Magnetosphere The magnetic field in the tail points towards the Earth in the northern half and away in the southern half, this geometry being supported by a cross-tail current (neutral sheet). The magnetopause is formed at a distance where the solar wind dynamic pressure equals the magnetic pressure of Earth's field. At this location, typically around 8 - 11 RE away on the Earth-Sun line, a large scale duskward current develops in the dayside magnetopause to cancel the Earth's field outside. At the same time, the dipole field inside is increased, being now about two times the nominal dipole value. Similar current flows around the magnetotail, but there the direction has to be reversed in order to cancel the field outside. This current is closed via the cross-tail current. The thickness of the current layer is typically from several hundred to a thousand kilometers. The magnetosphere presented above is of closed type. Even if simple, it can describe some dynamic events relating to Sun-Earth connection. However, when the effects of interplanetary magnetic field are taken into account, the magnetic reconnection complicates the physics of the magnetopause considerably by "opening" up the magnetosphere.

Convective Verses Co- Rotating Plasma Flow The sketchs left and below illustrate the flow of charged particles in the equatorial plane of the magnetosphere. The interaction of the solar wind with the magnetosphere (through reconnection and viscous processes) results in a bulk flow of plasma down the magnetotail. This flow is referred to as "convection," although this term is really a misnomer because convection is a thermal process and the flow of plasma is not, being governed instead by large-scale electric and magnetic fields. In the plasma sheet, the direction of the convective flow is sunward, perpendicular both to the direction of the Earth's magnetic field (out of the screen) and to the direction (dawn-to-dusk) of the electric field imposed on the magnetosphere by the solar wind interaction.

(The motion of the plasma perpendicular to both the electric and magnetic fields is known as "E-cross-B drift.") As coupling between the solar wind and the magnetosphere intensifies, sunward convection increases, and the boundary separating the convective and co-rotational flow regimes (known as the "separatrix") moves inward, freeing some of the plasma previously bound on "closed" Earth-encircling trajectories to follow "open" convective paths toward the dayside magnetopause. Weakening of convection enlarges the region of near-Earth plasma that co-rotates with the Earth and allows the magnetic field lines emptied of plasma during periods of high convection to refill. The magnetotail is formed by tail lobes (on open polar cap) and the plasma sheet (closed nightside auroral field lines). In the inner magnetosphere we have plasmasphere mapping to mid- and low-latitudes. Overlapping both plasmasphere and inner plasma sheet are radiation belts and ring current. Coupling The Sun/solar wind, magnetosphere, ionosphere, and upper atmosphere (thermosphere) are variously coupled to each other.

The coupling between the solar wind and the magnetosphere is via a magnetic reconnection process between the IMF component and the geomagnetic field. It creates the large scale magnetospheric convection electric field responsible for geomagnetic activity. Ionosphere and magnetosphere are closely linked together via magnetic field lines. Magnetospheric electric fields map down to the ionosphere, creating plasma convection, frictional heating and plasma instabilities. Auroral particle precipitation ionizes the high latitude atmosphere during nighttime, and heat can be conducted from the magnetosphere down to the ionosphere. Collisions between the convecting ionospheric plasma and the neutral atmosphere leads to generation of neutral winds and Joule heating of the neutral gas. Furthermore, the role of the newly discovered high-altitude atmospheric flashes in the ionospheric physics is still unknown. The two main sources of magnetospheric electric fields are the solar wind related, dawn-to-dusk directed ("convection") field, and the co-rotation electric field related to the rotation of the Earth. The electric fields have a strong effect on the drift paths of the magnetospheric plasma.

Convection field In the open magnetosphere model (1961), reconnection (or merging) of the interplanetary and geomagnetic field lines partially opens Earth�s magnetic field to the solar wind. For this to happen, the field lines must be oppositely directed: a southward interplanetary magnetic field (IMF) is thus needed to open Earth's closed dayside magnetic fields. The antisunward magnetospheric convection is produced when the reconnected, open field lines are swept over the polar caps at the solar wind speed. The dawn-to-dusk solar wind electric field maps along the field lines, and an antisunward drift of plasma is formed also in the polar cap ionosphere. When the northern and southern hemispheric field lines are streched into a magnetic tail by the solar wind, they eventually reconnect with each other deep in the antisunward region. This magnetic geometry has a tension that exerts a force on the plasma. Together with the pressure gradient and the potential difference applied across the magnetosphere by the flowing solar wind, these forces produce motion of the magnetospheric plasma on closed field lines towards the Sun, and an associated dawn- to-dusk magnetospheric electric field in the tail.

The mapping of this electric field into the ionosphere produce a dusk-to-dawn electric field and sunward plasma flow in the auroral zone: the familiar two-cell pattern of ionospheric convection is formed. If the southward IMF periods continues for several hours, the magnetosphere reaches a steady-state like configuration called steady magnetospheric convection Note that when the plasma flows toward Earth because of the convection field, a zonal charge separation opposing the field is created as electrons drift dawnward and protons duskward around the Earth. The inner magnetosphere is thus shielded from the magnetospheric electric field, and the electric field due to the rotation of the Earth is dominating. Inside this region, called the plasmasphere, cool dense plasma is flowing in concentric circles around the Earth. The Plasma sheet Plasma sheet is the region of closed field lines in the equatorial magnetotail. Plasma sheet is typically divided into plasma sheet and plasma sheet boundary layer (PSBL), the latter being at higher latitudes adjacent to the tail lobes. It is also divided into two parts by the cross-tail current sheet in the equatorial plane.

However, as the plasma sheet extends from the magnetotail to the geosynchronous orbit, the effects of the current are not as important in the inner plasma sheet as further tailward, and the magnetic field geometry has a transition region between the inner dipolar form and outer tail like field. [The geostationary or geosynchronous orbit is located at approximately 6.6 RE geocentric distance in the geographical equatorial plane. In it, centrifugal force just balances gravity for a spacecraft that is in a circular orbit with a period of one orbit per day (i.e., at fixed geographic longitude). The geosynchronous orbit tends to skim the inner boundary of the plasma sheet. This means that the surrounding plasma/magnetic field region varies a lot with the magnetic activity. The energetic electron fluxes at the geosynchronous orbit have been divided into two populations: Hard: 300-2000 keV, Temperature=200 keV, and Soft: 30-300 keV, T=25 keV, ] Plasma sheet is a very important region for auroral physics, since the night time auroral oval maps to it.

Plasma sheet particles are hot, having energies in the keV range. Plasma density is a slowly varying function of time, correlating with the solar wind density. This indicates that solar wind is - at least partly - providing the plasma. Ion temperature is about seven times the electron temperature. Because of the magnetospheric, large scale convection electric field, plasma is in continuous movement both toward Earth and toward the central cross-tail current region from tail lobes. At the inner plasma sheet boundary electrons and ions are on different convection paths, and electron plasma sheet does not always quite reach the geosynchronous orbit, while the ion plasma sheet does. During increased geomagnetic activity, O+ ion dominates in the inner magnetosphere. This is evident both in storms, and ring current. These ions flow upward from the ionosphere. Ionosphere Figure showing a north-south section through the structures that usually extend several hundreds of kilometers in the east-west direction. The blue lines represent the auroral current system. Together with the ionospheric closure current and the magnetospheric generator these form the complete auroral current circuit.

Magnetometers, which measure electric currents in the upper atmosphere, and their successors have played the role that barometers did for early weather forecasters. The pattern of pressure changes the instruments recorded tracked the passage of storms in the atmosphere. Now, it is storms in the magnetosphere that are recorded. The "cusp" which separates the magnetosphere's crown and tail is responsible for auroras. This is because it is at the cusp that magnetic field-lines stream down towards the ground, acting as paths for electrons and protons that have slipped past the bow shock. When these particles hit the upper atmosphere they generate light in the same way that electrons from the cathode of an old-fashioned television set do when they hit the phosphorescent dots of the screen. The auroras borealis and australis are the result of particles streaming in from the tail. But particles come in from the crown, as well, forming invisible daytime auroras. The Sondrestrom Upper Atmospheric Research Facility has a 32-metre-wide radar dish that measures conductivity from an altitude of 60km to 600km and has helped define the cusp's circuitry.

That is important because, although it is not technically part of the atmosphere, the plasma of charged particles in the magnetosphere experiences what might be referred to as weather. Like the weather on Earth, this space weather has consequences. If it gets nasty, communications satellites may be knocked out and radio communications within the atmosphere disrupted. Because of the Sun 's UV radiation, Earth 's upper atmosphere is partly (0.1% or less) ionized plasma at altitudes of 70-1500 km. This region, the ionosphere, is coupled to both the magnetosphere and the neutral atmosphere. It is of great practical importance because of its effect on radio waves. The existense of a conducting layer in the upper atmosphere results in interesting phenomena. A natural resonator is formed between the nearly perfectly conducting terrestrial surface and the ionosphere. Broadband electromagnetic impulses, like those from lightning flashes, fill this cavity, and create globally the so-called Schumann resonances at frequencies of 7.8, 14, 20, 26, 33, 39, and 45 Hz Ionospheric densities are higher during solar maximum years than during the minimum years.

Ionospheric electric fields are the main result of the coupling between the magnetosphere and ionosphere. While at low-latitudes the ionospheric plasma is co-rotating with the Earth, at higher latitudes it is convecting under the influence of the large scale magnetospheric electric field mapped to low altitudes. The convection pattern leads to ionospheric Hall currents and along the auroral oval so-called convection electrojets are formed at about 100 km altitude: eastward electrojet on the duskside, westward on the dawnside. Magnetic Field Reconnection The concept of merging or reconnection of magnetic field lines is widely used although the physics behind the process are still under debate. The concept was developed to explain particle acceleration in solar flares (1946). The basic idea behind reconnection is that (partly) antiparallel magnetic field lines can, when meeting, merge together and produce two topologically totally different field lines. Reconnection is thought to be the main link in the solar wind - magnetosphere coupling process, occurring at the dayside magnetopause between southward directed IMF and the northward directed geomagnetic field.

[I take this to mean the lines that arc from the sun's poles downward to the earth's pole vicinity (not the solar wind). If so what would pole reversal do?] It is the main process that transports mass, momentum, and energy from the solar wind into the magnetosphere, and it drives the large scale magnetospheric convection. There are several reasons to believe the existence of such a process: there is a clear correlation between the IMF direction and geomagnetic activity some measurements indicate the presence of unexplained processes at the magnetopause In addition to the dayside reconnection, similar process should be occurring in the far tail. Finally, some substorm models are also based on reconnection. This is quite natural, since the magnetic field lines above and below the cross-tail current sheet are oppositely directed, and the plasma sheet is typically thinning during the substorm growth phase. Reconnection allows the magnetic field of the Sun to connect to Earth's field, allowing energy and matter to flow from one to the other. The solar wind flows into Earth's magnetosphere through a narrow valve-like region a quarter of a million miles downwind of the planet, in a region known as the magnetic tail. Reconnection happens where it says engergy coupling region.

Depending upon the orientation of the solar wind and Earth's magnetic field, that valve opens and closes to allow plasma and energy from the Sun to enter Earth's space. On the sunlit side of Earth, the solar wind pushes Earth's magnetic field, but the plasma barely penetrates Earth's magnetic shell. It is in the distant reconnection region of the tail, on the night side of Earth, where the solar wind enters the magnetosphere. The flow of solar wind into the tail of the magnetosphere fills Earth's space with plasma and energy. This energy is stored like a battery until it is eventually released in bursts that cause auroras and other space weather phenomena. Magnetic reconnection was proposed more than forty years ago as the key process that allows this flow of solar wind into the magnetosphere. During reconnection, magnetic fields that are heading in opposite directions -- having opposite north or south polarities -- break and connect to each other. In April 1999, the "Wind" sattelite flew right through the reconnection region as the process was occurring. While flying tailward through the magnetosphere (away from Earth), Wind's instruments detected jets of plasma racing toward Earth.

Part-way through the journey, the jets stopped and Wind detected unusual electric currents and magnetic signatures that were predicted to occur at the point of reconnection. Some time later, as Wind kept flying away from Earth, the spacecraft detected that jets of plasma flow in the exact opposite direction of the previous flows, racing away from the planet. The event was comparable to a plane flying through the eerie, calm eye of a hurricane. "Reconnection is one of the fundamental physical processes in the universe. It occurs in so many places in the universe, yet the only place we can observe it directly in a natural environment is in our magnetosphere. It is absolutely crucial for understanding how the Sun connects to the Earth. This is how you populate the magnetosphere with plasma. The direct observation of reconnection has implications for many fields of physics. Reconnection on the Sun likely plays a role in the development of solar flares and of coronal mass ejections. Reconnection likely plays a role in the interaction of neighboring stars. And observations of reconnection in nature may aid the study of nuclear fusion and other plasma processes in the laboratory.

Plasmoids/flux ropes [Or Vortices] Tailward fast plasma flows with dipolar BZ signatures (north-then-south turning of Magnetic Field (B)) observed within plasma sheet are called plasmoids. Most plasmoids have also helical magnetic field structures (large By fields), called "flux ropes". Observations of plasmoids are highly correlated with substorm onsets. Accordingly, their existence is considered to support substorm models based on reconnection. This is because reconnection seems to be the most obvious mechanism for plasmoid creation. Plasmoids originate from the distance of about 20-30 RE, and have been observed up to about 200 RE downtail; tailward speeds are several hundreds of km/s. Of the heavier ions observed within the plasmoids, most of the oxygen ions are singly charged (O+, ionospheric origin), and most of the helium ions are doubly charged (He++, solar wind origin). Deeper Analysis The basic nature of the interaction between the solar wind and the Earth's magnetic field, leading to the concept of the Earth's magnetosphere, is based on two theoretical principles.

The first concerns the way in which plasmas and magnetic fields interact; they behave, approximately, as if they are "frozen" together. As a result of this freezing together, magnetic fields are transported by flowing plasmas; the field lines are bent and twisted as the flow bends and twists. The interplanetary magnetic field is wound into a large spiral structure by the Sun's rotation, and near the Earth it has a strength of about 5 nanoteslas (nT). This is a rather weak field, about one ten thousandth of the field at the Earth's surface, but nevertheless, it plays a crucial role in the Earth's interaction with the solar wind. The second principle concerns the force that the magnetic field exerts on the plasma, which usually opposes the bending and twisting of the field, or its compression, in the frozen-in flow. There are two components of this force. First, the field exerts an effective pressure on the plasma proportional to the square of the field strength. This force resists compressions or rarefactions of the magnetic field. Second, bent field lines exert a tension force on the plasma like stretched rubber bands. This force resists the bending and twisting of field lines.

Applying the first of these ideas to the interaction between the solar wind and the Earth, we recognize that the solar wind plasma is frozen to the interplanetary magnetic field, and the Earth's plasma (for example, from the ionosphere) to the Earth's field. When these plasmas meet each other, therefore, they do not mix, but instead they form distinct regions separated by a thin boundary. The solar wind thus confines the Earth's magnetic field to a cavity surrounding the planet, forming the Earth's magnetosphere. The structure of the Earth's magnetosphere in the noon-midnight meridian plane, showing a) the Chapman-Ferraro closed magnetosphere based on strict application of the frozen-in approximation, and b) the Dungey open magnetosphere, in which there is an essential breakdown of frozen-in flow at the dayside magnetopause and in the tail leading to the occurrence of reconnection. The arrowed solid lines indicate magnetic field lines, the arrowed dashed lines plasma streamlines, and the heavy long-dashed lines the principal boundaries (the bow shock and magnetopause).

The circled dots marked E in b) indicate the electric field associated with the flow, which is perpendicular to the flow and the field, and given in magnitude by their product. The size of the cavity is then determined by pressure balance at the boundary between the solar wind on one side, and the magnetic pressure of the planetary field on the other. Given a planetary "bar magnet" field that produces a field strength of about 30,000 nT at the Earth's surface at the equator, estimates place the boundary, called the magnetopause, at a geocentric distance of about 10 Earth radii on the upstream (day) side, and this is where it is generally observed. (The Earth's radius is about 6400 km.) On the downstream (night) side the cavity extends into a long magnetic tail. Across the magnetopause the magnetic field usually undergoes a sharp change in both strength and direction. Ampere's law then tells us that a sheet of electrical current must flow in the plasma in this interface, called the Chapman-Ferraro current.

A shock wave also stands in the flow upstream of the cavity, which forms because the speed of the solar wind relative to the Earth is much faster than that of wave propagation within it. Across the shock the flow is slowed, compressed, and heated, forming a layer of turbulent plasma outside the magnetopause called the magnetosheath. Inside the cavity the terrestrial plasma will approximately rotate with the Earth. This will occur because the Earth's field lines are frozen into the ionospheric plasma. There, co-rotation is enforced in the absence of other driving processes, by collisions between ions and atmospheric neutrals at heights of about 120-140 kilometers. Dungey's Open Magnetosphere The interplanetary magnetic field is compressed against the magnetopause and draped over it by the flow, but ultimately the field lines slip around the "sides" of the magnetosphere, frozen into the magnetosheath plasma. However, the "frozen-in" picture is only an approximation, and under some circumstances it will break down. One of those circumstances occurs when high current densities are present in the plasma, and we have seen that large currents will occur at the magnetopause boundary.

When the frozen-in condition is relaxed, the field will diffuse relative to the plasma in the magnetopause, allowing the interplanetary and terrestrial field lines to connect through the boundary. Dungey called this process magnetic reconnection. The distended loops of "open" magnetic flux formed by reconnection exert a magnetic tension force that accelerates the plasma in the boundary north and south away from the site where reconnection takes place, thus causing the open tubes to contract over the magnetopause toward the poles. The open tubes are then carried downstream by the magnetosheath flow, and stretched into a long cylindrical tail. Eventually, the open tubes close again by reconnecting in the center of the tail. This process forms distended closed flux tubes on one side of the tail reconnection site, which contract back toward the Earth and eventually flow to the dayside where the process can repeat. On the other side, "disconnected" field lines accelerate the tail plasma back into the solar wind. The key feature of the "open" magnetosphere is therefore the cyclical flow excited in the interior by these reconnection processes.

The plasma flow streamlines associated with the Dungey flow cycle (arrowed solid lines) and the main zones where plasma particles of differing characteristics flow down the Earth's magnetic field lines from the magnetosphere into the upper atmosphere and ionosphere. The interior heavy dashed circle shows the boundary between open field lines at high latitudes and closed field lines at lower latitudes (compare with the noon-midnight cross-section in Figure 1b).. The ionospheric image of the magnetospheric flow shows a view looking down onto the north pole of the Earth. It consists of twin flow vortices with open field lines that flow away from the Sun over the polar cap and closed field lines that flow toward the Sun at lower latitudes. Typical ionospheric flow speeds are several hundred meters per second. The overall flow cycle time is about 12 hours. From this the length of the tail can be estimated to be about 1000 Earth radii [16.6 times the distance to the moon]. The most important feature of the magnetosphere-ionosphere flow, however, is that its strength is modulated by variations in the direction and strength of the interplanetary magnetic field.

The dayside reconnection rate, and hence the magnetic flux throughput in the magnetosphere, is strong when the interplanetary field points south, opposite to the equatorial field of the Earth. When the interplanetary field points north, however, equatorial reconnection cannot occur, and the cyclical flow described above dies away. This dependence of the flow on the direction of the interplanetary field distinguishes Dungey's open magnetosphere from other possibilities (for example, flow excited in a closed Chapman-Ferraro system by "viscous" forces at the magnetopause), and has been demonstrated by many studies over the past 30 years. It is important to realize, however, that the magnetic flux throughput in the system, even at its strongest, amounts to no more than about 20% of the interplanetary magnetic flux brought up to the dayside magnetosphere by the solar wind. Most of the interplanetary flux is deflected around the magnetosphere. However, it is the breakdown of this picture at the 10-20% level due to reconnection at the magnetopause that is critical to the Earth's magnetospheric dynamics.

The contribution to the flux transport by other nonreconnection mechanisms appears to be much smaller than this. The cyclical flow driven by the solar wind is not, however, the only source of flow in the Earth's magnetosphere. In the absence of other effects the plasma would tend to rotate around with the Earth. When the flows caused by the Dungey cycle are added to the corotation flow, it is found that the Dungey cycle flows dominate in the outer part of the magnetosphere, while the corotation flow is confined to a small central core of dipole flux tubes, which typically extend in the equatorial plane to 4-6 Earth radii. This latter region is filled to relatively high densities with cold hydrogen/helium plasma from the ionosphere, forming a region called the plasmasphere. Outside this region the cold plasma density is much lower because of heating and loss of the ionospheric plasma during each cycle of the Dungey flow, and the medium is instead characterized by the presence of hot and tenuous plasma that is accelerated in the tail on closed field lines downstream from the tail reconnection site, and is then transported inward toward the Earth. The size of the plasmasphere is not constant, however.

The corotating region extends further from Earth than the average during intervals of weak Dungey cycle convection (northward interplanetary fields), and the ionosphere fills the outer flux tubes of this region toward equilibrium values while this condition persists. The corotating region shrinks when the Dungey cycle flow increases again (southward interplanetary fields), and the cold plasma that accumulated in the outer region is stripped away and flows to the dayside magnetopause, where much of it is heated and flows out into the solar wind. It is replaced by hot plasma flowing in from the tail. Magnetospheric Substorms The variability of the magnetospheric flow on timescales of minutes and hours that is associated with changes in the direction of the interplanetary magnetic field is a key feature. However, observations show that when dayside reconnection is enhanced, for example by a southward turn of the interplanetary field, the magnetosphere generally does not evolve smoothly toward a new steady state of enhanced convection. Instead, the system, particularly the tail, undergoes a characteristic evolution on a 1-2 hour timescale, called a magnetospheric substorm.

Suppose the magnetosphere is initially in a state of low flow during an interval of northward interplanetary field, and that the interplanetary field then turns south. Reconnection starts at the magnetopause, stripping magnetic flux off the dayside and adding it to the tail, so that the dayside magnetopause moves inward (up to an Earth radius), while the tail magnetopause moves out (and the field strength inside increases). These changes are accompanied by an excitation of the large-scale flow as the system adjusts to the altered field configuration. During this interval, called the "growth phase" of the substorm, the field lines in the tail become very distended and nondipolar. This development of the tail continues steadily over periods of several tens of minutes. Then, on a timescale of just a minute or two, these distended field lines collapse inward toward the Earth near the equator, and outward at higher latitudes, to a more dipolar form, (as shown in (b) below). Why the field suddenly reconfigures in this manner is at present unknown, and is the subject of much research worldwide. The development of the nightside tail magnetic field during the growth and expansion phases of a substorm.

Sketch a) shows the development of distended nondipolar field lines in the near-Earth tail during the growth phase, b) the onset of the field disruption at the beginning of the expansion phase and the rapid return of the inner tail field to a more dipolar form, while c) shows the down-tail propagation of this disruption and the subsequent induction of tail reconnection and plasmoid formation at larger distances from Earth. As the tail field lines collapse, the plasma that they contain is strongly heated and compressed. This produces a sudden intense flux of electrons at the top of the atmosphere at the feet of the field lines concerned, producing a brilliant auroral display as the atmospheric atoms are excited by collisions with the electrons at altitudes above 100 km, and then radiate. The upper atmosphere also becomes much more strongly ionized at these altitudes than before, and hence more electrically conducting, and strong electric currents flow whose magnetic effects can be observed on the ground. The tail field collapse usually starts in a restricted sector near the midnight meridian in the near-Earth end of the tail, and then propagates both across and down the tail, sometimes in a series of steps.

In the upper atmosphere the area of bright auroras and strong electric currents therefore expands to the east and west as well as poleward, such that this interval is called the "expansion phase" of the substorm. This field collapse often induces the onset of reconnection in the tail at distances of 20-40 Earth radii as it propagates downtail. When this happens, closed tail field lines containing hot plasma are "pinched off" to form a closed-loop "plasmoid" that propagates downtail and into the solar wind at speeds of 400-800 kilometers per second, as (as depicted in (c) above). Continued reconnection in the near-Earth tail then closes open tail field lines. After a few tens of minutes, however, the reconnection rate slackens and the reconnection region moves back down the tail, signaling the end of expansion. The subsequent "recovery phase" of the substorm typically lasts for many tens of minutes. During this period the system responds and adjusts to the input of magnetic flux and hot plasma on the nightside during the expansion phase.

The above sequence of events typically follows a southward turning of the interplanetary magnetic field that lasts for at least a few tens of minutes. Several such intervals and several substorms can occur in one day. If the southward field persists for significantly longer, however, the system often evolves through a series of substorm cycles each lasting about an hour. The hot tail plasma is then driven deep into the inner magnetosphere by the enhanced convection, this constituting the main characteristic of a magnetic storm proper. The positively charged ions in this plasma drift westward around the Earth, and the negative electrons drift eastward, such that both contribute to a westward-directed ring of current that encircles the Earth at distances, typically, of several Earth radii. The magnetic field produced by this current causes a worldwide decrease in the field strength on the ground at low and middle latitudes. The Ring Current Image of a portion of the Ring Current with Earth superimposed. The current is not smooth, and often does not completely encircle the equatorial zone of Earth. It is more prominent on the night time side (right in this figure), and as it moves into the dayside it breaks up and vanishes, possibly by losing particles into the magnetopause region.

A ring current is an electric current carried by charged particles trapped in a planet's magnetosphere. It is caused by the longitudinal drift of energetic (10-200 keV) particles. Earth's ring current is responsible for geomagnetic storms. The ring current system consists of a band, at a distance of 3-5 RE [11,889 - 19,815 miles], which lies in the equatorial plane and circulates clockwise around the Earth (when viewed from the north). The particles of this region produce a magnetic field in opposition to the Earth's magnetic field and so an Earthly observer would observe a decrease in the magnetic field in this area. The ring current energy is mainly carried by the ions, most of which are protons. However, one also sees alpha particles in the ring current, a type of ion that is plentiful in the solar wind. In addition, a certain percentage are O+ oxygen ions, similar to those in the ionosphere of Earth, though much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source. Ring current particle energies range from 0.05 MeV to 1 MeV.

During a geomagnetic storm, the number of particles in the ring current will increase. At the same time there is a decrease in the geomagnetic field. The motion of energetic ions and electrons through space is strongly constrained by the local magnetic field. The basic mode is rotation around magnetic field lines, while at the same time sliding along those lines, giving the particles a spiral trajectory. On typical field lines, attached to the Earth at both ends, such motion would soon lead the particles into the atmosphere, where they would collide and lose their energy. However, an additional feature of trapped motion usually prevents this from happening: the sliding motion slows down as the particle moves into regions where the magnetic field is strong, and it may even stop and reverse. It is as if the particles were repelled from such regions, an interesting contrast with iron, which is attracted to where the magnetic field is strong. The magnetic force is much stronger near the Earth than far away, and on any field line it is greatest at the ends, where the line enters the atmosphere. Thus electrons and ions can remain trapped for a long time, bouncing back and forth from one hemisphere to the other. In this way the Earth holds on to its radiation belts.

In addition to spiraling and bouncing, the trapped particles also slowly drift from one field line to another one like it, gradually going all the way around Earth. Ions drift one way (clockwise, viewed from north), electrons the other, and in either drift, the motion of electric charges is equivalent to an electric current circling the Earth clockwise. That is the so-called ring current, whose magnetic field slightly weakens the field observed over most of the Earth's surface. During magnetic storms the ring current receives many additional ions and electrons from the nightside "tail" of the magnetosphere and its effect increases, though at the Earth's surface it is always very small, only rarely exceeding 1% of the total magnetic field intensity Van Allan Belt The Van Allen radiation belt is a torus of energetic charged particles around Earth, trapped by Earth's magnetic field. The presence of a radiation belt had been theorized prior to the Space Age It consists of two belts around Earth, the inner radiation belt and the outer radiation belt. The particles are distributed such that the inner belt consists mostly of protons while the outer belt consists mostly of electrons. Within these belts are particles capable of penetrating 1 millimeter of lead. The Sun does not support long-term radiation belts.

The atmosphere limits the belts particles to regions above 200-1000 km. The belts are confined to an area which extends about 65� from the celestial equator. Van Allen belts are a result of the collision of Earth's magnetic field with the solar wind. Radiation from the solar wind then becomes trapped within the magnetosphere. The trapped particles are repelled from regions of stronger magnetic field, where field lines converge. This causes the particle to bounce back or "mirror." . Without this sort of "mirroring," ions and electrons would not be trapped in the Earth's magnetosphere, but would instead follow their guiding field lines into the atmosphere, where they would be absorbed and become lost. What happens instead is that every time a trapped particle approaches Earth, it is reflected back. It is thus confined to the more distant section of the field line. Now and then a violent outburst, known as a magnetic storm, drives tail plasma earthward, into the near-Earth magnetosphere. Electric fields (voltage differences) are essential to this process, to help tail particles break into trapped orbits and to drive them to higher energies.

When the outburst ends and the electric field dies away, the particles find themselves locked in trapped orbits of the ring current and the outer radiation belt. Lesser outbursts, known as magnetospheric substorms, occur quite frequently. Whereas the inner belt is marked by great stability, the ring current and outer belt constantly change. Sooner or later the particles are lost, e.g. by collision with the rarefied gas of the outermost atmosphere, and on the other hand, new ones are frequently injected from the tail. The electric fields which inject the new particles can also draw oxygen ions upwards from the ionosphere, and the ring current contains such ions, typically a few percent of the total, more during magnetic storms. The Inner Belt The inner radiation belt extends over altitudes of 650-6,300 km (1.1 - 3.3 RE). This ring is most concentrated in the Earth's equatorial plane. It consists mostly of protons on the order of 10-50 MeV (Flux maximum at about 2 RE), a by-product of collisions between cosmic ray ions and atoms of the atmosphere.

The belt also contains electrons, low-energy protons, and oxygen atoms with energies of 1-100 keV. When these electrons strike the atmosphere they cause the polar aurora. The intensity of the belt fluctuates, partly due to the influence of the solar cycle, and is strongest between 2-5,000 km. The number of cosmic ray ions is relatively small and the inner belt therefore accumulates slowly, but because the trapped protons are very stable in this belt (with particle lifetimes of up to ten years), high intensities are reached as they build up over many years. The belt was discovered by a Geiger counter on board the Explorer 1 satellite built by James Van Allen. The inner radiation belt owes its existence to the extraordinary stability of trapped orbits near the Earth. It is a by-product of the cosmic radiation, which by itself has a rather low intensity: the amount of energy received by Earth from cosmic rays is comparable to what it receives from starlight. Only by accumulating particles over the span of years does the inner belt reach its high intensity. [Can you believe this?] Cosmic rays are fast positive ions, bombarding Earth from all directions, probably filling our entire galaxy.

Though their numbers are small, the energy of each particle is quite high, so that when these ions smash into nuclei of atmospheric gases, fragments go flying off in different directions, some of them short-lived particles created by the collision. Most such fragments are absorbed by the atmosphere or by the ground, but a few are also splattered upwards, out of the atmosphere and into space. If these are electrically charged, e.g. electrons or ions, they will often end up trapped by the Earth's magnetic field. None of these however lasts very long, since trapped orbits which rise from the atmosphere must sooner or later enter the atmosphere again. Some of the fragments are however neutrons, particles similar to protons but without the electric charge; neutrons make up about half the weight of a typical atomic nucleus. Having no electric charge, neutrons are not affected by the Earth's magnetic field, and moving far too fast for gravity to hold them back, they usually escape into space. The free neutron is however radioactive: within about 10 minutes it breaks up into a proton, which captures most of the energy, an electron and a massless neutrino.

Ten minutes is a fairly long time for a fast particle, time enough for many neutrons to get halfway to Mars. However, decay times are spread out statistically, and while 10 minutes is the average, a few neutrons decay quite soon, while still inside the Earth's magnetic field. The energetic protons which then materialize are grabbed by the Earth's magnetic field, often on trapped orbits which do not return to the atmosphere, in which the proton can stay trapped for a rather long time. That is how the inner belt arises. [this sounds ridiculous] The Outer Belt The outer belt contains mainly electrons with energies up to 10 MeV. It is produced by injection and energization events following geomagnetic storms, which makes it much more dynamic than the inner belt (it is also subject to day-night variations). It has an equatorial distance of about 3 - 9 RE (10,000-65,000 km), with maximum for electrons above 1 MeV occurring at about X = 4 RE (14,500-19,000 km). 'Horns' of the outer belt dip sharply in towards the polar caps.

The outer belt is thought to consist of plasma trapped by the Earth's magnetosphere. There are very few particles of high energy within the outer belt. The electrons here have a high flux and along the outer edge and E > 40 Kev electrons can drop to normal interplanetary levels within about 100km (a decrease by a factor of 1000). This drop-off is a result of the solar wind. The particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic. The outer belt is larger and more diffuse than the inner, surrounded by a low-intensity region known as the ring current. Unlike the inner belt, the outer belt's particle population fluctuates widely and is generally weaker in intensity (less than 1 MeV), rising when magnetic storms inject fresh particles from the tail of the magnetosphere, and then falling off again. The outer radiation belt is nowadays seen as part of the plasma trapped in the magnetosphere.

The name "radiation belt" is usually applied to the more energetic part of that plasma population, e.g. ions of about 1 Mev of energy. The more numerous lower-energy particles are known as the "ring current", since they carry the current responsible for magnetic storms. Most of the ring current energy resides in the ions (typically, with 0.05 MeV) but energetic electrons can also be found. Cross-section of the two radiation belts. The intensity contours are banana-shaped, because they follow magnetic field lines to which the particles are attached. We now know that outer-belt ions and electrons probably come from the long "magnetic tail" of stretched field lines on the night side of the magnetosphere. A sudden increase in solar wind pressure can cause the radiation belts to change shape. In such an instance, particles on the sunward side of the planet will be carried inward (toward the planet), while particles on the far side of the planet will be carried further from the planet. This can give the radiation belts somewhat of a tear-drop shape. After such an incident, the belts tend to return to a more spherical shape.

The gas giant planets Jupiter, Saturn, Uranus and Neptune, all have intense magnetic fields with radiation belts similar to the Earth's outer belt. Jupiter's belt is the strongest, and is affected by its large moon Io, which loads it with many ions of sulfur and sodium from the moon's volcanoes. Saturn seems to have an "inner belt" similar to the Earth's, probably produced by cosmic rays which eject neutrons from Saturn's planetary rings. The Io Plasma Torus Within Jupiter's magnetosphere, there is a significant amount of hot, ionized gas, or plasma. This plasma moves along with Jupiter's rotating magnetic field, sweeping charged particles off the surfaces of its moons as it passes them. Io has a particularly significant impact on Jupiter's magnetosphere. Io's volcanoes continually expel an enormous amount of particles into space, and these are swept up by Jupiter's magnetic field at a rate of 1,000 kg/sec. This material becomes ionized in the magnetic field and forms a ring of plasma or doughnut-shaped track around Io's orbit called the Io Plasma Torus. The plasma consists mostly of oxygen and sulfur ions that emit radiation mostly in the extreme ultraviolet, but some emissions are observable by earth-based telescopes.

Radiation levels within the plasma torus are extreme and pose a serious threat to spacecraft. Of all the planets, only Jupiter gives off radio waves. This would indicate some active energy creation process exists. The torus is believed to be the cause of Jupiter's radio waves. This doesn't have an explanation yet. As Io circles around Jupiter and through the plasma torus, an enormous electrical current flows between them. Approximately 2 trillion watts of power is generated. The current follows the magnetic field lines to Jupiter's surface where it creates lightning in the upper atmosphere. Voyager instruments observed auroral discharges, similar to Earth's northern lights,in the northern regions, in ultraviolet and visible light. Pioneer 10 and 11 didn't see the ultraviolet discharges during their encounters.The auroral discharges appear to be related to material from Io that spirals along magnetic field lines to fall into Jupiter's atmosphere The first black and white Hubble Space Telescope image (top left) shows the flux tube, where Io and Jupiter are linked by an electrical current of charged particles. Volcanic emissions from Io flow along Jupiter's magnetic field lines, through Io, to Jupiter's north and south magnetic poles.

In the second black and white image, auroral emissions are visible at Jupiter's north and south poles. The ultraviolet image below shows how the structure and appearance of Jupiter's aurora changes at it rotates. This image of Io was taken while the moon was in Jupiter's shadow. The vivid colors are the result of collisions between Io's atmospheric gases and charged particles trapped in Jupiter's magnetic field. The red and green colors are probably produced by mechanisms similar to those that produce the aurora in Earth's polar regions. The blue light indicates locations where dense plumes of volcanic vapor rise into space. These may be places where Io is electrically connected to Jupiter. Jupiter's magnetosphere is the largest structure in the solar system. It is molded into a teardrop shape by a stream of energetic particles blowing away from the Sun called the solar wind. The side of the magnetosphere facing the Sun extends about 3 million kilometers from the planet, and its tail reaches out another 650 million kilometers, to the orbit of Saturn and beyond. If it were visible from Earth, it would appear several times larger in the sky than the full moon. Jupiter's magnetic field is generated deep within the planet.

Although its outer atmosphere is composed primarily of hydrogen gas, deep within the interior the pressure is so great that the hydrogen becomes a liquid. Because of the tremendous pressure, the electrons of the hydrogen atoms freely move from atom to atom, making it a very good conductor of electricity. Because of this property, it is referred to as liquid metallic hydrogen. Jupiter spins on its axis every 9.9 hours, generating convection currents in the liquid metallic hydrogen. This produces Jupiter's powerful magnetic field. This image taken on Jan. 4 and 5, 2001 by the Ion and Neutral Camera on NASA's Cassini spacecraft, makes the huge magnetosphere surrounding Jupiter visible in a way never seen before. Jupiter's magnetic field has been sketched over the image. The disk of Jupiter is shown by the black circle, and the approximate position of the Io plasma torus is represented by the yellow circles. StarTeach Astronomy Education you are here > Famous Astronomers Great Astronomers Thales (624-547 B.C., Ionian) was a Greek philosopher who traveled widely in Mesopotamia and Egypt, and brought astronomical records from these cultures back to Greece.

He believed that the Earth is a disk floating on an endless ocean. Legend has it that he correctly predicted a solar eclipse in the year 585 B.C. Anaximander (611-547 B.C., Ionian) was a Greek philosopher who made the first detailed maps of the Earth and the sky. He knew that the Earth was round, and believed that it was free-floating and unsupported. He measured its circumference, and was the first to put forward the idea that celestial bodies make full circles in their orbits. One of his greatest contributions was the fact that he was the first to conceptualize space as having depth. Pythagoras (569-475 B.C., Ionian) was a mathematician who put forward the idea that the universe is made of crystal spheres that encircle the Earth. According to him, the Sun, the Moon, the planets, and the stars travel in separate spheres. When the spheres touch each other, a 'music of the spheres' can be heard. Aristotle (384-322 B.C., Greek), the great philosopher, proved that the Earth is spherical, and believed that it was at the center of the universe.

His reason for believing this was actually quite scientific: he knew that if the Earth revolved around the Sun, then we should see the stars shift position throughout the year. Since he did not have the technology to detect this shift, as we do today, he concluded that Earth must rest at the center of the universe. According to him, the Sun, planets, and stars were located in spheres that revolved around the Earth. Aristarchus (310-230 B.C., Greek) was the first to put forward the idea that the Sun was actually in the center of the universe. His theory was considered far too radical. Unfortunately, history tends to forget that he came to this conclusion about 1,750 years before Copernicus did! He also attempted to measure the relative distances between the Earth and the Sun and the Earth and the Moon. Even though he used a reasonable method, his results were not very accurate, because he lacked the technological equipment to make a precise measurement. Hipparchus (190-120 B.C., Greek) is widely considered to be the greatest astronomer of ancient times.

He compiled the first known star catalog to organize astronomical objects, and also came up with a scale to define the brightnesses of stars. A version of this magnitude system is still used today. He measured the distance from the Earth to the Moon to be 29.5 Earth diameters (we know today that the real value is 30 Earth diameters). Perhaps his greatest discovery was the precession, or wobble, of the Earth's axis, which is caused by the gravitational pull of the Sun and Moon. Claudius Ptolemy (85-165 A.D., Greek) was an astronomer who used Hipparchus' extensive observations to develop a model that predicted the movements of the Sun, Moon, planets, and stars. His model, called the Ptolemaic system, visualized an Earth-centered universe and assumed that all astronomical objects move at constant speeds in circular orbits. The circle was considered by the ancients to be the perfect shape, and regardless of the evidence against circular orbits, Ptolemy built his model to fit this idea. The Ptolemaic model is one of the longest upheld scientific theories in history: it was the cornerstone of astronomy for 1,500 years.

al-Khwarizmi (780-850, Islamic) was the inventor of algebra. He developed this mathematical device completely in words, not mathematical expressions, but based the system on the Indian numbers borrowed by the Arabs (what we today call Arabic numerals). His work was translated into Latin hundreds of years later, and served as the European introduction to the Indian number system, complete with its concept of zero. Al-Khwarizmi performed detailed calculations of the positions of the Sun, Moon, and planets, and did a number of eclipse calculations. He constructed a table of the latitudes and longitudes of 2,402 cities and landmarks, forming the basis of an early world map. Omar Khayyam (1048-1131, Persian) was a great scientist, philosopher, and poet. He compiled many astronomical tables and performed a reformation of the calendar which was more accurate than the Julian and came close to the Gregorian. An amazing feat was his calculation of the year to be 365.24219858156 days long, which is accurate to the sixth decimal place!

Nicolaus Copernicus (1473-1543, Polish) began a new era of astronomy when he concluded that the Sun was the center of the universe instead of the Earth. Copernicus felt that the Ptolemaic system was contrived, but in his revisions of that model, he kept the orbits circular. The revolutionary idea was not popular with the Church, but several other astronomers such as Brahe and Galileo helped to eventually prove that this model of the universe more accurately portrayed reality. Tycho Brahe (1546-1601, Danish) built an observatory from which he made the most accurate astronomical observations up to that time. His observatory contained sophisticated equipment for mapping star positions, and for more than 20 years he made detailed records of his findings. He believed that the universe was a blend of the Ptolemaic and Copernican models, and created his own model in which the planets orbit the Sun and the Sun orbits the Earth. Galileo Galilei (1564-1642, Italian) is the father of observational astronomy.

In 1609, he heard about the Dutch invention of the telescope, and built one for himself. Even though his telescope was not very powerful compared to the amateur equipment available today, he was able to make a number of stunning discoveries which changed the face of astronomy. He saw the craters, mountains, and valleys of the Moon, noticed the huge number of stars making up the Milky Way, kept precise records of sunspot activity and the phases of Venus, and discovered four moons orbiting Jupiter. These moons are still called the Galilean Moons today, in honor of the earth-shattering scientific effects of the discovery. During a time when the Earth was still considered to be at the center of the universe, he publicized the fact that other astronomical bodies, such as Jupiter's moons, were clearly revolving around something other than the Earth. Galileo's support of the Copernican model of the universe frightened the Church, which put Galileo on trial in 1633. He was forced to renounce his Copernican views and was held under house arrest for the rest of his life. Johannes Kepler (1571-1630, German) was Tycho Brahe's assistant and student.

He inherited his teacher's extensive collection of astronomical records, and used them to develop three laws of planetary motion. He believed in the Copernican model of the universe, although he found it difficult to fit Tycho's observations of Mars into the model with a circular orbit. He therefore used the idea of elliptical orbits to describe the motions of the planets, which became known as Kepler's first law. His second law states that a line from the Sun to a planet sweeps out equal areas in equal amounts of time. The third law was a masterpiece of simplicity: the square of the number of years of a planet's orbital period is equal to the cube of that planet's average distance from the Sun. Giovanni Cassini (1625-1712, Italian) was the astronomer who first discovered the division in the rings of Saturn, today known as the Cassini division. He also found four moons orbiting Saturn, and measured the periods of rotation of Mars and Jupiter. The Cassini space mission currently on its way to Saturn was named after him. Isaac Newton (1643-1727, British) was a mathematician who developed extensive mathematics to describe the astronomical models of Copernicus and Kepler.

His Theory of Universal Gravitation was the foundation of Kepler's laws of planetary motion, but it also went further: Newton showed that the laws governing astronomical bodies were the same laws governing motion on the surface of the Earth. Newton's scientific ideas are so complete that they still offer an accurate description of physics today, except for certain cases in which 20th century physics must be used. Edmond Halley (1656-1742, British) became famous for predicting the 1682 appearance of a comet called Halley's Comet. He proved that the orbit of comets is periodic. Charles Messier (1730-1817, French) was a comet-hunter who published a list of 110 astronomical objects that should not be mistaken for comets. This list includes some of the most intriguing sights visible through small telescopes, including galaxies, nebulae, and star clusters. The M objects, as they are now called, are used today to identify the most brilliant objects in the sky. William Herschel (1738-1822, British) was the discover of Uranus and two of its moons.

He also discovered two more moons of Saturn and several asteroids, and made a catalog of 2,500 astronomical objects. He found the polar ice caps on Mars, which are today being studied by several satellites in the hopes of shedding light on the existence of water on Mars. Johann Bode (1747-1826, German) published a law now known as Bode's Law, which predicts mathematically the distances of the planets from the Sun. Using his law, he was able to determine that there should mathematically be another planet between Mars and Jupiter; this is where the asteroid belt is located. Joseph von Fraunhofer (1787-1826, German) discovered dark lines in the spectrum coming from the Sun. He carefully measured the positions of over 300 of these lines, creating a wavelength standard that is still in use today. Joseph Lockyer (1836-1920, British) was the astronomer who first discovered the element Helium when he was studying the Sun's atmosphere. He made detailed records of sunspot activity and also studied solar flares and prominences. He conducted several tours to places where solar eclipses would be visible.

He was also one of the first archaeoastronomers: he wrote a wonderful book called 'The Dawn of Astronomy', which investigates the astronomy of ancient cultures, in particular Egypt. Annie Jump Cannon (1863-1941, American) was a member of the famous group of Harvard astronomers called 'Pickering's Women'. The director of the Harvard College Observatory, Edward Pickering, hired a number of women to sort through and organize mounds of data on the stellar classification of stars. The stars were classified by their spectra, and Annie Cannon was the most prolific and careful of the workers. She single-handedly classified 400,000 stars into the scheme we use today (O B A F G K M), and discovered 300 variable stars. She paved the way for women entering the astronomical field. George Hale (1868-1938, American) discovered that sunspots have localized magnetic fields, which helped to explain an important phenomenon present in the Sun. Perhaps his greatest legacy was to found three important observatories: Yerkes, Mt. Wilson, and Palomar.

Henrietta Swan Levitt (1868-1921, American) was also a member of 'Pickering's Women' (see Annie Jump Cannon above). She discovered that a particular type of variable star known as a Cepheid could be used as a distance marker, making it possible to determine astronomical distances to objects. Ejnar Hertzsprung (1873-1967, Danish) was one of the inventors of the Hertzsprung-Russell diagram, which shows the relationship between the absolute magnitude and the spectral type of stars. He also made the contribution of finding the distance to the Small Magellanic Cloud, a galaxy visible from Earth's southern hemisphere. Karl Schwarzschild (1873-1916, German) was the first to study the theory of black holes. The Schwarzschild radius is the distance from a black hole at which bodies would have an escape velocity exceeding the speed of light and therefore would be invisible. He also wrote extensively on the curvature of space, based on Einstein's Theory of Relativity.

Henry Russell (1877-1957, American) was the one inventor of the Hertzsprung-Russell diagram describing the spectral types of stars. He measured the parallax of the stars photographically, allowing them to be properly placed on the H-R diagram. Albert Einstein (1879-1955, German) was probably the greatest mind of the twentieth century. His Special Theory of Relativity, proposed in 1905, extended Newtonian Mechanics to very large speeds close to the speed of light. It describes the changes in measurements of physical phenomena when viewed by observers who are in motion relative to the phenomena. In 1915, Einstein extended this further in the General Theory of Relativity, which includes the effects of gravitation. According to this theory, mass and energy determine the geometry of spacetime, and curvatures of spacetime manifest themselves in gravitational forces. Arthur Eddington (1882-1944, British) proved observationally that Einstein's prediction of light bending near the extreme mass of a star is scientifically accurate. He also explained the behavior of Cepheid variables, and discovered the relationship between the mass of a star and its luminosity.

Edwin Hubble (1889-1953, American) made an incredible contribution to astronomy and cosmology when he discovered that faraway galaxies are moving away from us. Known as Hubble's Law, the theory states that galaxies recede from each other at a rate proportional to their distance from each other. This concept is a cornerstone of the Big Bang model of the universe. Jan Oort (1900-1992, Dutch) first measured the distance between our solar system and the center of the Milky Way Galaxy and calculated the mass of the Milky Way. An enormous contribution of his was the proposal of a large number of icy comets left over from the formation of the solar system, now known as the Oort Cloud. George Gamow (1904-1968, Russian-born American) was the first to put forward the idea that solar energy comes from the process of nuclear fusion. Karl Jansky (1905-1950, American) discovered that radio waves are emanating from space, which led to the science of radio astronomy.

Gerard Kuiper (1905-1973, Dutch-born American) discovered a large number of comets at the edge of the solar system beyond Pluto's orbit, known as the Kuiper belt. He also discovered several moons in the outer solar system and the atmosphere of Saturn's moon Titan. Clyde Tombaugh (1906-1997, American) was the discoverer of the final planet in our solar system, Pluto. He found it photographically in 1930, using the telescope at the Lowell Observatory in Arizona. Subramanyan Chandrasekhar (1910-1995, Indian-born American) made important contributions to the theory of stellar evolution. He found that the limit, now called the Chandrasekhar limit, to the stability of white dwarf stars is 1.4 solar masses: any star larger than this cannot be stable as a white dwarf. James Van Allen (1914-, American) discovered the magnetosphere of the Earth. The belts of radiation surrounding the planet are called the Van Allen belts, and moderate the amount of solar radiation hitting Earth.

Fred Hoyle (1915-2001, British) was a believer in the steady-state model of the universe, and thus did not believe in the Big Bang Theory. He was, however, the one who coined the term 'Big Bang'. He also believed that early life forms are transported by comets, and that the interaction of a comet with the Earth is how life appeared on our planet. Robert Dicke (1916-1997, American) believed that it was possible to detect radiation left over from the Big Bang. He invented the microwave radiometer to detect this radiation, which has a wavelength of one centimeter. Alan Sandage (1926-, American) calculated the ages of many globular clusters, and discovered the first quasar. Roger Penrose (1931-, British) expanded the physics of black holes by showing that singularities in space were responsible for their existence. Arno Penzias (1933-, German-born American) was a co-discoverer of the cosmic microwave background radiation, which is radiation left over from the Big Bang.

Carl Sagan (1934-1996, American) could be called 'the astronomer of the people'. He popularized the science of astronomy with the general public, and revolutionized science fiction by believing that we are not alone in the universe. He championed the search for extraterrestrial intelligence, which continues today with a number of missions to Mars to search for signs of life on that planet. Robert Wilson (1936-, American) was a co-discoverer of cosmic microwave background radiation left over from the Big Bang. Kip Thorne (1940-, American) contributed to the understanding of black holes. Stephen Hawking (1942-, British) is another brilliant mind of the twentieth century. He combined the theory of general relativity and quantum theory in order to prove that black holes emit radiation and eventually evaporate. Despite being completely immobile as a result of Lou Gehrig's disease, he has written numerous books to bring astronomy, physics, math, and cosmology to the general public. Alan Guth (1940-, American) developed a new theory called the inflationary universe as an addition to the Big Bang Model. Inflation theory predicts that the universe is flat and infinite.

Famous Astronomers and Astrophysicists. ~manley/astronomers.html MacTutor History of Mathematics Archive. ~history/ Robert Burnham, Alan Dyer, and Jeff Kanipe, Astronomy: The Definitive Guide, Barnes and Noble Books, 2003.

Question: Who is best known for discovering the Earth's magnetosphere?
Answer: James A. Van Allen