From the Sun to the sky
How the solar wind, Earth's magnetic field and the auroral oval connect a storm on the Sun to the light above you.
The solar wind
The Sun's outer atmosphere, the corona, is so hot that it streams out into space in every direction as the solar wind. Near Earth it usually moves at 300 to 400 km/s, with a few protons in every cubic centimetre, and it carries a weak magnetic field from the Sun with it.
Earth's magnetic shield
Earth's magnetic field holds most of that wind off, carving out a protected region called the magnetosphere. On the side facing the Sun it's squeezed in; on the night side it stretches into a long tail.
The key moment is when the solar wind's magnetic field points south (a negative Bz). It can then join up with Earth's northward field at the front of the magnetosphere, a process called magnetic reconnection. Energy and particles pour in and are stored in the tail. When the tail can't hold any more, it snaps back and releases them towards Earth, along the field lines that lead down near the poles.
The auroral oval
Those field lines meet the atmosphere in a ring around each magnetic pole, the auroral oval. On a quiet night the oval is narrow and sits roughly 65 to 70° from the magnetic equator, over places like northern Norway, Iceland, Alaska and northern Canada. As activity rises, it widens and moves towards the equator, which is why big storms bring the aurora to places that rarely see it.
The Auroral oval map on the Now page shows this ring as NOAA forecasts it for the next 30 to 90 minutes. See Read the Now page.
Why you can see it from outside the oval
Aurora is high, 100 km and more. From a few hundred kilometres outside the oval you can still see its top, low over the horizon towards the pole. That's why the Your sky card looks for aurora between your place and about 6° of latitude towards the pole.