Modeling the Dawn/Dusk Asymmetry of Field Line Resonances

Wright, Andrew N.; Elsden, T.; Takahashi, Kazue

doi:10.1029/2018ja025638

Cited by 16 publications

(29 citation statements)

References 33 publications

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“…The results shown here demonstrate a decrease in the eigenfrequency with increased levels of geomagnetic activity, which suggests that during storm times the compressional waves can couple to field lines with lower footprint latitudes values and excite higher amplitudes (Degeling et al, ). Furthermore, the local time asymmetry in eigenfrequency and how this asymmetry changes with ring current intensity has important implications on the polarization of the Alfvén waves as well as the characteristics of FLRs (Elsden & Wright, ; Kabin et al, ; Wright et al, ). The MLT asymmetries are further demonstrated in Figure S1 in the supporting information, where we reproduce Figures a–d with line contours as opposed to a color contour.…”

Section: Discussionmentioning

confidence: 99%

Variations of Field Line Eigenfrequencies With Ring Current Intensity

2018

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We present results from the closed magnetosphere (5.9 ≤ L < 9.5 over all magnetic local times) to demonstrate and assess the variations in field line eigenfrequency with geomagnetic activity. Using the time-of-flight technique with realistic magnetic field and mass density models, the spatial distributions of field line eigenfrequencies are determined for a range of different geomagnetic activity levels, as defined by the Dst index. The results indicate that during geomagnetically active conditions, the eigenfrequency of a given field line is generally decreased compared to quiet times, in addition to variations in local asymmetries. By comparing the dependence to changes in the magnetic field and mass density distribution, it is established that the inflation and weakening of the geomagnetic field outweighs decreased plasma mass density and is the sole contributor to decreased eigenfrequencies with increased geomagnetic activity. We highlight the importance of considering the magnetic field, mass density, and average ion mass contributions when using observed eigenfrequencies to probe magnetospheric conditions. Furthermore, the estimates significantly improve upon existing time-of-flight results, through a consideration of mass density changes with geomagnetic activity. We also provide estimates of eigenfrequencies for a comparatively extended spatial region than available from prior direct observations of field line resonances. The results have clear implications for furthering our understanding of how wave energy propagates throughout the magnetosphere during geomagnetic storms. Plain Language SummaryThe Earth's magnetic field experiences resonant oscillations of individual magnetic field lines at discrete frequencies, known as eigenfrequencies, which play an important role in transporting energy throughout the Earth's space environment. The frequencies of these oscillations can be highly variable, and a key factor in this variability is the strength of the geomagnetic ring current. In this study, we employ realistic models describing the magnetic field configuration and the spatial distribution of the plasma mass density, which also account for variations with ring current intensity. This allows us to estimate how the eigenfrequencies vary for different levels of ring current strength. We explore changes in the magnitude as well as the spatial distribution. A key result is an observed decrease in the magnitude of the eigenfrequency with increased ring current strength. This result has important implications for understanding how energy can access regions closer to the Earth and consequently act to significantly energize the plasma.

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Section: Discussionmentioning

confidence: 99%

Variations of Field Line Eigenfrequencies With Ring Current Intensity

2018

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“…Consequently, these waves do not drive as significant a resonant response as those at noon. The red lines indicate a wave which has initially propagated toward noon, but due to refraction ends up propagating toward dusk, as also seen in Wright et al (2018). The right-hand panel deals with an antisymmetrically driven waveguide and will be discussed further in the following section.…”

Section: Changing the Driver Locationmentioning

confidence: 93%

“…In this work, we are interested in the structure of fast waveguide modes and how their structure affects the FLRs that they drive, particularly in a 3‐D inhomogeneous magnetospheric equilibrium. Understanding the structure of the fast mode which develops in an asymmetric equilibrium about noon has been used recently to explain the dawn‐dusk asymmetry of Pc5 wave observations (Wright et al, ). Further, the inclusion of azimuthal magnetic and density asymmetries has also been used to consider the effect of a plasmaspheric plume on ULF wave propagation (Degeling et al, ).…”

Section: Introductionmentioning

confidence: 99%

The Effect of Fast Normal Mode Structure and Magnetopause Forcing on FLRs in a 3‐D Waveguide

Elsden

Wright

2019

JGR Space Physics

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This paper investigates the excitation of waveguide modes in a nonuniform dipole equilibrium and, further, their coupling to field line resonances (FLRs). Waveguide modes are fast compressional ultralow frequency (ULF) waves, whose structure depends upon the magnetospheric equilibrium and the solar wind driving conditions. Using magnetohydrodynamic simulations, we consider how the structure of the excited waveguide mode is affected by various forms of magnetopause driving. We find that the waveguide supports a set of normal modes that are determined by the equilibrium. However, the particular normal modes that are excited are determined by the structure of the magnetopause driver. A full understanding of the spatial structure of the normal modes is required in order to predict where coupling to FLRs will occur. We show that symmetric pressure driving about the noon meridian can excite normal modes which remain around to drive resonances for longer than antisymmetric driving. Further, the critical quantity in terms of efficient coupling is the magnetic pressure gradient aligned with the resonance.

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“…The model used has been described at length in many previous studies (e.g., Elsden & Wright, 2017, 2018, 2019; Wright & Elsden, 2016; Wright et al, 2018) and will therefore only briefly be summarized here.…”

Section: Numerical Modelmentioning

confidence: 99%

Evolution of High‐m Poloidal Alfvén Waves in a Dipole Magnetic Field

Elsden

Wright

2020

JGR Space Physics

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We investigate how initially high‐m, poloidal Alfvén waves evolve using a numerical model solving the ideal, cold, linear magnetohydrodynamic (MHD) equations in a 2‐D dipole coordinate system. The curved magnetic geometry provides a key difference between the poloidal and toroidal Alfvén frequencies of any one field line. A polarization rotation from poloidal toward toroidal predicted from the Cartesian box model theory still occurs but now with the waves following contours of Alfvén frequency, which moves the Alfvén wave across field lines. The structure of these contours depends on the harmonic mode along the field line and the equilibrium. We find that the amplitude peak of the poloidal mode moves significantly radially outward in time. When the typically observed azimuthal phase motion of such waves is included, hodograms show a polarization rotation from purely poloidal to a mixed poloidal/toroidal polarization at all locations. Such features could be used to help interpret satellite observations of Pc4‐5 poloidal ultralow frequency (ULF) waves in Earth's magnetosphere.

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Modeling the Dawn/Dusk Asymmetry of Field Line Resonances

Cited by 16 publications

References 33 publications

Variations of Field Line Eigenfrequencies With Ring Current Intensity

Variations of Field Line Eigenfrequencies With Ring Current Intensity

The Effect of Fast Normal Mode Structure and Magnetopause Forcing on FLRs in a 3‐D Waveguide

Evolution of High‐m Poloidal Alfvén Waves in a Dipole Magnetic Field

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