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 , \approx 50 % 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 .