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