RHESSI, Hinode, and Spin

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Number: 57
1st Author: Säm Krucker
2nd Author: Steven Christe
Published: 6 January 2007
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RHESSI and Hinode

The Hinode observations began late in 2006, at solar minimum and yet in time to catch some of the last big flare events of Solar Cycle 23 (which will end, we hope, in 2007). We are watching activity eagerly because this preceding cycle helps with predictions for forthcoming ones. In Cycle 24 we'll be studying flares in depth with RHESSI and the new high-resolution (but low-energy) Hinode observations. In the meanwhile we get to practice on the 2006 flares. Hinode had great luck: launched on September 22, 2006, its powerful telescopes were deployed and functioning when a surprisingly energetic bout of solar activity, perhaps the last gasp of Cycle 23, broke out in December (it was to turn out to be not yet the last of the cycle).

The big flare of 13 Dec. 2006

Unfortunately, the most spectacular of the Hinode events, the flare of December 13, 2006, presented unusual problems for RHESSI. In this Nugget we publicize these problems because so many people will be curious about the RHESSI data, and because (infrequently) other flare events will inevitably have similar problems. We intend to return to this situation with a more general description in a future Nugget, and in the meanwhile we will show the problem for this one event.

The plots below (Figure 1) show the general circumstances of this flare. The time series at the top shows the GOES soft X-ray observations; the spectrogram in the middle the RHESSI data.

Figure 1: (Top) GOES soft X-ray time profiles; (middle) RHESSI X-ray spectrogram plot (the dashed line shows a night-day transition): (bottom) 4-second averaged position of the RHESSI rotation axis relative to the flare location. The bottom panel displays the bad news: this graph shows the distance from the RHESSI rotation axis to the flare. Because RHESSI imaging requires modulation of the image, which converts the image structure into a time series that can be analyzed, there is a blind spot: RHESSI cannot image sources exactly on the spin axis, since there is then no modulation. Figure 2 below shows the spin axis location during this event.
Figure 2:The location of the imaging axis over time during the flare. The black star is the location of the flare and the thick black line represents the average location of the imaging axis over time. When the instantaneous location of the imaging axis is too close to the flare then there is no modulation. The dynamic nature of the spin axis in this case may be due to constant shutter motions. RHESSI and rigid-body dynamics

A little background information about RHESSI is necessary. Its telescope rotates around the Sun-pointing axis, the longest dimension. In free space this kind of motion tends towards instability, such that any perturbing torque will cause nutation, a feature indeed of the Earth itself. A full mathematical description of this kind of motion, due largely to Poinsot, has been available since the 19th century. In RHESSI's case the basic motion is a spin with ~4-sec period. The spin axis is pointed directly at the Sun. The nutation wobbles the spin axis around a mean position on the Sun (see Figure 3 left). During eclipse, RHESSIs pointing system is turned off. During that time and due to the nature of its orbit, the spin axis drifts to the west. When, it's in dayline, the pointing system forces it to correct that drift by moving to the East. On average, RHESSIs spin axis is always slightly to the west (see figure 3, right).

Figure 3: Right: The location of the spin axis as a function of time during a typical RHESSI orbit. Again, the black line represents the average spin axis location. The circles around that line is the nutation. Note the drift from West to East. Left: The average location of the spin axis over a five year period. RHESSIs pointing is intentionally turned off whenever it is within about 4 arcmins from sun Center. Success!

But all is not lost. Because the spin axis nutates - a complexity, but a helpful one - there is still image modulation. It is just harder to analyze. Figure 3 shows RHESSI image contours overlaid on a magnetogram from Hinode, and this image confirms a seldom-observed phenomenon. The magnetogram shows a direct flare effect, the dark patch in the nearly circular umbra of the major sunspot. The magnetogram suggests that the line-of-sight magnetic field reverses direction here, which is physically implausible. Thus we explain the effect by the complicated response of the solar atmosphere to the flare disturbance, but the details remain murky. The RHESSI images shows that the magnetic perturbation has a close relationship to hard X-ray emission and thus to the high-energy electrons accelerated by the flare.

Figure 4: RHESSI 35-100 keV hard X-ray contours for 02:28:58 -02:29:10 UT overplotted on a Hinode magnetogram (02:28:42 UT). The contour levels are 25,35,45,55,65,75,85,95%. The magnetogram shows the flare effect very clearly in the umbra of the major spot, and the hard X-ray contours match this effect well with a small adjustment (-3", -9" in the x, y coordinates). The reader should compare the (x,y) coordinates of the images in Figure 4 with the spin-axis locations shown in the Figure 2.


It was a sad coincidence; this was perhaps the best-observed solar flare ever, since it benefited from good coverage by the world's first high-resolution solar space observatory (Hinode) and was also observed almost in its entirety by RHESSI.We had expected therefore to be able to compare low-energy and high-energy observations in a uniquely powerful way. Murphy's law struck yet again, but with patience and special processing, we are learning how to make use of the RHESSI data in spite of the bad luck in the flare's location.

Biographical note

Sa"m Krucker and Steven Christe are RHESSI team members at UC Berkeley.

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