A New Look at the Heliosphere and Solar Modulation
F.B. McDonald1, A.C. Cummings2, B.C. Heikkila3, N. Lal3, D.V. Reames1, E.C. Stone2, W.R. Webber4
1 Institute
for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA
2 California Institute of Technology, Pasadena, CA, USA
3 NASA/Goddard Space Flight Center, Greenbelt, MD, USA
4 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA
ACE / SOHO / STEREO / WIND
June 2010
Galactic Cosmic Ray Modulation
Cycle 23 Solar Minimum → Onset of Cycle 24
• Probes the large scale structure and dynamics of our heliosphere
• Cosmic Ray Modulation is dominated by the Sun
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Level of solar activity – (CME’s)
Tilt angle of the heliospheric neutral current sheet
Velocity (and density) of Solar Wind
Strength of IP magnetic field
Over the modern era (1951 – present) GCR intensity is at its lowest level over the past 1000 years
V1, V2 in the heliosheath will provide valuable insight into what is happening!
The deep, continuing solar minimum of cycle 23/24 provides an unprecedented opportunity for
modulation studies:
• To quantify the effects of reducing the strength of the Interplanetary Magnetic Field and of transit
changes in the current sheet tilt angle.
•
Better understanding of unusual epochs in the past such as the Sporer and Maunder minima.
Time Scales
• 11 year solar activity – dominated by level of solar activity – effects of ICMEs (reasonable
correlation with sunspot numbers)
• Heliomagnetic (22 years): very different time histories at 1 AU for qA> 0 minima (when particles
drift in over the solar poles and out along the current sheet. Flow pattern reversed for qA<0 (odd
cycle)
• q A<0 at 1 AU enhancement of GCRs > ~ 1 GV
Suppression of GCRs and ACRs < 1 GV
Cosmic Ray Drifts in the Heliosphere (qA > 0)
1 AU Solar Min. Energy Spectra (Cycles 19 – 22)
IPB is 28% below the previous minima of
cycles 19-22.
Heliospheric neutral current sheet approaches
minimum value at a slower rate than
previously observed.
Wang, Robbrecht and Sheely (2009) have
pointed out that these effects appear to be
related – the weaker polar fields result in the
apparent refusal of the heliospheric current
sheet to flatten closer to the equator.
Comparison of ACE, WIND and NM Data (2000 – 2010.3) with
IMP-8 Data Transposed from 20 Years Earlier
Table 1
Comparison of Cycle 21 and 22 with 23/24
Cycle 23/24
Cycle 21
Hermanus N.M (Rc = 4.9 GV)
SANAE NM Rate (Rc = 0.8 GV)
270 – 450 MeV/n GCR Fe
196 MeV/n GCR He
10 – 18 MeV/n ACR 0
10 – 18 MeV/n GCR C
Ulysses KET 2.5 GV electrons
Ulysses KET 2.5 GV protons
Russian Murmansk Balloon
Flights (Rc = 0.6 GV)
10-21 MeV/n ACR He
0.7%
0.96%
Cycle 23/24
Cycle 22
----15%
2.8%
3.6%
19.4± 2%
17±1.5 %
- 40 ± 3%
- 11 ± 1%
~ 30%
~0
13%
13%
15%
30 ±2 %
-20±4 %
The 23/24 Solar minimum intensities were from the end of 2009 except Ulysses
KET (end of 2008), the Murmansk Balloon Flights (April 2009) and ACE Iron
(2009.3)
Cosmic Ray response to 5° transient increase in the tilt angle of heliospheric neutral current sheet.
% of change from solar maximum to minimum
SANAE 4NMD
0.45%
3.1%
200 MeV/n GCR He
3.5%
0.7%
8-18 MeV ACR O
40%
0.2%
GCR He and H in the Heliosheath
• Radial Gradients < 0.2 %/AU over 2005-2009.2
• The V2 decrease starting in 2009.2 is not understood
Voyager-1
150 - 380 MeV/n GCR He
145 – 244 MeV/n GCR He
180 - 350 MeV GCR H
30 – 56 MeV/n ACR He
6 - 14 MeV GCR E
2006.14 – 2008.92
7.4
9.6
15.5
3.5
75.0
%/Year
%/Year
%/Year
%/Year
%/Year
% Below Webber/Higbie LIS
21%
30%
44%
Regression
Cosmic Ray Modulation
1 AU Comparison
Ken McCracken, 2007 ICRC
RESPONSE FUNCTION
MODEL HELIOSPHERE
a) 10 MeV Galactic Cosmic Rays are increasing
at a rate of 75%/yr.
b) No radial intensity gradient for 150-380
MeV/n GCR Helium (0.1 ± 0.2%/AU)
between V1 and V2.
c) Strong correlation between intensity increases
of Cosmic Ray 10 MeV Electrons and 265
MeV/n He.
d) There is expected to be a strong magnetic
barrier at the front and flanks of the
heliopause (The Axford-Cranfield effect).
Two Possible Solutions
i.
The heliopause is highly permeable to
energetic particles for reasons that are not
understood at this time
ii. The GCR Electrons and Medium energy ions
(< 350 MeV/n) gain entry into the heliosheath
and the heliosphere through the heliotail
region. Stay Tuned.
DISCUSSION and CONCLUSIONS
The ongoing period of very low solar activity has a multi-faceted effect on the modulation process.
• The increase in 135 - 250 MeV/n GCR He appears to be strongly related to the decrease in the
heliospheric magnetic field.
• The tilt angle changes of the heliospheric current sheet plays a major role for ACR O and for
Neutron Monitors.
• The lower IPB field will have a significant effect through-out the heliosphere. At the Voyagers in
the heliosheath there is an on-going increase of 265 MeV/n He (7.4%/year) and H (15.5%/year)
that is temporal and not spatial in nature. Since drift effects are not expected to be important in the
heliosheath, this increase is probably related to the lower B field.
• The lower solar wind velocity and pressure will have an effect on the dimensions of the
heliosphere and probably on the particle diffusion coefficients.
DISCUSSION and CONCLUSIONS
• Over the last 1000 years there have been previous epochs of low solar activity that have resulted
in significant increases in the GCR intensity. As measured by archival data from 10Be in polar ice
cores and 10C in tree rings.
• Caballero-Lopez et. al. modeled the cosmic ray intensity variations from 850-2000 AD by
varying the strength of the heliospheric magnetic field. At the Maunder Minimum a value as low
as 2nT was required for certain periods. Reductions in the solar wind speed and density could lead
to larger values of IPB for those periods.
• The current Quiet Sun period should provide insight into the changes that occurred on the Sun and
in the IP medium over those very unusual earlier periods. It is especially important to get
contemporaneous measurements of 10Be.