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Summer 2016 | VOLUME 58, NO. 2
SPACE WEATHER
Aviation
and
DRONES ARE COMING
– Space
Is the NAS Prepared?
Weather
The Effect of the Sun and Solar
Winds on Modern Aviation
By Michael Wiltberger, National Center for Atmospheric Research
Plus
T
The lesser-known fourth state of matter, plasma, is so hot
that the charged particles have become separated and form a quasineutral gas. In addition to emitting light, the sun is constantly emitting this plasma in the form of a solar wind. The conditions within
the solar wind are highly variable, depending in part on the solar
cycle, but the typical velocity is around 400 km/s, meaning that this
plasma will arrive to Earth a little more than four days after it leaves
the sun, significantly longer than the eight minutes it takes light to
travel from the sun to the Earth. Since the plasma in the solar wind
involves charged particles, it also pulls the solar magnetic field along
with it out into interplanetary space.
Figure 1 (p. 18) shows the beautiful yet complex interaction of
Space Weather
Just as the sun is a major driver of terrestrial weather, it is also the sun’s magnetic field and plasma. The plasma suspended above
the source of all space weather. While the intensity of the light the surface of the sun is held up by twists and turns of the compliemitted by the sun is relatively constant over time, our star is cated solar magnetic field. Motions of the plasma on the sun may
actually a very active body. As Galileo observed when he pointed twist and shear the magnetic field past a breaking point, resulting
his telescope at the sun, the surface is not smooth; it is dotted in an abrupt release of energy. The light and highly-charged partiwith dark spots, which we now call sunspots. These sunspots cles this process creates can f low out and reach the Earth in minare related to the emergence of magnetic fields from within the utes to hours. This process is commonly referred to as a solar f lare.
interior of the sun. The number and location of these sunspots go Often related to the solar f lare is a release of the plasma trapped
through a periodic cycle, with a period approximately equal to 11 by the magnetic field. This blob of plasma, often called a coronal
years. In fact, we have just passed the most recent solar sunspot mass ejection (CME), can also f low through interplanetary space
maximum. This periodic variation is commonly referred to as and reach Earth. These CMEs have higher velocities than the solar
the solar cycle and underlies the variation driving space weather wind, typically reaching Earth in a few days and usually carrying
with them much stronger magnetic fields.
toward the Earth.
The Journal of Air Traffic Control
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• The Effect ofhe
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Winds isonwell
Modern
weather presents within the enterprise. Space weather,
however, presents a relatively new challenge to aviation
operations. The impacts of space weather on our lives
and technology are driven by events occurring on the sun that propagate through interplanetary space and arrive at earth.
The most significant risks posed by space weather are related
to its impacts on Global Navigation Satellite Systems (GNSS).
Space weather can also degrade high-frequency (HF) radio communication and increase radiation exposure.
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SPACE WEATHER
Figure 1. Multispectral image of the sun showing a solar prominence
Image credit: NASA/SDO
Figure 2. WAAS vertical protection level during the Hallowen 2003
geomagnetic storm
Image credit: FAA
When the solar wind plasma arrives at Earth, it encounters
the Earth’s magnetic field, which provides some protection against
the entry of plasma into the near-Earth space environment. We
refer to this region of space as the magnetosphere. The solar wind
velocity is, in fact, supersonic, and its interaction with the Earth’s
magnetic field forms a shock wave, called the bow shock, upstream
of the Earth. Behind this shock, the solar wind plasma and magnetic field interact with the Earth’s field. If the solar wind field is
traveling in the opposite direction to the Earth’s field, the same
process of magnetic field breaking that released CMEs from the
sun can happen, allowing mass, momentum, and energy to f low
into the magnetosphere.
As previously mentioned, CMEs typically have higher speeds
and stronger magnetic fields than the ambient solar wind, resulting
in a compressed magnetosphere and, if the CME’s magnetic field is
opposite to the Earth’s field, deposition of a significant amount of
energy into the magnetosphere. This energy typically drives a phenomenon known as a geomagnetic storm, resulting in visible aurora
and enhancements to the Van Allen radiation belts.
The last piece of the system involved in space weather is called
the ionosphere. At high altitudes, above 80 km, the ultraviolet light
from the sun partially ionizes the atmosphere, resulting in a mix of
neutral gas and plasma. Electrical currents driven by the solar wind
interacting with magnetosphere complete their circuits through
this ionized gas. During geomagnetic storms these currents are
enhanced, which can result in the aurora borealis, the most beautiful space weather phenomena.
The aurora is more than a dynamic light show. The electrons
carrying the enhanced currents from the magnetosphere are energized and excite light from atoms, in the same fashion as a cathode-ray tube in an old TV. These electrons can also drive significant changes to the structure and composition of the ionosphere.
communication systems, as well as enhanced exposure to radiation.
Effects on Aviation
As the recently released National Space Weather Action Plan outlines, space weather poses risks to a broad range of technologies
that contribute to the nation’s security and economic vitality.[1]
Perhaps the most significant are the potential for currents driven in
the magnetosphere and through the ionosphere to disrupt the electric power grid over a wide geographic area. For the aviation community, changes in the ionosphere can cause risks to navigation and
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Summer 2016
Effects on GNSS
Global navigation satellite systems, such as the global positioning
system (GPS) operated by the United States, have become an essential part of aviation, and the FAA’s NextGen modernization plan
calls for even more utilization of this technology. In a simplified
fashion, this technology uses triangulation to determine the location of the receiver by calculating the location of multiple satellites
and how long it has taken the signal to reach the receiver. One of
the potential sources of error in the location calculation is delay in
the signal caused by the ionosphere. Modern GPS systems include
an ionospheric model to account for seasonal and solar cycle variations of the ionosphere.
One potential way to address errors introduced by the ionosphere is through the use of ground reference stations, such as
the WAAS set up by the FAA. The idea here is to use a series
of known locations on the ground to provide a corrective term to
WAAS-enabled GPS receivers to use in their location calculations.
Unfortunately, during strong geomagnetic storms, the ionosphere
can develop small-scale features that create errors beyond the ability of WAAS to correct.
As shown in Figure 2, a series of CMEs on the sun resulted
in a major geomagnetic storm on October 30, 2003. As illustrated by the graphic taken from the beginning of the geomagnetic
storm, the vertical protection level, i.e., the region assured to contain the indicated vertical position, is exceeding the limit of 50 m
allowed for precision approaches over most of the continental US
and Canada.[2] It is important to note that the system is behaving
correctly; it detects the extreme disturbance of the ionosphere and
indicates that it should not be used for precision approaches. In
fact, for a 15-hour period on October 29 and an 11-hour period
on October 30, commercial aircraft were unable to use WAAS for
precision approaches. During the most recent solar cycle, which has
been weaker than the previous one, several storms have disrupted
WAAS, including those on February 27, 2014, and March 17,
2015. It’s also worth pointing out that major geomagnetic storms
can occur at any point in the solar cycle; major ones occur more
frequently during the declining phase of the solar cycle.
Current forecasts of major geomagnetic storms with the
potential to disrupt WAAS are based upon detection of an
SPACE WEATHER
Figure 3. Graphic illustrating regions and frequencies affected by March
8, 2011, solar flare
Image credit: NOAA/SWPC
Figure 4. Radiation alert regions issued by the FAA during the 2003
Halloween storm
Image credit: FAA
earthward-directed CME by NASA research satellites. Forecasters
use this information to predict arrival times, within six hours,
about two to three days in advance. Unfortunately, the magnetic
field direction within the CME controls the size of the resulting
geomagnetic storm, and that is not detected until the CME passes solar wind monitors about 45 minutes upstream of the earth.
Support is needed for research required to predict the magnetic
field direction within the CME.
Figure 3 shows the impact of a solar flare event on March 8,
2011. The degraded frequencies over Australia are a result of the
radiation. The peak of this region is centered on the part of the Earth
currently facing the sun. The solar energetic particles (SEPs) emitted
by the flare are affected by the Earth’s magnetic field, so the region
affected by these particles is located near the magnetic poles.
The duration typically lasts between one to 10 hours and
requires utilization of frequency shifts or alternate communication
methods. This can be challenging for polar routes since communication with geosynchronous satellites may not be possible over the
entire route of f light due to line-of-sight issues between the aircraft
and satellite.
It is also important to point out that currently we do not have
good tools for forecasting solar f lares; communication systems are
already affected by the time of detection, typically by the GOES
X-ray sensor. Predicting the exact time of a solar f lare is beyond
our current physical understanding of the sun. Research is ongoing to improve probabilistic forecasts for solar f lares in order to
provide a higher quantification of the risks during an operating
interval.
Effects on HF Communications
High frequency (three to 30 MHz) radio communications are
commonly used in aviation, especially for communications with air
traffic control when over the oceans or poles, where short-range
VHF communication is not possible.
The energetic particles and radiation released during solar
f lares travel at or near the speed of light and arrive at the earth
eight minutes after the event occurs on the sun. Solar f lares typically last from minutes to hours and can create additional ionization
of the upper atmosphere, limiting which frequencies can be used
effectively for radio communications.
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The Journal of Air Traffic Control
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SPACE WEATHER
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Radiation Exposure
The upper atmosphere is affected by galactic cosmic, solar, and
magnetospheric radiation. The galactic cosmic radiation (GCR)
is omni-directional in nature and originates from supernovae
throughout the galaxy. The SEPs are created by solar f lares and
the shocks driven by CMEs. The magnetospheric radiation, a.k.a.
the Van Allen radiation belt, is often enhanced during geomagnetic
storms.
The intensity of this radiation increases with latitude and
altitude. The increase of radiation in higher altitudes is due to the
reduced absorption of ionizing radiation by the small amount of
atmosphere. The increase with latitude results from the shielding
effects of the Earth’s dipolar magnetic field. The FAA has estimated that aircrew exposures range from 0.2 to 9.1 mSv per year,
which is potentially larger than the 0.5 mSv per year exposure of
the average nuclear power worker. The exposure level, however,
is less than the internationally accepted annual dose limit of 20
mSv. The maximum radiation exposure from the GCR occurs at
solar minimum, the period in the 11-year solar cycle when solar
f lare and sunspots are at their lowest frequency. This is because
the “smoother” magnetic field structure of the sun and solar wind
at solar minimum presents fewer structures that can scatter the
incoming cosmic radiation and results in more radiation reaching
the Earth. Note: this variation is why NASA currently plans for
a manned mission to Mars to occur around a solar maximum to
reduce the overall radiation risks to the “colonists.”
During major geomagnetic storms, distortions of the Earth’s
magnetic field caused by the enhanced velocities and magnetic
fields present inside the CME allow SEPs and Van Allen Radiation
belt particles greater access to lower latitudes. During the 2003
Halloween storm, the FAA issued a radiation alert shown in Figure
4. During a lesser storm event, the alert regions would be located
near the geomagnetic poles; however, the magnetic field distortions’ magnitudes in this event pushed the regions where energetic
particles could reach aviation altitudes as far south as Miami. The
alert text noted that avoiding excessive radiation exposure is particularly important during pregnancy and that lowering f light altitude
can reduce the radiation dose rate.
Just like the GNSS impacts, the region at risk of higher radiation exposure is largely determined by the direction of the magnetic
field within the CME. It is possible to improve upon the FAA’s
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Summer 2016
relatively coarse alert regions by utilizing magnetic field modeling.
Additional data, especially from high latitude f lights, will be useful
in quantifying the exposure level on these f lights.
Conclusions
Space weather presents the aviation community with an array of
challenges. The changes to the ionosphere driven by solar f lares
and the geomagnetic storms following CMEs can affect radio
propagation. These impacts can degrade the accuracy of GPS systems and render certain HF bands ineffective for communications.
CMEs can drive short-term increases in the radiation exposure at
aviation altitudes. The increased exposure risk is greatest for high
latitude f lights, especially those f lying over the poles. The scientific community is working to develop tools to improve the forecast
accuracy and lead time of space weather impacts.
Anyone interested in knowing the current space weather
conditions and forecast can find that information on the Space
Weather Enthusiasts page provided by NOAA’s Space Weather
Prediction Center.[3] Those interested in taking a deeper dive into
the physics behind space weather are encouraged to complete the
Physics of the Aurora COMET MetEd training module.[4]
The National Science Foundation supports the National
Center for Atmospheric Research. The views expressed are those
of the author and do not necessarily represent the official policy or
position of the funding agencies. The thoughtful comments provided by Bruce Carmichael, Sarah Gibson, Robert Rutledge, and
Matthias Steiner during the preparation of this article are greatly
appreciated.
Dr. Michael Wiltberger is a scientist in the High Altitude Observatory
of the National Center for Atmospheric Research. His professional interests are in space physics with an emphasis on developing space weather
forecast models. Email: [email protected]
References
[1.] National Science and Technology Council, 2015, National Space Weather
Action Plan, Washington, DC.
[2.] http://www.nstb.tc.faa.gov/terms.html
[3.] http://www.swpc.noaa.gov/communities/space-weather-enthusiasts
[4.] https://www.meted.ucar.edu/training_module.php?id=161