Helmut
As easy one. They are called LIDS (lift improvement devices) and are fitted to all Harrier II aircraft if the guns are not being carried. They give about 1400 lb more VTO lift capability by trapping the so called 'fountain' of exhaust air that bounces back off the ground and goes up towards the fuselage. If in your pic the gear had been down you would have also seen the cross dam that comes down in front of the stakes making a three sided box to contain the fountain. The LIDS work well to about 8ft wheel height by which time you have upwards momentum and can corry on with the lift and into the transition. The other postive effect once you have started to go up from the ground is that the temp of the air finding its way into the intakes reduces with corresponding increase in thrust (some 100lb per deg C).