The New Multichannel Radiospectrograph ARTEMIS-IV/HECATE, of the University of Athens

We present the new solar radiospectrograph of the University of Athensoperating at the Thermopylae Station since 1996. Observations cover thefrequency range from 110 to 688 MHz. The radiospectrograph has a 7-meterparabolic antenna and two receivers operating in parallel. One is a sweepfrequency receiver and the other a multichannel acousto-optical receiver.The data acquisition system consists of a front-end VME based subsystem anda Sun Sparc-5 workstation connected through Ethernet. The two subsystems areoperated using the VxWorks real-time package. The daily operation is fullyautomated: pointing of the antenna to the sun, starting and stopping theobservations at pre-set times, data acquisition, data compression by`silence suppression', and archiving on DAT tapes. The instrument can beused either by itself to study the onset and evolution of solar radio bursts or in conjunction with other instruments including theNançay Decametric Array and the WIND/WAVES RAD1 and RAD2 low frequencyreceivers to study associated interplanetary phenomena.     

Tracing the Electron Density from the Corona to 1 au

We derive the electron density distribution in the ecliptic plane, from the corona to 1 AU, using observations from 13.8 MHz to a few kHz by the radio experiment WAVES aboard the spacecraft Wind. We concentrate on type III bursts whose trajectories intersect the spacecraft, as determined by the presence of burst-associated Langmuir waves, or by energetic electrons observed by the 3-D Plasma experiment. For these bursts we are able to determine the mode of emission, fundamental or harmonic, the electron density at 1 AU, the distance of emission regions along the spiral, and the time spent by the beams as they proceed from the low corona to 1 AU. For all of the bursts considered, the emission mode at burst onset was the fundamental; by contrast, in deriving many previous models, harmonic emission was assumed.By measuring the onset time of the burst at each frequency we are able to derive an electron density model all along the trajectory of the burst. Our density model, after normalizing the density at 1 AU to be ne(215 R0)=7.2 cm–3 (the average value at the minimum of solar activity when our measurements were made), is ne=3.3×105 r–2+4.1×106 r–4+8.0×107 r–6 cm–3, with r in units of R0. For other densities at 1 AU our result implies that the coefficients in the equation need to be multiplied by n _e (1 AU)/7.2.We compare this with existing models and those derived from direct, in-situ measurements (normalized to the same density at 1 AU) and find that it agrees very well with in-situ measurements and poorly with radio models based on apparent source positions or assumptions of the emission mode. One implication of our results is that isolated type III bursts do not usually propagate in dense regions of the corona and solar wind, as it is still sometimes assumed.     

Dust Detection by the Wave Instrument on STEREO: Nanoparticles Picked up by the Solar Wind?

Solar Physics - The STEREO wave instrument (S/WAVES) has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles...     

STEREO SECCHI and S/WAVES Observations of Spacecraft Debris Caused by Micron-Size Interplanetary Dust Impacts

Early in the STEREO mission observers noted that the white-light instruments of the SECCHI suite were detecting significantly more spacecraft-related “debris” than any previously flown coronagraphic instruments. Comparison of SECCHI “debris storms” with S/WAVES indicates that almost all are coincident with the most intense transient emissions observed by the radio and plasma waves instrument. We believe the debris is endogenous (i.e., from the spacecraft thermal blanketing), and the storms appear to be caused by impacts of large interplanetary dust grains that are detected by S/WAVES. Here we report the observations, compare them to interplanetary dust distributions, and document a reminder for future spacebased coronagraphic instrument builders.

     

Circular Polarization Observed in Interplanetary Type III Radio Storms

We report the detection and analysis of circular polarization in solar type III radio storms at hectometric-to-kilometric wavelengths. We find that a small (usually less than 5%), but statistically significant, degree of circular polarization is present in all interplanetary type III radio storms below 1 MHz. The sense of the polarization, which is right-hand circular for some storms and left-hand circular for others, is maintained for the entire duration of the type III storm (usually many days). For a given storm, the degree of circular polarization peaks near central meridian crossing of the associated active region. At a given time, the degree of circular polarization is found to generally vary as the logarithm of the observing frequency. The radiation characteristics, including the polarization, for one interplanetary type III storm exhibits an unusual 1.6 hour oscillation. Based on the standard plasma emission theory of type III radiation, we discuss the implications of these observations for the magnitude and radial dependence of the solar magnetic field above active regions on the Sun.     

Fine Structure of Solar Radio Bursts Observed at Decametric and Hectometric Waves

The analysis of WIND/WAVES RAD2 spectra with fine structure in the form of different fibers in 14 events covering 1997?–?2005 is carried out. A splitting of broad bands of the interplanetary (IP) type II bursts into narrow band fibers of different duration is observed. The instantaneous-frequency bandwidth of fibers is stable: 200?–?300 kHz for slow-drifting fibers in type II bursts, and 700?–?1000 kHz for fast-drifting fibers in type II?+?IV (continuum). Intermediate drift bursts (IDB or fiber bursts) and zebra patterns with variable frequency drift of stripes, typical for the metric range, were not found. Comparison of spectra with the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO C2) images shows a connection of the generation of the fiber structures with the passage of shock fronts through narrow jets in the wake of Coronal Mass Ejections (CME). Therefore the most probable emission mechanism of fibers in IP type II bursts appears to be resonance transition radiation (RTR) of fast particles at the boundary of two media with different refractive indices. The same mechanism is also valid for striae in the type III bursts. Taking into account a high-density contrast in the CME wake and the actually observed small-scale inhomogeneities, the effectiveness of the RTR mechanism in IP space must be considerably higher than in the meter or decimeter wavelengths. For the most part the fibers in the type IV continuum at frequencies of 14?–?8 MHz were seen as the direct expansion of similar fine structure (as fibers or “herringbone” structure) in the decametric range observed with the Nançay and IZMIRAN spectrographs.     

The digital multi-channel radiospectrograph in Nançay

We describe the new Solar Radio Spectrograph which has been operated at the Nançay Radio Astronomy Station since December 1978 for the analog part (which uses photographic film data acquisition) and since July 1979 using digital magnetic recording. This instrument was designed and built by the Space Research Department of the Paris Observatory and covers the range 469–110 MHz.The multichannel receiver yields a high sensitivity, as compared to a sweep-frequency receiver and the frequency windows where external interference is present can be eliminated from the data acquisition.The digital recording leads to convenient intensity calibration procedures and allows a modern data-handling over a large dynamic range: 50 dB with a 11 bit resolution.Intermodulation effects due to non linearities have been kept to a minimum by building the multiplexer as a tree and distributing the amplification along.The time resolution allows the data to be acquired at a rate of 100 samples per second per frequency channel. The frequency resolution can take two values: 120 channels 1 MHz-wide and 100 channels 200 kHz-wide can be positioned anywhere in the range 110–469 MHz.Some observations are shown including type V and type II-like bursts and harmonically related emission in hook structures. Some future plans are briefly mentioned aiming to perform circular polarization measurements in 120 frequency channels and real time data compression.Also at Department of Physics and Electronics, University of Athens, Greece.     

Bastille Day Event: A Radio Perspective

We describe the radio signatures that led up to and concluded the solar eruptive event of 14 July 2000 (Bastille Day Event). These radio signatures provide a means of remotely sensing the associated solar activity and transient phenomena. For many days prior to the Bastille Day Event kilometric Type III radio storm emissions were observed that were presumably associated with the active region NOAA 9077. These storm emissions continued until the X5.7 flare at ∼ 10 UT on 14 July 2000 that characterized the Bastille Day Event, then ceased abruptly. The Bastille Day Event itself produced very intense, complex, long-duration Type III-like radio emissions, which appear to have been associated with electrons generated (accelerated) deep in the solar corona. The coronal mass ejection (CME) associated with the Bastille Day Event generated decametric to kilometric Type II radio emissions as the CME propagated through the solar corona and interplanetary medium. The frequency drift of these Type II radio emissions are related to the dynamics of the propagating CME and indicate that the CME experienced significant deceleration as it propagated from the high corona into the interplanetary medium.     

Comparisons of interplanetary type III storm footpoints with solar features

The trajectories of 38 type III storms in the interplanetary medium have been deduced from ISEE-3 radio observations and extrapolated back to the Sun to determine the Carrington coordinates of their footpoints. The analysis assumes radial motion of the solar wind, and the trajectories are projected radially back toward the surface for the last few solar radii. To identify the storm sources, the footpoints were compared to a variety of solar features: to the large-scale neutral line at the base of the current sheet, to active regions, to the small-scale neutral lines and Hα filaments which trace out active regions, and to coronal holes. Most of the footpoints were found to lie near active regions, in agreement with metric storm locations. There is a weak correlation with Hα filaments, no apparent association with the current sheet, and an anticorrelation with coronal holes. There is a small excess of storms in the leading half of magnetic sectors.     

Farside explorer: unique science from a mission to the farside of the moon

Farside Explorer is a proposed Cosmic Vision medium-size mission to the farside of the Moon consisting of two landers and an instrumented relay satellite. The farside of the Moon is a unique scientific platform in that it is shielded from terrestrial radio-frequency interference, it recorded the primary differentiation and evolution of the Moon, it can be continuously monitored from the Earth–Moon L2 Lagrange point, and there is a complete lack of reflected solar illumination from the Earth. Farside Explorer will exploit these properties and make the first radio-astronomy measurements from the most radio-quiet region of near-Earth space, determine the internal structure and thermal evolution of the Moon, from crust to core, and quantify impact hazards in near-Earth space by the measurement of flashes generated by impact events. The Farside Explorer flight system includes two identical solar-powered landers and a science/telecommunications relay satellite to be placed in a halo orbit about the Earth–Moon L2 Lagrange point. One lander would explore the largest and oldest recognized impact basin in the Solar System— the South Pole–Aitken basin—and the other would investigate the primordial highlands crust. Radio astronomy, geophysical, and geochemical instruments would be deployed on the surface, and the relay satellite would continuously monitor the surface for impact events.