Gentle Chromospheric Evaporation Research Articles

Different Signatures of Chromospheric Evaporation in Two Solar Flares Observed with IRIS

Abstract We present different signatures of chromospheric evaporation in two solar flares observed by the Interface Region Imaging Spectrograph (IRIS). In the B1.6 flare on 2016 December 6 (SOL2016-12-06T10:40), the transition region Si iv line and the chromospheric C ii and Mg ii lines show blueshifts with low velocities up to 20 km s−1 at the flare loop footpoints in the rise phase, indicative of a gentle chromospheric evaporation. While in the C1.6 flare on 2015 December 19 (SOL2015-12-19T10:51), the Si iv, C ii, and Mg ii lines exhibit redshifts with velocities from several to tens of km s−1 at the footpoints, which might suggest an explosive chromospheric evaporation. Explosive evaporation has been observed in many flares that were captured by IRIS; however, gentle evaporation, especially manifested as blueshifts in the cool Si iv, C ii, and Mg ii lines, has scarcely been reported. Our results bring some new insights into chromospheric evaporation in the IRIS era.

Chromospheric evaporation in sympathetic coronal bright points

{Chromospheric evaporation is a key process in solar flares that has extensively been investigated using the spectroscopic observations. However, direct soft X-ray (SXR) imaging of the process is rare, especially in remote brightenings associated with the primary flares that have recently attracted dramatic attention.} {We intend to find the evidence for chromospheric evaporation and figure out the cause of the process in sympathetic coronal bright points (CBPs), i.e., remote brightenings induced by the primary CBP.} {We utilise the high-cadence and high-resolution SXR observations of CBPs from the X-ray Telescope (XRT) aboard the Hinode spacecraft on 2009 August 23.} {We discover thermal conduction front propagating from the primary CBP, i.e., BP1, to one of the sympathetic CBPs, i.e., BP2 that is 60$\arcsec$ away from BP1. The apparent velocity of the thermal conduction is $\sim$138 km s$^{-1}$. Afterwards, hot plasma flowed upwards into the loop connecting BP1 and BP2 at a speed of $\sim$76 km s$^{-1}$, a clear signature of chromospheric evaporation. Similar upflow was also observed in the loop connecting BP1 and the other sympathetic CBP, i.e., BP3 that is 80$\arcsec$ away from BP1, though less significant than BP2. The apparent velocity of the upflow is $\sim$47 km s$^{-1}$. The thermal conduction front propagating from BP1 to BP3 was not well identified except for the jet-like motion also originating from BP1.} {We propose that the gentle chromospheric evaporation in the sympathetic CBPs were caused by thermal conduction originating from the primary CBP.}

Spectral diagnostic of a microflare. Evidences of resonant scattering in C IV 1548 Å, 1550 Å lines

Aims. We study a microflare, classified as a GOES-A1 after background subtraction, which was observed in active region NOAA 8541 on May 15, 1999. Methods. We used TRACE filtergrams to study the morphology and time evolution. SUMER spectral lines were used to diagnose the chromospheric plasma (Si ii1533 A), transition region plasma (C iv 1548, 1550 A), and coronal plasma (Ne viii 770 A). Results. In the 171 A and 195 A filtergrams, we measure apparent mass motions along two small loops that compose the microflare from the eastern toward the western footpoints. In SUMER, the microflare is detected as a small (47 Mm 2 ), bright area at the western footpoints of the TRACE loops. The spectral profiles recorded over the bright area are complex. The Si ii1533 A line is self-reversed owing to opacity, and the coronal Ne viii line profile is composed of two Gaussian components, one of them systematically redshifted. The C iv1548 A and 1550 A profiles are badly distorted because of the temporary depression of the detector local gain caused by the very high count rates reached in the flaring region and we can only confirm the presence of strong blueshifts of � ‐200 km s −1 . Few, unaffected C iv profiles show two spectral components. In the northern part of the bright area, all SUMER spectral lines have at least one blueshifted spectral component. In the southern region of the bright area the spectral lines are redshifted. Adjacent to the microflare we measure, for the first time on the solar disk, an intensity ratio of the 1548 A line to 1550 A line with values of three to four, indicating that resonance scattering prevails in the lines formation. Moreover, the scattering region is found to be cospatial to a solar pore. Conclusions. The blueshifts in the footpoints of the microflare and the apparent mass motions observed with TRACE can be explained by a gentle chromospheric evaporation triggered by the microflare. The redshifted spectral components can be explained as cooling material that is falling back on the solar surface. The presence of resonant scattering can be explained by the low electron density expected in the transition region of a solar pore, combined with the high photon flux coming from the nearby microflare. We estimate that the lower limit of the electron density in the pore lies in the range 10 8 cm −3 to 10 9 cm −3 .

EXTREME-ULTRAVIOLET SPECTROSCOPIC OBSERVATION OF DIRECT CORONAL HEATING DURING A C-CLASS SOLAR FLARE

With the Coronal Diagnostic Spectrometer operating in rapid cadence (9.8 s) stare mode during a C6.6 flare on the solar disk, we observed a sudden brightening of Fe XIX line emission (formed at temperature T ≈ 8 MK) above the pre-flare noise without a corresponding brightening of emission from ions formed at lower temperatures, including He I (0.01 MK), O V (0.25 MK), and Si XII (2 MK). The sudden brightening persisted as a plateau of Fe XIX intensity that endured more than 11 minutes. The Fe XIX emission at the rise and during the life of the plateau showed no evidence of significant bulk velocity flows, and hence cannot be attributed to chromospheric evaporation. However, the line width showed a significant broadening at the rise of the plateau, corresponding to nonthermal velocities of at least 89 km s–1 due to reconnection outflows or turbulence. During the plateau He I, O V, and Si XII brightened at successively later times starting about 3.5 minutes after Fe XIX, which suggests that these brightenings were produced by thermal conduction from the plasma that produced the Fe XIX line emission; however, we cannot rule out the possibility that they were produced by a weak beam of nonthermal particles. We interpret an observed shortening of the O V wavelength for about 1.5 minutes toward the middle of the plateau to indicate new upward motions driven by the flare, as occurs during gentle chromospheric evaporation; relative to a quiescent interval shortly before the flare, the O V upward velocity was around –10 km s–1.

OBSERVATIONS OF THE THERMAL AND DYNAMIC EVOLUTION OF A SOLAR MICROFLARE

We observed a solar microflare over a wide temperature range with three instruments aboard the SOHO spacecraft (Coronal Diagnostic Spectrometer (CDS), Extreme-ultraviolet Imaging Telescope (EIT), and Michelson Doppler Imager (MDI)), TRACE (1600 A), GOES, and RHESSI. The microflare’s properties and behavior are those of a miniature flare undergoing gentle chromospheric evaporation, likely driven by nonthermal electrons. Extremeultraviolet spectra were obtained at a rapid cadence (9.8 s) with CDS in stare mode that included emission lines originating from the chromosphere (temperature of formation Tm ≈ 1 × 10 4 K) and transition region (TR), to coronal and flare (Tm ≈ 8 × 10 6 K) temperatures. Light curves derived from the CDS spectra and TRACE images (obtained with a variable cadence ≈ 34 s) 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 s −1 during the second precursor and shortly before the impulsive peak, indicating gentle chromospheric evaporation. Electron densities derived from an Oiv line intensity ratio (Tm ≈ 1.6 × 10 5 K) increased from 2.6 × 10 10 cm −3 during quiescent times to 5.2 × 10 11 cm −3 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 the 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 7 km 2 in the EIT image at 195 A, 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.

Multi-wavelength observations and modelling of a canonical solar flare

This paper investigates the temporal evolution of temperature, emission measure, energy loss and velocity in a C-class solar flare from both an observational and theoretical perspective. The properties of the flare were derived by following the systematic cooling of the plasma through the response functions of a number of instruments -- RHESSI (>5 MK), GOES-12 (5-30 MK), TRACE 171 A (1 MK) and SOHO/CDS (~0.03-8 MK). These measurements were studied in combination with simulations from the 0-D EBTEL model. At the flare on-set, upflows of ~90 km s-1 and low level emission were observed in Fe XIX, consistent with pre-flare heating and gentle chromospheric evaporation. During the impulsive phase, upflows of ~80 km s-1 in Fe XIX and simultaneous downflows of 20 km s-1 in He I and O V were observed, indicating explosive chromospheric evaporation. The plasma was subsequently found to reach a peak temperature of ~13 MK in approximately 10 minutes. Using EBTEL, conduction was found to be the dominant loss mechanism during the initial ~300s of the decay phase. It was also found to be responsible for driving gentle chromospheric evaporation during this period. As the temperature fell below ~8 MK, and for the next ~4,000s, radiative losses were determined to dominate over conductive losses. The radiative loss phase was accompanied by significant downflows of <40 km s-1 in O V. This is the first extensive study of the evolution of a canonical solar flare using both spectroscopic and broad-band instruments in conjunction with a hydrodynamic model. While our results are in broad agreement with the standard flare model, the simulations suggest that both conductive and non-thermal beam heating play important roles in heating the flare plasma during the impulsive phase of at least this event.

Rapid Cadence EUNIS‐06 Observations of a HeiiTransient Brightening in the Quiet Sun

We observed a transient brightening in the quiet Sun at rapid cadence (2.10 s) with the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS-06) sounding rocket instrument on 2006 April 12. The transient was visible only in He II at 303.78 A (T ≈ 5 × 104 K), and its maximum temperature T was <4 × 105 K. Taking its linear extent along the EUNIS slit to be the diameter of a circular feature, the transient's solar surface area was 7.8 × 107 km2. EUNIS observed the brightening to begin at 18:12:52 and peak at 18:13:29 UT; coordinated observations with SOHO's EIT confirm that EUNIS observed the onset of the brightening. EUNIS spectra yield maximum and average He II intensity enhancements of 2.09 and 1.46, respectively, relative to the pre-event quiet Sun. He II line profiles from EUNIS reveal that relative upflows were persistent during the transient (with a maximum speed around 20 km s−1) and that the upflow speed and intensity were positively correlated. Variations in the observed He II intensity and relative Doppler velocity were neither abrupt not impulsive, but occurred slowly compared to the EUNIS cadence. The local photospheric longitudinal magnetic field strength measured with SOHO's MDI revealed no significant variability. The transient's measured properties are consistent with its identification as a blinker or an elementary blinker, and its observed behavior suggests a formation mechanism involving gentle chromospheric evaporation.

Observational Evidence of Gentle Chromospheric Evaporation during the Impulsive Phase of a Solar Flare

Observational evidence of gentle chromospheric evaporation during the impulsive phase of a C9.1 solar flare is presented using data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager and the Coronal Diagnostic Spectrometer on board the Solar and Heliospheric Observatory. Until now, evidence of gentle evaporation has often been reported during the decay phase of solar flares, where thermal conduction is thought to be the driving mechanism. Here we show that the chromospheric response to a low flux of nonthermal electrons (≥5 × 109 ergs cm-2 s-1) results in plasma upflows of 13 ± 16, 16 ± 18, and 110 ± 58 km s-1 in the cool He I and O V emission lines and the 8 MK Fe XIX line, respectively. These findings, in conjunction with other recently reported work, now confirm that the dynamic response of the solar atmosphere is sensitively dependent on the flux of incident electrons.

Conduction-driven chromospheric evaporation in a solar flare

Observations of gentle chromospheric evaporation during the cooling phase of a solar flare are presented. Line profiles of the low-temperature (T of about 6 x 10 to the 6th K) coronal Mg XI line, observed with the X-Ray Polychromator on the Solar Maximum Mission, show a blueshift that persisted for several minutes after the impulsive heating phase. This result represents the first detection of an evaporation signature in a soft X-ray line formed at this low temperature. By combining the Mg XI blueshift velocity data with simultaneous measurements of the flare temperature derived from Ca XIX observations, it is demonstrated that the upward flux of enthalpy transported by this gently evaporating plasma varies linearly with the downward flux of thermal energy conducted from the corona. This relationship is consistent with models of solar flares in which thermal conduction drives chromospheric evaporation during the early part of the cooling phase.

Evidence for gentle chromospheric evaporation during the gradual phase of large solar flares

The Multichannel Subtractive Double Pass Spectrograph of the Meudon solar tower is used to obtain high spatial resolution H-alpha line profiles during the gradual phase of three solar flares. In all cases, small blueshifts lasting for several hours are observed in the flare ribbons. By contrast, the region between the two ribbons exhibits large redshifts that are typical of H-alpha post flare loops. The blueshifts in the ribbons is interpreted as upward chromospheric flows of 0.5-10 km/s, and the possible ambiguities of the interpretation are discussed. A preliminary analysis indicates that such upflows are sufficient to supply the greater than 10 to the 16th g of mass needed to maintain a dense H-alpha postflare loop system in the corona.