Sodium Guide Star Magnetometry - University of Arizona
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Sodium Guide Star Magnetometry R. Phillip Sco 1 | Michael Hart 1 | Tom Kane 2 | Paul D. Hillman 2 | Craig Denman 2 Abstract An experiment was devised to test the theory set forth in Higbie et. al. 3 proposing to measure Earth’s magnec field using a Sodium Guide Star. We expanded a Frequency Addion Source of Opcal Radiaon (FASOR) system operang at the Na D 2 a-line to a 4 inch beam and used an Acousto-Opc Modulator (AOM) to pulse the beam near the Larmor frequency of the Na atoms. By sweeping the pulse frequency over the Larmor resonance we were able to see an enhancement in the return signal. The frequency of the resonance is directly proporonal to the magnec field strength and can be used to remotely monitor magnec field strength and fluctuaons in the Earth’s mesosphere. Works Cited and Acknowledgments 1. Affiliated with the University of Arizona 2. Affiliated with FASORtronics LLC 3. Higbie James M., et.al, “Magnetometery with mesospheric sodium”, PNAS, March 1, 2011, Vol 108 no. 9, p. 3522. 4. Principle invesgators are with FASORtronics LLC with funding from the Office of Naval Research (ONR) under contract N00014-11-C-0314. Experimental Setup Conclusions and Future Work Background Na lasers have been used for years at several major observatories to create laser guide stars for Adapve Opcs (AO). Our goal was not to look at the return wavefront from the guide star, but rather to measure the variance in the return flux with relaon to pulse frequency. The sodium atoms are given a magnec moment via the circularly polarized FASOR beam. This magnec moment causes the atoms to precess in the earth’s magnec field. Pulsing the FASOR at the precession frequency thus yields an enhanced return. The Larmor precession frequency is directly proporonal to the magnec field strength as shown in Figure 2. We can use the frequency of the enhanced return signal to find the field strength at a given me and to monitor fluctuaons in the Earth’s magnec field. Figure 2: A parcle’s magnec moment (µ) precesses in a magnec field (B). The precession frequency (ω) is directly proporonal to the mag- nec field strength ω = -γB B µ Results We used an AOM to allow us to pulse the laser through the Larmor frequency resonance. The two waveplates allow us to circularly polarize our beam in order to give a magnec mo- ment to the sodium atoms. The combinaon of two beam expanders allows us to create a 4 inch beam which gives us a 4m spot within the sodium layer. • Using our setup we found the magnec field at the me of measurement to be 46.1 µT which is only slightly larger than the predicted field strength of 45,5 µT (WMM2010 model). • Fluctuaons in the signal can be monitored using a servo to lock onto the zero crossing between the negave and posive peaks. • We plan to correlate the signal with ground based measurements to show that this is a viable alternave to current measurement techniques. • We believe the signal can be monitored during daylight hours with a sufficiently narrow spectral filter. This would allow connuous monitoring of the field. Guide Star Rayleigh and Mie Backscaer Using the four inch beam we were able to create a ght Gaussian spot on the sodium layer about 4m in diameter shown in Figure 3. Comparing this to standard stars we found our guide star to have a photon flux of 110 s -1 cm -2 . Figure 3: The Guide star viewed through the Kuiper 61” Telescope on Mt. Bigelow Figure 4: By taking the difference in signal strength from two frequencies separated by 30 kHz we were able to find a magnec resonance from the sodium atoms. To analyze the return signal from the guide star we dithered between two pulse frequencies separated by a 30 kHz and looked at the difference in signal strength. When the difference is zero we know that the center frequency is at the peak of the Larmor resonance. As we scanned through a range of center frequencies we were able to see a large posive signal and a large negave signal as shown in Figure 4. This shows that we were able to find a resonance from the sodium atoms precessing around the magnec field. To track variances in the magnec field the modulaon will be opmized for a high zero-crossing slope while a servo tracks the driſt in Larmor frequency. This driſt directly corresponds to a change in field strength. Figure 1: Diagram 3 of the experiment. Larmor Resonance 322.5 kHz AOM Wavelplates Figure 5: Opcs table layout
Transcript of Sodium Guide Star Magnetometry - University of Arizona
Sodium Guide Star Magnetometry
R. Phillip Scott1 | Michael Hart1 | Tom Kane2 | Paul D. Hillman2 | Craig Denman2
Abstract
An experiment was devised to test the theory set forth in Higbie et. al.3 proposing to
measure Earth’s magnetic field using a Sodium Guide Star. We expanded a Frequency Addition
Source of Optical Radiation (FASOR) system operating at the Na D2a-line to a 4 inch beam and
used an Acousto-Optic Modulator (AOM) to pulse the beam near the Larmor frequency of the
Na atoms. By sweeping the pulse frequency over the Larmor resonance we were able to see an
enhancement in the return signal. The frequency of the resonance is directly proportional to
the magnetic field strength and can be used to remotely monitor magnetic field strength and
fluctuations in the Earth’s mesosphere.
Works Cited and Acknowledgments
1. Affiliated with the University of Arizona
2. Affiliated with FASORtronics LLC
3. Higbie James M., et.al, “Magnetometery with mesospheric sodium”, PNAS, March 1, 2011, Vol 108 no. 9, p. 3522.
4. Principle investigators are with FASORtronics LLC with funding from the Office of Naval
Research (ONR) under contract N00014-11-C-0314.
Experimental Setup
Conclusions and Future Work
Background
Na lasers have been used for years at several
major observatories to create laser guide stars for
Adaptive Optics (AO). Our goal was not to look at
the return wavefront from the guide star, but
rather to measure the variance in the return flux
with relation to pulse frequency. The sodium atoms
are given a magnetic moment via the circularly
polarized FASOR beam. This magnetic moment
causes the atoms to precess in the earth’s magnetic field. Pulsing the FASOR at the precession
frequency thus yields an enhanced return. The Larmor precession frequency is directly
proportional to the magnetic field strength as shown in Figure 2. We can use the frequency of
the enhanced return signal to find the field strength at a given time and to monitor fluctuations
in the Earth’s magnetic field.
Figure 2: A particle’s magnetic moment (µ)
precesses in a magnetic field (B). The precession
frequency (ω) is directly proportional to the mag-
netic field strength
ω = -γB
B µ
Results
We used an AOM to allow us to pulse the laser through the Larmor frequency resonance.
The two waveplates allow us to circularly polarize our beam in order to give a magnetic mo-
ment to the sodium atoms. The combination of two beam expanders allows us to create a 4
inch beam which gives us a 4m spot within the sodium layer.
• Using our setup we found the magnetic field at the time of measurement to be 46.1 µT
which is only slightly larger than the predicted field strength of 45,5 µT (WMM2010 model).
• Fluctuations in the signal can be monitored using a servo to lock onto the zero crossing
between the negative and positive peaks.
• We plan to correlate the signal with ground based measurements to show that this is a
viable alternative to current measurement techniques.
• We believe the signal can be monitored during daylight hours with a sufficiently narrow
spectral filter. This would allow continuous monitoring of the field.
Guide Star
Rayleigh and Mie
Backscatter
Using the four inch beam we were able to create a tight Gaussian spot on the sodium layer
about 4m in diameter shown in Figure 3. Comparing this to standard stars we found our guide
star to have a photon flux of 110 s-1cm-2.
Figure 3: The Guide star viewed through the Kuiper 61” Telescope on Mt. Bigelow
Figure 4: By taking the difference in signal strength from two frequencies separated
by 30 kHz we were able to find a magnetic resonance from the sodium atoms.
To analyze the return signal from the guide star we dithered between two pulse frequencies
separated by a 30 kHz and looked at the difference in signal strength. When the difference is
zero we know that the center frequency is at the peak of the Larmor resonance. As we scanned
through a range of center frequencies we were able to see a large positive signal and a large
negative signal as shown in Figure 4. This shows that we were able to find a resonance from the
sodium atoms precessing around the magnetic field. To track variances in the magnetic field
the modulation will be optimized for a high zero-crossing slope while a servo tracks the drift in
Larmor frequency. This drift directly corresponds to a change in field strength.
Figure 1: Diagram3 of the experiment.
Larmor Resonance
322.5 kHz
AOM
Wavelplates
Figure 5: Optics table layout
