1. T{\"o}r{\"o}k, T. and Kliem, B. and Titov, V. S. (2004) Ideal kink instability of a magnetic loop equilibrium. \aap 413: L27-L30 https://doi.org/10.1051/0004-6361:20031691, https://ui.adsabs.harvard.edu/abs/2004A &A...413L..27T, astro-ph, instabilities, magnetic fields, MHD, Sun: activity, Sun: corona, stars: coronae, Astrophysics, astro-ph/0311198, arXiv, The force-free coronal loop model by cite\{Tit:Dem-99\} is found to be unstable with respect to the ideal kink mode, which suggests this instability as a mechanism for the initiation of flares. The long-wavelength (m = 1) mode grows for average twists {\ensuremath{\Phi}} ⪆3.5{\ensuremath{\pi}} (at a loop aspect ratio of {\ensuremath{\approx}}5). The threshold of instability increases with increasing major loop radius, primarily because the aspect ratio then also increases. Numerically obtained equilibria at subcritical twist are very close to the approximate analytical equilibrium; they do not show indications of sigmoidal shape. The growth of kink perturbations is eventually slowed down by the surrounding potential field, which varies only slowly with radius in the model. With this field a global eruption is not obtained in the ideal MHD limit. Kink perturbations with a rising loop apex lead to the formation of a vertical current sheet below the apex, which does not occur in the cylindrical approximation., January
2. Baty, H. and Heyvaerts, J. (1996) Electric current concentration and kink instability in line-tied coronal loops.. \aap 308: 935-950 https://ui.adsabs.harvard.edu/abs/1996A &A...308..935B, SUN: CORONA, MHD, METHODS: NUMERICAL, INSTABILITIES, A fully three dimensional non-linear magnetohydrodynamic (MHD) simulation of the evolution of ideal kink modes in line-tied cylindrical coronal loops is presented. Two distinct initially unstable equilibria are considered: the uniform- twist force- free Gold-Hoyle profile, and a more realistic non force free field with variable and localized twist profile. In this latter case the instability non-linearly develops at the axial midplane of the loop a current concentration radially localized at a resonant point r\_s\_, where the condition \{vec\}(k).\{vec\}(B)=0 is satisfied, \{vec\}(k) being the local wave vector of the mode and \{vec\}(B) the equilibrium magnetic field. No such fine scale current structure forms for the Gold- Hoyle profile since \{vec\}(k).\{vec\}(B) then never vanishes. This current concentration extends along all the loop length down to the photosphere, and it takes the form of an helical ribbon of intense current with a weakly variable helicity along the axial direction. A kinked bifurcated equilibrium is reached in which the current concentration generated is non singular in the sense that this has a non-zero thickness. Moreover, the non- linear perturbed current seems to develop a filamentary structure superposed on the current layer. We discuss the resistive dissipation mechanism of such fine-scale structures., April
3. Griton, L. and Pantellini, F. and Meliani, Z. (2018) Three-Dimensional Magnetohydrodynamic Simulations of the Solar Wind Interaction With a Hyperfast-Rotating Uranus. Journal of Geophysical Research (Space Physics) 123(7): 5394-5406 https://doi.org/10.1029/2018JA025331, https://ui.adsabs.harvard.edu/abs/2018JGRA..123.5394G, magnetohydrodynamics, Uranus, solar wind, magnetosphere, Earth Science, We present magnetohydrodynamic simulations of a fast-rotating planetary magnetosphere reminiscent of the planet Uranus at solstice, that is, with the spin axis pointing to the Sun. We impose a 10 times faster rotation than for Uranus, in order to emphasize the effects of rotation on the magnetospheric tail without the need of an excessively large simulation domain while keeping the qualitative aspects of a supersonic magnetized solar wind interacting with a fast-rotating magnetosphere. We find that a complex helical Alfv{\'e}nic structure propagates downstream at a velocity exceeding the plasma velocity in the magnetosheath. Similarly, the reconnection regions, which mediate the interaction of the planetary magnetic field and the interplanetary magnetic field, do also form a helical structure with the same downstream velocity but a 2 times larger pitch. We speculate that the magnetic field of the helical structure connected to the interplanetary magnetic field asymptotically reduces the phase velocity of the helical structure toward the tailward velocity in the magnetosheath. For our simulations we use the MPI-AMRVAC code which we enhanced with a time-dependent background magnetic field in the splitting of the magnetic field., July
4. Suzana S. A. Silva and Gary Verth and Erico L. Rempel and Sergiy Shelyag and Luiz A. C. A. Schiavo and Viktor Fedun (2021) Solar Vortex Tubes. {II}. On the Origin of Magnetic Vortices. The Astrophysical Journal 915(1): 24 https://doi.org/10.3847/1538-4357/abfec2, American Astronomical Society, jun
5. Alexandrova, O. and Mangeney, A. and Maksimovic, M. and Cornilleau-Wehrlin, N. and Bosqued, J. M. and Andr{\'e}, M. (2006) Alfv{\'e}n vortex filaments observed in magnetosheath downstream of a quasi-perpendicular bow shock. Journal of Geophysical Research (Space Physics) 111(A12): A12208 https://doi.org/10.1029/2006JA011934, https://ui.adsabs.harvard.edu/abs/2006JGRA..11112208A, Magnetospheric Physics: Magnetosheath, Space Plasma Physics: MHD waves and instabilities (2149, 2752, 6050), Space Plasma Physics: Neutral particles (2151), Space Plasma Physics: Turbulence (4490), magnetic vortices, magnetosheath, turbulence, Earth Science, A12208, Magnetic field fluctuations in the frequency range [0.02-12.5] Hz are studied with the four Cluster satellites in the Earth magnetosheath downstream of a quasi-perpendicular bow shock. The turbulent spectrum presents a spectral break accompanied by a broad maximum usually interpreted as due to Alfv{\'e}n ion cyclotron waves. In this paper we establish that this spectral knee corresponds to space-localized coherent magnetic structures in the form of Alfv{\'e}n vortices. The Alfv{\'e}n vortex is a nonlinear cylindrical Alfv{\'e}n wave, quasi-parallel to the mean magnetic field B$$_{0}$$ and propagating in a plane perpendicular to B$_{0}$. In this plane the observed vortices are localized within 20c/{\ensuremath{\omega}}$$_{pi}$$. The frequent observations of such structures indicate their stability in the plasma. Therefore the Alfv{\'e}n vortices can be an important element in the magnetosheath turbulence. The possible origins of these vortices, such as a strong turbulence or the filamentation instability of an Alfv{\'e}n wave, are discussed., December