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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Modeling the quiet Sun cell and network emission with ALMA  

Costas Alissandrakis   Submitted: 2020-06-25 10:31

ALMA observations of the Sun at mm-lambda offer a unique opportunity to investigate the temperature structure of the solar chromosphere. In this article we expand our previous work on modeling the chromospheric temperature of the quiet Sun, by including measurements of the brightness temperature in the network and cell interiors, from high resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We also examine the absolute calibration of ALMA full-disk images. We suggest that the brightness temperature at the center of the solar disk in Band 6 is ~440 K above the value recommended by White et al. (2017) and we give improved results for the electron temperature variation of the average quiet Sun with optical depth, as well as the derived spectrum at the center of the disk. We found that the electron temperature in the network is considerably lower than predicted by model F of Fontenla et al. (1993) and that of the cell interior considerably higher than predicted by model A. Depending upon the network/cell segregation scheme, the electron temperature difference between network and cell at tau=1 (100 GHz) is from ~660 to ~1550 K, compared to ~3280 K predicted by the models; similarly, the Te ratio is from ~1.10, to 1.24, against 1.55 of the model prediction. We also found that the network/cell Te(τ) curves diverge as τ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior.

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

Publication Status: Accepted, Astronomy & Astrophysics
Last Modified: 2020-06-27 20:59
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Measurement of the Height of the Chromospheric Network Emission from Solar Dynamics Observatory Images  

Costas Alissandrakis   Submitted: 2019-11-05 09:48

We measured the height of the chromospheric network in the 1700, 1600, and 304 Å wavelength bands of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) from the shift of features on the disk with respect to corresponding features in SDO/Helioseismic and Magnetic Imager (HMI) images of the absolute value of the longitudinal magnetic field. We found that near the limb the 304 Å network emission forms 3.60±0.24 Mm above the 1600 Å emission, which, in turn, forms 0.48±0.10 Mm above the HMI (6173 A) level. At the center of the disk the corresponding height differences are 2.99±0.02 Mm and 0.33±0.06 Mm respectively. We also found that the 1600 Å network emission forms 0.25±0.02 Mm above the 1700 Å emission near the limb and 0.20±0.02 Mm at the disk center. Finally, we examined possible variations with the solar cycle. Our results can help to check and refine atmospheric models.

Authors: C. E. Alissandrakis
Projects: SDO-AIA,SDO-HMI

Publication Status: Acceped, Solar Physics
Last Modified: 2019-11-06 10:37
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Structure of the transition region and the low corona from TRACE and SDO observations near the limb  

Costas Alissandrakis   Submitted: 2019-07-21 00:57

We examined the structure near the solar limb in TRACE images of the continuum and in the 1600 and 171 Å bands as well as in SDO images in the continuum (from HMI) and all AIA bands. The images in different wavelength bands were carefully coaligned by using the position of Mercury for TRACE and Venus for SDO during their transit in front of the solar disk in 1999 and 2012 respectively. Chromospheric absorbing structures in the TRACE 171 Å band are best visible 7" above the white light limb, very close to the inner limb, defined as the inflection point of the rising part of the center-to-limb intensity variation. They are correlated with, but are not identical to spicules in emission, seen in the 1600 Å band. Similar results were obtained from AIA and SOT images. Tall spicules in 304 Å are not associated with any absorption in the higher temperature bands. Performing azimuthal averaging of the intensity over 15 degree sectors near the N, S, E and W limbs, we measured the height of the limb and of the peak intensity in all AIA bands. We found that the inner limb height in the transition region AIA bands increases with wavelength, consistent with a bound-free origin of the absorption from neutral H and He. From that we computed the column density and the density of neutral hydrogen as a function of height. We estimated a height of (2300 ? 500)km for the base of the transition region. Finally, we measured the scale height of the AIA emission of the corona and associated it with the temperature; we deduced a value of (1.24 ? 0.25) 10 6 K for the polar corona.

Authors: C. E. Alissandrakis, A. Valentino
Projects: SDO-AIA

Publication Status: In press, Solar Physics
Last Modified: 2019-07-21 23:47
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Modeling of the sunspot-associated microwave emission using a new method of DEM inversion  

Costas Alissandrakis   Submitted: 2018-12-17 08:24

We developed a method to compute the temperature and density structure along the line of sight by inversion of the differential emission measure (DEM), under the assumptions of stratification and hydrostatic equilibrium. We applied this method to the DEM obtained from AIA observations and used the results, together with potential extrapolations of the photospheric magnetic field, to compute the microwave emission of three sunspots, which we compared with observations from the RATAN-600 radio telescope and the Nobeyama Radioheliograph (NoRH). Our DEM based models reproduced very well the observations of the moderate-size spot on October 2011 and within 25% the data of a similar sized spot on March 2016, but predicted too low values for the big spot of April 14, 2016. The latter was better fitted by a constant conductive flux atmospheric model which, however, could not reproduce the peak brightness temperature of 4.7x10 6 K and the shape of the source at the NoRH frequency. We propose that these deviations could be due to low intensity non-thermal emission associated to a moving pore and to an opposite polarity light bridge. We also found that the double structure of the big spot at high RATAN-600 frequencies could be interpreted in terms of the variation of the angle between the magnetic field and the line of sight along the sunspot.

Authors: C. E. Alissandrakis, V. M. Bogod, T. I. Kaltman, S. Patsourakos, N. G. Peterova
Projects: Nobeyama Radioheliograph,RATAN-600,SDO-AIA

Publication Status: Acceped for publication in Solar Physics
Last Modified: 2018-12-19 12:30
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IRIS Observations of Spicules and Structures Near the Solar Limb  

Costas Alissandrakis   Submitted: 2018-01-12 00:07

We have analyzed IRIS spectral and slit-jaw observations of a quiet region near the South Pole. In this article we present an overview of the observations, the corrections, and the absolute calibration of the intensity. We focus on the average profiles of strong (Mg ii h and k, C ii and Si iv), as well as of weak spectral lines in the near ultraviolet (NUV) and the far ultraviolet (FUV), including the Mg ii triplet, thus probing the solar atmosphere from the low chromosphere to the transition region. We give the radial variation of bulk spectral parameters as well as line ratios and turbulent velocities. We present measurements of the formation height in lines and in the NUV continuum, from which we find a linear relationship between the position of the limb and the intensity scale height. We also find that low forming lines, such as the Mg ii triplet, show no temporal variations above the limb associated with spicules, suggesting that such lines are formed in a homogeneous atmospheric layer and, possibly, that spicules are formed above the height of 2 arc sec. We discuss the spatio-temporal structure near the limb from images of intensity as a function of position and time. In these images, we identify p-mode oscillations in the cores of lines formed at low heights above the photosphere, slow moving bright features in O i and fast moving bright features in C ii. Finally, we compare the Mg ii k and h line profiles, together with intensity values of the Balmer lines from the literature, with computations from the PROM57Mg non-LTE model developed at the Institut d'Astrophysique Spatiale and estimated values of the physical parameters. We obtain electron temperatures in the range of 8000 K at small heights to 20000 K at large heights, electron densities from 1.1x1011 to 4x1010 cm-3 and a turbulent velocity of about 24 km s-1.

Authors: C. E. Alissandrakis,  J.-C. Vial, A. Koukras,  E. Buchlin,  M. Chane-Yook
Projects: IRIS

Publication Status: In press, Solar Physics
Last Modified: 2018-01-14 00:01
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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 Nançay 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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Abstracts by Author
Modeling the quiet Sun cell and network emission with ALMA
Measurement of the Height of the Chromospheric Network Emission from Solar Dynamics Observatory Images
Structure of the transition region and the low corona from TRACE and SDO observations near the limb
Modeling of the sunspot-associated microwave emission using a new method of DEM inversion
IRIS Observations of Spicules and Structures Near the Solar Limb
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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