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SUMMARY:On the role of magnetic fluctuations in low magnetic Prandtl numbe
 r plasmas - Maarit Korpi-Lagg [Helsinki/Espoo]
DTSTART:20250512T130000Z
DTEND:20250512T140000Z
UID:TALK231358@talks.cam.ac.uk
CONTACT:Mattias Brynjell-Rahkola
DESCRIPTION:Magnetic fields on small scales are ubiquitous in the universe
 . For example\, the fluctuating magnetic fields in star-forming regions of
  galaxies are more than twice the strength of the magnetic fields coherent
  over large scales. On the solar surface\, magnetic fields are mostly conc
 entrated in medium and small-scale structures\, while the proportion compr
 ising the mean field strength is even lower than in galaxies. The generati
 on mechanisms of the fluctuating magnetic fields are not fully understood.
  One possibility is the so-called small-scale dynamo (SSD)\, the other is 
 tangling of the large-scale field structures through turbulence acting on 
 them. In the interstellar medium of galaxies\, the resistivity is much low
 er than the viscosity\, such that magnetic instabilities are easier to exc
 ite relative to the turbulence. SSD in such high magnetic Prandtl number (
 Pm\, i.e. the ratio between viscosity and resistivity) conditions has ther
 efore been predicted to be easily excited. In the Sun and cool stars\, Pm 
 is much lower\, namely in the range of 1e-6 to 1e-3. Both theoretically an
 d especially numerically\, SSD is more difficult to excite at such very lo
 w magnetic Prandtl numbers. Indeed\, some recent numerical studies has ind
 icated that the threshold for SSD excitation should systematically increas
 e with decreasing Pm\, concluding that SSD would be impossible in the Sun 
 and cool stars.\n\nAccelerating the magnetohydrodynamics solvers with grap
 hics processing units has recently opened an avenue to numerically study l
 ow-Pm flows. With these tools we have been able to perform simulations tha
 t approach the solar Pm-values\, studying both kinematic and non-linear re
 gimes.\nContrary to earlier findings\, the SSD turns out not only to be po
 ssible for Pms down to 0.0031\, but even to become increasingly easy to ex
 cite for Pm below approximately 0.05. We relate this behaviour to the know
 n hydrodynamic phenomenon\, referred to as the bottleneck effect. Extrapol
 ating our results to solar values of Pm indicates that an SSD would be pos
 sible under such conditions. The saturation strength of the SSD is of the 
 order of the turbulent kinetic energy independent of the Pm\, when the mag
 netic Reynolds number (Rm) is moderate (up to a few\nthousands). For highe
 r Rm the saturation strength rapidly diminishes and reaches levels of orde
 r of magnitude lower than turbulent kinetic energy\, casting a new doubt o
 f the SSD being important in the Sun and stars. Even higher resolution stu
 dies\, however\, would be required to verify this robustly. For such calcu
 lations\, however\, extraordinary resources/quantum computers are required
 .
LOCATION:MR14 DAMTP and online
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