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SAUND (ocean)

PBP, 2003. SAUND (ocean). ACOUSTIC. Propagation of ultrahigh-energy neutrino-produced acoustic waves in ice and salt The only affordable way to expand the collecting power of a future neutrino observatory by a factor ≥ 10 2 seems to be with radio and/or acoustic arrays.

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SAUND (ocean)

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  1. PBP, 2003 SAUND (ocean) ACOUSTIC

  2. Propagation of ultrahigh-energy neutrino-produced acoustic waves in ice and salt The only affordable way to expand the collecting power of a future neutrino observatory by a factor ≥ 102 seems to be with radio and/or acoustic arrays. Their much lower sensitivity to neutrino-induced cascades is an advantage when the goal is to detect neutrinos with energy ≥ 1018 eV. For acoustic arrays I will make the case that absorption and scattering lengths are orders of magnitude larger than for optical arrays.

  3. Peak frequency contours for a 1019 eV hadronic cascade in ice, in kHz (J. Vandenbroucke) em cascade  pancake-shaped pressure wave Peak pressure contours for a 1019eV hadronic cascade at 10 kHz in ice 2 4 x [km]

  4. Conversion of ionization energy into acoustic energy ocean S.P. ice NaCl T [ºC) 15º -51º 30º <vL> [m s-1] 1530 3920 4560  [m3 m-3 K-1] 25.5x10-5 12.5x10-5 11.6x10-5 CP [J kg-1 K-1] 3900 1720 839 Peak frequency 7.7 kHz20 kHz 42 kHz  <vL>2/CP0.153 1.12 2.87 Conversion efficiency ishighest for salt and lowest for ocean.

  5. Acoustic waves are scattered at grain boundaries, not at bubbles. Scattering depends on grain size, d, and frequency, f, not on temperature: s  d -3 f -4 in Rayleigh regime glacial ice at South Pole 10-2 1 km d = 1 cm 10-4 0.4 cm d ≈ 0.2 cm b = (scat)-1 = scattering coefficieint [m-1] 0.1 cm 103 km 10-6 10-8 energy concentrated here 10-10 104 103 105 frequency [Hz]

  6. Acoustic absorptivity,  [m-1], depends on T, not on d Dominant energy loss mechanism for acoustic waves in cold ice (T < -10ºC) is due to proton reorientation. Absorptivity: f 2  (1 + 4π2 f 2 2)v v = acoustic speed  = relaxation time between two possible configurations = 0 exp (U/kT) and U ≈ 0.58 eV  [m-1] 

  7. Consider two regimes: • SPATS (South Pole) T = -51 ºC, f >10 kHz, a ≈ 8.6 km or • Ross Ice Shelf T ≈ -28 ºC, f < 1 kHz, a≈ 500 m energy concentrated here

  8. Acoustic array on Ross Ice Shelf for GZK neutrinos?Advantages: • Flatness: acoustic waves can propagate by hopping along firn-air interface. • Cheap: deploy at the surface; no drilling required • Close to McMurdo; more accessible than South PoleDisadvantages: At T ≥ -28ºC, only waves with f <1 kHz have a > 500 m,and very little energy goes into such low-frequency waves.

  9. Site for ARIANNA? Ross Ice Shelf Temperature at 10-m depth Thickness Contours >500 m -28ºC -27º -27ºC <300m , Ross Sea Ross Sea

  10. Propagation in firn is analogous to propagation in lunar soil. Due to density gradient of firn, body waves follow curved paths and propagate in 2D if surface is flat. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • = hydrophones buried at ~1 m

  11. Cascade-induced acoustic pancakes are warped upward in firn (from Justin Vandenbroucke)  = 20º  = 10º  = 30º  = 40º

  12. Absorption in ice and salt In ice, acoustic waves lose energy by pulling protons (black dots) back and forth between bond sites. In NaCl acoustic waves lose energy by interactions of acoustic phonons with the thermal phonon background. D L

  13. Scattering and absorption in NaCl s 0.1 km 1 km 2 cm Scattering from grain boundaries 10 km a 103 km 104 km 105 km phonon-phonon absorption  (f)  f 2

  14. South Pole icevs ideal salt domes scattabs 10kHz 30 kHz 10kHz 30kHz Ice 0.2 cm 1650 km 20 km 8-12 km 8-12 km NaCl 0.75 cm 120 km 1.4 km 3x104 km 3300 km In typical salt domes, scattering is worse than in South Pole ice because grain size is larger. In salt domes, both scattering and absorption are dominated by impurities: clay, other minerals, and liquid inclusions. 3. Calculations of scattand absmust be checked withmeasurements at proposed sites. 4. Available volume of South Pole ice >> volume of any salt dome. 5. Drilling into ice is far cheaper than into salt domes. grain size

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