective phenomena.

The following questions are addressed:

- Data of photospheric faculae (D2.1). Sunspots cause irradiance deficit, whereas faculae are sources of irradiance excess. These data will be a new kind of input in irradiance studies (WP5).

- Sunspot data production. This has perhaps the longest history in solar physics but up to now there exists no catalogue containing all sunspots and all sunspot groups together, with data of positions, areas and magnetic fields, i.e. all relavant data. This project will provide the first such catalogue, the SDD, for the SOHO era (D2.2). We also endeavour to prolongue the existing datasets, to conduct their comparative studies and to digitize full-disc solar images (Task 2.3). The ultimate aim, beyond SOTERIA, is to achieve the most detailed data coverage of the era of solar observations.

- Magnetic data. Magnetograms of high temporal and spatial resolution are also needed (D2.3)

- Investigation of photospheric activity (D2.4). Precursors of some explosive events can be detected at the photospheric level, in these cases WP2 participates in the complex analysis of the active event. Otherwise we investigate properties of solar dynamo action, north-south asymmetries and phase lags, sunspot group tilts, differential rotation, questions of cycle forecast, and geoeffective connections of these phenomena.

The scientific profiles of the different institutes are necessarily specialized. The project SOTERIA offers a unique opportunity to bring together teams of different tools and expertise for the complex treatment of this important area. We cooperate with WP6 in establishing a virtual observatory.

Participating institutions are: KO as WP2 lead, HVAR, OBSPARIS, ROB, UNIGRAZ, UOulu.