Garuda purANa states that a kind of asuras called mandeha , who originally reside in an island called aruNa are trying to destroy Surya in the day at daybreak and sunset. The arghyam given at these times serves as a weapon to destroy them. These Mandehas are just the unwanted thoughts in our mind or the mind as a whole that is covering the shining Atma.

(Arghya Mantras are there for many devatas)
Like for Surya:

Bhano bhaskara martanda chandarashmi divakara
Arogya mayurvijayam sriyam putrancha dehime
Suryaya namaha idam arghyam samarpayami…

“A Brahmacharin shall bathe only once a day. Having performed the rite of Achamanam, and invoked the sacred pools therein, one shall take a bath in the river. Thirty million is the number of the malignant spirits, called Mandehas, who manifest a desire of devouring the sun at day break. He, who does not attend to his Sandhya rite at the meetings of the day and night, verily kills the sun, inasmuch as the libations of consecrated water (offered unto the sun-god in the course of a SandhyA) tend to consume these monsters (Mandehas) like streams of liquid fire.The Unions or meetings (Sandhya) of the day and the night, which are called “SandhyAs”, last for the period of two NAdikas till the sun or the stars appear in the sky.”