First Simultaneous Rocket and Radar Detections of Rare Low Summer Mesospheric Clouds

On 30 June 2016 a layer of dust, possibly meteoric smoke particles (MSPs), was observed with a rocket borne probe at 69.29 degrees N, 16.02 degrees E and altitudes of similar to 74km where patchy thin cloud layers, detected with the Middle Atmosphere Alomar Radar System, were present. The rocket traversed a layer with a net positive dust charge density of similar to 10(7) unit charges per cubic meters and a number density of neutral dust particles with sizes 4nm of similar to 10(8)m(-3). The positive charge density may require that elements that lower the photoelectric work function coat MSPs. The presence of this relatively large dust is consistent with smaller MSPs being swept out of the low mesospheric cloud region during the summer, while larger MSPs remain where their fall velocities equals the circulation updraught velocities. Large MSPs initially embedded in icy particles that subsequently sublimate may also fall until their fall velocities match the updraught velocities. Plain Language Summary A rocket and radar campaign was conducted in the summer of 2016 to investigate the clouds in the Earth's polar middle atmosphere and the role of meteoric smoke particles. They are produced by meteorites entering the atmosphere at high velocities, where they are heated by friction and ablate. We lack knowledge of the cloud transition phases from winter to summer conditions in late May and back in late August. Recent radar observations show that contrary to the belief a few years back, weak and low clouds are not totally absent in the summer season. One of the rockets flew through a very weak and low cloud, which also was observed by radar. The probability for this to happen is very low. Analysis shows that the cloud consists of 4- to 5-nm-sized meteoric smoke particles of number density a few times 10(8) particles m(-3) with a low positive dust charge density of similar to 10(7)m(-3). Our findings are consistent with size sorting being active and important in the low cloud region especially during the transition phases. The positive charge density apparently requires that the photoelectric properties of the smoke particles are affected by coating with or absorption of gases.