In astrophysics, bow shocks are shock waves that form in areas where the density and pressure change greatly because of stellar wind blowing from a star. A bow shock happens when the magnetosphere of an astrophysical object interacts with surrounding plasma, such as the solar wind. For Earth and other magnetized planets, this boundary is where the speed of the stellar wind suddenly decreases as it reaches the magnetopause. For stars, this boundary is usually the edge of the astrosphere, where the stellar wind meets the interstellar medium.

Description

The main feature of a shock wave is that the overall speed of the plasma changes from "supersonic" to "subsonic." The speed of sound in plasma, represented as $ c_s $, is calculated using the formula $ 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 challenge in astrophysics is the presence of magnetic fields. For example, charged particles in the solar wind move in spiral paths along magnetic field lines. The speed at which each particle circles around a field line can be compared to the average thermal speed of particles in a typical gas. In a typical gas, the average thermal speed is similar to the speed of sound. At the bow shock, the overall forward speed of the solar wind (the part of the velocity that moves parallel to the magnetic field lines) becomes slower than the speed at which the particles circle around the field lines.

Around the Earth

The most well-known example of a bow shock happens where the solar wind meets Earth's magnetopause. Bow shocks also form around all planets, whether they have strong magnetic fields, like Jupiter or Saturn, or not, like Mars and Venus. Earth's bow shock is about 17 kilometers (11 miles) thick and located approximately 90,000 kilometers (56,000 miles) from the planet.

At comets

Bow shocks form at comets when the solar wind interacts with the ionized gas around the comet. Far from the Sun, a comet is a frozen rock with no atmosphere. As it moves closer to the Sun, sunlight heats the comet, causing gas to escape from its surface. This gas creates a temporary atmosphere called a coma. Some of the gas in the coma becomes charged, or ionized, and when the solar wind passes through this ionized coma, a bow shock forms.

Scientists first observed bow shocks in the 1980s and 1990s as spacecraft passed by comets 21P/Giacobini–Zinner, 1P/Halley, and 26P/Grigg–Skjellerup. These observations showed that comet bow shocks are wider and more gradual compared to the sharp bow shocks seen around Earth. These studies happened when the comets were at their closest point to the Sun, where the bow shocks were fully developed.

The Rosetta spacecraft studied comet 67P/Churyumov–Gerasimenko as it traveled from a distance of 3.6 AU from the Sun to 1.24 AU, then back out. This allowed Rosetta to observe how the bow shock formed as the comet released more gas while moving closer to the Sun. In this early stage, the bow shock was called the "infant bow shock." The infant bow shock is not symmetrical and, compared to the distance from the comet’s center, is wider than fully developed bow shocks.

Around the Sun

For many years, scientists believed that the solar wind forms a bow shock at the edge of the heliosphere, where it meets the surrounding interstellar medium. As the solar wind moves away from the Sun, it becomes subsonic at the termination shock. The point where the solar wind and interstellar medium pressures balance is called the heliopause. The bow shock would be where the interstellar medium's flow also becomes subsonic. Previously, scientists thought the bow shock was located about 230 AU from the Sun, more than twice the distance of the termination shock as measured by the Voyager spacecraft.

In 2012, data from NASA's Interstellar Boundary Explorer (IBEX) showed that no solar bow shock exists. Supporting evidence from the Voyager spacecraft led scientists to revise their theories. Current understanding suggests that the bow shock does not form in the region of the galaxy where the Sun is located because of the strength of the local interstellar magnetic field and the speed at which the heliosphere moves through space.

Around other stars

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

Bow shocks are also often seen in Herbig Haro objects, where a focused stream of gas and dust from a young star interacts with the space between stars, creating bright bow shocks visible in optical light.

The Hubble Space Telescope took images of bow shocks made of dense gas and plasma in the Orion Nebula.

Bow shocks also form around six Cataclysmic variable stars (CVs) that have bright disks of material around them. These disks push fast streams of gas into space. The six CVs are: BZ Camelopardalis, V341 Ara, SY Cancri, ASASSN-V J205457.73+515731.9, LS Pegasi, and FY Vulpeculae.

Recent discoveries were partly made with the help of amateur astronomers. These nebulae are often found in areas where oxygen has lost two electrons and are located within larger H-alpha nebulae.

If a massive star is moving rapidly through space or if the space around it moves relative to the star, it can create an infrared bow shock that is visible in the 24-micron and sometimes 8-micron wavelengths of the Spitzer Space Telescope or the W3/W4-channels of WISE. In 2016, Kobulnicky et al. made the largest catalog of bow shocks using Spitzer and WISE data, listing 709 possible candidates. To expand this catalog, The Milky Way Project (a Citizen Science project) mapped infrared bow shocks in the Milky Way's plane. This effort identified 311 new candidates. This larger catalog will help scientists study the winds produced by massive stars.

The closest stars with infrared bow shocks are located within 130 parsecs.

Most of them belong to the Scorpius–Centaurus association.

Magnetic draping effect

A similar effect, called the magnetic draping effect, happens when a plasma flow that moves faster than the Alfvén speed (called super-Alfvénic) hits an object that is not magnetized, like when the solar wind reaches Venus's ionosphere. The flow bends around the object, wrapping the magnetic field along the wake behind it.

For the flow to be super-Alfvénic, the speed of the plasma (v) must be greater than the local Alfvén speed (V_A), which means the Alfvénic Mach number (M_A) is much larger than 1 (M_A ≫ 1). When the object is not magnetized but can conduct electricity, the surrounding magnetic field creates electric currents inside the object and in the surrounding plasma. These currents cause the flow to bend and slow down because the time it takes for the magnetic field to spread is much longer than the time it takes for the field to move with the plasma. These currents also create new magnetic fields that push the flow, forming a bow shock. For example, the ionospheres of Mars and Venus allow this interaction with the solar wind because they are conductive. If an object has no ionosphere, like the Moon, the solar wind's magnetized plasma is absorbed directly.

In magnetic draping, magnetic field lines wrap around the front side of the object, forming a narrow layer called a sheath. This sheath is similar to bow shocks found around planets. The magnetic field becomes stronger in this sheath until the pressure from the moving plasma (ram pressure) is about equal to the pressure from the magnetic field (magnetic pressure):

where ρ₀ is the plasma density, B₀ is the magnetic field near the object, and v is the speed of the plasma relative to the object. Magnetic draping has been observed around planets, moons, solar coronal mass ejections, and galaxies.