Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 40, 2002)

Science Nugget: October 4, 2002

Possible Solar Signatures of SEP Events


Solar energetic particle (SEP) events, especially those with high-energy ions, present a major space weather threat. For example, an alert is issued when the flux of protons > 10 MeV as measured by the GOES satellite (at geosynchronous) exceeds 10 particle flux unit (PFU, particles/s/cm/sr). We solar physicists want to know what observable solar signatures lead to major SEP events. But this is not straightforward, because an SEP event requires not just particle acceleration but also some conditions that allow the particles to get out to the interplanetary space.

For long time the flare had been the top suspect for an obvious reason. Before the spacecraft age, ground level events (GLEs) as observed with neutron monitors gave a primary source of information on the particles reaching the Earth. These GLEs, produced by protons in the GeV range, almost always followed intense flares. Several lines of theory were proposed so that particles could escape (or diffuse) from the flare site to the well-connected longitudes, from where they would propagate along the magnetic field in the form of the Parker spiral. This spiral field results from the solar rotation and the solar wind that brings the magnetic field out in the radial direction. It is easy to understand how the longitude of the field lines connecting the Sun and the Earth changes with the solar wind speed, as shown in this plot.. For fast solar wind, the well-connected longitudes move eastward from ~W60, corresponding to ~400 km/s, and the length of the spiral between the Sun and the Earth gets shorter.

The paradigm changed as a result of studies since the 1980s which established a better association of SEP events with CMEs. As shown in a cartoon taken from a review paper by Reames (1999), many researchers on SEPs seem to agree that intense and gradual SEP events are due to interplanetary shock waves driven by CMEs, and that particles accelerated in the flare (which are characterized by, for example, a high 3He/4He ratio) are observed only as a small and impulsive event (only when the flare is located at the well-connected longitude). Indeed, it would probably be impossible to understand the following example (from August 15-16 2001) without introducing wide shocks.

However, this change of view does not help us since many fast CMEs (e.g., > 1500 km/s, as observed by LASCO) have nothing to do with SEP events, and more importantly we still cannot isolate the solar signatures of shock-associated CMEs. Actually, CME initiation in general is still an open issue. It is our belief that a CME can be directly observed in X-ray/EUV images when there is enough temporal/spatial/temperature coverage, unless it is launched at very high altitudes. This nugget tries to associate those outward motions seen in the early phase of flares with the onset of fast CMEs and CME-driven shocks that are responsible for SEP events.


SEP events with quick onset

When we discuss the solar connection of SEP events, it is probably better to concentrate on those that do not take too long to start after the flare/CME, since the delayed onset implies acceleration/propagation away from the Sun, where the solar effect may be largely lost. It is known that such delayed events are often associated with activity in the eastern hemisphere.

We first pick up all the major SEP events during 1997-2001 that are included in the NOAA list (as maintained by Joe Kunches and others at ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SAT_ENVIRONMENT/PARTICLES/p_events.lst), and then rearrange the events in the order of the peak flux of > 10 MeV protons. We plot their time profiles with respect to GOES X-ray light curves. The following plot shows all the SEP events that started within two hours from the flare associated with the SEP-producing CME.

The association of the SEP events with the flares/CMEs is somewhat uncertain as we go to less intense events. For example, in the last event (7-May-2001), the C3 flare around 12:00 seems to have nothing to do with the CME/SEP, but there was a CME preceded by loop expansions from the backside around the time of the flare. This tells us that the CME-flare association without using coronal images could be erroneous.

An interesting thing from this plot is that some of those events whose peaks of proton flux come much later (e.g., with the shock arrival) have a component that indicates acceleration close to the Sun (e.g., 4-Nov-2001, 14-Jul-2000, 30-Sep-98, etc.) and that they are not always located at the well-connected longitudes, sometimes even in the eastern hemisphere. Conversely, several major events closer to the well-connected longitudes are missing from the plot (22-Nov-2001, 2-Apr-2001, etc.)


Flare ejections and SEP events

SXT data show many flares with ejections, and these flares are known to be associated with CMEs. Indeed, these ejections may be among the few things in imaging data that directly capture CMEs in the low corona -- other signatures are indirect and we have to wait longer to see them (dimming, arcade formation, etc.) Then we may ask if the properties of flare ejections can be used to predict SEP events, because they may give information on the shocks driven by fast motions. In order to observe fast motions, high time resolution is very important. SXT large field-of-view data in flare mode were often taken with the cadence of 10-30 seconds. Now we can also include TRACE data that have not only high spatial resolution but also high temporal resolution.

A preliminary survey of data for flare ejections indicates that they may usually represent coronal shocks (as manifested by metric type II bursts) and not necessarily SEP-producing CME shocks. For example, the following movie for the 2-Mar-2000 flare (click the thumbnail) shows fast ejections (up to 2000 km/s) near the well-connected longitude, and yet the associated CME was rather narrow (63 degrees) and there was no SEP event. This suggests that the possible shock driven by the flare ejections did not propagate large distances.

But there seem to be other cases where ejections in the early phase of the flare may indicate the relation with CME shocks. Look at the following movie for the Bastille Day 2000 event in the TRACE UV channel. We see a moving front ahead of the eruptive filament as early as 10:08 UT, with their legs appearing brighter. This front seems to be the earliest manifestation of the CME shock, and by moving westward from W07 as it accelerated particles it may have intersected the field lines connecting to the Earth at an early stage of the CME propagation. This example indicates that some of our flare ejections may represent the early manifestation of CME shocks and that their directions may be important in predicting the rising phase of SEP events.

Concluding remarks

By looking at flare ejections in many events in movie form, we tentatively conclude that those appearing as smooth curves tend to have close links with CME shocks and that those appearing as plasmoids or bullets tend to be confined in the corona. The 2-Mar-2000 event seems to belong to the second category, but there are many cases where the ejections do not go beyond certain distances, as shown in the following movie for another X-class flare.

This is an example of "confined ejection" and there was no associated CME or decametric type II burst. An interesting thing is that this event showed a metric type II burst between 12:13 and 12:18 UT in the range of 150-45 MHz (thanks to Dr. A. Klassen @ Potsdam), although it was not documented in the Solar Geophysical Data. These are essentially different from ejections that may have influence on CME shocks. Although some researchers keep explaining flare ejections in a unified manner (e.g., the often quoted cartoon), we should go beyond that and try to extract more information from the wealth of data.

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October 4, 2002

N. Nitta (NVN) (nitta@lmsal.com)