Does stimulating oneself during the daytime in Ramadan, resulting in the unintentional emission of semen with less than the usual stimulation, invalidate the fast and necessitate expiation?

It appears that the questioner caused the emission of semen by her action, and thereby her fasting was invalidated. She is only required to make up the fast (qada') and not to offer expiation (kaffarah). She must also hasten to repent to Allah Almighty and not return to this disobedience.