**Publication Date:** 2009.

**Language:** English.

**Abstract:**

Context. Magnetic clouds (MCs) are formed by magnetic flux ropes that are ejected from the Sun as coronal mass ejections. These structures generally have low plasma beta and travel through the interplanetary medium interacting with the surrounding solar wind. Thus, the dynamical evolution of the internal magnetic structure of a MC is a consequence of both the conditions of its environment and of its own dynamical laws, which are mainly dominated by magnetic forces.Aims. With in-situ observations the magnetic field is only measured along the trajectory of the spacecraft across the MC. Therefore, a magnetic model is needed to reconstruct the magnetic configuration of the encountered MC. The main aim of the present work is to extend the widely used cylindrical model to arbitrary cross-section shapes.Methods. The flux rope boundary is parametrized to account for a broad range of shapes. Then, the internal structure of the flux rope is computed by expressing the magnetic field as a series of modes of a linear force-free field.Results. We analyze the magnetic field profile along straight cuts through the flux rope, in order to simulate the spacecraft crossing through a MC. We find that the magnetic field orientation is only weakly affected by the shape of the MC boundary. Therefore, the MC axis can approximately be found by the typical methods previously used (e.g., minimum variance). The boundary shape affects the magnetic field strength most. The measurement of how much the field strength peaks along the crossing provides an estimation of the aspect ratio of the flux-rope cross-section. The asymmetry of the field strength between the front and the back of the MC, after correcting for the time evolution (i.e., its aging during the observation of the MC), provides an estimation of the cross-section global bending. A flat or/and bent cross-section requires a large anisotropy of the total pressure imposed at the MC boundary by the surrounding medium.Conclusions. The new theoretical model developed here relaxes the cylindrical symmetry hypothesis. It is designed to estimate the cross-section shape of the flux rope using the in-situ data of one spacecraft. This allows a more accurate determination of the global quantities, such as magnetic fluxes and helicity. These quantities are especially important for both linking an observed MC to its solar source and for understanding the corresponding evolution. © 2009 ESO.

**Author affiliation**: Dasso, S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Keywords:**
Interplanetary medium; Sun: coronal mass ejections (CMEs); Sun: magnetic fields; Arbitrary cross section; Boundary shapes; Coronal mass ejection; Cylindrical models; Cylindrical symmetry; Dynamical evolution; Field strengths; Flux ropes; Global quantities; Helicities; In-situ data; In-situ observations; Internal structure; Interplanetary medium; Large anisotropy; Magnetic clouds; Magnetic configuration; Magnetic field orientations; Magnetic field profile; Magnetic field strengths; Magnetic flux ropes; Magnetic models; Minimum variance; Solar source; Sun: coronal mass ejection; Sun: magnetic field; Theoretical models; Time evolutions; Total pressure; Aspect ratio; Astrophysics; Boundary layer flow; Interplanetary spacecraft; Magnetic fields; Magnetic flux; Magnetic structure; Planetary surface analysis; Solar wind; Sun; Semiconductor counters.

**Repository:** Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales

**Authors**:
Hubrig, S.; Schöller, M.; Kholtygin, A. F.; Gonzalez, Jorge Federico; Kharchenko, N. V.; Steffen, M.

**Publication Date:** 2012.

**Language:** English.

**Abstract:**

Context. Recent developments in observational techniques and theories revealed the potential significance of magnetic fields for stellar structure, evolution, and circumstellar environment. At present, the distribution of magnetic field strengths in massive stars from the zero-age main sequence to more evolved stages, which would shed light on the origin of the magnetic field, has not been studied. Aims. We searched for the presence of a magnetic field in the visually brightest early A-type supergiant HD 92207. Methods. Observations were obtained using the low-resolution spectropolarimetric mode of FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8 m Antu telescope of the VLT. For the mean longitudinal magnetic field measurements, we applied a linear regression analysis in two ways: using only the absorption hydrogen Balmer lines or using the entire spectrum including all available absorption lines. Results. A mean longitudinal magnetic field at a significance level of more than 3σ was detected in two out of three observations distributed over about one year. It is one of the rare cases where a field of about a few hundred Gauss is detected in an early A-type supergiant. All line profiles in the spectra of HD 92207 undergo distinct variations in radial velocities and intensities, probably caused by previously detected non-radial pulsations.

**Author affiliation**: Hubrig, S.. Leibniz Institut für Astrophysik Potsdam ; Alemania

**Author affiliation**: Schöller, M.. European Southern Observatory; Alemania

**Author affiliation**: Kholtygin, A. F.. Saint Petersburg State University; Rusia. Isaac Newton Institute of Chile. Saint Petersburg Branch; Rusia

**Author affiliation**: Gonzalez, Jorge Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentina

**Author affiliation**: Kharchenko, N. V.. Main Astronomical Observatory; Ucrania

**Author affiliation**: Steffen, M.. Leibniz Institut für Astrophysik Potsdam ; Alemania

**Repository:** CONICET Digital (CONICET). Consejo Nacional de Investigaciones Científicas y Técnicas

**Publication Date:** 2008.

**Language:** English.

**Abstract:**

Aims. We study the stability and modes of non - isothermal coronal loop models with different intensity values of equilibrium twisted magnetic field.Methods. We use an energy principle obtained by means of non - equilibrium thermodynamic arguments. The principle is expressed in terms of Hermitian operators and enables us to consider the coupled system of equations, the balance of energy equation and the equation of motion, to obtain modes and eigenmodes in a spectrum ranging from short to long-wavelength disturbances without using weak varying approximations of the equilibrium parameters. Long-wavelength perturbations introduce additional difficulties because the inhomogeneous nature of the medium produce disturbances corresponding to continuous intervals of eigenfrequencies, which cannot be considered as purely sinusoidal.Results. We analyze the modification of periods, modes structure, and stability when the helicity, the magnetic field strength, and the radius of the fluxtube are varied. The efficiency of the damping due to the resonant absorption mechanism is analyzed in terms of modes that can either impulsively release or store magnetic energy.Conclusions. We find that the onset of the instability is associated with a critical value of the helicity and the magnetic energy content has a determinant role on the instability of the system with respect to the stabilizing effect of the resonant absorption mechanism. © 2008 ESO.

**Author affiliation**: González, R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

**Keywords:**
Sun: corona; Sun: magnetic fields; Sun: oscillations; Absorption; Energy efficiency; Equations of motion; Magnetic field measurement; Magnetic fields; Magnetic materials; Mathematical operators; Mechanisms; Solar energy; Thermodynamic stability; Thermodynamics; Coronal loops; Coupled systems; Critical values; Eigen-frequencies; Eigen-modes; Energy equation; Energy principles; Equation of motion; Equilibrium parameters; Helicity; Hermitian operators; Intensity values; Irreversible energy; Long wavelengths; Magnetic energies; Magnetic field configurations; Magnetic field strengths; Mode analysis; Non-equilibrium; Non-isothermal; Resonant absorption; Solar coronal loops; Stabilizing effects; Sun: corona; Sun: magnetic fields; Sun: oscillations; System stability.

**Repository:** Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales

**Authors**:
Hubrig, S.; Schöller, M.; Savanov, I.; Gonzalez, Jorge Federico; Cowley, C. R.; Schütz, O.; Arlt, R.; Rüdiger, G.

**Publication Date:** 2010.

**Language:** English.

**Abstract:**

In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow-up studies of magnetism among young pre-main-sequence stars. We obtained high-resolution, high signal-to-noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5 kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb-Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot=13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non-statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current-driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate-mass stars could be an alternative to a frozen-in fossil field.

**Author affiliation**: Hubrig, S.. Astrophysikalisches Institut Potsdam; Alemania

**Author affiliation**: Schöller, M.. European Southern Observatory; Alemania

**Author affiliation**: Savanov, I.. Russian Academy of Sciences. Institute of Astronomy; Rusia

**Author affiliation**: Gonzalez, Jorge Federico. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico San Juan. Instituto de Ciencias Astronomicas de la Tierra y del Espacio; Argentina

**Author affiliation**: Cowley, C. R.. University Of Michigan; Estados Unidos

**Author affiliation**: Schütz, O.. European Southern Observatory; Chile

**Author affiliation**: Arlt, R.. Astrophysikalisches Institut Potsdam; Alemania

**Author affiliation**: Rüdiger, G.. Astrophysikalisches Institut Potsdam; Alemania

**Repository:** CONICET Digital (CONICET). Consejo Nacional de Investigaciones Científicas y Técnicas