One-dimensional Merging of Magnetic Fields with Cooling

Dorman, Victoria; Kulsrud, Russell M.

doi:10.1086/176097

Cited by 17 publications

(17 citation statements)

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“…Over the past 10 years, deep optical and UV observations have shown that many low metallicity globular clusters have bimodal or extended horizontal branch mass distributions which include objects at high temperature. Examples include NGC 2808 (Ferraro et al 1990;Sosin et al 1997), NGC 1851 (Walker 1992), M15 (Crocker, Rood, & O'Connell 1988), ω Cen (Whitney et al 1994), M79 (Hill et al 1996), NGC 6752 (Landsman et al 1996), and NGC 362 (Dorman et al 1997). Our UV observations of 47 Tuc and recent optical-band HST imaging of the centers of two other metal rich clusters NGC 6388 and NGC 6441 (Piotto et al 1997, Rich et al 1997 show that this phenomenon extends to clusters of high metallicity.…”

Section: Hot Starssupporting

confidence: 54%

“…A description of the UIT instrument and standard data reduction and calibration procedures is given in Stecher et al (1997) and Dorman et al (1997). The field of view is 40 ′ diameter.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

“…The metal-rich globulars therefore represent an important extreme which is important to understand in trying to identify the underlying mechanism of the UV-upturn phenomenon in galaxies. More discussion of these issues can be found in our companion paper on UV imaging of the cluster NGC 362 (Dorman et al 1997) and in Greggio & Renzini (1990) and Dorman et al (1995).…”

Section: Introductionmentioning

confidence: 98%

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Ultraviolet Imaging of the Globular Cluster 47 Tucanae

O’Connell¹,

Dorman²,

Shah³

et al. 1997

The Astronomical Journal

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We have used the Ultraviolet Imaging Telescope to obtain deep far-UV (1620 Angstrom), 40' diameter images of the prototypical metal-rich globular cluster 47 Tucanae. We find a population of about 20 hot (Teff > 9000 K) objects near or above the predicted UV luminosity of the hot horizontal branch (HB) and lying within two half-light radii of the cluster center. We believe these are normal hot HB or post-HB objects rather than interacting binaries or blue stragglers. IUE spectra of two are consistent with post-HB phases. These observations, and recent HST photometry of two other metal-rich clusters, demonstrate that populations with rich, cool HB's can nonetheless produce hot HB and post-HB stars. The cluster center also contains an unusual diffuse far-UV source which is more extended than its V-band light. It is possible that this is associated with an intracluster medium, for which there was earlier infrared and X-ray evidence, and is produced by C IV emission or scattered light from grains.Comment: 13 pages AASLaTeX including one postscript figure and one bitmapped image, JPEG format. Submitted to the Astronomical Jorunal. Full Postscript version available at http://www.astro.virginia.edu/~bd4r

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Section: Hot Starssupporting

confidence: 54%

“…A description of the UIT instrument and standard data reduction and calibration procedures is given in Stecher et al (1997) and Dorman et al (1997). The field of view is 40 ′ diameter.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

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Ultraviolet Imaging of the Globular Cluster 47 Tucanae

O’Connell¹,

Dorman²,

Shah³

et al. 1997

The Astronomical Journal

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“…Magnetic reconnection in weakly ionised plasmas is important in the solar photosphere/chromosphere (Sakai 1996;Bulanov & Sakai 1998;Litvinenko 1999;Furusawa & Sakai 2000;Sakai et al 2006), as well as in the interstellar medium (Dorman & Kulsrud 1995;Zweibel & Brandenburg 1997;Heitsch & Zweibel 2003). Solar observations of spectroscopic line shifts and non-thermal broadening confirm that the photosphere, chromosphere, and lower corona of the Sun are all in a highly dynamical state.…”

Section: Introductionmentioning

confidence: 96%

Chromospheric magnetic reconnection: two-fluid simulations of coalescing current loops

Smith¹,

Sakai

2008

A&A

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Aims. We investigate magnetic reconnection rates during the coalescence of two current loops in the solar chromosphere, by altering the neutral-hydrogen to proton density ratio, ionisation/recombination coefficients, collision frequency, and relative helicity of the loops. Methods. We used a newly developed two-fluid (ion-neutral) numerical code to perform 2.5D simulations of coalescing chromospheric current loops. Developed from the Artificial Wind scheme, the numerical code includes the effects of ion-neutral collisions, ionisation/recombination, thermal/resistive diffusivity, and collisional/resistive heating. Results. It was found that the rates of magnetic reconnection strongly depend on the neutral-hydrogen to proton density ratio: increasing the density ratio a thousandfold decreased the rate of magnetic reconnection twentyfold. This result implies that magnetic reconnection proceeds significantly faster in the upper chromosphere, where the density of ions (protons) and neutral-hydrogen is comparable, than in the lower chromosphere, where the density of neutral-hydrogen is over a thousand times the ion density. This result also implies that jets associated with fast magnetic reconnection tends to occur in the upper chromosphere / lower corona. The inclusion of ionisation/recombination, an important physical effect in the chromosphere, increases the total reconnected magnetic flux, but does not alter the rate of magnetic reconnection. Reductions in the ion-neutral collision frequency result in small increases to the rates of magnetic reconnection. The relative helicity of the two current loops was not observed to have any significant effect on the rates of magnetic reconnection. Comparisons of two-fluid and MHD (Magnetohydrodynamic) simulations show significant differences in the measured rates of magnetic reconnection, particularly for the higher neutral density cases which represent the lower chromosphere. This demonstrates that MHD is not an appropriate model for simulating magnetic reconnection in the solar chromosphere. Conclusions. The magnetic reconnection rates of coalescing current loops are strongly affected by the inclusion of neutral-hydrogen particles. It is therefore essential that ion-neutral collisions are included in future analytical/numerical models of chromospheric magnetic reconnection.

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“…Radiative cooling reduces the pressure in the current sheet, thus allowing the sheet to thin and, correspondingly, the current density to rise. This implies higher Lorentz forces accelerating the reconnection outflow, resulting in an enhancement over the Sweet-Parker rate of order A 1/2 , where A is the compression factor of the sheet due to the cooling (Dorman & Kulsrud 1995;Uzdensky & McKinney 2011). In the incompressible strong guide field case, the cooling leads to a weaker enhancement of the reconnection rate through the higher resistivity, but in a chromospheric environment this can hardly reach an order of magnitude.…”

Section: Introductionmentioning

confidence: 99%

Fast Magnetic Reconnection in the Solar Chromosphere Mediated by the Plasmoid Instability

Kliem

Lin

et al. 2015

ApJ

133

158

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Magnetic reconnection in the partially ionized solar chromosphere is studied in 2.5-dimensional magnetohydrodynamic simulations including radiative cooling and ambipolar diffusion. A Harris current sheet with and without a guide field is considered. Characteristic values of the parameters in the middle chromosphere imply a high magnetic Reynolds number of ∼ 10 6 -10 7 in the present simulations. Fast magnetic reconnection then develops as a consequence of the plasmoid instability without the need to invoke anomalous resistivity enhancements. Multiple levels of the instability are followed as it cascades to smaller scales, which approach the ion inertial length. The reconnection rate, normalized to the asymptotic values of magnetic field and Alfvén velocity in the inflow region, reaches values in the range ∼ 0.01-0.03 throughout the cascading plasmoid formation and for zero as well as for strong guide field. The out-flow velocity reaches ≈ 40 km s −1 . Slow-mode shocks extend from the X-points, heating the plasmoids up to ∼ 8 × 10 4 K. In the case of zero guide field, the inclusion of ambipolar diffusion and radiative cooling both cause a rapid thinning of the current sheet (down to ∼ 30 m) and early formation of secondary islands. Both of these processes have very little effect on the plasmoid instability for a strong guide field. The reconnection rates, temperature enhancements, and upward out-flow velocities from the vertical current sheet correspond well to their characteristic values in chromospheric jets.

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One-dimensional Merging of Magnetic Fields with Cooling

Cited by 17 publications

References 0 publications

Ultraviolet Imaging of the Globular Cluster 47 Tucanae

Ultraviolet Imaging of the Globular Cluster 47 Tucanae

Chromospheric magnetic reconnection: two-fluid simulations of coalescing current loops

Fast Magnetic Reconnection in the Solar Chromosphere Mediated by the Plasmoid Instability

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