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