Prasad Subramanian

Large-Angle Spectrometric Coronagraph Measurements of the Energetics of Coronal Mass Ejections

We examine the energetics of coronal mass ejections (CMEs) with data from the large-angle spectro- metric coronagraphs (LASCO) on SOHO. The LASCO observations provide fairly direct measurements of the mass, velocity, and dimensions of CMEs. Using these basic measurements, we determine the potential and kinetic energies and their evolution for several CMEs that exhibit —ux-rope morphologies. Assuming —ux conservation, we use observations of the magnetic —ux in a variety of magnetic clouds near the Earth to determine the magnetic —ux and magnetic energy in CMEs near the Sun. We —nd that the potential and kinetic energies increase at the expense of the magnetic energy as the CME moves out, keeping the total energy roughly constant. This demonstrates that —ux-rope CMEs are magnetically driven. Furthermore, since their total energy is constant, the —ux-rope parts of the CMEs can be con- sidered a closed system above D2 R _ . Subject headings: solar-terrestrial relationsSun: activitySun: coronaSun: magnetic —elds

Direct Detection of a Coronal Mass Ejection-Associated Shock in Large Angle and Spectrometric Coronagraph Experiment White-Light Images

The LASCO C2 and C3 coronagraphs recorded a unique coronal mass ejection on April 2, 1999. The event did not have the typical three-part CME structure and involved a small filament eruption without any visibile overlying streamer ejecta. The event exhibited an unusually clear signature of a wave propagating at the CME flanks. The speed and density of the CME front and flanks were consistent with the existence of a shock. To better establish the nature of the white light wave signature, we employed a simple MHD simulation using the LASCO measurements as constraints. Both the measurements and the simulation strongly suggest that the white light feature is the density enhancement from a fast-mode MHD shock. In addition, the LASCO images clearly show streamers being deflected when the shock impinges on them. It is the first direct imaging of this interaction.The Large Angle and Spectrometric Coronagraph Experiment (LASCO) C2 and C3 coronagraphs recorded a unique coronal mass ejection (CME) on 1999 April 2. The event did not have the typical three-part CME structure and involved a small-filament eruption without any visible overlying streamer ejecta. The event exhibited an unusually clear signature of a wave propagating at the CME flanks. The speed and density of the CME front and flanks were consistent with the existence of a shock. To better establish the nature of the white-light wave signature, we employed a simple MHD simulation using the LASCO measurements as constraints. Both the measurements and the simulation strongly suggest that the white-light feature is the density enhancement from a fast-mode MHD shock. In addition, the LASCO images clearly show streamers being deflected when the shock impinges on them. It is the first direct imaging of this interaction.

Source Regions of Coronal Mass Ejections

Observations of the solar corona with the Large Angle and Spectrometric Coronagraph Experiment (LASCO) and EUV Imaging Telescope (EIT) instruments on the Solar and Heliospheric Observatory (SOHO) provide an unprecedented opportunity to study coronal mass ejections (CMEs) from their initiation through their evolution out to 30 R☉. The objective of this study is to gain an understanding of the source regions from which the CMEs emanate. To this end, we have developed a list of 32 CMEs whose source regions are located on the solar disk and are well observed in EIT 195 A data during the period from solar minimum in 1996 January through the rising part of the cycle in 1998 May. We compare the EIT source regions with photospheric magnetograms from the Michelson Doppler Imager (MDI) instrument on SOHO and the National Solar Observatory at Kitt Peak and also with Hα data from various sources. The overall results of our study show that 41% of the CME-related transients observed are associated with active regions and have no prominence eruptions, 44% are associated with eruptions of prominences embedded in active regions, and 15% are associated with eruptions of prominences outside active regions. Those CMEs that do not involve prominence eruptions originate in active regions both with and without prominences. We describe six especially well observed events. These case studies suggest that active region CMEs (without eruptive prominences) are associated with active regions with lifetimes between 11 and 80 days. They are also often associated with small-scale emerging or canceling flux over timescales of 6-7 hr. CMEs associated with active region prominence eruptions, on the other hand, are typically associated with old active regions with lifetimes ~6-7 months.

FORMATION OF RELATIVISTIC OUTFLOWS IN SHEARING BLACK HOLE ACCRETION CORONAE

We examine the possibility that the relativistic jets observed in many active galactic nuclei may be powered by the Fermi acceleration of protons in a tenuous corona above a two-temperature accretion disk. In this picture the acceleration arises as a consequence of the shearing motion of the magnetic —eld in the corona, which is anchored in the underlying Keplerian disk. The protons in the corona have a power-law distribution because the density there is too low for proton-proton collisions to thermalize the energy supplied via Fermi acceleration. The same shear acceleration mechanism also operates in the disk itself; however, there the density is high enough for thermalization to occur and consequently the disk protons have a Maxwellian distribution. Particle acceleration in the corona leads to the development of a pressure-driven wind that passes through a critical point and subsequently transforms into a relativistic jet at large distances from the black hole. We combine the critical conditions for the wind with the structure equations for the disk and the corona to obtain a coupled disk-corona-wind model. Using the coupled model, we compute the asymptotic Lorentz factor of the jet as a function of the cylindrical starting radius at the base of the ! = out—ow, in the corona. Our results suggest that which is consistent with observations of super- ! = ( 10, luminal motion in blazars. We show that collisions between the jet and broad-line emission clouds can produce high-energy radiation with a luminosity sufficient to power the c-rays observed from blazars. Subject headings: acceleration of particlesaccretion, accretion disksgamma rays: theory ¨ radiation mechanisms: nonthermal

The relationship of coronal mass ejections to streamers

We have examined images from the Large-Angle and Spectrometric Coronagraph (LASCO) to study the relationship of coronal mass ejections (CMEs) to coronal streamers. We wish to test the suggestion [Low, 1996] that CMEs arise from flux ropes embedded in a streamer erupting and disrupting the streamer. The data span a period of 2 years near Sunspot minimum through a period of increased activity as Sunspot numbers increased. We have used LASCO data from the C2 coronagraph which records Thomson scattered white light from coronal electrons at heights between 1.5 and 6Rs. Maps of the coronal streamers have been constructed from LASCO C2 observations at a height of 2.5Rs at the east and west limbs. We have superposed the corresponding positions of CMEs observed with the C2 coronagraph onto the synoptic maps. We identified the different kinds of signatures CMEs leave on the streamer structure at this height (2.5Rs). We find four types of CMEs with respect to their effect on streamers: 1. CMEs that disrupt the streamer, 2. CMEs that have no effect on the streamer, even though they are related to it, 3. CMEs that create streamer-like structures and 4. CMEs that are latitudinally displaced from the streamer. CMEs in categories 3 and 4 are not related to the streamer structure. This is the most extensive observational study of the relation between CMEs and streamers to date. Previous studies using SMM data have made the general statement that CMEs are mostly associated with streamers and that they frequently disrupt it. However, we find that approximately 35% of the observed CMEs bear no relation to the preexisting streamer, while 46% have no effect on the observed streamer, even though they appear to be related to it. Our conclusions thus differ considerably from those of previous studies.

Combining visibilities from the giant meterwave radio telescope and the Nancay radio heliograph - High dynamic range snapshot images of the solar corona at 327 MHz

We report first results from an ongoing program of combining visibilities from the Giant Meterwave Radio Telescope (GMRT) and the Nancay Radio Heliograph (NRH) to produce composite snapshot images of the sun at meter wavelengths. We describe the data processing, including a specific multi-scale CLEAN algorithm. We present results of a) simulations for two models of the sun at 327 MHz, with differing complexity b) observations of a complex noise storm on the sun at 327 MHz on Aug. 27, 2002. Our results illustrate the capacity of this method to produce high dynamic range snapshot images when the solar corona has structures with scales ranging from the image resolution of 49 �� to the size of the whole sun. We emphasize that snapshot images of a complex object such as the sun, obtained by combining data from both instruments, are far better than images from either instrument alone, because their uv-coverages are very complementary.

High-rigidity Forbush decreases: due to CMEs or shocks?

Aims. We seek to identify the primary agents causing Forbush decreases (FDs) in high-rigidity cosmic rays observed from the Earth. In particular, we ask if these FDs are caused mainly by coronal mass ejections (CMEs) from the Sun that are directed towards the Earth, or by their associated shocks. Methods. We used the muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We selected those FD events that have a reasonably clean profile, and can be reasonably well associated with an Earth-directed CME and its associated shock. We employed two models: one that considers the CME as the sole cause of the FD (the CME-only model) and one that considers the shock as the only agent causing the FD (the shock-only model). We used an extensive set of observationally determined parameters for both models. The only free parameter in these models is the level of MHD turbulence in the sheath region, which mediates cosmic ray diffusion (into the CME for the CME-only model, and across the shock sheath for the shock-only model). Results. We find that good fits to the GRAPES-3 multi-rigidity data using the CME-only model require turbulence levels in the CME sheath region that are only slightly higher than those estimated for the quiescent solar wind. On the other hand, reasonable model fits with the shock-only model require turbulence levels in the sheath region that are an order of magnitude higher than those in the quiet solar wind. Conclusions. This observation naturally leads to the conclusion that the Earth-directed CMEs are the primary contributors to FDs observed in high-rigidity cosmic rays.

Radio Observations of Rapid Acceleration in a Slow Filament Eruption/Fast Coronal Mass Ejection Event

We discuss a filament eruption/coronal mass ejection (CME) event associated with a flare of GOES class M2.8 that occurred on 2001 November 17. This event was observed by the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz. NoRH observed the filament during its eruption both as a dark feature against the solar disk and a bright feature above the solar limb. The high cadence of the radio data allows us to follow the motion of the filament at high time resolution to a height of more than half a solar radius. The filament eruption shows a very gradual onset and then a rapid acceleration phase coincident with the launch of a fast halo CME. Soft X-ray and extreme-ultraviolet (EUV) images show heating in a long loop underneath the filament prior to the flare. The NoRH height-time plot of the filament shows a roughly constant gradual acceleration for 1 hr, followed by a very abrupt acceleration coincident with the impulsive phase of the associated flare, and then a phase of constant velocity or much slower acceleration. This pattern is identical to that recently found to occur in the motion of flare-associated CMEs, which also show a sharp acceleration phase closely tied to the impulsive phase of the flare. When the rapid acceleration occurs in this event, the flare site and the filament are separated by ~0.5 R☉, making it unlikely that a disturbance propagates from one location to the other. Models in which a disruption of the large-scale coronal magnetic field simultaneously permits the acceleration of the filament and the flare energy release seem to be a better explanation for this event.

A Study of Density Modulation Index in the Inner Heliospheric Solar Wind during Solar Cycle 23

An understanding of variations of density modulation index εN = ΔN/N, the ratio of the electron density fluctuation (ΔN) to the absolute solar wind density (N), in the inner heliosphere is of vital importance for understanding turbulent dissipation and consequent local heating of solar wind. In addition, the density modulation index plays crucial role in understanding the propagation of energetic electrons, through the heliosphere, produced by solar flares and other explosive solar surface phenomena. We have made a detailed study of εN in the inner heliosphere spanning the distance range from 0.2 to 0.8 AU, for the period 1998-2008, covering solar cycle 23. The rms electron density fluctuations (ΔN) have been deduced using ground-based interplanetary scintillation (IPS) observations at 327 MHz from the multi-station IPS observatory, at STEL, Japan. Before deriving ΔN, we have appropriately normalized scintillation measurements to remove the effect of finite source size. The absolute solar wind density (N), on the other hand, has been obtained from the space-borne Advanced Composition Explorer (ACE) mission. However, ACE density measurements are effectively at a distance of 1 AU at the Largangian point L1. Thus, for estimation of density at the location of the relevant scintillating sources, spreading over distances of 0.2-0.8 AU, the measured ACE densities at 1 AU are extrapolated in the sunward direction using an electron density model. Our analysis shows that εN does not vary with heliocentric distances r and the typical value of εN ranges from 1% to 10% which is is consistent with the earlier findings. A systematic decline in the solar wind electron density turbulence levels has been reported earlier for the period 1995 to 2008. Our investigation of the long-term temporal variations of εN over the distance range 0.2-0.8 AU have also shown a similar decline during the period 1998-2008. It therefore appears reasonable, from the linear relationship between the density fluctuations and magnetic field fluctuations, to conclude that the decrease in εN is connected to the unusual solar magnetic activity during the long and deep solar minimum at the end of the solar cycle 23.

Self-similar expansion of solar coronal mass ejections: Implications for Lorentz self-force driving

We examine the propagation of several coronal mass ejections (CMEs) with well-observed flux rope signatures in the field of view of the SECCHI coronagraphs on board the STEREO satellites using the graduated cylindrical shell fitting method of Thernisien et al. We find that the manner in which they propagate is approximately self-similar; i.e., the ratio (κ) of the flux rope minor radius to its major radius remains approximately constant with time. We use this observation of self-similarity to draw conclusions regarding the local pitch angle (γ) of the flux rope magnetic field and the misalignment angle (χ) between the current density J and the magnetic field B. Our results suggest that the magnetic field and current configurations inside flux ropes deviate substantially from a force-free state in typical coronagraph fields of view, validating the idea of CMEs being driven by Lorentz self-forces.