BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//ZContent.net//ZapCalLib 1.0//EN
CALSCALE:GREGORIAN
METHOD:PUBLISH
BEGIN:VEVENT
DTSTART;TZID=Atlantic/Canary:20180614T103000
DTEND;TZID=Atlantic/Canary:20180614T113000
UID:iactalks-1156
X-WR-CALNAME: IAC Talks: Open Astronomy Seminars
X-ORIGINAL-URL: /iactalks/Talks/view/1156
CREATED:2018-06-14T10:30:00+01:00
X-WR-CALDESC: IAC Talks upcomming talks
SUMMARY:Why supergranulation? The structure of the upper superadiabatic tra
 nsition in the Sun and stars
DESCRIPTION:Why supergranulation? The structure of the upper superadiabatic
  transition in the Sun and stars\nProf. Mark Rast\n\nTurbulent convection 
 in stellar envelopes is critical to heat transport and dynamo activity. Mo
 deling it well has proven surprisingly difficult, and recent solar and ste
 llar observations have raised questions about our understanding of the dyn
 amics of both the deep solar convection and the mean structure of the uppe
 r layers of convective stellar envelopes.  In particular, the amplitu
 de of low wavenumber convective motions in both local area radiative magne
 tohydrodynamic and global spherical shell magnetohydrodynamic simulations 
 of the Sun appear to be too high. In global simulations this results in we
 aker than needed rotational constraint and consequent non solar-like diffe
 rential rotation profiles. In deep local area simulations it yields strong
  horizontal flows in the photosphere on scales much larger than the observ
 ed supergranulation, leaving the origin of the solar supergranular scale e
 nigmatic. The problem is not confined to the Sun. When comparing computed 
 oscillation frequencies to observations, mixing length models of stellar c
 onvection show too sharp a transition to the interior adiabatic gradient. 
 This contributes to what asteroseismologists call the `surface effect' cor
 rection.\n We suggest that there is a common solution to these proble
 ms: convective motions in stellar envelopes are even more nonlocal than nu
 merical models suggest. Small scale photospherically driven motions domina
 te convective transport even at depth, descending through a very nearly ad
 iabatic or possible even somewhat subadiabatic deep convection zone. Conve
 ction of this form may meet Rossby number constraints set by global scale 
 motions, and implies that the solar supergranulation is the largest buoyan
 tly driven scale of motion in the Sun. We test this hypothesis using a sui
 te of three-dimensional stellar atmosphere models, and can use it to both 
 recover their mean stratification and estimate the supergranular scale on 
 other stars
END:VEVENT
END:VCALENDAR