Lecture 6 - The Edge of the Sun, and Beyond PDF

Lecture 6 – The edge of the Sun, and beyond Schematic diagram of the Heliosphere  The extended corona The “white light” corona – photospheric light is being scattered by electrons in the corona – bright features are higher density The solar wind Predicted...

Lecture 6 – The edge of the Sun, and beyond Schematic diagram of the Heliosphere  The extended corona The “white light” corona – photospheric light is being scattered by electrons in the corona – bright features are higher density The solar wind Predicted by E. Parker (1958) Confirmed by Soviet satellites Expected supersonic speeds at 4 solar radii SoHO measurements: supersonic velocities are reached much faster  thermodynamic expansion alone cannot be responsible for the acceleration Solar wind = supersonically expanding extension of the corona The solar wind Corona is too hot to be held back by gravity Expands out into space at supersonic speeds Above dark, low-density corona: Fast wind – low density, 700- 800 km/s, quite uniform Above bright corona: Slow wind - ~3x density of fast wind, 300-400 km/s, highly variable Solar wind = supersonically expanding extension of the corona  Both the fast and slow solar wind can be interrupted by CME-s moving at different speeds Clear relationship between bright emission from lower corona and high densities in upper (white-light) corona But not a 1:1 match!  Fast wind Originates mainly in polar dark corona (“coronal holes”) Outflow from smaller coronal holes (seems to be) slower (Probably) expands down to overlie “quiet Sun” by ~2 RSun What is the acceleration mechanism? Magnetic field of Sun, August 1996, modelled by Zoran Mikic and Jon Linker (SAIC) Fast wind Originates mainly in polar dark corona (“coronal holes”) Can be accelerated by magnetic (Alfven) waves (Alielden & Taroyan 2022, ApJ). Plots below show the radial wind speed for three different cases Background 100 wave events 200 wave events Alielden & Taroyan 2022, ApJ  Slow wind Originates mainly around the equator (“streamer belt”) Streamers produced by magnetic flux draping over closed field lines At solar maximum, the poles are also emitting a slow solar wind Exact origin still under debate Magnetic field of Sun, August 1996, modelled by Zoran Mikic and Jon Linker (SAIC) Solar wind is very different at solar minimum and maximum (ESA/NASA Ulysses mission) The Solar Wind extends out past the planets o Solar wind continues to expand outwards o Carries Sun’s magnetic field with it o Planets exist within the solar wind o Planets with magnetic fields carve out their own magnetic cavities – magnetospheres o Planets with no magnetic field but with atmospheres have surface screened from solar wind by atmosphere o Other bodies have direct interaction between solar wind and surface  A planet with a strong magnetic field (Earth, Jupiter, Saturn...) Computer-generated model of the Earth's Impact of a coronal mass ejection on magnetosphere. Semi-transparent surfaces the Earth’s magnetosphere, 18-20 represent particle density (red is high, blue is October 1995 low) and silvery tubes represent the magnetic (NASA scientific visualisation studio, http://svs.gsfc.nasa.gov/vis/a000000/a000000/a000088/index.html ) field (NASA scientific visualisation studio, http://svs.gsfc.nasa.gov/vis/a000000/a002300/a002391/index.html) If a planet’s magnetic field is weaker and solar wind pressure high (e.g. Mercury), magnetopause can be forced downwards – on Mercury solar wind can penetrate to surface at times A planet with an atmosphere but no magnetic field (e.g. Venus) Solar wind particles (hydrogen+, helium+ and helium++) charge-exchange with atmospheric gases New atmospheric ions follow solar magnetic field, flow away from planet with solar wind Ex-solar wind protons lost from planetary atmosphere Solar wind gradually strips atmosphere from planet Process seen at Mars and Venus Venera probe image No atmosphere and no magnetic field (e.g. Moon) Solar wind particles impact surface of object Rocky objects (e.g. Moon) made up of material containing much oxygen Impact of solar wind particles disrupts molecule bonds in rock grains Oxygen released by these impacts combines with solar wind hydrogen – producing water Water discovered in lunar soil by Chandrayaan-1 mission (announced 24th Sept. 2009) As solar wind expands, it exerts less pressure (no. particles/m3 x speed....) Eventually, pressure of solar wind = pressure of interstellar gas At this point, the solar wind can’t expand further Solar wind is supersonic “Stop” is sudden – the termination shock Shock front heats gas, accelerates energetic particles “Hot hydrogen” wall forms outside shock Simple calculation suggests termination shock at ~170 AU from Sun Not quite that simple... Solar system moving at 26 km/s (relative to background gas, ~16.5 km/s relative nearby stars, ~370 km/s relative to the “local” universe outside our local group of galaxies) roughly in the direction of Leo The overall shape of the heliosphere resembles that of a comet Heliosphere not symmetric, shock less dramatic than simple model predicts Heliosphere/interstellar medium coupling over a solar cycle (NASA visualisation studio, https://svs.gsfc.nasa.gov/2856/)  Voyager 2 passed through the termination shock (out of the supersonic solar wind) several times on 31 August – 1 September 2007 at ~84 AU from the Sun http://www.nature.com/nature/journal/v454/n7200/full/454038a.html The reason for multiple crossings is that the termination shock moves in and out as solar wind pressure changes (e.g. mass ejections, interaction of fast and slow streams of solar wind..) Beyond this is the heliosheath – a region of heated, turbulent, subsonically-expanding solar wind plasma The heliopause – the edge of the extended Sun – balance between IM SM pressure In August 2012, Voyager 1 exited the heliosphere and reached interstellar space ~ 122 AU "further 3D motion of the Heliopause (Chi Wang, MIT) than anyone, or anything, in history" http://web.mit.edu/afs/athena/org/s/space/www/voyager/voyager_science/voyager_science.html  Heliosphere Dominated by supersonic, outward-flowing solar atmosphere (solar plasma) In outer regions, neutral particles from interstellar space penetrate into heliosphere Termination shock Expansion of solar atmosphere becomes subsonic Much heating of solar plasma Heliosheath Region of heated, subsonically-expanding solar atmosphere outside termination shock Deflects interstellar plasma around solar system Modulates cosmic rays Heliopause Outer limit of solar atmosphere Pressure of interstellar gas = pressure of solar atmosphere Bow shock (rejected in 2012 by results from IBEX) Solar system is moving supersonically through interstellar gas – shock front forms ahead of it? Hydrogen wall Müller, 2004 Interstellar neutrals stream in freely Charge-exchange with solar protons Get “wall” of hydrogen Hydrogen wall https://soho.nascom.nasa.gov/data/summary/swan/  Preliminary Cassini data suggest the heliosphere may not have the comet-like shape predicted by existing models but that its shape may be a large, round bubble New results from the NASA IBEX mission (2009/10/15): IBEX's all-sky map of energetic neutral atom emission reveals a bright filament of unknown origin. V1 and V2 indicate the positions of the Voyager spacecraft A galactic magnetic field shapes the heliosphere as it drapes over it?  Recent results For the first time, the boundary of the heliosphere has been mapped, giving scientists a better understanding of how solar and interstellar winds interact. An updated model suggests the heliosphere (yellow), may be a deflated croissant shape, rather than the long-tailed comet shape suggested by other research: https://www.nasa.gov/solar-system/uncovering-our-solar-systems-shape/ Beyond the hydrogen wall.. Hydrogen wall at ~100-200 AU from Sun Outside the heliosphere - undisturbed interstellar material Solar system currently moving through region of relatively dense (1x10-7 particles/m3), cool (7000 K) gas – “local interstellar cloud” or “local fluff” Solar system entered local fluff ~ 10,000 years http://antwrp.gsfc.nasa.gov/apod/ap000411.html ago, will leave in ~1,900 years time Local fluff occupies a small region within a large volume of relatively empty space – “local bubble” Local bubble “peanut shaped”, about 300 light-years long Contains low-density (1x10-9 particles/m3), hot (~106 K) gas Carved out by supernova explosion ~5 million years ago in Scorpio-Centaurus cloud (about 130 light-years from Earth at the time) Confirmation in 2014 http://antwrp.gsfc.nasa.gov/apod/ap000411.htm l