radio science experiments on the lunar surface jan bergman & lennart Åhlén the “next”...
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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
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
Outline
Radio science objectives Lunar radio and plasma science Radio orbital angular momentum
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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!
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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.
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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!
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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:
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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!
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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.
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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.
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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!
Phy
sics
in S
pace
Physics in Space Programme, IRF, Uppsala, Sweden | www.phisp.irfu.se
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
jb@irfu.seala@irfu.se
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