太空|TAIKONG 国际空间科学研究所 - 北京 ISSI-BJ Magazine No

太空|TAIKONG 国际空间科学研究所 - 北京 ISSI-BJ Magazine No. 9 October 2016

LINK BETWEEN SOLAR WIND, MAGNETOSPHERE, AND IONOSPHERE FOREWORD

On July 6-7, 2016, the International recognized the very high scientifc Space Science Institute in value of the mission, and raised IMPRINT Beijing (ISSI-BJ) successfully constructive comments and organized a two-day Forum on suggestions on the mission concept, 太空 | TAIKONG “The Link between Solar Wind, payloads key techniques, and data ISSI-BJ Magazine Magnetosphere, Ionosphere”. product. They concluded that the ISSI-BJ Forums are informal, SMILE mission has complementary free debates, and brainstorming objectives to existing or future meetings among high-level solar space plasma missions. participants on open questions of Therefore, the SMILE mission is Address: No.1 Nanertiao, scientifc nature. In total, 28 leading yet another excellent example of Zhongguancun, scientists from eight countries how the Chinese Space Science Haidian District, participated in this Forum, which institutions can work together with Beijing, China was convened by Chi Wang the European Space Agency on Postcode: 100190 (NSSC, CAS), Graziella Branduardi- innovative and challenging, and Telephone: Raymont (MMSL-UCL, UK), Benoit complementary to the existing, +86-10-62582811 Lavraud (CNRS, France), Tony Lui missions. This offers signifcant Website: (APL, USA), and Maurizio Falanga opportunities for cooperation www.issibj.ac.cn (ISSI-BJ, China). through mission coordination and scientifc analysis that places The Forum’s main aims divided the Authors SMILE and China-Europe in a meeting into 4 sessions: Overview of central position, due to its unique the Solar Wind Magnetosphere and Graziella Branduardi - objectives and technology. Raymont (MSSL-UCL, UK), Ionosphere Coupling; Key Science C. Philippe Escoubet (ESA/ of the Solar wind, Magnetosphere, This TAIKONG magazine provides ESTEC, The Netherlands), Ionosphere Coupling; Instruments an overview of the scientifc Kip Kuntz (JHU/APL, USA), and Capability Required; Synergies objectives and the overall design Tony Lui (JHU/APL, USA), Complementary Missions and of the SMILE project, including Andy Read (Leicester U., International Collaborations. In spacecraft and instrumentation UK), David Sibeck (NASA/ this context, the European Space discussed during the Forum. Agency (ESA) and the Chinese GSFC, USA), Tianran Sun I wish to thank the conveners and Academy of Sciences (CAS) (NSSC/CAS, China), Brian organizers of the Forum, as well selected a joint small mission Walsh (Boston U., USA), as the ISSI-BJ staff, Lijuan En, (SMILE, to be launched in 2021) Chi Wang (NSSC/CAS, Anna Yang, and Xiaolong Dong, for to trace these processes from China) actively and cheerfully supporting beginning (the Sun) to end (the the organization of the Forum. In Earth's aurora), and investigate – in Editors particular, I wish to thank the authors, a way unmatched so far – how the who, with dedication, enthusiasm, solar wind interacts with the Earth's Anna Yang and seriousness, conducted the magnetic environment. Maurizio Falanga whole Forum and the editing of this The Forum started with an report. Let me also thank all those overview and goals of the who participated actively in this Front Cover SMILE mission. The participants stimulating Forum. discussed the interaction between the Earth's protective shield – Prof. Dr. Maurizio Falanga An artist's impression of the the magnetosphere – and the Solar-wind Magnetosphere supersonic solar wind. SMILE is Ionosphere Link Explorer expected to give an important (SMILE) mission. contribution to our understanding of space weather and, in particular, (Credit: CAS; the physical processes taking place Beijing Special thanks to: during the continuous interaction Chi Wang) between the solar wind and the October 2016 magnetosphere. The participants

2 太空|TAIKONG INTRODUCTION

Forum Overview

The interaction between the The Forum concentrated on Forum also reviewed the cur- solar wind and Earth’s mag- the main scientifc drivers for rent status and future plans for netosphere, and the geospace the Solar-wind Magnetosphere SMILE, the primary scientifc dynamics that result, comprise Ionosphere Link Explorer goals, the needed technolo- a fundamental driver of space (SMILE) mission, and how they gies, and how to best optimize weather, the conditions on the defne the mission specifca- international collaborations. Sun, in the solar wind, and tions, reviewed lessons learned in the magnetosphere, iono- from the previous in situ and The forum was sponsored by sphere and thermosphere, that imaging missions, discussed ISSI-BJ, with partial support can infuence the performance the SMILE mission for soft from the State Key Laboratory and reliability of technological X-ray magnetospheric imaging of Space Weather, National systems and endanger human and UV auroral imaging, com- Space Science Center (NSSC), life and health. Understanding pared the soft X-ray simulated and the Chinese Academy of how this vast system works results from different numeri- Sciences (CAS). requires knowledge of energy cal models, and examined op- and mass transport, and of the portunities for synergies with coupling both between regions complementary observations and between plasma and neu- from other space missions and tral populations. ground-based facilities. The

Background of the Solar Wind, Magnetosphere, Ionosphere

The solar wind is a stream of wind compresses charged particles (protons, the sunward side electrons, and heavier ionized of the magneto- atoms) released from the up- sphere but drags per atmosphere of the Sun. the nightside out The solar wind is divided into into a long mag- two components, respectively netotail. termed the slow solar wind and the fast solar wind. The slow The interaction solar wind has a velocity of of the solar wind about 400 km/s, a temperature with Earth leads of 1.4–1.6×106 K and a compo- to the formation sition that is a close match to of the magneto- the solar corona. By contrast, sphere, including the fast solar wind has a typi- the bow shock, cal velocity of 750 km/s, a tem- magnetosheath, perature of 8×105 K and it nearly cusps, mag- matches the composition of the netopause and Fig. 1: The dayside magnetosphere. The magne- Sun's photosphere. Near the the magnetotail topause represents the outer boundary of the mag- Earth, the solar wind encoun- (Figure 1). netosphere, and is compressed on the dayside. ters the Earth’s magnetic feld The bow shock compresses and defects the solar and the particles are defected wind so that it may fow around the magnetopause. by the Lorentz force. The solar

太空|TAIKONG 3 As shown in Figure 1, a col- wind plasma to penetrate deep and slow solar wind can be in- lisionless bow shock stands into the magnetosphere, all the terrupted by large, fast-mov- upstream from the magneto- way to the ionosphere. The ion- ing bursts of plasma called pause in the supersonic so- osphere is a region of Earth's interplanetary coronal mass lar wind. The shocked solar upper atmosphere, from about ejections, or CMEs. When a wind plasma fows around the 60 km to 1,000 km altitude. It is CME impacts the Earth's mag- magnetosphere through the ionized by solar radiation, plays netosphere, it temporarily de- magnetosheath. A relatively an important part in atmospher- forms the Earth's magnetic sharp transition from dense, ic electrical activity and forms feld, changing its direction and shocked, highly ionized solar the inner edge of the magneto- strength, and inducing large wind plasmas to tenuous, less sphere. electrical currents; this is called highly ionized magnetospher- a geomagnetic storm and it is ic plasmas marks the magne- The position and shape of the a global phenomenon. CME topause. High latitude cusps magnetopause change con- impacts can induce magnet- denote locations where feld stantly as the Earth’s magne- ic reconnection in the Earth's lines divide to close either in tosphere responds to varying magnetotail; this launches pro- the opposite hemisphere or far solar wind dynamic pressures tons and electrons downward down the magnetotail. Weak and interplanetary magnetic toward the Earth's atmosphere, feld strengths within the cusps feld orientations. Both the fast where they form the aurora. provide an opportunity for solar

GLOBAL MEASUREMENTS AND THE SOLAR WIND- MAGNETOSPHERE INTERACTION

Heliophysicists seek to un- A host of mechanisms have enhanced interactions during derstand, and model, the pro- been proposed to explain the intervals of southward inter- cesses governing the fow of nature of the solar wind-mag- planetary magnetic feld (IMF) solar wind mass, energy, and netosphere interaction, and in orientation. Therefore statistical momentum through the Sun - particular the entry into, stor- studies of remote observations Solar Wind - Magnetosphere - age within, and release from the demonstrating that ionospher- Ionosphere system. With this magnetosphere of solar wind ic convection, the strength of knowledge in hand, they will mass, energy, and momentum feld-aligned currents into and be able to forecast geomag- (Figure 2). Proposed mag- out of the ionosphere, the like- netic storms, the most haz- netopause entry mechanisms lihood of geomagnetic sub- ardous space weather events include solar wind pressure storms, and the magnitude in the near-Earth environment. variations battering the magne- of geomagnetic storms all in- Storms enhance the fuxes of tosphere, the Kelvin-Helmholtz crease for southward IMF ori- energetic particles within the (wind-over-water) instability on entations, point to reconnection magnetosphere to levels capa- the magnetopause, diffusion as the dominant mode of solar ble of harming spacecraft elec- driven by wave-particle inter- wind-magnetosphere interac- tronics, drive powerful currents actions, and magnetic recon- tion. Reconnection may be into the ionosphere that cause nection. Proposed magnetotail the cause or consequence of surges in electrical power line release mechanisms include a various plasma instabilities pro- transmission, enhance exo- host of plasma instabilities, e.g. posed to occur within the near- spheric densities and therefore ballooning or cross-tail current Earth magnetotail. drag on low-latitude space- driven instabilities, and mag- craft, and modify ionospheric netic reconnection. Reconnection is a microphys- densities in ways that severely ical process with macrophys- impact GPS navigation and sat- In contrast to all the other mech- ical consequences. The need ellite communication. anisms, reconnection predicts to understand the microscale

4 太空|TAIKONG physics underlying reconnec- microsatellites capable of mak- the amount of closed fux within tion has led to the launch of ing in situ measurements at all the dayside magnetosphere, multispacecraft missions like relevant locations. the rate of magnetopause ero- ISEE-1/2, Cluster, THEMIS, sion or recovery provides infor- and MMS with ever decreas- In the absence of any plans for mation concerning the steadi- ing interspacecraft separations. such a constellation, imagers ness of reconnection, while the These missions have confrmed can supply the global mea- location of the portion of the the presence of the acceler- surements needed to under- magnetopause that moves pro- ated plasma fows, magnetic stand the nature of the solar vides information concerning feld components normal to the wind-magnetosphere interac- the component or antiparallel magnetopause and magneto- tion. The boundaries seen in nature of reconnection. tail current sheet, streaming en- soft X-ray (and low energy neu- ergetic particles, and boundary tral atom) images correspond to Soft X-ray imagers can also be layers containing admixtures of plasma density structures like used to track the equatorward the particle populations on both the bow shock, magnetopause, motion of the cusps during the sides of reconnecting current and cusps. Thus soft X-ray substorm growth phase and sheets at the magnetopause imagers can be used to tract their poleward motion following and within the magnetotail, just the inward erosion of the day- onset. Just as in the case of the as predicted by reconnection side magnetopause during the magnetopause, cusp observa- models. growth phase of geomagnet- tions can be used to determine ic substorms and the outward the amount of closed fux with- While isolated single or close- motion of this boundary follow- in the dayside magnetosphere, ly-spaced multipoint in situ ing substorm onsets. The loca- the rates of erosion and recov- measurements can be used to tion of the magnetopause pro- ery, the steadiness of recon- identify reconnection events vides information concerning nection, and the equatorial or and study the microphysics of reconnection, they can- not be used to distinguish between models in which reconnection is predominantly patchy or global, transient or continuous, triggered by solar wind features or occurring in response to intrinsic current layer instabilities, component and occuring on the equatorial magnetopause or antiparallel and occurring on the high-latitude magnetopause. Nor can isolated measurements be used to determine the global state of the solar wind-magnetosphere interaction, as measured by the rate at which closed magnetic fux is Fig. 2: A snapshot of the complex plasma density structures generated by opened or open fux closed. the solar wind-magnetosphere interaction according to the Lyon-Fedder- For all of these tasks, and Mobarry (LFM) global magnetohydrodynamic simulation (C. Goodrich, per- many more, global observa- sonal communication). Color shading indicates the density in the noon-mid- tions are needed. It would, night meridional plane, while lines in the lower density inner magnetospheric however, be a major under- cavity suggest the magnetospheric magnetic feld confguration. The inset taking to launch a fotilla of in the lower right corner shows corresponding predictions for auroral activity in the northern hemisphere.

太空|TAIKONG 5 high-latitude location of recon- instability. Global imagers are Finally, measurements of the nection. needed to determine the oc- solar wind plasma and mag- currence rates and extents of netic feld input to the magne- Global auroral images from a these transients, which in turn tosphere by a monitor located high inclination, high altitude, determine their importance to near Earth are essential for the spacecraft provide an excel- the solar wind-magnetosphere above studies, because having lent complement to soft X-ray interaction. Observations of such a monitor reduces con- images. The dimensions of the the nightside auroral oval can cerns regarding the arrival times auroral oval indicate the open be used to pinpoint the time of of possible solar wind triggers magnetic fux within the Earth’s substorm onset, determine the for magnetospheric events and magnetotail. Poleward and extent of the reconnection line reduces concerns regarding the equatorward motions of the in the magnetotail, and distin- dimensions of solar wind struc- dayside and nightside auroral guish between steady, bursty, tures transverse to the Sun- oval provide crucial informa- and sawtooth modes of re- Earth line. In situ measure- tion concerning the occurrenc- connection in the magnetotail. ments from the same plasma es and rates of reconnection Global auroral images can be and magnetic feld instruments at the dayside magnetopause used to test the recently pro- on the observing spacecraft on and within the Earth’s magne- posed hypotheses that plasma an elliptical polar orbit can also totail. Ground-based auroral fows (and aurora) originating play a crucial role in validating imagers frequently observe within the dayside oval and the inferences concerning pro- transients in the dayside aurora streaming across the polar cap cesses at the magnetopause which can be interpreted as ev- trigger substorm onset when and in the magnetotail that are idence for bursty reconnection they reach the nightside oval. drawn from the soft X-ray and and/or the Kelvin-Helmholtz auroral imagers. A NOVEL METHOD TO IMAGE THE MAGNETOSPHERE

Solar wind charge-exchange SWCX is that due to the Earth’s dark side of the Moon suggest- (SWCX) occurs when highly ion- exosphere, which is primarily ed that the bulk of the emission ized species in the solar wind hydrogen, interacting with the was cis-lunar. The LTE rates interact with neutral atoms. shocked, compressed, solar were later shown to be strongly An electron from the neutral is wind in the magnetosheath. correlated with the solar wind transferred to the ion, initially in fux, and thus likely to be due a highly excited state. On relax- SWCX emission due to the to SWCX. ation to the ground state one or magnetosheath was frst ob- more photons are emitted, usu- served by ROSAT, though its The SWCX fux is given by the ally in the extreme ultraviolet source was a mystery at the integral along the line of sight or the soft (low energy) X-ray. time. ROSAT scanned great cir- of ς(nnnpvrel)=ςQ where nn is The energy band below 0.5 keV cles through the ecliptic poles, the density of neutral particles, is extraordinarily rich in SWCX with each scan overlapping np is the density of solar wind emission lines from a large ~95% of the previous scan. protons, vrel is their relative ve- number of ionization states of a Comparison of successive locities, and ς contains the in- large number of species, while scans revealed strong temporal formation about ion abundanc- the 0.5-2.0 keV band is dom- variations with scales of hours es, interaction cross-sections, inated by a few strong lines to days that were dubbed the branching ratios, etc. Q can be due to charge-exchange by “Long Term Enhancements” determined from MHD models O+7, O+8, Ne+9, and Mg+11. There (LTE). A large-scale minimiza- of the magnetosheath. However are many sources of SWCX tion routine was used to isolate the value of ς for strong lines is emission in the heliosphere, the LTE, though the absolute sometimes quite uncertain, and including comets and the neu- minimum level could not be de- for weak lines it is usually com- tral interstellar medium that termined. Comparison of the pletely unknown. A recent com- fows through the solar system. LTE rate during an observation parison of the ROSAT LTE rates Typically the brightest source of of the Moon to the fux from the with the Q determined from

6 太空|TAIKONG models for the solar wind during recent results from astrophysi- It should also be noted that the ROSAT observations has cal missions. studies of the X-ray emission led to a determination of ς for from the magnetosheath re- the ROSAT ¼ keV band, which SWCX emission from the mag- quire wide-feld imagery, an allows one to scale any MHD netosheath has been observed area of current interest in as- model of the magnetosheath by all recent astrophysical trophysics. For low to median to X-ray emissivity. Thus, one X-ray observatories. The XMM- solar wind conditions, the sig- can feel relatively confdent of Newton observatory is in high nal from the magnetosheath is the simulations of instrumental Earth orbit and sometimes ob- only a few times stronger than views of the magnetosheath. serves through the nose of the the soft X-ray background. magnetosheath. Given the ex- Thus, study of the X-ray emis- Three different groups have pected SWCX X-ray brightness sion from the magnetosheath been simulating the X-ray emis- of the dayside magnetosheath will require astrophysical tech- sion from the magnetosheath. such observations can serve as niques and expertise. In return, Although there has as yet not an important check on our sim- astrophysics is deeply interest- been a detailed comparison, it ulations. Discrepancies when ed in detecting the Warm Hot is clear that useful parameters, comparing predicted and ob- Intergalactic Medium through such as the magnetopause served emission strengths may O+6 and O+7 emission, and thus distance, can be determined be due to errors in the distanc- depends upon researches such for a large range of observing es to the magnetopause pre- as these to characterize and aspects, so long as the space- dicted by the MHD models. The remove the SWCX emission. craft is suffciently far from the differences in the underlying Understanding the SWCX from Earth. Determining the magne- MHD codes demonstrate the the magnetosheath will neces- topause distance is particular- need for X-ray observations to sarily require interdisciplinary ly interesting, not only for the constrain and validate the MHD study. science goals described above, results. but also given the divergent

AURORA AND SUBSTORM

A visible manifestation of the solar wind-magnetosphere-ionosphere coupling system is the aurora (Figure 3). A well-recognized analogy of how aurora provides a vivid image of this coupling is that of a cathode-ray tube in the old-fashioned television set. The ionosphere acts like the screen of the television set and the aurora represents the image formed by electron beams generated within the system due to its electromag- netic coupling activities. With this analogy in mind, one could extract valuable insights on the state as well as sites of disturbances of this coupled system. Fig. 3: Northern aurorae viewed by the IMAGE spacecraft (FUV instrument) on 15 July 2000 (Credit: NASA). The aurorae follow an oval approx- Although geomagnetic imately centred on the Earth magnetic pole. storms that last for days were

太空|TAIKONG 7 Sun

(a) quiet auroral (b) auroral breakup (c) substorm arcs (expansion onset) expansion

(d) late substorm (e) substorm (f) quiet auroral expansion recovery arcs

Fig. 4: A schematic diagram to illustrate the sequence of global auroral distribution viewed from above the North Pole during the progress of an auroral substorm. The concentric circles are the geomagnetic latitudes 10º apart. recognized early in space re- the auroral oval (Figure 4a). At interval when magnetospheric search as major disturbances, a substorm expansion onset, one energy is accumulated for later breakthrough came in the mid- of the parallel arcs, typically the release for substorm activities. 1960s suggesting that geo- most equatorward one, bright- Furthermore, it was later found magnetic storms seemed to be ens (Figure 4b) and breaks that the buildup of some geo- built up by a more fundamental up as the auroral activities ex- magnetic storms could occur disturbance period that lasts for pand poleward, westward as without having frequent sub- only a few hours based on auro- a surge-shape form, and east- storm occurrence. ral observations from a network ward as auroral patches (Figure of All-Sky-Cameras (ASCs) in 4c-4d). After about half an hour In spite of the discovery of the the polar region. Hence, that of these activities, auroral ac- substorm concept more than fundamental disturbance inter- tivities stop advancing pole- half a century ago, the physi- val was named substorm. In ward, westward and eastward, cal process for its development particular, the substorm interval followed by gradual retreat and as well as the possible solar can be broken down into two diminishing of former auroral wind features linking to its oc- phases initially, namely, expan- activities (Figure 4e) to return currence are still open ques- sion and recovery, as illustrated to the pre-substorm-expansion tions. There are some recent by the global auroral morphol- auroral distribution (Figure 4f). developments that may give us ogy depicted in Figure 4 when Later research indicated that the possibility to resolve these viewed from above the North this cyclical concept could be open questions when coor- Pole. Prior to substorm expan- extended to describe distur- dinated global observations, sion, auroras occur often as bances in the magnetosphere such as SMILE will return, and arcs aligned more or less paral- as a whole and an addition- ground-based auroral observa- lel to the geomagnetic latitudes al phase, called the growth tions are combined to address in a circumpolar belt known as phase, was added to mark the these issues.

8 太空|TAIKONG The frst development is the re- and disjoint features appear expansion onset. This feature cently proposed link based on when joining images from dif- was not recognized earlier due ground-based observations. ferent ASCs. Most importantly, to its low intensity and is shown This link suggests that some with simultaneous monitoring in Figure 4b. The auroral beads solar wind features initiate ion- of the solar wind impacting the have specifc wavelengths and ospheric disturbances on the dayside magnetosphere, the corresponding exponential dayside auroral oval. These dis- solar wind features that cause growth in the auroral intensity turbances are visible as auroral the initiation of the dayside ac- that are different from case to patches and/or ionospheric en- tivity that eventually leads to a case, apparently dependent on hanced fows in the polar region substorm development can be the state of the magnetosphere moving to the nightside away identifed with ease. This would just prior to substorm expan- from the Sunward direction. be a tremendous advance over sion onset. The characteris- When these features reach the the prevailing perception that tics of auroral beads revealed nightside poleward boundary southward IMF is generally fa- recently impose another set of the auroral oval, the aurora vorable for substorm develop- of rather severe observational at that location brightens and ment without the more refned constraints that discriminate sends equatorward another au- identifcation of any specifc so- among several potential sub- roral feature, known as auroral lar wind feature. storm onset processes under streamer, presumably related to consideration. Two potential fast plasma fows in the magne- A second recent development plasma instabilities that may totail. When this auroral stream- is the awareness of a low-in- account for these characteris- er reaches the equatorward tensity auroral feature called tics are the ballooning instabili- portion of the auroral oval and auroral beads that develop in ty and the cross-feld current in- touches a pre-existing auroral pre-breakup auroral arcs that stability. The latter was recently arc, it leads to the development eventually produce the ini- examined and was found to ac- of auroral substorm disturbanc- tial brightening and substorm count for the observed auroral es in association with magnetospheric disturbances such as plasma injections into the inner Auroral Beads (Motoba et al., 2012) magnetosphere to form the ring current that is responsible for the world-wide depression of equatorial geomagnetic feld at the Earth.

The check on the validity of this sequence of events can be improved drastically from what can be done presently by incorporating a global view of auroral observations from both the dayside and nightside. The global auroral imaging from a satellite such as SMILE would allow activities from these different local times to be moni- Current filamentation and field-aligned electron acceleration tored simultaneously. Present by cross-field current instability networks of ASCs do not cover Lui (2004) aurorae globally and even when Fig. 5: A schematic diagram to show (top) the conjugacy of auroral pieced together from individual beads by ground-based observations from both Northern and Southern ASCs images, the global view hemispheres, (middle) the wavelengths in the magnetospheric equatorial suffers distortion of aurorae due plane corresponding to auroral beads, and (bottom) the current flamen- to the fsh-eye lens used in ASC tation and electron acceleration arising from the excitation of the cross- feld current instability.

太空|TAIKONG 9 bead characteristics, as illus- is whether or not interplanetary cause the magnetosphere to be trated in Figure 5. The coordi- shocks impacting the magne- compressed but often do not nated global and ground-based tosphere can cause a substorm cause the sequence of auroral auroral observations around expansion. Traditionally, it was disturbances as schematically substorm expansion onset generally assumed that shocks illustrated in Figure 4. The si- would be ideal to test these could produce substorm ex- multaneous observations of the proposed plasma instabilities pansion. However, a more solar wind and the global auro- further. dedicated investigation on this ral morphology by the SMILE causality based on global au- mission would allow ample op- Finally, another outstand- roral observations from NASA’s portunities to address this out- ing question that can be ad- Polar satellite indicated that in- standing issue. dressed by the SMILE mission terplanetary shocks in general CUSP DYNAMICS

Since the frst spacecraft obser- means there are ample ingre- provides a schematic diagram vations identifying it nearly 40 dients for charge exchange be- giving an example of this pro- years ago, the cusp has been tween heavy solar wind ions cess. As magnetic feld lines widely recognized as the region and neutrals in this region. The in the solar wind (blue in fg- with the most direct entry of the result is the emission of soft ure) reconnect with feld lines solar wind into the Earth’s mag- X-rays from the cusps. in the magnetosphere (orange netosphere. Spatially, the cusps in fgure), plasma fows into the in the northern and southern Patterns of when, where, and magnetosphere. Depending on hemisphere are broad at high how much solar wind enters where reconnection occurs, the altitude, spanning several Earth the cusp also give valuable in- time-history effect will cause radii near the magnetopause, formation regarding how solar different density structures that and funnel down to just 10s of wind plasma and energy are can be imaged in soft X-rays. km in the ionosphere. Passage entering the Earth’s magneto- of solar wind plasma along feld sphere and ionosphere. The For years researchers have used lines down the throat of the converging magnetic topology the ion structures to discern be- cusp, to regions close to the of the cusp means the region tween different models of mag- Earth, means there is solar wind collects information about pro- netopause reconnection. The plasma penetrating deep into cesses occurring all along the structures or dispersions can the Earth's neutral atmosphere. dayside magnetopause. One provide information regarding A high neutral density in the low particular feature that provides when and where reconnection altitude atmosphere combined useful information is time-en- is initiated (poleward or equa- with the solar wind plasma ergy dispersions. Figure 6 torward of the cusp) as well as

t = 0 t = 1 t = 2 t = 3

High energy Low energy High density Cusp Low density

A. Solar wind with B. Reconnection opens C. Particles follow the D. Lowest energy southward IMF impacts cusp magnetic field tailward convecting field line particles precipitate last the magnetosphere lines - solar wind - highest energy particles depositing low density particles enter move fastest, depositing of particles largest density into cusp

Fig. 6: Diagram of solar wind-magnetosphere coupling and the generation of energy and density structures in the cusps.

10 太空|TAIKONG Single Dispersion: Overlapping Dispersions: Steady reconnection Ambiguous - multiple patches and/or bursts 7 7 10 10 10000 Decreasing 10000 6 6 energy with 10 10

time/latitude 5 5 ] 1000 10 ] 1000 10 [eV 4 [eV 4 HYDRA 10 HYDRA 10 Ion Energy 100 Ion Energy 100 Diff Energy Flux 103 103 Diff Energy Flux

10 102 10 102 2.5 4 2.0 Decreasing ]

density with ] 1.5 3 time/latitude [cm-3 HYDRA 1.0 [cm-3 2 HYDRA Ion Density Ion Density 0.5 1

0.0 ILAT [Deg]78.6 80.0 81.5 ILAT [Deg] 72.7 73.8 75.5 hhmm 0940 1000 1020 hhmm 0920 0940 1000 2001 Apr 21 2002 Apr 19

Fig. 7: Spacecraft measurements of a single cusp ion dispersion (left) and overlapping ion dispersions (right). The x-axis is time. The panels from top to bottom are ion energy spectra and ion density from the HYDRA instrument on NASA’s Polar mission. the time variability. Although latitude in the cusp. This pro- X-ray imaging of the cusp pro- in situ spacecraft measure- vides the classical picture of vides the potential to monitor ments of ion dispersions can continuous reconnection equa- the entire ion dispersion as a give information regarding torward of the cusp. On the right function of time. With knowl- the behavior of reconnection, side of the fgure is an example edge of the entire temporal point measurements have an of overlapping ion dispersions. and spatial history of cusp ion inherent space-time ambiguity. This may be providing evidence dispersions from upcoming Figure 7 shows two examples for a reconnection process that wide feld-of-view (FOV) soft of ion dispersions measured is occurring in many discontin- X-ray missions such as the from NASA’s Polar spacecraft uous patches. Or, the obser- Cusp Plasma Imaging Detector passing through the cusp on vations may be evidence for a (CuPID) Cubesat Observatory two different days illustrating single extended reconnection as well as SMILE, the commu- this challenge. line turning on and off. With nity may be able to separate point measurements from sin- the causes of these observa- On the left is a single ion dis- gle spacecraft in the cusp we tional signatures and develop a persion with decreasing den- are unable to separate these deeper understanding into how sity and particle energy as the models. reconnection is occurring at the spacecraft passes to higher dayside magnetopause.

MODELING SOLAR WIND-MAGNETOSPHERE INTERACTION AND FIELD OF VIEW

The solar wind-magnetosphere Center (NSSC), CAS. It solves The Earth’s dipole tilt is set to interaction can be modeled by the ideal MHD equations in the be zero and the ionosphere is global MHD codes, for exam- numerical domain extending simplifed as a spherical shell ple, the 3-D PPMLR (extend- from 30 to −300 RE along the x with a uniform Pedersen con- ed Lagrangian version of the axis and from -150 to 150 RE in ductance and a zero Hall con- piecewise parabolic method) y and z directions of the geo- ductance. MHD code jointly developed by centric solar magnetospheric the University of Science and (GSM) coordinate frame. The The X-ray intensity for a par- Technology of China (USTC) minimum grid spacing for the ticular line of sight can be es- and the National Space Science present simulation is 0.4 RE. timated by the line integration

太空|TAIKONG 11 of volume emission rate (P) optical/UV flter as required by and size of the cusps, and the [Cravens, 2000]: astrophysical X-ray telescopes. position, shape and size of the The combined QE of detector magnetopause – here assumed and flter is essentially identi- to be a Shue-model (Shue et cal to the EPIC-PN instrument al. 1997) shape with α=0.6, with the medium flter on XMM- with the nose of the magneto- Newton. The exposure time is pause at [10.0, 0.0, 0.0]. Initially where � is the effciency factor, 300 s for the simulations shown assumed to have a fxed posi- −15 which is taken to be 1×10 here. tion with an Earth radius size, eV cm2 in our simulations, n is H the cusps have recently been the number density of the exo- After the shock arrival (2nd row modeled using a more realistic spheric hydrogen and n that sw from the top of Figure 8) a shape that uses three mag- of the solar wind. Integration substantial increase of X-ray netic feld lines at 10, 12 and of P over the line of sight starts emission is observed as well 14 H MLT that change position from the satellite position to r as a compression of the mag- with the dipole tilt and the sea- =80 R . X-ray emission beyond E netosphere due to increase of son (from the Tsyganenko 1996 80 R is neglected as the den- E pressure at the magnetopause. model). The SMILE design pa- sity of exospheric hydrogen After the turning of the IMF rameters that are relevant are drops dramatically there. from northward to southward the FOV and orientation of both (4th row on Figure 8) the mag- the SXI (16° x 27°, short-side We use a solar storm on Sept. netopause is also observed to along the Earth-Sun line) and 12, 2014, with an ICME reach- move inward, most likely due to the UVI (10° x 10°), the offset ing the Earth at ~15:20 UT, to the erosion of the dayside mag- between these (22.8°), and simulate the response of the netosphere by magnetic recon- the SXI Earth avoidance angle Earth’s magnetosphere and its nection. (10°). The simulator takes a environment to the incoming spacecraft orbit and these pa- solar wind. The simulated X-ray The FOV of the X-ray instru- rameters and predicts what can intensity for the storm event ment is changing as the space- be observed during the orbit. It is shown in the left column of craft moves along its orbit. A is used to explore a number of Figure 8. From the top to the sophisticated simulator with high-ellipticity, high-inclination bottom, the panels show the specifed orbit, visibility and orbits in order to determine the X-ray intensity from the dayside FOV (Figure 9) was devel- optimal choice. magnetosheath and cusps be- oped for all general Earth/or- fore (top row) and after (second bit/magnetosheath/cusps etc. row from the top) the arrival of confgurations (see the SMILE the interplanetary shock, as well Website http://www.star.le.ac. as the response of the magne- uk/amr30/SMILE/ for examples tosphere to the interplanetary of the simulator outputs). It is magnetic feld turning from a very useful visualization tool, northward (third row) to south- able to create movies of what ward (fourth row). Furthermore, can be observed from a par- the above modeled X-ray inten- ticular orbit, and also able to sity is converted into observed calculate observability effcien- X-ray counts by using an X-ray cies, i.e. what percentage of the telescope simulator. The simu- considered orbit and confgura- lator assumes an optic which tion a certain target (nose, cusp uses square-channel micropore etc.) is visible or not due to the plates in a Lobster eye confgu- various constraints (e.g. bright ration. The detector is assumed Sun, bright or obscuring Earth, to have the quantum effcien- baffe considerations, radiation cy (QE) characteristics of a fux, etc.). Relevant environ- back-illuminated charge-cou- mental parameters that go into pled device (CCD) and has an the simulator are the position

12 太空|TAIKONG Fig. 8: Simulated dayside magnetosphere before (frst row) and after (second row) the arrival of an interplanetary shock, as well as its response to the interplanetary magnetic feld turning from northward (third row) to southward (fourth row). The fgure shows original MHD simulation data (left), the predicted soft X-ray counts (center) and the processed image (right).

太空|TAIKONG 13 Fig. 9: SXI instrument FOV (white rectangle) and UVI instrument FOV (yellow square). The nose of the magnetosphere is shown with a yellow oval at (10.0, 0.0, 0.0) RE and the cusps are shown by the red (North hemisphere) and pink (South hemisphere) spheres and by the red and pink magnetic feld lines. The SXI Earth avoidance baffe angle is shown with the orange dashed line. The sun avoidance angle is shown by the large yellow unbroken curve.

SMILE SCIENTIFIC OBJECTIVES AND MISSION OVERVIEW

The Solar wind Magnetosphere in November 2015. The launch diffusion, boundary instabilities, Ionosphere Link Explorer is planned for the end of 2021. turbulence, particle accelera- (SMILE) is a novel self-standing tion, etc.). Multi-point and multi- mission to be jointly developed Understanding and thus pre- scale missions such as Cluster, between the European Space dicting the non-linear global THEMIS and, more recently, Agency (ESA) and the Chinese system behaviour of the mag- MMS proceed down this path Academy of Sciences (CAS). netosphere has remained both with an ever-increasing focus A joint call for new missions the central objective and grand on the microscopic physics of was published in January 2015 challenge of solar-terrestrial space plasmas. However, piec- by ESA and CAS. The SMILE physics in particular (and space ing the individual parts togeth- mission was then proposed by plasma physics more generally) er to make a coherent overall an international team of scien- for more than 50 years. picture, capable of explaining tists, led by a Chinese and a and predicting the dynamics of European Principal Investigator, In situ data have dramatically the magnetosphere at the sys- in March 2015. Following a improved our understanding tem level has proved to be ex- technical and scientifc review, of the localised physical pro- tremely diffcult. This is due to the SMILE mission was selected cesses involved (reconnection, the fact that it is fundamentally

14 太空|TAIKONG impossible to determine the images of global auroral dis- SMILE will answer the following global behaviour of a complex tributions and simultaneous in science questions: system with sparse in situ mea- situ solar wind/magnetosheath surements, even with the sup- plasma and magnetic feld mea- • What are the fundamental port of increasingly sophisticat- surements. Remote sensing of modes of the dayside solar ed global computer models of dayside magnetosheath and wind/magnetosphere inter- the solar wind – magnetosphere the cusps with X-ray imaging is action? interaction. now possible thanks to the rel- • What defnes the substorm atively recent discovery of solar cycle? To address this global aspect, wind charge exchange (SWCX) SMILE will explore the solar X-ray emission, frst observed • How do CME-driven storms wind-magnetosphere coupling at comets, and subsequently arise and what is their rela- via X-ray images of the magne- found to occur in the vicinity of tionship to substorms? tosheath and polar cusps, UV the Earth’s magnetosphere.

What are the Fundamental Modes of the Dayside Solar Wind/Magnetosphere Interaction?

Dayside reconnection causes reconnection is more likely for magnetospheric magnetic feld, plasma to fow anti-sunward high beta solar wind conditions. their structure gives information through the magnetopause However, a simple confrmation about a larger context than any boundaries, the cusps, and of this hypothesis is obscured other structure within the mag- over the polar caps. On occa- by the fact that apparently un- netosphere. sions reconnection can persist steady magnetopause recon- for long times, both for north- nection may simply be directly The latitudinal location of the ward and southward interplan- driven by variations in the solar cusp depends on the level of etary magnetic feld (IMF) ori- wind parameters. interconnection of the Earth’s entations (e.g. Frey et al. 2003; dipole with the IMF (Newell et Phan et al. 2004). However, re- The peculiar magnetic topology al., 1989), i.e. the amount of connection can also be bursty of the cusps means that they recently ‘opened’ magneto- and time dependent, gener- also play a pivotal role in mag- spheric magnetic fux. When ating signifcant structure. For netospheric dynamics: they are the solar wind magnetic feld example, patchy reconnec- the sole locations where so- points southwards, magnetic tion (Russell and Elphic 1978), lar wind has direct access to reconnection at the magneto- bursty (i.e., time dependent) re- low altitudes (e.g. Cargill et al. pause opens closed dayside connection from a single X-line 2005). They are essentially the magnetic feld lines, causing (Scholer 1988; Southwood et boundary that separates mag- the region of open feld lines in al. 1988), and multiple X-line netic feld lines that close in the polar cap to expand to low- reconnection (Lee and Fu 1985; the dayside hemisphere from er latitudes. The latitudinal po- Raeder 2006; Omidi and Sibeck those that extend far down the sition of the cusp is also an in- 2007), may produce so-called magnetotail. During subsolar dicator of the Region 1 current fux transfer events (FTEs) reconnection solar wind ener- and how much magnetic fux is which, at the most basic level, gy, mass and momentum are being removed from the day- may be thought of as time de- transferred through the cusp side to fuel substorm behaviour pendent structures propagating into the magnetosphere. As on the nightside. When the IMF along the magnetopause. described above, this momen- turns northward, the cusps tum transfer is the major driver move poleward. This might be It is thought that steady recon- of large-scale magnetospheric because reconnection at the nection occurs for low beta (i.e. convection, refecting again the dayside magnetopause stops, magnetic feld pressure domi- pivotal role of these regions. while reconnection within the nated) solar wind and magne- Since the cusps are the end- magnetotail continues to close tosheath, whereas unsteady points of a large portion of the magnetic feld lines which then

太空|TAIKONG 15 convect to the dayside magne- assess with in situ measure- move equatorward (poleward) tosphere (e.g. Milan et al. 2003). ments because it occurs over a (e.g. Newell and Meng, 1994; large area of the magnetopause. Yamauchi et al. 1996; Palmroth However, it might also occur Therefore this critical parame- et al. 2001). because reconnection pole- ter determining the magnitude ward of both cusps appends of dynamical events accurately Reconnection is therefore magnetosheath magnetic feld has only been assessed with thought to cause the shape of lines to the dayside magneto- correlative studies. the magnetopause to become sphere, transforming open lobe blunter. By contrast, variations magnetic feld lines into closed Further complexity is intro- in the solar wind dynamic pres- dayside magnetic feld lines and duced by the east-west (or sure should cause self-similar allowing the immediate entry of dawn-dusk) component of the changes in magnetospheric di- dense magnetosheath particle IMF. The northern cusp moves mensions. Thus by measuring: fuxes into the magnetosphere duskward and the southern the curvature, size and absolute (Song and Russell 1992). The cusp dawnward during periods location of the magnetopause; cusps, being by defnition the of duskward IMF orientation, and the location (latitudinal po- boundary between open lobe and in the opposite directions sition), size, and shape of the and closed dayside feld lines, during intervals of dawnward cusps, it is possible to distin- should move to lower and high- IMF orientation (e.g. Newell et guish the differing effects of er latitudes as the open feld line al. 1989; Taguchi et al. 2009a). pressure changes and magnet- region expands and contracts, We understand these changes ic reconnection on the global respectively. The amount of in terms of magnetic reconnec- magnetospheric system. This open fux depends on the rate tion: when the IMF points dusk- would distinguish on a global of reconnection both on the ward, antiparallel reconnection level the nature of the solar wind dayside magnetopause and in is expected in the post-noon magnetosphere interaction, the the nightside magnetotail plas- northern hemisphere and pre- dominant driving mechanisms ma sheet. It is thus of key im- noon southern hemisphere. and modes of interaction. portance to measure how the Plasma enters the magneto- cusp responds to northward sphere from the cusp along the The measurements required to and southward turnings of the newly reconnected magnetic address the frst science objec- IMF, since this is intimately re- feld lines, which then move tive are as follow: lated to the strength of the so- anti-sunward in response to lar wind – magnetosphere cou- pressure gradient forces, but • steady/unsteady solar wind pling. often initially move towards lo- variations cal noon under the infuence • steady/unsteady motion of The cusp latitude is direct- of curvature forces (e.g. Smith the dayside magnetopause ly related to the level of open and Lockwood 1996). fux within the magnetosphere, • transient brightenings and which in turn is controlled by Finally, the solar wind dynam- equatorward leaps in the the main mechanism of energy ic pressure may play a role dayside auroral oval transfer, the reconnection pro- in determining cusp latitude. • transient brightenings and cess. Although crucial in under- LEO observations from e.g. equatorward leaps in the standing the system energet- DMSP and simulations predict cusp ics quantitatively, the amount that enhanced (reduced) pres- of energy transfer is diffcult to sures may cause the cusp to

What Defnes the Substorm Cycle?

We know that southward IMF the interval of southward IMF, plays in the subsequent onset is required to increase the en- the more energy is stored, but of geomagnetic activity is very ergy density of the magnetotail the precise nature of the en- controversial. For example one lobes, and the more prolonged ergy loading and the role it very fundamental question is

16 太空|TAIKONG whether each substorm re- Hubert et al. 2006, 2009; Milan persist for more prolonged in- quires its own interval of load- et al. 2004)? How large and rap- tervals where in situ satellite ing (growth phase), or whether id must these driving changes data are not available. During multiple substorms can occur be? Another viewpoint is that saw-tooth events, which are in response to a single growth the external solar wind condi- oscillations of energetic particle phase. tion provides only the general fuxes at geosynchronous orbit confguration of the magneto- recurring with a period of about The polar cap is an area of sphere for substorm expan- 2–4 h (e.g. Henderson et al., magnetic feld lines that are sion onset. When and where 2006), the auroral oval expands open to the solar wind and is it occurs depends on the ion- and contracts with a period of a readily identifed by the auroral ospheric conditions as well as few hours. It is not clear if this oval which bounds it. Auroral internal local magnetospheric is due to an intrinsic instability/ oval observations provide in- parameters. Furthermore the mode of dynamic behaviour or if formation about the ionospher- role of the prior history of the it corresponds to a series of re- ic footpoints of magnetopause magnetosphere in conditioning peating substorms. These may processes. Specifcally, the ex- the response is not well under- simply refect the same internal panding-contracting polar cap stood and there are reports of physics being driven differently paradigm utilises basic proper- substorms with no obvious ex- by the solar wind, or they may ties of the auroral oval to pro- ternal drivers (Huang 2002). represent fundamentally differ- vide direct measurements of the ent types of behaviour. state of the magnetosphere by Thus despite a plethora of in measuring the size of the polar situ observations, fundamental Disentangling these different cap (e.g. Milan et al. 2012). The questions remain unanswered. modes of behaviour follows on area of open fux within the po- If the onset of a substorm is from the frst question. Once lar cap changes directly in re- due to external driving, what is a substorm is triggered, what sponse to the amount of open the nature of the driving mech- controls its subsequent evolu- fux in the magnetotail lobes, anism, and how does this de- tion? To what extent is it sen- and the very dynamic changes pend on the precise confgura- sitive to changes in the solar that occur in this region are in tion of the magnetosphere? wind conditions, and how does response to different solar wind this sensitivity depend on the conditions. Although the substorm is per- internal state of the magneto- haps the most well-known type sphere (e.g. substorm phase, The trigger that leads to sub- of magnetospheric event, other amount of remaining stored en- storm onset remains controver- modes of magnetospheric be- ergy, etc.)? sial. Is the substorm triggered haviour are observed. During by changes in IMF orientation steady magnetospheric con- The measurements required to (related to a change in shape vection, anti-sunward iono- address the second science of the magnetopause due to re- spheric convection is observed, objective are as follow: confguration of magnetospher- and so fux is being transferred ic currents associated with from the dayside to the night- • location and motion of the dayside reconnection) (e.g. Hsu side, but the size of the polar dayside magnetopause and McPherron 2002; Lyons et cap does not change. Thus it boundary al. 1997; Morley and Freeman is thought that reconnection at • location and motion of the 2007; Wild et al. 2009)? Or do the day and night side is bal- auroral oval solar wind dynamic pressure anced. The solar wind drivers changes play the key role (by that enable steady magneto- • substorm brightenings of compressing the magnetotail) spheric convection are not well the auroral oval (e.g. Boudouridis et al. 2003; understood, because they can • solar wind input

太空|TAIKONG 17 How do CME-driven Storms Arise and What is Their Relationship to Substorms?

While intervals of southward Sometimes CMEs don’t have Finally although the question IMF occur naturally in the solar the expected effects associat- of how a storm starts has been wind, and so substorms occur ed with a geomagnetic storm. central to the scientifc studies on a daily basis (Borovsky et al. When the interplanetary mag- of the magnetosphere for as 1993), strong driving causing netic feld is northward the en- long as measurements have geomagnetic storms tends to ergy transmitted to the mag- been available, the question of occur in response to coherent netosphere is more limited. duration is growing in impor- solar wind structures, partic- However, when solar flaments tance, driven by the needs of the ularly Coronal Mass Ejections are contained in CMEs, there end-user in the space weather (CMEs) (Gonzales et al. 1999). can be some effects similar to context (i.e. confdence in is- superstorms such as the super- suing ‘all clear’). Does a storm The degree to which solar wind fountain in the equatorial iono- end because it has exhausted plasma, momentum and en- sphere, magnetotail stretching the reservoir of stored magnet- ergy enter the magnetosphere and strong joule heating in the ic energy in the magnetotail? Or is characterized by so-called polar ionosphere (Kozyra et al., does a storm stop because the solar wind coupling functions 2014). Furthermore, Turc et al. solar wind driving conditions (Gonzales 1990; Finch and (2014) showed that the Earth’s have changed? If both possi- Lockwood 2007). Physically, bow shock can, under certain bilities are observed to occur, magnetic reconnection at the conditions, modify the inter- which is the more important? dayside magnetopause is en- planetary magnetic feld direc- And once the solar wind driving hanced if there is a strong inter- tion contained in CMEs which is removed, how rapidly does planetary magnetic feld com- then do not have the predicted the magnetosphere recover? Is ponent opposite to the dayside effect on the magnetosphere. it more likely that the solar wind magnetospheric magnetic feld, conditions will change, or is the supplemented by fast solar Understanding the global CME/ stored magnetotail lobe ener- wind, for an extended period of magnetosphere interaction is gy depleted so rapidly that the time. crucial to understanding pre- changing solar wind plays only cisely how the structure of the a minor role? CMEs are transient eruptions CME is responsible for the dif- of material from the Sun’s co- ferent phases of geomagnetic The measurements required rona into space (Forbes 2000). storms. On a practical level, to address the third science CMEs propagate at super-mag- storms driven by CMEs have objective are the same as the netosonic speeds relative to the potentially severe space weath- ones for the second science ambient solar wind, and play a er consequences and represent objective but should be done particularly important role in the a signifcant threat to infrastruc- during a CME-driven storm: dynamics of the Earth's mag- ture resilience worldwide. netic feld because they can • location and motion of the contain long intervals of south- Very basic questions still re- dayside magnetopause ward IMF (e.g. Gonzales et al. main. Is the duration and mag- boundary 1999). In general, the largest nitude of solar wind driving the • location and motion of the geomagnetic disturbances are sole arbiter of whether a storm auroral oval associated with CMEs, with the will occur? What is the relation- level of activity being directly ship between the storm and • substorm brightenings of related to the fow speed, the substorm? Are storms always the auroral oval feld strength and the south- a separate phenomenon, or • solar wind input ward component of the mag- can they be considered as be- netic feld (Richardson et al. ing composed of multiple sub- 2001). storms?

18 太空|TAIKONG SMILE Mission Concept and Payload Design

As described above, SMILE (LIA) and the MAGnetometer will investigate the dynamic (MAG), which will establish response of the Earth’s mag- the solar wind/magnetosheath netosphere to the impact of properties simultaneously with the solar wind in a unique and the imaging instruments. global manner, never attempt- ed before. From a highly ellip- The SXI is a wide FOV Lobster- tical Earth polar orbit, SMILE eye telescope employing light will combine soft X-ray imaging weight (< 1 kg) micropore op- of the Earth’s magnetic bound- tic (MPO) to achieve soft X-ray aries and polar cusps with si- imaging with large spatial cov- multaneous UV imaging of the erage (16o x 27o FOV). At the Northern aurora, while self-suf- telescope focus are charge fciently measuring solar wind/ coupled devices (CCDs) pro- magnetosheath plasma and viding the good energy resolu- Fig. 10: CAD drawing of the SMILE SXI as viewed from the magnetic feld conditions in tion required to map the SWCX top. Clearly visible are the mi- situ. X-ray emission and character- cropore optic arrays, protect- ise the solar wind ionic popu- ed from stray light by the baf- For the frst time we will be lation generating it. The CCDs fe. X-rays are focussed onto able to trace and link the pro- need to be cooled to ~ -70oC CCDs located at the bottom cesses of solar wind injection in in order to operate properly in of the (black) optical bench. the magnetosphere with those the X-ray regime, and this is acting on the charged particles achieved by the use of a pas- the expected SXI count and precipitating into the cusps sive radiator. The SXI instru- processed images are shown in and eventually creating the au- ment development is led by the modeling section (Figure 8). rora. SMILE will shed light on the University of Leicester, UK In order to avoid straylight from the fundamental drivers of this (PI: Steve Sembay). Simulations the Sun and the bright Earth complex interaction, on how it of the modeled X-ray emissivity, penetrating to the focal plane takes place on a global scale, and how it evolves, which is still not understood. The science delivered by SMILE will have profound impact on our understanding of the way the solar wind interacts with the Earth’s environment, and will pave the way to future space weather monitoring and forecasting sat- ellites for which SMILE is an important scientifc precursor.

In order to achieve the scientific objectives set out above the SMILE payload comprises: the Soft X-ray Imager (SXI), which will map spectrally the Earth’s magnetic boundaries, magnetosheath and polar cusps; the Fig. 11: Conceptual mechanical : on the left hand side is the image in- UltraViolet Imager (UVI), ded- tensifer and on the front (right) is the baffe to control stray light levels. icated to imaging the auroral regions; the Light Ion Analyser

太空|TAIKONG 19 Fig. 12: Left panel – Example of the type of solar wind ion detector (fown on Chang’E-1/2) that will be adopted for SMILE LIA. Right panel – Example of fuxgate magnetometer (both from CAS/NSSC). the SXI incorporates an opti- for electron multiplication, a 360o FOV in azimuth, reach- cal/UV flter and a ~ 0.7 m long phosphor and a CMOS sensor. ing +/-45o in elevation by use of baffe (which is clearly visible The UVI, shown in Figure 11 is defector plates. MAG (Figure in Figure 10, giving a view of responsibility of the University 12 – Right panel) is a fuxgate the SXI from the top, and in of Calgary, Canada (PI: Eric type magnetometer measuring Figure 13, left panel, showing Donovan). both strength and direction of the SMILE fying confguration). the local magnetic feld. Its two The in situ package on board sensors will be mounted on a The UVI is a four mirror refective SMILE includes the LIA and boom some 2.5 m long, which telescope covering the band- MAG instruments. The LIA is seen in its deployed confg- pass 160 – 180 nm, which is se- (Figure 12 – Left panel) is a uration in Figure 13 (left panel). lected by appropriately coating top-hat analyser for protons Both instruments making up the optical surfaces and the de- and α particles, measuring their the in situ package are devel- tector. The detector is an image density, velocity and tempera- oped by CAS/NSSC, China (LIA intensifer comprising a photo- ture and working in the energy PI: Lei Dai, MAG PI: Lei Li). cathode, microchannel plates range 50 eV – 20 keV, with a

Fig. 13: Left panel – SMILE spacecraft fight confguration with its main elements labelled. Right panel – SMILE spacecraft including its propulsion module (from ESA-CAS Concurrent Design Facility)

20 太空|TAIKONG SMILE Spacecraft and Orbit

A CAD drawing of the SMILE passenger in a dual launch on passages near perigee (when spacecraft is shown in Figure a Soyuz rocket or could be the CCDs will be protected by 13 (left panel) with its main com- launched on its own on Vega C. closing a door mounted at the ponents labelled. Refecting the After spending some days or a bottom of the baffe). LIA and truly collaborative nature of this few months (depending on the MAG will be making measure- mission, the provision of the launch approach) in a low-Earth ments for most of the orbit. SMILE elements is shared be- parking orbit, SMILE will be in- tween CAS, ESA and nation- jected by its propulsion module During moderately strong so- al agencies. CAS provides the into a highly elliptical, high incli- -3 lar wind fux (NSW=12 cm and Propulsion Module, the Service nation (70o – 90o) orbit currently Vsw=400 km/s), the spacecraft Module, the spacecraft Prime baselined to have a period of will reach the solar wind near Contractor, Mission Operations ~50 hour and apogee altitude of apogee (Figure 14) and a direct (with ESA contribution) and the ~19 Earth radii; this allows ~41 comparison between the solar Chinese instruments. ESA pro- hour of SXI and UVI operations wind strength impacting the vides the Payload Module (the above an altitude of ~50,000 Earth and the X-ray images of interface plate between the km, selected in order to avoid the cusps/magnetostheath and service module and the instru- radiation damage to the SXI the auroral UV images will be ments and the sub-systems detectors during van Allen belt made. required to collect and download the scientifc data – see Figure 13, left panel), the launch- er and facilities for spacecraft integration and testing. The European/Canadian instruments will be provided by ESA member states and Canada. Science operations will be shared among the hardware institutes, ESA and CAS. An image of the SMILE spacecraft including the propulsion module, which will inject it into its operational highly elliptical polar orbit, is shown in Figure 13 (right panel).

Two possible ap- proaches to launch Fig. 14: SMILE orbit (red) on 1 April 2022 with an inclination of 67 deg. in

(scheduled for the X-ZGSE coordinate system. The Earth magnetic feld lines (black lines connect- end of 2021) are ed to the Earth), the magnetopause model from Shue et al., 1987 (grey) and being considered: the bow shock model from Merka et al., 2005 are shown for the solar wind SMILE could be parameters given in the left bottom corner (B=6 nT, Vsw=400 km/s, Nsw=12 cm-3).

太空|TAIKONG 21 SUMMARY AND RECOMMENDATIONS

of collaboration already proven in particular comparing results with Double Star. Moreover, the obtained with different codes imaging nature of two of the infor the same solar wind condi- struments in the SMILE payload tions, in order to establish the offers excellent potential for margin of error inherent to mod- outreach: the X-ray and UV im- el predictions. This is important ages and videos that SMILE will in order to verify that the SMILE return will captivate the public instruments can achieve the to science, to the physics of the science requirements of the Earth’s magnetic feld which mission. In addition, other mod- involves many processes that els such as Particle in Cell (PIC) are complex and essentially in- simulations, could be used to visible to the naked eye. SMILE obtain a complementary view will make visible the magneto- of the magnetosphere, in par- In summary, SMILE will turn spheric bubble shielding our ticular the polar cusps. It is what is an unwanted variable Earth from inclement solar wind recommended to approach sci- background ‘noise’ for soft conditions, and in doing so will entists developing such codes X-ray astronomical observa- make the science of solar-ter- and request their support. tions made along line of sights restrial interactions more un- crossing the Earth’s magne- derstandable and fascinating. Archived data (e.g. velocity, tosheath into a novel diagnos- temperature) collected by cur- tic tool of the conditions of geo- SMILE will break completely rent in situ missions such as space under the vagaries of the new ground in the way we ex- Cluster and THEMIS and more solar wind. SMILE will work in plore how the Earth environ- recently MMS, should also be synergy with other space mis- ment responds to activity on the used, in complement to MHD sions, current and forthcoming, Sun and in the solar wind, and models, to further develop the probing the microscale (such will open the way to future sys- instruments design and fx their as MMS, Cluster, Solar Orbiter, tematic and large scale moni- orientation on the spacecraft (in Solar Probe+, THOR …), and toring based on state-of-the-art particular LIA and SXI instru- with ground based observa- astronomical X-ray detection ments). tories in the polar regions, to and mapping techniques ap- lead to a comprehensive un- plied to terrestrial space plas- Such ISSI forum meetings are derstanding of solar-terrestrial ma science. ideal to exchange ideas and interactions. discuss recent developments Simulations of the magneto- on Sun-Earth connection sci- The cooperation of western na- sphere and its environment, ence and monitor the SMILE tions with China from mission through MHD modeling in mission implementation and design to launch and fight op- China, Europe and USA are be- it is recommended that such erations is another frst of the ing used to optimize the instru- meetings be organized at reg- SMILE mission, and a facet ment and mission design and ular intervals (once or twice a that makes it a brilliant show- the ISSI forum recommends year). case, building on and extend- continuing this effort during ing the successful experience the project development, and

22 太空|TAIKONG

太空|TAIKONG

Participants

Zhiming Cai Shanghai Engineering Center of Microsatellites, China Lei Dai National Space Science Center, CAS, China Alexei Dmitriev Institute of Space Science, National Central University, Chungli, Taiwan Malcolm Dunlop Space Science and Technology Department, RAL, UK Philippe Escoubet ESA/ESTEC, Netherlands Yuichiro Ezoe Tokyo Metropolitan University, Japan Maurizio Falanga ISSI-BJ, China Masaki Fujimoto Japan Aerospace Exploration Agency, Japan Hongqiao Hu Polar Research Institute of China, China Anders M. Jorgensen New Mexico Tech University, USA Kip Kuntz The Johns Hopkins University, USA Ziqian Liu NSSC, CAS, China Huawang Li Shanghai Engineering Center of Microsatellites, China Jing Li National Space Science Center, CAS, China Jann-Yenq Liu National Central University, Taiwan Tony Lui Applied Physics Laboratory, The Johns Hopkins University, USA Yoshizumi Miyoshi Nagoya University, Japan Zuyin Pu Peking University, China Walfried Raab ESA/ESTEC, Netherlands Graziella Branduardi-Raymont Mullard Space Science Laboratory, University College London, UK Andrew Read University of Leicester, UK Denis Rebuffat ESA/ESTEC, Netherlands Steve Sembay University of Leicester, UK David Sibeck NASA / GSFC, USA Tianran Sun National Space Science Center, CAS, China Brian Walsh Boston University, USA Chi Wang National Space Science Center, CAS, China Ji Wu National Space Science Center, CAS, China Xiaoxin Zhang National Center for Space Weather China Meteorological Administration, China Jianhua Zheng National Space Science Center, CAS, China Xiaocheng Zhu Shanghai Engineering Center of Microsatellites, China Qiugang Zong Peking University, China