Evidence of quiet Sun chromospheric activity related to an emerging small-scale magnetic loop

Aims: We investigate the temporal evolution of magnetic flux emergence in the quiet Sun atmosphere close to disk center. Methods: We combine high-resolution SoHO/MDI magnetograms with TRACE observations taken in the 1216 {\AA} channel in order to analyze the temporal evolution of an emerging small-scale magnetic loop and its traces in the chromosphere. Results: At first place, we find signatures of flux emergence very close to the edge of a supergranular network boundary located at disk center. The new emerging flux appears first in the MDI magnetograms in form of an asymmetric bipolar element, i.e. the patch with negative polarity is roughly two-times weaker than the corresponding patch with opposite polarity. The average values of magnetic flux and magnetic flux densities reach 1.6 x 10^18 Mx, -8.5 x 10^17 Mx, and 55 Mx cm^-2, -30 Mx cm^-2, respectively. The spatial distance between the opposite polarity patches of the emerged feature increases from about 2.5" to 5.0" during the lifetime of the loop which was not longer than 36 min. A more precise lifetime-estimate of the feature was not possible because of a gap in the temporal sequence of the MDI magnetograms. The chromospheric response to the emerged magnetic dipole occurs ~ 9 minutes later with respect to the photospheric magnetograms. It consists of a quasi-periodic sequence of time-localized brightenings visible in the 1216 {\AA} TRACE channel apparent for ~ 14 minutes and being co-spatial with the axis connecting the two patches of opposite magnetic polarity. Conclusions: We identify the observed event as a small-scale magnetic loop emerging at photospheric layers and subsequently rising up to the chromosphere. We discuss the possibility that the fluctuations detected in the chromospheric emission probably reflect magnetic field oscillations which propagate to the chromosphere in form of waves.Comment: 6 pages, 4 figures, 1 table, Astronomy and Astrophysics, in pres

Formation of a penumbra in a decaying sunspot

Context : Penumbrae are an important characteristic of sunspots, whose formation is intricately related to the nature of sub-photospheric magnetic fields. Aims : We study the formation of a penumbra in a decaying sunspot and compare its properties with those seen during the development of a proto-spot. Methods : High-resolution spectropolarimetric observations of active region NOAA 11283 were obtained from the spectro-polarimeter on board Hinode. These were complemented with full-disk filtergrams of continuum intensity, line-of-sight magnetograms, and dopplergrams from the Helioseismic and Magnetic Imager at high cadence. Results : The formation of a penumbra in the decaying sunspot occurs after the coalescence of the sunspot with a magnetic fragment/pore, which initially formed in the quiet Sun close to an emerging flux region. At first, a smaller set of penumbral filaments develop near the location of the merger with very bright penumbral grains with intensities of 1.2 I_QS, upflows of 4 km/s, and a lifetime of 10 hr. During the decay of these filaments, a larger segment of a penumbra forms at the location of the coalescence. These new filaments are characterized by nearly supersonic downflows of 6.5 km/s that change to a regular Evershed flow nearly 3 hr later. Conclusions : The coalescence of the pore with the decaying sunspot provided sufficient magnetic flux for the penumbra to form in the sunspot. The emerging flux region could have played a decisive role in this process because the formation occurred at the location of the merger and not on the opposite side of the sunspot.Comment: Accepted for publication in A&A Letters, 6 pages, 4 figure

Thin layer shearing of a highly plastic clay

International audienceShearing tests with a thin layer of clay between filter slabs render possible large and cyclic deformations with drainage. In the pressure range of 100 kPa they serve to validated visco-hypoplastic constitutive relations. This theory is also confirmed by tests with up to 14 MPa and super-imposed anti-plane cycles. After this kind of seismic disturbance the clay stabilizes if the ratio of permanent stresses is undercritical. Otherwise a spontaneous acceleration occurs after a delay. This could help to understand critical phenomena with clay smears in faults

Preferred sunspot longitudes: Non-axisymmetry and differential rotation

As recently found, the distribution of sunspots is non-axisymmetric and spot group formation implies the existence of two persistent active longitudes separated by 180 degrees. Here we quantitatively study the non-axisymmetry of sunspot occurrence. In a dynamic reference frame inferred from the differential rotation law, the raw sunspot data show a clear clustering around the persistent active longitudes. The differential rotation describing the dynamic frame is quantified in terms of the equatorial angular velocity and the differential rotation rate, which appear to be significantly different from those for individual sunspots. This implies that the active longitudes are not linked to the depth of sunspot anchoring. In order to quantify the observed effect, we introduce a measure of the non-axisymmetry of the sunspot distribution. The non-axisymmetric component is found to be highly significant, and the ratio of its strength to that of the axisymmetric one is roughly 1:10. This provides additional constraints for solar dynamo models.Comment: 7 pages, 8 figures, Astronomy and Astrophysics, in pres