Abstract
AbstractThe fundamental eigenfrequencies of standing Alfvén waves on closed geomagnetic field lines are estimated for the region spanning 5.9≤L < 9.5 over all MLT (Magnetic Local Time). The T96 magnetic field model and a realistic empirical plasma mass density model are employed using the time‐of‐flight approximation, refining previous calculations that assumed a relatively simplistic mass density model. An assessment of the implications of using different mass density models in the time‐of‐flight calculations is presented. The calculated frequencies exhibit dependences on field line footprint magnetic latitude and MLT, which are attributed to both magnetic field configuration and spatial variations in mass density. In order to assess the validity of the time‐of‐flight calculated frequencies, the estimates are compared to observations of FLR (Field Line Resonance) frequencies. Using IMAGE (International Monitor for Auroral Geomagnetic Effects) ground magnetometer observations obtained between 2001 and 2012, an automated FLR identification method is developed, based on the cross‐phase technique. The average FLR frequency is determined, including variations with footprint latitude and MLT, and compared to the time‐of‐flight analysis. The results show agreement in the latitudinal and local time dependences. Furthermore, with the use of the realistic mass density model in the time‐of‐flight calculations, closer agreement with the observed FLR frequencies is obtained. The study is limited by the latitudinal coverage of the IMAGE magnetometer array, and future work will aim to extend the ground magnetometer data used to include additional magnetometer arrays.
Highlights
Local excitation of a given field line by a source of wave energy at its eigenfrequency is known to result in the resonant oscillation of the field line [Dungey, 1954a, b]
In agreement with multiple previous FLR observations and time-of-flight calculations, the estimated frequency for the radial power law mass density model is observed to decrease with increasing footprint latitude, across all MLT sectors, which is a feature attributed to the magnetic field contribution, as field line length increases and magnetic field strength decreases for increasing footprint latitude [Obayashi and Jacobs, 1958; Orr and Matthew, 1971; Samson et al, 1971; Samson and Rostoker, 1972; Yumoto et al, 1983; Glassmeier et al, 1984; Poulter et al, 1984; Engebretson et al, 1986; Mathie et al, 1999; Takahashi et al, 2002, 2004; Wild et al, 2005; Plaschke et al, 2008; Liu et al, 2009; Takahashi et al, 2015]
The results show that, the range of footprint latitudes spans only a few degrees of latitude (Figure 7e), the time-of-flight calculations are generally in reasonable agreement with the average frequencies observed in the IMAGE data set
Summary
Local excitation of a given field line by a source of wave energy at its eigenfrequency is known to result in the resonant oscillation of the field line [Dungey, 1954a, b]. The local Alfvén speed is defined by the magnetic field strength and the plasma mass density. Since these values vary with radial distance, the fundamental eigenfrequency varies across the field lines, forming the Alfvén continuum. Large scale spatial variations in the frequency of standing Alfvén waves are attributed to the magnetic field configuration and the distribution of plasma mass density.
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