Dominik Utz

IMPULSIVE ACCELERATION OF CORONAL MASS EJECTIONS. I. STATISTICS AND CORONAL MASS EJECTION SOURCE REGION CHARACTERISTICS

We use high time cadence images acquired by the STEREO EUVI and COR instruments to study the evolution of coronal mass ejections (CMEs) from their initiation through impulsive acceleration to the propagation phase. For a set of 95 CMEs we derived detailed height, velocity, and acceleration profiles and statistically analyzed characteristic CME parameters: peak acceleration, peak velocity, acceleration duration, initiation height, height at peak velocity, height at peak acceleration, and size of the CME source region. The CME peak accelerations we derived range from 20 to 6800 m s–2 and are inversely correlated with the acceleration duration and the height at peak acceleration. Seventy-four percent of the events reach their peak acceleration at heights below 0.5 R ☉. CMEs that originate from compact sources low in the corona are more impulsive and reach higher peak accelerations at smaller heights. These findings can be explained by the Lorentz force, which drives the CME accelerations and decreases with height and CME size.

The size distribution of magnetic bright points derived from Hinode/SOT observations

Context. Magnetic bright points (MBPs) are small-scale magnetic features in the solar photosphere. They may be a possible source of coronal heating by rapid footpoint motions that cause magnetohydrodynamical waves. The number and size distribution are of vital importance in estimating the small scale-magnetic-field energy. Aims. The size distribution of MBPs is derived for G-band images acquired by the Hinode/SOT instrument. Methods. For identification purposes, a new automated segmentation and identification algorithm was developed. Results. For a sampling of 0.108 arcsec/pixel, we derived a mean diameter of (218 ±48) km for the MBPs. For the full resolved data set with a sampling of 0.054 arcsec/pixel, the size distribution shifted to a mean diameter of (166±31) km. The determined diameters are consistent with earlier published values. The shift is most probably due to the different spatial sampling. Conclusions. We conclude that the smallest magnetic elements in the solar photosphere cannot yet be resolved by G-band observations. The influence of discretisation effects (sampling) has also not yet been investigated sufficiently.

Dynamics of isolated magnetic bright points derived from Hinode/SOT G-band observations

Context. Small-scale magnetic fields in the solar photosphere can be identified in high-resolution magnetograms or in the G-band as magnetic bright points (MBPs). Rapid motions of these fields can cause magneto-hydrodynamical waves and can also lead to nanoflares by magnetic field braiding and twisting. The MBP velocity distribution is a crucial parameter for estimating the amplitudes of those waves and the amount of energy they can contribute to coronal heating. Aims. The velocity and lifetime distributions of MBPs are derived from solar G-band images of a quiet sun region acquired by the Hinode/SOT instrument with different temporal and spatial sampling rates. Methods. We developed an automatic segmentation, identification and tracking algorithm to analyse G-Band image sequences to obtain the lifetime and velocity distributions of MBPs. The influence of temporal/spatial sampling rates on these distributions is studied and used to correct the obtained lifetimes and velocity distributions for these digitalisation effects. Results. After the correction of algorithm effects, we obtained a mean MBP lifetime of (2.50 ± 0.05) min and mean MBP velocities, depending on smoothing processes, in the range of (1-2) km s -1 . Corrected for temporal sampling effects, we obtained for the effective velocity distribution a Rayleigh function with a coefficient of (1.62 ± 0.05) km s -1 . The x- and y-components of the velocity distributions are Gaussians. The lifetime distribution can be fitted by an exponential function.

Magnetic field strength distribution of magnetic bright points inferred from filtergrams and spectro-polarimetric data

Small scale magnetic fields can be observed on the Sun in G-band filtergrams as MBPs (magnetic bright points) or identified in spectro-polarimetric measurements due to enhanced signals of Stokes profiles. These magnetic fields and their dynamics play a crucial role in understanding the coronal heating problem and also in surface dynamo models. MBPs can theoretically be described to evolve out of a patch of a solar photospheric magnetic field with values below the equipartition field strength by the so-called convective collapse model. After the collapse, the magnetic field of MBPs reaches a higher stable magnetic field level. The magnetic field strength distribution of small scale magnetic fields as seen by MBPs is inferred. Furthermore, we want to test the model of convective collapse and the theoretically predicted stable value of about 1300 G. We used four different data sets of high-resolution Hinode/SOT observations that were recorded simultaneously with the broadband filter device (G-band, Ca II-H) and the spectro-polarimeter. To derive the magnetic field strength distribution of these small scale features, the spectropolarimeter (SP) data sets were treated by the Merlin inversion code. The four data sets comprise different solar surface types: active regions (a sunspot group and a region with pores), as well as quiet Sun. In all four cases the obtained magnetic field strength distribution of MBPs is similar and shows peaks around 1300 G. This agrees well with the theoretical prediction of the convective collapse model. The resulting magnetic field strength distribution can be fitted in each case by a model consisting of log-normal components. The important parameters, such as geometrical mean value and multiplicative standard deviation, are similar in all data sets, only the relative weighting of the components is different.

The Formation and Disintegration of Magnetic Bright Points Observed by Sunrise/IMaX

The evolution of the physical parameters of magnetic bright points (MBPs) located in the quiet Sun (mainly in the interwork) during their lifetime is studied. First, we concentrate on the detailed description of the magnetic field evolution of three MBPs. This reveals that individual features follow different, generally complex, and rather dynamic scenarios of evolution. Next, we apply statistical methods on roughly 200 observed MBP evolutionary tracks. MBPs are found to be formed by the strengthening of an equipartition field patch, which initially exhibits a moderate downflow. During the evolution, strong downdrafts with an average velocity of 2.4?km?s?1 set in. These flows, taken together with the concurrent strengthening of the field, suggest that we are witnessing the occurrence of convective collapses in these features, although only 30% of them reach kG field strengths. This fraction might turn out to be larger when the new 4?m class solar telescopes are operational as observations of MBPs with current state of the art instrumentation could still be suffering from resolution limitations. Finally, when the bright point disappears (although the magnetic field often continues to exist) the magnetic field strength has dropped to the equipartition level and is generally somewhat weaker than at the beginning of the MBPs evolution. Also, only relatively weak downflows are found on average at this stage of the evolution. Only 16% of the features display upflows at the time that the field weakens, or the MBP disappears. This speaks either for a very fast evolving dynamic process at the end of the lifetime, which could not be temporally resolved, or against strong upflows as the cause of the weakening of the field of these magnetic elements, as has been proposed based on simulation results. It is noteworthy that in about 10% of the cases, we observe in the vicinity of the downflows small-scale strong (exceeding 2?km?s?1) intergranular upflows related spatially and temporally to these downflows. The paper is complemented by a detailed discussion of aspects regarding the applied methods, the complementary literature, and in depth analysis of parameters like magnetic field strength and velocity distributions. An important difference to magnetic elements and associated bright structures in active region plage is that most of the quiet Sun bright points display significant downflows over a large fraction of their lifetime (i.e., in more than 46% of time instances/measurements they show downflows exceeding 1?km?s?1).

Long-term trends of magnetic bright points: I. Number of magnetic bright points at disc centre

The research was funded by the Austrian Science Fund (EWE): P20762, P23618 and J3176. Moreover, this work was supported by COST Action ES1005 (TOSCA). We are grateful to the Hinode team for the Opportunity to use their data. Hinode is a Japanese mission developed and launched by ISAS/JAXA, collaborating with NAOJ as a domestic partner, NASA and STFC (UK) as international partners. Scientific operation of the Hinode mission is conducted by the Hinode science team organised at ISAS/JAXA. This team mainly consists of scientists from institutes in the partner countries. Support for the post-launch operation is provided by JAXA and NAOJ (Japan), STFC (UK), NASA (USA), ESA, and NSC (Norway). D.U. and A.H. are grateful to the OAD (Osterreichiseher Austauschdienst) for financing a scientific stay at the Pic du Midi Observatory. R.M. is grateful to the Ministere des Affaires Etrangeres et Europeennes for financing a stay at the University of Graz. Furthermore, D.U. wishes to acknowledge again the OAD for providing financial means to conduct short research stays at the Astronomical Institute of the Czech Academy of Sciences as well as at the Slovak counterpart. Finally M. Barta is grateful to the MSMT for funding a research stay in Austria. Partial funding has also been obtained from the Spanish Ministerio de Economia through project ESP2013-47349-C6-1-R. Last but not least, all the authors wish to express their gratitude to the anonymous referee who helped to improve this study.

Variations of Magnetic Bright Point Properties with Longitude and Latitude as Observed by Hinode/SOT G-band Data

Small-scale magnetic fields can be observed on the Sun in high-resolution G-band filtergrams as magnetic bright points (MBPs). We study Hinode/Solar Optical Telescope (SOT) longitude and latitude scans of the quiet solar surface taken in the G-band in order to characterise the centre-to-limb dependence of MBP properties (size and intensity). We find that the MBP’s sizes increase and their intensities decrease from the solar centre towards the limb. The size distribution can be fitted using a log–normal function. The natural logarithm of the mean (μ parameter) of this function follows a second-order polynomial and the generalised standard deviation (σ parameter) follows a fourth-order polynomial or equally well (within statistical errors) a sine function. The brightness decrease of the features is smaller than one would expect from the normal solar centre-to-limb variation; that is to say, the ratio of a MBP’s brightness to the mean intensity of the image increases towards the limb. The centre-to-limb variations of the intensities of the MBPs and the quiet-Sun field can be fitted by a second-order polynomial. The detailed physical process that results in an increase of a MBP’s brightness and size from Sun centre to the limb is not yet understood and has to be studied in more detail in the future.

Two-dimensional segmentation of small convective patterns in radiation hydrodynamics simulations

Recent results from high-resolution solar granulation observations indicate the existence of a population of small granular cells that are smaller than 600 km in diameter. These small convective cells strongly contribute to the total area of granules and are located in the intergranular lanes, where they form clusters and chains. We study high-resolution radiation hydrodynamics simulations of the upper convection zone and photosphere to detect small granular cells, define their spatial alignment, and analyze their physical properties. We developed an automated image-segmentation algorithm specifically adapted to high-resolution simulations to identify granules. The resulting segmentation masks were applied to physical quantities, such as intensity and vertical velocity profiles, provided by the simulation. A new clustering algorithm was developed to study the alignment of small granular cells. This study shows that small granules make a distinct contribution to the total area of granules and form clusters of chain-like alignments. The simulation profiles demonstrate a different nature for small granular cells because they exhibit on average lower intensities, lower horizontal velocities, and are located deeper inside of convective layers than regular granules. Their intensity distribution deviates from a normal distribution as known for larger granules, and follows a Weibull distribution.

Latitude dependence of the solar granulation during the minimum of activity in 2009

Context. Knowledge of the latitude variation of the solar granulation properties (contrast and scale) is useful to better understand interactions between magnetic field, convection, differential rotation, and meridional circulation in the solar atmosphere. Aims. We investigated the latitude dependence of the contrast and scale of the solar granulation, with the help of HINODE/SOT blue continuum images taken in the frame of the HOP 79 program, along the central meridian and along the equator on a monthly basis in 2009 during the last solar minimum of activity. Methods. We selected the sharpest images in latitude and longitude intervals. The selected images in all the N-S and E-W scans taken in 2009 were combined to get statistically reliable results. Results. The contrast of the solar granulation decreases towards the poles and the scale increases, but not regularly since a perturbation occurs at around 60° where both quantities return close to their values at the disk center. Conclusions. Such a latitude variation in a period of minimum of activity (2009), is probably not due to magnetic field, neither the quiet magnetic field at the surface, nor the strong magnetic flux tubes associated with active regions, which could be embedded more or less deeply in the convection zone before they reach the surface. The decrease in contrast and increase in scale towards the pole seem to be related to the differential rotation and the perturbation around 60° to the meridional circulation.

Temporal variations in solar magnetic bright points intensity and plasma parameters

Magnetic bright points are one of the finest magnetic structures observed in the solar atmosphere. They possibly represent single flux tubes in quiet Sun regions. Their formation is described by the convective collapse model, while the decay phase of these structures is not well characterized yet. We attempt to follow the evolution of a few selected examples of MBPs and to study their changes in brightness and also the variations of plasma parameters during their lifetime. We use data from the Hinode satellite and the Sunrise mission. The G-band observations taken with a cadence of 30 seconds by the Hinode Solar Optical Telescope (SOT) show very fast changes of the maximum intensity of these structures. The complementary spectropolarimetric data, which are used to estimate the plasma parameters, were taken with a cadence of approximately two minutes. The variations of plasma parameters cannot be matched one to one to the changes in intensity due to the different temporal resolution. However, the slow changes of intensity with large amplitude are matched with variations of magnetic field strength and line-of-sight (LOS) velocity. The Sunrise/IMaX data have a temporal resolution of 32 seconds and show fast variations in the line wing intensity. These variations are associated with changes in the magnetic field strength and LOS velocity.