### Bow Shock

For other uses, see bow shock (aerodynamics).

A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium. In a planetary magnetosphere, the bow shock is the boundary at which the speed of the solar wind abruptly drops as a result of its approach to the magnetopause.[1] The best-studied example of a bow shock is that occurring where the solar wind encounters the Earth's magnetopause, although bow shocks occur around all magnetized planets. The Earth's bow shock is about 17 kilometres (11 mi) thick[2] and located about 90,000 kilometres (56,000 mi) from the Earth.[3]

For several decades, the solar wind from the Sun has been thought to form a bow shock when it collides with the surrounding interstellar medium. This long-held belief was called into question in 2012 when data from the Interstellar Boundary Explorer (IBEX) found the Solar System to be moving more slowly through the interstellar medium than previously believed.[4] This new finding suggests that beyond the termination shock and heliopause surrounding the Solar System there may be no, or very little bow shock.[4]

## Description

The defining criterion is that the bulk velocity of the fluid (in this case, the solar wind) drops from "supersonic" to "subsonic", where the speed of sound in plasma physics is defined as

$c_s^2 = \gamma p/ \rho$

where cs is the speed of sound, $\gamma$ is the ratio of specific heats, p is the pressure, and $\rho$ is the density of the plasma.

The particles making up the solar wind follow spiral paths along magnetic field lines. The velocity of each particle as it gyrates around a field line can be treated similarly to a thermal velocity in an ordinary gas, and in an ordinary gas, the mean thermal velocity is roughly the speed of sound. At the bow shock, the bulk forward velocity of the wind (which is the component of the velocity parallel to the field lines about which the particles gyrate) drops below the speed at which the particles are corkscrewing.

Bow shocks are also a common feature in Herbig Haro objects, in which a much stronger collimated outflow of gas and dust from the star interacts with the interstellar medium, producing bright bow shocks that are visible at optical wavelengths.

The following images show further evidence of bowshock existence from dense gases and plasma in the Orion Nebula.

## The Sun

It was hypothesised that the Sun also has a bow shock as it travels through the interstellar medium. This will occur if the interstellar medium is moving supersonically towards the Sun, since the Sun's solar wind is moving supersonically away from the Sun. The point where the interstellar medium becomes subsonic is the bow shock; the point where the interstellar medium and solar wind pressures balance is at the heliopause; the point where the solar wind becomes subsonic is the termination shock. According to Robert Nemiroff and Jerry Bonnell of NASA, the solar bow shock may lie at around 230 AU[5] from the Sun. However, data in 2012 from NASA's Interstellar Boundary Explorer (IBEX) and corroborated with results from the Voyagers, has found that due to refinements in the relative speed of the heliosphere and the local interstellar magnetic field strength it is believed the heliosphere is prevented from forming a bow shock in the region of our galaxy the Sun is currently passing through.

## In the infrared

In 2006, a far infrared bow shock was detected near the AGB star R Hydrae.[6]