S. J. Tappin

Low-energy solar electrons and ions observed at Ulysses February-April, 1991 - The inner heliosphere as a particle reservoir

Ulysses observations at 2.5 AU of 38–315 keV electrons and 61–4752 keV ions during February-April 1991 suggest in several ways that, during periods of sustained high solar activity, the inner heliosphere serves as a “reservoir” for low-energy solar particles. Particle increases were not associated one-to-one with large X-ray flares because of their poor magnetic connection, yet intensities in March-April remained well above their February levels. The rise phase of the particle event associated with the great flare of 2245UT March 22 lasted most of two days, while throughout the one-week decay phase, the lowest-energy ion fluxes were nearly equal at Ulysses and Earth (IMP-8).

LASCO observations of an Earth‐directed coronal mass ejection on May 12, 1997

Coronal mass ejections (CMEs) that occur near the center of the solar disk are most likely to impact Earth. Detection of such events as ‘halos’ in white-light coronagraphs has been somewhat controversial in recent years. We present observations from the LASCO coronagraphs on SOHO that provide convincing evidence of the detection of an Earth-directed CME on May 12, 1997. The event began at about 04:35 UT and propagated outwards from the Sun with a projected speed of around 250 km s−1. Using some reasonable assumptions about the geometry of the CME, we estimate the true speed to be around 600 km s−1. The onset of the event in LASCO is coincident (to within measurement uncertainties) with an eruptive event detected in extreme ultraviolet observations of the solar disk by the SOHO EIT. This is the first reported observation of a halo CME at projected distances greater than 10 R⊙, with a clearly identifiable solar origin. We discuss the possibility that at least some of the enhanced brightness observed by LASCO may be due to a compressional wave propagating in the corona.

Tracking a major interplanetary disturbance with SMEI

[1] We present the first clear observations of an Earth-directed interplanetary disturbance tracked by the Solar Mass Ejection Imager (SMEI). We find that this event can be related to two halo CMEs seen at the Sun about 2 days earlier, and which merged in transit to 1 AU. The disturbance was seen about 16 hours before it reached Earth, and caused a severe geomagnetic storm at the time which would have been predicted had SMEI been operating as a real-time monitor. It is concluded that SMEI is capable of giving many hours advance warning of the possible arrival of interplanetary disturbances.

Corotating particle enhancements out of the ecliptic plane

We have studied > 40 keV ion and electron enhancements seen at Ulysses from a series of corotating interaction regions (CIR), from mid-1992 until the end of 1993. The event intensity increased up to latitude 20°S; after that the ion flux maxima decreased by ∼23%/deg while the electron maxima decreased more erratically, and more slowly, during the last half of 1993. After April, 1993, the electron increases lagged the ions by up to four days. The electron energy spectrum hardened both towards the end of the events and as the latitude went above ∼35°. The electron/ion delays and the electron spectral hardening are related to the three-dimensional structure of the reverse shocks at high latitudes. The H/He ratio at ∼0.4-1.2 MeV/nucleon decreased towards the end of each event; the mean minimum value was 3.5±0.3, lower than observed in earlier studies of CIR-associated events near the ecliptic plane. The He increases have recurred with a period of 26.0 days. There have been two phase changes: +4.3 days (February 1993) and +2.4 days (August 1993), related to changes in the coronal structure of the source regions for the high speed solar wind streams.

First View of the Extended Green-Line Emission Corona At Solar Activity Minimum Using the Lasco-C1 Coronagraph on Soho

The newly developed C1 coronagraph as part of the Large-Angle Spectroscopic Coronagraph (LASCO) on board the SOHO spacecraft has been operating since January 29, 1996. We present observations obtained in the first three months of operation. The green-line emission corona can be made visible throughout the instruments full field of view, i.e., from 1.1 R⊙ out to 3.2 R⊙ (measured from Sun center). Quantitative evaluations based on calibrations cannot yet be performed, but some basic signatures show up even now: (1) There are often bright and apparently closed loop systems centered at latitudes of 30° to 45° in both hemispheres. Their helmet-like extensions are bent towards the equatorial plane. Farther out, they merge into one large equatorial ‘streamer sheet’ clearly discernible out to 32 R⊙. (2) At mid latitudes a more diffuse pattern is usually visible, well separated from the high-latitude loops and with very pronounced variability. (3) All high-latitude structures remain stable on time scales of several days, and no signature of transient disruption of high-latitude streamers was observed in these early data. (4) Within the first 4 months of observation, only one single ‘fast’ feature was observed moving outward at a speed of 70 km s-1 close to the equator. Faster events may have escaped attention because of data gaps. (5) The centers of high-latitude loops are usually found at the positions of magnetic neutral lines in photospheric magnetograms. The large-scale streamer structure follows the magnetic pattern fairly precisely. Based on our observations we conclude that the shape and stability of the heliospheric current sheet at solar activity minimum are probably due to high-latitude streamers rather than to the near-equatorial activity belt.

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.

The Relationship of Green-Line Transients to White-Light Coronal Mass Ejections

We report observations by the Large Angle Spectrometric Coronagraph (LASCO) on the SOHO spacecraft of three coronal green-line transients that could be clearly associated with coronal mass ejections (CMEs) detected in Thomson-scattered white light. Two of these events, with speeds >25 km s-1, may be classified as ‘whip-lite’ transients. They are associated with the core of the white-light CMEs, identified with erupting prominence material, rather than with the leading edge of the CMEs. The third green-line transient has a markedly different appearance and is more gradual than the other two, with a projected outward speed < 10 km s-1 . This event corresponds to the leading edge of a’ streamer blowout’ type of CME. A dark void is left behind in the emission-line corona following each of the fast eruptions. Both fast emission-line transients start off as a loop structure rising up from close to the solar surface. We suggest that the driving mechanism for these events may be the emergence of new bipolar magnetic regions on the surface of the Sun, which destabilize the ambient corona and cause an eruption. The possible relationship of these events to recent X-ray observations of CMEs is briefly discussed.

Co-rotating particle enhancements out of the ecliptic plane

The characteristics of the recurrent electron (38–53 keV) and ion (>∼0.5 MeV) enhancements observed by Ulysses from mid-1992 to April 1994 are presented. The magnitude of the ion flux increases reached a maximum at a latitude of ∼20°S and decreased afterwards by ∼23%/degree until early 1994. The magnitude of the electron increases showed a similar trend until May, 1993, after which time it became approximately constant, until it started to increase again in early 1994. The electron enhancements have lagged the protons by up to 5 days once Ulysses left the heliospheric current sheet (mid-1993). The electron spectral index tended to harden (a) during the decay of the event and (b) as the latitude increased, up to 50°S. The events have recurred on a 26.0 day period, but with significant phase shifts over the 25 rotations studied. The H/He ratio decreases across the maximum intensity. The mean minimum value for H/He was 3.5±0.3, lower than that measured in previous studies in the ecliptic plane.

THE THOMSON SURFACE. III. TRACKING FEATURES IN 3D

In this, the final installment in a three-part series on the Thomson surface, we present simulated observations of coronal mass ejections (CMEs) observed by a hypothetical polarizing white light heliospheric imager. Thomson scattering yields a polarization signal that can be exploited to locate observed features in three dimensions relative to the Thomson surface. We consider how the appearance of the CME changes with the direction of trajectory, using simulations of a simple geometrical shape and also of a more realistic CME generated using the ENLIL model. We compare the appearance in both unpolarized B and polarized pB light, and show that there is a quantifiable difference in the measured brightness of a CME between unpolarized and polarized observations. We demonstrate a technique for using this difference to extract the three-dimensional (3D) trajectory of large objects such as CMEs. We conclude with a discussion on how a polarizing heliospheric imager could be used to extract 3D trajectory information about CMEs or other observed features.

THE THOMSON SURFACE. II. POLARIZATION

The solar corona and heliosphere are visible via sunlight that is Thomson-scattered off free electrons, yielding a radiance against the celestial sphere. In this second part of a three-article series, we discuss linear polarization of this scattered light parallel and perpendicular to the plane of scatter in the context of heliospheric imaging far from the Sun. The difference between these two radiances (pB) varies quite differently with scattering angle, compared to the sum that would be detected in unpolarized light (B). The difference between these two quantities has long been used in a coronagraphic context for background subtraction and to extract some three-dimensional information about the corona; we explore how these effects differ in the wider-field heliospheric imaging case where small-angle approximations do not apply. We develop an appropriately simplified theory of polarized Thomson scattering in the heliosphere, discuss signal-to-noise considerations, invert the scattering equations analytically to solve the three-dimensional object location problem for small objects, discuss exploiting polarization for background subtraction, and generate simple forward models of several classes of heliospheric feature. We conclude that pB measurements of heliospheric material are much more localized to the Thomson surface than are B measurements, that the ratio pB/B can be used to track solar wind features in three dimensions for scientific and space weather applications better in the heliosphere than corona, and that,as an independent measurement of background signal, pB measurements may be used to reduce the effect of background radiances including the stably polarized zodiacal light.