How Do Death Eaters Send Messages Via the Dark Mark?

Question

In Deathly Hallows, Alecto Carrow sends a message to Snape -- Dark Mark to Dark Mark -- letting Snape know she's apprehended Harry Potter.

‘I was under the impression,’ said Snape, ‘that Alecto had apprehended an intruder.’

‘Really?’ said Professor McGonagall. ‘And what gave you that impression?’

Snape made a slight flexing movement of his left arm, where the Dark Mark was branded into his skin.

‘Oh, but naturally,’ said Professor McGonagall. ‘You Death Eaters have your own private means of communication, I forgot.’

_{Deathly Hallows - pages 480-481 - Bloomsbury - chapter 30, The Sacking of Severus Snape}

In previous books, the Dark Mark was used to summon the Death Eaters. Unless I'm remembering incorrectly, it isn't until Deathly Hallows that the Dark Mark is shown to be a means for direct communication between one Death Eater and another. It's clear that it could be used as a means of communication between a Death Eater and Voldemort, but I don't recall another instance of Death Eater to Death Eater communication besides the one between Snape and Alecto Carrow.

Now I'm picturing walkie-talkies. Magically, how do Death Eaters send each other messages using the Dark Mark? Would it be along the same lines as a Patronus, for example? (Note I say "along the same lines as a Patronus" -- I realize that Death Eaters, aside from Snape, cannot produce Patronuses) What kind of magic might be at the base of this?

Answer 1

There are 3 possibilities here - I'm not sure canon supports one over another though I have my pick:

1. It was NOT a direct communication. Alecto summoned Voldemort; and Voldemort contacted Snape as his top man on the spot in turn, to temporarily supervise over the situation (Carrows being not the sharpest knives in the drawer of DEs).

   To be honest, this sounds a lot more plausible to me than DE to DE radio - I seriously doubt V. would care to enable person-to-person communications between underlings. Wrong personality type.

2. Dark Marks are more of a broadcast than point-to-point. In other words, when Alecto summoned Voldemort, either ALL Dark Mark wearers get the signal, or at least nearby ones do (think having a powerful wireless phone and a baby monitor - the monitor will pick up the radio waves from nearby wireless phones sometimes). So Snape "overheard" the call to Voldemort.

   This possibility is feasible since the Voldemort call to DEs seems to be a broadcast in GoF - he summoned them ALL at once.

3. As NominSim said in a comment, they may have ALSO functioned similar to Protean -implemented DA coins. I can't find any unambiguous canon support for this possibility in canon though, given 2 possible explanations above that personally make more sense to me given Voldemort's command/personality style. (and furthermore, the timeline seems wrong for that as per NominSim's comment below)

Answer 2

I always assumed that Voldemort told the DEs to press their Dark Marks if they apprehended Harry and for nothing else, and so when Snape felt the Mark burn, he knew that was the only answer.

Answer 3

If the death eaters were summoned, they had to know certain information, like where to go. And it seemed to be a broadcast, not a point to point communication. If you are able to broadcast the information "come to X", then probably the information "mission completed, got HP" can also be broadcasted. It seems it worked both ways. Probably not on a conscios level.

Another and more probable explanation, that the Carrows knew they had to catch HP (Snape had given them the task, as McGonogall states when she allows the brother into the Ravenclaw tower), and they were allowed to disturb Lord Voldemort, if they completed the mission, so death eaters could only send their IDs and/or the location infos, and from it Voldemort knew, what happened, as he had given the tasks. The others just felt that xy in z summoned the dark lord, and moved only if xy=Voldemort, but then immediately. (Probably there was just two levels: xy = Voldemort and xy not= Voldemort.)

Answer 4

I think there is a canon based answer for this.

"Not after the Dark Lord gets hold of her! She's gone and sent for him, I felt me Mark burn, and he thinks we've got Potter!"
"'Got Potter'?" said Professor McGonagall sharply, "What do you mean, 'got Potter'?"
"He told us Potter might try and get inside Ravenclaw Tower, and to send for him if we caught him!"
"Why would Harry Potter try to get inside Ravenclaw Tower! Potter belongs in my House!"
Beneath the disbelief and anger, Harry heard a little strain of pride in her voice and affection for Minerva McGonagall gushed up inside him.
"We was told he might come in here!" said Carrow. "I dunno why, do I?"

So Voldemort told the Carrows about Harry entering the Ravenclaw Tower. He must have also informed Snape about this as he was the Headmaster. Flitwick allowed Alecto to enter the common room on his urgent request.

"Where are the Carrows?" he asked quietly.
"Wherever you told them to be, I expect, Severus," said Professor McGonagall.

This suggests that Snape knew why Alecto was inside Ravenclaw Common Room.

So there was no direct message sent by Alecto to Snape but rather Snape figured out that Alecto had touched her Dark Mark because she was stationed there on Voldemort's orders to nab the intruder, Potter.

So there is no direct message sending.