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

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

Testing THEMIS wave measurements against the cold plasma theory

International audienceThe THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission records a multitude of electromagnetic waves inside Earth's magnetosphere and provides data in the form of high-resolution electric and magnetic waveforms. We use multi-component measurements of whistler mode waves and test them against the theory of wave propagation in a cold plasma. The measured ratio cB/E (c is speed of light in vacuum, B is magnetic wave amplitude, E is electric wave amplitude) is compared to the same quantity calculated from cold plasma theory over linearized Faraday's law. The aim of this study is to get estimates for measurement uncertainties, especially with regard to the electric field and the cold plasma density, as well as evaluating the validity of cold plasma theory inside Earth's radiation belts

Poynting vector and wave vector directions of equatorial chorus

International audienceWe present new results on wave vectors and Poynting vectors of chorus rising and falling tones on the basis of 6 years of THEMIS (Time History of Events and Macroscale Interactions during Substorms) observations. The majority of wave vectors is closely aligned with the direction of the ambient magnetic field (B0). Oblique wave vectors are confined to the magnetic meridional plane, pointing away from Earth. Poynting vectors are found to be almost parallel to B0. We show, for the first time, that slightly oblique Poynting vectors are directed away from Earth for rising tones and toward Earth for falling tones. For the majority of lower band chorus elements, the mutual orientation between Poynting vectors and wave vectors can be explained by whistler mode dispersion in a homogeneous collisionless cold plasma. Upper band chorus seems to require inclusion of collisional processes or taking into account azimuthal anisotropies in the propagation medium. The latitudinal extension of the equatorial source region can be limited to ±6o around the B0 minimum or approximately ±5000 km along magnetic field lines. We find increasing Poynting flux and focusing of Poynting vectors on the B0 direction with increasing latitude. Also, wave vectors become most often more field aligned. A smaller group of chorus generated with very oblique wave normals tends to stay close to the whistler mode resonance cone. This suggests that close to the equatorial source region (within 20o latitude), a wave guidance mechanism is relevant, for example, in ducts of depleted or enhanced plasma density

Poynting flux analysis of whistler-mode chorus using THEMIS data

International audienceWe present a statistical analysis of Poynting fluxes for magnetospheric whistler-mode chorus emission observed on THEMIS (Time History of Events and Macroscale Interactions during Substorms). Results are shown for chorus rising and falling tones. The aim of this study is to clearly distinguish between chorus from equatorial source regions and from sources at higher magnetic latitudes, i.e., from dayside pockets of minimum magnetic field. Furthermore, the topic of magnetospherically reflected chorus as a possible process to produce falling tones will be discussed

Saturnian Low-Frequency Drifting Radio Bursts: Statistical Properties and Polarization. Planetary Radio Emissions| PLANETARY RADIO EMISSIONS VII 7|

After Cassini’s arrival at planet Saturn, its Radio and Plasma Wave Science (RPWS) experiment has performed numerous observations of a new type of planetary radio emissions in the lower kHz frequency range (< 50kHz). These bursty emissions have time scales of a few to 15 minutes and occur as slowly drifting events in the time-frequency spectrogram. They have neither been detected by the Voyager spacecraft nor by Ulysses. As a first approach to this new phenomenon, results of a statistical study with regard to the observer’s position, i.e. Cassini’s orbital position, will be presented. Furthermore, aspects of polarization will be highlighted as far as appropriate goniopolarimetric (3-antenna) observations are available

Local Time Occurrence of Solar Type III Bursts at Saturn’s Orbit. Planetary Radio Emissions| PLANETARY RADIO EMISSIONS VII 7|

We report on solar radio bursts observed by the RPWS experiment on board the Cassini spacecraft in the period from 1st January 2004 to 31st March 2010. In this time intervals of about six years a limited number of strong solar type III bursts, less than 300, has been recorded. This is mainly due to the solar activity which reaches its minimum in 2008–2009. In this study we consider type III solar bursts observed at frequencies lower than 1.2 MHz generated in the interplanetary medium. We analyse the solar bursts with the aim to estimate the Cassini local time (LT) occurrence rate, where the Kronian day has been divided into eight LT sectors. Our results are combined with the Cassini orbits where the LT and the distance to the planet are taken into consideration. We show that the type III burst occurrence rates depend on the solar activity, however the day side sector (midday to early afternoon) exhibits the lowest rate of occurrence