arXiv subjects

F. Santos

Publications and source records attributed to F. Santos.

7 recordsLinked to original sources

arXiv2025

Comparing observations of the closely located JUICE and STEREO-A spacecraft during the widespread solar energetic particle event of 2024 May 13

JUICE was launched in April 2023, and it is now in its cruise phase to Jupiter, where it is scheduled to arrive in July 2031. JUICE carries a radiation monitor, namely the RADiation hard Electron Monitor (RADEM) to measure protons, electrons, and ions, detecting particles coming mainly from the anti-Sun direction. On 2024 May 13, a large solar energetic particle (SEP) event took place in association with an eruption close to the western limb of the Sun as seen from Earth. Providentially, at that time JUICE was located very close to STEREO-A, being separated by 0.13 au in radial distance, 0.3 deg in latitude, and 1.6 deg in longitude. Methods. We analysed the IP context using in-situ measurements and studied the proton anisotropies measured by near-Earth spacecraft and STEREO-A. We focused on an isotropic period during the decay phase of the SEP event to compute the proton energy spectrum. We fit the STEREO-A spectrum and compared it to that measured by SOHO and JUICE. Conclusions. The RADEM instrument aboard JUICE is a valuable tool for measuring SEP events in the heliosphere, providing an excellent opportunity to study and characterise the energetic particle environment in the solar wind between 0.65 and 5.2 au. The intercalibration factors between the fluxes measured by STEREO-A and JUICE at the effective energies of 6.9 MeV, 13.3 MeV, 21.6 MeV, and 31.2 MeV are 1.02, 1.23, 1.12, and 0.95 respectively. These intercalibration factors are valid only until 2024 July 10, when the configuration of the RADEM instrument was changed.

astro-ph.SR↗

arXiv2021

The formation of planetary systems with SPICA

In this era of spatially resolved observations of planet forming disks with ALMA and large ground-based telescopes such as the VLT, Keck and Subaru, we still lack statistically relevant information on the quantity and composition of the material that is building the planets, such as the total disk gas mass, the ice content of dust, and the state of water in planetesimals. SPICA is an infrared space mission concept developed jointly by JAXA and ESA to address these questions. The key unique capabilities of SPICA that enable this research are (1) the wide spectral coverage 10-220 micron, (2) the high line detection sensitivity of (1-2) 10-19 W m-2 with R~2000-5000 in the far-IR (SAFARI) and 10-20 W m-2 with R~29000 in the mid-IR (SMI, spectrally resolving line profiles), (3) the high far-IR continuum sensitivity of 0.45 mJy (SAFARI), and (4) the observing efficiency for point source surveys. This paper details how mid- to far-IR infrared spectra will be unique in measuring the gas masses and water/ice content of disks and how these quantities evolve during the planet forming period. These observations will clarify the crucial transition when disks exhaust their primordial gas and further planet formation requires secondary gas produced from planetesimals. The high spectral resolution mid-IR is also unique for determining the location of the snowline dividing the rocky and icy mass reservoirs within the disk and how the divide evolves during the build-up of planetary systems. Infrared spectroscopy (mid- to far-IR) of key solid state bands is crucial for assessing whether extensive radial mixing, which is part of our Solar System history, is a general process occurring in most planetary systems and whether extrasolar planetesimals are similar to our Solar System comets/asteroids. ... (abbreviated)

astro-ph.EP↗

arXiv2020

The role of the magnetic field in the fragmentation process: the case of G14.225-0.506

B-fields are predicted to play a role in the formation of filamentary structures and their fragmentation process. We aim at investigating the role of the B-field in the process of core fragmentation toward the hub-filament systems in the IRDC G14.2, which present different fragmentation level. We performed observations of the thermal dust polarization at 350 μm using the CSO toward the hubs. We applied the polarization--intensity-gradient method to estimate the significance of the B-field over the G-force. The B-field in Hub-N shows a uniform structure along the E-W orientation, perpendicular to the major axis of the hub-filament system. The I-gradient in Hub-N displays a local minimum coinciding with the dust core MM1a detected with interferometric observations. The B-field orientation is perturbed when approaching the dust core. Hub-S shows 2 local minima, reflecting the bimodal distribution of the B-field. In Hub-N, both E and W of the hub-filament system, the I-gradient and the B-field are parallel whereas they tend to be perpendicular when penetrating the filaments and hub. The analysis of the δ- and Σ B-maps indicate that, the B-field cannot prevent the collapse, suggesting that the B-field is initially dragged by the infalling motion and aligned with it, or is channeling material toward the central ridge from both sides. Values of Σ B > 1 are found toward a N-S ridge encompassing the dust emission peak, indicating that in this region B-field dominates over G-force, or that with the current angular resolution we cannot resolve an hypothetical more complex structure. We estimated the B-field strength, the MtF ratio and the A-M number, and found differences between the 2 hubs. The different levels of fragmentation observed in these 2 hubs could arise from the differences in the B-field properties rather than from different intensity of the G-field.

astro-ph.GA↗

arXiv2010

Transforming triangulations on non planar-surfaces

We consider whether any two triangulations of a polygon or a point set on a non-planar surface with a given metric can be transformed into each other by a sequence of edge flips. The answer is negative in general with some remarkable exceptions, such as polygons on the cylinder, and on the flat torus, and certain configurations of points on the cylinder.

math.MG↗

arXiv2005

Lattice Delone simplices with super-exponential volume

In this short note we give a construction of an infinite series of Delone simplices whose relative volume grows super-exponentially with their dimension. This dramatically improves the previous best lower bound, which was linear.

math.CO↗

arXiv2004

On the rank of a tropical matrix

This is a foundational paper in tropical linear algebra, which is linear algebra over the min-plus semiring. We introduce and compare three natural definitions of the rank of a matrix, called the Barvinok rank, the Kapranov rank and the tropical rank. We demonstrate how these notions arise naturally in polyhedral and algebraic geometry, and we show that they differ in general. Realizability of matroids plays a crucial role here. Connections to optimization are also discussed.

math.CO↗

arXiv1997

Fiber polytopes for the projections between cyclic polytopes

The cyclic polytope $C(n,d)$ is the convex hull of any $n$ points on the moment curve ${(t,t^2,...,t^d):t \in \reals}$ in $\reals^d$. For $d' >d$, we consider the fiber polytope (in the sense of Billera and Sturmfels) associated to the natural projection of cyclic polytopes $π: C(n,d') \to C(n,d)$ which "forgets" the last $d'-d$ coordinates. It is known that this fiber polytope has face lattice indexed by the coherent polytopal subdivisions of $C(n,d)$ which are induced by the map $π$. Our main result characterizes the triples $(n,d,d')$ for which the fiber polytope is canonical in either of the following two senses: - all polytopal subdivisions induced by $π$ are coherent, - the structure of the fiber polytope does not depend upon the choice of points on the moment curve. We also discuss a new instance with a positive answer to the Generalized Baues Problem, namely that of a projection $π:P\to Q$ where $Q$ has only regular subdivisions and $P$ has two more vertices than its dimension.

math.CO↗