Lower Chromosphere Research Articles

Chromospheric models of a solar flare including velocity fields

We study the chromospheric structure of a small flare, before, during and after the rst hard X-ray spike. We construct 5 semiempirical models for dierent times, which will reproduce the proles of the H ,C a II Ka nd Si I 3905 lines during the flare evolution. In order to reproduce the intensity and the main characteristics of the line asymmetry, we introduce the velocity elds in the prole calculations. We nd that the whole chromosphere undergoes a strong upward motion within 1 s of the maximum of the rst hard X-ray spike, when the temperature and the density begin to rapidly increase. This seems the rst chromospheric response to the energy deposition and/or release. Only 6 s after the hard X-ray peak, a strong downflow begins in the low chromosphere and its velocity continues to increase even during the cooling phase.

Scattering polarization of molecular emission lines in the quiet solar chromosphere

We present scattering polarization measurements performed with THEMIS in July 2000 near the south polar limb. The low level of scattered light at THEMIS, which is the only large solar telescope to include a superpolished primary mirror, allows, in good seeing conditions, to observe the emission spectrum of the low chromosphere above the limb. These are, as far as we know, the first high spectral resolution observations of the intensity and the first measurements of the polarization of C 2 and MgH molecular lines in emission above the limb. Molecules are present in a thin layer in the region of the temperature minimum between the photosphere and the chromosphere. We present a very simple model for the formation of the polarized lines and we relate the observed polarization rates to the so-called intrinsic line polarizability coefficients W 2 . Those quantities may be derived from quantum mechanical computations involving the solution of the Schrodinger equation for the molecular potential. Solar observations provide a direct way of checking these heavy computations and contribute to the improvement of our knowledge in molecular physics. Nine C 2 molecular transitions and two MgH transitions are present in our spectral window; we find that for the C 2 transitions, the polarizability is between 0.13 and 0.26 and that it takes higher values, namely 0.41 and 0.46, for the two MgH transitions.

PRELIMINARY RESULTS ON THE SOLAR PHOTOSPHERIC DYNAMICS OBSERVED WITH VAMOS

The intensity and velocity fluctuations, observed simultaneously, are a powerful diagnostic tool of the dynamics of the solar atmosphere. The phase rela- tion between the fluctuations can improve our knowledge of the solar background, its relation with the acoustic sources, and its interaction with the solar acoustic oscillations. Furthermore, the opposite asymmetries observed along the p-mode line profiles in the intensity and velocity power spectra contain information about the source of the solar acoustic oscillations. For these reasons, it is relevant to study the height dependence of the asymmetries and phases in the solar atmosphere. In this paper, we present the results from the analysis of observations performed by the VAMOS instrument in the potassium 769.9nm line and NaID lines, and compare the measured phases with those obtained at dierent layers in the solar atmosphere by dierent instruments, spanning from the base of the photosphere to the low chromosphere.

Dynamics of the solar chromosphere

We analyze oscillations in the solar atmosphere using image sequences from the Transition Region and Coronal Explorer (TRACE) in three ultraviolet passbands which sample the upper solar photosphere and low chromosphere. We exploit the absence of atmospheric seeing in TRACE data to furnish comprehensive Fourier diagnostics (amplitude maps, phase-dierence spectra, spatio-temporal decomposition) for quiet-Sun network and internetwork areas with excellent sampling statistics. Comparison displays from the ground-based Ca ii Hs pec- trometry that was numerically reproduced by Carlsson & Stein are added to link our results to the acoustic shock dynamics in this simulation. The TRACE image sequences conrm the dichotomy in oscillatory behaviour between network and internetwork and show upward propagation above the cuto frequency, the onset of acoustic shock formation in the upper photosphere, phase-dierence contrast between pseudo-mode ridges and the inter- ridge background, enhanced three-minute modulation aureoles around network patches, a persistent low-intensity background pattern largely made up of internal gravity waves, ubiquitous magnetic flashers, and low-lying mag- netic canopies with much low-frequency modulation. The spatio-temporal occurrence pattern of internetwork grains is found to be dominated by acoustic and gravity wave interference. We nd no sign of the high-frequency sound waves that have been proposed to heat the quiet chromosphere, but such measurement is hampered by non-simultaneous imaging in dierent passbands. We also nd no signature of particular low-frequency fluxtube waves that have been proposed to heat the network. However, internal gravity waves may play a role in their excitation.

Temporal and spatial variations of the quiet upper chromosphere from SOHO/SUMER observations of hydrogen Lyman lines

We have analyzed a time series of spectra in the hydrogen Lyman lines and the Lyman continuum obtained by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the SOlar Heliospheric Observatory (SOHO). The time series of about 2 hours and 22 min was obtained on 1999 March 9 in a quiet region near the center of the solar disk. For our analysis, we have selected a Lyman continuum window around 907 A, and the ve Lyman lines: Ly5 (937.8 A), Ly7 (926.2 A), Ly9 (920.9 A), Ly11 (918.1 A), and Ly15 (915.3 A). We derived the central intensity of these lines for a large number of locations over the network and inter-network regions. We found from their line-center intensity time series and from the power spectrum analysis that the network and inter-network regions at the upper chromosphere are associated with 5{7 min and 3-min period of intensity oscillations, respectively. Phase dierence analysis shows that there is a preponder- ance of upward-propagating waves in the upper chromosphere. The phase velocity was estimated to be roughly 4{5 km s 1 in the network regions. In addition, a Fast Fourier analysis has been performed in the spatial domain, all along the slit length used, for all the spectra and for the total duration of the observations. We have detected signicant periodic spatial variations with Fourier transform power peaks around 9{10 arcsec and at 4 arcsec. They correspond to the scale of mesogranulation structure and the width of the supergranular boundary, respec- tively. This provides evidence for the existence of a meso-scale in the upper chromosphere, of the same size as observed in the photosphere and lower chromosphere by earlier studies. We notice that the size (9{10 arcsec) of the meso-scale structures appears to be the same in all Lyman lines and in the continuum, which are formed at dierent depths in the chromosphere.

The nature of network oscillations

We examine time-series of spectral data obtained from the Coronal Diagnostic Spectrometer (CDS) and the Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER) onboard the Solar Heliospheric Observatory (SOHO) spacecraft, in the period 30-31 July 1996. The observations were obtained in lines, ranging in temperature from 12 000 K to 106 K, covering the low chromosphere to the corona. We report here on a time series analysis, using wavelet methods, of small individual network regions in the quiet Sun. The wavelet analysis allows us to derive the duration as well as the periods of the oscillations. The statistical significance of the oscillations was estimated by using a randomisation method. The oscillations are considered to be due to waves, which are produced in short bursts with coherence times of about 10-20 min. The low chromospheric and transition region lines show intensity and velocity power in the 2-4 mHz range. The coronal line Mg x does not show any statistically significant power in this range. In general, it is thought likely that the chromosphere and possibly the transition region oscillates in response to forcing by the p-modes, but they are also influenced strongly by the presence of magnetic fields. The observed 2-4 mHz network oscillations can thus be interpreted in terms of kink and sausage waves propagating upwards along thin magnetic flux tubes. We perform a linear numerical computation comparing the results with our observations.

Ellerman Bombs, Type II White-light Flares and Magnetic Reconnection in the Solar Lower Atmosphere

Ellerman bombs and Type II white-light flares share many common features despite the large energy gap between them. Both are considered to result from local heating in the solar lower atmosphere. This paper presents numerical simulations of magnetic reconnection occurring in such a deep atmosphere, with the aim to account for the common features of the two phenomena. Our numerical results manifest the following two typical characteristics of the assumed reconnection process: (1) magnetic reconnection saturates in ~ 600--900 s, which is just the lifetime of the two phenomena; (2) ionization in the upper chromosphere consumes quite a large part of the energy released through reconnection, making the heating effect most significant in the lower chromosphere. The application of the reconnection model to the two phenomena is discussed in detail.

Polar surges and macrospicules

We continue our study of polar surges and macrospicules at the period of solar minimum, analyzing high resolution multiwavelength limb observations that provide a clearer picture of the dynamical phenomena occurring well above the chromosphere of a polar cap. The time sequence of an erupting and impulsive polar event is examined from the low chromosphere to coronal heights, deriving both proper motions and Doppler velocities. Our observations suggest that there is a close association of polar surges with explosive events, supporting the hypothesis that magnetic reconnection triggered by emerging flux provides the accelerative mechanism for this polar region event.

Lower chromosphere oscillations near 4 mHz

Abstract This paper is devoted to the study of spatial characteristics and height stratification of the 4-min oscillations. The line-of-sight velocity observations were made using a local differential method to ensure spatial filtering of the oscillations in a given range of wave numbers K. Particular emphasis is placed on the distinctive properties of the differential method, a knowledge of which can be a help in avoiding errors when interpreting observational results obtained by this method. To encompass the height range from the upper atmosphere to the lower chromosphere including the temperature minimum zone, a group of spectral lines was used Fe I λ 5434 A, Ba II λ 4554 A, Na DI λ 5896 A, and Hβ λ 4861 A. For each line, the observations were made in several parts of the wing. In the region of high values of K = 2–4 Mm−1, a maximum K ∼ 0.7 shows up. It is suggested that within the fine zone of temperature minimum there occurs a transformation of the oscillation frequency which ensures the upward propa...

On the Mechanism of Chromospheric Network Heating and the Condition for Its Onset in the Sun and Other Solar‐Type Stars

A mechanism for chromospheric network heating and a necessary and sufficient condition for its onset are presented. The heating mechanism consists of resistive dissipation of proton Pedersen currents, which flow orthogonal to the magnetic field in weakly ionized chromospheric plasma. The currents are driven by a convection electric field generated by velocity oscillations of linear, slow, longitudinal magnetoacoustic waves with frequencies ? 3.5 mHz in the lower chromosphere. The heating occurs in thin magnetic flux tubes and begins lower in the chromosphere in flux tubes with higher photospheric field strength. The lower chromosphere, which emits most of the net radiative loss in the network, is heated by flux tubes with photospheric field strengths ~700-1500 G. A typical field strength and core diameter for a flux tube in the lower chromosphere with a core heating rate of 107 ergs cm-2 s-1 are 170 G and 10 km. This core region is contained in a region with a diameter ~100 km in which the heating rate is an order of magnitude smaller. About N ~ 102 of these flux tubes distributed over the boundary region of a granule with a diameter ~103 km provide an average heating rate over the entire granule ~107 ergs cm-2 s-1. If the core heating rate is changed by a factor f, then N ~ f-1/2102. The condition for the onset of heating is that the ratio of the proton cyclotron frequency to the proton-hydrogen collision frequency equal unity. This ratio increases with height, and the condition is satisfied at a single height in a given flux tube. At this height, control of the proton dynamics begins to be dominated by the magnetic field rather than by collisions with hydrogen, and the anisotropic nature of the electrical conductivity begins to play a critical role in resistive dissipation. The protons become magnetized. Heating by dissipation of heavy ion and, to a lesser extent, proton Pedersen currents causes the temperature to start increasing. The heating increases hydrogen ionization. With increasing height, and hence proton magnetization, the Pedersen current density rapidly increases with hydrogen ionization via positive feedback, and the proton number density rapidly reaches and exceeds the heavy ion number density, resulting in an increase in heating rate by an order of magnitude over only 1 pressure scale height. During this process the protons rapidly dominate the Pedersen current. Heating by dissipation of magnetic field aligned currents is insignificant. Below the height in the atmosphere at which the onset condition is satisfied, any current orthogonal to the magnetic field must be primarily a Hall current, which is nondissipative. Heating by this mechanism must occur to some degree in the chromospheric network of all solar-type stars. It is proposed to be the dominant mechanism of chromospheric network heating, although viscous dissipation may also be important if the core heating rate is much larger than ~107 ergs cm-2 s-1 or if the linear MHD waves studied here evolve into shock waves with increasing height. Flux tubes in the quiet chromosphere are predicted to have two possible core diameters: ~10 km, corresponding to flux tubes in which network heating occurs, and ~104-105 km, perhaps corresponding to flux tubes in which active region heating might occur. The model has a singularity at the acoustic cutoff frequency, corresponding to periods near 3 minutes. Therefore, unless nonresistive damping mechanisms such as viscous dissipation and thermal conduction provide sufficiently strong damping, MHD oscillations with periods near 3 minutes in chromospheric magnetic flux tubes must be nonlinear.

Solar Spicules: A Review of Recent Models and Targets for Future Observations – (Invited Review)

Since their discovery over 100 years ago, there have been many suggestions for the origin and development of solar spicules. Because the velocities of spicules are comparable to the sound and Alfven speeds of the low chromosphere, linear theory cannot fully describe them. Consequently, detailed tests of theoretical ideas had to await the development of computing power that only became available during the 1970s. This work reviews theories for spicules and spicule-like features over approximately the past 25 years, with an emphasis on the models based on nonlinear numerical simulations. These models have given us physical insight into wave propagation in the solar atmosphere, and have helped elucidate how such waves, and associated shock waves, may be capable of creating motions and structures on magnetic flux tubes in the lower solar atmosphere. So far, however, it has been difficult to reproduce the most-commonly-quoted parameters for spicules with these models, using what appears to be the most suitable input parameters. A key impediment to developing satisfactory models has been the lack of reliable observational information, which is a consequence of the small angular size and transient lifetime of spicules. I close with a list of key observational questions to be addressed with space-based satellites, such as the currently operating TRACE satellite, and especially the upcoming Solar-B mission. Answers to these questions will help determine which, if any, of the current models correctly explains spicules.

On the Doppler Shifts of Solar Ultraviolet Emission Lines

We examine emission-line profiles observed with the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument during the roll of the SOHO spacecraft on 1997 March 20. SUMER data were acquired in selected wavelength bands including lines from the low chromosphere to the corona. Our main aim is to determine the center-to-limb behavior of emission lines formed in the chromosphere, transition region, and corona, especially of the observed Doppler shifts, to try to form a consistent picture of the basic kinematic properties of the emitting plasmas. To achieve this we combine the roll data with data from the full disk discussed elsewhere and fitted Gaussian profiles to the cores of the line profiles. The Doppler-shift data at large spatial scales (>50'') clearly reveal center-to-limb redshift behavior consistent with a cos ϑ variation in all transition region lines. The three "coronal" lines in the data set (of Ne VIII and Mg X) reveal center-to-limb behavior consistent with disk-center blueshifts, in contradiction to some previous work. The redshift to blueshift transition occurs at electron temperatures of about 5 × 105 K. Furthermore, we present evidence for an outflow of the fast solar wind from the coronal holes throughout the whole transition region. These results confirm and extend earlier work and point toward a (re-) measurement of rest wavelengths of lines formed at coronal temperatures in the laboratory. Together these results provide a firmer observational foundation for the development of classes of models to account for the well-known redshifts and point to the need to develop models that can also account for the coronal-line blueshifts.

Goddard High‐Resolution Spectrograph Observations of Cool Low‐Gravity Stars. IV. A Comparison of the K5 III stars α Tauri and γ Draconis

The Goddard High-Resolution Spectrograph (GHRS) has observed the K5 III star α Tau in the 2330 A region on three separate occasions. These spectra show marked changes with time, with the UV continuum varying by a factor of 2, and with the emission lines changing in flux by 30% or more, with the amount of change dependent upon the opacity of the line. The variations suggests a restructuring of the atmosphere rather than simply a change in the surface area covered by chromospheric material. Surprisingly, there was no detectable change in the chromospheric turbulence on timescales of hours or years. On average, the lower part of the atmosphere was found to be fairly static, with a slight infall of 1-2 km s-1. At higher altitudes, probed by observation of the stronger Fe II lines as well as of the O I (UV 2) and Mg II (UV 1) resonance lines, there is evidence for the acceleration of a slow wind, similar to that seen in the M giants γ Cru and μ Gem. This wind is much less massive than for the later type giants, however, since its effects are seen in only the most optically thick of the Fe II lines. Comparison of the α Tau observations with similar data for the K5 III hybrid star γ Dra shows remarkable similarity in the photosphere and lower chromosphere. Both stars have pronounced UV continua, identical turbulences and chromospheric densities, and very similar line fluxes and profiles for all lines formed in the lower chromosphere, including C II], Co I, Si II], and Fe II. A deep exposure near 1500 A also shows the first evidence for hot plasma in the atmosphere of α Tau through the detection of the C IV (UV 1) doublet with a surface flux about 30% of that observed in γ Dra. Most of the evidence for the stellar wind is in the Mg II (UV 1) and O I (UV 2) resonance lines. Modeling these lines using the Sobolev with Exact Integration (SEI) radiative transfer code shows that the wind in γ Dra accelerates faster and reaches a higher terminal velocity than does the wind in α Tau. However, the wind turbulent velocity in γ Dra is only about one-third of the value seen in α Tau. We conclude that the observations support the suggestion by Judge & Stencel that the processes that heat the chromosphere are distinct from those that drive the stellar winds.

THE LOWER SOLAR ATMOSPHERE : RAPPORTEUR PAPER II

This “rapporteur” report discusses the solar photosphere and low chromosphere in the context of chemical composition studies. The highly dynamical nature of the photosphere does not seem to jeopardize precise determination of solar abundances in classical fashion. It is still an open question how the highly dynamical nature of the low chromosphere contributes to first ionization potential (FIP) fractionation.

Impulsive phase He 10830 spectra of a large solar limb flare of 16 August 1989

We obtained simultaneously He i 10830 A spectra, Hα filtergrams and microwave data of a large limb flare (2N/X20) in 1989. In this paper we characterize He i 10830 spectra in relation to the impulsive phase. All the He i 10830 spectra, except those of the surge, show blue shift or blue asymmetry. The velocities inferred from the spectra range from a few to 160 km s−1, implying that the horizontal motion is very likely present in the structure of this flare at different heights. The He i 10830 profiles of a flare are relatively broad and cannot be simulated by the Doppler broadening mechanism with a uniform flare model atmosphere. It is most likely that these characteristics are related to rapid and localized heating in the low and middle chromosphere. Comparing the SXR and microwave data with the optical data leads to the following scenario: the corona was already heated to some extent before the flare onset, and in the first 2 minutes of the impulsive phase, heat conduction was the main source or, at least, a competitive source, for chromospheric heating. However, the impulsive event, associated with the unusually broadened He i 10830 line (Δλf>20 A) and temporally correlated with a microwave burst, is probably caused by electron-beam heating.

Does chromospheric emission mimic polar starspots in Doppler images?

We have used time-series spectra of the sodium doublet at 589.0 and 589.6 nm in AB Doradus to investigate the effects of a chromospheric temperature rise on Doppler images derived from strong lines whose cores are formed in the upper photosphere and lower chromosphere. On solar-type stars, the Na D lines probe a range of heights and tend to be much more sensitive to the chromospheric temperature structure than the weaker photospheric lines that are commonly used for Doppler imaging. Contrary to suggestions that polar spots may be an artefact arising from chromospheric emission reversals, we find that the images obtained from the NaD lines show less high-latitude structure, and give more reliable light-curve predictions, than images derived previously from fits to several weaker photospheric lines.

NONLINEAR MHD SIMULATIONS OF WAVE DISSIPATION IN FLUX TUBES

The phase mixing and resonant dissipation of Alfven waves is studied in both the 'closed' magnetic loops and the 'open' coronal holes observed in the hot solar corona. The resulting energy transfer from large to small length scales contributes to the heating of these magnetic structures. The nonlinear simulations show that the periodically varying shear flows that occur in the resonant layers are unstable. In coronal holes, the phase mixing of running Alfven waves is speeded up by the 'flaring out' of the magnetic field lines in the lower chromosphere.

Plasma Dynamics of Magnetic Flux Injection in the Solar Atmosphere

view Abstract Citations (5) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Plasma Dynamics of Magnetic Flux Injection in the Solar Atmosphere Huba, J. D. ; Chen, J. Abstract A computational analysis of magnetic flux injection from the solar photo sphere into the solar chromosphere and corona is presented. The model is one-dimensional and describes the evolution of the plasma and neutrals in the vertical x-direction; the magnetic field is assumed to be unidirectional in the horizontal plane. The initial magnetic field profile is a step function (a magnetic piston): B = B0 + ΔB for x < x0 and B = B0 for x > x0. The field is then allowed to relax self-consistently for t >0 and x > xx0: a shock is launched followed by the relaxation of the magnetic piston. Simulation results are presented for different initial positions x0 of the field jump in the chromosphere. When the piston (ΔB ∼ tens of gauss) is very low in the chromosphere, a weak shock is launched that results in upward plasma and neutral velocities V ≃ few km s-1 in the upper chromosphere and corona; the plasma and neutrals only attain velocities V ≃ tens of m s-1- in the lower chromosphere. A quasi-steady state follows, and the piston does not relax significantly because the pressure change across the magnetic piston is small. When the piston is initiated in the upper chromosphere, a stronger shock is launched into the corona; plasma and neutral velocities V ≃ tens of km s-1 are generated. Subsequently, the magnetic field piston relaxes and penetrates the transition layer into the corona; the upward velocities are V ≃ hundreds of km s-1. At the same time, plasma and neutrals are being swept into the corona. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177791 Bibcode: 1996ApJ...469..412H Keywords: MAGNETOHYDRODYNAMICS: MHD; SUN: CORONA; SUN: CHROMOSPHERE; SUN: MAGNETIC FIELDS full text sources ADS |

Observations of Solar Carbon Monoxide with an Imaging Infrared Spectrograph. I. Thermal Bifurcation Revisited

view Abstract Citations (64) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of Solar Carbon Monoxide with an Imaging Infrared Spectrograph. I. Thermal Bifurcation Revisited Ayres, Thomas R. ; Rabin, Douglas Abstract We describe long-slit spectroscopy of the solar 4.7 μm carbon monoxide (CO) Δυ = 1 bands at the Main spectrograph of the NSO McMath-Pierce telescope. We utilized stigmatic imaging of the temperature- and velocity-sensitive CO absorptions to map quiet regions near disk center and at the extreme limb. At Sun center the dominant long-lived spatial structures are small-scale hot spots associated with fragments of the supergranulation network seen in cotemporal Ca II filtergrams. Oscillatory thermal and velocity fluctuations of the global p-mode interference pattern are a pervasive feature of the maps, but the rms amplitudes (≍70 K and ≍240 m s-1) are perturbations on a relatively unstructured outer photosphere. We occasionally see small-scale transient cooling episodes longer lived than the p-mode wavepackets that might be overshooting granules or rising magnetic flux ropes. The events are too rare, however, to influence the global properties of the CO fundamental bands. Seeing-selected frames of the off-limb CO emissions show a typical extension of 0".6 for the strongest lines, with little variation along the limb. The off-limb extensions indicate the presence of cool gas up to 350 km above the "Tmin" of popular reference models of the solar chromosphere. We carried out two-dimensional model atmosphere simulations to study the effects of thermal inhomogeneities on the disk-center, extreme-limb, and off-limb behavior of the CO lines. The models are spherically symmetric, static, and in LTE. Our data favor a scenario in which the bulk of the low chromosphere below the base of the magnetic "canopy" is in reality a "COmosphere" dominated by gas colder than the minimum temperature in conventional models. The moderate-scale (≍5"), mild thermal perturbations of the p-mode pattern have little influence on the CO Δυ = 1 spectra. Small-scale (≍1") hot regions embedded in a cool average atmosphere are strongly "shadowed" at the extreme limb. The shadowing is of little consequence, however, because the atmosphere already is dominated by the cool component. The opposite scenario-small-scale cold regions in a warm average produce effective shadowing at the limb for granule-size (≍1"-2") dark points only if the covering fraction is relatively large (f &#8819 0.2). That scenario is ruled out: it predicts high-contrast dark spots at disk center, contrary to our observations. We also argue against the possibility of shadowing by even smaller, subresolution (≍0"3) cold spots with f &#8819 0.1. We show that multistep reactions, rather than direct radiative associations, dominate the gas-phase chemistry of CO molecules under conditions typical of the outer photosphere. The CO formation and radiative cooling timescales are fast enough that low-temperature plasma conditions can be restored following disruption by a localized heating event such as a Ca II K2v "flash." In cool giant stars, the chemical formation timescales are much longer than in dwarfs like the Sun. Nevertheless, the density dependence is such that the molecular cooling proceeds proportionately more rapidly than the gas dynamics, ensuring an even more important role for autocatalyzed "thermal bifurcation." Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177031 Bibcode: 1996ApJ...460.1042A Keywords: SUN: ABUNDANCES; SUN: ACTIVITY; SUN: ATMOSPHERE; SUN: INFRARED; SUN: OSCILLATIONS full text sources ADS |

Magnetic fields of sunspots based on combined optical and radio observations

In the present paper we present the results of measurement of magnetic fields in some sunspots at different heights in the solar atmosphere, based on simultaneous optical and radio measurements. The optical measurements were made by traditional photographic spectral observations of Zeeman splitting in a number of spectral lines orig-inating at different heights in the solar photosphere and chromosphere. Radio observa-tions of the spectra and polarization of the sunspot - associated sources were made in the wavelength range of 2–4 cm using large reflector-type radio telescope RATAN-600. The magnetic field penetrating the hot regions of the solar atmosphere were found from the shortest wavelength of generation of thermal cyclotron emission (presumably in the third harmonic of electron gyrofrequency). For all the eight cases under consideration we have found that magnetic field first drops with height, increases from the photosphere to lower chromosphere, and then decreases again as we proceed to higher chromosphere and chromosphere-corona transition region. Radio measurements were found to be well corre-lated with optical measurements of magnetic fields for the same sunspot. An alternative interpretation implies that different lines used for magnetic field measurements refer to different locations on the solar surface. If this is the case, then the inversion in vertical gra-dients of magnetic fields may not exist above the sunspots. Possible sources of systematic and random errors are also discussed.