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Information about STEREO-A close approach to Earth, August 2023

Anticipated science from STEREO observations of Comet ISON

The STEREO spacecraft are uniquely situated to observe the approach and perihelion passage of comet ISON in November 2013. The STEREO instrument teams — in particular the SECCHI team — are busy putting together observing plans to address a wide range of cometary and heliophysics science objectives. Some of the anticipated science from these observations are discussed below:

  • The dependence of the comet brightness on the angle between the direction to the Sun and the direction to the observer, known as the scattering phase angle, provides important information about the composition of the gas and dust particles in the comet's coma and tail. Simultaneous views from both STEREO spacecraft will sample different parts of the phase-angle vs. brightness relationship, giving a more complete picture than can be obtained from one viewing position alone, especially if these data can be combined with observations from other observatories such as the SOHO/LASCO coronagraph. The simultaneous polarized brightness observations from the STEREO COR1 and COR2 coronagraphs are of particular value for this analysis.

  • The multiple view-points offered by the SECCHI imagers and coronagraphs present a tremendous opportunity for studying the state of the nucleus, the dust production, brightness evolution and tail morphology as a function of heliocentric distance.

  • Comet ISON's passage coincides with a relatively high activity phase of the solar cycle, thereby offering a unique opportunity to study the interaction between solar wind transients (coronal mass ejections and co-rotating interaction regions) and the comet tail.

  • For solar wind studies, the comet tail reaction to the passage through fast and slow wind streams (particularly the interfaces between the two) acts as an in-situ probe allowing us to study the conditions at those areas. In that respect, comet ISON acts like a proxy for the Solar Probe Plus mission and the results of these studies may provide crucial information for the science planning for that mission.

  • Previous observations with the STEREO/EUVI and SDO/AIA telescopes of Comet C/2011 W3 (Lovejoy), combined with earlier observations of C/2011 N3 by SDO, opened up an entirely new window into the physics of the interactions between the dust particles in the comet tail and the solar atmosphere, and shed new light on the configuration of the magnetic field in the inner corona. High cadence, deep exposure observations of Comet ISON for several hours near perihelion, particularly at 171 A, can provide an opportunity to extend these observations to much higher altitudes above the solar surface.

To carry out these science objectives, the SECCHI team plans to combine their normal synoptic program with additional observations designed specifically for Comet ISON. These additional exposures will be interlaced with the regular exposures, so that the combination of the two will give a higher cadence than either would on their own. Shorter exposure times will be used to adjust for the anticipated high brightness in the coma. Some images will be taken at full resolution, as opposed to the reduced resolution used for the synoptic images at this point in the mission. Subfields concentrating on the comet will be used for many of the non-synoptic images to reduce the strain on the telemetry volume, which is the primary observational constraint.

During certain periods before and after the comet passes by the Sun, one or the other of the STEREO spacecraft will be rolled for 4 hours at a time to observe the comet in one of the Heliospheric Imager telescopes. These rolls are discussed in more detail on our observing geometry page.

The other STEREO instruments — IMPACT, PLASTIC, and SWAVES — will be carrying on their normal synoptic program during this entire period, though the effectiveness of some of the detectors will be reduced during the spacecraft rolls. There is, however, a possibility that some signatures of the comet may be detected in the in situ instruments a few days after the comet passes by the Sun. Possible signs of the comet are:

  • An increase in the solar wind mass density from cometary ions picked up by the solar wind. This may also be seen as a composition change.
  • Transverse Alfven waves.
  • A decrease in the proton speed and density, with a rise in kinetic temperatures, and draping by the magnetic field, would be a signature of passing through the central plasma tail.
These signatures, if seen, are expected to be of very short duration, from a few minutes to a few hours.

Last Revised: Tuesday, 01-Oct-2019 21:07:19 UTC
Responsible NASA Official: [email address: Therese.A.Kucera<at>nasa<dot>gov]
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