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.

Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of 10 fT Hz(-1/2), limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be 2x10(-18) T Hz(-1/2).

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 non-thermal electrons injected at the apex of a one-dimensional closed coronal loop, and include heating from thermal soft Xray, 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 upon previous simulations by incorporating more realistic non-thermal electron beam models and includes a more rigorous model of thermal XEUV heating. We find XEUV backwarming 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 non-existent. During the explosive phase, synthetic profiles for lines formed in the upper chromosphere and transition region show blue shifts corresponding to a plasma velocity of ∼120 km s −1 , and lines formed in the lower chromosphere show red shifts of ∼40 km s −1 .

We present a unified computational framework that 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.