THEMIS observations of electromagnetic ion cyclotron wave occurrence: Dependence on AE, SYMH, and solar wind dynamic pressure

Abstract

[1] Electromagnetic ion cyclotron (EMIC) waves are transverse plasma waves generated by anisotropic proton distributions with T perp > T para . They are believed to play an important role in the dynamics of the ring current and potentially, of the radiation belts. Therefore it is important to know their localization in the magnetosphere and the magnetospheric and solar wind conditions which lead to their generation. Our earlier observations from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes demonstrated that strong magnetospheric compressions associated with high solar wind dynamic pressure (P dyn ) may drive EMIC waves in the inner dayside magnetosphere, just inside the plasmapause. Previously, magnetospheric compressions were found to generate EMIC waves mainly close to the magnetopause. In this work we use an automated detection algorithm of EMIC Pc1 waves observed by THEMIS between May 2007 to December 2011 and present the occurrence rate of those waves as a function of L-shell, magnetic local time (MLT), P dyn , AE, and SYMH. Consistent with earlier studies we find that the dayside (sunward of the terminator) outer magnetosphere is a preferential location for EMIC activity, with the occurrence rate in this region being strongly controlled by solar wind dynamic pressure. High EMIC occurrence, preferentially at 12-15 MLT, is also associated with high AE. Our analysis of 26 magnetic storms with Dst < À50 nT showed that the storm-time EMIC occurrence rate in the inner magnetosphere remains low (<10%). This brings into question the importance of EMIC waves in influencing energetic particle dynamics in the inner magnetosphere during disturbed geomagnetic conditions. Citation: Usanova, M. E., I. R. Mann, J. Bortnik, L. Shao, and V. Angelopoulos (2012), THEMIS observations of electromagnetic ion cyclotron wave occurrence: Dependence on AE, SYMH, and solar wind dynamic pressure,

Electromagnetic cyclotron waves in the dayside subsolar outer magnetosphere generated by enhanced solar wind pressure: EMIC wave coherency

“…The outer zone, local noon EMIC waves reported here are consistent with a solar wind pressure pulse generation mechanism accelerating preexisting ∼10-100 keV protons in E ⟂ with consequential instability generating the waves [Olson and Lee, 1983;Anderson and Hamilton, 1993;Tsurutani et al, 2001;Engebretson et al, 2002;Usanova et al, 2012]. The waves shown in this paper lacked obvious rising tone structures.…”

“…EMIC wave properties and their external causes have been previously reported by Anderson and Hamilton [1993], Engebretson et al [2002], and Usanova et al [2012]. The compression of preexisting ∼10-100 keV electrons and protons induces a temperature anisotropy T ⟂ > T ∥ for both particles, and hence, simultaneous EMIC and chorus waves occur.…”

Resonant scattering of outer zone relativistic electrons by multiband EMIC waves and resultant electron loss time scales

“…While the intensity of EMIC waves varies with the level of geomagnetic activity and the spatial location [e.g., Usanova et al, 2012;Min et al, 2012;Meredith et al, 2014], in the present study, for simplicity, we have adopted a nominal intermediate amplitude of 1 nT for all three bands of EMIC waves. This value is consistent with the AMPTE survey results that the average power spectral density typically lies between 1 and 10 nT 2 /Hz [Anderson et al, 1992a[Anderson et al, , 1992b with a peak-to-peak wave amplitude of about 1.6 nT and between the CRRES survey results and the model results used by Kersten et al [2014].…”

“…Using the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer data, Anderson et al [1992a] reported that the occurrence rate of EMIC waves increases monotonically with L in the region L = 3-9, peaking at 10%-20% in the spatial region at 11:00-15:00 MLT within L = 7-9. Analyzing the Time History of Events and Macroscale Interactions Flux Gate Magnetometer data with an automated EMIC Pc1 wave detection algorithm, Usanova et al [2012] found that, in general, the EMIC wave occurrence rate increases with L in the dawn, noon, dusk, and midnight sectors, showing the highest occurrence rate (5%-8%) in the noon and dusk sectors and reaching its maximum at L~9 (see Figure 4 of Usanova et al [2012]). Such an MLT dependence of EMIC wave occurrence is consistent with the westward drift of energetic ions, which are commonly regarded as the free-energy source population for EMIC wave excitation.…”

Model of electromagnetic ion cyclotron waves in the inner magnetosphere

“…The rate of the electron loss is not simply controlled by the wave intensity but crucially depends on the wave spectral distribution [Glauert and Horne, 2005], and a properly specified global distribution of the EMIC wave power spectral density (PSD) is needed to accurately describe wave-particle interactions on the global magnetospheric scale throughout the 10.1002/2014JA020032 different storm phases. However, despite a number of statistical and case studies of EMIC waves [e.g., Anderson et al, 1992aAnderson et al, , 1992bFraser et al, 2010;Halford et al, 2010;Clausen et al, 2011;Usanova et al, 2012;Min et al, 2012], there is still no reliable global and dynamic model of the EMIC wave PSD that could be incorporated in the model of relativistic electrons in the outer RB. Currently, the EMIC wave PSD is poorly specified in all models of the outer RB relativistic electrons.…”