Anomalous cosmic rays within the inner heliosphere: Observations of helium by the High Energy Telescope onboard Solar Orbiter

This paper presents the first observations of anomalous cosmic ray helium by Solar Orbiter's High Energy Telescope between 0.3 and 1 au, deriving a radial gradient of approximately 22–32%/au and demonstrating that these gradients increase with enhanced solar modulation and a larger heliospheric current sheet tilt angle.

The Gist

The Cosmic "Wind" and the Solar Orbiter's Journey

Imagine the Sun isn't just a ball of fire, but a giant lighthouse in space. It constantly blows a "wind" made of charged particles (the solar wind) and has a massive, invisible magnetic field stretching out for billions of miles. This entire bubble is called the heliosphere.

Inside this bubble, there are two types of "space dust" floating around:

1. Galactic Cosmic Rays (GCRs): These are heavy, fast particles from deep space (supernovas) trying to crash into our solar system.
2. Anomalous Cosmic Rays (ACRs): These are the paper stars of this story. They start as neutral gas drifting in from deep space, get ionized (charged up) by the Sun, get pushed out to the edge of the solar system, get accelerated like a slingshot, and then come rushing back in toward the Sun.

The Mystery: Scientists have long wondered how these particles move. Do they drift smoothly? Do they get stuck? The movement depends heavily on the Sun's magnetic field, which flips its polarity (like a magnet switching North/South) every 11 years.

The New Detective: Solar Orbiter

For a long time, we only had a view of this "cosmic wind" from Earth's neighborhood (about 100 million miles away). But in 2020, the Solar Orbiter launched. Think of it as a spy plane that dives much closer to the Sun, getting as close as 29 million miles.

This paper is about what this spy plane found while it was diving toward the Sun between 2020 and 2022, specifically looking at Helium particles (a specific type of ACR).

The Experiment: Comparing the "Weather"

To understand how the particles change as they get closer to the Sun, the scientists needed a baseline. They used data from two other "weather stations":

• SOHO: A satellite hovering near Earth.
• ACE: Another satellite near Earth.
• Chang'e-4: A lander on the far side of the Moon (used for a quick check).

The Analogy: Imagine you are trying to figure out how the wind speed changes as you walk from the edge of a forest toward a tree.

• SOHO/ACE are people standing at the edge of the forest (1 AU away).
• Solar Orbiter is the person walking toward the tree (getting closer to 0.3 AU).
• The scientists compared what the walker felt vs. what the people at the edge felt.

The Big Findings

The team had to be very careful. They had to filter out "storms" (solar flares) that would mess up the data, just like you wouldn't measure the average wind speed during a hurricane. They also had to account for the fact that the Sun was waking up from a nap (solar minimum) and starting to get active again.

Here is what they discovered:

1. The Gradient is Steep (The "Hill" is Steeper than we thought)
Scientists measure the "radial gradient," which is basically how much the particle count changes as you get closer to the Sun.

• The Result: They found that the concentration of these Helium particles changes very rapidly as you get closer to the Sun.
• The Analogy: Imagine walking down a hill. If the hill is gentle, you don't feel much change in elevation. But if it's a steep cliff, you feel a huge change in just a few steps. The Solar Orbiter found that the "slope" of the particle density is much steeper in the inner solar system than we previously thought based on data from further away.
• The Numbers: They found a gradient of about 22% to 32% per Astronomical Unit (AU). This means for every step you take toward the Sun, the particle density changes significantly.

2. The Sun's "Magnetic Fence" is Getting Tangled
The paper notes that as the Sun became more active (more sunspots, more magnetic chaos), the gradient got even steeper.

• The Analogy: Think of the Sun's magnetic field as a fence. When the fence is straight and calm (solar minimum), particles can drift through easily. But when the fence gets twisted and tangled (solar maximum, high tilt angle), it becomes harder for particles to move in a straight line. They get "bunched up" or deflected more, creating a steeper difference in density between the edge and the center.

3. It Matches the "Spy Plane" from Before
The results matched what the Parker Solar Probe (PSP) saw a few years earlier. This is a big deal because it confirms that our understanding of how particles move in this "inner zone" is consistent, even though the two probes were there at slightly different times.

Why Does This Matter?

Think of the solar system as a giant highway.

• Before: We knew how traffic moved on the outer highways (far from the Sun).
• Now: We have a map of the inner city streets (close to the Sun).

The paper tells us that the "traffic rules" (physics) change as you get closer to the Sun. The magnetic fields act like a complex maze. If we want to protect future astronauts or understand how our solar system interacts with the galaxy, we need to know exactly how these particles behave in this inner maze.

The Bottom Line

The Solar Orbiter dove deep into the Sun's magnetic playground and found that Anomalous Cosmic Rays (Helium) pile up and change density much faster near the Sun than we expected. As the Sun gets "angrier" (more active), this effect gets even stronger. This helps scientists build better models of how the universe's "weather" affects our neighborhood.

Technical Summary

1. Problem Statement

Understanding the transport mechanisms of cosmic rays (CRs) within the heliosphere is critical for space physics. While the Parker transport equation describes CR propagation, the specific roles of diffusion, convection, and particle drifts in the inner heliosphere (within 1 au) remain poorly constrained due to a lack of in-situ measurements.

• The Gap: Previous observations of Anomalous Cosmic Rays (ACRs) were largely limited to the outer heliosphere (>1 au) or specific missions like the Parker Solar Probe (PSP).
• The Discrepancy: Recent PSP observations of ACR oxygen showed radial gradients inconsistent with models predicting smaller gradients during the positive magnetic polarity (A+) phase of the solar cycle. Instead, they resembled gradients from negative polarity (A-) cycles.
• The Goal: This study aims to derive the radial gradient of ACR helium in the inner heliosphere (0.3–1 au) using Solar Orbiter data to investigate how particle drift effects and large-scale magnetic fields influence transport, particularly during the transition from solar minimum to maximum.

2. Methodology

The study utilizes data from the High Energy Telescope (HET) onboard the Solar Orbiter (SolO) mission, covering the period from February 2020 to July 2022 (the second half of the solar minimum between cycles 24 and 25).

• Data Selection & Cleaning:
  • Quiet Time Selection: Periods of Solar Energetic Particle (SEP) events and Interplanetary Coronal Mass Ejections (ICMEs) were rigorously removed to isolate the background ACR and Galactic Cosmic Ray (GCR) populations.
  • Temporal Averaging: Data were averaged over Carrington rotation periods (defined from the Solar Orbiter's perspective, ranging 26.6–35.8 days) to mitigate short-term modulations caused by Stream Interaction Regions (SIRs) and Corotating Interaction Regions (CIRs).
• Instrument Cross-Comparison:
  • Helium spectra from SolO/HET were compared with SOHO/EPHIN (at L1), ACE/SIS (at L1), and Chang'e-4/LND (lunar surface) to validate instrument calibration and spectral consistency near 1 au.
  • SOHO/EPHIN served as the baseline for long-term solar modulation correction because it provided continuous coverage in the 10–50 MeV/nuc range.
• Gradient Calculation:
  • The radial gradient ($g_r$) was calculated using the formula: $g_r = \frac{\partial \ln f}{\partial r}$, where $f$ is the de-trended flux.
  • A linear fit was applied to the logarithm of the intensity ratio (SolO/SOHO) versus the radial distance ($r$) of Solar Orbiter.
• GCR Subtraction:
  • Since the energy range (11.1–49 MeV/nuc) contains both ACRs and GCRs, the authors used the Badhwar-O'Neill 2020 (BON2020) model to estimate and subtract the GCR contribution, isolating the pure ACR radial gradient.

3. Key Contributions

• First Inner Heliosphere ACR Helium Observations: This paper presents the first dedicated observations of ACR helium by Solar Orbiter/HET in the inner heliosphere (0.29–1 au) prior to the solar activity maximum.
• Quantification of Radial Gradients: It provides the first derived radial gradients for ACR helium specifically in the 0.3–1 au region, bridging the gap between L1 observations and deep-space measurements.
• Temporal Evolution Analysis: The study analyzes how radial gradients evolve as solar activity increases (from 2020 to 2022), linking these changes to the tilt angle of the Heliospheric Current Sheet (HCS) and sunspot numbers.
• Model Validation: It validates the BON2020 GCR model against in-situ penetrating channel data, confirming its reliability for subtracting GCR backgrounds in the inner heliosphere.

4. Key Results

• Spectral Consistency: Solar Orbiter/HET spectra (10–50 MeV/nuc) showed excellent agreement with SOHO/EPHIN and ACE/SIS when Solar Orbiter was near 1 au, with an average intensity ratio of $0.962 \pm 0.016$.
• Radial Gradient Values:
  • Raw Gradient (ACR + GCR): The average radial gradient between 11.1 and 49 MeV/nuc was $22 \pm 4\%$ /au.
  • Corrected Gradient (ACR only): After removing the GCR contribution, the average gradient for the 11.1–41.2 MeV/nuc range increased to $32 \pm 8\%$ /au.
  • Energy Dependence: The gradient was found to be energy-dependent, with values ranging from $17 \pm 9\%$ /au (29.5–41.2 MeV/nuc) to $35 \pm 8\%$ /au (11.1–19.4 MeV/nuc) after GCR subtraction.
• Comparison with PSP: The results are consistent with Parker Solar Probe (PSP) measurements (Rankin et al. 2021), which reported gradients of $\sim 25\%$/au (raw) and $\sim 34-45\%$/au (GCR-corrected) during the same solar minimum.
• Temporal Trends:
  • The radial gradients increased as the solar cycle progressed from minimum to maximum.
  • The average gradient rose from $15 \pm 5\%$ /au in the first orbit (2020) to $29.5 \pm 8\%$ /au in the third orbit (2021).
  • This trend correlates with the increasing tilt angle of the HCS (reaching $>70^\circ$) and rising sunspot numbers, suggesting that enhanced solar modulation and magnetic turbulence restrict particle drift, steepening the radial gradient.

5. Significance

• Transport Physics: The findings confirm that radial gradients in the inner heliosphere are significantly steeper than those in the outer heliosphere. This supports the theory that the magnetic field structure within 1 au (dominated by radial components) differs fundamentally from the outer heliosphere (dominated by transverse components), affecting cross-field diffusion and drift.
• Drift Effects: The increasing gradient with solar activity aligns with theoretical predictions that a highly tilted HCS (during high solar activity) inhibits the drift of ACRs into the inner heliosphere, effectively "trapping" them further out and creating a steeper intensity drop-off toward the Sun.
• Future Missions: These results provide essential constraints for cosmic ray transport models. They set the stage for future investigations into latitudinal gradients as Solar Orbiter moves to higher latitudes (post-2025) and for synergy with the upcoming IMAP mission, offering a comprehensive 3D view of particle transport from the equator to the poles and from the Sun to the outer heliosphere.