Spectral Characteristics of the He I D3 Line in a Quiescent Prominence Observed by THEMIS

We analyze the observations of a quiescent prominence acquired by the Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites Solaires (THEMIS) in the He I 5876 A (He I D3) multiplet aiming to measure the spectral characteristics of the He I D3 profiles and to find for them an adequate fitting model. The component characteristics of the He I D3 Stokes I profiles are measured by the fitting system approximating them with a double Gaussian. This model yields an He I D3 component peak intensity ratio of $5.5\pm0.4$, which differs from the value of 8 expected in the optically thin limit. Most of the measured Doppler velocities lie in the interval $\pm5$ km/s, with a standard deviation of $\pm1.7$ km/s around the peak value of 0.4 km/s. The wide distribution of the full-width at half maximum has two maxima at 0.25 A and 0.30 A for the He I D3 blue component and two maxima at 0.22 A and 0.31 A for the red component. The width ratio of the components is $1.04\pm0.18$. We show that the double-Gaussian model systematically underestimates the blue wing intensities. To solve this problem, we invoke a two-temperature multi-Gaussian model, consisting of two double-Gaussians, which provides a better representation of He I D3 that is free of the wing intensity deficit. This model suggests temperatures of 11.5 kK and 91 kK, respectively, for the cool and the hot component of the target prominence. The cool and hot components of a typical He I D3 profile have component peak intensity ratios of 6.6 and 8, implying a prominence geometrical width of 17 Mm and an optical thickness of 0.3 for the cool component, while the optical thickness of the hot component is negligible. These prominence parameters seem to be realistic, suggesting the physical adequacy of the multi-Gaussian model with important implications for interpreting He I D3 spectropolarimetry by current inversion codes.Comment: 25 pages,1 movie, 10 figures, 2 tables, 2 equations. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-017-1118-z The supplementary movie is available for viewing and download at https://www.dropbox.com/s/7tskvnc593tlbyv/Prominence_HeID3_GONG_AIA.mpg?dl=

The European Solar Telescope

The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems

The European Solar Telescope

The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems

Separation of drifting pulsating structures in a complex radio spectrum of the 2001 April 11 event

Aims. We present new method of separating a complex radio spectrum into single radio bursts. The method is used in the analysis of the 0.8–2.0 GHz radio spectrum of the 2001 April 11 event, which was rich in drifting pulsating structures. Methods. The method is based on the wavelet analysis technique, which separates different spatial-temporal components (radio bursts) that are difficult to recognize in the original radio spectrum. Results. We show with this method that the complex radio spectrum observed during the 2001 April 11 event consists of at least four drifting pulsating structures (DPSs). These structures were separated with respect to their different frequency drifts. The DPSs indicate at least four plasmoids that are supposed to be formed in a flaring current sheet

What causes the 24-day period observed in solar flares?

Previous studies report a ~24-day (synodic) period in the occurrence rate of solar flares for each of the solar cycles studied, Nos. 19–22 (Bai 1987, ApJ, 314, 795; Temmer et al. 2004, Sol. Phys. 221, 325). Here we study the 24-day period in the solar flare occurrence for solar cycles 21 and 22 by means of wavelet power spectra together with the solar flare locations in synoptic magnetic maps. We find that the 24-day peak revealed in the power spectra is just the result of a particular statistical clumping of data points, most probably caused by a characteristic longitudinal separation of about $+40^{\circ}$ to $+50^{\circ}$ of activity complexes in successive Carrington rotations. These complexes appear as parallel, diverging or converging branches in the synoptic magnetic maps and are particularly flare-productive

“Drifting tadpoles” in wavelet spectra of decimetric radio emission of fiber bursts

Aims. The solar decimetric radio emission of fiber bursts was investigated searching for the “drifting tadpole” structures proposed by theoretical studies. Methods. Characteristic periods with the tadpole pattern were searched for in the radio flux time series by wavelet analysis methods. Results. For the first time, we have found drifting tadpoles in the wavelet spectra of the decimetric radio emission associated with the fiber bursts observed in July 11, 2005. These tadpoles were detected at all radio frequencies in the 1602-1780 MHz frequency range. The characteristic period of the wavelet tadpole patterns was found to be 81.4 s and the frequency drift of the tadpole heads is -6.8 MHz s-1. These tadpoles are interpreted as a signature of the magnetoacoustic wave train moving along a dense flare waveguide and their frequency drift as a motion of the wave train modulating the radio emission produced by the plasma emission mechanism. Using the Aschwanden density model of the solar atmosphere, only low values of the Alfvén speed and the magnetic field strength in the loop guiding this wave train were derived which indicates a neutral current sheet as the guiding structure. The present analysis supports the model of fiber bursts based on whistler waves

Long period variations of dm-radio and X-ray fluxes in three X-class flares

ABSTRACT Aims. Long period (≥60 s) variations of the radio (0.8−4.5 GHz) and X-ray fluxes observed during the July 14, 2000, April 12, 2001, and April 15, 2001 flares by the Ondřejov radiospectrograph and Yohkoh spacecraft are studied by statistical methods. Methods. In the flares under study, characteristic periods are searched for by the Fourier and wavelet methods. To understand the origin of the 0.8−4.5 GHz drifting burst with long period variations, observed at the beginning of the April 15, 2001 flare, crosscorrelations, time shifts, coherence, and phase differences in its time series are computed. Results. The global statistical study of these flares revealed characteristic periods in the interval of 60−513 s in the radio (0.8−4.5 GHz) and 60−330 s in the X-ray Yohkoh fluxes. Cross-correlations between the radio fluxes at different frequencies helped us to determine the bursts generated by plasma or gyro-synchrotron mechanisms. In the April 12, 2001 flare, soft X-ray fluxes of the sources located at the loop-top and footpoints of a flare loop vary with the period of 60−320 s, and they are highly correlated. But their relation to the radio (1.1 GHz − plasma emission and 4.0 GHz − gyro-synchrotron emission) is complex. At the beginning of the April 15, 2001 flare, in the 0.8−4.5 GHz range, a broadband drifting radio burst with the time variation of 61−320 s was observed at times of flare loop ejection. Its detailed statistical analysis shows that this burst consists of two parts, and, that first part is generated by the plasma emission mechanism and the second, probably, by the gyro-synchrotron one. The characteristic period of about 300 s found in three X-class flares in their dm-radio and X-ray emissions is discussed