We found a match
Your institution may have rights to this item. Sign in to continue.
- Title
Magnetosonic ULF Waves With Anomalous Plasma–Magnetic Field Correlations: Standing Waves and Inhomogeneous Plasmas.
- Authors
Archer, M. O.; Southwood, D. J.; Hartinger, M. D.; Rastätter, L.; Nykyri, K.
- Abstract
Ultra‐low frequency (ULF) wave observations across the heliosphere often rely on the sign of correlations between plasma (density/pressure) and magnetic field perturbations to distinguish between fast and slow magnetosonic modes. However, the assumptions behind this magnetohydrodynamic result are not always valid, particularly within the magnetosphere which is inhomogeneous and supports standing waves along the geomagnetic field. Through theory and a global simulation, we find both effects can result in anomalous plasma–magnetic field correlations. The interference pattern in standing waves can lead both body and surface magnetosonic waves to have different cross‐phases than their constituent propagating waves. Furthermore, if the scale of gradients in the background are shorter than the wavelength or the waves are near‐incompressible, then advection by the wave of inhomogeneities can overcome the wave's inherent sense of compression. These effects need to be allowed for and taken into account when applying the typical diagnostic to observations. Plain Language Summary: Fluid plasma wave theory provides a key distinguishing feature between the two compressional wave modes: fast magnetosonic waves should have correlated fluctuations in the magnetic field strength and the plasma (density or pressure) whereas anticorrelation relates to the slow magnetosonic mode. This classic result is often used as a diagnostic for waves observed by spacecraft throughout the heliosphere. However, it is important to recognize that this result is derived under the assumption of a single traveling wave in a uniform background plasma. Planetary magnetospheres, in contrast, have spatially‐varying plasma conditions and can reflect waves off of boundaries to form standing modes. We investigate what influence these effects have on both freely propagating waves and those tied to a surface through analytic theory and a global magnetospheric simulation. We find it is possible for the wave's correlation between plasma and magnetic field to be fundamentally altered. For standing waves we show the interference pattern present can lead to this change, whereas for non‐uniform plasmas changes are due to the wave moving plasma with a significantly different background value. These effects need to be allowed for and taken into account when applying the typical plasma–magnetic field test to observed waves. Key Points: We test if the standard plasma–magnetic field relations for magnetosonic waves are altered by standing structure or inhomogeneous plasmaThe interference pattern present in standing (body or surface) waves can lead to anomalous results under certain plasma conditionsAdvection by the wave of plasma inhomogeneities can also result in reversed correlations if gradients are sufficiently large
- Subjects
INHOMOGENEOUS plasma; PLASMA waves; STANDING waves; MAGNETIC flux density; LONGITUDINAL waves; WAVES (Fluid mechanics); GEOMAGNETISM
- Publication
Geophysical Research Letters, 2023, Vol 50, Issue 16, p1
- ISSN
0094-8276
- Publication type
Article
- DOI
10.1029/2023GL104762