by RJN » Fri Nov 29, 2013 8:43 pm
Here are some speculative comments I have on Comet ISON's encounter with the Sun, given the latest SOHO images. Note that although I have published on comets once before, I have never published on sungrazing comets and don't really know much about how comet tails interact with the solar wind, particularly when near the Sun. Anyway, here goes:
Something has Survived
The SOHO images posted previously indeed do indicate that some part(s) of ISON survived.
ISON's Ion Tail
Why the ion tail will point somewhat away from the Sun. By eye, it seems to me, that ISON might well be outrunning the solar wind when within a solar diameter or two of the Sun's surface (a possibility mentioned above). But ISON's speed is almost completely horizontal and not out from the Sun at that time, therefore the ion tail which responds to the vertical wind should still point outward.
Since ISON is moving so fast, however, the ions expelled will move away from the place they were expelled, which will trail the comet's coma. The glowing ion trail should still appear to connect to the comet so long as it is expelling ions -- but now move outward from where ISON used to be -- but move outward from the Sun faster that the coma is moving. Therefore this ion tail will appear to "swing around" the coma as ISON recedes from the Sun. I think the newly developing ion tail being, for a while, nearly perpendicular to ISON's motion out from the Sun shows this effect in the SOHO C3 video. As ISON's motion shifts to more outward than sideways, the ion tail will keep on "swinging" until it (nearly) appears to point out from the coma in the direction away from the Sun.
ISON's Dust Tail
Why some of the dust tail will still point somewhat toward the Sun. Dust already trailing the nucleus(es) will continue to trail the nucleus(es) even after it rounded the Sun. This dust will not magically jump over the comet nucleus(es) after perihelion. That is also possibly why the orbit of the comet faintly persists (particularly in the SOHO C2 image) even hours after the comet has passed -- trailing dust is still trailing.
One caveat is that dust chunks small enough to evaporate and exposed to direct sunlight and solar wind may turn into plasma and no longer orbit as dust but as plasma.
After perihelion, however, newly emitted light dust will be pushed out from the coma by light pressure and so will not trail the coma. This dust will roughly follow ISON's orbit but be slightly further out from the Sun and move slightly faster.
Yes I am Making This Up
Again, almost all of this is speculation. I include it here partly to inform those interested and partly to invite discussion that may help to better illuminate what is actually going on with ISON and even possibly lead to timely and telling observations of this unique and brief opportunity in the life of this unusual comet. (e.g. There is no time for peer review in this case.)
- RJN
Here are some speculative comments I have on Comet ISON's encounter with the Sun, given the latest SOHO images. Note that although I have published on comets once before, I have never published on sungrazing comets and don't really know much about how comet tails interact with the solar wind, particularly when near the Sun. Anyway, here goes:
[b] Something has Survived [/b]
The SOHO images posted previously indeed do indicate that some part(s) of ISON survived.
[b] ISON's Ion Tail [/b]
[i] Why the ion tail will point somewhat away from the Sun. [/i] By eye, it seems to me, that ISON might well be outrunning the solar wind when within a solar diameter or two of the Sun's surface (a possibility mentioned above). But ISON's speed is almost completely horizontal and not out from the Sun at that time, therefore the ion tail which responds to the vertical wind should still point outward.
Since ISON is moving so fast, however, the ions expelled will move away from the place they were expelled, which will trail the comet's coma. The glowing ion trail should still appear to connect to the comet so long as it is expelling ions -- but now move outward from where ISON used to be -- but move outward from the Sun faster that the coma is moving. Therefore this ion tail will appear to "swing around" the coma as ISON recedes from the Sun. I think the newly developing ion tail being, for a while, nearly perpendicular to ISON's motion out from the Sun shows this effect in the SOHO C3 video. As ISON's motion shifts to more outward than sideways, the ion tail will keep on "swinging" until it (nearly) appears to point out from the coma in the direction away from the Sun.
[b] ISON's Dust Tail [/b]
[i] Why some of the dust tail will still point somewhat toward the Sun. [/i] Dust already trailing the nucleus(es) will continue to trail the nucleus(es) even after it rounded the Sun. This dust will not magically jump over the comet nucleus(es) after perihelion. That is also possibly why the orbit of the comet faintly persists (particularly in the SOHO C2 image) even hours after the comet has passed -- trailing dust is still trailing.
One caveat is that dust chunks small enough to evaporate and exposed to direct sunlight and solar wind may turn into plasma and no longer orbit as dust but as plasma.
After perihelion, however, newly emitted light dust will be pushed out from the coma by light pressure and so will not trail the coma. This dust will roughly follow ISON's orbit but be slightly further out from the Sun and move slightly faster.
[b] Yes I am Making This Up [/b]
[i] Again, almost all of this is speculation. [/i] I include it here partly to inform those interested and partly to invite discussion that may help to better illuminate what is actually going on with ISON and even possibly lead to timely and telling observations of this unique and brief opportunity in the life of this unusual comet. (e.g. There is no time for peer review in this case.)
- RJN