Why did we see northern blue over Moscow? 

On April 21, 2023 a giant solar filament erupted, hurling a powerfull coronal mass ejection toward Earth, which reached our planet in the evening of April 23, causing a strong geomagnetic storm and bright polar auroras seen even in Moscow yesterday night!

Why did this happen? Tatiana Podladchikova, Director of the Skoltech Center for Digital Engineering and Head of the Space Weather Laboratory, spoke about this.

Photo: Evelina SHajhrazeeva

"Polar aurora is the most beautiful natural phenomena of Space Weather. Approaching the Earth, a magnetic plasma cloud or a high-speed solar wind stream from a coronal hole can disrupt the magnetic field of our planet and initiate a magnetic reconnection with the interplanetary magnetic field (IMF) which is a component of the solar magnetic field. The Earth’s magnetosphere opens only if IMF dragged out from the solar corona by the solar wind is in the opposite direction to the Earth’s magnetic field. Solar wind continues to blow and reconnected magnetic lines drift to the night-side of the Earth, where they reconnect again. Part of the energy is ejected into space, and part is sent along the magnetic field lines to Earth’s magnetic poles. Charged particles travelling to the polar regions, excite oxygen and nitrogen there. And the radiation of excited atoms paints the sky with multi-colors and ignites the aurora.  So the polar oval exists permanently, 24 hours per day. However, during super storms, the polar oval expands to mid-latitudes and even to the equator. This is how the aurora can be seen even in Moscow. 
The Earth's magnetic field captures high-energy particles coming from the Sun and makes them move in a regular pattern. Particles spirally wind up on a magnetic line, approaching either the north or south magnetic pole, and at the same time drift around the Earth in a fraction of a second. In fact, the Earth is at the center of a gigantic electricity station. A ring current starts flowing around the Earth, weakening the Earth's magnetic field, and a magnetic storm covers our planet. Right now we are at the peak of the geomagnetic activity with the disturbance storm time index of around -200 nT. Such powerfull storms can seriously damage the modern technological systems such as radio communications, satellites, airplanes, power grids and pipelines".

This video shows the eruptive protuberance that is at the core of coronal mass ejection.

The video shows a propagation of a coronal mass ejection in interplanetary space in the direction to the Earth.