Spatially Resolved Observations of Solar Active Regions in Soft X-Rays and Centimetric Wavelengths

Soft X-ray images of solar active regions obtained by the S-054 experiment on Skylab have been compared with simultaneous interferometric observations at 2.8 cm. The radio data consist of one-dimensional scans with a spatial resolution of 16 arcseconds in the East-West direction. The resolution, although lower than the X-ray telescope resolution, is high enough for a detailed comparison.We have found that there is a general correspondence in position and size between X-ray and radio sources, but relevant differences are also present. In particular, very bright, narrow components at 2.8 cm appear coaligned with regions of very weak X-ray emission. These strong radio components appear to be localized directly above sunspot umbrae.Models of active regions are investigated both for the atmosphere directly over the sunspot umbra and for regions above the adjacent plage. The presence of the magnetic field is taken into account and its effects on the energy dissipation and on propagation of radio waves are discussed.

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Energy and spectral analysis of confined solar flares from radio and X-ray observations

Research in Astronomy and Astrophysics ◽

10.1088/1674-4527/21/11/274 ◽

2021 ◽

Vol 21 (11) ◽

pp. 274

Author(s):

Cheng-Ming Tan ◽

Karl Ludwig Klein ◽

Yi-Hua Yan ◽

Satoshi Masuda ◽

Bao-Lin Tan ◽

...

Keyword(s):

Solar Flares ◽

Peak Frequency ◽

X Rays ◽

Radio Waves ◽

X Ray ◽

High Energies ◽

Spectral Behaviour ◽

Solar Eruptions ◽

Different Characteristics ◽

The Magnetic Field

Abstract The energy and spectral shape of radio bursts may help us understand the generation mechanism of solar eruptions, including solar flares, coronal mass ejections, eruptive filaments, and various scales of jets. The different kinds of flares may have different characteristics of energy and spectral distribution. In this work, we selected 10 mostly confined flare events during October 2014 to investigate their overall spectral behaviour and the energy emitted in microwaves by using radio observations from microwaves to interplanetary radio waves, and X-ray observations of GOES, RHESSI, and Fermi/GBM. We found that: all the confined flare events were associated with a microwave continuum burst extending to frequencies of 9.4 ∼ 15.4 GHz, and the peak frequencies of all confined flare events are higher than 4.995 GHz and lower than or equal to 17 GHz. The median value is around 9 GHz. The microwave burst energy (or fluence) and the peak frequency are found to provide useful criteria to estimate the power of solar flares. The observations imply that the magnetic field in confined flares tends to be stronger than that in 412 flares studied by Nita et al. (2004). All 10 events studied did not produce detectable hard X-rays with energies above ∼300 keV indicating the lack of efficient acceleration of electrons to high energies in the confined flares.

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High Resolution Soft X-ray Spectroscopy and the Quest for the Hot (5–10 MK) Plasma in Solar Active Regions

Frontiers in Astronomy and Space Sciences ◽

10.3389/fspas.2021.638489 ◽

2021 ◽

Vol 8 ◽

Author(s):

Giulio Del Zanna ◽

Vincenzo Andretta ◽

Peter J. Cargill ◽

Alain J. Corso ◽

Adrian N. Daw ◽

...

Keyword(s):

High Resolution ◽

Active Regions ◽

High Resolution Spectroscopy ◽

X Rays ◽

X Ray ◽

Hot Plasma ◽

Exciting Opportunity ◽

Solar Active Regions ◽

Opening Up ◽

Narrow Spectral Region

We discuss the diagnostics available to study the 5–10 MK plasma in the solar corona, which is key to understanding the heating in the cores of solar active regions. We present several simulated spectra, and show that excellent diagnostics are available in the soft X-rays, around 100 Å, as six ionization stages of Fe can simultaneously be observed, and electron densities derived, within a narrow spectral region. As this spectral range is almost unexplored, we present an analysis of available and simulated spectra, to compare the hot emission with the cooler component. We adopt recently designed multilayers to present estimates of count rates in the hot lines, with a baseline spectrometer design. Excellent count rates are found, opening up the exciting opportunity to obtain high-resolution spectroscopy of hot plasma.

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Observations of energetic X-rays from quiescent solar active regions

Symposium - International Astronomical Union ◽

10.1017/s0074180900021872 ◽

1968 ◽

Vol 35 ◽

pp. 432-438 ◽

Cited By ~ 1

Author(s):

Loren W. Acton ◽

Philip C. Fisher

Keyword(s):

Active Regions ◽

Flare Activity ◽

X Rays ◽

X Ray ◽

Solar Active Regions ◽

Optical Flare

A measurement of the solar X-ray spectrum in the 1–30 KeV interval was made by rocket-borne instruments at 0015 UT on April 5, 1967. These measurements show that the solar X-ray spectrum, which falls off steeply with energy below 4 KeV, becomes less steep above this energy. These data are taken to indicate that energetic, non-thermal, electrons are present or continuously produced in quiescent solar active regions. No optical flare activity was reported for several hours around the time of these observations.

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Activated solar filaments and flares

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1980.0233 ◽

1980 ◽

Vol 297 (1433) ◽

pp. 575-585 ◽

Cited By ~ 1

Keyword(s):

Solar Flares ◽

Extreme Ultraviolet ◽

Active Regions ◽

X Rays ◽

X Ray ◽

Flare Loops ◽

Solar Filaments ◽

Loop Prominence ◽

Field Lines ◽

The Magnetic Field

Activations and disruptions of dark Ha filaments are very common phenomena on the Sun. They precede the most powerful two-ribbon solar flares, but they also appear far from any active region without any chromospheric flaring. Therefore, until very recently, filament disruptions were considered as interesting, but physically insignificant, flare precursors. Only Skylab observations have shown that the filament disruptions actually represent one of the basic and most important mechanisms of solar activity. These observations have revealed (1) that many coronal transients originate in eruptive filaments without chromospheric flares, (2) that Bruzek’s slow-mode waves originate in disrupted filaments and not in flares themselves, and (3) that many coronal X-ray enhancements outside active regions are also tops of newly formed loops, similar to the post-flare loops observed after filament disruptions in active regions. An interpretation of these data stems from Kopp & Pneuman’s theory of postflare loops: the process that disrupts a filament opens the magnetic field and causes a greatly enhanced mass-flow along the field lines. The open field lines subsequently reconnect, starting from the bottom of the corona and proceeding upwards. This process can last for many hours. Hot loops are first seen in X-rays, later in extreme ultraviolet (e.u.v.) lines, and, after an appropriate cooling time, in Hx as the loop prominence systems. The visibility of loops depends on plasma density. Several observed properties of solar flares indicate that the primary acceleration occurs as the field lines reconnect. Thus the process of particle acceleration in two ribbon flares can last for hours. Because reconnection is accomplished after essentially all filament disruptions, ‘disparitions brusques’ outside active regions should also accelerate particles.

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Radio Measurements of Coronal Magnetic Fields

International Astronomical Union Colloquium ◽

10.1017/s0252921100024933 ◽

1994 ◽

Vol 144 ◽

pp. 21-28 ◽

Cited By ~ 4

Author(s):

G. B. Gelfreikh

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Solar Corona ◽

Active Regions ◽

High Sensitivity ◽

Coronal Magnetic Field ◽

Transverse Magnetic ◽

Radio Sources ◽

Radio Waves ◽

Solar Active Regions

AbstractA review of methods of measuring magnetic fields in the solar corona using spectral-polarization observations at microwaves with high spatial resolution is presented. The methods are based on the theory of thermal bremsstrahlung, thermal cyclotron emission, propagation of radio waves in quasi-transverse magnetic field and Faraday rotation of the plane of polarization. The most explicit program of measurements of magnetic fields in the atmosphere of solar active regions has been carried out using radio observations performed on the large reflector radio telescope of the Russian Academy of Sciences — RATAN-600. This proved possible due to good wavelength coverage, multichannel spectrographs observations and high sensitivity to polarization of the instrument. Besides direct measurements of the strength of the magnetic fields in some cases the peculiar parameters of radio sources, such as very steep spectra and high brightness temperatures provide some information on a very complicated local structure of the coronal magnetic field. Of special interest are the results found from combined RATAN-600 and large antennas of aperture synthesis (VLA and WSRT), the latter giving more detailed information on twodimensional structure of radio sources. The bulk of the data obtained allows us to investigate themagnetospheresof the solar active regions as the space in the solar corona where the structures and physical processes are controlled both by the photospheric/underphotospheric currents and surrounding “quiet” corona.

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Emergence of Twisted Magnetic Flux Related Sigmoidal Brightening

International Astronomical Union Colloquium ◽

10.1017/s0252921100064630 ◽

2000 ◽

Vol 179 ◽

pp. 263-264

Author(s):

K. Sundara Raman ◽

K. B. Ramesh ◽

R. Selvendran ◽

P. S. M. Aleem ◽

K. M. Hiremath

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Flux ◽

Ultra Violet ◽

Active Regions ◽

Morphological Properties ◽

Energy Storage And Conversion ◽

The Sun ◽

X Rays ◽

The Magnetic Field

Extended AbstractWe have examined the morphological properties of a sigmoid associated with an SXR (soft X-ray) flare. The sigmoid is cospatial with the EUV (extreme ultra violet) images and in the optical part lies along an S-shaped Hαfilament. The photoheliogram shows flux emergence within an existingδtype sunspot which has caused the rotation of the umbrae giving rise to the sigmoidal brightening.It is now widely accepted that flares derive their energy from the magnetic fields of the active regions and coronal levels are considered to be the flare sites. But still a satisfactory understanding of the flare processes has not been achieved because of the difficulties encountered to predict and estimate the probability of flare eruptions. The convection flows and vortices below the photosphere transport and concentrate magnetic field, which subsequently appear as active regions in the photosphere (Rust & Kumar 1994 and the references therein). Successive emergence of magnetic flux, twist the field, creating flare productive magnetic shear and has been studied by many authors (Sundara Ramanet al.1998 and the references therein). Hence, it is considered that the flare is powered by the energy stored in the twisted magnetic flux tubes (Kurokawa 1996 and the references therein). Rust & Kumar (1996) named the S-shaped bright coronal loops that appear in soft X-rays as ‘Sigmoids’ and concluded that this S-shaped distortion is due to the twist developed in the magnetic field lines. These transient sigmoidal features tell a great deal about unstable coronal magnetic fields, as these regions are more likely to be eruptive (Canfieldet al.1999). As the magnetic fields of the active regions are deep rooted in the Sun, the twist developed in the subphotospheric flux tube penetrates the photosphere and extends in to the corona. Thus, it is essentially favourable for the subphotospheric twist to unwind the twist and transmit it through the photosphere to the corona. Therefore, it becomes essential to make complete observational descriptions of a flare from the magnetic field changes that are taking place in different atmospheric levels of the Sun, to pin down the energy storage and conversion process that trigger the flare phenomena.

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The Structure of Solar Active Regions from EUV and Soft X-Ray Observations

Solar Gamma-, X-, and EUV Radiation ◽

10.1007/978-94-010-1802-9_14 ◽

1975 ◽

pp. 109-131 ◽

Cited By ~ 11

Author(s):

Carole Jordan

Keyword(s):

Active Regions ◽

X Ray ◽

Solar Active Regions

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X-Ray Spectra of Solar Active Regions

Symposium - International Astronomical Union ◽

10.1017/s0074180900071539 ◽

1975 ◽

Vol 68 ◽

pp. 45-64 ◽

Cited By ~ 3

Author(s):

John H. Parkinson

Keyword(s):

Spectral Resolution ◽

Active Regions ◽

X Ray ◽

Quantitative Theory ◽

Great Progress ◽

Solar Active Regions ◽

Cascading Processes ◽

Weak Satellite

The last few years have seen great progress in our understanding of X-ray spectra of solar active regions. This paper demonstrates both the usefulness and the limitations of the techniques, both scientific and instrumental, that have recently become available. Improvements in spectral resolution led to the discovery of weak satellite lines to helium-like ions; the quantitative theory for these lines is also discussed. The observed intensities of the Fe XVII lines are also investigated and found to be in agreement with calculations that allow for cascading processes.

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First X-Ray Microprobe and Microspectroscopy Results From The Gsecars Sector At The Advanced Photon Source

Microscopy and Microanalysis ◽

10.1017/s1431927600011417 ◽

1997 ◽

Vol 3 (S2) ◽

pp. 905-906

Author(s):

Mark L. Rivers ◽

Stephen R. Sutton ◽

Peter Eng ◽

Matthew Newville

Keyword(s):

National Laboratory ◽

Argonne National Laboratory ◽

Third Generation ◽

X Rays ◽

Environmental Sciences ◽

X Ray ◽

State Determination ◽

X Ray Absorption ◽

The Magnetic Field ◽

Photon Source

The Advanced Photon Source (APS) at Argonne National Laboratory is a third-generation synchrotron x-ray source, optimized for producing x-rays from undulators. Such undulator sources provide extremely bright, quasi-monochromatic radiation which is ideal for an x-ray microprobe. Such microprobes can be used for trace element quantification with x-ray fluorescence, or for chemical state determination with x-ray absorption spectroscopy. The GeoSoilEnviroCARS (GSECARS) sector at the APS is building an x-ray microprobe for research in earth, planetary, soil and environmental sciences.The GSECARS undulator source is a standard APS Undulator “A” which is a 3.3 cm period device with 72 periods. The energies of the undulator peaks can be varied by adjusting the gap, and hence the magnetic field of the undulator. The energy of the first harmonic can be varied in this way from approximately 3.1 keV to 14 keV. A measured undulator spectrum is shown in Figure 1.

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