In the commonness of life on Earth, most would expect that the composition of space beyond our atmosphere might be planets, comets, asteroids, and an occasional meteor. In reality, the space between the planets and other larger objects of our solar system is richly composed of a variety of complex phenomena, which on Earth drive weather, affect communications, and provide beautiful displays with the aurora.
Long considered an empty void, the vast space between the planets and our Sun is actually filled with a tenuous gas comprised of neutral and ionized particles along with small dust grains. The source of the ionized particles comprising this gas is mostly outflows and outbursts of the Sun. Some of this gas is due to outflow of particles from planets, comets, and asteroids. Finally, some of this gas comes from the infall of particles of gas and dust from the surrounding interstellar space. In this chapter, we will explore the boundaries, composition, sour ces, and dynamics of the particles filling the interplanetary medium.
The Interplanetary Medium—Inner Boundary
The inner boundary of the interplanetary medium (IPM) is derived from specific models of gases in the outer atmosphere of the Sun called the corona. Because the Sun is an extremely dynamic object in space, the inner boundary of the Sun fluctuates with the modes of solar activities. In a simplistic argument, the boundary between the corona and the IPM can be defined as that point where the solar corona becomes less dense than other constituents of the IPM. This definition becomes too limited, though, when we realize that the interactions of solar plasmas are also governed by local magnetic fields, and hence trapped solar plasma can extend into the IPM significantly beyond the boundaries of the solar corona.
A possible alternative boundary point between the corona and the IPM is the point where the subsonic dynamics of the plasma of the corona transit into the supersonic flow, known as the solar wind. This boundary is best understood by examining the hydrostatic balances of gases that comprise the solar corona. Parker (1) showed in 1959 that the gas comprising the corona must expand due to pressure balances. Because the Sun is in pressure equilibrium, the outward thermal and magnetic pressures balance the gravitational attraction of the mass of the Sun. The solar wind derives from those particles that escape this boundary. The static equilibrium of the solar atmosphere is determined by the balance of gas pressure and solar gravity. Beyond a certain distance from the Sun, gas pressure exceeds gravity, and supersonic outflow ensues—the solar wind. In general, models show that in steady-state conditions, the exterior boundaries of the corona occur at around 1.01 to 10 solar radii depending on the values of the parameters used in solving the equations (2).
The Interplanetary Medium—Outer Boundary: the Heliosphere
The heliosphere is defined as the region that extends from the exterior boundaries of the Sun to the outermost reaches of the influence of the Sun. The heliosphere is a magnetic bubble formed by the effects of the Sun’s magnetic fields as it interacts with local interstellar winds. As the solar wind flows outward, it interacts with the flow of local interstellar wind and with infalling neutral particles and dust grains. Because the solar wind is a supersonic flow, the transition from the heliosphere into the local interstellar medium, it is believed, occurs as a shock called the termination shock. Farther out from the termination shock is the heliopause boundary layer. The termination shock is the backup of the pressure wave that develops from the heliopause boundary itself. It occurs due to the initial ”collision” of the plasma that composes the interstellar wind with the magnetic forces due to the Sun. The location of the termination shock and the heliopause varies significantly based on the activity of the Sun. During the solar maximum, the solar wind is weaker so that the external pressure on the heliopause forces the heliosphere to shrink. The most recent, high, solar activity levels give a potential opportunity for the far-flung Voyager I and Voyager II spacecraft to encounter the termination shock not just once but several times. (As of January 1, 2002, the Voyager I spacecraft was approximately 85 AU from the Sun, and the Voyager II spacecraft was approximately 67 AU from the Sun.) The shrinkage and expansion of the heliosphere occur much faster than the outward motion of the spacecraft. If the termination shock is currently shrinking past one or both of the spacecraft, as it expands, years later it will again pass across the spacecraft to be encountered again. There is the possibility of many such encounters that will provide the opportunity to understand the nature of the termination shock and the heliopause in great detail.
Figure 1 provides a graphical model of the magnetic bubble that forms the heliosphere along with the trajectory of the Voyager I and II spacecraft and the Pioneer 10 spacecraft, as well as the newly proposed NASA Interstellar Probe mission. The interstellar wind impacts the bow shock formed at the heliopause. The termination shock is shown within the bow shock. If current models (2) are accurate, the Voyager I and II spacecraft have a good opportunity to encounter the termination shock sometime on or before 2003. It is estimated that the termination shock at that time will occur at approximately 100 AU.
The Interplanetary Medium—Solar Inputs
The primary source of particles in the IPM is from the Sun in the form of the solar wind, coronal mass ejections (CMEs), and solar flares. The general composition of the plasma injected into the IPM is constrained by the composition of the corona. The primary constituents of the solar wind are approximately 95% protons, 4% alpha particles, and 1% minor ions including multiple ionization states of C, O, Si, and Fe. The solar wind also contains electrons in number approximately equal to the ions, and hence the solar wind is considered an electrically neutral plasma. The solar wind contains approximately 1-10 particles per cubic centimeter. The solar wind is a fast stream of particles that leaves the corona at approximately 400km/s in the ecliptic plane. The velocity of this stream varies significantly and ranges from 300-1000 km/s. At high heliolati-tudes (above 45°) during the solar minimum, the solar wind leaves the corona at approximately 800 km/s, again with a very large range of velocities. Figure 2 shows an image of the Sun (3). Superimposed on this image is a plot of the solar wind speed detected in one polar orbit of the Ulysses spacecraft around the Sun. The figure indicates that within approximately 30° of the ecliptic, the solar wind speed is characterized more by the slower solar wind speeds, although the figure shows that significant variations can occur. Above and below 30° the solar wind speed is characterized more by the faster speeds, again with some variations.
Figure 1. A model of the heliosphere as it interacts with local interstellar winds.
Other solar sources of particles include disturbances in the form of coronal mass ejections (CMEs) and solar flares. CMEs are large-scale bubbles of plasma and embedded magnetic fields that are released from the surface of the Sun. CMEs take hours to develop and are released abruptly. The release of a CME occurs across a large portion of the solar surface and can even affect the entire solar disk. On the other hand, solar flares are smaller scale explosions from the surface of the Sun that take just minutes to form. Solar flares tend to be localized to the area surrounding sunspots. Each of these phenomena transports large amounts of solar plasma into the IPM. The plasma released in a CME or solar flare is more energetic than the steady plasma flow of the solar wind. One last particle type emitted by the Sun is the solar energetic particle (SEP). SEPs are very high-energy ions and electrons that are accelerated by processes within the solar corona, including explosive solar flares. SEP energies are typically between 10 and 100 MeV but can exceed 1 GeV. SEPs provide to solar physicists the opportunity to investigate the composition of the Sun and to understand the accele-rative processes that energize the particles.
Figure 2. Solar wind speeds superimposed on an image of the Sun.
The Interplanetary Medium—Planetary ”Pickup Ions”
Another source of plasma injected into the IPM is due to the interaction of the interplanetary magnetic field with the magnetospheres of magnetized planets. The best examples come from what are called “upstream” particle bursts from the Jovian magnetosphere (4). Upstream events are characterized by enhancement of ions and electrons, compared to the solar wind. Voyagers I and II, Ulysses, and most recently the Cassini spacecraft have detected short-term enhancements in the overall density of particles as they approached and receded from the Jovian magnetosphere. In general, upstream events seem to occur when the local magnetic field of the IPM is pointed directly toward the planetary magnetosphere. The overall composition of ions during these events is best interpreted as planetary, not solar. The onset of these events is occasionally characterized by the reception of the faster, more energetic ions arriving at the spacecraft before the slower ions. In general, though, the detection of particles during an event occurs at the same time, indicating that the spacecraft is passing through a ”flux tube” of magnetically contained plasma that connects directly to the planetary magnetosphere. It is assumed that the magnetic fields associated with these flux tubes have become directly connected to the magnetic fields of the IPM and hence allow for transport of particles away from the planetary magnetosphere.
Figure 3 shows an example of an upstream event that occurred as the Cassini spacecraft was leaving the Jovian magnetosphere on day 37 of 2001. The figure is derived from data taken from the Cassini MIMI LEMMS (Low-Energy Magnetospheric Measurements System) detector and is plotted in distance from Jupiter in Jovian radii. At a distance of approximately 526.7 RJ (post-Jovian encounter—day 37 of 2001), an enhancement of the ion particle flux was noted. In this particular enhancement, the beginning of the event occurred in all of the lower ion energy channels at the same time. This is consistent with an explanation that the spacecraft flew through a magnetic flux tube that is connected to the Jovian magnetosphere on one end. The event lasted for approximately 1 day, and there was notable decay of ion flux in all but the lowest energy channel.
Figure 3. An upstream ion event on the outbound pass of the Cassini spacecraft at a distance of approximately 527 RJ.
The Interplanetary Medium—Interplanetary Magnetic Field
The other major component of the IPM is the interplanetary magnetic field (IMF) that fills the entire region of the heliosphere. The IMF is mostly due to the transport of solar magnetic structures into the IPM. As the outward moving plasma of the solar wind goes from subsonic in the corona to supersonic just outside the corona, the magnetic field becomes locked within the solar wind plasma. The solar wind then carries the coronal magnetic fields into the IPM. Parker (1) first developed a model of the way the coronal magnetic field is carried into the IPM. The overall model takes into account the shape of the magnetic field lines as the solar wind velocity and the rotation of the Sun determine them. If one assumes a purely radial solar wind and then considers this solar wind in the reference frame of the rotating Sun, then the solar wind has the following components:
where usw is the radial solar wind speed, Q© is the solar angular velocity, r is the distance from the sun, and 6 is the solar latitude. This solution assumes that the solar rotational rate on the surface of the Sun is constant instead of the observed latitudinal differential rotational rate. The solar magnetic field lines then follow velocity streamlines. The path is defined by the following equation:
As discussed in the first section, as the solar wind escapes from the solar surface, the solar wind speed becomes a constant at a critical distance from the solar surface. Using this fact, we can integrate equation 4 and derive the following solution:
where j© is the initial longitude where the solar wind developed on the surface of the Sun. This equation describes an Archimedean spiral where the magnetic field follows a spiraling path as it moves away from the Sun. For low latitudes (slow moving solar wind), the value of DR is approximately 6 AU — 1 AU farther than the orbit of Jupiter. For higher latitudes (fast moving solar wind), the value of DR is approximately 12-2 AU farther than the orbit of Saturn.
The Interplanetary Medium—Corotating Interaction Regions and Shocks
Based on the combined understanding of the latitudinal solar wind dependency coupled with the observed latitudinal differential rotational rate of the surface of the Sun, the overall structure of the IMF becomes complex. In specific regions, on the boundaries between fast moving solar wind and slow moving solar wind, interplanetary spacecraft have observed magnetically complex structures known as corotating interaction regions (CIRs). CIRs most generally occur when the fast moving solar wind overtakes the slower moving solar wind. CIRs are bounded by two shocks at the edges, called forward and reverse shocks. These shocks are characterized by changes in the magnetic field intensity, the particle density, and the overall magnetic pressure. The shocks associated with CIRs provide an explanation for energetic streams of ions propagating from interactive regions (5).
The effects are best understood by studying how shocks accelerate particles. Magnetized shocks in general are quite efficient in accelerating charged particles and producing an overall increase in the number of energetic ions and electrons in the local plasma environment. Acceleration of charged particles can occur through one of two processes: shock drift acceleration and diffusive shock acceleration. Shock drift acceleration occurs when a charged particle encounters a magnetized structure so that particle acceleration occurs due to induced electric fields parallel to the surface of the shock. The general understanding of shock drift acceleration comes from examining the motion of a charged particle as it encounters a shock. Because the magnetic field strength is larger inside the shock than outside, the gyroradius of a charged particle that impacts a shock decreases. In general, the motion of a particle includes a component of velocity parallel to the shock boundary. In the reference frame of the shock, the particle experiences an electric field parallel to the shock boundary that can either accelerate or decelerate the particle. Acceleration occurs while the particle is outside the shock, and deceleration occurs while the particle is within the shock. Because the gyroradius of the particle is larger outside the shock than inside the shock, the particle spends more time accelerating than decelerating. The factor that determines whether the particle is transmitted through the shock or reflected is based on conservation of the first adiabatic invariant. The condition for reflection is best seen from the following equation that describes the magnetic moment in terms of the pitch angle:
where subscript 1 refers to the conditions outside the shock, subscript 2 refers to the conditions inside the shock, p is the momentum of the particle, and a is the pitch angle of the particle. Under the condition
conservation of the first adiabatic invariant requires that sin a2> 1, which is not possible. This condition indicates that a particle is reflected from the shock boundary. If the condition is not true, then, the particle is transmitted through the shock. In either case, though, the motion of the particle includes acceleration while the particle is within the shock. In general, shock drift acceleration energizes a particle by a factor no larger than 10. However, repeated encounters with a particular shock and/or encounters with multiple shocks can allow the energy of a particle to increase dramatically. Another mitigating factor is the relationship between the direction of the shock normal and the direction of the magnetic field. If these are parallel, then shock drift acceleration is not effective in accelerating particles, but if these are perpendicular, shock drift acceleration is very effective in increasing a particle’s energy.
Diffusive shock acceleration occurs when a particle encounters a shock that is approaching. By examining the motion of the particle in the rest frame of the shock, it is seen that the energy of the particle is increased. Denote the velocity of the particle before and after the collision with the shock as v1 and v2, respectively. The velocity of the particle in the rest frame of the shock is indicated by primes:
In the rest frame, the velocity of the particle is simply reflected:
The change in energy is then given by combining Eqs. 6 and 7:
Depending on the value of v1ushock the particle either gains or loses energy. If v1«shock<0, then the particle gains energy; if v1Mshock>«2hock, then the particle loses energy.
When a particle encounters the shock multiple times (due to reflection from other magnetic anomalies outside of the shock), then, the particle can experience repeated acceleration. If a distribution of particles encounters a shock, individual collisions with the shock will be stochastic so that some of the particles are accelerated and some are decelerated. This process has a tendency to spread the particle velocity distribution function to include more slower and faster moving particles.
The Interplanetary Medium—Interstellar Sources of Particles: Pickup Ions
One final source of particles that comprise the constituents of the IPM is the infall of neutral and charged particles from the local interstellar medium. Because the heliopause is a shock boundary, it is difficult for charged particles to penetrate into the IPM. The main component of interstellar particles within the
IPM is neutral atoms. Because these particles are neutral, magnetic fields cannot deflect the motion of these particles. As these particles fall farther into the IPM toward the Sun, solar radiation ionizes them. As the particles become ionized, they become bound to the IPM’s magnetic field lines, and their overall motion becomes trapped within the outflowing solar wind. Blum and Fahr (6) originally proposed the concept of these particles, but they were not discovered until the Ulysses spacecraft entered into the quiet regions of the high latitude solar wind. The SWICS instrument (Solar Wind Ion Composition Spectrometer) of the Ulysses spacecraft provides the capability of capturing in situ distribution functions. A key discovery occurred when the SWICS instrument found an anomalous component of the solar wind distribution function. Figure 4 shows the distribution function from the SWICS instrument (7). The phase space density is shown as the dotted curve that has a maximum peak velocity of the solar wind velocity. Instead of the expected phase space density, the SWICS instrument showed that the local IPM was filled by a broad distribution of particles that form a slowing declining plateau in velocity space. This plateau, it is understood, indicates the existence of ionized interstellar hydrogen. The falloff of particles whose velocities are greater than 2 vsw is an expected result due to the filling-in nature of the pickup ions in velocity space. In the solar wind rest frame, the pickup ions are seen as an isotropic spherical shell whose radius is 1 vsw. When transformed into the rest frame of the detecting spacecraft, this filled-in shell appears as a distribution of particles whose velocities are between 0 and 2 vsw. Most interstellar neutrals are easily ionized by the solar radiative source. These particles penetrate to within 6 AU of the Sun. Because helium is more difficult to ionize than all other interstellar neutrals, helium penetrates more closely to the Sun at a cutoff distance of approximately 4.82 AU.
Figure 4. SWICS distribution function showing the 800-m/s solar wind but also indicating an unexpected plateau of particles indicating proton pickup ions.
Conclusion
Existing NASA satellite programs continue to provide a significant amount of data regarding the nature of the constituents of the interplanetary medium and transport processes. Proposed programs such as the Interstellar Probe (trajectory shown in Fig. 1) represent significant opportunities to clarify further the details of the overall structure of the heliosphere and provide in situ measurements of the plasma constituents within the interplanetary medium. To continue developing our understanding of the interplanetary medium and the overall heliosphere, we must maintain a continued presence in space.
