1. 太空科學 Space Sciences Sunny W. Y. Tam ( 談永頤 ) Institute of Space, Astrophysical and Plasma Sciences, National Cheng Kung University 「太空科學與衛星系統工程」 之 October 16, 2012

2. Outline Importance of missions to space sciences Research methods, research roles and collective research efforts An example of interactions in research: Solar wind –Basic knowledge of space physics and plasma physics

3. Importance of Missions to Space Sciences

4. Field of Space Sciences Study of the regions in space influenced by the magnetic fields of the Earth or the Sun –The solar-terrestrial (Sun-Earth) environment Extending to planetary studies within the solar system

5. The Solar-Terrestrial System Picture from Danish Meteorological Institute Interplanetary Space Solar Wind Magnetosphere

6. IONOSPHERE (white circle) From Physics of Space Plasmas: an Introduction by George K. Parks

7. Origination of Space Sciences The field was motivated by human curiosity to the Sun, auroral sightings, and the variations of the Earth’s magnetic field Photo by Mark Evans Aurora

8. Origination of Space Sciences Early studies around the first half of the 18 th century “Solar-terrestrial physics” ( 日地物理 ) instead of “space physics” ( 太空物理 ) or “space sciences” ( 太空科學 ) has been exclusively used as the name of the field until about half a century ago

9. Question: Why was the word “space” instead of “solar-terrestrial” eventually used? –Extension in the region of interest to farther away, e.g. Van Allen radiation belts and the geomagnetic tail, as a result of our capability of space exploration Space exploration has been playing a virtual role in the advancement of space sciences.

10. Research Methods, Research Roles and Collective Research Efforts

11. Goals –To explore the extended Sun-Earth space environment and to understand the reasons for the various phenomena in the system Primary branch of science –Plasma physics (more than 99.9% of known materials in space is in plasma state) Space physics Space Science Research

12. Research Methods & Different Roles of Space Scientists Methods 1.Observation 2.Theory Roles 1.Experimentalist 2.Data Analyst 3.Theoretician 4.Simulationist

13. 1.Experimentalist Build instruments to collect data in space 2.Data Analyst Analyze experimental data Look for anomalous features or behaviors Check for predicted features and behaviors 3.Theoretician Provide explanations for new observations Theorize unobserved features or behaviors 4.Simulationist Design and perform computer simulations to verify theories Try simulations to match new observations

14. Collective Efforts in Space Science Research The usual sequence of the involvement by the various kinds of space scientists in a research problem Observations Experimentalists Data Analysts Theories Theoreticians Simulationists Time Mission Approval of Mission Observation-guided research

15. Collective Efforts in Space Science Research A different sequence. A famous example is the research on the solar wind. Observations Experimentalists Data Analysts Theories Theoreticians/ Simulationists Time Mission Approval of Mission Theoreticians/Simulationists Theory Modifications Theory-guided research

16. An Example of Interactions in Research: Solar Wind

17. Classical Theory of the Formation of the Solar Wind Existence of solar wind suggested by Eugene Parker in 1958 Pressure difference suggested as the physical mechanism to drive the outflow from the Sun (pressure- driven solar wind) (No magnetic field in the model) [Astrophysical Journal, November 1958]

18. Luna 1: First Direct Observation of the Solar Wind Soviet spacecraft launched on January 2, 1959 Confirmed the existence of the solar wind A “failed” mission Failure in that it became an “artificial planet” orbiting around the Sun, while the original plan was for it to return to the Earth Luna 1 (Picture from nssdc.gsfc.nasa.gov)

19. The Helios Mission: Vast Contribution to Solar Wind Research Collaborative mission by US and West Germany Two spacecraft covering a wide range of positions in the ecliptic plane, reaching as close as 0.29 AU from the Sun Launch dates: December 10, 1974 (Helios 1) and January 15, 1976 (Helios 2) Large quantity of solar wind measurements One of the Helios space probes (Picture from www.ieap.uni-kiel.de)

20. Helios Measurements of Solar Wind Proton Velocity Distributions [Marsch et al., 1982] Solar wind of various speeds Proton velocity distributions in solar wind of lower speeds have more resemblance to Maxwellian distributions

21. Basic Knowledge of Space Physics and Plasma Physics Relevant to the Solar Wind

22. Motion of charged particles in uniform B field B Left-handed gyration Right-handed gyration ion (acceleration by Lorentz force) B Ion (positive charge) Guiding center

23. Let. Force-free motion parallel to the magnetic field Note that: Lorentz force alone does not change the kinetic energy of the particle

24. (Angular) Frequency of gyration,, known as gyrofrequency or cyclotron frequency Cyclotron period, Gyroradius (also known as Lamour radius), Note that: (1)Ω does not depend on the particle velocity, but r L does. (2)Particles of the same species have the same Ω.

25. 3-Dimensional Motion of charged particles in uniform B field B Motions of charged particles in the direction perpendicular to B are restricted due to gyration. In the direction parallel to B, charged particles respond to other forces (such as those due to electric and gravitational fields) as if the magnetic field were absent. The above is true also for non-uniform B (inhomogenous magnetic field). Helical ( 螺旋的 ) trajectory  Therefore, plasma flow is primarily along the magnetic field lines.

26. Fast Solar Wind as a Plasma Outflow Flow of plasma along open, inhomogeneous magnetic field lines originating from the Sun Adapted from www.sp.ph.ic.ac.uk Solar Magnetic Field open field line (fast solar wind) closed field line SOLAR WIND major ions: protons (H + ), alpha particles (He 2+ )

27. Situation with Inhomogeneous B Field 1.Conservation of magnetic moment For any charged particle, its magnetic moment is an adiabatic invariant ( 絕熱不變量 ) Conservation is no longer true if the particle is involved in non-adiabatic processes, such as heating due to waves

28.  If B is large enough such that, then the variation of the magnetic field in the perpendicular directions is not important (negligible perpendicular drift). 2.With non-uniform B, the variation of the magnetic field in the perpendicular direction is characterized by the scale length : The dependence of the Lamour radius with B is: B

29.  If B is large enough such that, then it is valid to use the guiding center to approximate a particle’s location (guiding-center approximation). The transition time scale, T tran, characterizes the time for a particle to travel the distance of a scale length: 3.The scale length in the parallel direction is characterized by : where s represents the distance along the field line. The dependence of the cyclotron period with B is:

30. General properties of solar outflow justify the following assumptions and approximations in theoretical research: Transport and dynamics important only in the direction parallel to B: one-dimensional plasma flow along the magnetic field line Gyrotropic assumption: no preferential direction in the plane transverse to the magnetic field Guiding-center approximation Steady state Quasi-neutrality: charge neutrality at the considered length scales no net electrical current (guided by observations)

31. Helios Observations: Experimental Indications of the Inadequacy of the Classical Solar Wind Theory

32. Inconsistency between Helios Observations and Classical Solar Wind Theory 1.Preferential acceleration of alpha particles compared with protons [Marsch et al., 1982] where is the alpha particle (He 2+ ) speed, and is the proton (H + ) speed Preferential acceleration of alpha particles, more apparent at higher solar wind speeds

33. Polarization electric field as a primary ion acceleration mechanism Upward velocity Ignore the effect of gravity, ion and electron have the same upward velocity ion Downward gravitational force stronger on the ion, charge separation would occur g An upward electric field arises to prevent the charge separation, maintaining quasi- neutrality in the plasma E g

34. In the classical theory, electric field is the primarily acceleration mechanism. In that case, at far away from the Sun: where is the total electric potential difference between the solar wind source and the particle’s location downstream. The average speed of a species should be proportional to its charge-to-mass ratio.  u α > u p but (q/m) α < (q/m) p, contrary to the prediction of the classical theory of the solar wind

35. 2.Anisotropic Protons with Higher Perpendicular Temperature at the Core Anisotropy at the core of the velocity distributions (wider contours in perpendicular direction) -- indication of perpendicular ion heating Inconsistency between Helios Observations and Classical Solar Wind Theory [Marsch et al., 1982]

36. In the classical theory, there is no particle heating (i.e. adiabatic). If Coulomb collisions were absent, a particle’s magnetic moment would be conserved as it flows out to large radial distances from the Sun. When a particle encounters a decrease in the magnetic field as it flows out, its, in principle, should decrease. The kinetic energy in the perpendicular directions is transferred to the parallel direction in the process, which is known as the mirror effect. In the absence of heating, the mirror effect suggests that the velocity spread (or temperature) in the perpendicular directions should be small compared with that in the parallel direction.

37. Strong Coulomb collisional effects may at most make the perpendicular temperature equal to the parallel temperature, but not higher.  Therefore, the observed anisotropy in the ion distributions suggests the presence of perpendicular heating of the ions.

38. Inconsistency between Helios Observations and Classical Solar Wind Theory 3.Double-peaked proton velocity distributions frequently observed at various heliocentric distances [Marsch et al., 1982]

39. Modifications of Theory of the Fast Solar Wind

40. Non-classical Theory of the Fast Solar Wind ( U SW ≥ 600 km/s ) Outflow not driven by plasma pressure gradient, but by turbulence at the solar wind source Wave-particle resonant interaction as a solar wind acceleration mechanism Ion Cyclotron Resonant Heating (ICRH), i.e. resonant heating by left-handed polarized (LHP) waves near the ion cyclotron frequency, as a probable candidate for the perpendicular ion heating

41. Left-handed polarized (LHP) wave Let ω be the frequency of the wave. Its period is T = 2π/ω. Let. In the figure, the polarization of is such that the magnitude of E x and E y are equal, with the phase of E y leading by 1/4 period. As a result, the vector traces out a circle in the orientation same as the fingers of a left hand with the thumb pointing in the direction of B. B, z x y In a given plane z = z 0

42. LHP waves have the same orientation of rotation as ions. If an ion encounters an LHP wave that rotates at the same frequency as itself, it will “see” the same phase of the electric field of the wave for an extended period, resulting in wave-particle resonant interaction. If the ion and the wave are in phase (ion perpendicular velocity and the electric field in the same direction), ion energy will increase. If they are out of phase ( and in opposite directions), ion energy will decrease. In general, Nevertheless, if the ions are evenly distributed over all phases, the overall ion perpendicular energy increases, hence the mechanism of ion cyclotron resonant heating. (in phase) (out of phase)

43. Resonance (in phase): Non-resonance Resonance (out of phase):

44. For ions evenly distributed over all phases, the overall ion perpendicular energy increases in the case of ion cyclotron resonance, hence the mechanism of ion cyclotron resonant heating.

45. Different ions (even of the same species) resonate with waves of different frequencies because those ions move with different speeds relative to the waves. Importance of kinetic effects Kinetic Ion Cyclotron Resonance Heating

46. Ion Cyclotron Resonance Recall that, the protons therefore have higher cyclotron frequency than the alpha particles. The LHP waves in resonance with the alpha particles generally have lower frequencies. Observations show that more wave power is available for cyclotron resonance at lower frequencies May preferentially energize the alpha particles Magnetic field spectral density (wave power) observed by Helios 2 [Bavassano et al., 1982] Power-law relationship

47. Acceleration of the Fast Solar Wind due to Kinetic Ion Cyclotron Resonant Heating Resonance between ions and LHP waves Ions heated, overall perpendicular kinetic energy increases Ion KE converted from perpendicular to parallel direction by the mirror effect as the plasma flows out: Ion acceleration Theory

48. Acceleration of the Fast Solar Wind due to Kinetic Ion Cyclotron Resonant Heating Simulations Assumptions on have to be made due to a lack of information about the turbulence: power spectra of the turbulence throughout the solar wind flow, including at the source (modeling) the fraction of LHP waves in the turbulence at given frequencies (a fixed parameter assumed)

49. Results of Simulation for the Fast Solar Wind Preferential acceleration of the alpha particles over the protons due to kinetic ion cyclotron resonance; heating in the transverse direction but mirror force converts the energy into the parallel direction. [Tam and Chang, 1999]

50. Results of Simulation for the Fast Solar Wind Formation of double-peaked proton velocity distribution [Tam and Chang, 1999]

51. Theory of Kinetic ICRH for the Fast Solar Wind Consistency with observations –Preferential acceleration of alpha particles over protons –Double-peaked proton velocity distributions Room for Improvement –Anisotropy at the proton core Given the uncertainty in the properties of the turbulence --- the most crucial element of the simulation, the theory seemed to be a step in the right direction

52. Summary Missions are essential for the advancement of space sciences Interactions among space scientists in different roles help solve research problems As an example, the research on the solar wind has demonstrated the importance of interactions among space scientists