Outflow speed structure of the low corona
revealed by spacecraft radio scintillation
observations in strong scattering
Takeshi Imamura1, Munetoshi Tokumaru2, Hiroki Ando1,Hiroaki Isobe3, Ayumi Asai3, Daikou Shiota5, Kentaro Yaji6,
Mayu Miyamoto4, Tomoaki Toda1, Alexander Nabatov7
1. Aerospace Exploration Agency, Japan. 2. Solar-Terrestrial Environment
Laboratory, Nagoya University. 3. Kyoto University, Japan.
4. The University of Tokyo, Japan, 5. RIKEN, Japan. 6. Rikkyo University,Japan. 7. Ukrainian Academy of Science, Ukraine. Flow speed measurements by radio scintillation
methodSummary of previous results(Armstrong & Woo, 1981)

IPS measurements(Tokumaru et al., 1991)

Measurements are scarce in the low corona (below ~5 Rs)
where strong scattering prevents reliable measurements.

Scintillation spectrum
Weak scatteringFresnelFrequency

Coles (1977)

Strong scattering

Rumsey (1975)
FresnelFrequencyFlow measurements bytracking moving coronalfeatures

Measurements in streamer belts(Sheeley et al., 1997) 

Running difference images obtainedwith coronagraphs of OHO O5+ velocity retrieval from Doppler dimming effectThree-fluid MHD model of a streamer(Ofman, 2000)
Velocity
Velocities in the equatorial streamer
belt (Strachan et al., 2002)

O5+1.8Rs

O5+5Rs
H+5RsH+1.8Rs

DensityO5+1.8Rs

O5+ H+
5Rs1.8Rs

H+5RsSolar wind

Venus orbiter
Akatsuki (USO)

X-band (8.4 GHz)
radio wave

Usuda DeepSpace Center,Japan

Angular position of Akatsuki relative to Sun as seen from Earthin June-July, 2011!"#$%&'!"-.%/
,"#!"#$%()%&*++

Observations
•!
•!
•!
•!

17 experiments at heliocentric distances of 1.5-20 Rs
X-band, open-loop recording (500 kHz sampling)
Recording length is 6-7 hours in each measurement
Simultaneous observations with a space solar telescope HINODE
were conducted on June 24, 25, 26, 27

Potential magnetic field seen from theEarth on June 25, 2011 calculated froma synoptic magnetic field map provided
by Global Oscillation Network Group Synoptic velocity map generated with IPS
measurements by STEL

Akatsuki trajectory

•! Mostly quiet-Sun, slow-wind regions were observed.
•! Observations near the North pole may be influenced by coronal holes.

012-34"5$%62$7489%:$;<8$%6":4897=#>%:97?>8<"#5%#<36$%@A$64B

Theoretical spectrum using the formula by Rino (1979) Refraction

Diffraction

krefkcharkchar =kdif

kref kdif

fchar =fref fdif

Examples of fitting linear functions toscintillation spectra in logarithmic
scale to find characteristic frequencies

Characteristic wavenumber from scintillation model : kchar
Characteristic frequency from observation : fchar!Solar wind velocity Vsw = 2! fchar/kchar Characteristic wavenumbers kchar for different density
perturbation parameters Cp and power-law exponents
Weakscattering" = 3

" = 11/3

" = 4

" = 3

" = 11/3

" = 4

Strongscattering•! kchar is roughly constant for each of the weak scattering regime
and the strong scattering regime.
•! The variability of kchar is included in the error estimation.

Derived solar wind speeds
DoublekneeSingle kneeDerived solar wind speeds
Acceleration in open field ?

Blob tracking(Sheeley et al., 1997) Supply of plasma to the solar wind in closed-fieldregions?Noci et al. (1997)

Summary•! Bulk outflow speeds of the low corona were obtained by spacecraft
radio scintillation observations including the strong scattering regime.
•! The speed profile shows slow motions (< 100 km s-1) at 1.5-6 RS, rapid
acceleration at 7-13 RS, and weaker acceleration at > 15 RS to reach~400 km s-1 at 20.5 RS. Such a speed profile is typical of the low
corona in the quiet Sun region: closed fields near the Sun suppress
radial acceleration, while rapid acceleration occurs at high altitudes inopen field lines.
•! Localized fast flows were also observed at 1.5 RS and 1.7 RS near theNorth pole probably due to the influence of open fields originated fromcoronal holes.
•! The ubiquitous outflow of several tens of km s-1 in the quiet-Sun low
corona region suggests that the supply of plasma to the slow solar
wind occurs continuously over an extended area. %Phase scintillation spectra 
1E+10

20.5 Rs
16.0 Rs
12.7 Rs
10.5 Rs
8.4 Rs
5.4 Rs3.5 Rs
1.9 Rs
1.5 Rs
1.7 Rs
2.4 Rs
4.1 Rs
5.9 Rs
8.6 Rs
9.5 Rs12.2 Rs
Kolmogorov

1E+9
1E+8
Spectral density (Hz-1)

1E+7
1E+6
1E+5
1E+4
1E+3
1E+2
1E+1
1E+0
1E-1
1E-2
1E-4

Kolmogorov
1E-3

1E-2
Frequency (Hz)

1E-1

1E+0

Scintillation indexConstant acceleration model
Sheeley et al., 1997

Parker’s solar wind modelMomentum eq.Mass conservation
Eq. of state
vs : sound speedCombining these, we have (Sheeley et al., 1997)

•! Fitting to ‘West’ datataken at > 6 Rs

T

Flow speed

vsRefractionDiffractionLrF ~ (L/k)0.5 !s!sk-1

sd
!s = 1/(k sd)U = rF /sd
sd = rF /U
U > 1 (rF > sd) Strong scattering
U < 1 (rF < sd) Weak scatteringsr
sr = L !s= rF2 k !s = rF2 /sd= U rF! sd sr ~ rF2(Rickett, 1990) 6/6 (20.7 Rs)6/26 (1.7 Rs)