L Versus Time Structures of Dayside Magnetic Pulsations Detected by the European Quasi‐Meridional Magnetometer Array

Abstract

We studied the spatiotemporal structure of ground magnetic pulsations on the dayside by displaying magnetic field perturbations detected by the European quasi‐Meridional Magnetometer Array (EMMA) as 2‐D images in the magnetic L value versus time space, called EMMAgrams. We generated EMMAgrams from observations made on 15 August 2015, including a previously studied pulsation event associated with an interplanetary shock. In addition to signatures of field line resonance driven by a cavity mode oscillation, we found poleward propagating structures with L‐independent periods in the Pc2 band. The Pc2 structures are attributed to periodic magnetohydrodynamic pulses (upstream waves) originating from the ion foreshock and propagating in the magnetosphere along the path proposed by Tamao. Ringing of local field lines at L‐dependent periods (transient pulsations) is also clearly detected as dispersive poleward propagating structures not only immediately after the shock impact but also during time periods of less obvious external disturbances. A transient pulsation decays after a few wave periods, and cross‐spectral analysis of transient pulsations detected at two stations with a small latitudinal separation indicates elevation of the cross phase in a band delimited by the field line resonance frequencies at the stations. Successive excitation of transient pulsations by variations of the solar wind dynamic pressure appears to contribute significantly to formation of similar cross‐phase peaks that are widely used in magnetoseismic studies.

Propagation of Ultralow‐Frequency Waves from the Ion Foreshock into the Magnetosphere During the Passage of a Magnetic Cloud

“…Large amplitude stepwise increases of the solar wind dynamic pressure (sudden impulses or sudden commencements) excite classical transient pulsations (Petrinec et al, 1996). However, Takahashi and Heilig (2019) revealed that transient pulsations are excited even in the absence of strong solar wind dynamic pressure pulses, which is true in the observations presented here. In Figures 9d and 9e, we find that B t and H exhibit similar periodicities but cannot unambiguously associate individual peaks between B t and H. This explains the low space-ground coherence shown in Figure 8.…”

“…To gain more insight into the space-ground relationship, we generated EMMA grams (Takahashi & Heilig, 2019). EMMA grams display high-pass filtered magnetic field time series from many (23 in the present study) stations as two-dimensional (2-D) images in the time versus L space.…”

“… (a) Perturbation of B t at THEMIS‐A about its 21‐s running mean. (b, c) EMMAgram (Takahashi & Heilig, 2019) for 1000 UT–1015 UT. The black arrow indicates the midpoint L between PPN and ZAG.…”

“…The degree of tilt varies with L , with larger tilt appearing mainly at L < 2.5 (see Figure 9e). Takahashi and Heilig (2019) noted that poleward propagating patterns can arise from field line resonance driven by monochromatic driver waves, eigenmode oscillation of field lines at L ‐dependent frequency (transient pulsations), or the travel time effect proposed by Tamao (1964). During transient pulsations, the tilt of the ridges (stripes) seen in the time‐ L plane increases with time (Poulter & Nielsen, 1982), whereas the other two maintain a constant tilt.…”

Nodal Structure of Toroidal Standing Alfvén Waves and Its Implication for Field Line Mass Density Distribution

“…Toroidal waves are excited by driver fast mode waves through steady field line resonance (Chen & Hasegawa, 1974; Southwood, 1974) or impulsive triggering (Takahashi & Heilig, 2019). The driver waves may originate from solar wind dynamic pressure pulses (Sarris et al., 2010), foreshock ULF waves (Clausen et al., 2009), or nightside disturbances associated with substorms (Takahashi et al., 1996).…”