Coulomb Collisions

How particle collisions shape solar wind evolution

Coulomb collisions act as friction on solar wind ions, gradually reducing the differential flows of alpha particles and proton beams during their journey from Sun to Earth. By extrapolating measured drifts to zero collisional age, we reveal the intrinsic velocity limits set by wave-particle interactions in the corona.

Extrapolated drift speeds reveal coronal launch conditions

A plot with collisional age on a logarithmic horizontal axis (from 10^-2 to 10^-1) and drift speed normalized by the anisotropic Alfven speed on the vertical axis (from 0 to 1.2). Dark blue square markers with vertical error bars trace alpha particle drift, curving steadily downward from about 0.62 at the youngest ages to about 0.38 at the oldest, following an exponential decay fit shown as a dark dotted line. Yellow plus markers with larger yellow error bars trace proton beam drift, remaining nearly flat around 1.0 to 1.05 across the same range. Inset text boxes show the fit equations and parameters for each species.

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Related Figures

Two overlapping histogram distributions plot drift speed normalized by the Alfven speed on the horizontal axis against measurement count on the vertical axis.

Proton beams drift faster than alphas relative to the Alfven speed

In fast, nearly collisionless solar wind, proton beams consistently drift about 60% faster than alpha particles relative to the main proton population.

Establishes the baseline observation that drifts cluster at characteristic Alfven speed fractions, motivating the collisional age analysis

Two overlapping histogram distributions plot the ratio of alpha particle drift speed to proton beam drift speed on the horizontal axis against normalized count on the vertical axis.

Alpha-to-beam speed ratio sharpens in youngest plasma

In the youngest, most collisionless solar wind measured at Earth, alpha particles drift at about 71% the speed of proton beams, compared to 62% when a broader range of weakly collisional data is included.

Shows that restricting to youngest plasma narrows the alpha-to-beam ratio, confirming collisional age as a major source of scatter

Alpha drift fades with collisions while beam drift persists

Alpha particle drift speed drops dramatically -- from about 80% to 40% of the Alfven speed -- as collisional age increases across the measured range.

Demonstrates the core finding that collisions dramatically slow alpha drift while leaving beam drift unchanged

Source

A Comparison of Alpha Particle and Proton Beam Differential Flows in Collisionally Young Solar Wind

The Astrophysical Journal (2018)

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