Ash-Shaykh Bin Baaz, may Allah have mercy upon him, said:
صلاة الإشراق هي صلاة الضحى في أول وقتها، والأفضل فعلها عند ارتفاع الضحى واشتداد الرمضاء
(As for) Salaatu Ishraaq (i.e. shurooq), (then) it is salaatu duhaa in its earliest time. And it is better to pray it during the forenoon (after sun-rise) and when the heat of the morning becomes severe.
Also Shaykh Bin Baaz has a similar answer found here [Reference removed since it no longer exists]
AbdulFattaah bin Uthman