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X-RAYS FROM SOLAR SYSTEM OBJECTS Evan Bray Astro 550 Introduction ■ The solar system is a natural laboratory, with a large variety of objects ■ Allows for close-up observations of phenomena that may be happening elsewhere ■ Typically associate X-Rays with ~1 million Kelvin plasmas – Planets/small bodies are 3-5 orders of magnitude colder! ■ Today we’ll focus on soft X-Rays (200 eV – 2 keV) Evan Bray 2 Brief History ■ 1950’s: X-Rays discovered from auroral emissions on Earth ■ 1962: Scorpius X-1 discovered during attempt to observe X-Rays from the Moon ■ 1970’s: Apollo 15 & 16 study fluorescently scattered X-Rays from the Moon ■ 1979: Einstein Observatory discovers X-Rays from Jupiter ■ 1996: ROSAT discovers X-Rays from the comet Hyakutake ■ 2000’s: Chandra/XMM-Newton confirm Mars, Venus, and many others as X-Ray emitters Evan Bray 3 Brief History Bhardwaj et al. 2010 Evan Bray 4 Important Terms ■ Auroral Region: High-latitude areas where charged particles collide with Earth’s atmosphere ■ Ionosphere: Upper region of Earth’s atmosphere. Dominated by particles ionized by solar radiation ■ Heliosphere: The region of space dominated by the Sun ■ Geocorona: Luminous part of outermost regions of Earth’s atmosphere. Extends to 16R⊕. Emits in FUV. ■ Io Plasma Torus: Ring-shaped cloud of ions around Jupiter caused by ionizing 1 ton of O per second from Io Evan Bray 5 Production Mechanisms 1. Collisional excitation of neutral species by charged particles (line emission) 2. Bremsstrahlung from electron collisions with neutrals and ions 3. Elastic & fluorescent scattering of solar X-Ray photons from neutrals 4. Solar wind charge exchange (SWCX) 5. Charge exchange of energetic heavy ions, or direct collisions of ions ■ Energy for these interactions comes from the Sun Evan Bray 6 Electron Collisions ■ In coronal-type plasmas, atoms are typically highly ionized (high Z) −13.6 𝑍 2 Energy 𝑒𝑉 ≈ 𝑛2 ■ Radiative recombination produces continuum emission ■ Can be followed by line emission if the bound state isn’t the ground state ■ Bound-free collisions are responsible for most of the planetary X-Ray luminosity Evan Bray 7 Electron Collisions – Bremsstrahlung ■ Produced by an electron accelerating in the electric field of an atomic nucleus ■ Key component of solar X-Ray continuum emission (thermal) ■ Responsible for hard X-Rays from terrestrial and Jovian auroras (nonthermal) – Fast electrons produced in magnetosphere – Travel along magnetic field lines into atmosphere Evan Bray 8 Solar Photon Scattering ■ X-Rays can be both elastically scattered and absorbed ■ Soft X-Rays are vastly more likely to be absorbed than scattered – Scattered X-Rays from Earth have still been observed – 𝛕~1 for 𝑁𝐻 = 1020 ■ X-Ray Scattering albedo for outer planets calculated to be 0.003 (Cravens 2006) – Total albedo of Jupiter = 0.343 ■ K-shell fluorescence usually results in emission of an Auger electron – Oxygen photon yield is only 0.2% => almost no fluorescence photons! ■ Fluorescence also important for solid surfaces, like the Moon and Saturn’s rings Evan Bray 9 Solar Wind Charge Exchange (SWCX) ■ Process of solar wind ions colliding with target neutrals – Proposed in 1997 to explain emissions from comet Hyakutake (Cravens 1997) – Energy derived from the million degree solar corona ■ Cross sections of ~10−15 𝑐𝑚2 – Several orders of magnitude higher than electron collisions! ■ Creates a cascade of photons as electrons drop to ground state Evan Bray 10 Solar Wind Charge Exchange (SWCX) ■ Heavy ions make up 0.1% of the solar wind 𝑂7+, 𝑂6+, 𝐶 6+, 𝐶 5+, 𝑁 6+, 𝑁𝑒 8+ , 𝑆𝑖 9+, 𝐹𝑒12+ – Composition is strongly variable in time and location ■ Volume emission rate is given by the following equation – Must be summed over all target and wind species ■ SWCX is an excellent probe of the solar wind for locations we can’t otherwise reach – Most notable contributions from Pioneer and Voyager probes Evan Bray 11 Earth – Auroral Emissions ■ Visible, UV, and X-Ray photons are emitted because of coupling between ionosphere and magnetosphere. ■ Majority are line emissions of N, O, and Ar. Confirmed by HEAO and Chandra – Small, noticeable bremsstrahlung component at high energies ■ Majority of X-Rays are directed normal to magnetic field – Leads to limb brightening in terrestrial X-Ray images ■ PIXIE instrument was critical in characterizing this phenomena ■ Directly related to solar activity Bhardwaj et al. 2010 Evan Bray 13 Evan Bray Bhardwaj et al. 2010 14 Earth – Non-Auroral Emissions ■ Two components: Thomson scattering of solar X-Rays, and absorption followed by emission of K-lines. ■ Short-lived (~1 ms) X-ray and γ-ray bursts detected above storm cells by CGRO in the 90’s – Bremsstrahlung from upward-propagating MeV electrons after a lightning strike – No conclusive evidence of mechanism yet. To be studied by future ESA missions. ■ 𝐿𝑥,𝑡𝑜𝑡𝑎𝑙 = 60 𝑀𝑊 Evan Bray 15 The Moon ■ Studied close-up by Apollo 15 & 16, and more distantly by ROSAT & Chandra ■ Dominated by fluorescent X-Rays – Small amounts of scatter solar X-Rays, almost no bremsstrahlung. – Excellent probe of lunar surface composition! ■ Greater fraction of Al/Si than Earth – Future missions will obtain ~20-km resolution maps of the surface ■ 𝐿𝑥 = 73 𝑘𝑊 ■ “Nightside emission” is actually SWCX with geocoronal H ■ Determining albedo variations with XMM-Newton? – Not possible at the moment. Need faster CCDs. Schmitt et al. 1991 Evan Bray 16 Venus ■ Difficult to observe due to distance from the Sun ■ Expected to be an X-Ray source due to proximity to the Sun and thick atmosphere ■ Observed in 2001 with ACIS-I and ACIS-S – Primarily fluorescent scattering of C and O. Some N and CO2 ■ Extent of limb brightening acts as probe of atmosphere structure, and response to solar activity ■ Optically bright sources can be challenging => Filter optical by dispersion! ■ 𝐿𝑥 = 55 𝑀𝑊 Dennerl et al. 2002 Evan Bray 17 Mars ■ First observed with ROSAT in ‘93 with no results. X-Rays detected by Chandra in 2001 – Dominated by a single O Kα emission line ■ Hypothesis: X-Ray scattering by attogram (10−18 𝑔) dust particles? – Dust storm was present during first half of observation – No significant change in X-Ray flux between halves ■ Martian halo detected out to ~3 Mars radii (35 excess photons) – Different spectral distribution than planetary disk – Indication of SWCX in Martian exosphere ■ 𝐿𝑥 = 8 𝑀𝑊 Dennerl 2002 Evan Bray 18 Jupiter – Auroral Emission ■ First observed by the International Ultraviolet Explorer, and confirmed by Voyager I ■ Electron vs. heavy ion precipitation – Early observations made distinction difficult – ROSAT showed slight preference for heavy ions ■ Spike in emission during collision with Shoemaker-Levy 9 ■ Chandra/XMM showed aurora particles originated 20-30 Jupiter radii away ■ Emissions came from higher latitudes than UV auroral zone – Both regions pulse with 40-minute period ■ Future Jupiter missions include in-situ observations of auroral emissions ■ 𝐿𝑥 = ~700 𝑀𝑊 Evan Bray 19 Bhardwaj et al. 2010 Evan Bray 20 Jupiter – Non-Auroral Emission ■ Low-latitude emissions discovered by ROSAT in 1997 – Initially thought to be S and O ions from the radiation belts ■ Later found to be scattered solar X-Rays ■ Correlation between local magnetic field strength and soft X-Ray emissions – Suggests a second component in addition to scattering? ■ Unlike auroral emissions, disk emissions do not significantly vary in time ■ 𝐿𝑥 = ~1 𝐺𝑊 Gladstone et al. 2002 Evan Bray 21 Saturn ■ First detected by XMM-Newton in 2002 ■ Concentrated in low-latitudes – No apparent polar emissions – Strong solar flares during observations imply scattered solar X-Rays ■ Chandra showed factor of 2-4 variability on the timescale of days ■ Composition largely similar to that of Jupiter ■ 𝐿𝑥 = 250 𝑀𝑊 Evan Bray 22 Evan Bray 23 Bhardwaj et al. 2005 Comets ■ Discovered by ROSAT in 1996 while observing comet Hyakutake ■ Initially theorized to be thermal bremsstrahlung from solar electrons – Predicted 𝐿𝑥 too low by factor of 100-1000 ■ Scattering from attogram dust grains? – Detected by PUMA dust monitor during Halley flyby – Can’t create the lines we see in spectra Lisse et al. 1996 ■ Emissions dominated by SWCX in tenuous atmosphere – No emission from tails Evan Bray 24 Comets ■ Tells us 4 important characteristics: 1. Spatial morphology 2. Composition 3. Solar wind properties 4. Source of local soft X-Ray background Bhardwaj et al. 2010 ■ Strong correlation with gas production rate and solar O ion flux – Zero correlation with dust production rate and solar X-Ray flux Evan Bray 25 Io Plasma Torus, Galilean Moons, and Rings of Saturn ■ IPT – Detected by Chandra in 1999 – Very soft spectra, predominantly O6+. Not too highly ionized – Nonthermal electron bremsstrahlung ■ Galilean Satellites – Io and Europa detected by Chandra in 1999 ■ Based on only ~10 photons each! – Ganymede and Callisto are likely emitters as well ■ Saturn’s Rings - Detected in 2004 by Chandra – Based on 42 photons, all in the Oxygen Kα Evan Bray 26 Diffuse X-Ray Background ■ Background is significantly variable in both time and space. ■ Thermal in origin, dominated by lines ■ Instrumental backgrounds can also be comparable to diffuse background ■ Angular size of source effects quality of data – Planets tend to span small portion of the FOV – Cometary emission can fill entire FOV. Makes background determination difficult Evan Bray 27 Evan Bray 28 Evan Bray 29 Summary ■ Almost all solar system X-Ray emission is powered by the Sun ■ Charge exchange, scattering, and bremsstrahlung are the dominant processes – Depends on size, composition, and distance from the Sun ■ Most emissions are highly dependent on stellar activity ■ Very low luminosities necessitate sensitive equipment and low backgrounds ■ XMM-Newton and Chandra are workhorses for small bodies – ROSAT useful for diffuse emissions from extended sources Evan Bray 30