Magnetic switchbacks are sudden reversals in the magnetic field of the solar wind. [1] They can also be described as traveling disturbances in the solar wind that caused the magnetic field to bend back on itself. They were first observed by the NASA-ESA mission Ulysses , the first spacecraft to fly over the Sun's poles. [2] [3] NASA's Parker Solar Probe and NASA/ESA Solar Orbiter both observed switchbacks.
Magnetic (or solar) switchback is a rapid polarity reversals of the radial heliospheric magnetic field. [4] These events have been termed "switchbacks", when referring to the change in magnetic field direction, or "velocity spikes", when referring to the sharp increase in solar wind speed. [3]
The switchbacks generate heat that warms solar corona. [5]
Helios 1 and 2 spacecraft observed sudden reversals of the Sun's magnetic field in 1970s. [6] Magnetic switchbacks were then observed by the Ulysses in 1995-1996, during the solar minimum, when the spacecraft detected numerous radial magnetic field polarity inversions. Similar structures were then observed by near-Earth heliospheric spacecraft such as Advanced Composition Explorer. Parker Solar Probe (PSP) observed first switchback on November 6, 2018. Similar effects were observed at distances around and below 0.3 AU, 1 AU, and up to 2.9 AU, and, as noted by Fedorov et al, "the question of whether all such observations relate to the same phenomenon is still open." [4]
On 27 September 2020, ESA/NASA Solar Orbiter (SolO) sampled a solar wind stream magnetically connected to a southern hemisphere coronal hole, while it was at 0.98 AU from Sun, and observed a fast solar wind with strong fluctuations of the magnetic field. The structures observed by SolO may effectively stand as the surviving remains of the switchbacks created near Sun and also observed by PSP. [4]
Given the phase of the solar cycle, if PSP was in the southern magnetic hemisphere, the solar wind magnetic field should always have had a magnetic polarity oriented inward toward the Sun. Instead, PSP observed thousands of intervals, ranging in duration from seconds to tens of minutes where the speed of the solar wind flow suddenly jumps and the magnetic field orientation rotates by nearly 180° in the most extreme cases, before returning just as quickly to the original solar wind conditions. [3]
SolO has found compelling clues as to the origin of magnetic switchbacks during its closest pass by the sun on 25 March 2022. Using the data of the Solar Orbiter Daniele Telloni and Gary Zank and their team came to the conclusion that the theory based on Ulysses data is correct, they "proved that switchbacks occur when there is an interaction between a region of open field lines and a region of closed field lines". [6] [7]
One theory, based on the Ulysses data, suggests that switchbacks are the result of a clash between open and closed magnetic fields. When an open magnetic field line brushes against a closed magnetic loop, they can reconfigure in a process called interchange reconnection – an explosive rearrangement of the magnetic fields that leads to a switchback shape. The open line snaps onto the closed loop, cutting free a hot burst of plasma from the loop, while "gluing" the two fields into a new configuration. That sudden snap throws an S-shaped kink into the open magnetic field line before the loop reseals. The Parker Solar Probe observed its first switchback on November 6, 2018. The observed switchback was close to the developed model. [2] [8]
A second theory agrees on the import of interchange reconnection, but differs on the nature of switchbacks themselves. Instead of viewing switchbacks as a kink in a magnetic field line, the second theory suggests it is the signature of a kind of magnetic structure, called a flux rope. [2] [9]
Another theory suggests that switchbacks form naturally as the solar wind expands into space. [2] [10]
The switchbacks, essentially S-shaped kinks in the magnetic field lines streaming from the Sun, seem to arise from a reconfiguration of open and looped magnetic field lines already in the Sun's atmosphere. When an open magnetic field line encounters a closed magnetic loop they can undergo a process called interchange reconnection. This allows the open magnetic field line to snap into the loop, and allows one side of the formerly closed magnetic loop to connect to solar magnetic field extending outwards into the solar system. This process would create an outward-flowing S-shaped kink in the newly formed open magnetic field line — a shape that tracks with the switchbacks measured by Parker Solar Probe. [1]
A corona is the outermost layer of a star's atmosphere. It is a hot but relatively dim region of plasma populated by intermittent coronal structures known as solar prominences or filaments.
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of particle species found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes 58Ni, 60Ni, and 62Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.
Ulysses was a robotic space probe whose primary mission was to orbit the Sun and study it at all latitudes. It was launched in 1990 and made three "fast latitude scans" of the Sun in 1994/1995, 2000/2001, and 2007/2008. In addition, the probe studied several comets. Ulysses was a joint venture of the European Space Agency (ESA) and the United States' National Aeronautics and Space Administration (NASA), under leadership of ESA with participation from Canada's National Research Council. The last day for mission operations on Ulysses was 30 June 2009.
A coronal mass ejection (CME) is a significant ejection of plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established.
In plasma physics, an Alfvén wave, named after Hannes Alfvén, is a type of plasma wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.
Magnetic reconnection is a physical process occurring in electrically conducting plasmas, in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnection involves plasma flows at a substantial fraction of the Alfvén wave speed, which is the fundamental speed for mechanical information flow in a magnetized plasma.
The Solar Orbiter (SolO) is a Sun-observing probe developed by the European Space Agency (ESA) with a National Aeronautics and Space Administration (NASA) contribution. Solar Orbiter, designed to obtain detailed measurements of the inner heliosphere and the nascent solar wind, will also perform close observations of the polar regions of the Sun which is difficult to do from Earth. These observations are important in investigating how the Sun creates and controls its heliosphere.
Cluster II was a space mission of the European Space Agency, with NASA participation, to study the Earth's magnetosphere over the course of nearly two solar cycles. The mission was composed of four identical spacecraft flying in a tetrahedral formation. As a replacement for the original Cluster spacecraft which were lost in a launch failure in 1996, the four Cluster II spacecraft were successfully launched in pairs in July and August 2000 onboard two Soyuz-Fregat rockets from Baikonur, Kazakhstan. In February 2011, Cluster II celebrated 10 years of successful scientific operations in space. In February 2021, Cluster II celebrated 20 years of successful scientific operations in space. As of March 2023, its mission was extended until September 2024. The China National Space Administration/ESA Double Star mission operated alongside Cluster II from 2004 to 2007.
Solar physics is the branch of astrophysics that specializes in the study of the Sun. It intersects with many disciplines of pure physics and astrophysics.
Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission began in February 2007 as a constellation of five NASA satellites to study energy releases from Earth's magnetosphere known as substorms, magnetic phenomena that intensify auroras near Earth's poles. The name of the mission is an acronym alluding to the Titan Themis.
In solar physics, a coronal loop is a well-defined arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes. Coronal loops begin and end at two footpoints on the photosphere and project into the transition region and lower corona. They typically form and dissipate over periods of seconds to days and may span anywhere from 1 to 1,000 megametres in length.
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Coronal seismology is a technique of studying the plasma of the Sun's corona with the use of magnetohydrodynamic (MHD) waves and oscillations. Magnetohydrodynamics studies the dynamics of electrically conducting fluids - in this case the fluid is the coronal plasma. Observed properties of the waves (e.g. period, wavelength, amplitude, temporal and spatial signatures, characteristic scenarios of the wave evolution, combined with a theoretical modelling of the wave phenomena, may reflect physical parameters of the corona which are not accessible in situ, such as the coronal magnetic field strength and Alfvén velocity and coronal dissipative coefficients. Originally, the method of MHD coronal seismology was suggested by Y. Uchida in 1970 for propagating waves, and B. Roberts et al. in 1984 for standing waves, but was not practically applied until the late 90s due to a lack of necessary observational resolution. Philosophically, coronal seismology is similar to the Earth's seismology, helioseismology, and MHD spectroscopy of laboratory plasma devices. In all these approaches, waves of various kind are used to probe a medium.
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Supra-arcade downflows (SADs) are sunward-traveling plasma voids that are sometimes observed in the Sun's outer atmosphere, or corona, during solar flares. In solar physics, arcade refers to a bundle of coronal loops, and the prefix supra indicates that the downflows appear above flare arcades. They were first described in 1999 using the Soft X-ray Telescope (SXT) on board the Yohkoh satellite. SADs are byproducts of the magnetic reconnection process that drives solar flares, but their precise cause remains unknown.
The Alfvén surface is the boundary separating a star's corona from the stellar wind defined as where the coronal plasma's Alfvén speed and the large-scale stellar wind speed are equal. It is named after Hannes Alfvén, and is also called Alfvén critical surface, Alfvén point, or Alfvén radius. In 2018, the Parker Solar Probe became the first spacecraft that crossed Alfvén surface of the Sun.
This article incorporates text from this source, which is in the public domain :Hatfield, Miles (29 April 2020). "New Insight Into Parker Solar Probe's Early Observations". NASA. This article incorporates text from this source, which is in the public domain :Hatfield, Miles (8 March 2021). "Switchbacks Science: Explaining Parker Solar Probe's Magnetic Puzzle". NASA. Retrieved 31 July 2022.
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