1,483 research outputs found
Unsupervised classification of fully kinetic simulations of plasmoid instability using Self-Organizing Maps (SOMs)
The growing amount of data produced by simulations and observations of space
physics processes encourages the use of methods rooted in Machine Learning for
data analysis and physical discovery. We apply a clustering method based on
Self-Organizing Maps (SOM) to fully kinetic simulations of plasmoid
instability, with the aim of assessing its suitability as a reliable analysis
tool for both simulated and observed data. We obtain clusters that map well, a
posteriori, to our knowledge of the process: the clusters clearly identify the
inflow region, the inner plasmoid region, the separatrices, and regions
associated with plasmoid merging. SOM-specific analysis tools, such as feature
maps and Unified Distance Matrix, provide one with valuable insights into both
the physics at work and specific spatial regions of interest. The method
appears as a promising option for the analysis of data, both from simulations
and from observations, and could also potentially be used to trigger the switch
to different simulation models or resolution in coupled codes for space
simulations
Particle-in-cell simulations of Alfv\'en wave parametric decay in a low-beta plasma
We study the parametric decay instability of parallel propagating Alfv\'en
wave in a low-beta plasma using one-dimensional fully kinetic simulations. We
focus for the first time on the conversion of the energy stored in the initial
Alfv\'en wave into particle internal energy, and on its partition between
particle species. We show that compressible fluctuations generated by the decay
of the pump wave into a secondary ion-acoustic mode and a reflected Alfv\'en
wave contribute to the gain of internal energy via two distinct mechanisms.
First, the ion-acoustic mode leads nonlinearly to proton trapping and proton
phase space mixing, in agreement with previous work based on hybrid
simulations. Second, during the nonlinear stage, a compressible front of the
fast type develops at the steepened edge of the backward Alfv\'en wave leading
to a field-aligned proton beam propagating backwards at the Alfv\'en speed. We
find that parametric decay heats preferentially protons, which gain about 50%
of the pump wave energy in the form of internal energy. However, we find that
electrons are also energized and that they contribute to the total energy
balance by gaining 10% of the pump wave energy. By investigating energy
partition and particle heating during parametric decay, our results contribute
to determine the role of compressible and kinetic effects in wave-driven models
of the solar wind
Generation of sub-ion scale magnetic holes from electron shear flow instabilities in plasma turbulence
Magnetic holes (MHs) are coherent structures associated with strong magnetic
field depressions in magnetized plasmas. They are observed in many
astrophysical environments at a wide range of scales but their origin is still
under debate. In this work we investigate the formation of sub-ion scale MHs
using a fully kinetic 2D simulation of plasma turbulence initialized with
parameters typical of the Earth's magnetosheath. Our analysis shows that the
turbulence is capable of generating sub-ion scale MHs from large scale
fluctuations via the following mechanism: first, the nonlinear large scale
dynamics spontaneously leads to the development of thin and elongated electron
velocity shears; these structures then become unstable to the electron
Kelvin-Helmholtz instability and break up into small scale electron vortices;
the electric current carried by these vortices locally reduces the magnetic
field, inducing the formation of sub-ion scale MHs. The MHs thus produced
exhibit features consistent with satellite observations and with previous
numerical studies. We finally discuss the kinetic properties of the observed
sub-ion scale MHs, showing that they are characterized by complex
non-Maxwellian electron velocity distributions exhibiting anisotropic and
agyrotropic features.Comment: Submitted to AP
Clustering of Global Magnetospheric Observations
The use of supervised methods in space science have demonstrated powerful
capability in classification tasks, but unsupervised methods have been less
utilized for the clustering of spacecraft observations. We use a combination of
unsupervised methods, being principal component analysis, self-organizing maps,
and hierarchical agglomerative clustering, to make predictions on if THEMIS and
MMS observations occurred in the magnetosphere, magnetosheath, or the solar
wind. The resulting predictions are validated visually by analyzing the
distribution of predictions and studying individual time series. Particular
nodes in the self organizing map are studied to see what data they represent.
The capability of deeper hierarchical analysis using this model is briefly
explored. Finally, the changes in region prediction can be used to infer
magnetopause and bow shock crossings, which can act as an additional method of
validation, and are saved for their utility in solar wind validation,
understanding magnetopause processes, and the potential to develop a bow shock
model.Comment: 36 pages, 22 figure
Domain of Influence analysis: implications for Data Assimilation in space weather forecasting
Solar activity, ranging from the background solar wind to energetic coronal
mass ejections (CMEs), is the main driver of the conditions in the
interplanetary space and in the terrestrial space environment, known as space
weather. A better understanding of the Sun-Earth connection carries enormous
potential to mitigate negative space weather effects with economic and social
benefits. Effective space weather forecasting relies on data and models. In
this paper, we discuss some of the most used space weather models, and propose
suitable locations for data gathering with space weather purposes. We report on
the application of \textit{Representer analysis (RA)} and \textit{Domain of
Influence (DOI) analysis} to three models simulating different stages of the
Sun-Earth connection: the OpenGGCM and Tsyganenko models, focusing on solar
wind - magnetosphere interaction, and the PLUTO model, used to simulate CME
propagation in interplanetary space. Our analysis is promising for space
weather purposes for several reasons. First, we obtain quantitative information
about the most useful locations of observation points, such as solar wind
monitors. For example, we find that the absolute values of the DOI are
extremely low in the magnetospheric plasma sheet. Since knowledge of that
particular sub-system is crucial for space weather, enhanced monitoring of the
region would be most beneficial. Second, we are able to better characterize the
models. Although the current analysis focuses on spatial rather than temporal
correlations, we find that time-independent models are less useful for Data
Assimilation activities than time-dependent models. Third, we take the first
steps towards the ambitious goal of identifying the most relevant heliospheric
parameters for modelling CME propagation in the heliosphere, their arrival
time, and their geoeffectiveness at Earth.Comment: Accepted for publication at Frontiers in Astronomy and Space Science
Collisionless Heat Flux Regulation via the Electron Firehose Instability in the Presence of a Core and Suprathermal Population in the Expanding Solar Wind
The evolution of the electron heat flux in the solar wind is regulated by the interplay between several effects: solar wind expansion, which can potentially drive velocity–space instabilities, turbulence, wave–particle interactions, and, possibly, collisions. Here we address the respective role played by the solar wind expansion and the electron firehose instability (EFI), developing in the presence of multiple electron populations, in regulating the heat flux. We carry out fully kinetic, expanding box model simulations and separately analyze the enthalpy, bulk, and velocity distribution function skewness contributions for each of the electron species. We observe that the key factor determining electron energy flux evolution is the reduction of the drift velocity of the electron populations in the rest frame of the solar wind. In our simulations, redistribution of the electron thermal energy from the parallel to the perpendicular direction after the onset of the EFI is observed. However, this process seems to impact energy flux evolution only minimally. Hence, reduction of the electron species drift velocity in the solar wind frame appears to directly correlate with efficiency for heat flux instabilities
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