J1.2 Where and Why Does Space Weather Occur?

J1.2 WHERE AND WHY DOES SPACE WEATHER OCCUR?

Patrick Market University of Missouri, Columbia, Missouri Delores J. Knipp* US Air Force Academy, Colorado,

1. INTRODUCTION Present and future aerospace leaders Modern aviators operate in a terrestrial need space-weather awareness and knowl- atmosphere, about which much is known. edge of potential impacts. The following vi- Despite this knowledge, some flights divert gnette illustrates space-weather effects on because of bad weather. On a seasonal today’s civil and military operations. basis, airport ramps full of aircraft are evacuated from a hurricane’s paths. Terres- Space Weather Vignette 2003 trial weather is monitored twenty-four hours a day, seven days a week. But what about Immediate Effects. In late October 2003 the space environment, home to critical when firestorms raged along the southern Cali- fornia coast, weary firefighters looked at the technology systems and billions of dollars in smoke-filtered Sun to see the origins of a storm space assets? Who monitors it? What of a different sort--a solar storm. Within minutes, should space professionals know about it? energetic-particle levels were so high that NASA What actions are taken to mitigate its ef- officials directed International Space Station as- fects? tronauts to take precautionary shelter. Airlines This paper provides a brief survey of the rerouted polar flights to avoid the high radiation primary agents and phenomena in space levels and communication blackout areas. Re- weather and space environment systems. It routed flights cost airlines $10,000 to $100,000 also introduces the basic terminologies un- per flight. From a tangled mass of magnetic field, the Sun hurled a cannonball of plasma earth- dergraduate students would encounter in the ward. Figure 2.1 shows one of a series of the first week of a space weather course. We outward propagating disturbances in the inter- start at the galactic level and work our way planetary medium. to Earth’s surface. We develop a framework Short-term Effects. Less than 24 hours af- that enables us to discuss space weather ter the fury on the Sun, utility companies reported effects. storm-induced currents over Northern Europe resulting in transformer problems and a subse- 2. SPACE IS NOT EMPTY! quent regional blackout. On October 28, the density of free electrons in the ionosphere on Earth’s sunlit side jumped by 25%. High- 2.1 Space is Not Benign frequency radio communication was disrupted. Effects of the space environment are Climbers on Mount Everest could not signal generally beyond sight and touch. These base-operations. Satellite navigation was com- effects occur in the realms of radio commu- promised in some areas. Auroras were visible in nication, spacecraft operations, high-altitude Houston, Texas, one of the more southern cities flight, extended power grids, and even com- in the United States. puter microchip manufacturing. A significant Long-term and Ongoing Effects. One part of civil and military operations now rely month later the same solar-active region sent on the “good” behavior of the natural space another blast toward Earth. Students at a west- ern US college, who were finishing a precision- environment. When the environment turns mapping project in a geography course, found hostile, space operators need to know. that satellite-derived position errors went from 5 meters to 75 meters in just a few minutes. Over *Corresponding author address: Delores J. Knipp, subsequent months, electric utility operators in Dept of Physics, Suite 2A25 Fairchild Hall, USAF the southern hemisphere discovered serious Academy, CO 80840: damage deep inside of power transformers. The e-mail: [email protected] cost to repair: Tens of millions of dollars. Space was not benign!

Figure 2.1. Mass and Energy Flowing from the Sun as Viewed by Three Instruments on the So- lar Heliosphereic Observatory Spacecraft. The scope of the image is ~ 16 solar radii in the hori- zontal. A mask covers the bright solar atmosphere out to a distance of one solar radius. To the right is evidence of a mass ejection. The mass outflow associated with this event was roughly equal to the mass of an average mountain on Earth. This was one of a series of events associated with the vignette discussed above. (Courtesy of NASA)

latter state, one in which a gas is so ener- 2.2 Defining Space Weather gized that its electrons and ions have sepa- Once generally viewed as a subset of rated, dominates space interactions. A very astronomy or perhaps astronautics, day-to- thin soup of plasma and energy constrains day interactions between Earth and the con- and interacts with near-Earth space creating stituents of interplanetary space are now space climate, space weather, and space commonly referred to as space weather or storms. Modeling this plasma environment space environment interactions. The sci- remains a challenge. ence behind these endeavors is space What constitutes the near-Earth space physics. Space weather describes the environment? For practical purposes we conditions in space that affect space-based define the region of space within 200 Earth 6 and ground-based technology systems as radii (200 RE = 1.28 x 10 km) as “near” well as Earth and its inhabitants. It is a con- Earth. By Earth standards, this is a huge sequence of the behavior of the Sun and volume in which over 8 million Earths would other stars, as well as the nature of Earth's fit, but by solar system standards, this vol- magnetic field and atmosphere, and our lo- ume is rather inconsequential. We often call cation in the solar system. As a practical this region simply the space environment. matter, space weather often means a dis- Does this mean Earth’s atmosphere is part turbed situation, whereas space environ- of the space environment? Most definitely it ment usually refers to normal or background is, but for the most part, we confine our dis- conditions. Space weather is created by cussions to atmospheric interactions occur- electromagnetic energy from the Sun and by ring above 50 km. We leave the atmos- the Sun’s out-flowing atmosphere that phere below that level (99.9% of atmos- streams by Earth (and all planets) at tre- pheric mass) in the capable hands of mete- mendous speeds. Earth’s magnetic field orologists. However, circumstances exist in and upper atmosphere dissipate, store, and which space weather processes spawn im- redirect some of this energy. The vast ma- pacts in the lower atmosphere and, occa- jority of energy and mass involved in space sionally, all the way to the surface. So, weather is from the Sun, but distant stars when necessary, we claim even the lower and closer objects, such as meteoroids, play atmosphere as space weather territory. a role too. The Sun is a huge, dense swirl of plasma, held together by its own gravitation. 2.3 Places and States of Space Weather It radiates away its excess thermonuclear and Space Environment energy as rivers of photons, neutrinos, and The material of space comes in several charged particles. The charged particles states: solid, liquid, gas, and plasma. The then form a dilute plasma that sweeps through interplanetary space. Thus, an un- This force accelerates charged particles derstanding of the environment of space along electric field lines. That is, ΣF = Σma hinges on understanding plasmas. Plasma = qE. is usually defined as an electrically neutral, If a magnetic field, B, rather than an ionized gas. electric field, is present and the charged par- Although the particles in plasma have ticle is moving, then we describe the force electric charges, the plasma at the macro- on a charged particle as the Lorentz force: scopic level remains electrically neutral, because the ions and free electrons, though FL =q(v x B) (1.2) not formally bound to each other, exist in a Where common hot “soup.” The electrons repel FL = Lorentz force vector [N] each other, and the ions also repel each v = particle’s velocity vector [m/s] other. The electrons are attracted to the B = magnetic induction vector [T]. ions, but the electrons are usually traveling By virtue of the cross product, this force so fast they don’t recombine with ions. Elec- acts perpendicular to v and B and creates a trons normally travel faster than the ions centripetal (toward the center) acceleration. because the thermal energy moves their This force is only active when particles are smaller masses more readily. in motion. In the space environment we of- Such a complex set of interactions ten find both fields acting on charged parti- makes plasma very dynamic. If plasma cles, creating what we call the generalized cools, the electrons slow and may recom- Lorentz force: bine with positive ions to form a gas. On a macroscopic scale, plasma is very fluid, FL = qE + qv x B = q(E + v x B) (1.3) much like a gas. However, unlike a gas, plasma exhibits one property that cooler Where FL is now the generalized Lorentz force vector on a charged particle created by E and B forms of matter rarely exhibit: it strongly fields. interacts with electromagnetic fields. This property sets plasma apart. Additionally, the The resultant motion for the charged parti- collective motions of charged particles gen- cles is a circular or spiral path, as shown in erate magnetic fields that, in turn, govern the Figure 2.1, depend- charged particle motions. In the presence of ing on the orienta- magnetic fields, individual charged particles tion of B and E. follow magnetically bent paths as they bounce between collisions. Figure 2.1. Helical Newton’s Second Law, ΣF = Σma, ap- Tracks of Two plies to individual particles and to collections Charged Particles. of particles. In our everyday lives we tend to Here we show an be more aware of forces acting on un- electron and posi- charged bodies, for example the sum of tron, created from gravity and lift, ΣF = -mg +L, that allows air- the decay of an en- craft to stay aloft. In space weather applica- ergetic photon. tions, we often ignore gravity because its Their motions are in influence is small; however we consider the presence of a other forces acting on charged particles, magnetic field. individually or collectively. Here we provide (Courtesy of European Organization for Nu- applications of Newton’s Second Law of Mo- clear Research [CERN]) tion for charged-particle and plasma behavior. The Coulomb force for charged parti- Net forces cause change in velocity, cles is which is really another way of saying that momentum is changed. Throughout discus- Fc = qE (1.1) sion of space weather, we will be interested Where in the causes of changes in momentum, en- Fc = Coulomb force vector [N] ergy and mass in the space environment q = charge on a particle [C], E,= electric field vector [V/m]. system resulting from space weather events.

3. SPACE WEATHER AGENTS unaware. Our atmosphere had protected us. It also protects us from particle out- 3.1 Other Stars Create Space Weather bursts of our own star, the Sun, which tend to occur during the active phase of the solar Stars other than our Sun are so distant magnetic cycle. that it is hard to imagine they could have an influence on Earth’s near-space environ- 3.2 The Sun Creates Space Weather ment. However, violent supernova explosions (stellar death) in our own galaxy and in The Nature of the Sun and Solar Dynam- distant galaxies accelerate material to near ics the speed of light. During such events, mil- The Sun is a nuclear furnace that, each lions to billions of volts of electrical energy second, converts millions of tons of hydro- are suddenly freed. In the tumult, electrons gen to helium in the solar core. In the proc- are stripped from their parent atoms. The ess, a tiny fraction of the mass belonging to remaining heavy nuclei travel for millions of the hydrogen atoms disappears only to re- years through interstellar space. These nu- appear as energy. The reservoir of energy cleonic remnants of distant and violent stel- creates temperature gradients that force an lar explosions, known as galactic cosmic outward-directed flow of energy. If the en- rays, arrive at Earth from all directions with ergy flowed out of the Sun without further extreme energies. Because of their high interaction with matter, we could end this energy, they penetrate through large book here. However, the high energy pho- amounts of shielding material. Thus, they tons created with each nuclear reaction are are dangerous to humans and equipment in captured by matter, emitted, and recaptured space. and reemitted billions of times on their path Cosmic rays are strongest during solar out of the Sun. With each capture, they give minimum when the heliospheric magnetic a bit of energy to their surroundings. By the field provides the least shielding. These time the photons reach the solar surface, particles speed through spacecraft, the at- most of them have degraded to the lower mosphere, and the otherwise significant pro- energy photons that we sense as yellow tection provided by Earth and interplanetary visible light. The visible light is only a small magnetic fields. Because these particles part of the total energy radiated from the are so energetic, they penetrate well into the 5800 K surface of the Sun. lowest levels of the ionosphere, where they Unlike Earth, the Sun does not rotate as often change radio signal propagation and a solid body. Rather, the Sun rotates fastest the effectiveness of some radar systems. at the equator (~25 Earth days per rotation) Numerous spacecraft malfunctions have and slowest toward the poles (over 32 Earth been associated with these ever-present days per rotation). This process is differen- particles. tial rotation and is one of the fundamental On rare occasions, violent gamma and drivers of space weather. Further, the outer X-ray upheavals on other stars have dis- layers of the Sun’s fluid interior convect turbed Earth’s upper atmosphere to the (boil) similar to water in a pot. The solar point that radio-wave propagation on Earth’s plasma motions twist and wind the Sun’s night side was impacted. Scientists have magnetic field into “islands” –known as sun- named the source of these impulsive stellar spots, whose field strength is hundreds or bursts, magnetars. In March 1979, widely even thousands of times stronger than separated Russian and US spacecraft de- Earth’s magnetic field. Thus, sunspots are tectors were blitzed by a front of energy compact storage zones for the Sun’s mag- moving through the solar system. Radiation netic energy. Sunspots usually appear in counts rose from 100 per second to 200,000 groups and are part of a large magnetic per second. Nearly 20 years later, a satis- structure known as an active region that factory explanation was finally formulated: often extends through the entire solar at- the spacecraft had been hit by bursts of en- mosphere. The differential twisting of the ergy from a neutron star from the Large Ma- solar plasma causes relative motion of one gellanic Cloud, a sister galaxy to our own sunspot with respect to a neighboring sun- Milky Way (Kouveliotou et al., 2003). At spot and sets the stage for explosive re- Earth’s surface, humans remained blissfully leases of energy. Observations of sunspots over many electromagnetic radiation released across a years revealed that the numbers of sunspots broad range of wavelengths called the solar change periodically. In some years as many spectrum. Each square meter of the top of as 250 sunspots are observed per day, in the Earth’s atmosphere receives 1366 Watts other years, none. This phenomenon is of power from the Sun. This value is the called the sunspot cycle and we illustrate it solar constant, which ultimately powers our in Figure 3.1. The plot shows that the num- atmosphere. We also see a quiet back- ber of sunspots varies with an average pe- ground outflow of tenuous and highly- riod of about 11.4 years. Some cycles are ionized plasma (100,000 K) called the solar as short as 8 years, others as long as 15. A wind. On average, the solar wind blows period of no or few observed sunspots is past Earth at a tremendous speed of ~ 400 called a sunspot minimum. Conversely, a km/s (about 1,000,000 mph in Earth's vicin- period of maximum number of sunspots is a ity), which is about ten times faster than the sunspot maximum. Beginning with the speed of sound in the rarified plasma. This minimum that occurred around 1755, sun- directed, or ram, flow of the solar wind has spot cycles have been numbered. The cycle kinetic energy. The random, omni- with its maximum in 2000 was cycle number directional motion of the 100,000 K plasma 23. particles is called thermal energy. Thermal The Sun has a variety of ways to re- energy is really just a form of non-directed, lease energy. Because of the huge number kinetic energy of the individual particles. of interaction between energy and solar mat- Physicists associate this phenomenon with ter, we see a relatively quiet background of the temperature of the solar wind.

Monthly Sunspot Number

300

250

18 21 200

150 Sunspot Number 12 100

0 1870 1890 1910 1930 1950 1970 1990 2010 Year

Figure 3.1. Sunspot numbers corresponding for the last 12 cycles. Selected cycle numbers are shown at the top. Data provided by the National Geophysical Data Center.

The solar wind consists of particles, turbances. Most of these disturbances oc- mostly ionized atoms of hydrogen and he- cur when energy stored in solar magnetic lium and traces of oxygen, carbon, iron, and fields converts to other forms. other elements. The radially flowing solar wind carries and stretches the Sun’s mag- • Solar flares—intense bursts of radia- netic field into space. In its stretched form, tive energy across the entire elec- the solar field is the interplanetary mag- tromagnetic spectrum, with the larg- netic field (IMF). However, one of the ends est burst enhancements in the X-ray, of these field lines stays rooted in the Sun. extreme ultraviolet (EUV), and radio Thus, as the Sun rotates (recall the mean portions of the spectrum. rotation period is 27 days), the IMF gets wrapped into a spiral, similar to a stream of • Solar energetic particles (SEP)— water from a rotating sprinkler. The Sun’s kinetic energy in the form of protons magnetic field arrives at Earth with an angle ejected with relativistic speeds near of about 45°. the flare site, or particles accelerated In the vicinity of Earth (1 Astronomical by a shock from the explosion site pushing into the solar wind. Unit [AU] from the Sun), each cubic meter of 6 space typically contains 5 x 10 positive ions • Coronal mass ejections (CME)— and an equal number of free electrons. This additional kinetic energy in the form is a very low particle content compared to of huge parcels of the Sun’s atmos- 25 the 10 molecules in each cubic meter of air phere accelerated (or ejected) into at sea level. Variations in the Sun's mag- interplanetary space. The parcels netic field are carried outward by the solar carry threads of the Sun’s magnetic wind and produce disturbances in the near- field into space as well. Much of the Earth environment called geomagnetic ejected plasma is from the Sun’s storms. Storms also accompany prolonged upper atmosphere, or corona. gusts of enhanced flow in the solar wind, called high-speed streams. Researchers Table 1.1 shows an impact grid for the now know that the Sun emits impulsive, important forms of the Sun’s emissions. transient bursts of particles and energy, as Even “quiet Sun” emissions produce geo- well as recurrent streams of fast flow. Oc- magnetic storms. casionally the mass ejections and high- Table 1.2 provides distances of interest speed streams merge to create a particularly in the space environment. potent storm. In addition to the background energy re- leases, we see forms of explosive energy release that create large space weather dis-

Table 1.1. Simplified Classifications of Space Weather Storm Effects. Here we list various forms of the Sun’s emissions and their characteristics.

Quiet Sun Emissions Time to Arrive at Earth Storm Impact Photons from ~5800 K Surface 8 min Normal Conditions No Solar Energetic Particles -- Normal Conditions Solar Wind Plasma 100 hr Normal Conditions -- With Strong Magnetic Field 60-100 hr Geomagnetic Storm -- With High Speed 30 hr Geomagnetic Storm Disturbed Solar Emissions Time to Arrive at Earth Storm Impact Solar Flare Photons (X-ray-radio) 8 min Radio Blackout Burst of Solar Energetic Particles 15 min – Several hr Radiation Storm Coronal Mass Ejection 20-120 hr Geomagnetic Storm

The Solar Environment minutes away. We rarely think of the Sun as Because the Sun is so close, we are an entity with an extended atmosphere, but able to study its behavior in detail. We are it has one, and it is large indeed, extending also benefactors (or victims) of its output. In well beyond Pluto’s orbit to a distance of our galaxy, the Sun is a middle-aged, yellow 120-150 AU. star, located about 150 million kilometers [1 AU] away from Earth—about 8.3 light-

Table 1.2. Some Distances in Space and the Space Environment. This table lists various distances from the Sun and from Earth.

Sun-centered Earth-centered Distance to galactic center Distance to solar wind monitor* 20 9 9 2 2.47 x 10 m = 1.65 x 10 AU 1.50 x 10 m = 2.35 x 10 Re Distance to nearest star Distance to the Moon 16 5 8 1 4.02 x 10 m = 2.68 x 10 AU 3.84 x 10 m = 6.03 x 10 Re Distance to heliopause* Distance to bowshock* 13 2 7 1 2.3 x 10 m = 1.53 x 10 AU ~9.60 x 10 m= 1.50 x 10 Re Sun-Pluto distance Distance to dayside magnetopause* 12 1 7 1 5.91 x 10 m = 3.94 x 10 AU ~6.37 x 10 m = 1.00 x 10 Re Sun-Earth distance Distance to geosynchronous orbit 11 7 1.50 x 10 m = 1 AU 4.20 x 10 m = 6.6 Re Solar radius Earth radius 8 -3 6 6.96 x 10 m = 4.64 x 10 AU 6.37 x 10 m = 1 Re *terms to be defined later in the paper

FOCUS ON: The Size of Things in the Solar System Most textbooks provide a drawing of the Sun and its planetary system that is not to scale. To give you a sense of the scaling of our solar system, consider the Sun to be the size of a 0.30 m (12 in) pie plate, which is also the approximate size of an adult foot. If you were to put the plate down and march about 110 “feet” beyond it you would reach Earth’s orbit at one AU. (Try this if your classroom is in a long building which has 12 in floor tiles.) In diameter, Earth is less than 1/100th the size of the Sun, typically the size of a pencil eraser in this scheme. To give you more perspective on the relative sizes of these bodies, it is useful to note that the entire Earth-Moon system would fit inside the radius of the Sun. Now that you know the Sun is large and distant, why does it matter? Because the disturbances that arise at the Sun’s surface and in its atmosphere are huge in comparison to Earth, if one of the disturbances interacts with Earth, it is likely to have spectacular and damaging results.

Figure 3.2. Relative sizes of the Sun and planets. (Image courtesy of N. Strobel, www.astronomynotes.com)

To put our entire solar system into perspective, we could scale the Sun to be the size of a large marble (about 2.5 cm in diameter). If we place our scaled Sun in Denver, Colorado, then the nearest star, Proxima Centauri, would be located in Chicago, Illinois. Alternatively, if we placed the scaled Sun in London, United Kingdom, then Proxima Centauri would be near Rome, Italy.

Heliosphere: The Extended Solar At- virtually all of the material in the heliosphere mosphere emanates from the Sun. At some distance The heliosphere is that region of space from the Sun (well beyond the orbit of Pluto), dominated by the Sun’s extended atmos- the supersonic solar wind slows to meet the phere. It is essentially a bubble in interstel- gases in the interstellar medium. At such a lar space produced by the solar wind (Figure location, the solar wind goes through a ter- 3.3). The distance to Pluto is about one mination shock to become subsonic. In third the distance to the near edge of the doing so, it slows and deflects in the direc- heliospheric boundary. The bubble forms tion of the interstellar flow to form a comet- because the Sun’s extended atmosphere is like tail beyond the Sun. Voyager I was in permeated by a magnetic field that is the vicinity of the termination shock in late roughly in the form of a dipole. When a 2004. The termination shock, where solar magnetic dipole is immersed in flowing magnetic fields bunch, appears to be a re- plasma from an outside source, in this case gion capable of accelerating newly ionized the interstellar medium, it deforms into a oxygen and helium into cosmic rays. Thus, tear-drop shape as in Figure 3.3. The outer it becomes a region of interest for those surface where the heliosphere meets the whose systems are vulnerable to cosmic interstellar medium is the heliopause. Here rays. the term “pause” indicates a break or dis- Inner Heliosphere In the interior continuity. The location of the discontinuity heliosphere, all planets behave as obstacles in solar plasma and magnetic field depends to the supersonic solar wind. A perturbation on the phase of the solar cycle. When the wave, or bowshock, forms upstream of Sun is more active the boundary is further each planet as a means of slowing and di- from the Sun because of the higher pressure verting the solar wind around the planet. generated by solar activity. In 2004, twenty- Smaller solar system bodies such as moons seven years after launch, the Voyager I and asteroids generally don’t have bow- spacecraft crossed the heliopause into the shocks. For a bowshock to form, the body interstellar wind, at ~94 AU. must have a magnetosphere, an iono- Outer Heliosphere We just introduced sphere, an atmosphere, or some combina- the idea of solar envelope, now we discuss tion thereof. Most moons and asteroids two general regions within this envelope. don’t qualify because they are too small to Although electrically neutral atoms from in- create or retain such spheres. terstellar space can penetrate this bubble,

A B

Figure 3.3. Heliospheres. A) An artist’s rendition of the heliosphere beyond the near-space environment. The small inner yellow is the Sun. B) This image is a Hubble Telescope view of the Bubble Nebula, a distant stellar system with its heliosphere and a stellar wind in excess of 2000 km/s. (A)Courtesy of NASA. B) Courtesy of Space Telescope Institute).

Earth’s bowshock forms ~15 RE up- of the magnetosphere must include these stream from Earth. Scientists have had a interactions. long-standing goal of positioning a space- Outer Magnetosphere Now we bring craft upwind of the bowshock so that incom- our tour closer to home. Just Earthward of ing solar wind could be monitored for analy- the bowshock is a zone of disordered solar sis and forecasting purposes. The Ad- wind created by the turbulent braking of so- vanced Composition Explorer (ACE) space- lar wind flow (Figure 3.5). In this region, craft has been about 230 RE upstream of called the magnetosheath, flow energy Earth since 1997. converts into turbulent eddies and ultimately into random thermal motion of the solar wind 3.3 The Earth’s Magnetosphere Re- constituents. Closer to Earth, the boundary sults From and Contributes to Space separating the frothy and weakly magnet- Weather ized solar wind from Earth’s magnetic do- In this section we describe Earth’s mag- main is called the magnetopause. This is netosphere as similar to that of the helio- the surface where the magnetosphere sphere. This similarity is not an accident. meets the interplanetary medium. The Magnetic fields immersed in moving, con- boundary is a fluid and magnetic membrane ducting fluids exhibit self-similar behavior that flexes in the dynamic solar wind. Gen- over many scales. erally the magnetopause is located at ~10 RE. Inside the magnetopause is that region Regions of the Magnetosphere of the space environment dominated by Earth’s magnetic field would resemble a Earth’s magnetic field; the magnetosphere. magnetic dipole at large distances from The local magnetospheric structure mirrors Earth, if the solar wind didn’t distort it into a the structure of the heliosphere-heliopause bullet-shaped magnetosphere (Figure 3.4). system. This similarity is no accident. Sys- Field lines distant from Earth become in- tems with differing plasma and magnetic creasingly stretched into a magnetotail that characteristics attempt to maintain their own extends well beyond lunar orbit (60 RE). identity. They do so by forming boundaries The distortion occurs because of field line, made of flowing charged particles (currents). current, and plasma interactions. As we If one system of magnetized plasma flows show in Chapter 8 an adequate description against another, the boundary often takes on a bullet or tear-drop shape.

Simple Dipole Distorted Dipole

Figure 3.4. The Distortion of Earth’s Magnetic Field by the Solar Wind. On the left is a simple dipole magnetic field configuration. On the right is Earth’s magnetic field as distorted by an external flow. The Earth’s north magnetic pole is currently at the south geographic pole and vice versa.

Figure 3.5. A Depiction of Earth’s Bow- shock, Magnetosphere, and Radiation Belts. The bowshock, indicated in dark blue, typically forms about 15 RE upstream of Earth and has a curved shape. This shock extends about 10 RE either side of Earth. Solar wind flow is depicted in yellow. Magnetospheric field lines are shown in red. As we show, the magnetosphere is not really a sphere. It is more like a hemisphere with an attached cylinder that ultimately takes on a bullet or tear-drop shape. (Courtesy of NASA)

Magnetospheric plasma originates in plasma. In 1958 an instrument onboard Ex- Earth's upper atmosphere and in the solar plorer 1, built by James Van Allen, meas- wind. Although much of the magnetosphere ured unexpectedly high values of ionizing is a near-vacuum by Earth standards, its radiation coming from a region within the high plasma density, compared to inter- dipolar magnetic field lines. Today we call planetary space, allows energy in the solar this region of near-space the radiation belts wind to drive electric currents and set or sometimes the Van Allen belts , (see plasma in motion within the magnetosphere Figure 3.5). The belts are doughnut-shaped and Earth’s upper atmosphere. We know and centered on Earth’s magnetic equator. two important effects of this behavior: 1) Spacecraft that operate in or near the radia- solar wind disturbances energize plasma tion belts must carry extra shielding to pre- even within Earth’s protective magnetic vent damage to instruments and compo- shield, and 2) the solar wind induces mag- nents from energetic particles trapped in the netospheric flows that are disconnected belts. from Earth’s rotation. Occupying much of the same region of Magnetotail Contained within the vol- space as the radiation belts is a torus ume of the magnetotail are regions of rela- (doughnut-shaped) region of higher density tively low and high densities of plasma. The but significantly lower temperature plasma magnetotail lobes, one in each hemi- with the hole aligned with Earth's magnetic sphere, have relatively low-density, cool axis called the plasmasphere. This plasma plasma. The plasma sheet within the cen- consists of hydrogen ions (protons) and tral magnetosphere has a denser and more electrons from Earth’s upper atmosphere. energetic plasma population. During peri- The plasmapause (sharp outer edge of the ods of geomagnetic storms, this plasma in- plasmasphere) is usually 4-6 Earth radii vades the orbit of geosynchronous satellites from Earth’s center (19,000-32,000 km (6.6 RE) and bathes the satellites in a soup above the surface). Inside the plasma- of energetic ions and electrons. This pause, the plasma rotates with Earth. The plasma bath leads to potentially lethal space inner edge of the plasmasphere, which in- weather impacts for instruments on geosyn- termixes with Earth’s upper atmosphere, is chronous satellites. the altitude at which protons replace oxygen Inner Magnetosphere Close to Earth as the dominant species in the atmospheric (within 5 RE), Earth’s magnetic field resem- plasma. This edge usually occurs at about bles a dipole associated with a giant bar 1000 km (600 mi) altitude. The plasmas- magnet. The relatively strong dipole field phere serves as a vital link between Earth’s lines near Earth act as a trap for energetic magnetic domain and Earth’s atmosphere.

Figure 3.6. A Depiction of Earth’s Plas- masheet, Plasmapause and Plasmas- phere. Here we show a cross-section from noon to midnight of the inner magnetosphere. The plasmasheet bridges the outer and inner magnetosphere. It is an inner magnetosphere feature at high latitudes. The lower-latitude and lower - energy region that bridges between the inner magnetosphere and upper atmosphere is the plasmasphere. The plasmapause is a variable boundary separating particles that are primarily from the Earth’s atmosphere from those primarily from the Earth’s magnetosphere. (Courtesy of NASA)

3.4 Earth’s Upper Atmosphere is Domi- gory in Figure 3.7 contains the word nated by Space Weather “sphere.” In this instance, the term is at least geometrically appropriate. Close to The Edge of Space Earth, gravity becomes a dominating force, Earth’s upper atmosphere occupies a and much of the material in the upper at- strategic position—the edge of space. For mosphere is stratified and constrained to a our purposes, it is a portion of the satellite spherical volume about Earth. (and rocket) operational arena, a source of matter for the regions of space outside the Atmospheric Classification sensible atmosphere, and a protective blan- ket shielding the regions below from many Mixing. The frequency of particle colli- energetic particles and photons. Many as- sions is the key to the mixing state of the tronautical engineers designate the edge of atmosphere. When collisions are numerous, space at about 130 km; because that is the the atmosphere is well mixed. Strong mix- lowest altitude at which a spacecraft can ing leads to a homogenous ratio of atmos- successfully make one full orbit. Nonethe- pheric constituents. Between Earth’s sur- less, we stick with our claim of the region at face and 100 km, the chemical mix of at- or above 50 km as near-space, because mospheric constituents is roughly 78% N2, interesting space physics-related effects 21% O2 and 1% other. The region below occur at this level and above. This shell of 100 km is the homosphere. Above 100 km molecules, atoms, and ions, extending down however, the lower density of particles leads to 50 km, interacts with (1) the Sun by pho- to fewer collisions. Therefore, atoms and tons, (2) the magnetosphere through plasma molecules tend to stratify with more massive and electromagnetic interactions, and (3) the particles remaining at relatively low altitudes, atmosphere below using waves and gravity. and the less massive ones diffusing to Driven by energy and momentum from higher levels. The stratified region is the above and below, the upper atmosphere’s heterosphere (as in heterogeneous). A behavior is quite complex and quite hot. To significant consequence of this stratification organize this complexity (and perhaps re- is the relative abundance of atomic oxygen duce it), we discuss four means by which above 300 km where many LEO satellites scientists typically categorize upper atmos- orbit. Atomic oxygen is exceedingly reac- pheric behavior: mixing, temperature, reten- tive. It attacks spacecraft components and tion, and degree of ionization. Each cate- reduces mission lifetime.

Near Space and Upper Atmosphere Characteristics Mixing Temperature Retention Ionization

10000 sphere Magneto sphere Plasma Exosphere 1000 Iono Heterosphere

Thermosphere sphere

Bound orbital

100 Mesopause sub orbital

Mesosphere Atmosphere Stratopause Height (km)

Stratosphere sphere Tropopause Neutral Atmo 10 Troposphere Homosphere

1 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Temperature (K) Figure 3.7. Classifications for the Upper Atmosphere. The central curve shows temperature variation with height. We note that the vertical scale is logarithmic.

Temperature. Figure 3.7 shows the full pheric region, the exosphere. Plasma par- temperature range of Earth’s thermal enve- ticles heated by various solar and magneto- lope. Temperatures near Earth’s surface spheric processes fight against gravity. are rather cool. Stratospheric temperatures Typically only the lighter atmospheric ele- show the influence of ozone absorption of ments occupy this upper domain. Some of solar ultraviolet (UV) radiation. Higher in the these particles may gain sufficient energy to mesosphere, efficient absorbers are largely achieve escape velocity from the atmos- absent, so radiative cooling creates low phere. Their ballistic trajectories allow them temperatures relative to the surrounding to exit to the plasmasphere and magneto- regions. Above 100 km, chemical species sphere. Below about 500 km, frequent colli- with an affinity for absorbing solar radiation sions and gravity thwart attempts at escape, again abound. Though the density of these and particles are retained unless energized species is low, solar photons are abundant, by extreme space weather storm effects. meaning individual particles in this region, The lower boundary of this region, known as called the thermosphere, share in a wealth the exobase, is typically near 500 km (300 of solar energy. With high available energy mi) but may be lower during higher solar per particle, temperatures rise rapidly. Exo- activity. The upper reaches of the exo- spheric temperatures exceed 1000 K, on sphere blend into the plasmasphere at average, over the solar cycle. As solar 10,000 km. Most low Earth-orbiting (LEO) emissions vary with the solar cycle, so do satellites operate in the exosphere. thermospheric temperatures and densities. Ionization. Existing within the thermo- When thermospheric densities vary, low- sphere is perhaps the most dynamic of the Earth orbits are perturbed by variations in upper atmosphere spheres: the ionosphere. atmospheric drag. Searing solar electromagnetic radiation Retention. Earth’s gravitational force is heats and excites atoms and molecules in greatly reduced in the most-distant atmos- Earth’s upper atmosphere. It also rips

FOCUS ON: The Ionosphere

The ionosphere serves as a high altitude reflector for short-wave broadcasting and long- range communication. During the occurrence of a solar flare, the enhanced X-ray radiation from the Sun causes the electron density in the lowest layer of the ionosphere to increase by a large factor. This increase results in a radio blackout–the immediate and large-scale absorption (rather than reflection) of high-frequency radio waves and subsequent disruption of short-wave communication over Earth’s sunlit hemisphere. In addition, beginning tens of hours after a solar flare, the global electron density in the upper levels of the ionosphere may undergo substantial variations because of the interaction of the disturbed solar wind with the near space environment. These variations can last for several days after disturbance onset. At high latitudes, a significant fraction of the ionization is produced by charged particles that have been dumped from the magnetosphere during times of magnetic storms and also by charged particles from the Sun that have been diverted to these latitudes by the geomagnetic field. During disturbed times, instabilities arise in the ionospheric plasma. These instabilities cause the initially (relatively) homogeneous plasma to develop density turbulence, typically magnetic field-aligned, with scale sizes on the order of centimeters to hundreds of kilometers. Electromagnetic waves propagating through this turbulence are scattered, resulting in scintillation or twinkling of signal sources, analogous to the twinkling of starlight by density turbulence in the atmosphere. These scintillations lead to strong disturbances in the radio band up to GHz frequencies, thereby disrupting communications and some navigation systems, such as the Global Positioning Satellite (GPS) system signals. In addition, the scattering can blind radar tracking (e.g., over-the-horizon radars) and disrupt (or sometimes improve) communications. Currently scientists are trying to find ways to use these scintillation and scattering effects as a diagnostic tool for space weather.

molecules apart and tears electrons from however. As the trail rapidly diffuses into some fraction of their parent particles. The the surrounding air, it quickly loses its ability free electrons and positive ions then form to reflect radio waves causing most reflec- several weakly ionized layers of plasma. tions to last less than 1 second. Occasion- These ionized layers constitute the iono- ally, a large meteor may create a trail capa- sphere. Because of the diurnal, (day- ble of reflecting radio waves for several min- night) nature of incident solar radiation, the utes. During times of enhanced meteor ac- ionization is more extensive during the day tivity, some forms of communication may be than during the night. Solar photons are not improved. On the other hand, radar beams the only culprit in ionization. Energetic parti- may experience anomalous reflection known cles flowing from the nightside magneto- as radar clutter. In the worst cases, some sphere also have a role in creating the high- radar systems are rendered ineffective. Me- latitude ionosphere. teoroids and meteors also represent a colli- sional hazard to spacecraft. Even tiny frag- 3.4 Meteors and Space Dust Affect ments endanger spacecraft components the Space Environment because they strike at such high speeds.

As Earth orbits the Sun, it constantly in- 3.6 Space Weather Effects in Earth’s tercepts the remains of old cometary tails Lower Atmosphere and at Earth’s Sur- and space dust. This material ablates as it face falls into the atmosphere and creates small ionization trails in its wake. We call these Galactic cosmic rays and their Sun- meteor trails or meteor echos. The elon- generated cousins, solar energetic parti- gated paraboloid of ionized air can be many cles (SEPs), penetrate Earth’s atmosphere. kilometers long. Occurring at an atmos- In the stratosphere and upper troposphere, pheric height of about 85-105 km (50-65 mi), these particles create radiation exposure to this ionized trail of meteor debris is capable avionics, flight crews, and passengers on of reflecting radio waves from transmitters high-latitude flights. The European Council on Earth. Meteor trail reflections are brief, has recommended that aircrews be treated as personnel receiving occupational radia- shortly, surface magnetic field disturbances tion exposure. As such, crew annual radia- provided one of the first historical clues tion dose must be monitored (Jansen, et al., about what we now call space weather. 2000). Space weather effects at the ground include Penetrating cosmic rays and their by- magnetically induced electrical surges in products may influence cloud formation by power lines that disrupt power grids and changing the ionization state of cloud con- similar effects in unprotected long distance densation nuclei. Solar cycle variations of communication lines on land or under the cloud cover are an active area of research sea. An extreme example of this resulted in (Tinsley, 2000). Energetic particles have the 1989 Hydro-Quebec power outage. been identified as a source of soft errors in Even fiber optic systems can be compro- stored data on computer systems. Com- mised if their signal-amplifying equipment is puter systems that operate in high, moun- powered by long metallic wires. tainous regions or where the magnetic field Now that we have completed a broad is slightly weak are more susceptible to description of the space environment, we cosmic ray damage. realize that particles, fields, and photons Extreme SEP events, associated with figure predominantly in space weather. very energetic solar flares are sensed in the troposphere and at the ground. The most energetic of these particles create chemical 4. SUMMARY compounds in the atmosphere that subse- quently drift to the ground. Snow in the po- Space is not empty, rather it contains a lar cap ice sheets becomes a recording de- tenuous gas of charged particles called vice for the great solar energetic particle plasma. Much of this plasma comes from events associated with the largest solar out-gassing of our local star, the Sun. While flares. Approximately 125 impulsive events the Sun yields a continuous flow of the solar have been identified in the polar ice cores material into the interplanetary environment, for the interval 1561-1950 (McCracken et al., it also has episodic events that flood the 2001). space environment with energetic particles Electrons energized in Earth’s magneto- and magnetic fields. The episodic events, sphere also penetrate into the stratosphere. called space weather, are known to interfere In the upper regions of the stratosphere, with modern technology systems on orbit energetic electrons alter ozone behavior and and sometimes on the ground. The con- create short-term reductions in ozone den- tinuous background emissions give rise to sity. Scientists are attempting to determine the space environment. if a solar cycle variation of upper level ozone The Sun’s influence extends to nearly exists. Researchers are quite certain a link 100 times the Sun-Earth distance. Within exists between stratospheric winds and the this volume of space the Sun’s radiation and solar cycle (Labitske and van Loon, 1987). outer atmosphere influence all planets, form- Solar cycle variations in ultraviolet wave- ing magnetospheres around many and iono- length radiation from the Sun appear to alter spheres around all. Our local ionosphere the amount of ozone and the distribution of and magnetosphere are the sites of consid- ozone heating between 30 and 50 km in erable energy exchange with the solar emis- Earth’s atmosphere. In turn, this heating sions. To date most space weather has affects the stratospheric circulation. Scien- been identified with these spheres of influ- tists are actively investigating the link be- ence. We are currently learning that the tween altered stratospheric circulation and energy does not stop at the ionosphere, but the location of the northern hemisphere win- rather extends into the atmosphere and all ter storm tracks. the way to the ground. Earth’s inhabitants are rather comforta- As space becomes ever more important bly shielded from almost all direct solar and to the conduct of daily life, space profes- solar wind effects. However, space weather sionals need to be aware of the many possi- disturbances of the magnetic field and ble impacts of space weather and space plasma in the magnetosphere and upper environment interactions. atmosphere propagate to the surface and even to the ocean floor. As we discuss 5. References Zhentao, X., Solar observations in ancient Jansen, F., P. Risto, and R. Fevre, Space China and solar variability, Phil. Trans R. Weather Hazard to Earth? Swiss Re Pub- Soc. Lond., A 330, 513, 1990. . lishing, Zurich, 2000. U.S. Department of Commerce, National Kouveliotou, C., R. Duncan, and C. Thomp- Oceanic and Atmospheric Administration, son. 2003. Magnetars, Scientific Ameri- Service Assessment Intense Space can, February, 2003. Weather Storms, October 19-November Labitzke, K., van Loon, H. Associations be- 07, 2003, National Weather Service, Silver tween the 11-year solar cycle, the QBO Spring Maryland,, 2004 and the atmosphere: I. the troposphere

and stratosphere in the northern winter. J. Selected Historical References Atmos. Terr. Sci. 50, 197–206, 1988. Barlow, W. H., On the Spontaneous Electricl Loomis. E., The Great Auroral Exhibition of th th Currents Observed in the Wires of the August 28 to September 4 , 1859. Electrical Telegraph, Phil. Trans. Roy. American Journal of Science and Arts, Soc., 139, 61-72, 1849. Vol. 78, Nov 28, 1859. Bartels, J., Solar eruptions and their iono- McCracken, K. G., Dreschhoff, G. A. M., spheric effects – A classical observation Zeller, E. J., Smart, D. F., Shea, M. A., and its new interpretation. Terr. Mag. Atmos. Solar cosmic ray events for the period Elect. 42, 235-239, 1937. 1561-1994, 1, Identification in polar ice, Carrington, R.C., On the distribution of the 1561-1950. J. Geophys. Res., 106, solar spots in latitude since the beginning 21,585-21,598, 2001. of the year 1854; with a map. Mon. Not. Tinsley, B., Influence of Solar Wind on the Roy. Astron. Soc., 19, 1-3,1859. Global Electric Circuit and Inferred Effects Carrington, R.C., Description of a singular on Cloud Microphysics, Temperature, and appearance seen in the Sun on Dynamics in the Troposphere., Space Sci. September 1, 1859. Mon Not. Roy. Astron. Reviews, 94, 231-258, 2000. Soc. 20, 13-14, 1860.

Hodgson, R., On a curious appearance seen Selected Further Readings in the Sun, Mon. Not. Roy. Astron. Soc., Cliver, E. W., The 1859 Space Weather 20,15,1860. Event: Then and Now, submitted to Ad- Loomis. E., The Great Auroral Exhibition of vances in Space Research, 2004. August 28th to September 4th, 1859. Carlowicz, M. and R. Lopez, Storms from American Journal of Science and Arts, the Sun The Emerging Science of Space Vol. 78, Nov 28, 1859. Weather, John Henry Press, Washington Maunder, E.W., Magnetic disturbances, as D. C., 2002. recorded at the Royal Observatory, Freeman, J. W., Storms in Space, Cam- Greenwich, and their association with sun- bridge Press, 2001. spots. Mon. Not. Roy. Astron. Soc., 65, 2- Siscoe, G., Space Weather Forecasting His- 34, 1905. torically Viewed through the Lens of Mete- Sabine, E., On periodical laws discoverable orology. Submitted to the Bulletin of the in the mean effects of the larger magnetic American Meteorological Society, 2005. disturbances. Phil. Trans., 142, 103-124, Tribble, A. C., The Space Environment Im- 1852. plications for Spacecraft Design, Princeton Schwabe, S. H., Solar observations during University Press, 2003. 1843. Astron. Nach. 21, No. 495, 233, Tsurutani, B. T., Gonzalez, W. D., Lakhina, 1844. G. S., Alex, S. The extreme magnetic Stewart, B., On the great magnetic distur- storm of September 1-2, 1859. J. Geo- bance which extended from August 28 to phys. Res. 108, No. A7, 1268 September 7, 1859, as recorded by pho- 10.1029/2002JA009504, 2003. tography at Kew Observatory. Phil. Trans., Tinsley, B., Influence of Solar Wind on the 151, 423-430, 1861. Global Electric Circuit and Inferred Effects von Humboldt, A. Kosmos: Entwurf einer on Cloud Microphysics, Temperature, and physichen Weltbeschreibung. Vol. 3, no. Dynamics in the Troposphere., Space Sci. 2, Cotta, Stuttgart, 1851. Reviews, 94, 231-258, 2000.