Hinode/EIS Nugget – Temporal evolution of microflares in bright points

by Suguru Kamio, MPS


Recent observations by Hinode during the solar activity minimum revealed that quiet regions and coronal holes are not really quiet. X-ray bright points are found all over the Sun, and they occasionally undergo impulsive brightenings or microflares. Improved resolution of Hinode EIS and XRT allows us to follow a temporal evolution of the microflares. It is interesting to note that microflares in bright points and large flares in active regions share some common characteristics.

Figure 1 shows lightcurves of different temperature emission lines as a microflare occurred in a quiet region. Hot emission lines (greater than 1 MK) exhibit primary peaks coincident with an X-ray peak. A noticeable increase in Fe XV emission, which is normally weak in quiet regions, indicates heating in the microflare. An electron density derived from the ratio of Fe XII emission lines indicates a density increase at the primary peak, which suggests chromospheric evaporation. Twelve minutes later, cool emission lines showed a delayed peak. Assuming a tiny loop with a diameter of the bright point, the estimated cooling time of the coronal loop agrees with the delayed cool emission. This kind of delayed cool emission is frequently observed in flare loops in active region. The properties of microflares nicely fit that of flares in active regions, though their scales are quite different.

The lightcurves of microflares in coronal holes and in quiet regions are basically the same, but there exists a difference in their surroundings. Figure 2 displays running difference of X-ray images in the course of two distinct microflares in a coronal hole (left panel) and in a quiet region (right panel).

A microflare in the coronal hole produces a jet above the bright point, indicating an ejection of hot material along open magnetic field lines. In contrast, the microflare exhibits coronal dimming in the quiet region, which is interpreted as rearrangement of closed magnetic fields triggered by the microflare, analogous to a large scale coronal dimming in active regions. The behavior of the surroundings reflects different magnetic field configurations in coronal holes and quiet regions.

For more details, please contact: Dr. Lucie Green.

Next EIS Nugget    »»  coming on Aug 01, 2011

Evidence of a Connection Between Active Region Outflows and the Solar Wind
by David H. Brooks (George Mason University) and Harry P. Warren (Naval Research Laboratory)

Last Revised: Monday, 30-June-2011

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