RAS Specialist Discussion Meeting report

Report on the RAS Specialist Discussion Meeting ‘System-scale observations and modelling of solar wind-magnetosphere-ionosphere-thermosphere (SW-M-I-T) coupling’.

Two Current Systems in the Preliminary Phase of Sudden Commencements in the Magnetosphere

The geomagnetic variations of the preliminary impulse (PI) of the sudden commencement (SC) are known to show a time delay of the peak displacement and longer duration time in the higher latitudes in the pre-noon and post-noon sectors of the polar region. This peculiar behavior of the PI geomagnetic variation is associated with temporal deformation of the ionospheric PI field-aligned current (FAC) distribution into a crescent shape; its lower-latitude edge extends toward the anti-sunward direction, and its higher-latitude edge almost stays on the same longitude near noon. Numerical simulations revealed that the deformation of the FAC distribution is derived from different behaviors of the two PI current systems. The first current system consists of the FAC connected to the PI FAC in the lower latitude side of the ionosphere, the cross-magnetopause current, and the magnetosheath current (type L current system). The cross-magnetopause current is the inertia current generated in the acceleration front of the solar wind due to the sudden compression of the magnetosheath. Thus, the longitudinal speed of the type L current system in the ionosphere is the solar wind speed in the magnetosheath projected into the ionosphere. In contrast, the PI current system connected to the PI FAC at higher latitude (type H current system) consists of the upward/downward FAC in the pre-noon/post-noon sector, respectively, and dawn-to-dusk field-perpendicular current (FPC) along the dayside magnetopause. The dawn-to-dusk FPC moves to the higher latitudes in the outer magnetosphere over time. The FAC of the type H current system is converted from the FPC due to convergence of the return FPC heading toward the sunward direction in the outer magnetosphere; the return FPC is the inertia current driven by the magnetospheric plasma flow associated with compression of the magnetopause behind the front region of the accelerated solar wind. The acceleration front spreads concentrically from the subsolar point. Consequently, as the return FPC is converted to the FAC of the type H current system, it does not move much in the longitudinal direction over time because the dawn-to-dusk FPC of the type H current system moves to the higher latitudes. Therefore, the high-latitude edge of the PI current distribution in the ionosphere moves only slightly. Finally, we clarified that the FPC-FAC conversion of the type L current system mainly occurs in the region where the Alfvén speed starts to increase toward the Earth. A region with a steep gradient of the Alfvén speed like the plasmapause is not always necessary for conversion from the FPC to the FAC. We also suggest the possible field-aligned structure of the standing Alfvén wave that may occur in the PI phase.

Echo state network model for analyzing solar-wind effects on the AU and AL indices

The properties of the auroral electrojets are examined on the basis of a trained machine-learning model. The relationships between solar-wind parameters and the AU and AL indices are modeled with an echo state network (ESN), a kind of recurrent neural network. We can consider this trained ESN model to represent nonlinear effects of the solar-wind inputs on the auroral electrojets. To identify the properties of auroral electrojets, we obtain various synthetic AU and AL data by using various artificial inputs with the trained ESN. The analyses of various synthetic data show that the AU and AL indices are mainly controlled by the solar-wind speed in addition to Bz of the interplanetary magnetic field (IMF) as suggested by the literature. The results also indicate that the solar-wind density effect is emphasized when solar-wind speed is high and when IMF Bz is near zero. This suggests some nonlinear effects of the solar-wind density.

Analysis of the distribution, rotation and scale characteristics of solar wind switchbacks: comparison between the first and second encounters of Parker Solar Probe

The S-shaped magnetic structure in the solar wind formed by the twisting of magnetic field lines is called a switchback, whose main characteristics are the reversal of the magnetic field and the significant increase in the solar wind radial velocity. We identify 242 switchbacks during the first two encounters of Parker Solar Probe (PSP). Statistics methods are applied to analyze the distribution and the rotation angle and direction of the magnetic field rotation of the switchbacks. The diameter of switchbacks is estimated with a minimum variance analysis (MVA) method based on the assumption of a cylindrical magnetic tube. We also make a comparison between switchbacks from inside and the boundary of coronal holes. The main conclusions are as follows: (1) the rotation angles of switchbacks observed during the first encounter seem larger than those of the switchbacks observed during the second encounter in general; (2) the tangential component of the velocity inside the switchbacks tends to be more positive (westward) than in the ambient solar wind; (3) switchbacks are more likely to rotate clockwise than anticlockwise, and the number of switchbacks with clockwise rotation is 1.48 and 2.65 times of those with anticlockwise rotation during the first and second encounters, respectively; (4) the diameter of switchbacks is about 10^5 km on average and across five orders of magnitude (10^3 – 10^7 km).