Search

Recent progress in laser-based high-repetition rate extreme ultraviolet (EUV) lightsources and multidimensional photoelectron spectroscopy enable the build-up of a new generation of time-resolved photoemission experiments. Here, we present a setup for time-resolved momentum microscopy driven by a 1 MHz femtosecond EUV table-top light source optimized for the generation of 26.5 eV photons. The setup provides simultaneous access to the temporal evolution of the photoelectron's kinetic energy and in-plane momentum. We discuss opportunities and limitations of our new experiment based on a series of static and time-resolved measurements on graphene.

We present energy-resolved photoelectron momentum maps for orbital tomography that have been collected with a novel and efficient time-of-flight momentum microscopy setup. This setup is combined with a 0.5 MHz table-top femtosecond extreme-ultraviolet light source, which enables unprecedented speed in data collection and paves the way towards time-resolved orbital imaging experiments in the future. Moreover, we take a significant step forward in the data analysis procedure for orbital imaging, and present a sparsity-driven approach to the required phase retrieval problem, which uses only the number of non-zero pixels in the orbital. Here, no knowledge of the object support is required, and the sparsity number can easily be determined from the measured data. Used in the relaxed averaged alternating reflections algorithm, this sparsity constraint enables fast and reliable phase retrieval for our experimental as well as noise-free and noisy simulated photoelectron momentum map data.

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, accepted

Context: The Sun's polar fields and open flux around the time of activity minima have been considered to be strongly correlated with the strength of the subsequent maximum of solar activity. Aims: We aim to investigate the behavior of a Babcock-Leighton dynamo with a source poloidal term that is based on the observed sunspot areas and tilts. In particular, we investigate whether the toroidal fields at the base of convection zone from the model are correlated with the observed solar cycle activity maxima. Methods: We used a flux transport dynamo model that includes convective pumping and a poloidal source term based on the historical record of sunspot group areas, locations, and tilt angles to simulate solar cycles 15 to 21. Results: We find that the polar fields near minima and the toroidal flux at the base of the convection zone are both highly correlated with the subsequent maxima of solar activity levels (r = 0.85 and r = 0.93, respectively). Conclusions: The Babcock-Leighton dynamo is consistent with the observationally inferred correlations., Comment: 8 pages, 5 figures, accepted for publication in A&A

The surface flux transport (SFT) model of solar magnetic fields involves empirically well-constrained velocity and magnetic fields. The basic evolution of the Sun's large-scale surface magnetic field is well described by this model. The azimuthally averaged evolution of the SFT model can be compared to the surface evolution of the flux transport dynamo (FTD), and the evolution of the SFT model can be used to constrain several near-surface properties of the FTD model. We compared the results of the FTD model with different upper boundary conditions and diffusivity profiles against the results of the SFT model. Among the ingredients of the FTD model, downward pumping of magnetic flux, related to a positive diffusivity gradient, has a significant effect in slowing down the diffusive radial transport of magnetic flux through the solar surface. Provided the pumping was strong enough to give rise to a downflow of a magnetic Reynolds number of 5 in the near-surface boundary layer, the FTD using a vertical boundary condition matches the SFT model based on the average velocities above the boundary layer. The FTD model with a potential field were unable to match the SFT results., Comment: Accepted for A&A

We solve the equations of motion of a one-dimensional planar Heisenberg (or Vaks-Larkin) model consisting of a system of interacting macro-spins aligned along a ring. Each spin has unit length and is described by its angle with respect to the rotational axis. The orientation of the spins can vary in time due to random forcing and spin-spin interaction. We statistically describe the behaviour of the sum of all spins for different parameters. The term "domino model" in the title refers to the interaction among the spins. We compare the model results with geomagnetic field reversals and find strikingly similar behaviour. The aggregate of all spins keeps the same direction for a long time and, once in a while, begins flipping to change the orientation by almost 180 degrees (mimicking a geomagnetic reversal) or to move back to the original direction (mimicking an excursion). Most of the time the spins are aligned or anti-aligned and deviate only slightly with respect to the rotational axis (mimicking the secular variation of the geomagnetic pole with respect to the geographic pole). Reversals are fast compared to the times in between and they occur at random times, both in the model and in the case of the Earth's magnetic field., Comment: accepted for Physical Review E, 18 December 2012

Geomagnetism is characterized by intermittent polarity reversals and rapid fluctuations. We have recently proposed a coupled macro-spin model to describe these dynamics based on the idea that the whole dynamo mechanism is described by the coherent interactions of many small dynamo elements. In this paper, we further develop this idea and construct a minimal model for magnetic variations. This simple model naturally yields many of the observed features of geomagnetism: its time evolution, the power spectrum, the frequency distribution of stable polarity periods, etc. This model has coexistent two phases; i.e. the cluster phase which determines the global dipole magnetic moment and the expanded phase which gives random perpetual perturbations that yield intermittent polarity flip of the dipole moment. This model can also describe the synchronization of the spin oscillation. This corresponds to the case of sun and the model well describes the quasi-regular cycles of the solar magnetism. Furthermore, by analyzing the relevant terms of MHD equation based on our model, we have obtained a scaling relation for the magnetism for planets, satellites, sun, and stars. Comparing it with various observations, we can estimate the scale of the macro-spins., Comment: 16 pages, 9 figures

We analyse mesogranular flow patterns in a three-dimensional hydrodynamical simulation of solar surface convection in order to determine its characteristics. We calculate divergence maps from horizontal velocities obtained with the Local Correlation Tracking (LCT) method. Mesogranules are identified as patches of positive velocity divergence. We track the mesogranules to obtain their size and lifetime distributions. We vary the analysis parameters to verify if the pattern has characteristic scales. The characteristics of the resulting flow patterns depend on the averaging time and length used in the analysis. We conclude that the mesogranular patterns do not exhibit intrinsic length and time scales.

We model the surface magnetic field and open flux of the Sun from 1913 to 1986 using a surface flux transport model, which includes the observed cycle-to-cycle variation of sunspot group tilts. The model reproduces the empirically derived time evolution of the solar open magnetic flux, and the reversal times of the polar fields. We find that both the polar field and the axial dipole moment resulting from this model around cycle minimum correlate with the strength of the following cycle., Comment: Accepted for publication by ApJ

We studied the effect of the perturbation of the meridional flow in the activity belts detected by local helioseismology on the development and strength of the surface magnetic field at the polar caps. We carried out simulations of synthetic solar cycles with a flux transport model, which follows the cyclic evolution of the surface field determined by flux emergence and advective transport by near-surface flows. In each hemisphere, an axisymmetric band of latitudinal flows converging towards the central latitude of the activity belt was superposed onto the background poleward meridional flow. The overall effect of the flow perturbation is to reduce the latitude separation of the magnetic polarities of a bipolar magnetic region and thus diminish its contribution to the polar field. As a result, the polar field maximum reached around cycle activity minimum is weakened by the presence of the meridional flow perturbation. For a flow perturbation consistent with helioseismic observations, the polar field is reduced by about 18% compared to the case without inflows. If the amplitude of the flow perturbation depends on the cycle strength, its effect on the polar field provides a nonlinearity that could contribute to limiting the amplitude of a Babcock-Leighton type dynamo., Comment: 16 pages, 5 figures, accepted by ApJ

The Spectral and Photometric Imaging Receiver (SPIRE), is the Herschel Space Observatory`s submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 microns, and an imaging Fourier Transform Spectrometer (FTS) which covers simultaneously its whole operating range of 194-671 microns (447-1550 GHz). The SPIRE detectors are arrays of feedhorn-coupled bolometers cooled to 0.3 K. The photometer has a field of view of 4' x 8', observed simultaneously in the three spectral bands. Its main operating mode is scan-mapping, whereby the field of view is scanned across the sky to achieve full spatial sampling and to cover large areas if desired. The spectrometer has an approximately circular field of view with a diameter of 2.6'. The spectral resolution can be adjusted between 1.2 and 25 GHz by changing the stroke length of the FTS scan mirror. Its main operating mode involves a fixed telescope pointing with multiple scans of the FTS mirror to acquire spectral data. For extended source measurements, multiple position offsets are implemented by means of an internal beam steering mirror to achieve the desired spatial sampling and by rastering of the telescope pointing to map areas larger than the field of view. The SPIRE instrument consists of a cold focal plane unit located inside the Herschel cryostat and warm electronics units, located on the spacecraft Service Module, for instrument control and data handling. Science data are transmitted to Earth with no on-board data compression, and processed by automatic pipelines to produce calibrated science products. The in-flight performance of the instrument matches or exceeds predictions based on pre-launch testing and modelling: the photometer sensitivity is comparable to or slightly better than estimated pre-launch, and the spectrometer sensitivity is also better by a factor of 1.5-2., Comment: Accepted for publication in Astronomy & Astrophyics (Herschel first results special issue)

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16x25 pixels, each, and two filled silicon bolometer arrays with 16x32 and 32x64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60-210\mu\ m wavelength regime. In photometry mode, it simultaneously images two bands, 60-85\mu\ m or 85-125\mu\m and 125-210\mu\ m, over a field of view of ~1.75'x3.5', with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47"x47", resolved into 5x5 pixels, with an instantaneous spectral coverage of ~1500km/s and a spectral resolution of ~175km/s. We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the Performance Verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions.

Various possibilities are currently under discussion to explain the observed weakness of the intrinsic magnetic field of planet Mercury. One of the possible dynamo scenarios is a dynamo with feedback from the magnetosphere. Due to its weak magnetic field Mercury exhibits a small magnetosphere whose subsolar magnetopause distance is only about 1.7 Hermean radii. We consider the magnetic field due to magnetopause currents in the dynamo region. Since the external field of magnetospheric origin is antiparallel to the dipole component of the dynamo field, a negative feedback results. For an alpha-omega-dynamo two stationary solutions of such a feedback dynamo emerge, one with a weak and the other with a strong magnetic field. The question, however, is how these solutions can be realized. To address this problem, we discuss various scenarios for a simple dynamo model and the conditions under which a steady weak magnetic field can be reached. We find that the feedback mechanism quenches the overall field to a low value of about 100 to 150 nT if the dynamo is not driven too strongly.

By coupling a solar surface flux transport model with an extrapolation of the heliospheric field, we simulate the evolution of the Sun's open magnetic flux and the heliospheric current sheet (HCS) based on observational data of sunspot groups since 1976. The results are consistent with measurements of the interplanetary magnetic field near Earth and with the tilt angle of the HCS as derived from extrapolation of the observed solar surface field. This opens the possibility for an improved reconstruction of the Sun's open flux and the HCS into the past on the basis of empirical sunspot data., Comment: 16 pages, 5 figures, Accepted for publication in ApJ

We present an elegant method of determining the eigensolutions of the induction and the dynamo equation in a fluid embedded in a vacuum. The magnetic field is expanded in a complete set of functions. The new method is based on the biorthogonality of the adjoint electric current and the vector potential with an inner product defined by a volume integral over the fluid domain. The advantage of this method is that the velocity and the dynamo coefficients of the induction and the dynamo equation do not have to be differentiated and thus even numerically determined tabulated values of the coefficients produce reasonable results. We provide test calculations and compare with published results obtained by the classical treatment based on the biorthogonality of the magnetic field and its adjoint. We especially consider dynamos with mean-field coefficients determined from direct numerical simulations of the geodynamo and compare with initial value calculations and the full MHD simulations., Comment: accepted for Astronomy & Astrophysics

In this study we address the question under which conditions a saturated velocity field stemming from geodynamo simulations leads to an exponential growth of the magnetic field in a corresponding kinematic calculation. We perform global self-consistent geodynamo simulations and calculate the evolution of a kinematically advanced tracer field. The self-consistent velocity field enters the induction equation in each time step, but the tracer field does not contribute to the Lorentz force. This experiment has been performed by Cattaneo & Tobias (2009) and is closely related to the test field method by Schrinner et al. (2005, 2007). We find two dynamo regimes in which the tracer field either grows exponentially or approaches a state aligned with the actual self-consistent magnetic field after an initial transition period. Both regimes can be distinguished by the Rossby number and coincide with the dipolar and multipolar dynamo regimes identified by Christensen & Aubert (2006). Dipolar dynamos with low Rossby number are kinematically stable whereas the tracer field grows exponentially in the multipolar dynamo regime. This difference in the saturation process for dynamos in both regimes comes along with differences in their time variability. Within our sample of 20 models, solely kinematically unstable dynamos show dipole reversals and large excursions. The complicated time behaviour of these dynamos presumably relates to the alternating growth of several competing dynamo modes. On the other hand, dynamos in the low Rossby number regime exhibit a rather simple time dependence and their saturation merely results in a fluctuation of the fundamental dynamo mode about its critical state., Comment: 6 pages, 8 figures

Flow-induced instabilities are relevant for the storage and dynamics of magnetic fields in stellar convection zones and possibly also in other astrophysical contexts. We continue the study started in the first paper of this series by considering the stability properties of longitudinal flows along magnetic flux tubes. A linear stability analysis was carried out to determine criteria for the onset of instability in the framework of the approximation of thin magnetic flux tubes. In the non-dissipative case, we find Kelvin-Helmholtz instability for flow velocities exceeding a critical speed that depends on the Alfv{\'e}n speed and on the ratio of the internal and external densities. Inclusion of a friction term proportional to the relative transverse velocity leads to a friction-driven instability connected with backward (or negative energy) waves. We discuss the physical nature of this instability. In the case of a stratified external medium, the Kelvin-Helmholtz instability and the friction-driven instability can set in for flow speeds significantly lower than the Alfv{\'e}n speed. Dissipative effects can excite flow-driven instability below the thresholds for the Kelvin-Helmholtz and the undulatory (Parker-type) instabilities. This may be important for magnetic flux storage in stellar convection zones and for the stability of astrophysical jets., Comment: accepted by Astronomy & Astrophysics

A comparison is made between mean-field models and direct numerical simulations of rotating magnetoconvection and the geodynamo. The mean-field coefficients are calculated with the fluid velocity taken from the direct numerical simulations. The magnetic fields resulting from mean-field models are then compared with the mean magnetic field from the direct numerical simulations.

Customary two-dimensional flux transport models for the evolution of the magnetic field at the solar surface do not account for the radial structure and the volume diffusion of the magnetic field. When considering the long-term evolution of magnetic flux, this omission can lead to an unrealistic long-term memory of the system and to the suppression of polar field reversals. In order to avoid such effects, we propose an extension of the flux transport model by a linear decay term derived consistently on the basis of the eigenmodes of the diffusion operator in a spherical shell. A decay rate for each eigenmode of the system is determined and applied to the corresponding surface part of the mode evolved in the flux transport model. The value of the volume diffusivity associated with this decay term can be estimated to be in the range 50--100 km^2/s by considering the reversals of the polar fields in comparison of flux transport simulations with observations. We show that the decay term prohibits a secular drift of the polar field in the case of cycles of varying strength, like those exhibited by the historical sunspot record., Comment: for further information visit: http://solweb.oma.be/users/baumann/

We address the question whether the properties of the observed latitude-time diagram of sunspot occurence (the butterfly diagram) provide evidence for the operation of a flux-transport dynamo, which explains the migration of the sunspot zones and the period of the solar cycle in terms of a deep equatorward meridional flow. We show that the properties of the butterfly diagram are equally well reproduced by a conventional dynamo model with migrating dynamo waves, but without transport of magnetic flux by a flow. These properties seem to be generic for an oscillatory and migratory field of dipole parity and thus do not permit an observational distinction between different dynamo approaches., Comment: 4 pages, 1 figure

Extensive magnetization and magnetostriction measurements were carried out on a single crystal of GdNi2B2C along the main tetragonal axes. Within the paramagnetic phase, the magnetic and strain susceptibilities revealed a weak anisotropy in the exchange couplings and two-ion tetragonal-preserving alpha-strain modes. Within the ordered phase, magnetization and magnetostriction revealed a relatively strong orthorhombic distortion mode and rich field-temperature phase diagrams. For H//(100) phase diagram, three field-induced transformations were observed, namely, at: Hd(T), related to the domain alignment; Hr(T), associated with reorientation of the moment towards the c-axis; and Hs(T), defining the saturation process wherein the exchange field is completely counterbalanced. On the other hand, For H//(001) phase diagram, only two field-induced transformations were observed, namely at: Hr(T) and Hs(T). For both phase diagrams, Hs(T) follows the relation Hs[1-(T/Tn)^2]^(1/2)kOe with Hs(T-->0)=128.5(5) kOe and Tn(H=0)=19.5 K. In contrast, the thermal evolution of Hr(T) along the c-axis (much simpler than along the a-axis) follows the relation Hr[1-T/Tr]^(1/3) kOe where Hr(T-->0)=33.5(5) kOe and Tr(H=0)=13.5 K. It is emphasized that the magnetoelastic interaction and the anisotropic exchange coupling are important perturbations and therefore should be explicitly considered if a complete analysis of the magnetic properties of the borocarbides is desired.