Active Region Plasma Research Articles

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

The detection of very hot plasma in the quiescent corona is important for diagnosing heating mechanisms. The presence and the amount of such hot plasma is currently debated. The SphinX instrument on-board CORONAS-PHOTON mission is sensitive to X-ray emission well above 1 keV and provides the opportunity to detect the hot plasma component. We analyzed the X-ray spectra of the solar corona collected by the SphinX spectrometer in May 2009 (when two active regions were present). We modelled the spectrum extracted from the whole Sun over a time window of 17 days in the 1.34-7 keV energy band by adopting the latest release of the APED database. The SphinX broadband spectrum cannot be modelled by a single isothermal component of optically thin plasma and two components are necessary. In particular, the high statistics and the accurate calibration of the spectrometer allowed us to detect a very hot component at ~7 million K with an emission measure of ~2.7 x 10^44 cm^-3. The X-ray emission from the hot plasma dominates the solar X-ray spectrum above 4 keV. We checked that this hot component is invariably present both at high and low emission regimes, i.e. even excluding resolvable microflares. We also present and discuss a possible non-thermal origin (compatible with a weak contribution from thick-target bremsstrahlung) for this hard emission component. Our results support the nanoflare scenario and might confirm that a minor flaring activity is ever-present in the quiescent corona, as also inferred for the coronae of other stars.

Very High-Resolution Solar X-Ray Imaging Using Diffractive Optics

This paper describes the development of X-ray diffractive optics for imaging solar flares with better than 0.1 arcsec angular resolution. X-ray images with this resolution of the \geq10 MK plasma in solar active regions and solar flares would allow the cross-sectional area of magnetic loops to be resolved and the coronal flare energy release region itself to be probed. The objective of this work is to obtain X-ray images in the iron-line complex at 6.7 keV observed during solar flares with an angular resolution as fine as 0.1 arcsec - over an order of magnitude finer than is now possible. This line emission is from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma at temperatures in excess of \approx10 MK. It provides information on the flare morphology, the iron abundance, and the distribution of the hot plasma. Studying how this plasma is heated to such high temperatures in such short times during solar flares is of critical importance in understanding these powerful transient events, one of the major objectives of solar physics. We describe the design, fabrication, and testing of phase zone plate X-ray lenses with focal lengths of \approx100 m at these energies that would be capable of achieving these objectives. We show how such lenses could be included on a two-spacecraft formation-flying mission with the lenses on the spacecraft closest to the Sun and an X-ray imaging array on the second spacecraft in the focal plane \approx100 m away. High resolution X-ray images could be obtained when the two spacecraft are aligned with the region of interest on the Sun. Requirements and constraints for the control of the two spacecraft are discussed together with the overall feasibility of such a formation-flying mission.

HINODE /EIS SPECTROSCOPIC VALIDATION OF VERY HOT PLASMA IMAGED WITH THE SOLAR DYNAMICS OBSERVATORY IN NON-FLARING ACTIVE REGION CORES

We use coronal imaging observations with SDO/AIA, and Hinode/EIS spectral data, to explore the potential of narrow band EUV imaging data for diagnosing the presence of hot (T >~5MK) coronal plasma in active regions. We analyze observations of two active regions (AR 11281, AR 11289) with simultaneous AIA imaging, and EIS spectral data, including the CaXVII line (at 192.8A) which is one of the few lines in the EIS spectral bands sensitive to hot coronal plasma even outside flares. After careful coalignment of the imaging and spectral data, we compare the morphology in a 3 color image combining the 171, 335, and 94A AIA spectral bands, with the image obtained for CaXVII emission from the analysis of EIS spectra. We find that in the selected active regions the CaXVII emission is strong only in very limited areas, showing striking similarities with the features bright in the 94A (and 335A) AIA channels and weak in the 171A band. We conclude that AIA imaging observations of the solar corona can be used to track hot plasma (6-8MK), and so to study its spatial variability and temporal evolution at high spatial and temporal resolution.

NEON INSIGHTS FROM OLD SOLAR X-RAYS: A PLASMA TEMPERATURE DEPENDENCE OF THE CORONAL NEON CONTENT

An analysis using modern atomic data of fluxes culled from the literature for O VIII and Ne IX lines observed in solar active regions by the P78 and Solar Maximum Mission satellites confirms that the coronal Ne/O abundance ratio varies by a factor of two or more, and finds an increase in Ne/O with increasing active region plasma temperature. The latter is reminiscent of evidence for increasing Ne/O with stellar activity in low-activity coronae that reaches a "neon saturation" in moderately active stars at approximately twice the historically accepted solar value of about 0.15 by number. We argue that neon saturation represents the underlying stellar photospheric compositions, and that low activity coronae, including that of the Sun, are generally depleted in neon. The implication would be that the solar Ne/O abundance ratio should be revised upward by a factor of about two to n(Ne)/n(O)~0.3. Diverse observations of neon in the local cosmos provide some support for such a revision. Neon would still be of some relevance for reconciling helioseismology with solar models computed using recently advocated chemical mixtures with lower metal content.

SUB-THz AND Hα ACTIVITY DURING THE PREFLARE AND MAIN PHASES OF AGOESCLASS M2 EVENT

Radio and optical observations of the evolution of flare-associated phenomena have shown an initial and rapid burst at 0.4 THz only followed subsequently by a localized chromospheric heating producing an H{\alpha} brightening with later heating of the whole active region. A major instability occurred several minutes later producing one impulsive burst at microwaves only, associated with an M2.0 GOES X-ray flare that exhibited the main H{\alpha} brightening at the same site as the first flash. The possible association between long-enduring time profiles at soft X-rays, microwaves, H{\alpha} and sub-THz wavelengths is discussed. In the decay phase the H{\alpha} movie shows a disrupting magnetic arch structure ejecting dark, presumably chromospheric, material upwards. The time sequence of events suggests genuine interdependent and possibly non-thermal instabilities triggering phenomena, with concurrent active region plasma heating and material ejection.

Solar 3.04 cm hydrogen line emission revealed in observations of the active region NOAA 10105

RATAN-600 observations of a microwave source located above the active region NOAA 10105 obtained on September 7–20, 2002 with a frequency resolution of ∼10% have revealed a spectral feature near 3.04 cm that can be interpreted as a neutral hydrogen line. This feature was observed September 11, 2002, in both absorption and emission, and was detected in the spectra of various portions of the source (sunspot, flocculus, and background). The maximum line depth of (35 ± 5)% of the source brightness was observed at the start of the observations (9.2h UT) in the flocculus in absorption. The line intensity decreased rapidly with time, becoming less than the measurement errors by 9.7h UT. It is most likely that the 3.04 cm emission is related to a 2B chromospheric flare (M2.2 X-ray burst) observed at ∼7.5h UT in the floccular field, near the main sunspot of NOAA 10105. In this case, the total duration of the event was about two hours. These observations are consistent with earlier statistical studies, and refines these based on data with higher spatial resolution. Recommendations for further observational studies of the solar 3.04-cm hydrogen line are presented; requirements for theories of the 3.04 cm line taking into account nonequilibrium states of the active-region plasma are indicated.

Spectroscopic characterization of an atmospheric pressure μ-jet plasma source

A new method for determination of plasma parameters under atmospheric pressure conditions is formulated and applied for characterization of a radio-frequency μ-jet plasma source using He/O2 mixture. By applying absolutely calibrated optical emission spectroscopy and numerical simulation, the gas temperature in the active plasma region and plasma parameters (electron density and electron distribution function) are determined. The steady-state concentrations of different species such as oxygen atom and ozone in the plasma channel and in the effluent of the plasma source are calculated using measured plasma parameters and gas temperature. On the other hand, spatial distribution of steady-state densities of these species are measured using emission and absorption spectroscopy. A comparison of the results thus obtained and the validation of the new method against two-photon absorption laser-induced fluorescence spectroscopy measurements are discussed. In addition, the influence of the surface processes and gas flow regime on the loss of the active species in the plasma source are discussed.

An iterative method in a probabilistic approach to the spectral inverse problem

Inverse problems are of great importance in astrophysics for deriving information about the physical characteristics of hot optically thin plasma sources from their EUV and X-ray spectra. We describe and test an iterative method developed within the framework of a probabilistic approach to the spectral inverse problem for determining the thermal structures of the emitting plasma. We also demonstrate applications of this method to both high resolution line spectra and broadband imaging data. Our so-called Bayesian iterative method (BIM) is an iterative procedure based on Bayes' theorem and is used to reconstruct differential emission measure (DEM) distributions. To demonstrate the abilities of the BIM, we performed various numerical tests and model simulations establishing its robustness and usefulness. We then applied the BIM to observable data for several active regions (AR) previously analyzed with other DEM diagnostic techniques: both SUMER/SOHO (Landi and Feldman, 2008) and SPIRIT/CORONAS-F (Shestov et al., 2010) line spectra data, and XRT/Hinode (Reale et al., 2009) broadband imaging data. The BIM results show that this method is an effective tool for determining the thermal structure of emitting plasma and can be successfully used for the DEM analysis of both line spectra and broadband imaging data. The BIM calculations correlate with recent studies confirming the existence of hot plasma in solar ARs. The BIM results also indicate that the coronal plasma may have the continuous distributions predicted by the nanoflare paradigm.

SDO/AIA response to coronal hole, quiet Sun, active region, and flare plasma

Aims. We examine the contribution of spectral lines and continuum emission to the EUV channels of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) in different regions of the solar atmosphere. Methods. Synthetic spectra were obtained using the CHIANTI atomic database and sample differential emission measures for coronal hole, quiet Sun, active region and flare plasma. These synthetic spectra were convolved with the effective area of each channel, in order to determine the dominant contribution in different regions of the solar atmosphere. Results. We highlight the contribution of particular spectral lines which under certain conditions can affect the interpretation of SDO/AIA data.

STEREO observations of quasi-periodically driven high velocity outflows in polar plumes

Plumes are one of the most ubiquitous features seen at the limb in polar coronal holes and are considered to be a source of high density plasma streams to the fast solar wind. We analyze STEREO observations of plumes and aim to reinterpret and place observations with previous generations of EUV imagers within a new context that was recently developed from Hinode observations. We exploit the higher signal-to-noise, spatial and temporal resolution of the EUVI telescopes over that of SOHO/EIT to study the temporal variation of polar plumes in high detail. We employ recently developed insight from imaging (and spectral) diagnostics of active region, plage, and quiet Sun plasmas to identify the presence of apparent motions as high-speed upflows in magnetic regions as opposed to previous interpretations of propagating waves. In almost all polar plumes observed at the limb in these STEREO sequences, in all coronal passbands, we observe high speed jets of plasma traveling along the structures with a mean velocity of 135km/s at a range of temperatures from 0.5-1.5MK. The jets have an apparent brightness enhancement of ~5% above that of the plumes they travel on and repeat quasi-periodically, with repeat-times ranging from five to twenty-five minutes. We also notice a very weak, fine scale, rapidly evolving, but ubiquitous companion of the plumes that covers the entire coronal hole limb. The observed jets are remarkably similar in intensity enhancement, periodicity and velocity to those observed in other magnetic regions of the solar atmosphere. They are multi-thermal in nature. We infer that the jets observed on the plumes are a source of heated mass to the fast solar wind. Further, based on the previous results that motivated this study, we suggest that these jets originated in the upper chromosphere.

Non-thermal atmospheric pressure HF plasma source: generation of nitric oxide and ozone for bio-medical applications

A new miniature high-frequency (HF) plasma source intended for bio-medical applications is studied using nitrogen/oxygen mixture at atmospheric pressure. This plasma source can be used as an element of a plasma source array for applications in dermatology and surgery. Nitric oxide and ozone which are produced in this plasma source are well-known agents for proliferation of the cells, inhalation therapy for newborn infants, disinfection of wounds and blood ozonation.Using optical emission spectroscopy, microphotography and numerical simulation, the gas temperature in the active plasma region and plasma parameters (electron density and electron distribution function) are determined for varied nitrogen/oxygen flows. The influence of the gas flows on the plasma conditions is studied. Ozone and nitric oxide concentrations in the effluent of the plasma source are measured using absorption spectroscopy and electro-chemical NO-detector at variable gas flows. Correlations between plasma parameters and concentrations of the particles in the effluent of the plasma source are discussed. By varying the gas flows, the HF plasma source can be optimized for nitric oxide or ozone production. Maximum concentrations of 2750 ppm and 400 ppm of NO and O3, correspondingly, are generated.

SOLAR MAGNETIC HELICITY INJECTED INTO THE HELIOSPHERE: MAGNITUDE, BALANCE, AND PERIODICITIES OVER SOLAR CYCLE 23

Relying purely on solar photospheric magnetic field measurements that cover most of solar cycle 23 (1996-2005), we calculate the total relative magnetic helicity injected into the solar atmosphere, and eventually shed into the heliosphere, over the latest cycle. Large active regions dominate the helicity injection process with ~5.7 × 1045 Mx2 of total injected helicity. The net helicity injected is 1% of the above output. Peculiar active-region plasma flows account for ~80% of this helicity; the remaining ~20% is due to solar differential rotation. The typical helicity per active-region CME ranges between (1.8-7) × 1042 Mx2 depending on the CME velocity. Accounting for various minor underestimation factors, we estimate a maximum helicity injection of ~6.6 × 1045 Mx2 for solar cycle 23. Although no significant net helicity exists over both solar hemispheres, we recover the well-known hemispheric helicity preference, which is significantly enhanced by the solar differential rotation. We also find that helicity injection in the solar atmosphere is an inherently disorganized, impulsive, and aperiodic process.

Atmospheric pressure dc corona discharges: operating regimes and potential applications

The operating regimes and the structures of dc corona discharges in air, nitrogen, helium and hydrogen–methane mixtures are studied for a point to plate electrode configuration. The characteristics of the dc negative corona discharge are investigated. In addition to the bright glow at the cathode (pin) region, a uniform diffuse glow is observed near the anode (plate) surface for the negative corona. This diffuse glow is observed in air and hydrogen–methane discharges only and not in nitrogen discharges. The presence of a glow near the planar anode is perhaps due to the increased electric field caused by a negative ion sheath formed only in electronegative gases. Optical emission spectroscopy (OES) was used to obtain species, spatially resolved temperature measurements and electric field estimations for the corona discharges in air. For the negative corona, the presence of a weak glow indicates an active plasma region near the positive planar electrode which may be useful for processing techniques such as plasma enhanced chemical vapor deposition. The dc negative corona discharge was observed to deposit films on the anode surface for air and methane.

Solar Observations of High‐Temperature Emission with the Extreme‐Ultraviolet Imaging Spectrometer onHinode

We present an analysis of solar coronal emission lines formed above 2.5 MK observed with the Extreme-Ultraviolet Imaging Spectrometer on Hinode during a small flare. Our main purpose is to evaluate the internal consistency of the atomic data available for the observed emission. We find that the observed emission from high-temperature Ca lines (Ca XIV, XV, XVI, and XVII) is generally consistent with the available atomic data. The observed Fe XVII emission at these wavelengths, in contrast, is more difficult to reconcile with current atomic calculations. The energy levels tabulated in the CHIANTI atomic physics database generally do not correspond to the observed wavelengths. After associating the calculated emissivities with the observed emission by hand, we find that the observed intensities are roughly consistent with what is predicted. However, the intensity of the strongest unblended line, Fe XVII 254.87 A, is not consistent with the intensities of the other Fe XVII lines at these wavelengths. Several of the Ca XV emission lines, which are formed at about 4 MK, form density-sensitive line ratios in the range log ne = 9–11 cm−3. Density measurements at these temperatures are potentially important for understanding the coronal heating mechanism. Our initial analysis suggests that high-temperature active region plasma is underdense relative to the predictions of steady heating models.

Three-dimensional MHD modeling of waves in active region loops

AbstractObservations show that MHD waves are one of the most important universal processes in the heliosphere. These waves are likely to play an important role in energy transfer in the heliosphere, and they can be used as a diagnostic tool of the properties of the local magneto-fluid environment. Recent observations by TRACE and Hinode satellites provide ample evidence of oscillations in coronal active region loops. The oscillations were interpreted as fast (kink), slow, and Alfvén modes, and the properties of the waves were used for coronal seismology. However, due to the complex interactions of the various modes in the inhomogeneous active region plasma, and due to nonlinearity, idealized linear theory is inadequate to properly describe the waves. To overcome this theoretical shortcoming we developed 3D MHD models of waves in active region loops. We investigated the effects of 3D active region magnetic and density structure on the oscillations and the wave dissipation, and we investigated the oscillation of individual loops. Some loops were constructed to contain several threads and twist. Here, we present the results of our models, and show how they can be used to understand better the properties of the waves, and of the active regions.

Density structure of an active region and associated moss using Hinode/EIS

Context: Studying the problem of active region heating requires precise measurements of physical plasma parameters such as electron density, temperature etc. It is also important to understand the relationship of coronal structures with the magnetic field. The Extreme-ultraviolet Imaging Spectrometer (EIS) aboard Hinode provides a rare opportunity to derive electron density simultaneously at different temperatures. Aims: MethodsWe study the density structure and characterise plasma in active regions and associated moss regions. In addition we study its relationship to the photospheric magnetic field. Methods: We used data recorded by the EIS, together with magnetic field measurements from the Michelson Doppler Imager (MDI) aboard SoHO and images recorded with the Transition Region And Coronal Explorer (TRACE) and X-Ray Telescope (XRT/Hinode). Results: We find that the hot core of the active region is densest with values as high as 10^10.5 cm^-3. The electron density estimated in specific regions in the active region moss decreases with increasing temperature. The moss areas were located primarily on one side of the active region, and they map the positive polarity regions almost exactly. The density within the moss region was highest at log T=5.8-6.1, with a value around 10^(10.0-10.5) cm^-3. The moss densities were highest in the strong positive magnetic field region. However, there was no such correlation for the negative polarity areas, where there was a large sunspot.

The Thermal Structure of an Active Region Observed Outside the Solar Disk

In the present work we analyze an extensive active region spectrum observed by the SUMER instrument on board SOHO with the aim of determining the thermal structure of the emitting plasma. We found that the plasma is made of three distinct, isothermal components, whose physical properties are similar to coronal hole, quiet-Sun, and active region plasmas. The temperatures of the coronal hole-like and quiet-Sun-like plasmas are in excellent agreement with previous measurements obtained outside active regions. We also used a DEM diagnostic technique to check the robustness of our results and found that the DEM curves are compatible with the presence of three distinct nearly isothermal plasmas if the individual DEM measurements are smoothed over a small temperature interval. Larger intervals lead the resulting DEM curves to a more multithermal behavior, raising the question of whether multithermal active region DEM curves available in the literature are real or an artifact of oversmoothing. The results are compared with measurements of the temperature of individual loop structures in the literature and discussed in light of a new picture of the solar corona.

EIT and TRACE responses to flare plasma

Aims: To understand the contribution of active region and flare plasmas to the $\lambda$195 channels of SOHO/EIT (Extreme-ultraviolet Imaging Telescope) and TRACE (Transition Region and Coronal Explorer). Methods: We have analysed an M8 flare simultaneously observed by the Coronal Diagnostic Spectrometer (CDS), EIT, TRACE and RHESSI. We obtained synthetic spectra for the flaring region and an outer region using the differential emission measures (DEM) of emitting plasma based on CDS and RHESSI observations and the CHIANTI atomic database. We then predicted the EIT and TRACE count rates. Results: For the flaring region, both EIT and TRACE images taken through the $\lambda$195 filter are dominated by Fe ${\rm XXIV}$ (formed at about 20 MK). However, in the outer region, the emission was primarily due to the Fe${\rm XII}$, with substantial contributions from other lines. The average count rate for the outer region was within 25% the observed value for EIT, while for TRACE it was a factor of two higher. For the flare region, the predicted count rate was a factor of two (in case of EIT) and a factor of three (in case of TRACE) higher than the actual count rate. Conclusions: During a solar flare, both TRACE and EIT $\lambda$195 channels are found to be dominated by Fe ${\rm XXIV}$ emission. Reasonable agreement between predictions and observations is found, however some discrepancies need to be further investigated.

The Transparency of Solar Coronal Active Regions

Resonance scattering has often been invoked to explain the disagreement between the observed and predicted line ratios of Fe XVII 15.01 A to Fe XVII 15.26 A (the ``3C/3D'' ratio). In this process photons of 15.01, with its much higher oscillator strength, are preferentially scattered out of the line of sight, thus reducing the observed line ratio. Recent laboratory measurements, however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26 Angstroms, suggesting that the observed 3C/3D ratio results from blending. Given our new understanding of the fundamental spectroscopy, we have re-examined the original solar spectra, identifying the Fe XVI 15.21 line and measuring its flux to account for the contribution of Fe XVI to the 15.26 flux. Deblending brings the 3C/3D ratio into good agreement with the experimental ratio; hence, we find no need to invoke resonance scattering. Low opacity in Fe XVII 15.01 also implies low opacity for Fe XV 284.2, ruling out resonance scattering as the cause of the fuzziness of TRACE and SOHO EIT 284-Angstrom images. The images must, instead, be unresolved due to the large number of structures at this temperature. Insignificant resonance scattering implies that future instruments with higher spatial resolution could resolve the active region plasma into its component loop structures.

Electron densities in EUV coronal bright points

Electron density measurements of six coronal bright points have been obtained using line ratio diagnostics of four ions in the temperature range 1.3–2.0 106 K. The results suggest that BP plasma has more similarities to active region plasma than to quiet Sun plasma. Nevertheless, they do not show the exact same behaviour: the increase of electron density at temperatures over ~ 6.2, observed in the core of active regions, has not been detected. The results based on new atomic data, although in better agreement decreasing more than a factor of two the densities obtained with older calculations, are still in some instances higher than those from . This could be a consequence of the inhomogeneity of the plasma in the observed volume. New measurements with a better constriction of the volumetric properties seem necessary to rule out the influence of other factors and confirm whether there is an inconsistency between the ions, perhaps due to line blending problems in the lines.