Observations of Thermal and Dynamic Evolution of a Solar Microflare

Transcription

Observations of Thermal and Dynamic Evolution of a Solar Microflare
Observations of the
Thermal and Dynamic
Evolution of a Solar
Microflare
J. W. Brosius (Catholic U. at NASA’s GSFC)
G. D. Holman (NASA/GSFC)
Abstract
We observed a microflare over a wide temperature range with SOHO (CDS, EIT,
MDI), TRACE, GOES, RHESSI. The microflare’s properties and behavior are those
of a miniature flare undergoing gentle chromospheric evaporation, likely driven by
nonthermal electrons. EUV spectra were obtained at rapid cadence (9.8 s) with CDS
in stare mode. Light curves derived from CDS spectra and TRACE images reveal two
precursor brightenings before the microflare. After the precursors, chromospheric and
TR emission are the first to increase, consistent with energy deposition by nonthermal
electrons. The initial slow rise is followed by a brief (20 s) impulsive EUV burst in the
chromospheric and TR lines, during which the coronal and hot flare emission gradually
begin to increase. Relative Doppler velocities measured with CDS are directed upward
with maximum values ≈ 20 km sˉ¹ during the second precursor and shortly before the
impulsive peak, indicating gentle chromospheric evaporation. Electron densities
derived from an O IV line intensity ratio increased by a factor of 20 from quiescent
times to 5.2×10¹¹ cmˉ³ at the impulsive peak. The X-ray emission observed by
RHESSI peaked after the impulsive peak at chromospheric and TR temperatures, and
revealed no evidence of emission from nonthermal electrons. Spectral fits to the
RHESSI data indicate a maximum temperature of ≈ 13 MK, consistent with a slightly
lower temperature deduced from GOES data. Magnetograms from MDI show that the
microflare occurred in and around a growing island of negative magnetic polarity
embedded in a large area of positive magnetic field. The microflare was compact,
covering an area of 4×10 km² in the EIT image at 195 Å, and appearing as a point
source located 7″ west of the EIT source in the RHESSI image. TRACE images
suggest that the microflare filled small loops.
Fig. 1: Compact GOES B2
Microflare Seen in EIT Image
Fig. 1: EIT images obtained on 2005 Nov. 16,
both displayed on the same negative intensity
scale. The microflare appears only in frame (b),
where it is indicated with an arrow and
highlighted with a contour that corresponds to
75% of the maximum intensity in the
microflare. The position of the CDS slit with its
twelve 4″×20″ pixels is outlined in red.
Fig. 2: Microflare Observed With
EIT, MDI, TRACE, RHESSI
Fig. 2: Four views of the microflare and its environs, including
(a) an EIT 195 Å image obtained at 08:11:51 UT, (b) an MDI
photospheric longitudinal magnetogram obtained at 08:03:02
UT, (c) a TRACE 1600 Å image obtained at 08:11:54 UT, and
(d) a RHESSI 3-10 keV pixon image integrated between 08:12:08
and 08:12:40 UT. Red lines outline the 4″×20″ CDS slit pixels,
and the microflare is indicated with an arrow. Contours in (a)
correspond to 75% and 90% of the maximum in the FOV;
contours in (c) and (d) correspond to 15% and 50% of the
FOV’s maximum. In (b) white represents outward (+) field and
black represents inward (-) field. The X’s indicate the locations
of the maxima observed by EIT (toward the left) and RHESSI
(toward the right).
Fig. 3: 1600 Å Images from TRACE
Fig. 3: Sequence of TRACE 1600 Å images of
the microflare covering the same (31.5″×31.5″)
field of view displayed in Fig. 2c. Time (UT) is
given in the upper left of each frame. Red lines
outline the 4″×20″ CDS slit pixels. Note that
the microflare appears to fill short, low-lying
loops in these images.
Fig. 4: Microflare EUV Light Curves
Obtained at 9.8 s Cadence With CDS
Fig. 4: Light curves derived from CDS spectra of EUV
emission lines formed over a wide range of temperature,
from the chromosphere (He I 584.3 Å) through the
transition region (O V 629.7 Å) and into the corona (Si
XII 520.7 Å), including hot flare emission (Fe XIX 592.2
Å). For clarity, the He I line intensity was reduced by a
factor of 1.5 while the Si XII and Fe XIX line intensities
were multiplied by factors of 7 and 10, respectively. The
microflare is preceded by two precursor brightenings
(07:58:16-08:00:42, indicated with dotted vertical lines,
and 08:01:51-08:08:23, dashed vertical lines) before itself
begins about 08:09:12 and ends about 08:16:03 UT (solid
vertical lines). The microflare is evident in its
chromospheric and TR emission for 2.5 minutes before
its coronal emission begins to increase, and 2.8 minutes
before its hot flare emission appears above the noise.
Fig. 5: Relative Doppler Velocities
Obtained at 9.8 s Cadence With CDS
Fig. 5: EUV relative Doppler velocities derived from
CDS spectra. The average wavelengths against which
the relative Doppler velocities were calculated are
derived from spectra obtained between 07:30:01 and
07:56:57 UT before the microflare (and before its
precursors), from the same 4″×20″ spatial pixel in which
the microflare was observed. We take the uncertainties
on the relative Doppler velocities in the He I and O V
lines to be the 1-sigma standard deviations (scatter) in
the relevant wavelengths within those intervals. For He
I this yields 1.6 km sˉ¹, and for O V it yields 4.6 km sˉ¹;
horizontal dashed lines indicate these uncertainties in
each frame. The precursors and the microflare itself are
delineated with vertical dotted, dashed, and solid lines as
in Fig. 4.
Fig. 6: Expanded View of Impulsive
CDS, RHESSI, GOES Light Curves
Fig. 6: EUV light curves (in ergs cmˉ² sˉ¹ sr ˉ¹)
of O V 629.7 Å, Si XII 520.7 Å (multiplied by a
factor of 8), and Fe XIX 592.2 Å (multiplied by
a factor of 20) from CDS, along with hard X-ray
light curves (arbitrary units) of 3-4 keV and 9-10
keV photons from RHESSI, plus a soft X-ray
light curve (arbitrary units) of 1.0-8.0 Å photons
from GOES.
Results

Two precursor brightenings preceded the microflare.

After precursors, chromospheric and TR emission are the
first to increase; initial slow rise is followed by a brief (20
s) impulsive EUV burst in chromospheric and TR lines,
during which the coronal and hot flare emission gradually
increase. This provides evidence for chromospheric
heating by nonthermal electron beams.

Relative Doppler velocities are directed upward with
maximum values ≈ 20 km/s during 2nd precursor and
shortly before impulsive peak; no intervals of redshifted
emission were observed. This indicates that gentle
chromospheric evaporation occurred not only during the
microflare’s precursors, but also during its impulsive rise.
Results (continued)

O IV 625.8/608.3  electron density = 5.2×10¹¹ cmˉ³, a factor of
20 increase over that during quiescent times.

The microflare was compact, covering 75 arcsec² (4×10 km²) in the
EIT image, and appearing as a point source to RHESSI.

The microflare is associated with an area of growing (in size,
strength) negative magnetic field embedded in a larger area of
positive field.

RHESSI spectra (limited to 3-10 keV) could be fitted with thermal
bremsstrahlung from an isothermal plasma, but not with single or
double power-law models. Thus, RHESSI observed no direct
evidence for an electron beam during the microflare.
Conclusions
Based on the microflare’s observed thermal and
dynamic behavior, it appears to be a miniature flare
undergoing gentle chromospheric evaporation, likely
driven by beamed electrons accelerated via magnetic
reconnection. Although RHESSI observed no direct
evidence for an electron beam during the microflare, it
may simply be that the nonthermal hard X-ray emission
associated with the microflare’s inferred electron beam
is below RHESSI’s level of detection.
Fig. 7: GOES B8 Microflare in AR
10652 at 18:00 UT 2004 July 27
Fig. 7: Observations of a GOES B8 microflare on
2004 July 27. The EIT 195 Å image on the left was
obtained at 18:00 UT. Light curves and relative
Doppler velocities derived from CDS spectra obtained
at 9.8 s cadence are shown in the two frames on the
right, within the 4"×20" slit pixel outlined in red.
Reference wavelengths for He I 584.3 Å, O V 629.7 Å,
and Si XII 520.7 Å were derived from spectra obtained
between 19:30 and 19:54 UT; that for Fe XIX 592.2 Å
was derived from spectra obtained between 20:54 and
21:18 UT. The standard deviation for all four lines is
less than 4.7 km sˉ¹, shown as horizontal solid lines.
The persistence of upward-directed relative Doppler
velocities, with no associated intervals of redshifted
emission, indicates gentle chromospheric evaporation.
Fig. 8: GOES M1.5 Flare in AR
10652 at 20:00 UT 2004 July 27
Fig. 8: Similar to Fig. 7, but for a GOES M1.5 flare.
This event clearly shows explosive evaporation near the
flare onset around 20:00 UT, as evidenced by upflows ≈
100 km sˉ¹ in Fe XIX (and ≈ 10 km sˉ¹ in Si XII) with
simultaneous downflows in the cooler lines. Later
during the event, around 20:16 UT, all lines show
upflows with none showing downflows, indicative of
gentle evaporation. Thus we observe a shift in type of
chromospheric evaporation, from explosive to gentle.
The downflow observed most prominently in Si XII
around 20:10 UT may be evidence for hot flare plasma
that is cooling and falling back down.

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