Mark Alan Weber

Evidence for Alfvén Waves in Solar X-ray Jets

Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed (∼800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 × 103 to 2 × 104 kilometers wide and 1 × 105 kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind.

SDO/AIA response to coronal hole, quiet Sun, active region, and flare plasma

Aims. We examine the contribution of spectral lines and continuum emission to the EUV channels of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) in different regions of the solar atmosphere. Methods. Synthetic spectra were obtained using the CHIANTI atomic database and sample differential emission measures for coronal hole, quiet Sun, active region and flare plasma. These synthetic spectra were convolved with the effective area of each channel, in order to determine the dominant contribution in different regions of the solar atmosphere. Results. We highlight the contribution of particular spectral lines which under certain conditions can affect the interpretation of SDO/AIA data.

Energy release in the solar corona from spatially resolved magnetic braids

It is now apparent that there are at least two heating mechanisms in the Sun’s outer atmosphere, or corona. Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000 K (refs 1, 2, 3). The active corona needs additional heating to reach 2,000,000–4,000,000 K; this heat has been theoretically proposed to come from the reconnection and unravelling of magnetic ‘braids’. Evidence favouring that process has been inferred, but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought to have an angular width of about 0.2 arc seconds have not been resolved. Fine-scale braiding has been seen in the chromosphere but not, until now, in the corona. Here we report observations, at a resolution of 0.2 arc seconds, of magnetic braids in a coronal active region that are reconnecting, relaxing and dissipating sufficient energy to heat the structures to about 4,000,000 K. Although our 5-minute observations cannot unambiguously identify the field reconnection and subsequent relaxation as the dominant heating mechanism throughout active regions, the energy available from the observed field relaxation in our example is ample for the observed heating.

Prevalence of small-scale jets from the networks of the solar transition region and chromosphere

As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.

HINODE X-RAY TELESCOPE DETECTION OF HOT EMISSION FROM QUIESCENT ACTIVE REGIONS: A NANOFLARE SIGNATURE?

The X-Ray Telescope (XRT) on the Japanese/USA/UK Hinode (Solar-B) spacecraft has detected emission from a quiescent active region core that is consistent with nanoflare heating. The fluxes from 10 broadband X-ray filters and filter combinations were used to construct differential emission measure (DEM) curves. In addition to the expected active region peak at log T = 6.3-6.5, we find a high-temperature component with significant emission measure at log T > 7.0. This emission measure is weak compared to the main peak—the DEM is down by almost three orders of magnitude—which accounts of the fact that it has not been observed with earlier instruments. It is also consistent with spectra of quiescent active regions: no Fe XIX lines are observed in a CHIANTI synthetic spectrum generated using the XRT DEM distribution. The DEM result is successfully reproduced with a simple two-component nanoflare model.

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.

Metastable Magnetic Configurations and Their Significance for Solar Eruptive Events

Solar flares and coronal mass ejections (CMEs) involve the sudden release of magnetic energy that can lead to the ejection from the Sun of large masses of gas with entrained magnetic field. In dynamical systems, such sudden events are characteristic of metastable configurations that are stable against small perturbations but unstable to sufficiently large perturbations. Linear stability analysis indicates whether or not the first requirement is met, and energetic analysis can indicate whether or not the second requirement is met: if a magnetic configuration that is stable against small perturbations can make a transition to a lower energy state, then it is metastable. In this paper, we consider a long twisted flux tube, anchored at both ends in the photosphere and restrained by an overlying magnetic arcade. We argue from a simple order-of-magnitude calculation that, for appropriate parameter values, it is energetically favorable for part of the flux tube to erupt into interplanetary space, even when the configuration is stable according to linear MHD stability theory. The properties of metastable magnetic configurations may be relevant to CMEs and to other explosive astrophysical events such as solar flares.

Hinode Calibration for Precise Image Co-Alignment between SOT and XRT (2006 November-2007 April)

To understand the physical mechanisms for activity and heating in the solar atmosphere, the magnetic coupling from the photosphere to the corona is an important piece of information from the Hinode observations, and therefore precise positional alignment is required among the data acquired by different telescopes. The Hinode spacecraft and its onboard telescopes were developed to allow us to investigate magnetic coupling with co-alignment accuracy better than 1 00 .U sing the Mercury transit observed on 2006 November 8 and co-alignment measurements regularly performed on a weekly basis, we have determined the information necessary for precise image co-alignment, and have confirmed that co-alignment better than 1 00 can be realized between Solar Optical Telescope (SOT) and X-Ray Telescope (XRT) with our baseline co-alignment method. This paper presents results from the calibration for precise co-alignment of CCD images from SOT and XRT.

DETECTING NANOFLARE HEATING EVENTS IN SUBARCSECOND INTER-MOSS LOOPS USING Hi-C

The High-resolution Coronal Imager (Hi-C) flew aboard a NASA sounding rocket on 2012 July 11 and captured roughly 345 s of high-spatial and temporal resolution images of the solar corona in a narrowband 193 A channel. In this paper, we analyze a set of rapidly evolving loops that appear in an inter-moss region. We select six loops that both appear in and fade out of the Hi-C images during the short flight. From the Hi-C data, we determine the size and lifetimes of the loops and characterize whether these loops appear simultaneously along their length or first appear at one footpoint before appearing at the other. Using co-aligned, co-temporal data from multiple channels of the Atmospheric Imaging Assembly on the Solar Dynamics Observatory, we determine the temperature and density of the loops. We find the loops consist of cool (~105 K), dense (~1010 cm–3) plasma. Their required thermal energy and their observed evolution suggest they result from impulsive heating similar in magnitude to nanoflares. Comparisons with advanced numerical simulations indicate that such dense, cold and short-lived loops are a natural consequence of impulsive magnetic energy release by reconnection of braided magnetic field at low heights in the solar atmosphere.

MULTI-STRANDED AND MULTI-THERMAL SOLAR CORONAL LOOPS: EVIDENCE FROM HINODE X-RAY TELESCOPE AND EUV IMAGING SPECTROMETER DATA

Data from the X-Ray Telescope (XRT) and the EUV Imaging Spectrometer (EIS) on the Japanese/USA/UK Hinode spacecraft were used to investigate the spatial and thermal properties of an isolated quiescent coronal loop. We constructed differential emission measure (DEM) curves using Monte Carlo based, iterative forward fitting algorithms. We studied the loop as a whole, in segments, in transverse cuts, and point-by-point, always with some form of background subtraction, and find that the loop DEM is neither isothermal nor extremely broad, with approximately 96% of the EM between 6.2 {<=}log T{<=} 6.7, and an EM-weighted temperature of log T = 6.48 {+-} 0.16. We find evidence for a gradual change in temperature along the loop, with log T increasing only by {approx}0.1 from the footpoints to the peak. The combine XRT-EIS data set does a good job of constraining the temperature distribution for coronal loop plasma. Our studies show that the strong constraints at high and low temperatures provided by the combined data set are crucial for obtaining reasonable solutions. These results confirm that the observations of at least some loops are not consistent with isothermal plasma, and therefore cannot be modeled with a single flux tube and must be multi-stranded.