Journal cover Journal topic
Annales Geophysicae Sun, Earth, planets, and planetary systems An interactive open-access journal of the European Geosciences Union
Ann. Geophys., 36, 59-69, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Regular paper
18 Jan 2018
Pulsating aurora and cosmic noise absorption associated with growth-phase arcs
Derek McKay1,2, Noora Partamies3,4, and Juha Vierinen1 1Department of Physics and Technology, University of Tromsø, Tromsø, Norway
2Sodankylä Geophysical Observatory, University of Oulu, Oulu, Finland
3UNIS University Centre in Svalbard, Svalbard, Norway
4Birkeland Centre for Space Science, Bergen, Norway
Abstract. The initial stage of a magnetospheric substorm is the growth phase, which typically lasts 1–2 h. During the growth phase, an equatorward moving, east–west extended, optical auroral arc is observed. This is called a growth-phase arc. This work aims to characterize the optical emission and riometer absorption signatures associated with growth-phase arcs of isolated substorms. This is done using simultaneous all-sky camera and imaging riometer observations. The optical and riometric observations allow determination of the location of the precipitation within growth-phase arcs of low- (< 10  keV) and high- (>  10 keV) energy electrons, respectively. The observations indicate that growth-phase arcs have the following characteristics:

1. The peak of the cosmic noise absorption (CNA) arc is equatorward of the optical emission arc. This CNA is contained within the region of diffuse aurora on the equatorward side.
2. Optical pulsating aurora are seen in the border region between the diffuse emission region on the equatorward side and the bright growth-phase arc on the poleward side. CNA is detected in the same region.
3. There is no evidence of pulsations in the CNA.
4. Once the equatorward drift starts, it proceeds at constant speed, with uniform separation between the growth-phase arc and CNA of 40 ± 10 km.

Optical pulsating aurora are known to be prominent in the post-onset phase of a substorm. The fact that pulsations are also seen in a fairly localized region during the growth phase shows that the substorm expansion-phase dynamics are not required to closely precede the pulsating aurora.

Citation: McKay, D., Partamies, N., and Vierinen, J.: Pulsating aurora and cosmic noise absorption associated with growth-phase arcs, Ann. Geophys., 36, 59-69,, 2018.
Publications Copernicus
Short summary
This study used the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) and Finnish Meteorological Institute's all-sky camera. It looked at radio absorption and optical emission of the long east–west band of aurora (known as the growth phase arc), which forms at the start of substorms. The study found that radio absorption was consistently south of the bright auroral arc and that optical pulsating aurora occurs in the boundary region between the radio absorption zone and the bright arc.
This study used the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) and Finnish...