Solar Observations with ALMA

Solar Observations with ALMA
Masumi Shimojo
Chile Observatory, NAOJ/NINS
and
Inter-ARC development team for solar observations
NRO-ALMA Science/Development Workshop@Nobeyama, 2015/07/28
Today’s Topics
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What can we see in solar mm/sub-mm images?
•
•
Main science targets of solar observations with
ALMA
Current status of solar observations with ALMA
•
Report of the commissioning (EOC/CSV) activities
related with solar observations.
2
mm/sub-mm emission
in solar atmosphere
•
When a flare has not occurred, there is no high-energy
electrons (> a few keV) in solar atmosphere.
•
•
Then, thermal emission dominates in the wavelength range.
During a flare, there is high-energy electrons (~ MeV) in
the flare loops (magnetic tube).
•
Then, non-thermal emission (gyro-synchrotron) dominates
even in sub-mm range.
3
mm/sub-mm emission
from solar atmosphere at “Quiet”
•
In solar atmosphere, the optical depth
-.
in the range depends on free-free & H
•
τ=1 layer is located at lower
chromosphere (Tphy = 4000~7000 K).
•
The layer satisfies LTE condition and
the source function is Planckian.
•
Rayleigh-Jean law can be applicable.
mm/sub-mm images are
thermograms of
lower chromosphere
Observing Freq. of ALMA
Solid line: The spectrum is calculated from VAL model
Vernazza, Averett & Loeser, 1973
4
Chromosphere is the hottest
layer in solar physics NOW!
IRIS
UV Imaging Spectrograph
G-band(Blue) — Ca II K(Orange) Movie
© NAOJ/ISAS
obtained with Solar Optical Telescope/Hinode
De Pontieu et al. (2014)
5
Chromosphere is the hottest
layer in solar physics NOW!
•
•
Hinode satellite launched by JAXA/ISAS in 2006 revealed dynamic chromosphere with fine structures.
•
Captured MHD waves, and numerous small jets produced by magnetic reconnection
•
However, Hinode doesn’t have an instrument for spectroscopy in chromosphere.
Based on the achievements of Hinode, IRIS (Interface Region Imaging Spectrograph) satellite was
launched by NASA in 2013.
•
The profile of UV lines emitted from dynamic phenomena is obtained with IRIS. However, the profile
and its evolution are too complex and it is not so easy to derive the physical parameters
(temperature, density) from the line profile.
Thermograms with high-spatial (⪍0.1”) / time (~1s) resolution obtained with
ALMA is the key observable for understanding complex chromosphere!
6
A part of science targets of solar
observations without flares.
•
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Reveal the thermal structure in chromosphere from a multi-band ALMA observation.
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CO lines (J=6➝5, 7➝6) might be a essential for detecting cool plasma (~3500K) predicted from IR observations.
•
The vertical structure of temperature above a sunspot is still puzzle.
Detect MHD waves and Vortex in an elemental magnetic flux (tube)
•
The size of an elemental flux is predicted as 0.2”~0.3”. ALMA with the higher band might be able to resolve the
flux and investigate its evolution for calculating the energy flux of waves propagated toward corona. (If we can
measure the polarization, the precision of estimating the flux becomes 25
very high.)
Wedemeyer-Bohm et al. 2015
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The sunspot observation with
Nobeyama 45m telescope
Iwai & Shimojo 2015
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Considering the contamination from the plage by the side lobes, the sunspot is darker than quiet region.
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The spatial resolution of NRO 45m is not enough for such a study even when we observe a large sunspot.
8
mm/sub-mm non-thermal emission
from MeV electrons.
Model Spectrum of Gyro-synchrotron
•
•
11.8GHz
Ecut
Assumptions
14.8GHz
18.5GHz
Ecut
35.0GHz
Energy spectrum of electrons
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Power-law (Index -4)
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Energy range: Ecut ~ 10 MeV
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Cut-off energy (Ecut ) is indicated in the
plot.
•
Ecut
86.0GHz
•
•
White & Kundu 1992
Ecut = lower-end energy of the
energy distribution)
Magnetic field
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Strength: 300 Gauss
•
The angle between the LoS and field:
45 degree
Note: In lower frequency (< 10GHz), the self
absorption by lower energy electrons is effective.
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Standard model of solar flares &
The evidences of magnetic reconnection in flares
Cusp & Plasmoid Eruption
(Tsuneta et al. 1992, Shibata et al.1995)
Reconnection Inflow
(Yokoyama et al. 2001)
[Next Question]
Where is the site of
particle acceleration?
Yokoyama & Shibata 2001
Reconnection Outflow
(Hara et al. 2011)
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Direct evidence of high-energy
electrons around the reconnection site
Narukage, Shimojo, Sakao (2014)
We found non-thermal emission around X-point
(reconnection site) from the 17 & 34 GHz data
obtained with Nobeyama Radioheliograph.
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Weak point of radio (& HXR) observations for solar flare studies:
Spatial resolution is significantly worse than the others.
GOES CLASS: X3.4 flare
The flare is one of the largest flares observed with Hinode satellite.
Color: Ca II H (Hinode), Blue contours: HXR(RHESSI), Green contours: 34 GHz (NoRH)
The source size of HXR and the width of the microwave loop are similar to the spatial resolution of each telescope.
The situation will be changed dramatically by ALMA!
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mm/sub-mm spectrum of solar flares
Raulin et al. 2004
X5.3 class flare on 25 Aug. 2001
SOLAR FLARE AT SUBMILLIMETER WAVELENGTHS
134
Kaufmann et al. 2009
191
X6.5 class flare on 6 Dec. 2006
P. Kaufmann et al.
Figure 2 Complete burst spectra, from decimeter to submillimeter waves, for the burst times 1 – 3 labeled at
Both
the observations were performed with the same telescopes
the top of Figure 1. Bars refer to an arbitrary uncertainty assumption of 10% changes in the optical depths. The
presence ofSubmillimeter-wave
the sub-THz component is particularly
well defined
the precursor-like
impulsive
(Solar
Telescope
& for
Owens
Valleystructure,
SolartheArray).
phase, and suggested for the following phase.
frequencies
change withof
time,solar
as it has been
known above
for other bursts
(Croom,
1971a;
•turnover
The
spectrum
flares
100
GHz
Roy, 1979; Nita, Gary, and Lee, 2004). Figure 3 shows the time variation of microwave
is afrequencies
land fof
confusion…
in some cases cannot be unambiguously determined because
turnover
s (which
apparently it exceeded the maximum OVSA limit), the GHz spectral index δ (defined as flux
∝ f −δ ) for 18 GHz ≥ f ≥ fs and the GHz flux at fs . In the bottom of the same figure we
the sub-THz spectral index, calculated as δsm = log[S(405)/S(212)]/ log[405/212],
•showFlare
observations with multi-bands of ALMA are desired.
with S(405) and S(212) the flux density at 405 and 212 GHz, respectively. Rapid superim5. Observed 3 – 405 GHz radio flux spectra at 16:31:13 UT (top) and 16:32:10
UTtime
(bottom).
posed
structures were present at both sub-THz frequencies and will be discussed in a
13
a from 1 – 18 GHz are from OVSA, and the vertical bars are the SST flux separate
densities and
their
paper.
f uncertainty. The curves represent the theoretical synchrotron spectra for magnetic fields
Main science targets of solar
observations with ALMA
•
Chromospheric Science
•
The images obtained with ALMA are thermograms in lower chromosphere.
•
They would show us the thermal structure of chromosphere.
•
The images with high-spatial/time resolution would reveal time evolution of magnetic fluxes.
•
•
The data would show us the energy flow in lower chromosphere, and it is crucial for
understanding heating of chromosphere and corona.
Flare Science
•
ALMA will provide us images of non-thermal emission sources (MeV electrons) with
unprecedented high resolution.
•
It would reveal the relation between the acceleration site and magnetic fields clearly.
•
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The relation is a essential for understanding particle acceleration in flares.
Spectrum in mm/sub-mm range might be clear?
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Difficulties of solar observations
Sun is too bright even in mm/sub-mm range.
• The receiver system of ALMA is saturated at solar observations?
• How to attenuate the input level to suitable level of the correlator?
• How to calibrate solar visibility data obtained with attenuation?
• The structure on solar disk is moving in the RA/DEC coordinate by differential solar rotation.
• The ephemeris of the Sun (sun center) installed in the antenna control system cannot be used for most
of scientific solar observations.
• The target (Sun) fills the main beam (& side-lobes) of an antenna.
• What array configuration and correlator usage is suitable for solar observations?
• How to guarantee the absolute flux density in the case?
• The WVR system cannot be used for solar observations.
• The receiver of WVR is saturated when the antenna directs to the Sun.
• Huge heat load from Sun
• It had already been solved at the design and construction of the antennas.
• We need to verify the performance of the antennas in actual solar observations
•
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Development history of solar
observations with ALMA
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2010 Summer: Started the development project of
solar observations in EA & JAO
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2011 Feb. : Single-dish solar observations by JAO
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2011 May: 1st Solar Campaign
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•
•
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First detection of solar visibility
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Verification of the attenuation level of the solar filter
(physical attenuation filter) using the prototype model.
2011 Dec.: 2nd Solar Campaign
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Test of the solar-filter control function
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Establish the phase calibration with solar filters
Test of the heterogenous array mode
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First attempt of solar image synthesis (Failed by the
shortage of the antennas)
2013 Oct.: 4th Solar Campaign
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Test the tracking of solar structures using external
ephemeris.
•
•
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First solar observation using 7m antennas.
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Verification of the pointing & focus stability during long
(> 4 hrs) solar observations.
Simultaneous single-dish observation
Test of the SIS mixer de-tuning Mode
— Succeed the image synthesis of a C1.9 flare!! —
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2014 Dec.: 5th Solar Campaign
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First campaign carried out by the inter- ARC development
team (EA: Shimojo/Iwai, NA:Bastian/White, EU:Roman/Barta)
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Establish the phase/flux/bandpass calibration with SIS
mixer de-tuning
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Solar observations with SIS mixer de-tuning
2012 Jun.: 3rd Solar Campaign
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Solar observations with solar filters
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Simultaneous single-dish observations with SIS mixer
de-tuning
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Our solutions for solar
observations with ALMA: 1
•
Sun is too bright even in mm/sub-mm range.
• The receiver system of ALMA is saturated at solar observations?
• At first, we establish solar observations with band3&6 that have large dynamic range.
• How to attenuate the input level to suitable level of the correlator?
• Idea 1: The input power to the receiver is reduced by a physical attenuating filter (solar filter:
-10dB@100GHz) located above the receiver cabin.
• Idea 2: The sensitivity of the receiver is reduced by the de-tuning of a SIS mixer (MD:
Change the basis voltage for a SIS mixer).
• How to calibrate solar visibility data obtained with attenuation?
• For Idea1: Remove the solar filter during the observation of calibrators.
• For Idea2: The attenuation level of the attenuator in the receiver system is reduced during the
observation of calibrators.
• The phase shift between solar and calibrator observations is measured in advance,
and the shift value is applied to the data at off-line.
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Solar filter vs. SIS mixer de-tuning (MD)
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Advantage
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Solar Filter: Tolerance for saturation in all bands
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MD: The calibration device in the antenna (amb/hot laods) is enabled.
Disadvantage
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Solar Filter: It is very hard to perform flux/bandpass calibration.
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The amb/hot loads cannot be used with solar filters.
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Most quasars cannot be used as a calibrator with solar filters.
MD: Tolerance for saturation is weaker than solar filter.
Our Decision: MD is used for solar observations with Band3/6 in Cycles.
KERR et al.: DEVELOPMENT OF THE ALMA BAND-3 AND BAND-6 SIDEBAND-SEPARATING SIS MIXERS
Solar Filter
204
207
Band3 SIS Mixer Assembly & SIS chip
IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 4, NO. 2, MARCH 2014
Fig. 8. Photographs of the Band-3 and -6 mixer chips (different scales). Dark
areas are the background seen through the quartz substrate. The suspended
stripline waveguide probe (to the left) connects through a broadband transition
to a capacitively loaded CPW. The dc/IF bonding pad is at the right. The dotted
Fig. 9. RF circuit of the Band-3 SIS mixer chip [8]. Dark or green regions are
red lines
of mixer
the broad-walls
of the
waveguide.
Figs. 9 and block
the background seen through the quartz substrate and contain no metallization.
Fig. 17.
One indicate
half ofthe
theposition
Band-3
assembly.
The
central aluminum
10 show more details.
base-electrode metallization is light gray; the darker (brown) concontains the waveguide branch-line hybrid, LO power divider, LO couplers, The
andlower Nb
ductors are
the 18.
Nb wiring
layer.front-end. The mixer assembly is at the top, behind the
Fig.
Band-3
(Kerry et al. 2014)
the image termination. The DSB mixer chips and LO terminations are in the left
and right modules. The RF signal enters through the lower waveguide and the
18
IF quadrature hybrid (aluminum). Below them are the IF isolators and
Our solutions for solar
observations with ALMA: 2
•
•
•
The structure on solar disk is moving in the RA/DEC coordinate by differential solar rotation.
• We use an external ephemeris file made for each solar target considering the differential rotation.
The target (Sun) fills the main beam (& side-lobes) of an antenna.
The WVR system cannot be used for solar observations.
• What array configuration and correlator usage is suitable for solar observations?
• To keep the coverage of the data in u-v plane, the compact array configuration (longest
baseline < 1 km) is used. The limitation is also reasonable for observations without WVR.
• For good u-v coverage and keeping simultaneity of the data, all antennas (12m & 7m) are
connected to the BaseLine correlator (12m + 7m Heterogeneous array).
• How to guarantee the absolute flux density in the case?
• We measure not only a system temperature(T ) but also an antenna temperature(T ) at the
sys
ant
target region, for calculating the system equivalent flux density.
• The single-dish observation and interferometric observation are performed simultaneously, for
obtaining the DC (background) component of the target and checking whether there is significant
bright regions (e.g. a flare) near the FoV of the interferometric observation.
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Summary
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Solar observations with ALMA will provide us essential knowledges for
understanding the fundamental problems of solar physics. For examples…
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Thermal structure of lower chromosphere
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Energy transport in lower chromosphere for heating solar atmosphere
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Particle acceleration in solar flares
The development of solar observations for open-use has reached a climax now.
•
To open solar observations to community in Cycle4, the international
development team of solar observations and JAO are working hard now.
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The start of solar observations at Cycle4 has NOT been decided yet.
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www.almaobservatory.org
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