July 2, 2009 | An experimental modeling study by a team of scientists that includes NCAR’s Hanli Liu (High Altitude Observatory) points to the propagation of waves upward from the lower atmosphere as a driver for variability in the ionosphere. The research is an important step toward better understanding space weather.

Scientists have long known that the ionosphere (uppermost atmospheric region) is strongly affected by the Sun. However, even when solar activity is at a minimum, the ionosphere still exhibits significant variability, a phenomenon that has puzzled scientists for decades.

The answer may lie in planetary waves—giant meanders in high-altitude winds that are generated near Earth’s surface and can rise into the stratosphere, where they can boost temperatures dramatically and change wind patterns. Observations have shown that these phenomena, known as sudden stratospheric warming events, impact weather as high as the ionosphere.

In January 2009, one of these events led to the stratosphere’s largest and longest-lasting temperature increase in 30 years and a major reversal of winds. It also occurred at a time when solar activity was at a lull, making it an ideal case study.

Analyzing data from the event, the team found that sudden stratospheric warming events affect many aspects of the ionosphere, including its electric field, electron density, and temperature. The magnitude of the variations, which can persist for several days, reaches 50 to 100% and is similar to that of a severe geomagnetic storm. The team presented its results at the American Geophysical Union's meeting in Toronto in May.

The model, which is made for simplified cases of stratospheric warming events, is a first step toward connecting the lower atmosphere and space weather. “Planetary waves from the lower atmosphere are very common, so it’s important to take them into account to understand the daily variability of space weather,” Liu says.

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