Monthly Archives: June 2019

Automatic Detection of Interplanetary Coronal Mass Ejections from In Situ Data: A Deep Learning Approach

Posted on by

Automatic Detection of Interplanetary Coronal Mass Ejections from In Situ Data: A Deep Learning Approach

Gautier Nguyen, Nicolas Aunai, Dominique Fontaine, Erwan Le Pennec, Joris Van den Bossche, Alexis Jeandet, Brice Bakkali, Louis Vignoli, and Bruno Regaldo-Saint Blancard The Astrophysical Journal, Volume 874, Number 2 doi : https://doi.org/10.3847/1538-4357/ab0d24

Decades of studies have suggested several criteria to detect interplanetary coronal mass ejections (ICME) in time series from in situ spacecraft measurements. Among them, the most common are an enhanced and smoothly rotating magnetic field, a low proton temperature, and a low plasma beta. However, these features are not all observed for each ICME due to their strong variability. Visual detection is time-consuming and biased by the observer interpretation, leading to non-exhaustive, subjective, and thus hardly reproducible catalogs. Using convolutional neural networks on sliding windows and peak detection, we provide a fast, automatic, and multiscale detection of ICMEs. The method has been tested on the in situ data from WIND between 1997 and 2015, and on the 657 ICMEs that were recorded during this period. The method offers an unambiguous visual proxy of ICMEs that gives an interpretation of the data similar to what an expert observer would give. We found at a maximum 197 of the 232 ICMEs of the 2010–2015 period (recall 84%±4.5%), including 90% of the ICMEs present in the lists of Nieves-Chinchilla et al. and Chi et al. The minimal number of False Positives was 25 out of 158 predicted ICMEs (precision 84%±2.6%). Although less accurate, the method also works with one or several missing input parameters. The method has the advantage of improving its performance by just increasing the amount of input data. The generality of the method paves the way for automatic detection of many different event signatures in spacecraft in situ measurements.

Signatures of Cold Ions in a Kinetic Simulation of the Reconnecting Magnetopause

Posted on by

Signatures of Cold Ions in a Kinetic Simulation of the Reconnecting Magnetopause

Dargent, J., Aunai, N., Lavraud, B., Toledo‐Redondo, S., Califano, F. ( 2019). Journal of Geophysical Research: Space Physics, 124, 2497– 2514.

At the Earth’s magnetopause, a low‐energy ion population of ionospheric origin is commonly observed at the magnetospheric side. In this work we use a 2‐D fully kinetic simulation to identify several original signatures related to the dynamics of cold ions involved in magnetic reconnection at the asymmetric dayside magnetopause. We identify several original signatures of the cold ions dynamics driven by the development of magnetic reconnection at the asymmetric dayside magnetopause. We find that cold ions tend to rarefy in the diffusion region, while their density is enhanced as a result of compression along magnetospheric separatrices. We also observe the formation of crescent‐shaped cold ion distribution functions along the separatrices in the near‐exhaust region, and we present an analytical model to explain this signature. Finally, we give evidence of a localized parallel heating of cold ions. These signatures should be detected with the magnetospheric multiscale mission high‐resolution observations.

Analyzing the magnetopause internal structure: New possibilities offered by MMS tested in a case study

Posted on by

Analyzing the magnetopause internal structure: New possibilities offered by MMS tested in a case studys

Rezeau, L., Belmont, G., Manuzzo, R., Aunai, N., Dargent, J. ( 2018). Journal of Geophysical Research: Space Physics, 123, 227– 241. doi : https://doi.org/10.1002/2017JA024526

We explore the structure of the magnetopause using a crossing observed by the Magnetospheric Multiscale (MMS) spacecraft on 16 October 2015. Several methods (minimum variance analysis, BV method, and constant velocity analysis) are first applied to compute the normal to the magnetopause considered as a whole. The different results obtained are not identical, and we show that the whole boundary is not stationary and not planar, so that basic assumptions of these methods are not well satisfied. We then analyze more finely the internal structure for investigating the departures from planarity. Using the basic mathematical definition of what is a one‐dimensional physical problem, we introduce a new single spacecraft method, called LNA (local normal analysis) for determining the varying normal, and we compare the results so obtained with those coming from the multispacecraft minimum directional derivative (MDD) tool developed by Shi et al. (2005). This last method gives the dimensionality of the magnetic variations from multipoint measurements and also allows estimating the direction of the local normal when the variations are locally 1‐D. This study shows that the magnetopause does include approximate one‐dimensional substructures but also two‐ and three‐dimensional structures. It also shows that the dimensionality of the magnetic variations can differ from the variations of other fields so that, at some places, the magnetic field can have a 1‐D structure although all the plasma variations do not verify the properties of a global one‐dimensional problem. A generalization of the MDD tool is proposed.

Perpendicular Current Reduction Caused by Cold Ions of Ionospheric Origin in Magnetic Reconnection at the Magnetopause: Particle‐in‐Cell Simulations and Spacecraft Observations

Posted on by

Perpendicular Current Reduction Caused by Cold Ions of Ionospheric Origin in Magnetic Reconnection at the Magnetopause: Particle‐in‐Cell Simulations and Spacecraft Observations

Geophysical Research Letters, 45, 10,033– 10,042. doi : https://doi.org/10.1029/2018GL079051

Sergio Toledo‐Redondo Jérémy Dargent Nicolas Aunai Benoit Lavraud Mats André Wenya Li Barbara Giles Per‐Arne Lindqvist Robert E. Ergun Christopher T. Russell James L. Burch

Cold ions of ionospheric origin are present throughout the Earth’s magnetosphere, including the dayside magnetopause, where they modify the properties of magnetic reconnection, a major coupling mechanism at work between the magnetosheath and the magnetosphere. We present Magnetospheric MultiScale (MMS) spacecraft observations of the reconnecting magnetopause with different amounts of cold ions and show that their presence reduces the Hall term in the Ohm’s law. Then, we compare two particle‐in‐cell simulations, with and without cold ions on the magnetospheric side. The cold ions remain magnetized inside the magnetospheric separatrix region, leading to the reduction of the perpendicular currents associated with the Hall effect. Moreover, this reduction is proportional to the relative number density of cold ions. And finally, the Hall electric field peak is reduced along the magnetospheric separatrix owing to cold ions. This should have an effect on energy conversion by reconnection from electromagnetic fields to kinetic energy of the particles.

Smilei : A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation

Posted on by

Smilei : A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation

Computer Physics Communications Volume 222, January 2018, Pages 351-373 DOI : https://doi.org/10.1016/j.cpc.2017.09.024

J.Derouillat, A. Beck, F. Pérez, T.Vinci, M. Chiaramello, A. Grassi, M.Flé, G.Bouchar, I. Plotnikov, N.Aunai, J.Dargent, C.Riconda, M.Grech

Smilei is a collaborative, open-source, object-oriented (C++) particle-in-cell code. To benefit from the latest advances in high-performance computing (HPC), Smilei is co-developed by both physicists and HPC experts. The code’s structures, capabilities, parallelization strategy and performances are discussed. Additional modules (e.g. to treat ionization or collisions), benchmarks and physics highlights are also presented. Multi-purpose and evolutive, Smilei is applied today to a wide range of physics studies, from relativistic laser–plasma interaction to astrophysical plasmas.