article: As the sun sets on the Saharan desert, its colours cast a glow on its vast sand dunes causing rainbow light to dance across the landscape. And Nasa has captured how this stunning show appears from space using its Landsat 7 satellite. It shows 'ribbons' of sand and patterned stripes known as linear dunes in the Erg Chech, a desolate sand sea in southwestern Algeria - but the exact cause of these longitudinal structures is not known. Linear dunes, like these captured by Nasa's Landsat 7 satellite, are straight ridges of sand that have been known to measure as long as 99 miles (160km). This image shows 'ribbons' of sand and patterned stripes in an area known as the Erg Chech, a desolate sand sea in southwestern Algeria - but the exact cause of these longitudinal structures is not known . The image was spotted by io9 and the colours were caused as rays of sunlight hit the sand and were reflected off. Linear dunes are straight ridges of sand that have been known to measure as long as 99 miles (160km). Ridges are categorised as linear when they are significantly longer than they are wide, and they occur on isolated ridges of dunes as well as, more commonly, as sets of parallel ridges separated by miles of sand and gravel. Linear dunes are straight ridges of sand that have been known to measure as long as 99 miles (160km). Ridges are categorised as linear when they are significantly longer than they are wide, and they occur on isolated ridges of dunes as well as, more commonly, as sets of parallel ridges separated by miles of sand, gravel. The dunes have also been known to merge and form so-called 'Y-shaped compound dunes', and the formation of these patterns are caused by the wind and movement of sand. But exactly how this wind movement forms the structures is disputed. The dunes have also been known to merge and form so-called 'Y-shaped compound dunes', and the formation of these patterns are caused by the wind and movement of sand. But exactly how this wind movement forms the structures is disputed. One theory, suggested by Steven Hanna in 1969 in his Formation of Longitudinal Sand Dunes by Large Helical Eddies in the Atmosphere paper, is that linear dunes are created by helical roll vortices. These are long rolls of air that spiral, or rotate one way before rotating another. They are commonly seen in clouds, creating what's known as 'cloud streets' that appear as cumulus-type clouds. As the wind moves and switches direction it causes the sand to move accordingly and create grooves and ridges in the sand. 'The dunes are aligned in the direction of the prevailing wind and are spaced 1.2 miles (2km apart)', explained Professor Hanna. 'Observations in the atmosphere and in the laboratory indicate that dominant forms of motion in the boundary layer of the atmosphere are counter-rotating helical roll vortices aligned along the wind. 'The necessary conditions for the formation of these roll vortices are fulfilled over large deserts and their spacings agree with the observed spacings of the dunes.' One theory, suggested by Steven Hanna in 1969, is that the linear dunes are created by helical roll vortices (illustrated left). These are long rolls of air that spiral, or rotate one way before rotating another. They are commonly seen in clouds, creating what's known as 'cloud streets' that appear as cumulus-type clouds align to be parallel to the wind (illustrated right) In his book, Geomorphology of Desert Dunes, Nicholas Lancaster from the Desert Research Institute argues that this theory has its flaws. He said Professor Hanna compared data on the spacing of linear dunes with the expected dimensions of helical roll vortices in subtropical desert areas, concluding their 'probable wavelength was 1.9 miles to 3.7 miles (2km to 6km).' But measured dimensions of roll vortices indicate their wavelength ranges between less than 50 metres and 31 miles (15 km). 'The observed scale of helical roll vortices is in many cases much larger than average dune spacing, suggesting that linear dunes are not the product of such atmospheric motions,' explained Mr Lancaster. 'Evidence for the existence of helical roll vortices in linear dune landscapes is slight. The theory is attractive but there is little empirical data to support it in dune area.' Ridges are categorised as linear when they are significantly longer than they are wide, and they occur on isolated ridges of dunes as well as sets of parallel ridges separated by miles of sand and gravel. Linear dunes in the Namib Naukluft National Park, Namibia are shown. The dunes have also been known to merge and form so-called 'Y-shaped compound dunes', and patterns are caused by the wind and movement of sand . Another theory believes Barchan evolution may play a role. 'Barchans' are crescent-shaped sand dunes (top left) that form in regions where there is almost unidirectional wind. If windblown sand comes from one direction, a dune will be a crescent-shaped barchan but if winds switch direction seasonally they become linear (pictured left centre and in Morocco right) While in the 'Formation of the complex linear dunes in the central Taklimakan sand sea, China' study, Chinese researcher Xunming Wang and his colleagues said that in the case of 'complex' linear dunes - that are a combination of two shapes - the theory of Barchan evolution may play a role. 'Barchans' are crescent-shaped sand dunes that form in regions where there is almost unidirectional wind. The shapes depend on the wind’s direction, and if windblown sand comes from one direction, for example, a dune will be a crescent-shaped barchan but if winds switch direction seasonally - coming from the southwest for half the year and from the southeast for the other half, for instance - this dune will be linear. If wind direction is more varied, star-shaped dunes appear. The Erg Chech features both compound and complex linear, as well as star dunes. Erg' comes from the Arabic word for 'field of sand dunes' Mr Wang said: 'Complex linear dune formation is shown to have five stages. 'Analysis clearly shows that under the control of wind regime, sand supply and other factors, the simple linear dunes move sideways while they evolve. ' The evolutionary processes of complex linear dunes in the region partly support the theory of ‘barchan evolution’ but do not support the ‘roll-vortex’ and ‘bimodal wind regime’ hypotheses.' The researchers continued that after the complex linear dunes develop,  local wind patterns and the amount of sand suggest it is possible for them to maintain their linear shape, rather than evolving into star shapes for example, even as wind direction changes. The Erg Chech features both compound and complex linear, as well as star dunes.
highlights: The image of the patterned stripes in the Saharan desert was captured by the Nasa's Landsat 7 satellite .
  They are examples of linear, or longitudinal, dunes in Erg Chech - a desolate sand sea in southwestern Algeria .
  'Erg' comes from the Arabic word for 'field of sand dunes' and the cause of such shapes is not known .
  Theories include helical roll vortices - rolls of rotating and spiralling air - and seasonal changes in wind direction .
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