Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSolar coronal observations at high frequencies
97
0
0.0
(
0
)
Added by
Athanassios Katsiyannis
Publication date
2001
fields
Physics
and research's language is
English
Authors
A.C. Katsiyannis
Ask ChatGPT about the research
No Arabic abstract
The Solar Eclipse Coronal Imaging System (SECIS) is a simple and extremely fast, high-resolution imaging instrument designed for studies of the solar corona. Light from the corona (during, for example, a total solar eclipse) is reflected off a heliostat and passes via a Schmidt-Cassegrain telescope and beam splitter to two CCD cameras capable of imaging at 60 frames a second. The cameras are attached via SCSI connections to a purpose-built PC that acts as the data acquisition and storage system. Each optical channel has a different filter allowing observations of the same events in both white light and in the green line (Fe XIV at 5303 A). Wavelet analysis of the stabilized images has revealed high frequency oscillations which may make a significant contribution on the coronal heating process. In this presentation we give an outline of the instrument and its future development.
rate research
Read More
Coronal jets represent important manifestations of ubiquitous solar transients, which may be the source of significant mass and energy input to the upper solar atmosphere and the solar wind. While the energy involved in a jet-like event is smaller than that of nominal solar flares and coronal mass ejections (CMEs), jets share many common properties with these phenomena, in particular, the explosive magnetically driven dynamics. Studies of jets could, therefore, provide critical insight for understanding the larger, more complex drivers of the solar activity. On the other side of the size-spectrum, the study of jets could also supply important clues on the physics of transients close or at the limit of the current spatial resolution such as spicules. Furthermore, jet phenomena may hint to basic process for heating the corona and accelerating the solar wind; consequently their study gives us the opportunity to attack a broad range of solar-heliospheric problems.
The weak, turbulent magnetic fields that supposedly permeate most of the solar photosphere are difficult to observe, because the Zeeman effect is virtually blind to them. The Hanle effect, acting on the scattering polarization in suitable lines, can in principle be used as a diagnostic for these fields. However, the prediction that the majority of the weak, turbulent field resides in intergranular lanes also poses significant challenges to scattering polarization observations because high spatial resolution is usually difficult to attain. We aim to measure the difference in scattering polarization between granules and intergranules. We present the respective center-to-limb variations, which may serve as input for future models. We perform full Stokes filter polarimetry at different solar limb positions with the CN band filter of the Hinode-SOT Broadband Filter Imager, which represents the first scattering polarization observations with sufficient spatial resolution to discern the granulation. Hinode-SOT offers unprecedented spatial resolution in combination with high polarimetric sensitivity. The CN band is known to have a significant scattering polarization signal, and is sensitive to the Hanle effect. We extend the instrumental polarization calibration routine to the observing wavelength, and correct for various systematic effects. The scattering polarization for granules (i.e., regions brighter than the median intensity of non-magnetic pixels) is significantly larger than for intergranules. We derive that the intergranules (i.e., the remaining non-magnetic pixels) exhibit (9.8 pm 3.0)% less scattering polarization for 0.2<u<0.3, although systematic effects cannot be completely excluded. These observations constrain MHD models in combination with (polarized) radiative transfer in terms of CN band line formation, radiation anisotropy, and magnetic fields.
Coronal rain is the well-known phenomenon in which hot plasma high in the Suns corona undergoes rapid cooling (from > 10^6 K to < 10^4 K), condenses, and falls to the surface. Coronal rain appears frequently in active region coronal loops and is very common in post-flare loops. This Letter presents discovery observations, which show that coronal rain is ubiquitous in the embedded bipole very near a coronal hole boundary. Our observed structures formed when the photospheric decay of active region leading sunspots resulted in a large parasitic polarity embedded in a background unipolar region. We observe coronal rain to appear within the legs of closed loops well under the fan surface, as well as preferentially near separatrices of the resulting coronal topology: the spine lines, null point, and fan surface. We analyze 3 events using SDO Atmospheric Imaging Assembly (AIA) observations in the 304, 171, and 211 {/AA} channels, as well as SDO Helioseismic and Magnetic Imager (HMI) magnetograms. The frequency of rain formation and the ease with which it is observed strongly suggests that this phenomenon is generally present in null-point topologies of this size scale. We argue that these rain events could be explained by the classic process of thermal nonequilibrium or via interchange reconnection at the null; it is also possible that both mechanisms are present. Further studies with higher spatial resolution data and MHD simulations will be required to determine the exact mechanism(s).
The central region of the Galaxy has been observed at 580, 620 and 1010 MHz with the Giant Metrewave Radio Telescope (GMRT). We detect emission from Sgr-A*, the compact object at the dynamical centre of the Galaxy, and estimate its flux density at 620 MHz to be 0.5 +/- 0.1 Jy. This is the first detection of Sgr A* below 1 GHz (Roy & Rao 2002, 2003), which along with a possible detection at 330 MHz (Nord et al. 2004) provides its spectrum below 1 GHz. Comparison of the 620 MHz map with maps made at other frequencies indicates that most parts of the Sgr A West HII region have optical depth 2. However, Sgr A*, which is seen in the same region in projection, shows a slightly inverted spectral index between 1010 MHz and 620 MHz. This is consistent with its high frequency spectral index, and indicates that Sgr A* is located in front of the Sgr A West complex, and rules out any low frequency turnover around 1 GHz, as suggested by Davies et al. (1976).
The results of the first observations of Type IV bursts at frequencies 10-30 MHz are presented. These observations were carried out at radio telescopes UTR-2 (Kharkov, Ukraine) and URAN-2 (Poltava, Ukraine) during the period 2003-2006. Detection of Type IV bursts in wide band from 10 to 30MHz with high sensitivity and time resolution allowed to study their properties in details. These bursts have fluxes 10-2000s.f.u. at maximum phase. Their durations are about 1-2 hours and even more. Some of Type IV bursts drift from high to low frequencies with drift rates about 10kHz/s. All observed Type IV bursts have fine structures in the form of sub-bursts with durations from 2s to 20s and frequency drift rates in a majority of 1-2MHz/s. In most cases, sub-bursts with negative drift rates were registered. Sometimes sub-bursts in absorption with durations 10-200s against Type IV burst background have been observed. The Type IV burst observed on July 22, 2004 had zebra structure, in which single zebra stripes had positive, negative and infinite drift rates.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
