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We analyze the micro-kinetic stability of the electron strahl in the solarwind depending on heliocentric distance. The oblique fast-magnetosonic/whistler(FM/W) instability has emerged in the literature as a key candidate mechanismfor the effective scattering of the electron strahl into the electron halopopulation. Using data from Parker Solar Probe (PSP) and Helios, we compare themeasured strahl properties with the analytical thresholds for the oblique FM/Winstability in the low- and high-$\beta_{\parallel c}$ regimes, where$\beta_{\parallel c}$ is the ratio of the core parallel thermal pressure to themagnetic pressure. Our PSP and Helios data show that the electron strahl is onaverage stable against the oblique FM/W instability in the inner heliosphere.Our analysis suggests that the instability, if at all, can only be excitedsporadically and on short timescales. We discuss the caveats of our analysisand potential alternative explanations for the observed scattering of theelectron strahl in the solar wind. Furthermore, we recommend the numericalevaluation of the stability of individual distributions in the future toaccount for any uncertainties in the validity of the analytical expressions forthe instability thresholds.

The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere