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Plasmapause surface waves (PSWs) near the plasmapause boundary are regarded to be the magnetospheric source of ionospheric auroral giant undulations (GUs) located at the equatorward boundary of diffuse aurora. However, the observational evidence of wave‐particle interaction connecting PSWs and GUs is absent. In this letter, we demonstrate GUs are driven by pitch‐angle scattering of time domain structures modulated by the PSWs, based on the conjugated ionospheric and magnetospheric observations. Specifically, ionospheric GUs are lighted by the pitch‐angle scattering of <1 keV thermal electron and ions and energetic ions with energy up to dozens of keV near the plasmapause. Further, the total fluxes during one PSW period and energy of scattered electron and ions determine the size and luminosity of GUs. Our research provides observational evidence that PSWs cause periodic electron precipitation via modulating the time domain structures rather than the previously predicted chorus or electron cyclotron harmonic waves. Plain Language Summary Boundary surface waves usually act as a kind of special oscillation along the boundary layer and are the widely existing physical phenomena in the universe. In our Earth, there are magnetopause surface wave and plasmapause surface wave. For the latter, the plasmapause surface wave has been confirmed to be a kind of sawtooth‐type auroral structures locating on the equatorial edge of aurora oval, named as giant undulations. But how can the plasmapause surface wave produce the auroral giant undulations is still unknown. Based on this question, we have provided the observational evidence of auroral giant undulations being driven by the periodic pitch‐angle scattering of time domain structures modulated by plasmapause surface waves. Our new results in this research would help us to better understand the energy conversion controlled by boundary dynamics and the crucial effect of boundary dynamics on the near‐surface space environment. Key Points Giant undulations (GUs) are lighted by the pitch‐angle scattering of <1 keV thermal electron and ions and energetic ions with energy up to dozens of keV Total fluxes during one plasmapause surface wave (PSW) period and energy of scattered electron and ions determine the size and luminosity of GUs PSWs can cause periodic electron precipitation by modulating time domain structures

Quasiperiodic (QP) emissions are whistler‐mode electromagnetic waves observed in the Earth's inner magnetosphere whose intensity has a nearly periodic time modulation with typical modulation periods on the order of minutes. Some events exhibit, on top of the main modulation period, an additional fine inner modulation with modulation periods on the order of seconds. We use high‐resolution multi‐component electromagnetic wave data obtained by the Van Allen Probes spacecraft to investigate one such event. Detailed wave analysis demonstrates that the fine inner modulation is due to a wave packet bouncing back and forth between the hemispheres. The presence of a density duct is important for the formation of the event, as demonstrated by the increased ratio of wave power propagating away from the equator (a tentative source region) within the duct. The main QP modulation period corresponds to the plasma number density modulation observed just outside the plasmasphere. Plain Language Summary The intensity of electromagnetic waves in the near‐Earth space, the magnetosphere, sometimes has a nearly periodic temporal modulation on the order of minutes. The origin of such waves, so‐called quasiperiodic emissions, is not yet fully understood. On top of the main modulation period, some events exhibit an additional fine inner modulation with periods on the order of seconds. We use wave propagation directions determined from the Van Allen Probes measurements to demonstrate that this shorter modulation corresponds to a wave packet bouncing in between the hemispheres. By examining the ratio of wave power propagating away from and toward the geomagnetic equator (a tentative source region), we further demonstrate that the presence of a region with enhanced density, guiding waves along a given magnetic field line, is important for the formation of the event. Additionally, the main modulation period of the event corresponds to the plasma number density modulation observed just outside the plasmasphere, possibly linked to a plasmapause surface wave. Our results, revealing the presence and origin of the fine inner structure of the waves, provide important observational constraints for models trying to explain the generation of quasiperiodic emissions. Key Points Whistler‐mode quasiperiodic event has a fine inner structure that is related to the wave bouncing between hemispheres Wave generation is related to the presence of a density duct, but the event can also be observed outside the duct Plasma number density just outside the plasmasphere is modulated with a period that corresponds to the period of the quasiperiodic event

Energy circulation in geospace lies at the heart of space weather research. In the inner magnetosphere, the steep plasmapause boundary separates the cold dense plasmasphere, which corotates with the planet, from the hot ring current/plasma sheet outside. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations, but direct evidence of the waves and their role in magnetospheric dynamics have not yet been detected. Here, we show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based measurements. The wave oscillates the plasmapause in the afternoon-dusk sector, triggers sawtooth auroral displays, and drives outward-propagating ultra-low frequency waves. We also show that the surface-wave-driven sawtooth auroras occurred in more than 90% of geomagnetic storms during 2014–2018, indicating that they are a systematic and crucial process in driving space energy dissipation. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations. Here, the authors show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based observations.

Boundary dynamics are crucial for the transport of energy, mass, and momentum in geospace. The recently discovered plasmapause surface wave (PSW) plays a key role in the inner magnetosphere dynamics. However, a comprehensive investigation of spatial variations of the PSW remains absent. In this study, we elucidate the spatial characteristics of a PSW through observations from multiple spacecrafts in the magnetosphere. Following the initiation of the PSW, quasi‐periodic injections of energetic ions, rather than electrons, are suggested to serve as energy source of the PSW. Based on the distinct wave and particle signatures, we categorize the PSW into four regions: seed region, growth region, stabilization region and decay region, spanning from nightside to afternoon plasmapause. These findings advance our understanding of universal boundary dynamics and contribute to a deeper comprehension of the pivotal roles of surface waves in the energy couplings within the magnetosphere‐plasmasphere‐ionosphere system. Plain Language Summary Much like the rhythmic beating of a drum, boundaries within the terrestrial system, which delimit distinct plasmas with varying temperatures and densities, can oscillate due to external impulses or internal instabilities, giving rise to surface waves. These surface waves travel along the boundary surface and establish standing wave structures in magnetic field lines. In the terrestrial space environment, two types of surface waves have been identified: the magnetopause surface wave and the plasmapause surface wave (PSW). These waves typically have frequencies ranging from fractions of milli Hertz to a few milli Hertz, with periods spanning from several to tens of minutes. They are believed to play a vital role in mass, energy, and momentum transport within Earth's magnetosphere. However, the exact source that excites the waves and the manner in which the waves evolve along the boundary remain elusive. In this study, we, for the first time, unveil the spatial evolutionary signatures of the PSW during a moderate geomagnetic storm accompanied by substorms, utilizing data from six magnetospheric spacecrafts positioned near the plasmapause. This research contributes to a better understanding of the physical mechanisms underlying the widespread existence of surface waves in the universe, shedding light on the processes of magnetosphere‐plasmasphere‐ionosphere energy couplings and wave‐particle interactions associated with surface waves. Key Points Spatial characteristics of plasmapause surface wave (PSW) are elucidate with multiple‐spacecraft observations in the magnetosphere It is suggested that quasi‐periodic injections of energetic ions rather than electrons sustain the PSW during the substorm The PSW is categorized into seed region, growth region, stabilization region, and decay region with different wave and particle properties

Giant undulations (GUs) have been well established to be the optical manifestation of the plasmapause surface wave (PSW) where the wave‐particle interactions provide particle sources to generate auroras. However, their detailed particles precipitation signatures in the ionosphere remain unclear. Here we analyze multi‐satellite conjugated observations in the ionosphere during a prominent GUs event, revealing the two‐zone precipitation pattern including energetic proton precipitations responsible for the main body of GUs and low‐energy electron precipitations for the edge of GUs. Interestingly, the occurrence of GUs is also accompanied with high‐energy particles precipitations of hundreds of keV and magnetic disturbances of three components. The sizes of sawtooth in the GUs correlate positively with the strength of adjacent subauroral polarization streams (SAPS). The two‐zone precipitation pattern and high‐energy particles precipitation over GUs are potentially related to the plasma sheet and very‐low frequency wave (VLF) modulation of the PSW, respectively. Plain Language Summary Planetary boundaries are mostly disturbed and dynamic, thus controlling energy, mass and momentum transfers for the whole active system. As a typical form of boundary dynamics, the surface wave inherently developed near the boundary when undergoing perturbation. For the earth, they are magnetopause surface wave on the magnetopause and plasmapause surface wave near the plasmapause from outer to inner region. Generally, the observation of surface wave is rare mostly due to sporadic magnetospheric probes. Recently, a series of works have confirmed the ionospheric giant undulations being the optical manifestation of plasmapause surface wave. Even though the close correlation between them, the important intermediate physical processes, including wave‐particle interaction, particle precipitation, ionospheric signatures, is hardly explored and thus unknown. Therefore, we have prudently shown the signatures of particles precipitation and magnetic field disturbance above giant undulations, and the close relation between the sizes of giant undulations and the velocities of their equatorward subauroral polarization streams. Our results would help to reveal the crucial middle processes between the originating plasmapause surface wave and resulting giant undulations, and to remotely monitor the boundary dynamics with higher temporal‐spatial resolution. Key Points The main body of GUs is lighted by energetic proton precipitation while the edge is lighted by low‐energy electron precipitation Occurrence of GUs is accompanied with particle precipitations of hundreds of keV and magnetic field three components perturbations It is suggested that the size of sawtooth correlates positively to their equatorward SAPS/SAID strength

We use multi-satellite and ground-based magnetic data to investigate the concurrent characteristics of Pc3 (22–100 mHz) and Pc4-5 (1–22 mHz) ultra-low-frequency (ULF) waves on the 31 October 2003 during the Halloween magnetic superstorm. ULF waves are seen in the Earth's magnetosphere, topside ionosphere, and Earth's surface, enabling an examination of their propagation characteristics. We employ a time–frequency analysis technique and examine data from when the Cluster and CHAMP spacecraft were in good local time (LT) conjunction near the dayside noon–midnight meridian. We find clear evidence of the excitation of both Pc3 and Pc4-5 waves, but more significantly we find a clear separation in the L shell of occurrence of the Pc4-5 and Pc3 waves in the equatorial inner magnetosphere, separated by the density gradients at the plasmapause boundary layer. A key finding of the wavelet spectral analysis of data collected from the Geotail, Cluster, and CHAMP spacecraft and the CARISMA and GIMA magnetometer networks was a remarkably clear transition of the waves' frequency into dominance in a higher-frequency regime within the Pc3 range. Analysis of the local field line resonance frequency suggests that the separation of the Pc4-5 and Pc3 emissions across the plasmapause is consistent with the structure of the inhomogeneous field line resonance Alfvén continuum. The Pc4-5 waves are consistent with direct excitation by the solar wind in the plasma trough, as well as Pc3 wave absorption in the plasmasphere following excitation by upstream waves originating at the bow shock in the local noon sector. However, despite good solar wind coverage, our study was not able to unambiguously identify a clear explanation for the sharp universal time (UT) onset of the discrete frequency and large-amplitude Pc3 wave power.

Nonthermal continuum (NTC) radiation is believed to be emitted at the plasmapause and near the magnetic equator. We present a particular type of NTC radiation, referred to as NTC patch, which appears over a wide frequency range and within a relatively short time interval. NTC patches are observed in all magnetospheric plasma environments of the Cluster 2 orbit and are shown to represent a quarter of the NTC events observed in 2003. A statistical analysis of the frequency pattern performed on the 2003 Cluster 2 Waves of High frequency and Sounder for Probing of Electron Density by Relaxation data indicates that the NTC patches can be divided into two classes: Those with banded emission in frequency are only observed close to the source region and are thus termed “plasmaspheric,” while the others, nonbanded, are termed “outer magnetospheric.” In an event on 26 September 2003, we localize the sources positions and study the expected propagation of each NTC frequency beam of a plasmaspheric patch. From the observations, we show that the sources are located very close to the satellite and to each other at positions projected on the XY GSE plane. Using a ray tracing code, we demonstrate that, close to the source regions, the satellite observes all frequency rays at the same time which overlap in the spectrogram making up the plasmaspheric patch. After the satellite crossing, the rays follow diverging paths and cannot therefore be observed further out by the same satellite simultaneously. Plasmaspheric patches are thus specific signatures of close and distorted source regions. Key Points NTC particular signature patches, two different patches can be observed Plasmaspheric NTC patches propagation disappear during propagation because of ray Study of the source position allows for plot of the shape of source region

Consecutive bursts of Pi2 pulsations are examined with magnetic field data obtained by the SMALL array in 1999. With reference to the H-component magnetic bays in the high-latitude magnetograms at CPMN, 10 events consisting of two consecutive Pi2 bursts simultaneously observed by the Beijing (BJI, L = 1.46) and Wuhan (WHN, L = 1.20) stations are identified as associated with substorm onsets. Owing to the same waveform seen by the CPMN and IGPP/LANL arrays, they are the global phenomena. Their occurrences are mostly in the 2100–2300 LT (local time) sector in which the dominant frequencies at WHN are higher than the mean frequency, but those at BJI are lower and close to the frequency of the surface wave at the plasmapause. Moreover, the LT dependence of azimuth and polarization of two consecutive Pi2 bursts at BJI and WHN are analyzed and consistent with the ULF waves theory by Itonaga and Yumoto (1998). GOES 8 and GOES 10 confirm the formation of the substorm current wedge after the onsets of two Pi2 bursts. Thus during substorm onsets, Pi2 pulsations at low latitudes may result from hydromagnetic waves driven by an impulsive source in the magnetotail which could commence in the longitude of 2230 LT and later propagate westward and eastward as well. Low-latitude Pi2 waves near the source site may be affected by several factors as they propagate by the stimulation of a surface wave at the plasmapause, by a localized field line oscillation inside the plasmapause, and by the magnetospheric/plasmaspheric cavity (resonance) mode.

We carried out a statistical analysis of Pi2 pulsations using the geomagnetic field data obtained at three ground stations. A local time dependence of the dominant frequency of Pi2 was found on the nightside. The frequency of mid-latitude Pi2 pulsations is lower on the dusk side than that on the dawn side. This tendency is attributed to the shape of the plasmasphere which bulges out to the dusk side. It was confirmed that the Pi2 frequency depends also on the geomagnetic activity measured with Kp index. During the disturbed periods, Pi2 pulsations have higher frequency than that in the quiet periods. This dependence is interpreted to be caused by the size of the plasmapause which is smaller under the disturbed conditions than that under the quiet conditions. The dominant frequency of Pi2 pulsations at lower latitudes has a peak in post-midnight, and a Kp dependence similar to that at mid-latitudes is also observed. However, the result for low-latitude Pi2’s is different from that for mid-latitude Pi2. We consider that the dominant mechanism of mid-latitude Pi2 is the plasmaspheric surface wave. In order to examine the idea that the surface wave on the plasmapause is the dominant mechanism of Pi2 pulsations at mid-latitudes, we estimated the resonance frequency of the surface wave on the plasmapause using a plasmaspheric model which includes the effect of the plasmaspheric bulge. The estimated frequency of the surface wave is higher on the dawn side than that on the dusk side, which is essentially consistent with the observational results. The predicted frequency under quiet conditions (Kp ≤ 3) is nearly equal to the observed Pi2 frequency at mid-latitudes. These results suggest that the dominant frequency of Pi2 pulsations at mid-latitudes depends on the structure of the plasmapause.

In this paper global surface wave modes supported by plasma discontinuities at both the magnetopause and the plasmapause are considered. The ionosphere at the ends of the magnetic field lines of the earth outer magnetosphere is considered as reflecting boundaries of the surface waves that propagate along the plasma boundaries. As a result a standing wave structure along the magnetic field fluxes of the outer magnetosphere, i.e. surface wave resonance structure can be formed. Due to quantized wavenumbers along the magnetic field lines, the surface wave resonance possesses quantified frequencies in a following way: f = nfres, where fres is frequency of the corresponding fundamental surface wave resonance and n is an integer. Global Pc5 pulsations have been observed and interpreted mostly as cavity modes of the earth magnetosphere. The global Pc5 pulsations however could alternatively be interpreted as ultra low-frequency surface wave resonances of the earth magnetosphere that do not necessarily involve the cavity mode-field aligned resonance transformation concept.