nrh and lofar scientific and technical aspects

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


November 8-9

NRH - LOFAR

• Comparisons / differences

– UV coverage– Software and data formats

• Science requirements

– Solar activity and bursts.– Quiet sun thermal emission.


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


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


November 8-9

NRH - LOFAR


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 !


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


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.


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.


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



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 (?)



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)



November 8-9

2005 Aug 15 : 432 410 327 226 164 151 MHz. Typical with central coronal hole


November 8-9

2008 Jun 6: very quiet sun

445 432

408 360 327 298

270 228 173 150 MHz.


November 8-9

2008 Jul 14 hole and large bright/dark

features

445 432

408 360 327 298

270 228 173 150 MHz.


November 8-9

2010 may 15 holes and NS large

features

445 432

408 361 327 299

271 228 173 151 MHz.


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



November 8-9

2004 jun 27: no saturation at Tc in the hole, the hole is not visible at

150 MHz


November 8-9

2007 may 23: No saturation at Tc in the hole, still cold at 150 MHz


November 8-9

2007 jun 10: bright feature spectrum needs a different model (structuration by B)


November 8-9

2008 jun 8: mean quiet sun is too hot at 450 MHz, the small « pore » can be fitted.


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)


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)


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 ?)


November 8-9

NRH - LOFAR

Thank you


November 8-9 Bastian et al. (2001)

nrh and lofar scientific and technical aspects

Documents

nrh lofar ksp

denser uv coveragewill

inverse of uv

nrh images

brownsnapshot uv coverage

greater uv coverage

nrh lofarnrhlight blue

poor uv sampling