P. K. Manoharan

Heliospheric tomography using interplanetary scintillation observations: 3. Correlation between speed and electron density fluctuations in the solar wind

We have examined the relationship between solar wind speed and electron density fluctuations on scale sizes around 100 km in the heliocentric distance range of 0.3 to 0.8 AU using interplanetary scintillation (IPS) data obtained at the Solar-Terrestrial Environment Laboratory. The solar wind properties derived from the IPS data are biased by line of sight integration through a three-dimensional structured solar wind. Therefore we have applied a computer-assisted tomography (CAT) method to deconvolve the line of sight integration and reconstruct the solar wind structure. The analysis was made for the solar wind speed V and electron density fluctuations δNe in the solar activity minimum phase when high-speed regions are separated from an equatorial low-speed region by a sharp velocity gradient. From results of the CAT analysis we derived the best fit power law relation of δNe ∝ V−γ with γ = 0.5 ± 0.15, indicating that fractional density fluctuations δNe/Ne in the high-speed wind are larger than those in the low-speed wind. Combining this relation with results of previous workers [Coles et al., 1995; Manoharan, 1993; Celnikier et al., 1987; Jackson et al., this issue], we suggest that the fractional density fluctuation level of the high-speed wind evolves with heliocentric distance.

Three-dimensional structure of the solar wind: Variation of density with the solar cycle

Interplanetary Scintillation (IPS) measurements obtained from a large number of compact radio sources (nearly 150 sources) distributed over the heliocentric distance range 15–175 solar radii (R(⊙) and heliographic latitude ∼75° N-75° S have been used to study the global three-dimensional density distribution of the solar wind plasma. Contours of constant electron-density fluctuations (ΔNe) in the heliospheric plasma obtained for both the solar minimum and maximum show a strong solar latitude dependence. During low solar activity, the equatorial density-fluctuation value decreases away from the equator towards higher latitudes and is reduced by ∼2.5 times at the poles; the level of turbulence is reduced by a factor of ∼7; the solar-wind mass flux density at the poles is ∼25% lower than the equatorial value. However, during high solar activity, the average distribution of density fluctuations becomes spherically symmetric. In the ecliptic, the variation of ΔNe with the heliocentric distance follows a power law of the formR−2.2 and it does not show any change with solar activity.

Radial Evolution and Turbulence Characteristics of a Coronal Mass Ejection

We investigate a coronal mass ejection (CME) associated with an X3.9 solar flare that occurred on 1992 June 25. This long-duration event showed a system of large postflare loops at the activity site throughout the period of the enhanced X-ray emission. The drift rate of the metric type IV radio burst observed near the X-ray maximum suggests the speed of the ejecta to be ~350 km s-1 at heights ≤2 solar radii. The solar proton intensities, in the energy range 1-100 MeV observed in the interplanetary medium, show gradual-decay profiles lasting for more than two days and suggest CME-driven acceleration near the Sun. The inference on the spatial and kinematical characteristics of the propagating CME in the inner heliosphere (0.2-1 AU) is primarily based on the interplanetary scintillation observations at 327 MHz, obtained from the Ooty Radio Telescope and the Solar-Terrestrial Environment Laboratory. The scintillation data show the deceleration of propagating disturbance speed, VCME ~ R-0.8, in the interplanetary medium. The speeds obtained from the radio and scintillation measurements also suggest that the coronal shock may not be directly related to the interplanetary shock. The size of the CME in the interplanetary medium seems to follow a simple scaling with distance from the Sun, indicating the pressure balance maintained between the ejecta and the ambient solar wind. The density turbulence spectrum of the plasma carried by the propagating disturbance seems to be flat, Φ ~ κ-2.8, also having a small dissipative scale length, Si(IPD) ≤ 5 km. The spectrum is significantly different from that of high-speed flow from coronal holes and low-speed wind originating above closed-field coronal streamers.

CME Propagation Characteristics from Radio Observations

Abstract We explore the relationship among three coronal mass ejections (CMEs), observed on 28 October 2003, 7 November 2004, and 20 January 2005, the type II burst-associated shock waves in the corona and solar wind, as well as the arrival of their related shock waves and magnetic clouds at 1 AU. Using six different coronal/interplanetary density models, we calculate the speeds of shocks from the frequency drifts observed in metric and decametric radio wave data. We compare these speeds with the velocity of the CMEs as observed in the plane-of-the-sky white-light observations and calculated with a cone model for the 7 November 2004 event. We then follow the propagation of the ejecta using Interplanetary Scintillation measurements, which were available for the 7 November 2004 and 20 January 2005 events. Finally, we calculate the travel time of the interplanetary shocks between the Sun and Earth and discuss the velocities obtained from the different data. This study highlights the difficulties in making velocity estimates that cover the full CME propagation time.

Magnetic Reconnection During the Two-phase Evolution of a Solar Eruptive Flare

We present a detailed multi-wavelength analysis and interpretation of the evolution of an M7.6 flare that occurred near the southeast limb on 2003 October 24. Pre-flare images at TRACE 195 A show that the bright and complex system of coronal loops already existed at the flaring site. The X-ray observations of the flare taken from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft reveal two phases of the flare evolution. The first phase is characterized by the altitude decrease of the X-ray looptop (LT) source for ~11 minutes. Such a long duration of the descending LT source motion is reported for the first time. The EUV loops, located below the X-ray LT source, also undergo contraction with similar speed (~15 km s–1) in this interval. During the second phase the two distinct hard X-ray footpoint (FP) sources are observed which correlate well with UV and Hα flare ribbons. The X-ray LT source now exhibits upward motion as anticipated from the standard flare model. The RHESSI spectra during the first phase are soft and indicative of hot thermal emission from flaring loops with temperatures T > 25 MK at the early stage. On the other hand, the spectra at high energies (e 25 keV) follow hard power laws during the second phase (γ = 2.6-2.8). We show that the observed motion of the LT and FP sources can be understood as a consequence of three-dimensional magnetic reconnection at a separator in the corona. During the first phase of the flare, the reconnection releases an excess of magnetic energy related to the magnetic tensions generated before a flare by the shear flows in the photosphere. The relaxation of the associated magnetic shear in the corona by the reconnection process explains the descending motion of the LT source. During the second phase, the ordinary reconnection process dominates describing the energy release in terms of the standard model of large eruptive flares with increasing FP separation and upward motion of the LT source.

Solar wind velocity and normalized scintillation index from single-station IPS observations

The recently refurbished Ooty Radio Telescope in southern India was used in a two-month campaign of interplanetary scintillation (IPS) observations in collaboration with the Cambridge IPS array in England during April–May 1992. The unique feature of this campaign was that, for the first time, scintillation enhancements were predicted in real time by observing solar events on 7–8 May, 1992 and then detected at both Ooty and Cambridge. Also, for the first time, high spatial resolution (∼ 100 sources sr−1) solar wind all-sky velocity maps were obtained at Ooty. Good consistency is found between the IPS observations from both observatories andin-situ shocks detected at Earth by IMP-8.Yohkoh soft X-ray images were used to infer the generation of a coronal mass ejection on 7 May, 1992.

EVOLUTION OF SOLAR MAGNETIC FIELD AND ASSOCIATED MULTIWAVELENGTH PHENOMENA: FLARE EVENTS ON 2003 NOVEMBER 20

We analyze Hα images, soft X-ray profiles, magnetograms, extreme ultra-violet images and, radio observations of two homologous flare events (M1.4/1N and M9.6/2B) on 2003 November 20 in the active region NOAA 10501 and study properties of reconnection between twisted filament systems, energy release, and associated launch of coronal mass ejections. During both events twisted filaments observed in Hα approached each other and initiated the flare processes. However, the second event showed the formation of cusp as the filaments interacted. The rotation of sunspots of opposite polarities, inferred from the magnetograms likely powered the twisted filaments and injection of helicity. Along the current sheet between these two opposite polarity sunspots, the shear was maximum, which could have caused the twist in the filament. At the time of interaction between filaments, the reconnection took place and flare emission in thermal and nonthermal energy ranges attained the maximum. The radio signatures revealed the opening of field lines resulting from the reconnection. The Hα images and radio data provide the inflow speed leading to reconnection and the scale size of the particle acceleration region. The first event produced a narrow and slow CME, whereas the later one was associated with a fast full halo CME. The halo CME signatures observed between the Sun and Earth using white-light and scintillation images and in situ measurements indicated the magnetic energy utilized in the expansion and propagation. The magnetic cloud signature at the Earth confirmed the flux rope ejected at the time of filament interaction and reconnection.

A study of density fluctuations in the solar wind acceleration region

Interplanetary scintillation (IPS) measurements of the density fluctuations in the fast and slow solar wind were made at 2 GHz and 8 GHz in 1994 at the Kashima Space Research Center, Communications Research Laboratory. Using the observations which cover the distance range from 5 to 76 solar radii (R s ), we investigate the radial evolution of the dissipation scale length of the density fluctuations, the so-called inner scale. Our IPS observations reveal that the inner scale shows different radial profiles between the inside and outside of the acceleration region. The size of the inner scale outside 25 R s increases linearly with radial distance, showing good agreement with previous observations made at r ≥ 50 R s . The inner scale inside 25 R s , on the other hand. deviates from the linear relation. We simulate the radial variation of the inner scale using a solar wind acceleration model and then compare the results to that of the observed inner scale. We find that, in the low-speed solar wind. the radial profile of the computed inner scale is in good agreement with that of the observed inner scale and that the solar wind acceleration causes the deviation of the inner scale from a linear relation. However, in the high-speed wind, we cannot reproduce the radial profile of the observed inner scale with the acceleration model, even though the line-of-sight integration effect on IPS measurements is taken into account. We propose that this disagreement is due to the density and magnetic field fluctuations not being correlated in the high-speed solar wind, as shown at greater heliocentric distances by Helios measurements.

The spectrum of electron density fluctuations in the solar wind and its variations with solar wind speed

We describe simultaneous interplanetary scintillation (IPS) measurements for the solar distance range 70-185 R⊙ made using the Ooty Radio Telescope and the Solar Terrestrial Environment Laboratory three-antenna system during the first half of the current solar cycle 22. These measurements have been used to establish the spectral characteristics of the electron density fluctuations in the solar wind, and we study the relation of these to the plasma flow speed. At times of low activity, for high-speed streams (Vsw ≈ 600 km s−1) the spectra (sensitive to the IPS temporal frequency range 1-10 Hz) are steep, with a power law exponent α ≈ 3.8. During high levels of activity for high-speed streams, the spectra are less steep with α ≈ 3.4. In the low-speed wind (Vsw ≈ 400 km s−1) the spectra are clearly flatter, α ≈ 2.9, and nearly invariant with solar activity. The average shape of the spectrum obtained from this study is consistent with results obtained via other techniques. By combining the present estimates with earlier spacecraft data, it is possible to consider the shape of the density fluctuation spectrum in the frequency range 10−5-10¹ Hz. It is shown that the spectrum has different slopes in different spectral domains and is well described by a three-component model as proposed by Coles and Harmon [1989]. Moreover, the high-frequency components of the spectra in the low- and high-speed streams involve different spatial frequency ranges.

IDENTIFICATION OF THE SOLAR SOURCE FOR THE 18 OCTOBER 1995 MAGNETIC CLOUD

It is necessary to identify signatures of solar sources in order to improve predictions of solar-caused geomagnetic activity. This is not a straightforward task as the relationship is not well understood. We apply an algorithm, derived from numerical simulations to identify the solar source of an interplanetary event that was observed by the WIND spacecraft on October 18, 1995 and was followed by a geomagnetic storm. No specific geomagnetic activity had been predicted at Space Weather Operations (SWO) in Boulder, CO, on the basis of earlier solar observations. The algorithm is used to estimate the time and location of the expected solar source of this interplanetary event. A review of solar observations prior to the WIND observations showed that solar activity precursors could be identified. A long-duration-event was seen by GOES in soft X-rays at the same time as a type IV burst was observed in metric radio wavelengths, and a rearrangement of fields was observed by the soft X-ray telescope on the Yohkoh satellite. This suggests that the algorithm is useful for post facto identification of solar sources, and that such combinations of solar activity should be further investigated for use in geomagnetic forecasting.