# Plasmas/Plasma objects/Io

The image at right represents "[t]he Jovian magnetosphere [magnetic field lines in blue], including the Io flux tube [in green], Jovian aurorae, the sodium cloud [in yellow], and sulfur torus [in red]."
"Io may be considered to be a unipolar generator which develops an emf [electromotive force] of 7 x 10$^5$ volts across its radial diameter (as seen from a coordinate frame fixed to Jupiter)."
"This voltage difference is transmitted along the magnetic flux tube which passes through Io. ... The current [in the flux tube] must be carried by keV electrons which are electrostatically accelerated at Io and at the top of Jupiter's ionosphere."
"Io's high density (4.1 g cm$^{-3}$) suggests a silicate composition. A reasonable guess for its electrical conductivity might be the conductivity of the Earth's upper mantle, 5 x 10$^{-5}$ ohm$^{-1}$ cm$^{-1}$ (Bullard 1967)."
As "a conducting body [transverses] a magnetic field [it] produces an induced electric field. ... The Jupiter-Io system ... operates as a unipolar inductor" ... Such unipolar inductors may be driven by electrical power, develop hotspots, and the "source of heating [may be] sufficient to account for the observed X-ray luminosity".
"The electrical surroundings of Io provide another energy source which has been estimated to be comparable with that of the [gravitational] tides (7). A current of 5 x 10$^6$ A is ... shunted across flux tubes of the Jovian field by the presence of Io (7-9)."
"[W]hen the currents [through Io] are large enough to cause ohmic heating ... currents ... contract down to narrow paths which can be kept hot, and along which the conductivity is high. Tidal heating [ensures] that the interior of Io has a very low eletrical resistance, causing a negligible extra amount of heat to be deposited by this current. ... [T]he outermost layers, kept cool by radiation into space [present] a large resistance and [result in] a concentration of the current into hotspots ... rock resistivity [and] contact resistance ... contribute to generate high temperatures on the surface. [These are the] conditions of electric arcs [that can produce] temperatures up to ionization levels ... several thousand kelvins".
"[T]he outbursts ... seen [on the surface may also be] the result of the large current ... flowing in and out of the domain of Io ... Most current spots are likely to be volcanic calderas, either provided by tectonic events within Io or generated by the current heating itself. ... [A]s in any electric arc, very high temperatures are generated, and the locally evaporated materials ... are ... turned into gas hot enough to expand at a speed of 1 km/s."

## Plasma objects

Plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms (reduce or increase the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions.
For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the "electron density", that is, the number of free electrons per unit volume. The degree of ionization of a plasma is the proportion of atoms that have lost or gained electrons, and is controlled mostly by the temperature. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e., response to magnetic fields and high electrical conductivity). The degree of ionization, α is defined as α = n$_i$/(n$_i$ + n$_a$) where n$_i$ is the number density of ions and n$_a$ is the number density of neutral atoms. The electron density is related to this by the average charge state <Z> of the ions through n$_e$ = <Z> n$_i$ where n$_e$ is the number density of electrons.
"Plasma is the fourth state of matter, consisting of electrons, ions and neutral atoms, usually at temperatures above 10$^4$ degrees Kelvin." "The sun and stars are plasmas; the earth's ionosphere, Van Allen belts, magnetosphere, etc., are all plasmas. Indeed, plasma makes up much of the known matter in the universe."

## X-rays

Weak X-ray signals have been detected from Jupiter's moon Io and from the Io Plasma Torus (IPT), a doughnut-shaped ring of energetic particles that circles Jupiter, shown in the image on the right.
"Io’s atmosphere arises from sublimation of surface SO$_2$ frost, sputtering of the surface by Jovian magnetospheric particles, and volcanic activity, with the latter perhaps be- ing the dominant contributor."
Gases escape from Io and are trapped in an orbit around Jupiter, where they are accelerated to high energies.
Collisions between the particles within the Io Plasma Torus and the surface of Io can account for the observed X-rays to within a factor of approximately three; i.e., are too low by a factor of three.
"The integrated ion energy flux at Europa for H, O, and S above 10 keV (Paranicas et al. 2001, Cooper et al. 2001) is ~ 10$^{10}$ keV/s-cm$^2$. The energetic ion flux at Io is ~5 times smaller, but is less well known than for Europa."
"Chandra X-ray Observatory observations have shown that Io and Europa, and probably Ganymede, emit soft x-rays with 0.25-2.0 keV luminosities ~1-2 MW. This emission is likely to result from bombardment of their surfaces by energetic (> 10 keV) H, O, and S ions from the region of the IPT. The IPT itself emits soft x-rays with a 0.25-1.0 keV luminosity ~0.1 GW. Most of this emission appears at the low end of the energy band, but an unresolved line or line complex is apparent in the spectrum at an energy consistent with oxygen. The origin of the IPT x-ray emission is uncertain, but bremsstrahlung from non-thermal electrons may account for a significant fraction of the continuum x-rays."
"The current-carrying electrons which strike [Io] will produce a flux of keV X-rays. At Earth this flux amounts to roughly 10$^{-7}$ photons cm$^{-2}$ s$^{-1}$, which is about 6 orders of magnitude below the sensitivity of current X-ray surveys [as of 1969]."
"Assuming the volume of a torus with semi-major axis equal to that of Io’s orbit and diameter equal to Jupiter’s equatorial diameter, and number densities in neutral oxygen and in ionized oxygen (mostly O$^+$) of 1000 cm$^{−3}$, we find a photon flux at the Earth of 2.6 × 10$^{−7}$ photons/s–cm$^2$ for the flaring active Sun. Our spectral fits require photon fluxes in the apparent line near 570 eV of ~3 × 10$^{−6}$ photons/s–cm$^2$, about an order of magnitude larger. We conclude that the observed x-ray emission from the IPT is not due to fluorescence excited by solar x-rays."
"The corresponding estimated energy flux at the telescope [is] 4.1 x 10$^{-16}$ erg/s-cm$^2$".
"Further X-ray observations and more detailed modeling are needed to probe more deeply into the origin and properties of the X-ray emission [from Io] and from the Io plasma torus."
The Hubble Space Telescope has observed that the movement of Io through Jupiter's magnetic field causes heating in the Jovian atmosphere.

## Hypotheses

* The volcanoes of Io originate from the current flowing through Io.