Andreas Magun

The Fully Polarimetric Imaging Radiometer SPIRA at 91 GHz

The Scanning Polarimetric Imaging Radiometer (SPIRA) is a versatile fully polarimetric imager operating at 91 GHz. It is designed for measurements of polarimetric signatures of the Earths surface and man-made objects. SPIRA combines a method for the measurement of the complete polarization state with a relatively fast high-resolution imager, which is suitable for a range of applications. The instrument measures all four Stokes parameters simultaneously and delivers images by mechanically scanning the scene with an elevation over azimuth scanner and an offset parabolic antenna. A two-channel heterodyne receiver is used for the reception of polarized radiation. The Stokes parameters are obtained by correlating two linear orthogonal-polarization components in a broadband analog adding correlator. In this paper, we present the design of the instrument and analyze its radiometric and polarimetric characteristics. The polarimetric calibration and a method for the characterization of the polarimetric calibration device are described. First polarimetric measurements are presented and discussed.

A multibeam antenna for solar MM-wave burst observations with high spatial and temporal resolution

In this paper a new method for the determination of the position of microwave burst sources on the Sun, its implementation and first observational results, are presented. The 13.7 m antenna at Itapetinga with a five-channel receiver operating at 48 GHz and with a time resolution of 1 ms is used. Five horn antennas clustered around the focus of the Cassegrain reflector provide 5 beams diverging by about 2′. This configuration allows the observation of different parts of an active region and the determination of the center of the burst position with an accuracy of 5″ to 20″ depending on the angular distance relative to the antenna axis. The field of view is ≈ 2′ by ≈ 4′. The time resolution of 1 ms is suitable to search for fast structures at 48 GHz. A total bandwidth of 400 MHz is used in order to achieve a sensitivity of 0.04 s.f.u. sufficient for the detection of weak bursts. First observational results of the flare on May 11, 1991 show a well-located source position during all stages.

A method for arc-second determination of solar burst emission centers with high time resolution and sensitivity at 48 GHz

A 48 GHz five-radiometer front end was installed at the Cassegrain focus of the 13.7-m Itapetinga antenna for the observation of solar bursts. The system works with five beam patterns partly overlapping. The five antenna temperatures are recorded with a temporal resolution of 1 millisecond, including time and antenna position. The ratios of the incoming antenna signals are used to determine the centroid of burst emission. Its coordinates are determined from groups of three receivers by using a least-square fit. In favourable observing conditions we obtain an angular accuracy of about 2 arc sec (r.m.s.), with a time resolution of 1 ms and a sensitivity of 0.05 s.f.u. The accuracy of the antenna tracking, the absolute pointing and the quality of radio seeing at Itapetinga are discussed. A preliminary analysis of an impulsive solar burst event is used to illustrate the capabilities of the method described here.

Spatial positions of fast-time structures of a solar burst observed at 48 GHz

The impulsive solar burst of October 28, 1992 showed temporal and spatial fine structures that were observed at 48 GHz with the multi-beam antenna of the Itapetinga Radio Observatory. The relative positions of burst centroids were determined with a spatial accuracy of 2″, with a temporal resolution of 1 millisecond. The burst intensity time profile shows fast pulses of about one second duration, superimposed by subsecond time structures. The spatial analysis of the fast pulses suggests that the emission originated from distinct locations, separated by about 5″. Our results favour the idea that impulsive solar bursts are a superposition of small elementary events spread both in time and space, probably resulting from discontinuous energy release processes.

On the relation between the peak frequency and the corresponding rise time of solar microwave impulsive bursts and the height dependence of magnetic fields

In the present paper we report the results of a correlation analysis for 57 microwave impulsive bursts observed at six frequencies in which we have obtained a regression line between the peak frequency and the corresponding rise time of microwave impulsive bursts: {ie361-01} (with a correlation coefficient of - 0.43). This can be explained in the frame of a thermal model. The magnetic field decrease with height has to be much slower than in a dipole field in order to explain the weak dependence of fp on tr. This decrease of magnetic field with height in burst sources is based on the relationship between fp and tr found by assuming a thermal flare model with a collisionless conduction front.

Great Microwave Bursts and hard X-rays from solar flares

The microwave and hard X-ray characteristics of 13 solar flares that produced microwave fluxes greater than 500 solar flux units have been analyzed. These Great Microwave Bursts were observed in the frequency range from 3 to 35 GHz at Bern, and simultaneous hard X-ray observations were made in the energy range from 30 to 500 keV with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission spacecraft. The principal aim of this analysis is to determine whether or not the same distribution of energetic electrons can explain both emissions. The temporal and spectral behaviors of the microwaves as a function of frequency and the X-rays as a function of energy were tested for correlations, with results suggesting that optically thick microwave emission, at a frequency near the peak frequency, originates in the same electron population that produces the hard X-rays. The microwave emission at lower frequencies, however, is poorly correlated with emission at the frequency which appears to characterize this common source. A single-temperature and a multitemperature model were tested for consistency with the coincident X-ray and microwave spectra at microwave burst maximum. Four events are inconsistent with both of the models tested, and neither of the models attempts to explain the high-frequency part of the microwave spectrum. A source area derived on the basis of the single-temperature model agrees to within the uncertainties with the observed area of the one burst for which spatially resolved X-ray images are available.

Fully-polarimetric passive scanning imager at millimeter wavelengths

In this paper we present SPIRA (Scanning Polarimetric Imaging Radiometer), a fully polarimetric and transportable imager at 91 GHz that is being developed at IAP. It will be used for measuring signatures of the Earth surface in all four Stokes parameters and can be transported and put into an imaging-ready status by two persons within hours. The mechanical imager scans in azimuth and elevation whereby an image of 120×120 pixels (field of view 30°× 30°) is acquired within less than 600 s. The instrument is controlled by a PC under Linux OS, achieving a nearly real-time performance by using a specially designed kernel module. The brightness temperature distribution of a scene is sampled with a 90° offset antenna at an angular resolution of 0.5°. The polarized signal is separated by an orthomode transducer into two orthogonal components and then down converted to an intermediate frequency band between 2 and 4 GHz by two sub-harmonic mixers that are pumped by a common local oscillator. The four Stokes parameters are extracted with an analog broadband correlator, consisting of a hybrid network and quadratic detectors. Raw images in the four Stokes parameters are immediately displayed and stored for postprocessing. Calibration is performed by adding noise from a switchable noise diode in each channel and by an external load at ambient temperature. Polarimetric calibration of the whole system is done by using coherent and thermal polarization generators.

First retrieval of vertical profiles of turbulence characteristics and horizontal wind velocity from solar transmission measurements at 212 and 405 GHz

We report on the investigation and successful application of vertical profiling of the structure parameter C2m and of the outer scale L0 of absorption fluctuations and of the horizontal wind velocity (vector) during daytime by the analysis of solar transmission measurements. The method is relatively simple and straightforward so that the presented (or a similar) technique could be used in the routine remote sensing of daytime C2m, L0, and wind profiles. It requires multiple beams pointing in different directions at the Sun. The retrieved profiles are consistent with the current knowledge of atmospheric physics. Simultaneous in situ wind velocity measurements agree with the retrieved wind velocity in the lowest 100 m above ground within the measurement uncertainties of less than +/-2 m/s. The derived values of C2m at 200 m above ground are in good agreement (within a factor of 1.5) with the findings of an earlier investigation at the same test site. Finally, it is shown that irradiance fluctuations of millimeter and submillimeter waves are dominantly affected by humidity fluctuations, even at a dry and elevated site.

Design and simulations of a nulling interferometer using a single Martin-Puplett interferometer for double-sideband operation

We propose a new quasi-optical setup for a nulling interferometer at short millimeter and submillimeter wavelengths employing a single Martin-Puplett interferometer (MPI) for sideband separation. The new design allows the simultaneous use of both sidebands, thus avoiding the loss of sensitivity from conventional sideband filters. Although intended for solar flare observations, the quasi-optical setup can be applied to cancel any extended circular source, while at the same time preserving the full sensitivity toward a point-like source on or near the extended one. Numerical simulations show that the sinusoidal transmission characteristic of the MPI limits the usable relative bandwidth to one-third of the IF. Compared to an instrument employing lossless sideband filters, an increase of the response to a point-like continuum source by a factor of /spl sim/1.6-2 is expected, depending on the source position and relative bandwidth.