Introduction To The Ionosphere

Transcription

1 Introduction To The Ionosphere John Bosco Habarulema Radar School September 2015, SANSA,

2 What is a radar? This being a radar school... RAdio Detection And Ranging To determine the range, R, R=Ct/2, C is the speed of light, and the factor of 2? We will come back to this later, Next Introduction to the ionosphere

3 Definitions thick shell of free electrons within the Earth s upper atmosphere

4 Definitions that region of the Earth s upper atmosphere covering an altitude range from ~ 50 to ~ 1000 km

5 Definitions a lightly ionized plasma. a propagation medium for HF communications

6 Atmospheric Structure Vertical structure based on temperature variation with altitude 5 layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere

7 Ionosphere sits between Mesosphere and exosphere Region between ~ 50 km to ~1000 km

8 Source of the ionosphere Ionised plasma created by EUV radiation and high energy solar particles

9 Sun Production of electrons Stripping of electrons from neutral atoms by EUV + A A + + e -

10 Loss of electrons recombination Dissociative + + A 2 + X + AX + + A + + AX + + e - A + X

11 Radiative recombination Direct combination of electrons with positively charged ions to form neutral atoms and also leads to photon emission + X + + e - X +Photon(Energy)

12 Summary of production and loss mechanisms Production of electrons: Photoionisation Loss of electrons: Recombination ( both dissociative and radiative processes) In the lower regions such as the D-region, attachment is the main process of losing electrons (electrons attach themselves to neutral atoms to form negatively charged ions) Attachment and recombination always take place within the ionosphere and are negative contributors to ionisation

13 Complex Ionospheric Structure Maximum density at some altitude Neutral atmosphere gets less dense Absorption by neutral atoms EUV weaker

14 Ionospheric formation summary

15 Ionospheric Structure Daytime structure has 4 regions: D region, km E region, km F1 region, km F2 region over 200 km Night time structure has 2 regions: E region F region

16 How were the layers labelled? Edward Appleton Nobel Prize, Physics, 1947 Asked how he came up with the labelling of the ionospheric layers, he gave the following answer The story of how I came to give the names D, E and F is really a simple one. In the early work with broadcasting wavelength, I obtained reflections from the Kennelly-Heaviside layer and I used on my diagrams the letter E for the electric vector of the down coming wave. When I found in winter 1925 that I could get reflections from a higher and completely different layer, I used the latter F for the electric vector of the down coming wave. Then about the same time I got occasionally reflections from a very low height and so naturally used the letter D for the electric vector of the returning waves. Then I suddenly realized that I must name these discrete layers and being rather fearful of assuming any finality about measurements, I felt I ought not to call these layers A, B and C, since there might be undiscovered layers, both below and above them. I therefore felt that the original designation for the electric field vector D, E and F might be used for the layers themselves. Worked with Robert Wattson-Watt on technology that led to the invention of RADAR

17 Dominant Ions Region Height [km] Ne max [cm -3 ] ion D x 10 4 (noon) absent at night NO + E x 10 5 (noon) < 1 x 10 4 (night) F x 10 5 (noon) absent at night F (noon) 10 5 (midnight) O 2 + NO + NO + O +

18 Ionospheric structure summary

19 Regions - Instruments Topside Ionosphere Satellites Bottomside Ionosphere Ionosondes, ISR Lower Ionosphere Rockets, Balloons

20 The ionosphere is variable (highly non-linear and changing medium) Not shown: Seasonal variation (1yr) Latitude dependence Diurnal variation (24 hrs) Solar activity variation (11 yrs) Magnetic activity variation

21 Latitudinal Variation Ionosphere has distinct features at some latitudes At lower latitudes, high electron concentration at approx ±20 off the magnetic equator Due to fountain effect High latitudes, ionosphere very complex because of its connection to the magnetosphere and interplanetary medium, through Earth s magnetic field

22 Diurnal Variation Electron density/frequencies great during the day than at night time F1 layer disappears at night, while F2 remain E region usually much weaker and may disappear Diurnal variation due to Sun movement across the sky

23 Seasonal Variation 14 Solar Maximum (2000) fof2, [MHz] Solar Minimum (1996) Seasonal variation associated with the revolution of the Earth around Sun Ionisation greater in summer than winter in E, F1 region but not F2 region seasonal anomaly Day Number

24 Solar Cycle Variation Solar Variation linked to 11-year solar activity, measured using sunspot numbers During high solar activity, ionisation great because solar radiation intensity is high thus enhancing electron concentration in ionosphere Solar disturbances (e.g. solar flares, CMEs) more frequent geomagnetic storms more frequent disturbances in the ionosphere more frequent

25 Magnetic Activity Variation -ve storm effect +ve storm effects

26 Summary The ionosphere is a highly complex medium Different layers (D, E, F1, F2) with different compositions and properties Can be measured by different techniques/instruments such as... Highly variable: Solar activity, magnetic activity, diurnal and seasonal variations Among the reasons we study the ionosphere are High Frequency (HF) (3-30 MHz) Radio propagation (Next Talk) Monitoring Space Weather (Next Talk) Radio Astronomy (SKA) Error correction for GPS applications, Navigation, Surveying, Geodesy Scientific Research

27 References Davies K., Ionospheric radio, Peter Peregrinus Ltd, 1990 McNamara L.F., The ionosphere: communications, surveillance, and direction finding, Krieger Publishing Company, Hofmann-Wellenhof., Lichtenegger, H., Collins, J.: GPS Theory and Practice (4th ed), Introduction to HF Radio Propagation, Australian Government IPS Radio and Space Services,

28 Thank You