On January 25th, 2018, Masanori Uchina (Japan) reported a small Kreutz-group comet in images from the STEREO-A spacecraft (see animation below). Not only was this Masanori’s first STEREO comet discovery, but it also coincidentally turned out to be STEREO’s 100th find! Masanori discovered his second STEREO comet only 17 days later, on February 9th, 2018 (STEREO-105)! STEREO-100 and STEREO-105 were both only two comets of a total of twenty-two recorded Kreutz-group sungrazing comets between January and mid-February 2018. Note that eight of them were observed over a period of only ten days, these all having been found (and solely observed) by STEREO!
Cropped animation of enhanced STEREO/SECCHI HI1-A images showing STEREO-100. Notice how this fuzzy comet brightens as it exits the animation (FOV). These were the last images of the comet before it disentigrated. Image credit: NASA/NRL STEREO/SECCHI HI1-A
STEREO-100: The story behind its discovery
Masanori Uchina spotted this comet on January 25th, 2018 when hunting the near-real time STEREO/SECCHI HI1-A images for Kreutz-group comets. Indeed, between November 2017 and March 2018 the STEREO-A spacecraft was well placed in space to observe the “Kreutz-group stream”, some coin this period the “[STEREO] Kreutz season”. Note that STEREO/SECCHI data is not quite real time (unlike SOHO/LASCO), as it takes a longer time to downlink the data from the spacecraft (2-3 days in average), hence the term “near-real time”. Indeed, Masanori found this comet in images taken on January 23rd. He reported his discovery to the STEREOHunters Yahoo Group forum shortly after spotting the comet (see his report below).
Original report for STEREO-100 and my (unofficial) confirmation message posted some moments later. The official confirmation was done after the end of the “STEREO Kreutz season” in April 2018 by Karl Battams (USA NRL). Image credit: STEREOHunters Yahoo chat group forum.
Consequently, as STEREO/SECCHI data is not real time, this means that one must make sure that any possible “new” STEREO comets haven’t first been spotted in SOHO/LASCO. Luckily there were no prior mentions of any Kreutz-group comets that alligned with Masanori’s discovery, hence his find was a definite “STEREO discovery”! The comet most likely went unnoticed in SOHO/LASCO due to its apparent faintness and due to the SOHO/LASCO occulting pylon (see further below for more details).
Like all small Kreutz-group sungrazers, STEREO-100 most likely disentigrated before reaching perihelion. I say “most likely” as there are no actual images showing the comet disentigrating (even though it’s obvious in the case of such small comets). In fact, the last (and only) images of this comet show it to still be brightening! (see animation of STEREO-100 above). This is because the STEREO/SECCHI HI instruments do not observe the Sun and its immediate surroundings. In fact, the FOV focuses on the Solar Outflow beyond a 15° offset from the Sun. Kreutz-group comets in average start disentigrating at about 10° (I think…). Hence, it’s the SOHO/LASCO and STEREO/SECCHI COR telescopes that usually get to observe the disentigration. However, as mentioned, STEREO-100 was too faint be observed by SOHO, thus making it even less likely to have been observed by the COR telescopes! Indeed, COR is poorly sensitive to comets, only brighter ones (that already appear obvious in SOHO) can be detected in these (see COR2-A animation of SOHO-3492 further below).
Cropped animation of enhanced SOHO/LASCO C2 images showing the disentigration Kreutz-group comet SOHO-2465. These were the last images of the comet. Image credit: ESA/NASA/NRL SOHO/LASCO C2.
On April 16th, 2018, long after the “STEREO Kreutz season” was over, Karl Battams posted the official confirmations of the most recent STEREO comets, in which he stated that Masanori’s comet was STEREO’s 100th comet discovery! “Many congratulations to Masanori for finding STEREO’s 100th comet!“.
Backstory: STEREO’s comets
STEREO’s 100th comet discovery is the result of eleven years of intense scrutinizing of the STEREO/SECCHI images, by both professional and amateur astronomers. The first ever STEREO comet discovery was made by amateur astronomer Alan Watson (Australia) in STEREO/SECCHI HI1-B images of 2007-08-31 and 2007-09-01. The comet is unintuitively designated STEREO-65.
STEREO-65 as seen in a strongly cropped STEREO/SECCHI HI1-B image from 2007-09-01. This was one of the last (and only) images of this comet. It was most likely too faint to be recovered in SOHO/LASCO. Image credit: NASA/NRL STEREO/SECCHI HI1-B.
This small Kreutz-group comet appeared very similar to STEREO-100, and was only officially confirmed a couple years ago, when re-inspecting the HI1-B images (hence its designation). At the time of its discovery, astronomers were still getting to know and “figuring out” STEREO/SECCHI. Consequently, because of the comet’s faintness, there was not enough evidence to support its existence (especially as the processing methods used back then were less adapted to the data). The first confirmed STEREO comet find, STEREO-1, was discovered quite a bit later, on 2008-02-19 (also by Alan Watson)! STEREO-1 was followed by STEREO-2, STEREO-3 and STEREO-4 only days later! All these four comets were of the Kreutz-group, like about 95% of STEREO’s comets! The remaining 5% are non-group comets, like STEREO-61 and STEREO-88 (see images below).
Comet STEREO-61 imaged on 2014-03-02. (c) Jean-François Soulier
Animation of comet STEREO-88 in STEREO/SECCHI HI1-A images arcing around the Sun. The comet is seen in the upper-left quadrant of these images. These were the last images of this comet. Image credit: NASA/NRL STEREO/SECCHI HI1-A.
January 2018: A flurry of faint Kreutz-group sungrazing comets!
As mentioned previously, STEREO-100 was far from being the only STEREO comet discovery of 2018. In fact, a total of fourteen Kreutz-group STEREO comets were discovered between January 6th and February 9th, with eight of these (including STEREO-100) having appeared between January 13th and January 23rd! Below are animations of a couple comets that appeared during this mild “comet storm”. As one can see in these GIFs, the comets were indeed faint. It is mainly due to this reason that they went unnoticed SOHO/LASCO.
Cropped animation of enhanced STEREO/SECCHI HI1-A images showing STEREO-94. These were the last images of the comet before it disentigrated. Image credit: NASA/NRL STEREO/SECCHI HI1-A
Cropped animation of enhanced STEREO/SECCHI HI1-A images showing STEREO-96. These were the last images of the comet before it disentigrated. Image credit: NASA/NRL STEREO/SECCHI HI1-A
A couple weeks after this small flurry of Kreutz-group sungrazers, a very bright comet (about mag +2) of the same family transited the STEREO/SECCHI and SOHO/LASCO FOVs. More about this comet (SOHO-3492) can be found in a previous blog post I wrote back in February: https://skyhuntblog.wordpress.com/2018/02/19/bright-sungrazer-observed-by-stereo/
The bright Kreutz-group comet SOHO-3492 as seen in STEREO/SECCHI COR2-A images of February 9th, 2018. Image credit: NASA/NRL STEREO/SECCHI COR2-A.Note that
January 2018: Why did STEREO suddenly discover so many Kreutz-group comets?
Interestingly, despite a similar number of Kreutz comets having been observed in December 2017 and January 2018, the proportion of these found either and SOHO or STEREO vary drastically! Indeed, only 13% of the 17 Kreutz comets observed in December were discovered in STEREO images. However, this proportion increased to 75% in January!
|December 2017||January 2018|
|SOHO Kreutz-group comets||15||4|
|STEREO Kreutz-group comets||2||12|
Table displaying the number of Kreutz-group comets discovered individually by SOHO and STEREO during December 2017 and January 2018.
This sudden and drastic increase of the STEREO discovery rate has nothing to do with the location or properties of the STEREO spacecraft. In fact, it is an effect almost enterily related to SOHO! The reason starts with the geometry of the Kreutz-group comet stream as seen from SOHO during December and January. Over the course of the year, as SOHO orbits the Sun, it observes the “Kreutz stream” from different angles, meaning that the geometry of the trajectories of Kreutz comets over the course of the year changes as seen from SOHO. These changes are already obvious from one month to another. In the case of December to January, the “Kreutz stream” moves East in the sky, further away from the SOHO/LASCO C2 FOV (see the schematic representation below). This impacts the amount of comets observed by SOHO, as the SOHO/LASCO C3 FOV has a much lower resolution than C2. Indeed, practically all Kreutz-group comet discovered in SOHO/LASCO in December are found in C2! This is because most Kreutz comets are infact too faint to show up in C3, due to the telescope’s resolution. On top of all that, the Kreutz stream in January was mostly masked by the Occulting pylon (the device holding up the SOHO/LASCO Coronagraph). Unlike SOHO, the apparent geometry of the Kreutz stream as seen from STEREO practically didn’t change between December and January, hence it had a very good view of the comets that SOHO was missing out on! Especially as the STEREO/SECCHI HI1-A images can observe comets just as faint as those solely visible in C2!
Schematic representation of the trajectory of typical Kreutz-group comets as seen in SOHO/LASCO from mid-December to mid-January. The length of the curves correspond to the part of the trajectory in which the comet are usually detectable. Notice how Kreutz-group comets observed in December [generally] survive long enough to appear in the SOHO/LASCO C2 FOV. In contrast, those observed in January rarely make it into the C2 FOV. Furthermore, notice how the trajectory of the January Kreutz comets are alot more obscured by the pylon, in comparison to the December comets. Image credit: ESA/NASA SOHO/LASCO and Trygve Prestgard.
With the reasons above, this explains why comets such as STEREO-100 went undetected by SOHO/LASCO in January, while comets of similar brightness such as SOHO-3481 and SOHO-3483 were first found and best seen in SOHO/LASCO*. Note that the latter two comets were only visible in HI1-A and C2 (see animations below).
Cropped animation of enhanced STEREO/SECCHI HI1-A images showing SOHO-3478, SOHO-3481, SOHO-3483. Despite these comets being just as bright the STEREO comets of January 2018, they better appeared in SOHO/LASCO! Image credit: NASA/NRL STEREO/SECCHI HI1-A
Animation of the bright Kreutz-group sungrazing comet SOHO-3478, and its four tiny fragments (marked by the black lines, in the floowing order): SOHO-3483, SOHO-3481, SOHO-3479 and SOHO-3480. These were some the last images of these comets before their demise. Had these comets appeared only a couple weeks later, it would have been unlikely that the four fragments would have been detected by SOHO at all! They would most likely solely have appeared in STEREO, similarly to STEREO-100.
*Note that I actually first discovered SOHO-3483 in STEREO/SECCHI HI1-A images of 2018-12-20 before reporting it in SOHO/LASCO C2. This is because I was very unsure about its appearance in STEREO. It appeared better in C2 images, hence it was easier to report it using those observations. More about this comet and it’s fragments can be found in this old blog post I wrote back in December 2017: https://skyhuntblog.wordpress.com/2017/12/23/solar-observatories-observe-the-demise-of-a-bright-sungrazer-and-its-many-fragments/