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Temperature Isotropization in Solar Flare Plasmas due to the Electron Firehose Instability  

Peter Messmer   Submitted: 2001-12-18 10:41

The isotropization process of a collisionless plasma with an electron temperature anisotropy along an external magnetic field (T_| ^e >> T_perp^e, | and perp with respect to the background magnetic field) and isotropic protons is investigated using a particle-in-cell (PIC) code. Restricting wave growth mainly parallel to the external magnetic field, the isotropization mechanism is identified to be the Electron Firehose Instability (EFI). The free energy in the electrons is first transformed into left-hand circularly polarized transverse low-frequency waves by a non-resonant interaction. Fast electrons can then be scattered towards higher perpendicular velocities by gyroresonance, leading finally to a complete isotropization of the velocity distribution. During this phase of the instability, Langmuir waves are generated which may lead to the emission of radio waves. A large fraction of the protons is resonant with the left-hand polarized electromagnetic waves, creating a proton temperature anisotropy T_|^p < T_perp^p. The parameters of the simulated plasma are chosen compatible to solar flare conditions. The results indicate the significance of this mechanism in the particle acceleration context: The EFI limits the anisotropy of the electron velocity distribution, and thus provides the necessary condition for further acceleration. It enhances the pitch-angle of the electrons and heats the ions.

Authors: Peter Messmer
Projects:

Publication Status: A&A (in press)
Last Modified: 2001-12-18 10:41
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The minimum bandwidth of narrowband spikes in solar flare decimetric radio  

Peter Messmer   Submitted: 1999-12-10 18:00

waveswavesThe minimum and the mean bandwidth of individual narrowband spikes in two events in decimetric radio waves is determined by means of multi-resolution analysis. Spikes of a few tens of millisecond duration occur at decimetric/microwave wavelength in the particle acceleration phase of solar flares. A first method determines the dominant spike bandwidth scale based on their scalegram, the mean squared wavelet coefficient at each frequency scale. This allows to measure the scale bandwidth independently of heuristic spike selection criteria, e.g. manual selection. The major drawback is a low resolution in the bandwidth. To overcome this uncertainty, a feature detection algorithm and a criterion for spike shape in the time-frequency plane is applied to locate the spikes. In that case, the bandwidth is measured by fitting an assumed spike profile into the denoised data.The smallest FWHM bandwidth of spikes was found at 0.17% and 0.41% of the center frequency in the two events. Knowing the shortest relevant bandwidth of spikes, the slope of the Fourier power spectrum of this two events was determined and no resemblance to a Kolmogorov spectrum detected. Additionally the correlation between spike peak flux and bandwidth was examined.

Authors: Messmer, P., Benz, A.O.
Projects:

Publication Status: A&A, 2000, (in press)
Last Modified: 1999-12-10 18:00
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PHOENIX-2: A New Broadband Spectrometer for Decimetric and Microwave Radio Bursts: Acoustic Spectrum Representation  

Peter Messmer   Submitted: 1999-11-15 14:03

A broad-band radio spectrometer measures radio flux density as a function of observing frequency and time. This data should be represented in such a way, that emission patterns, both in frequency and time as well as their dynamics, can be detected and analyzed. The common displaying method plots the color coded flux in a frequency-time plan. This representation is well adapted to the localization and feature detection capabilities of the human visual system. Here we present an acoustic rendering, exploiting the frequency analysis features of hearing.Although most people do not have the absolute pitch, they still can distinguish tones relatively, telling which note is higher or lower. Beside frequency resolution, differences in loudness are distinguishable, though merely on a logarithmic scale. The acoustic representation may therefore be a tool to detect the dynamics of radiation patterns. The above stated deficiencies of hearing prevent the acoustic representation from being an exploratory scientific tool. Nevertheless it might be a supplement in a pedagogical context, where different views of the same phenomenon can deepen the understanding more then accurate numbers.

Authors: Messmer, P., Benz, A.O., Monstein, C.
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Publication Status: Solar Physics 187 (2), Multimedia-Extension (CD-ROM), 1999
Last Modified: 1999-11-15 14:03
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PHOENIX-2: A New Broadband Spectrometer for Decimetric and Microwave Radio Bursts: First Results  

Peter Messmer   Submitted: 1999-11-13 16:00

 Radio Bursts: First Results A broadband radio spectrometer has been put into operation at Bleien, Switzerla- Switzerland, to register the flare emission of the full Sun. In the frequency range of operation, 0.1 to 4.0 GHz, both modes of circular polarization are recorded continuously. The new system, Phoenix-2, has been developed from the experience with the previous Phoenix spectrometer. Improved, computer-controlled focal hardware allows now a complete daily calibration, a more sophisticated calibration procedure, and monitoring of all essential instrumental and environ- environmental parameters. Calibrated data are now usually available the day after observation and are accessible through the Internet. The scientific improvements include a larger frequency range of observation, a larger number of completely recorded events due to full-day registration, more accurate measurem- measurements, particularly in circular polarization, and more reliable operation operation. First observations are presented and quantitative results comparing the calibration with single frequency instruments are reported.

Authors: Messmer, P., Benz, A.O., Monstein, C.
Projects:

Publication Status: Solar Physics 187 (2):335 - 345 (1999)
Last Modified: 1999-11-13 16:00
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Abstracts by Author
Temperature Isotropization in Solar Flare Plasmas due to the Electron Firehose Instability
The minimum bandwidth of narrowband spikes in solar flare decimetric radio
PHOENIX-2: A New Broadband Spectrometer for Decimetric and Microwave Radio Bursts: Acoustic Spectrum Representation
PHOENIX-2: A New Broadband Spectrometer for Decimetric and Microwave Radio Bursts: First Results

Related Pages
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