4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
4th LOFAR KSP meeting Potsdam 2010 November 8-9
NRH and LOFAR
Scientific and technical aspects
A. Kerdraon Observatoire de Paris - LESIA - USN
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
• Comparisons / differences
– UV coverage– Software and data formats
• Science requirements
– Solar activity and bursts.– Quiet sun thermal emission.
4th LOFAR KSP meeting Potsdam 2010 November 8-9
Nançay Radioheliograph vs LOFAR
• General characteristics (Lofar is red)
– Frequency range: 150 - 450 MHz ( 30 - 250 MHz)– 648 baselines from 50 to 3200m (25 to 4,800 to > 10000 – Spatial resolution: ~4 to 0.3 arcmin (depending on frequency, declination,
snapshot/synthesis) ( better than 0.2 arcmin at 150 MHz)– Field of view: from 10 to 3 degrees (> 3 degrees at 150 MHz))– Stokes I and V (4 Stokes)– Time resolution: 0.1 to 0.2 typ. ( could be < 0.1s ? Depending on the frequency
range ?)
• Works in progress on NRH:– Realtime computer: will need a > 1 month stop in 2011 (when?)– Correlator: will be changes in 2012. We consider a move to a more standard
data format after. Observations will stop a few month
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH -> LOFAR KSP: Interferometry
• U-V coverage– The solar corona is a broad complex source: u-v min < 30
• Main problem: negative bowl due to poor uv sampling around the origin. Strong impact on quiet Sun TB.
• Different size scales ( from 1 degree to a fraction of arcmin)• A dense coverage is needed at D < 1000m
– Diffusion of radio waves in the corona broadens sources: baselines > 10-20 km may not useful (probably, but…)
• Field of view:– Inverse of UV min -> largest structure observed, > 3 degrees– Primary beam: > 6 degrees -> field of view around the sun.
• CMEs, ejectas, may be observed at very high altitudes• To a lesser extent, type III also
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
4th LOFAR KSP meeting Potsdam 2010 November 8-9
Centre of UV plan
Lofar: orange
NRH= brown
Snapshot uv coverage: at common frequencies, cross-calibration should be easy.
LOFAR alone has not so many visibilities !
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
NRH
Light blue: snapshot coverage
Blue: denser uv coverage
Will LOFAR increase the spatial resolution on snapshots?Is it possible to have more baselines in the core ?Currently, the uv sampling is low at U,V > 1000m
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
NRHLight blue: rotationnal synthesis UV coverage
Dark blue: snapshot UV coverage
With rotationnal synthesis (not shown explicitly, let us imagine a 7h rotation), LOFAR has a much greater UV coverage then NRH -> images of the quiet sun
Again: the snapshot uv coverage is very sparse.
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
• Software aspects:– LOFAR: Casa or any Aips-like software. MS or uvfits
formats:• Lot of work to get 1 image. Solutions may be:
– Write python scripts– Export to Uvfits
• In any case, we have to make utilities to handle large numbers of images.
– NRH: proprietary visiblities files format. May export FITS images files.
• Has a lot of utilities to merge with other solar observations• Cannot export MS or uvfits format (to do ?)• Some capabilities to merge with other radiotelescopes (namely: GMRT)
with an uvfits import.
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH – LOFARSolar activity
• Expected science (from previous NRH observations): – T III bursts (electrons beams)
• In relation ship with flares and CMEs: they seem to be reasonnably connected to Interplanetary medium at 150 MHz
– Low frequency spectras– Direction finding in space– Comparisons with space coronagraphs, B extrapolations etc.
• But positions at higher altitudes may help ( 100 – 30 MHz range). Also an increase in spatial resolution and sensitivity.
• For type IIIs storms,( typ. < 80 MHz), and any particles acceleration high in the corona, everything has to be done
4th LOFAR KSP meeting Potsdam 2010 November 8-9
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH – LOFARSolar activity
• Shocks Type II radio bursts:– Sometimes at meter , often at decameter
• At meter wavelength– Often emission at 2nd or 3rd harmonic at a time.– Seams to be located on the CMEs sides.– May be difficult to compare with decameter, or interplanetary
shocks.
• With LOFAR:– Much more bursts.– More frequencies for a better understanding.– Connection with interplanetary shocks (?)
4th LOFAR KSP meeting Potsdam 2010 November 8-9
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
• Quiet sun emission: what benefits ?
– We are speaking of the slowly varying thermal emission, on which we can apply rotationnal synthesis
• We think now that increased spatial resolution make sense at 150 MHz: features are not spatially resolved, like bright lanes or small holes.
• Features (magnetic ?) are symplifying when the wavelength (i.e. the altitude) increases, probably this evolution with the altitude will continue in the 150 – 50 MHz range -> solar wind.
• All examples are from Mercier, C. and Chambe, G.• (to be published in Solar Phys. end of 2010)
4th LOFAR KSP meeting Potsdam 2010 November 8-9
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2005 Aug 15 : 432 410 327 226 164 151 MHz. Typical with central coronal hole
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2008 Jun 6: very quiet sun
445 432
408 360 327 298
270 228 173 150 MHz.
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2008 Jul 14 hole and large bright/dark
features
445 432
408 360 327 298
270 228 173 150 MHz.
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2010 may 15 holes and NS large
features
445 432
408 361 327 299
271 228 173 151 MHz.
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
• Models of quiet corona– 2 components:
• Corona ( Ne, T) isothermal, vertically stratified.
• Transition region: fitted to EUV emission measure.• TB < Tc at 400 MHz, TB = Tc at 100 (?) MHz
– Parameters may differ in different structures (holes, bright structures, “mean” quiet sun…)
– TB = TC (optically thick) may be reached in the meter range, but not everywhere on the sun.
– Optically thin regime is expected at f < 66 MHz
4th LOFAR KSP meeting Potsdam 2010 November 8-9
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2004 jun 27: no saturation at Tc in the hole, the hole is not visible at
150 MHz
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2007 may 23: No saturation at Tc in the hole, still cold at 150 MHz
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2007 jun 10: bright feature spectrum needs a different model (structuration by B)
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
2008 jun 8: mean quiet sun is too hot at 450 MHz, the small « pore » can be fitted.
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
• Quiet sun programm– Increase the frequency coverage at low
frequencies:• Better models: effect of the magnetic field on the
stratification, non isothermal models, solar wind acceleration…
– Increase the spatial resolution:• Better measurement of TB (there are a lot of samll scale
structures)
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR• Some conclusions
– Quiet sun: daily observations > 1 week in spring/summer, with at least a common frequency (150.9 MHz) with NRH and > 2 lower frequencies
– Active sun: on a good candidate region, a good set of frequencies and high time resolution (better than 1 sec).
– Small scale spectral features: high time resolution, what set of frequencies ? (zebra, pulsations and more at LF)
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH -> LOFAR KSP : time/frequency sampling for solar radio bursts
• 0.1 sec, 200 kHz bandwidth, 5 to 20 frequencies– With 20 frequencies, it is possible to have a raw spectrum of different
sources.
• With the 200 kHz bandwidth, only I and V are required– In a spectrograph mode (one or a few stations), 4 Stokes make sense if the
bandwidth is <10 kHz.
• Ability to obtain a large observing time:– This is mandatory to « get something ». If its possible only in the LF band of
Lofar, it is still OK, assuming that NRH is observing at the same time (calibration ?)
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9
NRH - LOFAR
Thank you
4th LOFAR solar KSP meeting Potsdam4th LOFAR KSP meeting Potsdam 2010
November 8-9 Bastian et al. (2001)