2000 February 24, SPS 1020 (Introduction to Space Sciences) - Read TNSS Chs. 14 and 15 (Jovian planet interiors and atmospheres) and Ch. 16 (Planetary Rings) for Tuesday, February 29 - Read TNSS Chs. 17 and 18 (Io and Europa) for Thursday, March 2. After Spring Break: Chapters 19 and 20, PBD Ch. 7 (Ganymede, Callisto, and Titan) --------------- Solar magnetism and Space Physics: --------------------------------- Prominences - buoyant tubes of magnetized gas, moving out from Sun (over hours/days) Can be 5e5 km high! Loop prominences: trace loops of magnetic field, from Sun DON'T CONFUSE WITH: Flares - much more powerful, and faster: sudden _short circuit_ in magnetic field (Magnetic *reconnection*) 1989 March flare: largest recorded, aged all spacecraft electronics by 5 years; outflowing radiation hit Earth, caused brilliant aurorae (including the one I saw in S. Arizona!) The Solar Corona: Can see it during solar eclipses, and through coronagraphs (special cameras that donŐt scatter light) Changes shape over Sun's 22-year magnetic cycle (2 x 11-year sunspot cycle) Dipole at solar minumum vs. higher multipole (i.e., roughly spherical, bunched-up magnetic field during solar max) => Big result from SOHO and esp. TRACE: corona likely heated by magnetic reconnection! (Long-standing mystery) Can now see field lines, see motion, follow reconnection events Coronal mass ejections (CMEs) - discovered by Skylab! Sudden, occur over hours Coronal holes - almost always present at poles of Sun Investigated by Ulysses spacecraft (flew over poles of Sun, 1994-1995). Ulysses found solar wind to be twice as fast (800 km/s) over poles, versus 450 km/s near ecliptic (and Earth): unknown why. The solar wind: -------------- Stream of charged particles (plasma) flowing out from Sun. Composed mainly of protons and electrons, some alpha (He4 nuclei) and heavier ions. => Protons have nearly all the mass: are the real radiation hazard Solar wind is NOT radiation pressure! (E = pc for a photon) (Solar sails work by catching radiation pressure, *not* the solar wind.) Solar wind's magnetic field strength drops off with 1/distance^2 from Sun. Magnetic field is trapped with the particles: it's spiral-shaped Planetary magnetospheres: ------------------------ A magnetosphere is the region around a planet where its magnetic field dominates the motion of charged particles Earth's magnetosphere: --------------------- Earth's magnetic field: The magnetic declination: difference between magnetic N (what a compass on Earth reads) and true N. It equals 2 degrees in Melbourne. Solar wind's field lines hit Earth at about a 45 degree angle. Bow shock: Solar wind is highly supersonic, so a _shock_ forms Solar wind is channeled into magnetic poles (N and S): Excites air molecules (as in fluorescent lamps), cases aurora borealis (N) and austrealis (S) Spread over Earth in _auroral ovals_ (Mary Hudson once got a valentine with a heart-shaped one!) The Van Allen belts: trapped radiation belts around Earth => Why Shuttle and ISS orbit only 400 km high; higher, the radiation gets nasty! Discovered serendipitously in 1958 by James Van Allen's *students* with Geiger counters (designed to look for cosmic rays) aboard the first successful U.S. satellite, Explorer 1. Ring current: made by circulating, trapped charged particles (Might charge up a spacecraft!) Magnetotail: extends past Moon; can sweep over it. Disconnection events (also called magnetic substorms): also observed in comet tails There is now a fleet of spacecraft studying Earth's magnetosphere, in a coordinated manner: Ulysses, SOHO, Wind, Polar, Equatorial, Geotail, ACE (for cosmic rays: see below). Last solar maximum: 1989-1991; next 2000-2002 The Earth's Moon: Solar wind directly strikes Moon's surface H, He4, other ions, He3 (v. rare) has built up in regolith for aeons (billions of years) => Might mine He3 for fusion reactors? (But what fusion reactors?) Creates cone-shaped _wake_ (umbra) behind it; among the best vacuums anywhere. Mars, Venus: no global magnetic fields, only remnant, fossil fields (although new claim, by Mars Global Surveyor) Solar wind does not strike Venus: it ionizes the tops of its atmosphere => Ionospheres, somewhat like Earth's Mars: weaker field, thinner atmosphere. Biogenic materials on surface must have been exposed to high radiation doses (solar ultraviolet light also gets through atmosphere: surface is bad place to look for life) Mercury: has weak magnetic field, but Solar wind is 10 times stronger there than at Earth. => No trapped particles, but there are magnetic substorms in magnetotail => magnetosphere is dynamic, unlike remnant fields of Venus and Mars Jupiter's magnetosphere: ------------------------ Largest "object" in Solar System; would have a 6-degrees angular diameter, if you could see it. (Angular diameter of Earth's Moon, from Earth, is 0.5 degrees.) - Known since 1940s from decametric radio emission (Jupiter among brightest objects in radio sky) - Trapped radiation fields: Pioneers 10 and 11 soaked up 1000s of times more than a lethal dose, for humans, electronics did fail; camera optics darkened - Io plasma torus: volcanoes on Io contribute sodium and sulfur ions, make current, affect aurorae - Magnetotail: can _wag_ by 45 degrees, extends past Saturn! - Europa, Ganymede also deep in Jovian magnetosphere => Ganymede: a magnetosphere within a magnetosphere! (Maybe also Callisto: very new, surprise result) No bow shock around Ganymede: particles in Jovian magnetosphere are subsonic, unlike Solar wind Saturn's magnetosphere: also trapped radiation belts, decametric radio emission, aurorae. Interaction with Titan's atmosphere? Uranus and Neptune: both magnetospheres asymmetrical and corkscrew-shaped, since tipped sideways! Magnetic declinations very large for both (59 degrees Uranus, 47 degrees Neptune), much more than any other Solar System objects (< 15 degrees); unknown why. The heliopause: the edge of the heliosphere: searching for the "edge" of the Solar System Indirect detection in 1992, from radio emission: at about 150 AU. (Pluto's at 40 AU.) Cosmic rays: high-energy charged particles from deep space. Accelerated by supernova shocks (?) - Provide information on the Universe (e.g., made almost entirely of matter, with little anti-matter) - At sea level: mostly muons (lifetime 2 milliseconds) - In orbit: mostly protons; on the way, they have nuclear reactions with air molecules (Proof of special relativity: they wouldn't make it to Earth, unless time dilated) - Interesting to high-energy physicists: much more energetic than the mightiest accelerators (although luminosity low) - Deflected greatly by solar wind, Earth's magnetosphere and atmosphere, heliosphere - Low-energy ones can come from solar active regions ("Solar cosmic rays")