THEMIS observations of compressional pulsations in the dawn-side magnetosphere: a case study

Abstract

Abstract. We present THEMIS-A low-and high-energy plasma, magnetic field, and energetic particle observations of long period (11-36 min) irregular compressional pulsations in the dawnside magnetosphere from 08:00 to 12:24 UT on 7 November 2007. We demonstrate that the pulsations maintain thermal and magnetic pressure balance, then employ finite gyroradius techniques to determine wave properties from the gyrophase distributions of 5-10 keV ions. The waves generally move sunward at velocities ∼10 km s −1 with the background plasma convection flow. Wavelengths range from 6700 to 23 300 km, corresponding to azimuthal wavenumbers m from 18 to 76. Wave periods decrease with increasing radial distance. Having determined the parameters describing the waves, we consider three previously proposed explanations: generation by substorm injection, generation by bounce or drift-bounce instabilities, and generation by the drift-mirror instability. The interval was quiet geomagnetically, arguing against any relationship to substorm injections. We found that ions with low energies of 69-628 eV or high energies of 28-615 keV would have been required to account for drift-bounce resonance during this interval, but inspection reveals ion fluxes at these energies near background levels during the time period considered. On the other hand, the criteria for the drift mirror instability are marginally satisfied. As predicted for the drift mirror instability, particle distributions peak more sharply near 90 • pitch angles during magnetic field strength enhancements than during strength depressions. At this point we therefore interpret the compressional pulsations observed by THEMIS A in terms of the drift mirror instability.

THEMIS observations of compressional poloidal pulsations in the dawnside magnetosphere: A case study

“…[18] We repeated the analyses of Korotova et al [2009] for the waves presented in this study and (in general) obtained similar results. By timing particle flux enhancements seen by duskward and dawnward looking sensors, we determine when flux enhancements reach locations 1 Rgyro (gyroradius) antisunward and sunward from the spacecraft and therefore the sunward component of the injection velocity, V = 2 Rgyro/ Figure 6.…”

“…There are several ways to calculate their velocity. Korotova et al [2009] described how the characteristics of compressional pulsations can be determined from magnetic field and gyrophase observations. From top to bottom, Figure 5 presents THEMIS D ESA 7-12 keV ion gyrophase distributions integrated over 45°-135°, ion pitch angle distributions, the FGM magnetic field, and ESA ion number density moment from 1740 to 1845 UT.…”

“…Korotova et al [2009] employed Thermal Emission Imaging System (THEMIS) A observations of long-period compressional pulsations in the dawnside magnetosphere from 1000 to 1330 UT on 7 November 2007 to test the three proposed explanations and showed that the wave characteristics and behavior of the energetic ions were most consistent with predictions for the drift mirror instability. Sibeck et al [2012] then showed that the characteristics of the pulsations originally reported by Korotova et al [2009] agreed well with the predictions of the kinetic-magnetohydrodynamic model [Cheng and Qian, 1994] for the linear magnetic field, electric potential, and current perturbations expected during the course of mixed balloon and mirror mode oscillations in a dipole magnetic field geometry.…”

Observations of a Pc5 global (cavity/waveguide) mode outside the plasmasphere by THEMIS

“…Finally, overall enhancements or decreases in wave energy ought to be associated with significant changes in the ion population, as would occur during a substorm injection or the recovery phase of a geomagnetic storm [e.g., Yang et al , 2010]. In the first panel in Figure 10, we show an ion energy flux spectrogram generated by the ESA and SST instruments on THD that includes energy ranges typically considered for drift bounce resonance [e.g., Korotova et al , 2009]. We find that there is a general decrease in the ion energy flux with time, with one peak occurring at ∼4 keV.…”

Van Allen Probe observations of drift-bounce resonances with Pc 4 pulsations and wave–particle interactions in the pre-midnight inner magnetosphere

“…Toroidal pulsations have been attributed to a wide variety of mechanisms, including field line resonances triggered by abrupt changes and sudden impulses in the solar wind dynamic pressure (e.g., Zhang et al, 2010;Sarris et al, 2010), the Kelvin-Helmholtz instability (KHI) at the magnetopause (e.g., Pu and Kivelson, 1983;Fujita et al, 1996), and bursty reconnection and the generation of flux transfer events on the magnetopause (Gillis et al, 1987). Poloidal pulsations have also been attributed to a number of mechanisms, most notably drift-bounce resonances, ballooning modes, and drift mirror mode instabilities (Hasegawa, 1969;Lanzerotti et al, 1969;Hughes et al, 1979;Engebretson et al, 1992;Korotova et al, 2009;Liu et al, 2013). Arthur and McPherron (1981) presented a statistical study of 215 magnetic pulsation events identified on the basis of waveform and period as Pc 4 (45 to 150 s period) observed at synchronous orbit by ATS 6.…”