University of Kansas X-Ray Emission in the Solar System

(DRAFT) X-Ray Emission from Comets and Planets T. E. Cravens

Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.

Introduction

An X-ray is a high energy photon with an energy approximately 0.1 keV or higher. Impact excitation of atoms or ions by fast charged particles followed by photon emission produces X-rays if the energies involved are high enough.

For example, consider hydrogen-like ions, which are fully stripped except for one electron. The bound state energies are given by E_n = - Z²13.6 eV / n² where n is the principle quantum number and Z is the charge of the nucleus. The energy of the transition from the n = 2 excited level from the n = 1 ground state is 10.2 Z² eV (which is 10.2 eV for H and 367 eV for C⁺⁵ ions). The excited ion/atom de-excites by emitting a 10.2 eV photon for H (Lyman alpha photons in the extreme ultraviolet) or 367 eV for C⁺⁵ (soft X-ray photons). X-ray production, as opposed to ultraviolet or visible radiation production, results when the transition energy is approximately 100 eV or higher.

As well as line emission, free-free (called bremsstrahlung) and bound-free transitions (going one way this is photoionization and the other this is radiative recombination) are possible and will result in continuum photon emission. The formula for the photon intensity due to bremsstrahlung for a Maxwellian electron distribution is given by (Northrop et al., 1997):

where N_e is the electron density, N_i the target ion density, Z the charge of the ion, T the temperature, n the photon frequency, h Planck's constant, and g is a statistical factor. Note that to produce X-ray photons (e.g., 1 keV) in this way requires charged particles (generally electrons) with energies in excess of the photon energy. Once created collisionally as just discussed, it is then possible for the X-ray photons to be scattered from a medium or to be absorbed by a medium followed by emission at lower energies (perhaps X-rays). This latter process is called fluorescence.

X-rays are typically produced in a hot gas, or plasma, where the particle energies are high enough to either excite X-ray line emission or to generate X-ray bremsstrahlung photons. Furthermore, in a hot plasma the ions will be more likely to be highly stripped, and have higher energy transitions than do less stripped ions. Examples of hot plasmas known to produce X-rays include the solar corona, the interstellar medium (both the intercloud medium and supernova remnants), and accretion disks surrounding "X-ray binaries." The plasma in the solar corona has temperatures of 1 - 3 x 10⁶ K, and line emission from highly stripped species is observed (Goldberg, 1969; Zirin, 1988). Figure 1 shows the temperature in the solar atmosphere and some species whose spectral signatures are measured.

Next: Solar System X-Rays

References

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Last modified January 7, 2004 Tizby Hunt-Ward tizby@ku.edu