1. Disturbance Dynamo Effects in the Low Latitude Ionosphere
Thomas J. Immel and the FUV Space Sciences Laboratory, Univ. of California, Berkeley
IMAGE Science Team Meeting
Yellowstone Park, Oct 12-14, 2005

2. Low Latitude Imaging Database
Day 75
The best opportunity of IMAGE mission for low latitude imaging is early 2002.
Seasonal preference for imaging nightside by IMAGE - Days are used here.
4-7 hours of imaging per orbit.
144 ionospheric bubbles are tracked. Preference for the deepest depressions in brightness (i.e. many more signatures are seen than characterized)
Day 96
Day 115

3. Determining Plasma Drift Speeds
The brightness depletions are tracked in magnetic longitude, and the drift speeds can be determined over several hours of magnetic local time.

4. Average Plasma Drift Speeds : Validation
Calculating the average drift speed at all MLTs, shows a peak at 20 MLT, and a general trend similar to the high solar flux average speeds measured at Jicamarca.

5. Effect of enhanced convection on drift speeds : Dst
A mean correlation of 0.86 over 6 hours of local time shows a remarkable correspondence between drift speed and Dst.

6. Figures from Fejer and Emmert, JGR, 2003
A problem with attribution of the Dst-plasma drift relationship to ring-current E-fields is that penetration of magnetospheric electric fields is usually short lived (20-60 minutes).

7. Scherliess and Fejer, JGR, 1997
A longer-lasting effect that is related to enhanced geomagnetic activity is the stormtime disturbance dynamo. This effect can lift equatorial plasma and also reduce its eastward drift speed, like penetrating fields but on a longer timescale.
Fejer and Scherliess, JGR, 1997

8. But what to use as a high latitude index?
The Kyoto Ae index is great, but takes years to come out in final, validated form. The last set is year If you have access to a better set of magnetometers, you can build your own Ae index. Joy.
Kp is not a high latitude index, nor is it a useful substitute.
B. J. Anderson et al. at APL have worked out an index for high latitude Birkeland currents from Iridium satellite data. It has the temporal/spatial resolution and sensitivity of the Iridium satellite network. As poor as that may be, it is not biased to any continent/local time, and available in final form for It is called Net_DB and I am going to use it.

9. Effect of Joule heating on drift speeds : Net_DB
A mean correlation of 0.83 is found between zonal speed and high latitude Birkeland currents with a delay of 6 hours

10. Time delay analysis of plasma drifts vs. Net_DB
The median Iridium Net_DB data (Anderson et al., JGR, 2002) in a sliding 2-hour window are compared to the plasma drift speeds for delays of -40 of +6 hours. A significant maximum in the correlations and slopes of linear fits is found around -6 hours. Another is evident 24 hours earlier.

11. Evidence of Distubance Dynamo effect at Low Latitudes
The correlation between plasma drift speed and Net_DB reaches a peak with a delay of 6 to 10 hours in the 3 hours after sunset. This delay can be attributed to the response time of the thermosphere to high latitude heating. The secondary peaks 1 day later are expected due to the slow decay of disturbance dynamo effects.
The slopes show similar peaks at MLT, but indicate insignificant dependence of velocity on Net_DB beyond the MLT, -10 hour peak.

12. Increasing Delay
Comparisons to Jicamarca radar observations show a good correspondence in the effects seen by IMAGE at LT and the Jicamarca radar in the post-midnight sector.

13. Disturbance Dynamo Efficiency
Equatorial radar results show the strongest disturbance dynamo effects in the post-midnight sector.
We find significant effects in the 19-20h local time sector.
IMAGE-FUV measures *zonal* drifts, where the radar-determined efficiencies are calculated from *vertical* drifts.
Are the meridional and zonal disturbance electric fields effective in different local time sectors?

14. Summary
Tracking of ionospheric bubbles for 6+ hours and the observation of the global dynamics of the equatorial ionosphere under the influence of geomagnetic disturbances is a previously unrealized capability of high altitude imaging platforms.
The reduction in zonal drift speeds observed with increasing high latitude Joule heating (using Net_DB) is consistent with thermospheric-wind driven disturbance dynamo effects.
A delay time of 6-10 hours (post-sunset) between input and maximum influence at low latitudes is found. This is more than the 2-4 hour delay in vertical ionospheric drift influence of Joule heating found by Scherliess and Fejer [JGR, 1998] (near-midnight).
Residual effects 24 hours after the first peak in “efficiency” of high latitude heating further suggest the observation of disturbance dynamo effects.

15. Questions
Do the vertical Jicamarca and zonal IMAGE-FUV drifts offer complimentary views of disturbance dynamo effects? Or are these observations contradictory?