The ionosphere has a non-uniform index of refraction to radio waves propagating through it, since its electron density is a complex function of space and time. At each change of refraction index, a propagating radio wave's direction changes according to Snell's Law (think of the bending of light at an air-water interface).
To model a radio wave path through the ionosphere, a ray tracing code models the ionosphere as a medium with a varying refractive index, and then applies Snell's law at each small change to compute the ray's final path.
VOACAP is one example of a ray tracing code developed over a number of years by a shortwave broadcaster (Voice of America = US government external broadcasts). It is freely available - for more information, see
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Think of the magnetic fields of Earth being concentrated at the North and South poles. So if there's a magnetic disruption, you'd expect it to effect the magnetic poles the most... therefore "polar paths."
An additional memory tool is to look at the "A" and "K" indexes from the question. AK = Alaska, Alaska has Polar Bears...Polar Paths.
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The orientation of the magnetic field in a coronal mass ejection from the sun is a huge factor in determining the strength of the event's effects here at Earth.
When coronal mass ejections occur on the Sun, a huge amount of energy is released from violent reconfigurations of the complex magnetic field surrounding the star. These events cause high energy charged particles to stream outwards from the sun's outer atmosphere, or corona, into the solar system. Since the particles are charged, they carry with them magnetic field perturbations as they move outwards that are partially (but not completely) reflective of the conditions at the sun when they were released.
If the CME's particles encounter Earth, we have a geomagnetic storm. One of the biggest regulators of the strength of effects of the particles on our magnetosphere and ionosphere is in the disturbance magnetic field's Z orientation (\(+\) = toward our north pole, \(-\) = toward our south pole), labeled as Bz.
If Bz is southward in the incoming particle cloud, this is opposite to our Earth's normal magnetic field - think of the bar magnet repelling a magnetic with opposite polarity. In this case, the CME associated Bz then pushes on the Earth's magnetic field and compresses it, dumping energy into the magnetosphere. The energy input can lead to oscillations in the magnetic field as the Earth 'fights back'.
Since the ionosphere is tightly bound to the background magnetic field lines, it too begins to move and oscillate, quickly developing lots of irregularities, scintillation, and other structure that degrades long distance HF propagation. This is why Bz is a key factor in determining geomagnetic storm strength at Earth.
[FYI, NASA maintains spacecraft monitors just upstream of earth - the Advanced Composition Explorer (ACE) and the new DSCOVR satellite - that have comprehensive space environment monitors, and one of the key variables measured there is the orientation of B.]
Think when things "go south" something bad is happening.
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VHF and UHF have a relatively short range due to the fact that the high frequencies can't get bent by the atmosphere, which makes the range about seeing distance.
You can then guess that the radio waves will extend only slightly farther than the horizon, which is 15% that distance.
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Solar flares are ranked A, B, C, M, and X with "A" being the lowest and "X" the highest. Intensity increases in alphabetic order.
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The NOAA Geomagnetic Storm Scale indicates the severity of geomagnetic storms. It is denoted by a G followed by a number from 1 to 5, with 1 being a minor event, and 5 being an extreme event. Power systems: Weak power grid fluctuations can occur.
G5 is an extreme geomagnetic storm with the following effects:
Power systems: Widespread voltage control problems and protective system problems can occur; some grid systems may experience complete collapse or blackouts. Transformers may experience damage.
Spacecraft operations: May experience extensive surface charging and problems with orientation, uplink/downlink, and tracking satellites.
Other systems: Pipeline currents can reach hundreds of amps, HF (high frequency) radio propagation may be impossible in many areas for one to two days, satellite navigation may be degraded for days, low-frequency radio navigation can be out for hours, and auroras have been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.).
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The expected answer is twice as great, so that is what you should choose on the exam. This is known to be incorrect but unfortunately is the value in the official exam; we can hope it will be changed or removed, but keep that in mind when you test. (you can submit feedback to email@example.com)
The A, B, C, M, X scale is logarithmic, where each letter in the index represents a 10-fold increase. So an X-class flare is 10 times more powerful than an M-class flare, and 100 times more powerful than a C-class flare.
Within each class there is a linear scale that goes from 1–9.
(NOTE: as there is no letter beyond X, scientists continue the linear scale beyond X9. A very powerful flare in 2003 measured X28 before the sensors cut out!)
Imagine that this is how big an X1 is:
That would make an X2 this big:
And an X3 would be this big:
In other words, X3 is not twice as big as X2, but only 50% bigger.
Wikipedia's article on Solar flares is actually a little confusing. I'd suggest the NASA and UNC links above.
Note: The last great X2 occurred in October 2013.
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304A can be thought of as 304 Å (Angstroms). The word angstroms only occurs in the correct answer: UV emissions at 304 angstroms, correlated to solar flux index.
But what is the Solar Flux Index, and what does it have to do with Ultraviolet Light at 304 Å?
Solar Flux Index (SFI) is measured in solar flux units (SFUs). SFI is defined as the amount of flux (radio noise) emitted at 2800 MHz. Because 2800 MHz correlates with a wavelength of 10.7 cm, it is also referred to as the 10.7 cm Flux Index.
An ångström or angstrom unit (Symbol: Å or A) is a unit of length equal to 10–10 meters, used principally to express the wavelengths of electromagnetic radiation. It is named after Swedish physicist Anders Jonas Ångström.
This problem refers to λ = 304 Å = 30.4 nm.
The 304A Index, often shortened to just 304A, is a
"NOAA reported value from 0 to unknown. Indicates relative strength of total solar radiation at a wavelength of 304 angstroms (or 30.4 nm), emitted primarily by ionized helium in the sun's photosphere. Two measurements are available for this parameter, one measured by the Solar Dynamics Observatory, using the EVE instrument, and the other, using data from the SOHO satellite, using its SEM instrument. Responsible for about half of all the ionization of the F layer in the ionosphere. 304A looseley correlates to SFI. The background level - at solar minimums - will typically be around 134, and at solar maxima can exceed 200 or more. Updated hourly."
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VOACAP stands for Voice of America Coverage Analysis Program. It is used to predict HF propagation.
VOA is the clue contained in the question. As a shortwave broadcast operation, HF propagation would be the one thing they might be most interested in.
See HF predictions map.
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Ground waves are a result of interaction of the radio signal with the ground. The interaction has the effect of causing the signal to follow the curvature of the earth. Ground wave propagation is most useful on the 1.8MHz and 3.5MHz bands during daytime.
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Ground waves are a result of interaction of the radio signal with the ground. The interaction has the effect of causing the signal to follow the curvature of the earth. Ground wave propagation is most useful on the 1.8 MHz and 3.5 MHz bands during daytime.
One way to remember this answer is to consider commercial AM broadcast: ground-wave propagation via vertical towers that polarize vertically.
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The effect is caused by inversion layers (density changes) in the atmosphere. This effect is more commonly referred to as tropospheric ducting. An excellent write up can be found here: https://en.wikipedia.org/wiki/Tropospheric_propagation
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