Separation and Quantification of Ionospheric Convection Sources: 2. The Dipole Tilt Angle Influence on Reverse Convection Cells During Northward IMF

Abstract

This paper investigates the influence of Earth's dipole tilt angle on the reverse convection cells (sometimes referred to as lobe cells) in the Northern Hemisphere ionosphere during northward IMF, which we relate to high‐latitude reconnection. Super Dual Auroral Radar Network plasma drift observations in 2010–2016 are used to quantify the ionospheric convection. A novel technique based on Spherical Elementary Convection Systems (SECS) that was presented in our companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026634) is used to isolate and quantify the reverse convection cells. We find that the dipole tilt angle has a linear influence on the reverse cell potential. In the Northern Hemisphere the reverse cell potential is typically two times higher in summer than in winter. This change is interpreted as the change in interplanetary magnetic field‐lobe reconnection rate due to the orientation of the dipole tilt. Hence, the dipole tilt influence on reverse ionospheric convection can be a significant modification of the more known influence from vswBz. These results could be adopted by the scientific community as key input parameters for lobe reconnection coupling functions.

Quantifying the Lobe Reconnection Rate During Dominant IMF By Periods and Different Dipole Tilt Orientations

“…As will be discussed toward the end of the paper, we interpret the polar cap plasma circulation (Φ lobe ), as a result of lobe reconnection in a quantitative manner, similar as Reistad, Laundal, Østgaard, Ohma, Thomas, et al. (2019) did for pure northward IMF. The total magnetic flux transport rate is quantified by the cross polar cap potential (potential difference between the global maximum and minimum locations), and will reflect the contribution from all sources, which we assume to be the day‐ and nightside reconnection processes (Dungey cycle) in addition to lobe reconnection.…”

“…The quantification of the potential (total magnetic flux transport rate [Wb/s = V]) associated with the polar cap plasma circulation, Φ lobe , can be illustrated by the number of closed contours of the electric potential inside the polar cap, and will be described in detail in Section 3. As will be discussed toward the end of the paper, we interpret the polar cap plasma circulation (Φ lobe ), as a result of lobe reconnection in a quantitative manner, similar as Reistad, Laundal, Østgaard, Ohma, Thomas, et al (2019) did for pure northward IMF. The total magnetic flux transport rate is quantified by the cross polar cap potential (potential difference between the global maximum and minimum locations), and will reflect the contribution from all sources, which we assume to be the day-and nightside reconnection processes (Dungey cycle) in addition to lobe reconnection.…”

“…(2019); Reistad, Laundal, Østgaard, Ohma, Thomas, et al. (2019) to represent the average convection pattern based on the selection of observations described above using Spherical Elementary Current Systems (SECS) (Amm, 1997; Amm & Viljanen, 1999; Amm et al., 2010). Similar to Reistad, Laundal, Østgaard, Ohma, Thomas, et al.…”

“…Understanding the cause of the dipole tilt effect on the climatology of global convection (e.g., Pettigrew et al, 2010;Thomas & Shepherd, 2018) is of great scientific interest. It has been pointed out that the lobe reconnection process is likely responsible for hemispheric asymmetries in plasma circulation at polar latitudes, as the lobe reconnection process is not bound by the same north-south symmetry constraints as dayside reconnection, and can hence operate independently in the two hemispheres (e.g., Chisham et al, 2004;Reistad, Laundal, Østgaard, Ohma, Thomas, et al, 2019). This leads to hemispheric differences in the ionospheric flux transport, often quantified by the cross polar cap potential, for example, Pettigrew et al (2010); Thomas and Shepherd (2018).…”

“…Comparing the lobe reconnection rate inferred during pure northward IMF byReistad, Laundal, Østgaard, Ohma, Thomas, et al (2019) (4 kV in winter, 8 kV in summer) to the lobe reconnection rate during the IMF B y intervals considered here, it appears that that the Wilder et al (2008) coupling parameter is not applicable when |IMF B y | ≳ |IMF B z |. However, we note that the Wilder et al (2008) coupling parameter (E-field [V/m]) is not directly comparable to the lobe reconnection rate which we address here (rate of magnetic flux transport inside the polar cap [Wb/s = V]) as the length of the lobe reconnection x-line may vary with IMF clock angle.…”