Production of Energy-dependent Time Delays in Impulsive Solar Flare Hard X-Ray Emission by Short-Duration Spectral Index Variations

Cross-correlation techniques have been used recently to study the relative timing of solar flare hard X-ray emission at different energies. These studies find that for the majority of the impulsive flares observed with BATSE there is a systematic time delay of a few tens of milliseconds between low (approximate to 50 keV) and higher energy emission (approximate to 100 keV). These time delays have been interpreted as energy-dependent time-of-flight differences for electron propagation from the corona, where they are accelerated, to the chromosphere, where the bulk of the hard X-rays are emitted. We show in this paper that crosscorrelation methods fail if the spectral index of the flare is not constant. BATSE channel ratios typically display variations of factors of 2 to 5 over time intervals as short as a few seconds. Using simulated and observed data, we demonstrate that cross-correlating energy channels with identical timing characteristics, but with variations in the amplitudes of one or a small number of relatively strong emission spikes, produces asymmetric time delays of either sign. The reported time delays are therefore largely due to spectral index variations and are not signatures of time-of-flight effects

Mechanisms for the Origin of Turbulence in Non-Star-forming Clouds: The Translucent Cloud MBM 40

We present a multiline, high spatial and velocity resolution CO, H I, and IRAS 100 μm study of the high-latitude, low-mass, non-star-forming, translucent molecular cloud MBM 40. The cloud mass is distributed into two ridges, or filaments, that form a hairpin structure. Velocity channel maps indicate a highly ordered flow in the molecular gas, with the northeastern part of the filament moving away from and the southwestern filament moving toward the observer relative to the mean cloud radial velocity. Significant changes in emissivity occur over 0.03 pc, indicating large transverse density gradients along the ridges. However, the velocity field appears to be continuous, showing no evidence for shock compression. The neutral hydrogen at the same velocity envelops the molecular gas but shows a decrease along the hairpin, indicating that the atomic hydrogen has converted to H2; the strongest 100 μm emission coincides with the CO, not the H I, emission peak. These results indicate that MBM 40 is condensing out of a larger scale flow and is structured by thermal instability and shear flow turbulence. This externally driven turbulence does not produce large compression and may explain why gravitational collapse and star formation do not occur in MBM 40

Stochastic Electron Acceleration by Cascading Fast Mode Waves in Impulsive Solar Flares

We present a model for the acceleration of electrons from thermal to ultrarelativistic energies during an energy release fragment in an impulsive solar flare. Long-wavelength low-amplitude fast mode waves are assumed to be generated during the initial flare energy release (by, for example, large-scale restructuring of the magnetic field). These waves nonlinearly cascade to higher wavenumbers and eventually reach the dissipation range, whereupon they are transit-time damped by electrons in the tail of the thermal distribution. The electrons, in turn, are energized out of the tail and into substantially higher energies. We find that for turbulence energy densities much smaller than the ambient magnetic field energy density and comparable to the thermal particle energy density, and for a wide range of initial wavelengths, a sufficient number of electrons are accelerated to hard X-ray-producing energies on observed timescales. We suggest that MHD turbulence unifies electron and proton acceleration in impulsive solar flares, since a preceding study established that a second MHD mode (the shear Alfven wave) preferentially accelerates protons from thermal to gamma-ray line-producing energies

A New System of Parallel Isolated Nonthermal Filaments near the Galactic Center: Evidence for a Local Magnetic Field Gradient

We report the discovery of a system of isolated nonthermal filaments approximately 0fdg5 northwest (75 pc in projection) of Sgr A. Unlike other isolated nonthermal filaments which show subfilamentation, braiding of subfilaments, and flaring at their ends, these filaments are simple linear structures and more closely resemble the parallel bundled filaments in the Galactic center radio arc. However, the most unusual feature of these filaments is that the 20/90 cm spectral index uniformly decreases as a function of length, in contrast to all other nonthermal filaments in the Galactic center. This spectral gradient may not be due to simple particle aging but could be explained by a curved electron energy spectrum embedded in a diverging magnetic field. If so, the scale of the magnetic gradient is not consistent with a large scale magnetic field centered on Sgr A* suggesting that this filament system is tracing a local magnetic field

The Wall of Reconnection-Driven Magnetohydrodynamic Turbulence in a Large Solar-Flare

LaRosa & Moore (1993) recently proposed that the bulk dissipation of magnetic field that is required for the electron energization in the explosive phase of solar flares occurs in a \u27\u27fat current sheet,\u27\u27 a wall of cascading MHD turbulence sustained by highly disordered driven reconnection of opposing magnetic fields impacting at a turbulent boundary layer. In two-ribbon eruptive flares, this turbulent reconnection wall is supposed to develop at the usual reconnection site in the standard model for these flares; that is, the reconnection wall stands in the vertical magnetic rent made by the eruption of the sheared core of the preflare closed bipolar field. Here, we use the well-observed great two-ribbon eruptive flare of 1984 April 24/25 to assess the feasibility of both (1) the standard model for the overall three-dimensional form and action of the magnetic field and (2) the turbulent reconnection wall within it. The observed aspects of this flare that we use are (1) the preflare photospheric vector magnetic field; (2) the occurrence of a flare spray and the size, form, and spreading of the chromospheric flare ribbons; and (3) the rate of production of hard (greater than or similar to 25 keV) X-rays in the explosive phase of the flare. We find (1) that the morphology of this flare closely matched that of the standard model; (2) the preflare sheared core field had enough nonpotential magnetic energy to power the flare; (3) the model turbulent wall required to achieve the flare\u27s peak dissipative power easily fit within the overall span of the flaring magnetic field; (4) this wall was thick enough to have turbulent eddies large enough (diameters similar to 10(8) cm) to produce the similar to 10(26) ergs energy release fragments typically observed in the explosive phase of flares; (5) the aspect ratio (thickness/vertical extent) of the turbulent reconnection wall was in the 0.1-1 range expected by (Packer 1973). We therefore conclude that the viability of our version of the standard model (i.e., having the magnetic field dissipation occur in our turbulent reconnection wall) is well confirmed by this typical great two-ribbon eruptive flare

New Promise for Electron Bulk Energization in Solar Flares: Preferential Fermi Acceleration of Electrons over Protons in Reconnection-driven Magnetohydrodynamic Turbulence

The hard X-ray luminosity of impulsive solar flares indicates that electrons in the low corona are bulk energized to energies of order 25 keV. LaRosa & Moore pointed out that the required bulk energization could be produced by cascading MHD turbulence generated by Alfvénic outflows from sites of strongly driven reconnection. LaRosa, Moore, & Shore proposed that the compressive component of the cascading turbulence dissipates into the electrons via Fermi acceleration. However, for this to be a viable electron bulk energization mechanism, the rate of proton energization by the same turbulence cannot exceed the electron energization rate. In this paper we estimate the relative efficiency of electron and proton Fermi acceleration in the compressive MHD turbulence expected in the reconnection outflows in impulsive solar flares. We find that the protons pose no threat to the electron energization. Particles extract energy from the MHD turbulence by mirroring on magnetic compressions moving along the magnetic field at the Alfvén speed. The mirroring rate, and hence the energization rate, is a sensitive function of the particle velocity distribution. In particular, there is a lower speed limit Vmin ≍ VA, below which the pitch-angle distribution of the particles is so highly collapsed to the magnetic field in the frame of the magnetic compressions that there is no mirroring and hence no Fermi acceleration. For coronal conditions, the proton thermal speed is much less than the Alfvén speed and proton Fermi acceleration is negligible. In contrast, nearly all of the electrons are super-Alfvénic, so their pitch-angle distribution is nearly isotropic in the frame of the magnetic compressions. Consequently, the electrons are so vigorously mirrored that they are Fermi accelerated to hard X-ray energies in a few tenths of a second by the magnetic compressions on scales of 105-103 cm in the cascading MHD turbulence. We conclude that dissipation of reconnection-generated MHD turbulence by electron Fermi acceleration plausibly accounts for the electron bulk energization in solar flares

The Strength and Structure of the Galactic Center Magnetic Field

This paper summarizes recently obtained, strong evidence for a weak global field in the Galactic center (GC): the existence of a large-scale region of diffuse, low-frequency, nonthermal emission coincident with the central molecular zone. The overall energetics of this emission, considered along with constraints on GC cosmic ray energy density and diffusion, indicate clearly that the magnetic field pervading this region is ∼ 10 μG. For completeness, additional points on the orientation of the GC nonthermal filaments, rotation measures of extragalactic sources seen through the GC, and comparison with other normal spiral galaxies are also reviewed

The Galactic Center Nonthermal Filaments: Recent Observations and Theory

The large-scale topology and strength of the Galactic Center magnetic field have been inferred from radio imaging of the nonthermal filaments (NTFs). These objects, which seem to be unique to the Galactic center, are defined by extreme aspect ratios and a high degree of polarization. Recent high resolution, wide-field VLA imaging of the GC at 90 cm has revealed new candidate NTFs with a wide range of orientations relative to the Galactic plane. We present follow up 6 cm polarization observations of 6 of these candidates and confirm 4 as new NTFs. Together the new 90 and 6 cm results complicate the previous picture of largely perpendicular filaments that trace a globally ordered magnetic field. NTF observations in general do not rule out any particular models for the origin of the NTFs. Hence we explore the idea that the NTFs are local, individual structures: magnetic wakes generated through the interaction of molecular clouds with a Galactic Center wind. Numerical simulations of the evolution of a magnetized wake will be discussed and compared with NTF observations

Evidence of a Weak Galactic Center Magnetic Field from Diffuse Low-Frequency Nonthermal Radio Emission

New low-frequency 74 and 330 MHz observations of the Galactic center (GC) region reveal the presence of a large-scale (6° × 2°) diffuse source of nonthermal synchrotron emission. A minimum-energy analysis of this emission yields a total energy of ~(phi4/7f3/7) × 1052 ergs and a magnetic field strength of ~6(phi/f)2/7 μG (where phi is the proton to electron energy ratio and f is the filling factor of the synchrotron emitting gas). The equipartition particle energy density is 1.2(phi/f)2/7 eV cm-3, a value consistent with cosmic-ray data. However, the derived magnetic field is several orders of magnitude below the 1 mG field commonly invoked for the GC. With this field the source can be maintained with the supernova rate inferred from the GC star formation. Furthermore, a strong magnetic field implies an abnormally low GC cosmic-ray energy density. We conclude that the mean magnetic field in the GC region must be weak, of order 10 μG (at least on size scales gtrsim125\u27\u27)