Ion Cyclotron Waves (ICWs)
in the Solar Wind
of the Inner Heliosphere
Lan K. Jian1,2
1Dept.
of Astronomy, Univ. of Maryland, College Park, MD
2Heliophysics Science Div., NASA GSFC, Greenbelt, MD
Thanks to C.T. Russell, H.Y. Wei, J.G. Luhmann, N. Omidi ,
P. Isenberg, X. Blanco-Cano, R. Strangeway, M. Cowee, J. Leisner
Supported by NASA Grants NNX12AB29G, NNX13AI65G
Russell Symposium
UCLA
May 8, 2013
History of the Study
• In 2008, before I graduated, when I was studying the solar
wind magnetic field data from STEREO mission, I noticed that
there were short-duration, coherent, nearly circularly
polarized waves, where the transverse power dominated
• In Russell’s group, there had been studies of ICWs in various
planetary environment. I found the characteristics of these
waves in the solar wind mimicked the ones observed in the
planetary environment. Chris confirmed they were ICWs and
we published the results in a 2009 ApJ Letter
• Since then, we have studied the ICWs using the data from the
MESSENGER, Helios, Venus Express, and Wind missions. We
are investigating the radial variations of these waves and their
relation with the solar wind plasma properties to understand
the generation and energy dissipation of these ICWs. They
may play an important role in the solar wind heating
History of the Study (cont.)
• In 2012, advised by John Podesta, we found some earlier
work of these waves
– Kenneth Behannon’s thesis in 1976 (shortly before his retirement)
• Showed the observation of highly coherent, polarized fluctuations at
frequencies between 0.01-10 Hz at 0.46-1 AU by Mariner 10
• Suggested that these waves might be associated with instabilities driven
by temperature anisotropy
– Tsurutani et al. (1994)
• Reported the electromagnetic waves with frequencies near the local
proton gyrofrequency at 1 AU using ISEE-3 observations
• Found the Parker spiral orientation is the most common case (likely due
to a small amount of events)
• Speculated the waves are generated by solar wind pickup of freshly
created hydrogen ions, and possible sources for the hydrogen are Earth’s
atmosphere, CMEs, comets and interstellar neutral atoms
– Neither of them pursued in any detail of these waves. They did
not explain many of the wave properties or emphasize their
importance
Criteria of ICWs
Ellipticity in s/c frame: -0.95
95% polarized
Propagating 1o away from B
(Jian et al., 2009)
13-band Average
ICW
Criteria
1. Transverse power being dominant
2. |Ellipticity| > 0.7
3. Percent polarization (signal-to-noise ratio) > 70%
4. Long axis of its perturbation ellipse within 10o of the
direction perp. to both B and k, i.e., the B-k plane  LH
waves in plasma frame (Stix, 1962; Lacombe et al., 1990;
Blanco-Cano, 1995)
Observations of ICW Storms
fpc
Observations of ICW Storms (cont.)
To find out the periods of ICW storms
1. Do the dynamic spectrum analysis of transverse power, ellipticity, the
coherence between two transverse field components, and the
propagation angle for each 12-hour period using Means (1972)
quadrature spectrum technique
2. Find the periods where the transverse power is enhanced, ellipticity
and coherence are high
3. Inspect the time series of B for these periods, and do the detailed wave
analysis
4. Select intervals satisfying the ICW criteria
Distribution of ICW Storms at STEREO A in 2008
focusing cone
data from
B. Klecker
•
•
•
ICWs tend to appear as storms. We find 245 (78) ICW storms >= 10 (20) min in 2008 STA
data. The ICW storms >= 10 (20) min take about 1% (0.5%) of the whole year
ICW storms can appear LH and RH in s/c frame closely, but one polarity dominates at one
time
The distribution of ICW storms do not seem to have the focusing cone effect as the
interstellar particle has. The power of ICW storm does NOT depend on the pickup He+ flux
Solar Wind Stream Conditions of ICW Storms
•
•
Most of the ICW storms are observed in the trailing part of fast wind streams,
sometimes in close contact with the slow wind streams
In the trailing part of not all but more than a half of fast wind streams during 2008,
we find ICW storms
Distribution of B-R Angle
• ICW storms are preferentially observed when B is more radial
Angular Variations of ICW
Occurrence and Wave Power (0.3 AU)
20-second
more radial
(Jian et al., 2010)
LH vs. RH in Spacecraft Frame (0.3 AU)
~72%
~28%
stronger
Not due to local pickup
higher
lower
(Jian et al., 2010)
At 0.7 and 1 AU, the LH waves in s/c frame are stronger than RH ones too
Radial Variations of Frequency
fsc / fpc
fsw / fpc
• fsw < fpc: consistent with the fact that the waves have not been
resonantly damped by local protons yet
• fsw / fpc is mainly 0.3-0.5, nearly constant with distance
Radial Variation of ICW Parameters
Heliocentric distance
0.3 AU
0.7 AU
1 AU
Occurrence rate [/day]
33.7
28.9
15.4
Duration [second]
20.0
59.0
51.5
0.94
1.79
1.18
18.2
26.7
25.1
4.0
5.3
4.5
0.03
0.03
0.03
Wave power (δB)2 [nT2]
0.532
0.044
0.016
(δB)2×r [nT2AU]
0.160
0.031
0.016
fsc [Hz]
0.59
0.28
0.28
fsw [Hz]
0.144
0.052
0.030
ICW cumulative time
fraction [%]
Angle between B and R
[o]
Propagation angle from B
[ o]
Relative wave amplitude
δB/B
fewer waves at a greater distance
almost parallel propagating
Conservation of Poynting flux
δB×E×r2 = constant
 (δB)2 VA r2 = constant
 (δB)2 B n-1/2 r2 = constant
 (δB)2 r = constant
Elimination of Several
Generation Mechanisms

The ICWs in the solar wind are ubiquitous in the inner
heliosphere. They exist in regions far away from any
planetary and cometary influence

No shocks occurred within our observation window

The field direction is close to the solar wind velocity, and the
wave frequency in the s/c frame is higher than the local fpc,
so the waves are not locally generated by pickup ions
 The ICWs must be generated at a location closer to the Sun
than the s/c
Waves Generated by Pickup Ions

When the IMF and solar wind velocity are perpendicular, pickup
ions can be accelerated by the –V×B electric field and form a ring
beam distribution in velocity space, which is unstable to the
generation of the ICWs


These waves are LH polarized and near the ion cyclotron
frequency in the plasma frame. These waves can propagate
both directions along IMF, but they will all be carried outward by
the super-Alfvénic solar wind. Hence, they can appear either LH
or RH polarized in the s/c frame
When the IMF is more aligned with the flow, the pickup ions have a
large parallel drift velocity relative to the solar wind and will
generate waves that are RH polarized in the plasma frame and
propagating toward the Sun. Because the s/c is essentially in the
frame moving with the pickup ion beam, the waves appear LH
polarized in the s/c frame
 The ICWs we see is possibly due to the perpendicular pickup closer
to the Sun
Propagation Scenario of
the ICWs in the Solar Wind
After the ICWs are generated, they can propagate either inward or outward
along the magnetic field. However, they will both be carried out by the superAlfvénic solar wind. As they convect outward, much of the ICW energy has
damped and the energy remaining may be only a very small remnant
The appearance of LH and RH waves in the s/c frame should be due to outward
and inward propagation. The lower power and smaller occurrence rate of RH
waves are consistent with longer travel time and greater damping experienced
by these inward-propagating waves.
k2
The enhanced occurrence rate of ICWs when the
IMF is more radial is likely due to minimal damping
associated with parallel-to-B propagation since
the wave normal angle is constantly being pulled
toward the radial direction by refraction
θ2
B2
r
k1 θ1
VA1 > VA2
B1
Refraction Effect
Summary and Discussion
Strong narrow-band ICWs are detected extensively and discretely
from at least 0.3 to 1 AU in the solar wind, far away from the
influence of any planet
ICWs tend to appear as storms, which can last 10 – 30 min. The
storms are preferentially observed in the trailing part of fast wind
streams and when the field is radial
ICWs propagate close to B. Their fsw is below the local fpc and close to
the local He+ gyro-frequency. However, the ICW occurrence and
power do not correlate with He+ flux
ICWs can appear both LH and RH in the s/c frame due to the Doppler
shift. Our closer-to-Sun generation and outward propagation scenario
is consistent with the facts
LH ones are observed more often than RH ones; they are stronger
and have lower fsw
Radial decrease of the wave power and Poynting flux
fsc / fpc > 1, and the ratio increases with heliocentric distance
Summary and Discussion (cont.)
As the ICWs approach local fpc at a greater heliocentric distance,
they can provide an energy source for extended solar wind heating
There are also speculations that these waves might be generated by
plasma temperature anisotropy or ion beam (such as α particle
drift). We will check temperature anisotropy, drift velocity of α
particles to study if there is anything special in the ICW time
The Solar Orbiter and Solar Probe Plus missions flying closer to the
Sun should be able to see many more such ICWs with stronger wave
power. More observations and coordinated theoretical work are
needed in time to better understand these ICWs