View
222
Download
1
Tags:
Embed Size (px)
Citation preview
Introducing Solar Radiophysics
Dynamic spectrum & Frequency drift
• A dynamic spectrum describes the flux density in terms of frequency and time.
• The time rate of change of frequency is called “frequency drift”. That is
Frequency drift =
dfdt
Understanding physical mechanisms
Spectral types
Available theories
Convincing or not
I few no
II few no
III manymostly irrelevant
IV no no
V no no
Dynamic Spectrum
A conceptual interpretation
f
t
A simple picture of dynamic spectrum
f
R0v
pf
Envisioned Source Region Situation
f
pf
t0t t
0 0pfdf f
dt
v vR R
Observed Dynamic Spectrum
“Plasma Emission”
In general it involves four processes:• Generation of enhanced Langmuir wav
es• Partial conversion of Langmuir waves i
nto fundamental em waves• Production of backward Langmuir wav
es• Generation of second harmonic em wa
ves
Development of Theories of “Plasma Emission”
• Ginzburg & Zheleznyakov (1958)• Tsytovich (1967) and Kaplan & Tsytovich
(1968)• Melrose (1980) and others
Classification of Spectral Types of Radio Emission
Spec. type
Nature Source
I StormPre-flare, decay phase
II Bursts CME & shock wave
IIIBursts & storm
Flare-assoc. electrons
IV Continuum Behind shock wave
V Bursts After type III bursts
Difficulties with “plasma emission” hypothesis
Summary of F-wave theories
• Scattering of Langmuir waves by ions (Ginzburg & Zheleznyakov, 1958; Tsytovich 1967)
• Scattering by Ion sound waves (Melrose 1980)
• Collapse of Langmuir wave packets (Goldman 1980)
Summary of H-wave theories
• Coalescence of two Langmuir waves (Ginzburg & Zheleznyakov 1958)
• Collapse of Langmuir wave soliton (Goldman et al. 1980)
Difficulties with the plasma emission scenario (1)
• H/F ratio = 1.6 ~ 1.9
f
R
H
FHf
Ff
F-H waves are generated at the same time in the source region according to plasma emission theories.
H/F frequency ratio at a given time
f
t
H
FHf
Ff
1.6 2.0H
F
ff
Difficulties with the plasma emission scenario (2)
• H/F ratio = 1.6 ~ 1.9• Temporal delay of F component
Initial delay of F waves
f
t
H
F
Hf
Ff
Moreover…
• Observations show that the starting H wave frequency is often more than twice the starting frequency of F waves.
• In some cases initially F wave frequency is only one third of that of the H wave.
• Statistically the starting frequencies of H waves peak around 200 MHz whereas those of F waves peak around 60 MHz.
Difficulties with the plasma emission scenario (3)
• H/F ratio = 1.6 ~ 1.9• Temporal delay of F component• Only a fraction of type III events
have F-H pair.
Difficulties with the plasma emission scenario (4)
• H/F ratio = 1.6 ~ 1.9• Temporal delay of F component• Only a fraction of type III events
have F-H pair emission.• F component waves are more
directive than H component waves.
Difficulties with the plasma emission scenario (5)
• H/F ratio = 1.6 ~ 1.9• Temporal delay of F component• Only a fraction of type III events
have F-H pair emission.• F component waves are more
directive than H component waves. • Coincidental source regions of H-F
waves with same frequency
Expected Source Regions
f
R
H
F
sf
HRFR
Stewart, R. T., Proc. Astron. Soc. Aust., 2, 100 (1972)
Interplanetary type III emission
Additional unresolved issues
Low-frequency interplanetarytype III emission
• Interplanetary type III emission was not known until late 1970s.
• It is not observable by ground facilities.
• Because it is observed with satellites the results must be interpreted accordingly.
Comments on satellite observations
• When a satellite is in the source region, in principle, it can measure the distribution function of the beam electrons. However, the angular resolution is often limited.
• The observations enable us to examine the role of Langmuir waves in the emission process. However, we usually cannot pin point the actual source position of the waves.
Consensus & standard explanation
• In general, because of subjective reasons, researchers believe that plasma emission is the generation mechanism.
• However, there are difficult issues which have puzzled and mystified scientists for years.
Few of the difficult issues
• A clear electron beam is rarely observed. The best result is a weak trace of a beam which is marginally unstable according to plasma kinetic theory.
Observation of Langmuir waves
Energetic electron distribution function
Few of the difficult issues
• A clear electron beam is rarely observed. The best result is a weak trace of a beam which is marginally unstable according to plasma kinetic theory.
• The emission often stops suddenly in the solar wind.
Few of the difficult issues
• A clear electron beam is rarely observed. The best result is a weak trace of a beam which is marginally unstable according to plasma kinetic theory.
• The emission often stops suddenly in the solar wind.
• In some cases the emission actual began in interplanetary space.
• At very low frequencies (f < 100 kHz) the source size becomes very large.
• At very low frequencies (f < 100 kHz) the source size becomes very large.
• The emission durations of the very low frequency radiation can be exceedingly long.
Energetics
• It is established that
• Thus the kinetic energy density of beam electrons is about
• If this total amount of energy density is converted to Langmuir waves, the waves would have an electric field ~100 mV/m.
50/ 10bn n
40 5 10 /v km s
13 36 10 /erg cm
Summary of major results of CMI
• Both O-mode and X-mode waves may be amplified.
• The amplified waves have frequencies close to electron gyro-frequency and its second harmonic.
• It turns out that O-mode is unimportant.
• Amplification of X-mode waves depends on the ratio of plasma frequency to gyro frequency.
Further Remarks
• In the region where
both F-H waves are emitted.• In the region where
H waves are emitted.
0 0.2p
g
f
f
0.2 1.3p
g
f
f
Simultaneous observations by Wind & Ulysses spacecraft
Other four types of solar radio emissio
• type I storms,
• type II bursts,
• type IV emission, and
• type V bursts
Type I Storms
• J. S. Hey first observed the radiation in 1946.
• It is found that the radiation is connected with large sunspots.
• It consists of narrow band, spiky bursts and a broadband continuum.
• The radiation is not related to flares.• It occurs for days after the appearance of
large active regions. The noise storms is due to change of coronal magnetic field.
An example of type I storms
(Continuation)
• Occasionally there are type III storms at frequencies below type I bursts.
• The type I storm continuum may be due to nonthermal electrons trapped in loops.
• Type I bursts differs from type III bursts in that it is strongly polarized and has no harmonic band.
• The key issue is what produces the bursts.
Storms are usually associated with large sunspots
Type I bursts from a bipolar region
For comparison, light lines show areas of plus and minus magnetic field based on Mount Wilson data.
A proposed model to account for erratic movement of sources
Type II bursts
• It was first identified in coronal shock wave by R. Payne-Scott and coworkers in 1947.
• Extremely intense and narrow bands. • Fundamental and harmonic
components• Slow frequency drift which suggests
a beam speed ~ 1000 km/s.• Frequencies are close to local
plasma frequency and its harmonic.
Payne-Scott et al. (1947): First measurement of type II bursts. Note the progressive time delay in the onset of the outburst on different frequencies.
Frequency drift of four type II bursts. The dotted line represents a constant drift rate of 0.22 MHz per second
Type II bursts with herringbone structure.
(Continuation)
• Backbone and herringbone structures• The backbone is co-moving with a
shock.• The herringbone structure is
interpreted as signatures of a beam of fast electrons associated with the shock.
• But herringbone structures appear only in about 20% of type II bursts.
• Only 65% of the shocks observed as a fast CME radiate type II bursts.
f
t
Backbone
Herringbone
Schematic description of a dynamic spectrum
(Continuation)
• The frequency ratio of H/F bands is closer to 2 than in the case of type III bursts.
• The source regions of F and H bands with a given frequency basically coincide.
• Lowest frequency is about 20 MHz.• Type II emission usually occurs about
one minute after the peak of flare associated hard X-rays.
H/F frequency ratio
Unshaded: F-H pair
Shaded : One band
All four type II bursts contain two harmonic and split bands
Starting frequency of fundamental bands of type II bursts
Study of a compound type II and type III bursts
Type II bursts with harmonic feature
Type IV emission
• May be grouped into three sub-classes1. Stationary type IV emission2. Moving type IV emission3. Decimetric type IV emission
• Early explanation: synchrotron radiation• Difficulties: (i) bandwidth
(ii) energetic electrons• More recent notion: trapped electrons
Moving plasmoids scenario
• Loops and their evolution have important implications to the understanding of flare physics and radiophysics.
• Dulk & Altschuler (1971) has inferred that type IV bursts might be due to moving plasmoid.
• The key question is how the plasmoid id formed.
A suggested scenario of type VIm emission
H flare ribbons
Filament
Type IV bursts
Moving type IV emission
• Brightness temperature K• Emission is evidently due to some
kind of induced process.• Most likely the emission is attributed
to non-thermal trapped electrons.• Moving type IV bursts is moving with
nearly constant speed of a few hundred km/s.
9 1010 10
Type V bursts
• Usually occurs immediately after type III bursts.
• Often has opposite sense of polarization.
• In general frequencies are lower than 60 MHz.
A type V bursts event