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Insight into why scientists are having difficulty predicting this solar cycle




 
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Author Topic: Insight into why scientists are having difficulty predicting this solar cycle  (Read 2720 times)
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W1VD
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« on: August 07, 2011, 10:59:38 AM »

Interesting read. Copied over from the 600 meter mailing list.

_________

Gang, (warning: very long)

As most of you know, I 've worked at the Very Large Array (VLA) radio
telescope, now for 34 years. Let me share a few things I know about what's
going on with the sun and solar observing efforts.

Keep in mind, these are my personal opinions and experiences, and I'm not
representing the observatory.

This description is fairly long, as I want as many as possible to understand
some of the complicated aspects of solar physics, and why this prolonged
solar minimum has been so hard to predict.

Let me first add that over the years, I have shared posts, charts and graphs
posted by hams on solar related HF propagation with our scientific staff,
the PhD astronomers. Stories of when a solar flare or geomagnetic storm
hit; rudimentary “S-meter” measurements, and strange skip conditions.
They have always marveled that a bunch of hams (mostly QRPers) have such an
interest in “Solar Weather” and have bothered to learn some of the solar
physics required to understand it as a hobby. They are impressed. Solar
physics is a VERY small group of people. Hams are well recognized by this
small group.

Here's the main problem: what really causes sunspots, flares, CMEs, the
solar cycle, etc. is not well understood. It is known they are probably
linked to magnetic fields and other phenomenon going on inside the
photosphere or underneath it (inside the sun's interior), and magnetic
processes and plasma physics in the hot corona. The photosphere is the
“surface” of the sun where light originates. The corona is the solar
“atmosphere” above it.

Solar astronomers have numerous problems trying to figure out solar physics.

First, information to see or map (make images) of magnetic and plasma
physics are more dominant at relatively low frequencies, defined here as
below 1 GHz, in the 100s of MHz in the meter wavelengths, in the 10s of MHz
- the decameter wavelengths, and even down to the 10s of Khz.

In optical astronomy, including our own eyes, we see light emitted from the
sun from both the photosphere and from the corona. The corona is very
bright, and why you can't see the actual solar disk (the photosphere)
without special filters or equipment. You've probably seen photos of the
sun during a total eclipse with all the splinters of light extending outward
when blocked by the moon at totality. This is one of the few cases where
you can see just the corona while the moon is blocking the photosphere.
Otherwise, the sun is just a big, fuzzy ball of bright light, hard to
discriminate the photosphere from the corona.

We can penetrate the bright corona to see the photosphere at very short
wavelengths, in the 10s of GHZ region with radio, and shorter wavelengths in
the optical or near-optical domain. This reveals the thermal processes
going on at the surface of the photosphere, but not the non-thermal,
magnetic or some plasma processes going on.

To “see” what is going on near the photosphere with magnetic fields and
other physics, or to look at what are called the electron plasma and
electron cyclotron frequencies, we need to observe at low frequencies (LF),
typically around 400 MHz to around 10 MHz. Also remember, that the MUF
(maximum usable frequency), below which our HF signals bounce back to earth,
works in reverse … low frequency emissions from the sun lower than the MUF
are also bounced off the E-F layers back into space and do not reach earth
borne instruments. So our MUF tends to be the lower radio limit of
observing the sun. What we call the MUF, astronomers call the “plasma
frequency.” Some LF solar emissions, in the KHz region, also manage to
penetrate our ionosphere.

So the obvious question is “so why don't you build a low frequency radio
telescope to peer deep inside the sun?”

Such low frequency solar arrays have been built since the late 1950s,
limited in use by the technology of the time. Most have been built by
universities with limited budgets yielding instruments with resolutions of
10 arc minutes or worse (making fairly course contour lines for a fairly
crude map), and using very narrow bandwidths to avoid RFI. Still, they have
produced some useful science. The Clark Lake Radio Telescope in California,
designed and built by Dr. Bill Erickson, was the first major
science-producing solar array – though long since decommissioned and
dismantled. Most LF arrays, once used for solar science, are all gone due
to the limited science they produced and lack of funding. Only the Culgoora
array in Australia and the Nancay Array in France remain operational at low
frequencies, though mostly used for deep space science over solar observing.
The Giant Meterwave Radio Telescope (GMRT) in Pune, India also has
capabilities at 50, 153, 233, 325, 610 and 1420 MHz, though also seldom used
for solar observing. The Owens Valley Radio Observatory (OVRO) in
California also did some solar burst work until recently.

You can do a search on the above observatories for more information.

Far more useful science is derived in the microwave frequencies than at LF,
such that this is where the money has gone into instruments over the past 40
years. Basically, bigger bang for the buck. As a result, LF astronomy has
been almost totally neglected, and astronomers are beginning to complain
about this situation.

There are some recent projects to build some LF instruments such as the Long
Wavelength Array (LWA), for which two arrays are now built and operating on
a limited basis at the VLA, the Frequency Agile Solar Radio telescope (FASR
– pronounced “phasor”) though currently un-funded, and a couple of
others on the drawing board. These are years away from producing science.

So what about the VLA? Why don't we add a low frequency component to the
VLA dishes? Actually, we have and it produced some very useful science,
including the first ever low frequency sky survey conducted at 74 MHz and
mapping the sun's Faraday rotation. The antenna was a cross-polarized
dipole (2 dipoles at right angles to sense differences in linear
polarization) suspended between the quadrapod legs that holds up the apex
and subreflector. They were positioned in an attempt to “focus” the
dipoles between the dish and using the subreflector as a virtual ground.
Using the 88-foot diameter VLA dishes as a giant reflector, these basic wire
antennas produced about 26dB of gain at 74 and 327 MHz!

In 2007, we began a major upgrade to the original 1975 VLA electronics,
converting the old narrow band receivers to wide band cryogenic receivers
cooled to 15K, a completely new suite of receiver feed horns, going from a
50 MHz IF to a 4 GHz IF, with a 4 GHz sampler and correlator, fiber optic
interfaces, etc. Converting all 28 antennas, without interrupting
observing, was completed in January of this year (2011). This project is
called the EVLA – Expanded Very Large Array. We now have complete
coverage from 1-50 GHz with a suite of 8 cryo cooled receivers and greatly
enhanced sensitivity and resolution. The receiver bands are the entire L,
S, C, X, Ka, K, Ku and Q bands.

Unfortunately, the EVLA transition electronics (2007-2011), and the final
EVLA configuration, was not compatible with the old 74 MHz and P-band
(300-400 MHz) receivers. As a result, we pulled the dipoles and receivers
off the antennas during the EVLA retrofits and ceased accepted LF observing
proposals at the end of 2006. There has been no LF solar observing since.
Even worse, many of the older solar LF arrays also ceased operation during
this time, leaving the astronomical community with virtually no low
frequency solar imaging instruments to this day.

So, why not design a new set of LF receivers to be compatible with the EVLA?
Well, actually, that's what we're doing right now. In fact, this is the
project I'm now working on called LBR for Low Band Receiver. It is a joint
project between us (National Radio Astronomy Observatory, NRAO) and the
Naval Research Laboratory (NRL). I am designing the NRAO portion and I
should have the first receiver prototype built in about a month. It is a
4-channel receiver (times two polarizations) and will have continuous
coverage from 50-500 MHz with selectable filters, both analog and digital,
for filtering out RFI. It also has a switching noise source for constantly
calibrating the noise temperature of the electronics to derive the actual
temperature of the signal. For now, we'll be using the same old dipoles hung
on the dishes. A great ham radio project, and I'm getting paid to do it :-)

We're hoping to have all 28 receivers built and installed on the VLA
antenna, and tested with our new correlator, when we go to the A-array in
September 2012. The A-array is the most spread out antenna configuration,
with antennas extending out the full 13 miles on each of the three arms of
the VLA. We need this wide separation between antennas to properly
“fringe” the antennas at these low frequencies to act as an
interferometer. It is expected the resolution of this configuration will be
in the 10s of arc SECONDS for making fairly detailed images. I don't think
any LF solar instrument has anything better than an arc-MINUTE resolution,
so this will be an improvement by an order of magnitude.

The LBR will be used for solar physics and deep space science. For deep
space science, mapping the low frequency plasma and magnetic fields at the
center of our galaxy, a black hole, is one of the goals. Finding magnetic
fields with no optical counterpart could identify planets around other suns.

Another goal is to detect the “Epoch of Reionization.” This is
theoretically where the fast moving electrons from the big bang are slowing
down, if not halting, to form a giant wall of electrons some 13.7 billion
light years away. These slowing free electrons would then be recombining to
form ionized hydrogen molecules (hence, "reionization"). Theoretical
astronomers estimate the 1432 MHz spectral line of hydrogen has been red
shifted down to about 196 MHz in this region. Now that is a major red shift
and would be a significant scientific discovery. If detected, it might
answer the age old question if our universe is still expanding. Of course,
these signals would be extremely weak, probably in the order of 20K, or
0.3dB above the galactic background. (Hint: 1dB noise figure (NF) = 75
degrees Kelvin of noise temperature). If the LBR system on the VLA can't
detect it, there is currently no other instrument on Earth capable of
detecting it either.

And finally, of interest to QRPers, one purpose of the LBR on the VLA will
be to look through the ionosphere at a bright, distant calibrator source,
such as Cas A, and using interferometry of the 28 VLA antennas, measure the
exact stratified layers of electron densities of our ionosphere. In other
words, it is hoped this will produce dynamic, real time 3D images of our
ionosphere … including exactly where the E and F layers are currently at
(their heights above ground for skip distance and where signals would likely
land), their electron densities (for coefficient of reflection at different
frequencies) and the plasma frequency (i.e., the MUF). The LWA project will
be able to do the same ionospheric imaging. At present, our scientists are
excited about this capability, but believe it has little outside interest.
I am certainly encouraging the folks upstairs that if successful, please
make the data available online in real time! At least for us QRPers -hi.

BACK TO THE SOLAR STUFF

We are experiencing a very unusual solar cycle best described by four-letter
words. It is not known why this very quiet and prolonged solar cycle is
occurring. One of the reasons, is there are currently no low frequency
solar imaging radio telescopes “on line,” leaving a fairly large hole in
studying the sun during this unusual cycle.

Scientists are doing their best using existing instruments, but these are
almost exclusively high frequency radio (>1 GHz) and optical and near
optical instruments. Satellites also provide gamma and x-ray emissions,
measure the solar wind, electron and proton emissions, and image the sun at
various wavelengths. Just about every wavelength you can image, except the
long wavelengths. From this information, they infer what must be going on
in the magnetic and plasma domains. But so far, with an absence of real,
quantitative data, nothing has been found to suggest why the sun is so
inactive, nor any history to compare it with.

Even when the LBR receivers at the VLA become operational, and the LWA
array, there are still some problems in doing solar science for what
astronomers would like to see.

First, with much of the interest in solar imaging for the magnetic dynamo
and plasma physics in the low frequencies, finding a “clear channel” to
observe in the 10-400 MHz or so is difficult. These frequencies are filled
with man made transmissions. RFI is not only a problem in finding clear
frequencies, but also many earth generated transmissions in this spectrum
are “loud” and can easy saturate the sensitive receivers.

For some of this science, the widest bandwidth one can find is needed.
We're certainly hoping to exploit the old analog TV band, now vacant, for as
long as that condition exists.

And finally, the biggest problem of all. I will quote from an internal
proposal regarding observing in the 10-400 MHz spectrum. “Coronal
magnetic fields have heretofore been inaccessible to quantitative study
(i.e., ability to make direct measurements). Quantitative knowledge of
coronal magnetic fields is crucial to virtually all solar physics above the
photosphere, including the structure and evolution of active regions,
flares, filaments, and coronal mass ejections. … Radio observations
provide the means of both directly and indirectly measuring magnetic fields
in the corona. However, such measurements require a broadband imaging
capability."

Now the kicker: “One of the fundamental questions in solar physics is how
the solar corona maintains its high temperature of several million degrees
Kelvin above a surface of [only] 6000 K.” It is not known where the power
needed to produce this heating comes from, with things like nano-flares to
some sort of resonant wave heating being suspected.

This is why it is so important to attempt to look inside this hot corona at
low frequencies where the plasma physics is taking place. This is where the
extreme heat ionizes electrons, and further heating generates the plasma.
But, as stated above, it is not known what physics keeps the corona so darn
hot. What causes the transition of electrons to plasma, other than heat?
What are the magnetic fields doing? Might it have something to do with the
stalled solar minimum? Plus, of course, trying to image these same things
on the photosphere.

Again, the main problem is to see the photosphere, magnetic fields, etc.,
basically sitting at 6000 degrees Kelvin, when you have to look through a
two million degree furnace first. Kinda like trying to see that oncoming
car when you're driving into a blinding sun – and trying to read the car's
license plate number. It is hoped the new LF receivers, the wide spacings
of the VLA antenna, and new techniques when correlating the signals, will
allow us to probe deeper into the corona and hopefully resolve the
photosphere. And of course, to provide new information that can be used to
predict solar behavior.

I know all of this discussion does not explain why our solar cycle is such a
dud. But hopefully, it will explain why nobody else is offering an
explanations either. There was just an unfortunate chain of events that
left solar scientists with a lack of instruments that might be offering a
clue. And the excellent instruments that are being used are, as of yet, not
sensing anything unusual. And lastly, letting you know there are new
exciting instruments that will hopefully be operational in about a year, and
hopefully those instruments will be available to perform a new level of
solar observing, and making the data and images available to the public real
time – especially the 3D imaging of our E and F layers. There is no doubt
QRPers will learn to interpret such information to our advantage when it
becomes available. We'll share it with the solar astronomers!

If anything does crop up within scientific circles that might explain our
current solar inactivity, I will certainly pass it on. But otherwise, I am
as much at a loss to explain it as you. We can only hope it improves.

For what it's worth … best DX :-(

72, Paul NA5N
Socorro, NM


       
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Steve - K4HX
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« Reply #1 on: August 07, 2011, 11:29:30 AM »

Excellent! Thanks for sharing.
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W3SLK
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Just another member member.


« Reply #2 on: August 07, 2011, 12:40:55 PM »

Its kind of like the dynamics of predicting weather precisely, because Mother Nature, and her cousin 'Ole Sol' tend to throw a monkey wrench in to preclude conventional wisdom  Wink
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Mike(y)/W3SLK
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« Reply #3 on: August 07, 2011, 01:12:34 PM »

Apparently, from the article, radio astronomers use different terminology from that used by communications engineers. What is described as "LF" would fall into what we call HF/VHF/UHF.

Back in the late 60s during the heyday of the Apollo era, as the film version of Arthur C. Clarke's "Space Odyssey" was captivating the public's imagination, conventional wisdom was that by now we would have permanent outposts on the lunar surface. If, in order to finance the ongoing war to be followed by more wars to come, the lunar program hadn't been de-funded and scrapped once we showed the world that we beat the Russians, the lunar surface might have made an excellent platform for those "low frequency" radio observatories, well away from the shield of the ionosphere and RFI from earthly telecommunications.
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« Reply #4 on: August 07, 2011, 03:37:07 PM »

Our New Mexico cabin site is on a high mesa (8000ft) overlooking the valley where the VLA is located.

We took the tour a few years back, it is indeed an amazing place, doing amazing things to map our heavens.

In a word the place is HUGE!  It takes two sets of full-gage railroad tracks to move the individual dishes in a three-legged pattern, miles across.

73DG
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« Reply #5 on: August 07, 2011, 05:54:21 PM »

the lunar surface might have made an excellent platform for those "low frequency" radio observatories, well away from the shield of the ionosphere and RFI from earthly telecommunications.

Why stick with a terrestrial sort of HF antenna system?

In 0-G space, you could build wire Yagis as large as you wanted.  20 elements at 100 KHz, anyone? How about dipoles for 10 HZ?
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« Reply #6 on: August 08, 2011, 01:00:23 AM »

If you haven't seen the VLA site in NM it's worth the trip, and the drive out there is spectacular!

Great post! Thanks!
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