norh observations of rhessi microflares m.r. kundu, dept. of astronomy, university of maryland,...
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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, MDP.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 includingAR 9934 were involved. We shall discuss only some events observed 2002 May2-6. AR 9934 was a complex region which contained a sunspot with astrong negative polarity of the magnetic field on the north side and abipolar 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 thecomplex loop connecting the north sunspot to the south side ofthe region.
NoRH 17-GHz maps show a maximum above the sunspot and there isalso emission on the south side of the AR. RHESSI mapssuperimposed on the NoRH maps show that X-ray emission in the range3-25 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 at3-6 keV and two footpoints of the loop in the 6-12 and 12-25 keVranges. These footpoints are located above opposite magneticpolarities 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 regionswith 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 12-25 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 6 1046 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 & 12-25 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
May 02 The RHESSI and NoRH images at 4 different times The maps are superposed on a MDI magnetogram.
2002 May 0317 GHz time profile
GOES 1-8 A profile
RHESSI spectogram
RHESSI 3-25 keV
May 03
In HXR these micro-flares are single, andeach is displaced fromthe 17 GHz sources.
Each HXR micro-flare has significantemission in the 12-25 keVband.
Faint 17-GHzfeature in all 3 cases
May 03 The 03:55 microflare difference map (bottom row) shows a loop-likemicrowave feature connecting the HXR source to the bright, compact 17 GHz component ~100” to the north.
The 03:58 microflare in all HXRbands 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 offsetbut nearly coincident withthe brightest 17 GHz source.
It is possible that the 17GHz is at one end/footpointof the RHESSI loop-likestructure.
2002 May 03Left: 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 microflare at 03:58 is shownin 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 6 1046 cm-3. Slope of the power law is -3.2.
2002 May 0417 GHz time profile
GOES 1-8 A profile
RHESSI 3-25 keV
RHESSI spectogram
2002 May 04
The HXR source liesclose to a weak 17 GHz source
The HXR source is visible inall 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 05:43and 05:45 show thatRHESSI sources in the twolower bands are close toa 17 GHz source
2002 May 04Left: 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 concernedstarts 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 0517 GHz time profile
GOES 1-8 A profile
RHESSI 3-25 keV
RHESSI spectogram
May 05
Note that microflares atdifferent times can comefrom different componentsof active regions.
Time profiles of differentcomponents of 17 GHzmicroflare-producingregion.
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 03 2002 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 0305:06:00
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-componentfit for a 12-s time bin using thermal, atomic line emission and nonthermalbremsstrahlung.
Time bin 1 2 3 4
T1 (MK) 16.3 16.5 17.8 17.3
EM (1046 cm-3) 7.25 9.55 9.68 13.7
3.62 3.33 4.48 6.03
F20 0.134 0.349 0.219 0.153
Eturn (keV) 12.3 11.3 12.6 12.8
The components are:Temperature (T), emission measure(E), spectral index (), 20-keV flux (F20), and lower cutoff (Eturn)
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*1046 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)