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(Upper and lower) tangent arcs are called circumscribed halos too, depending on solar altitude. They surround / are tangent to 22 degree halos. With different solar altitude, they take different shapes.

Upper Tangent arc Circumscribed halos Lower Tangent Arc
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All images by Jerry Xiaojin Zhu unless noted otherwise. Feel free to use them for your personal enjoyment. For other usage please contact the author at jerryzhu@gmail.com