Regular solar observer & correspondent Harry Roberts reports on active region AR12422, a complex Delta Group:
AR12422: Complex Delta Group. Harry Roberts
Hale’s invention of the visual magnetograph and the Hale Classification of sunspots is a heroic tale, but we can only touch on it here. He found that spots were magnetic entities, shaped by hidden flux sources: sources that reversed polarity at successive solar cycles. And, sometime soon, current spots will be joined by others, of reversed polarity, as a new cycle begins.
Or will they? Research and data seem to show a steady decline in overall solar activity. The writer’s plot of current flaring against that of past cycles shows SC24 flaring is an order of magnitude weaker in X-ray flux than the previous three cycles (SC21-23). Yet past flare activity was seen to peak some time after the sunspot peak; i.e. peak flaring occurred some years after the sunspot maximum. We have yet to reach that point in cycle SC24. What will happen then?
With this in mind, the writer maps the more ‘active-looking’ spot groups in hope of flare activity: Delta groups are watched most closely. Sometimes nature cooperates.
Delta spots. Hale did not recognize the delta mix of fields: i.e. spots of opposite ‘sign’ in one penumbra; Künzel added that class in 1960. In white light, Delta groups may look like a blend of two bipolar groups- with say, twin (p) or (f) spots, or spots in long chains, or distorted penumbrae: kind of sunspot “train wrecks”! Not surprisingly, Delta groups are the chief ‘flarers’. In fact, all but one of the SC24 groups, thus far, that hosted an X-class flare, were class Delta.
AR12422. The writer has few logs of it due to cloud. Sighted first on Sep. 27 (04:00UT) and logged in detail 18h later (Fig1, 27th,22:00), it looked Delta class. But what would the magnetograms show? Two kinds are posted on-line: the ‘robot’ ‘scopes (GONG, SDO, etc) log ‘global field’ in real-time –with flux range 0 – 100G using an Fe line in IR (of low spatial resolution). Also we still have Hale’s own scope at Mt Wilson, using a red Fe line in the visual: flux range 1000-3000G+. With high spatial resolution, it gives polarity and flux in individual spot umbrae (i.e. umbral fields) – and is likely what Künzel had in mind in 1960. Global fields and umbral fields don’t always agree on a Delta classification for a given group: but they did for AR12422.
Umbral fields. On the 28th the strongest field logged at MtW was 2400G in the (f) umbrae, with 2100G in (p) spots; no Delta mix was detected by MtW, while NOAA assigned Delta by this time from SDO, GONG etc. global data. We see (Fig 1) complex (p) and (f) spot clusters with linear umbrae and spot chains attached. It was actively flaring, the strongest an M7.6 on the 28th – but the writer logged none during a 62 min patrol.
At the limb. Fig2 Shows the group 4d later (after more cloud) at the west limb, when it had grown to a huge ~20° in length! In disbelief, the log was confirmed with SDO’s HMI 6173Å continuum image. Groups this long are rare. Its (p) spots had migrated 5° west over the 4d period, while the (f) drifted just 2° eastward. Or had new (p) spots emerged ahead of the group? The spots are now in complex chains, albeit compressed by limb curvature.
Delta mix. Mixed polarity is seen in the group’s umbrae (Fig2) and MtW now assigns class delta. Flux strength is down, but measurement at the limb detects transverse components of flux; often less than vertical. Strongest field is seen in the (f) spots.
Flaring. (Fig3)32 mins into the H-alpha patrol a sub-flare erupted (00:08UT) near -25,100 and rapidly brightening to GOES M5.5 at 00:13, as flare material (ejecta?) spread to the limb at -25,120. At 00:14 a bright arc rose above the limb, but was dark against the chromosphere: likely an ejecting filament or post flare loop. It soon became a fine display of such loops, with footpoints that shifted about amongst the spot umbrae. At 00:20, clumpy spray-like ejecta rose bright above the limb in places. Motion in the ejecting filament was timed at ~200km.sec-1
Next day (not shown) saw the trailing 10° of AR12422 still visible, with large bright surges above the limb at the latitude of the now unseen (p) spots. Despite an 80min H-alpha patrol no other transients arose.
It is likely that remnants of the group will reappear ~2015 Oct 16. This delta group was a good example of its class, but while it hosted many GOES M-class flares, the M7.6 was strongest and no X-class were produced. We wait: hoping that the late peak in X-class flaring seen in SC23 will be repeated in coming months.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.