### 1. HXR Ribbons

One of the best examples of the use of RHESSI's imaging spectroscopy
with the finest grids is the work done on the M8.0 flare on May 13, 2005
(Dennis et al. 2007; Liu et al. 2007a, b, 2008; Jing et al. 2007). This
is a particular event associated with an EUV sigmoid eruption, where HXR
sources exhibit an elongated shape. Dennis et al. (2007) demonstrated
that adding grids 1 and 2 can enhance the image quality depending on the
source structure, and the extended HXR sources in this event is such a
suitable case. Together with natural weighting, in which counts from all
detectors are given equal weights, a more physically reasonable image
morphology can be obtained. Liu et al. (2007a) showed that until the
flare maximum (intervals a - c), HXR emissions appear as point-like
compact sources located in regions with strong magnetic field within the
chromospheric ribbons; after the flare maximum (intervals d - f)), the
HXR sources, however, become elongated to form a ribbon structure almost
along the entire section of chromospheric ribbons (see Fig. 1).

![](media/image1.jpeg){width="5.763888888888889in"
height="3.921527777777778in"}**Figure 1**. Time sequence of 25--50 keV
images covering the HXR rise to decay phases of the 2005 May 13 M8.0
flare. The green contours show flux at levels of 0.1, 0.115, and 0.13
photons cm**^-2^** s^‑1^ arcsec^‑2^. Panel **f** also shows a RHESSI 6
-- 12 keV image with yellow contours. The white contours outline the
TRACE 1600 Å ribbons taken near the center of each RHESSI 1-minute time
interval.

By measuring the local ribbon expansion velocity and the variation of
HXR flux and magnetic field strength, Jing et al. (2007) found that a
good spatial correlation between the HXR flux and the reconnection rate
or energy release rate in support of the standard 2D reconnection model
only exists when associated with footpoint-like HXR sources earlier in
the event. Multiwavelength observations carried out by Liu et al.
(2007b) corroborated the suggestion that the sigmoid-to-arcade
transformation can reorganize the coronal magnetic fields along the
third dimension unspecified in the 2D model, and explains not only the
ribbon-like HXR at a time but why the footpoint HXR appears first and
then evolved to the ribbon-like HXR. Namely, the tether-cutting
reconnection firstly occurs between the two elbows in the middle of the
sigmoid, during which the HXR sources can appear footpoint-like as
usual. Later on, the electrons accelerated in the magnetic arcade can
bombard the chromosphere along the footpoints of the arcade, resulting
in the ribbon-like HXR emissions. Exploiting RHESSI's capability of
imaging spectroscopy in each pixel, Liu et al. (2008) further found a
spatial anticorrelation relationship between the local HXR flux and the
local spectral index. They suggested this as a spatial analog of the
temporal soft-hard-soft spectral evolution pattern of the integrated HXR
flux, which may contribute to explaining the confined nature of HXR
sources compared with extended H\[EQUATION\]/UV ribbons. They also found
an anticorrelation between the HXR spectral index and the local electric
field along the ribbon, which suggests electron acceleration by the
electric field during flares.

Dennis, B. R., Liu, C., Schwartz, R. A., & Tolbert, A. K. 2007, in AAS
Meeting Abs., Vol. 210, 93.06

Jing, J., Lee, J., Liu, C., Gary, D. E., & Wang, H. 2007, ApJL, 664,
L127

Liu, C., Lee, J., Gary, D. E., & Wang, H. 2007a, ApJL, 658, L127

Liu, C., Lee, J., Jing, J., Gary, D. E., & Wang, H. 2008, ApJL, 672, L69

Liu, C., Lee, J., Yurchyshyn, V., Deng, N., Cho, K.-S., Karlický, M., &
Wang, H. 2007b, ApJ, 669, 1372