In this project we have investigated the processes that support the upflow and partly escape of air into space at high latitudes, as schematically shown in the attached figure. For the first time the dynamics of neutral and charged particles are investigated together in this project “Role of the geomagnetic field in atmospheric escape from Earth” of the DFG Priority Programme “Planetary Magnetism.” Our studies are mainly based on satellite observations sampled, e.g., by CHAMP and the DMSP fleet. In the first part we focus on the conditions that lead to an upwelling of neutral air. Most prominent for this phenomenon is the polar cusp region on the dayside. All the geomagnetic field lines reaching out to the magnetopause (boundary between magnetosphere and open space) have their ionospheric footprint in the cusp. Air upwelling events are identified in CHAMP observations as local peaks in mass density data.With the help of a superposed epoch analyses we could reveal the typical evolution of other parameters during the events.
We find peaks collocated with the density anomaly in small-scale field-aligned currents (SSFACs), electron temperature, and upward ion velocity. All these signals are particularly strong in the cusp region. But they show different dependencies on season. While the amplitude of density anomaly is practically constant over the year, most intense SSFACs are observed in summer. Conversely, electron temperature and ion upward drift peaks are largest in winter. Interestingly, these two quantities show a high degree of correlation between their cusp-related peaks. This suggests that an ambipolar electric field set up by the hot electrons accelerates the ions upward.
We also checked a possible dependence on the IMF By component but could not find any effect on the density anomaly. This is quite different for large-scale field-aligned current (LSFAC) accompanying the density peaks. Generally the anomalies occur halfway between oppositely directed R1 and R0 FAC sheets. The direction of the R0 FAC is closely controlled by the IMF By orientation and at the same time the R1/R2 pattern is rotated in local time. We find downward R1 (upward R0) for positive IMF By and upward R1 (downward R0) for negative IMF By associated with our events. This is the typical FAC configuration bracketing the cusp region.Also the thermospheric wind is strongly influenced by IMF By. For positive IMF By, peaks in westward wind velocity are accompanying the density anomalies. Conversely, for negative IMF By, poleward winds are dominating. All these observations infer a strong Joule heating as the driver for the density anomaly.
Likewise we also investigated the evolution of related quantities accompanying peak in major ion outflows. From a superposed epoch analysis, we revealed that either negative peaks in IMF Bz or enlarged IMF By values preceded about 30 min the appearance of upflow events. This again confirms the importance of magnetic reconnection for the escape. Similar to the air upwelling we find highest rates of ion upflow between the R1 and R0 FAC sheets. This qualifies again the cusp region as major source for the events. In case of vertical velocity a clear seasonal dependence emerged with highest value in winter and lowest in summer. But for the ion flow rate (density X velocity) this dependence largely disappears. This latter quantity is more suitable for describing the atmospheric loss.
In our opinion Joule heating fueled by SSFACs is the major mechanism for driving the air upwelling. In case of ion upflow, we consider an ambipolar electric field set up by hot electrons as an important driver. In both cases intense precipitation of soft electrons (i.e., <100 eV) is needed in addition. These electrons enhance the ionospheric conductivity at the location of density anomaly, and in case of ion upflow facilitate the heating of electron gas. We think that the required contribution of precipitating electrons in both cases is the reason for the collocation of neutral air and ion upwelling.
2011 - 2017
DFG priority programme SPP 1488 - Planetary Magnetism
The CHAMP and GRACE missions were sponsored by the Space Agency of the German Aerospace Center (DLR) through funds of the Federal Ministry of Economics and Technology. The Center for Space Sciences at the University of Texas at Dallas and the US Air Force are gratefully acknowledged for making available the DMSP thermal plasma data. The authors gratefully acknowledge the use of NASA/GSFC’s Space Physics Data Facility’s OMNIWeb service and OMNI IMF and Solar wind data. The Deutsche Forschungsgemeinschaft (DFG) supported Guram Kervalishvili through the Priority Programme “Planetary Magnetism” SPP 1488.