Relation between the asymmetric ring current effect and the anti-sunward auroral currents, as deduced from CHAMP observations

During magnetically active periods the storm-time disturbance signal on the ground commonly develops an azimuthal asymmetry. Negative deflections of the magnetic horizontal (H) component are enhanced in the 18:00 local time sector and smallest in the morning sector. This is commonly attributed to the asymmetric ring current effect. In this study we investigate the average characteristics of anti-sunward net currents that are not closing in the ionosphere. Their intensity is growing proportionally with the amount of solar wind input to the magnetosphere. There is almost twice as much current flowing across the polar region in the winter hemisphere as on the summer side. This seasonal dependence is more pronounced in the dusk sector than in the dawn sector. Event studies reveal that anti-sunward currents are closely related to the main phase of a magnetic storm. Since the asymmetry of storm-time disturbances also builds up during the main phase, we suggest a relation between these two phenomena. From a statistical study of ground-based disturbance levels during magnetically active periods, we obtain support for our suggestion. We propose a new 3D current system responsible for the zonally asymmetric storm-time disturbance signal that does not involve the ring current. The high-latitude anti-sunward currents are connected at their noon and midnight ends to field-aligned currents that lead the currents to the outer magnetosphere. The auroral net current branch on the morning side is closed along the dawn flank near the magnetopause, and the evening side currents flow along the dusk flank magnetosphere. Regardless through which loop the current is flowing, near-Earth storm-time disturbance levels will in both cases be reduced in the morning sector and enhanced in the evening.

Relation between the asymmetric ring current effect and the anti-sunward auroral currents, as deduced from CHAMP observations

During magnetically active periods the storm-time disturbance signal on ground develops commonly an azimuthal asymmetry. Negative deflections of the magnetic horizontal (H) component are enhanced in the 18:00 local time sector and smallest in the morning sector. This is commonly attributed to the asymmetric ring current effect. In this study we are investigating the average characteristics of anti-sunward net currents that are not closing in the ionosphere. Their intensity is growing proportionally with the amount of solar wind input to the magnetosphere. There is almost twice as much current flowing in the winter hemisphere as on the summer side. This seasonal dependence is more pronounced on the dusk than on the dawn side. Event studies reveal that anti-sunward currents are closely related to the main phase of a magnetic storm. Since also the asymmetry of storm-time disturbances build up during the main phase, we suggest a relation between these two phenomena. From a statistical study of ground-based disturbance levels during magnetically active periods we obtain support for our suggestion. Observed storm-time disturbance amplitudes are clearly smaller in the summer hemisphere than in the winter part. This difference increases toward higher latitudes. We propose a new 3D current system responsible for the zonally asymmetric storm-time disturbance signal that does not involve the ring current. The high-latitude anti-sunward currents are connected at their noon and midnight ends to field-aligned currents that lead the currents to the outer magnetosphere. The net current branch on the morning side is closed along the dawn flank plasmapause, and the evening side currents along the dusk flank magnetopause. Regardless through which loop the current is flowing, near-Earth storm-time disturbance level will in both cases be reduced in the morning sector and enhanced in the evening.

Inferring the source regions of pulsating auroras

The aurora is an essential tool for remote sensing the large-scale dynamics of the magnetosphere which are to difficult to observe in situ and impossible to recreate in a lab. Despite pulsating aurora being a common and widespread morning-sector phenomenon, the processes responsible for its differentiation are not understood. In situ measurements of the pulsating aurora source regions are difficult to associate with specific auroral features, yet such observations will be necessary for identifying the unique causes of pulsating auroras. This study reports a method of inferring when a spacecraft is passing through the source region of patchy aurora (PA) based on the structuring of chorus. The locations of longer-lived chorus packets are found to correspond to the region of PA occurrence reported by Grono et al. (2019b). This result constrains the region where the structuring mechanisms and conditions responsible for PA and patchy pulsating aurora (PPA) can exist.

Equivalent currents associated with morning-sector geomagnetic Pc5 pulsations during auroral substorms

Space and time variations of equivalent currents during morning-sector Pc5 pulsations (T ∼ 2–8 min) on 2 days (18 January and 19 February 2008) are studied in the context of substorm activity with THEMIS and MIRACLE ground-based instruments and THEMIS P3, P5, and P2 probes. These instruments covered the 22:00–07:00 magnetic local time during the analyzed events. In these cases abrupt changes in the Pc5 amplitudes, intensifications and/or weakenings, were recorded some minutes after auroral breakups in the midnight sector. We analyze three examples of Pc5 changes with the goal to resolve whether substorm activity can have an effect on Pc5 amplitude or not. In two cases (on 19 February) the most likely explanation for Pc5 amplitude changes comes from the solar wind (changes in the sign of interplanetary magnetic field Bz). In the third case (on 18 January) equivalent current patterns in the morning sector show an antisunward-propagating vortex which replaced the Pc5-related smaller vortices and consequently the pulsations weakened. We associate the large vortex with a field-aligned current system due to a sudden, although small, drop in solar wind pressure (from 1 to 0.2 nPa). However, the potential impact of midnight substorm activity cannot be totally excluded in this case, because enhanced fluxes of electrons with high enough energies (∼ 280 keV) to reach the region of Pc5 within the observed delay were observed by THEMIS P2 at longitudes between the midnight and morning-sector instrumentation.