La simulation auto-cohérente de l'interaction entre un vent solaire turbulent et différents corps du système solaire est rendue possible grace à un nouveau solveur hybrid-PIC multi-GPU et une approche spécifique en deux étapes. Ce nouveau type de simulation et ses premiers résultats sont ici présentés, en utilisant l'exemple d'une comète de moyenne activité. Cette première application démontre le potentiel de cette approche pour aller au-delà de l'hypothèse classique d'un vent laminaire en amont de l'obstacle. Nous démontrons comment les fluctuations du champ magnetique évoluent lors de l'interaction avec l'atmosphère cométaire, résultant en des structures dans la queue cométaire autrement absente dans le cas d'un vent laminaire. La magnétosphère est réduite en taille, et la région la plus interne de la coma fortement réduite en densité. L'applicabilité de cette approche est discutée pour d'autre corps du système solaire, en particulier Mercure et Mars.

Parker Solar Probe (PSP) data recorded within a heliocentric radial distance of 0.3~AU have revealed a magnetic field dominated by Alfvénic structures that undergo large local variations or even reversals of the radial magnetic field. They are called magnetic switchbacks, they are consistent with folds in magnetic field lines within a same magnetic sector, and are associated with velocity spikes during an otherwise calmer background. They are thought to originate either in the low solar atmosphere through magnetic reconnection processes, or result from the evolution of turbulence or velocity shears in the expanding solar wind. In this work, we investigate the temporal and spatial characteristic scales of magnetic switchback patches. We define switchbacks as a deviation from the nominal Parker spiral direction and detect them automatically for PSP encounters 1, 2, 4 and 5. We focus in particular on a 5.1-day interval dominated by switchbacks during E5. We perform a wavelet transform of the solid angle between the magnetic field and the Parker spiral and find periodic spatial modulations with two distinct wavelengths, respectively consistent with solar granulation and supergranulation scales. In addition we find that switchback occurrence and spectral properties seem to depend on the source region of the solar wind rather than on the radial distance of PSP. These results suggest that switchbacks are formed in the low corona and modulated by the solar surface convection pattern.

Small-scale magnetic fields in the quiet Sun have been shown to contain in total more flux than active regions and represent an important reservoir of magnetic energy. But the origin and evolution of these fields still remain largely unknown. We present a study of the solar-cycle and center-to-limb variations of the magnetic-flux structures at small scales in the solar internetwork. We used Hinode SOT/Spectropolarimetric data from the irradiance program  from 2008 to 2016 and applied a deconvolution to the intensity and polarization profiles to correct them from the smearing due the Point Spread Function of the telescope. Then we performed a spectral analysis of the spatial fluctuations of the magnetic-flux density in 10''x10'' internetwork regions spanning a wide range of latitudes. At low and mid latitudes the power spectra do not vary significantly with the solar cycle, except in some enhanced network regions at solar maximum where we detect a marginal increase of the power of the fluctuations at granular scales. At high latitudes variations in opposition of phase with the solar cycle are observed at granular scales. Whatever the latitude the power of the magnetic fluctuations at scales smaller than 0.5'' remain constant throughout the solar cycle. These results are in favor of a small-scale dynamo that operates in the internetwork.

Recent MMS observations (e.g. [Holmes et al, 2018, Steinvall et al., 2019]) exploring various regions of the magnetosphere have found solitary potential structures call Electron phase-space Hole (EH). These structures have kinetic scale (dozens of Debye lengths) and persist during long time (dozens of plasma frequency periods). EH are characterized by a bipolar electric field parallel to ambient magnetic field and fastly propagate along this latter (a few tenths of speed light). We have created a 3D Bernstein-Greene-Kruskal (BGK) model (as [Chen et al, 2004]) adapted to various magnetospheric ambient magnetic fields. BGK model results depend on choice of potential shape and passing distribution function at infinity (before EH potential interaction).

2D-3V Particle-In-Cell simulations have been developed with the fully kinetic code Smilei [Derouillat et al, 2017], using real magnetosphere plasma parameters. Solitary waves in the magnetotail are cylindrical three-dimensional potentials which can be generated through nonlinear evolution of an electron beam instability (or bump on tail). The simulated EH are comparable to the EH observed in the magnetosphere with the same parameters.

We have also investigated the EH formation with density inhomogeneities using a BGK stability model we have developed. Indeed, density inhomogeneities exist notably in interplanetary plasmas. As a result taking into account the background density inhomogeneities, significantly alters the stabilty criteria. We have performed 2D-3V PIC simulations with realistic inhomogeneous density background (smaller than 10% of mean density) to understand such a type of EH formation.

We compared the free decay of spectral evolution during ten nonlinear times, successively with and without expansion, in the quasi-incompressible limit. In this case one has two separated cascades, resp. for the dominant energy (Alfvén waves coming from the Sun) and sub-dominant one (reflected waves). Results are as follows.

(1) Without expansion we find the when the ratio $b_{rms}/B_0$ is not too small, the system adopts rapidly the Alfvénic version of the Iroshnikov-Kraichnan phenomenology (weak isotropic turbulence), corresponding to spectra ``pinned'' at small scales with the sum of the two spectral indices being -3 (Grappin Pouquet Léorat 1983).

(2) With expansion, turbulence evolves much more slowly away from its initial form (and not at all if the initial spectrum is short-ranged, Grappin, SolarWind8 1996). The two spectra are quasi-parallel, the asymptotic scaling being determined by the initial one. The final index is -5/3 if the initial index is in between -3/2 and -5/3. If the initial index is closer to -1, the asymptotic spectrum will be flatter, with index between -1.4 and -1.55, depending on the initial index.

Several of these properties are observed in the solar wind, in particular the steepening with distance as found in both Helios and PSP missions. It remains to explain these properties, how expansion can change evolution so strongly, and undestand what the systematic change of spectral index with distance implies in terms of turbulent heating.

The Earth's near-space environment is very dynamic with transient phenomena triggered by interaction between the solar wind and the Earth's magnetopause. The solar wind carries along an interplanetary magnetic field (IMF) whose orientation determines the dynamics of the interaction. When the IMF is southward, magnetic reconnection can be triggered at the Earth's magnetopause on the dayside. A flux transfer event (FTE) is a transient portal that allows the bursty transfer of solar wind into the Earth's magnetosphere. An FTE is envisaged as a twisted magnetic field structure with helical field that looks like a rope. We study the relationship between the IMF orientations and the twist direction of an ensemble of FTEs observed by NASA's Magnetospheric Multiscale mission (MMS) and ESA’s Cluster by modelling FTEs as magnetic flux ropes. We found that the majority of the flux rope twist is controlled by the IMF orientation, such that the rope is twisted in the left-handed or right-handed sense depending on the east-west component of the IMF. This result supports the formation of FTEs by a multiple reconnection mechanism. However, we also found a significant population in which the flux rope twist is not expected from such model. In this talk, we will discuss alternative, possible FTE formation mechanisms and the importance of magnetic helicity studies in the magnetosphere.

The Nançay Radioheliograph is dedicated to imaging the solar corona at decimetre-to-metre wavelengths. The imaged structures are the quiet corona, through thermal bremsstrahlung, and bright collective emissions due to electrons accelerated in quiescent, flaring and eruptive active regions. The instrument produced nearly daily maps of the Sun between 1996 and 2015, at several frequencies in the 150-450 MHz range with sub-second cadence. The observations were stopped in 2015 for a major technical upgrade through the replacement of the correlator and the data acquisition system. They were resumed in November 2020. This contribution will give a brief overview of the technical changes and present observations at eight frequencies of solar activity since November 2020, including bursts produced by electron beams and bursts associated with coronal mass ejections. Emphasis will be laid on observations conducted during the two perihelion passages of the Parker Solar Probe mission in January and April, as well as during periods of interest to the Solar Orbiter mission.

We will review the very latest observations and results obtained by the Radio and Plasma Waves (RPW) Instrument on the recently launched Solar Orbiter mission. RPW is designed to measure in-situ magnetic and electric fields and waves from 'DC' to a few hundreds of kHz. RPW is also capable of measuring solar radio emissions up to 16 MHz and link them to solar flares observed by the onboard remote sensing instruments. These latest results concern a wide range of phenomena including: Whistler Waves, dust impacts, Langmuir waves, Type III bursts and observations during the recently visited Venus environment.

Authors: Q. Noraz, A.S. Brun,  A. Strugarek

Abstract:

The solar magnetic field is generated and sustained through an internal dynamo. In stars, this process is determined by the combined action of turbulent convective motions and the differential rotation profile. It can sometimes lead to magnetic cyclic variabilities, like in the Sun with the 11 years cycle. Traces of magnetic cycles have been detected for other solar-like stars as well, ranging from a few years to a few tens of years. How are these cycles controlled? During their life, the rotation of stars is subject to complex evolution. Recent 3D numerical simulations of solar-like stars show that different regimes of differential rotation can be characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby number (slow rotators). If this regime occurs during the main sequence, and in general for slow rotators, we may wonder how the magnetic generation through dynamo process will be impacted, more especially if our Sun is in such a transition. In particular, can slowly rotating stars have magnetic cycles?
We present a numerical multi-D study with the STELEM and ASH codes to understand the magnetic field generation of solar-like stars under various differential rotation regimes, and focus on the existence of magnetic cycles.

We find that short cycles are favoured for small Rossby numbers (fast rotators), and long cycles for intermediate (solar-like) Rossby numbers. Slow rotators (high Rossby number) are found to produce only steady dynamo with no cyclic activity in most cases considered. Magnetic cycles can be produced with anti-solar differential rotation only if the alpha effect is fine tuned for this purpose.

A detection of magnetic cycles for such stars (or lack of thereof) would therefore be a tremendous constrain on deciphering what type of dynamo is actually acting in the Sun and solar-like stars. In addition, the future out-of-the-ecliptic observations of Solar Orbiter will give fantastic constraints on the magnetic field of the solar polar caps, and thus on which type of dynamo our Sun is the most likely to operate. Combined together, these will allow to better understand the physics in action in the complex present-day solar dynamo.

The 3D MHD modeling is a powerful tool to investigate the physical process responsible for the formation and evolution of the heliosphere and the solar wind. To fully understand the role of each physical ingredient making the model correct, we need a validation process where the output from the simulations are compared to the data.
In this work, we present the results from the validation process of a global 3D MHD modeling of the solar corona where the heating originates from wave turbulence. We analyze three simulations results which differ for the initial electron density and plasma velocity.
We use the density and temperature 3D distributions, output from the simulations, to produce synthetic UV an WL pB images. For these tests we selected AIA 193 UV emission, MLSO K-Cor and LASCO C2 pB images during 6 and 7 November 2018. We then make quantitative comparisons of the disk and off limb corona. Our tests clearly identify the set of parameters that provides the best quantitative reproduction of the observed corona.

B. Perri (1), A. S. Brun (2), A. Strugarek (2), V. Réville (3) 

(1) CmPA, KU Leuven (2) AIM, CEA (3) IRAP

Though generated deep inside the convection zone, the solar magnetic field has a direct impact on the Earth space environment via various mechanisms. In particular, it strongly modulates the solar wind in the whole heliosphere: observations have shown that the 11-year cycle created by the dynamo inside the Sun affects the latitudinal speed distribution of the solar wind over the years. However, the wind also influences the topology of the coronal magnetic field by opening the magnetic field lines in the coronal holes, which can affect the inner magnetic field of the star by altering the dynamo boundary conditions. This coupling is especially difficult to model because it covers a large variety of spatio-temporal scales. Quasi-static studies have begun to help us unveil this how the dynamical dynamo magnetic field shapes the wind. Nevertheless, the full interplay between the solar dynamo and the solar wind still eludes our understanding.

We use the compressible magnetohydrodynamical code PLUTO to compute simultaneously in 2.5D the generation and evolution of magnetic field inside the star via an alpha-omega dynamo process and the corresponding evolution of the corona over a dynamo cycle. A multi-layered internal boundary condition at the surface of the star connects the inner and outer stellar layers, allowing both to adapt and update in real time. We focus on young suns to test the parameter range in which the coupling is effective, with dynamo cycle periods between 5 weeks and 5 days. Our coupled dynamo-wind model allows us to characterize how the solar wind conditions change as a function of the cycle phase, and also to quantify the evolution of the Alfvén surface, mass and angular momentum losses with the changing dynamo field. We further assess for the first time the impact of the solar wind on the dynamo itself by testing different levels of feedback and comparing them to no feedback at all. Finally, we characterize the exchange of information between inner and outer stellar layers in terms of helicity.

This gives us a new tool to better understand the Sun-Earth connection at various moments of the solar cycle in a space weather perspective and to eventually extend it to other stars with different rotation and level of activity.

Global MHD models of the corona and solar wind are a key tool for the interpretation of the new generation of inner heliospheric spacecraft's data. They can be used as a self-consistent framework to validate both the sources of the solar wind plasma measured in situ by Parker Solar Probe and Solar Orbiter and a given theory of the solar wind birth and acceleration. Over the past decade, several Alfvén wave turbulence driven MHD models have been developed for this purpose with varying levels of complexity. We present here such a model which has been able to reproduce most of PSP in situ large scale measurements. We show that in addition to the wind speed, density and magnetic field polarity, the amplitude of the perturbations is well recovered. This suggests that switchbacks, which have been observed to be a significant part of the magnetic field perturbations in the first PSP encounters, are fully part of the solar wind turbulence. Full MHD models are also very important to understand the global dynamics of the corona. For instance, using a simplified 2.5D configuration of our model, we have been able to explain some periodicities of flux ropes generated at the tip of helmet streamers through a tearing instability. We finally look at the creation of such flux rope events in 3D simulations and compare with the data of PSP encounter 5 and Solar Orbiter first perihelion.

In certain structures of the solar corona such as active regions for example we see an enhancement in the abundances of elements with a low First Ionization Potential or low FIP elements. This is known as the FIP effect. Because this is taking place only in some structures and because we also see these abundance enhancements in the solar wind, the FIP effect can allow us to trace back the source of heliospheric plasma. Elemental abundances are however difficult to measure.

We present a way to re-analyze EIS/Hinode observations in order to obtain FIP biases for more that half the available EIS data. By using the Linear Combination Ratio method developed to obtain relative FIP biases from optimized linear combinations of lines, we can use only a few spectral lines to obtain these measurements. This method is fast and simple, it is easy to automate. With as little as four lines we can make some relative FIP bias measurements. It is DEM independent but still accurate. 

This will allow us to go back and take a look at the abundances even when the observations carried out were not intended for it in the first place. We could study the distribution of relative FIP biases in different solar structures such as active regions or coronal hole boundaries. We could use some catalogs of events. Something that could be very interesting is looking at changes in abundances during the solar cycle, the link between magnetic activity and abundances and, of course, to pinpoint the source regions of different solar wind structures.