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Solar-Wind Structure and Turbulence

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Solar-Wind Structure and Turbulence Solar-Wind Structure and Turbulence Ben Chandran University of New Hampshire Heliophysics Summer School, UCAR, July, 2019 Outline I. 3D Structure of the Solar Wind II. Introduction to Turbulence III. Measurements of Solar-Wind Turbulence IV. Magnetohydrodynamic (MHD) Turbulence V. Reflection-Driven MHD Turbulence and the Origin of the Solar Wind. Collaborators • Dartmouth: B. Li, B. Rogers • Florida Institute of Technology: J. Perez • Oxford: A. Schekochihin • Princeton: L. Arzamasskiy, M. Kunz • U. Arizona: K. Klein • UC Berkeley: S. Bale, A. Mallet, , M. Pulupa, E. Quataert, C. Salem • University College London: S.-Y. Jeong, D. Verscharen • U. Michigan: J. Kasper, K. Klein, B. van der Holst • UNH: D. Borovikov, S. Chernoglazov, K. Germaschewski, J. Hollweg, P. Isenberg, I. Hoppock, M. Lee, S. Markovskii, B. Vasquez • Special acknowledgement: R. Kulsrud. What is the Solar Wind? (ESA) • Quasi-continuous, radial outflow of particles from the Sun • Fast (300 - 800 km/s), hot and dilute (105 K, 5 cm-3 at 1 AU) • Plasma: behaves like a fluid, but it generates and is in turn influenced by electromagnetic fields The Solar Wind in Relation to the Sun • it is the extension of the solar atmosphere • represents about 10-6 of the energy output of the Sun • mass loss rate is -14 -1 about 10 Msun yr (NASA) Solar-Wind Structure Depends on the Solar Cycle G. Piopol/stargazer.net • “Solar minimum” - very few sunspots. • “Solar maximum” - many sunspots, solar flares, and coronal mass ejections. 3D Structure Near Solar Minimum • Fast wind (700-800 km/s) is the basic state of the flow near solar minimum • Fast wind emanates primarily from open-field-line regions near the poles - “polar coronal holes” • Slow wind (300-500 km/s) is confined to low latitudes (less so at solar maximum) (McComas et al 2003) 3D Structure Near Solar Maximum Ulysses • much more complex • alternates between fast and slow wind at virtually all heliographic latitudes (McComas et al 2003) What Causes the Different Speeds of the Fast Wind and Slow Wind? (Cranmer 2005) • Near the Sun, the solar wind flows along magnetic flux tubes. • When the flux-tube cross section increases rapidly with increasing r, the outflow remains fairly slow in the low corona. This allows more heat to be conducted back to the Sun, removing energy from the wind, and reducing the asymptotic wind speed. Outline I. 3D Structure of the Solar Wind II. Introduction to Turbulence III. Measurements of Solar-Wind Turbulence IV. Magnetohydrodynamic (MHD) Turbulence V. Reflection-Driven MHD Turbulence and the Origin of the Solar Wind. What Is Turbulence? Operational definition for turbulence in plasmas and fluids: turbulence consists of disordered motions spanning a large range of length scales and/or time scales. “Energy Cascade” in Hydrodynamic Turbulence Canonical picture: larger eddies break up into smaller eddies ENERGY INPUT LARGE ENERGY CASCADE SMALL SCALES SCALES DISSIPATION OF FLUCTUATION ENERGY Plasma Turbulence Vs. Hydro Turbulence • In plasmas such as the solar wind, turbulence involves electric and magnetic fields as well as velocity fluctuations. • In some cases the basic building blocks of turbulence are not eddies but plasma waves or wave packets. Where Does Turbulence Occur? • Atmosphere (think about your last plane flight). • Oceans. • Sun, solar wind, interstellar medium, intracluster plasmas in clusters of galaxies... What Causes Turbulence? • Instabilities: some source of free energy causes the amplification of fluctuations which become turbulent. Example: convection in stars. • Stirring. Example: stirring cream into coffee. • Requirement: the medium can’t be too viscous. (Stirring a cup of coffee causes turbulence, but stirring a jar of honey does not.) What Does Turbulence Do? • Turbulent diffusion or mixing. Examples: cream in coffee, pollutants in the atmosphere. • Turbulent heating. When small-scale eddies (or wave packets) dissipate, their energy is converted into heat. Example: the solar wind. Outline I. 3D Structure of the Solar Wind II. Introduction to Turbulence III. Measurements of Solar-Wind Turbulence IV. Magnetohydrodynamic (MHD) Turbulence V. Reflection-Driven MHD Turbulence and the Origin of the Solar Wind. In Situ Measurements solar wind N T RTN coordinates R spacecraft (E.g., Helios, ACE, Wind, STEREO...) Quantities measured include v, B, E, n, and T Spacecraft Measurements of the Magnetic Field and Velocity A•,•v• WAvrs x• So•,Aa 3537 '• I I I I I I I I I • I . bR 0 m OVR -4 -25 +4 I , 25 -4 5 bN O. 0 vN -4 5 B N B o I ! I I I I I I I I I oN 8 12 16 20 24 TIME (HRS) Data from the Mariner 5 spacecraft (Belcher & Davis 1971) corrected for aberration due to the spacecraft ropy in the pressure.This requires that 4•r motion). The two lower curves on the plot are {•, -- p•_)/Bo• be 0.40. The averageduring proton number density and magnetic field this periodof (2kTo/m•)•%the most probable strength((B) = 5.3 % (N) = 5.4 cm-3).This protonvelocity, was observedto be 47 km/sec, period is one of the better examplesof the waves whichcorresponds to 4•'p•/B 0 • = 0.5, wherep• is and illustrates their most characteristic features: the mean proton pressure. With reasonable closecorrelation between b and v, variationsin b valuesof the electronand a pressuresand of the comparableto the field strength, and relatively pressureanisotropy [Hundhausen et al., 1967],the little variation in field strength or density. In requiredvalue of •a seemsentirely reasonable. this case the average magnetic field is inward On other occasionswhen /• = 8•rp•/Bo• is along the spiral and the correlation between smaller,values of • closerto unity wouldbe b and v is positive; when the magnetic field is expected. outward the correlationin periodsof goodwaves Wavesversus discontinuities. Figure 2 is an is negative. This indicatesoutward propagation expandedplot of three particular 10-rainperiods (seeequation 1). indicated on Figure 1, where the crossesare the The scaleratio used for plotting the magnetic basic magnetometer data (one reading in field and velocity variations in Figure i corre- approximately4 sec)and the linesare the plasma spondsto a value of D• -• of 6.4 km data (1 per 5.04 rain), scaledin the sameratio as This was determined by the condition that, in Figure 1. On this time scale,the wavesmay be when this ratio is used for a fixed area plot eithergradual (2b) or discontinuous(2a, 2c), with of v• versusb• for all the data, the stun of the abrupt changeswithin 4 sec.As discussedbelow, squares of the perpendicular distances from we feel that all three examplesare Alfv•nic, the points to a line of unit slope is minimized. with continuousmagnetic field lines, but with a (Mathematicallythis gives D• -• = discontinuity in direction in cases 2a and 2c. the ratio of the standard deviations.) The Suchabrupt changes occur at a rate of about i per average values of N and N, during this per- hour and are enmeshedin more gradual changes. iod are 5.4 and 0.4 cm-3, respectively; thus The visual appearanceof the field fluctuations equationi with • = i givesD• -• = 8.2 km is qualitatively different on the time scalesof sec-•/-F.We feel that the discrepancybetween Figuresi and 2. With the scaleused in Figure 2, this predictedvalue and the observedvalue of 6.4 the most prominent structuresare the abrupt is significantand probably is due to the anisot- changesthat tend to be precededand followedby A•,•v• WAvrs x• So•,Aa 3537 '• I I I I I I I I I • I . bR 0 m OVR -4 -25 +4 I , 25 Is This -4 5 Turbulence? bN O. 0 vN -4 5 B N B o I ! I I I I I I I I I oN 8 12 16 20 24 TIME (HRS) • The velocity and magnetic field in these measurements corrected for aberration due to the spacecraft ropy in the pressure.This requires that 4•r motion). The twoappear lower curves on theto plot fluctuate are {•, -- p•_)/Bo• inbe 0.40. a Therandom averageduring or disordered fashion. proton number density and magnetic field this periodof (2kTo/m•)•%the most probable strength((B) = 5.3 % (N) = 5.4 cm-3).This protonvelocity, was observedto be 47 km/sec, period is one of the better examplesof the waves whichcorresponds to 4•'p•/B 0 • = 0.5, wherep• is and illustrates their most characteristic features: the mean proton pressure. With reasonable closecorrelation • betweenBut b andhow v, variations doin b wevalues tellof the electron whetherand a pressures and there of the are “velocity comparableto the field strength, and relatively pressureanisotropy [Hundhausen et al., 1967],the little variation influctuations field strength or density. In spanningrequiredvalue of •a seems a entirelylarge reasonable. range of scales,” as in our this case the average magnetic field is inward On other occasionswhen /• = 8•rp•/Bo• is along the spiral and the correlation between smaller,values of • closerto unity wouldbe b and v is positive;operational when the magnetic field is definition expected. of turbulence? outward the correlationin periodsof goodwaves Wavesversus discontinuities. Figure 2 is an is negative. This indicatesoutward propagation expandedplot of three particular 10-rainperiods (seeequation 1). indicated on Figure 1, where the crossesare the The scaleratio One used for plotting way: the magnetic by basicexamining magnetometer data the(one reading power in spectrum of the field and• velocity variations in Figure i corre- approximately4 sec)and the linesare the plasma spondsto a value of D• -• of 6.4 km data (1 per 5.04 rain), scaledin the sameratio as This was determinedfluctuations. by the condition that, in Figure 1. On this time scale,the wavesmay be when this ratio is used for a fixed area plot eithergradual (2b) or discontinuous(2a, 2c), with of v• versusb• for all the data, the stun of the abrupt changeswithin 4 sec.As discussedbelow, squares of the perpendicular distances from we feel that all three examplesare Alfv•nic, the points to a line of unit slope is minimized.
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