Enhanced ionization of the Martian nightside ionosphere during solar energetic particle events

Electron densities in the Martian nightside ionosphere are more than 90% of time too low to be detected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft. However, the relative number of ionograms with peak electron density high enough to be detected represents a good statistical proxy of the ionospheric density. We focus on solar energetic particle (SEP) events, and we analyze their effects on ionospheric formation. SEP time intervals were identified in situ using the background counts recorded by the ion sensor of the ASPERA-3 instrument on board Mars Express. We show that peak electron densities during the SEP events are large enough to be detected in more than 30% of measurements, and, moreover, the reflections of the sounding signal from the ground almost entirely disappear. Nightside electron densities during SEP events are thus substantially increased as compared to normal nightside conditions

Experimental observation of the optical spin-orbit torque

Spin polarized carriers electrically injected into a magnet from an external polarizer can exert a spin transfer torque (STT) on the magnetization. The phe- nomenon belongs to the area of spintronics research focusing on manipulating magnetic moments by electric fields and is the basis of the emerging technologies for scalable magnetoresistive random access memories. In our previous work we have reported experimental observation of the optical counterpart of STT in which a circularly polarized pump laser pulse acts as the external polarizer, allowing to study and utilize the phenomenon on several orders of magnitude shorter timescales than in the electric current induced STT. Recently it has been theoretically proposed and experimentally demonstrated that in the absence of an external polarizer, carriers in a magnet under applied electric field can develop a non-equilibrium spin polarization due to the relativistic spin-orbit coupling, resulting in a current induced spin-orbit torque (SOT) acting on the magnetization. In this paper we report the observation of the optical counterpart of SOT. At picosecond time-scales, we detect excitations of magnetization of a ferromagnetic semiconductor (Ga,Mn)As which are independent of the polarization of the pump laser pulses and are induced by non-equilibrium spin-orbit coupled photo-holes.Comment: 4 figure, supplementary information. arXiv admin note: text overlap with arXiv:1101.104

Voigt effect-based wide-field magneto-optical microscope integrated in a pump-probe experimental setup

In this work, we describe an experimental setup for a spatially resolved pump-probe experiment with an integrated wide-field magneto-optical (MO) microscope. The MO microscope can be used to study ferromagnetic materials with both perpendicular-to-plane and in-plane magnetic anisotropy via polar Kerr and Voigt effects, respectively. The functionality of the Voigt effect-based microscope was tested using an in-plane magnetized ferromagnetic semiconductor (Ga,Mn)As. It was revealed that the presence of mechanical defects in the (Ga,Mn)As epilayer alters significantly the magnetic anisotropy in their proximity. The importance of MO experiments with simultaneous temporal and spatial resolutions was demonstrated using a (Ga,Mn)As sample attached to a piezoelectric transducer, which produces a voltage-controlled strain. We observed a considerably different behavior in different parts of the sample that enabled us to identify sample parts where the epilayer magnetic anisotropy was significantly modified by the presence of the piezoelectric transducer and where it was not. Finally, we discuss the possible applicability of our experimental setup for the research of compensated antiferromagnets, where only MO effects even in magnetic moments are present

Anisotropy of Magnetic Field and Velocity Fluctuations in the Solar Wind

We present a large statistical study of the fluctuation anisotropy in minimum variance (MV) frames of the magnetic field and solar wind velocity. We use 2, 10, 20, and 40 minute intervals of simultaneous magnetic field (the Wind spacecraft) and velocity (the Spektr-R spacecraft) observations. Our study confirms that magnetic turbulence is a composite of fluctuations varying along the mean magnetic field and those changing in the direction perpendicular to the mean field. Regardless of the length scale within the studied range of spacecraft-frame frequencies, ≈90% of the observed magnetic field fluctuations exhibit an MV direction aligned with the mean magnetic field, ≈10% of events have the MV direction perpendicular to the background field, and a negligible portion of fluctuations has no preferential direction. On the other hand, the MV direction of velocity fluctuations tends to be distributed more uniformly. An analysis of magnetic compressibility and density fluctuations suggests that the fluctuations resemble properties of Alfvénic fluctuations if the MV direction is aligned with background magnetic field whereas slow-mode-like fluctuations have the MV direction perpendicular to the background field. The proportion between Alfvénic and slow-mode-like fluctuations depends on plasma β and length scale: the dependence on the solar wind speed is weak. We present 3D numerical MHD simulations and show that the numerical results are compatible with our experimental results

Evolution of Magnetic Field Fluctuations and Their Spectral Properties within the Heliosphere: Statistical Approach

We present the first comprehensive statistical study of the evolution of compressive and noncompressive magnetic field fluctuations in the inner heliosphere. Based on Parker Solar Probe (PSP) and Solar Orbiter data at various distances from the Sun, we show the general trends and compare them with Wind observations near 1 au. The paper analyzes solar wind power spectra of magnetic field fluctuations in the inertial and kinetic ranges of frequencies. We find a systematic steepening of the spectrum in the inertial range with the spectral index of around −3/2 at closest approach to the Sun toward −5/3 at larger distances (above 0.4 au), the spectrum of the field component perpendicular to the background field being steeper at all distances. In the kinetic range, the spectral indices increase with distance from −4.8 at closest PSP approach to ≈−3 at 0.4 au and this value remains approximately constant toward 1 au. We show that the radial profiles of spectral slopes, fluctuation amplitudes, spectral breaks, and their mutual relations undergo rapid changes near 0.4 au

Nightside ionosphere of Mars studied with local electron densities: A general overview and electron density depressions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95308/1/jgra21490.pd

Spectral features of lightning-induced ion cyclotron waves at low latitudes: DEMETER observations and simulation

International audience[1] We use a comprehensive analysis of 6-component ELF wave data from the DEMETER satellite to study proton whistlers, placing emphasis on low-latitude events originating from lightning strokes in the hemisphere opposite to the hemisphere of observation. In this case, the formation of proton whistlers does not involve mode conversion caused by a strong mode coupling at a crossover frequency, although a polarization reversal remains an important element in formation of the phenomenon. DEMETER measurements of the six electromagnetic field components in the frequency band below 1000 Hz make it possible to determine not only the dynamic spectrum, but also the wave polarization, the wave normal angle, and the normalized parallel component of the Poynting vector. This permits us to address fine features of proton whistlers, in particular, we show that the deviation of the upper cutoff frequency from the equatorial cyclotron frequency is related to the Doppler shift. Experimental study of proton whistlers is supplemented by an investigation of ion cyclotron wave propagation in a multicomponent magnetoplasma and by numerical modeling of spectrograms, both in the frame of geometrical optics

Direct measurement of the three dimensional magnetization vector trajectory in GaMnAs by a magneto-optical pump-and-probe method

We report on a quantitative experimental determination of the three-dimensional magnetization vector trajectory in GaMnAs by means of the static and time-resolved pump-and-probe magneto-optical measurements. The experiments are performed in a normal incidence geometry and the time evolution of the magnetization vector is obtained without any numerical modeling of magnetization dynamics. Our experimental method utilizes different polarization dependences of the polar Kerr effect and magnetic linear dichroism to disentangle the pump-induced out-of-plane and in-plane motions of magnetization, respectively. We demonstrate that the method is sensitive enough to allow for the determination of small angle excitations of the magnetization in GaMnAs. The method is readily applicable to other magnetic materials with sufficiently strong circular and linear magneto-optical effects.Comment: main paper: 7 pages, 3 figures; supplementary information: 11 pages, 6 figure

Exciton spin dynamics in spherical CdS quantum dots

Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wavefunction in the lowest exciton state are of S-type. This rules out the spatial-symmetry-induced origin of the dark exciton in CdS quantum dots. The exciton bleaching dynamics is studied using time- and polarization-resolved transient absorption technique of ultrafast laser spectroscopy. Several samples with a different mean size of CdS quantum dots in different glass matrices were investigated. This enabled the separation of effects that are typical for one particular sample from those that are general for this type of material. The experimentally determined dependence of the electron spin relaxation rate on the radius of quantum dots agrees well with that computed theoretically.Comment: 24 pages, 10 figure

Multiple-stable anisotropic magnetoresistance memory in antiferromagnetic MnTe

Commercial magnetic memories rely on the bistability of ordered spins in ferromagnetic materials. Recently, experimental bistable memories have been realized using fully compensated antiferromagnetic metals. Here we demonstrate a multiple-stable memory device in epitaxial MnTe, an antiferromagnetic counterpart of common II–VI semiconductors. Favourable micromagnetic characteristics of MnTe allow us to demonstrate a smoothly varying zero-field antiferromagnetic anisotropic magnetoresistance (AMR) with a harmonic angular dependence on the writing magnetic field angle, analogous to ferromagnets. The continuously varying AMR provides means for the electrical read-out of multiple-stable antiferromagnetic memory states, which we set by heat-assisted magneto recording and by changing the writing field direction. The multiple stability in our memory is ascribed to different distributions of domains with the Neel vector aligned along one of the three magnetic easy axes. The robustness against strong magnetic field perturbations combined with the multiple stability of the magnetic memory states are unique properties of antiferromagnets