An observational study of the response of the upper atmosphere of Mars to lower atmospheric dust storms

The effects of dust storms on densities, temperatures, and winds in the lower atmosphere of Mars are substantial. Here we use upper atmospheric observations to investigate how dust storms affect the upper atmosphere of Mars. We use aerobraking accelerometer, ultraviolet stellar occultation, and radio occultation datasets to examine the magnitudes and timescales associated with upper atmospheric density changes during dust storms. We find that: (1) Upper atmospheric conditions can be perturbed by dust storms outside the classical “dust storm season” of Ls = 180-360°. (2) The upper atmospheric regions affected by even a small dust event can include nearly all latitudes. (3) Atmospheric temperatures can be affected by dust storms at altitudes as high as 160 km. (4) The onset of the upper atmospheric response to a distant dust event can be a few days or less. (5) The characteristic timescale for the decay of the upper atmospheric response to a dust event can be 20-120° of Ls, and it may differ from the corresponding timescale for the lower atmosphere. (6) Average upper atmospheric densities can change by factors of a few during mere regional dust storms and an order of magnitude change is possible for the largest storms: these are general trends and individual density measurements may be greater than suggested by a general trend by a factor of two due to the intrinsic variability of the upper atmosphere. The decay timescale and magnitude of the upper atmospheric response depend on altitude, and larger events have shorter decay timescales. The substantial effects seen in the upper atmosphere illuminate the vertical extent of modified atmospheric circulation patterns and associated adiabatic heating/cooling during extreme dust loading, timescales for the onset and decay of the upper atmospheric response, and highlight potential dangers to spacecraft operations.