The Kristian Birkeland Medal 2019

Doctor Bruce Tadashi Tsurutani

Dr. Bruce Tsurutani, is principal scientist at Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. Dr. Tsurutani is renowned in multiple research fields including space weather, nonlinear plasma waves, plasma instabilities and wave-particle interactions, auroral particle precipitation, dayside ionospheric physics, solar wind interactions with magnetosphere and ionosphere, geomagnetism. Among them, space weather has provoked his lifelong attention and dedication. He published around 700 papers peer reviewed journal.

In his Ph.D. thesis, the study results suggested that the main triggering mechanism of magnetospheric substorms is the southward turning of interplanetary magnetic field (IMF). During 1974-79, Dr. Tsurutani found the post-midnight relativistic during substorms and evidence of power line stimulation by chorus waves.In 1984 he revealed that although the IMF southward turning may trigger geomagnetic activity substorms, the substorms can be further intensified by heliospheric current sheet crossing the magnetosphere.In 1987 Dr. Tsurutani found that High Intensity Long Duration (T > 2 days), and continuous aurora1 activity (HILDCAA) events are caused by outward (from the sun) propagating interplanetary Alfvén wave trains.

After studying major magnetic storms during the maximum of the 20th solar cycle, Dr. Tsurutani found that quantitative predictions of storm intensities based on solar observations appear to be very difficult but that the extreme values of the southward interplanetary magnetic fields, rather than solar wind speeds, are the primary causes of great magnetic storms. He soon found out that a possible mechanism for generating the intense interplanetary Bs, which is responsible for the subsequent intense magnetic storms, is the shock compression of preexisting southwardly- directed IMF Bz (Bs).

After studying a historical super storm that occurred in September 1859, he noted that the most intense magnetic storms are indeed related to intense solar flares, and that the two phenomena have a common cause: magnetic reconnection at the Sun. Dr. Tsurutani found that during an interplanetary/solar event, extremely complex magnetic storms can occur in the post-solar maximum phase as well. He discovered the “superfountain effect” of the dayside ionospheric total electron content (TEC), a consequence of the “prompt penetration” of interplanetary/polar cap electric fields to the equatorial and near-equatorial ionosphere.

He also explored the ionospheric effects of prompt penetration electric fields for a variety of interplanetary magnetic field directions. He categorized the positive- phase ionospheric storms that occur in the dayside (i.e., the superfountain), and negative-phase ionospheric storms that occur on the nightside (with a TEC reduction). Later Dr. Tsurutani reported a different mechanism that also causes the dayside TEC enhancement; extreme EUV solar flares can result in extreme ionospheric effects, including a sudden, intense, and long-lasting ionospheric TEC enhancement in the dayside ionosphere, and may affect radio navigation and communications.