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Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica
Polar Science xxx (2017) 1e8
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Polar Science j o u r n a l h o m e p a g e : h t t p s : / / w w w . e v i s e . c o m / p r o fi l e / # / J R N L _ P O L A R / l o g i n
Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica Ze-Jun Hu*, Fang He, Jian-Jun Liu, De-Hong Huang, De-Sheng Han, Hong-Qiao Hu, Bei-Chen Zhang, Hui-Gen Yang, Zhuo-Tian Chen, Bin Li, Xiang-Cai Chen SOA Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2017 Received in revised form 27 June 2017 Accepted 8 September 2017 Available online xxx
The Chinese Antarctic Zhongshan Station (ZHS) is located in a unique geographical and geomagnetic site that is suitable for observations on the cusp and of the post-noon dayside aurora and the poleward boundary of the nightside auroral oval. Since 2010, a unique, advanced synthetically auroral observation system has been deployed at ZHS, composed of a multi-wavelength all-sky imager, a multi-scale imager, a spectrum imager, and a radio imager. This system can record auroral forms from large (~100 km) to small scales (~10 m), auroral spectral lines from 400 to 730 nm, and auroral characteristics in the radio spectrum. Using this system, we have investigated the visible characteristics of UV ‘bright spots,’ the variation characteristics of the dayside shock auroras and convection, and the methodology for the quietday curve (QDC), among others. These investigations have enhanced our understanding of the solar wind emagnetosphereeionosphere coupling process. © 2017 Elsevier B.V. and NIPR. All rights reserved.
Keywords: Multi-wavelength aurora Multi-scale aurora Polar region Ionosphere
1. Introduction In the South and North Polar Regions, the aurora (Aurora Australis and Aurora Borealis, respectively) is one of the most magnificent phenomena of nature. It is manifested by optical emission produced by particle collisions between neutral particles in the upper atmosphere in the polar regions and precipitating particles from the solar wind and the magnetosphere, mainly electrons and protons. Different energies of precipitating particles and different elements in the upper atmosphere cause auroral emission at different wavelengths. Therefore, by monitoring the form and spectrum of the aurora, investigators can study the energy coupling process between the solar wind and the terrestrial magnetosphere, the physical processes associated with particle precipitation from the solar wind and the magnetosphere to the polar ionosphere, and variations in the composition of the upper atmosphere (Hu et al., 2009, 2010, 2012, 2013, 2014). The auroral oval is a manifestation of ionospheric precipitation of the various magnetospheric regions; e.g., the mantle, the cusp, the low-latitude boundary layer (LLBL), the boundary plasma sheet
* Corresponding author. No. 451, Jinqiao Road, Polar Research Institute of China, Shanghai 200136, China. E-mail address: [email protected] (Z.-J. Hu).
(BPS), and the central plasma sheet (CPS) (Newell and Meng, 1992; Newell et al., 2004). The Chinese Antarctic Zhongshan Station (ZHS) (labeled ‘1’ in Fig. 1) is located at (69.37 S, 76.38 E), corresponding to 74.66 geomagnetic latitude (MLAT) and 96.80 geomagnetic longitude (MLON) in corrected geomagnetic (CGM) coordinates. The magnetic local time (MLT) is approximately equal to Universal Time (UT) plus 1.7 h. Owing to the Earth's rotation, ZHS passes through the precipitation region of the cusp during the daytime and through the polar cap region at night, and this makes its location suitable for observations of the dayside cusp aurora, the post-noon aurora, and the poleward boundary of the nightside auroral oval. Moreover, the Chinese Arctic Yellow River Station (78.92 N, 11.93 E; 76.24 MLAT, 110.52 MLON; labeled ‘5’ in Fig. 1), which is also located in the cusp region in the Northern Hemisphere, represents a geomagnetic conjugate observatory pair at the cusp latitude with the ZHS (the geomagnetic conjugate point of ZHS is labeled ‘6’ in Fig. 1). The two stations are suitable for carrying out collaborative observations for comparison studies of the polar space environment in the Southern and Northern Hemispheres. Auroral observations at ZHS started in 1995 using an optical allsky television camera, a multi-wavelength scanning spectrophotometer, and an all-sky CCD imager (Yang et al., 2000). Recently, auroral observations at ZHS have been further improved with the help of funding support from the Chinese Polar Research and Expedition Project, the International Polar Year China action plan,
https://doi.org/10.1016/j.polar.2017.09.001 1873-9652/© 2017 Elsevier B.V. and NIPR. All rights reserved.
Please cite this article in press as: Hu, Z.-J., et al., Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica, Polar Science (2017), https://doi.org/10.1016/j.polar.2017.09.001
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Fig. 1. Locations of cusp-latitude auroral observatories in the Southern and Northern Hemispheres. 1: Chinese Zhongshan Station (69.37 S, 76.38 E); 2: Chinese Taishan Station (73 510 S, 76.97 E); 3: Chinese Kunlun Station (80.42 S, 77.12 E); 4: South Pole Station; 5: Chinese Yellow River Station at Ny-Ålesund, Svalbard (78.92 N, 11.93 E); 6: Geomagnetic conjugate site of ZHS. The solid and dashed black circles around the stations are the FOVs of the all-sky imagers. There are no all-sky imagers at locations 2, 3, and 6, so their FOVs are indicated by dashed circles. The solid and dashed red lines indicate 90 , 80 , 70 , and 60 magnetic latitude, and 0 , 45 , 90 , 135 , 180 , 225 , 270 , and 315 magnetic longitude, respectively.
and the Meridian Space Weather Monitoring Project. In 2010, new auroral observation instruments were installed in a new building (Fig. 2) at ZHS, which contains six optical cabins with glass domes on a flat roof (inset in Fig. 2), while several old observation systems have been updated. This paper provides a detailed introduction to the auroral observation system at ZHS, and summarizes recent research achievements based on auroral observations at ZHS. 2. Auroral observation system at Zhongshan Station Auroras are particular space physics phenomena in the polar regions that can be observed with the naked eye. Therefore, visible observations of auroras represent a popular method of study. Auroral observations at ZHS are mainly undertaken at visible
wavelengths using imaging observations in all-sky mode as well as of small-scale fields of view (FOVs), and through spectral observations. However, optical-wavelength observations can be affected by interference from sunlight and weather conditions. Since the intensity of sunlight is far greater than that of the aurora, the optical observation system cannot observe auroral emission when the solar elevation is greater than 8 (therefore, the optical observation season of ZHS only runs from the end of February to the beginning of October, between 16:00 LT and the following day at 07:00 LT). Auroral emission occurs at altitudes of 100e400 km. Weather conditions such as clouds, snow, or rain can significantly interfere with the transmission of auroral emission to the ground. A radio detector can be used to monitor the aurora in the presence of bright sunlight and in poor weather conditions. The auroral observation system at ZHS consists of four major subsystems: a multi-wavelength all-sky auroral imaging (MWASI) observation component, a multi-scale auroral imaging observation system, an auroral spectrum observation set-up, and an auroral radio observation station. Details of each subsystem are described in the following sections. 2.1. Multiple-wavelength all-sky auroral imaging observation
Fig. 2. Aurora Australis above the Aurora Observatory (green building) at the Chinese Zhongshan Station, Antarctica. (Photo: Ze-Jun Hu, Polar Research Institute of China; Date: 21 July 2014). The horizontal profile of the Aurora Observatory is an equilateral hexagon, and the six cabins are distributed symmetrically within the hexagon, as shown in the inset.
Since 2010, a MWASI has been available at ZHS, consisting of seven parts (see Fig. 3): (1) a Mamiya 645 24 mm/f 4.0 fish-eye lens with a 180 FOV, (2) a mechanical shutter, (3) a collimator lens, (4) a filter wheel with six filters (interference filters centered at 427.8, 432.0, 540.0, 557.7, 620.0, and 630.0 nm with widths of 2 nm each), (5) a smart motor system (used to control and drive the filter wheel), (6) relay optics, and (7) an EMCCD detector (1024 1024 pixels). Through automatic filter-wheel changes, the MWASI can capture the all-sky auroral forms and luminosities at specific wavelengths. The exposure time of each image is 4 s, and the interval time between each pair of auroral images is 10 s. The diameter of the all-sky FOV at an altitude of 150 km is about
Please cite this article in press as: Hu, Z.-J., et al., Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica, Polar Science (2017), https://doi.org/10.1016/j.polar.2017.09.001
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2.2. Multi-scale aurora imaging observations
Fig. 3. Multi-wavelength all-sky CCD imager at ZHS. The imager is composed of seven parts: (1) a Mamiya 645 24 mm/f 4.0 fish-eye lens, (2) a mechanical shutter, (3) a collimator lens, (4) a filter wheel, (5) a smart motor system, (6) relay optics, and (7) an EMCCD detector.
1000 km. The spatial resolution of the auroral images associated with the EMCCD detector ranges from ~0.5 km pixel1 at zenith to ~18 km pixel1 on the horizon, both at an altitude of 150 km. In the visible auroral spectrum, the intensities of the spectral 1 1 lines at Nþ 2 (1NG) 427.8 nm, O( S) 557.7 nm, and O( D) 630.0 nm are stronger than those of other emission lines. They are excited by electrons with E > 1 keV, 0.5 keV < E < 1 keV, and E < 0.5 keV, respectively, where E is the characteristic energy of the precipitating electrons. These electrons of different energy levels originate from different magnetospheric regions (Newell and Meng, 1992; Newell et al., 2004). They precipitate into the polar ionosphere through various dynamical processes and accelerating mechanisms, where they excite the various auroral luminosities and forms (Newell and Meng, 1992; Newell et al., 1996; Hu et al., 2009). Therefore, the auroral forms, spectra, and luminosities offer both qualitative and quantitative clues not only as to the magnetospheric structure, but also to the prevailing dynamical processes in the magnetosphere. Fig. 4 shows Keograms of the temporal variations in auroral emission intensities along the magnetic meridian at 427.8, 557.7, and 630.0 nm. The observation period is from 12:00 to 24:00 UT (i.e., 13:42e01:42 MLT), which covers midday, post-noon, and nightside auroral activity. The auroral form and intensity exhibit different characteristics. Before 13:10 UT, the auroral intensity is weak at 427.8 and 557.7 nm but significant at 630.0 nm (Fig. 4a), which implies that ZHS is located in the midday gap of the dayside oval (Hu et al., 2009). At this time, the aurora is observed as a dayside corona aurora characterized by a ray structure (Fig. 4b; Hu et al., 2009), which is caused by the broad-band electron precipin waves (Newell et al., 2009). tation associated with kinetic Alfve During the period 13:10e15:34 UT, ZHS is located in the post-noon ‘hot spot’ region (Newell et al., 1996; Liou et al., 1997; Hu et al., 2009, 2010, 2012), since the auroral intensity at all wavelengths exhibits greater intensity. At 14:28 UT (Fig. 4c), the aurora shows a brightened spiral structure (Hu et al., 2013), which occurs for the 427.8, 557.7, and 630.0 nm excitations. This auroral form is related to the KelvineHelmholtz (KHI) and current-sheet instabilities (Lysak and Song, 1996; Hu et al., 2013). At 17:57e22:48 UT, the auroral intensity at 427.8 and 557.7 nm is significantly enhanced, and the poleward boundary of the auroras shows periodic poleward and equatorward movement. These optical characteristics may be related to the expansion and recovery phases of auroral substorms, respectively (Akasofu, 1964). At 21:06 UT, the all-sky image captures a poleward aurora arc (Fig. 4d). This structure is related to the inverted-V precipitation cone (Newell et al., 2009).
The multi-scale aurora imaging system consists of three smallscale auroral imagers (SSAI) with FOVs of 47, 19 , and 8 . The major difference between the MWASI and the SSAI is that the SSAI consists of a front lens with a small FOV, a single BG3 glass filter, relay optics, and an EMCCD detector (1024 1024 pixels). Because the SSAI FOV is narrow and cannot cover the whole sky, the FOV center of each SSAI is focused only on the local magnetic zenith (at the magnetic meridian, the magnetic zenith is located approximately 12.13 from the geographic zenith), and the maximum spatial resolutions are approximately 20, 49, and 128 m pixel1 at an altitude of 150 km. The three SSAIs and the MWASI constitute a unique optical system for auroral observations that can zoom into the auroral structure step by step. In the current observation mode, the exposure time of every SSAI is about 0.06 s, and the sampling frequency is 14.27 Hz. These imagers, with their high-speed sampling rate, can capture rapid auroral movements at small scales and temporal fluctuations in the intensity signature. Small-scale auroras are associated with acceleration processes at altitudes of 2e3 RE (Earth radii). The auroral structure, motion, and evolution reflect the dynamical processes in the acceleration region (Galperin, 2002). Using the SSAIs at ZHS, we can obtain a highly accurate value for the scale, the lifetime, and the twodimensional evolution of these small-scale auroral structures and investigate small-scale physical processes in the acceleration ren) waves, the KHI, and the quasi-static gion, such as nonlinear (Alfve electric field (Stasiewicz et al., 2000). For instance, the spiral arm of the auroral spiral shown in Fig. 4 exhibits some wavelike structures, which are called curls or folds (see the red circle in the 47 image in Fig. 5a). These may result from electrostatic instabilities of the magnetized, precipitating electron sheet (Ivchenko et al., 2005). In addition, the brightened arc in Fig. 4d has a quasi-static arc structure (47 snapshot in Fig. 5b). The general consensus is that the formation of quasi-static (or quiet) arcs is related to inverted-V precipitation (or acceleration), which is also called quasi-static acceleration owing to the presence of an upward parallel electric field of converging electric field structure (Frank and Ackerson, 1971; Mozer et al., 1977; Ergun et al., 1998).
2.3. Auroral spectrum observation Detailed auroral spectra can provide significant information on the sources and mechanisms responsible for the auroral emission, the energy distribution and fluxes of the precipitating particles, and the dynamical and chemical conditions in the polar upper atmosphere. A meridian imaging auroral spectrograph (ASG; Fig. 6a) was installed at ZHS in May 2010 to measure the auroral spectrum in the visible region along a magnetic meridian. The ASG is composed of an objective lens (a fish-eye lens with 180 FOV), a slit, a grism, a focusing lens, and an EMCCD detector (1024 1024 pixels; Fig. 6b). The slit, with a width of 70 mm and oriented along the meridian, is placed in the focal plane of the fish-eye lens. The grism is used as the ASG's dispersive element, dispersing 400e730 nm auroral spectral lines into the light-sensitive area of the EMCCD detector. The ASG's spectral resolution is ~0.3e0.4 nm pixel1 at 550 nm. Fig. 6c presents an image of the auroral spectrum taken at 21:06:00 UT on 21 July 2014. There is clear auroral emission at 470.9 nm (Nþ 2 ), 589.0 nm (NaI), and 636.4 nm (O), as well as at 427.8, 557.7, and 630.0 nm. Other spectral lines of auroral emission are also apparent in the ASG image, although they are vague because their intensities are lower than those of the bright spectral lines.
Please cite this article in press as: Hu, Z.-J., et al., Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica, Polar Science (2017), https://doi.org/10.1016/j.polar.2017.09.001
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Fig. 4. (a) Keograms of temporal variations of the emission intensities along the magnetic meridian at 427.8, 557.7, and 630.0 nm observed with the multi-wavelength all-sky imager at 12:00e24:00 UT on 21 July 2014. The left-hand vertical axis indicates the zenith angle (ZA) on the magnetic meridian (M.S. and M.N. indicate magnetic south and north, respectively), while the horizontal axis shows the time in UT. (b) and (c) All-sky images of three auroral emission events at 12:27, 14:28, and 21:06 UT. The vertical center line in each image is the magnetic meridian. M.S. is up and M.N. is down; the left- and right-hand sides of each image are magnetic west and east, respectively. The red circles in the 557.7 nm images show the FOV of the 47 imager.
2.4. Auroral radio observation Cosmic radio noise (CRN) from distant stars and galaxies is a
radio-frequency electromagnetic wave characterized by a wide spectrum. It is generally considered to be constant outside the Earth's atmosphere. When it passes through the ionospheric D
Please cite this article in press as: Hu, Z.-J., et al., Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica, Polar Science (2017), https://doi.org/10.1016/j.polar.2017.09.001
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Fig. 5. Images of the multi-scale auroras at (a) 14:28:06 UT and (b) 21:06:06 UT on 21 July 2014. The left, middle, and right columns show images with FOVs of 47, 19 , and 8 , respectively. The red arrows in the 47 images show the directions corresponding to the red arrows in the 557.7 nm images of Fig. 4. The red circles in the 47 and 19 images show the respective FOVs.
Fig. 6. (a) Auroral spectrograph at Zhongshan Station. (b) Optical diagram of the auroral spectrograph. The meridional plane is perpendicular to this plane. (c) Image of the auroral spectrograph at 21:06:00 UT on 21 July 2014.
region (60e90 km) and the lower E region (90e140 km), the freeelectron motions in these regions are affected by the CRN. Because of the high density of neutral particles in the lower regions of the ionosphere, a fraction of the energy of the free electrons will be transferred to the neutral particles; i.e., a fraction of the energy of the CRN is transferred to the neutral particles. As a result, the amplitude of the CRN recorded by a ground-based receiver is lower than that outside the atmosphere. This process is called cosmic noise absorption (CNA; Browne et al., 1995). Auroral particle precipitation can result in ionization of the neutral atmosphere and subsequently affect the CNA. Therefore, CRN observations provide information on auroral particle precipitation. CRN observations are not affected by sunlight, so this is a good approach to investigate auroral particle precipitation during daylight. In 1997, an imaging riometer was installed at ZHS (Yamagishi et al., 2000). In 2012, it was updated with a new recording
system and a global positioning service receiver. With an 8 8 antenna array working at 38.2 MHz (Fig. 7a), the imaging riometer can uncover the two-dimensional distribution of the CNA at high spatial resolution once a second. The FOV of the antenna array is 45 at zenith, so that the width of the FOV is 200 km at a height of 90 km, and the spatial resolution of the central beam is 20 km at a height of 90 km (Fig. 7b). Fig. 8a presents the CNA Keogram for 12:00e24:00 UT on 21 July 2014 (this period is the same as that for the multi-wavelength auroral Keograms in Fig. 4a). At 14:00e15:00 UT and 21:00e21:45 UT, the CNA is significant, at more than 0.6 dB. Compared with the auroral MWASI Keograms (Fig. 4a), during both periods intense aurora emission occurs within the range from 30 to 30 from the zenith at ZHS, which is also shown in the FOV of the imaging riometer. The two-dimensional CNA images at 12:27:26, 14:28:06, and 21:06:06 UT are shown in Fig. 8b, corresponding to
Please cite this article in press as: Hu, Z.-J., et al., Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica, Polar Science (2017), https://doi.org/10.1016/j.polar.2017.09.001
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Fig. 7. (a) Antenna array of the Imaging Riometer at ZHS. (b) Lobe pattern of the Imaging Riometer at a height of 90 km (‘þ’: lobe center).
Fig. 8. (a) CNA Keogram along the magnetic meridian (from 73 to 76 magnetic latitude) acquired from the Riometer at ZHS, Antarctica for 12:00e24:00 UT on 21 July 2014 (in units of dB). (b) CNA at 12:27:26 UT, 14:28:06 UT, and 21:06:06 UT (M.E., M.W. represent magnetic east and west, respectively).
the 557.7 nm all-sky auroral images in Fig. 4b, c, and 4d, respectively. The red circles in the 557.7 nm images show the approximate FOV of the imaging riometer. At 12:27:26 UT, the two-dimensional CNA is weak, and there is no obvious aurora at ZHS zenith (Fig. 4b). At 14:28:06 UT (Fig. 4c) and 21:06:06 UT (Fig. 4d), a bright spiral and an intense auroral arc appear at the ZHS zenith, respectively. Correspondingly, the two-dimensional CNA at these times (Fig. 8b) is significant.
3. Recent progress in aurora studies at Zhongshan Station Using optical and radio auroral data obtained with the auroral instruments at ZHS, and combining with other radio and satellite data, the following studies were carried out during the past five years. Ultraviolet imagers (UVIs) aboard the Polar and Viking satellites showed that spatially periodic bright spots appear frequently in a ‘string of pearls’ configuration in the post-noon oval (1400e1600 MLT sector; Lui et al., 1989). The mechanism responsible for the
post-noon bright spots and the visible form observed by groundbased imagers is unclear. Using the ASI at ZHS in Antarctica and the Polar UVI in Arctic, Hu et al. (2013) showed that multiple bright auroral spirals in post-noon auroral arcs are seen in the FOV of ASI at ZHS in the Southern Hemisphere, while multiple bright spots are seen by the Polar UVI at the conjugate FOV of ASI in the post-noon auroral oval in the Northern Hemisphere. These observations confirm that auroral spirals are the visible, ground-based characteristics of post-noon UV bright spots, and it has been suggested that the current-sheet instability (Lysak and Song, 1996) above the parallel electric-field region is a possible cause of the bright spots in the ionosphere. Shock auroras, which are caused by interplanetary shocks or solar-wind pressure pulses, are one of the most significant visible indications of the dynamical processes related to the solar windemagnetosphereeionosphere coupling process (Zhou et al., 2003). Using auroral observations at ZHS and the South Pole Station, combined with ionospheric convection observed by the SuperDARN radars, Liu et al. (2011) showed that the auroral intensity
Please cite this article in press as: Hu, Z.-J., et al., Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica, Polar Science (2017), https://doi.org/10.1016/j.polar.2017.09.001
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decreases in the post-noon oval but increases in the pre-noon oval when the solar wind pressure is amplified. At the same time, the direction of the ionospheric plasma flow is reversed anti-sunward in the post-noon oval. These results could be understood with reference to the physical model of sudden commencement proposed by Araki (1994). Based on long-term studies of the CNA observed with the riometers, it has been found that the CNA is measured relative to the baseline of the cosmic noise signal level recorded under quiet ionospheric conditions. Hence, it is important to obtain a reliable quiet-day curve (QDC). The cosmic noise detected not only shows diurnal and seasonal changes, but it is also strongly affected by solar activity, geomagnetic disturbances, and man-made electromagnetic interference. Based on previous research, He et al. (2014, 2015) used data acquired with the ZHS imaging riometer to develop a new technique for estimating the QDC. Compared with the previous methodology for measuring the QDC, He's technique produced a better QDC, which will result in a more consistent correspondence between the CNA and the in situ aurora, and a better expression of the small-scale structures of the weak absorption regions (He et al., 2015). 4. Summary and prospects The Chinese Antarctic Zhongshan Station is located at a unique geographical site that is well suited to observe the cusp, the postnoon dayside, and the nightside aurora, and also to perform cusplatitude conjugate observations with the Chinese Arctic Yellow River Station. An advanced synthetic auroral observation system was deployed at ZHS in 2010, including a multi-wavelength all-sky auroral imaging observation component, a multi-scale auroral imaging observation system, an auroral spectrum observation set-up, and an auroral radio observation station. This auroral observation system has been used to perform several investigations of the visible characteristics of UV ‘bright spots,’ variations in the dayside shock auroras and convection, and the methodology for obtaining the QDC, among others. Based on these analyses, our understanding of the solar windemagnetosphereeionosphere coupling process has been enhanced. Geospace is a significant part of the environment of importance for the survival and development of mankind. Space weather has a marked effect on human activities. As a direct, significant indicator of the solar windemagnetosphereeionosphere coupling process, auroras are increasingly important for the monitoring of space weather. At Taishan (labeled ‘2’ in Fig. 1) and Kunlun (labeled ‘3’ in Fig. 1) stations (which are also located at the cusp-latitude auroral oval and are suitable for observations of the dayside aurora), an aurora monitoring system will be installed in the next five years, which will establish a dayside aurora-monitoring chain in Antarctica (including the South Pole Station, labeled ‘4’ in Fig. 1). The auroral monitoring chain almost overlaps with the Chinese Antarctic magnetometer chain at the cusp latitude (Liu et al., 2016) and will greatly promote future research and monitoring capabilities of space weather, auroral physics, and the solar windemagnetosphereeionosphere coupling process. Acknowledgments This work was supported by the Polar Environment Comprehensive Investigation and Assessment Programs (CHINARE2017), the National Natural Science Foundation of China (grants 41274164, 41431072, 41674169, 41374161, 41474146, and 41374159), the Pudong Development of Science and Technology Program (PKC2013-207), the Chinese Meridian Project, and the National Program for Support of Top-Notch Young Professionals. Data were
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Liu, J.J., Hu, H.Q., Han, D.S., Araki, T., Hu, Z.J., Zhang, Q.H., Yang, H.G., Sato, N., Yukimatu, A.S., Ebihara, Y., 2011. Decrease of auroral intensity associated with reversal of plasma convection in response to an interplanetary shock as observed over Zhongshan station in Antarctica. J. Geophys. Res. 116, A03210. http://dx.doi.org/10.1029/2010JA016156. Liu, Y.H., Hu, H.Q., Yang, H.G., et al., 2016. Chinese antarctic magnetometer chain at the cusp latitude. Adv. Polar Sci. 27, 102e106. http://dx.doi.org/10.13679/ j.advps.2016.2.00102. Lui, A.T.Y., Venkatesan, D., Murphree, J.S., 1989. Auroral bright spots on the dayside oval. J. Geophys. Res. 94 (A5), 5515e5522. Lysak, R.L., Song, Y., 1996. Coupling of KelvineHelmholtz and current sheet instabilities to the ionosphere: a dynamic theory of auroral spirals. J. Geophys. Res. 101 (A7), 15411e15422. Mozer, F., et al., 1977. Observations of paired electrostatic shocks in the polar magnetosphere. Phys. Rev. Lett. 38, 292e295. http://dx.doi.org/10.1103/ PhysRevLett.38.292. Newell, P.T., Meng, C.-I., 1992. Mapping the dayside ionosphere to the magnetosphere according to particle precipitation characteristics. Geophys. Res. Lett. 19 (06), 609e612. Newell, P.T., Lyons, K.M., Meng, C.-I., 1996. A large survey of electron acceleration events. J. Geophys. Res. 101 (A2), 2599e2914. Newell, P.T., Ruohoniemi, J.M., Meng, C.-I., 2004. Maps of precipitation by source region, binned by IMF, with inertial convection streamlines. J. Geophys. Res. 109, A10206. http://dx.doi.org/10.1029/2004JA010499. Newell, P.T., Sotirelis, T., Wing, S., 2009. Diffuse, monoenergetic, and broadband aurora: the global precipitation budget. J. Geophys. Res. Space Phys. 114 (A9),
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A09207. http://dx.doi.org/10.1029/2009JA014326. Stasiewicz, K., Bellan, P., Chaston, C., Kletzing, C., Lysak, R., Maggs, J., Pokhotelov, O., Seyler, C., Shukla, P., Stenflo, L., Streltsov, A., Wahlund, J.-E., 2000. Small scale nic structure in the aurora. Space Sci. Rev. 92, 423e533. Alfve Yamagishi, H., Fujita, Y., Sato, N., Nishino, M., Stauning, P., Liu, R., Saemundsson, Th, 2000. Interhemispheric conjugacy of auroral poleward expansion observed by conjugate imaging riometers at ~67 and 75 e77 invariant latitude. Adv. Polar Up. Atmos. Res. 14, 12e33.
Yang, H., Sato, N., Makita, K., Kikuchi, M., Kadokura, A., Ayukawa, M., Hu, H.Q., €m, I., 2000. Synoptic observations of auroras along the Liu, R.Y., H€ aggstro postnoon oval: a survey with all sky TV observations at Zhongshan Antarctica. J. Atmos. Sol.eTerr. Phys. 62, 787e797. Zhou, X.-Y., Strangeway, R.J., Anderson, P.C., Sibeck, D.G., Tsurutani, B.T., Haerendel, G., Frey, H.U., Arballo, J.K., 2003. Shock aurora: FAST and DMSP observations. J. Geophys. Res. 108 (A4), 8019,. http://dx.doi.org/10.1029/ 2002JA009701.
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Polar Science j o u r n a l h o m e p a g e : h t t p s : / / w w w . e v i s e . c o m / p r o fi l e / # / J R N L _ P O L A R / l o g i n
Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica Ze-Jun Hu*, Fang He, Jian-Jun Liu, De-Hong Huang, De-Sheng Han, Hong-Qiao Hu, Bei-Chen Zhang, Hui-Gen Yang, Zhuo-Tian Chen, Bin Li, Xiang-Cai Chen SOA Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2017 Received in revised form 27 June 2017 Accepted 8 September 2017 Available online xxx
The Chinese Antarctic Zhongshan Station (ZHS) is located in a unique geographical and geomagnetic site that is suitable for observations on the cusp and of the post-noon dayside aurora and the poleward boundary of the nightside auroral oval. Since 2010, a unique, advanced synthetically auroral observation system has been deployed at ZHS, composed of a multi-wavelength all-sky imager, a multi-scale imager, a spectrum imager, and a radio imager. This system can record auroral forms from large (~100 km) to small scales (~10 m), auroral spectral lines from 400 to 730 nm, and auroral characteristics in the radio spectrum. Using this system, we have investigated the visible characteristics of UV ‘bright spots,’ the variation characteristics of the dayside shock auroras and convection, and the methodology for the quietday curve (QDC), among others. These investigations have enhanced our understanding of the solar wind emagnetosphereeionosphere coupling process. © 2017 Elsevier B.V. and NIPR. All rights reserved.
Keywords: Multi-wavelength aurora Multi-scale aurora Polar region Ionosphere
1. Introduction In the South and North Polar Regions, the aurora (Aurora Australis and Aurora Borealis, respectively) is one of the most magnificent phenomena of nature. It is manifested by optical emission produced by particle collisions between neutral particles in the upper atmosphere in the polar regions and precipitating particles from the solar wind and the magnetosphere, mainly electrons and protons. Different energies of precipitating particles and different elements in the upper atmosphere cause auroral emission at different wavelengths. Therefore, by monitoring the form and spectrum of the aurora, investigators can study the energy coupling process between the solar wind and the terrestrial magnetosphere, the physical processes associated with particle precipitation from the solar wind and the magnetosphere to the polar ionosphere, and variations in the composition of the upper atmosphere (Hu et al., 2009, 2010, 2012, 2013, 2014). The auroral oval is a manifestation of ionospheric precipitation of the various magnetospheric regions; e.g., the mantle, the cusp, the low-latitude boundary layer (LLBL), the boundary plasma sheet
* Corresponding author. No. 451, Jinqiao Road, Polar Research Institute of China, Shanghai 200136, China. E-mail address: [email protected] (Z.-J. Hu).
(BPS), and the central plasma sheet (CPS) (Newell and Meng, 1992; Newell et al., 2004). The Chinese Antarctic Zhongshan Station (ZHS) (labeled ‘1’ in Fig. 1) is located at (69.37 S, 76.38 E), corresponding to 74.66 geomagnetic latitude (MLAT) and 96.80 geomagnetic longitude (MLON) in corrected geomagnetic (CGM) coordinates. The magnetic local time (MLT) is approximately equal to Universal Time (UT) plus 1.7 h. Owing to the Earth's rotation, ZHS passes through the precipitation region of the cusp during the daytime and through the polar cap region at night, and this makes its location suitable for observations of the dayside cusp aurora, the post-noon aurora, and the poleward boundary of the nightside auroral oval. Moreover, the Chinese Arctic Yellow River Station (78.92 N, 11.93 E; 76.24 MLAT, 110.52 MLON; labeled ‘5’ in Fig. 1), which is also located in the cusp region in the Northern Hemisphere, represents a geomagnetic conjugate observatory pair at the cusp latitude with the ZHS (the geomagnetic conjugate point of ZHS is labeled ‘6’ in Fig. 1). The two stations are suitable for carrying out collaborative observations for comparison studies of the polar space environment in the Southern and Northern Hemispheres. Auroral observations at ZHS started in 1995 using an optical allsky television camera, a multi-wavelength scanning spectrophotometer, and an all-sky CCD imager (Yang et al., 2000). Recently, auroral observations at ZHS have been further improved with the help of funding support from the Chinese Polar Research and Expedition Project, the International Polar Year China action plan,
https://doi.org/10.1016/j.polar.2017.09.001 1873-9652/© 2017 Elsevier B.V. and NIPR. All rights reserved.
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Fig. 1. Locations of cusp-latitude auroral observatories in the Southern and Northern Hemispheres. 1: Chinese Zhongshan Station (69.37 S, 76.38 E); 2: Chinese Taishan Station (73 510 S, 76.97 E); 3: Chinese Kunlun Station (80.42 S, 77.12 E); 4: South Pole Station; 5: Chinese Yellow River Station at Ny-Ålesund, Svalbard (78.92 N, 11.93 E); 6: Geomagnetic conjugate site of ZHS. The solid and dashed black circles around the stations are the FOVs of the all-sky imagers. There are no all-sky imagers at locations 2, 3, and 6, so their FOVs are indicated by dashed circles. The solid and dashed red lines indicate 90 , 80 , 70 , and 60 magnetic latitude, and 0 , 45 , 90 , 135 , 180 , 225 , 270 , and 315 magnetic longitude, respectively.
and the Meridian Space Weather Monitoring Project. In 2010, new auroral observation instruments were installed in a new building (Fig. 2) at ZHS, which contains six optical cabins with glass domes on a flat roof (inset in Fig. 2), while several old observation systems have been updated. This paper provides a detailed introduction to the auroral observation system at ZHS, and summarizes recent research achievements based on auroral observations at ZHS. 2. Auroral observation system at Zhongshan Station Auroras are particular space physics phenomena in the polar regions that can be observed with the naked eye. Therefore, visible observations of auroras represent a popular method of study. Auroral observations at ZHS are mainly undertaken at visible
wavelengths using imaging observations in all-sky mode as well as of small-scale fields of view (FOVs), and through spectral observations. However, optical-wavelength observations can be affected by interference from sunlight and weather conditions. Since the intensity of sunlight is far greater than that of the aurora, the optical observation system cannot observe auroral emission when the solar elevation is greater than 8 (therefore, the optical observation season of ZHS only runs from the end of February to the beginning of October, between 16:00 LT and the following day at 07:00 LT). Auroral emission occurs at altitudes of 100e400 km. Weather conditions such as clouds, snow, or rain can significantly interfere with the transmission of auroral emission to the ground. A radio detector can be used to monitor the aurora in the presence of bright sunlight and in poor weather conditions. The auroral observation system at ZHS consists of four major subsystems: a multi-wavelength all-sky auroral imaging (MWASI) observation component, a multi-scale auroral imaging observation system, an auroral spectrum observation set-up, and an auroral radio observation station. Details of each subsystem are described in the following sections. 2.1. Multiple-wavelength all-sky auroral imaging observation
Fig. 2. Aurora Australis above the Aurora Observatory (green building) at the Chinese Zhongshan Station, Antarctica. (Photo: Ze-Jun Hu, Polar Research Institute of China; Date: 21 July 2014). The horizontal profile of the Aurora Observatory is an equilateral hexagon, and the six cabins are distributed symmetrically within the hexagon, as shown in the inset.
Since 2010, a MWASI has been available at ZHS, consisting of seven parts (see Fig. 3): (1) a Mamiya 645 24 mm/f 4.0 fish-eye lens with a 180 FOV, (2) a mechanical shutter, (3) a collimator lens, (4) a filter wheel with six filters (interference filters centered at 427.8, 432.0, 540.0, 557.7, 620.0, and 630.0 nm with widths of 2 nm each), (5) a smart motor system (used to control and drive the filter wheel), (6) relay optics, and (7) an EMCCD detector (1024 1024 pixels). Through automatic filter-wheel changes, the MWASI can capture the all-sky auroral forms and luminosities at specific wavelengths. The exposure time of each image is 4 s, and the interval time between each pair of auroral images is 10 s. The diameter of the all-sky FOV at an altitude of 150 km is about
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2.2. Multi-scale aurora imaging observations
Fig. 3. Multi-wavelength all-sky CCD imager at ZHS. The imager is composed of seven parts: (1) a Mamiya 645 24 mm/f 4.0 fish-eye lens, (2) a mechanical shutter, (3) a collimator lens, (4) a filter wheel, (5) a smart motor system, (6) relay optics, and (7) an EMCCD detector.
1000 km. The spatial resolution of the auroral images associated with the EMCCD detector ranges from ~0.5 km pixel1 at zenith to ~18 km pixel1 on the horizon, both at an altitude of 150 km. In the visible auroral spectrum, the intensities of the spectral 1 1 lines at Nþ 2 (1NG) 427.8 nm, O( S) 557.7 nm, and O( D) 630.0 nm are stronger than those of other emission lines. They are excited by electrons with E > 1 keV, 0.5 keV < E < 1 keV, and E < 0.5 keV, respectively, where E is the characteristic energy of the precipitating electrons. These electrons of different energy levels originate from different magnetospheric regions (Newell and Meng, 1992; Newell et al., 2004). They precipitate into the polar ionosphere through various dynamical processes and accelerating mechanisms, where they excite the various auroral luminosities and forms (Newell and Meng, 1992; Newell et al., 1996; Hu et al., 2009). Therefore, the auroral forms, spectra, and luminosities offer both qualitative and quantitative clues not only as to the magnetospheric structure, but also to the prevailing dynamical processes in the magnetosphere. Fig. 4 shows Keograms of the temporal variations in auroral emission intensities along the magnetic meridian at 427.8, 557.7, and 630.0 nm. The observation period is from 12:00 to 24:00 UT (i.e., 13:42e01:42 MLT), which covers midday, post-noon, and nightside auroral activity. The auroral form and intensity exhibit different characteristics. Before 13:10 UT, the auroral intensity is weak at 427.8 and 557.7 nm but significant at 630.0 nm (Fig. 4a), which implies that ZHS is located in the midday gap of the dayside oval (Hu et al., 2009). At this time, the aurora is observed as a dayside corona aurora characterized by a ray structure (Fig. 4b; Hu et al., 2009), which is caused by the broad-band electron precipin waves (Newell et al., 2009). tation associated with kinetic Alfve During the period 13:10e15:34 UT, ZHS is located in the post-noon ‘hot spot’ region (Newell et al., 1996; Liou et al., 1997; Hu et al., 2009, 2010, 2012), since the auroral intensity at all wavelengths exhibits greater intensity. At 14:28 UT (Fig. 4c), the aurora shows a brightened spiral structure (Hu et al., 2013), which occurs for the 427.8, 557.7, and 630.0 nm excitations. This auroral form is related to the KelvineHelmholtz (KHI) and current-sheet instabilities (Lysak and Song, 1996; Hu et al., 2013). At 17:57e22:48 UT, the auroral intensity at 427.8 and 557.7 nm is significantly enhanced, and the poleward boundary of the auroras shows periodic poleward and equatorward movement. These optical characteristics may be related to the expansion and recovery phases of auroral substorms, respectively (Akasofu, 1964). At 21:06 UT, the all-sky image captures a poleward aurora arc (Fig. 4d). This structure is related to the inverted-V precipitation cone (Newell et al., 2009).
The multi-scale aurora imaging system consists of three smallscale auroral imagers (SSAI) with FOVs of 47, 19 , and 8 . The major difference between the MWASI and the SSAI is that the SSAI consists of a front lens with a small FOV, a single BG3 glass filter, relay optics, and an EMCCD detector (1024 1024 pixels). Because the SSAI FOV is narrow and cannot cover the whole sky, the FOV center of each SSAI is focused only on the local magnetic zenith (at the magnetic meridian, the magnetic zenith is located approximately 12.13 from the geographic zenith), and the maximum spatial resolutions are approximately 20, 49, and 128 m pixel1 at an altitude of 150 km. The three SSAIs and the MWASI constitute a unique optical system for auroral observations that can zoom into the auroral structure step by step. In the current observation mode, the exposure time of every SSAI is about 0.06 s, and the sampling frequency is 14.27 Hz. These imagers, with their high-speed sampling rate, can capture rapid auroral movements at small scales and temporal fluctuations in the intensity signature. Small-scale auroras are associated with acceleration processes at altitudes of 2e3 RE (Earth radii). The auroral structure, motion, and evolution reflect the dynamical processes in the acceleration region (Galperin, 2002). Using the SSAIs at ZHS, we can obtain a highly accurate value for the scale, the lifetime, and the twodimensional evolution of these small-scale auroral structures and investigate small-scale physical processes in the acceleration ren) waves, the KHI, and the quasi-static gion, such as nonlinear (Alfve electric field (Stasiewicz et al., 2000). For instance, the spiral arm of the auroral spiral shown in Fig. 4 exhibits some wavelike structures, which are called curls or folds (see the red circle in the 47 image in Fig. 5a). These may result from electrostatic instabilities of the magnetized, precipitating electron sheet (Ivchenko et al., 2005). In addition, the brightened arc in Fig. 4d has a quasi-static arc structure (47 snapshot in Fig. 5b). The general consensus is that the formation of quasi-static (or quiet) arcs is related to inverted-V precipitation (or acceleration), which is also called quasi-static acceleration owing to the presence of an upward parallel electric field of converging electric field structure (Frank and Ackerson, 1971; Mozer et al., 1977; Ergun et al., 1998).
2.3. Auroral spectrum observation Detailed auroral spectra can provide significant information on the sources and mechanisms responsible for the auroral emission, the energy distribution and fluxes of the precipitating particles, and the dynamical and chemical conditions in the polar upper atmosphere. A meridian imaging auroral spectrograph (ASG; Fig. 6a) was installed at ZHS in May 2010 to measure the auroral spectrum in the visible region along a magnetic meridian. The ASG is composed of an objective lens (a fish-eye lens with 180 FOV), a slit, a grism, a focusing lens, and an EMCCD detector (1024 1024 pixels; Fig. 6b). The slit, with a width of 70 mm and oriented along the meridian, is placed in the focal plane of the fish-eye lens. The grism is used as the ASG's dispersive element, dispersing 400e730 nm auroral spectral lines into the light-sensitive area of the EMCCD detector. The ASG's spectral resolution is ~0.3e0.4 nm pixel1 at 550 nm. Fig. 6c presents an image of the auroral spectrum taken at 21:06:00 UT on 21 July 2014. There is clear auroral emission at 470.9 nm (Nþ 2 ), 589.0 nm (NaI), and 636.4 nm (O), as well as at 427.8, 557.7, and 630.0 nm. Other spectral lines of auroral emission are also apparent in the ASG image, although they are vague because their intensities are lower than those of the bright spectral lines.
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Fig. 4. (a) Keograms of temporal variations of the emission intensities along the magnetic meridian at 427.8, 557.7, and 630.0 nm observed with the multi-wavelength all-sky imager at 12:00e24:00 UT on 21 July 2014. The left-hand vertical axis indicates the zenith angle (ZA) on the magnetic meridian (M.S. and M.N. indicate magnetic south and north, respectively), while the horizontal axis shows the time in UT. (b) and (c) All-sky images of three auroral emission events at 12:27, 14:28, and 21:06 UT. The vertical center line in each image is the magnetic meridian. M.S. is up and M.N. is down; the left- and right-hand sides of each image are magnetic west and east, respectively. The red circles in the 557.7 nm images show the FOV of the 47 imager.
2.4. Auroral radio observation Cosmic radio noise (CRN) from distant stars and galaxies is a
radio-frequency electromagnetic wave characterized by a wide spectrum. It is generally considered to be constant outside the Earth's atmosphere. When it passes through the ionospheric D
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Fig. 5. Images of the multi-scale auroras at (a) 14:28:06 UT and (b) 21:06:06 UT on 21 July 2014. The left, middle, and right columns show images with FOVs of 47, 19 , and 8 , respectively. The red arrows in the 47 images show the directions corresponding to the red arrows in the 557.7 nm images of Fig. 4. The red circles in the 47 and 19 images show the respective FOVs.
Fig. 6. (a) Auroral spectrograph at Zhongshan Station. (b) Optical diagram of the auroral spectrograph. The meridional plane is perpendicular to this plane. (c) Image of the auroral spectrograph at 21:06:00 UT on 21 July 2014.
region (60e90 km) and the lower E region (90e140 km), the freeelectron motions in these regions are affected by the CRN. Because of the high density of neutral particles in the lower regions of the ionosphere, a fraction of the energy of the free electrons will be transferred to the neutral particles; i.e., a fraction of the energy of the CRN is transferred to the neutral particles. As a result, the amplitude of the CRN recorded by a ground-based receiver is lower than that outside the atmosphere. This process is called cosmic noise absorption (CNA; Browne et al., 1995). Auroral particle precipitation can result in ionization of the neutral atmosphere and subsequently affect the CNA. Therefore, CRN observations provide information on auroral particle precipitation. CRN observations are not affected by sunlight, so this is a good approach to investigate auroral particle precipitation during daylight. In 1997, an imaging riometer was installed at ZHS (Yamagishi et al., 2000). In 2012, it was updated with a new recording
system and a global positioning service receiver. With an 8 8 antenna array working at 38.2 MHz (Fig. 7a), the imaging riometer can uncover the two-dimensional distribution of the CNA at high spatial resolution once a second. The FOV of the antenna array is 45 at zenith, so that the width of the FOV is 200 km at a height of 90 km, and the spatial resolution of the central beam is 20 km at a height of 90 km (Fig. 7b). Fig. 8a presents the CNA Keogram for 12:00e24:00 UT on 21 July 2014 (this period is the same as that for the multi-wavelength auroral Keograms in Fig. 4a). At 14:00e15:00 UT and 21:00e21:45 UT, the CNA is significant, at more than 0.6 dB. Compared with the auroral MWASI Keograms (Fig. 4a), during both periods intense aurora emission occurs within the range from 30 to 30 from the zenith at ZHS, which is also shown in the FOV of the imaging riometer. The two-dimensional CNA images at 12:27:26, 14:28:06, and 21:06:06 UT are shown in Fig. 8b, corresponding to
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Fig. 7. (a) Antenna array of the Imaging Riometer at ZHS. (b) Lobe pattern of the Imaging Riometer at a height of 90 km (‘þ’: lobe center).
Fig. 8. (a) CNA Keogram along the magnetic meridian (from 73 to 76 magnetic latitude) acquired from the Riometer at ZHS, Antarctica for 12:00e24:00 UT on 21 July 2014 (in units of dB). (b) CNA at 12:27:26 UT, 14:28:06 UT, and 21:06:06 UT (M.E., M.W. represent magnetic east and west, respectively).
the 557.7 nm all-sky auroral images in Fig. 4b, c, and 4d, respectively. The red circles in the 557.7 nm images show the approximate FOV of the imaging riometer. At 12:27:26 UT, the two-dimensional CNA is weak, and there is no obvious aurora at ZHS zenith (Fig. 4b). At 14:28:06 UT (Fig. 4c) and 21:06:06 UT (Fig. 4d), a bright spiral and an intense auroral arc appear at the ZHS zenith, respectively. Correspondingly, the two-dimensional CNA at these times (Fig. 8b) is significant.
3. Recent progress in aurora studies at Zhongshan Station Using optical and radio auroral data obtained with the auroral instruments at ZHS, and combining with other radio and satellite data, the following studies were carried out during the past five years. Ultraviolet imagers (UVIs) aboard the Polar and Viking satellites showed that spatially periodic bright spots appear frequently in a ‘string of pearls’ configuration in the post-noon oval (1400e1600 MLT sector; Lui et al., 1989). The mechanism responsible for the
post-noon bright spots and the visible form observed by groundbased imagers is unclear. Using the ASI at ZHS in Antarctica and the Polar UVI in Arctic, Hu et al. (2013) showed that multiple bright auroral spirals in post-noon auroral arcs are seen in the FOV of ASI at ZHS in the Southern Hemisphere, while multiple bright spots are seen by the Polar UVI at the conjugate FOV of ASI in the post-noon auroral oval in the Northern Hemisphere. These observations confirm that auroral spirals are the visible, ground-based characteristics of post-noon UV bright spots, and it has been suggested that the current-sheet instability (Lysak and Song, 1996) above the parallel electric-field region is a possible cause of the bright spots in the ionosphere. Shock auroras, which are caused by interplanetary shocks or solar-wind pressure pulses, are one of the most significant visible indications of the dynamical processes related to the solar windemagnetosphereeionosphere coupling process (Zhou et al., 2003). Using auroral observations at ZHS and the South Pole Station, combined with ionospheric convection observed by the SuperDARN radars, Liu et al. (2011) showed that the auroral intensity
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decreases in the post-noon oval but increases in the pre-noon oval when the solar wind pressure is amplified. At the same time, the direction of the ionospheric plasma flow is reversed anti-sunward in the post-noon oval. These results could be understood with reference to the physical model of sudden commencement proposed by Araki (1994). Based on long-term studies of the CNA observed with the riometers, it has been found that the CNA is measured relative to the baseline of the cosmic noise signal level recorded under quiet ionospheric conditions. Hence, it is important to obtain a reliable quiet-day curve (QDC). The cosmic noise detected not only shows diurnal and seasonal changes, but it is also strongly affected by solar activity, geomagnetic disturbances, and man-made electromagnetic interference. Based on previous research, He et al. (2014, 2015) used data acquired with the ZHS imaging riometer to develop a new technique for estimating the QDC. Compared with the previous methodology for measuring the QDC, He's technique produced a better QDC, which will result in a more consistent correspondence between the CNA and the in situ aurora, and a better expression of the small-scale structures of the weak absorption regions (He et al., 2015). 4. Summary and prospects The Chinese Antarctic Zhongshan Station is located at a unique geographical site that is well suited to observe the cusp, the postnoon dayside, and the nightside aurora, and also to perform cusplatitude conjugate observations with the Chinese Arctic Yellow River Station. An advanced synthetic auroral observation system was deployed at ZHS in 2010, including a multi-wavelength all-sky auroral imaging observation component, a multi-scale auroral imaging observation system, an auroral spectrum observation set-up, and an auroral radio observation station. This auroral observation system has been used to perform several investigations of the visible characteristics of UV ‘bright spots,’ variations in the dayside shock auroras and convection, and the methodology for obtaining the QDC, among others. Based on these analyses, our understanding of the solar windemagnetosphereeionosphere coupling process has been enhanced. Geospace is a significant part of the environment of importance for the survival and development of mankind. Space weather has a marked effect on human activities. As a direct, significant indicator of the solar windemagnetosphereeionosphere coupling process, auroras are increasingly important for the monitoring of space weather. At Taishan (labeled ‘2’ in Fig. 1) and Kunlun (labeled ‘3’ in Fig. 1) stations (which are also located at the cusp-latitude auroral oval and are suitable for observations of the dayside aurora), an aurora monitoring system will be installed in the next five years, which will establish a dayside aurora-monitoring chain in Antarctica (including the South Pole Station, labeled ‘4’ in Fig. 1). The auroral monitoring chain almost overlaps with the Chinese Antarctic magnetometer chain at the cusp latitude (Liu et al., 2016) and will greatly promote future research and monitoring capabilities of space weather, auroral physics, and the solar windemagnetosphereeionosphere coupling process. Acknowledgments This work was supported by the Polar Environment Comprehensive Investigation and Assessment Programs (CHINARE2017), the National Natural Science Foundation of China (grants 41274164, 41431072, 41674169, 41374161, 41474146, and 41374159), the Pudong Development of Science and Technology Program (PKC2013-207), the Chinese Meridian Project, and the National Program for Support of Top-Notch Young Professionals. Data were
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Please cite this article in press as: Hu, Z.-J., et al., Multi-wavelength and multi-scale aurora observations at the Chinese Zhongshan Station in Antarctica, Polar Science (2017), https://doi.org/10.1016/j.polar.2017.09.001