Tunable Transient Decay Times in Nonlinear Systems: Application to Magnetic Precession

The dynamical motion of the magnetization plays a key role in the properties of magnetic materials. If the magnetization is initially away from the equilibrium direction in a magnetic nanoparticle, it will precess at a natural frequency and, with some damping present, will decay to the equilibrium position in a short lifetime. Here we investigate a simple but important situation where a magnetic nanoparticle is driven non-resonantly by an oscillating magnetic field, not at the natural frequency. We find a surprising result that the lifetime of the transient motion is strongly tunable, by factors of over 10,000, by varying the amplitude of the driving field.Comment: EPL Preprin

The UARS microwave limb sounder version 5 data set: Theory, characterization, and validation

Nitric acid (HNO3) is a major player in processes controlling the springtime depletion of polar ozone. It is the main constituent of the Polar Stratospheric Clouds (PSCs) and a primary reservoir for reactive nitrogen. Potential variations in the stratospheric circulation and temperature may alter the extent and duration of PSCs activity, influencing the future ozone levels significantly. Monitoring HNO3 and its long-term variability, especially in polar region, is then crucial for better understanding issues related to ozone decline and expected recovery. In this study we present an intercomparison between ground based HNO3 measurements, carried out by means of the Ground-Based Millimeter-wave Spectrometer (GBMS), and two satellite data sets produced by the two NASA/JPL Microwave Limb Sounder (MLS) experiments. In particular, we compare UARS MLS measurements (1991-1999) with those carried out by the GBMS at South Pole, Antarctica (90°S), Fall of 1993 and 1995. A similar intercomparison is made between Aura MLS HNO3 observations (2004 - to date) and GBMS measurements obtained during the period February 2004 - March 2007, at the mid-latitudes/high altitudes station of Testa Grigia (45.9° N, 7.7° E, elev. 3500 m), and during polar winters 2008/09 and 2009/2010 at Thule Air Base (76.5°N 68.8°W), Greenland. We assess systematic differences between GBMS and both UARS and Aura HNO3 data sets at seven potential temperature levels (θ) spanning the range 465 – 960 K. The UARS data set advected to the South Pole shows a low bias, within 20% for all θ levels but the 960 K, with respect to GBMS measurements. A very good agreement, within 5%, is obtained between Aura and GBMS observations at Testa Grigia, while larger differences, possibly due to latitude dependent effects, are observed over Thule. These differences are under further investigations but a preliminary comparison over Thule among MLS v3, GBMS, and ACE-FTS measurements suggests that GBMS measurements carried out during winter 2009 might not be reliable. These comparisons have been performed in the framework of the NASA JPL GOZCARDS project, which is aimed at developing a long-term, global data record of the relevant stratospheric constituents in the context of ozone decline. GBMS has been selected in GOZCARDS since its HNO3 dataset, although sampling different latitudes in different years, is the only one spanning a sufficiently long time interval for cross-calibrating HNO3 measurements by the UARS and Aura MLS experiments

Using magnetic hyperbolic metamaterials as high frequency tunable filters

Metamaterials have enabled a series of major advances in optical devices in the past decade. Here, we suggest a type of hyperbolic metamaterial based on spin canting in magnetic multi-layers. We show that these structures have unique features in microwave waveguides that act as tunable filters. In the resulting band pass filter, we demonstrate an exceptional frequency tunability of 30 GHz with external fields smaller than 500 Oe. Unlike single metallic ferromagnetic films, we also demonstrate a high-frequency band-stop filter at very low fields

The effect of transverse magnetic correlations on a coupled order parameter: shifted transition temperatures and thermal hysteresis

We use a Green's function method with Random Phase Approximation to show how magnetic correlations may affect electric polarization in multiferroic materials with magnetic-exchange-type magnetoelectric coupling. We use a model spin 1/2 ferromagnetic ferroelectric system but our results are expected to apply to multiferroic materials with more complex magnetic structures. In particular, we find that transverse magnetic correlations result in a change in the free energy of the ferroelectric solutions leading to the possibility for thermal hysteresis of the electric polarization above the magnetic Curie temperature. Although we are motivated by multiferroic materials, this problem represents a more general calculation of the effect of fluctuations on coupled order parameters

Comparing learned predictiveness effects within and across compound discriminations

In four human learning experiments, we examined the extent to which learned predictiveness depends upon direct comparison between relatively good and poor predictors. Participants initially solved (1) linear compound discriminations in which one or both of the stimuli in each compound were predictive of the correct outcome, (2) biconditional discriminations where only the configurations of the stimuli were predictive of the correct outcome, or (3) pseudo-discriminations in which no stimulus features were predictive. In each experiment, subsequent learning and test stages were used to assay changes in the associability of each stimulus brought about by its role in the initial discriminations. Although learned predictiveness effects were observed in all experiments (i.e. previously predictive cues were more readily associated with a new outcome than previously non-predictive cues), the same changes in associability were observed regardless of whether the stimulus was initially learned about in the presence of an equally predictive, more predictive, or less predictive stimulus. The results suggest that learned associability is not controlled by competitive allocation of attention, but rather by the absolute predictiveness of each individual cue.Australian Research Council, and the German Academic Exchange Service (DAAD

Characterizing sampling and quality screening biases in infrared and microwave limb sounding

This study investigates orbital sampling biases and evaluates the additional impact caused by data quality screening for the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and the Aura Microwave Limb Sounder (MLS). MIPAS acts as a proxy for typical infrared limb emission sounders, while MLS acts as a proxy for microwave limb sounders. These biases were calculated for temperature and several trace gases by interpolating model fields to real sampling patterns and, additionally, screening those locations as directed by their corresponding quality criteria. Both instruments have dense uniform sampling patterns typical of limb emission sounders, producing almost identical sampling biases. However, there is a substantial difference between the number of locations discarded. MIPAS, as a mid-infrared instrument, is very sensitive to clouds, and measurements affected by them are thus rejected from the analysis. For example, in the tropics, the MIPAS yield is strongly affected by clouds, while MLS is mostly unaffected. The results show that upper-tropospheric sampling biases in zonally averaged data, for both instruments, can be up to 10 to 30 %, depending on the species, and up to 3 K for temperature. For MIPAS, the sampling reduction due to quality screening worsens the biases, leading to values as large as 30 to 100 % for the trace gases and expanding the 3 K bias region for temperature. This type of sampling bias is largely induced by the geophysical origins of the screening (e.g. clouds). Further, analysis of long-term time series reveals that these additional quality screening biases may affect the ability to accurately detect upper-tropospheric long-term changes using such data. In contrast, MLS data quality screening removes sufficiently few points that no additional bias is introduced, although its penetration is limited to the upper troposphere, while MIPAS may cover well into the mid-troposphere in cloud-free scenarios. We emphasize that the results of this study refer only to the representativeness of the respective data, not to their intrinsic quality

A linear CO chemistry parameterization in a chemistry-transport model: evaluation and application to data assimilation

This paper presents an evaluation of a new linear parameterization valid for the troposphere and the stratosphere, based on a first order approximation of the carbon monoxide (CO) continuity equation. This linear scheme (hereinafter noted LINCO) has been implemented in the 3-D Chemical Transport Model (CTM) MOCAGE (MOdèle de Chimie Atmospherique Grande Echelle). First, a one and a half years of LINCO simulation has been compared to output obtained from a detailed chemical scheme output. The mean differences between both schemes are about ±25 ppbv (part per billion by volume) or 15% in the troposphere and ±10 ppbv or 100% in the stratosphere. Second, LINCO has been compared to diverse observations from satellite instruments covering the troposphere (Measurements Of Pollution In The Troposphere: MOPITT) and the stratosphere (Microwave Limb Sounder: MLS) and also from aircraft (Measurements of ozone and water vapour by Airbus in-service aircraft: MOZAIC programme) mostly flying in the upper troposphere and lower stratosphere (UTLS). In the troposphere, the LINCO seasonal variations as well as the vertical and horizontal distributions are quite close to MOPITT CO observations. However, a bias of ~−40 ppbv is observed at 700 Pa between LINCO and MOPITT. In the stratosphere, MLS and LINCO present similar large-scale patterns, except over the poles where the CO concentration is underestimated by the model. In the UTLS, LINCO presents small biases less than 2% compared to independent MOZAIC profiles. Third, we assimilated MOPITT CO using a variational 3D-FGAT (First Guess at Appropriate Time) method in conjunction with MOCAGE for a long run of one and a half years. The data assimilation greatly improves the vertical CO distribution in the troposphere from 700 to 350 hPa compared to independent MOZAIC profiles. At 146 hPa, the assimilated CO distribution is also improved compared to MLS observations by reducing the bias up to a factor of 2 in the tropics. This study confirms that the linear scheme is able to simulate reasonably well the CO distribution in the troposphere and in the lower stratosphere. Therefore, the low computing cost of the linear scheme opens new perspectives to make free runs and CO data assimilation runs at high resolution and over periods of several years