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In a rotation-dominated magnetosphere, there is a region where closed field lines rotate around the planet, and also a region where the open field lines stretch away from the planet, forming the lobes of the magnetotail. This paper shows that there could be a third, significantly different region, where the closed field lines form twisted vortex structures anchored in the magnetotail. Such patterns form when there are significant plasma sources inside the magnetosphere and the time scale of the plasmoid formation process is substantially larger than the planetary rotation period. In the presence of vortices, the Dungey and Vasyliunas cycles act differently. The Dungey flow does not penetrate the central region of the polar cap. Tail reconnection events are rare, thus leaving the plasma time enough to participate in the essentially 3-dimensional vortex-forming plasma motion. The above conditions are fulfilled for Saturn. We discovered vortex-like patterns in the plasma and magnetic field data measured by the Cassini spacecraft in the nightside magnetosphere of Saturn. The plasma whirling around in these vortices never reaches the dayside, instead, it performs a retrograde motion in the high latitude regions of the magnetotail. Low-energy plasma data suggest that the observed patterns correspond to the closed field line vortices., Comment: to be published in MNRAS

Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system for the first time. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [Ir$^{\rm III}$(ppy)$_2$(bpy)]$^+$ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe$_3$(CO)$_{12}$ as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale. The present findings enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance., Comment: revised version

Disentangling the dynamics of electrons and nuclei during nonadiabatic molecular transformations remains a considerable experimental challenge. Here we have investigated photoinduced electron transfer dynamics following a metal-to-ligand charge-transfer (MLCT) excitation of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1- ylidine)-pyridine, with simultaneous femtosecond-resolution Fe K{\alpha} and K\b{eta} X-ray Emission Spectroscopy (XES) and Wide Angle X-ray Scattering (WAXS). This measurement clearly shows temporal oscillations in the XES and WAXS difference signals with the same 278 fs period oscillation. The oscillatory signal originates from an Fe-ligand stretching mode vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. The vibrational wavepacket is created by 40% of the excited population that undergoes electron transfer from the non-equilibrium MLCT excited state to the 3MC excited state with a 110 fs time constant, while the other 60% relaxes to a 3MLCT excited state in parallel. The sensitivity of the K{\alpha} XES spectrum to molecular structure results from core-level vibronic coupling, due to a 0.7% average Fe-ligand bond length difference in the lowest energy geometry of the 1s and 2p core-ionized states. These results highlight the importance of vibronic effects in time-resolved XES experiments and demonstrate the role of metal-centered excited states in the electronic excited state relaxation dynamics of an Fe carbene photosensitizer.

Hot flow anomalies (HFAs) are studied using observations of the magnetometer and the plasma instrument aboard the four Cluster spacecraft. We study several specific features of tangential discontinuities on the basis of Cluster measurements from the time periods of February-April 2003, December 2005-April 2006 and January-April 2007, when the separation distance of spacecraft was large. The previously discovered condition (Facsko et al., 2008) for forming HFAs is confirmed, i.e. that the solar wind speed and fast magnetosonic Mach number values are higher than average. Furthermore, this constraint is independent of the Schwartz et al. (2000)s condition for HFA formation. The existence of this new condition is confirmed by simultaneous ACE magnetic field and solar wind plasma observations at the L1 point, at 1.4 million km distance from the Earth. The temperature, particle density and pressure parameters observed at the time of HFA formation are also studied and compared to average values of the solar wind plasma. The size of the region affected by the HFA was estimated by using two different methods. We found that the size is mainly influenced by the magnetic shear and the angle between the discontinuity normal and the Sun-Earth direction. The size grows with the shear and (up to a certain point) with the angle as well. After that point it starts decreasing. The results are compared with the outcome of recent hybrid simulations., Comment: 20 pages, 12 figure, published in 2009

High energy resolution, hard X-ray spectroscopies are powerful element selective probes of the electronic and local structure of matter, with diverse applications in chemistry, physics, biology and materials science. The routine application of these techniques is hindered by the complicated and slow access to synchrotron radiation facilities. Here we propose a new, economic, easily operated laboratory high resolution von H\'amos type X-ray spectrometer, which offers rapid transmission experiments for X-ray absorption, and is also capable of recording X-ray emission spectra. The use of a cylindrical analyzer crystal and a position sensitive detector enabled us to build a maintenance free, flexible setup with low operational costs, while delivering synchrotron grade signal to noise measurements in reasonable acquisition times. We demonstrate the proof of principle and give examples for both measurement types. Finally, tracking of a several day long chemical transformation, a case better suited for laboratory than synchrotron investigation, is also presented.

We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ~ 7 ps.

The nanoscopic magneto-electronic phase separation in doped La1-xSrxCoO3 perovskites was studied with local probes. The phase separation is directly observed by M\"ossbauer spectroscopy in the studied doping range of 0.05 <= x <= 0.25 both at room-temperature as well as in the low temperature magnetic phase. Extended with current synchrotron based X-ray spectroscopies, these data help to characterize the volume as well as the local electric and magnetic properties of the distinct phases. A simple model based on a random distribution of the doping Sr ions describes well both the evolution of the separated phases as well as the variation of the Co spin state. The experiments suggest that Sr doping initiates small droplets and a high degree of doping driven cobalt spin-state transition, while the Sr-free second phase vanishes rapidly with increasing Sr content.

Let $s:[1,\infty) \to \C $ be a locally integrable function in Lebesgue's sense. The logarithmic (also called harmonic) mean of the function $s$ is defined by [\tau(t) := \frac 1{\log t} \int_1^t \frac {s(x)}{x} dx, \qquad t>1,] where the logarithm is to base $e$. Besides the ordinary limit $\lim_{x\to \infty} s(x)$, we also use the notion of the so-called statistical limit of $s$ at $\infty$, in notation: $ \stlim_{x\to \infty} s(x)=\ell $, by which we mean that for every $\e>0$, [\lim_{b\to \infty} \frac 1b \Big | \Big {x\in(1,b): |s(x)-\ell| >\e \Big} \Big| = 0.] We also use the ordinary limit $\lim_{t\to\infty} \tau(t)$ as well as the statistical limit $\stlim_{t\to\infty} \tau(t)$. We will prove the following Tauberian theorem: Suppose that the real-valued function $s$ is slowly decreasing or the complex-valued $s$ is slowly oscillating. If the statistical limit $\stlim_{t\to\infty} \tau(t) =\ell $ exists, then the ordinary limit $\lim_{x\to\infty} s(x) = \ell $ also exists.

We report the continuous variation of the spin moment of cobalt in LaCoO3 across its temperature and pressure-induced spin transitions evidenced with K\beta emission spectra. The first thermal transition is best described by a transition to an orbitally nondegenerate intermediate spin (S=1) state. In parallel, continuous redistribution of the 3d electrons is also indicated by partial fluorescence yield X-ray absorption spectra. At high pressure, our study confirms that the material becomes low spin between 40 and 70 kbar at room temperature.

Using the persistent current I induced by an Aharonov-Bohm flux in square lattices with random potentials, we study the interplay between electronic correlations and disorder upon the ground state (GS) of a few polarized electrons (spinless fermions) with Coulomb repulsion. K being the total momentum, we show that I is proportional to K in the continuum limit. We use this relation to distinguish between the continuum regimes, where the lattice GS behaves as in the continuum limit and I is independent of the interaction strength U when K is conserved, and the lattice regimes where I decays as U increases. Changing the disorder strength W and U, we obtain many regimes which we study using the map of local currents carried by three spinless fermions.

The ground state of an electron gas is characterized by the interparticle spacing to the effective Bohr radius ratio r_s=a/a_B*. For polarized electrons on a two dimensional square lattice with Coulomb repulsion, we study the threshold value r_s* below which the lattice spacing s becomes a relevant scale and r_s ceases to be the scaling parameter. For systems of small ratios s/a_B*, s becomes only relevant at small r_s (large densities) where one has a quantum fluid with a deformed Fermi surface. For systems of large s/a_B*, s plays also a role at large r_s (small densities) where one has a Wigner solid, the lattice limiting its harmonic vibrations. The thermodynamic limit of physical systems of different a_B* is qualitatively discussed, before quantitatively studying the lattice effects occurring at large r_s. Using a few particle system, we compare exact numerical results obtained with a lattice and analytical perturbative expansions obtained in the continuum limit. Three criteria giving similar values for the lattice threshold $r_s^*$ are proposed. The first one is a delocalization criterion in the Fock basis of lattice site orbitals. The second one uses the persistent current which can depend on the interaction in a lattice, while it becomes independent of the interaction in the continuum limit. The third one takes into account the limit imposed by the lattice to the harmonic vibrations of the electron solid.

For an interacting system of N electrons, we study the conditions under which a lattice model of size L with nearest neighbor hopping t and U/r Coulomb repulsion has the same ground state as a continuum model. For a fixed value of N, one gets identical results when the inter-electron spacing to the Bohr radius ratio r_s < r_s^*. Above r_s^*, the persistent current created by an enclosed flux begins to decay and r_s ceases to be the scaling parameter. Three criteria giving similar r_s^* are proposed and checked using square lattices., Comment: 7 pages, 5 postscript figures

In this second paper, using N=3 polarized electrons (spinless fermions) interacting via a U/r Coulomb repulsion on a two dimensional L * L square lattice with periodic boundary conditions and nearest neighbor hopping, we show that a single unpaired fermion can co-exist with a correlated two particle Wigner molecule for intermediate values of the Coulomb energy to kinetic energy ratio r_s., Comment: 15 pages, 28 postscript figures

We consider three spinless fermions free to move on 2d square lattice with periodic boundary conditions and interacting via a U/r Coulomb repulsion. When the Coulomb energy to kinetic energy ratio r_s is large, a rigid Wigner molecule is formed. As r_s decreases, we show that melting proceeds via an intermediate regime where a floppy two particle molecule coexists with a partially delocalized particle. A simple ansatz is given to describe the ground state of this mesoscopic solid-liquid regime., Comment: to appear in Europhysics Letters