Chromospheric Oscillations Research Articles

The solar chromosphere as seen in high-resolution millimeter observations

We report on chromospheric oscillations studied with 15 sec cadence observations obtained at a wavelength of 3.5 mm with the Berkeley-Illinois-Maryland Array (BIMA). Dif- ferent solar structures, including active and quiet-Sun regions, are analyzed. We compare the high-resolution millimeter 2-D images with Ca II K line images from Big Bear Solar Observa- tory. Using Fourier and wavelet time series analysis techniques we find signatures of intensity oscillations with periods in the range of 2-7 minutes. We discuss the results in the context of the dynamical model of the solar atmosphere by Carlsson & Stein (1995, 1997).

On the Formation of Extreme‐Ultraviolet Helium Lines in the Sun: Analysis ofSOHOData

The resonance lines of in the Sun are several times stronger than expected, relative to lines of other ions. To explore the origins of this helium enhancement,'' we have studied data from the SUMER, CDS, MDI, and EIT instruments on the Solar and Heliospheric Observatory (SOHO). Time series data obtained in a quiet region and a coronal hole indicate that the spatio-temporal properties of the He I 584 Angstrom, and He II 304 and 1084 Angstrom lines are qualitatively unlike other chromospheric and transition region lines. Helium line intensities vary slowly compared to chromospheric oscillations and to transient transition region brightenings seen in other lines, yet they vary rapidly (both spatially and temporally) compared to the coronal radiation. This suggests that photoionization/recombination plays a minor excitation role in these lines. The Doppler shift of the 584 Angstrom line shows a remarkably clear 4-8 mHz oscillation, with no accompanying line intensity oscillation. Transient brightenings are used to show that nonradial photon scattering might explain the previously found diffuse appearance of the network but cannot account for the reduced network/internetwork intensity contrast in the 584 Angstrom line. We propose a new enhancement mechanism, based on arguments in a companion paper, in which predominantly neutral species such as diffuse across magnetic field lines into regions of hot coronal plasma, but charged ions do not. The enhanced internetwork emission may arise from the diffusion of atoms across the chromospheric canopy; the enhanced network emission may have contributions from diffusion from spicules. This mechanism may naturally explain why the 584 Angstrom line shows Doppler shift, but not intensity, oscillations. Finally, if a mechanism such as velocity redistribution'' dominates emission, spectroheliograms of lines will be bright in regions of large temperature gradients parallel to the magnetic field. Differences between and other spectroheliograms may then reveal the relative contributions of classical'' and cool loop'' models to the observed emission. (Less)

Characteristic Scales of Chromospheric Oscillation Wave Packets

We use wavelet transforms to study the characteristic time scales of chromospheric oscillation that are observed in Transition Region and Coronal Explorer (TRACE) ultraviolet continuum image time series. Using several data sets, we investigate the statistical, spatial, and temporal intermittence of the number, duration, mean frequency, and delay (wait time) between wave packets in the time series data. Further, we demonstrate that these characteristic values are consistent with newly developed pictures of the wave-mode suppression and conversion by the chromospheric magnetic canopy. We propose that wavelet analysis may be fruitfully used in diagnosing the structure of the chromosphere and in identifying chromospheric oscillation wave packets temporally and spatially with their photospheric sources.

Intensity Oscillations in the Upper Transition Region above Active Region Plage

Although there are now many observations showing the presence of oscillations in the corona, almost no observational studies have focused on the bright upper transition region (TR) emission (the so-called moss) above active region plage. Here we report on a wavelet analysis of observations (made with the Transition Region and Coronal Explorer) of strong (~5%-15%) intensity oscillations in the upper TR footpoints of hot coronal loops. They show a range of periods from 200 to 600 s, typically persisting for 4-7 cycles. These oscillations are not associated with sunspots, as they usually occur at the periphery of plage regions. A preliminary comparison to photospheric vertical velocities (using the Michelson Doppler Imager on board the Solar and Heliospheric Observatory) reveals that some upper TR oscillations show a correlation with p-modes in the photosphere. In addition, a majority of the upper TR oscillations are directly associated with upper chromospheric oscillations observed in Hα, i.e., periodic flows in spicular structures. The presence of such strong oscillations at low heights (of the order of 3000 km) provides an ideal opportunity to study the propagation of oscillations from photosphere and chromosphere into the TR and corona. It can also help us understand the magnetic connectivity in the chromosphere and TR and shed light on the source of chromospheric mass flows such as spicules.

Wavelet Analysis of Umbral Oscillations

We study the temporal behavior of the intensity and velocity chromospheric umbral oscillations, applying wavelet analysis techniques to four sets of observations in the Hα line and one set of simultaneous observations in the Hα and the nonmagnetic Fe I (5576.099 A) line. The wavelet and Fourier power spectra of the intensity and the velocity at chromospheric levels show both 3 and 5 minute oscillations. Oscillations in the 5 minute band are prominent in the intensity power spectra; they are significantly reduced in the velocity power spectra. We observe multiple peaks of closely spaced cospatial frequencies in the 3 minute band (5-8 mHz). Typically, there are three oscillating modes present: (1) a major one near 5.5 mHz, (2) a secondary near 6.3 mHz, and (3) oscillations with time-varying frequencies around 7.5 mHz that are present for limited time intervals. In the frame of current theories, the oscillating mode near 5.5 mHz should be considered as a fingerprint of the photospheric resonator, while the other two modes can be better explained by the chromospheric resonator. The wavelet spectra show a dynamic temporal behavior of the 3 minute oscillations. We observed (1) frequency drifts, (2) modes that are stable over a long time and then fade away or split up into two oscillation modes, and (3) suppression of frequencies for short time intervals. This behavior can be explained by the coupling between modes closely spaced in frequency or/and by long-term variations of the driving source of the resonators.

Investigating the role of plasma topography on chromospheric oscillations observed by TRACE

We present the results of an investigation into the interaction of the topographic structure of the solar chromospheric plasma with the wave modes manifesting themselves in the UV continua formed there. We show that there is a distinct correla- tion between the inferred plasma topography, the phase-dierences between and suppression of oscillations in dierent levels of the solar atmosphere. We interpret these factors as evidence of interaction between the oscillations and the extended magnetic canopy. This work is based on the analysis of joint observations made by the Solar and Heliospheric Observatory (SOHO) spacecraft and the Transition Region and Coronal Explorer (TRACE).

Photospheric and chromospheric oscillations in the base of coronal holes

In this paper we carry out an analysis of the spatial–temporal line-of-sight velocity variations measured in the chromospheric (Hα, Hβ) and photospheric (Fei 6569 A, Fei 4864 A, Nii 4857 A) lines at the base of 17 coronal holes. Time series of a duration from 43 to 120 min were recorded with the CCD line-array and the CCD matrix. Rather frequently we observed quasi-stationary upward flows with a measured velocity of up to 1 km s−1 in the photosphere and up to 4–5 km s−1 in the chromosphere (equivalent radial velocity of up to 3 km s−1 and up to 12–15 km s−1 accordingly) near dark points on the chromospheric network boundary inside polar CH. Line-of-sight velocity fluctuation spectra contain meaningful maxima in the low-frequency region clustering around the values 0.4, 0.75, and 1 mHz. Usually, the spatial localization of these maxima mutually coincides and, in our opinion, coincides with the chromospheric network boundary. Acoustic 3- and 5-min oscillations are enhanced in the coronal hole region and reach 1 km s−1 in the photosphere and 3–4 km s−1 in the chromosphere. These oscillations are not localized spatially and are distinguished throughout the entire region observed.

Long-Period Chromospheric Oscillations in Network Bright Points

The spatial variation of chromospheric oscillations in network bright points (NBPs) is studied using high-resolution observations in Ca II K3. Light curves and hence power spectra were created by isolating distinct regions of the NBP via a simple intensity thresholding technique. Using this technique, it was possible to identify peaks in the power spectra with particular spatial positions within the NBPs. In particular, long-period waves with periods of 4-15 minutes (1-4 mHz) were found in the central portions of each NBP, indicating that these waves are certainly not acoustic but possibly due to magnetoacoustic or magnetogravity wave modes. We also show that spatially averaged or low spatial resolution power spectra can lead to an inability to detect such long-period waves.

Spatially Resolved STIS Spectroscopy of α Orionis: Evidence for Nonradial Chromospheric Oscillation from Detailed Modeling

Four spatially resolved near-UV raster scans across the chromospheric disk of α Ori, obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, reveal mean chromospheric infall from 1998 January to April, which reversed to upflow in deeper layers between 1998 September and 1999 March. In 1998 September we detect systematic reversals in the component maxima of four double-peaked emission lines of Si I (UV 1), Fe II (UV 36), Fe II (UV 61), and Al II] (UV 1), when scanning across the UV disk. Detailed modeling of the Si I λ2516 resonance line with radiative transport calculations in spherical geometry constrain the mean radial velocity structure in the projected slit area (25 × 100 mas) for different aperture positions, observed off-limb to 157.5 mas. Hence, we determine with semiempirical models that these spatial reversals of emission-line components correspond to average opposite flow velocities of ~2 km s-1 across the chromospheric disk. We determine that the chromospheric velocity field cannot be represented by a unique radial velocity structure across the stellar disk in order to match the observed peak ratios of this raster scan. These subsonic velocities indicate (local) nonradial movements of chromospheric fluid in confined regions during a chromospheric oscillation phase, which reverses from global contraction into expansion over this monitoring period of 15 months.

A Study of Chromospheric Oscillations Using theSOHOandTRACESpacecraft

We analyze line and continuum time-series data of the solar atmosphere, with between 10 and 60 s cadence, using the MDI and SUMER instruments on the SOHO spacecraft and the UV bandpasses on the T RACE satellite. The co-aligned data sets sample spectral features formed from photosphere to the middle transition region, spanning —ve decades in pressure, under quiet-Sun and plage conditions. We discuss power, phase diUerence, and coherence spectra, and examine data in the time domain. The observed photospheric and chromospheric oscillations are strongly coupled for frequencies between 2 and 8 mHz. Phase coherences decrease with increasing height, with only occasional periods and locations of observable coherence up to heights where transition region emission lines are formed. The middle chromosphere (in the SUMER continua) oscillates in several megameter (Mm) diameter coherent patches with power predominantly in the 5¨7 mHz range. The T RACE data, formed in the upper photosphere, show smaller patterns superimposed on these large-scale oscillations, resulting (at least in part) from granulation. At the observed spatial scales, all the observed properties point to p-modes, especially the ii pseudomodes ˇˇ just above the acoustic cutoU frequency, as the dominant mode of the chromospheric dynamics. Smaller scale ii acoustic event ˇˇ drivers, associated with granular dynamics, appear to be less important. The predominant internetwork chromospheric oscillations arise from regions much larger horizontally than vertically. If propagating largely vertically, this can naturally explain why the one-dimensional simulations of Carlsson & Stein might be more successful than expected. The chromospheric response to the p-mode driving is, however, intermittent in space and time. Some of the intermittency appears to result from the interaction of the upward-propagating waves with magnetic —elds. Evidence for this includes suppressed intensities and oscillations near quiet-Sun network elements (which we dub ii magnetic shadows ˇˇ), absence of oscillations in internetwork regions neighboring plage magnetic —elds, and a change in character of the quiet-Sun internetwork oscillations between the 119 and 104 nm continua formed at 1 and 1.2 Mm. The latter might be caused by canopy —elds that form between these heights under typical quiet-Sun conditions. A SUMER-only data set reported by et al. has a Wikst‘l factor of 3 more oscillatory power in the 104 nm continuum than the data analyzed here, with stronger coherences extending into the solar transition region. Together, these data support the general picture that the chromosphere oscillates primarily in response to forcing by the p-modes, they are therefore large-scale (several Mm across) waves, and they are often strongly in—uenced by magnetic eUects (internetwork —elds, or the overlying canopy), before the oscillations even reach the transition region.

A Time‐dependent Semiempirical Model of the Chromospheric Umbral Oscillation

We present a time-dependent semiempirical model of the chromospheric umbral oscillation in sunspots. This model has been obtained by applying recently developed non-LTE inversion techniques to a time series of spectropolarimetric observations. The model consists of two optically thick unresolved atmospheric components: a quiet component with downward velocities that covers most of the resolution element and an component with upward velocities as high as 10 km s-1 that covers a smaller filling factor and has a higher temperature at the same chromospheric optical depth. This semiempirical model accounts for all the observational signatures of the chromospheric oscillation when the filling factor of the active component oscillates between a few percent and 20% of the resolution element. We discuss a plausible physical scenario in which upward-propagating waves in a downflowing magnetized environment lead to periodic mass ejections from the atmospheric layers where the waves become nonlinear.

Dynamics of a Quiescent Solar Prominence Observed with the SUMER/SOHO Instrument

We present the results obtained from analyzing SUMER/SOHO observational data of a quiescent solar prominence. The studied prominence is made of complex structures. From the 1-hr data set, we derive characteristic frequencies in terms of intensity and velocity oscillations, as measured in 4 transition-region lines. The presence of different types of frequencies is detected: chromospheric oscillations and intermediate periods (6 min to 12 min). This result suggests that these oscillations are transmitted by the magnetic fields from the chromosphere to the transition region.

Fine‐Scale Magnetic Effects onp‐Modes and Higher Frequency Acoustic Waves in a Solar Active Region

We present results from a 5 hr time series of simultaneous high-resolution measurements of oscillations in the photosphere and the chromosphere on fine spatial scales for a rapidly evolving solar active region and examine their relation to the vector magnetic field. The photospheric oscillations are determined from Doppler shifts in the Zeeman-insensitive spectral line Fe I 557.6 nm, whereas the chromospheric oscillations are determined from the intensity fluctuations seen in the Ca II K line. The vector magnetic field configurations just prior to and just after the time-series measurements are obtained from the full Stokes inversion of the line profiles of Fe I 630.15 and 630.25 nm. In both the photosphere and the chromosphere, p-mode power is suppressed by the magnetic field, decreasing with increasing field strength. At higher frequencies (above the acoustic cutoff) power is also suppressed at the highest magnetic field strengths in the photosphere and the chromosphere but is enhanced in the photosphere in localized patches (halos) of intermediate field strength surrounding the patches of highest field strength (as first reported by Brown et al.). The evidence for similar halos in the chromosphere (as first seen by Braun et al. and Toner & LaBonte) and their association with the photospheric halos is less clear. We find seismic evidence, in the forms of suppression of p-mode and higher frequency power and a halo of enhanced higher frequency power, for a developing pore prior to its appearance as a dark patch in Fe I 557.6 nm core intensity. This result shows that p-mode power suppression cannot be due primarily to the greater transparency of the cooler pore atmosphere.

Anomalous polarization profiles in sunspots: possible origin of umbral flashes

We present time-series spectropolarimetric observations of sunspots in the Ca II infrared triplet lines, which show a periodic occurrence of anomalous, asymmetric, circular polarization profiles in the umbral chromosphere. The profiles may be caused by the periodic development of an unresolved atmospheric component in a downward flowing magnetized environment. This active component with upward directed velocities as high as 10 kilometers per second is connected to the umbral flash (UF) phenomenon. We can explain the observations with a semiempirical model of the chromospheric oscillation and of the sunspot magnetized atmospheric plasma during a UF event.

Chromospheric and Transition Region Internetwork Oscillations: A Signature of Upward‐propagating Waves

We analyze spectral time series obtained on 1997 April 25 with the SUMER instrument on SOHO. Line and continuum data near 1037 A were acquired at a cadence of 16 s. This spectral region was chosen because it contains strong emission lines of C II, formed in the upper chromosphere/lower transition region; O VI, formed in the upper transition region; and neighboring continuum emission formed in the middle chromosphere. The time series reveal oscillatory behavior. Subsonic (3-5 km s-1 amplitude) Doppler velocity oscillations in the C II and O VI lines, with periods between 120 and 200 s, are prominent. They are seen as large-scale coherent oscillations, typically of 3-7 Mm length scale, occasionally approaching 15 Mm, visible most clearly in internetwork regions. The Doppler velocity oscillations are related to oscillations seen in the continuum intensity, which precede upward velocity in C II by 40-60 s. The C II Doppler shift precedes the O VI Doppler shift by 3-10 s. Oscillations are also present in the line intensities, but the intensity amplitudes associated with the oscillations are small. The continuum intensity precedes the C II intensity by 30-50 s. Phase difference analysis shows that there is a preponderance of upward-propagating waves in the upper chromosphere that drive an oscillation in the transition region plasma, thus extending the evidence for upward-propagating waves from the photosphere up to the base of the corona.

Oscillations of the upper chromosphere

Variations of intensity and wavelength in several UV lines have been observed with the SUMER spectroheliometer onboard SOHO, and they have been analysed to obtain oscillation spectra and phase differences between lines of different ions. Lines intensities of neutral or singly ionized atoms (with temperature of formation ≤ 30 000 K) exhibit an increase of oscillatory power between 2.5 and 7 mHz, which may be considered as the signature of p modes. Lines of highly ionized elements (with a temperature of formation ≥ 50 000 K) yield power spectra which are continuously decreasing with frequency. Brightness variations of the continuum at different wavelengths between 1000 and 1400 A present oscillations in the same frequency range. Thus, p modes seem to be efficiently stopped by the transition region. No clear evidence is found for the existence of a chromospheric oscillation mode. Phase comparisons between lines formed at different altitudes (in particular Si i and Si ii) indicate that these lines oscillate in phase, within the precision of the measurements.

Velocity and Magnetic Field Fluctuations in the Photosphere of a Sunspot

We use a data set of exceptionally high quality to measure oscillations of Doppler velocity, intensity, and the vector magnetic field at photospheric heights in a sunspot. Based on the full Stokes inversion of the line profiles of Fe I 630.15 and 630.25 nm, in the sunspot umbra we find upper limits of 4 G (root mean square [rms]) for the amplitude of 5 minute oscillations in magnetic field strength and 009 (rms) for the corresponding oscillations of the inclination of the magnetic field to the line of sight. Our measured magnitude of the oscillation in magnetic field strength is considerably lower than that found in 1997 by Horn, Staude, & Landgraf. Moreover, we find it likely that our measured magnetic field oscillation is at least partly due to instrumental and inversion cross talk between the velocity and magnetic signals, so that the actual magnetic field strength fluctuations are even weaker than 4 G. In support of this we show, on the basis of the eigenmodes of oscillation in a theoretical model of the sunspot umbra, that magnetic field variations of at most 0.5 G are all that is to be expected. The theoretical model also provides an explanation of the shift of power peaks in Doppler velocity to the 3 minute band in chromospheric umbral oscillations, as a natural consequence of the drastic change in character of the eigenmodes of oscillation between frequencies of about 4.5 and 5.0 mHz due to increased tunneling through the acoustic cutoff-frequency barrier. Using measurements of the phase of velocity oscillations above the acoustic cutoff frequency, we determine the relative velocity response height in the umbra of four different photospheric spectral lines from the phase differences between velocities in these lines, assuming that the oscillations propagate vertically at the local sound speed. In spacetime maps of fluctuations in continuum intensity, Doppler velocity, magnetic field strength, and field inclination, we see distinct features that migrate radially inward from the inner penumbra all the way to the center of the umbra, at speeds of a few tenths of a kilometer per second. These moving features are probably a signature of the convective interchange of magnetic flux tubes in the sunspot, although we failed to find any strong correlation among the features in the different quantities, indicating that these features have not been fully resolved.

Chromospheric Oscillations in K[TINF]2v[/TINF] Bright Points

The 3 minute waves observed in K2v emission from bright points in the quiet solar chromosphere are shown to be oscillations of the atmosphere at the cutoff period of acoustic waves, implying impulsive excitation. Other interpretations of the period are shown not to be viable since they fail the dual requirement of spatial intermittence, in which most of the supergranulation cell interior is inert over the course of an hour, and temporal intermittence, in which the active locations in the cell interior are active at most half the time. It is speculated that the episodic excitation of the K2v oscillations is due to collisions of granules with magnetic elements of mixed polarity with a field strength of at least 0.5 kG.

Oscillations of the Magnetic Network

AbstractWe present high-quality power spectra of oscillations in the quiet solar chromosphere measured from the Ca II H line. They show unambiguously that the network does not participate in chromospheric three-minute oscillations, but that instead it oscillates in slow wave motions which are neither present in the surrounding non-magnetic chromosphere, nor in the underlying photosphere.

Photoelectric observations of chromospheric sunspot oscillations. III - Spatial distribution of power and frequency in umbrae

view Abstract Citations (47) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric Observations of Chromospheric Sunspot Oscillations. III. Spatial Distribution of Power and Frequency in Umbrae Lites, B. W. Abstract On the basis of analyses of the spatial distribution of oscillation power and frequency of data from Lites (1984), the multiple peaks frequently observed in average power spectra are noted to be due to the presence of cospatial modes with different frequencies. Changes in these modes' amplitudes and frequencies with time and place within an umbra appear to arise from an interference of modes in the driving force in the photosphere. High oscillatory power regions in the 3-min band are uncorrelated with high oscillatory power regions in the 5-min band within the umbrae, suggesting that the 5-min oscillations in the photosphere do not drive the 3-min chromospheric oscillations. These results are used to develop a consistent view of the umbral oscillation mechanism: attention is given to a photospheric resonator for fast magnetoatmospheric waves, driving the chromospheric resonator through nonlinear wave interaction. Publication: The Astrophysical Journal Pub Date: February 1986 DOI: 10.1086/163963 Bibcode: 1986ApJ...301..992L Keywords: Chromosphere; Electrophotometry; Resonant Frequencies; Solar Oscillations; Spatial Distribution; Sunspots; Power Spectra; Spectral Energy Distribution; Stellar Spectrophotometry; Umbras; Solar Physics; SUN: CHROMOSPHERE; SUN: OSCILLATIONS; SUN: SUNSPOTS full text sources ADS | Related Materials (4) Part 1: 1982ApJ...253..386L Part 2: 1984ApJ...277..874L Part 4: 1986ApJ...301.1005L Part 5: 1988ApJ...334.1054L