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Antenna for the detection of electromagnetic audio-band disturbances on-board LISA

This paper characterizes the magnetic sensor coils used in LISA Pathfinder, demonstrating their superior performance in the audio frequency band with noise levels significantly below LISA mission requirements, thereby validating their necessity for monitoring high-frequency electromagnetic disturbances on board the spacecraft.

Original authors: D. Serrano, A. Pérez-Ortega, D. Roma-Dollase, J. Salvans-Tort, J. Ho-Zhang, J. Ramos-Castro, M. Nofrarias

Published 2026-02-09

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CC BY 4.0

Original authors: D. Serrano, A. Pérez-Ortega, D. Roma-Dollase, J. Salvans-Tort, J. Ho-Zhang, J. Ramos-Castro, M. Nofrarias

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the LISA mission as a giant, ultra-sensitive ear floating in space, designed to listen to the faint whispers of gravitational waves—ripples in the fabric of the universe caused by massive cosmic events like colliding black holes. This "ear" is incredibly delicate; even the tiniest nudge can throw off its hearing.

The problem is that the spacecraft itself is full of electronics. Just like your home appliances create a hum of electromagnetic noise, the computers and instruments on the spacecraft generate magnetic "static." If this static is too loud, it could shake the spacecraft's test masses (the free-floating weights inside) just enough to drown out the cosmic whispers the mission is trying to hear.

The "Antenna" Solution
The authors of this paper looked at a piece of hardware left over from a previous mission called LISA Pathfinder. Originally, this hardware was a set of copper coils designed to push on the test masses with magnetic fields (like a speaker pushing air). However, the team realized these same coils could be flipped around to listen to magnetic fields instead.

Think of it like a microphone. A speaker pushes air to make sound; a microphone catches sound waves to create electricity. These coils are being repurposed from "speakers" into highly sensitive "microphones" for magnetic fields.

Why Listen to "Audio" Frequencies?
LISA is designed to listen to very low-frequency gravitational waves (millihertz). However, the electronics on the spacecraft generate noise at much higher frequencies (the "audio" range, from 50 Hz to 500 Hz).

The authors explain that even though the spacecraft is shielded, high-frequency magnetic noise can sometimes "leak" through or get modulated in a way that creates a slow, annoying hum that interferes with the main measurement. It's like trying to hear a violin solo in a room where someone is constantly tapping a drum nearby; even if the drum isn't playing the same note, the vibration can ruin the recording.

How Good is the Microphone?
The team tested these repurposed coils in a lab, wrapping them in special magnetic blankets (mu-metal shields) to block out Earth's own magnetic field and the hum of the building's electricity.

They found the coils are incredibly sensitive:

The Performance: They can detect magnetic whispers as faint as 0.17 picotesla (that's 0.00000000000017 Tesla) at 500 Hz.
The Comparison: This performance is ten times better than what the LISA mission actually requires. It's like having a microphone that is so good it can hear a pin drop, while the mission only needed it to hear a whisper.

The "Blind Spots" and the Fix
One challenge with a single coil is that it has "blind spots." If a magnetic disturbance comes from a specific angle (like a sound coming from directly above a microphone), the coil might not hear it at all.

To solve this, the paper suggests placing two of these coils on the spacecraft, positioned at an angle to each other (about 60 degrees apart, following the shape of the spacecraft's telescopes).

The Analogy: Imagine two ears. If a sound comes from one side, one ear hears it clearly while the other might hear it less. By having two ears (coils) facing different directions, the spacecraft can "hear" magnetic disturbances coming from almost any angle.
The Result: The authors calculated that with this two-coil setup, the spacecraft can detect magnetic noise from any electronic device on board, ensuring that no "static" goes unnoticed.

The Bottom Line
The paper concludes that by reusing the old coils from the Pathfinder mission as magnetic "ears," the LISA mission will have a safety net. This system can detect and flag high-frequency magnetic disturbances, ensuring that the spacecraft knows exactly when its own electronics are making too much noise, allowing scientists to ignore those bad data points and keep listening for the true sounds of the universe.

Technical Summary: Antenna for the detection of audio-band electromagnetic disturbances on-board LISA

Problem Statement
The Laser Interferometer Space Antenna (LISA) mission aims to detect gravitational waves in the millihertz frequency band. While the primary science band is low-frequency, high-frequency magnetic fields (audio-band, 20 Hz–20 kHz) generated by on-board electronics pose a potential threat to instrument sensitivity. These fields can induce forces on the free-falling test masses (TMs) through mechanisms such as the down-conversion of high-frequency fields via the $B \cdot \nabla B$ term or amplitude modulation coupling into the millihertz band. Although the TM vacuum enclosure provides shielding, it is less effective at the lower end of the audio band (hundreds of Hz). Consequently, there is a need for a sensor capable of monitoring these high-frequency magnetic disturbances to raise quality flags during in-flight operations. The LISA mission specifies a sensitivity requirement for such a sensor of $S^{1/2}_{B,req} < \sqrt{1 + 10^6/f^2}$ pT/ $\sqrt{\text{Hz}}$ for frequencies between 50 Hz and 500 Hz.

Methodology
The authors propose repurposing the injection coils originally designed for the LISA Pathfinder (LPF) mission. In LPF, these coils were used to apply controlled magnetic forces to test masses; for LISA, where such force experiments are not planned, they are re-evaluated as Search Coil Sensors (SCS) for the audio band.

The study involves:

Theoretical Modeling: Deriving the magnetic force on TMs from high-frequency sources and establishing the sensitivity requirements. The authors model the LPF coil as an air-core SCS, avoiding ferromagnetic cores to prevent low-frequency noise and magnetic field gradients.
Circuit Characterization: An equivalent RLC circuit model is developed, including the coil's resistance ( $R$ ), inductance ( $L$ ), and self-capacitance ( $C$ ), along with a parallel load resistor ( $R_0$ ) to flatten the frequency response and a non-inverting operational amplifier for signal conditioning. Noise budgets are calculated by summing the contributions of the amplifier, resistors, and data acquisition unit (DAU).
Experimental Validation: The LPF coil (2400 turns of copper wire on a titanium frame) was tested in a laboratory environment. A three-layer mu-metal shield was used to suppress Earth's magnetic field and building mains noise. A Helmholtz coil generated controlled sinusoidal magnetic fields for sensitivity calibration, while a fluxgate magnetometer provided reference measurements. Noise performance was measured using a high-sampling-rate DAU (400 kHz) and analyzed via Welch's method.

Key Contributions and Results

Performance Exceeding Requirements: The characterization demonstrates that the LPF coils, when used as SCS, significantly outperform the current LISA requirements. The measured magnetic noise floor is 1.45 pT/ $\sqrt{\text{Hz}}$ at 50 Hz and 0.17 pT/ $\sqrt{\text{Hz}}$ at 500 Hz. This represents a performance one order of magnitude better than the required sensitivity in the 50–500 Hz band.
High-Frequency Capability: Beyond the audio band, the coils achieve a noise floor of 0.1 pT/ $\sqrt{\text{Hz}}$ at frequencies above 1 kHz, despite being air-core sensors.
Detection Capability: Experimental tests confirmed the ability to detect a 50 pT magnetic signal at 50 Hz with a Signal-to-Noise Ratio (SNR) of 20.2 ± 1.4, validating the sensor's ability to identify potential disturbances.
Detection Range Analysis: The authors modeled the detection range based on the orientation of magnetic dipoles relative to the coil axis. They conclude that the proposed placement of two coils (one per telescope) within the LISA spacecraft, oriented approximately 60° apart, allows for the detection of high-frequency magnetic sources from on-board electronics with high SNR across the entire spacecraft volume, mitigating "blind spots" associated with single-coil orientation.

Significance
The paper establishes that repurposing the heritage LPF injection coils as audio-band magnetometers is a viable and highly effective solution for the LISA mission's Science Diagnostics Subsystem (SDS). The study confirms that these sensors can monitor high-frequency magnetic disturbances originating from spacecraft electronics, which are otherwise undetectable by the low-frequency fluxgate magnetometers used on LPF. By providing a sensor with sensitivity well below the mission requirements, the work offers a robust precautionary measure to identify non-nominal conditions that could degrade the gravitational wave measurement, ensuring the integrity of the LISA mission's scientific output.

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