Harry reports on coronal loops in the sunspot group AR12422

Post flare loops in H-alpha above AR12422
Post flare loops in H-alpha above AR12422. Sketch and copyright Harry Roberts ©, all rights reserved.

Regular solar observer & correspondent Harry Roberts previously reported on the complex Delta Group of sunspots, AR12422. Here he follows up with a report on the group’s coronal loops.

AR12422: Coronal Loops.                                                           Harry Roberts.

Recently, when AR12422 hosted a GOES M5.5 flare, a fine display of post flare loops (PFL) ensued: we followed their development over just 8min. While they occur at every major flare, such loops are rare as they can only be seen at the Sun’s limbs, against the dark background of space. For this reason Hale Delta class spots are watched closely as they approach the Sun’s limbs.

Coronal loops. Post flare loops are a special kind of coronal loop. Coronal  loops form above most big spot groups as magnetic flux ‘ropes’ emerge from the solar interior, forming the sunspots. While much bigger than PFL, coronal loops are invisible in any spectral band unless they are filled with plasma. Since they are so large they occupy the corona from the so-called ‘Transition Zone’ at  ~2Mm high to the corona above 100Mm. This is a hot but tenuous atmospheric region, at ~106K, and little or no H-alpha light is emitted. Yet extreme UV light is emitted – and of the various bands sampled by satellites, that of Fe IX at 171Å (T=6.3×105K) seems to best reveal coronal loops. Other bands at 304Å HeII, etc. show lower structures.

Schrijver & Zwaan: “Solar and Stellar Magnetic Activity” P189 state: “It is remarkable that no bright coronal loops end in the dark cores of spot umbrae; some bright loops appear to end in umbral light bridges. Penumbral fields, in contrast, lie at the base of bright loops”

Post flare loops. While of the same origin, the difference between a PF loop and a coronal loop is one of altitude and temperature. The PFL’s are seen when a solar flare heats a ‘bubble’ above a spot group at an altitude where H-alpha plasma can condense on the (otherwise invisible) coronal loops – making them briefly visible. In contrast to Ha (6563Å) with ionization T ~5000K, EUV 171Å, at higher temperature and altitude, reveals taller bigger loops that extend over vast areas above the spot group (Fig2).

M5.5 flare. Our earlier discussion of PFL’s above this flare (on 2015 Oct2 ~00:11UT) showed four configurations of loops to a height above the limb of ~22Mm: while dark loop arches faintly seen against the disc suggested a total height above loop footpoints (amid the sunspots) of 66Mm. A further log (Fig1) made at 00:34UT, about 15min later, shows complex multiple loops now 78Mm above the spot group. Note, this group is still some 20° of longitude from the limb and the loops are not yet tangential to our line-of-sight. This height, 78,000 km (~6 Earth diam.) is typical of PFL’s.

Coronal loops above AR12422
Coronal loops above AR12422. Sketch and copyright Harry Roberts ©, all rights reserved.

Coronal loops. What did the SDO AIA 171Å show of coronal loops above the site? Fig2.1 shows 171Å images of the region above the site soon after the flare (28min later) and ~45hr later (Fig2.2) when the spot group has rotated out of sight. Note the 171Å images show coronal loops to be much higher than the PF loops:  a at 60Mm high, b at 93Mm and c at 156Mm. Note the sites of AR12422 (p) and (f) spots plotted on the disc below the loops. The ‘a’ coronal loops are twice the height of our H-alpha PF loops. The coronal loops are filled with plasma and persist much longer than the PF loops. Note that the SDO images have been rotated in Po to match the H-alpha record. At times, H-alpha PF loops show well on SDO AIA 304Å HeII images.

So the H-alpha band shows events of the chromosphere, and at times up to ~100Mm altitude: the writer once followed an Moon-sized globule of ejecta to a height of 600Mm from the limb – but now realizes such events are very rare. In EUV the drama doesn’t end with the loops seen in 171Å: indeed it’s only the beginning. The coronal loops often form the base for helmet streamers that then flow deep into the solar system as a solar wind, and the tenuous heliosphere.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.

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