20 research outputs found
Preliminary Analysis of SOHO/STEREO Observations of Sungrazing Comet ISON (C/2012 S1) Around Perihelion
We present photometric and morphological analysis of the behavior of
sungrazing comet C/2012 S1 ISON in SOHO and STEREO images around its perihelion
on 2013 November 28.779 UT. ISON brightened gradually November 20-26 with a
superimposed outburst on November 21.3-23.5. The slope of brightening changed
about November 26.7 and was significantly steeper in SOHO's orange and clear
filter images until November 27.9 when it began to flatten out, reaching a peak
about November 28.1 (), then fading before brightening
again from November 28.6 () until disappearing behind the
occulting disc. ISON brightened continuously as it approached perihelion while
visible in all other telescopes/filters. The central condensation disappeared
about November 28.5 and the leading edge became progressively more elongated
until perihelion. These photometric and morphological behaviors are reminiscent
of the tens of meter sized Kreutz comets regularly observed by SOHO and STEREO
and strongly suggest that the nucleus of ISON was destroyed prior to
perihelion. This is much too small to support published gas production rates
and implies significant mass loss and/or disruption in the days and weeks
leading up to perihelion. No central condensation was seen post-perihelion. The
post-perihelion lightcurve was nearly identical in all telescopes/filters and
fell slightly steeper than . This implies that the brightness was
dominated by reflected solar continuum off of remnant dust in the coma/tail and
that any remaining active nucleus was <10 m in radius.Comment: Accepted by ApJL; 11 pages of text (pre-print style), 3 figures, 1
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Sodium Brightening of (3200) Phaethon Near Perihelion
Sunskirting asteroid (3200) Phaethon has been repeatedly observed in STEREO
HI1 imagery to anomalously brighten and produce an antisunward tail for a few
days near each perihelion passage, phenomena previously attributed to the
ejection of micron-sized dust grains. Color imaging by the SOHO LASCO
coronagraphs during the 2022 May apparition indicate that the observed
brightening and tail development instead capture the release of sodium atoms,
which resonantly fluoresce at the 589.0/589.6 nm D lines. While HI1's design
bandpass nominally excludes the D lines, filter degradation has substantially
increased its D line sensitivity, as quantified by the brightness of Mercury's
sodium tail in HI1 imagery. Furthermore, the expected fluorescence efficiency
and acceleration of sodium atoms under solar radiation readily reproduce both
the photometric and morphological behaviors observed by LASCO and HI1 during
the 2022 apparition and the 17 earlier apparitions since 1997. This finding
connects Phaethon to the broader population of sunskirting and sungrazing
comets observed by SOHO, which often also exhibit bright sodium emission with
minimal visible dust, but distinguishes it from other sunskirting asteroids
without detectable sodium production under comparable solar heating. These
differences may reflect variations in the degree of sodium depletion of
near-surface material, and thus the extent and/or timing of any past or present
resurfacing activity.Comment: 30 pages, 14 figures, 6 tables; PSJ, in pres
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Fine-Scale Structure in Cometary Dust Tails I::Analysis of Striae in Comet C/2006 P1 (McNaught) through Temporal Mapping
Striated features, or striae, form in cometary dust tails due to an as-yet unconstrained process or processes. For the first time we directly display the formation of striae, at C/2006 P1 McNaught, using data from the SOHO LASCO C3 coronagraph. The nature of this formation suggests both fragmentation and shadowing effects are important in the formation process. Using the SOHO data with STEREO-A and B data from the HI-1 and HI-2 instruments, we display the evolution of these striae for two weeks, with a temporal resolution of two hours or better. This includes a period of morphological change on 2007 January 13–14 that we attribute to Lorentz forces caused by the comet’s dust tail crossing the heliospheric current sheet. The nature of this interaction also implies a mixing of different sized dust along the striae, implying that fragmentation must be continuous or cascading. To enable this analysis, we have developed a new technique – temporal mapping – that displays cometary dust tails directly in the radiation beta (ratio of radiation pressure to gravity) and dust ejection time phase space. This allows for the combination of various data sets and the removal of transient motion and scaling effects
The EUV Emission in Comet-Solar Corona Interactions
The Atmospheric Imaging Assembly (AlA) on the Solar Dynamics Observatory (SDO) viewed a comet as it passed through the solar corona on 2011 July 5. This was the first sighting of a comet by a EUV telescope. For 20 minutes, enhanced emission in several of the AlA wavelength bands marked the path of the comet. We explain this EUV emission by considering the evolution of the cometary atmosphere as it interacts with the ambient solar atmosphere. Water ice in the comet rapidly sublimates as it approaches the Sun. This water vapor is then photodissociated, primarily by Ly-alpha, by the solar radiation field to create atomic Hand O. Other molecules present in the comet also evaporate and dissociate to give atomic Fe and other metals. Subsequent ionization of these atoms can be achieved by a number of means, including photoionization, electron impact, and charge exchange with coronal protons and other highly-charged species. Finally, particles from the cometary atmosphere are thermalized to the background temperature of the corona. Each step could cause emission in the AlA bandpasses. We will report here on their relative contribution to the emission seen in the AlA telescopes
Using the EUV to Weigh a Sun-Grazing Comet as it Disappears in the Solar Corona
On July 6,2011, the Atmospheric Imaging Assembly (AlA) on the Solar Dynamics Observatory (SDO) observed a comet in most of its EUY passbands. The comet disappeared while moving through the solar corona. The comet penetrated to 0.146 solar radii (\simapprox.100,000 km) above the photosphere before its EUY faded. Before then, the comet's coma and a tail were observed in absorption and emission, respectively. The material in the variable tail quickly fell behind the nucleus. An estimate of the comet's mass based on this effect, one derived from insolation, and one using the tail's EUY brightness, all yield \sim 50$ giga-grams some 10 minutes prior to the end of its visibility. These unique first observations herald a new era in the study of Sun-grazing comets close to their perihelia and of the conditions in the solar corona and solar wind. We will discuss the observations and interpretation of the comet by SDO as well as the coronagraph observations from SOHO and STEREO. A search of the SOHO comet archive for other comets that could be observed in the SDO; AlA EUY channels will be describe