Transparency
Where the data comes from
Every number on Aurora Tonight traces back to a publicly available source. This page explains what those sources are, what they provide, and why they can be relied on.
NASA DONKI
Space Weather Database Of Notifications, Knowledge, Information
DONKI is operated by NASA's Community Coordinated Modeling Center (CCMC) at Goddard Space Flight Center. It is the primary public database for space weather events - solar flares, coronal mass ejections, and geomagnetic storms - issued and annotated by space weather forecasters.
Geomagnetic storms
Active storm events with Kp index readings every 3 hours. This is the primary source for current and recent Kp values shown on the site.
CME analysis
Coronal mass ejections (CMEs) are large solar eruptions that can trigger aurora. DONKI records analysed CMEs and, where applicable, their predicted arrival time at Earth.
Solar flares
M-class and X-class flare events are listed with their peak times and intensity classifications. Strong flares often accompany the CMEs that drive major aurora storms.
The forecast updates every 30 minutes by querying the DONKI API. NASA's involvement means the underlying data has been reviewed by professional space weather analysts, not generated automatically.
NASA DONKI websiteNOAA Space Weather Prediction Center
Real-time solar wind data from the DSCOVR satellite
NOAA - the National Oceanic and Atmospheric Administration - is the US federal agency responsible for weather forecasting, including space weather. Its Space Weather Prediction Center (SWPC) in Boulder, Colorado operates 24 hours a day and issues the official geomagnetic storm watches, warnings, and alerts used by governments and utilities worldwide.
The real-time solar wind measurements come from the DSCOVR satellite (Deep Space Climate Observatory), which sits at the L1 Lagrange point - around 1.5 million kilometres between the Earth and Sun. From this position it can measure the incoming solar wind roughly 15-60 minutes before it reaches Earth, giving just enough warning to be useful.
Bz (southward component)
The Bz value measures whether the solar wind's magnetic field is pointing north or south. A sustained negative (southward) Bz allows the solar wind to couple with Earth's magnetosphere and drives geomagnetic activity. This is the single most important real-time indicator for aurora.
Bt (total field strength)
The total magnetic field strength of the solar wind. A high Bt combined with negative Bz is the most favourable combination for aurora activity.
Solar wind speed
Measured in km/s. Faster solar wind - especially in the 600-800+ km/s range following a CME - increases the energy available to drive geomagnetic disturbances.
Proton density
The number of protons per cubic centimetre in the solar wind. Higher density, combined with high speed and southward Bz, increases the pressure on Earth's magnetosphere.
These readings update every minute at the source. Aurora Tonight fetches a 2-hour rolling window and displays the most recent valid reading, refreshing roughly every minute on the forecast page.
NOAA SWPC websiteOpen-Meteo
Cloud cover forecasts for location pages
Knowing that a Kp 7 storm is in progress is only half the picture - if there is 100% cloud cover overhead, you won't see anything. Open-Meteo is an open-source weather API that provides hourly and daily cloud cover forecasts, drawing on data from national meteorological services including ECMWF, GFS, and others depending on the region.
On individual location pages, Aurora Tonight fetches cloud cover data directly from Open-Meteo in your browser using the page's coordinates. This means the cloud forecast is always pulled fresh for the specific latitude and longitude of that location - not a generic regional figure.
Hourly cloud cover
Cloud cover percentage for the current hour and the hours ahead. Shown on location forecast pages as a sky condition indicator alongside the Kp reading.
7-day daily average
Mean cloud cover for each day of the coming week, used on location overview pages to give a broader picture of viewing conditions for the days ahead.
Open-Meteo is free to use and does not require an API key. It is widely used in weather and astronomy applications. The underlying forecast models are the same ones used by professional meteorologists - Open-Meteo simply makes them accessible via a clean API without the licensing restrictions of commercial weather services.
Open-Meteo websiteHow the forecast is built
The Kp index readings come from NASA DONKI's geomagnetic storm records. Kp is a global measure of geomagnetic disturbance on a scale from 0 (completely quiet) to 9 (extreme storm). The site uses the latest reported Kp value to determine storm level and to calculate the approximate latitude at which aurora should be visible, if skies are clear.
Upcoming CME arrival times - where DONKI analysts have modelled an Earth-directed event - are displayed alongside the current conditions. These are estimates: CME travel times from the Sun to Earth range from around 15 hours for fast events to 3-4 days for slower ones, and the actual impact can vary by several hours either side.
The live solar wind panel (Bz, Bt, speed, density) is drawn directly from NOAA's DSCOVR measurements and gives a more immediate picture of current conditions. When Bz turns strongly negative and holds there, aurora activity typically follows within an hour.
Cloud cover data comes from Open-Meteo and is fetched client-side on each location page. It sits alongside the Kp forecast as a practical check - strong geomagnetic conditions only translate to a sighting if the sky is clear. The two pieces of data together give a more complete answer to the question most people actually have: is it worth going out tonight?
Space weather forecasting is not deterministic. Even with the best available data, conditions can change rapidly and aurora can appear - or fail to appear - without clear warning. The data on this site reflects what authoritative sources are currently reporting; it does not guarantee a sighting. Read more about the site and its approach on the about page.
