RT info:eu-repo/semantics/masterThesis
T1 Síntesis de líneas espectrales de simulaciones numéricas de magnetoconvección
A1 Dorantes Monteagudo, Antonio Jesús
A2 Máster Universitario en Astrofísica
AB Nowadays, the studies carried out by the Sun are diverse and their purpose is simple: having abetter aknowledgement of our star.Spectroscopy and spectropolarimetry are the most important observational techniques in thisfield of Astrophysics, but in our case we will not use observational data. We will take a numericalsimulation of the Sun and we will compare the data provided by it with those obtained throughthe observations to know how accurate is the simulation.Studies of the Sun by numerical simulations provides numerous advantages. One of the mostimportant is that it allows to identify physical processes that occur there. When the syntheticspectrum of the simulation is obtained, it is compared with those obtained with the observations,but, previously, we have to ad the imperfections introduced by the telescope or spectrograph inthe data. If the comparison with the observations is good, we can infer that the properties andphenomena given in the simulation occur as well in the Sun. Then we have the option to reproducethis phenomenon every time we want and change it to study other posibilities.In the presence of magnetic fields, the lines suffer modifications due to splitting of the energylevels, resulting new transitions. This is known as the Zeeman effect. When the magnetic fieldis weak (the splitting is lower than the width of the line), it may not have visible effects on theintensity profiles. However we can see changes in the circular polarization profiles. This case ischaracterized by the weak field equation, which is the solution of the radiative tranfer equation(RTE) under these conditions. This is why under weak field conditions we do not obtain theinformation of the field from the intensity profiles, we do it from the circular polarization profiles.In this work we will synthesize the Fe 5247Å, 5250Å, 6173Å, 6301Å, 6302Å, 15648Å, 15652Ålines from a simulation of the quiet Sun. The simulation will contain the values of magnetic fieldcomponents, speed or temperature in each point. We will have to do an interpolation to get a modelthat is equidistant in tau on a logarithmic scale, instead of being equidistant in height. After this,we will give the model to SIR and it will carry out the synthesis and it will return us the intensity,linear polarization and circular polarization profiles.SIR perform the synthesis solving the radiative transfer equation for τ = τ5 (optical depthof the continuum at 5000Å), using a Hermite algorithm. It also works under conditions of LocalThermodynamic Equilibrium (which allows to use Saha’s and Botzmann’s equation to computethe populations) in order to calculate the absorption matrix and the source function. SIR assumeHydrostatic Equilibrium in each iteration.We will only work with circular intensity and polarization profiles since the simulation willfulfill the weak field regime and there is no linear polarization. For the former, For Stokes I, wewill make a study of the displacement of the lines which will give us an estimation of the materialvelocity at each point using the Doppler effect formula. Comparing with the intensity of the continuum, we see that for the intergranular lanes we obtain positive speeds (descending material)whose absolute value is greater than the ones for granular zones, where we obtain negative speeds(ascending material). This is because the gravity action support the descending movement while,in the ascending one, it goes against.We also do a study of the bisector of the lines but, in this case, we make a separation between theintergranular lanes and granular areas. This separation is done using the intensity of the continuum.We will calculate the mean value and defining the granules as those areas which exceed a sigmaof this average value. The intergranular lanes are defined as the areas which has associated anintensity lower or equal than one sigma of the mean value. We obtain for the granules a bisectorthat is blueshifted, as is expected, since as we have said, the speeds are negative, while for theintergranular lanes, the bisector is redshifted (positive velocities).To finish with this part, we will calculate the spatial average of the intensity profiles andcompare it with the lines of the atlas, obtaining a fairly similar fit with all lines.Now, for the circular polarization profiles (Stokes V ), we will calculate their amplitudes andasymmetries. Previously we will make a classification of all profiles and we will choose for the studyonly those whose class will be 1, 2, or 3, according to this classification. We generate amplitudehistograms where we can see that for the seven lines most of the profiles have amplitudes between0.001 and 0.01.We have made an analysis of the amplitude and area asymmetries and with them we willdetermine the gradients of speed and magnetic field. If we have very asymmetric profiles, thesegradients will be larger because they tend to cause a deformation in the circular polarization. Onthe contrary, if they are not very asymmetric we will know that the gradients are smaller. Wefound the first case in the higher energy visible lines because they are formed at higher altitudes,while for infrared lines we have smaller asymmetries, because its formation occurs closer to thecontinuum.We will also calculate the spatial average of the circular polarization profiles and we study ifwe are in a weak field regime. In order to do this, we will look for the proportionality coefficientbetween the average Stokes V and the derivative of the average intensity minimizing residuals. Ifwe represent both, we can see that we are in a weak field regime since both profiles are proportional.Some lines, specially infrared ones, are at the limit of the approach, but it is considered that, evenin the limit, we are in weak field regime. Using this comparison, we will calculate the average ofthe longitudinal magnetic field and comparing it with the obtained from the simulation, we aregoing to make an estimation of the height where the lines are formed.Finally, using the classification that we have done previously, we will take the infrared linesand we will do a study of the classes with the amplitude and with the intensity of the continuum.From the first one, we conclude that the classes with a higher frequency predominate the largeramplitudes and as they become less frequent, they are more significant at lower amplitudes. Fromthe study of the continuum we find that the most frequent class predominates in the granules(higher intensities), the next one abounds in intergranular lanes (lower intensities) and the rest ispresent everywhere.
YR 2021
FD 2021
LK http://riull.ull.es/xmlui/handle/915/23657
UL http://riull.ull.es/xmlui/handle/915/23657
LA es
DS Repositorio institucional de la Universidad de La Laguna
RD 15-ago-2024