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Robbrecht E.,Royal Observatory of Belgium | Wang Y.-M.,U.S. Navy
Astrophysical Journal Letters | Year: 2010

The opening-up of the magnetic field during solar eruptive events is often accompanied by a dimming of the local coronal emission. From observations of filament eruptions recorded with the Extreme-Ultraviolet Imager on STEREO during 2008-2009, it is evident that these dimmings are much more pronounced in 19.5 nm than in the lower-temperature line 17.1 nm, as viewed either on the disk or above the limb. We conclude that most of the cooler coronal plasma is not ejected but remains gravitationally bound when the loops open up. This result is consistent with Doppler measurements by Imada and coworkers, who found that the upflow speeds in a transient coronal hole increased dramatically above a temperature of 1 MK; it is also consistent with the quasistatic behavior of polar plumes, as compared with the hotter interplume regions that are the main source of the fast solar wind. When the open flux reconnects and closes down again, the trapped plasma is initially heated to such high temperatures that it is no longer visible at Fe ix 17.1 nm. Correspondingly, 17.1 nm images show a dark ribbon or "heat wave" propagating away from the polarity inversion line and coinciding with the brightened Fe xv 28.4 nm and Fe xii 19.5 nm post-eruptive loops and their footpoint areas. Such dark ribbons provide a clear example of dimmings that are not caused by a density depletion. The propagation of the "heat wave" is driven by the closing-down, not the opening-up, of the flux and can be observed both off-limb and on-disk. © 2010. The American Astronomical Society. All rights reserved. Source

Wang Y.-M.,U.S. Navy | Robbrecht E.,Royal Observatory of Belgium
Astrophysical Journal | Year: 2011

Observations of the interplanetary magnetic field (IMF) have suggested a statistical tendency for the heliospheric current sheet (HCS) to be shifted a few degrees southward of the heliographic equator during the period 1965-2010, particularly in the years near sunspot minimum. Using potential-field source-surface extrapolations and photospheric flux-transport simulations, we demonstrate that this southward displacement follows from Joy's law and the observed hemispheric asymmetry in the sunspot numbers, with activity being stronger in the southern (northern) hemisphere during the declining (rising) phase of cycles 20-23. The hemispheric asymmetry gives rise to an axisymmetric quadrupole field, whose equatorial zone has the sign of the leading-polarity flux in the dominant hemisphere; during the last four cycles, the polarity of the IMF around the equator thus tended to match that of the north polar field both before and after polar field reversal. However, large fluctuations are introduced by the nonaxisymmetric field components, which depend on the longitudinal distribution of sunspot activity in either hemisphere. Consistent with this model, the HCS showed an average northward displacement during cycle 19, when the "usual" alternation was reversed and the northern hemisphere became far more active than the southern hemisphere during the declining phase of the cycle. We propose a new method for determining the north-south displacement of the HCS from coronal streamer observations. © 2011. The American Astronomical Society. All rights reserved. Source

Lobel A.,Royal Observatory of Belgium
Canadian Journal of Physics | Year: 2011

Herein we develop a new method to determine oscillator strength values of atomic absorption lines with state-of-the-art detailed spectral synthesis calculations of the optical spectrum of the Sun and of standard spectral reference stars. We update the log(gf) values of 911 neutral lines observed in the KPNO-FTS flux spectrum of the Sun and high-resolution echelle spectra (R = 80 000) of Procyon (F5 IV-V) and ε Eri (K2 V) observed with large signal-to-noise (S/N) ratios of ∼2000 using the new Mercator-Hermes spectrograph at La Palma Observatory (Spain). We find for 483 Fe i, 85 Ni i, and 51 Si i absorption lines in the sample a systematic overestimation of the literature log(gf) values with central line depths below 15%. We employ a curve-of-growth analysis technique to test the accuracy of the new oscillator strength values and compare calculated equivalent line widths to the Moore, Minnaert, and Houtgast atlas of the Sun. The online SpectroWeb database at spectra.freeshell.org interactively displays the observed and synthetic spectra and provides the new log(gf) values together with important atomic line data. The graphical database is under development for stellar reference spectra of every spectral sub-class observed with large spectral resolution and S/N ratios. © 2011 Published by NRC Research Press. Source

Lapenta G.,Catholic University of Leuven | Bettarini L.,Royal Observatory of Belgium
Geophysical Research Letters | Year: 2011

We report a 3D magnetohydrodynamics simulation that studies the formation of dipolarization fronts during magnetotail reconnection. The crucial new feature uncovered in the present 3D simulation is that the process of reconnection produces flux ropes developing within the reconnection region. These flux ropes are unstable to the kink mode and introduce a spontaneous structure in the dawn-dusk direction. The dipolarization fronts forming downstream of reconnection are strongly affected by the kinking ropes. At the fronts, a density gradient is present with opposite direction to that of the acceleration field and leads to an interchange instability. We present evidence for a causal link where the perturbations of the kinking flux ropes with their natural and well defined scales drive and select the scales for the interchange mode in the dipolarization fronts. The results of the simulation are validated against measured structures observed by the THEMIS mission. Copyright 2011 by the American Geophysical Union. Source

Rosenblatt P.,Royal Observatory of Belgium
Astronomy and Astrophysics Review | Year: 2011

The origin of the Martian moons, Phobos and Deimos, is still an open issue: either they are asteroids captured by Mars or they formed in situ from a circum-Mars debris disk. The capture scenario mainly relies on the remote-sensing observations of their surfaces, which suggest that the moon material is similar to outer-belt asteroid material. This scenario, however, requires high tidal dissipation rates inside the moons to account for their current orbits around Mars. Although the in situ formation scenarios have not been studied in great details, no observational constraints argue against them. Little attention has been paid to the internal structure of the moons, yet it is pertinent for explaining their origin. The low density of the moons indicates that their interior contains significant amounts of porous material and/or water ice. The porous content is estimated to be in the range of 30-60% of the volume for both moons. This high porosity enhances the tidal dissipation rate but not sufficiently to meet the requirement of the capture scenario. On the other hand, a large porosity is a natural consequence of re-accretion of debris at Mars' orbit, thus providing support to the in situ formation scenarios. The low density also allows for abundant water ice inside the moons, which might significantly increase the tidal dissipation rate in their interiors, possibly to a sufficient level for the capture scenario. Precise measurements of the rotation and gravity field of the moons are needed to tightly constrain their internal structure in order to help answering the question of the origin. © Springer-Verlag 2011. Source

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