南極昭和基地で観測されたSI時における数十keV
電子降下現象とVLF応答

Hiroyasu TADOKORO (1)Y.Miyoshi(3) H.Yamagishi(2) H. Miyaoka(2)Y. Tanaka(2)(1) Tohoku University	 (2) National Institute of Polar Research (NIPR)	 
(3) Nagoya University	 
1	Abstract
The relationship between dayside chorus emissions and energetic electron
precipitation with several tens of keV at a sudden impulse are examined. We examine
the spatiotemporal evolution of CNA and VLF emissions observed at Syowa Station(-69.00 S, 39.58 E, and L = 6.1) in Antarctica and electron temperature anisotropyobtained from LANL associated with sudden impulses from 1999 to 2007. The
detailed analyses of variations focusing on 12 MLT that VLF at 2.0 kHz always
increases at the sudden impulses. However, only 44% of events (7 of 16) show clear
enhancements of CNA and the remaining 56% of events do not show enhancements.
The CNA enhancement events suggest that the compression of the daysidemagnetosphere cause the enhancement of temperature anisotropy of hot electrons in
the dayside, which includes the enhancement of whistler mode waves as well as theenhanced precipitation of energetic electrons. The cause of no enhancement events ofCNA is probably due to absence of energetic electrons with several tens of keV to ahundred keV, while tens keV electrons that cause the VLF wave excitations shouldexist. It is suggested that resonance at the off-equator between the VLF waves and 30– 100 keV electrons contributes to the electron precipitation at several tens of keV,which can be observed as the intensification of CNA, while the resonance near themagnetic equator likely produces the enhancement of the dayside aurora by theprecipitation of several keV electrons.	
2	  Energetic electron precipitation by W-P interactions
Energetic ele. precipitaion into the atmosphere is
one of the most important loss processes of themagnetpsheric energetic ele.
! Dawn chorus(strong dependence of geomagnetic activity)(e.g., Lam et al., 2010)	

! Dayside chorus (this study)
・dependence of geomagnetic activity @ equator・independence of geomagnetic acvtivity @off-equator
Minimum-B pockets(Polar chorus)	
(Lam et al., 2010)	

(Tsurutani and Smith, 1979)	

(Kawashima and Fukushima, 1964)	

3	  Previous studies (Response to the solar wind)・Salvati et al., 2000… the solar wind dynamic pressure controls the dayside chorus
emissions which were observed at the ground –based AGO stations(L>8).
・Meurant et al., 2003… electron and proton auroral emissions obtained by
IMAGE-FUV during a sudden increase of solar wind pressure start to intensify at thepost noon sector and expand to the night side for ~ 6 minutes.

・Fu et al., 2000… Chorus emissions observed by
THEMIS sat. are intensified in response to a sudden
impulse.
・Gail et al., 1990… 12 events of 16 events at thedayside showed that riometer absorption and VLF
enhancement are associated with sc.
Observed two ground-stations is higher lat. thanSyowa station.

(Meurant et al., 2003)	

4	  Purpose
We investigate statistically dayside VLF wave and CNA activities
associated with sudden impulses using the long-term data at Syowa
Station, Antarctica (-69.00 S, 39.58 E, and L = 6.1). Moreover, we
examine the temperature anisotropy of hot electrons in themagnetosphere, which drives the whistler mode wave excitation, usingthe Los Alamos National Laboratory (LANL) satellite data.

Specific advantage to the previous studies☆The relationship of variations between CNA and VLF at the dayside
associated with sudden impulses is not clear.
ー＞to examine responses of CNA and VLF to a scale of solar winddynamic pressure.
☆To statistically examine electron temperature anisotropies and VLF
intensities as function of MLT associated with sudden impulses.
5	  Data(1999-2007)! Syowa station @L=6.1! CNA(Cosmic Noise Absorption) data・alternative measure of several tens ofkeV ele. precipitation! ELF/VLF data (9 ch.)Smith et al., 2004

・（350Hz, 750Hz, 1.2kHz, 2.0kHz, 4.0kHz, 8.0kHz, 30kHz, 60kHz, 95kHz）
0.5fce~1.9kHz (lower band chorus) , 750Hz-2.0kHz are used in this study.

! WDC2! SYM-H! LANL@L=6.6! MPA data, electron temperature (.03-45keV)! OMNI-2! Solar wind parameters

6	Data analysis(1999-2007)! Superposed epoch analysis
Y1999-2007
SI(198 events)
・Restriction of the events○Available data set in the ELF/VLF receiver at Syowa Station○SYM-H > -10nT during the period from -30 to 60 min.
○Enhancements of SYM-H of > 5nT for 10 min.
○without storm main and recovery phases

7	750Hz

Statistical result(1999-2007: 198 SI:median)

1.2kHz! 0 epoch time--->SYM-H max.

2.0kHz! IMF Bz>0Tperp/
Tpara! VLF depends on freq.

Vx! Temp. anisotropy

IMF BzDayside > Nightside
Duration ~20min.

Psw--->focus on 12 MLT

SYM-H
8	

12 MLT(16 events)
Thick lines: median values
Thin lines: each event	! 7 events (red lines) →(Discussion 1,2)-->enhancements of CNA (44%)
750Hz->enhancement ?
1.2kHz, 2.0kHz  enhancements
The intensities at three freq. become weakassociated with the enhancements after theVLF max.
Absorption effect of VLF [Smith et al., 2004;
Ozaki et al., 2009]! 9 events (blue lines) →(Discussion 3)-->NO enhancements of CNA (56%)
750Hz->enhancement ?
1.2kHz, 2.0kHz  enhancements☆variation of Psw
9	CNA enhancements > NO CNA enhancements

Discussion, Summary! We have examined spatiotemporal evolution of CNA, VLF emissions,and temp. anisotropy.
・Those evolutions at 12 MLT were almost correlated with the suddenimpulse.
・The maximum CNA and the temperature anisotropy evolution haveweak correlation with the SYM-H, i.e., the solar wind dynamicpressure. On the other hand, the dependences of the VLF waves are
not clear on the dynamic pressure, and each frequency show thedifferent variations.
Energetic ele. precipitation at the dayside due to Psw enhancement.

=>The results are consistent with the event study by Salvati et al. [2000], at theAGO stations (L>8).

10	

Discussion1 (Wave-particle interactions)! Dayside energetic electron precipitation through w-p interactions[e.g., Tsurutani et al., 2001]

1. Compression of the dayside magnetosphere due to an increase in the solar winddynamic pressure.	
2. Intensification of the electron temperature anisotropy by adiabatic acceleration.	
->enhancement of dayside electron temperature anisotropy observed by LANL in
statistical result.
3. Intensification of VLF emissions in the magnetosphere due to the instability of theelectron temperature anisotropy distribution.	
->enhancement of VLF emissions at (750Hz,) 1.2kHz, and 2.0kHz observed at
Syowa station.
4. Energetic electron precipitation into the atmosphere through wave-particle
interactions with the enhanced VLF emissions.
->enhancement of CNA (6 events of 15 events, 40%) observed at Syowa station.	
11	

Discussion2 (Resonance condition)! Resonance conditions@equator
1keV~12keV(12keV with 750Hz, 5keV with 1.2kHz, 1keV with 2kHz)
dayside aurora@off-equator
13-28 mag. Lat.(30keV~100keV with 750Hz-2kHz)CNA
2B wce ⎛w ⎞⎜⎜1 −⎟⎟ER =2µ0 N w ⎝ wce ⎠3(Kennel and Petschek, 1966)	

ER: Parallel resonant electron energy	

B: Ambient magnetic field	

µ0: Permeability of a vacuum	

N: Plasma density [Sheeley et al., 2000]	

w: Frequency of whistler mode waves	

wce : Local cyclotron frequency	

12	

Discussion3 (NO CNA enhancement)! Possible mechanism B: effect of adiabatic heating☆variation of Psw during the sudden impulses
CNA enhancements(0.232nPa/min.) > NO CNA enhancements(0.084nPa/min.)⇒ Adiabatic heating(assumption: conservation of 1st. adiabatic invariant: Tpara. = no variation during the sudden impulse)
variation of Tperp./Tpara.
@CNA enhancements-> 1.13 times (if 1.55 before SI, then 1.75)@NO CNA enhancements -> 1.06 times (if 1.55 before SI, then 1.64)☆ observation of Tperp./Tpara. at the daysideobserved Tperp./Tpara. at the before the sudden impulse -> 1.55
observed enhancement of Tperp./Tpara. associated with the sudden impulse -> 1.7	13	

Conclusion

14