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At least 559 records · Page 31

Observations and 3D MHD Modeling of Small-scale Solar Prominence Oscillations

Small scaleoscillations were recently observed in prominences with high resolution Hinode/SOT and IRIS SJI in unprecedented detailand identified as fast magnetosonic waves. The high-resolution observations provide evidence of flows of cool material, fluid instabilities such as Kelvin-Helmholtz and Rayleigh-Taylor, and the spectroscopic information provides evidence of Doppler shifts, heating, and cooling of the prominence material. Recently nonlinear fast magnetosonic waves were observed withperiods in the range of 5–11 minutes and wavelengths in the plane of the sky (POS) of ~2000kmand flows, and the flows in narrow threads withPOS speed ∼16–46 km/s. Thenonlinear fast magnetosonic waves and flows were modeled using 2.5D MHD (Ofman & Kucera 2020). Here, we extend the studyof the nonlinear fast magnetosonic waves using more realistic 3D MHD model. The model includes the 3D structure of the density and the magnetic fieldas well as mode coupling anddemonstrate the propagating density compressions associated with the nonlinear fast magnetosonicwaves, as well as the guided modesin the pillar. The 3DMHD modelingimprove understanding ofthe POSeffects in limb prominence oscillations observations.The results of the modeling are useful for coronal seismology of the prominence structure.

Leon Ofman↗

Heating of solar and stellar chromospheres and coronae by MHD waves

The two general classes of models that deal with the required heating of stellar chromospheres and coronae assume that outer stellar atmospheres are heated by hydrodynamic or by magnetohydrodynamic (MHD) waves and that these waves are generated by turbulent motions in the stellar convection zones. This paper considers the types of MHD waves and the source of these waves in stars like sun, the efficiency of the generation of MHD waves, and the manner of propagation and energy dissipation of MHD waves. It is shown that the basic criteria for the validity of any theory of MHD wave heating must account for the mean level of heating observed in stellar chromospheres and coronae, and for the range of radiative losses observed for a given spectral type. It is also required that the MHD wave heating theory accounts for the existence of inhomogeneities in stellar atmospheres. The results obtained indicate that magnetic tube waves might supply enough energy for the chromospheric and coronal heating and might also account for the observed range of variations of stellar radiative losses for a given spectral type.

Musielak, Z. E.↗

Observations of solar X-ray bursts in the energy range 5-15 keV

Bursts of solar X-rays in the energy range 5-15 keV are associated with flares and are due to thermal emission from a hot coronal plasma. The results of the first study of a large sample of separate bursts, 197 events associated with subflares, and of a few events of importance 1 are presented. The observations were made by a proportional counter on the satellite OSO-7 from October, 1971 to June, 1972. In most cases, the temperature characterizing the X-ray spectrum rises impulsively at the onset of the burst and then declines slowly throughout the remainder of the burst. The emission measure rises exponentially with a time scale of 30-100 sec and then declines slowly on a time scale of the order of 1,000 sec. It is shown that the growth of the thermal energy in the flare plasma throughout the burst can be due to the heating of new cool material.

Datlowe, D. W.↗

Understanding Coronal Heating with Emission Measure Distributions

It is widely believed that the cross-field spatial scale of coronal heating is small, so that the fundamental plasma structures (loop strands) are spatially unresolved. We therefore must appeal to diagnostic techniques that are not strongly affected by spatial averaging. One valuable observable is the emission measure distribution, EM(T), which indicates how much material is present at each temperature. Using data from the Extreme-ultraviolet Imaging Spectrograph on the Hinode mission, we have determined emission measure distributions in the cores of two active regions. The distributions have power law slopes of approximately 2.4 coolward of the peak. We compare these slopes, as well as the amount of emission measure at very high temperature, with the predictions of a series of models. The models assume impulsive heating (nanoflares) in unresolved strands and take full account of non equilibrium ionization. A variety of nanoflare properties and initial conditions are considered. We also comment on the selection of spectral lines for upcoming missions like Solar Orbiter.

Klimchik, James A.↗

Kilometre-Scale Structures in the Sun's Corona

Knowledge of the structure of the Sun's corona is important for our understanding of how this high-temperature plasma is heated, and of the processes involved in the acceleration of the solar wind. The structure can be investigated directly by imaging at optical and shorter wavelengths, or indirectly through the effects of changing electron density on the propagation of radio waves (scattering and scintillation). Radio measurements have established many of the characteristics of the density fluctuations in the corona and solar wind, but the fundamental nature of these structures is not yet fully understood. Two specific features that have proved difficult to explain are an abrupt increase in anisotropy of the irregularities close to the Sun, and a break in the power-law spectrum describing the density fluctuations. Here I argue that these features are the manifestation of a transition from small ray-like or filamentary structures in the corona that rotate with the Sun to turbulent density irregularities convecting with the solar wind. I estimate the size of the smallest filamentary structure within coronal holes to be about I km at the Sun, approximately three orders of magnitude smaller than the smallest filamentary structures observed in images of different wavelengths.

Woo, Richard↗

Coronal plasma parameters in a long-duration X-ray event observed by Skylab

Physical conditions in the solar coronal X-ray-emitting plasma are analyzed for an event observed by the S-056 X-ray telescope on Skylab in August 1973. The event was located at the west limb and consisted of a series of high looplike structures that underwent dramatic changes in structure and intensity for more than 50 hours. A total X-ray energy (8 to 20 A) of about 5 by 10 to the 28th power erg is obtained by integrating over the lifetime and is shown to be equivalent to that reported previously for a 'typical' flare. It is found that a first major peak in total X-ray flux and thermal energy density was due to an increase in temperature, a major secondary peak in these parameters corresponded to the time of maximum in density, and conductivity energy losses near maximum intensity were much greater than radiative losses. Models dealing with sources of heating and energy balance are evaluated.

Vorpahl, J. A.↗

Impulsive phase soft X-ray blueshifts at a loop footpoint

Solar Maximum Mission (SMM) observations of a solar flare that occurred on May 24, 1987 are described. The event was noteworthy in that it was observed during the impulsive phase with the SMM X-ray Poly-chromator (XRP) pointed at a location associated with the chromospheric footpoints of a system of coronal loops. Density-sensitive line ratios at the flare site imply an initially large electron density of 5 x 10 to the 12th/cu cm, which decreased an order of magnitude during the flare. Spectral scans of the soft X-ray Mg XI line at the site reveal asymmetric blueshifted (200 km/s) profiles concurrent with impulsive hard X-ray emission. The blueshift amplitude was correlated with the intensity of hard X-rays (with a phase delay of about 30 s) and showed fluctuations on a time scale comparable with the variation of hard X-ray emission. These observations are interpreted as evidence for chromospheric evaporation produced by heating and expansion of footpoint plasma.

Zarro, Dominic M.↗

Photospheric Current Spikes and Their Possible Association with Flares - Results from an HMI Data Driven Model

A data driven, near photospheric magnetohydrodynamic model predicts spikes in the horizontal current density, and associated resistive heating rate per unit volume Q. The spikes appear as increases by orders of magnitude above background values in neutral line regions (NLRs) of active regions (ARs). The largest spikes typically occur a few hours to a few days prior to M or X flares. The spikes correspond to large vertical derivatives of the horizontal magnetic field. The model takes as input the photospheric magnetic field observed by the Helioseismic & Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO) satellite. This 2.5 D field is used to determine an analytic expression for a 3 D magnetic field, from which the current density, vector potential, and electric field are computed in every AR pixel for 14 ARs. The field is not assumed to be force-free. The spurious 6, 12, and 24 hour Doppler periods due to SDO orbital motion are filtered out of the time series of the HMI magnetic field for each pixel using a band pass filter. The subset of spikes analyzed at the pixel level are found to occur on HMI and granulation scales of 1 arcsec and 12 minutes. Spikes are found in ARs with and without M or X flares, and outside as well as inside NLRs, but the largest spikes are localized in the NLRs of ARs with M or X flares. The energy to drive the heating associated with the largest current spikes comes from bulk flow kinetic energy, not the electromagnetic field, and the current density is highly non-force free. The results suggest that, in combination with the model, HMI is revealing strong, convection driven, non-force free heating events on granulation scales, and that it is plausible these events are correlated with subsequent M or X flares. More and longer time series need to be analyzed to determine if such a correlation exists. Above an AR dependent threshold value of Q, the number of events N(Q) with heating rates greater than or equal to Q obeys a scale invariant power law distribution for each AR given by N(Q) varies Q(sup -s), where 0.40 less than or equal to S less than or equal to 0.53, with a mean and standard deviation across the 14 ARs of 0.47 and 0.045, showing there is little variation of S from one AR to another. These properties of N(Q) are in close agreement with those of the distribution N(E) for the total energy E of solar flares, determined from observations to be N(E) = constant x E(sup -alpha). From observations of nanoflares in the 0.7 to 4 MK range, and from observations of flares in hard X-rays, it is found that 0.51 less than or equal to alpha less than or equal to 0.57, and 0.4 less than or equal to alpha less than or equal to 0.6, respectively (Crosby et al. 1993, Sol. Phys., 143, 275; Aschwanden & Parnell 2002, ApJ, 572, 1048). Observations also show that, as is found here for the exponent S, there is little variation of alpha with AR (Wheatland 2000, ApJ, 532, 1209), indicating N(E) and N(Q) are largely independent of individual properties of ARs such as area, total magnetic flux, and distribution of current density (i.e. non-potentiality). Therefore the power law scaling of the photospheric heating rate Q computed here on granulation scales is essentially identical to that found for coronal observations of flare energies on scales 1-2 orders of magnitude larger. This suggests the physical mechanisms that cause Q and coronal flares are closely related. It seems likely that Q is the signature of a magnetic reconnection process in an energy range and volume orders of magnitude smaller than those of flares. In this context, at least the larger spikes in Q might be signatures of UV photospheric or lower chromospheric bombs in which plasma is heated to temperatures approximately 10(exp -5) K (Peter et al. 2014, Science 346, 1255726; Judge 2015, ApJ, 808, 116). In addition, lattice based avalanche simulations of flare energy release predict 0.4 less than or equal to alpha less than or equal to 0.5, while analytic, fractal-diffusive self-organized criticality models predict 0.4 less than or equal to alpha less than or equal to 0.67, in excellent agreement with observations, and the results presented here (Aschwanden & Parnell 2002, ApJ, 572, 1048; Aschwanden 2012, A&A, 539, A2; Aschwanden 2013, in "Self Organized Criticality Systems"; Aschwanden et al. 2016, SSR, 198, 47).

Goodman, Michael↗

The sun and interplanetary magnetic field

The interplanetary magnetic field (IMF) serves as a link between the sun, the response of the earth to solar activity and variations in galactic cosmic radiation. The IMF originates as a solar-coronal magnetic field that is transported into space by the solar wind. The close connection between solar magnetic fields and the origin and structure of the solar wind is described. The solar wind forms the heliosphere, a cavity containing the magnetized solar plasma from which the interstellar plasma and field are excluded. The entry of galactic cosmic rays into the heliosphere and their strong interaction with the IMF are discussed, this topic being of primary importance to the production and temporal variations of radiogenic elements. The profound influence of the IMF on geomagnetic activity and the aurora is discussed within the context of merging or reconnection with the planetary field. The physical connection is thus established between solar magnetic fields, magnetic storms and aurora. The state of the solar wind and IMF during the Maunder minimum is considered and an explanation for the (relative) absence of sunspots and aurora is proposed. The mechanism is an interruption of the oscillatory solar dynamo, a consequent reduction in the heating of the corona, a cessation of the supersonic solar wind and a weakening or absence of southward-directed magnetic fields in the vicinity of the earth.

Smith, Edward J.↗

Tether-Cutting Energetics of a Solar Quiet Region Prominence Eruption

We study the morphology and energetics of a slowly evolving quiet-region solar prominence eruption occurring on 1999 February 8-9 in the solar north polar crown region, using soft X-ray data from the soft X-ray telescope (SXT) on Yohkoh and Fexv EUV 284 Angstrom data from the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). After rising at approximately equal to l kilometer per second for about six hours, the prominence accelerates to a velocity of approximately equal to 10 kilometers per second, leaving behind EUV and soft X-ray loop arcades of a weak flare in its source region. Intensity dimmings occur in the eruption region cospatially in EUV and soft X-rays, indicating that the dimmings result from a depletion of material. Over the first two hours of the prominences rapid rise, flare-like brightenings occur beneath the rising prominence that might correspond to tether-cutting magnetic reconnection. These brightenings have heating requirements of up to approximately 10(exp 28)-10(exp 29) ergs, and this is comparable to the mechanical energy required for the rising prominence over the same time period. If the ratio of mechanical energy to heating energy remains constant through the early phase of the eruption, then we infer that coronal signatures for the tether cutting may not be apparent at or shortly after the start of the fast phase in this or similar low-energy eruptions, since the plasma-heating energy levels would not exceed that of the background corona.

Sterling, Alphonse C.↗

Observation and Modeling of the Solar Transition Region: A Quasi-Static Loops Model with Implications for Heating the Lower Transition Region - 1

We report on observations of the solar atmosphere in several extreme ultraviolet and far-ultraviolet bandpasses obtained by the Multi-Spectral Solar Telescope Array, a rocket borne spectroheliograph, on flights in 1987, 1991, and 1994, spanning the last solar maximum. Quiet sun emission observed in the 171 A - 175 A bandpass, which includes lines of 0Ov, O vi, Fe ix, and Fe x, has been analyzed to test models of the temperatures and geometries of the structures responsible for this emission. Analyses of intensity variations above the solar limb reveal scale heights consistent with a quiet sun plasma temperature of 500 000 K less than or equal to T(sub e) less than or equal to 800 000 K. Intensity modulations in the quiet sun are observed to occur on a scale comparable to the supergranular scale. The structures responsible for the quiet sun EUV emission are modeled as small quasi-static loops. We find that the emission predicted by loop models with maximum temperatures between 700 000 K and 900 000 K are consistent with our observations. We also present a preliminary comparison of the predictions of our models with observations of diagnostic spectral line ratios obtained from previous observers. We discuss the implications a distribution of loops of the type we model here would have for heating the lower transition region. Finally, in fight of the models we calculate here, we briefly review the current state of knowledge concerning the contributions thermal conduction from coronal (T(sub e) greater than or equal to 10(exp 6) K) and upper transition region (10(exp 5) K less than T(sub e) less than 10(esp 6) K) structures make to lower transition region emission. We argue that the evidence which has lead many authors to conclude that the interface of hotter and cooler plasmas makes a negligible contribution to lower transition region emission is much less compelling in light of recent observations and analyses. We further argue that it is the interface of chromospheric material with structures such as loops that have sub-coronal peak temperatures (i.e. less than 900 000 K) that makes the dominant contribution to lower transition region emission in the quiet sun.

Oluseyi, Hakeem M.↗

Linear tearing modes of a forced current-sheet equilibrium

Linear tearing modes are studied in a nearly singular forced current-sheet equilibrium, such as could result from global magnetic forces in the solar corona. Growth rates for the tearing modes, determined by solving the linearized reduced MHD (Strauss) equations numerically, were found to scale as (gamma)tau(d) = about S exp 4/5 k(y) exp 4/5, where S is the Lundquist number, k(y) is the wavenumber, and tau(d) is the classical resistive diffusion time. This scaling is in agreement with predictions from analytical theory. Because of the faster S scaling of these modes compared to the tearing modes of a diffuse current-sheet equilibrium, the modes have much higher growth rates (by a factor of about 10,000) for coronal values of S (about 10 to the 12th). For coronal parameters, the growth times of these new modes are estimated to be on the order of several hours to days, as compared to growth times of months to years for the tearing modes in a diffuse current-shear equilibrium. The growth times are comparable to reconnection times scales required in models of coronal heating by magnetic field dissipation.

Liewer, Paulett C.↗

Prominence Eruption Initiated by Helical Kink Instability of an Embedded Flux Rope

We study the triggering mechanism of a limb-prominence eruption and the associated coronal mass ejection (CME) near AR 12342 using Solar Dynamics Observatory and Large Angle and Spectrometric Coronagraph/Solar Heliospheric Observatory observations. The prominence is seen with an embedded flux thread (FT) at one end and bifurcates from the middle to a different footpoint location. The morphological evolution of the FT is similar to that of an unstable flux rope (FR), which we regard as a prominence-embedded FR. The FR twist exceeds the critical value. In addition, the morphology of the prominence plasma in 304 Å images marks the helical nature of the magnetic skeleton, with a total of 2.96 turns along arc length. The potential field extrapolation model indicates that the critical height of the background magnetic field gradient falls within the inner corona (105 Mm), which is consistent with the extent of coronal plasma loops. These results suggest that the helical kink instability in the embedded FR caused the slow rise of the prominence to the height of the torus instability domain. Moreover, the differential emission measure analysis unveils heating of the prominence plasma to coronal temperatures during an eruption, suggesting reconnection-related heating underneath the upward rising embedded FR. The prominence starts with a slow rise motion of 10 km s(exp. -1), which is followed by fast and slow acceleration phases that have an average acceleration of 28.9 m s(exp. -2) and 2.4 m s(exp. -2) in C2 and C3 field of view, respectively. As predicted by previous numerical simulations, the observed synchronous kinematic profiles of the CME leading edge and the core support the involved FR instability in the prominence initiation.

Vemareddy, P.↗

Origins of the Solar Wind

This paper presented the differential emission measure analysis of SUMER observations of a coroner streamer. We found that: The coroner streamer is isothermal at all heights. This suggests that the loops comprising the streamer must have very flat temperature profiles. The coroner streamer is "overdense" relative to the predictions of hydrostatic equilibrium at most heights. At the lowest heights the streamer is actually "underdense". The SUMER temperature measurements are not consistent with those derived from simultaneous SXT observations. SXT indicates systematically higher temperatures as well as a strong temperature gradient. These SUMER measurements yield somewhat lower temperatures and no gradient in the temperature with height. Previous work has suggested that there may be a hot component to the streamer that is preferentially observed with SXT. Our analysis shows that high temperature emission lines would be observed with SUMER if this were true and thus discounts this possibility. We suggested that scattered light in SXT might produce spurious temperature measurements. The temperature and density structure of this coroner streamer are very similar to "TRACE" active region loops (flat temperature profiles, overdense relative to uniform heating, and relatively cool temperatures).

Warren, Harry↗

Transient microwave brightenings in solar active regions: Comparison between VLA and Yohkoh observations

We report observations of transient microwave (2 cm) brightenings and their relationship with brightenings in soft X-rays. The peak flux of the microwave brightenings observed by the Very Large Array (VLA) is smaller than the previously reported fluxes by two orders of magnitude. The microwave sources were highly polarized (up to 100%) and were situated on the periphery of a sunspot umbra. Among the many transients observed in X-rays by Yohkoh, two were observed simultaneously in microwaves. The microwave sources were found to be closer to the umbra of the sunspot than were the X-ray loops. It seems that the microwave sources are located at the footpoints of the looplike X-ray transients. Using the combined VLA, Yohkoh, and Mees data set, we determine the physical parameters of the loop in which the brightenings occur. We find that an increase in emission measure accompanied by small-scale heating can account for the X-ray brightening. The microwave emission can be interpreted as thermal gyroresonance or nonthermal gyrosynchrotron processes during the X-ray brightening. The magnetic field in the microwave-source region is found to be 1200-1800 G. The observations also provide evidence for temperature gradient in the coronal loops.

Gopalswamy, N.↗

Core and Wing Densities of Asymmetric Coronal Spectral Profiles: Implications for the Mass Supply of the Solar Corona

Recent solar spectroscopic observations have shown that coronal spectral lines can exhibit asymmetric profiles, with enhanced emissions at their blue wings. These asymmetries correspond to rapidly upflowing plasmas at speeds exceeding approximately equal to 50 km per sec. Here, we perform a study of the density of the rapidly upflowing material and compare it with that of the line core that corresponds to the bulk of the plasma. For this task, we use spectroscopic observations of several active regions taken by the Extreme Ultraviolet Imaging Spectrometer of the Hinode mission. The density sensitive ratio of the Fe(sub XIV) lines at 264.78 and 274.20 Angstroms is used to determine wing and core densities.We compute the ratio of the blue wing density to the core density and find that most values are of order unity. This is consistent with the predictions for coronal nanoflares if most of the observed coronal mass is supplied by chromospheric evaporation driven by the nanoflares. However, much larger blue wing-to-core density ratios are predicted if most of the coronal mass is supplied by heated material ejected with type II spicules. Our measurements do not rule out a spicule origin for the blue wing emission, but they argue against spicules being a primary source of the hot plasma in the corona. We note that only about 40% of the pixels where line blends could be safely ignored have blue wing asymmetries in both Fe(sub XIV) lines. Anticipated sub-arcsecond spatial resolution spectroscopic observations in future missions could shed more light on the origin of blue, red, and mixed asymmetries.

coronal spectral profiles↗

Tether-Cutting Energetics of a Solar Quiet Region Prominence Eruption

We study the morphology and energetics of a slowly-evolving quiet region solar prominence eruption occurring on 1999 February 8-9 in the solar north polar crown region, using soft X-ray data from the soft X-ray telescope (SXT) on Yohkoh and Fe xv EUV 284 A data from the EUV Imaging Telescope (EIT) on SOHO. After rising at approx. 1 km/s for about six hours, the prominence accelerates to a velocity of approx. 10 km/s, leaving behind EUV and soft X-ray loop arcades of a weak flare in its source region. Intensity dimmings occur in the eruption region cospatially in EUV and soft X-rays, indicating that the dimmings result from a depletion of material. Over the first two hours of the prominence s rapid rise, flare-like brightenings occur beneath the rising prominence which may correspond to "tether cutting" magnetic reconnection. These brightenings have heating requirements of up to approx. 10(exp 28)-10(exp 29) ergs, and this is comparable to the mechanical energy required for the rising prominence over the same time period. If the ratio of mechanical energy to heating energy remains constant through the early phase of the eruption, then we infer that coronal signatures for the tether cutting may not be apparent at or shortly after the start of the fast phase in this or similar low-energy eruptions, since the plasma-heating energy levels would not exceed that of the background corona.

Sterling, Alphonse C.↗

The Third Flight of the Marshall Grazing Incidence X-Ray Spectrometer (MaGIXS-3)

The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) is the first X-ray slitless imaging spectrograph sounding rocket instrument designed to observe spectrally dispersed soft X-ray images of the solar corona over a wide field-of-view. During the first flight of MaGIXS (MaGIXS-1), occurred on 30 July 2021, several emission lines from coronal structures including X-ray bright points were observed. Further, MaGIXS-1 analysis also demonstrated the successful inversion of overlappograms using robust unfolding algorithms. Given the demonstrated success of MaGIXS-1, the second flight of the instrument with a simplified optical design, MaGIXS-2, is scheduled for 2024 to observe high temperature diagnostic emission lines. Results from MaGIXS-1 discovered dominant missing emission lines near 15A, arising from relatively cool plasma that peaks around 2 MK. This wavelength region hosts several closely spaced satellite lines of Fe XVII, Fe XVI and Fe XV ions, which are expected to be enhanced at lower temperatures and are currently unmodeled in the CHIANTI atomic database. This wavelength region offers one of the most unique diagnostics to measure electron temperature, as well as signatures of equilibrium state of the plasma in active regions, which has not been studied so far. The goal of MaGIXS-3 mission is the to determine coronal heating parameters, such as the spatial and temporal properties of coronal heating events, by measuring discriminating observations, such as electron temperature, effective temperature, density, abundance, and departures from thermal equilibrium of the plasma, by observing the Sun in the SXR wavelength range. To meet this goal, the MaGIXS instrument will be upgraded with a new X-ray telescope mirror that will provide higher spatial and spectral resolution and throughput, allowing for spectral lines to be observed at the relevant spatial and temporal scales. In addition, MaGIXS-3 will also carry The Resolving Inversion Context X-ray Spectrometer (TRICXS), a high dispersion Bragg crystal spectrometer to spectrally resolve the lines near 14.9 to 15.9A, critical to unlock the full diagnostic potential of this wavelength range. Here we will present the preliminary concept design of MaGIXS-3 and discuss the potential observations.

X-ray Imaging↗