Weimer05FAC_Radial-LinePlots_IMFClockAngle

Below are the conjugate ionospheric field aligned currents (FAC) maps for -90 deg. and 90 deg. IMF Clock Angle respectively. 

From these maps, we are particularly interested in explaining the increase in post-noon/pre-dusk SP auroral hiss occurrence rate for increasing By. -90deg. IMF Clock Angle plots (top one) corresponds to the -ve By and the bottom one, for +90deg IMF Clock Angle, corresponds to +ve By.  Also note, that blue color represent the FAC coming out of the ionosphere (i.e. upwards and have -ve sign).

-90deg. IMF Clock Angle:

+90deg. IMF Clock Angle:

Note specifically that the FAC maps for the Southern Hemisphere, which show that for -ve By, upward FAC around SP latitude range increase as we move from Noon towards Dusk and then towards Midnight in MLT. 

But of particular interest to us are the comparative magnitudes of these upwards FAC in the pre-dusk sector for -ve and +ve By conditions. For this reason we draw line plots of integrated FAC against MLT (Noon to Midnight), with FAC integrated over +/- 3deg. or 4 deg. around SP geomagnetic latitude of -74 deg.

Below are these Integrated FAC line plots:

-90deg. IMF Clock Angle:

+90deg. IMF Clock Angle:

This shows that the upward FAC do increase in the post-noon/pre-dusk sector for an increase in By (as the FAC are more negative). This might indicate that the downward electron precipitation increases with increased By in the pre-dusk sector hence resulting in a higher pre-dusk hiss occurrence rate associated with +ve By.

 We also investigate this for different IMF Clock Angles, especially when the Bz is also -ve. This is shown below for IMF Clock Angles -135deg (Bz -ve and By -ve) and +135deg (Bz+ve and By -ve).

-135deg. IMF Clock Angle:

+135deg. IMF Clock Angle:

And Integrated FAC line plots are as follows:

 As can be readily seen upward FAC increase for post-noon sector as By goes from -ve to +ve but there is not much of an increase in upward FAC for times right before the dusk. This also correlates well with our observations from the Hiss occurrence rate plots against the IMF Clock Angle. We saw that pre-dusk hiss was more correlated with increasing By when Bz was kept small. Otherwise pre-dusk hiss doesn't show large enough occurrence rate.