The phenomenon behind this event is called Joule (or Ohmic) heating. Our planet has a magnetic field that spreads to the near-Earth space. This magnetosphere includes many electric current systems, of which some connect the magnetosphere to the ionized upper atmosphere, called the ionosphere. These Birkeland currents close within the ionosphere, causing Joule heating.
Effectively, Joule heating is similar to a situation, where one conveys electric current through water, creating heat (don’t try this at home). The closure of magnetospheric currents within the ionosphere creates heating that lifts the atmosphere upwards like steam in a sauna. If a satellite is flying low, it will suddenly feel more friction, causing it to drop in altitude if the orbit is not actively maintained.
I encountered Joule heating already more than two decades ago while writing my PhD thesis. Then, my goal was to understand energy transfer and dissipation within the solar wind – magnetosphere – ionosphere system. Joule heating is the main dissipation mechanism depositing solar wind energy to the ionosphere. Many times people asked “yes but what is the benefit of these studies.” Well, I said: At some point in the future, Joule heating can bring down satellites.
One interesting example was the European Space Agency’s Goce-satellite, which had used its fuel and started to re-enter in November 2013. During the Goce de-orbiting phase, a minor geomagnetic activity occurred, causing the orbit to decrease in altitude faster. Finally, it ended up near Falkland Islands, however, the original predicted landing site and the final touchdown location were separated possibly by at least six hours. With a 90-minute orbit that means 4 rounds around the globe. In other words, nobody knew where it was going to land, and all this uncertainty was due to Joule heating.
Last year, I gave a presentation in European Union Space Traffic Management hearing meeting with a title “Space weather and space debris”. I also noted Joule heating in this context and warned the participants that we might start to see uncontrolled de-orbitings if we don’t understand Joule heating.
To understand Joule heating, we need to understand why the currents are caused, how strong they are, what makes them intensify, and what are the characteristics of the medium through which they close. Additionally, ionospheric winds influence Joule heating. In these cases, Joule heating can continue even if the currents would decay. This phenomenon is called “the fly-wheel effect”.
Energy transfer and dissipation were – and still are – unsolved due to lack of proper in situ measurements and accurate modelling. The problem in understanding Joule heating very much highlights the general problem in space physics. Normally, we only have a few measurements in the vast space at any given time. This is exactly the reason I wanted to develop world’s most accurate space environment simulation Vlasiator.
Joule heating is especially difficult to assess. Some studies say it is overestimated; some studies claim it’s underestimated. The problem is that Joule heating maximises at around 120 km altitude but reaches above 200 km altitude. The 80 – 200 km altitude range is often called “the ignorosphere”, because it’s too low for spacecraft (they burn) and too high for ground-based measurements. Hence modelling is crucial. My modelling-based PhD thesis concluded that Joule heating is underestimated by at least 400%.
A couple of years back, an international group to which I belonged to suggested to the European Space Agency to fly a satellite through the ignorosphere to really measure for the first time the amount of Joule heating. Obviously, this is an incredibly difficult mission because satellites won’t last in the ignorosphere. However, we presented in our opinion a plausible technology to achieve this, but; in the end our mission was not selected.
Feb 9, 2022 showed that the selection of Daedalus would have been a wise choice. I hope something good will come out of the Starlink losses, and the international agencies reconsider to fly Daedalus. As satellites are being launched more than ever, it would be very cost-efficient to invest into one science mission and related modelling to finally understand Joule heating. This is the way we can protect our space-borne assets.
Figure 1: From Palmroth+ 2021, https://doi.org/10.5194/angeo-39-189-2021, 2021: Schematic view of the high-latitude ionospheric current system, showing the configuration of the magnetospheric (Birkeland) currents depicted with blue and red vertical arrows. They close horizontally through the yellow region, depicting the auroral zone. In this closure, heating is created. As the figure shows, Joule heating maximises in the dawn and dusk sectors. Sometimes intense Joule heating occurs within the midnight sector as well, especially during substorms that occur daily, but which are still one of the top mysteries in space physics. Solar direction is to bottom left, the midnight is in top right.