Flow instabilities of magnetic flux tubes IV. Flux storage in the solar overshoot region

We consider the effects of material flows on the dynamics of toroidal magnetic flux tubes located close to the base of the solar convection zone, initially within the overshoot region. The problem is to find the physical conditions in which magnetic flux can be stored for periods comparable to the dynamo amplification time, which is of the order of a few years. We carry out nonlinear numerical simulations to investigate the stability and dynamics of thin flux tubes subject to perpendicular and longitudinal flows. We compare the simulations with the results of simplified analytical approximations. We determine ranges of the flow parameters for which a linearly Parker-stable magnetic flux tube is stored in the middle of the overshoot region for a period comparable to the dynamo amplification time. The residence time for magnetic flux tubes with fluxes of 2x10^{21} Mx in the convective overshoot layer is comparable to the dynamo amplification time, provided that the average speed and the duration of the downflow do not exceed about 50 m/s and 100 days, respectively, and that the lateral extension of the flow is smaller than about 10 degrees.Comment: Accepted to be published in Astronomy and Astrophysics. 16 pages, 16 figures. To access GIF animations, use http://www.mps.mpg.de/homes/ishik/flute/frict_inst.gif, http://www.mps.mpg.de/homes/ishik/flute/TF60.gif and http://www.mps.mpg.de/homes/ishik/flute/TF180.gi

Molecular Basis for poly(A) RNP Architecture and Recognition by the Pan2-Pan3 Deadenylase

The stability of eukaryotic mRNAs is dependent on a ribonucleoprotein (RNP) complex of poly(A)-binding proteins (PABPC1/Pab1) organized on the poly(A) tail. This poly(A) RNP not only protects mRNAs from premature degradation but also stimulates the Pan2-Pan3 deadenylase complex to catalyze the first step of poly(A) tail shortening. We reconstituted this process in vitro using recombinant proteins and show that Pan2-Pan3 associates with and degrades poly(A) RNPs containing two or more Pab1 molecules. The cryo-EM structure of Pan2-Pan3 in complex with a poly(A) RNP composed of 90 adenosines and three Pab1 protomers shows how the oligomerization interfaces of Pab1 are recognized by conserved features of the deadenylase and thread the poly(A) RNA substrate into the nuclease active site. The structure reveals the basis for the periodic repeating architecture at the 3' end of cytoplasmic mRNAs. This illustrates mechanistically how RNA-bound Pab1 oligomers act as rulers for poly(A) tail length over the mRNAs' lifetime.We would like to thank ... the MPIB cryo-EM, and core facilities ..

Limits to solar cycle predictability: Cross-equatorial flux plumes

Within the Babcock-Leighton framework for the solar dynamo, the strength of a cycle is expected to depend on the strength of the dipole moment or net hemispheric flux during the preceding minimum, which depends on how much flux was present in each hemisphere at the start of the previous cycle and how much net magnetic flux was transported across the equator during the cycle. Some of this transport is associated with the random walk of magnetic flux tubes subject to granular and supergranular buffeting, some of it is due to the advection caused by systematic cross-equatorial flows such as those associated with the inflows into active regions, and some crosses the equator during the emergence process. We aim to determine how much of the cross-equatorial transport is due to small-scale disorganized motions (treated as diffusion) compared with other processes such as emergence flux across the equator. We measure the cross-equatorial flux transport using Kitt Peak synoptic magnetograms, estimating both the total and diffusive fluxes. Occasionally a large sunspot group, with a large tilt angle emerges crossing the equator, with flux from the two polarities in opposite hemispheres. The largest of these events carry a substantial amount of flux across the equator (compared to the magnetic flux near the poles). We call such events cross-equatorial flux plumes. There are very few such large events during a cycle, which introduces an uncertainty into the determination of the amount of magnetic flux transported across the equator in any particular cycle. As the amount of flux which crosses the equator determines the amount of net flux in each hemisphere, it follows that the cross-equatorial plumes introduce an uncertainty in the prediction of the net flux in each hemisphere. This leads to an uncertainty in predictions of the strength of the following cycle.Comment: A&A, accepte

Magnetic flux generation and transport in cool stars

The Sun and other cool stars harbouring outer convection zones manifest magnetic activity in their atmospheres. The connection between this activity and the properties of a deep-seated dynamo generating the magnetic flux is not well understood. By employing physical models, we study the spatial and temporal characteristics of the observable surface field for various stellar parameters. We combine models for magnetic flux generation, buoyancy instability, and transport, which encompass the entire convection zone. The model components are: (1) a thin-layer alpha-Omega dynamo at the base of the convection zone; (2) buoyancy instabilities and the rise of flux tubes through the convection zone in 3D, which provides a physically consistent determination of emergence latitudes and tilt angles; and (3) horizontal flux transport at the surface. For solar-type stars and rotation periods longer than about 10 days, the latitudinal dynamo waves generated by the deep-seated alpha-Omega dynamo are faithfully reflected by the surface distribution of magnetic flux. For rotation periods of the order of two days, however, Coriolis acceleration of rising flux loops leads to surface flux emergence at much higher latitudes than the dynamo waves at the bottom of the convection zone reach. A similar result is found for a K0V star with a rotation period of two days. In the case of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy activity cycles and mixed-polarity fields at high latitudes.Comment: 14 pages, 14 figures. Accepted for publication in Astronomy & Astrophysic

No evidence for synchronization of the solar cycle by a "clock"

The length of the solar activity cycle fluctuates considerably. The temporal evolution of the corresponding cycle phase, that is, the deviation of the epochs of activity minima or maxima from strict periodicity, provides relevant information concerning the physical mechanism underlying the cyclic magnetic activity. An underlying strictly periodic process (akin to a perfect "clock"), with the observer seeing a superposition of the perfect clock and a small random phase perturbation, leads to long-term phase stability in the observations. Such behavior would be expected if cycles were synchronized by tides caused by orbiting planets or by a hypothetical torsional oscillation in the solar radiative interior. Alternatively, in the absence of such synchronization, phase fluctuations accumulate and a random walk of the phase ensues, which is a typical property of randomly perturbed dynamo models. Based on the sunspot record and the reconstruction of solar cycles from cosmogenic C14, we carried out rigorous statistical tests in order to decipher whether there exists phase synchronization or random walk. Synchronization is rejected at significance levels of between 95% (28 cycles from sunspot data) and beyond 99% (84 cycles reconstructed from C14, while the existence of random walk in the phases is consistent with all data sets. This result strongly supports randomly perturbed dynamo models with little inter-cycle memory.Comment: Astronomy & Astrophysics, in pres

Observational Evidence for Coronal Twisted Flux Rope

Multi-instrument data sets of NOAA AR10938 on Jan. 16, 2007, (e.g., {\emph{Hinode}}, {\it{STEREO}}, {\it{GOES}}, {\it{MLSO}} and {\it{ISOON}} H$\alpha$) are utilized to study the fine structure and evolution of a magnetic loop system exhibiting multiple crossing threads, whose arrangement and individual shapes are very suggestive of individual field lines in a flux rope. The footpoints of the magnetic threads are closely rooted into pores and plage areas. A C-class flare recorded by {\it{GOES}} at approximately 2:35 UT near one of the footpoints of the multi-thread system (along with a wisp of loop material shown by EUV data) led to the brightening of the magnetic structure revealing its fine structure with several threads that indicate a high degree of linking (suggesting a left-handed helical pattern as shown by the filament structure formed later-on). EUV observations by {\emph{Hinode}}/EIS of hot spectral lines at 2:46 UT show a complex structure of coronal loops. The same features were observed about 20 minutes later in X-ray images from {\emph{Hinode}}/XRT and about 30 minutes further in EUV images of {\it{STEREO}}/SECCHI/EUVI with much better resolution. H$\alpha$ and 304 {\AA} images revealed the presence of several filament fibrils in the same area. They evolved a few hours later into a denser structure seemingly showing helical structure, which persistently lasted for several days forming a segment of a larger scale filament. The present observations provide an important indication for a flux robe as a precursor of a solar filament.Comment: 13 pages, 4 figure

Phospho-dependent and phospho-independent interactions of the helicase UPF1 with the NMD factors SMG5-SMG7 and SMG6

Nonsense-mediated mRNA decay (NMD) is a eukaryotic surveillance pathway that recognizes mRNAs with premature stop codons and targets them for rapid degradation. Evidence from previous studies has converged on UPF1 as the central NMD factor. In human cells, the SMG1 kinase phosphorylates UPF1 at the N-terminal and C-terminal tails, in turn allowing the recruitment of the NMD factors SMG5, SMG6 and SMG7. To understand the molecular mechanisms, we recapitulated these steps of NMD in vitro using purified components. We find that a short C-terminal segment of phosphorylated UPF1 containing the last two Ser-Gln motifs is recognized by the heterodimer of SMG5 and SMG7 14-3-3-like proteins. In contrast, the SMG6 14-3-3-like domain is a monomer. The crystal structure indicates that the phosphoserine binding site of the SMG6 14-3-3-like domain is similar to that of SMG5 and can mediate a weak phospho-dependent interaction with UPF1. The dominant SMG6-UPF1 interaction is mediated by a low-complexity region bordering the 14-3-3-like domain of SMG6 and by the helicase domain and C-terminal tail of UPF1. This interaction is phosphorylation independent. Our study demonstrates that SMG5-SMG7 and SMG6 exhibit different and non-overlapping modes of UPF1 recognition, thus pointing at distinguished roles in integrating the complex NMD interaction network

MpTCP1 controls cell proliferation and redox processes in Marchantia polymorpha

TCP transcription factors are key regulators of angiosperm cell proliferation processes. It is unknown whether their regulatory growth capacities are conserved across land plants, which we examined in liverworts, one of the earliest diverging land plant lineages. We generated knockout mutants for MpTCP1, the single TCP‐P clade gene in Marchantia polymorpha, and characterized its function conducting cell proliferation and morphological analyses as well as mRNA expression, transcriptome, chemical and DNA binding studies. Mptcp1ge lines show a reduced vegetative thallus growth and extra tissue formation in female reproductive structures. Additionally, mutant plants reveal increased H2O2 levels and an enhanced pigmentation in the thallus caused by formation of secondary metabolites, such as aminochromes. MpTCP1 proteins interact redox‐dependently with DNA and regulate the expression of a comprehensive redox network, comprising enzymes involved in H2O2 metabolism. MpTCP1 regulates Marchantia growth context‐dependently. Redox sensitivity of the DNA binding capacity of MpTCP1 proteins provides a mechanism to respond to altered redox conditions. Our data suggest that MpTCP1 activity could thereby have contributed to diversification of land plant morphologies and to adaptations to abiotic and biotic challenges, experienced by liverworts during early land plant colonization

Geochemistry of metabasites and gabbroic rocks from the Tepla-Domazlice zone.

Various amphibolites, metagabbros and eclogitic relics of the Mariimske Lazne complex, and amphibolites from the Cerna Hora Massif exhibit an uniform geochemical character which compares well with modern mid-ocean ridge basalts. Geochemically these metabasites are similar to the amphibolites of the My to area and to schistose. partly striped amphibolites of the neighbouring Tirschenreuth-Mahring Zone and the Erbendorf-Vohenstrauss Zone (Bavaria). Greenschists and amphibolites from the Domailice metamorphic complex show an alkaline-basaltic tendency conforming to modern within-plate basalts or basalts from anomalous midocean ridge segments. In their chemical character, these metabasites compare well with the flaseramphibolites of the Erbendorf-Vohenstrauss Zone. Fine-grained amphibolites in the Warzenrieth area and gabbroic amphiboltes in the Blatterberg-Hoher Bogen area show normal MORB character. The metamorphosed gabbroic rocks in the southern part of the Neukirchen-Kdyne (meta-) igneous complex are subalkaline-tholeiitic and exhibit a magmatic differentiation trend. They differ from the neighbouring amphibolites by generally lower contents of incompatible elements