LISA Pathfinder: These magnetic results will attract your attention

Title: Magnetic-induced force noise in LISA Pathfinder free-falling test masses

Authors: Michele Armano et al.

First Author’s Institution: European Space Agency

Status: Accepted Oct 2024, Physical Review Letters

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In my wavy era

We are well and truly in the age of gravitational wave detector science, kicked off in 2015 by the LIGO detection of two black holes merging. Just last year, the NANOGrav collaboration presented evidence of a gravitational wave background formed by the collective hum of orbiting supermassive black holes. However, existing detectors can only cover a certain range on the spectrum of gravitational wave frequencies, with ground-based detectors like LIGO on the higher end at frequencies of hundreds of Hz, and pulsar timing array detectors like NANOGrav on the low end, at nanohertz frequencies. To try and cover this unexamined region, the European Space Agency is developing a space-based detector, the Laser Interferometer Space Antenna (LISA), in collaboration with NASA. LISA will cover frequencies from 0.1 mHz to 0.1 Hz, which will aid in the study of gravitational wave events from compact objects like merging white dwarfs (see Figure 1). 

Like all gravitational wave detectors, LISA operates on the basis of general relativity, in this case a thought experiment originating from Einstein. The idea is that if you can send a photon between two free-falling objects, the frequency of the photon may change due to the curvature of spacetime. If you can measure the change in frequency, you can measure the curvature. A gravitational wave passing by will change the local spacetime curvature and its effect can consequently be measured through the change in frequency. The full scale LISA mission will harness this effect in the form of a giant interferometer in space, with three spacecraft separated by millions of kilometers acting as the interferometers mirrors. 

To test key technologies and concepts in preparation for the LISA mission, LISA Pathfinder was launched in 2015.  LISA Pathfinder effectively shrank LISA down to a single spacecraft, with the separation between the two free-falling test masses reduced to centimeters. The main goal was to show that the two test masses will stay along their geodesic (a straight line in general relativity). LISA will only work if we can keep them along their geodesics, otherwise different forces pushing them around could wash out any gravitational wave signature. Thus these test masses must have a very low relative acceleration. Measuring this acceleration was the main goal of LISA Pathfinder. 

The spacecraft used sensors in combination with a feedback loop to essentially move in response to the falling masses; the spacecraft moved around the masses without touching them. It also shielded the masses from many outside effects, but internal forces could still affect results and increase noise, such as temperature changes, magnetic forces, and radiation. A schematic can be seen in Figure 2. The authors of today’s letter focus on the magnetic diagnostics of LISA Pathfinder and present a measurement of the noise contribution from magnetic effects.

One massive test

The two test masses used were made of a gold and platinum alloy. This combination made for a very dense test mass, which helped reduce changes to its acceleration and for a low magnetic susceptibility. Magnetic susceptibility is a way to quantify how much a material will magnetize in the presence of an applied magnetic field. We don’t want to disturb our test masses with additional magnetic forces, so the lower the magnetic susceptibility, the better. 

Treating the test masses as dipoles in an external magnetic field, the authors predict that the dominant term from magnetic noise in the low frequency regime will be caused by their magnetic susceptibility and the gradient of the local magnetic field, generated by magnetic contributions from various equipment within the spacecraft. Since the susceptibility is already fixed (we can’t change the test masses now that they’re in space), the main effect comes from changes in the local magnetic field. 

Using on-board induction coils, the LISA Pathfinder team generated magnetic fields within the spacecraft to measure the magnetic properties of the test masses. They were able to measure these to precisions an order of magnitude greater than when done on the ground pre-launch. Now we have estimate of the noise caused by these magnetic forces! 

A positive outcome

As seen in Figure 3, the relative acceleration between the test masses is below the noise level required by Pathfinder and LISA, which is great news. And the contribution from magnetic forces is only about 1% of noise power, at 0.1 mHz. 

The authors also find that the interplanetary magnetic field, originating from the Sun, can change this noise value during extreme space weather conditions. By examining the low end of the interplanetary magnetic field fluctuations, they noticed non-stationary behavior (statistical parameters, such as mean, changing over time). This behaviour closely matched the change in solar weather parameters, like the solar wind speed. They find that as a result, the noise contribution from magnetic noise could increase by 4.6 times during extreme solar wind conditions. But, the authors also emphasize that these non-stationary effects should not be directly correlated to the overall fluctuations in relative acceleration between the test masses, since magnetic noise is just one contribution to the full noise model. 

These results are close to initial estimates and meet the requirements of the mission! The authors suggest that in the future the local magnetic field gradient within the spacecraft could be tuned to help reduce this noise, since this gradient is what pairs with the interplanetary magnetic field to cause the bulk magnetic noise. Based on their measured values, they posit that the onboard temperature sensors could be contributing to the local field, but this shouldn’t be an issue in most cases and they could be replaced by platinum sensors in the future, to have no magnetic contribution. With this result, ESA is one step closer towards getting LISA launched and detecting gravitational waves!

Astrobite edited by Nathalie Korhonen Cuestas

Featured image credit: ESA