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Ionospheric Variability


Notwithstanding the ionosphere is just the one percent of the atmosphere above 100 km,  it is very important because of its influence on the passage of radio waves. The propagation of the radio waves throughout the ionosphere is strongly influenced by its regular variations (diurnal, seasonal, solar cycle) as well as by different short and long term changes. Short term variations are mainly caused by the impact on the upper atmosphere of  charged particles fluxes (electrons and protons) continuously emitted by the Sun and that constitute the “solar wind”. When the Sun is active extraordinary  particles emission events can occur causing magnetic storms and ionospheric disturbances (e.g. absorption phenomena in the lower ionosphere, Spread F, ionospheric storms, ionospheric irregularities and scintillations, etc…). The duration of an ionospheric disturbance is variable  (from few minutes to 1-2 days) depending on the latitude, the energy of the particles emission event,  the simultaneous occurrence of other solar events such as solar flares, Coronal Mass Ejection (CME), etc…Different events can also be uncorrelated each other and can occur during a period of low solar activity.  
The polar ionosphere is directly connected to the outer space by the geomagnetic field lines configuration and then particularly sensitive to the perturbation events mentioned above. In Figure 1 ionospheric absorption events are shown occurred in the period 7-16 September 2005, connected to X-ray emission and Interplanetary CME (ICME).  Absorption data are derived from the experimental observation of the cosmic noise by a riometer at 30 Mhz deployed at the Italian Antarctic Base “Mario Zucchelli”.

Figure 1 (GOES 11 satellite) Variation of solar X-rays (10 - 80 nm), differential solar proton flux, and ionospheric absorption, during September 7-16 2005. Five events of Interplanetary Coronal Mass Ejection (ICME - 1AU) can be recognized superimposed on the Interplanetary Magnetic Field behavior (IMF).
- 1st event , - 2nd event , - 3rd event , - 4th event , - 5th event)

Another example of short term irregular variability is given in figure 2 concerning a scintillation event over both Arctic and Antarctica the 14th April 2006.  Traces show the amplitude scintillation index S4 for all satellites in view as observed by the GISTM (GPS Ionospheric Scintillation and TEC Monitor) receivers deployed at Ny Alesund (Svalbard, Norway) and Mario Zucchelli Station (Antarctica).

Figure 2 S4 Amplitude scintillation index observed on April 14, 2006 between 20 and 23 UT over the North Pole, from GISTM (GPS Ionospheric Scintillation and TEC Monitor) receiver at Ny Alesund (Svalbard, Norway) (on the left), and over the South Pole, from Mario Zucchelli Station (Terra Nova Bay, Antarctica).


The secular trends of the ionosphere are matter of debate since 90’s when the hypothesis of a possible link with the modifications induced by the greenhouse effect on the upper atmosphere was introduced. More recently the discussion has been renewed including the secular variation of the geomagnetic field among the possible natural origins causing the observed change on the ionosphere. Some investigations  on this subject highlighted a possible excursion and/or inversion of the geomagnetic field  faster in Antarctica (about 300 yrs) than on the global scale (about 1000 yrs). As the well known geomagnetic control on the F ionospheric layer, the time scale of the secular decreasing of the electron density in the ionosphere has been  investigated. The analysis, carried out using about 30 years of data coming from high latitude ionosonde stations in the Southern Hemisphere, seems to confirm the link between the secular variation of both geomagnetic field and electron density, being of about 500 years  the estimated time scale for the polar ionosphere decay. This finding could be considered an evidence for a chaotic state of the ionosphere possibly due to a geomagnetic polarity transition, leading to a weakening of the magnetosphere shield. 

Figure 3 Information Content vs year applied to foF2 monthly medians from a set of stations at high latitude in the Southern Hemisphere. The time scale Tk  of about 500 years is comparable with that obtained for the geomagnetic secular variation over Antarctica.

 


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