skip to main content

This content will become publicly available on December 15, 2023

Title: Identifying the Structure and Propagation of Dawnside Pc5 ULF Waves Using Space‐Ground Conjunctions

Pc5 ultralow frequency waves are important for transferring energy between the magnetosphere and ionosphere. While many observations have been performed on Pc5 waves properties, it has been difficult to determine the source region, signal propagation path, and the two‐dimensional structure of Pc5 waves beyond coverage by a small number of satellites. Pc5 waves often show a dawn‐dusk asymmetry, but the cause of the asymmetry is under debate. To address these issues, we used conjunction events between the THEMIS satellites and all‐sky imagers and analyzed two Pc5 wave events that were stronger on the dawnside. For both events, the Pc5 waves propagated from dawnside magnetopause toward the nightside magnetosphere. The Pc5 waves were also associated with dawnside magnetopause surface waves, which were probably induced by the Kelvin‐Helmholtz instability. The ionospheric equivalent currents identified multiple vortices on the dawnside associated with quasi‐periodic auroral arcs and much weaker perturbations on the duskside. Global auroral imaging also presented a similar dawn‐dusk asymmetry with multiple arcs on the dawnside, while only one or two major arcs existed on the duskside. Pc5 waves in the magnetosphere had an anti‐phase relation between the total magnetic field and thermal pressure, with a slower propagation velocity compared with more » magnetohydrodynamic waves. The Poynting flux was anti‐sunward with an oscillating field‐aligned component. These properties suggest that Pc5 waves were slow or drift mirror mode waves coupled with standing Alfven waves. The ground‐based and multi‐satellite observations provide crucial information for determining the Pc5 waves properties, possible source region, and signal propagation path.

« less
 ;  ;  ;  ;  ;  ;  ;  
Award ID(s):
Publication Date:
Journal Name:
Journal of Geophysical Research: Space Physics
DOI PREFIX: 10.1029
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Pc5 (2–7 mHz) ultralow frequency (ULF) waves play a significant role in resonating with particles and transferring energy in the coupled magnetospheric and ionospheric system. Recent studies found that Pc5 ULF waves can be triggered by foreshock transients which can perturb the magnetopause through dynamic pressure variation. However, whether foreshock transient‐driven Pc5 ULF waves are geoeffective and can propagate globally is still poorly understood. In this study, we take advantage of the conjunction between in situ (by the THEMIS probes, Geotail satellite, GOES satellites, and Van Allen probes) and ground‐based (by the all‐sky imager at South Pole and ground‐based magnetometers) observations to simultaneously analyze the waves from the foreshock region to the dayside and nightside magnetosphere. Both of our two events show that the Pc5 ULF waves are generated by foreshock transients in the dayside magnetosphere. The in situ observations by THEMIS A and D and the 2‐D auroral signatures show that the compressional mode waves are likely broadband and coupled to the FLRs with different frequencies and different azimuthal phase speeds. This is the first report that foreshock transients can drive both low‐ and high‐m FLRs, with the azimuthal wave numbers varying from ~5 to ~23. Moreover, themore »Pc5 ULF waves propagated antisunward to midnight, this can potentially modulate magnetospheric and ionospheric dynamics globally.

    « less
  2. Abstract

    Recent studies of Pc5‐band (150–600 s) ultralow frequency waves found that foreshock disturbances can be a driver of dayside compressional waves and field line resonance, which are two typical Pc5 wave modes in the dayside magnetosphere. However, it is difficult to find spatial structure of dayside Pc5 waves using a small number of satellites or ground magnetometers. This study determines 2‐D structure of dayside Pc5 waves and their driver by utilizing coordinated observations by the THEMIS satellites and the all‐sky imager at South Pole during two series of Pc5 waves on 29 June 2008. These Pc5 waves were found to be field line resonances (FLRs) and driven by foreshock disturbances. The ground‐based all‐sky imager at South Pole shows that periodic poleward moving arcs occurred simultaneously with the FLRs near the satellite footprints over ~3°latitude and had the same frequencies as FLRs. This indicates that they are the auroral signature of the FLRs. The azimuthal distribution of the FLRs in the magnetosphere and their north‐south width in the ionosphere were further determined in the 2‐D images. In the first case, the FLRs distribute symmetrically in the prenoon and postnoon regions with out‐of‐phase oscillation as the odd toroidal mode in themore »equatorial plane. In the second case, the azimuthal wavelengths of the 350–500 s and 300–450 s period waves were ~8.0 and ~5.2 Re in the equatorial plane. It also shows a fine azimuthal structure embedded in the large‐scale arcs, indicating that a high azimuthal wave number (m~ 140) mode wave coupled with the low‐wave number FLRs.

    « less
  3. Abstract

    We analyze horizontal plasma drifts measured by the Defense Meteorological Satellite Program satellites during two intense magnetic storms. It is found, for the first time, that westward plasma flows associated with subauroral polarization streams (SAPS) in the dusk‐evening sector penetrate continuously to equatorial latitudes. The westward ion drifts between subauroral and equatorial latitudes occur nearly simultaneously. The latitudinal profile of the westward ion drifts at low latitudes (approximately within ±30° magnetic latitude [MLat]) is relatively flat, and the westward ion drifts at the magnetic equator reach 200–300 m s−1. In the dawn‐morning sector, eastward ion drifts at subauroral latitudes are also SAPS. The storm‐time dawnside auroral boundary moves to ∼±55° MLat, and the dawnside SAPS penetrate to ∼±20° MLat at 0930 local time. A dawnside SAPS flow channel appears to exist, although it is not as well defined as the duskside SAPS flow channel. Thermospheric wind data measured by the Challenging Minisatellite Payload satellite are analyzed, and zonal disturbance winds are derived. Disturbance winds can reach equatorial latitudes rapidly near midnight but are limited to ±40° geographic latitude or higher near noon. The effects of disturbance winds on the zonal ion drifts at middle and low latitudes are discussed. It ismore »suggested that both the westward ion drifts at middle and low latitudes in the dusk‐evening sector and the eastward ion drifts at middle and lower latitudes in the dawn‐morning sector are caused primarily by penetration of the SAPS and auroral electric fields.

    « less
  4. Abstract

    During the main phase of geomagnetic storms, the horizontal ground magnetic component is more depressed at dusk than at dawn, which was originally explained by the partial ring current model. Later studies questioned the idea that the ring current intensifies primarily at dusk, and the cause of this dawn‐dusk asymmetry still remains to be understood. The present study targets this old issue by statistically examining SuperMAG geomagnetic (sub‐)indices. The results are summarized as follows: (1) the dawn‐dusk asymmetry is correlated with the enhancement of the dawnside westward auroral electrojet (AEJ); (2) its correlation with the duskside eastward AEJ, which was considered by the classical partial ring current model, is insignificant; (3) the magnetic depression tends to be larger on the dayside than on the nightside, and the asymmetry is correlated with the westward AEJ in the midnight sector; (4) the dawn‐dusk asymmetry enhances typically for 1–2 h, and the day‐night asymmetry for <1 h; (5) the dawn‐dusk and day‐night asymmetries are not correlated with each other. It is suggested that the dawn‐dusk asymmetry is associated with a dawnside wedge current system with an enhanced westward AEJ (1 & 2), and it is suggested to be driven by a substorm‐like process, andmore »possibly also by enhanced global convection (4). The day‐night asymmetry is probably associated with the formation of the substorm wedge current system (3 & 4). Although the associated dawnside and midnight westward AEJs may often enhance simultaneously, they are possibly driven by different processes (4 & 5).

    « less
  5. Abstract

    During geomagnetic storms, some fraction of the solar wind energy is coupled via reconnection at the dayside magnetopause, a process that requires a southward interplanetary magnetic fieldBz. Through a complex sequence of events, some of this energy ultimately drives the generation of electromagnetic ion cyclotron (EMIC) waves, which can then scatter energetic electrons and ions from the radiation belts. In the event described in this paper, the interplanetary magnetic field remained northward throughout the event, a condition unfavorable for solar wind energy coupling through low‐latitude reconnection. While this resulted in SYM/H remaining positive throughout the event (so this may not be considered a storm, in spite of the very high solar wind densities), pressure fluctuations were directly transferred into and then propagated throughout the magnetosphere, generating EMIC waves on global scales. The generation mechanism presumably involved the development of temperature anisotropies via perpendicular pressure perturbations, as evidenced by strong correlations between the pressure variations and the intensifications of the waves globally. Electron precipitation was recorded by the Balloon Array for RBSP Relativistic Electron Losses balloons, although it did not have the same widespread signatures as the waves and, in fact, appears to have been quite patchy in character. Observationsmore »from Van Allen Probe A satellite (at postmidnight local time) showed clear butterfly distributions, and it may be possible that the EMIC waves contributed to the development of these distribution functions. Ion precipitation was also recorded by the Polar‐orbiting Operational Environmental Satellite satellites, though tended to be confined to the dawn‐dusk meridians.

    « less