Important notice about STEREO Behind
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
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
- 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
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:
These signatures, if seen, are expected to be of very short duration, from a
few minutes to a few hours.
- 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.
Last Revised: Tuesday, 03-Sep-2013 21:25:55 GMT
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