Multipole radiation is a theoretical framework for the description of electromagnetic or gravitational radiation from time-dependent distributions of distant sources. These tools are applied to physical phenomena which occur at a variety of length scales - from gravitational waves due to galaxy collisions to gamma radiation resulting from nuclear decay. Multipole radiation is analyzed using similar multipole expansion techniques that describe fields from static sources, however there are important differences in the details of the analysis because multipole radiation fields behave quite differently from static fields. This article is primarily concerned with electromagnetic multipole radiation, although the treatment of gravitational waves is similar.
Electromagnetic radiation depends on structural details of the source system of electric charge and electric current. Direct analysis can be intractable if the structure is unknown or complicated. Multipole analysis offers a way to separate the radiation into moments of increasing complexity. Since the electromagnetic field depends more heavily on lower-order moments than on higher-order moments, the electromagnetic field can be approximated without knowing the structure in detail.
Properties of multipole radiation
Linearity of moments
Since Maxwell's equations are linear, the electric field and magnetic field depend linearly on source distributions. Linearity allows the fields from various multipole moments to be calculated independently and added together to give the total field of the system. This is the well-known principle of superposition.
Origin dependence of multipole moments
Multipole moments are calculated with respect to a fixed expansion point which is taken to be the origin of a given coordinate system. Translating the origin changes the multipole moments of the system with the exception of the first non-vanishing moment. For example, the monopole moment of charge is simply the total charge in the system. Changing the origin will never change this moment. If the monopole moment is zero then the dipole moment of the system will be translation invariant. If both the monopole and dipole moments are zero then the quadrupole moment is translation invariant, and so forth. Because higher-order moments depend on the position of the origin, they cannot be regarded as invariant properties of the system.
Field dependence on distance
The field from a multipole moment depends on both the distance from the origin and the angular orientation of the evaluation point with respect to the coordinate system. In particular, the radial dependence of the electromagnetic field from a stationary -pole scales as . That is, the electric field from the electric monopole moment scales as inverse distance squared. Likewise, the electric dipole moment creates a field that scales as inverse distance cubed, and so on. As distance increases, the contribution of high-order moments becomes much smaller than the contribution from low-order moments, so high-order moments can be ignored to simplify calculations.
The radial dependence of radiation waves is different from static fields because these waves carry energy away from the system. Since energy must be conserved, simple geometric analysis shows that the energy density of spherical radiation, radius , must scale as . As a spherical wave expands, the fixed energy of the wave must spread out over an expanding sphere of surface area . Accordingly, every time-dependent multipole moment must contribute radiant energy density that scales as , regardless of the order of the moment. Hence, high-order moments cannot be discarded as easily as in static case. Even so, the multipole coefficients of a system generally diminish with increasing order, usually as , so radiation fields can still be approximated by truncating high-order moments.
Time-dependent electromagnetic fields
Time-dependent source distributions can be expressed using Fourier analysis. This allows separate frequencies to be analyzed independently. Charge density is given by
and current density by
For convenience, only a single angular frequency ω is considered from this point forward; thus
The superposition principle may be applied to generalize results for multiple frequencies. Vector quantities appear in bold. The standard convention of taking the real part of complex quantities to represent physical quantities is used.
The intrinsic angular momentum of elementary particles (see Spin (physics)) may also affect electromagnetic radiation from some source materials. To account for these effects, the intrinsic magnetization of the system would have to be taken into account. For simplicity however, these effects will be deferred to the discussion of generalized multipole radiation.
In these formulas c is the speed of light in vacuum, is the Dirac delta function, and is the Euclidean distance from the source point x′ to the evaluation point x. Integrating the time-dependent source distributions above yields
where k=ω/c. These formulas provide the basis for analyzing multipole radiation.
Multipole expansion in near field
The near field is the region around a source where the electromagnetic field can be evaluated quasi-statically. If target distance from the multipole origin is much smaller than the radiation wavelength , then . As a result, the exponential can be approximated in this region as:
See Taylor expansion. By using this approximation, the remaining x′ dependence is the same as it is for a static system, the same analysis applies. Essentially, the potentials can be evaluated in the near field at a given instant by simply taking a snapshot of the system and treating it as though it were static - hence it is called quasi-static. See near and far field and multipole expansion. In particular, the inverse distance is expanded using spherical harmonics which are integrated separately to obtain spherical multipole coefficients.
Multipole expansion in far field: Multipole radiation
At large distances from a high frequency source, , the following approximations hold:
Since only the first-order term in is significant at large distances, the expansions combine to give
Each power of corresponds to a different multipole moment. The first few moments are evaluated directly below.
Electric monopole radiation, nonexistence
The zeroth order term, , applied to the scalar potential gives
where the total charge is the electric monopole moment oscillating at frequency ω. Conservation of charge requires q=0 since
Electric dipole radiation
Electric dipole potential
Integration by parts yields
and the charge continuity equation shows
It follows that
Similar results can be obtained by applying the first-order term, to the scalar potential. The amplitude of the electric dipole moment of the system is , which allows the potentials to be expressed as
Electric dipole fields
or, in a source-free region of space, the relationship between the magnetic field and the electric field can be used to obtain
where is the impedance of free space. The electric and magnetic fields that correspond to the potentials above are
Pure electric dipole power
The dot product with extracts the emission magnitude and the factor of 1/2 comes from averaging over time. As explained above, the cancels the radial dependence of radiation energy density. Application to a pure electric dipole gives
Magnetic dipole radiation
Magnetic dipole potential
The integrand can be separated into symmetric and anti-symmetric parts in n and x′
The second term contains the effective magnetization due to the current and integration gives the magnetic dipole moment.
Notice that has a similar form to . That means the magnetic field from a magnetic dipole behaves similarly to the electric field from an electric dipole. Likewise, the electric field from a magnetic dipole behaves like the magnetic field from an electric dipole. Taking the transformations
Electric quadrupole radiation
Electric quadrupole potential
This corresponds to the traceless electric quadrupole moment tensor . Contracting the second index with the normal vector allows the vector potential to be expressed as
Generalized multipole radiation
As the multipole moment of a source distribution increases, the direct calculations employed so far become too cumbersome to continue. Analysis of higher moments requires more general theoretical machinery. Just as before, a single source frequency is considered. Hence the charge, current, and intrinsic magnetization densities are given by
respectively. The resulting electric and magnetic fields share the same time-dependence as the sources.
Using these definitions and the continuity equation allows Maxwell's equations to be written as
These equations can be combined by taking the curl of the last equations and applying the identity . This gives the vector forms of the non-homogeneous Helmholtz equation.
Solutions of the wave equation
The homogeneous wave equations that describes electromagnetic radiation with frequency in a source-free region have the form.
The wave function can be expressed as a sum of vector spherical harmonics
Where are the normalized vector spherical harmonics and and are spherical Hankel functions. See spherical Bessel functions. The differential operator is the angular momentum operator with the property . The coefficients and correspond to expanding and contracting waves respectively. So for radiation. To determine the other coefficients, the Green's function for the wave equation is applied. If the source equation is
then the solution is:
The Green function can be expressed in vector spherical harmonics.
Note that is a differential operator that acts on the source function . Thus, the solution to the wave equation is:
Electric multipole fields
Applying the above solution to the electric multipole wave equation
The electric field is:
The formula can be simplified by applying the identities
The spherical bessel function can also be simplified by assuming that the radiation length scale is much larger than the source length scale, which is true for most antennas.
is the same as the electric multipole moment in the static case if it were applied to the static charge distribution whereas corresponds to an induced electric multipole moment from the intrinsic magnetization of the source material.
Magnetic multipole fields
Applying the above solution to the magnetic multipole wave equation
The magnetic field is:
As before, the forumula simplifies to:
is the magnetic multipole moment from the effective magnetization while corresponds to the intrinsic magnetization .
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