Abstract
Abstract. We use magnetic field and plasma observations from the Van Allen Probes, Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Geostationary Operational Environmental Satellite system (GOES) spacecraft to study the spatial and temporal characteristics of long-lasting poloidal Pc4 pulsations in the dayside magnetosphere. The pulsations were observed after the main phase of a moderate storm during low geomagnetic activity. The pulsations occurred during various interplanetary conditions and the solar wind parameters do not seem to control the occurrence of the pulsations. The most striking feature of the Pc4 magnetic field pulsations was their occurrence at similar locations during three of four successive orbits. We used this information to study the latitudinal nodal structure of the pulsations and demonstrated that the latitudinal extent of the magnetic field pulsations did not exceed 2 Earth radii (RE). A phase shift between the azimuthal and radial components of the electric and magnetic fields was observed from ZSM = 0.30 RE to ZSM = −0.16 RE. We used magnetic and electric field data from Van Allen Probes to determine the structure of ULF waves. We showed that the Pc4 magnetic field pulsations were radially polarized and are the second-mode harmonic waves. We suggest that the spacecraft were near a magnetic field null during the second orbit when they failed to observe the magnetic field pulsations at the local times where pulsations were observed on previous and successive orbits. We investigated the spectral structure of the Pc4 pulsations. Each spacecraft observed a decrease of the dominant period as it moved to a smaller L shell (stronger magnetic field strength). We demonstrated that higher frequencies occurred at times and locations where Alfvén velocities were greater, i.e., on Orbit 1. There is some evidence that the periods of the pulsations increased during the plasmasphere refilling following the storm.
Highlights
Poloidal Pc4 pulsations (7 mHz < f < 22 mHz) characterized by strong displacements of the magnetic field lines in the radial direction are common in both the dayside and nightside magnetosphere (e.g., Hughes et al, 1978)
We studied long-lasting poloidal Pc4 pulsations observed by multi-spacecraft in the dayside magnetosphere
The most striking feature of the Pc4 magnetic field pulsations was their occurrence at similar locations during three of four successive orbits of Van Allen Probes, each 9 h apart
Summary
Poloidal Pc4 pulsations (7 mHz < f < 22 mHz) characterized by strong displacements of the magnetic field lines in the radial direction are common in both the dayside and nightside magnetosphere (e.g., Hughes et al, 1978). These poloidal pulsations have been attributed to a number of mechanisms, most to field line resonances (Cramm et al, 2000), drift–bounce resonances, ballooning modes and drift mirror mode instabilities (e.g., Engebretson et al, 1992). Eriksson et al (2005) reported three cases of poloidal Pc5 pulsations with large azimuthal number (m = 100) waves observed by Cluster that occurred in the dayside magnetosphere during magnetically quiet times from L = 4 to L = 6. They studied observations from LANL satellites for signs of proton injection prior to, or during the observed ULF events but none of the events showed any sign of enhanced proton flux. Though no obvious cause for their generation was found, the study of their spatial and temporal characteristics helps us understand the conditions that are favorable for triggering these pulsations
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