Vishnu is the soul of Surya deva. This is mentioned throughout vedas . Which is one of the reasons why Surya is referred to as Surya -Narayana.
The wheel of Surya bhagawan moves by the strength of Vishnu
- ādityarathavegena viṣṇorbāhubalena ca |(Ashwalayana Samhita 1/191/18)
- viṣṇave tvā devastvā savitā madhvānaktvoṣadhe |(Kāthaka Samhitā 1/3/2)
Shatapatha brahmana mentions about being within the sun is the inner controller of all.
- य आदित्ये तिष्ठन्। आदित्यादन्तरो यमादित्यो न वेद यस्यादित्यः शरीरं य आदित्यमन्तरो यमयति स त आत्मान्तर्याम्यमृतः
- Meaning: He who is dwelling in the SUN, is within the SUN, whom Sun doesn’t know, whose body is the SUN, Who internally rules the Sun.
Maitrayani samhita directly attributes devata of Surya mandala to Vishnu
- हिरण्मयेन पात्रेण सत्यस्यापिहितं मुखम् तत्त्वं पूषन्नपावृणु सत्यधर्माय विष्णवे योऽसा आदित्ये पुरुषः सोऽसा अहम्
- ❝The mouth of the true (Brahman) is covered with a golden lid; open that, O Pushan (sun), that we may go to the true one, Viṣṇu. He who is the person in the sun.❞~Maitrāyaṇī Upaniṣad/samhita 6.35
- यश्चैषोऽग्नौ यश्चायं हृदये यश्चासावादित्ये स एष एकः ।
- “He who is here in the fire, he who is here in the heart, and he who is yonder in the Sun—he indeed is One.”
Sayana refers to Vishnu in Tad vishno parama padam as the spiritual sun that dwells within the Surya Mandala .
- तद् विष्णोः परमं पदम् ...
- दिवीव चक्षुराततम्
- “Viṣṇu's supreme station … like an eye extended in heaven.”
- AV 7.27.7 commentary
- सर्वेषां चक्षुःस्थानीयं सूर्यमण्डलम्
- “the solar orb, which occupies the place of the eye of all.”
Narayana in rig vidhanam is referred to as the solar orb which dwells in savitar mandala .
- ध्यायेत् सदा सवितृमण्डलमध्यवर्ती
- नारायणः सरसिजासनसन्निविष्टः ।
- “One should always meditate upon Nārāyaṇa, seated upon a lotus, who is situated in the midst of Savitṛ's solar orb.”
Maitrayani samhita also mentions that Sattva is the inner self of the sun and achyuta is the inner self of Sattva
- रविमध्ये स्थितः सोमः सोममध्ये हुताशनः ।
- तेजोमध्ये स्थितं सत्त्वं सत्त्वमध्ये स्थितोऽच्युतः ॥
- “Soma is situated in the midst of Ravi; Agni is situated in the midst of Soma; Sattva is situated in the midst of Tejas; and Acyuta is situated in the midst of Sattva.”
- शुद्धः सत्त्वान्तरस्थमचलममृतमच्युतं ध्रुवं विष्णुसंज्ञितम् ।
- “He sees the pure, immovable, immortal, Acyuta, steadfast, called Viṣṇu, abiding within [the] Sattva…”
Vishnu is the indweller of Surya .