EMIC Waves in the Outer Magnetosphere: Observations of an Off‐Equator Source Region. Issue 11 (11th June 2019)

• Record Type: Journal Article
• Authors: Vines, S. K.; Allen, R. C.; Anderson, B. J.; Engebretson, M. J.; Fuselier, S. A.; Russell, C. T.; Strangeway, R. J.; Ergun, R. E.; Lindqvist, P. A.; Torbert, R. B.; Burch, J. L.
• Is Part Of: Geophysical research letters. Volume 46:Issue 11(2019)
• Page Start: 5707
• Page End: 5716
• Publication Date: 2019-06-11
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL082152
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Electromagnetic ion cyclotron (EMIC) waves at large L shells were observed away from the magnetic equator by the Magnetospheric MultiScale (MMS) mission nearly continuously for over four hours on 28 October 2015. During this event, the wave Poynting vector direction systematically changed from parallel to the magnetic field (toward the equator), to bidirectional, to antiparallel (away from the equator). These changes coincide with the shift in the location of the minimum in the magnetic field in the southern hemisphere from poleward to equatorward of MMS. The local plasma conditions measured with the EMIC waves also suggest that the outer magnetospheric region sampled during this event was generally unstable to EMIC wave growth. Together, these observations indicate that the bidirectionally propagating wave packets were not a result of reflection at high latitudes but that MMS passed through an off‐equator EMIC wave source region associated with the local minimum in the magnetic field. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are a fundamental plasma instability in space environments. In near‐Earth space, these waves act as one mechanism for energetic electrons in the radiation belts to be lost to the atmosphere. Because EMIC waves are important for the transport of energy throughout the magnetosphere, understanding where and how these waves are generated, as well as how the waves move along a magnetic field line, is necessary for understanding the full cycle of energization and loss of plasma. The two previous case studies of EMIC waves at high latitudes in the outer magnetosphere were not able to determine if the waves were generated at those high latitudes or if the wave signatures were due to reflection of the waves back toward the magnetic equator, which has important implications for waves seen from the ground. The observations presented here show EMIC waves in the outer magnetosphere away from the equator nearly continuously over several hours. Using the wave Poynting flux direction (which indicates how the waves are moving along the magnetic field), we show unambiguously for the first time that these EMIC waves are from a local source region at higher latitudes. Key Points: Several hours of EMIC wave activity were observed off‐equator in the outer magnetosphere with plasma conditions favorable for local growth Changes in direction of the wave Poynting vector indicate transition of source region from poleward, to local, to equatorward of spacecraft Observations confirm association of EMIC wave source region with local minimum‐B of the field line, possibly related to Shabansky orbits