First Observational Evidence for Meridional Flow-like Profile in 17 GHz: Magnetic Trees on the Sun? by Srinjana Routh et al.

Decrypting the behaviour of large-scale flows in the Sun, namely the differential rotation and the meridional circulation, is detrimental to the understanding of the evolution of the solar magnetic field in the global context. Among them, the meridional flow has been notoriously difficult to measure, owing to the fact that it is an order of magnitude lower than the differential rotation measurements, with its existence in the higher solar atmosphere being a long-standing mystery. The meridional flow profile is often mathematically described as the following symmetric profile (Yeats et al. 2013),

(Eq.1) \[u_{\theta} = -R_{\odot}\Delta_{u}cos\theta sin^{p}\theta\]

And, asymmetric (Routh et al. (2026)),

(Eq.2) \[u_{\theta} = -R_{\odot}\Delta_{u}cos\theta sin^{p}\theta + \kappa sin\theta\]

Where, $p=cos^{-1}((\sqrt{1+p})^{-1})$ is the concentration parameter, $\Delta_{u}$ is the divergence parameter and $\kappa$ is the asymmetry parameter. The recent study Routh et al. (2026) harnesses a tracer-independent image correlation approach using radio images at 17 GHz from the Nobeyama Radioheliograph (NoRH), (see NoRH CESRA nuggets) sampling a relatively well-defined height in the upper chromosphere (~3000 ± 500 km), where the magnetic field is expected to dominate $\betaRouth et al. (2024) and Routh et al. (2025).

Data and analysis

28 years of daily full-disc 17 GHz radio images (1992–2020) were used, and a tracer-independent, automated image-correlation technique was applied. By dividing each solar image into overlapping latitudinal bins (15$^{\circ}$ wide) and maximising 2D cross-correlation of temporally separated segments, the method determines movement in either latitudinal or longitudinal direction without relying on the presence of visible features such as sunspots or plages. Importantly, the method performs well even during solar minimum, when features are sparse. In this study, movement in the latitudinal direction is used to calculate the flow profile.

Figure 1. Cycle-averaged flow profile in the latitudinal direction as obtained from 17 GHz data.

Upon obtaining the latitudinal flow profile of the solar chromosphere, we clearly see a poleward flow profile consistent across the cycles 23 and 24 (Fig.1). Further, the amplitude of this flow profile decreases during the rising phase  (Fig. 2, Left Panel)  and relatively increases in the declining phase (Fig. 2, Right Panel). An asymmetric behaviour is observed in the flow profile, which is consistent with the activity dominance of the corresponding hemisphere. This increase in flow amplitude with a decrease in activity, either overall or in a particular hemisphere, and vice versa, is consistent with the long-standing understanding that the sub-surface meridional flow amplitude is inversely affected by activity.

Figure 2. Flow profile in the rising and declining phases of the cycles, completely (cycles 23 and 24) and partially (cycle 22) covered by the NoRH dataset.

On comparing the movement of the features of higher brightness temperatures (e.g., active regions and decaying small-scale elements; see Fig.3) with the temporal distribution of magnetic elements we find the movement of these features align well with the latter. Since the poleward movement of these magnetic features in the photosphere is a signature of the meridional flow, the coalignment of the same motion with that of the upper atmospheric features suggest that the sub-surface motion is reflected well into the upper atmosphere.

Figure 3. Brightness Temperature Contours from 17 GHz corresponding to two specific temperatures (11300 K in orange and 12000 K in green) overplotted on the magnetic butterfly diagram obtained from Kitt Peak, SOHO/MDI and SDO/HMI. The vertical dashed lines in black corresponds to the start of the time where the movement of these features can be seen to correlate well with movement of magnetic features in the photosphere.

Conclusions

Our findings present the very first observational evidence of a flow signature similar to that of subsurface meridional flow in characteristics. The presence of the signature of a plasma dominated phenomenon suggests that the flow signature determined from the movement of the features observed in 17 GHz reflects that of the sub-surface rooting of the same, thereby lending additional support to the magnetic tree hypothesis. Future coordinated studies at wavelengths observing similar heights will be helpful in unveiling the extent of this phenomenon and might help in understanding the coupling of different layers of the solar atmosphere.

Additional info

Based on the recent study by Routh, S., “Radio Signature of Higher Atmospheric Meridional Flow and Implications for Magnetic Trees in the Sun”, The Astrophysical Journal, vol. 1004, Art. no. 1, 2026 doi:10.3847/1538-4357/ae69dc

References

Routh, S., et. al, “Exploring the Dynamic Rotational Profile of the Hotter Solar Atmosphere: A Multi-wavelength Approach Using SDO/AIA Data”, The Astrophysical Journal, vol. 975, 158, IOP, 2024

Routh, S., “Insights into chromospheric large-scale flows using Nobeyama 17 GHz radio observations: I. The differential rotation profile”, Astronomy and Astrophysics Letters, vol. 700, Art. no. L3, 2025