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Eastwood, J. P., Kataria, D. O., McInnes, C. R., Barnes, N. C., and Mulligan, P. (2015) Sunjammer. Weather, 70(1). pp. 27-30.

Copyright © 2015 The Authors

http://eprints.gla.ac.uk/102774/

Deposited on: 18 February 2015

Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Sunjammer

1 Space weather is ultimately driven by solar wind structure moving at typical speeds of Weather 2015, Vol. – January 70, No. 1 J. P. Eastwood , −1 2 activity (Schwenn, 2006; Eastwood, 2008). 400kms , this provides just over 1h of lead D. O. Kataria , There are two basic types of space weather time before the event arrives at Earth. In the C. R. McInnes3, N. C. drivers generated by the Sun: solar flares, case of an extremely fast moving CME, e.g. −1 4 5 powerful transient emissions of electromag- observed by ACE to be moving at 1000kms , Barnes and P. Mulligan netic energy at a wide range of wavelengths, the transit time to Earth is reduced to ~24min. 1The Blackett Laboratory, Imperial and coronal mass ejections (CMEs), erup- To increase the warning time, measurements College London tions of material from the solar corona that made further upstream from the Earth are 2Department of Space and Climate have initial speeds typically 500–2000kms−1, required but the basic laws of orbital mechan- Physics, Mullard Space Science and extremes of ~3000kms−1. If a CME ics make this extremely difficult. Laboratory, University College London is Earth-directed, its interaction with the The solution may come from what seems 3Department of Mechanical and Earth’s magnetosphere can cause a geomag- to be the realm of science fiction: solar sailing, netic storm. However, the ability of a CME where the pressure of sunlight itself enables Aerospace Engineering, University of to generate a storm strongly depends on a spacecraft to station-keep far upstream of Strathclyde, Glasgow its internal magnetic field structure and this the Earth indefinitely. Families of so-called 4 L.Garde Inc., Tustin, CA, USA can only be established by satellites mak- artificial Lagrange points for solar sail space- 5 Office of Systems Development, ing in situ measurements. As such, whilst craft were devised in the UK some time ago National Oceanographic and we can image CMEs leaving the Sun and (McInnes et al., 1994). Sunjammer is a mis- Atmospheric Administration, Silver track their progress into the interplanetary sion concept that would use a solar sail and Spring, MD, USA medium (see Harrison and Davies, 2014), it the pressure of sunlight to station-keep at is simply not yet possible to know immedi- such an artificial Lagrange point, sunward ately after the launch of a CME whether it of the L1 Lagrange point. It would carry a Introduction will harmlessly pass the Earth or whether space weather payload, designed and built Space weather describes how conditions in it will cause a geomagnetic storm. In fact, in the UK, validating new miniaturised space can affect human activity and tech- remote determination of the magnetic field sensor technology, and measuring both nology both in space and on the ground is now recognised as a key scientific chal- the solar wind and its magnetic field. The (National Research Council, 2008), and has lenge, stimulating research into alternative payload builds on the UK’s strong heritage been actively monitored for a number of techniques that include measurements of in observing space weather phenomena, years now, most notably by the National the source magnetic field at the Sun and particularly in developing instrumentation Oceanic and Atmospheric Administration forecasts of its evolution as the CME travels for satellites studying solar-terrestrial phys- (NOAA) Space Weather Prediction Center to Earth, and adaptation of radio astronomy ics such as STEREO, Cluster, SOHO and the (SWPC)1. In the past few years space weather techniques for remote sensing magnetic upcoming Solar Orbiter mission. monitoring activities in both Europe and the fields in space. However, in situ measure- ments are still the only reliable way to estab- UK have developed rapidly, most recently Geomagnetic Storms with the formation of the space weather lish the magnetic field. service at the Met Office, described else- In the context of space weather these and Coronal Mass Ejections where in this special issue. Space weather are currently provided by NASA’s Advanced The Earth’s magnetic field extends into can affect power grids, pipelines, trans- Composition Explorer (ACE) satellite, just space where it forms the magnetosphere, oceanic communication cables, satellites, upstream of the Earth at the L1 Lagrange a magnetic ‘bubble’ in near Earth space that aviation, global navigation satellite sys- point, which is located on the Sun–Earth line is confined by the solar wind (see Figure 1). 6 tems, telecommunications and radio com- ~1.5 × 10 km sunward of the Earth (approxi- The solar wind is a continuous supersonic munications, to list just the major impacts mately 1% of the distance from the Earth stream of material emitted by the Sun in all identified by a recent Royal Academy of to the Sun). At L1, the gravitational pull of directions. Both the solar wind and the mag- Engineering study (Cannon et al., 2013). the Sun and the Earth conspire to ensure a netosphere are plasmas (i.e., quasi-neutral As a specific example, in 2003 a series of satellite placed there will maintain an orbital gases of electrons and ions, mainly protons), 2 strong space weather events had a very period equal to that of the Earth . From this and the solar wind–magnetosphere inter- wide ranging impact, documented by a sub- location the solar wind can be monitored action is ultimately governed by plasma sequent NOAA Service Assessment (Balch, in situ before its arrival at Earth. For solar physics. The solar wind is magnetised, and 2004). Space weather is now included in the this magnetic field behaves as if it is embed- 2 For more information about the Lagrange Cabinet Office National Risk Register of Civil points (also referred to as Lagrangian points) ded in the solar wind flow. At Earth the Emergencies (Cabinet Office, 2013). see http://en.wikipedia.org/wiki/Lagrangian_ strength and orientation of the solar wind point or http://www.nasa.gov/missions/ magnetic field continuously varies on time 1 http://www.swpc.noaa.gov solarsystem/f-lagrange.html scales of minutes to hours. 27 For example, we can image a CME leav- 13 ing the Sun and estimate its arrival time. 4 But, we cannot say whether it will cause a significant storm until we can measure its magnetic field. In particular, it is quite 5 possible that the CME magnetic field will rotate northward, in which case weaker 2 7 geomagnetic activity will be experienced. 68 This is not uncommon; for example, on 8–9 January 2014 there was notable UK media coverage about an impending CME arrival, but in the event its magnetic field did not Sunjammer rotate southward and the geomagnetic field was only weakly disturbed.

Figure 1. The Earth’s magnetosphere. The Sun is to the left, and solar wind (yellow) flows from Measuring the solar wind left to right. Since the solar wind is supersonic, a bow shock forms (short-dashed line), and in situ the shocked solar wind (orange) flows around the magnetosphere (white). The magnetopause To date, real-time monitoring of the solar (long-dashed line) separates the solar wind and the magnetosphere. If the solar wind magnetic wind is provided by the ACE satellite3, field points ‘southward’, opposite to the Earth’s magnetic field at the nose of the magnetosphere, orbiting the L1 Lagrange point. ACE, which then magnetic reconnection will occur, allowing the solar wind to enter the magnetosphere. This interaction is described in more detail in the text. An animated version (produced by NASA) can launched in 1997, will soon be joined by the be found at http://www.youtube.com/watch?v=mgUZwoR0gcE. DSCOVR spacecraft which is set to launch into orbit around L1 in early 2015. Weather – January 2015, Vol. 70, No. 1 70, No. – JanuaryVol. 2015, Weather The solar wind plasma cannot easily pen- slightly more complicated than outlined here Placing spacecraft monitors on the Earth- etrate the magnetosphere and the boundary (for a more technical review see Eastwood Sun line closer to the Sun than L1 is extremely between the solar wind and the magneto- et al., 2014a), the basic qualitative features of difficult because of the fundamental equa- sphere is known as the magnetopause. plasma entry, storage and subsequent explo- tions governing orbital mechanics. A satellite However, under certain conditions the mag- sive release lie at the heart of geomagnetic closer to the Sun will naturally orbit faster, netopause can ‘break down’, allowing solar activity. This mechanism is known as the and so will move away from the Earth–Sun wind plasma and energy to enter and be Dungey cycle, named after the UK scientist line. One elegant solution is to use a solar stored in the magnetosphere. The most Jim Dungey who first proposed the concept sail, which employs the pressure of sunlight important plasma physics process control- of the open magnetosphere (Dungey, 1961). itself to generate a continuous thrust that ling the entry of plasma across the magne- In the context of space weather it is there- enables artificial Lagrange points closer to topause is ‘magnetic reconnection’. This is fore of primary concern to determine when the Sun as we now discuss. most likely to occur on the magnetopause and for how long southward magnetic field It is well known that electromagnetic at locations where the solar wind magnetic conditions will persist at the magnetopause, radiation exerts a pressure and as such, a field points in the opposite direction to that together with the solar wind speed. There reflective surface experiences a force. This of the Earth. In particular, if the solar wind is natural variability in the solar wind mag- force is small: at Earth’s orbit, the pressure of magnetic field has a southward orientation netic field orientation, and so small loading/ sunlight is ~9μPa, one millionth of the wind (blue field line ‘1’ in Figure 1), opposite to the unloading events called substorms occur pressure from a gentle breeze. Nevertheless, Earth’s magnetic field (red field line ‘2’), then on an almost daily basis. Strong driving (i.e. although the pressure is small, it acts con- reconnection will occur at the nose of the long durations of southward interplanetary tinuously and over time it can significantly magnetosphere at an X-line. As illustrated in magnetic field (IMF) together with fast solar change a satellite’s orbit. The effects of Figure 1, the consequence of this is a recon- wind flow) tends to be caused by discrete solar radiation on satellite orbits has been figuration of the magnetic field topology events – in particular by CMEs because known for some time, from the early bal- so that the solar wind and magnetospheric they often contain a helical magnetic field loon satellites in the 1960s (e.g. ECHO and magnetic fields link (or ‘reconnect’), creating structure. As the CME passes over the Earth, PAGEOS) to more recent interplanetary mis- a so-called open magnetic field line (pur- this manifests itself as a steady rotation in sions such as NASA’s MESSENGER mission to ple field line ‘3’). This open magnetic flux the solar wind magnetic field orientation at Mercury. When making its initial fly-bys of is convected by the solar wind flow over the magnetopause, which can take a day or the planet Mercury, mission planners tilted the poles of the Earth (purple field line ‘4’), more in the largest events. A second impor- MESSENGER’s solar panels and used solar adding magnetic flux and associated stored tant driver of geomagnetic storms are stream radiation pressure to fine tune the trajec- energy to the magnetotail. This energy interaction regions where regions of fast solar tory, saving fuel in the process. cannot be stored indefinitely, and is explo- wind catch up with slower solar wind, form- Evidently, to maximise this effect one sively released during another reconnec- ing compressed regions following a spiral requires an extremely light spacecraft and tion event in the magnetotail, where open pattern in interplanetary space (Balogh et al., an extremely large reflective area. In fact, field lines (purple field line ‘5’) reconnect to 1999). These regions contain large-amplitude a solar sail can modify orbits in several dif- form closed field lines in the tail (red field waves, which can have long duration inter- ferent ways (McInnes, 2004). For example, line ‘6’) energising and driving plasma into vals of southward IMF and cause moderate Sun-centred orbits can be displaced above the inner magnetosphere (red field line ‘7’) geomagnetic storms. These are common in the ecliptic plane. Alternatively, equipped where it enhances the ring current. Plasma the declining phase of solar cycles, and will with a solar sail, one can place a spacecraft particles and energy are also delivered along become important in the next few years. into a solar polar orbit that is synchronous field lines to the polar regions, where bright As described in the introduction, meas- to the Earth. However, most relevant to the auroral displays can be observed. Magnetic urements have to be made in situ by a sat- problem of space weather monitoring is the flux is also returned to the solar wind (blue ellite upstream of the Earth. This can lead 28 field line ‘8’). Whilst in reality the process is to some confusion about warning times. 3 http://www.srl.caltech.edu/ACE/ace_mission.html existence of another family of orbit solu- tions that enable artificial Lagrange points closer to the Sun. In fact, a continuum of solutions can be generated, which depends on the attitude of the sail and the inherent properties of the spacecraft (effectively its mass and surface area). With a sail it is thus possible to fly and station-keep closer to the Sun, and thus provide longer lead time warning of space weather events. Sunjammer The first serious proposals for space mis- sions that would use a solar sail to achieve their objectives were developed in the 1970s. Particular effort was expended by NASA’s Jet Propulsion Laboratory (JPL) in developing Weather 2015, Vol. – January 70, No. 1 a rendezvous mission with comet Halley, but unfortunately this mission was not selected for flight. The first successful solar sail mission was the Japanese Interplanetary Kite-craft Accelerated by Radiation Of the Figure 2. The successful ground deployment test of the 318m2 sail in 2005. (Image Credit: NASA.) Sun (IKAROS) mission, launched in 2010 as a secondary payload accompanying the Akatsuki spacecraft to Venus. IKAROS suc- cessfully deployed a 200m2 sail in inter- planetary space, and demonstrated ‘photon propulsion’ for the first time.

The Sunjammer mission concept The first concrete ideas for a sub-L1 space weather monitor using solar sail technol- ogy date to the 1990s when the Geostorm Warning Mission was investigated by NASA’s JPL at the request of NOAA. This study showed that existing and emerging technology could be used to build a solar sail spacecraft capable of station-keeping near the Sun–Earth line, at ~3 × 106km upstream from the Earth, doubling the L1 warning time and also validating solar sail technology. This then led to the Geostorm Warning Mission proposal, submitted in 1999 to NASA’s New Millenium Program Space Technology-5 flight validation opportunity (West, 2004). Led by JPL, the design of the solar sail itself was developed by L.Garde, Inc. Whilst the proposal ultimately was not selected, development work continued, and successful ground deployment tests of an 83m2 and then a 318m2 sail were completed in 2004 and 2005 respectively (see Figure 2). Figure 3. Artist’s impression of Sunjammer . The sail is approximately 40m2; the spacecraft itself is The Sunjammer mission concept has since relatively small and located within the centre of the sail. (Image credit: L.Garde, Inc.) been developed by L.Garde, Inc. (Barnes et al., 2014). It is named (with permis- torques that will cause it to rotate). The sail booms are slowly inflated with gas, and sion) after the Arthur C. Clarke short story membrane itself is a 5μm thick Kapton film, as they extend they draw the sail out with describing a solar-sail yacht race and an coated with Aluminium on the sun-side and them. The material of the boom is such that artist’s impression of Sunjammer is shown chromium on the reverse. The sail area is when it drops below a certain temperature, in Figure 3. Four beams extend from the ~1200m2, four times larger than that shown it becomes very rigid. While stowed, the central spacecraft body with the four sail in Figure 2, but weighs only 8.5kg. thermal design ensures that they remain quadrants suspended between them. The Perhaps the biggest challenge of any above this temperature, but once exposed triangular section at the end of each beam solar sail mission is the deployment of the to space they will cool and thus harden. is a vane – a control surface whose orienta- large sail structure once in space. In the Once fully deployed, the booms become tion can be adjusted to alter the attitude Sunjammer concept the sail and the booms rigid, the inflatant is expelled and the sup- of the spacecraft, and can be set to ensure would be stowed into the spacecraft itself, port module containing the gas canisters passive stability (i.e. ensure that once the in a package about 0.5m3. The deployment and associated deployment mechanisms orientation of the sail is set, there are no then occurs in a controlled fashion. The can then be discarded. 29 A key requirement of any solar sail mis- fields and capacitive charging, which could demonstration, in Advances in Solar sion is that the instruments are as light contaminate the space weather measure- Sailing. Macdonald M (ed.). Springer: as possible. The UK Space Agency has ments, the sail design includes numerous Berlin. pp 115–126. funded the engineering development of ‘grounding’ devices to reduce this possibility. Brown P, Beek T, Carr C et al. 2012. two highly-miniaturised instruments that Successful in-flight demonstration will open Magnetoresistive magnetometer for space would be suitable for a sub-L1 solar sail mis- a path to an operational mission using space science applications. Meas. Sci. Technol. 23: sion. MAGIC (MAGnetometer from Imperial weather technology. 025902. College) is designed to measure the solar Cabinet Office. 2013. National Risk Register of Civil Emergencies, 2013 Edition, wind magnetic field, and SWAN (Solar Wind Summary Crown copyright. Cabinet Office: London. ANalyser) is designed to measure the solar Of all the space weather phenomena that Cannon P, Angling M, Barclay L et al. wind itself. The instruments have been pose a risk to our technology and activity 2013. Extreme Space Weather: Impacts developed by Imperial College London and on Engineered Systems and Infrastructure. Sunjammer in space and on the ground, geomagnetic University College London’s Mullard Space Royal Academy of Engineering: London. storms are perhaps one of the most impor- Science Laboratory (MSSL), respectively, and Dungey JW. 1961. Interplanetary mag- tant. The strongest storms are associated are world-leading in terms of their minimal netic field and the auroral zones. Phys. Rev. with CMEs. However, as we have seen, their mass and power requirements. Lett. 6: 47–48. ‘geoeffectiveness’ fundamentally depends The MAGIC magnetometer uses magne- Eastwood JP. 2008. The science of space on their internal plasma structure and weather. Philos. Trans. R. Soc. London A toresistive (MR) sensor technology which magnetic field, which must be measured 366: 4489–4500. enables a 100× reduction in both the sen- in situ. To increase warning times, monitor- Eastwood JP, Hietala H, Toth G et al. sor mass and volume compared to the typi- ing platforms operating further upstream of 2014a. What controls the structure and cal fluxgate magnetometers that are used dynamics of the Earth’s magnetosphere? the Earth than existing satellites are needed. for science missions (Brown et al., 2012). Space Sci. Rev. doi:10.1007/s11214-014- Solar sail missions are a potential game- MR sensors are solid state devices, and 0050-x changer for space weather monitoring work on the principle that in the presence Eastwood JP, Brown P, Oddy TM et al.

Weather – January 2015, Vol. 70, No. 1 70, No. – JanuaryVol. 2015, Weather because they can station-keep further 2014b. Magnetic field measurements from of an external magnetic field, their resist- upstream of the Earth than existing a solar sail platform with space weather ance changes. A possible implementation satellites such as ACE. The Sunjammer mis- applications, in Advances in Solar Sailing. of MAGIC on Sunjammer is derived from a Macdonald M (ed.). Springer: Berlin. sion concept sees as part of its payload two previous instrument that has flown in low pp 185–200. UK-built instruments that measure the solar Earth orbit on a CubeSat called Cubesat for Harrison RA, Davies JA. 2014. wind and its magnetic field, and is founded Ions, NEutrals and MAgnetic fields (CINEMA), Demonstrating the power of heliospheric on UK innovations in orbital dynamics and imaging for space weather: tracking solar and is described in more detail by Eastwood mission design. This payload is highly min- ejecta from Sun to Earth. Weather 69: et al. (2014b). A number of instrument iaturised and thus optimised for solar sail 246–249. doi: 10.1002/wea.2354 developments have been made, such as applications. Ultimately, key goals include Kataria DO, Hailey M, Hu H. 2014. Solar the use of two boom-mounted sensors investigation of the feasibility of using solar Wind Analyser (SWAN) for the Sunjammer so as to better characterise the spacecraft solar sail mission, in Advances in Solar sail platforms for making operational space magnetic field and remove its effects. The Sailing. Macdonald M (ed.). Springer: weather measurements, and conducting Berlin. pp 201–210. design is also more robust in the use of high scientific studies of the solar wind and its reliability components where possible. McInnes CR. 2004. Solar Sailing: magnetic field to better understand the Technology, Dynamics and Mission The SWAN plasma detector is designed fundamental sources and causes of space Applications. Springer: Berlin. to measure the properties of the solar wind weather. Finally, although solar sails will McInnes CR, McDonald AJC, Simmons plasma itself (Kataria et al., 2014). SWAN is an pave the way for future space weather JFL et al. 1994. Solar sail parking in electrostatic analyser; such instruments use restricted three-body systems. J. Guid. monitoring systems, this technology has electric fields to bend incoming particles Control Dyn. 17(2): 399–406. applications in many diverse areas of space onto a detector plane, and by rapidly step- National Research Council. 2008. Severe exploration and exploitation, ranging from ping the voltage, one can quickly measure Space Weather Events – Understanding interstellar probes to Earth observation. Societal and Economic Impacts: A Workshop the flux of particles as a function of energy. Report. The National Academies Press: MSSL has considerable heritage and experi- Acknowledgements Washington, DC. ence in developing such instruments, hav- Schwenn R. 2006. Space weather: the solar ing built similar instruments flying on both Development of the MAGIC and SWAN perspective. Living Rev. Sol. Phys. 3(2): 1–72. the Cluster and Cassini spacecraft. SWAN is instruments is supported by the UK Space West J. 2004. The geostorm warning mis- based on both the ChaPS plasma detector Agency. We thank the reviewers for their sion: enhanced opportunity based on new which launched on the TechDemoSat tech- comments and suggestions that improved technology, 14th AAS/AIAA Space Flight Mechanics Conference, Volume 102. AAS: nology demonstrator satellite earlier this the manuscript. Maui, HI, pp 8–12. year, and the solar wind experiment that will fly on European Space Agency’s (ESA’s) References Solar Orbiter. SWAN also contains a solid Correspondence to: Jonathan P. Eastwood state detector for energetic particles (ions Balch C. 2004. Intense Space Weather [email protected] > 5MeV, electrons > 250keV). Storms October 19–November 07, 2003. Service Assessment. National Oceanic and Any in-flight sail demonstration must ini- © 2015 The Authors. Weather published by Atmospheric Administration: Silver Spring, John Wiley & Sons Ltd on behalf of Royal tially aim to understand and evaluate the sail MD. Meteorological Society performance and the performance of novel Balogh A, Bothmer V, Crooker NU miniaturised space weather instrumentation. et al. 1999. The solar origin of corotating This is an open access article under the For example, the photoelectric effect means interaction regions and their formation in terms of the Creative Commons Attribution that the sunward facing side of the sail will the inner heliosphere. Space Sci. Rev. 89: License, which permits use, distribution and 141–178. tend to become positively charged relative to reproduction in any medium, provided the

the anti-sunward side. In the knowledge that Barnes NC, Derbes WC, Player CJ original work is properly cited. et al. 2014. Sunjammer: a solar sail this could lead to electric currents, magnetic 30 doi:10.1002/wea.2438