A Khalifa University research group has investigated the tenuous plasma environments surrounding Mars and Saturn to better understand plasmas in the Earthbound laboratory and beyond our solar system.
Plasma is an interesting research challenge to scientists across disciplines. So much of the universe is made of plasma — it comprises over 99 percent of the visible universe in environments as diverse as stars or distant nebulas, and is manifested in the form of auroras, lightning, and even in such technological applications as neon signs.
Plasma is often called the “fourth state of matter” after solid, liquid, and gas. When gas is sufficiently heated, the molecules get more energetic and excitable, moving around more and more freely. At a high enough temperature, the atoms themselves will break apart, with electrons separating from their nuclei, leaving behind charged particles known as ions amid a swirl of electrons. This is plasma.
Plasmas are found throughout the solar system and beyond: in the solar corona and in the solar wind from the sun, in the magnetospheres of Earth and other planets, in tails of comets, and in the interstellar and intergalactic media.
Dr. Ioannis Kourakis, Associate Professor of Mathematics and Theme Leader for Magnetospheric Modeling at Khalifa University’s Space and Planetary Science Center (SPSC), studies plasma on Earth and in the solar system. His research group recently investigated the tenuous plasma environment surrounding Mars and Saturn in their magnetospheres to better understand the structure of the plasma environment surrounding these planets. They also examined the morphology of plasma waves in the aurora of Earth.
The research group comprises Dr. Kuldeep Singh, Dr. Nikos Lazaridis, Dr. Steffy Varghese and Dr. Hans Huybrighs, assisted by a number of visitors and collaborating students. This group has undertaken further studies of the plasma environment around Venus.
Solitary Waves in the Martian Magnetosphere
Magnetic fields are generated by electric currents flowing in a planet’s liquid outer core. They may extend far into space, where they meet interplanetary magnetic field lines, which are carried throughout the solar system by the solar wind. The region of space containing the planet’s magnetic field is known as its magnetosphere. Plasma in the Earth’s magnetosphere produces the auroras (also known as the Northern Lights, due to their early observation in latitudes near the North Pole) when charged particles interact with the plasma. The magnetosphere shields the planet from the solar wind and ionizing particles and also helps prevent the solar wind from entering the atmosphere over time. Research indicates that Mars lost its atmosphere, which may also be associated with its lack of a strong magnetic field.
The solar magnetic-field lines hitting the planet’s magnetosphere create a shape known as the bow shock. The velocity of the plasma in the solar winds drops as this plasma is forced around the shape of the planet. This creates the classic bow shape with the solar wind curving around the area of impact and extending beyond the sides of the planet.
“Mars does not have an intrinsic magnetic field, but the properties of the atmosphere it does have act as an obstacle to the solar wind, functioning as an induced magnetosphere,” Dr. Kourakis said. “Recently, the Mars Atmosphere and Volatile Evolution (MAVEN) mission provided an excellent opportunity to explore plasma process in the Martian quasi-magnetosphere. Different plasma waves were observed in the upper atmosphere, which suggests that although Mars only possesses an induced mini magnetosphere, it is highly dynamic and capable of generating various plasma waves. Investigating this could provide meaningful information about the different plasma-wave processes operating in such a dynamic region.”
Data from previous missions show that the Martian magnetosheath is filled with compressive waves, which gradually evolve to multiple shocks. These occur when the solar wind hits the magnetosphere, causing shock waves.
Current understanding of the Martian environment suggests the presence of solitary waves in the Martian upper atmosphere, where individual electrostatic wave pulses, known as electrostatic solitary waves, propagated in tenuous plasma move through the magnetosphere. Even though these had not yet been observed, recent analysis of data from the MAVEN mission did detect pulses in the bipolar electric field of the Martian magnetosheath region, i.e. the region of space between the place of impact with the magnetosphere and the bow shock of the planet.
In a paper published in The Astrophysical Journal, a research team led and coordinated by Dr. Kourakis used theory and numerical (data) analysis to identify these structures and to investigate the propagation characteristics of these pulses. Dr. Kourakis collaborated with researchers from the Indian Institute of Geomagnetism to provide an efficient interpretation of the observed data, which suggest that these pulses are actually solitary plasma waves. This is among the first studies to report and model solitary-wave structures in the Martian magnetosheath.
The magnetosheath is an important and dynamic region of turbulent plasma flow that may play a role in the structure of the bow shock and dictate the flow of energetic particles across the magnetosphere. The solitary pulses captured by the MAVEN mission were observed in the magnetosheath, and when modeled with simulations and theory, the data suggest these pulses were ion-acoustic solitary-wave structures — i.e., spatially localized electrostatic waveforms propagating in the plasma. Electrostatic solitary waves are important as they offer an insight to the nonlinear features of localized electrostatic disturbances in a plasma medium. Researchers across disciplines can use these insights to understand the inherent properties of matter (dispersion, nonlinearity, gain/loss mechanisms) for applications in, for example, transmitting information across large distances without distortion or loss in intensity. Understanding solitary waves in space plasmas can offer a new concept of nonlinear-plasma dynamics in space and help us better understand the physics of our universe.
Saturn’s Dusty Plasma
Dusty plasmas are plasmas containing solid particles at the nanometer up to micron scale. These particles acquire an electric charge by collecting electrons and ions from the plasma, in turn affecting the plasma properties. Dusty plasmas are a common occurrence in a number of natural environments, including planetary rings and comet tails, as well as in the technological components used to manufacture semiconductor chips and magnetic-fusion devices.
“Dust is a ubiquitous ingredient in space and astrophysical environments,” Dr. Kourakis said. “The physics of dusty plasma interest researchers because of their essential role in space and astrophysical plasmas, but also in laboratory plasmas in fusion devices, solar cells, and semiconductor chips. Satellite missions have established that space plasmas tacitly deviate from expected behavior, as they possess highly energetic particles (ions or electrons) that affect their dynamics and lead to an “abnormal” (non-Maxwell/Boltzmann type) statistical profile. These have been found in the solar wind and magnetosphere around Earth, and data from the Voyager 1 and 2 spacecraft indicate similar patterns in Saturn’s magnetosphere.”
The Radio and Plasma Wave Science instrument onboard the Cassini mission to Saturn returned data suggesting that charged dust grains in Saturn’s rings interact with the surrounding plasma of Saturn’s magnetosphere. Different charges lead to changes in the generation of waves in the plasma, according to models created by the SPSC team.
Research into space plasmas in our solar system is motivated by the desire to understand how the solar wind interacts with our own planet’s magnetic field, particularly in how geomagnetic storms can impact satellites and endanger astronauts. Solar storms are recognized as a risk factor representing a threat for e.g., telecommunications on Earth, thus establishing space weather and space plasma science as a priority area of research. Additionally, understanding environments in our solar system helps in understanding plasma environments beyond it, and also in the laboratory (where relevant phenomena may be reproduced at a smaller scale).
5 December 2022