19
$\begingroup$

In the Wikipedia article about lightining, the following explanation is given about the electrification process in clouds:

The details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm electrification. The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward rapidly (updraft) and temperatures range from −15 to −25 °C (5 to −13 °F); see Figure 1. In that area, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets (small water droplets below freezing), small ice crystals, and graupel (soft hail). The updraft carries the super-cooled cloud droplets and very small ice crystals upward. At the same time, the graupel, which is considerably larger and denser, tends to fall or be suspended in the rising air.

The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged; see Figure 2. The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm.

This mechanism seems to rule out the possibility of lightning in absence of a condensed phase (liquid droplets/solid crystals). That makes me wonder, is lightning truly impossible in stars hot enough to be fully gaseous, like our Sun? Or could a alternative mechanism generate lightning in such environment.

Even in absence of any alternative mechanisms, I think lightning still could form at least in small, cold stars, or in old brown dwarfs. What would be the size/age threshold?

$\endgroup$
5
38
$\begingroup$

Stars are not actually gaseous, they are plasmas, i.e., highly ionized. Hence, the entire star is highly conductive and does not easily develop the voltage difference via friction needed for lightning like that in our atmosphere. On the other hand, stellar atmospheres have strong magnetic fields which reconnect, and that causes strong voltage spikes that generate high energy electrons. On the Earth we would call streams of high energy electrons lightning, but on the sun we call them flares.

$\endgroup$
9
$\begingroup$

An essential feature of the lightning is the electrical breakdown - an insulator (air) becomes a conductor for a while, relatively high current flows in the conducting channel for a short while, then stops. The conducting channel is an insulator again.

This requires

  1. insulating (dielectric) medium and
  2. a means of creating an intense electric fields.

Given enough temperature and/or pressure, everything becomes a conductor. So you need a cool enough star (way cooler than our Sun is). The sun is electrically conductive from its Corona all the way to the center.

Point 2 is easier as the Sun (and with somewhat lesser certainity all other stars that rotate and have convective zone) run some kind of hydromagnetic dynamo . These processes do create spectacular events (magnetic reconnections), but they are not lightnings in the true sense of the word as they happen in an already conducting medium.

The cooler brown dwarfs, as well as gas giant planets, do have layers of atmosphere that are sufficiently dielectric. Well, no hydromagnetic dynamo in these conditions (it may as well run deep below in the hotter and denser layers, but no ligthnings because of the reasons stated above). Some of upper layers are cool enough to host water and other clouds, giving them the right conditions for separating spatially a great deal of electric charges, just like our thunderstorms do.

Both Jupiter and Saturn are known to host powerful thunderstorms, detectable both in radio waves and in visible light. I see no reason why the same processes can't happen in a brown dwarf, but I am not aware of any scientific research on the matter.

$\endgroup$
2
  • $\begingroup$ Another possible location that comes to mind are the envelopes of red giants. Probably too thin (near vacuum), and I'm not sure if they are cold enough to avoid full ionization, but they are surely hot enough to preclude water droplets. Yet, I imagine high melting point oxides and carbides could possibly form dust grains and get charged. $\endgroup$
    – ksousa
    Oct 26 '20 at 15:18
  • 1
    $\begingroup$ Red giants pose another set of problems. First, the envelope pressure is quite low. A recent question about the lightnings on Mars concluded that the density is not enough. Low pressure enables glow discharge instead of sparks. And the only solid matter near a red giant I can imagine is the soot around the carbon stars. Itself a conductor (and I am not sure if for thunderstorm-type electrification the solid phase should also be a dielectric). High melting point carbides/oxides require even heavier elements that don't surface until the final show. $\endgroup$
    – fraxinus
    Oct 26 '20 at 19:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.