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. 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. 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. 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. 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 Å) through the transition region (O V Å) and into the corona (Si XII Å), including hot flare emission (Fe XIX 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. 4: Light curves derived from CDS spectra of EUV emission lines formed over a wide range of temperature, from the chromosphere (He I Å) through the transition region (O V Å) and into the corona (Si XII Å), including hot flare emission (Fe XIX 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. 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 Å, Si XII Å (multiplied by a factor of 8), and Fe XIX 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 Å photons from GOES. Fig. 6: EUV light curves (in ergs cmˉ² sˉ¹ sr ˉ¹) of O V Å, Si XII Å (multiplied by a factor of 8), and Fe XIX 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 Å photons from GOES.

Results Two precursor brightenings preceded the microflare. 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. 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 2 nd 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. Relative Doppler velocities are directed upward with maximum values ≈ 20 km/s during 2 nd 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. 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 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. 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. 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. 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 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 Å, O V Å, and Si XII Å were derived from spectra obtained between 19:30 and 19:54 UT; that for Fe XIX 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. 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 Å, O V Å, and Si XII Å were derived from spectra obtained between 19:30 and 19:54 UT; that for Fe XIX 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 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. 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.