If I was an astronaut and took off my helmet, would I hear these plasma waves?
No! Aside from this being a terrible idea, you also wouldn’t hear anything. There are many different types of sound waves, only a few of which you can hear with your ears. There are sound waves in the air of the Earth’s atmosphere that we’re all familiar with, and there are sound waves in water that you might have heard when swimming in a pool. There’s also sound waves in the solid Earth that seismologists study, sound waves and other types of waves in plasmas that we’re studying, and many more examples of waves that you can’t hear with your ears but that can still be measured and audified as we do with the HARP project. The plasma waves we’re studying are too low in pitch to hear, and they’re also too faint; even if our ears could detect plasma in the same way they can detect the motion of air, there’s not enough plasma to make a loud noise in the near vacuum of space. Read more about sound and plasma waves here, and see this video for other examples of space audio.
Why are there silent times and black spaces in the spectra?
Silent times and black spaces means that there are no suitable measurements available to study the “harp” that we’re interested in. This occurs for a few reasons: 1) When the THEMIS satellite is farthest away from the Earth it often leaves the region known as the magnetosphere, where the “harp” strings exist. Even though it still collects data at these times, the data can’t be used to study the magnetospheric “harp” that we’re interested in; 2) When the THEMIS satellite is closest to the Earth the magnetic field gets much larger. Because of this our satellite instruments need to measure the magnetic field at a lower accuracy. Also as the satellite speeds up close to the Earth, it moves too quickly through the different “harp” strings. Both of these effects prevent us from being able to study the harp close to Earth; 3) The magnetic field measurements aren’t available or have data quality issues. Read more information about the THEMIS mission here, with significantly more information about each instrument, including the fluxgate magnetometer that we’re using for HARP, here.
Where did we obtain the data? What is the THEMIS mission?
We’re using data from the fluxgate magnetometer instrument on NASA’s THEMIS mission, which is used to measure magnetic fields. THEMIS originally consisted of five satellites orbiting the Earth, but two are now in lunar orbit in what’s referred to as the ARTEMIS mission. We’re using just one of the THEMIS satellites that’s remained in orbit around the Earth. All data from THEMIS is publicly available. We obtained the data for HARP from a widely used repository known as the Coordinated Data Analysis Web hosted by NASA’s Space Physics Data Facility.
How are the sounds recorded and can I make my own?
The sounds you’re hearing are measurements of magnetic field variations from the THEMIS satellite. You can’t hear magnetic fields with your ears, but in plasmas the magnetic field moves similarly to the particles. By using its “magnetic ears” theTHEMIS satellites can more easily detect waves that are present than instruments designed to measure the particles directly. Through a technique known as audification, you are able to hear sounds related to the magnetospheric “harp” and other phenomena in near-Earth space. Audification is a direct conversion of these measurements to sound. There’s only a few steps needed to convert the magnetic field measurements to the sound used in this project, as described further in this study: (1) Obtain THEMIS magnetic field measurements that can be used to study the “harp” (see other FAQ entries), (2) Remove magnetic fields corresponding to slowly varying trends since we’re interested in waves and variations that change on timescales of minutes and seconds, (3) Resample the time series so the waves fall in the audible range, (4) Slow down playback without changing the pitch through a technique known as time-stretching and create sounds. The steps are described further here. The software we use to generate the sounds is publicly available using open-source python software.
You can also generate space sounds from many satellites, including THEMIS, using NASA’s CDAWeb interface. Read more here.
What causes these sounds in space?
These sounds aren’t caused by the common space objects that you might be used to, like planets, asteroids, and comets. They are ultimately caused by the solar wind interacting with Earth’s magnetic shield, the magnetosphere. The sounds themselves are wave-like vibrations of the plasma present within the magnetosphere, driven by solar wind energy. Scientists are still studying the wide variety of different waves that can be produced, trying to better understand how this happens and how they can affect us and our technology.