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interstellar molecules

with F being the density of particles in a given magnetic flux tube, t the time, v the particle speed, s the distance along the magnetic field spiral, p the momentum in the solar wind frame, vsowi the solar wind speed, ψ the garden hose angle between the magnetic field line and the radial direction, ζ the focusing length, and E /E = 1 − µvsowiv secψ/c2 the ratio of the total energy in the solar wind frame to that in a fixed frame.

This transport equation can be solved only numerically. Different approximations exist: the focused transport equation can be applied to MeV particles in the inner heliosphere (see focused transport equation); the diffusion convection equation, on the other hand, is applied to particles with energies in the keV range, to observations beyond 1 AU, and to the modulation of galactic cosmic rays (see diffusionconvection equation).

interplanetary scintillation (IPS) Solar wind induced scintillation of cosmic radio signals.

interplanetary scintillation (IPS) observations Remote-sensing observations of the inner solar wind, based on analysis of how a radio beam from a distant source (natural or from a spacecraft) is disturbed as it passes near the sun.

interplanetary sector structure

See helio-

spheric magnetic field.

 

 

 

interplanetary shock wave

See hydromag-

netic shock wave.

 

 

 

interplanetary stream structure

See helio-

spheric stream structure.

 

 

 

interplate earthquake

An earthquake that

occurs on the boundary fault dividing two lithospheric plates, as opposed to intraplate earthquakes that occur within a plate.

interstellar clouds Dark material along the spiral arms of our galaxy, consisting of clouds of neutral hydrogen, or of interstellar molecules.

interstellar dust Small dust grains of size 5×10−9 m to 2×10−7 m, apparently consisting

of refractory materials (silicates and graphite) (≈ 108 atoms/grain) with a power law distribution n(r) r−3.5 where r is the radius of the particle. Additional components consisting of polyaromatic hydrocarbons, and of water ice, are also present. Interstellar dust is responsible for extinction (especially visible in dark nebulae), and for reddening (a general phenomenon) of starlight.

Dust production presumably occurs in cool red giants, which are old stars with enhanced metals and substantial stellar winds, and also in supernova explosions, which can transport large amounts of metals into the interstellar environment.

interstellar gas The gas lying between stars in the plane of the galaxy, composed principally (≈ 90%) of atomic hydrogen, H , approximately half of which is neutral (H I), observable by its 21 cm radiation, and half is ionized (H II, ionized by hot O and B stars). The number density varies from 10−2 to 106 cm3, averaging about 1 cm3. There is a small admixture of “metals”.

interstellar medium The matter composed of dust and gas in diffuse form that fills the space between the stars of a galaxy. Interstellar matter varies widely in temperature, density, and chemical composition. The interstellar medium reaches to the outer layers of the atmosphere of stars where the density and temperature of the gas increase steeply, but far from the stars its density drops to less than 1 particle cm−3. However, in a galaxy there are many places where the density of interstellar matter increases significantly. These concentrations or regions of the interstellar medium, sometimes called clouds, or shells because of their appearance, have different physical and chemical characteristics that depend on the amount of light and energy that they receive.

interstellar molecules Inorganic (silicates); and especially organic molecules identified via infrared spectroscopy in cool dense dark interstellar clouds (of typically solar system size) in the nucleus and along the spiral arms of the galaxy; about 10−4 the interstellar mass. At present (AD 2000) over 100 organic molecules have been fairly securely identified, involving H,

© 2001 by CRC Press LLC

intertropical convergence zone (ITCZ)

C, N, O, S, ranging from the simplest CO (carbon dioxide), NO, OH, to H2O (water), C2S, and NH3, to molecules with up to eight carbons, and unusual (or surprising) small molecules like CN, CP, CSi, HCl, KCl, HF, MgCN, and SiH4.

intertropical convergence zone (ITCZ)

A

fairly narrow band where surface southeasterly and northeasterly trade winds meet and converge. The convergence forces moist surface air to rise and as a result, water vapor condenses, forming high clouds and releasing condensational heat. Thus, the ITCZ also appears as a longitudinally oriented, latitudinally narrow band of high clouds, making itself one of the most visible features of the Earth from space. Over continents, the ITCZ moves back and forth seasonally across the equator following the sun, while the oceanic ITCZ’s position is determined by the underlying sea surface temperature. In particular, the ITCZ over the eastern Pacific and Atlantic Oceans stays north of the equator for most times of a year (5◦ to 10◦ N) because sea surface temperatures are generally higher north of the equator than south. The development of equatorial asymmetry in Earth’s climate despite the nearly symmetric distribution of annual-mean solar radiation is a result of ocean-atmosphere interactions and land-sea distributions.

intraplate earthquake Any earthquake that cannot be associated with plate margin processes. Three very large intraplate earthquakes, with magnitudes of about eight, occurred near New Madrid, Missouri in the winter of 1811– 1812.

intrinsic permeability (k) The ability of a porous medium to transmit fluid, independent of the fluid properties themselves, increases with increasing size and shape of the pore space, k = Nd2, where N is a dimensionless constant that accounts for the shape of the pore space and d is the mean grain diameter. In practice, the intrinsic permeability is measured with a device

called a permeameter, ranging in natural systems from 10−21 to 10−9m2.

intrusion In geophysics, a magma body that has solidified at depth.

invariant See adiabatic invariant, longitudi- nal invariant.

inverse Compton effect Compton scattering of a photon by a particle, typically an electron, whose kinetic energy is comparable to, or larger than, the energy of the photon. In contrast to the usual Compton scattering, where the photon loses energy, in inverse Compton scattering part of the kinetic energy of the particle is transferred to the photon. The emerging photon has its energy increased by up to a factor γ 2 where γ 2 = 1/(1 − (v/c)2), where c is the speed of light. If electrons are relativistic v/c 1, then γ 1. In this case inverse Compton scattering can dramatically increase the energy of initially low-energy photons. Inverse Compton scattering has been proposed as the mechanism producing hard X-rays and gamma rays in active galactic nuclei. See Compton scattering.

inverse Compton radiation The radiation produced when a high energy (relativistic) electron interacts with a much slower moving photon thereby transferring energy from the electron to the photon. Potentially important in large solar flares where the ambient population of low energy thermal photons interact with the high energy non-thermal electrons accelerated in the flare.

inverse problems A general classification for problems in which (ir)radiance measurements are used to infer inherent optical properties of the medium and sources.

inverse theory A model of the world can be tested against measurements from the real world by evaluating the difference between the measurements and predictions from the model for the measurements. Predicting measurements from models is termed “forward modeling”, while the reverse — using data to create models, by using the data to infer values for parameters in the model — is termed “inverse modeling”. A very simple case is least squares fitting of a straight line to data. A slightly more complex case would be the least squares fit of a model with many parameters to data, where the function that predicts the data d from the model parameters p is linear, and can be written as a ma-

© 2001 by CRC Press LLC

ionogram

trix G so that:

d = Gp

in which case a least-squares solution p can be obtained via:

p = GT G−1GT d .

However, it may be that other solutions fit the data equally well, in which case the inversion is said to be nonunique. Complications may be introduced by a nonlinear relationship between p and d, by constraints on the parameters, or by biases concerning the model (e.g., a preference for simple models). See nonuniqueness.

inverted barometer effect (inverse barometer response) The inverse response of sea level to changes in atmospheric pressure. A static reduction of 1.005 mb in atmospheric pressure will cause a stationary rise of 1 cm in sea level. The dynamic response, especially at periods shorter than a few days is not constant.

Io Moon of Jupiter, also designated JI. Discovered by Galileo in 1610, it is one of the four Galilean satellites. Its orbit has an eccentricity of 0.004, an inclination of 0.04◦, a precession of 48.6◦ yr−1, and a semimajor axis of

4.22 × 105 km. Its radius is 1815 km, its mass 8.89 × 1022 kg, and its density 3.55 g cm−3. It

has a geometric albedo of 0.61, and orbits Jupiter once every 1.769 Earth days. Io is tidally heated by Jupiter, and has active volcanoes as a result.

ion acoustic waves Ion acoustic waves are among the simplest wave motions supported by a plasma. They behave in a fashion similar to normal sound waves in neutral gas, with one principal difference: the ion acoustic wave speed is determined by the ion’s inertia and the combined pressure of both ions and electrons in the plasma. In this wave, the ions and electrons oscillate synchronously, coupled by the electric field generated.

ionization The process by which ions are produced, typically occurring by collisions with atoms or electrons (“collisional ionization”), or by interaction with electromagnetic radiation (“photoionization”).

ionization equilibrium A plasma at a given temperature is in ionization equilibrium if the ratio of the number of ions in a given ionization state for a given element, nion, to the total number of ions of that element, nel, is governed by the equations of thermodynamic equilibrium. Departures from ionization equilibrium imply changes in the temperature of the peak abundance of the various ions, so that the corresponding lines would be formed at temperatures significantly different from the formation temperature deduced from equilibrium.

ionizing radiation Electromagnetic radiation with energy exceeding the typical binding energy of electrons in molecules; electromagnetic radiation with wavelength shorter than that of ultraviolet light; also applied to particulate radiation of similarly high energy.

ionogram The conventional display obtained from an ionosonde which contains information about the ionosphere. An ionogram is constructed by displaying the returned signal as a function of frequency (on the horizontal axis) and time delay (on the vertical axis). The vertical axis, called the virtual height, is the height that the returned signal would have reached had it traveled in free space and been reflected from a perfect reflector above the ionosonde. The E and F layers of the ionosphere can be recognized easily on most middle and low latitude ionograms, recognition being more complex at high latitudes due to the presence of particle effects. The layers of the ionosphere can be parameterized in terms of their peak electron density (measured in megahertz, the units used on an ionogram for measuring electron density) and base height (measured in virtual height units, usually kilometers), both of which can be readily measured from ionograms. Conventionally, these are only recorded for the ordinary ray polarization, information from the extraordinary ray, when present on the ionogram, being used to improve these estimates. The most familiar internationally agreed parameters are hE, foE (the E region), hF, foF1 (the daytime F1 region), and hF2, foF2 (the daytime F2 region). At night these become just hF and foF2 (the nighttime F region). When the F1 region is not visible, the daytime F region parameters are hF and foF2.

© 2001 by CRC Press LLC

ionosonde

Each sporadic E layer can be described by three parameters: hEs, foEs, and fbEs. Finally, two further important parameters are fmin, the minimum frequency returned from the ionosphere and recorded on the ionogram and M(3000)F2. These parameters are tabulated hourly, together with other information, and exchanged between World Data Centers. There is now a serious attempt to collect these data in real time and construct global maps of the ionosphere. To obtain the electron density profile in real height units, sometimes called true height, it is necessary to invert the ionogram, and correct for the time delay caused by the ionosphere. See criti- cal frequency, ionosonde, ionospheric sounding, plasma frequency.

ionosonde A swept frequency pulsed radio transmitter on the ground cosited with a receiver forming a vertically directed radar system. A single frequency pulse is transmitted upwards and may be reflected from the ionosphere, the reflected signal being observed with the receiver. This process is repeated at a sequence of frequencies usually starting at a frequency of 1.0 MHz and finishing at around 20 MHz. A picture of the ionization above the ionosonde is then built up into an ionogram. This is constructed by displaying the returned signal as a function of frequency (on the horizontal axis) and time delay (on the vertical axis). See ionogram.

ionosphere The layer of the Earth’s atmosphere extending from the top of the mesosphere (typically 80 to 90 km above the surface) to about 500 km. The thermosphere, or ionosphere, is the uppermost layer of a planetary atmosphere. It is also often called the upper atmosphere. It is characterized by low density ( 3 × 10−9gm/cm3; number density7 × 10−11/cm3) and pressure ( 0.1 Pa), and substantial ionization of the atoms, resulting in significant free electron density, which is a function of solar activity. The temperature increases rapidly with increasing altitude up to about 200 km, followed by a leveling off in the 300 to 500 km region. This heating is the result of photoionization and photodissociation of molecules into atoms and ions by direct absorption of solar photons.

ionospheric absorption Radio waves passing through the ionosphere experience interactions with the free electrons and may lose energy by these interactions if the electrons experience collisions with the neutral atmosphere. This process is called absorption, or sometimes non-deviative absorption, because the radiowave path through the absorbing region is not affected by the absorption. Collisions between electrons and the neutral atmosphere increase with decreasing altitude, so the D region is particularly important for absorption processes. Absorption varies as the inverse square of the frequency, so the losses are larger at low HF frequencies. In normal circumstances, during the daytime, ionospheric absorption prevents radio frequencies greater than about 2 or 3 MHz propagating via the ionosphere unless very large powers are transmitted. During nighttime, when the D-region ionization drops to very low levels, these same transmissions can propagate by the ionosphere to large distances. During a solar flare, X-ray radiation from the sun can increase significantly, resulting in greater ionization of the D region and subsequently larger absorption of radio waves. These increases may be sufficiently large to prevent all HF signals propagating by the ionosphere. See D region, short wave fadeout, solar flare.

ionospheric index An index usually based on observations of the F region critical frequency (foF2), which gives a measure of the solar cycle effects on the ionosphere. There are a variety of ionospheric indices of which the best known are IF2 and T. Both these indices are calculated in similar ways. First, past data are used to estimate calibration curves then, using current data with the calibration curves, an index is calculated that best represents the solar cycle effects for this epoch. Ionospheric indices are effective because the F region is strongly dependent on changes in solar activity. The correlation between the monthly median hourly foF2 and the smoothed sunspot number, for a location, can be higher than 0.95. Ionospheric indices are often used with HF propagation models for predicting propagation conditions. See ionospheric radio propagation path.

© 2001 by CRC Press LLC

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