MHD Clumps: Heterogeneity and Radiative Cooling

We seek to understand the role magnetic field alignment and clump-clump spacing (filling fraction) have on the evolution of a host of randomly-distributed clumps overrun by a steady supersonic wind.

We do so by imposing a magnetic field in two configurations: parallel (Bx) and perpendicular (By) to the shock front, both with a plasma beta value of 1. We undertake an additional case in which there is no magnetic field, for comparison. We choose two configurations of clumps: sparse and dense, which are characterized by a filling fraction,

.\qquad\qquad f_{frac}=...$

Finally, we include the effects of radiative cooling. Based on \cite{Frank et al. 2011, Yirak et al. 2010} and references therein, we choose initial densities and temperatures such that both clump bow shocks and transmitted shocks have cooling time scales less than their respective dynamic timescales. In particular, the cloud-crushing time \citep{KMC94} given by

.\qquad\qquad t_{cc} = \frac{r_c}{\sqrt{\chi}v_s}$

where r_c is the radius of the clump, \chi the ratio of clump to preshock ambient density, and v_s the shock (wind) velocity.

As expected from \cite[][e.g.]{fragile2005, shin2008} and others, the Bx and hydro cases look relatively similar, while the By case is most disparate. Owing to stretching and bending of magnetic field lines, we find an amplification of field strength in _._._ of _._._. In case By, the shock speed is reduced by a factor of _._._, compared to _._._ for Bx (both relative to the hydro case).

We further discuss results below.

Note: I [Kris] am recalling the assignments below from memory so they may not be correct.

Hydro Case: Setting the Scene

Hydro sparse

Done: Brandon

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Hydro dense

Done: Brandon

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Bx Case: Strands Dominate

Bx sparse

Done: Shule

movie​

Bx dense

Done: Shule

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By Case: Transitions to a Wall

By sparse

Jonathan: As of 12/20, needs roughly three weeks more for flow to reach edge of domain. At frame ~110 (out of an estimated 140).

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{{{!comment

• Click here for a discussion on the investigation of spurious artifacts in the By case with AMR.

}}}

By dense

Erica: Restarted from beginning with fixed grid. As of 12/20, produced about 150 frames in 4 days. Here is frame 154:

movie​