Polar wind

History

In 1966 Bauer and, separately, Dessler and Michel noted that because the geomagnetic field above the poles forms a long tail beyond the Moon's orbit, ions should flow from the higher pressure region in the ionosphere into space. The term "polar wind" was coined in 1968 in articles by Banks and Holzer and by Ian Axford. Axford suggested the term because the process resembles the solar wind. Early experimental characterization came from Explorer 33 in 1966 and especially the ISIS-2 satellite in 1974.

Data from the 1981 Dynamics Explorer introduced uncertainty about the role of cool O+ ions, which was resolved by the 1989 Akebono satellite and the 1996 Polar satellite. The concept originated from the terrestrial helium budget paradox: helium produced by radioactive decay of uranium and thorium escapes slower than it is produced. The realization that ionized helium could escape along open magnetic field lines near the poles offered a possible solution.

Causes

For decades, scientists worked to understand the polar wind. One key cause is now known to be ambipolar outflow, where a polarization electric field in the ionosphere accelerates thermal plasma. This type of field was first suggested in the 1920s for stars. It arises from gravitational charge separation and points opposite to gravity. Though very weak, in the sparse high-altitude plasma it becomes stronger than gravity for light ions. In the polar wind region, expanding ionospheric plasma allows gravity to pull ions downward relative to electrons. This charge separation creates the electric field, which then drives some ions upward and out of the atmosphere. This process is called ambipolar outflow. Other causes include solar photoelectrons accelerating ions along magnetic field lines. Ions on open field lines are pushed away from the Sun by the solar wind.

Measurements

Numerous investigations have been conducted, including ISIS-2, Dynamics Explorer, Akebono, and Polar satellites. The primary ingredients are electrons, H+, He+, and O+ ions. O+ dominates below 4000 km. All three ion species reach supersonic velocities above 7000 km, with velocities exceeding Mach 2 above 50,000 km. Polar wind velocity increases with altitude and is higher on the dayside. In 2022, the NASA Endurance sounding rocket launched from Svalbard directly measured the ambipolar electric field, finding a potential difference of +0.55 volt with an uncertainty of 0.09 volt between 250 km and 768 km altitude.