HAL-OBSPM
Not a member yet
37663 research outputs found
Sort by
Diurnal Temperature Variations and Migrating Thermal Tides in the Martian Lower Atmosphere Observed by the Emirates Mars InfraRed Spectrometer
No full textInternational audienceAbstract The Martian atmosphere experiences large diurnal variations due to the ∼24.6 hr planetary rotation and its low heat capacity. Understanding such variations on a planetary scale is limited due to the lack of observations, which are greatly addressed with the recent advent of the Emirates Mars Mission (EMM). As a result of its unique high‐altitude orbit, instruments onboard are capable of obtaining a full geographic and local time coverage of the Martian atmosphere every 9–10 Martian days, approximately ∼5° in solar longitude ( L S ). This enables investigations of the diurnal variation of the current climate on Mars on a planetary scale without significant local time (LT) gaps or confusions from correlated seasonal variations. Here, we present the results of diurnal temperature variations and thermal tides in the Martian atmosphere using temperature profiles retrieved from the Emirates Mars InfraRed Spectrometer (EMIRS) observations. The data during the primary mission is included, covering an entire Martian Year (MY) starting from MY 36 L S = 49°. The diurnal temperature patterns suggest a dominant diurnal tide in most seasons, while the semi‐diurnal tide presents a similar amplitude near the perihelion. The seasonal variation of the diurnal tide latitudinal distribution is well explained by the total vorticity due to zonal wind, while that of the semi‐diurnal tide following both dust and water ice clouds, and the ter‐diurnal tide following only dust. Comparison with the updated Mars Planetary Climate Model (PCM, version 6) suggests improvements in simulating the dust and water cycles, as well as their radiative processes
All-sky search for short gravitational-wave bursts in the first part of the fourth LIGO-Virgo-KAGRA observing run
No full textInternational audienceWe present a search for short-duration gravitational-wave transients in data from the first eight months of Advanced LIGO-Virgo-KAGRA's fourth observing run, denoted O4a. We use four analyses which are sensitive to a wide range of potential signals lasting up to a few seconds in the 16-4096 Hz band. Excluding binary black hole merger candidates that were already identified by low-latency analyses, we find no statistically significant evidence for other gravitational-wave transients. We measure the sensitivity of the search for representative signals, including sine-Gaussians, Gaussian pulses, and white-noise bursts with different frequencies and durations, adopting a false alarm rate of 1 per 100 years as detection threshold. Depending on signal type, we find improvements over previous searches by factors of 2 to 10 in terms of sensitivity to strain amplitude and of 90% confidence upper limit on the rate density of sources. We also evaluate a variety of core-collapse supernova models and find that, for some models, the search could have detected gravitational waves from stellar core-collapse throughout the Milky Way. Finally, we consider neutron star f-modes associated with pulsar glitches and find that, assuming a source similar to the Vela Pulsar, the search could have detected a gravitational-wave signal from a glitch with fractional frequency change as small as to depending on the neutron star mass
The First Detection of Molecular Activity in the Largest Known Oort Cloud Comet: ALMA Imaging of C/2014 UN271 (Bernardinelli–Bernstein) at 16.6 au from the Sun
No full textInternational audienceWe report observations of comet C/2014 UN271 (Bernardinelli-Bernstein) carried out on UT 2024 March 8 and 17 at a heliocentric distance (r H ) of 16.6 au using the Atacama Large Millimeter/Submillimeter Array (ALMA). The CO (J =2-1) line at 230 GHz was detected along with continuum emission from its dust coma and large (∼140 km) nucleus, revealing the nature of the activity drivers and outgassing kinematics of the largest Oort cloud comet discovered to date. This work presents spectrally integrated flux maps, autocorrelation spectra, production rates, and parent scale lengths for CO and a stringent upper limit for the H 2 CO production rate. CO outgassing displayed multiple active jets which evolved from one epoch to the next. The continuum emission was compact and spatially unresolved, and is consistent with thermal emission from the large nucleus and a tentative detection of a dust coma. Complementary optical observations provided activity context for the ALMA epochs, indicating that UN271 underwent an outburst in late February before returning to a quiescent brightness in mid-late March. These results represent the first secure detection of molecular activity reported in the literature for C/2014 UN271 and highlight the dynamic nature of this distantly active small world
Non-Gaussianity in SMICA
No full textInternational audienceWe develop a new formalism for the component separation method Spectral Matching Independent Component Analysis (SMICA) in order to include the information contained in the foregrounds beyond second-order statistics. We also develop a binned bispectrum estimator that works directly using maps of different frequency channels, capable of determining the bispectrum of multiple components at the same time, shifting the traditional approach to non-Gaussianity estimation from a cleaned map to the component separation step, for a better handling of foreground uncertainty. We test our method on 400 E and B polarization simulations based on the LiteBIRD experiment, containing the two main sources of contamination for CMB polarization experiments: polarized dust and synchrotron emission. We show that the bispectrum does not improve the precision of the power spectrum estimation or of the spectral parameters. However, we are capable of recovering the correct 3-point correlator of the foregrounds and standard constraints on primordial non-Gaussianity in a coherent multi-frequency and multi-component framework. The advantage of our approach is that it combines data in an optimal way accounting for the power spectrum and the bispectrum of the various components, which is not true for the standard approach
Identifying the Growth Phase of Magnetic Reconnection using Pressure-Strain Interaction
No full textInternational audienceMagnetic reconnection often initiates abruptly and then rapidly progresses to a nonlinear quasi-steady state. While satellites frequently detect reconnection events, ascertaining whether the system has achieved steady-state or is still evolving in time remains challenging. Here, we propose that the relatively rapid opening of the reconnection separatrices within the electron diffusion region serves as an indicator of the growth phase of reconnection. The opening of the separatrices is produced by electron flows diverging away from the neutral line downstream of the X-line and flowing around a dipolarization front. This flow pattern leads to characteristic spatial structures in the electron pressure-strain interaction that could be a useful indicator for the growth phase of a reconnection event. We employ two-dimensional particle-in-cell numerical simulations of anti-parallel magnetic reconnection to validate this prediction. We find that the signature discussed here, alongside traditional reconnection indicators, can serve as a marker of the growth phase. This signature is potentially accessible using multi-spacecraft single-point measurements, such as with NASA's Magnetospheric Multiscale satellites in Earth's magnetotail. Applications to other settings where reconnection occurs are also discussed
Défis pour la métrologie du temps et des fréquences : la stratégie du CCTF
No full textThis paper is an introduction to the Metrologia Special Focus issue on the CCTF Strategy. It presents the main CCTF activities in different fields of Time and Frequency metrology: primary and secondary frequency standards, time and frequency transfer techniques, and atomic time scales. This includes cross-cutting "hot topics" that deserve special attention such as: redefinition of the SI second, continuous UTC and leap seconds, mutual benefits between UTC and GNSS. The CCTF strategy aims to ensure the best services to the Time and Frequency community, including focused training based on resource sharing to improve national and international timekeeping.</div
SPECTCOL: A VAMDC tool for molecular spectroscopy and collisional data
No full textInternational audienceContext: Modeling of atomic and molecular processes in the interstellar medium, especially in the case of nonlocal thermodynamic equilibrium requires combining spectroscopic and collisional data. This combination provides a so-called matching file that is an input file for radiative transfer codes.Aims: We present the main functionalities of the SPECTCOL software that was developed in the environment of VAMDC standards: retrieving and exporting information from the CDMS, JPL, or HITRAN spectroscopic databases and the BASECOL collisional database, combining spectroscopic data and collisional data, and exporting the matching result in various formats.Methods: The SPECTCOL software is a Java client.Results: All molecules in the BASECOL database were tested with the SPECTCOL software, and the resulting matching files are available in a repository. Conclusions . The SPECTCOL software and associated tutorials are available on the VAMDC website
On the eigenvalues of a central configuration
No full textThe equations of the Newtonian n-body problem have a matrix form, where an n x n matrix depending on the masses and on the mutual distances appears as a factor. The eigenvalues of this matrix are real and nonnegative. In a motion of relative equilibrium, the configuration, called central, has constant mutual distances. The matrix is constant. We prove that in a relative equilibrium of 5 bodies the two nontrivial eigenvalues are strictly greater than the three trivial ones. This result improves published inequalities about the central configurations, which belong to two independent lines of research. One starts with Williams in 1938 and concerns constraints on the shape of the configuration. The other concerns the Hessian of the potential and its index, and applies to the linear stability of the self-similar motions and to the possible bifurcations. We also considerably clarify the very useful identities with which Williams discusses his inequalities
Determination of the accuracy of actinometry and line ratio techniques in an O 2 glow discharge: part I, comparison of absolute oxygen atom densities with CRDS measurements
No full textInternational audienceThe accuracy of oxygen atom density measurements in plasmas by optical emission actinometry was tested by comparison to simultaneous direct absorption measurements on the 1 D 2 -> 3 P 2 transition by cavity ringdown spectroscopy (CRDS). The accuracy of the latter technique depends only on the accuracy to which the transition probability is known. Measurements were performed on a glow discharge in O 2 operating between 0.5 and 5 Torr, and using both Ar and Xe as the actinometer gas. The rate constants for electron impact excitation, and thus the actinometry calibration factors, were calculated from the (measured) reduced electric field using a Boltzmann equation solver (Loki-B). Several sets of cross-section were tested for the EEDF calculation and for the electron impact excitation to the specific levels of O, Ar and Xe used for actinometry. The best results were obtained with the IST Lisbon cross-section set for O 2 and O, and the BSR500 excitation cross-sections for Ar and Xe. Good agreement with the CRDS trends and absolute values was observed when using Xe as the actinometer gas, whereas with Ar the trends were well reproduced but it was necessary to increase the electron impact cross-section of the transition Ar( 1 S 0-> 2p 1 ) of the BSR500 database by a factor of 3˘0.3 to reproduce the absolute values
Euclid: From Galaxies to Gravitational Waves – Forecasting Stochastic Gravitational Wave Background Anisotropies and Their Cross-Correlation
No full textInternational audienceWe estimate the amplitude and spatial anisotropy in the stochastic gravitational wave background (SGWB) energy density due to compact binary coalescence (CBC) events: binary black holes (BBH), binary neutron stars (BNS), and black hole-neutron star (BHNS) mergers. Our starting point is the Flagship Simulation Galaxy Catalogue developed by the Euclid Consortium. For each galaxy in the Catalogue, we use the simulated mass and starformation to constrain the galaxy's star-formation history, and predict its contribution to the gravitational-wave energy density through CBC mergers. Combining such contributions from all galaxies in the Catalogue results in a prediction for the frequency spectrum and spatial anisotropy of the CBC SGWB. We also compare this prediction to semi-analytical models of SGWB generated by compact binaries. We identify a set of effective parameters that capture the key features of these models, and we apply a Bayesian framework to infer these parameters assuming an ideal scenario of cosmic variance-limited search. This represents the first step toward developing a comprehensive framework that will eventually enable the correlation of SGWB anisotropy and \textit{Euclid} galaxy data, potentially allowing us to extract valuable astrophysical information from this new observable