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The MWA is capable of a wide range of science investigations. These planned investigations are described in detail by Bowman et al. (2012). Here, we review the four key science themes that encompass the planned investigations and that have driven the design of the array. The key science themes for the array are: 1) detection of fluctuations in the brightness temperature of the diffuse redshifted 21 cm line of neutral hydrogen from the epoch of reionization (EoR); 2) studies of Galactic and extragalactic (GEG) processes based on a deep, confusion-limited survey of the full sky visible to the array; 3) time domain astrophysics through exploration of the variable radio sky (transients); and 4) solar heliosphere and ionosphere (SHI) imaging and characterization via propagation effects on background radio sources.

1. Epoch of Reionization

Exploration of the Cosmic Dawn, the period when the first stars and galaxies formed in the early Universe, has been identified as an important area for new discoveries within the next decade. The MWA is one of the first radio interferometers to attempt to detect redshifted 21 cm line emission from neutral hydrogen gas in the intergalactic medium (IGM) during this period. The array has been designed to optimize its ability to detect brightness temperature fluctuations in the 21 cm line emission during the EoR in the redshift range 6 < z < 10. During the EoR, primordial neutral hydrogen begins to be ionized by the radiation from the first luminous sources. The MWA has sufficient thermal sensitivity to detect the presence of the large ionized bubbles that form during reionization through measurements of the power spectrum and other statistical properties of the fluctuations to a significance level of 14 sigma (Beardsley et al. 2012). In order to achieve this objective, the MWA will be a testbed to develop and demonstrate techniques to subtract the bright radio foregrounds that obscure the 21 cm background.

Animation - Simulation of reionization by Alvarez et al. (2009) showing the expected "swiss cheese" signature in intergalactic hydrogen gas during the EoR. The MWA is attempting to detect this signature using statistical methods by observing the 21 cm radio line from the gas.

2. Galactic Science

Radio emission from the Galaxy and from extragalactic sources is both a complicating foreground for EoR observations and an interesting scientific target that forms the second key science theme for the MWA. The MWA will be unique in its ability to conduct an arc-minute resolution, confusion-limited survey of the full Southern Hemisphere sky below 10 degrees declination over the 80 to 300 MHz frequency range. The survey will include the Galactic Center and the Large and Small Magellanic Clouds. At the low observing frequencies of the MWA, non-thermal processes and Faraday rotation (and depolarization) effects will be prominent.

The MWA should be particularly well suited to identifying the missing population of old and faint supernova remnants (SNRs) in the Galaxy, closing the gap between the 300 known SNRs and the expected 1000 to 2000, and thereby providing a critical measurement of the total energy budget of the interstellar medium. Additional experiments planned for the MWA target radio relics and clusters, the cosmic web, and Faraday tomography to probe magnetic fields. Cosmic ray mapping may be also be possible along sight-lines where sufficiently dense HII regions have become optically thick in the MWA frequency band, blocking synchrotron emission from the Galaxy behind them.

Figure - Map showing turbulence in the gas and plasma between stars in our Milky Way galaxy (Gaensler et al. 2012). The MWA enables astronomers to study the interstellar medium with remarkable new precision.

3. Time Domain Astrophysics

With its high survey efficiency, as well as planned long integrations on EoR target fields, the MWA will enable sensitive transient and variable searches for both rare and faint events on timescales from seconds to days. The MWA will perform blind searches and target known transient sources, including low-mass stars and brown dwarfs, pulsars, X-ray binaries, and isolated neutron stars. At time resolution in the nanosecond range, the voltage capture capability of the MWA will allow studies of pulsars at low frequencies and searches for fast transients (e.g. Wayth et al. 2012).

4. Space Weather

The final key science theme for the MWA encompasses the field of space weather, targeting multiple aspects of solar bursts as they travel from the surface of the Sun to the Earth. The primary science focuses on high-dynamic range spectroscopic imaging to map the frequency, spatial, and time evolution of radio bursts occurring in the solar corona at heights of approximately 1 to 4 solar radii. Interplanetary scintillation will be used to constrain the density and turbulence of the interstellar wind in the inner heliosphere. The MWA may also enable measurements of the magnetic field of the heliosphere plasma if it proves possible to track changes in the polarization angles of background sources. Lastly, the calibration solutions of the MWA will yield near-real time corrections for ionospheric distortions, providing a new window into variability in the Earth's ionosphere.

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