Multipoint 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.

Characteristics and Sources of Intense Geoelectric Fields in the United States: Comparative Analysis of Multiple Geomagnetic Storms

“…Another point we would like to emphasize is the unique role of ULF wave activity as a direct driver of intense geoelectric fields during geomagnetic storms. Note that the drivers of ULF waves vary from external sources such as IP shocks to internal sources such as particle injections during substorms (Shi et al, 2018(Shi et al, , 2020. However, these external and internal sources can also drive intense geoelectric fields via mechanisms other than ULF waves.…”

Characterization of High‐m ULF Wave Signatures in GPS TEC Data

“…These are low‐m toroidal waves which have been investigated in Shi et al. (2020) and are beyond the scope of this study.…”

“…Note that Pc5 waves were present in GPS TEC (Figures 2g and 2h), ground magnetometer (Figures 2g-2i), and GOES observations (Figure 3a). These are low-m toroidal waves which have been investigated in Shi et al (2020) and are beyond the scope of this study.…”

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

“…The cusp has been the subject of numerous observational (e.g., Lavraud et al., 2005 ; Nykyri et al., 2012 ; Pitout et al., 2009 ; Taylor et al., 2004 ) and simulation studies (Omidi & Sibeck, 2007 ; Palmroth et al., 2013 ; X. Y. Wang et al., 2009 ) 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 observations have shown the structure of the cusp and its boundaries under various IMF conditions (Lavraud et al., 2004 , 2005 ; Pitout et al., 2012 ), and statistical studies of cusp structure and motion are shown in (Pitout et al., 2006 , 2009 ).…”