scholarly journals Scaling laws of quasi-periodic pulsations in solar flares

2019 ◽  
Vol 624 ◽  
pp. A65 ◽  
Author(s):  
C. E. Pugh ◽  
A.-M. Broomhall ◽  
V. M. Nakariakov
Keyword(s):  
Magnetic Flux ◽  
Solar Flares ◽  
Scaling Laws ◽  
Simulated Data ◽  
Theoretical Models ◽  
Separation Distance ◽  
Magnetic Polarity ◽  
Disc Centre ◽  
Flare Ribbon ◽  
Positive Correlations

Context. Quasi-periodic pulsations (QPPs) are a common feature of solar flares, but there has previously been a lack of observational evidence to support any of the theoretical models that might explain the origin of these QPPs. Aims. We aimed to determine if there are any relationships between the QPP period and other properties of the flaring region, using a previously assembled sample of flares with QPPs. If any relationships exist, then these can be compared with scaling laws for the theoretical QPP mechanisms. Methods. To obtain the flaring region properties, we made use of the Atmospheric Imaging Assembly (AIA) 1600 Å and Helioseismic and Magnetic Imager (HMI) data. The flare ribbons are visible in AIA 1600 Å images, and the positive and negative magnetic polarity ribbons can be distinguished and the magnetic properties determined in the HMI magnetograms. The ribbon properties calculated in this study were the ribbon separation distance, area, total unsigned magnetic flux, and average magnetic field strength. Only the flares that occurred within ±60° of the solar disc centre were included, which meant a sample of 20 flares with 22 QPP signals. Results. Positive correlations were found between the QPP period and the ribbon properties. The strongest correlations were with the separation distance and magnetic flux. Because these ribbon properties also correlate with the flare duration and because the relationship between the QPP period and flare duration may be influenced by observational bias, we also made use of simulated data to determine whether artificial correlations were introduced. These simulations show that although QPPs cannot be detected for certain combinations of QPP period and flare duration, this does not introduce an apparent correlation. Conclusions. There is evidence of relationships between the QPP period and flare ribbon properties, and in the future, the derived scaling laws between these properties can be compared to equivalent scaling laws for theoretical QPP mechanisms.

X-ray Spectral Synthesis in Hydrodynamic Flare Models

1990 ◽  
Vol 115 ◽  
pp. 126-131
Author(s):  
S. Serio ◽  
E. Antonucci ◽  
M.A. Dodero ◽  
G. Peres ◽  
F. Reale
Keyword(s):  
Solar Flares ◽  
Energy Deposition ◽  
Electron Beams ◽  
Spectral Synthesis ◽  
Theoretical Models ◽  
Hydrodynamic Models ◽  
X Ray ◽  
Relativistic Electron Beams ◽  
Stellar Flares ◽  
Magnetically Confined

AbstractCompact solar flares are triggered by sudden energy release in magnetically confined plasma. This class of flares is well suited to be studied with numerical hydrodynamic models. In particular, one can compare the evolution of observed and synthetic X-ray spectra, computed under various assumptions for the mechanism of impulsive energy deposition, to constrain theoretical models and their parameter space. We discuss recent results on solar flares along this line, non thermal to models of energy depositions by relativistic electron beams. We shall also discuss possible applications of X-ray spectral synthesis to stellar flares.

Skin-layer of the eruptive magnetic flux rope in large solar flares

2016 ◽  
Vol 56 (4) ◽  
pp. 393-400
Author(s):  
G. N. Kichigin ◽  
L. I. Miroshnichenko ◽  
V. I. Sidorov ◽  
S. A. Yazev
Keyword(s):  
Magnetic Flux ◽  
Solar Flares ◽  
Flux Rope ◽  
Skin Layer ◽  
Magnetic Flux Rope

scholarly journals DEFINITION OF THE RECONNECTION RATE OF MOST POWERFUL SOLAR FLARES

2020 ◽  
Vol 6 (334) ◽  
pp. 61-67
Author(s):  
A. Sarsembayeva ◽  
◽  
F. Belisarova ◽  
M. Odsuren ◽  
A. Sarsembay ◽  
...  
Keyword(s):  
Magnetic Flux ◽  
Flux Density ◽  
Solar Flares ◽  
Universal Time ◽  
Magnetic Flux Density ◽  
Physical Parameters ◽  
Reconnection Rate ◽  
Scale Size ◽  
Definition Of ◽  
Physical Quantities

We observed top 10 X-class solar flares registered in the period November 1997 - September 2017. We measure physical parameters of 10 solar flares, such as the temporal scale, size, and magnetic flux density, and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also made a brief analysis of solar flares registered in these days, also has shown the duration of time and peak of solar flares in Universal time. We have identified several physical quantities of solar flares and estimated reconnection rate of solar flares. To determine the physical parameters we used images taken with the AIA instrument on board SDO satellite at wavelengths 131 Å, 174 Å, 193 Å, 211 Å, 335 Å, 1600 Å, 1700 Å, 4500 Å, SXT - pictures, HMI Magnetogram, SOLIS Chromospheric Magnetogram, GOES XRT-data. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using the observed values. The inflow velocities are distributed from a few km s-1 to several tens of km s-1, and the Alfven velocities in the corona are in the range from 103 to 104 km s-1. Hence, the reconnection rate is 10-3. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases.

scholarly journals Numerical study of the propulsive performance of two-dimensional pitching foils at very high frequencies: scaling laws and turbulence effects

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Enrique Sanmiguel-Rojas ◽  
Ramon Fernandez-Feria
Keyword(s):  
Numerical Simulations ◽  
Scaling Laws ◽  
Theoretical Models ◽  
Linear Potential ◽  
Propulsive Efficiency ◽  
Content Type ◽  
High Frequencies ◽  
Linear Potential Theory ◽  
Very High Frequencies ◽  
Very High

Purpose This paper aims to analyze the propulsive performance of small-amplitude pitching foils at very high frequencies with double objectives: to find out scaling laws for the time-averaged thrust and propulsive efficiency at very high frequencies; and to characterize the Strouhal number above which the effect of turbulence on the mean values cannot be neglected. Design/methodology/approach The thrust force and propulsive efficiency of a pitching NACA0012 foil at high reduced frequencies (k) and a Reynolds number Re = 16 000 are analyzed using accurate numerical simulations, both assuming laminar flow and using a transition turbulence model. The time-averaged results are validated with available experimental data for k up to about 12 (Strouhal number, St, up to 0.6). This study also compares the present numerical results with the predictions of theoretical models and existing numerical results. For a foil pitching about its quarter chord with amplitude α0 = 8o, the reduced frequency is varied here up to k = 30 (St up to 2), much higher than in any previous numerical or experimental work. Findings For this pitch amplitude, turbulence effects are found negligible for St ≲ 0.8, and affecting less than 10% to the time-averaged thrust coefficient CT¯ for larger St Linear potential theory fails for very large k, even for the small pitch amplitude considered, particularly for the power coefficient, and therefore for the propulsive efficiency. It is found that CT¯ ∼ St2 for large St, in agreement with recent models, and the propulsive efficiency decays as 1/k, in disagreement with the linear potential theory. Originality/value Pitching foils are increasingly studied as efficient propellers and energy harvesting devices. Their performance at very high reduced frequencies has not been sufficiently analyzed before. The authors provide accurate numerical simulations to discern when turbulence is relevant for the computation of the time-averaged thrust and efficiency and how their scaling with the reduced frequency is affected in relation to the laminar-flow predictions. This is relevant because some small-amplitude theoretical models predict high propulsive efficiency of pitching foils at very high frequencies over certain ranges of the structural parameters, and only very accurate numerical simulations may decide on these predictions.

scholarly journals Application of GISAXS in the Investigation of Three-Dimensional Lattices of Nanostructures

Crystals ◽  
2019 ◽  
Vol 9 (9) ◽  
pp. 479 ◽  
Author(s):  
Lovro Basioli ◽  
Krešimir Salamon ◽  
Marija Tkalčević ◽  
Igor Mekterović ◽  
Sigrid Bernstorff ◽  
...  
Keyword(s):  
Ion Beam ◽  
Three Dimensional ◽  
Simulated Data ◽  
Theoretical Models ◽  
Grazing Incidence ◽  
Material Structure ◽  
Suitable Model ◽  
X Ray Scattering ◽  
Ion Beam Treatment ◽  
Ordered Nanostructures

The application of the grazing-incidence small-angle X-ray scattering (GISAXS) technique for the investigation of three-dimensional lattices of nanostructures is demonstrated. A successful analysis of three-dimensionally ordered nanostructures requires applying a suitable model for the description of the nanostructure ordering. Otherwise, it is possible to get a good agreement between the experimental and the simulated data, but the parameters obtained by fitting may be completely incorrect. In this paper, we theoretically examine systems having different types of nanostructure ordering, and we show how the choice of the correct model for the description of ordering influences the analysis results. Several theoretical models are compared in order to show how to use GISAXS in the investigation of self-assembled arrays of nanoparticles, and also in arrays of nanostructures obtained by ion-beam treatment of thin films or surfaces. All models are supported by experimental data, and the possibilities and limitations of GISAXS for the determination of material structure are discussed.

scholarly journals Flows, Evolution of Magnetic Fields, and Flares

1993 ◽  
Vol 141 ◽  
pp. 323-332 ◽  
Author(s):  
Haimin Wang
Keyword(s):  
Magnetic Fields ◽  
Magnetic Structure ◽  
Solar Flares ◽  
Active Regions ◽  
Magnetic Polarity ◽  
Flux Emergence ◽  
Electric Currents ◽  
Magnetic Shear ◽  
Before And After

AbstractThis paper reviews observations on the evolution of magnetic fields and flows in active regions which produce major flares. It includes the following topics: (1) Relationship between magnetic shear and flares; (2) Relationship between electric currents and flares; (3) Flows in active regions, particularly the emergence of new flux inside sheared penumbrae, and the mixed magnetic polarity nature of this kind of flux emergence; and (4) Changes of magnetic structure immediately before and after major solar flares; in particular, I will describe some recent findings that shear may increase after major flares.

scholarly journals Heating of Stellar Chromospheres and Transition Regions

2004 ◽  
Vol 219 ◽  
pp. 437-448
Author(s):  
Zdzislaw E. Musielak
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Acoustic Waves ◽  
Theoretical Models ◽  
Flux Tubes ◽  
Stellar Convection ◽  
Wave Heating ◽  
Active Stars ◽  
Magnetic Flux Tubes ◽  
Magnetic Waves

To explain the heating of stellar chromospheres and transition regions, two classes of heating mechanisms have been considered: dissipation of acoustic and magnetic waves generated in stellar convection zones; and dissipation of currents generated by photospheric motions of surface magnetic fields. The focus of this paper is on the wave heating mechanisms and on recent results which demonstrate that theoretical models of stellar chromospheres based on the wave heating can explain the “basal flux” and the observed Ca II emission in most stars but cannot account for the observed Mg II emission in active stars. The obtained results clearly show that the base of stellar chromospheres is heated by acoustic waves, the heating of the middle and upper chromospheric layers is dominated by magnetic waves associated with magnetic flux tubes, and that other non-wave heating mechanisms are required to explain the structure of the highest layers of stellar chromospheres and transition regions.

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