#ion tail

From Astronomy Picture of the Day; October 6, 2025:

The Changing Ion Tail of Comet Lemmon Images Credit & Copyright: Victor Sabet & Julien De Winter How does a comet tail change? It depends on the comet. The ion tail of Comet C/2025 A6 (Lemmon) has been changing markedly, as detailed in the featured image sequenced over five days between September 25 and October 3 (left to right) from Texas, USA. On some days, the comet's ion tail was relatively more complex than other days. Reasons for tail changes include the rate of ejection of material from the comet's nucleus, the strength and complexity of the passing solar wind, and the rotation rate of the comet. Sometimes, over the course of a week, apparent differences even result from a change of perspective from the Earth. In general, a comet's ion tail will point away from the Sun, as gas expelled is pushed out by the Sun's wind. Comet Lemmon is still inbound and brightening, passing nearest the Earth on October 21 and nearest the Sun on November 8.

The tails of Comet NEOWISE!! Comet’s usually have 2 tails that always point away from the Sun.

Here, NEOWISE’s blue ion tail on the left points directly away from the Sun and is pushed out by the flowing and charged solar wind. Structure in the ion tail comes from different rates of expelled blue-glowing ions from the comet's nucleus, as well as the always changing structure of our Sun's wind.

In this photo the very gas-rich comet shows numerous structures in its ion tail.

The difficulty with comet imaging is not only to compensate for the movement of the comet in front of the stars, but also to consider the rapid changes in the structures during the exposure time.

Photographer: GERALD RHEMANN

This Is Why Comets Glow An Eerie Green Color

“But even though things aren't looking particularly good for this comet, there's always a chance it will surprise us. Furthermore, the features that you can expect for this comet — the ion tail, the dust tail, the coma, and the nucleus — are common to practically all comets that enter our inner Solar System. When a comet gets warm enough, it creates an extended, gas-rich cloud known as a coma around its nucleus. If the coma contains carbon-nitrogen and carbon-carbon bonds, the Sun's ultraviolet light will excite the electrons inside it, causing them to emit a green glow when they drop down in energy. And whenever you see that green glow, know that there's a chance of the comet's nucleus splitting apart. It may not happen this time, or even most times, but there's a chance for a visually spectacular show. When it comes to skywatching, it's hard to ask for more.”

We typically think of comets as frozen mixes of ice and rock, but they’re so much more. There’s dust and volatile compounds present, and when the light and heat from the Sun interacts with the surface, it kicks up molecules into a gaseous, diffuse coma. This coma then gets struck by the ultraviolet light from the Sun. The dust particles get accelerated, creating the main comet tail you’re used to, but there are gas particles in the coma that simply get kicked to higher energies. If there’s enough cyanogen (CN) and diatomic carbon (C2) molecules present, they’ll create a green color due to their atomic transitions. And whenever you see that color, know that there’s the potential for the cometary nucleus to split, creating a spectacular outburst.