radio science experiments on the lunar surface jan bergman & lennart Åhlén the “next”...

Radio Science ExperimentsRadio Science Experimentson the Lunar Surfaceon the Lunar Surface

Jan Bergman & Lennart Åhlén

The “NEXT” Lunar Radio Explorer

Workshop

ESTEC, Noordwijk, December 7, 2007

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Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se

Outline

Radio science objectives Lunar radio and plasma science Radio orbital angular momentum

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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!

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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.

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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

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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!

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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

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Lunar ionosphere 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

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)

nf p 9 kHz (for n in cm-3)

~400 kHz

ALSEP – Apollo Lunar Surface Exploration Package

The plasma frequency:

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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!

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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

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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.

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Radio detection of UHECν Primary produces 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 UHECν and 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.

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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!

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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

ErBBrEBErL

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.

Thank’s for your attention!Thank’s for your attention!

Jan Bergman & Lennart Åhlén

[email protected]@irfu.se

radio science experiments on the lunar surface jan bergman & lennart Åhlén the “next”...

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