Multiradar observations of substorm‐driven ULF waves

Abstract

A recent statistical study of ULF waves driven by substorm‐injected particles observed using Super Dual Auroral Radar Network (SuperDARN) found that the phase characteristics of these waves varied depending on where the wave was observed relative to the substorm. Typically, positive azimuthal wave numbers, m, were observed in waves generated to the east of the substorms and negative m to the west. The magnitude of m typically increased with the azimuthal separation between the wave observation and the substorm location. The energies estimated for the driving particles for these 83 wave events were found to be highest when the waves were observed closer to the substorm and lowest farther away. Each of the 83 events studied by James et al. (2013) involved just a single wave observation per substorm. Here a study of three individual substorm events are presented, with associated observations of multiple ULF waves using various different SuperDARN radars. We demonstrate that a single substorm is capable of driving a number of wave events characterized by different azimuthal scale lengths and wave periods, associated with different energies, W, in the driving particle population. We find that similar trends in m and W exist for multiple wave events with a single substorm as was seen in the single wave events of James et al. (2013). The variety of wave periods present on similar L shells in this study may also be evidence for the detection of both poloidal Alfvén and drift compressional mode waves driven by substorm‐injected particles.

Contribution of ULF Wave Activity to the Global Recovery of the Outer Radiation Belt During the Passage of a High‐Speed Solar Wind Stream Observed in September 2014

“…During the global electron flux recovery, both Alfvénic fluctuations in the three components of the IMF and a southward turning of the average IMF Bz component contributed to provide energy input to the Earth's inner magnetosphere via dayside reconnection, which in turn may drive enhanced substorm activity (see, e.g., Gonzalez et al, ). Also, it has been shown in the literature that substorm activity may induce ULF wave generation (see, e.g., James et al, , ). While James et al () suggest that ULF waves can be driven by energy sources internal to the magnetosphere, James et al () suggest that the ULF waves can be driven by energy sources coming from both internal and external to the magnetosphere.…”

“…Also, it has been shown in the literature that substorm activity may induce ULF wave generation (see, e.g., James et al, , ). While James et al () suggest that ULF waves can be driven by energy sources internal to the magnetosphere, James et al () suggest that the ULF waves can be driven by energy sources coming from both internal and external to the magnetosphere. According to James et al (), the external sources can be Kelvin‐Helmholtz instability on the magnetopause or solar wind buffeting.…”

“…James et al () performed a statistical analysis of 83 substorm events, and they showed that substorm activity can function as an internal (i.e., within the Earth's magnetosphere) source of poloidal‐mode (high‐ m ) ULF waves. On the other hand, James et al () analyzed three individual substorm events and showed that the magnetospheric ULF waves can be driven by sources both internal and external to the magnetosphere. They suggested that the waves driven by energy sources external to the magnetosphere (e.g., Kelvin‐Helmholtz instability on the magnetopause or solar wind buffeting) are often characterized by low azimuthal wave numbers ( m ) or large azimuthal scale sizes.…”

Conjugate Ionosphere‐Magnetosphere Observations of a Sub‐Alfvénic Compressional Intermediate‐m Wave: A Case Study Using EKB Radar and Van Allen Probes

“…The wave are observed as velocity variations of small‐scale field‐aligned ionospheric irregularities along the beam direction. The velocity variations occur due to the drift of the ionospheric plasma caused by magnetospheric ULF waves (Berngardt, ; Bland et al, ; Chelpanov et al, , ; James et al, ; Ponomarenko et al, , ; Wright & Yeoman, ; Yeoman et al, ). Due to the close arrangement of these beams the wave propagation direction and length can be determined.…”

Ballooning Instability in the Magnetospheric Plasma: Two‐Dimensional Eigenmode Analysis

“…The resulting plasma pressure, plasma pressure gradient, and profiles are shown in Figure 4 (two upper panels) for L fall = 7. These values are typical for the azimuthally small-scale waves observed in the magnetosphere (e.g., Yeoman et al, 2012;James et al, 2016;Rubtsov, Agapitov, et al, 2018;Takahashi et al, 2018). The calculated Ω 2 − (L) profile is shown in the lower panel of this figure.…”