How Do Ultra‐Low Frequency Waves Access the Inner Magnetosphere During Geomagnetic Storms?. Issue 19 (11th October 2019)

• Record Type: Journal Article
• Authors: Rae, I. Jonathan; Murphy, Kyle R.; Watt, Clare E.J.; Sandhu, Jasmine K.; Georgiou, Marina; Degeling, Alex W.; Forsyth, Colin; Bentley, Sarah N.; Staples, Frances A.; Shi, Quanqi
• Is Part Of: Geophysical research letters. Volume 46:Issue 19(2019)
• Page Start: 10699
• Page End: 10709
• Publication Date: 2019-10-11
• Subjects: ULF waves -- storm time -- radiation belt -- wave penetration; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL082395
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Wave‐particle interactions play a key role in radiation belt dynamics. Traditionally, ultra‐low frequency (ULF) wave‐particle interaction is parameterized statistically by a small number of controlling factors for given solar wind driving conditions or geomagnetic activity levels. Here we investigate solar wind driving of ULF wave power and the role of the magnetosphere in screening that power from penetrating deep into the inner magnetosphere. We demonstrate that during enhanced ring current intensity, the Alfvén continuum plummets, allowing lower frequency waves to penetrate deeper into the magnetosphere than during quiet periods. With this penetration, ULF wave power is able to accumulate closer to the Earth than characterized by statistical models. During periods of enhanced solar wind driving such as coronal mass ejection driven storms, where ring current intensities maximize, the observed penetration provides a simple physics‐based reason for why storm time ULF wave power is different compared to nonstorm time waves. Plain Language Summary: Geomagnetic storms are the most dynamic and unpredictable phenomena in near‐Earth space. During geomagnetic storms, the Van Allen Radiation Belts can be significantly enhanced, via a number of physical processes. One of these processes is the action of large‐scale ultra‐low frequency waves, which are in large part directly related to the prevailing solar wind conditions. In this study, we show that the conditions and internal structuring in near‐Earth space during a geomagnetic storm dictate how close to the Earth these large‐scale waves can reach. Through a combination of ground‐based and in situ measurements, we show how magnetic field strength and heavy ions control where these waves can access. We show that conditions both internal and external to near‐Earth space must be taken into account to understand the behavior of waves, and therefore radiation belt particle dynamics, during geomagnetic storms. Key Points: We determine the Alfvén continuum and enhancement of global ultra‐low frequency (ULF) waves during the 2013 St. Patrick's Day geomagnetic storm When the Alfvén continuum plummets, lower frequency waves are able to penetrate far deeper into the magnetosphere than expected Both solar wind and internal geomagnetic conditions must be considered for the penetration of ULF waves into the inner magnetosphere