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Ionospheric Perturbations

A loop prominence appeared on Nov 9, 2000 and that gave rise to a M7.4-class event. It was followed by a CME of G3 class that impacted Earth 31 hours later, and sparking a strong geomagnetic storm. Pictured in EUV by TRACE orbital observatory.

Perturbations associated to disturbed conditions (IV)

When we speak of disturbed conditions we must turn to the sun because it is the main actor, the great perturbator of the terrestrial environment.

We saw previously that the sun emits two kinds of radiations :

- electromagnetic radiations : light, EUV, X-rays, etc

- particle emissions : high-energy protons, fast electrons, helions, etc.

Due to their different mass and the electric state for some, both types of radiations are not released at the same time from the solar surface. Coming from the depths of the sun, some take time to reach the surface as they interact with gaz and plasma before espacing into space.

Electromagnetic radiations being massless but carrying energy (for example 18 eV for EUV, a few MeV for X-rays) they leave the sun at first and arrive on Earth 8.3 minutes after their release. Conversely, particles showing a more or less important mass or energy at rest (up to 100 MeV for the heaviest protons), they travel the same distance much slower, but at speeds that sometimes exceed 1000 km/s. The fastest arrive thus on Earth 41 hours or 1.7 days after their release. The slowest need of about 10 days to cover the same distance.

Carrying more of less energy, not all radiations produce the same effects. But in all cases, charged particles and ionizing radiations are events that affect the strongest the ionosphere, some carried by geomagnetospheric currents to polar caps and to the equator via ring currents, other striking directly the upper atmosphere without embellishment or further ado.

Effects of X rays : Shortwave Fadeout, SWF

SWF occurs after a strong X-flare is released by an active region of the Sun. This event is sometimes followed with a CME (Coronal mass Ejection). This material is ejected into space at speed reaching 1000 km/s and takes about 2 days to reach the Earth.

A typical X-ray sunflare in a solar active region, source of disturbances in LF, MF and HF bands. Document NASA/MSFC.

A sunflare can reach an energy level of 1025 Joules (~ 1032 ergs) equivalent to 10 billion megatonnes of TNT or 2 million times the world's nuclear arsenal (~ 20 Gt of TNT in 1968) ! This phenomenal energy is equivalent to some 1010 eV/nucleon and to a flux which power reaches 10-4 Watt/m2 for a M-class flare which can be multiplied by 10 and more (>10-4 Watt/m2) during an eruption of X-class, an energy to report on a surface sometimes tens of times bigger than the Earth !

In the radio spectrum, the class-X9.3 flare that erupted on september 6, 2017 (see the next videos on YouTube recorded by SDO at 171, 193, 211/193/171 and 1600 ) reached a flux density of 12000 sfu (peak) or 1.2x108 Jy at 10.7 cm of wavelenght what was 26 times more than a X2-class flare. At 12h12 UT, the speed of the solar wind reached 1962 km/s, a value rarely reached. This intense activity last 3 minutes.

Once the plasma ejected, after some hours the active region become quiet again and does no more look bright as during the eruption but this process can repeat during a few days like it was the case between September 4 and 8, 2017.

The X-ray flux of such an eruption is so strong that radiations reaching the Earth penetrate down to the D region of the Earth atmosphere and increase the photoionization process up to a factor of 10, increasing highly the density of free electrons.

During about 1 hour or more, the D layer sees is aborption potential decupled. This reaction increases as the frequency decreases and creates a huge fade on low frequencies. During an SWF, the signal strength can drop of 5 dB on 20 MHz, 15 dB and more on 7 MHz and over 30 dB on 3.5 MHz and down.

X-rays propagating only in straight line, they affect essentially the side of Earth under the daylight. The SWF will be the most effective at local noon and low or equatorial latitudes or over location with a small solar zenith angle since they will receive a higher energy per unit area (flux). For this reason, disturbing only daytime propagation, it is also know as "daylight fadeout".

Note that during DX operations, sky waves can be disturbed by SWF as well. On their way to the E or F layer, they must pass twice through the D region. If these areas are affected by an absorption due to a X-flare, you might experiment a SWF, even if your communication circuit connects stations far from the equatorial region.

The effect of a severe and moderate SWFs on a circuit over Australia (UTC+12). At left the usual propagation conditions, at right during fadeouts. At around 03 UTC we observe a complete loss of communications (blackout) for about 20 minutes. The recovery starts at the highest frequencies. A minor SWF occurs around 21 UTC affecting only the lowest frequencies.

Second effect of X-rays : Sudden Ionospheric Disturbances, SID

The D-layer absorption is well-known from amateurs working on low bands. When the sun releases a CME or an X-flare, sending X-rays and gammas ray to the Earth, a couple of days later the D-layer see its ionization level increases drastically, absorbing all short frequencies. The result is a deep shortwave fadeout (see below) . Shortly after we can assist to a complete communications blackout : these are sudden ionospheric disturbances, aka SID.

Recording of a X18-class sunflare on September 7, 2005 between 17h08-17h47 UT at 18.7 MHz by Thomas Ashcraft. This strong emission of X-rays is associated to a Sudden Ionospheric Disturbance, much noises and blackouts on HF bands. Here is the audiofile associated to this event.

This phenomenon affects mainly low HF bands in degrading the MUF over about half the Earth hemisphere, the one under the sunlight. Generally the phenomenon occurs in the evening or at night and interrupts all communications on the 160 and 80 m bands. During periods of  high absorption, when geomagnetic storms occur,  communications can be interrupted on all HF spectrum, up to the 10 m band. The attenuation is however limited to a few dB in the 20-10 m band vs. 20+ dB on 80 m and exceeding sometimes 40 dB on 160 m, better than what could do your transceiver Attenuation knob, HI!

In the same time, while X-rays and EUV radiations disturb drastically LF, MF, and HF bands, the ionization level is enforced in the D-Region, making it a better reflector in lowering its effective height. This phenomenon enhances VLF propagation during a few days. This enforcement is apparent with solar flares of Class M and X.

Map of the D-Region Absorption Prediction. Compared to a situation where there is not the least attenuation (at right), it is rare that this map turns so red. It was calculated on September 11, 2017, some days after several sun flares of class X and M associated to several emissions of CME by the Sun, among which one was directed toward the Earth two days earlier. When high energy solar particles reached the geomagnetosphere, they affected a large part of the ionosphere. That day, V/UHF bands were closed at high latitudes with auroras in Alaska and up to mid latitudes (>55 with Kp=8). This time the effects of the CME created a radio blackout up to 15 MHz where the attenuation reached till ~35 dB; a S7 signal looked to a S1 signal ! You can get real-time update on SWPC website.

Monitoring both part of the spectrum, we can see an exact correlation between HF blackouts and VLF openings, signing the occurrence of a solar flare event in a near past (last 2 days or so). If you want participating in such a monitoring, check the next link to the AAVSO website to monitor SIDs in VLF. Several articles will help you building your receiver and antenna.

Effects of heavy protons : Polar Cap Absorption, PCA

Their occurrence means no propagation. During a geomagnetic storm, the ionospheric region located around the Earth's polar caps acts as a radio waves absorber, like is the D-layer at lower altitudes. This phenomenon may last several days and interrupts all radio communications via both polar caps. All these emissions warm the upper atmosphere and increase its density.

Due to the high energy of particles, this radiation hazard affects also orbiting satellites in damaging their surface or destroying their components (memory, CPU, etc). Personnel orbiting the Earth avoids extravehicular activities (EVA) during these periods and shelters in the command module. The aviation, civil and military, recommends also to their pilots flighing over 30000 ft to avoid missions at low latitudes above the tropopause during these events. During a two-hour flight the crew can indeed double his daily radiation dose (from 6mSv at sea level to 12 mSv at 80000 ft). This situation may last a few days and disturb many services as well as the economy.

Effects of plasma clouds : Ionospheric storm

Associated to effects of sunflares, an ionospheric storm can disturbe HF propagation conditions more stronger than SWF or PCA. Physicists speak of "storm" to mean that this event contains more unusual phenomena than "normally" disturbed ionosphere.

When a plasma cloud (any solar eruption releasing matter into space) moves away from the sun surface and hits the Earth, it affect the geomagnetic field in which the ionosphere is embedded as well as its chemical constitution over large scales at F2 level.

In such circumstances the Maximum Usable Frequency (MUF) can increase or decrease at a particular location. The change depends on the status of the ionosphere (time of day, season, latitude, duration, etc).

As the plasma cloud needs some times to reach the Earth, an ionospheric storm normally begins between 2 and 4 days after we recorded a X-ray sunflare, a time large enough to permit operators monitoring these events to prepare and send warnings and other alert messages to all people concerned by radio activities (scientist using trans-horizon radars, militaries working in HF, ships and aicrafts pilots, hams, broadcasts, mobile services, etc).

The effect of an ionospheric storm on a circuit over Australia (UTC+12). At left, the usual propagation conditions. At right, during the storm the MUF decreases at night from about 24 to around 10 MHz. Note that the LUF is not affected by the storm.

The decrease of the critical frequency is the most important because it affect the MUF, and thus all long distance HF communications. During an ionospheric storm the F2-layer can drop by a factor of two, pulling with it all the MUF that can lost more than half of its height (for example 24 to 10 MHz or 10 to 5 MHz). Generally, during a ionospheric storm the height of the D, E and F1 layers are not affected. To be complete say that during severe depletion of the F2 layer, the critical frequency of the F2 can drop below that of the F1 layer, and thus the F2 layer is no more measurable from the ground. In this case one say that the ionosphere is in a G condition and the highest supported frequency becomes the F1 layer instead of the F2.

Effects of ionospheric storms are the most important at equinoxes and in summer and at the higher latitudes. At equatorial latitudes and in winter the main effect of storm is to increase the the critical frequency foF2 (and MUFs), although a severe storm can results in a following decrease of these same frequencies. We can also observe an increasing absorption ad mid-latitudes due to ionization of the D-layer by charged particles (mainly electrons). At high latitude such absorptions will be mch stronger and severe on most occasions.

Blackout

In the worst conditions, if a CME reaches Class-X or after a strong X-flare and the release of fast electrons and heavy protons to Earth, arriving to the Earth orbit these radiations become by altering electronic devices onboard all satellites that had the misfortune to be on daylight side of the Earth. The next image at right shows the spectacular effect of such an event on SOHO's imager that was hit head on by a plasma cloud released by a CME. Then, in striking the ionosphere these charged particles can produce a complete blackout of telecommunications in most polar countries and sometimes at mid-latitudes (35). At these occasions Kp index jumps up to 7 and 9. This event can last some days and is much stronger than a simple SWF that last a few hours at worst.

At left, this extended loop prominence looks harmless but it was at the source of a "megaflare" of X28 class, the strongest event ever recorded, on October 28,  2003 over the active region AR10486. At right, the kind of jam that occured on SOHO's CCD camera when plasma constituted of heavy protons and ions reached the Earth orbit and disturbed not only communications but electronic devices too. This event was recorded on July 14, 2004 at 1328 UTC. Documents TRACE and SOHO.

As the surface activity of the sun is affected by its magnetic internal activity, it is common that we observe a radio blackout as long as a large sunspot group is transiting on the Sun disk. The mean solar rotation being of about 28 days, the blackout can last up to 2 weeks during the paroxysms of the solar activity. However we avoid most of these blackouts as these eruptions have to occur on the trajectory to the Earth, thus practically on the center of the Sun disk to produce its effects. All other flare and eruptions not emitted toward the Earth are simply lost in space, feeding the solar cosmic rays.

Perturbations associated to solar features

Sudden Disappearing Filaments, SDF

Solar filaments (dark) recorded in Hα light (red) on April 4, 2000 with the Big Bear solar telescope (BBSO) of 1.60 m in diameter. Filaments are in fact prominences that are not viewed on the sun limb.

Solar filaments are another name given to prominences appearing on the sun surface and that are not viewed on the sun limb from Earth. Colder and less contrasted than the other active regions of the solar surface (sunspots, facular and flare areas) they appear darker in front of the chromosphere surface as shows the picture at right recorded in Hα light.

As some occasions, following the activity of the sun magnetic field, these dark filaments disappear in a few hours or from one day to another. In fact, like a magnetic that displays an open field over caps, the arch of plasma went out of the solar magnetic influence over active regions, and moved away into space in a fashion similar to solar flares. This event is known as Sudden Disappearing Filaments or SDF.

Moving at speeds ranging between 300 and 800 km/s, up to twice slower than X-rays, the plasma cloud is preceeded by a shock wave. This latter hits the Earth bow shock and the geomagnetic field 3 to 4 days after its emission while the filament cloud reaches the Earth in 3 to 10 days only, physicists having all the time to forecast its effects and warn concerned people.

Main consequences of this event are double. While the shock wave is penetrating at high speed the geomagnetoshere's bow shock, compressing geomagnetosphere currents on the daylight side of the Earth, its gives rise to a sudden commencement magnetic storm at high and mid latitudes. The horizontal component of the geomagnetic field (mainly Bz, also named Hp because it is oriented to Earth, parallel to the rotation axis) shakes and become disturbed during one or two days according to it is really sudden or gradual; after a short increase of low amplitude, it quickly decreases and shows a negatif value for a while, sign of a forecoming geomagnetic storm. Its amplitude can reach 200 nT during a few hours.

Then arrives the plasma cloud itself. Like an X-ray flare, it can cause a magnetic and possible ionospheric storm, and is at the origin of auroras at high latitudes.

Variations of the geomagnetic components at the geosynchronous orbit on April 4-7, 2000, associated to an active period of auroras from April 6. Note the crossing of the solar wind shock wave on April 6 by 16h UTC with a brutal orientation change of the Hn component that usually points to the north (blue, and pointing south). Document SPIDR-NGDC-NOAA.

SDF follows the solar cycle, and at the minimum of its activity the sun displays not more sunspot in white light, no chromospheric detail in Hydrogen-alpha light, and nothing more in UV light; the sun activity is quiet. SDF is thus a feature that only appears during periods of high solar activity, essentially during the two years preceeding and following the solar maximum.

High speed solar wind stream, HSSWS

A spectaculaire coronal hole (dark area) pictured in EUV by SDO on October 13, 2015. Here is the analyssis by Spaceweather. In a coronal hole, the magnetic field is open (like on Earth's polar caps), permitting ionized matter like the solar wind to escape into space, otherwhise trapped in the solar magnetic field.

As its name suggests, HSSWS, that can also be read "hiss-wis", is a high speed stream of ionized material released by coronal holes of the sun. This one is accelerated by the sun magnetic field, faster over coronal holes. Where the typical solar wind speed is 300 km/s and qualified as slow, it can reach 500 km/s for a HSSWS. At that speed it needs about 4 days to reach the Earth.

Due to the sun rotation, if a coronal hole is visible in the solar corona in X-ray "light" straight on on the sun disk, in spite of the fact that the orientation of the stream is radial with respect to the sun surface, it will probably not be directed to Earth. Indeed, like the previous water sprinkler image, at the Earth orbit the solar wind stream shows a component directed to the East as seen from the Earth surface, rather than head-on (radial). Knowing that slow streams are more curved than fast streams, this "sprinkler effect" makes that the solar material that left the sun earlier is delayed with respect to the material released later.

As a HSSWS sweeps over the Earth it can cause ionospheric storms, just as plasma cloud released by X-ray sunflares. However, its effects is not as devastating as the one of sunflares or CME as particles do not move as fast as these plasma clouds. The effects are also longer than the of a large sunflare because the HSSWS takes time to sweep over the Earth  A typical HSSWS "sweep" of the Earth can last a few days. 

Like all the sun activity, HSSWS follow the sun cycle. Knowing that coronal holes develop at best during periods of quiet sun, darkening all the sun surface in X-ray "light" due to their low radiation, HSSWS is a sign of decreasing of the sun activity.

Geomagnetic effects

Geomagnetic storm

We explained in page 2 that a geomagnetic storm occurs when the daily Ap index exceeds 29. The level of disturbance of a geomagnetic storm is not all comparable to conditions that we can experiment during a ionospheric storm or a CME event. Here we speak of field strenght variations that rarely exceed 200 nanoteslas (nT) out of a total of about 30000 nT. By comparison, a "simple" ionospheric storm can drop the MUF by 50% and a CME can create a blackout for hours.

The Earth's geomagnetism interest first geophysicists prospectors who measure the local variations of the geomagnetic field. In the frame of ionospheric physics, its effects interest us because it is more easy to measure its strenght and its effects to estimate then what we can expectfrom solar disturbances in the ionosphere.

Geomagnetic storms occur in conjunction with ionospheric storms and have the same origin : sunflare, SDF and HSSWS. It usuall consists in a well-defined processus showing an increase of the geomagnetic field called the initial phase, during which there is at the surface of the Earth an increase of the mid-latitude horizontal component of the geomagnetic field (H). The initial phase can last between some hours to a day, but some storms proceed directly into the main phase without showing an initial phase.

The sky of Toronto (43N) pictures by Todd Carlson during and after the backout that occured in Canada during the night of 13 to 14 March 1989 for the greatest pleasure of astronomers, or almost, Hi!

This phase is followed by a large decrease called the main phase. During that period the horizontal magnetic field at mid-latitudes is generally decreasing, while we observe an increasing of the westward-flowing magnetospheric ring current. The northward component can be depressed as much as several hundred nanoteslas in severe storms. The main phase generally last less than one day and can be as short as an hours.

At last we can assist to a recovery phase when the depressed northward geomagnetic field component returns to normal levels. Recovery is typically complete in one to two days, but can take longer.

When a geomagnetic storm is causes by sunflares, it usually starts with a sudden commencement (SC) when the shock wave hit the Earth's bow shock. A storm caused by a HSSWS starts gradually and is described as a gradual commencement. Of stroms are recurrents due to period crossing of HSSWS, we speak of recurrent storms.

We will see on the next page that the intensity level of the geomagnetic field is defined through geomagnetic indices, Ap and Kp, called planetary indices.

These are geomagnetic storms that affect dramatically human infrastructures. They induce electric currents in pipelines leading to erosion and arcing, and power outages at high latitudes. Among the largest power distribution failure name the events of August 1972 and Mars 1989 in USA and Canada.

Auroral E

During a strong geomagnetic storm, the availability of Auroral-E trafic is improved, but it generates a high Doppler spread. This phenomenon mainly concerns VHF propagation. The same events create Sporadic-E propagation. Note that both auroral oval and Doppler spread can be simulated in recent programs, like DXAtlas (aurora) and IONOS (Doppler spread and other ionospheric effects).

South Atlantic Anomaly, SAA

We have seen about the radio propagation, that the F2 region is located near 300 km of altitude where is the peak of the F2 ionization layer. This region is also crossed by the lower limit of Van Allen belts. Recall that Van Allen belts are two radiations zones of the geomagnetosphere encircling the Earth respectively at 3000 and 20000 km above the equator. They are constituted of charged particles trapped by the geomagnetic field, and the ring currents. This latter arise when particles of opposite electric charge from the solar wind flow in opposite directions around the Earth at the equator. This is this ring current that creates geomagnetic storms.

At left N-S profile of the SAA. Over South America its lower limits reaches 200 km instead of 500 km. At right the first SAA radiometric spectrum recorded by Terra satellite. Dark bands in the SAA are areas were data were not collected. Documents ESA and NASA/MISR adapted by the author.

From a geomagnetic point of view, between South America and the South Atlantic ocean, there is a large area of inverted magnetic flux, an large scale anomaly as we can see on this geomagnetic map (in red) prepared by DMI and based on IGRF2000 model. The lower part of Van Allen belts goes down to 200 km of altitude only instead of staying at its nominal height of 500 km. This area is known as the South Atlantic Anomaly (SAA).

After strong solar X-flares this area traps "killer electrons" of more than 2 MeV and protons over 50 MeV, disturbing the functioning of all satellites with a serious health hazard for all astronauts in orbit crossing the SAA between 100 and about 1500 km of altitude.

The first damages recorded were calculation errors on GOES 4 satellite in the '80s, then on TDRS-1. This latter was even nicknamed "the first solid-state cosmic rays detector"... Damages can be so serious that in 1993 the Hubble Space Telescope (HST) had to be repaired at 610 km of altitude, an unusual height for the space shuttle; HST optronic couplers (New Technology) experimented temporary failures each time the telescope crossed the SAA. The instrument had to be shut down during 7 orbits over 16 each day, astronomers loosing 50 to 60% of their working time, some having to delay their study to the next year. In July 2000, real-time data and images received by orbital observatories were also affected by the SAA, optical systems and onboard electronic having been damaged by heavy protons.

Hopefully for us, outside periods of geomagnetic or ionospheric storms that affect all the propagation at high and mid-latitudes, during quiet days, and in spite it is constituted of charged particles, the SAA is almost inactive and doesn't affect propagation conditions over South America.

To check to NOAA: Space Weather Scales and severity of effects

This ends the long list of solar and geomagnetic perturbations. Now that the theory is over, let's review all the solar and geomagnetic indices affecting propagation. We are going to see how they progress and interact and what are their main effects.

Next chapter

Solar and geomagnetic indices

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