Caius L. Selhorst

Association of Radio Polar Cap Brightening with Bright Patches and Coronal Holes

Radio-bright regions near the solar poles are frequently observed in Nobeyama Radioheliograph (NoRH) maps at 17 GHz, and often in association with coronal holes. However, the origin of these polar brightening has not been established yet. We propose that small magnetic loops are the source of these bright patches, and present modeling results that reproduce the main observational characteristics of the polar brightening within coronal holes at 17 GHz. The simulations were carried out by calculating the radio emission of the small loops, with several temperature and density profiles, within a 2D coronal hole atmospheric model. If located at high latitudes, the size of the simulated bright patches are much smaller than the beam size and they present the instrument beam size when observed. The larger bright patches can be generated by a great number of small magnetic loops unresolved by the NoRH beam. Loop models that reproduce bright patches contain denser and hotter plasma near the upper chromosphere and lower corona. On the other hand, loops with increased plasma density and temperature only in the corona do not contribute to the emission at 17 GHz. This could explain the absence of a one-to-one association between the 17 GHz bright patches and those observed in extreme ultraviolet. Moreover, the emission arising from small magnetic loops located close to the limb may merge with the usual limb brightening profile, increasing its brightness temperature and width.

The behavior of the spotless active regions during the solar minimum 23-24

In this work, we analysed the physical parameters of the spotless actives regions observed during solar minimum 23 - 24 (2007 - 2010). The study was based on radio maps at 17~GHz obtained by the Nobeyama Radioheliograph (NoRH) and magnetograms provided by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). The results shows that the spotless active regions presents the same radio characteristics of a ordinary one, they can live in the solar surface for long periods (>10 days), and also can present small flares.

Analysis Of Kepler-71 Activity Through Planetary Transit

An exoplanet transiting in front of the disk of its parent star may hide a dark starspot causing a detectable change in the light curve, that allows to infer physical characteristics of the spot such as size and intensity. We have analysed the Kepler Space Telescope observations of the star Kepler-71 in order to search for variabilities in 28 transit light curves. Kepler-71 is a star with 0.923Ms and 0.816Rs orbited by the hot Jupiter planet Kepler-71b with radius of 1.0452RJ. The physical parameters of the starspots are determined by fitting the data with a model that simulates planetary transits and enables the inclusion of spots on the stellar surface with different sizes, intensities, and positions. The results show that Kepler-71 is a very active star, with several spot detections, with a mean value of 6 spots per transit with size 0.6Rp and 0.5 Ic, as a function of stellar intensity at disk center (maximum value).

The influence of eclipses in the stellar radio emission

Here we simulate the shape of a planetary transit observed at radio wavelengths. The simulations use a light curve of the K4 star HAT-P-11 and its hot Jupiter companion as proxy. From the HAT-P-11 optical light curve, a prominent spot was identified (1.10 Rp and 0.6 Ic). On the radio regime, the limb brighting of 30% was simulated by a quadratic function, and the active region was assumed to have the same size of the optical spot. Considering that the planet size is 6.35% of the the stellar radius, for the quiet star regions the transit depth is smaller than 0.5%, however, this value can increase to ~2% when covering an active region with 5.0 times the quiet star brightness temperature.

Study of active region temporal evolution at 17GHz

We present a statistical study of the evolution of active regions observed at microwave wavelengths during the 23rd solar cycle (1992–2004). Active regions are sites of increased magnetic fields, thus the radio emission at microwave wavelengths is mostly due to gyro‐resonance of thermal electrons around relatively strong magnetic fields. We analyze active regions observed in full Sun images obtained daily by the Nobeyama Radioheliograph (NoRH) at 17 GHz. Additional information is obtained from photospheric magnetograms of the same day. Also a multiwavelength study provides a global view of the solar atmosphere above active regions, the radio images will be further compared to ultraviolet (SOHO) and H‐alpha images. The active regions are characterized by: position, area, maximum and mean brightness temperatures, and magnetic field. The inferred magnetic fields are used to estimate the contribution to the emission at 17 GHz. We discuss the results of the correlation among these physical parameters.