Fe XIV green line loops in the inner corona as seen by LASCO/C1 on June 01, 1998 at 04:12 UT plus three different reconstruction schemes based on an 8 first-level scales plus continuum, each scale further subdivided in 4 scales plus continuum

Towards unambiguous characterization of coronal structures A new approach based on a wavelet packets equalization technique

Mierla M.1, Stenborg G.2, Schwenn R.1, Cobelli P.3, Podlipnik B.1

1.MPAe, 2.CUA / NASA GSFC , 3.UBA, contact address mierla@linmpi.mpg.de

By means of a multi-resolution image processing technique based on wavelet packets, we can enhance both: the boundaries and the internal details of originally faint and diffuse structures. This approach provides a means for the quantification of stationary and dynamic coronal structures required for conducting morphological studies.

Method description and examplesWith advent of the LASCO coronagraphs on SOHO, a new quality of coronal observations

could be obtained. The field of view now extends from 1.1 R⊙ (C1) to 30 R⊙ (C3), with

sufficient sensitivity to make visible even the almost continuous solar wind outflow in the

streamer belt. However, these coronal features are very faint and diffuse. Special

techniques must be applied in order to reveal these features and to track unambiguously

their time evolution. One of them is the wavelet packets equalization technique. It

consists in decomposition of a chosen image in wavelet scales with the first wavelet scales

containing the higher frequency components and the last scales containing lower

frequency signatures [1]. Every one of these scales can be decomposed further in

subsequent scales. By assigning different weight to these levels and subsequently

recombining them (plus a smoothed array, called continuum), a very good contrast

enhanced image can be obtained.

0 1 … 4

0 1 1 1 1

1 4 4 4 4

2 4 4 4 4

3 4 4 4 4

4 4 4 4 4

... 0 0 0 0

8 0 0 0 0

0 1 … 4

0 1 1 1 1

1 4 4 4 4

2 4 4 4 4

3 4 4 4 4

4 4 4 4 4

... 4 4 4 4

8 4 4 4 4

0 1 … 4

0 1 1 1 1

1 0 0 0 0

2 4 10 7 4

3 4 4 7 4

4 4 4 4 4

... 0 0 0 0

8 0 0 0 0

Note how the low frequency component present in both the original image and the one

in the opposite corner is suppressed in the other two images, thus the enhancing

contrast for close-to-limb fine structures. On the bottom right image we put more

weight on the high frequency components and the structures appear sharper .

The example chosen for C2 is to show how the proper selection of the reconstruction

strategies can reveal details hidden or not resolved in the untreated image. The upper

right image is equivalent to unsharp masking obtained by giving to every wavelet scale

the same weight and removing the continuum component. In the bottom left image the

continuum at the first decomposition level was removed and the high frequency

components were given more weight. This leads to the enhancement of internal details.

For the bottom right picture only the first-level continuum and the second-level highest

frequencies were used. Thus, even minor details, not seen in the original image, are

made visible.

A CME observed by LASCO-C2 coronagraph on August 13, 2002 plus three different reconstruction schemes based on an 8 first-level scales plus continuum, each scale further subdivided in 3 scales plus continuum.

In a further step we reorganize the

contrast-enhanced images. By placing

side by side several profiles taken at

different times for a given angular

position, a height-time diagram is

obtained. Further on, for a given radial

distance, angle-time diagrams are

constructed. The two diagrams allow

the accurate determination of kinematic

quantities (speed, acceleration) and

angular extent of the events [2].

Application of this method on a particular event

The event chosen is a blob moving out radially as seen in LASCO/C2 images on June 2-

3, 1998. The steps are:

1) Process images using wavelet packets equalization technique

We selected 10 scales for the first decomposition and 3 scales for the second one, with

a weight of 1 for the first levels and 5 for the last ones.

2) Build the height time and angular time diagrams for C2 processed images:

Conclusion By applying the wavelet packet equalization technique to LASCO

images:

- diffuse close-to-limb magnetic field structures are better discerned,

- faint structures hidden in the background can be revealed,

- the unseen internal details of coronal transients are revealed.

This approach allows unambiguous image interpretation and provides a

mean for the quantification of stationary and dynamic coronal

structures.

References

[1] Stenborg G., Cobelli P. J. 2003, A&A , 398, 1185-1193

[2] Sheeley, N. R. et al. 1999, JGR, 104, 24739-24767

Outflow, seen in C2 images, on June, 2-3 1998 on the NW limb. The arrows point to the blob, and the blue dashed lines delimitate it.

1998/06/02 23:28 UT 1998/06/02 23:56 UT

1998/06/03 00:28UT 1998/06/03 00:58 UT

PA = 40º

R = 2.85 R⊙

PA = 40º

R = 3.00 R⊙

PA = 40º

R = 3.30 R⊙ PA = 40º

R = 3.70 R⊙

The LASCO/C2 images show the blob moving out radially. From the height time diagram

we can see that the blob moves with a constant speed up to 4.1 R⊙. At this distance it

accelerates and then it moves again with a constant velocity. From the angular time

diagram an angular width of approximate 7º is derived.

LASCOimages

time

time

I

time

R

PA

R

PA

m

The sketch showing the building of height time diagram

4.1 R⊙

ORIGINAL

ORIGINAL4.1 R⊙

The height-time diagram for a polar angle of 40º The angle-time diagram for the radial distance of 3 R⊙

Time [in hours]20 30