MMS Observations of Field Line Resonances Under Disturbed Solar Wind Conditions

Abstract

We report an observational study of magnetospheric field‐line resonances (FLRs) using multi‐point data from the Magnetospheric Multiscale (MMS) mission. We examine well‐defined FLR events in the frequency range ∼2–10 mHz observed under disturbed solar wind conditions, such as high‐speed solar wind streams and solar wind pressure pulses. We focus on their azimuthal wave numbers (m‐numbers) and spatial variations of the wave frequencies, and compare their occurrence characteristics and wave properties for insights into their energy sources. Under disturbed solar wind conditions, we have found supporting observations for both external and internal energy sources for the generation of FLRs. The FLRs associated with different energy sources appear to have different wave characteristics. Although mode coupling is very common (i.e., waves generally have both toroidal and poloidal components with variable relative amplitudes), our observations show that solar wind disturbances directly drive low‐m, toroidal mode dominated FLRs, but internal energy sources are inferred for high‐m, poloidal mode dominated FLRs. The frequencies of the toroidal and poloidal waves have different spatial variations as a function of the L‐value. The frequency of the toroidal mode waves can change continuously with L while the frequency of poloidal waves exhibits discrete spatial structure along L.Key PointsEvidence for both external and internal energy sources of field‐line resonances (FLRs) under disturbed solar wind conditionsCharacteristics of azimuthal wave numbers with different energy sourcesCharacteristic of frequency changing with L for toroidal and poloidal waves