2 Takushoku University, Tokyo 193, Janpan;
3 Polar Research Institute of China, Shanghai 200129, China
Transient magnetospheric phenomena and their relationship to the conditions of the solar wind are very important to investigate the energy transfer process from the solar wind to the Earth's magnetosphere. It has long been known that sudden positive changes of the solar wind dynamic pressure can trigger the geomagnetic sudden commencement (SC) and long period magnetic pulsations (Kaufman and Walker, 1974). Now it is becoming increasingly clear that large amplitude impulsive variations of the solar wind dynamic pressure are quite common (Sibeck et al. 1989) and that tinese variations can drive magnetopause motion, magnetospheric magnetic field compression, transient oscillations in high-latitude ionospheric flows, ground magnetic field pulsations, increased ULF/VLF wave activity (Hirasawa, 1981) and cosmic noise absorption (Matsushita, 1961). However, the study on the phenomena associated with the solar wind negative pressure impulse (SI-) is still very limited (Araki and Nagano, 1988), especially, as far as we know, no report for their relation to the optical aurora.
The field of views of the Syowa East HF radar covers over Zhongshan Station. The geomagnetic latitude of Zhongshan Station, as same as that of South Pole Station, is ~ 74. 50S, which is the average location under the cusp region. We have examined the relation between HF radar signatures and optical aurora by using the data on 3 August, 1997. High time resolution camping beam of the HF radar with the sampling period of 17's over Zhongshan Station has operated under clear sky and geomagnetically disturbed condition. A geomagnetic negative sudden impulse (SI-) occurred at ~1432 UT on 3 August, 1997 was associated with the sudden decrease of solar wind plasma density. In association with this events a sudden southward turning of the interplanetary magnetic field (IMF) Bz also occurred. We examine in details on this event by using simultaneous observations of HF radar and optical aurora, comparing with data from WIND satellite and ground based magnetograms.2 Instruments and data sources
The field of views of the Syowa East HF radar covers Zhongshan Station. The Syowa East HF radar is a Japan-operated component of the extended network of HF radars called Super DARN (Super Dual Auroral Radar Network) (Greenwald et al. 1995). The field of view of this radar extend from ~ 65°S up to ~ 85°S in magnetic latitude and covers up to ~4, 5 h in magnetic local time. The radar employs linear phased arrays of 16 log-periodic antennas and its operational frequency is between 8 MHz and 20 MHz. The radar forms a single beam which is narrow in azimuth (between 2. 5° and 6% depending on the transmitted frequency) but broad in elevation (up to ~40, at 8 MHz). A radar scan over a 52° angular segment is completed in each 60 s or 120 s by sweeping the single beam through 16 successive positions differing by 3. 25°. In near-real time, the backscatter power, line-of-sight Doppler velocity, and Doppler spectral width are found at each beam position by fitting the autocorrelation functions for 70 range bins of width 45 km, starting at a slant range of 180 km. The field of view of the Syowa East and other Super DARN radars in the southern hemisphere are shown in Figure 1. Also shown are the locations of ground-based observatories including Zhongshan Station and South Pole Stations.
In this paper the data from a panchromatic all-sky TV cameras at Zhongshan Station, ground based magnetograms at Zhongshan Station and Syowa Station in the southern hemisphere and the International Monitor for Auroral Geomagnetic Effects (IMAGE) (Liihr et al.1998) array in the Scandinavian region are analyzed and compared with the data of interplanetary magnetic field, solar wind dynamic pressure and plasma density onboard WIND satellite (Oglivie and Parks, 1996).3 Results
The top panel of Figure 2 (see Plate 1) shows Keogram produced from all-sky TV images. In order to compare the spatial and temporal relation between the optical aurora and HF radar signatures, the vertical axis of this Keogram shows direction along the beam 6 of the Syowa East radar. The middle panel of Figure 2 shows the HF backscatter power of the beam 6. The bottom panel shows the H component of magnetogram at Zhongshan Station. It is clearly found that optical auroral intensity and HF backscatter power enhanced simultaneously at 1432 -1439 UT. It is also found that the intensity enhanced region moved to poleward with time for both of optical aurora and HF backscatter with almost same manner. Then both of the HF backscatter power and optical emission ceased at ~1439 UT. It is very important to point out again thal the spatial and temporal variation signatures for both of the phenomena look very similar. It is also found that the H-component of magnetic variation showed very similar variations as the optical auroral luminosity variations during the time interval for the time period of 1432-1445 UT.
Figure 3 shows the all-sky TV images observed at Zhongshan Station at every 1- minute interval from, 1435:00 UT to, 1440:00 UT. The directions of up, down, right and left in the all-sky image data indicate the magnetic southward, northward, eastward and westward, respectively. It is found that the feint auroral emission with east-west aligned band type aurora was verified at 1435:00 UT. Then the emission intensity started to enhance with time, and it reached the maximum intensity at ~, 1439:00 U T. Looking carefully the movement of the fine structure of the aurora, one can see that the emission peak was moving eastward and poleward.
Figure 4 shows the X components of magnetic variation observed by IMAGE magnetometer array in the northern hemisphere, which locates very close to geomagnetic conjugate meridian of Zhongshan Station. The initial signatures of magnetic variainon associated with SI- occurred simultaneously at all stations at ~1432 UT. It is very interesting that the positive peaks of the magnetic variation showed a time lag between BJNand NAL, which locate in the cusp geomagnetic latitude. On the other hand, the time lag was very small between SOR and PEL which locate in the auroral and sub- auroral zone. It is worth noting that the maximum intensity variation was found at HOR.
Figure 5 shows IMF data observed onboard WIND satellite. The spacecraft was at about (79. 7, -60. 2, -14. 8) Re in GSE coordinates during this time interval. The panel from top to bottom shows the total intensity and 3-components of IMF (B｣, Bx, By, Bz), electron density and ion pressure, respectively. It is clearly found that the solar wind electron density and ion pressure showed negative impulse from ~23 el. /cm; lo ~7 el./cm3 and ~17 nPa to ~4 nPa, respectively at ~1340 UT. Correspondingly, the Bt, By and Bz changed from ~11 nT to 14 nT, ~-5 nT to +3 nT and ~1 nT to -9 nT, respectively. Such discontinuity occurred within the time interval of 2 min. It suggests that the sudden impulses of HF radar backscatter powers the optical auroral enhancement and magnetic variations observed in the ionosphere and on the ground should be caused by the impulsive discontinuity of solar wind negative pressure when we are taking in account the transportation time of solar wind between the position of WIND satellite and the Earths magnetopause.4 Discussion
The most interesting signature of this event is that one to one correlations between HF radar and optical aurora are found in spatial and temporal variations associated with the SI. It could be expected that the negative solar wind pressure impulse could cause the expansion of the magnetopause. The question is why the rarefaction of the magnetopause and magnetosphere can lead to the optical auroral enhancement and HF radar irregularities. Or does the sudden negative change of IMF Bz cause the direct effect of auroral emission enhancement without the relation to solar wind negative pressure? In order to separate the different causes we have to analyze other events, under solar wind conditions such as negative pressure impulses associated with stable positive or negative IMF Bz. Another interesting signature of this event is that quasi-periodic variations with period of ~10 min are found for both of optical auroral intensity variations and magnetic variations. It suggests that solar wind sudden negative pressure impulse enhances the long period magnetic pulsations. The optical aurora also enhanced quasi-periodically with a close correlation to the magnetic pulsations. It has been well known that sudden positive changes in the solar wind dynamic pressure can trigger the long period magnetic pulsations. However, there are no evidences that a negative pressure impulse can also trigger the long periodic optical aurora and HF radar irregularities with a close correlation to magnetic pulsations, in our knowledge.Acknowledgements Syowa East HF radar and optical instruments at Zhongshan Suuion are supported by the Ministry of Education, Science, Sports and Culture of Japan (MONBUSHO). The operation of instruments at Zhongshan Station is supported by the Chinese Arctic and Antarctic Administration. The optical observation at Zhongshan Station during over winter in, 1997has carried out by the 13th CHINARE. The 38th JARE has carried out the HF radar operation at Syowa Station. We thank the institutes that maintain the IMAGE magnetometer array. The plasma data onboard WIND saielline are provided by Dr. K. W. Oglivie at NASA/GSFC. The authors thank Prof. S. Okano, Drs. T. Ono and M. Taguchi for their efforts on optical instruments installed at Zhongshan Station.
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