Research

Simulations of dynamic magnetar magnetospheres

Where do magnetar bursts and giant flares come from? Here we study what happens when a rotating magnetar’s crust is slowly deformed, by differential rotation through large angles. The surface motion twists the magnetosphere, leading to outward field inflation and eruptive events similar to solar coronal mass ejections, terminated by catastrophic reconnection. These events come in two flavours: gradual and explosive; the latter can be as brief as a few light crossing times of the light cylinder. These simulations show that very fast magnetospheric dynamics need not be triggered by any stellar behaviour on short time scales.

twistLetter.png

Figure: twist-induced flare from a rotating magnetar. Colour is toroidal magnetic field times the spherical radial coordinate (gold is positive, blue negative); the red field line projection indicates the field line closing at the light cylinder for the untwisted rotating star. From: ADS.

The eruption and reconnection events are accompanied by large increases in the spindown torque on the star. An explosive event, involving a significant fraction of the star’s magnetic flux, could be responsible for the August 27th 1998 giant flare, and coincident “anti-glitch”, from SGR 1900+14.

Dynamics of Strongly Twisted Relativistic Magnetospheres
Parfrey, Beloborodov & Hui, 2013, ApJ, arXiv, ADS

Twisting, reconnecting magnetospheres and magnetar spindown
Parfrey, Beloborodov & Hui, 2012, ApJLarXiv, ADS


Testing general relativity with galaxy bias

The galaxy (or dark matter halo) bias—the ratio of galaxy overdensity to matter overdensity—should be the same at all spatial averaging scales in general relativity. Exotic physics, like modified gravity or clustering dark energy, can imprint a distinctive scale-dependence on the galaxy distribution. This can happen even if the collapse dynamics are Newtonian, due to the effect of a scale-dependent cosmological growth factor. Constraints can be placed on theories by measuring the galaxy bias at a single snapshot, for example using the LRG power spectrum, or by comparing two different galaxy populations at different stages of bias relaxation.

birthneg

Bias versus scale of averaging region (Fourier wavenumber or region mass) for haloes forming at zero redshift, where gravity is stronger than GR on scales greater than about 10 Mpc/h. Halo masses, in Msun/h, are between 5e14 (most biased) and 1e12 (least biased).

Scale-dependent halo bias from scale-dependent growth
Parfrey, Hui & Sheth, 2011, Phys. Rev. DarXiv, ADS

Evolution of galaxy bias, generalized
Hui & Parfrey, 2008, Phys. Rev. DarXiv, ADS


MHD instabilities and the solar dynamo

Sunspots are generally confined to near the solar equator, at latitudes less than about 35 degrees. An analysis of the triple-diffusive magneto-rotational instability (MRI) shows that the solar tachocline—the high shear region between the radiative and convective zones—should be MRI-unstable near the poles, at latitudes greater than about 38 degrees. This could explain why sunspots aren’t seen there: small-scale MRI-driven turbulence prevents coherent (Babcock-Leighton) dynamo action in the tachocline, except near the equator. It is also circumstantial evidence that the solar dynamo operates primarily in, or in somehow fundamentally tied to, the tachocline.

sunspots.png

Growth rate of the fastest growing MRI mode in units of the rotation rate, for latitudinal, radial, and equally latitudinal and radial magnetic fields. The lines above the plot show the latitudes unstable to the Tayler instability.

The origin of solar activity in the tachocline
Parfrey & Menou, 2007, ApJLarXiv, ADS

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