Geomagnetosphere activity

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Chorus recorded by Cluster 2 satellite from data collected on July 9, 2001. Chorus consists of brief, rising-frequency tones that sound like the chorus of birds singing at sunrise, hence the name "chorus" or "dawn chorus". Chorus at Earth is generated by electrons in Earth's Van Allen radiation belts. Once generated, the chorus waves affect the motions of the electrons through a process called a wave-particle interaction. Wave-particle interactions disturb the trajectories of the radiation belt electrons and cause the electrons to hit the upper atmosphere. Document Donald A. Gurnett/U/Iowa.

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"Saucers" recorded by Dynamics Explorer spacecraft. Document Donald A. Gurnett/U/Iowa.

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Short but spectacular whistler recorded by Cluster 3 and 4 satellites from data collected on Feb 4, 2001. Whistlers are produced by lightning. This energy is radiated as electromagnetic waves over a very broad spectrum of frequencies, from very low-frequency radio waves to visible light. Some of these radio waves propagate upward into the ionized gas located above the Earth's atmosphere guided along geomagnetic field, and often echo back and forth between the northern and southern hemispheres. The waves travel faster at higher frequencies and slower at lower frequencies. Therefore, a spacecraft will first detect the higher frequencies and later the lower frequencies. The result is a whistling tone, hence the name "whistler". Document Donald A. Gurnett/U/Iowa.

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Pure whistlers of lightnings recorded with a VLF receiver. The note whistler has traveled along a signal magnetic field line. It is heard as a clear whistling sound. Pulses that we heard over 10 kHz at beginning, at 8 and 9 sec are OMEGA signals. Several other faint whislters are audible in the sound sample. These whistlers are sferics dispersed even more than tweeks. The sound of a whistler is a musical descending tone that lasts for a second or more. The high frequencies arrive first, followed by lower ones. Document IMAGE/INSPIRE.

Additional information can be found on Stephen P. McGreevy's website.

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Lightning whistlers propagating in the magnetosphere. Lightning-generated whistlers came into regular use as remote probes of the radial distribution of electron density in the Earth's geomagnetic equatorial plane.

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Diffuse whistlers. The signals traveled along a set of magnetic field lines that are not all of the same length. The sound is "breathy" or "swooshy". Document IMAGE/INSPIRE.

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Sferics, short for "atmospherics", are impulsive signals emitted by lightnings striking within a thousand kilometers or so of the receiver. Their signals cover simultaneous all audio VLF frequencies. Document IMAGE/INSPIRE.

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Whistle echo trains. They result when the radio wave bounces back and forth between magnetic conjugate points. Each time the signal bounces off the ionosphere, some of the energy leaks down in the lower atmosphere and is heard as a whistler. All of the whistlers in the train are the result of a single lightning stroke. Successive "hops" of the whistler are seen with increasing dispersion time as the distance travelled grows with each bounce. Document IMAGE/INSPIRE.

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Two-hop whistlers originated near the receiver site. The signal that travels along the magnetic field line bounces off the ionosphere in the other magnetic hemisphere and returns to be heard as a whistler near where the original lightning stroke occurred. Two-hop whistlers can be identified by the presence of a strong "local" sferic (within 2000 km) between one and two seconds before the whistler is heard. Document IMAGE/INSPIRE.

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Spectacular whistlings recorded in NW Alberta, Canada, on June 2, 1996 at approx. 1030 UTC (4.30 a.m. MDT). Pitched whistling and whooping are wildly varying upward and downward. 

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Pure whistlers recorded during a beautiful colorful dawn at Great Basin National Park on Sept 16, 1994. Taped using a WR-4B VLF receiver connected to a 150-meter longwire (500 ft.) strung to the north-east at about 2-4m (6-15 ft) above the ground in the aspen and fir trees. These whistlers and associated lightning sferics were probably occurring from nasty T-storms pummeling Dallas, Texas and eastern Nebraska (Omaha area). The geomagnetic indices were also rather low (A=6-7, K=0-1, SFI~80). Additional information can be found on Stephen P. McGreevy's website. Document McGreevy.

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T-storms. They are multitude of very hissy (diffuse) whistlers recorded on June 13, 1993 near Berlin, Nevada at approx. 1330 UTC (local dawn) with a WR-4B VLP receiver conencted to 5 meters (15 ft) vertical wire. Additional information can be found on Stephen P. McGreevy's website. Document McGreevy.

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Nice mix of pure whistlers and tweeks that occurred all night. Recorded on April 1, 1994 at 11:30 UTC (3h30 a.m. PST), 160 km north of San Francisco, CA, in Mendocino County. Additional information can be found on Stephen P. McGreevy's website. Document McGreevy.

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Magnetosphere whistlers recorded on the nightside on March 26, 1996 at 07:59 UTC by Don Gurnett from U.Iowa. Initially the wideband receiver (VLF) was connected to the electric Eu antenna, but was switched to the Bu magnetic search coil antenna at 07:59:06 UTC. A series of brief whistlers is evident throughout this interval below 1.5 kHz. These sounds are audio frequency electromagnetic waves produced by lightning. Once produced, these waves travel along closed magnetic field lines from one hemisphere to the other in the right-hand polarized, whistler mode of propagation. The duration of the whistling tone is related to the length of the propagation path. Because of anisotropies in the index of refraction, the wave energy is confined within a cone that makes an angle of 19°28' with respect to the local magnetic field.

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Magnetosphere whistlers recorded on May 10, 1996 at 00:16 UTC by Don Gurnett from U.Iowa using a wideband spectrogram taken from the Earth dayside. The wideband receiver was connected to a magnetic loop antenna throughout this interval. Two clusters of whistlers of varying duration are seen below 8 kHz at 00:16:25 UTC and 00:16:44 UTC.

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Magnetosphere whistlers recorded on June 12, 1996 at 13:58 UTC by Don Gurnett  from U.Iowa using a wideband spectrogram taken from the Earth nightside. The wideband receiver was again connected to a magnetic loop antenna. Some whistlers can be seen up to 9 kHz (13:58:24 UTC and 13:58:29 UTC) and several more below 4 kHz (13:58:32 UTC and 13:58:44 UTC).

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Lightning whistlers and tweeks propagating in the magnetosphere recorded by NASA scientists.

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Lightning whistlers propagating in the magnetosphere recorded by NASA scientists.

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Whistlers in the magnetosphere. Document U.Iowa.

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Lightning whistlers recorded in stereo. Document U.Iowa.

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Earth proton whistlers. Document U.Iowa.

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Earth multi-hop whistlers. Document U.Iowa.

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Lightning tweeks recorded between 100 Hz and 30 kHz by Altaïr.

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Magnetosphere cusp waves recorded on May 29, 1996 at 02:56 UTC by Don Gurnett from U.Iowa. The magnetic field lines of the Earth can be divided into two parts according to their location on the sunward or tailward side of the planet. Between these two parts on both hemispheres are funnel-shaped areas with near zero magnetic field magnetitude called the polar cusps. They provide a direct entry for the magnetosheath plasma into the magnetosphere.

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Magnetosphere chorus recorded on May 31, 1996 at 06:47 UTC by Don Gurnett  from U.Iowa. Chorus emissions are electromagnetic widebands emissions propagating in the right-hand polarized whistler mode. They are among the most intense plasma waves in the outer magnetosphere. 

Chorus emissions are observed at intermediate invariant latitudes, between L=4 and L=10, and over a wide range of local times with a peak in the distribution near local dawn.

This record was taken at daytime at latitudes just below the dayside auroral zone. The receiver was connected to a magnetic loop antenna. The discrete tones characteristic of chorus can be seen as a dense population of short, very intense rising tones between 500 Hz and 1.2 kHz.

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Chorus. Occasionally, especially in the quiet times of the morning, chorus can be heard. They sounds like many birds calling in turn. Chorus seems to be the result of many brief, short-path whistler-like emissions occurring at almost the same time. Document IMAGE/INSPIRE.

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Loud dawn chorus and hiss recorded on August 18, 1993 in SE Oregon's Alvord Desert at 14:30 UTC (7h30 a.m. PDT). A major magnetic storm was in progress. Magnetic field "micro-pulsations" are very evident (slow undulations in the hiss/chorus every 3-4 sec similar to ocean waves. Additional information can be found on Stephen P. McGreevy's website. Document McGreevy.

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Dawn chorus with evident magnetic field micro-pulsations (undulations in the chorus trains). Recorded by Michael Mideke in 1990 or 1991.

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Geomagnetic storm recorded on Feb 21, 1994 by Southpole.

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Earth chorus. Document U.Iowa.