Solar Flares at Millimeter and Submillimeter
Wavelengths – Instrumental Techniques
and Observations
Colloquium, Institute of Astronomy, ETH Zürich
June 29, 2004
Thomas Lüthi
Institute of Applied Physics
University of Bern
http://www.iapmw.unibe.ch
Contents
• Introduction
• April 12, 2001: First observation of a solar flare at submillimeter wavelengths
• BEMRAK – The Bernese Multibeam Radiometer for
KOSMA
• October 28, 2003: First measurement of the effective angular size of a solar flare at 210 GHz
• Summary
Solar Flares
Hα
10.04.2001
• duration: ∼minutes
• Emax ≈ 1025 J
• energy from the magnetic field
in the solar atmosphere
→ particle acceleration
Kawabe Cosmic Park (J)
→ emission over the whole
e/m spectrum
Simplified Emission Model
Yohkoh SXT 13.01.1992
Soft X−rays
Microwaves
mm/submm Emission
Energy Release
Corona
e−
Transition Zone
Chromosphere
Photosphere
e−
Hard X−rays
Gamma−rays
EUV Lines
Hα
Motivation for mm/submm observations
• energy spectrum of accelerated electrons
→ important parameter for flare models
• relativistic (&1 MeV) electrons
– bremsstrahlung
→ hard X-rays/γ-rays
– (gyro) synchrotron emission
→ mm/submm-waves
• thermal bremsstrahlung of the hot flare plasma
Observational Challenges
• Earth’s atmosphere
– increasing attenuation
– increasing variability of attenuation
• rising background flux density (S ∝ ν 2 )
• (mostly) decreasing flare flux density
→ strongly decreasing S/N
Observational Challenges
Instrumentation
Bumishus
• 8.4–50 GHz
• HPBW ≈ 60’
Nulling-Interferometer
• 89.4 GHz
• quiet sun background
is cancelled
KOSMA
• 230/345 GHz
• HPBW ≈ 1.5’
12.04.2001: First Observation of a Solar Flare
in the Submillimeter Range with KOSMA
T. Lüthi et al., Astronomy & Astrophysics,
415, 1123-1132, March 2004
12.04.2001: First Observation of a Solar Flare
in the Submillimeter Range with KOSMA
T. Lüthi et al., Astronomy & Astrophysics,
415, 1123-1132, March 2004
12.04.2001: First Observation of a Solar Flare
in the Submillimeter Range with KOSMA
T. Lüthi et al., Astronomy & Astrophysics,
415, 1123-1132, March 2004
12.04.2001: First Observation of a Solar Flare
in the Submillimeter Range with KOSMA
T. Lüthi et al., Astronomy & Astrophysics,
415, 1123-1132, March 2004
Problem: Absolute Flux Density Calibration
Pn
?
ϕ
• Source missed by ∼1.5 arcminutes
• Relative source position only known within ±0.5’
→ absolute flux density calibration inaccurate (±40%)
Solution: Multibeam Receiver
• Gaussian source → 4 parameters
position [ϑ,ϕ], diameter [D],
flux density [S]
• Sobsi ∝ TB ⊗ Pni
• S2 /S1 → ϕ
• accuracy HPBW
• 4 beams → (ϑ, ϕ), D, S
Previous Multibeam Systems
6 arcmin
212 GHz
405 GHz
Itapetinga (1989)
48 GHz
SST (1999)
212 & 405 GHz
BEMRAK (Bernese Multibeam
Radiometer for KOSMA)
• 4 beams at 210 GHz
τ = 2.3 ms, ∆S ≈ 2 sfu
• beam synthesis
→ ideally located 4th beam
→ cost-effective
• simultaneous observations
at 230 & 345 GHz
Beam Synthesis
LO distribution
network
IF combining
network
LO
USynth
feedhorns
U2
U1
mixer
ADC
U3
Quasioptics
focal plane
BEMRAK
230/345
GHz
telescope mount
28.10.2003: GOES X17.2 flare
10000
8000
ch 1
6000
ch 2
ch 3
synth
2000
0
10000
8000
6000
4000
2000
a
0
source size [arcsec]
flux density [sfu]
4000
80
60
40
20
0
T. Lüthi et al., A&A, 420, 361–370, 2004
28.10.2003: GOES X17.2 flare
10000
8000
ch 1
6000
ch 2
ch 3
synth
2000
0
10000
8000
6000
4000
2000
a
0
source size [arcsec]
flux density [sfu]
4000
80
60
40
20
0
T. Lüthi et al., A&A, 420, 361–370, 2004
28.10.2003: Images
flux density
TB
Preliminary Interpretation
t
ϕ
ϕ
Summary
• mm/submm-waves: - relativistic electrons (&1 MeV)
- hot solar flare plasma
• experimental challenge
• 3 years of observations with KOSMA
• first solar flare observation at submm-wavelengths
• problem: calibration
→ development of an improved multibeam receiver
→ first measurement of the effective source diameter
at 210 GHz