Climatology of Vertical Ion Velocity and its Relationship with Large-scale FAC Based on DMSP and CHAMP Observations: Seasonal and IMF By Dependence

Abstract:

In this study we present climatology of vertical ion drift and its relationship with large-scale field-aligned current (LSFAC) in the Northern Hemisphere cusp region using the superposed epoch analysis (SEA) method. The dependence on the interplanetary magnetic field (IMF) B_ycomponent orientation and the local season is also investigated. The three local seasons of 130 days each are defined as follows: local winter (1 January ± 65 days), combined equinoxes (1 April and 1 October ± 32 days), and local summer (1 July ± 65 days). Our results are based on DMSP (F13 and F15) and CHAMP satellite observations and NASA/GSFC’s OMNI online data set during the years 2001-2005. The time and location of the vertical ion velocity peaks (> 100 m/s for upflow and <-100 m/s for downflow) in the cusp region are used as reference parameters (time and location of the event) for the SEA method.

Event number distribution based on the ion velocity peaks shows no significant variations for IMF B_y component orientation and local season. Corresponding averaged profiles of total magnetic field B_t and IMF components, taken for flow events from both DMSP satellites and all seasons together, also show no differences in temporal variations (only negligible changes in amplitude) between upflow and downflow events. There is no systematic sign change related to flow direction in any of the three IMF components and IMF B_z is always negative. IMF B_y and B_z components are peaking about half an hour before the event for all considered cases.

The main findings of our SEA analysis, can be summarized as follows:

• Vertical plasma flow, taken for both DMSP satellites together, shows no dependence on IMF B_y component orientation. Its amplitude is increasing towards local summer.
• Vertical plasma velocity is much higher in winter than during combined equinox or summer. It seems the ion density is low in winter and increases towards local summer.
• The plasma upflow is generally stronger than the downflow in all given cases.
• LSFAC shows a clear dependence on IMF B_y component orientation and local season. Amplitude of LSFAC is also increasing towards local summer.
• LSFAC shows an IMF B_y dependent regular pattern for upflow cases. Conversely, there are no regular patterns for downflow cases in all seasons and both IMF By signs. The peak in upward flow occurs at latitudes of the Region 1 FACs.