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NASA NTRS2014

Spatiotemporal Organization of Energy Release Events in the Quiet Solar Corona

Using data from the STEREO and SOHO spacecraft, we show that temporal organization of energy release events in the quiet solar corona is close to random, in contrast to the clustered behavior of flaring times in solar active regions. The locations of the quiet-Sun events follow the meso- and supergranulation pattern of the underling photosphere. Together with earlier reports of the scale-free event size statistics, our findings suggest that quiet solar regions responsible for bulk coronal heating operate in a driven self-organized critical state, possibly involving long-range Alfvenic interactions.

sun activity↗

NASA NTRS2018

Solar Surfing: Final Report on a Phase I NASA Innovative Advanced Concepts Study

The Sun sustains life on Earth and NASA has made its study one of the four pillars of the Science Mission Directorate. A specific area of study, the coronal heating problem, has been of significant concern for nearly 80years; namely how does the 5800 K surface of the Sun heat the nearby corona to over 1,000,000 K. Differing theories have been proposed to explain this process, but verification by actual measurement would not only resolve this issue, it would provide close-up measurements of the Sun never before obtained. However, this requires the development of a solar shield that can protect a satellite located less than 10,000 km from the Sun's surface. Steps towards that capability are the goal of this NIAC project. The current state-of-the-art in solar shielding is best shown by the upcoming Parker Solar Probe Mission, so the approach taken by that satellite is discussed and used as a starting point; allowing a distance of 9.5 solar radii from the Sun's center to be reached. It is then shown that state-of-the-art solar reflectors do not improve this performance. Next, we review the use of pressed powder as a better solar reflector and show that there is some improvement, but not sufficient to reach the Sun's surface. We spend some time on this architecture because the Parker Solar Probe has a thin scattering layer on its solar shield and it is important to discuss the advantages and disadvantages of this feature.

solar surfing↗

NASA NTRS2019

Solar Activity Modeling: From Subgranular Dynamical Scales to the Solar Cycles

The dynamical effects of solar magnetoconvection span a wide range spatial and temporal scales that extend from the interior to the corona and from fast turbulent motions to global magnetic activity. To study the solar activity on short temporal scales (from minutes to hours), we use 3D radiative MHD simulations that allow us to investigate complex turbulent interactions that drive various phenomena, such as plasma eruptions, spontaneous formation of magnetic structures, funnel-like structures and magnetic loops in the corona, and others. In particular, we focus on multi-scale processes of energy exchange across layers of the solar interior and atmosphere, which contribute to coronal heating and eruptive dynamics. For modeling global-scale activity, we use a data assimilation approach that has demonstrated great potential for building reliable long-term forecasts of solar activity. In particular, it has been shown that the Ensemble Kalman Filter (EnKF) method applied to the Parker-Kleeorin-Ruzmakin dynamo model is capable of predicting solar activity up to one sunspot cycle ahead in time, as well as estimating the properties of the next cycle a few years before it begins. In this presentation, using the available magnetogram data, we discuss development of the methodology and forecast quality criteria (including forecast uncertainties and sources of errors). We demonstrate the influence of observational limitations on prediction accuracy, and we present the EnKF predictions of the upcoming Solar Cycle (25) based on both the sunspot number series and observed magnetic fields and discuss the uncertainties and potential of the data assimilation approach for modeling and forecasting solar activity.

Kitiashvili, I. N.↗

NASA NTRSDate not supplied

Milli-Arcsecond (MAS) Imaging with SmallSat Enabled Super-resolution

The objective of this project is to demonstrate a new imaging technology with unprecedented capabilities for scientific exploration. This technology innovation will enable extremely high-resolution imaging capability, otherwise unattainable with conventional approaches. It is based on a unique combination of novel diffractive optical sensing (photon sieve), advanced image processing, and small satellite formation flying. The spatial resolution of current solar instruments are more than 100 times coarser than the size of energy dissipation features in coronal heating (<100 km) since conventional optics cannot be figured to near diffraction-limited accuracy at extreme ultraviolet (EUV) wavelengths. This project will demonstrate the feasibility of imaging of a few milli-arcseconds (MAS) to study the structure and development of these dissipation regions.

Farzad Kamalabadi↗

NASA NTRSDate not supplied

Preliminary Results from the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS)

The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) is a sounding rocket mission that aims to observe the soft x-ray solar spectrum (0.6 – 2.5 nm) with both spatial and spectral resolution over a substantial field of view. This wavelength range has several high temperature and abundance diagnostics that can be used to assist in diagnosing the coronal heating mechanism. MaGIXS launched from White Sands Missile Range on July 30, 2021 and successfully observed the Sun through a 4’ x 33’ effective slot, producing ``overlappograms’’, where the spatial and spectral information are overlapped and must be unfolded. In this presentation, I will report on the MaGIXS launch and data collection and provide preliminary analysis of MaGIXS data.

Amy Winebarger↗

NASA NTRSDate not supplied

Preliminary Results from the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS)

Amy Winebarger↗

NASA NTRSDate not supplied

The Thickness of Electric Current Sheets

The thickness of electric current sheets is extremely important, especially as it relates to the onset of fast magnetic reconnection. Onset determines how much magnetic free energy can build up in a field before it is explosively released. This has implications for coronal heating, CMEs, and many other phenomena on the Sun and throughout the universe. Significant effort has been devoted to the question of whether equilibrium current sheets in realistic geometries have finite or zero thickness. Using a simple force balance analysis, we show why current sheets without a guide field (2D) and with a guide field that is invariant in the guide field direction (2.5D) cannot be in equilibrium if they have both finite thickness and finite width. We then estimate the conditions under which the tension of a curved line-tied guide field can produce a 3D equilibrium sheet that is finite in all dimensions. Finally, we speculate that some quasi-statically evolving current sheets undergoing slow stressing may reach a critical point at which equilibrium is no longer possible. They then rapidly thin and reconnect.

James A Klimchuk↗

NASA NTRS1995

Acceleration of solar wind in polar coronal holes by induction heating

The universal induction heating mechanism supplying with the energy all the processes of coronal heating and the solar wind acceleration is developed. The observed relative 'trembling' of photospheric super-large scale magnetic fields with quasi-periods of 1-4 days amounts 30-40 percent in amplitude. The inductive electric field appears in the corona. The electric currents cause the Joule dissipation. The uneven heating leads to the solar wind acceleration. A model is suggested in which high-speed streams in space are caused by the combination of the enhanced inductive energy flux from the solar coronal active regions; the work against the regular magnetic field; losses from coronal emission. The consideration is made in terms of the dissipative solar wind theory with the finite electrical conductivity of plasma. The leakage of plasma and the energy flux across the magnetic field, caused by the induction heating processes, are taken into account. The polar coronal holes (and the mid-latitude ones) are indicators of energy transfer balance but not direct sources of high-speed streams in the solar wind.

Chertkov, A. D.↗

NASA NTRS1987

Solar transition region and coronal response to heating rate perturbations

Observations of Doppler shifts in UV emission lines formed in the solar transition region show continual plasma downflows and impulsive plasma upflows. Using numerical simulations, the authors examine the conjecture that areas of downflowing plasma are the base regions of coronal loops in which the heating is gradually decreasing and that areas of upflowing plasma are the base regions of coronal loops in which the heating rate is gradually increasing. Beginning with a coronal loop in equilibrium, the heating rate is reduced on time scales of 100, 1000, and 2000 s to 10 percent and 1 percent of the initial value, and the loop is allowed to evolve to a new equilibrium. The heating rate for the cooled models is then increased back to the initial value on the same time scales. While significant mass motions do develop in the simulations, both the emission measure and the velocity at 100,000 K do not show the characteristics present in UV observations.

Mariska, John T.↗

NASA NTRS1984

Radio-frequency heating of the coronal plasma during flares

A model is developed for the radio-frequency (RF) heating of soft X-ray emitting plasma in solar flares due to absorption of amplified cyclotron radiation. The radiation, carrying approximately 10 to the 27th to approximately 10 to the 30th erg/s, is generated through maser emission following partial precipitation of electrons in one or more flaring loops. The maser operates in a large number of small regions, each producing an 'elementary burst' (EB) of short duration. This radiation propagates either directly or after reflection to the second-harmonic absorption layer, where it is absorbed by thermal electrons. The properties of EBs and the heating of the electrons in the absorption layer are discussed in detail. RF heating and evaporation models for the production of soft X-ray emitting plasma are compared. Properties of the RF heating model that explain observed features are energy transport across field lines, rapid heating (in approximately 1 s) of coronal plasma to approximately 3 x 10 to the 7th K, and instigation of turbulent velocities up to the ion sound speed.

Melrose, D. B.↗

NASA NTRS1987

Solar transition region and coronal response to heating rate perturbations

The solar transition region is in a dynamic state characterized by impulsively upflowing plasma and continually downflowing plasma. Using numerical simulations, the conjecture that the areas of downflowing plasma are simply the base regions of coronal loops in which the heating rate is gradually decreasing and the areas of upflowing plasma are the base regions of coronal loops in which the heating rate is gradually increasing is examined. The calculations suggest that gradually reducing or increasing the heating in a magnetic flux tube will not result in plasma motions that are similar to those that are observed at high spatial resolution in the UV.

Mariska, John T.↗

NASA NTRS1980

Resonant electrodynamic heating of stellar coronal loops: An LRC circuit analogue

The electrodynamic coupling of stellar coronal loops to underlying beta velocity fields. A rigorous analysis revealed that the physics can be represented by a simple yet equivalent LRC circuit analogue. This analogue points to the existence of global structure oscillations which resonantly excite internal field line oscillations at a spatial resonance within the coronal loop. Although the width of this spatial resonance, as well as the induced currents and coronal velocity field, explicitly depend upon viscosity and resistivity, the resonant form of the generalized electrodynamic heating function is virtually independent of irreversibilities. This is a classic feature of high quality resonators that are externally driven by a broad band source of spectral power. Applications to solar coronal loops result in remarkable agreement with observations.

Ionson, J. A.↗

NASA NTRS1982

Resonant electrodynamic heating of stellar coronal loops - An LRC circuit analog

The problem of electrodynamic coupling of stellar coronal loops where beta is less than 1 to underlying velocity fields where beta is greater than approximately 1 is treated. A rigorous analysis reveals that the physics can be represented by a simple yet equivalent LRC circuit analog. This derived analog suggests the existence of global structure oscillations which resonantly excite internal field line oscillations at a spatial resonance within the coronal loop. Even though the width of this spatial resonance, as well as the induced currents and coronal velocity field, within the resonance region explicitly depends on viscosity and resistivity, the resonant form of the generalized electrodynamic heating functions is virtually independent of irreversibilities. This is a classic feature of high-quality resonators that are driven externally by a broad-band source of spectral power.

Ionson, J. A.↗

NASA NTRS1978

Resonant absorption of Alfvenic surface waves and the heating of solar coronal loops

A first-order mass- and energy-balance model is developed for steady-state EUV 'coronal rain' loops that are not associated with postflare events and are often seen over sunspot umbrae. The model disregards variations in a loop's thermodynamic and magnetic properties along magnetic-field lines and yields average properties of the resonant absorption sheath, the boundary layer, and the loop's interior. Both irreversible heating by Alfvenic surface waves and energy transport via induced boundary-layer convection are taken into account in the analysis. Results that include predictions of the temperatures of the sheath and the boundary-layer plasma, the temperature of the interior plasma, the radiative output of the loop, and the filling factor associated with this radiation are given in terms of the period and velocity amplitude of the relevant surface wave as well as various parameters that characterize a typical 'coronal rain' loop. These results are shown to be consistent with the observed minimum and maximum temperatures as well as the radiative output of typical 'coronal rain' loops.

Ionson, J. A.↗

NASA NTRS2007

Shell Models of RMHD Turbulence and the Heating of Solar Coronal Loops

A simplified nonlinear numerical model for the development of incompressible magnetohydrodynamics in the presence of a strong magnetic field B|| and stratification, nicknamed 'Shell-Atm,' is presented. In planes orthogonal to the mean field, the nonlinear incompressible dynamics is replaced by two-dimensional shell models for the complex variables u and b, allowing one to reach large Reynolds numbers while at the same time carrying out sufficiently long integrations to obtain good statistics at moderate computational cost. The shell models of different planes are coupled by Alfve'n waves propagating along B||. The model may be applied to open or closed magnetic field configurations where the axial field dominates and the plasma pressure is low; here we apply it to the specific case of a magnetic loop of the solar corona heated by means of turbulence driven by photospheric motions, and we use statistics for its analysis. The Alfven waves interact nonlinearly and form turbulent spectra in the directions perpendicular and, through propagation, also parallel to the mean field. A heating function is obtained and shown to be intermittent; the average heating is consistent with values required for sustaining a hot corona and is proportional to the aspect ratio of the loop to the -1.5 power, and haracteristic properties of heating events are distributed as power laws. Crosscorrelations show a delay of dissipation compared with energy content.

corona↗

NASA NTRS2006

Thermodynamic Structure of Collision-Dominated Expanding Plasma: Heating of Interplanetary Coronal Mass Injections

We investigate the thermodynamic structure of interplanetary coronal mass ejections (ICMEs) using combined surveys of the ejecta between 0.3 and 20 AU. ICMEs are shown to have a moderate expansion in the solar wind compared with theoretical predictions. The expansion seems to be governed by a polytrope with gamma approx. 1.3 in this distance range. We find that Coulomb collisions are important contributors to the ion-ion equilibration process in the ICME plasma. The alpha-proton differential speed quickly drops to below 10 km/s due to strong Coulomb collisions. However, the two species of particles are far from thermal equilibrium with a temperature ratio T(sub alpha/T(sub p) = 4-6, suggestive of a preferential heating of alpha particles. The plasma heating rate as a function of heliocentric &stance required for the temperature profile is deduced by taking into account the expansion and energy transfer between protons and alphas via Coulomb collisions. The turbulence dissipation rate is also inferred from the inertial range power spectrum of magnetic fluctuations within ICMEs. Comparison of the turbulence dissipation rate with the required heating rate shows that turbulence dissipation seems sufficient to explain the ICME heating. Sources powering the turbulence are also investigated by examining the instabilities induced by temperature anisotropies and energy deposition by pickup ions.

Liu, Y.↗

NASA NTRS1978

Determination of the structure and heating mechanisms of coronal loops from soft X-ray observations with the solar probe

High resolution soft X-ray imaging from the solar probe is justified in terms of the expected scientific returns which include the determination of the temperature and density structure of a coronal loop. The advantages of the grazing incidence telescope over the multiple pinhole camera are discussed. An instrument package is described which includes a grazing incidence mirror, a thermal prefilter, a three position filter wheel and a focal plane detector baselined as an 800 by 800 back-illuminated charge coupled device. The structural assembly together with the data processing equipment would draw heavily on the designs being developed for the Solar Polar Mission.

Davis, J. M.↗

NASA NTRS1996

Coronal structure and heating: Comparison between SXT/Yohkoh observations of an active region and magnetogram

We have studied the magnetic structure in AR 7150 (S09E06) observed on 29 April 1992 by the Soft X-Ray Telescope (SXT) on Yohkoh. The observed X-ray images are compared with force-free magnetic fields with different values of alpha, extrapolated from the Marshall Space Flight Center (MSFC) photospheric magnetogram observed at the same time. The results show that the magnetic field of the active region is not potential. Different groups of loops are characterized by different values of alpha. Bright loops correlation between the brightness of individual loops with the amount of twist. Further investigation of the magnetic state of the loop structure requires accurate nonlinear force-free calculations.

Cheng, C.-C.↗