arXiv subjects

G. Lapenta

Publications and source records attributed to G. Lapenta.

At least 19 recordsLinked to original sources

arXiv2024

Validation of EUHFORIA cone and spheromak Coronal Mass Ejection Models

Aims. We present the validation results for arrival times and geomagnetic impact of Coronal Mass Ejections (CMEs), using the cone and spheromak CME models implemented in EUropean Heliospheric FORecasting Information Asset (EUHFORIA). Validating numerical models is crucial in ensuring their accuracy and performance with respect to real data. Methods. We compare CME plasma and magnetic field signatures, measured in situ by satellites at the L1 point, with the simulation output of EUHFORIA. The validation of this model was carried out by using two datasets in order to ensure a comprehensive evaluation. The first dataset focuses on 16 CMEs that arrived at the Earth, offering specific insights into the model's accuracy in predicting arrival time and geomagnetic impact. Meanwhile, the second dataset encompasses all CMEs observed over eight months within Solar Cycle 24, regardless of whether they arrived at Earth, covering periods of both solar minimum and maximum activity. This second dataset enables a more comprehensive evaluation of the model's predictive precision in term of CME arrivals and misses. Results. Our results show that EUHFORIA provides good estimates in terms of arrival times, with root mean square errors (RMSE) values of 9 hours. Regarding the number of correctly predicted ICME arrivals and misses, we find a 75% probability of detection in a 12 hours time window and 100% probability of detection in a 24 hours time window. The geomagnetic impact forecasts, measured by the $K_p$ index, provide different degrees of accuracy, ranging from 31% to 69%. These results validate the use of cone and spheromak CMEs for real-time space weather forecasting.

astro-ph.SR↗

arXiv2022

Heliophysics Discovery Tools for the 21st Century: Data Science and Machine Learning Structures and Recommendations for 2020-2050

Three main points: 1. Data Science (DS) will be increasingly important to heliophysics; 2. Methods of heliophysics science discovery will continually evolve, requiring the use of learning technologies [e.g., machine learning (ML)] that are applied rigorously and that are capable of supporting discovery; and 3. To grow with the pace of data, technology, and workforce changes, heliophysics requires a new approach to the representation of knowledge.

astro-ph.IM↗

arXiv2022

Mixing the solar wind proton and electron scales. Theory and 2D-PIC simulations of firehose instability

Firehose-like instabilities (FIs) are cited in multiple astrophysical applications. Of particular interest are the kinetic manifestations in weakly-collisional or even collisionless plasmas, where these instabilities are expected to contribute to the evolution of macroscopic parameters. Relatively recent studies have initiated a realistic description of FIs, as induced by the interplay of both species, electrons and protons, dominant in the solar wind plasma. This work complements the current knowledge with new insights from linear theory and the first disclosures from 2D PIC simulations, identifying the fastest growing modes near the instability thresholds and their long-run consequences on the anisotropic distributions. Thus, unlike previous setups, these conditions are favorable to those aperiodic branches that propagate obliquely to the uniform magnetic field, with (maximum) growth rates higher than periodic, quasi-parallel modes. Theoretical predictions are, in general, confirmed by the simulations. The aperiodic electron FI (a-EFI) remains unaffected by the proton anisotropy, and saturates rapidly at low-level fluctuations. Regarding the firehose instability at proton scales, we see a stronger competition between the periodic and aperiodic branches. For the parameters chosen in our analysis, the a-PFI is excited before than the p-PFI, with the latter reaching a significantly higher fluctuation power. However, both branches are significantly enhanced by the presence of anisotropic electrons. The interplay between EFIs and PFIs also produces a more pronounced proton isotropization.

astro-ph.SR↗

arXiv2021

Turbulent magneto-genesis in a collisionless plasma

We investigate an efficient mechanism for generating magnetic fields in turbulent, collisionless plasmas. By using fully kinetic, particle-in-cell simulations of an initially non-magnetized plasma, we inspect the genesis of magnetization, in a nonlinear regime. The complex motion is initiated via a Taylor-Green vortex, and the plasma locally develops strong electron temperature anisotropy, due to the strain tensor of the turbulent flow. Subsequently, in a domino effect, the anisotropy triggers a Weibel instability, localized in space. In such active wave-particle interaction regions, the magnetic field seed grows exponentially and spreads to larger scales due to the interaction with the underlying stirring motion. Such a self-feeding process might explain magneto-genesis in a variety of astrophysical plasmas, wherever turbulence is present.

physics.plasm-ph↗

arXiv2021

On the role of solar wind expansion as a source of whistler waves: scattering of suprathermal electrons and heat flux regulation in the inner heliosphere

The role of solar wind expansion in generating whistler waves is investigated using the EB-iPic3D code, which models solar wind expansion self-consistently within a fully kinetic semi-implicit approach. The simulation is initialized with an electron velocity distribution function modeled after Parker Solar Probe observations during its first perihelion at 0.166 au, consisting of a dense core and an anti-sunward strahl. This distribution function is initially stable with respect to kinetic instabilities. Expansion drives the solar wind into successive regimes where whistler heat flux instabilities are triggered. These instabilities produce sunward whistler waves initially characterized by predominantly oblique propagation with respect to the interplanetary magnetic field. The excited waves interact with the electrons via resonant scattering processes. As a consequence, the strahl pitch angle distribution broadens and its drift velocity reduces. Strahl electrons are scattered in the direction perpendicular to the magnetic field, and an electron halo is formed. At a later stage, resonant electron firehose instability is triggered and further affects the electron temperature anisotropy as the solar wind expands. Wave-particle interaction processes are accompanied by a substantial reduction of the solar wind heat flux. The simulated whistler waves are in qualitative agreement with observations in terms of wave frequencies, amplitudes and propagation angles. Our work proposes an explanation for the observations of oblique and parallel whistler waves in the solar wind. We conclude that solar wind expansion has to be factored in when trying to explain kinetic processes at different heliocentric distances.

astro-ph.SR↗

arXiv2020

Particle-In-Cell simulation of whistler heat flux instabilities in the solar wind: heat flux regulation and electron halo formation

We present results of two-dimensional fully kinetic Particle-In-Cell simulation in order to shed light on the role of whistler waves in the scattering of strahl electrons and in the heat flux regulation in the solar wind. We model the electron velocity distribution function as initially composed of core and strahl populations as typically encountered in the near-Sun solar wind as observed by Parker Solar Probe. We demonstrate that, as a consequence of the evolution of the electron velocity distribution function, two branches of the whistler heat flux instability can be excited, which can drive whistler waves propagating in the direction oblique or parallel to the background magnetic field. First, oblique whistler waves induce pitch-angle scattering of strahl electrons, towards higher perpendicular velocities. This leads to the broadening of the strahl pitch angle distribution and hence to the formation of a halo-like population at the expense of the strahl. Later on, the electron velocity distribution function experiences the effect of parallel whistler waves, which contributes to the redistribution of the particles scattered in the perpendicular direction into a more symmetric halo, in agreement with observations. Simulation results show a remarkable agreement with the linear theory of the oblique whistler heat flux instability. The process is accompanied by a significant decrease of the heat flux carried by the strahl population.

astro-ph.SR↗

arXiv2020

Multi-beam Energy Moments of Multibeam Particle Velocity Distributions

High resolution electron and ion velocity distributions, f(v), which consist of N effectively disjoint beams, have been measured by NASA's Magnetospheric Multi-Scale Mission (MMS) observatories and in reconnection simulations. Commonly used standard velocity moments generally assume a single mean-flow-velocity for the entire distribution, which can lead to counterintuitive results for a multibeam f(v). An example is the (false) standard thermal energy moment of a pair of equal and opposite cold particle beams, which is nonzero even though each beam has zero thermal energy. By contrast, a multibeam moment of two or more beams has no false thermal energy. A multibeam moment is obtained by taking a standard moment of each beam and then summing over beams. In this paper we will generalize these notions, explore their consequences and apply them to an f(v) which is sum of tri-Maxwellians. Both standard and multibeam energy moments have coherent and incoherent forms. Examples of incoherent moments are the thermal energy density, the pressure and the thermal energy flux (enthalpy flux plus heat flux). Corresponding coherent moments are the bulk kinetic energy density, the RAM pressure and the bulk kinetic energy flux. The false part of an incoherent moment is defined as the difference between the standard incoherent moment and the corresponding multibeam moment. The sum of a pair of corresponding coherent and incoherent moments will be called the undecomposed moment. Undecomposed moments are independent of whether the sum is standard or multibeam and therefore have advantages when studying moments of measured f(v).

physics.plasm-ph↗

arXiv2020

Particle-In-Cell simulations of the parallel proton firehose instability influenced by the electron temperature anisotropy in solar wind conditions

In situ observations of the solar wind show a limited level of particle temperature anisotropy with respect to the interplanetary magnetic field direction. Kinetic electromagnetic instabilities are efficient to prevent the excessive growth of the anisotropy of particle velocity distribution functions. Among them, the firehose instabilities are often considered to prevent the increase of the parallel temperature and hence to shape the velocity distribution functions of electrons and protons in the solar wind. We present a non-linear modeling of the parallel firehose instability, retaining a kinetic description for both the electrons and protons. One-dimensional (1D) fully kinetic Particle-In-Cell simulations using the Energy Conserving semi-implicit method (ECsim) are performed to clarify the role of the electron temperature anisotropy in the development of the parallel proton firehose instability. We found that in the presence of an electron temperature anisotropy, such that the temperature parallel to the background magnetic field is higher than the temperature in the perpendicular direction, the onset of the parallel proton firehose instability occurs earlier and its growth rate is faster. The enhanced wave fluctuations contribute to the particle scattering reducing the temperature anisotropy to a stable, nearly isotropic state. The simulation results compare well with linear theory. A test case of 1D simulations at oblique angles with respect to the magnetic field is also considered, as a first step to study the cumulative effect of protons and electrons on the full spectrum of instabilities.

physics.plasm-ph↗

arXiv2019

Local regimes of turbulence in 3D magnetic reconnection

The process of magnetic reconnection when studied in Nature or when modeled in 3D simulations differs in one key way from the standard 2D paradigmatic cartoon: it is accompanied by much fluctuations in the electromagnetic fields and plasma properties. We developed a diagnostics to study the spectrum of fluctuations in the various regions around a reconnection site. We define the regions in terms of the local value of the flux function that determines the distance form the reconnection site, with positive values in the outflow and negative values in the inflow. We find that fluctuations belong to two very different regimes depending on the local plasma beta (defined as the ratio of plasma and magnetic pressure). The first regime develops in the reconnection outflows where beta is high and is characterized by a strong link between plasma and electromagnetic fluctuations leading to momentum and energy exchanges via anomalous viscosity and resistivity. But there is a second, low beta regime: it develops in the inflow and in the region around the separatrix surfaces, including the reconnection electron diffusion region itself. It is remarkable that this low beta plasma, where the magnetic pressure dominates, remain laminar even though the electromagnetic fields are turbulent.

astro-ph.SR↗

arXiv2019

Particle-in-cell simulations of the whistler heat-flux instability in the solar wind conditions

In collision-poor plasmas from space, e.g., solar wind or stellar outflows, the heat-flux carried by the strahl or beaming electrons is expected to be regulated by the self-generated instabilities. Recently, simultaneous field and particle observations have indeed revealed enhanced whistler-like fluctuations in the presence of counter-beaming populations of electrons, connecting these fluctuations to the whistler heat-flux instability (WHFI). This instability is predicted only for limited conditions of electron beam-plasmas, and was not captured in numerical simulations yet. In this letter we report the first simulations of WHFI in particle-in-cell (PIC) setups, realistic for the solar wind conditions, and without temperature gradients or anisotropies to trigger the instability in the initiation phase. The velocity distributions have a complex reaction to the enhanced whistler fluctuations conditioning the instability saturation by a decrease of the relative drifts combined with induced (effective) temperature anisotropies (heating the core electrons and pitch-angle and energy scattering the strahl). These results are in good agreement with a recent quasilinear approach, and support therefore a largely accepted belief that WHFI saturates at moderate amplitudes. In anti-sunward direction the strahl becomes skewed with a pitch-angle distribution decreasing in width as electron energy increases, that seems to be characteristic to self-generated whistlers and not to small-scale turbulence.

physics.plasm-ph↗

arXiv2019

Interaction between electrostatic collisionless shocks generates strong magnetic fields

The head-on collision between electrostatic shocks is studied via multi-dimensional Particle-In-Cell simulations. It is found that the shock velocities drop significantly and a strong magnetic field is generated after the interaction. This transverse magnetic field is due to the Weibel instability caused by pressure anisotropies due to longitudinal electron heating while the shocks approach each other. Finally, it is shown that this phenomenon can be explored in the laboratory with current laser facilities within a significant parameter range.

physics.plasm-ph↗

arXiv2019

A violin sonata for reconnection

The process of magnetic reconnection when studied in Nature or when modeled in 3D simulations differs in one key way from the standard 2D paradigmatic cartoon: it is accompanied by much fluctuations in the electromagnetic fields and plasma properties. We developed a new diagnostics, the topographical fluctuations analysis (TFA) to study the spectrum of fluctuations in the various regions around a reconnection site. We find that fluctuations belong to two very different regimes. The first regime is better known, it develops in the reconnection outflows and is characterized by a strong link between plasma and electromagnetic fluctuations leading to momentum and energy exchanges via anomalous viscosity and resistivity. But there is a second, new, regime: it develops in the inflow and in the region around the separatrix surfaces, including the reconnection diffusion region itself. In this new regime the plasma remains laminar but the electromagnetic fields fluctuates strongly. We present an analogy with the smooth continuous motion of the bow of a violin producing the vibrations of the strings to emit music.

physics.plasm-ph↗

arXiv2018

Properties of turbulence in the reconnection exhaust: numerical simulations compared with observations

The properties of the turbulence which develops in the outflows of magnetic reconnection have been investigated using self-consistent plasma simulations, in three dimensions. As commonly observed in space plasmas, magnetic reconnection is characterized by the presence of turbulence. Here we provide a direct comparison of our simulations with reported observations of reconnection events in the magnetotail investigating the properties of the electromagnetic field and the energy conversion mechanisms. In particular, simulations show the development of a turbulent cascade consistent with spacecraft observations, statistics of the the dissipation mechanisms in the turbulent outflows similar to the one observed in reconnection jets in the magnetotail, and that the properties of turbulence vary as a function of the distance from the reconnecting X-line.

physics.space-ph↗

arXiv2018

Generation of turbulence in colliding reconnection jets

The collision of magnetic reconnection jets is studied by means of a three dimensional numerical simulation at kinetic scale, in the presence of a strong guide field. We show that turbulence develops due to the jets collision producing several current sheets in reconnection outflows, aligned with the guide field direction. The turbulence is mainly two-dimensional, with stronger gradients in the plane perpendicular to the guide field and a low wave-like activity in the parallel direction. First, we provide a numerical method to isolate the central turbulent region. Second, we analyze spatial second-order structure function and prove that turbulence is confined in this region. Finally, we compute local magnetic and electric frequency spectra, finding a trend in the sub-ion range that differs from typical cases for which the Taylor hypothesis is valid, as well as wave activity in the range between ion and electron cyclotron frequencies. Our results are relevant to understand observations of reconnection jets collisions in space plasmas.

astro-ph.SR↗

arXiv2017

On the origin of the crescent-shaped distributions observed by MMS at the magnetopause

MMS observations recently confirmed that crescent-shaped electron velocity distributions in the plane perpendicular to the magnetic field occur in the electron diffusion region near reconnection sites at Earth's magnetopause. In this paper, we re-examine the origin of the crescent-shaped distributions in the light of our new finding that ions and electrons are drifting in opposite directions when displayed in magnetopause boundary-normal coordinates. Therefore, ExB drifts cannot cause the crescent shapes. We performed a high-resolution multi-scale simulation capturing sub-electron skin depth scales. The results suggest that the crescent-shaped distributions are caused by meandering orbits without necessarily requiring any additional processes found at the magnetopause such as the highly asymmetric magnetopause ambipolar electric field. We use an adiabatic Hamiltonian model of particle motion to confirm that conservation of canonical momentum in the presence of magnetic field gradients causes the formation of crescent shapes without invoking asymmetries or the presence of an ExB drift. An important consequence of this finding is that we expect crescent-shaped distributions also to be observed in the magnetotail, a prediction that MMS will soon be able to test.

physics.space-ph↗

arXiv2015

Particle Control in Phase Space by Global K-Means Clustering

We devise and explore an iterative optimization procedure for controlling particle populations in particle-in-cell (PIC) codes via merging and splitting of computational macro-particles. Our approach, is to compute an optimal representation of the global particle phase space structure while decreasing or increasing the entire particle population, based on k-means clustering of the data. In essence the procedure amounts to merging or splitting particles by statistical means, throughout the entire simulation volume in question, while minimizing a 6-dimensional total distance measure to preserve the physics. Particle merging is by far the most demanding procedure when considering conservation laws of physics; it amounts to lossy compression of particle phase space data. We demonstrate that our k-means approach conserves energy and momentum to high accuracy, even for high compression ratios, $\mathcal{R} \approx 3$ --- \emph{i.e.}, $N_{f} \lesssim 0.33N_{i}$. Interestingly, we find that an accurate particle splitting step can be performed using k-means as well; this from an argument of symmetry. The split solution, using k-means, places splitted particles optimally, to obtain maximal spanning on the phase space manifold. Implementation and testing is done using an electromagnetic PIC code, the \ppcode. Nonetheless, the k-means framework is general; it is not limited to Vlasov-Maxwell type PIC codes. We discuss advantages and drawbacks of this optimal phase space reconstruction.

astro-ph.IM↗

arXiv2015

Where should MMS look for electron diffusion regions?

A great possible achievement for the MMS mission would be crossing electron diffusion regions (EDR). EDR are regions in proximity of reconnection sites where electrons decouple from field lines, breaking the frozen in condition. Decades of research on reconnection have produced a widely shared map of where EDRs are. We expect reconnection to take place around a so called x-point formed by the intersection of the separatrices dividing inflowing from outflowing plasma. The EDR forms around this x-point as a small electron scale box nested inside a larger ion diffusion region. But this point of view is based on a 2D mentality. We have recently proposed that once the problem is considered in full 3D, secondary reconnection events can form [Lapenta et al., Nature Physics, 11, 690, 2015] in the outflow regions even far downstream from the primary reconnection site. We revisit here this new idea confirming that even using additional indicators of reconnection and even considering longer periods and wider distances the conclusion remains true: secondary reconnection sites form downstream of a reconnection outflow causing a sort of chain reaction of cascading reconnection sites. If we are right, MMS will have an interesting journey even when not crossing necessarily the primary site. The chances are greatly increased that even if missing a primary site during an orbit, MMS could stumble instead on one of these secondary sites.

physics.space-ph↗

arXiv2014

Signatures of Secondary Collisionless Magnetic Reconnection Driven by Kink Instability of a Flux Rope

The kinetic features of secondary magnetic reconnection in a single flux rope undergoing internal kink instability are studied by means of three-dimensional Particle-in-Cell simulations. Several signatures of secondary magnetic reconnection are identified in the plane perpendicular to the flux rope: a quadrupolar electron and ion density structure and a bipolar Hall magnetic field develop in proximity of the reconnection region. The most intense electric fields form perpendicularly to the local magnetic field, and a reconnection electric field is identified in the plane perpendicular to the flux rope. An electron current develops along the reconnection line in the opposite direction of the electron current supporting the flux rope magnetic field structure. Along the reconnection line, several bipolar structures of the electric field parallel to the magnetic field occur making the magnetic reconnection region turbulent. The reported signatures of secondary magnetic reconnection can help to localize magnetic reconnection events in space, astrophysical and fusion plasmas.

physics.plasm-ph↗