Download Bray - X-rays from Solar System Objects

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)
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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
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Brief History
Bhardwaj et al. 2010
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Bhardwaj et al. 2010
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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
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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
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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
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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
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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
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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
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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
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