Variations of Field Line Eigenfrequencies With Ring Current Intensity

Abstract

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.

How Do Ultra‐Low Frequency Waves Access the Inner Magnetosphere During Geomagnetic Storms?

“…It is important to remember that when using a proxy such as Dst index, two very different intervals are averaged, decreasing Dst during the main phase and increasing Dst during the recovery phase even though both phases pass through the same values of Dst. However, Sandhu, Yeoman, and Rae () model provides useful context for interpreting our results. We now consider the role of the ring current itself in reducing the Alfvén continuum in the inner magnetosphere.…”

“…Certainly heavy ions must play a role. However, Sandhu, Yeoman, and Rae () constructed a statistical model of the average mass densities as a function of Dst index. Sandhu, Yeoman, and Rae () found that although the average ion mass did increase significantly with increasingly negative Dst index, the electron densities in the inner magnetosphere reduced.…”

“…Hence, on average, lower Dst index values reduce the plasma mass density, rather than increasing it as previously thought. Sandhu, Yeoman, and Rae () concluded that the changes in the magnetic field drove the changes in eigenfrequency; during sudden increases in dayside compression, the geomagnetic field strength in the outer magnetosphere increases across the dayside. It is important to remember that when using a proxy such as Dst index, two very different intervals are averaged, decreasing Dst during the main phase and increasing Dst during the recovery phase even though both phases pass through the same values of Dst.…”

ULF Wave Driven Radial Diffusion During Geomagnetic Storms: A Statistical Analysis of Van Allen Probes Observations

“…The assumption of a dipole magnetic field configuration may also contribute to discrepancies. During the main phase of a geomagnetic storm, the enhanced ring current significantly distorts and inflates the magnetic field configuration in the inner magnetosphere and field lines with a given footprint latitude can map to larger L values than during non‐storm periods (Ganushkina et al., 2010; Sandhu et al., 2018). The electric field power on the dayside tends to increase with L so the magnetometer measurements would be overestimating values.…”

“…Studies, in particular Wharton et al (2020), show that Eigen frequencies across a large range of L values are significantly depressed during the main phase of the storm. This is attributed to a weakening of the local magnetic field strength due to the magnetic field perturbations associated with an enhanced ring current population (Jorgensen et al, 2004;Sandhu et al, 2018), as well as an increase in the local plasma mass density due to enhanced concentrations of heavy ions (e.g., Kale et al, 2009;Kistler & Mouikis, 2016;Sandhu et al, 2017;Yue et al, 2019). Furthermore, studies such as Kale et al (2009), Katus et al (2015) show that during the main phase of the storm the plasmapause boundary moves to lower L values.…”