Michael E. Contadakis

Fast colorimetry of the flare star EV lacertae from UBVRI observations in 2004

Aims. We report results of a quantitative colorimetric UBVRI analysis of two flare events on the red dwarf EV Lac. The photometric data were obtained in September 2004, during the multi-site synchronous monitoring from the four observatories in Ukraine, Russia, Greece, and Bulgaria. These observations confirmed the presence of small-scale high-frequency oscillations (HFO) initially detected by Rodono (1974, A&A, 32, 337) and recently reconfirmed by the authors. Here we discuss the color characteristics of flares and HFO. Methods. Colorimetric analysis had been performed with the help of the time tracks in the UBVRI color-color diagrams from the earliest phase of flare development. Digital filtering technique was used to evaluate the time-dependent color indices. Results. As can be clearly seen in the diagrams, color indices oscillate on a time scale of seconds, far exceeding instrumental errors. Regarding the HFO, we conclude that the bulk of a flare oscillates during a major part of its lifetime between the states of hydrogen plasma opaque and transparent in the Balmer continuum. We find that at the peaks of oscillations the color tracks drift into the regions of color-color diagrams corresponding to a blackbody radiation, which provides an estimate of color temperatures from 17 000 to 22 000 K. We also find that flares cover ∼1% of the stellar disc.

Atmospheric tides in the area of Thessaloniki

Abstract The tidal changes of the barometric pressure in the area of Thessaloniki, Greece were investigated by analyzing a sample of 10.75 years of hourly measurements. The tidal analysis showed that the signal-to-noise ratio varies from 16.8 to 1 for the long period waves (Sa, Ssa, Mm, Mf). For the short period constituents, the corresponding quantity was negligible with the exception of the diurnal P1, S1, K1 and semidiurnal waves S2, K2 where values of 14, 93, 15 and 61 and 7 were found respectively. The amplitude and the phase difference of the above tidal waves were determined with an accuracy of 0.008–0.18 hPa and 1–3 ° correspondingly. There is a considerable correlation of the surface temperature to the changes of the pressure affecting mainly the Ssa, S1 and S2 waves. The barometric changes computed in the area of Thessaloniki using the amplitude and phase difference determined through the tidal analysis reach a maximum amplitude equal to 10 hPa per year. It is also shown that the tidal changes of the barometric pressure in the test area correspond to about 37% of the total changes. A Fourier analysis of the daily mean values of the barometric pressure for the last 44 years showed significant amplitudes of the power spectral density (PSD) at the non-tidal periods of 8 and 52.6 days.

Hydrologic Changes as Possible Earthquake Precursor in Greece

Although reliable reports on pre-, co- andpost-seismic variations of the hydrologiccharacteristics of a seismic active area were knownfrom the antiquity in Greece, it was only the last twodecades that systematic research focused its intereston the hydrologic precursory phenomena. This researchis summarized in this paper.On the basis of the results of the research forearthquake precursory phenomena on the undergroundwater level and temperature at selected areas of highseismic risk, which were performed the last fifteenyears in Greece, it can be supported that monitoringthe shallow underground water level and temperaturefor detecting earthquake precursory phenomena mayprove a useful method in the frame of aninterdisciplinary research for earthquake prediction.

Assessment of recent tidal models in the Mediterranean Sea

Ocean tides especially in closed sea areas can deviate considerably from the theoretical values due to unequal water depths and to the fact that the continents impede the movement of water. Satellite altimetry enabled the development of improved tidal models event in closed sea areas, by assimilating altimeter data into hydrodynamic models. The Mediterranean Sea due to its morphology is an ideal test field for the assessment of tidal models, based on this technique. An attempt to assess the recent tidal models TPXO.6, GOT00.2 and NAO.99b was based on (a) an inter-comparison of tidal heights computed from the three models at different time moments on the same 15′ × 15′ grid covering the Mediterranean and (b) on the comparison of the statistics of a crossover analysis of nearly 2.3 years of JASON-1 altimeter data (Cycle1-86), before and after the tidal correction, using the three tidal models. The inter-comparison in terms of mean value and standard deviation of the differences between the tidal heights on the 15′ × 15′ grid resulted in mean values up to 3 mm and standard deviations ranging from 16 to 26 mm. However, maximum values of differences exceed several dm. On the other hand, the statistics of the crossover analysis showed a 17% decrease of the standard deviation of the JASON-1 crossover differences after the tidal correction. These results show a good agreement between the three tidal models.

Further investigation of the high frequency optical oscillations during the flare phase of the red dwarf EV Lac

In this paper we present the results of the analysis of the U‐light curve for a flare of magnitude 1.2, which was observed on September 2004, with the help of the 30‐inch Cassegrain telescope of the Stephanion Observatory. The results of both, DFT‐analysis and wavelet analysis, indicate that (a) Oscillations with period 1 to 1.5min, 11s, 7.5s and 5.5s appear during the pre‐flare state and persist during the whole flare state, (b) From the flare maximum phase on, a progressive increase of oscillations with periods 20s up to 4.0s is markedly indicated and (c) At the end of the flare only the oscillation of the pre‐flare state do remain. This result is in favour of (or does not contradict) the suggested explanation, i.e. the evolution of a fast mode magneto‐acoustic wave generated at the impulsive phase of the flare and traveling through the magnetic loop.