such particle is used with impersonal verbs in Esperanto:

Pluvas, it rains, it is raining.
Negxis hieraux, it snowed yesterday.


VERBS PRECEDING THEIR SUBJECTS.

51. When the verb in a sentence precedes its subject, English often uses
an introductory particle, such as "there," "it." In Esperanto no such
particles are needed:

Estas floroj sur la tablo, there are flowers on the table.
Estis Johano, ne Alfredo, en la gxardeno,
it was John, not Alfred, in the garden.
Estas domo en la kampo, there is a house in the field.
Estis mi, it was I.


COORDINATING CONJUNCTIONS.

52. Words like "aux", "kaj", "nek", "sed", which join words,
word-groups, or sentences together are conjunctions. All the
conjunctions given so far connect words, phrases, or sentences of
similar rank or kind. These are called coordinating conjunctions,
and the words, phrases, or sentences connected by them are said
to be coordinate:

Cxu vi marsxas aux kuras?
("Aux" connects the verbs.)