Investigating Low Energy X-rays from the Sun: Challenges and Innovations

1. The Sun in X-rays “Low” Energies To Be Further Investigated Marino Maiorino

2. Summary • The Sun and Its Structure • Light as a Measurement Tool • The Corona Mistery • The Present Theories • Watch the Sun! • Going to Space • X-ray Imaging Detectors • New Technologies • Conclusions

3. The Sun and Its Structure • Core • p-p reaction • 1.5·107 °K • Radiative Zone • Convective Mantle • Outer, “atmosphere” layers: • Photosphere (5800 °K) • Chromosphere • Transition region • Corona

4. Light as a Measurement Tool • Black body spectrum and a bunch of laws: • Temperature as a Function of Depth • lmaxT = constant!

5. The Corona Mystery • What’s Corona? • Outer atmosphere of the Sun. • Thickness is a million km, but is possibly even larger. • The Corona is too hot to emit visible light. • How can we see it? • During a solar eclipse, observe light reflected from the dust particles associated with it. • Directly, using an X-ray telescope.

7. The Corona Mystery • Temperatures in Corona • Temperature > 106 °K: emission in X-rays!

8. The Corona Mystery • In literature you can find: • “The reason for the enormous temperatures in the corona is not well understood, but may relate to heating by the Sun's magnetic field lines…” • Theories: • Wave dissipation • Nano-flares • … • Also see: • http://arxiv.org/PS_cache/astro-ph/pdf/0203/0203167.pdf

9. The Present Theories • Wave Dissipation • Acoustic heating: ruled out due to dependency of Lx on rotational speed; • Magneto-acoustic heating: heating by waves and/or particles induced from magnetic fields at the bottom of the corona.

10. The Present Theories • Nano-Flares • Sun emits strong bursts of X-rays before the UV lightning of flares • A large number of very small “nano-flares” was proposed as a mechanism to heat Corona • Theory: at 250’000 °K some magnetic reconnection takes place between chromosphere and corona • SOHO saw low temp. gas (<400’000°K) going down; hotter gas going up • Nowadaysit is thought to be one of the mechanisms involved in coronal heating, NOT the most important one

11. Watch the Sun! • Usefulness of a Sun-staring mission to monitor “Space weather”: • Short radio waves propagation (e.g.: cellphones, radars and antennas) • Artificial satellite communications (research, GPS-navigation satellites) • Artificial satellite orbits (Earth atmosphere inflation) • Electric power grids (tranformers overload) • Health-damaging radiations (astronauts, jet passengers)

12. Going to Space • Our atmosphere, again...

13. X-ray Imaging Detectors • History of X-ray detection in space • Uhuru (US), 1970-73, 2-20 keV, proportional counters • OSO7 (US), 1971-74, 1 keV – 10 MeV, prop. counters + X-ray telescope • SAS3 (US), 1975-79, 0.1-60 keV, prop. counters collimated • HEAO1 (US), 1977-79, 0.2 keV – 10MeV, plenty of experiments • Einstein (US), 1978-81, 0.2-20 keV, grazing incidence telescope • Ginga (JAP), 1987-91, 1-500 keV, prop. counters • GRANAT (RUS-EU), 1989-98, 2 keV – 100 MeV, coded mask telescope • ROSAT (D-US-UK), 1990-99, 0.1-2.5 keV, X-ray telescope + high resolution imager • XMM-Newton (EU), 1999, 0.1-15 keV, grazing incidence X-ray imaging telescope • Chandra (US), 1999, 0.1-10 keV, grazing incidence telescope

14. X-ray Imaging Detectors • So “low” energies? • 2·106 °K / 11604 = 172 eV (a few nm) • In the “soft” X-ray band (0.12 - 5 keV) the Sun is the brightest X-ray source (by a factor of 106): X-ray Sun Luminosity = 1027 erg/s!

15. X-ray Imaging Detectors • Focusing Technologies: • X-ray (grazing) telescopes • Coded mask aperture telescopes • Detection Technologies: • Imaging Proportional Counters • Microchannel Plates • CCD Spectrometers • Imaging Gas Scintillation Proportional Counters

16. X-ray Imaging Detectors • Existing Projects • SOHO (NASA-ESA) • Yohkoh (JAP) • GOES-12 (NASA-NOAA) • TRACE (NASA) 171-1600 Å • RHESSI (NASA) 3 keV – 20 MeV

18. New Technologies • Single-photon counters (Medipix): • An X-ray photon generates two particle showers (h+ and e-) in a solid-state detector • By suitably biasing the detector, one shower is sent to an electronic device, able to see it as a pulse • The electronic device is divided in picture elements: any of these pixels is able to count the incoming pulses • Lower energy limit: a few keV

19. New Technologies • 3D detectors: • Electrodes are close • Low full depletion bias • Low collection distance • Thickness NOT related to collection distance • No charge spreading • Fast charge collection • Micron scale • USE Latest MEM techniques • Pixel device • Readout each p+ column

20. New Technologies • Josephson Junction Arrays: • Two superconductors, separated by an insulating layer, form a Josephson Junction. • Cooper pairs can flow from one side to the other • If a small voltage (a few millivolts) is applied, an alternating current of frequency in the microwave range results • Tecnology already used in SQUID detectors (sensitive to magnetic fields) • By coupling the junction material to a particle absorber, you can see particles (infrared photons!) hitting the detector by monitoring the voltage in the circuit

21. Conclusions • Sun, our nearest star, still hides secrets... even there where it’s more transparent...; • Study of Sun is straightforward (well... let’s get to space, first...); • Imaging technologies are ready for new challenges; • A Sun-staring mission offers unusual economical points of interest!