Edward E. DeLuca

Continuous plasma outflows from the edge of a solar active region as a possible source of solar wind

The Sun continuously expels a huge amount of ionized material into interplanetary space as the solar wind. Despite its influence on the heliospheric environment, the origin of the solar wind has yet to be well identified. In this paper, we report Hinode X-ray Telescope observations of a solar active region. At the edge of the active region, located adjacent to a coronal hole, a pattern of continuous outflow of soft-x-ray–emitting plasmas was identified emanating along apparently open magnetic field lines and into the upper corona. Estimates of temperature and density for the outflowing plasmas suggest a mass loss rate that amounts to ∼1/4 of the total mass loss rate of the solar wind. These outflows may be indicative of one of the solar wind sources at the Sun.

Temperature and Emission-Measure Profiles along Long-lived Solar Coronal Loops Observed with the Transition Region and Coronal Explorer

We report an initial study of temperature and emission-measure distributions along four steady loops observed with the Transition Region and Coronal Explorer at the limb of the Sun. The temperature diagnostic is the filter ratio of the extreme-ultraviolet 171 and 195 A passbands. The emission-measure diagnostic is the count rate in the 171 A passband. We find essentially no temperature variation along the loops. We compare the observed loop structure with theoretical isothermal and nonisothermal static loop structure.

Steady Flows Detected in Extreme-Ultraviolet Loops

Recent Transition Region and Coronal Explorer (TRACE) observations have detected a class of active region loops whose physical properties are inconsistent with previous hydrostatic loop models. In this Letter we present the first co-aligned TRACE and the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) observations of these loops. Although these loops appear static in the TRACE images, SUMER detects line-of-sight flows along the loops of up to 40 km s-1. The presence of flows could imply an asymmetric heating function; such a heating function would be expected for heating that is proportional to (often asymmetric) footpoint field strength. We compare a steady flow solution resulting from an asymmetric heating function to a static solution resulting from a uniform heating function in a hypothetical coronal loop. We find that the characteristics associated with the asymmetrically heated loop better compare with the characteristics of the loops observed in the TRACE data.

Hinode, TRACE, SOHO, and Ground-based Observations of a Quiescent Prominence

A quiescent prominence was observed by several instruments on 2007 April 25. The temporal evolution was recorded in Hα by the Hinode SOT, in X-rays by the Hinode XRT, and in the 195 A channel by TRACE. Moreover, ground-based observatories (GBOs) provided calibrated Hα intensities. Simultaneous extreme-UV (EUV) data were also taken by the Hinode EIS and SOHO SUMER and CDS instruments. Here we have selected the SOT Hα image taken at 13:19 UT, which nicely shows the prominence fine structure. We compare this image with cotemporaneous ones taken by the XRT and TRACE and show the intensity variations along several cuts parallel to the solar limb. EIS spectra were obtained about half an hour later. Dark prominence structure clearly seen in the TRACE and EIS 195 A images is due to the prominence absorption in H I, He I, and He II resonance continua plus the coronal emissivity blocking due to the prominence void (cavity). The void clearly visible in the XRT images is entirely due to X-ray emissivity blocking. We use TRACE, EIS, and XRT data to estimate the amount of absorption and blocking. The Hα integrated intensities independently provide us with an estimate of the Hα opacity, which is related to the opacity of resonance continua as follows from the non-LTE radiative-transfer modeling. However, spatial averaging of the Hα and EUV data have quite different natures, which must be taken into account when evaluating the true opacities. We demonstrate this important effect here for the first time. Finally, based on this multiwavelength analysis, we discuss the determination of the column densities and the ionization degree of hydrogen in the prominence.

MAGNETOHYDRODYNAMIC TURBULENCE OF CORONAL ACTIVE REGIONS AND THE DISTRIBUTION OF NANOFLARES

We present results from numerical simulations of an externally driven two-dimensional magnetohydrodynamic system over extended periods of time, used to model the dynamics of a transverse section of a solar coronal loop. A stationary forcing was imposed to model the photospheric motions at the loop footpoints. After several photospheric turnover times, a turbulent stationary regime is reached that has an energy dissipation rate consistent with the heating requirements of coronal loops. The turbulent velocities obtained in our simulations are consistent with those derived from the nonthermal broadening of coronal spectral lines. We also show the development of small scales in the spatial distribution of electric currents, which are responsible for most of the energy dissipation. The energy dissipation rate as a function of time displays an intermittent behavior, in the form of impulsive events, that is a direct consequence of the strong nonlinearity of the system. We associate these impulsive events of magnetic energy dissipation with the so-called nanoflares. A statistical analysis of these events yields a power-law distribution as a function of their energies with a negative slope of 1.5, consistent with those obtained for flare energy distributions reported from X-ray observations. A simple model of dissipative structures, based on Kraichnans theory for MHD turbulence, is also presented.

Slipping Magnetic Reconnection in Coronal Loops

Magnetic reconnection of solar coronal loops is the main process that causes solar flares and possibly coronal heating. In the standard model, magnetic field lines break and reconnect instantaneously at places where the field mapping is discontinuous. However, another mode may operate where the magnetic field mapping is continuous but shows steep gradients: The field lines may slip across each other. Soft x-ray observations of fast bidirectional motions of coronal loops, observed by the Hinode spacecraft, support the existence of this slipping magnetic reconnection regime in the Suns corona. This basic process should be considered when interpreting reconnection, both on the Sun and in laboratory-based plasma experiments.

Angular momentum transport and dynamo action in the sun - Implications of recent oscillation measurements

The implications of a newly proposed picture of the suns internal rotation (Brown et al., 1989; Morrow, 1988) for the distribution and transport of angular momentum and for the solar dynamo are considered. The new results, derived from an analysis of solar acoustic oscillations, affect understanding of how momentum is cycled in the sun and provide clues as to how and where the solar dynamo is driven. The data imply that the only significant radial gradient of angular velocity exists in a transitional region between the bottom of the convection zone, which is rotating like the solar surface, and the top of the deep interior, which is rotating rigidly at a rate intermediate between the equatorial and polar rates at the surface. Thus the radial gradient must change sign at the latitude where the angular velocity of the surface matches that of the interior. These inferences suggest that the cycle of angular momentum that produces the observed latitudinal differential rotation in the convection zone may be coupled to layers of the interior beneath the convection zone. 35 refs.

SOLAR CYCLE PROPAGATION, MEMORY, AND PREDICTION: INSIGHTS FROM A CENTURY OF MAGNETIC PROXIES

The solar cycle and its associated magnetic activity are the main drivers behind changes in the interplanetary environment and Earths upper atmosphere (commonly referred to as space weather). These changes have a direct impact on the lifetime of space-based assets and can create hazards to astronauts in space. In recent years there has been an effort to develop accurate solar cycle predictions (with aims at predicting the long-term evolution of space weather), leading to nearly a hundred widely spread predictions for the amplitude of solar cycle 24. A major contributor to the disagreement is the lack of direct long-term databases covering different components of the solar magnetic field (toroidal versus poloidal). Here, we use sunspot area and polar faculae measurements spanning a full century (as our toroidal and poloidal field proxies) to study solar cycle propagation, memory, and prediction. Our results substantiate predictions based on the polar magnetic fields, whereas we find sunspot area to be uncorrelated with cycle amplitude unless multiplied by area-weighted average tilt. This suggests that the joint assimilation of tilt and sunspot area is a better choice (with aims to cycle prediction) than sunspot area alone, and adds to the evidence in favor of active region emergence and decay as the main mechanism of poloidal field generation (i.e., the Babcock-Leighton mechanism). Finally, by looking at the correlation between our poloidal and toroidal proxies across multiple cycles, we find solar cycle memory to be limited to only one cycle.

OBSERVATIONS AND NONLINEAR FORCE-FREE FIELD MODELING OF ACTIVE REGION 10953

We present multiwavelength observations of a simple bipolar active region (NOAA 10953), which produced several small flares ( mostly B class and one C8.5 class) and filament activations from April 30 to May 3 in 2007. We also explore nonlinear force-free field (NLFFF) modeling of this region prior to the C8.5 flare on May 2, using magnetograph data from SOHO/MDI and Hinode/SOT. A series of NLFFF models are constructed using the flux-rope insertion method. By comparing the modeled field lines with multiple X-ray loops observed by Hinode/XRT, we find that the axial flux of the flux rope in the best-fit models is ( 7 +/- 2) x 10(20) Mx, while the poloidal flux has a wider range of (0.1-10) x 10(10) Mx cm(-1). The axial flux in the best-fit model is well below the upper limit (similar to 15 x 10(20) Mx) for stable force-free configurations, which is consistent with the fact that no successful full filament eruption occurred in this active region. From multiwavelength observations of the C8.5 flare, we find that the X-ray brightenings ( in both RHESSI and XRT) appeared about 20 minutes earlier than the EUV brightenings seen in TRACE 171 angstrom images and filament activations seen in MLSO H alpha images. This is interpreted as an indication that the X-ray emission may be caused by direct coronal heating due to reconnection, and the energy transported down to the chromosphere may be too low to produce EUV brightenings. This flare started from nearly unsheared flare loop, unlike most two-ribbon flares that begin with highly sheared footpoint brightenings. By comparing with our NLFFF model, we find that the early flare loop is located above the flux rope that has a sharp boundary. We suggest that this flare started near the outer edge of the flux rope, not at the inner side or at the bottom as in the standard two-ribbon flare model.

Apparent Flows above an Active Region Observed with the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer (TRACE) observed Active Region 8395 on 1998 December 1 from 1:30:00 to 3:00:00 UT at high cadence in the Fe IX/Fe X channel (log Te ≈ 6.0). Throughout the observing time, brightness variations along a dense bundle of coronal field lines in the southwest corner of the active region were observed. Movies made of this region give the impression of continuous intermittent outflow in this bundle of coronal loops; such apparent outflow is often seen in the TRACE data. In this Letter, we present an analysis of four separate flow events occurring in three different loops. These events are used as tracers of the flow in order to characterize its physical properties, such as apparent velocity. The projected velocities of the intensity fronts of these flows (and hence lower limits of true velocities) are between 5 and 20 km s-1. Comparisons of the observed intensities with those predicted by a quasi-static model suggest that the events can be explained only by a mass flow from the chromosphere into the corona. The persistence of the flows, and their ubiquity in the TRACE observations, indicates that hydrostatic loops models are not applicable to this class of coronal structures.