Toward detection and characterization of exoplanetary magnetic fields via low frequency radio observation
Author(s)Knapp, Mary (Mary E.)
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences.
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Low frequency radio emission from planets is produced by the interaction of energetic charged particles from the planet's ionosphere and/or the solar wind with the planet's magnetic field. The temporal and spectral characteristics and variability of planetary radio emission encode information about a planet's magnetic field strength and morphology, rotation, and interior. This thesis describes three distinct approaches to detecting radio emission from extrasolar planets ( exoplanets). The first is a novel approach using 'big data' and computer aided discovery tools to mine radio survey images for faint radio emission from the location of nearby stars. The flexible approach described in this thesis produced upper limits of rv lOOx Jovian radio flux for a large sample of nearby stars and known exoplanet systems. The sensitivity is sufficient that large radio bursts from nearby stars or planets could have been detected if they took place during the survey observation(s). The framework developed here can be used for automated exoplanet radio emission searches in future radio survey data. The second approach described herein is a blind survey of the nearest Northern hemisphere stars across a broad range of frequencies in order to detect as-yet unknown planets or set tight constraints on radio emission from the stars and possible substellar companions. The survey approach used here is novel because it makes no assumptions about which stars are most likely to host radio emitting planets and it covers frequencies from 30 MHz to 4 GHz. This survey produced a detection of multiple rv50% circularly polarized flares from the M dwarf binary system Ross 614 as well as limits at the lOx Jovian flux level for the remaining stars observed. The limits attained from this survey are the first published at 1-4 GHz for these objects and the only available radio limits for a newly discovered cool (T9) brown dwarf. The limits from this survey place a preliminary constraint on the magnetic field of the brown dwarf at <350 G. The third approach focuses on known exoplanet systems and targets key orbital phases where intense radio emission is predicted. In the case of eccentric hot Jupiter HD 80606 b, radio flux from the planet is expected to increase by a factor of up to 3000 compared to the quiescent flux as the planet passes within 6 Rof its host at periastron due to high density stellar wind impinging on HD 80606 b's putative magnetosphere. Data obtained from LOFAR LBA is used to set the lowest limits to date on radio flux from HD 80606 b near planetary periastron. The same concept of orbital phase targeting is used to optimize an observing strategy for recently-discovered multiple planet host TRAPPIST-I. In the case of TRAPPIST-1, the quadrature phases of planets TRAPPIST-1 b and TRAPPIST-1 c are targeted to maximize the chance of observing Io-Jupiter like planetary modulation of stellar radio emission. The quadrature phase targeting approach is new to this field. The thesis concludes with a discussion of the benefits of space-based observation for exoplanetary radio searches. Ground-based observations are limited by the plasma frequency of the ionosphere, so planets with Earth-like magnetic fields cannot be observed. Telescopes on the ground also suffer from ionospheric phase errors that are difficult to fully calibrate. Space-based observation does not suffer from the effects of the ionosphere and can therefore support lower frequency observations than ground-based instruments. A novel instrument, the vector sensor, optimized for space-based radio interferometry is introduced. New algorithms for all-sky vector sensor imaging have been developed and tested in simulation and on sky data with encouraging results. Finally, the prospects for detecting Earth- or Jupiter-analogs in the solar neighborhood, either from the ground or from space, are assessed. Very large space-based arrays are required to detect either Jupiter or the Earth at 10 pc; at least 105 -106 antennas are needed for sufficient sensitivity.
Thesis: Ph. D. in Planetary Sciences, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 225-256).
DepartmentMassachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences.
Massachusetts Institute of Technology
Earth, Atmospheric, and Planetary Sciences.