35 research outputs found
In orbit degradation of EUV optical components in the wavelength range 10-40 nm AO 138-3
A complement of EUV optical components, including mirrors and thin film filters, was flown as part of the Long Duration Exposure Facility (LDEF) AO 138-3. The most original amongst these components were multilayered interference reflectors for the 10-40 nm wavelength range. Very moderate degradation was observed for those components which were exposed to the sun. The degradation is compatible with the deposition of a few nanometers of absorbing material on the surface of the samples
EIT: Solar corona synoptic observations from SOHO with an Extreme-ultraviolet Imaging Telescope
The Extreme-ultraviolet Imaging Telescope (EIT) of SOHO (solar and heliospheric observatory) will provide full disk images in emission lines formed at temperatures that map solar structures ranging from the chromospheric network to the hot magnetically confined plasma in the corona. Images in four narrow bandpasses will be obtained using normal incidence multilayered optics deposited on quadrants of a Ritchey-Chretien telescope. The EIT is capable of providing a uniform one arc second resolution over its entire 50 by 50 arc min field of view. Data from the EIT will be extremely valuable for identifying and interpreting the spatial and temperature fine structures of the solar atmosphere. Temporal analysis will provide information on the stability of these structures and identify dynamical processes. EIT images, issued daily, will provide the global corona context for aid in unifying the investigations and in forming the observing plans for SOHO coronal instruments
Flows in the solar atmosphere due to the eruptions on the 15th July, 2002
<p>Which kind of flows are present during flares? Are they compatible with the present understanding of energy release and which model best describes the observations? We analyze successive flare events in order to answer these questions. The flares were observed in the magnetically complex NOAA active region (AR) 10030 on 15 July 2002. One of them is of GOES X-class. The description of these flares and how they relate to the break-out model is presented in Gary & Moore (2004). The Coronal Diagnostic Spectrometer on board SOHO observed this active region for around 14 h. The observed emission lines provided data from the transition region to the corona with a field of view covering more than half of the active region. In this paper we analyse the spatially resolved flows seen in the atmosphere from the preflare to the flare stages. We find evidence for evaporation occurring before the impulsive phase. During the main phase, the ongoing magnetic reconnection is demonstrated by upflows located at the edges of the flare loops (while downflows are found in the flare loops themselves). We also report the impact of a filament eruption on the atmosphere, with flows up to 300 km s<sup>-1</sup> observed at transition-region temperatures in regions well away from the location of the pre-eruptive filament. Our results are consistent with the predictions of the break out model before the impulsive phase of the flare; while, as the flare progresses, the directions of the flows are consistent with flare models invoking evaporation followed by cooling and downward plasma motions in the flare loops.</p>
Parameters of the Magnetic Flux inside Coronal Holes
Parameters of magnetic flux distribution inside low-latitude coronal holes
(CHs) were analyzed. A statistical study of 44 CHs based on Solar and
Heliospheric Observatory (SOHO)/MDI full disk magnetograms and SOHO/EIT 284\AA
images showed that the density of the net magnetic flux, , does
not correlate with the associated solar wind speeds, . Both the area and
net flux of CHs correlate with the solar wind speed and the corresponding
spatial Pearson correlation coefficients are 0.75 and 0.71, respectively. A
possible explanation for the low correlation between and
is proposed. The observed non-correlation might be rooted in the structural
complexity of the magnetic field. As a measure of complexity of the magnetic
field, the filling factor, , was calculated as a function of spatial
scales. In CHs, was found to be nearly constant at scales above 2 Mm,
which indicates a monofractal structural organization and smooth temporal
evolution. The magnitude of the filling factor is 0.04 from the Hinode SOT/SP
data and 0.07 from the MDI/HR data. The Hinode data show that at scales smaller
than 2 Mm, the filling factor decreases rapidly, which means a mutlifractal
structure and highly intermittent, burst-like energy release regime. The
absence of necessary complexity in CH magnetic fields at scales above 2 Mm
seems to be the most plausible reason why the net magnetic flux density does
not seem to be related to the solar wind speed: the energy release dynamics,
needed for solar wind acceleration, appears to occur at small scales below 1
Mm.Comment: 6 figures, approximately 23 pages. Accepted in Solar Physic
Multi-wavelength observations of an unusual impulsive flare associated with CME
We present the results of a detailed analysis of multi-wavelength
observations of a very impulsive solar flare 1B/M6.7, which occurred on 10
March, 2001 in NOAA AR 9368 (N27 W42). The observations show that the flare is
very impulsive with very hard spectrum in HXR that reveal non-thermal emission
was most dominant. On the other hand this flare also produced type II radio
burst and coronal mass ejections (CME), which are not general characteristics
for impulsive flares. In H we observed the bright mass ejecta (BME)
followed by drak mass ejecta (DME). Based on the consistence of the onset times
and direction of BME and CME, we conclude that these two phenomena are closely
associated. It is inferred that the energy build-up took place due to
photospheric reconnection between emerging positive parasitic polarity and
predominant negative polarity, which resulted as a consequence of flux
cancellation. The shear increased to due to further emergence of
positive parasitic polarity causing strongly enhanced cancellation of flux. It
appears that such enhanced magnetic flux cancellation in a strongly sheared
region triggered the impulsive flare.Comment: 14 pages, 8 Figures, Accepted for the publication in Solar Physic
Coronal Field Opens at Lower Height During the Solar Cycles 22 and 23 Minimum Periods: IMF Comparison Suggests the Source Surface Should Be Lowered
Coronal Dimmings and Energetic CMEs
We have analyzed the coronal dimmings for seven fast (> 600 km/s) coronal mass ejections (CMEs) occurring between 23 April and 9 May which were associated with flares from NOAA active region (AR) 8210. These dimming regions were identified by their strong depletion in coronal emission within a half hour of the estimated time of CME lift-off. They included areas which were as dark as quiescent coronal holes as well as other regions with weaker brightness depletions. We found that the extended dimming areas in these events generally mapped out the apparent "footprint" of the CME. In two of the seven cases, a pair of dimmings were more or less symmetrically positioned north and south of the flare site. In the five remaining cases, the dimmings were most prominent to the north of AR 8210 (approximately S15 latitude) and extended well north of the solar equator, consistent with the locations of the CMEs. We discuss the implications of these results for the sigmoid/double dimming/flux rope model of CMEs
The Preflight Photometric Calibration of the Extreme-Ultraviolet Imaging Telescope EIT
This paper presents the preflight photometric calibration of the Extreme-ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO). The EIT consists of a Ritchey-Chretien telescope with multilayer coatings applied to four quadrants of the primary and secondary mirrors, several filters and a backside-thinned CCD detector. The quadrants of the EIT optics were used to observe the Sun in 4 wavelength bands that peak near 171, 195, 284, and 304 . Before the launch of SOHO, the EIT mirror reflectivities, the filter transmissivities and the CCD quantum efficiency were measured and these values are described here. The instrumental throughput in terms of an effective area is presented for each of the various mirror quadrant and filter wheel combinations. The response to a coronal plasma as a function of temperature is also determined and the expected count rates are compared to the count rates observed in a coronal hole, the quiet Sun and an active region