OVSA Science Highlight No. 4: When the Sun Meets the Crab
Contributed by Peijin Zhang1, 2 (1Center for Solar-Terrestrial Research, New Jersey Institute of Technology, 323 Martin Luther King Jr Blvd., Newark, NJ 07102-1982, USA; 2 Cooperative Programs for the Advancement of Earth System Science, University Corporation for Atmospheric Research, Boulder, CO, USA); Edited by Bin Chen; Posted on September 11, 2025.
Being situated in the Zodiac constellation Taurus, the Crab Nebula (also known as Tau A), is in conjunction with the Sun in mid-June every year. During the conjunction, the crab is completely outshone by the Sun in optical wavelengths. However, at meter-decameter wavelengths, the Sun is no longer dazzlingly bright anymore; the radio brightness of the Crab Nebula can be up to one-tenth that of the quiet Sun! Thus, the two can be observed together with a radio telescope that has a sufficiently large field of view and a dynamic range that is adequate for imaging both of them.
During such conjunctions, the radio image of the Crab Nebula is broadened and distorted by the heliospheric plasma as the radio waves propagate through it. The broadening/distortion effect is more profound as the Crab Nebula gets closer to the Sun. These observations, in turn, provide us with a unique opportunity to study the turbulent structures in the extended solar corona, which is otherwise challenging to measure remotely.
This effect was predicted not long after the birth of radio astronomy. In Figure 1, Hewish (1958) showed his prediction of what the Crab Nebula would look like as it approaches the Sun (Figure 1).
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| Figure 1: Left: isotropic‑scattering prediction for a compact background source near the Sun (Hewish 1958). Right: 2024 OVRO‑LWA observation of the Crab during conjunction showing arc‑like broadening and substructure. |
We observed the Crab Nebula (Tau A) during its 2024 solar conjunction with the Owens Valley Radio Observatory’s Long Wavelength Array (OVRO‑LWA), imaging 30–80 MHz at projected heliocentric distances of ≈5–27 R⊙. OVRO-LWA has both a large field of view and high image dynamic range – the right combination for studying Crab's solar conjunctions. These observations produced daily, wide‑field images that were used to quantify how scattering varies with frequency and impact parameter, sampling three key dates near closest approach (e.g., Jun 9, Jun 14, and Jun 22). Figure 2 shows such multi-frequency "time-lapse" images of the Crab Nebula's 2024 solar conjunction as observed by OVRO-LWA.
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| Figure 2: OVRO-LWA's multi-frequency time-lapse of the Crab Nebula during its solar conjunction from June 9, 2024, to June 22, 2024. |
With the angular broadening measurement, we found that the anisotropic ratio decreased along the radial direction. Also, near the closest approach, the image became arc‑like and showed substructure at the lowest frequencies, especially when the line of sight crossed dense coronal streamers (features that align with contemporaneous white‑light streamer structure), which we interpreted as evidence for the presence of mesoscale density inhomogeneities. In addition, the observed total flux decreased toward lower frequency and smaller impact parameter, consistent with stronger scattering/absorption in denser plasma.
These measurements have helped quantify how magnetic‑field‑guided turbulence varies with heliocentric distance and how it tangentially broadens background sources (affecting apparent position and flux). They offer important and unique diagnostic value to inform radio‑wave propagation models and improve the interpretation of heliospheric radio observations.
Based on the recent paper by Zhang, P., Mondal, S., Chen, B., Yu, S., Gary, D. et al. (2025), "Probing the Turbulent Corona and Heliosphere Using Radio Spectral Imaging Observation during the Solar Conjunction of Crab Nebula," The Astrophysical Journal, in press. Arxiv: 2506.01632