🔭 astrophysics

Constraining the onset height of coronal mass ejection driven shocks using near-Sun observations in visible and radio wavelengths

This paper demonstrates how joint observations from the ADITYA-L1 mission's VELC instrument and Gauribidanur radio facilities can constrain the heliocentric distance where coronal mass ejection-driven shocks form, addressing the debate over type II radio burst onsets caused by the lack of routine white-light CME data below 1.5 solar radii.

Original authors: C. Kathiravan, V. Muthupriyal, R. Ramesh

Published 2026-02-25

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CC0 1.0

Original authors: C. Kathiravan, V. Muthupriyal, R. Ramesh

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ 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 Sun as a massive, fiery factory that occasionally spits out huge clouds of superheated gas and magnetic fields. Scientists call these clouds Coronal Mass Ejections (CMEs). When these clouds shoot out into space, they can act like a supersonic jet breaking the sound barrier, creating a massive shockwave in the solar atmosphere.

This paper is about solving a long-standing mystery: Exactly how close to the Sun's surface do these shockwaves form?

For a long time, scientists had a blind spot. They could see the shockwaves (which show up as radio signals) and they could see the gas clouds far out in space, but they couldn't see the gas clouds right next to the Sun because the Sun's glare was too bright. It was like trying to watch a car start its engine from 100 miles away; you could hear the engine roar (the radio shock), but you couldn't see the car itself until it was far down the road.

Here is how the authors solved this puzzle, explained simply:

1. The New "Super-Telescope" (VELC)

The researchers used a brand-new instrument on India's ADITYA-L1 space mission called VELC. Think of VELC as a pair of high-tech sunglasses with a tiny, perfectly placed sticker (an "occulter") that blocks out the blinding center of the Sun.

The Magic: Because this sticker is so small, VELC can look at the Sun's atmosphere just 0.15 solar radii above the surface. Previous telescopes had to block out a much larger chunk of the Sun, leaving a "blind spot" right where the action happens.

2. The "Radio Siren" (Gauribidanur)

On the ground in India, the Gauribidanur observatory was listening to the Sun's radio signals. When a CME creates a shockwave, it screams out a specific type of radio noise called a Type II burst.

The Clue: Just like a police siren changes pitch as an ambulance drives away, these radio bursts change frequency as the shockwave moves away from the Sun. By listening to the pitch, scientists can calculate exactly how far away the shockwave is.

3. The "Aha!" Moment

On May 27, 2024, a massive solar flare happened.

The Radio: The ground station heard the "siren" (Type II burst) start at a specific frequency. This told them the shockwave was about 1.19 times the Sun's radius away from the center.
The Eye: At that exact same moment, the VELC telescope looked at that same spot and saw a bright, expanding cloud of gas (the CME) right there!

The Big Discovery

By combining the "ears" (radio) and the "eyes" (VELC), the team proved that the shockwave formed incredibly close to the Sun—just 0.19 solar radii above the surface.

Why does this matter?
Think of the Sun's atmosphere like a crowded dance floor.

The Old View: Scientists thought the "shockwave" (the bouncer clearing the dance floor) only started forming once the CME (the rowdy group) had already pushed its way out to the edge of the room.
The New View: This paper shows the bouncer starts clearing the floor immediately as the group steps onto the dance floor, right near the DJ booth.

This is a huge deal because:

Particle Acceleration: These shockwaves are like giant particle accelerators. They blast dangerous particles (Solar Energetic Particles) toward Earth. Knowing exactly where the shock starts helps us predict when these dangerous particles will arrive.
Filling the Gap: It proves we no longer need to guess where the CME starts. We can actually see it happening in real-time, right next to the Sun's surface.

In a Nutshell

This paper is like finally getting a security camera that can see right up to the front door of a house, rather than just the driveway. By using a new space telescope and ground-based radio listeners, scientists have finally caught a solar shockwave in the act of being born, proving it happens much closer to the Sun than we ever thought possible.

1. Problem Statement

A significant debate in solar physics concerns the heliocentric distance ( $r$ ) at which Magnetohydrodynamic (MHD) shocks, associated with Coronal Mass Ejections (CMEs), are first formed. These shocks drive Type II radio bursts, which are signatures of shock-accelerated electrons.

The Gap: Routine white-light CME observations (e.g., via SOHO/LASCO C2 & C3) are limited to distances $r \gtrsim 2.2 R_\odot$ due to the large size of the coronagraph occulter.
The Consequence: Because the onset of Type II bursts typically occurs at $r \approx 1.2 - 2.0 R_\odot$ (corresponding to frequencies of 200–50 MHz), the exact location of the CME at the moment the shock forms is usually inferred only through extrapolation of height-time plots. This introduces uncertainty regarding the precise onset height of the shock.

2. Methodology

The authors utilized a multi-wavelength approach combining near-Sun visible spectroscopy and radio spectroscopy to directly observe the CME and the associated shock onset simultaneously.

Visible Observations (VELC/ADITYA-L1):
- Instrument: The Visible Emission Line Coronagraph (VELC) on the ADITYA-L1 mission.
- Capability: An internally occulted coronagraph with an occulter radius of $1.05 R_\odot$ , allowing observations as close as $r \approx 1.13 R_\odot$ .
- Mode: Sit-and-stare mode using the 5303 Å (green line) spectroscopy channel on the 1st slit.
- Target: A CME associated with an X2.9 class flare on May 27, 2024. The slit was positioned to observe the East limb ( $\theta \approx 90^\circ - 104^\circ$ ).
- Analysis: Peak intensity of the 5303 Å emission line was computed to track the CME's leading edge and density enhancements.
Radio Observations (Gauribidanur):
- Instruments: Gauribidanur LOw-frequency Solar Spectrograph (GLOSS) and Gauribidanur RAdio Spec-troPolarimeter (GRASP).
- Data: Dynamic spectra covering 330–10 MHz.
- Target: Identification of Type II radio bursts occurring simultaneously with the CME observations.
Correlation Analysis:
- The authors compared the time of enhanced emission in the VELC data with the start time of the Type II radio burst.
- They calculated the electron density ( $N_e$ ) from the 5303 Å flux and compared it with the density derived from the radio burst frequency using the Newkirk (1961) two-fold electron density model.
- They verified the CME's radial propagation using position angle consistency and speed estimates.

3. Key Contributions

First Joint Observation: This study presents one of the first successful joint observations of a CME and a Type II radio burst using ADITYA-L1/VELC (visible) and ground-based radio facilities (Gauribidanur).
Overcoming Occulter Limits: By utilizing VELC's smaller occulter ( $1.05 R_\odot$ ), the authors directly observed the CME at heights previously inaccessible to routine coronagraphs, eliminating the need for extrapolation for this specific event.
Validation of Density Models: The study empirically validated the use of the Newkirk (1961) density model for active region coronae by cross-referencing optical flux-derived densities with radio-frequency-derived densities.

4. Results

Event Timing:
- CME Onset: Enhanced 5303 Å emission was detected starting at ~07:04 UT at a heliocentric distance of $r \approx 1.18 R_\odot$ .
- Radio Onset: The first Type II radio burst began at ~07:04 UT with a start frequency of ~174 MHz.
Shock Height Determination:
- Using the Newkirk model, the plasma level corresponding to 174 MHz is located at $r \approx 1.19 R_\odot$ .
- The separation between the CME leading edge ( $1.18 R_\odot$ ) and the shock front ( $1.19 R_\odot$ ) is approximately $0.01 R_\odot$ , consistent with theoretical expectations for the "shock standoff distance" near the Sun.
Density Consistency:
- Electron density derived from the 5303 Å flux: $N_e \approx 2.2 \times 10^8 \text{ cm}^{-3}$ .
- Electron density derived from the 174 MHz plasma frequency: $N_e \approx 3.7 \times 10^8 \text{ cm}^{-3}$ .
- The authors note these values are reasonably close, confirming that the optical enhancement and the radio burst originate from the same CME-driven shock.
Propagation: The CME propagated radially, as indicated by the constant position angle and the source region's location at the solar limb (S18E89), minimizing projection effects.

5. Significance

Shock Formation Height: The study conclusively demonstrates that CME-driven MHD shocks can form as close as $r \approx 1.19 R_\odot$ (approx. $0.19 R_\odot$ above the solar limb). This refines the understanding of where particle acceleration begins.
Solar Energetic Particles (SEPs): Knowing the precise onset height of shocks is critical for modeling the release time and acceleration mechanisms of SEPs, which pose hazards to space assets and astronauts.
Future Synergy: The paper highlights the potential of combining ADITYA-L1/VELC with SDO/AIA and other radio spectrographs to study shock signatures across different wavelengths, particularly for limb events.
Methodological Benchmark: It establishes a robust framework for using near-Sun visible spectroscopy to constrain radio burst onset heights, reducing reliance on extrapolated CME kinematics.

In summary, this paper provides direct observational evidence that CMEs can drive MHD shocks very close to the solar surface, resolving a long-standing uncertainty in solar physics through the unique capabilities of the ADITYA-L1 mission.