A Statistical Study of Pi2 Pulsations Observed in the Upper Ionosphere Using Swarm Magnetic Field Data

Abstract

The properties of Pi2 pulsations observed in the upper ionosphere are studied using magnetic field data acquired by the Swarm A spacecraft in low Earth orbit and at the low-latitude Bohyun ground station (BOH, L = 1.3) for January 2014 to June 2015. From time intervals when Swarm A was on the nightside (magnetic local time (MLT) = 1800-0600 hr) and the BOH station was near midnight (MLT = 2100-0300 hr), we identified 621 Pi2 events in the horizontal H component of the BOH data. For each event we examined the coherence between the horizontal H component on the ground and the B x (radial), B y (azimuthal), or B z (compressional) components at Swarm A. Out of 621 events, the B x −H high-coherence (> 0.7) events are ∼ 6%, the B y −H high-coherence events are ∼ 2%, and the B z −H high-coherence events are ∼ 25%. The ground satellite high-coherence events occurred when the spacecraft was located at magnetic latitudes between −50 • and 50 • . Using the ground satellite high-coherence events, we statistically examined the latitudinal structure of the relative amplitude and phase of the ionospheric Pi2 pulsations and found that their latitudinal variations is consistent with the north-south mode structure expected from the plasmaspheric resonance model. Our statistical results indicate that the source of ionospheric Pi2 pulsations is the plasmaspheric resonance.

Statistical Analysis of Pi2 Pulsations Observed by Van Allen Probes

“…Neither E ϕ nor B ν exhibits an amplitude minimum in the L domain covered by the spacecraft. These results are consistent with the structures of the simulated f 2_sim mode (Figures 14j-14l) and confirm previous statistical results and their cavity mode interpretation (Park et al, 2020;Takahashi, Lee, et al, 2003;Takahashi et al, 1995;Teramoto et al, 2011Teramoto et al, , 2022.…”

“…The disturbances can be bursty bulk flows with random periodicity. Previous statistical studies using satellite data found radially standing structures of compressional Pi2 waves in the inner magnetosphere, in favor of the cavity mode interpretation (Park et al., 2020; Takahashi, Lee, et al., 2003; Takahashi et al., 1995; Teramoto et al., 2011, 2022). However, there are Pi2 models that assume earthward‐propagating fast mode waves in the inner magnetosphere (Uozumi et al., 2009, 2020).…”

“…The disturbances can be bursty bulk flows with random periodicity. Previous statistical studies using satellite data found radially standing structures of compressional Pi2 waves in the inner magnetosphere, in favor of the cavity mode interpretation (Park et al, 2020;…”

“…For comparison with the RBSP observations, the maps are shown for the L <7 domain. Because ground H at low latitudes and magnetospheric B μ at L <2 oscillate in phase with very similar amplitudes during Pi2 events (e.g., Park et al., 2020), we can make a direct comparison of the maps from the observation and the simulation. There is a strong day‐night asymmetry in the amplitude maps, which is a direct consequence of the source disturbance being localized to the nightside, combined with the boundary conditions that allow escape of wave energy to the magnetotail before azimuthally uniform mode structures are established.…”

“…The main point of the present study is that we use data from Van Allen Probes (Radiation Belt Storm Probes, RBSP). To analyze the RBSP data, we use a methodology similar to those applied to data from Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) (Takahashi et al., 1995), Combined Release and Radiation Effects Satellite (CRRES) (Takahashi, Lee, et al., 2003), Dynamics Explorer (DE)‐1 (Teramoto et al., 2011), Swarm (Park et al., 2020), and Arase (Teramoto et al., 2022). The difference between the previous and present studies is that the RBSP data include the electric field ( E ), the magnetic field ( B ), and the electron number density ( n e ) for a 7‐year period, during which the spacecraft completed three orbital precessions in magnetic local time (MLT).…”