The ionosphere of the Earth
F. J. Gordillo-Vázquez Instituto de Astrofísica de Andalucía (IAA), Granada, CSIC, Spain
[email protected]
International Summer School – Workshop Spectroscopy of the atmospheres (SPECAT 09) Jaca, Spain June 28 – July 10 2009
Outline Introduction
The ionospheric layers Ionospheric chemistry Perturbations in the ionosphere The ionosphere and radio wave propagation Life in the upper atmosphere
Conclusions
Introduction The precursors C. F. Gauss (1777 – 1855)
Both Gauss and Stewart suggested the “conducting layer” hypothesis in 1839 and 1883 to explain small daily variations In the Earth’s magntenic field
B. Stewart (1828 – 1887)
Pioneers of wireless communications
E. V. Appleton (1892 – 1965)
O. Lodge (1836 – 1920) A. E. Kennelly (1861 – 1939)
O. Heaviside (1875 – 1950)
Systematic research of the ionosphere by the teams of Breit and Tuve in the USA and Appleton and Barnett in the UK began in 1924 with measurements of heights of reflecting layers
In 1902, Lodge, Kennelly and Heaviside, independently suggested at almost the same time that radio waves reflected by an electrically conducting layer in the upper atmosphere
Introduction The term “ionosphere” was first used by Watson-Watt (one of the radar developers) in 1926. It appeared printed for the first time in 1929.
Watson att W (1892 – 1973)
E. V. Appleton (1892 – 1965)
The term “ionosphere” came into use widely in the 1932-1934 period when Watson-Watt, Appleton and others began to use it commonly.
The discovery of at least TWO separate ionised layers is credited to Appleton who later on introduced the terms “E” and “F” for the layers, which have been used since then; the layer “D” was discovered later. Appleton was awarded a Nobel prize in 1947 for his confirmation in 1927 of the existence of the ionosphere.
Important data
In modern terms, the ionosphere is a weakly ionised plasma embedded in the Earth thermosphere Solar radiation at λ < 100 nm liberates electrons and turns most of the upper atmosphere into a partially ionised plasma The ionosphere major species are O+, O2+ and NO+. Below 90 – 100 km, ion density decrease very fast As a partially ionised plasma, the ionosphere interacts strongly with radio signals, can affect telecommunications The ionosphere plays an importan role in atmospheric electricity. It can also affect GPS when it becomes turbulent
The ionospheric layers The ionosphere starts to be sensible at about 50 km to 60 km and it reaches as high as about 1000 km, being the solar radiation (mainly at ultraviolet (UV) and short X-Ray wavelengths) the one that produces ionization
The air at an altitude of about 50 km is one-thousandth as dense at Earth’s surface. Moreover, the density of the Earth’s atmosphere decreases exponentially with altitude, so the upper atmosphere, the region between 50 km and about 1500 km above the surface, includes less than 1 % of the total atmospheric mass
H (km)
500 F2 Layer
300
The ionosphere is divided in layers according to their electron density. Due to different molecules and atoms in the atmosphere and their different rates of absorption, a series of distinct regions or layers of electron density exist. These are denoted by letters D, E, F1 and F2 and usually are collectively referred to as the bottom side of the ionosphere.
Night
F1 Layer
200
E Layer 100
Day
D Layer
50 7
8 9 10 11 12 Electron density in Log10 Ne (m-3)
The ionospheric layers The part of the ionosphere between the F2 layer and the upper boundary of the ionosphere is termed the topside of the ionosphere. It is the F2 layer where usually the maximum electron density occurs as a consequence of the combination of the absorption of the EUV light and increase of neutral atmospheric density as the altitude decreases upper boundary of the ionosphere H (km)
The D, E, and F1 layers show a strong diurnal, seasonal and latitudinal variation. The diurnal variation of the D, E, and F1
500
layers also implies that they tend to vanish or greatly reduce in size at night. The F1 layer disappears in winter time
F2 Layer
300 Night
F1 Layer
200
E Layer 100
Day
D Layer
50 7
8 9 10 11 12 Electron density in Log10 Ne (m-3)
The D layer (the mesosphere) The D region comprises the mesosphere. At night, the electrons may become attached to atoms and molecules forming negative ions that cause the D layer to disappear. By day, as a consequence of sun’s radiation, the electrons tend to detach themselves from the ions causing the D layer electrons to re-appear. As a consequence of that, at the altitude of about 60 to 70 km, the D layer electrons are present by day but not by night causing a distinct diurnal variation in the electron density.
The D region is a poorly understood chemical mix of positive, negative, and cluster ions.
D - layer
Temperature
D - layer
The E layer The behaviour of the E layer almost entirely depends on the level of solar activity and the zenith angle of the sun. The E layer is free of disturbances unlike the D and F layers and is only present by day (the ionization of the E layer rapidly decays at night).
Near UV 300 - 400 nm
IR cooling
Middle UV 200 - 300 nm IR cooling Visible & IR
Far UV 120 - 200 nm Extreme UV 10 -120 nm X-ray
The primary source of ionization is the sun’s X-ray emissions resulting in electron densities showing distinct solar-cycle, seasonal and daily variations.
F2 Region (250 – 400 Km) F1 Region (130 – 250 Km) E Region (95 – 130 Km)
In the E-layer, positive ions are mostly molecular. Some charges are attached to trace metal atoms to form very long-lived ions which are probably the main ionic component of mid-latitude Sporadic E layer
D Region (50 – 90 Km) STRATOSPHERE (10 – 50 Km) TROPOSPHERE (0 – 10 Km) Earth
The F1 layer Near UV 300 - 400 nm
The main source of ionization in the F1 layer is the EUV light. The F1 layer is only observed during the day since the electron densities are primarily controlled by the zenith angle of the sun.
IR cooling
Middle UV 200 - 300 nm IR cooling Visible & IR
Far UV 120 - 200 nm Elve Extreme UV 10 -120 nm Halo X-ray
Altitude (km)
Giant Blue jet
When the F1 layer is present, it changes rapidly in a matter of minutes. It is more pronounced during the summer than during the winter months for low solar sunspot numbers and for periods with ionospheric storms
100
Sprite
F2 Region (250 – 400 Km) 50
F1 Region (130 – 250 Km)
E Region (95 – 130 Km) D Region (50 – 90 Km) STRATOSPHERE (10 – 50 Km) 0
TROPOSPHERE (0 – 10 Km)km Earth
The F2 layer Near UV 300 - 400 nm
The F2 layer is the most important ionospheric layer from the point of view of High Frequency (HF) or Short Wave (SW) radio signals propagation. The F2 layer does not follow the solar zenith angle dependence. It is the F2 layer, with the greatest density of free electrons that is potentially the most effective reflector of radio waves
IR cooling
Middle UV 200 - 300 nm IR cooling Visible & IR
Far UV 120 - 200 nm Extreme UV 10 -120 nm X-ray
F2 Region (250 – 400 Km) F1 Region (130 – 250 Km)
However, the variability of the F2 layer in height and density, its strange day/night and seasonal behaviour, and its complex response to geomagnetic disturbances have long puzzled scientists
E Region (95 – 130 Km) D Region (50 – 90 Km) STRATOSPHERE (10 – 50 Km) TROPOSPHERE (0 – 10 Km) Earth
Ionospheric chemistry Radiative and collisional heating transfer, along with diffusive transport and chemical reactions, determine the basic thermal (and chemical) structure of the upper atmosphere
100
1000
104
103
Altitude (km)
Absorption of solar UV radiation is the primary heat source; shorter wavelength photons generally deposit their energy at higher altitudes. An essential difference between the lower and the upper atmosphere is that the heating and the cooling processes in the upper atmosphere are far removed from local thermodynamic equilibrium. Energy is partitioned among ionization, molecular dissociation, internal atomic and molecular excitation, and direct heating. Much of the energy that produces internal excitation is radiated as airglow at various wavelengths from IR to extreme UV
Temperature (K)
F-region O+, N+, e-
Neutral Temp.
Ti = Tn e- Temp.
Ion Temp.
102
D-region: H+(H2O)n CO3-, NO3-, eCharge density 100
1000
104
105
E-region O2+, NO+, e106
Charge density (cm-3) Local Thermodynamic Equilibrium: Tions = Tneutrals = Telectrons ONLY below 100 Km
Ionospheric chemistry The primary mechanism by which the upper atmosphere is cooled involves IR emission by CO2 (mainly below 120 km) and NO (mainly at or near 150 km). Heat deposited above 150 km is conducted downward, transferred via collisions to excited states of those cooling agents, emitted as IR radiation, and lost to space. Cooling is inefficient at higher altitudes because the major species there, mainly O, O2 and N2, do not radiate efficiently in the IR. Consequently the upper atmosphere above 150 km is very warm with temperatures varying between 600 K and 1800 K
Near UV 300 - 400 nm
IR cooling
CO2* Middle UV
200 - 300 nm
IR cooling Visible & IR
NO*
Far UV 120 - 200 nm Extreme UV 10 -120 nm X-ray
400 km C H E M I S T R 50 km Y
F2 Region (250 – 400 Km) F1 Region (130 – 250 Km) E Region (95 – 130 Km) D Region (50 – 90 Km) STRATOSPHERE (10 – 50 Km) TROPOSPHERE (0 – 10 Km) Earth
Chemistry of the F-layer The chemistry of the F-region centres on the conversion of O+, formed as the primary ion, to secondary molecular ions (O2+, NO+) that can recombine with electrons + + Charge exchange O + O2 → O + O2
Dissociative recombination
O2+
557.78 nm
+ e → O* + O
Green airglow
630 nm
O+ + N2 → N + NO+ Dissociative recombination (most important neutratization path)
NO+ + e → N* + O At F-region altitudes, the concentration of N2 is much higher than that of O2 because photodissociation of O2 is almost complete
Protonosphere Above the F2 electron density peak (~ 250 km), the region dominated by O+ gives way to the so-called protonosphere, a region dominated by H+
O+ + H ↔ O + H+
Chemistry of the E-layer The molecular ions O2+ and NO+ dominate in E-layer 1000
N2+ + O → N* + NO+
e
H (km)
O+
N2+ + O2 → N2 + O2+
He+ 500 H+ 300
NO production in E-layer
F2 layer
250 F1 layer N+
200
O2+ N2+
150
102
O+ e
NO+ O2+
E layer
100
Neutral nitride oxide (NO) begins to play an important part in atmospheric chemistry at E region altitudes
NO+
103
104 Ion density (cm-3)
105
106
Measured positive ions and electrons in the E and F layers during daytime at solar minimum
N (2D) + O2 → NO + O O2+ + N2 → NO + NO+
Chemistry of the E-layer Sporadic E phenomena 130 km
From time to time, unusual propagation of short-wave radio signals suggests the presence of local areas of increased ionization in the E region. These effects are known as Sporadic E phenomena. One manifestation of Sporadic E is longdistance reception of TV pictures that have arrived by a reflected path that is usually absent. Narrow (1 – 3 km) localized layers of metal ions are often associated with sporadic E
H (km)
Increases in metal ion concentrations have been observed during meteor showers Ionospheric chemistry is very much affected by the presence of metals because they have low ionization potentials (< 7 eV) and are monoatomic
e-
NO+ O2+ 95 km
Metal ions (Fe+, Mg+…)
O2+ + Mg → O2 + Mg+ NO+ + Mg → NO + Mg+
Monoatomic metal ions do NOT recombine easily & reactions of metal oxide ions (like MgO+) are very fast reproducing atomic ions. Thus, the distribution of metal ions is controlled by physical processes
Chemistry of the D-layer Chemical complexity characterizes the D region of the ionosphere. Low temperatures (the lowest in the entire atmosphere (down to 180 K)), relatively high pressures (between 0.01 and 1 mbar), and a wide range of minor trace reactants permit a multitude of reactions. At the same time, in-situ experimental study of the D- region is difficult because ion and electron concentrations are low and variable; high pressures hinder sampling into mass spectrometers
100 km 90 km
82 – 85 km
75 km
H+(H2O)n + e → H + nH2O
Hydrated protons, H+(H2O)n, with n ranging from 2 to at least 8, and occasionally even 20 are common below 82 km
70 km
Sharp decrease (~ one order of magnitude) in ELECTRON DENSITY in few km e + O2 + M → O2- + M
NO+ and O2+ ions dominate above 82 - 85 km
50 km
Transition region from electrons (dominant > 82 km) to negative ions as carriers of negative charge
Perturbations in the Ionosphere (X-rays - sudden ionospheric disturbances (SID)) The activity on the Sun can cause dramatic sudden changes to the ionosphere. The Sun can unexpectedly erupt with a solar flare, a violent explosion in the Sun’s atmosphere caused by huge magnetic activity. These sudden flares produce large amounts of X-rays and EUV energy, which travel to the Earth (and other planets) at the speed of light Electrons will penetrate to the D-region and will increase absorption causing a HF (3 – 30 MHz) radio blackout. Also VLF (3- 30 kHz) signals will become reflected by the D-region Increase ionization from latitudes 45º to 72º
X - ray Last 10’s of minutes
ee-
X - ray Last for several days
Perturbations in the Ionosphere (Protons - solar proton events (SPE) and polar cap absorption (PCA)) Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The proton spiral around and down the magnetic field lines of the Earth penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers at high magnetic latitudes (> 60 º) causing radio blackouts that can last up to several days (but the average is usually around 24 to 36 hours)
High energy PROTONS
High energy PROTONS
+60º
Hit the Earth upper Atmosphere 15’ – 2 h after solar flare
In almost all disturbances (SID, electron and SPE) of the D region, the primary ions are almost 100 per cent O2+, since the major constituent O2 is capable of being ionized
Perturbations in the Ionosphere (Geomagnetic storms) A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar coronal mass ejections (CME), coronal holes, or solar flares, a geomagnetic storm is caused by a solar wind shock wave which typically strikes the Earth's magnetic field 24 to 36 hours after the event. This only happens if the shock wave travels in a direction toward Earth. NO-3 core ions Solar wind
Perturbations in the Ionosphere (Lightning) Lightning can cause ionospheric perturbations in the D-region in three possible ways: VLF radio waves launched into magnetosphere. This
so-called “whistler” mode waves can interact with radiation belt particles and can cause them to precipitate onto the ionosphere. The latter are called Lighting induced Electron Precipitation (LEP) events
Additional
ionization can occur from direct heating as a result of huge motions of charge in lighting strikes. These events are called Early/Fast
Positive
(and some negative) Cloud to Ground, and intra-cloud lightning can cause Transient Luminous Events (TLE) in the D-region
The ionosphere and radio wave propagation
The electron density concentration in the IONOSPHERE conditionates the propagation of radio waves
The ionosphere and radio wave propagation Since diffraction cannot possibly account for the travel of radio waves across the Atlantic, Marconi’s success, in 1901, in sending radio signal of about 500 kHz (AM) from England to America demanded some explanation When radio broadcasting started in the early 1920s, it was found that, at night, the signal strengths at distances of about 100 km varied widely over a few minutes, sometimes disappearing completely. This fading of the signals was ascribed to interference effects between reflected sky wave and the direct ground wave. Reflections of the frequencies used for broadcasting thus seemed possible even for nearly vertical incidence, but only at night
Marconi's antenna system at Poldhu (UK), December 1901
IONOSPHERE SK
A YW
GROUND WAVE
VE
The ionosphere and radio wave propagation Ionization at low altitudes (D-region) leads to absorption of broadcast frequencies, and prevents reflection from higher altitudes during the day. At night, the absorption is much diminished (Dregion disappears), and the various fading and interference effects become pronounced. The D region is more involved with absorption, rather than reflection, of radio waves. The presence of electrons in the D region is responsible for the attenuation/absorption of reflected radio waves (in Elayer) of frequencies up to ~ 1 MHz used for broadcasting. Thus, the ionosphere is important for radio wave (mainly AM) propagation AMPLITUDE MODULATED (AM) radio (540 – 1600 kHz) o Ground wave – received (local reception) o Sky wave DAY: absorbed in D-region of ionosphere NIGHT: reflected by E- region ¾ Interference from distant transmitters ¾ Fading in/out of long distance reception IONOSPHERE SHORT WAVE (SW) radio (2 – 26 MHz) o Ground wave – received (local reception) o Sky wave – reflected by F-region (enables long distance reception) FREQUENCY MODULATED (FM) radio (88 – 108 MHz) o Ground wave – received (local reception) o Sky wave – transmitted through ionosphere into space
SK
A YW
GROUND WAVE
VE
The ionosphere and radio wave propagation F layer
E layer
T
R
T
R
T
R
T
R
Increasing frequency 1000 750
Reflection altitude (km)
F1 penetration
E penetration
500 250
F2 penetration
fcritical (F1)
fcritical (F2)
fcritical (E) 2
3
4 5 Frequency (MHz)
6
7
8
9 10
Electron densities in the ionosphere have been most commonly measured from the ground by a device called an ionosonde. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30 MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer
Life in the upper atmosphere 1970s Studies by Russian scientists established the fact that culturable bacteria and fungi can be isolated from heights of 60 – 70 km, including bacteria of the genera Micrococcus and Mycobacterium and fungi such as, Circinella muscae, Aspergillus niger and Penicillium notatum Micrococcus FUNGI – Aspergillus bacteria niger
FUNGI - Penicillium notatum
2003
Culturable bacteria were claimed to have been found by an anglo-indian team in the upper atmosphere (up to 41 km). However, some important questions remain unanswered: How do such bacteria cross the tropopause to reach such heights?. Although possible mechanisms have been suggested, such transfer remains difficult to explain. It is however, likely to be more easily achieved by sub-micron bacteria
Life in the upper atmosphere 2009
Very recently (March 2009) an Indian Balloon experiment flown at altitudes between 21 and 41 km has identified three bacterial colonies, namely, PVAS-1, B3 W22 and B8 W22 that seem to be totally new species Indian Balloon experiment (March 2009) The studies recently reported by Indian teams do not conclusively establish the extra-terrestrial origin of microorganisms
The importance of sub-micron size bacteria It is becoming increasingly clear that sub-micron bacteria exist in the environment. The importance of this fact to astrobiology lies in the fact that the smaller the bacterium, the more likely it is to a) be elevated from Earth (and presumably other planets) to the stratosphere, and b) to slowly fall back to Earth and c) fall through planetary atmospheres without burning up
Some (personal) conclusions The chemistry of the ionosphere is relatively well-known except for
the complex processes in the D-region Prediction of perturbations in the ionosphere is NOT yet reliable
enough. Further work is needed to avoid communication (including GPS ) and power blackouts
The impact of Transient Luminous Events (TLE) on the energetic
balance and chemical properties of the mesosphere (D-region) needs further research. Several space missions from ESA, CNES and other space agencies are planned for next years (ASIM-2013, TARANIS2012, …) Further research is required about the upper boundaries of the
biosphere. Are the “living aerosols” recently detected in the upper atmosphere (up to 41 km) from the Earth?. If not, how have they reach our planet, what’s their nature?. Where do they come from?
