When do finite sample effects significantly affect entropy estimates ?

An expression is proposed for determining the error caused on entropy estimates by finite sample effects. This expression is based on the Ansatz that the ranked distribution of probabilities tends to follow an empirical Zipf law.Comment: 10 pages, 2 figure

Synoptic solar radio observations as proxies for upper atmosphere modelling

The specification of the upper atmosphere strongly relies on solar proxies that can properly reproduce the solar energetic input in the UV. Whilst the microwave flux at 10.7 cm (also called F10.7 index) has been routinely used as a solar proxy, we show that the radio flux at other wavelengths provides valuable complementary information that enhances their value for upper atmospheric modelling. We merged daily observations from various observatories into a single homogeneous data set of fluxes at wavelengths of 30, 15, 10.7, 8 and 3.2 cm, spanning from 1957 to today. Using blind source separation (BSS), we show that their rotational modulation contains three contributions, which can be interpreted in terms of thermal bremsstrahlung and gyro-resonance emissions. The latter account for 90% of the rotational variability in the F10.7 index. Most solar proxies, such as the MgII index, are remarkably well reconstructed by simple linear combination of radio fluxes at various wavelengths. The flux at 30 cm stands out as an excellent proxy and is better suited than the F10.7 index for the modelling the thermosphere-ionosphere system, most probably because it receives a stronger contribution from thermal bremsstrahlung. This better performance is illustrated here through comparison between the observed thermospheric density, and reconstructions by the Drag Temperature Model.Comment: 13 page

Towards the definition of a solar forcing dataset for CMIP7

The solar forcing prepared for Phase 6 of the Coupled Model Intercomparison Project (CMIP6) has been used extensively in climate model experiments and has been tested in various intercomparison studies. Recently, an International Space Science Institute (ISSI) working group has been established to revisit the solar forcing recommendations, based on the lessons learned from CMIP6, and to assess new datasets that have become available, in order to define a road map for building a revised and extended historical solar forcing dataset for the upcoming Phase 7 of CMIP. This paper identifies the possible improvements required and outlines a strategy to address them in the planned new solar forcing dataset. Proposed major changes include the adoption of the new Total and Spectral Solar Irradiance Sensor (TSIS-1) solar reference spectrum for solar spectral irradiance and an improved description of top-of-the-atmosphere energetic electron fluxes, as well as their reconstruction back to 1850 by means of geomagnetic proxy data. In addition, there is an urgent need to consider the proposed updates in the ozone forcing dataset in order to ensure a self-consistent solar forcing in coupled models without interactive chemistry. Regarding future solar forcing, we propose consideration of stochastic ensemble forcing scenarios, ideally in concert with other natural forcings, in order to allow for realistic projections of natural forcing uncertainties

Waveforms of Langmuir turbulence in inhomogeneous solar wind plasmas

International audienceModulated Langmuir waveforms have been observed by several spacecraft in various regions of the heliosphere, such as the solar wind, the electron foreshock, the magnetotail, or the auroral ionosphere. Many observations revealed the bursty nature of these waves, which appear to be highly modulated, localized, and clumped into spikes with peak amplitudes typically 3 orders of magnitude above the mean. The paper presents Langmuir waveforms calculated using a Hamiltonian model describing self-consistently the resonant interaction of an electron beam with Langmuir wave packets in a plasma with random density fluctuations. These waveforms, obtained for different profiles of density fluctuations and ranges of parameters relevant to solar type III electron beams and plasmas measured at 1 AU, are presented in the form they would appear if recorded by a satellite moving in the solar wind. Comparison with recent measurements by the STEREO and WIND satellites shows that their characteristic features are very similar to the observations

Recovering coefficients of the complex Ginzburg-Landau equation from experimental spatio-temporal data: two examples from hydrodynamics

International audienceThere are many examples where the description of the complexity of flows can only be achieved by the use of simple models. These models, obtained usually from phenomenological arguments, need in general the knowledge of some parameters. The challenge is then to determine the values of these parameters from experiments. We will give two examples where we have been able to evaluate the coefficients of the complex Ginzburg-Landau equation (CGLE) from space-time chaotic data applied to first a row of coupled cylinder wakes and then to wave propagation in the Ekman layer of a rotating disk. In the first case, our analysis is based on a proper decomposition of experimental chaotic flow fields, followed by a projection of the CGLE onto the proper directions. We show that our method is able to recover the parameters of the model which permits to reconstruct the spatio-temporal chaos observed in the experiment. The second physical system under consideration is the flow above a rotating disk and its cross-flow instability. Our aim is to study the properties of the wavefield through a Volterra series equation. The kernels of the Volterra expansion, which contain relevant physical information about the system, are estimated by fitting two-point measurements via a nonlinear parametric model. We then consider describing the wavefield with the CGLE, and derive analytical relations which express the coefficients of the Ginzburg-Landau equation in terms of the kernels of the Volterra expansion

Searching for chaotic deterministic features in laboratory water surface waves

International audienceThe dynamic evolution of laboratory water surface waves has been studied within the framework of dynamical systems with the aim to identify stochastic or deterministic nonlinear features. Three different regimes are considered: pure wind waves, pure mechanical waves and mixed (wind and mechanical) waves. These three regimes show different dynamics. The results on wind waves do not clearly support the recently proposed idea that a deterministic Stokes-like component dominate the evolution of such waves; they are more appropriately described by a similarity-like approach that includes a random character. Cubic resonant interactions are clearly identified in pure mechanical waves using tricoherence functions. However, detailed aspects of the interactions do not fully agree with existing theoretical models. Finally, a deterministic motion is observed in mixed waves, which therefore are best described by a low dimensional nonlinear deterministic process

The EUV Sun as the superposition of elementary Suns

International audienceAims. Many studies assume that the solar irradiance in the EUV can be decomposed into different contributions, which makes modelling the spectral variability considerably easier. We consider a different approach in which these contributions are not imposed a priori but effectively and robustly inferred from spectral irradiance measurements. Methods. This is a source separation problem with a positivity constraint, for which we use a Bayesian solution. Results. Using five years of daily EUV spectra recorded by the TIMED/SEE satellite, we show that the spectral irradiance can be decomposed into three elementary spectra. Our results suggest that they describe different layers of the solar atmosphere rather than specific regions. The temporal variability of these spectra is discussed

On aspects of the measurement of non-linear turbulence processes using the cluster wave experiments

International audienceThe ESA/NASA Cluster mission has four identical satellites and is due for launch at the end of 1995. It will provide a unique opportunity to study medium scale processes in the region from inside the magnetopause to the solar wind. The polar orbit will allow measurements in the cusp and along auroral field lines, both regions where turbulence is to be expected. Five of the eleven instruments on each payload form the Wave Experiment Consortium (WEC); EFW, STAFF, VMISPER, WBD and DWP. The WEC is capable of a wide variety of wave and turbulence measurements. This paper outlines these capabilities and describes the form of the data which will be collected. The paper gives a discussion of how the WEC data may be analysed so as to give an insight into the non-linear processes which occur in these regions of the space plasmas. There are many ways in which a plasma may be considered to behave in a non-linear manner. We concentrate on how the spatio-temporal turbulence in the plasma may be investigated so as to yield the energy spectrum with respect to both the frequency and wave number

The effect of flares on total solar irradiance

Flares are powerful energy releases occurring in stellar atmospheres. Solar flares, the most intense energy bursts in the solar system, are however hardly noticeable in the total solar luminosity. Consequently, the total amount of energy they radiate 1) remains largely unknown and 2) has been overlooked as a potential contributor to variations in the Total Solar Irradiance (TSI), i.e. the total solar flux received at Earth. Here, we report on the detection of the flare signal in the TSI even for moderate flares. We find that the total energy radiated by flares exceeds the soft X-ray emission by two orders of magnitude, with an important contribution in the visible domain. These results have implications for the physics of flares and the variability of our star.Comment: accepted in Nature Physic