The role of magnetic handedness in magnetic cloud propagation

We investigate the propagation of magnetic clouds (MCs) through the inner heliosphere using 2.5-D ideal magnetohydrodynamic (MHD) simulations. A numerical solution is obtained on a spherical grid, either in a meridional plane or in an equatorial plane, by using a Roe-type approximate Riemann solver in the frame of a finite volume approach. The structured background solar wind is simulated for a solar activity minimum phase. In the frame of MC propagation, special emphasis is placed on the role of the initial magnetic handedness of the MC\u27s force-free magnetic field because this parameter strongly influences the efficiency of magnetic reconnection between the MC\u27s magnetic field and the interplanetary magnetic field. Magnetic clouds with an axis oriented perpendicular to the equatorial plane develop into an elliptic shape, and the ellipse drifts into azimuthal direction. A new feature seen in our simulations is an additional tilt of the ellipse with respect to the direction of propagation as a direct consequence of magnetic reconnection. During propagation in a meridional plane, the initial circular cross section develops a concave-outward shape. Depending on the initial handedness, the cloud\u27s magnetic field may reconnect along its backside flanks to the ambient interplanetary magnetic field (IMF), thereby losing magnetic flux to the IMF. Such a process in combination with a structured ambient solar wind has never been analyzed in detail before. Furthermore, we address the topics of force-free magnetic field conservation and the development of equatorward flows ahead of a concave-outward shaped MC. Detailed profiles are presented for the radial evolution of magnetoplasma and geometrical parameters. The principal features seen in our MHD simulations are in good agreement with in-situ measurements performed by spacecraft. The 2.5-D studies presented here may serve as a basis under more simple geometrical conditions to understand more complicated effects seen in 3-D simulations

Self-consistent formation of a 0.5 cyclotron frequency gap in magnetospheric whistler-mode waves

Decades of in-situ observations of whistler-mode waves in Earth's magnetosphere reveal that there is frequently a gap in the spectral power at around half the local electron gyrofrequency. Recent theoretical and kinetic simulation studies have suggested that the gap arises due to the presence of temperature anisotropy in both a ``warm' and a ``hot' electron population, leading to two separate (and independent) regions of wave growth in frequency space. We present two-dimensional kinetic plasma simulations using the powerful EPOCH particle-in-cell code that offer an alternative explanation. After an initial linear-growth period, our simulations show self-consistent formation of a gap feature. In most cases this arises where linear theory predicts the maximum growth rate, and is associated with subtle local structuring of the hot electron distributions. This feature persists in multiple simulations with varying hot electron parameters. We discuss these results in the context of in-situ observations of both waves and electron distribution functions and argue that the rapid reorganisation of electron distributions in a small, but key, region of phase-space during the growth of whistler-mode waves naturally results in the spectral gap often observed at half the electron gyrofrequency

Observations of Radiation Belt Losses Due to Cyclotron Wave-Particle Interactions

Electron loss to the atmosphere plays a critical role in driving dynamics of the Earths Van Allen radiation belts and slot region. This is a review of atmospheric loss of radiation belt electrons caused by plasma wave scattering via Doppler-shifted cyclotron resonance. In particular, the focus is on observational signatures of electron loss, which include direct measurements of precipitating electrons, measured properties of waves that drive precipitation, and variations in the trapped population resulting from loss. We discuss wave and precipitation measurements from recent missions, including simultaneous multi-payload observations, which have provided new insight into the dynamic nature of the radiation belts

Magnetic clouds in the solar wind: A numerical assessment study of analytical models

Magnetic clouds (MCs) are "magnetized plasma clouds" moving in the solar wind. MCs transport magnetic flux and helicity away from the Sun. These structures are not stationary but feature temporal evolution as they propagate in the solar wind. Simplified analytical models are frequently used for the description of MCs, and fit certain observational data well. The goal of the present study is to investigate numerically the validity of an analytical model which is widely used for the description of MCs, and to determine under which conditions this model's implied assumptions cease to be valid. A numerical approach is applied. Analytical solutions that have been derived in previous studies are implemented in a \textbf{3-D magnetohydrodynamic} simulation code as initial conditions. Initially, the analytical model represents the main observational features of the MCs. However, these characteristics prevail only if the structure moves with a velocity close to the velocity of the background flow. In this case an MC's evolution can quite accurately be described using an analytic, self-similar approach. The dynamics of the magnetic structures which move with a velocity significantly above or below that of the velocity of the solar wind is investigated in detail. Besides the standard case in which MCs only expand and propagate in the solar wind, the case of an MC rotating around its axis of symmetry is also considered, and the resulting influence on the MC's dynamics is studied

Equatorial electron loss by double resonance with oblique and parallel intense chorus waves

International audiencePuzzling satellite observations of butterfly pitch angle distributions and rapid dropouts of 30–150 keV electrons are widespread in the Earth’s radiation belts. Several mechanisms have been proposed to explain these observations, such as enhanced outward radial diffusion combined withmagnetopause shadowing or scattering by intense magnetosonic waves, but their effectiveness is mainly limited to storm times. Moreover, the scattering of 30–150 keV electrons via cyclotron resonance with intense parallel chorus waves should be limited to particles with equatorial pitch angle smaller than 70∘–75∘, leaving unaffected a large portion of the population. In this paper, we investigate the possible effects of oblique whistler mode waves, noting, in particular, that Landau resonance with very oblique waves can occur up to ∼89∘. We demonstrate that such very oblique chorus waves with realistic amplitudes can very efficiently nonlinearly transport nearly equatorially mirroring electrons toward smaller pitch angleswhere nonlinear scattering (phase bunching) via cyclotron resonance with quasi-parallel waves can take over and quickly send them to much lower pitch angles <40∘. The proposed double resonance mechanism could therefore explain the formation of butterfly pitch angle distributions as well as contribute to some fast dropouts of 30–150 keV electrons occurring during moderate geomagnetic disturbances at L = 4–6. Since 30–150 keV electrons represent a seed population for a further acceleration to relativistic energies by intense parallel chorus waves during storms or substorms, the proposed mechanism may have important consequences on the dynamics of 100 keV to MeV electron fluxes in the radiation belts

Electron diffusion and advection during nonlinear interactions with whistler‐mode waves

Radiation belt codes evolve electron dynamics due to resonant wave‐particle interactions. It is not known how to best incorporate electron dynamics in the case of a wave power spectrum that varies considerably on a ‘sub‐grid' timescale shorter than the computational time‐step of the radiation belt model ΔtRBM, particularly if the wave amplitude reaches high values. Timescales associated with the growth rate of thermal instabilities are very short, and are typically much shorter than ΔtRBM. We use a kinetic code to study electron interactions with whistler‐mode waves in the presence of a thermally anisotropic background. For ‘low' values of anisotropy, instabilities are not triggered and we observe similar results to those obtained in Allanson et al. (2020, https://doi.org/10.1029/2020JA027949), for which the diffusion roughly matched the quasilinear theory over short timescales. For ‘high' levels of anisotropy, wave growth via instability is triggered. Dynamics are not well described by the quasilinear theory when calculated using the average wave power. Strong electron diffusion and advection occurs during the growth phase ( ≈ 100ms). These dynamics ‘saturate' as the wave power saturates at ≈ 1nT, and the advective motions dominate over the diffusive processes. The growth phase facilitates significant advection in pitch angle space via successive resonant interactions with waves of different frequencies. We suggest that this rapid advective transport during the wave growth phase may have a role to play in the electron microburst mechanism. This motivates future work on macroscopic effects of short‐timescale nonlinear processes in radiation belt modelling

The role of magnetic handedness in magnetic cloud propagation

We investigate the propagation of magnetic clouds (MCs) through the inner heliosphere using 2.5-D ideal magnetohydrodynamic (MHD) simulations. A numerical solution is obtained on a spherical grid, either in a meridional plane or in an equatorial plane, by using a Roe-type approximate Riemann solver in the frame of a finite volume approach. The structured background solar wind is simulated for a solar activity minimum phase. In the frame of MC propagation, special emphasis is placed on the role of the initial magnetic handedness of the MC's force-free magnetic field because this parameter strongly influences the efficiency of magnetic reconnection between the MC's magnetic field and the interplanetary magnetic field. Magnetic clouds with an axis oriented perpendicular to the equatorial plane develop into an elliptic shape, and the ellipse drifts into azimuthal direction. A new feature seen in our simulations is an additional tilt of the ellipse with respect to the direction of propagation as a direct consequence of magnetic reconnection. During propagation in a meridional plane, the initial circular cross section develops a concave-outward shape. Depending on the initial handedness, the cloud's magnetic field may reconnect along its backside flanks to the ambient interplanetary magnetic field (IMF), thereby losing magnetic flux to the IMF. Such a process in combination with a structured ambient solar wind has never been analyzed in detail before. Furthermore, we address the topics of force-free magnetic field conservation and the development of equatorward flows ahead of a concave-outward shaped MC. Detailed profiles are presented for the radial evolution of magnetoplasma and geometrical parameters. The principal features seen in our MHD simulations are in good agreement with in-situ measurements performed by spacecraft. The 2.5-D studies presented here may serve as a basis under more simple geometrical conditions to understand more complicated effects seen in 3-D simulations

Estimation of the Chorus Group Velocity from THEMIS Wave Observations

International audienceChorus waves can play an important role for the energy budget of Earth's radiation belts. Some nonlinear analytical models describing chorus generation and energy transfer between waves and energetic electrons need the wave group velocity as a model parameter. The group velocity is the propagation velocity of the amplitude envelope of a wave packet. Theoretically, its absolute value is derived from a derivative of frequency (om) with respect to wave number (k), i.e. v_g = dₒm/dₖ. It is difficult, if not impossible, to infer this quantity directly from electromagnetic wave observations in space. We propose to take a "detour" over the Poynting velocity (vₚ), which is related to the Poynting vector (S) and the average wave energy density (W). Both, S and W can be inferred from a combination of electric and magnetic signals measured by triaxial antenna systems. We demonstrate the concept and show first results from an application to chorus observations made by the THEMIS spacecraft