Special live online demo version; see GitHub page. Use arrow keys to navigate • Figures and movies are copied from many sources; I have made best effort to credit and will rectify any oversights.
Peter K. G. Williams ·
@pkgw ·
http://newton.cx/~peter/
Bucknell University Physics Colloquium · 2013 Sep 19
Hallinan et al., NRAO/AUI/NSF
PKGW, Edo Berger, Ben Cook, John Gizis, Ashley Zauderer
… and it’s troublesome.
“The uncertainty scales with the strength of the magnetic field.” — truism
⇕
“What happens when you add in a magnetic field?” — generic seminar question
Magnetism plays a role on all sorts of scales:
Average surface field ∼1 Gauss (∼$10^{-4}$ Tesla).
NASA Solar Dynamics Observatory AIA — YouTube
Dynamo: converts mechanical to electromagnetic energy.
Solar dynamo believed to be an “α–Ω” mechanism.
This dissipation powers flares, coronal mass ejections (CMEs), and underlies the mystery of coronal heating: how do you get >10$^6$ K from 7000 K?
The dominant dissipation mechanism, but a huge challenge to understand.
NASA SDO — YouTube
Average surface field ∼0.5 G (but radius is much smaller than Sun!).
NASA / Goddard Space Flight Center
NASA — YouTube
Field is much more dipolar than Sun.
Deflects CMEs and the solar wind — vital protection!
NASA — YouTube
CMEs routinely force satellite safe-moding
Geomagnetic storm of 1989: nine-hour, Quebec-wide blackout
“Carrington Event” of 1859:
Maunder Minimum / “Little Ice Age” tied to famines, wars, witch hunts (Behringer 1999)
NASA
Both phenomenologically and theoretically.
NASA / David Hathaway
Kitt Peak Solar Observatory
NASA / WISE
The very smallest stars (hydrogen-burning) and brown dwarfs (BDs; no H fusion). Temperatures $\lesssim$3000 K.
Steven Dutch / UW Green Bay
A best-effort sample of objects within 8 parsecs (26 lightyears):
Courtesy Z. Berta.
Jupiter is ∼320 ($10^{2.5}$) Earth masses, 11 Earth radii.
The Sun is ∼1000 Jupiter masses ($10^{5.5}$ Earth masses), 10 Jupiter radii.
Recall: the solar dynamo depends crucially on the “tachocline” interface layer between the radiative and convective zones!
Wikipedia / Xenoforme
What happens to the dynamo?!
mailmagazine24.com (climate change denialists)
Corona: X-rays, radio
Transition region: ultraviolet
Chromosphere: optical spectral lines (e.g. Hα, 6563 Å)
Photosphere: optical broadband variability
PKGW+ (2013)
Berger+ (2010)
Bright, bursty, circularly-polarized emission strongly indicates electron cyclotron maser instability (ECMI).
$$\nu_\textrm{cyc} = \frac{e B}{2 \pi m_e c}$$
Easy handle on the characteristic magnetic field strength:
$$B \approx (360\textrm{ G}) \frac{\nu}{1\textrm{ GHz}}$$
Recall: solar surface field is ∼1 G.
This remains true down to extremely cool (900 K) objects (Route & Wolszczan, 2012).
Most observations have obeyed $L_R \propto L_X^{\sim 3/4}$.
PKGW+ (2013)
Another sign of a different regime of dynamo activity.
Observations will speak directly to the habitability of these planets.
In fact, it seems to shoot up past the transition to full convection.
Yet another new behavior toward the UCD regime.
Observationally:
This makes sense in standard dynamo models.
Stars “spin down” via magnetized winds, but strength of effect decreases with radius (recent realization — Reiners & Mohanty, 2012)
Fast rotation and a falloff of X-ray emission.
Cook, PKGW+ (2013)
The big puzzle: is this really due to rotation, or is it just a correlation effect with mass/temperature?
We observed objects with similar masses, different rotational velocities; also carefully collected published data.
Statistical analysis, accounting for upper limits: significant correlation in the narrow mass range as well as the broader sample.
Cook, PKGW+ (2013)
This could still be correlation without causation.
Measure a mean field strength with complicated spectropolarimetric “Zeeman-Doppler imaging” (ZDI) technique.
Cook, PKGW+ (2013)
Caveat: ZDI data are not sensitive to tangled fields, and only detect ≲15% of the total field.
Stronger, larger-scale field → more X-ray?
Dynamo bimodality in fastest rotators (Gastine+, 2013)
Thanks for
listening! Questions?
Me: Peter K. G. Williams · @pkgw · http://newton.cx/~peter/
Design credits: Hakim El Hattab (“night” theme), Julieta Ulanovsky (Montserrat font), Steve Matteson (Open Sans font).
Tech credits: git, reveal.js, MathJax, pdf.js, Firefox developer tools.
Acknowledgments: this work is supported in part by the NSF REU and DOD ASSURE programs under NSF grant no. 1262851, the Smithsonian Institution, NSF grant AST-1008361, and NASA Chandra Award Number G02-13007A issued by the Chandra X-ray Observatory Center, operated by the Smithsonian Astrophysical Observatory and NASA under contract NAS8-03060.
NASA / David Hathaway