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Center-to-limb observations of the Sun with ALMA  

Costas Alissandrakis   Submitted: 2017-05-26 09:11

We measured the center-to-limb variation of the brightness temperature, Tb, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, Te, as a function of optical depth, tau. The ALMA images are very similar to AIA images at 1600 Å. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is 961.1?2.5 arcsec and 964.1?4.5 arcsec at 239 and 100 GHz respectively. A slight but statistically significant limb brightening is observed at both frequencies. The inversion of the center-to-limb curves shows that Te varies linearly with the logarithm of optical depth for tau(100GHz) between 0.34 and 12, with a slope dlnTe/dtau=-608 K. Our results are 5% lower than predicted by the average quiet sun model C of Fontenla et al. (1993), but do not confirm previous reports that the mm-lambda solar spectrum is better fitted with models of the cell interior.

Authors: C. E. Alissandrakis, S. Patsourakos, A. Nindos, T. S. Bastian
Projects: ALMA

Publication Status: Accepted for publication in Astronomy and Astrophysics
Last Modified: 2017-05-31 14:42
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A tiny event producing an interplanetary type III burst  

Costas Alissandrakis   Submitted: 2015-08-01 01:33

We investigate the conditions under which small scale energy release events in the low corona gave rise to strong interplanetary (IP) type III bursts. We analyze observations of three tiny events, detected by the Nancay Radio Heliograph (NRH), two of which produced IP type IIIs. We took advantage of the NRH positioning information and of the high cadence of AIA/SDO data to identify the associated EUV emissions. We measured positions and time profiles of the metric and EUV sources. We found that the EUV events that produced IP type IIIs were located near a coronal hole boundary, while the one that did not was located in a closed magnetic field region. In all three cases tiny flaring loops were involved, without any associated mass eruption. In the best observed case the radio emission at the highest frequency (435 MHz) was displaced by ~55" with respect to the small flaring loop. The metric type III emission shows a complex structure in space and in time, indicative of multiple electron beams, despite the low intensity of the events. From the combined analysis of dynamic spectra and NRH images we derived the electron beam velocity as well as the height, ambient plasma temperature and density at the level of formation of the 160 MHz emission. From the analysis of the differential emission measure derived from the AIA images we found that the first evidence of energy release was at the footpoints and this was followed by the development of flaring loops and subsequent cooling. We conclude that even small energy release events can accelerate enough electrons to give rise to powerful IP type III bursts. The proximity of the electron acceleration site to open magnetic field lines facilitates the escape of the electrons into the interplanetary space. The offset between the site of energy release and the metric type III location warrants further investigation.

Authors: C. E. Alissandrakis, A. Nindos, S. Patsourakos, A. Kontogeorgos, P. Tsitsipis
Projects: Nançay Radioheliograph,SDO-AIA,STEREO,Wind

Publication Status: Accepted for publication in Astronomy & Astrophysics
Last Modified: 2015-08-03 11:57
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RATAN-600 Observations of Small Scale Structures with High Spectral Resolution  

Costas Alissandrakis   Submitted: 2014-04-02 14:19

We present observations of quiet-sun small-scale structures (SSS) in the microwave range with the Radio Astronomical Telescope of the Academy of Sciences 600 (RATAN-600) spectral-polarization facility in a wide range of frequencies. SSS are regularly recorded in routine observations of the large reflector-type radio telescope and represent manifestations in the radio range of various structures of the quiet-sun: supergranulation network, bright points, plage patches and so on. A comparison with with images from the Solar Dynamics Observatory (SDO) showed that the microwave emission comes from a region extending from the chromosphere to the low transition region. We measured the properties of the SSS as well as the degree of circular polarization averaged over the beam of the radio telescope and from this we estimated the magnetic field at the formation level of the radiation.

Authors: V. M. Bogod, C. E. Alissandrakis, T. I. Kaltman, S. Kh. Tokhchukova
Projects: None

Publication Status: Solar Physics, in pres
Last Modified: 2014-04-07 10:55
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Center-to-limb observations of the Sun with ALMA
A tiny event producing an interplanetary type III burst
RATAN-600 Observations of Small Scale Structures with High Spectral Resolution

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