S.Ananthakrishnan

Electronic Science, Pune University, Pune 411008, India

A Colloquium in NAOC on 20 June 2012

EIT195 EIT304 SOHO/LASCO

Photosphere and outer layers as seen at different wavelengths

Manifestations of the Sun

VMG around solar minimum

Explosive and Stormy Sun: Flares and CMEs

Filaments Prominence

Quiet Sun Active Sun Dynamic Sun

The inconstant Sun

Significant temporal and spatial variations are observed with solar activity cycle in

Sunspot number (and area) Magnetic flux Flare index 10.7 cm radio flux P-mode and f-mode oscillation

frequencies Rotation rate in convection zone Solar RMS magnetic fields Solar irradiance Coronal line intensity Coronal mass ejection rate and speed

Waves and oscillations in the Solar atmosphere

Waves and motions near prominences/flare

Hinode

Velocity oscillations in quiet photosphere

Wave motion in sunspot penumbra

Shocks and blast waves 06DEC2006/X6 Flare

Blast waves and Moreton waves

Travelling waves from flare site or solar quakes observed {kosovichev98, kosovichev99, Donea99, Kosovichev 2005}.

The outermost rarefied layer of the solar atmosphere. This is also invisible to usual ground instruments except during total solar eclipse. Structures appear to be magnetically controlled.

What is the solar corona?

Coronal holes: dark regions in corona, associated with "open" magnetic field lines, often found at the Sun's poles. Source of high-speed solar wind (Coles,--,Ananthakrishnan, et al., Nature 1980)

The temperature of the corona is ~ million degrees, adequate to emit X-rays

Active region

Coronal hole

loops

Corona is the hot plasma, visible during total eclipses and also from space-borne UV, EUV, X-ray telescopes (Skylab, Yohkoh, SoHO, TRACE, SXI

Coronal hole

System of coronal loops showing twisted structures and loops Highly structured in magnetic field, plasma density and temperature. Coronal magnetic fields – closed (denser loops – active regions) and open (hot and rarefied, coronal holes)

Magnetized structures in the corona

A natural plasma lab where a variety of conditions and processes occur – MHD, wave dynamics, turbulence, magnetic reconnection, etc.

Structure and dynamics completely dominated by magnetic fields which is generated by dynamo mechanism in the interior and emerges through surface to corona.

Physical processes in corona play crucial role in solar-terrestrial connection – space weather.

Why do we study the Corona?

Fundamental puzzles of solar corona:

What mechanisms heat the corona? – waves (acoustic or magnetic); reconnection or annihilation of coronal magnetic field! Details not known. What accelerates the coronal plasma – fast solar wind – to speeds exceeding 800 km/s? Slow solar wind is well described by the Parker solution, but physics of fast solar wind not clear -- connected with MHD waves! Several questions remain. What are physical processes behind solar flares and CMEs? Lots of details available, but mechanism for rapid conversion of magnetic energy into heat and particle acceleration still not understood.

How do we study it? Historically by total solar eclipses with ground-based telescopes.

(disadvantage - eclipses are rare and do not last longer than a few minutes to study dynamic events). Not too many Coronagraphs available.

Direct routine observations from ground difficult. (some high altitude coronagraph exist – e.g. Sac Peak Observatory, USA)

Space-borne UV, EUV, X-ray instruments provide major information on dynamic phenomena – Yohkoh, SOHO, TRACE, Hinode, STEREO, The Indian ADITYA is expected by 2014-2015.

How about waves in the corona? Physical processes in corona are governed by MHD theory, with the conditions: low plasma , speeds v << c, characteristic time >>collision time etc.

Coronal Mass Ejections

A Coronograph is a man-made eclipse with an occulting disc

blocking out the visible surface of the Sun (the

Photosphere).

Allows us to observe the hot solar atmosphere, the Corona

Continuous outflow of ionised gas (“plasma”), The Solar

Wind, 1014 kg per day

Events CMEs eject ~1013 kg at about 350 km s-1

What is a CME? A prominence eruption that becomes

CME core (in microwaves, Nobeyama)

SOHO/LASCO sees the CME

later in the corona with the core

Brightening on the disk is the associated flare

Emergence of Coronal Magnetic Flux

Loops of magnetic flux

emerge through the surface in

active (sunspot) regions

Some of this flux is “open” rises through

the corona and is frozen-in to the solar wind

outflow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Sun

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Solar wind flow is radial

Solar rotation and radial solar wind generates a Parker spiral field structure

Field is “frozen-in” to the solar wind flow

Parker Spiral (an example of frozen-in)

Trees On The Shore

(1970)

Sub-storm Initiation:

1 2

3 4

5

20 -30 RE.

?

Earth’s Geotail is virtually unstudied at radio

wavelengths.

Southward Bz

20 - 30 RE.

?

Storm Initiation

When the solar wind hits the earth’s magnetic field, it squashes it to an onion shape and flows around it, but high energy particles find their way into the atmosphere via the polar regions and create a variety of effects including Auroras. Thus there is an intimate link between the Earth and the Sun.

The background to Interplanetary scintillation (IPS):

The electron density irregularities in the solar wind plasma scatter radio waves coming towards an observer from a distant radio source and produce random diffraction pattern on the ground resulting in intensity fluctuation of the radio source signal. ( see next Fig. 1)

The resulting geometry is shown in Fig. 2 and 3 taken from Balasubramanian et al. (2003) and Manoharan et al.(1995).

A schematic of the

IPS phenomenon

from the STELAB,

Nagoya web site.

Fig. 1

Interplanetary Scintillation (IPS)

rmsmIndexillationSc 2)(int

rmserms NaLr 2/14/1

2 Scintillation Index (m) =

Fig. shows a schematic of

the observing geometry for

a typical IPS observation.

The ecliptic plane is

shown by the lightly

shaded semi-ellipse.

Letters `S' and `E' denote

Sun and Earth. The line

from `E' through point `P'

is the Line-of-sight (LOS)

to a radio source. The line

from `S' through `P' refers

to the radialy directed

solar wind across LOS.

The lines other than SE

and SA are outside the

ecliptic plane. The angle ε

refers to the solar

elongation and γ, to the

heliographic latitude. Balasubramanian et al. (2003)

Fig. 2

Geometry of IPS

observations as shown

in the previous slide.

The Sun-Earth-Radio

source angle is the

solar elongation angle ε.

The r distance from

the Sun to the closest

approach to the LOS

(Line-Of-Sight) is,

p = sin ε.

Manoharan,

Ananthakrishnan et al.

(1995) Fig. 3

I

On source

Fig. 4

Fig. 4 a

I

Off

source

Weak

source

Strong

source

The Earth orbiting around

the Sun changes the

elongation angle changing

the amplitude of the

fluctuation phase. The

scintillation index, m,

gives an estimate of the

peak to peak random

fluctuations and is defined

as the rms variation of the

fluctuations to the mean

intensity (I/<I> ) (Fig. 4a).

m = I/<I>

= sin ε

Fig. 4 b m-p curve of quasar

1148-001 over the years 1976-

1991. ( Manoharan,

Ananthakrishnan et al. (1995))

Since the amplitude of the fluctuation

phase changes as the elongation

angle changes, hence m also

changes reaching a peak at some

particular distance from the Sun,

which is decided by the transition

between weak and strong

scattering at the frequency of

observation. Over several years

these m-p curves have become

standardized (Fig. 4).

g = mdate/ <m > elong

~ 1 for quiet solar wind,

> 1 for disturbed conditions;

< 1 for depletion regions

As stated by Hewish,

Based on a terminology called `g’ introduced by Hewish,

any deviations from such `normally expected IPS behavior' in the m-p curve can be normalized in terms of a value

g = mdate/ <m>elong= S/<S>.

g ~ 1 for quiet solar wind; > 1 for disturbed conditions; < 1 for depletion regions

By observing a large number of scintillating radio sources in the sky and plotting their deviations via the quantity g, g-maps have been produced to study changes in the large scale structure of the interplanetary medium. g values > 1 indicate enhanced turbulence while values <1 indicate depletion regions of electron density, since g is related to ne by g ~ ne/9.

(Gapper et al, 1982, Hewish and Bravo 1986, Tappin, 1986, Manoharan et al., 1995, Janardhan et al. ,1996, Balasubramanian et al., 2003, Manoharan, 2006)

Fig. 5 a. Ooty Radio Telescope

Single antenna of 530m x 30 m

Ooty Radio Telescope.

has provided excellent data for many of these results.

IPS telescope at Thaltej,

Ahmedabad (similar to Hewish Array)

and a record of IPS signals at the

bottom at 103 MHz. This has been

dismantled in 2000 due to heavy

radio interference

Fig. 5b.

In order to find the relation between the phase fluctuations on the solar wind screen and the intensity fluctuations on the ground, the electric field at any point x in the observer's plane can be found. The intensity and its auto correlation function can also be derived and the power spectrum is given by Mz

2(q)= 4 Φo2(q) sin²(zq²/2k), where

q=qF=2k/z is the Fresnel frequency below which the intensity fluctuations are heavily attenuated (Salpeter 1967).

As the screen moves across the LOS with a velocity u, the temporal intensity It is related to the spatial intensity Ix by It(t) = Ix(ut).

Hence, Mt²(τ)= Mz²(uτ,0) and the temporal spectrum P(f) FT Mz²(uτ,0)

If the radio source is of finite angular size Mext ²(q) = Mo²(q) |V(qz/2π)|², where V(qz/2π) is called the visibility function =dθ e iqz/2πθ b(θ).

Thus, it is possible to derive the solar wind velocity given θ and vice-versa by suitable model-fitting (Fig. 6 & 7). This is the basis on which the single station velocities have been derived in Ooty. These estimates have been then compared with three station velocities and found to be in good agreement (Fig. 8) (Manoharan and Ananthakrishnan 1990, Moran et al. 2000).

By fixing the velocity, the compact angular sizes of extragalactic sources have also been derived.

(Pramesh Rao, Ananthakrishnan and colleagues started work in this area from 1971 onwards)

Figure shows

model fitting of

the temporal

spectra using

the parameters

of source size,

solar wind

velocity, axial

ratio, power law

index and

power level for

each elongation.

These have

been now

automated at

Ooty.

Fig. 6

The model fits to the power spectra show that care is required in fitting models..

Fig. 7

Fig. 8

Ooty Single

station

velocities

compared with

STE Lab.

three station

velocities. It

shows fair

agreement.

Repeating the

earlier slides,

Australian web

site showing

the propagation

of a CME from

the Sun and

enhancement

of scintillation at

Earth.

Fig. 10

Janardhan et al. (1996) Fig.11a

Janardhan et al. (1996)

Fig. 11b

Fig. 12

Fig. 13

Manoharan (2006)

Fast CME on April 3-4, 2001

Ananthakrishnan (2000) Fig. 15

Murray (Dryer), Tom (Detman) and I have been fascinated by the idea of making a 3 D MHD model of a transient which can be vetted by IPS and other observations. Figure 15 shows our idea at a schematic level. This is very interesting if it can be implemented.

The next result that I want to mention is the very interesting Solar disappearance event of May 11, 1999 (Balasubramanian et al., 2003, Janardhan et al., 2005), shown in Fig. 16 & 17.

Solar wind disappearance event in 1999 Fig. 16

May 11,

1999 g-

values as

measured

by Ooty IPS,

showing

that the

solar wind

density

fluctuations

had

become

extremely

low.

Ooty results

(Balasubramanian et

al. , JGR, (2003) )

Is it due to

magnetic

field

reversal or

due to a

transient

Coronal

Hole

( Janardha

n et al.

2005)

Fig. 17

The velocity vs g plot for May

1999, May 1995 and August

1998

The source of solar wind disappearance was shown later by my colleague Janardhan and others to be unipolar, stable flows from the boundary between large active regions and coronal holes located at central meridian.

The characteristic features of such disappearance events are that of highly non-radial, low density, low velocity solar wind flows associated with large flux expansion factors and extended Alfven radii.

Dynamic evolution of active region open fields that `pinch off ’ the coronal hole outflow to eject a low-density bubble that later engulfs the earth seems to be the mechanism.

My last topic is Magnetic fields that impact us.

From Zeeman Splitting of the FeI

1564.8nm line. Yearly mean values

are shown (906 data points).

The decrease in the avg. magnetic

field strength is 52 Gauss /year.

Continuum intensity of sunspot umbral

spectra normalized to the intensity of

the nearby quiet sun.

The increase in average

temperature is about 73 K/year

Below 1500 Gauss as measured with the

Fe I 1564.8nm line, photospheric magnetic

fields do not produce perceptible darkening.

1998 – 2005

If the trend

continues,

sunspots will

disappear

from the solar

surface

beyond 2015!!

Livingstone & Penn 2006

Are Sunspots Disappearing?

Solar Polar Fields

Solar wind measurements from 1995-2009 (Janardhan et

al, 2010, Jian et. al., 2011) have shown that the solar minimum in

2008—2009 has experienced the slowest solar wind with the

weakest solar wind dynamic pressure and magnetic field as

compared to the earlier 3 cycles.

Extended Cycle 23

Minimum

Janardhan et al., 2010

Norm

aliz

ed S

cin

tilla

tion Index

Year

Solar Wind Microturbulence

Janardhan, Bishoi, Ananthakrishnan et al.2011

The extended Minimum in 2008-2009?

Earliest signatures in the IPM as far

back as 1995 - Janardhan, Bishoi,

Ananthakrishnan et al. 2011, GRL

Are sunspots disappearing?? I don’t

think so, but don’t know!

The occurrence of extreme events that can have

catastrophic effects on modern technology:

There is some indication that such events may actually

occur more often in weaker cycles.

Three of the five largest solar energetic proton events

and two of the eight strongest storms in the last 150 years

occurred during solar cycles 13 and 14 respectively. Both

cycles were relatively weak.

MYSTERIES FROM THE SUN SEEM NEVER ENDING!

Extreme Events ?

The quest goes on!

THANK YOU VERY

MUCH FOR YOUR

ATTENTION.

Abstract:

Studying the Sun has been fascinating for me for more than 45 years now. I have learnt a few things about the solar flares, solar radio bursts, solar wind, CMEs and Sun’s magnetic fields. It has been a never ending quest. In this talk I will share some of these exciting details with you.

Manoharan, P.K. Highlights of Astronomy, Volume 15, 2010

Heating mechanisms for solar corona

Two types of mechanisms are proposed:

•Waves (acoustic and magnetic) •Magnetic reconnections

Acoustic waves do not reach far into the chromosphere, let alone the transition region or corona, before steepening into shocks in the rapidly decreasing density regions. Slow mode magnetoacoustic waves can propagate further but again they too form shocks and dissipate in the chromosphere. Fast modes, however, manage to reach transition region between chromosphere and corona, but they either dissipate or get reflected back. More recently, there are evidences of coronal waves detection, and also that they may carry adequate energy to heat the corona.

Waves in the Corona Systematic observational investigations of various waves and oscillations in the

corona now achieved due to high spatial and temporal resolution of coronal instruments, space-borne as well as ground-based

Theory of interaction of MHD modes with plasma structures (MHD modes of a magnetic cylinder) provides the theoretical basis for interpretation of the phenomena.

Observationally determined properties of MHD modes of solar coronal structures, such as active region loops, polar plumes, and other open structures, show excellent agreement with theory.

The combination of observational knowledge with MHD wave theory gives rise to a new method of investigation of coronal plasmas, the MHD seismology of the corona.

Coronal MHD waves and oscillations – coronal seismology - a rapidly developing, and promising branch of solar physics.

Related important topics, such as the coronal Moreton or EIT wave, oscillations in coronal bright points, sub-second radio pulsations, etc.

The visible skin of the Sun

Photoheliogram: A white-light image of the Sun taken on October 28, 2003 showing Limb Darkening Effect, sharp solar edge and Sunspots

The photosphere – 100 km

thick layer of hot gases, T~ 6000 K. Emits 99.99% of energy generated in the solar interior, mostly in visible spectral range centered at ~ 5000 Å or 500 nm.

In 1992, we started looking at the possibility of producing the g-maps and v-maps from Ooty data. M.Dryer and myself suggested a `picket fence' approach (Manoharan et al. (1995)), which puts the telescope at various solar elongations so that interplanetary transients are observed based on a STOA prediction model (Fig. 9 a, b), be they due to flares or CME's or transient Coronal Holes. The expected increase in scintillation from a large number of sources, as they are intercepted along the LOS by the transients could be observed by the telescope at each elongation. Manoharan et al. (2000) effectively used the picket fence approach on a 1992 June CME/flare event.

Fig. 9 a

STOA

model

assumes

that the

shock is

initially at a

constant

velocity

(piston-

driven), but

decelerates

at 1/R

based on

blast theory.

Fig. 9 b

Ooty 3-D G-MAP of the CME on April 4, 2001

Fig. 14 Manoharan (2006)

Jian et al., 2011 Solar Physics

Animation of Halloween 2003 Events

… to illustrate their heliospheric impact