Stormy Space Weather Ahead?=20

Janet Luhmann=20

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=
U
 S 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, Berkele=
y=20

Figure Captions=20

Figure 1. Composite image from the SOHO spacecraft of the coronal white lig=
ht 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/bestof=
soho). The white light comes from sunlight scattered by coronal electrons t=
hat are trapped within magnetic field loops and ropes. Coronagraphs work by=
 blocking the bright disk to produce artificial total eclipses. The EUV dis=
k image shows the locations of hotspots or active regions that may play a r=
ole
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 2=
3 sunspot number based on studies of previous cycles, while the bracketing =
lines indicate the range of that prediction. The ragged line shows the actu=
al sunspot number.

Figure 3. An illustration of the magnetic "bubble" called the magnetosphere=
 that defines near-Earth space. The magnetosphere contains the upper atmosp=
here 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 fi=
lled 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 magne=
tosphere known as the magnetosheath (from the Goddard Space Flight Center I=
STP eb site http://www-istp.gsfc.nasa.gov/istp/outreach).

Figure 4. Illustration of the solar wind, the outermost solar atmosphere, f=
lowing radially from the Sun but containing an embedded spiral interplaneta=
ry field due to solar rotation. The magnetosphere deflects most of the sola=
r wind plasma around the Earth and its atmosphere at altitudes above about =
10 Earth radii - roughly 64000 km, in a region called the magnetosheath tha=
t 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 obtain=
ed during total eclipses under solar minimum (left) and solar maximum (righ=
t) activity conditions. (images from the High Altitude Observatory, Boulder=
 website http://hao.ucar.edu). As noted earlier, these are produced by sunl=
ight scattered from coronal electrons that are trapped in the coronal magne=
tic field. The changes in the shapes of the densest (brightest) parts of th=
e 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 eruptin=
g 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.gs=
fc.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 g=
lowing 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 Obser=
vatory. The dark features are the prominences existing at the time the X-ra=
y image in Figure 8 was obtained (from the Mullard Space Sciences Laborator=
y Web site  a Cohttp://mssly1.mssl.ucl. ac.uk/ydac/nuggets). As explained i=
n the text, prominences are clouds of photospheric material suspended above=
 the main photosphere by coronal magnetic fields. They are sometimes seen t=
o disappear at the time of a CME, thus providing evidence a CME has occurre=
d even when coronal images are not available.

Figure 10. Image from the Extreme Ultraviolet Telescope on the SOHO spacecr=
aft, 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 (f=
rom the Goddard Space Flight Center Solar Data Analysis Center Web site htt=
p://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 Laborato=
ry Web site http://vestige.lmsal.com/TRACE).

Figure 12. A comparison of sunspot number and the occurrence of geomagnetic=
ally stormy days during the previous six solar cycles shows their general c=
orrespondence (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 ahe=
ad, 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/io=
nosphere responses, while the energized particles can enter along interconn=
ected interplanetary and magnetospheric field lines (Figures 14 and 15).

Figure 14. Figure illustrating Professor James Dungey's key concept that fi=
rst explained the role in solar-terrestrial energy transfer played by the S=
outhward interplanetary magnetic field. Dipolar field lines, representing t=
he Earth's magnetic field, are vectorially added to straight external field=
 lines representing a uniform interplanetary field. When this external fiel=
d points Southward (left panel) with respect to the dipole moment, the fiel=
ds exhibit greater topological interconnection than does the example having=
 Northward external field (right panel). The greater interconnection of Sou=
thward interplanetary fields with the magnetopheric fields allows entry of =
solar particles, and enables the solar wind to directly drive the convectio=
n 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 fi=
elds, and the antisunward motion of the interplanetary field with the solar=
 wind. The numbers suggest the time-sequence of a particular field line's p=
osition. (Picture from the textbook by Kivelson and Russell).

Figure 16. Illustration of solar wind-driven convection inside the magnetos=
phere, 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 cr=
oss-section in Figure 15. The currents circulating around the Earth are res=
ponsible for the magnetic field decreases measured at low latitudes on the =
ground when magnetic storms enhance them. (NOTE YOU SHOULD REMOVE ALL LABEL=
S NOT USED SUCH AS MAGNETOPAUSE, PLASMA SHEET, ETC.)=20

Figure 17. Auroral oval as observed in visible light by the POLAR spacecraf=
t, superposed on the globe for scale (from the Goddard Space Fight Center I=
STP Web site http://www-istp.gsfc.nasa.gov/istp/outreach). The oval roughly=
 traces the boundary of the interconnected interplanetary and terrestrial m=
agnetic fields (Figure 15).



Dr Valerie Jamieson
Features Editor, Physics World

Tel. +44 (0)117 930 1226 direct
Fax +44 (0)117 925 1942
e-mail valerie.jamieson@ioppublishing.co.uk
URL http://physicsweb.org/


**********************************************************************
IOP Publishing Limited
Registered in England under Registration No 467514.
Registered Office: Dirac House, Temple Back, Bristol BS1 6BE England

This e-mail message has been checked by MIMEsweeper using
Sophos Sweep for the presence of computer viruses.
**********************************************************************