Red Arc with Green Diffuse Aurora (RAGDA), is a two-component form of northern lights that occurs south of the oval.
Both parts of the aurora are formed when positive particles from the magnetosphere hit the Earth's upper atmosphere. The phenomenon occurs before magnetic midnight during large aurora substorms and is best distinguished when it deteches southward from the bright rays of the substorm aurora.
The phenomenon consists of a faint red arc, which looks a lot like a Stable Auroral Red (SAR)-arc. The common factor for these two red arcs is the reaction of the ring current with the substorm.
A Red Arc with Green Diffuse Aurora
Photo: Antero Ohranen
Below the red arc are green diffuse patches or sausage-like shapes. RAGDA's green aurora is essentially featureless and without any rays. It may appear slightly more bluish in camera images compared to the green aurora of the oval.
RAGDA's green has slightly more bluish shade than the rest of the oval.
Photo: Lasse Nurminen
Sometimes the red arc and the green patches are clearly separated from the aurora oval and sometimes almost in contact with the southward edge of the oval.
During an active substorm, the green northern lights can sometimes be seen with red tops. These usually have rays that RAGDA's green aurora lacks.
An emissionless gap without any aurora light can be observed between the red arc and green diffuse aurora. The two features don't seem to be connected. Of the two aligned structures, the red one is located ~ 100 kilometers higher than the green aurora.
There seems to be an empty area without any aurora light between the red and green aurora.
Photo: Markku Ruonala
The event is dynamic. It sometimes starts with green blobs, then a red arc appears. These two can also appear in the sky at the same time. The shapes move often from east to west. Then the Red Arc with Green Diffuse Aurora fades away and only the red arc remains visible in the sky. The red arc is recognized as the SAR arc.
Observing this phenomenon is easier when the night sky is clear and dark before magnetic midnight. When looking for the red arc with green diffuse aurora, the best results can be achieved by pointing the camera towards south or southwest of the brightest part of the oval.
A wide-angle lens is recommended for photographing the shape of the red arc going over the sky, but a regular lens can also be used. The red arc is quite dim, so the exposure times needed are typically longer than the ones for northern lights.
Photo: Pirjo Koski
If the red arc cannot be distinguished in the images, the phenomenon identification 'Diffuse green auroras' should be used for isolated hazy green blobs/arc.
Dunes in RAGDA's green aurora
Photo: Matias Takala