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Detecting Thermals Remotely
Nilton Renno, Stephen Rogacki, Michael Parker, Brian Russell, Robb Gillespie, and William Rogers e-mail: [email protected]
Outline • Fair weather electric fields
– The global electric circuit – Electric field in thermals
• Previous investigations • Theoretical predictions
• A new electric field sensor
• Measurements – From the ground – Airborne
• Conclusions
Fair weather electric fields
The global electric circuit
+ + + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - - - - - - -
2 pA/m2
500 kV
~ 100 lightning strikes per sec
Ionosphere
Surface
I
Electric fields in thermals
- - - - - - - - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + +
+ + - + + + + - + + - + + + +
- + - + + + + + +
E E
Results of previous investigations
Markson, 1975 see also Kohl, 1968
Theoretical predictions
• Assuming that thermals are uniformly charged, infinitely long cylinders of ~100 m of radius, we get
where d is the distance from the thermal (in m).
• Taking the thermal charge density to be -100 e/cm3, we find that the electric field
at 1 km from the thermal is ~10 V/m.
E ≈104
dVm
A new electric field sensor
- - - - - - -
- - - - - - - - - - - - - - - - - + + + +
Requirements
• To distinguish the ambient space field from the effects of charged particles colliding 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 from the surface – Instrument diameter ~ 1 cm
Our sensor (patent pending)
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 will be developed – We plan to study the idea on installing it in a
winglet
Installation on EP
horizontal plane
vertical plane
Measurements
Measurements at the ground
Ground measurements on May 28, 2008 from 14:16:25 to 16:11:00 local time
1 2 3
1 2 3
3 2 1
strong thermals selected for study
Thermal #1 (vertical plane)
reduction in magnitude and rotation of the electric field
thermal gusts
Thermal #1 (horizontal plane) increases in the magnitude of the electric field correlated
with thermal gusts
Thermal #2 (vertical plane)
variations similar to that of thermal #1
Thermal #2 (horizontal plane)
variations similar to that of thermal #1
Flights at the TuSC in Arizona
Flight Path July 02, 2008
July 02, 2008 Start of flight End of flight
1 2
July 02, 2008 (2 - 60 s average)
Start of flight End of flight
1 2
July 02, 2008 Thermal #1
~ 2 km
July 02, 2008 Thermal #1 (2 - 60 s average)
Indication that the thermal is not centered?
July 02, 2008 Thermal #2
~ 1 km
July 02, 2008 Thermal #2 (2 - 60 s average)
~ 1 km
Flight path July 04, 2008
July 04, 2008
1 2 3
July 04, 2008 (2 - 60 s average)
1 2 3
July 04, 2008 Thermal #1
Left wing on inside
Right wing on inside
July 04, 2008 Thermal #1 (2 - 60 s average)
~ 2 km
July 04, 2008 Thermal #2
~ 1.2 km
July 04, 2008 Thermal #2 (2 - 60 s average)
July 04, 2008 Thermal #3
July 04, 2008 Thermal #3 (2 - 60 s average)
~ 1.2 km
Conclusions
• Our measurements suggests that thermals can be detected remotely with a passive electric field sensor
• The 60 s average electric field appears to increase steadily 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
• The electric field might also be used for centering thermals – There is a maximum in the vertical component of
the electric field at the center of the thermal
Plans for the future
• Additional measurements will be conducted: – In various regions and weather conditions – With a data logger recording the electric field, GPS
position, and TE rate of climb – With measurements of the gradient and AC
components of the field near thermals
• The position of the glider with respect to the thermals will be analyzed – The electric field will be used to locate the direction of
the thermals – The idea of using the electric field for centering
thermals will be tested
Thanks!