I used 燠 to represent embers or live charcoal, as I feel like watching the embers of a fire flicker makes people thoughtful, and perhaps be reminded of the ephemerality of life. 虚 was a better kanji to use than 空 in this context, as I felt like 虚 gives a more empty feeling of a vast, empty sky, much like outer space. When combined together, I feel like someone could imagine the soft flickering of burning embers amongst a void of nothing, which gives both a peaceful and existential feeling. I thought it might paint a picture of transience. What do you think?

Oh, you write such a deep meaning in your Haiku !
Haiku sometimes uses special words to express the feeling of the writer.
Actually I don't know much about Haiku, and could not understand it....but after reading your explanation, I think it's good for you to select the kanji.

I'm glad you think so, thank you!