Posts Tagged ‘nature’
“One man’s signal is another man’s noise,” began Dr. Kudeki as he derived incoherent scatter radar theory from Nyquist’s noise theorem in ECE458. I think of that statement often, whether it be QRM on the ham bands or sifting through the pocket litter of web users looking for their consumption preferences.
This morning, I admired just such an example of signal and noise while watching the NOAA Doppler weather radar. Undesired targets of a radar that return echoes are termed “clutter” in the radar parlance and one simplistic way of eliminating clutter, especially when you expect the desired scatterers (“targets”) to move, is to assume that all of the stationary returns are clutter. In the weather radar, we get clutter from all sorts of stationary things like trees, hills, and buildings. Of course, what causes the clutter to move?
You see, it was one of those humid August mornings when a ham’s mind wanders to…tropospheric ducting. Yes, indeed the clutter returns were moving, intensifying before and after sunrise. I was fixated on this and watched the loop over and over again before noticing an even more interesting bit of clutter!
Beginning at 0958 UT on 4 August 2014, there is a small ring forming out over the Elk River area. The ring, which is indicated by the downward-pointing vertical arrows, expanded over the next >40 minutes. I was puzzled and watched the loop over and over. I considered and discarded a number of theories before resorting to Google. Apparently, it’s very likely a flock of birds. Sure enough, the epicenter of the ring is Elk Neck State Park. Fascinating.
The slanted arrows in the figure above indicates the ground clutter that I was originally noticing as a signature of tropo ducting, obviously now of secondary interest in this sequence of images!
Epilogue: I sent these frames to my father, who is an avid observer of the natural world. He passed them along to two friends back home who are birders. At press time, one reported that he had learned of these “bird circles” from Greg Miller, another birder from the area who got famous as one of the subjects of the book (and movie of the same title) The Big Year. I haven’t read/seen it, but I guess they went to Adak, which has a special place in my heart. Anyhow, it’s a funny small and interesting world in which we live.
At 522,000,000 miles per watt, Voyager could be the ultimate in QRP … if you have the right antenna.
I was enjoying a leisurely sweltering summer Sunday afternoon in the back yard with Evan, Sarah, two Adirondack chairs, a kiddie pool, and the schematics for an IC-290A I have on the bench. I came in to get a glass of water and while I was inside, I checked my e-mail (since I have some equipment for sale). No prospective buyers, but I did have a message from Sean, KX9X, that he was working aurora on 6 and 2 meters. I quickly plugged in the 2-meter rig and swung the beam around to the north. Sure enough, there were raspy aurora signals all over two meters. I quickly put N9GX (EN60) in the log for my first ever aurora QSO. This was at least as cool as working K5QE on 2-meter Es with 10 watts.
So, I fumbled around a drawer and pulled out a cable to connect the TS-700S to the computer and fired up Audacity. I made this interesting recording of KA1ZE/3. I started out with the beam to the NE (45 degrees azimuth) with a strong auroral buzz on Stan’s signal. Then I swung the beam around to the NNW direct path (345 degrees azimuth). I’m in FM19la and he’s in FN01xt, which is exactly 200 miles (322 km) direct path. On the direct (forward tropo scatter) path, there is still a hint of aurora, but the tone is a bit purer. When I turn the rotor there is pretty bad hum from a (not unexpected) ground loop.
In order to better visualize a few things, I ran a short-time Fourier transform (this is the actual technical term for a “waterfall”) on the audio file. I need to code-up a polyphase implementation of the FFT like that used in Rocky, but there are only so many hours in a day. Click on the image for full-size.
There are lots of interesting details here. First, you can see that the auroral scatter is both Doppler-shifted (lower in frequency) and Doppler-broadened (spread out from the central frequency) compared with the direct tropo scatter signal. Second, you can see the ground-loop-induced hum at the low-frequency end. Auroral backscatter comes from field-aligned plasma density irregularities embedded in the auroral convection flow. Because most readers will be allergic to the vector math, we make the (somewhat gross) approximation that KA1ZE and I are transmitting and receiving from the same location. Now, we can take a stab at estimating the flow velocity from the following equation:
Where delta-f is the Doppler shifted frequency (about 300-Hz from these data), c0 is the speed of light (300,000,000 m/s), f is the carrier frequency (144 MHz), and vflow is the flow velocity. While we’re making approximations, if we round f up to 150 MHz, the twos cancel and we get the Doppler shift of 300 Hz corresponding to a flow velocity of 300 m/s (670 mi/hr). Fast! Because it is lower in frequency than the direct signal, we can also infer that the flow was directed away from us.
There you have it! Science fair projects with your ham radio.
An interesting article appeared on physorg.com yesterday regarding changes in the Earth’s magnetic field and its relation to solar activity. Although short on detail it hints at significant changes going on within our sun.
Since the mid-1800s, scientists have been systematically measuring changes in the Earth’s magnetic field and the occurrence of geomagnetic activity. Such long- term investigation has uncovered a number of cyclical changes, including a signal associated with 27-day solar rotation.
This is most clearly seen during the declining phase and minimum of each 11-year solar cycle, when the Sun’s magnetic dipole is sometimes tilted with respect to the Sun’s rotational axis. With the Sun’s rotation and the emission of solar wind along field lines from either end of the solar magnetic dipole, an outward propagating spiral-like pattern is formed in the solar wind and the interplanetary magnetic field that can drive 27-day, and occasionally 13.5-day, recurrent geomagnetic activity.
Recurrent geomagnetic activity can also be driven by isolated and semipersistent coronal holes, from which concentrated streams of solar wind can be emitted.
During the most recent solar minimum, which took place from 2006 to 2010, however, several researcher groups noticed 6.7-day and 9-day recurrent changes in geomagnetic activity, and similar patterns in the interplanetary magnetic field, and the solar wind. Using modern data covering the previous two solar minima, these higher-frequency occurrences were judged to be unusual.
Love et al. analyzed historical geomagnetic activity records from 1868 to 2011 and find that the 6.7-day and 9-day recurrent changes were actually unique in the past 140 years.They suggest that the higher-frequency changes in geomagnetic activity are due to an unusual transient asymmetry in the solar dynamo, the turbulent, rotating plasma deep within the sun which generates the magnetic field.
More information: Geomagnetic detection of the sectorial solar magnetic field and the historical peculiarity of minimum 23-24 Geophysical Research Letters, doi:10.1029/2011GL050702 , 2012
Provided by American Geophysical Union
“Geomagnetic data reveal unusual nature of recent solar minimum.”
March 19th, 2012. http://www.physorg.com/news/2012-03-geomagnetic-reveal-unusual-nature-solar.html