Your Smartphone Can Tell Us About Solar Storms

Have you ever noticed that your map's little blue GPS dot sometimes seems to wander around? One day, it’s remarkably accurate, but the next, it’s placing you several blocks away from where you’re standing. These quirks don’t just affect your Google Maps - they can throw off navigation for aircraft, ships, agricultural equipment and more. And when you consider just how many parts of modern life rely on that drifting blue dot, it starts to become clear how deeply our world is intertwined with satellite navigation.

There are several reasons for these inconsistencies, but one of the most fascinating involves a surprising cast of characters: the Sun, the Earth’s upper atmosphere, its magnetic field  - and your smartphone.

One of the fellow PhD candidates in our cohort brought a recent (November 2024) study published in Nature to my attention that piqued my interest in this (thank you, Tommy Marshman!). The paper reveals how millions of mobile devices scattered across the planet could actually help solve this problem on a global scale.

The upper layer of Earth’s atmosphere is constantly bombarded by ultraviolet and X-ray radiation from the Sun. This high-energy radiation strips electrons from atoms and molecules, ionising them and creating a soup of charged particles.

As this is a global effect, this region is known as the ionosphere - a highly dynamic environment, strongly influenced by solar activity. When the Sun is active, it sends more radiation our way, increasing the number of free electrons in the ionosphere. These changes can happen over minutes or hours, and even differ depending on whether it’s the day or night side of the planet.

Most of our navigation tools - from Google Maps to aircraft systems - rely on GPS or other Global Navigation Satellite Systems (GNSS) to pinpoint location, and these signals must pass through our atmosphere, in particular our ionosphere, where they are affected and disrupted.

To correct for the ionosphere’s impact on these signals (that wondering blue dot), a global network of ground-based GNSS monitoring stations measures the Total Electron Content (TEC) in the sky above them. But this is no small feat. The highly dynamic ionosphere is changing with time and location, as Earth spins and the Sun’s radiation shifts across the globe. As such,  different regions of the ionosphere react in different ways.

These monitoring stations are at fixed locations, rely on specialised scientific instruments, and only observe the sky above them. That means large swathes of the planet, especially remote or isolated regions, often go unmonitored, leaving critical gaps in our global understanding.

But there might be a solution to this, and that solution might be the very device you are reading this on right now.

In this new study, the authors propose a fascinating idea with promising results: using data from millions of mobile devices to track changes in the ionosphere’s Total Electron Content (TEC). And it turns out there’s a surprisingly elegant way to do it.

Smartphones come equipped with built-in GPS capabilities. At any given moment, your location is triangulated by signals from at least four GPS satellites, using two frequency bands. These radio signals, however, don’t travel through a vacuum — they pass through Earth’s ionosphere, a magnetised and ionised medium that can alter the speed of the signals depending on frequency. Lower-frequency signals experience more delay than higher ones, a phenomenon known as dispersion. The amount of dispersion is directly tied to the amount of TEC along the signal’s path: more electrons, more delay.

Interestingly, radio astronomers see something similar when observing pulsars. As the linearly polarised light from pulsars passes through the ionosphere, it undergoes a twist known as a rotation measure. These twists fluctuate throughout the year — and even across the 11-year solar cycle — reflecting changes in solar activity and its impact on Earth’s upper atmosphere.

By harnessing the countless devices across Earth that constantly send and receive GNSS signals, scientists have effectively created a planet-wide network of sensors — a distributed “telescope” capable of monitoring the ionosphere in real time, across both space and time.

Voilà — that solves the coverage problem.

In this study, the researchers tapped into publicly available data from millions of Android phones to monitor how the GNSS signals were being distorted by the ionosphere. And what they found was fascinating.

Firstly, the authors reported that they were able to double the global coverage of TEC measurements by utilising mobile devices, in particular in regions on Earth where the GNSS stations are limited (such as in Eastern Europe, India, South Asia, parts of Africa and much of South America).

On top of this, they also captured the day-night ionospheric cycle, including a crest of ionisation that lies to both the north and south of the geomagnetic equator, with a unique dip southwards towards South America.

However, the most intriguing results showcase the day the Earth was hit by a powerful, class G5 geomagnetic storm on 10 - 11 May 2024, the most powerful solar storm that occurred in the last 20 years. When that extreme geomagnetic storm arrived on Earth, it really shook up Earth’s magnetic field and stirred up dramatic variations in the TEC. This caused the phone network to detect anomalous disturbances over the Caribbean and caused a narrow plume of ionisation over North America. On another occasion (5 November 2023), phone measurements revealed a depletion of ionisation - the mid-latitude ionospheric trough - over Europe.

This was not without its challenges, though. Smartphones have small, non-scientific antennas and GNSS hardware that is built to make do for everyday usage and not as professional as the scientific instruments mounted on the GNSS stations. This resulted in higher uncertainties and noisier measurements. Additionally, mobile devices don’t always have a clear view of the sky and are often inside buildings or within large cities where numerous skyscrapers are blocking the line of sight in certain directions. On top of this, each phone has its unique quirks with calibration and individual biases (e.g. older operating systems, etc.) that could vary results - all of which needed to be accounted for.

Interestingly, this effort wasn’t driven purely by scientific curiosity. The authors (including researchers from Google) were researching how to reduce ionospheric-induced GPS errors and improve positioning services and the user experience for the Android user community around the globe.

It showcases the incredible potential of our pocket-sized supercomputers. Smartphones, as part of distributed sensor networks, are increasingly being used in other case studies, such as using networks of phones to detect earthquakes (using the in-built accelerometers) and even weather measurements using barometric tools built into these devices.

It highlights a humble, yet fascinating point of how technology can be serendipitously used for scientific purposes - the devices that help regulate and navigate (literally) our daily lives, are also helping us learn more about the dynamical interactions between our local star, our planet and the protective layers that stand guard between.