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1. Lateral Motion of Mantle Plumes in 3‐D Geodynamic Models

   https://doi.org/10.1029/2018GL081404  

   Li, Mingming; Zhong, Shijie (May 2019, Geophysical Research Letters)

   Abstract It has been proposed that hot spot tracks are caused by moving rigid plates above relatively stationary hot spots. However, the fixity of hot spots remains under debate. Here, we perform 3‐D very high resolution (<25 km laterally) global mantle convection models with realistic convection vigor to investigate the lateral motion of mantle plumes. We find that the lateral motion of plumes beneath the Pacific plate is statistically similar to that beneath the Indo‐Atlantic plates. In the past 80 Ma, the majority (>90%) of plumes move laterally with an average speed of 0–20 mm/year under the no‐net‐rotation reference frame, and there are a small portion (~10–20%) of plumes whose lateral motion is less than 5 mm/year. The geodynamic modeling results are statistically in a good agreement with the hot spot motions in the last 5 Ma estimated from observation‐based kinematic models. Our results suggest a small‐to‐moderate (0–20 mm/year) lateral motion of most plume‐induced hot spots. 

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2. Torus-stable zone above starspots

   https://doi.org/10.1093/mnras/stab3249  

   Sun; Török, Tibor; DeRosa, Marc L. (November 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Whilst intense solar flares are almost always accompanied by a coronal mass ejection (CME), reports on stellar CMEs are rare, despite the frequent detection of stellar ‘super flares’. The torus instability of magnetic flux ropes is believed to be one of the main driving mechanisms of solar CMEs. Suppression of the torus instability, due to a confining background coronal magnetic field that decreases sufficiently slowly with height, may contribute to the lack of stellar CME detection. Here, we use the solar magnetic field as a template to estimate the vertical extent of this ‘torus-stable zone’ (TSZ) above a stellar active region. For an idealized potential field model comprising the fields of a local bipole (mimicking a pair of starspots) and a global dipole, we show that the upper bound of the TSZ increases with the bipole size, the dipole strength, and the source surface radius where the coronal field becomes radial. The boundaries of the TSZ depend on the interplay between the spots’ and the dipole’s magnetic fields, which provide the local- and global-scale confinement, respectively. They range from about half the bipole size to a significant fraction of the stellar radius. For smaller spots and an intermediate dipole field, a secondary TSZ arises at a higher altitude, which may increase the likelihood of ‘failed eruptions’. Our results suggest that the low apparent CME occurrence rate on cool stars is, at least partially, due to the presence of extended TSZs. 

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3. Deformations of acid-mediated invasive tumors in a model with Allee effect

   https://doi.org/10.1007/s00285-025-02209-w  

   Carter, Paul; Doelman, Arjen; van_Heijster, Peter; Levy, Daniel; Maini, Philip; Okey, Erin; Yeung, Paige (May 2025, Journal of Mathematical Biology)

   Abstract We consider a Gatenby–Gawlinski-type model of invasive tumors in the presence of an Allee effect. We describe the construction of bistable one-dimensional traveling fronts using singular perturbation techniques in different parameter regimes corresponding to tumor interfaces with, or without, an acellular gap. By extending the front as a planar interface, we perform a stability analysis to long wavelength perturbations transverse to the direction of front propagation and derive a simple stability criterion for the front in two spatial dimensions. In particular we find that in general the presence of the acellular gap indicates transversal instability of the associated planar front, which can lead to complex interfacial dynamics such as the development of finger-like protrusions and/or different invasion speeds. 

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4. Modeling Stellar Surface Features on a Subgiant Star with an M-dwarf Companion

   https://doi.org/10.3847/1538-3881/ac70ca  

   Schutte, Maria C.; Hebb, Leslie; Lowry, Simon; Wisniewski, John; Hawley, Suzanne L.; Mahadevan, Suvrath; Morris, Brett M.; Robertson, Paul; Rohn, Graeme; Stefansson, Gudmundur (June 2022, The Astronomical Journal)

   Abstract Understanding magnetic activity on the surface of stars other than the Sun is important for exoplanet analyses to properly characterize an exoplanet’s atmosphere and to further characterize stellar activity on a wide range of stars. Modeling stellar surface features of a variety of spectral types and rotation rates is key to understanding the magnetic activity of these stars. Using data from Kepler, we use the starspot modeling program STarSPot (STSP) to measure the position and size of spots for KOI-340, which is an eclipsing binary consisting of a subgiant star (T_eff= 5593 ± 27 K,R_⋆= 1.98 ± 0.05R_⊙) with an M-dwarf companion (M_⋆= 0.214 ± 0.006M_⊙).STSPuses a novel technique to measure the spot positions and radii by using the transiting secondary to study and model individual active regions on the stellar surface using high-precision photometry. We find that the average size of spot features on KOI-340's primary is ∼10% the radius of the star, i.e., two times larger than the mean size of solar-maximum sunspots. The spots on KOI-340 are present at every longitude and show possible signs of differential rotation. The minimum fractional spotted area of KOI-340's primary is $2_{-2}^{+12}\mathrm{\%}$, while the spotted area of the Sun is at most 0.2%. One transit of KOI-340 shows a signal in the transit consistent with a plage; this plage occurs right before a dark spot, indicating that the plage and spot might be colocated on the surface of the star. 

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5. Instability mechanisms of repelling peak solutions in a multi-variable activator–inhibitor system

   https://doi.org/10.1063/5.0125535  

   Knobloch, Edgar; Yochelis, Arik (December 2022, Chaos: An Interdisciplinary Journal of Nonlinear Science)

   We study the linear stability properties of spatially localized single- and multi-peak states generated in a subcritical Turing bifurcation in the Meinhardt model of branching. In one spatial dimension, these states are organized in a foliated snaking structure owing to peak–peak repulsion but are shown to be all linearly unstable, with the number of unstable modes increasing with the number of peaks present. Despite this, in two spatial dimensions, direct numerical simulations reveal the presence of stable single- and multi-spot states whose properties depend on the repulsion from nearby spots as well as the shape of the domain and the boundary conditions imposed thereon. Front propagation is shown to trigger the growth of new spots while destabilizing others. The results indicate that multi-variable models may support new types of behavior that are absent from typical two-variable models. 

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