Introduction to the Sun and the Sun-Earth System Robert Fear1,2 [email protected] 1 Space Environment Physics group University of Southampton 2 Radio & Space Plasma Physics group University of Leicester The solar-terrestrial system Corona is so hot that the Sun’s gravity cannot hold it down – it flows outwards as the solar wind Alfvén’s theorem states that plasma and magnetic field are tied – Sun’s magnetic field carried into heliosphere to form IMF A break-down of Alfvén’s theorem is sufficient to drive the dynamics of the magnetosphere Alfvén’s theorem also means that plasmas of different origins cannot mix – the solar wind and Earth’s environment are segregated Part 1 Basics of space plasma physics and How this determines the structure of the magnetosphere Part 2 The solar wind is highly variable… … and consequently so is the Earth’s geomagnetic activity Plasma gas plasma proton electron how to make a plasma Heat the gas (solar corona) Photoionisation (ionosphere) 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 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 Density and temperature profile Solar structure After Babcock (1961) The magnetospheric cavity • 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 • Inside the magnetic field strength is greater than in the solar wind, but the plasma density is much lower: the magnetosphere is a cavity Earth radii magnetosphere Earth radii Magnetic field strength Particle density Solar wind 7 nT 7 cm-3 Outer magnetosphere 20-60 nT 0.01-1 cm-3 Magnetic field lines are distorted: currents must flow Chapman-Ferraro currents As the solar wind compresses the magnetosphere a current layer must form Bdipole Bsheet undisturbed dipole field Bdipole falls off as r -3 solar wind magnetosphere j current sheet field Ampére’s Law: curl B 0 j magnetopause electron B proton For field strength to (almost) cancel out in solar wind Bsheet ≈ Bdipole Thus, just inside magnetopause the field strength is “compressed” to 2Bdipole 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 Pdyn m pV nV nm pV 2 If solar wind n = 7 cm-3, and V = 450 km s-1, then Pdyn = 2.5 nPa (cf. you blow with a dynamic pressure of ~1 Pa) The mass striking the dayside magnetopause (assuming radius of ~10 RE) is ~60 kg s-1 The kinetic energy carried by these protons is ~6x1012 W (cf. sunlight falling on Earth’s surface ~1017 W) A magnetic field exerts a pressure equal to 2 Pmag B 20 Bdipole falls off as r -3, so Pmag 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 RE 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 Not all magnetospheres are created equal The size of a magnetosphere depends on: - the strength of the magnetic field of the planet Sun - the ram pressure of the solar wind 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 The magnetotail These calculations explain the shape of the near-Earth magnetosphere Earth radii Overall the magnetosphere should be rain-drop shaped, but is observed to have a long “tail”, perhaps 1000 RE 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 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 What happens when the solar wind encounters Earth? Bow shock If the IMF is southward… Magnetopause 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 Location of reconnection IMF Bz < 0, By = 0 IMF Bz > 0, By > 0 Reconnection with closed field lines Reconnection with open field lines The Dungey cycle: The open magnetosphere Interplanetary Magnetic Field [IMF] Sun Magnetic flux is “opened” Solar wind flow The Dungey cycle: The open magnetosphere Interplanetary Magnetic Field [IMF] Sun Magnetic flux is “opened” Solar wind flow The Dungey cycle: The open magnetosphere Interplanetary Magnetic Field [IMF] Sun Magnetic flux is “opened” Solar wind flow The Dungey cycle: The open magnetosphere Interplanetary Magnetic Field [IMF] Sun Magnetic flux is “opened” Solar wind flow The Dungey cycle: The open magnetosphere Interplanetary Magnetic Field [IMF] Solar wind flow Sun Magnetic flux is “opened” “Open” flux is “closed” The Dungey cycle: The open magnetosphere Interplanetary Magnetic Field [IMF] Solar wind flow Sun Magnetic flux is “opened” “Open” flux is “closed” The Dungey cycle: The open magnetosphere Interplanetary Magnetic Field [IMF] Solar wind flow Sun Magnetic flux is “opened” “Open” flux is “closed” The Dungey cycle Closed Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Aurora at the footprint of these field lines are the signature of plasma entry due to reconnection Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Closed The Dungey cycle Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Closed The Dungey cycle Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Closed The Dungey cycle The Dungey cycle Closed Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Tail reconnection occurs explosively in a process known as the substorm - Earth’s most intense aurorae occur here Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Closed The Dungey cycle Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Closed The Dungey cycle Sun Magnetic flux is “opened” “Open” flux is “closed” Open Closed Closed The Dungey cycle Open Closed Closed The Dungey cycle The Cluster mission EISCAT Ionospheric radars SPEAR CUTLASS and SuperDARN 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 (BZ < 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 • Ionospheric convection of order hundreds m s-1 • Solar wind speed of order 1000 times larger • Therefore geomagnetic tail ~ 1000 RE (Dungey, 1965) Open Closed • Distance across polar cap ~1 RE Closed How long is the magnetotail? The disconnected tail Sun Solar wind flow • Disconnected field lines unkink at ~1.2 VSW • Disconnected tail is ~5 times longer than connected tail (5103 RE, or 0.2 AU) Cowley (1991) • Distance across polar cap ~1 RE • 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 Open Closed • Using similar arguments to Dungey (1965): Closed How long does this all take? 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 Dungey cycle Corotation 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 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 Plasma populations in the magnetosphere Magnetosheath Plasmasphere Plasma sheet Ring current aurora borealis aurora australis The auroral ovals (aurora polaris) Substorms The dynamic auroral oval The theta aurora (transpolar arc) IMAGE data courtesy of Stephen Mende, Harald Frey and the IMAGE FUV team IMAGE FUV Figure courtesy Milan et al. (2012) The shape of the magnetosphere Fopen = BIAI = BlobeAlobe lobe Blobe BI Pmag = B 2/2µ0 Blobe plasma sheet 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 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) 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 Polar cap convection: Non-steady-state Faraday (1831) Siscoe and Huang (1985) Cowley and Lockwood (1992) 𝑑𝐹𝑃𝐶 = Φ𝐷 − Φ𝑵 𝑑𝑡 The auroral substorm Substorm 5 June 1998 Substorm FPC 0.9 GWb 0.6 GWb 0.3 GWb 0.0 GWb Other planets have aurora, too
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