Ionosphere

Source: Wikipedia, the free encyclopedia.

The ionosphere (

above sea level,[3] a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on Earth.[4] It also affects GPS signals
that travel through this layer.

Relationship of the atmosphere and ionosphere

History of discovery

As early as 1839, the German mathematician and physicist

Glace Bay, Nova Scotia, one year later.[7]

In 1902,

Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties.[9]

In 1912, the

amateur radio operators, limiting their operations to frequencies above 1.5 MHz (wavelength 200 meters or smaller). The government thought those frequencies were useless. This led to the discovery of HF radio propagation via the ionosphere in 1923.[10]

In 1925, observations during a solar eclipse in New York by Dr. Alfred N. Goldsmith and his team demonstrated the influence of sunlight on radio wave propagation, revealing that short waves became weak or inaudible while long waves steadied during the eclipse, thus contributing to the understanding of the ionosphere's role in radio transmission.[11]

In 1926, Scottish physicist Robert Watson-Watt introduced the term ionosphere in a letter published only in 1969 in Nature:[12]

We have in quite recent years seen the universal adoption of the term 'stratosphere'..and..the companion term 'troposphere'... The term 'ionosphere', for the region in which the main characteristic is large scale ionisation with considerable mean free paths, appears appropriate as an addition to this series.

In the early 1930s, test transmissions of

HAARP ran a series of experiments in 2017 using the eponymous Luxembourg Effect.[13]

Maurice V. Wilkes and J. A. Ratcliffe researched the topic of radio propagation of very long radio waves in the ionosphere. Vitaly Ginzburg
has developed a theory of electromagnetic wave propagation in plasmas such as the ionosphere.

In 1962, the Canadian satellite Alouette 1 was launched to study the ionosphere. Following its success were Alouette 2 in 1965 and the two ISIS satellites in 1969 and 1971, further AEROS-A and -B in 1972 and 1975, all for measuring the ionosphere.

On July 26, 1963 the first operational geosynchronous satellite Syncom 2 was launched.[14] On board radio beacons on this satellite (and its successors) enabled – for the first time – the measurement of total electron content (TEC) variation along a radio beam from geostationary orbit to an earth receiver. (The rotation of the plane of polarization directly measures TEC along the path.) Australian geophysicist Elizabeth Essex-Cohen from 1969 onwards was using this technique to monitor the atmosphere above Australia and Antarctica.[15]

Geophysics

The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth,[16] stretching from a height of about 50 km (30 mi) to more than 1,000 km (600 mi). It exists primarily due to ultraviolet radiation from the Sun.

The lowest part of the

plasma
which is referred to as the ionosphere.

recombination
, in which a free electron is "captured" by a positive ion. Recombination occurs spontaneously, and causes the emission of a photon carrying away the energy produced upon recombination. As gas density increases at lower altitudes, the recombination process prevails, since the gas molecules and ions are closer together. The balance between these two processes determines the quantity of ionization present.

Ionization depends primarily on the Sun and its Extreme Ultraviolet (EUV) and X-ray irradiance which varies strongly with

mid-latitudes
, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization.

Sydney Chapman proposed that the region below the ionosphere be called neutrosphere[17] (the neutral atmosphere).[18][19]

Layers of ionization

Ionospheric layers.

At night the F layer is the only layer of significant ionization present, while the ionization in the E and D layers is extremely low. During the day, the D and E layers become much more heavily ionized, as does the F layer, which develops an additional, weaker region of ionisation known as the F1 layer. The F2 layer persists by day and night and is the main region responsible for the refraction and reflection of radio waves.

Ionospheric sub-layers from night to day indicating their approximate altitudes
Lightning sprites.

D layer

The D layer is the innermost layer, 48 km (30 mi) to 90 km (56 mi) above the surface of the Earth. Ionization here is due to

solar flares
can generate hard X-rays (wavelength < 1 nm) that ionize N2 and O2. Recombination rates are high in the D layer, so there are many more neutral air molecules than ions.

Medium frequency (MF) and lower high frequency (HF)

cosmic rays. A common example of the D layer in action is the disappearance of distant AM broadcast band
stations in the daytime.

During

solar proton events, ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare events are known as Polar Cap Absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region.[20]
In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours.

E layer

Main article: Kennelly–Heaviside layer

The

sporadic E
events, the Es layer can reflect frequencies up to 50 MHz and higher. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer weakens because the primary source of ionization is no longer present. After sunset an increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.

This region is also known as the

Miles Barnett
.

Es layer

The Es layer (

radio amateurs
very exciting when long distance propagation paths that are generally unreachable "open up" to two-way communication. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs every day during June and July in northern hemisphere mid-latitudes when high signal levels are often reached. The skip distances are generally around 1,640 km (1,020 mi). Distances for one hop propagation can be anywhere from 900 km (560 mi) to 2,500 km (1,600 mi). Multi-hop propagation over 3,500 km (2,200 mi) is also common, sometimes to distances of 15,000 km (9,300 mi) or more.

F layer

Main article: F region
Main gases of the ionosphere (about 50 km; 30 miles and above on this chart) vary considerably by altitude

The

shortwave
) radio communications.

Above the F layer, the number of oxygen ions decreases and lighter ions such as hydrogen and helium become dominant. This region above the F layer peak and below the plasmasphere is called the topside ionosphere.

From 1972 to 1975 NASA launched the AEROS and AEROS B satellites to study the F region.[21]

Ionospheric model

An ionospheric model is a mathematical description of the ionosphere as a function of location, altitude, day of year, phase of the sunspot cycle and geomagnetic activity. Geophysically, the state of the ionospheric plasma may be described by four parameters: electron density, electron and ion temperature and, since several species of ions are present, ionic composition. Radio propagation depends uniquely on electron density.

Models are usually expressed as computer programs. The model may be based on basic physics of the interactions of the ions and electrons with the neutral atmosphere and sunlight, or it may be a statistical description based on a large number of observations or a combination of physics and observations. One of the most widely used models is the International Reference Ionosphere (IRI),[22] which is based on data and specifies the four parameters just mentioned. The IRI is an international project sponsored by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI).[23] The major data sources are the worldwide network of ionosondes, the powerful incoherent scatter radars (Jicamarca, Arecibo, Millstone Hill, Malvern, St Santin), the ISIS and Alouette topside sounders, and in situ instruments on several satellites and rockets. IRI is updated yearly. IRI is more accurate in describing the variation of the electron density from bottom of the ionosphere to the altitude of maximum density than in describing the total electron content (TEC). Since 1999 this model is "International Standard" for the terrestrial ionosphere (standard TS16457).

Persistent anomalies to the idealized model

Overview of ionosphere phenomena

Ionograms allow deducing, via computation, the true shape of the different layers. Nonhomogeneous structure of the electron/ion-plasma
produces rough echo traces, seen predominantly at night and at higher latitudes, and during disturbed conditions.

Winter anomaly

At mid-latitudes, the F2 layer daytime ion production is higher in the summer, as expected, since the Sun shines more directly on the Earth. However, there are seasonal changes in the molecular-to-atomic ratio of the neutral atmosphere that cause the summer ion loss rate to be even higher. The result is that the increase in the summertime loss overwhelms the increase in summertime production, and total F2 ionization is actually lower in the local summer months. This effect is known as the winter anomaly. The anomaly is always present in the northern hemisphere, but is usually absent in the southern hemisphere during periods of low solar activity.

Equatorial anomaly

Electric currents created in sunward ionosphere.

Within approximately ± 20 degrees of the magnetic equator, is the

horizontal
magnetic field, forces ionization up into the F layer, concentrating at ± 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain.

Equatorial electrojet

The worldwide solar-driven wind results in the so-called Sq (solar quiet) current system in the E region of the Earth's ionosphere (ionospheric dynamo region) (100–130 km (60–80 mi) altitude). Resulting from this current is an electrostatic field directed west–east (dawn–dusk) in the equatorial day side of the ionosphere. At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow within ± 3 degrees of the magnetic equator, known as the equatorial electrojet.

Ephemeral ionospheric perturbations

X-rays: sudden ionospheric disturbances (SID)

Main article: Sudden ionospheric disturbance

When the Sun is active, strong solar flares can occur that hit the sunlit side of Earth with hard X-rays. The X-rays penetrate to the D-region, releasing electrons that rapidly increase absorption, causing a high frequency (3–30 MHz) radio blackout that can persist for many hours after strong flares. During this time very low frequency (3–30 kHz) signals will be reflected by the D layer instead of the E layer, where the increased atmospheric density will usually increase the absorption of the wave and thus dampen it. As soon as the X-rays end, the sudden ionospheric disturbance (SID) or radio black-out steadily declines as the electrons in the D-region recombine rapidly and propagation gradually returns to pre-flare conditions over minutes to hours depending on the solar flare strength and frequency.

Protons: polar cap absorption (PCA)

Further information: Solar particle event § Polar cap absorption events

Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours. Coronal mass ejections can also release energetic protons that enhance D-region absorption in the polar regions.

Storms

Main articles: Geomagnetic storm and Ionospheric storm

Geomagnetic storms and ionospheric storms are temporary and intense disturbances of the Earth's magnetosphere and ionosphere.

During a geomagnetic storm the F₂ layer will become unstable, fragment, and may even disappear completely. In the Northern and Southern polar regions of the Earth

aurorae
will be observable in the night sky.

Lightning

Further information: Lightning

Lightning can cause ionospheric perturbations in the D-region in one of two ways. The first is through VLF (very low frequency) radio waves launched into the magnetosphere. These so-called "whistler" mode waves can interact with radiation belt particles and cause them to precipitate onto the ionosphere, adding ionization to the D-region. These disturbances are called "lightning-induced electron precipitation" (LEP) events.

Additional ionization can also occur from direct heating/ionization as a result of huge motions of charge in lightning strikes. These events are called early/fast.

In 1925, C. T. R. Wilson proposed a mechanism by which electrical discharge from lightning storms could propagate upwards from clouds to the ionosphere. Around the same time, Robert Watson-Watt, working at the Radio Research Station in Slough, UK, suggested that the ionospheric sporadic E layer (Es) appeared to be enhanced as a result of lightning but that more work was needed. In 2005, C. Davis and C. Johnson, working at the Rutherford Appleton Laboratory in Oxfordshire, UK, demonstrated that the Es layer was indeed enhanced as a result of lightning activity. Their subsequent research has focused on the mechanism by which this process can occur.

Applications

Radio communication

Due to the ability of ionized atmospheric gases to

radio amateur hobbyists for private recreational contacts and to assist with emergency communications during natural disasters. Armed forces use shortwave so as to be independent of vulnerable infrastructure, including satellites, and the low latency of shortwave communications make it attractive to stock traders, where milliseconds count.[24]

Mechanism of refraction

When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio-frequency energy is given up to this resonant oscillation. The oscillating electrons will then either be lost to recombination or will re-radiate the original wave energy. Total refraction can occur when the collision frequency of the ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough.

A qualitative understanding of how an electromagnetic wave propagates through the ionosphere can be obtained by recalling

geometric optics. Since the ionosphere is a plasma, it can be shown that the refractive index is less than unity. Hence, the electromagnetic "ray" is bent away from the normal rather than toward the normal as would be indicated when the refractive index is greater than unity. It can also be shown that the refractive index of a plasma, and hence the ionosphere, is frequency-dependent, see Dispersion (optics).[25]

The

plasma frequency
of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below:

where N = electron density per m3 and fcritical is in Hz.

The Maximum Usable Frequency (MUF) is defined as the upper frequency limit that can be used for transmission between two points at a specified time.

where =

sine
function.

The cutoff frequency is the frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by refraction from the layer.

GPS/GNSS ionospheric correction

See also: Total electron content

There are a number of models used to understand the effects of the ionosphere on global navigation satellite systems. The

GPS. This model was developed at the US Air Force Geophysical Research Laboratory circa 1974 by John (Jack) Klobuchar.[26] The Galileo navigation system uses the NeQuick model.[27]

Other applications

The open system electrodynamic tether, which uses the ionosphere, is being researched. The space tether uses plasma contactors and the ionosphere as parts of a circuit to extract energy from the Earth's magnetic field by electromagnetic induction.

Measurements

Properties of Earth’s Upper Atmosphere

Overview

Scientists explore the structure of the ionosphere by a wide variety of methods. They include:

A variety of experiments, such as HAARP (

High Frequency Active Auroral Research Program
), involve high power radio transmitters to modify the properties of the ionosphere. These investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and military purposes. HAARP was started in 1993 as a proposed twenty-year experiment, and is currently active near Gakona, Alaska.

The SuperDARN radar project researches the high- and mid-latitudes using coherent backscatter of radio waves in the 8 to 20 MHz range. Coherent backscatter is similar to Bragg scattering in crystals and involves the constructive interference of scattering from ionospheric density irregularities. The project involves more than 11 countries and multiple radars in both hemispheres.

Scientists are also examining the ionosphere by the changes to radio waves, from satellites and stars, passing through it. The Arecibo Telescope located in Puerto Rico, was originally intended to study Earth's ionosphere.

Ionograms

Main article:
Ionogram

Ionograms show the virtual heights and critical frequencies of the ionospheric layers and which are measured by an ionosonde. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30 MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave is refracted less by the ionization in the layer, and so each penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer. Tracings of the reflected high frequency radio pulses are known as ionograms. Reduction rules are given in: "URSI Handbook of Ionogram Interpretation and Reduction", edited by William Roy Piggott and Karl Rawer, Elsevier Amsterdam, 1961 (translations into Chinese, French, Japanese and Russian are available).

Incoherent scatter radars

Incoherent scatter radars operate above the critical frequencies. Therefore, the technique allows probing the ionosphere, unlike ionosondes, also above the electron density peaks. The thermal fluctuations of the electron density scattering the transmitted signals lack coherence, which gave the technique its name. Their power spectrum contains information not only on the density, but also on the ion and electron temperatures, ion masses and drift velocities.

GNSS radio occultation

GNSS signal tangentially scrapes the Earth, passing through the atmosphere, and is received by a Low Earth Orbit (LEO) satellite. As the signal passes through the atmosphere, it is refracted, curved and delayed. An LEO satellite samples the total electron content and bending angle of many such signal paths as it watches the GNSS satellite rise or set behind the Earth. Using an Inverse Abel's transform, a radial profile
of refractivity at that tangent point on earth can be reconstructed.

Major GNSS radio occultation missions include the

GRACE, CHAMP, and COSMIC
.

Indices of the ionosphere

In empirical models of the ionosphere such as Nequick, the following indices are used as indirect indicators of the state of the ionosphere.

Solar intensity

F10.7 and R12 are two indices commonly used in ionospheric modelling. Both are valuable for their long historical records covering multiple solar cycles. F10.7 is a measurement of the intensity of solar radio emissions at a frequency of 2800 MHz made using a ground radio telescope. R12 is a 12 months average of daily sunspot numbers. The two indices have been shown to be correlated with each other.

However, both indices are only indirect indicators of solar ultraviolet and X-ray emissions, which are primarily responsible for causing ionization in the Earth's upper atmosphere. We now have data from the

GOES
spacecraft that measures the background X-ray flux from the Sun, a parameter more closely related to the ionization levels in the ionosphere.

Geomagnetic disturbances

Ionospheres of other planets and natural satellites

Objects in the Solar System that have appreciable atmospheres (i.e., all of the major planets and many of the larger natural satellites) generally produce ionospheres.[28] Planets known to have ionospheres include Venus, Mars,[29] Jupiter, Saturn, Uranus, Neptune and Pluto.

The atmosphere of Titan includes an ionosphere that ranges from about 880 km (550 mi) to 1,300 km (810 mi) in altitude and contains carbon compounds.[30] Ionospheres have also been observed at Io, Europa, Ganymede, and Triton.

See also

Notes

  1. ISBN 978-3-12-539683-8
    .
  2. ^ "ionosphere". Merriam-Webster.com Dictionary.
  3. ^ Zell, Holly (2 March 2015). "Earth's Atmospheric Layers". NASA. Retrieved 2020-10-23.
  4. ISBN 0-7923-0775-5
    .
  5. ^ Gauss, Carl Friedrich (1839). "Allgemeine Theorie des Erdmagnetismus [General theory of terrestrial magnetism]". In Gauss, Carl Friedrich; Weber, Wilhelm (eds.). Resultate aus den Beobachtungen des Magnetischen Vereins im Jahre 1838 [Findings from the Observations of the Magnetic Society in the Year 1838] (in German). Leipzig, (Germany): Weidmanns' Bookshop. pp. 1–57. Gauss speculated that magnetic forces might be generated not only by electrical currents flowing through the Earth's interior but also by some sort of electrical current(s) flowing through the atmosphere. From p. 50: "§ 36. Ein anderer Theil unserer Theorie, über welchen ein Zweifel Statt finden kann, ist die Voraussetzung, … zu untersuchen, wie die aus denselben hervorgehende magnetische Wirkung auf der Erdoberfläche sich gestalten würde." (Another part of our theory about which doubt may arise is the assumption that the agents of terrestrial magnetic force have their source exclusively in the interior of the Earth. If the immediate causes [of terrestrial magnetism] should be sought entirely or in part outside [the Earth's interior], then we can — in so far as we exclude baseless fantasies and we want to restrict ourselves to the scientifically known [facts] — consider only galvanic currents. Atmospheric air is not a conductor of such currents; empty space also is not: thus our knowledge fails us when we seek a carrier for galvanic currents in the upper regions [of the atmosphere]. Only the enigmatic phenomena of the northern lights — in which by all appearances electricity in motion plays a major role — prohibits us from simply denying the possibility of such currents just on account of that ignorance, and in any case it remains interesting to investigate how the magnetic effect resulting from [those currents] would manifest itself on the Earth's surface.)
  6. ^ John S. Belrose, "Fessenden and Marconi: Their Differing Technologies and Transatlantic Experiments During the First Decade of this Century Archived 2009-01-23 at the Wayback Machine". International Conference on 100 Years of Radio, 5–7 September 1995.
  7. ^ "Marconi and the History of Radio". IEEE Antennas and Propagation Magazine. 46.
  8. ^ Heaviside, Oliver (1902). "Telegraphy". Encyclopaedia Britannica. Vol. 33 (10th ed.). pp. 213–235. Speaking of wireless telegraphy, Heaviside speculated about the propagation of Hertzian (radio) waves through the atmosphere. From p. 215: "There may possibly be a sufficiently conducting layer in the upper air. If so, the waves will, so to speak, catch on to it more or less. Then the guidance will be the sea on one side and the upper layer on the other."
  9. ^ Kennelly, A.E. (15 March 1902). "On the elevation of the electrically conducting strata of the earth's atmosphere". The Electrical World and Engineer. 39 (11): 473.
  10. ^ worldradiohistory.com: Broadcast listening in the pioneer days of radio on the short waves, 1923 1945 Jerome S. Berg Quote: "...In addition to having to obtain licenses - a constraint to which they adapted only slowly - the amateurs were, with some exceptions, restricted to the range below 200 meters (that is, above 1500 kc.), bands that were largely unexplored and thought to be of little value. The navy attributed most interference to the amateurs, and was happy to see them on the road to a hoped - for extinction. From the amateurs' point of view, their development of the shortwave spectrum began less as a love affair than a shotgun marriage. However, all that would change...It took several years before experimenters ventured above 2-3 mc. and started to understand such things as shortwave propagation and directionality. The short waves, as they were called, were surrounded with mystery...Also in 1928 Radio News publisher Hugo Gernsback began shortwave broadcasting on 9700 kc. from his station, WRNY, New York, using the call W2XAL. "A reader in New South Wales, Aus- tralia," reported Gernsback, "writes us that while he was writing his letter he was listening to WRNY's short-wave transmitter, 2XAL, on a three-tube set; and had to turn down the volume, otherwise he would wake up his family. All this at a distance of some 10,000 miles! Yet 2XAL ...uses less than 500 watts; a quite negligible amount of power. "6...The 1930s were the golden age of shortwave broadcasting...Shortwave also facilitated communication with people in remote areas. Amateur radio became a basic ingredient of all expeditions...The term shortwave was generally taken to refer to anything above 1.5 mc., without upper limit...", backup
  11. ^ "Sun Affects Radio, Observations Show". The New York Times. No. 24473. The New York Times Company. 25 January 1925. pp. 1, 4. Retrieved 25 January 2024.
  12. ^ The letter, dated 8 November 1926, was addressed to the Secretary of the Radio Research Board.
  13. ^ "Gakona HAARPoon 2017". 2017-02-19. Archived from the original on 2017-02-20.
  14. ^ "Firsts in the Space Race. From an Australian perspective". harveycohen.net. Archived from the original on 11 September 2017. Retrieved 8 May 2018.
  15. ^ "Elizabeth A. Essex-Cohen Ionospheric Physics Papers etc". harveycohen.net. Archived from the original on 11 September 2017. Retrieved 8 May 2018.
  16. ^ "The Ionosphere | Center for Science Education". scied.ucar.edu. Retrieved 2023-04-05.
  17. ISSN 0148-0227
    .
  18. ISBN 9783319215815
    .
  19. ^ "Neutrosphere - Glossary of Meteorology". Glossary.ametsoc.org. 2012-01-26. Retrieved 2022-08-12.
  20. S2CID 122220113
    .
  21. ISBN 978-0-671-07580-4
    . p. 12 AEROS
  22. ^ Bilitza, 2001
  23. ^ "International Reference Ionosphere". Ccmc.gsfc.nasa.gov. Archived from the original on 2011-02-23. Retrieved 2011-11-08.
  24. S2CID 233990323
    .
  25. ^ Lied, Finn (1967). High Frequency Radio Communications with Emphasis on Polar Problems. Advisory Group for Aerospace Research and Development. pp. 1–6.
  26. ^ "ION Fellow - Mr. John A. Klobuchar". www.ion.org. Archived from the original on 4 October 2017. Retrieved 8 May 2018.
  27. ^ "Ionospheric Correction Algorithm for Galileo Single Frequency Users" (PDF). Galileo Open Service. Archived (PDF) from the original on 10 February 2018. Retrieved 9 February 2018.
  28. ^ "Planetary ionospheres". Department of Physics and Astronomy. Uppsala University. Retrieved 2023-06-04.
  29. ^ "Mars Express: First global map of martian ionosphere". Archived from the original on 2015-09-10. Retrieved 2015-10-31.
  30. ^ NASA/JPL: Titan's upper atmosphere Archived 2011-05-11 at the Wayback Machine Accessed 2010-08-25

References

External links

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