SummaryWe present spectral diagnostics for the fluxes of emission lines, in the spectral range 3600-4400 Å, during the cooling phase of stellar flares on dMe stars. Using these diagnostics, electron temperatures have been computed for flares on AD Leonis, Proxima Centauri and UV Ceti. This preliminary model assumes a single flare loop containing a homogeneous, stationary optically-thin flare plasma.

Abstract. In this paper we present the results of a different approach in the study of the so-called rotation-activity connection, which is a well established correlation between rotation and magnetic activity at chromospheric and outer atmospheric levels. The present study concerns the photospheric level and was carried out by using V -band photometric light curve amplitudes as indicators of starspot coverage and of magnetic activity. A high degree of correlation between the envelope of maximum V -band light curve amplitudes and the rotation period is found for the active star members of young open clusters (IC 2602, IC 2391, Alpha Persei, Pleiades and Hyades), as well as for active field stars. This correlation shows a different behaviour in two different rotation period ranges. Moreover, some evidence of a possible activity "saturation" is found among the most rapidly rotating stars of the sample. Additional correlations between photospheric and other magnetic activity indicators in the chromosphere, transition region and corona are also investigated. The results presented here can be considered as an extension of the well established rotation-activity connection valid from the corona, transition region and chromosphere, down to the photosphere.

Abstract.The maximum amplitude (A max ) of spot-induced brightness variations from long-term V-band photometry and the ratio L X /L bol between X-ray and bolometric luminosities are suitable indicators of the level of magnetic activity in the photosphere and in the corona of late-type stars, respectively. By using these activity indicators we investigate the dependence of coronal X-ray emission on the level of photospheric starspot activity in a homogeneous sample of low mass main sequence field and cluster stars of different ages (IC 2602, IC 4665, IC 2391, α Persei, Pleiades and Hyades). First, the activity-rotation connection at the photospheric level is re-analysed, as well as its dependence on spectral type and age. The upper envelope of A max increases monotonically with decreasing rotational period (P) and Rossby number (R 0 ) showing a break around 1.1 d that separates two rotation regimes where the starspot activity shows different behaviours. The A max -P and A max -R 0 relations are fitted with linear, exponential and power laws to look for the function which best represents the trend of the data. The highest values of A max are found among K-type stars and at the ages of α Persei and Pleiades. We also analyse the activity-rotation connection at the coronal level as well as its dependence on spectral type. The level of X-ray emission increases with increasing rotation rate up to a saturation level. The rotational period at which saturation occurs is colour-dependent and increases with advancing spectral type. Also the L X /L bol -P and L X /L bol -R 0 relations are fitted with linear, exponential and power laws to look for the best fitting function. Among the fastest rotating stars (P ≤ 0.3 d) there is evidence of super-saturation. Also the highest values of L X /L bol are found among K-type stars. Finally, the photospheric-coronal activity connection is investigated by using for the first time the largest ever sample of light curve amplitudes as indicators of the magnetic filling factor. The activity parameters L X /L bol and A max are found to be correlated with each other, thus confirming the dependence of coronal activity on photospheric magnetic fields. More precisely, the L X /L bol -A max distribution shows the presence of an upper envelope, which is constant at the L X /L bol −3.0 saturation level, and of a lower envelope. The best fit to the lower envelope is given by a power law with steepness decreasing from F-G to M spectral types. However, it is considered a tentative result, since the fit reduced chi-squares are large. Such spectral-type dependence may be related to a colour dependence of A max on the total starspot filling factor, as well as to the coronal emission being possibly more sensitive to starspot activity variations in F-and G-type than in M-type stars. The L X /L bol -A max mean values for each cluster in our sample decrease monotonically with increasing age, showing that the levels of photospheric and coronal activity evolve in time according to a single power law till the S...

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