On differentiating multiple types of ULF magnetospheric waves in
response to solar wind periodic density structures
Abstract
Identifying the nature and source of Ultra Low Frequencies (ULF) waves
(f ≤ 4 mHz) at discrete frequencies in the Earth’s magnetosphere is a
complex task. The challenge comes from the simultaneous occurrence of
externally and internally generated waves, and the ability to robustly
identify such perturbations. Using a recently developed robust spectral
analysis procedure, we study an interval that exhibited in magnetic
field measurements at geosynchronous orbit and in ground magnetic
observatories both internally supported and externally generated ULF
waves. The event occurred on November 9, 2002 during the interaction of
the magnetosphere with two interplanetary shocks that were followed by a
train of 90 min solar wind periodic density structures. Using the
Wang-Sheeley-Arge model, we mapped the source of this solar wind stream
to an active region and a mid-latitude coronal hole just prior to
crossing the Heliospheric current sheet. In both the solar wind density
and magnetospheric field fluctuations, we separated broad power
increases from enhancements at specific frequencies. For the waves at
discrete frequencies, we used the combination of satellite and ground
magnetometer observations to identify differences in frequency,
polarization, and observed magnetospheric locations. The magnetospheric
response was characterized by: (i) forced breathing by periodic solar
wind dynamic pressure variations below ≈ 1 mHz; (ii) a combination of
directly driven oscillations and wave modes triggered by additional
mechanisms (e.g., shock and interplanetary magnetic field discontinuity
impact, and substorm activity) between ≈ 1 and ≈ 4 mHz; and (iii)
largely triggered modes above ≈ 4 mHz.