Image-Optimized Coronal Magnetic Field Models [SSA]

We have reported previously on a new method we are developing for using image-based information to improve global coronal magnetic field models. In that work we presented early tests of the method which proved its capability to improve global models based on flawed synoptic magnetograms, given excellent constraints on the field in the model volume. In this follow-up paper we present the results of similar tests given field constraints of a nature that could realistically be obtained from quality white-light coronagraph images of the lower corona. We pay particular attention to difficulties associated with the line-of-sight projection of features outside of the assumed coronagraph image plane, and the effect on the outcome of the optimization of errors in localization of constraints. We find that substantial improvement in the model field can be achieved with this type of constraints, even when magnetic features in the images are located outside of the image plane.

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S. Jones, J. Davila and V. Uritsky
Fri, 23 Jun 17

Comments: N/A

Using eigenmode-mixing to measure or constrain the Sun's interior B-field [SSA]

Understanding the generation and distribution of the Sun’s interior magnetic (B-) field is a longstanding challenge. Here we describe how measurements of the Sun’s oscillation eigenfunctions might be used to measure the Sun’s interior B-field. The B-field induces mode-mode couplings, causing the angular patterns of the eigenfunctions to differ from simple Y_{lm}’s We concentrate on the magnetic coupling between modes with the same (n,l) values and different but nearby $m$-values, since these non-axisymmetric couplings clearly cannot be due to the Sun’s axisymmetric rotation and since for these cases, mode mixing is enhanced by the near-degeneracy of the mode frequencies. We analyze magnetically-induced mode mixing in two stages of increasing complexity: first neglecting mode damping, and then incorporating realistic damping rates. We introduce a novel detection statistic that tests for the presence of non-axisymmetric mode-mixing in Solar Doppler data. We show that our detection statistic is naturally robust against spatial aliasing. We estimate our statistic’s signal-to-noise ratio (SNR) as a function of the mode-mixing amplitude. While B-induced mode-mixing is probably not detectable in a single mode pair, we argue that the phase of the B-induced mixing should be approximately the same across a wide range of modes. The total SNR then grows roughly as Np^{1/2}, where Np is the number of mode pairs. We conclude that B-induced mode-mixing should be detectable for a fairly wide range of B-field magnitudes and geometries.

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C. Cutler
Fri, 23 Jun 17

Comments: 12 pages, 1 figure, submitted to ApJ

Imaging Spectroscopy of Type U and J Solar Radio Bursts with LOFAR [SSA]

Radio U-bursts and J-bursts are signatures of electron beams propagating along magnetic loops confined to the corona. The more commonly observed type III radio bursts are signatures of electron beams propagating along magnetic loops that extend into interplanetary space. Given the prevalence of solar magnetic flux to be closed in the corona, it is an outstanding question why type III bursts are more frequently observed than U-bursts or J-bursts. We use LOFAR imaging spectroscopy between 30-80 MHz of low-frequency U-bursts and J-bursts, for the first time, to understand why electron beams travelling along coronal loops produce radio emission less often. The different radio source positions were used to model the spatial structure of the guiding magnetic flux tube and then deduce the energy range of the exciting electron beams without the assumption of a standard density model. The radio sources infer a magnetic loop 1 solar radius in altitude, with the highest frequency sources starting around 0.6 solar radii. Electron velocities were found between 0.13 c and 0.24 c, with the front of the electron beam travelling faster than the back of the electron beam. The velocities correspond to energy ranges within the beam from 0.7-11 keV to 0.7-43 keV. The density along the loop is higher than typical coronal density models and the density gradient is smaller. We found that a more restrictive range of accelerated beam and background plasma parameters can result in U-bursts or J-bursts, causing type III bursts to be more frequently observed. The large instability distances required before Langmuir waves are produced by some electron beams, and the small magnitude of the background density gradients make closed loops less facilitating for radio emission than loops that extend into interplanetary space.

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H. Reid and E. Kontar
Fri, 23 Jun 17

Comments: 9 pages, 7 figures

Disc truncation in embedded star clusters: Dynamical encounters versus face-on accretion [GA]

Observations indicate that the dispersal of protoplanetary discs in star clusters occurs on time scales of about 5 Myr. Several processes are thought to be responsible for this disc dispersal. Here we compare two of these processes: dynamical encounters and interaction with the interstellar medium, which includes face-on accretion and ram pressure stripping. We perform simulations of embedded star clusters with parameterisations for both processes to determine the environment in which either of these processes is dominant. We find that face-on accretion, including ram pressure stripping, is the dominant disc truncation process if the fraction of the total cluster mass in stars is $\lesssim 30\,\%$ regardless of the cluster mass and radius. Dynamical encounters require stellar densities $\gtrsim 10^4$ pc$^{-3}$ combined with a mass fraction in stars of $\approx 90\,\%$ to become the dominant process. Our results show that during the embedded phase of the cluster, the truncation of the discs is dominated by face-on accretion and dynamical encounters become dominant when the intra-cluster gas has been expelled. As a result of face-on accretion the protoplanetary discs become compact and their surface density increases. In contrast, dynamical encounters lead to discs that are less massive and remain larger.

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T. Wijnen, O. Pols, F. Pelupessy, et. al.
Fri, 23 Jun 17

Comments: Accepted for publication in A&A, 14 pages, 8 figures, 1 table

Pulsed Accretion in the T Tauri Binary TWA 3A [SSA]

TWA 3A is the most recent addition to a small group of young binary systems that both actively accrete from a circumbinary disk and have spectroscopic orbital solutions. As such, it provides a unique opportunity to test binary accretion theory in a well-constrained setting. To examine TWA 3A’s time-variable accretion behavior, we have conducted a two-year, optical photometric monitoring campaign, obtaining dense orbital phase coverage (~20 observations per orbit) for ~15 orbital periods. From U-band measurements we derive the time-dependent binary mass accretion rate, finding bursts of accretion near each periastron passage. On average, these enhanced accretion events evolve over orbital phases 0.85 to 1.05, reaching their peak at periastron. The specific accretion rate increases above the quiescent value by a factor of ~4 on average but the peak can be as high as an order of magnitude in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in good agreement with numerical simulations of binary accretion with similar orbital parameters. In these simulations, periastron accretion bursts are fueled by periodic streams of material from the circumbinary disk that are driven by the binary orbit. We find that TWA 3A’s average accretion behavior is remarkably similar to DQ Tau, another T Tauri binary with similar orbital parameters, but with significantly less variability from orbit to orbit. This is only the second clear case of orbital-phase-dependent accretion in a T Tauri binary.

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B. Tofflemire, R. Mathieu, G. Herczeg, et. al.
Fri, 23 Jun 17

Comments: 6 pages, 4 figures

Magnetohydrodynamic Simulations for Studying Solar Flare Trigger Mechanism [SSA]

In order to understand the flare trigger mechanism, we conducted three-dimensional magnetohydrodynamic simulations using a coronal magnetic field model derived from data observed by the Hinode satellite. Several types of magnetic bipoles were imposed into the photospheric boundary of the Non-linear Force-Free Field (NLFFF) model of Active Region NOAA 10930 on 2006 December 13 to investigate what kind of magnetic disturbance may trigger the flare. As a result, we confirm that certain small bipole fields, which emerge into the highly sheared global magnetic field of an active region, can effectively trigger a flare. These bipole fields can be classified into two groups based on their orientation relative to the polarity inversion line: the so called opposite polarity (OP) and reversed shear (RS) structures as it was suggested by Kusano et al. (2012). We also investigated the structure of the footpoints of reconnected field lines. By comparing the distribution of reconstructed field lines and the observed flare ribbons, the trigger structure of the flare can be inferred. Our simulation suggests that the data-constrained simulation taking into account both the large-scale magnetic structure and the small-scale magnetic disturbance such as emerging fluxes is a good way to find out a flare productive active region for space weather prediction.

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J. Muhamad, K. Kusano, S. Inoue, et. al.
Fri, 23 Jun 17

Comments: 28 pages, 10 figures

The Multiplicity of M-Dwarfs in Young Moving Groups [SSA]

We image 104 newly identified low-mass (mostly M-dwarf) pre-main sequence members of nearby young moving groups with Magellan Adaptive Optics (MagAO) and identify 27 binaries with instantaneous projected separation as small as 40 mas. 15 were previously unknown. The total number of multiple systems in this sample including spectroscopic and visual binaries from the literature is 36, giving a raw multiplicity rate of at least $35^{+5}{-4}\%$ for this population. In the separation range of roughly 1 – 300 AU in which infrared AO imaging is most sensitive, the raw multiplicity rate is at least $24^{+5}{-4}\%$ for binaries resolved by the MagAO infrared camera (Clio). The M-star sub-sample of 87 stars yields a raw multiplicity of at least $30^{+5}{-4}\%$ over all separations, $21^{+5}{-4}\%$ for secondary companions resolved by Clio from 1 to 300 AU ($23^{+5}{-4}\%$ for all known binaries in this separation range). A combined analysis with binaries discovered by the Search for Associations Containing Young stars shows that multiplicity fraction as a function of mass and age over the range of 0.2 to 1.2 $M\odot$ and 10 – 200 Myr appears to be linearly flat in both parameters and across YMGs. This suggests that multiplicity rates are largely set by 100 Myr without appreciable evolution thereafter. After bias corrections are applied, the multiplicity fraction of low-mass YMG members ($< 0.6 M_\odot$) is in excess of the field.

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Y. Shan, J. Yee, P. Bowler, et. al.
Fri, 23 Jun 17

Comments: 25 pages