NoRH Observations of RHESSI Microflares M.R. Kundu, Dept. of Astronomy, University of Maryland, College Park, MD E.J.Schmahl, Dept. of Astronomy, University of Maryland, College Park, MD and Lab for Astronomy and Solar Physics, NASA/GSFC V.I. Garaimov, Dept. of Astronomy, University of Maryland, College Park, MD P.C. Grigis, Institute of Astronomy, ETH Zurich, CH-8092 Zurich, Switzerland

ABSTRACT  We present a summary of the analysis of more than two dozen microflares, observed simultaneously by RHESSI in hard X-rays and Nobeyama RadioHeliograph (NoRH) in microwaves (17 GHz). The RHESSI microflares are observed in the energy range 3-25 keV. The observations were made 2002, May 2-6.  We describe the imaging characteristics of these microflares including their locations in hard X-rays and microwaves and the relative positions of the micro-flaring sources relative to MDI magnetograms.  We discuss the brightness temperatures, emission measures and their hard X-ray spectral properties. One sees the mini flaring loops clearly in NoRH images. The microwave emission often seems to come from the RHESSI foot points (for higher energies), and from the entire small (mini) flaring loop (for lower energies).  Sometimes the two (microwave & HXR) sources coincide, at other times they are at opposite ends of the mini flaring loop.  Typically, the hard X-ray spectrum of the microwave associated RHESSI microflares can be fit by a thermal component at low energies(3-6 keV) and a nonthermal component at higher energies (12-25 keV).

 More than two dozen microwave events corresponding to RHESSI microflares were observed in our May 2002 observing period. Several active regions including AR 9934 were involved. We shall discuss only some events observed 2002 May2-6.  AR 9934 was a complex region which contained a sunspot with a strong negative polarity of the magnetic field on the north side and a bipolar region on the south side.  MDI images show fast evolution of the south part of the region. TRACE images show many small loops in the south and the complex loop connecting the north sunspot to the south side of the region.  NoRH 17-GHz maps show a maximum above the sunspot and there is also emission on the south side of the AR. RHESSI maps superimposed on the NoRH maps show that X-ray emission in the range 3-25 keV are located inside the radio contours. THE 2002 MAY EVENTS OBSERVED AT NOBEYAMA

 During all three events HXR emission was located in the south part of the AR. RHESSI maps for 2002 May 3 (event 1) clearly show an X-ray loop at 3-6 keV and two footpoints of the loop in the 6-12 and keV ranges. These footpoints are located above opposite magnetic polarities as seen in overlays of hard X-ray images on the MDI image.  For 2002 May 4 (event 2) HXR images show a small X-ray loop, located close to the same position as previous event. Footpoints of the X-ray loop are not resolved.  Overlays of HXR images on the MDI image shows that the X-ray loop was located above the magnetic neutral line and it connected two regions with opposite magnetic polarities.  During both events HXR emission was observed below 25 keV.  Total radio flux from the X-ray emitting active region was less than 0.5 sfu.  No significant polarization of the radio emission was observed in either case.

 The 2002 May 2 01:52 event is one of several microflares observed on this day, which originated in the NE part of the active region. At 01:52:10 UT RHESSI images in 3 energy bands 3-6 keV, 6-12 keV and keV overlie a 17-GHz NE source which occupies mostly an MDI negative polarity, implying that the HXR source may be situated above the strongest microwave source -- probably one footpoint of the microwave flaring loop.  The NE source and another SW source seem to contribute to the microflare emission at the same time as judged from the time profiles.  For the maximum phase of the 2002 May 3,03:58 microflare, HXR spectrum was calculated. It could be fitted by three components: thermal bremsstrahlung, atomic emission lines, and power law spectrum. Temperature of the thermal component was about 1.6 keV; the emission measure was about cm-3. Slope of the power law is -3.2.

 May 03 May 02

May 03 May 04

May 05 May 06

2002 May 02  17 GHz time profile  RHESSI 3-25 keV  RHESSI spectrogram

May 02  Grayscale 17 GHz, contours RHESSI bands 3-6, 6-12 & keV (top to bottom)  Bottom row shows background-subtracted 17 GHz maps  In all these events, the HXR source appears close to, but somewhat displaced limbward of the 17 GHz source

May 02

 The RHESSI and NoRH images at 4 different times  The maps are superposed on a MDI magnetogram.

2002 May 03  17 GHz time profile  GOES 1-8 A profile  RHESSI spectogram  RHESSI 3-25 keV

May 03  In HXR these micro- flares are single, and each is displaced from the 17 GHz sources.  Each HXR micro- flare has significant emission in the keV band.  Faint 17-GHz feature in all 3 cases

May 03  The 03:55 microflare difference map (bottom row) shows a loop-like microwave feature connecting the HXR source to the bright, compact 17 GHz component ~100” to the north.  The 03:58 microflare in all HXR bands shows a loop and footpoints. This RHESSI loop is probably the same as the 03:55 microwave “loop”.  In the 04:15 event, the HXR source is single & elongated, suggestive of a loop displaced eastward of the earlier loop.

May 03  The microflare at 04:01 very compact, unresolved, and is exactly co-spatial in HXR and 17 GHz.  The other two HXR micro- flares are slightly offset but nearly coincident with the brightest 17 GHz source.  It is possible that the 17 GHz is at one end/footpoint of the RHESSI loop-like structure.

2002 May 03 Left: time profiles at 17 GHz and for GOES and RHESSI (3-25 keV) Right: contour maps at 17 GHz and HXR superimposed on MDI magnetogram The The The microflare at 03:58 is shown in the bottom row. HXR loop at 3-6 keV; two footpoints in 6-25 keV.

2002 May 3 03:58 X-ray spectrum fitted by three components: thermal bremsstrahlung, atomic lines, and power law Temperature of the thermal component is about 1.6 keV; emission measure is about cm -3. Slope of the power law is -3.2.

2002 May 04  17 GHz time profile  GOES 1-8 A profile  RHESSI 3-25 keV  RHESSI spectogram

2002 May 04  The HXR source lies close to a weak 17 GHz source  The HXR source is visible in all three bands in two events

May 04  The FOV of the 17-GHz map does not overlap the RHESSI microflare at 05:54 UT.  The background- subtracted maps at 05:43 and 05:45 show that RHESSI sources in the two lower bands are close to a 17 GHz source

2002 May 04 Left: time profiles at 17 GHz and for GOES and RHESSI (3-25 keV) Right: contour maps at 17 GHz and HXR superimposed on MDI magnetogram The event concerned starts at 0:508 UT (first row). Note a small X- ray loop close to the May 3 location. The HXR source is compact with unresolved footpoints.

2002 May 05  17 GHz time profile  GOES 1-8 A profile  RHESSI 3-25 keV  RHESSI spectogram

May 05  Note that microflares at different times can come from different components of active regions.  Time profiles of different components of 17 GHz microflare-producing region.

2002 May 05  MDI images along with 17 GHz and RHESSI contours  Note the RHESSI loop-like structure and single sources at the boundary between positive & negative polarities, suggestive of mini-loops

2002 May 06  17 GHz time profile  RHESSI 3-25 keV  RHESSI spectogram

May 06

TRACE images of the AR 9934 with MDI contours 2002 May May 04

MDI images with 17 GHz and RHESSI contours of AR May May 04

RHESSI SPECTRA 2002 May 02 Model fits of thermal, line, & power law components 01:46:30 01:50:00 01:51:30 01:53:40

RHESSI SPECTRA 2002 May 03 03:57:30 04:00:00 04:00:30 04:03:00

2002 May 03 05:06:00 05:09:00 Model fits of thermal, line, & power law components

RHESSI HXR spectra for the May 3, 03:58 event. Each panel shows a 5-component fit for a 12-s time bin using thermal, atomic line emission and nonthermal bremsstrahlung. Time bin1234 T1 (MK) EM (10 46 cm -3 )  F E turn (keV) The components are: Temperature (T), emission measure (E), spectral index (  ), 20-keV flux (F20), and lower cutoff (E turn )

Conclusions Microwave (17GHz) micro-events are always associated with RHESSI microflares observed in the energy range 3-25 keV. This is certainly true for all stronger microflares considered here. RHESSI microflares have loop-like structures in lower energy bands (3-6,6-12 keV) and foot point emission usually in higher energy band (12-25 keV). The microwave emission comes from the foot points, and from the entire small (mini) flaring loop (for lower energy HXR events). The relative positions of microwaves and hard X-rays in all energy bands are similar to what is observed in normal flares. Sometimes the two (microwave & hard X- ray) sources coincide, at other times the two are at opposite ends of the mini flaring loop. Sometimes one sees the mini flaring loops clearly in NoRH images.

Time profiles of different components of the 17GHz AR show that successive events may come from different component sources. The hard X-ray spectrum of a typical microwave- associated RHESSI micro flare can be fit by a thermal component (EM~6*10 46 cm -3 at 3-6 keV) at low energies and (sometimes) a nonthermal component (with slope -3.2) at higher energies. Sometimes one observes compact, unresolved co- located HXR & microwave micro-events. To be continued at nbym_06 Conclusions (continued)