The Relationship Between Plasma Flow Doppler Velocities and Magnetic Field Parameters During the Emergence of Active Regions at the Solar Photospheric Level

A statistical study has been carried out of the relationship between plasma flow Doppler velocities and magnetic field parameters during the emergence of active regions at the solar photospheric level with data acquired by the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO). We have investigated 224 emerging active regions with different spatial scales and positions on the solar disc. The following relationships for the first hours of the emergence of active regions have been analysed: i) of peak negative Doppler velocities with the position of the emerging active regions on the solar disc; ii) of peak plasma upflow and downflow Doppler velocities with the magnetic flux growth rate and magnetic field strength for the active regions emerging near the solar disc centre (the vertical component of plasma flows); iii) of peak positive and negative Doppler velocities with the magnetic flux growth rate and magnetic field strength for the active regions emerging near the limb (the horizontal component of plasma flows); iv) of the magnetic flux growth rate with the density of emerging magnetic flux; v) of the Doppler velocities and magnetic field parameters for the first hours of the appearance of active regions with the total unsigned magnetic flux at the maximum of their development.     

Atmosphere Dynamics of the Active Region NOAA 11024

We present results of the study of chromospheric and photospheric line-of-sight velocity fields in the young active region NOAA 11024. Multi-layer, multi-wavelength observational data were used for the analysis of the emerging flux in this active region. Spectropolarimetric observations were carried out with the telescope THEMIS on Tenerife (Canary Islands) on 4 July 2009. In addition, space-borne data from SOHO/MDI, STEREO and GOES were also considered. The combination of data from ground- and space-based telescopes allowed us to study the dynamics of the lower atmosphere of the active region with high spatial, spectral, and temporal resolutions. THEMIS spectra show strong temporal variations of the velocity in the chromosphere and photosphere for various activity features: two pores, active and quiet plage regions, and two surges. The range of variations of the chromospheric line-of-sight velocity at the heights of the formation of the Hα core was extremely large. Both upward and downward motions were observed in these layers. In particular, a surge with upward velocities up to ?73 km?s^?1 was detected. In the photosphere, predominantly upward motions were found, varying from ?3.1 km?s^?1 upflows to 1.4 km?s^?1 downflows in different structures. The velocity variations at different levels in the lower atmosphere are compatible with the emergence of magnetic flux.     

Solar Dynamics Observatory and Hinode Observations of a Blowout Jet in a Coronal Hole

A blowout jet occurred within the south coronal hole on 9 February 2011 at 09:00 UT and was observed by the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory, and by the EUV Imaging Spectrometer (EIS) and X-Ray Telescope (XRT) onboard the Hinode spacecraft during coronal-hole monitoring performed as part of Hinode Operations Program No. 177. Images from AIA show expanding hot and cold loops from a small bright point with plasma ejected in a curtain up to 30 Mm wide. The initial intensity front of the jet had a projected velocity of 200 km?s^?1, and the line-of-sight (LOS) velocities measured by EIS are between 100 and 250 km?s^?1. The LOS velocities increased along the jet, implying that an acceleration mechanism operates within the body of the jet. The jet plasma had a density of 2.7×10⁸ cm^?3 and a temperature of 1.4 MK. During the event a number of bright kernels were seen at the base of the bright point. The kernels have sizes of ≈?1000 km, are variable in brightness, and have lifetimes of 1?–?15 minutes. An XRT filter ratio yields temperatures of 1.5?–?3.0 MK for the kernels. The bright point existed for at least ten hours, but disappeared within two hours after the jet, which lasted for 30 minutes. HMI data reveal converging photospheric flows at the location of the bright point, and the mixed-polarity magnetic flux canceled over a period of four hours on either side of the jet.     

Magnetic Causes of the Eruption of a Quiescent Filament

During the JOP178 campaign in August 2006, we observed the disappearance of our target, a large quiescent filament located at S25°, after an observation time of three days (24 August to 26 August). Multi-wavelength instruments were operating: THEMIS/MTR (“MulTi-Raies”) vector magnetograph, TRACE (“Transition Region and Coronal Explorer”) at 171 Å and 1600 Å and Hida Domeless Solar telescope. Counter-streaming flows (+/?10 km?s^?1) in the filament were detected more than 24 hours before its eruption. A slow rise of the global structure started during this time period with a velocity estimated to be of the order of 1 km?s^?1. During the hour before the eruption (26 August around 09:00 UT) the velocity reached 5 km?s^?1. The filament eruption is suspected to be responsible for a slow CME observed by LASCO around 21:00 UT on 26 August. No brightening in Hα or in coronal lines, no new emerging polarities in the filament channel, even with the high polarimetry sensitivity of THEMIS, were detected. We measured a relatively large decrease of the photospheric magnetic field strength of the network (from 400 G to 100 G), whose downward magnetic tension provides stability to the underlying stressed filament magnetic fields. According to some MHD models based on turbulent photospheric diffusion, this gentle decrease of magnetic strength (the tension) could act as the destabilizing mechanism which first leads to the slow filament rise and its fast eruption.     

Time – Distance Analysis of the Emerging Active Region NOAA 10790

We investigate the emergence of Active Region NOAA 10790 by means of time?–?distance helioseismology. Shallow regions of increased sound speed at the location of increased magnetic activity are observed, with regions becoming deeper at the locations of sunspot pores. We also see a long-lasting region of decreased sound speed located underneath the region of the flux emergence, possibly relating to a temperature perturbation due to magnetic quenching of eddy diffusivity, or to a dense flux tube. We detect and track an object in the subsurface layers of the Sun characterised by increased sound speed which could be related to emerging magnetic-flux and thus obtain a provisional estimate of the speed of emergence of around 1 km?s^?1.     

Sequential Coronal Mass Ejections from AR8038 in May 1997

Three homologous coronal mass ejections (CMEs) occurred on 5, 12 and 16 May 1997 from the single magnetic polarity inversion line (PIL) of AR8038. The three events together provide STEREO-like quadrature views of the 12 May 1997 CME and EIT double dimming. The recurrent CMEs with the nearly identical post-CME potential state and the ‘sigmoid to arcade to sigmoid’ transformations indicate a repeatable store?–?release?–?restore process of the free energy. How was the free magnetic energy re-introduced to the potential state corona after each release in this decaying active region? Making use of the known time interval bounded by the adjacent homologous CMEs, we made the following measures. The unsigned magnetic flux of AR8038, Φ_AR, decreased by approximately 18% during 66 h, while the unsigned flux, Φ_PIL, in a Gaussian-weighted PIL-region containing the flare site increased by about 50% during 36 hrs prior to the C1.3 flare on 12 May 1997. The significant increase of Φ_PIL demonstrates the magnetic gradient increase and the build-up of free energy in the PIL-region during the time leading to the eruption. Fourier local correlation tracking (FLCT) flow speed in AR8038 ranges from 0 to 292.8 m?s^?1 with a mean value of 63.2 m?s^?1. The flow field contains a persistent converging flow towards the flaring PIL and an effective shear flow distributed in the AR. Minor angular motions were found. An integrated proxy Poynting flux S _h estimates the energy input to the corona to be on the order of 1.15×10³² erg during the 66 hrs before the C1.3 flare. It suggests that sufficient energy for a flare/CME can be introduced to the corona on the order of several days by the flows deduced from photospheric magnetic field motions in this small decaying active region.     

Velocities and Temperatures of an Ellerman Bomb and Its Associated Features

We investigated the velocity and temperature characteristics of an Ellerman bomb (EB) and its associated features based on observations made with the Fast Imaging Solar Spectrograph (FISS) and a broadband TiO filter of the 1.6 meter New Solar Telescope at Big Bear Solar Observatory. In the TiO images of the photospheric level, we found a granular cell expanding in two opposite directions near the site of the EB. When one end of this granule reached the EB site, the transverse speed of the tip of the expanding granule rapidly decreased and the EB brightened. The wings of the Hα profile of the EB indicated that the EB was blueshifted up to 7 km?s^?1. About 260 s after the EB brightening, a surge was seen in absorption and varied from a blueshift of 20 km?s^?1 to a redshift of 40 km?s^?1 seen in the Hα and Ca ii 8542 Å lines. From the Doppler absorption width of the two lines determined by applying the cloud model, we estimated the mean temperature of the surge material to be about 29000 K and the mean speed of nonthermal motion to be about 11 km?s^?1. We discuss the physical implications of our results in terms of magnetic reconnection and processes related to it.     

Statistical Analysis of Large-Scale EUV Waves Observed by STEREO/EUVI

We statistically analyzed the kinematical evolution and wave pulse characteristics of 60 strong large-scale EUV wave events that occurred during January 2007 to February 2011 with the STEREO twin spacecraft. For the start velocity, the arithmetic mean is 312±115 km?s^?1 (within a range of 100?–?630 km?s^?1). For the mean (linear) velocity, the arithmetic mean is 254±76 km?s^?1 (within a range of 130?–?470 km?s^?1). 52 % of all waves under study show a distinct deceleration during their propagation (a≤?50 m?s^?2), the other 48 % are consistent with a constant speed within the uncertainties (?50≤a≤50 m?s^?2). The start velocity and the acceleration are strongly anticorrelated with c≈?0.8, i.e. initially faster events undergo stronger deceleration than slower events. The (smooth) transition between constant propagation for slow events and deceleration in faster events occurs at an EUV wave start-velocity of v≈230 km?s^?1, which corresponds well to the fast-mode speed in the quiet corona. These findings provide strong evidence that the EUV waves under study are indeed large-amplitude fast-mode MHD waves. This interpretation is also supported by the correlations obtained between the peak velocity and the peak amplitude, impulsiveness, and build-up time of the disturbance. We obtained the following association rates of EUV wave events with other solar phenomena: 95 % are associated with a coronal mass ejection (CME), 74 % to a solar flare, 15 % to interplanetary type II bursts, and 22 % to coronal type II bursts. These findings are consistent with the interpretation that the associated CMEs are the driving agents of the EUV waves.     

Large-scale Structure of the Chromospheric Doppler Velocities on the Solar Disk from 2D-spectroscopy within the He I 10830 Å Line

A distribution map of the large-scale chromospheric Doppler velocities on the solar disk for 5 June 2002 is presented. The map was obtained using a 2D-spectroscopy technique within the He?i 10830 Å line. The spatial resolution of the map is about 30 arc sec. The map demonstrates a downflow in the chromosphere over active regions, especially significant around the spots and inside the plages. Positive Doppler velocities correspond to strong magnetic field areas, regardless of the field sign. Three major chromospheric outflow zones are observed: an equatorial and two polar ones. Each area of substantial negative Doppler velocities matches a zone of weak intensity of inner corona observed within the Fe?ix-x 171 Å line by the SOHO spacecraft. A Doppler velocity histogram and the dependence of the Doppler velocities on the cosine of the heliocentric angle for the solar disk are calculated. The total mass outflow from the upper chromosphere is estimated as 2×10¹³ g s^?1. Four percent of this amount is sufficient to produce the fast solar wind.

     

Vorticity of Subsurface Flows of Emerging and Decaying Active Regions

We study the temporal variation of the vorticity of subsurface flows of 828 active regions and 977 quiet regions. The vorticity of these flows is derived from measured subsurface velocities. The horizontal flows are determined by analyzing high-resolution Global Oscillation Network Group Doppler data with ring-diagram analysis covering a range of depths from the surface to about 16 Mm. The vertical velocity component is derived from the divergence of the measured horizontal flows using mass conservation. We determine the change in unsigned magnetic flux density during the disk passage of each active region using Michelson Doppler Imager (MDI) magnetograms binned to the ring-diagram grid with centers spaced by 7.5° ranging ± 52.5° in latitude and central meridian distance with an effective diameter of 15° after apodization. We then sort the data by their flux change from decaying to emerging flux and divide the data into five subsets of equal size. We find that the vorticity of subsurface flows increases during flux emergence and decreases when active regions decay. For flux emergence, the absolute values of the zonal and meridional vorticity components show the most coherent variation with activity, while for flux decrease the strongest signature is in the absolute values of the meridional and vertical vorticity components. The temporal variation of the enstrophy (residual vorticity squared) is thus a good indicator for either flux increase or decrease. There are some indications that the increase in vorticity during flux emergence happens about a day later at depths below about 8 Mm compared to layers shallower than about 4 Mm. This timing difference might imply that the vorticity signal analyzed here is caused by the interaction between magnetic flux and turbulent flows near the solar surface. There are also hints that the vorticity decrease during flux decay begins about a day earlier at layers deeper than about 8 Mm compared to shallower ones. However, the timing difference between the change at different depths is comparable to the time step of the analysis.     

Horizontal velocities in the solar photosphere

Horizontal macroscopic velocities V _hor in the photosphere are studied. High-resolution spectrograms of quiet regions are analyzed for center-limb variation of rms Doppler shifts. The data are treated to assure that the observed velocities refer to constant size volumes on the Sun (800 × × 3000 × 250 km), independent of μ. Using known height variation of vertical velocities and calculated line formation heights, the height dependence of 〈V _hor〉 is obtained. From a value around 450 m s^?1 it decreases rapidly with increasing height. To study also small-scale velocities, the time evolution of subarcsecond size elements in the photospheric network (solar filigree) is studied on filtergrams. It is concluded that they show proper motions implying 〈V _hor〉 about 1 km s^?1.     

Determination of the True Shape of Coronal Loops

Using line of sight velocity measurements from the SUMER and CDS instruments aboard SOHO, in conjunction with a simple geometrical model, we reconstructed the true, 3D shape and the velocity of plasma flow along coronal loops. The projection of the loop on the sky and the position of the footpoints define a family of curves. Assuming that the loop is located on a plane, the line of sight velocity can be used to select the most plausible solution. For two loops, observed in the Ne viii? 770 Å and O v? 630 Å spectral lines, we find asymmetric, subsonic uni-directional flows, with velocity maxima of ≈?80 km?s^?1 near the footpoints. The loops are highly inclined with respect to the vertical, by 55^○ and 70^○, respectively; thus the true height of the loop tops from the photospheric level is ≈?20^′′, comparable to the isothermal scale height.     

Subsurface Supergranular Vertical Flows as Measured Using Large Distance Separations in Time–Distance Helioseismology

As large-distance rays (say, 10?–?24°) approach the solar surface approximately vertically, travel times measured from surface pairs for these large separations are mostly sensitive to vertical flows, at least for shallow flows within a few Mm of the solar surface. All previous analyses of supergranulation have used smaller separations and have been hampered by the difficulty of separating the horizontal and vertical flow components. We find that the large-separation travel times associated with supergranulation cannot be studied using the standard phase-speed filters of time–distance helioseismology. These filters, whose use is based upon a refractive model of the perturbations, reduce the resultant travel-time signal by at least an order of magnitude at some distances. More effective filters are derived. Modeling suggests that the center–annulus travel-time difference [δt _oi] in the separation range Δ=10?–?24^° is insensitive to the horizontally diverging flow from the centers of the supergranules and should lead to a constant signal from the vertical flow. Our measurement of this quantity, 5.1±0.1 seconds, is constant over the distance range. This magnitude of the signal cannot be caused by the level of upflow at cell centers seen at the photosphere of 10 m?s^?1 extended in depth. It requires the vertical flow to increase with depth. A simple Gaussian model of the increase with depth implies a peak upward flow of 240 m?s^?1 at a depth of 2.3 Mm and a peak horizontal flow of 700 m?s^?1 at a depth of 1.6 Mm.     

Simulations of the sun's polar magnetic fields during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, we have simulated the evolution of the radial component of the solar photospheric magnetic field during 1976–1984 and derived the corresponding evolution of the line-of-sight polar fields as seen from Earth. The observed timing and strength of the polar-field reversal during cycle 21 can be accounted for by supergranular diffusion alone, for a diffusion coefficient of 800 km² s^-1. For an assumed 300 km² s^-1 rate of diffusion, on the other hand, a poleward meridional flow with a moderately broad profile and a peak speed of 10 m s^-1 reached at about 5° latitude is required to obtain agreement between the simulated and observed fields. Such a flow accelerates the transport of following-polarity flux to the polar caps, but also inhibits the diffusion of leading-polarity flux across the equator. For flows faster than about 10 m s^-1 the latter effect dominates, and the simulated polar fields reverse increasingly later and more weakly than the observed fields.Laboratory for Computational Physics and Fluid Dynamics.E. O. Hulburt Center for Space Research.     

Photospheric Plasma Flows Around a Solar Spot

We study photospheric plasma flows in an active region NOAA 8375, by using uninterrupted high-resolution SOHO/MDI observations (137 intensity images, 44 hours of observations). The active region consists of a stable large spot and many small spots and pores. Analyzing horizontal flow maps, obtained with local correlation tracking technique, we found a system of stable persistent plasma flows existing in the active region. The flows start on either side of the sunspot and extend over 100′′ to the east. Our measurements show that the speed of small sunspots and pores, averaged over 44 hours, was about 100 m s⁻¹, which corresponds to root-mean-square longitudinal drifts of sunspots of 0.67°–0.76° day⁻¹. We conclude that these large-scale flows are due to faster proper motion of the large sunspot relative to the ambient photospheric plasma. We suggest that the flows may be a good carrier to transport magnetic flux from eroding sunspots into the outer part of an active region.     

Meridional circulation and turbulence in surface layers of the stars

The meridional circulation is considered in the surface layers of the stars where the optical depth τ?1. It is shown that the radial component of circulation velocity reaches its maximum value at τ≈1 and decreases at τ→0. The tangential velocity reverses its sign at τ≈1 — i.e., the meridional flows are closed in stellar atmospheres. The tangential velocities can be of the order of 10⁶–10⁷ cm s^?1 in atmospheres of O-B-A stars. Such hydrodynamical motions can result in the generation of turbulence in the surface layers. Characteristic turbulent velocities are of the order of 10⁵–10⁶ cm s^?1 in early-type stars. The small-scale turbulent motions generate the acoustic waves and the flux of such waves may be the source of energy to heat coronae in O and B stars.     

Quiescent Reconnection Rate Between Emerging Active Regions and Preexisting Field,with Associated Heating: NOAA AR 11112

When magnetic flux emerges from beneath the photosphere, it displaces the preexisting field in the corona, and a current sheet generally forms at the boundary between the old and new magnetic domains. Reconnection in the current sheet relaxes this highly stressed configuration to a lower energy state. This scenario is most familiar and most often studied in flares, where the flux transfer is rapid. We present here a study of steady, quiescent flux transfer occurring at a rate three orders of magnitude lower than that in a large flare. In particular, we quantify the reconnection rate and the related energy release that occurred as the new polarity emerged to form NOAA Active Region 11112 (SOL16 October 2010T00:00:00L205C117) within a region of preexisting flux. A bright, low-lying kernel of coronal loops above the emerging polarity, observed with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and the X-ray Telescope onboard Hinode, originally showed magnetic connectivity only between regions of newly emerged flux when overlaid on magnetograms from the Helioseismic and Magnetic Imager. Over the course of several days, this bright kernel advanced into the preexisting flux. The advancement of an easily visible boundary into the old flux regions allows measuring the rate of reconnection between old and new magnetic domains. We compare the reconnection rate with the inferred heating of the coronal plasma. To our knowledge, this is the first measurement of steady, quiescent heating related to reconnection. We determined that the newly emerged flux reconnects at a fairly steady rate of 0.38×10¹⁶ Mx?s^?1 over two days, while the radiated power varies between (2?–?8)×10²⁵ erg?s^?1 over the same time. We found that as much as 40 % of the total emerged flux at any given time may have reconnected. The total amounts of transferred flux (～?1×10²¹ Mx) and radiated energy (～?7.2×10³⁰ ergs) are comparable to that of a large M- or small X-class flare, but are stretched out over 45 hours.     

Small-scale motions over concentrated magnetic regions of the quiet Sun

We have used a 5.5 min time-sequence of spectra in the Fe i lines λ5576 (magnetically insensitive), λ6301.5 and λ6302.5 (magnetically sensitive) to study the association of concentrated magnetic regions and velocity in the quiet Sun. After the elimination of photospheric oscillations we found downflows of 100–300 m s ^?1, displaced by about 2″ from the peaks of the magnetic field; this velocity is comparable to downflow velocity associated with the granulation and of the same order or smaller than the oscillation amplitude. Quasi-periodic time variations of the vertical component of the magnetic field up to ± 40% were also found with a period near 250 s, close to the values found for the velocity field. Finally we report a possible association of intensity maxima at the line center with peaks of the oscillation amplitude.     

The Association of Solar Flares with Coronal Mass Ejections During the Extended Solar Minimum

We study the association of solar flares with coronal mass ejections (CMEs) during the deep, extended solar minimum of 2007?–?2009, using extreme-ultraviolet (EUV) and white-light (coronagraph) images from the Solar Terrestrial Relations Observatory (STEREO). Although all of the fast (v>900 km?s^?1), wide (θ>100^°) CMEs are associated with a flare that is at least identified in GOES soft X-ray light curves, a majority of flares with relatively high X-ray intensity for the deep solar minimum (e.g. ?1×10^?6 W?m^?2 or C1) are not associated with CMEs. Intense flares tend to occur in active regions with a strong and complex photospheric magnetic field, but the active regions that produce CME-associated flares tend to be small, including those that have no sunspots and therefore no NOAA active-region numbers. Other factors on scales similar to and larger than active regions seem to exist that contribute to the association of flares with CMEs. We find the possible low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves, and Type III bursts, in 91 %, 74 %, 57 %, and 74 %, respectively, of the 35 flares that we associate with CMEs. None of these observables can fully replace direct observations of CMEs by coronagraphs.     

The Evolution of Barbs of a Polar Crown Filament Observed by SDO

From 16 to 21 August 2010, a northern (???N60) polar crown filament was observed by Solar Dynamics Observatory (SDO). Employing the six-day SDO/AIA data, we identify 69 barbs, and select 58 of them, which appeared away from the western solar limb (???W60), as our sample. We systematically investigate the evolution of filament barbs. Three different types of apparent formation of barbs are detected, including i)?the convergence of surrounding moving plasma condensations, comprised 55.2?% of our sample, ii)?the flows of plasma condensations from the filament, comprised 37.9?%, and iii)?the plasma injections from the neighboring brightening regions, comprised 6.9?%. We also find three different ways that barb disappear, involving: i)?bi-lateral movements (44.8?%), and ii)?outflowing of barb plasma (27.6?%) results in the disappearance of a barb, as well as iii)?disappearance of a barb is associated with a neighboring brightening (27.6?%). The evolution of the magnetic fields, e.g. emergence and cancellation of magnetic flux, may cause the formation or disappearance of the barb magnetic structures. Barbs exchange plasma condensations with the surrounding atmosphere, filament, and nearby brightenings, leading to the increase or drainage of barb material. Furthermore, we find that all the barbs undergo oscillations. The average oscillation period, amplitude, and velocity are 30?min, 2.4?Mm, and 5.7?km?s^?1, respectively. Besides the oscillations, 21 (36?%) barbs manifested sideward motions having an average speed of 0.45?km?s^?1. Small-scale wave-like propagating disturbances caused by small-scale brightenings are detected, and the barb oscillations associated with these disturbances are also found. We propose that the kinematics of barbs are influenced or even caused by the evolution of the neighboring photospheric magnetic fields.