During World War I, when the soldiers used long telephone wires in order to listen to their enemy, they occasionally picked up sounds that were similar to the sounds people make when they whistle. Because the initial intention of those long telephone wires was not to pick up these sounds, these strange noises confused the soldiers, therefore they let researchers analyze the source of those signals. After researching, scientists took a spectrogram from the sounds soldiers discovered, then they concluded that the sounds were from plasma. Later on, the researchers named these noises the whistlers.

Plasmas are a state of matter in which consists mostly of evenly positively charged protons and negatively charged electrons(NASA). Plasmas are the most common phase of ordinary matter in the universe. In fact, there are plasmas in our solar system. In the Sun’s atmosphere, because the temperature is very high, atoms are moving and colliding with each other very fast. This collision can cause a large portion of the atoms to be ionized (loss an electron), in which the gas acts as a plasma. Not only is plasma very common in the Sun, but it also is common on Earth. There are plasmas in lightning, the Earth’s ionosphere, and the Earth’s magnetosphere. Because of the detection of the whistlers in WWI, people can use whistler-mode waves to detect plasmas or any other ionized gas. As it turns out, this whistler-mode wave can be helpful in the field of astronomy.  With the understanding of the whistler-mode wave, scientists can gain more understanding of Earth’s near-space environment and our solar system. 

To study the Earth’s near-space environment, the Van Allen Probes mission from NASA uses the whistler-mode wave to study the two extreme and dynamic regions of space known as the Van Allen Radiation Belts that surround Earth. These two belts are concentric rings that filed with high energy particles that can sometimes shoot down into Earth’s atmosphere. The radiation belts can swell and shrink over time as part of a larger space weather system driven by energy and can affect our entire solar system(Zell). Since space weather can affect space exploration, it is important for people to understand these radiation belts which can affect space weather directly. The Van Allen Probes used the newest instruments to detect the whistler-mode wave from the radiation belts. The Electric and Magnetic Field Instruments Suite and Integrated Science can differentiate processes that help provide energy and speed to the particles in the belt or lead to their ejection from the belts. The instrument focuses specifically on the magnetic fields and plasma waves(Garner). During a geomagnetic storm, the chance of astronauts hit by damaging particles from the radiation belts increases. The Van Allen Probes can develop predictive models to prevent astronauts’ damage. In 2019, the Van Allen Probes mission was completed. During this seven-year-long research, scientists gained many new understandings of the Van Allen Radiation Belts. 

With the help of whistler-mode waves, we can not only study Earth’s near-space environment but can also confirm and validate laboratory studies. In 2017, a team at the University of California, Los Angeles (UCLA) Large Plasma Device (LAPD) developed the appropriate laboratory equipment to excite and image chorus wave growth. Later, they used information from their laboratory observations to develop predictions compared with the observations from Van Allen Probes mission. The observation from space validated the prediction from LAPD. This understanding along with the mission of Van Allen Probes will help humans to create whistler-mode waves that can decrease the damaging effects of high radiation fluxes that can occur either naturally or from other countries. In the future, the team plans to use weak waves to inject into the solar system to determine if the free energy in the plasma will nonlinearly amplify them(NASA).

Since scientists gained lots of success to apply whistler-mode waves near Earth, scientists started to look deeply into the solar system. Because of our ionosphere block Saturn’s radiation, so the scientist needs to let a spacecraft to travel near Saturn and listen from there. The Cassini mission from NASA uses the spacecraft to detect and study the radio and plasma waves around Saturn to get a better understanding of Saturn’s relationship with its moons and rings. Cassini used Cassini’s Plasma Spectrometer (CAPS) to detect and analyze plasma, which helped scientists to understand the composition, density, flow, velocity, and temperature of ions and electrons in Saturn’s magnetosphere.  By measuring the plasmas in Saturn, Cassini discovered that most of the ions in Saturn come from water ejected by Saturn’s moon Enceladus. Once a particle is charged, it responds to the rotation of the planet's magnetosphere, so the particles from Enceladus begin to circling Saturn under the influence of the magnetic field (NASA). With the help of our understanding of whistler-mode waves, this Cassini mission opens up our understanding of Saturn and its system.

With a better comprehension of one gas giant planet in our solar system, scientists who had the aid of whistler-mode waves aimed their goal to another gas giant, Jupiter. Scientists also use whistler-mode waves to understand Jupiter. From the flyby survey of Voyager 1, scientists understand how Jupiter interacts with its moons. Io is the third-largest moon of Jupiter, also it is the most volcanically moon in Jupiter. The discovery of Io’s volcanism shows that it can affect the whole Jovian system. Also, Io appears to be the primary source of matter that pervades the Jovian magnetosphere. From Nature, researchers conclude that the generated whistler-mode waves from Jupiter’s moon are capable of significantly modifying the energetic particle environment, accelerating particles to very high energies, or producing depletions in phase space density. Observations of Jupiter’s magnetosphere provide a unique opportunity to observe how objects with an internal magnetic field can interact with particles trapped in magnetic fields of larger-scale objects (Shprits, Y. Y., et al.). With whistler-mode waves, scientists can have more understanding of Jupiter and its moons. 

Even though we used the whistler-mode wave to understand more about our neighborhood in the solar system, there is still more information that we need to discover to understand these planets fully. From the accidental discovery of the soldiers from WWI to modern technology, the whistler-mode wave helps humans a lot. In the future, the whistler-mode wave will definitely increase its usage in astronomy for further understanding. 

Work Cited

[1]. “Plasma.” NASA, NASA, www-spof.gsfc.nasa.gov/Education/wplasma.html.

[2]. Zell, Holly. “Van Allen Probes Mission Overview.” NASA, NASA, 18 Mar. 2015, www.nasa.gov/mission_pages/rbsp/mission/index.html.

[3].Garner, Rob. “Van Allen Probes - Spacecraft and Instruments.” NASA, NASA, 18 May 2015, www.nasa.gov/mission_pages/rbsp/spacecraft/index.html.

[4].“Exciting Chirping Whistler Waves: From the Laboratory to Space.” NASA, NASA, science.nasa.gov/technology/technology-stories/exciting-chirping-whistler-waves.

[5]. “Cassini Plasma Spectrometer (CAPS).” NASA, NASA, solarsystem.nasa.gov/missions/cassini/mission/spacecraft/cassini-orbiter/cassini-plasma-spectrometer/.

[6]. Shprits, Y. Y., et al. “Strong Whistler Mode Waves Observed in the Vicinity of Jupiter's Moons.” Nature News, Nature Publishing Group, 7 Aug. 2018, www.nature.com/articles/s41467-018-05431-x.