Unveiling Lunar Radio Science: Insights & Discoveries

1. Radio Science Experimentson the Lunar Surface Jan Bergman & Lennart Åhlén The “NEXT” Lunar Radio Explorer Workshop ESTEC, Noordwijk, December 7, 2007

2. Outline • Radio science objectives • Lunar radio and plasma science • Radio orbital angular momentum

3. Radio Science Objectives • Gain knowledge of the lunar EM environment • Necessary before larger radio observatories from the Moon can be realised • Radio measurements are versatile • Study the Moon itself • Use the Moon as a shield • Use the Moon as a target • Should attract many scientists well beyond the radio astronomy community!

4. Radio Noise Environment • Sun and the solar wind • Planetary radio emissions, AKR • Man-made sources • Galactic noise • More exotic • Askaryan radio pulses • Dust and meteorites • Magnetotelluric waves? • “One man’s signal is another man’s noise” Typical man-made interference received by the WAVES instrument on board WIND, averaged over 24 hours.

5. Radio silent Moon • Most radio quiet site within reachin our solar system • Lunar LF radio observatory • A dreamfor many decades • The only low frequency radio map of the universe was made by RAE-2 using a single dipole Lunar occultation of Earth observed by the RAE-2 satellite, 1973. The top frame is a computer generated spectrogram; the other plots display intensity vs. time variations at frequencies where terrestrial noise levels are often observed

6. RAE-2 all sky image at 2 MHz RAE-2 all sky image at ~2 MHz. From J. C. Novaco and L. W. Brown. Nonthermal galactic emission below 10 MHz. Astrophysical Journal, 221:114-123, April 1978 A relatively simple but modern digital radio receiver on the lunar surface could do wonders!

7. Pristine Moon • Investigate the pristine lunar environment • Lunar exosphere and plasma • Lunar landers expel large amounts of gas and dust (at speeds up to 2 km/s!) • Mechanical wear – Moon dust sandblasting • Contaminates the lunar environment • Apollo’s neutral mass spectrometers severely hampered. The effect lasted several months after the mission. • How will this affect lunar radio science? • Do the necessary recordings quickly! • Before frequent landings make such studies futile • And then, keep track of the lunar “climate” changes

8. Lunar ionosphere The plasma frequency: kHz (for n in cm-3) • Photoelectron layer near surface on the day side • Apollo ALSEP observed plasma densities reaching 10000 cm-3 extending several 100 meters • The Luna and Apollo measurements are the only attempts so far to diagnose the near-surface plasma ~400 kHz Dual frequency radar measurements from the Luna 22 spacecraft give (reasonably good) evidence that an ionised layer builds up on the illuminated side of the Moon (Vyshlov, 1976) ALSEP – Apollo Lunar Surface Exploration Package

9. Lunar wake plasma dynamics • Only modern (1996) lunar wake plasma measurements by WIND at 6.5 RM • WIND revealed a lunar wake density cavity • Electron density: 0.01 cm-3 • Temperature: ~100 eV • Plasma emissions in the wake • Crossed ion wake flow • Cross wake current has to close somewhere near the Moon! • Carried by conductive photoelectron layer near the dayside lunar surface? • No progress expected unless new measurements are made • A lander and an orbiter equipped with magnetometers and radio antennas (thermal noise receiver) could do the job!

10. Interactions with the geotail • Moon plasma effects on the geomagnetic tail and on near Earth magnetospheric processes are unknown • If a well developed lunar ionosphere exists, magnetospheric effects should be significant • Would act both as a mass load and a diversion of electrical currents in the geotail • If true, mass loading of the geotail could lead to large magnetospheric disturbances, even causing auroral storms Magnetospheric boundary as seen from the Moon in soft X-rays. Artist’s conception. LRX/NASA/Rob Kilgore The Moon, at 60 RE, is well within the Earth’s magnetosphere, which extends out to ~250 RE

11. Ultrahigh energy cosmic rays • Cosmic rays interact with CMB photons above 1020 eV • Intergalactic medium no longer transparent – the GZK limit • Still, there seems to exist particles beyond the GZK limit! • Their origin is unknown • GZK cut-off should produce neutrinos • No GZK neutrinos observed • UHECR at E > 1020 eV might already be neutrinos! UHECν • UHECν flux is very low • Probably ~1 particle/km2 and year • Huge detector volumes required The so called GZK limit on cosmic rays. A handful of super GZK events have been observed, shown here in red. From AGASA.

12. Radio detection of UHECν • Primaryproduces a charged particle shower • Yields incoherent Cherenkov emissions at optical wavelengths • Output power scales only linearly with primary energy • In radio, the emission becomes coherent • Output power scales quadratically with primary energy • Askaryan 1962: • “... use of ice, permafrost, very dry rock etc.” • “Very dry rock” is plentiful in upper layers of the Moon • For E > 1016 eV the Moon becomes opaque to neutrinos • Detection by antennas on the surface or from an orbiter • How to separate UHECνand other UHECR? O. Stål, J. E. S. Bergman, B. Thidé, L. K. S. Daldorff, and G. Ingelman. Prospects for lunar satellite detection of radio pulses from ultrahigh energy neutrinos interacting with the Moon. Phys. Rev. Lett., 98(7):071103, 16 February 2007.

13. Micrometeorites and dust • Micrometeorites hitting the Moon or dust hitting antennas also produce radio pulses • Cassini ring crossing, June 30, 2004 • Over 100000 dust hits detected in less than 5 minutes • Distinguished from neutrino induced pulses because of their much longer pulse duration • µs rather than 10’s of ns • A lunar radio receiver should be fast enough and have transient detection capabilities!

14. Detection of radio orbital angular momentum • L is conserved • Radial O(1/r2) fields carry information about the source rotation to infinity • Simple to generate using a small phased array [PRL, 24 Aug. 2007] • Could radio OAM be detected from a point measurement of E and B? • Large, AU wide, stationary beams • Measure during one year • Pulsars or other transient signals • Measure the beam profile as it sweeps by the receiver Synthesized radio La Guerre-Gauss (LG) beams using a circular array of ten tripoles. Upper panel shows an l=1 and lower panel shows an l=3 beam.

15. Thank’s for your attention! Jan Bergman & Lennart Åhlén jb@irfu.se ala@irfu.se