We are observing PCA phenomena among most interesting events of last days.Well,what is this phenomena?
Polar Cap Absorption Events - Massive Short Wave Communications Blackouts
Solar proton events produce several major consequences at Earth:High energy protons penetrate the Earth's magnetic field in the polar regions, crash into atmospheric particles and produce ion and electron pairs that temporarily increase the density in the lowest regions of the ionosphere. This causes absorption of short wave radio signals and wide-spread blackout of communications, called a polar cap absorption event.Dangerous levels of high energy particle radiation build up in the magnetosphere that can damage spacecraft microelectronics and pose a serious threat to the safety of astronauts.
The Trigger: The Arrival of a Solar Proton Event
Occasionally the Sun blows clouds of very high velocity protons at the Earth (called a solar proton event). Solar protons can penetrate into the Earth's magnetosphere near the magnetic poles. Solar protons with energies up to about 10 million electron volts bombard the ionosphere, knocking electrons free during collisions with air particles and increasing the density of the ionosphere at low altitudes (in the D- and E-regions).
Occasionally the Sun blows clouds of very high velocity protons at the Earth (called a solar proton event). Solar protons can penetrate into the Earth's magnetosphere near the magnetic poles. Solar protons with energies up to about 10 million electron volts bombard the ionosphere, knocking electrons free during collisions with air particles and increasing the density of the ionosphere at low altitudes (in the D- and E-regions).
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Solar Proton Event
Solar proton events produce several major consequences at Earth:High energy protons penetrate the Earth's magnetic field in the polar regions, crash into atmospheric particles and produce ion and electron pairs that temporarily increase the density in the lowest regions of the ionosphere. This causes absorption of short wave radio signals and wide-spread blackout of communications, called a polar cap absorption event.Dangerous levels of high energy particle radiation build up in the magnetosphere that can damage spacecraft microelectronics and pose a serious threat to the safety of astronauts.
Minutes to Hours Later .....
Solar protons are accelerated to very high velocities in the explosive release of energy associated with a solar flare. These particles first make their way out of the corona. Then they cannot take a straight line path to the Earth but must travel along the Sun's extended magnetic field lines (called the interplanetary magnetic field or IMF). These field lines have a structure resembling the spiral traced out by a spinning lawn sprinkler, which is referred to as a Parker spiral.
The image on the right shows how flare location on the surface of the Sun affects the intensity and arrival time of the high speed solar particles at the Earth. In the first case, the flare occurred in the center of the Sun on a direct line-of-sight path to the Earth. In the second case, the flare site is on the limb of the Sun, directly connected to the Earth along a curved magnetic field line. The line plot in each case gives the history of the flux (in logarithmic units) as it arrives at Earth. The flare particles, blown out at the limb of the Sun, arrive more rapidly and in greater numbers (flux is almost 10 times higher) than those that began their journey from the center of the solar disk.
Solar protons are accelerated to very high velocities in the explosive release of energy associated with a solar flare. These particles first make their way out of the corona. Then they cannot take a straight line path to the Earth but must travel along the Sun's extended magnetic field lines (called the interplanetary magnetic field or IMF). These field lines have a structure resembling the spiral traced out by a spinning lawn sprinkler, which is referred to as a Parker spiral.
The image on the right shows how flare location on the surface of the Sun affects the intensity and arrival time of the high speed solar particles at the Earth. In the first case, the flare occurred in the center of the Sun on a direct line-of-sight path to the Earth. In the second case, the flare site is on the limb of the Sun, directly connected to the Earth along a curved magnetic field line. The line plot in each case gives the history of the flux (in logarithmic units) as it arrives at Earth. The flare particles, blown out at the limb of the Sun, arrive more rapidly and in greater numbers (flux is almost 10 times higher) than those that began their journey from the center of the solar disk.
D- and E-regions
The Effects on Radio Waves
Short wave radio waves at relatively high frequencies (in the HF range) are absorbed by the increased particles in the low altitude ionosphere causing a complete black out of radio communications. This is called a Polar Cap Absorption (PCA) event. Lower frequency waves that would normally reflect off the low altitude ( D- or E-region) ionospheric layers now do so at lower than normal altitudes changing dramatically their propagation paths.
How Long Does a PCA Last?
PCA events are particularly devastating to radio communications because they can last for days depending on the size and location of the magnetic disturbance on the Sun that produces them.
Short wave radio waves at relatively high frequencies (in the HF range) are absorbed by the increased particles in the low altitude ionosphere causing a complete black out of radio communications. This is called a Polar Cap Absorption (PCA) event. Lower frequency waves that would normally reflect off the low altitude ( D- or E-region) ionospheric layers now do so at lower than normal altitudes changing dramatically their propagation paths.
How Long Does a PCA Last?
PCA events are particularly devastating to radio communications because they can last for days depending on the size and location of the magnetic disturbance on the Sun that produces them.
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