Ripples in the pond of magnetic field reconnection

Most of the visible matter in the Universe is made up of charged particles or plasmas that can develop magnetic field reconnection (MR) at points where the direction of the magnetic field shows abrupt changes. Through MR, the energy of the magnetic field can be effectively transferred into the kinetic and thermal energies of plasmas, causing many explosive plasma phenomena that occur on the Sun, planetary magnetospheres and pulsars and even black holes.

The interface or magnetopause between the solar wind and the Earth’s magnetosphere (approximately 70,000 km from Earth) is one of the most likely sites in our solar system for MR to occur between interplanetary and terrestrial magnetic fields. Earth’s magnetopause is also easily accessible for in situ observations from spacecraft that cannot be performed on the Sun and other astronomical environments.

Magnetic reconnection can create cracks at the edge of the magnetopause to prevent the conducting magnetosphere from perfectly shielding Earth’s space environments from the solar wind. In the central regions of MR the magnetic field with different directions intersects, forming an X line. Identifying MR signatures in space environments has long been a theoretical and observational challenge due to the fact that the positions of the X lines cannot be predetermined and the spacecraft can only view limited portions of the structures. NASA’s Magnetospheric Multiscale (MMS) spacecraft made up of four satellites 15 km apart and launched in 2015 is a pioneering mission to study the multiscale physics of MR.

Mirror waves with ripples such as plasma and magnetic fields have been widely observed in the solar system which are the product of the mirror instability that occurs under circumstances of high temperature anisotropy. In particular, when the temperature perpendicular to the magnetic field far exceeds the parallel temperature, the plasma can easily develop mirror instability. Such anisotropic temperature characteristics are clearly evidenced by MMS observations that contributed to the results of small-scale mirror waves in the solar wind not seen in previous spacecraft missions.

Recently a research team led by Professor Lin-Ni Hau of National Central University (Taiwan) used data from NASA’s MMS space probe along with theoretical models to reveal for the first time the overall geometry of magnetic reconnection (MR) with the presence of an X Line within the space domain of 2000 km x 2000 km. Within 15-30 seconds of crossing Earth’s magnetopause, all four MMS spacecraft with an exceptionally high time resolution of 0.15 seconds captured, for the first time, signs of the mirror waves surrounding the X line.

The two MR events are located 70,000 km and 150,000 km from Earth, respectively, and show common characteristics of magnetic field and plasma ripples in the MR pond with probe paths less than 30 km from the X lines. The coexistence of MR and mirror waves supports the earlier theoretical prediction of mixed MR and mirror instability which can produce more drastic processes of energy conversion and plasma acceleration. The new discovery published in the October issue of The Astrophysical Journal Letters (ApJL) by Hau et al. may have shed light on the possible mechanism of explosive magnetic reconnection phenomena occurring in space, solar and astronomical environments.

Provided by the National Central University