Multi-point Conjugate Observations of Dayside ULF Waves during an Extended Period of Radial IMF

Abstract

Long-lasting Pc5 ultralow frequency (ULF) waves spanning the dayside and extending from L ∼ 5.5 into the polar cap region were observed by conjugate ground magnetometers. Observations from MMS satellites in the magnetosphere and magnetometers on the ground confirmed that the ULF waves on closed field lines were due to fundamental toroidal standing Alfvén waves. Monochromatic waves at lower latitudes tended to maximize their power away from noon in both the morning and afternoon sectors, while more broadband waves at higher latitudes tended to have a wave power maximum near noon. The wave power distribution and MMS satellite observations during the magnetopause crossing indicate surface waves on a Kelvin-Helmholtz (KH) unstable magnetopause coupled with standing Alfvén waves. The more turbulent ion foreshock during an extended period of radial interplanetary magnetic field (IMF) likely plays an important role in providing seed perturbations for the growth of the KH waves. These results indicate that the Pc5 waves observed on closed field lines and on the open field lines of the polar cap were from the same source.Plain Language Summary The Earth's magnetic field lines can oscillate at ultralow frequencies (ULF: 1 mHz to 5 Hz). These natural oscillations of closed magnetic field lines, analogous to vibrations on a stretched string, are also called geomagnetic pulsations or ULF waves. ULF waves play a key role in the transfer of energy from terrestrial space to Earth's upper atmosphere. In this study, we report a long-lasting large spatial scale ULF wave event observed by ground observatories from both hemispheres. Together with satellite measurements in space, we are able to confirm that these waves were driven by upstream turbulent structures due to the interaction between matter and electromagnetic fields emitted from the Sun and the Earth's outer atmosphere and magnetic field.

Hybrid Simulations of the Cusp and Dayside Magnetosheath Dynamics Under Quasi‐Radial Interplanetary Magnetic Fields

“…The region of largest δB 2 z has a width of approximately 14 d i (∼ 1.2R E ). In addition to the dipole tilt, which places the northern cusp further towards the dayside, the north-south asymmetry in fluctuations in both runs is also affected by the different locations of the quasi-parallel shock, consistent with observations of magnetic field perturbations under quasi-radial IMF conditions (Shi et al, 2020). The structure of magnetic field lines in both cases at selected times is shown in Fig.…”

“…Wang et al, 2009;Omidi & Sibeck, 2007;Palmroth et al, 2013) under different IMF conditions. Under IMF with a strong radial component, the cusp has been observed to be highly dynamic (Taylor et al, 2004), and it has been shown that the turbulence in the ion foreshock can lead to ULF waves on the open and closed field lines of the polar cap (Shi et al, 2020). Other statistical observa-tions have shown the structure of the cusp and its boundaries under various IMF conditions (Lavraud et al, 2004(Lavraud et al, , 2005Pitout et al, 2012), and statistical studies of cusp structure and motion are shown in (Pitout et al, 2006(Pitout et al, , 2009.…”