Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
So long as the hijab covers the ears, it is sufficient to wipe over the hijab and there is no need to put the hands under the hijab to wipe them.
Answering your question, the Fatwa Center at Islam Q and A, states:
The well-known view in the school of Imam Ahmad is that a woman may wipe over her hijab if it goes under her chin, because it was narrated that some of the wives of the Companions (may Allah be pleased with them) did that.
Whatever the case, if there is some difficulty, either because the weather is cold or because it is difficult to take it off and put it on again, then there is nothing wrong with wiping in such cases, otherwise it is better not to do that. (Fatawa al-Taharah, p. 171)
It says in Sharh Muntaha al-Iradat (1/60): “It is also valid for a woman to wipe over a hijab that goes under the chin, because Umm Salamah used to wipe over her hijab, as was mentioned by Ibn Al-Mundhir.”
So long as the hijab covers the ears, it is sufficient to wipe over the hijab and there is no need to put the hands under the hijab to wipe them. Similarly, if a man is wearing a turban, he does not have to wipe his ear too, even if they are not covered; rather that is recommended only.
Shiekh Ibn `Uthaymeen (may Allah have mercy on him) said: “It is also Sunnah to wipe whatever part of the head is uncovered, such as the forelock, the sides of the head and the ears.” (Fatawa al-Taharah, p. 170)
Almighty Allah knows best.
Source: www.islamqa.info