Musar do Wyjścia 1:6

Since Joseph was elevated to the position of king and rode in the מרכבת המשנה, this is a reference to the שר מצרים, the celestial representative of Egypt, who is the most highly placed of all the celestial representatives of the seventy gentile nations. Our sages in Ketuvot 112 have said that Egypt is second only to ארץ ישראל, hence "the second best chariot" is the vehicle suited for the angel representing Egypt in the Heavens. Even though Joseph rose greatly after having been elevated from the status of a slave to that of a temporal king, he did not rise sufficiently for his מרכבה to equal the level of that of his father until the Holy Spirit returned to Jacob. This is alluded to in אלה תולדות יעקב יוסף, that when Jacob was re-united with Joseph, the cherubs representing the male and female (in all their innocence) (cf. Kings I 7,36) were able to unite through the covenant of circumcision in the emanation of יסוד. Although Jacob and family resided in Egypt, they did not feel themselves in exile as long as Joseph ruled. Once a new King arose, as per Exodus 1,8, who reportedly "did not know Joseph," the situation changed. The mystical dimension of Jacob as "one" and of Joseph as "one," means that all twelve tribes are inextricably linked to both Jacob and Joseph. The letter א contains the secret of this unity.