HAO 2010 PROFILES IN SCIENCE: Dr. Keith MacGregor

Contact

303-494-1557
kmac@ucar.edu

Dr. Keith MacGregor is a Senior Scientist in the High Altitude Observatory of the National Center for Atmospheric Research. He received his PhD in Physics in 1977 from the Massachusetts Institute of Technology. His initial work with HAO began in 1981 as a Scientist I. His research is focused on hydrodynamics, magnetohydrodynamics, and radiative transfer, as applied to the structure and dynamics of solar/stellar interiors and atmospheres.

Publication:

(1) Emission of Alfven Waves by Planets in Close Orbits

Abstract:

The detection of numerous, Jupiter-like, extrasolar planets in close orbits about their parent stars has generated considerable interest in the nature of possible star-planet interactions. Keith MacGregor (HAO) and scientific visitor Mark Pinsonneault (Ohio State University) have examined the electrodynamics of a conducting planet orbiting within a magnetized wind that emanates from its parent star. When the orbital motion differs from corotation with the star, an electric field exists in the rest frame of the planet, inducing a charge separation in its ionosphere. Because the planet is immersed in a plasma, this charge can flow away from it along the stellar magnetic field lines it successively contacts in its orbit. For sufficiently rapid orbital motion, a current system can be formed that is closed by Alfvenic disturbances that propagate along field lines away from the planet. Using a simple model for the wind from a Sun-like star, MacGregor and Pinsonneault have surveyed the conditions under which Alfven wave emission can occur, and have estimated the power radiated in the form of linear waves for a range of stellar, planetary, and wind properties. For a Jupiter-like planet in a close (a < 0.10 AU) orbit about a solar-type star, the emitted wave power can be as large as 1027 erg/s. While only a small influence on the planet's orbital dynamics, a wave power of this magnitude may have consequences for wind dynamics and localized heating of the stellar atmosphere. These effects are currently under investigation.

Team: Keith MacGregor (HAO/NCAR), Mark Pinsonneault (Ohio State University)

Emission of Alfven waves by a close-in conducting planet immersed in the wind outflow emanating from its parent star
Figure: Emission of Alfven waves by a close-in conducting planet immersed in the wind outflow emanating from its parent star. The results depicted were obtained by assuming a star of solar mass and radius emits a thermally driven wind from a coronal reference radius r0 where the temperature is 106 K, the number density is 2 x 108 cm-3, and the radial magnetic field strength is 1 G. Upper panel: the wind speed (uW), Alfven speed (uA), and orbital speed of a Jupiter-mass planet (uorb) as functions of the assumed orbital radius in units of r0. Only the region interior to the Alfven radius (where uW = uA) is shown. In this region, inwardly propagating waves can (in principle) reach the reference radius in the stellar corona. The waves emitted as a result of the star-planet interaction are linear where uorb << uA. Middle panel: The Alfven transit time tA, defined to be the time required for an Alfven wave to travel in the wind from rorb to r0 and back, as a function of the assumed orbital radius. Also shown is the time required for the planet to travel a distance equal to its diameter, tD = D/uorb, where the planetary diameter D is assumed to be that of Jupiter. Emission of Alfven waves can occur for conditions such that tD << t_A. Lower planel: the power PA radiated in the form of linear Alfven waves as a consequence of the planetary motion through the magnetized wind.