Modelling spacecraft-emitted electrons measured by SWA-EAS experiment on board Solar Orbiter mission

This paper utilizes numerical simulations with the Spacecraft Plasma Interaction Software to model and validate how photo- and secondary electron emissions from the Solar Orbiter spacecraft contaminate low-energy electron measurements by the SWA-EAS instrument, revealing that such contamination extends well above the spacecraft potential threshold due to emissions from distant surfaces and highlighting a discrepancy between the simulated and observed spectral breaks that suggests a difference between the detector's actual and measured potentials.

The Gist

The Big Picture: A Spacecraft in a Storm of Particles

Imagine the Solar Orbiter spacecraft as a giant, floating house traveling through the solar wind. The solar wind isn't empty space; it's a constant, invisible storm of tiny, charged particles (mostly electrons and protons) rushing past the ship at supersonic speeds.

The scientists on board have a very sensitive instrument called SWA-EAS. Think of this instrument as a "particle rain gauge." Its job is to count the rain (electrons) coming from the sky (the solar wind) to understand the weather (plasma conditions) outside.

The Problem:
The "house" (the spacecraft) isn't just a passive observer. Because it's moving through this storm and being hit by sunlight, the house itself starts to act like a battery. It gets electrically charged (usually positively, like a balloon rubbed on your hair).

This charge causes two messy problems for the rain gauge:

1. The "Static Cling" Effect: The charged house attracts the rain (electrons) from far away, speeding them up so they hit the gauge harder than they naturally would.
2. The "Spitback" Effect: The house itself starts spitting out its own tiny electrons (like static electricity making your hair stand up). These "house electrons" are cold and slow, and they mix in with the "sky electrons," making it hard to tell which is which.

The scientists wanted to know: How much of what the rain gauge sees is real weather, and how much is just the house messing things up?

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The Experiment: Building a Virtual Twin

Since they couldn't stop the real spacecraft to clean the sensor, they built a digital twin (a computer simulation) of the Solar Orbiter.

• The Simulation: They used a software called SPIS (Spacecraft Plasma Interaction Software). Imagine this as a super-advanced video game engine that simulates physics. They built a 3D model of the spacecraft, complete with its solar panels, antennas, and the rain gauge (SWA-EAS) sitting on a long boom at the back.
• The Setup: They set the "weather" in the simulation to match two specific real-life moments when the Solar Orbiter was flying near the Sun (about 0.3 AU away).
  • Case A: A "crowded" day with lots of solar wind particles (low spacecraft charge).
  • Case B: A "sparse" day with fewer particles (high spacecraft charge).

They then let the simulation run and watched how the virtual rain gauge measured the electrons.

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The Surprising Discovery: The "Ghost" Electrons

In simple physics, you might expect a clear line in the data. You'd think: "Electrons below 5 volts are from the house; electrons above 5 volts are from the sky." The "break" in the data should happen exactly where the spacecraft's electrical charge ends.

But the real data was weird.
The scientists found that the "house electrons" were showing up in the data way above the expected energy limit. It was as if the house was spitting out electrons that were somehow gaining enough energy to look like they came from the sky.

The Simulation Revealed the "Why":
The computer model acted like an X-ray vision, allowing them to see exactly where every electron came from. They discovered a clever trick of geometry and electricity:

1. The Distant Spitters: Electrons weren't just coming from the sensor itself. They were being emitted from distant parts of the spacecraft, like the solar panels or the heat shield, which are far away from the sensor.
2. The Roller Coaster Ride: Imagine an electron leaving a solar panel. It starts slow. As it travels toward the sensor, it has to navigate the spacecraft's complex electric field.
   • It might get slowed down as it leaves the panel (climbing a hill).
   • But then, as it approaches the sensor, the positive charge of the main spacecraft body yanks it forward, accelerating it like a roller coaster dropping down a hill.
3. The Result: By the time these "distant house electrons" hit the sensor, they have gained enough speed to look like "sky electrons." They cross the energy threshold that scientists usually use to separate the two groups.

Analogy: Imagine you are standing in a field (the sensor) trying to count birds flying over (the solar wind). But your house (the spacecraft) has a fan blowing out the back door. The fan blows a leaf (a house electron) out. The leaf gets caught in a wind tunnel created by the house's shape, speeds up, and flies over your head looking exactly like a bird. You count it as a bird, but it's actually a leaf from your own house.

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Why This Matters

The paper concludes with two major takeaways:

1. The "Break" is a Liar: Scientists often use the point where the data "breaks" (the energy level where house electrons stop and sky electrons start) to guess the spacecraft's voltage. This study shows that for the Solar Orbiter, that method is unreliable. The "break" happens at a higher energy than the actual voltage because of those distant, accelerating electrons.
2. The Sensor Might Be Different: The simulation showed that the sensor itself might have a slightly different electrical charge than the main body of the spacecraft. This is like if the front door of your house had a different static charge than the back door. This difference could explain why the real data didn't perfectly match the simulation.

The Bottom Line

The Solar Orbiter is a brilliant machine, but it's not a perfect observer. It's like a person trying to listen to a quiet conversation in a room while they are also shouting and moving around.

This study used a computer to figure out exactly how the spacecraft's own "shouting" (electron emissions) is distorting the "conversation" (solar wind data). They found that the spacecraft is sneaky: it sends electrons from far away that get a free ride to the sensor, tricking the instruments.

The lesson for the future: When scientists analyze data from the Solar Orbiter, they can no longer just subtract a simple number to fix the data. They have to account for this complex "roller coaster" effect where the spacecraft's own shape and distant surfaces are secretly boosting the energy of its own electrons.

Technical Summary

1. Problem Statement

Spacecraft immersed in plasma environments inevitably emit cold electrons due to photoelectric effects (sunlight) and secondary electron emission (impact by ambient plasma). These emissions, combined with spacecraft charging, significantly distort local plasma properties and contaminate in-situ measurements of ambient electron fluxes.

• The Specific Issue: For the Solar Orbiter mission, previous observations (Štverák et al., 2025) revealed a discrepancy: the "break" in the electron energy spectrum (the point where spacecraft-emitted electrons transition to ambient solar wind electrons) often occurs at energies significantly higher than the measured spacecraft potential ($\Phi_{SC}$).
• The Challenge: Standard theoretical models assume that spacecraft-emitted electrons are trapped below the potential energy threshold ($e\Phi_{SC}$) and that the spectral break occurs exactly at this threshold. The observed deviation suggests that electrons emitted from distant spacecraft surfaces can reach the detector with energies above the local potential, complicating the derivation of unperturbed plasma parameters (density, temperature, bulk velocity).

2. Methodology

The authors employed numerical simulations to replicate the interaction between the Solar Orbiter spacecraft and the solar wind plasma, aiming to isolate and quantify the sources of electron contamination.

• Simulation Tool: The study utilized Spacecraft Plasma Interaction Software (SPIS), a particle-in-cell (PIC) code capable of modeling plasma-surface interactions.
• Geometry & Materials: A simplified but representative 3D model of the Solar Orbiter was constructed, including the main body, sun shield, solar arrays, high-gain antenna (HGA), payload boom, and the SWA-EAS detector. Different surface materials (e.g., Black Kapton, solar cells, steel) were assigned specific electrical nodes and emission properties.
• Virtual Detector: A virtual spherical detector was placed at the tip of the payload boom (mimicking the real SWA-EAS position) to record incoming particle fluxes. Crucially, this detector could distinguish between:
  • Ambient Electrons (AE)
  • Photoelectrons (PE)
  • Secondary Electrons (SE) from electron impact
  • Secondary Electrons from ion impact (SI)
  • Source Location: The model could trace detected electrons back to their specific emission surface (e.g., solar panels vs. the detector baffle).
• Case Studies: Two simulation runs were performed corresponding to real Solar Orbiter data samples taken at ~0.3 AU:
  • Run A: High plasma density ($30 \text{ cm}^{-3}$), low spacecraft potential ($\sim 2.3 \text{ V}$).
  • Run B: Low plasma density ($4.3 \text{ cm}^{-3}$), high spacecraft potential ($\sim 8.0 \text{ V}$).
• Parameters: Initial plasma conditions (density, temperature, bulk speed) were derived from RPW and SWA-PAS instruments. The model included a finite electrical resistance ($1 \text{ M}\Omega$) between structural nodes to simulate imperfect grounding, a key factor in achieving realistic potentials.

3. Key Contributions

• Source Separation: Unlike real instruments, the SPIS model successfully separated the total electron flux into contributions from specific spacecraft surfaces, proving that contamination is not a monolithic population but a superposition of fluxes from diverse locations.
• Mechanism Explanation: The study provided the first direct numerical evidence explaining why spacecraft electrons are detected above the potential threshold. It demonstrated that electrons emitted from distant, highly charged surfaces (like solar panels) are decelerated as they leave the surface but are subsequently re-accelerated by the positive potential of the main spacecraft body and the detector, allowing them to impact the detector with energies exceeding the local potential.
• Potential Profile Analysis: The simulations revealed non-monotonic potential profiles (including negative potential wells in the wake) under certain conditions (Run A), which act as additional barriers or reflectors for low-energy electrons, further altering the measured spectra.

4. Key Results

• Qualitative Agreement: The simulated energy spectra matched real SWA-EAS data well, confirming that cold electron contamination persists well above the spacecraft potential energy threshold.
• Break Location Discrepancy:
  • In simulations, the peak of the spacecraft-emitted flux occurred exactly at the spacecraft potential ($e\Phi_{SC}$).
  • In real data, the peak often occurred significantly higher (e.g., 5–6 V vs. 2–3 V in Run A).
  • Interpretation: This suggests the potential of the SWA-EAS detector itself may differ from the potential of the main spacecraft body measured by the RPW antennas, likely due to imperfect electrical coupling.
• Source-Dependent Spectra:
  • Photoelectrons: Emitted from sun-lit surfaces (solar panels, heat shield). Their energy distribution is narrow and peaks near the spacecraft potential because only particles with specific trajectories and sufficient initial energy can escape the source and reach the detector.
  • Secondary Electrons: Emitted from all surfaces, including those close to the detector (baffles, boom). Their distribution is broad (spanning 0–20 eV) and dominates the low-energy spectrum.
• Potential Barriers: In the high-density/low-potential case (Run A), a negative potential sheath formed in the spacecraft wake. This acted as a barrier, reflecting some escaping electrons back toward the detector, effectively modifying the energy threshold for detection.

5. Significance and Implications

• Re-evaluation of Spacecraft Potential: The study challenges the assumption that the spacecraft potential measured by RPW is identical to the potential of the SWA-EAS detector. Using the RPW value to correct SWA-EAS data may introduce systematic errors in derived plasma parameters.
• Limitations of the "Break" Proxy: The spectral break cannot be used as a reliable proxy for the spacecraft potential on Solar Orbiter because it is shifted by the complex geometry and multi-source nature of electron emissions.
• Data Processing: The findings imply that deriving unperturbed ambient plasma properties (density, temperature, heat flux) from SWA-EAS data requires more sophisticated correction models that account for:
  1. The specific geometry of the spacecraft and detector.
  2. The potential difference between the detector and the main bus.
  3. The distinct contributions of photo- and secondary electrons from various surfaces.
• Future Directions: The authors propose using directional analysis of 3D velocity distributions to identify "clean" viewing angles (uncontaminated by distant surfaces) to better estimate the true detector potential. They also suggest future model improvements, such as including the interplanetary magnetic field and refining the spacecraft geometry.

In summary, this paper uses high-fidelity numerical modeling to resolve a long-standing discrepancy in Solar Orbiter electron data, demonstrating that spacecraft geometry and multi-surface emission physics fundamentally alter the measured electron energy spectra, necessitating a revision of standard data correction protocols.