Transcript Solar Energetic Particle and Cosmic Ray Cutoffs During

Solar Energetic Particle and Cosmic Ray Cutoffs During Geomagnetic Storms II
B. T. Kress, Dartmouth College, Hanover, NH
4) Case Studies of Cutoff Variations During Storms:
1) Introduction
3) Geomagnetic cutoffs
The Earth's magnetic field usually shields latitudes below
~60o from direct penetration by solar energetic particles
(SEPs). During geomagnetic storms, a suppression in
geomagnetic shielding can lower the cutoff latitude up to
~15o, increasing the area of the polar cap region to which
particles have access by a factor of two to three
(Labrador et al., 2002). It has been shown that this
reduction in geomagnetic shielding is correlated with the
depression and recovery of the disturbance storm time
(Dst) index (Leske et al., 2001), and can thus be
attributed to a reduction in field strength in the inner
magnetosphere due to ring current buildup. Observations
and model results show that changes in solar wind
conditions may also significantly modify SEP cutoffs on a
timescale of minutes well before the main phase of a
storm (Kress et al., 2004); however, the effect of solar
wind dynamic pressure and IMF on the Earth’s energetic
particle cutoffs remains poorly understood. In several
case studies we here examine the role of changes in
solar wind conditions on SAMPEX energetic particle
cutoffs. In future work, a systematic comparison will be
made between SAMPEX observations and SEP cutoffs,
modeled by computing energetic particle trajectories in
magnetospheric model fields from the coupled CISM
(Center for Integrated Space Weather Modeling) codes.
The envelope of the green trace in Figure 2 shows the
intensity of SEPs in the solar wind (measured in the polar
cap regions). The rapid variations are due to SAMPEX’s ~90
min. orbital period (in low Earth orbit), during which it passes
through the North and South polar cap regions. While
SAMPEX is at lower latitudes the energetic ion fluxes are
cut off due to geomagnetic shielding.
SEP event
Figure 2. SAMPEX 19-27 MeV protons during an SEP event
In Figure 3 we expand the time scale of Figure 2 to see just a
few North and South polar cap passes. In this work, the
cutoff latitude is taken to be where the SEP flux falls below
½ its mean polar cap value, shown by the black trace.
2) Background
Average polar cap flux
Using conservation of energy and the azimuthal
component of the generalized momentum of a charged
particle in a dipole magnetic field, Störmer (1955) showed
the existence forbidden and allowed regions for particle
orbits.
r
Summary
5) A Global View
SAA
4/17 6:09 UT
South cap
North cap
South cap…
particle data. Visualization tools have also been developed, for studying and
comparing variations in Geomagnetic cutoffs.
• During severe geomagnetic storms the cutoff latitude typically varies up or down
~10o between 50o and 70o magnetic latitude.
Mq
cos 2 
mvc 1  1  cos 3 
South exit
South enter
(need to modify search so we are here)
• The relationship between geomagnetic cutoff and solar wind dynamic pressure is
complex, with the cutoff latitude sometimes increasing and sometimes decreasing
with an increase in solar wind dynamic pressure. Future study is warranted.
Cutoff latitude on Earth’s surface
Acknowledgments
Figure 3. Expanding time scale in Figure 2, several polar cap crossings are shown
r
M
• An algorithm has been developed for extracting cutoffs from SAMPEX energetic
Figure 4 shows cutoff latitude vs. time for the 18-19 Apr 2001
event. Note also the appearance of the SAA near ~20o Lat.
λ
SAMPEX/PET energetic particle data is provided by Glenn Mason (JHU/APL); ACE
Magnetic Field data is from N. Ness (Bartol Research Institute); ACE/SWEPAM
Solar Wind data is from D. J. McComas (SWRI); Dst index from the WDC-C2
KYOTO Dst index service. This material is based upon work supported in part by
the STC Program of the National Science Foundation under Agreement Number
ATM-0120950. Additional funding is from NASA LWS grant NAG5-12202.
Forbidden
Allowed
Figure 1.
where M is the dipole moment, λ is the latitude and r is
the radial distance from the center of the dipole. Note that
for a constant dipole moment M, the size of the shielded
region is determined by the rigidity (mvc/q) of the particle.
As the rigidity is increased, the size of the forbidden
region becomes smaller and the cutoff latitude is lower.
Although Störmer's analytic result is derived in a pure
dipole, well defined cutoffs are observed in geospace.
The intersection between the boundary of the forbidden
region and the Earth’s surface is at the cutoff latitude.
References
4/18 6:08 UT
Kress, B. K., M. K. Hudson, K. L. Perry and P. L. Slocum, Dynamic Modeling of
Geomagnetic Cutoff for the 23-24 November 2001 Solar Energetic Particle Event,
Geophys. Res. Lett., 31, L04808, 2004.
SAA
Labrador, A. W., R. A. Leske, S. Kanekal, B. Klecker, M. Looper, J. Mazur and R. A.
Mewaldt, SAMPEX Measurements of Geomagnetic-Cutoffs during the April 21,
2002 Solar Energetic Particle Event, Storms 2 Workshop, APL August 19-21, 2003
Cutoff
Figure 4. SAMPEX energetic particle cutoffs vs. time.
Figure 5. Red stars indicate observed cutoff latitudes for 1927 MeV protons as SAMPEX exits the North polar cap region
on 17 & 18 Apr 2001. Blue contours show several IGRF
L-shells.
Leske, R. A., R. A. Mewaldt, and E.C. Stone, Observations of geomagnetic cutoff
variations during solar energetic particle events and implications for the radiation
environment at the Space Station, J. Geophys. Res. 106, 30,011--30,022, 2001.
Störmer, C., "The Polar Aurora", Oxford University Press, 1950.