Is the bijakshara Aim - (ऐं) related to indra?

In spiritual realm bijaksharas have a vital place. Many mantras carry bIjaksharas, apart from the deity's name.

For example: chamunda mantra

“Om Aim Hreem Kleem Chamundaye Viche”

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In general aim (ऐं)  bIjakshara is related to Saraswati.

In Rig Veda, Saraswati Suktam appears, wherein Saraswati was eulogised as eliminator of majic spells and demons, as Indra did.

Rig Veda 6.61.5

यस्त्वा देवि सरस्वत्युपब्रूते धने हिते । इन्द्रं न वृत्रतूर्ये ॥
yas tvā devi sarasvaty upabrūte dhane hite | indraṃ na vṛtratūrye ||

English translation:

Whoever makes appeal to you, goddess Sarasvatī, when the stake is set,

as if to Indra at the overcoming of Vr̥tra,

Rig Veda 6.61.6

त्वं देवि सरस्वत्यवा वाजेषु वाजिनि । रदा पूषेव नः सनिम् ॥
tvaṃ devi sarasvaty avā vājeṣu vājini | radā pūṣeva naḥ sanim ||

English translation:

Aid (him), goddess Sarasvatī, at the prize (contests), you prizewinner.

Channel gain to us, like Pūṣan.

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In Rig Veda Indra was eulogized as the eliminator of many demons like Vritra.

विश्वमित्सवनं सुतमिन्द्रो मदाय गच्छति । वृत्रहा सोमपीतये ॥
viśvam it savanaṃ sutam indro madāya gacchati | vṛtrahā somapītaye ||

English translation:

“Indra, the destroyer of Vritra, repairs assuredly to every ceremony where the libation is poured out, to drink the Soma juice for (his) exhilaration.”

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aindra indicates that related to indra. 

Hence, the bIja Aim (ऐं) relates Indra only.

An Agamic mantra (or Tantric mantra) dedicated to Lord Indra (Mantradeva Prakashika).

Im Indraya namaha

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Hence, the bija aim (ऐं)  relates to both Indra and Saraswati.

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Shri Aurobino says, in his “The secret of the Veda” as follows:

 

It is true that in the Vedic writings Saraswati is spoken of as the secret self of Indra,—an expression, we may observe, that is void of sense if Saraswati is only a northern river and Indra the god of the sky, but has a very profound and striking significance if Indra be the illumined Mind and Saraswati the inspiration that proceeds from the hidden plane of the supramental Truth.

 it has been said of Saraswati that she is the impeller of happy truths, codayitrī sūnṛtānāṃ and an awakener of the consciousness to right thinkings or right states of mind, cetantī sumatīnām

 Rig Veda 1.3.11

 

चोदयित्री सूनृतानां चेतन्ती सुमतीनाम् ।

यज्ञं दधे सरस्वती ॥
codayitrī sūnṛtānāṃ cetantī sumatīnām |

yajñaṃ dadhe sarasvatī ||

English translation:

The impeller of happy truths, an awakener of the consciousness to right thinkings,

Sarasvatī has received our sacrifice.