by Chris Peterson » Sat Feb 17, 2024 4:51 am
johnnydeep wrote: ↑Fri Feb 16, 2024 6:34 pm
JimB wrote: ↑Fri Feb 16, 2024 9:17 am
I wonder why the tail of the comet is streaming away from the sun, "buffeted by the solar wind" as in the description, but there seems to be a separate halo around the head of the comet which is more or less smoothly distributed and unaffected by the solar wind?
Probably due to a difference in the make up of the material in the coma (more gas than dust?) versus the tail (more dust than gas?), though the tail is definitely just former coma material. Hard to say exactly from the description of comet structure in Wikipedia though I'm sure others more knowledgeable can clarify. (Hmm, my Chris Peterson sense is remembering that the dust is affected more by the charged particles in the solar wind and photon pressure, whereas the ionic gas is directed by the magnetic field lines in the Sun's magnetosphere. Or something like that.)
https://en.wikipedia.org/wiki/Comet_tail#Tail_formation wrote:Tail formation
In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations,[1][2] but these detections have been questioned,[3][4] and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.
The streams of dust and gas each form their own distinct tails, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.[5]
Both the coma and the ion tail are primarily gas (although as the comet gets closer to the Sun, the amount of dust in the coma increases). But the two gas regions behave very differently in response to the solar wind (which is made up of charged particles), because the coma is composed of neutral gas, while the ion trail is (you guessed it) ionized. The solar wind is sweeping the gas that gets ionized out into the tail. Leaving neutral gas in place. The dust tail is driven primarily by radiation pressure.
[quote=johnnydeep post_id=337163 time=1708108446 user_id=132061]
[quote=JimB post_id=337154 time=1708075078 user_id=147260]
I wonder why the tail of the comet is streaming away from the sun, "buffeted by the solar wind" as in the description, but there seems to be a separate halo around the head of the comet which is more or less smoothly distributed and unaffected by the solar wind?
[/quote]
Probably due to a difference in the make up of the material in the coma (more gas than dust?) versus the tail (more dust than gas?), though the tail is definitely just former coma material. Hard to say exactly from the description of comet structure in Wikipedia though I'm sure others more knowledgeable can clarify. (Hmm, my Chris Peterson sense is remembering that the dust is affected more by the charged particles in the solar wind and photon pressure, whereas the ionic gas is directed by the magnetic field lines in the Sun's magnetosphere. Or something like that.)
[quote=https://en.wikipedia.org/wiki/Comet_tail#Tail_formation][size=150]Tail formation[/size]
In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations,[1][2] but these detections have been questioned,[3][4] and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.
The streams of dust and gas each form their own distinct tails, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.[5][/quote]
[/quote]
Both the coma and the ion tail are primarily gas (although as the comet gets closer to the Sun, the amount of dust in the coma increases). But the two gas regions behave very differently in response to the solar wind (which is made up of charged particles), because the coma is composed of neutral gas, while the ion trail is (you guessed it) ionized. The solar wind is sweeping the gas that gets ionized out into the tail. Leaving neutral gas in place. The dust tail is driven primarily by radiation pressure.