Clouds From Space Shuttle Exhaust Persists In Atmosphere, Cause Noctilucent Clouds

  

Image: U.S. Naval Research Laboratory

Understanding the rapid transport of high altitude exhaust plumes near 105 km is providing new insight into the effects of winds at the bottom edge of the space weather regime towards improved forecasts of the co-located E-region of the ionosphere. This knowledge is critical for improving models of communication signal propagation and over-the-horizon-radar, explains Dr. Stevens, a Research Physicist in NRL’s Space Science Division. Current theories suggest that the plumes are rapidly transported because of narrow, high-speed wind shears. These wind shears are also linked to the occurrence of so-called Sporadic E events, thus establishing a possible link between plume transport and the lower ionosphere.

During every launch, the space shuttle injects about 350 tons of water vapor from its three main engines off the east coast of the United States between 100 and 115 km altitude. Many studies have now shown that the poleward transport of this water vapor is much faster than global-scale models predict, and a few have furthermore shown that bursts of PMCs near 83 km altitude can result. These observations are forcing researchers to reexamine their understanding of global wind patterns in the lower thermosphere.

The long-term PMC record is also likely to be modified by increased space traffic, which is important because PMCs have been implicated as indicators of upper atmospheric climate change, explains Dr. Stevens. By assembling a suite of satellite and ground-based observations following the space shuttle’s final launch, the NRL-led research team has revealed the nature of these shuttle clouds for the first time. The observations from both European and American collaborators show not only the rapid poleward transport of the plume and ensuing PMC formation, but that shuttle clouds are brighter than over 99% of all other PMCs and that the ice particles are larger at higher altitudes, which is the opposite of conventional models.

 

“Polar mesospheric clouds are the highest clouds on Earth,” says space scientist Michael Stevens at the Naval Research Laboratory, Washington, who is first author on the paper. “They shine brightly when the sun is just below the horizon and typically occur over polar regions in the summer. There is some evidence that they are increasing in number and people want to know if this is indicative of climate change or something else that we don’t understand.”

Since they shine at night, PMCs are also known as noctilucent clouds, and they can serve as an indicator not just of temperature changes, but also of how currents and waves move high in Earth’s atmosphere. A visible cloud of water vapor from something like the shuttle also offers a serendipitous way to observe such motions in the upper winds.

“The plume from the shuttle becomes a ready-made experiment to observe the movement in the atmosphere,” says Charles Jackman, a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. who is the project scientist for a NASA mission called Aeronomy Ice in the Mesosphere (AIM) that specifically observes PMCs. “What this team found is interesting since the plume moved so quickly to the pole, indicating that the winds appear much stronger at those latitudes than was thought.”

To track the plume across the sky, the scientists collated seven sets of observations, including data from AIM. The first two sets of instruments to see the plume were on a NASA spacecraft called TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics). Next the plume was viewed through the Sub-Millimeter Radiometer on the Swedish Odin satellite. When the plume reached higher latitudes, it was picked up by the ground-based Microwave Spectrometer at the Institute of Atmospheric Physics in Kühlungsborn, Germany as well as an identical ground-based water vapor instrument called cWASPAM1 at the Arctic Lidar Observatory for Middle Atmospheric Research (ALOMAR) in Andenes, Norway. The plume collated into its final shape over the arctic, as a new, extremely bright PMC on July 9, 2011 and there, it could be observed from above by the AIM satellite flying overhead, and from below by another instrument at ALOMAR called the RMR lidar.

Over the course of the plume’s travels, these observations showed it spreading horizontally over a distance of some 2000 to 2500 miles. Those parts that drifted into the high latitudes near the North Pole formed ice particles which settled into layers of PMCs down at about 55 miles above Earth’s surface. The speed with which the plume arrived at the arctic was a surprise.

“The speed of the movement in the upper atmosphere gives us new information for our models,” says Stevens. “As you get higher up in the atmosphere, we just don’t have as many measurements of wind speeds or temperatures. The take-away message here is that we need to improve the models of that region.”

Noctilucent clouds – also known as polar mesospheric clouds (PMCs) over Kühlungsborn, Germany on July 9, 2011. These clouds shine brightly even during the night. Shuttle exhaust made of water vapor formed particularly bright PMCs on July 9, 2011 over Scandanavia.

 Credit: Leibniz-Institute of Atmospheric Physics 

Since observations of PMCs may be connected to global climate, it’s important to subtract out sporadic effects such as shuttle exhaust from other consistent, long-term effects.

“One of AIM’s big goals is to find out how much of the cloud’s behavior is naturally induced versus man-made,” says Jackman. “This last shuttle launch will help researchers separate the shuttle exhaust from the rest of the observations.”

Indeed, the AIM observations showed a clear difference between typical PMCs and this shuttle-made one. Normally smaller particles exist at the top, with larger ones at the bottom. The shuttle plume PMC showed a reversed configuration, with larger particles at the top, and smaller at the bottom – offering a way to separate out such clouds in the historical record 

By allowing researchers to distinguish the shuttle PMCs from more typical clouds, these results will ultimately enable a search of the historical record to separate the anthropogenic PMCs from the natural PMCs.

The research results are reported in a paper in press that was posted on the website of the Journal of Geophysical Research or August 27, 2012.