Zhong Shan

The Space Physics Group of Oulu has developed an auroral photometer to be operated in the Chinese Zhong Shan station in Antarctic. This station is closely conjugated with the Svalbard area.

Important note: This message was received on 8 May 1996 from China:

The trip to Zhongshan of the Chinese Antactic Research Expedition had met some trouble in the way. The ice breaker "Snow Dragon" was already arrived the Great Wall station in the end of 1995. Some part of the ship was gone wrong. Most of the expedition members have to back with the ship. The photometer and other instruments including Japanese instruments have been returned to China too. We and the photometer have to wait next trip to Zhongshan.

Introduction

Of the two Chinese scientific stations in Antarctic, the one in Zhong Shan is closely conjugated with the Svalbard area in the northern hemisphere. This makes it a valuable place for coordinated measurements both with the EISCAT Svalbard Radar (ESR) and the Polar satellite. In 1991 a cooperation agreement was signed between the Ionospheric Laboratory of the China Research Institute of Radiowave Propagation (CRIRP) in Xinxiang, China, and the Department of Physical Sciences, University of Oulu, Finland, to build an auroral photometer system for the Zhong Shan station (Kaila et al, 1997). The multichannel scanning photometer agreed upon was constructed in Oulu by April 1995, and delivered to China in May. Measurements were supposed to start in March 1997, to be continued during the local winter time until October (the same schedule of measurements is planned for every year). However, due to problems mentioned above, the current status of the system is unknown!

The Zhong Shan station

Figure 1. Locations of Zhong Shan station (ZHO), South Pole (SP) and Magnetic South Pole (MSP). The line drawn over the station shows the scanning area at the altitude of 250 km.

The Zhong Shan station is located at 69.4°S, 76.4°E (Figure 1). Table 1 lists the most important figures related to this location. Note that the Zhong Shan station is at high enough latitude to be, most of the time, inside the polar cap or at the polarward edge of the auroral oval.

Table 1: The Zhong Shan station
Geographic location	69.4°S, 76.4°E	Geomagnetic latitude, MLT midnight, and geographic conjugate point are calculated with geocgm s/w from NSSDCA B field azimuth is calculated towards west from south
Geomagnetic latitude	-76.8°
MLT midnight	21:28 UT
B elevation and azimuth	71.5°, 102.5°
Geogr. conjugate point	78.2°N, 0.6°W

Figure 2a shows the height of the sun in Zhong Shan as a function of time of year and UT in isocontour lines of constant elevation angles of 0°, 6°, 12°, and 18° below horizon. The best conditions for optical measurements are when the sun is lower than -18° from the horizon, i.e., during the nights between March and September. In mid-winter this makes over 13 hours of measurements per night. By extending the measurements to cover elevations up to -12° during winter daytime one reaches the cusp/LLBL region in the afternoon hours (11:00 UT = 16:00 LT = 13:30 MLT).

Figure 2. Degrees of light and darkness in a) Zhong Shan and b) Kilpisjärvi during a year as indicated by the height of the sun from the horizon.

The Zhong Shan station is magnetically conjugated with the Svalbard area, the conjugate point being about 400 km west of the island (Figure 3). Note that, with increasing magnetic activity, this point moves southward by few degrees. The EISCAT Svalbard Radar (ESR) can cover approximately the same area. Several groups will also provide, e.g., CCD-based all-sky cameras on Svalbard and in Greenland’s east coast covering the conjugate area of Zhong Shan.

Figure 3. EISCAT Svalbard Radar (ESR), Kilpisjärvi (KIL) and the Zhong Shan conjugate point.

It is interesting to check if there are common periods of darkness between Northern Scandinavia and Zhong Shan. The map of sun’s height for Kilpisjärvi, Finland, is drawn in Figure 2b in the same format as was done for Zhong Shan. It can be seen that there are two periods, one in spring (March) and one in fall (September), when the stations can make simultaneous optical measurements. These periods occur around the local magnetic midnights, and can be several hours long when allowing measurements in the not optimal -12° sun elevation angles. The common periods get shorter closer to the modelled conjugate point.

The photometer constructed in Oulu will not be the only instrument to be operated in Zhong Shan. In addition to normal magnetic measurements made by the Chinese, the National Institute of Polar Research (NIPR) in Japan has, in cooperation with the Polar Research Institute in Shanghai, China, a TV-camera, an all-sky-camera, a photometer, and an imaging riometer in Zhong Shan.

Instrument

The instrument installed in Zhong Shan is a scanning, five channel auroral photometer with the possibility to measure the background light by tilting the individual filters at preset intervals. The wavelengths measured are listed in Table 2.

Table 2: Photometer channels
Wave- length (nm)	Source	Sensiti- vity p/Rs
425.2	N2+ 1NG (0-1) R-band	5.7
426.7	N2+ 1NG (0-1) R-band	7.5
427.8	N2+ 1NG (0-1) P-band	8.3
486.1	H+	10.
630.0	O(1D)	1.4

The basic lines are the blue nitrogen line 427.8 nm and the red oxygen line 630.0 nm, an often used combination to measure the energy characteristics of the precitating electrons (Rees and Luckey, 1974; Rees and Roble, 1986). The proton line 486.1 nm can be used to study the proton precipitation. In addition, the N2+ 1 NG band shape depends on the rotational temperature of neutral molecules in the emission source region. By measuring the P-branch and two parts of the R-branch we can determine this temperature and use it to define yet another energy parameter for the precipitating electrons. The field of views of the channels are 3° for the proton line, and 2° for the others. The proton precipitation is typically much more diffuse than the electron precipitation, and we can thus improve the sensitivity of the channel by using wider spatial integration.

The photometer is operating in pulse counting mode and it is controlled from a standard 386 PC. The accumulated pulses are read from the counter card with a sampling frequency that can be as high as 100 samples/second. The absolute time is kept correct with a GPS clock, and the elevation angle (measurement direction) with an absolute angle encoder. Data is written into a hard disk in a binary format.

Measurements

The photometer will be measuring continuously during the dark periods. This will total up to about 2210 hours of measurements per year from March the 2nd to October the 13th (full moon periods are also included). Measurements will be made either at fixed direction towards the magnetic zenith or with a scanning angle of 160° (higher than 10° above the horizon). The scanning plane, determined by the magnetic field direction and realized by proper orientation of the instrument, is drawn in Figure 1 as a line at the altitude of 250 km, which is typical height for the 630.0 nm emission. For the N2+ lines the region covered is about half of this. Several different modes in which the instrument can be run are listed in Table 3.

Table 3: Measurement modes
Mode #	Description	Integration- times (sec)
1	Normal mode; fixed direction or scan	0.05-0.5
2	Fast mode with individual integrationon times for different channels; fixed direction	0.01-1
3	Midex mode; both fixed direction and scan	0.1-0.3
11	Mode 1 with fast scanning speed	0.1-0.3
13	Mode 3 with fast scanning speed	0.1-0.3

The instrument will be operated mostly in modes 1 and 11, either with a fixed direction or with a scan. Sometimes the photometer will be operated in the mixed modes 3 and 13, in which 800 seconds of field aligned measurements alternate with a full back and forth scan of 160°. Mode 2 will be used only in special events.

Since it is not possible to get the data from Zhong Shan during the Antarctic winter time, the related satellite studies will suffer from a considerable time delay. However, it is planned that the raw data products, e.g., in the form of quick-look-plots, will be made publicly available as soon as possible, and interested parties with complementing data sets are invited to take part in the subsequent data analysis.

Discussion

As already mentioned, the Zhong Shan station will be most of the time in the polar cap region or at the polarward edge of the auroral oval. Also the optical cusp/LLBL region can be accessed in the afternoon sector. Accordingly, different optical phenomena can be studied as the photometer measures several important emission lines simultaneously with high temporal resolution. In the dayside these include, e.g., dayside auroral breakups and their relation to reconnection processes (e.g. Farrugia et al., 1995), cusp related Pc 3 range optical pulsations (Engebretson et al., 1990), and the 1400-1600 MLT sector bright spots (e.g., Vo and Murphree, 1995). The ability of the instrument to measure the background light makes the measurements more reliable also during these less optimal dayside hours. Because of the GPS timing the data can be compared with satellite and other measurements without timing errors.

The Zhong Shan measurements have a value of their own. However, together with other measurements, either by ground-based instruments in the conjugate northern hemisphere or by satellites, they are even more important. For example, the EISCAT Svalbard Radar measurements may obtain optical support from the Zhong Shan station during the light Arctic summer, when similar measurements are not possible in the northern hemisphere. One may also try to study conjugate optical phenomena during the short periods of common darkness in Zhong Shan and Northern Scandinavia. The Oulu group plans coordinated optical measurements between Zhong Shan and Kilpisjärvi in March and September each year. Finally, the Cluster and other satellite projects will provide possibilities for further coordinated studies.

Acknowledgements

The financial support of the University of Oulu for the photometer hardware is gratefully acknowledged. Collaboration between the University of Oulu and CRIRP, China, has been possible due to the grants for visiting scientists from the Academy of Finland.