Swami Vivekananda has written that he doesn't know either.
We now come to a peculiar doctrine. I do not understand much of it
myself. If you can make something out of it, I will read it to you.
When a man dies, who has by meditation purified himself and got
knowledge, he first goes to light, then from light to day, from day to
the light half of the moon, from that to the six months when the sun
goes to the north, from that to the year, from the year to the sun,
from the sun to the moon, from the moon to the lightning, and when he
comes to the sphere of lightning, he meets a person who is not human,
and that person leads him to (the conditioned) Brahman. This is the
way of the gods. When sages and wise persons die, they go that way and
they do not return. What is meant by this month and year, and all
these things, no one understands clearly. Each one gives his own
meaning, and some say it is all nonsense. What is meant by going to
the world of the moon and of the sun, and this person who comes to
help the soul after it has reached the sphere of lightning, no one
knows. There is an idea among the Hindus that the moon is a place
where life exists, and we shall see how life has come from there.
Those that have not attained to knowledge, but have done good work in
this life, first go, when they die, through smoke, then to night, then
to the dark fifteen days, then to the six months when the sun goes to
the south, and from that they go to the region of their forefathers,
then to ether, then to the region of the moon, and there become the
food of the gods, and later, are born as gods and live there so long
as their good works will permit. And when the effect of the good work
has been finished, they come back to earth by the same route. They
first become ether, and then air, and then smoke, and then mist, then
cloud, and then fall upon the earth as raindrops; then they get into
food, which is eaten up by human beings, and finally become their
children. Those whose works have been very good take birth in good
families, and those whose works have been bad take bad births, even in
animal bodies. Animals are continually coming to and going from this
earth. That is why the earth is neither full nor empty.
The Complete Works of Swami Vivekananda, Volume 2, Practical Vedanta and other Lectures, Practical Vedanta: Part II
Sphere of Lighting or Electric Sphere is what? Who resides there?
Let us check the actual scriptural source of lightning mentioned by Swami Vivekananda. The lightning term is mentioned in Chandogya Upanishad 4.15.5.
Now [1], as for such persons, whether the cremation rites are performed or
not [2], they go to light [3]; from light to day; from the day to the bright
fortnight; from the bright fortnight to those six months during which
(the sun) rises towards the north; from the months to the year; from
the year to the sun; from the sun to the moon; from the moon to the
lightning. (From the region of Brahman) a person, who is other than
human, (comes and) causes them, existing there, to realize Brahman [4].
This is the path of the gods and the path to Brahman. Those who go by
this path do not return to this [5] human whirlpool - yea they do not
return.
Chandogya Upanisad 4.15.5
[1] The path which is to be taken by the aspirant who is devoted to
meditations and practices described above follows now.
[2] Here the idea is to praise the meditation and not to deprecate the
funeral rites. The scripture itself cannot blame or show the futility
of any of its own injunctions. This indifference to rites is only to
point out the importance of the particular topic dealt with. Here it
has been said that through the performance of rites the soul is
neither helped nor retarded in its progress to perfection (Br U
4.4.23).
[3] That is, they are absorbed in the deities of light, day, months,
etc.
[4]That is, the Brahman in Satyaloka and not the highest principle of
the Upanisads, Parabrahman or Paramatman. For, realization of It does
not require traversing a path or reaching a goal outside one's self.
(cf, Mu 3.2.8-9; Ch 6.10.1)
[5] The word 'this' indicates that they will not return in this cycle;
but may come in another cycle. It is to be noted that though the
result of meditation of those who go to the Brahmaloka gets exhausted,
they become liberated through His grace and never come back. But those
who, instead of meditation on God, perform the spiritual disciplines
like the knowledge of the five fires, Asvamedha sacrifice, or strict
Brahmacarya, to to the Brahmaloka, but come back at the end of the
cycle (Brahma Sutra 4.3.10; 4.4.22)
Commentary by Swami Swahananda
Swami Swahananda does not say anything about lightning. I checked Swami Nikhilananda and Radhakrishnan and they are also silent about lightning.
I checked Swami Gambhirananda's Bengali translation. I an giving my free translation of his Bengali translation of the Chandogya 4.15.5 line 'from the sun to the moon; from the moon to the lightning. (From the region of Brahman) a person, who is other than human, (comes and) causes them, existing there, to realize Brahman.'
My free translation of Swami Gambhirananda:
from the sun to the moon; from the moon he realizes the noble deity of
lightning (Vidyutabhimani devata). From Brahmaloka a non-human person comes to Lightning-loka
(Vidyut-loka) and causes them to realize Brahman.
According to Swami Gambhirananda, Vidyut-loka or lightning-loka is a place where resides Vidyuta deity or deity of lightning
