THE SUN AND THE EARTH: DOUBLESTAR

Despite the interest of Zhao Jiuzhang in the space environment, dedicated spacecraft were slow to emerge. The first space environment program, started in 1988 and called Meridian, was ground-based using 15 locations, including Zhongshan base at the South Pole, and later extended to sounding rockets. Europe offered a new opportunity. China and Europe had first agreed a cooperation program in 1980 (Chapter 3), which took concrete form 12 years later when China made arrange-

ments with the European Space Agency to take data from the Cluster project, an upcoming major venture with four satellites to study the Sun’s interaction with the Earth’s magnetosphere. China may have spotted an opportunity to participate in an international scientific program at relatively low cost and, in 1997, China proposed its own complementary project, Doublestar, called Tan Ce or “explorer” in Chinese. A feasibihty study concluded in 1999 and the program was approved by the Chinese government in 2000, leading on 9th July 2001 to an agreement in Paris with the European Space Agency. Cluster was originally to fly in 1996, but the probes were blown apart when Europe’s Ariane 5 exploded on its maiden mission. The backup models were taken out of storage to fly into orbit on the Russian Soyuz in summer 2000, so Tan Ce was very timely.

The notion of multiple satellites to explore the magnetosphere was well established, the main example being the Russian Interball project in which two sets of satellites had explored the magnetosphere from 1994 to 1995. Like Interball, China’s Doublestar system also comprised two satellites – hence the title “Double­star” – and proposed, using similar instruments, that their findings be cross – referenced to those of Cluster. Doublestar was a complementary mission insofar as the Chinese planned to reach regions of the sky inaccessible to the Cluster probes and build up a three-dimensional picture. Tan Ce 1 was originally to orbit out to 8 Earth radii, but Chinese scientist Zuyin Pu proposed that be lengthened to 12 Earth radii so as to extend the Cluster data. Their orbits were synchronized in such a way that all six satellites would, from time to time, be in the same line to observe solar activity.

The Doublestar mission design was for a first, equatorial satellite concentrated on the Earth’s magnetic tail, while the second, polar satellite checked out the magnetic poles and the resulting auroras. The mission aimed to improve scientists’ knowledge of magnetic storms which can upset communications, radar, and navigation systems on the Earth. It was anticipated that each mission would last a year, this short length determined by the damage resulting from regular passage through intense radiation belts. The European Space Agency contributed a modest €8m to the mission in return for four hours a day of data over the planned 18 months of the missions. The instrumentation is detailed in Table 7.4.

These were small satellites, about 350 kg in weight, 1.2 m high, 2.1 m in diameter, with a solar array of 6.33 m2 able to generate 280 W, with a design life of 12­18 months. Ground receiving stations were configured to receive data in Beijing, Shanghai, and Villafranca, Spain, while data centers were established in Beijing, China; Toulouse, France; Noordwijk, the Netherlands; Didcot, Britain; and Graz, Austria. The program got under way very quickly, despite interruptions from the Severe Acute Respiratory Syndrome (SARS) medical emergency.

The equatorial satellite was launched first, lifting off from Taiyuan on a Long March 2C on 29th December 2003, broadcast on Chinese TV. It entered a highly elliptical orbit of 570-78,948 km, the furthest orbit ever achieved by China, inclination 28.5°. One boom did not deploy but this did not have a large negative impact. It made its first observations on 21st January 2004, a 6.1 solar flare. The next day, 12.6 Earth radii out, it noted that the pressure of the solar wind had

Table 7.4. Tan Ce instruments.

Both spacecraft

Fluxgate magnetometer	Britain
Plasma electron current experiment	Britain
High-energy electron detector	China
High-energy proton detector	China
Heavy-ion detector	China

TC-1 equatorial/ tail

Active space potential controller Austria

Hot-ion analyzer France

TC-2 polar

Energetic neutral atom imager Ireland

Low-frequency electromagnetic wave detector China

Tan Ce instrument testing. Courtesy: Susan McKenna-Lawlor.

increased by five times. Tan Ce 2 was duly launched on 25th July 2004, entering a somewhat different orbit, of 560-38,278 km, 90°, circling the Earth every 7.3 hr. By operating with Cluster, data could be collected from six data points. In August

2004, for example, Tan Ce 1 and 2 were in the trapped region behind the Earth, while the four Cluster satellites were further behind in the neutral sheet. In February

2005, by contrast, Tan Ce 1 was on the sunward side, Tac Ce above the Earth at the cusp, and Cluster in the magnetosheath.

The initial mission lasted a year to August 2005. By May 2006, ground controllers had received 175 GB from Tan Ce 1 and 145 GB from Tan Ce 2. Tan Ce l’s backup attitude controller failed during a big magnetic storm, but ground controllers were able to keep the spacecraft under control. Both missions were extended to September 2007, the official mission termination point. Tan Ce 1 decayed on 14th October 2007. Tan Ce 2 was lost in August 2007 but, to some surprise, was recovered that November.

The Tan Ce and Cluster missions led to at least 1,000 scientific papers. The International Academy of Astronautics (IAA) conferred the prestigious Laurels for Team Achievement Award on the Double Star/Cluster Team for providing unprecedented measurement capability and discoveries in geospace. The main fields covered by the two spacecraft were geomagnetic storms; magnetospheric sub-storms;

This shows the two Tan Ce satellites, though they flew far apart. Courtesy: ESA.

magnetic reconnection; the interaction of the solar wind with the magnetosphere and ionosphere; changes in the radiation belt, the ring current, and the plasmasphere; the plasma sheet; geomagnetic pulsations; space plasma; and the magnetosheath, magnetopause, cusp, and polar cap.

The first results of the mission were presented at a symposium on Cluster and Doublestar in Noordwijk, the Netherlands, in September 2005, which took in the results of 21 simultaneous magnetopause crossings. The initial scientific results from Tan Ce were:

• they confirmed the theory of magnetic reconnection in the Earth’s magneto­sphere; they found multiple reconnection sites and flux ropes 6.3 Earth radii out in the Earth’s fragmented magnetotail; Flux Transfer Events (FTEs) were noted at the points of reconnection, speeding at between 170 and 250 km/sec;

• they discovered 140 ion density holes in the solar wind upstream of the bow shock, several thousands of kilometers apart, in upstreaming particles;

• Tan Се 1 detected cracks in a neutron star crust during a starquake;

• they found density holes ahead of the bow shock;

• ultra-low-frequency waves made the magnetic field lines wobble; and

• low latitude is the best place for ultra-low-frequency waves to assist solar wind particles to penetrate the magnetopause [6].

Later, two detailed mission reports were issued [7]. The principal highlights were:

• Tan Се 1 recorded 516 tailward flow events at between 7 and 13 Earth radii and found eight magnetic flux ropes;

• individual magnetic storms were studied in detail, such as the “Halloween storm” of 31st October 2003 and the violent storm of 21st-22nd January 2005 (mach 5.4); Tan Се 1 observed a sub-storm on 12th October 2004, noting low-density, high-temperature ions originating from the ionosphere and flowing along the magnetic field – observations matched with the American Geotail;

• two oxygen-rich Bursty Bulk flows (BBFs) were observed during the magnetic storm of 8th November 2004;

• Tan Се 1 measured bursts of flows from the Sun, typically 48-103 sec during storms, their velocity (rising from 390 km/sec to 520 km/sec), and ion densities (ranging from 0.14 cm-3 to 0.28 cm-3);

• Tan Ce 2 observed 14 dawn chorus events in November 2004, the outbreak of radio noise associated with solar storms near the equatorial plane and their spreading to the mid and higher latitudes; and

• the polar spacecraft found vortex-like plasma flows at the boundary of the outer radiation belt and the ring current, going in opposite rotational directions.

FTEs were a feature of particular interest. Between February and April 2004, Tan Се 1 detected 27 FTEs, mainly at low latitudes, moving along the sides of the magnetosphere into the magnetotail, this time being matched with Europe’s Cluster spacecraft. Individual FTEs that affected both the four Cluster spacecraft and Tan Се 1 were studied, such as the 10-min FTE of 13th March 2004, enabling the profiling of an individual event in extraordinary detail. The typical duration of an FTE was measured: 130 sec.

A scientist who won particular recognition for his part in the mission was Zuyin Pu, who was awarded the 2010 COSPAR Vikram Sarabhai gold medal for his work on the anti-parallel reconnection of the magnetosphere at low and high latitudes, magnetic nulls, and energy transport from the solar wind to the magnetosphere, generating micro-pulsations [8]. Another was Susan McKenna-Lawlor of Space Technology Ireland, located on the campus of the National University of Ireland, Maynooth, who was responsible for Tan Ce 2’s NeUtral Atom Detector Unit (NUADU – the name of a Celtic warrior). NUADU was designed to monitor the ring current during geomagnetic storms and data were received up to mission end.

Tan Ce results: the ring current (right). Courtesy: Susan McKenna-Lawlor.

The unit featured the capability to record four Energetic Neutral Atom (ENA) distributions. These ENA data were used to remotely monitor the evolution of the terrestrial ring current during significant geomagnetic storms, thereby providing new insights into solar-related dynamic magnetospheric processes [9]. Bright ENA emissions recorded at the feet of terrestrial magnetic field lines during magnetic storm events indicated the presence of strong related increases in the fluxes of trapped energetic charged particles. ENA data recorded by NUADU and by NASA’s IMAGE/HENA instrument while viewing the northern and southern hemispheres during a major magnetic storm provided the first views of the ring current to be simultaneously obtained in both hemispheres.

The successor to Doublestar is the MIT mission, which stands for Magnetosphere- Ionosphere-Thermosphere, now in development. The purpose of MIT is to study:

The frame of the upcoming MIT mission. Courtesy: Susan McKenna-Lawlor.

• the processes that trigger magnetospheric storms and enable their recovery;

• the transport of ionospheric ions in the magnetosphere;

• the behavior of electrical fields during storms, with their temporal and spatial parameters;

• temperature variations during geomagnetic storms, their seasonal and diurnal variations; and

• the generation, propagation, and dissipation of large-scale gravity waves during storms.

Four satellites are involved: two in perpendicular polar orbits about the Earth at 600 km, called the thermosphere satellites (T1 and T2); a magnetospheric satellite in polar orbit between 1 and 7 Earth radii (M); and a solar wind satellite (S), in an equatorial orbit of 3-25 Earth radii. The instrument package has already been indicated and is outlined in Table 7.5.

Table 7.5. MIT instruments.

Magnetic field detector Electrical field detector Neutral particle spectrometer Plasma analysis system Neutral atom imaging suite Aurora imager

Limb aurora and airglow imager Atmospheric wind and temperature remote sensor

One will carry a new Neutral Atom Detector Unit following NUADU (NAIS-H) but featuring higher spatial resolution combined with a Low Energy Neutral Atom Imager (NAIS-L).