Finite Lifetime Fragment Model 4 for Striae Formation in the Dust Tails of Comets (FLM 4) Acceleration by Lorenz-force

The striations in the dust tails of comets are referred to as striae, and their origin has long been a mystery. We introduce a new dynamic model to describe the forms of the striae observed in comets Hale-Bopp (C/1995 O1), West (C/1975 V1), and Seki-Lines (C/1962 C1). Charged particles made of refractory materials, with radii less than 0.5micrometer, are expelled from the comet’s nucleus and accelerated by Lorentz forces near the nucleus. These particles decay many times to form striae, which have a lifespan of less than about 100 days at a distance of 1 astronomical unit from the sun. Over time, they continue to decay and eventually disappear from view. The following dynamic model explains these material science processes. Particles expelled from the comet’s nucleus are subjected to three forces: solar gravity, solar radiation pressure, and Lorentz forces near the nucleus. As these particles decrease in size, the Lorentz forces and radiation pressure cause fluctuations, increasing and decreasing to form striae. This model, which is less of a dynamic approximation than previous theories (FLM3), explains the structure of the striae, enables predictions of their luminosity, and clarifies their origin.

💡 Research Summary

The paper presents a new theoretical framework, the Finite Lifetime Fragment Model 4 (FLM‑4), to explain the formation of striae—thin, parallel filaments observed in cometary dust tails. Building on a series of earlier models (Sekanina‑Farrell, FLM‑1, FLM‑2, FLM‑3), the authors aim to reduce the number of free parameters, incorporate the Lorentz force explicitly, and provide a quantitative method for predicting both the morphology and brightness of striae.

Key physical assumptions: (1) Dust particles ejected from the comet nucleus are sub‑micron (≤0.5 µm) silicate grains that acquire a net electric charge. (2) Near the nucleus, the solar wind drags magnetic field lines into a U‑shaped configuration; a virtual “acceleration cylinder” of radius CL, aligned with the Sun‑comet line, defines the region where the Lorentz force acts on the charged grains. (3) The particles experience three accelerations: solar gravity (βg = –1), solar radiation pressure (βf), and the Lorentz acceleration (βi). The Lorentz term is given by A ≈ 0.004 R/S² (cm s⁻²) with R≈0.4763, leading to βi = 6.74527 × 10⁻³ R/S². βi is non‑zero only while the particle resides inside the acceleration cylinder; it drops to zero after a time ti. The radiation‑pressure term βf depends non‑linearly on particle radius S, with separate empirical formulas for S ≤ 0.6 µm and S > 0.6 µm, multiplied by a fitting constant hSi.

The total acceleration ratio is β(t) = βf(t)+βi(t). As particles decay (see below), βf rises to a maximum near S ≈ 0.24 µm and then declines, while βi produces a sharp, transient increase when the particle passes through the cylinder. The resulting time‑varying β drives periodic changes in the particle’s orbital elements (perihelion distance, eccentricity, argument of perihelion). The authors argue that this “rise‑fall‑rise” pattern in β is the fundamental driver of the observed striae: each particle group repeatedly accelerates and decelerates, producing a thin, bright filament that persists until the particles become sub‑visible (<0.005 µm) and lose their solid character.

Particle decay is modeled as an exponential size reduction driven by accumulated solar energy: S(t) = S₀ exp