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Cosmic Rays: Messengers from the Distant Galaxy and Probes of the Heliosphere

Cosmic rays are exceptionally energetic particles that often travel at almost the speed of light. There are three classes of cosmic ray:

  • Galactic cosmic rays (a), or GCRs, originate beyond the heliosphere
  • Solar energetic particles (b), or SEPs, are produced by the Sun
  • Heliospheric cosmic rays (c), which include anomalous cosmic rays, are accelerated within the heliosphere

a. Image
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b. Image
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c. Image
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Left: The supernova remnant Cassiopeia A.

Middle: TRACE image of a solar flare

Right:Schematic of the life cycle of a neutral atom. Interstellar neutral atoms stream into the supersonic solar wind, where some experience charge exchange to produce fast, outwardly streaming neutrals and pickup ions. The pickup ions are convected to the shock, where some are energized at the termination shock and become anomalous cosmic rays.

Cosmic rays are of great interest for at least two reasons:

  1. They are hazardous to humans and radiation-sensitive systems
  2. They carry information about the large-scale properties and character of the heliosphere and galaxy

When cosmic rays collide with atoms from the upper atmosphere, a cascade of secondary cosmic rays showers down to the Earth's surface. The Earth's magnetosphere protects us from the primary cosmic rays, but thousands of secondary cosmic rays, although less energetic, pass through our bodies every minute. These secondary cosmic rays, with an average intensity of about 100 per square meter per second, represent only a few percent of the natural background radiation to which we are exposed (d-e).

d. Image
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e. Image
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A cosmic ray or gamma ray enters the Earth's atmosphere where it collides with an air molecule. This produces a shower of secondary particles, some of which collide with more air molecules, and so on. The broad shower of particles striking the Earth can then be measured by ground-based detectors, from which the energy of the original cosmic ray can then be determined. Two examples of air showers generated by a cosmic or gamma ray are illustrated.

However, the highly energetic cosmic rays in space pose serious threats to spacecraft and high-altitude planes. Computer hardware and sensitive electronic instruments on craft traveling in outer space must be shielded from cosmic rays to prevent them from passing through electronic chips and "flipping" the logical states. Astronauts are also at risk of high-level radiation from cosmic rays, especially during the active phase of the Sun (f).

f. Image
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The Ulysses spacecraft made a latitudinal survey of the solar wind during both solar minimum and solar maximum. The wind velocity and magnetic field strength and polarity are superimposed on an image of the sun during solar maximum. The wind speed, although subject to large fluctuations, is approximately 400 km/s at all latitudes.

In order to understand the second reason why cosmic rays are important, we must comprehend how cosmic rays propagate and how they reach the Earth.

Cosmic rays have enormous velocities and, as they stream collectively, they excite Alfvénic waves, which are magnetic irregularities. Charged cosmic rays follow helical orbits as they gyrate around the large-scale heliospheric or galactic magnetic field. However, the presence of both self-induced and pre-existing magnetic irregularities causes the helical orbit of the charged cosmic ray to change continuously in, typically, small increments. Since magnetic irregularities convect with the large-scale flow, the cosmic ray, although very energetic and fast-moving, experiences constant buffeting and, therefore, the net movement of the particle tends to be with the flow, on average. We can picture this by comparing the cosmic ray with an energetic, fast-moving person in a crowd, whose progress is impeded by the dense crowd, so that he tends to move at the speed and with the flow of the crowd (g-h).

g. Image
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h. Image
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Left: The solar wind admits fluctuations on almost all conceivable scales. This series of plots illustrates the point by showing solar wind magnetic field data that has been averaged using four different time intervals. The top left plot shows ACE data at 1 AU using 8-hour averaging. Large fluctuations are always present even at these scales for the full year of 2000. The bottom left shows smaller scales using a 1-hour averaging and one sees as much structure as at the larger time scale. The top right plot now uses 10-minute averaging and high frequency fluctuations are still present. Finally, in the bottom right plot, going to the highest frequencies using 1-minute averaging, we see that fluctuations persist on these very small scales. Note too that the amplitudes of the fluctuations do not become smaller as the scales become finer.

Right: The spectral density of the power in the interplanetary magnetic field magnitude (B_mag) and in the trace of the magnetic field as a function of frequency at 1 AU. Fluctuations are clearly present on all frequency scales, ranging from very low to very high. Plotted in a log-log format, parts of the spectrum lie on a straight line indicating a power law dependence of the spectral density with frequency. Figure courtesy of C.W. Smith.

The ability of cosmic rays to diffuse by scattering across vast regions of interplanetary and interstellar space allows us to use them to probe both the distant regions in which they were born and the intervening space. The composition of cosmic rays can therefore tell us about the gas that surrounds stars before they become supernovae or, in the case of anomalous cosmic rays, about the interstellar gas that surrounds our heliosphere (i-j).

i. Image
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j. Image
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Left: In nearby galaxy NGC 6822, this glowing emission nebula complex surrounds bright, massive, newborn stars. A mere 4 million years young, these stars condensed from the galaxy's interstellar gas and dust clouds. Credit: C. R. O'Dell (Vanderbilt University) et al., Hubble Heritage Team, NASA

Right: Elliptical galaxies are unlike spiral galaxies and hence unlike our own Milky Way Galaxy. The giant elliptical galaxy named NGC 4881 on the upper left lies at the edge of the giant Coma Cluster of Galaxies. Elliptical galaxies are ellipsoidal in shape, contain no spiral arms, contain little interstellar gas or dust, and are found mostly in rich clusters of galaxies. Elliptical galaxies appear typically yellow-red, as opposed to spirals, which have spiral arms that appear quite blue. Much speculation continues on how each type of galaxy can form, on whether ellipticals can evolve from colliding spirals, or spirals can be created from colliding ellipticals, or both. Besides the spiral galaxy on the right, all other images in this picture are of galaxies that lie well behind the Coma Cluster. Credit: W. A. Baum (U. Washington), WFPC2, HST, NASA

Because the regular and disordered interplanetary and interstellar magnetic field both act to guide and scatter charged cosmic rays, observations of cosmic rays serve as a means of probing magnetic fields on both the largest of scales and the smallest (k-l).

k. Image
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l. Image
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Left: The solar wind admits fluctuations on almost all conceivable scales. This series of plots illustrates the point by showing solar wind magnetic field data that has been averaged using four different time intervals. The top left plot shows ACE data at 1 AU using 8-hour averaging. Large fluctuations are always present even at these scales for the full year of 2000. The bottom left shows smaller scales using a 1-hour averaging and one sees as much structure as at the larger time scale. The top right plot now uses 10-minute averaging and high frequency fluctuations are still present. Finally, in the bottom right plot, going to the highest frequencies using 1-minute averaging, we see that fluctuations persist on these very small scales. Note too that the amplitudes of the fluctuations do not become smaller as the scales become finer.

Right: The Parker spiral. The field is radial close to the Sun, about 45 degrees from radial at 1 AU, and almost 90 degrees from radial beyond 15 AU. Figure courtesy of Steve Suess.

Especially important, and an area in which Voyager has made seminal contributions, is how the solar wind, its disturbances, the interplanetary magnetic field, and turbulence affect the propagation of cosmic rays. Low-energy cosmic rays experience great difficulty "swimming upstream" into the solar wind, whereas more energetic particles find it much easier. As a result, the spectrum that we observe is always more depressed at low energies than at high energies. This effect is called modulation. Of course, as the solar wind varies, especially as the solar magnetic field reverses, the intensity of cosmic ray modulation changes dramatically (m-n).

m. Image
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n. Image
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Left: The flux of anomalous cosmic rays in the heliosphere can be modeled numerically and observed by spacecraft such as Voyager. This figure shows the accelerated spectrum at the termination shock (where the cosmic rays are accelerated) and at the Voyager location in 1996. The corresponding Voyager observations are also plotted.

Right: A composition of the effects, shown extended over a period of 30 years, to illustrate the ability of the combined drift/GMIR approach to simulate complete 11 and 22 year cycles in the modulation of galactic cosmic rays in the heliosphere.

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