LWS TR&T Focus Teams:
Factors that Control the Highly Variable Intensity and Evolution of Solar Particle Events
Team Leader: Nat Gopalswamy (GSFC)
Target Description: It is widely believed that the largest solar energetic particle (SEP) events are caused by CME-driven shock acceleration (although other processes may also contribute). However, observationally, the efficiency of this process appears to be highly variable. As an example, a 2001 study found more than a thousand-fold spread in the intensity of >20 MeV protons accelerated by CMEs of the same velocity. On the other hand, a 2004 statistical study suggested that CMEs that erupt soon after a previous CME from the same active region are much more efficient in accelerating particles than those erupting into a pristine environment. Evidently, once a large eruption occurs, coronal and interplanetary properties play a key role, along with CME properties, in determining how intense the SEP event will be. This could be due to a stronger turbulence level or a larger population of seed particles at the second shock; other suggested explanations include differences in the open and closed field-line geometry, or a lowering of the Alfven velocity, leading to the formation of a stronger shock. Among the additional factors that likely affect acceleration and transport efficiency are shock geometry, global IMF structure, connection longitude, proton-amplified Alfven waves, and streaming limits.
This FST is timely. First, there are ~100 cycle-23 SEP events in the available database with broad SEP and solar-wind/ICME coverage (ACE, Cluster, GOES, SAMPEX, SOHO, Ulysses and Wind), and excellent near-Earth CME and other imaging (SOHO, RHESSI, TRACE, Hinode). With experience from these events as a guide, it will be possible to take full advantage of new, multispacecraft data from STEREO and near- Earth assets. For cycle-24 events there is a unique opportunity to make multipoint measurements of SEP, solar wind, and ICME properties, providing much greater detail on coronal/interplanetary initial conditions, and on the resulting longitudinal and temporal evolution of SEP events. In addition, for the first time, three-point CME imaging will provide higher precision and more detailed CME properties, along with multipoint coronal imaging. Finally, SDO will enable greatly improved capabilities to characterize the dynamic solar activity and its effects on the inner-heliosphere. Never before have such an array of distributed in situ, imaging, and modeling assets been available for this focused study.
Goals and Measures of Success: The scientific goal of this FST is to identify the key properties that characterize when (a) SEP acceleration is efficient (large, intense events with rapid onsets) and when it is not (small, slowly developing events), and (b) to identify the conditions that facilitate efficient SEP radial and longitudinal transport; and develop a physical understanding of how these key properties function with theory, modeling, and simulations. The practical goal is to enable a forecaster, during the first one-two hours following an eruption, to use multipoint real-time data; knowledge of initial coronal and interplanetary conditions; models and experience to make more accurate predictions of how intense, long-lasting, and far-reaching the SEP event will (or will not) be. The goal here is not to predict how or when an active region will erupt. The primary measures of success of this work would be quantifying and then improving our current ability to combine real-time data (CME, radio, X and gamma-ray, and other imaging), along with data on initial conditions (IMF, solar wind, magnetic configuration), to forecast, within the first one-two hours following an eruption, the resulting peak intensity, fluence, composition, spatial evolution, and duration of accelerated particles, including the possibility of a large shock-spike event or the possibility of an early łall-clear˛ announcement. Measures of success are the following: Development of improved models of E and F region plasma instabilities and turbulence; Establishment of the connection (if any) between E and F region irregularities; Identification of the causes of day-to-day variability of irregularities; Understanding of the connection between large-scale ionospheric processes and the development of electron density irregularities (e.g., equatorial spread F); and € Development of a predictive capability for irregularity onset and evolution.
Types of Investigations: