The 12th Hellenic Astronomical Conference

A granule seen in the far wings of the H-alpha line:

exceptional darkening before fragmentation

Sung-Hong Park1, Georgia Tsiropoula1, Ioannis Kontogiannis1,

Kostas Tziotziou1, Eamon Scullion2, Gerry Doyle3

1 IAASARS/NOA, 2 Trinity College Dublin, 3 Armagh Observatory

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Hα -1.03Å Hα +1.03Å

1. Introduction: Solar Interior

Solar convection (red/blue: down/up flows; 15 hours)

Courtesy of Chris Henze

20 M

m

48 Mm

Courtesy of Kelvin Song

1. Introduction: Solar Granulation

Almeida et al. 2004

G-band (10.8Å filter at 4305.6Å)

Typical characteristics of granules

- Size: 1 Mm - Lifetime: ~16 min

- Doppler velocity: ~1 km/s at the disk center - Microturbulent velocity: ~2km/s

- Radial expansion speed: ~1-2 km/s - Temperature: 5,000-10,000 K

SST/CRISP Hα wideband movie obtained by Luc Rouppe van der Voort

1. Introduction: Evolution of Granules

1. Introduction: Motivation & Objectives

• Although our knowledge of granules and their evolution at the bottom of the

photosphere is quite precise from mainly white-light or photospheric line

observations, their temporal evolution throughout the photosphere is not

well known.

• To this extent Hα far wing observations can be useful for the diagnosis of

vertical flows and temperatures in the upper photospheric layers of granules

in relation to their evolution.

• Using time series of two-dimensional (2D) high-resolution, high-cadence, Hα

observations, we investigate temporal variations of some spectral properties

of a quiet Sun granule.

• The study will provide a better understanding of the physics underlying the

evolution of an individual granule at the near-surface layers of the Sun.

2. Observations: Swedish 1-m Solar Telescope (SST)

Obs. time 07:32-08:21 UT on 2014 June 07

Wavelength

-1.03, -0.77, -0.26Å,

Hα center,

+0.26, +0.77, +1.03Å,

Hα Wideband (bandpass: 4.9Å)

Ca II 8542Å Wideband

Cadence 4 sec

Resolution 0.059”

(~43 km)

Field-of-

view 84”×84”

Crisp Imaging Spectropolarimeter (CRISP)

Note that high resolution Hα images were

achieved with the aid of the adaptive optics

system (Scharmer et al. 2003b) and with image

restoration using the Multi-Object Multi-

Frame Blind De- convolution (MOMFBD;

van Noort et al. 2005) method. Photograph by Miguel Claro

CRISP Hα Full FOV

3. Results: Darkening before Fragmentation

Period of Investigation: 2014 June 07 08:03-08:12 UT (~10 min)

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CRISP Hα+1.03Å Movie

Two Spectral Properties: (1) Doppler Signal

- Doppler Signal (DS) provides a qualitative picture of upward (positive

DS) and downward (negative DS) moving material.

- We calculate DS from the ±0.77Å and ±1.03Å image pairs, respectively,

using the following formula:

- Note that a zero reference of DS is defined as the mean DS value of

a neighboring quiet Sun region.

Two Spectral Properties: (2) Full-Width at Half-Maximum

- We estimate the Full-Width at Half-Maximum (FWHM) of the Hα profiles

of the granule area under study.

- The average line profile in the same neighboring quiet Sun region as used

for determining the zero reference of DS is used to find the continuum

intensity IC by fitting it to a reference Hα line profile (up to ±30Å from the

line centre) published by David (1961).

- Then, FWHM is the wavelength separation between two wavelengths on

either side of the profile where the spectrum intensity I = 0.5 × ( IC + Imin ).

The Granule’s Evolution I

- Formed at ~08:00 UT.

- During the initial phase (08:03

UT – 08:05 UT), the DS maps

show mainly downflows in the

intergranular lane, while

upflows in the granule.

- From ~08:05 UT to 08:09 UT,

the granule gets darker as

observed in Hα wideband and

far wing images. The DS and

FWHM maps show downflows

and important broadening of the

spectral line.

- After the darkening finishes, the

granule fragments into several

small granules (e.g. at 8:11:47

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Wideband I (−1.03 Å) I (−0.77 Å) I (+0.77 Å) I (+1.03 Å)−0.05 0 0.05

DS ±0.77 Å0.97 1.25FWHM [Å]

The Granule’s Evolution II: Space-time Slice Images

- Cloud events (marked by 1-5)

very low intensity - high

positive DS (high-speed

upflow) - very high FWHM

values

- Granule

low intensity – negative DS

(downflow) - high FWHM in

the granule area.

- Rapid shrinkage

Granule’s western boundary at

~08:07 UT.

08:03

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(b)

(c)

The Granule’s Evolution III: Hα Line Profiles

−1.0 −0.5 0.0 0.5 1.0D l [Å]

0.2

0.4

0.6

0.8

I /

I C

08:08:39 UT (Upflow #4)08:03:01 UT (Reference)

08:03:01 UT08:05:48 UT08:08:27 UT

4. Summary and Discussion

The behaviour of the Hα line profiles in the quiet Sun can be summarized as follows.

1. Initially, the granular region has its normal appearance, i.e. bright granule, dark

intergranular lanes.

2. Most of the region in the granule gets dark at both far blue and red wings of the

Hα line, during a period of ~5 minutes.

3. During the darkening the line profiles in the granule are redshifted, asymmetric

and broader relative to a quiet Sun mean profile.

4. The granule remains constant in size for most of the time, and then it shrinks

before its fragmentation.

1. After the darkening phase the granule becomes fragmented into a number of

small granules, while the line profile gets closer to the quiet Sun mean profile.

Why this granule appears so dark in both Hα far wings?

• Of course, because of the broadening of the Hα line profile!

• Related to thermal and non-thermal broadening

• Non-thermal broadening is influenced by the unresolved granular motions

associated with the so-called microturbulent velocities.

• Previous studies, however, indicate that granules have microturbulent velocities of

the order of ~2 km/s at the photosphere that even decrease with height (see, e.g.,

Stodilka & Malynych 2006; Fontenla et al. 2008), so it is not expected that the

increase of the FWHM is due to an increase of the microturbulent velocity.

• We conclude, therefore, that the increase of the Hα line width is mainly due to

thermal broadening and thus to temperature increase caused by adiabatic

compression associated with the downflowing material and the granule’s rapid

shrinkage.

• In addition, it is conjectured that the granular fragmentation can be associated with

the downflows in the granule, as well as the shrinkage of the granule.

Thank you for your attention!

Any comments or questions are welcome.