Rachel A. Hock

HOT SPINE LOOPS AND THE NATURE OF A LATE-PHASE SOLAR FLARE

The fan-spine magnetic topology is believed to be responsible for many curious emission signatures in solar explosive events. A spine field line links topologically distinct flux domains, but direct observation of such structure has been rare. Here we report a unique event observed by the Solar Dynamic Observatory (SDO) where a set of hot coronal loops (over 10 MK) that developed during the rising phase of a flare connected to a quasi-circular ribbon at one end and a remote brightening at the other. Magnetic field extrapolation suggests these loops are partly tracers of the evolving spine field line. The sequential brightening of the ribbon, the apparent shuffling loop motion, and the increasing volume occupied by the hot loops suggest that continuous slipping- and null-point-type reconnections were at work, energizing the loop plasma and transferring magnetic flux within and across the dome-shaped, fan quasi-separatrix layer (QSL). We argue that the initial reconnection is of the “breakout” type, which then transitioned to a more violent flare reconnection nearing the flare peak with an eruption from the fan dome. Significant magnetic field changes are expected and indeed ensued, which include a change of the horizontal photospheric field, a shift of the QSL footprint, and reduction in shear of the coronal loops. This event also features an extreme-ultraviolet (EUV) late phase, i.e. a secondary emission peak observed in warm EUV lines (about 2‐7 MK) as much as 1‐2 hours after the soft X-ray peak. We show that this peak comes from the large post-reconnection loops beside and above the compact fan dome, a direct product of eruption in such topological settings. Cooling of these “late-phase arcades” naturally explains the sequential delay of the late-phase peaks in increasingly cooler EUV lines. The long cooling time of the large arcades contributes to the long delay; additional heating may also be required. Our result demonstrates the critical nature of cross-scale magnetic coupling ‐ minor topological change in a sub-system may lead to explosions on a much larger scale. Subject headings: Sun: activity — Sun: corona — Sun: flares — Sun: surface magnetism — Sun: magnetic topology

MECHANISMS AND OBSERVATIONS OF CORONAL DIMMING FOR THE 2010 AUGUST 7 EVENT

Coronal dimming of extreme ultraviolet (EUV) emission has the potential to be a useful forecaster of coronal mass ejections (CMEs). As emitting material leaves the corona, a temporary void is left behind which can be observed in spectral images and irradiance measurements. The velocity and mass of the CMEs should impact the character of those observations. However, other physical processes can confuse the observations. We describe these processes and the expected observational signature, with special emphasis placed on the differences. We then apply this understanding to a coronal dimming event with an associated CME that occurred on 2010 August 7. Data from the Solar Dynamics Observatorys Atmospheric Imaging Assembly and EUV Variability Experiment (EVE) are used for observations of the dimming, while the Solar and Heliospheric Observatorys Large Angle and Spectrometric Coronagraph and the Solar Terrestrial Relations Observatorys COR1 and COR2 are used to obtain velocity and mass estimates for the associated CME. We develop a technique for mitigating temperature effects in coronal dimming from full-disk irradiance measurements taken by EVE. We find that for this event, nearly 100% of the dimming is due to mass loss in the corona.

EUV variability experiment (EVE), multiple EUV grating spectrographs (MEGS), radiometric calibrations and results

The NASA Solar Dynamics Observatory (SDO), scheduled for launch in early 2009, incorporates a suite of instruments including the EUV Variability Experiment (EVE). Two channels of EVE, the Multiple EUV Grating Spectrograph (MEGS) A and B channels use concave reflection gratings to image solar spectra onto CCDs to measure the solar extreme ultraviolet (EUV) irradiance from 5 to 105 nm. MEGS provides these spectra at 0.1nm spectral resolution every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-year mission. The calibration of the MEGS channels in order to convert the instrument counts in to physical units of W/m2/nm was performed at the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility III (SURF III) located in Gaithersburg, Maryland. Although the final post-environmental calibrations have yet to be performed, preliminary results from the pre-environmental calibrations show very good agreement with the theoretical optical design given by Crotser et al. Further analysis is still needed in regards to the higher order contributions to determine the final first order QT for all channels, but two techniques are currently being analyzed and show promising results.

Forecasting solar extreme and far ultraviolet irradiance

A new method is presented to forecast the solar irradiance of selected wavelength ranges within the extreme ultraviolet (EUV) and far ultraviolet (FUV) bands. The technique is similar to a method recently published by Henney et al. (2012) to predict solar 10.7 cm (2.8 GHz) radio flux, abbreviated F10.7, utilizing advanced predictions of the global solar magnetic field generated by a flux transport model. In this and the previous study, we find good correlation between the absolute value of the observed photospheric magnetic field and selected EUV/FUV spectral bands. By evolving solar magnetic maps forward 1 to 7 days with a flux transport model, estimations of the Earth side solar magnetic field distribution are generated and used to forecast irradiance. For example, Pearson correlation coefficient values of 0.99, 0.99, and 0.98 are found for 1 day, 3 day, and 7 day predictions, respectively, of the EUV band from 29 to 32 nm. In the FUV, for example, the 160 to 165 nm spectral band, correlation values of 0.98, 0.97, and 0.96 are found for 1 day, 3 day, and 7 day predictions, respectively. In the previous study, the observed F10.7 signal is found to correlate well with strong magnetic field (i.e., sunspot) regions. Here we find that solar EUV and FUV signals are significantly correlated with the weaker magnetic fields associated with plage regions, suggesting that solar magnetic indices may provide an improved indicator (relative to the widely used F10.7 signal) of EUV and FUV nonflaring irradiance variability as input to ionospheric and thermospheric models.

MEASUREMENTS AND MODELING OF TOTAL SOLAR IRRADIANCE IN X-CLASS SOLAR FLARES

The Total Irradiance Monitor (TIM) from NASAs SOlar Radiation and Climate Experiment can detect changes in the total solar irradiance (TSI) to a precision of 2 ppm, allowing observations of variations due to the largest X-class solar flares for the first time. Presented here is a robust algorithm for determining the radiative output in the TIM TSI measurements, in both the impulsive and gradual phases, for the four solar flares presented in Woods et al., as well as an additional flare measured on 2006 December 6. The radiative outputs for both phases of these five flares are then compared to the vacuum ultraviolet (VUV) irradiance output from the Flare Irradiance Spectral Model (FISM) in order to derive an empirical relationship between the FISM VUV model and the TIM TSI data output to estimate the TSI radiative output for eight other X-class flares. This model provides the basis for the bolometric energy estimates for the solar flares analyzed in the Emslie et al. study.

Detecting coronal holes for solar activity modeling

Solar image analysis relies on the detection of coronal holes for predicting disruptions to earths magnetic field. This paper introduces a level-set method for detecting coronal holes based on the processing of extreme ultra-violet images (EUVI) and magnetic images. For validating the approach, two independent manual annotations were combined to produce a set of 46 consensus maps. Overall, the level-set method produces significant improvements over the currently used approach. Future work needs to focus on validating the approach on larger datasets, the integration of more imaging modalities, and an analysis of inter-rater and intra-rater variability.