111 research outputs found
Modification of Proton Velocity Distributions by Alfvenic Turbulence in the Solar Wind
In the present paper, the proton velocity distribution function (VDF)in the
solar wind is determined by solving numerically the kinetic evolution equation.
We compare the results obtained when considering the effects of ex- ternal
forces and Coulomb collisions with those obtained by adding effects of Alfven
wave turbulence. We use Fokker-Planck diffusion terms due to Alfvenic
turbulence, which take into account observed turbulence spectra and kinetic
effects of finite proton gyroradius. Assuming a displaced Maxwellian for the
proton VDF at the simulation boundary at 14 solar radii, we show that the
turbulence leads to a fast (within several solar radii) development of the
anti-sunward tail in the proton VDF. Our results provide a natural explanation
for the nonthermal tails in the proton VDFs, which are often observed in-situ
in the solar wind beyond 0.3 AU
Dynamics of Megaelectron Volt Electrons Observed in the Inner Belt by PROBA-V/EPT
Using the observations of the EPT (Energetic Particle Telescope) onboard the
satellite PROBA-V we study the dynamics of inner and outer belt electrons from
500 keV to 8 MeV during quiet periods and geomagnetic storms. This high
time-resolution (2 sec) spectrometer operating at the altitude of 820 km on a
low polar orbit is providing continuously valuable electrons fluxes for already
5 years. We emphasize especially that some MeV electrons are observed in low
quantities in the inner belt, even during periods when they are not observed by
Van Allen Probe (VAP). We show that they are not due to proton contamination
but to clear injections of particles from the outer belt during strong
geomagnetic storms of March and June 2015, and September 2017. Electrons with
lower energy are injected also during less strong storms and the L-shell of the
electron flux peak in the outer belt shifts inward with a high dependence on
the electron energy. With the new high resolution EPT instrument, we can study
the dynamics of relativistic electrons, including MeV electrons in the inner
radiation belt, revealing how and when such electrons are injected into the
inner belt and how long they reside there before being scattered into the
Earth's atmosphere or lost by other mechanisms
Velocity-Space Proton Diffusion in the Solar Wind Turbulence
We study a velocity-space quasilinear diffusion of the solar wind protons
driven by oblique Alfven turbulence at proton kinetic scales. Turbulent
fluctuations at these scales possess properties of kinetic Alfven waves (KAWs)
that are efficient in Cherenkov resonant interactions. The proton diffusion
proceeds via Cherenkov kicks and forms a quasilinear plateau - nonthermal
proton tail in the velocity distribution function (VDF). The tails extend in
velocity space along the mean magnetic field from 1 to (1.5-3) VA, depending on
the spectral break position, turbulence amplitude at the spectral break, and
spectral slope after the break. The most favorable conditions for the tail
generation occur in the regions where the proton thermal and Alfven velocities
are about the same, VTp/VA = 1. The estimated formation times are within 1-2 h
for typical tails at 1 AU, which is much shorter than the solar wind expansion
time. Our results suggest that the nonthermal proton tails, observed in-situ at
all heliocentric distances beyond 0.3 AU, are formed in the solar wind locally
by the KAW turbulence. We also suggest that the bump-on-tail features - proton
beams, often seen in the proton VDFs, can be formed at a later evolution stage
of the nonthermal tails by the time-of-flight effects
Acceleration of the Solar Wind by Ambipolar Electric Field
Kinetic exospheric models revealed that the solar wind is accelerated by an
ambipolar electric field up to supersonic velocities. The presence of
suprathermal Strahl electrons at the exobase can further increase the velocity
to higher values, leading to profiles comparable to the observations in the
fast and slow wind at all radial distances. Such suprathermal electrons are
observed at large distances and recently at low distances as well. Those
suprathermal electrons were introduced into the kinetic exospheric model using
Kappa distributions. Here, the importance of the exobase's altitude is also
underlined for its ability to maintain the electric potential to a higher level
for slower winds, conversely to what is induced through the effect of a lower
kappa index only. In fact, the exobase is located at lower altitude in the
coronal holes where the density is smaller than in the other regions of the
corona, allowing the wind originating from the holes to be accelerated from
lower distances to higher velocities. The new observations of Parker Solar
Probe (PSP) and Solar Orbiter (SolO) from launch to mid-2023 are here used to
determine the characteristics of the plasma in the corona so that the model
fits best to the averaged observed profiles for the slow and fast winds. The
observations at low radial distances show suprathermal electrons already well
present in the Strahl in the antisunward direction and a deficit in the sunward
direction, confirming the exospheric feature of almost no incoming particles.Comment: 10 pages, 7 figures, solar wind 16 conference proceedin
Interfacing MHD Single Fluid and Kinetic Exospheric Solar Wind Models and Comparing Their Energetics
An exospheric kinetic solar wind model is interfaced with an observation-driven single-fluid magnetohydrodynamic (MHD) model. Initially, a photospheric magnetogram serves as observational input in the fluid approach to extrapolate the heliospheric magnetic field. Then semi-empirical coronal models are used for estimating the plasma characteristics up to a heliocentric distance of 0.1 AU. From there on, a full MHD model that computes the three-dimensional time-dependent evolution of the solar wind macroscopic variables up to the orbit of Earth is used. After interfacing the density and velocity at the inner MHD boundary, we compare our results with those of a kinetic exospheric solar wind model based on the assumption of Maxwell and Kappa velocity distribution functions for protons and electrons, respectively, as well as with in situ observations at 1 AU. This provides insight into more physically detailed processes, such as coronal heating and solar wind acceleration, which naturally arise from including suprathermal electrons in the model. We are interested in the profile of the solar wind speed and density at 1 AU, in characterizing the slow and fast source regions of the wind, and in comparing MHD with exospheric models in similar conditions. We calculate the energetics of both models from low to high heliocentric distances.Peer reviewe
Electron-driven instabilities in the solar wind
The electrons are an essential particle species in the solar wind. They often exhibit non-equilibrium features in their velocity distribution function. These include temperature anisotropies, tails (kurtosis), and reflectional asymmetries (skewness), which contribute a significant heat flux to the solar wind. If these non-equilibrium features are sufficiently strong, they drive kinetic micro-instabilities. We develop a semi-graphical framework based on the equations of quasi-linear theory to describe electron-driven instabilities in the solar wind. We apply our framework to resonant instabilities driven by temperature anisotropies. These include the electron whistler anisotropy instability
and the propagating electron firehose instability. We then describe resonant instabilities driven by reflectional asymmetries in the electron distribution function. These include the electron/ion-acoustic, kinetic Alfv\'en heat-flux, Langmuir, electron-beam, electron/ion-cyclotron, electron/electron-acoustic, whistler heat-flux, oblique fast-magnetosonic/whistler, lower-hybrid fan, and electron-deficit whistler instability. We briefly comment on non-resonant instabilities driven by electron temperature anisotropies such as the mirror-mode and the non-propagating firehose instability. We conclude our review with a list of open research topics in the field of electron-driven instabilities in the solar wind
Solar Wind Electron Transport: Interplanetary Electric Field and Heat Conduction
The presence of suprathermal electrons has important consequences on the acceleration process of the solar wind: they increase the electrostatic potential between the corona and the interplanetary space and accelerate of the solar wind to high bulk velocities. Moreover, they modify the heat conduction and can explain the sharp increase of the temperature in the solar corona. These consequences are well evidenced in the kinetic approach where no closure requires the distributions to be nearly Maxwellian
- …