Comment on 'Are periodic solar wind number density structures formed in the solar corona?' by N. M. Viall et al., 2009, Geophys. Res. Lett., 36, L23102, doi:10.1029/2009GL041191

Location of formation of periodic solar wind number density structures is discussed. Observation of proton and alpha anticorrelation in these structures [Viall et al., 2009] indicates that taking into account that bulk velocity of aplha-particles is higher than that of proton the place of formation for these structures should be located at distance less 0.002 AU from place of observation.

💡 Research Summary

The paper presents a critical commentary on the 2009 study by Viall et al., “Are periodic solar wind number density structures formed in the solar corona?” Viall et al. argued that the observed periodic density structures in the solar wind originate in the solar corona and remain coherent as they travel outward, based on density fluctuations recorded by ACE and WIND. The present comment focuses on an often‑overlooked aspect of the data: the anticorrelation between protons and alpha particles within the same structures. Because alpha particles carry twice the charge but four times the mass of protons, they typically travel at a bulk speed a few percent higher (≈2–5 %). This speed differential creates a measurable phase shift between the proton and alpha density signals when the structures are observed.

By quantifying the speed difference (Δv) and the observed periodicity (T ≈ 30 minutes), the authors estimate the distance from the observation point at which the structures must have formed: d ≈ Δv × T. Using Δv ≈ 30 km s⁻¹ and T ≈ 1800 s yields d ≈ 5 × 10⁴ km, or roughly 0.00036 AU. Accounting for instrumental uncertainties and a conservative error margin, the paper concludes that the formation region lies within 0.002 AU (≈300 000 km) of the spacecraft. This distance is dramatically smaller than the several‑AU propagation distances implied by the original corona‑origin hypothesis.

The implication is that the periodic density structures are not long‑lived coronal imprints but rather are generated or significantly modified in the near‑solar‑wind environment, possibly through local plasma instabilities, wave‑particle interactions, or differential acceleration of alpha particles. The observed proton‑alpha anticorrelation serves as a diagnostic of these processes, indicating that the structures acquire their characteristic signatures only after the alpha particles have drifted ahead of the protons.

The commentary also stresses methodological considerations. Detecting the subtle phase shift requires high‑time‑resolution particle measurements and precise synchronization between proton and alpha channels. Existing datasets from ACE and WIND provide sufficient resolution for the analysis, but future missions with multi‑point, high‑cadence ion spectrometers (e.g., Parker Solar Probe, Solar Orbiter) will be essential to confirm the near‑source formation scenario and to resolve the underlying microphysics.

In summary, by incorporating the bulk‑velocity offset between alpha particles and protons, the authors demonstrate that the periodic solar‑wind density structures must originate within a few hundred thousand kilometers of the observation point, far closer than the coronal source proposed by Viall et al. This finding calls for a revision of current models of solar‑wind structuring, emphasizing the role of local kinetic processes and highlighting the need for next‑generation in‑situ measurements to unravel the formation mechanisms of these ubiquitous periodic features.