ACE News Archives | ACE News #148 - February 28, 2012 |
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Figure 1: Longitudinal distribution of (a) 71-112 keV electron and (b) 25-53
MeV proton peak intensities. Black symbols indicate SEP events measured near
Earth by ACE (left) and SoHO (right). Red and blue symbols indicate events
measured by STEREO-A, STEREO-B, respectively. Events for which no intensity
increase was observed are plotted at the bottom horizontal axes. Magenta lines
connect events observed simultaneously by two or three spacecraft with
intensity enhancements. Orange lines connect those events for which no
intensity enhancement was observed. The gray area follows the functional form
j ~ exp[-(Φ-Φ0)
2/2σ2].
See text for best-fit parameter values.
The current fleet of spacecraft distributed throughout the inner heliosphere
allows the study of solar energetic particle (SEP) events during the rising
phase of solar cycle 24 from multiple vantage points
(see ACE News #139).
Simultaneous observations of SEP events by ACE, SoHO and the two STEREO
spacecraft allow us to estimate the longitudinal dependence of peak SEP
intensities. We selected 26 SEP events observed during the 3-year period
2009-2011 for which (1) a 25-53 MeV proton intensity enhancement was detected;
(2) a 71-112 keV electron intensity enhancement was observed by at least two
spacecraft; and (3) a solar origin (i.e. solar flare and/or filament eruption)
has been confidently identified.
Figure 1 shows (a) the 72-112 keV electron and (b) the 25-53 MeV proton peak
intensity observed in these events by ACE or SoHO (black dots), STEREO-A (red
circles) and STEREO-B (blue circles). Horizontal axes show the longitude
difference between the parent solar event and the footpoint of the nominal
magnetic field line connecting each spacecraft with the Sun (assuming a Parker
spiral with a 400 km/s solar wind speed). The magenta lines connect events
simultaneously observed by two or three spacecraft. The orange lines connect
events observed by three spacecraft, but in which one of the spacecraft did
not detect any particle increase (artificially plotted in the low horizontal
axes of the Figure).
We use the functional form
j = j0exp[-(Φ-Φ0)
2/2σ2].
to describe the
longitudinal distribution of peak intensities. By combining three pairs of
spacecraft for events observed by three spacecraft and one pair for events
observed by two spacecraft (i.e. using only those events connected by magenta
lines in the Figure) we obtain
Φ0=-18°±5°
and
σ=51°±2°
for 71-112 keV electrons and
Φ0=-14°±4°
and
σ=46°±2°
for 25-53 MeV protons.
These values are directly comparable to previous studies performed
with Helios and IMP-8 data from solar cycle 21 that obtained
Φ0=-11°±3°
and
σ=36°±2°
for 27-37 MeV protons (Lario et al., 2006; ApJ 653, 1531-1544).
As solar cycle 24 progresses we expect a large number of SEP events that will
increase the statistical significance of the results of the study.
This item was contributed by
David Lario and Edmond C. Roelof of the Johns
Hopkins University Applied Physics Laboratory.
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Last modified 28 Feb 2012.