The Dependence of Solar Energetic Particle Event Characteristics on Heliographic Longitude

Details The Dependence of Solar Energetic Particle Event Characteristics on Heliographic Longitude The Dependence of Solar Energetic Particle Event Characteristics on Heliographic Longitude

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh12a..04w&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SH12A-04

Astronomy and Astrophysics

Astrophysics

[2101] Interplanetary Physics / Coronal Mass Ejections, [2134] Interplanetary Physics / Interplanetary Magnetic Fields, [7514] Solar Physics, Astrophysics, And Astronomy / Energetic Particles, [7519] Solar Physics, Astrophysics, And Astronomy / Flares

Scientific paper

Characteristics of solar energetic particle (SEP) events observed at 1 AU from the Sun are known to depend on the magnetic connection of the observer to the acceleration region as well as on whether the acceleration is associated with a shock ("gradual" SEP events) or with flare reconnection ("impulsive" events). Much of our present knowledge of the dependence of SEP event characteristics on heliographic longitude has been the result of single-spacecraft statistical studies that identified systematic correlations of various event properties (e.g., rise time, peak intensity, composition) with the location of the observer relative to the magnetic field line having the best connection to the solar source region. Several studies combining SEP observations made near Earth with data from spacecraft at other heliographic longitudes (Helios and various planetary missions) or latitudes (Ulysses) have also contributed. The two STEREO spacecraft, in conjunction with ACE and Wind operating near Earth, have enabled the first systematic study of the longitudinal dependence of SEP characteristics in individual events at 1 AU. The years 2007 through 2010, during which longitudinal separations of the STEREOs from Earth increased from 0o to nearly 90o, were characterized by a very low level of solar activity. These conditions allowed unusually sensitive measurements of a number of impulsive SEP events and led to the realization that particles accelerated in these events are frequently detectable over a range of longitudes much greater than expected from simple models in which flare-accelerated particles escape into the heliosphere along small clusters of open field lines involved in a reconnection event. Particle fluences, however, were found to have a strong dependence on separation from the well-connected longitude. Since early 2011, as solar activity has been on the increase, a number of gradual SEP events have been observed over the >180o range of heliographic longitudes now spanned by the STEREOs and ACE, enabling multipoint studies of this type of event as well. The interpretation of the SEP data, which are sensitive to both acceleration and transport effects, is benefitting significantly from the 3D observations of coronal activity and CMEs that are now being provided by STEREO, SDO, and SOHO. We will present recent observations of the longitudinal dependence of SEP event characteristics and discuss how they are beginning to modify our understanding of these events. This work was supported by NASA at Caltech (under grants NNX08AI11G, NNX10AQ68G, and NNX10AQ68G, and through UC Berkeley under contract NAS5-03131), JPL, APL (under grant NNX10AT75G), and LMSAL (under grants NNX07AN13G and NNX08AB23G).

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