Plasma in the Ionosphere Ionization and Recombination

Transcription

1 Plasma in the Ionosphere Ionization and Recombination Agabi E Oshiorenoya July, 2004 Space Physics 5P Umeå Universitet Department of Physics Umeå, Sweden

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3 Contents 1 Introduction 6 2 Ionization and Recombination 8 3 Layer by layer D -Layer E Layer E s F Layer 13 3

4 List of Figures 11 Ionospheric profile 7 21 Ionization and recombination 9 22 Variation of reactions with day and night 9 23 Ionization and recombination rates 10 4

5 List of Tables 31 Summary of Ionization conditions 11 5

6 1 Introduction The ionosphere is the region of the Earth s atmosphere between 60 and 1000 km In this region free electrons and ions can exist for a considerable period of time, resulting in the formation of a plasma gas containing charged particles This plasma is not stationary, but is constantly circulating around Flow patterns develop which can, for example, carry plasma from the dayside to the night side or vice-versa The plasma flow pattern is related to the space weather, and thus to the strength and direction of the interplanetary magnetic field and the solar wind So where did the ionosphere come from? Well it is created from the ambient neutral atmosphere by ionization However, the source of magnetospheric plasma is a much more complicated question: although it seems obvious that both solar wind and ionosphere feed it, the relative importance of these two sources is subject of ongoing research Furthermore, it seems also that plasmasphere can provide part of the plasma in plasma sheet; although it originates, in the end, from ionosphere, the transport mechanism is quite different than in direct ionospheric outflow That issue of origin sorted, it is known based on electron density observations that the ionosphere is subdivided into three layers: D-layer (60 to 90 km), the E and E s -layers (90 to 140 km), and F 1 and F 2 -layer (above 140 km) Since electron density increases nearly continuously with altitude to a maximum at an average level close to 300 km, D- E-, F-layers are not discrete, rather differ each other on the reflection of radio waves The radio waves reflection being the case as the ionosphere comes in handy for radio commucations Yes, it is easily construed that plasmas are only to be found in exotic places (not only in the Ionosphere for the earth but also places like the plasmasphere and frankly 99% of the cosmos is in the plasma state) but plasmas are just a gas containing charged particles This is obtainable in ordinary places just as well as exotic places, in ordinary places plasmas can be created by collisions of energetic particles, strong electric fields acting on bond electrons, or ionising radiation 6

7 1 Introduction 7 Fig 11: Ionospheric profile So in order to demystify the exoticness of the plasma without trivializing the nature of its phenomena, it is clear that plasmas are quite abundant and more so in the ionosphere So what happens to the plasma in ionosphere? What are its characteristics as distinct or similar to other plasmas [1]?

8 2 Ionization and Recombination Ionization and recombination is at the heart of the ionosphere, solar radiation at ultraviolet (UV) and shorter X-Ray wavelengths is considered to be ionizing since photons of energy at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision, this implying that the temperatures are hot enough At the same time, however, an opposing process called recombination begins to take place in which a free electron is captured by a positive ion if it moves close enough to it Ionization has a threshold energy, recombination has not but is much less probable However, not all radiations from the sun (which is the main actor in the ionosphere variations) produce the same effects on the ionosphere 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 u pper atmosphere without embellishment Threshold is ionization energy (136eV, H) X i The Integral over Maxwellian distribution gives rate coefficients (reaction rates) Because of the tail of the Maxwellian distribution, the ionization rate extends below T = X i and in equilibrium when n ions /n neutrals =< σ i > v/< σ r > v The percentage of ions is in the range of 100% if electron temprature: T e X i /10 eg H 2 is ionized for T e 1eV As the gas density increases at lower altitudes, the recombination process accelerates since the gas molecules and ions are closer together The point of balance between these two processes determines the degree of ionization present at any given time The ionization depends primarily on the Sun and its activity The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the sun After sunset the molecular ions eg H 2 and He in the D, E and F 1 layer vanish quickly through dissociative recombination of the type; 8

9 2 Ionization and Recombination 9 Fig 21: Ionization and recombination Fig 22: Variation of reactions with day and night

10 2 Ionization and Recombination 10 Fig 23: Ionization and recombination rates XY + + e X + Y Which on yielding two atoms these processes have high reaction probabilities The F 2 is a much harder process because and X + + e X + ω X + + e + M X + M for ω being a photon and M a third reactant which is much slower and hence the F 2 layer more stable at night Generally ionization and combination reactions play an important role in conductivity of the ionosphere With lower collision frequencies resulting in lower conductivities but also depending on the proportion of neutral molecules that abound Three kind of conditivities are distinguishable parallel conductivity, Pedersen conductivity and Hall Conductivity which are determined by the electron and ion collisions probabilities

11 3 Layer by layer It is clear up this point that one can summarize fig31 [2] the ionosphere as a region that is primarily bombarded by the ionizing rays of the sun (broadband radiation) However as expected this will vary from layer to though one can essentialy say the entire ionosphere is built of a plasma, the reactions will vary Since we are earth bound lets imagine we are to fly into space with a very slow rocket and a bag where we put the following premises; The earth has a magnectic field which is stronger at the poles, we know this because we have measured it with compass needless It also has a property called gravity, which we know because we are using an anti-gravition generator to propel us into space We know the air density decreases because we used weather baloons and hence taking our own life support We have also determined by advanced spectroscopy the composition of the air around us We know some quantum theory and electromagnectic theory because well lets just say we know that As we journey upwards the AI on board which is tuned to verbose tells of decreasing pressure which it explains as since more than half of the atmosphere s molecules are located below an altitude of 55 km, atmospheric pressure Layer D E F 1 F 2 A ca 1500 NED < X X 10 5 DED X X X 10 6 IS NO + O + 2 NO + O + 2 NO + O + 2 O + O + He + H + CI Lmnα (1215nm)Lmnβ (1025nm)X-Rays UV UV Tab 31: Summary of Ionization conditions 11

12 3 Layer by layer 12 decreases roughly 50% (to around 500 mb) within the lowest 55 km Above 55 km, the pressure continues to decrease but at an increasingly slower rate But we are going much more higher than this soon we pass the ozone which the AI also explains away as being created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O 2 ), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O 2 to create ozone, O3 The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle 31 D -Layer When we reach an altitude of about 50km we pay some attention to the range of holographic data being displayed in the cockpit, the AI explains that this is the D layer which is the innermost layer, 50 km to 90 km above the surface of the Earth The Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 1215 nanometre(nm) ionizing nitric oxide (NO) In addition, when the sun is active with 50 or more sunspots, Hard X-rays (wavelength 1 nm) ionize the air (N 2, O 2 ) During the night cosmic rays produce a residual amount of ionization Recombination is high in this layer, thus the net ionization effect is very low and as a result the high-frequency (HF) radio waves aren t reflected by the D layer The frequency of collision between electrons and other particles in this region during the day is about 10 million collisions per second! The D layer is mainly responsible for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher The absorption is small at night and greatest about midday The layer reduces greatly after sunset primarily due to recombination, but remains due to reionizing galactic cosmic rays A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime 32 E Layer After covering a distance of about 30km we encounter the E layer which is the middle layer, 90km to 120km above the surface of the Earth Ionization is due to Soft X-Ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O 2 ) This layer can only reflect radio waves having frequencies less than 10 MHz It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves During the daytime the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves On the night side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection

13 3 Layer by layer E s The Es layer or sporadic E-layer Sporadic E propagation is characterized by small clouds of intense ionization 34 F Layer The F layer or region, also known as the Appleton layer, is 120km to 400km above the surface of the Earth Here extreme ultraviolet (UV) ( nm) solar radiation ionizes molecular oxygen (O 2 ) The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2 The F layers are responsible for most skywave propagation of radio waves, and are thickest and most reflective of radio on the side of the Earth facing the sun and merges with the magnetosphere, whose plasmas are generally more rarefied but also much hotter The ions and electrons of the magnetospheric plasma come in part from the ionosphere below, in part from the solar wind

14 References [1] wwwgooglecom In most of what has followed i have found information from the internet from numrous sources [2] Kjell Rönmark Lecture notes on space physics UmeåUniversitet, March