Alexey Kuznetsov Armagh Observatory
Outline of the talk Solar radio emission History Instruments and methods Results of observations Radio emission of planets Overview / history / instruments Radio emission of Jupiter Radio emission of other planets Radio emission of brown dwarfs Overview / instruments Results of observations 21 September 2012 Armagh Observatory 2
1890-1940: first attempts to detect radio emission from the Sun 1890: Thomas Edison 1893-1896: Johannes Wilsing & Julius Scheiner (Potsdam Observatory) 1901: Charles Nordmann (Observatory of Nice) 1931-1933: Karl Jansky (Bell Telephone Laboratories) 1933-1934: John Kraus & Arthur Adel (University of Michigan) No emission was detected! Reasons: Insufficient sensitivity. The experiments were performed during solar minima. 21 September 2012 Armagh Observatory 3
1942: discovery of solar radio emission February 26-27, 1942: radar stations along the south coast of England (at the frequency of about 60 MHz) were jammed by radio signals from an unknown source. James Hey (Army Operational Research Group) identified the Sun as the emission source. He also found that the emission was associated with a large group of sunspots. 1942: George Southworth (Bell Telephone Laboratories) detected solar radio emission at 3 and 10 GHz. 1943: Grote Reber (an amateur radio astronomer) detected solar radio emission at 160 MHz. He was the first to publish solar radio observations. 21 September 2012 Armagh Observatory 4
After 1945: rapid progress in solar radio astronomy!" # $% & 21 September 2012 Armagh Observatory 5
Why solar radio observations are important? What can we learn from them? Radio observations allow us to study the parameters of magnetic field and plasma in the solar corona, where observations at other wavelengths usually fail due to low plasma density. Radio emission of solar flares is produced by the high-energy electrons that are the key factor in development of the flares. Therefore, radio observations allow us to study the parameters of these electrons. Disadvantages of radio observations: Spatial resolution is usually low. Emission mechanisms are rather complicated. 21 September 2012 Armagh Observatory 6
Frequencies of solar radio bursts: from ~10 MHz to tens of GHz. Angular resolution of a telescope: λ θ, d where is the wavelength and d is the diameter of the antenna (mirror). To achieve the resolution of 10 arcsec, we need:!' ( $ ) Total intensity of solar radio bursts: W max < 10-7 W / m 2. W radio / W optical < 10-10. Frequency 100 MHz 1 GHz 10 GHz Diameter 62 km 6.2 km 620 m 21 September 2012 Armagh Observatory 7
Radiometers / radiopolarimeters Full-disk observations * + " +, '- &. $/ $ 0 $ $ 0 $ $1 2 3 4 5 6 7 &, '' 21 September 2012 Armagh Observatory 8
Spectrographs / spectropolarimeters Full-disk observations A number of frequency channels 1-2 GHz 5.2-7.6 GHz 2.6-3.8 GHz 4 & & % 8 ' & ' 21 September 2012 Armagh Observatory 9
Radioheliographs (radio interferometers) Multi-antenna instruments Interferometric methods are used to reconstruct images * + " 7 1. 2 9/ / 2 # +, '- &. 0 3 4 5 &. 2 % 8 '* " 4 7 0 3 4 5 $ 21 September 2012 Armagh Observatory 10
Radioheliographs (radio interferometers) Multi-antenna instruments Interferometric methods are used to reconstruct images + " : /.) / 2 9) / 2 # +, '- &. 0 3 4 5 &. % 8 ' " : 7 0 3 4 5 $ 21 September 2012 Armagh Observatory 11
Radioheliographs (radio interferometers) Multi-antenna instruments Interferometric methods are used to reconstruct images * ; " 7. / 2 2 9/ 2 # +, '- &. 2 < 2 = 4 5 &. < / % 8 '* " 4 7 ) = 4 5 $ 21 September 2012 Armagh Observatory 12
Components of solar radio emission 1. Emission of the quiet Sun. free-free emission of thermal electrons (T ~ 10 6 K). 2. Slowly-varying component. is produced due to cyclotron radiation of thermal electrons in strong magnetic fields of the active regions; demonstrates a very good correlation with the sunspot number. 3. Sporadic radio emission. is associated with flares; is produced by nonthermal electrons. 21 September 2012 Armagh Observatory 13
> 7 Standard model of a solar flare % " ' Electron energy: up to ~10 MeV. 21 September 2012 Armagh Observatory 14
Schematic spectrum of solar radio bursts (Dulk 1985) plasma emission gyrosynchrotron emission 21 September 2012 Armagh Observatory 15
Low-frequency (metric) emission The longest history of observations. Five basic spectral classes of metric bursts are known (type I, II, III, IV, and V). Type II and III bursts can be indicators of geoeffective factors. > 8 ' & 7 ', ' + & ( ) 21 September 2012 Armagh Observatory 16
Type III bursts Short narrowband bursts with fast frequency drift (towards lower frequencies). Harmonic structure is sometimes observed. Are produced by escaping subrelativistic (0.1c 0.5c) electron beams on open magnetic field lines. Can extend to very low frequencies (into interplanetary space).? & @!!!+ & +, + 4 7!!!!+ & 21 September 2012 Armagh Observatory 17
Type II bursts Short narrowband bursts with slow frequency drift (towards lower frequencies). Harmonic structure is often observed. Most likely, are produced at the fronts of coronal mass ejections (CMEs).? & @!!+ & +, + 4 7 A +, + 6 % # # 4 # 21 September 2012 Armagh Observatory 18
High-frequency (microwave) emission Incoherent gyrosynchrotron mechanism dominates. The emission is produced by trapped electrons in closed magnetic configurations. & ''@ 7 21 September 2012 Armagh Observatory 19
NoRH images of the solar flare on 22 August 2005 0 3 4 5 $ 3 4 5 $ 0 3 4 5 $5 7 21 September 2012 Armagh Observatory 20
NoRH images of the solar flare on 25 September 2011 0 3 4 5 $ 3 4 5 $ 0 3 4 5 $5 7 21 September 2012 Armagh Observatory 21
Typical spectrum of solar gyrosynchrotron burst * " B & ''/ = / 2 2 21 September 2012 Armagh Observatory 22
Decimetric radio bursts (300 MHz 5 GHz) The emission is produced by trapped electrons in closed magnetic configurations. Plasma radiation mechanism dominates: unstable electron beam plasma waves radio emission. % 8 ' & & + & +, + 4 7 21 September 2012 Armagh Observatory 23
Fine spectral structure of decimetric radio bursts: zebra patterns C " + & @ 5 + +, + - & % 7 " : & + : + C " + & @ 5 + +, + 4 & & 21 September 2012 Armagh Observatory 24
Fine spectral structure of decimetric radio bursts: fibers? & @ & ''+ + & +, + 4 & & 21 September 2012 Armagh Observatory 25
Fine spectral structure of decimetric radio bursts: spikes? & @ & ' +, + B 8 /? & @ & ' +, + : & D 7 21 September 2012 Armagh Observatory 26
Coming soon New or upgraded solar-oriented radio telescopes: Upgraded Siberian Solar Radio Telescope Expanded Owens Valley Solar Array Chinese Spectral Radioheliograph General-purpose radio instruments: Expanded Very Large Array Low-Frequency Array 21 September 2012 Armagh Observatory 27
Planetary radio emission Ionospheric cutoff f < f c f c 10 MHz f > f c Ionosphere 21 September 2012 Armagh Observatory 28
Planetary radio emission Typical frequencies Earth: Jupiter: Saturn: Uranus: ~30 800 khz up to 40 MHz (maximum at ~15 MHz) ~20 khz 1.2 MHz ~15 750 khz Neptune: ~20 700 khz Mercury, Venus, Mars: no significant radio emission Comparative spectra of planetary auroral radio emissions (Zarka 1998). 21 September 2012 Armagh Observatory 29
Discovery of Jovian radio emission 1955: Bernard Burke and Kenneth Franklin (Carnegie Institution) accidentally discovered radio emission from Jupiter (at the frequency of 22.2 MHz) using the Mills Cross Array. The discovery was made during observations of the Crab Nebula (as a test source). However, at that time Jupiter was located nearby Later, it was found that Jovian radio emission was observed earlier by different researchers, but not recognized. = % 21 September 2012 Armagh Observatory 30
Instruments for planetary radio astronomy * ;? % - & +. 2 / 2 = 4 5 % 8 ' & ', 21 September 2012 Armagh Observatory 31
Instruments for planetary radio astronomy E : " : E : " / / 2 1 - & +.1 / = 4 5 % 8 ' & ', @ + & 21 September 2012 Armagh Observatory 32
Jovian decametric radiation (DAM) Frequency range: ~3 40 GHz. Average intensity (normalized to 1 AU distance): 10-19 W m -2 Hz -1 = 10 3 sfu. 100% elliptical polarization. Strongly modulated by the planet rotation and the orbital phase of Io. % & ', +, + * ;? % F / 2 2 0 21 September 2012 Armagh Observatory 33
Jovian decametric radiation (DAM) C!& +, + * ;? % ( G(, 4 / 2 2 1 C # & + + '? % = '& '!+ & 21 September 2012 Armagh Observatory 34
Jovian decametric radiation: source model The emission is generated at the Io flux tube. Emission mechanism: electron-cyclotron maser instability. Emission frequency: ff B = 2.80 10 6 B (the magnetic field of B = 14 G corresponds to the cyclotron frequency of f B = 39.2 MHz). Plasma density is very low ( 5 cm -3 ). 21 September 2012 Armagh Observatory 35
Fine spectral structure of Jovian decametric radiation: S-bursts Duration (at a fixed frequency): tens of ms. Bandwidth (at a fixed time): up to 200 khz. Fast frequency drift (usually towards lower frequencies). Intensity (normalized to 1 AU distance): up to 10-17 W m -2 Hz -1 = 10 5 sfu. > 8 ' @ + & 21 September 2012 Armagh Observatory 36
Other components of Jovian radio emission Hectometric and kilometric radiations maser emission plasma emission (?) + & ', +, + & 7 & '+ 21 September 2012 Armagh Observatory 37
Other components of Jovian radio emission Decimetric radiation Frequency range: 100 MHz 10 GHz. Intensity: ~10 5 times lower than that of DAM. Produced by high-energy (tens of MeV) electrons in Jovian radiation belts. Emission mechanism: synchrotron. 5 GHz : @ 7 ', +, @ H 6 % / 2 2 21 September 2012 Armagh Observatory 38
Instruments for planetary radio astronomy Voyager 1 & 2 FAST Cassini 21 September 2012 Armagh Observatory 39
Auroral kilometric radiation (AKR) of the Earth +, '% " B / 2 2 / Emission frequency: ~30 800 khz (maximum at ~250 khz). +, '% " = & / 2 2 ) Produced at the heights of 2000 20000 km. Produced at the magnetic longitudes of 60 78. 21 September 2012 Armagh Observatory 40
Auroral kilometric radiation of Saturn (SKR) Emission frequency: ~20 1200 khz (maximum at 100 400 khz). Produced at the heights of 6000 180000 km. +, ' " 6 / 2 Produced at the magnetic longitudes of 60 85. 21 September 2012 Armagh Observatory 41
Auroral kilometric radiations of Uranus and Neptune +, E & +, * & 21 September 2012 Armagh Observatory 42
Radio emission from exoplanets Not yet detected! But is expected to be detected soon with the Low-Frequency Array (LOFAR) 21 September 2012 Armagh Observatory 43
Radio emission from ultracool dwarfs Very low-mass stars (spectral class M7, temperature < 2700 K). Brown dwarfs (spectral classes L, T, Y). Ultracool dwarfs Neutral planetary-like atmospheres instead of ionized solar- and stellar-like coronae. Radio emission was occasionally detected in 2001. To date, ~10% of M7-L3 objects have been detected in radio. The emission frequency: a few GHz (currently detected at 1-20 GHz). The emission intensity can be very high much higher than that of the solar or planetary radio emission. 21 September 2012 Armagh Observatory 44
Physical characteristics of ultracool dwarfs Radius: R ~ R Jupiter (~ 0.1 R ); Mass: M < 100 M Jupiter (< 0.1 M ). 21 September 2012 Armagh Observatory 45
Instruments for stellar radio astronomy H 6 7 % H 6 % 21 September 2012 Armagh Observatory 46
Instruments for stellar radio astronomy % + " 21 September 2012 Armagh Observatory 47
M9 dwarf TVLM513-46646 1 3 4 5 ) 1 3 4 5 C " 7 &, +, @ H 6 % Emission intensity (normalized to 1 AU distance): up to 3 10-16 W m -2 Hz -1 = 3 10 6 sfu. Total emission power: 10 4 times higher than that of Jupiter. Pulse period (1.96 hr) coincides with the stellar rotation period. ~100% circular polarization. 21 September 2012 Armagh Observatory 48
L3.5 dwarf 2MASS J00361617+1821104 Temperature of the dwarf: ~1900 K. 1 1 3 4 5 1 1 3 4 5 Emission intensity (normalized to 1 AU distance): up to 2 10-17 W m -2 Hz -1 = 2 10 5 sfu. Polarization degree: > 60%. The emission period (3.08 hr) seems to correspond to the rotation period. " 7 &, +, @ H 6 % 4 / 2 2 1 21 September 2012 Armagh Observatory 49
M7 dwarf 2MASS J13142039+1320011 Average emission intensity (normalized to 1 AU distance): 10-16 W m -2 Hz -1 = 10 6 sfu. Polarization degree: ~25%. The emission period (3.89 hr) seems to correspond to the rotation period. " 7 &, +, @ H 6 % = 6 / 2 21 September 2012 Armagh Observatory 50
T6.5 dwarf 2MASS J10475385+2124234 Temperature of the dwarf: ~900 K. Emission intensity (normalized to 1 AU distance): 10-16 W m -2 Hz -1 = 10 6 sfu. Polarization degree: up to 90%.? 7 &, +, @ % + " & I ( 5 5 / 2 / 21 September 2012 Armagh Observatory 51
M8.5 dwarf DENIS-P J104814.9-395604 Power-law spectrum indicates the gyrosynchrotron emission mechanism. " & + @ % & : % ", / 2 21 September 2012 Armagh Observatory 52
Source model Radio emission from ultracool dwarfs: a scaled-up analogue of the planetary auroral radio emissions? Magnetic field is highly asymmetric (similar to that of Uranus or Neptune). Short periodic radio bursts: maser emission mechanism; required magnetic field: 3000 G. Quiescent or slowly varying component: gyrosynchrotron emission mechanism; required magnetic field: ~100 G. 21 September 2012 Armagh Observatory 53
Why radio observations of ultracool dwarfs are important? What can we learn from them? Radio (and some optical) observations indicate the presence of very strong magnetic fields at ultracool dwarfs. The origin of these magnetic fields is unknown. The mechanisms proposed for the Sun do not work at low-mass stars and brown dwarfs. Radio observations are currently the only way to study the magnetic field structure at ultracool dwarfs. These results are very important for the further development of the stellar (and, possibly, solar) dynamo theory. 21 September 2012 Armagh Observatory 54