For meringues, place egg whites in a bowl and begin beating until very frothy but not stiff. Slowly add brown sugar, about a tablespoon at a time, followed by icing sugar. Beat until thick and glossy. Place nine spoonfuls onto a parchment-lined tray and bake for about 1 hour 15 minutes to 1 hour 30 minutes. Cool on a rack.
Remove ice cream from freezer to soften slightly. Line a 25cm by 12cm loaf tin with cling film or parchment paper. Crumble meringues roughly and mix with the ice cream. Pack this mixture into the terrine mould and smooth the surface. Cover with cling film and freeze until firm.
For caramel oranges, make a dark caramel with the sugar and water. Whilst this is happening, carefully slice peel and pith off oranges, then slice into circles and place in a bowl. Collect any juices in the bowl and squeeze any excess out of the peel.
When the caramel is ready, remove from heat and add the oranges along with their juice. Working very quickly, swirl them around in the caramel, then tip them back into the bowl and cool.
Serve slices of frozen terrine with orange in caramel sauce.