M is for Magnifique

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Contents

Introduction

On October 12th, new magnetic flux began to emerge in the middle of the negative-polarity region of active region 11112. This emerging flux continued to grow over the next few days stressing the already existing fields and presumably building up magnetic energy. A few days later, on October 16th, the pent-up energy was finally released through an M-class flare (SOL2010-10-16T19:12). The region went on to produce a few C-class events before rotating around the limb, but in a sense its master work was over. This nugget concerns this M-class flare which was simultaneously observed by [RHESSI] and SDO. Here, we focus on the AIA observations, which are unique due to the fact that this is the first flare for which AIA's automatic exposure control was enabled. In a future Nuggets, we will take a look at the SDO EVE observations, a subject which we have already visited in a previous Nugget, and other topics such as loop oscillations which AIA is particularly good at observing.

Figure 1: The SDO spacecraft with instruments highlighted.
Figure 2: The four AIA telescopes.

Intro to AIA

First a quick introduction to SDO's AIA since we have not yet discussed this important new instrument. The Atmospheric Imaging Assembly (AIA) is one of three instruments on board the Solar Dynamics Observatory (SDO). AIA consists of four telescopes, each imaging at multiple wavelengths, and provides full disk solar images with a pixel size of 0.6 arcseconds and a time cadence of around 12 seconds. Unlike RHESSI, SDO observes the Sun continuously, barring occasional eclipse periods. These guaranteed observations coupled with the unprecedented combination of spatial and temporal resolution have opened up a treasure trove of possibilities for solar wave observations. AIA takes solar images at the traditional SOHO/EIT wavelengths of 171 &#197 and 193 &#197, but also in the 94 &#197, 131 &#197, 211 &#197, 304 &#197 and 335 &#197 bands. Thus, for the first time, simultaneous consistent high-resolution full disk imaging of the Sun at a broad range of different temperatures is possible (see Figure 1). Combined with RHESSI observations, interesting new discoveries are surely on the horizon.


Channel name Primary ion(s) Region of atmosphere Char. log(T)
white light continuum photosphere 3.7
1700Å continuum temperature minimum, photosphere 3.7
304Å He  II chromosphere, transition region 4.7
1600Å C  IV+cont. transition region + upper

photosphere

5.0
171Å Fe  IX quiet corona, upper

transition region

5.8
193Å Fe  XII,

XXIV

corona and hot flare plasma 6.1, 7.3
211Å Fe  XIV active-region corona 6.3
335Å Fe  XVI active-region corona 6.4
94Å Fe  XVIII flaring regions 6.8
131Å Fe  VIII, XX, XXIII flaring regions 5.6, 7.0, 7.2
Figure 3: Temperature response for the various AIA channels.

Automatic Exposure Compensation (AEC)

The large increase in flux during flares poses a problem for most detectors. In RHESSI's case, we use movable metal shutters which come in automatically when needed and block out the large thermal fluxes at low energies. For an EUV imager such as AIA, high fluxes can be mitigated by simply changing the exposure time of the images just like in optical photography. The October 16th flare is the first flare for which automatic exposure compensation (AEC) was on therefore it provides an interesting test case for the SDO's team current strategy for AEC. The following movie shows AEC in action in 171 angstrom.

The flickering is due to the fact that AEC is only enabled every other frame. This strategic choice is to make sure that there is always an overexposed frame available throughout the flare in order to be able to see the fainter global structure. Comparing different channels, it is also clear that AEC works on every AIA channel independently. A closer look at an underexposed compared to overexposed frame can be seen in Figure 1.

Waves?

Maintaining information on the global structure can be important since flares are known to create traveling disturbances such as Moreton or EIT waves, which can propagate across the solar disk and disturb nearby magnetic field structures, setting up local standing waves. All of these waves, both standing and traveling, are interesting in that they can give us information which is usually hard to come by such as the magnetic field strength. In addition, AIA, with its steady cadence, full sun field of view and good sensitivity, is an excellent platform to spot this types of events. Taking a look at 94 Angstrom, a higher temperature channel, it is clear that this flare is associated with some sort of propagating disturbance (check out the following movie to see it in action). A quick back of the envelope calculation suggests that this disturbance is traveling close to 1000 km/s! This value is strangely close to CME speeds yet a quick look at coronagraph data suggest no CMEs are associated with this time. We will make sure to take a closer look at this and associated waves in a future nugget.

Conclusion

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