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Adv. Space Res. Vol. 27, No. 8, pp. 1429-1432, 2001 © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved

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HIGH LATITUDE Pi3 PULSATIONS OBSERVED BY THE EISCAT VHF RADAR

T. Nagatsuma 1, S. Nozawa 2, S. C. Buchert 2, and R. Fujii 2

I Hiraiso Solar Terrestrial Research Center, CRL, 3601 lsozaki, Hitachinaka, lbaraki 311-1202, JAPAN

2Solar-Terrestrial Environment Laboratory, Nagoya Univ., Furo-cho, Chikusa-ku, Nagoya 464-8601, JAPAN

ABSTRACT

Long period irregular magnetic pulsations, Pi3, are found from EISCAT/VHF radar observations at the high latitude in the dawn sector on Sept. 29, 1996. From the comparison between the radar and ground magnetometer data, it is suggested that the meandering of the auroral electrojet causes these pulsations. We have found that this meandering structure propagates duskward. The propagation velocity is estimated to be about 700 m/s. The scale of meandering structure is about 1000 km. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION

In the case of northward IMF, quasi-periodic phenomena are observed at the morning sector. From observation by all-sky camera, Shiokawa et al. (1996) reported that quasi-periodic sun-aligned arcs moving duskward in the

morningside polar cap region. The velocities of the arcs are 400-500 m/s. Spatially quasi-periodic plasma signature were observed by the Akebono and DMSP satellites (Hirahara et al., 1998). Highly structured ionospheric convections are observed by DE-2 satellite (Taguchi et al., 1995). Ohtani et al. (1994) examined quasi-periodic, meandering auroral electrojets propagating westward, and also northward, using Freja and ground station data. While these studies st,: .~sted that wavy structures at the dawnside flank caused by the Kelvin-Helmholtz instability are the origin of these quasi-periodic phenomena, the global structure and time evolution of these phenomena is still unknown. To study the global dynamics of the geomagnetic field variations and the plasma convection corresponding to these quasi-periodic phenomena, the dawnside high latitude Pi3 observed on Sept. 29,

1996 are analyzed in this work.

INSTRUMENTATION

The observations presented in this paper were made using the EISCAT VHF radar and the IMAGE magnetometer network. The locations of the IMAGE Magnetometer stations used in this paper and the Common Program Four

(CP-4) pointing directions are shown in Figure 1.

The data from the VHF radar were obtained using the CP-4-B experiment. The concept of the experiment is essentially the same as the UK-POLAR experiment described by van Eyken et al. (1984). In this experiment the radar beams point at a low elevation (el. = 30 deg.) far to the north. While the UK-POLAR experiment measures two directions mutually using a beam-swinging technique with the UHF radar, the CP-4-B experiment measures

1429

1430 T. Nagatsuma et al.

two directions independently using two simultaneous beams from the VHF radar. Therefore, the CP-4-B experiment is better suited to observe high latitude ionospheric convections and polar cap phenomena with high time resolution.

The IMAGE magnetometer network consists of 15 ground-based magnetometers covering the range from sub-anroral to polar cap region. A detailed description of the IMAGE magnetometer network was presented by Luhr et ah (1998). Six high latitude stations data (SOR, BJN, HOP, HOR, LYR, NAL) are used in this paper.

OBSERVATION

The IMAGE Magnetometer Network

Figure 2 shows three components of the magnetic field variations observed by the IMAGE magnetometer network on Sept. 29, 1996. X, Y, and Z components are defined as geographical northward, eastward, and downward, respectively. Irregular magnetic field fluctuations, Pi3, with period 30-40 minutes are observed between 01:20 and 04:20 UT. They occur in the morning sector since the magnetic local time of this meridian is about UT+3 hour. The amplitude of the Pi3 becomes larger with time. The occurrence of the Pi3 is limited to higher latitude stations from BJN to NAL, Pi3 is not observed at the lowest latitude station, SOR. The X and Y components of Pi3 show anti-correlation. The phase delay of the Pi3 increases with increasing latitude. However, no phase difference is seen between BJN and HOP.

Equivalent current plots are shown in Figure 3. The vectors at the top of this panel show the sun-aligned direction. As suggested from Figure 2, the equivalent current flows northeastward and southwestward, and the flow directions are close to the sun-aligned. The characteristics of the current flow transition show a sheer rather than a vortex.

Solar Wind

The solar wind conditions observed by the WIND satellite during the occurrence of the Pi3 are Bx<0, By-0, and Bz-0, respectively. Solar wind velocity is about 470 km/s. Number density is 10/cc. (These data are not shown here.) There is no significant variation of dynamic pressure during this period.

The EISCAT VHF Radar

Figure 4a shows the line-of-sight ion drift velocities observed by the EISCAT VHF radar. The seven gates close to the radar site are shown. Thick and thin lines show the northward beam (AZ.=0 deg.) and westward beam (Az.=-I 5 deg.), respectively. Positive velocity means that the plasma moves towards the radar site. Pulsations in the ion drift velocities are enhanced earlier than those in the magnetic field variations. The phase delay of the Pi3 increases with increasing latitude, in the same way as the magnetic field observations. Comparing the northward and westward beams, the drift velocity pulsations of the westward beam has a phase delay. A comparison between the third gate of CP-4 and the Y component of magnetic field at BJN, the footpoint of the third gate, is shown in Figure 4b. It is apparent that the variations correspond well. The ratio of the drift velocities to the magnetic field variations depends on the local time.

DISCUSSION AND SUMMARY

These Pi3 pulsations can be interpreted as a meandering of the auroral electrojet. Since the line-of-sight ion drift

velocities are well correlated with the orthogonal component of magnetic field, it is suggested that these pulsations

are caused by sheer flows of the Hall current. There is no significant magnetospheric compression due to the solar

High Latitude Pi3 Pulsations 1431

E I S C A T C P - 4 & I M A G E Magnetometer Chain

o . .... ........... ..... , o

8O

78

76

-a 74

8 70

68

66

EQUIVALENT CURRENT PLOT 50nT/div. 1996/09/29

-LYR ~ ~ ~ ~I

~HOR ~ . . . . , , . , , . _ ~ ~ . , _ , ~ _ ~ =

~JN X

_ _ Y SON . . . . . . . . . . • .JJ~ww.,i,.,- . . . . . ' 7r~ ' r ~rzr,." " . , ~ . . . . . . . .

1 2 3 4 Time(UT)

Fig. 1 Geographic locations of IMAGE magnetometer stations used in this paper and pointing directions of the CP-4 experiment.

Fig. 3. Equivalent current vector plot of Pi3 pulsations.

IMAGE PLOT 10nT/div. 1996/09/29

i ¥co oneat . . . . . . . . . .

~ ~ ~ ~ ~ ' ~ .

~ i ~ t i J i i i i ~ I ~ i i i J j i B : ~ : ,, : : : : : _

NAL

LYR

HOR

HOP

BJN

SOR

NAL

LYR

HOR

HOP

BJN

SOR

00 01 02 03 04 05 Time(UT)

NAL

LYR

HOR

HOP

BJN

SOR

Fig. 2. Three components of magnetic field observations from the IMAGE network on Sept. 29, 1996. X , Y , and Z components are defined as geographical northward, eastward, and downward, respectively. Only the six high latitude stations are shown.

E

>

EISCAT CP-4 200 m/s ! d i v . 1996/09/29

' ' - L~='.,;:;,~,.',,)' ' ' - ,d. '0,,&,~ ' '

~ ~ G2 CGMLat.=70., deg.

• ~ ~ "J G1 CGMLat.=70.4 deg.

, , , , ,0L1, , , , , 2 , , , , , i . . . . . i , , , , , 00 0 03 04 05

Time(UT)

CP-4 & BJN 200/div.

_ _ ~ C P - 4 North G3

00 01 02 03 04 05 Time(UT)

Fig. 4a. Line of sight Ion drift velocities observed by the EISCAT VHF radar (top). Fig. 4b. Comparison between ion drift velocities of the third gate and Y component of magnetic field variations at BJN (bottom).