Modulation of chorus wave intensity by ULF waves from Van Allen Probes Observation Lunjin Chen 1, Zhiyang Xia 1, Lei Dai 2 1 Physics Dept., The University of Texas at Dallas 2 National Space Science Center, Chinese Academy of Sciences Acknowledgement: Van Allen Probes EMFISIS, EFW, ECT teams.

Introduction on ULF modulation Location Distribution of ULF modulating chorus events from THEMIS. [Li et al., 2011] Many mechanisms on generation of magnetospheric ULF waves:  External: magnetopause surface wave; solar wind variation.  Internal instability associated with temperature anisotropy, plasma inhomogeneity, drift resonance, bounce and drift resonance. ULF waves are capable of modulating  magnetospheric VLF waves [Kimura 1974], electrons and ions [Kremer et al., 1981].  ground measurement of electron precipitations [Spanswick et al., 2005] and VLF waves [Sato et al., 1974].

The ULF modulation event, observed by Van Allen Probe B, occurs during main phase of a strong geomagnetic storm (CME-driven). 13:40-14:30 UT, Jul Probe B L=5.2 MLT=21 MLAT=-1 0 Probe A L=3 MLT=17 MLAT=-2 0 Jul 2013 We analyze a ULF event having many modulation signatures in the deep magnetosphere. Bz negative (down to -10nT) for an entire day.

Br Radial Ba Azimuthal Bp parallel Er Ea Br Ba Bp Er Ea The ULF wave is likely due to coupling of drift- mirror instability and a poloidal mode. Plasma density  Period ~2 min (Pc4-5)  parallel comp. δBp dominant ~ 10 nT  drift-mirror mode [Hasegawa 1969] plasma density variation, out of phase with δBp. δB/B~9%, δN/N~10%  Second harmonic poloidal mode [Singer et al., 1982] o Electric field node at the equator. δE/(V A δB) ~ 0.02 <<1 o δBr is finite (~1/3 δBp). Van Allen Probe B

 At density crests (Bp troughs), e- and H+ fluxes increase and peak at 90 o pitch angle. The fluxes decrease at density troughs.  The clear modulation is present for e- (0.2-6 keV) and H+ (2-51 keV).  This PAD variation can be explained by gyrokinetic theory [Takahashi et al., 1990]. Both electrons and protons are modulated by ULF wave phase. e-, 469 eV e-, 1.0 keV e-, 2.6 keV H+, 9.6 keV H+, 15.2 keV H+, 38.1 keV

VLF chorus wave excitation responds to ULF wave phase. Electric PSD Magnetic PSD Wave Normal Angle  Lower and upper band chorus intensity tends to enhance near ULF wave Bp troughs.  Linear growth rate due to observed electron distribution is consistent with chorus observation at lower band but not at upper band. fce fce/2 Ellipticity Linear Growth Rate γ/fce e-, 469 eV fce fce/2 fce fce/2

chorus fine structure from wave form near the green vertical line last slide. no clear connection between upper band and lower band chorus is found, suggesting different generation mechanism. 0.5fce fce 0.1fce 0.5fce fce 0.1fce

Summary The ULF wave event in the inner magnetosphere exhibits the coupling between drift mirror mode and second harmonic poloidal mode. The ULF wave shows many clear modulation signatures: plasma density, energetic electron and ion pitch angle distribution, and VLF chorus wave intensity. Linear growth rate is consistent with the lower band wave observation. No linear free energy is available for exciting the upper band.

The ULF modulation event is a spatially local process. Probe B L=5.2 MLT=21 MLAT=-1 0 Probe A L=3 MLT=17 MLAT=-2 0 e-, 1.0 keV H+, 15.2 keV

The ULF event does not last long (1-8 hrs). RBSP A 4 hrs before the event RBSP A 4 hrs after the event e-, 1.0 keV H+, 15.2 keV e-, 1.0 keV H+, 15.2 keV