Introduction to the Sun and the Sun-Earth System

Transcription

1 Introduction to the Sun and the Sun-Earth System Robert Fear 1,2 1 Space Environment Physics group University of Southampton 2 Radio & Space Plasma Physics group University of Leicester

2 The solar-terrestrial system Corona is so hot that the Sun s gravity cannot hold it down it flows outwards as the solar wind A break-down of Alfvén s theorem is sufficient to drive the dynamics of the magnetosphere Alfvén s theorem states that plasma and magnetic field are tied Sun s magnetic field carried into heliosphere to form IMF Alfvén s theorem also means that plasmas of different origins cannot mix the solar wind and Earth s environment are segregated

3 Part 1 Basics of space plasma physics and How this determines the structure of the magnetosphere

4 Part 2 The solar wind is highly variable and consequently so is the Earth s geomagnetic activity

5 Plasma gas how to make a plasma Heat the gas (solar corona) proton electron plasma Photoionisation (ionosphere)

6 Moving charges make currents - + The result is a + collective motion of the charges and the magnetic field: the frozen-in theory Currents generate magnetic fields The magnetic field modifies the trajectories of the charges

7 The Sun 6,000 K The Sun is not just a ball of hot gas, but is a highly dynamic plasma threaded by constantlyvarying electromagnetic fields The Sun converts 4 million tonnes of its mass into photons every second - sunlight It also blows off 1 million tonnes per second from the corona to form the solar wind

8 Density and temperature profile Solar structure

9

10

11 After Babcock (1961)

12

13

14

15

16

17 Earth radii The magnetospheric cavity magnetosphere The Earth s magnetic field and plasma environment provide an impenetrable obstacle to the outward flow of the solar wind The dipolar magnetic field of the Earth is distorted by the impinging solar wind Earth radii Inside the magnetic field strength is greater than in the solar wind, but the plasma density is much lower: the magnetosphere is a cavity Solar wind Outer magnetosphere Magnetic field strength 7 nt nt Particle density 7 cm cm -3

18 Magnetic field lines are distorted: currents must flow

19 Chapman-Ferraro currents As the solar wind compresses the magnetosphere a current layer must form B dipole B sheet solar wind magnetosphere undisturbed dipole field B dipole falls off as r -3 current sheet field j magnetopause Ampére s Law: curl B 0 j electron proton B For field strength to (almost) cancel out in solar wind B sheet B dipole Thus, just inside magnetopause the field strength is compressed to 2B dipole

20 The location of the boundary (the magnetopause) is determined by magnetic pressure on the inside and particle ram pressure on the outside ram (dynamic) pressure = momentum crossing unit area in unit time P m V nv nm V 2 dyn p p If solar wind n = 7 cm -3, and V = 450 km s -1, then P dyn = 2.5 npa (cf. you blow with a dynamic pressure of ~1 Pa) The mass striking the dayside magnetopause (assuming radius of ~10 R E ) is ~60 kg s -1 The kinetic energy carried by these protons is ~6x10 12 W (cf. sunlight falling on Earth s surface ~10 17 W)

21 A magnetic field exerts a pressure equal to P mag B B dipole falls off as r -3, so P mag falls off as r -6 The magnetosphere compresses until the magnetic pressure just inside the magnetopause balances the solar wind ram pressure At the nose of the magnetosphere the dipole field must be compressed to a field strength of ~60 nt to give Pmag = 2.5 npa This occurs where the magnetopause is pushed in to a stand-off distance of ~10 R E Away from the nose, the solar wind strikes the magnetopause obliquely, so the normal component of the ram pressure decreases Hence the magnetosphere flares outwards

22 Not all magnetospheres are created equal The size of a magnetosphere depends on: - the strength of the magnetic field of the planet - the ram pressure of the solar wind Sun Jupiter is 5 times further from the Sun than the Earth, so the solar wind pressure is reduced by a factor of 1/25 The magnetic field of Jupiter is over 10 times stronger than the Earth s Jupiter s magnetosphere is 5 times larger than the Sun

23

24 Earth radii The magnetotail These calculations explain the shape of the near-earth magnetosphere Overall the magnetosphere should be rain-drop shaped, but is observed to have a long tail, perhaps 1000 R E or more in length This indicates that a viscous-like interaction must take place between the solar wind and the magnetopause to stretch it into a magnetotail : Solar wind-magnetosphere coupling Earth radii ~1/20 AU

25 Solar wind-magnetosphere coupling: Magnetic reconnection In most solar system environments magnetic fields are frozen to the plasma - different plasmas cannot mix At thin boundaries the frozen-in approximation can break down, leading to magnetic reconnection and plasma, momentum and energy exchange between otherwise segregated regions

26 What happens when the solar wind encounters Earth? If the IMF is southward Bow shock Magnetopause

27 The open magnetosphere Open flux Magnetic reconnection results in an open magnetosphere Closed flux Where reconnection occurs on the magnetopause depends on the relative orientation between the incoming interplanetary magnetic field (IMF) and field lines at the magnetopause

28 Location of reconnection IMF B z < 0, B y = 0 IMF B z > 0, B y > 0 Reconnection with closed field lines Reconnection with open field lines

29 The Dungey cycle: The open magnetosphere Sun Interplanetary Magnetic Field [IMF] Solar wind flow Magnetic flux is opened

30 The Dungey cycle: The open magnetosphere Sun Interplanetary Magnetic Field [IMF] Solar wind flow Magnetic flux is opened

31 The Dungey cycle: The open magnetosphere Sun Interplanetary Magnetic Field [IMF] Solar wind flow Magnetic flux is opened

32 The Dungey cycle: The open magnetosphere Sun Interplanetary Magnetic Field [IMF] Solar wind flow Magnetic flux is opened

33 The Dungey cycle: The open magnetosphere Sun Interplanetary Magnetic Field [IMF] Solar wind flow Magnetic flux is opened Open flux is closed

34 The Dungey cycle: The open magnetosphere Sun Interplanetary Magnetic Field [IMF] Solar wind flow Magnetic flux is opened Open flux is closed

35 The Dungey cycle: The open magnetosphere Sun Interplanetary Magnetic Field [IMF] Solar wind flow Magnetic flux is opened Open flux is closed

36 Closed The Dungey cycle Aurora at the footprint of these field lines are the signature of plasma entry due to reconnection Open Closed Sun Magnetic flux is opened Open flux is closed

37 Closed The Dungey cycle Open Closed Sun Magnetic flux is opened Open flux is closed

38 Closed The Dungey cycle Open Closed Sun Magnetic flux is opened Open flux is closed

39 Closed The Dungey cycle Open Closed Sun Magnetic flux is opened Open flux is closed

40 Closed The Dungey cycle Tail reconnection occurs explosively in a process known as the substorm - Earth s most intense aurorae occur here Open Closed Sun Magnetic flux is opened Open flux is closed

41 Closed The Dungey cycle Open Closed Sun Magnetic flux is opened Open flux is closed

42 Closed The Dungey cycle Open Closed Sun Magnetic flux is opened Open flux is closed

43 Closed The Dungey cycle Open Closed Sun Magnetic flux is opened Open flux is closed

44 Closed The Dungey cycle Open Closed

45

46 The Cluster mission

47 EISCAT Ionospheric radars SPEAR CUTLASS and SuperDARN

48 Evidence for the Dungey cycle Evidence for reconnection in the magnetosphere (Dungey cycle) includes: Geomagnetic activity (auroral displays and magnetic field activity) correlates with southward IMF (B Z < 0) Accelerated flows seen at magnetopause Voltage associated with convection increases for southward IMF Dayside magnetopause erodes and magnetotail flares when IMF southward Magnetosheath ions and electrons gain access in the cusp ions show dispersion feature, and range of energies indicates extended source (open magnetosphere) and much more

49 Closed How long is the magnetotail? Distance across polar cap ~1 R E Ionospheric convection of order hundreds m s -1 Open Closed Solar wind speed of order 1000 times larger Therefore geomagnetic tail ~ 1000 R E (Dungey, 1965)

50

51 The disconnected tail Cowley (1991) Sun Solar wind flow Disconnected field lines unkink at ~1.2 V SW Disconnected tail is ~5 times longer than connected tail ( R E, or 0.2 AU)

52 Closed How long does this all take? Using similar arguments to Dungey (1965): Distance across polar cap ~1 R E Open Closed Ionospheric convection of order hundreds m s -1 Time for field line to convect from dayside reconnection site to nightside reconnection site comes out at ~4 hours

53 Corotation The rotation of the planet also imparts momentum to the magnetospheric plasma Ionospheric plasma is frictionally coupled to the neutral atmosphere The magnetic field lines, frozen to this plasma, attempt to rotate with the planet In turn, the magnetospheric plasma is frozen to the corotating magnetic field

54 Dungey cycle Corotation

55 Plasma populations in the magnetosphere magnetosheath The solar wind (mainly H + and e - ) populates the hot, low density (~ 1 cm -3 ) plasma sheet This is in pressure balance with the very low density (~0.01 cm -3 ) lobes

56 Plasma populations in the magnetosphere The ionosphere populates the cold, high density (~ 100 cm-3) plasmasphere (say, O+ and e-) Outside of this region, very high energy particles comprise the Van Allen belts

57 Plasma populations in the magnetosphere Magnetosheath Plasmasphere Plasma sheet Ring current

58

59

60

61

62

63

64 aurora borealis The auroral ovals (aurora polaris) aurora australis

65 Substorms

66 The theta aurora (transpolar arc) The dynamic auroral oval

67 IMAGE FUV IMAGE data courtesy of Stephen Mende, Harald Frey and the IMAGE FUV team

68 Figure courtesy Milan et al. (2012)

69 The shape of the magnetosphere F open = B I A I = B lobe A lobe P mag = B 2 /2µ 0 B I lobe plasma sheet B lobe B lobe lobe The shape of the magnetosphere is determined by pressure balance with the out-flowing solar wind The magnetic field is compressed until the magnetic pressure balances the normal stress exerted by the solar wind ram pressure The magnetosphere is most compressed at the sub-solar point and flares out as the solar wind strikes at grazing incidence

70 Convection flows Not existence of open flux per se which generates flows it is the creation/destruction of open flux (Cowley & Lockwood, 1992) Dayside reconnection removes flux from day side and adds it to lobe/polar cap area of polar cap increases Results in non-aerodynamic shape of magnetopause Solar wind pressure acts to restore aerodynamic shape B B B Plasma sheet B Cowley & Lockwood (1992)

71 Convection flows Not existence of open flux per se which generates flows it is the creation/destruction of open flux (Cowley & Lockwood, 1992) Nightside reconnection removes flux from lobe/polar cap area of polar cap decreases Pressure balance acts to restore If dayside & nightside reconnection rates equal steady state

72 Polar cap convection: Non-steady-state Faraday (1831) Siscoe and Huang (1985) Cowley and Lockwood (1992) df PC dt = Φ D Φ N

73 The auroral substorm

74 5 June 1998 Substorm Substorm F PC 0.9 GWb 0.6 GWb 0.3 GWb 0.0 GWb

75 Other planets have aurora, too