University:
Boston University
Course:
CAS AS 311 | Planetary Physics
Academic year:

2022
Views:

81

Pages:

14
Author:

Hjerkinnckh4

Outline I. Introduction to Turbulence II. Measurements of Turbulence in the Solar Wind. III. Magnetohydrodynamics (MHD) IV. Alfven Waves, the Origin of the Solar Wind, and Solar Probe Plus V. Introduction to phenomenological theories of MHD turbulence. Coronal Heating and Solar-Wind Acceleration by Alfven Waves wave reflection He++ k´ k k5 Sun k4 k3 wave launching k1 k2 wave coupling & turbulence SOLAR WIND H+ e- wave-particle interactions field lines (Lee/Donahue/Chandran) 1. Alfven Waves are launched by the Sun and transport energy outwards 2. The waves become turbulent, which causes wave energy to ‘cascade’ from long wavelengths to short wavelengths 3. Short-wavelength waves dissipate, heating the corona and launching the solar wind. Coronal Heating and Solar-Wind Acceleration by Alfven Waves wave reflection He++ k´ k k5 Sun k4 k3 wave launching k1 k2 wave coupling & turbulence SOLAR WIND H+ e- wave-particle interactions field lines (Lee/Donahue/Chandran) 1. Alfven Waves are launched by the Sun and transport energy outwards 2. The waves become turbulent, which causes wave energy to ‘cascade’ from long wavelengths to short wavelengths 3. Short-wavelength waves dissipate, heating the corona and launching the solar wind. Mechanisms for Launching Waves 1. Magnetic reconnection (e.g., Axford & McKenzie 1992) 2. Motions of the footpoints of coronal magnetic field lines (e.g., Cranmer & van Ballegooijen 2005) Alfven Waves in the Chromosphere (Data from Hinode - DePontieu et al 2007) • calculated Alfven-wave energy flux is sufficient to power solar wind • Alfven waves propagating away from the Sun are also observed in the corona (Tomczyk et al 2007). Coronal Heating and Solar-Wind Acceleration by Alfven Waves wave reflection He++ k´ k k5 Sun k4 k3 wave launching k1 k2 wave coupling & turbulence SOLAR WIND H+ e- wave-particle interactions field lines (Lee/Donahue/Chandran) 1. Alfven Waves are launched by the Sun and transport energy outwards 2. The waves become turbulent, which causes wave energy to ‘cascade’ from long wavelengths to short wavelengths 3. Short-wavelength waves dissipate, heating the corona and launching the solar wind. Analogy to Hydrodynamics Canonical picture: larger eddies break up into smaller eddies ENERGY INPUT LARGE SCALES ENERGY CASCADE SMALL SCALES DISSIPATION OF FLUCTUATION ENERGY Coronal Heating and Solar-Wind Acceleration by Alfven Waves wave reflection He++ k´ k k5 Sun k4 k3 wave launching k1 k2 wave coupling & turbulence SOLAR WIND H+ e- wave-particle interactions field lines (Lee/Donahue/Chandran) 1. Alfven Waves are launched by the Sun and transport energy outwards 2. The waves become turbulent, which causes wave energy to ‘cascade’ from long wavelengths to short wavelengths 3. Short-wavelength waves dissipate, heating the corona and launching the solar wind. Not viscosity, but some type of collisionless dissipation. • Farther from the Sun, AW turbulence likely plays an important role in heating the solar wind and determining the solarwind temperature profile. For reasons of time, I won’t go into this in detail. The following references are a starting point for learning more about this: Cranmer et al (2009), Vasquez et al (2007). Returning to the question of the solar wind’s origin, how can we determine if Alfven-wave turbulence really is responsible for generating the solar wind? Returning to the question of the solar wind’s origin, how can we determine if Alfven-wave turbulence really is responsible for generating the solar wind? New frontier in heliospheric physics: in situ measurements of the solar-wind acceleration region... Solar Probe Plus 1. Several passes to within 10 solar radii of Sun. 2. Planned launch: 2018. 3. In situ measurements will greatly clarify the physics responsible for the solar wind’s origin. FIELDS Experiment on Solar Probe Plus Electric-field antennae Boom for two flux-gate magnetometers and one search-coil magnetometer FIELDS will provide in-situ measurements of electric fields and magnetic fields in the solar-wind acceleration region SWEAP Experiment on Solar Probe Plus SWEAP will provide in-situ measurements of densities, flow speeds (including velocity fluctuations), and temperatures for electrons, alpha particles, and protons.

Alfven Waves, the Origin of the Solar

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