questions concerning the formation and evolution of galaxies, black holes, and the intergalactic medium.

Frequency Agile Solar Radiotelescope. The Frequency Agile Solar Radiotelescope (FASR) consists of three arrays of radio telescopes operating across a broad range of frequency (from 50 megahertz to 20 gigahertz). Its overall scientific program is to conduct time-domain mapping of the solar atmosphere in a campaign mode, delivering data products to the solar physics community. These will be used to study the nature and evolution of the Sun’s magnetic field, to understand solar flares, improve the ability to predict “space weather” caused by solar activity, and better understand the quiet sun.

Hydrogen Epoch of Reionization Array. The Hydrogen Epoch of Reionization Array (HERA) is a multistage project in radio astronomy to understand how hydrogen is ionized after the first stars start to shine. The first phase (HERA I) is under way and will demonstrate the feasibility of the technical approach. The second phase (HERA II) would serve as a pathfinder for an eventual worldwide effort in the following decade to construct a facility with a total collecting area of a square kilometer and the power to make detailed maps of this critical epoch in the history of the universe. Proceeding with HERA II should be subject to HERA I meeting stringent performance requirements in its ability to achieve system calibration and the removal of cosmic foreground emission.

North American Nanohertz Observatory for Gravitational Waves. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) would utilize the naturally occurring population of precision astronomical “clocks” called pulsars (rapidly spinning neutron stars) to detect very low frequency gravitational waves using upgraded capabilities of the existing Arecibo and Robert Byrd radio telescopes. The pulsar timing should also be able to detect the formation and collision of massive black holes with signals at periods of months to several years. This facility might even be able to detect relic gravitational waves from the very early universe (which is otherwise inaccessible to direct observations).


Cosmic Microwave Background Initiatives. NSF has invested wisely in the past in ground-based telescopes that have considerably advanced the understanding of the fluctuations, polarization, and distortion of the cosmic microwave background through the Sunyaev-Zeldovich effect and gravitational lensing in a way that has complemented the suborbital and space program especially by working at smaller angular scales. Thanks to the development of new detector technology, ground-

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