Janet Luhmann
To casual observers, the Sun may appear calm and unchanging. But the behavi= our and energy output of our nearest star varies according to a cycle that = lasts11 years or so. This behaviour has been followed for centuries by foll= owing the movements of sunspots -- dark regions that appear and disappear o= n the visible solar "surface" [Query:OK]. But the rise and fall in the numb= er of sunspots -- and the accompanying solar activity -- have recently attr= acted renewed interest as the Sun approaches a peak in its 11-year cycle. T= his interest has also been fuelled by the recent spectacular images from sp= acecraft, such as the Solar and Heliospheric Observatory (SOHO).
As we become ever more reliant on space-based technology, such as the globa= l positioning system for navigation and remote monitoring, are we also beco= ming more vulnerable to the major disturbances that solar activity can trig= ger?
The events that occurred at the height of activity during the two previous = solar cycles altered our perception of so-called space weather. In August 1= 972, some of the largest ever fluxes of energetic particle radiation from t= CAN WE CHANGE "LARGEST EVER (etc.)" TO "largest fluxes of energetic solar particle radiation on record were detected." he Sun were detected. Fortunately these blasts of radiation occurred in bet= ween two Apollo lunar landings: had the astronauts been outside of their ve= hicle during these events, they would have received a potentially lethal ra= diation dose. Instead, one of the major telecommunications lines in the US = failed briefly, and satellites in the popular geosynchronous orbit some 40:= :000;;km above the Earth were exposed to solar energetic particles that aff= ected the efficiencies of their solar-cell power systems.
More recently, during a magnetic storm in March 1989, transformers at the H= ydro-Quebec power stations in Canada reacted to a current surge that was in= duced by the changing magnetic fields at ground level. The surge lead to po= wer blackouts throughout Quebec that lasted for several hours, and the powe= r company lost more than 21::500 Megawatts of its production capacity. In a= ddition, a transformer at a nuclear power plant in New Jersey was damaged b= eyond repair as a result of the induced current.
Geomagnetic observatories confirmed that these incidents were related to th= e occurrence of a major disturbance in space. However, previous solar cycle= s have also taught us that one or two extraordinary episodes usually occur = during the solar cycle without notable fanfare.
Far-reaching effects
So what is space weather anyway and what, if anything, can be done to forec= ast it and/or reduce its potentially harmful effects? Space weather is driv= en by the Sun and describes the conditions that prevail above the stratosphere, AT ALTITUDES ABOVE about 50;;km, where responses to solar influences are= particularly strong. These regions affected by these conditions include th= e Earth's upper atmosphere, the ionosphere -- the ionized region of the upp= er atmosphere that extends a few hundred kilometres -- and the radiations b= elts in the magnetosphere, the region beyond the ionosphere where electrons= and ions are trapped by the Earth's magnetic field. Space weather IN FACT RESULTS FROM SOLAR ACTIVITY'S INFLUENCES ON the interplanetary space beyond the reach of = the Earth's magnetic field. This region between the planets is filled with = a stream of protons and electrons from the outer solar atmosphere together = with its magnetic field, which collectively form the solar wind.
Stormy space-weather conditions are usually inferred from deviations in the= magnetic fields that are measured by satellites and ground-based instrumen= ts. These disturbances can also be deduced from conditions in the upper atmosphere and ionosphere, WHERE ONE FINDS INTENSIFIED auroras and airglows. And at high a= ltitudes the intensity of both electromagnetic and particle radiation incre= ases, which can pose a hazard for astronauts and possibly even for air crew= travelling on high-altitude supersonic routes (see "Cosmic rays: an in-fli= ght hazard?" by Denis O'Sullivan Physics World May 2000 pXX).
In addition to power and communications failures on the ground, disturbed s= pace weather increases the risk to spacecraft on several counts. During sto= rmy space weather, the geosynchronous communication and weather satellites = that reside on the outer edges of the radiation belt can suddenly find them= selves in an environment that differs from the one for which they were designed. The increase in the intensity of ELECTROMAGNETIC RADIATION AT EXTREME ULTRAVIOLET WAVELENGTHS (LESS THAN 1000 ANGSTROMS) HEATS AND THEREBY INFLATES THE UPPE ATMOSPHERE. THE RESULTING INCREASED DENSITIES OF ATMOSPHERIC GASES AT SATELLITE ALTITUDES can increase the a= tmospheric friction or drag on the spacecraft, which can affect the orbit. = In the 1970s, the Skylab space station re-entered the Earth's atmosphere prematurely during an unexpectedly high level of solar activity DUE TO THIS EFFECT. (ALSO DELETE NEXT SENT.) Indeed, this type of at= mospheric drag is an ongoing concern for the Hubble Space Telescope. Moreover, all satellites send and receive their communication signals throu= gh the ionosphere, which can be dramatically altered by space weather. The = instructions sent to the satellite from a control centre run an increased r= isk of errors. Recently the performance of several expensive satellites, including the Int= elstat and GOES-8 satellites in 1995 and Telesat in 1996, DEGRADED (OR DIMINISHED) at times dur= ing or after a sequence of space weather events. More seriously, the Telsta= r satellite was lost in 1997.
Learning from observations Much more is now known about the phenomena that underlie space-weather dist= urbances thanks to research in solar and space physics, which routinely com= bines analyses and interpretations of solar, interplanetary, and Earth-spac= e observations. Indeed, an international constellation of satellites and ne= tworks of ground-based aeronomy and solar observatories have joined forces = to both monitor and unravel the physics of space weather. Among the satellites, SOHO, Yohkoh and the Transition Region and Coronal Ex= plorer (TRACE) watch the Sun. In addition, SOHO, together with the Advances= Composition Explorer (ACE) and NASA's Wind satellite measure the interplan= etary environment near the Earth. Meanwhile, the state of the magnetosphere= is monitored by the Polar, Interball and Geotail satellites, and the Solar= Anomalous Magnetospheric Particle Explorer (SAMPEX). And various satellite= s -- including the Fast Auroral Snapshot satellite, the Student Nitric Oxid= e Explorer and Defence Meteorological Satellite program -- probe the response and interactions of the upper atmosphere TO what is happening above.
Back on Earth, optical and radio telescopes at sites ranging from Learmouth= in Australia to Kitt Peak in Arizona keep their instrumental eyes on the S= un. Meanwhile, aeronomy observatories in Alaska, Greenland and Sweden monit= or the conditions in the upper atmosphere and ionosphere using radar, camer= as and photometers designed for auroral observations. And magnetic-field ob= servatories throughout the world provide "geomagnetic indices" that are use= d as a measure of the severity of space weather, in much the same way that = the Richter scale is used to characterize earthquakes.
The magnetic or geomagnetic storms that dominate space weather during activ= e solar periods are now widely recognized as the Earth's response to a part= icular type of solar activity, known as coronal mass ejections. This recogn= ition is due, in part, to the publication of consciousness-raising reports = by Jack Gosling of Los Alamos Laboratory in the 1990s. Coronal mass ejections came to light decades ago during observations of tot= al solar eclipses. But the fleeting nature of these observations and qualit= ative documentation makes it difficult to know who to credit the initial di= scovery to. The Sun's outer atmosphere, called the corona, is composed of h= ighly ionized hydrogen and is normally invisible due to the blinding bright= ness of light from the photosphere, the apparent surface of the Sun. During= a total solar eclipse, however, the corona becomes visible through the Tho= mson scattering of white light from the photosphere by electrons in the cor= ona. Multiple ray-like structures are seen where the electron density is hi= gher than average (see Total solar eclipses: magic, science and wonder by F= rancisco Diego Physics World August 1999 pp31--36). This structure changes = with time and is most complex when the Sun is active.
The structure of the corona is controlled by the Sun's changing magnetic fi= eld. The field is simple and approximately dipolar around the solar minimum= but becomes dominated by higher-order harmonics around the sunspot maximum= . These harmonics form many localized loops that are related to the active = regionS. Indeed, many eclipse observers witnessed coronal features resemblin= g rising blobs or tongues of material from the Sun.
Detailed studies of the corona became possible following the invention of t= he coronagraph by French astronomer Bernard Lyot in the 1930s. Lyot's instr= ument created false eclipses, which meant that observations of the corona c= ould be made even when the Sun was not in eclipse. Later observations of bo= th the coronal structure and what appeared to be coronal eruptions were con= firmed and greatly improved thanks to the improvement of low light-level im= agers and the flight of coronagraphs into space above Earth's light-scatter= ing atmosphere (figure 6).
The appearance of coronal mass ejections, together with the generally accep= ted coronal physics, suggests that the ejections are localized, large scale= changes in the corona. These changes involve the expulsion of material fro= m the corona, mainly in the form of proton--electron plasma that exceeds th= e normal level of plasma carried outward in the solar wind. Furthermore, th= e appearance of the eruptions suggests that twisted magnetic "loops", which= expand outward into interplanetary space, are involved. The speeds of the = loopS have been measured from sequences of coronagraph images. The initial v= elocities range from tens of kilometres per second to around 2000;;kilometr= es per second. In comparison, typical speed of the solar wind is 350--400;;= km::s--1. (LAST WORDS DELETED).
The eruptions are accompanied by bursts of radio waves and soft X-rays that= last for several hours. The X-rays are thermal emissions from glowing arcades , a series of loops IN WHICH THE PLASMA HAS BEEN HEATED (figure 8). The relationship between these arcades and coronal = mass ejections is still under study. Visible light is also present in the a= rcade emissions but is too faint to see against the bright visible disk of = the photosphere. Meanwhile, the radio emissions are from gyrating electrons= that have been accelerated in the corona, or in association with the inter= action between the coronal mass ejection and the solar wind as it travels t= oward the Earth.
Elongated clouds of photospheric gas are another solar feature affected by = coronal mass ejections. These so-called filaments, (DELETED ARE, ADD COMMA) suspended above the bright photosphere, (ADD COMMA, DELETE AND) are (DELETE OFTEN) called prominences when they are observed n= ear the limb or edge of the visible disk. Prominences are often seen erupt with some coronal mass ejections. (DELETE OTHER) Filaments appear as dark feature= s in filtered images where only radiation from the hydrogen Lyman-alpha (12= 15.7;;Angstrom) line is allowed to pass through (Figure 9). Such observatio= ns have revealed that the filaments often disappear when coronal mass eject= ions erupt in their vicinity. This suggests that changes occur in the invis= ible coronal magnetic fields which surround a filament during a coronal mas= s ejection. The changes destroy the balance of forces that keeps the clouds= of gas aloft.
A flare is a distinct, smaller-scale solar activity that energizes the coronal plasma in the vicinity of complex groups of evolving sunspotS OR ACTIVE regions. Flares are sometimes, though not always, associated with cor= onal mass ejections. Although it is generally agreed that the mass ejection= s involve changes in the large-scale coronal magnetic fields, the exact phy= sical causes of these phenomena remains a subject of intense investigation. The structure of the coronal magnetic field can be inferred from coronagrap= h images. Observations show that the magnetic field of the corona, together= with the field of the underlying photosphere, evolves rapidly when the Sun= is active and as new groups of sunspots emerge and disperse. CORONAL mass ejections occur on timescales ranging from a few a week, duri= ng times of low solar activity, to a few a day around the sunspot maximum. Only a fraction of the coronal mass ejections seen at the Sun have a size a= nd direction that leads to interplanetary effects in the vicinity of Earth.= However, the imprint of the solar cycle on the frequency of magnetic storm= s provides the first line of evidence that sunspots, coronal mass ejections= and magnetic storms are all somehow related. Although the physics connecting sunspots and coronal mass ejections is not = completely understood in detail, it is at least based logically on their co= mmon ties to the Sun's magnetic field.
Stormy space weather
But how do coronal mass ejections lead to magnetic storms? The answer is in= the disturbances they produce in interplanetary space. The interplanetary = counterparts of coronal mass ejections that ARE detected by plasma and field de= tectors on spacecraft like SOHO, Wind and ACE appear as large bubbles or he= lices of twisted magnetic fields a few million kilometres across. The bubbl= es and helices are still partially rooted at the Sun,which is consistent wi= th the coronal picture (Figure 7). A few of these features move at speeds (DELETE PHRASE SINCE SPEEDS OF CMES WERE STATED EARLER) in excess of = the normal solar wind speed. This leads to the production of shock waves th= at accelerate a small fraction of the solar wind particles ahead of the eje= cta cloud to high energies (figure 13). Protons reach energies of around 10= 0;;MeV, while electrons reach hundreds of kiloelectron volts.
The magnetosphere is contained within -- and controlled by -- the boundary = conditions imposed on it from the outside by interplanetary conditions (Fig= ure 3). These are normally determined by the magnetic field embedded in the= quiet solar wind. This magnetic field of solar wind results from the combi= nation of the solar-field at its base and the 27-day rotation of the Sun. T= he rotation winds the field into a spiral even though the solar-wind plasma= flows radially, as illustrated in figure 4. The overall size of and bullet shape of the magnetosphere is determined pri= marily by the balance between the dynamic pressure of the solar wind (which= is given by the solar wind mass density times its velocity squared) and th= e pressure of Earth's magnetic field. The nose of the magnetosphere is normally located at around 64::000;;km (i.= e. about 10 Earth radii). But at times when the dynamic pressure of the sol= ar wind is extraordinarily high, the edge of the magnetosphere can move ins= ide the geosynchronous satellite orbit (at 6.6 Earth radii), leaving satell= ites exposed to interplanetary particles and fields. Meanwhile, the interpl= anetary magnetic field controls the transfer of solar wind energy and momen= tum into the magnetosphere.
The role of the interplanetary field, and the importance of its orientation= , was first explained by James Dungey of Imperial College in 1961. Observat= ions clearly showed that the occurrence of interplanetary magnetic fields p= ointing Southward relative to Earth's magnetic dipole axis (which is tilted= roughly 11 degrees from the rotation axis) in large part dictated the stre= ngth of geomagnetic activity. The reason is that this Southward interplanetary field is opposite to Earth= 's magnetic field on the nose of the magnetosphere, allowing magnetic inter= connection between the interplanetary and magnetospheric magnetic fields th= at does not occur for Northward-directed interplanetary field (Figure 14). This magnetic interconnection engages, like a clutch, the magnetosphere to = interplanetary space whenever the interplanetary field assumes a Southward = orientation. It allows the access of solar particles, and the imposition of= interplanetary electric fields along the nearly equipotential field lines = into the magnetosphere where they drive circulation and currents in the hig= h-latitude ionosphere (Figure 15).
The particles that are accelerated by the shock reach Earth ahead of the sh= ock wave itself, and can enter the magnetosphere along the interconnected m= agnetic field lines (Figure 15). This initial radiation increase in the hig= h latitude magnetosphere is the first sign of an imminent major magnetic st= orm. (Although the associated increases in the number of X-rays arrive from= the Sun at the speed of light, photons are less reliable indicators of the= approaching fast-moving ejecta.) Because the solar wind behind the shock i= s compressed by the ejecta moving through it, a plug of dense solar wind pu= shes the nose of the magnetosphere closer to Earth (Figure 3). The embedded= interplanetary magnetic field is also compressed and draped around the eje= cted material. This draping produces interplanetary fields with stronger No= rth--South orientations than are usual in the undisturbed solar wind (Figur= e 4), as does the twisted ejecta field following it (Figure 7). Thus the st= age is set for the interaction of the interplanetary disturbance with the m= agnetosphere -- the key factor in producing a strong storm.
When a magnetic storm occurs, the electric field associated with the inter- planetary field moving past the Earth penetrates deep into the m= agnetosphere along the interconnected fields. As this electric field drives= strong convection in (DELETE THE) both the magnetosphere and high latitude ionosphe= re (Figure 16), some of the magnetospheric and ionospheric plasma is energi= zed and injected toward the Earth to form a "ring current". The ring curren= t is the cause of the magnetic field decrease measured on the ground at mid= and low-latitudes that is the hallmark of a magnetic storm. In the same time period, several physical processes related to the passage = of the interplanetary disturbance increase the particle content and energie= s in the radiation belts. A distorted version of the interplanetary shock w= ave propagates through the magnetosphere, accelerating some of the resident= radiation belt particles as it travels. Magnetohydrodynamic waves excited = in the magnetosphere by the disturbance cause additional particle accelerat= ion. Some of the particles accelerated out of the solar wind plasma by the = interplanetary shock (or in a solar flare, which often accompanies the fast= est coronal mass ejections seen at the Sun) penetrate into the radiation be= lt, adding to the local energetic particle population.
The compression of the magnetosphere causes particles to be energized as th= ey are forced into regions of stronger magnetic field, and dumped into the = upper atmosphere in greater numbers than is normal. The strong magnetospher= ic and ionospheric convection is likewise accompanied by the intensified pr= ecipitation of particles into the upper atmosphere, especially near the Nor= th and South poles where the interconnected magnetic fields converge (Figure 15). These particles collide with gas molecules in the upper = atmosphere, (DELETE GASES THEREBY)exciting them into a higher energy state. These states then dexcite by emitting radiation AT WAVELENGTHS CHARACTERISTIC OF THE ENERGY LEVELS OF THE EXCITED ATOMS OR MOLECULES(E.G. ATOMIC OXYGEN AT 6300 AND 5577 ANGSTROMS). ( Figure 17). The ionosphere is further disturbed by the intensification of the currents = flowing through it, especially where it is made more conducting due to addi= tional ionization by the precipitating particles. The polar upper atmospher= e, coupled by collisions of its neutral gas particles to ions in the enhanc= ed and dynamic auroral ionosphere, and undergoing compositional changes tha= t lead to heating, expands and in so doing launches large-scale atmospheric= waves from the polar regions towards the equator.
So a magnetic storm is a complicated and multifaceted response to a coronal= mass ejection launched towards the Earth from the Sun. Moreover, each stor= m has its own idiosyncracies depending on the prior states of the magnetosp= here and ionosphere, and the details of the interplanetary disturbance that= reaches Earth's location. For example, sometimes dense material associated= with a disappearing filament on the Sun arrives as a dense mass of helium-= rich plasma trailing the main ejecta. The resulting incident dynamic pressu= re can temporarily increase tenfold, adding a late phase to the storm that = includes a second magnetospheric compression and associated geospace respon= ses- a "one-two punch". "Superstorms" can result if the interplanetary dist= urbance produced by a CME is reinforced enroute to Earth by interaction wit= h another disturbance.
Space weather forecasts
Can magnetic storms be forecast, and if they can be, how would forecasts be= used? Although the space weather system is a complex physical system, enou= gh is now known to begin building sophisticated models for simulating magne= tic storms and their effects. There are two basic types of models: empirica= l models and numerical simulations, and two basic philosophies.=20 The first approach (DELETE IS) involves modelling near-Earth space based on OBSERVED PATTERNS OF BEHAVIOR IN RESPONSE TO PARTICULAR INTERPLANETARY CONDITIONS. An example of a Near-Earth s= pace empirical model developed at Rice University provides characterization= s of radiation belt and inner magnetosphere behavior for specific levels of= geomagnetic activity. Examples of Near-Earth space numerical models are th= e global magnetospheric simulations using magnetohydrodynamic approximation= s by groups at Dartmouth College, UCLA and The University of Michigan in the US, and upper= atmosphere/ionosphere models developed at University College, London and N= CAR High Altitude Observatory in Colorado. Access to information about thes= e models can be found via the space physics web site directory available at http:// = espsun.space.swri.edu/SPA). The alternative approach is a "cradle-to-grave" system modeling of the comp= lete solar, interplanetary, magnetospheric, and upper atmosphere/ionosphere= system based on solar observations. The full system models, from the Sun t= o the upper atmosphere, are still in the concept development stage. All req= uire further improvements in physical understanding, and perhaps data assim= ilation techniques such as those used in traditional meteorology, to improv= e their performance.
Taking into account the several hundred to ~1000 km/s speeds of the CME-rel= ated interplanetary disturbances, it is easy to calculate the possible adva= nce warning times for storm forcasts. Interplanetary information based mode= ls can provide a Space Weather forecast about an hour ahead by using real-t= ime information from spacecraft like ACE at the Earth/Sun libration point (= ~200 Earth radii upstream of Earth). Only solar observation based models ha= ve the potential of providing forecasts several days ahead. Currently, the = only solar observation based models used on a regular basis involve semi-em= pirical schemes for predicting interplanetary shock arrival and solar energ= etic particles. Perhaps the greatest barrier to these longest lead-time sto= rm predictions is the lack of knowledge regarding CME initiation. Many of t= he models under development in both academic institutions and research labo= ratories worldwide are ultimately destined for customer-oriented organizati= ons such as the National Oceanic and Atmospheric Administration Space Envir= onment Center in the U.S. (http://sec.noaa.gov), the Hiraiso Solar Terrestr= ial Research Center in Japan (http://hiraiso.crl. go.jp), IPS Radio and Spa= ce Services in Australia (http://www.ips.gov.au), and the Lund Space Weathe= r Center in Sweden (http://www.irfl.lu.se), who will be the likely provider= s of the resulting Space Weather forecasts.
How will such forecasts be used? Space Weather information users who alread= y take advantage of the aforementioned organizations' offerings include com= munications and power industries, geophysical surveyors using magnetic fiel= d measurements and GPS for resource prospecting and monitoring, launch serv= ice providers, NASA's Human Exploration Division, national defense agencies, and radio hobbyists. These users can a= lso choose to exploit the knowledge of the physics behind the phenomenology= of Space Weather storms to design robust systems and procedures that are m= inimally compromised. Most of the models described above are not yet ready = for use during the solar maximum that is upon us in 2000-2001, but the detailed observations obtained this solar maximum will p= rovide the test cases and impetus for what is to come. Indeed, some of the = largest Space Weather events on record can occur during the declining solar= activity phase. Best of all, the framework of the models will reveal more = of the intricate physics that makes up the connected Sun-Earth system, broa= dening our horizons on our environment in space and our relationship with o= ur own variable star.
Further reading=20
J W Dungey 1961 INTERPLANETARY MAGNETIC FIELD AND THE AURORAL ZONES Phys. Rev. Lett. 6 47
J T Gosling THE SOLAR FLARE MYTH 1993 J. Geophysical Res. 98 (CORRECT PAGE NO.)18,937
M G Kivelson and C T Russell 1995 Introduction to Space Physics (Cambridge = University Press)
Exploratorium Solar Maximum Web page http://www.exploratorium.edu/solarmax=
US National Research Council Space Weather Research Perspective Web page ht= tp://www.nas.edu/ssb/cover.html=20
Janet Luhmann, Space Sciences Laboratory, University of California, Berkeley
Figure Captions
Figure 1 Composite image from the SOHO spacecraft of the coronal white light imaged around the disk by the LASCO coronagraph, and on the disk by the EIT telescope in the extreme ultraviolet, during a Coronal Mass Ejection or CME (from the SOHO Web site http:// sohowww.nascom.nasa.gov/gallery/bestofsoho). The white light comes from sunlight scattered by coronal electrons that are trapped within magnetic field loops and ropes. Coronagraphs work by blocking the bright disk to produce artificial total eclipses. The EUV disk image shows the locations of hotspots or active regions that may play a role in launching this eruption. (see text for details). CMEs are thought to be the cause of disturbed space weather, or magnetic storms, on the Earth.
Figure 2. The increasing observed sunspot number, the standard indicator of solar activity, compared with a prediction for the current solar cycle 23 (from Marshall Space Flight Center Space Sciences Web site http:// science. msfc.nasa.gov/ssl). The smooth central line is a prediction of the cycle 23 sunspot number based on studies of previous cycles, while the bracketing lines indicate the range of that prediction. The ragged line shows the actual sunspot number.
Figure 3 An illustration of the magnetic "bubble" called the magnetosphere that defines near-Earth space. The magnetosphere contains the upper atmosphere with its ionized component, the ionosphere, which occupy a thin layer over the Earth on this scale. The space between the Sun and the Earth is filled with a largely hydrogen (proton and electron) plasma called the solar wind, which is the outward-flowing outermost atmosphere of the Sun. A shock wave or bow shock forms upstream of the Earth in the solar wind, which is diverted around the magnetosphere in the region between the shock and magnetosphere known as the magnetosheath (from the Goddard Space Flight Center ISTP eb site http://www-istp.gsfc.nasa.gov/istp/outreach).
Figure 4 Illustration of the solar wind, the outermost solar atmosphere, flowing radially from the Sun but containing an embedded spiral interplanetary field due to solar rotation. The magnetosphere deflects most of the solar wind plasma around the Earth and its atmosphere at altitudes above about 10 Earth radii - roughly 64000 km, in a region called the magnetosheath that is bounded on its outer side by a collisionless shock wave or bow shock (Figure 3).
Figure 5 White light images of the Sun's outer atmosphere or corona obtained during total eclipses under solar minimum (left) and solar maximum (right) activity conditions. (images from the High Altitude Observatory, Boulder website http://hao.ucar.edu). As noted earlier, these are produced by sunlight scattered from coronal electrons that are trapped in the coronal magnetic field. The changes in the shapes of the densest (brightest) parts of the corona reflect the solar cycle changes in the coronal magnetic field from an approximately dipolar to a complex configuration as the sunspot number increases.
Figure 6 White light coronagraph images of a CME in the process of erupting obtained during the Solar Maximum Mission, flown in the mid-80s (from the archives of the NCAR High Altitude Observatory, Boulder).
Figure 7 Sketch showing a prevailing view of the magnetic field lines in a CME (from the Goddard Space Flight Center ISTP web site http://www-istp.gsfc.nasa.gov/istp/outreach). This "rope" of magnetic field lines is what is thought to thread structures such as that seen in the white light image in Figure 6.
Figure 8 Soft X-ray image of the Sun from the Yohkoh spacecraft, showing glowing arcades associated with the complex sunspot regions known as active regions (from the Mullard Space Sciences Laboratory Web site http://mssly1.mssl.ucl.ac.uk/ydac/nuggets).
Figure 9 Solar image in the Lyman alpha line of Hydrogen from Meudon Observatory. The dark features are the prominences existing at the time the X-ray image in Figure 8 was obtained (from the Mullard Space Sciences Laboratory Web site http://mssly1.mssl.ucl. ac.uk/ydac/nuggets). As explained in the text, prominences are clouds of photospheric material suspended above the main photosphere by coronal magnetic fields. They are sometimes seen to disappear at the time of a CME, thus providing evidence a CME has occurred even when coronal images are not available.
Figure 10 Image from the Extreme Ultraviolet Telescope on the SOHO spacecraft, illust rating the scale of a solar prominence eruption observed in the 304 Angstrom line of ionized helium in comparison with the Earth's size (from the Goddard Space Flight Center Solar Data Analysis Center Web site http://umbra. nascom.nasa.gov). Prominence eruptions are sometimes associated with the occurrence of coronal mass ejections, the cause of magnetic storms on the Earth, but are not a direct cause by themselves.
Figure 11 Solar image showing the scale of a flare, for comparison to the scale of a CME (Figure 1). (From the Lockheed Martin Astrophysical Laboratory Web site http://vestige.lmsal.com/TRACE).
Figure 12 A comparison of sunspot number and the occurrence of geomagnetically stormy days during the previous six solar cycles shows their general correspondence (from the National Geophysical Data Center, Boulder, Colorado, USA).
Figure 13 Illustration of a CME-driven interplanetary shock wave's effects, including the compression of the solar wind plasma and magnetic field ahead, and the acceleration of some of the ambient solar wind particles. When this structure encounters the magnetosphere, the strong magnetic fields and high densities produce a panoply of magnetospheric and upper atmosphere/ionosphere responses, while the energized particles can enter along interconnected interplanetary and magnetospheric field lines (Figures 14 and 15).
Figure 14 Figure illustrating Professor James Dungey's key concept that first explained the role in solar-terrestrial energy transfer played by the Southward interplanetary magnetic field. Dipolar field lines, representing the Earth's magnetic field, are vectorially added to straight external field lines representing a uniform interplanetary field. When this external field points Southward (left panel) with respect to the dipole moment, the fields exhibit greater topological interconnection than does the example having Northward external field (right panel). The greater interconnection of Southward interplanetary fields with the magnetopheric fields allows entry of solar particles, and enables the solar wind to directly drive the convection of plasmas in the magnetosphere and ionosphere.
Figure 15 The concepts of magnetospheric and ionospheric convection driven by the combination of the interconnected terrestrial and interplanetary fields, and the antisunward motion of the interplanetary field with the solar wind. The numbers suggest the time-sequence of a particular field line's position. (Picture from the textbook by Kivelson and Russell).
Figure 16 Illustration of solar wind-driven convection inside the magnetosphere, and the currents that result from it (from the textbook by Kivelson and Russell). This may be viewed as the 3-dimensional counterpart of the cross-section in Figure 15. The currents circulating around the Earth are responsible for the magnetic field decreases measured at low latitudes on the ground when magnetic storms enhance them. (NOTE YOU SHOULD REMOVE ALL LABELS NOT USED SUCH AS MAGNETOPAUSE, PLASMA SHEET, ETC.)
Figure 17 Auroral oval as observed in visible light by the POLAR spacecraft, superposed on the globe for scale (from the Goddard Space Fight Center ISTP Web site http://www-istp.gsfc.nasa.gov/istp/outreach). The oval roughly traces the boundary of the interconnected interplanetary and terrestrial magnetic fields (Figure 15).
