Bow shock

In astrophysics, a bow shock triggers when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the border at which the speed of the stellar wind suddenly drops as a result of its approach to the magnetopause. For stars, this border is usually the edge of the astrosphere, where the stellar wind meets the interstellar medium.

LL Orionis bow shock in Orion nebula. The star's wind smashes together with the nebula flow. Hubble, 1995

Description

The defining judging requirement of a shock wave is that the bulk velocity (speed) of the plasma drops from "supersonic" to "subsonic", where the speed of sound c_s is defined by $c_{s}^{2}=\gamma p/\rho$ where $\gamma$ is the ratio of specific heats, $p$ is the pressure, and $\rho$ is the density of the plasma.

A common difficulty in astrophysics is the presence of a magnetic field. For example, the charged particles making up the solar wind follow spiral paths along magnetic field lines. The velocity (speed) of each particle as it moves 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 about the speed of sound. At the bow shock, the bulk forward velocity of the wind (which is the part of the velocity parallel to the field lines about which the particles moving in a spiral path.) drops below the speed at which the particles are moving at a spiral path.

Around the Earth

The best-studied example of a bow shock is that it occurs where the Sun's wind encounters Earth's magnetopause, although bow shocks occur around all planets, also unmagnetized, such as Mars and Venus and magnetized, such as Jupiter or Saturn. Earth's bow shock is about 17 kilometres thick and located about 90,000 km from the planet.

At comets

Bow shocks form at comets as a result of the interaction between the solar wind and the cometary ionosphere. Far away from the Sun, a comet is an icy boulder without an atmosphere. As it approaches the Sun, the heat of the sunlight causes gas to be released from the cometary nucleus, creating an atmosphere called a coma. The coma is partially ionized by the sunlight, and when the solar wind passes through this ion coma, the bow shock appears.

The first observations were made in the 1980s and 90s as several spacecraft flew by comets which were 21P/Giacobini–Zinner, Halley's Comet, and 26P/Grigg–Skjellerup. It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at for example Earth. These observations were all made near perihelion when the bow shocks already were fully developed.

The Rosetta spacecraft followed comet 67P/Churyumov–Gerasimenko from far out in the solar system, at a distance of 3.6 AU from the Sun, in toward perihelion at 1.24 AU, and back out again. This allowed Rosetta to observe the bow shock as it formed when the outgassing increased during the comet's journey toward the Sun. In this early state of development the shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.

Around the Sun

For many decades, the solar wind has been thought to form a bow shock at the edge of the heliosphere, where it collides with the surrounding interstellar medium. Moving away from the Sun, the point where the solar wind flow becomes subsonic is the termination shock, the point where the interstellar medium and solar wind pressures balance is the heliopause, and the point where the flow of the interstellar medium becomes subsonic would be the bow shock. This solar bow shock was thought to lie at a distance around 230 AU from the Sun, more than twice the distance of the termination shock as encountered by the Voyager spacecraft.

However, data obtained in 2012 from NASA's Interstellar Boundary Explorer Spacecraft indicates the lack of any solar bow shock. Along with present results from the Voyager spacecraft, these findings have motivated some theoretical improvement; current thinking is that formation of a bow shock is prevented, at least in the galactic region through which the Sun is passing, by a combination of the strength of the local interstellar magnetic-field and of the relative velocity of the heliosphere.

Around other stars

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

The bow shock that was seen around R Hydrae

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.

Around massive stars

If a massive star is a runaway star, it can form an infrared bow-shock that is detectable in 24 μm and sometimes in 8μm of the Spitzer Space Telescope or the W3/W4-channels of the satelite, WISE. In 2016 Kobulnicky et al. did create the largest spitzer/WISE bow-shock catalog to date with 709 bow-shock candidates. To get a larger bow-shock catalog The Milky Way Project (a Citizen Science project) aims to map infrared bow-shocks in the galactic plane. This larger catalog will help to understand the stellar wind of massive stars.

Zeta Ophiuchi is the most famous bow shock of a massive star. Image is from the Spitzer Space Telescope.

The closest stars with bow-shocks are:

Name	Distance (light years)	Spectral type	Belongs to
Beta Crucis	277 ly	B1IV	Lower Centaurus–Crux subgroup
Alpha Muscae	316 ly	B2IV	Lower Centaurus–Crux subgroup
Alpha Crucis	322 ly	B1V+B0.5IV	Lower Centaurus–Crux subgroup
Zeta Ophiuchi	365 ly	O9.2IVnn	Upper Scorpius subgroup
Theta Carinae	456 ly	B0Vp	IC 2602
Tau Scorpii	472 ly	B0.2V	Upper Scorpius subgroup
Delta Scorpii	489 ly	B0.3IV	Upper Scorpius subgroup
Epsilon Persei	603 ly	B1.5III
Sigma Scorpii	697 ly	O9.5(V)+B7(V)	Upper Scorpius subgroup

Magnetic draping effect

A similar effect, known as the magnetic draping effect, occurs when a super-Alfvenic plasma flow impacts an unmagnetized object such as what happens when the solar wind reaches the ionosphere of Venus: the flow deflects around the object draping the magnetic field along the wake flow.

The condition for the flow to be super-Alfvenic means that the relative velocity between the flow and object, $v$, is larger than the local Alfven velocity $V_{A}$ which means a large Alfvenic Mach number: $M_{A}\gg 1$. For unmagnetized and electrically conductive objects, the ambient field creates electric currents inside the object, and into the surrounding plasma, such that the flow is deflected and slowed as the time scale of magnetic dissipation is much longer than the time scale of magnetic field advection. The induced currents in turn generate magnetic fields that deflect the flow creating a bow shock. For example, the ionospheres of Mars and Venus provide the conductive environments for the interaction with the solar wind. Without an ionosphere, the flowing magnetized plasma is absorbed by the non-conductive body. The latter occurs, for example, when the solar wind interacts with Moon which has no ionosphere. In magnetic draping, the field lines are wrapped and draped around the leading side of the object creating a narrow sheath which is similar to the bow shocks in the planetary magnetospheres. The concentrated magnetic field increases until the ram pressure becomes comparable to the magnetic pressure in the sheath:

$\rho _{0}v^{2}={\frac {B_{0}^{2}}{2\mu _{0}}},$

where $\rho _{0}$ is the density of the plasma, $B_{0}$ is the draped magnetic field near the object, and $v$ is the relative speed between the plasma and the object. Magnetic draping has been detected around planets, moons, solar coronal mass ejections, and galaxies.