Simulating the in situ condensation process of solar prominences

Prominences in the solar corona are a hundredfold cooler and denser than their surroundings, with a total mass of 1013 up to 1015 g. Here, we report on the first comprehensive simulations of three-dimensional, thermally and gravitationally stratified magnetic flux ropes where in situ condensation to a prominence occurs due to radiative losses. After a gradual thermodynamic adjustment, we witness a phase where runaway cooling occurs while counter-streaming shearing flows drain off mass along helical field lines. After this drainage, a prominence-like condensation resides in concave upward field regions, and this prominence retains its overall characteristics for more than two hours. While condensing, the prominence establishes a prominence-corona transition region where magnetic field-aligned thermal conduction is operative during the runaway cooling. The prominence structure represents a force-balanced state in a helical flux rope. The simulated condensation demonstrates a right-bearing barb, as a remnant of the drainage. Synthetic images at extreme ultraviolet wavelengths follow the onset of the condensation, and confirm the appearance of horns and a three-part structure for the stable prominence state, as often seen in erupting prominences. This naturally explains recent Solar Dynamics Observatory views with the Atmospheric Imaging Assembly on prominences in coronal cavities demonstrating horns.Publisher PDFPeer reviewe

Organizational Alignment and Performance: Past, Present and Future

Purpose – The purpose of this paper is to identify and define the types of organizational alignment – vertical and horizontal; to examine the evidence for the alignment‐performance relationship, and propose research questions and practical implications to advance the theory and practice of managing alignment. Design/methodology/approach – The study is a conceptual examination based on a thorough review of both theoretical and empirical research. Findings – The paper finds that vertical alignment has received considerably more attention in the literature. Studies of horizontal alignment within organizations are less common. When horizontal alignment is studied, the focus tends to be dyadic – between two functional areas. The limitations posed by the dyadic approach suggest gaps in the research and opportunities for future research. As firms grow and diversify, becoming multi‐business organizations, the importance of horizontal alignment will be elevated. Research limitations/implications – Research on vertical alignment should focus on developing larger sets of moderating variables, such as the morale of the workforce, or the life cycle of the firm or industry. Research on horizontal alignment should explore multi‐point horizontal alignment. Practical implications – Managers in organizations with multiple strategic business units could use the application questions in the study to assess the state of alignment in their respective units and the organization as a whole. Originality/value – The paper documents existing literature on the concept of organizational alignment and identifies new opportunities to continue to build and expand the research stream. It also provides a list of application questions that may be used to assess organizational alignment in organizations

A search for pulsars around Sgr A* in the first Event Horizon Telescope data set

In 2017 the Event Horizon Telescope (EHT) observed the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz (λ = 1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT data sets. The high observing frequency means that pulsars—which typically exhibit steep emission spectra—are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope, and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the fast folding algorithm, and single-pulse searches targeting both pulsars and burst-like transient emission. We use the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction (≲2.2 of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region

Three-dimensional magnetohydrodynamic simulations of the evolution of magnetic fields in Fanaroff-Riley class II radio sources

Radio observations of Fanaroff-Riley class II sources often show correlations between the synchrotron emission and the linear-polarimetric distributions. Magnetic position vectors seem to align with the projected emission of both the radio jets and the sources' edges. Using statistics we study such relation as well as its unknown time evolution via synthetic polarisation maps of model FR II sources formed in 3D-MHD numerical simulations of bipolar, hypersonic and weakly magnetised jets. The magnetic field is initially random with a Kolmogorov power spectrum, everywhere. We investigate the structure and evolution of magnetic fields in the sources as a function of the power of jets and the observational viewing angle. Our synthetic polarisation maps agree with observations, showing B-field vectors which are predominantly aligned with the jet axis, and show that magnetic fields inside sources are shaped by the jets' backflow. Polarimetry is found to correlate with time, the viewing angle and the jet-to-ambient density contrast. The magnetic structure inside thin elongated sources is more uniform than inside more spherical ones. We see jets increase the magnetic energy in cocoons in proportion to the jet velocity and the cocoon width. Filaments in the synthetic emission maps suggest turbulence develops in evolved sources.Comment: Accepted for publication in the MNRAS. 21 pages, 11 figure