\contentsline {chapter}{\numberline {1}OVERVIEW}{1}
\contentsline {section}{\numberline {1.1}INTRODUCTION}{1}
\contentsline {section}{\numberline {1.2}TECHNICAL OVERVIEW}{2}
\contentsline {subsection}{\numberline {1.2.1}Antenna and Receiver Improvements}{3}
\contentsline {subsubsection}{Improved Low Noise Receivers}{3}
\contentsline {subsubsection}{New Observing Bands at the Cassegrain Focus}{3}
\contentsline {subsubsection}{New Prime Focus Systems}{4}
\contentsline {subsubsection}{Sensitivity Goals}{4}
\contentsline {subsection}{\numberline {1.2.2}A New LO/IF Transmission System}{4}
\contentsline {subsection}{\numberline {1.2.3}A New Correlator}{5}
\contentsline {subsection}{\numberline {1.2.4}Improved Surface-Brightness Sensitivity}{5}
\contentsline {subsection}{\numberline {1.2.5}High Angular Resolution---The A+ Configuration}{6}
\contentsline {subsection}{\numberline {1.2.6}Other Capabilities}{7}
\contentsline {section}{\numberline {1.3}NEW SCIENCE WITH THE ENHANCED VLA}{7}
\contentsline {subsection}{\numberline {1.3.1}Imaging-Spectroscopy of Solar Radio Bursts}{8}
\contentsline {subsection}{\numberline {1.3.2}Bistatic Radar Observations of Planets and Minor Bodies}{8}
\contentsline {subsection}{\numberline {1.3.3}High-resolution Imaging of Thermal Emission}{8}
\contentsline {subsection}{\numberline {1.3.4}Imaging Proto-planetary Disks}{9}
\contentsline {subsection}{\numberline {1.3.5}Transient Phenomena}{9}
\contentsline {subsection}{\numberline {1.3.6}Galactic \hbox {H\hskip 1truept{\psc i} }Survey}{10}
\contentsline {subsection}{\numberline {1.3.7}Extragalactic \hbox {H\hskip 1truept{\psc i} }Surveys}{10}
\contentsline {subsection}{\numberline {1.3.8}Clusters of Galaxies}{11}
\contentsline {subsection}{\numberline {1.3.9}Objects at High Redshift}{11}
\contentsline {section}{\numberline {1.4}IMPACT ON SCIENCE WITH THE VLBA}{12}
\contentsline {section}{\numberline {1.5}SUMMATION}{14}
\contentsline {chapter}{\numberline {2}THE SOLAR SYSTEM}{17}
\contentsline {section}{\numberline {2.1}SOLAR PHYSICS}{17}
\contentsline {subsection}{\numberline {2.1.1}The Active Sun}{18}
\contentsline {subsubsection}{Solar Flares}{18}
\contentsline {subsubsection}{Radio Bursts}{19}
\contentsline {subsubsection}{Large-scale Transient Phenomena}{21}
\contentsline {subsubsection}{Active Regions}{22}
\contentsline {subsection}{\numberline {2.1.2}The Quiet Sun}{22}
\contentsline {subsubsection}{Chromospheric Structure}{22}
\contentsline {subsubsection}{Prominences and Filaments}{24}
\contentsline {subsubsection}{Coronal Holes}{25}
\contentsline {section}{\numberline {2.2}THE INTERPLANETARY MEDIUM}{25}
\contentsline {section}{\numberline {2.3}PLANETARY SCIENCE}{26}
\contentsline {subsection}{\numberline {2.3.1}Bistatic Planetary Radar}{26}
\contentsline {subsection}{\numberline {2.3.2}The Giant Planets}{29}
\contentsline {subsection}{\numberline {2.3.3}The Terrestrial Planets}{30}
\contentsline {subsubsection}{Mercury}{30}
\contentsline {subsubsection}{Mars}{32}
\contentsline {section}{\numberline {2.4}MINOR BODIES}{32}
\contentsline {subsection}{\numberline {2.4.1}Asteroids}{32}
\contentsline {subsection}{\numberline {2.4.2}The Galilean Satellites and Titan}{33}
\contentsline {subsection}{\numberline {2.4.3}Comets}{33}
\contentsline {section}{\numberline {2.5}TECHNICAL REQUIREMENTS}{34}
\contentsline {chapter}{\numberline {3}THE MILKY WAY GALAXY}{37}
\contentsline {section}{\numberline {3.1}INTRODUCTION}{37}
\contentsline {section}{\numberline {3.2}STAR FORMATION}{38}
\contentsline {subsection}{\numberline {3.2.1}Gravitational Infall}{38}
\contentsline {subsection}{\numberline {3.2.2}Proto-stellar Disks }{38}
\contentsline {subsection}{\numberline {3.2.3}Proto-planetary disks }{39}
\contentsline {section}{\numberline {3.3}MASSIVE STAR FORMING REGIONS}{40}
\contentsline {section}{\numberline {3.4}PMS AND MS STARS}{41}
\contentsline {section}{\numberline {3.5}ACTIVE STARS}{43}
\contentsline {section}{\numberline {3.6}STELLAR ASTROMETRY}{44}
\contentsline {section}{\numberline {3.7}IMAGING THERMAL EMISSION}{45}
\contentsline {section}{\numberline {3.8}CIRCUMSTELLAR MASERS}{47}
\contentsline {section}{\numberline {3.9}RADIO RECOMBINATION LINES}{49}
\contentsline {section}{\numberline {3.10}MOLECULAR LINES}{50}
\contentsline {section}{\numberline {3.11}X-RAY TRANSIENTS}{51}
\contentsline {section}{\numberline {3.12}SUPERNOVA REMNANTS}{53}
\contentsline {subsection}{\numberline {3.12.1}Spectral Imaging}{53}
\contentsline {subsection}{\numberline {3.12.2}Integrated Spectrum Studies}{54}
\contentsline {section}{\numberline {3.13}PULSARS}{54}
\contentsline {section}{\numberline {3.14}LARGE-SCALE STRUCTURE}{55}
\contentsline {subsection}{\numberline {3.14.1}The Galactic Center }{55}
\contentsline {subsection}{\numberline {3.14.2}The Galactic Magnetic Field }{57}
\contentsline {subsection}{\numberline {3.14.3}Galactic Nonthermal Continuum Emission }{57}
\contentsline {subsection}{\numberline {3.14.4}Galactic \hbox {H\hskip 1truept{\psc i} }}{58}
\contentsline {chapter}{\numberline {4}EXTRAGALACTIC RESEARCH AND COSMOLOGY}{59}
\contentsline {section}{\numberline {4.1}INTRODUCTION}{59}
\contentsline {section}{\numberline {4.2}NEARBY GALAXIES}{60}
\contentsline {subsection}{\numberline {4.2.1}Supernovae}{60}
\contentsline {subsubsection}{Monitoring: early radio light curves}{60}
\contentsline {subsubsection}{Monitoring: broader frequency coverage}{60}
\contentsline {subsubsection}{Monitoring: fainter sources}{61}
\contentsline {subsubsection}{SNe searches: going off the deep end}{61}
\contentsline {subsection}{\numberline {4.2.2}Supernova Remnants}{61}
\contentsline {subsection}{\numberline {4.2.3}\hbox {H\hskip 1truept{\psc ii} }Regions}{63}
\contentsline {subsection}{\numberline {4.2.4}Extended Emission: Spectral Imaging}{63}
\contentsline {subsection}{\numberline {4.2.5}Polarization}{65}
\contentsline {subsubsection}{Polarization of SNR/SNe}{65}
\contentsline {subsubsection}{Radio halos}{66}
\contentsline {subsubsection}{Galaxy disks}{66}
\contentsline {subsection}{\numberline {4.2.6}Imaging Starburst Galaxies}{66}
\contentsline {subsection}{\numberline {4.2.7}Instrumental Requirements}{67}
\contentsline {section}{\numberline {4.3}SPECTROSCOPY OF NORMAL GALAXIES}{68}
\contentsline {subsection}{\numberline {4.3.1}Neutral Hydrogen in Individual Galaxies}{68}
\contentsline {subsection}{\numberline {4.3.2}Neutral Hydrogen Surveys}{68}
\contentsline {subsubsection}{An all-sky \hbox {H\hskip 1truept{\psc i} }survey in 0 $<z<$ 0.1}{69}
\contentsline {subsubsection}{The structure of a cluster of galaxies, combined with a pencil beam survey}{69}
\contentsline {subsubsection}{The evolution of gas content at 0.2 $<z<$ 0.8}{70}
\contentsline {subsection}{\numberline {4.3.3}Instrumental Requirements for Neutral Hydrogen Surveys}{70}
\contentsline {subsubsection}{Comparison with existing and planned instruments}{70}
\contentsline {subsection}{\numberline {4.3.4}Radio Recombination Lines from Extragalactic Sources}{71}
\contentsline {subsection}{\numberline {4.3.5}OH Masers in Nearby Galaxies: the Birth of O and B Stars}{71}
\contentsline {subsection}{\numberline {4.3.6}Water MegaMasers: Nuclear Dynamics in Nearby Galaxies}{73}
\contentsline {subsection}{\numberline {4.3.7}Molecules in Nearby Galaxies}{73}
\contentsline {section}{\numberline {4.4}ACTIVE GALACTIC NUCLEI}{74}
\contentsline {subsection}{\numberline {4.4.1}The State of Radio-galaxy Physics}{74}
\contentsline {subsection}{\numberline {4.4.2}Jets}{75}
\contentsline {subsubsection}{Low power sources: jet deceleration?}{75}
\contentsline {subsubsection}{High power sources: high-velocity ``spines''?}{76}
\contentsline {subsubsection}{Particle acceleration}{76}
\contentsline {subsubsection}{Stability and confinement}{76}
\contentsline {subsection}{\numberline {4.4.3}Hot Spots}{77}
\contentsline {subsection}{\numberline {4.4.4}Lobes of Radio-loud AGNs}{77}
\contentsline {subsection}{\numberline {4.4.5}Black Hole ``Beacons'' in Nearby Galactic Nuclei?}{78}
\contentsline {subsection}{\numberline {4.4.6}Instrumental Requirements for AGN Continuum Studies}{78}
\contentsline {subsection}{\numberline {4.4.7}Absorption against AGNs}{79}
\contentsline {section}{\numberline {4.5}CLUSTER ENVIRONMENTS}{79}
\contentsline {subsection}{\numberline {4.5.1}Cluster Radio Halos}{80}
\contentsline {subsection}{\numberline {4.5.2}Faraday Rotation}{80}
\contentsline {subsection}{\numberline {4.5.3}The Sunyaev-Zeldovich Effect}{81}
\contentsline {subsection}{\numberline {4.5.4}Radio Galaxies in Clusters}{81}
\contentsline {subsection}{\numberline {4.5.5}Gravitational Lensing}{81}
\contentsline {section}{\numberline {4.6}OBJECTS AT HIGH REDSHIFT}{82}
\contentsline {subsection}{\numberline {4.6.1}Source Populations}{83}
\contentsline {subsection}{\numberline {4.6.2}Molecules, Dust and Free-Free Emission}{84}
\contentsline {subsection}{\numberline {4.6.3}Starbursts at High Redshifts}{85}
\contentsline {subsection}{\numberline {4.6.4}Faraday Rotation}{86}
\contentsline {chapter}{\numberline {5}TECHNICAL ISSUES AND BUDGET}{87}
\contentsline {section}{\numberline {5.1}INTRODUCTION}{87}
\contentsline {section}{\numberline {5.2}ANTENNAS AND RECEIVERS -- TRADEOFFS}{87}
\contentsline {subsection}{\numberline {5.2.1}Polarization vs. Wide Bandwidths}{87}
\contentsline {subsection}{\numberline {5.2.2}Spillover}{87}
\contentsline {subsection}{\numberline {5.2.3}Prime Focus Systems vs. Cassegrain Performance}{88}
\contentsline {subsection}{\numberline {5.2.4}Antenna Modifications}{91}
\contentsline {subsection}{\numberline {5.2.5}A Robust Total-Power System}{91}