Joel C. Allred

A subfemtotesla multichannel atomic magnetometer

The magnetic field is one of the most fundamental and ubiquitous physical observables, carrying information about all electromagnetic phenomena. For the past 30 years, superconducting quantum interference devices (SQUIDs) operating at 4 K have been unchallenged as ultrahigh-sensitivity magnetic field detectors, with a sensitivity reaching down to 1 fT Hz-1/2 (1 fT = 10-15 T). They have enabled, for example, mapping of the magnetic fields produced by the brain, and localization of the underlying electrical activity (magnetoencephalography). Atomic magnetometers, based on detection of Larmor spin precession of optically pumped atoms, have approached similar levels of sensitivity using large measurement volumes, but have much lower sensitivity in the more compact designs required for magnetic imaging applications. Higher sensitivity and spatial resolution combined with non-cryogenic operation of atomic magnetometers would enable new applications, including the possibility of mapping non-invasively the cortical modules in the brain. Here we describe a new spin-exchange relaxation-free (SERF) atomic magnetometer, and demonstrate magnetic field sensitivity of 0.54 fT Hz-1/2 with a measurement volume of only 0.3 cm3. Theoretical analysis shows that fundamental sensitivity limits of this device are below 0.01 fT Hz-1/2. We also demonstrate simple multichannel operation of the magnetometer, and localization of magnetic field sources with a resolution of 2 mm.

RADIATIVE HYDRODYNAMIC MODELS OF THE OPTICAL AND ULTRAVIOLET EMISSION FROM SOLAR FLARES

We report on radiative hydrodynamic simulations of moderate and strong solar flares. The flares were simulated by calculating the atmospheric response to a beam of nonthermal electrons injected at the apex of a one-dimensional closed coronal loop and include heating from thermal soft X-ray, extreme ultraviolet, and ultraviolet (XEUV) emission. The equations of radiative transfer and statistical equilibrium were treated in non-LTE and solved for numerous transitions of hydrogen, helium, and Ca II, allowing the calculation of detailed line profiles and continuum emission. This work improves on previous simulations by incorporating more realistic nonthermal electron beam models and includes a more rigorous model of thermal XEUV heating. We find that XEUV back-warming contributes less than 10% of the heating, even in strong flares. The simulations show elevated coronal and transition region densities resulting in dramatic increases in line and continuum emission in both the UV and optical regions. The optical continuum reaches a peak increase of several percent, which is consistent with enhancements observed in solar white-light flares. For a moderate flare (~M class), the dynamics are characterized by a long gentle phase of near balance between flare heating and radiative cooling, followed by an explosive phase with beam heating dominating over cooling and characterized by strong hydrodynamic waves. For a strong flare (~X class), the gentle phase is much shorter, and we speculate that for even stronger flares the gentle phase may be essentially nonexistent. During the explosive phase, synthetic profiles for lines formed in the upper chromosphere and transition region show blueshifts corresponding to a plasma velocity of ~120 km s-1, and lines formed in the lower chromosphere show redshifts of ~40 km s-1.

Multiwavelength Observations of Flares on AD Leonis

We report results from a multiwavelength observing campaign conducted during 2000 March on the flare star AD Leo. Simultaneous data were obtained from several ground- and space-based observatories, including observations of eight sizable flares. We discuss the correlation of line and continuum emission in the optical and ultraviolet wavelength regimes, as well as the flare energy budget, and we find that the emission properties are remarkably similar even for flares of very different evolutionary morphology. This suggests a common heating mechanism and atmospheric structure that are independent of the detailed evolution of individual flares. We also discuss the Neupert effect, chromospheric line broadening, and velocity fields observed in several transition region emission lines. The latter show significant downflows during and shortly after the flare impulsive phase. Our observations are broadly consistent with the solar model of chromospheric evaporation and condensation following impulsive heating by a flux of nonthermal electrons. These data place strong constraints on the next generation of radiative hydrodynamic models of stellar flares.

FROM RADIO TO X-RAY: FLARES ON THE dMe FLARE STAR EV LACERTAE

We present the results of a campaign to observe flares on the M dwarf flare star EV Lacertae over the course of two days in 2001 September, utilizing a combination of radio continuum, optical photometric and spectroscopic, ultraviolet spectroscopic, and X-ray spectroscopic observations to characterize the multiwavelength nature of flares from this active, single, late-type star. We find flares in every wavelength region in which we observed. A large radio flare from the star was observed at both 3.6 and 6 cm and is the most luminous example of a gyrosynchrotron flare yet observed on a dMe flare star. The radio flare can be explained as encompassing a large magnetic volume, comparable to the stellar disk, and involving trapped electrons that decay over timescales of hours. Flux enhancements at 6 cm accompanied by highly negatively circularly polarized emission (?c ? -100%) imply that a coherent emission mechanism is operating in the corona of EV Lac. There are numerous optical white-light flares, and yet no signature of emission-line response from the chromosphere appears. Two small ultraviolet enhancements differ in the amount of nonthermal broadening present. There are numerous X-ray flares occurring throughout the observation, and an analysis of undispersed photons and grating events reveals no evidence for abundance variations. Higher temperatures are present during some flares; however, the maximum temperature achieved varies from flare to flare. There is no evidence for density variations during any flare intervals. In the multiwavelength context, the start of the intense radio flare is coincident with an impulsive optical U-band flare, to within 1 minute, and yet there is no signature of an X-ray response. There are other intervals of time when optical flaring and UV flaring is occurring, but these cannot be related to the contemporaneous X-ray flaring: the time-integrated luminosities do not match the instantaneous X-ray flare luminosity, as one would expect for the Neupert effect. We investigate the probability of chance occurrences of flares from disparate wavelength regions producing temporal coincidences but find that not all the flare associations can be explained by a superposition of flares due to a high flaring rate. We caution against making causal associations of multiwavelength flares based solely on temporal correlations for high flaring rate stars such as EV Lac.

Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares

We report on radiative hydrodynamic simulations of M dwarf stellar flares and compare the model predictions to observations of several flares. The flares were simulated by calculating the hydrodynamic response of a model M dwarf atmosphere to a beam of nonthermal electrons. Radiative back-warming through numerous soft X-ray, extreme-ultraviolet, and ultraviolet transitions are also included. The equations of radiative transfer and statistical equilibrium are treated in non-LTE for many transitions of hydrogen, helium, and the Ca II ion, allowing the calculation of detailed line profiles and continuum radiation. Two simulations were carried out, with electron beam fluxes corresponding to moderate and strong beam heating. In both cases we find that the dynamics can be naturally divided into two phases: an initial gentle phase in which hydrogen and helium radiate away much of the beam energy and an explosive phase characterized by large hydrodynamic waves. During the initial phase, lower chromospheric material is evaporated into higher regions of the atmosphere, causing many lines and continua to brighten dramatically. The He II 304 line is especially enhanced, becoming the brightest line in the flaring spectrum. The hydrogen Balmer lines also become much brighter and show very broad line widths, in agreement with observations. We compare our predicted Balmer decrements to decrements calculated for several flare observations and find the predictions to be in general agreement with the observations. During the explosive phase both condensation and evaporation waves are produced. The moderate flare simulation predicts a peak evaporation wave of ~130 km s-1 and a condensation wave of ~30 km s-1. The velocity of the condensation wave matches velocities observed in several transition region lines. The optical continuum also greatly intensifies, reaching a peak increase of 130% (at 6000 A) for the strong flare, but does not match observed white-light spectra.

A Unified Computational Model for Solar and Stellar Flares

We present a unified computational framework which can be used to describe impulsive flares on the Sun and on dMe stars. The models assume that the flare impulsive phase is caused by a beam of charged particles that is accelerated in the corona and propagates downward depositing energy and momentum along the way. This rapidly heats the lower stellar atmosphere causing it to explosively expand and dramatically brighten. Our models consist of flux tubes that extend from the sub-photosphere into the corona. We simulate how flare-accelerated charged particles propagate down one-dimensional flux tubes and heat the stellar atmosphere using the Fokker-Planck kinetic theory. Detailed radiative transfer is included so that model predictions can be directly compared with observations. The flux of flare-accelerated particles drives return currents which additionally heat the stellar atmosphere. These effects are also included in our models. We examine the impact of the flare-accelerated particle beams on model solar and dMe stellar atmospheres and perform parameter studies varying the injected particle energy spectra. We find the atmospheric response is strongly dependent on the accelerated particle cutoff energy and spectral index.

From Radio to X-Ray: The Quiescent Atmosphere of the dMe Flare Star EV Lacertae

We report on multiwavelength observations spanning radio to X-ray wavelengths of the M dwarf flare star EV Lacertae and probing the characteristics of the outer atmospheric plasma from the upper chromosphere to the corona. We detect the star at a wavelength of 2 cm (15 GHz) for the first time. UV and FUV line profiles show evidence of nonthermal broadening, and the velocity width appears to peak at lower temperatures than in the Sun; this trend is confirmed in another active M dwarf flare star. Electron density measurements indicate nearly constant electron pressures between log T = 5.2 and 6.4. At higher coronal temperatures, there is a sharp increase of 2 orders of magnitude in density (ne ~ 1013 cm-3 at log T = 6.9). X-ray, EUV, FUV, and NUV spectra constrain the differential emission measure (DEM) from the upper chromosphere through the corona. The coronal pressures are inconsistent with the assumption of hydrostatic equilibrium, either through emission measure (EM) modeling or application of scaling laws, and imply large conductive loss rates and a large energy input at the highest temperatures. The timescales for radiative and conductive losses in EV Lacs upper atmosphere imply that significant continued heating must occur for the corona to maintain its quiescent properties. The high-frequency radio detection requires the high-temperature X-ray-emitting coronal plasma to be spatially distinct from the radio emission source. Length scales in the low-temperature corona are markedly larger than those in the high-temperature corona, further suggestions of an inhomogeneous mixture of thermal and nonthermal coronal plasma.

Hα line profile asymmetries and the chromospheric flare velocity field

The asymmetries observed in the line profiles of solar flares can provide important diagnostics of the properties and dynamics of the flaring atmosphere. In this paper the evolution of the Halpha and Ca II 8542 {\AA} lines are studied using high spatial, temporal and spectral resolution ground-based observations of an M1.1 flare obtained with the Swedish 1-m Solar Telescope. The temporal evolution of the Halpha line profiles from the flare kernel shows excess emission in the red wing (red asymmetry) before flare maximum, and excess in the blue wing (blue asymmetry) after maximum. However, the Ca II 8542 {\AA} line does not follow the same pattern, showing only a weak red asymmetry during the flare. RADYN simulations are used to synthesise spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. We show that the red asymmetry observed in Halpha is not necessarily associated with plasma downflows, and the blue asymmetry may not be related to plasma upflows. Indeed, we conclude that the steep velocity gradients in the flaring chromosphere modifies the wavelength of the central reversal in the Halpha line profile. The shift in the wavelength of maximum opacity to shorter and longer wavelengths generates the red and blue asymmetries, respectively.

DATA-DRIVEN RADIATIVE HYDRODYNAMIC MODELING OF THE 2014 MARCH 29 X1.0 SOLAR FLARE

Spectroscopic observations of solar flares provide critical diagnostics of the physical conditions in the flaring atmosphere. Some key features in observed spectra have not yet been accounted for in existing flare models. Here we report a data-driven simulation of the well-observed X1.0 flare on 2014 March 29 that can reconcile some well-known spectral discrepancies. We analyzed spectra of the flaring region from the Interface Region Imaging Spectrograph (IRIS) in MgII h&k, the Interferometric BIdimensional Spectropolarimeter at the Dunn Solar Telescope (DST/IBIS) in H

Simulations of the Mg II k and Ca II 8542 lines from an Alfvén wave-heated flare chromosphere

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