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We examine the radial evolution of correlation lengths perpendicular(\(\lambda_C^{\perp}\)) and parallel (\(\lambda_C^{\parallel}\)) to themagnetic-field direction, computed from solar wind magnetic-field data measuredby Parker Solar Probe (PSP) during its first eight orbits, Helios 1, AdvancedComposition Explorer (ACE), WIND, and Voyager 1 spacecraft. Correlation lengthsare grouped by an interval's alignment angle; the angle between themagnetic-field and solar wind velocity vectors (\(\Theta_{\rm BV}\)). Paralleland perpendicular angular channels correspond to angles\(0^{\circ}~<~\Theta_{\rm BV}~<~40^{\circ}\) and \(50^{\circ}~<~\Theta_{\rmBV}~<~90^{\circ}\), respectively. We observe an anisotropy in the innerheliosphere within 0.40~au, with \(\lambda_C^{\parallel} / \lambda_C^{\perp}\approx 0.75\) at 0.10~au. This anisotropy reduces with increasing heliocentricdistance and the correlation lengths roughly isotropize within 1~au. Resultsfrom ACE and WIND support a reversal of the anisotropy, such that\(\lambda_C^{\parallel} /\lambda_C^{\perp} \approx 1.29\) at 1~au. The ratiodoes not appear to change significantly beyond 1~au, although the small numberof parallel intervals in the Voyager dataset precludes unambiguous conclusionsfrom being drawn. This study provides insights regarding the radial evolutionof the large, most energetic interacting turbulent fluctuations in theheliosphere. We also emphasize the importance of tracking the changes insampling direction in PSP measurements as the spacecraft approaches the Sun,when using these data to study the radial evolution of turbulence. This canprove to be vital in understanding the more complex dynamics of the solar windin the inner heliosphere and can assist in improving related simulations.

Isotropization and Evolution of Energy-Containing Eddies in Solar Wind Turbulence: Parker Solar Probe, Helios 1, ACE, WIND, and Voyager 1