detecting thermals remotely: initial results · detecting thermals remotely: initial results nilton...
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Detecting ThermalsRemotely:
Initial Results
Nilton Renno, Stephen Rogacki,Michael Parker, Brian Russell, RobbGillespie, and William Rogerse-mail: [email protected]
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Outline• Fair weather electric fields
– The global electric circuit– Electric field in thermals
• Previous investigations• Theoretical predictions
• Our new electric field sensor• Measurements
– From the ground– Airborne
• Conclusions
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Fair weatherelectric fields
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The global electric circuit
+ + + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - - - - - - -
2 pA/m2
500 kV
~ 100 lightning strikes per sec
Ionosphere
Surface
I
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Electric fields in thermals
- - - - - - - - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + +
+ + - + + + + - + + - + + + +
- + - + ++ + + +
EE
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Results of previousinvestigations
Markson, 1975see also Kohl, 1968
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Theoretical predictions
• Assuming that thermals are uniformlycharged (-100 e/cm3), infinitely longcylinders of ~100 m of radius, we get
where d is the distance from the thermal (in m).
• Thus, the electric field at 1 km from thethermal is ~10 V/m.
E !10
4
d
V
m
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Our new electricfield sensor
- -- - -- -
- - -- - -- - - -- - --- - -+ + + +
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Requirements• To distinguish the ambient space field
from the effects of charged particlescolliding with the sensor– Vary the rotation rate during measurements
(Maruvada et al. 1983)– Add sharp points to limit the sensor potential
• To measure the electric fields ~ 1 cm fromthe surface– Instrument diameter ~ 1 cm
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Our sensor(patent pending)
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Sensor characteristics• DC to > 10 Hz• Range: 1 to 106 V/m• Resolution: 1 V/m• 2-d vector field (plane of rotation)
• A version of the sensor for gliders might bedeveloped– The idea of installing it inside a winglet or the fin
might be studied
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Installation on EP
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horizontal plane
vertical plane
Measurements
relative to the glider
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Measurements at theground
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Ground MeasurementsMay 28, 2008
from 14:16:25 to 16:11:00 local time
1 2 3
1 2 3
321Thermals selected for study
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Flights at the TuSC inArizona
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Flight Path July 02, 2008
12
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July 02, 2008Start of flight End of flight
1 2
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July 02, 2008Thermal #1
~ 2 km
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July 02, 2008Thermal #2
~ 1 km
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Flight path July 04, 2008
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July 04, 2008
1 2
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July 04, 2008 Thermal #1
~ 1.2 km
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July 04, 2008Thermal #2
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Conclusions
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• These were our first measurements– There were problems with the direction of the e-field– The frame of reference was the glider
• Theory and our initial measurementssuggest that thermals can be detectedremotely with a passive electric field sensor– The signal is much larger than the sensitivity of the
sensor
• The 60 s average electric field increasessteadily toward thermals– The first derivative of the electric field might be a
good indicator of the approach of a thermal– The direction of the local field might be a good
indicator of the direction of the thermal
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• There are theoretical andobservational indications that theelectric field contain informationabout the nature (e.g., dust andmoisture content) of the local air-mass– It might contain information about regional
circulations (shear lines, organization ofthermals, sea breezes, etc)
– This will be investigated
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Plans for the future
• Additional measurements will beconducted:– In an inertial frame of reference– Including the direction of the electric field– In various regions and weather conditions
• The position of the glider with respectto the thermals will be analyzed– The idea of using the electric field to locate
(find the direction) the thermals will be tested
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Thanks!