🔭 astrophysics

Visualizing the Magnetic Structure in Interplanetary Coronal Mass Ejections with ATHARV

This paper introduces ATHARV, a publicly available tool that converts in-situ multipoint observations into a three-dimensional spatial representation of the locally sampled plasma and magnetic field along spacecraft trajectories, enabling the inference of mesoscale inhomogeneities and flux-rope distortions through comparisons between these representations.

Original authors: Vivek Menon, Jyoti Sheoran, Vaibhav Pant, Dipankar Banerjee

Published 2026-06-02

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CC BY 4.0

Original authors: Vivek Menon, Jyoti Sheoran, Vaibhav Pant, Dipankar Banerjee

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

The Problem: The "Single-Track" Mystery

Imagine you are trying to figure out the shape of a giant, invisible, spinning balloon floating through space. You can't see the whole thing; you can only fly a tiny drone through it. As your drone flies along its specific path, it records data every second: "Here the wind is fast," "Here the magnetic field is strong," "Here the wind is slow."

This is exactly what scientists face with Coronal Mass Ejections (CMEs). These are massive bubbles of magnetized plasma (super-hot gas) shot out by the Sun. When they hit Earth, they can cause auroras or mess up satellites.

The problem is that our spacecraft only sample these bubbles along their own specific trajectory. They give us a one-dimensional time-series slice of a three-dimensional object. It's like trying to guess the shape of a complex sculpture just by looking at the shadow it casts on a wall, or by running your hand along a single curve of the statue. You might think it's a simple circle, but it could actually be a twisted pretzel.

Furthermore, these solar bubbles aren't static; they are expanding like a rising loaf of bread as they travel. If you don't account for the bread rising while you are tracing its surface, your measurements of the loaf's size and shape will be wrong.

The Solution: ATHARV (The "Local Map" Maker)

The authors created a new tool called ATHARV. Think of ATHARV as a special mapmaker that takes the "drone's" one-dimensional flight log and reconstructs a local 3D picture of the specific part of the solar bubble the spacecraft passed through.

Here is how it works:

Rewinding Time: ATHARV takes the data recorded as the spacecraft moved along its path and mathematically "rewinds" the motion. It asks: "If the bubble was smaller and the spacecraft was in a different spot, where would this specific piece of gas have been?"
Accounting for Expansion: It knows the bubble is stretching. It adjusts the map to show how the gas parcels moved apart from each other as the bubble grew.
The Result: Instead of a flat line of data, ATHARV produces a 3D visualization (a "quiver plot") of the local region the spacecraft sampled. You can see arrows representing the magnetic field, showing exactly how they twist and turn in that specific slice of space.

The Experiment: A Double-Check with Two Drones

To test this tool, the scientists looked at a specific solar event that happened on April 23–24, 2023. They had a unique advantage: two spacecraft (STEREO-A and Wind) flew through the same solar bubble at almost the same time, but from slightly different angles.

Think of it like two people walking through a giant, twisting tunnel from slightly different paths.

Person A (STEREO-A) walked through the center.
Person B (Wind) walked through the side.

What They Found

Using ATHARV, they reconstructed the local magnetic structure of the bubble for both spacecraft.

The "Messy" Front: The front part of the bubble (the "sheath") was chaotic. The magnetic arrows were pointing everywhere, like a pile of tangled headphones. This is expected because the bubble crashes into the solar wind, creating turbulence.
The "Twisted" Core: The main body of the bubble (the "Magnetic Ejecta") was much more organized. The magnetic field lines were wrapped around a central axis like a spiral staircase or a twisted rope. This is called a "flux rope."
The Twist in the Tale:
- Wind's View: The spacecraft saw a clean, simple twist. The magnetic field rotated about 180 degrees (a half-turn), which looks like a standard, neat rope.
- STEREO-A's View: This spacecraft saw something wilder. The magnetic field rotated 330 degrees (almost a full turn). It looked like the rope was writhe or kinked.

The Big Reveal: Because the two spacecraft saw such different things in their local samples, the scientists realized the solar bubble wasn't a perfect, straight cylinder. It was likely distorted, bent, or twisted in 3D space. If they had only looked at the data from one spacecraft, they would have thought the bubble was either a simple rope (if they only saw Wind's data) or a broken, complex mess (if they only saw STEREO-A's data).

The Takeaway

The paper concludes that single-point measurements are dangerous because they can trick you. A solar bubble might look simple from one angle and complex from another.

ATHARV is the tool that helps scientists stop guessing and start visualizing the local 3D structure of these solar storms. By comparing the local 3D pictures from multiple spacecraft, we can infer mesoscale inhomogeneities and distortions in the storm's structure. This is crucial for predicting how these complex, twisted structures will affect our technology on Earth.

In short: ATHARV turns a flat, confusing line of numbers into a 3D movie of the specific slice of a solar storm a spacecraft passed through. By comparing these local slices from different angles, we can reveal that what looks like a simple rope from one side might actually be a knotted, twisted mess from another.

Technical Summary: ATHARV – A Tool for Spatial Reconstruction of ICME Magnetic Structure

Problem Statement
Interplanetary Coronal Mass Ejections (ICMEs) are primary drivers of space weather, with their geoeffectiveness dictated by the orientation and structure of their internal magnetic fields. However, in-situ spacecraft observations are inherently limited to one-dimensional time-series sampling along a specific trajectory. This limitation obscures the three-dimensional magnetic structure and evolution of ICMEs. Furthermore, conventional methods for estimating ICME size and expansion often fail to account for the combined effects of the ICME's intrinsic expansion and the spacecraft's relative motion. This can lead to significant overestimations of radial sizes, particularly for fast-moving missions or structures with strong expansion signatures. Additionally, interpreting magnetic configurations from single-point measurements often misses mesoscale inhomogeneities and structural distortions that are critical for understanding ICME evolution.

Methodology: The ATHARV Framework
To address these limitations, the authors introduce the Analysis Tool for Heliospheric Arrangement of Remapped Vectors (ATHARV). This framework remaps in-situ time-series data into spatial coordinates to reconstruct a three-dimensional spatial representation of the plasma and magnetic field along the spacecraft trajectory (the locally sampled region), rather than the full 3D structure of the entire ICME. The methodology operates under the assumption that the magnetic field is frozen into the plasma and does not evolve appreciably during the spacecraft passage.

The tool employs two distinct remapping strategies based on the observed velocity profiles:

Case A (Uniform Positive Expansion): For ICMEs exhibiting a linear decline in bulk velocity from the leading to the trailing edge (indicating self-similar expansion), ATHARV assumes uniform expansion with potentially different rates along three orthogonal directions. It calculates the position of plasma parcels at a reference time ( $t_0$ ) by solving for the displacement based on the center propagation velocity ( $v_{cm}$ ) and expansion velocity ( $v_{exp}$ ).
Case B (Non-uniform or Negative Expansion): For sheaths or dynamically perturbed ejecta where velocity profiles are non-linear or indicate compression (negative expansion), the tool treats measured in-situ velocities as proxies for Lagrangian velocities. It traces parcel trajectories backward in time to estimate their positions at $t_0$ .

The framework integrates these spatial reconstructions with complementary diagnostics, including hodograms and magnetic-field orientation angles, to assess magnetic coherence and rotation. The tool is implemented as an online web interface accessible to the heliophysics community.

Application and Results
The authors demonstrated ATHARV using multipoint in-situ observations of an ICME detected on April 23–24, 2023, by the STEREO-A and Wind spacecraft near 1 au. The spacecraft were separated by small radial ( $\sim$ 0.04 au) and longitudinal ( $\sim$ 10.2 $^\circ$ ) distances, allowing for mesoscale sampling. Applying ATHARV to multiple spacecraft and comparing the resulting spatial representations enabled the inference of mesoscale inhomogeneities and flux-rope distortions.

Spatial Reconstruction: ATHARV reconstructed the ICME structure in a non-rotating heliocentric coordinate system. The tool estimated the radial sizes of the sheath and Magnetic Ejecta (ME) by accounting for expansion.
- Sheath: Estimated at $\sim$ 0.085 au (STEREO-A) and $\sim$ 0.12 au (Wind).
- ME: Estimated at $\sim$ 0.28 au (STEREO-A) and $\sim$ 0.25 au (Wind).
- Comparison: Conventional duration-based estimates yielded significantly larger ME sizes (0.40 au at STEREO-A and 0.27 au at Wind), particularly at STEREO-A where expansion was stronger. This confirms that conventional methods overestimate the true radial size of expanding MEs.
Magnetic Structure:
- The sheath regions at both spacecraft displayed highly variable and disordered magnetic fields, consistent with turbulence.
- The ME regions exhibited coherent magnetic field rotations consistent with a south–west–north (SWN) flux-rope configuration with right-handed helicity.
- Mesoscale Inhomogeneity: Despite the consistent global configuration, significant differences were observed between the two spacecraft.
  - Wind: Showed a gradual decline in magnetic field magnitude and a total rotation angle of $\sim$ 180 $^\circ$ , consistent with a classical, simple flux-rope traversal.
  - STEREO-A: Displayed a more symmetric magnetic field profile and a total rotation angle exceeding 180 $^\circ$ (reaching $\sim$ 330 $^\circ$ ). This suggests a double rotation or a more complex path through the structure.

Significance and Claims
The paper claims that ATHARV provides a consistent framework for interpreting in-situ ICME observations by explicitly separating temporal evolution from spatial structure. The key findings and claims include:

Correction of Size Estimates: The tool demonstrates that ignoring expansion and spacecraft motion leads to systematic overestimation of ICME radial sizes. ATHARV offers a physically more consistent method for determining true spatial extents.
Revealing 3D Complexity: The multipoint application of ATHARV revealed that ICMEs can possess mesoscale inhomogeneities. The same event appeared as a "simple ejecta" at Wind but a "complex ejecta" (with distorted geometry and multiple rotations) at STEREO-A, despite their close proximity. This highlights the limitations of single-point measurements in inferring global magnetic configurations.
Structural Interpretation: The large rotation angle and symmetric profile at STEREO-A suggest the spacecraft sampled a portion of the ejecta with a writhed or distorted flux-rope geometry, where the axis itself is curved, rather than a simple straight invariant axis.
Community Resource: The authors emphasize that ATHARV is available online to facilitate the investigation of ICME spatial structure and evolution, supporting future studies that integrate data from multiple spacecraft and missions (including Aditya-L1).

The paper concludes that while idealized flux-rope models are useful, the reality of ICME structure involves significant spatial distortions and inhomogeneities that require multipoint observations and tools like ATHARV to fully characterize.

The Problem: The "Single-Track" Mystery

The Solution: ATHARV (The "Local Map" Maker)

The Experiment: A Double-Check with Two Drones

What They Found

The Takeaway

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