Fast Halo Coronal Mass Ejections Research Articles

Solar Fast‐Wind Regions as Sources of Shock Energetic Particle Production

If the production of solar energetic particles (SEPs) near the Sun is due to shocks driven by coronal mass ejections (CMEs), then two factors might favor SEP acceleration in slow rather than fast solar wind streams. The first is that both the MHD fast-mode and solar wind flow speeds are higher in the fast-wind streams. The second is that shock seed populations in the fast-wind streams consist of weak suprathermal ion tails with soft spectra. An earlier study used observed fast west-limb or halo CMEs and solar wind O+7/O+6 ratios to compare associated SEP events and fast CMEs in fast wind with those in slow wind. Properties of the few fast CMEs in fast-wind regions suggested a bias in terms of associated SEP event size and/or CME speed against shock production of SEPs in fast-wind regions. Here the previous 1998-2000 study is expanded through 2002. The results of the much larger event sample now show no significant bias against SEP production in fast-wind regions. We discuss the implications of this result for shock seed populations and for shock propagation in coronal fast-wind regions.

Observational Evidence of a Magnetic Flux Rope Eruption Associated with the X3 Flare on 2002 July 15

We present the study of an eruption from the low solar atmosphere (photosphere/chromosphere) as seen in Transition Region and Coronal Explorer 1600 Angstrom images and with the Solar and Heliospheric Observatory Michelson Doppler Imager. The eruption reached its maximum at 20: 08 UT on 2002 July 15 in the NOAA Active Region 10030 ( N19degrees, W01degrees), accompanied by an X3 flare and followed by a fast-halo coronal mass ejection. The main observational results from the data are as follows: ( 1) the erupting plasma was in a rapidly rising, twisted ropelike structure; ( 2) the eruption occurred just preceding the onset of its driven flare; and (3) the morphology and magnetic flux of one slender footpoint (similar to9000 km in length) of the rope developed rapidly on the photosphere. This structure disappeared in white light and in the magnetograms within 60 minutes. This evidence supports the erupting flux rope model. Our data favor the idea that a catastrophic loss of MHD equilibrium can be the primary driving mechanism for the rapid ejection of a flux rope. This conclusion is based on the judgment that the ambient fields of the flux rope were partly opened as a result of the magnetic reconnection.

Solar Energetic Particle Production by Coronal Mass Ejection–driven Shocks in Solar Fast‐Wind Regions

Gradual solar energetic particle (SEP) events at 1 AU are produced by coronal/interplanetary shocks driven by coronal mass ejections (CMEs). Fast (vCME 900 km s-1) CMEs might produce stronger shocks in solar slow-wind regions, where the flow and fast-mode MHD wave speeds are low, than in fast-wind regions, where those speeds are much higher. At 1 AU the O+7/O+6 ratios distinguish between those two kinds of wind streams. We use the 20 MeV proton event intensities from the EPACT instrument on Wind, the associated CMEs observed with the LASCO coronagraph on SOHO, and the ACE SWICS solar wind values of O+7/O+6 to look for variations of peak SEP intensities as a function of O+7/O+6. No significant dependence of the SEP intensities on O+7/O+6 is found for either poorly connected or well-connected CME source regions or for different CME speed ranges. However, in the 3 yr study period we find only five cases of SEP events in fast wind, defined by regions of O+7/O+6 < 0.15. We suggest that in coronal holes SEP acceleration may take place only in the plume regions, where the flow and Alfv?n speeds are low. A broad range of angular widths are associated with fast (vCME ? 900 km s-1) CMEs, but we find that no fast CMEs with widths less than 60? are associated with SEP events. On the other hand, nearly all fast halo CMEs are associated with SEP events. Thus, the CME widths are more important in SEP production than previously thought, but the speed of the solar wind source regions in which SEPs are produced may not be a factor.

Telescopic and Microscopic views of the magnetosphere: Multispacecraft observations

The magnetospheric research community has long sought the capability to view the Sun-Earth system in a global way and to probe concurrently the microphysical details of key physical regions. This objective has now been substantially realized with the combination of the CLUSTER and IMAGE missions. With the additional use of SOHO, ACE, FAST, SAMPEX, POLAR, and geostationary orbit spacecraft, there is a remarkable ability to apply both telescopic and microscopic principles. As an example, a bright active region on the Sun gave rise on 29 March 2001 to a fast halo coronal mass ejection (CME) event observed by SOHO instruments. Subsequently on 31 March, a strong interplanetary shock wave ahead of a magnetic cloud (probably arising from the earlier CME) passed the ACE spacecraft and hit the Earth’s magnetosphere. This driver compressed the subsolar magnetopause to ≤ 4 RE geocentric distance and initiated a powerful geomagnetic storm (minimum Dst ~ −360 nT). The CLUSTER set of four spacecraft were located in the midnight sector of the magnetosphere near perigee (r~4 RE) at ~0635 UT and observed a dispersionless injection of energetic (E ≥ 20 keV) electrons in association with a large magnetospheric substorm expansion phase onset (AE ~1200 nT). Concurrent to these in situ observations, the IMAGE spacecraft was returning a sequence of global Energetic Neutral Atom (ENA) images from the medium-energy (MENA) and high-energy (HENA) sensor systems. These data showed a very prominent injection of substorm ions in the premidnight (and postdusk) sector of the inner magnetosphere. This event is consistent with a substorm onset that pushed the substorm ‘injection boundary’ far inside of geostationary orbit and far toward the dusk sector. In another event on August 27, 2001, the IMAGE-CLUSTER combination provided evidence that magnetic reconnection began in the mid-tail plasma sheet some 7 min prior to auroral onset and brightening. Alternative interpretations may also be possible even with the multis-pacecraft data available. Notwithstanding, these data gave an unprecedented view both telescopically and microscopically of a magnetospheric substorm onset and help establish key process timing in the magnetosphere. Such available events reveal the power of multispacecraft observations.

Lasco and eit Observations of the Bastille day 2000 Solar Storm

The period of 10–14 July 2000 saw a large number of energetic solar events ending with a very energetic flare that was associated with a large solar energetic particle event and a fast halo coronal mass ejection (CME) that produced the largest geomagnetic disturbance since 1989. This paper tries to summarize the complex coronal activity observed during this period, in order to establish a background for a number of papers in this topical issue. The GOES X-ray data are presented. Data animations of observations from EIT and LASCO C2 and C3 are presented on the accompanying CD-ROM. The observations around the time of the three X-class flares are considered. EIT observations of the Bastille Day flare show coronal brightening followed by dimming. LASCO had good data coverage for all three events. For one of the flares, no coronal response was seen. The other two flares are associated with halo CMEs. The timing suggests that the start of the flares and CMEs are simultaneous to approximately 30 min. Analysis of the LASCO and EIT images following the Bastille Day flare show the arrival of energetic particles at SOHO at approximately 10:41 UT on 14 July. Individual features of these CMEs have been tracked and the height–time plots used to estimate the dynamics of the CMEs. The initial speed and deceleration of the halo CMEs estimated from the fitting of height–time plots are compared with the in-situ observations at L1. The three flares are identified as the solar sources of three shocks observed at 1 AU. Finally, it is stressed that global heliospheric effects during periods of exceptional activity should consider a cumulative scenario rather than events in isolation.

14 July 2000, a near-global coronal event and its association with energetic electron events detected in the interplanetary medium

On 14 July 2000, the LASCO coronagraphs showed a very fast halo coronal mass ejection in association with the radio bursts seen shortly after 10:00 UT. Radio imaging observations by the Nancay radioheliograph (NRH) of these bursts showed a very complex event that can be regarded as global: the sources encompassed all the visible range in longitude and a huge span in latitude. Another interesting feature of the radio event is its recurrent nature: after the most intense phase shortly after 10:00 UT, two other strong outbursts are detected, one at about 12:50 UT and another at about 13:48 UT. All of these sub-events showed similar development and likely evidence for CMEs. The launch of a CME in association with the 14:00 UT sub-event is inferred from WIND/WAVES, with interplanetary type II signatures in the hectometric wavelength range at that time. These later events were not detected by LASCO due to energetic particles hitting the CCD. During the Bastille Day event, energetic particle observations measured in situ by ACE/EPAM are dominated by energetic electrons. Changes in anisotropy and energy spectrum of the ∼38–350 keV electrons suggest a good correlation with the coronal radio observations. In addition to the three main radio events and particle observations, the NRH data reveal moving features in the southern hemisphere. These moving features, located at about 45 deg south and with an angular extent of about 45 deg, are illuminated by non-thermal electrons and are seen at distances up to 2.5 solar radii from the Sun center. More generally, we interpret the global and recurrent coronal activity, revealed by the radio data, as responsible for populating the interplanetary medium with energetic electrons.