Polarization of solar active regions at 9.5 mm wavelength

A study of the circular polarization structure of solar active regions has been made from data obtained at 9.5 mm wavelength, using the 85 ft reflector and polarimeter at the Naval Research Laboratory Maryland Point Observatory. The angular resolution of the telescope at this wavelength is 1.6. All important active regions observed at 9.5 mm are bipolar in nature, the degree of polarization is about the same for both right and left circular components and it ranges up to about 4%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates clearly the neutral line between the regions of opposite polarity on magnetograms. Unipolar regions in magnetograms also show up as unipolar regions at 9.5 mm. Magnetic fields as low as 5–10 G on magnetograms manifest as distinctly polarized regions on 9.5 mm maps. A line of zero polarization seems to delineate the extent of absorption features observed at 9.5 mm in coincidence with H dark filaments.     

Transient sonar signal classification using hidden Markov modelsand neural nets

In ocean surveillance, a number of different types of transient signals are observed. These sonar signals are waveforms in one dimension (1-D). The hidden Markov model (HMM) is well suited to classification of 1-D signals such as speech. In HMM methodology, the signal is divided into a sequence of frames, and each frame is represented by a feature vector. This sequence of feature vectors is then modeled by one HMM. Thus, the HMM methodology is highly suitable for classifying the patterns that are made of concatenated sequences of micro patterns. The sonar transient signals often display an evolutionary pattern over the time scale. Following this intuition, the application of HMM's to sonar transient classification is proposed and discussed in this paper. Toward this goal, three different feature vectors based on an autoregressive (AR) model, Fourier power spectra, and wavelet transforms are considered in our work. In our implementation, one HMM is developed for each class of signals. During testing, the signal to be recognized is matched against all models. The best matched model identifies the signal class. The neural net (NN) classifier has been successfully used previously for sonar transient classification. The same set of features as mentioned above is then used with a multilayer perceptron NN classifier. Some experimental results using “DARPA standard data set I” with HMM and MLP-NN classification schemes are presented. A combined NN/HMM classifier is proposed, and its performance is evaluated with respect to individual classifiers     

Seamount subduction and rupture characteristics of the March 11, 2011, Tohoku earthquake

We suggest that the spatial location of the 2011 Tohoku earthquake rupture and slip distribution on it was strongly influenced by the subduction of seamount chains. Subduction of seamounts across the Japan trench caused weak coupling on the plate interface which acted as barriers to the 2011 Tohoku earthquake rupture and thus delimited it.     

Propagation of electrons emitting weak type III bursts in coronal streamers

We report the observations of weak type III bursts at 73.8, 57.5, 50.0, and 38.5 MHz from Clark Lake Radio Observatory on four days and discuss their characteristics. In addition to Clark Lake data, the magnetogram and sunspot/active region data and the coronal streamer data obtained by HAO's Coronagraph/Polarimeter aboard SMM satellite are used to study the location of the burst sources with respect to the coronal streamers emanating from active regions. It is shown that the bursts occur within or close to the edge of dense coronal streamers implying that the coronal streamers contain open magnetic field lines along which the electrons generating the bursts propagate. The positional analysis of the bursts is used to estimate the variation of coronal electron density with radial distance.On leave from the Indian Institute of Astrophysics, Kodaikanal, India.     

Imaging observations of the evolution of meter-decameter burst emission during a major flare

We present the results of a study of the evolution of 3 February, 1986 flare at meter-decameter wavelengths using the two dimensional imaging observations made with the Clark Lake multifrequency radioheliograph. The flare was complex and produced various types of meter-decameter bursts. The preflare activity was observed in the form of type III bursts some tens of minutes prior to the impulsive onset. From the positional analysis of the preflare and impulsive phase type III bursts and other measured characteristics we discuss the characteristics of energy release and possible magnetic field configurations in the vicinity of energy release region. From positional and temporal studies of the flare continuum and type II burst in relation to the microwave and hard X-ray emissions, we discuss the possible magnetic field structures in which the accelerated particles are confined or along which they propagate. We develop a schematic model of the flaring region based upon our study.On leave from Indian Institute of Astrophysics, Kodaikanal, India.     

A decameter type II burst associated with a behind-the-limb flare

The characteristics of a decameter type II burst associated with a possible behind-the-limb flare are discussed. The burst source had an unusually high velocity. Assuming that the disturbance propagated as an MHD wave, the magnetic field strength at the 40 MHz plasma level is estimated to be 5.6 gauss.     

Radioheliograph and white-light coronagraph studies of a coronal mass ejection event

We analyze the radioheliograph and SMM-C/P observations of 1986 November 3 mass ejection event. The metric radio emissions are the only detected activity associated with the mass ejection, but are adequate to study the evolution of the event. The start time of the ejection seems to precede a possible flare behind the limb indicated by the early type III bursts. We discuss the physical relation between various types of bursts and the CME. We interpret moving type IV bursts as a plasma emission process. It is also shown using white-light coronagraph data that the density in the source region of the moving type IV is sufficient to support second harmonic plasma emission at the observed frequency of 50 MHz.     

Decameter type IV bursts associated with coronal transients

Recent observations of neutral line absorbing features in the solar atmosphere may give an important clue to the mechanism whereby both type III solar radiobursts and solar flares are triggered. It is suggested that as new satellite magnetic flux emerges at the edge of an active region in an area of opposite polarity a neutral sheet builds up between the new and old flux. When the sheet has a length of about a megametre its thermal insulation from the surrounding plasma is effective enough for a thermal instability to occur. The resulting compression and inflow of plasma is observed in H on the disc as a neutral line absorbing feature. Furthermore, the electric field of the accompanying collisionless tearing mode instability in a thin slab near the centre of the sheet exceeds the runaway field; it may therefore accelerate electrons to high enough energies to produce the type III burst which usually occurs at the same time as the absorbing feature. Perhaps the flare which sometimes ensues is triggered when the quasi-equilibrium state is destroyed by the development of turbulence in the neutral sheet.     

An eruptive prominence and associated cm-mm emission outside the solar limb

We present radio maps at 22 and 44 GHz which show the emission before and after the eruption of a quiescent prominence located at the west limb. The observed radiation following the eruption is not consistent with thermal bremsstrahlung mechanism. It can be interpreted as due to gyrosynchrotron emission of nonthermal electrons. Our observations appear to be similar to the microwave radiation observed in post-flare loops; this radiation is due to nonthermal electrons trapped in the closed magnetic structures formed after the prominence eruption.