Multispacecraft Observation of the Presubstorm Long‐Lasting Poloidal ULF Wave

Abstract

Magnetic azimuthally small-scale (azimuthal wave numbers m ≫ 1) pulsations in Pc4-5 bands (45-600 s periods; Jacobs et al., 1964) on the dayside of the Earth's magnetosphere are intensively studied in recent years. The high-m waves are observed by satellites, high-frequency radars (Shi et al., 2018, and references therein), and optical manifestations of auroral undulations (Motoba et al., 2015). Generally, these waves propagate westward (m < 0; see Chelpanov et al., 2018), but some authors reported eastward propagating waves (e.g., Chelpanov et al., 2019;Yamamoto et al., 2019). These waves are believed to be excited through drift or drift-bounce resonance with ∼1-100 keV protons (Min et al., 2017;Takahashi et al., 2018). Electron flux oscillations in a wide energy range were found to correlate with Pc4-5 waves as well (Ren et al., 2017(Ren et al., , 2018. The energy transfer from particles to the wave going through internal instabilities caused by non-Maxwellian proton distribution or phase space density radial gradient (Southwood et al., 1969). Usually, Pc4-5 waves are associated with the MHD Alfvén waves (e.g., Dai et al., 2013), although sometimes they can be identified with the drift-compressional modes (Mager et al., 2019;Rubtsov et al., 2018). According to previous studies, the dayside high-m Pc4-5 waves usually appear as a consequence of magnetic storms (

Polarization and Spatial Distribution Features of Pc4 and Pc5 Waves in the Magnetosphere

“…L‐MLAT distributions of poloidal waves in the day and dawn sectors shown in Figures 4b and 4d are more uniform than those of toroidal waves (Figures 2b and 2d). This fact indicates both even and odd harmonics excitation, matching previous observations of dayside second harmonic poloidal waves (Liu et al., 2013; Rubtsov et al., 2021; Shi et al., 2018; Takahashi et al., 2018). The distribution of ϕ is more uniform than that for the toroidal waves (Figure 4h).…”

“…Poloidal waves are often associated with FLR, but with discrete frequency change with L ‐shell (Le et al., 2017, 2021; Rubtsov et al., 2021). A 〈 f wave 〉 increase toward the Earth shown in Figure 5a is not as prominent as toroidal waves (see Figure 3a) and occurs mostly at the dayside (see Figure S2 in Supporting Information ).…”

“…We merge wave events with the same magnetic field oscillation component and the wave frequency when the time difference between the end time of the first event and the start time of the following event is less than 60 min. It is useful for long-lasting waves detection, when beads are observed (e.g., Korotova et al, 2016;Rubtsov et al, 2021).…”

Resonant Generation of an Alfvén Wave by a Substorm Injected Electron Cloud: A Van Allen Probe Case Study

“…Most of the observed poloidal waves are the second harmonic of the Alfvén wave. They are asymmetric in the electric field relative to the equator and are generated by the drift‐bounce resonance (Chen & Hasegawa, 1994; Hughes et al., 1978; Min et al., 2017; Rubtsov et al., 2021; Southwood et al., 1969; Takahashi et al., 1990; Takahashi, Oimatsu, et al., 2018). It should be noted that the drift‐bounce resonance is impossible with energetic electrons, since due to their small mass the frequencies of their bouncing motion along field lines is much higher than the wave frequency.…”

Resonant generation of an Alfven wave by a substorm injected electron cloud: a Van Allen probe case study

“…They are asymmetric in the electric field relative to the equator and are generated by the drift-bounce resonance (Southwood et al, 1969;Hughes et al, 1978;Takahashi et al, 1990;Chen & Hasegawa, 1994;Min et al, 2017;Takahashi, Oimatsu, et al, 2018;Rubtsov et al, 2021). It should be noted, that the drift-bounce resonance is impossible with energetic electrons, since due to their small mass the frequencies of their bouncing motion along field lines is much higher than the wave frequency.…”