T. A. Howard

ON THE MAGNETIC FLUX BUDGET IN LOW-CORONA MAGNETIC RECONNECTION AND INTERPLANETARY CORONAL MASS EJECTIONS

Wepresentthefirstquantitativecomparisonbetweenthetotalmagneticreconnectionfluxinthelowcoronainthe wake of coronal mass ejections (CMEs) and the magnetic flux in magnetic clouds (MCs) that reach 1 AU 2Y3 days after CME onset. The total reconnection flux is measured from flare ribbons, and the MC flux is computed using in situ observations at 1 AU, all ranging from 10 20 to 10 22 Mx. It is found that for the nine studied events in which the association between flares, CMEs,and MCs isidentified, the MCflux iscorrelatedwiththe total reconnection flux! r. Further, the poloidal (azimuthal) MC flux ! p is comparable with the reconnection flux ! r, and the toroidal (axial) MC flux ! t is a fraction of ! r. Events associated with filament eruption do not exhibit a different ! t, p-! r relation from events not accompanied by erupting filaments. The relations revealed between these independently measured physical quantities suggest that for the studied samples, the magnetic flux and twist of interplanetary magnetic flux ropes, reflected by MCs, are highly relevant to low-corona magnetic reconnection during the eruption. We discuss the implications of this result for the formation mechanism of twisted magnetic flux ropes, namely, whether the helical structure of themagnetic flux ropeis largelypre-existing or formed in situ by low-coronamagnetic reconnection.We alsomeasuremagneticfluxencompassedincoronaldimmingregions(! d)anddiscussitsrelationtothereconnection flux inferred from flare ribbons and MC flux.

On the Evolution of Coronal Mass Ejections in the Interplanetary Medium

Two coronal mass ejections (CMEs) are presented which were tracked through the LASCO field of view (FOV) within 30 R? and later as interplanetary CMEs (ICMEs) through the SMEI FOV from 80 to 150 R?. They were also associated with erupting filaments observed by EIT, providing information on trajectory of propagation. This allowed three-dimensional reconstructions of CME/ICME geometry, along with corrected (not sky plane projected) measurements of distance-time (DT) plots for each event to ~0.5 AU. An investigation of morphology was conducted. The results suggest that fine structures of the CMEs are eroded by the solar wind, and curvature becomes more sharply convex outward, suggesting that ICME footpoints remain fixed to the Sun even at 0.5 AU. We also present two models describing the evolution of CMEs/ICMEs at large distances from the Sun (far from the launch mechanism and effects of gravity and solar pressure) and consider two drag models: aerodynamic drag and snowplow. There was little difference between these, and their DT profiles matched well with the SMEI data for event 1. Event 2 showed a net acceleration between the LASCO and SMEI FOVs and we could match the data for this event well by introducing a driving Lorentz force. ICME mass almost doubled as a result of swept-up solar wind material from the snowplow model. Finally, we compared the geometry and kinematics of the ICME with that produced by the HAFv2 model and found that the model reasonably matched the geometry, but overestimated the ICME speed.

Inner Heliospheric Flux Rope Evolution via Imaging of Coronal Mass Ejections

Understanding the evolution of flux ropes in coronal mass ejections (CMEs) is of importance both to the scientific and technological communities. Scientifically their presence is critical to models describing CME launch and they likely play a role in CME evolution. Technologically they are the major contributor to severe geomagnetic storms. Using a new processing technique on the STEREO/SECCHI heliospheric imaging data, we have tracked a magnetic flux rope observed by the Wind spacecraft in December 2008 to its origins observed by coronagraphs. We thereby establish that the cavity in the classic three-part coronagraph CME is the feature that becomes the magnetic cloud. This implies that the bright material ahead of the cavity is piled-up coronal or solar wind material. We track the evolution of the cavity en-route and find that its structure transforms from concave inward (curving away from the Sun) to concave outward (toward the Sun) around 0.065 AU from the Sun. The pileup was tracked and its leading edge remained concave inward throughout its journey. Two other CMEs in January 2009 are also inspected and a similar cavity is observed in each, suggesting that they too each contained a flux rope. The results presented here are the first direct observation, through continuous tracking, associating a particular flux rope observed in situ with the same flux rope before ejection from the corona. We speculate that detailed heliospheric imagery of CMEs may lead to a means by which flux ropes can be identified remotely in the heliosphere.

Statistical survey of earthbound interplanetary shocks, associated coronal mass ejections and their space weather consequences

A comprehensive statistical analysis of events relevant to space weather over the 80 month period from January 1998 to August 2004 is presented. A database has been constructed using data from instruments from the SOHO, ACE, WIND and GOES spacecraft, as well as ground magnetometer data. Parameters investigated include times and epochs of halo and partial halo coronal mass ejections (HCMEs) along with details of the interplanetary shock at L1 (0.99 AU), namely the changes in the interplanetary magnetic field and solar wind density, and shock speed. Transit time to the Earth and average transient speed have also been determined, along wilh the projected speed and angular width of the HCME at the Sun. An estimate is made of the acceleration of the transients on their passage from the Sun to the Earth, and associated solar flare data are considered. Finally, the geoeffectiveness of the events are analysed using A p , Dst and sudden commencement data. We found that just over a quarter of the 938 HCMEs observed by LASCO were associated with a forward shock near L1, suggesting that around half of the Earthbound HCMEs are either deflected away from the Sun-Earth line or do not form a shock. Around half of the shocks went on to cause a geomagnetic storm, consistent with a southward B IMF occurring 50% of the time. There was a general tendency for HCME and shock speeds to be more varied (with more events at higher speeds) around solar maximum, and most events decelerated in transit to the Earth, implying a speed equalisation between the HCME shock and surrounding solar wind, although an assumption of a constant acceleration appears to be invalid. Only around 40% of the shock/storms were associated with an X or M class flare, and there appears to be no relationship between flare intensity and any physical parameter close to the Earth, except in extreme cases. There was a tendency for HCME speed near the Sun to increase with flare intensity. This casts doubt on the validity of using flare data alone as an effective space weather forecaster.

DIRECT OBSERVATION OF A COROTATING INTERACTION REGION BY THREE SPACECRAFT

White-light observations of interplanetary disturbances have been dominated by interplanetary coronal mass ejections (ICMEs). This is because the other type of disturbance, the corotating interaction region (CIR), has proved difficult to detect using white-light imagers. Recently, a number of papers have appeared presenting CIR observations using the Solar Terrestrial Relations Observatory (STEREO) Heliospheric Imagers (HIs), but have mostly only focused on a single spacecraft and imager. In this paper, we present observations of a single CIR that was observed by all three current white-light heliospheric imagers (SMEI and both STEREO HIs), as well as the in situ instruments on both STEREO satellites and ACE. We begin with a discussion of the geometry of the CIR structure, and show how the apparent leading edge structure is expected to change as it corotates relative to the observer. We use these calculations to predict elongation-time profiles for CIRs of different speeds for each of the imagers, and also to predict the arrival times at the in situ instruments. We show that although all three measured different parts, they combine to produce a self-consistent picture of the CIR. Finally, we offer some thoughts on why CIRs have proved so difficult to detect in white-light heliospheric images.

ON THE CORONAL MASS EJECTION ONSET AND CORONAL DIMMING

A comprehensive case and statistical study of CME onsets has been conducted on the solar limb using the CDS, LASCO and EIT instruments aboard the SOHO spacecraft. This is the first dedicated campaign to establish firmly the EUV signatures of CME onsets and is based on a series of low-corona observing campaigns made in 2002. The event database consisted of 36 multiple emission line sequences observed with CDS and the study builds, in particular, on studies of EUV coronal dimming which have been associated with CME onsets. We witness a range of dimming events in EUV coronal emission line data. Shorter events, commonly of duration < 4 hours, we find are indirectly associated with CME onsets whereas longer-duration dimmings (> 4 hours) appear to be either due to coronal evolution or rotational effects. However, for some CME onsets, where the CDS pointing was appropriate, no dimming was observed. Dimming observed in EIT typically occurred immediately after the launch of a loop or prominence, and in 5 out of 9 events there is evidence of a matter buildup within the loop before launch. A total of 10 events occurred where CDS was used to directly observe the CME footprint, but no relationship between these events was found. The results suggest that the response of the corona to a CME launch differs between the low (1.0 R⊙≤R≤1.2 R⊙) and middle (1.2 R⊙

SOLAR MAGNETIC TRACKING. IV. THE DEATH OF MAGNETIC FEATURES

The removal of magnetic flux from the quiet-Sun photosphere is important for maintaining the statistical steady state of the magnetic field there, for determining the magnetic flux budget of the Sun, and for estimating the rate of energy injected into the upper solar atmosphere. Magnetic feature death is a measurable proxy for the removal of detectable flux, either by cancellation (submerging or rising loops, or reconnection in the photosphere) or by dispersal of flux. We used the SWAMIS feature tracking code to understand how nearly 2 ? 104 magnetic features die in an hour-long sequence of Hinode/SOT/NFI magnetograms of a region of the quiet Sun. Of the feature deaths that remove visible magnetic flux from the photosphere, the vast majority do so by a process that merely disperses the previously detected flux so that it is too small and too weak to be detected, rather than completely eliminating it. The behavior of the ensemble average of these dispersals is not consistent with a model of simple planar diffusion, suggesting that the dispersal is constrained by the evolving photospheric velocity field. We introduce the concept of the partial lifetime of magnetic features, and show that the partial lifetime due to Cancellation of magnetic flux, 22?hr, is three times slower than previous measurements of the flux turnover time. This indicates that prior feature-based estimates of the flux replacement time may be too short, in contrast with the tendency for this quantity to decrease as resolution and instrumentation have improved. This suggests that dispersal of flux to smaller scales is more important for the replacement of magnetic fields in the quiet Sun than observed bipolar cancellation. We conclude that processes on spatial scales smaller than those visible to Hinode dominate the processes of flux emergence and cancellation, and therefore also the quantity of magnetic flux that threads the photosphere.

INBOUND WAVES IN THE SOLAR CORONA: A DIRECT INDICATOR OF ALFVÉN SURFACE LOCATION

The tenuous supersonic solar wind that streams from the top of the corona passes through a natural boundary—the Alfven surface—that marks the causal disconnection of individual packets of plasma and magnetic flux from the Sun itself. The Alfven surface is the locus where the radial motion of the accelerating solar wind passes the radial Alfven speed, and therefore any displacement of material cannot carry information back down into the corona. It is thus the natural outer boundary of the solar corona and the inner boundary of interplanetary space. Using a new and unique motion analysis to separate inbound and outbound motions in synoptic visible-light image sequences from the COR2 coronagraph on board the STEREO-A spacecraft, we have identified inbound wave motion in the outer corona beyond 6 solar radii for the first time and used it to determine that the Alfven surface is at least 12 solar radii from the Sun over the polar coronal holes and 15 solar radii in the streamer belt, well beyond the distance planned for NASAs upcoming Solar Probe Plus mission. To our knowledge, this is the first measurement of inbound waves in the outer solar corona and the first direct measurement of lower bounds for the Alfven surface.

Coronal Mass Ejections : An Introduction

Chapter 1: Introduction.- Chapter 2: History.- Chapter 3: Summary of Spacecraft.- Chapter 4: How We Observe CMEs.- Chapter 5: Geometry of CMEs and ICMEs.- Chapter 6: Radio Astronomical Techniques.- Chapter 7: Associated Phenomena.- Chapter 8: CME Onset and Initial Acceleration.- Chapter 9: CME Evolution.- Chapter 10: Interaction With the Earth and Other Planets - Contribution to Space Weather.- Chapter 11: Summary and Discussion.- Glossary.- Index.

Disconnecting open solar magnetic flux

Disconnection of open magnetic flux by reconnection is requi red to balance the injection of open flux by CMEs and other eruptive events. Making use of recent advances in heliospheric background subtraction, we have imaged many abrupt disconnection events. These events produce dense plasma clouds whose distinctive shape can now be traced from the corona across the inner solar system via heliospheric imaging. The morphology of each initial event is characteristic of magnetic reconnection across a current sheet, and the newly-disconnected flux takes the form of a “U”-shaped lo op that moves outward, accreting coronal and solar wind material. We analyzed one such event on 2008 December 18 as it formed and accelerated at 20 m s −2 to 320 km s −1 , expanding self-similarly until it exited our field of view 1 .2 AU from the Sun. From acceleration and photometric mass estimates we derive the coronal magnetic field strength to be 8 μT , 6R⊙ above the photosphere, and the entrained flux to be 1 .6× 10 11 W b (1.6× 10 19 Mx). We model the feature’s propagation by balancing inferred magnetic tension force against accretion drag. This model is consistent with the feature’s behavior and accepted solar wind parameters. By counting events over a 36 day window, we estimate a global event rate of 1 d −1 and a global solar minimum unsigned flux disconnection rate of 6 ×10 13 W b y −1 (6×10 21 Mx y −1 ) by this mechanism. That rate corresponds to ∼ −0.2 nT y −1 change in the radial heliospheric field at 1 AU, indicating th at the mechanism is important to the heliospheric flux balance.