Highly charged ions (HCI)
exist in many areas in astrophysics. Anywhere there are extreme conditions of
temperature or plasma activity, they are sure to be found. They exist on the sun,
around Jupiter, around comets, and throughout the galaxy in stars, supernova
remnants, the interstellar medium and of course in
more distant external galaxies. They are important because they provide a
diagnostic of temperature and plasma density in the region in which they exist.
In this article they’re use in the study of comets, the Jupiter-Io system and
the Sun will be discussed as a brief introduction to the field.
Basically, a highly charged ion is a particular
species that has had several electrons added or removed so that it is in a high
charge state. This can occur in regions where there is enough thermal energy to
remove several electrons by collisional processes or the photon energy density
is high enough to produce photoionization. As a result, the particular emissions of
photons from the atom during transitions are affected because of the increase
in the nuclear Coulomb field. For Δn=1 transitions in a hydrogen-like ion (that is, it has been
ionised to the point where it has only one electron) the photon energy scales
as Z2, the atomic number. This brings many normally optical or UV
transitions in hydrogen into the X-ray region (see Fig 1)..
In fact, X-ray emission is often (but not always) indicative of transitions in
HCI. X-ray detection is the main method of analysis of HCI in astrophysics.
Satellites such as
Fig 1: The Z2
scaling law in action on hydrogen-like oxygen.
Comets
It
came as a surprise in 1996 when the ROSAT satellite discovered X-rays being
emitted from both comet Hyakutake and Hale-Bopp. This was not expected as comets are cold, and
emission in this region is only expected from hot (> ~106 K)
plasmas. After the discovery, more comets were investigated and it is now
generally accepted that comets do generally emit X-rays. Several mechanisms
were reported in Science post-1996; the mechanism now commonly accepted
is that of Solar Wind Charge Exchange (SWCX). The solar wind is made up of ions
in high charge states such as O7+, C6+, Ne8+
and Fe12+ and originates in the solar corona at temperatures of the
order of 106 K. These are blown out away from the sun, but when they
arrive at the comet they interact with neutral atoms in the comet’s atmosphere.
It is here charge exchange takes place. For example, suppose an oxygen ion, O7+,
collides with neutral water in the comet. The O7+ will capture an
electron into an excited state from the water forming H2O+
and (O6+)*. The oxygen will then decay
from its excited state and emit a photon in the X-ray region. Such work in
X-rays using HCI is improving the MHD models of cometary
nuclei to lead to a better understanding of the conditions close to the centre
of the cometary nucleus. Recently, the emissions have
been used to study HCI abundance in the solar wind.
Jupiter-Io System
The
environment around Jupiter is one of the most interesting and dynamic in the
solar system. The extreme volcanic activity on Jupiter’s closest Galilean moon,
Io throws material into its orbit. Also, as Io passes through the intense
magnetic field of Jupiter it generates a current of several million amps.
Material is swept up in the orbit of Io and rapidly ionised. Charge exchange processes
similar to what occurs in the case of the comet occurs for low energy ions at
the outer edges of the torus (see fig 2). These
become neutrals and escape from the Jupiter system, as observed by the Cassini
spacecraft during its fly-by. Higher energy ions can however penetrate much
further into the torus and are driven into higher
charge states, in some cases becoming fully stripped of all their electrons.
These then collide with neutrals in the torus, charge
exchange into an excited state and emit photons in the X-ray region. Unresolved
line spectra suggest oxygen ions from SO2 originating on Io are the
cause.
However this does not explain the much higher level of
X-ray flux emanating from the region. Another contributing factor for the
production of continuum X-rays in the region is bremsstrahlung
radiation, where free electrons radiate some energy as they are accelerated
in the electric field close to ions. Even this does not account for the much
higher observed radiated power.
So, much work has to be done to determine other
mechanisms for X-ray production from the Jupiter-Io torus.
Fig 3. The Jupiter – Io plasma
torus http://www.planetaryexploration.net/jupiter/io/io_plasma_torus.html
Solar Diagnostics
Much
of the X-ray radiation emitted from the sun is from the solar corona. Here the
plasma density is low at ~108 cm-3, and the temperature
at 2x106 K. Due to the extreme temperature many atoms are ionised to
high charge states, seen as spectral lines, and UV/X-ray continuum from bremsstrahlung radiation. Investigating the solar plasma is
incredibly important, as it provides a laboratory for hot plasmas and highly
charged ions.
In this particular environment, it is
important that the sun is studied; it is the only long term self-sustaining
nuclear fusion reaction and so has implications for the development of high
temperature plasmas for nuclear fusion here on earth. By looking at regions
where there are highly charged ions such as Fe25+ in solar flares,
temperature, electron density, collisional and radiative parameters can be defined. Fe25+ emits
X-ray photons due to its very high charge state; these can be studied
spectroscopically to determine these parameters Here
on earth, we can then attempt to mimic these conditions to better understand
nuclear fusion.
In terms of solar diagnostics, an
equation relating temperature, electron density and the relative levels of
ionisation (the Saha equation)is used in conjunction with the spectral lines to determine
the temperature and the electron density of a region, which is very important.
For example, the solar wind is emanated from the corona. The nature of the
corona can be determined from the spectral line ratios of excited states of
highly charged atoms. For a particular species, e.g. the highly charged OVI
hydrogen-like ion, the relative ratios in the brightness of say, the n=3 to
ground state and the n=2 to ground state transitions will give a diagnostic of
the temperature changes of the region of where the ion exists. So very sensitive measurements are possible. The same is
true of determining the density of electrons in the area. In this case, two
lines are chosen so as to be around the same level, one of them having an
excited state lifetime which is very long (a metastable state). Making some assumptions and using the Saha
equation allows changes in the density to be monitored using the ratio of line
fluxes of the metastable state to ground state transition and the normal state
to ground state transition. Such changes are fundamental to understanding the
internal workings of the sun.
So highly charged ions are very important in both very violent
regions in astrophysics and very cool ones. Their role
as a diagnostic is without doubt one of the most powerful in the field.
References
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X-ray Emission from Io, Europa, and the Io Plasma Torus
ApJ 572 1077
(2002)
3. Chandra X-ray Observatory Website Chandra.harvard.edu/
4. Silver,E.
Laboratory Astrophysics Survey of Key
X-ray Diagnostic Lines Using a Microcalorimeter on an
EBIT
5. Behar,E.
;
6. Brown, G.V. et al. Measurements of Atomic Parameters of Highly Charged Ions for
Interpreting Astronomical Spectra (2001)
7. Cravens, T.E. X-ray
Emission from Comets Science 296 (2002)
8. Hoekstra, R.; Morgenstern, R. Exciting Highly charged Ions in the Universe
9. Gillaspy,
10. Chutjian,A.
Collision Phenomena involving Highly
Charged Ions in Astronomical Objects