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Carbary J.F.,Johns Hopkins University | Mitchell D.G.,Johns Hopkins University | Krimigis S.M.,Johns Hopkins University | Krupp N.,Max Planck Institute For Sonnensystemforshung
Geophysical Research Letters | Year: 2011

Since Saturn's vernal equinox in August 2009 (day 223), energetic electrons (110-365 keV) have exhibited a variety of periodic and aperiodic behavior within a spectral window of 5-15 hours. From late 2009 through the end of 2010, when the observed at dusk, a single period near 10.7 hours dominated the Lomb spectra of these particles. Near the end of 2010, however, the energetic electrons displayed multiple periods, with the strongest at 10.65 hours. The periodicity observed after equinox has a mean value of 10.69 0.06 hours and agreed closely with that of Saturn kilometric radio (south) emissions. By early 2011, when the observer had moved to the dayside, the periodicities abruptly disappeared and the Lomb spectra show no periodicity. This behavior may suggest changes in Saturn's ionosphere as a result of seasonal change, or may alternately imply a local time dependence of periodicity caused by magnetodisk thickness asymmetry. Copyright © 2011 by the American Geophysical Union.

Ermolli I.,National institute for astrophysics | Criscuoli S.,National institute for astrophysics | Uitenbroek H.,U.S. National Solar Observatory | Giorgi F.,National institute for astrophysics | And 3 more authors.
Astronomy and Astrophysics | Year: 2010

Context.The intensity of the Ca II K resonance line observed with spectrographs and Lyot-type filters has long served as a diagnostic of the solar chromosphere. However, the literature contains a relative lack of photometric measurements of solar features observed at this spectral range. Aims.We study the radiative emission of various types of solar features, such as quiet Sun, enhanced network, plage, and bright plage regions, identified on filtergrams taken in the Ca II K line. Methods.We analysed full-disk images obtained with the PSPT, by using three interference filters that sample the Ca II K line with different bandpasses. We studied the dependence of the radiative emission of disk features on the filter bandpass. We also performed a non-local thermal equilibrium (NLTE) spectral synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The synthesis was carried out by utilizing the partial frequency redistribution (PRD) with the most recent set of semi-empirical atmosphere models in the literature and some earlier atmosphere models. As the studied models were computed by assuming the complete redistribution formalism (CRD), we also performed simulations with this approximation for comparison. Results. We measured the center-to-limb variation of intensity values for various solar features identified on PSPT images and compared the results obtained with those derived from the synthesis. We find that CRD calculations derived using the most recent quiet Sun model, on average, reproduce the measured values of the quiet Sun regions slightly more accurately than PRD computations with the same model. This may reflect that the utilized atmospheric model was computed assuming CRD. Calculations with PRD on earlier quiet Sun model atmospheres reproduce measured quantities with a similar accuracy as to that achieved here by applying CRD to the recent model. We also find that the median contrast values measured for most of the identified bright features, disk positions, and filter bandpasses are, on average, a factor ̃1.9 lower than those derived from PRD simulations performed using the recent bright feature models. The discrepancy between measured and modeled values decreases by ̃12% after taking into account straylight effects on PSPT images. When moving towards the limb, PRD computations display closer agreement with the data than performed in CRD. Moreover, PRD computations on either the most recent or the earlier atmosphere models of bright features reproduce measurements from plage and bright plage regions with a similar accuracy. © 2010 ESO.

Borrero J.M.,Max Planck Institute For Sonnensystemforshung | Solanki S.K.,Max Planck Institute For Sonnensystemforshung | Solanki S.K.,Kyung Hee University
Astrophysical Journal | Year: 2010

We have employed a penumbral model, which includes the Evershed flow and convective motions inside penumbral filaments, to reproduce the azimuthal variation of the net circular polarization (NCP) in sunspot penumbrae at different heliocentric angles for two different spectral lines. The theoretical NCP fits the observations as satisfactorily as penumbral models based on flux tubes. The reason for this is that the effect of convective motions on the NCP is very small compared to the effect of the Evershed flow. In addition, the NCP generated by convective upflows cancels out the NCP generated by the downflows. We have also found that, in order to fit the observed NCP, the strength of the magnetic field inside penumbral filaments must be very close to 1000 G. In particular, field-free or weak-field filaments fail to reproduce both the correct sign of the NCP and its dependence on the azimuthal and heliocentric angles. © 2010 The American Astronomical Society.

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