  Observations of the sungrazing comet C/2012 S1 (ISON) were carried out using
the Atacama Large Millimeter/submillimeter Array (ALMA) at a heliocentric
distance of 0.58-0.54 AU (pre-perihelion) on 2013 November 16-17. Temporally
resolved measurements of the coma distributions of HNC, CH$_3$OH, H$_2$CO and
dust were obtained over the course of about an hour on each day. During the
period UT 10:10-11:00 on Nov. 16, the comet displayed a remarkable drop in
activity, manifested as a $>42$% decline in the molecular line and continuum
fluxes. The H$_2$CO observations are consistent with an abrupt, $\approx50$%
reduction in the cometary gas production rate soon after the start of our
observations. On Nov. 17, the total observed fluxes remained relatively
constant during a similar period, but strong variations in the morphology of
the HNC distribution were detected as a function of time, indicative of a
clumpy, intermittent outflow for this species. Our observations suggest that at
least part of the detected HNC originated from degradation of nitrogen-rich
organic refractory material, released intermittently from confined regions of
the nucleus. By contrast, the distributions of CH$_3$OH and H$_2$CO during the
Nov. 17 observations were relatively uniform, consistent with isotropic outflow
and stable activity levels for these species. These results highlight a large
degree of variability in the production of gas and dust from comet ISON during
its pre-perihelion outburst, consistent with repeated disruption of the nucleus
interspersed with periods of relative quiescence.
